WO2023115388A1 - 正极活性材料及其制备方法、正极极片、二次电池、电池模块、电池包和用电装置 - Google Patents

正极活性材料及其制备方法、正极极片、二次电池、电池模块、电池包和用电装置 Download PDF

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WO2023115388A1
WO2023115388A1 PCT/CN2021/140462 CN2021140462W WO2023115388A1 WO 2023115388 A1 WO2023115388 A1 WO 2023115388A1 CN 2021140462 W CN2021140462 W CN 2021140462W WO 2023115388 A1 WO2023115388 A1 WO 2023115388A1
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Prior art keywords
positive electrode
active material
electrode active
core
optionally
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PCT/CN2021/140462
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English (en)
French (fr)
Inventor
蒋耀
欧阳楚英
张欣欣
邓斌
王志强
徐波
袁天赐
刘少军
陈尚栋
赵旭山
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宁德时代新能源科技股份有限公司
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Priority to PCT/CN2021/140462 priority Critical patent/WO2023115388A1/zh
Priority to CN202180098462.XA priority patent/CN117378059A/zh
Priority to KR1020247008222A priority patent/KR20240048003A/ko
Priority to JP2023543271A priority patent/JP2024505446A/ja
Priority to AU2022371736A priority patent/AU2022371736A1/en
Priority to PCT/CN2022/126838 priority patent/WO2023066394A1/zh
Priority to CN202280013385.8A priority patent/CN117121236A/zh
Priority to KR1020247007998A priority patent/KR20240046889A/ko
Priority to PCT/CN2022/126778 priority patent/WO2023066386A1/zh
Priority to CN202280013384.3A priority patent/CN116964781A/zh
Priority to EP22882990.9A priority patent/EP4261946A1/en
Priority to CN202280007637.6A priority patent/CN116547835A/zh
Priority to KR1020237024611A priority patent/KR20230122108A/ko
Priority to PCT/CN2022/126829 priority patent/WO2023066393A1/zh
Publication of WO2023115388A1 publication Critical patent/WO2023115388A1/zh
Priority to US18/351,925 priority patent/US20230361296A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present application relates to the technical field of lithium batteries, in particular to a positive electrode active material and a preparation method thereof, a positive electrode sheet containing the same, a secondary battery, a battery module, a battery pack, and an electrical device.
  • lithium-ion batteries are widely used in various large-scale power devices, energy storage systems and various consumer products due to their excellent electrochemical performance, no memory effect, and low environmental pollution.
  • lithium manganese phosphate positive electrode active material has the advantages of high working voltage, wide range of raw material sources and less environmental pollution, and is considered to be a positive electrode active material that is expected to replace lithium iron phosphate and become a power lithium-ion battery.
  • the present application is carried out in view of the above-mentioned problems, and its purpose is to provide a new type of doped manganese phosphate lithium positive electrode active material with a core-shell structure, so that the secondary battery using the positive electrode active material has a higher grammage. Capacity, good cycle performance and safety performance.
  • the first aspect of the present application provides a positive electrode active material with a core-shell structure, which includes an inner core and a shell covering the inner core,
  • the chemical formula of the inner core is Li 1+x Mn 1-y A y P 1-z R z O 4 , where x is any value within the range of -0.100-0.100, and y is any value within the range of 0.001-0.500 , z is any value in the range of 0.001-0.100, and the A is selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb , one or more elements in Nb and Ge, can be selected as one or more elements in Fe, Ti, V, Ni, Co and Mg, and the R is selected from B, Si, N and S One or more elements, optionally, the R is an element selected from B, Si, N and S;
  • the shell includes a first cladding layer covering the inner core, a second cladding layer covering the first cladding layer, and a third cladding layer covering the second cladding layer, wherein,
  • the first cladding layer includes crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , wherein,
  • the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li a MP 2 O 7 or M b (P 2 O 7 ) c maintains electrical neutrality,
  • M in the crystalline pyrophosphate Li a MP 2 O 7 and M b (P 2 O 7 ) c are each independently selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, One or more elements in Nb or Al,
  • the second cladding layer includes crystalline phosphate XPO 4 , wherein X is one selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al or multiple elements;
  • the third cladding layer is carbon.
  • the crystalline state means that the degree of crystallinity is above 50%, that is, 50%-100%. A crystallinity of less than 50% is called a glassy state.
  • the crystalline pyrophosphate and crystalline phosphate salts described herein have a degree of crystallinity ranging from 50% to 100%. Pyrophosphate and phosphate with a certain degree of crystallinity are not only conducive to giving full play to the ability of the pyrophosphate coating layer to hinder the dissolution of manganese and the excellent ability of the phosphate coating layer to conduct lithium ions, and to reduce the interface side reactions.
  • the phosphate coating layer and the phosphate coating layer can perform better lattice matching, so that a tight combination between the coating layer and the coating layer can be achieved.
  • the present application provides a new type of lithium manganese phosphate by doping the element A at the manganese position and doping the element R at the phosphorus position to obtain a doped lithium manganese phosphate inner core and sequentially performing three-layer coating on the surface of the inner core.
  • the lithium manganese phosphate positive electrode active material with a core-shell structure, and applying the positive electrode active material to a secondary battery can significantly improve the high-temperature cycle performance, cycle stability and high-temperature storage performance of the secondary battery.
  • the interplanar distance of the crystalline pyrophosphate in the first cladding layer is in the range of 0.293-0.470 nm, and the included angle of the crystal direction (111) is in the range of 18.00°-32.00°;
  • the crystal plane distance of the crystalline phosphate in the secondary cladding layer is in the range of 0.244-0.425nm, and the angle range of the crystal direction (111) is in the range of 20.00°-37.00°.
  • Both the first cladding layer and the second cladding layer in the positive electrode active material described in the present application use crystalline substances, and their interplanar spacing and included angle ranges are within the above-mentioned ranges. As a result, the impurity phase in the cladding layer can be effectively avoided, thereby improving the gram capacity, cycle performance and rate performance of the material.
  • the ratio of y to 1-y is 1:10 to 1:1, optionally 1:4 to 1:1.
  • the cycle performance and rate performance of the secondary battery are further improved.
  • the ratio of z to 1-z is 1:999 to 1:9, optionally 1:499 to 1:249.
  • the cycle performance and rate performance of the secondary battery are further improved.
  • the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon, and optionally, the molar ratio of SP2 carbon to SP3 carbon is in the range of 0.1-10 Any value, can be any value in the range of 2.0-3.0.
  • the overall performance of the secondary battery is improved by limiting the molar ratio of the SP2 form carbon to the SP3 form carbon within the above range.
  • the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more optionally greater than 0 and less than or equal to equal to 2% by weight, based on the weight of the inner core; and/or
  • the coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more optionally 2-4% by weight, based on the content of the inner core weighing scale; and/or
  • the coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more optionally greater than 0 and less than or equal to 2% by weight, based on The weight of the kernel.
  • the coating amount of the three-layer coating layer is preferably within the above range, so that the inner core can be fully coated without sacrificing the positive electrode activity. Under the premise of the gram capacity of the material, the kinetic performance and safety performance of the secondary battery are further improved.
  • the thickness of the first cladding layer is 1-10 nm; and/or
  • the thickness of the second cladding layer is 2-15nm; and/or
  • the thickness of the third cladding layer is 2-25nm.
  • the thickness of the first cladding layer when the thickness of the first cladding layer is in the range of 1-10nm, it can avoid the adverse effect on the kinetic properties of the material that may be caused when it is too thick, and can avoid that the transition metal cannot be effectively blocked when it is too thin.
  • the problem of ion migration when the thickness of the first cladding layer is in the range of 1-10nm, it can avoid the adverse effect on the kinetic properties of the material that may be caused when it is too thick, and can avoid that the transition metal cannot be effectively blocked when it is too thin.
  • the thickness of the second coating layer is in the range of 2-15nm, the surface structure of the second coating layer is stable, and the side reaction with the electrolyte is small, so the side reaction at the interface can be effectively reduced, thereby improving the secondary High temperature performance of the battery.
  • the electrical conductivity of the material can be improved and the compacted density performance of the battery electrode sheet prepared by using the positive electrode active material can be improved.
  • the manganese element content is in the range of 10% by weight to 35% by weight, optionally in the range of 15% by weight to 30% by weight, more
  • the content of phosphorus element is within the range of 12% by weight to 25% by weight, and optionally within the range of 15% by weight to 20% by weight, the weight ratio range of manganese and phosphorus elements 0.90-1.25, optional 0.95-1.20.
  • the content of the manganese element is within the above range, which can effectively avoid the deterioration of the material structure stability and the decrease in density that may be caused if the content of the manganese element is too large. problems, thereby improving the performance of the cycle, storage and compaction density of the secondary battery; and can avoid problems such as low voltage platform that may be caused if the manganese content is too small, thereby improving the energy density of the secondary battery.
  • the content of the phosphorus element is within the above range, which can effectively avoid the following situation: if the content of the phosphorus element is too large, it may cause excessive covalency of P-O. Strongly affect the conduction of small polarons, thereby affecting the electrical conductivity of the material; if the content of phosphorus element is too small, it may make the pyrophosphate in the inner core, the first cladding layer and/or the second The stability of the phosphate lattice structure in the cladding layer decreases, thereby affecting the overall stability of the material.
  • the weight ratio of the manganese element to the phosphorus element is within the above range, which can effectively avoid the following situation: if the weight ratio is too large, it may cause transition metal dissolution If the weight ratio increases, it will affect the stability of the material and the cycle and storage performance of the secondary battery; if the weight ratio is too small, the discharge voltage platform of the material may decrease, thereby reducing the energy density of the secondary battery.
  • the lattice change rate of the positive electrode active material having a core-shell structure before and after complete lithium extraction is 4% or less, optionally 3.8% or less, more preferably 2.0-3.8%.
  • the positive electrode active material with a core-shell structure described in the present application can achieve a lattice change rate of less than 4% before and after lithium intercalation and deintercalation. Therefore, the use of the positive electrode active material can improve the gram capacity and rate performance of the secondary battery.
  • the Li/Mn antisite defect concentration of the positive electrode active material having a core-shell structure is less than 4%, optionally less than 2.2%, more preferably 1.5-2.2%. With the Li/Mn antisite defect concentration within the above range, Mn 2+ can be prevented from hindering the transport of Li + , and at the same time, the gram capacity and rate performance of the positive electrode active material can be improved.
  • the positive electrode active material having a core-shell structure has a compacted density of 2.2 g/cm or more at 3T (ton), optionally 2.2 g/cm or more and 2.8 g/cm 3 or less.
  • increasing the compaction density increases the weight of the active material per unit volume, which is more conducive to increasing the volumetric energy density of the secondary battery.
  • the surface oxygen valence state of the positive electrode active material having a core-shell structure is below -1.90, optionally between -1.90 and -1.98. Therefore, by limiting the surface oxygen valence state of the positive electrode active material within the above range, the interface side reaction between the positive electrode material and the electrolyte can be reduced, thereby improving the performance of the battery cell cycle, high temperature storage and gas production.
  • a second aspect of the present application provides a method for preparing a positive electrode active material, comprising the following steps:
  • the chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O 4 , where x is any value in the range of -0.100-0.100, and y is in the range of 0.001-0.500 Any value in the range, z is any value in the range of 0.001-0.100, and the A is selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, One or more elements of Ga, Sn, Sb, Nb and Ge, optionally one or more elements of Fe, Ti, V, Ni, Co and Mg, the R is selected from B, Si , one or more elements in N and S, optionally, the R is an element selected from B, Si, N and S;
  • Coating step Li a MP 2 O 7 and/or M b (P 2 O 7 ) c and XPO 4 suspensions are provided respectively, the core material is added to the above suspensions and mixed, and the positive electrode is obtained by sintering Active material, wherein 0 ⁇ a ⁇ 2, 1 ⁇ b ⁇ 4, 1 ⁇ c ⁇ 6, the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li a MP 2 O 7 or M b (P 2 O 7 ) c maintains electrical neutrality; the M is each independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al ; The X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al;
  • the positive electrode active material has a core-shell structure, which includes the inner core and a shell covering the inner core, and the shell includes a first cladding layer covering the inner core, and a shell covering the first cladding layer.
  • the second coating layer includes crystalline phosphate XPO 4
  • the third coating layer is carbon.
  • the step of providing core material comprises the following steps:
  • Step (1) mixing and stirring a manganese source, a dopant of element A, and an acid in a container to obtain manganese salt particles doped with element A;
  • Step (2) Mix the manganese salt particles doped with element A with the dopant of lithium source, phosphorus source and element R in a solvent to obtain a slurry, and sinter under the protection of an inert gas atmosphere to obtain a doped
  • the inner core doped with element A and element R is Li 1+x Mn 1-y A y P 1-z R z O 4 , wherein x is between -0.100- Any value within the range of 0.100, y is any value within the range of 0.001-0.500, z is any value within the range of 0.001-0.100
  • the A is selected from Zn, Al, Na, K, Mg, Mo, W , one or more elements of Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and one or more of Fe, Ti, V, Ni, Co and Mg Multiple elements
  • the R is one or more elements selected from B, Si, N and S, optionally, the
  • the dopant of the element A is selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb
  • elemental substances carbonates, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of one or more elements in Ge.
  • the dopant of the element R is one or more elements selected from B, Si, N and S, respectively inorganic acid, subacid, organic acid, sulfate, chloride, One or more of nitrates, organic acid salts, oxides, hydroxides;
  • the step (1) is mixed at a temperature of 20-120°C, optionally 40-120°C; and/or
  • the stirring in the step (1) is carried out at 400-700rpm for 1-9h, optionally for 3-7h.
  • the prepared inner core and the positive electrode active material produced by it will have fewer lattice defects, which is conducive to inhibiting the dissolution of manganese and reducing the interaction between the positive electrode active material and the electrolysis process. Interfacial side reactions of the liquid, thereby improving the cycle performance and safety performance of the secondary battery.
  • the step (2) is mixed at a temperature of 20-120°C, optionally 40-120°C, for 1-12h.
  • the coating step includes:
  • the first coating step the source of the element M, the phosphorus source and the acid and optionally the lithium source are used to obtain the suspension of the first coating layer; the inner core obtained in the inner core step is combined with the first coating obtained in the first coating step.
  • the coating layer suspension is fully mixed, dried, and then sintered to obtain the material coated with the first coating layer;
  • the second coating step dissolving the source of element X, phosphorus source and acid in a solvent to obtain a suspension of the second coating layer; combining the material coated with the first coating layer obtained in the first coating step with The suspension of the second coating layer obtained in the second coating step is fully mixed, dried, and then sintered to obtain a material coated with two coating layers;
  • the third coating step dissolve the carbon source in the solvent, fully dissolve to obtain the third coating layer solution; then add the materials coated with the two coating layers obtained in the second coating step to the third coating layer solution, mixed uniformly, dried, and then sintered to obtain a material covered by three layers of coating layers, that is, a positive electrode active material.
  • the source of the element M is the elemental substance of one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al, carbonic acid One or more of salts, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides.
  • the source of the element X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al.
  • each source of elements A, R, M, and X depends on the target doping amount, and the ratio of the amounts of lithium source, manganese source and phosphorus source conforms to the stoichiometric ratio.
  • control the pH of the solution dissolved with the source of element M, phosphorus source and acid, and optionally lithium source to be 3.5-6.5, then stir and react for 1-5h, and then The solution is heated to 50-120° C. and maintained at this temperature for 2-10 hours, and/or sintering is carried out at 650-800° C. for 2-6 hours.
  • the gram capacity and its cycle and high temperature storage performance, gram capacity and rate performance of the secondary battery prepared using the positive electrode active material can be guaranteed or even improved.
  • the second coating step after dissolving the source of element X, the phosphorus source and the acid in the solvent, stirring and reacting for 1-10 hours, then raising the temperature of the solution to 60-150°C, and This temperature is maintained for 2-10 hours, and/or, sintering is carried out at 500-700° C. for 6-10 hours.
  • the following situation can be avoided by selecting the reaction temperature and the reaction time as described above: when the reaction temperature is too low, the reaction cannot take place Or the reaction rate is slow; when the temperature is too high, the product decomposes or forms a heterogeneous phase; when the reaction time is too long, the product particle size is large, which may increase the time and difficulty of the subsequent process; when the reaction time is too short, the reaction is incomplete , less product is obtained.
  • the sintering in the third coating step is carried out at 700-800° C. for 6-10 hours.
  • the preparation method of the positive electrode active material described in the application has wide sources of raw materials, low cost and simple process, which is conducive to realizing industrialization.
  • the third aspect of the present application provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer includes the core- A positive electrode active material with a shell structure or a positive electrode active material prepared by the method described in the second aspect of the present application, and the content of the positive electrode active material in the positive electrode film layer is 90-99.5% by weight, preferably 95-99.5% by weight % by weight, based on the total weight of the positive electrode film layer.
  • the positive electrode sheet described in the application is used in batteries, which improves the high-temperature cycle performance, rate performance and safety performance of the secondary battery.
  • a fourth aspect of the present application provides a secondary battery, which includes the positive electrode sheet described in the third aspect of the present application.
  • the secondary battery provided by the present application has high rate performance, good cycle performance, good safety performance and large battery capacity.
  • a fifth aspect of the present application provides a battery module including the secondary battery described in the fourth aspect of the present application.
  • a sixth aspect of the present application provides a battery pack, which includes the battery module described in the fifth aspect of the present application.
  • the seventh aspect of the present application provides an electric device, which includes the secondary battery described in the fourth aspect of the present application, the battery module described in the fifth aspect of the present application, or the battery pack described in the sixth aspect of the present application. at least one.
  • the positive electrode sheet, secondary battery, battery module, battery pack and electrical device described in the present application include the positive electrode active material described in the present application, so they have high gram capacity, good cycle performance and safety performance.
  • FIG. 1 is a schematic diagram of a positive electrode active material having a core-shell structure described in the present application.
  • FIG. 2 is a schematic diagram of a secondary battery according to an embodiment of the present application.
  • FIG. 3 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG. 2 .
  • FIG. 4 is a schematic diagram of a battery module according to an embodiment of the present application.
  • FIG. 5 is a schematic diagram of a battery pack according to an embodiment of the present application.
  • FIG. 6 is an exploded view of the battery pack according to one embodiment of the present application shown in FIG. 5 .
  • FIG. 7 is a schematic diagram of an electrical device in which a secondary battery is used as a power source according to an embodiment of the present application.
  • ranges disclosed herein are defined in terms of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit that define the boundaries of the specified range. Ranges defined in this manner may be inclusive or exclusive and may be combined arbitrarily, ie any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are contemplated. Additionally, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, the following ranges are all expected: 1-3, 1-4, 1-5, 2- 3, 2-4 and 2-5.
  • the numerical range "a-b” represents an abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers.
  • the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article, and "0-5" is only an abbreviated representation of the combination of these values.
  • a certain parameter is an integer ⁇ 2
  • the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed in sequence, and may also include steps (b) and (a) performed in sequence.
  • step (c) means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c) , may also include steps (a), (c) and (b), may also include steps (c), (a) and (b) and so on.
  • the “comprising” and “comprising” mentioned in this application mean open or closed.
  • the “comprising” and “comprising” may mean that other components not listed may be included or included, or only listed components may be included or included.
  • the term "or” is inclusive unless otherwise stated.
  • the phrase "A or B” means “A, B, or both A and B.” More specifically, the condition "A or B” is satisfied by either of the following: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists) ; or both A and B are true (or exist).
  • the term “coating layer” refers to a material layer coated on the lithium manganese phosphate core, and the material layer can completely or partially cover the lithium manganese phosphate core, and the use of “coating layer ” is just for convenience of description and is not intended to limit the present invention.
  • the term “thickness of the cladding layer” refers to the thickness of the material layer coated on the lithium manganese phosphate core in the radial direction of the lithium manganese phosphate core.
  • the inventors of the present application have found in actual operation that the existing lithium manganese phosphate positive electrode active material suffers from severe manganese dissolution during the deep charge and discharge process. Although there are attempts in the prior art to coat lithium manganese phosphate with lithium iron phosphate to reduce interfacial side reactions, this coating cannot prevent the dissolved manganese from continuing to migrate into the electrolyte. The dissolved manganese is reduced to metal manganese after migrating to the negative electrode.
  • the metal manganese produced in this way is equivalent to a "catalyst", which can catalyze the decomposition of the SEI film (solid electrolyte interphase, solid electrolyte interphase film) on the surface of the negative electrode and produce by-products; a part of the by-products is gas, so it will cause secondary battery damage.
  • another part of the by-product is deposited on the surface of the negative electrode, which will hinder the passage of lithium ions into and out of the negative electrode, causing the impedance of the secondary battery to increase, thereby affecting the kinetic performance of the secondary battery.
  • the electrolyte and the active lithium inside the battery are continuously consumed, which will have an irreversible impact on the capacity retention of the secondary battery.
  • the inventors found that by modifying lithium manganese phosphate and coating lithium manganese phosphate with multiple layers, a new type of positive electrode active material with a core-shell structure can be obtained, and the positive electrode active material can Significantly reduced manganese leaching and reduced lattice change rate are realized, and when used in secondary batteries, the cycle performance, rate performance, and safety performance of the battery can be improved and the capacity of the battery can be increased.
  • the first aspect of the present application provides a novel positive electrode active material with a core-shell structure, which includes an inner core and a shell covering the inner core, and the chemical formula of the inner core is Li 1+x Mn 1-y A y P 1-z R z O 4 , where x is any value within the range of -0.100-0.100, y is any value within the range of 0.001-0.500, z is any value within the range of 0.001-0.100, the A One or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, can be One or more elements of Fe, Ti, V, Ni, Co and Mg, said R is one or more elements selected from B, Si, N and S, optionally, said R is selected from One element from B, Si, N and S;
  • the shell includes a first cladding layer covering the inner core, a second cladding layer covering the first cladding layer, and a third cladding layer covering the second cladding layer, wherein,
  • the first cladding layer includes crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , wherein,
  • the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li a MP 2 O 7 or M b (P 2 O 7 ) c maintains electrical neutrality,
  • M in the crystalline pyrophosphate Li a MP 2 O 7 and M b (P 2 O 7 ) c are each independently selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, One or more elements in Nb or Al,
  • the second cladding layer comprises crystalline phosphate XPO 4 , wherein,
  • the X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al;
  • the third cladding layer is carbon.
  • the above-mentioned limitation on the numerical range of y is not only a limitation on the stoichiometric number of each element as A, but also a limitation on each element as A.
  • Limitation of sum of stoichiometric numbers of elements For example, when A is two or more elements A1, A2...An, the stoichiometric numbers y1, y2...yn of A1, A2...An each need to fall within the numerical range defined by the application for y, and y1 The sum of , y2 ... yn also needs to fall within this value range.
  • the limitation of the numerical range of the stoichiometric number of R in this application also has the above meaning.
  • A when A is selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge
  • Q, D, E, K are each independently selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb , one of Nb and Ge, optionally, at least one of Q, D, E, K is Fe.
  • one of n1, n2, n3, n4 is zero, and the rest are not zero; more optionally, two of n1, n2, n3, n4 are zero, and the rest are not zero; also optionally, Three of n1, n2, n3, n4 are zero, and the rest are not zero.
  • the inner core Li 1+x Mn 1-y A y P 1-z R z O 4 it is advantageous to dope one, two, three or four of the above-mentioned A elements at the manganese site.
  • Doping one, two or three of the above-mentioned A elements in addition, it is advantageous to dope one or two R elements at the phosphorus site, which is conducive to uniform distribution of doping elements.
  • the size of x is affected by the valence size of A and R and the size of y and z, so as to ensure that the whole system is electrically neutral sex. If the value of x is too small, the lithium content of the entire inner core system will decrease, which will affect the gram capacity of the material. The value of y will limit the total amount of all doping elements. If y is too small, that is, the amount of doping is too small, the doping elements will have no effect. If y exceeds 0.5, the Mn content in the system will be less, which will affect the quality of the material. voltage platform.
  • the R element is doped at the position of P. Since the PO tetrahedron is relatively stable, but too much z value will affect the stability of the material, so the z value is limited to 0.001-0.100.
  • the cathode active material of the present application can improve the gram capacity, cycle performance and safety performance of the secondary battery.
  • the lithium manganese phosphate positive electrode active material of the present application has a core-shell structure, wherein by doping the manganese site and phosphorus site of the lithium manganese phosphate core with element A and element R respectively, not only can effectively reduce The dissolution of manganese, thereby reducing the migration of manganese ions to the negative electrode, reducing the consumption of electrolyte due to the decomposition of the SEI film, improving the cycle performance and safety performance of the secondary battery, can also promote the adjustment of Mn-O bonds, and reduce the migration barrier of lithium ions.
  • the core by coating the core with the first coating layer including crystalline pyrophosphate, the migration resistance of manganese can be further increased, its dissolution can be reduced, and the content of lithium impurities on the surface can be reduced , reduce the contact between the inner core and the electrolyte, thereby reducing the side reaction at the interface, reducing gas production, and improving the high-temperature storage performance, cycle performance and safety performance of the secondary battery; by further coating the crystalline phosphate with excellent ability to conduct lithium ions
  • the coating layer can effectively reduce the interface side reactions on the surface of the positive electrode active material, thereby improving the high-temperature cycle and storage performance of the secondary battery; by further coating the carbon layer as the third coating layer, the secondary battery can be further improved. safety performance and dynamic performance.
  • the element A doped at the manganese position of lithium manganese phosphate also helps to reduce the lattice change rate of lithium manganese phosphate in the process of lithium manganese phosphate intercalation, and improves the performance of the lithium manganese phosphate positive electrode material.
  • Structural stability greatly reducing the dissolution of manganese and reducing the oxygen activity on the particle surface; the element R doped at the phosphorus site also helps to change the difficulty of changing the Mn-O bond length, thereby improving electronic conductance and reducing lithium ion migration Potential barrier, promote the migration of lithium ions, and improve the rate performance of the secondary battery.
  • the entire inner core system maintains electrical neutrality, which can ensure that the defects and impurity phases in the positive electrode active material are as small as possible. If there is an excess of transition metals (such as manganese) in the positive electrode active material, since the structure of the material system itself is relatively stable, the excess transition metals are likely to be precipitated in the form of simple substances, or form impurity phases inside the lattice to maintain electrical neutrality. Sex can make such impurity as little as possible. In addition, ensuring the electrical neutrality of the system can also generate lithium vacancies in the material in some cases, so that the kinetic performance of the material is better.
  • transition metals such as manganese
  • the average particle size range of the inner core prepared by the present application is 50-500nm, and the Dv50 is 200-300nm.
  • the primary particle size of the core is in the range of 50-500nm, and the Dv50 is 200-300nm. If the average particle diameter of the inner core is too large (more than 500nm), the gram capacity of the secondary battery using the material will be affected; if the average particle diameter of the inner core is too small, the specific surface area will be large and easy to agglomerate. Difficult to achieve uniform coating.
  • the median particle diameter Dv50 refers to the particle diameter corresponding to when the cumulative volume distribution percentage of the material reaches 50%.
  • the median particle size Dv50 of the material can be measured by laser diffraction particle size analysis. For example, with reference to the standard GB/T 19077-2016, a laser particle size analyzer (such as Malvern Master Size 3000) is used for measurement.
  • crystalline state means that the degree of crystallinity is above 50%, that is, 50%-100%. A crystallinity of less than 50% is called a glassy state.
  • the crystalline pyrophosphate and crystalline phosphate salts described herein have a degree of crystallinity ranging from 50% to 100%. Pyrophosphate and phosphate with a certain degree of crystallinity are not only conducive to giving full play to the ability of the pyrophosphate coating layer to hinder the dissolution of manganese and the excellent ability of the phosphate coating layer to conduct lithium ions, and to reduce the interface side reactions. Phosphate cladding and phosphate cladding enable better lattice matching, thereby enabling tighter bonding of the cladding layers.
  • the crystallinity of the first coating layer material crystalline pyrophosphate and the second coating layer material crystalline phosphate of the positive electrode active material can be tested by conventional technical means in this field, for example, by density method, infrared spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance absorption methods, and can also be tested by, for example, X-ray diffraction methods.
  • the specific X-ray diffraction method for testing the crystallinity of the first coating layer crystalline pyrophosphate and the second coating layer crystalline phosphate of the positive electrode active material may include the following steps:
  • the total scattering intensity by X-rays which is the sum of the scattering intensity of the entire space material, which is only related to the intensity of primary rays, the chemical structure of positive electrode active material powder, and the total number of electrons participating in the diffraction.
  • the quality is related to the order state of the sample; then the crystalline scattering and the non-crystalline scattering are separated from the diffraction pattern, and the crystallinity is the ratio of the crystalline partial scattering to the total scattering intensity.
  • the crystallinity of pyrophosphate and phosphate in the cladding layer can be adjusted, for example, by adjusting the process conditions of the sintering process, such as sintering temperature, sintering time, and the like.
  • the pyrophosphate as the first coating layer can effectively isolate the doped metal ions from the electrolyte.
  • the structure of crystalline pyrophosphate is stable, therefore, the coating of crystalline pyrophosphate can effectively inhibit the dissolution of transition metals and improve cycle performance.
  • the bond between the first cladding layer and the core is similar to a heterojunction, and the firmness of the bond is limited by the degree of lattice matching.
  • the lattice mismatch is below 5%, the lattice matching is better, and the two are easy to combine closely.
  • the tight combination can ensure that the coating layer will not fall off in the subsequent cycle process, which is beneficial to ensure the long-term stability of the material.
  • the measurement of the bonding degree between the first cladding layer and the core is mainly performed by calculating the mismatch degree of each lattice constant between the core and the cladding.
  • the matching degree between the inner core and the first cladding layer is improved compared with the undoped elements, and the distance between the inner core and the pyrophosphate cladding layer is can be brought together more closely.
  • the crystalline phosphate is selected as the second coating layer, firstly, because it has a higher degree of lattice matching with the first layer of coating crystalline pyrophosphate (the mismatch is only 3%); secondly, the phosphate
  • the stability of itself is better than that of pyrophosphate, and the use of it to coat pyrophosphate is beneficial to improve the stability of the material.
  • the structure of crystalline phosphate is very stable, and it has an excellent ability to conduct lithium ions. Therefore, coating with crystalline phosphate can effectively reduce the interface side reactions on the surface of the positive electrode active material, thereby improving the high temperature of the secondary battery. Cycling and storage performance.
  • the lattice matching mode between the second cladding layer and the first cladding layer, etc. is similar to the above-mentioned combination between the first cladding layer and the core.
  • the lattice mismatch is below 5%, the lattice matching is relatively strong. Well, the two are easy to combine tightly.
  • carbon is used as the third coating layer. Since the electrochemical reaction occurs in the application of the secondary battery, the participation of electrons is required. Therefore, in order to increase the electron transport between particles and the electron transport at different positions on the particles, it is possible to use a material with excellent conductivity. Carbon is used to coat the positive electrode active material. Carbon coating can effectively improve the electrical conductivity and desolvation ability of cathode active materials.
  • FIG. 1 is a schematic diagram of an ideal positive electrode active material with a three-layer cladding structure.
  • the innermost circle schematically represents the inner core, and the first cladding layer, the second cladding layer, and the third cladding layer are sequentially arranged from the inside to the outside. What this figure shows is the ideal state where each layer is completely covered. In practice, each layer of coating can be completely covered or partially covered.
  • the average particle diameter of the primary particles of the positive active material is in the range of 50-500 nm, and the volume median particle diameter Dv50 is in the range of 200-300 nm. Since the particles will be agglomerated, the actual measured secondary particle size after agglomeration may be 500-40000nm.
  • the particle size of the cathode active material affects the processing of the material and the compacted density performance of the electrode sheet.
  • the average particle size of the primary particles of the positive electrode active material is too small, which may cause particle agglomeration, difficulty in dispersing, and requires more bonding agent, resulting in poor brittleness of the pole piece; the average particle size of the primary particles of the positive electrode active material is too large, which may cause larger gaps between the particles and reduce the compacted density.
  • the lattice change rate and Mn dissolution of lithium manganese phosphate during lithium intercalation and deintercalation can be effectively suppressed, thereby improving the high-temperature cycle stability and high-temperature storage performance of the secondary battery.
  • the interplanar distance of the crystalline pyrophosphate in the first cladding layer is in the range of 0.293-0.470 nm, and the included angle of the crystal direction (111) is in the range of 18.00°-32.00°;
  • the crystal plane distance of the crystalline phosphate in the secondary cladding layer is in the range of 0.244-0.425nm, and the angle range of the crystal direction (111) is in the range of 20.00°-37.00°.
  • Both the first coating layer and the second coating layer in the positive electrode active material described in the present application use crystalline substances.
  • the crystalline pyrophosphate and crystalline phosphate in the cladding layer can be characterized by conventional technical means in the art, and can also be characterized, for example, by means of a transmission electron microscope (TEM). Under TEM, the inner core and the cladding layer can be distinguished by measuring the interplanar spacing.
  • TEM transmission electron microscope
  • the concrete test method of the interplanar spacing and the included angle of the crystalline pyrophosphate in the cladding layer and the crystalline phosphate can comprise the following steps:
  • interplanar spacing range of crystalline pyrophosphate The difference between the interplanar spacing range of crystalline pyrophosphate and the existence of crystalline phosphate can be directly judged by the value of interplanar spacing.
  • Crystalline pyrophosphate and crystalline phosphate within the range of the above-mentioned interplanar spacing and included angle can more effectively inhibit the lattice change rate and Mn dissolution of lithium manganese phosphate during lithium intercalation and deintercalation, thereby improving the performance of secondary batteries.
  • the ratio of y to 1-y is 1:10 to 1:1, optionally 1:4 to 1:1.
  • y represents the sum of the stoichiometric numbers of the Mn-site doping elements A.
  • the ratio of z to 1-z is 1:9 to 1:999, optionally 1:499 to 1:249.
  • z represents the sum of the stoichiometric numbers of the P-site doping elements R.
  • the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon, and optionally, the molar ratio of SP2 carbon to SP3 carbon is in the range of 0.1-10 Any value, can be any value in the range of 2.0-3.0.
  • the molar ratio of SP2 form carbon to SP3 form carbon can be about 0.1, about 0.2, about 03, about 0.4, about 0.5, about 0.6, about 0.7, about 0.8, about 0.9, about 1, About 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, or about 10, or within any range of any of the above values.
  • the comprehensive electrical performance of the secondary battery is improved.
  • the carbon in the coating layer is all amorphous SP3 If it is in the form of graphitized SP2, the conductivity is poor; although the conductivity is good, there are few lithium ion channels, which is not conducive to the deintercalation of lithium.
  • limiting the molar ratio of SP2 carbon to SP3 carbon within the above range can not only achieve good electrical conductivity, but also ensure the passage of lithium ions, which is beneficial to the realization of the function of the secondary battery and its cycle performance.
  • the mixing ratio of the SP2 form and the SP3 form of the carbon of the third cladding layer can be controlled by sintering conditions such as sintering temperature and sintering time.
  • sintering conditions such as sintering temperature and sintering time.
  • sucrose as the carbon source to prepare the third coating
  • subjecting the sucrose to pyrolysis at high temperature, deposited on the second coating while at high temperature will result in both SP3 and SP2 forms carbon coating.
  • the proportion of SP2 carbon and SP3 carbon can be adjusted by selecting pyrolysis conditions and sintering conditions.
  • the structure and characteristics of the carbon in the third cladding layer can be measured by Raman (Raman) spectrum, and the specific test method is as follows: by dividing the energy spectrum of the Raman test, Id/Ig (wherein Id is SP3 form carbon Peak intensity, Ig is the peak intensity of SP2 form carbon), thereby confirming the molar ratio of the two.
  • the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more optionally greater than 0 and less than or equal to equal to 2% by weight, based on the weight of the inner core; and/or
  • the coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more optionally 2-4% by weight, based on the content of the inner core weighing scale; and/or
  • the coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more optionally greater than 0 and less than or equal to 2% by weight, based on the inner core weight gauge.
  • the coating amount of each layer is not zero.
  • the coating amount of the three-layer coating layer is preferably within the above range, so that the inner core can be fully coated without sacrificing the positive electrode activity. Under the premise of the gram capacity of the material, the kinetic performance and safety performance of the secondary battery are further improved.
  • too little cladding amount means that the thickness of the cladding layer is relatively thin, which may not be able to effectively hinder the migration of transition metals; If the amount is too large, it means that the cladding layer is too thick, which will affect the migration of Li + , and then affect the rate performance of the material.
  • the cladding amount is within the above range, the following situations can be avoided: if the cladding amount is too much, the platform voltage of the material as a whole may be affected; if the cladding amount is too small, it may not be possible to achieve sufficient wrapping effect.
  • the carbon coating mainly plays the role of enhancing the electron transport between particles.
  • the structure also contains a large amount of amorphous carbon, the carbon density is low. Therefore, if the coating amount is too large Large, it will affect the compaction density of the pole piece.
  • the thickness of the first cladding layer is 1-10 nm; and/or
  • the thickness of the second cladding layer is 2-15nm; and/or
  • the thickness of the third cladding layer is 2-25nm.
  • the thickness of the first cladding layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm or about 10 nm, or within any of the above values. within any range.
  • the thickness of the second cladding layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, About 13nm, about 14nm, about 15nm, or within any range of any of the above numerical values.
  • the thickness of the third cladding layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm , about 13nm, about 14nm, about 15nm, about 16nm, about 17nm, about 18nm, about 19nm, about 20nm, about 21nm, about 22nm, about 23nm, about 24nm or about 25nm, or within any range of any of the above numerical values.
  • the thickness of the first cladding layer is in the range of 1-10 nm, it can avoid the adverse effect on the dynamic performance of the material that may be generated when it is too thick, and can avoid the possibility that the migration of transition metal ions cannot be effectively hindered when it is too thin. The problem.
  • the thickness of the second coating layer is in the range of 2-15nm, the surface structure of the second coating layer is stable, and the side reaction with the electrolyte is small, so the side reaction at the interface can be effectively reduced, thereby improving the secondary High temperature performance of the battery.
  • the electrical conductivity of the material can be improved and the compaction performance of the battery electrode sheet prepared by using the positive electrode active material can be improved.
  • the thickness test of the coating layer is mainly carried out by FIB.
  • the specific method may include the following steps: randomly select a single particle from the positive electrode active material powder to be tested, cut a thin slice with a thickness of about 100 nm from the middle position of the selected particle or near the middle position, and then Carry out TEM test on the sheet, measure the thickness of the cladding layer, measure 3-5 positions, and take the average value.
  • the manganese content is in the range of 10% by weight to 35% by weight, optionally in the range of 15% by weight to 30% by weight, more optionally in the range of 17% by weight to 20% by weight.
  • the content of phosphorus element is in the range of 12 weight %-25 weight %, optionally in the range of 15 weight %-20 weight %, the weight ratio of manganese element and phosphorus element is in the range of 0.90-1.25, can be selected as 0.95-1.20.
  • the content of manganese may correspond to that of the inner core.
  • limiting the content of the manganese element to the above range can effectively avoid problems such as poor material structure stability and density drop that may be caused if the manganese element content is too large, thereby improving the cycle life of the secondary battery. , storage and compaction performance; and can avoid problems such as low voltage platform that may be caused if the manganese content is too small, thereby improving the energy density of the secondary battery.
  • limiting the content of the phosphorus element within the above range can effectively avoid the following situation: if the content of the phosphorus element is too large, it may cause the covalency of P-O to be too strong and affect the conduction of small polarons, thereby Affects the electrical conductivity of the material; if the phosphorus content is too small, it may destabilize the lattice structure of the inner core, the pyrophosphate in the first cladding layer and/or the phosphate in the second cladding layer The property decreases, thus affecting the stability of the material as a whole.
  • the weight ratio of manganese to phosphorus content has the following effects on the performance of the secondary battery: if the weight ratio is too large, it means that there is too much manganese, and the dissolution of manganese increases, which affects the stability of the positive electrode active material and the performance of the gram capacity, thereby affecting the secondary battery.
  • the measurement of manganese and phosphorus elements can be carried out by conventional technical means in this field. Particularly, adopt the following method to determine the content of manganese element and phosphorus element: material is dissolved in dilute hydrochloric acid (concentration 10-30%), utilize the content of each element of ICP test solution, then measure and convert the content of manganese element, Get its weight ratio.
  • the lattice change rate of the positive electrode active material with a core-shell structure is less than 4%, optionally less than 3.8%, and more preferably 2.0-3.8%.
  • the lithium manganese phosphate (LiMnPO 4 ) deintercalation process is a two-phase reaction.
  • the interfacial stress of the two phases is determined by the lattice change rate before and after lithium intercalation.
  • the positive electrode active material with a core-shell structure described in the present application can achieve a lattice change rate of less than 4% before and after lithium intercalation, so the use of the positive electrode active material can improve the rate performance of the secondary battery.
  • the rate of lattice change can be measured by methods known in the art, such as X-ray diffraction (XRD).
  • the Li/Mn antisite defect concentration of the positive electrode active material having a core-shell structure is less than 4%, optionally less than 2.2%, more preferably 1.5-2.2%.
  • the Li/Mn antisite defect mentioned in this application refers to the exchange of the positions of Li + and Mn 2+ in the LiMnPO 4 lattice.
  • the Li/Mn antisite defect concentration refers to the percentage of Li + exchanged with Mn 2+ to the total Li + .
  • the concentration of Li/Mn antisite defects can be measured according to JIS K 0131-1996, for example.
  • the positive electrode active material with a core-shell structure described in the present application can realize the aforementioned lower Li/Mn antisite defect concentration.
  • the mechanism is not very clear, the inventors of the present application speculate that since Li + and Mn 2+ will exchange positions in the LiMnPO 4 lattice, and the Li + transport channel is a one-dimensional channel, so Mn 2+ is in the Li + It will be difficult to migrate in the channel, thereby hindering the transport of Li + .
  • the positive electrode active material with the core-shell structure described in the present application has a relatively low concentration of Li/Mn antisite defects, which is within the above range, so Mn 2+ can be avoided from hindering the transport of Li + , and the positive electrode activity can be improved at the same time. Gram capacity play and rate performance of the material.
  • the positive electrode active material has a compacted density at 3T of not less than 2.2 g/cm 3 , optionally not less than 2.2 g/cm 3 and not more than 2.8 g/cm 3 .
  • the compacted density can be measured according to GB/T 24533-2009.
  • the surface oxygen valence state of the positive electrode active material is below -1.90, optionally between -1.90 and -1.98.
  • the stable valence state of oxygen is -2.
  • the surface valence is below -1.7.
  • by limiting the surface oxygen valence state of the positive electrode active material within the above range it is possible to reduce the interface side reaction between the positive electrode material and the electrolyte, thereby improving the cycle of the battery cell, high temperature storage and gas production and other performance.
  • the surface oxygen valence state can be measured by methods known in the art, for example by electron energy loss spectroscopy (EELS).
  • EELS electron energy loss spectroscopy
  • a second aspect of the present application provides a method for preparing a positive electrode active material, comprising the following steps:
  • the chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O 4 , where x is any value in the range of -0.100-0.100, and y is in the range of 0.001-0.500 Any value in the range, z is any value in the range of 0.001-0.100, and the A is selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, One or more elements of Ga, Sn, Sb, Nb and Ge, optionally one or more elements of Fe, Ti, V, Ni, Co and Mg, the R is selected from B, Si , one or more elements in N and S, optionally, the R is an element selected from B, Si, N and S;
  • Coating step Li a MP 2 O 7 and/or M b (P 2 O 7 ) c and XPO 4 suspensions are provided respectively, the core material is added to the above suspensions and mixed, and the positive electrode is obtained by sintering Active material, wherein 0 ⁇ a ⁇ 2, 1 ⁇ b ⁇ 4, 1 ⁇ c ⁇ 6, the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li a MP 2 O 7 or M b (P 2 O 7 ) c maintains electrical neutrality; M is each independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al; X One or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al;
  • the positive electrode active material has a core-shell structure, which includes the inner core and a shell covering the inner core, and the shell includes a first cladding layer covering the inner core, and a shell covering the first cladding layer.
  • the second coating layer includes crystalline phosphate XPO 4
  • the third coating layer is carbon.
  • the step of providing core material comprises the steps of:
  • Step (1) mixing and stirring a manganese source, a dopant of element A, and an acid in a container to obtain manganese salt particles doped with element A;
  • Step (2) Mix the manganese salt particles doped with element A with the dopant of lithium source, phosphorus source and element R in a solvent to obtain a slurry, and sinter under the protection of an inert gas atmosphere to obtain a doped
  • the inner core doped with element A and element R is Li 1+x Mn 1-y A y P 1-z R z O 4 , wherein x is between -0.100- Any value within the range of 0.100, y is any value within the range of 0.001-0.500, z is any value within the range of 0.001-0.100
  • the A is selected from Zn, Al, Na, K, Mg, Mo, W , one or more elements of Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and one or more of Fe, Ti, V, Ni, Co and Mg Multiple elements
  • the R is one or more elements selected from B, Si, N and S, optionally, the
  • the preparation method of the present application is not particularly limited to the source of the material, and the source of a certain element may include one of the simple substance, sulfate, halide, nitrate, organic acid salt, oxide or hydroxide of the element or more, the precursor is the source that can realize the purpose of the preparation method of the present application.
  • the dopant of element A is selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb
  • elemental substances carbonates, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of one or more elements in Ge.
  • the dopant of the element R is an inorganic acid, a subacid, an organic acid, a sulfate, a chloride, or a respective one or more elements selected from B, Si, N, and S One or more of nitrates, organic acid salts, oxides, hydroxides;
  • the manganese source may be a manganese-containing substance known in the art that can be used to prepare lithium manganese phosphate.
  • the manganese source may be one or more selected from elemental manganese, manganese dioxide, manganese phosphate, manganese oxalate, and manganese carbonate.
  • the acid may be one or more selected from organic acids such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, silicic acid, and silicic acid, and organic acids such as oxalic acid.
  • the acid is a dilute organic acid having a concentration of 60% by weight or less.
  • the lithium source may be a lithium-containing substance known in the art that can be used to prepare lithium manganese phosphate.
  • the lithium source is one or more selected from lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.
  • the phosphorus source can be a phosphorus-containing substance known in the art that can be used to prepare lithium manganese phosphate.
  • the phosphorus source is one or more selected from diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate and phosphoric acid.
  • the dopant of element A and the acid are reacted in a solvent to obtain a manganese salt suspension doped with element A
  • the suspension is filtered, drying and sanding to obtain manganese salt particles doped with element A with a particle size of 50-200 nm.
  • the slurry in step (2) is dried to obtain a powder, and then the powder is sintered to obtain an inner core doped with elements A and R.
  • the step (1) is mixed at a temperature of 20-120°C, optionally 40-120°C; and/or
  • the stirring in the step (1) is carried out at 400-700rpm for 1-9h, optionally for 3-7h.
  • the reaction temperature in the step (1) can be at about 30°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C °C; the stirring in the step (1) is carried out for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours or about 9 hours; optionally,
  • the reaction temperature and stirring time in the step (1) can be within any range of the above-mentioned arbitrary values.
  • the step (2) is mixed at a temperature of 20-120°C, optionally 40-120°C, for 1-12h.
  • the reaction temperature in the step (2) can be at about 30°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C °C; the mixing described in the step (2) was carried out for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, about 10 hours, About 11 hours or about 12 hours;
  • the reaction temperature and mixing time in the step (2) can be within any range of any of the above-mentioned values.
  • the prepared inner core and the positive electrode active material prepared therefrom have fewer lattice defects, which is beneficial to inhibit the dissolution of manganese and reduce the contact between the positive electrode active material and the electrolyte. Interfacial side reactions, thereby improving the cycle performance and safety performance of secondary batteries.
  • the pH of the solution is controlled to be 3.5-6, optionally, the pH of the solution is controlled to be 4-6, more preferably Optionally, the pH of the solution is controlled to be 4-5. It should be noted that in this application, the pH of the resulting mixture can be adjusted by methods commonly used in the art, for example, by adding acid or base.
  • the molar ratio of the manganese salt particles to the lithium source and the phosphorus source is 1:0.5-2.1:0.5-2.1, more optionally, the doped
  • the molar ratio of manganese salt particles doped with element A to lithium source and phosphorus source is about 1:1:1.
  • the sintering conditions during the preparation of lithium manganese phosphate doped with elements A and R are: sintering at 600-950°C for 4-10 hours under an atmosphere of inert gas or a mixture of inert gas and hydrogen. hours; alternatively, the sintering can be performed at about 650°C, about 700°C, about 750°C, about 800°C, about 850°C or about 900°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours; optionally, the sintering temperature and sintering time can be within any range of any of the above values.
  • the protective atmosphere is a mixed gas of 70-90 vol% nitrogen and 10-30 vol% hydrogen.
  • the coating step includes:
  • the first coating step the source of the element M, the phosphorus source and the acid, and optionally the lithium source are dissolved in a solvent to obtain a suspension of the first coating layer; the inner core obtained in the inner core step and the first coating The suspension of the first coating layer obtained in the step is fully mixed, dried, and then sintered to obtain the material coated with the first coating layer;
  • the second coating step dissolving the source of element X, phosphorus source and acid in a solvent to obtain a suspension of the second coating layer; combining the material coated with the first coating layer obtained in the first coating step with The suspension of the second coating layer obtained in the second coating step is fully mixed, dried, and then sintered to obtain a material coated with two coating layers;
  • the third coating step dissolve the carbon source in the solvent, fully dissolve to obtain the third coating layer solution; then add the materials coated with the two coating layers obtained in the second coating step to the third coating layer solution, mixed uniformly, dried, and then sintered to obtain a material covered by three layers of coating layers, that is, a positive electrode active material.
  • the source of the element M is selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al.
  • the source of the element X is a single substance of one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al, One or more of carbonates, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides.
  • each source of the elements A, R, M, and X depends on the target doping amount, and the ratio of the amounts of the lithium source, the manganese source and the phosphorus source conforms to the stoichiometric ratio.
  • the carbon source is one or more selected from starch, sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and citric acid.
  • the pH of the solution dissolved with the source of the element M, the phosphorus source and the acid, and optionally the lithium source is controlled to be 3.5-6.5, then stirred and reacted for 1-5h, and then The solution is heated to 50-120° C. and maintained at this temperature for 2-10 hours, and/or sintering is carried out at 650-800° C. for 2-6 hours.
  • the reaction proceeds substantially.
  • the reaction is performed for about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours or about 5 hours.
  • the reaction time of the reaction may be within any range of any value mentioned above.
  • the pH of the solution is controlled to be 4-6.
  • the solution is heated to about 55°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C, and maintained at this temperature for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours; optionally, the first coating In the step, the temperature and holding time of the heating can be within any range of any of the above-mentioned values.
  • the sintering may be performed at about 650°C, about 700°C, about 750°C, or about 800°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours or about 6 hours; optionally, the sintering temperature and sintering time can be within any range of any of the above-mentioned values.
  • the first cladding step by controlling the sintering temperature and time within the above range, the following situation can be avoided: when the sintering temperature in the first cladding step is too low and the sintering time is too short, it will cause The crystallinity of the first cladding layer is low, and there are many amorphous substances, which will lead to a decrease in the effect of inhibiting metal dissolution, thereby affecting the cycle performance and high-temperature storage performance of the secondary battery; and when the sintering temperature is too high, it will cause the second The appearance of impurity phases in the first coating layer will also affect its effect of inhibiting metal dissolution, thereby affecting the cycle and high-temperature storage performance of the secondary battery; when the sintering time is too long, the thickness of the first coating layer will increase, affecting The migration of Li + affects the gram capacity and rate performance of the material.
  • the second coating step after dissolving the source of element X, phosphorus source and acid in a solvent, stirring and reacting for 1-10 hours, then raising the temperature of the solution to 60-150° C., and This temperature is maintained for 2-10 hours, and/or, sintering is carried out at 500-700° C. for 6-10 hours.
  • the reaction proceeds sufficiently.
  • the reaction is carried out for about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours hours, about 9 hours or about 10 hours.
  • the reaction time of the reaction may be within any range of any value mentioned above.
  • the solution is heated to about 65°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C, about 120°C, about 130°C, about 140°C or about 150°C and maintaining at that temperature for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours;
  • the heating temperature and holding time may be within any range of any of the above-mentioned values.
  • step of providing the core material and the first cladding step and the second cladding step, before sintering, that is, in the preparation of the chemically reacted core material (steps (1)-(2) )) and in the preparation of the first coating layer suspension and the second coating layer suspension by selecting the appropriate reaction temperature and reaction time as mentioned above, the following situation can be avoided: when the reaction temperature is too low, the reaction cannot occur or the reaction rate is slow; when the temperature is too high, the product decomposes or forms a heterogeneous phase; when the reaction time is too long, the product particle size is large, which may increase the time and difficulty of the subsequent process; Incomplete, less product is obtained.
  • the sintering may be sintering at about 550°C, about 600°C or about 700°C for about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours;
  • the sintering temperature and sintering time may be within any range of any of the above-mentioned values.
  • the second cladding step by controlling the sintering temperature and time within the above range, the following situation can be avoided: when the sintering temperature in the second cladding step is too low and the sintering time is too short, it will cause The crystallinity of the second cladding layer is low, and the amorphous state is more, which reduces the performance of reducing the surface reactivity of the material, thereby affecting the cycle and high-temperature storage performance of the secondary battery; and when the sintering temperature is too high, it will cause the second cladding layer
  • the appearance of impurity phases in the coating will also affect its effect of reducing the surface reactivity of the material, thus affecting the cycle and high-temperature storage performance of the secondary battery; when the sintering time is too long, the thickness of the second coating layer will increase, affecting The voltage platform of the material, thereby reducing the energy density of the material, etc.
  • the sintering in the third cladding step is performed at 700-800° C. for 6-10 hours.
  • the sintering may be performed at about 700°C, about 750°C or about 800°C for about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours;
  • the sintering temperature and sintering time may be within any range of any of the above-mentioned values.
  • the third cladding step by controlling the sintering temperature and time within the above range, the following situation can be avoided: when the sintering temperature in the third cladding step is too low, it will cause the third cladding layer The degree of graphitization of the third cladding layer decreases, which affects its conductivity, thereby affecting the gram capacity of the material; when the sintering temperature is too high, the degree of graphitization of the third cladding layer is too high, which affects the transmission of Li + , thereby affecting the gram capacity of the material.
  • the drying temperature is from 100°C to 200°C, optionally from 110°C to 190°C, more preferably from 120°C to 180°C , even more preferably at a drying temperature of 120°C to 170°C, most preferably at a drying temperature of 120°C to 160°C, and the drying time is 3-9h, preferably 4-8h, more preferably 5-7h, most preferably Optionally about 6h.
  • the positive electrode active material prepared by the preparation method of the positive electrode active material described in the present application has a reduced dissolution amount of Mn and Mn doping elements after cycling, and the high temperature stability, high temperature cycle performance and rate Performance is improved.
  • the source of raw materials is extensive, the cost is low, and the process is simple, which is conducive to the realization of industrialization.
  • the third aspect of the present application provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, the positive electrode film layer includes the core-shell electrode described in the first aspect of the present application.
  • the positive electrode active material of the structure or the positive electrode active material prepared by the method described in the second aspect of the present application, and the content of the positive electrode active material in the positive electrode film layer is more than 10% by weight, optionally 90-99.5% by weight %, more optionally 95-99.5% by weight, based on the total weight of the positive electrode film layer.
  • the positive electrode current collector has two opposing surfaces in its own thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposing surfaces of the positive electrode current collector.
  • the positive electrode current collector can be a metal foil or a composite current collector.
  • aluminum foil can be used as the metal foil.
  • the composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base.
  • the composite current collector can be formed by forming metal materials (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyethylene terephthalic acid It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
  • PP polypropylene
  • PET polyethylene glycol ester
  • PBT polybutylene terephthalate
  • PS polystyrene
  • PE polyethylene
  • the positive electrode active material adopts the positive electrode active material described in this application.
  • the positive electrode film layer may further optionally include a binder.
  • the binder may include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene At least one of ethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.
  • the positive electrode film layer may also optionally include a conductive agent.
  • the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.
  • the binder accounts for 0.4-5.5% by weight, optionally 0.4-4.5% by weight
  • the conductive agent accounts for 0.1-2.5% by weight, optionally 0.1% by weight. -0.5% by weight
  • other additives account for 0.001-1% by weight, based on the total weight of the positive electrode membrane.
  • the coating weight of the positive pole piece is 0.28-0.45g/1540.25mm 2 , and the compacted density reaches above 2.0g/cm 3 , optionally 2.2-2.8g/cm 3 .
  • the positive electrode sheet can be prepared in the following manner: the above-mentioned components used to prepare the positive electrode sheet, such as positive electrode active material, conductive agent, binder and any other components, are dispersed in a solvent (such as N -methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.
  • a solvent such as N -methylpyrrolidone
  • the negative electrode sheet includes a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, and the negative electrode film layer includes a negative electrode active material.
  • the negative electrode current collector has two opposing surfaces in its own thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposing surfaces of the negative electrode current collector.
  • the negative electrode current collector can use a metal foil or a composite current collector.
  • copper foil can be used as the metal foil.
  • the composite current collector may include a base layer of polymer material and a metal layer formed on at least one surface of the base material of polymer material.
  • Composite current collectors can be formed by metal materials (copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver and silver alloys, etc.) on polymer material substrates (such as polypropylene (PP), polyethylene terephthalic acid It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
  • the negative electrode active material can be a negative electrode active material known in the art for secondary batteries.
  • the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, lithium titanate, and the like.
  • the silicon-based material may be at least one selected from elemental silicon, silicon-oxygen compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.
  • the tin-based material may be at least one selected from elemental tin, tin oxide compounds and tin alloys.
  • the present application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials of batteries can also be used. These negative electrode active materials may be used alone or in combination of two or more.
  • the negative electrode film layer may further optionally include a binder.
  • the binder can be selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), At least one of polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).
  • the negative electrode film layer may also optionally include a conductive agent.
  • the conductive agent may be at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.
  • the negative electrode film layer may optionally include other additives, such as thickeners (such as sodium carboxymethylcellulose (CMC-Na)) and the like.
  • thickeners such as sodium carboxymethylcellulose (CMC-Na)
  • CMC-Na sodium carboxymethylcellulose
  • the negative electrode sheet can be prepared in the following manner: the above-mentioned components used to prepare the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.
  • a solvent such as deionized water
  • the electrolyte plays the role of conducting ions between the positive pole piece and the negative pole piece.
  • the present application has no specific limitation on the type of electrolyte, which can be selected according to requirements.
  • electrolytes can be liquid, gel or all solid.
  • the electrolyte is an electrolytic solution.
  • the electrolyte solution includes an electrolyte salt and a solvent.
  • the electrolyte salt may be selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, trifluoro At least one of lithium methanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium difluorooxalate borate, lithium difluorodifluorooxalatephosphate and lithium tetrafluorooxalatephosphate.
  • the solvent may be selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene propyl carbonate , butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, butyric acid At least one of ethyl ester, 1,4-butyrolactone, sulfolane, dimethylsulfone, methylethylsulfone and diethylsulfone.
  • the electrolyte may optionally include additives.
  • additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain performances of the battery, such as additives that improve battery overcharge performance, additives that improve high-temperature or low-temperature performance of batteries, and the like.
  • a separator is further included in the secondary battery.
  • the present application has no particular limitation on the type of the isolation membrane, and any known porous structure isolation membrane with good chemical stability and mechanical stability can be selected.
  • the material of the isolation film may be at least one selected from glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.
  • the separator can be a single-layer film or a multi-layer composite film, without any particular limitation. When the separator is a multilayer composite film, the materials of each layer may be the same or different, and there is no particular limitation.
  • the positive pole piece, the negative pole piece and the separator can be made into an electrode assembly through a winding process or a lamination process.
  • the fourth aspect of the present application provides a secondary battery, which includes the positive active material with a core-shell structure described in the first aspect of the present application or the positive active material prepared by the method described in the second aspect of the present application or the present application The positive pole piece described in the third aspect.
  • the secondary battery includes the positive pole piece, the negative pole piece, the electrolyte and the separator according to the present application.
  • active ions are intercalated and extracted back and forth between the positive electrode and the negative electrode.
  • the electrolyte plays the role of conducting ions between the positive pole piece and the negative pole piece.
  • the separator is arranged between the positive pole piece and the negative pole piece, which mainly plays a role in preventing the short circuit of the positive and negative poles, and at the same time allows ions to pass through.
  • a lithium ion secondary battery may include an outer package.
  • the outer package can be used to package the above-mentioned electrode assembly and electrolyte.
  • the outer package of the lithium-ion secondary battery may be a hard case, such as a hard plastic case, aluminum case, steel case, and the like.
  • the outer packaging of the lithium-ion secondary battery may also be a soft bag, such as a pouch-type soft bag.
  • the material of the soft bag may be plastic, and examples of plastic include polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).
  • FIG. 2 shows a square-shaped secondary battery 5 as an example.
  • the outer package may include a housing 51 and a cover 53 .
  • the housing 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plates enclose to form an accommodating cavity.
  • the housing 51 has an opening communicating with the accommodating cavity, and the cover plate 53 can cover the opening to close the accommodating cavity.
  • the positive pole piece, the negative pole piece and the separator can be formed into an electrode assembly 52 through a winding process or a lamination process.
  • the electrode assembly 52 is packaged in the accommodating chamber. Electrolyte is infiltrated in the electrode assembly 52 .
  • the number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.
  • the lithium-ion secondary battery can be assembled into a battery module, and the number of lithium-ion batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.
  • a fifth aspect of the present application provides a battery module including the secondary battery according to the fourth aspect of the present application.
  • FIG. 4 is a battery module 4 as an example.
  • a plurality of lithium-ion batteries 5 can be arranged sequentially along the length direction of the battery module 4 .
  • the plurality of lithium ion batteries 5 can be fixed by fasteners.
  • the battery module 4 may also include a housing with an accommodating space, and a plurality of lithium-ion batteries 5 are accommodated in the accommodating space.
  • the above-mentioned battery modules can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.
  • a sixth aspect of the present application provides a battery pack, which includes the battery module according to the fifth aspect of the present application.
  • the battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box.
  • the battery box includes an upper box body 2 and a lower box body 3 , the upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery module 4 .
  • Multiple battery modules 4 can be arranged in the battery box in any manner.
  • the seventh aspect of the present application provides an electric device, which includes the secondary battery according to the fourth aspect of the present application, the battery module according to the fifth aspect of the present application, or the battery according to the sixth aspect of the present application At least one of the packages.
  • the secondary battery, battery module, or battery pack can be used as a power source of the electric device, and can also be used as an energy storage unit of the electric device.
  • the electric devices may include mobile devices (such as mobile phones, notebook computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, etc.) , electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but not limited thereto.
  • a secondary battery, a battery module or a battery pack can be selected according to its use requirements.
  • FIG. 7 is an example of an electrical device.
  • the electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle.
  • a battery pack or a battery module may be used.
  • a device may be a cell phone, tablet, laptop, or the like.
  • the device is generally required to be light and thin, and a secondary battery can be used as a power source.
  • the positive electrode sheet, secondary battery, battery module, battery pack and electrical device described in the present application include the positive electrode active material described in the present application, so they have high gram capacity, good cycle performance and safety performance.
  • Step S1 Preparation of Fe, Co, V and S co-doped manganese oxalate
  • Step S2 Prepare inner core Li 0.997 Mn 0.60 Fe 0.393 V 0.004 Co 0.003 P 0.997 S 0.003 O 4
  • Step S3 Preparation of the first coating layer suspension
  • Li2FeP2O7 solution To prepare Li2FeP2O7 solution, dissolve 7.4 g of lithium carbonate, 11.6 g of ferrous carbonate, 23.0 g of ammonium dihydrogen phosphate and 12.6 g of oxalic acid dihydrate in 500 mL of deionized water, control the pH to 5, then stir and The reaction was carried out for 2 hours to obtain a solution, and then the solution was heated to 80° C. and maintained at this temperature for 4 hours to obtain a suspension of the first coating layer.
  • Step S4 Coating of the first coating layer
  • step S2 Add 1571.9 g of the doped lithium manganese phosphate core material obtained in step S2 to the suspension of the first coating layer obtained in step S3 (the content of the coating substance is 15.7 g), stir and mix thoroughly for 6 hours, and mix evenly Afterwards, it was dried in an oven at 120°C for 6 hours, and then sintered at 650°C for 6 hours to obtain a pyrophosphate-coated material.
  • Step S5 Preparation of the second coating layer suspension
  • Step S6 Coating of the second coating layer
  • step S4 Add 1586.8 g of the pyrophosphate-coated material obtained in step S4 to the second coating layer suspension (coating substance content: 47.1 g) obtained in step S5, stir and mix thoroughly for 6 hours, and mix well Afterwards, it was dried in an oven at 120°C for 6 hours, and then sintered at 700°C for 8 hours to obtain a two-layer coated material.
  • Step S7 Preparation of the third coating layer aqueous solution
  • Step S8 Coating of the third coating layer
  • step S6 Add 1633.9 g of the two-layer coated material obtained in step S6 to the sucrose solution obtained in step S7, stir and mix together for 6 hours, after mixing evenly, transfer to an oven at 150°C to dry for 6 hours, and then sinter at 700°C for 10 hours A three-layer coated material is obtained.
  • the negative electrode slurry was evenly coated on the copper foil of the negative electrode current collector at a ratio of 0.117g/1540.25mm 2 , and then dried, cold pressed, and cut to obtain the negative electrode sheet.
  • a commercially available PP-PE copolymer microporous film with a thickness of 20 ⁇ m and an average pore diameter of 80 nm was used.
  • the above obtained positive electrode sheet, separator, and negative electrode sheet are stacked in order, so that the separator is in the middle of the positive and negative electrodes to play the role of isolation, and the bare cell is obtained by winding. Place the bare cell in the outer package, inject the above electrolyte and package it to obtain a full battery (hereinafter also referred to as "full battery").
  • the positive electrode active material prepared above, polyvinylidene fluoride (PVDF), and acetylene black were added to N-methylpyrrolidone (NMP) at a weight ratio of 90:5:5, and stirred in a drying room to form a slurry.
  • NMP N-methylpyrrolidone
  • the above slurry is coated on the aluminum foil, dried and cold pressed to form a positive electrode sheet.
  • the coating amount was 0.2 g/cm 2
  • the compacted density was 2.0 g/cm 3 .
  • a lithium sheet is used as the negative electrode, and a solution of 1 mol/L LiPF 6 in ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) with a volume ratio of 1:1:1 is used as the electrolyte , and assembled into a button battery (hereinafter also referred to as "button") in a button box together with the above-mentioned positive pole piece prepared.
  • EC ethylene carbonate
  • DEC diethyl carbonate
  • DMC dimethyl carbonate
  • positive electrode active materials and batteries in Examples 2 to 27 and Comparative Examples 1 to 19 were prepared.
  • the differences in the preparation of positive electrode active materials are shown in Table 1-6, wherein Comparative Examples 1-2 , 4-10 and 12 are not coated with the first layer, so there are no steps S3 and S4; Comparative Examples 1-11 are not coated with the second layer, so there are no steps S5-S6.
  • the first cladding layer material and/or the second cladding layer material used are all crystalline by default.
  • the positive electrode active material sample was prepared as a button charge, and the above button charge was charged at a small rate of 0.05C until the current was reduced to 0.01C. Then take out the positive pole piece in the button battery, and soak in dimethyl carbonate (DMC) for 8 hours. Then dry, scrape the powder, and screen out the particles whose particle size is less than 500nm. Take a sample and calculate its unit cell volume v1 in the same way as the above-mentioned test fresh sample, and use (v0-v1)/v0 ⁇ 100% as the lattice change rate (unit cell volume change rate) before and after it completely deintercalates lithium. in the table.
  • DMC dimethyl carbonate
  • the Li/Mn antisite defect concentration is obtained. Specifically, import the XRD results tested in the "Measurement Method of Lattice Change Rate” into the General Structural Analysis System (GSAS) software, and automatically obtain the refined results, which include the occupancy of different atoms. By reading the refined As a result, the Li/Mn antisite defect concentration is obtained.
  • GSAS General Structural Analysis System
  • the fresh full batteries prepared in the above-mentioned examples and comparative examples were left to stand for 5 minutes, and discharged to 2.5V at 1/3C. Stand still for 5 minutes, charge to 4.3V according to 1/3C, and then charge at constant voltage at 4.3V until the current is less than or equal to 0.05mA. Stand still for 5 minutes, and record the charging capacity at this time as C0. Discharge to 2.5V according to 1/3C, let stand for 5min, then charge to 4.3V according to 3C, let stand for 5min, record the charging capacity at this time as C1. 3C charging constant current ratio is C1/C0 ⁇ 100%.
  • the full cells prepared in the above-mentioned examples and comparative examples were discharged to a cut-off voltage of 2.0V at a rate of 0.1C. Then the battery was disassembled, and the negative pole piece was taken out. On the negative pole piece, 30 discs with a unit area (1540.25mm 2 ) were randomly selected, and the inductively coupled plasma emission spectrum (ICP) was tested with Agilent ICP-OES730. According to the ICP results, the amounts of Fe (if the Mn site of the positive electrode active material is doped with Fe) and Mn are calculated, so as to calculate the dissolution amount of Mn (and Fe doped at the Mn site) after cycling.
  • the test standard is based on EPA-6010D-2014.
  • the positive electrode active material sample prepared above Take 5 g of the positive electrode active material sample prepared above to prepare a button electrode according to the button electrode preparation method described in the above examples. Charge the button with a small rate of 0.05C until the current decreases to 0.01C. Then take out the positive pole piece in the battery, and soak it in DMC for 8 hours. Then dry, scrape the powder, and screen out the particles whose particle size is less than 500nm. The obtained particles were measured by electron energy loss spectroscopy (EELS, the instrument model used was Talos F200S), and the energy loss near-edge structure (ELNES) was obtained, which reflected the density of states and energy level distribution of the elements. According to the density of states and energy level distribution, the number of occupied electrons is calculated by integrating the data of the valence band density of states, so as to calculate the valence state of the charged surface oxygen.
  • EELS electron energy loss spectroscopy
  • ELNES energy loss near-edge structure
  • Dissolve 5 g of the positive electrode active material prepared above in 100 ml of aqua regia (concentrated hydrochloric acid: concentrated nitric acid 1:3) (concentration of concentrated hydrochloric acid ⁇ 37%, concentration of concentrated nitric acid ⁇ 65%), and use ICP to test the elements of the solution. content, and then measure and convert the content of manganese or phosphorus (amount of manganese or phosphorus/amount of positive electrode active material*100%) to obtain its weight ratio.
  • the full cells prepared in the above-mentioned examples and comparative examples with a state of charge (SOC) of 100% were stored.
  • the open circuit voltage (OCV) and AC internal resistance (IMP) of the cell are measured before, after and during storage to monitor the SOC, and the volume of the cell is measured.
  • the full battery was taken out after every 48 hours of storage, and the open circuit voltage (OCV) and internal resistance (IMP) were tested after standing for 1 hour, and the cell volume was measured by the drainage method after cooling to room temperature.
  • the batteries of all the examples kept the SOC above 99% during the experiment until the end of storage.
  • the thickness test of the coating layer is mainly to cut a thin slice with a thickness of about 100nm from the middle of the single particle of the positive electrode active material prepared above through FIB, and then conduct a TEM test on the thin slice to obtain the original picture of the TEM test, and save the original picture format (xx.dm3) .
  • the thickness was measured at three locations on the selected particle and the average value was taken.
  • the test is performed by Raman spectroscopy. By splitting the energy spectrum of the Raman test, Id/Ig is obtained, where Id is the peak intensity of SP3 form carbon, and Ig is the peak intensity of SP2 form carbon, thereby confirming the molar ratio of the two.
  • Compacted density and the performance of the prepared battery (capacity, high temperature cycle performance, high temperature storage performance) are excellent.
  • the present application is not limited to the above-mentioned embodiments.
  • the above-mentioned embodiments are merely examples, and within the scope of the technical solutions of the present application, embodiments that have substantially the same configuration as the technical idea and exert the same effects are included in the technical scope of the present application.
  • various modifications conceivable by those skilled in the art are added to the embodiments, and other forms constructed by combining some components in the embodiments are also included in the scope of the present application. .

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Abstract

本申请提供具有核-壳结构的正极活性材料及其制备方法、包含其的正极极片、二次电池、电池模块、电池包和用电装置，所述正极活性材料包括内核及包覆所述内核的壳，所述内核包括Li1+xMn1-yAyP1-zRzO4，所述壳包括包覆所述内核的第一包覆层、包覆所述第一包覆层的第二包覆层以及包覆所述第二包覆层的第三包覆层，其中，所述第一包覆层包括晶态焦磷酸盐LiaMP2O7和/或Mb(P2O7)c，所述晶态焦磷酸盐为电中性；所述第二包覆层包括晶态磷酸盐XPO4；所述第三包覆层为碳。

Description

正极活性材料及其制备方法、正极极片、二次电池、电池模块、电池包和用电装置 技术领域
本申请涉及锂电池技术领域，尤其涉及正极活性材料及其制备方法、包含其的正极极片、二次电池、电池模块、电池包和用电装置。
背景技术
随着新能源领域的快速发展，锂离子电池凭借其优良的电化学性能、无记忆效应、环境污染小等优势广泛应用于各类大型动力装置、储能***以及各类消费类产品中，尤其广泛应用于纯电动汽车、混合电动汽车等新能源汽车领域。其中，磷酸锰锂正极活性材料具有工作电压高、原料来源广泛和对环境污染小等优点，被认为是有望取代磷酸铁锂而成为动力锂离子电池的正极活性材料。
然而，在现有技术中，采用磷酸锰锂正极活性材料的二次电池的循环性能、高温存储性能和安全性能一直无法得到综合提升，这极大地限制了磷酸锰锂电池更大范围的应用。因此，业界尚期待设计出一款兼具较高的克容量、良好的循环性能和安全性能的磷酸锰锂正极活性材料。
发明内容
本申请是鉴于上述课题而进行的，其目的在于，提供一种新型的具有核-壳结构的掺杂磷酸锰锂正极活性材料，使得应用所述正极活性材料的二次电池具有较高的克容量、良好的循环性能和安全性能。
为了达到上述目的，本申请的第一方面提供了一种具有核-壳结构的正极活性材料，其包括内核及包覆所述内核的壳，
所述内核的化学式为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、 Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中一种或多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素；
所述x、y和z的值满足以下条件：使整个内核保持电中性；
所述壳包括包覆所述内核的第一包覆层、包覆所述第一包覆层的第二包覆层以及包覆所述第二包覆层的第三包覆层，其中，
所述第一包覆层包括晶态焦磷酸盐Li aMP 2O 7和/或M b(P 2O 7) c，其中，
0≤a≤2，1≤b≤4，1≤c≤6，所述a、b和c的值满足以下条件：使晶态焦磷酸盐Li aMP 2O 7或M b(P 2O 7) c保持电中性，
所述晶态焦磷酸盐Li aMP 2O 7和M b(P 2O 7) c中的M各自独立地为选自Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素，
所述第二包覆层包括晶态磷酸盐XPO 4，其中，所述X为选自Li、Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；
所述第三包覆层为碳。
本文中，晶态意指结晶度在50％以上，即50％-100％。结晶度小于50％的称为玻璃态。本申请所述的晶态焦磷酸盐和晶态磷酸盐的结晶度为50％至100％。具备一定结晶度的焦磷酸盐和磷酸盐不但有利于充分发挥焦磷酸盐包覆层阻碍锰溶出和磷酸盐包覆层优异的导锂离子的能力、减少界面副反应的功能，而且能够使得焦磷酸盐包覆层和磷酸盐包覆层能够更好的进行晶格匹配，从而能够实现包覆层和包覆层之间紧密的结合。
本申请通过在磷酸锰锂的锰位掺杂元素A并在磷位掺杂元素R得到掺杂的磷酸锰锂内核并在所述内核表面依次进行三层包覆，提供了一种新型的具有核-壳结构的磷酸锰锂正极活性材料，将所述正极活性材料应用于二次电池中，能够显著改善二次电池的高温循环性能、循环稳定性和高温储存性能。
在任意实施方式中，所述第一包覆层中的晶态焦磷酸盐的晶面间距范围为0.293-0.470nm，晶向(111)的夹角范围为18.00°-32.00°；所述第二包覆层中的晶态磷酸盐的晶面间距范围为0.244-0.425nm，晶向(111)的夹角范围为20.00°-37.00°。
本申请所述的正极活性材料中的第一包覆层和第二包覆层均使用晶态物质，它们的晶面间距和夹角范围在上述范围内。由此，能够有效避免包覆层中的杂质相，从而提升材料的克容量，循环性能和倍率性能。
在任意实施方式中，在所述内核中，y与1-y的比值为1：10至1：1，可选为1：4至1：1。由此，进一步提升二次电池的循环性能和倍率性能。
在任意实施方式中，在所述内核中，z与1-z的比值为1：999至1：9，可选为1：499至1：249。由此，进一步提升二次电池的循环性能和倍率性能。
在任意实施方式中，所述第三包覆层的碳为SP2形态碳与SP3形态碳的混合物，可选地，所述SP2形态碳与SP3形态碳的摩尔比为在0.1-10范围内的任意数值，可选为在2.0-3.0范围内的任意数值。
本申请通过将SP2形态碳与SP3形态碳的摩尔比限制在上述范围内，提升了二次电池的综合性能。
在任意实施方式中，所述第一包覆层的包覆量为大于0且小于或等于6重量％，可选为大于0且小于或等于5.5重量％，更可选为大于0且小于或等于2重量％，基于所述内核的重量计；和/或
所述第二包覆层的包覆量为大于0且小于或等于6重量％，可选为大于0且小于或等于5.5重量％，更可选为2-4重量％，基于所述内核的重量计；和/或
所述第三包覆层的包覆量为大于0且小于或等于6重量％，可选为大于0且小于或等于5.5重量％，更可选为大于0且小于或等于2重量％，基于所述内核的重量计。
本申请所述的具有核-壳结构的正极活性材料中，三层包覆层的包覆量优选在上述范围内，由此能够对所述内核进行充分包覆，并同时在不 牺牲正极活性材料克容量的前提下，进一步改善二次电池的动力学性能和安全性能。
在任意实施方式中，所述第一包覆层的厚度为1-10nm；和/或
所述第二包覆层的厚度为2-15nm；和/或
所述第三包覆层的厚度为2-25nm。
本申请中，当所述第一包覆层的厚度范围为1-10nm时，能够避免过厚时可能产生的对材料的动力学性能的不利影响，且能够避免过薄时不能有效阻碍过渡金属离子的迁移的问题。
当所述第二包覆层的厚度在2-15nm范围内时，所述第二包覆层的表面结构稳定，与电解液的副反应小，因此能够有效减轻界面副反应，从而提升二次电池的高温性能。
当所述第三包覆层的厚度范围为2-20nm时，能够提升材料的电导性能并且改善使用所述正极活性材料制备的电池极片的压实密度性能。
在任意实施方式中，基于所述具有核-壳结构的正极活性材料的重量计，锰元素含量在10重量％-35重量％范围内，可选在15重量％-30重量％范围内，更可选在17重量％-20重量％范围内，磷元素的含量在12重量％-25重量％范围内，可选在15重量％-20重量％范围内，锰元素和磷元素的重量比范围为0.90-1.25，可选为0.95-1.20。
本申请所述的具有核-壳结构的正极活性材料中，所述锰元素的含量在上述范围内，能够有效避免若锰元素含量过大可能会引起的材料结构稳定性变差、密度下降等问题，从而提升二次电池的循环、存储和压实密度等性能；且能够避免若锰元含量过小可能会导致的电压平台低等问题，从而提升二次电池的能量密度。
本申请所述的具有核-壳结构的正极活性材料中，所述磷元素的含量在上述范围内，能够有效避免以下情况：若磷元素的含量过大，可能会导致P-O的共价性过强而影响小极化子导电，从而影响材料的电导率；若磷元素的含量过小，可能会使所述内核、所述第一包覆层中的焦磷酸盐和/或所述第二包覆层中的磷酸盐晶格结构的稳定性下降，从而影响材料整体的稳定性。
本申请所述的具有核-壳结构的正极活性材料中，所述锰元素与磷元素的重量比在上述范围内，能够有效避免以下情况：若该重量比过大，可能会导致过渡金属溶出增加，影响材料的稳定性和二次电池的循环及存储性能；若该重量比过小，可能会使材料的放电电压平台下降，从而使二次电池的能量密度降低。
在任意实施方式中，所述具有核-壳结构的正极活性材料在完全脱嵌锂前后的晶格变化率为4％以下，可选为3.8％以下，更可选为2.0-3.8％。
本申请所述的具有核-壳结构的正极活性材料能够实现4％以下的脱嵌锂前后的晶格变化率。因此使用所述正极活性材料能够改善二次电池的克容量和倍率性能。
在任意实施方式中，所述具有核-壳结构的正极活性材料的Li/Mn反位缺陷浓度为4％以下，可选为2.2％以下，更可选为1.5-2.2％。通过Li/Mn反位缺陷浓度在上述范围内，能够避免Mn 2+阻碍Li +的传输，同时提升正极活性材料的克容量和倍率性能。
在任意实施方式中，所述具有核-壳结构的正极活性材料在3T(吨)下的压实密度为2.2g/cm 3以上，可选地为2.2g/cm 3以上且2.8g/cm 3以下。由此，提高压实密度，则单位体积活性材料的重量增大，越有利于提高二次电池的体积能量密度。
在任意实施方式中，所述具有核-壳结构的正极活性材料的表面氧价态为-1.90以下，可选地为-1.90至-1.98。由此，通过如上所述将正极活性材料的表面氧价态限定在上述范围内，能够减轻正极材料与电解液的界面副反应，从而改善电芯的循环，高温存储产气等性能。
本申请的第二方面提供一种正极活性材料的制备方法，包括以下步骤：
提供内核材料的步骤：所述内核化学式为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中的一种或 多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素；
包覆步骤：分别提供Li aMP 2O 7和/或M b(P 2O 7) c以及XPO 4悬浊液，将所述内核材料加入到上述悬浊液中并混合，经烧结获得正极活性材料，其中0≤a≤2，1≤b≤4，1≤c≤6，所述a、b和c的值满足以下条件：使晶态焦磷酸盐Li aMP 2O 7或M b(P 2O 7) c保持电中性；所述M各自独立地为选自Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；所述X为选自Li、Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；
其中，所述正极活性材料具有核-壳结构，其包括所述内核及包覆所述内核的壳，所述壳包括包覆所述内核的第一包覆层、包覆所述第一包覆层的第二包覆层以及包覆所述第二包覆层的第三包覆层，所述第一包覆层包括晶态焦磷酸盐Li aMP 2O 7和/或M b(P 2O 7) c，所述第二包覆层包括晶态磷酸盐XPO 4，所述第三包覆层为碳。
在任意实施方式中，所述提供内核材料的步骤包括以下步骤：
步骤(1)：将锰源、元素A的掺杂剂和酸在容器中混合并搅拌，得到掺杂有元素A的锰盐颗粒；
步骤(2)：将所述掺杂有元素A的锰盐颗粒与锂源、磷源和元素R的掺杂剂在溶剂中混合并得到浆料，在惰性气体气氛保护下烧结后得到掺杂有元素A和元素R的内核，其中，所述掺杂有元素A和元素R的内核为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中的一种或多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素。
在任意实施方式中，所述元素A的掺杂剂为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的 一种或多种元素各自的单质、碳酸盐、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种。
在任意实施方式中，所述元素R的掺杂剂为选自B、Si、N和S中的一种或多种元素各自的无机酸、亚酸、有机酸、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种；
在任意实施方式中，所述步骤(1)在20-120℃、可选为40-120℃的温度下进行混合；和/或
所述步骤(1)中所述搅拌在400-700rpm下进行1-9h，可选地为3-7h。
当内核颗粒制备过程中的加热温度和搅拌时间处于上述范围内时，制备获得的内核以及由其制得的正极活性材料的晶格缺陷较少，有利于抑制锰溶出，减少正极活性材料与电解液的界面副反应，从而改善二次电池的循环性能和安全性能。
在任意实施方式中，所述步骤(2)在20-120℃、可选为40-120℃的温度下进行混合1-12h。
在任意实施方式中，所述包覆步骤包括：
第一包覆步骤：将元素M的源、磷源和酸以及任选地锂源，得到第一包覆层悬浊液；将内核步骤中获得的内核与第一包覆步骤获得的第一包覆层悬浊液充分混合，干燥，然后烧结，得到第一包覆层包覆的材料；
第二包覆步骤：将元素X的源、磷源和酸溶于溶剂中，得到第二包覆层悬浊液；将第一包覆步骤中获得的第一包覆层包覆的材料与第二包覆步骤获得的第二包覆层悬浊液充分混合，干燥，然后烧结，得到两层包覆层包覆的材料；
第三包覆步骤：将碳源溶于溶剂中，充分溶解得到第三包覆层溶液；然后将第二包覆步骤中获得的两层包覆层包覆的材料加入所述第三包覆层溶液中，混合均匀，干燥，然后烧结得到三层包覆层包覆的材料，即正极活性材料。
在任意实施方式中，所述元素M的源为选自Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素各自的单质、碳酸盐、 硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种。
在任意实施方式中，所述元素X的源为选自Li、Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素各自的单质、碳酸盐、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种。
本申请中，元素A、R、M、X各自的源的加入量取决于目标掺杂量，锂源、锰源和磷源的用量之比符合化学计量比。
在任意实施方式中，所述第一包覆步骤中，控制溶解有元素M的源、磷源和酸以及任选地锂源的溶液pH为3.5-6.5，然后搅拌并反应1-5h，然后将所述溶液升温至50-120℃，并保持该温度2-10h，和/或，烧结在650-800℃下进行2-6小时。
通过将所述第一包覆步骤的条件控制在上述范围内，能够保证甚至改善使用所述正极活性材料制备的二次电池的克容量及其循环和高温存储性能以及克容量和倍率性能等。
在任意实施方式中，所述第二包覆步骤中，将元素X的源、磷源和酸溶于溶剂后，搅拌并反应1-10h，然后将所述溶液升温至60-150℃，并保持该温度2-10h，和/或，烧结在500-700℃下进行6-10小时。
在所述提供内核材料的步骤和所述第一包覆步骤和所述第二包覆步骤中，在烧结之前，即，在发生化学反应的内核材料的制备中(步骤(1)-(2))以及在第一包覆层悬浮液和第二包覆层悬浮液的制备中，通过如上所述选择的反应温度和反应时间，能够避免以下情况：反应温度过低时，则反应无法发生或反应速率较慢；温度过高时，产物分解或形成杂相；反应时间过长时，产物粒径较大，可能会增加后续工艺的时间和难度；反应时间过短时，则反应不完全，获得的产物较少。
任意实施方式中，所述第三包覆步骤中的烧结在700-800℃下进行6-10小时。
通过将所述第三包覆步骤的条件控制在上述范围内，能够确保甚至改善使用所述正极活性材料制备的二次电池的克容量和压实密度。
本申请所述的正极活性材料的制备方法，原料来源广泛、成本低廉，工艺简单，有利于实现工业化。
本申请的第三方面提供一种正极极片，其包括正极集流体以及设置在正极集流体至少一个表面上的正极膜层，所述正极膜层包括本申请第一方面所述的具有核-壳结构的正极活性材料或通过本申请第二方面所述的方法制备的正极活性材料，并且所述正极活性材料在所述正极膜层中的含量为90-99.5重量％，优选为95-99.5重量％，基于所述正极膜层的总重量计。
本申请所述的正极极片用于电池中，改善了二次电池的高温循环性能、倍率性能和安全性能。
本申请的第四方面提供一种二次电池，其包括本申请第三方面所述的正极极片。本申请提供的二次电池的倍率性能高、循环性能好、安全性能好并且电池容量大。
本申请的第五方面提供一种电池模块，其包括本申请第四方面所述的二次电池。
本申请的第六方面提供一种电池包，其包括本申请第五方面所述的电池模块。
本申请的第七方面提供一种用电装置，其包括本申请第四方面所述的二次电池、本申请第五方面所述的电池模块或本申请第六方面所述的电池包中的至少一种。
本申请所述正极极片、二次电池、电池模块、电池包和用电装置包括本申请所述的正极活性材料，因而具有较高的克容量、良好的循环性能和安全性能。
附图说明
图1为本申请所述的具有核-壳结构的正极活性材料的示意图。
图2为本申请一实施方式的二次电池的示意图。
图3为图2所示的本申请一实施方式的二次电池的分解图。
图4为本申请一实施方式的电池模块的示意图。
图5为本申请一实施方式的电池包的示意图。
图6为图5所示的本申请一实施方式的电池包的分解图。
图7为本申请一实施方式的二次电池用作电源的用电装置的示意图。
附图标记说明：
1电池包；2上箱体；3下箱体；4电池模块；5二次电池；51壳体；52电极组件；53顶盖组件
具体实施方式
以下，适当地参照附图详细说明具体公开了本申请的正极活性材料及其制造方法、正极极片、二次电池、电池模块、电池包和用电装置的实施方式。但是会有省略不必要的详细说明的情况。例如，有省略对已众所周知的事项的详细说明、实际相同结构的重复说明的情况。这是为了避免以下的说明不必要地变得冗长，便于本领域技术人员的理解。此外，附图及以下说明是为了本领域技术人员充分理解本申请而提供的，并不旨在限定权利要求书所记载的主题。
本申请所公开的“范围”以下限和上限的形式来限定，给定范围是通过选定一个下限和一个上限进行限定的，选定的下限和上限限定了特定范围的边界。这种方式进行限定的范围可以是包括端值或不包括端值的，并且可以进行任意地组合，即任何下限可以与任何上限组合形成一个范围。例如，如果针对特定参数列出了60-120和80-110的范围，理解为60-110和80-120的范围也是预料到的。此外，如果列出的最小范围值1和2，和如果列出了最大范围值3，4和5，则下面的范围可全部预料到：1-3、1-4、1-5、2-3、2-4和2-5。在本申请中，除非有其他说明，数值范围“a-b”表示a到b之间的任意实数组合的缩略表示，其中a和b都是实数。例如数值范围“0-5”表示本文中已经全部列出了“0-5”之间的全部实数，“0-5”只是这些数值组合的缩略表示。另外，当表述某个参数为≥2的整数，则相当于公开了该参数为例如整数2、3、4、5、6、7、8、9、10、11、12等。
如果没有特别的说明，本申请的所有实施方式以及可选实施方式可以相互组合形成新的技术方案。
如果没有特别的说明，本申请的所有技术特征以及可选技术特征可以相互组合形成新的技术方案。
如果没有特别的说明，本申请的所有步骤可以顺序进行，也可以随机进行，可选是顺序进行的。例如，所述方法包括步骤(a)和(b)，表示所述方法可包括顺序进行的步骤(a)和(b)，也可以包括顺序进行的步骤(b)和(a)。例如，所述提到所述方法还可包括步骤(c)，表示步骤(c)可以任意顺序加入到所述方法，例如，所述方法可以包括步骤(a)、(b)和(c)，也可包括步骤(a)、(c)和(b)，也可以包括步骤(c)、(a)和(b)等。
如果没有特别的说明，本申请所提到的“包括”和“包含”表示开放式，也可以是封闭式。例如，所述“包括”和“包含”可以表示还可以包括或包含没有列出的其他组分，也可以仅包括或包含列出的组分。
本申请中使用的术语“以上”、“以下”包含本数，例如“一种以上”是指一种或多种，“A和B中的一种以上”是指“A”、“B”或“A和B”。
如果没有特别的说明，在本申请中，术语“或”是包括性的。举例来说，短语“A或B”表示“A，B，或A和B两者”。更具体地，以下任一条件均满足条件“A或B”：A为真(或存在)并且B为假(或不存在)；A为假(或不存在)而B为真(或存在)；或A和B都为真(或存在)。
需要说明的是，在本文中，术语“包覆层”是指包覆在磷酸锰锂内核上的物质层，所述物质层可以完全或部分地包覆磷酸锰锂内核，使用“包覆层”只是为了便于描述，并不意图限制本发明。同样地，术语“包覆层的厚度”是指包覆在磷酸锰锂内核上的所述物质层在磷酸锰锂内核径向上的厚度。
本申请发明人在实际作业中发现，目前现有的磷酸锰锂正极活性材料在深度充放电过程中，锰溶出比较严重。虽然现有技术中有尝试对磷酸锰锂进行磷酸铁锂包覆，从而减少界面副反应，但这种包覆无法阻止溶出的锰继续向电解液中迁移。溶出的锰在迁移到负极后，被还原成金属锰。这样产生的金属锰相当于“催化剂”，能够催化负极表面的SEI膜(solid electrolyte interphase，固态电解质界面膜)分解，产生副产物； 所述副产物的一部分为气体，因此导致会二次电池发生膨胀，影响二次电池的安全性能；另外，所述副产物的另一部分沉积在负极表面，会阻碍锂离子进出负极的通道，造成二次电池阻抗增加，从而影响二次电池的动力学性能。此外，为补充损失的SEI膜，电解液和电池内部的活性锂被不断消耗，会给二次电池容量保持率带来不可逆的影响。
发明人在进行大量研究后发现，通过对磷酸锰锂进行改性以及对磷酸锰锂的多层包覆，能够得到一种新型的具有核-壳结构的正极活性材料，所述正极活性材料能够实现显著降低的锰溶出以及降低的晶格变化率，其用于二次电池中，能够改善电池的循环性能、倍率性能、安全性能并且提高电池的容量。
[正极活性材料]
本申请的第一方面提供了一种新型的具有核-壳结构的正极活性材料，其包括内核及包覆所述内核的壳，所述内核的化学式为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中一种或多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素；
所述x、y和z的值满足以下条件：使整个内核保持电中性；
所述壳包括包覆所述内核的第一包覆层、包覆所述第一包覆层的第二包覆层以及包覆所述第二包覆层的第三包覆层，其中，
所述第一包覆层包括晶态焦磷酸盐Li aMP 2O 7和/或M b(P 2O 7) c，其中，
0≤a≤2，1≤b≤4，1≤c≤6，所述a、b和c的值满足以下条件：使晶态焦磷酸盐Li aMP 2O 7或M b(P 2O 7) c保持电中性，
所述晶态焦磷酸盐Li aMP 2O 7和M b(P 2O 7) c中的M各自独立地为选自Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素，
所述第二包覆层包括晶态磷酸盐XPO 4，其中，
所述X为选自Li、Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；
所述第三包覆层为碳。
除非另有说明，否则上述内核的化学式中，当A为两种以上元素时，上述对于y数值范围的限定不仅是对每种作为A的元素的化学计量数的限定，也是对各个作为A的元素的化学计量数之和的限定。例如当A为两种以上元素A1、A2……An时，A1、A2……An各自的化学计量数y1、y2……yn各自均需落入本申请对y限定的数值范围内，且y1、y2……yn之和也需落入该数值范围内。类似地，对于R为两种以上元素的情况，本申请中对R化学计量数的数值范围的限定也具有上述含义。
在一个可选的实施方式中，当A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种、两种、三种或四种元素时，A y为Q n1D n2E n3K n4，其中n1+n2+n3+n4＝y，且n1、n2、n3、n4均为正数且不同时为零，Q、D、E、K各自独立地为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge的一种，可选地，Q、D、E、K中至少一个为Fe。可选地，n1、n2、n3、n4之一为零，其余不为零；更可选地，n1、n2、n3、n4中的两个为零，其余不为零；还可选地，n1、n2、n3、n4中的三个为零，其余不为零。所述内核Li 1+xMn 1-yA yP 1-zR zO 4中，在锰位掺杂一种、两种、三种或四种上述A元素是有利的，可选地，掺杂一种、两种或三种上述A元素；此外，在磷位掺杂一种或两种R元素是有利的，这样有利于使掺杂元素均匀分布。
所述内核Li 1+xMn 1-yA yP 1-zR zO 4中，x的大小受A和R的价态大小以及y和z的大小的影响，以保证整个体系呈现电中性。如果x的值过小，会导致整个内核体系的含锂量降低，影响材料的克容量发挥。y值会限制所有掺杂元素的总量，如果y过小，即掺杂量过少，掺杂元素起不到作用，如果y超过0.5，会导致体系中的Mn含量较少，影响材料的电压平台。所述R元素掺杂在P的位置，由于P-O四面体较稳定，而z值过大会影响材料的稳定性，因此将z值限定为0.001-0.100。
本申请的正极活性材料能够提高二次电池的克容量、循环性能和安全性能。虽然机理尚不清楚，但推测是本申请的磷酸锰锂正极活性材料为核-壳结构，其中通过对磷酸锰锂内核的锰位和磷位分别掺杂元素A和元素R，不仅可有效减少锰溶出，进而减少迁移到负极的锰离子，减少因SEI膜分解而消耗的电解液，提高二次电池的循环性能和安全性能，还能够促进Mn-O键调整，降低锂离子迁移势垒，促进锂离子迁移，提高二次电池的倍率性能；通过对内核包覆包括晶态焦磷酸盐的第一包覆层，能够进一步增大锰的迁移阻力，减少其溶出，并减少表面杂锂含量、减少内核与电解液的接触，从而减少界面副反应、减少产气，提高二次电池的高温存储性能、循环性能和安全性能；通过进一步包覆具有优异导锂离子的能力的晶态磷酸盐包覆层，可以使正极活性材料的表面的界面副反应有效降低，进而改善二次电池的高温循环及存储性能；通过再进一步包覆碳层作为第三包覆层，能够进一步提升二次电池的安全性能和动力学性能。
此外，在所述内核中，在磷酸锰锂的锰位掺杂的元素A还有助于减小该材料在脱嵌锂过程中磷酸锰锂的晶格变化率，提高磷酸锰锂正极材料的结构稳定性，大大减少锰的溶出并降低颗粒表面的氧活性；在磷位掺杂的元素R还有助于改变Mn-O键长变化的难易程度，从而改善电子电导并降低锂离子迁移势垒，促进锂离子迁移，提高二次电池的倍率性能。
另外，整个内核体系保持电中性，能够保证正极活性材料中的缺陷和杂相尽量少。如果正极活性材料中存在过量的过渡金属(例如锰)，由于该材料体系本身结构较稳定，那么多余的过渡金属很可能会以单质的形式析出，或在晶格内部形成杂相，保持电中性可使这样的杂相尽量少。另外，保证体系电中性还可以在部分情况下使材料中产生锂空位，从而使材料的动力学性能更优异。
本申请制备的内核的平均粒径范围为50-500nm，Dv50为200-300nm。内核的一次颗粒大小均在50-500nm的范围内，Dv50为200-300nm。如果所述内核平均粒径过大(超过500nm)，则使用该材料的二次电池的 克容量发挥会受到影响；如果所述内核平均粒径过小，则其比表面积较大，容易团聚，难以实现均匀包覆。
本申请中，中值粒径Dv50是指材料累计体积分布百分数达到50％时所对应的粒径。在本申请中，材料的中值粒径Dv50可采用激光衍射粒度分析法测定。例如参照标准GB/T 19077-2016，使用激光粒度分析仪(例如Malvern Master Size 3000)进行测定。
通过工艺控制(例如，对各种源的材料进行充分混合、研磨)，能够保证各元素在晶格中均匀分布，不出现聚集的情况。A元素和R元素掺杂后的磷酸锰锂的XRD图中的主要特征峰位置与未掺杂的LiMnPO 4的一致，说明掺杂过程没有引入杂质相，因此，所述内核性能的改善主要是来自元素掺杂，而不是杂相导致的。本申请发明人在制备本申请所述的正极活性材料后，通过聚焦离子束(简称FIB)切取已制备好的正极活性材料颗粒的中间区域，通过透射电子显微镜(简称TEM)以及X射线能谱分析(简称EDS)进行测试发现，各元素分布均匀，未出现聚集。
本申请中，晶态意指结晶度在50％以上，即50％-100％。结晶度小于50％的称为玻璃态。本申请所述的晶态焦磷酸盐和晶态磷酸盐的结晶度为50％至100％。具备一定结晶度的焦磷酸盐和磷酸盐不但有利于充分发挥焦磷酸盐包覆层阻碍锰溶出和磷酸盐包覆层优异的导锂离子的能力、减少界面副反应的功能，而且能够使得焦磷酸盐包覆层和磷酸盐包覆层能够更好的进行晶格匹配，从而能够实现包覆层更紧密的结合。
本申请中，所述正极活性材料的第一包覆层物质晶态焦磷酸盐和第二包覆层物质晶态磷酸盐的结晶度可以通过本领域中常规的技术手段来测试，例如通过密度法、红外光谱法、差示扫描量热法和核磁共振吸收方法测量，也可以通过例如，X射线衍射法来测试。
具体的X射线衍射法测试正极活性材料的第一包覆层晶态焦磷酸盐和第二包覆层晶态磷酸盐的结晶度的方法可以包括以下步骤：
取一定量的正极活性材料粉末，通过X射线测得总散射强度，它是整个空间物质的散射强度之和，只与初级射线的强度、正极活性材料粉 末化学结构、参加衍射的总电子数即质量多少有关，而与样品的序态无关；然后从衍射图上将结晶散射和非结晶散射分开，结晶度即是结晶部分散射对散射总强度之比。
需要说明的是，在本申请中，包覆层中的焦磷酸盐和磷酸盐的结晶度例如可通过调整烧结过程的工艺条件例如烧结温度、烧结时间等进行调节。
本申请中，由于金属离子在焦磷酸盐中难以迁移，因此焦磷酸盐作为第一包覆层可以将掺杂金属离子与电解液进行有效隔离。晶态焦磷酸盐的结构稳定，因此，晶态焦磷酸盐包覆能够有效抑制过渡金属的溶出，改善循环性能。
第一包覆层与核之间的结合类似于异质结，其结合的牢固程度受晶格匹配程度的限制。晶格失配在5％以下时，晶格匹配较好，两者容易结合紧密。紧密的结合能够保证在后续的循环过程中，包覆层不会脱落，有利于保证材料的长期稳定性。第一包覆层与核之间的结合程度的衡量主要通过计算核与包覆各晶格常数的失配度来进行。本申请中，在所述内核中掺杂了A和R元素后，与不掺杂元素相比，所述内核与第一包覆层的匹配度得到改善，内核与焦磷酸盐包覆层之间能够更紧密地结合在一起。
选择晶态磷酸盐作为第二包覆层，首先，是因为它与第一层包覆物晶态焦磷酸盐的晶格匹配度较高(失配度仅为3％)；其次，磷酸盐本身的稳定性好于焦磷酸盐，用其包覆焦磷酸盐有利于提高材料的稳定性。晶态磷酸盐的结构很稳定，其具有优异导锂离子的能力，因此，使用晶态磷酸盐进行包覆能够使正极活性材料的表面的界面副反应得到有效降低，从而改善二次电池的高温循环及存储性能。第二包覆层和第一包覆层之间的晶格匹配方式等，与上述第一包覆层和核之间的结合情况相似，晶格失配在5％以下时，晶格匹配较好，两者容易结合紧密。
碳作为第三层包覆的主要原因是碳层的电子导电性较好。由于在二次电池中应用时发生的是电化学反应，需要有电子的参与，因此，为了增加颗粒与颗粒之间的电子传输，以及颗粒上不同位置的电子传输，可 以使用具有优异导电性能的碳来对正极活性材料进行包覆。碳包覆可有效改善正极活性材料的导电性能和去溶剂化能力。
图1为理想中的三层包覆结构的正极活性材料的示意图。如图所示，最里面的圆示意表示内核，由内向外依次为第一包覆层、第二包覆层、第三包覆层。该图表示的是每层均完全包覆的理想状态，实践中，每一层包覆层可以是完全包覆，也可以是部分包覆。
在一些实施方式中，所述正极活性材料的一次颗粒的平均粒径范围为50-500nm，体积中值粒径Dv50在200-300nm范围内。由于颗粒会发生团聚，因此实际测得团聚后的二次颗粒大小可能为500-40000nm。正极活性材料颗粒的大小会影响材料的加工和极片的压实密度性能。通过选择一次颗粒的平均粒径在上述范围内，从而能够避免以下情况：所述正极活性材料的一次颗粒的平均粒径太小，可能会引起颗粒团聚，分散困难，并且需要较多的粘结剂，导致极片脆性较差；所述正极活性材料的一次颗粒的平均粒径太大，可能会使颗粒间的空隙较大，压实密度降低。
通过上述方案，能够有效抑制脱嵌锂过程中磷酸锰锂的晶格变化率和Mn溶出，从而提升二次电池的高温循环稳定性和高温储存性能。
在一些实施方式中，所述第一包覆层中的晶态焦磷酸盐的晶面间距范围为0.293-0.470nm，晶向(111)的夹角范围为18.00°-32.00°；所述第二包覆层中的晶态磷酸盐的晶面间距范围为0.244-0.425nm，晶向(111)的夹角范围为20.00°-37.00°。
本申请所述的正极活性材料中的第一包覆层和第二包覆层均使用晶态物质。对于包覆层中的晶态焦磷酸盐和晶态磷酸盐，可通过本领域中常规的技术手段进行表征，也可以例如借助透射电镜(TEM)进行表征。在TEM下，通过测试晶面间距可以区分内核和包覆层。
包覆层中的晶态焦磷酸盐和晶态磷酸盐的晶面间距和夹角的具体测试方法可以包括以下步骤：
取一定量的经包覆的正极活性材料样品粉末于试管中，并在试管中注入溶剂如酒精，然后进行充分搅拌分散，然后用干净的一次性塑料吸 管取适量上述溶液滴加在300目铜网上，此时，部分粉末将在铜网上残留，将铜网连带样品转移至TEM样品腔中进行测试，得到TEM测试原始图片，保存原始图片。
将上述TEM测试所得原始图片在衍射仪软件中打开，并进行傅里叶变换得到衍射花样，量取衍射花样中衍射光斑到中心位置的距离，即可得到晶面间距，夹角根据布拉格方程进行计算得到。
晶态焦磷酸盐的晶面间距范围和晶态磷酸盐的存在差异，可通过晶面间距的数值直接进行判断。
在上述晶面间距和夹角范围内的晶态焦磷酸盐和晶态磷酸盐，能够更有效地抑制脱嵌锂过程中磷酸锰锂的晶格变化率和Mn溶出，从而提升二次电池的高温循环性能、循环稳定性和高温储存性能。
在一些实施方式中，在所述内核中，y与1-y的比值为1：10至1：1，可选为1：4至1：1。此处y表示Mn位掺杂元素A的化学计量数之和。在满足上述条件时，使用所述正极活性材料的二次电池的能量密度和循环性能可进一步提升。
在一些实施方式中，在所述内核中，z与1-z的比值为1：9至1：999，可选为1：499至1：249。此处z表示P位掺杂元素R的化学计量数之和。在满足上述条件时，使用所述正极活性材料的二次电池的能量密度和循环性能可进一步提升。
在一些实施方式中，所述第三包覆层的碳为SP2形态碳与SP3形态碳的混合物，可选地，所述SP2形态碳与SP3形态碳的摩尔比为在0.1-10范围内的任意数值，可选为在2.0-3.0范围内的任意数值。
在一些实施方式中，所述SP2形态碳与SP3形态碳的摩尔比可为约0.1、约0.2、约03、约0.4、约0.5、约0.6、约0.7、约0.8、约0.9、约1、约2、约3、约4、约5、约6、约7、约8、约9或约10，或在上述任意值的任意范围内。
本申请中，“约”某个数值表示一个范围，表示该数值±10％的范围。
通过选择碳包覆层中碳的形态，从而提升二次电池的综合电性能。具体来说，通过使用SP2形态碳和SP3形态碳的混合形态并将SP2形态 碳和SP3形态碳的比例限制在一定范围内，能够避免以下情况：如果包覆层中的碳都是无定形SP3形态，则导电性差；如果都是石墨化的SP2形态，则虽然导电性良好，但是锂离子通路少，不利于锂的脱嵌。另外，将SP2形态碳与SP3形态碳的摩尔比限制在上述范围内，既能实现良好的导电性，又能保证锂离子的通路，因此有利于二次电池功能的实现及其循环性能。
所述第三包覆层碳的SP2形态和SP3形态的混合比可以通过烧结条件例如烧结温度和烧结时间来控制。例如，在使用蔗糖作为碳源制备第三包覆层的情况下，使蔗糖在高温下进行裂解后，在第二包覆层上沉积同时在高温作用下，会产生既有SP3形态也有SP2形态的碳包覆层。SP2形态碳和SP3形态碳的比例可以通过选择高温裂解条件和烧结条件来调控。
所述第三包覆层碳的结构和特征可通过拉曼(Raman)光谱进行测定，具体测试方法如下：通过对Raman测试的能谱进行分峰，得到Id/Ig(其中Id为SP3形态碳的峰强度，Ig为SP2形态碳的峰强度)，从而确认两者的摩尔比。
在一些实施方式中，所述第一包覆层的包覆量为大于0且小于或等于6重量％，可选为大于0且小于或等于5.5重量％，更可选为大于0且小于或等于2重量％，基于所述内核的重量计；和/或
所述第二包覆层的包覆量为大于0且小于或等于6重量％，可选为大于0且小于或等于5.5重量％，更可选为2-4重量％，基于所述内核的重量计；和/或
所述第三包覆层的包覆量为大于0小于或等于6重量％，可选为大于0小于或等于5.5重量％，更可选为大于0小于或等于2重量％，基于所述内核的重量计。
本申请中，每一层的包覆量均不为零。
本申请所述的具有核-壳结构的正极活性材料中，三层包覆层的包覆量优选在上述范围内，由此能够对所述内核进行充分包覆，并同时在不 牺牲正极活性材料克容量的前提下，进一步改善二次电池的动力学性能和安全性能。
对于第一包覆层而言，通过包覆量在上述范围内，则能够避免以下情况：包覆量过少则意味着包覆层厚度较薄，可能无法有效阻碍过渡金属的迁移；包覆量过大则意味着包覆层过厚，会影响Li +的迁移，进而影响材料的倍率性能。
对于第二包覆层而言，通过包覆量在上述范围内，则能够避免以下情况：包覆量过多，可能会影响材料整体的平台电压；包覆量过少，可能无法实现足够的包覆效果。
对于第三包覆层而言，碳包覆主要起到增强颗粒间的电子传输的作用，然而由于结构中还含有大量的无定形碳，因此碳的密度较低，因此，如果包覆量过大，会影响极片的压实密度。
在一些实施方式中，所述第一包覆层的厚度为1-10nm；和/或
所述第二包覆层的厚度为2-15nm；和/或
所述第三包覆层的厚度为2-25nm。
在一些实施方式中，所述第一包覆层的厚度可为约2nm、约3nm、约4nm、约5nm、约6nm、约7nm、约8nm、约9nm或约10nm，或在上述任意数值的任意范围内。
在一些实施方式中，所述第二包覆层的厚度可为约2nm、约3nm、约4nm、约5nm、约6nm、约7nm、约8nm、约9nm、约10nm、约11nm、约12nm、约13nm、约14nm、约15nm，或在上述任意数值的任意范围内。
在一些实施方式中，所述第三层包覆层的厚度可为约2nm、约3nm、约4nm、约5nm、约6nm、约7nm、约8nm、约9nm、约10nm、约11nm、约12nm、约13nm、约14nm、约15nm、约16nm、约17nm、约18nm、约19nm、约20nm、约21nm、约22nm、约23nm、约24nm或约25nm，或在上述任意数值的任意范围内。
当所述第一包覆层的厚度范围为1-10nm时，能够避免过厚时可能产生的对材料的动力学性能的不利影响，且能够避免过薄时可能无法有效阻碍过渡金属离子的迁移的问题。
当所述第二包覆层的厚度在2-15nm范围内时，所述第二包覆层的表面结构稳定，与电解液的副反应小，因此能够有效减轻界面副反应，从而提升二次电池的高温性能。
当所述第三包覆层的厚度范围为2-25nm时，能够提升材料的电导性能并且改善使用所述正极活性材料制备的电池极片的压密性能。
包覆层的厚度大小测试主要通过FIB进行，具体方法可以包括以下步骤：从待测正极活性材料粉末中随机选取单个颗粒，从所选颗粒中间位置或中间位置附近切取100nm左右厚度的薄片，然后对薄片进行TEM测试，量取包覆层的厚度，测量3-5个位置，取平均值。
在一些实施方式中，基于正极活性材料的重量计，锰元素含量在10重量％-35重量％范围内，可选在15重量％-30重量％范围内，更可选在17重量％-20重量％范围内，磷元素的含量在12重量％-25重量％范围内，可选在15重量％-20重量％范围内，锰元素和磷元素的重量比范围为0.90-1.25，可选为0.95-1.20。
在本申请中，在仅正极活性材料的内核中含有锰的情况下，锰的含量可与内核的含量相对应。
在本申请中，将所述锰元素的含量限制在上述范围内，能够有效避免若锰元素含量过大可能会引起的材料结构稳定性变差、密度下降等问题，从而提升二次电池的循环、存储和压密等性能；且能够避免若锰元含量过小可能会导致的电压平台低等问题，从而提升二次电池的能量密度。
本申请中，将所述磷元素的含量限制在上述范围内，能够有效避免以下情况：若磷元素的含量过大，可能会导致P-O的共价性过强而影响小极化子导电，从而影响材料的电导率；若磷含量过小，可能会使所述内核、所述第一包覆层中的焦磷酸盐和/或所述第二包覆层中的磷酸盐晶格结构的稳定性下降，从而影响材料整体的稳定性。
锰与磷含量重量比大小对二次电池的性能具有以下影响：该重量比过大，意味着锰元素过多，锰溶出增加，影响正极活性材料的稳定性和克容量发挥，进而影响二次电池的循环性能及存储性能；该重量比过小，意味着磷元素过多，则容易形成杂相，会使材料的放电电压平台下降，从而使二次电池的能量密度降低。
锰元素和磷元素的测量可采用本领域中常规的技术手段进行。特别地，采用以下方法测定锰元素和磷元素的含量：将材料在稀盐酸中(浓度10-30％)溶解，利用ICP测试溶液各元素的含量，然后对锰元素的含量进行测量和换算，得到其重量占比。
在一些实施方式中，所述具有核-壳结构的正极活性材料在完全脱嵌锂前后的晶格变化率为4％以下，可选为3.8％以下，更可选为2.0-3.8％。
磷酸锰锂(LiMnPO 4)的脱嵌锂过程是两相反应。两相的界面应力由脱嵌锂前后的晶格变化率大小决定，晶格变化率越小，界面应力越小，Li +传输越容易。因此，减小内核的晶格变化率将有利于增强Li +的传输能力，从而改善二次电池的倍率性能。本申请所述的具有核-壳结构的正极活性材料能够实现4％以下的脱嵌锂前后的晶格变化率，因此使用所述正极活性材料能够改善二次电池的倍率性能。晶格变化率可通过本领域中已知的方法，例如X射线衍射图谱(XRD)测得。
在一些实施方式中，所述具有核-壳结构的正极活性材料的Li/Mn反位缺陷浓度为4％以下，可选为2.2％以下，更可选为1.5-2.2％。
本申请所述的Li/Mn反位缺陷，指的是LiMnPO 4晶格中，Li +与Mn 2+的位置发生互换。相应地，Li/Mn反位缺陷浓度指的是与Mn 2+发生互换的Li +占Li +总量的百分比。本申请中，Li/Mn反位缺陷浓度例如，可以依据JIS K 0131-1996进行测试。
本申请所述的具有核-壳结构的正极活性材料能够实现上述较低的Li/Mn反位缺陷浓度。虽然机理尚不十分清楚，但本申请发明人推测，由于LiMnPO 4晶格中，Li +与Mn 2+会发生位置互换，而Li +传输通道为一维通道，因此Mn 2+在Li +通道中将难以迁移，进而阻碍Li +的传输。由此，本申请所述的具有核-壳结构的正极活性材料由于Li/Mn反位缺陷浓度较 低，在上述范围内，因此，能够避免Mn 2+阻碍Li +的传输，同时提升正极活性材料的克容量发挥和倍率性能。
在一些实施方式中，所述正极活性材料在3T下的压实密度为2.2g/cm 3以上，可选地为2.2g/cm 3以上且2.8g/cm 3以下。压实密度越高，单位体积活性材料的重量越大，因此提高压实密度有利于提高电芯的体积能量密度。压实密度可依据GB/T 24533-2009测量。
在一些实施方式中，所述正极活性材料的表面氧价态为-1.90以下，可选地为-1.90至-1.98。
氧的稳定价态本为-2价，价态越接近-2价，其得电子能力越强，即氧化性越强，通常情况下，其表面价态在-1.7以下。本申请通过如上所述将正极活性材料的表面氧价态限定在上述范围内，能够减轻正极材料与电解液的界面副反应，从而改善电芯的循环，高温存储产气等性能。
表面氧价态可通过本领域中已知的方法测量，例如通过电子能量损失谱(EELS)测量。
本申请的第二方面提供一种正极活性材料的制备方法，包括以下步骤：
提供内核材料的步骤：所述内核化学式为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中的一种或多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素；
包覆步骤：分别提供Li aMP 2O 7和/或M b(P 2O 7) c以及XPO 4悬浊液，将所述内核材料加入到上述悬浊液中并混合，经烧结获得正极活性材料，其中0≤a≤2，1≤b≤4，1≤c≤6，所述a、b和c的值满足以下条件：使晶态焦磷酸盐Li aMP 2O 7或M b(P 2O 7) c保持电中性；M各自独立地为选自Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；X 为选自Li、Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；
其中，所述正极活性材料具有核-壳结构，其包括所述内核及包覆所述内核的壳，所述壳包括包覆所述内核的第一包覆层、包覆所述第一包覆层的第二包覆层以及包覆所述第二包覆层的第三包覆层，所述第一包覆层包括晶态焦磷酸盐Li aMP 2O 7和/或M b(P 2O 7) c，所述第二包覆层包括晶态磷酸盐XPO 4，所述第三包覆层为碳。
在一些实施方式中，所述提供内核材料的步骤包括以下步骤：
步骤(1)：将锰源、元素A的掺杂剂和酸在容器中混合并搅拌，得到掺杂有元素A的锰盐颗粒；
步骤(2)：将所述掺杂有元素A的锰盐颗粒与锂源、磷源和元素R的掺杂剂在溶剂中混合并得到浆料，在惰性气体气氛保护下烧结后得到掺杂有元素A和元素R的内核，其中，所述掺杂有元素A和元素R的内核为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中的一种或多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素。
本申请的制备方法对材料的来源并没有特别的限制，某种元素的来源可包括该元素的单质、硫酸盐、卤化物、硝酸盐、有机酸盐、氧化物或氢氧化物中的一种或多种，前体是该来源可实现本申请制备方法的目的。
在一些实施方式中，所述元素A的掺杂剂为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素各自的单质、碳酸盐、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种。
在一些实施方式中，所述元素R的掺杂剂为选自B、Si、N和S中的一种或多种元素各自的无机酸、亚酸、有机酸、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种；
本申请中，锰源可为本领域已知的可用于制备磷酸锰锂的含锰物质。作为示例，所述锰源可为选自单质锰、二氧化锰、磷酸锰、草酸锰、碳酸锰中的一种或多种。
本申请中，酸可为选自盐酸、硫酸、硝酸、磷酸、硅酸、亚硅酸等有机酸和有机酸如草酸中的一种或多种。在一些实施方式中，所述酸为浓度为60重量％以下的稀的有机酸。
本申请中，锂源可为本领域已知的可用于制备磷酸锰锂的含锂物质。作为示例，所述锂源为选自碳酸锂、氢氧化锂、磷酸锂、磷酸二氢锂中的一种或多种。
本申请中，磷源可为本领域已知的可用于制备磷酸锰锂的含磷物质。作为示例，所述磷源为选自磷酸氢二铵、磷酸二氢铵、磷酸铵和磷酸中的一种或多种。
在一些可选实施方式中，在所述锰源、所述元素A的掺杂剂与所述酸在溶剂中反应得到掺杂有元素A的锰盐悬浮液后，将所述悬浮液过滤，烘干，并进行砂磨以得到粒径为50-200nm的经元素A掺杂的锰盐颗粒。
在一些可选实施方式中，将步骤(2)中的浆料进行干燥得到粉料，然后将粉料烧结得到掺杂有元素A和元素R的内核。
在一些实施方式中，所述步骤(1)在20-120℃、可选为40-120℃的温度下进行混合；和/或
所述步骤(1)中所述搅拌在400-700rpm下进行1-9h，可选地为3-7h。
可选地，所述步骤(1)中的反应温度可在约30℃、约50℃、约60℃、约70℃、约80℃、约90℃、约100℃、约110℃或约120℃进行；所述步骤(1)中所述搅拌进行约2小时、约3小时、约4小时、约5小时、约6小时、约7小时、约8小时或约9小时；可选地，所述步骤(1)中的反应温度、搅拌时间可在上述任意数值的任意范围内。
在一些实施方式中，所述步骤(2)在20-120℃、可选为40-120℃的温度下进行混合1-12h。可选地，所述步骤(2)中的反应温度可在约30℃、约50℃、约60℃、约70℃、约80℃、约90℃、约100℃、约110℃或约120℃进行；所述步骤(2)中所述混合进行约2小时、约3小时、约4小时、约5小时、约6小时、约7小时、约8小时、约9小时、约10小时、约11小时或约12小时；可选地，所述步骤(2)中的反应温度、混合时间可在上述任意数值的任意范围内。
当内核颗粒制备过程中的温度和时间处于上述范围内时，制备获得的内核以及由其制得的正极活性材料的晶格缺陷较少，有利于抑制锰溶出，减少正极活性材料与电解液的界面副反应，从而改善二次电池的循环性能和安全性能。
在一些实施方式中，可选地，在制备A元素和R元素掺杂的稀酸锰颗粒的过程中，控制溶液pH为3.5-6，可选地，控制溶液pH为4-6，更可选地，控制溶液pH为4-5。需要说明的是，在本申请中可通过本领域通常使用的方法调节所得混合物的pH，例如可通过添加酸或碱。
在一些实施方式中，可选地，在步骤(2)中，所述锰盐颗粒与锂源、磷源的摩尔比为1：0.5-2.1：0.5-2.1，更可选地，所述掺杂有元素A的锰盐颗粒与锂源、磷源的摩尔比为约1：1：1。
在一些实施方式中，可选地，制备A元素和R元素掺杂的磷酸锰锂过程中的烧结条件为：在惰性气体或惰性气体与氢气混合气氛下在600-950℃下烧结4-10小时；可选地，所述烧结可在约650℃、约700℃、约750℃、约800℃、约850℃或约900℃下烧结约2小时、约3小时、约4小时、约5小时、约6小时、约7小时、约8小时、约9小时或约10小时；可选地，所述烧结的温度、烧结时间可在上述任意数值的任意范围内。在制备A元素和R元素掺杂的磷酸锰锂过程中，烧结温度过低以及烧结时间过短时，会导致材料内核的结晶度较低，会影响整体的性能发挥，而烧结温度过高时，材料内核中容易出现杂相，从而影响整体的性能发挥；烧结时间过长时，材料内核颗粒长的较大，从而影响克容量发挥，压实密度和倍率性能等。
在一些可选实施方式中，可选地，保护气氛为70-90体积％氮气和10-30体积％氢气的混合气体。
在一些实施方式中，所述包覆步骤包括：
第一包覆步骤：将元素M的源、磷源和酸以及任选地锂源，溶于溶剂中，得到第一包覆层悬浊液；将内核步骤中获得的内核与第一包覆步骤获得的第一包覆层悬浊液充分混合，干燥，然后烧结，得到第一包覆层包覆的材料；
第二包覆步骤：将元素X的源、磷源和酸溶于溶剂中，得到第二包覆层悬浊液；将第一包覆步骤中获得的第一包覆层包覆的材料与第二包覆步骤获得的第二包覆层悬浊液充分混合，干燥，然后烧结，得到两层包覆层包覆的材料；
第三包覆步骤：将碳源溶于溶剂中，充分溶解得到第三包覆层溶液；然后将第二包覆步骤中获得的两层包覆层包覆的材料加入所述第三包覆层溶液中，混合均匀，干燥，然后烧结得到三层包覆层包覆的材料，即正极活性材料。
在一些实施方式中，所述元素M的源为选自Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素各自的单质、碳酸盐、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种。
在一些实施方式中，所述元素X的源为选自Li、Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素各自的单质、碳酸盐、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种。
所述元素A、R、M、X各自的源的加入量取决于目标掺杂量，锂源、锰源和磷源的用量之比符合化学计量比。
作为示例，所述碳源为选自淀粉、蔗糖、葡萄糖、聚乙烯醇、聚乙二醇、柠檬酸中的一种或多种。
在一些实施方式中，所述第一包覆步骤中，控制溶解有元素M的源、磷源和酸以及任选地锂源的溶液pH为3.5-6.5，然后搅拌并反应1-5h， 然后将所述溶液升温至50-120℃，并保持该温度2-10h，和/或，烧结在650-800℃下进行2-6小时。
可选地，在第一包覆步骤中，所述反应充分进行。可选地，在第一包覆步骤中，所述反应进行约1.5小时、约2小时、约3小时、约4小时、约4.5小时或约5小时。可选地，第一包覆步骤中，所述反应的反应时间可在上述任意数值的任意范围内。
可选地，在第一包覆步骤中，控制溶液pH为4-6。可选地，在第一包覆步骤中，将所述溶液升温至约55℃、约60℃、约70℃、约80℃、约90℃、约100℃、约110℃或约120℃，并在该温度下保持约2小时、约3小时、约4小时、约5小时、约6小时、约7小时、约8小时、约9小时或约10小时；可选地，第一包覆步骤中，所述升温的温度和保持时间可在上述任意数值的任意范围内。
可选地，在所述第一包覆步骤中，所述烧结可在约650℃、约700℃、约750℃、或约800℃下烧结约2小时、约3小时、约4小时、约5小时或约6小时；可选地，所述烧结的温度、烧结时间可在上述任意数值的任意范围内。
在所述第一包覆步骤中，通过将烧结温度和时间控制在以上范围内，可以避免以下情况：当所述第一包覆步骤中的烧结温度过低以及烧结时间过短时，会导致第一包覆层的结晶度低，非晶态物质较多，这样会导致抑制金属溶出的效果下降，从而影响二次电池的循环性能和高温存储性能；而烧结温度过高时，会导致第一包覆层出现杂相，也会影响到其抑制金属溶出的效果，从而影响二次电池的循环和高温存储性能等；烧结时间过长时，会使第一包覆层的厚度增加，影响Li +的迁移，从而影响材料的克容量发挥和倍率性能等。
在一些实施方式中，所述第二包覆步骤中，将元素X的源、磷源和酸溶于溶剂后，搅拌并反应1-10h，然后将所述溶液升温至60-150℃，并保持该温度2-10h，和/或，烧结在500-700℃下进行6-10小时。
可选地，在第二包覆步骤中，所述反应充分进行。可选地，在第二包覆步骤中，所述反应进行约1.5小时、约2小时、约3小时、约4小 时、约4.5小时、约5小时、约6小时、约7小时、约8小时、约9小时或约10小时。可选地，第二包覆步骤中，所述反应的反应时间可在上述任意数值的任意范围内。
可选地，在第二包覆步骤中，将所述溶液升温至约65℃、约70℃、约80℃、约90℃、约100℃、约110℃、约120℃、约130℃、约140℃或约150℃，并在该温度下保持约2小时、约3小时、约4小时、约5小时、约6小时、约7小时、约8小时、约9小时或约10小时；可选地，第二包覆步骤中，所述升温的温度和保持时间可在上述任意数值的任意范围内。
在所述提供内核材料的步骤和所述第一包覆步骤和所述第二包覆步骤中，在烧结之前，即，在发生化学反应的内核材料的制备中(步骤(1)-(2))以及在第一包覆层悬浮液和第二包覆层悬浮液的制备中，通过如上所述选择适当的反应温度和反应时间，从而能够避免以下情况：反应温度过低时，则反应无法发生或反应速率较慢；温度过高时，产物分解或形成杂相；反应时间过长时，产物粒径较大，可能会增加后续工艺的时间和难度；反应时间过短时，则反应不完全，获得的产物较少。
可选地，在第二包覆步骤中，所述烧结可在约550℃、约600℃或约700℃下烧结约6小时、约7小时、约8小时、约9小时或约10小时；可选地，所述烧结的温度、烧结时间可在上述任意数值的任意范围内。
在所述第二包覆步骤中，通过将烧结温度和时间控制在以上范围内，可以避免以下情况：当所述第二包覆步骤中的烧结温度过低以及烧结时间过短时，会导致第二包覆层的结晶度低，非晶态较多，降低材料表面反应活性的性能下降，从而影响二次电池的循环和高温存储性能等；而烧结温度过高时，会导致第二包覆层出现杂相，也会影响到其降低材料表面反应活性的效果，从而影响二次电池的循环和高温存储性能等；烧结时间过长时，会使第二包覆层的厚度增加，影响材料的电压平台，从而使材料的能量密度下降等。
在一些实施方式中，所述第三包覆步骤中的烧结在700-800℃下进行6-10小时。可选地，在第三包覆步骤中，所述烧结可在约700℃、约750℃ 或约800℃下烧结约6小时、约7小时、约8小时、约9小时或约10小时；可选地，所述烧结的温度、烧结时间可在上述任意数值的任意范围内。
在所述第三包覆步骤中，通过将烧结温度和时间控制在以上范围内，可以避免以下情况：当所述第三包覆步骤中的烧结温度过低时，会导致第三包覆层的石墨化程度下降，影响其导电性，从而影响材料的克容量发挥；烧结温度过高时，会造成第三包覆层的石墨化程度过高，影响Li +的传输，从而影响材料的克容量发挥等；烧结时间过短时，会导致包覆层过薄，影响其导电性，从而影响材料的克容量发挥；烧结时间过长时，会导致包覆层过厚，影响材料的压实密度等。
在上述第一包覆步骤、第二包覆步骤、第三包覆步骤中，所述干燥均在100℃至200℃、可选为110℃至190℃、更可选为120℃至180℃、甚至更可选为120℃至170℃、最可选为120℃至160℃的干燥温度下进行，干燥时间为3-9h、可选为4-8h，更可选为5-7h，最可选为约6h。
通过本申请所述的正极活性材料的制备方法所制备的正极活性材料，其制备的二次电池在循环后Mn与Mn位掺杂元素的溶出量降低，且高温稳定性、高温循环性能和倍率性能得到改善。另外，原料来源广泛、成本低廉，工艺简单，有利于实现工业化。
[正极极片]
本申请的第三方面提供一种正极极片，其包括正极集流体以及设置在正极集流体至少一个表面的正极膜层，所述正极膜层包括本申请第一方面所述的具有核-壳结构的正极活性材料或通过本申请第二方面所述的方法制备的正极活性材料，并且所述正极活性材料在所述正极膜层中的含量在10重量％以上，可选为90-99.5重量％，更可选为95-99.5重量％，基于所述正极膜层的总重量计。
本申请中，作为示例，正极集流体具有在其自身厚度方向相对的两个表面，正极膜层设置在正极集流体相对的两个表面的其中任意一者或两者上。
在一些实施方式中，所述正极集流体可采用金属箔片或复合集流体。例如，作为金属箔片，可采用铝箔。复合集流体可包括高分子材料基层和形成于高分子材料基层至少一个表面上的金属层。复合集流体可通过将金属材料(铝、铝合金、镍、镍合金、钛、钛合金、银及银合金等)形成在高分子材料基材(如聚丙烯(PP)、聚对苯二甲酸乙二醇酯(PET)、聚对苯二甲酸丁二醇酯(PBT)、聚苯乙烯(PS)、聚乙烯(PE)等的基材)上而形成。
本申请中，正极活性材料采用本申请所述的正极活性材料。
在一些实施方式中，正极膜层还可选地包括粘结剂。作为示例，所述粘结剂可以包括聚偏氟乙烯(PVDF)、聚四氟乙烯(PTFE)、偏氟乙烯-四氟乙烯-丙烯三元共聚物、偏氟乙烯-六氟丙烯-四氟乙烯三元共聚物、四氟乙烯-六氟丙烯共聚物及含氟丙烯酸酯树脂中的至少一种。
在一些实施方式中，正极膜层还可选地包括导电剂。作为示例，所述导电剂可以包括超导碳、乙炔黑、炭黑、科琴黑、碳点、碳纳米管、石墨烯及碳纳米纤维中的至少一种。
在一些实施方式中，在所述正极极片中，粘结剂占比为0.4-5.5重量％，可选为0.4-4.5重量％，导电剂占比为0.1-2.5重量％，可选为0.1-0.5重量％，其他添加剂占比为0.001-1重量％，基于所述正极膜片的总重量计。
在一些实施方式中，正极极片的涂覆重量为0.28-0.45g/1540.25mm 2，压实密度达到2.0g/cm 3以上，可选为2.2-2.8g/cm 3。
在一些实施方式中，可以通过以下方式制备正极极片：将上述用于制备正极极片的组分，例如正极活性材料、导电剂、粘结剂和任意其他的组分分散于溶剂(例如N-甲基吡咯烷酮)中，形成正极浆料；将正极浆料涂覆在正极集流体上，经烘干、冷压等工序后，即可得到正极极片。
[负极极片]
负极极片包括负极集流体以及设置在负极集流体至少一个表面上的负极膜层，所述负极膜层包括负极活性材料。
作为示例，负极集流体具有在其自身厚度方向相对的两个表面，负极膜层设置在负极集流体相对的两个表面中的任意一者或两者上。
在一些实施方式中，所述负极集流体可采用金属箔片或复合集流体。例如，作为金属箔片，可以采用铜箔。复合集流体可包括高分子材料基层和形成于高分子材料基材至少一个表面上的金属层。复合集流体可通过将金属材料(铜、铜合金、镍、镍合金、钛、钛合金、银及银合金等)形成在高分子材料基材(如聚丙烯(PP)、聚对苯二甲酸乙二醇酯(PET)、聚对苯二甲酸丁二醇酯(PBT)、聚苯乙烯(PS)、聚乙烯(PE)等的基材)上而形成。
在一些实施方式中，负极活性材料可采用本领域公知的用于二次电池的负极活性材料。作为示例，负极活性材料可包括以下材料中的至少一种：人造石墨、天然石墨、软炭、硬炭、硅基材料、锡基材料和钛酸锂等。所述硅基材料可为选自单质硅、硅氧化合物、硅碳复合物、硅氮复合物以及硅合金中的至少一种。所述锡基材料可为选自单质锡、锡氧化合物以及锡合金中的至少一种。但本申请并不限定于这些材料，还可以使用其他可被用作电池负极活性材料的传统材料。这些负极活性材料可以仅单独使用一种，也可以将两种以上组合使用。
在一些实施方式中，负极膜层还可选地包括粘结剂。所述粘结剂可为选自丁苯橡胶(SBR)、聚丙烯酸(PAA)、聚丙烯酸钠(PAAS)、聚丙烯酰胺(PAM)、聚乙烯醇(PVA)、海藻酸钠(SA)、聚甲基丙烯酸(PMAA)及羧甲基壳聚糖(CMCS)中的至少一种。
在一些实施方式中，负极膜层还可选地包括导电剂。导电剂可为选自超导碳、乙炔黑、炭黑、科琴黑、碳点、碳纳米管、石墨烯及碳纳米纤维中的至少一种。
在一些实施方式中，负极膜层还可选地包括其他助剂，例如增稠剂(如羧甲基纤维素钠(CMC-Na))等。
在一些实施方式中，可以通过以下方式制备负极极片：将上述用于制备负极极片的组分，例如负极活性材料、导电剂、粘结剂和任意其他组分分散于溶剂(例如去离子水)中，形成负极浆料；将负极浆料涂覆在负极集流体上，经烘干、冷压等工序后，即可得到负极极片。
[电解质]
电解质在正极极片和负极极片之间起到传导离子的作用。本申请对电解质的种类没有具体的限制，可根据需求进行选择。例如，电解质可以是液态的、凝胶态的或全固态的。
在一些实施方式中，所述电解质采用电解液。所述电解液包括电解质盐和溶剂。
在一些实施方式中，电解质盐可为选自六氟磷酸锂、四氟硼酸锂、高氯酸锂、六氟砷酸锂、双氟磺酰亚胺锂、双三氟甲磺酰亚胺锂、三氟甲磺酸锂、二氟磷酸锂、二氟草酸硼酸锂、二草酸硼酸锂、二氟二草酸磷酸锂及四氟草酸磷酸锂中的至少一种。
在一些实施方式中，溶剂可为选自碳酸亚乙酯、碳酸亚丙酯、碳酸甲乙酯、碳酸二乙酯、碳酸二甲酯、碳酸二丙酯、碳酸甲丙酯、碳酸乙丙酯、碳酸亚丁酯、氟代碳酸亚乙酯、甲酸甲酯、乙酸甲酯、乙酸乙酯、乙酸丙酯、丙酸甲酯、丙酸乙酯、丙酸丙酯、丁酸甲酯、丁酸乙酯、1,4-丁内酯、环丁砜、二甲砜、甲乙砜及二乙砜中的至少一种。
在一些实施方式中，所述电解液还可选地包括添加剂。例如添加剂可以包括负极成膜添加剂、正极成膜添加剂，还可以包括能够改善电池某些性能的添加剂，例如改善电池过充性能的添加剂、改善电池高温或低温性能的添加剂等。
[隔离膜]
在一些实施方式中，二次电池中还包括隔离膜。本申请对隔离膜的种类没有特别的限制，可以选用任意公知的具有良好的化学稳定性和机械稳定性的多孔结构隔离膜。
在一些实施方式中，隔离膜的材质可为选自玻璃纤维、无纺布、聚乙烯、聚丙烯及聚偏二氟乙烯中的至少一种。隔离膜可以是单层薄膜，也可以是多层复合薄膜，没有特别限制。在隔离膜为多层复合薄膜时，各层的材料可以相同或不同，没有特别限制。
在一些实施方式中，正极极片、负极极片和隔离膜可通过卷绕工艺或叠片工艺制成电极组件。
[二次电池]
本申请第四方面提供一种二次电池，其包括本申请第一方面所述的具有核-壳结构的正极活性材料或通过本申请第二方面所述的方法制备的正极活性材料或本申请第三方面所述的正极极片。
所述二次电池包括根据本申请所述的正极极片、负极极片、电解质和隔离膜。在电池充放电过程中，活性离子在正极极片和负极极片之间往返嵌入和脱出。电解质在正极极片和负极极片之间起到传导离子的作用。隔离膜设置在正极极片和负极极片之间，主要起到防止正负极短路的作用，同时可以使离子通过。
在一些实施方式中，锂离子二次电池可包括外包装。该外包装可用于封装上述电极组件及电解质。
在一些实施方式中，锂离子二次电池的外包装可以是硬壳，例如硬塑料壳、铝壳、钢壳等。锂离子二次电池的外包装也可以是软包，例如袋式软包。软包的材质可以是塑料，作为塑料，可列举出聚丙烯(PP)、聚对苯二甲酸丁二醇酯(PBT)以及聚丁二酸丁二醇酯(PBS)等。
下文中将适当参照附图对本申请的二次电池、电池模块、电池包和用电装置进行说明。
本申请对二次电池的形状没有特别的限制，其可以是圆柱形、方形或其他任意的形状。例如，图2是作为一个示例的方形结构的二次电池5。
在一些实施方式中，参照图3，外包装可包括壳体51和盖板53。其中，壳体51可包括底板和连接于底板上的侧板，底板和侧板围合形成容纳腔。壳体51具有与容纳腔连通的开口，盖板53能够盖设于所述开口，以封闭所述容纳腔。正极极片、负极极片和隔离膜可经卷绕工艺或叠片工艺形成电极组件52。电极组件52封装于所述容纳腔内。电解液浸润于电极组件52中。二次电池5所含电极组件52的数量可以为一个或多个，本领域技术人员可根据具体实际需求进行选择。
在一些实施方式中，锂离子二次电池可以组装成电池模块，电池模块所含锂离子电池的数量可以为一个或多个，具体数量本领域技术人员可根据电池模块的应用和容量进行选择。
因此，本申请的第五方面提供一种电池模块，其包括根据本申请第四方面所述的二次电池。
图4是作为一个示例的电池模块4。参照图4，在电池模块4中，多个锂离子电池5可以沿电池模块4的长度方向依次排列设置。当然，也可以按照其他任意的方式进行排布。进一步可以通过紧固件将该多个锂离子电池5进行固定。
可选地，电池模块4还可以包括具有容纳空间的外壳，多个锂离子电池5容纳于该容纳空间。
在一些实施方式中，上述电池模块还可以组装成电池包，电池包所含电池模块的数量可以为一个或多个，具体数量本领域技术人员可根据电池包的应用和容量进行选择。
因此，本申请的第六方面提供一种电池包，其包括根据本申请第五所述的电池模块。
图5和图6是作为一个示例的电池包1。参照图5和图6，在电池包1中可以包括电池箱和设置于电池箱中的多个电池模块4。电池箱包括上箱体2和下箱体3，上箱体2能够盖设于下箱体3，并形成用于容纳电池模块4的封闭空间。多个电池模块4可以按照任意的方式排布于电池箱中。
本申请的第七方面提供一种用电装置，其包括根据本申请第四方面所述的二次电池、根据本申请第五方面所述的电池模块或根据本申请第六方面所述的电池包中的至少一种。所述二次电池、电池模块、或电池包可以用作所述用电装置的电源，也可以用作所述用电装置的能量存储单元。所述用电装置可以包括移动设备(例如手机、笔记本电脑等)、电动车辆(例如纯电动车、混合动力电动车、插电式混合动力电动车、电动自行车、电动踏板车、电动高尔夫球车、电动卡车等)、电气列车、船舶及卫星、储能***等，但不限于此。
作为所述用电装置，可以根据其使用需求来选择二次电池、电池模块或电池包。
图7是作为一个示例的用电装置。该用电装置为纯电动车、混合动力电动车、或插电式混合动力电动车等。为了满足该用电装置对二次电池的高功率和高能量密度的需求，可以采用电池包或电池模块。
作为另一个示例的装置可以是手机、平板电脑、笔记本电脑等。该装置通常要求轻薄化，可以采用二次电池作为电源。
本申请所述正极极片、二次电池、电池模块、电池包和用电装置包括本申请所述的正极活性材料，因而具有较高的克容量、良好的循环性能和安全性能。
实施例
以下，说明本申请的实施例。下面描述的实施例是示例性的，仅用于解释本申请，而不能理解为对本申请的限制。实施例中未注明具体技术或条件的，按照本领域内的文献所描述的技术或条件或者按照产品说明书进行。所用试剂或仪器未注明生产厂商者，均为可以通过市购获得的常规产品。本发明实施例中各成分的含量，如果没有特别说明，均以不含结晶水的质量计。
本申请实施例涉及的原材料来源如下：
Figure PCTCN2021140462-appb-000001
Figure PCTCN2021140462-appb-000002
I.电池制备
实施例1：
步骤1：正极活性材料的制备
步骤S1：制备Fe、Co、V和S共掺杂的草酸锰
将689.6g碳酸锰、455.27g碳酸亚铁、4.65g硫酸钴、4.87g二氯化钒加入混料机中充分混合6h。然后将得到的混合物转入反应釜中，并加入5L去离子水和1260.6g二水合草酸，加热至80℃，以500rpm的转速充分搅拌6h，混合均匀，直至反应终止无气泡产生，得到Fe、Co、和V共掺杂的草酸锰悬浮液。然后将悬浮液过滤，在120℃下烘干，再进行砂磨，得到粒径为100nm的草酸锰颗粒。
步骤S2：制备内核Li 0.997Mn 0.60Fe 0.393V 0.004Co 0.003P 0.997S 0.003O 4
取(1)中制备的草酸锰1793.1g以及368.3g碳酸锂、1146.6g磷酸二氢铵和4.9g稀硫酸，将它们加入到20L去离子水中，充分搅拌，在80℃下均匀混合反应10h，得到浆料。将所述浆料转入喷雾干燥设备中进行喷雾干燥造粒，在250℃的温度下进行干燥，得到粉料。在保护气氛(90％氮气和10％氢气)中，在700℃下将所述粉料在辊道窑中进行烧结4h，得到上述内核材料。
步骤S3：第一包覆层悬浊液的制备
制备Li 2FeP 2O 7溶液，将7.4g碳酸锂，11.6g碳酸亚铁，23.0g磷酸二氢铵和12.6g二水合草酸溶于500mL去离子水中，控制pH为5，然后搅拌并在室温下反应2h得到溶液，之后将该溶液升温到80℃并保持此温度4h，得到第一包覆层悬浊液。
步骤S4：第一包覆层的包覆
将步骤S2中获得的掺杂后的1571.9g磷酸锰锂内核材料加入到步骤S3中获得的第一包覆层悬浊液(包覆物质含量为15.7g)中，充分搅拌混合6h，混合均匀后，转入120℃烘箱中干燥6h，然后在650℃下烧结6h得到焦磷酸盐包覆后的材料。
步骤S5：第二包覆层悬浊液的制备
将3.7g碳酸锂、11.6g碳酸亚铁、11.5g磷酸二氢铵和12.6g二水合草酸溶于1500mL去离子水中，然后搅拌并反应6h得到溶液，之后将该溶液升温到120℃并保持此温度6h，得到第二包覆层悬浊液。
步骤S6：第二包覆层的包覆
将步骤S4中获得的1586.8g的焦磷酸盐包覆后的材料加入到步骤S5中得到的第二包覆层悬浊液(包覆物质含量为47.1g)中，充分搅拌混合6h，混合均匀后，转入120℃烘箱中干燥6h，然后700℃烧结8h得到两层包覆后的材料。
步骤S7：第三包覆层水溶液的制备
将37.3g蔗糖溶于500g去离子水中，然后搅拌并充分溶解，得到蔗糖水溶液。
步骤S8：第三包覆层的包覆
将步骤S6中获得的两层包覆的材料1633.9g加入到步骤S7中得到的蔗糖溶液中，一同搅拌混合6h，混合均匀后，转入150℃烘箱中干燥6h，然后在700℃下烧结10h得到三层包覆后的材料。
步骤2：正极极片的制备
将上述制备的三层包覆后的正极活性材料、导电剂乙炔黑、粘结剂聚偏二氟乙烯(PVDF)按重量比为97.0：1.2：1.8加入到N-甲基吡咯烷酮(NMP)中，搅拌混合均匀，得到正极浆料。然后将正极浆料按0.280g/1540.25mm 2均匀涂覆于铝箔上，经烘干、冷压、分切，得到正极极片。
步骤3：负极极片的制备
将负极活性物质人造石墨、硬碳、导电剂乙炔黑、粘结剂丁苯橡胶(SBR)、增稠剂羧甲基纤维素钠(CMC)按照重量比为90：5：2：2： 1溶于溶剂去离子水中，搅拌混合均匀后制备成负极浆料。将负极浆料按0.117g/1540.25mm 2均匀涂覆在负极集流体铜箔上，经过烘干、冷压、分切，得到负极极片。
步骤4：电解液的制备
在氩气气氛手套箱中(H 2O<0.1ppm，O 2<0.1ppm)，将有机溶剂碳酸乙烯酯(EC)/碳酸甲乙酯(EMC)按照体积比3/7混合均匀，加入12.5重量％(基于碳酸乙烯酯/碳酸甲乙酯溶剂的重量计)LiPF 6溶解于上述有机溶剂中，搅拌均匀，得到电解液。
步骤5：隔离膜的制备
使用市售的厚度为20μm、平均孔径为80nm的PP-PE共聚物微孔薄膜(来自卓高电子科技公司，型号20)。
步骤6：全电池的制备
将上述获得的正极极片、隔离膜、负极极片按顺序叠好，使隔离膜处于正负极中间起到隔离的作用，并卷绕得到裸电芯。将裸电芯置于外包装中，注入上述电解液并封装，得到全电池(下文也称“全电”)。
【扣式电池的制备】
将上述制备的正极活性材料、聚偏二氟乙烯(PVDF)、乙炔黑以90：5：5的重量比加入至N-甲基吡咯烷酮(NMP)中，在干燥房中搅拌制成浆料。在铝箔上涂覆上述浆料，干燥、冷压制成正极极片。涂覆量为0.2g/cm 2，压实密度为2.0g/cm 3。
采用锂片作为负极，采用1mol/L的LiPF 6在体积比1：1：1的碳酸乙烯酯(EC)、碳酸二乙酯(DEC)和碳酸二甲酯(DMC)中的溶液为电解液，与上述制备的正极极片一起在扣电箱中组装成扣式电池(下文也称“扣电”)。
实施例2至27和对比例1至19
以类似于实施例1的方式制备实施例2至27和对比例1至19中的正极活性材料和电池，正极活性材料的制备中的不同之处参见表1-6，其 中对比例1-2、4-10和12未包覆第一层，因此没有步骤S3、S4；对比例1-11未包覆第二层，因此没有步骤S5-S6。
注：本申请所有实施例和对比例中，如未标明，则使用的第一包覆层物质和/或第二包覆层物质均默认为晶态。
表1：内核的制备原料
Figure PCTCN2021140462-appb-000003
Figure PCTCN2021140462-appb-000004
Figure PCTCN2021140462-appb-000005
Figure PCTCN2021140462-appb-000006
Figure PCTCN2021140462-appb-000007
Figure PCTCN2021140462-appb-000008
Figure PCTCN2021140462-appb-000009
Figure PCTCN2021140462-appb-000010
Figure PCTCN2021140462-appb-000011
Figure PCTCN2021140462-appb-000012
II.性能评价
1.晶格变化率测试方法：
在25℃恒温环境下，将正极活性材料样品置于XRD(型号为Bruker D8 Discover)中，采用1°/min对样品进行测试，并对测试数据进行整理分析，参照标准PDF卡片，计算出此时的晶格常数a0、b0、c0和v0(a0,b0和c0表示晶胞各个方面上的长度大小，v0表示晶胞体积，可通过XRD精修结果直接获取)。
采用上述实施例中扣电制备方法，将所述正极活性材料样品制备成扣电，并对上述扣电以0.05C小倍率进行充电，直至电流减小至0.01C。然后将扣电中的正极极片取出，并置于碳酸二甲酯(DMC)中浸泡8小时。然后烘干，刮粉，并筛选出其中粒径小于500nm的颗粒。取样并按照与上述测试新鲜样品同样的方式计算出其晶胞体积v1，将(v0-v1)/v0×100％作为其完全脱嵌锂前后的晶格变化率(晶胞体积变化率)示于表中。
2.Li/Mn反位缺陷浓度
将“晶格变化率测量方法”中测试的XRD结果与标准晶体的PDF(Powder Diffraction File)卡片对比，得出Li/Mn反位缺陷浓度。具体而言，将“晶格变化率测量方法”中测试的XRD结果导入通用结构分析***(GSAS)软件中，自动获得精修结果，其中包含了不同原子的占位情况，通过读取精修结果获得Li/Mn反位缺陷浓度。
3.压实密度
取5g的上述制得的正极活性材料粉末放于压实专用模具(美国CARVER模具，型号13mm)中，然后将模具放在压实密度仪器上。施加3T的压力，在设备上读出压力下粉末的厚度(卸压后的厚度)，通过ρ＝m/v，计算出压实密度，其中使用的面积值为标准的小图片面积1540.25mm 2。
4. 3C充电恒流比
在25℃恒温环境下，将上述各个实施例和对比例制备的新鲜全电池静置5min，按照1/3C放电至2.5V。静置5min，按照1/3C充电 至4.3V，然后在4.3V下恒压充电至电流小于等于0.05mA。静置5min，记录此时的充电容量为C0。按照1/3C放电至2.5V，静置5min，再按照3C充电至4.3V，静置5min，记录此时的充电容量为C1。3C充电恒流比即为C1/C0×100％。
3C充电恒流比越高，说明二次电池的倍率性能越好。
5.过渡金属Mn(以及Mn位掺杂的Fe)溶出测试
将45℃下循环至容量衰减至80％后的上述各个实施例和对比例制备的全电池采用0.1C倍率进行放电至截止电压2.0V。然后将电池拆开，取出负极极片，在负极极片上随机取30个单位面积(1540.25mm 2)的圆片，用Agilent ICP-OES730测试电感耦合等离子体发射光谱(ICP)。根据ICP结果计算其中Fe(如果正极活性材料的Mn位掺杂有Fe的话)和Mn的量，从而计算循环后Mn(以及Mn位掺杂的Fe)的溶出量。测试标准依据EPA-6010D-2014。
6.表面氧价态
取5g上述制得的正极活性材料样品按照上述实施例中所述扣电制备方法制备成扣电。对扣电采用0.05C小倍率进行充电，直至电流减小至0.01C。然后将扣电中的正极极片取出，并置于DMC中浸泡8小时。然后烘干，刮粉，并筛选出其中粒径小于500nm的颗粒。将所得颗粒用电子能量损失谱(EELS，所用仪器型号为Talos F200S)进行测量，获取能量损失近边结构(ELNES)，其反映元素的态密度和能级分布情况。根据态密度和能级分布，通过对价带态密度数据进行积分，算出占据的电子数，从而推算出充电后的表面氧的价态。
7.正极活性材料中锰元素和磷元素的测量
将5g上述制得的正极活性材料在100ml逆王水(浓盐酸：浓硝酸＝1：3)中(浓盐酸浓度～37％，浓硝酸浓度～65％)溶解，利用ICP测试溶液各元素的含量，然后对锰元素或磷元素的含量进行测量和换算(锰元素或磷元素的量/正极活性材料的量*100％)，得到其重量占比。
8.扣式电池初始克容量测量方法
在2.5-4.3V下，将上述各实施例和对比例制备的扣式电池按照0.1C充电至4.3V，然后在4.3V下恒压充电至电流小于等于0.05mA，静置5min，然后按照0.1C放电至2.0V，此时的放电容量为初始克容量，记为D0。
10.全电池60℃存储30天电芯膨胀测试：
在60℃下，存储100％充电状态(SOC)的上述各个实施例和对比例制备的全电池。在存储前后及过程中测量电芯的开路电压(OCV)和交流内阻(IMP)以监控SOC，并测量电芯的体积。其中在每存储48h后取出全电池，静置1h后测试开路电压(OCV)、内阻(IMP)，并在冷却至室温后用排水法测量电芯体积。排水法即先用表盘数据自动进行单位转换的天平单独测量电芯的重力F 1，然后将电芯完全置于去离子水(密度已知为1g/cm 3)中，测量此时的电芯的重力F 2，电芯受到的浮力F 浮即为F 1-F 2，然后根据阿基米德原理F 浮＝ρ×g×V 排，计算得到电芯体积V＝(F 1-F 2)/(ρ×g)。
由OCV、IMP测试结果来看，本实验过程中直至存储结束，全部实施例的电池始终保持99％以上的SOC。
存储30天后，测量电芯体积，并计算相对于存储前的电芯体积，存储后的电芯体积增加的百分比。
11.全电池45℃下循环性能测试
在45℃的恒温环境下，在2.5-4.3V下，按照1C充电至4.3V，然后在4.3V下恒压充电至电流≤0.05mA，静置5min，然后按照1C放电至2.5V，容量记为D n(n＝0,1,2,……)。重复前述过程，直至容量衰减(fading)到80％，记录此时的重复次数，即为45℃下80％容量保持率对应的循环圈数。
12.晶面间距和夹角测试
取1g上述制得的各正极活性材料粉末于50mL的试管中，并在试管中注入10mL质量分数为75％的酒精，然后进行充分搅拌分散30分钟，然后用干净的一次性塑料吸管取适量上述溶液滴加在300目铜网上，此时，部分粉末将在铜网上残留，将铜网连带样品转移至 TEM(Talos F200s G2)样品腔中进行测试，得到TEM测试原始图片，保存原始图片格式(xx.dm3)。
将上述TEM测试所得原始图片在DigitalMicrograph软件中打开，并进行傅里叶变换(点击操作后由软件自动完成)得到衍射花样，量取衍射花样中衍射光斑到中心位置的距离，即可得到晶面间距，夹角根据布拉格方程进行计算得到。
通过得到的晶面间距和相应夹角数据，与其标准值比对，即可对包覆层的不同物质进行识别。
13.包覆层厚度测试
包覆层的厚度大小测试主要通过FIB从上述制得的正极活性材料单个颗粒中间切取100nm左右厚度的薄片，然后对薄片进行TEM测试，得到TEM测试原始图片，保存原始图片格式(xx.dm3)。
将上述TEM测试所得原始图片在DigitalMicrograph软件中打开，通过晶格间距和夹角信息，识别出包覆层，量取包覆层的厚度。
对所选颗粒测量三个位置处的厚度，取平均值。
14.第三层包覆层碳中SP2形态和SP3形态摩尔比的测定
本测试通过拉曼(Raman)光谱进行。通过对Raman测试的能谱进行分峰，得到Id/Ig，其中Id为SP3形态碳的峰强度，Ig为SP2形态碳的峰强度，从而确认两者的摩尔比。
所有实施例和对比例的性能测试结果参见下面的表格。
Figure PCTCN2021140462-appb-000013
Figure PCTCN2021140462-appb-000014
Figure PCTCN2021140462-appb-000015
Figure PCTCN2021140462-appb-000016
Figure PCTCN2021140462-appb-000017
Figure PCTCN2021140462-appb-000018
Figure PCTCN2021140462-appb-000019
Figure PCTCN2021140462-appb-000020
Figure PCTCN2021140462-appb-000021
Figure PCTCN2021140462-appb-000022
Figure PCTCN2021140462-appb-000023
压实密度)和所制备的电池性能(电容量、高温循环性能、高温存储性能)均表现优异。
需要说明的是，本申请不限定于上述实施方式。上述实施方式仅为示例，在本申请的技术方案范围内具有与技术思想实质相同的构成、发挥相同作用效果的实施方式均包含在本申请的技术范围内。此外，在不脱离本申请主旨的范围内，对实施方式施加本领域技术人员能够想到的各种变形、将实施方式中的一部分构成要素加以组合而构筑的其它方式也包含在本申请的范围内。

Claims (26)

  1. 一种具有核-壳结构的正极活性材料，其包括内核及包覆所述内核的壳，
    所述内核的化学式为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中一种或多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素；
    所述x、y和z的值满足以下条件：使整个内核保持电中性；
    所述壳包括包覆所述内核的第一包覆层、包覆所述第一包覆层的第二包覆层以及包覆所述第二包覆层的第三包覆层，其中，
    所述第一包覆层包括晶态焦磷酸盐Li aMP 2O 7和/或M b(P 2O 7) c，其中，0≤a≤2，1≤b≤4，1≤c≤6，所述a、b和c的值满足以下条件：使晶态焦磷酸盐Li aMP 2O 7或M b(P 2O 7) c保持电中性，
    所述晶态焦磷酸盐Li aMP 2O 7和M b(P 2O 7) c中的M各自独立地为选自Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素，
    所述第二包覆层包括晶态磷酸盐XPO 4，其中，所述X为选自Li、Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；
    所述第三包覆层为碳。
  2. 根据权利要求1所述的具有核-壳结构的正极活性材料，其中，
    所述第一包覆层中的晶态焦磷酸盐的晶面间距范围为0.293-0.470nm，晶向(111)的夹角范围为18.00°-32.00°；所述第二包覆层中的晶态磷酸盐的晶面间距范围为0.244-0.425nm，晶向(111)的夹角范围为20.00°-37.00°。
  3. 根据权利要求1或2所述的具有核-壳结构的正极活性材料，其中，在所述内核中，y与1-y的比值为1：10至1：1，可选为1：4至1：1。
  4. 根据权利要求1-4中任一项所述的具有核-壳结构的正极活性材料，其中，在所述内核中，z与1-z的比值为1：9至1：999，可选为1：499至1：249。
  5. 根据权利要求1-4中任一项所述的具有核-壳结构的正极活性材料，其中，所述第三包覆层的碳为SP2形态碳与SP3形态碳的混合物，可选地，所述SP2形态碳与SP3形态碳的摩尔比为在0.1-10范围内的任意数值，可选为在2.0-3.0范围内的任意数值。
  6. 根据权利要求1-5中任一项所述的具有核-壳结构的正极活性材料，其中，
    所述第一包覆层的包覆量为大于0且小于或等于6重量％，可选为大于0且小于或等于5.5重量％，更可选为大于0且小于或等于2重量％，基于所述内核的重量计；和/或
    所述第二包覆层的包覆量为大于0且小于或等于6重量％，可选为大于0且小于或等于5.5重量％，更可选为2-4重量％，基于所述内核的重量计；和/或
    所述第三包覆层的包覆量为大于0且小于或等于6重量％，可选为大于0且小于或等于5.5重量％，更可选为大于0且小于或等于2重量％，基于所述内核的重量计。
  7. 根据权利要求1-6中任一项所述的具有核-壳结构的正极活性材料，其中，所述第一包覆层的厚度为1-10nm；和/或
    所述第二包覆层的厚度为2-15nm；和/或
    所述第三包覆层的厚度为2-25nm。
  8. 根据权利要求1-7中任一项所述的具有核-壳结构的正极活性材料，其中，
    基于正极活性材料的重量计，锰元素含量在10重量％-35重量％范围内，可选在15重量％-30重量％范围内，更可选在17重量％-20重量％范围内，磷元素的含量在12重量％-25重量％范围内，可选在15重量％-20 重量％范围内，锰元素和磷元素的重量比范围为0.90-1.25，可选为0.95-1.20。
  9. 根据权利要求1-8中任一项所述的具有核-壳结构的正极活性材料，其中，所述具有核-壳结构的正极活性材料在完全脱嵌锂前后的晶格变化率为4％以下，可选为3.8％以下，更可选为2.0-3.8％。
  10. 根据权利要求1-9中任一项所述的具有核-壳结构的正极活性材料，其中，所述具有核-壳结构的正极活性材料的Li/Mn反位缺陷浓度为4％以下，可选为2.2％以下，更可选为1.5-2.2％。
  11. 根据权利要求1-10中任一项所述的具有核-壳结构的正极活性材料，其中，所述具有核-壳结构的正极活性材料在3T下的压实密度为2.2g/cm 3以上，可选地为2.2g/cm 3以上且2.8g/cm 3以下。
  12. 根据权利要求1-11中任一项所述的具有核-壳结构的正极活性材料，其中，所述具有核-壳结构的正极活性材料的表面氧价态为-1.90以下，可选地为-1.90至-1.98。
  13. 一种正极活性材料的制备方法，包括以下步骤：
    提供内核材料的步骤：所述内核化学式为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中的一种或多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素；
    包覆步骤：分别提供Li aMP 2O 7和/或M b(P 2O 7) c以及XPO 4悬浊液，将所述内核材料加入到上述悬浊液中并混合，经烧结获得正极活性材料，其中0≤a≤2，1≤b≤4，1≤c≤6，所述a、b和c的值满足以下条件：使晶态焦磷酸盐Li aMP 2O 7或M b(P 2O 7) c保持电中性；所述M各自独立地为选自Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；所述X为选自Li、Fe、Ni、Mg、Co、Cu、Zn、Ti、Ag、Zr、Nb或Al中的一种或多种元素；
    其中，所述正极活性材料具有核-壳结构，其包括所述内核及包覆所述内核的壳，所述壳包括包覆所述内核的第一包覆层、包覆所述第一包覆层的第二包覆层以及包覆所述第二包覆层的第三包覆层，所述第一包覆层包括晶态焦磷酸盐Li aMP 2O 7和/或M b(P 2O 7) c，所述第二包覆层包括晶态磷酸盐XPO 4，所述第三包覆层为碳。
  14. 根据权利要求13所述的正极活性材料的制备方法，所述提供内核材料的步骤包括以下步骤：
    步骤(1)：将锰源、元素A的掺杂剂和酸在容器中混合并搅拌，得到掺杂有元素A的锰盐颗粒；
    步骤(2)：将所述掺杂有元素A的锰盐颗粒与锂源、磷源和元素R的掺杂剂在溶剂中混合并得到浆料，在惰性气体气氛保护下烧结后得到掺杂有元素A和元素R的内核，其中，所述掺杂有元素A和元素R的内核为Li 1+xMn 1-yA yP 1-zR zO 4，其中x为在-0.100-0.100范围内的任意数值，y为在0.001-0.500范围内的任意数值，z为在0.001-0.100范围内的任意数值，所述A为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素，可选为Fe、Ti、V、Ni、Co和Mg中的一种或多种元素，所述R为选自B、Si、N和S中的一种或多种元素，可选地，所述R为选自B、Si、N和S中的一种元素。
  15. 根据权利要求14所述的正极活性材料的制备方法，其中，
    所述步骤(1)在20-120℃、可选为40-120℃的温度下进行混合；和/或
    所述步骤(1)中所述搅拌在400-700rpm下进行1-9h，可选地为3-7h。
  16. 根据权利要求14所述的正极活性材料的制备方法，其中，所述步骤(2)在20-120℃、可选为40-120℃的温度下进行混合1-10h。
  17. 根据权利要求14-16中任一项所述的正极活性材料的制备方法，其中，
    所述元素A的掺杂剂为选自Zn、Al、Na、K、Mg、Mo、W、Ti、V、Zr、Fe、Ni、Co、Ga、Sn、Sb、Nb和Ge中的一种或多种元素各自的单质、碳酸盐、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种；和/或，
    所述元素R的掺杂剂为选自B、Si、N和S中的一种或多种元素各自的无机酸、有机酸、硫酸盐、氯化盐、硝酸盐、有机酸盐、氧化物、氢氧化物中的一种或多种。
  18. 根据权利要求13-17中任一项所述的正极活性材料的制备方法，其中，所述包覆步骤包括：
    第一包覆步骤：将元素M的源、磷源和酸以及任选地锂源，溶于溶剂中，得到第一包覆层悬浊液；将内核步骤中获得的内核与第一包覆步骤获得的第一包覆层悬浊液充分混合，干燥，然后烧结，得到第一包覆层包覆的材料；
    第二包覆步骤：将元素X的源、磷源和酸溶于溶剂中，得到第二包覆层悬浊液；将第一包覆步骤中获得的第一包覆层包覆的材料与第二包覆步骤获得的第二包覆层悬浊液充分混合，干燥，然后烧结，得到两层包覆层包覆的材料；
    第三包覆步骤：将碳源溶于溶剂中，充分溶解得到第三包覆层溶液；然后将第二包覆步骤中获得的两层包覆层包覆的材料加入所述第三包覆层溶液中，混合均匀，干燥，然后烧结得到三层包覆层包覆的材料，即正极活性材料。
  19. 根据权利要求18所述的正极活性材料的制备方法，其中，
    所述第一包覆步骤中，控制溶解有元素M的源、磷源和酸以及任选地锂源的溶液pH为3.5-6.5，然后搅拌并反应1-5h，然后将所述溶液升温至50-120℃，并保持该温度2-10h，和/或，
    所述烧结在650-800℃下进行2-6小时。
  20. 根据权利要求18-19中任一项所述的正极活性材料的制备方法，其中，
    所述第二包覆步骤中，将元素X的源、磷源和酸溶于溶剂后，搅拌并反应1-10h，然后将所述溶液升温至60-150℃，并保持该温度2-10h，和/或，
    烧结在500-700℃下进行6-10小时。
  21. 根据权利要求18-20中任一项所述的正极活性材料的制备方法，其中，所述第三包覆步骤中的烧结在700-800℃下进行6-10小时。
  22. 一种正极极片，其包括正极集流体以及设置在正极集流体至少一个表面的正极膜层，所述正极膜层包括权利要求1-12中任一项所述的具有核-壳结构的正极活性材料或通过权利要求13-21中任一项所述的正极活性材料制备方法制备的正极活性材料，并且所述正极活性材料在所述正极膜层中的含量为90-99.5重量％，可选为95-99.5重量％，基于所述正极膜层的总重量计。
  23. 一种二次电池，包括权利要求1-12中任一项所述的具有核-壳结构的正极活性材料或通过权利要求13-21中任一项所述的正极活性材料制备方法制备的正极活性材料或权利要求22所述的正极极片。
  24. 一种电池模块，其包括权利要求23所述的二次电池。
  25. 一种电池包，其包括权利要求24所述的电池模块。
  26. 一种用电装置，其包括选自权利要求23所述的二次电池、权利要求24所述的电池模块或权利要求25所述的电池包中的至少一种。
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