CN111525100B - Preparation method of porous carbon coated LiFePO4 positive electrode material with surface having pre-stress - Google Patents

Preparation method of porous carbon coated LiFePO4 positive electrode material with surface having pre-stress Download PDF

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CN111525100B
CN111525100B CN201911225877.XA CN201911225877A CN111525100B CN 111525100 B CN111525100 B CN 111525100B CN 201911225877 A CN201911225877 A CN 201911225877A CN 111525100 B CN111525100 B CN 111525100B
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lithium iron
porous carbon
iron phosphate
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stress
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CN111525100A (en
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黄剑锋
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Nantong Dingxin Battery Co ltd
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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
    • H01M4/362—Composites
    • H01M4/364—Composites as mixtures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • 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
    • H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • 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
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • 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

Abstract

The invention discloses porous carbon coated LiFePO with a surface having a pre-stress4The preparation method of the anode material can realize the combination of a solid-phase reaction and a solvothermal method, adopts commercial phenolic resin as a carbon source through a two-step synthesis method, obtains a highly-crosslinked porous carbon structure after the phenolic resin is pyrolyzed, firstly prepares carbon-coated ferrous phosphate, and then converts the carbon-coated ferrous phosphate into carbon-coated lithium iron phosphate in situ, and the conversion can prepare porous carbon-coated LiFePO with the surface having pre-stress4The anode material introduces a brand-new micro-airflow impact device to assist carbon coating in the process, so that the coating strength of the carbon layer on the lithium iron phosphate is improved, the generation of pre-stress is facilitated, the lithium iron phosphate is not easy to fall off or pulverize due to the expansion of the lithium iron phosphate, the conductivity of the lithium iron phosphate is improved to a certain extent, and the lithium iron phosphate is prevented from causing capacity loss due to dissolution in an electrolyte in the circulating process, so that the conductivity and the circulating stability of the lithium iron phosphate are improved, and the service life of the lithium iron phosphate is prolonged.

Description

Porous carbon coated LiFePO with surface having pre-stress4Preparation method of positive electrode material
Technical Field
The invention relates to the technical field of preparation of lithium ion battery anode materials, in particular to porous carbon coated LiFePO with a pre-stress surface4A preparation method of the cathode material.
Background
The air pollution and resource exhaustion are caused by the mass combustion of fossil fuel, and the dependence of people on the fossil fuel can be greatly reduced by the popularization and research of the lithium secondary battery. The research on the positive electrode material of the lithium ion battery as a lithium source supplier and a core component of the battery has an important role in improving the overall capacity of the battery. The lithium iron phosphate material has wide raw material distribution, is safe and pollution-free, and is widely applied to the anode material of the lithium ion battery. But its development is limited by its low electronic conductivity and low lithium ion diffusion rate.
The lithium iron phosphate is in an orthogonal olivine structure, and the space group is Pnma. In 1997, Goodenough et al discovered this structure and applied it to lithium ion batteries. Due to LiFePO4Middle PO4The tetrahedral P-O bond is stronger, and the structural stability is ensured. But the diffusion channel is single, so that the ion diffusion rate is low and is less than 10 at room temperature-9S·cm-1Much lower than LiCoO2(10-3S·cm-1) And LiMn2O4(10-5S·cm-1). Lower ion diffusionThe rate and electronic conductivity make the material have poor rate capability, severely limiting its development. Therefore, it becomes crucial to select a suitable conductive material to increase the conductivity of lithium iron phosphate.
The materials commonly used at present for improving the electrical conductivity are mainly carbon materials, and carbon sources include glucose, sucrose, ascorbic acid, citric acid, polydopamine and the like. The coating method comprises an in-situ coating method and an ex-situ coating method. The material can effectively improve the conductivity of the lithium iron phosphate on one hand, and can avoid the capacity loss caused by direct contact of the lithium iron phosphate and the electrolyte on the other hand. However, the preparation process is complex and expensive, and the special olivine structure of the lithium iron phosphate cannot be tightly combined with the conductive carbon, so that the lithium iron phosphate is easy to cause volume expansion and further fall off or pulverize in the charging and discharging processes, thereby reducing the cycle stability and the service life of the lithium iron phosphate.
Disclosure of Invention
1. Technical problem to be solved
Aiming at the problems in the prior art, the invention aims to provide a porous carbon coated LiFePO with a surface having a pre-stress4The preparation method of the anode material can realize the combination of solid-phase reaction and a solvothermal method, adopts commercial phenolic resin as a carbon source through a two-step synthesis method, obtains a highly cross-linked porous carbon structure after the phenolic resin is pyrolyzed, firstly prepares carbon-coated ferrous phosphate, and then converts the carbon-coated ferrous phosphate into carbon-coated lithium iron phosphate in situ, and the conversion can prepare porous carbon-coated LiFePO with the surface having pre-stress4The anode material introduces a brand-new micro-airflow impact device to assist carbon coating in the process, so that the coating strength of the carbon layer on the lithium iron phosphate is improved, the generation of pre-stress is facilitated, the lithium iron phosphate is not easy to fall off or pulverize due to the expansion of the lithium iron phosphate, the conductivity of the lithium iron phosphate is improved to a certain extent, and the lithium iron phosphate is prevented from causing capacity loss due to dissolution in an electrolyte in the circulating process, so that the conductivity and the circulating stability of the lithium iron phosphate are improved, and the service life of the lithium iron phosphate is prolonged.
2. Technical scheme
In order to solve the above problems, the present invention adopts the following technical solutions.
Porous carbon coated LiFePO with surface having pre-stress4The preparation method of the cathode material comprises the following steps:
s1, weighing 9-18g of analytically pure ferrous nitrate, dissolving in 50-100mL of distilled water,
obtaining a solution A;
s2, taking 5-20ml of analytically pure phosphoric acid, and adding 55-40ml of distilled water to obtain a diluted phosphoric acid solution B;
s3, dropwise adding the solution B into the solution A while stirring, centrifugally separating the generated precipitate, washing the precipitate for 3 times by using distilled water, and drying the precipitate in an oven at the temperature of 60 ℃ to obtain a white precipitate C;
s4, dissolving phenolic resin in 50ml of analytical pure ethanol to prepare a solution D with the concentration of 30-40%;
s5, soaking the C in the solution D for 5-10min, performing suction filtration, drying in an oven at 60-80 ℃, performing pressing and pore-forming through a micro-airflow impact device during the process, and repeating the process for 3-5 times to obtain a product E;
s6, exhausting the gas in the micro-airflow impact device before drying, and storing the preheated air in the oven in advance;
s7, calcining the product E in a tubular atmosphere furnace at 480-550 ℃,
keeping the temperature for 1-3h to obtain a product F;
s8, placing the F in a lithium phosphate solution with the mass fraction of 20-50%, reacting in a microwave hydrothermal reaction kettle, controlling the reaction temperature at 160-190 ℃ and the reaction time at 3-6h, naturally cooling to room temperature after the reaction is finished, separating solid phase substances, washing for 2-3 times in a 75% ethanol solution, and drying at 60-80 ℃ to obtain a final product G.
Further, in the step S3, the stirring is performed by mechanical stirring, the stirring time is 15-20min, and the stirring speed is 60-80 r.min-1And after the dripping of the solution B is finished, the stirring is continued for not less than 5min, so that the reaction completeness is improved, and the precipitation of precipitates is promoted.
Further, CAS number of the phenolic resin: 9003-35-4, molecular weight 134.133.
Furthermore, in the step S6, the micro air flow starts to occur 3-5min after the start of drying and continues until the end of drying, the micro air flow is generated early so that the phenolic resin solution easily falls off from the surface of the ferrous phosphate precipitate, the coating effect is reduced, the micro air flow impact pressing and pore-forming effects after the phenolic resin is cured are poor, the micro air flow impact is performed only when the phenolic resin is coated on the surface of the ferrous phosphate precipitate and is cured to a certain degree, not only can the heat exchange speed be increased and the drying is assisted, but also a certain pressing effect can be achieved, the coating strength of the phenolic resin is improved, the coating with the ferrous phosphate precipitate is more compact, the micro pores can be punched on the phenolic resin coating layer, the contact area with the phenolic resin in the subsequent soaking process is increased, and the subsequent phenolic resin coating layer is more compact in connection with the previous phenolic resin coating layer, is not easy to fall off independently.
Further, the atmosphere in the step S7 is nitrogen, and the flow rate of nitrogen is 100 sccm.min-1。
Further, the temperature increase rate in the step S7 is started to be 10-20 ℃ min-1And the temperature is increased to 400 ℃ and then is reduced to 5-10 ℃ min-1The preheating decomposition is mild, so the heating speed can be low, the pyrolysis is severe, and the heating speed is preferably slow.
Further, the suspension system in the step S8 contains lithium phosphate in a molar ratio of Fe2+:Li+:PO4-1: (1-1.2): 1, preparing the mixture.
Further, little air current strikes device includes the bi-pass piston barrel that all link up from top to bottom, a pair of gas shield of bi-pass piston barrel inner fixedly connected with, it has a plurality of evenly distributed's impact micropore, a pair of to cut on the gas shield is close to the equal fixedly connected with in one end each other and places the net, the piston of a pair of symmetry is gone back to sliding connection in bi-pass piston barrel inner, and interference fit between piston and the bi-pass piston barrel, the equal fixedly connected with U type mounting panel in both ends about the bi-pass piston barrel, U type mounting panel is close to a pair of electric putter of piston one end fixedly connected with, and electric putter's output and piston fixed connection, both ends are all cut about the bi-pass piston barrel and have the gas port, and the gas port just to a pair of placing between the net.
Furthermore, the diameter of the impact micropores is uniformly reduced along the direction close to the placing net, the vertical section is in an inverted trapezoid shape, the effect of accelerating micro airflow is achieved, and the pressing and pore-forming effects are improved.
Furthermore, the diameter of the impact micropores is 0.1-1 μm, and in view of the particle size of the ferrous phosphate precipitate, if the diameter of the impact micropores is too small, the pressing effect is poor, the pore-forming is too dense and tiny, the connection effect between the front and rear coating layers is difficult to play, if the diameter is too large, the phenolic resin coating layer is easy to impact and fall off, the pore-forming is difficult, and the pressing and pore-forming effects can be achieved only with a proper diameter.
3. Advantageous effects
Compared with the prior art, the invention has the advantages that:
(1) the scheme can combine a solid-phase reaction with a solvothermal method, and adopts a two-step synthesis method, namely, taking commercial phenolic resin as a carbon source, obtaining a highly-crosslinked porous carbon structure after pyrolysis of the phenolic resin, firstly preparing carbon-coated ferrous phosphate, and then converting the carbon-coated ferrous phosphate into carbon-coated lithium iron phosphate in situ, wherein the conversion can be used for preparing porous carbon-coated LiFePO with the surface having pre-stress4The anode material introduces a brand-new micro-airflow impact device to assist carbon coating in the process, so that the coating strength of the carbon layer on the lithium iron phosphate is improved, the generation of pre-stress is facilitated, the lithium iron phosphate is not easy to fall off or pulverize due to the expansion of the lithium iron phosphate, the conductivity of the lithium iron phosphate is improved to a certain extent, and the lithium iron phosphate is prevented from causing capacity loss due to dissolution in an electrolyte in the circulating process, so that the conductivity and the circulating stability of the lithium iron phosphate are improved, and the service life of the lithium iron phosphate is prolonged.
(2) The invention firstly prepares porous carbon-coated ferrous phosphate, and then the porous carbon-coated ferrous phosphate is mixed with Li through the ferrous phosphate+The volume of the carbon-coated lithium iron phosphate obtained by the reaction can expand in the lithiation reaction process, and the expansion is suppressed by the outer carbon layer, so that the surface compressive stress of the lithium iron phosphate crystal grains exists, and the compressive stress can effectively improve the cycle of the lithium iron phosphate crystal grainsAnd (4) stability. Meanwhile, in the process of lithium removal/lithium insertion, the porous carbon-coated lithium iron phosphate structure prepared by the invention has different internal and external expansion degrees, the volume expansion rate of carbon is smaller than that of lithium iron phosphate, and the porous carbon shell can continuously generate compressive stress on the internal lithium iron phosphate so as to protect the lithium iron phosphate from pulverization and capacity attenuation caused by volume change.
(3) The method fully combines the advantages of high theoretical capacity of the lithium iron phosphate, stable voltage platform and good conductivity of the porous carbon obtained from the phenolic resin, the obtained porous carbon-coated lithium iron phosphate structure has excellent cycling stability, and compared with a porous carbon-coated lithium iron phosphate material without pre-stress, the cycling stability and the service life of the porous carbon-coated lithium iron phosphate material can be improved by 40%.
Drawings
FIG. 1 is a schematic flow diagram of the main process of the present invention;
FIG. 2 is a schematic view of the internal structure of the micro-airflow impact device of the present invention;
FIG. 3 is a schematic view of the structure of the micro-flow of the present invention in an impact state;
FIG. 4 shows X for the product of example 2 of the present invention-A ray diffraction profile;
FIG. 5 is a TEM image of the product under the conditions of example 2 of the present invention.
The reference numbers in the figures illustrate:
1 bi-pass piston cylinder, 2 gas barriers, 3 placing nets, 4 impact micropores, 5 pistons, 6U-shaped mounting plates, 7 electric push rods and 8 gas ports.
Detailed Description
The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention; it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments, and all other embodiments obtained by those skilled in the art without any inventive work are within the scope of the present invention.
In the description of the present invention, it should be noted that the terms "upper", "lower", "inner", "outer", "top/bottom", and the like indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience of description and simplification of description, but do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and thus should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
In the description of the present invention, it should be noted that, unless otherwise specifically stated or limited, the terms "mounted," "disposed," "sleeved/connected," "connected," and the like, should be construed broadly, such as "connected," which may be fixedly connected, detachably connected, or integrally connected, mechanically connected, electrically connected, directly connected, indirectly connected through an intermediate medium, or connected between two elements.
Example 1:
referring to fig. 1, S1, 9g of analytically pure ferrous nitrate was weighed and dissolved in 50mL of distilled water to obtain solution a;
s2, taking 5ml of analytically pure phosphoric acid, and adding 55ml of distilled water to obtain a diluted phosphoric acid solution B;
s3, dropwise adding the solution B into the solution A while stirring, and mechanically stirring for 15-20min at a stirring speed of 60-80 r.min-1After the solution B is dropwise added, continuously stirring for not less than 5min, improving the reaction completeness, promoting the precipitation of precipitates, centrifugally separating the generated precipitates, washing the precipitates for 3 times by using distilled water, and drying the precipitates in a 60 ℃ drying oven to obtain white precipitates C;
s4, dissolving phenolic resin (CAS number: 9003-35-4, molecular weight 134.133) in 50ml of analytically pure ethanol to prepare a solution D with the concentration of 30%;
s5, soaking the C in the solution D for 5min, performing suction filtration, drying in an oven at 80 ℃, performing pressing and pore-forming through a micro-airflow impact device during the process, and repeating the process for 3 times to obtain a product E;
s6, exhausting the gas in the micro-airflow impact device before drying, and storing the preheated air in the oven in advance;
s7, calcining the product E in a tubular atmosphere furnace at 480 ℃ and 10 ℃ min at the beginning in nitrogen atmosphere-1And the temperature is increased to 400 ℃ and then is reduced to 5 ℃ min-1The preheating decomposition is mild, so the temperature rising speed can be small and fast, the pyrolysis is severe, the temperature rising speed is preferably slow, the heat preservation time is 2 hours, and the nitrogen flow rate is 100 sccm.min-1Obtaining a product F;
s8, placing F in a lithium phosphate solution with the mass fraction of 20%, wherein the ion proportion in a suspension system is Fe according to the mol ratio2+:Li+:PO4-1: 1: 1, reacting in a microwave hydrothermal reaction kettle, controlling the reaction temperature to be 180 ℃, controlling the reaction time to be 5 hours, naturally cooling to room temperature after the reaction is finished, separating solid-phase substances, washing for 2 times in 75% ethanol solution, and drying at 60 ℃ to obtain a final product G, namely the porous carbon coated LiFePO with the pre-stress on the surface4。
Referring to fig. 2, the micro-airflow impact device includes a bi-pass piston cylinder 1 which is through from top to bottom, a pair of air barriers 2 are fixedly connected to the inner end of the bi-pass piston cylinder 1, a plurality of impact micro-holes 4 are uniformly distributed on the air barriers 2, the diameters of the impact micro-holes 4 are uniformly reduced along the direction close to the placing net 3, the vertical cross-sectional shape is an inverted trapezoid, the micro-airflow acceleration effect is achieved, the pressing and pore-forming effects are improved, the diameters of the impact micro-holes 4 are 0.1-1 μm, when the particle size of ferrous phosphate precipitates is considered, the pressing effect is poor when the diameters of the impact micro-holes 4 are too small, the pore-forming is too dense and tiny, the connection effect between the front and rear coating layers is difficult to be achieved, when the diameters are too large, the phenolic resin coating layers are easy to impact and fall off, the pore-forming is difficult to be achieved, only a proper diameter can achieve the pressing and pore-forming effects, the placing net 3 is fixedly connected to one end of the pair of the air barriers 2 which is close to each other, the device is used for placing white sediment C, a phenolic resin coating layer on the surface of the white sediment C can be impacted by micro air flow uniformly, the inner end of a two-way piston cylinder 1 is further connected with a pair of symmetrical pistons 5 in a sliding mode, the pistons 5 and the two-way piston cylinder 1 are in interference fit and used for extruding hot air, due to the fact that the extruded area of the hot air in the two-way piston cylinder 1 is different from the accessible area of an impact micro hole 4, the hot air can be accelerated to form independent micro air flow to impact when passing through the impact micro hole 4, the upper end and the lower end of the two-way piston cylinder 1 are fixedly connected with U-shaped mounting plates 6, one ends, close to the pistons 5, of the U-shaped mounting plates 6 are fixedly connected with a pair of electric push rods 7, the output ends of the electric push rods 7 are fixedly connected with the pistons 5, air ports 8 are drilled at the left end and the right end of the two-way piston cylinder 1, and the air ports 8 are over against a pair of placing nets 3.
Referring to fig. 3, in step S6, the micro-airflow should start to occur 3-5min after the start of drying and continue until the end of drying, the micro-airflow is generated early, which easily causes the phenol formaldehyde resin solution to fall off from the surface of the ferrous phosphate precipitate, and reduces the coating effect, and the micro-airflow impact pressing and pore-forming effect after the phenol formaldehyde resin is cured is poor, and the micro-airflow impact is performed only when the phenol formaldehyde resin is coated on the surface of the ferrous phosphate precipitate and cured to a certain degree, so as to achieve the effects of accelerating the heat exchange speed and assisting drying, and simultaneously achieve a certain pressing effect, improve the coating strength of the phenol formaldehyde resin, and make the coating with the ferrous phosphate precipitate more compact, and also punch micro-pores on the phenol formaldehyde resin coating layer, increase the contact area with the phenol formaldehyde resin in the subsequent soaking process, and make the subsequent phenol formaldehyde resin coating layer more compact with the previous phenol formaldehyde resin coating layer, is not easy to fall off independently.
Example 2:
referring to fig. 1, S1, weighing and analyzing 18g of pure ferrous nitrate, and dissolving in 100mL of distilled water to obtain a solution a;
s2, taking 20ml of analytically pure phosphoric acid, and adding 40m of distilled water to obtain a diluted phosphoric acid solution B;
s3, dropwise adding the solution B into the solution A while stirring, and mechanically stirring for 15-20min at a stirring speed of 60-80 r.min-1The solution B is continuously stirred for not less than 5min after the dropwise addition is finished, the reaction completeness is improved, the precipitation of precipitates is promoted, and the generated precipitates are centrifugally separated and washed by distilled waterWashing for 3 times, and drying in a drying oven at 60 ℃ to obtain a white precipitate C;
s4, dissolving phenolic resin (CAS number: 9003-35-4, molecular weight 134.133) in 50ml of analytically pure ethanol to prepare a solution D with the concentration of 40%;
s5, soaking the C in the solution D for 10min, filtering, drying in an oven at 80 ℃, and repeating for 5 times to obtain a product E;
s6, exhausting the gas in the micro-airflow impact device before drying, and storing the preheated air in the oven in advance;
s7, calcining the product E in a tubular atmosphere furnace in the atmosphere of nitrogen at 550 ℃ and at the temperature rise rate of 20 ℃ min-1And the temperature is increased to 400 ℃ and then is reduced to 10 ℃ min-1The preheating decomposition is mild, so the temperature rising speed can be small and fast, the pyrolysis is severe, the temperature rising speed is preferably slowed, the heat preservation time is 3h, and the nitrogen flow rate is 100 sccm.min-1Obtaining a product F;
s8, placing the F in a lithium phosphate solution with the mass fraction of 30%, and placing the lithium phosphate in a suspension system according to the mol ratio Fe2+:Li+:PO4-1: 1.2: 1, reacting in a microwave hydrothermal reaction kettle, controlling the reaction temperature to be 180 ℃, controlling the reaction time to be 5 hours, naturally cooling to room temperature after the reaction is finished, separating solid-phase substances, washing in 75% ethanol solution for 3 times, and drying at 80 ℃ to obtain a final product G, namely the porous carbon coated LiFePO with the surface having the pre-stress4。
The remainder was in accordance with example 1.
Example 3:
referring to fig. 1, S1, weighing and analyzing 15g of pure ferrous nitrate, and dissolving in 75mL of distilled water to obtain a solution a;
s2, taking 15ml of analytically pure phosphoric acid, and adding 50ml of distilled water to obtain a diluted phosphoric acid solution B;
s3, dropwise adding the solution B into the solution A while stirring, and mechanically stirring for 15-20min at a stirring speed of 60-80 r.min-1The time for continuously stirring after the solution B is dripped is not less than 5min, so that the reaction is improvedPromoting the precipitation of precipitate, centrifugally separating the generated precipitate, washing the precipitate for 3 times by using distilled water, and drying the precipitate in a 60 ℃ drying oven to obtain white precipitate C;
s4, dissolving phenolic resin (CAS number: 9003-35-4, molecular weight 134.133) in 50ml of analytically pure ethanol to prepare a solution D with the concentration of 35%;
s5, soaking the C in the solution D for 8min, filtering, drying in an oven at 70 ℃, and repeating for 4 times to obtain a product E;
s6, exhausting the gas in the micro-airflow impact device before drying, and storing the preheated air in the oven in advance;
s7, calcining the product E in a tubular atmosphere furnace at 500 ℃ in nitrogen atmosphere and 15 ℃ min at the beginning-1And the temperature is increased to 400 ℃ and then is reduced to 8 ℃ min-1The preheating decomposition is mild, so the temperature rising speed can be small and fast, the pyrolysis is severe, the temperature rising speed is preferably slow, the heat preservation time is 2 hours, and the nitrogen flow rate is 100 sccm.min-1Obtaining a product F;
s8, placing the F into a lithium phosphate solution with the mass fraction of 35%, wherein lithium phosphate in a suspension system is Fe according to the mol ratio2+:Li+:PO4-1: 1.1: 1, reacting in a microwave hydrothermal reaction kettle, controlling the reaction temperature to 190 ℃ and the reaction time to 3-6h, naturally cooling to room temperature after the reaction is finished, separating solid-phase substances, washing for 2 times in 75% ethanol solution, and drying at 70 ℃ to obtain a final product G, namely the porous carbon coated LiFePO with the surface having pre-stress4。
The remainder was in accordance with example 1.
Example 4:
referring to fig. 1, S1, weighing and analyzing 18g of pure ferrous nitrate, and dissolving in 50mL of distilled water to obtain a solution a;
s2, taking 20ml of analytically pure phosphoric acid, and adding 40ml of distilled water to obtain a diluted phosphoric acid solution B;
s3, dropwise adding the solution B into the solution A while stirring, and mechanically stirring for 15-20min at a stirring speed60-80r·min-1After the solution B is dropwise added, continuously stirring for not less than 5min, improving the reaction completeness, promoting the precipitation of precipitates, centrifugally separating the generated precipitates, washing the precipitates for 3 times by using distilled water, and drying the precipitates in a 60 ℃ drying oven to obtain white precipitates C;
s4, dissolving phenolic resin (CAS number: 9003-35-4, molecular weight 134.133) in 50ml of analytically pure ethanol to prepare a solution D with the concentration of 35%;
s5, soaking the C in the solution D for 10min, filtering, drying in an oven at 60 ℃, and repeating for 5 times to obtain a product E;
s6, exhausting the gas in the micro-airflow impact device before drying, and storing the preheated air in the oven in advance;
s7, calcining the product E in a tubular atmosphere furnace at the temperature of 520 ℃ and the temperature rise rate of 20 ℃ per minute in nitrogen atmosphere-1And the temperature is increased to 400 ℃ and then is reduced to 10 ℃ min-1The preheating decomposition is mild, so the heating rate can be small and fast, the pyrolysis is severe, the heating rate is preferably slow, the heat preservation time is 1h, and the nitrogen flow rate is 100 sccm.min-1Obtaining a product F;
s8, placing the F into a lithium phosphate solution with the mass fraction of 40%, wherein lithium phosphate in a suspension system is Fe according to the mol ratio2+:Li+:PO4-1: 1.1: 1, reacting in a microwave hydrothermal reaction kettle, controlling the reaction temperature to 170 ℃, reacting for 4 hours, naturally cooling to room temperature after the reaction is finished, separating solid-phase substances, washing in 75% ethanol solution for 3 times, and drying at 70 ℃ to obtain a final product G, namely the porous carbon coated LiFePO with the surface having pre-stress4。
The remainder was in accordance with example 1.
Example 5:
referring to fig. 1, S1, weighing 10g of analytically pure ferrous nitrate, and dissolving in 80mL of distilled water to obtain a solution a;
s2, taking 18ml of analytically pure phosphoric acid, and adding 42ml of distilled water to obtain a diluted phosphoric acid solution B;
s3, dropwise adding into the solution AStirring the solution B while dropwise adding, wherein mechanical stirring is adopted, the stirring time is 15-20min, and the stirring speed is 60-80 r.min-1After the solution B is dropwise added, continuously stirring for not less than 5min, improving the reaction completeness, promoting the precipitation of precipitates, centrifugally separating the generated precipitates, washing the precipitates for 3 times by using distilled water, and drying the precipitates in a 60 ℃ drying oven to obtain white precipitates C;
s4, dissolving phenolic resin (CAS number: 9003-35-4, molecular weight 134.133) in 50ml of analytically pure ethanol to prepare a solution D with the concentration of 32%;
s5, soaking the C in the solution D for 8min, filtering, drying in an oven at 60 ℃, and repeating the steps for 4 times to obtain a product E;
s6, exhausting the gas in the micro-airflow impact device before drying, and storing the preheated air in the oven in advance;
s7, calcining the product E in a tubular atmosphere furnace in nitrogen atmosphere at 490 ℃ and at a temperature rise rate of 18 ℃ per minute-1And the temperature is increased to 400 ℃ and then is reduced to 9 ℃ min-1The preheating decomposition is mild, so the temperature rise speed can be small and fast, the pyrolysis is severe, the temperature rise speed is preferably slow, the heat preservation time is 1.5h, and the nitrogen flow rate is 100 sccm.min-1Obtaining a product F;
s8, placing the F into a lithium phosphate solution with the mass fraction of 45%, wherein lithium phosphate in a suspension system is Fe according to the mol ratio2+:Li+:PO4-1: 1.05: 1, reacting in a microwave hydrothermal reaction kettle, controlling the reaction temperature to be 190 ℃, controlling the reaction time to be 4h, naturally cooling to room temperature after the reaction is finished, separating solid-phase substances, washing in 75% ethanol solution for 3 times, and drying at 75 ℃ to obtain a final product G, namely the porous carbon coated LiFePO with the surface having the pre-stress4。
The remainder was in accordance with example 1.
Example 6:
referring to fig. 1, S1, weighing and analyzing 12g of pure ferrous nitrate, and dissolving in 65mL of distilled water to obtain a solution a;
s2, taking 12ml of analytically pure phosphoric acid, and adding 48ml of distilled water to obtain a diluted phosphoric acid solution B;
s3, dropwise adding the solution B into the solution A while stirring, and mechanically stirring for 15-20min at a stirring speed of 60-80 r.min-1After the solution B is dropwise added, continuously stirring for not less than 5min, improving the reaction completeness, promoting the precipitation of precipitates, centrifugally separating the generated precipitates, washing the precipitates for 3 times by using distilled water, and drying the precipitates in a 60 ℃ drying oven to obtain white precipitates C;
s4, dissolving phenolic resin (CAS number: 9003-35-4, molecular weight 134.133) in 50ml of analytically pure ethanol to prepare a solution D with the concentration of 36%;
s5, soaking the C in the solution D for 7min, carrying out suction filtration, drying in an oven at 65 ℃, and repeating for 4 times to obtain a product E;
s6, exhausting the gas in the micro-airflow impact device before drying, and storing the preheated air in the oven in advance;
s7, calcining the product E in a tubular atmosphere furnace at the temperature of 530 ℃ and the temperature rise rate of 12 ℃ min in nitrogen atmosphere-1And the temperature is increased to 400 ℃ and then is reduced to 7 ℃ min-1The preheating decomposition is mild, so the temperature rise speed can be small and fast, the pyrolysis is severe, the temperature rise speed is preferably slow, the heat preservation time is 2.5h, and the nitrogen flow rate is 100 sccm.min-1Obtaining a product F;
s8, placing the F into a lithium phosphate solution with the mass fraction of 50%, wherein lithium phosphate in a suspension system is Fe according to the mol ratio2+:Li+:PO4-1: 1.15: 1, reacting in a microwave hydrothermal reaction kettle, controlling the reaction temperature to be 180 ℃, controlling the reaction time to be 5 hours, naturally cooling to room temperature after the reaction is finished, separating solid-phase substances, washing for 2 times in 75% ethanol solution, and drying at 70 ℃ to obtain a final product G, namely the porous carbon coated LiFePO with the surface having pre-stress4。
According to the invention, porous carbon-coated ferrous phosphate is prepared firstly, and then the ferrous phosphate reacts with Li < + > to obtain carbon-coated lithium iron phosphate, the volume of the carbon-coated lithium iron phosphate expands in the lithiation reaction process, and the expansion of the carbon-coated lithium iron phosphate is suppressed by the outer carbon layer, so that the surface compressive stress of lithium iron phosphate crystal grains exists, and the compressive stress can effectively improve the cycle stability of the lithium iron phosphate. Meanwhile, the porous carbon-coated lithium iron phosphate structure prepared by the invention has different internal and external expansion degrees in the lithium removal/lithium insertion process, the volume expansion rate of carbon is less than that of lithium iron phosphate, the porous carbon shell can continuously generate compressive stress on the internal lithium iron phosphate to protect the lithium iron phosphate from pulverization and capacity attenuation caused by volume change, and the micro-airflow impact device in the preparation process can play a role in strengthening the porous carbon shell, is not easy to fall off under the stress action of the high-expansion-rate lithium iron phosphate and can play a good protection role.
Please refer to fig. 4, which is an X-ray diffraction analysis diagram of the product in example 2, it can be seen from the diagram that lithium iron phosphate is crystalline, the surface-coated porous carbon layer is amorphous, no corresponding diffraction peak appears on the diffraction diagram, and comparison with data of a standard lithium iron phosphate JCPDS card shows that the diffraction peak of the lithium iron phosphate prepared in example 2 has obvious drift toward a high angle direction, which indicates that the surface of the lithium iron phosphate has compressive stress, resulting in the drift of the diffraction peak.
Please refer to fig. 5, which is a TEM image of a product obtained under the conditions of example 2, it can be seen from the graph that a porous carbon layer is completely coated on the surface of lithium iron phosphate, and when the concentration of the phenolic resin solution is 40%, the prepared porous carbon can be tightly coated on the surface of lithium iron phosphate, so as to generate an expected compressive stress, and the existence of the compressive stress can significantly alleviate the volume expansion of lithium iron phosphate in the charging and discharging process, improve the cycle stability and prolong the service life of the lithium iron phosphate.
The method fully combines the advantages of high theoretical capacity of the lithium iron phosphate, stable voltage platform and good conductivity of the porous carbon obtained from the phenolic resin, the obtained porous carbon-coated lithium iron phosphate structure has excellent cycling stability, and compared with a porous carbon-coated lithium iron phosphate material without pre-stress, the cycling stability and the service life of the porous carbon-coated lithium iron phosphate material can be improved by 40%.
The invention can realize the combination of solid-phase reaction and solvothermal method, adopts a two-step synthesis method and adopts commercialThe method comprises the steps of taking the phenolic aldehyde resin as a carbon source, obtaining a highly cross-linked porous carbon structure after pyrolysis of the phenolic aldehyde resin, preparing carbon-coated ferrous phosphate, and converting the carbon-coated ferrous phosphate into carbon-coated lithium iron phosphate in situ, wherein the conversion can be used for preparing porous carbon-coated LiFePO with the surface having pre-stress4The anode material introduces a brand-new micro-airflow impact device to assist carbon coating in the process, so that the coating strength of the carbon layer on the lithium iron phosphate is improved, the generation of pre-stress is facilitated, the lithium iron phosphate is not easy to fall off or pulverize due to the expansion of the lithium iron phosphate, the conductivity of the lithium iron phosphate is improved to a certain extent, and the lithium iron phosphate is prevented from causing capacity loss due to dissolution in an electrolyte in the circulating process, so that the conductivity and the circulating stability of the lithium iron phosphate are improved, and the service life of the lithium iron phosphate is prolonged.
The foregoing is only a preferred embodiment of the present invention; the scope of the invention is not limited thereto. Any person skilled in the art should also be able to cover the technical scope of the present invention by the equivalent or modified embodiments and the modified concepts of the present invention.

Claims (10)

1. Porous carbon coated LiFePO with surface having pre-stress4The preparation method of the anode material is characterized by comprising the following steps: the method comprises the following steps:
s1, weighing 9-18g of analytically pure ferrous nitrate, and dissolving in 50-100mL of distilled water to obtain a solution A;
s2, taking 5-20ml of analytically pure phosphoric acid, and adding 55-40ml of distilled water to obtain a diluted phosphoric acid solution B;
s3, dropwise adding the solution B into the solution A while stirring, centrifugally separating the generated precipitate, washing the precipitate for 3 times by using distilled water, and drying the precipitate in an oven at the temperature of 60 ℃ to obtain a white precipitate C;
s4, dissolving phenolic resin in 50ml of analytical pure ethanol to prepare a solution D with the concentration of 30-40%;
s5, soaking the C in the solution D for 5-10min, performing suction filtration, drying in an oven at 60-80 ℃, performing pressing and pore-forming through a micro-airflow impact device, and repeating the process of the step S5 for 3-5 times to obtain a product E;
s6, exhausting the gas in the micro-airflow impact device before drying, and storing the preheated air in the oven in advance;
s7, placing the product E in a tubular atmosphere furnace for calcination at 480-550 ℃ for 1-3h to obtain a product F;
s8, placing the F in a lithium phosphate solution with the mass fraction of 20-50%, reacting in a microwave hydrothermal reaction kettle, controlling the reaction temperature at 160-190 ℃ and the reaction time at 3-6h, naturally cooling to room temperature after the reaction is finished, separating solid phase substances, washing for 2-3 times in a 75% ethanol solution, and drying at 60-80 ℃ to obtain a final product G.
2. The porous carbon coated LiFePO with pre-stress surface according to claim 14The preparation method of the anode material is characterized by comprising the following steps: in the step S3, mechanical stirring is adopted, the stirring time is 15-20min, and the stirring speed is 60-80 r.min-1And after the dripping of the solution B is finished, continuously stirring for not less than 5 min.
3. The porous carbon coated LiFePO with pre-stress surface according to claim 14The preparation method of the anode material is characterized by comprising the following steps: CAS number of the phenolic resin: 9003-35-4, molecular weight 134.133.
4. The porous carbon coated LiFePO with pre-stress surface according to claim 14The preparation method of the anode material is characterized by comprising the following steps: in the step S6, the micro air flow should start to occur 3-5min after the drying starts, and continue until the drying ends.
5. The porous carbon coated LiFePO with pre-stress surface according to claim 14The preparation method of the anode material is characterized by comprising the following steps: the atmosphere in the step S7 is nitrogen gas, and the flow rate of nitrogen gas is 100 sccm.min-1。
6. According to claim1 the porous carbon coated LiFePO with the surface having the pre-stress4The preparation method of the anode material is characterized by comprising the following steps: the temperature rise rate in the step S7 is 10-20 ℃ min-1And the temperature is increased to 400 ℃ and then is reduced to 5-10 ℃ min-1。
7. The porous carbon coated LiFePO with pre-stress surface according to claim 14The preparation method of the anode material is characterized by comprising the following steps: the suspension system in step S8 contains lithium phosphate in a molar ratio of Fe2+:Li+:PO4-1: (1-1.2): 1, preparing the mixture.
8. The porous carbon coated LiFePO with pre-stress surface according to claim 14The preparation method of the anode material is characterized by comprising the following steps: little air current impact device includes bi-pass piston cylinder (1) that all link up from top to bottom, a pair of gas shield (2) of bi-pass piston cylinder (1) inner fixedly connected with, dig impact micropore (4) that have a plurality of evenly distributed on gas shield (2), it is a pair of gas shield (2) are close to the equal fixedly connected with in one end each other and place net (3), bi-pass piston cylinder (1) inner still sliding connection has piston (5) of a pair of symmetry, and interference fit between piston (5) and bi-pass piston cylinder (1), the equal fixedly connected with U type mounting panel (6) in both ends about bi-pass piston cylinder (1), U type mounting panel (6) are close to a pair of electric putter (7) of piston (5) one end fixedly connected with, and the output and piston (5) fixed connection of electric putter (7), both ends are all dug and have gas feed mouth (8) about bi-pass piston cylinder (1), and the gas port (8) is opposite to the space between the pair of placing nets (3).
9. The porous carbon coated LiFePO with pre-stress surface according to claim 84The preparation method of the anode material is characterized by comprising the following steps: the diameter of the impact micropores (4) is uniformly reduced along the direction close to the placing net (3), and the vertical section is in an inverted trapezoid shape.
10. The method of claim 8Porous carbon coated LiFePO with surface having pre-stress4The preparation method of the anode material is characterized by comprising the following steps: the diameter of the impact micropores (4) is 0.1-1 μm.
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CN103733396A (en) * 2012-08-08 2014-04-16 北京科瑞沅科技有限公司 Doped secondary battery positive electrode material and preparation method thereof

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