CN113594456B

CN113594456B - Positive electrode slurry, preparation method thereof, positive plate and lithium ion battery

Info

Publication number: CN113594456B
Application number: CN202110745881.XA
Authority: CN
Inventors: 卢周广; 李志强; 黄贺; 尹利红; 廖科萌; 程化
Original assignee: Southwest University of Science and Technology
Current assignee: Southwest University of Science and Technology
Priority date: 2021-07-01
Filing date: 2021-07-01
Publication date: 2022-12-13
Anticipated expiration: 2041-07-01
Also published as: CN113594456A

Abstract

The invention belongs to the field of lithium batteries, and particularly relates to anode slurry which comprises an anode material and a binder, wherein the binder is dextran lithium sulfonate. The invention also discloses a preparation method of the anode material, an anode plate coated with the anode slurry and a lithium ion battery. The invention effectively inhibits the growth of lithium dendrite in the lithium metal battery and the phenomenon of puncturing the battery diaphragm, and improves the performance of the lithium battery; the surface of the positive electrode material particles is coated with a uniform binder interface layer with higher structure, electrochemical stability and excellent ionic and electronic conductivity, so that the problem of surface stability of the positive electrode material in a high-voltage charging process is effectively solved, and the performance of the lithium battery is improved.

Description

Positive electrode slurry, preparation method thereof, positive plate and lithium ion battery

Technical Field

The invention belongs to the field of lithium batteries, and particularly relates to anode slurry, a preparation method thereof, an anode sheet coated with the anode slurry and a lithium ion battery.

Background

Since 1991, commercial lithium ion batteries have emerged to date, and lithium batteries have gradually reached the peak of commercialization, and are widely used in portable electronic devices and electric vehicles, such as smart phones, notebook computers, digital cameras, and other consumer electronic devices. Nowadays, the requirements for light weight and long endurance of products are further improved, the energy density (especially the energy density of a positive electrode material) of the current lithium battery cannot meet the requirements, and the development of a novel battery or the improvement of the capacity of the existing battery to improve the energy density and the cycle life of the lithium ion battery are urgently needed. However, since the theoretical capacity of the graphite negative electrode currently commercialized is only 372mA · h/g, the application of the battery is limited. Recent research results show that a plurality of negative electrode materials such as silicon, tin, transition metal oxides and the like can be used for extractingReplacing the graphite negative electrode which is commercialized at present. In addition to these materials, lithium metal is a very promising high energy density anode material because of its theoretical capacity up to 3860 mA-h/g and low energy density (0.534 g/cm) ³ ) And has a very low oxidation-reduction potential (-3.04V relative to the standard hydrogen electrode), so that the lithium metal can meet the requirements of electric automobiles and electronic equipment on high-energy density batteries. However, highly active lithium metal reacts with the electrolyte during battery charge and discharge cycles, resulting in interfacial instability and formation of lithium dendrites, and continued electrolyte consumption resulting in low coulombic efficiency and limited cycle life. Furthermore, the generation of lithium dendrites and dead lithium may cause safety problems such as thermal runaway and even combustion or explosion, preventing the lithium metal negative electrode from being used for a chargeable lithium battery. Therefore, stabilizing the Solid Electrolyte Interface (SEI) structure of the lithium metal surface is a key to the development of safe lithium metal negative electrodes. For naturally formed SEI, inorganic substances act as fast lithium ion channels and organic substances act as highly elastic soft substrates that can buffer volume changes. However, such SEI is composed of irregular inorganic matter on the surface of lithium metal and interfacial organic matter, forming a fragile structure, resulting in repeated breakage of SEI and formation of a surface layer. The artificial SEI is a method for effectively improving interface stability and prolonging the service life of a lithium metal negative electrode, and the inorganic SEI comprises lithium fluoride, lithium nitride, lithium phosphate, lithium alloy and the like, and can form high-lithium-affinity and high-elasticity organic SEI through in-situ chemical reaction, spin coating of organic matters and the like. To produce an ideal artificial SEI, the organic and inorganic components should be rearranged into reasonable structures and have a synergistic effect of high flexibility and ionic conductivity. In summary, the unstable SEI structure problem is an urgent problem to be solved for the development of lithium rechargeable batteries based on lithium metal negative electrodes.

The prior art has limited improvement on unstable SEI at lithium metal interface, and has complex process or high cost, and the prior art can not be applied to high-flux industrial production on a large scale, such as using LiF and LiN in representative Lu Yingying subject group _X O _y And Li ₂ O is used as inorganic SEI, and the average coulombic efficiency of the lithium metal battery in 450 cycles reaches 99.1 percent. However, galant et al in the article show that the ionic conductivity of LiF is only 10 ^-13 ～10 ^-14 S/cm, and LiF also lacks mechanical integrity, so LiF cannot be used as SEI to protect lithium metal on its own, but can only be used in artificial SEI in combination with other inorganic materials or organic polymers. Others such as Li ₃ N, lithium alloys, etc. although they can improve the cycle stability of lithium metal, they generate gas (N) under a large current ₂ ) Or the contact reaction of the lithium metal with the electrolyte cannot be prevented. Organic lithium salts, as artificial SEI, need to have several advantages: first, it has high elasticity and toughness, and in the case of a lithium salt containing hydroxyl groups and carboxyl groups, it is required that the Young's modulus is greater than 1GPa per 5 carbon atoms, and at least one lithium alkoxide or lithium carboxylate per 2 to 12 carbon atoms, and it can prevent the volume expansion of lithium metal to within 20%. Second, the thickness of the SEI after fabrication is thin and controllable, limited to a few hundred nanometers to a few micrometers. And thirdly, the lithium ion battery has more lithium-philic groups, contains more carboxyl lithium-philic groups than hydroxyl lithium salt, and is tightly combined with a lithium sheet and not easy to fall off. Fourthly, after the artificial SEI is activated, the artificial SEI hardly reacts with lithium metal and electrolyte and has high passivity. The artificial SEIs reported at present do not have all the advantages, and some composite artificial SEIs have the advantages, but the components and the structure are complex and expensive, and the defects limit the popularization and the application of the technologies in industrial production.

The capacity of the anode material in the lithium ion battery is low, and the priority of improving the capacity of the anode of the lithium ion battery is urgently needed. With lithium cobaltate (LiCoO) ₂ LCO) cathode material, as an example, has an unexceptive market position in the consumer electronics market due to its excellent stability and minimum energy volume density as the earliest lithium ion battery cathode material to realize commercialization. The theoretical capacity of lithium cobaltate is 274mAh/g, but when polyvinylidene fluoride (PVDF) is used as a binder in practical application, the lithium cobaltate is charged to 4.3V and only has 140mAh/g capacity, because irreversible laminated structure collapse can occur under high voltage (4.35-4.8V) to damage the lithium cobaltate structure, reduce the cycle performance, cause rapid capacity decline, and limit the capacity of the battery. And high pressureLithium cobaltate can improve the charge cut-off voltage to about 4.35V, the specific capacity of the lithium cobaltate can be exerted to about 165mAh/g, but compared with the theoretical capacity of 274mAh/g, the lithium cobaltate still has a large promotion space, so that the charge cut-off voltage of the lithium cobaltate is further promoted, and particularly the high-voltage charge cut-off voltage of 4.6V is concerned by a plurality of scientific researchers. If the cut-off voltage of lithium cobaltate can be increased to 4.6V vs Li ⁺ Then its theoretical capacity would reach 220mA h/g. However, as the charging is further increased to the voltage, a series of interface side reactions occur due to the phase transition of O3 → H1-3 → O1 near 4.55V, the dissolution of Co element, the release of oxygen in crystal lattice and the abnormal growth of the interface on the surface of the positive electrode, and these high-voltage side reactions cause the problems of the increase of the resistance and the attenuation of the capacity of the battery, thereby greatly limiting the practical application of the battery. Currently, many modification strategies have been reported, such as doping of elements and surface coating, which can occupy some vacancies formed after high-pressure delithiation of LCO by doping Al, mg, zr, fe, ti, cr, mn, etc., thereby stabilizing its structure. Secondly, the anode/electrolyte interface phase (CEI) on the surface of the LCO is made more stable by surface modification, such as oxides, fluorides, phosphates, etc., so that the entire LCO structure is stable at high voltage, improving battery performance and capacity. Although commercial doping and coating of elements is a good method for improving the high voltage performance of LCO, few coatings can achieve both binder and structural stability, and excessive doping and coating can increase the complexity of the process, thereby limiting the commercialization of high voltage lithium cobaltate, and similar problems are also severe for other cathode materials. Therefore, it is a current goal of the battery industry to improve high voltage LCO, to obtain a high energy density positive electrode material that is stable and safe and can be cycled for long periods of time. In practical application, the positive electrode of the lithium ion battery is composed of an active material, a conductive agent and a binder (additive), wherein the positive electrode binder additive is used as an important component of the lithium ion battery and is required to be capable of well binding the active material and the conductive agent in the positive electrode on a current collector. However, with the development and research of the cathode binder in recent years, the cathode binder is found to influence and even improve the performance of the cathode material to a great extent, and the development is highPerformance lithium ion battery positive electrode binders (additives) are becoming the focus of research.

Disclosure of Invention

The invention provides positive electrode slurry capable of stabilizing a positive electrode material and improving the performance of the positive electrode material, which comprises the positive electrode material and a binder, wherein the binder is glucan lithium sulfonate.

Further, the positive electrode material is selected from lithium cobaltate (LiCoO) ₂ ) Lithium-rich lithium manganate (Li) _1.2 MnO ₂ ) Lithium manganate (LiMnO) ₂ ) Lithium iron phosphate (LiFePO) ₄ ) And NCM811 ternary cathode Material (LiNi) _0.8 Co _0.1 Mn _0.1 O ₂ ) NCM622 ternary cathode Material (LiNi) _0.6 Co _0.2 Mn _0.2 O ₂ ) NCM532 ternary cathode Material (LiNi) _0.5 Co _0.3 Mn _0.2 O ₂ ) At least one of (1).

Further, the lithium dextran sulfonate coats the powder particle surface of the positive electrode material.

Further, the positive electrode slurry includes a conductive agent.

Further, the conductive agent is selected from at least one of carbon black, conductive graphite, carbon fiber, carbon nanotube and graphene.

Further, the positive electrode slurry includes a dispersant.

Further, the dispersing agent is DMSO or water, namely the dextran sulfonic acid lithium is prepared into a DMSO or water solution for use, or water or DMSO is added as the dispersing agent when in use.

Further, the mass ratio of the positive electrode material to the dextran lithium sulfonate is 80:1 to 10; the mass ratio of the positive electrode material to the conductive agent is 80:1 to 20.

The second aspect of the invention provides a preparation method of the anode slurry, which comprises the following steps: and uniformly mixing the positive electrode material and the binder.

The third aspect of the invention provides a positive plate, which is an aluminum foil coated with the positive slurry.

The first preparation method of the positive plate comprises the following steps: the positive plate coated with the DSL modification is manufactured by the ball milling technology and is suitable for various positive materials.

DSL is used as a high-voltage anode binder, and the surface of anode material powder particles is coated by DSL powder through a ball milling technology. Preparing 1-20 mg/mL DSL DMSO solution in a glove box, taking a proper amount of solution, mixing with anode material powder and conductive agent slurry, performing ball milling uniformly, then uniformly coating the anode material slurry on an aluminum foil by using a coating machine, and finally placing in a vacuum oven for vacuum drying at 100-120 ℃ for 10-15 hours for later use.

And a second preparation method of the positive plate: the DSL modified and coated positive plate is manufactured by a homogenate smear technology and is suitable for positive materials insensitive to water, such as lithium iron phosphate and lithium cobaltate.

DSL is used as a bonding agent, and the surface of the positive electrode material powder particles is coated by DSL modification through a homogenate smear technology and is stuck on an aluminum foil. Firstly, preparing 3-30 mg/mL DSL aqueous solution, taking a proper amount of the solution, anode material powder and conductive agent to be mixed and evenly ground, then evenly coating the anode material slurry on an aluminum foil by a coating machine, finally putting the aluminum foil on the aluminum foil to be dried for 10-15 hours in vacuum at 100-120 ℃, and storing the dried DSL coated anode plate in a water-free and oxygen-free glove box for later use.

The third preparation method of the positive plate comprises the following steps: and uniformly mixing the positive electrode material, the conductive agent and the glucan lithium sulfonate in a proper mass ratio by a high-speed vibration mixer, adding a proper amount of ultrapure water as a dispersing agent, and stirring at normal temperature until the slurry is completely and uniformly mixed. Coating the slurry on an aluminum foil by using a coating machine, transferring the electrode plate into a forced air drying oven for forced air drying at 50-80 ℃ for 10-15 hours to remove a large amount of moisture, taking out the electrode plate, transferring the electrode plate into a vacuum drying oven for vacuum drying at 100-120 ℃ for 10-15 hours to remove residual moisture; and finally, compacting the dried pole piece on a roller machine, punching into a lithium cobaltate positive plate, and storing in a glove box for later use.

By taking lithium cobaltate LCO as an example, the LCO is coated by the DSL, so that the LCO is prevented from directly contacting with the electrolyte, cobalt ions can not be dissolved in the electrolyte, the LCO can not collapse in an irreversible structure under high voltage, and the LCO particles are not easy to fall off due to the high viscosity of the DSL, thereby greatly inhibiting the change of the structure of the LCO under the high voltage.

In a fourth aspect of the present invention, a negative electrode sheet is provided, wherein a current collector of the negative electrode sheet is coated or reaction-modified with lithium dextran sulfonate.

Further, the current collector is a copper foil or a lithium foil.

The load capacity of the negative plate is 0.01-20 mAh/cm ^-2 。

In a fifth aspect of the present invention, a method for preparing a negative electrode sheet is provided.

The first preparation method of the negative plate comprises the following steps: the lithium dextran sulfonate solution was spin coated on the current collector and dried.

And (3) taking a copper sheet as a current collector, and spin-coating a layer of lithium dextran sulfonate on the negative copper sheet to serve as artificial SEI (solid electrolyte interphase) by a spin-coating technology. Firstly, preparing 20-100 mg/mL lithium dextran sulfonate DMSO solution, spin-coating 100-500 mu L lithium dextran sulfonate solution on a clean copper sheet, then uniformly spin-coating the liquid on the copper sheet by a spin-coating machine, finally putting the copper sheet into a vacuum oven for vacuum drying at 100-120 ℃ for 10-15 hours, and assembling a battery to deposit lithium metal to be used as a lithium metal cathode.

And the second preparation method of the negative plate comprises the following steps: and (3) soaking the current collector in a dextran lithium sulfonate solution, taking out the current collector and drying.

And (3) taking a lithium sheet as a current collector, and modifying artificial SEI on the surface of lithium metal by using a lithium sheet soaking technology. Firstly, preparing 0.1-200 mg/mL dextran sulfonic acid lithium DMSO solution in a glove box, then putting a clean lithium sheet into the DSL-DMSO solution for soaking, stirring for 10-15 hours in the glove box, and finally drying in a transition bin of the glove box to obtain the lithium metal negative electrode sheet for modifying the DSL-DMSO artificial SEI.

The third preparation method of the negative plate comprises the following steps: adding the glucan lithium sulfonate into electrolyte, forming a battery by using a current collector and the electrolyte, performing charge-discharge circulation, taking out the current collector, and drying.

And (3) taking a lithium sheet as a current collector, and modifying the artificial SEI on the surface of the lithium metal through chemical reaction. First, a DMSO electrolyte (containing 0.8M LiNO) was prepared ₃ And 0.2M LiPF ₆ ) Adding dextran sulfatePreparing 1-20 mg/mL DSL-DMSO solution by lithium oxide, then forming a symmetrical battery by using a lithium sheet and the lithium sheet, using the electrolyte to perform constant current charging and discharging for 0.5-2 hours respectively, circulating for 10-40 circles, finally taking out the lithium sheet from the battery, cleaning by using dimethyl carbonate and drying to obtain the lithium metal negative electrode sheet for modifying DSL-DMSO artificial SEI.

The negative plate obtained in the way is used for assembling a lithium ion battery, lithium is preferentially deposited between a DSL film and lithium metal or on the surface of copper foil in the process of charging the battery, and the DSL film inhibits the growth of lithium dendrite.

In a sixth aspect of the present invention, there is provided a lithium ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a casing, wherein the positive electrode sheet and/or the negative electrode sheet are as described above.

The surface of a negative electrode obtained by the lithium ion battery through the circulation reaction contains lithium sulfide and lithium sulfonate, and the lithium sulfide and the lithium sulfonate are embedded into dextran lithium sulfonate modified on the surface of a current collector; the related studies confirmed that lithium sulfide salts have a high affinity with lithium metal and rapidly provide conductive lithium ions, and thus the resultant lithium sulfide and lithium sulfonate salts have increased lithium affinity and ionic conductivity.

The negative electrode sheet may be used for other rechargeable batteries, such as sodium ion batteries, potassium ion batteries, magnesium ion batteries, zinc ion batteries, and aluminum ion batteries, in addition to lithium ion batteries.

In the face of the problems of unstable SEI structure of a lithium metal interface, poor long-cycle stability of high-pressure lithium cobaltate and difficult industrialization, the natural polymer is used as a binder to stabilize the high-pressure high-energy-density positive electrode material, and is also used for improving the metal negative electrode interface and inhibiting dendritic crystal growth.

On the one hand, starting from the optimization and the structural improvement of a lithium cobaltate interface, the DSL is used as a high-voltage lithium cobaltate battery positive binder to replace the traditional polyvinylidene fluoride (PVDF), and the DSL is closely attached to the surface of a positive material to form an even coating layer, so that the high-voltage side reaction between electrolyte and lithium cobaltate can be reduced, the surface of the material is protected from being corroded by the electrolyte, the precipitation of oxygen and the decomposition of cobalt elements are reduced, and the effect of stabilizing the long circulation of the 4.6V high-voltage lithium cobaltate is achieved.

On the other hand, the negative electrode (copper sheet or lithium sheet) of the battery is improved by adopting glucan lithium sulfonate (DSL, 50 ten thousand molecular weight) as artificial SEI, and the DSL has high elasticity, and the lithium sulfonate structure in the structure has fluidity and is easy to dissolve, so that the DSL has high ionic conductivity. Meanwhile, in the charging and discharging process, DSL can generate local slow reaction with lithium metal to form Li ₂ S and Li ₂ SO _x The inorganic layer of (a) increases toughness and ionic conductivity of the SEI, and finally improves the performance of the battery.

The invention has the following beneficial effects:

the sulfonated DLS has high ionic conductivity, high elasticity and Young modulus up to 6GPa, and after the negative electrode is modified by spin coating, the lithium metal battery has high stability and safety; after the anode is modified by spin coating, the uniform coating layer can reduce the high-pressure side reaction between electrolyte and an anode material (such as LCO), protect the surface of the material from being corroded by the electrolyte, and reduce the precipitation of oxygen and the decomposition of cobalt element, so that the lithium battery has high-pressure stability, high energy density and long cycle life.

The method has the advantages that the process is mature and stable, the raw materials are low in price, the thickness of the coating agent can be accurately controlled, the coating agent can be manufactured from nano-scale to micron-scale thicknesses, the negative plate and the positive plate can be industrially produced in a large scale, and the commercial mass production is easy to realize.

The invention effectively inhibits the growth of lithium dendrite in the lithium metal battery and the phenomenon of puncturing the battery diaphragm, and improves the performance of the lithium battery; the surface of the positive electrode material particles is coated with a uniform binder interface layer with higher structure, electrochemical stability and excellent ionic and electronic conductivity, so that the problem of surface stability of the positive electrode material in a high-voltage charging process is effectively solved, and the performance of the lithium battery is improved.

Drawings

FIG. 1 is a thermogravimetric plot of lithium dextran sulfonate;

FIG. 2 is a graph of the Fourier transform infrared spectra of lithium dextran sulfonate and dextran;

FIG. 3 is a digital photograph of a prepared lithium dextran sulfonate film;

FIG. 4 is an SEM photograph of the positive electrode sheet of example 1;

FIG. 5 is a topographical view of the positive plate of example 1 under 3D contour machine characterization;

fig. 6 is a high resolution TEM characterization of the positive plate of example 1 after cycling;

fig. 7 is a coulombic efficiency graph of the full cell of example 1;

FIG. 8 is a graph of rate performance and high temperature cycle performance for the cell of example 1;

FIG. 9 is an SEM photograph of a copper wafer of example 3 after spin-coating lithium dextran sulfonate;

fig. 10 is an SEM image of lithium deposition on a generic copper sheet current collector of example 3;

fig. 11 is an SEM image of lithium deposition on a DSL-coated copper sheet current collector of example 3;

FIG. 12 is an SEM image of a modified lithium dextran sulfonate on a lithium plate of example 4;

FIG. 13 is a lithium metal anode surface XPS characterization after cycling for a lithium metal battery of example 4;

fig. 14 is a graph of cyclic polarization of a lithium metal battery of example 4;

fig. 15 is a coulombic efficiency graph of the lithium metal battery of example 4;

FIG. 16 is a graph of the rate cycling of the full cell of example 5;

fig. 17 is a cycle performance chart of the full cell of example 5.

Detailed Description

The present invention will be further described with reference to the following specific examples.

Synthesis of dextran lithium sulfonate: adding pyridine (or formamide) into a reaction tank, dropwise adding chlorosulfonic acid while stirring at 25 ℃, heating to 45-65 ℃, adding glucan, wherein the ratio of glucan (m): pyridine (V): chlorosulfonic acid (V) =1:6:1.48, preserving the heat for 2.5-4 h, standing for layering, and pouring the upper pyridine to obtain pasty esterified substance. Stirring and cooling the pasty ester, adding a proper amount of water at about 30 ℃, neutralizing with 400g/L (40%) of LiOH solution until the pH value is 10-11, standing and layering,adding appropriate amount of water and ethanol into the subnatant to make its mass concentration reach 455g/L (45.5%), maintaining the temperature at 37 deg.C, stirring, washing for 30min, standing, separating the subnatant, and repeating the above operations and washing twice. Obtaining the dextran sulfonate lithium salt solution. Adding appropriate amount of water and medicinal active carbon, decolorizing at 50 deg.C, press filtering, adding appropriate amount of water and ethanol into the filtrate to make ethanol reach 47.5%, stirring at 37 deg.C for 30min, standing, collecting the lower layer solution, and freeze drying the lower layer solution to obtain dextran lithium sulfonate. The finally obtained sulfonated glucan has rich organic functional groups, and particularly can realize the bonding and uniform coating of the glucan lithium sulfonate on a matrix through the strong hydrogen bond interaction of the sulfonated functional groups and the surfaces of the aluminum foil and the positive electrode material. Meanwhile, three high-conductivity lithium ion sulfonate groups are arranged on the ring of the sulfonated glucan monomer, and the sulfonated glucan monomer has high ionic conductivity (more than 1.4 multiplied by 10) ^-3 S/cm), high elasticity and Young modulus as high as 6GPa, can bear high voltage of over 5V under the actual charging test, and has strong high-voltage decomposition resistance; conventional dextrans do not have these excellent physicochemical properties. Fig. 1 is a thermogravimetric plot of DSL, showing that lithium dextran sulfonate has a very high decomposition temperature; FIG. 2 is a Fourier change infrared spectrogram of dextran lithium sulfonate and dextran, and it can be seen that the sulfonated dextran has rich sulfur functional groups, and can interact with strong hydrogen bonds on the surfaces of aluminum foil and positive electrode material, so as to realize the adhesion and uniform coating of dextran lithium sulfonate on the substrate; FIG. 3 is a digital photograph of the prepared lithium dextran sulfonate film, illustrating the high elasticity of the lithium dextran sulfonate film.

Example 1

Preparation of positive plate and lithium battery

Taking the preparation of a lithium cobaltate-lithium metal half-cell as an example, the slurry coating method is adopted to prepare the positive plate.

The positive electrode active material (lithium cobaltate powder), the conductive agent (Super P), and lithium dextran sulfonate were mixed at a ratio of 80:10:2, uniformly mixing the materials by a high-speed vibration mixer, adding a proper amount of ultrapure water serving as a dispersing agent, and stirring at normal temperature until the slurry is completely and uniformly mixed.

By coating machinesThe slurry was coated on an aluminum foil for a battery having a thickness of about 0.1mm, and the electrode sheet was transferred to a forced air drying oven for 12 hours at 60 ℃ to remove a large amount of moisture. After being taken out, the mixture is transferred into a vacuum drying oven to be dried in vacuum for 12 hours at the temperature of 110 ℃ to remove residual moisture. Finally, compacting the dried pole piece on a roller machine, punching into a 12mm lithium cobaltate positive plate, putting the lithium cobaltate positive plate into a glove box for storage for later use, wherein the active material loading capacity of the finally obtained positive plate is about 3mg/cm ² 。

The button cell is assembled by adopting a CR2032 type button cell case according to the sequence of a positive plate case, a stainless steel gasket, a positive plate, a diaphragm, a negative plate (lithium plate), the stainless steel gasket, a stainless steel spring piece and a negative plate case. The electrolyte consumption of each button cell is about 90 microlitres, and the electrolyte is required to be dripped on two sides of the diaphragm in equal quantity, so that the diaphragm is fully soaked. The whole button cell assembly process is completed in a glove box in an argon atmosphere.

For comparison, PVDF was chosen as the binder and tested and characterized in the same way.

By SEI characterization, fig. 4 (a) shows that PVDF binder cannot coat all lithium cobaltate particles, as shown in fig. 4, and only the underside of the particles are connected; while DSL served as a binder, fig. 4 (b) SEM characterization showed that few individual bare particles were visible, and almost all particles were covered, making the LCO pole piece denser.

As shown in fig. 5, under a 3d profilometer, the PVDF bonded LCO surface of fig. 5 (a) is very rough and super, while the DSL bonded LCO surface of fig. 5 (b) is very smooth. These factors result in PVDF-bound LCO being prone to separation after multiple cycles, while DSL-bound LCO is more cohesive and denser, resulting in a tight bond without dusting.

By high resolution TEM characterization, it was found that lithium dextran sulfonate coating on the DSL-bonded LCO particle surface was very uniform with a thickness around 9 nm.

After the cell was cycled, the positive electrode was again characterized by high resolution TEM, as shown in fig. 6, with some salt crystals (e.g., li) appearing at the PVDF-bonded LCO edge in fig. 6 (a) ₂ CO ₃ ) Resulting in poor lithium ion transport and delithiated cobaltThe lithium structure is unstable, resulting in the reduction of the LCO battery performance; the LCO bonded by DSL in fig. 6 (b) still maintains uniform surface coating after cell cycling, making the LCO structure more stable.

To confirm whether the DSL coating structure can stabilize the structure of high-voltage lithium cobaltate, they were assembled with lithium sheets to form full cells, charged to 4.6V, and discharged to 2.8V, respectively. As shown in fig. 7, the PVDF is used as the binder, so that the LCO cell decays quickly under high voltage, and the discharge capacity is reduced by almost 30% when the cell is charged and discharged to 100 circles; and the LCO bonded by the DSL still retains more than 94% of capacity after 100 circles of charge and discharge.

In order to further verify the practical use performance of the lithium cobaltate battery with DSL as a binder, rate performance test and high-temperature performance test were carried out on the lithium cobaltate battery, and the results are shown in fig. 8. Fig. 8 (a), under the high-current rate performance test, the discharge capacity of the DSL-LCO electrode is reduced less compared with that of the PVDF-LCO electrode under the same high-current condition, which proves that the DSL binder modified lithium cobalt oxide battery can well meet the market demand for high-current charge and discharge batteries. Fig. 8 (b), in a high temperature test at 45 ℃, DSL-LCO shows more excellent cycle performance, and still shows near 120mAh/g capacity retention after 100 cycles of charge and discharge cycles, which is much larger than the capacity retention of less than 100mAh/g after 100 cycles of conventional PVDF-LCO battery cycles, which proves that the DSL binder modified lithium cobalt oxide battery has a wider application range and can adapt to more complex use environments.

The experiment verifies the feasibility of using the high-pressure lithium cobaltate to improve the energy density, and the method of coating LCO by the DSL binder can produce a large amount of commercial high-pressure lithium cobaltate batteries and improve the LCO capacity and the energy density applied to the market at present.

Example 2

The DSL modified and coated positive plate is manufactured by a ball milling technology and is suitable for various positive electrode materials, such as lithium cobaltate (LiCoO) ₂ ) Lithium-rich lithium manganate (Li) _1.2 MnO ₂ ) Lithium manganate (LiMnO) ₂ ) Lithium iron phosphate (LiFePO) ₄ ) NCM811 ternary cathode Material (LiNi) _0.8 Co _0.1 Mn _0.1 O ₂ ) NCM622 ternary positiveElectrode material (LiNi) _0.6 Co _0.2 Mn _0.2 O ₂ ) NCM532 ternary cathode Material (LiNi) _0.5 Co _0.3 Mn _0.2 O ₂ ). And the DSL is used as a binder and a high-voltage stabilizer, and the DSL powder is coated on the surface of the anode material powder particles by a ball milling technology.

Taking an NCM811 ternary positive electrode material as an example, preparing a 5mg/mL DSL DMSO solution in a glove box, taking a proper amount of the solution, NCM811 ternary positive electrode material powder and a conductive agent mixed slurry (the mass ratio of the positive electrode material, the conductive agent and the dextran lithium sulfonate is 80.

In addition, the DSL modified coated positive plate can be manufactured by a homogenate smear technology, and is suitable for water-insensitive positive materials, such as lithium iron phosphate and lithium cobaltate. DSL is used as a binder, and the surface of the positive electrode material powder particles is coated by DSL modification through a homogenate smear technology and is stuck on an aluminum foil.

Taking lithium iron phosphate as an example, firstly preparing a 10mg/mL DSL aqueous solution, taking a proper amount of the solution, mixing with lithium iron phosphate powder and a conductive agent (the mass ratio of the lithium iron phosphate to the conductive agent to the dextran sulfonate is 80.

Example 3

Preparation of negative plate and lithium battery

And (3) spinning a layer of lithium dextran sulfonate on a negative copper sheet to serve as artificial SEI by a spinning technology.

Taking a 16mm round copper sheet as an example, firstly, ultrasonically cleaning the copper sheet for 0.5h by using ethanol, acetone and 0.1M hydrochloric acid, and cutting the copper sheet into 2.0cm ² Drying the small round blocks and putting intoThe glove box is ready for use.

Firstly, preparing 50mg/mL dextran sulfonic acid lithium DMSO solution, spin-coating 200 muL dextran sulfonic acid lithium solution on a copper sheet, then uniformly spin-coating liquid on the copper sheet through a spin coating machine, and finally putting the copper sheet into a vacuum oven for vacuum drying for 12 hours at 110 ℃ to obtain a copper negative electrode sheet (copper current collector), wherein the thickness of the dextran sulfonic acid lithium is 1μm, the SEM of the copper current collector is shown in figure 9, and the pictures a, b and c are shot pictures under a scanning electron microscope of 4 thousand times, 5 thousand times and 10 thousand times respectively; the text in the electron microscope image herein has no meaning other than the necessary information such as magnification and length.

The copper negative plate and the lithium plate are assembled into a button cell, wherein the cell shell is 2025 type, stainless steel gaskets are added on two sides, the thickness of the lithium plate is 500 μ M, the diameter of the lithium plate is 16mm, and the electrolyte is 1M LiTFSI dissolved in DOL/DME (1).

The assembled cell was left for 12 hours and then at 0.5mA/cm ² Performing charge and discharge (charge and discharge time is 4 h) test under current density, circulating for 100 circles, and finally depositing lithium on the copper sheet with the capacity of about 2mAh/cm ² . And then transferring the copper current collector into a glove box to disassemble the battery, taking out the copper current collector, rinsing the copper current collector with dimethyl carbonate, then drying the copper current collector, and representing the copper current collector with a scanning electron microscope.

For comparison, a copper negative plate which is not treated by the dextran sulfonic acid lithium DMSO solution is selected to be assembled into a battery, and the battery is tested and characterized according to the same method.

Comparing the lithium deposition on the current collectors of the common copper sheet and the DSL-coated copper sheet, it can be observed that, when the common copper sheet is used as the current collector, as shown in fig. 10, fig. 10 (a) is an SEM image magnified by 2.9 kilo times, fig. 10 (b) is a magnified image magnified by 9 kilo times, and fig. 10 (c) is a magnified image magnified by 22 kilo times under an electron microscope, and fig. 10 (a) shows that the deposition of lithium metal on the copper surface is unstable and uniform, fig. 10 (b) can clearly observe that lithium dendrites exist locally, and fig. 10 (c) is a locally magnified image, and the size of the dendrites are clearly observed. Furthermore, as the deposition time increases, lithium dendrites become very pronounced and it can be observed that the surface is almost entirely composed of dendrites.

When using a DSL coated copper sheet as the current collector, FIG. 11 (a) is an enlargement, as shown in FIG. 11A 2 k plane SEM, fig. 11 (b), (c) are cross-sectional SEM images of different areas magnified 5 k times, it can be observed that a smooth lithium sheet on the current collector, lithium is preferentially deposited on the lower layer of DSL so that the surface is smooth and uniform without any lithium dendrites; as can be seen from fig. 11 (b), the deposited lithium metal is tightly connected to the Cu foil and the DSL film without excess voids, and the thickness of the DSL film is approximately 1.2 μm. After lithium deposition, the cells were charged and discharged at the same current density (1 mAh/cm) ² ) After 100 cycles, as shown in FIG. 11 (c), no lithium dendrite was observed. This demonstrates that over a limited range of lithium deposition and cycling, the copper current collector coated with DSL can significantly suppress the generation of lithium dendrites.

Example 4

Preparation of negative plate and lithium battery

Artificial SEI was modified on the lithium metal surface by the immersion lithium sheet technique.

Firstly, preparing a 5mg/mL dextran sulfonic acid lithium DMSO solution in a glove box, then putting a clean lithium sheet into the DSL-DMSO solution for soaking, stirring for 12 hours in the glove box, and finally drying in a transition bin of the glove box to obtain a lithium metal negative electrode sheet for modifying the DSL-DMSO artificial SEI, wherein the thickness of the dextran sulfonic acid lithium is 1.8 mu m. SEM of the lithium collector is shown in fig. 12, where fig. 12 (a) is a planar SEM at 2.5 k times magnification and fig. 12 (b) is a planar SEM at 15 k times magnification. Assembling the modified lithium sheet and lithium iron phosphate into a battery, circulating for 20 circles, disassembling the battery, taking out the battery, cleaning, drying and the like, and finally carrying out deep analysis and test on the surface of the negative electrode by using X-ray photoelectron spectroscopy (XPS) to obtain an XPS test result after analysis of 100nm, wherein as shown in figure 13, a black line is an XPS fitting graph and an original graph, a light gray line is a fitting graph of each peak, and XPS characterization shows that dextran lithium sulfonate is on the surface of lithium metal and is embedded with Li ₂ S and Li ₂ SO _x Which can significantly improve ion conductivity and lithium affinity.

To demonstrate that DSL-modified lithium metal surfaces significantly improve the performance and stability of lithium metal batteries, symmetric batteries were assembled for testing: the Li-Li symmetrical battery formed by the common lithium sheet and the DSL modified lithium sheet and the lithium sheet respectively has the current load of 1mA/cm ² Respectively charging and discharging at current density2h, repeatedly and circularly measuring a polarization curve, as shown in fig. 14, a black line is the polarization curve of the DSL-modified lithium sheet, a light gray line is the polarization curve of a common lithium sheet, the overpotential of a symmetric battery assembled by the DSL-modified lithium sheet is small, and the stable overpotential can be maintained after 200 circles of circulation, which indicates that the DSL-modified lithium metal interface is very stable and can prevent the reaction of electrolyte and lithium metal; however, the overpotential increased significantly after 100 cycles for a symmetrical cell assembled with a normal lithium sheet because the surface SEI was unstable.

The symmetric battery is cycled for 400 times, the coulombic efficiency curve is shown in fig. 15, and the DSL modified lithium sheet keeps high coulombic efficiency and is hardly reduced; the coulombic efficiency of a conventional lithium sheet rapidly decays to a short circuit at 240 cycles because the lithium metal and the electrolyte continuously react, and the internal stress of lithium deposition causes the SEI to be continuously broken and recombined, thereby degrading the battery performance.

Example 5

Preparation of negative plate and lithium battery

The lithium metal surface modifies the artificial SEI by chemical reaction.

First, a DMSO electrolyte (containing 0.8M LiNO) was prepared ₃ And 0.2M LiPF ₆ ) And then adding lithium dextran sulfonate to prepare a 5mg/mL DSL-DMSO solution, then forming a symmetrical battery by using a lithium sheet and a lithium sheet, using the electrolyte to perform constant current charging and discharging for 1 hour respectively, circulating for 20 circles, finally taking out the lithium sheet from the battery, cleaning by using dimethyl carbonate and drying to obtain the lithium metal negative electrode sheet for modifying the DSL-DMSO artificial SEI.

The lithium metal negative plate modified by DSL and the common lithium plate are respectively assembled with lithium iron phosphate (LFP) to form the full battery, the performance of the full battery under different multiplying powers is tested, and a multiplying power cycle curve is shown in figure 16. Under low rate, the discharge capacity of DSL-lithium negative electrodes is similar to that of common lithium negative electrodes such as 0.1C and 0.2C, the discharge capacity of the battery is kept at a high level of 160mAh/g, the overpotential is very low, and when the current density of charging and discharging is gradually increased from 0.5C to 5C, the DSL-Li I LFP battery presents very low overpotential and high discharge capacity, and the discharge capacity is reduced to 148mAh/g from 160 mAh/g. For a Cu-Li I LFP battery assembled by a common lithium sheet, the overvoltage change is very large, so that the charging is insufficient, the capacity is reduced, and finally the discharge capacity is greatly reduced from 160mAh/g to 50mAh/g.

The DSL modified lithium metal negative plate and the common lithium plate are respectively assembled with lithium iron phosphate (LFP) to form the whole battery, and the whole battery is respectively charged and discharged under the rate of 1C, the cycle stability of the whole battery is shown in figure 17, the DSL-Cu-Li | LFP battery presents stable discharge capacity, and after the battery is cycled for 250 circles, the capacity retention rate of more than 90 percent can be still maintained, which shows that the modification of the DSL on the lithium metal surface can obviously improve the long-term cyclicity of the battery. After the Cu-Li I LFP battery is circulated for 60 circles, the capacity of the battery is reduced very quickly, and finally the battery fails, because lithium metal is consumed due to continuous fracture and regeneration of an interface SEI, and the impedance is greatly increased. Therefore, the DSL modified lithium metal negative electrode battery can keep stable discharge capacity under the working state of different capacities of the battery.

Example 6

The battery was assembled using the positive electrode sheet of example 1 and the negative electrode sheet of example 3, and an excellent initial capacity of approximately 210mAh/g at an initial capacity of 0.1C. The battery has the advantages that the capacity retention rate reaches over 95.4% after the battery circulates for 100 circles, and the capacity retention rate can also reach over 70% after the battery circulates for 500 circles, compared with the battery assembled by the LCO positive plate coated with the PVDF as the binder and the common copper plate, the battery has the capacity retention rates of less than 70% and 40%, and the results prove that the irreversible structural collapse of the LCO and the dissolution of cobalt ions and the occurrence of side reactions are prevented by the DSL-coated positive plate and the DSL-modified negative plate in the battery circulation process, and the capacity retention rate and the cycle life of the battery are improved.

The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are also within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. The positive electrode slurry comprises a positive electrode material and a binder, and is characterized in that the binder is glucan lithium sulfonate.

2. The positive electrode slurry according to claim 1, wherein the positive electrode material is at least one selected from lithium cobaltate, lithium-rich lithium manganate, lithium iron phosphate, NCM811 ternary positive electrode material, NCM622 ternary positive electrode material, and NCM532 ternary positive electrode material.

3. The positive electrode slurry according to claim 1, wherein the lithium dextran sulfonate coats a surface of a powder particle of the positive electrode material.

4. The positive electrode slurry according to claim 1, wherein the mass ratio of the positive electrode material to the lithium dextran sulfonate is 80:1 to 10.

5. The positive electrode slurry according to claim 1, further comprising a conductive agent and a dispersant.

6. The positive electrode slurry according to claim 5, wherein the dispersant is DMSO or water.

7. The positive electrode slurry according to claim 5, wherein the conductive agent is at least one selected from carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene.

8. The method for producing a positive electrode slurry according to any one of claims 1 to 7, wherein the positive electrode material and the binder are uniformly mixed.

9. A positive electrode sheet, characterized in that the positive electrode sheet is an aluminum foil coated with the positive electrode slurry according to any one of claims 1 to 7.

10. A lithium ion battery comprising a positive plate, a negative plate, a separator, an electrolyte and a case, wherein the positive plate is according to claim 9.