Positive electrode composition, aqueous positive electrode slurry and method for preparing the same, aqueous positive electrode sheet, secondary battery, and electric device

Abstract

The application provides a positive electrode composition, an aqueous positive electrode slurry and a preparation method thereof, an aqueous positive electrode sheet, a secondary battery and an electric device. The positive electrode composition comprises a lithium-containing positive electrode active material and an organic solvent coating covering at least part of the surface of the lithium-containing positive electrode active material, wherein the lithium-containing positive electrode active material comprises lithium and a transition metal element, the boiling point T of the organic solvent coating is less than 100 DEG C, and the solubility of the organic solvent coating in water at room temperature is less than 10%. m The application can reduce the irreversible loss of lithium and transition metal elements in the positive electrode sheet, and make the secondary battery have high energy density and good cycle performance, rate performance and safety performance at the same time.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a positive electrode composition, an aqueous positive electrode slurry and its preparation method, an aqueous positive electrode sheet, a secondary battery and an electrical device. Background Technology

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. With the application and promotion of secondary batteries, their cost and environmental pollution issues have received increasing attention. The positive electrode is one of the key factors determining the performance of secondary batteries. The solvents used in existing positive electrode slurries are usually oil-based solvents, such as N-methylpyrrolidone (NMP). However, NMP has drawbacks such as high usage, high volatility, difficulty in recycling, high toxicity, and high cost. Therefore, it is necessary to develop a low-cost, environmentally friendly, and high-performance aqueous positive electrode. Summary of the Invention

[0003] The purpose of this application is to provide a positive electrode composition, an aqueous positive electrode slurry and its preparation method, an aqueous positive electrode sheet, a secondary battery and an electrical device, which can reduce the irreversible loss of lithium and transition metal elements in the positive electrode sheet, and enable the secondary battery to simultaneously achieve high energy density, good cycle performance, rate performance and safety performance.

[0004] A first aspect of this application provides a cathode composition comprising a lithium-containing cathode active material and an organic solvent coating on at least a portion of the surface of the lithium-containing cathode active material, wherein the lithium-containing cathode active material comprises lithium and a transition metal element, and the organic solvent coating has a boiling point T. m The organic solvent coating has a solubility of less than 10% in water at temperatures below 100°C and at room temperature.

[0005] T m When an organic solvent coating with a solubility of less than 10% at room temperature is applied to at least a portion of the surface of lithium-containing cathode active material particles, bound water is less likely to form on the surface of the particles, and the dissolution of lithium and transition metal elements from the particle surface is reduced. This results in less irreversible loss of lithium and transition metal elements in the cathode electrode prepared from this coating, thereby improving the energy density and cycle performance of the secondary battery. Furthermore, the cathode electrode prepared from this coating has a lower moisture content, which reduces the risk of bulging, corrosion, and severe self-discharge in the secondary battery, while also lowering the internal resistance. Therefore, a secondary battery using the cathode composition provided in this application can simultaneously achieve high energy density, good cycle performance, rate performance, and safety performance.

[0006] In some embodiments, the transition metal element includes one or more of Fe, Ni, Co, Mn, Al, Cu, Zn, and Ti.

[0007] In some embodiments, the lithium-containing cathode active material includes LiFe. m Mn 1-m PO4, Li(Ni) x Co y Mn z Al a Cu b Zn c Ti d One or more of O2, 0≤m≤1, x+y+z+a+b+c+d=1, 0.5≤x<1, 0.05≤y<1, 0≤z<0.5, 0≤a≤0.1, 0≤b≤0.1, 0≤c≤0.1, 0≤d≤0.1.

[0008] In some embodiments, the boiling point T of the organic solvent coating is... m The temperature range is between 30°C and 100°C, and optionally between 40°C and 60°C. When the boiling point of the organic solvent coating is within a suitable range, it is beneficial to further reduce the internal resistance of the secondary battery and significantly improve its rate performance and cycle performance.

[0009] In some embodiments, the solubility of the organic solvent coating in water at room temperature is less than 1%, optionally 0.01%-1%. When the solubility of the organic solvent coating is within a suitable range, it is beneficial to fully exert its protective effect on the surface of lithium-containing cathode active material particles, thereby further reducing the internal resistance of the secondary battery and significantly improving the rate performance and cycle performance of the secondary battery.

[0010] In some embodiments, the organic solvent coating comprises one or more of aromatic hydrocarbons, aliphatic hydrocarbons, alicyclic hydrocarbons, halogenated hydrocarbons, alcohols, phenols, esters, ketones, ethers, alcohol ethers, ester ethers, nitriles, and sulfides. Optionally, the organic solvent coating comprises one or more of benzene, hexane, isohexane, n-heptane, isooctane, 2,2-dimethylpentane, 3-methylpentane, cyclopentane, cyclohexane, dichloromethane, trichloromethane, carbon tetrachloride, 1,2-dichloroethane, carbon disulfide, methyl acetate, and ethyl acetate. These organic solvent coatings have suitable boiling points and solubilities, which can better prevent the formation of bound water by combining the surface of lithium-containing cathode active material particles with solvent water in the slurry, while effectively reducing the dissolution of lithium and transition metal elements from the surface of lithium-containing cathode active material particles. In this case, the secondary battery can better balance high energy density and good cycle performance and rate performance. In addition, these organic solvent coatings can also accelerate water removal efficiency, reduce the moisture content of the positive electrode sheet, and alleviate the cracking of the positive electrode film.

[0011] In some embodiments, the positive electrode composition further includes one or more of a dispersant, an aqueous binder, and a conductive agent.

[0012] In some embodiments, the lithium-containing positive electrode active material has a mass percentage of 88%-99%, optionally 90%-94%, based on the total mass of the positive electrode composition.

[0013] In some embodiments, the organic solvent coating content is 0.01%-10% by mass, optionally 3%-7%, based on the total mass of the cathode composition. When the content of the organic solvent coating is within a suitable range, it can better prevent the formation of bound water by the combination of the lithium-containing cathode active material particles with the solvent water in the slurry, while further reducing the dissolution of lithium and transition metal elements from the surface of the lithium-containing cathode active material particles, thereby further improving the energy density and cycle performance of the secondary battery.

[0014] In some embodiments, the mass percentage of the dispersant is less than 0.7% based on the total mass of the cathode composition, optionally 0.1% to 0.5%.

[0015] In some embodiments, the aqueous binder comprises less than 5% by mass based on the total mass of the cathode composition, optionally 2%-4%.

[0016] In some embodiments, the conductive agent has a mass percentage of less than 5% based on the total mass of the positive electrode composition, optionally between 0.5% and 3%.

[0017] In some embodiments, the dispersant includes one or more of anionic dispersants, cationic dispersants, nonionic dispersants, amphoteric dispersants, electrically neutral dispersants, polymeric dispersants, and controlled radical dispersants, and optionally includes one or more of polyethyleneimine, sodium dodecyl sulfonate, polyvinyl alcohol, and polyethylene glycol octylphenyl ether.

[0018] In some embodiments, based on the total mass of the cathode composition, the aqueous binder includes methylcellulose and its salts, xanthan gum and its salts, chitosan and its salts, alginate and its salts, polyethyleneimine and its salts, polyacrylamide, acrylonitrile-acrylic acid copolymer and its derivatives, or mixtures thereof.

[0019] In some embodiments, the aqueous adhesive comprises a compound mixture of xanthan gum and polyethyleneimine.

[0020] Optionally, the mass ratio of xanthan gum to polyethyleneimine is 2:1 to 0.2:2.8.

[0021] Optionally, the number average molecular weight of the xanthan gum is 300,000-2,000,000, and the number average molecular weight of the polyethyleneimine is 2,000-50,000.

[0022] In some embodiments, the aqueous binder comprises a blend of acrylonitrile-(meth)acrylic acid copolymer and polyethyleneimine, based on the total mass of the cathode composition.

[0023] Optionally, the mass ratio of the acrylonitrile-(meth)acrylic acid copolymer to the polyethyleneimine is 2:1 to 0.2:2.8.

[0024] Optionally, the number average molecular weight of the acrylonitrile-(meth)acrylic acid copolymer is 300,000-2,000,000, and the number average molecular weight of the polyethyleneimine is 2,000-70,000.

[0025] The second aspect of this application provides an aqueous positive electrode slurry, including solvent water and the positive electrode composition of the first aspect of this application.

[0026] In some embodiments, the solid content of the aqueous cathode slurry is 40%-90%, optionally 50%-70%. When the solid content of the aqueous cathode slurry is within a suitable range, the aqueous cathode slurry has good dispersibility and high stability, and the cathode sheet prepared from it has low moisture content and the film layer is not prone to cracking.

[0027] In some embodiments, the viscosity of the aqueous cathode slurry is 100cp-10000cp, optionally 3000cp-7000cp. When the viscosity of the aqueous cathode slurry is within a suitable range, the aqueous cathode slurry has good dispersibility and high stability, and the cathode sheet prepared from it has low moisture content and the film layer is not prone to cracking.

[0028] The third aspect of this application provides a method for preparing an aqueous cathode slurry, which is used to prepare the aqueous cathode slurry of the second aspect of this application, including the steps of: S100, mixing lithium-containing cathode active material and organic solvent coating material uniformly to form a clump; S200, mixing the obtained clump, conductive agent, aqueous binder and solvent water uniformly to obtain an aqueous cathode slurry.

[0029] Add T during the pre-mixing process of the slurry mThe method of preparing an organic solvent coating material with a solubility of less than 10% at room temperature (below 100°C) and coating at least a portion of the surface of lithium-containing cathode active material particles before mixing with other components can prevent the formation of bound water by the solvent water in the slurry on the surface of the lithium-containing cathode active material particles. This also reduces the dissolution of lithium and transition metal elements from the surface of the lithium-containing cathode active material particles, resulting in less irreversible loss of lithium and transition metal elements in the cathode electrode prepared by this method. Consequently, the secondary battery exhibits higher energy density and better cycle performance. Furthermore, the cathode electrode prepared by this method has lower moisture content and less residual organic solvent coating material, which can also reduce the risk of bulging, corrosion, and severe self-discharge in the secondary battery, and lower the internal resistance of the secondary battery.

[0030] The fourth aspect of this application provides an aqueous positive electrode sheet, including a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, wherein the positive electrode film layer is a layer formed after drying the aqueous positive electrode slurry of the second aspect of this application, or a layer formed after drying the aqueous positive electrode slurry obtained by the method of the third aspect of this application.

[0031] In some embodiments, the ratio of the mass fraction W1 of the transition metal element in the region 100 nm from the surface of the lithium-containing cathode active material particle to the mass fraction W2 of the transition metal element in the region 100 nm from the center of the lithium-containing cathode active material particle is α, where α is 60%-100%, and optionally 70%-100%.

[0032] In some embodiments, the thickness of the positive electrode film is H, and the ratio of the porosity P1 in the region H / 3 of the positive electrode film from the surface to the porosity P2 in the region H / 3 of the positive electrode current collector is β, where β is 1.15-2.0, optionally 1.40-1.60. The positive electrode film of this application exhibits a significantly higher porosity in the surface region than in the inner region, which facilitates better electrolyte wetting of the positive electrode film, accelerates lithium-ion transport, and reduces battery internal resistance. Therefore, it can further improve the energy density, cycle performance, and rate performance of the secondary battery.

[0033] In some embodiments, the mass content of the organic solvent coating is below 5000 ppm, and optionally below 200 ppm, based on the total mass of the aqueous positive electrode sheet.

[0034] In some embodiments, the water content of the aqueous positive electrode sheet after vacuum drying at 80°C for 6 hours is below 400 ppm, and optionally below 200 ppm.

[0035] The fifth aspect of this application provides a secondary battery, including the aqueous positive electrode sheet of the fourth aspect of this application.

[0036] The sixth aspect of this application provides an electrical device, including the secondary battery of the fifth aspect of this application.

[0037] The aqueous positive electrode sheet of this application has a lower moisture content and residual organic solvent coating, resulting in less irreversible loss of lithium and transition metal elements. Simultaneously, the positive electrode film of this application exhibits a significantly higher porosity in the surface region than in the inner region, which facilitates better electrolyte wetting of the positive electrode film, accelerates lithium-ion transport, and reduces battery internal resistance. Therefore, the secondary battery using the aqueous positive electrode sheet of this application can simultaneously achieve high energy density, good cycle performance, rate performance, and safety performance. The electrical device of this application includes the secondary battery provided in this application, and thus possesses at least the same advantages as the secondary battery. Attached Figure Description

[0038] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0039] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0040] Figure 2 yes Figure 1 An exploded view of the implementation method of the secondary battery.

[0041] Figure 3 This is a schematic diagram of one embodiment of the battery module of this application.

[0042] Figure 4 This is a schematic diagram of one embodiment of the battery pack of this application.

[0043] Figure 5 yes Figure 4 An exploded view of an embodiment of the battery pack shown.

[0044] Figure 6 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0045] Figure 7 This is a scanning electron microscope (SEM) image of the positive electrode sheet prepared in Example 1.

[0046] Figure 8 This is an X-ray energy dispersive spectroscopy (EDS) spectrum of the positive electrode sheet prepared in Example 1.

[0047] The accompanying drawings are not necessarily drawn to scale. The reference numerals are as follows: 1 Battery pack, 2 Upper casing, 3 Lower casing, 4 Battery module, 5 Secondary battery, 51 Housing, 52 Electrode assembly, 53 Cover plate. Detailed Implementation

[0048] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode composition, aqueous positive electrode slurry, preparation method thereof, aqueous positive electrode sheet, secondary battery, and power device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0049] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0050] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0051] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0052] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0053] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0054] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0055] In this description, unless otherwise stated, "above" and "below" include the stated number.

[0056] In this description, unless otherwise stated, “multiple,” “many,” or “more” means two, two, or more than two.

[0057] In this application, the term "room temperature" means 20℃±2℃.

[0058] A secondary battery, also known as a rechargeable battery or accumulator, is a battery that can be recharged after discharge to reactivate its active materials and continue to be used. Typically, a secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits while allowing active ions to pass through. The electrolyte, situated between the positive and negative electrodes, conducts the active ions.

[0059] With the application and promotion of secondary batteries, their cost and environmental pollution issues have received increasing attention. Aqueous cathode slurries using water as a solvent have attracted growing interest from researchers due to their low cost and environmental friendliness. However, the inventors have found that lithium ions and transition metal ions easily precipitate on the surface of cathode active material particles in the presence of water, affecting the energy density and cycle performance of secondary batteries. In addition, aqueous cathode slurries also suffer from poor fluidity and difficulty in dispersion, and cathode sheets prepared using aqueous cathode slurries also exhibit defects such as difficulty in water removal, high moisture content, and easy cracking of the cathode film.

[0060] In view of this, the inventors of this application, through extensive research and practice, have proposed a positive electrode composition suitable for use in aqueous positive electrode slurries.

[0061] Positive electrode composition

[0062] A first aspect of this application provides a positive electrode composition comprising a lithium-containing positive electrode active material and an organic solvent coating coating at least a portion of the surface of the lithium-containing positive electrode active material. The lithium-containing positive electrode active material comprises lithium and a transition metal element, and the organic solvent coating has a boiling point T. m The organic solvent coating has a solubility of less than 10% in water at temperatures below 100°C and at room temperature.

[0063] The inventors discovered through extensive research and practice that T m When an organic solvent coating with a solubility of less than 10% at room temperature is applied to at least a portion of the surface of lithium-containing cathode active material particles, it can reduce the dissolution of lithium and transition metal elements from the surface of the lithium-containing cathode active material particles and simultaneously prevent the formation of bound water by the combination of the surface of the lithium-containing cathode active material particles with the solvent water in the slurry.

[0064] Meanwhile, since the organic solvent coating used in this application has the characteristics of low boiling point and low solubility, it can also lower the overall boiling point of the slurry, thereby increasing the water removal efficiency of the positive electrode and reducing the moisture content of the positive electrode. In addition, the organic solvent coating of this application can also play a certain plasticizing role in the slurry, thereby improving the slurry dispersibility and alleviating the problem of positive electrode film cracking.

[0065] Therefore, T mWhen an organic solvent coating with a solubility of less than 10% at room temperature is applied to at least a portion of the surface of lithium-containing cathode active material particles, bound water is less likely to form on the surface of the particles, and the dissolution of lithium and transition metal elements from the particle surface is reduced. This results in less irreversible loss of lithium and transition metal elements in the cathode electrode prepared from this coating, thereby improving the energy density and cycle performance of the secondary battery. Furthermore, the cathode electrode prepared from this coating has a lower moisture content, which reduces the risk of bulging, corrosion, and severe self-discharge in the secondary battery, while also lowering the internal resistance. Therefore, a secondary battery using the cathode composition provided in this application can simultaneously achieve high energy density, good cycle performance, rate performance, and safety performance.

[0066] Boiling point T of organic solvent-coated products m At temperatures below 100°C, the residual amount of the organic solvent in the positive electrode can be reduced, which is beneficial for reducing the internal resistance of the secondary battery and improving its rate performance and cycle performance. In some embodiments, the boiling point T of the organic solvent coating is... m The boiling points of the organic solvent coatings can be within the following ranges: 30℃ to 100℃, 30℃ to 90℃, 30℃ to 80℃, 30℃ to 70℃, 30℃ to 60℃, 40℃ to 100℃, 40℃ to 90℃, 40℃ to 80℃, 40℃ to 70℃, and 40℃ to 60℃. When the boiling points of the organic solvent coatings are within a suitable range, it is beneficial to further reduce the internal resistance of the secondary battery and significantly improve its rate performance and cycle performance.

[0067] The solubility of the organic solvent coating in water at room temperature is below 10%, which helps ensure that the organic solvent coating remains on the surface of the lithium-containing positive electrode active material particles during the stirring and dispersion of the positive electrode slurry. This prevents the organic solvent coating from losing its protective effect on the surface of the lithium-containing positive electrode active material particles due to excessive dissolution in the solvent water. In some embodiments, the solubility of the organic solvent coating in water at room temperature may be below 8%, 5%, 3%, 2%, or 1%. Optionally, the solubility of the organic solvent coating in water at room temperature is 0.01%-5%, 0.01%-3%, 0.01%-2%, or 0.01%-1%. When the solubility of the organic solvent coating is within a suitable range, it helps to fully exert its protective effect on the surface of the lithium-containing positive electrode active material particles, thereby further reducing the internal resistance of the battery and significantly improving the rate performance and cycle performance of the secondary battery.

[0068] In some embodiments, the boiling point T of the organic solvent coating is... mBetween 40°C and 60°C, the solubility of the organic solvent coating in water at room temperature is 0.01%-1%. At this temperature, the organic solvent coating can fully protect the surface of the lithium-containing cathode active material particles and reduce the internal resistance of the battery. As a result, the secondary battery can have high energy density while also exhibiting significantly improved rate performance and cycle performance.

[0069] In some embodiments, the organic solvent coating comprises one or more of aromatic hydrocarbons, aliphatic hydrocarbons, alicyclic hydrocarbons, halogenated hydrocarbons, alcohols, phenols, esters, ketones, ethers, alcohol ethers, ester ethers, nitriles, and sulfides.

[0070] Optionally, the organic solvent coating comprises one or more of benzene, hexane, isohexane, n-heptane, isooctane, 2,2-dimethylpentane, 3-methylpentane, cyclopentane, cyclohexane, dichloromethane, trichloromethane, carbon tetrachloride, 1,2-dichloroethane, carbon disulfide, methyl acetate, and ethyl acetate. Further, the organic solvent coating comprises one or more of hexane, isohexane, cyclopentane, carbon tetrachloride, carbon disulfide, methyl acetate, and ethyl acetate. In particular, the organic solvent coating comprises one or more of isohexane, cyclopentane, and carbon disulfide.

[0071] These organic solvent coatings possess suitable boiling points and solubilities, effectively preventing the formation of bound water by combining with solvent water in the slurry on the surface of lithium-containing cathode active material particles. Simultaneously, they effectively reduce the dissolution of lithium and transition metal elements from the surface of the lithium-containing cathode active material particles. This allows the secondary battery to better balance high energy density with good cycle and rate performance. Furthermore, these organic solvent coatings can also accelerate water removal efficiency, reduce the moisture content of the cathode electrode, and alleviate cathode film cracking.

[0072] In some embodiments, the organic solvent coating may coat more than 70% of the surface of the lithium-containing cathode active material. Optionally, the organic solvent coating may coat more than 80% of the surface of the lithium-containing cathode active material. Further, the organic solvent coating may coat more than 90% of the surface of the lithium-containing cathode active material. In particular, the organic solvent coating may completely coat the surface of the lithium-containing cathode active material.

[0073] When organic solvent coatings are located on most or even all of the surface of lithium-containing cathode active materials, they can better prevent the formation of bound water by the combination of the lithium-containing cathode active material particles with the solvent water in the slurry. At the same time, they can further reduce the dissolution of lithium and transition metal elements from the surface of lithium-containing cathode active material particles, thereby further improving the energy density and cycle performance of secondary batteries.

[0074] In some embodiments, the transition metal element includes one or more of Fe, Ni, Co, Mn, Al, Cu, Zn, and Ti.

[0075] This application does not impose any particular limitation on the type of lithium-containing cathode active material. In some embodiments, the lithium-containing cathode active material includes, but is not limited to, LiFe. m Mn 1-m PO4, Li(Ni) x Co y Mn z Al a Cu b Zn c Ti d One or more of O2, 0≤m≤1, x+y+z+a+b+c+d=1, 0.5≤x<1, 0.05≤y<1, 0≤z<0.5, 0≤a≤0.1, 0≤b≤0.1, 0≤c≤0.1, 0≤d≤0.1. Optionally, the lithium-containing positive electrode active material includes LiFePO4(LFP), LiMnPO4, LiFe 0.9 Mn 0.1 PO4, LiFe 0.8 Mn 0.2 PO4, LiFe 0.7 Mn 0.3 PO4, LiFe 0.6 Mn 0.4 PO4, LiFe 0.5 Mn 0.5 PO4, Li(Ni) 0.5 Co 0.2 Mn 0.3 O2(NCM523), Li(Ni) 0.6 Co 0.2 Mn 0.2 O2(NCM622), Li(Ni) 0.8 Co 0.1 Mn 0.1 O2(NCM811), Li(Ni) 0.8 Co 0.15 Al 0.05 One or more of O2.

[0076] In some embodiments, the positive electrode composition further includes an aqueous binder. The aqueous binder can bond the positive electrode active material, conductive agent, etc., to the current collector, enhancing the contact between the positive electrode active material and the conductive agent, as well as between the positive electrode active material and the current collector, and stabilizing the structure of the positive electrode sheet. Compared to oil-based binders, such as polyvinylidene fluoride, aqueous binders are lower in cost, more environmentally friendly, and safer to use.

[0077] The aqueous adhesive may comprise an aqueous dispersion or emulsion with a solid component content of 5% or more. The aqueous adhesive may also comprise a solid that can form a stable dispersion with a solid component content of 1% or more with water.

[0078] In some embodiments, the aqueous adhesive comprises soluble polysaccharides and their derivatives, water-soluble or aqueous dispersion polymers, or mixtures thereof. For example, the aqueous adhesive includes, but is not limited to, methylcellulose and its salts (e.g., lithium methylcellulose, sodium methylcellulose, potassium methylcellulose, etc.), xanthan gum and its salts, chitosan and its salts, alginate and its salts (e.g., lithium alginate, sodium alginate, potassium alginate, etc.), polyethyleneimine and its salts, polyacrylamide, acrylonitrile-(meth)acrylic acid copolymers and their derivatives, or mixtures thereof.

[0079] In some embodiments, the aqueous adhesive comprises a compound mixture of xanthan gum and polyethyleneimine. Optionally, the mass ratio of xanthan gum to polyethyleneimine is 2:1 to 0.2:2.8. Optionally, the number average molecular weight of the xanthan gum is 300,000-2,000,000, and the number average molecular weight of the polyethyleneimine is 2,000-50,000.

[0080] In some embodiments, the aqueous adhesive comprises a blend of acrylonitrile-(meth)acrylic acid copolymer and polyethyleneimine. Optionally, the mass ratio of the acrylonitrile-(meth)acrylic acid copolymer to the polyethyleneimine is 2:1 to 0.2:2.8. Optionally, the number average molecular weight of the acrylonitrile-(meth)acrylic acid copolymer is 300,000-2,000,000, and the number average molecular weight of the polyethyleneimine is 2,000-70,000.

[0081] In some embodiments, the positive electrode composition further includes a conductive agent. This application does not impose any particular limitation on the type of conductive agent, which may include, for example, one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0082] In some embodiments, the positive electrode composition further includes a dispersant that can further improve the stability of the slurry, making it less prone to sedimentation. The dispersant includes one or more of anionic dispersants, cationic dispersants, nonionic dispersants, amphoteric dispersants, electrically neutral dispersants, polymeric dispersants, and controlled radical dispersants. As an example, the dispersant includes, but is not limited to, one or more of polyethyleneimine, sodium dodecyl sulfonate, polyvinyl alcohol, and polyethylene glycol octylphenyl ether.

[0083] In some embodiments, the positive electrode composition further includes two or more of an aqueous binder, a conductive agent, and a dispersant.

[0084] In some embodiments, the lithium-containing positive electrode active material has a mass percentage content of 88%-99% based on the total mass of the positive electrode composition. Optionally, the lithium-containing positive electrode active material has a mass percentage content of 90%-94%.

[0085] In some embodiments, the organic solvent coating content is 0.01%-10% by mass based on the total mass of the cathode composition. Optionally, the organic solvent coating content is 3%-7% by mass. When the content of the organic solvent coating is within a suitable range, it can better prevent the formation of bound water by the combination of the lithium-containing cathode active material particles with the solvent water in the slurry, while further reducing the dissolution of lithium and transition metal elements from the surface of the lithium-containing cathode active material particles, thereby further improving the energy density and cycle performance of the secondary battery.

[0086] The content of organic solvent coating should not be too high, as this may increase the residual amount of organic solvent coating in the prepared positive electrode sheet, which may affect the internal resistance of the secondary battery. When the content of organic solvent coating is high, the stability of the slurry may decrease, for example, it is more likely to settle. In addition, when the content of organic solvent coating is high, the slurry drying time is shortened and the drying speed is accelerated, which may make the film layer in the prepared positive electrode sheet more prone to cracking.

[0087] In some embodiments, the mass percentage of the dispersant is less than 0.7% based on the total mass of the positive electrode composition. Optionally, the mass percentage of the dispersant is 0.1%-0.5%.

[0088] In some embodiments, the water-based adhesive comprises less than 5% by mass based on the total mass of the positive electrode composition. Optionally, the water-based adhesive comprises 2%-4% by mass.

[0089] In some embodiments, the mass percentage of the conductive agent is less than 5% based on the total mass of the positive electrode composition. Optionally, the mass percentage of the conductive agent is 0.5%-3%.

[0090] Aqueous cathode slurry

[0091] The second aspect of this application provides an aqueous positive electrode slurry, including solvent water and the positive electrode composition of the first aspect of this application.

[0092] In some embodiments, the solid content of the aqueous cathode slurry is 40%-90%. Optionally, the solid content of the aqueous cathode slurry is 50%-70%. When the solid content of the aqueous cathode slurry is within a suitable range, the aqueous cathode slurry has good dispersibility and high stability, and the cathode sheet prepared from it has low moisture content and the film layer is not prone to cracking.

[0093] When the solid content of an aqueous cathode slurry is low, its stability may decrease, making it more prone to sedimentation. Furthermore, a lower solid content in an aqueous cathode slurry results in a higher solvent content, leading to longer drying times and higher drying temperatures. This can cause the film layer in the prepared cathode sheet to crack more easily. Conversely, a higher solid content in an aqueous cathode slurry can worsen its uniformity and make coating more difficult.

[0094] In some embodiments, the viscosity of the aqueous cathode slurry (viscosity at room temperature in this application) is 100 cp to 10000 cp. Optionally, the viscosity of the aqueous cathode slurry is 3000 cp to 7000 cp. When the viscosity of the aqueous cathode slurry is within a suitable range, the aqueous cathode slurry exhibits good dispersibility and high stability, while the cathode sheet prepared from it has low moisture content and the film layer is less prone to cracking.

[0095] When the viscosity of an aqueous cathode slurry is low, its stability may decrease, making it more prone to sedimentation. Furthermore, lower viscosity slurries contain more solvent, resulting in longer drying times and higher drying temperatures, which can make the film layer in the prepared cathode sheet more susceptible to cracking. Conversely, higher viscosity aqueous cathode slurries can make coating more difficult.

[0096] The viscosity of the slurry has a well-known meaning in the art and can be measured using instruments and methods well-known in the art. For example, it can be measured with reference to GB / T 2794-2013 "Determination of Viscosity of Adhesives - Single Cylinder Rotation Viscometer Method".

[0097] Preparation method of aqueous positive electrode slurry

[0098] The third aspect of this application provides a method for preparing an aqueous cathode slurry, comprising the steps of: S100, mixing a lithium-containing cathode active material and an organic solvent coating to form a clump; S200, mixing the obtained clump, a conductive agent, an aqueous binder, and water solvent to obtain an aqueous cathode slurry.

[0099] The method for preparing aqueous cathode slurry provided in this application is capable of preparing the aqueous cathode slurry of the second aspect of the embodiments of this application.

[0100] Add T during the pre-mixing process of the slurrym The method of preparing an organic solvent coating material with a solubility of less than 10% at room temperature (below 100°C) and coating at least a portion of the surface of lithium-containing cathode active material particles before mixing with other components can prevent the formation of bound water by the solvent water in the slurry on the surface of the lithium-containing cathode active material particles. This also reduces the dissolution of lithium and transition metal elements from the surface of the lithium-containing cathode active material particles, resulting in less irreversible loss of lithium and transition metal elements in the cathode electrode prepared by this method. Consequently, the secondary battery exhibits higher energy density and better cycle performance. Furthermore, the cathode electrode prepared by this method has lower moisture content and less residual organic solvent coating material, which can also reduce the risk of bulging, corrosion, and severe self-discharge in the secondary battery, and lower the internal resistance of the secondary battery.

[0101] In some embodiments, a dispersant may also be added to S200.

[0102] Water-based positive electrode sheet

[0103] A fourth aspect of this application provides an aqueous positive electrode sheet, comprising a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector. The positive electrode film layer is a layer formed by drying an aqueous positive electrode slurry according to a second aspect of this application, or a layer formed by drying an aqueous positive electrode slurry prepared by a third aspect of this application. The positive electrode film layer can be formed by coating the aforementioned aqueous positive electrode slurry onto the positive current collector and then performing processes such as drying and cold pressing. The positive current collector has two surfaces opposite each other in its thickness direction, and the positive electrode film layer is disposed on either or both of these opposing surfaces.

[0104] The aqueous positive electrode sheet of this application has a low moisture content and low residual organic solvent coating, resulting in less irreversible loss of lithium and transition metal elements. Simultaneously, the positive electrode film of this application exhibits a significantly higher porosity in the surface region than in the inner region, which facilitates better electrolyte wetting of the positive electrode film, accelerates lithium-ion transport, and reduces battery internal resistance. Therefore, secondary batteries using the aqueous positive electrode sheet of this application can simultaneously achieve high energy density, good cycle performance, rate performance, and safety performance.

[0105] In some embodiments, in the aqueous positive electrode sheet, the ratio of the mass fraction W1 of transition metal elements in the region 100 nm from the surface of the lithium-containing positive electrode active material particles to the mass fraction W2 of transition metal elements in the region 100 nm from the center of the particles is α, where α is 60%-100%. For example, α can be 70%-100%, 75%-100%, or 80%-100%. A higher α value indicates that the surface of the lithium-containing positive electrode active material particles is less affected by the solvent water, resulting in less dissolution of transition metals and a more stable particle crystal structure. In the positive electrode sheet of this application, α is above 60%, above 70%, above 75%, or even above 80%. Therefore, the secondary battery using it can simultaneously achieve high energy density and good cycle performance.

[0106] In this application, when the particles are spherical or near-spherical, the "particle center" refers to the center of the sphere; when the positive electrode active material particles are irregularly shaped, the intersection of the longest and shortest diagonals of the particle can be taken as the particle center.

[0107] The parameter α of the lithium-containing positive electrode active material in the aqueous positive electrode sheet can be obtained by the following test method.

[0108] A sample of a certain area (e.g., 4cm to 8cm in length and 4cm to 8cm in width) is cut from the aqueous positive electrode sheet and polished using a CP ion cross-section polisher (e.g., IB-19500) to obtain a polished sample with a cut surface. SEM-EDS testing is performed using a scanning electron microscope (SEM, e.g., ZEISS sigma 300) and an X-ray energy dispersive spectroscopy (EDS, e.g., Horiba 7021-H), for example, referring to JY / T 010-1996, to observe the elemental distribution in the sample. A cross-section of the lithium-containing positive electrode active material particle is selected from the SEM-EDX image, and the mass fraction W1 of the transition metal element in any region 100nm from the surface of this cross-section is measured. Then, the mass fraction W2 of the transition metal element in any region 100nm from the particle center is measured, with parameter α = (W1 / W2) × 100%. To ensure the accuracy of the test results, multiple regions of the cross-section of the lithium-containing cathode active material particles can be tested, and then the average value can be taken.

[0109] It should be noted that the above tests on the positive electrode sheet can be performed by sampling during the secondary battery fabrication process or by sampling from the fabricated secondary battery. As an example, when the test samples are taken from the fabricated secondary battery, the sampling can be performed as follows:

[0110] Discharge the secondary battery (for safety reasons, the battery is usually fully discharged); after removing the battery, take out the positive electrode and soak it in dimethyl carbonate (DMC) for a certain period of time (e.g., 2-10 hours); then take out the positive electrode and dry it at a certain temperature and time (e.g., 60℃, 4h), and then take out the positive electrode.

[0111] Meanwhile, when testing the positive electrode sample taken from the prepared secondary battery, since a protective film of a certain thickness may have formed on the surface of the lithium-containing positive electrode active material particles, the mass fraction of transition metal elements in the region 120nm to 220nm from the surface of the cross section of the lithium-containing positive electrode active material particles can be taken as W1 to ensure the accuracy of the test results.

[0112] In some embodiments, the thickness H of the positive electrode film is not specifically limited, and can be, for example, 50 μm-500 μm. In this application, the thickness of the positive electrode film has a meaning known in the art and can be tested using methods known in the art, such as measuring it using a micrometer.

[0113] In some embodiments, the thickness of the positive electrode film is H, and the ratio of the porosity P1 in the region H / 3 of the positive electrode film from the surface to the porosity P2 in the region H / 3 of the positive electrode current collector is β, where β is 1.15-2.0. Optionally, β is 1.20-2.0, 1.25-2.0, 1.30-2.0, 1.35-2.0, 1.40-2.0, 1.20-1.70, 1.25-1.70, 1.30-1.70, 1.35-1.70, 1.40-1.70, 1.20-1.60, 1.25-1.60, 1.30-1.60, 1.35-1.60, or 1.40-1.60.

[0114] The positive electrode film layer of this application exhibits a significantly higher porosity in the surface region than in the inner region. This facilitates better electrolyte wetting of the positive electrode film layer, accelerates lithium-ion transport, and reduces battery internal resistance, thereby further improving the energy density, cycle performance, and rate performance of the secondary battery. A possible reason for this is that the organic solvent coating material used in this application has low boiling point and low solubility, making it easier to float during the slurry drying process. Therefore, the porosity of the surface region of the positive electrode film layer is significantly higher than that of the inner region.

[0115] In this application, the porosity of the positive electrode film has a meaning known in the art and can be tested using methods known in the art, such as GB / T 24586-2009.

[0116] In some embodiments, based on the total mass of the aqueous positive electrode, the mass content (i.e., residual amount) of the organic solvent coating is below 5000 ppm. Optionally, the mass content of the organic solvent coating is below 2000 ppm, below 1000 ppm, below 500 ppm, below 200 ppm, below 100 ppm, or even below 80 ppm. In this application, the mass content of the organic solvent coating can be determined using gas chromatography-mass spectrometry (GC-MS).

[0117] In some embodiments, the moisture content (i.e., residual amount) of the positive electrode sheet after vacuum drying at 80°C for 6 hours can be below 400 ppm. Optionally, the moisture content of the positive electrode sheet after vacuum drying at 80°C for 6 hours can be below 300 ppm, below 200 ppm, or even below 180 ppm.

[0118] In this application, the moisture content of the positive electrode sheet after vacuum drying at 80°C for 6 hours can be tested as follows: Aqueous positive electrode slurry is coated onto the positive electrode current collector, and then placed in a vacuum oven in a drying room (dew point < -30°C) for vacuum drying at 80°C for 6 hours to obtain the dried positive electrode film layer; 0.6g-1g of powder sample is weighed from the dried positive electrode film layer (e.g., by scraping powder with a blade) and placed in a moisture testing chamber to detect the moisture content. The moisture testing chamber is a Karl Fischer moisture analyzer, and the weighing instrument can be an electronic balance.

[0119] In some embodiments, the positive current collector may be a metal foil or a composite current collector. An example of a metal foil is aluminum foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. An example of a metal material may be one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. An example of a polymer substrate may be one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0120] It should be noted that, unless otherwise stated, all the above tests for the positive electrode sheet can be performed by sampling during the secondary battery preparation process or by sampling from the prepared secondary battery. Furthermore, the test methods provided in this application are for the positive electrode film layer on one side of the positive electrode current collector. When the positive electrode film layer is disposed on both sides of the positive electrode current collector, if the test results of the positive electrode film layer on either side meet the requirements of this application, it is considered to fall within the protection scope of this application.

[0121] Secondary batteries

[0122] A fifth aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte. During the charging and discharging process of the secondary battery, active ions are inserted and extracted back and forth between the positive electrode and the negative electrode, and the electrolyte plays a role in conducting active ions between the positive electrode and the negative electrode.

[0123] [Positive electrode plate]

[0124] The positive electrode used in the secondary battery of this application is the aqueous positive electrode of the fourth aspect of the embodiments of this application.

[0125] [Negative electrode plate]

[0126] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. For example, the negative current collector has two surfaces opposite each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0127] The negative electrode current collector can be a metal foil or a composite current collector. As an example of a metal foil, copper foil can be used. The composite current collector may include a polymer material substrate and a metal material layer formed on at least one surface of the polymer material substrate. As an example, the metal material may include one or more of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer material substrate may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0128] The negative electrode film typically comprises a negative electrode active material, optional binder, optional conductive agent, and other optional additives. The negative electrode film is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry coating is typically formed by dispersing the negative electrode active material, optional conductive agent, optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP) or water, but is not limited to these. As an example, the binder used for the negative electrode film may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, aqueous acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). As an example, the conductive agent used for the negative electrode film may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Other optional additives may include thickeners (e.g., sodium carboxymethyl cellulose, CMC), PTC thermistor materials, etc.

[0129] The negative electrode active material may be any negative electrode active material known in the art for use in secondary batteries. As examples, the negative electrode active material may include one or more of natural graphite, artificial graphite, soft carbon, hard carbon, mesophase microcarbon spheres, silicon-based materials, tin-based materials, lithium titanate, Li-Sn alloys, Li-Sn-O alloys, and Li-Al alloys. Silicon-based materials may include one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxides, and tin alloys. This application is not limited to these materials; other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0130] The negative electrode sheet of this application does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application further includes a conductive undercoating layer (e.g., composed of a conductive agent and an adhesive) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application further includes a protective layer covering the surface of the negative electrode film layer.

[0131] [Electrolytes]

[0132] This application does not impose specific limitations on the type of electrolyte, which can be selected according to requirements. For example, the electrolyte may include one or more of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions).

[0133] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent. The types of electrolyte salt and solvent are not specifically limited and can be selected according to actual needs.

[0134] As an example, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0135] As an example, the solvent may include one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl ester carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0136] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives capable of improving certain battery performance, such as additives improving battery overcharge performance, battery high-temperature performance, and battery low-temperature power performance. As an example, the additives may include, but are not limited to, one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), and 1,3-propanesulfonate lactone (PS).

[0137] [Isolation membrane]

[0138] Secondary batteries using electrolytes, as well as some secondary batteries using solid electrolytes, also include a separator. The separator is disposed between the positive and negative electrodes, primarily serving to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0139] In some embodiments, the material of the separator can be selected from one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different.

[0140] In some embodiments, the positive electrode sheet, the multilayer composite separator, and the negative electrode sheet can be fabricated into an electrode assembly using a winding process or a stacking process.

[0141] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0142] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0143] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0144] In some embodiments, such as Figure 2 As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0145] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. The number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0146] Figure 3 This is a schematic diagram of battery module 4 as an example. Figure 3 As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0147] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0148] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0149] Figure 4 and Figure 5 This is a schematic diagram of battery pack 1 as an example. Figure 4 and Figure 5As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0150] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is then placed in an outer packaging, dried, and injected with an electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0151] Electrical appliances

[0152] A sixth aspect of this application provides an electrical device. The electrical device includes one or more of the secondary battery, battery module, or battery pack described in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0153] The electrical device can be equipped with a secondary battery, battery module, or battery pack according to its usage requirements.

[0154] Figure 6 This is a schematic diagram of an example electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0155] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0156] Example

[0157] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0158] Example 1

[0159] Preparation of positive electrode sheet

[0160] 92 parts by mass of LiFePO4 (LFP, lithium-containing positive electrode active material) and 4 parts by mass of isohexane (organic solvent coating) were mixed evenly to form a lumpy substance. Then, 2 parts by mass of conductive carbon black (SuperP) and 2 parts by mass of water-based binder were mixed evenly. An appropriate amount of solvent water was added and stirred to disperse the mixture evenly, thus preparing a positive electrode slurry with a solid content of 50%. The positive electrode slurry was evenly coated onto the surface of the positive electrode current collector aluminum foil, and then placed in a vacuum oven in a drying room (dew point < -30℃) and vacuum dried at 80℃ for 6 hours. After cold pressing and slitting, the positive electrode sheet was obtained. The water-based binder was a 1:1 mass mixture of acrylonitrile-acrylic acid copolymer LA-133 and polyethyleneimine.

[0161] Preparation of negative electrode sheet

[0162] Artificial graphite (anode active material), carbon black (Super P) (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC) (thickener) are mixed thoroughly in an appropriate amount of deionized water at a mass ratio of 96.2:0.8:0.8:1.2 to form a negative electrode slurry. The negative electrode slurry is then uniformly coated onto the surface of copper foil (anode current collector), and after drying, cold pressing, and slitting, a negative electrode sheet is obtained.

[0163] Preparation of electrolyte

[0164] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3:7 to obtain an organic solvent; LiPF6 was uniformly dissolved in the above organic solvent to obtain an electrolyte, wherein the mass fraction of LiPF6 was 12.5%.

[0165] Preparation of the separating membrane

[0166] Polypropylene film is used as the separator.

[0167] Preparation of secondary batteries

[0168] The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is placed in an outer aluminum shell, baked at 80°C, and then the electrolyte is added. After vacuum sealing, standing, hot and cold pressing, formation, shaping, and capacity testing, a secondary battery is obtained.

[0169] Examples 2-17 and Comparative Examples 2-7

[0170] The preparation method of the secondary battery is similar to that in Example 1, except that the preparation parameters of the positive electrode are different, as detailed in Table 1.

[0171] Comparative Example 1

[0172] The preparation method of the secondary battery is similar to that in Example 1, except that the organic solvent coating material isohexane was not added during the preparation of the positive electrode.

[0173] Test section

[0174] (1) Moisture content test of positive electrode sheet

[0175] Weigh 0.6g-1g of powder sample from the positive electrode film layer after vacuum drying at 80℃ for 6 hours and before cold pressing. Then, place the powder sample in a moisture testing chamber to determine its moisture content. The moisture testing chamber is a Karl Fischer moisture analyzer, and the weighing instrument can be an electronic balance.

[0176] (2) Test of residual organic solvent coating on positive electrode sheet

[0177] The secondary battery is discharged (generally, it is fully discharged for safety reasons); after disassembling the battery, the positive electrode is removed and soaked in dimethyl carbonate (DMC) for 4 hours; then the positive electrode is removed and dried at 60℃ for 4 hours. After drying, the positive electrode is scraped to obtain powder, which is then sent to a gas chromatograph-mass spectrometer (GC-MS) for analysis. Specifically, the powder is placed in an Al2O3 crucible, leveled, and covered. The parameters are set as follows: nitrogen atmosphere, purge gas 60 mL / min, protective gas 20 mL / min. The temperature rise program is set to 10℃ / min, from 35℃ to 600℃.

[0178] (3) Elemental distribution test in positive electrode active material particles

[0179] Samples of approximately 6cm × 6cm were cut from freshly prepared positive electrode sheets and polished using an IB-19500CP ion section polisher to obtain polished samples with cut surfaces. Following JY / T 010-1996, SEM-EDS analysis was performed using a ZEISS Sigma 300 scanning electron microscope and a Horiba 7021-H X-ray energy dispersive spectrometer to observe the elemental distribution in the samples. A cross-section of the positive electrode active material particle was selected from the SEM-EDX image. The mass fraction W1 of transition metal elements in the region 100nm from the surface of this cross-section was measured, and then the mass fraction W2 of transition metal elements in the region 100nm from the particle center was measured. The parameter α = W1 / W2. The higher the α, the less transition metal elements are lost from the surface of the positive electrode active material particle. To ensure the accuracy of the test results, multiple points (e.g., 5 to 10) were randomly selected from the corresponding regions on the surface and at the center of the positive electrode active material particle for testing, and the average value was taken as the test result.

[0180] (4) Porosity test of positive electrode sheet

[0181] The positive electrode film layer was peeled off with adhesive tape. The porosity P1 in the region H / 3 from the surface of the positive electrode film layer and the porosity P2 in the region H / 3 from the positive electrode current collector in the positive electrode film layer were tested according to GB / T 24586-2009. The parameter β = P1 / P2.

[0182] (5) Initial specific capacity test of positive electrode active material

[0183] At 25℃, the secondary battery was charged to 3.65V at a constant current of 1 / 3C, and then charged to 0.05C at a constant voltage of 3.65V. After standing for 5 minutes, it was discharged to 2.5V at a constant current of 1 / 3C to obtain the initial capacity of the secondary battery.

[0184] The initial specific capacity (mAh / g) of the positive electrode active material = the initial capacity of the secondary battery / the mass of the positive electrode active material.

[0185] (6) Cycle performance test of secondary batteries

[0186] At 25℃, the secondary battery was charged at a constant current of 1 / 3C to 3.65V, and then charged at a constant voltage of 3.65V to a current of 0.05C. After standing for 5 minutes, it was discharged at a constant current of 1 / 3C to 2.5V. The resulting discharge capacity was recorded as the initial capacity of the secondary battery. The secondary battery was subjected to a cyclic charge-discharge test according to the above method, and the discharge capacity after each cycle was recorded.

[0187] The capacity retention rate of a secondary battery after 300 cycles = (discharge capacity after 300 cycles / initial capacity) × 100%.

[0188] (7) DC impedance test of secondary battery

[0189] At 25℃, the secondary battery is charged to 3.65V at a constant current of 1 / 3C, and then charged to 0.05C at a constant voltage of 3.65V. After standing for 5 minutes, the voltage V1 is recorded. The secondary battery is discharged at a constant current of 1 / 3C for 30 seconds, and the voltage V2 is recorded. The internal resistance after the first cycle is represented by (V2-V1) / (1 / 3C).

[0190] Repeat the above steps, and record the internal resistance of the secondary battery after each cycle.

[0191] The internal resistance growth rate of a secondary battery after 300 cycles = (internal resistance after the 300th cycle / internal resistance after the first cycle) × 100%.

[0192] Table 1 presents the test results for Examples 1-17 and Comparative Examples 1-7.

[0193] As can be seen from the test results in Table 1, adding T during the pre-mixing process of the slurry... m By using organic solvent coatings with a solubility of less than 10% at room temperature below 100°C, and coating at least a portion of the surface of lithium-containing cathode active material particles before mixing with other components, a secondary battery can simultaneously possess high energy density, good cycle performance, and low internal resistance. This is likely because the organic solvent coatings can prevent the formation of bound water by combining the surface of the lithium-containing cathode active material particles with the solvent water in the slurry, while reducing the dissolution of lithium and transition metal elements from the surface of the lithium-containing cathode active material particles. This results in less irreversible loss of lithium and transition metal elements in the cathode electrode sheet prepared from it. Furthermore, the cathode electrode sheet of this application has low moisture content and low residual organic solvent coating content, and the cathode film exhibits a significantly higher porosity in the surface region than in the inner region. This facilitates better electrolyte wetting of the cathode film layer, accelerates lithium-ion transport, and reduces battery internal resistance.

[0194] Figure 7 and Figure 8 The SEM-EDS test results of the positive electrode sheet prepared in Example 1 are shown. Region a represents the area approximately 100 nm from the surface of the positive electrode active material particles, and region b represents the area approximately 100 nm from the center of the particles. Figure 8 It can be seen that the distribution of Fe element in the positive electrode active material particles of the positive electrode sheet prepared in this application is relatively uniform. In particular, the Fe element content on the particle surface is still relatively high, and no large amount of Fe element dissolution occurs.

[0195] The test results from Examples 1-16 and Comparative Examples 2-4 show that when the boiling point T of the organic solvent-coated material added during the pre-mixing process of the slurry is... mAbove 100℃, although less lithium and transition metal elements dissolve from the surface of the lithium-containing cathode active material particles, the moisture content and residual organic solvent coating of the cathode sheet are both high. Furthermore, the porosity of the surface region of the cathode film is close to that of the inner region, thus failing to effectively promote electrolyte wetting, accelerate lithium-ion transport, or reduce battery internal resistance. Therefore, at this temperature, the secondary battery cannot simultaneously possess high energy density, good cycle performance, and low internal resistance.

[0196] The test results from Examples 1-16 and Comparative Examples 5-7 show that when the room temperature solubility of the organic solvent coating added in the pre-stirring process of the slurry is higher than 10%, the organic solvent coating dissolves in a large amount of solvent water and loses its protective effect on the surface of the lithium-containing cathode active material particles. As a result, more lithium and transition metal elements are dissolved from the surface of the lithium-containing cathode active material particles. Therefore, the secondary battery cannot simultaneously have high energy density, good cycle performance and low internal resistance.

[0197] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

[0198]

Claims

1. A positive electrode composition comprising a lithium-containing positive electrode active material and an organic solvent coating material covering at least a portion of the surface of the lithium-containing positive electrode active material. The lithium-containing positive electrode active material includes lithium and transition metal elements, wherein the transition metal elements include one or more selected from Fe, Ni, Co, Mn, Al, Cu, Zn, and Ti. The boiling point T of the organic solvent coating m The solubility of the organic solvent-coated material in water is less than 10% at temperatures below 100°C and at room temperature. The organic solvent coating includes one or more of the following: benzene, hexane, isohexane, n-heptane, isooctane, 2,2-dimethylpentane, 3-methylpentane, dichloromethane, trichloromethane, carbon tetrachloride, 1,2-dichloroethane, carbon disulfide, methyl acetate, and ethyl acetate. Based on the total mass of the cathode composition, the organic solvent coating content is 4%-10% by mass.

2. The positive electrode composition according to claim 1, wherein, The lithium-containing positive electrode active material includes LiFe m Mn 1-m PO4, Li(Ni) x Co y Mn z Al a Cu b Zn c Ti d One or more of O2, 0≤m≤1, x+y+z+a+b+c+d=1, 0.5≤x<1, 0.05≤y<1, 0≤z<0.5, 0≤a≤0.1, 0≤b≤0.1, 0≤c≤0.1, 0≤d≤0.

1.

3. The positive electrode composition according to claim 1, wherein, The boiling point T of the organic solvent coating m Between 30°C and 100°C; and / or, The solubility of the organic solvent-coated material in water at room temperature is less than 1%.

4. The positive electrode composition according to claim 3, wherein, The boiling point T of the organic solvent coating m Between 40°C and 60°C; and / or, The solubility of the organic solvent-coated material in water at room temperature is 0.01%-1%.

5. The positive electrode composition according to claim 1, wherein, The organic solvent coating has a mass percentage of 4%-7%.

6. The positive electrode composition according to claim 1, wherein, The positive electrode composition further includes one or more of a dispersant, an aqueous binder, and a conductive agent.

7. The positive electrode composition according to claim 6, wherein, Based on the total mass of the positive electrode composition The lithium-containing positive electrode active material has a mass percentage content of 88%-99%; and / or, The dispersant has a mass percentage content of less than 0.7%; and / or, The water-based adhesive has a mass percentage content of less than 5%; and / or, The conductive agent has a mass percentage content of less than 5%.

8. The positive electrode composition according to claim 7, wherein, Based on the total mass of the positive electrode composition The lithium-containing positive electrode active material has a mass percentage content of 90%-94%; and / or, The dispersant has a mass percentage content of 0.1%-0.5%; and / or, The water-based adhesive has a mass percentage content of 2%-4%; and / or, The conductive agent has a mass percentage content of 0.5%-3%.

9. The positive electrode composition according to any one of claims 6-8, wherein, The dispersant includes one or more of polyethyleneimine, sodium dodecyl sulfonate, polyvinyl alcohol, and polyethylene glycol octylphenyl ether.

10. The positive electrode composition according to any one of claims 6-8, wherein, The aqueous adhesive includes methylcellulose and its salts, xanthan gum and its salts, chitosan and its salts, alginate and its salts, polyethyleneimine and its salts, polyacrylamide, acrylonitrile-acrylic acid copolymer and its derivatives, or mixtures thereof.

11. The positive electrode composition according to claim 10, wherein, The water-based adhesive comprises a compound mixture of xanthan gum and polyethyleneimine.

12. The positive electrode composition according to claim 11, wherein, The mass ratio of xanthan gum to polyethyleneimine is 2:1 to 0.2:2.

8.

13. The positive electrode composition according to claim 11, wherein, The number average molecular weight of the xanthan gum is 300,000-2,000,000, and the number average molecular weight of the polyethyleneimine is 2,000-50,000.

14. The positive electrode composition according to claim 10, wherein, The water-based adhesive comprises a blend of acrylonitrile-(meth)acrylic acid copolymer and polyethyleneimine.

15. The positive electrode composition according to claim 14, wherein, The mass ratio of the acrylonitrile-(meth)acrylic acid copolymer to the polyethyleneimine is 2:1-0.2:2.

8.

16. The positive electrode composition according to claim 14, wherein, The number-average molecular weight of the acrylonitrile-(meth)acrylic acid copolymer is 300,000-2,000,000, and the number-average molecular weight of the polyethyleneimine is 2,000-70,000.

17. An aqueous positive electrode slurry, comprising water as a solvent and a positive electrode composition according to any one of claims 1-16.

18. The aqueous positive electrode slurry according to claim 17, wherein, The solid content of the aqueous positive electrode slurry is 40%-90%; and / or, The viscosity of the aqueous positive electrode slurry is 100cp-10000cp.

19. The aqueous positive electrode slurry according to claim 18, wherein, The solid content of the aqueous positive electrode slurry is 50%-70%; and / or, The viscosity of the aqueous cathode slurry is 3000cp-7000cp.

20. A method for preparing an aqueous cathode slurry, used to prepare the aqueous cathode slurry according to any one of claims 17-19, comprising the steps of: S100, lithium-containing positive electrode active material and organic solvent coating are mixed evenly to form a clump; S200: The obtained lumps, conductive agent, water-based binder and solvent water are mixed evenly to obtain an aqueous positive electrode slurry.

21. An aqueous positive electrode sheet, comprising a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, wherein, The positive electrode film layer is a layer formed after drying the aqueous positive electrode slurry according to any one of claims 17-19, or a layer formed after drying the aqueous positive electrode slurry obtained by the method of claim 20.

22. The aqueous positive electrode sheet according to claim 21, wherein, The ratio of the mass fraction W1 of the transition metal element in the region 100 nm from the surface of the lithium-containing positive electrode active material particle to the mass fraction W2 of the transition metal element in the region 100 nm from the center of the lithium-containing positive electrode active material particle is α, where α is 60%-100%.

23. The aqueous positive electrode sheet according to claim 22, wherein, α is 70%-100%.

24. The aqueous positive electrode sheet according to any one of claims 21-23, wherein, The thickness of the positive electrode film is H, and the ratio of the porosity P1 in the region H / 3 away from the surface of the positive electrode film to the porosity P2 in the region H / 3 away from the positive electrode current collector in the positive electrode film is β, where β is 1.15-2.

0.

25. The aqueous positive electrode sheet according to claim 24, wherein, β is 1.40-1.

60.

26. The aqueous positive electrode sheet according to claim 21, wherein, Based on the total mass of the aqueous positive electrode sheet, the mass content of the organic solvent coating is below 5000 ppm; and / or, The water-based positive electrode sheet, after being vacuum dried at 80°C for 6 hours, has a moisture content of less than 400 ppm.

27. The aqueous positive electrode sheet according to claim 26, wherein, Based on the total mass of the aqueous positive electrode sheet, the mass content of the organic solvent coating is below 200 ppm; and / or, The water-based positive electrode sheet, after being vacuum dried at 80°C for 6 hours, has a moisture content of less than 200 ppm.

28. A secondary battery comprising an aqueous positive electrode sheet according to any one of claims 21-27.

29. An electrical device comprising a secondary battery according to claim 28.

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