Lithium ion battery, positive electrode slurry, dispersing agent and preparation method of dispersing agent

Abstract

The invention provides a lithium ion battery, positive electrode slurry, a dispersing agent and a preparation method of the dispersing agent, the dispersing agent comprises a multi-chain polymer composed of a polyether main chain, an aliphatic chain grafted to the polyether main chain and a polyester chain, the mass ratio of the polyether main chain to the aliphatic chain to the polyester chain in the multi-chain polymer is (40-50): (25-30): (15-20), and the mass ratio of the polyether main chain to the aliphatic chain to the polyester chain in the multi-chain polymer is (40-50): (25-30): (15-20). The molecular weight of the polyether main chain is 2500-5000, the number of carbon atoms of the aliphatic chain is 8-22, carboxyl groups are further grafted on the polyester chain, and the molecular weight of the multi-chain polymer is 30000 + / -10000. The microscopic multilevel structure of the dispersing agent is optimized, the cracking risk and interface resistance of the positive electrode active material layer are reduced, and the cycle performance of the battery is improved.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and particularly relates to a lithium-ion battery, a positive electrode paste, a dispersant and a preparation method of the dispersant. Background Art

[0002] Due to its advantages of high safety, long cycle life and low cost, the cathode material of the lithium iron phosphate system has become the mainstream cathode material in the fields of power batteries and energy storage. The cathode material of the lithium iron phosphate system is usually nano-scale particles with a high specific surface area. The physical effects such as van der Waals force and electrostatic attraction between the particles are significant, and it is extremely easy to spontaneously agglomerate in the positive electrode paste system, resulting in very poor dispersion uniformity. In addition, the dispersion uniformity of the positive electrode paste will directly affect the flexibility of the electrode sheet. Especially for cylindrical batteries with a winding structure, higher requirements are imposed on the flexibility of the positive electrode material layer after the positive electrode paste is cured.

[0003] In the prior art, for example, in Patent CN202411196004.1, a linear polyether chain is used, which can reduce the viscosity of the positive electrode paste to a certain extent. However, due to its linear molecular structure, the steric hindrance effect that can be created is insufficient, and it is difficult to improve the phenomenon of particle agglomeration in the positive electrode paste. Patent CN202311037221.1 uses a polyacrylic acid-based dispersant to improve the rheology of the positive electrode paste, and to a certain extent, it improves the uniformity of the lithium iron phosphate system cathode material particles in the positive electrode paste. However, during the subsequent winding process, there are still problems of winding cracking caused by insufficient flexibility of the cured positive electrode paste. Moreover, the applicant has proved through experiments that the probability of cracks appearing in the positive electrode sheet prepared by the polyacrylic acid-based dispersant solution exceeds 30% after being bent 180°.

[0004] On the other hand, in order to make up for the disadvantage of the low energy density of the cathode material of the lithium iron phosphate system, the compaction density of the positive electrode material layer is also increased to improve the energy density of the cathode material of the lithium iron phosphate system. However, when the dispersion uniformity of the positive electrode paste is poor, it is also difficult to increase the compaction density of the positive electrode material layer, which further affects the overall performance of the battery. Summary of the Invention

[0005] To solve the above problems and enhance the uniformity of the positive electrode paste and the flexibility of the positive electrode material layer after its curing, a first aspect of the present application provides a dispersant, which includes a multi-chain polymer composed of a polyether main chain, an aliphatic chain grafted to the polyether main chain and a polyester chain. The mass ratio of the polyether main chain, the aliphatic chain and the polyester chain in the multi-chain polymer is: (40 - 50):(25 - 30):(15 - 20). The molecular weight of the polyether main chain is: 2500 - 5000. The number of carbon atoms of the aliphatic chain is 8 - 22. A carboxyl group is also grafted on the polyester chain. The molecular weight of the multi-chain polymer is: 30000 ± 10000.

[0006] In some alternative embodiments, the grafting rate of the polyether main chain is 40-45%.

[0007] In some alternative embodiments, an anchoring group is further connected to the fatty chain. The anchoring group includes one or more of a polar group, a benzene ring-containing group, and a conjugated π-bond-containing group. The substitution degree of the anchoring group on the fatty chain is 0.1-0.4.

[0008] In some alternative embodiments, a glycidyl methacrylate (GMA) chain with a degree of polymerization of 10-50 is also grafted onto the polyether main chain. The fatty chain is indirectly grafted onto the polyether main chain by being incorporated into the end of the GMA chain through a chain transfer reaction.

[0009] In some alternative embodiments, the degree of polymerization of the polyester chain is 12-20, and the proportion of the polyester chain grafted onto the polyether main chain is 0.1-0.5; the carboxyl group is connected to the end of the polyester chain through a chain transfer, and the substitution degree of the carboxyl group on the polyester chain is 0.4-0.9.

[0010] The second aspect of the present application provides a method for preparing a dispersant. The method is used to prepare the dispersant according to any one of the above, and includes the following steps:

[0011] S1: Using dimethyl carbonate (DMC) as a catalyst, glycerol as an initiator, and ethylene oxide and / or propylene oxide as monomers, set the reaction temperature to 125±2°C to prepare a polyether main chain;

[0012] S2: Using ammonium persulfate as an initiator, GMA as a monomer, and fatty chain mercaptan as a chain terminator, GMA polymerizes to form a GMA chain with a degree of polymerization of 10-50 and a free radical at the end. The fatty chain mercaptan undergoes a chain transfer reaction with the free radical at the end of the GMA chain to terminate the growth of the GMA chain and graft the fatty chain onto the end of the GMA chain;

[0013] S3: Using adipic acid and 1,4-butanediol as reactants, β-cyclodextrin as a catalyst, set the reaction temperature to 200°C, and melt-polycondense to synthesize a polyester chain. Dissolve the polyester chain in an aqueous solution of an organic acid, add the initiator ammonium persulfate, and react at 70-80°C for 3-5 hours. The carboxyl group on the organic acid is grafted onto the polyester chain through a free radical chain transfer;

[0014] S4: Add the polyether main chain, the GMA chain with the fatty chain connected at the end, and the polyester chain to NMP in a mass ratio of (40-50):(25-30):(15-20), use ammonium persulfate as an initiator, react at 45-75°C for 1-3 hours, and filter through a 0.2μm PTFE membrane to obtain the multi-chain polymer; and

[0015] S5: Add NMP to the multi-chain polymer and adjust the solid content to 3 wt% to obtain the dispersant.

[0016] In some optional preparation methods, in S1, when ethylene oxide and propylene oxide are used as monomers simultaneously, the feeding ratio of ethylene oxide to propylene oxide is: molar ratio 2:1 - 5:1.

[0017] In some optional preparation methods, in S2, the feeding ratio of GMA, the aliphatic chain mercaptan to ammonium persulfate is: molar ratio 100:(2 - 10):(0.1 - 0.5), the initiation temperature is 65 °C, the reaction temperature is 75 °C, and the reaction time is 2 h.

[0018] In some optional preparation methods, in S3, the feeding ratio of adipic acid to 1,4 - butanediol is: molar ratio 1:1.05; the addition amount of β - cyclodextrin is 0.2 - 0.5 wt% of the weight of adipic acid; the feeding ratio of the organic acid to the polyester chain is: molar ratio 1:(2 - 5); the addition amount of the initiator ammonium persulfate is 0.1 - 0.5 wt% of the weight of the organic acid.

[0019] The third aspect of the present application provides a positive electrode slurry, comprising a positive electrode active material, a positive electrode conductive agent, a dispersant, and a positive electrode binder, characterized in that the dispersant is the dispersant according to any one of the above, and the mass addition ratio is: positive electrode active material: positive electrode conductive agent: dispersant: positive electrode binder = (94 - 98):(1 - 3):(0.2 - 1):(0.8 - 2), and the addition ratio of the dispersant is measured by the multi-chain polymer.

[0020] In some optional embodiments, the 24 - h static viscosity rebound rate of the positive electrode slurry ≤ 30%.

[0021] In some optional embodiments, the particle size of the positive electrode active material satisfies: 1.1 ≤ (Dv90 - Dv10) / Dv50 ≤ 1.8.

[0022] The fourth aspect of the present application provides a lithium - ion battery, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer is formed by curing the positive electrode slurry coated on the surface of the positive electrode current collector, characterized in that the positive electrode slurry is the positive electrode slurry according to the above.

[0023] In some optional embodiments, the apparent compaction density c of the positive electrode sheet satisfies: 2.4 g / cm 3 ≤ c ≤ 2.8 g / cm3 , the areal density of the positive electrode sheet is: 10-20 mg / cm 2 , the contact resistance between the positive electrode active material layer and the positive electrode current collector ≤ 0.3 Ω·cm 2 .

[0024] This application has at least the following technical effects:

[0025] 1) In the first aspect of this application, a dispersant is provided, which includes a multi-chain polymer with a multi-level microstructure composed of a polyether main chain, an aliphatic chain grafted on the polyether main chain, and a polyester chain. On the one hand, the polyether main chain has a strong affinity for the solvent and is easy to dissolve and diffuse in the positive electrode slurry system. On the other hand, its large molecular weight and the grafting of the aliphatic chain and polyester chain endow it with a large steric hindrance, effectively preventing the conductive agent particles in the positive electrode slurry from agglomerating with the positive electrode active material, thereby improving the dispersion uniformity of the positive electrode slurry. At the same time, the aliphatic chain with 8-22 carbon atoms has a toughening effect. When subjected to external force, the aliphatic chain absorbs energy through the slip and orientation of molecular chains, inhibiting cracking. Further, the rigidity of the polyester chain can complement the flexibility of the polyether main chain, forming a "rigid-flexible combination" molecular configuration, which not only provides mechanical stability for the multi-chain polymer but also maintains its dynamic dispersion ability. The grafted carboxyl group on it can, on the one hand, reduce the activation energy during the solvation and desolvation reactions of lithium ions, improving the battery kinetics. On the other hand, it also forms a dipole interaction or hydrogen bond with the outer-coated carbon of the positive electrode active material, enhancing the anchoring ability for the positive electrode active material particles and preventing particle sedimentation. At the same time, controlling the mass ratio of the polyether main chain, aliphatic chain, and polyester chain in the multi-chain polymer to be: (40-50):(25-30):(15-20), balancing the mechanical stability and flexibility of the molecular configuration of the multi-chain polymer, the dispersion uniformity and stability of each material in the positive electrode slurry, weaving to form a stable three-dimensional cross-linked dispersion network, adapting to positive electrode active materials such as the lithium iron phosphate system, optimizing the performance of the electrode sheet, and improving the rate performance and energy density of the battery.

[0026] 2) In the second aspect of this application, a preparation method of a dispersant is provided. By stepwise preparing the polyether main chain, GMA chain, grafting of the aliphatic chain on the GMA chain, polyester chain, and substitution of the carboxyl group on the polyester chain, it is convenient to control the degree of polymerization of each polymer chain, as well as the grafting rate of the branched chain and the degree of group substitution, balancing the mechanical properties, flexible properties, and anchoring properties of the dispersant, improving the viscosity stability of the positive electrode slurry, the particle dispersion uniformity, and the flexibility of the cured positive electrode active material layer, and it does not crack after 180° bending, adapting to winding batteries such as cylindrical batteries.

[0027] 3) The third aspect of the present application provides a positive electrode paste. By adjusting the ratio of the positive electrode active material, the positive electrode conductive agent, the dispersant described above, and the positive electrode binder, a positive electrode paste suitable for a lithium iron battery system is prepared. It has good coating uniformity. After drying and rolling, a positive electrode sheet is obtained, which has a high surface flatness, is not easy to crack, and has a uniform distribution of positive electrode particles, ensuring the transmission of lithium ions and being beneficial to improving the rate performance.

[0028] 4) The fourth aspect of the present application provides a lithium ion battery, which includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer is formed after the above-mentioned positive electrode paste is coated on the surface of the positive electrode current collector and cured. The areal density of the obtained positive electrode sheet is: 10 - 20 mg / cm 2 , and the apparent compaction density c satisfies: 2.4 g / cm 3 ≤ c ≤ 2.8 g / cm 3 , improving the volume energy density of the positive electrode active material layer and being beneficial to improving the battery capacity. Detailed Embodiments

[0029] The following details the embodiments of this embodiment. Elements with the same or similar functions are described below. The following embodiments are exemplary and are only used to explain this embodiment, and should not be construed as a limitation of this embodiment.

[0030] In the description of this embodiment, it should be understood that for the orientation description, such as up, down, front, back, left, right, etc., the indicated orientation or positional relationship is based on the shown orientation or positional relationship. It is only for the convenience of describing this embodiment and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this embodiment.

[0031] In the description of this embodiment, the meaning of several is one or more, the meaning of multiple is more than two, greater than, less than, exceeding, etc. are understood as not including the present number, and above, below, within, etc. are understood as including the present number. If there is a description of first and second, it is only for the purpose of distinguishing technical features and should not be construed as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features or implicitly indicating the sequence relationship of the indicated technical features.

[0032] In the description of this embodiment, unless otherwise clearly defined, words such as setting, installing, connecting, etc. should be understood in a broad sense. Those skilled in the art can reasonably determine the specific meanings of the above words in this embodiment in combination with the specific content of the technical solution.

[0033] The common structure of existing lithium-ion batteries includes a housing with one end open, a wound core assembled into the housing from the opening, an electrolyte injected into the housing, and a cap covering the opening of the housing. It can be understood that the lithium-ion battery can be a cylindrical lithium-ion battery or a square lithium-ion battery.

[0034] The lithium-ion battery includes a wound core and an electrolyte. The wound core is formed by winding a positive electrode sheet, a separator, and a negative electrode sheet stacked together. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer is formed by curing a positive electrode paste coated on the surface of the positive electrode current collector. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector. The negative electrode active material layer is formed by curing a negative electrode paste coated on the surface of the negative electrode current collector. The negative electrode paste includes a negative electrode active substance, a negative electrode conductive agent, and a negative electrode binder. The negative electrode active substance is selected from at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon carbon, silicon oxide, and pre-lithiated silicon oxide, and the silicon content in the negative electrode active substance is 1.0-25.0 wt%. The negative electrode conductive agent is selected from carbon nanotubes or a composition of carbon nanotubes and carbon black. The negative electrode binder is selected from at least one of polyacrylic acid, polyacrylonitrile, and styrene-acrylic acid. The apparent compaction density d of the negative electrode sheet satisfies 1.2 g / cm 3 ≤d≤1.8 g / cm 3 。 The electrolyte includes: a lithium salt, a solvent, and an additive; the lithium salt is selected from any one or a combination of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorooxalate borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; the solvent is selected from any one or a combination of dimethyl carbonate, diethyl carbonate, ethylene carbonate, and ethyl methyl carbonate; the additive is selected from any one or a combination of propylene carbonate, butylene carbonate, ethyl acetate, ethyl methyl carbonate, and fluoroethylene carbonate.

[0035] In the first aspect of the present application, a dispersant is provided, which includes a multi-chain polymer composed of a polyether main chain, an aliphatic chain grafted to the polyether main chain, and a polyester chain. The mass ratio of the polyether main chain, the aliphatic chain, and the polyester chain in the multi-chain polymer is: (40-50):(25-30):(15-20). The molecular weight of the polyether main chain is: 2500-5000. The number of carbon atoms in the aliphatic chain is 8-22. A carboxyl group is also grafted on the polyester chain. The molecular weight of the multi-chain polymer is: 30000±10000.

[0036] On the one hand, the polyether main chain has a strong affinity for solvents, is easy to dissolve and disperse in the cathode slurry system, and has strong wetting performance. On the other hand, its relatively large molecular weight and the grafting of the aliphatic chain and polyester chain endow it with a large steric hindrance, effectively preventing the agglomeration of cathode particles and conductive agent particles in the cathode slurry, thereby improving the dispersion uniformity of the cathode slurry. At the same time, the aliphatic chain with 8-22 carbon atoms has a toughening effect. When subjected to external forces, the aliphatic chain absorbs energy through the slippage and orientation of molecular chains, inhibiting cracking. Further, the rigidity of the polyester chain can complement the flexibility of the polyether main chain, forming a "rigid-flexible combination" molecular configuration, which not only provides mechanical stability for the multi-chain polymer but also maintains its dynamic dispersion ability. The grafted carboxyl group can, on the one hand, reduce the activation energy during the solvation and desolvation reactions of lithium ions, improving the battery kinetics, and on the other hand, form hydrogen bonds with the cathode active material, enhancing the anchoring ability of the multi-chain polymer to the cathode active material particles and preventing particle sedimentation. At the same time, controlling the mass ratio of the polyether main chain, aliphatic chain, and polyester chain in the multi-chain polymer to be (40-50):(25-30):(15-20) balances the mechanical stability and flexibility of the molecular configuration of the multi-chain polymer, the dispersion uniformity and stability of each material in the cathode slurry, weaves and constructs a stable dispersion network, adapts to cathode active materials such as the lithium iron phosphate system, and improves the energy density of the cathode active material.

[0037] Further, an anchoring group is also connected to the aliphatic chain. The anchoring group includes one or more of a polar group, a benzene ring-containing group, and a conjugated π-bond-containing group. The substitution degree of the anchoring group on the aliphatic chain is 0.1-0.4. The anchoring group can be covalently connected to the aliphatic chain through a substitution reaction or indirectly connected to the aliphatic chain through a branched chain introduced. The anchoring group can be a polar group, such as a hydroxyl group, a carboxyl group, an amino group, etc. The polar group forms a hydrogen bond with the cathode active material as an anchoring mechanism to reduce particle agglomeration. The anchoring group can also be a benzene ring-containing group and / or a conjugated π-bond-containing group. The delocalized π-ring and conjugated π-bond in the benzene ring can form a π-π stacking interaction with the sp 2 hybrid carbon coating layer, having both van der Waals forces and local charge interactions. The multi-chain polymer is adsorbed on the surface of the cathode active material particles through the anchoring group and exerts a steric hindrance effect, spatially isolating the cathode particles, thereby avoiding particle agglomeration. In addition, the conjugated π-bond of the anchoring group and the sp 2 carbon layer on the surface of the cathode active material particles can form a continuous electron transport channel, reducing the interfacial resistance of the cathode active material layer, significantly improving the overall conductivity of the cathode sheet, reducing the battery internal resistance, and increasing the capacity discharge rate of the battery. Specifically, the contact resistance between the cathode active material layer and the cathode current collector is ≤0.3 Ω·cm 2 .

[0038] Further, the grafting rate of the polyether main chain is 40-45%. By controlling the grafting rate of the polyether main chain, the molecular configuration of the multi-chain polymer is defined, thereby ensuring the steric hindrance effect of the multi-chain polymer. It should be noted that in the specific implementation process, the grafting density of the aliphatic chain and the grafting density of the polyester chain can be obtained by separately monitoring the intensities of the C-H stretching vibration peak (2850 cm-1) and the -COOH peak (1700 cm-1) through FTIR, and then the grafting rate of the polyether main chain can be obtained.

[0039] In some alternative embodiments, a glycidyl methacrylate GMA chain with a degree of polymerization of 10-50 is also grafted on the polyether main chain, and the aliphatic chain is indirectly grafted on the polyether main chain by accessing to the end of the GMA chain through a chain transfer reaction. Further, the degree of polymerization of the GMA chain is 20. The chain transfer reaction between the GMA chain and the aliphatic chain can form a comb-like branched chain structure, further increasing the steric hindrance effect of the multi-chain polymer. And the aliphatic chain, as a flexible side chain, on the one hand dilutes the entanglement density of the main chain, reduces the friction between molecular chains, thereby improving the fluidity. On the other hand, the outward expansion of the aliphatic chain can play its toughness characteristics such as intramolecular slip and orientation structure conversion in the positive electrode slurry system to resist external forces, improve the anti-fracture performance of the positive electrode active material layer, and reduce the cracking risk during winding.

[0040] In some alternative embodiments, the degree of polymerization of the polyester chain is 12-20, and the proportion of the polyester chain grafted on the polyether main chain is: 0.1-0.5; the carboxyl group is connected to the end of the polyester chain through a chain transfer, and the substitution degree of the carboxyl group on the polyester chain is 0.4-0.9. The rigid characteristics of the polyester chain provide mechanical stability. For the grafted carboxyl group, on the one hand, as a polar group, the carboxyl group can form dynamic interactions with water molecules or organic solvents in the electrolyte through hydrogen bonds, change the solvation layer structure of lithium ions, and reduce the activation energy during the solvation and desolvation reactions of lithium ions, thereby enhancing the battery kinetics. Specifically, the carboxyl group weakens the strong hydrogen bond network in the electrolyte system, reduces the energy barrier of ion migration, and thus accelerates the desolvation process of lithium ions. On the other hand, the carboxyl group can also form hydrogen bonds with the positive electrode active material, enhancing the anchoring ability of the multi-chain polymer to the positive electrode active material particles and preventing particle sedimentation.

[0041] The second aspect of the present application provides a preparation method of a dispersant, which is used to prepare the dispersant according to any one of the above, and includes the following steps:

[0042] S1: Using dimethyl carbonate DMC as a catalyst, glycerol as an initiator, and ethylene oxide and / or propylene oxide as monomers, setting the reaction temperature to 125±2°C to prepare a polyether main chain;

[0043] S2: Using ammonium persulfate as the initiator, glycidyl methacrylate (GMA) as the monomer, and fatty chain mercaptan as the chain terminator, GMA polymerizes to form a GMA chain with a degree of polymerization of 10 - 50 and a free radical at the end. The fatty chain mercaptan undergoes a chain transfer reaction with the free radical at the end of the GMA chain through -SH, terminating the growth of the GMA chain and grafting the fatty chain onto the end of the GMA chain;

[0044] S3: Using adipic acid and 1,4 - butanediol as reactants, and β - cyclodextrin as the catalyst, set the reaction temperature at 200°C, and conduct melt polycondensation to synthesize a polyester chain. Dissolve the polyester chain in an aqueous solution of organic acid, add the initiator ammonium persulfate, and react at 70 - 80°C for 3 - 5 h. The carboxyl group on the organic acid is grafted onto the polyester chain through free radical chain transfer;

[0045] S4: Add the polyether main chain, the GMA chain with the fatty chain connected at the end, and the polyester chain to NMP at a mass ratio of (40 - 50):(25 - 30):(15 - 20). Using ammonium persulfate as the initiator, react at 45 - 75°C for 1 - 3 hours, and filter through a 0.2 μm PTFE membrane to obtain the multi - chain polymer; and

[0046] S5: Add NMP to the multi - chain polymer and adjust the solid content to 3 wt% to obtain the dispersant.

[0047] By step - by - step preparation of the polyether main chain, GMA chain, grafting of the fatty chain on the GMA chain, polyester chain, and substitution of the carboxyl group on the polyester chain, it is convenient to control the degree of polymerization of each polymer chain, as well as the grafting rate of the branch chain and the degree of group substitution, balance the mechanical properties, flexible properties, and anchoring properties of the dispersant, improve the viscosity stability of the positive electrode slurry, the uniformity of particle dispersion, and the flexibility of the cured positive electrode active material layer, and it does not crack after 180° bending, being suitable for wound batteries such as cylindrical batteries.

[0048] Further, in S1, when ethylene oxide and propylene oxide are used as monomers simultaneously, the feeding ratio of ethylene oxide to propylene oxide is: molar ratio 2:1 - 5:1. By regulating the feeding ratio of ethylene oxide to propylene oxide, the hydrophilicity of the polyether main chain is controlled to adapt to the positive electrode slurry system and is more easily dissolved to exert the dispersion function.

[0049] Further, in S2, the feeding ratio of GMA, the fatty chain mercaptan, and ammonium persulfate is: molar ratio 100:(2 - 10):(0.1 - 0.5), the initiation temperature is 65°C, the reaction temperature is 75°C, and the reaction time is 2 h. In the specific implementation process, the fatty chain mercaptan is straight - chain hexadecyl mercaptan.

[0050] Further, in the step S3, the feeding ratio of adipic acid to 1,4-butanediol is: molar ratio 1:1.05; the addition amount of β-cyclodextrin is 0.2-0.5 wt% of the weight of adipic acid; the feeding ratio of the organic acid to the polyester chain is: molar ratio 1:(2-5); the addition amount of the initiator ammonium persulfate is 0.1-0.5 wt% of the weight of the organic acid. The cavity structure of β-cyclodextrin is used to encapsulate adipic acid and 1,4-butanediol molecules, promote the orientation arrangement to facilitate the esterification reaction, and also act as a catalyst to promote the polycondensation reaction and reduce side reactions. Ammonium persulfate attacks the organic acid to form organic acid monomer free radicals, and the organic acid monomer free radicals are transferred to the end of the polyester chain through a chain transfer reaction. In the specific implementation process, the organic acid is acrylic acid, and the acrylic acid monomer free radicals can further polymerize to form acrylic acid chain free radicals, and further undergo chain transfer and graft onto the polyester chain.

[0051] A positive electrode slurry includes a positive electrode active material, a positive electrode conductive agent, a dispersant, and a positive electrode binder. The dispersant is the dispersant according to any one of the above, and the mass addition ratio is: positive electrode active material: positive electrode conductive agent: dispersant: positive electrode binder = (94-97):(0.5-1.5):(0.2-0.5):(0.3-1), and the addition ratio of the dispersant is measured based on the multi-chain polymer. Specifically, the positive electrode active material is lithium iron phosphate, the positive electrode conductive agent is selected from conductive carbon black and / or carbon nanotubes, and the positive electrode binder is selected from polyvinylidene fluoride.

[0052] Further, the viscosity rebound rate of the positive electrode slurry after standing for 24 h is ≤30%. By controlling the viscosity rebound rate of the positive electrode slurry after standing for 24 hours, the stability of the dispersion network of the positive electrode slurry is restricted, and the electrode sheet defects can be effectively reduced, and the energy density and cycle life of the battery can be improved.

[0053] It should be noted that according to GB / T 22235, the characterization method of the viscosity of the positive electrode slurry is: using a rotary viscometer (model: DV2T), rotor CPE-40, rotation speed 50 rpm, viscosity 5000±500 mPa·s at 25°C, measuring the viscosity η0 of the positive electrode slurry at 0 h and the viscosity η of the positive electrode slurry after standing for 24 hours respectively, and calculating the viscosity rebound rate r through the formula: r = (η-η0) / η0.

[0054] Further, the particle size of the positive electrode active material satisfies: 1.1≤(Dv90-Dv10) / Dv50≤1.8. Controlling the particle size span of the positive electrode active material particles between 1.1 and 1.8 indicates that the particle distribution is neither too concentrated to avoid the agglomeration of small particles, nor too dispersed to prevent pores or component stratification during the coating of the positive electrode slurry. The appropriate distribution of the particle size is beneficial to balancing the packing density of the positive electrode active material layer and the ion transport efficiency.

[0055] A third aspect of the present application provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer is formed by curing a positive electrode slurry coated on the surface of the positive electrode current collector, and the positive electrode slurry is the positive electrode slurry described in any one of the above. Through the above positive electrode slurry system, the winding performance and capacity density of the positive electrode sheet are improved, and the upper limit of the capacity of the positive electrode material in the lithium iron system is further increased.

[0056] Further, the apparent compaction density c of the positive electrode sheet satisfies: 2.4 g / cm 3 ≤c≤2.8 g / cm 3 , the areal density of the positive electrode sheet is: 10 - 20 mg / cm 2 , and the contact resistance between the positive electrode active material layer and the positive electrode current collector is ≤0.3 Ω·cm 2 . By regulating the parameters of the apparent compaction density and areal density of the positive electrode sheet, combined with the optimization of the flexibility of the electrode sheet, it can be well matched with the cylindrical battery core with a diameter of 18 - 46 mm. At the same time, the electron transport channel formed between the conjugated π bond of the anchoring group and the sp 2 hybrid carbon layer coated on the surface of the positive electrode active material particles greatly reduces the interfacial resistance and effectively improves the cycle performance and stability of the battery. In the specific implementation process, correspondingly, the apparent compaction density of the negative electrode sheet is: 1.2 g / cm 3 ≤d≤1.8 g / cm 3 , and the capacity N / P ratio of the negative electrode sheet to the positive electrode sheet is 1:(1.02 - 1.1).

[0057] The following describes the technical solutions of the present application in combination with Examples 1 - 14 and Comparative Examples 1 - 11.

[0058] Example 1

[0059] Example 1 provides a lithium-ion battery, which includes:

[0060] A battery core, which is formed by winding a positive electrode sheet, a separator and a negative electrode sheet stacked together. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer formed on at least one side surface of the positive electrode current collector. The positive electrode active material layer is formed by curing a positive electrode slurry coated on the surface of the positive electrode current collector. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer formed on at least one side surface of the negative electrode current collector. The negative electrode active material layer is formed by curing a negative electrode slurry coated on the surface of the negative electrode current collector. The positive electrode slurry includes a positive electrode active substance, a positive electrode conductive agent and a positive electrode binder. The negative electrode slurry includes a negative electrode active substance, a negative electrode conductive agent, a negative electrode binder and a thickening agent; and

[0061] The electrolyte includes: a lithium salt, a solvent, and an additive.

[0062] The manufacturing method of the above lithium-ion battery includes the following specific steps:

[0063] I. Preparation of the dispersant:

[0064] Using dimethyl carbonate (DMC) as a catalyst, glycerol as an initiator, adding ethylene oxide (EO) and propylene oxide (PO) in a molar ratio of 2.5:1, setting the reaction temperature at 125 ± 2 °C, synthesizing the polyether main chain, and monitoring the molecular weight of the polyether main chain to be approximately 3800;

[0065] Mixing GMA (glycidyl methacrylate) monomer, hexadecyl mercaptan (C16-SH), and ammonium persulfate (APS) in a molar ratio of 100:5:0.3, after initiation at 65 °C, maintaining at 75 °C for 2 h to graft the C16 fatty chain. The product is a polymer molecule with a main chain of GMA chains with a polymerization degree of approximately 20 and C16 fatty chains as side chains, presenting an overall comb-like structure. The reaction process is as follows: ammonium persulfate decomposes to generate free radicals (SO4 -· ), attacking the double bond of GMA, initiating the polymerization reaction to form GMA chain radicals. C16-SH undergoes a chain transfer reaction with the free radical chain through the thiol group (-SH), terminating the growth of the main chain and introducing the C16 fatty chain into the system;

[0066] Feeding adipic acid and 1,4-butanediol (BDO) in a molar ratio of 1:1.05, adding β-cyclodextrin at a proportion of 0.2 - 0.5 wt% of the mass of adipic acid, carrying out melt polycondensation at 200 °C to synthesize a polyester chain with a polymerization degree of 12 - 20. Dissolving the polyester chain in an aqueous acrylic acid solution, controlling the molar ratio of acrylic acid monomer to polyester chain molecules to be 1:5 - 1:2, adding an APS initiator at a proportion of 0.1 - 0.5 wt% of the feeding mass of the acrylic acid monomer, and reacting at 70 - 80 °C for 3 - 5 h. The carboxyl group on the acrylic acid monomer is grafted onto the polyester chain through free radical chain transfer to complete the operation of introducing carboxyl groups into the polyester branches;

[0067] Adding the polyether main chain: GMA-fatty chain: carboxyl-polyester chain in a mass ratio of 45:30:25 to N-methylpyrrolidone (NMP), using APS as an initiator, after polymerization for 2 h, filtering with a 0.2 μm PTFE membrane to obtain a multi-chain polymer with a molecular weight of 30000. Among them, the proportion of polyester chains grafted on the polyether main chain is 0.2, and the substitution degree of carboxyl groups on the polyester chain is 0.8. Using NMP to regulate the solid content to 3 wt% to obtain the dispersant.

[0068] II. Fabrication of the positive electrode sheet:

[0069] The positive electrode active material lithium iron phosphate, conductive carbon black, the above-prepared dispersant and polyvinylidene fluoride (PVDF) were taken and mixed in an N-methylpyrrolidone solvent system at a mass ratio of 96.5:1.5:0.3:1.7 to obtain a positive electrode slurry with a solid content of 65%. The positive electrode slurry was then coated on a 12.0 μm thick positive electrode collector (aluminum foil), and after drying and cold pressing, a positive electrode sheet was obtained. The single-side surface density of the electrode sheet was 15 mg / cm 2 , the pole piece compaction is 2.75g / cm 3 Specifically, the apparent compaction density of the raw material powder of the selected positive electrode active material after compaction is 2.45g / cm 3 , particle size (Dv90-Dv10) / Dv50=1.5, electrode compaction density-raw material powder compaction density=2.75-2.45=0.3.

[0070] 3. Production of negative electrode sheet:

[0071] The negative electrode sheet includes a negative electrode current collector (copper foil) and a negative electrode slurry coated on both sides of the copper foil. Calculated by mass percentage, the negative electrode slurry includes 96.0% silicon-carbon material (Si content 10.0wt%), 1.5% carbon nanotubes, 1.0% thickener sodium carboxymethyl cellulose (CMC) and 1.5% negative electrode binder polyacrylic acid (PAA). The above substances are added to deionized water and stirred to form a negative electrode slurry with a solid content of 40%. The negative electrode slurry is then coated on both sides of the copper foil, dried and cold pressed to form a negative electrode sheet with a compaction density of 1.5g / cm 3 ;

[0072] 4. Preparation of electrolyte:

[0073] Lithium hexafluorophosphate is mixed with an organic solvent to obtain an electrolyte, wherein the concentration of lithium hexafluorophosphate in the electrolyte is 1.0 mol / L, and the organic solvent comprises components in the following ratios: ethylene carbonate (EC): fluoroethylene carbonate (FEC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) = 15:15:20:50.

[0074] 5. Diaphragm:

[0075] A high-porosity diaphragm is selected, in which the thickness of the base film PE is 9 μm, the thickness of the ceramic coating on both sides of the base film is 1.0 μm, and the thickness of the PVDF coating is 1.0 μm. The air permeability of the diaphragm is ≤100s / 100mL.

[0076] 6. Assembly of lithium-ion batteries:

[0077] The positive electrode sheet and the negative electrode sheet are respectively rolled, slit, and then wound together with the separator to obtain a 21700 cylindrical battery core. Subsequently, after the battery core is welded to the connecting piece, it is installed in the battery case. After completing the electrolyte injection, sealing, and formation processes, the lithium-ion battery of Example 1 is obtained. The case of this lithium-ion battery is a cylinder, and its dimensional parameters are: diameter: 16 - 55 mm, length 63 - 140 mm.

[0078] Example 2

[0079] Example 2 provides a lithium-ion battery. The difference between Example 2 and Example 1 is that: polyether main chain : aliphatic chain = 5 : 2, and the other conditions are the same as those in Example 1.

[0080] Example 3

[0081] Example 3 provides a lithium-ion battery. The difference between Example 3 and Example 1 is that: polyether main chain : aliphatic chain = 3 : 1, and the other conditions are the same as those in Example 1.

[0082] Example 4

[0083] Example 4 provides a lithium-ion battery. The difference between Example 4 and Example 1 is that the substitution degree of the carboxyl group on the polyester chain is 0.5, and the other conditions are the same as those in Example 1.

[0084] Example 5

[0085] Example 5 provides a lithium-ion battery. The difference between Example 5 and Example 1 is that the proportion of grafting the polyester chain on the polyether main chain is: 0.3, and the other conditions are the same as those in Example 1.

[0086] Example 6

[0087] Example 6 provides a lithium-ion battery. The difference between Example 6 and Example 1 is that the proportion of grafting the polyester chain on the polyether main chain is: 0.4, and the other conditions are the same as those in Example 1.

[0088] Example 7

[0089] Example 7 provides a lithium-ion battery. The difference between Example 7 and Example 1 is that the proportion of grafting the polyester chain on the polyether main chain is: 0.5, and the other conditions are the same as those in Example 1.

[0090] Example 8

[0091] Example 8 provides a lithium-ion battery. The difference between Example 8 and Example 1 is that the molecular weight of the multi-chain polymer is 20000, and the other conditions are the same as those in Example 1.

[0092] Example 9

[0093] Example 9 provides a lithium-ion battery. The difference between Example 9 and Example 1 is that the molecular weight of the multi-chain polymer is 40,000, and the other conditions are the same as those in Example 1.

[0094] Example 10

[0095] Example 10 provides a lithium-ion battery. The difference between Example 10 and Example 1 is that the particle size of the positive electrode active material (Dv90 - Dv10) / Dv50 = 1.1.

[0096] Example 11

[0097] Example 11 provides a lithium-ion battery. The difference between Example 11 and Example 1 is that the particle size of the positive electrode active material (Dv90 - Dv10) / Dv50 = 1.3.

[0098] Example 12

[0099] Example 12 provides a lithium-ion battery. The difference between Example 12 and Example 1 is that the particle size of the positive electrode active material (Dv90 - Dv10) / Dv50 = 1.8.

[0100] Example 13

[0101] Example 13 provides a lithium-ion battery. The difference between Example 13 and Example 1 is that the apparent compaction density of the positive electrode active material raw powder is 2.21 g / cm 3 , and the apparent compaction density of the positive electrode sheet is 2.43 g / cm 3 .

[0102] Example 14

[0103] Example 14 provides a lithium-ion battery. The difference between Example 14 and Example 1 is that the apparent compaction density of the positive electrode active material raw powder is 2.55 g / cm 3 , and the apparent compaction density of the positive electrode sheet is 2.77 g / cm 3 .

[0104] Comparative Example 1

[0105] Comparative Example 1 provides a lithium-ion battery. The difference between Comparative Example 1 and Example 1 is that the polyether main chain: aliphatic chain = 7:1, and the other conditions are the same as those in Example 1.

[0106] Comparative Example 2

[0107] Comparative Example 2 provides a lithium-ion battery. The difference between Comparative Example 2 and Example 1 is that the polyether main chain: aliphatic chain = 1:4, and the other conditions are the same as those in Example 1.

[0108] Comparative Example 3

[0109] Comparative Example 3 provides a lithium-ion battery. The difference between Comparative Example 3 and Example 1 is that the substitution degree of the carboxyl group on the polyester chain is 0, and the other conditions are the same as those in Example 1.

[0110] Comparative Example 4

[0111] Comparative Example 4 provides a lithium-ion battery. The difference between Comparative Example 4 and Example 1 is that the substitution degree of the carboxyl group on the polyester chain is 0.1, and the other conditions are the same as those in Example 1.

[0112] Comparative Example 5

[0113] Comparative Example 5 provides a lithium-ion battery. The difference between Comparative Example 5 and Example 1 is that the substitution degree of the carboxyl group on the polyester chain is 0.3, and the other conditions are the same as those in Example 1.

[0114] Comparative Example 6

[0115] Comparative Example 6 provides a lithium-ion battery. The difference between Comparative Example 6 and Example 1 is that the substitution degree of the carboxyl group on the polyester chain is 1.3, and the other conditions are the same as those in Example 1.

[0116] Comparative Example 7

[0117] Comparative Example 7 provides a lithium-ion battery. The difference between Comparative Example 7 and Example 1 is that the substitution degree of the carboxyl group on the polyester chain is 1.6, and the other conditions are the same as those in Example 1.

[0118] Comparative Example 8

[0119] Comparative Example 8 provides a lithium-ion battery. The difference between Comparative Example 8 and Example 1 is that the molecular weight of the multi-chain polymer is 4000, and the other conditions are the same as those in Example 1.

[0120] Comparative Example 9

[0121] Comparative Example 9 provides a lithium-ion battery. The difference between Comparative Example 9 and Example 1 is that the molecular weight of the multi-chain polymer is 6000, and the other conditions are the same as those in Example 1.

[0122] Comparative Example 10

[0123] Comparative Example 10 provides a lithium-ion battery. The difference between Comparative Example 10 and Example 1 is that the molecular weight of the multi-chain polymer is 65000, and the other conditions are the same as those in Example 1.

[0124] Comparative Example 11

[0125] Comparative Example 11 provides a lithium-ion battery. The difference between Comparative Example 11 and Example 1 is that: the apparent tap density of the positive electrode active material particles is 2.45 g / cm 3 , and the apparent tap density of the positive electrode sheet is 2.82 g / cm 3 , and the other conditions are the same as those in Example 1.

[0126] Examples 1-14 and Comparative Examples 1-11 were respectively subjected to the following tests:

[0127] (1) Test method for the sheet resistance of the positive electrode sheet: The 46-probe method of the RM2610 resistance test system was used (45 probes are arranged in a square matrix, and 1 probe is used as the ground probe to measure the sheet resistance of the positive electrode sheets in Examples 1-14 and Comparative Examples 1-11. The specific process is as follows:

[0128] ① The positive electrode sheet that has been rinsed with dimethyl carbonate and vacuum dried at 60 °C for 6 hours was divided into 40 square grid samples of 1.0 cm × 1.0 cm to ensure the flatness of the sample surface;

[0129] ② Select a square grid sample, place the sample on the test device and use a pressure gauge to adjust the pressure applied by the probe to ensure good contact between the probe and the sample, and the contact area is 0.01 cm 2 . During the test, 20 peripheral probes apply a constant current to make the current flow through the surface, interface and current collector of the positive electrode sheet. At the same time, 25 middle probes measure the voltage change in real time. Since the surface, interface and current collector of the positive electrode sheet have significantly different resistances, the measured voltage will reflect these differences;

[0130] ③ Denote the sheet resistance of the grid sample measured in step ② as R1, and then use the same method to randomly select another 9 square grids on the above-mentioned electrode sheet for sheet resistance measurement. The obtained values are respectively denoted as R2, R3, R4, R5, R6, R7, R8, R9, R10. Finally, by calculating the arithmetic mean of these values, the average sheet resistivity R of the actually tested positive electrode sheet is obtained β (R β =(R1 + R2 + R3 + R4 + R5 + R6 + R7 + R8 + R9 + R10) / 10), in order to comprehensively evaluate the conductivity and uniformity of the positive electrode sheet.

[0131] (2) Test method for energy density:

[0132] The lithium-ion batteries of Examples 1-14 and Comparative Examples 1-11 were placed in a constant temperature oven at 25 °C for 4 hours and tested according to the following steps:

[0133] 1. Constant current and constant voltage charge to 4.2V under the condition of 0.1C, with the cut-off current of 0.01C, and stand still for 10 min;

[0134] 2. Constant current discharge to 2.5V cut-off under the condition of 0.1C, and stand still for 10 min to obtain the discharge capacity C and the average discharge voltage V;

[0135] 3. Weigh the battery and record the weight as G, energy density = C·V / G

[0136] (3) Battery rate performance test method: Place the battery in a constant temperature oven at 25°C for 4 hours and conduct the test according to the following steps:

[0137] 1. Constant current and constant voltage charge to 3.65V under the condition of 0.1C, with the cut-off current of 0.01C, and stand still for 10 min;

[0138] 2. Constant current discharge to 2.0V cut-off under the condition of 0.1C, and stand still for 10 min;

[0139] 3. Constant current and constant voltage charge to 3.65V under the condition of 1C, with the cut-off current of 0.01C, and stand still for 10 min;

[0140] 4. Constant current discharge to 2.0V cut-off under the condition of 1C, and stand still for 10 min, record the capacity S1;

[0141] 5. Repeat steps 3 and 4 for 600 times, record the capacity S2.

[0142] 6. Capacity retention rate = S2 / S1*100%

[0143] The arithmetic average diaphragm resistivity R actually measured according to the above method β 、The results of the energy density of the battery and the cycle capacity retention rate of the battery are shown in the following table:

[0144]

[0145]

[0146] From the results of Examples 1-3 and Comparative Examples 1-2, it can be seen that the diaphragm resistance value of the positive electrode sheet in Example 1 is the lowest. At this time, the side chains grafted on the main chain of the dispersant have a polyether chain:aliphatic chain ratio of 3:2. Deviating from this ratio will lead to an increase in the diaphragm resistance of the positive electrode sheet, and a decrease in the energy density and cycle capacity retention rate. This is because at the appropriate ratio, the proportion of the polyether chain can ensure that the surface tension of the positive electrode active material particles in the positive electrode slurry is reduced. The polyether chain plays a lubricating role, reducing the slip resistance between particles, promoting the movement and filling of small particles during the compaction process, increasing the compaction density of the positive electrode sheet, and effectively increasing the energy density. The aliphatic chain provides elastic support and provides an appropriate steric hindrance effect to balance dispersion and flexibility.

[0147] From the results of Examples 1 and 4-7 and Comparative Examples 3-7, it can be seen that with the change of the carboxyl substitution degree, the resistance of the positive electrode sheet, the energy density of the battery, and the cycle performance also change accordingly. The carboxyl group substitution degree of Example 1 is 0.8, and the polyester chain grafting degree is 0.2. At this time, the comprehensive performance of the battery cell is the best. This is because the increase in the carboxyl substitution degree can reduce the energy barrier of lithium ion migration, accelerate the desolvation of lithium ions, and reduce the interfacial resistance. At the same time, a certain amount of -COOH can promote the anchoring effect of the dispersant on LFP through dipole interaction or hydrogen bonding. However, too high a substitution degree will lead to a weakening of the adhesion force between active particles and a decrease in the adsorption efficiency of binder molecules, which is not conducive to long-term cycle performance. If the proportion of the polyester chain is insufficient, the effect of reducing the surface tension of the slurry will be poor, the slurry will be unevenly dispersed, the conductivity network uniformity of the positive electrode active material layer will be poor, the resistance of the electrode sheet will increase, and the cycle rate performance will decrease. If the proportion of the polyester chain is too high, the binding energy between it and the surface of the positive electrode active material particles will be too large, isolating the contact between the positive electrode active material particles and the conductive agent, thereby increasing the resistance of the positive electrode sheet.

[0148] From the results of Examples 1, 8-9 and Comparative Examples 8-10, it can be seen that when the molecular weight of the dispersant is too high or too low, the resistance of the positive electrode sheet will increase and the cycle performance will decrease. This is because it is difficult to depolymerize when the molecular weight of the multi-chain polymer is too high, while when the molecular weight is low, the molecular chain length of the dispersant is short and the chain length cannot form a good steric hindrance, both of which cannot achieve a good dispersion effect, resulting in too high viscosity of the positive electrode slurry, uneven coating of the positive electrode sheet, and a significant increase in the diaphragm resistance of the positive electrode sheet.

[0149] As can be seen from the results of Example 1 and Examples 10-12, among the cathode materials of the lithium iron system with different particle size distributions, the cathode sheet resistance, energy density, and cycle capacity retention rate can all be maintained at a relatively high level, indicating that the obtained dispersant system is suitable for the lithium iron phosphate system with different particle sizes. At the same time, by comparing Examples 13-14 with Comparative Example 11, it can be obtained that due to the different designs of the LFP powder particles, on the premise that there are differences in the apparent compaction density of the raw material powder, the dispersant can significantly improve the apparent compaction density of the cathode sheet, and can increase the energy density and maintain the cycle capacity retention rate within a certain range of apparent compaction density. However, when the apparent compaction density exceeds a certain window, the overvoltage of the cathode sheet will significantly increase the resistivity of the cathode sheet, thereby reducing the cycle capacity retention rate. This is because too high compaction density will cause some cathode active material particles to break, breaking the electrical conductivity network connectivity of the cathode material and resulting in an increase in the resistance of the cathode sheet. At the same time, the broken cathode active material particles will expose more active sites, resulting in an increase in side reactions during the cycle. At the same time, as the number of cycles increases, the cathode binder cannot bind these roll-pressed and broken particles, ultimately leading to a decrease in the cycle capacity retention rate.

[0150] In summary, for the lithium-ion battery provided in this application, a dispersant with a special multi-level microstructure is applied to the cathode slurry to form a cathode active material layer. The wettability of the polyether chain can reduce the surface tension, increase the slip effect during the rolling of cathode particles, and improve the compaction density. The flexibility of the fatty chain can reduce the cracking risk, and the anchoring group can reduce the interfacial resistance, optimize the structure of the cathode sheet, improve the compaction, thereby increasing the energy density of the battery, reducing the resistance, and increasing the cycle capacity retention rate.

[0151] In the description of this specification, references to terms such as "some embodiments", "an embodiment", or similar descriptions mean that the specific features, structures, materials, or characteristics described in connection with the embodiment are included in at least one embodiment or example. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0152] Although the embodiments of this embodiment have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and purposes of this embodiment. The scope of this embodiment is defined by the claims and their equivalents.

Claims

1. A dispersant, characterized in that, It includes a multi-chain polymer composed of a polyether main chain, an aliphatic chain grafted to the polyether main chain, and a polyester chain. The mass ratio of the polyether main chain, the aliphatic chain, and the polyester chain in the multi-chain polymer is: (40 - 50):(25 - 30):(15 - 20). The molecular weight of the polyether main chain is: 2500 - 5000. The number of carbon atoms in the aliphatic chain is 8 - 22. A carboxyl group is also grafted on the polyester chain. The molecular weight of the multi-chain polymer is: 30000 ± 10000.

2. The dispersant according to claim 1, characterized in that, The grafting rate of the polyether main chain is 40 - 45%.

3. The dispersant according to claim 1, wherein An anchoring group is also connected to the aliphatic chain. The anchoring group includes one or more of a polar group, a benzene ring-containing group, and a conjugated π-bond-containing group. The substitution degree of the anchoring group on the aliphatic chain is: 0.1 - 0.

4.

4. The dispersant according to claim 1, wherein A glycidyl methacrylate GMA chain with a degree of polymerization of 10 - 50 is also grafted on the polyether main chain. The aliphatic chain is indirectly grafted to the polyether main chain by accessing the end of the GMA chain through a chain transfer reaction.

5. The dispersant according to claim 1, characterized in that, The degree of polymerization of the polyester chain is 12 - 20. The proportion of the polyester chain grafted on the polyether main chain is: 0.1 - 0.

5. The carboxyl group is connected to the end of the polyester chain through a chain transfer, and the substitution degree of the carboxyl group on the polyester chain is 0.4 - 0.

9.

6. A preparation method of a dispersant, characterized in that, The method is used to prepare the dispersant according to any one of claims 1 to 5, and includes the following steps: S1: Using dimethyl carbonate DMC as a catalyst, glycerol as an initiator, ethylene oxide and / or propylene oxide as monomers, setting the reaction temperature at 125 ± 2 °C to prepare the polyether main chain; S2: Using ammonium persulfate as an initiator, GMA as a monomer, and aliphatic chain mercaptan as a chain terminator, GMA polymerizes to form a GMA chain with a degree of polymerization of 10 - 50 and a free radical at the end. The aliphatic chain mercaptan undergoes a chain transfer reaction with the free radical at the end of the GMA chain to terminate the growth of the GMA chain and graft the aliphatic chain to the end of the GMA chain; S3: Using adipic acid and 1,4-butanediol as reactants, β-cyclodextrin as a catalyst, setting the reaction temperature at 200 °C, melt polycondensing to synthesize the polyester chain, dissolving the polyester chain in an aqueous organic acid solution, adding the initiator ammonium persulfate, and reacting at 70 - 80 °C for 3 - 5 h. The carboxyl group on the organic acid is grafted to the polyester chain through a free radical chain transfer; S4: Adding the polyether main chain, the GMA chain with the aliphatic chain connected at the end, and the polyester chain in a mass ratio of (40 - 50):(25 - 30):(15 - 20) to NMP, using ammonium persulfate as an initiator, reacting at 45 - 75 °C for 1 - 3 hours, and filtering through a 0.2 μm PTFE membrane to obtain the multi-chain polymer; and S5: Adding NMP to the multi-chain polymer and adjusting the solid content to 3 wt% to obtain the dispersant.

7. The preparation method according to claim 6, characterized in that, In the S1, when ethylene oxide and propylene oxide are used as monomers simultaneously, the feeding ratio of ethylene oxide to propylene oxide is: molar ratio 2:1 - 5:

1.

8. The preparation method according to claim 6, characterized in that, In the step S2, the feeding ratio of the GMA, the fatty chain mercaptan and the ammonium persulfate is as follows: molar ratio 100:(2 - 10):(0.1 - 0.5), the initiation temperature is 65 °C, the reaction temperature is 75 °C, and the reaction time is 2 h.

9. The preparation method according to claim 6, characterized in that, In the step S3, the feeding ratio of the adipic acid and the 1,4 - butanediol is as follows: molar ratio 1:1.05; the addition amount of the β - cyclodextrin is 0.2 - 0.5 wt% of the weight of the adipic acid; the feeding ratio of the organic acid and the polyester chain is as follows: molar ratio 1:(2 - 5); the addition amount of the initiator ammonium persulfate is 0.1 - 0.5 wt% of the weight of the organic acid.

10. A positive electrode paste, comprising a positive electrode active material, a positive electrode conductive agent, a dispersant, and a positive electrode binder, characterized in that, The dispersant is the dispersant according to any one of claims 1 to 5, and the mass addition ratio is: positive electrode active material: positive electrode conductive agent: dispersant: positive electrode binder = (94 - 98):(1 - 3):(0.2 - 1):(0.8 - 2), and the addition ratio of the dispersant is measured by the multi - chain polymer.

11. The positive electrode paste according to claim 10, characterized in that, The 24 - h static viscosity rebound rate of the positive electrode paste is ≤30%.

12. The positive electrode paste according to claim 10, wherein, The particle size of the positive electrode active material satisfies: 1.1 ≤ (Dv90 - Dv10) / Dv50 ≤ 1.

8.

13. A lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer is formed by curing a positive electrode paste coated on the surface of the positive electrode current collector, and is characterized in that, The positive electrode paste is the positive electrode paste according to claim 10 or 11.

14. The lithium ion battery according to claim 13, wherein, The apparent tap density c of the positive electrode sheet satisfies: 2.4 g / cm 3 ≤ c ≤ 2.8 g / cm 3 , the areal density of the positive electrode sheet is: 10 - 20 mg / cm 2 , and the contact resistance between the positive electrode active material layer and the positive electrode current collector ≤ 0.3 Ω·cm 2 .

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