A negative electrode slurry, a negative electrode sheet, and a battery

Abstract

In view of the poor stability and adhesion of the existing negative electrode slurry, the application provides a negative electrode slurry, a negative electrode sheet and a battery. The negative electrode slurry comprises a negative electrode active material, a conductive agent and a binder, the binder comprises a first binder, a second binder and a third binder, the first binder is selected from water-soluble cellulose polymers and / or their salts, the second binder is an alkali metalized water-soluble carboxylic acid polymer, and the third binder comprises a polymer, the polymer comprises a vinyl aromatic structural unit and a conjugated diene structural unit, the neutralization degree x of the second binder is greater than or equal to 80%, and the Zeta potential of the third binder is Z, and |Z|=40-50 mV. The negative electrode slurry provided by the application can improve the interfacial adhesion of the negative electrode sheet and the stability of the negative electrode slurry, and realizes the combination of strong adhesion and high stability.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, specifically relating to a negative electrode slurry, a negative electrode sheet, and a battery. Background Technology

[0002] Lithium-ion batteries have been widely used in new energy vehicles, portable electronic devices, energy storage systems and other fields due to their advantages such as high energy density, long cycle life and environmental friendliness. Their performance is closely related to the stability and reliability of the manufacturing process.

[0003] Currently, the aqueous manufacturing system for lithium-ion battery anodes generally uses a composite binder composed of sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) emulsion as the core system. However, in order to enhance the interfacial interaction with the active material and improve cycle stability, lithium-ionized Li-PAA is used or added to the anode slurry as a binder, and its neutralization degree is increased to strengthen the interfacial bonding force. However, high neutralization degree will significantly increase the ionic strength of the system, compress the electrode bilayer, and reduce electrostatic repulsion, causing the SBR emulsion and active material particles to flocculate and agglomerate, reducing the stability of the slurry, and thus causing electrode coating defects and reduced peel strength.

[0004] Therefore, there is an urgent need for a negative electrode slurry with good stability and strong adhesion to improve the peel strength of the negative electrode sheet and improve the cycle stability of the battery. Summary of the Invention

[0005] This invention addresses the problems of poor stability and adhesion of existing negative electrode slurries by providing a negative electrode slurry, a negative electrode sheet, and a battery.

[0006] To solve the above-mentioned technical problems, the present invention provides a negative electrode slurry, which includes a negative electrode active material, a conductive agent, and a binder. The binder includes a first binder, a second binder, and a third binder. The first binder is selected from water-soluble cellulose polymers and / or their salts. The second binder is an alkali-metallized water-soluble carboxylic acid polymer. The third binder includes a polymer comprising vinyl aromatic structural units and conjugated diene structural units. The neutralization degree of the second binder is x, where x ≥ 80%. The zeta potential of the third adhesive is Z, |Z| = 40~50 mV.

[0007] Preferably, the gelation rate of the third adhesive is less than 0.01.

[0008] Preferably, the particle size of the third binder satisfies the following condition: D10 is 50~120nm, D50 is 180~400nm, and D90 is 450~590nm.

[0009] Preferably, the particle size distribution of the third binder is PDI = (D90-D10) / D50, and PDI = 1.2~2.0.

[0010] Preferably, the viscosity of the negative electrode slurry is 3000~5000 mPa·s; and / or; The viscosity drift rate of the negative electrode slurry after standing at 20~30℃ for 24 hours is ≤7%.

[0011] Preferably, the fineness of the negative electrode slurry is 15~25μm.

[0012] Preferably, the glass transition temperature of the third adhesive is -30℃ to 30℃.

[0013] Preferably, the mass ratio of the first adhesive, the second adhesive, and the third adhesive is (0.4~1):(1.2~2.5):(1~3).

[0014] Preferably, the polymer in the third adhesive further includes acrylate structural units, acrylonitrile structural units, acrylic structural units and acrylamide structural units, and the mass ratio of the vinyl aromatic structural units, conjugated diene structural units, acrylate structural units, acrylonitrile structural units, acrylic structural units and acrylamide structural units is (10~50):(10~60):(10~60):(1~20):(0.1~30):(0.1~30).

[0015] Secondly, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, the negative electrode active material layer being formed from the negative electrode slurry as described in any of the above claims.

[0016] Thirdly, this application provides a battery including the negative electrode sheet as described above.

[0017] In this application, when the neutralization degree x of the second binder (alkali-metallated water-soluble carboxylic acid polymer) is ≥80%, it can improve the ionic strength and conductivity of the negative electrode slurry, while forming stable chemical bonds with the current collector surface, significantly enhancing the interfacial bonding ability of the binder. Furthermore, by limiting the Zeta potential |Z| of the third binder to the range of 40~50mV, the negative electrode slurry can maintain strong electrostatic repulsion even under the high ionic strength environment caused by the high neutralization degree of the second binder. This effectively resists the compression effect of increased ionic strength on the electric double layer, preventing the aggregation of third binder particles due to insufficient electrostatic repulsion. It also prevents the flocculation of third binder particles with active materials and conductive agent particles, thereby ensuring the dispersion stability of the negative electrode slurry. The synergistic effect of these two factors improves the interfacial bonding force of the negative electrode sheet and the stability of the negative electrode slurry, achieving a balance between strong adhesion and high stability. Detailed Implementation

[0018] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] One embodiment of this application provides a negative electrode slurry, which includes a negative electrode active material, a conductive agent, and a binder. The binder includes a first binder, a second binder, and a third binder. The first binder is selected from water-soluble cellulose polymers and / or their salts. The second binder is an alkali-metallized water-soluble carboxylic acid polymer. The third binder includes a polymer comprising vinyl aromatic structural units and conjugated diene structural units. The neutralization degree of the second binder is x, where x ≥ 80%. The zeta potential of the third adhesive is Z, |Z| = 40~50 mV.

[0020] In this embodiment, when the neutralization degree x of the second binder (alkali-metallated water-soluble carboxylic acid polymer) is ≥80%, it can improve the ionic strength and conductivity of the negative electrode slurry, and simultaneously form stable chemical bonds with the current collector surface, significantly enhancing the interfacial bonding ability of the binder. Furthermore, by limiting the Zeta potential |Z| of the third binder to the range of 40~50 mV, the negative electrode slurry can maintain strong electrostatic repulsion even under the high ionic strength environment caused by the high neutralization degree of the second binder. This effectively resists the compression effect of increased ionic strength on the electric double layer, preventing the aggregation of third binder particles due to insufficient electrostatic repulsion. It also prevents the flocculation of third binder particles with active materials and conductive agent particles, thereby ensuring the dispersion stability of the negative electrode slurry. The synergistic effect of these two factors improves both the interfacial bonding force of the negative electrode sheet and the stability of the slurry, achieving a balance between strong adhesion and high stability.

[0021] Specifically, the electric double layer refers to a structure at the solid-liquid interface or the electrode-electrolyte interface, consisting of a charged surface and a corresponding counterion layer.

[0022] In this invention, the first binder is selected from water-soluble cellulose polymers and / or their salts commonly used in the art. For example, the water-soluble cellulose polymers and / or their salts are preferably one or more of methylcellulose, ethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, lithium carboxymethylcellulose, and sodium carboxymethylcellulose.

[0023] The second binder is a water-soluble carboxylic acid polymer with alkali metal oxide commonly used in the art, such as a lithium or sodium salt of a polycarboxylic acid polymer. In this invention, the second binder is preferably lithium-modified polyacrylic acid.

[0024] Lithium-modified polyacrylic acid refers to polyacrylic acid (PAA) that has been neutralized with lithium alkali to form lithium salt. The degree of neutralization of lithium-modified polyacrylic acid microparticles is x = [n(LiOH) / n(-COOH)] × 100%, where n is the number of moles, which is the amount of substance.

[0025] Specifically, the total number of carboxyl groups in PAA = n (COOH), and the target degree of neutralization = x. Therefore, the number of moles of LiOH to be added is: n(LiOH) = x * n(COOH) Here, x is represented as a decimal. For example: 80% neutralization: n(LiOH) = 0.80*n(COOH); 90% neutralization: n(LiOH) = 0.90*n(COOH); 100% neutralization degree: n(LiOH) = 1.00 * n(COOH); 110% neutralization degree: n(LiOH) = 1.10*n(COOH).

[0026] The third binder comprises a polymer, which includes vinyl aromatic structural units and conjugated diene structural units. The zeta potential (|Z|) of the third binder includes, but is not limited to, 40 mV, 41 mV, 42 mV, 43 mV, 44 mV, 45 mV, 46 mV, 47 mV, 48 mV, 49 mV, or 50 mV. The zeta potential is determined by adjusting the third binder to a solid content of 0.01%-0.10% using deionized water, and then testing it using a potentiometer. The zeta potential determines the level of electrostatic repulsion between emulsion particles, and the level of electrostatic repulsion directly determines whether the emulsion is prone to aggregation, flocculation, and instability. Within a certain range, the higher the potential, the better the emulsion stability; this relationship is particularly evident in the high-lithiation second binder system of this invention.

[0027] In a preferred embodiment, the neutralization degree x of the second binder is 80% to 110%. When the neutralization degree is within the range of 80% to 110%, a large number of carboxyl groups in the second binder can be converted into lithium carboxylate groups. On the one hand, this improves the ionization degree and dispersion ability of the second binder in the aqueous system, making it easier to form a stable adsorption layer on the surface of graphite, conductive agent, and binder particles. On the other hand, the introduction of lithium ions helps to improve the ion transport environment in the negative electrode, thereby reducing interfacial impedance and improving rate performance and low-temperature DC internal resistance. At the same time, an appropriate proportion of incompletely neutralized carboxyl groups can be retained within this range, which can maintain the polar interaction and adhesion between the second binder and the surface of the active material / current collector, thereby achieving a better balance between dispersibility, stability, and adhesion in the slurry system.

[0028] When the neutralization degree is below 80%, a high proportion of carboxyl groups in the second binder are not converted into lithium salt form. The ionization degree of the second binder is insufficient, and the extension of the molecular chains in water is limited, making it difficult to fully exert its dispersion and stabilization effect on graphite particles and conductive agents. At the same time, the lithium ion content provided by the second binder is low, which is not conducive to improving the ion transport environment in the negative electrode slurry and negative electrode sheet. This can easily lead to large viscosity fluctuations in the negative electrode slurry, decreased storage stability, and easy particle agglomeration, thereby affecting the uniformity of electrode coating, bonding consistency, and the rate performance and low temperature performance of the battery.

[0029] When the neutralization degree exceeds 110%, the lithium source in the second binder system is excessive relative to the carboxyl group content, leading to increased local alkalinity in the negative electrode slurry system or an increase in residual free lithium salt / small molecule alkali. This alters the interfacial interaction between the second and third binder particles and the graphite surface, compresses the colloidal double layer, disrupts the original dispersion balance, and easily causes problems such as slurry thickening, flocculation, and decreased storage stability. Excessively high neutralization degrees may also weaken the hydrogen bonding and polar adsorption effects provided by the residual carboxyl groups in the second binder, reducing its adhesion to the negative electrode active material and copper foil, thus negatively impacting the mechanical and cycle stability of the negative electrode sheet.

[0030] Specifically, the neutralization degree x of the second adhesive includes, but is not limited to, 80%, 85%, 90%, 95%, 100%, 105%, or 110%.

[0031] In some embodiments, the gelation rate of the third adhesive is less than 0.01.

[0032] Specifically, the gelation rate of the third binder reflects the degree of formation of insoluble cross-linked networks in the system, which is related to the composition of polymer structural units, particle interface stability, and polymerization process conditions. When the gelation rate increases, the number of insoluble gel particles in the system increases. These particles are prone to act as flocculation nuclei in the slurry, adsorbing third binder emulsion particles, negative electrode active material particles, and conductive agent particles, resulting in poorer slurry fineness, decreased storage stability, and poorer electrode bonding uniformity.

[0033] The method for controlling the gelation rate of the polymer in the third binder is well known in the art. For example, in this invention, it is adjusted by controlling the proportion range of vinyl aromatic structural units, conjugated diene structural units, acrylate structural units, acrylonitrile structural units, acrylic structural units and acrylamide structural units in the third binder.

[0034] The present invention found that when the gelation rate of the third binder is controlled below 0.01, the coarsening and flocculation effects of insoluble gel particles on the slurry system are significantly reduced, and the fineness of the slurry, storage stability and electrode bonding uniformity are significantly improved. Therefore, 0.01 is determined as the preferred upper limit for gelation rate control in the present invention.

[0035] The gel ratio of the third binder is the proportion of insoluble gel remaining in the third binder after toluene extraction. If the gel ratio is too high, a large number of insoluble cross-linked network particles will form. These particles are large in size and have high surface activity, making them highly susceptible to becoming flocculation nuclei. They adsorb polymer particles, active material particles, and conductive agent particles from the third binder, leading to poor slurry fineness, large particle agglomeration, and consequently, coating defects. Furthermore, the insoluble residue will reduce the density and bonding uniformity of the electrode sheet.

[0036] By limiting the gelation rate of the third binder to less than 0.01, the formation of insoluble cross-linked networks is reduced, preventing insoluble cross-linked networks from becoming the core of flocculation and coarsening in the slurry. This ensures that the slurry dispersion and rheological stability can still be maintained when compounded with lithium-ionized polyacrylic acid with a higher degree of neutralization.

[0037] In some embodiments, the particle size of the third binder satisfies the following condition: D10 is 50~120nm, D50 is 180~400nm, and D90 is 450~590nm.

[0038] By limiting the size range of the third binder particles, it can be uniformly dispersed in the slurry, while adapting to the high ionic strength environment of the neutralized second binder, thus balancing the dispersion stability of the slurry and the adhesion density of the electrode.

[0039] Specifically, D10 is 50~120nm, ensuring a certain amount of small particles in the emulsion to fill the tiny gaps between the active material and the conductive agent, improving the density and uniformity of the electrode. D50 is 180~400nm, which is the optimal median particle size for high ionic strength environments. Particles of this size can ensure sufficient specific surface area to form a good interfacial bond with the negative electrode active material, the first binder, and the second binder, while avoiding the problem of excessively high surface energy and easy agglomeration caused by excessively small particles. At the same time, it can also avoid abnormal slurry viscosity and uneven coating caused by excessively large particles. D90 is 450~590nm, which limits the size of the largest particles in the emulsion, avoiding defects such as poor slurry fineness and coating pinholes caused by the presence of large particles. At the same time, it ensures that the emulsion particles can uniformly cover the surface of the active material to form a continuous bonding network. The third binder in this particle size range, in synergy with its high zeta potential and low gelation rate, can maintain stable dispersion in high ionic strength environments, while improving the bonding strength and compactness of the electrode, reducing internal porosity, improving ion conduction efficiency, and thus improving the rate performance and cycle stability of the battery.

[0040] Specifically, D10 includes, but is not limited to, 20nm, 40nm, 60nm, 80nm, 100nm, or 120nm.

[0041] D50 includes, but is not limited to, 180nm, 210nm, 240nm, 270nm, 300nm, 330nm, 360nm, 380nm, or 400nm.

[0042] D90 includes, but is not limited to, 450nm, 470nm, 490nm, 510nm, 530nm, 550nm, 570nm, or 590nm.

[0043] In some embodiments, the particle size distribution (PDI) of the third binder is (D90-D10) / D50, and PDI = 1.2~2.0. By limiting the particle size distribution width of the third binder particles, the uniformity of the third binder particle size is ensured, which avoids particle agglomeration and achieves sufficient coverage of the surface of the active material and filling of gaps.

[0044] Furthermore, in some embodiments, the particle size of the third binder is 600-700 nm for D97 and 900-1500 nm for D99.

[0045] D97 includes, but is not limited to, 600nm, 610nm, 620nm, 630nm, 640nm, 650nm, 670nm, 680nm, 690nm, or 700nm.

[0046] D99 includes, but is not limited to, 900nm, 950nm, 1000nm, 1050nm, 1100nm, 1150nm, 1200nm, 1250nm, 1300nm, 1350nm, 1400nm, 1450nm, or 1500nm.

[0047] In some embodiments, the polymer in the third adhesive further includes acrylate structural units, acrylonitrile structural units, acrylic structural units and acrylamide structural units, and the mass ratio of the vinyl aromatic structural units, conjugated diene structural units, acrylate structural units, acrylonitrile structural units, acrylic structural units and acrylamide structural units is (10~50):(10~60):(10~60):(1~20):(0.1~30):(0.1~30).

[0048] By limiting the proportion range of each structural unit in the third binder, the interfacial adsorption performance, flexibility, adhesion and resistance to ion interference of the third binder are optimized, making it suitable for the high ion strength environment of the high neutralization degree second binder.

[0049] Vinyl aromatic structural units and conjugated diene structural units provide the third binder with an elastic framework and improve its film-forming continuity. Acrylic ester structural units are used to further adjust the glass transition temperature (Tg) and film toughness of the third binder. Acrylonitrile structural units enhance the polarity and cohesive strength of the third binder, reducing swelling damage from the electrolyte. Acrylic and acrylamide structural units introduce polar groups such as carboxyl and amide groups, enhancing the adsorption and anchoring of the negative electrode active material, conductive agent, and current collector surface oxide layer, adjusting the zeta potential of the third binder, and improving its aqueous dispersion stability. By limiting the mass ratio of each structural unit within the above range, a balance between rigidity, flexibility, polarity, and stability can be achieved in the third binder. This results in strong adhesion and interfacial adsorption capacity, stability under high ionic strength conditions, and good synergistic effects with the first binder and the high-neutralization second binder, further improving the stability of the slurry and the bonding reliability of the electrode. In addition, the third binder in this composition has excellent film-forming properties, and can form a continuous and dense adhesive film, thereby improving the conductivity uniformity and cycle stability of the electrode.

[0050] Specifically, the mass ratios of vinyl aromatic structural units, conjugated diene structural units, acrylate structural units, acrylonitrile structural units, acrylic structural units, and acrylamide structural units include, but are not limited to, 10:60:10:20:30:0.1, 30:60:10:20:30:0.1, 50:60:10:20:30:0.1, 10:10:10:20:30:0.1, 10:35:10:20:30:0.1, and 1 0:60:45:20:30:0.1, 10:60:60:20:30:0.1, 10:60:10:1:30:0.1, 10:60:10:10:30:0.1, 10:60:10:20:0.1:0.1, 10:60:10:20:15:0.1, 10:60:10:20:30:15, 10:60:10:20:30:30 or 30:35:45:10:15:15.

[0051] In some embodiments, the glass transition temperature (Tg) of the third binder is -30°C to 30°C. The glass transition temperature is obtained by differential scanning calorimetry.

[0052] In some embodiments, the vinyl aromatic structural unit is a structural unit obtained by polymerizing vinyl aromatic monomers, including but not limited to styrene.

[0053] The conjugated diene structural unit is a structural unit obtained by the polymerization of a conjugated diene, and the conjugated diene includes, but is not limited to, butadiene.

[0054] The acrylonitrile structural unit is a structural unit obtained by polymerizing acrylonitrile monomers, including but not limited to acrylonitrile.

[0055] The acrylic structural unit is a structural unit obtained by polymerization of carboxyl-containing unsaturated monomers, including but not limited to at least one of acrylic acid, methacrylic acid, itaconic acid, maleic acid, and fumaric acid.

[0056] The acrylamide structural unit is a structural unit obtained by polymerization of amide-unsaturated monomers, including but not limited to at least one of acrylamide, N,N-dimethylacrylamide, and N,N-methylenebisacrylamide.

[0057] The acrylate structural unit is a structural unit obtained by polymerizing acrylate monomers. Specifically, the glass transition temperature (Tg) of the homopolymer corresponding to the acrylate monomers used is -60℃ to 110℃.

[0058] The acrylate monomers include, but are not limited to, at least one of methyl methacrylate, ethyl acrylate, propyl acrylate, isopropyl methyl acrylate, caprolactone acrylate, cyclotrimethylolpropane methyl acetal acrylate, benzyl acrylate, ethoxyphenol acrylate, polyethylene glycol diacrylate, butyl acrylate, hexyl acrylate, heptyl acrylate, nonyl acrylate, octyl acrylate, undecyl acrylate, pentyl acrylate, dodecyl acrylate, and tridecyl acrylate.

[0059] In some embodiments, the mass ratio of the first binder, the second binder, and the third binder is (0.4~1):(1.2~2.5):(1~3). This limits the proportions of each component in the composite binder to maximize the synergistic effect of the first, second, and third binders, while also considering the dispersion stability, rheological properties, and bonding strength of the electrode.

[0060] In this invention, CMC is used as the first binder, which can form a stable dispersion network in the aqueous phase, effectively dispersing the negative electrode active material and conductive agent, laying the foundation for the stability of the slurry. If the amount of CMC is too small, the dispersion effect is insufficient, which can easily lead to the agglomeration of the active material; if the amount of CMC is too large, the slurry viscosity will be too high, affecting the coating performance. Li-PAA is used as the second binder, which can ensure sufficient interfacial bonding force, while avoiding excessive ionic strength and slurry agglomeration due to excessive dosage. If the amount of Li-PAA is too small, the interfacial bonding force is insufficient, and the electrode peel strength will decrease; if the amount is too large, the ionic strength will be too high, and even if the third binder has a high zeta potential and low gelation rate, it will be difficult to completely avoid agglomeration. The third binder can provide sufficient adhesion and flexibility, and form a continuous bonding network in synergy with CMC and Li-PAA; if the amount of the third binder is too small, the adhesion force is insufficient, and the electrode is prone to powdering and cracking; if the amount is too large, it will lead to an increase in electrode resistance, affecting the rate performance of the battery. When the mass ratio of the three components is controlled within the above range, the synergistic optimization of "dispersion-bonding-stabilization" can be achieved. CMC ensures the dispersion basis, Li-PAA enhances the interfacial bonding force, and the third binder enhances the bonding flexibility and anti-agglomeration ability, together ensuring the stability of the slurry in a high ionic strength environment, while improving the peel strength and manufacturing consistency of the electrode.

[0061] In some embodiments, the mass ratio of the negative electrode active material, the conductive agent, and the third binder is (50~100):(0.6~3):(1~3). This limits the proportion of the core functional components in the slurry, balances the energy density of the active material, the conductivity of the conductive agent, and the binding performance of the third binder, ensuring optimal overall battery performance.

[0062] Furthermore, the negative electrode slurry also includes a bactericide and a defoamer, and the mass ratio of the third binder, bactericide and defoamer is (1~80):(0.001~5):(0.001~5).

[0063] In some embodiments, the solid content of the negative electrode slurry is 54% to 56%. Controlling the solid content at 54% to 56% is the optimal range that balances coating performance and electrode density: if the solid content is too low, the slurry viscosity is insufficient, leading to defects such as sagging and uneven thickness during coating, and the electrode will have excessively high porosity after drying, resulting in decreased bonding strength and conductivity; if the solid content is too high, the slurry viscosity is too high, resulting in poor fluidity, making uniform coating difficult, and easily causing clogging of the coating equipment.

[0064] In some embodiments, the viscosity of the negative electrode slurry is 3000~5000 mPa·s. Controlling the viscosity to 3000~5000 mPa·s ensures the slurry has good thixotropic and fluid properties, facilitating stirring and dispersion while maintaining shape stability during coating, preventing sagging or breakage, and improving coating uniformity. If the viscosity is too low, the slurry's fluidity is too strong, making it difficult to control the coating thickness; if the viscosity is too high, coating resistance increases, easily leading to defects such as streaks and pinholes.

[0065] In some embodiments, the viscosity drift rate of the negative electrode slurry after standing for 24 hours at 20-30°C is ≤7%. In this invention, the Zeta potential of the third binder reflects the interfacial electrical properties of the emulsion particle surface and the level of electrostatic repulsion between particles. Since a high degree of neutralization of the second binder (e.g., Li-PAA) increases the ionic strength of the negative electrode slurry system, thereby compressing the electric double layer on the surface of the colloidal particles and weakening the electrostatic repulsion between particles, if the |Zeta| potential of the third binder is insufficient, the polymer particles of the third binder are more prone to aggregation, flocculation, or coarsening under high ionic strength conditions, leading to the reconstruction of the internal network structure of the slurry. This manifests as an increase in viscosity, a higher viscosity change rate, and poorer storage stability after standing. Conversely, when the |Zeta| potential of the third binder is controlled in a higher range, especially 40-50 mV, the polymer particles of the third binder can still maintain strong electrostatic repulsion in the high ionic strength second binder system, reducing irreversible aggregation and flocculation between particles, which is beneficial to maintaining the structural stability of the slurry system during the standing process, thus making the slurry exhibit a smaller viscosity change rate and better viscosity stability.

[0066] In some embodiments, the fineness of the negative electrode slurry is 15-25 μm. Controlling the fineness to 15-25 μm ensures that there are no large particle agglomerates in the slurry, avoiding defects such as scratching the coating roller and pinholes during coating, while also ensuring the uniformity of the electrode's microstructure, improving bonding strength and conductivity uniformity. If the fineness is too high, the presence of large particles in the slurry will lead to a rough electrode surface and uneven porosity, affecting the battery's cycle performance. If the fineness is too low, it will increase the slurry preparation cost and may cause abnormal viscosity.

[0067] Furthermore, the fineness of the negative electrode slurry includes, but is not limited to, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm or 25μm.

[0068] By limiting the key process parameters of the negative electrode slurry, we can ensure that the slurry has good rheological and coating properties, making it suitable for industrial production, while also ensuring the quality consistency of the electrode sheets.

[0069] Specifically, the fineness was obtained by using a Hegman scraper fineness tester.

[0070] In some embodiments, the negative electrode active material includes at least one of silicon, silicon carbide, and graphite.

[0071] Conductive agents include, but are not limited to, at least one of carbon black, graphene, and carbon nanotubes.

[0072] Furthermore, one embodiment of this application provides a method for preparing a negative electrode slurry, comprising the following steps: Preparation of the third binder: Vinyl aromatic monomers, conjugated dienes, acrylate monomers, acrylonitrile monomers, acrylic monomers, acrylamide monomers, anionic emulsifiers, nonionic emulsifiers, and stabilizers are emulsified in an emulsification kettle at a mass ratio of (10~50):(10~60):(10~60):(1~20):(0.1~30):(0.1~30):(0.0001-0.5):(0.0001-0.5):(0.001-5). After emulsification, 20 parts of the emulsion are transferred to a reaction kettle. Nitrogen gas is introduced to remove active oxygen from the reaction kettle and pipelines. Add the initiator and react at 50-70℃ for 1-5 hours. Then raise the temperature to 80-90℃, add the remaining emulsion while stirring, and add it over 1-6 hours. Then continue to react at 80-90℃ for 1-4 hours, cool and discharge to obtain the third binder.

[0073] The stabilizer is an additive used to improve the stability, storage stability, mechanical stability, or electrolyte stability of the emulsion polymerization process. Preferably, the stabilizer includes one or more of the following: protective colloids, buffers, and antiflocculation agents; more preferably, the stabilizer may be selected from one or more of the following: polyvinyl alcohol, polyvinylpyrrolidone, hydroxyethyl cellulose, carboxymethyl cellulose salt, polyacrylate, sodium hexametaphosphate, sodium pyrophosphate, sodium bicarbonate, and phosphate buffers.

[0074] The anionic emulsifier includes at least one of fatty acid salt compounds, rosin soap compounds, alkyl sulfate compounds, alkyl sulfonate compounds, alkylbenzene sulfonate compounds, sulfosuccinate compounds, ether sulfate compounds, and phosphate salt compounds; preferably, the anionic emulsifier is selected from at least one of sodium stearate, sodium oleate, sodium laurate, disproportionated rosin potassium soap, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, sodium α-olefin sulfonate, sodium dioctyl sulfosuccinate, sodium fatty alcohol polyoxyethylene ether sulfate, and alkyl phosphate salts.

[0075] The nonionic emulsifier includes at least one of fatty alcohol polyoxyethylene ether compounds, alkylphenol polyoxyethylene ether compounds, polyoxyethylene fatty acid ester compounds, sorbitol ester compounds, and their polyoxyethylene adducts.

[0076] The reactive emulsifier includes a surfactant containing polymerizable unsaturated double bonds and also having a hydrophilic group. Preferably, the reactive emulsifier is selected from at least one of allyl sulfonate compounds, styrene sulfonate compounds, allyloxyhydroxypropyl sulfonate compounds, and phosphate ester surfactants containing polymerizable double bonds.

[0077] By rationally selecting the type and ratio of emulsifiers, a stable interface layer can be formed on the surface of the obtained third binder particles, thereby improving the absolute value of the zeta potential and the interface stability of the emulsion. This enhances its dispersion and retention ability in the high ionic strength negative electrode slurry system, reduces the tendency of emulsion particles to flocculate and agglomerate, and is beneficial to improving the slurry storage stability, electrode uniformity, and the cycle performance and low-temperature performance of the battery.

[0078] The mass ratio of vinyl aromatic structural units, anionic emulsifiers, nonionic emulsifiers, and stabilizers is (10~50):(0.0001~0.5):(0.0001~0.5):(0.001~5).

[0079] Initiators include at least one of ammonium persulfate (APS), potassium persulfate (KPS), and sodium persulfate.

[0080] As is known to those skilled in the art, the reaction in preparing the third binder is a conventional free radical polymerization reaction, and the specific method and reaction conditions are the free radical polymerization methods commonly used in the prior art, which will not be described in detail in this invention.

[0081] The first binder (e.g., carboxymethyl cellulose (CMC)), conductive agent, and negative electrode active material are added to water for dispersion. Then, a second binder (e.g., Li-PAA) with a neutralization degree ≥80% is introduced, and finally, a third binder is added for dispersion to obtain the negative electrode slurry.

[0082] The mass ratio of the first adhesive, the second adhesive, and the third adhesive is (0.4-1):(1.2-2.5):(1-3).

[0083] The mass ratio of the negative electrode active material, the conductive agent and the third binder is (50-100):(0.6-3):(1-3).

[0084] The above preparation method first establishes the aqueous phase dispersion and rheological basis with a first binder and completes the dispersion of the conductive agent and negative electrode active material. Then, a second binder with x≥80% is introduced to bring the system into the target ionic strength and interfacial electrical environment. Finally, the third binder in the above embodiment is added and homogenized, thereby maintaining the dispersion and rheological stability of the negative electrode slurry under the impact of the highly lithium-ionized second binder. If the third binder is added to the first binder system first, and then the second binder is added, the instantaneous salt impact will cause the negative electrode slurry to flocculate in one go, affecting the coating of the negative electrode slurry.

[0085] An embodiment of the present invention also provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer is formed from the negative electrode slurry described in any of the above embodiments. Applying the optimized negative electrode slurry to the preparation of the negative electrode sheet achieves the transformation of slurry properties into electrode sheet properties, thereby improving the quality and reliability of the negative electrode sheet.

[0086] An embodiment of the present invention also provides a battery, including the negative electrode sheet as described above. Applying the above-described high-performance negative electrode sheet to battery assembly improves the overall performance of the battery by enhancing the performance of the negative electrode sheet, thus solving the problems of poor cycle stability and short lifespan in existing batteries.

[0087] The present invention will be further illustrated by the following examples.

[0088] Specifically, this invention discloses the negative electrode slurry, negative electrode sheet, and battery.

[0089] Example 1 1. Negative electrode slurry 30 parts styrene, 32 parts butadiene, 20 parts methyl methacrylate, 10 parts acrylonitrile, 5 parts acrylic acid, 3 parts acrylamide, 0.5 parts sodium dodecylbenzenesulfonate, 0.2 parts OP-10, and 0.2 parts sodium pyrophosphate were emulsified in an emulsification kettle with high-speed stirring. After emulsification, 20 parts of the emulsion were transferred to a reaction kettle. Nitrogen gas was introduced to purge active oxygen from the reaction kettle and pipelines. 0.3 parts of potassium persulfate initiator were added, and the reaction was carried out at 60°C for 3 hours. Then, the temperature was raised to 85°C, and the remaining emulsion was added while stirring over a period of 3 hours. The reaction was then continued at 85°C for 2 hours. After cooling, the material was discharged to obtain the third binder.

[0090] 0.7 parts of carboxymethyl cellulose (CMC), 0.6 parts of conductive agent super P, and 96.6 parts of graphite were dispersed in water, then 1.8 parts of Li-PAA with a neutralization degree of 80% were introduced, and finally 2.0 parts of a third binder were added for dispersion to obtain the negative electrode slurry.

[0091] 2. Negative electrode plate The negative electrode slurry is coated onto copper foil, dried, and then cut into negative electrode sheets of a fixed size.

[0092] 3. Battery After assembling the negative electrode, separator, and NCM811 positive electrode into a dry cell, a lithium battery is obtained by electrolyte injection. The electrolyte is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 mass ratio, and then LiPF6 is added until the molar concentration is 1.0 mol / L.

[0093] Examples 2-14, Comparative Examples 1-6 Examples 2-14, Comparative Examples 1-6, and Example 1 follow most of the same steps, except that the third adhesive uses the formulation in Table 1, and its performance test results are shown in Table 2.

[0094] Table 1 Table 2 In Table 2, the glass transition temperature of the third adhesive was obtained by differential scanning calorimetry, and the test method is as follows: After drying the emulsion samples into a film, the glass transition temperature (Tg) of the binders in the examples and comparative examples was measured using a differential scanning calorimeter (Shanghai Qunhong Instrument Equipment Co., Ltd., model: DSC-100). The steps included: turning on high-purity nitrogen, setting the nitrogen flow rate to 0.5 L / min to 0.6 L / min, turning on the DSC power, and running the desktop program; setting the temperature to -60℃, holding it at that temperature for 10 min, with a temperature range of -60℃ to 120℃, and a heating rate of 10 K / min; after setting these parameters, placing the prepared binder sample on the heating furnace, closing the furnace protective cover, adding an appropriate amount of liquid nitrogen to the constant temperature container, and running the test when the sample temperature reached -60℃.

[0095] The Zeta potential of the third binder was obtained by testing using the following method: Take a third adhesive sample and dilute it with deionized water to a solid content of 0.1%. Perform the test using a potentiometric analyzer (ELS / laser Doppler electrophoresis) equipped with a disposable or washable electrophoresis tank and a temperature control module (25±0.5℃).

[0096] The gelation rate of the third adhesive was obtained by testing using the following method: Prepare a dry gel film using the third binder and extract it using a Soxhlet extractor or a constant-temperature reflux extraction device; prepare a filter bag / metal mesh (with pore size sufficient to retain the insoluble gel). Weigh the dry sample mass m0 (accurate to 0.1 mg) and place it in the filter bag; reflux extract with toluene for 72 h to ensure complete extraction of soluble components; remove the filter bag and rinse with toluene until no residue adheres; dry at 105℃ or under vacuum to constant weight to obtain the mass of insoluble residue m1; the gelation rate is calculated using the formula (m1 / m0) × 100%.

[0097] Examples 15-21, Comparative Examples 6-12 Examples 15-21, Comparative Examples 6-12 and Example 1 are mostly the same in terms of steps, except that the negative electrode slurry uses the formulation in Table 3.

[0098] Table 3 In Table 3, the slurry fineness (Hegman) was obtained by testing using the following methods: Fineness testing was conducted using a Hegman scraper fineness meter. During testing, an appropriate amount of the slurry to be tested was added to the deeper end of the groove in the scraper fineness meter at 20-30℃. A straight scraper was used to scrape the slurry along its length in a single, uniform, and continuous motion, forming a coating with gradually decreasing thickness on the groove surface. After scraping, the location where obvious particles, coarse streaks, agglomerated streaks, or continuous uneven marks first appeared on the sample surface was visually observed, and the corresponding scale value was recorded as the fineness value of the slurry. Two to three parallel tests were performed, and the average value was taken.

[0099] The negative electrode slurry, negative electrode sheet, and battery prepared in the above embodiments and comparative examples were tested as follows.

[0100] 1. Third binder compatibility stability test: Viscosity test was conducted before and after the addition of the third binder. When the viscosity difference of the negative electrode slurry was within ±1000 cP, it was considered that the compatibility stability of the third binder was high.

[0101] 2. Stability test of negative electrode slurry: After degassing, the prepared negative electrode slurry was allowed to stand for 30 min to eliminate the influence of air bubbles; the initial viscosity η0 was measured at 25℃; the negative electrode slurry was sealed and allowed to stand at 25℃ for 24 h (without stirring); after removal, it was slowly inverted 5 times, and the viscosity η was measured again. 24 ; Calculate viscosity drift ratio: [|(η 24 -η0)| / η0]×100%; The microstructure of the negative electrode slurry will undergo sedimentation / network reconstruction over time, and viscosity drift over time is an important indicator of stability.

[0102] 3. Peel strength test: The coated single-sided negative electrode sheet was tested at 1.6 g / cm². 3 After compaction, the electrode is cut into 20cm long x 3cm wide pieces. 3M double-sided tape is attached to the steel plate, and the coated side of the electrode is fixed to the tape on the steel plate with the coated side facing down. After rolling back and forth 6 times with a 2.5kg roller, a tensile testing machine with a range of 40N is used. The upper plate clamps the copper foil side, and the coating and copper foil are torn apart at a speed of 50mm / min and a tensile test at 180°. The data of the stable tensile section is recorded as the peel strength (N / m).

[0103] 4. Battery DCIR test: Charging: 0.5C constant current and constant voltage to 4.2V, 0.05C cutoff; Discharging: 0.5C constant current discharge to 3.0V, 5 cycles; ① Test temperature: 12℃, 25℃; ② SOC: 10%, 30%, 50%, 70%, 90%, 100%; ③ DCIR test method: 1C discharge for 10S. 5. Cyclic performance test: At 25℃, charge at a constant current of 0.5C to 4.35V, and then charge at a constant voltage to the cutoff current of 0.05C; then discharge at a discharge rate of 1C to 3.0V, and record the initial discharge capacity as Q0; cycle 500 times according to the above charge and discharge mechanism, and record the discharge capacity after 500 cycles as Q1. Then the capacity retention rate of the battery after 500 cycles = (Q1 / Q0) × 100%.

[0104] The test results are shown in Table 4.

[0105] Table 4 As shown in Table 4, the test results of Examples 1 to 21, Comparative Examples 1 to 6, and Comparative Examples 11 to 12 indicate that when the negative electrode slurry contains a third binder and the Zeta potential |Z| of the third binder is in the range of 40-50mV, the negative electrode slurry exhibits high dispersion stability. Furthermore, the negative electrode slurry contains Li-PAA with a neutralization degree x ≥ 80%. The synergistic effect of these two components improves the peel strength of the negative electrode sheet, achieving a balance between strong adhesion and high stability. At the same time, it reduces the internal resistance of the battery and improves the capacity retention rate of the battery.

[0106] The test results from Examples 1, 15 to 21, and Comparative Examples 7 to 10 show that when the mass ratio of carboxymethyl cellulose, lithium-modified polyacrylic acid, and the third binder in the negative electrode slurry is (0.4 to 1): (1.2 to 2.5): (1 to 3), the negative electrode slurry has high stability and a slurry fineness of 15-25 μm, making the conductive network and pore structure more continuous and uniform, reducing local contact resistance and transmission bottlenecks, thereby reducing DCIR and further improving the cycle performance of the battery.

[0107] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode slurry, characterized in that, The negative electrode slurry includes a negative electrode active material, a conductive agent, and a binder. The binder includes a first binder, a second binder, and a third binder. The first binder is selected from water-soluble cellulose polymers and / or their salts. The second binder is an alkali-metallated water-soluble carboxylic acid polymer. The third binder includes a polymer comprising vinyl aromatic structural units and conjugated diene structural units. The neutralization degree of the second binder is x, where x ≥ 80%. The zeta potential of the third adhesive is Z, |Z| = 40~50 mV.

2. The negative electrode slurry according to claim 1, characterized in that, The gelation rate of the third adhesive is less than 0.

01.

3. The negative electrode slurry according to claim 1, characterized in that, The particle size of the third binder meets the following condition: D10 is 50~120nm, D50 is 180~400nm, and D90 is 450~590nm; and / or, The particle size distribution of the third binder is PDI = (D90-D10) / D50, and PDI = 1.2~2.

0.

4. The negative electrode slurry according to claim 1, characterized in that, The viscosity of the negative electrode slurry is 3000~5000 mPa·s; and / or; The viscosity drift rate of the negative electrode slurry after standing at 20~30℃ for 24 hours is ≤7%.

5. The negative electrode slurry according to claim 1, characterized in that, The fineness of the negative electrode slurry is 15-25 μm.

6. The negative electrode slurry according to claim 1, characterized in that, The glass transition temperature of the third adhesive is -30℃ to 30℃.

7. The negative electrode slurry according to any one of claims 1-6, characterized in that, The mass ratio of the first adhesive, the second adhesive, and the third adhesive is (0.4~1):(1.2~2.5):(1~3).

8. The negative electrode slurry according to claim 1, characterized in that, The polymer in the third adhesive further includes acrylate structural units, acrylonitrile structural units, acrylic structural units and acrylamide structural units, and the mass ratio of the vinyl aromatic structural units, conjugated diene structural units, acrylate structural units, acrylonitrile structural units, acrylic structural units and acrylamide structural units is (10~50):(10~60):(10~60):(1~20):(0.1~30):(0.1~30).

9. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer is formed from the negative electrode slurry according to any one of claims 1 to 8.

10. A battery, characterized in that, Includes the negative electrode sheet as described in claim 9.

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