Binder for preparing anode sheet by dry process, anode sheet and preparation method therefor, and secondary battery

Abstract

The present invention provides a binder for preparing an anode sheet by a dry process, an anode sheet and a preparation method therefor, and a secondary battery. By mass fraction, the binder comprises 33-100 wt% of gelatin and 0-67 wt% of polytetrafluoroethylene (PTFE). Compared with a conventional PTFE binder, in the binder for preparing an anode sheet by a dry process provided by the present invention, gelatin is used to replace part or all of PTFE as a binder, and the gelatin has electrochemical stability at a low potential / voltage; thus, the amount of PTFE used is reduced, so that reduction reactions caused by low LUMO orbital thereof can be reduced, thereby ensuring the initial coulombic efficiency of batteries, and solving the problems of low initial coulombic efficiency of batteries and decreased battery capacity due to the use of PTFE as a binder in current preparation of anode sheets by a dry process.

Description

A binder for dry-process anode sheet, anode sheet and preparation method thereof, and secondary battery Technical Field

[0001] The present invention relates to the field of secondary batteries, and in particular to a binder for dry-process anode sheet production, an anode sheet and a preparation method thereof, and a secondary battery. Background Art

[0002] As a core component of secondary batteries, the electrode sheet's manufacturing process determines its quality and directly impacts various battery performance characteristics. Currently, electrode sheet manufacturing processes include wet coating and dry coating, with wet coating being the mainstream. However, wet coating requires solvents, which poses issues such as solvent volatilization, environmental pollution, and residual electrode sheet residue. While the wet coating process is mature and can effectively recover solvents, it places high demands on equipment capabilities and results in high process costs.

[0003] Compared to the wet coating process, the dry process is not only simpler but also avoids solvent volatilization issues. Furthermore, under high temperature and electrolyte conditions, electrodes prepared using the dry process also exhibit better bonding and adhesion, which can improve the overall performance of the battery. However, the self-supporting anode electrode membranes currently produced using the dry process primarily use polytetrafluoroethylene (PTFE) as a binder, and the content is relatively high. PTFE, due to its relatively low LUMO orbital, readily accepts electrons, making it electrochemically unstable in the anode environment. Side reactions are more likely to occur at low voltages, lowering the initial coulombic efficiency and resulting in a decrease in battery capacity.

[0004] In view of this, it is indeed necessary to provide a technical solution to the above problems.

[0005] Summary of the Invention

[0006] One of the purposes of the present invention is to address the deficiencies of the prior art and provide a binder for dry-process anode sheets to solve the problem that the use of polytetrafluoroethylene (PTFE) as a binder in the current dry-process anode sheets leads to low initial coulombic efficiency of the battery and a decrease in battery capacity.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A binder for dry-process anode sheet production comprises, by mass fraction, 33wt% to 100wt% of gelatin and 0wt% to 67wt% of polytetrafluoroethylene.

[0009] A second object of the present invention is to provide an anode sheet, comprising an anode active sheet, wherein the anode active sheet comprises an anode active material and a binder, wherein the binder is the binder for dry-process anode sheet production as described above.

[0010] Preferably, the mass of the binder accounts for 1% to 5% of the mass of the anode active sheet.

[0011] Preferably, the mass of gelatin in the binder accounts for 1% to 4.5% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 0.5% to 2% of the mass of the anode active sheet.

[0012] Preferably, the mass of the anode active material accounts for 95% to 98% of the mass of the anode active sheet; the anode active sheet further includes a conductive agent, and the mass of the conductive agent accounts for 0% to 2% of the mass of the anode active sheet.

[0013] Preferably, the thickness of the anode active sheet is 35-70 μm.

[0014] Preferably, the anode sheet further comprises an anode current collector, and the anode active sheet is disposed on at least one surface of the anode current collector.

[0015] A third object of the present invention is to provide a method for preparing the above-mentioned anode sheet, comprising the following steps:

[0016] The anode active material is first mixed with gelatin, and then polytetrafluoroethylene is added and mixed continuously to obtain a premix;

[0017] The obtained premix is ​​subjected to fiberization treatment at 80° C. to 120° C., and then rolled into a film, and thinned to obtain an anode active sheet, thereby completing the preparation of the anode sheet.

[0018] Preferably, the method for preparing the anode sheet further comprises compounding the obtained anode active sheet with an anode current collector to complete the preparation of the anode sheet.

[0019] The fourth object of the present invention is to provide a secondary battery comprising a cathode sheet, an anode sheet and a separator separated between the cathode sheet and the anode sheet, wherein the anode sheet is the anode sheet described above or an anode sheet prepared by the anode sheet preparation method described above.

[0020] The beneficial effects of the present invention are as follows: compared to the conventionally used polytetrafluoroethylene (PTFE) binder, the dry-process anode sheet binder provided by the present invention uses gelatin to replace part or all of polytetrafluoroethylene as a binder. Gelatin has electrochemical stability at lower potentials / voltages, reduces the amount of polytetrafluoroethylene (PTFE), and can reduce the reduction reaction caused by its relatively low LUMO orbit, thereby ensuring the first coulombic efficiency of the battery, solving the problem that the use of polytetrafluoroethylene (PTFE) as a binder in the current dry-process anode sheet leads to low first coulombic efficiency of the battery and decreased battery capacity. In addition, due to the hydrophobic and oleophobic properties of PTFE, there are problems with electrolyte infiltration, and gelatin contains rich hydrophilic groups, which can be effectively attached to the surface of the electrode material particles, so that the electrode has a more stable material interface and a smaller lithium ion diffusion barrier, thereby ensuring the structural integrity of the electrode material and improving the electrochemical performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG1 is a schematic diagram of an anode active sheet according to Example 1 of the present invention.

[0022] FIG2 is a schematic diagram of an anode active sheet according to Example 2 of the present invention.

[0023] FIG3 is a schematic diagram of an anode active sheet according to Example 3 of the present invention.

[0024] FIG4 is a schematic diagram of the infiltration effect of Comparative Example 1 of the present invention.

[0025] FIG5 is a schematic diagram of the infiltration effect of Example 1 of the present invention.

[0026] FIG6 is a schematic diagram of the wetting effect of Example 5 of the present invention. DETAILED DESCRIPTION

[0027] To make the technical solutions and advantages of the present invention more clear, the present invention and its beneficial effects will be further described in detail below in conjunction with specific implementation methods, but the implementation methods of the present invention are not limited thereto.

[0028] 1. Binder

[0029] The first aspect of the present invention is to provide a binder for dry-process anode sheet production, comprising 33 wt% to 100 wt% gelatin and 0 wt% to 67 wt% polytetrafluoroethylene by mass fraction.

[0030] The dry-process anode sheet production described in this invention refers to solvent-free anode sheet production. Compared to conventional dry-process anode sheet production methods, this invention reduces the polytetrafluoroethylene binder content from 100% to 0% to 67% by weight. This quantitative reduction reduces the reaction of polytetrafluoroethylene in the battery system, thereby reducing its impact on the battery's initial coulombic efficiency. Furthermore, to ensure the electrode sheet's adhesion and the ability to be dry-processed, gelatin, a more electrochemically stable binder, is added as a binder. This replaces the reduced polytetrafluoroethylene to ensure the electrode sheet's adhesion.

[0031] In addition, the added gelatin is a macromolecular polymer formed by cross-linking of multiple amino acids and peptides. It contains rich hydrophilic groups and can effectively adhere to the surface of the anode material particles to ensure the structural integrity of the electrode material. The electrode with the addition of gelatin has a more stable material interface, a smaller lithium ion diffusion energy barrier, and smoother lithium ion transport, which can improve the electrochemical performance of the battery.

[0032] It should be noted that the gelatin binder provided by the present invention is intended for dry-process preparation. In wet-process preparation, if gelatin is also used as a binder, although it is insoluble in water, it will expand in volume when immersed in water and dissolve into a colloid when heated. Adding it to the slurry can seriously affect the slurry's fluidity and viscosity, thereby affecting the slurry's performance. However, if used in a dry process, the fiberization temperature can soften the gel and increase its viscosity during the fiberization process, which in turn facilitates the fiberization process and forms a membrane with uniform thickness and stable structure.

[0033] Specifically, calculated by mass fraction, the gelatin content may be 33wt% to 40wt%, 40wt% to 50wt%, 50wt% to 60wt%, 60wt% to 70wt%, 70wt% to 80wt%, 80wt% to 90wt% or 90wt% to 100wt%, and the corresponding content of gelatin + polytetrafluoroethylene = 100%. The content of polytetrafluoroethylene can be calculated based on the content of gelatin. The specific content of polytetrafluoroethylene may be 0 to 10wt%, 10wt% to 20wt%, 20wt% to 30wt%, 30wt% to 40wt%, 40wt% to 50wt%, 50wt% to 60wt% or 60wt% to 67wt%.

[0034] Controlling the contents of gelatin and polytetrafluoroethylene within the above ranges avoids the inability to effectively reduce the side reactions of polytetrafluoroethylene at the anode due to excessive polytetrafluoroethylene content, and also avoids the inability to obtain a structurally complete anode diaphragm due to insufficient tensile strength of the diaphragm due to insufficient gelatin content.

[0035] Preferably, the composition comprises 33 wt% to 90 wt% of gelatin and 10 wt% to 67 wt% of polytetrafluoroethylene, calculated by mass fraction.

[0036] More preferably, the gelatin content is 33 wt% to 67 wt% and the polytetrafluoroethylene content is 33 wt% to 67 wt% by mass. More preferably, the gelatin content is 50 wt% to 67 wt% and the polytetrafluoroethylene content is 33 wt% to 50 wt% by mass.

[0037] 2. Anode

[0038] The second aspect of the present invention is to provide an anode sheet, including an anode active sheet, wherein the anode active sheet includes an anode active material and a binder, and the binder is the binder for dry-process anode sheet production as described above.

[0039] In some embodiments, the binder comprises 1% to 5% of the mass of the anode active sheet. Specifically, the binder may comprise 1% to 2%, 2% to 3%, 3% to 4%, or 4% to 5% of the mass of the anode active sheet. Preferably, the binder comprises 2% to 4% of the mass of the anode active sheet. Controlling the binder content within the above range ensures sufficient bonding performance without affecting the proportion of the anode active material due to excessive content.

[0040] In some embodiments, the mass of gelatin in the binder accounts for 1% to 4.5% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 0.5% to 2% of the mass of the anode active sheet. Specifically, the mass of gelatin in the binder can be 1% to 1.5%, 1.5% to 2%, 2% to 2.5%, 2.4% to 3%, 3% to 3.5%, 3.5% to 4%, or 4% to 4.5% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene can be 0.5% to 1% or 1% to 2% of the mass of the anode active sheet.

[0041] In the anode active sheet, the gelatin content is controlled between 1% and 4.5%. On the one hand, this avoids the problem that the gelatin content is too low, which results in insufficient tensile strength of the anode diaphragm and inability to form a complete anode sheet; on the other hand, it also avoids the problem that the gelatin content is too high, which relatively leads to a decrease in the mass fraction of the anode active material and affects the battery energy density.

[0042] At the same time, based on the aforementioned gelatin content, the polytetrafluoroethylene content is adjusted between 0.5% and 2%, which can avoid the problem of reduced initial coulombic efficiency of the battery due to excessive polytetrafluoroethylene content. Furthermore, the inventors have discovered that the polytetrafluoroethylene content should be at least 0.5%. If the polytetrafluoroethylene content is too low, it will significantly affect the film quality of the anode sheet, and in severe cases, it may even prevent the anode sheet from forming a film. Therefore, the polytetrafluoroethylene content is adjusted between 0.5% and 2%.

[0043] Preferably, the mass of gelatin in the binder accounts for 1% to 3% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 1% to 2% of the mass of the anode active sheet.

[0044] Furthermore, while ensuring the membrane quality meets requirements, the higher the active material mass fraction, the better. Therefore, preferably, the polytetrafluoroethylene content plus the gelatin content is ≤ 3% of the anode active sheet content. For example, when the gelatin content is 2%, the polytetrafluoroethylene content can be up to 1%; when the gelatin content is 1%, the polytetrafluoroethylene content can be up to 2%.

[0045] Preferably, the mass of gelatin in the binder accounts for 1% to 2% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 1% to 2% of the mass of the anode active sheet. More preferably, the mass of gelatin in the binder accounts for 2% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 1% of the mass of the anode active sheet.

[0046] In some embodiments, the mass of the anode active material accounts for 95% to 98% of the mass of the anode active sheet, specifically, the mass of the anode active material accounts for 95% to 96%, 96% to 97%, or 97% to 98% of the mass of the anode active sheet.

[0047] In some embodiments, the anode active material may include, but is not limited to, one or more of graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbeads, silicon-based materials, tin-based materials, lithium titanate, or other metals capable of forming alloys with lithium. The graphite may be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; and the tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys.

[0048] In some embodiments, the anode active sheet further includes a conductive agent, the mass of which accounts for 0% to 2% of the mass of the anode active sheet. When the conductive agent is also included, while maintaining a constant binder content, the ratio of the anode active material can be adjusted so that the ratio of the anode active material, the conductive agent, and the binder is 100%. Setting the conductive agent within the range of 0% to 2% avoids affecting the ratio of the anode active material due to excessive content, while further improving the conductivity of the anode.

[0049] In some embodiments, the conductive agent is at least one of conductive carbon black SP, Ketjen black, conductive graphite, carbon fiber, carbon nanotube, graphene, and superconducting carbon black. Preferably, the conductive agent is conductive carbon black SP.

[0050] In some embodiments, the thickness of the anode active sheet is 35 to 70 μm. Specifically, the thickness of the anode active sheet can be 35 to 40 μm, 40 to 45 μm, 45 to 50 μm, 50 to 55 μm, 55 to 60 μm, 60 to 65 μm, or 65 to 70 μm. Controlling the thickness of the anode active sheet within the above range, on the one hand, avoids the risk of holes due to the membrane thickness being too thin, which poses a safety hazard; on the other hand, it avoids the membrane being too thick, which makes it difficult to be well matched with the existing wet-process cathode sheet and battery structure, and the problem of electrolyte infiltration, thereby affecting the electrochemical performance of the battery. Preferably, based on the above thickness range, the anode sheet obtained by the present invention can control the thickness of the anode active sheet to be less than 50 μm and still have good membrane surface performance. Relatively speaking, the lower the thickness, the more conducive it is to electrolyte infiltration, and the more significant the improvement in the electrochemical performance of the battery.

[0051] In some embodiments, the anode sheet further includes an anode current collector, and the anode active sheet is disposed on at least one surface of the anode current collector.

[0052] Specifically, the anode current collector is generally a structure or part that collects current. The anode current collector can be any material suitable for use as an anode current collector for lithium-ion batteries in the art. For example, the anode current collector can include but is not limited to metal foil, and more specifically can include but is not limited to copper foil.

[0053] The third aspect of the present invention is to provide a method for preparing the anode sheet, comprising the following steps:

[0054] The anode active material is first mixed with gelatin, and then polytetrafluoroethylene is added and mixed continuously to obtain a premix;

[0055] The obtained premix is ​​subjected to fiberization treatment at 80° C. to 120° C., and then rolled into a film, and thinned to obtain an anode active sheet, thereby completing the preparation of the anode sheet.

[0056] During the preparation process, the anode active material is first mixed with gelatin, and then polytetrafluoroethylene is added and continued to mix, which can ensure the tensile properties of the obtained anode sheet.

[0057] During the fiberization process, the temperature is controlled between 80°C and 120°C, avoiding the problem of insufficient fiberization due to too low a temperature, and also avoiding the problem of too high a temperature causing the anode active sheet to become too hard and unfavorable for subsequent thinning. This fiberization temperature range is more compatible with the binder described in the present invention, resulting in anode active sheets with a better film surface at a lower thickness.

[0058] In some embodiments, the method for preparing the anode sheet further comprises laminating the obtained anode active sheet with the anode current collector to complete the preparation of the anode sheet. Specifically, the lamination can be performed by rolling, or other lamination methods can be used to combine the two to form a whole.

[0059] 3. Secondary batteries

[0060] The fourth aspect of the present invention is to provide a secondary battery comprising a cathode sheet, an anode sheet and a separator separated between the cathode sheet and the anode sheet, wherein the anode sheet is the anode sheet described above or an anode sheet prepared by the anode sheet preparation method described above.

[0061] In some embodiments, the cathode sheet includes a cathode current collector and a cathode active layer coated on at least one surface of the cathode current collector, the cathode active layer further includes a cathode active material, and the cathode active material can be, but is not limited to, a chemical formula such as Li x Ni h Co y M z O 2-d N d (wherein 0.95≤x≤1.2, h>0, y≥0, z≥0, and h+y+z=1, 0≤d≤1, M is selected from a combination of one or more of Mn and Al, and N is selected from a combination of one or more of F, P, and S), the cathode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The cathode active material may also be subjected to a modification treatment. The method for modifying the cathode active material should be known to those skilled in the art. For example, the cathode active material may be modified by coating, doping, etc. The materials used for the modification treatment may include but are not limited to a combination of one or more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The cathode active material may also be at least one of a sodium transition metal oxide, a sodium polyanion compound, and a sodium Prussian blue analogue; the sodium-based cathode active material is at least one of a sodium transition metal oxide, a sodium polyanion compound, and a sodium Prussian blue analogue; wherein the sodium transition metal oxide NaCoO2, NaFeO2, NaNiO2, NaNiFeMnO2, NaCuFeMnO2, NaNi 0.5Mn 0.5 O2; the polyanion compound of sodium may be any one of Na3V2(PO4)3, NaFePO4, Na2Fe2(SO4)3, Na2Fe2P2O7; the Prussian blue analogue of sodium may be any one of NaFeFe(CN)6, Na2CoFe(CN)6, Na2NiFe(CN)6.

[0062] The cathode current collector can be any material suitable for use as a cathode current collector for lithium-ion batteries in the art. For example, the cathode current collector can include but is not limited to metal foil, and more specifically can include but is not limited to aluminum foil.

[0063] The separator can be made of various materials suitable for lithium-ion battery separators in the art, for example, it can be a combination of one or more materials including but not limited to polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fiber.

[0064] The secondary battery also includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt, and additives. The electrolyte lithium salt can be LiPF6 and / or LiBOB, as used in high-temperature electrolytes; can also be at least one of LiBF4, LiBOB, and LiPF6, as used in low-temperature electrolytes; can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI, as used in overcharge-preventing electrolytes; or can be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; a chain carbonate, including DFC, DMC, or EMC; or a carboxylic acid ester, including MF, MA, EA, and MP. Additives include, but are not limited to, at least one of a film-forming additive, a conductive additive, a flame retardant additive, an overcharge prevention additive, an additive for controlling the H2O and HF content in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

[0065] To make the technical solutions and advantages of the present invention more clear, the present invention and its beneficial effects will be further described in detail below with reference to specific embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0066] Example 1

[0067] A binder for dry-process anode sheet production comprises, by mass fraction, 67 wt% gelatin and 33 wt% polytetrafluoroethylene.

[0068] The binder is used in an anode sheet, which includes an anode active sheet and an anode current collector. The anode active sheet is arranged on both surfaces of the anode current collector. The anode active sheet includes an anode active material and the above-mentioned binder for dry-process anode sheet production, wherein the mass of gelatin accounts for 2% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 1% of the mass of the anode active sheet (taking the binder as a whole, the mass of gelatin accounts for 67wt% and the mass of polytetrafluoroethylene accounts for 33wt% after conversion); the anode active material is graphite Gr, and its mass accounts for 97% of the mass of the anode active sheet.

[0069] The preparation method of the anode sheet is as follows:

[0070] 1) using a mixer to mix the anode active material and gelatin, and then adding polytetrafluoroethylene and continuing to mix to obtain a premix;

[0071] 2) The obtained premix is ​​subjected to a fiberizing treatment at 100° C., and then rolled into a film, and thinned to a thickness of 45 μm using a double-roll mill to obtain an anode active sheet;

[0072] 3) Compounding the obtained anode active sheet with copper foil to obtain an anode sheet.

[0073] Example 2

[0074] The difference from Example 1 is the thickness of the anode active sheet during the preparation of the anode sheet. The thickness of this example is reduced to 30 μm.

[0075] The rest is the same as in Example 1 and will not be described again here.

[0076] Example 3

[0077] The difference from Example 1 is the thickness of the anode active sheet during the preparation of the anode sheet. The thickness of this example is reduced to 80 μm.

[0078] The rest is the same as in Example 1 and will not be described again here.

[0079] The anode sheets obtained in Examples 1 to 3 above were calculated based on a CB value of 1.05 for the corresponding cathode single-surface density. The results are shown in Table 1 and Figures 1 to 3 below.

[0080] Table 1

[0081] The experimental results above show that, as shown in Figure 2, when the thickness of the anode active sheet is reduced to less than 30 μm, the thinning thickness is too low, resulting in membrane holes and making it impossible to produce a complete anode sheet. As shown in Figure 3, when the thickness of the anode active sheet is reduced to more than 80 μm, the thinning thickness is too high, resulting in a higher cathode areal density. Cathode sheets produced at this density often have a low process yield. Therefore, thinning thicknesses above 80 μm are not suitable for conventional batteries.

[0082] The inventors have conducted numerous tests and verified that a thinned anode active sheet thickness of 35 to 70 μm can meet applicable requirements, and the thickness can be selected based on actual needs. In particular, a thinned anode active sheet thickness of 45 μm, as shown in FIG1 , is relatively low but still meets the application requirements, and the anode sheet exhibits relatively excellent performance.

[0083] Therefore, the following Examples 4 to 10 are based on the thinning thickness of the anode active sheet being 45 μm, and other embodiments are listed to further describe the features of the present invention in detail.

[0084] Example 4

[0085] The difference from Example 1 is the content of the binder. In this embodiment, the mass of gelatin accounts for 1.5% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 1.5% of the mass of the anode active sheet (that is, based on the binder as a whole, the mass of gelatin accounts for 50wt%, and the mass of polytetrafluoroethylene accounts for 50wt%); the anode active material is graphite, and its mass accounts for 97% of the mass of the anode active sheet.

[0086] The rest is the same as in Example 1 and will not be described again here.

[0087] Example 5

[0088] The difference from Example 1 is the content of the binder. In this embodiment, the mass of gelatin accounts for 1% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 2% of the mass of the anode active sheet (that is, based on the binder as a whole, the mass of gelatin accounts for 33wt%, and the mass of polytetrafluoroethylene accounts for 67wt%); the anode active material is graphite, and its mass accounts for 97% of the mass of the anode active sheet.

[0089] The rest is the same as in Example 1 and will not be described again here.

[0090] Example 6

[0091] The difference from Example 1 is the content of the binder. In this embodiment, the mass of gelatin accounts for 2.5% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 0.5% of the mass of the anode active sheet (that is, based on the binder as a whole, the mass of gelatin accounts for 83.3wt%, and the mass of polytetrafluoroethylene accounts for 16.7wt%). The anode active material is graphite, and its mass accounts for 97% of the mass of the anode active sheet.

[0092] The rest is the same as in Example 1 and will not be described again here.

[0093] Example 7

[0094] The difference from Example 1 is the content of the binder. In this embodiment, the mass of gelatin accounts for 1.5% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 0.5% of the mass of the anode active sheet (that is, based on the binder as a whole, the mass of gelatin accounts for 75wt%, and the mass of polytetrafluoroethylene accounts for 25wt%). The anode active material is graphite, and its mass accounts for 98% of the mass of the anode active sheet.

[0095] The rest is the same as in Example 1 and will not be described again here.

[0096] Example 8

[0097] The difference from Example 1 is the content of the binder. In this embodiment, the mass of gelatin accounts for 2% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 2% of the mass of the anode active sheet (that is, based on the binder as a whole, the mass of gelatin accounts for 50wt%, and the mass of polytetrafluoroethylene accounts for 50wt%); the anode active material is graphite, and its mass accounts for 96% of the mass of the anode active sheet.

[0098] The rest is the same as in Example 1 and will not be described again here.

[0099] Example 9

[0100] The difference from Example 1 is the content of the binder. In this embodiment, the mass of gelatin accounts for 4.5% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 0.5% of the mass of the anode active sheet (that is, based on the binder as a whole, the mass of gelatin accounts for 90wt% and the mass of polytetrafluoroethylene accounts for 10wt%). The anode active material is graphite, and its mass accounts for 95% of the mass of the anode active sheet.

[0101] The rest is the same as in Example 1 and will not be described again here.

[0102] Example 10

[0103] The difference from Example 1 is the content of the binder. In this embodiment, the mass of gelatin accounts for 1% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 1% of the mass of the anode active sheet (that is, based on the binder as a whole, the mass of gelatin accounts for 50wt%, and the mass of polytetrafluoroethylene accounts for 50wt%); the anode active material is graphite, and its mass accounts for 98% of the mass of the anode active sheet.

[0104] The rest is the same as in Example 1 and will not be described again here.

[0105] Comparative Example 1

[0106] The difference from Example 1 is the content of the binder. In this comparative example, the mass of polytetrafluoroethylene accounts for 3% of the mass of the anode active sheet (that is, based on the binder as a whole, the mass of polytetrafluoroethylene after conversion accounts for 100%); the anode active material is graphite, and its mass accounts for 97% of the mass of the anode active sheet.

[0107] The rest is the same as in Example 1 and will not be described again here.

[0108] Comparative Example 2

[0109] The difference from Example 1 is the preparation method of the premix. In this comparative example, the anode active material, gelatin and polytetrafluoroethylene are mixed together using a mixer to obtain the premix.

[0110] The rest is the same as in Example 1 and will not be described again here.

[0111] Comparative Example 3

[0112] The difference from Example 1 is the preparation method of the premix. In this comparative example, gelatin and polytetrafluoroethylene are first mixed with a mixer, and then the anode active material is added and mixed continuously to obtain the premix.

[0113] The rest is the same as in Example 1 and will not be described again here.

[0114] The above Examples 1 to 10 and Comparative Example 1 are sorted out as shown in Table 1.

[0115] Table 1

[0116] The electrolyte wetting performance of the anode sheets obtained in Examples 1 and 5 and Comparative Example 1 was tested. A drop of electrolyte was dripped onto the surface of a 3mm x 3mm electrode sheet using a rubber-tipped dropper. The differences in electrolyte wetting rates were observed over a 5-minute period. The effect diagrams in Figures 4-6 demonstrate that replacing at least a portion of the polytetrafluoroethylene binder with gelatin improves electrolyte wetting performance. This demonstrates that the combined use of gelatin and PTFE as a binder in the present invention effectively improves the hydrophobic and oleophobic properties of PTFE, resolving the electrolyte wetting issues associated with using PTFE alone as a binder.

[0117] In addition, the anode sheets obtained in Examples 1, 4 to 10 and Comparative Examples 1 to 3 were also used in lithium-ion batteries. The preparation method is as follows:

[0118] 1) Cathode sheet: After mixing the active material LiCoO2 (manufacturer: Xiamen Tungsten New Energy): conductive agent Super P: polyvinylidene fluoride PVDF in a weight ratio of 96.7:1.8:1.5, add it to N-methylpyrrolidone (NMP) and mix it evenly to prepare a lithium-ion battery positive electrode slurry; the positive electrode slurry is coated on both sides of the current collector aluminum foil, dried at 85°C and cold pressed, and then trimmed, cut and slit, and dried under vacuum conditions at 85°C for 4 hours. The tabs are welded to prepare a lithium-ion battery cathode sheet.

[0119] 2) Use polyethylene double-sided coated alumina ceramic as the diaphragm (Shanghai Enjie New Material Technology Co., Ltd. is selected).

[0120] 3) The cathode sheet, anode sheet, and separator are wound or stacked to form a roll core or bare cell, which is then sealed in an aluminum-plastic film and injected with electrolyte. The cell is then allowed to stand to form a soft-pack lithium-ion battery.

[0121] The obtained lithium-ion battery was subjected to a cycle performance test.

[0122] The cycle test method is: charging: 3.0C CC CV 4.25V, 2.0C CC CV 4.35V, 1.5C 4.50V, CV to 0.05C; discharging: 0.7C 3.0V.

[0123] The test results are shown in Table 2 below.

[0124] Table 2

[0125] From the comparison of the results in Table 2 above, it can be seen that compared with Comparative Example 1, the present invention can improve the initial coulombic efficiency of the battery and ensure the capacity retention rate after long cycles after replacing at least a part of the polytetrafluoroethylene binder with gelatin.

[0126] Furthermore, a comparison of Examples 1, 4, and 5 shows that, when the total amount of binder added is constant, increasing the gelatin content, that is, reducing the polytetrafluoroethylene content, is more conducive to improving the battery's initial coulombic efficiency. However, the results of Example 6 show that a higher gelatin content is not necessarily better; combining gelatin with a portion of polytetrafluoroethylene as a binder is more effective in improving anode sheet performance.

[0127] In addition, it can be seen from the comparison of Examples 1 and 7 to 9 that controlling the total amount of binder within a certain range to avoid excessively high binder content affecting the proportion of active materials, or to avoid excessively low binder content reducing the bonding performance of the anode sheet system, is more conducive to improving the battery's initial coulombic efficiency and long-cycle capacity retention rate.

[0128] In summary, the binder provided by the present invention, by replacing at least a portion of the polytetrafluoroethylene binder with gelatin, solves the problem that the use of polytetrafluoroethylene (PTFE) as a binder in the current dry process for making anode sheets leads to low initial coulombic efficiency of the battery and thus reduced battery capacity.

[0129] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention fall within the scope of protection of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A binder for dry-process anode sheets, characterized in that, By mass fraction, it includes 33 wt% to 100 wt% of gelatin and 0 wt% to 67 wt% of polytetrafluoroethylene.

2. An anode sheet, characterized in that, It includes an anode active sheet, and the anode active sheet includes an anode active material and a binder, and the binder is the binder for dry-processed anode sheets described in claim 1.

3. The anode sheet according to claim 2, characterized in that, The mass of the binder accounts for 1% to 5% of the mass of the anode active sheet.

4. The anode sheet according to claim 3, characterized in that, The mass of gelatin in the binder accounts for 1% to 4.5% of the mass of the anode active sheet, and the mass of polytetrafluoroethylene accounts for 0.5% to 2% of the mass of the anode active sheet.

5. The anode sheet according to any one of claims 2 to 4, characterized in that, The mass of the anode active material accounts for 95% to 98% of the mass of the anode active sheet; the anode active sheet further includes a conductive agent, and the mass of the conductive agent accounts for 0% to 2% of the mass of the anode active sheet.

6. The anode sheet according to claim 2, characterized in that, The thickness of the anode active sheet is 35 to 70 μm.

7. The anode sheet according to claim 2, characterized in that, It further includes an anode current collector, and the anode active sheet is disposed on at least one surface of the anode current collector.

8. A method for preparing the anode sheet according to any one of claims 2 to 7, characterized in that, It includes the following steps: First mix the anode active material with gelatin, and then add polytetrafluoroethylene and continue to mix to obtain a premix. Fibrillate the obtained premix at 80°C to 120°C, then roll it into a film and thin it to obtain the anode active sheet, thus completing the preparation of the anode sheet.

9. The method for preparing the anode sheet according to claim 8, wherein, It further includes compounding the obtained anode active sheet with the anode current collector to complete the preparation of the anode sheet.

10. A secondary battery, comprising a cathode sheet, an anode sheet, and a separator interposed between the cathode sheet and the anode sheet, characterized in that, The anode sheet is the anode sheet described in any one of claims 2 to 7 or the anode sheet prepared by the preparation method of the anode sheet described in claim 8 or 9.

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