Negative pole piece, preparation method thereof and lithium ion battery

Abstract

The invention provides a negative pole piece, a preparation method thereof and a lithium ion battery. The negative pole piece comprises a negative current collector and a negative active substance layer arranged on at least one side of the negative current collector, the negative active substance layer comprises a negative active material and an additive, the negative active material comprises a silicon-based active substance, and the additive comprises bacterial cellulose. The bacterial cellulose is added into the negative electrode active material layer, so that the volume expansion phenomenon among silicon-based active material particles is effectively relieved, the structural strength of the negative electrode plate is improved, the expansion rate of the negative electrode plate is reduced, and meanwhile, the cycle performance of the lithium ion battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and particularly relates to a negative electrode sheet, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] At present, the negative electrode material is the carrier of metal ions and electrons during the charging process of lithium-ion batteries, and plays an important role in energy storage and release. As one of the main factors affecting the energy density of lithium-ion batteries, the future research and development directions mainly focus on improving capacity, energy density, cycle performance, etc.

[0003] In recent years, silicon-based negative electrode materials have been considered as one of the ideal materials for improving the energy density of lithium-ion batteries due to their high theoretical specific capacity (4200 mAh / g). However, as the negative electrode material of lithium-ion batteries, silicon-based materials will generate large volume expansion stress during charge and discharge processes, resulting in the cracking and pulverization of silicon particles, thereby affecting the stability of the electrode structure of lithium-ion batteries and the cycle performance of the batteries. To address this problem, it is usually necessary to add a binder to the silicon-based negative electrode material to provide the necessary cohesive force, thereby restraining the volume expansion of silicon particles and maintaining the integrity of the electrode structure. However, too high a content of the binder will reduce the loading capacity of the active material and increase the impedance of the negative electrode sheet, thereby deteriorating the electrical performance of the lithium-ion battery.

[0004] Therefore, there is an urgent need to develop a negative electrode sheet that can improve the volume expansion stress of silicon-based negative electrode materials, ensure its stable electrode structure and excellent battery cycle performance, so as to solve the above problems. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a negative electrode sheet, a preparation method thereof, and a lithium-ion battery. By adding bacterial cellulose to the negative electrode active material layer, the present invention not only effectively alleviates the volume expansion phenomenon between silicon-based active material particles, but also facilitates the rapid transmission of ions and electrons, thereby comprehensively improving the cycle performance and rate performance of lithium-ion batteries.

[0006] To achieve the purpose of this invention, the following technical solutions are adopted:

[0007] In the first aspect, the present invention provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and an additive, the negative electrode active material includes a silicon-based active material, and the additive includes bacterial cellulose.

[0008] Preferably, based on the total mass of the materials of the negative electrode active material layer being 100%, the mass percentage content of the bacterial cellulose is 0.5%-2%.

[0009] Preferably, the surface of the bacterial cellulose has hydroxyl groups, and the content of the hydroxyl groups is 3×10 4 -1×10 5 pieces.

[0010] Preferably, the bacterial cellulose has a three-dimensional network structure.

[0011] Preferably, the length of the bacterial cellulose is 20 μm - 200 μm, the diameter of the bacterial cellulose is 50 nm - 100 nm, and the aspect ratio of the bacterial cellulose is ≥200.

[0012] Preferably, the bacterial cellulose satisfies at least one or more of the following conditions:

[0013] a. The degree of polymerization of the bacterial cellulose is 15,000 - 20,000;

[0014] b. The tensile strength of the bacterial cellulose is 200 MPa - 300 MPa;

[0015] c. The Young's modulus of the bacterial cellulose is 15 GPa - 35 GPa.

[0016] Preferably, the negative electrode active material layer further includes a conductive agent and a binder.

[0017] Preferably, based on the total mass of the materials of the negative electrode active material layer being 100%, the mass percentage content of the negative electrode active material is 91.8% - 93.3%, and the mass percentage content of the binder is 4% - 6%.

[0018] In a second aspect, the present invention provides a method for preparing the negative electrode plate described in the first aspect, and the method includes the following steps:

[0019] Mix the negative electrode active material, the conductive agent, the binder, and the additive to prepare a slurry for the negative electrode active material layer, and coat the slurry for the negative electrode active material layer on at least one side of the negative electrode current collector, and obtain the negative electrode plate after drying.

[0020] In a third aspect, the present invention provides a lithium-ion battery, and the lithium-ion battery includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte, and the negative electrode plate is the negative electrode plate described in the first aspect or the negative electrode plate prepared according to the method for preparing the negative electrode plate in the second aspect.

[0021] The numerical ranges described in the present invention not only include the point values exemplified above, but also include any point values between the above numerical ranges not exemplified. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the ranges.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention provides a negative electrode plate, which has the following advantages by introducing bacterial cellulose additive into the negative active material layer:

[0024] The bacterial cellulose introduced in the present invention has hydroxyl (-OH) groups on its surface, which can form hydrogen bonds with silicon dioxide (SiO x ) and silanol (-Si-OH) groups on the surface of the silicon-based active material, enhancing the binding force between the silicon-based active materials and the acting force between the silicon-based active material and the negative electrode current collector. Moreover, the bacterial cellulose has a three-dimensional network structure, which can further reduce the situation that the silicon-based active material falls off from the current collector due to the expansion stress during the charge and discharge process of the battery, enhancing the stability of the electrode structure. At the same time, the three-dimensional network structure provides a large number of pores, which can serve as the transmission channels for metal ions, thereby improving the rate performance of the lithium-ion battery. In addition, the bacterial cellulose in the present invention has an ultra-high aspect ratio, which endows it with excellent mechanical properties, making the negative electrode plate provided by the present invention have good mechanical stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is the scanning electron microscope image of bacterial cellulose.

[0026] Figure 2 is the cross-sectional scanning electron microscope image of the negative electrode plate provided in Example 1.

[0027] Figure 3 is the cross-sectional scanning electron microscope image of the negative electrode plate provided in Comparative Example 1.

[0028] Figure 4 is the comparison chart of the cycle performance of the lithium-ion batteries provided in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The technical solution of the present invention will be further described below by combining the accompanying drawings and specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations to the present invention.

[0030] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0031] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meanings as commonly understood by those skilled in the technical field to which the present invention pertains; the terms used in the specific embodiments are only for the purpose of describing specific embodiments and are not intended to limit the present invention; the terms "comprising" and "having" and any variations thereof in the description of the specification, claims and the above drawings of the present invention are intended to cover non-exclusive inclusion.

[0032] In the description of the specific embodiments of the present invention, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity, specific order or primary-secondary relationship of the indicated technical features. In the description of the embodiments of the present invention, the meaning of "a plurality of" is more than two, unless otherwise specifically defined.

[0033] Referring to "embodiments" in the present invention means that the specific features, structures or characteristics described in connection with the embodiments can be included in at least one embodiment of the present invention. The phrase appears at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in the present invention can be combined with other embodiments.

[0034] In the description of the embodiments of the present invention, the term "and / or" is only a description of the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in the present invention generally represents an "or" relationship between the associated objects before and after.

[0035] Throughout the present invention, numerical values represent approximate measures or limits of ranges, covering minor deviations from a given value and embodiments having about the mentioned value and embodiments having the exact value mentioned. Except for the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., quantities or conditions) in this specification (including the appended claims) should be understood to be modified in all cases by the term "about", whether or not "about" actually appears before the numerical value. "About" indicates that the stated numerical value allows for some minor imprecision (reasonably close to the exact value of the stated value; approximately or reasonably close to the stated value; almost). If the imprecision provided by "about" is not otherwise understood in the art in this ordinary sense, then "about" as used in the present invention indicates at least the variations that can be produced by ordinary methods of measuring and using such parameters. For example, "about" can include variations of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in some aspects, optionally less than or equal to 0.1%.

[0036] In addition, the disclosure of a range includes the disclosure of all values within the entire range and further divided ranges, including the endpoints and sub-ranges given for these ranges.

[0037] In one embodiment of the present invention, a negative electrode plate is provided. The negative electrode plate includes a negative electrode current collector and a negative electrode active material layer provided on at least one side of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and an additive. The negative electrode active material includes a silicon-based active material, and the additive includes bacterial cellulose.

[0038] Specifically, the present invention places no particular limitation on the material of the negative electrode current collector, as long as it has conductivity and does not cause chemical changes in the battery. Exemplarily, the negative electrode current collector includes, but is not limited to, any one of copper, nickel, or stainless steel. For example, the negative electrode current collector is a copper foil.

[0039] Specifically, the present invention places no particular limitation on the shape of the negative electrode current collector. Exemplarily, the shape of the negative electrode current collector includes, but is not limited to, metal foil, metal grid, metal mesh, or metal foam, etc.

[0040] Specifically, the silicon-based active material in the present invention can be selected according to actual production needs. Exemplarily, it includes single crystal silicon particles, silicon powder, silicon oxide materials, or silicon-carbon materials, etc.

[0041] It can be understood that the negative electrode active material of the present invention uses a silicon-based active material with a high theoretical specific capacity. However, due to the problem of large volume expansion during charge and discharge, and the problem of the need to add a high content of binder to inhibit the adverse effects caused by the volume expansion between silicon-based material particles, therefore, while using the silicon-based active material, the present invention uses bacterial cellulose as an additive. Bacterial cellulose has the advantages of good mechanical properties, rich hydroxyl groups, and a three-dimensional network structure, which helps to improve the metal ion conduction and electron conduction properties of the silicon-based negative electrode material, and reduces the structural damage caused by volume expansion of the silicon-based active material during charge and discharge, thereby improving the cycle stability of the lithium-ion battery.

[0042] In one embodiment, based on the total mass of the materials of the negative electrode active material layer being 100%, the mass percentage content of bacterial cellulose is 0.5% - 2%.

[0043] Specifically, the mass percentage content of bacterial cellulose can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, etc. The above values are only examples and are not limiting.

[0044] It can be understood that if the cellulose content is too high, it is easy to have too high viscosity, which will lead to uneven dispersion of the silicon-based active material. If the cellulose content is too low, it cannot bond and restrain the silicon-based active material, which will affect the expansion rate of the negative electrode sheet.

[0045] In one embodiment, the surface of bacterial cellulose has hydroxyl groups, and the content of hydroxyl groups is 3×10 4 -1×10 5 pieces.

[0046] Specifically, the content of hydroxyl groups can be 3×10 4 , 4×10 4 , 5×10 4 pieces, 6×10 4 pieces, 7×10 4 pieces, 8×10 4 pieces, 9×10 4 pieces, 1×10 5 pieces, etc. The above values are only examples and are not limiting.

[0047] It can be understood that bacterial cellulose has hydroxyl (-OH) groups, including free hydroxyl groups on the surface that have not formed hydrogen bonds and hydroxyl groups that have formed hydrogen bonds inside the bacterial cellulose polymer. It should be noted that the hydroxyl group content here refers to the total content of free hydroxyl groups on the surface of bacterial cellulose that have not formed hydrogen bonds.

[0048] It can be understood that the free hydroxyl groups on the surface of bacterial cellulose can form hydrogen bonds with the silica (SiO x ) and silanol (-Si-OH) groups on the surface of the silicon-based active material. Therefore, the binding force between the silicon-based active materials and the force between the silicon-based active material and the negative current collector can be enhanced, and the situation where the silicon-based active material falls off from the current collector due to expansion stress during the charge and discharge of the battery can be reduced, enhancing the stability of the electrode structure.

[0049] It can be understood that it is necessary to control the hydroxyl content on the surface of bacterial cellulose to be sufficient so that it can form hydrogen bonds with the silicon-based active material. However, if the hydroxyl content is too high, swelling is likely to occur. Therefore, it is necessary to control the hydroxyl content on the surface of bacterial cellulose within the above range, which can form a large number of hydrogen bonds with the silicon-based active material, which is very beneficial for maintaining the stability of the electrode structure.

[0050] In one embodiment, the bacterial cellulose has a three-dimensional network structure.

[0051] It can be understood that the bacterial cellulose in the present invention has a developed three-dimensional network structure, and the silicon-based active material can be bound and entangled in the network structure, reducing the situation where it falls off from the negative current collector during charge and discharge due to expansion, thereby preventing the sudden drop in the battery's electrical performance caused by its falling off, further ensuring the stability of the electrode sheet structure. This three-dimensional network structure has a high porosity, providing a transmission channel for lithium ions, which has a certain promoting effect on the rate performance of the battery. This three-dimensional network structure can also enhance the electron flow ability and uniformity by forming a cross-linked network with the conductive agent. This three-dimensional network structure can also make the binder distribute more evenly, helping to improve the binding performance of the binder and stabilizing the silicon-based active material particles, further reducing the expansion rate of the negative electrode sheet.

[0052] In one embodiment, the length of the bacterial cellulose is 20 μm - 200 μm, the diameter of the bacterial cellulose is 50 nm - 100 nm, and the aspect ratio of the bacterial cellulose is ≥200.

[0053] Specifically, the length of the bacterial cellulose can be 20 μm, 30 μm, 50 μm, 80 μm, 100 μm, 120 μm, 150 μm, 180 μm, 200 μm, etc. The diameter of the bacterial cellulose can be 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, etc. The above values are only examples and are not limiting.

[0054] It can be understood that if the length of bacterial cellulose is too short, it is not conducive to providing buffering for the expansion of the silicon-based active material during expansion, affecting the expansion rate of the battery. If the length of bacterial cellulose is too long, it is not conducive to its arrangement and dispersion in the electrode sheet, affecting the cycle performance of the battery.

[0055] Specifically, the aspect ratio of bacterial cellulose can be 200, 300, 500, 800, 1000, 1500, 2000, 3000, 4000, etc. The above data are only examples and not limitations.

[0056] It can be understood that the ultra-high aspect ratio of bacterial cellulose in the present invention makes its mechanical strength far exceed that of commonly used cellulose materials such as carboxymethyl cellulose. It has excellent mechanical properties. When the aspect ratio of bacterial cellulose is too small, bacterial cellulose will present a "rod-like" morphology and cannot play a binding role during the charging and discharging process when the silicon-based active material expands. An excessive aspect ratio may affect its processing performance.

[0057] In one embodiment, the bacterial cellulose satisfies at least one or more of the following conditions:

[0058] a. The degree of polymerization of bacterial cellulose is 15000 - 20000;

[0059] b. The tensile strength of bacterial cellulose is 200 MPa - 300 MPa;

[0060] c. The Young's modulus of bacterial cellulose is 15 GPa - 35 GPa.

[0061] Specifically, the polymer monomer structure of the bacterial cellulose in this embodiment is as follows:

[0062]

[0063] There are 6 hydroxyl groups (-OH) on the polymer monomer, and a large number of hydrogen bonds are formed intra- and intermolecularly. A large number of -OH exist in the entire molecular structure formed, and the structure is as follows:

[0064]

[0065] The degree of polymerization of the bacterial cellulose in this embodiment can reach 15000 - 20000, metabolically synthesizing a closely related lamellar structure, making it have a high tensile strength and elastic modulus. Its tensile strength is 200 MPa - 300 MPa. Due to the existence of a large number of hydrogen bonds in its molecular structure, the Young's modulus of the membrane plane is 15 GPa - 35 GPa.

[0066] In one embodiment, the negative electrode active material layer further includes a conductive agent and a binder.

[0067] It is understandable that the present invention does not specifically limit the type of conductive agent. Exemplarily, it includes at least one of carbon-based materials, powdered nickel or other metal particles, or conductive polymers. For example, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, acetylene black, carbon nanotubes, carbon nanofibers, polyaniline, polythiophene, polyacetylene, polypyrrole, poly(3,4-ethylenedioxythiophene), poly(sulfonated styrene), etc.

[0068] It is understandable that the present invention also does not specifically limit the type of binder. Based on the concept of the present invention, binders that can bond and hold the negative electrode active material, enhance the contact between the negative electrode active material and the negative electrode conductive agent, and between the negative electrode active material and the negative electrode current collector, thereby stabilizing the structure of the negative electrode sheet, are within the protection scope of the present invention. Exemplarily, it includes thermoplastic resins or thermosetting resins. For example, polytetrafluoroethylene (PTFE), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), nitrile rubber (NBR), styrene-ethylene-butylene-styrene copolymer (SEBS), styrene-butadiene-styrene copolymer (SBS), lithium polyacrylate (LiPAA), sodium polyacrylate (NaPAA), sodium alginate, lithium alginate, etc.

[0069] The present invention also provides a method for preparing the above-mentioned negative electrode sheet, which includes the following steps:

[0070] Mix the negative electrode active material, conductive agent, binder, and additive to prepare a slurry for the negative electrode active material layer, and coat the slurry for the negative electrode active material layer on at least one side of the negative electrode current collector, and obtain the negative electrode sheet after drying.

[0071] In one embodiment, based on the total mass of the materials of the negative electrode active material layer being 100%, the mass percentage of the negative electrode active material is 91.8% - 93.3%, and the mass percentage of the binder is 4% - 6%.

[0072] It is understandable that limiting the mass of the negative electrode active material within this range in the present invention can ensure that the energy density, capacity, etc. of the battery meet the required standards, while the binder within this range can prevent the electrode from cracking or falling off due to volume change during charge and discharge, maintaining the integrity of the electrode structure. A conductive agent is also provided in the negative electrode active material layer to improve the electron transfer efficiency of the electrode, improve the rate performance and cycle stability, and its content can be adjusted adaptively according to the actually selected content of the negative electrode active material, binder, and additive.

[0073] The present invention also provides a lithium-ion battery, which includes a positive electrode sheet, the above-mentioned negative electrode sheet, a separator, and an electrolyte.

[0074] In one embodiment, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer covering the surface of the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material, a binder, and a conductive agent.

[0075] Regarding the related content of the negative electrode plate, reference can be made to the above introduction, and it will not be elaborated here one by one.

[0076] In one embodiment, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer covering at least one side surface of the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material, a binder, and a conductive agent.

[0077] It can be understood that the present invention does not particularly limit the positive electrode current collector material, as long as it has conductivity and does not cause chemical changes in the battery. Exemplarily, the positive electrode current collector includes, but is not limited to, any one of aluminum, nickel, or stainless steel. For example, the positive electrode current collector is aluminum foil.

[0078] It can be understood that the present invention does not particularly limit the shape of the positive electrode current collector. Exemplarily, the shape of the positive electrode current collector includes, but is not limited to, metal foil, metal grid, metal mesh, metal foam, etc.

[0079] It can be understood that the positive electrode active material is a compound that reversibly intercalates and deintercalates lithium, and the positive electrode active material in the present invention can be any positive electrode active material known in the art. Exemplarily, the positive electrode active material can be selected from lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium manganate, lithium iron manganese phosphate, lithium vanadium phosphate, lithium vanadium oxide phosphate, lithium iron phosphate, lithium titanate, or lithium-rich manganese-based elastic materials, etc.

[0080] It can be understood that in order to prevent short circuits inside the battery, a separator is also required in the lithium-ion battery. The separator is located between the positive electrode plate and the negative electrode plate to block the transmission of electrons inside the battery. The separator in the present invention is any one of various separators used in lithium-ion batteries, and the separator includes materials with low resistance to ion migration of the electrolyte and good electrolyte retention ability. Exemplarily, the separator includes, but is not limited to, one or more of polypropylene, polyethylene, polyvinylidene fluoride, polyimide, and polyacrylonitrile, and users can select according to actual needs. It should be noted here that when a solid electrolyte is used, the solid electrolyte acts as a separator or is coated on the separator, and in this case, a conventional separator may no longer be required.

[0081] The electrolyte includes a liquid electrolyte and a solid electrolyte. The liquid electrolyte includes a lithium salt and a non-aqueous solvent. Exemplarily, the lithium salt includes LiPF 6 、LiBF 4 、LiAsF 6 、LiClO 4 、LiB(C 6 H 5) 4 、LiCH 3 SO 3 、LiCF 3 SO 3 、LiN(SO 2 CF 3 ) 2 、LiC(SO 2 CF 3 ) 3 、Li 2 SiF 6 、LiBOB、LiBF 2 (C 2 O 4 ) one or more of the following, and users can select according to actual needs. In the embodiments of the present invention, the lithium salt is preferably LiPF 6 , because LiPF 6 can have high ionic conductivity and can improve the cycling characteristics of the battery.

[0082] Exemplarily, the non-aqueous solvent includes one or more of carbonate compounds, carboxylate compounds, and ether compounds. Further, the carbonate compounds include linear carbonate compounds, cyclic carbonate compounds, and fluorinated carbonate compounds. Specifically, the linear carbonate compounds include, but are not limited to, diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), etc.; the cyclic carbonate compounds include, but are not limited to, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VEC), etc.; the fluorinated carbonate compounds include, but are not limited to, fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethyl ethyl carbonate, etc.

[0083] The carboxylate compounds include, but are not limited to, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, methyl formate, etc.

[0084] The ether compounds include, but are not limited to, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc.

[0085] Furthermore, the non-aqueous solvent may also include one or more of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters.

[0086] In the embodiments of the present invention, the solid electrolyte includes, but is not limited to, polymer solid electrolytes, oxide solid electrolytes, sulfide solid electrolytes, etc.

[0087] Exemplarily, the sulfide solid electrolyte may be selected from: Li 2 S-P 2 S 5 、Li 2 S-P 2 S 5 -MS x (where M is Si, Ge, and Sn and 0 ≤ x ≤ 2), Li 3.4 Si 0.4 P 0.6 S 4 、Li 10 GeP 2 S 11.7 O 0.3 、Li 9.6 P 3 S 12 、Li 7 P 3 S 11 、Li 9 P 3 S 9 O 3 、Li 10.35 Si 1.35 P 1.65 S 12 、Li 9.81 Sn 0.81 P 2.19 S 12 、Li 10 (Si 0.5 Ge 0.5 )P 2 S 12 、Li(Ge 0.5 Sn 0.5 )P 2 S 12 、Li(Si 0.5 Sn 0.5 )P 2 S 12 、Li 10 GeP 2 S 12(LGPS), Li 6 PS 5 X (where X is Cl, Br, or I), Li 7 P 2 S 8 I, Li 10.35 Ge 1.35 P 1.65 S 12 , Li 3.25 Ge 0.25 P 0.75 S 4 , Li 10 SnP 2 S 12 , Li 10 SiP 2 S 12 , Li 9.54 Si 1.74 P 1.44 S 11.7 C l0.3 , (1 - x)P 2 S 5-x Li 2 S (where 0.5 ≤ x ≤ 0.7), at least one of which can be selected by the user according to actual needs.

[0088] Exemplarily, the halide solid electrolyte can be selected from: Li 2 CdC l4 , Li 2 MgC l4 , Li 2 Cd I4 , Li 2 ZnI 4 , Li 3 OCl, LiI, Li 5 ZnI 4 , Li 3 OCl 1-x Br x (where 0 < x < 1), at least one of which can be selected by the user according to actual needs.

[0089] Exemplarily, the oxide solid electrolyte can be selected from: perovskite type, garnet type, LISICON type, NASICON type. Among them, the perovskite type solid electrolyte material is preferably LLTO (lithium lanthanum titanate / lithium titanate lanthanum, Li 0.33 La 0.56 TiO 3 ), the garnet type solid electrolyte material is preferably LLZO (lithium lanthanum zirconate / zirconate lanthanum lithium, Li 7 La 3 Zr 2 O12 ), NASICON (sodium superionic conductor) type solid electrolyte materials are preferably at least one of LATP (lithium aluminum titanium phosphate, Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 ), and users can select according to actual needs.

[0090] The embodiments of the present invention will be described more specifically below through examples. However, the embodiments of the present invention are not limited to these examples.

[0091] Example 1

[0092] 1. Preparation of negative electrode sheet

[0093] The negative electrode sheet includes a copper foil current collector and a negative electrode active material layer provided on one side of the copper foil current collector. Based on the total mass of the materials of the negative electrode active material layer being 100%, the negative electrode active material layer includes the following components: 93% of negative electrode active material, 0.2% of conductive carbon black (Super P), 1% of single-walled carbon nanotubes (SWCNT), 4.5% of polyacrylic acid (PAA), 0.5% of carboxymethyl cellulose (CMC), and 0.8% of bacterial cellulose additive. Among them, the negative electrode active material is composed of 50% silicon oxide (SiO x ) and 50% graphite by mass.

[0094] The structure of bacterial cellulose is as Figure 1 shown. The hydroxyl content of bacterial cellulose is 6×10 4 pieces, the length is 50μm - 150μm, the diameter is 70nm - 80nm, and the aspect ratio is ≥625.

[0095] This example also provides a method for preparing the above-mentioned negative electrode sheet, which includes the following steps:

[0096] Mix the above-mentioned formulated amounts of negative electrode active material, Super P, SWCNT, PPA, CMC, and bacterial cellulose additive to prepare a slurry for the negative electrode active material layer, and then coat the slurry for the negative electrode active material layer on the surface of the negative electrode current collector through a coater, and then form a negative electrode active material layer on the surface of the negative electrode current collector. After baking and rolling, the negative electrode sheet as shown in Figure 2 is obtained.

[0097] 2. Preparation of positive electrode sheet

[0098] The positive electrode plate includes an aluminum foil current collector and a positive electrode active material layer provided on one side of the aluminum foil current collector. Taking the total mass of the materials of the positive electrode active material layer as 100%, the positive electrode active material layer includes the following components: 97.64% of a 9-series high-nickel ternary positive electrode material (NCM), 0.5% of conductive carbon black (Super P), 0.6% of multi-walled carbon nanotubes (MWCNT), 0.06% of single-walled carbon nanotubes (SWCNT), and 1.2% of polyvinylidene fluoride (PVDF). Mix the above-formulated amounts of NCM, Super P, MWCNT, SWCNT, and PVDF to prepare a slurry for the positive electrode active material layer, and then coat the slurry for the positive electrode active material layer on the surface of the positive electrode current collector through a coater, thereby forming a positive electrode active material layer on the surface of the positive electrode current collector. After baking and rolling, the positive electrode plate is obtained.

[0099] 3. Preparation of Lithium-Ion Batteries

[0100] Stack the above-prepared positive electrode plate, negative electrode plate, and separator in sequence, with the separator positioned between the positive and negative electrodes to obtain a bare battery cell. Place the bare battery cell in an outer packaging shell, inject the electrolyte, and after vacuum packaging, standing, forming, shaping, and other processes, a lithium-ion battery is obtained.

[0101] Example 2

[0102] The difference between this example and Example 1 is that taking the total mass of the materials of the negative electrode active material layer as 100%, the mass percentage content of the bacterial cellulose additive is 0.1%, and the mass percentage content of the negative electrode active material is adaptively adjusted to 93.7%. Others are the same as in Example 1.

[0103] Example 3

[0104] The difference between this example and Example 1 is that taking the total mass of the materials of the negative electrode active material layer as 100%, the mass percentage content of the bacterial cellulose additive is 5%, and the mass percentage content of the negative electrode active material is adaptively adjusted to 88.8%. Others are the same as in Example 1.

[0105] Example 4

[0106] The difference between this example and Example 1 is that the hydroxyl content of the bacterial cellulose is 1×10 4 pieces, and others are the same as in Example 1.

[0107] Example 5

[0108] The difference between this example and Example 1 is that the hydroxyl content of the bacterial cellulose is 3×10 5 pieces, and others are the same as in Example 1.

[0109] Example 6

[0110] The difference between this embodiment and Embodiment 1 is that the length of the bacterial cellulose is 2 μm, and the others are the same as those in Embodiment 1.

[0111] Embodiment 7

[0112] The difference between this embodiment and Embodiment 1 is that the length of the bacterial cellulose is 500 μm, and the others are the same as those in Embodiment 1.

[0113] Embodiment 8

[0114] The difference between this embodiment and Embodiment 1 is that the aspect ratio of the bacterial cellulose is ≤20, and the others are the same as those in Embodiment 1.

[0115] Comparative Example 1

[0116] This comparative example provides a negative electrode plate, as Figure 3 shown. The difference between this comparative example and Embodiment 1 is that, based on the total mass of the materials of the negative electrode active material layer being 100%, the negative electrode active material layer includes the following components: 93.8% of negative electrode active material, 0.2% of conductive carbon black (Super P), 1% of single-walled carbon nanotubes (SWCNT), 4.5% of polyacrylic acid (PAA), and 0.5% of carboxymethyl cellulose (CMC), and the others are the same as those in Embodiment 1.

[0117] Test conditions

[0118] 1. Perform performance tests on the lithium-ion batteries provided in Embodiments 1 to 8 and Comparative Example 1, and the test methods are as follows:

[0119] (1) Cycle performance test

[0120] Perform cycle performance tests on the lithium-ion batteries prepared in the above embodiments and comparative examples. At a temperature of 25°C ± 2°C, in the first step, charge at a current of 1C until the upper voltage limit (4.2V), terminate the voltage, the cut-off current is 0.05C, and let it stand for 30 min; in the second step, discharge at a current of 1C until the lower voltage limit (3V), record the discharge capacity, and let it stand for 30 min; cycle the first and second steps, and test the electrochemical performance of the lithium-ion battery after 500 cycles. The test results are shown in Table 1.

[0121] (2) Expansion rate test of the negative electrode plate

[0122] First, take the rolled negative electrode plate and measure its thickness, and record the average value as d 1 , and then disassemble the lithium-ion battery after cycling (after cycling charge and discharge 500 times at a current of 1C at a temperature of 25 ± 2°C), measure the thickness of the negative electrode plate therein, and record the average value as d 2, where the thickness of the copper foil current collector is denoted as d 0 , the expansion rate of the negative electrode plate is calculated according to the following formula, and the test results are shown in Table 1

[0123]

[0124] In the formula, α--the expansion rate of the negative electrode plate, unit: %.

[0125] 2. The swelling test is carried out on the negative electrode plates provided in Example 1 and Example 5, and the test method is as follows:

[0126] Take the negative electrode plate after rolling and baking and soak it in the electrolyte at room temperature (25±2°C), measure the swelling degree, record the mass before and after liquid absorption, and calculate the swelling degree of the negative electrode plate according to the following formula

[0127]

[0128] In the formula, σ--the swelling degree of the negative electrode plate, unit: %; W t --the mass of the negative electrode plate that reaches equilibrium after soaking, unit: g; W 0 --the mass of the negative electrode plate before soaking, unit: g;

[0129] The test results are: the swelling degree of the negative electrode plate of Example 1 is 10.3%, and the swelling degree of the negative electrode plate of Example 5 is 22.6%.

[0130] Table 1

[0131]

[0132]

[0133] It can be seen from Example 1 and Comparative Example 1 in Table 1 and Figure 4 It can be seen that introducing bacterial cellulose as an additive into the negative electrode plate of the present invention reduces the expansion rate of the negative electrode plate and improves the cycle performance of the battery. It can be known from Example 1 and Examples 2-8 that when the content of the introduced bacterial cellulose additive in the negative electrode active material layer, the content of hydroxyl groups on the surface of bacterial cellulose, the length of bacterial cellulose, and the aspect ratio are within the range limited by the present invention, the improvement effect on the expansion rate of the negative electrode plate and the cycle performance of the battery is better.

[0134] In addition, by testing the swelling degrees of the negative electrode plates in Example 1 and Example 5, it can be further seen that the swelling degree of the negative electrode plate in Example 1 is much smaller than that of the negative electrode plate in Example 5. It can be shown that too high a hydroxyl content will bring too high a swelling degree, causing the bacterial cellulose to absorb liquid and swell and deform, and its effect of improving the expansion rate and cycling performance of the negative electrode will be greatly reduced. Therefore, it is further proved that the more hydroxyl groups on the surface of bacterial cellulose is not necessarily better, but within the scope limited by the present invention, better effects will be achieved.

[0135] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector, characterized in that: The negative electrode active material layer includes a negative electrode active material and an additive, wherein the negative electrode active material includes a silicon-based active material, and the additive includes bacterial cellulose.

2. The negative electrode sheet according to claim 1, characterized in that: Taking the total mass of the material of the negative electrode active material layer as 100%, the mass percentage of the bacterial cellulose is 0.5%-2%.

3. The negative electrode sheet according to claim 1, characterized in that: The bacterial cellulose surface has hydroxyl groups, and the content of the hydroxyl groups is 3×10 4 -1×10 5 indivual.

4. The negative electrode sheet according to claim 1, characterized in that: The bacterial cellulose has a three-dimensional network structure.

5. The negative electrode sheet according to claim 1, characterized in that: The length of the bacterial cellulose is 20 μm-200 μm, the diameter of the bacterial cellulose is 50 nm-100 nm, and the aspect ratio of the bacterial cellulose is ≥200.

6. The negative electrode sheet according to claim 1, characterized in that: The bacterial cellulose at least meets one or more of the following conditions: a. The degree of polymerization of the bacterial cellulose is 15000-20000; b. The tensile strength of the bacterial cellulose is 200MPa-300MPa; c. The Young's modulus of the bacterial cellulose is 15 GPa-35 GPa.

7. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material layer further includes a conductive agent and a binder.

8. The negative electrode sheet according to claim 7, characterized in that: Taking the total mass of the material of the negative electrode active material layer as 100%, the mass percentage of the negative electrode active material is 91.8%-93.3%, and the mass percentage of the binder is 4%-6%.

9. A method for preparing a negative electrode sheet according to any one of claims 1 to 8, characterized in that: The method comprises the following steps: The negative electrode active material, the conductive agent, the binder and the additive are mixed to prepare the negative electrode active material layer slurry, the negative electrode active material layer slurry is coated on at least one side of the negative electrode current collector, and the negative electrode sheet is obtained after drying.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The negative electrode sheet is the negative electrode sheet according to any one of claims 1 to 8 or the negative electrode sheet is prepared according to the method for preparing the negative electrode sheet according to claim 9.

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