Multilayer negative electrode and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium batteries, in particular to a multi-layer negative electrode and a preparation method and application thereof.The base foil of the multi-layer negative electrode is provided with a bottom layer, the bottom layer is provided with a surface layer, and the surface layer comprises surface layer glue, surface layer conductive carbon and a surface layer active substance; the peel strength of the bottom layer is recorded as Rb, the peel strength of the surface layer is recorded as Rs, the rebound correction coefficient of the surface layer is recorded as Ee, the mass percent of surface layer glue in the surface layer is recorded as Cc, Rb is larger than or equal to 60N / m and smaller than or equal to 320N / m, Rs is larger than or equal to 5.6 N / m and smaller than or equal to 42N / m, Ee is larger than or equal to 0.233 and smaller than or equal to 1.45, Cc is larger than or equal to 1.2% and smaller than or equal to 7.5%, and [In (Rb / Rs-0. 4)] * (Ee / Cc) 1 / 2 is larger than or equal to 0.12 and smaller than or equal to 26.67. According to the invention, the negative electrode is designed into a multi-layer structure and is supplemented with the surface glue with excellent performance, so that the anti-expansion performance of the multi-layer negative electrode is improved under the synergistic effect, and the rapid charging performance and the long cycle performance of the lithium ion battery can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a multi-layer negative electrode and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have balanced performance, cost, and safety to become the mainstream technology in the current energy storage field. Silicon-based materials have received widespread attention for their ultra-high theoretical specific capacity and low cost. However, the volume expansion of silicon-based negative electrodes during the cycle process will lead to pulverization of the electrode structure and peeling of the current collector, which will in turn lead to loss of negative electrode active materials, ion / electron connectivity failure, and continuous deterioration of the interface between the electrolyte and the newly exposed silicon-based surface, resulting in poor cycle life and low first efficiency. Moreover, silicon is a semiconductor and needs to rely on carbon coating and other treatments to improve conductivity. At present, silicon-based materials are mostly used in combination with graphite, such as silicon-carbon negative electrode Si / C, silicon-oxygen-carbon negative electrode SiO x / C, although this method can alleviate the expansion problem, the effect is minimal.

[0003] Anode binders play a crucial role in suppressing silicon anode expansion. They enhance cycling stability by enhancing the electrode's mechanical strength, maintaining the integrity of the conductive network, and suppressing the pulverization of active particles. Traditional binders include PVDF and CMC / SBR. PVDF relies on van der Waals forces, while CMC / SBR exhibits insufficient adhesion to silicon materials. Consequently, both cannot tolerate large volume changes, leading to cracking or shedding of the anode electrode after cycling. In the prior art, PAA and its derivatives, due to their excellent adhesion and expansion resistance, have gradually become the mainstream binders for suppressing the expansion of silicon-based materials and are widely used in high-silicon anodes. However, simply improving the binder alone cannot effectively suppress silicon anode expansion. To meet industrialization needs, it is necessary not only to develop new binders to improve tolerance to silicon expansion, but also to design the anode electrode structure to reduce internal stress, thereby stabilizing the anode electrode structure, ensuring mechanical strength and recoverability, and preventing anode electrode failure and shedding of the anode active material.

[0004] For this purpose, this application is filed. Summary of the Invention

[0005] In view of the deficiencies of the prior art, the present invention provides a multi-layer negative electrode and a preparation method and application thereof.

[0006] In order to achieve the above objectives, the main technical solutions adopted by the present invention include:

[0007] First, the present invention provides a multi-layer negative electrode, comprising a base foil, wherein a bottom layer is provided on the front side or / and the back side of the base foil, and a surface layer is provided on the bottom layer, wherein the bottom layer comprises a bottom layer glue and a bottom layer conductive carbon, and the surface layer comprises a surface layer glue, a surface layer conductive carbon and a surface layer active material;

[0008] The peel strength of the bottom layer is recorded as Rb, the peel strength of the surface layer is recorded as Rs, the rebound correction coefficient of the surface layer is recorded as Ee, and the instantaneous thickness of the surface layer after the multi-layer negative electrode is rolled is recorded as H. 压实 The thickness of the multilayer negative electrode when it is lithiated to a potential of <0.005V is recorded as H 反弹 , Ee=(H 反弹 -H 压实 ) / H 压实 , the mass percentage of the surface glue in the surface layer is recorded as Cc, then the above technical parameters meet the following requirements: 60N / m≤Rb≤320N / m, 5.6N / m≤Rs≤42N / m, 0.233≤Ee≤1.45, 1.2%≤Cc≤7.5%, 0.15≤[In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 ≤26.5.

[0009] Compared with the prior art, the present invention designs the negative electrode into a multi-layer structure of base foil, bottom layer and surface layer:

[0010] (1) The surface of the base foil is coated with a bottom layer, and the bottom conductive carbon in the bottom layer forms a uniform loose porous structure, which can provide more contact sites for the surface active material, increase the contact area between the surface active material and the base foil, and strengthen the adhesion between the surface active material and the base foil, thereby improving the peeling strength and reducing the risk of surface shedding. The bottom layer peeling strength Rb reflects the bonding force between the surface active material and the base foil. When 60N / m≤Rb≤320N / m, it can not only inhibit the warping of the pole piece and the peeling of the current collector caused by silicon expansion, but also will not cause local stress concentration, taking into account both bonding strength and toughness.

[0011] (2) The surface peel strength Rs reflects the bonding force between the surface layer and the bottom layer. Appropriate surface peel strength can maintain the interface integrity of the multilayer structure and prevent interlayer peeling. When 5.6N / m≤Rs≤42N / m, the difference in peel strength between the surface layer and the bottom layer causes the expansion stress to be preferentially transmitted to the bottom layer. The high elasticity of the surface layer allows the stress to be released in time, avoiding local stress concentration leading to crack expansion and further reducing the overall expansion rate.

[0012] (3) The rebound correction coefficient Ee of the surface layer reflects the elasticity of the surface binder. The smaller the rebound correction coefficient, the stronger the ability of the surface layer to absorb expansion stress and the lower the expansion rate in the thickness direction of the negative electrode. When 0.233≤Ee≤1.45, the elasticity of the surface layer is moderate, which not only helps to recover in time after the completion of lithiation, but also does not have an adverse effect on the compaction density.

[0013] (4) The proportion of surface adhesive in the surface layer Cc will directly affect the elasticity of the surface layer. The larger the proportion of surface adhesive, the denser the bonding network in the surface layer and the lower the expansion rate. However, if the proportion of surface adhesive is too large, it will block the pores and reduce the ion transmission rate. Therefore, in order to balance the elasticity and electrolyte wettability, it is required that 1.2% ≤ Cc ≤ 7.5%;

[0014] In summary, a bottom layer whose peel strength Rb meets the above requirements is the basis for suppressing interface failure. A surface layer whose peel strength Rs, rebound correction coefficient Ee, and surface adhesive mass percentage Cc meet the above requirements can effectively consume expansion stress and achieve multi-level dissipation of expansion stress. When Rb, Rs, Ee, and Cc meet the following relationship: 0.15≤[In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 When ≤26.5, after the multi-layer structure design, the parameters are coordinated and regulated, influence each other, and work together to improve the anti-expansion performance of the negative electrode, reduce the internal stress, stabilize the negative electrode structure, and avoid the failure of the negative electrode and the shedding of the negative active material.

[0015] Furthermore, the present invention found that the performance of the multilayer negative electrode is better when Rb, Rs, Ee, and Cc meet the following conditions: 70 N / m ≤ Rb ≤ 180 N / m, 7 N / m ≤ Rs ≤ 30 N / m, 0.3 ≤ Ee ≤ 1.2, 2% ≤ Cc ≤ 5.4%, and 1.15 ≤ [In (Rb / Rs - 0.4)] × (Ee / Cc) 1 / 2 ≤3.12; it is speculated that under this constraint, the various parameters achieve the best match, so that the performance of the multi-layer negative electrode is optimized to a greater extent, which is more conducive to improving the fast charging performance and long cycle performance of lithium-ion batteries.

[0016] Furthermore, the surface adhesive is obtained by polymerizing acrylic acid monomer, acrylonitrile monomer and mercapto-containing acrylate, wherein the mass percentages of the raw materials during the polymerization are: acrylic acid monomer 4%-10%, acrylonitrile monomer 75%-95% and mercapto-containing acrylate 1%-21%;

[0017] In the prior art, acrylonitrile is a relatively common polymerization monomer of acrylic binders and is generally used in combination with acrylic acid monomers. Given that the acrylonitrile polymerization product has strong rigidity but is inferior to acrylic acid in terms of polymerization degree, softness, and toughness, when it is used to prepare binders, especially negative electrode binders, its addition should not be too much. Otherwise, the elastic buffering properties of the resulting binder will deteriorate, making it unable to adapt to the volume expansion of the negative electrode, and the viscosity and fluidity will deteriorate, resulting in an increase in glass transition temperature, brittle and easy cracking of the electrode, reduced elongation, and poor softness and toughness. In addition, its highly polar cyano group also increases the surface energy of the electrode, deteriorates electrolyte wettability, hinders lithium ion transmission, and affects rate performance. After being used in the electrode, chain segment shrinkage may occur during drying, leading to electrode warping, etc.

[0018] The present invention has discovered that when a mercapto-containing acrylate is used, the amount of acrylonitrile added can be significantly increased. By rationally matching the content of acrylic acid monomer, acrylonitrile monomer, and mercapto-containing acrylate, not only can the adverse effects of high acrylonitrile content on the binder preparation process and binder properties be avoided, but also a lithium battery negative electrode binder with good adhesion, strong cohesion, good elastic cushioning, high mechanical strength, and good thermal and electrochemical stability can be obtained. The present invention speculates that the reason for this is that during the polymerization reaction of the mercapto-containing acrylate, the mercapto group (SH) may undergo a nucleophilic addition reaction with the cyano group (CN) of acrylonitrile to form a thioamide structure, which in turn reduces the order and crystallinity of the polyacrylonitrile and changes the thermodynamic properties of the polymer, such as the glass transition temperature. Ultimately, the adhesion of the copolymer is improved, resulting in good mechanical strength and strong cohesion, as well as good softness, elasticity, and toughness.

[0019] It should be noted that the surface adhesive may also be selected from existing technologies, such as polyacrylic acid-butadiene-acrylonitrile copolymer, polyacrylic acid-methyl / ethyl / propyl acrylate-acrylonitrile-styrene copolymer, polystyrene-butadiene-acrylic acid-acrylonitrile copolymer, polystyrene-butadiene-methyl / ethyl / propyl acrylate copolymer, polystyrene-butadiene-acrylic acid-methyl / ethyl / propyl acrylate copolymer, polystyrene-butadiene-acrylic acid-methyl / ethyl / propyl acrylate-acrylonitrile copolymer, etc., as long as the mass percentage Cc of the surface adhesive in the surface layer and the obtained surface rebound correction coefficient Ee and peel strength Rs meet the requirements of the present invention;

[0020] Preferably, in the bottom layer, the mass percentages of the bottom layer glue and the bottom layer conductive carbon are 80%-99.5% and 0.5%-20% respectively;

[0021] Preferably, in the surface layer, the mass percentages of the surface glue, the surface conductive carbon, and the surface active material are 1.2%-7.5%, 0.3%-4%, and 88.5%-98.5% respectively;

[0022] Preferably, the bottom conductive carbon and the surface conductive carbon are both selected from at least one of conductive carbon black, graphene, acetylene black, Ketjen black, conductive graphite, carbon nanotubes, carbon microtubes, carbon nanofibers, and carbon microfibers; the surface active material is at least one of artificial graphite negative electrode material, natural graphite negative electrode material, modified artificial graphite negative electrode material, modified natural graphite negative electrode material, lithium metal negative electrode material, hard carbon negative electrode material, lithium titanate negative electrode material, lithium nitrogen compound, lithium aluminum alloy material, lithium titanium alloy material, lithium silicon alloy material, and silicon-carbon composite material, deposited silicon-carbon negative electrode material, silicon oxide negative electrode material, pre-lithium silicon oxide negative electrode material, and pre-magnesium silicon oxide negative electrode material; the bottom glue is selected from polyacrylic acid, polyacrylic acid-acrylate, polyacrylonitrile, polyurethane resin, phenolic resin, epoxy resin or acrylic resin, polymethyl methacrylate, polyacrylic acid-methyl / ethyl / propyl acrylate-acrylonitrile copolymer, polyacrylic acid-acrylonitrile copolymer, polyacrylic acid-butadiene-acrylonitrile copolymer.

[0023] Furthermore, the mercapto-containing acrylate monomer is mercapto polyethylene glycol acrylate and / or mercapto methacrylate;

[0024] Preferably, at least a portion of the acrylic acid monomer is pre-neutralized with a base such as NaOH, KOH, etc. to form a carboxylate. Pre-neutralization can significantly improve the solubility of the acrylic acid monomer, regulate the polymerization reaction, and optimize the properties of the polymer.

[0025] Preferably, the molecular weight of the mercapto polyethylene glycol acrylate is 2000-5000. Considering the flexibility of the copolymer product alone, the longer the PEG chain length in the mercapto polyethylene glycol acrylate, the better, that is, the larger the molecular weight, the better. However, too long a chain length will excessively sacrifice mechanical strength. Through repeated experiments, the present invention found that when the molecular weight of the mercapto polyethylene glycol acrylate is 2000-5000, the copolymer product has both good mechanical strength and flexibility.

[0026] Preferably, the specific operation of the polymerization is: adding mercapto-containing acrylate, acrylonitrile monomer, and acrylic acid monomer to a hydroalcohol solvent in order according to a proportion, then adding an emulsifier and a dispersant and stirring to obtain a suspension; heating the suspension and adding an initiator dropwise while stirring to polymerize under an inert atmosphere; filtering, washing, drying, crushing, and sieving; wherein: the hydroalcohol solvent is a mixed solvent of water and ethanol, and the volume of water is greater than the volume of ethanol; the emulsifier is at least one of sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, Span 80, and Tween 20; the dispersant is polyethylene At least one of ethylene pyrrolidone, polyvinyl alcohol, hydroxypropyl cellulose, gelatin, and polyacrylate; the initiator is one of potassium persulfate, sodium persulfate, azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate; the added mass of the initiator is 0.2%-1% of the total mass of the monomers; the polymerization temperature is 60-80°C, and the polymerization time is 12-18 hours. It should be noted that this preparation method is only one method, and other preparation methods in the art that can obtain the copolymerization of the above three components are also possible. This specific operation is only for example and not for limitation.

[0027] In this embodiment, mercapto polyethylene glycol acrylate and / or mercapto methacrylate are copolymerized with acrylic acid monomer and acrylonitrile monomer. When the three monomers are mixed in the required ratio, the resulting product has high elastic modulus, tensile strength and elongation at break after forming a film, and rebounds in time during the battery cell cycle, which is beneficial to the construction of a cyclic conductive network and the reduction of the battery cell expansion rate. In addition, further tests found that the molecular weight of the polymer product is 15W-25W. The binder within this molecular weight range has good solubility in oily solvents, which helps to obtain a binder solution with moderate viscosity and good fluidity. When the binder glue is used in the preparation of negative electrode plates, it has good adhesion and coating properties; the glass transition temperature of the polymer product is 85-110°C, which can provide sufficient bonding strength to prevent the plate from cracking, and maintain a certain toughness to avoid embrittlement. During the charge and discharge process, when the active material, such as the active material containing silicon, expands in volume, the binder with this Tg value can buffer the stress through elastic deformation and reduce the damage to the plate structure, and can also have an appropriate degree of swelling in the electrolyte, ensuring that the electrolyte fully infiltrates the pores of the plate without causing bonding failure, and at the same time forming a stable SEI to reduce side reactions.

[0028] Furthermore, the base foil has a thickness of 3-16 μm, the bottom layer has a thickness of 0.3-5 μm, and the surface layer has a thickness of 45-180 μm;

[0029] Preferably, the bottom layer accounts for 30%-100% of the area of ​​the front side or the back side of the base foil, that is, the bottom layer can be provided on one side or both sides of the base foil, and the bottom layer can fully cover the surface of the base foil or partially cover the surface of the base foil, preferably a full coverage coating method; the surface layer accounts for 85%-100% of the area of ​​the bottom layer, that is, the surface layer should cover most of the bottom layer, preferably a full coverage coating method;

[0030] Preferably, the base foil is selected from one of copper-based foil, nickel-based foil, iron-based foil, and copper-containing alloy-based foil, and has a surface wetting tension of 52-96 mN / m, which helps the primer to spread evenly and firmly coat on its surface, with few defects such as shrinkage cavities and fisheyes and high bonding strength; its tensile strength is 480-660 MPa, which can not only resist the tension of the coating machine, reduce tensile deformation, and avoid fluctuations in coating thickness or breakage due to elongation, but also withstand high pressure during rolling to prevent wrinkling or breakage of the electrode sheet, and can also inhibit lithium dendrite puncture during use, especially during fast charging, reducing the risk of short circuit;

[0031] Preferably, the true density of the surface layer is 1.98-2.23 cm 3 / g, which can not only increase the negative electrode's gram capacity, but also will not cause pore closure due to excessive compaction, hindering the infiltration of the electrolyte; the porosity of the surface layer is 24%-56%, which can not only accelerate wetting, but also accommodate the volume change during charging and discharging, and prevent the electrode from cracking; the cohesive strength of the surface layer is 640-980N / m, the active material on the surface is not easy to detach, the conductive network structure of the surface is stable, and thus the cycle stability is good, and it is not easy to lose powder during slitting and winding, and will not contaminate the battery cell; by regulating the true density, porosity and cohesive strength of the surface layer, the energy density, rate performance and cycle life of the multi-layer negative electrode can be balanced.

[0032] Secondly, the present invention provides a method for preparing a multilayer negative electrode, comprising the following steps:

[0033] S1, mixing the base glue and the base conductive carbon in proportion and adding a solvent to prepare a base coating, then coating the base coating on the base foil and drying it to obtain a base foil provided with a base layer;

[0034] S2, mixing the surface glue, the surface conductive carbon and the surface active material in proportion and adding a solvent to prepare a surface coating, and then applying the surface coating on the base layer and drying;

[0035] S3. Roller pressing.

[0036] Furthermore, in step S1, the solid content of the primer is 3.5%-35% and the fineness is no greater than 45 μm;

[0037] Preferably, the solid content of the primer is 6.5%-18%;

[0038] Preferably, the solvent is selected from at least one of toluene, ethylbenzene, xylene, dimethylacetamide, N-methylpyrrolidone, acetone, dimethyl sulfoxide, tetrahydrofuran, water, ethanol, isopropanol, and n-butanol.

[0039] Furthermore, in step S2, the surface adhesive is first prepared into a surface adhesive liquid, the solid content of the surface adhesive liquid is preferably 3%-9%, and then the mixing is performed. The specific operation of the mixing is: first, the surface conductive carbon and the surface active material are pre-mixed uniformly, and then 20%-70% of the surface adhesive liquid and the solvent are added, stirred until uniform and the solid content is controlled to be 30%-55%, and finally, the remaining surface adhesive liquid and solvent are added, and stirred again until uniform and the solid content is controlled to be 32%-48%.

[0040] Preferably, the surface glue solution is obtained by dispersing the surface glue in an oily solvent and applying glue at the dew point, wherein the solid content is 3%-9%, the dew point is ≤-35°C, and the oily solvent is selected from one of dimethylacetamide, N-methylpyrrolidone, acetone, and dimethyl sulfoxide;

[0041] Preferably, the drying temperature is 70-105°C.

[0042] Thirdly, the present invention provides a lithium battery comprising a positive electrode sheet, a separator, an electrolyte and a multi-layer negative electrode.

[0043] Furthermore, the positive electrode sheet is made of positive electrode slurry with a surface density of 0.006-0.027 g / cm 2 The positive electrode slurry is prepared by coating the positive electrode glue, positive electrode conductive carbon, and positive electrode active material in the following mass percentages of 0.3%-7.5%, 0.2%-4%, and 88.5%-99.5% in sequence and stirring evenly in a solvent. The positive electrode slurry has a solid content of 60%-80% and a fineness of no more than 45 μm.

[0044] Preferably, a portion of the positive electrode conductive carbon and all of the positive electrode active material are pre-mixed uniformly, and then 30%-70% of the positive electrode glue and the solvent are added and stirred until uniform, and then the remaining positive electrode conductive carbon, the remaining positive electrode glue and the solvent are added and stirred until uniform and the solid content is controlled to be 65%-85%, and finally the solvent is added and stirred again until uniform and the solid content is controlled to be 68%-80%;

[0045] Preferably, the positive electrode glue is selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, hydrogenated nitrile rubber, and carboxylated polyvinylidene fluoride;

[0046] Preferably, the positive electrode active material is selected from any one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, lithium-rich manganese oxide, lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium cobalt oxide, and lithium nickel manganese oxide;

[0047] Preferably, the solvent is selected from at least one of toluene, ethylbenzene, xylene, dimethylacetamide, N-methylpyrrolidone, acetone, dimethyl sulfoxide, and tetrahydrofuran.

[0048] Compared with the existing technology, the present invention designs the negative electrode into a multi-layer structure and supplements it with a surface glue with excellent performance. The various parameters are synergistically regulated, influence each other, and work together to improve the anti-expansion performance of the negative electrode, reduce internal stress, stabilize the negative electrode structure, avoid failure of the negative electrode and shedding of the negative electrode active material, and ultimately improve the fast charging performance and long cycle performance of the lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0050] Figure 1 25° C. cycle performance comparison of the lithium batteries obtained in Example 1 of the present invention and Comparative Example 1.

[0051] Figure 2 This is a comparison chart of the cell expansion performance during the cycling process at 25°C of the lithium batteries obtained in Example 1 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0052] The following will be combined with the accompanying drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0053] The chemical raw materials used in the following examples and comparative examples were all commercially available. Some of the raw materials were purchased from the following suppliers: mercaptopolyethylene glycol acrylate was purchased from Shaanxi Xingbei Aike Biotechnology Co., Ltd.; mercapto methacrylate was purchased from Tianmen Hengchang Chemical Co., Ltd.; 2-methylthioethyl acrylate was purchased from Alpha Chemical Co., Ltd.; thioacetate was purchased from Shandong Xiya Chemical Co., Ltd.; and sulfoethyl methacrylate was purchased from Shanghai Saikerui Biotechnology Co., Ltd. Any preparation or testing procedures not described in detail in the following examples and comparative examples are conventional in the art and are well known to those skilled in the art.

[0054] Example 1

[0055] A multilayer negative electrode comprises a 3-μm-thick copper foil (tested to have a surface wetting tension of 85 mN / m and a tensile strength of 620 MPa). The front and back sides of the copper foil are both provided with a base layer, and a surface layer is provided on the base layer. The base layer comprises a base glue and a base conductive carbon (80% polyacrylic acid: 20% conductive carbon black, 1 μm thick). The surface layer comprises a surface glue, a surface conductive carbon, and a surface active substance (2% surface glue, 0.3% surface conductive carbon, and 97.7% surface active substance, 75 μm thick).

[0056] The preparation method of the multilayer negative electrode is as follows:

[0057] S1. Mixing a base glue and a base conductive carbon in a certain proportion and adding N-methylpyrrolidone to prepare a base coating having a solid content of 15% and a fineness of no more than 45 μm. Then, applying the base coating to both surfaces of the copper foil and drying the base coating to obtain a copper foil provided with a base;

[0058] S2. First, the surface glue is made into a surface glue liquid, and then mixed with the surface conductive carbon (specifically graphene) and the surface active material (specifically a silicon-carbon and graphite composite material, with a silicon content of 20wt%) in proportion and a solvent is added to make a surface coating. The specific operation is: first, the surface conductive carbon and the surface active material are pre-mixed evenly, and then 50% of the surface glue liquid and N-methylpyrrolidone are added, stirred until uniform and the solid content is controlled to 45%, and finally, the remaining surface glue liquid and N-methylpyrrolidone are added, stirred again until uniform and the solid content is controlled to 40% to obtain a surface coating; and then the surface coating is applied to the surfaces of the upper and lower bottom layers and dried so that a surface layer with a thickness of 55μm is provided on each bottom layer;

[0059] The surface adhesive is obtained by polymerizing acrylic acid monomer, acrylonitrile monomer and mercapto methacrylate monomer, and the ratio of the three monomers is: 4wt% acrylic acid monomer, 90wt% acrylonitrile monomer and 6wt% mercapto methacrylate monomer. The specific preparation method is as follows: mercapto methacrylate monomer, acrylonitrile monomer and acrylic acid monomer are added in order in proportion to a water-ethanol solvent with a volume ratio of 6:4, and then sodium dodecylbenzenesulfonate (the addition amount is 0.7% of the total weight of the monomers) and polyvinylpyrrolidone (the addition amount is 1% of the total weight of the monomers) are added and stirred to obtain a suspension; the suspension is heated to 60°C and azobisisoheptanenitrile is added dropwise while stirring and polymerizing under an inert atmosphere for 16 hours, and the addition amount of azobisisoheptanenitrile is 0.5% of the total weight of the monomers; the mixture is filtered, washed, dried, crushed and sieved to obtain a powdered product; the preparation method of the surface adhesive solution is as follows: the obtained powdered product is mixed with NMP, the solid content is controlled to 6wt%, stirred, and glued at a dew point of -35°C;

[0060] S3, rolling to obtain a multilayer negative electrode, the surface true density of which is 2.15cm 3 / g, porosity of 42%, and cohesive strength of 880N / m; the multilayer negative electrode is cut, slotted, and die-cut for later use.

[0061] The peel strength of the bottom layer of the multilayer negative electrode is recorded as Rb, the peel strength of the surface layer is recorded as Rs, the rebound correction coefficient of the surface layer is recorded as Ee, and the instantaneous thickness of the surface layer after the multilayer negative electrode is rolled is recorded as H. 压实 The thickness of the multilayer negative electrode when it is lithiated to a potential of <0.005V is recorded as H 反弹 , Ee=(H 反弹 -H 压实 ) / H 压实 , the mass percentage of the surface glue in the surface layer is recorded as Cc. After testing, the above parameters and [In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 The calculation results are shown in Table 2.

[0062] A lithium battery is assembled into a semi-solid lithium battery by assembling the above-mentioned multi-layer negative electrode and positive electrode sheet, electrolyte, and separator for electrochemical performance testing. The positive electrode sheet is made of positive electrode slurry with a surface density of 0.02g / cm 2The positive electrode slurry is coated on aluminum foil and dried. The positive electrode slurry is made by adding 95% by weight of lithium nickel cobalt manganese oxide (LNCMnO), 2% by weight of a conductive agent, and 3% by weight of polyvinylidene fluoride to N-methylpyrrolidone. The solid content is 65% and the fineness is no greater than 45 μm. The electrolyte is a 12.5% ​​by weight LiPF6 solution (a mixed solution of fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 as the solvent, and 3% by weight of succinonitrile (SN) is added). The separator is a polyolefin-based double-sided ceramic-coated separator. During assembly, the positive and negative electrode sheets are first die-cut into individual sheets. These are then stacked with a separator with one ceramic side and one adhesive side, with the ceramic side corresponding to the positive electrode. The bare cell is placed in a shell and sealed on the top and side. The cell is then baked, filled with liquid, and allowed to stand to form a cell. The cell is then charged to capacity. The pressure during charging is 0.2 MPa and the temperature is 60°C.

[0063] Example 2

[0064] Compared with Example 1, there are the following differences: (1) the raw material ratio of the surface glue is adjusted to 10 wt% of acrylic acid monomer, 75 wt% of acrylonitrile monomer and 15 wt% of mercapto methacrylate monomer; (2) the mass percentages of the bottom glue and the bottom conductive carbon are adjusted to 99.5% and 0.5% respectively, and the thickness of the bottom layer is 1.5 μm; (3) the mass percentages of the surface glue, the surface conductive carbon and the surface active material are adjusted to 3.5%, 4% and 92.5% respectively, and the thickness of the surface layer is 100 μm; (4) the thickness of the copper foil is adjusted to 10 μm; the rest are consistent with Example 1.

[0065] Example 3

[0066] Compared with Example 1, there are the following differences: (1) the raw material ratio of the surface adhesive is adjusted to 4 wt% of acrylic acid monomer, 75 wt% of acrylonitrile monomer and 21 wt% of mercapto methacrylate monomer; (2) the mass percentages of the bottom adhesive and the bottom conductive carbon are adjusted to 90% and 10% respectively, and the thickness of the bottom layer is 0.3 μm; (3) the mass percentages of the surface adhesive, the surface conductive carbon and the surface active material are adjusted to 1.2%, 2.5% and 96.3% respectively, and the thickness of the surface layer is 180 μm; (4) the thickness of the copper foil is adjusted to 16 μm; the rest are consistent with Example 1.

[0067] Example 4

[0068] Compared with Example 1, there are the following differences: (1) the raw material ratio of the surface adhesive is adjusted to 7 wt% of acrylic acid monomer, 80 wt% of acrylonitrile monomer and 13 wt% of mercapto methacrylate monomer; (2) the mass percentages of the bottom adhesive and the bottom conductive carbon are adjusted to 85% and 15% respectively, and the thickness of the bottom layer is 0.8 μm; (3) the mass percentages of the surface adhesive, the surface conductive carbon and the surface active material are adjusted to 1.8%, 4% and 94.2% respectively, and the thickness of the surface layer is 120 μm; (4) the thickness of the copper foil is adjusted to 5 μm; the rest are consistent with Example 1.

[0069] Example 5

[0070] Compared with Example 1, there are the following differences: (1) the raw material ratio of the surface glue is adjusted to 4 wt% of acrylic acid monomer, 95 wt% of acrylonitrile monomer and 1 wt% of mercapto methacrylate monomer; (2) the mass percentages of the bottom glue and the bottom conductive carbon are adjusted to 95% and 5% respectively, and the thickness of the bottom layer is 5 μm; (3) the mass percentages of the surface glue, the surface conductive carbon and the surface active material are adjusted to 1.2%, 5% and 93.8% respectively, and the thickness of the surface layer is 45 μm; (4) the thickness of the copper foil is adjusted to 12 μm; the rest are consistent with Example 1.

[0071] Example 6

[0072] Compared with Example 2, the preparation method of the surface adhesive and the mass percentage of the surface adhesive in the surface layer are adjusted: (1) acrylic acid ester monomer, acrylonitrile monomer, and mercaptomethyl acrylate monomer are added in proportion in sequence to a water-ethanol solvent with a volume ratio of 6:4, and then sodium lauryl sulfate (added in an amount of 0.7% of the total mass of the monomers) and polyvinyl alcohol (added in an amount of 1% of the total mass of the monomers) are added and stirred to obtain a suspension; the suspension is heated to 75°C and azobisisobutyronitrile is added dropwise while stirring and polymerized under an inert atmosphere for 16 hours, wherein the added amount of azobisisobutyronitrile is 0.6% of the total mass of the monomers; the suspension is filtered, washed, dried, crushed, and sieved to obtain a powdery product; (2) in the surface layer, the mass percentages of the surface adhesive, surface conductive carbon, and surface active material are 7.5%, 0.3%, and 92.2%, respectively; the rest are consistent with Example 2.

[0073] Example 7

[0074] Compared with Example 2, the preparation method of the surface glue and the mass percentage of the surface glue in the surface layer are adjusted: (1) to a water-ethanol solvent with a volume ratio of 6:4, acrylate monomers, acrylonitrile monomers, and mercapto methacrylate are added in proportion in sequence, and then a mixture of Span 80 and Tween 20 in a mass ratio of 1:1 (the added amount is 0.7% of the total mass of the monomers) and hydroxypropyl cellulose (the added amount is 1% of the total mass of the monomers) are added and stirred to obtain a suspension; the suspension is heated to 80°C and azobisisobutyronitrile is added dropwise while stirring and polymerized under an inert atmosphere for 16 hours, the added amount of azobisisobutyronitrile is 0.8% of the total mass of the monomers; the product is filtered, washed, dried, crushed, and sieved to obtain a powdery product; (2) in the surface layer, the mass percentages of the surface glue, surface conductive carbon, and surface active material are 6%, 1.5%, and 92.5%, respectively; the rest are consistent with Example 2.

[0075] Example 8

[0076] Compared with Example 2, the raw material mercapto methacrylate monomer of the surface glue is adjusted to an equal amount of mercapto polyethylene glycol acrylate with a molecular weight of 2000, and the mass percentages of the surface glue, surface conductive carbon, and surface active substance in the surface layer are 4%, 3.5%, and 92.5%, respectively; the rest are consistent with Example 2.

[0077] Example 9

[0078] Compared with Example 2, the raw material mercapto methacrylate monomer of the surface glue is adjusted to an equal amount of mercapto polyethylene glycol acrylate with a molecular weight of 3400, and the mass percentages of the surface glue, surface conductive carbon, and surface active substance in the surface layer are 2.5%, 4%, and 93.5%, respectively; the rest are consistent with Example 2.

[0079] Example 10

[0080] Compared with Example 2, the following differences exist: (1) the raw material mercapto methacrylate monomer of the surface adhesive is adjusted to an equal amount of mercapto polyethylene glycol acrylate with a molecular weight of 5000; (2) the mass percentages of the surface adhesive, surface conductive carbon, and surface active material in the surface layer are 1.5%, 5%, and 93.5%, respectively; (3) the thickness of the bottom layer is 2 μm; the rest are consistent with Example 2.

[0081] Example 11

[0082] Compared with Example 2, there are the following differences: (1) the mass percentages of surface glue, surface conductive carbon, and surface active material in the surface layer are 7%, 0.8%, and 92.2%, respectively; (2) the thickness of the bottom layer is 2.5 μm; the rest are consistent with Example 2.

[0083] Example 12

[0084] Compared with Example 1, there are the following differences: (1) the raw material ratio of the surface adhesive is adjusted to 9 wt% of acrylic acid monomer, 78 wt% of acrylonitrile monomer and 13 wt% of mercapto methacrylate monomer; (2) the mass percentages of the surface adhesive, surface conductive carbon and surface active material are adjusted to 3.5%, 4% and 92.6% respectively; the rest are consistent with Example 1.

[0085] Example 13

[0086] Compared with Example 2, the surface layer thickness is 65 μm, and the rest is consistent with Example 2.

[0087] Example 14

[0088] Compared with Example 2, there are the following differences: (1) the mass percentages of surface glue, surface conductive carbon, and surface active material in the surface layer are 5%, 2.5%, and 92.5%, respectively; (2) the thickness of the bottom layer is 3 μm, and the rest are consistent with Example 2.

[0089] Example 15

[0090] Compared with Example 3, the raw material for preparing the surface glue was adjusted from mercapto methacrylate to an equal amount of mercapto polyethylene glycol acrylate, and the rest remained the same as Example 3.

[0091] Example 16

[0092] Compared with Example 1, there are the following differences: (1) the raw material ratio of the surface glue is adjusted: 7 wt% of acrylic acid monomer, 79 wt% of acrylonitrile monomer and 13 wt% of mercapto methacrylate monomer; (2) the thickness of the surface layer is 70 μm; the rest are consistent with Example 1.

[0093] Comparative Example 1

[0094] A mixture of aqueous binder CMC (sodium carboxymethyl cellulose, molecular weight of about 300 kDa) and SBR (styrene-butadiene rubber) in a mass ratio of 1:2.5 was used as the surface glue (the amount added was consistent with that in Example 1), and the oily solvent was adjusted to water. The lithium battery negative electrode sheet was prepared by an operation similar to that in Example 1 and assembled into a lithium battery cell.

[0095] Comparative Example 2

[0096] Compared with Example 2, the mercapto methacrylate in the raw material for preparing the surface glue was replaced with an equal amount of 2-methylthioethyl acrylate, and the rest remained the same as in Example 2.

[0097] Comparative Example 3

[0098] Compared with Example 2, the mercapto methacrylate in the raw material for preparing the surface glue was replaced with an equal amount of allyl thioacetate, and the rest remained the same as in Example 2.

[0099] Comparative Example 4

[0100] Compared with Example 2, the mercapto methacrylate in the raw material for preparing the surface adhesive was replaced with an equal amount of sulfoethyl methacrylate, and the rest remained the same as in Example 2.

[0101] Comparative Example 5

[0102] Compared with Example 2, mercapto methacrylate in the raw materials for preparing the surface adhesive was removed, and the rest remained the same as in Example 2.

[0103] Comparative Example 6

[0104] Compared with Example 2, the raw material for preparing the surface adhesive, mercapto methacrylate monomer, was adjusted to an equal amount of mercapto polyethylene glycol acrylate with a molecular weight of 1000, and the preparation method was also adjusted accordingly. All other aspects remained the same as in Example 2.

[0105] Comparative Example 7

[0106] Compared with Example 2, the raw material for preparing the surface adhesive, mercapto methacrylate monomer, was adjusted to an equal amount of mercapto polyethylene glycol acrylate with a molecular weight of 6000, and the preparation method was also adjusted accordingly.

[0107] The main technical parameters involved in the above examples and comparative examples are shown in Table 1 below. For ease of listing, the mass ratio of acrylic acid monomer, acrylonitrile monomer, and mercapto-containing acrylic ester is expressed as PAA:PAN:mercapto-PAN ester; Example 1, Example 2, etc. are represented by S1, S2, etc., respectively; Comparative Example 1, Comparative Example 2, etc. are represented by D1, D2, etc., respectively; [In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 Referred to as relational expression.

[0108] Table 1 Main technical parameters of each embodiment and comparative example

[0109]

[0110]

[0111] The Rb and Rs of the multilayer negative electrode lithium obtained in each embodiment and each comparative example were tested using a universal tensile tester. The testing method was conventional. Ee: The thickness of the negative electrode after multiple rolling was measured. The prepared battery cell was lithiumed to a potential of <0.005V. The thickness of the corresponding multilayer negative electrode obtained by disassembling the battery cell and lithiumed to a potential of <0.005V was recorded as H. 反弹, Ee=(H 反弹 -H 压实 ) / H 压实 , Cc (known). And calculate the relationship ([In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 ) value; the battery cells obtained from each embodiment and comparative example were tested at a voltage of 3-4.5V and 4C fast charge cycles. Cycle performance: the number of cycles at which the cutoff retention rate reached 80%; 2C discharge capacity = 2C discharge capacity at 4.5V to 3V capacity / 0.2C discharge capacity at 4.5V to 3V capacity; volumetric energy density = 0.2C discharge capacity during capacity division / cell volume during 0.2C capacity division. The final test results are shown in Table 2.

[0112] Table 2 Test results of various embodiments and comparative examples

[0113]

[0114]

[0115] like Figure 1-Figure 2 , Table 1 and Table 2 show together:

[0116] From the test results of Examples 1 to 17, it can be seen that the present invention designs the negative electrode into a multi-layer structure consisting of a base foil, a bottom layer, and a surface layer, and limits the bottom layer peel strength Rb, the surface layer peel strength Rs, the surface layer rebound correction coefficient Ee, the mass percentage of the surface layer glue in the surface layer Cc, and the above four parameters meet 60N / m≤Rb≤320N / m, 5.6N / m≤Rs≤42N / m, 0.233≤Ee≤1.45, 1.2%≤Cc≤7.5%, 0.15≤[In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 ≤26.5, it is helpful to obtain a lithium-ion battery with good fast charging performance and long cycle performance. By comparing the test results between various embodiments, the present invention found that when Rb, Rs, Ee, and Cc further meet 70N / m≤Rb≤180N / m, 7N / m≤Rs≤30N / m, 0.3≤Ee≤1.2, 2%≤Cc≤5.4%, 1.15≤[In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 When ≤3.12, the various parameters achieve optimal matching, the performance of the multi-layer negative electrode is optimized to the greatest extent, and the resulting lithium battery has excellent fast charging performance and long cycle performance.

[0117] The multilayer negative electrode proposed in the present invention requires a surface adhesive with excellent performance to achieve a stable structure and good anti-expansion performance. Figure 1 and Figure 2As shown in the figure, when the surface adhesive used in the present invention is replaced with a mixture of aqueous binders CMC and SBR in a mass ratio of 1:2.5, at the same addition amount, the cycle performance, 2C discharge rate performance and volume energy density of the assembled lithium battery are significantly deteriorated.

[0118] The test results of Comparative Examples 2 to 7 can further confirm that the raw materials for preparing the surface glue proposed in the present invention play a key role in performance. The use of these two substances in combination with acrylic acid monomer and acrylonitrile monomer can obtain a high-performance negative electrode binder. After its application in the present invention, the multi-layer negative electrode can meet the design requirements. If these two substances are replaced by other sulfur-containing substances or mercapto polyethylene glycol acrylates with a molecular weight not within the specified range of the present invention, the resulting product can be used as a surface glue in the present invention, and a multi-layer negative electrode that meets the design requirements cannot be obtained.

[0119] In summary: The present invention designs the negative electrode into a multi-layer structure and supplements it with a surface glue with excellent performance. The various parameters are synergistically regulated, influence each other, and work together to improve the anti-expansion performance of the negative electrode and stabilize the negative electrode structure. Its use in the negative electrode of lithium batteries helps to build a circulating conductive network and effectively inhibit the expansion of the battery cell, thereby improving the cycle life and fast charging performance of the lithium battery.

[0120] Although embodiments of the present invention have been shown and described above, it should be understood that the above embodiments are illustrative and are not to be construed as limiting the present invention. Those skilled in the art may alter, modify, replace, and modify the above embodiments within the scope of the present invention. Furthermore, those skilled in the art may combine and incorporate the various embodiments or examples described in this specification, as well as features thereof, without conflicting requirements.

Claims

1. A multi-layer negative electrode, comprising a base foil, wherein a bottom layer is provided on the front side or / and the back side of the base foil, and a surface layer is provided on the bottom layer, characterized in that: The bottom layer includes bottom layer glue and bottom layer conductive carbon, and the surface layer includes surface layer glue, surface layer conductive carbon and surface layer active material; The peel strength of the bottom layer is recorded as Rb, the peel strength of the surface layer is recorded as Rs, the rebound correction coefficient of the surface layer is recorded as Ee, and the instantaneous thickness of the surface layer after the multi-layer negative electrode is rolled is recorded as H. 压实 The thickness of the multilayer negative electrode when it is lithiated to a potential of <0.005V is recorded as H 反弹 , Ee=(H 反弹 -H 压实 ) / H 压实 , the mass percentage of the surface glue in the surface layer is recorded as Cc, then the above technical parameters meet the following requirements: 60N / m≤Rb≤320N / m, 5.6N / m≤Rs≤42N / m, 0.233≤Ee≤1.45, 1.2%≤Cc≤7.5%, 0.15≤[In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 ≤26.

5.

2. The multilayer negative electrode according to claim 1, characterized in that The technical parameters meet the following requirements: 70N / m≤Rb≤180N / m, 7N / m≤Rs≤30N / m, 0.3≤Ee≤1.2, 2%≤Cc≤5.4%, 1.15≤[In(Rb / Rs-0.4)]×(Ee / Cc) 1 / 2 ≤3.

12.

3. The multi-layer negative electrode according to claim 1, characterized in that The surface adhesive is obtained by polymerizing acrylic acid monomer, acrylonitrile monomer and mercapto-containing acrylate, wherein the mass percentage of each raw material during polymerization is: acrylic acid monomer 4%-10%, acrylonitrile monomer 75%-95% and mercapto-containing acrylate 1%-21%; Preferably, in the bottom layer, the mass percentages of the bottom layer glue and the bottom layer conductive carbon are 80%-99.5% and 0.5%-20% respectively; Preferably, in the surface layer, the mass percentages of the surface glue, the surface conductive carbon, and the surface active material are 1.2%-7.5%, 0.3%-4%, and 88.5%-98.5% respectively; Preferably, the bottom conductive carbon and the surface conductive carbon are both selected from at least one of conductive carbon black, graphene, acetylene black, Ketjen black, conductive graphite, carbon nanotubes, carbon microtubes, carbon nanofibers, and carbon microfibers; the surface active material is at least one of artificial graphite negative electrode material, natural graphite negative electrode material, modified artificial graphite negative electrode material, modified natural graphite negative electrode material, lithium metal negative electrode material, hard carbon negative electrode material, lithium titanate negative electrode material, lithium nitrogen compound, lithium aluminum alloy material, lithium titanium alloy material, lithium silicon alloy material, and silicon-carbon composite material, deposited silicon-carbon negative electrode material, silicon oxide negative electrode material, pre-lithium silicon oxide negative electrode material, and pre-magnesium silicon oxide negative electrode material; the bottom glue is selected from polyacrylic acid, polyacrylic acid-acrylate, polyacrylonitrile, polyurethane resin, phenolic resin, epoxy resin or acrylic resin, polymethyl methacrylate, polyacrylic acid-methyl / ethyl / propyl acrylate-acrylonitrile copolymer, polyacrylic acid-acrylonitrile copolymer, polyacrylic acid-butadiene-acrylonitrile copolymer.

4. The multi-layer negative electrode according to claim 3, characterized in that The mercapto-containing acrylate monomer is mercapto polyethylene glycol acrylate and / or mercapto methacrylate; Preferably, the acrylic acid monomer is at least partially pre-neutralized; Preferably, the molecular weight of mercapto polyethylene glycol acrylate is 2000-5000; Preferably, the specific operation of the polymerization is: adding mercapto-containing acrylate, acrylonitrile monomer, and acrylic acid monomer to a hydroalcohol solvent in proportion in sequence, then adding an emulsifier and a dispersant and stirring to obtain a suspension; heating the suspension and adding an initiator dropwise while stirring to polymerize under an inert atmosphere; filtering, washing, drying, crushing, and sieving; wherein: the hydroalcohol solvent is a mixed solvent of water and ethanol and the volume of water is greater than the volume of ethanol; the emulsifier is at least one of sodium lauryl sulfate, sodium dodecylbenzenesulfonate, Span 80, and Tween 20; the dispersant is at least one of polyvinyl pyrrolidone, polyvinyl alcohol, hydroxypropyl cellulose, gelatin, and polyacrylate; the initiator is one of potassium persulfate, sodium persulfate, azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate; the added mass of the initiator is 0.2%-1% of the total mass of the monomers; the polymerization temperature is 60-80°C, and the polymerization time is 12-18 hours.

5. The multi-layer negative electrode according to claim 1, characterized in that: The base foil has a thickness of 3-16 μm, the bottom layer has a thickness of 0.3-5 μm, and the surface layer has a thickness of 45-180 μm; Preferably, the bottom layer accounts for 30%-100% of the area of ​​the front side of the base foil or the back side of the base foil, and the surface layer accounts for 85%-100% of the area of ​​the bottom layer; Preferably, the base foil is selected from one of copper-based foil, nickel-based foil, iron-based foil, and copper-containing alloy-based foil, and has a surface wetting tension of 52-96 mN / m and a tensile strength of 480-660 MPa; Preferably, the true density of the surface layer is 1.98-2.23 cm 3 / g, porosity is 24%-56%, and cohesive strength is 640-980N / m.

6. A method for preparing a multilayer negative electrode according to any one of claims 1 to 5, characterized in that: The steps are as follows: S1, mixing the base glue and the base conductive carbon in proportion and adding a solvent to prepare a base coating, then coating the base coating on the base foil and drying it to obtain a base foil provided with a base layer; S2, mixing the surface glue, the surface conductive carbon and the surface active material in proportion and adding a solvent to prepare a surface coating, and then applying the surface coating on the base layer and drying; S3. Roller pressing.

7. The preparation method according to claim 6, characterized in that In step S1, the solid content of the primer is 3.5%-35% and the fineness is not greater than 45 μm; Preferably, the solid content of the primer is 6.5%-18%; Preferably, the solvent is selected from at least one of toluene, ethylbenzene, xylene, dimethylacetamide, N-methylpyrrolidone, acetone, dimethyl sulfoxide, tetrahydrofuran, water, ethanol, isopropanol, and n-butanol.

8. The preparation method according to claim 6, characterized in that In step S2, the surface adhesive is first prepared into a surface adhesive liquid, and then the mixing is performed. The specific operation of the mixing is: first, the surface conductive carbon and the surface active material are pre-mixed uniformly, then 20%-70% of the surface adhesive liquid and the solvent are added, stirred until uniform and the solid content is controlled to be 30%-55%, and finally, the remaining surface adhesive liquid and the solvent are added, stirred again until uniform and the solid content is controlled to be 32%-48%; Preferably, the surface glue solution is obtained by dispersing the surface glue in an oily solvent and applying glue at the dew point, wherein the solid content is 3%-9%, the dew point is ≤-35°C, and the oily solvent is selected from one of dimethylacetamide, N-methylpyrrolidone, acetone, and dimethyl sulfoxide; Preferably, the drying temperature is 70-105°C.

9. A lithium battery, characterized in that: The invention comprises a positive electrode sheet, a separator, an electrolyte and a multi-layer negative electrode according to any one of claims 1 to 5 or a multi-layer negative electrode prepared according to the preparation method according to any one of claims 6 to 8.

10. The lithium battery according to claim 9, characterized in that The positive electrode sheet is made of positive electrode slurry with a surface density of 0.006-0.027 g / cm 2 The positive electrode slurry is prepared by coating the positive electrode glue, positive electrode conductive carbon, and positive electrode active material in the following mass percentages of 0.3%-7.5%, 0.2%-4%, and 88.5%-99.5% in sequence and stirring evenly in a solvent. The positive electrode slurry has a solid content of 60%-80% and a fineness of no more than 45 μm. Preferably, a portion of the positive electrode conductive carbon and all of the positive electrode active material are pre-mixed uniformly, and then 30%-70% of the positive electrode glue and the solvent are added and stirred until uniform, and then the remaining positive electrode conductive carbon, the remaining positive electrode glue and the solvent are added and stirred until uniform and the solid content is controlled to be 65%-85%, and finally the solvent is added and stirred again until uniform and the solid content is controlled to be 68%-80%; Preferably, the positive electrode glue is selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, hydrogenated nitrile rubber, and carboxylated polyvinylidene fluoride; Preferably, the positive electrode active material is selected from any one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, lithium-rich manganese base, lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium cobalt oxide, and lithium nickel manganese oxide; Preferably, the solvent is selected from at least one of toluene, ethylbenzene, xylene, dimethylacetamide, N-methylpyrrolidone, acetone, dimethyl sulfoxide, and tetrahydrofuran.

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