Negative pole piece, preparation method thereof and secondary battery

Abstract

The invention relates to a negative pole piece, a preparation method thereof and a secondary battery. The negative pole piece comprises a negative current collector; the first active material layer is arranged on at least one side surface of the negative electrode current collector; the second active material layer is arranged on the surface of one side, far away from the negative electrode current collector, of the first active material layer; the first active substance layer comprises a first active material coated by a first modified polymer, and the second active substance layer comprises a second active material coated by a second modified polymer; one of the first modified polymer and the second modified polymer includes at least a furan group, and the other includes at least a maleimide group. According to the negative pole piece provided by the invention, a self-repairable polymer molecule is formed by utilizing a DA reaction between furan and maleimide, so that the interlayer stripping force in the later cycle period is recovered, the coating falling is reduced, and the cycle capacity retention rate is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a negative electrode sheet and a preparation method thereof, and a secondary battery. Background Art

[0002] Secondary batteries have the advantages of high energy density and long service life, and are widely used in electric vehicles and energy storage.

[0003] In order to improve the energy density of secondary batteries, the proportion of active materials can be increased by manufacturing thick electrodes. However, thick electrodes have large ion diffusion resistance, are prone to capacity loss at high charge and discharge rates, and have poor rate performance.

[0004] In traditional technology, double-layer coating technology is used to design the parameters of the electrode coating, thereby taking into account both high energy density and rate performance.

[0005] However, during the charging and discharging process, due to the different full-charge expansion rates of the negative electrode coating, local stress will be generated between the coating layers. As the cycle progresses, the interlayer stress becomes more and more uneven, which can easily lead to irreversible partial detachment of the upper coating, thereby causing the internal resistance of the battery cell to increase and the capacity to drop sharply. Summary of the invention

[0006] Based on this, it is necessary to provide a negative electrode plate and its preparation method and a secondary battery to repair and bond the active material layers, thereby restoring the interlayer peeling force in the later stage of the cycle, reducing coating shedding, reducing the internal resistance of the battery cell, and improving the cycle capacity retention rate.

[0007] The first aspect of the present application provides a negative electrode plate, which includes: a negative electrode collector; a first active material layer, arranged on at least one side surface of the negative electrode collector; and a second active material layer, arranged on the surface of the first active material layer away from the negative electrode collector; the first active material layer includes a first active material coated with a first modified polymer, and the second active material layer includes a second active material coated with a second modified polymer; one of the first modified polymer and the second modified polymer includes at least a furan group, and the other includes at least a maleimide group.

[0008] In some embodiments, the mass ratio of the first modified polymer to the first active material in the first active material layer is (0.03-0.2):1; and / or the mass ratio of the second modified polymer to the second active material in the second active material layer is (0.03-0.2):1.

[0009] In some embodiments, the mass ratio of the first modified polymer to the first active material in the first active material layer is equal to the mass ratio of the second modified polymer to the second active material in the second active material layer.

[0010] In some embodiments, the first polymer matrix material of the first modified polymer and the second polymer matrix material of the second modified polymer are the same or different and are independently selected from one or more of polymethyl methacrylate, polyethyl methacrylate, poly-N,N-methylenebisacrylamide, and polyacrylamide.

[0011] In some embodiments, the surface density of the first active material layer is lower than the surface density of the second active material layer, and the compaction density of the first active material layer is higher than the compaction density of the second active material layer.

[0012] In some embodiments, the surface density of the first active material layer is 60 g / cm 2 ~90g / cm 2 The surface density of the second active material layer is 90 g / cm 2 ~150g / cm 2 .

[0013] In some embodiments, the compaction density of the first active material layer is 1.55 g / cm 3 ~1.7g / cm 3 The compaction density of the second active material layer is 1.45 g / cm 3 ~1.55g / cm 3 .

[0014] The second aspect of the present application provides a method for preparing a negative electrode sheet, the method comprising the following steps:

[0015] The first active material and the first modified polymer undergo a modification reaction to obtain a first active material coated by the first modified polymer, and then the first active material coated by the first modified polymer is prepared into a first slurry; the second active material and the second modified polymer undergo a modification reaction to obtain a second active material coated by the second modified polymer, and then the second active material coated by the second modified polymer is prepared into a second slurry; the first slurry and the second slurry are sequentially coated on at least one side of the surface of the negative electrode collector, and after drying, a first active material layer arranged on the surface of the negative electrode collector and a second active material layer arranged on the surface of the first active material layer away from the negative electrode collector are formed to obtain a negative electrode sheet; wherein, one of the first modified polymer and the second modified polymer includes at least a furan group, and the other includes at least a maleimide group.

[0016] In some embodiments, the first modified polymer and the second modified polymer are prepared by the following steps: one of the furan modifier and the maleimide modifier is subjected to a graft copolymerization reaction with a first polymer monomer solution and an initiator to obtain a first modified polymer; the other of the furan modifier and the maleimide modifier is subjected to a graft copolymerization reaction with a second polymer monomer solution and an initiator to obtain a second modified polymer.

[0017] A third aspect of the present application provides a secondary battery, which includes the negative electrode sheet provided by the first aspect or the negative electrode sheet prepared by the preparation method provided by the second aspect.

[0018] Compared with the traditional technology, this application has at least the following beneficial effects:

[0019] The negative electrode plate provided in the present application is formed by coating the first active material and the second active material with the first modified polymer and the second modified polymer respectively, and utilizing the Diels-Alder reaction between the furan group and the maleimide group in the first modified polymer and the second modified polymer to form a self-repairing polymer molecule between the first active material layer and the second active material layer, and reconstructing the molecular structure at the fault site between the active material layers through the dynamic covalent bonds, hydrogen bonds and other structures inside the polymer molecules, thereby playing a role in repairing and bonding, thereby restoring the interlayer peeling force in the later stage of the cycle, reducing the shedding of the coating, reducing the internal resistance of the battery cell, and improving the cycle capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Flow chart of a method for preparing a negative electrode sheet in one embodiment of the present application.

[0021] Figure 2 These are SEM images of the cross-sectional morphology of the negative electrode plates before and after cycling in Example 1 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0022] References to embodiments of the present application will now be provided in detail, one or more examples of which are described below. Each example is provided as an explanation rather than a limitation of the present application. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made to the present application without departing from the scope or spirit of the present application. For example, a feature described or described as part of one embodiment may be used in another embodiment to produce a further embodiment.

[0023] Therefore, it is intended that the present application covers such modifications and variations that fall within the scope of the appended claims and their equivalents. Other objects, features and aspects of the present application are disclosed in or are apparent from the following detailed description. It will be appreciated by those of ordinary skill in the art that this discussion is merely a description of exemplary embodiments and is not intended to limit the broader aspects of the present application.

[0024] In the present application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0025] In this application, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are deemed to be continuous and include the minimum and maximum values ​​of the range, as well as each value between such minimum and maximum values. Further, when a range refers to an integer, each integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0026] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0027] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0028] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, it is mentioned that the method may also include step (c), which means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0029] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0030] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0031] The first aspect of the present application provides a negative electrode plate, which includes a negative electrode current collector, a first active material layer and a second active material layer. The first active material layer is arranged on at least one side surface of the negative electrode current collector. The second active material layer is arranged on the surface of the first active material layer away from the negative electrode current collector. The first active material layer includes a first active material coated with a first modified polymer, and the second active material layer includes a second active material coated with a second modified polymer. One of the first modified polymer and the second modified polymer includes at least a furan group, and the other includes at least a maleimide group.

[0032] The negative electrode plate provided in the present application is formed by coating the first active material and the second active material with the first modified polymer and the second modified polymer respectively, and utilizing the Diels-Alder reaction between the furan group and the maleimide group in the first modified polymer and the second modified polymer to form a self-repairing polymer molecule between the first active material layer and the second active material layer, and reconstructing the molecular structure at the fault site between the active material layers through the dynamic covalent bonds, hydrogen bonds and other structures inside the polymer molecules, thereby playing a role in repairing and bonding, thereby restoring the interlayer peeling force in the later stage of the cycle, reducing the shedding of the coating, reducing the internal resistance of the battery cell, and improving the cycle capacity retention rate.

[0033] In some embodiments, the first modified polymer includes at least a furan group, and the second modified polymer includes at least a maleimide group.

[0034] In some embodiments, the first modified polymer includes at least a maleimide group, and the second modified polymer includes at least a furan group.

[0035] In some embodiments, the first active material and the second active material are the same or different and are independently selected from natural graphite, artificial graphite, hard carbon, soft carbon, mesophase carbon microspheres, elemental silicon, silicon oxide SiO x (0<x≤2) or one or more of silicon-carbon materials.

[0036] In some embodiments, the mass ratio of the first modified polymer to the first active material in the first active material layer is (0.03-0.2):1, including but not limited to 0.03:1, 0.05:1, 0.1:1, and 0.2:1. Within the above mass ratio range, while suppressing the volume expansion of the first active material, the first active material layer is ensured to have a high theoretical gram capacity and ionic conductivity, thereby not only improving the cycle performance of the secondary battery, but also ensuring the high energy density and rate performance of the battery.

[0037] In some embodiments, the mass ratio of the second modified polymer to the second active material in the second active material layer is (0.03-0.2):1, including but not limited to 0.03:1, 0.05:1, 0.1:1, and 0.2:1. Within the above mass ratio range, while suppressing the volume expansion of the second active material, the second active material layer is ensured to have a high theoretical gram capacity and ionic conductivity, thereby not only improving the cycle performance of the secondary battery, but also ensuring the high energy density and rate performance of the battery.

[0038] In some embodiments, the first polymer matrix material of the first modified polymer and the second polymer matrix material of the second modified polymer are the same or different, and are independently selected from one or more of polymethyl methacrylate (PMMA), polyethyl methacrylate (PEMA), and poly N,N-methylenebisacrylamide (PMBA).

[0039] It is understandable that PMMA, PEMA and PMBA, in addition to being used to load furan groups and maleimide groups, also have certain bonding properties, further improving the mechanical stability of the first active material layer and the second active material layer, thereby improving the cycle performance of the secondary battery. At the same time, the above materials have a good lithium ion transport mechanism when used in lithium-ion batteries, reducing the risk of lithium plating at the negative electrode.

[0040] Furthermore, the first polymer matrix material and the second polymer matrix material are both polyethyl methacrylate (PEMA). PEMA has good flexibility, which is conducive to the self-repairing reaction, further improves the recovery rate of the interlayer peeling force, and improves the cycle capacity retention rate.

[0041] In some embodiments, the thickness of the negative electrode sheet is 100 μm to 170 μm.

[0042] In some embodiments, the surface density of the first active material layer is lower than the surface density of the second active material layer, and the compaction density of the first active material layer is higher than the compaction density of the second active material layer. The present application improves the liquid absorption rate and the electrical conductivity of the negative electrode sheet by performing the above-mentioned arrangement on the first active material layer and the second active material layer of the negative electrode sheet.

[0043] In some specific embodiments, the surface density of the first active material layer is 60 g / cm 2 ~90g / cm 2 The surface density of the second active material layer is 90 g / cm 2 ~150g / cm 2 Furthermore, the surface density of the first active material layer is 75 g / cm 2 The surface density of the second active material layer is 120 g / cm 2 .

[0044] In some specific embodiments, the compaction density of the first active material layer is 1.55 g / cm 3 ~1.7g / cm 3 The compaction density of the second active material layer is 1.45 g / cm 3 ~1.55g / cm 3 Furthermore, the compaction density of the first active material layer is 1.63 g / cm 3 The compaction density of the second active material layer is 1.50 g / cm 3 .

[0045] In some embodiments, the first active material layer further includes a negative electrode conductor, a negative electrode binder, and a negative electrode dispersant, and the second active material layer further includes a negative electrode binder, a negative electrode conductor, and a negative electrode dispersant.

[0046] The present application has no particular restrictions on the types of negative electrode conductors, negative electrode binders and negative electrode dispersants, as long as the purpose of the present application can be achieved. For example, the negative electrode conductor may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers, and the conductive carbon black may include but is not limited to at least one of acetylene black or Ketjen black. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nano-carbon fibers. The above-mentioned metal materials may include but are not limited to metal powders and / or metal fibers, specifically, metals may include but are not limited to at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymers may include but are not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. For example, the negative electrode binder may include but is not limited to at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, polyamide-imide, styrene-butadiene rubber or polyvinylidene fluoride. For example, the negative electrode dispersant may include, but is not limited to, one or more of sodium hydroxymethyl cellulose, polyvinyl pyrrolidone, sodium carboxymethyl cellulose, and polyacrylic acid.

[0047] The present application has no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or composite current collector. For example, the composite current collector may be a lithium copper composite current collector, a carbon copper composite current collector, a nickel copper composite current collector, a titanium copper composite current collector, etc. The present application has no particular restrictions on the thickness of the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector may be 4 μm to 15 μm.

[0048] The second aspect of the present application provides a method for preparing a negative electrode sheet, such as Figure 1 As shown, the preparation method comprises the following steps:

[0049] S1. A first active material and a first modified polymer undergo a modification reaction to obtain a first active material coated with the first modified polymer, and then the first active material coated with the first modified polymer is prepared into a first slurry.

[0050] S2, the second active material and the second modified polymer undergo a modification reaction to obtain a second active material coated by the second modified polymer, and then the second active material coated by the second modified polymer is prepared into a second slurry.

[0051] S3. Coating the first slurry and the second slurry in sequence on at least one side of the negative electrode current collector, and after drying, forming a first active material layer arranged on the surface of the negative electrode current collector and a second active material layer arranged on the surface of the first active material layer away from the negative electrode current collector to obtain a negative electrode sheet.

[0052] One of the first modified polymer and the second modified polymer includes at least a furan group, and the other includes at least a maleimide group.

[0053] The preparation method of the negative electrode plate provided in the present application is that the first active material and the second active material are respectively coated with the first modified polymer and the second modified polymer through a modification reaction. When drying after coating, the furan group and the maleimide group in the first modified polymer and the second modified polymer undergo a Diels-Alder reaction to form a polymer molecule. The polymer molecule contains structures such as dynamic covalent bonds and hydrogen bonds. The molecular structure is reconstructed at the interlayer fracture site to play a repair and bonding role, thereby restoring the interlayer peeling force in the later stage of the cycle, reducing the shedding of the coating, reducing the internal resistance of the battery cell, and improving the cycle capacity retention rate of the secondary battery.

[0054] In some embodiments, in step S1 or step S2, the temperature of the modification reaction is 100°C to 200°C, including but not limited to 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, and 200°C.

[0055] In some embodiments, in step S1 or step S2, the modification reaction time is 0.5 h to 1.5 h, including but not limited to 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h, 1 h, 1.1 h, 1.2 h, 1.3 h, 1.4 h, and 1.5 h.

[0056] In some embodiments, in step S3, the first slurry and the second slurry coated on at least one side of the negative electrode current collector are dried at 25° C. to 200° C. Further, the first slurry and the second slurry coated on at least one side of the negative electrode current collector are dried at 60° C.

[0057] In some embodiments, in step S3, the first slurry and the second slurry coated on at least one side of the negative electrode current collector are dried for 5 to 60 minutes. Further, in step S3, the first slurry and the second slurry coated on at least one side of the negative electrode current collector are dried for 10 minutes.

[0058] In some embodiments, in step S3, the surface density of the first active material layer is 60 g / cm 2 ~90g / cm 2 , including but not limited to 60g / cm 2 , 70g / cm 2 、80g / cm 2 , 90g / cm 2 The surface density of the second active material layer is 90 g / cm 2 ~150g / cm 2 , including but not limited to 90g / cm 2 , 100g / cm 2 , 110g / cm 2 , 120g / cm 2 , 130g / cm 2 , 140g / cm 2 , 150g / cm 2 .

[0059] In some embodiments, in step S3, the compaction density of the first active material layer is 1.55 g / cm 3 ~1.7g / cm 3 , including but not limited to 1.55cm 3 、1.6cm3 、1.65cm 3 、1.7cm 3 The compaction density of the second active material layer is 1.45 g / cm 3 ~1.55g / cm 3 , including but not limited to 1.45cm 3 、1.5cm 3 、1.55cm 3 .

[0060] In some embodiments, the first modified polymer and the second modified polymer are prepared by the following steps:

[0061] S01, one of the furan modifier and the maleimide modifier, the first polymer monomer solution and the initiator are subjected to graft copolymerization reaction to obtain a first modified polymer.

[0062] S02, the other one of the furan modifier and the maleimide modifier, the second polymer monomer solution and the initiator are subjected to graft copolymerization reaction to obtain a second modified polymer.

[0063] In some embodiments, the first polymer monomer and the second polymer monomer are the same or different and are independently selected from one or more of methyl methacrylate (MMA), ethyl methacrylate (EMA), and N,N-methylenebisacrylamide (MBA). Further, the first polymer monomer and the second polymer monomer are both ethyl methacrylate (EMA).

[0064] In some embodiments, the furan modifier includes one or more of a furan group-containing silane coupling agent (KH560-FA), furan diacid polyester, furan diol polyester, furan cyclopentadiene epoxy resin, and furan-modified bisphenol A epoxy resin.

[0065] In some embodiments, the maleimide modifier includes one or more of maleimide, N-phenylmaleimide, dichloromaleimide, IV-(4-carboxyphenyl)maleimide, and fluorine-containing maleimide derivatives.

[0066] In some embodiments, the initiator includes one or more of benzoyl peroxide (BPO), dicumyl peroxide (DCP), di-tert-butyl peroxide (DTBP), azobisisobutyronitrile (AIBN) and azobisisoheptanenitrile (ABVN).

[0067] In some embodiments, in step S01 or step S02, the temperature of the graft copolymerization reaction is 40°C to 80°C.

[0068] In some embodiments, in step S01 or step S02, the graft copolymerization reaction time is 2 hours to 6 hours.

[0069] In some embodiments, in step S01 or step S02, the graft copolymerization reaction is carried out under an inert atmosphere. Further, the graft copolymerization reaction is carried out under a nitrogen atmosphere.

[0070] A third aspect of the present application provides a secondary battery, which includes the negative electrode sheet provided by the first aspect or the negative electrode sheet prepared by the preparation method provided by the second aspect.

[0071] In some embodiments, the secondary battery further includes a positive electrode sheet, which includes a positive electrode collector and a positive electrode material layer disposed on at least one surface of the positive electrode collector. The present application has no particular restrictions on the positive electrode collector, as long as the purpose of the present application can be achieved. For example, the positive electrode collector may include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector). The present application has no particular restrictions on the thickness of the positive electrode collector, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode collector may be 5 μm to 20 μm.

[0072] In some embodiments, the positive electrode material layer includes a positive electrode active material. The present application has no particular restrictions on the positive electrode active material, as long as the purpose of the present application can be achieved. For example, the positive electrode active material may include but is not limited to lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, lithium iron manganese phosphate, or lithium titanate. At least one.

[0073] In some embodiments, the positive electrode material layer may further include a positive electrode conductor and a positive electrode binder. The present application does not particularly limit the types of the positive electrode conductor and the positive electrode binder, as long as the purpose of the present application can be achieved. For example, it may be at least one of the above-mentioned negative electrode conductor and the above-mentioned negative electrode binder. The present application does not particularly limit the mass ratio of the positive electrode active material, the positive electrode conductor, and the positive electrode binder in the positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved.

[0074] In some embodiments, the secondary battery further includes a separator. The present application has no particular restrictions on the separator, as long as the purpose of the present application can be achieved. For example, the material of the separator may include, but is not limited to, polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET) films), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of separator may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane.

[0075] In some embodiments, the isolation membrane includes a substrate layer. The substrate layer may be a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. In the present application, there is no particular limitation on the thickness of the isolation membrane, as long as the purpose of the present application can be achieved. For example, the thickness of the isolation membrane may be 3 μm to 30 μm.

[0076] In some embodiments, the secondary battery further includes an electrolyte, and the electrolyte includes a lithium salt and a non-aqueous solvent. The present application has no particular restrictions on lithium salts, as long as the purpose of the present application can be achieved. For example, the lithium salt may include but is not limited to at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalate borate) (LiBOB), and lithium difluoroborate. The present application has no particular restrictions on the content of lithium salts in the electrolyte, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on non-aqueous solvents, as long as the purpose of the present application can be achieved, for example, non-aqueous solvents may include but are not limited to at least one of carbonate compounds, carboxylate compounds, ether compounds or other organic solvents. The above-mentioned carbonate compounds may include but are not limited to at least one of chain carbonate compounds, cyclic carbonate compounds or fluorocarbonate compounds. The above-mentioned linear carbonate compound may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC) or ethyl methyl carbonate (EMC). The above-mentioned cyclic carbonate may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The present application has no particular restrictions on the content of the non-aqueous solvent in the electrolyte, as long as the purpose of the present application can be achieved.

[0077] In the present application, the secondary battery also includes a shell for accommodating a positive electrode plate, a separator, a negative electrode plate and an electrolyte, as well as other components known in the field of secondary batteries, and the present application does not limit the above-mentioned other components. The present application has no special restrictions on the shell, and it can be a shell known in the art, as long as the purpose of the present application can be achieved. For example, the shell can be a hard shell or a flexible shell. The material of the hard shell can be metal, and the present application does not limit the type of metal. A metal hard shell known in the art can be used, as long as the purpose of the present application can be achieved. The flexible shell can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0078] The preparation process of the secondary battery of the present application is well known to those skilled in the art and is not particularly limited in the present application. For example, the preparation process of the secondary battery may include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding and folding them as needed to obtain an electrode assembly of a winding structure, placing the electrode assembly in a housing, injecting an electrolyte into the housing and sealing it to obtain a secondary battery.

[0079] The present application will be further described below with reference to specific embodiments and comparative examples.

[0080] Example 1

[0081] Negative electrode:

[0082] (1) 0.4 g of furan modifier-furan group-containing silane coupling agent (KH560-FA) was subjected to graft copolymerization reaction with 100 mL of first polymer monomer ethyl methacrylate (EMA) solution and initiator benzoyl peroxide (BPO) under nitrogen atmosphere and heated at 60°C for 4 h. The unreacted EMA and BPO were washed with acetone three times to obtain a first modified polymer modified with furan groups.

[0083] The concentration of the ethyl methacrylate solution was 20% (w / v), and the mass of BPO was 0.1 g.

[0084] (2) 2 g of maleimide modifier-maleimide and 150 mL of second polymer monomer ethyl methacrylate (EMA) solution and initiator benzoyl peroxide (BPO) were heated at 60°C under nitrogen atmosphere for 3 h to carry out graft copolymerization reaction, and the unreacted EMA and BPO were removed by washing with acetone three times to obtain a second modified polymer modified with maleimide groups.

[0085] The concentration of the ethyl methacrylate solution was 30% (w / v), and the mass of BPO was 0.5 g.

[0086] (3) The first active material graphite and the first modified polymer were subjected to modification reaction in an oil bath at 120° C. for 1 hour to obtain the first active material coated with the first modified polymer layer.

[0087] Wherein, the mass ratio of the first modified polymer to the first active material graphite is 0.05:1.

[0088] (4) The first active material coated by the first modified polymer layer, the negative electrode conductive agent conductive carbon black (Super-P), the negative electrode binder styrene-butadiene rubber (SBR), and the negative electrode dispersant sodium carboxymethyl cellulose (CMC) are added into a stirring tank in a mass ratio of 96:1:1.2:1.8, and the solvent is deionized water to prepare a first slurry.

[0089] (5) The second active material graphite and the second modified polymer were subjected to modification reaction in an oil bath at 120° C. for 1 hour to obtain a second active material coated with a second modified polymer layer.

[0090] The mass ratio of the second modified polymer to the second active material graphite is 0.05:1.

[0091] (6) The second active material coated by the second modified polymer layer, the negative electrode conductive agent conductive carbon black (Super-P), the negative electrode binder styrene-butadiene rubber (SBR), and the negative electrode dispersant sodium carboxymethyl cellulose (CMC) are added into a stirring tank in a mass ratio of 96:1:1.2:1.8, and the solvent is deionized water to prepare a second slurry.

[0092] (7) The first slurry and the second slurry are sequentially coated on both side surfaces of the negative electrode current collector copper foil, and dried at 60° C. for 10 min to form a first active material layer disposed on the surface of the negative electrode current collector and a second active material layer disposed on the surface of the first active material layer away from the negative electrode current collector, thereby obtaining a negative electrode sheet.

[0093] The surface density of the first active material layer is 75 g / cm 2 , compacted density is 1.63g / cm 3 The surface density of the second active material layer is 120 g / cm 2 , compacted density is 1.50g / cm 3 ; The thickness of the negative electrode sheet is 143μm.

[0094] Secondary battery:

[0095] The lithium iron phosphate positive electrode sheet, PP / PE / PP separator and the above negative electrode sheet are stacked together to form a bare battery cell and placed in an aluminum-plastic film. After welding the top cover, the electrolyte (EC:EMC volume ratio is 3:7, LiPF6 is 1 mol / L) is injected, followed by sealing, formation and other processes to obtain a lithium-ion secondary battery.

[0096] Example 2

[0097] The method for preparing the negative electrode plate and the secondary battery in this embodiment is basically the same as that in Embodiment 1, except that:

[0098] In step (1), the first polymer monomer is replaced by methyl methacrylate (MMA);

[0099] In step (2), the second polymer monomer is replaced by methyl methacrylate (MMA).

[0100] Example 3

[0101] The method for preparing the negative electrode plate and the secondary battery in this embodiment is basically the same as that in Embodiment 1, except that:

[0102] In step (3), the mass ratio of the first modified polymer to the first active material graphite is 0.1:1.

[0103] In step (5), the mass ratio of the second modified polymer to the second active material graphite is 0.05:1.

[0104] Example 4

[0105] The method for preparing the negative electrode plate and the secondary battery in this embodiment is basically the same as that in Embodiment 1, except that:

[0106] In step (3), the mass ratio of the first modified polymer to the first active material graphite is 0.05:1.

[0107] In step (5), the mass ratio of the second modified polymer to the second active material graphite is 0.1:1.

[0108] Example 5

[0109] The method for preparing the negative electrode plate and the secondary battery in this embodiment is basically the same as that in Embodiment 1, except that:

[0110] In step (3), the mass ratio of the first modified polymer to the first active material graphite is 0.2:1.

[0111] In step (5), the mass ratio of the second modified polymer to the second active material graphite is 0.2:1.

[0112] Example 6

[0113] The method for preparing the negative electrode plate and the secondary battery in this embodiment is basically the same as that in Embodiment 1, except that:

[0114] In step (3), the mass ratio of the first modified polymer to the first active material graphite is 0.03:1.

[0115] In step (5), the mass ratio of the second modified polymer to the second active material graphite is 0.03:1.

[0116] Example 7

[0117] The method for preparing the negative electrode plate and the secondary battery in this embodiment is basically the same as that in Embodiment 1, except that:

[0118] In step (7), the surface density of the first active material layer is 75 g / cm 2 , compacted density is 1.63g / cm 3 The surface density of the second active material layer is 120 g / cm 2 , compacted density is 1.63g / cm 3 .

[0119] Example 8

[0120] The method for preparing the negative electrode plate and the secondary battery in this embodiment is basically the same as that in Embodiment 1, except that:

[0121] In step (7), the surface density of the first active material layer is 120 g / cm 2 , compacted density is 1.63g / cm 3 The surface density of the second active material layer is 120 g / cm 2 , compacted density is 1.50g / cm 3 .

[0122] Comparative Example 1

[0123] Negative electrode:

[0124] (1) The first active material graphite, the negative electrode conductive agent conductive carbon black (Super-P), the negative electrode binder styrene-butadiene rubber (SBR), and the negative electrode dispersant sodium carboxymethyl cellulose (CMC) were added into a stirring tank at a mass ratio of 96:1:1.2:1.8, and the solvent was deionized water to prepare a first slurry.

[0125] (2) The second active material graphite, the negative electrode conductive agent conductive carbon black (Super-P), the negative electrode binder styrene-butadiene rubber (SBR), and the negative electrode dispersant sodium carboxymethyl cellulose (CMC) were added to a stirring tank in a mass ratio of 96:1:1.2:1.8, and the solvent was deionized water to prepare a second slurry.

[0126] (7) The first slurry and the second slurry are sequentially coated on both sides of the negative electrode current collector copper foil, and dried at 60° C. for 10 min to form a first active material layer disposed on the surface of the negative electrode current collector and a second active material layer disposed on the surface of the first active material layer away from the negative electrode current collector, and then rolled to obtain a negative electrode sheet.

[0127] The surface density of the first active material layer is 75 g / cm 2 , compacted density is 1.63g / cm 3 The surface density of the second active material layer is 120 g / cm 2 , compacted density is 1.50g / cm 3 ; The thickness of the negative electrode sheet is 143μm.

[0128] Secondary battery:

[0129] The lithium iron phosphate positive electrode sheet, PP / PE / PP separator and the above negative electrode sheet are stacked together to form a bare battery cell and placed in an aluminum-plastic film. After welding the top cover, the electrolyte (EC:EMC volume ratio is 3:7, LiPF6 is 1 mol / L) is injected, followed by sealing, formation and other processes to obtain a lithium-ion secondary battery.

[0130] Performance Testing

[0131] (1) Cycle test

[0132] The batteries obtained from the above embodiments and comparative examples were subjected to constant current and constant voltage charge and discharge cycle test at a rate of 1C. The charge cutoff condition was 3.65V constant voltage until the current was less than 0.05C, and the discharge cutoff condition was 2.5V voltage cutoff. The protection voltages for charge and discharge were 3.75V and 2.4V respectively, and the cycle test was carried out for 2000 weeks.

[0133] Calculate the cycle capacity retention rate (%) = (discharge capacity of the 2000th cycle / discharge capacity of the first cycle) × 100%. The test results are shown in Table 1.

[0134] (2) Pole cross-sectional morphology

[0135] The negative electrode sheets obtained in the above-mentioned Example 1 and Comparative Example 1 and the disassembled electrode sheets after the cycle test were cut by the confocal ion beam technology, and the electron microscope photos with a magnification of 500 times were taken at an accelerating voltage of 3kV to 5kV and a working distance of 5.5mm to 6mm to observe the peeling of the upper and lower graphite layers. The test results are shown in Figure 1 .

[0136] (3) Peel force test

[0137] The negative electrode sheets obtained by the above embodiment and comparative example were subjected to cycle tests for 200, 500, and 1000 weeks, respectively. The sheets were disassembled and cut into strips with a width of 24 mm and a length of >150 mm. The strips were pasted on a flat thin steel plate and fixed with double-sided tape. The strips were peeled off at an angle of 180° using a universal tensile testing machine, and the peeling force values ​​were recorded. The test results are shown in Table 1.

[0138] (4) Liquid absorption test

[0139] Liquid absorption rate = (pole sheet mass after liquid absorption - pole sheet mass before liquid absorption) / pole sheet mass; the liquid absorption rate of the negative electrode sheet is measured and calculated, and the test results are shown in Table 1.

[0140] (5) DCR test

[0141] The batteries in the embodiment and the comparative example were subjected to 1C discharge DCR (direct current resistance) test. The test results are shown in Table 1.

[0142] Table 1

[0143]

[0144] As shown in Table 1, by comparing Examples 1 to 8 and Comparative Example 1, it can be seen that the negative electrode plate provided by the present application improves the cycle retention rate, reduces the attenuation of the peeling force during the cycle, reduces the full-charge expansion rate, and ensures the conductivity of the negative electrode plate.

[0145] Combination Figure 1 As shown, by comparing the cross-sectional morphology of Example 1 and Comparative Example 1, it can be seen that since the active material layer in Comparative Example 1 is not coated with the modified polymer layer, the interlayer structure cannot be self-repaired, resulting in the gradual peeling of the upper and lower graphite layers in the later stage of the cycle.

[0146] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0147] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the technical concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A negative electrode plate, characterized in that: include: Anode current collector; A first active material layer is disposed on at least one side of the negative electrode current collector; as well as A second active material layer is disposed on a surface of the first active material layer away from the negative electrode current collector; The first active material layer includes a first active material coated by a first modified polymer, and the second active material layer includes a second active material coated by a second modified polymer; One of the first modified polymer and the second modified polymer includes at least a furan group, and the other includes at least a maleimide group.

2. The negative electrode sheet according to claim 1, characterized in that: The mass ratio of the first modified polymer to the first active material in the first active material layer is (0.03-0.2):1; and / or, The mass ratio of the second modified polymer to the second active material in the second active material layer is (0.03-0.2):

1.

3. The negative electrode sheet according to claim 2, characterized in that: A mass ratio of the first modified polymer to the first active material in the first active material layer is equal to a mass ratio of the second modified polymer to the second active material in the second active material layer.

4. The negative electrode sheet according to claim 1, characterized in that: The first polymer matrix material of the first modified polymer and the second polymer matrix material of the second modified polymer are the same or different and are independently selected from one or more of polymethyl methacrylate, polyethyl methacrylate and poly-N,N-methylenebisacrylamide.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The surface density of the first active material layer is lower than the surface density of the second active material layer, and the compaction density of the first active material layer is higher than the compaction density of the second active material layer.

6. The negative electrode sheet according to claim 5, characterized in that: The surface density of the first active material layer is 60 g / cm 2 ~90g / cm 2 The surface density of the second active material layer is 90 g / cm 2 ~150g / cm 2 .

7. The negative electrode sheet according to claim 5, characterized in that: The compaction density of the first active material layer is 1.55 g / cm 3 ~1.7g / cm 3 The compaction density of the second active material layer is 1.45 g / cm 3 ~1.55g / cm 3 .

8. A method for preparing a negative electrode sheet, characterized in that: The following steps are involved: The first active material and the first modified polymer undergo a modification reaction to obtain a first active material coated with the first modified polymer, and the first active material coated with the first modified polymer is prepared into a first slurry; The second active material and the second modified polymer undergo a modification reaction to obtain a second active material coated with the second modified polymer, and the second active material coated with the second modified polymer is prepared into a second slurry; sequentially coating the first slurry and the second slurry on at least one side of the negative electrode current collector, and drying to form a first active material layer disposed on the surface of the negative electrode current collector and a second active material layer disposed on a side of the first active material layer away from the negative electrode current collector, to obtain a negative electrode sheet; Wherein, one of the first modified polymer and the second modified polymer at least includes a furan group, and the other at least includes a maleimide group.

9. The method for preparing a negative electrode sheet according to claim 8, characterized in that: The first modified polymer and the second modified polymer are prepared by the following steps: One of the furan modifier and the maleimide modifier is subjected to graft copolymerization reaction with the first polymer monomer solution and the initiator to obtain the first modified polymer; The other of the furan modifier and the maleimide modifier is subjected to graft copolymerization reaction with the second polymer monomer solution and an initiator to obtain the second modified polymer.

10. A secondary battery, characterized in that: The secondary battery comprises the negative electrode sheet according to any one of claims 1 to 7 or the negative electrode sheet prepared by the preparation method according to claim 8 or 9.

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