Negative electrode sheet and secondary battery

Abstract

The present invention belongs to the technical field of secondary batteries, and in particular relates to a negative electrode plate and a secondary battery, wherein the negative electrode plate includes a negative electrode current collector, a first active layer, and a second active layer. The single-sided coating surface density of the negative electrode plate is A, the percentage of the first constant current and constant voltage charging capacity to the total capacity is a, the single-sided coating surface density of the first active layer is B, and the single-sided coating surface density of the second active layer is C. Then, the single-sided coating surface density of the first active layer and the single-sided coating surface density of the second active layer satisfy the following relationship: B = A*a%, C = A-B. The negative electrode plate of the present invention is provided with two coating layers, and the surface density of the two coating layers is set according to the charging mode, so that the first active layer close to the negative electrode current collector can achieve rapid lithium insertion, thereby increasing the charging rate, and the second active layer is away from the negative electrode current collector, reducing the impact of large currents. At the same time, the two are used in combination to improve the cycle capacity retention rate and thickness expansion rate of the plate.

Description

Technical Field

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

[0002] Batteries are widely used as energy storage components in electric vehicles. Compared to traditional lead-acid and nickel-cadmium batteries, lithium-ion batteries offer high specific energy, long life, low pollution, and high operating voltage, making them widely used. Lithium-ion batteries are "rocking chair batteries" that use lithium-intercalating compounds with low lithium insertion potentials, such as graphite, hard carbon, and silicon oxide, as their negative electrodes. As the number of cycles increases, the thickness of lithium-ion batteries also increases accordingly. As demand for charging time increases, the charge rate of lithium-ion batteries continues to increase, and charging methods are constantly evolving, from traditional single-stage constant current and constant voltage charging to multi-stage constant current and constant voltage charging, significantly increasing the charging rate. As the charging rate increases, the impact and damage to the negative electrode material during charging increases, reducing the cycle capacity retention and increasing thickness expansion, which is particularly noticeable for batteries with silicon-carbon negative electrodes. Summary of the Invention

[0003] One of the purposes of the present invention is to address the shortcomings of the existing technology and provide a negative electrode plate with two layers of coating. The surface density of the two layers of coating is designed according to the charging system, so that the negative electrode plate can achieve rapid charge and discharge while maintaining a high cycle capacity retention rate and thickness expansion rate.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A negative electrode plate includes a negative electrode current collector, a first active layer disposed on at least one surface of the negative electrode current collector, and a second active layer disposed on a side of the first active layer away from the negative electrode current collector. The single-sided coating area density of the negative electrode plate is A, the percentage of the first section constant current and constant voltage charging capacity to the total capacity is a, the single-sided coating area density of the first active layer is B, and the single-sided coating area density of the second active layer is C. Then, the single-sided coating area density of the first active layer and the single-sided coating area density of the second active layer satisfy the following relationship: B = A*a%, C = AB.

[0006] Preferably, the first active layer includes one or more of artificial graphite, conductive carbon black, coke, carbon fiber, and carbon nanotubes.

[0007] Preferably, the second active layer includes carbon materials and silicon materials, the carbon materials include one or more of graphite, conductive carbon black, coke, carbon fiber, and carbon nanotubes, and the silicon materials include one or more of silicon monoxide and silicon oxide.

[0008] Preferably, the mass ratio of carbon material to silicon material in the second active layer is 1-3:2-5.

[0009] Preferably, the single-sided coating density A of the negative electrode sheet is in the range of 0.002 g / cm 2 ~0.12g / cm 2 .

[0010] Preferably, the percentage of the first constant current and constant pressure charging capacity to the total capacity is a value ranging from 30% to 80%.

[0011] Preferably, the single-sided coating density B of the first active layer is in the range of 0.002 g / cm 2 ~0.08g / cm 2 .

[0012] Preferably, the single-side coating density C of the second active layer is in the range of 0.002 g / cm 2 ~0.08g / cm 2 .

[0013] Preferably, the thickness ratio of the first active layer to the second active layer is 1-6:1-2.

[0014] A second object of the present invention is to provide a secondary battery that addresses the deficiencies of the prior art and can achieve rapid charge and discharge and has good capacity retention and thickness expansion rate.

[0015] In order to achieve the above object, the present invention adopts the following technical solutions:

[0016] A secondary battery comprises the above-mentioned negative electrode plate.

[0017] Compared with the prior art, the beneficial effect of the present invention is that the negative electrode plate of the present invention is provided with two coating layers, and the surface density of the two coating layers is set according to the charging mode, so that the first active layer close to the negative electrode current collector can realize rapid lithium insertion, thereby improving the charging rate, and the second active layer is far away from the negative electrode current collector, reducing the impact of large current. At the same time, the two are used in combination to improve the cycle capacity retention rate and thickness expansion rate of the plate, thereby improving the energy density. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of the negative electrode plate of the present invention.

[0019] Figure 2 It is a comparison chart of the capacity retention rates of Example 1 of the present invention and Comparative Example 1.

[0020] Figure 3 3 is a comparison chart of the thickness expansion rates of Example 1 of the present invention and Comparative Example 1.

[0021] Among them: 1. negative electrode current collector; 2. first active layer; 3. second active layer. DETAILED DESCRIPTION

[0022] The present invention will be described in further detail below with reference to specific implementation methods and the accompanying drawings, but the implementation methods of the present invention are not limited thereto.

[0023] A negative electrode plate includes a negative electrode current collector 1, a first active layer 2 disposed on at least one surface of the negative electrode current collector 1, and a second active layer 3 disposed on a side of the first active layer 2 away from the negative electrode current collector 1. The single-sided coating area density of the negative electrode plate is A, the percentage of the first section constant current and constant voltage charging capacity to the total capacity is a, the single-sided coating area density of the first active layer 2 is B, and the single-sided coating area density of the second active layer 3 is C. Then, the single-sided coating area density of the first active layer 2 and the single-sided coating area density of the second active layer 3 satisfy the following relationship: B = A*a%, C = AB.

[0024] The negative electrode plate of the present invention is provided with two coating layers. The surface density of the two coating layers is set according to the charging mode, so that the first active layer 2 close to the negative electrode current collector 1 can realize rapid lithium insertion, thereby improving the charging rate. The second active layer 3 is far away from the negative electrode current collector 1, reducing the impact of large current. At the same time, the two are used in combination to improve the cycle capacity retention rate and thickness expansion rate of the plate, thereby improving the energy density.

[0025] The first stage of a multi-stage constant current and constant voltage charging system is often high-current constant current and constant voltage charging, while the lithium insertion of the negative electrode is driven by an external force, and lithium insertion occurs preferentially closer to the negative electrode current collector 1 copper foil. The present invention uses a double-layer coating technology to match the charging system to calculate the coating surface density of the first active layer 2 and the second active layer 3. Excessive lithium insertion and excessive expansion of the silicon-carbon composite material in the bottom layer will cause cracking and inactivation of the active material, thereby improving the cycle capacity retention rate of the silicon-carbon composite battery and reducing the thickness expansion rate. Through the above precise design, the proportion of silicon composite material in the negative electrode active material can be increased, further improving the battery energy density.

[0026] In some embodiments, the first active layer 2 comprises one or more of artificial graphite, conductive carbon black, coke, carbon fiber, and carbon nanotubes. The first active layer 2 comprises a carbon active material, a conductive agent, and a binder. The carbon active material is primarily artificial graphite, conductive carbon black, coke, carbon fiber, carbon nanotubes, etc., and can rapidly intercalate lithium with the negative electrode current collector 1 without causing significant expansion.

[0027] In some embodiments, the second active layer 3 includes carbon materials and silicon materials, wherein the carbon materials include one or more of graphite, conductive carbon black, coke, carbon fiber, and carbon nanotubes, and the silicon materials include one or more of silicon monoxide and silicon oxide. Compared with the first active layer 2, the second active layer 3 mainly contains silicon materials, thereby increasing the specific capacity of the negative electrode sheet, and the first active layer 2 is provided between the second active layer 3 and the negative electrode current collector 1 to prevent the silicon material in the second active layer 3 and the silicon-carbon composite material from being excessively embedded in the bottom layer with lithium, resulting in volume expansion and inactivation of the active material. The combination of the first active layer 2 and the second active layer 3 improves the cycle capacity of the silicon-carbon composite material battery and reduces the thickness expansion rate, while also improving the energy density of the battery.

[0028] In some embodiments, the mass ratio of the carbon material to the silicon material in the second active layer 3 is 1-3:2-5. The mass ratio of the carbon material to the silicon material in the second active layer 3 is set to ensure that the electrode has both a certain charge and discharge rate performance and a certain gram capacity. Preferably, the mass ratio of the carbon material to the silicon material is 1:2, 1:3, 1:4, 1:5, 2:3, 2:4, 2:5, 3:2, 3:3, 3:4, or 3:5. Preferably, the silicon material accounts for 3% to 15% of the negative electrode.

[0029] In some embodiments, the single-sided coating density A of the negative electrode sheet is in the range of 0.002 g / cm 2 ~0.12g / cm 2 Preferably, the single-sided coating density A of the negative electrode sheet is 0.002 g / cm 2 , 0.05g / cm 2 , 0.06g / cm 2 , 0.07g / cm 2 , 0.08g / cm 2 , 0.09g / cm 2 , 0.1g / cm 2 , 0.11g / cm 2 , 0.12g / cm 2 .

[0030] In some embodiments, the percentage a of the first constant current and constant pressure charging capacity to the total capacity ranges from 30% to 80%. Preferably, the percentage a of the first constant current and constant pressure charging capacity to the total capacity ranges from 30%, 35%, 40%, 45%, 50%, 55%, 60%, 70%, or 80%.

[0031] In some embodiments, the single-sided coating density B of the first active layer 2 is in the range of 0.002 g / cm 2 ~0.08g / cm2 Preferably, the single-sided coating density B of the first active layer 2 is in the range of 0.002 g / cm 2 , 0.008g / cm 2 , 0.02g / cm 2 , 0.03g / cm 2 , 0.04g / cm 2 , 0.05g / cm 2 , 0.06g / cm 2 , 0.07g / cm 2 , 0.08g / cm 2 .

[0032] In some embodiments, the single-sided coating density C of the second active layer 3 is in the range of 0.02 g / cm 2 ~0.08g / cm 2 Preferably, the single-sided coating density C of the second active layer 3 is 0.002 g / cm 2 , 0.008g / cm 2 , 0.02g / cm 2 , 0.03g / cm 2 , 0.04g / cm 2 , 0.05g / cm 2 , 0.06g / cm 2 , 0.07g / cm 2 , 0.08g / cm 2 .

[0033] In some embodiments, the thickness ratio of the first active layer 2 to the second active layer 3 is 1 to 6:1 to 2. Preferably, the thickness ratio of the first active layer 2 to the second active layer 3 is 1:1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 2:1, 2:1.3, 2:1.4, 2:1.5, 3:1.3, 3:1.5, 6:1, 5:1, or 5:2.

[0034] A secondary battery can realize rapid charge and discharge and has good capacity retention rate and thickness expansion rate.

[0035] A secondary battery comprises the above-mentioned negative electrode plate.

[0036] The secondary battery can be a lithium-ion battery, a sodium-ion battery, a potassium-ion battery, an aluminum-ion battery, a magnesium-ion battery, or a calcium-ion battery. The following secondary battery uses a lithium-ion battery as an example. Specifically, a lithium-ion battery includes a positive electrode sheet, a separator, a negative electrode sheet, an electrolyte, and a housing. The separator separates the positive electrode sheet from the negative electrode sheet, and the housing is used to house and encapsulate the positive electrode sheet, negative electrode sheet, separator, and electrolyte. The negative electrode sheet is the negative electrode sheet described above.

[0037] positive electrode

[0038] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material may include but is not limited to a chemical formula such as Li a Ni x Co y M z O 2-b N b (wherein 0.95≤a≤1.2, x>0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from a combination of one or more of Mn and Al, and N is selected from a combination of one or more of F, P, and S), the positive electrode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 The positive electrode active material may be a combination of one or more of O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material may also be subjected to a modification treatment. Methods for modifying the positive electrode active material should be known to those skilled in the art. For example, the positive electrode active material may be modified by coating, doping, etc. The materials used for the modification treatment may include, but are not limited to, a combination of one or more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive electrode current collector is generally a structure or part for collecting current. The positive electrode current collector may be any material suitable for use as a positive electrode current collector for lithium-ion batteries in the art. For example, the positive electrode current collector may include, but is not limited to, metal foil, and more specifically, may include, but is not limited to, aluminum foil.

[0039] negative electrode

[0040] The negative electrode sheet includes a negative electrode current collector 1 and a negative electrode active material layer disposed on the surface of the negative electrode current collector 1. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material may include, but is not limited to, one or more of graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbeads, silicon-based materials, tin-based materials, lithium titanate, or other metals capable of forming alloys with lithium. The graphite may be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; and the tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector 1 is typically a structure or component for collecting current. The negative electrode current collector 1 may be any material suitable for use as a negative electrode current collector 1 in lithium-ion batteries. For example, the negative electrode current collector 1 may include, but is not limited to, metal foil, and more specifically, copper foil.

[0041] electrolyte

[0042] The lithium-ion battery also includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt, and additives. The electrolyte lithium salt can be LiPF6 and / or LiBOB, as used in high-temperature electrolytes; can also be at least one of LiBF4, LiBOB, and LiPF6, as used in low-temperature electrolytes; can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI, as used in overcharge-preventing electrolytes; or can be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; a chain carbonate, including DFC, DMC, or EMC; or a carboxylic acid ester, including MF, MA, EA, and MP. Additives include, but are not limited to, at least one of a film-forming additive, a conductive additive, a flame retardant additive, an overcharge prevention additive, an additive to control the H2O and HF content in the electrolyte, an additive to improve low-temperature performance, and a multifunctional additive.

[0043] The separator can be any material suitable for lithium-ion battery separators in the art, for example, it can be a combination of one or more materials including but not limited to polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fibers.

[0044] Preferably, the shell is made of stainless steel or aluminum-plastic film. More preferably, the shell is made of aluminum-plastic film.

[0045] Example 1

[0046] According to the multi-stage constant current and constant voltage charging system, the existing battery of the same system is fully charged, and the full charge capacity is 5000mAh. The first stage of constant current and constant voltage charging is 2000mAh, accounting for 40% of the total charge capacity. According to the user's design requirements, the total anode single-side coating surface density is A = 0.085g / cm 2 , of which silicon composite material accounts for 5%; the single-sided coating density of the first layer of pure graphite coating is B = 0.085g / cm 2 *40% = 0.034 g / cm 2 , the second layer single-sided coating surface density C=AB=0.051g / cm 2 , the silicon composite material accounts for (0.085*5%) / 0.051=8.33% in B, and the active material content of the silicon-carbon composite material in the negative electrode sheet is controlled to remain unchanged at 5%.

[0047] A method for preparing a negative electrode sheet for improving the cycle performance of a lithium-ion soft-pack silicon-carbon battery comprises the following steps:

[0048] Step 1: Coat both sides of the negative electrode current collector 1 with a surface density of 0.034 g / cm 2 The pure graphite layer is the first active layer 2, wherein the active material accounts for 96%, the thickener CMC-Na (sodium hydroxymethyl cellulose) accounts for 2%, and the adhesive SBR (styrene-butadiene latex) accounts for 2%.

[0049] Step 2: Coat the first active layer 2 with a surface density of 0.051 g / cm 2 The silicon-carbon composite material is dried to form a second active layer 3 to prepare a negative electrode sheet, wherein the silicon composite active material accounts for 8.33% of the active material; in the second layer of active material, the active material accounts for 96%, the thickener CMC-Na (sodium hydroxymethyl cellulose) accounts for 2%, and the adhesive SBR (styrene-butadiene latex) accounts for 2%, wherein the mass ratio of carbon material to silicon material is 2:3.

[0050] Step 3: Roll the above-mentioned electrode to a thickness of 110 μm.

[0051] Preparation of positive electrode:

[0052] Lithium cobalt oxide, conductive agent superconducting carbon (Super-P), and binder polyvinylidene fluoride (PVDF) are mixed evenly in a mass ratio of 97:1.5:1.5 to prepare a lithium-ion battery positive electrode slurry with a certain viscosity. The slurry is coated on a current collector aluminum foil, dried at 85°C, and then cold pressed; then the slurry is trimmed, cut, and striped. After stripping, it is dried at 110°C under vacuum conditions for 4 hours, and the tabs are welded to make lithium-ion battery positive electrode sheets.

[0053] Preparation of electrolyte:

[0054] Lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent consisting of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (the mass ratio of the three was 1:2:1) to obtain an electrolyte with a concentration of 1 mol / L.

[0055] The isolation film was selected from a polypropylene-based film having a thickness of 8 μm.

[0056] Preparation of lithium-ion batteries:

[0057] The positive electrode sheet, separator and negative electrode sheet are wound into a battery cell, with the separator located between the positive and negative electrode sheets. The positive electrode is lead out with an aluminum tab by spot welding, and the negative electrode is lead out with a nickel tab by spot welding. The battery cell is then placed in an aluminum-plastic film shell, and the above-mentioned electrolyte is injected. After packaging, formation and capacity processes, a lithium-ion battery is made.

[0058] Example 2

[0059] The difference from Example 1 is that the mass ratio of the carbon material to the silicon material in the second active layer 3 is 1:2.

[0060] The rest is the same as in Example 1 and will not be described again here.

[0061] Example 3

[0062] The difference from Example 1 is that the mass ratio of the carbon material to the silicon material in the second active layer 3 is 1:5.

[0063] The rest is the same as in Example 1 and will not be described again here.

[0064] Example 4

[0065] The difference from Example 1 is that the mass ratio of the carbon material to the silicon material in the second active layer 3 is 2:5.

[0066] The rest is the same as in Example 1 and will not be described again here.

[0067] Example 5

[0068] The difference from Example 1 is that the mass ratio of the carbon material to the silicon material in the second active layer 3 is 3:5.

[0069] The rest is the same as in Example 1 and will not be described again here.

[0070] Example 6

[0071] The difference from Example 1 is that the single-sided coating density A of the negative electrode sheet is 0.09 g / cm 2 The percentage a of the first constant current and constant voltage charging capacity to the total capacity is 40%, and the single-sided coating density B of the first active layer 2 is 0.036 g / cm 2The single-sided coating density C of the second active layer 3 is 0.064 g / cm 2 .

[0072] The rest is the same as in Example 1 and will not be described again here.

[0073] Example 7

[0074] The difference from Example 1 is that the single-sided coating density A of the negative electrode sheet is 0.095 g / cm 2 The percentage a of the first constant current constant voltage charging capacity to the total capacity is 40%, and the single-sided coating density B of the first active layer 2 is 0.038 g / cm 2 The single-sided coating density C of the second active layer 3 is 0.057 g / cm 2 .

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

[0076] Example 8

[0077] The difference from Example 1 is that the single-sided coating density A of the negative electrode sheet is 0.098 g / cm 2 The percentage a of the first constant current and constant voltage charging capacity to the total capacity is 40%, and the single-sided coating density B of the first active layer 2 is 0.0392 g / cm 2 The single-sided coating density C of the second active layer 3 is 0.0588 g / cm 2 .

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

[0079] Example 9

[0080] The difference from Example 1 is that the single-sided coating density A of the negative electrode sheet is 0.1 g / cm 2 The percentage a of the first constant current and constant voltage charging capacity to the total capacity is 40%, and the single-sided coating density B of the first active layer 2 is 0.04 g / cm 2 The single-sided coating density C of the second active layer 3 is 0.06 g / cm 2 .

[0081] The rest is the same as in Example 1 and will not be described again here.

[0082] Example 10

[0083] The difference from Example 1 is that the single-sided coating density A of the negative electrode sheet is 0.07 g / cm 2 The percentage a of the first constant current and constant voltage charging capacity to the total capacity is 40%, and the single-sided coating density B of the first active layer 2 is 0.028 g / cm2 The single-sided coating density C of the second active layer 3 is 0.042 g / cm 2 .

[0084] The rest is the same as in Example 1 and will not be described again here.

[0085] Comparative Example 1

[0086] Step 1: Coat both sides of the negative electrode current collector 1 with a surface density of 0.085 g / cm 2 The silicon-carbon composite material layer comprises 5% of the active material of the silicon composite material; in the coating, the active material accounts for 96%, the thickener CMC-Na (sodium hydroxymethyl cellulose) accounts for 2%, and the adhesive SBR (styrene-butadiene latex) accounts for 2%.

[0087] Step 2: Roll the above-mentioned electrode to a thickness of 110 μm.

[0088] A negative electrode sheet for a lithium-ion soft-pack silicon-carbon battery not matching a charging system is obtained using the above preparation method. A lithium-ion soft-pack silicon-carbon battery is obtained by slitting and sheeting the negative electrode sheet, winding the sheeted negative electrode sheet with the positive electrode sheet to form a roll core, and then packaging, baking, injecting liquid, forming, resealing, and sorting the roll core.

[0089] Comparative Example 2

[0090] The difference from Example 1 is that the single-sided coating density A of the negative electrode sheet is 0.15 g / cm 2 The percentage a of the first constant current and constant voltage charging capacity to the total capacity is 40%, and the single-sided coating density B of the first active layer 2 is 0.06 g / cm 2 The single-sided coating density C of the second active layer 3 is 0.09 g / cm 2 hour

[0091] The rest is the same as in Example 1 and will not be described again here.

[0092] Performance test: The secondary batteries of Examples 1-10 and the battery of Comparative Example 1 were subjected to performance tests. The test results are recorded in Tables 1 and 2.

[0093] Table 1

[0094]

[0095]

[0096] Table 2

[0097] Week 100 200 300 400 500 600 700 800 Example 1 4.1％ 5.1％ 5.8％ 6.2％ 6.8％ 7.1％ 7.5％ 7.7％ Example 2 4.3％ 5.3％ 5.9％ 6.4％ 6.9％ 7.2％ 7.6％ 7.9％ Example 3 4.2％ 5.4％ 6.1％ 6.3％ 7.0％ 7.3％ 7.7％ 7.8％ Example 4 4.4％ 5.3％ 5.9％ 6.4％ 6.9％ 7.3％ 7.6％ 7.9％ Example 5 4.3％ 5.2％ 5.9％ 6.5％ 6.9％ 7.4％ 7.6％ 7.9％ Example 6 4.3％ 5.3％ 5.9％ 6.4％ 7.0％ 7.3％ 7.6％ 8.2％ Example 7 4.4％ 5.4％ 5.9％ 6.3％ 6.9％ 7.2％ 7.6％ 7.8％ Example 8 4.3％ 5.2％ 5.9％ 6.4％ 6.9％ 7.3％ 7.7％ 7.9％ Example 9 4.4％ 5.3％ 5.9％ 6.5％ 6.9％ 7.3％ 7.8％ 8.0％ Example 10 4.3％ 5.3％ 6.1％ 6.3％ 6.9％ 7.3％ 7.7％ 7.9％ Comparative Example 1 5.7％ 7.2％ 8.3％ 9.2％ 10.4％ 11.8％ 13.2％ 14.6％ Comparative Example 2 5.8％ 7.1％ 9.6％ 10.2％ 11.8％ 12.6％ 14.7％ 16.5％

[0098] According to Table 1, the secondary battery of the present invention has better performance than the battery of Comparative Example 1, with a capacity retention rate of more than 88.3% after 800 charge and discharge cycles, and a thickness expansion rate of less than 8.2% after 800 charge and discharge cycles. By comparison with Examples 1-5, it can be seen that when the mass ratio of carbon material to silicon material in the second active layer 3 is set to 2:3, the performance of the prepared secondary battery is better. By comparison with Examples 1, 6-10, it can be seen that when the single-sided coating surface density A of the negative electrode plate is set to 0.085 g / cm 2 The percentage a of the first constant current and constant voltage charging capacity to the total capacity is 40%, and the single-sided coating density B of the first active layer 2 is 0.034 g / cm 2 The single-sided coating density C of the second active layer 3 is 0.051 g / cm 2 The secondary battery prepared has better performance. Figure 1 and Figure 2 , from the comparison between Example 1 and Comparative Example 1, it can be seen that when only one layer of active coating is provided, the active coating is easy to expand during multiple cycles, resulting in a large volume change of up to 14.6%. Due to the volume expansion, the pressure between the pole pieces increases, which in turn affects the capacity retention rate, making the capacity retention rate low, only 82%. Figure 1 and Figure 2 It can be concluded that compared with Comparative Example 1, the capacity retention rate of Example 1 of the present invention is increased by 8%, the thickness expansion rate is reduced by 47.2%, and the performance is significantly improved.

[0099] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention fall within the scope of protection of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A negative electrode plate, characterized in that: The invention comprises a negative electrode current collector, a first active layer provided on at least one surface of the negative electrode current collector, and a second active layer provided on a side of the first active layer away from the negative electrode current collector, wherein the single-side coating area density of the negative electrode electrode sheet is A, the percentage of the first constant current and constant voltage charging capacity to the total capacity is a, a is 40%, the single-side coating area density of the first active layer is B, and the single-side coating area density of the second active layer is C. Then, the single-side coating area density of the first active layer and the single-side coating area density of the second active layer satisfy the following relationship: B = A*a%, C = AB; The second active layer comprises a carbon material and a silicon material, and the mass ratio of the carbon material to the silicon material in the second active layer is 1 to 3:2 to 5; The single-sided coating density A of the negative electrode sheet is in the range of 0.05 g / cm 2 ~0.12g / cm 2 ; The single-sided coating density of the first active layer is B, and the value range is 0.02 g / cm 2 ~0.08g / cm 2 ; The single-sided coating density of the second active layer is C, which has a value range of 0.02 g / cm 2 ~0.08g / cm 2 .

2. The negative electrode sheet according to claim 1, characterized in that: The first active layer includes one or more of artificial graphite, conductive carbon black, coke, carbon fiber, and carbon nanotubes.

3. The negative electrode sheet according to claim 1 or 2, characterized in that: The second active layer includes carbon materials and silicon materials. The carbon materials include one or more of graphite, conductive carbon black, coke, carbon fiber, and carbon nanotubes. The silicon materials include one or more of silicon monoxide and silicon oxide.

4. The negative electrode sheet according to claim 1, characterized in that: The thickness ratio of the first active layer to the second active layer is 1-6:1-2.

5. A secondary battery, characterized in that: The negative electrode comprises the negative electrode sheet according to any one of claims 1 to 4.

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