Negative plate and lithium ion battery

Abstract

The invention discloses a negative electrode plate and a lithium ion battery. The negative electrode plate comprises a negative electrode current collector, a conductive coating arranged on at least one side surface of the negative electrode current collector, and a negative electrode active material layer arranged on the surface, deviating from the negative electrode current collector, of the conductive coating, wherein the ratio of the diffraction peak intensity (111) to the diffraction peak intensity (220) of the negative electrode current collector is greater than or equal to 2, and the negative electrode active material layer comprises a negative electrode active material containing a silicon-based material; and the relation coefficient K between the mass percentage P of the silicon-based material in the negative electrode active material layer and the elastic modulus E of the negative electrode current collector and the compaction density rho of the negative electrode active material layer is equal to E / [P (rho + 1) + 1] greater than or equal to 50000. Therefore, the expansion and deformation of the negative electrode active material layer parallel to the XY direction of the negative electrode current collector in the charge-discharge cycle process can be effectively reduced, and the risk of current collector fragment after cycle is reduced, so that the cycle expansion rate of the lithium ion battery is reduced, and the cycle performance and the safety performance of the battery are 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 lithium-ion battery. Background Art

[0002] Lithium-ion batteries, with their numerous advantages, including high voltage, high energy, long cycle life, and lack of memory effect, have found widespread application in consumer electronics, power tools, medical electronics, energy storage, and other fields. Within a battery's structure, the anode material is a key factor influencing its performance. Currently, the primary anode material for commercial lithium-ion batteries is graphite, which has a theoretical capacity of 372 mAh / g and an actual capacity of approximately 350 mAh / g, approaching its theoretical capacity per gram. However, as electronic devices increasingly demand higher battery capacity and energy density, graphite alone is no longer sufficient to meet the increasing demand.

[0003] Silicon-based materials are characterized by high discharge capacity, environmental friendliness, and abundant reserves. Silicon, with a theoretical capacity of up to 4200 mAh / g, is approximately 10 times that of commercially available graphite, attracting widespread attention. To increase battery energy density, doping graphite with silicon is one approach. However, silicon expands significantly during battery charging, with its own expansion rate reaching as high as 300%. This leads to a sharp increase in cell expansion after silicon-doping the negative electrode. Specifically, after discharge, the silicon material structure cannot recover and may even collapse. This structural damage severely impacts the battery's cycling performance, limiting its commercial application. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a negative electrode sheet and a lithium ion battery.

[0005] In a first aspect of the present invention, a negative electrode sheet is provided, comprising:

[0006] A negative electrode current collector, wherein the ratio of the diffraction peak (111) peak intensity to the diffraction peak (220) of the negative electrode current collector is ≥2;

[0007] A conductive coating is provided on at least one side of the negative electrode current collector;

[0008] A negative electrode active material layer is provided on a surface of the conductive coating layer away from the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material;

[0009] The elastic modulus of the negative electrode current collector is E, the mass percentage of the silicon-based material in the negative electrode active material layer is P, the compaction density of the negative electrode active material layer is ρ, and the relationship coefficient K among E, P and ρ satisfies: K=E / [P(ρ+1)+1]≥50000.

[0010] According to the negative electrode sheet of the embodiment of the present invention, there are at least the following beneficial effects: the negative electrode sheet controls the ratio of the peak intensity of the negative electrode collector diffraction peak (111) to the peak intensity of (220) to be ≥2. The greater the (111) peak intensity, the greater the proportion of the (111) texture, and the greater the strength of the negative electrode collector, which helps to reduce the risk of the silicon-containing negative electrode sheet breaking and the risk of the battery cell deformation under the action of cyclic expansion stress, and improves the cycle performance and safety performance; in addition, the negative electrode active material of the negative electrode active material layer in the negative electrode sheet includes a silicon-based material, and the higher the amount of silicon doped in the negative electrode sheet, the greater the negative electrode sheet charging expansion, and the greater the expansion force generated by the negative electrode sheet, thereby restraining the expansion of the negative electrode sheet in the XY direction parallel to the negative electrode collector, and the elastic modulus of the collector is required to be The higher the demand, the greater the compaction density of the negative electrode sheet, the lower the porosity of the electrode sheet, the lower the kinetic performance, and the greater the charging expansion. Therefore, while controlling the ratio of the diffraction peak (111) peak intensity to the (220) peak intensity of the negative electrode collector to be ≥2, by limiting the relationship coefficient K between the elastic modulus E of the negative electrode collector and the mass percentage P of the silicon-based material in the negative electrode active material layer and the compaction density ρ of the negative electrode active material layer to satisfy K=E / [P(ρ+1)+1]≥50000, the expansion and deformation of the negative electrode active material layer parallel to the XY direction of the negative electrode collector during the charge and discharge cycle can be effectively reduced, the risk of the collector breaking after the cycle can be reduced, and the cycle expansion rate of the lithium-ion battery can be reduced, and the cycle performance and safety performance of the battery can be improved.

[0011] In some embodiments of the present invention, the negative electrode sheet satisfies at least one of the following conditions:

[0012] 1) The ratio of the diffraction peak (111) intensity to the diffraction peak (220) intensity of the negative electrode current collector is 2 to 10;

[0013] 2) The elastic modulus E of the negative electrode current collector is ≥ 60000 N / m;

[0014] 3) The mass percentage of the silicon-based material in the negative electrode active material layer is P = 1% to 75%;

[0015] 4) The compaction density of the negative electrode active material layer is ρ≤1.9g / cm 3 .

[0016] If the peak intensity of the negative electrode current collector diffraction peak (111) is too large, the current collector's bending resistance will deteriorate, and the current collector will easily break in the later stage of the cycle. Therefore, by controlling the ratio of the peak intensity of the negative electrode current collector diffraction peak (111) to the peak intensity of (220) to be 2 to 10, the strength and bending resistance of the negative electrode current collector can be guaranteed at the same time, thereby improving the cycle performance and safety of the battery. Furthermore, the ratio of the peak intensity of the negative electrode current collector diffraction peak (111) to the peak intensity of (220) can be controlled to be any value among 2, 2.5, 3, 3.6, 4, 4.5, 5, 5.5, 6, 7, 7.5, 8, 9, 10, or any range of two values.

[0017] By controlling the elastic modulus E of the negative electrode current collector to ≥ 60,000 N / m, the expansion of the negative electrode sheet in the XY direction parallel to the negative electrode current collector can be effectively restrained, thereby improving the battery cycle performance and safety performance. Furthermore, the elastic modulus of the negative electrode current collector may be greater than or equal to any value of 60000 N / m, 62000 N / m, 65000 N / m, 68000 N / m, 70000 N / m, 72000 N / m, 75000 N / m, 80000 N / m, 82000 N / m, 85000 N / m, 86000 N / m, 90000 N / m, 95000 N / m, 98000 N / m, 100000 N / m, 120000 N / m, 150000 N / m, 160000 N / m, 180000 N / m, 200000 N / m, and 250000 N / m, or equal to any two of the values ​​in the range.

[0018] Among them, the elastic modulus control of the negative electrode current collector can be achieved by controlling the negative electrode current collector production process. For example, for the electrolytic copper foil manufacturing process, the addition and dosage of additives (such as refiners, high resistance agents, etc.) in the copper foil electrolyte can be controlled to control the copper foil grain size. The grain size affects the elastic modulus, thereby obtaining negative electrode current collector copper foils with different elastic moduli. As for the peak intensity ratio control of the negative electrode current collector, the ratio of the negative electrode current collector diffraction peak (111) peak intensity to the (220) peak intensity can also be controlled by controlling additives and production processes, parameters, etc. For example, for the electrolytic copper foil manufacturing process, the additive combination, electrodeposition parameters and post-processing process will affect the copper ion deposition kinetics and crystal nucleus growth direction, and then by controlling the additive combination, electrodeposition parameters and post-processing process, their synergistic effect can be used to obtain negative electrode current collector copper foils with different diffraction peak (111) peak intensity to (220) peak intensity ratios. Specifically, according to demand, the negative electrode current collector can be customized to the current collector manufacturer.

[0019] By controlling the mass percentage P of the silicon-based material in the negative electrode active material layer to be between 1% and 75%, the energy density of the battery can be effectively improved while controlling the expansion of the negative electrode sheet. Furthermore, the mass percentage P of the silicon-based material in the negative electrode active material layer can be any value among 1%, 5%, 8%, 10%, 15%, 17%, 20%, 22%, 25%, 30%, 34%, 36%, 40%, 45%, 48%, 50%, 52%, 55%, 57%, 60%, 63%, 65%, 68%, 70%, 72%, 75%, or any range of any two values.

[0020] By controlling the compaction density of the negative electrode active material layer ρ≤1.9g / cm 3 , to ensure the porosity of the electrode, thereby improving the dynamic performance and reducing the charging expansion. Furthermore, the compaction density of the active material layer of the negative electrode can be controlled at 0.2g / cm 3 , 0.5g / cm 3 , 0.7g / cm 3 , 0.8g / cm 3 , 1g / cm 3 , 1.2g / cm 3 , 1.25g / cm 3 , 1.3g / cm 3 , 1.4g / cm 3 , 1.45g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.65g / cm 3 , 1.7g / cm 3 , 1.75g / cm 3 , 1.8g / cm 3 , 1.85g / cm 3 , 1.9g / cm 3 The range of values ​​for either or both of .

[0021] In some embodiments of the present invention, the elastic modulus E of the negative electrode current collector is calculated as the tensile force corresponding to a 0.5 mm displacement divided by the displacement on the tensile force-displacement curve of the negative electrode current collector. Because the local deformation of the negative electrode sheet in the XY directions parallel to the negative electrode current collector is less than 0.5 mm, a larger elastic modulus of the negative electrode current collector indicates a better effect in restraining the expansion of the electrode sheet in the XY directions.

[0022] Furthermore, the elastic modulus E of the negative electrode current collector can be tested by the following method: using a tensile testing machine with a range of 0N to 1000N and an indication error of ±1%, and a vernier caliper with a range of 0mm to 300mm and a minimum graduation value of 0.02mm or a measuring tool of corresponding accuracy; cutting a sample with a length of 150±0.5mm and a width of 15±0.25mm from the object to be tested; taking 5 samples in the longitudinal and transverse directions along the width direction of the object to be tested, and then placing the sample in the tensile testing machine, wherein the chuck The distance is 50 ± 0.1 mm, the chuck tensioning speed is 50 mm / min, and the test temperature is 20 ± 10°C; the sample to be tested is continuously loaded in the longitudinal direction until it breaks, and a tensile force-displacement curve is calculated by software. The value on the tensile force-displacement curve is then read and the elastic modulus E is calculated according to the formula E = F / L, where F is the tensile force corresponding to a displacement of 0.5 mm on the tensile force-displacement curve, and L is the width of the sample to be tested. The arithmetic mean of the test results of 10 specimens is taken to obtain the elastic modulus E of the test object.

[0023] In some embodiments of the present invention, the silicon-based material is selected from at least one of silicon carbide and silicon oxide.

[0024] In some embodiments of the present invention, the negative electrode current collector is selected from copper foil. Further, the negative electrode current collector can be at least one of electrolytic copper foil and rolled copper foil.

[0025] In some embodiments of the present invention, the thickness of the negative electrode current collector is 1 mm to 15 mm. For example, the thickness of the negative electrode current collector can be any value among 1 mm, 2 mm, 5 mm, 6 mm, 7 mm, 8 mm, 10 mm, 12 mm, 14 mm, and 15 mm, or any range of two values.

[0026] In some embodiments of the present invention, the negative electrode active material layer further includes a first conductive agent and a first binder.

[0027] In some embodiments of the present invention, the mass percentage of the first conductive agent in the negative electrode active material layer is 1% to 5%; and / or the mass percentage of the first binder in the negative electrode active material layer is 1% to 10%. Furthermore, the mass percentage of the first conductive agent in the negative electrode active material layer may be any one of 1%, 1.2%, 1.3%, 1.5%, 1.6%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.4%, 3.5%, 3.7%, 4%, 4.2%, 4.5%, 4.8%, 5%, or any range of two of them; the mass percentage of the first binder in the negative electrode active material layer is any one of 1%, 1.5%, 2%, 2.5%, 2.8%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.8%, 7%, 7.5%, 8%, 8.2%, 8.5%, 8.6%, 9%, 9.5%, 10%, or any range of two of them.

[0028] In some embodiments of the present invention, the first conductive agent is selected from at least one of conductive carbon black, carbon nanotubes, carbon fibers, and graphene; and / or the first binder is selected from at least one of polyacrylic acid, sodium polyacrylate, sodium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, and polyvinylidene fluoride.

[0029] In some embodiments of the present invention, the negative electrode active material in the negative electrode active material layer further includes graphite.

[0030] In some embodiments of the present invention, the conductive coating includes a second conductive agent and a second binder.

[0031] In some embodiments of the present invention, the weight percentage of the second conductive agent in the conductive coating is 30% to 80%; and / or the weight percentage of the second binder in the conductive coating is 20% to 70%. Furthermore, the weight percentage of the second conductive agent in the conductive coating is any one of 30%, 35%, 40%, 42%, 45%, 50%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, 75%, 80%, or any range of two values; the weight percentage of the second binder in the conductive coating is any one of 20%, 25%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 50%, 55%, 58%, 60%, 65%, 70%, or any range of two values.

[0032] In some embodiments of the present invention, the second conductive agent is selected from at least one of conductive carbon black, carbon nanotubes, and carbon fibers; and / or the second adhesive is selected from at least one of polyvinylidene fluoride, sodium carboxymethyl cellulose, and styrene-butadiene rubber.

[0033] In some embodiments of the present invention, the conductive coating is provided on two opposite surfaces of the negative electrode current collector.

[0034] In a second aspect of the present invention, a lithium-ion battery is provided, which includes a positive electrode sheet, a negative electrode sheet and a separator. The negative electrode sheet adopts any of the aforementioned negative electrode sheets of the present invention, and the separator is sandwiched between the positive electrode sheet and the negative electrode sheet.

[0035] The lithium-ion battery includes any of the aforementioned negative electrode sheets of the present invention, thereby having all the beneficial effects of the aforementioned negative electrode sheets. Specifically, the negative electrode active material of the negative electrode active material layer of the negative electrode sheet includes a silicon-based material, and by limiting the relationship coefficient K between the elastic modulus E of the negative electrode current collector, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the negative electrode sheet compaction density ρ to satisfy K=E / [P(ρ+1)+1]≥50000, and the ratio of the diffraction peak (111) peak intensity to the (220) peak intensity of the negative electrode current collector is ≥2, the expansion and deformation of the negative electrode active material layer parallel to the XY direction of the negative electrode current collector during the charge and discharge cycle can be effectively reduced, the risk of the current collector breaking after the cycle can be reduced, and the cycle expansion rate of the lithium-ion battery can be reduced, thereby improving the cycle performance and safety performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0037] Figure 1 This is the XRD diffraction pattern of the negative electrode current collector used in Example 1;

[0038] Figure 2 1 is a cycle curve diagram of the lithium ion battery of Example 11 and Comparative Example 14. DETAILED DESCRIPTION

[0039] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0040] First of all, it should be noted that the negative electrode current collector copper foil used in the following cases was purchased from the copper foil manufacturer - Changchun Chemical (Panjin) Co., Ltd.

[0041] Example 1

[0042] This embodiment provides a negative electrode sheet comprising a negative electrode current collector, a conductive coating, and a negative electrode active material layer. The conductive coating is disposed on two opposing surfaces of the negative electrode current collector, and the negative electrode active material layer is disposed on the surface of the conductive coating facing away from the negative electrode current collector.

[0043] The negative electrode current collector specifically uses a copper foil with a thickness of 6 μm, and its elastic modulus is 80,000 N / m, and the ratio of the crystal diffraction peak (111) peak intensity I(111) to the (220) peak intensity I(220) I(111) / I(220) is 3.2.

[0044] The above negative electrode current collector was purchased from the copper foil manufacturer, Changchun Chemical (Panjin) Co., Ltd. X-ray diffraction test was performed using the following method:

[0045] An X-ray diffraction tester (Bruker, D8) was used to perform the test according to the "General Rules for X-ray Diffraction Analysis" standard: JJSK0131-1996. The specific test settings were as follows: the test voltage was 40 kV, the current was 30 mA, the scanning angle range was 10° to 80°, the scanning step was 0.017°, and the time set for each step was 0.5 s to obtain the XRD diffraction pattern, as shown in Figure 2. Figure 1 The highest diffraction peak intensity I(111) attributable to 43.0°-44.0° and the highest diffraction peak intensity I(220) attributable to 74.5°-74.5° were recorded, and the ratio of I(111) / I(220) was calculated to be 3.2.

[0046] The negative electrode active material layer comprises a negative electrode active material, a conductive agent, and a binder in a mass ratio of 96:2:2. The negative electrode active material comprises a silicon-based material and artificial graphite. The silicon-based material is specifically silicon-carbon material SiC, and the mass percentage P of the silicon-based material in the negative electrode active material layer is 10%. The conductive agent is conductive carbon black, and the binder is polyacrylic acid. The thickness of the negative electrode active material layer is 80 μm, and the compaction density ρ is 1.70 g / cm 3 .

[0047] The conductive coating includes a conductive agent and a binder, wherein the conductive agent is carbon nanotubes, and the binder is sodium carboxymethyl cellulose and styrene-butadiene rubber with a glass transition temperature (Tg) less than 10°C. The mass ratio of carbon nanotubes, sodium carboxymethyl cellulose and styrene-butadiene rubber is 50:3:47.

[0048] The relationship coefficient K=E / [P(ρ+1)+1]=62992 among the elastic modulus E of the negative electrode collector in the negative electrode sheet, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0049] The negative electrode sheet is prepared by a preparation method comprising the following steps:

[0050] S1. Add silicon-carbon material SiC and artificial graphite to a blender and mix to form a negative electrode active material. Subsequently, add the binder polyacrylic acid and the conductive agent conductive carbon black to the stirred negative electrode active material (the mass ratio of the negative electrode active material, conductive carbon black, and polyacrylic acid is 96:2:2, of which the weight proportion of the silicon-carbon material SiC is 10%), stir for 60 minutes at a revolution speed of 20 laps / min and a rotation speed of 200 laps / min, and then add deionized water at a solid content ratio of 36% and stir for 120 minutes at a revolution speed of 20 laps / min and a rotation speed of 1800 laps / min to obtain a mixed slurry.

[0051] S2. Take conductive carbon nanotubes, binder sodium carboxymethyl cellulose and styrene-butadiene rubber (Tg < 10°C) in a mass ratio of 50:3:47, add the conductive carbon nanotubes and binder sodium carboxymethyl cellulose into a blender and mix, then add binder styrene-butadiene rubber, and then add deionized water at a solid content ratio of 9%, and stir for 60 minutes at an orbital speed of 10 revolutions / minute and a rotation speed of 1600 revolutions / minute to obtain a conductive slurry.

[0052] S3. Use a copper foil with a thickness of 6 μm as the negative electrode current collector, and its elastic modulus is 80000 N / m, I(111) / I(220)=3.2; apply the conductive slurry prepared in step S2 on the two opposite surfaces of the negative electrode current collector, and leave an empty foil area in the length direction, and then dry it to obtain a negative electrode current collector with a conductive coating; then apply the mixed slurry prepared in step S1 on the surface of the conductive coating away from the negative electrode current collector, and dry it; after drying, perform cold pressing to obtain a negative electrode active material layer, wherein the thickness of the negative electrode active material layer is 80 μm and the compaction density is 1.70 g / cm 3 , and then cut to obtain the negative electrode sheet.

[0053] Example 2

[0054] This embodiment provides a negative electrode sheet, which differs from Example 1 in that: the mass percentage P of the silicon-based material (silicon-carbon material Si) in the negative electrode active material layer in this embodiment is 3%, and the rest is basically the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=74006 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this embodiment, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0055] Example 3

[0056] This embodiment provides a negative electrode sheet, which differs from Example 1 in that: the mass percentage P of the silicon-based material (silicon-carbon material Si) in the negative electrode active material layer in this embodiment is 5%, and the rest is the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=70485 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this embodiment, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0057] Example 4

[0058] This embodiment provides a negative electrode sheet, which differs from Example 1 in that: the mass percentage P of the silicon-based material (silicon-carbon material Si) in the negative electrode active material layer in this embodiment is 15%, and the rest is the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=56940 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this embodiment, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0059] Example 5

[0060] This embodiment provides a negative electrode sheet, which differs from Example 1 in that: the mass percentage P of the silicon-based material (silicon-carbon material Si) in the negative electrode active material layer in this embodiment is 20%, and the rest is the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=51948 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this embodiment, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0061] Example 6

[0062] This embodiment provides a negative electrode sheet, which differs from Example 1 in that the elastic modulus E of the negative electrode current collector used in this embodiment is 90,000 N / m, and the rest is basically the same as in Example 1. Furthermore, the relationship coefficient between the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this embodiment, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer is K=E / [P(ρ+1)+1]=70866.

[0063] Example 7

[0064] This embodiment provides a negative electrode sheet, which differs from Example 1 in that the elastic modulus E of the negative electrode current collector used in this embodiment is 70,000 N / m, and the rest is basically the same as in Example 1. Furthermore, the relationship coefficient between the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this embodiment, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer is K=E / [P(ρ+1)+1]=55118.

[0065] Example 8

[0066] This embodiment provides a negative electrode sheet, which differs from Example 1 in that the ratio of the crystal diffraction peak (111) peak intensity I(111) to the (220) peak intensity I(220) of the negative electrode current collector used in this embodiment is 4.6, and the rest is the same as in Example 1.

[0067] Example 9

[0068] This embodiment provides a negative electrode sheet, which differs from Example 1 in that the ratio of the crystal diffraction peak (111) peak intensity I(111) to the (220) peak intensity I(220) of the negative electrode current collector used in this embodiment is 2.6, and the rest is the same as in Example 1.

[0069] Example 10

[0070] This embodiment provides a negative electrode sheet, which differs from Example 1 in that the ratio of the crystal diffraction peak (111) peak intensity I(111) to the (220) peak intensity I(220) of the negative electrode current collector used in this embodiment is 2.3, and the rest is the same as in Example 1.

[0071] Comparative Example 1

[0072] This comparative example provides a negative electrode sheet, which differs from Example 1 in that: in this comparative example, the mass percentage P of the silicon-based material (silicon-carbon material Si) in the negative electrode active material layer is 30%, and the rest is the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=44199 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this comparative example, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0073] Comparative Example 2

[0074] This comparative example provides a negative electrode sheet, which differs from Example 1 in that: in this comparative example, the mass percentage P of the silicon-based material (silicon-carbon material Si) in the negative electrode active material layer is 50%, and the rest is the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=34043 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this comparative example, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0075] Comparative Example 3

[0076] This comparative example provides a negative electrode sheet, which differs from Example 1 in that: in this comparative example, the mass percentage P of the silicon-based material (silicon-carbon material Si) in the negative electrode active material layer is 70%, and the rest is the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=27682 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this comparative example, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0077] Comparative Example 4

[0078] This comparative example provides a negative electrode sheet, which differs from Example 1 in that the elastic modulus E of the negative electrode current collector used in this comparative example is 60,000 N / m, and the rest is basically the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=47244 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this comparative example, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0079] Comparative Example 5

[0080] This comparative example provides a negative electrode sheet, which differs from Example 1 in that the elastic modulus E of the negative electrode current collector used in this comparative example is 50,000 N / m, and the rest is basically the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=39370 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this comparative example, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0081] Comparative Example 6

[0082] This comparative example provides a negative electrode sheet, which differs from Example 1 in that the elastic modulus E of the negative electrode current collector used in this comparative example is 45000 N / m, and the rest is basically the same as in Example 1. Furthermore, the relationship coefficient K=E / [P(ρ+1)+1]=35433 among the elastic modulus E of the negative electrode current collector in the negative electrode sheet of this comparative example, the mass percentage P of the silicon-based material in the negative electrode active material layer, and the compaction density ρ of the negative electrode active material layer.

[0083] Comparative Example 7

[0084] This comparative example provides a negative electrode sheet, which differs from Example 1 in that the ratio of the crystal diffraction peak (111) peak intensity I(111) to the (220) peak intensity I(220) of the negative electrode current collector used in this comparative example is 1.5, and the rest is the same as Example 1.

[0085] Example 11

[0086] This embodiment provides a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, and a separator, wherein the negative electrode sheet adopts the negative electrode sheet of embodiment 1, and the separator is sandwiched between the positive electrode sheet and the negative electrode sheet.

[0087] The lithium-ion battery is prepared by a preparation method comprising the following steps:

[0088] Preparation of the positive electrode sheet: The positive electrode active material NCM523, the conductive agent acetylene black, and the binder PVDF were mixed in a mass ratio of 97:2:1, and the solvent NMP was added. The mixture was stirred in a vacuum mixer until the system was uniform to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. The positive electrode sheets were then cold pressed and cut into pieces.

[0089] Preparation of the electrolyte: Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0090] Assembly of a lithium-ion battery: A polyethylene film is used as a separator, and the negative electrode sheet of Example 1 is used; the positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrodes to serve as an isolation film, and then wound to obtain a bare cell; the bare cell is placed in an outer packaging shell, dried, and then injected with an electrolyte. After vacuum packaging, standing, forming, and shaping, a lithium-ion battery is obtained.

[0091] Examples 12 to 20

[0092] Examples 12 to 20 respectively provide a lithium-ion battery, which differs from Example 11 in that Examples 12 to 20 respectively use the negative electrode sheets of Examples 2 to 10 to replace the negative electrode sheet used in Example 11, and are otherwise the same as Example 11.

[0093] Comparative Examples 8 to 14

[0094] Comparative Examples 8 to 14 respectively provide a lithium ion battery, which differs from Example 11 in that: Comparative Examples 8 to 14 respectively use the negative electrode sheets of Comparative Examples 1 to 7 instead of the negative electrode sheet used in Example 11, and the rest are the same as Example 11.

[0095] Performance testing:

[0096] 1. Cycle performance test:

[0097] In a constant temperature box at 25°C±3°C, the lithium ion batteries of the following examples and comparative examples were respectively left to stand for 1 hour, charged at a constant current of 1.5C to 4.4V, with a cutoff rate of 1.2C; then charged at a constant current and constant voltage of 1.2C to 4.48V, with a cutoff rate of 0.05C, and finally discharged at a constant current of 0.7C to 3.0V. This was one charge and discharge cycle process, and the discharge capacity of the lithium ion battery in the first cycle at 25°C±3°C was recorded; the charge and discharge cycle process was then repeated according to the above method, and the discharge capacity of each charge and discharge cycle process was recorded, and then compared with the discharge capacity of the first cycle to obtain a cycle capacity curve.

[0098] Five lithium-ion batteries were taken from each group, and the average value of the capacity retention rate of the lithium-ion batteries was calculated.

[0099] 25° C. cycle retention rate (%) of lithium-ion battery = discharge capacity (mAh) at the 400th cycle / discharge capacity (mAh) after the first cycle × 100%.

[0100] 2. Cyclic XY direction expansion rate test:

[0101] The width of the negative electrode active material-coated region and the width of the negative electrode uncoated region of each of the above Examples and Comparative Examples lithium-ion batteries, after disassembly while fully charged at the 400th cycle, were recorded using a measuring instrument. The XY expansion ratio (%) of the lithium-ion battery at the 400th cycle = (width of the negative electrode active material-coated region at the 400th cycle - width of the negative electrode uncoated region at the 400th cycle) / width of the negative electrode uncoated region at the 400th cycle × 100%.

[0102] Cyclic thickness expansion test:

[0103] A flat-plate thickness gauge with a pressure of 600 g was used to measure the thickness of the lithium-ion battery of the embodiment at the first cycle under full charge and at the 400th cycle under full charge. The thickness expansion rate (%) of the lithium-ion battery at the 400th cycle = (full charge thickness at the 400th cycle / first full charge thickness - 1) × 100%.

[0104] The settings of each embodiment and comparative example are shown in Table 1. The performance of the lithium ion batteries of the above embodiments 11 to 20 and comparative examples 8 to 14 were tested according to the above test methods. The results are shown in Table 2:

[0105] Table 1

[0106]

[0107] Table 2

[0108]

[0109]

[0110] It can be seen from Table 1 and Table 2 that, compared with Comparative Examples 8 to 14, the relationship coefficient K between the mass percentage P of the silicon-based material in the negative electrode active material layer of the negative electrode sheet of the lithium ion battery of Examples 11 to 20 and the elastic modulus E of the negative electrode current collector and the compaction density ρ of the negative electrode active material layer satisfies K=E / [P(ρ+1)+1]≥50000, and the ratio of the diffraction peak (111) peak intensity to the (220) peak intensity of the negative electrode current collector is ≥2. The lithium ion battery can maintain a 25°C 400 cycle retention rate of more than 80%, and at the same time can maintain a 25°C 400 cycle thickness expansion rate of less than 9.5% and a 25°C 400 cycle XY expansion rate of less than 0.35%, and can ensure that the single-sided area of ​​the negative electrode sheet is not broken.

[0111] Comparison of the lithium-ion battery performance test results of Examples 11-15 and Comparative Examples 8-10 shows that a higher silicon-based material content in the negative electrode active material layer significantly increases the cycle expansion rate of the lithium-ion battery. Referring to Comparative Examples 8-10, it can be seen that when the silicon-based material content P in the negative electrode active material layer is too high, and the relationship coefficient K between P and the elastic modulus E of the negative electrode current collector and the compacted density ρ of the negative electrode active material layer exceeds the specified range of this application, the elastic modulus of the negative electrode current collector is insufficient to contain the expansion of the silicon-based material, which leads to a significant increase in the XY expansion rate of the negative electrode active material layer, causing expansion and deformation of the lithium-ion battery, and the risk of current collector fragmentation, thereby affecting the cycle performance and safety performance of the lithium-ion battery.

[0112] Comparative Examples 11, 16-17, and Comparative Examples 11-13 demonstrate that, when the negative electrode active material is the same, the different elastic moduli of the negative electrode current collector reflect the stress intensity it can withstand from the negative electrode active material layer, and thus the degree to which the negative electrode suppresses expansion and deformation in the XY directions parallel to the negative electrode current collector. A smaller elastic modulus of the negative electrode current collector increases the expansion and deformation rate of the negative electrode sheet in the XY directions parallel to the negative electrode current collector, which in turn deteriorates the cycling performance of the lithium-ion battery and reduces the capacity retention rate after the same number of cycles.

[0113] By comparing Examples 13, 18 to 20 and Comparative Example 14, it can be seen that as the value of I(111) / I(220) continues to decrease, the cyclic XY expansion rate of the lithium ion battery increases, and when the ratio of I(111) / I(220) is less than 2, the fragmentation phenomenon occurs. The value of I(111) / I(220) reflects the proportion of the two orientation grains in the material. The smaller the ratio, the smaller the proportion of the current collector (111) grains, the lower the current collector strength, and the lithium ion battery expands more with the increase in the number of cycles, and the stress generated also increases accordingly. The current collector cannot withstand the stress generated and thus breaks.

[0114] Therefore, through the comparison of the above embodiments and comparative examples, it can be clearly understood that the present invention can effectively reduce the expansion and deformation of the negative electrode sheet in the lithium ion battery by limiting the relationship coefficient K=E / [P(ρ+1)+1]≥50000 between the mass percentage of the silicon-based material in the negative electrode sheet and the elastic modulus of the negative electrode current collector and the compaction density of the negative electrode active material layer, and the ratio of the diffraction peak (111) peak intensity to the (220) peak intensity of the negative electrode current collector to ≥2. At the same time, it can also improve the phenomenon of single-sided fragmentation of the negative electrode sheet to a certain extent, thereby improving the cycle performance and safety performance of the lithium ion battery.

[0115] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that: include: A negative electrode current collector, wherein the ratio of the diffraction peak (111) peak intensity to the diffraction peak (220) of the negative electrode current collector is ≥2; A conductive coating is provided on at least one side of the negative electrode current collector; A negative electrode active material layer is provided on a surface of the conductive coating layer away from the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material; The elastic modulus of the negative electrode current collector is E, the mass percentage of the silicon-based material in the negative electrode active material layer is P, the compaction density of the negative electrode active material layer is ρ, and the relationship coefficient K among E, P and ρ satisfies: K=E / [P(ρ+1)+1]≥50000.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode sheet meets at least one of the following conditions: 1) The ratio of the diffraction peak (111) intensity to the diffraction peak (220) intensity of the negative electrode current collector is 2 to 10; 2) The elastic modulus E of the negative electrode current collector is ≥ 60000 N / m; 3) The mass percentage of the silicon-based material in the negative electrode active material layer is P = 1% to 75%; 4) The compaction density of the negative electrode active material layer is ρ≤1.9g / cm 3 .

3. The negative electrode sheet according to claim 1, characterized in that: The silicon-based material is selected from at least one of silicon carbide and silicon oxide; And / or, the negative electrode current collector satisfies at least one of the following conditions: 1) The negative electrode current collector is selected from copper foil; 2) The thickness of the negative electrode current collector is 1 mm to 15 mm.

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

5. The negative electrode sheet according to claim 4, characterized in that: The mass percentage of the first conductive agent in the negative electrode active material layer is 1% to 5%; and / or the mass percentage of the first binder in the negative electrode active material layer is 1% to 10%.

6. The negative electrode sheet according to claim 4, characterized in that: The first conductive agent is selected from at least one of conductive carbon black, carbon nanotubes, carbon fibers, and graphene; and / or the first binder is selected from at least one of polyacrylic acid, sodium polyacrylate, sodium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, and polyvinylidene fluoride.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: The conductive coating includes a second conductive agent and a second binder.

8. The negative electrode sheet according to claim 7, characterized in that: The mass percentage of the second conductive agent in the conductive coating is 30% to 80%; and / or the mass percentage of the second binder in the conductive coating is 20% to 70%.

9. The negative electrode sheet according to claim 7, characterized in that: The second conductive agent is selected from at least one of conductive carbon black, carbon nanotubes, and carbon fibers; and / or the second adhesive is selected from at least one of polyvinylidene fluoride, sodium carboxymethyl cellulose, and styrene-butadiene rubber.

10. A lithium ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet and a separator, wherein the negative electrode sheet is the negative electrode sheet according to any one of claims 1 to 9, and the separator is sandwiched between the positive electrode sheet and the negative electrode sheet.

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