Lithium ion battery

Abstract

The invention provides a lithium ion battery which meets the condition that (2M + 3) 2 (S + 2) 2 / W is greater than or equal to 20, M = exp (m1 / d1) + (d1-2) (m1-0. 26), m1 is the tensile strength of a negative current collector in the length direction, and d1 is the thickness of the negative current collector; s = exp (m < 2 > / d2) + (d2-2) (m < 2 >-0.16), m < 2 > is the puncture strength of the diaphragm, and d2 is the thickness of a base material in the diaphragm; w = ((exp (-10q)) / (phi-6) + exp (10q)) (d3-13.5) (rho-3. 5) / 100, d3 is the thickness of the negative electrode active layer, phi is the OI value of the negative electrode active layer, q is the mass percentage of the silicon-based negative electrode material in the negative electrode active layer, and rho is the surface density of the negative electrode active layer, so that the extrusion effect on the battery shell caused by the volume expansion of the negative electrode material can be effectively solved, and the service life of the battery shell is prolonged. The cycle life and the safety performance of the battery can be effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology and relates to a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries are widely used in new energy vehicles, portable electronic devices, and energy storage due to their advantages such as high voltage, wide operating temperature range, fast charging and discharging, high charging efficiency, high output power, no memory effect, and being green and pollution-free. However, during the charging and discharging process, the lattice spacing of the negative electrode active material in lithium-ion batteries changes as lithium ions are inserted, leading to the formation of microscopic internal stress. This causes the negative electrode to expand, which manifests as an increase or decrease in the thickness and / or width of the negative electrode coating at the macroscopic level. This process, repeated with charge and discharge cycles, can easily compress the battery casing, causing damage, creating safety hazards, and affecting the battery's cycle life. Summary of the Invention

[0003] To address the aforementioned shortcomings, this invention provides a lithium-ion battery that effectively solves the squeezing effect on the battery casing caused by the volume expansion of the negative electrode material, thereby effectively improving the battery's cycle life and safety performance.

[0004] This invention provides a lithium-ion battery, the lithium-ion battery comprising a negative electrode sheet and a separator, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer disposed on at least one functional surface of the negative electrode current collector, the negative electrode active layer comprising a negative electrode active material;

[0005] The lithium-ion battery satisfies Equation 1.

[0006] (2M+3) 2 (S+2) 2 / W≥20 Equation 1;

[0007] In Equation 1, M = exp(m1 / d1) + (d1-2)(m1-0.26), where m1 is the tensile strength of the negative electrode current collector in the length direction, in GPa; and d1 is the thickness of the negative electrode current collector, in μm.

[0008] S = exp(m2 / d2) + (d2-2)(m2-0.16), where m2 is the puncture strength of the diaphragm in kgf; and d2 is the thickness of the substrate in the diaphragm in μm.

[0009] d3 is the thickness of the negative electrode active layer, in μm; ρ is the OI value of the negative electrode active layer; q is the mass percentage of the silicon-based negative electrode material in the negative electrode active material; ρ is the areal density of the negative electrode active layer, in mg / cm³. 2 .

[0010] Furthermore, the lithium-ion battery satisfies Equation 2.

[0011] (2M+3) 2 (S+2) 2 / W≥50 Equation 2.

[0012] Furthermore, m1 is 0.3 GPa to 0.8 GPa, and d1 is 3 μm to 9 μm.

[0013] Furthermore, M ranges from 1.2 to 4.9.

[0014] Furthermore, m2 is 0.18 kgf to 0.6 kgf, and d2 is 3.5 μm to 7 μm.

[0015] Furthermore, S ranges from 1.1 to 3.3.

[0016] Furthermore, q ranges from 0 to 0.3, and d3 ranges from 50 μm to 80 μm. The value is 10–30, and the density is 7 mg / cm³. 2 ~12mg / cm 2 .

[0017] Furthermore, W ranges from 2 to 80.

[0018] Furthermore, the diaphragm also includes adhesive layers disposed on both sides of the substrate;

[0019] The thickness of the adhesive layer on one side is 0.5 to 3 μm.

[0020] Furthermore, the diaphragm also includes a ceramic layer disposed on at least one surface of the adhesive layer away from the substrate;

[0021] The thickness of the ceramic layer on one side is 0.5 to 3 μm.

[0022] This invention controls the tensile strength and thickness of the negative electrode current collector, the puncture strength and thickness of the separator, the thickness of the substrate in the separator, the thickness of the negative electrode active layer, the OI value, the areal density, and the mass percentage of silicon-based negative electrode material in the lithium-ion battery, so that the lithium-ion battery satisfies the above formula 1. This effectively suppresses the squeezing effect of the volume expansion of the negative electrode material on the battery shell, thereby avoiding corner breakage and leakage of the battery and improving the safety and cycle performance of the battery. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0024] This invention provides a lithium-ion battery, which includes a negative electrode sheet and a separator. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one functional surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material.

[0025] Lithium-ion batteries satisfy Equation 1.

[0026] (2M+3) 2 (S+2) 2 / W≥20 Equation 1;

[0027] In Equation 1, M = exp(m1 / d1) + (d1-2)(m1-0.26), where m1 is the tensile strength of the negative electrode current collector in the length direction, in GPa; and d1 is the thickness of the negative electrode current collector, in μm.

[0028] S = exp(m2 / d2) + (d2-2)(m2-0.16), where m2 is the puncture strength of the diaphragm in kgf and d2 is the thickness of the substrate in the diaphragm in μm.

[0029] d3 is the thickness of the negative electrode active layer, in μm; ρ is the OI value of the negative electrode active layer; q is the mass percentage of silicon-based negative electrode material in the negative electrode active material; ρ is the areal density of the negative electrode active layer, in mg / cm³. 2 .

[0030] Specifically, the negative electrode current collector has two functional surfaces in the thickness direction. In one embodiment, the negative electrode active layer is disposed on one functional surface of the negative electrode current collector, and no functional layer is disposed on the other functional surface. In this case, the thickness d3 of the negative electrode active layer refers to the overall thickness of the negative electrode active layer, and the areal density of the negative electrode active layer refers to the overall areal density of the negative electrode active layer. In another embodiment, the negative electrode active layer is disposed on both functional surfaces of the negative electrode current collector. In this case, the thickness d3 of the negative electrode active layer refers to the thickness of one side of the negative electrode active layer, and the areal density of the negative electrode active layer refers to the areal density of one side of the negative electrode active layer.

[0031] The tensile strength m1 of the negative electrode current collector in the length direction of the present invention is obtained by testing it using the following method: the negative electrode current collector is cut into a rectangular sample with a length of 200 mm and a width of 13 mm. The length direction is taken as the tensile direction. The load area A0 of the rectangular sample perpendicular to the tensile direction is calculated. The tensile tester is used to test it. The tensile rate is 50 mm / min. The load force when the rectangular sample breaks is recorded as F0. Then the tensile strength m1 is F0 / A0, and the unit is GPa.

[0032] The thickness d1 of the negative electrode current collector in this invention is measured using a micrometer, and the unit is μm.

[0033] The puncture strength m2 of the diaphragm in this invention is obtained by testing according to the GB / T 36363-2018 standard, and the unit is kgf.

[0034] The thickness d2 of the substrate in the diaphragm of the present invention is obtained by performing SEM testing on the cross-section of the diaphragm to obtain a cross-sectional SEM image, and measuring the thickness value of the substrate in the cross-sectional SEM image, with the unit being μm.

[0035] The thickness d3 of the negative electrode active layer in this invention is obtained by measuring the following steps: Before fabricating the negative electrode sheet, the thickness of the negative electrode current collector is measured using a micrometer. Then, the negative electrode active layer is coated on at least one functional surface of the negative electrode current collector. After drying and compaction, the negative electrode sheet is obtained, and the thickness of the negative electrode sheet is measured using a micrometer. When the negative electrode current collector has a negative electrode active layer on only one functional surface, the thickness d3 of the negative electrode active layer is the thickness of the negative electrode sheet minus the thickness of the negative electrode current collector. When the negative electrode active layer is provided on both functional surfaces of the negative electrode current collector, the thickness d3 of the negative electrode active layer is (thickness of the negative electrode sheet - thickness of the negative electrode current collector) / 2, in μm.

[0036] OI value in this invention The OI value is obtained by XRD testing of the negative electrode. The ratio of the peak intensity of the diffraction peak of the (004) crystal plane to that of the (110) crystal plane is the OI value. The 2θ of the diffraction peak of the (004) crystal plane is 53.7°~55.7°, and the 2θ of the diffraction peak of the (110) crystal plane is 76.4°~78.4°.

[0037] The areal density of the negative electrode active layer in this invention is measured through the following steps: Before fabricating the negative electrode sheet, the weight of the negative electrode current collector per unit area is weighed. Then, the negative electrode active layer is coated on at least one functional surface of the negative electrode current collector. After drying and compaction, the negative electrode sheet is obtained, and the weight of the negative electrode sheet per unit area is weighed. When the negative electrode current collector has a negative electrode active layer on only one functional surface, the areal density ρ of the negative electrode active layer is the weight of the negative electrode sheet per unit area minus the weight of the negative electrode current collector per unit area. When the negative electrode current collector has a negative electrode active layer on both functional surfaces, the areal density ρ of the negative electrode active layer is (weight of the negative electrode sheet per unit area - weight of the negative electrode current collector per unit area) / 2, with units of mg / cm³. 2 .

[0038] The present invention does not specifically limit the type of negative electrode current collector. For example, it can be selected from any one of copper current collector, copper-PET composite current collector, copper-polypropylene composite current collector, and copper-polyimide composite current collector, with copper current collector being preferred.

[0039] The present invention does not specifically limit the type of substrate. For example, it may be selected from any one of polyethylene substrate, polypropylene substrate, polyethylene / polypropylene hybrid substrate, polypropylene / ethylene / propylene multilayer co-extruded substrate, nonwoven substrate, polyimide substrate, and aramid substrate, preferably polyethylene substrate.

[0040] The present invention does not specifically limit the source of the negative electrode current collector and the substrate; products prepared by commercially available products or conventional preparation methods known to those skilled in the art are acceptable.

[0041] According to the technical solution provided by the present invention, by making the lithium-ion battery satisfy Equation 1, the safety performance and cycle life of the battery can be effectively improved. The inventors analyzed this principle and believe that the reason may be that by controlling the tensile strength of the negative electrode current collector in the length direction, the thickness of the negative electrode current collector, the puncture strength of the separator, the thickness of the substrate in the separator, the thickness and OI value of the negative electrode active layer, the mass content of silicon-based negative electrode material in the negative electrode active material, and the areal density of the negative electrode active layer, the aforementioned parameters can satisfy Equation 1, which can effectively reduce the initial expansion of the negative electrode active layer in the thickness direction, while ensuring that the separator and the negative electrode current collector can support the expansion of the negative electrode active layer in the width direction. This can significantly reduce the extrusion pressure on the casing caused by the expansion of the negative electrode material, especially when it contains silicon-based materials. Therefore, the lithium-ion battery of the present invention can be cycled 800 times at room temperature (25-35°C) without corner leakage and has a good cycle capacity retention rate.

[0042] Furthermore, we found that when a lithium-ion battery satisfies Formula 1, the burrs generated by the copper foil can be effectively reduced, and the separator can effectively resist foreign objects from piercing the separator, thereby effectively mitigating physical self-discharge and improving the self-discharge capability of the lithium-ion battery.

[0043] In one specific implementation, the lithium-ion battery satisfies Equation 2.

[0044] (2M+3) 2 (S+2) 2 / W≥50 Equation 2. Specifically, by further controlling the tensile strength of the negative electrode current collector in the length direction, the thickness of the negative electrode current collector, the puncture strength of the separator, the thickness of the substrate in the separator, the thickness and OI value of the negative electrode active layer, the mass content of silicon-based negative electrode material in the negative electrode active material, and the areal density of the negative electrode active layer, the M value, S value, and W value can be further controlled to make the lithium-ion battery satisfy Equation 2. When the lithium-ion battery satisfies Equation 2, the expansion of the negative electrode active layer in the thickness direction can be further suppressed, and the supporting effect of the separator and negative electrode current collector on the negative electrode sheet can be improved. Therefore, the battery can still not experience corner breakage and leakage after 500 cycles at a high temperature of 35-45°C, and has a better cycle capacity retention rate.

[0045] In one specific embodiment, m1 is 0.3 GPa to 0.8 GPa, and d1 is 3 μm to 9 μm. For example, m1 is 0.3 GPa, 0.4 GPa, 0.5 GPa, 0.6 GPa, 0.7 GPa, or 0.8 GPa; and d1 is 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or 9 μm. Within this range, not only can the negative electrode current collector have high tensile strength, providing sufficient support for the negative electrode active layer, but the thickness of the negative electrode current collector is also relatively moderate, which can reduce energy density loss to a certain extent.

[0046] In one specific embodiment, M is 1.2 to 4.9. For example, M can be 1.2, 1.6, 2.0, 2.4, 2.8, 3.2, 3.6, 4.0, 4.4, or 4.9. Within this range, the supporting effect of the negative electrode current collector on the negative electrode active layer can be further enhanced, ensuring that the negative electrode active layer will not detach due to volume expansion, thus affecting the cycle performance of the battery. Furthermore, the compression effect of the volume expansion of the negative electrode material on the battery casing can be further alleviated, avoiding corner breakage and leakage problems.

[0047] In one specific embodiment, m2 is 0.18 kgf to 0.6 kgf, and d2 is 3.5 μm to 7 μm. For example, m2 is 0.18 kgf, 0.2 kgf, 0.3 kgf, 0.4 kgf, 0.5 kgf, or 0.6 kgf; and d2 is 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, or 7 μm. This not only ensures that the separator has high puncture strength and provides sufficient support for the negative electrode active layer, but also that the thickness of the substrate in the separator is moderate, which can reduce energy density loss to a certain extent.

[0048] In one specific embodiment, S is 1.1 to 3.3. For example, S is 1.1, 1.3, 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, 2.8, 3.1, or 3.3. Within this range, the supporting effect of the separator on the negative electrode active layer can be further improved, ensuring that the negative electrode sheet will not detach from the separator due to the volume expansion of the negative electrode active material, and preventing separator rupture, which would affect the cycle performance and safety performance of the battery. Furthermore, it can alleviate the squeezing effect of the volume expansion of the negative electrode material on the battery casing, avoiding corner breakage and leakage problems.

[0049] In one specific embodiment, q is 0 to 0.3, and d3 is 50 μm to 80 μm. The value is 10–30, and the density is 7 mg / cm³. 2 ~12mg / cm 2 For example, q is 0, 0.05, 0.1, 0.15, 0.2, 0.25 or 0.3; d3 is 50μm, 55μm, 60μm, 65μm, 70μm, 75μm or 80μm; The values ​​are 10, 15, 20, 25, or 30; ρ is 7 mg / cm³. 2 8mg / cm 2 9mg / cm 2 10mg / cm 2 11mg / cm 2 Or 12mg / cm 2 At this point, the volume expansion of the negative electrode active layer can be further suppressed, thereby further reducing the initial expansion of the negative electrode active layer in the thickness and width directions, avoiding excessive expansion of the negative electrode sheet that could compress the separator and the outer casing, causing corner breakage and leakage; at the same time, it can also reduce the loss of energy density and ensure the high energy density of the battery.

[0050] In one specific embodiment, W is 2 to 80. For example, W is 2, 10, 20, 30, 40, 50, 60, 70, or 80. Within this range, not only can the volume expansion of the negative electrode active material be further suppressed, and the squeezing effect of the negative electrode material on the battery casing be alleviated, thus giving the battery a higher cycle life, but also the high energy density of the battery can be guaranteed.

[0051] In one specific embodiment, the diaphragm further includes an adhesive layer disposed on both sides of the substrate; the thickness of the adhesive layer is 0.5 μm to 3 μm.

[0052] For example, the thickness of the adhesive layer is 0.5μm, 1.0μm, 1.5μm, 2.0μm, 2.5μm or 3.0μm.

[0053] The thickness of the adhesive layer in this invention refers to the thickness of the adhesive layer on one side.

[0054] The present invention does not limit the type of adhesive layer, for example, it may be selected from at least one of polyvinylidene fluoride, polymethyl methacrylate, polyimide, polyetherimide, and polyamideimide.

[0055] When the thickness of the adhesive layer is within the aforementioned range, it can not only enhance the interaction force between the separator and the negative electrode, ensuring that the separator and the negative electrode will not peel off due to the expansion of the negative electrode active material, thus helping to improve the cycle stability of the battery, but also reduce the loss of energy density.

[0056] In one embodiment, the diaphragm further includes a ceramic layer disposed on at least one surface of the adhesive layer away from the substrate;

[0057] The thickness of the ceramic layer is 0.5μm to 3μm.

[0058] For example, the thickness of the ceramic layer is 0.5μm, 1.0μm, 1.5μm, 2.0μm, 2.5μm or 3.0μm.

[0059] The present invention does not limit the type of ceramic layer, but may include at least one of alumina, boehmite, magnesium oxide, silicon oxide, aluminum nitride, magnesium hydroxide, and barium sulfate.

[0060] When the separator includes a ceramic layer, it can increase the separator's absorption of electrolyte and reduce thermal shrinkage, thereby improving the battery's cycle performance and safety. When the thickness of the ceramic layer is within the aforementioned range, it can reduce energy density loss.

[0061] The lithium-ion battery of the present invention will be described in detail below through specific embodiments.

[0062] Example 1

[0063] 1) The negative electrode active materials, artificial graphite and silicon carbide (mass ratio 85:15), conductive agent carbon black, binder styrene-butadiene rubber, and dispersant sodium carboxymethyl cellulose, were dispersed in an appropriate amount of deionized water at a mass fraction of 97.2:0.5:1:1.3. After thorough stirring to form a uniform negative electrode slurry, the negative electrode slurry was uniformly coated onto the surface of a copper foil with a thickness of 6 μm and a tensile strength of 0.55 GPa in the length direction using a coating machine. After drying, rolling, and cutting, a single-sided density of 9 mg / cm³ was obtained. 2 The single-sided compaction density is 1.73 g / cm³. 3 A negative electrode with an OI value of 20, wherein the thickness of the negative electrode active layer on one side is 65 μm;

[0064] 2) The positive electrode active material lithium cobalt oxide, conductive agent carbon black, and binder polyvinylidene fluoride were dispersed in an appropriate amount of N-methylpyrrolidone at a mass ratio of 97.6:1.2:1.2. After thorough stirring, a uniform positive electrode slurry was formed. The positive electrode slurry was coated onto the surface of an aluminum foil with a thickness of 10 μm using a coating machine. After drying, rolling, and cutting, a single-sided surface density of 16.7 mg / cm³ was obtained. 2 The single-sided compaction density is 4.12 g / cm³. 3 The positive electrode plate.

[0065] 3) The diaphragm comprises a polyethylene layer with a thickness of 5.3 μm and polyvinylidene fluoride layers coated on both sides of the polyethylene layer, with an alumina ceramic layer disposed on the surface of one side of the polyvinylidene fluoride layer. The alumina ceramic layer has a thickness of 2 μm, the polyvinylidene fluoride layers on both sides are 1 μm thick, and the puncture strength of the polyethylene substrate is 0.39 kgf.

[0066] 4) The pre-cut negative electrode sheet, separator, and positive electrode sheet are stacked in sequence, with the ceramic layer side of the separator facing the positive electrode sheet. A core is then formed by winding the core. The casing is an aluminum-plastic film containing a cavity with a length of 79.2 mm and a width of 60.8 mm. The core is centrally fixed within the cavity of the aluminum-plastic film and is impregnated with an electrolyte solution. The electrolyte solution comprises a mixture of ethylene carbonate (EC), propylene carbonate (PC), propyl propionate (PP), lithium hexafluorophosphate (LiPF6), fluoroethylene carbonate (FEC), and 1,3-propane sulpholol (PS) in a weight ratio of 16:16:48:15:3:2. After formation, secondary sealing, and sorting, the battery has a voltage of 3.75V, an initial thickness of 3.34 mm, and a capacity of 3001 mAh.

[0067] Example 2

[0068] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the thickness of the copper foil is adjusted to 4 μm and the tensile strength is adjusted to 0.43 GPa; the single-sided thickness of the negative electrode active layer is adjusted to 76 μm, the OI value is adjusted to 14.5, and the single-sided density is adjusted to 9.67 mg / cm³. 2 ;

[0069] In step 3), the thickness of the polyethylene layer is adjusted to 4.2 μm and the puncture strength of the polyethylene layer is adjusted to 0.22 kgf.

[0070] Example 3

[0071] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the mass ratio of artificial graphite to silicon carbide is adjusted to 92:8, the thickness of the copper foil is adjusted to 4 μm, and the tensile strength is adjusted to 0.43 GPa; the single-sided thickness of the negative electrode active layer is adjusted to 67 μm, the OI value is adjusted to 12.9, and the single-sided density is adjusted to 9.25 mg / cm³. 2 ;

[0072] In step 3), the thickness of the polyethylene layer is adjusted to 4.2 μm, and the puncture strength of the polyethylene base layer is adjusted to 0.22 kgf.

[0073] Example 4

[0074] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the mass ratio of artificial graphite to silicon carbide is adjusted to 96:4, the thickness of the copper foil is adjusted to 5 μm, and the tensile strength is adjusted to 0.48 GPa; the single-sided thickness of the negative electrode active layer is adjusted to 67 μm, the OI value is adjusted to 12.9, and the single-sided density is adjusted to 9.24 mg / cm³. 2 ;

[0075] In step 3), the thickness of the polyethylene layer is adjusted to 4.7 μm and the puncture strength of the polyethylene layer is adjusted to 0.28 kgf.

[0076] Example 5

[0077] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the tensile strength of the copper foil is adjusted to 0.3 GPa.

[0078] Example 6

[0079] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the tensile strength of the copper foil is adjusted to 0.8 GPa.

[0080] Example 7

[0081] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the thickness of the copper foil is adjusted to 3 μm.

[0082] Example 8

[0083] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the thickness of the copper foil is adjusted to 9 μm.

[0084] Example 9

[0085] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the thickness of the copper foil is adjusted to 4 μm and the tensile strength is adjusted to 0.33 GPa.

[0086] Example 10

[0087] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the thickness of the copper foil is adjusted to 9 μm and the tensile strength is adjusted to 0.8 GPa.

[0088] Example 11

[0089] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 3), the puncture strength of the polyethylene layer is adjusted to 0.18 kgf.

[0090] Example 12

[0091] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 3), the puncture strength of the polyethylene layer is adjusted to 0.6 kgf.

[0092] Example 13

[0093] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 3), the thickness of the polyethylene layer is adjusted to 3.5 μm.

[0094] Example 14

[0095] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 3), the thickness of the polyethylene layer is adjusted to 7 μm.

[0096] Example 15

[0097] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 3), the thickness of the polyethylene layer is adjusted to 3.7 μm and the puncture strength of the polyethylene layer is adjusted to 0.18 kgf.

[0098] Example 16

[0099] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 3), the thickness of the polyethylene layer is adjusted to 7 μm and the puncture strength of the polyethylene layer is adjusted to 0.6 kgf.

[0100] Example 17

[0101] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the negative electrode active material is replaced with pure artificial graphite, and silicon carbide is not used.

[0102] Example 18

[0103] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the thickness of the negative electrode active layer on one side is 50 μm and the OI value of the negative electrode sheet is adjusted to 13.

[0104] Example 19

[0105] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the thickness of the negative electrode active layer on one side is 80 μm and the OI value of the negative electrode sheet is adjusted to 26.

[0106] Example 20

[0107] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the density of the negative electrode sheet on one side is adjusted to 7 mg / cm³. 2 .

[0108] Example 21

[0109] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the density of the negative electrode sheet on one side is adjusted to 12 mg / cm³. 2 .

[0110] Example 22

[0111] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the negative electrode active material is replaced with pure artificial graphite instead of silicon carbide; the single-sided thickness of the negative electrode active layer is 53 μm, the OI value of the negative electrode sheet is adjusted to 23; and the single-sided density of the negative electrode sheet is adjusted to 8.3 mg / cm³. 2 .

[0112] Example 23

[0113] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the single-sided thickness of the negative electrode active layer is 75 μm, the OI value is 18, the negative electrode active materials are artificial graphite and silicon carbide (mass ratio of 70:30), the thickness of the copper foil is adjusted to 7 μm, the tensile strength is adjusted to 0.6 GPa, the OI value of the negative electrode sheet is adjusted to 18, and the single-sided density is adjusted to 9.97 mg / cm³. 2 ;

[0114] In step 3), the thickness of the polyethylene layer is adjusted to 7 μm, and the puncture strength of the polyethylene base layer is adjusted to 0.6 kgf.

[0115] Example 24

[0116] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the single-sided thickness of the negative electrode active layer is 83 μm, the OI value is 35, the negative electrode active material is artificial graphite and silicon carbide (mass ratio of 64:36), the thickness of the copper foil is adjusted to 10.00 μm, the tensile strength is adjusted to 0.85 GPa, the OI value of the negative electrode sheet is adjusted to 35.0, and the single-sided density is adjusted to 12.42 mg / cm³. 2 ;

[0117] In step 3), the thickness of the polyethylene layer is adjusted to 9.0 μm, and the puncture strength of the polyethylene base layer is adjusted to 0.73 kgf.

[0118] Comparative Example 1

[0119] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the mass ratio of artificial graphite to silicon carbide is adjusted to 70:30; the single-sided thickness of the negative electrode active layer is 75 μm, the OI value of the negative electrode sheet is adjusted to 18; and the single-sided density of the negative electrode sheet is adjusted to 9.97 mg / cm³. 2 .

[0120] Comparative Example 2

[0121] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), the mass ratio of artificial graphite to silicon carbide is adjusted to 89:11, the thickness of the copper foil is adjusted to 2.8 μm, and the tensile strength is adjusted to 0.21 GPa; the single-sided thickness of the negative electrode active layer is adjusted to 84 μm, the OI value is adjusted to 35, and the single-sided density is adjusted to 13 mg / cm³. 2 ;

[0122] In step 3), the thickness of the polyethylene layer is adjusted to 3 μm, and the puncture strength of the polyethylene base layer is adjusted to 0.15 kgf.

[0123] Test case

[0124] 1. The tensile strength of the negative electrode current collector in the length direction, the thickness of the negative electrode current collector, the puncture strength of the diaphragm, the thickness of the substrate in the diaphragm, the thickness of the negative electrode active layer, and the OI value of the negative electrode active layer in the above embodiments and comparative examples were tested:

[0125] (1) Tensile strength of the negative electrode current collector in the length direction

[0126] The negative electrode current collector is cut into rectangular samples with a length of 200 mm and a width of 13 mm. The length direction is taken as the tensile direction. The load area A0 of the rectangular sample perpendicular to the tensile direction is calculated. The tensile tester is used to test the sample at a tensile rate of 50 mm / min. The load force when the rectangular sample breaks is recorded as F0. The tensile strength m1 is F0 / A0, and the unit is GPa.

[0127] (2) Thickness of the negative electrode current collector

[0128] The thickness of the negative electrode current collector was measured using a micrometer, with the unit being μm.

[0129] (3) Puncture strength of the diaphragm

[0130] The puncture strength of the diaphragm was tested according to GB / T 36363-2018 standard, with the unit being kgf.

[0131] (4) Thickness of the substrate in the diaphragm

[0132] The substrate was subjected to SEM testing, and its thickness was obtained by observing its cross-section, with the unit being μm.

[0133] (5) Thickness of the negative electrode active layer

[0134] The thickness of the negative electrode current collector is measured using a micrometer, and then the thickness of the negative electrode sheet including the negative electrode current collector is measured using a micrometer. The thickness of the negative electrode active layer is (thickness of the negative electrode sheet - thickness of the negative electrode current collector) / 2, in μm.

[0135] (6) OI value of negative electrode active layer

[0136] The OI value is obtained by XRD testing of the negative electrode. The ratio of the peak intensity of the diffraction peak of the (004) crystal plane to that of the (110) crystal plane is 53.7°~55.7°, and the 2θ of the diffraction peak of the (004) crystal plane is 76.4°~78.4°.

[0137] The test results and calculation results are shown in Table 1 and Table 2.

[0138] Table 1

[0139]

[0140]

[0141] Table 2

[0142] M S W <![CDATA[((M+50) 2 +S 2 ) / (W 2 ×1000)]]> Example 1 2.3 1.8 12.74 65 Example 2 1.5 1.2 17.38 20 Example 3 1.5 1.2 7.05 50 Example 4 1.8 1.4 4.88 100 Example 5 1.2 1.8 12.74 34 Example 6 3.3 1.8 12.74 107 Example 7 1.5 1.8 12.74 41 Example 8 3.1 1.8 12.74 97 Example 9 1.2 1.8 12.74 34 Example 10 4.9 1.8 12.74 188 Example 11 2.3 1.1 12.74 43 Example 12 2.3 2.6 12.74 93 Example 13 2.3 1.5 12.74 53 Example 14 2.3 2.2 12.74 78 Example 15 2.3 1.1 12.74 42 Example 16 2.3 3.3 12.74 124 Example 17 2.3 1.8 3.03 274 Example 18 2.3 1.8 9.06 92 Example 19 2.3 1.8 16.43 51 Example 20 2.3 1.8 8.11 102 Example 21 2.3 1.8 19.69 42 Example 22 2.3 1.8 2.01 413 Example 23 2.8 3.3 79.94 26 Example 24 5.8 5.1 226.89 47 Comparative Example 1 2.3 1.8 79.94 10 Comparative Example 2 1.0 1.0 20.20 12

[0143] From Table 1 and Table 2, we can see that:

[0144] The lithium-ion batteries in Examples 1-26 all satisfy Equation 1, while Examples 1, 3, 4, 6, 8, 10, 12-14, 16-20, and 22 all satisfy Equation 2; while Comparative Examples 1 and 2 do not satisfy Equation 1.

[0145] 2. The leakage, cycle performance, energy density, rate performance, and self-discharge capacity of the lithium-ion batteries in the above embodiments and comparative examples were tested:

[0146] (1) Circulation performance and leakage of corner rupture fluid

[0147] Room temperature cycling performance: The lithium-ion batteries prepared in the above examples and comparative examples were placed in an environment of 25±2℃ and charged at a constant current of 1C until the cutoff current was 0.05C. After the battery was fully charged, it was left to stand for 5 minutes, and then discharged at a constant current of 0.5C until the cutoff voltage was 3.0V. The charging and discharging mechanism was repeated three times. The highest discharge capacity in the first three cycles was recorded as the initial capacity Q0. After 800 cycles, the discharge capacity Q1 of the battery was recorded. The high temperature capacity retention rate of the battery is Q1 / Q0×100%. In addition, during the process, the appearance of the battery was checked every 50 cycles. It was observed whether there was any cracking at the four corners of the battery and the results were recorded.

[0148] High-temperature cycling performance: The lithium-ion batteries prepared in the above examples and comparative examples were placed in an environment of 45±2℃ and left to stand. When the battery body reached 45±2℃, the battery was charged at a constant current of 1.2C to 4.25V, then charged at 0.7C to the upper limit voltage of 4.53V, and then charged at a constant voltage to 0.025C. After standing for 10 minutes, the battery was discharged at 0.5C to 3V and left to stand for 10 minutes. Cyclic tests were performed according to the above charge and discharge steps. After 500 cycles, the discharge capacity Q3 of the battery was recorded. The high-temperature capacity retention rate of the battery is Q2 / Q3×100%. In addition, during this process, the appearance of the battery was checked every 50 cycles to observe whether there was any corner breakage and the results were recorded.

[0149] (2) Self-discharge capability

[0150] In this invention, the K-value is used to evaluate the self-discharge capacity of the battery. Specifically, after the cells prepared in the above embodiments and comparative examples are subjected to capacity testing, they are placed in a 45°C high-temperature chamber for 48 hours. After the initial settling period, they are placed in a 25°C environment for another 36 hours. The cell voltage is recorded as V1 (mV) after the settling period. The cells are then placed for another T hours (approximately 72 hours) and the cell voltage is recorded as V2 (mV) after the settling period. The K-value is (V1-V2) / T, with the unit being mV / h. The test results are shown in Table 3.

[0151] Table 3

[0152]

[0153]

[0154] From Tables 1-3, we can see that:

[0155] The lithium-ion batteries in Examples 1-24 exhibit better cycle performance and a lower probability of corner leakage compared to Comparative Examples 1 and 2. Specifically, the lithium-ion battery in Example 22 maintains a capacity retention of 84.33% after 800 cycles at room temperature and 75.42% after 500 cycles at high temperature, without any corner leakage, with a K-value of 0.0136. In contrast, the comparative examples show a capacity retention of only 45.12% after 800 cycles at room temperature and 48.52% after 500 cycles at high temperature, but corner leakage occurs after 200 cycles at room temperature and 150 cycles at high temperature. Therefore, the lithium-ion battery of this invention effectively solves the problem of compression on the battery casing caused by the volume expansion of the negative electrode material, thereby effectively improving the battery's cycle life and safety performance.

[0156] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium-ion battery, characterized by, The lithium ion battery comprises a negative electrode sheet, a separator, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer arranged on at least one functional surface of the negative electrode current collector, and the negative electrode active layer comprises a negative electrode active material; The lithium ion battery satisfies formula 1, (2M+3) 2 (S+2) 2 W≥20 Equation 1; In formula 1, M = exp(m1 / d1) + (d1-2)(m1-0.26), m1 is the tensile strength of the negative electrode current collector in the length direction, and the unit is GPa; d1 is the thickness of the negative electrode current collector, and the unit is μm; S = exp(m2 / d2) + (d2-2)(m2-0.16), m2 is the puncture strength of the separator, and the unit is kgf; d2 is the thickness of the base material in the separator, and the unit is μm; W = ((exp(-10q) / (φ-6) + exp(10q))(d3-13.5)(p-3.5) / 100, d3 is the thickness of the negative active layer, unit: pm; φ is the OI value of the negative active layer; q is the mass percentage of silicon-based negative electrode material in the negative active material; p is the area density of the negative active layer, unit: mg / cm2 2 .

2. The lithium-ion battery of claim 1, wherein, The lithium ion battery satisfies formula 2, (2M+3) 2 (S+2) 2 W≥ 50 Formula 2.

3. The lithium-ion battery according to claim 1 or 2, characterized in that m1 is 0.3 GPa to 0.8 GPa, and d1 is 3 μm to 9 μm.

4. The lithium-ion battery of claim 3, wherein, M is 1.2 to 4.

9.

5. The lithium-ion battery according to any one of claims 1 to 4, characterized in that, m2 is 0.18 kgf to 0.6 kgf, and d2 is 3.5 μm to 7 μm.

6. The lithium-ion battery of claim 5, wherein, S is 1.1 to 3.

3.

7. The lithium-ion battery according to any one of claims 1 to 6, characterized in that, q is 0 to 0.3, d3 is 50 μm to 80 μm, φ is 10 to 30, and p is 7 mg / cm 2 ~ 12 mg / cm 2 .

8. The lithium-ion battery of claim 7, wherein, W is 2 to 80.

9. The lithium-ion battery according to any one of claims 1 to 8, characterized in that, The separator further comprises an adhesive layer arranged on both sides of the base material; The thickness of the adhesive layer is 0.5 to 3 μm.

10. The lithium-ion battery of claim 9, wherein, The separator further comprises a ceramic layer, and the ceramic layer is arranged on at least one surface of the adhesive layer away from the base material; The single-side thickness of the ceramic layer is 0.5 to 3 μm.

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