Battery

Abstract

The invention relates to the technical field of batteries, in particular to a battery which comprises a negative plate, a positive plate and electrolyte, and the negative plate comprises a negative current collector and a negative active coating located on at least one side surface of the negative current collector; a conductive bottom coating is arranged between the negative electrode active coating and the negative electrode current collector; the negative electrode active coating comprises a negative electrode active material, and the negative electrode active material comprises a spherical silicon carbon material. The invention can solve the problem that the active material at the bottom layer of the pole piece cannot be effectively utilized, improve lithium precipitation on the surface of the negative electrode in the later period of circulation, solve the problem of expansion of the silicon-containing negative electrode and prolong the cycle life of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a battery. Background Art

[0002] With the advent of the 5G era and the rapid development of lithium-ion battery technology, people have put forward higher requirements for the energy density, fast charging ability, and charge-discharge rate of lithium-ion batteries. Fast-charging lithium batteries with high energy density are also the development trend of consumer lithium-ion batteries. However, with the increase in energy density, the thickness of the positive and negative electrode sheets of lithium batteries becomes thicker and thicker, and the transmission distance of lithium ions becomes longer and longer. In addition, after the charging speed becomes faster, lithium ions are more likely to accumulate and congest on the surface of the electrode sheet during the charge-discharge process, and lithium ions cannot quickly embed into the bottom active material far from the surface of the electrode sheet and close to the current collector of the electrode sheet, resulting in the ineffective utilization of the active material at the bottom of the electrode sheet. In addition, the potential change causes lithium deposition on the surface of the electrode sheet in the later stage of the cycle. Especially for the silicon-doped negative electrode sheet, with the increase in the thickness of the electrode sheet, the lithium deposition and swelling are more serious, seriously affecting the service life of lithium-ion batteries. Summary of the Invention

[0003] In view of this, the present invention is committed to providing a battery that can fully solve the problem of ineffective utilization of the active material at the bottom of the electrode sheet, improve lithium deposition on the surface of the negative electrode in the later stage of the cycle, and at the same time can effectively solve the swelling problem of the silicon-containing negative electrode sheet, improving the cycle life and cycle expansion of the battery.

[0004] The present invention provides a battery, which includes a negative electrode sheet, a positive electrode sheet, and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode active coating located on at least one surface of the negative electrode current collector; a conductive bottom coating is provided between the negative electrode active coating and the negative electrode current collector;

[0005] The negative electrode active coating includes negative electrode active materials, and the negative electrode active materials include spherical silicon-carbon materials. The particle size Dv50 of the spherical silicon-carbon materials is D;

[0006] The thickness of the negative electrode current collector is H0, and the thickness of the conductive bottom coating is H1. H0 and H1 satisfy 0.1H0 ≤ H1 ≤ 0.9H0;

[0007] After the first charge-discharge, in the battery in the fully charged state, H0, H1, H2, and D satisfy 0.20 ≤ D / (H0 + H1 + H2) ≤ 0.28, where H2 is the thickness of the negative electrode active coating.

[0008] Through the above technical solutions, the present invention has at least the following advantages compared with the prior art:

[0009] (1) The battery of the present invention can enhance the electron conduction of the active material near the negative electrode current collector side (i.e., the bottom layer), fully solve the problem that the active material at the bottom layer of the electrode sheet cannot be effectively utilized, and improve the lithium deposition on the surface of the negative electrode in the later stage of cycling;

[0010] (2) The battery of the present invention can avoid the influence of the conductive bottom coating provided at the bottom layer of the negative electrode active coating on the expansion of the silicon negative electrode, buffer the expansion stress of the spherical silicon-carbon material, effectively solve the expansion problem of the silicon-containing negative electrode sheet, and thus improve the cycle life and cycle expansion of the battery.

[0011] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. Description of the Drawings

[0012] Figure 1 The cross-sectional structural schematic diagram of the negative electrode sheet in an example of the present invention is shown;

[0013] Figure 2 The schematic diagram of the lithium deposition situation on the surface of the negative electrode sheet after the battery is disassembled in an example of the present invention is shown. Detailed Embodiments

[0014] The following details the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not used to limit the present invention.

[0015] The present invention provides a battery, which includes a negative electrode sheet, a positive electrode sheet and an electrolyte. Among them, the cross-sectional structural schematic diagram of the negative electrode sheet is as Figure 1 shown. The negative electrode sheet includes a negative electrode current collector 100 and a negative electrode active coating 110 located on at least one surface of the negative electrode current collector 100; a conductive bottom coating 120 is provided between the negative electrode active coating 110 and the negative electrode current collector 100;

[0016] The negative electrode active coating 110 includes a negative electrode active material, and the negative electrode active material includes a spherical silicon-carbon material, and the particle size Dv50 of the spherical silicon-carbon material is D;

[0017] The thickness of the negative electrode current collector 100 is H0, the thickness of the conductive bottom coating 120 is H1, and H0 and H1 satisfy 0.1H0 ≤ H1 ≤ 0.9H0, for example, 0.1H0, 0.2H0, 0.3H0, 0.4H0, 0.5H0, 0.6H0, 0.7H0, 0.8H0 or 0.9H0;

[0018] After the first charge and discharge, in the battery in a fully charged state, H0, H1, H2, and D satisfy 0.20 ≤ D / (H0 + H1 + H2) ≤ 0.28, such as 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, or 0.28, where H2 is the thickness of the negative electrode active coating 110.

[0019] According to Figure 1 As shown in the schematic cross-sectional structure diagram of the negative electrode sheet, the negative electrode sheet includes a negative electrode current collector 100 and a negative electrode active coating 110 covering one side thereof, and a conductive bottom coating 120 is provided therebetween. It should be noted that although the figure only shows the cross-sectional structure of the single-sided coating of the negative electrode current collector, this is a typical simplified expression of the schematic diagram; in actual applications, based on the battery design requirements, the negative electrode active coating 110 and the conductive bottom coating 120 can exist on both sides of the negative electrode current collector 100 at the same time (i.e., a double-sided coating structure is formed), or only a single-sided coating is retained according to specific performance requirements; the setting of the conductive bottom coating 120 is not limited to the single side shown in the figure, and its double-sided coating is also applicable to the present invention; in addition, Figure 1 The display of the single-sided coating is only used to clarify the relative position relationship of each layer of material, and does not constitute a limitation on the coating direction of the negative electrode sheet.

[0020] In traditional negative electrode sheets, when the negative electrode current collector is in direct contact with the negative electrode active coating, the utilization rate of the bottom active material near the negative electrode current collector is low due to the too long electron transmission path during the charge and discharge process. In addition, the potential change will cause lithium deposition on the negative electrode surface in the later stage of the cycle; in the present invention, by introducing a conductive bottom coating as an intermediate layer on the basis of the negative electrode current collector and the negative electrode active coating, the electron conduction of the active material near the negative electrode current collector side (i.e., the bottom layer) can be enhanced, and the problem that the bottom active material of the electrode sheet cannot be effectively utilized can be fully solved, and lithium deposition on the negative electrode surface in the later stage of the cycle can be improved; moreover, by further controlling the thickness relationship, the thickness H1 of the conductive bottom coating and the thickness H0 of the negative electrode current collector satisfy 0.1H0 ≤ H1 ≤ 0.9H0, which can avoid the decrease in the loading amount of the active material due to the too thick conductive bottom coating, and at the same time ensure the coating amount of the conductive bottom coating, enhance the adhesion between the negative electrode active coating and the negative electrode current collector, and thus play a role in buffering the expansion stress of the spherical silicon carbon material;

[0021] However, due to the difference in elastic modulus between the conductive coating and the negative active coating, internal stress will be generated in the silicon-doped negative electrode sheet during the charge-discharge volume expansion process, which may lead to problems such as cracking of the conductive coating or delamination at the interface between the conductive coating and the negative active coating, thus the expansion problem of the negative electrode sheet in the later stage of cycling has not been effectively solved. Based on this, the present invention further controls the relationship between the particle size Dv50 of the spherical silicon-carbon material in the battery after the first charge-discharge and the thickness of the electrode sheet, satisfying 0.20 ≤ D / (H0 + H1 + H2) ≤ 0.28, where H0 is the thickness of the negative current collector, H1 is the thickness of the conductive bottom coating, and H2 is the thickness of the negative active coating. By reasonably reducing the particle size of the spherical silicon-carbon material, the diffusion path of lithium ions can be shortened, and at the same time, the internal stress generated in the negative active coating during cyclic expansion can be reduced, which will not affect the electron conduction of the active material on the negative current collector side (i.e., the bottom layer), and can also effectively alleviate the problems of coating cracking or interface delamination in the later stage of cycling. However, if the thickness of the negative electrode sheet is too thin at this time, the expansion space of the negative active coating will be compressed, which will instead lead to increased expansion of the negative electrode sheet. If the thickness of the negative electrode sheet is too thick, the coating structure stability of the negative electrode sheet will decrease due to the too small particle size of the spherical silicon-carbon material, resulting in capacity loss of the negative active material. Therefore, by controlling the ratio of the particle size Dv50 of the spherical silicon-carbon material to the thickness of the electrode sheet within a certain range, the present invention can solve the expansion problem in the later stage of cycling caused by the setting of the conductive bottom coating, ensure the effective utilization of the bottom layer active material, improve the lithium deposition problem on the negative electrode surface in the later stage of cycling, thereby further improving the cycling performance of the battery, improving the cycling life and cyclic expansion of the battery, and prolonging the service life of the battery.

[0022] In summary, the negative electrode sheet of the present invention can enhance the electron conduction of the active material near the negative current collector side (i.e., the bottom layer), fully solve the problem that the active material at the bottom layer of the electrode sheet cannot be effectively utilized, and improve the lithium deposition on the negative electrode surface in the later stage of cycling. Moreover, it can avoid the influence of the conductive bottom coating arranged at the bottom layer of the negative active coating on the expansion of the silicon negative electrode, buffer the expansion stress of the spherical silicon-carbon material, effectively solve the expansion problem of the silicon-containing negative electrode sheet, and thus improve the cycling life and cyclic expansion of the battery.

[0023] The thickness H1 of the conductive bottom coating is the thickness of the conductive bottom coating on either side of the negative electrode current collector. If the conductive bottom coating is provided only on one side of the negative electrode current collector, then H1 is equal to the thickness of the single-sided conductive bottom coating. If the conductive bottom coating is provided on both sides of the negative electrode current collector simultaneously, then H1 is equal to the average value of the thicknesses of the conductive bottom coatings on both sides. Similarly for the thickness H2 of the negative electrode active coating. Therefore, H0 + H1 + H2 can actually be understood as the total thickness of the negative electrode current collector, the conductive bottom coating, and the negative electrode active coating on one side of the negative electrode current collector, which can be represented by H, and H = H0 + H1 + H2. In addition, the test methods for the thickness H0 of the negative electrode current collector, the thickness H1 of the conductive bottom coating, and the thickness H2 of the negative electrode active coating are as follows: After the battery of the present invention undergoes the first charge and discharge, it is then fully charged, and then the battery is disassembled in a closed environment with an ambient humidity less than 2%. A cross-section of the negative electrode sheet along the width direction of the negative electrode current collector is obtained, and the cross-section SEM of this cross-section is tested in a vacuum chamber. By analyzing the cross-section SEM image obtained through software, the thickness parameters of the negative electrode sheet of the present invention are determined, including the thickness H0 of the negative electrode current collector, the thickness H1 of the conductive bottom coating, and the thickness H2 of the negative electrode active coating. Additionally, the particle size Dv50 of the spherical silicon-carbon material can also be obtained by observing and analyzing the cross-section SEM image of the above-mentioned negative electrode sheet. Furthermore, it should be noted that after the battery undergoes the first charge and discharge and is then fully charged, it has almost no effect on the changes in the thickness H0 of the negative electrode current collector and the thickness H1 of the conductive bottom coating. Therefore, the thickness H0 of the negative electrode current collector set before the first charge and the thickness H1 of the conductive bottom coating applied can be considered to be the same as the thicknesses after the first charge.

[0024] The specific operation steps for the first charge and discharge of the battery of the present invention are as follows: Use a Blue-Energy test device to perform the first charge and discharge on the battery. Specifically, charge the battery to 4.53V at a constant current of 2C and a constant voltage, with the current cutoff at 0.05C; then discharge at 1C to 3.0V to complete the first charge and discharge (1 complete charge and discharge cycle); then charge at 1C with a constant current and a constant voltage to 4.53V, and the battery can be subjected to subsequent tests after being fully charged with the current cutoff at 0.05C. In addition, the fully charged state of the battery can be understood as 100% SOC, indicating that the state of charge (State of Charge, abbreviated as SOC) of the battery has reached 100%, that is, the battery has reached the fully charged state.

[0025] In one example, 0.2H0 ≤ H1 ≤ 0.6H0.

[0026] In one example, after 50T cycles, in the battery in the fully charged state, H0, H1, H2, and D satisfy 0.1 ≤ D / (H0 + H1 + H2) ≤ 0.2, such as 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or 0.2.

[0027] In one example, after 200 cycles of 200T, in the battery in a fully charged state, H0, H1, H2, and D satisfy 0.05 ≤ D / (H0 + H1 + H2) ≤ 0.18, such as 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, or 0.18.

[0028] After the battery has been cycled 50T and 200T, the battery in a fully charged state refers to the battery that has been charged and discharged 50 times and 200 times in accordance with the specific charging and discharging operation steps of the battery of the present invention above, and then the battery is charged at a constant current and constant voltage of 1C to 4.53V, with the current cut off at 0.05C to reach the fully charged state.

[0029] As the number of cycle periods increases, the degree of volume expansion of the silicon-containing negative electrode sheet will increase accordingly, and the problems of coating cracking or interface delamination caused by the difference in elastic modulus between the conductive coating and the negative electrode active coating will become more severe. Therefore, on the basis of controlling the relationship between the particle size Dv50 of the spherical silicon carbide material and the electrode sheet thickness after the first charge and discharge of the battery, further controlling the relationship between the particle size Dv50 of the spherical silicon carbide material and the electrode sheet thickness after the battery has been cycled 50T and 200T can, corresponding to different cycle periods, more reasonably adjust the appropriate particle size of the spherical silicon carbide material, thereby further shortening the diffusion path of lithium ions, reducing the internal stress generated in the negative electrode active coating during cyclic expansion, and adjusting an appropriate expansion space for the negative electrode active material, improving the coating structure stability of the negative electrode sheet, thereby further solving the expansion problem in the later stage of cycling caused by the setting of the conductive bottom coating, improving the expansion problem of the negative electrode in the later stage of cycling, enhancing the cycle life and cyclic expansion of the battery, and extending the service life of the battery.

[0030] In one example, 4μm ≤ H0 ≤ 10μm, such as 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, or 10μm.

[0031] In one example, 0.4μm ≤ H1 ≤ 9μm, such as 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, or 9μm.

[0032] In one example, 25μm ≤ H2 ≤ 45μm, such as 25μm, 26μm, 27μm, 28μm, 29μm, 30μm, 31μm, 32μm, 33μm, 34μm, 35μm, 36μm, 37μm, 38μm, 39μm, 40μm, 41μm, 42μm, 43μm, 44μm, 45μm.

[0033] In one example, the conductive bottom coating comprises a conductive agent, an adhesive, a leveling agent, and a thickening agent.

[0034] In one example, the conductive agent includes at least one of conductive carbon black, single-walled conductive carbon nanotubes, multi-walled conductive carbon nanotubes, and graphene.

[0035] In one example, the adhesive includes at least one of styrene-butadiene rubber, PAA, polyacrylic acid, and PVDF;

[0036] In one example, the leveling agent includes thiourea.

[0037] In one example, the thickening agent includes at least one of CMC and CMC-Li.

[0038] In one example, the mass ratio between the conductive agent, the adhesive, the leveling agent, and the thickening agent is (10% - 25%):(40% - 70%):(0.2% - 0.5%):(5% - 15%).

[0039] In one example, the mass content of the conductive agent is 10% - 25%, such as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, or 25%.

[0040] In one example, the mass content of the adhesive is 40% - 70%, such as 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, or 70%.

[0041] In one example, the mass content of the leveling agent is 0.2% - 0.5%, such as 0.2%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.3%, 0.31%, 0.32%, 0.33%, 0.34%, 0.35%, 0.36%, 0.37%, 0.38%, 0.39%, 0.4%, 0.41%, 0.42%, 0.43%, 0.44%, 0.45%, 0.46%, 0.47%, 0.48%, 0.49%, or 0.5%.

[0042] In one example, the mass content of the thickening agent is 5% - 15%, such as 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%.

[0043] In the conductive bottom coating, the content of the conductive agent is relatively high, accounting for 10%-25% by mass. Its main function is to improve the conductivity between the negative electrode current collector and the negative electrode active coating. At the same time, the conductive agent has a lubricating effect, which can prevent the particles of the negative electrode active material at the bottom layer from squeezing the negative electrode current collector during production, resulting in damage to the current collector; the content of the binder is relatively high, accounting for 40%-70% by mass. Its main function is to increase the adhesion between the negative electrode active material and the negative electrode current collector, that is, it can be used to improve the peel strength between the negative electrode active coating and the negative electrode current collector, reduce the demolding risk, and at the same time can also buffer the expansion stress of the spherical silicon carbon material, thereby alleviating the cyclic expansion problem of the battery; the leveling agent in the conductive bottom coating is used to prevent the conductive bottom coating from agglomerating or missing coating during coating, making the conductive bottom coating smoother; the thickener in the conductive coating functions to increase the adhesion of the conductive bottom coating in part and increase the viscosity of the slurry in part, making the coating more smooth.

[0044] In one example, the peel strength between the conductive bottom coating and the negative electrode current collector is 10 gf - 55 gf, such as 10 gf, 15 gf, 20 gf, 25 gf, 30 gf, 35 gf, 40 gf, 45 gf, 50 gf or 55 gf. The test method for the peel strength between the conductive bottom coating and the negative electrode current collector is as follows: discharge the available battery to 3.0 V, disassemble the battery, then cut the disassembled negative electrode sheet into a test sample electrode sheet with a width of 3 cm and a length of 8 cm after removing the negative electrode active coating, and use a QJ210A peel strength test device to test the peel strength between the conductive bottom coating and the negative electrode current collector of the test sample electrode sheet. By further controlling the peel strength between the conductive bottom coating and the negative electrode current collector, the expansion stress generated during the charge and discharge process of the battery can be more effectively dispersed, further improving the cyclic expansion performance of the battery; if the peel strength is too large, this stress will concentrate at the interface between the conductive bottom coating and the negative electrode current collector, easily causing the coating to crack or delaminate from the negative electrode current collector.

[0045] In one example, the tensile strength of the negative electrode current collector is A, and the thickness H1 of the conductive bottom coating and the tensile strength A of the negative electrode current collector satisfy 0.002A ≤ H1 ≤ 0.007A. For example, H1 can be 0.002A, 0.003A, 0.004A, 0.005A, 0.006A, or 0.007A, where A is 400 MPa - 950 MPa. For example, A can be 400 MPa, 450 MPa, 500 MPa, 550 MPa, 600 MPa, 650 MPa, 700 MPa, 750 MPa, 800 MPa, 850 MPa, 900 MPa, or 950 MPa. The test method for the tensile strength A of the negative electrode current collector is as follows: Discharge the available battery to 3.0V, then remove the negative electrode active coating and the conductive bottom coating from the disassembled negative electrode sheet. Cut the foil serving as the negative electrode current collector into test samples with a length of 20 cm and a width of 5 cm. Test the tensile strength of the test sample foil according to ASTM standard B152M-19, with the unit of MPa, denoted as A. By controlling A and H1 to satisfy the above relationship, the anti-extension rate of the negative electrode current collector can be further improved, preventing brittle fracture of the negative electrode current collector after the conductive bottom coating is set, reducing the risk of electrode sheet fracture, and enhancing the safety performance of the negative electrode sheet and the battery. This is because when H1 is greater than 0.007A, the thickness of the conductive bottom coating is too thick, and large internal stresses will be generated during the coating and drying processes due to factors such as solvent evaporation and material shrinkage. If the internal stress cannot be effectively released, it will cause cracking of the conductive bottom coating, or if the tensile strength A of the negative electrode current collector is too low, it will increase the risk of electrode sheet fracture. When H1 is less than 0.002A, the thickness of the conductive bottom coating is too small, which will hinder the electron conduction of the underlying active material, increasing the risk of lithium deposition on the negative electrode. Or if the tensile strength A of the negative electrode current collector is too high, it will lead to a decrease in the anti-extension rate of the negative electrode current collector, thereby reducing the buffer performance of the negative electrode current collector for the expansion stress of the negative electrode active material.

[0046] In one example, the elongation of the negative electrode sheet is 5% - 8%. For example, it can be 5%, 6%, 7%, or 8%. The test method for the elongation of the negative electrode sheet is as follows: Discharge the available battery to 3.0V, then directly cut the disassembled negative electrode sheet into test samples with a length of 20 cm and a width of 5 cm. Test the elongation (%) of the test sample according to ASTM standard B152M-19. By testing the elongation of the negative electrode sheet, the electrode sheet extension caused by the expansion of the negative electrode sheet can be improved, avoiding problems such as the detachment of the negative electrode active material from the negative electrode current collector and even the fracture of the current collector.

[0047] In one example, the porosity of the negative electrode sheet is P, and the thickness H of the negative electrode sheet and the porosity P of the negative electrode sheet satisfy 1.7 ≤ H / P ≤ 2.1, for example 1.7, 1.8, 1.9, 2 or 2.1, where P is 23% - 48%, for example 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47% or 48%. The test method for the porosity P of the negative electrode sheet is as follows: Discharge the available battery to 3.0V, then take the cross-section of the disassembled negative electrode sheet along the thickness direction of the electrode sheet to prepare a scanning electron microscope test sample, take a cross-sectional electron microscope image, take an area of a specific length and width, such as an area of 100μm * 100μm, use Image J software for color rendering to distinguish the pores between the negative active materials, and then the area S of the specific length and width area taken from the negative electrode sheet can be obtained through software analysis 总 and the total area S of the regions where the pores are located 孔 ; Finally, the porosity can be calculated using the formula, porosity P(%) = S 孔 / S 总 ; The porosity P of the negative electrode sheet has the following characteristics. The unit of the porosity P is a percentage, and its relationship with the total thickness H (H = H0 + H1 + H2) of the negative electrode current collector, the conductive bottom coating, and the negative electrode active coating on one side of the negative electrode current collector satisfies 1.7 ≤ H / P ≤ 2.1, and it satisfies that as the thickness H of the negative electrode sheet increases, the porosity P of the negative electrode sheet also increases; this can not only improve the kinetic performance of the negative electrode sheet, but also effectively inhibit the swelling of the negative electrode sheet, thereby further improving the cycle life of the battery; this is because when H / P is too large, the thickness of the electrode sheet is too large, or the porosity P of the negative electrode sheet is too small, which is not conducive to the transport rate of lithium ions in the coating and the electrolyte, resulting in a decrease in the kinetic performance of the negative electrode sheet and an impact on the cycle life; when H / P is too small, the thickness of the electrode sheet is too small, which will compress the swelling space of the negative electrode active material, or the porosity P of the negative electrode sheet is too large, which will lead to an increase in the side reactions of the negative electrode active material, and the solid by-products and gases generated by the side reactions will further compress the swelling space of the negative electrode active material, thus affecting the cycle life and swelling performance of the battery

[0048] In one example, the particle size Dv10 of the spherical silicon-carbon material is 4 μm - 6 μm, such as 4 μm, 5 μm or 6 μm; the particle size Dv50 of the spherical silicon-carbon material is 6 μm - 15 μm, such as 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm; the particle size Dv90 of the spherical silicon-carbon material is 13 μm - 20 μm, such as 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm. The test method for the particle size Dv10 and Dv90 of the spherical silicon-carbon material is the same as that for the particle size Dv50 of the spherical silicon-carbon material, which will not be elaborated here. By controlling the particle size of the spherical silicon-carbon material under different volume distributions, the diffusion path of lithium ions can be further shortened, the electron conduction of the active material on the negative electrode current collector side (i.e., the bottom layer) can be improved, thereby improving the problem of lithium deposition on the surface of the negative electrode sheet. At the same time, the internal stress generated during the cyclic expansion of the negative electrode active coating can be reduced, effectively slowing down the coating cracking or interface delamination problems in the later stage of cycling, and improving the cycle life and expansion of the battery.

[0049] In one example, in the cross-section of the negative electrode active coating along the width direction of the negative electrode current collector on one side, within the area obtained by multiplying any 100 μm along the width direction of the negative electrode current collector by the thickness of the negative electrode active coating on one side, the number of the spherical silicon-carbon materials is 3 - 10, such as 3, 4, 5, 6, 7, 8, 9 or 10; among them, the number of the spherical silicon-carbon materials with the particle size Dv10 of 4 μm - 6 μm is 3 - 6, such as 3, 4, 5 or 6; the number of the spherical silicon-carbon materials with the particle size Dv50 of 6 μm - 15 μm is 2 - 4, such as 2, 3 or 4; the number of the spherical silicon-carbon materials with the particle size Dv90 of 13 μm - 20 μm is 1 - 3, such as 1, 2 or 3.

[0050] The cross-section of the negative electrode active coating along the width direction of the negative electrode current collector can be located on either side of the negative electrode current collector; the position of the cross-section of the negative electrode active coating along the width direction of the negative electrode current collector is arbitrary, as long as it can completely present the cross-section of the negative electrode active coating along the width direction; further, in the present invention, a detection area for the number of spherical silicon-carbon materials is demarcated in this cross-section, and the area calculation formula of this area = any 100 μm along the width direction of the negative electrode current collector × the thickness of the single-sided negative electrode active coating, where the 100 μm along the width direction of the negative electrode current collector can be randomly selected. In actual detection, generally 3-5 segments of 100 μm along the width direction of the negative electrode current collector are selected, and then the number of spherical silicon-carbon material particles in these obtained areas is counted and averaged to obtain the number of spherical silicon-carbon material particles in this area, where the method for observing the number of spherical silicon-carbon material particles is SEM. The method for observing the number of spherical silicon-carbon material particles with different particle size dimensions is to continue to analyze the number of spherical silicon-carbon material particles with different particle size dimensions through software after counting the total number of particles. The selection of the detection area is the same as the above method and will not be elaborated here.

[0051] By controlling the number of spherical silicon-carbon materials and the number of spherical silicon-carbon materials with different sizes, the present invention can make the spherical silicon-carbon materials more evenly distributed in the thickness direction of the negative electrode sheet, which is more conducive to dispersing the expansion of spherical silicon-carbon material particles in the vertical and horizontal directions of the negative electrode sheet, thereby being conducive to improving the overall expansion problem of the battery; at the same time, it can also improve the problem of demoulding of the negative electrode active coating of the negative electrode sheet.

[0052] In one example, the mass content of the spherical silicon-carbon material in the negative electrode active coating is 5%-35%, such as 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34% or 35%.

[0053] In one example, the oil absorption value of the spherical silicon-carbon material is 10 mL / 100 g - 30 mL / 100 g, such as 10 mL / 100 g, 11 mL / 100 g, 12 mL / 100 g, 13 mL / 100 g, 14 mL / 100 g, 15 mL / 100 g, 16 mL / 100 g, 17 mL / 100 g, 18 mL / 100 g, 19 mL / 100 g, 20 mL / 100 g, 21 mL / 100 g, 22 mL / 100 g, 23 mL / 100 g, 24 mL / 100 g, 25 mL / 100 g, 26 mL / 100 g, 27 mL / 100 g, 28 mL / 100 g, 29 mL / 100 g or 30 mL / 100 g; the specific surface area of the spherical silicon-carbon material is 100 m 2 / g - 150 m 2 / g, such as 100 m 2 / g, 105 m 2 / g, 110 m 2 / g, 115 m 2 / g, 120 m 2 / g, 125 m 2 / g, 130 m 2 / g, 135 m 2 / g, 140 m 2 / g, 145 m 2 / g or 150 m 2 / g. By controlling the oil absorption value of the spherical silicon-carbon material within this range and simultaneously controlling its specific surface area within this range, the present invention can effectively improve the absorption of the spherical silicon-carbon material to the electrolyte, ensure sufficient wetting on the surface of the spherical silicon-carbon material particles, reduce the solid-liquid transfer impedance on the surface of the spherical silicon-carbon material, and improve the kinetic performance of the battery. Moreover, after improving the wettability of the surface of the spherical silicon-carbon material particles, the diffusion rate of lithium ions in the electrolyte will also increase, reducing side reactions caused by excessive or too low local ion concentration, thereby improving the cycle performance of the battery; in addition, sufficient wetting helps the expansion stress received by the spherical silicon-carbon material particles in all directions to be more uniform, thus improving the expansion performance.

[0054] In one example, after 50T cycles, in the battery in a fully charged state, in a cross-section of the negative electrode active coating along the width direction of the negative electrode current collector on one side thereof, within an area obtained by multiplying any 100 μm along the width direction of the negative electrode current collector by the thickness of the negative electrode active coating on one side, the number of spherical silicon-carbon materials with broken particles accounts for 10%-30% of the total number of spherical silicon-carbon materials in this area, such as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29% or 30%. The broken area of a single spherical silicon-carbon material with a broken particle accounts for 5%-25%, such as 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24% or 25%. The spherical silicon-carbon materials with broken particles refer to spherical silicon-carbon materials with incomplete particles or cracks on the particle surface. The broken area refers to the sum of the area covered by the incomplete part of the particle and the area of the cracks on the particle surface. The method for testing the percentage of the number of spherical silicon-carbon materials with broken particles in the total number of spherical silicon-carbon materials in this area and the broken area ratio of a single spherical silicon-carbon material with a broken particle can refer to the method for observing the number of particles of spherical silicon-carbon materials. First, obtain a cross-sectional electron micrograph of the negative electrode active material layer, and then for a specific area to be tested (the area obtained by multiplying any 100 μm along the width direction of the negative electrode current collector by the thickness of the negative electrode active coating on one side), use Image J software for color rendering, so that the spherical silicon-carbon materials with broken particles and the spherical silicon-carbon materials without broken particles can be distinguished, and then the percentage (%) of the number of spherical silicon-carbon materials with broken particles in the total number of spherical silicon-carbon materials in this area can be counted; in addition, by using Image J software for color rendering, the actually broken area and the unbroken area on the spherical silicon-carbon materials with broken particles can also be distinguished, and then the broken area of the spherical silicon-carbon materials with broken particles can be obtained through software analysis, and then the broken area ratio of a single spherical silicon-carbon material with a broken particle can be calculated = the broken area of a single spherical silicon-carbon material with a broken particle / the cross-sectional area of a single spherical silicon-carbon material × 100%. It should be noted that since the observed electron micrograph is a cross-sectional electron micrograph, the counted broken area is only half of the broken area of a single spherical silicon-carbon material, which is used to represent the broken area ratio of the entire spherical silicon-carbon material with a broken particle. By controlling the number of spherical silicon-carbon materials with broken particles and the broken area of a single broken spherical silicon-carbon material, the present invention can avoid generating more new interfaces when there are too many spherical silicon-carbon material particles with broken particles, increasing the consumption of the electrolyte and causing the formation of a SEI film with a greater impedance, deteriorating the cycle life of the battery.

[0055] In one example, the electrolyte contains fluoroethylene carbonate (FEC). Based on the total mass of the electrolyte, the mass content of fluoroethylene carbonate is 7%-20%, for example, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%. Adding a certain amount of FEC to the electrolyte helps to form a SEI film on the surface of the negative electrode and can also improve the stability of the SEI film on the negative electrode, thereby enhancing the cycling performance of the battery and extending the cycle life.

[0056] In one example, the electrolyte contains vinylene carbonate (VC). Based on the total mass of the electrolyte, the mass content B of vinylene carbonate is 0.1%-5%, for example, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%. The electrolyte contains VC, and the mass content range of VC is B, where B satisfies: 0.1%-5%. This is beneficial to improving the interfacial stability between the positive electrode active material and the electrolyte, helps to reduce the decomposition of the electrolyte, forms a more stable SEI film, and reduces the dissolution of transition metals in the positive electrode material, thereby further enhancing the cycling performance of the battery and extending the cycle life. Further, when the content B of VC is too low, it is not conducive to film formation on the positive electrode and cannot effectively inhibit the dissolution of transition metals in the positive electrode active material, which will damage the SEI film on the negative electrode and reduce the cycling stability of the battery; while when the content B of VC is too high, it will cause an excessive reaction of the negative electrode active material, resulting in too fast decomposition of the electrolyte. The decomposition will release gas, increasing the internal pressure of the battery. If these gases cannot be released in time, it may cause the battery to expand or undergo thermal runaway, and may even cause fire or explosion in severe cases, affecting the safety performance of the battery. Therefore, by controlling the mass content B of vinylene carbonate within a suitable range, the cycling performance of the battery can be enhanced while avoiding safety problems.

[0057] In one example, the sphericity of the spherical silicon-carbon material is S, and the mass content of vinylene carbonate in the electrolyte is B. The S and B satisfy 0.2 ≤ S / B ≤ 8, where S is 0.6 - 1, for example, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95 or 1. When the sphericity of the spherical silicon-carbon material and the content B of VC are simultaneously adjusted to satisfy 0.2 ≤ S / B ≤ 8, it is beneficial to alleviate the swelling of the negative electrode sheet and improve the cycle performance of the battery. This is because the more perfect the spherical structure of the spherical silicon-carbon material is, when subjected to internal stress, the stress distribution will be more uniform, so that the negative electrode swelling can be better alleviated. At the same time, using the property that VC can form a film on the positive electrode, the harm of the transition metal dissolution of the positive electrode material to the negative electrode SEI film can be reduced, thereby improving the stability of the negative electrode SEI film, and further maintaining the structural integrity of the silicon-containing negative electrode; generally speaking, due to the improved swelling uniformity of the spherical silicon-carbon material, the frequency of rupture and repair of the negative electrode SEI film is reduced, and the harm of transition metal dissolution to the negative electrode SEI film is reduced, which is beneficial to improving the swelling and cycle of the negative electrode sheet at the same time; and the addition of VC also improves the film-forming stability of the positive electrode material, thereby improving the cycle performance and cycle life of the battery as a whole.

[0058] In one example, the sphericity S of the spherical silicon-carbon material is 0.7 - 0.95.

[0059] In one example, the positive electrode sheet includes a positive electrode active coating, the positive electrode active coating contains a positive electrode active material, and the positive electrode active material includes a substance with the chemical formula Li a Ni x Co y Mn z M k O2, 0.9 ≤ a ≤ 1.1 (0.9, 0.95, 1, 1.05, 1.1 or 1.11), 0.6 ≤ x ≤ 0.95 (0.6, 0.65, 0.7, 0.71, 0.8, 0.85, 0.9 or 0.95), 0 < y ≤ 0.2 (0.01, 0.05, 0.1, 0.15 or 0.2), 0 < z ≤ 0.2 (0.01, 0.05, 0.1, 0.15 or 0.2), 0 ≤ k ≤ 0.05 (0, 0.01, 0.02, 0.03, 0.04 or 0.05), and M is selected from at least one of Al, Zr, Mg, Y, Ti, B, Sr, W, Si, La and Nb. In the positive electrode active material of the present invention, the doping element M can be included, and the stability of the positive electrode active material can be further improved by doping the M element, and the cycle performance of the battery can be improved.

[0060] In one example, the total content of Al element, Mg element and Zr element in the positive electrode active coating ranges from 1000 ppm to 7000 ppm, such as 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm or 7000 ppm. Further, the doping of Al, Mg, and Zr elements is beneficial to improving the ionic conductivity, reducing the migration resistance of the positive electrode active material, avoiding the generation of lithium plating on the surface of the electrode sheet, and improving the cycle life of the lithium-ion battery. However, the total content of the three elements of Al element, Mg element and Zr element should not be too high, as too high will cause harm to the negative electrode SEI film and affect the cycle life of the negative electrode active material. The mass content of Al element, Mg element and Zr element in the positive electrode active coating can be measured by ICP method (inductively coupled plasma method). The specific method is: discharge the battery of the present invention to 3.0 V and disassemble it, and perform ICP element content detection on the disassembled positive electrode active coating to obtain the content of each element in the positive electrode active coating.

[0061] In one example, the chemical formula is Li a Ni x Co y Mn z M k The substance of O2 includes single crystal materials and polycrystalline materials. The mass content ratio of the single crystal material is 0% - 100%, such as 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100%. The higher the proportion of the single crystal material, the more it can improve the structural stability of the positive electrode material on the positive electrode side and further improve the cycle life of the battery.

[0062] In one example, the mass content ratio of the single crystal material is 50% - 80%.

[0063] In one example, the mass content ratio of Ni element included in the single crystal material is 85% - 99%, such as 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%. By controlling the mass content ratio of Ni element included in the single crystal material to be 85% - 99%, it is beneficial to improve the energy density of the positive electrode active material and improve the cycle life of the battery cell.

[0064] In one example, after 10T cycles, it is discharged to 3.0V and disassembled. The content of Ni element in the negative active coating is 2%-7%, such as 2%, 3%, 4%, 5%, 6% or 7%. The content of Mn element in the negative active coating is 0.5%-5%, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%. The mass contents of Ni element and Mn element in the negative active coating can also be measured by ICP method (Inductively Coupled Plasma method). The specific method is as follows: After the battery completes 50 full charge-discharge cycles according to the specific operation steps of the first charge-discharge of the battery of the present invention, it is discharged to 3.0V and disassembled. The ICP element content detection is carried out on the disassembled negative active coating, and the content of each element in the negative active coating can be obtained. The Ni element and Mn element in the negative active coating come from the transition metal elements that are slightly decomposed from the positive active material during the charge-discharge process and migrate to the negative electrode to participate in the formation process of SEI on the surface of the negative active material. By controlling the element contents of Ni element and Mn element in the negative active coating, the stability of the negative SEI can be further improved, the damage of SEI during the cycle can be reduced, and the generation of new SEI can be avoided, and the consumption of electrolyte by the negative film formation can be reduced, thereby further improving the cycle life of the lithium battery.

[0065] The present invention will be described in detail below through examples. The examples described in the present invention are only a part of the examples of the present invention, rather than all the examples. All other examples obtained by those of ordinary skill in the art based on the examples in the present invention without creative efforts belong to the scope of protection of the present invention.

[0066] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0067] The following examples are used to illustrate the present invention.

[0068] Example 1

[0069] Prepared according to the following method:

[0070] (1) Prepare the positive electrode sheet

[0071] The ternary material powder (chemical formula is LiNi 0.9 Co 0.01 Mn 0.1 M 0.02O2, the mass content ratio of the single crystal material in the ternary material is 60%, and the mass content ratio of the Ni element included in the single crystal material is 90%). Polyvinylidene fluoride, acetylene black, and carbon nanotubes are put into a vacuum mixer according to a mass ratio of 96:2:1.5:0.5, and N-methylpyrrolidone (NMP) is added. Under the action of the vacuum mixer, they are fully mixed until a uniform and well-flowing positive electrode active coating slurry is formed. The above positive electrode active coating slurry is evenly coated on aluminum foil, dried, rolled, slit, and punched to obtain the positive electrode sheet of Example 1.

[0072] (2) Prepare the negative electrode sheet

[0073] Prepare the negative electrode active coating slurry

[0074] Put artificial graphite, spherical silicon carbide material (particle size Dv50, D = 10μm, Dv10 = 5μm, Dv90 = 15μm; the oil absorption value of the spherical silicon carbide material is 15 mL / 100g; the specific surface area of the spherical silicon carbide material is 122m 2 / g; the sphericity S of the spherical silicon carbide material is 0.9), styrene-butadiene rubber (SBR), lithiated polyacrylic acid (PAA), and acetylene black into a vacuum mixer according to a mass ratio of 86.5:10:1:1:1.5, add deionized water, and under the action of the vacuum mixer, they are fully mixed to finally form a uniform and well-flowing negative electrode active coating slurry;

[0075] Prepare the conductive bottom coating slurry

[0076] Put conductive carbon black, styrene-butadiene rubber (SBR), thiourea, and CMC-Li into a vacuum mixer according to a mass ratio of 25:64.7:0.3:10, and under the action of the vacuum mixer, they are fully mixed to finally form a uniform and well-flowing conductive bottom coating slurry;

[0077] The above conductive bottom coating slurry is evenly coated on a copper foil with a thickness of 8μm (H0 = 8μm). Among them, both sides of the copper foil are coated with the conductive bottom coating slurry, and the coating thickness of the conductive bottom coating on one side is 2.4μm (H1 = 2.4μm). Then, it is dried. After drying, the above negative electrode active coating slurry is continuously and evenly coated on the surface of the conductive bottom coating on both sides, and the coating thickness of the negative electrode active coating on one side is 35μm (H2 = 35μm). Then, it is dried, rolled, and die-cut to obtain the negative electrode sheet of Example 1;

[0078] Among them, the peel force between the conductive bottom coating and the negative electrode current collector is measured to be 30 gf, the tensile strength A of the negative electrode current collector is 500 MPa, and the porosity P of the negative electrode sheet is 23%;

[0079] The measured particle size Dv10 of the spherical silicon-carbon material is 5 μm, and the particle size Dv90 of the spherical silicon-carbon material is 15 μm;

[0080] In the cross-section of the negative electrode active coating on one side along the width direction of the negative electrode current collector, in any area obtained by multiplying 100 μm along the width direction of the negative electrode current collector by the thickness of the single-sided negative electrode active coating, the number of spherical silicon-carbon materials is 5. Among them, the number of spherical silicon-carbon materials with a particle size Dv10 of 4 μm - 6 μm is 2, the number of spherical silicon-carbon materials with a particle size Dv50 of 6 μm - 15 μm is 2, and the number of spherical silicon-carbon materials with a particle size Dv90 of 13 μm - 20 μm is 1; the number of spherical silicon-carbon materials with broken particles accounts for 20% of the total number of spherical silicon-carbon materials in this area, and the proportion of the broken area of a single broken spherical silicon-carbon material is 8.

[0081] (3) Preparation of electrolyte

[0082] In an argon glove box with a water content < 0.1 ppm and an oxygen content < 0.1 ppm, fluoroethylene carbonate (FEC), PC, PP, and EP are mixed evenly according to a mass ratio of 15:25:30:10. Sufficiently dried lithium hexafluorophosphate (LiPF6) is added thereto and stirred for dissolution. The added mass of LiPF6 is 16.5% of the total mass of the electrolyte. Vinylene carbonate (VC) based on 3.5% of the total mass of the electrolyte is added and stirred evenly. After passing the physical property test, the electrolyte of Example 1 is obtained.

[0083] (4) Preparation of battery

[0084] The positive electrode sheet obtained in step (1), the negative electrode sheet obtained in step (2), and the separator (PP) are wound to obtain a bare battery cell; the electrolyte prepared in step (3) is injected into the dried and qualified battery cell, and after processes such as standing, aging, formation, degassing, aging, and sorting, the battery is obtained;

[0085] Among them, the total content of Al element, Mg element, and Zr element in the positive electrode active coating of the obtained battery is measured to be 6100 ppm; after the obtained battery is cycled 10T and discharged to 3.0V and disassembled, the content of Ni element in the negative electrode active coating is 5%, and the content of Mn element in the negative electrode active coating is 2%.

[0086] Example 2 group

[0087] In this group of examples, the preparation method of the battery refers to Example 1. The only difference is that the thickness H1 of the conductive bottom coating is changed, as specifically shown in Table 1-3.

[0088] Example 3 group

[0089] The preparation method of the battery in this group of examples refers to Example 1. The only difference is that the thickness H2 of the negative electrode active coating is changed. For details, see Table 1-3.

[0090] Example 4 group

[0091] The preparation method of the battery in this group of examples refers to Example 1. The only difference is that the particle size Dv50 D of the spherical silicon-carbon material is changed. Specifically:

[0092] Example 4-1, particle size Dv50, D = 6 μm, Dv10 = 4 μm, Dv90 = 13 μm;

[0093] Example 4-2, particle size Dv50, D = 15 μm, Dv10 = 6 μm, Dv90 = 20 μm;

[0094] For details, see Table 1-3.

[0095] Comparative Example 1 group

[0096] The preparation method of the battery in this group of comparative examples refers to Example 1. The only difference is that the thickness H1 of the conductive bottom coating is changed. For details, see Table 1-3.

[0097] Comparative Example 2 group

[0098] The preparation method of the battery in this group of comparative examples refers to Example 1. The only difference is that the particle size Dv50 D of the spherical silicon-carbon material is changed. Specifically:

[0099] Comparative Example 2-1, particle size Dv50, D = 5 μm, Dv10 = 3 μm, Dv90 = 12 μm;

[0100] Comparative Example 2-2, particle size Dv50, D = 17 μm, Dv10 = 7 μm, Dv90 = 22 μm;

[0101] For details, see Table 1-3.

[0102] Comparative Example 3

[0103] The preparation method of the battery in this group of comparative examples refers to Example 1. The only difference is that during the preparation of the negative electrode sheet, a conductive bottom coating is not provided between the negative electrode active coating and the negative electrode current collector. Instead, the above-mentioned negative electrode active coating slurry is directly coated on both sides of the negative electrode current collector to form a negative electrode active layer. For details, see Table 1-3.

[0104] Example 5 group

[0105] The preparation method of the battery in this group of examples refers to Example 1. The only difference is that the mass content B of vinylene carbonate in the electrolyte is changed. For details, see Table 1-3.

[0106] Example Group 6

[0107] In this group of examples, the preparation method of the battery refers to Example 1. The only difference is that the sphericity S of the spherical silicon-carbon material is changed. See Table 1-3 for details.

[0108] Test Example 1

[0109] For the batteries prepared in each of the above examples and comparative examples, several batteries were prepared for each example / comparative example. Then, the several batteries prepared for each group of examples / comparative examples were divided into three batches. After the first charge and discharge of the first batch, the thickness parameters (including H0, H1, and H2) of the negative electrode sheet were measured in the fully charged state. This thickness parameter is the average value of the test results of all batteries in the same batch; the second batch was after 50 cycles, and the thickness parameters (including H0, H1, and H2) of the negative electrode sheet were measured in the fully charged state. Similarly, it is also an average value; the third batch was after 200 cycles, and the thickness parameters (including H0, H1, and H2) of the negative electrode sheet were measured in the fully charged state. Similarly, it is also an average value. All test results were recorded in Table 1-3.

[0110] Table 1

[0111]

[0112] D / (H0 + H1 + H2) in Table 1 首充 represents the relationship between the thickness parameter of the negative electrode sheet measured in the fully charged state after the first charge and discharge of the above first batch of batteries and the particle size Dv50 of the spherical silicon-carbon material, D / (H0 + H1 + H2) 50T 、D / (H0 + H1 + H2) 200T Similarly, they respectively represent the relationships between the thickness parameters of the negative electrode sheet measured in the fully charged state after 50 times and 200 times of charge and discharge of the above second and third batches of batteries and the particle size Dv50 of the spherical silicon-carbon material. It should be noted that the sum of the thicknesses of the electrode sheets H0 + H1 + H2 and the particle size Dv50 of the spherical silicon-carbon material will change with the continuous increase of the number of cycles. The thickness of the electrode sheet will increase, and the particle size of the spherical silicon-carbon material will increase. Moreover, the thickness and particle size after the first charge and discharge will also be different from the thickness of the electrode sheet and the particle size of the spherical silicon-carbon material set during battery preparation. Therefore, when substituting into the formula D / (H0 + H1 + H2) 首充 、D / (H0 + H1 + H2) 50T and D / (H0 + H1 + H2) 200T for calculation, it is necessary to pay attention to substituting the thickness of the electrode sheet (H0 + H1 + H2) in the same cycle period and the corresponding particle size Dv50 of the spherical silicon-carbon material.

[0113] Table 2

[0114]

[0115] Table 3

[0116]

[0117]

[0118] Test Example 2

[0119] (1) Lithium plating test on the negative electrode surface

[0120] The observation method for the lithium plating situation on the negative electrode surface is as follows: For the batteries obtained from the above-mentioned examples and comparative examples, at 25°C ± 2°C, first perform constant current discharge, discharge at 0.2C to the lower limit voltage, and stand for 5 min; then perform constant current and constant voltage charging, charge at 0.5C to the upper limit voltage, cut off at 0.025C, stand for 10 min, discharge at 0.2C to the lower limit voltage, and then repeat this charge-discharge cycle process 800 times. After the end, fully charge the battery, disassemble the battery cell in a dry environment, and observe the lithium plating situation on the negative electrode surface. As Figure 2 shown, it is a schematic diagram of the lithium plating situation on the negative electrode sheet surface after the battery is disassembled. The lithium plating degrees are divided into four grades: no lithium plating, slight lithium plating at the edge, lithium plating at the edge, and severe lithium plating at the edge. No lithium plating means that no gray or silver lithium is generated on the negative electrode sheet surface, such as Figure 2 Example 1 in; slight lithium plating at the edge means that linear lithium plating appears at the edge of the negative electrode sheet, and the lithium plating area accounts for less than 10% of the area of the negative electrode sheet, showing gray; lithium plating at the edge means that on the basis of slight lithium plating at the edge of the negative electrode sheet, the lithium plating area spreads towards the center of the negative electrode sheet, and the lithium plating area accounts for 10 - 30% of the area of the negative electrode sheet, also showing gray; severe lithium plating at the edge means that on the basis of lithium plating at the edge, it has spread towards the center of the electrode sheet, the lithium plating area accounts for > 30% of the area of the negative electrode sheet, and the lithium precipitated at the edge shows silver, such as Figure 2 Comparative Example 3 in.

[0121] (2) Test on the cycle capacity retention rate and cycle expansion rate of the battery

[0122] Test the cycle capacity retention rate and cycle expansion rate of the batteries obtained from the examples and comparative examples. The specific test method is as follows: First measure the thickness M0 of the fully charged battery cell before the test; at 25°C, perform constant current charging at a charging rate of 2C to 4.53V, with a cut-off current of 0.05C, and then discharge at a discharge rate of 1C to 3.0V; perform cycling according to the aforementioned charge-discharge mechanism, with the number of cycles being 800 times. Then fully charge the battery, take out the battery cell, stand at room temperature for 1 h, and measure the thickness M1 of the fully charged battery cell again;

[0123] Calculate the ratio of the discharge capacity of the last cycle of the above cycle to the first discharge capacity to obtain the capacity retention rate / % of the battery after 800T cycles, and record it in Table 4;

[0124] Calculate the expansion rate of the battery after 800 cycles according to the following formula: Battery cycle 800T expansion rate (%) = (M1 - M0) / M0 * 100%, and record it in Table 4.

[0125] Record the above test results in Table 4.

[0126] Table 4

[0127]

[0128] Compared with Example 1, for the second group of Examples, the thickness H1 of the conductive bottom coating changes. When H1 is close to 0.1H0, the thickness of the conductive bottom coating becomes relatively low, which will lead to a decrease in the utilization rate of the underlying active material, resulting in slight lithium deposition at the edge of the negative electrode sheet in the later stage of the cycle, as shown in Example 2-3; when H1 is close to 0.9H0, the thickness of the conductive bottom coating becomes relatively thick, which will lead to a decrease in the loading amount of the negative active material, a decrease in the cycle capacity retention rate and an increase in the expansion rate, as shown in Example 2-4; relatively, if H1 is less than 0.1H0, the thickness of the conductive bottom coating is too thin, and the lithium deposition problem at the negative electrode edge will be aggravated, as shown in Comparative Example 1-1. If H1 is greater than 0.9H0, the thickness of the conductive bottom coating is too thick, and the impact on the battery cycle capacity retention rate and expansion rate will also be greater, as shown in Comparative Example 1-2.

[0129] According to Example 1, the second to fourth groups of Examples, and Comparative Examples 2-1 and 2-2, controlling the value of D / (H0 + H1 + H2) within the above range after the first charge and discharge, 50 cycles and 200 cycles can solve the expansion problem in the later stage of the cycle caused by the setting of the conductive bottom coating, ensure the effective utilization of the underlying active material, improve the lithium deposition problem on the surface of the negative electrode in the later stage of the cycle, thereby further improving the cycle performance of the battery, effectively improving the cycle life and cycle expansion of the battery, and extending the service life of the battery.

[0130] In Comparative Example 3, no conductive bottom coating is provided, and the lithium deposition problem at the edge of the negative electrode sheet in the later stage of the cycle will become more serious. The utilization rate of the underlying active material is not high, and the cycle performance and expansion problem will also be affected by the charge and discharge of the battery, seriously affecting the service life of the lithium-ion battery.

[0131] Comparing Example 5-1 with Example 1: The mass content B of vinylene carbonate in the electrolyte is too small, and S / B is too large, which is not conducive to the formation of a film on the positive electrode and cannot effectively inhibit the dissolution of transition metals in the positive electrode active material. As a result, it will damage the negative electrode SEI film and reduce the cycle stability of the battery. Comparing Example 5-2 with Example 1: The mass content B of vinylene carbonate in the electrolyte is too large, and S / B is too small, which will cause excessive reaction of the negative electrode active material and the battery to expand. In addition, comparing Example 6-1 with Example 1, it can be seen that the sphericity of the spherical silicon-carbon material particles is too low, which will lead to an increase in the frequency of rupture and repair of the SEI film formed by vinylene carbonate on the negative electrode. S / B is too small, which is also not conducive to improving the expansion and cycle of the battery at the same time.

[0132] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A battery, characterized in that: The battery comprises a negative electrode sheet, a positive electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating located on at least one side of the negative electrode current collector; a conductive primer is disposed between the negative electrode active coating and the negative electrode current collector; The negative electrode active coating comprises a negative electrode active material, the negative electrode active material comprises a spherical silicon-carbon material, and the particle size Dv50 of the spherical silicon-carbon material is D; The thickness of the negative electrode current collector is H0, the thickness of the conductive primer layer is H1, and H0 and H1 satisfy 0.1H0≤H1≤0.9H0; After the first charge and discharge, in the battery in a fully charged state, H0, H1, H2 and D satisfy 0.20≤D / (H0+H1+H2)≤0.28, wherein H2 is the thickness of the negative electrode active coating.

2. The battery according to claim 1, characterized in that 0.2H0≤H1≤0.6H0; and / or, after 50T cycles, in the fully charged battery, H0, H1, H2 and D satisfy 0.1≤D / (H0+H1+H2)≤0.2; And / or, after 200T cycles, in the battery in a fully charged state, H0, H1, H2 and D satisfy 0.05≤D / (H0+H1+H2)≤0.

18.

3. The battery according to claim 1 or 2, characterized in that: The conductive primer layer comprises a conductive agent, an adhesive, a leveling agent and a thickener; Preferably, the conductive agent includes at least one of conductive carbon black, single-walled conductive carbon tubes, multi-walled conductive carbon tubes, and graphene; Preferably, the adhesive includes at least one of styrene-butadiene rubber, PAA, polyacrylic acid, and PVDF; Preferably, the leveling agent comprises thiourea; Preferably, the thickener comprises at least one of CMC and CMC-Li; Preferably, the mass ratio of the conductive agent, the adhesive, the leveling agent and the thickener is (10%-25%): (40%-70%): (0.2%-0.5%): (5%-15%).

4. The battery according to claim 1 or 2, characterized in that: The peeling force between the conductive primer layer and the negative electrode current collector is 10gf-55gf; And / or, the tensile strength of the negative electrode current collector is A, and the thickness H1 of the conductive primer layer and the tensile strength A of the negative electrode current collector satisfy 0.002A≤H1≤0.007A, wherein A is 400MPa-950MPa; And / or, the porosity of the negative electrode sheet is P, and the thickness H of the negative electrode sheet and the porosity P of the negative electrode sheet satisfy 1.7≤H / P≤2.1, wherein P is 23%-48%.

5. The battery according to claim 1 or 2, characterized in that: The particle size Dv10 of the spherical silicon-carbon material is 4 μm-6 μm, the particle size Dv50 of the spherical silicon-carbon material is 6 μm-15 μm, and the particle size Dv90 of the spherical silicon-carbon material is 13 μm-20 μm; And / or, in the cross section of the negative electrode active coating on one side along the width direction of the negative electrode current collector, in the area obtained by any 100μm along the width direction of the negative electrode current collector×the thickness of the negative electrode active coating on one side, the number of the spherical silicon-carbon materials is 3-10, wherein the number of the spherical silicon-carbon materials with a particle size Dv10 of 4μm-6μm is 3-6, the number of the spherical silicon-carbon materials with a particle size Dv50 of 6μm-15μm is 2-4, and the number of the spherical silicon-carbon materials with a particle size Dv90 of 13μm-20μm is 1-3.

6. The battery according to claim 1 or 2, characterized in that: The mass content of the spherical silicon-carbon material in the negative electrode active coating is 5%-35%; And / or, the oil absorption value of the spherical silicon-carbon material is 10mL / 100g-30mL / 100g, and the specific surface area of ​​the spherical silicon-carbon material is 100m 2 / g-150m 2 / g.

7. The battery according to claim 1 or 2, characterized in that: After 50T cycles, in the battery in a fully charged state, in a cross section of the negative electrode active coating on one side along the width direction of the negative electrode current collector, in an area obtained by any 100 μm along the width direction of the negative electrode current collector×the thickness of the negative electrode active coating on one side, the number of spherical silicon-carbon materials with broken particles accounts for 10%-30% of the total number of the spherical silicon-carbon materials in the area, and the damaged area of ​​a single spherical silicon-carbon material with broken particles accounts for 5%-25%.

8. The battery according to claim 1, characterized in that The electrolyte contains fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate is 7%-20% based on the total mass of the electrolyte; And / or, the electrolyte contains vinylene carbonate, and the mass content B of the vinylene carbonate is 0.1%-5% based on the total mass of the electrolyte; And / or, the sphericity of the spherical silicon-carbon material is S, the mass content of vinylene carbonate in the electrolyte is B, and S and B satisfy 0.2≤S / B≤8, wherein S is 0.6-1.

9. The battery according to claim 1, characterized in that The positive electrode sheet includes a positive electrode active coating, the positive electrode active coating contains a positive electrode active material, and the positive electrode active material includes a substance with the chemical formula Li a Ni x Co y Mn z M k O2, where 0.9 ≤ a ≤ 1.1, 0.6 ≤ x ≤ 0.95, 0 < y ≤ 0.2, 0 < z ≤ 0.2, 0 ≤ k ≤ 0.05, and M is selected from at least one of Al, Zr, Mg, Y, Ti, B, Sr, W, Si, La, and Nb.

10. The battery according to claim 9, characterized in that The total content of Al, Mg and Zr in the positive electrode active coating is in the range of 1000ppm-7000ppm; And / or, the chemical formula is Li a Ni x Co y Mn z M k The material of O2 includes single crystal material and polycrystalline material, the mass content of the single crystal material accounts for 0%-100%; the mass content of the Ni element included in the single crystal material accounts for 85%-99%; And / or, after 10T of cycling, the battery is discharged to 3.0V and disassembled, and the content of Ni in the negative electrode active coating is 2%-7%, and the content of Mn in the negative electrode active coating is 0.5%-5%.

Images