Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate and a lithium ion secondary battery comprising the same. The lithium ion secondary battery comprises a positive plate and a negative plate, the area, exceeding the positive plate, of the edge of one side in the width direction of the negative plate is an overhang area, and the width OH of the overhang area is 0.5 mm-2mm; the negative plate comprises a negative current collector and a negative active material layer, the negative active material layer comprises a negative active material, the negative active material comprises a silicon material, the silicon material comprises a silicon element, the content of the silicon element is 0-60% based on the total weight of the negative active material layer, and the sphericity degree of the second binder is 0.8-1; and the negative plate meets the following relational expression: 0.5 < = B / (A-B) < = 3.5. The lithium ion secondary battery disclosed by the invention has relatively good quick charge capacity, relatively high cycle performance and relatively high safety performance.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a lithium-ion secondary battery. Background Art

[0002] As an important component of sustainable energy, battery technology has greatly promoted the sustainable development of society and the convenience and intelligence of human life. Among them, lithium-ion batteries have the advantages of high energy density and long cycle life, and have been widely used in mobile phones, laptops, power tools, new energy vehicles and energy storage. According to market research, mileage anxiety, service life, charging speed and safety issues greatly affect the user experience of new energy vehicles, which shows that power batteries are in urgent need of targeted technological innovation.

[0003] Research has found that the use of high-gram capacity silicon-based negative electrode materials is conducive to improving the energy density of batteries, but the silicon negative electrode will undergo severe volume expansion during the charging process. During the discharge process, as lithium ions are released, the volume of the silicon negative electrode will gradually shrink. This "breathing effect" will continuously load-unload-load stress on the pole piece, causing metal fatigue of the metal current collector, and the pole piece will break at the overhang position of the negative electrode (that is, the area where the edge of one side of the negative electrode sheet exceeds the positive electrode sheet in the width direction of the negative electrode sheet). Finally, the battery cell fails due to the obstruction of electron transmission. In addition, the breathing effect of the silicon negative electrode will also expose more electrochemical active sites, aggravate the vicious cycle of SEI film destruction-recombination-destruction, hinder electron / ion transmission, and deteriorate the fast charging ability and cycle stability of the battery. Especially under high-rate charging conditions, aging cells are prone to lithium precipitation, and the temperature of the cell continues to rise. When heat continues to accumulate and cannot be released, it will cause safety accidents, greatly affecting the safety performance of the battery.

[0004] Therefore, it is very important to invent a battery that combines high energy density, long cycle life, fast charging performance and high safety. Summary of the invention

[0005] To solve the above technical problems, the present invention provides a lithium-ion secondary battery. The negative electrode sheet of the present invention can ensure the normal transmission of electrons and lithium ions in the entire life cycle of the lithium-ion secondary battery, thereby inhibiting the expansion of the lithium-ion secondary battery during the cycle process and improving the fast charging capability, cycle performance and safety performance of the lithium-ion secondary battery.

[0006] To achieve the above object, the present invention provides a lithium ion secondary battery, the lithium ion secondary battery comprising a positive electrode sheet and a negative electrode sheet, the dimension of the negative electrode sheet in the width direction is greater than the dimension of the positive electrode sheet in the width direction, the area where one side edge of the negative electrode sheet in the width direction exceeds the positive electrode sheet is an overhang area, and the width OH of the overhang area is 0.5mm-2mm;

[0007] The negative electrode sheet includes a negative electrode current collector and negative electrode active material layers located on one or both sides of the negative electrode current collector. The negative electrode active material layers include negative electrode active materials and a second binder. The negative electrode active materials include silicon materials, and the silicon materials include silicon elements. Based on the total weight of the negative electrode active material layer, the content of the silicon element is 0-60%, and the sphericity of the second binder is 0.8-1;

[0008] Then, the negative electrode sheet satisfies the following relational expression: 0.5 ≤ B / (A - B) ≤ 3.5, where A is the weight loss rate of the weight loss peak located at 350°C - 650°C in the thermogravimetric analysis spectrum of the negative electrode active material layer after the negative electrode sheet is soaked in DMC at 0% SOC, with the unit of wt%, and B is the weight loss rate of the weight loss peak located at 350°C - 650°C in the thermogravimetric analysis spectrum of the negative electrode active material layer after the negative electrode sheet is soaked in DMC and deionized water at 0% SOC, with the unit of wt%; the thermogravimetric analysis spectrum is obtained by performing thermogravimetric analysis on the negative electrode active material layer at a heating rate of 10°C / min in a nitrogen atmosphere.

[0009] By the above technical solutions, the present invention has at least the following advantages compared with the prior art:

[0010] By controlling the relationship between the weight loss rates of the negative electrode sheet, it is possible to construct a three-dimensional elastic bonding network combining points and lines in the negative electrode sheet while ensuring the normal processing of the negative electrode slurry. This bonding network can effectively inhibit the expansion of the silicon material, especially the expansion of the silicon material, maintain the stability of the structure of the negative electrode sheet during the cycling process, and ensure the integrity of the conductive network, providing a channel for the smooth transmission of electrons and ions, thereby improving the cycling stability, fast charging performance, and safety performance of the lithium-ion secondary battery during its entire life cycle.

[0011] Other features and advantages of the present invention will be described in detail in the subsequent specific implementation section.

[0012] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they 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

[0013] Figure 1 The following shows the thermogravimetric curve of the negative electrode sheet after being soaked in DMC in an embodiment of the present invention.

[0014] Figure 2 The figure shows the thermogravimetric curve of the negative electrode sheet after being soaked in DMC and deionized water in an embodiment of the present invention.

[0015] Figure 3 The figure shows the top view of the positive electrode sheet and the negative electrode sheet in the lithium-ion secondary battery of the present invention. Detailed Embodiments

[0016] The following provides a detailed description of the detailed embodiments of the present invention. It should be understood that the detailed embodiments described herein are only for explaining and illustrating the present invention, and are not used to limit the present invention. In this document, unless otherwise specified, the data range includes the endpoints.

[0017] It should be noted that the numerical representation methods such as "first" and "second" in this disclosure are only used to distinguish different substances or usage methods, and do not represent the difference in order.

[0018] The present invention provides a lithium-ion secondary battery, which includes a positive electrode sheet and a negative electrode sheet. The size of the negative electrode sheet in the width direction is larger than that of the positive electrode sheet in the width direction. The area where one side edge of the negative electrode sheet in the width direction exceeds the positive electrode sheet is the overhang area, and the width OH of the overhang area is 0.5 mm - 2 mm;

[0019] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on one or both sides of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and a second binder. The negative electrode active material includes a silicon material, and the silicon material includes silicon element. Based on the total weight of the negative electrode active material layer, the content of the silicon element is 0 - 60% (for example, 0, 0.1%, 0.5%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or 60%), and the sphericity of the second binder is 0.8 - 1 (for example, 0.8, 0.85, 0.9, 0.95 or 1);

[0020] Then the negative electrode sheet satisfies the following relational expression: 0.5 ≤ B / (A - B) ≤ 3.5, where A is the weight loss rate of the weight loss peak located at 350°C - 650°C in the thermogravimetric analysis spectrum of the negative electrode active material layer after the negative electrode sheet is soaked in DMC at 0% SOC (as shown in Figure 1 the figure), in wt%, and B is the weight loss rate of the weight loss peak located at 350°C - 650°C in the thermogravimetric analysis spectrum of the negative electrode active material layer after the negative electrode sheet is soaked in DMC and deionized water at 0% SOC (as shown in Figure 2as shown), the weight loss rate of the weight loss peak at 350°C - 650°C, with the unit of wt%; the thermogravimetric analysis spectrum is obtained by performing thermogravimetric analysis on the negative electrode active material layer at a heating rate of 10°C / min in a nitrogen atmosphere.

[0021] The negative electrode sheet can satisfy the following relational expression: 0.5 ≤ B / (A - B) ≤ 3.5 (for example, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5).

[0022] Through research, it is found that the second binder has good mechanical elasticity and can change with the change of the volume of the silicon material. When the volume of the silicon material expands, it can expand together to relieve the stress generated by the volume expansion. When the volume of the silicon material shrinks, it can shrink together. At the same time, when the sphericity of the second binder is controlled within the above range, the second binder can be evenly distributed on the surface and between the particles of the silicon material, providing sufficient bonding sites for it, forming a firm bonding network, inhibiting the expansion of the silicon material, and avoiding the peeling of the negative electrode active layer.

[0023] The negative electrode sheet of the present invention can form a three-dimensional elastic bonding network combining points and lines in the negative electrode sheet by controlling the relationship between the weight loss rates of the negative electrode sheet. On the one hand, this three-dimensional elastic bonding network can inhibit the expansion of the silicon material with the silicon element weight content in a specific range and shrink with the shrinkage of the negative electrode active material, thereby relieving the stress generated by the "breathing effect" and reducing the influence of the stress on the negative electrode current collector. On the other hand, it can also keep the bonding network intact during the "breathing effect" process. Especially when the silicon material shrinks, through the cooperation of the elastic bonding network and the rigid bonding network, a smooth electron and lithium ion transmission channel is constructed, reducing the polarization degree of the lithium ion secondary battery, thereby improving the fast charging performance, cycle performance, and safety performance of the lithium ion secondary battery. At the same time, by controlling the relationship between the weight loss rates of the negative electrode sheet after being soaked in DMC and soaked in DMC and deionized water respectively at 0% SOC, a better adaptability between the first binder and the second binder can be achieved, thereby realizing the performance advantages of the three-dimensional bonding network and further improving the fast charging performance, cycle performance, and safety performance of the lithium ion secondary battery.

[0024] When B / (A - B) > 3.5, the stability of the negative electrode slurry is poor and it is difficult to process normally; when B / (A - B) < 0.5, the bonding network in the negative electrode sheet is rigid, and it is difficult to maintain the integrity of the bonding network when the negative electrode active material shrinks. After long-term cycling, it will exacerbate the polarization of the battery cell and deteriorate the cycle life and fast charging ability of the battery cell.

[0025] such as Figure 3The size of the negative electrode sheet 1 in the width direction is larger than that of the positive electrode sheet 2 in the width direction. The area where one side edge of the negative electrode sheet 1 in the width direction exceeds the positive electrode sheet 2 is the overhang area 3, and the width OH of the overhang area 3 is 0.5 mm - 2 mm (for example, 0.5 mm, 1 mm, 1.5 mm or 2 mm). When controlling OH within the above range, it can ensure that the negative electrode sheet completely covers the positive electrode sheet, thereby avoiding lithium deposition on the negative electrode sheet, enabling the lithium-ion secondary battery to have a higher energy density and a longer cycle life. At the same time, it is also beneficial to relieve the stress generated by the volume expansion of the silicon material, maintain the structural stability of the negative electrode sheet, and improve the cycle performance of the lithium-ion secondary battery. When OH < 0.5 mm, the negative electrode does not cover the positive electrode well, increasing the risk of lithium deposition and making battery assembly difficult. When OH > 2 mm, the utilization rate of the negative electrode active material is low, which is not conducive to improving the energy density of the lithium-ion secondary battery. Moreover, the excessive negative electrode active material is likely to consume more electrolyte, exacerbating side reactions at high temperatures and deteriorating the cycle performance and service life of the lithium-ion secondary battery.

[0026] In the present invention, the weight content of silicon element in the negative electrode active material layer can be measured by conventional methods, such as EDS or ICP.

[0027] In the present invention, in the two side edge regions along the width direction, the width of the negative electrode sheet is larger than that of the positive electrode sheet. It can be understood that in the width direction, the size by which the width of the negative electrode sheet on one side edge exceeds the width of the positive electrode sheet is the same as the size by which the width of the negative electrode sheet on the other side edge exceeds the width of the positive electrode sheet. The size of the overhang of the negative electrode sheet is the size by which the width of the negative electrode sheet in the width direction on either side exceeds the width of the positive electrode sheet.

[0028] In the present invention, by controlling the relationship between the weight loss rates of the weight loss peaks of the negative electrode sheet after being soaked in DMC or DMC and deionized water at 0% SOC, compared with the prior art, the cycle stability, fast charging performance, and safety performance of the lithium-ion secondary battery have been improved. To further enhance the effect, one or more technical features can be further optimized.

[0029] In the present invention, the "negative electrode sheet at 0% SOC" refers to the negative electrode sheet when the battery is discharged to the lower limit voltage [for example, for ternary high-nickel materials (the molar content of element nickel is 50% or more), it is 2.5 V].

[0030] In one example, the negative electrode sheet satisfies the following relationship: 1 ≤ B / (A - B) ≤ 2.5.

[0031] In one example, A wt% is 2wt%-6wt% (e.g., 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt% or 6wt%). Controlling A wt% within the above range can ensure strong adhesion between silicon materials, between silicon materials and negative electrode conductive agents, and between negative electrode active material layers and negative electrode current collectors, thereby avoiding demolding of negative electrode active material layers due to excessive expansion of silicon materials during charging and discharging, thereby improving the cycle stability and safety performance of lithium-ion secondary batteries. When A wt%>6wt%, it will hinder the transmission of electrons and ions to a certain extent, which is not conducive to the performance of the capacity of the positive electrode active material, thereby reducing the energy density of the lithium-ion secondary battery, and also compromising the rate performance and cycle performance of the lithium-ion secondary battery, while increasing the risk of lithium plating during charging and discharging of the lithium-ion secondary battery, and deteriorating the safety performance of the lithium-ion secondary battery. When Awt%<2wt%, the negative electrode active material layer will be demolded from the surface of the negative electrode current collector, affecting the structural stability of the lithium-ion secondary battery, and further affecting the effective transmission of electrons and ions in the negative electrode sheet, deteriorating the cycle performance, rate performance and safety performance of the lithium-ion secondary battery; and the obstruction of electron and ion transmission will also increase the risk of lithium precipitation on the surface of the negative electrode sheet, which is not conducive to the utilization of the capacity of the positive electrode active material and reduces the energy density of the battery.

[0032] In one example, A wt% is 2.5 wt%-5 wt%.

[0033] In one example, B wt% is 1wt%-4wt% (e.g., 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt% or 4wt%). When B wt% is controlled within the above range, it is helpful to construct an elastic bonding network in the negative electrode sheet, thereby effectively alleviating the expansion of the silicon material, slowing down the side reaction between the electrolyte and the silicon material, reducing the polarization of the lithium ion secondary battery, improving the cycle stability of the lithium ion secondary battery, and extending the cycle life of the lithium ion secondary battery. When B wt% is less than 1wt%, the amount of the second binder is insufficient to construct an elastic bonding network, and the expansion of the silicon material is still difficult to be suppressed. After multiple charge and discharge cycles of the lithium ion secondary battery, the SEI film of the negative electrode sheet undergoes multiple destruction-reorganization, the polarization degree of the lithium ion secondary battery system is aggravated, and the kinetic performance is insufficient to support high-rate charge and discharge, which can easily lead to failure or safety loss of control of the lithium ion secondary battery. When B wt% is greater than 4wt%, the excessive amount of the second binder will not effectively improve the bonding strength to the electrode sheet. On the contrary, the electrochemically inert second binder component will hinder the electron / ion transmission, reduce the kinetic performance of the negative electrode side, deteriorate the fast charging capability and cycle stability of the lithium-ion secondary battery, and cause lithium deposition at the negative electrode, causing safety hazards.

[0034] In one example, B wt% is 1.5 wt%-3 wt%.

[0035] In one example, A wt% is 2.5wt%-5wt%, B wt% is 1.5wt%-3wt%, and the negative electrode sheet satisfies the following relationship: 1≤B / (AB)≤2.5.

[0036] In one example, based on the total weight of the negative electrode active material layer, the content of the silicon element is 0-50%.

[0037] In one example, the silicon material includes one or more of silicon-carbon material, silicon-oxygen material, silicon element and silicon alloy.

[0038] In one example, the silicon-carbon material includes a material in which silicon particles are filled in pores of porous amorphous carbon or porous crystalline carbon, and / or a material in which silicon particles are mixed with amorphous carbon or crystalline carbon.

[0039] In one example, the silicon material has an average particle size of 5 μm-14 μm (eg, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, or 14 μm).

[0040] In one example, the average particle size of the silicon material is 7 μm-12 μm.

[0041] In the present invention, the average particle size can be obtained by scanning electron microscopy (SEM) testing. An electrolytic emission scanning electron microscope (S-3400N manufactured by Hitachi, Ltd.) is used to observe the surface of the porous layer at a magnification of 50,000 times. The image size at this time is 2.5μm×1.8μm. It should be noted that the number of pixels is 1,280 pixels×960 pixels, and the size of 1 pixel is 2nm×1.9nm. For the average particle size, a square or rectangle with the smallest area that completely surrounds one particle is drawn on the obtained image, that is, a square or rectangle in which the end of the particle is connected to the four sides of the square or rectangle. In the case of a square, the length of one side is set as the particle size, and in the case of a rectangle, the length of the long side (major axis diameter) is set as the particle size. For any 81 particles, the particle size is measured, and the number average is set as the average particle size. It should be noted that when more than 81 particles are observed in the captured image, the number average of any 81 particle sizes in the image is set as the average particle size. When 81 particles are not observed in the image, multiple images are captured and the number average of the total 81 particle sizes is set as the average particle size.

[0042] In one example, the specific surface area of ​​the silicon material is 0.8 m 2 / g-8m 2 / g (for example, 0.8m2 / g, 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g or 8 m 2 / g).

[0043] In one example, the specific surface area of the silicon material is 1 m 2 / g - 6 m 2 / g.

[0044] In one example, the negative electrode active material layer includes a negative electrode active material, a first binder, a second binder, and a negative electrode conductive agent. The negative electrode active material includes a silicon material and a carbon material.

[0045] In one example, the carbon material includes one or more of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, and soft carbon.

[0046] In one example, based on the total weight of the negative electrode active material, the weight content of the silicon material is 1 wt% - 30 wt% (for example, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 12 wt%, 15 wt%, 16 wt%, 18 wt%, 20 wt%, 22 wt%, 25 wt%, 26 wt%, 28 wt%, or 30 wt%). When controlling the weight content of the silicon material in the negative electrode active material within the above range, it is possible to maintain the stability of the silicon negative electrode sheet structure while ensuring a good electron / ion transport network in the negative electrode sheet, suppress the volume expansion of the negative electrode active material, especially the silicon material, and at the same time ensure that the lithium-ion secondary battery has a high energy density, excellent cycle performance, and safety performance.

[0047] In one example, the second binder includes one or more of styrene-butadiene rubber, styrene-acrylate copolymer, acrylate copolymer, and polyurethane.

[0048] In one example, the elastic modulus of the second binder is 2 MPa - 8 MPa (for example, 2 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa, or 8 MPa). Controlling the elastic modulus of the second binder within the above range is beneficial for the second binder to construct an elastic bonding network in the negative electrode sheet, thereby effectively suppressing the expansion of the silicon material, slowing down the side reaction between the silicon material and the electrolyte, and further extending the cycle life of the battery cell. When the elastic modulus of the second binder is less than 2 MPa, the elasticity of the bonding network in the negative electrode sheet is low, and powder falling is likely to occur during the slitting or winding of the electrode sheet, exacerbating the self-discharge of the battery. When the elastic modulus of the second binder is greater than 8 MPa, it is difficult to prepare the second binder in batches, and it cannot meet the requirements of industrial battery production.

[0049] In one example, the elastic modulus of the second binder is 3 MPa - 6 MPa.

[0050] In the present invention, the elastic modulus can be obtained by testing in the following manner, specifically as follows: Use a stamping machine to punch the binder film into a spindle-shaped specimen, measure the cross-sectional area of the spindle-shaped specimen, fix both ends of the specimen to the upper and lower clamps of a tensile testing machine, and then stretch the specimen at a speed of 50 mm / min until fracture occurs, recording the displacement and the applied force during the process. Divide the displacement at fracture by the specimen length to obtain the elongation at break of the film, and divide the applied force at fracture by the cross-sectional area of the spindle-shaped specimen to obtain the tensile strength; the ratio of the tensile strength to the elongation at break is the elastic modulus.

[0051] In one example, the average particle size of the second binder is 0.06 μm - 0.8 μm (for example, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, or 0.8 μm).

[0052] In one example, the average particle size of the second binder is 0.1 μm - 0.6 μm.

[0053] In one example, the negative electrode active material layer includes a first binder, and the first binder includes one or more of polyacrylic acid, polyacrylate, polyacrylonitrile, polyacrylamide, and their copolymers.

[0054] In one example, the negative electrode active material layer includes a first binder and a second binder. Through research, it is found that the first binder has both the functions of bonding and dispersion. In the negative electrode sheet, it can improve the bonding force of the negative electrode sheet, and the glass transition temperature of the first binder is relatively high. The bonding network formed by the first binder has a certain rigidity. However, it is still difficult to inhibit the volume expansion of the silicon material during charge and discharge when using the first binder alone, and this bonding network cannot change when the volume of the silicon material shrinks, resulting in a reduction in the integrity of the bonding network. The second binder plays a bonding role through point contact. Simple point bonding is difficult to fully contact with the negative electrode active material and is not sufficient to inhibit the volume expansion of the silicon material for a long time. Through the synergistic cooperation of the second binder and the first binder, it can not only solve the problem that the rigid bonding network formed by the first binder cannot change with the shrinkage of the silicon material, but also rely on the point contact between the second binder and the negative electrode conductive agent and the silicon material to play a bonding role, realizing the further combination of the elastic bonding network and the rigid bonding network, thereby further improving the fast charging performance, cycle performance, and safety performance of the lithium-ion secondary battery.

[0055] In one example, the first binder includes alkali metal elements, and the alkali metal elements include lithium element (Li element) and sodium element (Na element). For example, the first binder can be lithiated, such as one or more of lithiated polyacrylate, lithiated polyacrylonitrile, lithiated polyacrylamide, and their copolymers.

[0056] In one example, based on the total weight of the first binder, the weight content of the alkali metal elements is 1.5% - 8% (for example, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, or 8%).

[0057] In one example, based on the total weight of the first binder, the weight content of the alkali metal elements is 2.5% - 6%

[0058] In one example, the negative electrode active material layer includes lithium element and sodium element.

[0059] In one example, at 0% SOC, the weight content of lithium element in the negative electrode active material layer > 400 ppm (for example, 450 ppm, 500 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, or 2500 ppm), and the weight content of sodium element < 1% (for example, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, 0.1%, 0.05%, 0.01%, or 0).

[0060] In one example, the first binder further includes sodium carboxymethyl cellulose (CMC-Na) and / or lithium carboxymethyl cellulose (CMC-Li). Based on the total weight of the negative electrode active material layer, the weight content of sodium carboxymethyl cellulose and / or lithium carboxymethyl cellulose is 0%-0.7% (for example, 0%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6% or 0.7%).

[0061] In the present invention, the weight contents of lithium element and sodium element in the negative electrode active material layer after the negative electrode sheet is soaked in DMC at 0% SOC can be obtained by conventional methods. For example, after the negative electrode sheet is soaked in DMC at 0% SOC, it is tested by using Thermo Scientific TM iCAP TM PRO XP ICP-OES inductively coupled plasma optical emission spectrometer (ICP).

[0062] In one example, after the negative electrode sheet is soaked in DMC at 0% SOC, there is one weight loss peak at 200°C - 350°C.

[0063] It has been found through research that when solely using the second binder, carboxymethyl cellulose is required to disperse the negative electrode slurry. However, when the negative electrode active material includes silicon material, the silicon material will directly react with carboxymethyl cellulose. While consuming the solvent and continuously undergoing side reactions, it will also have an adverse impact on the transmission of lithium ions, deteriorating the fast charging performance and cycle stability of the lithium ion secondary battery. Moreover, the negative electrode slurry containing silicon material is alkaline, and carboxymethyl cellulose will continuously decompose in the alkaline slurry, thereby causing carboxymethyl cellulose to lose its suspension stability and resulting in abnormal processing of the negative electrode slurry. However, in the present invention, a small amount of carboxymethyl cellulose can be included in the negative electrode active material layer because the first binder has a dispersing effect, which can stabilize the suspension while reducing the influence of the silicon material on carboxymethyl cellulose, so that the negative electrode sheet slurry can still be normally processed while including carboxymethyl cellulose. In addition, the presence of lithium carboxymethyl cellulose can provide sufficient lithium ions, improving the initial efficiency and fast charging performance of the lithium ion secondary battery.

[0064] In one example, based on the total weight of the first binder, the weight content of the Li element is 1.5% - 8% (for example, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, or 8%). Controlling the weight content of the Li element in the first binder within the above range can form a locally high-concentration lithium-ion distribution near the negative electrode active material, achieve rapid lithium-ion transport, thereby improving the kinetics on the negative electrode side and enhancing the fast charging performance of the battery cell. When the weight content of the Li element is less than 1.5%, a locally high-concentration lithium-ion distribution cannot be formed near the negative electrode active material, and the ion transport cannot be significantly improved, resulting in an insignificant improvement in the fast charging ability of the battery cell. When the weight content of the Li element is greater than 8%, the excessive Li element will undergo side reactions with the electrolyte at high temperatures, causing the battery cell to bulge, increasing the risk of safety hazards, and the excessive weight content of the Li element will make the first binder alkaline, which is not conducive to the processing of the silicon negative electrode slurry.

[0065] In one example, based on the total weight of the first binder, the weight content of the Li element is 2.5% - 6%.

[0066] In one example, the molecular weight of the first binder is 500,000 - 1,500,000 (for example, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, 1,400,000, or 1,500,000). Controlling the molecular weight of the first binder within the above range, the first binder can have excellent adhesion, which is beneficial to stabilizing the structure of the negative electrode sheet, inhibiting the expansion of the silicon material, and can also promote the uniform dispersion of the negative electrode active material in the slurry, while maintaining good slurry viscosity stability and improving slurry coating processing. When the molecular weight of the first binder is lower than 500,000, the thickening effect of the first binder on the slurry is poor, which is not conducive to the preparation of the negative electrode slurry, resulting in poor coating stability of the negative electrode active material layer and uneven thickness distribution, and lithium deposition is likely to occur at the position where the negative electrode active material layer is thinner. When the molecular weight is greater than 1,500,000, the molecular weight of the first binder is prone to entanglement, resulting in poor viscosity stability of the negative electrode slurry and unable to meet the requirements of battery industrial production.

[0067] In one example, the molecular weight of the first binder is 700,000 - 1,300,000.

[0068] According to a specific embodiment, the negative electrode sheet satisfies the following relational expression: 0 ≤ D1 - D2 ≤ 0.3 Ω·cm, where D1 is the maximum conductivity in the unit of Ω·cm, and D2 is the minimum conductivity in the unit of Ω·cm. The conductivity is measured at five position points equidistantly on the center line in the width direction along the length direction of the negative electrode sheet. Among them, D1 is the maximum conductivity among the five test sites, and D2 is the minimum conductivity among the five test sites. Equidistantly means that the distance between any two adjacent position points among the five position points is equal. On the center line in the width direction of the negative electrode sheet, the conductivity is measured at five position points A1, A2, A3, A4, and A5 equidistantly along the length direction. Equidistantly means that the distance between A1 and A2 is equal to the distance between A2 and A3, equal to the distance between A3 and A4, and equal to the distance between A4 and A5. Within this range, it can ensure that the negative electrode sheet has a good electron transport network and bonding network, which is beneficial to the structural stability of the negative electrode sheet during charge and discharge, ensures smooth electron / ion transport in the system, and improves the cycle stability and rate performance of the battery cell. When D1 - D2 is greater than 0.3 Ω·cm, there is a situation of uneven distribution of the binder or conductive agent inside the electrode sheet. Insufficient local bonding sites and lack of conductive sites will lead to hindered electron / ion transport, which will seriously reduce the cycle performance and fast charging ability of the battery cell.

[0069] According to a specific embodiment, the negative electrode sheet satisfies the following relationship at the same time: 10≤P / M≤70 (for example, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or 70), 0.2≤Z / M≤1.5 (for example, 0.2, 0.5, 0.8, 1, 1.2 or 1.5), wherein P is the tensile strength of the negative electrode collector, in MPa, Z is the elongation of the negative electrode collector, in %, and M is the OI value of the negative electrode sheet. The study found that when the OI value of the negative electrode sheet is low, the direction of the graphite sheet is perpendicular to the negative electrode current collector, and there are fewer bonding sites between the negative electrode current collector and the negative electrode active material layer. In addition, the expansion direction of the graphite sheet in the negative electrode sheet with a lower OI value tends to be parallel to the negative electrode current collector. At this time, even if a negative electrode current collector with high tensile strength and elongation is used, it is difficult to solve the expansion problem of the negative electrode sheet. Increasing the bonding sites between the negative electrode and the current collector is a more effective strategy to inhibit expansion. When the OI value of the negative electrode sheet is high, the direction of the graphite sheet is parallel to the negative electrode current collector, and there are more bonding sites between the negative electrode current collector and the negative electrode active material layer. At this time, the graphite sheet in the negative electrode sheet with a higher OI value expands more in the direction perpendicular to the negative electrode current collector. The larger tensile strength and elongation of the negative electrode current collector can resist the stress exerted by the expansion of the negative electrode active material, avoid the destruction of the structure of the negative electrode sheet, hinder the electron / ion transmission, and ultimately deteriorate the cycle performance and increase safety hazards. When the negative electrode sheet satisfies the above relationship, the negative electrode current collector (for example, copper foil) has suitable tensile strength and ductility, and can also have good adaptability with silicon materials, inhibiting the anisotropic volume expansion of the negative electrode sheet during the charge and discharge process, ensuring the structural stability of the negative electrode sheet during the cycle process, and the integrity of the electron / ion transmission pathway, thereby achieving the advantages of the cycle performance, rate performance, and safety performance of lithium-ion secondary batteries.

[0070] In one example, the negative electrode sheet satisfies the following relationships simultaneously: 20≤P / M≤40, 0.5≤Z / M≤1.

[0071] In one example, the tensile strength P MPa of the negative electrode current collector is 300 MPa - 800 MPa (for example, 300 MPa, 400 MPa, 500 MPa, 600 MPa, 700 MPa, or 800 MPa). Controlling the tensile strength of the negative electrode current collector within the above range enables the negative electrode current collector to withstand the stress generated by the volume expansion of the silicon material, ensuring that the electrode sheet does not break during the cycling process, extending the cycle life of the lithium-ion secondary battery, improving the fast charging ability, and enhancing the safety performance. When the tensile strength of the negative electrode current collector is greater than 800 MPa, the negative electrode current collector, such as copper foil, has fine and uneven grains, with a large number of small-angle grain boundaries and twin boundaries, resulting in poor corrosion resistance of the copper foil, which is not conducive to the manufacture of lithium-ion secondary batteries; when the tensile strength of the negative electrode current collector is less than 300 MPa, the tensile resistance and compressive deformation ability of the negative electrode current collector decrease, and during the rolling and slitting processes, the negative electrode current collector is prone to breakage, affecting the manufacturing process of the lithium-ion secondary battery. Additionally, during the cycling process, the negative electrode current collector is also prone to metal fatigue due to the breathing effect of the silicon material, which may further cause the negative electrode current collector to break, deteriorating the cycle performance, fast charging ability, and safety performance of the lithium-ion secondary battery.

[0072] In one example, the tensile strength P MPa of the negative electrode current collector is 400 MPa - 700 MPa.

[0073] In the present invention, the tensile strength can be tested using an electronic universal testing machine. Specifically: Samples are taken along the transverse and longitudinal directions of the negative electrode current collector (copper foil), cut into strips with a length of 200 ± 0.5 mm and a width of 15 ± 0.25 mm. The distance between the grips of the testing machine is 125 + 0.1 mm, the gauge length is 50 mm, and a tensile test is carried out at a speed of 50 mm / min. The test results of the tensile strength can be obtained from the electronic universal testing machine.

[0074] In one example, the elongation rate Z% of the negative electrode current collector is 3% - 18% (for example, 3%, 5%, 6%, 8%, 10%, 12%, 15%, 16%, or 18%). Controlling the elongation rate of the negative electrode current collector within the above range enables good adaptation of the elongation between the negative electrode current collector and the negative electrode active material layer, ensuring normal rolling processing of the electrode sheet. At the same time, the negative electrode current collector with an appropriate elongation rate can relieve the expansion of the silicon material during charge and discharge through stress release, maintaining the structural stability of the electrode sheet, and ensuring the cycle performance, fast charging performance, and safety performance of the lithium-ion secondary battery. When the elongation rate of the negative electrode current collector is less than 6%, it is difficult to meet the rolling processing of the electrode sheet, resulting in the inability to produce lithium-ion secondary batteries normally; when the elongation rate of the negative electrode current collector is greater than 18%, the grains of the negative electrode current collector are coarse, the distance between atoms becomes larger, and the electron transmission path becomes longer, leading to a decrease in the conductivity of the negative electrode current collector and a reduction in the rate performance of the lithium-ion secondary battery.

[0075] In the present invention, the elongation rate can be obtained by testing with an electronic universal testing machine. Specifically: Samples are taken along the transverse and longitudinal directions of the negative current collector (copper foil), cut into strips with a length of 200 ± 0.5 mm and a width of 15 ± 0.25 mm. The distance between the grips of the testing machine is 125 + 0.1 mm, the gauge length is 50 mm, and a tensile test is carried out at a speed of 50 mm / min. The test results of the elongation rate can be obtained on the electronic universal testing machine.

[0076] In one example, the OI value M of the negative electrode sheet is 10 - 40 (for example, 10, 12, 15, 18, 20, 25, 30, 35 or 40). When the OI value of the negative electrode sheet is controlled within the above range, the elongation performance of the negative electrode sheet is relatively good, there will be no powder dropping phenomenon, and at the same time, the orientation degree of the negative electrode sheet can be improved, the cycle performance and rate performance of the lithium-ion secondary battery can be improved, and at this time, the swelling performance of the negative electrode sheet is moderate, which can avoid problems such as too high swelling rate of the lithium-ion secondary battery caused by too high OI value. When the OI value of the negative electrode sheet < 10, there are too few bonding sites between the negative electrode active material layer and the negative current collector, resulting in powder dropping of the electrode sheet and unable to be processed into a battery. When the OI value of the negative electrode sheet > 40, during the lithium intercalation process of the negative electrode sheet, the swelling perpendicular to the electrode sheet direction will be higher, easily causing the negative electrode sheet to break at the overhang position, hindering electron / ion transport, increasing the polarization of the battery system, resulting in serious deterioration of the rate performance and cycle performance of the battery, and may also cause lithium deposition problems.

[0077] In one example, the OI value M of the negative electrode sheet is 15 - 25.

[0078] In the present invention, the OI value refers to the ratio of the peak area of (004) to the peak area of (110) of the negative electrode sheet in the X-ray diffraction pattern, that is, OI value = S I004 / S I110 . The test conditions of the X-ray diffraction pattern are to use a Thermo Scientific T ARL TM EQUINOX Pro vertical X-ray diffractometer for testing, the diffraction angle range is 10° - 90°, and the scanning speed is 2° / min. Among them, the characteristic peak 004 is located at 52° - 57°, and the characteristic peak 110 is located at 73° - 78°.

[0079] In one example, the tensile strength P MPa of the negative current collector is 400 MPa - 700 MPa, the elongation rate Z% of the negative current collector is 6% - 18%, and the OI value M of the negative electrode sheet is 15 - 25. Then the negative electrode sheet simultaneously satisfies the following relational expressions: 10 ≤ P / M ≤ 70, 0.2 ≤ Z / M ≤ 1.5.

[0080] In one example, the areal density S mg / cm of the negative electrode sheet2 is 6 mg / cm 2 -15 mg / cm 2 (e.g., 6 mg / cm 2 , 7 mg / cm 2 , 8 mg / cm 2 , 9 mg / cm 2 , 10 mg / cm 2 , 11 mg / cm 2 , 12 mg / cm 2 , 13 mg / cm 2 , 14 mg / cm 2 or 15 mg / cm 2 ). When the areal density of the negative electrode sheet is controlled within the above range, a high-energy-density battery with excellent cycling performance and rate performance can be obtained. When the areal density of the negative electrode sheet is greater than 15 mg / cm 2 , the coating amount of the negative electrode active material increases, the electron / ion transport path becomes longer, the rate performance and cycling performance of the lithium-ion secondary battery deteriorate. At the same time, the stress generated by the expansion of the silicon material is greater, which easily causes the deformation of the electrode sheet and the rupture of the negative electrode current collector, affecting the safety performance of the lithium-ion secondary battery. When the areal density of the negative electrode sheet is less than 6 mg / cm 2 , the capacity of the negative electrode sheet is too low, resulting in an imbalance in the matching with the positive electrode sheet, which causes the negative electrode sheet to easily have the problem of lithium deposition, resulting in the abnormal use of the lithium-ion secondary battery.

[0081] In the present invention, the areal density represents the areal density of a single side. For example, for the areal density of the negative electrode sheet, when there is a negative electrode active material layer on one side of the negative electrode current collector, the areal density of the negative electrode sheet is the areal density of this side (i.e., the side with the negative electrode active material layer); when there are negative electrode active material layers on both sides of the negative electrode current collector, the areal densities of both sides of the negative electrode sheet are the same, and the areal density of the negative electrode sheet is the areal density of any one side.

[0082] In the present invention, the areal density can be measured by the following method: After taking out the electrode sheet from the fully discharged battery cell, soak it in the solvent DMC for 30 min and rinse it with DMC multiple times, and then vacuum dry it at 150 °C for 12 h. Use a punching machine to punch the dried electrode sheet into small circles with an area of 15.4 cm 2 . Then use an electronic balance to weigh the mass of the circular sheet as M mg; observe the cross-section of the electrode sheet with a scanning electron microscope to obtain the thickness of the electrode current collector as H cm; based on this, the areal density CW (mg / cm 2 ) of the single side of the electrode sheet can be calculated.

[0083] Among them, ρ represents the density of the current collector.

[0084] In one example, the tap density Y of the negative electrode sheet is 1.3 g / cm 3 -1.8 g / cm 3 (for example, 1.3 g / cm 3 , 1.4 g / cm 3 , 1.5 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 3 or 1.8 g / cm 3 ). By controlling the tap density of the negative electrode sheet within the above range, the first binder and the second binder can fully bond the negative electrode active material, the negative electrode conductive agent and the negative electrode current collector together, constructing a long-term effective and firm bonding network, suppressing the volume expansion of the silicon material during charge and discharge, and also being able to shorten the electron transmission path, improve the wettability of the electrolyte to the negative electrode sheet, make full use of the electrochemical active area of the negative electrode sheet, increase the ion transmission channels, improve the rate performance and cycle stability of the lithium-ion secondary battery, and avoid lithium deposition in the lithium-ion secondary battery under high-rate usage conditions. When the tap density of the negative electrode sheet is less than 1.3 g / cc, although the liquid retention amount on the negative electrode side can be increased and the ionic conductivity can be improved, the lithium-ion secondary battery after working for a period of time will have an overly loose structure of the negative electrode sheet, and the side reaction between the negative electrode sheet and the electrolyte will intensify, resulting in the deformation of the lithium-ion secondary battery and deteriorating the safety performance. Moreover, too low a tap density is also not conducive to improving the volumetric energy density of the battery cell. When the tap density of the negative electrode sheet is greater than 1.8 g / cc, the wettability of the electrolyte to the negative electrode sheet is poor, the ion transmission is blocked, and during charge and discharge, the utilization rate of the negative electrode active material is low, which easily leads to lithium deposition on the negative electrode side, deteriorating the cycle performance and safety performance of the lithium-ion secondary battery.

[0085] In one example, the thickness T μm of the negative electrode current collector is 5 μm - 10 μm (for example, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm). By controlling the thickness of the negative electrode current collector within the above range, the negative electrode current collector can withstand the stress generated by the volume expansion of the negative electrode active material such as silicon material, ensure that the electrode sheet does not break during the cycle, and at the same time take into account the energy density of the lithium-ion secondary battery. When the thickness of the negative electrode current collector is less than 5 μm, the tensile strength and compressive deformation resistance of the negative electrode current collector decrease. During the charge and discharge process of the lithium-ion secondary battery, the negative electrode current collector undergoes continuous applied-release stress, and at this time, the possibility of the negative electrode current collector breaking or cracking is relatively large, affecting the service life and safety of the lithium-ion secondary battery.

[0086] In one example, the negative electrode current collector includes a copper foil.

[0087] In one example, the negative electrode active material layer further includes a negative electrode conductive agent.

[0088] In one example, the negative electrode conductive agent includes at least one of electrothermal carbon black (SP), Ketjen black, acetylene black, graphite conductive agents (KS-6, KS-15, S-O, SEG-6), carbon fiber (VGCG), carbon nanotube (CNT), and graphene.

[0089] In one example, the carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0090] In one example, the negative electrode conductive agent includes single-walled carbon nanotubes, which can improve the conductivity of the negative electrode sheet, thereby improving the rate performance of the lithium-ion secondary battery.

[0091] In one example, based on the total weight of the negative electrode active material layer, the weight content of the negative electrode active material is 85% - 99.5% (for example, 85%, 88%, 90%, 92%, 94%, 96%, 98%, or 99.5%), the weight content of the first binder is 0.2% - 5% (for example, 0.2%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, or 5%), the weight content of the second binder is 0.2% - 5% (for example, 0.2%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, or 5%), and the weight content of the negative electrode conductive agent is 0.1% - 5% (for example, 0.1%, 0.2%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, or 5%).

[0092] In one example, based on the total weight of the negative electrode active material layer, the weight content of the negative electrode active material is 90% - 97.5%, the weight content of the first binder is 1% - 4%, the weight content of the second binder is 1% - 4%, and the weight content of the negative electrode conductive agent is 0.5% - 2%.

[0093] The lithium-ion secondary battery of the present invention further includes a positive electrode sheet, a separator, and an electrolyte.

[0094] In one example, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer located on one or both surfaces of the positive electrode current collector.

[0095] In one example, the positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder.

[0096] In one example, the positive electrode active material includes a ternary material, and the chemical formula of the ternary material is Li a Ni x Co y Mn z A kO2, where 0.9 ≤ a ≤ 1.1, 0.5 ≤ x ≤ 0.95, 0 < y ≤ 0.2, 0 < z ≤ 0.3, 0 ≤ k ≤ 0.05, A is a doping element, and A includes one or more of Al, Mg, Ti, Zr, B, P, and Y.

[0097] The positive electrode active material may include a positive electrode active material having a layered structure.

[0098] In one example, the ternary material includes a single crystal structure and / or a polycrystalline structure.

[0099] In one example, the ternary material includes a single crystal structure and a polycrystalline structure. When the single crystal structure and the polycrystalline structure are blended, it is beneficial to fill the gaps between the positive electrode active materials, ensuring that the lithium ion secondary battery has a high energy density.

[0100] In one example, the positive electrode conductive agent includes at least one of conductive carbon black (SP), Ketjen black, acetylene black, graphite conductive agents (KS-6, KS-15, S-O, SEG-6), carbon fiber (VGCG), carbon nanotube (CNT), and graphene.

[0101] In one example, the positive electrode conductive agent includes carbon nanotubes and conductive carbon black, which can improve the conductivity of the positive electrode sheet and thus improve the rate performance of the lithium ion secondary battery.

[0102] In one example, the positive electrode binder includes one or more of polyacrylonitrile, polyimide, and perfluorosulfonic acid ionomer binders.

[0103] In one example, the positive electrode binder includes one or more of PVDF, PVDF-HFP, polytetrafluoroethylene, polyacrylonitrile, and polyimide.

[0104] In one example, based on the total weight of the positive electrode active material layer, the weight content of the positive electrode active material is 80% - 99.8% (for example, 80%, 85%, 90%, 95%, or 99.8%), the weight content of the positive electrode conductive agent is 0.1% - 10% (for example, 0.1%, 1%, 2%, 4%, 6%, 8%, or 10%), and the weight content of the positive electrode binder is 0.1% - 10% (for example, 0.1%, 1%, 2%, 4%, 6%, 8%, or 10%).

[0105] In one example, based on the total weight of the positive electrode active material layer, the weight content of the positive electrode active material is 90% - 99%, the weight content of the positive electrode conductive agent is 0.5% - 5%, and the weight content of the positive electrode binder is 0.5% - 5%.

[0106] The electrolyte can be a conventional electrolyte in the art. For example, the electrolyte includes a lithium salt, an organic solvent, and an additive.

[0107] In one example, the lithium salt includes one or more of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium difluoro(dioxalato)borate (LiDFOP), and lithium bis(trifluoromethylsulfonyl)imide.

[0108] In one example, the organic solvent includes one or more of ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, diethyl carbonate (DMC), ethyl formate, ethyl acetate (EA), ethyl propionate (EP), and propyl propionate (PP).

[0109] In one example, the additive includes one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene vinyl carbonate (VEC), methylene bis(methanesulfonate) (MMDS), and 1,3-propane sultone (PS).

[0110] In one example, based on the total weight of the electrolyte, the weight content of the lithium salt is 10% - 20% (e.g., 10%, 13%, 15%, 18%, or 20%), the weight content of the organic solvent is 75% - 89.99% (e.g., 75%, 78%, 80%, 83%, 85%, 88%, or 89.99%), and the weight content of the additive is 0.01% - 10% (e.g., 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, or 5%).

[0111] In one example, based on the total weight of the electrolyte, the weight content of the lithium salt is 12% - 16%, the weight content of the organic solvent is 79% - 87.95%, and the weight content of the additive is 0.05% - 5%.

[0112] The separator can be a conventional separator in the art. For example, the separator includes a base film and a porous layer.

[0113] The base film can include at least one of polyethylene, polypropylene, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyethylene terephthalate, polyimide, and aramid.

[0114] In one example, the thickness of the separator is 3 μm - 12 μm.

[0115] In one example, the porous layer is disposed on at least one surface of the base film.

[0116] In one example, the porous layer includes inorganic particles and a first separator binder.

[0117] In one example, the inorganic particles include at least one of alumina, silica, magnesia, titania, calcium oxide, zirconia, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate.

[0118] In one example, the separator further includes a glue layer, and the glue layer is located on the surface of the porous layer or the surface of the base film.

[0119] In one example, the glue layer includes a second separator binder.

[0120] The first separator binder and the second separator binder may each independently include one or more of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, carboxymethyl cellulose salt, polyvinylpyrrolidone, and polymethyl methacrylate.

[0121] In one example, the cut-off voltage of the lithium-ion secondary battery is ≥4.2V. Controlling the cut-off voltage of the lithium-ion secondary battery can improve the fast charging performance of the lithium-ion secondary battery at high voltages.

[0122] Since the lithium-ion secondary battery described in the present invention includes the negative electrode sheet described in the first aspect of the present invention, the cycle stability, fast charging performance, and safety performance of the lithium-ion secondary battery are improved.

[0123] The present invention will be described in detail below through embodiments. The embodiments described in the present invention are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0124] The following embodiments are used to illustrate the negative electrode sheet of the present invention.

[0125] Example 1

[0126] (1) Component preparation

[0127] Negative electrode current collector: copper foil (the tensile strength P of the negative electrode current collector is 650 MPa, and the elongation rate Z of the negative electrode current collector is 15%);

[0128] Negative electrode active material layer: Negative electrode active material: 81.2 parts by weight of graphite, 14.3 parts by weight of silicon-carbon material (the weight content of silicon element in the negative electrode active material layer is 25%); First binder: lithium polyacrylate (the weight content of Li element in the first binder is 3%, and the molecular weight of the first binder is 900,000), 1.5 parts by weight; Second binder: styrene-butadiene rubber latex (the elastic modulus of the second binder is 4 MPa, the average particle size of the second binder is 0.21 μm, and the sphericity of the second binder is 0.9), 2 parts by weight; Negative electrode conductive agent: carbon black and carbon nanotubes (the weight ratio of carbon black to carbon nanotubes is 5:1), 1 part by weight.

[0129] (2) Preparation of negative electrode sheet

[0130] Mix the negative electrode active material, the first binder, the second binder and the negative electrode conductive agent, and obtain a uniformly dispersed mixture through high-speed stirring. The mixture is made into a negative electrode active material slurry using water as a solvent, and the solid content in the slurry is 50 wt%. Coat the slurry evenly on both side surfaces of the negative electrode current collector, and obtain the negative electrode sheet after drying and rolling.

[0131] Among them, A is 3.8%, B is 2.2%, B / (A - B)=2.2 / (3.8 - 2.2)=1.38, D1 - D2 = 0.08 Ω·cm, the OI value M of the negative electrode sheet is 18.2, (P×Z) / (100×M)=(650×15) / (100×18.2)=5.36. The surface density S of the negative electrode sheet is 8.5 mg / cm 2 , the compaction density Y is 1.6 g / cm 3 , the thickness T of the negative electrode current collector is 6 μm, S×Y / T = 8.5×1.6 / 6 = 2.27, and the overhang size OH of the negative electrode sheet is 1 mm.

[0132] Example 2

[0133] (1) Component preparation

[0134] Negative electrode current collector: copper foil (the tensile strength P of the negative electrode current collector is 650 MPa, and the elongation rate Z of the negative electrode current collector is 15%);

[0135] Negative electrode active material layer: Negative electrode active material: 81.2 parts by weight of graphite, 14.3 parts by weight of silicon-carbon material, First binder: 1 part by weight of lithium polyacrylate, 0.5 part by weight of lithium carboxymethyl cellulose, totaling 1.5 parts by weight (the weight content of Li element in the first binder is 3.2%, and the molecular weight of the first binder is 1.1 million), Second binder: styrene-butadiene rubber latex (the elastic modulus of the second binder is 4 MPa, the average particle size of the second binder is 0.21 μm, and the sphericity of the second binder is 0.9), 2 parts by weight, Negative electrode conductive agent: carbon black and carbon nanotubes (the weight ratio of carbon black to carbon nanotubes is 5:1), 1 part by weight.

[0136] (2) The preparation of the negative electrode sheet is carried out with reference to Example 1.

[0137] Among them, A is 3.3%, B is 2.3%, B / (A - B) = 2.3 / (3.3 - 2.3) = 2.3, D1 - D2 = 0.03 Ω·cm, the OI value M of the negative electrode sheet is 18.2, (P×Z) / (100×M) = (650×15) / (100×18.2) = 5.36. The areal density S of the negative electrode sheet is 8.5 mg / cm 2 , and the tap density Y is 1.6 g / cm 3 , the thickness T of the negative electrode current collector is 6 μm, S×Y / T = 8.5×1.6 / 6 = 2.27, and the overhang size OH of the negative electrode sheet is 1 mm.

[0138] Example 3

[0139] Negative electrode current collector: copper foil (the tensile strength P of the negative electrode current collector is 770 MPa, and the elongation rate Z of the negative electrode current collector is 10.4%);

[0140] Negative electrode active material layer: Negative electrode active material: 80 parts by weight of graphite, 14 parts by weight of silicon-carbon material, First binder: lithium polyacrylate (the weight content of Li element in the first binder is 6.7%, and the molecular weight of the first binder is 1.4 million), 2 parts by weight, Second binder: styrene-butadiene rubber latex (the elastic modulus of the second binder is 6.3 MPa, the average particle size of the second binder is 0.76 μm, and the sphericity of the second binder is 0.8), 3 parts by weight, Negative electrode conductive agent: carbon black and carbon nanotubes (the weight ratio of carbon black to carbon nanotubes is 5:1), 1 part by weight.

[0141] (2) The preparation of the negative electrode sheet is carried out with reference to Example 1.

[0142] Among them, A is 5.5%, B is 3.4%, B / (A - B) = 3.4 / (5.5 - 3.4) = 1.62, D1 - D2 = 0.11 Ω·cm, the OI value M of the negative electrode sheet is 28.7, (P×Z) / (100×M) = (770×10.4) / (100×28.7) = 2.79. The areal density S of the negative electrode sheet is 8.5 mg / cm 2 , the tap density Y is 1.3 g / cm 3 , the thickness T of the negative electrode current collector is 9 μm, S×Y / T = 8.5×1.3 / 9 = 1.23, and the size OH of the overhang of the negative electrode sheet is 1 mm.

[0143] Example 4

[0144] (1) Component preparation

[0145] Negative electrode current collector: copper foil (the tensile strength P of the negative electrode current collector is 770 MPa, and the elongation rate Z of the negative electrode current collector is 10.4%);

[0146] Negative electrode active material layer: Negative electrode active material: 79.7 parts by weight of graphite, 14 parts by weight of silicon-carbon material, First binder: 2 parts by weight of lithium polyacrylate, 0.3 parts by weight of lithium carboxymethyl cellulose, a total of 2.3 parts by weight (the weight content of Li element in the first binder is 6.6%, and the molecular weight of the first binder is 1.5 million), Second binder: styrene-butadiene rubber latex (the elastic modulus of the second binder is 6.3 MPa, the average particle size of the second binder is 0.76 μm, and the sphericity of the second binder is 0.8), 3 parts by weight, Negative electrode conductive agent: carbon black and carbon nanotubes (the weight ratio of carbon black to carbon nanotubes is 5:1), 1 part by weight.

[0147] (2) The preparation of the negative electrode sheet is carried out with reference to Example 1.

[0148] Among them, A is 5.3%, B is 3.1%, B / (A - B) = 3.1 / (5.3 - 3.1) = 1.41, D1 - D2 = 0.21 Ω·cm, the OI value M of the negative electrode sheet is 28.7, (P×Z) / (100×M) = (770×10.4) / (100×28.7) = 2.79. The areal density S of the negative electrode sheet is 8.5 mg / cm 2 , the tap density Y is 1.3 g / cm 3 , the thickness T of the negative electrode current collector is 9 μm, S×Y / T = 8.5×1.3 / 9 = 1.23, and the size OH of the overhang of the negative electrode sheet is 1 mm.

[0149] Example 5 group

[0150] Example 5a

[0151] Performed with reference to Example 1, except that the weight portion of the first binder is 1; the weight portion of the second binder is 1.3, A is 2.5%, B is 1.5%, B / (A - B) = 1.5 / (2.5 - 1.5) = 1.5, D1 - D2 = 0.15 Ω·cm.

[0152] Example 5b

[0153] Performed with reference to Example 1, except that the weight portion of the first binder is 2.7; the weight portion of the second binder is 2, A is 5%, B is 2.2%, B / (A - B) = 2.2 / (5 - 2.2) = 0.79, D1 - D2 = 0.22 Ω·cm.

[0154] Example 5c

[0155] Performed with reference to Example 1, except that the weight portion of the first binder is 1.1; the weight portion of the second binder is 1.1, A is 2%, B is 1%, B / (A - B) = 1 / (2 - 1) = 1, D1 - D2 = 0.26 Ω·cm.

[0156] Example 5d

[0157] Performed with reference to Example 1, except that the weight portion of the first binder is 4; the weight portion of the second binder is 2, A is 6%, B is 2.2%, B / (A - B) = 2.2 / (6 - 2.2) = 0.58, D1 - D2 = 0.35 Ω·cm.

[0158] Example 5e

[0159] Performed with reference to Example 1, except that the weight portion of the first binder is 1; the weight portion of the second binder is 2.7, A is 4%, B is 3%, B / (A - B) = 3 / (4 - 3) = 3, D1 - D2 = 0.1 Ω·cm.

[0160] Example 5f

[0161] Performed with reference to Example 1, except that the weight portion of the first binder is 2; the weight portion of the second binder is 3.8, A is 6%, B is 4%, B / (A - B) = 4 / (6 - 4) = 2, D1 - D2 = 0.06 Ω·cm.

[0162] Example 5g

[0163] Performed with reference to Example 1, except that the weight portion of the first binder is 2; the weight portion of the second binder is 5, A is 7%, B is 5.2%, B / (A - B) = 5.2 / (7 - 5.2) = 2.89, D1 - D2 = 0.52 Ω·cm.

[0164] Example 6 Group

[0165] This set of examples is used to illustrate the effects produced when the weight content of Li element in the first binder changes.

[0166] Example 6a

[0167] Carried out with reference to Example 1, the difference is that the weight content of Li element in the first binder is 2.5%, and D1 - D2 = 0.08 Ω·cm.

[0168] Example 6b

[0169] Carried out with reference to Example 1, the difference is that the weight content of Li element in the first binder is 6%, and D1 - D2 = 0.05 Ω·cm.

[0170] Example 6c

[0171] Carried out with reference to Example 1, the difference is that the weight content of Li element in the first binder is 1.5%, and D1 - D2 = 0.07 Ω·cm.

[0172] Example 6d

[0173] Carried out with reference to Example 1, the difference is that the weight content of Li element in the first binder is 8%, and D1 - D2 = 0.06 Ω·cm.

[0174] Example 6e

[0175] Carried out with reference to Example 1, the difference is that the weight content of Li element in the first binder is 0.5%, and D1 - D2 = 0.09 Ω·cm.

[0176] Example 6f

[0177] Carried out with reference to Example 1, the difference is that the weight content of Li element in the first binder is 8.4%, and D1 - D2 = 0.07 Ω·cm.

[0178] Example 7 group

[0179] This set of examples is used to illustrate the effects produced when the molecular weight of the first binder changes.

[0180] Example 7a

[0181] Carried out with reference to Example 1, the difference is that the molecular weight of the first binder is 700,000, and D1 - D2 = 0.22 Ω·cm.

[0182] Example 7b

[0183] Carried out with reference to Example 1, the difference is that the molecular weight of the first binder is 1.3 million, and D1 - D2 = 0.06 Ω·cm.

[0184] Example 7c

[0185] Performed with reference to Example 1, except that the molecular weight of the first binder is 500,000 and D1 - D2 = 0.26 Ω·cm.

[0186] Example 7d

[0187] Performed with reference to Example 1, except that the molecular weight of the first binder is 1,500,000 and D1 - D2 = 0.23 Ω·cm.

[0188] Example 7e

[0189] Performed with reference to Example 1, except that the molecular weight of the first binder is 400,000 and D1 - D2 = 0.35 Ω·cm.

[0190] Example 7f

[0191] Performed with reference to Example 1, except that the molecular weight of the first binder is 1,600,000 and D1 - D2 = 0.4 Ω·cm.

[0192] Example 8 group

[0193] This group of examples is used to illustrate the effects produced when the weight content of carboxymethyl cellulose in the negative electrode active material changes.

[0194] Example 8a

[0195] Performed with reference to Example 2, except that the weight content of carboxymethyl cellulose in the negative electrode active material is 0.2% and D1 - D2 = 0.03 Ω·cm.

[0196] Example 8b

[0197] Performed with reference to Example 2, except that the weight content of carboxymethyl cellulose in the negative electrode active material is 0.7% and D1 - D2 = 0.22 Ω·cm.

[0198] Example 8c

[0199] Performed with reference to Example 2, except that the weight content of carboxymethyl cellulose in the negative electrode active material is 0.5% and D1 - D2 = 0.05 Ω·cm.

[0200] Example 9 group

[0201] This group of examples is used to illustrate the effects produced when the elastic modulus of the second binder changes.

[0202] Example 9a

[0203] Performed in accordance with Example 1, except that the elastic modulus of the second binder is 6 MPa and D1 - D2 = 0.05 Ω·cm.

[0204] Example 9b

[0205] Performed in accordance with Example 1, except that the elastic modulus of the second binder is 2.1 MPa and D1 - D2 = 0.07 Ω·cm.

[0206] Example 9c

[0207] Performed in accordance with Example 1, except that the elastic modulus of the second binder is 7.7 MPa and D1 - D2 = 0.02 Ω·cm.

[0208] Example 9d

[0209] Performed in accordance with Example 1, except that the elastic modulus of the second binder is 1.4 MPa and D1 - D2 = 0.08 Ω·cm.

[0210] Example 10 group

[0211] This group of examples is used to illustrate the effects when (P × Z) / (100 × M) changes.

[0212] Example 10a

[0213] Performed in accordance with Example 1, except that the tensile strength P of the negative electrode current collector is 400 MPa, (P × Z) / (100 × M) = (400 × 15) / (100 × 18.2) = 3.3, and D1 - D2 = 0.09 Ω·cm.

[0214] Example 10b

[0215] Performed in accordance with Example 1, except that the tensile strength P of the negative electrode current collector is 700 MPa, (P × Z) / (100 × M) = (700 × 15) / (100 × 18.2) = 5.77, and D1 - D2 = 0.12 Ω·cm.

[0216] Example 10c

[0217] Performed in accordance with Example 1, except that the tensile strength P of the negative electrode current collector is 800 MPa, (P × Z) / (100 × M) = (800 × 15) / (100 × 18.2) = 6.59, and D1 - D2 = 0.08 Ω·cm.

[0218] Example 10d

[0219] Performed in reference to Example 1, with the difference that the tensile strength P of the negative electrode current collector is 300 MPa, (P×Z) / (100×M) = (300×15) / (100×18.2) = 2.47, and D1 - D2 = 0.06 Ω·cm.

[0220] Example 10e

[0221] Performed in reference to Example 1, with the difference that the elongation rate Z of the negative electrode current collector is 6.3%, (P×Z) / (100×M) = (650×6.3) / (100×18.2) = 2.25, and D1 - D2 = 0.05 Ω·cm.

[0222] Example 10f

[0223] Performed in reference to Example 1, with the difference that the elongation rate Z of the negative electrode current collector is 17.8%, (P×Z) / (100×M) = (650×17.8) / (100×18.2) = 6.36, and D1 - D2 = 0.1 Ω·cm.

[0224] Example 10g

[0225] Performed in reference to Example 1, with the difference that the OI value M of the negative electrode sheet is 15, (P×Z) / (100×M) = (650×15) / (100×15) = 6.5, and D1 - D2 = 0.12 Ω·cm.

[0226] Example 10h

[0227] Performed in reference to Example 1, with the difference that the OI value M of the negative electrode sheet is 25, (P×Z) / (100×M) = (650×15) / (100×25) = 3.9, and D1 - D2 = 0.09 Ω·cm.

[0228] Example 10i

[0229] Performed in reference to Example 1, with the difference that the OI value M of the negative electrode sheet is 10, (P×Z) / (100×M) = (650×15) / (100×10) = 9.75, and D1 - D2 = 0.08 Ω·cm.

[0230] Example 10j

[0231] Performed in reference to Example 1, with the difference that the OI value M of the negative electrode sheet is 40, (P×Z) / (100×M) = (650×15) / (100×40) = 2.44, and D1 - D2 = 0.06 Ω·cm.

[0232] Example 10k

[0233] It is carried out with reference to Example 1, except that the tensile strength P of the negative electrode current collector is 900 MPa, the elongation rate Z of the negative electrode current collector is 20%, the OI value M of the negative electrode sheet is 15, (P×Z) / (100×M) = (900×20) / (100×15) = 12, and D1 - D2 = 0.07 Ω·cm.

[0234] Example 10l

[0235] It is carried out with reference to Example 1, except that the elongation rate Z of the negative electrode current collector is 6.3%, the OI value M of the negative electrode sheet is 40, (P×Z) / (100×M) = (650×6.3) / (100×40) = 1.02, and D1 - D2 = 0.08 Ω·cm.

[0236] Example 10m

[0237] It is carried out with reference to Example 1, except that the tensile strength P of the negative electrode current collector is 900 MPa, the elongation rate Z of the negative electrode current collector is 10%, the OI value M of the negative electrode sheet is 45, (P×Z) / (100×M) = (900×10) / (100×45) = 2, and D1 - D2 = 0.1 Ω·cm.

[0238] Example 11 group

[0239] This group of examples is used to illustrate the weight content of silicon element in the negative electrode active material layer.

[0240] Example 11a

[0241] It is carried out with reference to Example 1, except that the weight content of silicon element in the negative electrode active material layer is 50%.

[0242] Example 11b

[0243] It is carried out with reference to Example 1, except that the weight content of silicon element in the negative electrode active material layer is 60%.

[0244] Comparative Example 1

[0245] Negative electrode current collector: copper foil (the tensile strength P of the negative electrode current collector is 650 MPa, and the elongation rate Z of the negative electrode current collector is 15%);

[0246] Negative electrode active material layer: Negative electrode active material: 81.2 parts by weight of graphite, 14.3 parts by weight of silicon-carbon material, 2 parts by weight of styrene-butadiene rubber latex (the elastic modulus of the styrene-butadiene rubber latex is 4 MPa, the average particle size of the styrene-butadiene rubber latex is 0.21 μm, and the sphericity of the styrene-butadiene rubber latex is 0.9), 1.5 parts by weight of lithium carboxymethyl cellulose (the weight content of Li element in lithium carboxymethyl cellulose is 2%), Negative electrode conductive agent: carbon black and carbon nanotubes (the weight ratio of carbon black to carbon nanotubes is 5:1), 1 part by weight.

[0247] (2) The preparation of the negative electrode sheet was carried out with reference to Example 1.

[0248] Among them, D1 - D2 = 0.5 Ω·cm.

[0249] Comparative Example 2

[0250] It was carried out with reference to Example 1, the difference being that the parts by weight of the first binder was 4; the parts by weight of the second binder was 1, A was 5, B was 1.2, B / (A - B) = 1.2 / (5 - 1.2) = 0.32, D1 - D2 = 0.42 Ω·cm.

[0251] Comparative Example 3

[0252] It was carried out with reference to Example 1, the difference being that the parts by weight of the first binder was 1; the parts by weight of the second binder was 3, A was 4, B was 3.2, B / (A - B) = 3.2 / (4 - 3.2) = 4, D1 - D2 = 0.6 Ω·cm.

[0253] Comparative Example 4

[0254] It was carried out with reference to Example 1, the difference being that the weight content of silicon element in the negative electrode active material layer was 70%.

[0255] Preparation Example

[0256] The negative electrode sheets prepared in the examples and comparative examples were used to prepare lithium ion secondary batteries respectively in the following manner.

[0257] (1) Positive electrode sheet

[0258] The positive electrode active material (29.1 parts by weight of single crystal LiNi 0.9 Co 0.04 Mn 0.04 Al 0.02 O2, 67.9 parts by weight of polycrystalline LiNi 0.92 Co 0.02 Mn 0.04 Al 0.02O2), a cathode binder (polyvinylidene fluoride (PVDF), 1 part by weight), and a cathode conductive agent (1 part by weight of conductive carbon black and 1 part by weight of carbon nanotubes) were mixed and uniformly dispersed by high-speed stirring to obtain a mixture. The mixture was made into a cathode active material slurry using N-methylpyrrolidone as a solvent, and the solid content in the slurry was 70 wt%. The slurry was uniformly coated on both sides of the current collector, and after drying and rolling, a cathode sheet was obtained.

[0259] (2) Anode sheet

[0260] Anode sheets prepared using the above-mentioned examples and comparative examples were used respectively.

[0261] (3) Electrolyte

[0262] In a glove box filled with argon (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvents were uniformly mixed, and then a sufficiently dried lithium salt was quickly added thereto, and an additive was added, and stirred for 10 h to fully dissolve it, and the required electrolyte could be obtained. Among them, the lithium salt: lithium hexafluorophosphate (LiPF6), 15 parts by weight; organic solvents: 80 parts by weight, wherein the weight ratio of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DMC), and dimethyl carbonate (DEC) = 6:22:26:26; additive: 5 parts by weight of fluoroethylene carbonate.

[0263] (4) Separator

[0264] PP (7 μm) porous layer (thickness 2 μm, PVDF + alumina) was selected.

[0265] (5) Lithium-ion secondary battery

[0266] The cathode sheet in step (1), the separator in step (4), and the anode sheet in step (2) were wound to form a stacked core or a wound core, and after welding, casing, encapsulation, injecting the electrolyte in step (3), forming, and sorting, a lithium-ion secondary battery was obtained.

[0267] Test example

[0268] The batteries prepared in the examples and comparative examples were tested as follows:

[0269] (1) Test of OI value

[0270] The anode sheet was cut into a suitable size, attached to a sample plate, and the X-ray diffraction pattern of the anode was obtained by using an X-ray diffractometer. The excitation source of the X-ray diffractometer was CuKα, the scanning angle range was 10° - 90°, and the scanning speed was 2° / min. The OI value was the ratio of the peak area of (004) to the peak area of (110). The sample was subjected to thermogravimetric testing under the above conditions.

[0271] (2) Thermogravimetric (TG) Test

[0272] After removing the electrode from the fully discharged battery cell, soak it in solvent DMC for 8 h and rinse it with DMC multiple times. Then, dry it in vacuum at 80 °C for 12 h. Scrape the paste from the current collector and collect the paste powder as the sample for the thermogravimetric test. The thermogravimetric test is carried out under a nitrogen atmosphere, with the temperature range from room temperature to 800 °C and the heating rate of 10 °C / min. Conduct the thermogravimetric test on the sample under the above conditions.

[0273] (3) Room Temperature Cycling Test

[0274] At 25 °C, perform 1C / 1C charge-discharge cycling within the charge-discharge window of 4.25 V to 2.5 V. The test process is as follows: First, charge at a constant current of 1C to 4.25 V, then charge at a constant voltage with a cut-off current of 0.05C, and finally discharge at a constant current of 1C to 2.5 V. Conduct such cyclic tests. Record the thickness of the battery cell at the first discharge as H1. Record the number of cycles when the ratio of the discharge capacity to the first discharge capacity (capacity retention rate) reaches 80% in Table 1. At the same time, record the thickness H2 of the fully charged battery cell at the end of the cycle. The battery cell cycle expansion rate = [(H2 - H1) / H1] × 100%.

[0275] (4) Overcharge Test

[0276] At 25 °C, charge the battery at a constant current of 2C to 4.25 V, then charge at a constant voltage with a cut-off current of 0.05C. After leaving it standing for 2 h, charge the battery at a constant current of 1C until the voltage reaches 5.53 V or the time reaches 1 h to stop the test; Observe for 1 h or until the highest temperature on the battery surface drops to the peak temperature of 10 °C and below to stop the test. Observe whether the sample battery catches fire, explodes or leaks during the test. If none of these three situations occur, it means passing the test. If at least one of the above three situations occurs, it means failing the test. A total of 10 battery samples are tested, and the results are expressed as "the number of samples passing the test / 10". For example, "1 / 10" means that only 1 out of 10 battery samples passes the test.

[0277] (5) Discharge Rate Test (5C / 0.1C)

[0278] At 25 °C, charge the battery cell at a constant current and constant voltage to 4.2 V (cut-off current is 0.05C); Let it stand for 0.5 h and discharge at a constant current of nC to 2.5 V (n = 0.1, 5), and record the discharge capacity at different discharge rates. The 5C rate performance = (discharge capacity at 5C / discharge capacity at 0.1C) × 100%.

[0279] (6) Lithium Deposition Test

[0280] At room temperature, the battery cells nC (n = 1, 2, 3, 4, 5) are charged to 4.2V and then charged at a constant voltage, with a cut-off current of 0.05C, and left standing for 30 minutes; discharged at 1C to 2.5V and left standing for 30 minutes; such charge and discharge cycles are repeated 20 times, and finally the battery cells nC are charged at a constant current and constant voltage to 4.2V to end the test. Then, the fully charged battery cells are dissected to observe whether lithium plating occurs on the negative electrode side under different DC charging rates, and the maximum charging rate for testing is 5C. The results are expressed in terms of the lithium plating window and the lithium plating situation. For example, if lithium plating starts at 4C, the result is expressed as "lithium plating at 4C", and if there is no lithium plating at 5C, the result is expressed as "no lithium plating at 5C".

[0281] Record the obtained results in Table 2.

[0282] Table 2

[0283]

[0284]

[0285]

[0286] It can be seen from Table 1 that by comparing the comparative examples and the examples, it can be seen that the lithium plating window of the battery made from the negative electrode sheet of the examples is significantly widened, the overcharge test performance is improved, the number of cycles with a capacity retention rate of 80% is significantly increased, and the thickness expansion rate is significantly reduced. This shows that by the cooperation of the first binder and the second binder and controlling the relationship between the weight loss rate of the weight loss peak of the negative electrode sheet at 0% SOC after being soaked in DMC or DMC and deionized water, the cycle stability, fast charging performance and safety performance of the battery are improved.

[0287] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope 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 lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises a positive electrode sheet and a negative electrode sheet, wherein the negative electrode sheet has a larger dimension in the width direction than the positive electrode sheet, and an area where one side edge of the negative electrode sheet exceeds the positive electrode sheet in the width direction is an overhang area, and a width OH of the overhang area is 0.5 mm to 2 mm; The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer located on one side or both sides of the negative electrode current collector, the negative electrode active material layer comprises a negative electrode active material and a second binder, the negative electrode active material comprises a silicon material, the silicon material comprises silicon element, based on the total weight of the negative electrode active material layer, the content of the silicon element is 0-60%, and the sphericity of the second binder is 0.8-1; Then the negative electrode sheet satisfies the following relationship: 0.5≤B / (AB)≤3.5, Among them, A is the weight loss rate of the weight loss peak located at 350°C-650°C in the thermogravimetric analysis spectrum of the negative electrode active material layer after the negative electrode sheet is soaked in DMC at 0% SOC, and the unit is wt%; B is the weight loss rate of the weight loss peak located at 350°C-650°C in the thermogravimetric analysis spectrum of the negative electrode active material layer after the negative electrode sheet is soaked in DMC and deionized water at 0% SOC, and the unit is wt%; the thermogravimetric analysis spectrum is obtained by performing thermogravimetric analysis on the negative electrode active material layer at a heating rate of 10°C / min in a nitrogen atmosphere.

2. The lithium ion secondary battery according to claim 1, wherein The negative electrode sheet satisfies the following relationship: 1≤B / (AB)≤2.5; and / or, A wt% is 2wt%-6wt%, preferably 2.5wt%-5wt%; and / or, B wt% is 1wt%-4wt%, preferably 1.5wt%-3wt%; And / or, the negative electrode active material layer includes a first binder, and the first binder includes one or more of polyacrylic acid, polyacrylic acid salt, polyacrylonitrile, polyacrylamide and copolymers thereof; and / or, the second binder comprises one or more of styrene-butadiene rubber, styrene-acrylate copolymer, acrylate copolymer and polyurethane; and / or, the glass transition temperature of the first binder is 60° C.-150° C.; And / or, the glass transition temperature of the second binder is -40°C to 40°C.

3. The lithium ion secondary battery according to claim 2, wherein: The first binder includes an alkali metal element, and the weight content of the alkali metal element is 1.5%-8%, preferably 2.5%-6%, based on the total weight of the first binder; and / or, the molecular weight of the first binder is 500,000-1.5 million, preferably 700,000-1.3 million; and / or, the elastic modulus of the second binder is 2MPa-8MPa, preferably 3MPa-6MPa; And / or, the average particle size of the second binder is 0.06 μm-0.8 μm, preferably 0.1 μm-0.6 μm.

4. The lithium ion secondary battery according to claim 3, wherein: At 0% SOC, the weight content of Li element in the negative electrode active material layer is ≥400ppm, and the weight content of sodium element is <1%; The first binder further comprises sodium carboxymethyl cellulose and / or lithium carboxymethyl cellulose, and the weight content of the sodium carboxymethyl cellulose and / or lithium carboxymethyl cellulose is 0%-0.7% based on the total weight of the negative electrode active material layer; And / or, the negative electrode sheet satisfies the following relationship: 0≤D1-D2≤0.3Ω·cm, wherein D1 is the maximum conductivity, in units of Ω·cm, and D2 is the minimum conductivity, in units of Ω·cm.

5. The lithium ion secondary battery according to claim 4, wherein: The negative electrode sheet satisfies the following relationship at the same time: 10≤P / M≤70, 0.2≤Z / M≤1.5, wherein P is the tensile strength of the negative electrode current collector, in MPa, Z is the elongation of the negative electrode current collector, in %, and M is the OI value of the negative electrode sheet; Preferably, the negative electrode sheet satisfies the following relationship simultaneously: 20≤P / M≤40, 0.5≤Z / M≤1.

6. The lithium ion secondary battery according to claim 5, wherein: The tensile strength P of the negative electrode current collector is 300MPa-800MPa; And / or, the elongation Z% of the negative electrode current collector is 3%-18%; And / or, the OI value M of the negative electrode sheet is 10-40, preferably 15-25.

7. The lithium ion secondary battery according to claim 1, wherein The silicon material includes one or more of silicon-carbon material, silicon-oxygen material, silicon element and silicon alloy; And / or, the negative electrode active material further comprises a carbon material, wherein the carbon material comprises one or more of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon and soft carbon; and / or, the average particle size of the silicon material is 5 μm-14 μm, preferably 7 μm-12 μm; And / or, the specific surface area of ​​the silicon material is 0.8m 2 / g-8m 2 / g, preferably 1m 2 / g-6m 2 / g.

8. The lithium ion secondary battery according to claim 7, wherein: The silicon-carbon material includes a material in which silicon particles are filled in pores of porous amorphous carbon or porous crystalline carbon, and / or a material in which silicon particles are mixed with amorphous carbon or crystalline carbon.

9. The lithium ion secondary battery according to any one of claims 1 to 8, wherein: The surface density of the negative electrode sheet is Smg / cm 2 6mg / cm 2 -15mg / cm 2 ; And / or, the compaction density Y of the negative electrode sheet is 1.3 g / cm 3 -1.8g / cm 3 ; And / or, the thickness T μm of the negative electrode current collector is 5 μm-10 μm.

10. The lithium ion secondary battery according to claim 1, wherein The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer located on both sides of one side of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active substance. The positive electrode active substance includes a ternary material. The chemical formula of the ternary material is Li a Ni x Co y Mn z A k O2, wherein 0.9≤a≤1.1, 0.5≤x≤0.95, 0<y≤0.2, 0<z≤0.3, 0≤k≤0.05, A is a doping element, and A includes one or more of Al, Mg, Ti, Zr, B, P and Y; Preferably, the ternary material comprises a single crystal structure and / or a polycrystalline structure.

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