Lithium ion battery

Abstract

The invention relates to the technical field of energy storage lithium ion batteries, in particular to a lithium ion battery which comprises a negative plate, the negative plate comprises a negative active material, the negative active material comprises a graphite matrix and a coating layer, and the coating layer comprises an amorphous carbon material; the amorphous carbon material comprises soft carbon and / or hard carbon; based on the total mass of the negative electrode active material, the mass content of the coating layer is 0.5-5%; when the negative electrode active material is measured by a Raman spectrometer, the intensity ratio of the peak intensity Id to the peak intensity Ig meets the condition that Id / Ig is more than or equal to 0.1 and less than or equal to 0.8. The lithium ion battery provided by the invention has relatively good charge-discharge cycle performance and safety performance in a low-temperature scene.

Description

Technical Field

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

[0002] A lithium-ion battery is an electrochemical energy storage device. During the charging process, electrical energy is converted into chemical energy, and during discharging, chemical energy is converted into electrical energy. Therefore, lithium-ion batteries can store electrical energy for repeated use and can currently be used as an energy storage device in life scenarios. However, the lithium-ion battery energy storage device has certain limitations on the temperature of the usage scenario. When used in a low-temperature scenario (below 0°C), the charge-discharge cycle performance of the battery becomes poor, and lithium plating is likely to occur. In severe cases, it will cause safety problems such as battery short circuit, fire, and even explosion. Therefore, there is an urgent need to develop a lithium-ion battery that can be normally charged and discharged without lithium plating in a low-temperature scenario. Summary of the Invention

[0003] The purpose of the present invention is to overcome the above problems existing in the prior art and provide a lithium-ion battery. The lithium-ion battery of the present invention has good charge-discharge cycle performance and safety performance in a low-temperature scenario.

[0004] The first aspect of the present invention provides a lithium-ion battery, and the lithium-ion battery includes a negative electrode sheet;

[0005] The negative electrode sheet includes a negative electrode active material, the negative electrode active material includes a graphite matrix and a coating layer, and the coating layer includes an amorphous carbon material;

[0006] The amorphous carbon material includes soft carbon and / or hard carbon;

[0007] Based on the total mass of the negative electrode active material, the mass content of the coating layer is 0.5%-5%;

[0008] The negative electrode active material is measured by a Raman spectrometer, and the peak intensity I -1 in the range of 1300 cm -1 -1400 cm d and the peak intensity I -1 in the range of 1550 cm -1 -1650 cm g satisfy the following intensity ratio: 0.1 ≤ I d / I g ≤ 0.8.

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

[0010] (1) By coating an amorphous carbon material coating on the surface of the graphite matrix, the present invention can improve the charge-discharge cycle performance and safety performance of lithium-ion batteries in low-temperature scenarios;

[0011] (2) By adjusting the mass content of the coating layer and the uniformity of the coated amorphous carbon material, the present invention can improve the high-temperature performance of lithium-ion batteries while improving the low-temperature performance of lithium ions.

[0012] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, 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 It is the SEM image of the surface of the negative electrode sheet of the lithium-ion battery prepared in Example 1 of the present invention;

[0014] Figure 2 It is the capacity retention rate test chart of the lithium-ion battery prepared in Example 1 of the present invention when cycled 300 times at -20 °C and 0.2C / 0.2C;

[0015] Figure 3 It is the process capability report chart of the peak intensity of I d / I g of the negative electrode active material prepared in Example 1 of the present invention. Detailed Embodiments

[0016] The following details the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not used to limit the present invention.

[0017] Unless otherwise defined, all scientific and technical terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention pertains.

[0018] Lithium-ion battery energy storage devices are commonly used in room temperature (25 °C) and high temperature (45 °C) scenarios. For use in low-temperature scenarios (below 0 °C), their charge-discharge performance and cycle performance will become poor. Especially when charging it in a low-temperature scenario, at this time, due to the low temperature, the electrochemical reaction polarization is large, and the potential of the negative electrode will be low during charging, even lower than the potential of lithium metal precipitation (Potential Li / Li +Less than 0 V), which easily causes lithium plating; moreover, since the precipitation of lithium metal often exists in the form of metal dendrites, when the negative electrode expands greatly, the lithium dendrites are likely to pierce the separator, resulting in battery short circuit and triggering safety problems such as fire and explosion.

[0019] Therefore, to solve the problem of normal charge and discharge without lithium plating of lithium-ion batteries in low-temperature scenarios, it is necessary to optimize the design of the positive and negative electrodes and the electrolyte of the battery, so as to improve the charge and discharge polarization of lithium-ion batteries, reduce the risk of lithium plating during charge and discharge, and improve its safety performance.

[0020] Based on the above problems, the first aspect of the present invention provides a lithium-ion battery, which can improve the charge and discharge polarization of lithium-ion batteries in low-temperature scenarios, enhance the low-temperature cycle performance of lithium-ion batteries, and can reduce the risk of lithium plating during charge and discharge and improve its safety performance.

[0021] In the present invention, the lithium-ion battery includes a negative electrode; the negative electrode includes a negative electrode active material, the negative electrode active material includes a graphite matrix and a coating layer, and the coating layer includes an amorphous carbon material; the amorphous carbon material includes soft carbon and / or hard carbon;

[0022] Based on the total mass of the negative electrode active material, the mass content of the coating layer is 0.5%-5%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%;

[0023] The negative electrode active material is measured by a Raman spectrometer, and the peak intensity I -1 -1400 cm -1 in the range d and the peak intensity I -1 -1650 cm -1 in the range g satisfy: 0.1 ≤ I d / I g ≤ 0.8, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or 0.8.

[0024] Graphite materials have the characteristics of anisotropy, with large impedance and polarization during low-temperature charging. By coating a coating layer of amorphous carbon material on the surface of the graphite matrix, the lithium-ion insertion active sites on its surface can be increased, and the isotropy of the graphite matrix material can be enhanced, thereby greatly reducing the lithium-ion insertion impedance, improving the negative electrode polarization, and enhancing the low-temperature charging performance of the negative electrode, and further enabling the lithium-ion battery to have good charge and discharge cycle performance and safety performance in low-temperature scenarios.

[0025] However, the amorphous carbon material coated on the surface of the graphite matrix will increase its electrochemical reaction activity, and side reactions will occur in high-temperature scenarios. In order to improve the high-temperature performance of lithium-ion batteries while improving the low-temperature performance of lithium ions, the present invention further adjusts the mass content of the coating layer to be 1%-5% based on the total mass of the negative electrode active material, so as to avoid excessive mass content of the amorphous carbon material and affect the high-temperature performance of lithium-ion batteries. In one example, based on the total mass of the negative electrode active material, the mass content of the coating layer is 2.5%-4.5%.

[0026] Among them, based on the total mass of the negative electrode active material, the mass content of the coating layer can be measured by thermogravimetric analysis (TGA). Specifically, the mass changes of the negative electrode active material layer of the negative electrode sheet during heating are measured to distinguish the mass content of different components. Among them, the principle is as follows: amorphous carbon materials and graphite matrices will have different thermal decomposition behaviors during heating. By analyzing the thermogravimetric curve, the mass changes of the two can be distinguished, so as to determine the mass content of different components, and then the mass content of the coating layer in the negative electrode active material can be calculated.

[0027] Furthermore, although controlling the mass content of the coating layer can avoid the aggravation of side reactions of excessive amorphous carbon materials in high-temperature scenarios, if the uniformity of the amorphous carbon material coated on the surface of the graphite matrix is low, it will also lead to the occurrence of side reactions of the negative electrode active material. Therefore, the present invention uses a Raman spectrometer to measure the negative electrode active material, so that the peak intensity I -1 -1400cm -1 in the range satisfies: 0.1 ≤ I d / I -1 -1650cm -1 in the range satisfies: 0.1 ≤ I g / I d / I g ≤ 0.8, which can further improve the uniformity of the amorphous carbon material of the coating layer on the surface of the graphite matrix, thereby balancing the charge and discharge performance of the negative electrode active material at high and low temperatures, and improving the high and low temperature cycle performance and safety performance of lithium-ion batteries.

[0028] It should be noted that the peak intensity I -1 -1400cm -1 of the D peak in the range of Raman testing can represent the defect degree on the surface of the negative electrode active material, and the peak intensity I d of the G peak in the range of 1550cm -1 -1650cm -1 can represent the graphitization degree of the negative electrode active material. gIt can represent the graphitization degree of the negative electrode active material. When measuring the negative electrode active material with a Raman spectrometer, specifically using Raman mapping, several groups of point tests will be carried out on the surface of the negative electrode active material, and a lot of peak intensity data of Raman tests will be statistically analyzed. Therefore, the peak intensity I d and the peak intensity I g The intensity ratio of can be used to evaluate the coating uniformity on the surface of the graphite matrix particles. When the I d / I g value of the Raman test is less than 0.1, it indicates that the degree of defects on the surface of the negative electrode active material is low, representing that the content of amorphous carbon coated on the surface of the graphite matrix is less or there is no coating. In this way, the polarization of the graphite particles during low-temperature charge and discharge is large, and the low-temperature charge cycle performance is poor. When the I d / I g value of the Raman test is greater than 0.8, it indicates that the degree of defects on the surface of the negative electrode active material is high, and the content of amorphous carbon coated on the surface is large, which will lead to a decrease in the capacity, tap density and initial efficiency of the negative electrode active material. At the same time, too many defects on the surface of the graphite matrix will also cause the side reactions to intensify in a high-temperature environment, resulting in performance deterioration. Therefore, it is necessary to reasonably control the range of the I d / I g value of the Raman test, improve the uniformity of the amorphous carbon material of the coating layer on the surface of the graphite matrix, so as to balance the high and low temperature charge and discharge performance of the negative electrode active material, and improve the high and low temperature cycle performance and safety performance of the lithium-ion battery.

[0029] In the present invention, the specific Raman test method for measuring the negative electrode active material with a Raman spectrometer is as follows: using an in-situ micro laser confocal Raman spectrometer, the model of which is HORIBA HR800, and equipped with four lasers with wavelengths of 325nm, 532nm, 633nm, and 785nm respectively. The Raman shift range is 100cm -1 -4000cm -1 , the peak position of the D peak is 1300cm -1 -1400cm -1 , the peak position of the G peak is 1550cm -1 -1650cm -1 , the diffraction intensity of the D peak is represented by I d , and the diffraction intensity of the G peak is represented by I g .

[0030] In an example, the peak intensity I d and the peak intensity I g satisfy: 0.3 ≤ I d / I g ≤ 0.6.

[0031] In an example, the peak intensity I dwith the peak intensity I g The numerical distribution ≥ 90%. The peak intensity I d and the peak intensity I g The numerical distribution refers to the distribution of the Raman test data of the peak intensity I d and the peak intensity I g . When the numerical distribution ≥ 90%, it represents that the peak intensity data distribution is relatively concentrated, indicating that the coating layer on the surface of the graphite matrix particles is more uniform, and the amorphous carbon material is more evenly distributed on the graphite matrix surface. Therefore, by controlling the numerical distribution of the peak intensity I d and the peak intensity I g ≥ 90%, the charge and discharge performance of the negative electrode active material at high and low temperatures can be further balanced, and the high and low temperature cycle performance and safety performance of the lithium-ion battery can be improved.

[0032] In one example, the numerical distribution of the peak intensity I d and the peak intensity I g ≥ 95%.

[0033] In the present invention, the Dv50 of the negative electrode active material is 5 μm - 20 μm, for example, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm. The Dv50 of the negative electrode active material refers to the particle size when the cumulative volume distribution of the negative electrode active material accounts for 50%. When the Dv50 of the negative electrode active material is less than 5 μm, the Dv50 particle size of the negative electrode active material is small, and the migration path of lithium ions embedded in the negative electrode active material particles will become shorter, which helps to reduce the impedance inside the negative electrode. However, there are more defects on the surface of the negative electrode active material with small particle size, and the electrochemical reaction activity is too high, which will lead to a decrease in the capacity and compaction density of the negative electrode, thus easily affecting the high temperature cycle and high temperature storage performance of the lithium-ion battery; when the Dv50 of the negative electrode active material is greater than 20 μm, the Dv50 particle size of the negative electrode active material is large, and the migration path of lithium ions embedded in the negative electrode active material particles will become longer, resulting in an increase in the impedance inside the negative electrode, which is not conducive to the improvement of the charging ability at low temperatures. Therefore, by controlling the Dv50 of the negative electrode active material within a suitable range, the charge and discharge performance of the negative electrode active material at high and low temperatures can be further balanced, and the high and low temperature cycle performance and safety performance of the lithium-ion battery can be improved. The Dv50 of the negative electrode active material can be measured using a Malvern particle size analyzer. The test steps are as follows: Disperse the negative electrode active material particles in deionized water containing a dispersant (such as nonylphenol polyoxyethylene ether, with a content of 0.02 wt% - 0.03 wt%) to form a mixture, ultrasonicate the mixture for about 2 minutes, and then put it into the Malvern particle size analyzer for testing to obtain the median particle size Dv50 of the negative electrode active material particles.

[0034] In one example, the Dv50 of the negative electrode active material is 8 μm - 15 μm.

[0035] In the present invention, the Dv90 / Dv10 of the negative electrode active material is 2.5 - 3.5, for example, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4 or 3.5. The Dv90 and Dv10 of the negative electrode active material respectively represent the particle sizes when the cumulative volume distribution of the negative electrode active material accounts for 90% and 10%. Therefore, by controlling the Dv90 / Dv10 of the negative electrode active material within a suitable range, the uniformity of the particle size distribution of the negative electrode active material can be improved, the particle size of the particles with a cumulative volume distribution of 90% can be prevented from being too small, the capacity and tap density of the negative electrode active material can be increased, and at the same time, the particle size of the particles with a cumulative volume distribution of 90% can be prevented from being too large, improving the charging performance at low temperatures; in addition, by controlling the Dv90 / Dv10 of the negative electrode active material within a suitable range, the specific surface area of the negative electrode active material can also be reduced, reducing the occurrence of side reactions of the negative electrode active material, thereby further improving the high-temperature performance of the graphite matrix negative electrode active material. When the Dv90 / Dv10 is within a certain range, the size of the Dv10 can also be controlled to prevent the proportion of small particles in the negative electrode active material particles from being large due to a small Dv10. The surface activity of the small-particle negative electrode active material is high, which will affect the high-temperature performance of the negative electrode active material.

[0036] In one example, the Dv90 / Dv10 of the negative electrode active material is 2.8 - 3.3.

[0037] In the present invention, the Dv10 of the negative electrode active material is 2 μm - 10 μm, for example, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.

[0038] In one example, the Dv10 of the negative electrode active material is 3 μm - 8 μm.

[0039] In the present invention, the Dv90 of the negative electrode active material is 7 μm - 35 μm, for example, 7 μm, 8 μm, 9 μm, 10 μm, 13 μm, 15 μm, 17 μm, 19 μm, 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, 30 μm, 31 μm, 33 μm or 35 μm.

[0040] In one example, the Dv90 of the negative electrode active material is 10 μm - 25 μm.

[0041] In the present invention, Dn10 of the negative electrode active material is ≥ 1.0 μm, such as 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, 3.8 μm, 3.9 μm, 4 μm, 4.1 μm, 4.2 μm, 4.3 μm, 4.4 μm, 4.5 μm, 4.6 μm, 4.7 μm, 4.8 μm, 4.9 μm or 5 μm. Dn10 of the negative electrode active material refers to the particle size of the negative electrode active material when the cumulative quantity distribution accounts for 10%. When Dn10 of the negative electrode active material is small, it means that the proportion of small particles in the negative electrode particles is relatively large, which will increase the content of fine powder in the negative electrode. The electrochemical reaction activity of small particles is strong, which will deteriorate the high-temperature performance of the lithium battery. Therefore, it is necessary to control Dn10 of the negative electrode active material ≥ 1.0 μm.

[0042] However, if the particle size of the negative electrode active material is too large, it is not conducive to improving the low-temperature charging ability.

[0043] In one example, Dn10 of the negative electrode active material is 2.5 μm - 5.0 μm.

[0044] It should be noted that the particle sizes Dv50, Dv90, Dv10, Dn10 of the above negative electrode active material can be measured by the laser particle size method, and the instrument model used is Mastersizer 3000.

[0045] In the present invention, the specific surface area of the negative electrode active material is 0.5 m 2 / g - 1.5 m 2 / g, such as 0.5 m 2 / g, 0.6 m 2 / g, 0.7 m 2 / g, 0.8 m 2 / g, 0.9 m 2 / g, 1 m 2 / g, 1.1 m 2 / g, 1.2 m 2 / g, 1.3 m 2 / g, 1.4 m 2 / g or 1.5 m 2 / g. When the specific surface area of the negative electrode active material is too large, it will lead to an increase in surface defects of the material, an increase in the electro-chemical reaction activity, and an intensification of side reactions at high temperatures, resulting in the deterioration of the charging performance; when the specific surface area of the negative electrode active material is relatively low, the side reactions at high temperatures can be reduced, but the specific surface area of the negative electrode active material should not be too low, as it is likely to cause a significant drop in the battery capacity. The specific surface area of the negative electrode active material can be measured using the TriStar II 3020Plus high-throughput specific surface area and pore size analyzer produced by Micromeritics, USA.

[0046] In one example, the specific surface area of the negative electrode active material is 0.8 m 2 / g - 1.4 m 2 / g.

[0047] In the present invention, the specific capacity of the negative electrode active material is 320 mAh / g - 355 mAh / g, for example, 320 mAh / g, 325 mAh / g, 330 mAh / g, 335 mAh / g, 340 mAh / g, 345 mAh / g, 350 mAh / g or 355 mAh / g. When the specific capacity of the negative electrode active material is relatively low, with the thickness of the positive electrode remaining unchanged, the thickness of the negative electrode side increases, the low-temperature charging performance deteriorates, and the energy density of the lithium-ion battery will be reduced; when the specific capacity of the negative electrode active material is relatively high, the degree of graphitization of the negative electrode active material is high, but its fast charging ability will be somewhat reduced, which is not conducive to the improvement of the low-temperature charging ability.

[0048] In one example, the specific capacity of the negative electrode active material is 340 mAh / g - 353 mAh / g.

[0049] In the present invention, in the XRD pattern of the negative electrode sheet, the intensity of the diffraction peak of the (004) crystal plane where the diffraction angle 2θ is located at 54 ± 1° is D004, and the intensity of the diffraction peak of the (110) crystal plane where the diffraction angle 2θ is located at 78 ± 1° is D110. D004 and D110 satisfy: 1.0 ≤ D004 / D110 ≤ 10.0, for example, 1.0, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5 or 10.0. The XRD pattern of the negative electrode sheet is obtained using Thermo Scientific TM ARL TMTest with EQUINOX Pro vertical X-ray diffractometer, the diffraction angle range is 10 - 90°. The ratio of D004 / D110 represents the ratio of the intensity of the graphite crystal plane parallel to the negative current collector to the intensity of the graphite crystal plane perpendicular to the negative current collector. When D004 / D110 is larger, it means the intensity of the crystal plane parallel to the negative current collector is high, then the isotropy of the graphite matrix is poor, and the charge polarization in the negative electrode sheet is large, which is not conducive to the improvement of low-temperature charging ability. While when D004 / D110 is smaller, it means the intensity of the graphite crystal plane perpendicular to the negative current collector is high, and the isotropy of the graphite matrix is good, but this will reduce the peeling force between the graphite matrix particles and the negative current collector, and there is a risk of active material falling off from the surface of the current collector in the negative electrode sheet, which is not conducive to the long cycle performance of the lithium-ion battery.

[0050] In one example, 3.0 ≤ D004 / D110 ≤ 7.0.

[0051] In the present invention, the tap density range of the negative electrode sheet is 1.2 g / cm 3 - 1.65 g / cm 3 , for example, 1.2 g / cm 3 , 1.25 g / cm 3 , 1.3 g / cm 3 , 1.35 g / cm 3 , 1.4 g / cm 3 , 1.45 g / cm 3 , 1.5 g / cm 3 , 1.55 g / cm 3 , 1.6 g / cm 3 or 1.65 g / cm 3 . By controlling the tap density of the negative electrode sheet within this range, the porosity of the negative electrode sheet can be relatively high, which is conducive to the infiltration of the electrolyte, thereby improving the ion transport efficiency of the negative electrode sheet and further improving the cycle performance of the lithium-ion battery.

[0052] In one example, the tap density range of the negative electrode sheet is 1.3 g / cm 3 - 1.55 g / cm 3 .

[0053] In the present invention, the negative electrode sheet includes a negative electrode binder, and the negative electrode binder includes a lithiated acrylic acid copolymer (PAA). The lithiated acrylic acid copolymer contains more lithium, which can serve as active sites for ion conduction, providing more transport sites for lithium ions at low temperatures, thereby being able to reduce the lithium ion migration impedance, reduce the negative electrode polarization, and improve the charge and discharge performance in a low-temperature environment.

[0054] In one example, the monomers of the lithiated acrylic acid copolymer include at least two monomers among acrylic acid, acrylonitrile, acrylamide, vinyl alcohol, and vinylidene fluoride.

[0055] In the present invention, the mass content of lithium element in the lithiated acrylic acid copolymer is 1.5% - 4.0%, such as 1.5%, 2%, 2.5%, 3%, 3.5%, 4.0%. The mass content of lithium element in the lithiated acrylic acid copolymer can be obtained by testing the surface of the negative electrode sheet through Inductively Coupled Plasma Optical Emission Spectrometer (ICP - OES) technology. Specifically, first, obtain the mass content of lithium element in the negative electrode active material layer on the surface of the negative electrode sheet, and then, through the proportion of the lithiated acrylic acid copolymer in the negative electrode active material layer, the mass content of lithium element in the lithiated acrylic acid copolymer can be calculated. The mass content of lithium element in the lithiated acrylic acid copolymer is related to the electrical performance of the battery. If it is less, there are fewer active sites for ion conduction in the negative electrode sheet, and the charging polarization cannot be effectively reduced, resulting in poor effects. When the mass content of lithium element is more, PAA itself is slightly alkaline, which is not conducive to the processing of the slurry, and there are many side reactions at high temperatures, easily causing the deterioration of battery performance.

[0056] In one example, the mass content of lithium element in the lithiated acrylic acid copolymer is 2.0% - 3.5%.

[0057] In the present invention, the lithiated acrylic acid copolymer includes acrylonitrile groups. Based on the mass of the lithiated acrylic acid copolymer, the mass content of the acrylonitrile groups is 10% - 45%, such as 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45%. The infrared characteristic peak of the cyano group in the acrylonitrile group is at 2000 cm -1 - 2500 cm -1Within a certain range, the characteristic absorption peak of cyano groups (-CN) in the lithiated acrylic acid copolymer is measured by infrared spectroscopy (IR), the absorption value ratio of the sample is calculated, and combined with the standard working curve, the mass content of acrylonitrile groups can be determined. When the mass content of acrylonitrile groups in the lithiated acrylic acid copolymer is within a suitable range, it is beneficial to the lithium ion transport between the negative electrode active materials, which can improve the kinetic performance of the negative electrode active materials. If the content is too low, the impedance of the negative electrode sheet will be relatively large due to the addition of the negative electrode binder, which is not conducive to improving the charging ability of the negative electrode. However, when the mass content of acrylonitrile groups in the lithiated acrylic acid copolymer is too high, the adhesion between the negative electrode active material particles and the negative electrode current collector will be weak, resulting in poor long-cycle performance of the lithium ion battery. Therefore, the performance of the negative electrode binder can be improved by controlling the mass content of acrylonitrile groups in the lithiated acrylic acid copolymer within a suitable range.

[0058] In one example, the content of the acrylonitrile groups is 25%-40%.

[0059] In one example, the glass transition temperature of the lithiated acrylic acid copolymer is 60°C - 100°C, such as 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C. The glass transition temperature (Tg) of the lithiated acrylic acid copolymer is relatively high. The negative electrode binder with a high Tg can still maintain good physical properties at low temperatures, thus reducing the collapse of the negative electrode structure and the loss of active substances, avoiding the reduction of the kinetic characteristics of the internal reaction of the battery, and improving the electrochemical performance of the battery. Moreover, the negative electrode binder with a high Tg can still maintain excellent solid-state characteristics at relatively high temperatures, which helps to maintain the structural stability of the negative electrode, thereby reducing the deformation and cracks of the negative electrode during use and improving the cycle life of the battery.

[0060] In the present invention, the negative electrode binder further includes acrylate-modified styrene-butadiene rubber. Although the lithiated acrylic acid copolymer can provide more transport sites for lithium ions at low temperatures, reducing impedance and polarization, its binding ability as a negative electrode binder is relatively weak. Therefore, small particles of styrene-butadiene rubber need to be added for bonding the negative electrode active materials. The small particles of styrene-butadiene rubber have more contact points and better bonding force, which can ensure that the active material of the negative electrode sheet can be stably bonded to the current collector and is not easy to fall off. Moreover, although the bonding force of styrene-butadiene rubber is strong, its impedance is also relatively large. After being modified with acrylate, its impedance can be significantly reduced, and the ion-conducting ability of the negative electrode binder can be improved, thereby improving the fast charging kinetics of the negative electrode and further enhancing the low-temperature charging performance of the battery.

[0061] In one example, the acrylate modifier of the acrylate-modified styrene-butadiene rubber includes at least one of butyl methacrylate, butyl acrylate, methyl methacrylate, methyl acrylate, ethyl methacrylate, ethyl acrylate, n-octyl methacrylate, n-octyl acrylate, isooctyl methacrylate, isooctyl acrylate, dodecyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl methacrylate, and 2-hydroxypropyl acrylate.

[0062] In the present invention, based on the total mass of the acrylate-modified styrene-butadiene rubber, the mass content ratio of the acrylate group is 10%-60%, such as 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%. The mass content ratio of the acrylate can be measured by infrared testing method. Specifically, it is tested using the Thermo Fisher Nicolet iS20 Fourier transform infrared spectrometer, and the scanning range is 4000-400 cm -1 , and the infrared functional group characteristic peak of the acrylate group is located in the range of 1680-1720 cm -1 band, and the mass content ratio of the acrylate group can be obtained by calculation. After the styrene-butadiene rubber is modified with acrylate, its impedance can be significantly reduced, and the ion-conducting ability of the negative electrode binder can be improved. However, the modification degree of the acrylate cannot be too high, which will lead to a decrease in the adhesion of the styrene-butadiene rubber. Therefore, by adjusting the mass content ratio of the acrylate group, the over-high modification degree of the styrene-butadiene rubber can be avoided, thereby improving the fast charging kinetics of the negative electrode and enhancing the low-temperature charging performance of the battery. At the same time, it can ensure that the active material of the negative electrode sheet can be stably adhered to the current collector and is not easy to fall off.

[0063] In one example, based on the total mass of the negative electrode binder, the mass content ratio of the acrylate group is 20%-50%.

[0064] In one example, the glass transition temperature of the acrylate-modified styrene-butadiene rubber is -40°C to 40°C, such as -40°C, -35°C, -30°C, -25°C, -20°C, -15°C, -10°C, -5°C, 0°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, or 40°C. The low glass transition temperature (Tg) of the acrylate-modified styrene-butadiene rubber enables the styrene-butadiene rubber to maintain good flexibility and elasticity in a low-temperature environment. When it is used as a negative electrode binder, it can reduce the embrittlement of the negative electrode active material at low temperatures and improve the flexibility of the negative electrode sheet, thereby maintaining good electrochemical performance and cycle stability.

[0065] In one example, the Dv50 of the acrylate-modified styrene-butadiene rubber is 150 nm - 600 nm, such as 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm or 600 nm. When the particle size of the acrylate-modified styrene-butadiene rubber is large, its swelling effect in contact with the electrolyte is large, resulting in weak adhesion, but the fast charging kinetics is strong. When the particle size of the acrylate-modified styrene-butadiene rubber is small, it can provide more adhesion sites, thereby improving the adhesion between the negative electrode active material particles and the negative electrode current collector, and the swelling effect with the electrolyte is also small, but its fast charging kinetics decreases. Therefore, the adhesion effect of the negative electrode active material particles, the swelling performance with the electrolyte, and the fast charging kinetics performance can be balanced by adjusting the Dv50 of the acrylate-modified styrene-butadiene rubber within a suitable range.

[0066] In the present invention, in the negative electrode binder, the mass ratio of the lithiated acrylic acid copolymer to the acrylate-modified styrene-butadiene rubber is 2:1 - 1:1, such as 2:1, 1.9:1, 1.8:1, 1.7:1, 1.6:1, 1.5:1, 1.4:1, 1.3:1, 1.2:1 or 1:1. The mass ratio of the lithiated acrylic acid copolymer to the acrylate-modified styrene-butadiene rubber can be understood as the ratio of the addition amounts of two different types of negative electrode binders in the negative electrode binder, and it can generally be obtained by calculation when adding the binder of the negative electrode active material layer. The lithiated acrylic acid copolymer has good kinetic performance and is good for low-temperature lithium ion transmission, so its content can be relatively high compared to the acrylate-modified styrene-butadiene rubber. However, when the mass ratio is greater than 2:1, the content of the lithiated acrylic acid copolymer is too high and the content of the acrylate-modified styrene-butadiene rubber is too low, and the peel strength of the negative electrode sheet will deteriorate, which is not conducive to the long cycle of the lithium ion battery; when the mass ratio is less than 1:1, the mass content of the lithiated acrylic acid copolymer is too low and the mass content of the acrylate-modified styrene-butadiene rubber is too high, then the impedance of the negative electrode sheet is large and the charging kinetic performance will deteriorate.

[0067] In one example, based on the total mass of the negative electrode sheet, the total mass ratio of the lithiated acrylic acid copolymer to the acrylate-modified styrene-butadiene rubber is 2% - 6%, such as 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5% or 6%. The total amount of the two negative electrode binders should not be too much, which is likely to cause an increase in the internal resistance of the negative electrode active material, reduce the energy density of the battery, and generate more heat during the charge and discharge process, affecting the safety of the battery; moreover, when the total mass ratio of the negative electrode binder is too high, it will also reduce the fast charging ability of the negative electrode active material, resulting in poor low-temperature charging effect of the battery.

[0068] In one example, the temperature of the weight loss peak of the negative electrode sheet is 400°C - 600°C, such as 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C or 600°C. The thermogravimetric analysis of the negative electrode sheet can be tested using the METTLER TOLEDO thermal analysis system DSC3, and the test temperature range is 25 - 700°C. The temperature range of the weight loss peak of the negative electrode sheet is related to the total mass ratio of the negative electrode binder lithium acrylate copolymer and acrylate-modified styrene-butadiene rubber in the negative electrode sheet, and the mass ratio between these two negative electrode binders. Therefore, by controlling the temperature of the weight loss peak of the negative electrode sheet to be 400°C - 600°C, the total mass ratio of the two negative electrode binders and the mass ratio between them can be further balanced, thereby further improving the charge kinetics performance and long cycle performance of the battery.

[0069] In the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode conductive agent. In the present invention, the negative electrode active material, the negative electrode binder and the negative electrode conductive agent form a negative electrode active material layer, and the negative electrode active material layer is located on either one or both sides of the negative electrode current collector.

[0070] In one example, the negative electrode current collector is at least one of copper foil, chromium foil, nickel foil, and titanium foil.

[0071] In one example, the negative electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0072] In the present invention, the lithium ion battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes lithium iron phosphate.

[0073] Furthermore, the lithium iron phosphate is single crystal lithium iron phosphate. The capacity and tap density of single crystal lithium iron phosphate are higher than those of polycrystalline, and the high temperature cycle and high temperature storage performance of single crystal are better than those of polycrystalline. Applied to the energy storage system, considering the long cycle and storage performance, the lithium iron phosphate of the present invention can be single crystal lithium iron phosphate (LiFePO4). The single crystal lithium iron phosphate can be prepared by a high temperature solid state method.

[0074] In one example, the Dv50 of the positive electrode active material is 0.5 μm - 5 μm, such as 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 2 μm, 3 μm, 4 μm or 5 μm. The ionic conductivity of lithium iron phosphate is also low and anisotropic. If its particle size is small, the ionic migration impedance inside the lithium iron particles can be reduced, and its charge and discharge ability at low temperature can be improved.

[0075] In one example, the specific surface area of the positive electrode active material is 5.0 m 2 / g - 20.0 m 2 / g, for example, 5 m 2 / g, 10 m 2 / g, 15 m 2 / g or 20 m 2 / g.

[0076] In one example, the lithium iron phosphate includes a carbon coating layer; based on the total mass of the lithium iron phosphate, the mass content of carbon element is 1.0% - 1.5%. For example, it is 1.0%, 1.1%, 1.2%, 1.3%, 1.4% or 1.5%. The carbon element content is the content of amorphous carbon coated on the surface of the lithium iron phosphate, which can be obtained by testing with a carbon-sulfur analyzer. The lithium iron phosphate has poor conductivity and low conductivity. Coating the surface with a carbon coating layer can significantly improve the conductive ability of the lithium iron phosphate, reduce the impedance of the positive electrode sheet, and thus improve its charge and discharge ability at low temperatures.

[0077] The positive electrode uses single crystal lithium iron phosphate, which has a smaller particle size and a higher carbon coating content, and can reduce the charge and discharge impedance of the lithium ion battery at low temperatures. Therefore, based on the above design, the present invention can achieve stable charge and discharge of the lithium ion battery in low temperature scenarios.

[0078] In one example, based on the total mass of the lithium iron phosphate, the mass content of lithium element is 1% - 10% (1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%), the mass content of iron element is 25% - 40% (25%, 27%, 29%, 30%, 31%, 33%, 35%, 37%, 39% or 40%), and the mass content of phosphorus element is 10% - 20% (10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%). The proportion of Li, Fe and P elements in the lithium iron phosphate material is tested using Thermo Scientific TM iCAP TM PRO XP ICP-OES inductively coupled plasma emission spectrometer. The mass content ratio of lithium element, iron element and phosphorus element in the lithium iron phosphate will affect the performance of the lithium iron phosphate. When the mass content of lithium is less, its capacity is low, resulting in a low energy density of the lithium ion battery; when the mass content of lithium is more, the rate performance of the lithium iron phosphate is poor, deteriorating the low temperature charging performance; when the mass content of iron and phosphorus elements is within this range, the positive electrode active material can have a high energy density and low temperature performance.

[0079] In the present invention, the thickness of the single-sided active layer of the positive electrode sheet is 50 μm - 110 μm, such as 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm or 110 μm; the thickness of the single-sided active layer of the negative electrode sheet is 40 μm - 100 μm, such as 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm or 100 μm.

[0080] In one example, the ratio of the thickness of the single-sided active layer of the positive electrode sheet to the thickness of the single-sided active layer of the negative electrode sheet is 1:1 - 1:1.5, such as 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4 or 1:1.5.

[0081] Only when the ratio of the thickness of the single-sided active layers of the positive and negative electrode sheets is within the designed range can the battery have good cycling performance at low and high temperatures. When the ratio of the thickness of the single-sided active material layers of the positive and negative electrode sheets is relatively large, the thickness of the positive electrode sheet is small and the thickness of the negative electrode sheet is large, resulting in an excessive amount of negative active material, which will lead to poor high-temperature cycling and storage performance of the battery. When the ratio of the thickness of the single-sided active material layers of the positive and negative electrode sheets is relatively small, the thickness of the positive electrode sheet is large and the thickness of the negative electrode sheet is small, resulting in a relatively small amount of negative active material and a small CB (Cell Balance) value of the negative electrode sheet relative to the positive electrode sheet. The excessive amount of negative active material cannot support the long-term cycling of the battery, leading to deterioration of its long-cycle performance.

[0082] In the present invention, the shape and the material of the outer shell of the lithium-ion battery can be unrestricted. The shape of the lithium-ion battery can include cylindrical, square, polygonal, thin-sheet, C-shaped battery, D-shaped battery, polygonal battery, etc.; the material of the outer shell of the lithium-ion battery can be an aluminum shell, a steel shell, an aluminum-plastic composite film, a plastic shell, etc.

[0083] In one example, the lithium-ion battery is a square aluminum-shell battery. This is because the square aluminum-shell battery has a higher volume energy density and a larger amount of residual electrolyte in the battery, which can ensure sufficient wetting of the electrode sheets, thereby further ensuring the long-cycle performance of the battery.

[0084] The materials, reagents, etc. used in the following examples can be obtained from commercial sources without special instructions.

[0085] The present invention will be described in detail below with reference to specific examples, which are used for understanding rather than limiting the present invention.

[0086] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but should not be regarded as specific limitations of the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement or improvement made under the guidance of the spirit and principle of the present invention shall be included within the protection scope of the present invention.

[0087] In the methods of the following embodiments, unless otherwise specified, they are all conventional methods; the experimental materials used, unless otherwise specified, are directly purchased from a conventional biochemical reagent factory and used without any purification.

[0088] Example 1

[0089] For preparing the lithium-ion battery of the present invention:

[0090] Preparation of the negative electrode active material:

[0091] The petroleum coke is crushed and shaped to obtain artificial graphite raw materials with a certain particle size distribution. After heat treatment in a rotary kiln at 500 - 600 °C, semi-finished artificial graphite materials are obtained. The semi-finished products are loaded into a graphite crucible for graphitization treatment. The graphitization temperature is 2300 - 2500 °C. After the material cools down, artificial graphite materials are obtained. The artificial graphite materials are fully mixed with asphalt fine powder, charged into the crucible, and heat-treated at 1000 - 1300 °C (in an N2 atmosphere). After completion, the material is discharged, and the material is mixed, screened, and demagnetized to obtain the negative electrode active material. The particle size of the obtained negative electrode active material is Dv10 = 7.5 μm, Dv50 = 13.5 μm, Dv90 = 23.5 μm, Dv90 / Dv10 = 3.13 μm, Dn10 = 3.6 μm, the specific surface area is 0.8 m 2 / g, the specific capacity is 345 mAh / g, and the peak intensity I d / I g value is 0.45, as Figure 3 shown, Figure 3 is the I d / I g peak intensity process capability report chart of the negative electrode active material prepared in Example 1 of the present invention, and the numerical distribution of the peak intensity ratio satisfying 0.3 ≤ I d / I g ≤ 0.6 is 95%; calculated based on the total mass of the negative electrode active material, the mass content of the coating layer is 3%.

[0092] Design of the negative electrode sheet:

[0093] The formulation of the negative electrode active paste is as follows: the negative electrode active material prepared above: conductive agent: CMC: lithiated acrylic acid copolymer: acrylate-modified styrene-butadiene rubber = 95.0:1.0:0.8:1.7:1.5; the mass content of lithium element in the lithiated acrylic acid copolymer is 2.5%, the mass content of acrylonitrile group is 30%, and the position of the infrared characteristic peak is 2200 cm -1 , the glass transition temperature is 78 °C, and the copolymer monomers of the lithiated acrylic acid copolymer include acrylonitrile and acrylic acid; the position of the infrared characteristic peak of the acrylate-modified styrene-butadiene rubber is 1695 cm -1 , its particle size Dv50 = 350 nm, the glass transition temperature is 16 °C, and the mass content ratio of the acrylate group is 40%; the thermal weight loss peak of the tested negative electrode sheet is 470 °C, the thickness of the single-sided active material layer of the negative electrode sheet is 95 μm, and the compaction density is 1.45 g / cm 3 , the value of the intensity of the crystal plane diffraction peak D004 / D110 of the negative electrode sheet tested by XRD is 5.7; the negative electrode active paste is evenly coated on a 6-μm copper foil of the negative electrode current collector, and after rolling and slitting, a negative electrode sheet is obtained. Among them, the SEM image of the surface of the negative electrode sheet is as shown in Figure 1 .

[0094] Design of the positive electrode sheet:

[0095] The positive electrode active material is single-crystal lithium iron phosphate, which is prepared by a high-temperature solid-phase method. Its particle size Dv50 = 0.8 μm, specific surface area is 7.8 m 2 / g, the mass content of carbon element in LiFePO4 is 1.2%, the mass content of Li element is 6%, the mass content of Fe element is 36%, and the mass content of P element is 14%. The formulation of the positive electrode active paste used in the preparation of the positive electrode sheet is positive electrode active material: conductive agent: PVDF = 96.5%:1.5%:2.0%. The thickness of the single-sided active material layer of the positive electrode sheet is 100 μm; the positive electrode active paste is evenly coated on a 6-μm copper foil of the positive electrode current collector, and after rolling and slitting, a positive electrode sheet is obtained.

[0096] Preparation of the battery:

[0097] The above positive electrode sheet, negative electrode sheet and separator (polyethylene separator) are wound and assembled into a lithium battery dry cell core, and then assembled into a square aluminum shell battery, and the processes of liquid injection, formation and sorting are carried out to obtain a square aluminum shell lithium battery; among them, the full battery electrolyte is LBC001 (purchased from Shenzhen Capchem Technology Co., Ltd.), and the size of the square aluminum shell battery is: length 48 - 52 mm * width 155 - 165 mm * height 113 - 120 mm.

[0098] Specifically, please refer to Table 1 and Table 2.

[0099] Example 2 group

[0100] This group of examples refers to the preparation method of Example 1. The only difference is that the process parameters in the preparation of the negative electrode active material are changed. Specifically, the mass ratio of artificial graphite material to pitch fine powder, the stirring rate during mixing, and the heat treatment temperature are adjusted, so that the mass content of the coating layer on the graphite matrix surface and the coating uniformity of the amorphous carbon material on the graphite matrix surface are changed. For specific parameter changes, please refer to Table 1 and Table 2.

[0101] Comparative Example 1 group

[0102] This group of comparative examples refers to the preparation method of Example 1. The only difference is that the process parameters in the preparation of the negative electrode active material are changed. Specifically, the mass ratio of artificial graphite material to pitch fine powder, the stirring rate during mixing, and the heat treatment temperature are adjusted, so that the mass content of the coating layer on the graphite matrix surface and the coating uniformity of the amorphous carbon material on the graphite matrix surface are changed. Specifically:

[0103] Comparative Example 1-1: During the preparation of the negative electrode active material, no pitch fine powder is added, and the negative electrode active material is pure artificial graphite material;

[0104] Comparative Example 1-2: During the preparation of the negative electrode active material, the mass content of the coating layer is 0.3%;

[0105] Comparative Example 1-3: During the preparation of the negative electrode active material, the mass content of the coating layer is 5.3%;

[0106] Comparative Example 1-4: The negative electrode active material uses pure soft carbon. At this time, the mass content of its coating layer can be considered 100%;

[0107] For specific parameter changes, please refer to Table 1 and Table 2.

[0108] Table 1

[0109]

[0110]

[0111] Table 2

[0112]

[0113] In Table 2, the diffraction peak intensity D004 / D110 of the negative electrode sheet in Comparative Example 1-4 is represented by " / " because the diffraction peak intensity value of the soft carbon material cannot be measured.

[0114] Example 3 group

[0115] This group of examples refers to the preparation method of Example 1. The only difference is that the mass content of lithium element or the mass content of acrylonitrile group in the lithiated acrylic acid copolymer is changed. Specifically:

[0116] Example 3-1: The mass content of lithium element in the lithiated acrylic acid copolymer is changed to 2%;

[0117] Example 3-2: The mass content of lithium element in the lithiated acrylic acid copolymer is changed to 3.5%;

[0118] Example 3-3: The mass content of lithium element in the lithiated acrylic acid copolymer is changed to 1.5%;

[0119] Example 3-4: The mass content of lithium element in the lithiated acrylic acid copolymer is changed to 4%;

[0120] Example 3-5: The mass content of lithium element in the lithiated acrylic acid copolymer is changed to 1%;

[0121] Example 3-6: The mass content of lithium element in the lithiated acrylic acid copolymer is changed to 5%;

[0122] Example 3-7: The mass content of acrylonitrile group is changed to 22%;

[0123] Example 3-8: The mass content of acrylonitrile group is changed to 46%.

[0124] Example 4 group

[0125] This group of examples refers to the preparation method of Example 1. The only difference is that the Dv50 of the acrylate-modified styrene-butadiene rubber is changed. Specifically:

[0126] Example 4-1: Dv50 of the acrylate-modified styrene-butadiene rubber = 150 nm, and its glass transition temperature is 14 °C;

[0127] Example 4-2: Dv50 of the acrylate-modified styrene-butadiene rubber = 600 nm, and its glass transition temperature is 18 °C;

[0128] Example 4-3: Dv50 of the acrylate-modified styrene-butadiene rubber = 100 nm, and its glass transition temperature is 13 °C;

[0129] Example 4-3: Dv50 of the acrylate-modified styrene-butadiene rubber = 650 nm, and its glass transition temperature is 20 °C.

[0130] Example 5 group

[0131] This group of embodiments refers to the preparation method of Embodiment 1. The only difference is that the total amount and mass ratio of the lithiated acrylic acid copolymer and the acrylate-modified styrene-butadiene rubber are changed. Specifically:

[0132] Example 5-1, negative electrode active material: conductive agent: CMC: lithiated acrylic acid copolymer: acrylate-modified styrene-butadiene rubber = 96.2: 1.0: 0.8: 1.33: 0.67;

[0133] Example 5-2, negative electrode active material: conductive agent: CMC: lithiated acrylic acid copolymer: acrylate-modified styrene-butadiene rubber = 92.3: 1.0: 0.8: 3: 3.

[0134] Test Example

[0135] (1) Capacity test and charge-discharge test of the negative electrode button cell:

[0136] The negative electrode active materials prepared in the above examples and comparative examples, conductive carbon black, CMC, and SBR are made into a negative electrode slurry according to a mass ratio of 95.5%: 1.5%: 1.5%: 1.5%. The current collector is copper foil, and the coating surface density is 10 mg / cm 2 , and the designed compaction density is 1.40 g / cm 3 . The lithium sheet is used as the counter electrode for the button cell, the polyethylene diaphragm is used as the diaphragm, and the electrolyte is a solution composed of 1 mol / L LiPF6 with a solvent of EC / DMC / EMC in a volume ratio of 1: 1: 1. A button cell is assembled, the battery model is CR2430, and an Arbin BT2000 type battery tester is used for the battery charge-discharge test. The charge-discharge range is set to 0 V to 2.0 V, which can be used to measure the specific capacity and the first charge-discharge efficiency of the negative electrode active material of the present invention.

[0137] Specifically, the capacity and charge-discharge test methods of the negative electrode button cell are as follows:

[0138] The assembled negative electrode button cell is subjected to a charge-discharge test according to the following steps:

[0139] 1) Stand still for 10 min;

[0140] 2) Discharge the button cell at a current density of 0.6 mA until the cut-off voltage is 5 mV;

[0141] 3) Stand still for 10 min;

[0142] 4) Discharge the button cell at a current density of 0.1 mA until the cut-off voltage is 5 mV;

[0143] 5) Stand still for 10 min;

[0144] 6) Charge the coin cell at a current density of 0.6 mA until the cut-off voltage reaches 2 V.

[0145] The specific capacity of the negative electrode of the coin cell = the capacity measured in step 6) divided by the total mass of the negative active material in the negative electrode sheet of the coin cell, with the unit of mAh / g.

[0146] The first charge-discharge efficiency (%) of the negative electrode of the coin cell = the first discharge specific capacity of the coin cell / the first charge specific capacity of the coin cell × 100%.

[0147] (2) Testing the mass energy density and volume energy density of the prismatic aluminum shell battery:

[0148] Use an Arbin BT2000 battery tester to test the mass energy density and volume energy density of the prismatic aluminum shell batteries prepared in the above examples and comparative examples. Set the charge-discharge interval to 3.65 V - 2.2 V, measure the capacity and average voltage of the prismatic aluminum shell batteries, and calculate the mass energy density (unit: Wh / Kg) and volume energy density (unit: Wh / L) of the batteries based on the mass and volume of the batteries respectively.

[0149] (4) Testing the degree of lithium plating on the negative electrode sheet and the low-temperature charging performance of the battery:

[0150] 1) Let the prismatic aluminum shell batteries prepared in the above examples and comparative examples stand in an environment of 25 °C for 1 hour.

[0151] 2) Charge the battery at a current density of 0.33C until 3.65 V, then perform constant voltage charging with a cut-off current of 0.05C.

[0152] 3) Stand for 30 min.

[0153] 4) Discharge the battery at a current density of 0.33C until 2.2 V.

[0154] 5) Stand for 30 min.

[0155] 6) Move the battery to an environment of -20 °C and stand for 4 hours.

[0156] 7) Charge the battery at a current of 0.2C until 3.65 V, then perform constant voltage charging with a cut-off current of 0.05C.

[0157] 8) Stand for 30 min.

[0158] 9) Discharge the battery at a current of 0.2C until 2.2 V.

[0159] 10) Stand for 30 min.

[0160] 11) Repeat the test steps of 7 - 10 until 50 charge-discharge cycles.

[0161] 12) Take out the battery that has undergone the above 50 low-temperature cycles after being fully charged, perform a full-charge disassembly, and observe the degree of lithium deposition on the negative electrode (from the smallest degree of lithium deposition to the largest, it is no lithium deposition, slight lithium deposition, and severe lithium deposition).

[0162] (5) Low-temperature charge-discharge cycle performance test of the battery:

[0163] Refer to the test steps of the above-mentioned test on the degree of lithium deposition on the negative electrode, repeat the test steps of 7-10 until 300 charge-discharge cycles. Let the discharge capacity of the first cycle be C1 and the discharge capacity of the 300th cycle be C2. Then the capacity retention rate of the battery for 300 cycles at -20°C, 0.2C / 0.2C is C2 / C1 * 100%. Among them, the test chart of the capacity retention rate of the battery in Example 1 for 300 cycles at -20°C, 0.2C / 0.2C is as Figure 2 shown.

[0164] (6) High-temperature cycle performance test of the battery:

[0165] 1) Let the prismatic aluminum shell batteries prepared in the above examples and comparative examples stand still in an environment of 25°C for 1 hour;

[0166] 2) Charge the battery at a current density of 0.33C until 3.65V, and then perform constant-voltage charging with a cut-off current of 0.05C;

[0167] 3) Stand still for 30 min;

[0168] 4) Discharge the battery at a current density of 0.33C until 2.2V;

[0169] 5) Stand still for 30 min;

[0170] 6) Move the battery to an environment of 45°C and stand still for 4 hours;

[0171] 7) Charge the battery at a current of 1C until 3.65V, and then perform constant-voltage charging with a cut-off current of 0.05C;

[0172] 8) Stand still for 30 min;

[0173] 9) Discharge the battery at a current of 1C until 2.2V;

[0174] 10) Stand still for 30 min;

[0175] 11) Repeat the test steps of 7-10 until 1000 charge-discharge cycles. Let the discharge capacity of the first cycle be H1 and the discharge capacity of the 1000th cycle be H2. Then the capacity retention rate of the battery for 1000 cycles at 45°C, 1C / 1C is H2 / H1 * 100%.

[0176] Record the test results of all the above tests in Table 3.

[0177] Table 4

[0178]

[0179]

[0180]

[0181] It can be seen from the test results in Table 4 that in the embodiment, the surface of the negative electrode active material is coated with an amorphous carbon material, and the mass content of the coating layer and the uniformity of the coated amorphous carbon material are adjusted, which improves the isotropy of the negative electrode active material, reduces the charge and discharge impedance in a low-temperature environment, and can improve the high-temperature performance of the lithium-ion battery while improving the low-temperature performance of lithium ions; in the negative electrode active slurry formula of the negative electrode sheet, a lithiated acrylic acid multi-component copolymer and a small-particle-size acrylate-modified styrene-butadiene rubber are used as the negative electrode binder, which can ensure the processing performance of the negative electrode active material and further reduce the charge and discharge polarization of the negative electrode at low temperature, reduce the lithium-ion migration impedance, and improve the charge and discharge performance in a low-temperature environment. The square aluminum-shell lithium-ion battery of the present invention simultaneously has excellent low-temperature charge and discharge cycle performance and high-temperature charge and discharge cycle performance, and this design has a relatively broad application field.

[0182] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution 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 battery, characterized in that, The lithium-ion battery includes a negative electrode sheet; The negative electrode sheet includes a negative electrode active material, the negative electrode active material includes a graphite matrix and a coating layer, and the coating layer includes an amorphous carbon material; The amorphous carbon material includes soft carbon and / or hard carbon; Based on the total mass of the negative electrode active material, the mass content of the coating layer is 0.5%-5%; The negative electrode active material is measured by a Raman spectrometer, and the peak intensity I within the range of 1300 cm -1 -1400 cm -1 and the peak intensity I d within the range of 1550 cm -1 -1650 cm -1 satisfy the following intensity ratio: 0.1 ≤ I g / I d / I g ≤ 0.

8.

2. The lithium-ion battery according to claim 1, characterized in that, The peak intensity I d and the peak intensity I g satisfy: 0.3 ≤ I d / I g ≤ 0.6; Preferably, the peak intensity I d and the peak intensity I g have a numerical distribution of ≥ 90%.

3. The lithium-ion battery according to claim 1, characterized in that, The negative electrode active material satisfies at least one of the following conditions: (i) Dv50 is 5μm - 20μm; (ii) Dv90 / Dv10 is 2.5 - 3.5; (iii) Dn10 ≥ 1.0μm; (iv) Specific surface area is 0.5 m 2 / g - 1.5 m 2 / g; (v) The specific capacity is 320mAh / g - 355mAh / g.

4. The lithium ion battery according to claim 1, wherein In the XRD pattern of the negative electrode sheet, the intensity of the diffraction peak of the (004) crystal plane where the diffraction angle 2θ is located at 54 ± 1° is D004, and the intensity of the diffraction peak of the (110) crystal plane where the diffraction angle 2θ is located at 78 ± 1° is D110. D004 and D110 satisfy: 1.0 ≤ D004 / D110 ≤ 10.0; and / or, the compaction density of the negative electrode sheet ranges from 1.2 g / cm 3 - 1.65 g / cm 3 .

5. The lithium-ion battery according to claim 1, characterized in that, The negative electrode sheet includes a negative electrode binder, and the negative electrode binder includes a lithiated acrylic acid copolymer; Preferably, the monomers of the lithiated acrylic acid copolymer include at least two monomers among acrylic acid, acrylonitrile, acrylamide, vinyl alcohol, and vinylidene fluoride; Preferably, the mass content of lithium element in the lithiated acrylic acid copolymer is 1.5%-4.0%; Preferably, the lithiated acrylic acid copolymer includes acrylonitrile groups, and based on the mass of the lithiated acrylic acid copolymer, the mass content of the acrylonitrile groups is 10%-45%; Preferably, the glass transition temperature of the lithiated acrylic acid copolymer is 60°C - 100°C.

6. The lithium ion battery according to claim 5, characterized in that, The negative electrode binder further includes an acrylate-modified styrene-butadiene rubber; Preferably, based on the total mass of the acrylate-modified styrene-butadiene rubber, the mass content ratio of the acrylate groups is 10%-60%; Preferably, the glass transition temperature of the acrylate-modified styrene-butadiene rubber is -40°C - 40°C; Preferably, the Dv50 of the acrylate-modified styrene-butadiene rubber is 150nm - 600nm.

7. The lithium-ion battery according to claim 6, characterized in that, In the negative electrode binder, the mass ratio of the lithiated acrylic acid copolymer to the acrylate-modified styrene-butadiene rubber is 2:1 - 1:1; Preferably, based on the total mass of the negative electrode sheet, the total mass content ratio of the lithiated acrylic acid copolymer and the acrylate-modified styrene-butadiene rubber is 2%-6%; Preferably, the temperature of the weight loss peak of the negative electrode sheet is 400°C - 600°C.

8. The lithium ion battery according to claim 1, wherein The negative electrode sheet includes a negative electrode current collector and a negative electrode conductive agent; Preferably, the negative electrode current collector is at least one of copper foil, chromium foil, nickel foil, and titanium foil; Preferably, the negative electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

9. The lithium ion battery according to any one of claims 1-8, characterized in that, The lithium-ion battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes lithium iron phosphate; the lithium iron phosphate is preferably single-crystal lithium iron phosphate; Preferably, the Dv50 of the positive electrode active material is 0.5μm - 5μm; Preferably, the specific surface area of the positive electrode active material is 5.0 m 2 / g - 20.0 m 2 / g; Preferably, the lithium iron phosphate includes a carbon coating layer; based on the total mass of the lithium iron phosphate, the mass content of carbon element is 1.0% - 1.5%; Preferably, based on the total mass of the lithium iron phosphate, the mass content of lithium element is 1% - 10%, the mass content of iron element is 25 - 40%, and the mass content of phosphorus element is 10% - 20%.

10. The lithium-ion battery according to claim 9, characterized in that, The thickness of the single-sided active layer of the positive electrode sheet is 50μm - 110μm, and the thickness of the single-sided active layer of the negative electrode sheet is 40μm - 100μm; Preferably, the ratio of the thickness of the single-sided active layer of the positive electrode sheet to the thickness of the single-sided active layer of the negative electrode sheet is 1:1 - 1:1.5.

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