Negative electrode active material, negative electrode sheet, secondary battery, and electronic device

Abstract

Provided are a negative electrode active material, a negative electrode sheet, a secondary battery, and an electronic device. The negative electrode active material comprises a silicon-carbon material, and a particle of the silicon-carbon material comprises a carbon carrier, elemental silicon, and a carbon coating layer. An average thickness of the carbon coating layer ranges from 2.5 nm to 12 nm. When X-ray photoelectron spectroscopy analysis is performed on the carbon coating layer, on the basis of the sum of a peak area of a C-C corresponding peak, a peak area of a C-O corresponding peak, and a peak area of a C=O corresponding peak, the proportion of the peak area of the C-C corresponding peak ranges from 60% to 95%. By means of the configurations, an overdischarge gas generation voltage during cycling of the secondary battery can be reduced, thereby improving the safety performance and cycle performance of the secondary battery.

Description

Negative electrode active material, negative electrode sheet, secondary battery and electronic device

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on March 31, 2024, with application number 202410383577.9 and invention name “A negative electrode active material, a negative electrode sheet, a secondary battery and an electronic device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of electrochemical technology, and in particular to a negative electrode active material, a negative electrode plate, a secondary battery, and an electronic device. Background Art

[0003] Secondary batteries represented by lithium-ion batteries have outstanding characteristics such as high energy density, long cycle life, low pollution, and no memory effect. However, the negative electrode active materials of traditional commercial lithium-ion batteries are basically limited to graphite materials, and their low capacity (LiC6, 372mAh / g) greatly hinders the improvement of energy density. In recent years, silicon-based materials have been widely used due to their high specific capacity (Li 15 Si4, 3579mAh / g, rich resource reserves, reasonable lithiation platform (<0.4V vs.Li / Li + ) is gradually entering the industrialization stage of lithium-ion battery anodes and is considered the most promising next-generation anode active material. However, silicon-based materials undergo significant volume changes during the lithium insertion / extraction process, resulting in reduced cycling performance of lithium-ion batteries and the failure of anode active material particles to differentiate, hindering the full electrochemical performance of silicon-based materials.

[0004] To address this issue, existing technologies typically nanosize silicon-based materials. While this can prevent the breakage of negative electrode active material particles to a certain extent, the high specific surface area of ​​nanosized silicon-based materials increases interfacial side reactions during cycling, continuously forming a large amount of solid electrolyte (SEI) film. The SEI film contains organic or inorganic components such as lithium carbonate, CO, C=O, and CH. During over-discharge, the SEI film on the surface of the graphite-based negative electrode in the range of 0% state of charge (SOC) to -10% SOC will degrade and produce gas, causing the lithium-ion battery to swell and affect safety performance. In addition, because the silicon-lithium alloy formed by the lithiation of silicon-based materials has high reactivity, it is more likely to react with the electrolyte to form an SEI film containing the above-mentioned organic or inorganic components. Furthermore, the SEI film continues to form on the surface of the silicon-based material during cycling. Therefore, the use of silicon-based materials will result in a higher over-discharge gassing voltage of the lithium-ion battery than that of graphite materials. Excessively high over-discharge gassing voltage makes the lithium-ion battery more prone to gassing during over-discharge, reducing the safety performance of the lithium-ion battery. Summary of the Invention

[0005] The purpose of this application is to provide a negative electrode active material, a negative electrode plate, a secondary battery, and an electronic device to reduce the over-discharge gassing voltage during the secondary battery cycle, thereby improving the cycle performance and safety performance of the secondary battery. The specific technical solution is as follows:

[0006] It should be noted that in the invention content of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0007] The first aspect of the present application provides a negative electrode active material, comprising a silicon-carbon material, wherein particles of the silicon-carbon material include a carbon carrier, elemental silicon, and a carbon coating layer, wherein the average thickness of the carbon coating layer is 2.5 nm to 12 nm, and X-ray photoelectron spectroscopy analysis of the carbon coating layer shows that, based on the sum of the peak areas of the CC peak, the CO peak, and the C=O peak, the CC peak accounts for 60% to 95%. This configuration can reduce the over-discharge gassing voltage during the cycling of a secondary battery, thereby improving the cycling performance and safety of the secondary battery.

[0008] In one embodiment of the present application, the carbon coating layer includes a film-like or granular distribution. When the coating form is within the above range, it is beneficial to reduce the over-discharge gas generation voltage during the cycle of the secondary battery, thereby improving the safety and cycle performance of the secondary battery.

[0009] In one embodiment of the present application, in the Raman spectrum of the carbon coating layer, 1200 cm -1 to 1400cm -1 The peak area of ​​the characteristic peak in the range is I D , 1500cm -1 to 1700cm -1 The peak area of ​​the characteristic peak in the range is I G , satisfying: 0.8≤I D / I G ≤1.2. By regulating I D / I G When the value of is within the above range, the graphitization degree of the carbon coating layer is high, which is beneficial to improving the cycle performance and rate performance of the secondary battery. Therefore, the secondary battery has good safety performance and cycle performance.

[0010] In one embodiment of the present application, the (002) interplanar spacing of the carbon coating layer is between 0.335 nm and 0.352 nm. By regulating the (002) interplanar spacing of the carbon coating layer within this range, the over-discharge gassing voltage during the secondary battery cycle is reduced, thereby improving the safety and cycling performance of the secondary battery.

[0011] In one embodiment of the present application, the elastic modulus of the carbon coating layer is 12 GPa to 20 GPa. By regulating the elastic modulus of the carbon coating layer within the above range, it is beneficial to reduce the over-discharge gassing voltage during battery cycling, thereby improving the safety and cycling performance of the secondary battery.

[0012] In one embodiment of the present application, the specific surface area of ​​the silicon carbon material particles is 1m 2 / g to 3.5m 2 / g, preferably, the specific surface area of ​​the silicon carbon material particles is 1.5m 2 / g to 3m 2 By regulating the specific surface area of ​​the silicon-carbon material particles within the above range, it is beneficial to reduce the over-discharge gas generation voltage during the cycle of the secondary battery, thereby improving the safety performance and cycle performance of the secondary battery.

[0013] In one embodiment of the present application, the Dv50 of the silicon-carbon material is 5μm to 10μm, and the Dv99 of the silicon-carbon material is 15μm to 20μm. By regulating the Dv50 and Dv99 of the silicon-carbon material within the above ranges, the spatial distribution of the negative electrode active material in the negative electrode material layer is more reasonable, further reducing the contact between the carbon coating layer of the silicon-carbon material and the electrolyte, reducing the possibility of excessive SEI film formation, thereby reducing the over-discharge gassing voltage during the secondary battery cycle, and further improving the safety and cycle performance of the secondary battery.

[0014] In one embodiment of the present application, Dv10 of the silicon-carbon material is D1 μm, Dn10 of the silicon-carbon material is D2 μm, 1.5 ≤ D1 / D2 ≤ 2, and 3 ≤ D1 ≤ 6. By regulating the values ​​of D1 / D2 and D1 within the above ranges, the spatial distribution of the negative electrode active material in the negative electrode material layer is made more reasonable, further reducing the contact between the carbon coating layer of the silicon-carbon material and the electrolyte, and reducing the possibility of excessive SEI film formation, thereby reducing the over-discharge gassing voltage during the secondary battery cycle, thereby improving the safety and cycle performance of the secondary battery.

[0015] In one embodiment of the present application, the negative electrode active material further comprises graphite, wherein the graphite comprises at least one of artificial graphite or natural graphite. Based on the mass of the negative electrode active material, the mass percentage of the silicon-carbon material is 5% to 30%, and the mass percentage of the graphite is 70% to 95%. By regulating the mass percentage of the silicon-carbon material and the mass percentage of the graphite within the above ranges, the high surface stability of the graphite can be utilized to protect the interface of the silicon-carbon material particles to a certain extent, thereby reducing the excessive formation of the SEI film, thereby reducing the over-discharge gassing voltage during the cycle of the secondary battery, and thereby improving the safety and cycle performance of the secondary battery.

[0016] A second aspect of the present application provides a method for preparing a negative electrode active material, which comprises the following steps:

[0017] (1) heating the carbon support to 400° C. to 480° C. in an inert atmosphere at a heating rate of 5° C. / min to 20° C. / min, then maintaining the temperature, and then depositing the carbon support in a silane gas mixture for 2 to 30 hours to obtain an intermediate; wherein the silane gas mixture is composed of silane and an inert gas, and based on the mass of the silane gas mixture, the mass percentage of silane is 10% to 50%, and the mass percentage of the inert gas is 50% to 90%.

[0018] (2) The intermediate is subjected to carbon coating treatment to obtain a silicon-carbon material; wherein the carbon coating treatment method includes any one of a liquid phase method, a gas phase method or a gas-liquid combined method, the carbon source includes at least one of coal tar pitch, petroleum pitch, acetylene, propylene, natural gas, ethylene or cyclohexane, the carbon coating treatment temperature is 620°C to 810°C, and the carbon coating treatment time is 1h to 6h.

[0019] In one embodiment of the present application, the method of carbon coating treatment includes a liquid phase method, which includes the following steps: placing the intermediate in an organic solvent containing a carbon source to obtain an organic solution, volatilizing the organic solution, and performing carbon coating treatment under an inert atmosphere to obtain a silicon-carbon material; wherein the carbon source includes at least one of petroleum asphalt or coal tar asphalt, the organic solvent includes at least one of tetrahydrofuran, n-hexane or xylene, the temperature of the carbon coating treatment is 700°C to 810°C, and the time of the carbon coating treatment is 1h to 2h.

[0020] In one embodiment of the present application, the method of carbon coating treatment includes a gas phase method, which includes the following steps: carbon coating the intermediate in a reducing atmosphere to obtain a silicon-carbon material; wherein the reducing atmosphere includes at least one of acetylene, propylene, natural gas, ethylene or cyclohexane, the temperature of the carbon coating treatment is 620°C to 700°C, and the time of the carbon coating treatment is 2h to 6h.

[0021] The negative electrode active material is prepared by the above method. The temperature of the carbon coating treatment is relatively high, which can regulate the average thickness of the carbon coating layer in the particles of the silicon-carbon material and improve its quality. It can also increase the degree of graphitization of the carbon coating layer, thereby improving the quality and content of the SEI film, effectively protecting the contact between the silicon-lithium alloy and the electrolyte during the cycle of the secondary battery, reducing the possibility of repeated growth of the SEI film, and reducing the over-discharge gas production voltage of the secondary battery, thereby improving the safety performance and cycle performance of the secondary battery.

[0022] The third aspect of the present application provides a negative electrode plate, comprising a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises the negative electrode active material provided in the first aspect of the present application or the negative electrode active material prepared according to the preparation method provided in the second aspect of the present application. Application of the negative electrode plate of the present application to a secondary battery can reduce the over-discharge gassing voltage during the secondary battery cycle, thereby improving the safety and cycling performance of the secondary battery.

[0023] A fourth aspect of the present application provides a secondary battery comprising the negative electrode sheet of any of the aforementioned embodiments. The secondary battery provided in the fourth aspect of the present application has a low over-discharge gas generation voltage during cycling, as well as good safety performance and cycling performance.

[0024] In one embodiment of the present application, a secondary battery includes an electrolyte, the electrolyte including compound A, and compound A includes at least one of fluoroethylene carbonate, bisfluoroethylene carbonate, 1,3-propane sultone, or vinyl sulfate. In another embodiment of the present application, the secondary battery includes an electrolyte, the electrolyte including compound A, and the weight percentage of compound A is 8% to 18% based on the weight of the electrolyte. Selecting the above-mentioned type of compound A and regulating the weight percentage of compound A within the above-mentioned range can preferentially reduce and form lithium fluoride and lithium sulfate, which are difficult to decompose and produce gas, and distribute them on the surface of the carbon layer before the carbonate solvent, thereby increasing the proportion of difficult-to-gas substances in the SEI film and stabilizing the structure of the SEI, thereby reducing the over-discharge gassing voltage during the secondary battery cycle and improving the safety and cycling performance of the secondary battery.

[0025] The fifth aspect of the present application provides an electronic device comprising the secondary battery of any of the aforementioned embodiments. The secondary battery of the present application has good safety performance and cycle performance, and therefore, the electronic device of the present application has a long service life.

[0026] Beneficial effects of this application:

[0027] The present application provides a negative electrode active material, including a silicon-carbon material. The silicon-carbon material particles include a carbon carrier, elemental silicon, and a carbon coating layer. The average thickness of the carbon coating layer is 2.5 nm to 12 nm. X-ray photoelectron spectroscopy analysis of the carbon coating layer shows that, based on the sum of the peak areas of the CC peak, the CO peak, and the C=O peak, the CC peak accounts for 60% to 95%. The present application improves the quality of the SEI film, reduces the over-discharge gassing voltage during the cycling of the secondary battery, and improves the safety and cycling performance of the secondary battery by designing the particle structure of the silicon-carbon material and regulating the average thickness of the carbon coating layer and the CC peak area ratio within the scope of the present application.

[0028] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0030] FIG1 is an XPS spectrum of the silicon-carbon material of Example 1-1 of the present application;

[0031] FIG2 is a HRTEM image of the silicon-carbon material of Example 1-1 of the present application;

[0032] FIG3 is an XRD spectrum of the silicon-carbon material of Example 1-1 of the present application;

[0033] FIG4 is a SEM image of the silicon-carbon material of Example 1-1 of the present application;

[0034] FIG5 is an SEM image of the silicon-carbon material of Examples 1-15 of the present application. DETAILED DESCRIPTION

[0035] The following will be combined with the accompanying drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0036] It should be noted that, in the specific embodiments of the present application, lithium-ion batteries are used as an example of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0037] A first aspect of the present application provides a negative electrode active material, the negative electrode active material includes a silicon-carbon material, the particles of the silicon-carbon material include a carbon carrier, elemental silicon and a carbon coating layer, the average thickness H of the carbon coating layer is 2.5nm to 12nm, for example, the average thickness H of the carbon coating layer can be 2.5nm, 2.8nm, 3nm, 3.2nm, 3.5nm, 3.8nm, 4nm, 4.2nm, 4.5nm, 4.8nm, 5nm, 5.2nm, 5.5nm, 5.8nm, 6nm, 6.2nm, 6.5nm, 6.8nm, 7nm, 7.2nm, 7.5nm, 7.8nm, 8nm, 8.2nm, 8.5nm, 8.8nm, 9nm, 9.2nm, 9.5nm, 9.8nm, 10nm, 10.2nm, 10.5nm, 10.8nm, 11nm, 11.2nm, 11.5nm, 11.8nm, 12nm or a range consisting of any two of these values. The carbon coating is subjected to X-ray photoelectron spectroscopy analysis. Based on the sum of the peak area of ​​the CC corresponding peak, the peak area of ​​the CO corresponding peak and the peak area of ​​the C=O corresponding peak, the peak area of ​​the CC corresponding peak accounts for 60% to 95%. For example, the peak area of ​​the CC corresponding peak may be 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 95% or a range consisting of any two of these values.

[0038] When the average thickness of the carbon coating is too small, that is, less than the lower limit of the carbon coating of the present application, the carbon coating is prone to rupture during the cycle of the secondary battery, causing the Si inside the silicon-carbon material particles to contact the electrolyte, aggravating the interface side reactions during the cycle, causing the secondary battery to produce gas and expand, and reducing the safety performance of the secondary battery. When the average thickness of the carbon coating is too large, that is, greater than the upper limit of the carbon coating of the present application, the carbon content in the particles of the silicon-carbon material is too high and the silicon content is low, resulting in a decrease in the specific capacity of the silicon-carbon material. Regulating the average thickness of the carbon coating within the scope of the present application can effectively protect the contact between the silicon-lithium alloy and the electrolyte during the cycle of the secondary battery, reduce the generation of organic substances such as organic carbonate lithium, lithium carbonate and proton electrolyte in the SEI film, reduce the over-discharge gas production voltage of the secondary battery, and thus improve the safety performance of the secondary battery. When the peak area ratio of the CC corresponding peak is too low, that is, lower than the lower limit of the peak area ratio of the CC corresponding peak in this application, it indicates that there are too many CO and / or C=O on the surface of the carbon coating layer, which is easy to adsorb a large amount of organic matter such as lithium carbonate or proton electrolyte that is easy to produce gas during the formation of the SEI film. The over-discharge gas production voltage of the secondary battery is too high, which reduces the safety performance of the secondary battery. When the peak area ratio of the CC corresponding peak is too high, that is, higher than the upper limit of the peak area ratio of the CC corresponding peak in this application, it indicates that there are too few CO and / or C=O on the surface of the carbon coating layer. CO and / or C=O themselves have a certain electronegativity, which is conducive to the adsorption of SEI film components and enhances the bonding between the carbon coating layer and the SEI film. Therefore, when the CO and / or C=O ratio on the surface of the carbon coating layer is too low, the adsorption / bonding of the carbon coating layer to the SEI film will be reduced, thereby reducing the cycle performance of the secondary battery. By regulating the peak area ratio of the CC corresponding peak within the scope of this application, the ratio of CO and / or C=O on the surface of the carbon coating layer can be effectively controlled within an appropriate range, reducing the generation of CO, CO2 and H2 in the over-discharge gas, improving the adsorption / binding of the carbon coating layer to the SEI film, further reducing the over-discharge gas production voltage during the secondary battery cycle, and improving the cycle performance and safety performance of the secondary battery. This application designs the particle structure of the silicon-carbon material to regulate the average thickness of the carbon coating layer and the peak area ratio of the CC corresponding peak within the scope of this application, thereby improving the quality of the SEI film, reducing the over-discharge gas production voltage during the secondary battery cycle, and improving the safety performance and cycle performance of the secondary battery. In this application, the carbon coating layer is completely coated on the particle surface of the silicon-carbon material. At least part of the above-mentioned "complete coating" is a homogeneous carbon layer coating, that is, at least part of the carbon coating layer is uniformly coated on the particle surface of the silicon-carbon material with uniform thickness.

[0039] In the present application, when X-ray photoelectron spectroscopy (XPS) is used to analyze the components of the carbon coating of the silicon-carbon material, the silicon-carbon material can be stuck on the double-sided tape on the sample stage and placed in the sample chamber for testing. Conductive carbon tape cannot be used to prevent detection problems caused by the carbon on the conductive carbon tape. The following methods can be used to perform peak calculations on the final carbon spectrum: charge calibration; peak fitting of the XPS peak; peak area and ratio calculation of the spectrum after peak fitting using scientific drawing software (Origin) to obtain the ratio of CC, CO and / or C=O. Peak area refers to the integrated area of ​​the peak. The peak area ratio of CC = peak area of ​​the peak corresponding to CC / (peak area of ​​the peak corresponding to CC + peak area of ​​the peak corresponding to CO + peak area of ​​the peak corresponding to C=O).

[0040] In one embodiment of the present application, the coating form in the carbon coating layer includes a film distribution or a granular distribution. The film distribution makes the particle surface of the silicon-carbon material uniformly distributed, thereby homogenizing the formation of the SEI film, reducing the risk of the carbon coating layer being too thin and breaking, resulting in contact between the silicon-lithium alloy and the electrolyte. The granular distribution makes the particle surface of the silicon-carbon material have a certain roughness, which can improve the bonding between the particle surface of the silicon-carbon material and the negative electrode binder, thereby further enhancing the stability and adhesion of the SEI film on the particle surface of the silicon-carbon material. When the coating form within the above range is adopted, it is beneficial to reduce the risk of excessive contact area between the silicon-lithium alloy exposed by the carbon coating layer and the electrolyte during the cycle of the secondary battery, thereby reducing the over-discharge gas production voltage during the cycle of the secondary battery and improving the safety performance and cycle performance of the secondary battery. In the present application, the film distribution refers to the carbon coating layer covering the particle surface of the silicon-carbon material in the form of a thin film without protrusions. Granular distribution means that carbon particles are connected to cover the particle surface of the silicon-carbon material, wherein in a scanning electron microscope (SEM) area of ​​1000nm×1000nm, the number of carbon particles is 800 to 3000, the minimum particle size of the carbon particles can be 2.5nm, and the maximum particle size of the carbon particles can be 12nm.

[0041] In one embodiment of the present application, in the Raman spectrum of the carbon coating layer, 1200 cm -1 to 1400cm -1 The peak area of ​​the characteristic peak in the range is I D , 1500cm -1 to 1700cm -1 The peak area of ​​the characteristic peak in the range is I G , satisfying: 0.8≤I D / I G ≤1.2. For example, I D / I GThe value of can be 0.8, 0.82, 0.85, 0.88, 0.9, 0.92, 0.95, 0.98, 1.0, 1.02, 1.05, 1.08, 1.1, 1.12, 1.15, 1.18, 1.2 or a range consisting of any two of them. By regulating I D / I G The value of is within the above range, so that the degree of graphitization of the carbon coating is high. The carbon coating with a high degree of graphitization has good flexibility and conductivity, which is beneficial to restraining the expansion of silicon inside the silicon-carbon material particles, reducing the possibility of excessive contact with the electrolyte to generate a large amount of SEI film that is prone to over-discharge and gas production, reducing the over-discharge and gas production voltage during the cycle of the secondary battery, thereby improving the safety performance of the secondary battery. In addition, the improvement of the quality of the carbon coating is beneficial to improving the cycle performance and rate performance of the secondary battery. Therefore, the secondary battery has good safety performance and cycle performance. In the present application, it can be understood by those skilled in the art that since the Raman test is a scattering spectrum surface test, its detection depth is only 10nm at most, and since the average thickness of the carbon coating of the present application is 2.5nm to 12nm, and in the silicon-carbon material particles, the carbon coating is not directly adjacent to the carbon support, but the elemental silicon on the surface of the carbon support, so the test results obtained by Raman testing the silicon-carbon material can be identified as the test results obtained by Raman testing the carbon coating.

[0042] In one embodiment of the present application, the (002) interplanar spacing of the carbon coating layer is 0.335 nm to 0.352 nm. For example, the (002) interplanar spacing of the carbon coating layer can be 0.335 nm, 0.338 nm, 0.340 nm, 0.342 nm, 0.345 nm, 0.348 nm, 0.350 nm, 0.352 nm, or a range consisting of any two of these values. In the present application, the peak area ratio of CC and I D / I G The value of regulates the graphitization degree of the carbon coating layer, thereby affecting the (002) crystal plane spacing of the carbon coating layer. By regulating the (002) crystal plane spacing of the carbon coating layer within the above range, it is beneficial to improve the order of the carbon coating layer and play a role in screening the solvent. When the solvated lithium ions in the electrolyte enter the negative electrode active material particles, the (002) crystal plane spacing within the scope of this application can block the solvent outside the silicon-carbon material particles, reducing the contact between the solvent and the silicon-lithium alloy inside the silicon-carbon material particles, thereby reducing the possibility of repeated growth of SEI, reducing the over-discharge gas generation voltage during the secondary battery cycle, and thus improving the safety performance and cycle performance of the secondary battery.

[0043] In one embodiment of the present application, the elastic modulus of the carbon coating layer is 12 GPa to 20 GPa. For example, the elastic modulus of the carbon coating layer can be 12 GPa, 13 GPa, 14 GPa, 15 GPa, 16 GPa, 17 GPa, 18 GPa, 19 GPa, 20 GPa or a range consisting of any two values ​​therein. In the present application, the peak area ratio of CC and I D / I G The value of affects the elastic modulus of the carbon coating layer. When the peak area ratio of CC is higher, I D / I G A smaller value indicates fewer surface impurity groups on the carbon coating, a higher degree of graphitization, and a higher elastic modulus. By regulating the elastic modulus of the carbon coating within this range, the likelihood of the carbon coating rupturing during secondary battery cycling due to the significant expansion stress generated by silicon within the silicon-carbon material particles is reduced. This further reduces contact between the solvent and the silicon-lithium alloy within the silicon-carbon material particles, thereby reducing the likelihood of repeated SEI growth and lowering the over-discharge gassing voltage during secondary battery cycling, thereby improving the safety and cycling performance of the secondary battery.

[0044] In one embodiment of the present application, the specific surface area of ​​the silicon carbon material particles is 1m 2 / g to 3.5m 2 / g, preferably, the specific surface area of ​​the silicon carbon material particles is 1.5m 2 / g to 3m 2 / g. For example, the specific surface area of ​​silicon carbon material particles can be 1m 2 / g, 1.2m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2m 2 / g, 2.2m 2 / g, 2.5m 2 / g, 2.8m 2 / g、3m 2 / g, 3.2m 2 / g, 3.5m 2 / g or a range consisting of any two of the values. In the present application, the amount of carbon coating on the surface of the silicon-carbon material particles affects the average thickness of the carbon coating layer, and thus affects the specific surface area of ​​the silicon-carbon material particles. By regulating the specific surface area of ​​the silicon-carbon material particles within the above range, it is beneficial to control the contact area between the carbon coating layer and the electrolyte within a suitable range, thereby reducing the excessive content of the carbon coating layer that reacts with the electrolyte to form an SEI film, resulting in the possibility of increased degradation and gas production of the SEI film when the secondary battery is over-discharged, thereby reducing the over-discharge gas production voltage during the cycle of the secondary battery, and thus improving the safety performance and cycle performance of the secondary battery.

[0045] In one embodiment of the present application, the Dv50 of the silicon-carbon material is 5 μm to 10 μm, and the Dv99 of the silicon-carbon material is 15 μm to 20 μm. For example, the Dv50 of the silicon-carbon material can be 5 μm, 5.2 μm, 5.5 μm, 5.8 μm, 6 μm, 6.2 μm, 6.5 μm, 6.8 μm, 7 μm, 7.2 μm, 7.5 μm, 7.8 μm, 8 μm, 8.2 μm, 8.5 μm, 8.8 μm, 9.2 μm, 9.5 μm, 9.8 μm, 10 μm, or a range consisting of any two of these values; the Dv99 of the silicon-carbon material can be 15 μm to 20 μm. 9 can be 15μm, 15.2μm, 15.5μm, 15.8μm, 16μm, 16.2μm, 16.5μm, 16.8μm, 17μm, 17.2μm, 17.5μm, 17.8μm, 18μm, 18.2μm, 18.5μm, 18.8μm, 19.2μm, 19.5μm, 19.8μm, 20μm or a range consisting of any two of these values. By regulating the Dv50 and Dv99 of the silicon-carbon material within the above range, during the process of preparing the negative electrode sheet, that is, after stirring the negative electrode slurry and coating it on the negative electrode current collector, the silicon-carbon material within the above particle size range is mixed with graphite, so that the spatial distribution of the negative electrode active material in the negative electrode material layer is more reasonable, and the face-to-face contact protection of the graphite particles on the silicon-carbon material particles is maximized, further reducing the contact between the carbon coating layer of the silicon-carbon material and the electrolyte, reducing the possibility of excessive SEI film formation, thereby reducing the over-discharge gassing voltage during the secondary battery cycle, and thus improving the safety and cycle performance of the secondary battery. In this application, Dv50 refers to the particle size that reaches 50% of the volume cumulative value from the smallest particle size in the volume-based particle size distribution of the silicon-carbon material; Dv99 refers to the particle size that reaches 99% of the volume cumulative value from the smallest particle size in the volume-based particle size distribution of the silicon-carbon material.

[0046] In one embodiment of the present application, Dv10 of the silicon-carbon material is D1 μm, Dn10 of the silicon-carbon material is D2 μm, 1.5≤D1 / D2≤2, and 3≤D1≤6. For example, the value of D1 / D2 may be 1.5, 1.52, 1.55, 1.58, 1.6, 1.62, 1.65, 1.68, 1.7, 1.72, 1.75, 1.78, 1.8, 1.82, 1.85, 1.88, 1.9, 1.92, 1.95, 1.98, 2, or a range consisting of any two values ​​thereof, and the value of D1 may be 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, 5.2, 5.5, 5.8, 6, or a range consisting of any two values ​​thereof. By regulating the values ​​of D1 / D2 and D1 within the above ranges, during the preparation of the negative electrode sheet, i.e., after stirring the negative electrode slurry and coating it on the negative electrode current collector, the silicon-carbon material within the above particle size range is mixed with graphite, so that the spatial distribution of the negative electrode active material in the negative electrode material layer is more reasonable, maximizing the face-to-face contact protection of the graphite particles on the silicon-carbon material particles, further reducing the contact between the carbon coating layer of the silicon-carbon material and the electrolyte, and reducing the possibility of excessive SEI film formation, thereby reducing the over-discharge gassing voltage during the secondary battery cycle, thereby improving the safety and cycle performance of the secondary battery. In this application, Dv10 refers to the particle size that reaches 10% of the volume cumulative value, measured from the small particle size, in the volume-based particle size distribution of the silicon-carbon material; Dn10 refers to the particle size that reaches 10% of the number cumulative value, measured from the small particle size, in the number-based particle size distribution of the silicon-carbon material.

[0047] In one embodiment of the present application, the negative electrode active material further comprises graphite, and the graphite comprises at least one of artificial graphite or natural graphite. Based on the mass of the negative electrode active material, the mass percentage of the silicon-carbon material is 5% to 30%, and the mass percentage of the graphite is 70% to 95%. For example, the mass percentage of the silicon-carbon material can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, or a range consisting of any two of these values, and the mass percentage of the graphite can be 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 95%, or a range consisting of any two of these values. By regulating the mass percentage of silicon-carbon material and the mass percentage of graphite within the above range, the high surface stability of graphite can be used to protect the interface of silicon-carbon material particles to a certain extent, and the surface of the graphite particles is in contact with the carbon coating, which is beneficial to reduce the contact area between the carbon coating and the electrolyte, thereby reducing the excessive generation of SEI film, thereby reducing the over-discharge gas production voltage during the cycle of the secondary battery, and thus improving the safety performance and cycle performance of the secondary battery. The present application does not particularly limit the method for regulating the mass percentage of silicon-carbon material and the mass percentage of graphite, as long as the purpose of the present application can be achieved. For example, the mass percentage of silicon-carbon material and the mass percentage of graphite can be regulated by regulating the mass ratio of silicon-carbon material and graphite in the negative electrode active material.

[0048] A second aspect of the present application provides a method for preparing a negative electrode active material, which comprises the following steps:

[0049] (1) The carbon support is heated in an inert atmosphere at a heating rate v of 5°C / min to 20°C / min to a temperature T1 of 400°C to 480°C, and then kept warm. For example, the heating rate v in the inert atmosphere can be 5°C / min, 8°C / min, 10°C / min, 12°C / min, 15°C / min, 18°C / min, 20°C / min, or a range consisting of any two values ​​therein, and the carbon support can be heated to a temperature T1 of 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, or a range consisting of any two values ​​therein. The deposition time t1 in the silane gas mixture is then 2 to 30 hours. For example, the deposition time t1 can be 2 hours, 5 hours, 8 hours, 10 hours, 12 hours, 15 hours, 18 hours, 20 hours, 22 hours, 25 hours, 28 hours, 30 hours, or a range consisting of any two of these values, to obtain an intermediate. The silane gas mixture comprises silane and an inert gas, and the silane content is 10% to 50% by weight, and the inert gas content is 50% to 90% by weight, based on the mass of the silane gas mixture. For example, the mass percentage of silane can be 10%, 13%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, 33%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, or a range consisting of any two of the values ​​therein; the mass percentage of inert gas can be 50%, 53%, 55%, 58%, 60%, 63%, 65%, 68%, 70%, 73%, 75%, 78%, 80%, 83%, 85%, 88%, 90%, or a range consisting of any two of the values ​​therein.

[0050] (2) The intermediate is subjected to carbon coating treatment to obtain a silicon-carbon material; wherein the carbon coating treatment method includes any one of a liquid phase method, a gas phase method or a gas-liquid combined method, the carbon source includes at least one of coal tar pitch, petroleum asphalt, acetylene, propylene, natural gas, ethylene or cyclohexane, the carbon coating treatment temperature T2 is 620°C to 810°C, and the carbon coating treatment time t2 is 1h to 6h. For example, the temperature T2 of the carbon coating treatment can be 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, 810℃ or a range consisting of any two of the values ​​therein, and the time t2 of the carbon coating treatment can be 1h, 1.2h, 1.5h, 1.8h, 2h, 2.2h, 2.5h, 2.8h, 3h, 3.2h, 3.5h, 3.8h, 4h, 4.2h, 4.5h, 4.8h, 5h, 5.2h, 5.5h, 5.8h, 6h or a range consisting of any two of the values ​​therein.

[0051] The present application has no particular restriction on the holding time of the carbon carrier after the temperature is raised to T1 of 400°C to 480°C at a heating rate v of 5°C / min to 20°C / min in an inert atmosphere, as long as the purpose of the present application can be achieved. For example, the holding time can be 10min to 60min. The present application has no particular restriction on the carbon carrier, as long as the purpose of the present application can be achieved. In the present application, the gas-liquid combination method refers to pre-coating by gas phase method first, and then transferring to the liquid phase system for liquid phase coating, which includes the following steps: pre-coating the intermediate with carbon in a reducing atmosphere; wherein the reducing atmosphere includes at least one of acetylene, propylene, natural gas, ethylene or cyclohexane, the temperature of the carbon coating treatment is 500°C to 600°C, and the time of the carbon coating treatment is 1h to 3h. The pre-coated intermediate is then placed in an organic solvent containing a carbon source to obtain an organic solution, the organic solution is volatilized, and carbon coating treatment is performed under inert gas to obtain silicon-carbon composite particles; wherein the carbon source includes at least one of petroleum asphalt or coal tar asphalt, the organic solvent includes at least one of tetrahydrofuran, n-hexane or xylene, the temperature of the carbon coating treatment is 700°C to 810°C, and the time of the carbon coating treatment is 1 hour to 2 hours.

[0052] In one embodiment of the present application, the method of carbon coating treatment includes a liquid phase method, which includes the following steps: placing the intermediate in an organic solvent containing a carbon source to obtain an organic solution, volatilizing the organic solution, and performing carbon coating treatment under an inert atmosphere to obtain a silicon-carbon material; wherein the carbon source includes at least one of petroleum asphalt or coal tar asphalt, the organic solvent includes at least one of tetrahydrofuran, n-hexane or xylene, the temperature of the carbon coating treatment is 700°C to 810°C, and the time of the carbon coating treatment is 1h to 2h. For example, the temperature of the carbon coating treatment can be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, 810°C or a range consisting of any two of the values ​​therein, and the time of the carbon coating treatment can be 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h or a range consisting of any two of the values ​​therein.

[0053] In one embodiment of the present application, the method of carbon coating treatment includes a gas phase method, which includes the following steps: carbon coating the intermediate in a reducing atmosphere to obtain a silicon-carbon material; wherein the reducing atmosphere includes at least one of acetylene, propylene, natural gas, ethylene or cyclohexane, the temperature of the carbon coating treatment is 620°C to 700°C, and the time of the carbon coating treatment is 2h to 6h. For example, the temperature of the carbon coating treatment can be 620°C, 630°C, 640°C, 650°C, 660°C, 670°C, 680°C, 690°C, 700°C or a range consisting of any two values ​​therein, and the time of the carbon coating treatment can be 2h, 2.2h, 2.5h, 2.8h, 3h, 3.2h, 3.5h, 3.8h, 4h, 4.2h, 4.5h, 4.8h, 5h, 5.2h, 5.5h, 5.8h, 6h or a range consisting of any two values ​​therein.

[0054] Compared with the liquid phase method, the carbon coating layer formed when the gas phase method is used for carbon coating is more uniform. When the two methods are used to prepare negative electrode active materials, the temperature of the carbon coating treatment is higher, which can regulate the average thickness of the carbon coating layer in the silicon-carbon material particles and improve its quality. It can also increase the degree of graphitization of the carbon coating layer, thereby improving the quality and content of the SEI film, effectively protecting the contact between the silicon-lithium alloy and the electrolyte during the secondary battery cycle, reducing the possibility of repeated growth of the SEI film, reducing the amount of organic matter such as organic carbonate lithium, lithium carbonate and proton electrolyte in the solid electrolyte (SEI) film, reducing the over-discharge gas generation voltage of the secondary battery, and thus improving the safety performance and cycle performance of the secondary battery.

[0055] The present application does not impose any particular restrictions on the method for regulating the average thickness of the carbon coating layer, as long as the purpose of the present application can be achieved. For example, when the carbon coating treatment is performed by a vapor phase method, the average thickness of the carbon coating layer can be regulated by regulating the time t2 of the carbon coating treatment when preparing the negative electrode active material. For example, when other conditions remain unchanged, the average thickness of the carbon coating layer increases by extending the time t2 of the carbon coating treatment; the average thickness of the carbon coating layer decreases by shortening the time t2 of the carbon coating treatment. The present application does not impose any particular restrictions on the method for regulating the peak area ratio of the CC corresponding peak, as long as the purpose of the present application can be achieved. For example, the peak area ratio of the CC corresponding peak can be regulated by regulating the carbon source or the temperature T2 of the carbon coating treatment. For example, when the carbon coating treatment is performed by a vapor phase method, at a higher temperature, when the reducing atmosphere is acetylene, the peak area ratio of the CC corresponding peak is higher; when other conditions remain unchanged, the peak area ratio of the CC corresponding peak increases when the temperature T2 of the carbon coating treatment is increased; the peak area ratio of the CC corresponding peak decreases when the temperature T2 of the carbon coating treatment is decreased. In the present application, the coating form in the carbon coating layer can be regulated by regulating the carbon coating treatment method. For example, when the carbon coating treatment is performed by a gas phase method, the coating form in the carbon coating layer is a film-like distribution, and when the carbon coating treatment is performed by a liquid phase method, the coating form in the carbon coating layer is a granular distribution. D / I G There is no particular limitation on the control method, as long as the purpose of this application can be achieved. For example, the temperature T2 of the carbon coating treatment can be controlled. D / I G For example, when other conditions remain unchanged, the temperature T2 of the carbon coating treatment is increased. D / I G The value of the carbon coating treatment decreases; reduce the temperature T2, I D / I GThe value increases. The present application has no particular restrictions on the method for regulating the (002) interplanar spacing, as long as the purpose of the present application can be achieved. For example, the (002) interplanar spacing can be regulated by regulating the temperature T2 of the carbon coating treatment. For example, when other conditions remain unchanged, as the temperature T2 of the carbon coating treatment increases, the value of the (002) interplanar spacing will be closer to the value of the theoretical (002) interplanar spacing of graphite. The present application has no particular restrictions on the method for regulating the elastic modulus, as long as the purpose of the present application can be achieved. For example, the elastic modulus can be regulated by regulating the parameters of the carbon coating treatment method. For example, when the liquid phase method is used for carbon coating, the elastic modulus can be regulated by whether to apply pressure and the size of the pressure. When other conditions remain unchanged and no pressure treatment is performed, the elastic modulus of the carbon coating layer is smaller. When the gas phase method is used for carbon coating, the elastic modulus can be regulated by the temperature T2 of the carbon coating treatment. When other conditions remain unchanged, increasing the temperature T2 of the carbon coating treatment increases the elastic modulus; decreasing the temperature T2 of the carbon coating treatment decreases the elastic modulus. The present application has no particular restrictions on the method for regulating the specific surface area, as long as the purpose of the present application can be achieved. For example, when the carbon coating treatment is carried out by the gas phase method, the specific surface area can be regulated by regulating the time t2 of the carbon coating treatment. For example, when other conditions remain unchanged, the specific surface area decreases by extending the time t2 of the carbon coating treatment; the specific surface area increases by shortening the time t2 of the carbon coating treatment. In addition, it can be understood that the specific surface area of ​​the silicon-carbon material is also correlated with the Dv50 and Dv99 of the silicon-carbon material. For example, when other conditions remain unchanged, when the Dv50 and Dv99 of the silicon-carbon material are larger, the specific surface area is smaller; when the Dv50 and Dv99 of the silicon-carbon material are smaller, the specific surface area is larger. The present application has no particular restrictions on the method for regulating the Dv50, Dv99, Dv10 and Dn10 of the silicon-carbon material, as long as the purpose of the present application can be achieved. For example, the Dv50, Dv99, Dv10 and Dn10 of the silicon-carbon material can be regulated by regulating the particle size of the carbon carrier. The Dv50, Dv99, Dv10, and Dn10 of the silicon-carbon material can also be controlled by classifying and grinding the silicon-carbon material particles. For example, when other conditions remain unchanged, extending the grinding time reduces the Dv50, Dv99, Dv10, and Dn10 of the silicon-carbon material; shortening the grinding time increases the Dv50, Dv99, Dv10, and Dn10 of the silicon-carbon material.

[0056] The third aspect of the present application provides a negative electrode plate, comprising a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises the negative electrode active material provided in the first aspect of the present application or the negative electrode active material prepared according to the preparation method provided in the second aspect of the present application. Application of the negative electrode plate of the present application to a secondary battery can reduce the over-discharge gassing voltage during the secondary battery cycle, thereby improving the safety and cycling performance of the secondary battery.

[0057] In the present application, the above-mentioned "negative electrode material layer located on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be located on one surface of the negative electrode current collector along its thickness direction, or on both surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of ​​the surface of the negative electrode current collector, or it can be a partial area of ​​the surface of the negative electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or a composite current collector (such as a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.). In the present application, there is no special restriction on the thickness of the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode current collector is 4μm to 20μm. Optionally, the negative electrode material layer may also include a negative electrode binder and a conductive agent. The present application does not particularly limit the type of negative electrode binder in the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the negative electrode binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon. The present application does not particularly limit the type of conductive agent in the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the conductive agent may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, metal materials or conductive polymers. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The above-mentioned metal material may include, but is not limited to, metal powder and / or metal fiber. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The present application has no particular restrictions on the mass ratio of the negative electrode active material, conductive agent, and negative electrode binder in the negative electrode material layer, as long as the purpose of the present application can be achieved. Optionally, the negative electrode active material layer also includes a thickener, which may include, but is not limited to, sodium carboxymethyl cellulose.

[0058] This application does not impose any particular restrictions on the preparation method of the negative electrode sheet, as long as the objectives of this application can be achieved. For example, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: dispersing silicon-carbon material with graphite, a conductive agent, a negative electrode binder, and a thickener in a solvent to form a uniform negative electrode slurry, coating the negative electrode slurry on the negative electrode current collector, and obtaining the negative electrode sheet through processes such as drying and cold pressing. This application does not impose any particular restrictions on the solvent, as long as the objectives of this application can be achieved. For example, the solvent may include deionized water. This application does not impose any particular restrictions on the solid content of the negative electrode slurry, as long as the objectives of this application can be achieved.

[0059] A fourth aspect of the present application provides a secondary battery comprising the negative electrode sheet of any of the aforementioned embodiments. The secondary battery provided in the fourth aspect of the present application has a low over-discharge gas generation voltage during cycling, as well as good safety performance and cycling performance.

[0060] In one embodiment of the present application, a secondary battery includes an electrolyte, the electrolyte including compound A, and compound A includes at least one of fluoroethylene carbonate (FEC), bisfluoroethylene carbonate (DFEC), 1,3-propane sultone, or vinyl sulfate. Compound A selected from the above types has a high reduction potential. During the secondary battery cycle, compound A can be reduced to lithium fluoride and lithium sulfate, which are difficult to decompose and produce gas, in preference to carbonate solvents during the secondary battery cycle. These lithium fluoride and lithium sulfate are distributed on the surface of the carbon layer, increasing the proportion of difficult-to-gas substances in the SEI film and stabilizing the SEI structure, thereby reducing the over-discharge gassing voltage during the secondary battery cycle and improving the safety and cycle performance of the secondary battery.

[0061] In one embodiment of the present application, the secondary battery includes an electrolyte, the electrolyte includes compound A, and the mass percentage of compound A is 8% to 18% based on the mass of the electrolyte. For example, the mass percentage of compound A can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18% or a range consisting of any two values ​​therein. By regulating the mass percentage of compound A within the above range, the reduction potential of compound A is relatively high. During the cycle of the secondary battery, lithium fluoride and lithium sulfate that are difficult to decompose and produce gas can be reduced to the surface of the carbon layer in preference to carbonate solvents, thereby increasing the proportion of difficult-to-gas substances in the SEI film and stabilizing the structure of the SEI, thereby reducing the over-discharge gas production voltage during the cycle of the secondary battery and improving the safety and cycle performance of the secondary battery.

[0062] In the present application, in addition to compound A, the electrolyte also includes a lithium salt and a non-aqueous solvent. The lithium salt may include at least one of LiPF6, LiPO2F2, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalatoborate) (LiBOB), lithium bis(trifluoromethanesulfonyl imide) (LiTFSI) or lithium difluoroborate. The present application does not limit the content of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, the non-aqueous solvent may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvents. The above-mentioned carbonate compound may include but is not limited to at least one of a chain carbonate compound, a cyclic carbonate compound or a fluorinated carbonate compound. Above-mentioned linear carbonate compound can include but not limited to at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate or methyl ethyl carbonate.Above-mentioned cyclic carbonate can include but not limited to at least one of propylene carbonate (PC), butylene carbonate or vinyl ethylene carbonate.Fluorinated carbonate compound can include but not limited to at least one of carbonic acid-1,2-difluoro ethylene, carbonic acid-1,1-difluoro ethylene, carbonic acid-1,1,2-trifluoro ethylene, carbonic acid-1,1,2,2-tetrafluoro ethylene, carbonic acid-1-fluoro-2-methyl ethylene, carbonic acid-1-fluoro-1-methyl ethylene, carbonic acid-1,2-difluoro-1-methyl ethylene, carbonic acid-1,1,2-trifluoro-2-methyl ethylene or carbonic acid trifluoromethyl ethylene at least one. The carboxylate compound may include but is not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid, valerolactone or caprolactone. The ether compound may include but is not limited to at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate.

[0063] The secondary battery of the present application includes a positive electrode sheet. The present application has no particular restrictions on the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the positive electrode sheet includes a positive electrode collector and a positive electrode material layer located on at least one surface of the positive electrode collector. The above-mentioned "positive electrode material layer located on at least one surface of the positive electrode collector" means that the positive electrode material layer can be located on one surface of the positive electrode collector along the thickness direction of itself, or on two surfaces of the positive electrode collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of ​​the surface of the positive electrode collector, or it can be a partial area of ​​the surface of the positive electrode collector. The present application has no particular restrictions, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the positive electrode collector, as long as the purpose of the present application can be achieved. For example, the positive electrode collector can include aluminum foil, aluminum alloy foil or a composite current collector, etc. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate. The positive electrode material layer of the present application comprises a positive electrode active material. The present application has no particular restrictions on the type of positive electrode active material, as long as it can achieve the purpose of the present application. For example, the positive electrode active material may comprise lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 O2 (NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganate, lithium manganese iron phosphate or lithium titanate, etc. At least one of the following. In the present application, the positive electrode active material may also contain non-metallic elements, for example, the non-metallic elements include at least one of fluorine, phosphorus, boron, chlorine, silicon or sulfur. In the present application, there is no particular restriction on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm, and the thickness of the single-sided positive electrode material layer is 30μm to 120μm. In the present application, the positive electrode material layer may also include a positive electrode binder and a conductive agent. The present application does not particularly limit the type of positive electrode binder in the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the positive electrode binder can be the same type as the positive electrode binder in the above-mentioned negative electrode material layer. The present application does not particularly limit the type of conductive agent in the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the conductive agent can be the same type as the conductive agent in the above-mentioned negative electrode material layer. The present application does not particularly limit the mass ratio of the positive electrode active material, conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved.

[0064] In the present application, the secondary battery also includes a separator. The present application has no particular restrictions on the separator, as long as the purpose of the present application can be achieved. For example, the material of the separator may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of separator may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane.

[0065] In some embodiments of the present application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. In some embodiments of the present application, the inorganic layer includes inorganic particles and a binder. The present application has no particular restrictions on the inorganic particles. For example, the inorganic particles may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application has no particular restrictions on the binder, for example, the binder can be at least one of the above-mentioned binders. In some embodiments of the present application, the polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl ether or polyvinylidene fluoride or poly (vinylidene fluoride-hexafluoropropylene). In the present application, the thickness of the diaphragm is not particularly limited, as long as the purpose of the present application can be achieved, for example, the thickness of the diaphragm can be 3 μm to 30 μm.

[0066] The secondary battery of this application also includes a packaging bag for containing the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte, as well as other components of the secondary battery known in the art. This application does not limit these other components. This application does not specifically limit the packaging bag and can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0067] The present application does not particularly limit the preparation method of the secondary battery, and the battery can be prepared by methods known to those skilled in the art, as long as the purpose of the present application can be achieved. For example, the preparation method of the secondary battery may include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, as needed, an overcurrent protection element, a guide plate, etc. can be placed in the packaging bag to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0068] The secondary battery of the present application is not particularly limited and may include any device that generates an electrochemical reaction. In one embodiment of the present application, the secondary battery may include, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery (lithium ion polymer battery).

[0069] The fifth aspect of the present application provides an electronic device comprising the secondary battery of any of the aforementioned embodiments. The secondary battery of the present application has good safety performance and cycle performance, and therefore, the electronic device of the present application has a long service life.

[0070] The electronic device of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0071] Example

[0072] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0073] Test methods and equipment:

[0074] Average thickness of carbon coating layer and (002) crystal plane spacing test:

[0075] The silicon-carbon material was sliced ​​using a focused ion beam (FIB) and characterized using a high-resolution transmission electron microscope (HRTEM, Talos F200X). Within the same selected area (500,000x magnification), the thickness of the carbon coating was measured at five randomly selected locations within the silicon-carbon material particles. The arithmetic mean of the thicknesses at the five different locations was calculated to obtain the average thickness of the carbon coating. The lattice fringes of the carbon coating were observed at a higher magnification (1,000,000x magnification). Combined with transmission electron microscopy analysis (i.e., digital micrograph image processing), the Fourier transform (FFT) image was obtained, and the (002) interplanar spacing of the carbon coating was calculated.

[0076] Peak area ratio test of CC corresponding peak:

[0077] XPS was used to analyze the components of the carbon coating of the silicon-carbon material. The silicon-carbon material was attached to the double-sided tape on the sample stage and placed in the sample chamber for testing.

[0078] Peak calculation: (1) Charge calibration. (2) Perform peak fitting of the XPS peaks. (3) Calculate the integrated area ratio of the peak-fitted spectrum using scientific graphics software (Origin) to obtain the ratios of CC, CO, and C=O. The peak area ratio of the CC peak = integrated area of ​​CC / (integrated area of ​​CC + integrated area of ​​CO + integrated area of ​​C=O).

[0079] For example, Figure 1 is an XPS spectrum of Example 1-1 after peak fitting, wherein the blue line (i.e., the black solid dotted line in the figure) is the baseline, the black line (i.e., the black solid line in the figure) represents the original test curve of Example 1-1, and the red line (i.e., the black hollow dashed line in the figure) is the curve after fitting of Example 1-1. It can be seen from the figure that the carbon coating layer on the surface of the silicon-carbon material contains CC, CO and C=O. The peak area ratio of the peak corresponding to CC = the peak area of ​​the peak corresponding to CC / (the peak area of ​​the peak corresponding to CC + the peak area of ​​the peak corresponding to CO + the peak area of ​​the peak corresponding to C=O), that is, the peak area ratio of the peak corresponding to CC of Example 1-1 is 85.2%. The peak area refers to the integrated area of ​​the peak.

[0080] Raman testing and I D / I G Calculation:

[0081] The silicon carbon material is loaded on a flat glass slide for Raman testing, with a test range of 100 cm -1 Up to 2000cm -1;After the test is completed, observe 1200cm -1 to 1400cm -1 Between the carbon D peak and 1500cm -1 to 1700cm -1 The peak position of the peak is taken as the maximum point of the peak, and the half-peak width is taken as the half-peak width of the peak. The Raman spectrum is subjected to peak fitting and I D / I G Integral area ratio calculation.

[0082] Calculation formula: I D / I G =D peak integrated area / G peak integrated area.

[0083] Carbon coating morphology test:

[0084] Scanning electron microscopy (SEM) was used to characterize the particle surface of the silicon-carbon material and observe the coating form of the carbon coating layer.

[0085] Elastic modulus test:

[0086] The elastic modulus of the carbon coating of the silicon-carbon material was tested using an atomic force microscope (AFM). The test temperature was 25°C and the humidity was 50%. A single particle of silicon-carbon material was selected under the microscope, and a probe was placed in contact with the carbon coating on the particle's surface. The stress changes in the carbon coating were measured by controlling the probe's position and force. By measuring the stress change curve of the carbon coating, the elastic modulus of the carbon coating of the silicon-carbon material particle can be obtained. Using atomic force microscope data analysis software (Nanoscope8.1), the experimental indentation stress curve data was fitted to the tip-sample contact model, allowing direct analysis and reading of the elastic modulus data.

[0087] Specific surface area test:

[0088] The specific surface area of ​​the silicon-carbon material particles was measured using a specific surface area measuring instrument (model: TriStar II 3020) by means of the gas adsorption BET method.

[0089] Test of mass percentage of silicon-carbon material and mass percentage of graphite:

[0090] The mass percentage of silicon-carbon material and the mass percentage of graphite were determined by alkaline etching, high-temperature oxidation and thermogravimetric analysis. 0.5g of a negative electrode active material mixed with silicon-carbon material and graphite, as well as 1.5g of sodium hydroxide, were added to a crucible. After heating to 300°C and then cooling to below 100°C, the reactants were placed in deionized water at 80°C, washed, filtered, dried and weighed. The mass loss was the mass of silicon. The dried powder was then subjected to thermogravimetric analysis in an air atmosphere (model: STA499C). The sample experienced mass loss before 700°C, which was the mass loss of carbon in the silicon-carbon material. The mass loss ratio of silicon plus the mass loss ratio between 300°C and 700°C is the mass percentage of silicon-carbon material. Continue heating to 1200°C, and a second mass loss will occur between 700°C and 1200°C. The mass loss ratio at this stage is the mass percentage of graphite.

[0091] Particle size test:

[0092] In a 50mL clean beaker, add 0.02g of silicon-carbon material particles and 20mL of deionized water. Ultrasonicate for 5 minutes using a 120W ultrasonic cleaner to completely disperse the silicon-carbon material particles in the deionized water to obtain a sample dispersion. This sample dispersion was then measured using a particle size analyzer (MasterSizer 2000) to determine the particle sizes Dv50, Dv99, Dv10, and Dn10 of the silicon-carbon material.

[0093] XRD test:

[0094] The silicon-carbon material was tested using an X-ray powder diffractometer (instrument model: Bruker D8 ADVANCE) with a Cu Kα target; the voltage was 40 kV, the current was 40 mA, the scanning angle range was 10° to 80°, and the scanning rate was 2° / min.

[0095] Over-discharge gas production voltage test:

[0096] At 25°C, the prepared lithium-ion battery was charged to 4.5V at a constant current rate of 0.1C, then charged to a current of 0.05C at a constant voltage of 4.5V, and then allowed to stand for 5 minutes. The charge capacity at this time was recorded, which was the first cycle charge capacity; then discharged to 3.0V at a constant current rate of 0.1C, and then allowed to stand for 5 minutes. The above charge and discharge process was the first cycle activation process. The subsequent electrochemical cycle was carried out under a fast charge cycle program of charging to 4.5V at a constant current of 1C and discharging to 3.0V at a constant current of 0.5C. After 100 cycles, an over-discharge gas production test was performed, and the thickness of the lithium-ion battery at this time was recorded as TH1; then the battery was over-discharged at a discharge rate of 0.01C to 3.0V, discharged at 10mA to 1.0V, and then discharged at 5mA to 1.0V. The thickness of the lithium-ion battery at this time was recorded as TH2. During this period, the thickness was measured every 30s, and the voltage when the thickness of the lithium-ion battery suddenly changed was recorded as the over-discharge gas production voltage.

[0097] Cyclic performance test:

[0098] The test temperature was 25°C. The lithium-ion batteries in the examples and comparative examples were charged at a constant current of 0.1V to 4.5V, then at a constant voltage of 4.5V to 0.05V. After standing for 5 minutes, they were discharged at a rate of 0.1V to 3.0V. The capacity obtained in this step was taken as the initial capacity. Cycling tests were performed according to the above cycle process. The capacity of the lithium-ion battery was measured after each cycle. The capacity of each cycle was compared to the initial capacity to obtain a capacity decay curve. The charge and discharge cycles were repeated with the initial discharge capacity as 100%. The test was stopped when the discharge capacity retention rate decayed to 80% of the initial discharge capacity, and the number of cycles was recorded.

[0099] Example 1-1

[0100] <Preparation of Silicon Carbon Materials>

[0101] 100 g of porous carbon skeleton (Dv50 = 8.7 μm, Dv99 = 18.9 μm) was heated at a heating rate v of 10°C / min to 450°C (T1) under an argon atmosphere, then held for 10 minutes. The atmosphere was then switched to a silane mixture, and deposition was performed in the silane mixture to obtain an intermediate. The deposition time t1 was 10 hours. The silane mixture consisted of silane and argon, with the mass percentage of silane being 20% ​​and the mass percentage of argon being 80% based on the mass of the silane mixture.

[0102] The atmosphere was then switched to a reducing atmosphere, and the intermediate was carbon-coated. The carbon-coating temperature T2 was 650°C, and the carbon-coating time t2 was 2.5 hours. The intermediate was then cooled to room temperature to obtain a black silicon-carbon material. The reducing atmosphere consisted of propylene and argon, with the weight percentage of propylene being 20% ​​and the weight percentage of argon being 80% based on the weight of the reducing atmosphere.

[0103] The prepared silicon-carbon material includes a porous carbon skeleton, elemental silicon and a carbon coating layer. The average thickness H of the carbon coating layer is 3.5 nm, the coating form in the carbon coating layer is a film-like distribution, the (002) crystal plane spacing of the carbon coating layer is 0.338 nm, the elastic modulus of the carbon coating layer is 16.2 GPa, and the specific surface area of ​​the silicon-carbon material particles is 1.13 m 2 / g, Dv50 of silicon-carbon material is 9μm, Dv99 of silicon-carbon material is 19μm, Dv10 of silicon-carbon material is 3μm, that is, D1 is 3μm, Dn10 of silicon-carbon material is 2μm, that is, D2 is 2μm, and D1 / D2 is 1.5.

[0104] <Preparation of negative electrode sheet>

[0105] The negative electrode active material, styrene-butadiene rubber (SBP), and sodium carboxymethyl cellulose (CMC) were mixed in a weight ratio of 97:2:1, and deionized water was added as a solvent to prepare a negative electrode slurry with a solid content of 40 wt%. The negative electrode slurry was then stirred evenly in a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on one surface of a 12 μm thick negative electrode current collector copper foil and dried at 85°C to obtain a negative electrode sheet coated on one side with a negative electrode material layer. The coating weight of the negative electrode material layer during coating was 10 mg / cm 2 The above steps are then repeated on the other surface of the copper foil to obtain a negative electrode sheet coated with a negative electrode material layer on both sides. After cold pressing, cutting, slitting, and drying under vacuum conditions at 120°C for 12 hours, a negative electrode sheet with a specification of 78mm×875mm is obtained for use. The compacted density of the negative electrode material layer after cold pressing is 1.7g / cc; the negative electrode active material includes artificial graphite and the silicon-carbon material prepared above, and the mass ratio of artificial graphite to silicon-carbon composite particles is 80:20.

[0106] <Preparation of positive electrode sheet>

[0107] The positive electrode active material lithium cobalt oxide (LiCoO2), conductive carbon black Super P, and positive electrode binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1.4:1.6, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a positive electrode slurry with a solid content of 72wt%. After vacuum stirring, the positive electrode slurry was obtained. The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 12μm and dried at 85℃ to obtain a positive electrode sheet with a single-sided positive electrode material layer. The coating weight of the positive electrode material layer during coating was 19mg / cm 2The above steps are then repeated on the other side of the aluminum foil to obtain a double-sided positive electrode sheet coated with a positive electrode material layer. This is followed by cold pressing, cutting, slitting, and drying under vacuum conditions at 85°C for 4 hours to obtain a 74mm x 867mm positive electrode sheet ready for use. The compacted density of the positive electrode material layer after cold pressing is 4.2g / cc.

[0108] <Preparation of Electrolyte>

[0109] In an argon atmosphere glove box with a water content of less than 10 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a weight ratio of EC:EMC:DEC = 30:50:20 to obtain a base solvent. Lithium hexafluorophosphate (LiPF6) was then added to the base solvent and mixed to obtain an electrolyte. The lithium salt content was 12.5% ​​by weight of the electrolyte, with the remainder being the base solvent.

[0110] <Preparation of Separator>

[0111] A porous polyethylene (PE) film with a thickness of 7 μm (provided by Celgard) was used as the separator.

[0112] <Preparation of lithium-ion batteries>

[0113] The positive electrode sheet, separator, negative electrode sheet, and separator are stacked in order, with the separator positioned between the positive and negative electrodes to provide insulation. The electrodes are then wound to form an electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag, dried at 80°C, and then injected with electrolyte. The soft-pack lithium-ion battery is then produced through vacuum packaging, resting, formation, shaping, and capacity testing.

[0114] Example 1-2 to Example 1-11

[0115] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0116] Examples 1-12

[0117] The process is the same as Example 1-1 except that the porous carbon skeleton is broken by air flow for 5 hours before deposition in the silane mixed gas to convert the porous carbon skeleton into small-particle porous carbon in the preparation of silicon-carbon material.

[0118] Example 1-13 to Example 1-14

[0119] Except that the prepared silicon-carbon material is classified and ground in <Preparation of Silicon-Carbon Material> so that Dv50, Dv99, D1, and D1 / D2 of the silicon-carbon material are as shown in Table 1, the rest is the same as Example 1-1.

[0120] Examples 1-15

[0121] Except for preparing the silicon-carbon material according to the following steps, the rest is the same as Example 1-1.

[0122] <Preparation of Silicon Carbon Materials>

[0123] 100 g of porous carbon skeleton (Dv50 = 8.7 μm, Dv99 = 18.9 μm) was heated at a heating rate v of 10°C / min to 450°C (T1) under an argon atmosphere, then held for 10 minutes. The atmosphere was then switched to a silane mixture, and deposition was performed in the silane mixture to obtain an intermediate. The deposition time t1 was 10 hours. The silane mixture consisted of silane and argon, with the mass percentage of silane being 20% ​​and the mass percentage of argon being 80% based on the mass of the silane mixture.

[0124] The pre-coated silicon-carbon particles were placed in a sealed high-temperature autoclave, and a tetrahydrofuran solution of petroleum asphalt was added for liquid-phase coating. The solvent was then evaporated by rotary evaporation. The carbon coating was then performed by calcining the particles at 700°C under 0.5 MPa argon gas for 1.5 hours. The particles were then cooled to room temperature to obtain a black silicon-carbon material. The carbon coating temperature T2 was 700°C, and the carbon coating time t2 was 1.5 hours.

[0125] The prepared silicon-carbon material includes a porous carbon skeleton, elemental silicon and a carbon coating layer. The average thickness H of the carbon coating layer is 5.1 nm, the coating form in the carbon coating layer is granular, the (002) crystal plane spacing of the carbon coating layer is 0.339 nm, the elastic modulus of the carbon coating layer is 18.5 GPa, and the specific surface area of ​​the silicon-carbon material particles is 2.2 m 2 / g, Dv50 of silicon-carbon material is 8.9μm, Dv99 of silicon-carbon material is 19μm, Dv10 of silicon-carbon material is 3μm, that is, D1 is 3μm, Dn10 of silicon-carbon material is 2μm, that is, D2 is 2μm, and D1 / D2 is 1.5.

[0126] Example 1-16 to Example 1-21

[0127] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as Examples 1-15.

[0128] Examples 1-22

[0129] Except that no pressure was applied during the carbon coating treatment in <Preparation of Silicon-Carbon Material>, the rest was the same as in Example 1-15.

[0130] Example 2-1 to Example 2-4

[0131] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-2.

[0132] Example 3-1 to Example 3-7

[0133] The process was the same as in Example 1-2, except that the first compound was added in the preparation of the electrolyte and the relevant preparation parameters were adjusted according to Table 3. When the mass percentage of the added first compound changed, the mass percentage of the base solvent also changed. The mass ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in the base solvent remained unchanged, and the mass percentage of the lithium salt remained unchanged.

[0134] Comparative Examples 1 to 3

[0135] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0136] Comparative Example 4

[0137] The preparation process was the same as in Example 1-15, except that the pre-coated silicon-carbon particles were transferred into a sealed high-temperature autoclave, only petroleum asphalt was added for solid-phase ball milling, and the relevant preparation parameters were adjusted according to Table 1.

[0138] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0139] As can be seen from Examples 1-1 to 1-22 and Comparative Examples 1 to 4, regulating the average thickness of the carbon coating layer and the peak area ratio of the CC corresponding peak within the range of this application reduces the over-discharge gassing voltage of the lithium-ion battery and increases the number of cycles of the lithium-ion battery, indicating that the lithium-ion battery of this application has good safety and cycling performance. However, in Comparative Examples 1 to 4, the average thickness of the carbon coating layer or the peak area ratio of the CC corresponding peak is outside the range of this application, and the lithium-ion battery has a high over-discharge gassing voltage and a low number of cycles, indicating that the safety and cycling performance of the lithium-ion battery are poor.

[0140] As can be seen from Figure 2, the carbon carrier, elemental silicon and carbon coating layer in the particles of the silicon-carbon material of Example 1-1 have relatively obvious boundaries with each other. As can be seen from Figure 3, the silicon-carbon material includes elemental silicon and amorphous carbon. By comparing Figures 4 and 5, it can be seen that the plane layer in the figure is the carbon coating layer of the silicon-carbon material. Compared with Figure 5, the carbon coating layer in Figure 4 is smoother, and the carbon coating layer in Figure 5 has more particles, indicating that the surface coating of the silicon-carbon material of Example 1-1 is a film-like distribution, and the surface coating of the silicon-carbon material of Example 1-15 is a granular distribution.

[0141] The coating form of the carbon coating layer generally affects the safety and cycling performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-22, when the coating form of the carbon coating layer is within the scope of this application, the lithium-ion battery has a low over-discharge gassing voltage and a high number of cycles, demonstrating that the lithium-ion battery provided by this application has good safety and cycling performance.

[0142] I D / I G The value of usually affects the safety performance and cycle performance of lithium ion batteries. From Example 1-1 to Example 122, it can be seen that when I D / I G The value of is within the scope of this application, the over-discharge gas production voltage of the lithium-ion battery is low, and the number of cycles of the lithium-ion battery is large, which shows that the lithium-ion battery provided in this application has good safety performance and cycle performance.

[0143] The (002) interplanar spacing of the carbon coating layer typically affects the safety and cycling performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-22, when the (002) interplanar spacing of the carbon coating layer is within the range of this application, the lithium-ion battery has a low over-discharge gassing voltage and a high number of cycles, demonstrating that the lithium-ion battery provided by this application has good safety and cycling performance.

[0144] The elastic modulus of the carbon coating typically affects the safety and cycling performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-22, when the elastic modulus of the carbon coating is within the range of this application, the lithium-ion battery's over-discharge gassing voltage is low and the lithium-ion battery has a high number of cycles, demonstrating that the lithium-ion battery provided herein has good safety and cycling performance.

[0145] The specific surface area of ​​silicon-carbon material particles generally affects the safety and cycling performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-22, when the specific surface area of ​​the silicon-carbon material particles is within the range of this application, the over-discharge gassing voltage of the lithium-ion battery is low and the number of cycles of the lithium-ion battery is high, thus demonstrating that the lithium-ion battery provided by this application has good safety and cycling performance.

[0146] The Dv50 and Dv99 values ​​of silicon-carbon materials typically affect the safety and cycling performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-22, when the Dv50 and Dv99 values ​​of the silicon-carbon materials are within the ranges of this application, the over-discharge gassing voltage of the lithium-ion battery is low and the number of cycles of the lithium-ion battery is high, demonstrating that the lithium-ion battery provided by this application has good safety and cycling performance.

[0147] The values ​​of D1 / D2 and D1 generally affect the safety and cycling performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-22, when the values ​​of D1 / D2 and D1 are within the ranges of this application, the over-discharge gassing voltage of the lithium-ion battery is low and the number of cycles of the lithium-ion battery is high, indicating that the lithium-ion battery provided by this application has good safety and cycling performance.

[0148] Table 2

[0149] The mass percentage of silicon-carbon material and the mass percentage of graphite generally affect the safety and cycle performance of lithium-ion batteries. It can be seen from Examples 1-2 and 2-1 to 2-4 that when the mass percentage of silicon-carbon material and the mass percentage of graphite are within the range of this application, the over-discharge gas production voltage of the lithium-ion battery is low and the number of cycles of the lithium-ion battery is high, thereby demonstrating that the lithium-ion battery provided by this application has good safety and cycle performance. Among them, the content of silicon-carbon material in Example 2-3 is relatively low, and when applied to a lithium-ion battery, the energy density of the lithium-ion battery is relatively low.

[0150] Table 3

[0151] Note: “ / ” in Table 3 indicates no relevant parameters.

[0152] The type and content of Compound A generally affect the safety and cycling performance of lithium-ion batteries. As can be seen from Examples 1-2 and 3-1 to 3-7, when the type and content of Compound A are within the range of this application, the lithium-ion battery's over-discharge gassing voltage is low and the number of cycles is high, demonstrating that the lithium-ion battery provided in this application has good safety and cycling performance.

[0153] The terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed or inherent to such process, method, or article.

[0154] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0155] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A negative electrode active material comprising a silicon-carbon material, wherein particles of the silicon-carbon material comprise a carbon support, elemental silicon, and a carbon coating layer, wherein the average thickness of the carbon coating layer is 2.5 nm to 12 nm, and X-ray photoelectron spectroscopy analysis of the carbon coating layer shows that, based on the sum of the peak area of ​​the peak corresponding to CC, the peak area of ​​the peak corresponding to CO, and the peak area of ​​the peak corresponding to C=O, the peak area corresponding to CC accounts for 60% to 95%.

2. The negative electrode active material according to claim 1, wherein The coating form in the carbon coating layer includes film-like distribution or particle-like distribution.

3. The negative electrode active material according to claim 1, wherein In the Raman spectrum of the carbon coating layer, 1200 cm -1 to 1400cm -1 The peak area of ​​the characteristic peak in the range is I D , 1500cm -1 to 1700cm -1 The peak area of ​​the characteristic peak in the range is I G , satisfying: 0.8≤I D / I G ≤1.

2.

4. The negative electrode active material according to claim 1, wherein The silicon-carbon material satisfies at least one of the following characteristics: (1) The (002) interplanar spacing of the carbon coating layer is 0.335 nm to 0.352 nm; (2) The elastic modulus of the carbon coating layer is 12 GPa to 20 GPa; (3) The specific surface area of ​​the silicon-carbon material particles is 1m 2 / g to 3.5m 2 / g, preferably, the specific surface area of ​​the silicon-carbon material particles is 1.5m 2 / g to 3m 2 / g.

5. The negative electrode active material according to claim 1, wherein In the particle size distribution of the silicon-carbon material, the silicon-carbon material satisfies at least one of the following characteristics: (1) The Dv50 of the silicon-carbon material is 5 μm to 10 μm, and the Dv99 of the silicon-carbon material is 15 μm to 20 μm; (2) The Dv10 of the silicon-carbon material is D1 μm, the Dn10 of the silicon-carbon material is D2 μm, 1.5≤D1 / D2≤2, 3≤D1≤6.

6. The negative electrode active material according to claim 1, wherein The negative electrode active material also includes graphite, which includes at least one of artificial graphite and natural graphite. Based on the mass of the negative electrode active material, the mass percentage of the silicon-carbon material is 5% to 30%, and the mass percentage of the graphite is 70% to 95%.

7. A method for preparing the negative electrode active material according to any one of claims 1 to 6, comprising the following steps: (1) heating the carbon support to 400° C. to 480° C. in an inert atmosphere at a heating rate of 5° C. / min to 20° C. / min, then maintaining the temperature, and then depositing the carbon support in a silane gas mixture for 2 to 30 hours to obtain an intermediate; wherein the silane gas mixture is composed of silane and an inert gas, and based on the mass of the silane gas mixture, the mass percentage of the silane is 10% to 50%, and the mass percentage of the inert gas is 50% to 90%; (2) performing carbon coating treatment on the intermediate to obtain the silicon-carbon material; wherein the carbon coating treatment method includes any one of a liquid phase method, a gas phase method or a gas-liquid combined method, the carbon source includes at least one of coal tar pitch, petroleum asphalt, acetylene, propylene, natural gas, ethylene or cyclohexane, the temperature of the carbon coating treatment is 620°C to 810°C, and the time of the carbon coating treatment is 1h to 6h.

8. The preparation method according to claim 7, wherein The carbon coating treatment method includes a liquid phase method, which includes the following steps: placing the intermediate in an organic solvent containing a carbon source to obtain an organic solution, volatilizing the organic solution, and performing the carbon coating treatment under the inert atmosphere to obtain the silicon-carbon material; wherein the carbon source includes at least one of petroleum asphalt or coal tar asphalt, the organic solvent includes at least one of tetrahydrofuran, n-hexane or xylene, the temperature of the carbon coating treatment is 700°C to 810°C, and the time of the carbon coating treatment is 1 hour to 2 hours.

9. The preparation method according to claim 7, wherein The carbon coating treatment method includes a gas phase method, which includes the following steps: subjecting the intermediate to the carbon coating treatment in a reducing atmosphere to obtain the silicon-carbon material; wherein the reducing atmosphere includes at least one of acetylene, propylene, natural gas, ethylene or cyclohexane, the temperature of the carbon coating treatment is 620°C to 700°C, and the time of the carbon coating treatment is 2h to 6h.

10. A negative electrode sheet comprising a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises the negative electrode active material according to any one of claims 1 to 6 or the negative electrode active material prepared by the preparation method according to claims 7 to 9. A secondary battery comprising the negative electrode sheet according to claim 10 .

12. The secondary battery according to claim 11, comprising an electrolyte, the electrolyte comprising compound A, the compound A satisfying at least one of the following characteristics: (1) Compound A comprises at least one of fluoroethylene carbonate, difluoroethylene carbonate, 1,3-propane sultone or vinyl sulfate; (2) Based on the mass of the electrolyte, the mass percentage of the compound A is 8% to 18%. 13 . An electronic device comprising the secondary battery according to claim 11 .

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