Silicon-carbon composite material and secondary battery

Abstract

The invention relates to a silicon-carbon composite material, a negative pole piece and a secondary battery, the silicon-carbon composite material comprises monatomic silicon and silicon nitride, the silicon-carbon composite material satisfies: 2 < = alpha < = 3, the alpha value represents the peak intensity ratio of ISi2p to IN1s, and ISi2p and IN1s are the peak intensities of characteristic peaks of X-ray photoelectron spectroscopy of the silicon-carbon composite material at 103 + / -0.5 eV and 399 + / -0.5 eV respectively. The silicon-carbon composite material has high specific capacity and also has excellent rate capability, cycle performance and expansion performance.

Description

Silicon-carbon composite material and secondary battery TECHNICAL FIELD

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a silicon-carbon composite material and a secondary battery using the same. BACKGROUND

[0002] Secondary batteries such as lithium ion batteries have been rapidly developed in the field of new energy vehicles and large-scale energy storage. However, as the negative electrode material of traditional commercial lithium ion batteries, the low capacity of graphite and the safety hazard of lithium dendrites hinder its further application. Developing a lithium ion battery negative electrode material with high energy density and high safety is the focus of current lithium battery technology development. Compared with carbon-based materials such as graphite, elemental silicon is considered to be a lithium battery negative electrode material that can replace graphite due to its high theoretical specific capacity and suitable working voltage. However, the poor electrical conductivity of elemental silicon and the huge volume expansion during alloying / de-alloying seriously restrict the large-scale application of elemental silicon in lithium ion batteries.

[0003] SUMMARY

[0004] The present application provides a silicon-carbon composite material and a secondary battery.

[0005] The first aspect of the present application provides a silicon-carbon composite material, which comprises elemental silicon and silicon nitride, and satisfies 2≤α≤3, where the value of α represents the peak intensity ratio of I Si2p and I N1s , I Si2p and I N1s are the peak intensities of the characteristic peaks of the silicon-carbon composite material at 103±0.5 eV and 399±0.5 eV, respectively, in X-ray photoelectron spectroscopy.

[0006] In the present application, the silicon-carbon composite material contains silicon nitride, which can react with lithium ions to generate lithium nitride with high ionic conductivity after lithium intercalation of the silicon-carbon composite material. Lithium nitride can provide a stable transport channel for lithium ions, which helps to improve the rate performance of the silicon-carbon composite material. Moreover, silicon nitride can also buffer the stress change of the silicon-carbon composite material during charging and discharging, thereby improving the cycle performance and expansion performance of the silicon-carbon composite material. At the same time, the value of α can reflect the content of silicon nitride in the silicon-carbon composite material. When the value of α is within the above range, it is beneficial to make the silicon nitride react with lithium ions to generate an appropriate amount of lithium nitride after lithium intercalation, so that the silicon-carbon composite material has high specific capacity, and also has excellent rate performance, cycle performance and expansion performance.

[0007] Based on the first aspect, in some possible implementations, 2.2≤α≤2.7, which is beneficial to further enable the silicon-carbon composite material to have excellent rate performance, cycle performance and expansion performance.

[0008] In some possible implementation manners based on the first aspect, the silicon-carbon composite material further satisfies: 0.18≤β≤0.26, where β represents the peak intensity ratio of I N1s and I C1s , and I C1s represents the peak intensity of the characteristic peak of the silicon-carbon composite material at 283±1 eV in X-ray photoelectron spectroscopy. When I N1s and I C1s satisfy the above relationship, the silicon-carbon composite material contains a suitable content of nitrogen element, which is conducive to generating a suitable amount of silicon nitride, so as to obtain a suitable amount of lithium nitride, thereby improving the rate performance, cycle performance and expansion performance of the silicon-carbon composite material.

[0009] In some possible implementation manners based on the first aspect, 0.20≤β≤0.24, which is conducive to generating a suitable amount of silicon nitride, and further improving the rate performance, cycle performance and expansion performance of the silicon-carbon composite material.

[0010] In some possible implementation manners based on the first aspect, the silicon-carbon composite material comprises a core and a shell located outside the core, the shell comprises a carbon material, the core comprises a porous carbon substrate, and elemental silicon and silicon nitride are dispersed in the porous carbon substrate. The porous carbon substrate serves as the skeleton of the silicon-carbon composite material, so that the silicon-carbon composite material has good electrical conductivity and cycle stability, and the porous carbon substrate serves as the matrix for deposition of the silicon material, which can inhibit the volume expansion of the silicon material during lithium intercalation, reduce the expansion stress, and further improve the cycle stability of the silicon-carbon composite material.

[0011] In some possible implementation manners based on the first aspect, the content of carbon element is 40wt% to 70wt% based on the mass of the silicon-carbon composite material. This is conducive to improving the electrical conductivity of the silicon-carbon composite material, so that the silicon-carbon composite material has good rate performance, cycle performance and expansion performance.

[0012] In some possible implementation manners based on the first aspect, the content of silicon element is 20wt% to 60wt% based on the mass of the silicon-carbon composite material. This is conducive to improving the specific capacity of the silicon-carbon composite material, and further improving the energy density.

[0013] In some possible implementation manners based on the first aspect, the content of nitrogen element is 1wt% to 5wt% based on the mass of the silicon-carbon composite material. This is conducive to improving the rate performance, cycle performance and expansion performance of the silicon-carbon composite material.

[0014] In some possible implementation manners based on the first aspect, 5≤γ≤30, where γ represents the content ratio of silicon element and nitrogen element in the silicon-carbon composite material. This is conducive to making the silicon-carbon composite material have excellent specific capacity, rate performance, cycle performance and expansion performance.

[0015] In some possible implementation manners based on the first aspect, the size of the silicon grains in the silicon-carbon composite material is 0.8 nm to 1 nm. This is beneficial to buffering the deformation stress of the silicon-carbon composite material in the charging and discharging process, and thus improves the cycle performance and expansion performance of the silicon-carbon composite material.

[0016] In some possible implementation manners based on the first aspect, the sphericity of the silicon-carbon composite material is 0.6 to 0.8. This is beneficial to improving the compaction density of the silicon-carbon composite material, and thus improves the energy density of the secondary battery.

[0017] In some possible implementation manners based on the first aspect, the silicon-carbon composite material satisfies: 1.3≤δ≤1.6, the δ value represents the ratio of the main peak intensity to the sub-peak intensity in the differential capacity curve of the delithiation, the main peak intensity of dQ / dV is the intensity of the characteristic peak located at 0.25 V to 0.3 V, and the sub-peak intensity of dQ / dV is the intensity of the characteristic peak located at 0.4 V to 0.45 V. The main peak of dQ / dV is caused by the delithiation reaction of lithium-silicon alloy (Li x Si), and the sub-peak of dQ / dV is related to the delithiation reaction of crystalline Li 15 Si4. The greater the peak intensity of the sub-peak of dQ / dV, the higher the proportion of the delithiation reaction of crystalline Li 15 Si4. The appropriate amount of crystalline Li 15 Si4 can improve the specific capacity of the silicon-carbon composite material. When the δ value is in the above range, the silicon-carbon composite material has excellent cycle performance and expansion performance while having a high specific capacity.

[0018] In some possible implementation manners based on the first aspect, the particle size D V 50 of the silicon-carbon composite material is 5 μm to 10 μm, and the particle size D V 99 is 15 μm to 25 μm. This can reduce the risk of increased consumption of electrolyte and low material compaction density, and also reduce the risk of low ionic conductivity and reduced rate performance.

[0019] In some possible implementation manners based on the first aspect, the specific surface area of the silicon-carbon composite material is 1 m 2 / g to 50 m 2 / g. This is beneficial to reducing the side reaction between the silicon-carbon composite material and the electrolyte, and improving the first coulombic efficiency and cycle performance of the silicon-carbon composite material.

[0020] In some possible implementation manners based on the first aspect, the compaction density of the silicon-carbon composite material is 1.05 g / cm 3 3 to 1.15 g / cm 3 3 under a mass of 5 tons. This is beneficial to improving the compaction density of the negative electrode sheet, and thus improving the energy density of the secondary battery.

[0021] Based on the first aspect, in some possible implementations, the silicon-carbon composite material has a first delithiation specific capacity of 500 mAh / g to 2500 mAh / g. This is beneficial to improve the cycle performance, expansion performance and rate performance of the secondary battery.

[0022] The second aspect of the present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer arranged on at least one surface of the negative electrode current collector. The negative electrode active layer contains a negative electrode active material, and the negative electrode active material includes a silicon-carbon composite material. In the present application, the silicon-carbon composite material contains silicon nitride. After the silicon-carbon composite material is embedded with lithium, the silicon nitride can react with lithium ions to generate lithium nitride with high ionic conductivity, which helps to improve the rate performance of the silicon-carbon composite material. The silicon nitride can also act as a buffer material to alleviate the stress change of the silicon-carbon composite material during charging and discharging, thereby improving the cycle performance and expansion performance of the silicon-carbon composite material. The value of a is within the above range, which is beneficial to make the silicon nitride react with lithium ions to generate an appropriate amount of lithium nitride after being embedded with lithium. In this way, the silicon-carbon composite material can have high specific capacity, excellent rate performance, cycle performance and expansion performance, thereby improving the rate performance, cycle performance and expansion performance of the negative electrode sheet.

[0023] Based on the second aspect, in some possible implementations, the negative electrode active material contains graphite and the silicon-carbon composite material, and the mass percentage of the graphite in the negative electrode active material is 35wt% to 95wt%. The mass percentage of the graphite is within the above range, which can further improve the cycle performance of the negative electrode active layer, thereby improving the cycle performance of the secondary battery. BRIEF DESCRIPTION OF DRAWINGS

[0024] The above and / or additional aspects and advantages of the present application will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which:

[0025] FIG. 1 is an X-ray photoelectron spectrogram of the silicon-carbon composite material prepared in Example 1.

[0026] FIG. 2 is a structural schematic diagram of the silicon-carbon composite material prepared in Example 1.

[0027] FIG. 3 is an X-ray diffraction diagram of the silicon-carbon composite material prepared in Example 1.

[0028] FIG. 4 is an electron scanning microscope diagram of the silicon-carbon composite material prepared in Example 1.

[0029] FIG. 5 is a differential capacity curve diagram of the silicon-carbon composite material prepared in Example 1. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present application will be described clearly and in detail below. Obviously, the described embodiments are some of the embodiments of the present application, rather than all the embodiments. Unless otherwise defined, all the technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing the specific embodiments and are not intended to limit the present application.

[0031] In the related art, in order to buffer the volume expansion of elemental silicon, methods such as silicon nanocrystallization, porous silicon, and introduction of transition metal oxides can be used. However, the high specific surface area and low tap density of silicon nanocrystallization and porous silicon limit their large-scale application. Although the introduction of transition metal oxides can also reduce the volume expansion of silicon, the mechanical properties and chemical stability of metal oxides are not outstanding, and therefore they are not good buffer media. Although the specific capacity of carbon materials as negative electrode materials is small, due to the advantages of low price, good electrical conductivity, outstanding chemical and thermal stability, etc., carbon materials can be used as a good conductive medium and buffer matrix for silicon-based materials. Therefore, a silicon-carbon composite material with low volume expansion is prepared by combining silicon-based materials and carbon materials. However, the cycle performance, expansion performance, and rate performance of the silicon-carbon composite material still need to be improved.

[0032] In view of the above problems, a silicon-carbon composite material, a negative electrode sheet, and a secondary battery are provided in the present application.

[0033] An embodiment of the present application provides a secondary battery, which includes a shell, an electrode assembly, and an electrolyte. The shell can be a packaging bag obtained by packaging with a packaging film (such as an aluminum plastic film), and the secondary battery can be a soft package battery. In other embodiments, the secondary battery can also be a steel shell battery or an aluminum shell battery.

[0034] The electrode assembly includes a positive electrode sheet, a negative electrode sheet, and a separator film. The separator film is arranged between the positive electrode sheet and the negative electrode sheet. The electrode assembly can be a stacked structure formed by alternately stacking the positive electrode sheet, the separator film, and the negative electrode sheet. In other embodiments, the electrode assembly can also be a wound structure formed by winding the positive electrode sheet, the separator film, and the negative electrode sheet after being stacked.

[0035] In some embodiments, the secondary battery includes a lithium ion battery or a sodium ion battery.

[0036] Negative electrode sheet

[0037] The negative electrode includes a negative current collector and a negative active layer disposed on the negative current collector. The negative current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or carbon-based current collector, or any composite current collector disclosed in the prior art, such as, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The negative active layer contains a negative active material, including silicon-carbon composite materials.

[0038] Silicon-carbon composite materials include elemental silicon and silicon nitride. Silicon-carbon composite materials satisfy the condition: 2 ≤ α ≤ 3, where α represents the silicon content. Si2p and I N1s The peak intensity ratio, I Si2p and I N1s The peak intensities are the characteristic peaks at 103±0.5 eV and 399±0.5 eV, respectively, of the X-ray photoelectron spectroscopy of the silicon-carbon composite material. In this application, the silicon-carbon composite material contains silicon nitride. After lithium intercalation, the silicon nitride reacts with lithium ions to generate lithium nitride with high ionic conductivity. Lithium nitride provides a stable transport channel for lithium ions, which helps improve the rate performance of the silicon-carbon composite material. Furthermore, silicon nitride can buffer stress changes during charge-discharge processes, thereby improving the cycle performance and expansion performance of the silicon-carbon composite material. In this application, the α value reflects the ratio of silicon to nitrogen in the silicon-carbon composite material, thus reflecting the content of silicon nitride. An α value within the above range is beneficial for silicon nitride to react with lithium ions after lithium intercalation to generate an appropriate amount of lithium nitride, resulting in a silicon-carbon composite material with high specific capacity while also exhibiting excellent rate performance, cycle performance, and expansion performance.

[0039] If the α value is small, such as α < 2, the silicon-carbon composite material contains more silicon nitride, which will form excessive lithium nitride, thereby increasing lithium ion consumption and reducing the specific capacity of the silicon-carbon composite material. A small α value also reduces the content of elemental silicon, thus reducing the capacity contribution of elemental silicon and lowering the specific capacity of the silicon-carbon composite material. If α > 3, the silicon-carbon composite material contains less silicon nitride, resulting in less lithium nitride formation, which will reduce the lithium-ion conductivity of the silicon-carbon composite material, causing a decrease in its rate performance, cycle performance, and expansion performance. In some embodiments, the α value can be 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, or any value within the range of any two of the above values.

[0040] In some embodiments, 2.2≤α≤2.7 helps silicon nitride react with lithium ions after lithium intercalation to generate a more appropriate amount of lithium nitride, further improving the rate performance, cycle performance and expansion performance of silicon-carbon composite materials.

[0041] In some embodiments, the silicon-carbon composite material further satisfies: 0.18≤β≤0.26, where the value of β represents the peak intensity ratio of I N1s and I C1s , where I C1s represents the peak intensity of the characteristic peak at 283±1 eV in the X-ray photoelectron spectrum of the silicon-carbon composite material. The value of β can reflect the content ratio of nitrogen element and carbon element in the silicon-carbon composite material, and thus can be used to determine the approximate content of nitrogen element in the silicon-carbon composite material. When I N1s and I C1s satisfy the above relationship, the silicon-carbon composite material contains a suitable content of nitrogen element, which is conducive to generating a suitable amount of silicon nitride, so as to facilitate obtaining a suitable amount of lithium nitride, thereby improving the rate performance, cycle performance and expansion performance of the silicon-carbon composite material. In some embodiments, β can be 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26 or any value within the range formed by any two of the above values.

[0042] In some embodiments, 0.20≤β≤0.24, which is conducive to generating a suitable amount of silicon nitride, and further improves the rate performance, cycle performance and expansion performance of the silicon-carbon composite material.

[0043] Referring to FIG. 1, the X-ray photoelectron spectrum of the silicon-carbon composite material further has an O1s characteristic peak at 532±1 eV.

[0044] Referring to FIG. 2, in some embodiments, the silicon-carbon composite material includes a core and a shell located outside the core, the shell includes a carbon material, the core includes a porous carbon substrate, and elemental silicon and silicon nitride are dispersed in the porous carbon substrate. In some embodiments, the elemental silicon and silicon nitride can be dispersed in the pores of the porous carbon substrate and / or outside the porous carbon substrate. In the silicon-carbon composite material, the porous carbon substrate serves as the skeleton of the silicon-carbon composite material, so that the silicon-carbon composite material has good electrical conductivity and cycle stability. Moreover, the porous carbon substrate serves as the matrix for deposition of the elemental silicon and silicon nitride, which can inhibit the volume expansion of the silicon material during lithium intercalation, thereby reducing the expansion stress. Meanwhile, the pore structure in the porous carbon substrate can also disperse the expansion stress of the silicon material, reduce the volume expansion rate of the silicon-carbon composite material, and further improve the cycle performance and expansion performance of the silicon-carbon composite material. In some embodiments, the elemental silicon and silicon nitride are dispersed in the pores of the porous carbon substrate and / or outside the porous carbon substrate, the elemental silicon provides more capacity contribution to the silicon-carbon composite material, and the silicon nitride generates lithium nitride with high ionic conductivity after lithium intercalation, thereby improving the rate performance of the silicon-carbon composite material. Meanwhile, the silicon nitride also serves to buffer the stress change of the silicon during the charging and discharging process, thereby improving the cycle performance and expansion performance of the silicon-carbon composite material. The shell formed by the carbon material can on the one hand reduce the specific surface area of the silicon-carbon composite material, reduce the side reactions between the silicon-carbon composite material and the electrolyte, and improve the initial coulombic efficiency, and on the other hand can further improve the electrical conductivity of the silicon-carbon composite material, which is conducive to further improving the rate performance. In some embodiments, the elemental silicon and silicon nitride are uniformly dispersed in the pores of the porous carbon substrate and / or outside the porous carbon substrate, which can more favorably improve the electrical conductivity of the silicon-carbon material and improve the initial coulombic efficiency, and further improve the rate performance.

[0045] In some embodiments, the content of carbon element is 40wt% to 70wt% based on the mass of the silicon-carbon composite material, which is conducive to improving the electrical conductivity of the silicon-carbon composite material, so that the silicon-carbon composite material has good rate performance, cycle performance and expansion performance. If the content of carbon element in the silicon-carbon composite material is too high, the content of silicon is relatively reduced, which can reduce the specific capacity of the silicon-carbon composite material. If the content of carbon element in the silicon-carbon composite material is too low, it can affect the rate performance, cycle performance and expansion performance of the silicon-carbon composite material. For example, the content of carbon element in the silicon-carbon composite material can be 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 70wt% or any value within the range between any two of the above values. Preferably, the content of carbon element in the silicon-carbon composite material is 55wt% to 70wt%.

[0046] In some embodiments, the content of silicon element is 20wt% to 60wt% based on the mass of the silicon-carbon composite material, which is beneficial to improve the specific capacity of the silicon-carbon composite material. By controlling the content of carbon element and silicon material in the above range, the silicon-carbon composite material can have high specific capacity, rate performance, cycle performance and expansion performance. In some embodiments, the content of silicon element in the silicon-carbon composite material is 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt% or any value within the range formed by any two of the above values. Preferably, the content of silicon element in the silicon-carbon composite material is 30wt% to 45wt%.

[0047] In some embodiments, the content of nitrogen element is 1wt% to 5wt% based on the mass of the silicon-carbon composite material, which is beneficial to improve the rate performance, cycle performance and expansion performance of the silicon-carbon composite material. In some embodiments, the content of nitrogen element in the silicon-carbon composite material is 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt% or any value within the range formed by any two of the above values. Preferably, the content of nitrogen element in the silicon-carbon composite material is 2wt% to 4wt%.

[0048] In some embodiments, 5≤γ≤30, where γ represents the ratio of the content of silicon element and nitrogen element in the silicon-carbon composite material. The ratio of silicon element and nitrogen element in the appropriate range is beneficial to further improve the specific capacity, rate performance, cycle performance and expansion performance of the silicon-carbon composite material. In some embodiments, the value of γ can be 5, 10, 15, 20, 25, 30 or any value within the range formed by any two of the above values.

[0049] In some embodiments, the size of silicon grains in the silicon-carbon composite material is 0.8nm to 1nm. A small enough size of silicon grains can buffer the deformation stress of the silicon-carbon composite material during charging and discharging, thereby improving the cycle performance and expansion performance of the silicon-carbon composite material. For example, the particle size of the silicon material can be 0.8nm, 0.82nm, 0.84nm, 0.86nm, 0.88nm, 0.9nm, 0.92nm, 0.94nm, 0.96nm, 0.98, 1nm or any value within the range formed by any two of the above values.

[0050] In some embodiments, the sphericity of the silicon-carbon composite material is 0.6 to 0.8, which is beneficial to improve the compaction density of the silicon-carbon composite material, thereby improving the energy density of the secondary battery. For example, the sphericity of the silicon-carbon composite material can be 0.6, 0.65, 0.7, 0.75, 0.8 or any value within the range formed by any two of the above values.

[0051] In some embodiments, the silicon-carbon composite material satisfies: 1.3≤δ≤1.6, where the value of δ represents the ratio of the main peak intensity and the sub-peak intensity in the differential capacity-voltage curve of the silicon-carbon composite material during delithiation, the main peak intensity is the intensity of the characteristic peak located at 0.25 V to 0.3 V, and the sub-peak intensity is the intensity of the characteristic peak located at 0.4 V to 0.45 V, as shown in FIG. 5.

[0052] The differential capacity-voltage curve of the silicon-carbon composite material during delithiation has the meaning known in the art, which is obtained by taking the first derivative of the delithiation capacity of the silicon-carbon composite material with respect to the voltage, with the charge-discharge capacity of the silicon-carbon composite material as the horizontal coordinate and the voltage as the vertical coordinate. The differential capacity-voltage curve can reflect the capacity contained in a unit range of the silicon-carbon composite material. If the capacity at a certain voltage plateau is high, it means that a very small voltage fluctuation range will contribute a lot of capacity, and a characteristic peak will appear on the curve. Each characteristic peak represents an electrochemical reaction, and the peak height of each characteristic peak represents the contribution of the corresponding electrochemical reaction to the capacity.

[0053] In this application, the main peak of dQ / dV is caused by the delithiation reaction of lithium-silicon alloy (Li x Si), and the sub-peak of dQ / dV is related to the delithiation reaction of crystalline Li 15 Si4. The greater the peak intensity of the sub-peak of dQ / dV, the higher the proportion of the delithiation reaction of crystalline Li 15 Si4. An appropriate amount of crystalline Li 15 Si4 can improve the specific capacity of the silicon-carbon composite material, but excessive crystalline Li 15 Si4 will reduce the cycle performance and expansion performance of the silicon-carbon composite material. When silicon nitride is introduced into the silicon-carbon composite material, it indirectly inhibits the size of silicon grains, affects the delithiation reaction of Li 15 Si4, reduces the peak intensity of the sub-peak of dQ / dV, and thus affects the performance of the silicon-carbon composite material. When δ is between 1.3 and 1.6, it is beneficial to control the silicon-carbon composite material to have high specific capacity while also having excellent cycle performance and expansion performance. In some embodiments, δ can be 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, or any value within the range between any two of the above values. Preferably, 1.4≤δ≤1.6.

[0054] In some embodiments, the particle size D V 50 of the silicon-carbon composite material is 5 μm to 10 μm, and the particle size D V99 is 15 μm to 25 μm. When the particle size of the silicon-carbon composite material is within the above range, on the one hand, not only can the risk of larger specific surface area, increased consumption of electrolyte and lower material compaction density caused by smaller particle size of the silicon-carbon composite material be reduced, but also the problems of low ionic conductivity and poor rate performance caused by larger particle size can be improved; on the other hand, it is also conducive to shortening the deposition path of silicon and improving the uniformity of silicon deposition, thereby reducing the volume expansion of the silicon-carbon composite material during lithium intercalation, and in particular, when the prepared silicon-carbon composite material is applied to the negative electrode sheet, the uniform dispersibility in the negative electrode slurry preparation process can be improved. For example, the particle size D V 50 can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or any value within the range consisting of any two of the above values. The particle size D V 99 can be 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm or any value within the range consisting of any two of the above values.

[0055] D V 50 is also referred to as "median particle size" and represents the particle size of the silicon-carbon composite material particles in the particle size distribution on a volume basis, which reaches 50% of the volume accumulation from the small particle size, that is, the volume of the silicon-carbon composite material particles smaller than this particle size accounts for 50% of the total volume of the silicon-carbon composite material particles. D V 99 represents the particle size of the silicon-carbon composite material particles in the particle size distribution on a volume basis, which reaches 99% of the volume accumulation from the small particle size.

[0056] In some embodiments, the specific surface area of the silicon-carbon composite material is 1 m 2 / g to 50 m 2 / g, which is conducive to reducing the side reaction of the silicon-carbon composite material with the electrolyte and improving the first coulombic efficiency and cycle performance of the silicon-carbon composite material. The specific surface area of the silicon-carbon composite material can be 1 m 2 / g, 5 m 2 / g, 10 m 2 / g, 15 m 2 / g, 20 m 2 / g, 25 m 2 / g, 30 m 2 / g, 35 m 2 / g, 40 m 2 / g, 45 m 2 / g, 50 m 2 / g or any value within the range consisting of any two of the above values.

[0057] In some embodiments, the silicon-carbon composite material has a compacted density of 1.05 g / cm3 measured at a mass of 5 tons. 3 to 1.15 g / cm3 3 This is beneficial to improve the compacted density of the negative electrode sheet, thereby improving the energy density of the secondary battery. For example, the silicon-carbon composite material can have a compacted density of 1.05 g / cm3, 1.06 g / cm3, 1.07 g / cm3, 1.08 g / cm3, 1.09 g / cm3, 1.10 g / cm3, 1.11 g / cm3, 1.12 g / cm3, 1.13 g / cm3, 1.14 g / cm3, 1.15 g / cm3, or any value within a range defined by any two of the above values, measured at a mass of 5 tons. 3 3 3 3 3 3 3 3 3 3 3 or any value within a range defined by any two of the above values.

[0058] In some embodiments, the silicon-carbon composite material has a first delithiation specific capacity of 500 mAh / g to 2500 mAh / g, which is beneficial to improve the cycle performance, swelling performance, and rate performance of the secondary battery. For example, the silicon-carbon composite material can have a first delithiation specific capacity of 500 mAh / g, 600 mAh / g, 700 mAh / g, 800 mAh / g, 900 mAh / g, 1000 mAh / g, 1100 mAh / g, 1200 mAh / g, 1300 mAh / g, 1400 mAh / g, 1500 mAh / g, 1600 mAh / g, 1700 mAh / g, 1800 mAh / g, 1900 mAh / g, 2000 mAh / g, 2100 mAh / g, 2200 mAh / g, 2300 mAh / g, 2400 mAh / g, 2500 mAh / g, or any value within a range defined by any two of the above values.

[0059] The method for preparing the silicon-carbon composite material is not particularly limited in the present application. Illustratively, the method for preparing the silicon-carbon composite material can include, but is not limited to, the following steps: placing the porous carbon in a reaction cavity, introducing a silicon source gas and a nitrogen source gas into the reaction cavity, thermally depositing the silicon source gas and the nitrogen source gas to form elemental silicon and silicon nitride inside or outside the pores of the porous carbon, and then introducing a carbon source gas to pyrolyze and deposit amorphous carbon, thereby obtaining the silicon-carbon composite material.

[0060] ​​​​​​​​​​The silicon source gas can include, but is not limited to, at least one of monosilane, disilane, trisilane, tetrasilane, chlorosilane, dichlorosilane, trichlorosilane, or tetrachlorosilane. The nitrogen source gas is generally ammonia. The carbon source gas can include, but is not limited to, at least one of methane, acetylene, ethylene, ethane, propyne, propylene, propane, butyne, butylene, or butane.

[0061] By adjusting the flow rate ratio of the silicon source gas and the nitrogen source gas, the pyrolysis temperature of the silicon source gas and the nitrogen source gas, and the pyrolysis time, the peak intensity ratio a of I Si2p and I N1s and the peak intensity ratio β of I N1s and I C1s are controlled. For example, increasing the pyrolysis temperature, the nitrogen element content in the silicon-carbon composite material increases, the a value decreases, and the β value increases; decreasing the pyrolysis temperature, the a value increases, and the β value decreases. Increasing the flow rate ratio of the silicon source gas and the nitrogen source gas, the a value increases; decreasing the flow rate ratio of the silicon source gas and the nitrogen source gas, the a value decreases, and the β value is proportional to the flow rate of the nitrogen source gas; extending or shortening the time of the silicon source gas and the nitrogen source gas, the a value does not change significantly, and the β value is proportional to the time of the nitrogen source gas. The skilled person can adjust the pyrolysis temperature of the silicon source gas and the nitrogen source gas, the gas flow rate of the silicon source gas or the nitrogen source gas, and the time of the silicon source gas or the nitrogen source gas according to the needs, for example, the pyrolysis temperature of the silicon source gas and the nitrogen source gas is 400-600°C, the gas flow rate ratio of the silicon source gas and the nitrogen source gas is 0.55-2.5, the gas flow rate of the silicon source gas is 1-10 L / min, and the time of the silicon source gas and the nitrogen source gas is 2-10 h.

[0062] In this application, the time of the carbon source gas is 1-20 h, and the flow rate is 1-10 L / min.

[0063] The negative electrode active layer further comprises a binder for binding the negative electrode active material particles to facilitate the formation of a film layer, and also to improve the bonding force between the negative electrode active layer and the negative electrode current collector. In some embodiments, the binder can include, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene rubber, acrylated butadiene rubber, epoxy resin, or nylon, etc.

[0064] The negative active layer can further include a conductive material, which includes but is not limited to a carbon-based material, a metal-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material can include but is not limited to natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material can include but is not limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0065] In some embodiments, the negative active material further includes graphite. Since graphite has a certain flexibility, it can alleviate the volume expansion of the negative active layer as a whole in cooperation with the silicon-carbon composite material. At the same time, graphite and silicon-carbon composite material as negative active material can make full use of the advantages of both silicon-carbon composite material and graphite to make the secondary battery achieve better electrochemical performance. In the negative active layer, the mass fraction of graphite is 35wt% to 95wt% based on the mass of the negative active material. The mass fraction of graphite in the negative active layer within the above range can further improve the cycle performance of the negative active layer, thereby improving the cycle performance of the secondary battery.

[0066] In some embodiments, the particle size D V 50 of the negative active material is 5μm to 15μm, D V 99 is 15μm to 40μm.

[0067] In some embodiments, the specific surface area of the negative active material is 1m 2 / g to 10m 2 / g.

[0068] In some embodiments, the negative active material has a compacted density of 1.35g / cm 3 3 to 1.8g / cm 3 3 under a mass of 5 tons.

[0069] In some embodiments, the first delithiation specific capacity of the negative active material is 400mAh / g to 1000mAh / g.

[0070] The particle size, specific surface area, powder compacted density, and first delithiation specific capacity of the negative active material controlled within the above range can further improve the energy density, first coulombic efficiency, cycle performance, and rate capability of the secondary battery.

[0071] The separator

[0072] The material and shape of the separator used in the secondary battery of the present application are not particularly limited and can be any of the techniques disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic material formed of a material stable to the electrolyte of the present application, etc.

[0073] The separation film can include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, a film, or a composite film having a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

[0074] The surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinylalkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer includes a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinylalkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0075] Electrolyte solution

[0076] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and optionally an additive. The organic solvent in the electrolyte of the present application can be any organic solvent known in the art that can be used as a solvent for an electrolyte. The electrolyte used in the electrolyte of the present application is not limited and can be any electrolyte known in the art. The additive of the electrolyte of the present application can be any additive known in the art that can be used as an additive for an electrolyte. In some embodiments, the organic solvent includes, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate. In some embodiments, the organic solvent includes an ether-based solvent, such as including at least one of 1,3-dioxolane (DOL) and dimethoxyethane (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis-trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalato)borate LiB(C2O4)2 (LiBOB), or lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0077] Positive electrode tab

[0078] The positive electrode tab includes a positive current collector and a positive active layer disposed on the positive current collector. The positive current collector can use an aluminum foil or a nickel foil, etc., and can be any composite current collector disclosed in the art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive active layer includes a positive active material, which includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive active material can include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive active material can include, but is not limited to, at least one of lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganate, spinel-type lithium nickel manganate, and lithium titanate.

[0079] The positive active layer can further include a binder to bind the positive active material particles to facilitate formation of a film layer and to improve the adhesion between the positive active layer and the positive current collector. In some embodiments, the binder can include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon.

[0080] The positive active layer can further include a conductive material, which can include, but is not limited to, a carbon-based material, a metal-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material can include, but is not limited to, at least one of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material can include, but is not limited to, metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0081] The secondary battery described above can be applied to an electronic device to supply power to a load in the electronic device. Moreover, the silicon-carbon composite material in the secondary battery has excellent cycle performance, expansion performance, and rate performance, which is beneficial to improve the service life of the electronic device. The electronic device can include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0082] The present application is described below by way of specific examples and comparative examples. Those skilled in the art will understand that the preparation methods described in the present application are merely examples, and any other suitable preparation method is within the scope of the present application.

[0083] Example 1

[0084] (1) A porous carbon material (the specific surface area of the porous carbon is 2000 m 2 / g, and the pore volume is 0.9 cm 3The 1000 g of the treated porous carbon was placed in a rotary furnace, heated to 200℃ under argon atmosphere, vacuumized to -95 kPa for 1 h to remove the trace amount of water remained in the porous carbon. The temperature was raised to 500℃ at a rate of 10℃ / min under argon atmosphere, and the gas phase pressure was kept at micro-positive pressure (3 kPa), while the flow rate of 100% concentration of silane gas and 10% concentration of ammonia / argon gas was set to 1 L / min and 1 L / min respectively, and the reaction was continued for 5 h, so that the silane and ammonia gas were simultaneously adsorbed and deposited in the pores of the porous carbon to form elemental silicon and a small amount of silicon nitride.

[0085] (2) The same argon atmosphere and temperature were used, 50% concentration of acetylene / argon was used as carbon source, and the flow rate was set to 1 L / min for 5 h to make acetylene-derived carbon deposited on the outside and part of the pores of the material in step (1).

[0086] (3) The temperature was lowered to room temperature and the product was discharged to obtain the final silicon-carbon composite material.

[0087] Examples 2 to 8 and Comparative Examples 1 to 4

[0088] Examples 2 to 8 are different from Example 1 in that the flow rate of ammonia, pyrolysis temperature or pyrolysis time is adjusted. Except for the adjustment of the preparation parameters in Table 1, the rest is the same as Example 1.

[0089] Comparative Example 5

[0090] The micron silicon powder and graphite after 400 mesh screening and magnetic removal pretreatment were mixed in a mass ratio of 45:55 and ground under inert atmosphere to obtain a composite material.

[0091] The preparation conditions of each example and comparative example are shown in Table 1.

[0092] Table 1

[0093] Note: “ / ” in the table means not introduced.

[0094] The silicon-carbon composite materials prepared in Examples 1 to 8 and Comparative Examples 1 to 5 were tested for silicon element content, nitrogen element content, carbon element content, silicon grain size, sphericity, D V 50 / D V 99, specific surface area and compacted density.

[0095] Test method

[0096] (1) XPS test of silicon-carbon composite material:

[0097] The surface elements of the silicon-carbon composite material were characterized by using the EscaLab 250Xi X-ray photoelectron spectrometer of Thermo Fisher. Before the XPS test, the silicon-carbon composite material was subjected to 300 seconds of Ar + Etching to remove part of the carbon layer on the surface. X-rays (Al Ka = 1486.6 eV) were used to interact with the surface of the silicon-carbon composite material, and the photoelectric effect was used to excite the silicon-carbon composite material surface to emit photoelectrons. The kinetic energy (K.E) of the photoelectrons was measured by an energy analyzer in the energy analysis range of 0 to 2000 eV, and then the binding energy (B.E) of the excited electrons was obtained.

[0098] (2) Characterization method of silicon content of silicon-carbon composite material:

[0099] The silicon content a in the silicon-carbon composite material was characterized by ICP (inductively coupled plasma optical spectrometer) technology.

[0100] (3) Characterization of nitrogen content, oxygen content and carbon content of silicon-carbon composite material:

[0101] The nitrogen content b and oxygen element content d in the silicon-carbon composite material were tested by using an oxygen-nitrogen-hydrogen content analyzer (HORIBA: EMGA-830), and the test was carried out according to the national standard GB / T 14265-2017 "General method for analysis of hydrogen, oxygen, nitrogen, carbon and sulfur in metal materials". Then the carbon content c in the silicon-carbon composite material = 1-a-b-d.

[0102] (4) XRD test and calculation method of silicon microcrystal size in silicon-carbon composite material:

[0103] The D8 Advance device was used, with Cu target ( ) as the target material, and the test was carried out at a voltage of 60 kV from an angle range of 10° to 80° of 2θ. After obtaining the XRD spectrum of the silicon-carbon composite material, the Debye-Scherrer formula: D = Kλ / βcosθ was used at 2θ = 28.4° to calculate the size of the silicon microcrystals in the silicon-carbon composite material. Wherein, K is the Scherrer constant, D is the size of the silicon microcrystals, B is the measured sample diffraction peak half-width, θ is the Bragg diffraction angle, and γ is the X-ray wavelength.

[0104] (5) Test of sphericity of silicon-carbon composite material:

[0105] Firstly, the silicon-carbon composite material to be tested is polished by IB-09010CP / ion polisher (voltage is 6 kV), and then tested by JEOL-JSM-6700F type scanning electron microscope under the voltage of 5 kV and the current of 0.8 nA and in the back scattering mode to obtain the scanning electron microscope image of the silicon-carbon composite material in the back scattering mode. The sphericity is the ratio of the surface area of the same volume sphere of the object to the surface area of the object, which is a parameter for characterizing the morphology of particles. The closer the particle morphology is to a sphere, the closer the sphericity is to 1. The calculation formula of the sphericity of any particle is as follows:

[0106] Wherein, Vp is the volume of the particle, and Sp is the surface area of the particle.

[0107] (6) SEM test:

[0108] The silicon-carbon composite material is tested by JEOL-JSM-6700F type scanning electron microscope under the voltage of 5 kV and the current of 0.8 nA.

[0109] (7) SEM test in back scattering mode:

[0110] Firstly, the silicon-carbon composite material is polished by IB-09010CP / ion polisher (voltage is 6 kV), and then tested by JEOL-JSM-6700F type scanning electron microscope under the voltage of 5 kV and the current of 0.8 nA and in the back scattering mode.

[0111] (8) Test of silicon-carbon composite material D V 50 / D V 99:

[0112] In a 50 ml clean beaker, 0.02 g of the powder sample of the silicon-carbon composite material in each example and the comparative example is added, 20 ml of deionized water is added, and a surfactant is added dropwise to completely disperse the powder sample in water. Ultrasonic cleaning is performed in a 120 W ultrasonic cleaner for 5 min. The silicon-carbon composite material is tested by using MasterSizer 2000 equipment from 0° to 135° detection angle. According to the particle size distribution laser diffraction method GB / T19077-2016, the particle size distribution is measured to obtain the D V 50 and D V 99 value of the silicon-carbon composite material.

[0113] (9) Test of specific surface area of silicon-carbon composite material:

[0114] The BET specific surface area of the silicon-carbon composite material is tested by using a TriStar II 3020 device, and the test is based on the national standard GB / T 19587-2017 "Gas adsorption BET method for determining the specific surface area of solid substances". Specifically, the silicon-carbon composite material is taken as the sample, the sample tube is immersed in liquid nitrogen at-196℃, and the adsorption amount of nitrogen on the solid surface under different pressures is measured under 0.05-0.30 relative pressure, and the monolayer adsorption amount of the sample is obtained based on the BET multilayer adsorption theory and its formula, so as to calculate the specific surface area of the solid.

[0115] (10) Test of the compacted density of the silicon-carbon composite material:

[0116] The test standard of the compacted density of the powder refers to GB / T 24533-2009 "Lithium ion battery graphite negative electrode material". The specific test method is as follows:

[0117] 1.0000±0.0500g of the silicon-carbon composite material sample is weighed and placed in the test mold (CARVER #3619(13mm)), and then the sample is placed in the test device, which is a three-dimensional UTM7305, the test tonnage is 5.0 tons, the pressure increasing rate is 10mm / min, the pressure holding time is 30s, the pressure releasing rate is 30mm / min, and the pressure releasing holding time is 10s.

[0118] The calculation formula of the compacted density is: compacted density = mass of the sample / (force receiving area of the sample x thickness of the sample after compaction).

[0119] Table 2

[0120] The X-ray photoelectron spectrogram of the silicon-carbon composite material prepared in Example 1 is shown in Figure 1, and the XPS spectrogram appears a wide peak at 103±0.5eV, 283±1eV, 399±0.5eV and 532±1eV, which respectively corresponds to the characteristic peaks of Si2p, C1s, N1s and O1s.

[0121] The XRD spectrogram of the silicon-carbon composite material prepared in Example 1 is shown in Figure 3, and it can be seen from Figure 3 that carbon and elemental silicon exist in the silicon-carbon composite material.

[0122] The SEM graph of the silicon-carbon composite material prepared in Example 1 is shown in Figure 4, and it can be seen from Figure 4 that the particles in the silicon-carbon composite material are uniformly dispersed and present a spherical morphology.

[0123] The preparation process of the button half-cell includes:

[0124] (1) Preparation of the negative electrode sheet: The silicon-carbon composite material, acetylene black and polyacrylic acid binder prepared in the above examples were added to deionized water according to a mass ratio of 70:20:10, and after being stirred thoroughly, a slurry was formed. A coating layer with a thickness of 100 pm was coated using a doctor blade, and after being dried in a vacuum drying oven at 85 °C for 12 hours, the coating layer was cut into a circular sheet with a diameter of 1 cm using a punch press in a dry environment to obtain a negative electrode sheet.

[0125] Electrolyte and separator: In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), propylene carbonate (PC), ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a ratio of 1:1:1 to obtain a base solvent, and LiPF6 was added to the base solvent. After being stirred uniformly, an electrolyte was obtained, wherein the mass percentage of LiPF6 was 12% based on the mass of the electrolyte. An 8 pm polypropylene / polyethylene composite film was used as a separator.

[0126] (2) Assembly of the button-type half-cell:

[0127] The above negative electrode sheet was assembled into a button-type half-cell in the glove box with lithium sheet as the counter electrode in the order of negative electrode sheet, separator and lithium sheet with lithium sheet as the counter electrode.

[0128] The button-type batteries assembled with the silicon-carbon composite materials prepared in Examples 1 to 8 and Comparative Examples 1 to 5 were tested for the first delithiation specific capacity, the first coulombic efficiency, the capacity retention rate, the rate performance and the expansion rate, and the specific testing methods were as follows:

[0129] The testing method for the first delithiation specific capacity and the first coulombic efficiency included:

[0130] The above assembled button-type half-cell was subjected to charge-discharge testing on a LAND battery testing system (LAND CT2001A). The half-cell testing was performed in a working voltage range of 0.01 V to 2.0 V, and the half-cell was discharged at 0.1 C to 0.01 V, rested for 5 min, discharged at 50 pA to 0.01 V, rested for 5 min, and charged at 0.1 C to 2.0 V, rested for 5 min. The first discharge capacity and the first charge capacity of the half-cell were recorded as the first lithiation capacity and the first delithiation specific capacity, respectively.

[0131] The first coulombic efficiency (%) = (first delithiation capacity / first lithiation capacity) x 100%.

[0132] The differential capacity-voltage curve of the silicon-carbon composite material during delithiation was obtained by taking the first derivative of the delithiation specific capacity of the silicon-carbon composite material with respect to voltage as the vertical coordinate and voltage as the horizontal coordinate.

[0133] The testing method for the capacity retention rate (%) of the half-cell after 50 cycles included:

[0134] The assembled button cell half-battery was discharged at 0.5C to 0.01V at 25°C, rested for 5 min, then discharged at 50μA to 0.01V, rested for 5 min, charged at 0.5C to 2.0V, and rested for 5 min. The discharge capacity of the first cycle was recorded. Then the same procedure was repeated for 50 cycles, and the discharge capacity of the 50th cycle was recorded.

[0135] Half-battery 50-cycle capacity retention rate (%) = (discharge capacity of the 50th cycle / discharge capacity of the first cycle) x 100%.

[0136] The test method for the rate performance of the half-battery includes:

[0137] The button cell half-battery prepared above was subjected to rate testing on a LAND CT2001A battery test system. The working voltage range was 0.01V-2V, the half-battery was discharged at 0.5C to 0.01V, rested for 5 min, then charged at 0.5C to 2.0V, and rested for 5 min. The charge capacity of the button cell half-battery was recorded, which was the delithiation capacity at 0.5C. Similarly, the same procedure was repeated at 1.0C, 2.0C and 5.0C, respectively, to obtain the delithiation capacities of the half-battery at 1.0C, 2.0C and 5.0C, respectively. The rate performance of the silicon-carbon composite material was judged by comparing the delithiation capacity at 5.0C.

[0138] The test method for the thickness expansion rate of the negative electrode tab includes:

[0139] The button cell half-battery electrochemical device before and after 50 cycles was disassembled, and the negative electrode tab was obtained. The thickness of the tab was measured 12 times with a vernier caliper and the average value was obtained. If the thickness of the copper foil was l, the thickness of the tab before 50 cycles was m, and the thickness of the tab after 50 cycles was n, then the thickness expansion rate k of the negative electrode tab after 50 cycles was k = (n-m) / (m-l) x 100%.

[0140] The differential capacity curve of the silicon-carbon composite material prepared in Example 1 is shown in FIG. 5. As can be seen from FIG. 5, the differential capacity curve has wide peaks at 0.25V-0.3V and 0.4V-0.45V, respectively.

[0141] The first delithiation specific capacity, the first coulombic efficiency, the rate performance, the cycle performance and the expansion performance of the silicon-carbon composite materials prepared in the examples and comparative examples were recorded in Table 3.

[0142] Table 3

[0143] In combination with Tables 1-3, in Comparative Examples 2 and 5 (physical mixing), no silicon nitride was introduced, and the peak intensity ratio a values of the corresponding silicon-carbon composites were relatively large. In the examples and other comparative examples, silicon nitride was introduced into the silicon-carbon composites, which significantly improved the rate performance, cycle performance, and expansion performance of the silicon-carbon composites. This indicates that the introduction of silicon nitride into the silicon-carbon composites enables the silicon nitride to react with lithium ions to generate lithium nitride with high ionic conductivity, which helps to improve the rate performance of the silicon-carbon composites. The silicon nitride can also act as a buffer material to buffer the stress changes of the silicon-carbon composites during the charging and discharging process, thereby improving the cycle performance and expansion performance of the silicon-carbon composites. Si2p and I N1s the peak intensity ratio a values were relatively large. In the examples and other comparative examples, silicon nitride was introduced into the silicon-carbon composites, which significantly improved the rate performance, cycle performance, and expansion performance of the silicon-carbon composites. This indicates that the introduction of silicon nitride into the silicon-carbon composites enables the silicon nitride to react with lithium ions to generate lithium nitride with high ionic conductivity, which helps to improve the rate performance of the silicon-carbon composites. The silicon nitride can also act as a buffer material to buffer the stress changes of the silicon-carbon composites during the charging and discharging process, thereby improving the cycle performance and expansion performance of the silicon-carbon composites.

[0144] Compared with Comparative Examples 1-5, in Examples 1-8, when the a value is in the range of 2-3, the prepared silicon-carbon composites have excellent rate performance, cycle performance, and expansion performance in addition to high specific capacity. When the a value is in the range of 2-3, as the a value gradually increases, the specific capacity of the silicon-carbon composites gradually increases, and the silicon-carbon composites can also have excellent rate performance, cycle performance, and expansion performance.

[0145] In Examples 1-6, when the β value is in the range of 0.18-0.26, the silicon-carbon composites can further have excellent rate performance, cycle performance, and expansion performance in addition to high specific capacity.

[0146] Compared with Comparative Examples 1-5, in Examples 1-8, the ratio δ of the main peak intensity to the secondary peak intensity of dQ / dV is between 1.3 and 1.6, and the silicon-carbon composites can have excellent rate performance, cycle performance, and expansion performance in addition to high specific capacity.

[0147] The above disclosure is merely the preferred embodiments of the present application, and of course cannot be used to limit the present application, and therefore equivalent changes made in accordance with the present application still fall within the scope of the present application.

Claims

1. A silicon-carbon composite material, wherein, The silicon-carbon composite material comprises elemental silicon and silicon nitride, and the silicon-carbon composite material satisfies: 2≤α≤3, where the value of α represents I. Si2p and I N1s The peak intensity ratio, I Si2p and I N1s These are the peak intensities of the characteristic peaks of the X-ray photoelectron spectrum of the silicon-carbon composite material at 103±0.5eV and 399±0.5eV, respectively.

2. The silicon-carbon composite material of claim 1, wherein, 2.2≤α≤2.7。 3. The silicon-carbon composite material of claim 1 or 2, wherein, The silicon-carbon composite also satisfies: 0.18≤β≤0.26, where the value of β represents the peak intensity ratio of I N1s and I C1s , where I C1s represents the peak intensity of the characteristic peak at 283±1 eV in the X-ray photoelectron spectrum of the silicon-carbon composite.

4. The silicon-carbon composite material of claim 3, wherein, 0.20≤β≤0.24。 5. The silicon-carbon composite material of any one of claims 1 to 4, wherein, The silicon-carbon composite material comprises a core and a shell located outside the core, the shell comprises a carbon material, the core comprises a porous carbon substrate, and the elemental silicon and the silicon nitride are dispersed in the porous carbon substrate.

6. The silicon-carbon composite material of any one of claims 1 to 5, wherein, The silicon-carbon composite material satisfies at least one of the following conditions: (1) the content of carbon element is 40wt% to 70wt% based on the mass of the silicon-carbon composite material; (2) the content of silicon element is 20wt% to 60wt% based on the mass of the silicon-carbon composite material; (3) the content of nitrogen element is 1wt% to 5wt% based on the mass of the silicon-carbon composite material; (4) 5≤γ≤30, the value of γ represents the ratio of the content of silicon element and nitrogen element in the silicon-carbon composite material; (5) the size of silicon crystal grains in the silicon-carbon composite material is 0.8nm to 1nm; (6) the sphericity of the silicon-carbon composite material is 0.6 to 0.

8.

7. The silicon-carbon composite material of any one of claims 1 to 6, wherein, The silicon-carbon composite material satisfies 1.3≤δ≤1.6, the value of δ represents the ratio of the main peak intensity and the secondary peak intensity in the differential capacity curve of delithiation, the main peak intensity of dQ / dV is the intensity of the characteristic peak located at 0.25V to 0.3V, and the secondary peak intensity of dQ / dV is the intensity of the characteristic peak located at 0.4V to 0.45V.

8. The silicon-carbon composite material of any one of claims 1 to 7, wherein, The silicon-carbon composite material satisfies at least one of the following conditions: (1) the particle size D of the silicon-carbon composite material V 50 is 5 μm to 10 μm, D V 99 is 15 μm to 25 μm; (2) the silicon-carbon composite material has a specific surface area of 1 m 2 / g to 50 m 2 / g; (3) the silicon-carbon composite material has a compacted density of 1.05 g / cm3 or more, as measured at a mass of 5 tons 3 to 1.15 g / cm3 3 ; (4) the specific capacity of the silicon-carbon composite material in the first delithiation is 500mAh / g to 2500mAh / g.

9. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, wherein, The negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon composite material according to any one of claims 1 to 8.

10. The secondary battery according to claim 9, wherein The negative electrode active material comprises graphite and the silicon-carbon composite material, and the mass ratio of the graphite is 35wt% to 95wt% based on the mass of the negative electrode active material.

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