A silicon-carbon composite material, a negative electrode sheet, an electrochemical device, and an electronic device

Abstract

The application belongs to the technical field of electrochemistry, and relates to a silicon-carbon composite material, a negative electrode sheet, an electrochemical device and an electronic device. The Raman spectrum of the silicon-carbon composite material satisfies 0.61 < R / R' < 0.78; wherein R is the ratio of I A and I B ; R' is the ratio of I D and I G ; I A represents the intensity of a peak at 521±5 cm ‑1 in the Raman spectrum of the silicon-carbon composite material; I B represents the intensity of a peak at 480±5 cm ‑1 in the Raman spectrum of the silicon-carbon composite material; I D represents the intensity of a peak at 1360±5 cm ‑1 in the Raman spectrum of the silicon-carbon composite material; and I G represents the intensity of a peak at 1580±5 cm ‑1 in the Raman spectrum of the silicon-carbon composite material. The silicon-carbon composite material with the above characteristics can be applied to a secondary battery negative electrode material, so that the secondary battery has obviously improved conductivity, low-temperature performance, cycle performance and expansion performance under the premise of maintaining a high specific capacity.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemistry, and relates to a silicon-carbon composite material, a negative electrode sheet, an electrochemical device, and an electronic device. Background Art

[0002] Electrochemical devices such as lithium-ion batteries are widely used in various aspects of today's life due to their advantages such as no memory effect, small size, light weight, and environmental friendliness. In recent years, electrochemical devices have developed rapidly in the fields of new energy vehicles and large-scale energy storage. However, in the negative electrode materials of traditional commercial electrochemical devices, taking lithium-ion batteries as an example, carbon-based materials such as graphite have a low capacity, resulting in a low energy density of lithium-ion batteries; while silicon-based materials are prone to expansion, resulting in poor cycling performance of lithium-ion batteries, thus greatly limiting their large-scale application in electrochemical devices.

[0003] Although carbon materials have a small specific capacity as negative electrode materials, due to their advantages such as low price, good conductivity, and outstanding chemical and thermal stability, they can be used as good conductive media and buffer matrices for silicon-based materials. Therefore, combining silicon oxide and carbon to prepare a silicon-carbon composite negative electrode material is an effective method. For example, Yang et al. prepared uniform silicon nanoparticle@phenolic resin-based carbon composite materials with adjustable carbon layer thickness by a simple and scalable production method. Son et al. deposited a uniform nanosilicon layer on the surface of carbon materials by CVD method using silane gas as the silicon source, and obtained a low-expansion silicon-carbon composite material.

[0004] Although there are a large number of studies on silicon-carbon composite materials at present, there are few studies on the internal relationship between the characteristics of the materials themselves and the cycling performance and expansion performance. Summary of the Invention

[0005] The purpose of the present invention is to provide a silicon-carbon composite material, a negative electrode sheet, an electrochemical device, and an electronic device. Applying the silicon-carbon composite material provided by the present invention to the negative electrode material of a secondary battery can enable the secondary battery to have significantly improved conductivity, low-temperature performance, cycling performance, and expansion performance while maintaining a high specific capacity.

[0006] According to the first aspect of the present invention, the present invention provides a silicon-carbon composite material, the silicon-carbon composite material includes elemental silicon and a carbon material; the Raman spectrum of the silicon-carbon composite material satisfies: 0.61 < R / R' < 0.78; where the R value is the ratio of I A to I B ; the R' value is the ratio of I D to I G ; I A represents the intensity of the peak at 521 ± 5 cm -1 in the Raman spectrum of the silicon-carbon composite material; IB The Raman spectrum of the silicon-carbon composite material at 480±5 cm⁻¹ represents... -1 The intensity of the peak at that location; I D The Raman spectrum of the silicon-carbon composite material is 1360±5 cm⁻¹. -1 The intensity of the peak at that location; I G The Raman spectrum of the silicon-carbon composite material is 1580±5 cm⁻¹. -1 The intensity of the peak at that location.

[0007] Through experimental investigation, this invention unexpectedly discovered that when the R / R' value of the silicon-carbon composite material is in the range of 0.61 to 0.78, the secondary battery exhibits improved conductivity, low-temperature performance, cycle performance, and expansion performance while maintaining a high specific capacity. In other words, when the silicon-carbon composite material provided by this invention satisfies the above characteristics, the secondary battery exhibits significantly improved conductivity, low-temperature performance, cycle performance, and expansion performance while maintaining a high specific capacity.

[0008] In some embodiments of the present invention, the range of R in the Raman spectral characteristics of the silicon-carbon composite material is: 0.78≤R≤0.9; the range of R' in the Raman spectral characteristics of the silicon-carbon composite material is 1.2≤R'≤1.25.

[0009] Further experimental investigation revealed that when the R value of the silicon-carbon composite material provided by this invention is within the aforementioned range, the secondary battery exhibits significantly improved cycle performance and expansion performance while maintaining a high specific capacity. Simultaneously, when the R' value of the silicon-carbon composite material is within the aforementioned range, the secondary battery demonstrates improved cycle performance while maintaining good conductivity and low-temperature performance. In other words, when both the R value and the R' value in the Raman spectral characteristics of the silicon-carbon composite material provided by this invention satisfy the aforementioned range characteristics, the secondary battery exhibits significantly improved conductivity, low-temperature performance, cycle performance, and expansion performance while maintaining a high specific capacity.

[0010] In some embodiments of the present invention, the elemental silicon comprises at least one of silicon nanoparticles, silicon submicron particles, or silicon nanofilms. In some embodiments of the present invention, the carbon material comprises a porous carbon framework and a carbon coating layer. In the silicon-carbon composite material provided by the present invention, the elemental silicon in the silicon-carbon composite material imparts a high specific capacity to the silicon-carbon composite material, and the carbon material in the silicon-carbon composite material not only buffers the volume expansion of silicon to a certain extent, but also enhances the conductivity of the silicon-carbon composite material.

[0011] In some embodiments of the present application, the ratio of the content a of carbon element in the silicon-carbon composite material to the content b of silicon element in the silicon-carbon composite material ranges from 0.5 to 10 based on the mass of the silicon-carbon composite material; and / or, the content a of carbon element in the silicon-carbon composite material ranges from 40 wt% to 90 wt%, and the content b of silicon element in the silicon-carbon composite material ranges from 10 wt% to 60 wt%; preferably, the ratio of the content a of carbon element in the silicon-carbon composite material to the content b of silicon element in the silicon-carbon composite material ranges from 1.07 to 2.03 based on the mass of the silicon-carbon composite material; and / or, the content a of carbon element in the silicon-carbon composite material ranges from 50 wt% to 64.8 wt%, and the content b of silicon element in the silicon-carbon composite material ranges from 31.9 wt% to 46.6 wt%; preferably, the ratio of the content a of carbon element in the silicon-carbon composite material to the content b of silicon element in the silicon-carbon composite material ranges from 1.1 to 2.02, and the content a of carbon element in the silicon-carbon composite material ranges from 52 wt% to 64 wt%, and the content b of silicon element in the silicon-carbon composite material ranges from 32 wt% to 44 wt%.

[0012] In the technical solutions provided by the present application, by regulating the ratio of a to b and the values of a and b within the above ranges, the silicon-carbon composite material can have high specific capacity, can buffer the volume expansion of the silicon-carbon composite material to a certain extent, and can enhance the conductivity of the silicon-carbon composite material.

[0013] In some embodiments of the present application, the internal cross-section SEM image of the silicon-carbon composite material shows that the internal surface of the particles is flat, and the pore diameter of the pores in the internal particles is less than 50 nm. The SEM image of the cross-section of the silicon-carbon composite material provided by the present application in the back scattering mode shows that the internal particles are flat and smooth, have an internal structure of pores with small pore diameters, have no large pore structure, and the external surface of the particles has a certain silicon enrichment phenomenon. The flat and smooth internal structure of the particles with small pore diameters can improve the deposition uniformity in the internal particles of the silicon-carbon composite material, and avoid the phenomenon of low capacity caused by the absence of silicon deposition in the internal particles of the silicon-carbon composite material due to the presence of large pores; at the same time, the certain silicon enrichment on the external surface of the particles can make the silicon-carbon composite material have high specific capacity and high initial coulombic efficiency.

[0014] In some embodiments of the present application, the silicon-carbon composite material satisfies at least one of the following conditions 1) to 5):

[0015] 1) The size of the silicon microcrystal in the silicon-carbon composite material is less than 1 nm; preferably, the size of the silicon microcrystal in the silicon-carbon composite material is 0.8-0.95 nm;

[0016] 2) the particle size D V 50 is 5-10 μm, D V 99 is 15-25 μm; preferably, the particle size D V 50 is 5-6.5 μm, D V 99 is 15-19 μm;

[0017] 3) the specific surface area of the silicon-carbon composite material is 1-50 m 2 / g; preferably, the specific surface area of the silicon-carbon composite material is 4-8 m 2 / g; 2 / g; preferably, the specific surface area of the silicon-carbon composite material is 4-8 m 2 / g;

[0018] 4) the oxygen element content in the silicon-carbon composite material is 1.5-4.0 wt%; the oxygen element content in the silicon-carbon composite material is 1.2-3 wt%;

[0019] 5) the first delithiation specific capacity of the silicon-carbon composite material is 500-2500 mAh / g; preferably, the first delithiation specific capacity of the silicon-carbon composite material is 1400-2000 mAh / g.

[0020] By regulating the size of the silicon microcrystals in the silicon-carbon composite material within the above range, the lithium extraction activity of the silicon microcrystals can be improved, and the transport of active ions can be accelerated. Meanwhile, the smaller size of the silicon microcrystals can buffer the deformation stress during the charging and discharging process, thereby improving the cycle performance and expansion performance of the silicon-carbon composite material. By regulating the particle sizes D v 50 and D v 99 of the silicon-carbon composite material within the above range, the dispersion uniformity of the slurry can be improved, and the transport of active ions can be improved, thereby being conducive to improving the cycle performance and expansion performance of the secondary battery. By regulating the specific surface area of the silicon-carbon composite material within the above range, the side reaction between the silicon-carbon composite material and the electrolyte can be reduced, thereby being conducive to improving the cycle performance and expansion performance of the secondary battery. By regulating the oxygen element content of the silicon-carbon composite material within the above range, the specific capacity, cycle performance and expansion performance of the lithium ion battery can be further balanced, thereby improving the overall performance of the lithium ion battery. When the first delithiation specific capacity of the silicon-carbon composite material is within the above range, the tap density performance, cycle performance and expansion performance of the silicon-carbon composite material can be considered.

[0021] According to a second aspect of the present application, the present application also provides a method for preparing the silicon-carbon composite material, the method comprising: regulating the R value of the silicon-carbon composite material by changing at least one parameter of the pyrolysis temperature of the silicon-containing gas, the flow rate of the silicon-containing gas, the time for introducing the silicon-containing gas, the time for micro-oxidation, the temperature for micro-oxidation, or the flow rate of the oxygen; and / or regulating the R' value of the silicon-carbon composite material by changing at least one parameter of the pyrolysis temperature of the carbon source gas, the flow rate of the carbon source gas, or the time for introducing the carbon source gas.

[0022] In some embodiments of the present application, the method comprises the following steps:

[0023] (1) Silicon deposition: using a porous carbon material as a precursor and a silicon-containing gas as a silicon source, elemental nano-silicon is adsorbed and deposited in the pores of the porous carbon material by chemical vapor deposition;

[0024] (2) Micro-oxidation: introducing oxygen to micro-oxidize the nano-silicon on the superficial and external surfaces of the porous carbon;

[0025] (3) Carbon deposition: after the micro-oxidation treatment, introducing a carbon source gas to deposit the carbon source gas on the external surface of the material obtained in step (2) and form a carbon coating layer, thereby obtaining a silicon-carbon material coated with a carbon coating layer;

[0026] wherein, steps (1)-(3) are all carried out under the protection of an inert gas atmosphere and a micro-positive pressure gas phase pressure condition; the micro-positive pressure condition is 0.1-5 kPa.

[0027] In some embodiments of the present application, the method further comprises:

[0028] (4) grinding and sieving the silicon-carbon composite material obtained in step (3) to obtain the silicon-carbon composite material.

[0029] In some embodiments of the present application, in step (1), the micro-positive pressure condition can be 1.5-2.5 kPa; in step (2), the micro-positive pressure condition is 0.5-1.5 kPa; and in step (3), the micro-positive pressure condition is 2.0-3.0 kPa.

[0030] In some embodiments of the present application, the inert gas is argon.

[0031] In some embodiments of the present application, the silicon-containing gas can include, but is not limited to, at least one of monosilane, disilane, trisilane, tetrasilane, chlorosilane, dichlorosilane, trichlorosilane, or tetrachlorosilane; 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; the porous carbon material can include, but is not limited to, at least one of activated carbon, expanded graphite, carbon molecular sieve, carbon nanofiber, or carbon nanotube; preferably, the activated carbon is obtained by activation of biocarbon, resin carbon, coke, or the like.

[0032] In some embodiments of the present application, the specific surface area of the porous carbon material is 1000 m 2 / g to 2500 m 2 / g; the pore volume of the porous carbon material is 0.5 cm 3 / g to 1.5 cm 3 / g; the porous carbon material contains 0-30% ultramicropores, 0-30% submicropores, 0-60% macropores, and 0-20% mesopores.

[0033] According to a third aspect of the present application, the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material on at least one surface of the negative electrode current collector, wherein the negative electrode active material comprises the silicon-carbon composite material according to the first aspect of the present application.

[0034] In some embodiments of the present application, the negative electrode active material further comprises graphite, a conductive agent, and a binder; the mass fraction of the graphite in the negative electrode active material is 35 wt.% to 95 wt.%.

[0035] In some embodiments of the present application, the negative electrode active material satisfies at least one of the following conditions (I) to (III):

[0036] (I) the particle size D V 50 is 5 μm to 15 μm, D V 99 is 15 μm to 40 μm;

[0037] (II) the specific surface area of the negative electrode active material is 1 m 2 / g to 10 m 2 / g;

[0038] (III) the initial delithiation specific capacity of the negative electrode active material is 400 mAh / g to 1000 mAh / g.

[0039] According to a fourth aspect of the present application, the present application provides an electrochemical device, comprising the negative electrode sheet according to the third aspect of the present application.

[0040] According to a fifth aspect of the present application, the present application provides an electronic device comprising the electrochemical device according to the fourth aspect of the present application.

[0041] Compared with the prior art, the present application has the following beneficial effects:

[0042] The present application provides a silicon-carbon composite material, a negative electrode sheet containing the silicon-carbon composite material, an electrochemical device and an electronic device. The silicon-carbon composite material provided by the present application has an I A The ratio R / R' of the ratio R of I B The ratio R' of I D The ratio R' of I G When the ratio R / R' of the ratio R of I BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The XRD pattern of the silicon-carbon composite material obtained in Example 1; the XRD pattern shows that the elemental silicon in the silicon-carbon composite material obtained in the present application exists in an amorphous form;

[0044] Figure 2 The Raman spectrum of the silicon-carbon composite material obtained in Example 1;

[0045] Figure 3 The SEM image of the cross-section of the silicon-carbon composite material obtained in Example 1 under backscattered mode;

[0046] Figure 4 The first cycle charge-discharge curve of the silicon-carbon composite material obtained in Example 1. DETAILED DESCRIPTION

[0047] The technical solutions of the present application are further illustrated by specific examples below, and the specific examples do not represent a limitation on the protection scope of the present application. Some non-essential modifications and adjustments made by others according to the concept of the present application still fall within the protection scope of the present application.

[0048] It should be noted that in the following content, the present application is explained by taking lithium-ion batteries as an example of secondary batteries, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0049] According to a first aspect of the present application, the present application provides a silicon-carbon composite material, the silicon-carbon composite material comprising elemental silicon and a carbon material; a Raman spectrum of the silicon-carbon composite material satisfies: 0.61 < R / R' < 0.78; wherein R is a ratio of I A to I B ; R' is a ratio of I D to I G ; I A represents an intensity of a peak at 521 ± 5 cm -1 in a Raman spectrum of the silicon-carbon composite material; I B represents an intensity of a peak at 480 ± 5 cm -1 in a Raman spectrum of the silicon-carbon composite material; I D represents an intensity of a peak at 1360 ± 5 cm -1 in a Raman spectrum of the silicon-carbon composite material; and I G represents an intensity of a peak at 1580 ± 5 cm -1 in a Raman spectrum of the silicon-carbon composite material.

[0050] I A represents an intensity of a characteristic peak at 521 ± 5 cm -1 in a Raman spectrum of crystalline silicon, the silicon-silicon bond is a symmetrical structure, can produce strong Raman scattering, the crystalline silicon has highly consistent bond angle and bond length, is arranged in order, can form a Raman sharp peak, and the characteristic strong scattering band is located at 521 ± 5 cm -1 ; I B represents an intensity of a characteristic peak at 480 ± 5 cm -1 in a Raman spectrum of amorphous silicon, the amorphous silicon structure is relatively disordered, the bond angle, bond energy, bond length and swing range are large, and a relatively wide 480 ± 5 cm -1 Raman peak is formed, which is obviously different from the characteristic peak of the crystalline silicon. Based on the intensity ratio of the peaks at 521 ± 5 cm -1 and 480 ± 5 cm -1 in the Raman spectrum, that is, the ratio R of I A and I B , the ratio of the contents of the two forms of silicon, the crystalline silicon and the amorphous silicon, in the silicon-carbon composite material of the present application can be characterized.

[0051] I A represents an intensity of a characteristic peak at 521 ± 5 cm -1 in a Raman spectrum of the silicon-carbon composite material. Specifically, I A represents an intensity of a corresponding characteristic peak between 516 and 527 cm -1 in a Raman spectrum of the silicon-carbon composite material, for example, I A represents an intensity of a characteristic peak at 516 cm -1, 517 cm -1 , 518 cm -1 , 519 cm -1 , 520 cm -1 , 521 cm -1 , 522 cm -1 , 523 cm -1 , 524 cm -1 , 525 cm -1 , 526 cm -1 , or 527 cm -1 , or the intensity of the characteristic peak at any two of the above wavelength values.

[0052] Similarly, I B represents the intensity of the peak at 480 ± 5 cm -1 in the Raman spectrum of the silicon-carbon composite material. Specifically, I B represents the intensity of the characteristic peak between 475 and 485 cm -1 in the Raman spectrum of the silicon-carbon composite material, for example, I B represents the intensity of the characteristic peak at 475 cm -1 , 476 cm -1 , 477 cm -1 , 478 cm -1 , 479 cm -1 , 480 cm -1 , 481 cm -1 , 482 cm -1 , 483 cm -1 , 484 cm -1 , or 485 cm -1 , or the intensity of the characteristic peak between any two of the above wavelength values.

[0053] The D peak and the G peak of the Raman spectrum are characteristic peaks of carbon atom crystals, near 1360 ± 5 cm -1 and 1580 ± 5 cm -1 , respectively. The D peak represents the defects of carbon atom crystals, which is inversely proportional to the degree of ordering of the carbon structure; the G peak represents the in-plane stretching vibration of sp 2 hybridization of carbon atoms, which represents the graphitization degree of the carbon structure, and the intensity ratio of the D peak and the G peak of the Raman spectrum can be used to reflect the degree of disorder of carbon. In the present application, I D represents the intensity of the peak at 1360 ± 5 cm -1 in the Raman spectrum of the silicon-carbon composite material, and I G represents the intensity of the peak at 1580 ± 5 cm -1the intensity of the peak at 1360 ± 5 cm -1 and 1580 ± 5 cm -1 in the Raman spectrum of the carbon material, i.e. the ratio R’ of the intensity of the peak at 1360 ± 5 cm D and the intensity of the peak at 1580 ± 5 cm G in the Raman spectrum of the carbon material.

[0054] I D represents the intensity of the peak at 1360 ± 5 cm -1 in the Raman spectrum of the silicon-carbon composite material. In particular, I D represents the intensity of the peak corresponding to a wavelength comprised between 1355 and 1365 cm -1 in the Raman spectrum of the silicon-carbon composite material, such as I D represents the intensity of the peak corresponding to a wavelength of 1355 cm -1 , 1356 cm -1 , 1357 cm -1 , 1358 cm -1 , 1359 cm -1 , 1360 cm -1 , 1361 cm -1 , 1362 cm -1 , 1363 cm -1 , 1364 cm -1 , or 1365 cm -1 in the Raman spectrum of the silicon-carbon composite material.

[0055] I G represents the intensity of the peak at 1580 ± 5 cm -1 in the Raman spectrum of the silicon-carbon composite material. In particular, I G represents the intensity of the peak corresponding to a wavelength comprised between 1575 and 1585 cm -1 in the Raman spectrum of the silicon-carbon composite material, such as I G represents the intensity of the peak corresponding to a wavelength of 1575 cm -1 , 1576 cm -1 , 1577 cm -1 , 1578 cm -1 , 1579 cm -1 , 1579 cm -1 , 1580 cm -1 , 1581 cm -1 , 1582 cm -1 , 1583 cm -1 , 1584 cm -1 , or 1585 cm -1The intensity of the characteristic peak at any wavelength, or the intensity of the corresponding characteristic peak between the range consisting of any two of the above-mentioned wavelength values.

[0056] The inventors have found that when the ratio of R to R' in the Raman spectrum characteristics of the silicon-carbon composite material is within the scope of the present application, the secondary battery has significantly improved conductivity, low-temperature performance, cycle performance and expansion performance while maintaining a high specific capacity. Therefore, in the design of the silicon-carbon composite material of the present application, the long-term performance of the silicon-carbon composite material can be predicted by the ratio of R to R', and it is found that when the R / R' value of the silicon-carbon composite material is within the range of 0.61-0.78, the silicon-carbon composite material has the best balanced ratio between crystalline silicon and amorphous silicon, and between carbon defects and carbon graphitization, so that the silicon-carbon composite material has significantly improved conductivity, low-temperature performance, cycle performance and expansion performance while maintaining a high specific capacity.

[0057] Specifically, the ratio of R / R' can be 0.611, 0.612, 0.613, 0.615, 0.516, 0.617, 0.618, 0.619, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.771, 0.772, 0.773, 0.774, 0.775, 0.776, 0.777, 0.778, or a range consisting of any two of the above-mentioned values. When the R / R' value is too low, such as 0.61 or lower, although the cycle performance and expansion performance of the lithium ion battery are excellent, the specific capacity, the first coulomb efficiency and the conductivity are all significantly decreased; when the R / R' value is too large, such as 0.78 or higher, although the lithium ion battery has relatively high specific capacity, the first coulomb efficiency and the conductivity, the cycle performance and the expansion performance are significantly reduced.

[0058] In some embodiments of the present application, the range of R in the Raman spectrum characteristics of the silicon-carbon composite material is 0.78≤R≤0.9; the range of R' in the Raman spectrum characteristics of the silicon-carbon composite material is 1.2≤R'≤1.25. It is found that when the ratio of I A to I B , the R value of I D / I G and the R' value are within the scope of the present application, the secondary battery has significantly improved conductivity, low-temperature performance, cycle performance and expansion performance while maintaining a high specific capacity. Therefore, in the design of the silicon-carbon composite material of the present application, the long-term performance of the silicon-carbon composite material can be predicted by the ratio of I A to I B , the ratio R of ID / I G R' value to estimate the long-term performance of the silicon-carbon composite material, and it is found that when the ratio R of I A / I B and I D / I G is within the range of 0.78-0.9 and the R' value of I A / I B is within the range of 1.2-1.25, the silicon-carbon composite material has the best balanced ratio between crystalline silicon and amorphous silicon, and between carbon defects and carbon graphitization, so that the silicon-carbon composite material has significantly improved conductivity, low-temperature performance, cycle performance and expansion performance while maintaining a high specific capacity.

[0059] Specifically, the ratio R of I D / I G may be 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90 or a range consisting of any two of the above values. When the R value is too large, the crystalline silicon content of the silicon-carbon composite material is too high, which will deteriorate its cycle performance and expansion performance, and when the R value is too small, the amorphous silicon content of the silicon-carbon composite material is too high, which is not conducive to the specific capacity and the first coulomb efficiency of the silicon-carbon composite material. Specifically, the ratio R' of I A / I B may be 1.2, 1.21, 1.22, 1.23, 1.24, 1.25 or a range consisting of any two of the above values. When the R' value is too large, the defect degree of the silicon-carbon composite material is too high, which will increase the side reaction, reduce the conductivity, and affect its cycle performance and low-temperature performance; when the R' value is too small, although the conductivity of the silicon-carbon composite material is improved, the defect degree is too low, which is not conducive to the volume expansion of the buffer material.

[0060] That is, the ratio R of I D / I G and the ratio R' of I A / I B of the silicon-carbon composite material provided by the present application are too large or too small, which will significantly affect the performance of the silicon-carbon composite material. For example, when the ratio R of I D / I G and the ratio R' of I A / I B of the silicon-carbon composite material are too low, although the cycle performance and expansion performance of the lithium ion battery are excellent, its specific capacity, first coulomb efficiency and conductivity are significantly reduced; when the ratio R of I D / IG When the ratio R' is too large, although the lithium ion battery has relatively high specific capacity, first coulomb efficiency and conductivity, its cycle performance and expansion performance are significantly reduced, and at the same time, the large R value and R' value also deteriorate the low-temperature performance of the silicon-carbon composite material.

[0061] In some embodiments of the present application, the elemental silicon includes at least one of silicon nanoparticles, silicon sub-micron particles or silicon nanometer thin film. In some embodiments of the present application, the carbon material includes a porous carbon skeleton and a carbon coating layer. In the silicon-carbon composite material provided by the present application, the elemental silicon endows the silicon-carbon composite material with high specific capacity, and the carbon material of the silicon-carbon composite material not only buffers the volume expansion of silicon to a certain extent, but also enhances the conductivity of the silicon-carbon composite material.

[0062] In some embodiments of the present application, the ratio of the content a of carbon element in the silicon-carbon composite material to the content b of silicon element in the silicon-carbon composite material ranges from 0.5 to 10 based on the mass of the silicon-carbon composite material; preferably, the ratio of the content a of carbon element in the silicon-carbon composite material to the content b of silicon element in the silicon-carbon composite material ranges from 1.07 to 2.03, and more preferably, the ratio of the content a of carbon element in the silicon-carbon composite material to the content b of silicon element in the silicon-carbon composite material ranges from 1.1 to 2. Specifically, a / b can be 1.08, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, 1.70, 1.75, 1.80, 1.85, 1.90, 1.95, 2.0, 2.01, 2.02 or a range formed by any two of the above values. By adjusting the ratio of a and b within the above range, the silicon-carbon composite material can have high specific capacity, while buffering the volume expansion of the silicon-carbon composite material to a certain extent and enhancing the conductivity of the silicon-carbon composite material. When the ratio of a and b is too large, the silicon-carbon composite material has small volume expansion and high conductivity, but low specific capacity; when the ratio of a and b is too small, the silicon-carbon composite material has high specific capacity, but low expansion performance and conductivity.

[0063] In some embodiments of the present application, the content a of carbon element in the silicon-carbon composite material ranges from 40wt% to 90wt% based on the mass of the silicon-carbon composite material, and the content b of silicon element in the silicon-carbon composite material ranges from 10wt% to 60wt%; preferably, the content a of carbon element in the silicon-carbon composite material ranges from 50wt% to 64.8wt%, and the content b of silicon element in the silicon-carbon composite material ranges from 31.9wt% to 46.6wt%; more preferably, the content a of carbon element in the silicon-carbon composite material ranges from 52wt% to 64wt%, and the content b of silicon element in the silicon-carbon composite material ranges from 32wt% to 44wt%. Specifically, the content a of carbon element in the silicon-carbon composite material can be 50.1wt%, 50.5wt%, 51wt%, 52wt%, 52.1wt%, 52.5wt%, 53wt%, 54wt%, 55wt%, 56wt%, 57wt%, 58wt%, 59wt%, 60wt%, 61wt%, 62wt%, 63wt%, 64wt%, 64.5wt%, 64.6wt%, 64.7wt%, or a range between any two of the above values. Specifically, the content b of silicon element in the silicon-carbon composite material can be 31.9wt% to 46.6wt%, 32wt%, 33wt%, 34wt%, 35wt%, 36wt%, 37wt%, 38wt%, 39wt%, 40wt%, 41wt%, 42wt%, 43wt%, 44wt%, 45wt%, 46wt%, 46.1wt%, 46.2wt%, 46.3wt%, 46.4wt%, 46.5wt%, or a range between any two of the above values. By adjusting the values of a and b within the above ranges, the silicon-carbon composite material can have high specific capacity, while buffering the volume expansion of the silicon-carbon composite material to some extent, and enhancing the conductivity of the silicon-carbon composite material. When the content a is too large, the specific capacity of the silicon-carbon composite material will deteriorate; when the content a is too small, the expansion performance and conductivity of the silicon-carbon composite material will decrease. Conversely, when the content b is too large, the expansion performance and conductivity of the silicon-carbon composite material will decrease; when the content b is too small, the specific capacity of the silicon-carbon composite material will deteriorate.

[0064] In some embodiments of the present application, the SEM image of the internal cross-section of the silicon-carbon composite material in backscattered mode shows that the internal surface of the particles is smooth, the pore size of the pores in the particles is less than 50nm, and there is a certain silicon-rich phenomenon on the external surface of the particles. Exemplarily, Figure 3The SEM image of the cross section of the silicon-carbon composite material in backscattered mode is shown. The SEM image of the cross section of the silicon-carbon composite material in backscattered mode is observed, the inside of the particle is smooth and smooth, the internal structure of the pore with a small pore size, no large pore structure, and a certain silicon-rich phenomenon exists on the outer surface of the particle. The smooth and smooth internal structure with a small pore size can improve the deposition uniformity of the silicon-carbon composite material particle, and avoid the occurrence of large pores to make the internal silicon deposition and cause the phenomenon of low capacity; at the same time, the silicon-rich on the outer surface of the particle to a certain extent can make the silicon-carbon composite material have higher specific capacity and first coulomb efficiency.

[0065] In some embodiments of the present application, the size of the silicon microcrystal in the silicon-carbon composite material is less than 1 nm; preferably, the size of the silicon microcrystal in the silicon-carbon composite material is 0.8-0.95 nm. Specifically, the size of the silicon microcrystal in the silicon-carbon composite material can be 0.99 nm, 0.98 nm, 0.97 nm, 0.96 nm, 0.95 nm, 0.94 nm, 0.93 nm, 0.92 nm, 0.91 nm, 0.90 nm, 0.89 nm, 0.88 nm, 0.87 nm, 0.86 nm, 0.85 nm, 0.84 nm, 0.83 nm, 0.82 nm, 0.81 nm, 0.80 nm, 0.70 nm, 0.60 nm, 0.50 nm, 0.40 nm, 0.30 nm, 0.20 nm, or a range composed of any two of the above. By adjusting the size of the silicon microcrystal in the silicon-carbon composite material within the above range, the deintercalation activity of the silicon microcrystal can be improved, and the transmission of active ions can be accelerated. At the same time, the small size of the silicon microcrystal can buffer the deformation stress in the charging and discharging process, thereby improving the cycle performance and swelling performance of the silicon-carbon composite material.

[0066] In some embodiments of the present application, the particle size D V 50 is 5 μm-10 μm, D V 99 is 15 μm-25 μm; preferably, the particle size D V 50 is 5 μm-6.5 μm, D V 99 is 15 μm-19 μm. Specifically, the particle size D V 50 can be 5 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, 8.5 μm, 9.0 μm, 9.5 μm, 10.0 μm, or a range composed of any two of the above. D V99 can be 15 μm, 15.5 μm, 16 μm, 16.5 μm, 17 μm, 17.5 μm, 18 μm, 18.5 μm, 19 μm, 19.5 μm, 20 μm, 20.5 μm, 21 μm, 21.5 μm, 22 μm, 22.5 μm, 23 μm, 23.5 μm, 24 μm, 24.5 μm, 25 μm, or a range between any two of the above values. By adjusting the particle size D v 50 and D v 99 is within the above range, the dispersion uniformity of the slurry can be improved, and the transmission of active ions is improved, thereby improving the cycle performance and swelling performance of the secondary battery.

[0067] In the present application, D v 50 represents the particle size reaching 50% of the volume accumulation in the particle size distribution of the silicon-carbon composite material on a volume basis, starting from the small particle size. v 99 represents the particle size reaching 99% of the volume accumulation in the particle size distribution of the material on a volume basis, starting from the small particle size.

[0068] In some embodiments of the present application, the specific surface area of the silicon-carbon composite material is 1 m 2 / g to 50 m 2 / g; preferably, the specific surface area of the silicon-carbon composite material ranges from 4 m 2 / g to 8 m 2 / g. For example, the specific surface area of the silicon-carbon composite material can be 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 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 a range between any two of the above values. The silicon-carbon composite material having a specific surface area within the above range can reduce the side reaction between the silicon-carbon composite material and the electrolyte, thereby improving the cycle performance and swelling performance of the secondary battery.

[0069] In some embodiments of the present application, the content of oxygen element in the silicon-carbon composite material is 1.0wt% to 4.0wt%; preferably, the content of oxygen element in the silicon-carbon composite material ranges from 1.2wt% to 3wt%. Specifically, the content of oxygen element in the silicon-carbon composite material can be 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, or a range consisting of any two of the above values. By regulating the content of oxygen element in the silicon-carbon composite material within the above range, the specific capacity, cycle performance and swelling performance of the lithium ion battery can be further balanced, thereby improving the overall performance of the lithium ion battery.

[0070] In some embodiments of the present application, the first delithiation specific capacity of the silicon-carbon composite material is 500mAh / g to 2500mAh / g; preferably, the first delithiation specific capacity of the silicon-carbon composite material is 1400mAh / g to 2000mAh / g. Specifically, the first delithiation specific capacity of the silicon-carbon composite material can be 500mAh / g, 600mAh / g, 700mAh / g, 800mAh / g, 900mAh / g, 1000mAh / g, 1100mAh / g, 1200mAh / g, 1300mAh / g, 1400mAh / g, 1500mAh / g, 1600mAh / g, 1700mAh / g, 1800mAh / g, 1900mAh / g, 2000mAh / g, 2100mAh / g, 2200mAh / g, 2300mAh / g, 2400mAh / g, 2500mAh / g, or a range consisting of any two of the above values. When the first delithiation specific capacity of the silicon-carbon composite material is within the above range, the specific capacity, cycle performance and swelling performance of the silicon-carbon composite material can be considered.

[0071] According to a second aspect of the present application, the present application provides a preparation method of the silicon-carbon composite material, which comprises, but is not limited to, the following steps: regulating the R value of the silicon-carbon composite material by changing at least one parameter of the pyrolysis temperature of the silicon-containing gas, the flow rate of the silicon-containing gas, the time of passing the silicon-containing gas, the time of micro-oxidation, the temperature of micro-oxidation, or the flow rate of oxygen; and / or, regulating the R' value of the silicon-carbon composite material by changing at least one parameter of the pyrolysis temperature of the carbon source gas, the flow rate of the carbon source gas, or the time of passing the carbon source gas.

[0072] In some embodiments of the present application, the preparation method comprises, but is not limited to, the following steps:

[0073] (1) Silicon deposition: porous carbon material is used as a precursor, and a silicon-containing gas is used as a silicon source. By chemical vapor deposition, elemental nanosilicon is adsorbed and deposited in the pores of the porous carbon;

[0074] (2) Micro-oxidation: oxygen is introduced to perform micro-oxidation on the nanosilicon on the surface and outer surface of the porous carbon;

[0075] (3) Carbon deposition: after the micro-oxidation treatment, a carbon source gas is introduced to cause the carbon source gas to deposit on the outer surface of the material obtained in step (2) and form a carbon coating layer, thereby obtaining a silicon-carbon material coated with a carbon coating layer;

[0076] (4) The silicon-carbon composite material obtained in step (3) is ground and sieved to obtain the silicon-carbon composite material.

[0077] The steps (1)-(3) are all carried out in an inert gas protection atmosphere and under a micro-positive pressure gas phase pressure condition, and the micro-positive pressure condition is 0.1-5 kPa. Specifically, the micro-positive pressure condition can be 0.1 kPa, 0.5 kPa, 1.0 kPa, 1.5 kPa, 2.0 kPa, 2.5 kPa, 3.0 kPa, 3.5 kPa, 4.0 kPa, 4.5 kPa, 5.0 kPa or a range composed of any two of the above values.

[0078] In some embodiments of the present application, in step (1), the micro-positive pressure condition can be 1.5-2.5 kPa. In step (2), the micro-positive pressure condition is 0.5-1.5 kPa. In step (3), the micro-positive pressure condition is 2.0-3.0 kPa.

[0079] In some embodiments, the inert gas can be argon.

[0080] In the above preparation method, by changing the pyrolysis temperature, the flow rate of the silicon-containing gas and the time of introducing the silicon-containing gas, the R value of the silicon-carbon composite material can be adjusted. For example, increasing the pyrolysis temperature increases the R value of the silicon-carbon composite material; decreasing the pyrolysis temperature decreases the R value of the silicon-carbon composite material; increasing the flow rate of the silicon-containing gas increases the R value of the silicon-carbon composite material; decreasing the flow rate of the silicon-containing gas decreases the R value of the silicon-carbon composite material; extending the time of introducing the silicon-containing gas increases the R value of the silicon-carbon composite material; shortening the time of introducing the silicon-containing gas decreases the R value of the silicon-carbon composite material. At the same time, the R value of the silicon-carbon composite material can also be adjusted by changing the micro-oxidation time and the flow rate of the introduced oxygen. For example, extending the micro-oxidation time decreases the R value of the silicon-carbon composite material; shortening the micro-oxidation time increases the R value of the silicon-carbon composite material.

[0081] Similarly, the R' value of the silicon-carbon composite material can be regulated by changing the pyrolysis temperature, the flow rate of the carbon source gas, and the time of passing the carbon source gas. For example, increasing the pyrolysis temperature decreases the R' value of the silicon-carbon composite material; decreasing the pyrolysis temperature increases the R' value of the silicon-carbon composite material; increasing the flow rate of the carbon source gas increases the R' value of the silicon-carbon composite material; decreasing the flow rate of the carbon source gas slightly decreases the R' value of the silicon-carbon composite material; extending the time of passing the carbon source gas increases the R' value of the silicon-carbon composite material; and shortening the time of passing the carbon source gas slightly decreases the R' value of the silicon-carbon composite material.

[0082] The skilled person can adjust the pyrolysis temperature of the silicon-containing gas or the carbon source gas, the flow rate of the silicon-containing gas or the carbon source gas, the time of passing the silicon-containing gas or the carbon source gas, the temperature of micro-oxidation, the time of micro-oxidation, and the flow rate of the oxygen, etc. according to the needs. For example, the pyrolysis temperature of the silicon-containing gas or the carbon source gas is 400-800°C, the flow rate of the silicon-containing gas or the carbon source gas is 100-800 sccm, the time of passing the silicon-containing gas or the carbon source gas is 1-20 h, the flow rate of the oxygen is 50-200 sccm, the time of micro-oxidation is 1-20 h, and the temperature of micro-oxidation is 50-200°C.

[0083] In some embodiments of the present application, the silicon-containing gas can include, but is not limited to, at least one of monosilane, disilane, trisilane, tetrasilane, chlorosilane, dichlorosilane, trichlorosilane, or tetrachlorosilane; the carbon source gas can include, but is not limited to, at least one of methane, acetylene, ethylene, ethane, propyne, propylene, propane, butyne, butene, or butane; and the porous carbon material can include, but is not limited to, at least one of activated carbon, expanded graphite, carbon molecular sieve, carbon nanofiber, or carbon nanotube. The activated carbon can be obtained by activating biocarbon, resin carbon, coke, etc.

[0084] In some embodiments of the present application, the specific surface area of the porous carbon material is 1000 m 2 / g to 2500 m 2 / g. Specifically, the specific surface area of the porous carbon material can be 1000 m 2 / g, 1100 m 2 / g, 1200 m 2 / g, 1300 m 2 / g, 1400 m 2 / g, 1500 m 2 / g, 1600 m 2 / g, 1700 m 2 / g, 1800 m 2 / g, 1900 m 2 / g, 2000 m 2 / g, 2100 m 2 / g, 2200 m 2 / g, 2300 m 2 / g, 2400 m 2 / g, 2500 m 2 / g, or a range consisting of any two of the above values. The pore volume of the porous carbon material can be 0.5 cm 3 / g to 1.5 cm 3 / g. Specifically, the pore volume of the porous carbon material can be 0.5 cm 3 / g, 0.6 cm 3 / g, 0.7 cm 3 / g, 0.8 cm 3 / g, 0.9 cm 3 / g, 1.0 cm 3 / g, 1.1 cm 3 / g, 1.2 cm 3 / g, 1.3 cm 3 / g, 1.4 cm 3 / g, 1.5 cm 3 / g, or a range consisting of any two of the above values. In addition, the pore size distribution of the porous carbon material can also be changed. In some embodiments of the present application, the porous carbon material comprises 0-30% ultramicropores, 0-30% submicropores, 0-60% macropores, and 0-20% mesopores. Among them, the ultramicropore is defined as a pore with a pore size less than 0.7 nm, the submicropore is defined as a pore with a pore size between 0.7 nm and 1.0 nm, the macropore is defined as a pore with a pore size between 1.0 nm and 2.0 nm, and the mesopore is defined as a pore with a pore size greater than 2 nm. When the specific surface area, pore volume, and pore size distribution of the porous carbon material are within the above ranges, the adsorption and deposition of the silicon-containing gas can be facilitated.

[0085] In some embodiments of the present application, the pyrolysis temperature of the silicon-containing gas can be 400-800°C; preferably, the pyrolysis temperature of the silicon-containing gas or the carbon source gas can be 300-500°C. Specifically, the pyrolysis temperature of the silicon-containing gas can be 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, or a range consisting of any two of the above values.

[0086] In some embodiments of the present application, the flow rate of the silicon-containing gas can be 100 sccm to 700 sccm; preferably, the flow rate of the silicon-containing gas can be 400 sccm to 600 sccm. Specifically, the flow rate of the silicon-containing gas can be 100 sccm, 150 sccm, 200 sccm, 250 sccm, 300 sccm, 350 sccm, 400 sccm, 450 sccm, 500 sccm, 550 sccm, 600 sccm, 650 sccm, 700 sccm, or a range between any two of the foregoing values.

[0087] In some embodiments of the present application, the time for passing the silicon-containing gas can be 1 h to 20 h; preferably, the time for passing the silicon-containing gas can be 7 h to 15 h. Specifically, the time for passing the silicon-containing gas can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, or a range between any two of the foregoing values.

[0088] In some embodiments of the present application, the flow rate of the oxygen gas can be 50 sccm to 200 sccm; preferably, the flow rate of the oxygen gas can be 100 sccm to 150 sccm. Specifically, the flow rate of the oxygen gas can be 50 sccm, 60 sccm, 70 sccm, 80 sccm, 90 sccm, 100 sccm, 110 sccm, 120 sccm, 130 sccm, 140 sccm, 150 sccm, 160 sccm, 170 sccm, 180 sccm, 190 sccm, 200 sccm, or a range between any two of the foregoing values.

[0089] In some embodiments of the present application, the time for passing the oxygen gas can be 1 h to 20 h; preferably, the time for passing the oxygen gas can be 3 h to 8 h. Specifically, the time for passing the silicon-containing gas can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, or a range between any two of the foregoing values.

[0090] In some embodiments of the present application, the temperature of the micro-oxidation is 50-200°C; preferably, the temperature of the micro-oxidation is 50-150°C. Specifically, the temperature of the micro-oxidation can be 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, or a range between any two of the aforementioned values.

[0091] In some embodiments of the present application, the pyrolysis temperature of the carbon source gas is 400-800°C; preferably, the pyrolysis temperature of the carbon source gas is 450-600°C. Specifically, the pyrolysis temperature of the carbon source gas can be 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, or a range between any two of the aforementioned values.

[0092] In some embodiments of the present application, the flow rate of the carbon source gas is 200-800sccm; preferably, the flow rate of the carbon source gas is 450-700sccm. Specifically, the flow rate of the carbon source gas can be 200sccm, 250sccm, 300sccm, 350sccm, 400sccm, 450sccm, 500sccm, 550sccm, 600sccm, 650sccm, 700sccm, 750sccm, 800sccm, or a range between any two of the aforementioned values.

[0093] In some embodiments of the present application, the time of the carbon source gas is 1-20h; preferably, the time of the carbon source gas is 3-10h. Specifically, the time of the carbon source gas can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, or a range between any two of the aforementioned values.

[0094] In some embodiments of the present application, in step (1), the volume mixing ratio of silane gas to inert gas can be 5-20:80-100. In step (3), the volume mixing ratio of carbon source gas to inert gas can be 40-60:40-60.

[0095] In some embodiments of the present application, in step (4), when the obtained silicon-carbon composite material is ground and sieved, the mesh size used is 300-500 mesh. Specifically, the mesh size can be 300 mesh, 350 mesh, 400 mesh, 450 mesh, 450 mesh, 500 mesh, or a range between any two of the aforementioned values.

[0096] According to a third aspect of the present application, the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material on at least one surface of the negative electrode current collector, wherein the negative electrode active material comprises the silicon-carbon composite material in any of the preceding embodiments. Therefore, the negative electrode sheet comprising the silicon-carbon composite material has significantly improved conductivity, low-temperature performance, cycle performance and expansion performance while maintaining a high specific capacity.

[0097] In some embodiments of the present application, the negative electrode current collector in the negative electrode sheet of the present application can include, but is not limited to, at least one of a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a foamed nickel, a foamed copper or a composite current collector (e.g., a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.). The thickness of the negative electrode current collector in the negative electrode sheet of the present application can include, but is not limited to, 6 μm to 12 μm. The thickness of the negative electrode sheet of the present application can include, but is not limited to, 50 μm to 150 μm.

[0098] In some embodiments of the present application, the negative electrode active material of the present application can further include a conductive agent, a binder. The conductive agent can include, but is not limited to, at least one of acetylene black, conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black or graphene, etc. The binder can include, but is not limited to, at least one of sodium alginate, polyacrylate, polyimide, polyamide, polyamide-imide, polyvinylidene fluoride, polystyrene butadiene copolymer (styrene butadiene rubber), polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium hydroxymethyl cellulose or potassium hydroxymethyl cellulose. The mass ratio between the silicon-carbon composite material, the conductive agent and the binder in the negative electrode sheet of the present application can be selected by those skilled in the art according to actual needs.

[0099] In some embodiments of the present application, the negative electrode active material of the present application can further include graphite, and the mass percentage of graphite in the negative electrode active material is 35 wt.% to 95 wt.%. Specifically, the mass percentage of graphite in the negative electrode active material can be 35 wt.%, 40 wt.%, 45 wt.%, 50 wt.%, 55 wt.%, 60 wt.%, 65 wt.%, 70 wt.%, 75 wt.%, 80 wt.%, 85 wt.%, 95 wt.% or a range consisting of any two of the above values. When the mass percentage of graphite in the negative electrode active material is within the above range, the energy density and rate performance of the secondary battery can be considered, and the internal resistance can be reduced.

[0100] In some embodiments of the present application, the particle size D V 50 is 5 μm to 15 μm, DV 99 is 15 μm to 40 μm. Specifically, the particle diameter D V 50 can be 5 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, 8.5 μm, 9.0 μm, 9.5 μm, 10.0 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 12.5 μm, 13 μm, 13.5 μm, 14 μm, 14.5 μm, 15 μm, or a range between any two of the aforementioned values. 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, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, 31 μm, 32 μm, 33 μm, 34 μm, 35 μm, 36 μm, 37 μm, 38 μm, 39 μm, 40 μm, or a range between any two of the aforementioned values. In the present application, D v 50 represents the particle diameter at which the volume accumulation is 50% in the particle size distribution of the negative electrode active material layer on a volume basis, D v 99 represents the particle diameter at which the volume accumulation is 99% in the particle size distribution of the negative electrode active material on a volume basis.

[0101] In some embodiments of the present application, the specific surface area of the negative electrode active material is 1 m 2 / g to 10 m 2 / g. Specifically, the specific surface area of the negative electrode active material can be 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g, or a range between any two of the aforementioned values. The negative electrode active material having the specific surface area in the above range can reduce the side reaction between the negative electrode active material and the electrolyte, and is advantageous in improving the cycle performance and the swelling performance of the secondary battery.

[0102] In some embodiments of the present application, the specific capacity of the negative active material in the first lithium extraction is 400 mAh / g to 1000 mAh / g. Specifically, the specific capacity of the negative active material in the first lithium extraction can be 400 mAh / g, 500 mAh / g, 600 mAh / g, 700 mAh / g, 800 mAh / g, 900 mAh / g, 1000 mAh / g, or a range formed by any two of the above values. When the specific capacity of the negative active material in the first lithium extraction is within the above range, the energy density of the secondary battery and the cycle performance and expansion performance can be balanced.

[0103] In some embodiments of the present application, the negative electrode sheet can be prepared by a method known in the art, including but not limited to: mixing the silicon-carbon composite material provided by the present application with a conductive agent and a binder to obtain a mixture slurry, uniformly coating the mixture slurry on a negative electrode current collector, and drying to obtain a negative electrode sheet.

[0104] According to a fourth aspect of the present application, the present application provides an electrochemical device comprising the negative electrode sheet according to any of the preceding embodiments.

[0105] In some embodiments of the present application, the electrochemical device of the present application includes, but is not limited to: all kinds of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors. In some embodiments, the electrochemical device is a lithium secondary battery.

[0106] In some embodiments of the present application, the lithium secondary battery includes, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0107] In some embodiments of the present application, the secondary battery provided by the present application includes a positive electrode sheet, a separator, an electrolyte, and a negative electrode sheet according to any of the preceding embodiments. Therefore, the secondary battery provided by the present application has significantly improved conductivity, low-temperature performance, cycle performance, and expansion performance while maintaining a high specific capacity.

[0108] In some embodiments, the positive electrode tab includes a positive current collector and a positive active material disposed on at least one surface of the positive current collector. The positive current collector can include, but is not limited to, at least one of an aluminum foil, an aluminum alloy foil, or a composite current collector (e.g., an aluminum-carbon composite current collector). The positive active material includes a positive active material, which can include, but is not limited to, at least one of a nickel-cobalt-manganese lithium acid (e.g., commonly known as NCM811, NCM622, NCM523, NCM111), a nickel-cobalt-aluminum lithium acid, a lithium iron phosphate, a lithium-rich manganese-based material, a lithium cobalt acid (LiCo02), a lithium manganese acid, a lithium manganese iron phosphate, or a lithium titanate. The positive active material further includes a conductive agent and a binder, which can be at least one of the conductive agent and the binder selected for the negative active material described above.

[0109] In some embodiments, the positive electrode tab can be prepared by a preparation method known in the art, which can include, but is not limited to, the steps of mixing the positive active material, the conductive agent, and the binder in a solvent to obtain an active material mixture slurry, and uniformly coating the mixture slurry on the current collector to obtain the positive electrode tab. In some embodiments, the solvent can include, but is not limited to, N-methyl pyrrolidone.

[0110] The separator of the secondary battery is used to separate the positive electrode tab and the negative electrode tab, prevent internal short circuit of the secondary battery, allow free passage of electrolyte ions, and not affect the progress of the electrochemical charging and discharging process. In some embodiments, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP)-based polyolefin (PO), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator can include, but is not limited to, at least one of a woven film, a non-woven film, a microporous film, a composite film, a calendered film, or a spunlaid film.

[0111] In some embodiments, the separator of the secondary battery can include a substrate layer and a surface treatment layer on at least one surface of the substrate layer. The substrate layer can be a nonwoven fabric, a film, or a composite film having a porous structure, and the material of the substrate layer can include, but is not limited to, at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. 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 can include inorganic particles and a binder, and the inorganic particles can include, but are not limited to, 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, or barium sulfate, and the binder can be at least one of the above-described binders. The polymer layer can include a polymer, and the material of the polymer can include, but is not limited to, at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0112] In some embodiments, the electrolyte of the secondary battery can include a lithium salt and a non-aqueous solvent. The lithium salt can be selected from at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2. In some embodiments, the mass percentage of the lithium salt can be 8% to 15% based on the mass of the electrolyte, and specifically, the mass percentage of the lithium salt in the electrolyte can be 8%, 11%, 12.5%, 13%, 15%, or a range between any two of the above-described values. The non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or another organic solvent.

[0113] In some embodiments, the secondary battery provided by the present application further includes a packaging bag for containing the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte, and other components known in the art, and the present application does not limit the above-described other components. The packaging bag is not particularly limited and can be a packaging bag known in the art.

[0114] In some embodiments, the method for preparing the secondary battery provided by the present application can include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in sequence, and winding, folding or the like as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly into a packaging bag, injecting electrolyte into the packaging bag and sealing the bag to obtain the secondary battery. Alternatively, the method for preparing the secondary battery can include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in sequence, then fixing the four corners of the entire stack structure with a tape to obtain an electrode assembly with a stack structure, placing the electrode assembly into a packaging bag, injecting electrolyte into the packaging bag and sealing the bag to obtain the secondary battery. In addition, a current-preventing element, a guide plate or the like can be placed in the packaging bag as needed to prevent the pressure inside the secondary battery from rising and overcharging or discharging.

[0115] According to a fifth aspect of the present application, the present application provides an electronic device comprising the electrochemical device of any one of the preceding embodiments. In some embodiments, the electronic device comprises, but is not limited to, at least one of 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, an illuminating appliance, a toy, a game machine, a timepiece, an electric tool, a flashlight, a camera, a household large storage battery, or a lithium ion capacitor.

[0116] Embodiments

[0117] Hereinafter, the present application will be described more specifically by way of specific examples and comparative examples.

[0118] The test methods used are as follows:

[0119] Test of parameters related to silicon-carbon composite material

[0120] (1) Method for characterizing the content of silicon element in silicon-carbon composite material:

[0121] The content of silicon element in silicon-carbon composite material is characterized by means of ICP (inductively coupled plasma spectrometer).

[0122] (2) Method for characterizing the content of carbon element in silicon-carbon composite material:

[0123] The content of carbon element in silicon-carbon composite material is characterized by means of ICP (inductively coupled plasma spectrometer).

[0124] (3) Method for characterizing the oxygen content in the silicon-carbon composite material:

[0125] The secondary battery after 100 cycles was disassembled to obtain the negative electrode sheet, which was naturally dried in a glove box, then the powder was carefully scraped off with a knife, and finally the collected powder was tested by a German Elementar elemental analyzer to obtain the oxygen content in the silicon-carbon composite material.

[0126] (4) Method for calculating the size of silicon crystallites in the silicon-carbon composite material:

[0127] XRD test: D8 Advance equipment was used, with a Cu target as the target material, and the test was performed at a voltage of 60 kV from an angle range of 10° to 80° of 2θ. After obtaining the XRD pattern of the silicon-carbon composite material, the Debye-Scherrer formula was used at 2θ = 28.4° to calculate the size of the silicon crystallites in the silicon-carbon composite material. Among them, K is the Scherrer constant, D is the size of the silicon crystallites, B is the measured sample diffraction peak half-width, θ is the Bragg diffraction angle, and γ is the X-ray wavelength.

[0128] (5) Raman spectrum test: HR Evolution equipment was used, with a solid-state laser of 532 nm (1 MHz, power of 100 mV) for testing. After obtaining the Raman spectrum of the silicon-carbon composite material, the intensity of the peaks at 521±5 cm -1 , 480±5 cm -1 , 1360±5 cm -1 and 1580±5 cm -1 was taken, i.e. I A , I B , I D and I G .

[0129] (6) SEM test: First, the silicon-carbon composite material was polished by IB-09010CP / ion polisher (voltage of 6 kV), then the JEOL-JSM-6700F type scanning electron microscope was used to test the silicon-carbon composite material at a voltage of 5 kV and a current of 0.8 nA, and in back scattering mode.

[0130] (7) D V 50 / D V 99: MasterSizer 2000 equipment was used to test the silicon-carbon composite material from 0° to 135° of the detection angle, to obtain the D V 50 and D V 99 values of the silicon-carbon composite material.

[0131] (8) Specific surface area: the BET specific surface area of the silicon-carbon composite material is tested by using a TriStar II 3020 device.

[0132] (9) Conductivity test: the powder conductivity of the silicon-carbon composite material is tested by using a Suzhou Jizhe Electronics ST-2255 type resistivity tester.

[0133] Test of electric performance related parameters

[0134] (1) Preparation process of half battery and full battery:

[0135] Preparation process of negative electrode sheet:

[0136] The silicon-carbon composite material in the application is used as the negative active material, conductive carbon black is used as the conductive agent, and polyacrylic acid is used as the binder. The mass ratio of the negative active material, the conductive carbon black and the polyacrylic acid is 70:20:10. The negative active material, the conductive agent and the binder aqueous solution are fully mixed to obtain a mixture slurry, and the mixture slurry is uniformly coated on a copper foil and dried to obtain a negative electrode sheet.

[0137] Preparation process of positive electrode sheet:

[0138] Super P is used as the conductive agent, and PVDF is used as the binder. The mass ratio of the positive active material (LiFePO4), Super P and PVDF is 70:20:10. The positive active material, Super P and the PVDF solution are fully mixed to obtain a mixture slurry, and the mixture slurry is uniformly coated on an aluminum foil and dried to obtain a positive electrode sheet.

[0139] Electrolyte and separator:

[0140] In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed uniformly at a volume ratio of 1:1, LiPF6 and fluoroethylene carbonate (FEC) are slowly added to the mixed solution, and the mixture is stirred uniformly to obtain a non-aqueous electrolyte; Celgard 2400 separator is used as the separator.

[0141] Assembly of half battery and full battery:

[0142] The above negative electrode sheet is assembled into a button-type half battery in the glove box with lithium sheet or positive electrode sheet as the counter electrode in the order of negative electrode sheet, separator, lithium sheet or positive electrode sheet, and the above prepared positive electrode sheet is assembled into a button-type full battery as the counter electrode.

[0143] (2) First delithiation specific capacity test:

[0144] The half-cell and full-cell assembled above were respectively subjected to charge-discharge test on a LAND CT2001A battery test system. The half-cell test adopted a working voltage range of 0.01 V to 2 V, was discharged at a constant current of 0.1 C to 0.01 V, was rested for 5 min, was discharged at a constant current of 50 μA to 0.01 V, was rested for 5 min, was charged at a constant current of 0.1 C to 2.0 V, and was rested for 5 min, and the first charge capacity of the half-cell was recorded as the first delithium specific capacity.

[0145] (3) Full-cell first discharge specific capacity test:

[0146] The full-cell test adopted a working voltage range of 2.4 V to 3.8 V, was charged at a constant current of 0.1 C to 3.8 V, was charged at a constant voltage of 3.8 V to 50 μA cutoff, was rested for 5 min, was discharged at a constant current of 0.1 C to 2.4 V, and was rested for 5 min, and the first discharge capacity of the full-cell was recorded, and the full-cell first discharge specific capacity = the first discharge capacity of the full-cell / the mass of the positive active material (LiFePO4).

[0147] (4) Thickness expansion rate test of negative electrode tab:

[0148] The full-cell electrochemical device assembled above before and after 100 cycles was disassembled, and the negative electrode tab was obtained. The thickness of the tab was measured 12 times by a vernier caliper and the average value was obtained, if the thickness of the copper foil was a, the thickness of the tab before 100 cycles was b, and the thickness of the tab after 100 cycles was c, then the thickness expansion rate k of the negative electrode tab after 100 cycles was k = (c-b) / (b-a) x 100%.

[0149] (5) Low-temperature discharge capacity retention rate test:

[0150] The half-cell assembled above was discharged at a constant current of 0.5 C to 0.01 V at 25 °C, was rested for 5 min, was discharged at a constant current of 50 μA to 0.01 V, was rested for 5 min, was charged at a constant current of 0.5 C to 2.0 V, and was rested for 5 min. After three cycles, the discharge capacity of the third cycle was recorded, and then the cell was stored at -10 °C for 24 h, was discharged at a constant current of 0.5 C to 0.01 V, was rested for 5 min, and was discharged at a constant current of 50 μA to 0.01 V, and the discharge capacity under low-temperature condition was recorded.

[0151] Low-temperature discharge capacity retention rate (%) = (discharge capacity under low-temperature condition / third cycle discharge capacity) x 100%.

[0152] (6) Half-cell 50 cycle capacity retention rate (%):

[0153] The above assembled half-cell was discharged at 0.5C constant current to 0.01V at 25°C, and then rested for 5min, and then discharged at 50μA constant current to 0.01V, and then rested for 5min, and then charged at 0.5C constant current to 2.0V, and then rested for 5min, and then recorded the discharge capacity of the first cycle. Then the same step was carried out for 50 cycles of charging and discharging, and then recorded the discharge capacity of the 50th cycle.

[0154] Half-cell 50 cycle capacity retention rate (%) = (discharge capacity of the 50th cycle / discharge capacity of the first cycle) x 100%.

[0155] (7) Full-cell 100 cycle capacity retention rate (%):

[0156] The above assembled full-cell was charged at 0.5C constant current to 3.8V at 25°C, and then charged at 3.8V constant voltage to 50μA cutoff, and then rested for 5min, and then discharged at 0.5C constant current to 2.4V, and then rested for 5min, and then recorded the discharge capacity of the first cycle. Then the same step was carried out for 100 cycles of charging and discharging, and then recorded the discharge capacity of the 100th cycle.

[0157] Full-cell 100 cycle capacity retention rate (%) = (discharge capacity of the 100th cycle / discharge capacity of the first cycle) x 100%.

[0158] Example 1

[0159] I. Preparation of silicon-carbon composite material

[0160] (1) Resin-based activated carbon with a specific surface area of 1804m 2 / g, a pore volume of 0.78cm 3 / g, ultramicropores of 21%, submicropores of 22%, macropores of 52%, and mesopores of 5% was selected. The resin-based activated carbon was dried and sieved to 400 mesh, and 50g of the treated porous carbon material was placed in a chemical vapor deposition furnace. Under an argon protective atmosphere, the temperature was raised to 450°C, and under a micro-positive pressure (2kPa) gas phase pressure, a mixed gas of 10:90 volume ratio of silane and argon was used as the silicon source, and the flow rate was set to 500sccm for continuous reaction for 10h, to obtain a silicon-carbon material in which elemental nanosilicon was adsorbed and deposited in the pores of the porous carbon.

[0161] (2) In the same vapor deposition furnace, the temperature was lowered to 100°C, and under a micro-positive pressure (1kPa) gas phase pressure, oxygen was slowly introduced at a flow rate of 100sccm for 5h, to perform micro-oxidation treatment on the nanosilicon on the surface and outer surface of the porous carbon.

[0162] (3) In the same vapor deposition furnace, continue to heat to 500℃ under the protection of argon atmosphere, and under the micro-positive pressure (2.5 kPa) of the gas phase pressure, use the mixed gas of acetylene and argon with a volume ratio of 50:50 as the carbon source, set the flow rate of 650 sccm, and continuously react for 5 h, so that the acetylene-derived carbon is deposited on the outer surface of the silicon-carbon material obtained in step (2) and forms a carbon-coated layer coated silicon-carbon material;

[0163] (4) Grind and sieve the silicon-carbon material obtained in step (3) to 400 meshes to obtain the final silicon-carbon composite material.

[0164] II. Preparation of negative electrode sheet

[0165] The silicon-carbon composite material prepared by the method is used as the negative electrode active material, conductive carbon black is used as the conductive agent, and polyacrylic acid is used as the binder. The silicon-carbon composite material, the conductive carbon black, and the polyacrylic acid are fully mixed and stirred in a mass ratio of 7:2:1, and then a proper amount of deionized water is fully stirred and mixed to form a uniform negative electrode slurry with a certain viscosity. The negative electrode slurry is uniformly coated on a negative electrode current collector copper foil, and then the negative electrode slurry is fully dried at 100℃ and punched to obtain a negative electrode sheet.

[0166] III. Preparation of positive electrode sheet

[0167] LiFePO4 is used as the positive electrode active material, Super P is used as the conductive agent, and PVDF is used as the binder. LiFePO4, Super P, and PVDF are fully mixed and stirred in a mass ratio of 7:2:1, and then a proper amount of N-methyl pyrrolidone (NMP) is fully stirred and mixed to form a uniform positive electrode slurry with a certain viscosity. The positive electrode slurry is uniformly coated on a positive electrode current collector aluminum foil, and then the positive electrode slurry is fully dried at 100℃ and punched to obtain a positive electrode sheet.

[0168] IV. Preparation of electrolyte

[0169] In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed uniformly in a volume ratio of 1:1, and then LiPF6 and fluoroethylene carbonate (FEC) are slowly added to the mixed solution and stirred uniformly to obtain a non-aqueous electrolyte. The mass percentage of LiPF6 in the non-aqueous electrolyte is 12.5%, and the mass percentage of FEC is 4.5% based on the mass of the electrolyte.

[0170] V. Separation membrane

[0171] Celgard 24000 separation membrane is used as the separation membrane.

[0172] VI. Preparation of lithium ion battery

[0173] The above negative electrode sheet was assembled into a button-type half-cell (lithium sheet as the counter electrode) and a full-cell (positive electrode sheet as the positive electrode) in the glove box, respectively, with the lithium sheet and the positive electrode sheet as the counter electrode, in the order of negative electrode sheet, separator, lithium sheet or positive electrode sheet.

[0174] Examples 2 to 8

[0175] On the basis of Example 1, except that in the step of "I. Preparation of silicon-carbon composite material", the relevant preparation parameters were adjusted according to Table 1, and the rest were the same as Example 1.

[0176] Comparative Examples 1 to 6

[0177] On the basis of Example 1, except that in the step of "I. Preparation of silicon-carbon composite material", the relevant preparation parameters were adjusted according to Table 1, and the rest were the same as Example 1.

[0178] The preparation parameters, powder performance parameters and electrical performance parameters of each example and comparative example are shown in Table 1, Table 2 and Table 3.

[0179] Table 1: Experimental process parameters of Examples 1-8 and Comparative Examples 1-6

[0180]

[0181] Table 2: Powder data of Examples 1-8 and Comparative Examples 1-6

[0182]

[0183] Table 3: Half-cell and full-cell electrochemical performance of Examples 1-8 and Comparative Examples 1-6

[0184]

[0185]

[0186] Referring to Table 1, Table 2 and Table 3, from the example and comparative example data, it can be seen that when the R / R' value is too large, such as in Comparative Example 1 and Comparative Example 2, the R / R' value of the silicon-carbon composite material is 0.87 and 0.83, respectively, although the lithium ion battery has a high first delithiation specific capacity and conductivity, the corresponding capacity retention rate (%) and thickness expansion rate (%) are significantly decreased, i.e. the cycle performance and expansion performance of the silicon-carbon composite material are significantly decreased; and when the R / R' value is too low, such as in Comparative Example 3 and Comparative Example 4, the R / R' value of the silicon-carbon composite material is 0.56 and 0.50, respectively, although the lithium ion battery has relatively good 50-cycle capacity retention rate (%) and 100-cycle capacity retention rate (%), the corresponding specific capacity, conductivity, low-temperature discharge capacity retention rate, first discharge specific capacity, thickness expansion rate and other performances are significantly decreased.

[0187] Further, when the R value of the silicon-carbon composite material is too large, such as in Comparative Example 1 and Comparative Example 2, the R value of the silicon-carbon composite material is 0.99 and 0.96, respectively, although the lithium ion battery has a high first delithiation specific capacity and conductivity, the cycle performance (capacity retention rate) and expansion performance (thickness expansion rate) of the lithium ion battery are significantly decreased; when the R value is too small, such as in Comparative Example 3 and Comparative Example 4, the R value of the silicon-carbon composite material is 0.71 and 0.64, respectively, although the lithium ion battery has relatively good cycle performance (capacity retention rate), the specific capacity, conductivity, low-temperature discharge capacity retention rate, first discharge specific capacity, and thickness expansion rate are significantly decreased. When the R' value is too large, such as in Comparative Example 3 and Comparative Example 4, the R' value of the silicon-carbon composite material is 1.26 and 1.28, respectively, although the lithium ion battery has relatively good cycle performance (capacity retention rate), the specific capacity, conductivity, low-temperature discharge capacity retention rate, first discharge specific capacity, and thickness expansion rate are significantly decreased; and when the R' value is too low, such as in Comparative Example 1 and Comparative Example 2, the R' value of the silicon-carbon composite material is 1.14 and 1.16, respectively, although the lithium ion battery has a high first delithiation specific capacity and conductivity, the cycle performance (capacity retention rate) and expansion performance (thickness expansion rate) of the lithium ion battery are significantly decreased. At the same time, too low R value or too high R' value can further deteriorate the low-temperature performance (low-temperature discharge capacity retention rate) of the silicon-carbon composite material, such as in Comparative Example 3 and Comparative Example 4.

[0188] In addition, it can be seen from Comparative Example 5 that when the R' value of the silicon-carbon composite material is 1.24, i.e. within the range of the present application, and the R value is 0.97, i.e. too large and not within the range of the present application, although the lithium ion battery has a high specific capacity, the conductivity, low-temperature performance, cycle performance, and expansion performance of the lithium ion battery are decreased. It can also be seen from Comparative Example 6 that when the R' value of the silicon-carbon composite material is 1.30, i.e. too large and not within the range of the present application, and the R value is 0.79, i.e. within the range of the present application, although the lithium ion battery has good cycle performance and expansion performance, the conductivity, specific capacity, and low-temperature performance are significantly decreased.

[0189] It has been found through research that, by simultaneously regulating the R / R' value of the silicon-carbon composite material and the corresponding R value and R' value within the scope of the present application, the lithium ion battery has higher specific capacity and improved conductivity, low-temperature performance, cycle performance, and expansion performance. That is, by regulating the silicon-carbon composite material to simultaneously satisfy the R / R' value in the range of greater than 0.61 and less than 0.78 (such as the R / R' value in the examples: 0.62, 0.66, 0.68, 0.71, 0.72, 0.74, 0.75), the corresponding R value in the range of greater than 0.78 and less than 0.9 (such as the R value in the examples: 0.78, 0.79, 0.82, 0.86, 0.87, 0.88, 0.89, 0.90), and the corresponding R' value in the range of greater than or equal to 1.2 and less than or equal to 1.25 (such as the R' value in the examples: 1.20, 1.21, 1.22, 1.23, 1.25), the silicon-carbon composite material has the best balance ratio between crystalline silicon and amorphous silicon, and between carbon defects and carbon graphitization, thereby having significantly improved conductivity, low-temperature performance, cycle performance, and expansion performance while maintaining high specific capacity.

[0190] As can also be seen from Examples 1 to 8, compared to the existing silicon-carbon composite material or the silicon-carbon composite material in the comparative examples, when the R / R' value of the silicon-carbon composite material and the corresponding R value and R' value are simultaneously regulated within the scope provided by the present application, the obtained silicon-carbon composite material has higher specific capacity and more excellent conductivity, low-temperature performance, cycle performance, and expansion performance, thereby exhibiting more outstanding lithium storage performance. After the lithium ion battery is cycled, the oxygen element content of the silicon-carbon composite material in the negative electrode sheet also affects the cycle performance of the lithium ion battery. As can be seen from Examples 1 to 8, by regulating the oxygen element content of the silicon-carbon composite material in the negative electrode sheet after cycling within the scope of the present application, it is beneficial to further balance the specific capacity, cycle performance, and expansion performance of the lithium ion battery, thereby improving the overall performance of the lithium ion battery. The ratio of the carbon material to elemental silicon of the silicon-carbon composite material, the size of the silicon microcrystals, the oxygen element content, the D V 50、D V 99、The specific surface area and the oxygen element content also generally affect the performance of the lithium ion battery. As can be seen from Examples 1 to 8, by controlling the content ratio of the carbon material to elemental silicon of the silicon-carbon composite material, the size of the silicon microcrystals, the oxygen element content, the D V 50、D V 99、The specific surface area and the oxygen element content also generally affect the performance of the lithium ion battery. As can be seen from Examples 1 to 8, by controlling the content ratio of the carbon material to elemental silicon of the silicon-carbon composite material, the size of the silicon microcrystals, the oxygen element content, the D

[0191] It is to be understood that the present application is described by way of example only, and that modifications or alterations can be made to the features and embodiments described without departing from the spirit and scope of the application. In addition, modifications can be made to the features and embodiments described to accommodate specific situations and materials without departing from the spirit and scope of the application. Accordingly, the application is not limited to the specific embodiments disclosed herein, but rather, the scope of the application includes all embodiments falling within the scope of the claims.

Claims

1. A silicon-carbon composite material comprising elemental silicon and a carbon material, characterized in that, The Raman spectrum of the silicon-carbon composite material satisfies: 0.61 < R / R' < 0.78; wherein, R value is the ratio of I A and I B ; R' value is the ratio of I D and I G ; I A represents the intensity of the peak at 521±5cm -1 in the Raman spectrum of the silicon-carbon composite material; I B represents the intensity of the peak at 480±5cm -1 in the Raman spectrum of the silicon-carbon composite material; I D represents the intensity of the peak at 1360±5cm -1 in the Raman spectrum of the silicon-carbon composite material; I G represents the intensity of the peak at 1580±5cm -1 in the Raman spectrum of the silicon-carbon composite material.

2. The silicon-carbon composite material of claim 1, wherein, R in the Raman spectrum of the silicon-carbon composite material ranges from 0.78 to 0.9; R' in the Raman spectrum of the silicon-carbon composite material ranges from 1.2 to 1.

25.

3. The silicon-carbon composite of claim 1, wherein, The elemental silicon includes at least one of silicon nanoparticles, silicon submicron particles, or silicon nanofilms; and the carbon material includes a porous carbon framework and a carbon coating layer.

4. The silicon-carbon composite of claim 1, wherein, The ratio of the content a of carbon in the silicon-carbon composite material to the content b of silicon in the silicon-carbon composite material ranges from 0.5 to 10; and / or, The content a of carbon in the silicon-carbon composite material ranges from 40 wt% to 90 wt%, and the content b of silicon in the silicon-carbon composite material ranges from 10 wt% to 60 wt%.

5. The silicon-carbon composite of claim 1, wherein, The ratio of the content a of carbon in the silicon-carbon composite material to the content b of silicon in the silicon-carbon composite material ranges from 1.07 to 2.03; and / or, The content a of carbon in the silicon-carbon composite material ranges from 50 wt% to 64.8 wt%, and the content b of silicon in the silicon-carbon composite material ranges from 31.9 wt% to 46.6 wt%.

6. The silicon-carbon composite of claim 4, wherein, 1.18 < a / b < 2.

01.

7. The silicon-carbon composite of claim 4, wherein, The content a of carbon in the silicon-carbon composite material ranges from 51.8 wt% to 63.6 wt%, and the content b of silicon in the silicon-carbon composite material ranges from 31.6 wt% to 44.0 wt%.

8. The silicon-carbon composite of claim 1, wherein, The internal cross-sectional SEM image of the silicon-carbon composite material shows that the internal surface of the particles is flat, and the pore size of the pores in the particles is less than 50 nm.

9. The silicon-carbon composite of any one of claims 1 to 8, wherein, The silicon-carbon composite material satisfies at least one of the following conditions 1) to 5): 1) the size of the silicon microcrystals in the silicon-carbon composite material is less than 1 nm; or, the size of the silicon microcrystals in the silicon-carbon composite material is 0.8 nm to 0.95 nm; 2) the particle size D of the silicon-carbon composite material V 50 is in the range of 5 µm to 10 µm, D V 99 is in the range of 15 µm to 25 µm; Alternatively, the particle size D V 50 is in the range of 5 µm to 6.5 µm, D V 99 is in the range of 15 µm to 19 µm; 3) the silicon-carbon composite material has a specific surface area ranging from 1 m 2 / g to 50 m 2 / g; or, the silicon-carbon composite material has a specific surface area ranging from 4 m 2 / g to 8 m 2 / g. 4) the content of oxygen in the silicon-carbon composite material ranges from 1.0 wt% to 4.0 wt%; or, the content of oxygen in the silicon-carbon composite material ranges from 1.2 wt% to 3 wt%; 5) the first delithiation specific capacity of the silicon-carbon composite material is 500 mAh / g to 2500 mAh / g; or, the first delithiation specific capacity of the silicon-carbon composite material is 1400 mAh / g to 2000 mAh / g.

10. The silicon-carbon composite material of any one of claims 1 to 9, wherein, 0.62 < R / R' < 0.

75.

11. The silicon-carbon composite of claim 9, wherein, The size of the silicon microcrystals in the silicon-carbon composite material is 0.8 nm to 0.92 nm.

12. The silicon-carbon composite of claim 9, wherein, The content of oxygen in the silicon-carbon composite material ranges from 1.3 wt% to 2.6 wt%.

13. A method of making a silicon-carbon composite material as claimed in any one of claims 1 to 12, characterised in that, The preparation method comprises: adjusting the R value of the silicon-carbon composite material by changing at least one of the pyrolysis temperature of the silicon-containing gas, the flow rate of the silicon-containing gas, the time for introducing the silicon-containing gas, the time for micro-oxidation, the temperature for micro-oxidation, or the flow rate of the oxygen; and / or, The R' value of the silicon-carbon composite material is regulated by changing at least one of a pyrolysis temperature of the carbon source gas, a flow rate of the carbon source gas, or a time for inputting the carbon source gas.

14. The method of claim 13, wherein, The preparation method comprises the following steps: (1) Silicon deposition: taking a porous carbon material as a precursor and a silicon-containing gas as a silicon source, elemental nanosilicon is adsorbed and deposited in the pores of the porous carbon by chemical vapor deposition; (2) Micro-oxidation: oxygen is inputted to micro-oxidize the nanosilicon on the superficial surface and the outer surface of the porous carbon; (3) Carbon deposition: after the micro-oxidation treatment, a carbon source gas is inputted to make the carbon source gas deposit on the outer surface of the material obtained in step (2) and form a carbon coating layer, thereby obtaining a silicon-carbon material coated with the carbon coating layer; wherein, the steps (1)-(3) are all carried out under the protection of an inert gas atmosphere and a micro-positive pressure gas phase pressure condition; the micro-positive pressure condition is 0.1-5 kPa.

15. The preparation method according to claim 14, characterized in that, The preparation method further comprises: (4) The silicon-carbon composite material obtained in step (3) is ground and sieved to obtain the silicon-carbon composite material.

16. The method of claim 14, wherein, In the step (1), the micro-positive pressure condition is 1.5-2.5 kPa; in the step (2), the micro-positive pressure condition is 0.5-1.5 kPa; and in the step (3), the micro-positive pressure condition is 2.0-3.0 kPa.

17. A negative electrode sheet characterized by comprising: The negative electrode tab comprises a negative electrode current collector and a negative electrode active material located on at least one surface of the negative electrode current collector, and the negative electrode active material comprises the silicon-carbon composite material according to any one of claims 1-12.

18. The negative electrode sheet according to claim 17, wherein The negative electrode active material further comprises graphite, a conductive agent, and a binder; the mass ratio of the graphite in the negative electrode active material is 35 wt% to 95 wt%.

19. The negative electrode sheet according to claim 17, wherein The negative electrode active material satisfies at least one of the following conditions (I)-(III): (I) particle size D V 50 is 5 µm to 15 µm, D V 99 is 15 µm to 40 µm; (II) the specific surface area of the negative electrode active material is 1 m 2 / g~10 m 2 / g; (III) The first delithiation specific capacity of the negative electrode active material is 400 mAh / g-1000 mAh / g.

20. An electrochemical device, characterized by, The electrochemical device comprises the negative electrode tab according to any one of claims 17-19.

21. An electronic device, comprising: The electrochemical device comprises the negative electrode tab according to any one of claims 17-19.

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