Silicon-carbon negative electrode material, electrochemical device, and electronic device

Abstract

The present application relates to a kind of silicon-carbon negative electrode material, electrochemical device and electronic device.The silicon-carbon negative electrode material includes silicon-carbon composite and the coating layer on the surface of at least a part of silicon-carbon composite, and the coating layer includes carbon nitride and carbon nanotube.The silicon-carbon negative electrode material can improve cycle life and cycle structure stability as lithium ion battery negative electrode material.

Description

Technical Field

[0001] This application relates to the field of lithium-ion batteries. Specifically, this application relates to a silicon-carbon anode material and a method for preparing the same. This application also relates to an anode, an electrochemical device, and an electronic device comprising the silicon-carbon anode material. Background Technology

[0002] Silicon, as a negative electrode material for lithium-ion batteries, boasts a theoretical specific capacity as high as 4200 mAh / g and is considered the most promising next-generation lithium-ion negative electrode material to replace traditional graphite, thereby improving the energy density of lithium-ion batteries. However, during charge and discharge, as lithium ions intercalate into the silicon lattice, silicon-silicon bonds break and lithium-silicon bonds gradually form. This causes a significant volume expansion (300% to 400%) in silicon particles during lithium intercalation. This massive volume expansion is unavoidable during the lithium intercalation process. During cycling, this volume change can lead to silicon particle fragmentation, resulting in electrode pulverization and the formation of an unstable SEI film, causing rapid capacity decay. Due to the numerous drawbacks of silicon itself, extensive research and practical production have focused on combining silicon with other materials through appropriate preparation methods to obtain silicon-based composite materials. These composites utilize the physical properties of other materials to improve the electrochemical performance of elemental silicon. A particularly ideal approach is to combine nano-silicon particles with a structurally stable and highly conductive matrix material. This approach fully leverages the high capacity of silicon while mitigating the volume expansion effect of silicon and providing electron and lithium-ion transport channels. Carbon-based materials possess excellent flexibility, conductivity, mechanical strength, and cycle stability, and are abundant and inexpensive. Extensive research has shown that carbon materials such as graphite, carbon nanotubes, graphene, and graphene oxide can be combined with silicon through various preparation methods to effectively mitigate the volume expansion effect of silicon and improve lithium-ion and electron conductivity, thus significantly enhancing electrochemical performance. While silicon-carbon materials can mitigate silicon particle expansion to some extent through structural design, this sacrifices the proportion of silicon, limiting its content in secondary particles to below 20%; otherwise, it cannot effectively suppress silicon expansion. Therefore, the high specific capacity advantage of silicon materials cannot be fully utilized to improve the energy density of lithium-ion batteries. Furthermore, amorphous carbon and graphite are relatively reactive; with lithium insertion / extraction in the battery, they readily react with lithium ions and the electrolyte, forming an SEI film, leading to continuous electrolyte consumption, capacity decay, and increased internal resistance. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this application provides a silicon-carbon anode material that, as a lithium-ion battery anode material, can improve cycle life and cycle structure stability.

[0004] In a first aspect, the silicon-carbon anode material provided in this application includes a silicon-carbon composite and a coating layer located on at least a portion of the surface of the silicon-carbon composite, wherein the coating layer includes carbon nitride and carbon nanotubes. The use of a carbon nitride and carbon nanotube composite layer to confine the internal silicon-carbon composite ensures good electrical contact between the nano-silicon and conductive carbon. The outer carbon nitride and carbon nanotube composite layer, possessing high stability and high strength, alleviates volume expansion while also stabilizing the SEI film, thereby improving the cycle stability of the lithium-ion battery.

[0005] According to some embodiments of this application, the mass content of the carbon nitride is 50% to 80% based on the mass of the coating layer.

[0006] According to some embodiments of this application, the particle size Dv50 of the carbon-silicon composite is a nm, and the particle size Dv50 of the silicon-carbon anode material is b nm, where 0.7 ≤ a / b ≤ 0.9; and 50 ≤ a ≤ 500, 60 ≤ b ≤ 700. In this application, Dv50 represents the median particle size in the volume distribution.

[0007] According to some embodiments of this application, the silicon-carbon anode material has a silicon content of 30% to 80% by mass. According to some embodiments of this application, the silicon-carbon composite comprises silicon particles with a gap between the silicon particles greater than or equal to 2 nm.

[0008] According to some embodiments of this application, the silicon-carbon anode material satisfies at least one of the following conditions (a) to (b): (a) the porosity of the silicon-carbon anode material is 1% to 10%; (b) the porosity of the silicon-carbon anode material is I D / I G It is 1.5 to 2, where I D For Raman spectra located at 1300 cm⁻¹ -1 Up to 1400cm -1 The peak intensity within the range of I G Located at 1580 cm⁻¹ in the Raman spectrum -1 Up to 1620cm -1 The peak intensity range.

[0009] According to some embodiments of this application, the diameter of the carbon nanotubes is 2 nm to 10 nm.

[0010] In a second aspect, this application provides an electrochemical device comprising a negative electrode, the negative electrode comprising a current collector and a negative electrode active material layer, the negative electrode active material layer comprising the silicon-carbon negative electrode material described in the first aspect.

[0011] According to some embodiments of this application, the electrochemical device satisfies at least one of the following conditions (c) to (f): (c) the resistance of the negative electrode active material layer is 0.2Ω to 2Ω; (d) the adhesion strength of the negative electrode is 30N / m to 50N / m; (e) the compaction density of the negative electrode is 1.5g / cm³. 3 Up to 2g / cm 3 (f) The porosity of the negative electrode is 10% to 20%.

[0012] In a third aspect, this application provides an electronic device comprising the electrochemical device described in the second aspect.

[0013] The silicon-carbon anode material includes a silicon-carbon composite and a coating layer located on at least a portion of the surface of the silicon-carbon composite. The coating layer includes carbon nitride and carbon nanotubes, which can improve cycle life and cycle structure stability. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the structure of a silicon-carbon anode material according to an embodiment of this application. Detailed Implementation

[0015] For the sake of brevity, this article only discloses a few specific numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0016] In this description, unless otherwise stated, "above" and "below" include the stated number.

[0017] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0018] The term "about" is used to describe and indicate small variations. When used in conjunction with an event or situation, the term may refer to examples in which the event or situation occurred precisely or in examples in which the event or situation occurred very approximately. For example, when used in conjunction with numerical values, the term may refer to a range of variation less than or equal to ±10% of the numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. Additionally, quantities, ratios, and other numerical values ​​are sometimes presented in range format herein. It should be understood that such range format is for convenience and brevity and should be interpreted flexibly to include not only numerical values ​​explicitly specified as range limits but also all individual numerical values ​​or subranges covered within the range, as if each numerical value and subrange were explicitly specified.

[0019] The list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another instance, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0020] Mitigating volume expansion and resolving the SEI film problem are two major aspects that need to be addressed for the commercial application of silicon anode materials. To solve these technical problems, this application provides a silicon anode material and its preparation method, as well as an electrochemical device and an electronic device including the silicon anode material.

[0021] I. Silicon-Carbon Anode Materials

[0022] The silicon-carbon anode material provided in this application includes a silicon-carbon composite and a coating layer located on at least a portion of the surface of the silicon-carbon composite, wherein the coating layer includes carbon nitride and carbon nanotubes. On the one hand, by confining the silicon-carbon composite within the shell through the outer carbon nitride and carbon nanotube composite layer, the volume change effect of silicon particles can be mitigated, ensuring the stability of the electrode structure. The interaction between carbon nitride and carbon nanotubes can induce the formation of electron-hole pairs, which is beneficial for the rapid transport of electrons from carbon nanotubes to carbon nitride. In addition, the outer coating layer can isolate the contact between the electrolyte and silicon particles, and the highly stable carbon nitride is less likely to react with the electrolyte, thus avoiding electrolyte consumption. On the other hand, as a whole, the elastic carbon material between silicon particles of the silicon-carbon composite is conducive to absorbing volume expansion, ensuring good electrical contact between nano-silicon and conductive carbon.

[0023] According to some embodiments of this application, the mass content of carbon nitride is 50% to 80% based on the mass of the coating layer. In some embodiments of this application, the mass content of carbon nitride is 53%, 57%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 72%, 74%, 77%, or any value between them. Increasing the carbon nitride content can improve the mechanical strength of the carbon nitride and carbon nanotube composite layer, which is beneficial for alleviating the stress generated in the silicon-carbon composite core and maintaining the stability of the material structure during cycling. However, when the carbon nitride content increases to a certain level, the carbon nanotube content in the composite layer decreases, and the overall conductivity of the composite layer decreases, which is detrimental to electron and ion transport and affects rate performance. Conversely, when the carbon nitride content in the composite layer decreases, the mechanical strength of the composite layer decreases, and the increased carbon nanotubes are prone to side reactions with lithium ions and electrolytes during lithium insertion / extraction processes, causing cycling effects.

[0024] According to some embodiments of this application, the particle size Dv50 of the carbon-silicon composite is a nm, and the particle size Dv50 of the silicon-carbon anode material is b nm, where 0.7 ≤ a / b ≤ 0.9, 50 ≤ a ≤ 500, and 60 ≤ b ≤ 700. In this application, Dv50 represents the median particle size in the volume distribution.

[0025] According to some embodiments of this application, the particle size Dv50 of the carbon-silicon composite is a nm, and the particle size Dv50 of the silicon-carbon anode material is b nm, where 0.7 ≤ a / b ≤ 0.9. In some embodiments of this application, a / b is 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, or any value between them. When a / b is too high, the thickness of the outer carbon nitride and carbon nanotube composite layer is relatively thin. The thinner carbon nitride and carbon nanotube composite shell cannot effectively restrain the expansion of the silicon-carbon core, causing structural damage and exposing the nano-silicon to the electrolyte, preventing the formation of a stable SEI film and affecting cycle performance. When the a / b ratio is too low, the thickness of the carbon nitride and carbon nanotube composite layer increases. While this can improve the restraint on the expansion of the silicon-carbon core, an excessively thick carbon nitride and carbon nanotube composite layer is not conducive to the transport of ions and electrons. At the same time, increasing the content of carbon nitride and carbon nanotubes will reduce the silicon content, which is not conducive to fully utilizing the high energy density advantage of silicon, resulting in poor electrochemical performance of the material.

[0026] According to some embodiments of this application, the particle size Dv50 of the silicon carbide composite is a nm, where 50 ≤ a ≤ 500. In some embodiments of this application, a is 70, 100, 150, 200, 250, 300, 350, 400, 450, 480, or any value between them.

[0027] According to some embodiments of this application, the particle size Dv50 of the silicon-carbon anode material is b nm, where 60 ≤ b ≤ 700. In some embodiments of this application, b is 80, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 680, or any value between them.

[0028] According to some embodiments of this application, the silicon-carbon composite comprises silicon particles, and the gap between the silicon particles is greater than or equal to 2 nm. In this application, the silicon-carbon composite comprises silicon particles and a carbon layer filling the spaces between adjacent silicon particles, the size of the gap between the silicon particles being the thickness of the carbon layer filling the spaces between the silicon particles. The silicon content is directly related to the size of the gap between the silicon particles; a larger gap between silicon particles means a thicker carbon layer, and a higher carbon content means a lower silicon content. In this application, the gap between the silicon particles is the shortest distance between the silicon particles. In some embodiments, the shortest distance between the silicon particles is the shortest distance between the edges of adjacent silicon particles in the SEM image at the cross-section in a CROSS-SECTION test. In some embodiments of this application, the gap between the silicon particles is 3 nm to 30 nm.

[0029] According to some embodiments of this application, the silicon content of the silicon-carbon anode material is 30% to 80% by mass. In some embodiments of this application, the silicon content is 35%, 40%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 57%, 60%, 63%, 65%, 67%, 70%, 73%, 75%, 78%, or any value between them. Higher silicon content results in higher specific capacity of the material. However, due to the increased silicon content, under otherwise identical conditions, the carbon content decreases, the carbon layer thickness decreases, meaning the gaps between silicon particles decrease, the buffer space between nano-silicon particles decreases, and the expansion of the silicon-carbon core increases. This may damage the outer carbon nitride and carbon nanotube composite shell, causing structural damage, allowing electrolyte to penetrate into the silicon-carbon core and causing side reactions, thus affecting cycle performance. When the silicon content is reduced, i.e. the carbon content is increased, the carbon material fills the porous structure during the composite preparation process, serving as a gap between adjacent silicon particles. When the carbon layer thickness increases, it helps to absorb the stress generated by the expansion of silicon lithiation and maintain electrical contact during the charging and discharging process. However, if the silicon content is too low, there is no gap between the silicon particles. As a pure silicon core, the expansion increases significantly, which has already broken through the outer composite shell, causing a severe decline in the material's cycle performance.

[0030] According to some embodiments of this application, the porosity of the silicon-carbon anode material is 1% to 10%. In some embodiments of this application, the porosity of the silicon-carbon anode material is 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or any value between them. Increased porosity provides better expansion space for the silicon-carbon composite, which is beneficial for improving expansion performance; however, more voids affect electrical contact and are detrimental to rate performance. Too low a porosity is not conducive to buffering the expansion stress generated during lithiation, resulting in poor expansion performance during cycling.

[0031] According to some embodiments of this application, the silicon-carbon anode material I... D / I G The range is 1.5 to 2, for example, 1.6, 1.7, 1.8, 1.9, etc., where I D For Raman spectra located at 1300 cm⁻¹ -1 Up to 1400cm -1 The peak intensity within the range of I G Located at 1580 cm⁻¹ in the Raman spectrum -1 Up to 1620cm -1 The peak intensity range. In the embodiments of this application, carbon nitride is formed by in-situ high-temperature sintering in the presence of carbon nanotubes. During the reaction, the interaction between carbon nitride and carbon nanotubes can induce the formation of electron-hole pairs, resulting in an increase in defects and disorder in the carbon nanotubes, i.e., I D / I G The ratio increases, resulting in a higher I. D / I G This precisely demonstrates the interaction between carbon nitride and carbon nanotubes, forming electron-hole pairs that facilitate the rapid transport of electrons from carbon nanotubes to carbon nitride. However, when I... D / I G As the ratio increases further, the increased disorder of carbon nanotubes will affect conductivity. Therefore, controlling it within this range can provide better conductivity for the carbon nitride and carbon nanotube composite layer.

[0032] According to some embodiments of this application, the diameter of the carbon nanotubes is from 2 nm to 10 nm. In some embodiments of this application, the diameter of the carbon nanotubes is 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, or any value between them. When the diameter of the carbon nanotubes increases, the mechanical strength of the carbon nanotubes and carbon nitride decreases, which is not conducive to the confinement of the expansion of the silicon-carbon core and affects the expansion performance. When the diameter of the carbon nanotubes decreases, the carbon nanotubes are not easily dispersed in the composite and cannot form a uniform distribution with the carbon carbide, which is not conducive to improving the overall conductivity of the composite.

[0033] II. Preparation methods of silicon-carbon anode materials

[0034] The method for preparing the silicon-carbon anode material provided in this application includes mixing a silicon-carbon composite, carbon nanotubes, and a nitrogen-containing carbon source, followed by sintering under an inert atmosphere. In some embodiments of this application, the sintering temperature can be a temperature within the range of 300°C to 800°C. In some embodiments of this application, the sintering time can be 2 hours to 8 hours.

[0035] According to some embodiments of this application, the nitrogen-containing carbon source is selected from at least one of ammonia nitrile, polycyanamide, and urea.

[0036] According to some embodiments of this application, the preparation method of the silicon-carbon composite includes the following steps:

[0037] Step A: After mixing porous silica and carbon precursor, sintering is carried out under an inert atmosphere to obtain the first solid composite.

[0038] Step B: After mixing the first solid composite with the reducing agent, sinter the mixture under a hydrogen-argon mixed atmosphere to obtain the second solid composite.

[0039] Step C: Mix the second solid composite with acid, filter, and dry to obtain the silicon-carbon composite.

[0040] In some embodiments of the above preparation method, in step A, the carbon precursor is selected from phenolic resins, etc. According to some embodiments of this application, in step B, the reducing agent includes a metal reducing agent, such as magnesium powder. According to some embodiments of this application, in step C, the acid can be one or a combination of hydrochloric acid, sulfuric acid, nitric acid, oxalic acid, hydrofluoric acid, and phosphoric acid.

[0041] In some embodiments of the above preparation method, the sintering temperature in step A can be a temperature within the range of 500℃ to 1200℃. The sintering time can be 1h to 5h.

[0042] In some embodiments of the above preparation method, the sintering temperature in step B can be a temperature within the range of 600℃ to 800℃. The sintering time can be 3h to 10h.

[0043] In some embodiments of the above preparation method, the inert atmosphere may be one or a combination of nitrogen, argon, and helium.

[0044] III. Negative Electrode

[0045] This application also provides a negative electrode, which includes a negative electrode. According to some embodiments, the negative electrode includes a current collector and a negative electrode active material layer, the negative electrode active material layer comprising the silicon-carbon negative electrode material described in the first aspect.

[0046] In some embodiments, the resistance of the negative electrode active material layer is 0.2Ω to 2Ω. In some embodiments, the adhesion strength of the negative electrode is 30N / m to 50N / m. Improving the adhesion strength of the electrode sheet results in better contact between particles and the substrate, and between particles themselves, leading to better buffering of expansion stress. Simultaneously, it can reduce the internal resistance of the battery and improve its rate performance, cycle life, and expansion performance.

[0047] In some embodiments, the compaction density of the negative electrode is 1.5 g / cm³. 3 Up to 2g / cm 3 In some embodiments, the porosity of the negative electrode is 10% to 20%.

[0048] In some embodiments, the current collector includes: copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0049] In some embodiments, the adhesive includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0050] In some embodiments, the conductive agent includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0051] The negative electrode of this application can be prepared using methods known in the art. Typically, the negative electrode active material, along with optional conductive agents (such as carbon materials like carbon black and metal particles), binders (such as SBR), and other optional additives (such as PTC thermistor materials), are mixed and dispersed in a solvent (such as deionized water). After thorough stirring, the mixture is uniformly coated onto a negative electrode current collector, and then dried to obtain the negative electrode containing the negative electrode film. Metal foil or porous metal plates can be used as the negative electrode current collector.

[0052] IV. Electrochemical Device

[0053] An embodiment of this application provides an electrochemical device, which includes a negative electrode, a positive electrode, an electrolyte, and a separating membrane.

[0054] negative electrode

[0055] The negative electrode in the electrochemical device of this application is the negative electrode described in the third aspect of this application.

[0056] positive electrode

[0057] The materials, composition, and manufacturing methods of the positive electrode that can be used in the embodiments of this application include any techniques disclosed in the prior art.

[0058] In some embodiments, the positive electrode includes a current collector and a layer of positive electrode active material located on the current collector.

[0059] In some embodiments, the positive electrode active material includes, but is not limited to: lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese (NCM) ternary material, lithium iron phosphate (LiFePO4), or lithium manganese oxide (LiMn2O4).

[0060] In some embodiments, the positive electrode active material layer further includes a binder and optionally a conductive material. The binder improves the bonding between the positive electrode active material particles and also improves the bonding between the positive electrode active material and the current collector.

[0061] In some embodiments, the adhesive includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0062] In some embodiments, the conductive material includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0063] In some embodiments, the current collector may include, but is not limited to, aluminum.

[0064] The positive electrode can be prepared by methods known in the art. For example, the positive electrode can be obtained by mixing an active material, a conductive material, and a binder in a solvent to prepare an active material composition, and then coating the active material composition onto a current collector. In some embodiments, the solvent may include, but is not limited to, N-methylpyrrolidone.

[0065] electrolyte

[0066] The electrolyte that can be used in the embodiments of this application can be an electrolyte known in the prior art.

[0067] In some embodiments, the electrolyte comprises an organic solvent, a lithium salt, and additives. The organic solvent of the electrolyte according to this application may be any organic solvent known in the art that can be used as an electrolyte solvent. There are no limitations on the electrolyte used in the electrolyte according to this application; it may be any electrolyte known in the art. The additives of the electrolyte according to this application may be any additives known in the art that can be used as electrolyte additives.

[0068] In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate.

[0069] In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt.

[0070] In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalateborate)borate LiB(C2O4)2 (LiBOB), or lithium difluorooxalateborate LiBF2(C2O4) (LIDFOB).

[0071] In some embodiments, the concentration of lithium salt in the electrolyte is: about 0.5 mol / L to 3 mol / L, about 0.5 mol / L to 2 mol / L, or about 0.8 mol / L to 1.5 mol / L.

[0072] Separating membrane

[0073] In some embodiments, a separator is provided between the positive and negative electrodes to prevent short circuits. The material and shape of the separator used in the embodiments of this application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator comprises a polymer or inorganic material formed from a material stable to the electrolyte of this application.

[0074] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected.

[0075] A surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0076] The inorganic layer comprises inorganic particles and a binder. The inorganic particles include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene.

[0077] The polymer layer contains a polymer, the polymer material of which includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0078] In some embodiments, the electrochemical device of this application includes, but is not limited to, all types of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors.

[0079] In some embodiments, the electrochemical device is a lithium secondary battery.

[0080] In some embodiments, 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.

[0081] V. Electronic Devices

[0082] The electronic device described in this application can be any device that uses the electrochemical device described in the fourth aspect of this application.

[0083] In some embodiments, the electronic device includes, but is not limited to: laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries or lithium-ion capacitors, etc.

[0084] Test method:

[0085] Particle size test: Add about 0.02g of powder sample to a 50ml clean beaker, add about 20ml of deionized water, and then add a few drops of 1% surfactant to completely disperse the powder in the water. Sonicate in a 120W ultrasonic cleaner for 5 minutes and test the particle size distribution using a MasterSizer 2000.

[0086] Silicon content testing: First, the carbon nitride content of the samples was tested using a thermogravimetric analyzer (STA449F3-QMS403C) on both pure carbon nitride and the silicon-carbon anode material. The testing conditions were consistent: an inert atmosphere, a heating rate of 10℃ / min, a temperature rise to 800℃ and held for 2 hours, and an atmosphere pressure of 0.01MPa to 0.04MPa. Since carbon nitride decomposes almost 100% above 750℃, leaving almost no residual carbon nitride, the difference in the remaining mass percentage of the two samples represents the mass percentage of other substances in the silicon-carbon anode material after removing carbon nitride. Subtracting 100 from the obtained value gives the mass percentage of carbon nitride. Then, the carbon content of the remaining sample after heat treatment was tested. Subtracting 100 from the obtained carbon nitride and carbon content values ​​gives the silicon content percentage. Carbon content testing was performed as follows: After heat treatment, the remaining sample was heated at high temperature in a high-frequency furnace under oxygen-enriched conditions to oxidize carbon and sulfur into carbon dioxide and sulfur dioxide. This gas, after treatment, entered the corresponding absorption cell, absorbed the corresponding infrared radiation, and was then converted into a corresponding signal by a detector. This signal was sampled by a computer, linearly corrected, and converted into a value proportional to the concentrations of carbon dioxide and sulfur dioxide. The values ​​from the entire analysis process were then accumulated. After the analysis, this accumulated value was divided by the weight value in the computer, multiplied by the correction factor, and the blank was subtracted to obtain the percentage carbon and sulfur content in the sample. The sample was tested using a high-frequency infrared carbon-sulfur analyzer (Shanghai Dekai HCS-140).

[0087] Carbon nitride content test of the coating layer: The test method was to use a thermogravimetric analyzer (STA449F3-QMS403C) to test pure carbon nitride and the silicon-carbon anode material samples respectively. The test conditions were the same: inert atmosphere, heating rate of 10℃ / min, heating to 800℃ and holding for 2h, and atmosphere pressure set to 0.01MPa to 0.04MPa. Since carbon nitride decomposes almost 100% above 750℃, with almost no residual carbon nitride remaining, the difference in the remaining mass percentage of the two can be taken as the mass percentage 'a' of other substances in the silicon-carbon composite after removing carbon nitride in the silicon-carbon anode material. Thus, the mass percentage 'b' of carbon nitride in the silicon-carbon anode material can be determined, i.e., a + b = 1. The silicon-carbon anode material was stirred in concentrated hydrofluoric acid for 2 hours. After washing and drying, the mass percentage (c) of the other substances in the silicon-carbon anode material after removing silicon particles was determined by weighing. The mass percentage (d) of carbon nitride in the other substances after removing silicon was then obtained, i.e., d = a / c. The composite material after silicon removal was then tested using a thermogravimetric analyzer (STA449F3-QMS403C). The test conditions were: air atmosphere, heating rate of 10℃ / min, heating to 500℃ and holding for 2 hours, atmosphere pressure set to 0.01MPa to 0.04MPa, with only amorphous carbon volatilizing under these stable conditions. The mass percentage (e) of amorphous carbon in the composite material after silicon removal could be calculated, and the mass percentage (f) of carbon nanotubes in the composite material after silicon removal was obtained, i.e., f = 1 - de. The mass percentage (g) of carbon nitride in the outer coating layer was then determined, i.e., g = d / (d + f).

[0088] SEM testing: The microstructure of powder was observed using a scanning electron microscope to characterize the surface coating of the material. The selected testing instrument was an OXFORD EDS (X-max-20mm2), with an accelerating voltage of 10KV. The focal length was adjusted, and the observation magnification was from 50K for high magnification and from low magnification (500-2000) to observe the particle agglomeration.

[0089] TEM testing: Characterization was performed using a JEOL JEM-2010 transmission electron microscope at an operating voltage of 200 kV.

[0090] Raman spectroscopy was performed using a Jobin Yvon LabRAM HR spectrometer with a 532 nm light source and a measurement range of 0 cm⁻¹. -1 Up to 4000cm -1 The test range is 100μm × 100μm, and the results are obtained by statistically analyzing 100 I values. D / I G Worthy of the final I D / I G value.

[0091] Porosity test: The porosity of the silicon-carbon material and the negative electrode is tested by gas displacement method. The silicon-carbon material is first cross-sectioned and the porosity is tested using the cross-section. The calculation method is: the percentage of the sample pore volume to the total area, P = (V-V0) / V×100%, where V0 is the true volume and V is the apparent volume.

[0092] CROSS-SECTION Test: The cross-section polishing instrument uses an ion source to ionize inert gas to generate inert ions. After acceleration and focusing, the high-speed inert ions bombard atoms or molecules on the sample surface, achieving ion polishing. After CP cutting, the sample is placed on a dedicated SEM stage for SEM testing. The instrument model is IB-09010CP, with an ion acceleration voltage of 2kV to 6kV, and argon gas is used. This application uses CROSS-SECTION testing to cut the silicon-carbon anode material, allowing for the measurement of the coating thickness at the cross-section and the gaps between silicon particles at the cross-section.

[0093] Negative electrode adhesion test: An MX-0580 electrode adhesion tester with an accuracy class of 0.5 or lower, a rated load range of 1N to 5000N, and a stress control rate range of 0.005%FS / S to 6%FS / S was used. The test method was as follows: For attaching and fixing the electrode, take a flat, thin steel plate approximately 200mm to 300mm in length and 40mm to 60mm in width. First, apply a strip of double-sided tape (longer than the sample test length and the same width as the electrode) to the center of the steel plate, smoothing it firmly to ensure a tight fit. Remove the double-sided tape and attach the electrode to the tape, ensuring a perfect match between the electrode and the tape. The tensile stress-strain curve is then analyzed.

[0094] Negative electrode compaction density test: A UTM7305 powder compaction density meter was used. During the compression process under external force, as the powder moves and deforms, larger voids are filled, increasing the contact area between particles. This generates attractive forces between atoms and strengthens the mechanical wedging effect between particles, thus forming a compact with density and strength. The maximum pressure is 12 tons, the opening height is 0cm to 13.75cm, the slider stroke is 12.5cm to 31.25cm, and the working area of ​​the circular lower pressure plate is 10cm². In this case, the compaction density under 5 tons of pressure was used as the compaction density value of the negative electrode.

[0095] Resistance testing of the negative electrode active material layer: A resistivity meter (Suzhou Jinglü Electronics ST-2255A) was used. The negative electrode was taken and subjected to a constant pressure of 5000 kg ± 2 kg using an electronic press for 15 to 25 seconds. The sample was then placed between the electrodes of the meter. The sample height was h (cm), the voltage across the two ends was U, the current was I, and the resistance was R (kΩ). The area of ​​the powder-pressed sheet was S = 3.14 cm². 2The electronic conductivity of the powder is calculated using the formula δ=h / (S×R) / 1000, with units of S / m.

[0096] The preparation process of the silicon-carbon anode material in the examples and comparative examples is as follows:

[0097] 1. Take a certain amount of hexadecyltrimethylammonium bromide (CTAB) and disperse it in a mixed solution of water and ethanol in a volume ratio of 2:1. Add ammonia water, stir ultrasonically to disperse evenly, add tetraethyl orthosilicate (TEOS) in a certain proportion with ammonia water, stir and react at room temperature for 1 to 6 hours, and centrifuge to obtain a solid complex.

[0098] 2. Disperse the solid substance obtained in step 1 in a mixed solution of ammonium nitrate and alcohol, and stir at 60°C for 2 to 10 hours to remove CTAB.

[0099] 3. Disperse the solid material obtained in step 2 in an alcohol solution, add phenolic resin in a certain proportion to the solid material, stir at room temperature until the alcohol evaporates to dryness, and sinter at 500℃ to 1200℃ for 1 to 5 hours under an inert atmosphere, with a heating rate of 0.2℃ / min to 20℃ / min.

[0100] 4. Grind the solid material obtained in step 3 and magnesium powder in a ceramic boat at a certain mass ratio until homogeneous. Place the mixture in a stainless steel sleeve and sinter at 600℃ to 800℃ for 3 to 10 hours under a hydrogen-argon mixed atmosphere at a sintering rate of 0.2℃ / min to 20℃ / min. Then disperse the mixture in dilute acid and stir for 1 to 5 hours to remove magnesium oxide. After washing with anhydrous alcohol, freeze-dry to obtain the silicon-carbon composite.

[0101] 5. Take carbon nanotubes and ammonia nitrile in a certain mass ratio, mix them at 30℃ to 100℃ for 1h to 5h, add the above silicon-carbon composite after mixing evenly, continue stirring and mixing under the same conditions for 1h to 5h, cool to room temperature after mixing evenly, grind thoroughly, and sinter at 300℃ to 800℃ for 2h to 8h under an inert atmosphere, with a heating rate of 0.2℃ / min to 20℃ / min.

[0102] Full battery evaluation

[0103] (1) Preparation of lithium-ion batteries

[0104] Preparation of the positive electrode: LiCoO2, conductive carbon black, and polyvinylidene fluoride (PVDF) were thoroughly mixed in an N-methylpyrrolidone solvent system at a weight ratio of approximately 95:2.5:2.5 to obtain a positive electrode slurry. The obtained positive electrode slurry was coated onto a positive electrode current collector aluminum foil, dried, and cold-pressed to obtain the positive electrode.

[0105] Preparation of the negative electrode: graphite, silicon-carbon negative electrode material prepared according to the examples and comparative examples, conductive agent (conductive carbon black, Super) The slurry is mixed with binder PAA in a weight ratio of approximately 70:15:5:10, and an appropriate amount of water is added. The mixture is kneaded at a solid content of approximately 55 wt% to 70 wt%. An appropriate amount of water is added to adjust the viscosity of the slurry to approximately 4000 Pa·s to 6000 Pa·s, thus preparing the negative electrode slurry. The prepared negative electrode slurry is coated onto the negative electrode current collector copper foil, dried, and cold-pressed to obtain the negative electrode.

[0106] Preparation of electrolyte: Under a dry argon atmosphere, LiPF6 was added to a solvent composed of propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) in a weight ratio of approximately 1:1:1 and mixed thoroughly. The concentration of LiPF6 was approximately 1.15 mol / L. Then, approximately 12.5 wt% of fluoroethylene carbonate (FEC) was added and mixed thoroughly to obtain the electrolyte.

[0107] Preparation of the separator: PE porous polymer film was used as the separator.

[0108] Lithium-ion battery fabrication: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes for isolation. The cells are then wound to obtain bare cells. These bare cells are placed in outer packaging, injected with electrolyte, and sealed. After formation, degassing, and edge trimming processes, the lithium-ion battery is obtained.

[0109] (2) Cyclic performance test

[0110] The test temperatures were 25℃ / 45℃. The battery was charged at a constant current of 0.7C to 4.4V, then charged at a constant voltage of 0.025C, and after a 5-minute rest period, discharged at 0.5C to 3.0V. The capacity obtained from this process was used as the initial capacity. Cyclic tests were then performed using 0.7C charging / 0.5C discharging. The capacity decay curve was obtained by comparing the capacity at each step with the initial capacity. The number of cycles at 25℃ until 90% capacity retention was recorded as the battery's room temperature cycle performance, and the number of cycles at 45℃ until 80% capacity retention was recorded as the battery's high-temperature cycle performance. The cycle performance of the material was compared by comparing the number of cycles under these two conditions.

[0111] (3) Discharge rate test

[0112] At 25℃, the capacitor was discharged to 3.0V at 0.2C, allowed to stand for 5 minutes, charged to 4.45V at 0.5C, and then charged at a constant voltage to 0.05C, allowed to stand for 5 minutes. The discharge rate was adjusted, and discharge tests were conducted at 0.2C, 0.5C, 1C, 1.5C, and 2.0C respectively. The discharge capacity was obtained at each rate and compared with the capacity obtained at 0.2C. The rate performance was compared by comparing the ratio of 2C to 0.2C.

[0113] (4) Battery full charge expansion rate test

[0114] The thickness of a fresh battery at half charge (50% SOC) is measured using a micrometer. After 400 cycles, the battery is fully charged (100% SOC). The thickness of the battery at this time is measured again using a micrometer. By comparing the thickness of the fresh battery at the initial half charge (50% SOC), the expansion rate of the fully charged (100% SOC) battery can be obtained.

[0115] Table 1 shows the effect of carbon nitride content on performance in the carbon nitride and carbon nanotube composite coating. In Table 1, the same silicon-carbon composite core is used for all examples. Examples 1 to 5 are coated with carbon nitride and carbon nanotubes. The difference is that in Comparative Example 1, only carbon nanotubes are coated on the surface.

[0116] Table 1

[0117]

[0118] The comparison between Examples 1 to 3 and Examples 4 and 5 illustrates the effect of the mass percentage of carbon nitride in the coating layer of the silicon-carbon anode material on the material performance. Under otherwise identical conditions, increasing the carbon nitride content can improve the mechanical strength of the carbon nitride and carbon nanotube composite shell, which helps alleviate the stress generated by the silicon-carbon core and maintain the stability of the material structure during cycling. However, when the carbon nitride content increases to a certain level, the carbon nanotube content in the composite shell decreases, and the overall conductivity of the composite shell decreases, which is detrimental to electron and ion transport and affects rate performance. Conversely, when the carbon nitride content in the composite shell decreases, the mechanical strength of the composite shell decreases, and the added carbon nanotubes are prone to side reactions with lithium ions and electrolytes during lithium insertion / extraction, causing cycling performance issues. Example 4 directly demonstrates that when the carbon nitride content in the outer layer decreases to 40%, the expansion constraint force of the composite shell on the silicon-carbon core is greatly reduced, exhibiting poor cycling performance and high expansion. Example 5 directly illustrates that when the mass percentage of carbon nitride in the outer layer increases to 90%, the conductivity of the composite shell decreases significantly, resulting in poor rate performance. Comparative Example 1 directly illustrates the comparison of strength between the carbon shell and the composite shell when carbon material is used as the composite shell, showing that the carbon shell has a weaker binding force on the expansion of the internal silicon-carbon material, leading to greater material expansion. Therefore, when the mass content of carbon nitride is between 40% and 90%, the cycle performance and discharge rate performance are better than those of Comparative Example 1. Preferably, the mass content of carbon nitride is between 50% and 80%, and more preferably, it is between 55% and 75%.

[0119] Table 2 shows the influence of the Dv50 of the silicon-carbon core composite and the Dv50 of the silicon-carbon anode active material on performance. The Dv50 of the silicon-carbon core composite is a nm, and the Dv50 of the silicon-carbon anode active material is b nm.

[0120] Table 2

[0121]

[0122] The comparisons between Examples 2, 6, 7, 8, and 9 illustrate the influence of the ratio a / b (dv50 of the silicon-carbon core particles and the Dv50 of the silicon-carbon anode material) on material performance. Under otherwise identical conditions, when a / b is increased, as in Example 6 when it is increased to approximately 0.9, the outer composite shell becomes too thin. This thinner carbon nitride and carbon nanotube composite shell cannot effectively restrain the expansion of the silicon-carbon core, causing structural damage and exposing the nano-silicon to the electrolyte, preventing the formation of a stable SEI film and affecting cycle performance. Conversely, as shown in Example 7, decreasing the a / b value to approximately 0.7, i.e., increasing the thickness of the carbon nitride and carbon nanotube composite shell, while improving restraint on the silicon-carbon core expansion, results in an excessively thick carbon nitride and carbon nanotube composite layer, which hinders ion and electron transport. Furthermore, increasing the carbon nitride and carbon nanotube content reduces the silicon content, which is detrimental to fully utilizing the high energy density of silicon. In Examples 8 and 9, when a / b was reduced to 0.57 and increased to 0.95 respectively, the materials exhibited poor electrochemical performance.

[0123] Table 3 shows the effect of the mass percentage of silicon in the silicon-carbon anode active material on its performance. Based on Example 2, we further investigated the effect of the mass percentage of silicon in the silicon-carbon anode material on the anode active material.

[0124] Table 3

[0125]

[0126] The comparison between Examples 2, 10, and 11 and Comparative Example 2 illustrates the effect of silicon content in silicon-carbon materials on material performance. Higher silicon content results in higher specific capacity. However, with increased silicon content, under otherwise identical conditions, lower carbon content leads to a thinner carbon layer, meaning less space between silicon particles and less buffer space between nano-silicon particles. This increases the expansion of the silicon-carbon core, potentially damaging the outer carbon nitride and carbon nanotube composite shell, causing structural damage, and allowing electrolyte penetration into the silicon-carbon core, leading to side reactions and affecting cycle performance. Conversely, lowering the silicon content (i.e., increasing the carbon content) allows carbon material to fill the porous structure during composite preparation, acting as gaps between adjacent silicon particles. Increased carbon layer thickness helps absorb the stress generated by the expansion of lithiation, maintaining electrical contact during charge and discharge. Comparative Example 2 directly demonstrates that at a high silicon load of 85%, there are no gaps between silicon particles, resulting in significant expansion of the pure silicon core, which breaks through the outer composite shell, causing severe degradation in material cycle performance.

[0127] Table 4 shows the effect of porosity on the performance of silicon-carbon anode active materials. Examples 12 to 15 differ from Example 2 only in porosity.

[0128] Table 4

[0129]

[0130]

[0131] The comparison between Examples 2, 12, and 13 and Examples 14 and 15 illustrates the effect of porosity at the cross-section of the silicon-carbon anode material on the material properties: Under otherwise identical conditions, when the porosity of the silicon-carbon anode material is increased, as shown in Example 13, when the porosity is increased to 9%, the composite has better expansion space, which is beneficial to improving expansion performance. However, the larger voids affect electrical contact and are not conducive to rate performance. When the porosity is reduced to 2%, as shown in Example 12, the lower porosity is not conducive to buffering the expansion stress generated during lithiation, resulting in poor expansion performance during cycling. In Examples 14 and 15, when the porosity is reduced to 0.1% and increased to 15%, respectively, the expansion performance and rate performance of the material are greatly affected.

[0132] Table 5 shows the effect of carbon nanotube diameter on performance. Except for the diameter of the carbon nanotubes in Examples 16 to 19, which differs from that in Example 2 (see Table 5 for details), the other parameters are the same as in Example 2.

[0133] Table 5

[0134]

[0135] The comparison between Examples 2, 16, and 17 and Examples 18 and 19 illustrates the influence of the diameter of carbon nanotubes in the outer coating layer on material properties. Under otherwise identical conditions, when the carbon nanotube diameter increases to 8 nm, as shown in Example 17, although there is an interaction between the carbon nanotubes and carbon nitride, the larger carbon nanotubes composited in carbon nitride result in lower mechanical strength compared to pure carbon nitride, which is detrimental to the binding of the expanding silicon-carbon core and affects expansion performance. Conversely, when the carbon nanotube diameter decreases to 2 nm, as shown in Example 16, the smaller diameter carbon nanotubes are not easily dispersed in the composite and cannot form a uniform distribution with the carbon nitride, which is detrimental to improving the overall conductivity of the composite. Examples 18 and 19, which reduce the carbon nanotube diameter to 1 nm and increase it to 15 nm respectively, exhibit poor rate performance and expansion performance.

[0136] Table 6 shows the effect of the size of the gap between silicon particles on performance. In Examples 20 to 22, the gap between silicon particles and the silicon content are different from those in Example 2. In Comparative Example 3, a pure silicon core is used instead of a silicon-carbon composite.

[0137] Table 6

[0138]

[0139] The test results from Examples 2, 20 to 22, and Comparative Example 3 show that, under the premise of keeping other conditions unchanged, increasing the gap between adjacent silicon particles reduces the silicon content and the specific capacity of the anode material. The increased carbon layer thickness, acting as a buffer against silicon particle expansion stress, effectively improves cycle, expansion, and rate performance. Comparative Example 3 directly demonstrates that when there is no carbon buffer layer between silicon particles, the battery exhibits poor performance due to the influence of silicon expansion stress. Example 24 shows that when the carbon layer thickness is increased to 50 nm, the battery exhibits better electrochemical performance, but the silicon content is low, and the specific capacity is only 950 mAh / g, which cannot fully utilize the high energy density advantage of the silicon anode.

[0140] Table 7 shows I D / I G The effect of the resistance of the negative electrode active material layer on performance. In Examples 23 to 26, I... D / I G The resistance of the negative electrode material is different from that in Example 2 (see Table 7 for details).

[0141] Table 7

[0142]

[0143] The test results from Examples 2 and 23 to 26 show that, while keeping other conditions unchanged, the improvement in I... D / I G This indicates that the interaction between carbon nitride and carbon nanotubes induces the formation of more electron-hole pairs, leading to an increase in defects and disorder in the carbon nanotubes. This facilitates the rapid transport of electrons from the carbon nanotubes to carbon nitride. However, if I... D / I G As the ratio increases further, the increased disorder of carbon nanotubes affects conductivity. Therefore, controlling it within this range can provide good conductivity for the carbon nitride and carbon nanotube composite layer. For example, in Example 26, I... D / I G When the resistance is increased to 2.1, the resistance of the negative electrode active material layer is at its maximum, and the battery also exhibits poor rate performance.

[0144] Table 8 shows the effect of negative electrode adhesion and the resistance of the negative electrode active material layer on performance. The adhesion and resistance of the negative electrode active material layer in Examples 27 to 30 differ from those in Example 2 (see Table 8 for details).

[0145] Table 8

[0146]

[0147] The test results from Examples 2 and 27 to 30 show that, under the premise of keeping other conditions unchanged, improving the adhesion of the electrode sheet results in better contact between particles and the substrate, and between particles themselves, and a better buffering effect on expansion stress. At the same time, it can reduce the internal resistance of the battery and improve the rate, cycle and expansion performance of the battery. For example, in Example 30, when the adhesion of the electrode sheet is increased by 50 N / m, the battery can achieve a rate performance of 89.7%.

[0148] Table 9 shows the effect of negative electrode porosity on performance. The porosity and compaction density of the negative electrodes in Examples 31 to 34 differ from those in Example 2 (see Table 9 for details).

[0149] Table 9

[0150]

[0151] The test results from Examples 2 and 31 to 34 show that, under the premise of keeping other conditions unchanged, increasing the porosity of the electrode will reduce the compaction density of the electrode. For example, in Example 32, when the porosity is increased to 18%, the composite has a better expansion space, which is beneficial to improving the expansion performance. However, the large number of pores affects the electrical contact and is not conducive to the rate performance. When the porosity is reduced to 10%, as shown in Example 31, the low porosity is not conducive to buffering the expansion stress generated during lithiation, resulting in poor expansion performance during cycling. In Examples 33 and 34, when the porosity is reduced to 5% and increased to 25%, respectively, the expansion performance and rate performance of the material are greatly affected.

[0152] While some exemplary embodiments of this application have been described and illustrated, this application is not limited to the disclosed embodiments. Rather, those skilled in the art will recognize that modifications and changes may be made to the described embodiments without departing from the spirit and scope of this application as described in the appended claims.

Claims

1. A silicon-carbon negative electrode material, comprising a silicon-carbon composite and a coating layer on at least a part of a surface of the silicon-carbon composite, the coating layer comprising carbon nitride and carbon nanotubes, the silicon-carbon composite comprising silicon particles, gaps between the silicon particles being 10 to 21 nm;the carbon nanotubes having a diameter of 3 nm to 7 nm, the mass content of the carbon nitride being 50% to 80% based on the mass of the coating layer;the silicon-carbon negative electrode material having I D / I G of 1.5 to 2, wherein I D is the peak intensity in the range of 1300 cm -1 to 1400 cm -1 in a Raman spectrum, and I G is the peak intensity in the range of 1580 cm -1 to 1620 cm -1 in a Raman spectrum.

2. The silicon-carbon negative electrode material of claim 1, wherein, The mass content of the carbon nitride is 50% to 80% based on the mass of the coating layer.

3. The silicon-carbon negative electrode material of claim 1, wherein, The particle size Dv50 of the silicon-carbon composite is a nm, the particle size Dv50 of the silicon-carbon negative electrode material is b nm, 0.7≤a / b≤0.9; and 50≤a≤500, 60≤b≤700.

4. The silicon-carbon negative electrode material of claim 1, wherein, The mass content of the silicon element is 30% to 80% based on the silicon-carbon negative electrode material.

5. The silicon-carbon negative electrode material of claim 1, wherein, The silicon-carbon negative electrode material satisfies the following condition (a): (a) The porosity of the silicon-carbon negative electrode material is 1% to 10%.

6. An electrochemical device comprising a negative electrode, the negative electrode comprising a current collector and a negative electrode active material layer, the negative electrode active material layer comprising the silicon-carbon negative electrode material according to any one of claims 1 to 5.

7. The electrochemical device of claim 6, wherein, The electrochemical device satisfies at least one of the following conditions (c) to (f): (c) The electrical resistance of the negative electrode active material layer is 0.2 Ω to 2 Ω; (d) The adhesion of the negative electrode is 30 N / m to 50 N / m; (e) the compacted density of the negative electrode is 1.5 g / cm 3 to 2 g / cm 3 ; (f) The porosity of the negative electrode is 10% to 20%.

8. An electronic device comprising the electrochemical device according to claim 6 or 7.

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