Silicon-carbon negative electrode material, preparation method and application thereof

Abstract

The application discloses a silicon-carbon negative electrode material, which comprises a core and a shell, wherein the core comprises a porous carbon framework and silicon dispersed in the pores of the porous carbon framework; the porous carbon framework is wrapped and dispersed with carbon nanotubes; the content of silicon in the silicon-carbon negative electrode material is 25 wt% to 55 wt%; and the shell comprises a carbon material. In the linear scanning electron microscope energy spectrum of the cross section of the silicon-carbon negative electrode material, the standard deviation of the content of the silicon along the direction from the center to the edge of the cross section is not more than 200. The application also provides a negative electrode sheet, an electrochemical device and a preparation method of the silicon-carbon negative electrode material. The application can improve the conductivity and reduce the expansion, thereby prolonging the service life.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, and in particular to a silicon-carbon anode material and its preparation method, an anode sheet using the silicon-carbon anode material, and an electrochemical device using the anode sheet. Background Technology

[0002] Lithium-ion batteries possess advantages such as high volumetric and gravimetric energy density, environmental friendliness, high operating voltage, small size, light weight, and long cycle life, making them widely used in portable consumer electronics. With the rapid development of electric vehicles and mobile electronic devices in recent years, the demands for battery energy density, safety, and cycle performance have increased significantly, leading to expectations for new lithium-ion batteries with comprehensively improved performance. Among these, energy density and cycle performance have become critical technical issues that urgently need to be addressed, and improving the active materials in the electrodes is one research direction for solving these problems.

[0003] Currently, graphite is the most widely used anode material, boasting advantages such as high efficiency and a stable charge-discharge platform. However, the performance of commercial graphite has been almost fully developed, with its relatively low capacity and the safety hazards posed by lithium dendrites hindering its further application. Compared to graphite as an anode material, elemental silicon, due to its ultra-high theoretical specific capacity and suitable operating voltage, is considered the most promising lithium-ion battery anode material to replace graphite. However, its low conductivity and the significant volume expansion during alloying / dealloying severely limit the large-scale application of elemental silicon in lithium-ion batteries. Summary of the Invention

[0004] This application provides a silicon-carbon anode material that can improve conductivity and reduce expansion.

[0005] In addition, this application also provides a negative electrode sheet using the aforementioned silicon-carbon negative electrode material and an electrochemical device using the aforementioned negative electrode sheet. This application also provides a method for preparing the aforementioned silicon-carbon negative electrode material.

[0006] This application provides a silicon-carbon anode material, comprising a core and a shell. The core includes a porous carbon framework and silicon dispersed within the pores of the porous carbon framework. Carbon nanotubes are encapsulated and dispersed within the porous carbon framework. The silicon content in the silicon-carbon anode material is 25 wt% to 55 wt%. The shell comprises carbon material. In the linear scanning electron microscopy energy dispersive spectroscopy (ESI) of a cross-section of the silicon-carbon anode material, the standard deviation of the silicon content variation along the direction from the center to the edge of the cross-section does not exceed 200.

[0007] The silicon-carbon anode material of this application, due to the excellent conductivity and mechanical properties of its carbon nanotubes, exhibits that a specific amount of carbon nanotubes dispersed within the porous carbon framework can enhance both the conductivity and mechanical properties of the silicon-carbon anode material, while also restraining its expansion, thus improving structural stability. Furthermore, linear scanning electron microscopy (SEM) energy dispersive spectroscopy of the cross-section of the silicon-carbon anode material reveals a uniform distribution of silicon, further enhancing its conductivity and mechanical properties. When this silicon-carbon anode material is applied to the anode sheet in an electrochemical device, the improved conductivity increases the probability of lithium capture within the material, thereby improving the delithiation capacity and initial charge-discharge performance of the electrochemical device. Simultaneously, the carbon nanotubes can alleviate the volume expansion of silicon-carbon during lithium intercalation, improving its expansion, pulverization, and structural stability during charge-discharge cycles, thus enhancing its cycle performance.

[0008] Based on the first aspect, in some possible embodiments, the conductivity of the silicon-carbon anode material is between 9 S / cm and 30 S / cm. In the above possible embodiments, a conductivity within a specific range is beneficial for ensuring the conductivity of the silicon-carbon anode material, further improving the cycle performance of the electrochemical device using the silicon-carbon anode material.

[0009] Based on the first aspect, in some possible implementations, the particle elastic modulus of the silicon-carbon anode material is 4 GPa to 10 GPa. A particle elastic modulus within a specific range is beneficial to improving the stability of the silicon-carbon anode material structure and to further improving the expansion and cycle performance of electrochemical devices using the silicon-carbon anode material.

[0010] Based on the first aspect, in the silicon-carbon anode material, the content of carbon nanotubes is from 0.2wt% to 7.0wt%. The carbon nanotube content within this range can further alleviate the volume expansion of silicon-carbon during lithium intercalation, and can control the formation of pores during the preparation of porous carbon framework, reduce the proportion of macropores, improve the uniformity of silicon deposition, further improve the cycle performance of electrochemical device and reduce the cycle expansion rate of electrochemical device.

[0011] Based on the first aspect, in some possible embodiments, the silicon-carbon anode material satisfies at least one of the following conditions: (1) the X-ray diffraction pattern of the silicon-carbon anode material has a characteristic peak in the range of 20° to 30°, and the full width at half maximum (FWHM) of the characteristic peak is greater than 2°; (2) the Raman spectrum of the silicon-carbon anode material has a peak at 450 cm⁻¹. -1 Up to 500cm -1(3) The particle size Dv50 of the silicon-carbon anode material is 3μm to 20μm, and the particle size Dv99 of the silicon-carbon anode material is 3μm to 20μm.

[0012] In the above possible embodiments, the X-ray diffraction pattern of the silicon-carbon anode material reveals that the pores in the porous carbon framework are micropores, meaning that 90% of the pores in the porous carbon framework have a diameter of less than 2 nanometers. This facilitates the embedding of silicon within the porous carbon framework into the silicon-carbon anode material, resulting in a majority of silicon particles smaller than 2 nanometers, thereby further reducing the expansion of the silicon-carbon anode material. The Raman spectroscopy of the silicon-carbon anode material also reveals that the silicon particles are small and amorphous, further reducing the expansion of the silicon-carbon anode material. Furthermore, silicon-carbon anode materials with a specific particle size range experience smoother processing in subsequent stirring, coating, and other anode sheet fabrication processes, and are easier to combine with graphite.

[0013] Based on the first aspect, in some possible embodiments, the pore volume of pores with a diameter greater than 2 nm in the silicon-carbon anode material is greater than the pore volume of pores with a diameter not exceeding 2 nm. In some possible embodiments, the pore volume of pores with a diameter greater than 2 nm ranges from 0.04 to 0.20. In the above possible embodiments, since micropores have better adsorption effects, silicon preferentially adsorbs and deposits in micropores. Considering that 90% of the pores in the porous carbon framework have a diameter less than 2 nm, it can be understood that the size of silicon in the silicon-carbon anode material is mostly less than 2 nm, which is beneficial for further reducing the expansion of the silicon-carbon anode material.

[0014] A second aspect of this application provides a negative electrode sheet, including a current collector and a negative electrode active layer. The negative electrode active layer includes a negative electrode active material, which includes the silicon-carbon negative electrode material described above.

[0015] The negative electrode of this application improves the conductivity of the silicon-carbon negative electrode material through the uniform distribution of carbon nanotubes and silicon elements, which can improve the probability of lithium being captured inside the silicon-carbon negative electrode material, thereby improving the delithiation capacity of the electrochemical device and enhancing the first charge-discharge performance. At the same time, carbon nanotubes can also alleviate the volume expansion of silicon-carbon during lithium intercalation. Combined with the uniform distribution of silicon elements, it can improve the expansion and pulverization of silicon-carbon and structural stability during charge-discharge, thereby improving its cycle performance.

[0016] Based on the second aspect, in some possible embodiments, the negative electrode active material further includes graphite, wherein the content of the silicon-carbon negative electrode material is 5 wt% to 40 wt%, and the content of the graphite is 95 wt% to 60 wt%.

[0017] In the above possible implementations, the silicon-carbon anode material with the specific content mentioned above can effectively reduce the impact of silicon expansion on the anode sheet while fully utilizing the ultra-high theoretical specific capacity and suitable operating voltage of silicon, which is beneficial to improving the initial coulombic efficiency, energy density, and cycle performance of the anode sheet. Specifically, if the silicon-carbon anode material content is too high, the volume expansion of the anode active layer is significant, resulting in poor cycle performance; if the silicon-carbon anode material content is too low, it is not conducive to improving the initial coulombic efficiency. Since graphite has a certain degree of flexibility, its combination with silicon-carbon anode material can alleviate the volume expansion of the anode active layer. In addition, the anode sheet can also fully utilize the advantages of both silicon-carbon anode material and graphite to achieve better electrochemical performance.

[0018] A third aspect of this application provides an electrochemical device including a negative electrode as described above.

[0019] The electrochemical device of this application improves the conductivity of the silicon-carbon anode material by uniformly distributing carbon nanotubes and silicon elements in the anode sheet, thereby increasing the probability of lithium being captured inside the silicon-carbon anode material, thus improving the delithiation capacity of the electrochemical device and enhancing the first charge-discharge performance. At the same time, carbon nanotubes can also alleviate the volume expansion of silicon-carbon during lithium intercalation, and the uniform distribution of silicon elements can improve the expansion and pulverization and structural stability of silicon-carbon during charge-discharge, thereby improving its cycle performance.

[0020] The fifth aspect of this application provides a method for preparing the silicon-carbon anode material as described above, comprising: mixing resin and carbon nanotubes to form a mixture and curing it; carbonizing and activating the cured mixture to obtain a porous carbon framework; and depositing silanes into the porous carbon framework to form a core and then forming a shell with alkanes.

[0021] The method for preparing the silicon-carbon anode material described in this application involves first mixing resin and carbon nanotubes to form a mixture, then solidifying it and carbonizing it to form a porous framework. This facilitates the dispersion of the carbon nanotubes within the porous framework, thereby enhancing the binding effect of the carbon nanotubes on the prepared silicon-carbon anode material. This alleviates the volume expansion of the silicon-carbon anode material, improving the expansion and pulverization of silicon-carbon during charge and discharge, as well as its structural stability and cycle performance. Simultaneously, it also improves the overall conductivity of the silicon-carbon anode material, thereby increasing the probability of lithium being captured within the silicon-carbon anode material, improving the delithiation capacity of the electrochemical device, and enhancing the first charge-discharge performance.

[0022] Based on the second aspect, in some possible implementations, the carbonization conditions are heating to 700°C to 1100°C and holding at that temperature for 1 to 5 hours, and the activation specifically involves activating the carbonized material by carbon dioxide, water vapor, sodium hydroxide, potassium hydroxide, or phosphoric acid after cooling down.

[0023] Based on the second aspect, in some possible implementations, the step "depositing silane to form a core on the porous carbon framework and then forming a shell with alkane" specifically involves: slowly heating the porous carbon framework to 400°C to 600°C in an inert atmosphere and holding it at that temperature; then switching the atmosphere to a silane mixture for deposition for 1 to 20 hours, wherein the silane mixture contains 2% to 20% silane and 80% to 98% inert gas by mass; then switching the atmosphere to an alkane mixture at 500°C to 1000°C and holding it for 10 hours to form a shell; finally switching the atmosphere back to an inert atmosphere and cooling it to room temperature, wherein the alkane mixture contains 5% to 100% alkane and 95% to 0% inert gas by mass. Attached Figure Description

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

[0025] Figure 1 The figures shown are tables and diagrams illustrating various data from the embodiments and comparative examples of this application. Detailed Implementation

[0026] The technical solutions in the embodiments of this application are described clearly and in detail below. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0027] The embodiments of this application will be described in detail below. However, this application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided to provide a thorough and detailed understanding of this application to those skilled in the art.

[0028] Additionally, for simplicity and clarity, the dimensions or thicknesses of various components and layers may be enlarged in the accompanying drawings. Throughout the text, the same values ​​refer to the same elements.

[0029] Furthermore, when describing the implementation of this application, the word "may" refers to "one or more implementations of this application".

[0030] The technical terms used herein are for the purpose of describing particular embodiments and are not intended to limit this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. It should be further understood that the term "comprising," when used in this specification, means the presence of the described features, values, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, values, steps, operations, elements, components, and / or combinations thereof. A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may 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.

[0031] Spatial terms, such as "above," may be used herein for convenience in describing the relationship between one element or feature and another element (or feature) or feature (or feature) illustrated in the figure. It should be understood that, in addition to the orientations depicted in the figure, spatial terms are intended to include different orientations of the device or apparatus in use or operation. For example, if the device in the figure is flipped, an element described as "above" or "on" other elements or features would be oriented "below" or "under" other elements or features. Therefore, the exemplary term "above" can include both above and below orientations.

[0032] In this application, the design relationships of greater than, less than, or not equal to parameter values ​​need to exclude reasonable errors of the measuring equipment.

[0033] One embodiment of this application provides an electrochemical device, which includes a positive electrode, a negative electrode, and a separator. The separator is disposed between the positive electrode and the negative electrode. The positive electrode, the separator, and the negative electrode can be sequentially and alternately stacked to form a stacked electrode assembly, or the positive electrode, the separator, and the negative electrode can be sequentially stacked and then wound to form a wound electrode assembly.

[0034] The electrochemical device further includes a housing and an electrolyte (not shown), wherein the positive electrode, the negative electrode, the separator, and the electrolyte are housed within the housing. The housing may be a packaging bag encapsulated with an encapsulation film, such as, but not limited to, aluminum-plastic film; that is, the electrochemical device may be a pouch battery. The housing may also be, but is not limited to, housings disclosed in the prior art such as steel-cased batteries and aluminum-cased batteries.

[0035] The positive electrode includes a positive current collector and a positive active layer disposed on the positive current collector. The positive current collector can be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive active layer contains a positive active material, which includes a compound that reversibly inserts and extracts lithium ions (i.e., a lithiation intercalation compound). In some embodiments, the positive active material may include a lithium transition metal composite oxide. This lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive active material may include, but is not limited to, lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary materials (NCM), lithium manganese oxide (LiMn2O4), and lithium nickel manganese oxide (LiNi). 0.5 Mn 1.5 At least one of lithium iron phosphate (LiFePO4) or lithium iron phosphate (LiFePO4).

[0036] The positive electrode active layer also includes an adhesive to bond the positive electrode active material particles, thereby facilitating the formation of a film layer, and also improving the bonding force between the positive electrode active layer and the positive electrode current collector.

[0037] In some embodiments, the adhesive may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0038] The positive electrode active layer may further comprise a conductive material, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based material may include, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material may include, but is not limited to, metal powder or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0039] The negative electrode sheet includes a negative current collector and a negative active layer disposed on the negative current collector. The negative current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or carbon-based current collector, or any composite current collector disclosed in the prior art, such as, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The negative active layer contains a negative active material, including silicon-carbon negative electrode material.

[0040] The silicon-carbon anode material comprises a core and a shell. The core includes a porous carbon framework and silicon dispersed within the pores of the porous carbon framework. Carbon nanotubes are encapsulated and dispersed within the porous carbon framework. The silicon content in the silicon-carbon anode material is 25 wt% to 55 wt%. The shell comprises carbon material. In the linear scanning electron microscopy (SEM) energy dispersive spectroscopy (EDS) of the cross-section of the silicon-carbon anode material, the standard deviation of the silicon content variation along the direction from the center to the edge of the cross-section does not exceed 200. Specifically, the silicon-carbon anode material particles are cut into a cross-section using ion polishing as the test surface. The cut silicon-carbon anode material is transferred into a field emission scanning electron microscope (FET), and the test surface is linearly scanned from the center outwards to record the content distribution of each element. For example, 100 data points of silicon content obtained from the linear scan are taken at equal intervals, and statistical standard deviation analysis is performed on the 100 data points to obtain the standard deviation of silicon content variation.

[0041] The aforementioned silicon-carbon anode material, due to the excellent conductivity and mechanical properties of its carbon nanotubes, exhibits that a specific amount of carbon nanotubes dispersed within the porous carbon framework can enhance both the conductivity and mechanical properties of the silicon-carbon anode material, while also restraining its expansion and thus improving structural stability. Furthermore, linear scanning electron microscopy (SEM) energy dispersive spectroscopy of the cross-section of the silicon-carbon anode material reveals a uniform distribution of silicon. The dispersion of carbon nanotubes and the uniform distribution of silicon enhance the conductivity and strength of the silicon-carbon anode material, reduce lithium capture within it, alleviate its expansion, and improve the initial coulombic efficiency of the electrochemical device using this anode, thereby increasing the energy density and cycle performance of the electrochemical device.

[0042] In some embodiments, the silicon content is 20% to 45%, which can further reduce the expansion of silicon-carbon anode materials and improve the cycle performance of electrochemical devices.

[0043] In some embodiments, the conductivity of the silicon-carbon anode material is from 9 S / cm to 30 S / cm. In some embodiments, the conductivity of the silicon-carbon anode material is from 11 S / cm to 25 S / cm. In some embodiments, the conductivity of the silicon-carbon anode material is 9 S / cm, 11 S / cm, 14 S / cm, 20 S / cm, 25 S / cm, 30 S / cm, or a value within any two of the above ranges. When the conductivity of the silicon-carbon anode material is within the above ranges, the conductivity of the silicon-carbon anode material can be guaranteed, further improving the cycle performance of the electrochemical device.

[0044] In some embodiments, the particle elastic modulus of the silicon-carbon anode material is 4 GPa to 10 GPa, which helps to ensure the mechanical strength of the silicon-carbon anode material to alleviate internal stress, further reduce the expansion of the silicon-carbon anode material and improve the cycle performance of the electrochemical device.

[0045] In some embodiments, the carbon nanotube content in the silicon-carbon anode material is from 0.2 wt% to 7.0 wt%. In some embodiments, the carbon nanotube content is from 1.0 wt% to 6.0 wt%. In some embodiments, the carbon nanotube content is 0.2 wt%, 1.0 wt%, 1.5 wt%, 2.5 wt%, 4.5 wt%, 6.0 wt%, 6.5 wt%, 7.0 wt%, or a value within any two of the above values. Carbon nanotube content within the above range helps to further mitigate the volume expansion of silicon-carbon during lithium intercalation, and can control the formation of pores during the preparation of the porous carbon framework, reduce the proportion of macropores, improve the uniformity of silicon deposition, ensure the initial efficiency (i.e., initial coulombic efficiency) of the electrochemical device, further improve the cycle performance of the electrochemical device, and reduce the cycle expansion rate of the electrochemical device.

[0046] In some embodiments, the X-ray diffraction pattern of the silicon-carbon anode material has a characteristic peak in the range of 20° to 30°, and the full width at half maximum (FWHM) of the characteristic peak is greater than 2°, meaning that the pores in the porous carbon framework are micropores, and 90% of the pores in the porous carbon framework have a pore size of less than 2 nanometers. This means that the silicon embedded in the porous carbon framework in the silicon-carbon anode material is mostly smaller than 2 nanometers, thereby further reducing the expansion of the silicon-carbon anode material.

[0047] Furthermore, in the silicon-carbon anode material, the pore volume of pores with a diameter greater than 2 nm is greater than that of pores with a diameter not exceeding 2 nm. Because micropores have better adsorption effects, silicon preferentially adsorbs and deposits in the micropores. Therefore, the silicon size in the silicon-carbon anode material is mostly less than 2 nm, which helps to further reduce the expansion of the silicon-carbon anode material. In some embodiments, the pore volume of pores with a diameter greater than 2 nm ranges from 0.04 to 0.20.

[0048] In some embodiments, the Raman spectrum of the silicon-carbon anode material is at 450 cm⁻¹. -1 Up to 500cm -1 The silicon in the silicon-carbon anode material has a characteristic peak within a certain range, indicating that the silicon is small in size and amorphous, which helps to further reduce the expansion of the silicon-carbon anode material.

[0049] The particle size Dv50 of the silicon-carbon anode material can be from 3 μm to 20 μm, and the particle size Dv99 of the silicon-carbon anode material can be from 3 μm to 20 μm, so as to facilitate smoother mixing and coating with other materials (such as binders) to form the anode active layer, and to facilitate mixing and matching with other materials.

[0050] The negative electrode active material may further include graphite. Due to the flexibility of graphite, its combination with the silicon-carbon negative electrode material can alleviate the volume expansion of the negative electrode active layer. Simultaneously, the simultaneous use of graphite and the silicon-carbon negative electrode material as active materials helps reduce the overall expansion of the negative electrode active layer and fully utilizes the advantages of both materials to achieve better electrochemical performance.

[0051] In the negative electrode active material, the content of the silicon-carbon negative electrode material can be from 5 wt% to 40 wt%. Further, the content of graphite can be from 95 wt% to 60 wt%.

[0052] The negative electrode active layer also includes a binder to bond the positive electrode active material particles, thereby facilitating the formation of a film layer, and also improving the bonding force between the negative electrode active layer and the negative electrode current collector.

[0053] In some embodiments, the adhesive may include, 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.

[0054] The negative electrode active layer may further include a conductive material, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based material may include, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material may include, but is not limited to, metal powder or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0055] The separator membrane includes a membrane layer with a porous structure, and its material includes, but is not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the separator membrane may be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane, etc.

[0056] The electrolyte can be in one or more of the following states: gel, solid, and liquid. In some embodiments, the liquid electrolyte includes a lithium salt and an organic solvent. The lithium salt may be selected from, but is not limited to, one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium tris(trifluoromethanesulfonyl)methyllithium (LiC(SO2CF3)3), lithium dioxolaneborate (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is selected as the lithium salt because it provides high ionic conductivity and improves cycling characteristics. The organic solvent may be a carbonate compound, a carboxylic acid ester compound, or an ether. Compounds, nitrile compounds, other organic solvents, or combinations thereof. Examples of carbonate compounds include, but are not limited to, diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0057] The aforementioned electrochemical device is applied to electronic devices to power other electronic components within those devices. Because the silicon-carbon anode material in the electrochemical device improves cycle performance and energy density, it contributes to extending the lifespan of the electronic device. These electronic devices may include, but are not limited to, laptops, pen-based 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, and lithium-ion capacitors, etc.

[0058] This application also provides a method for preparing a silicon-carbon anode material, which includes the following steps:

[0059] Step S1: Mix the resin and carbon nanotubes to form a mixture and then cure it.

[0060] Methods for forming the mixture include, but are not limited to, ball milling. Organic solvents may also be added during mixing to facilitate the dispersion of carbon nanotubes. The curing process involves drying and shaping the mixture, using curing conditions commonly found in existing technologies, which will not be elaborated upon here.

[0061] Step S2: The solidified mixture is carbonized and activated to obtain a porous carbon skeleton.

[0062] Specifically, the cured mixture is heated to 700°C to 1100°C and carbonized for 1 to 5 hours in an inert atmosphere (e.g., but not limited to nitrogen). After cooling to room temperature, it is activated to form a porous carbon framework. Activation can be performed using carbon dioxide or water vapor, or by alkaline etching or acid etching. Alkaline etching typically uses sodium hydroxide or potassium hydroxide, while acid etching typically uses phosphoric acid.

[0063] Step S3: After depositing silane to form a core in the porous carbon framework, the core is then covered with a shell formed by alkane to form a carbon coating layer.

[0064] Specifically, the porous carbon framework is slowly heated to 400°C to 600°C in an inert atmosphere (e.g., but not limited to argon), and then the atmosphere is switched to a silane mixture for deposition over 1 hour to 20 hours to form a nucleus. The slow heating rate can be, but is not limited to, 0.5°C / min to 5°C / min, and the silane mixture comprises 2% to 20% silane and 80% to 98% inert gas (e.g., but not limited to argon) by mass.

[0065] Then, at a temperature of 500°C to 1000°C, the atmosphere is switched to an alkane mixture and maintained for 2 to 20 hours to form a shell covering the core with a carbon coating layer. After that, the atmosphere is switched to an inert atmosphere (e.g., but not limited to nitrogen atmosphere) and cooled to room temperature to finally obtain the silicon-carbon anode material. The alkane mixture contains 5% to 100% by mass of alkanes (e.g., but not limited to acetylene) and 0% to 95% by mass of an inert gas (e.g., but not limited to argon).

[0066] Example 1

[0067] Preparation of negative electrode sheet:

[0068] 1) Preparation of silicon-carbon anode material: Linear phenolic resin (RF), hexamethylenetetramine (HMT), and carbon nanotubes (CNTs) were mixed in the weight ratio (described in Table 1) to form a mixture. The mixing method was ball milling at a speed of 500 r / min for 6 h. The ball-milled mixture, which was dark in color, was heated to 130℃ and held at that temperature for 10 h for curing. The cured mixture was transferred to a box furnace and heated to 900℃ for carbonization under a nitrogen atmosphere for 2 h. After cooling, it was transferred to a rotary kiln for activation under a carbon dioxide atmosphere for 9 h to obtain a porous carbon framework. The porous carbon framework was heated to 500°C at a rate of 2°C / min under an argon atmosphere. The atmosphere was then switched to a silane mixture (20% silane and 80% argon by mass percentage) and deposited at 500°C for 10 hours to form a nucleus. The atmosphere was then switched to an acetylene mixture (20% acetylene and 80% argon by mass percentage) and deposited at 500°C for another 10 hours. The atmosphere was then switched to nitrogen and cooled to room temperature (25°C) to obtain the silicon-carbon anode material.

[0069] 2) Using a mixture of graphite and the aforementioned silicon-carbon anode material at a weight ratio of 80:20 as the anode active material, the anode active material, styrene-butadiene rubber (SBP), and sodium carboxymethyl cellulose (CMC) are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 97:2:1 to form a uniform anode slurry, wherein the solid content of the anode slurry is 40 wt%. This slurry is coated onto the copper foil of the anode current collector, dried at 85°C, and then cold-pressed, cut, and slit before being dried under vacuum at 120°C for 12 hours to obtain the anode sheet.

[0070] Preparation of the positive electrode sheet: Lithium cobalt oxide (LiCoO2), conductive carbon black Super P, and polyvinylidene fluoride (PVDF) were thoroughly mixed in an appropriate amount of N-methylpyrrolidone (NMP) solvent at a weight ratio of 97:1.4:1.6 to form a uniform positive electrode slurry with a solid content of 72 wt%. This slurry was coated onto aluminum foil for the positive electrode current collector, dried at 85°C, and then cold-pressed, cut, and slit before being dried under vacuum at 85°C for 4 hours to obtain the positive electrode sheet.

[0071] Preparation of electrolyte: In a dry argon atmosphere glove box, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed evenly in a mass ratio of EC:EMC:DEC=30:50:20. Then, lithium salt LiPF6 is added and mixed evenly to obtain the electrolyte. The mass percentage of LiPF6 in the electrolyte is 12.5%.

[0072] Preparation of the separator: A 7-micron thick porous polyethylene (PE) polymer film was used as the separator.

[0073] Preparation of lithium-ion batteries: The above-mentioned positive electrode sheet, separator, and negative electrode sheet are sequentially stacked, wound, and welded with tabs, and then placed in an aluminum-plastic film packaging bag. Electrolyte is then injected, and the batteries undergo vacuum sealing, settling, formation, shaping, and capacity testing to obtain a soft-pack lithium-ion battery.

[0074] The specific steps for preparing the lithium-ion batteries in Examples 2-12 and Comparative Examples 1-3 are the same as those in Example 1, and the differences can be found in Table 1.

[0075] Table 1

[0076]

[0077] The first efficiency test of lithium-ion batteries was conducted on the button batteries formed by the negative electrode sheets of the above embodiments and comparative examples. The specific test method is as follows: Take the single-sided coated negative electrode sheet prepared in the corresponding embodiment or comparative example, and cut it into pieces with an area of ​​1.54 cm². 2 The lithium sheet was used as the working electrode, followed by a lithium foil as the counter electrode and a porous polyethylene membrane as the separator. Electrolyte was injected and the cells were assembled to form a button cell. The button cell was first discharged to 0V using three stages of low current (0.05C / 50μA / 20μA), and the initial discharge capacity was recorded. Then, it was charged to 2.0V using a constant current of 0.1C, and the initial charge capacity was recorded. The initial efficiency was calculated as (initial charge capacity / initial discharge capacity) × 100%. The initial reversible specific capacity of the negative electrode active material from 0V to 2.0V was calculated as (initial charge capacity of the button cell / mass of the negative electrode active material). The electrolyte was a 12.5% ​​(w / w) LiPF6 solution, and the solvent could be obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 (w / w) mass ratio. The initial efficiencies of the button cells corresponding to each embodiment and comparative example, obtained through testing, are recorded in [the relevant section]. Figure 1 middle.

[0078] The silicon-carbon anode materials of the above embodiments and comparative examples were subjected to scanning electron microscopy (SEM) line scan elemental analysis. The specific method is as follows: The anode sheets prepared in the corresponding embodiments or comparative examples were cut into cross-sections using ion polishing. The cut cross-sections of the silicon-carbon anode materials were then used as the test surface of the sample, transferred into a field emission scanning electron microscope (FET), and tested after focusing. During testing, a line scan was performed from the center of the particle outwards to measure the silicon element content in the silicon-carbon anode material, and the silicon element content distribution was recorded. Then, the scanned values ​​were subjected to variance analysis to obtain the standard deviation, which was recorded. Figure 1 middle.

[0079] The silicon content and carbon nanotube content of the silicon-carbon anode materials in the above embodiments and comparative examples were tested.

[0080] The silicon content in the silicon-carbon anode material was characterized and tested using ICP (inductively coupled plasma spectroscopy).

[0081] The specific method for testing carbon nanotube content is as follows: First, based on the total amount of RF and HMT added in Comparative Example 1 and the weight of the resulting porous carbon framework, calculate the yield of RF+HMT in the mixing / curing / carbonization / and activation process, denoted as d%; then record the yield of RF+HMT+CNTs in the mixing / curing / carbonization / and activation process in other examples, denoted as e%; then the proportion of CNTs in the porous carbon framework of the examples is f% = (ed) / e × 100%, and the remaining porous carbon framework components are g% = 1 - f%; then the CNTs content in the silicon-carbon anode material of the examples is h% = f / (f + g + b) × 100%, and recorded in [the relevant section]. Figure 1 In the remaining porous carbon framework, the content of the components is i% = 1 - h% - b.

[0082] Conductivity tests were performed on the silicon-carbon anode materials of each embodiment and comparative example. The specific test methods are as follows: The conductivity of the silicon-carbon anode material powder was tested using a powder conductivity meter (model FT-8100) based on the four-probe test principle, referring to standard GB / T1552-1995. A known amount of silicon-carbon anode material powder was used, and its volume was compressed to a set pressure value or intensity under hydraulic power. The conductivity of the silicon-carbon anode material powder was measured online, and the data were recorded. Figure 1 middle.

[0083] The particle strength of the silicon-carbon anode materials in each embodiment and comparative example was tested. The specific testing method is as follows: The hardness and elastic modulus of individual silicon-carbon anode material particles were tested using a nanoindenter (Hysitron TI 950), and the testing standard was JB / T 12721-2016. Before testing, the silicon-carbon anode material powder was dispersed in epoxy resin and cured. The cured resin was cut by ion polishing, and pressure was applied to individual particles using a nano-probe. The indentation depth on the particle surface was monitored, and the elastic modulus of the particle was calculated. The elastic modulus of five particles of the same sample were tested in parallel, and the average value was taken to obtain the particle elastic modulus of the silicon-carbon anode material, which was recorded in [the relevant documentation]. Figure 1 middle.

[0084] Cycle performance tests and full charge expansion rate tests were conducted on the soft-pack lithium-ion batteries of each embodiment and comparative example.

[0085] The specific method for cycle performance testing is as follows: Charge at a constant current of 0.7C to 4.4V, then charge at a constant voltage of 0.025C, allow to stand for 5 minutes, and then discharge at 0.5C to 3.0V. Use the capacity obtained in this step as the initial capacity, and perform cycle tests at 0.7C charge / 0.5C discharge. Calculate the ratio of the capacity at each step to the initial capacity to obtain the capacity decay curve. Cycle at 25℃ until the capacity retention reaches 90% (recorded in...). Figure 1 The cell's room temperature cycling performance (CLP) is defined as the number of cycles at 45°C until 80% capacity retention is achieved (as recorded in [reference needed]). Figure 1 The high-temperature cycle performance of the battery is denoted as (in Chinese). The cycle performance of the material is compared by comparing the number of cycles under the two conditions mentioned above.

[0086] The specific method for testing the full-charge expansion rate of a battery is as follows: Use a micrometer to measure the thickness of a fresh pouch lithium-ion battery at half-charge (50% State of Charge (SOC)). After 400 cycles, the battery is in a fully charged (100% SOC) state. Measure the battery thickness again with the micrometer and compare it to the thickness of the fresh battery at the initial half-charge (50% SOC) state. This will give you the full-charge (100% SOC) battery expansion rate, which is then recorded. Figure 1 middle.

[0087] Particle size test:

[0088] Add approximately 0.02g of silicon-carbon anode material powder sample to a 50ml clean beaker, add approximately 20ml of deionized water, and then add 3 drops of 1% surfactant to completely disperse the silicon-carbon anode material powder in the water. Use a 120W ultrasonic cleaner to sonicate for 5 minutes, and use a MasterSizer 2000 to test its particle size.

[0089] Raman test

[0090] The silicon-carbon anode material was mounted on a flat glass slide for Raman testing, with a test range of 100 cm. -1 Up to 1200cm -1 After the test, pay attention to 450cm. -1 Up to 550cm -1 For characteristic peaks within the range, the point with the maximum peak value is taken as the peak position, and half of the difference between the x-coordinates of half the peak value is taken as the half-peak width of the peak.

[0091] XRD test

[0092] The silicon-carbon anode material was loaded onto a sample stage for powder XRD testing. The testing range was 10° to 90°, and the scanning speed was 5° / min. After the test, the characteristic peaks in the range of 15° to 35° were observed. The point with the maximum peak value was taken as the peak position, and half of the difference in the x-coordinate of half the peak value was taken as the half-peak width of the peak.

[0093] Determination / Testing Method of Pore Capacity

[0094] The pore volume of the silicon-carbon anode material was measured using the N2 gas adsorption method. After obtaining the adsorption / desorption data, the pore structure was fitted using the NRDFT model to obtain pore volume data for <2nm and >2nm, respectively.

[0095] From Table 1 and Figure 1 The recorded data shows that, especially compared to Example 1 and other embodiments, the strength of the silicon-carbon anode material is improved after the addition of carbon nanotubes, thus enhancing its ability to buffer battery volume expansion and consequently improving battery cycle performance. Furthermore, the conductivity of the silicon-carbon anode material is also improved after the addition of carbon nanotubes, improving its electron diffusion capability and reducing the probability of lithium ions being trapped (trapping) within the silicon-carbon structure, thereby increasing its initial charge efficiency and energy density. However, excessively high carbon nanotube content can negatively impact the initial charge efficiency. Specifically, excessively high carbon nanotube content leads to an excessively high proportion of large pores during the formation of the porous carbon framework, making subsequent silicon deposition difficult. This results in an increased specific surface area of ​​the final product, leading to increased SEI film formation in the electrochemical device and consequently affecting the initial charge efficiency. Similarly, in Comparative Example 2, the low silicon content results in insufficient filling of pores in the porous carbon framework, leading to an increased specific surface area of ​​the final product, which in turn increases SEI film formation in the electrochemical device and affects the initial charge efficiency. As can be seen from the comparison of Comparative Example 3 and other embodiments, the silicon content in the silicon-carbon anode material is too high, the binding ability of carbon nanotubes is limited, the battery expands severely, and the cycle performance deteriorates.

[0096] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Therefore, any equivalent variations made in accordance with this application are still within the scope of this application.

Claims

1. A silicon-carbon anode material, wherein, The material includes a core and a shell. The core comprises a porous carbon framework and silicon dispersed in the pores of the porous carbon framework. Carbon nanotubes are encapsulated and dispersed within the porous carbon framework. In the silicon-carbon anode material, the silicon content is 25 wt% to 55 wt%, and the carbon nanotube content is 0.2 wt% to 7 wt%. The shell comprises carbon material. In the linear scanning electron microscopy energy dispersive spectroscopy of the cross section of the silicon-carbon anode material, the standard deviation of the silicon content variation along the direction from the center to the edge of the cross section is 149 to 199.

2. The silicon-carbon anode material as described in claim 1, wherein, The conductivity of the silicon-carbon anode material is 9 S / cm to 30 S / cm, and / or the particle elastic modulus of the silicon-carbon anode material is 4 GPa to 10 GPa.

3. The silicon-carbon anode material as described in claim 1, wherein, The silicon-carbon anode material satisfies at least one of the following conditions: (1) The X-ray diffraction pattern of the silicon-carbon anode material has a characteristic peak in the range of 20° to 30°, and the full width at half maximum (FWHM) of the characteristic peak is greater than 2°. (2) The Raman spectrum of the silicon-carbon anode material at 450 cm⁻¹ -1 Up to 500cm -1 It has a characteristic peak within the range; (3) The particle size Dv50 of the silicon-carbon anode material is 3μm to 20μm, and the particle size Dv99 of the silicon-carbon anode material is 3μm to 20μm.

4. The silicon-carbon anode material as described in claim 1, wherein, In the silicon-carbon anode material, the pore volume of pores with a diameter greater than 2 nm is greater than that of pores with a diameter not exceeding 2 nm.

5. A negative electrode sheet, comprising a current collector and a negative electrode active layer, wherein the negative electrode active layer comprises a negative electrode active material, wherein, The negative electrode active material includes the silicon-carbon negative electrode material as described in any one of claims 1 to 4.

6. The negative electrode sheet as described in claim 5, wherein, The negative electrode active material further includes graphite, wherein the content of the silicon-carbon negative electrode material is 5 wt% to 40 wt%, and the content of the graphite is 95 wt% to 60 wt%.

7. An electrochemical device, wherein, Includes the negative electrode sheet as described in any one of claims 5 to 6.

8. A method for preparing the silicon-carbon anode material as described in claim 1, comprising: The resin and carbon nanotubes are mixed to form a mixture and then cured. The solidified mixture was carbonized and activated to obtain a porous carbon framework. as well as The porous carbon framework is deposited with silane to form a core, and then an alkane is used to form a shell.

9. The method for preparing the silicon-carbon anode material as described in claim 8, wherein, The carbonization conditions are to raise the temperature to 700°C to 1100°C and hold it for 1 to 5 hours. The activation is specifically carried out by carbon dioxide, water vapor, sodium hydroxide, potassium hydroxide or phosphoric acid after the carbonization is cooled down.

10. The method for preparing the silicon-carbon anode material as described in claim 8, wherein, The step "depositing silane to form a core on the porous carbon framework and then forming a shell with alkanes" specifically involves: The porous carbon framework is slowly heated to 400°C to 600°C under an inert atmosphere and held at that temperature. Then, the atmosphere is switched to a silane mixture gas for deposition for 1 to 20 hours. The silane mixture gas contains 2% to 20% silane and 80% to 98% inert gas by mass. At a temperature of 500°C to 1000°C, the atmosphere is switched to an alkane mixture gas and held for 10 hours to form a shell. Then, the atmosphere is switched back to an inert atmosphere and cooled to room temperature. The alkane mixture gas contains 5% to 100% alkanes and 0% to 95% inert gas by mass.

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