Silicon carbon anode material, method for preparing the same, and use

Abstract

A silicon-carbon anode material comprising a core and a shell is provided, wherein the core comprises a porous carbon skeleton and silicon dispersed within the pores of the porous carbon skeleton. Carbon nanotubes are encapsulated and dispersed within the porous carbon skeleton, and in the silicon-carbon anode material, the silicon element content is 25 wt% to 55 wt%, and the shell comprises a carbon material. In the energy spectrum obtained by linear scanning electron microscopy of a cross-section of the silicon-carbon anode material, the standard deviation of the change in the silicon element content along the direction from the center to the edge of the cross-section is 200 or less. An electrochemical apparatus and a method for preparing a silicon-carbon anode material are provided.

Description

[Technical Field]

[0001] This invention claims priority to the Chinese patent application filed on June 29, 2023, with the National Intellectual Property Administration of China Patent Office, application number 202310786717.2, with the title "Silicon Carbon Anode Material, Method for Preparation Thereof, and Use Thereof." All contents contained in the said Chinese patent application are incorporated herein by reference.

[0002] The present invention relates to the field of electrochemical energy storage, and more particularly to silicon-carbon anode materials and methods for preparing the same, anode pieces to which the silicon-carbon anode material is applied, and electrochemical apparatus to which the anode piece is applied. [Background technology]

[0003] Lithium-ion batteries have advantages such as high volumetric and mass energy density, environmental friendliness, high operating voltage, small volume, light weight, and long cycle life, and are widely used in the field of portable home electronics. In recent years, with the rapid development of electric vehicles and mobile electronic devices, the demands on energy density, safety, and cycle characteristics of batteries have been increasing, and the emergence of new lithium-ion batteries that comprehensively improve overall performance is expected. Among these, energy density and cycle characteristics are important technical problems that need to be solved urgently, and improving the active material in electrodes is one direction of research to solve the above problems.

[0004] Currently, graphite is the most widely used anode material, offering advantages such as high efficiency and a stable charge / discharge platform. However, the performance of commercially available graphite has been nearly maximized, and its low capacity and the safety risks associated with lithium dendrites are hindering its further use. On the other hand, compared to graphite as an anode material, elemental silicon is considered the most promising alternative to graphite as a lithium-ion anode material due to its very high theoretical specific capacity and suitable operating voltage. However, its low conductivity and enormous volume expansion during the alloying / dealloying process severely limit the large-scale application of elemental silicon in lithium-ion batteries. [Overview of the Initiative]

[0005] This invention provides a silicon-carbon anode material that can improve conductivity and reduce expansion.

[0006] Furthermore, the present invention further provides a negative electrode piece of the silicon-carbon negative electrode material and an electrochemical apparatus to which the negative electrode piece is applied, and the present invention further provides a method for preparing the silicon-carbon negative electrode material described above.

[0007] A first aspect of the present invention provides a silicon-carbon anode material comprising a core and a shell, wherein the core comprises a porous carbon skeleton and silicon dispersed within the pores of the porous carbon skeleton. Carbon nanotubes are encapsulated and dispersed within the porous carbon skeleton, the silicon element content in the silicon-carbon anode material is 25 wt% to 55 wt%, and the shell comprises a carbon material. In the energy spectrum obtained by linear scanning electron microscopy of a cross-section of the silicon-carbon anode material, the standard deviation of the change in the silicon element content along the direction from the center to the edge of the cross-section is 200 or less.

[0008] The silicon-carbon anode material of the present invention has carbon nanotubes within it that possess good conductivity and mechanical properties. Therefore, a specific amount of carbon nanotubes dispersed in the porous carbon framework can improve the conductivity of the silicon-carbon anode material, while also improving its mechanical properties and suppressing its expansion, thereby contributing to improved structural stability. Energy spectra obtained from a linear scanning electron microscope of a cross-section of the silicon-carbon anode material show that the silicon element is uniformly distributed within the material. This is advantageous for further improving the conductivity and mechanical properties of the silicon-carbon anode material. When the above silicon-carbon anode material is applied to an anode piece in an electrochemical apparatus, the improved conductivity of the silicon-carbon anode material improves the probability of lithium being trapped within the silicon-carbon anode material. This improves the lithium desorption capacity of the electrochemical device and enhances its initial charge-discharge characteristics. Furthermore, carbon nanotubes mitigate the volume expansion of silicon-carbon during the lithium insertion process, improving the expansion, powdering, and structural stability of silicon-carbon during charge and discharge, thereby improving its cycle characteristics.

[0009] According to the first embodiment, in several possible embodiments, the electrical conductivity of the silicon-carbon anode material is 9 S / cm to 30 S / cm. In the above possible embodiments, an electrical conductivity within a specific range is advantageous for ensuring the conductivity of the silicon-carbon anode material and for further improving the cycle characteristics of the electrochemical apparatus to which the silicon-carbon anode material is applied.

[0010] According to the first embodiment, in several possible embodiments, the elastic modulus of the particles of the silicon-carbon anode material is 4 GPa to 10 GPa, and the elastic modulus of the particles within a specific range is advantageous for improving the structural stability of the silicon-carbon anode material and for further improving the expansion and cycling characteristics of the electrochemical apparatus to which the silicon-carbon anode material is applied.

[0011] According to the first embodiment, in the silicon-carbon anode material, the carbon nanotube content is 0.2 wt% to 7.0 wt%, and a carbon nanotube content within this range can further mitigate the volume expansion of silicon-carbon during the lithium insertion process, control pore formation during the preparation of the porous carbon skeleton, reduce the proportion of micropores, and improve the uniformity of silicon deposition, thereby further improving the cycle characteristics of the electrochemical apparatus and reducing the cycle expansion rate of the electrochemical apparatus.

[0012] According to the first aspect, in several possible embodiments, the silicon-carbon anode material (1) has one characteristic peak in the X-ray diffraction pattern of the silicon-carbon anode material within the range of 20° to 30°, and the full width at half maximum of the characteristic peak is greater than 2°, and (2) has a Raman spectrum of 450 cm² -1 ~500cm -1 (3) The material has one characteristic peak within the specified range, and (4) the particle size Dv50 of the silicon-carbon anode material is 3 μm to 20 μm, and the particle size Dv99 of the silicon-carbon anode material is 3 μm to 20 μm, satisfying at least one of these conditions.

[0013] In the above possible embodiments, as can be seen from the X-ray diffraction pattern of the silicon-carbon anode material, the pores in the porous carbon framework are micropores. That is, the pore diameter of 90% of the pores in the porous carbon framework is less than 2 nm, which is advantageous in that the size of the silicon adsorbed into the porous carbon framework in the silicon-carbon anode material is almost less than 2 nm. This is advantageous in further reducing the expansion of the silicon-carbon anode material. From the Raman spectrum of the silicon-carbon anode material, it can be seen that the size of the silicon in the silicon-carbon anode material is small and amorphous, which is advantageous in further reducing the expansion of the silicon-carbon anode material. On the other hand, the silicon-carbon anode material within a specific particle size range makes the processing process for preparing anode pieces by subsequent stirring and coating smoother and facilitates compounding with graphite.

[0014] According to the first embodiment, in several 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 of 2 nm or less. In several possible embodiments, the pore volume range of pores with a diameter greater than 2 nm is 0.04 to 0.20. In the above possible embodiments, silicon is preferentially adsorbed and deposited in the micropores because the adsorption effect of the micropores is superior. Combining this with the premise that the pore diameter of 90% of the pores in the porous carbon skeleton is less than 2 nm, it is found that the size of silicon in the silicon-carbon anode material is almost less than 2 nm, which is advantageous for further reducing the expansion of the silicon-carbon anode material.

[0015] A second aspect of the present invention provides a negative electrode piece including a current collector and a negative electrode active layer. The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon negative electrode material as described above.

[0016] In the negative electrode piece of the present invention, the conductivity of the silicon-carbon negative electrode material is improved by the uniform distribution of carbon nanotubes and silicon elements. This improves the probability of lithium being trapped inside the silicon-carbon negative electrode material, thereby improving the lithium desorption capacity of the electrochemical device and enhancing the initial charge-discharge characteristics. Furthermore, the carbon nanotubes can mitigate the volume expansion of silicon-carbon during the lithium insertion process, and combined with the uniform distribution of silicon elements, improves the expansion and powdering of silicon-carbon during charge and discharge and improves its structural stability, thereby enhancing its cycle characteristics.

[0017] According to a second embodiment, in some possible embodiments, the negative electrode active material further contains graphite, wherein the content of the silicon-carbon negative electrode material in the negative electrode active material is 5 wt% to 40 wt%, and the content of the graphite is 95 wt% to 60 wt%.

[0018] In the above possible embodiments, the specific amount of silicon-carbon anode material fully utilizes the characteristics of silicon, such as its extremely high theoretical specific capacitance and appropriate operating voltage, while effectively reducing the impact of silicon expansion on the anode piece, which is advantageous for improving the initial Coulomb efficiency, energy density, and cycle characteristics of the anode piece. Specifically, if the content of silicon-carbon anode material is too high, the volume expansion of the anode active layer becomes significant, resulting in poor cycle characteristics, while if the content of silicon-carbon anode material is too low, it is disadvantageous for improving the initial Coulomb efficiency. Since graphite has a certain degree of flexibility, when compounded with the silicon-carbon anode material, the volume expansion of the anode active layer can be mitigated. Furthermore, by fully utilizing the advantages of both the silicon-carbon anode material and graphite, the anode piece can achieve good electrochemical properties.

[0019] A third aspect of the present invention provides an electrochemical apparatus including the negative electrode piece described above.

[0020] The negative electrode piece of the electrochemical apparatus of the present invention improves the conductivity of the silicon-carbon negative electrode material by uniformly distributing carbon nanotubes and silicon elements. This improves the probability of lithium being trapped inside the silicon-carbon negative electrode material, improves the lithium desorption capacity of the electrochemical apparatus, and enhances the initial charge-discharge characteristics. Furthermore, the carbon nanotubes can mitigate the volume expansion of silicon-carbon during the lithium insertion process, and combined with the uniform distribution of silicon elements, improves the expansion and powdering of silicon-carbon during charge-discharge and improves its structural stability, thereby improving its cycle characteristics.

[0021] A fifth aspect of the present invention provides a method for preparing a silicon-carbon anode material as described above. The preparation method includes the steps of mixing a resin and carbon nanotubes to form a mixture and curing the mixture, carbonizing the cured mixture and then activating it to obtain a porous carbon skeleton, and depositing silane on the porous carbon skeleton to form a core, and then forming a shell with an alkane.

[0022] The method for preparing the silicon-carbon negative electrode material of the present invention first mixes a resin and carbon nanotubes to form a mixture. After curing the mixture, it is further carbonized to form a porous skeleton, which facilitates the dispersion of the carbon nanotubes in the porous skeleton. This is advantageous for improving the binding of the carbon nanotubes to the prepared silicon-carbon negative electrode material, further alleviating the volume expansion of the silicon-carbon negative electrode material, improving the expansion pulverization and structural stability of silicon-carbon during charge and discharge, and improving its cycle characteristics. At the same time, this is advantageous for improving the conductivity of the entire silicon-carbon negative electrode material and further improving the probability that lithium is captured inside the silicon-carbon negative electrode material, improving the lithium desorption capacity of the electrochemical device, and improving the initial charge-discharge characteristics.

[0023] According to the fifth aspect, in some possible embodiments, the conditions for carbonization are to raise the temperature to 700 °C to 1100 °C and keep it warm for 1 hour to 5 hours, and the activation is specifically carried out by carbon dioxide, water vapor, sodium hydroxide, potassium hydroxide or phosphoric acid after the temperature drops after carbonization.

[0024] According to the fifth aspect, in some possible embodiments, the step of "depositing silane on the porous carbon skeleton to form a core and then forming a shell with an alkane" is specifically as follows. The porous carbon skeleton is gradually heated to 400 °C to 600 °C in an inert atmosphere and then kept warm. Then, the atmosphere is switched to a silane mixed gas and deposited for 1 hour to 20 hours. The silane mixed gas contains 2% to 20% silane and 80% to 98% inert gas by mass percentage. Further, the atmosphere is switched to an alkane mixed gas at a temperature of 500 °C to 1000 °C and held for 10 hours to form a shell. Then, the atmosphere is switched to an inert atmosphere and cooled to room temperature. The alkane mixed gas contains 5% to 100% alkane and 95% to 0% inert gas by mass percentage.

Embodiments for Carrying out the Invention

[0025] The following clearly and detailedly describes the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are some embodiments of the present invention, not all embodiments. Unless otherwise defined, all technical terms and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention are only for the purpose of explaining specific embodiments and are not intended to limit the present invention.

[0026] The following will detail the embodiments of the present invention. However, the present invention can be embodied in many different forms and should not be construed as limited to the exemplary embodiments described herein. Rather, by providing these exemplary embodiments, the present invention is fully and detailedly conveyed to those skilled in the art.

[0027] Furthermore, when using "possible" in describing the embodiments of the present invention, it means "one or more embodiments of the present invention".

[0028] The terminology used herein is for the purpose of describing specific embodiments and is not intended to limit the invention. Where used herein, the singular form is intended to include the plural form unless the context specifically indicates otherwise. Furthermore, it should be understood that the term “including,” where used herein, means the presence of the described features, figures, steps, operations, elements and / or components, but does not exclude the presence or increase of one or more other features, figures, steps, operations, elements, components and / or combinations thereof. Lists of items connected by the terms “at least one of,” “at least one of,” “at least one kind of,” or other similar terms mean any combination of the listed items. For example, if items A and B are listed, the phrase “at least one of A and B” means A only, B only, or A and B. In other instances, when items A, B, and C are listed, the phrase "at least one of A, B, and C" means A only, or B only, C only, A and B (excluding C), A and C (excluding B), B and C (excluding A), or all of A, B, and C.

[0029] In this invention, the design relationships between parameter values ​​that are greater than, less than, or unequal are necessary to eliminate reasonable errors in the measuring device.

[0030] One embodiment of the present invention provides an electrochemical apparatus including a positive electrode piece, a negative electrode piece, and a separator. The separator is placed between the positive electrode piece and the negative electrode piece. The positive electrode piece, the separator, and the negative electrode piece are sequentially stacked alternately to form a stacked electrode assembly, or the positive electrode piece, the separator, and the negative electrode piece are sequentially stacked and then wound together to form a wound electrode assembly.

[0031] The electrochemical apparatus further comprises a case and an electrolyte, wherein the positive electrode piece, the negative electrode piece, the separator, and the electrolyte are housed within the case. The case may be, but is not limited to, a packaging bag wrapped in a packaging film, such as an aluminum plastic film; that is, the electrochemical apparatus may be a soft pack battery. The case may be, but is not limited to, cases disclosed in the prior art, such as steel case batteries or aluminum case batteries.

[0032] The positive electrode piece includes a positive electrode current collector and a positive electrode active layer installed on the positive electrode current collector. The positive electrode current collector may be made of aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, for example, a current collector combined with the conductive foil and polymer substrate, but is not limited thereto. The positive electrode active layer includes a positive electrode active material, which includes a lithium ion compound (i.e., a lithiated intercalate compound) that reversibly intercalates and releases lithium ions. In some embodiments, the positive electrode 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 electrode active material is lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary material (NCM), lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 It may contain, but is not limited to, at least one of O4 and lithium iron phosphate (LiFePO4).

[0033] The positive electrode active layer further includes a binder that binds positive electrode active material particles to facilitate the formation of a film layer and enhances the bonding force between the positive electrode active layer and the positive electrode current collector.

[0034] In some embodiments, the binder may include, but is not limited to, at least one of the following: polyvinyl alcohol, hydroxypropyl cellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyfluoroethylene, ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic acid (esterified) styrene-butadiene rubber, epoxy resin, and nylon.

[0035] The positive electrode active layer may further contain a conductive material, which includes, but is not limited to, carbon-based materials, metallic 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 fibers, or any combination thereof. In some embodiments, the metallic material may include, but is not limited to, metal powders or metal fibers such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0036] The negative electrode piece includes a negative electrode current collector and a negative electrode active layer installed on the negative electrode current collector. The negative electrode current collector may be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, and carbon-based current collectors, or any composite current collector disclosed in the prior art, for example, a current collector combined with the conductive foil and polymer substrate. The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon negative electrode material.

[0037] The silicon-carbon anode material comprises a core and a shell, the core comprising a porous carbon skeleton and silicon dispersed within the pores of the porous carbon skeleton. Carbon nanotubes are encapsulated and dispersed within the porous carbon skeleton, the silicon content in the silicon-carbon anode material is 25 wt% to 55 wt%, and the shell comprises a carbon material. In the energy spectrum of the cross-section of the silicon-carbon anode material obtained by linear scanning electron microscopy, the standard deviation of the change in silicon content along the direction from the center to the edge of the cross-section is 200 or less. Specifically, the cross-section of the silicon-carbon anode material particles is cut out by ion polishing to create the measurement surface. The cut silicon-carbon anode material is transferred to a field emission scanning electron microscope, and a linear scan is performed on the measurement surface from the center outward to record the distribution of each element's content. For example, data on silicon content obtained by linear scanning are divided into 100 equidistant data points. A mathematical statistical standard deviation analysis is then performed on these 100 data points to obtain the standard deviation of the change in silicon content.

[0038] The silicon-carbon anode material described above has good conductivity and mechanical properties due to the carbon nanotubes it contains. Therefore, a specific amount of carbon nanotubes dispersed in the porous carbon framework can improve the conductivity of the silicon-carbon anode material, improve its mechanical properties, and constrain its expansion, thereby contributing to improved structural stability. On the other hand, energy spectra obtained from a linear scanning electron microscope of the cross-section of the silicon-carbon anode material show that the silicon element is uniformly distributed in the silicon-carbon anode material. The dispersion of carbon nanotubes and the uniform distribution of the silicon element improve the conductivity and strength of the silicon-carbon anode material, reduce lithium trapping in the silicon-carbon anode material, mitigate the expansion of the silicon-carbon anode material, increase the initial Coulomb efficiency of the electrochemical apparatus to which the anode piece is applied, and further improve the energy density and cycle characteristics of the electrochemical apparatus.

[0039] In some embodiments, a silicon element content of 20% to 45% can further reduce the expansion of the silicon-carbon anode material and improve the cycle characteristics of the electrochemical apparatus.

[0040] In some embodiments, the electrical conductivity of the silicon-carbon anode material is 9 S / cm to 30 S / cm. In some embodiments, the electrical conductivity of the silicon-carbon anode material is 11 S / cm to 25 S / cm. In some embodiments, the electrical conductivity of the silicon-carbon anode material is within the range of 9 S / cm, 11 S / cm, 14 S / cm, 20 S / cm, 25 S / cm, 30 S / cm, or any two of the above values. When the electrical conductivity of the silicon-carbon anode material is within the above range, the conductivity of the silicon-carbon anode material can be ensured, and the cycle characteristics of the electrochemical apparatus can be further improved.

[0041] In some embodiments, the elastic modulus of the silicon-carbon anode material particles being 4 GPa to 10 GPa ensures the mechanical strength of the silicon-carbon anode material, relieves internal stress, and further reduces the expansion of the silicon-carbon anode material, which is advantageous for improving the cycle characteristics of the electrochemical apparatus.

[0042] In some examples, the carbon nanotube content in the silicon-carbon anode material is 0.2 wt% to 7.0 wt%. In some examples, the carbon nanotube content is 1.0 wt% to 6.0 wt%. In some examples, the carbon nanotube content is within the range of 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 any two of the above values. A carbon nanotube content within the above range helps to further mitigate the volume expansion of silicon-carbon during the lithium insertion process and allows for control of pore formation during the preparation of the porous carbon skeleton, thereby reducing the proportion of micropores, improving the uniformity of silicon deposition, ensuring the initial efficiency (i.e., initial Coulomb efficiency) of the electrochemical apparatus, further improving the cycle characteristics of the electrochemical apparatus, and reducing the cycle expansion rate of the electrochemical apparatus.

[0043] In some embodiments, the silicon-carbon anode material exhibits a single characteristic peak in the X-ray diffraction pattern within the range of 20° to 30°, with a full width at half maximum (FWHM) greater than 2°. This means that the pores in the porous carbon framework are micropores, and 90% of the pores in the porous carbon framework have a pore diameter of less than 2 nm. This is advantageous because the size of the silicon adsorbed into the porous carbon framework in the silicon-carbon anode material is almost 2 nm or less, which is beneficial for further reducing the expansion of the silicon-carbon anode material.

[0044] Furthermore, 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 of 2 nm or less. Because the adsorption effect of micropores is superior, silicon is preferentially adsorbed and deposited in the micropores. Therefore, since the size of silicon in the silicon-carbon anode material is mostly less than 2 nm, it is advantageous for further reducing the expansion of the silicon-carbon anode material. In some examples, the pore volume range of pores with a diameter greater than 2 nm is 0.04 to 0.20.

[0045] In some embodiments, the Raman spectrum of the silicon-carbon anode material was 450 cm⁻¹. -1 ~500cm -1 It has one characteristic peak within the range, meaning that the silicon size in the silicon-carbon anode material is small and amorphous, which is advantageous for further reducing the expansion of the silicon-carbon anode material.

[0046] The particle size Dv50 of the silicon-carbon anode material may be 3 μm to 20 μm, and the particle size Dv99 of the silicon-carbon anode material may also be 3 μm to 20 μm. This makes the process smoother and easier when forming the anode active layer by mixing and coating with other materials (e.g., a binder).

[0047] The anode active material may further contain graphite, and since graphite has a certain degree of flexibility, when combined with the silicon-carbon anode material, it can mitigate the volume expansion of the anode active layer. At the same time, using both graphite and the silicon-carbon anode material as active materials is advantageous in reducing the overall expansion of the anode active layer, and it is possible to achieve excellent electrochemical properties by fully utilizing the advantages of both the silicon-carbon anode material and graphite.

[0048] In the anode active material, the content of the silicon-carbon anode material may be 5 wt% to 40 wt%. Furthermore, the content of the graphite may be 95 wt% to 60 wt%.

[0049] The negative electrode active layer further includes a binder that facilitates the formation of a film layer by binding positive electrode active material particles, and also improves the bonding force between the negative electrode active layer and the negative electrode current collector.

[0050] In some embodiments, the binder may include, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyfluoroethylene, ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic acid (esterified) styrene-butadiene rubber, epoxy resin, or nylon.

[0051] The negative electrode active layer may further contain a conductive material, which includes, but is not limited to, carbon-based materials, metallic 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 fibers, or any combination thereof. In some embodiments, the metallic material may include, but is not limited to, metal powders or metal fibers such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0052] The separator includes a film layer having a porous structure, and its material includes, but is not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, and aramid. For example, the separator may be a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film.

[0053] The state of the electrolyte may be one or more of a gel, solid, and liquid state. In some examples, the liquid electrolyte includes a lithium salt and an organic solvent. The lithium salts include 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), and lithium bis(trifluoromethylsulfonyl)imide (LiN(SO2CF3)). 2. One or more of the following may be selected, but are not limited to: tri(trifluoromethylsulfonylimide)methyllithium (LiC(SO2CF3)3), lithium bis(oxalate)borate (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, as a lithium salt, LiPF6 can be used, for example, because it can provide high ionic conductivity and improve cycle characteristics. The organic solvent may be a carbonate compound, a carboxylic acid ester compound, an ether compound, a nitrile compound, another organic solvent, or a combination thereof.Examples of carbonate compounds include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), and 1,2-difluoroethylene carbonate. This includes, but is not limited to, ionate, 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.

[0054] The above electrochemical apparatus is applied to an electronic device to supply power to other electronic elements in the electronic device. The silicon-carbon anode material in the above electrochemical apparatus is advantageous for improving cycle characteristics and energy density, and therefore advantageous for improving the service life of the electronic device. The electronic device may include, but is not limited to, notebook computers, pen-input computers, mobile computers, e-book players, mobile phones, portable facsimile machines, portable copiers, portable printers, stereo headsets, video recorders, LCD televisions, portable cleaners, portable CD players, MiniDiscs, transceivers, electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric assist bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, strobes, cameras, large household storage batteries, and lithium-ion capacitors.

[0055] The present invention further provides a method for preparing a silicon-carbon anode material, the preparation method comprising the following steps.

[0056] Step S1: The resin and carbon nanotubes are mixed to form a mixture, and the mixture is cured. The method of forming the mixture includes, but is not limited to, a ball mill. To facilitate the dispersion of carbon nanotubes, an organic solvent may be added during mixing. The curing described above involves drying and molding the mixture, and can be done by applying conventional curing conditions, which will not be explained here.

[0057] Step S2: The hardened mixture described above is carbonized and then activated to obtain a porous carbon skeleton. Specifically, the hardened mixture is heated to 700°C to 1100°C in an inert atmosphere (for example, a nitrogen atmosphere, but not limited thereto) for 1 to 5 hours to carbonize, then cooled to room temperature and activated to form a porous carbon skeleton. The activation may be performed by activating the carbonized structure with carbon dioxide or water vapor, or by activating the carbonized structure with alkaline etching or acid etching. The alkaline etching may usually be performed using sodium hydroxide or potassium hydroxide, and the acid etching may usually be performed using phosphoric acid.

[0058] Step S3: After depositing silane onto the porous carbon skeleton to form a core, an alkane is used to form a carbon coating layer shell to cover the core. Specifically, the porous carbon skeleton is gradually heated to 400°C to 600°C in an inert atmosphere (for example, an argon atmosphere, but not limited thereto), then the atmosphere is switched to a silane mixed gas, and the core is formed by deposition for 1 to 20 hours. The gradual heating rate may be 0.5°C / min to 5°C / min, but not limited thereto, and the silane mixed gas contains 2% to 20% silane and 80% to 98% inert gas (for example, argon gas, but not limited thereto) by mass percentage. Subsequently, the atmosphere was switched to an alkane mixed gas at a temperature of 500°C to 1000°C and held for 2 to 20 hours to form a shell as a carbon coating layer covering the core. After that, the atmosphere was switched to an inert atmosphere (e.g., a nitrogen gas atmosphere, but not limited thereto), and the temperature was lowered to room temperature to finally obtain the silicon-carbon anode material. The alkane mixed gas contained 5% to 100% by mass of alkanes (e.g., acetylene, but not limited thereto) and 0% to 95% by mass of an inert gas (e.g., argon gas, but not limited thereto).

[0059] Example 1 Preparation of the negative electrode piece: 1) Preparation of silicon-carbon anode material: Linear phenolic resin (RF), urotropin (HMT), and carbon nanotubes (CNTs) were mixed in a weight ratio (see Table 1) to form a mixture. The mixing method was a ball mill, with a ball mill rotation speed of 500 r / min and a ball milling time of 6 hours. The ball-milled mixture, which was dark in color, was heated to 130°C and held at this temperature for 10 hours to cure. The cured mixture was transferred to a box-type furnace, heated to 900°C under a nitrogen atmosphere for 2 hours to carbonize, then cooled, and finally transferred to a rotary kiln furnace and activated under a carbon dioxide atmosphere for 9 hours to obtain a porous carbon skeleton. A porous carbon skeleton was heated to 500°C at a rate of 2°C / min under an argon atmosphere. Then, the atmosphere was switched to a silane gas mixture (20% silane and 80% argon by mass percentage), and deposition was carried out for 10 hours at 500°C to form a core. The atmosphere was then switched to an acetylene gas mixture (20% acetylene and 80% argon by mass percentage), and deposition continued for another 10 hours at 500°C. Finally, the atmosphere was switched to nitrogen gas, and the temperature was lowered to room temperature of 25°C to obtain a silicon-carbon anode material.

[0060] 2) A mixture of graphite and the silicon-carbon anode material in a weight ratio of 80:20 was prepared as an anode active material. This anode active material, styrene-butadiene rubber (SBP), and sodium carboxymethylcellulose (CMC) were mixed with an appropriate amount of deionized water in a weight ratio of 97:2:1 and thoroughly stirred to obtain a homogeneous anode slurry. The solid content of the anode slurry was 40 wt%. This slurry was coated onto a copper foil anode current collector, dried at 85°C, then cold-pressed, cut, and slit, and finally dried under vacuum conditions at 120°C for 12 hours to obtain an anode piece.

[0061] Preparation of positive electrode pieces: Lithium cobalt oxide (LiCoO2), the positive electrode active material, Super P, a conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed thoroughly with an appropriate amount of N-methylpyrrolidone (NMP) solvent in a weight ratio of 97:1.4:1.6 to obtain a homogeneous positive electrode slurry, the solid content of which was 72 wt%. This slurry was coated onto aluminum foil, which was the positive electrode current collector, dried at 85°C, then cold-pressed, cut, and slit, and finally dried under vacuum conditions at 85°C for 4 hours to obtain positive electrode pieces.

[0062] Preparation of electrolyte: In a glove box under a dry argon atmosphere, ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed in a mass ratio of EC:EMC:DEC = 30:50:20. Then, LiPF6, a lithium salt, was added and uniformly mixed to obtain the electrolyte. In the electrolyte, the mass percentage of LiPF6 was 12.5%.

[0063] Separator preparation: A 7 μm thick porous polyethylene (PE) polymer film was used as the separator. Preparation of lithium-ion battery: The positive electrode piece, the separator, and the negative electrode piece described above were stacked in this order, wound up, and tabs were welded together before being placed in an aluminum plastic film packaging bag. Subsequently, the electrolyte was injected, and after processes such as vacuum sealing, standing, formation, trimming, and capacity measurement were performed to obtain a soft-pack lithium-ion battery. The specific preparation steps for the lithium-ion batteries in Examples 2-12 and Comparative Examples 1-3 refer to Example 1, and the differences can be seen in Table 1.

[0064] [Table 1]

[0065] For the button batteries formed from the negative electrode pieces in each of the above examples and comparative examples, the initial efficiency of the lithium-ion battery was measured, and the specific measurement method is as follows: Take the single-sided coated negative electrode piece prepared in the corresponding example or comparative example, and measure the area of ​​1.54 cm². 2 A button cell was obtained by cutting the material into pieces, using the working electrode, then using a lithium sheet as the counter electrode, a porous polyethylene film as the separator, injecting the electrolyte, and assembling the components. The button cell was first discharged to 0V with three small currents of 0.05C / 50μA / 20μA, and the initial discharge capacity of the button cell was recorded. Then, it was charged to 2.0V with a constant current of 0.1C, and the initial charge capacity of the button cell was recorded. The initial efficiency was calculated as: initial charge capacity / initial discharge capacity × 100%. The initial reversible gram capacity of the negative electrode active material at 0V to 2.0V was calculated as: initial charge capacity of the button cell / mass of the negative electrode active material. The electrolyte contained 12.5% ​​by mass of lithium salt LiPF6, and the solvent was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a mass ratio of 1:1. The initial efficiencies of the button cells corresponding to each measured example and comparative example are shown in Table 2.

[0066] [Table 2]

[0067] Linear scanning elemental analysis was performed on the silicon-carbon anode materials of each of the above examples and comparative examples using a scanning electron microscope. The specific method is as follows: After cutting a cross-section of the anode piece prepared in the corresponding example or comparative example by ion polishing, the cut cross-section of the silicon-carbon anode material was used as the measurement surface of the test sample, and it was transferred to a field emission scanning electron microscope. After focusing, measurements were taken. During measurement, linear scanning was performed from the center of the particle outward to measure the silicon element in the silicon-carbon anode material, and the distribution of silicon element content was recorded. Subsequently, analysis of variance was performed on the scanned values ​​to obtain the standard deviation, which is shown in Table 2.

[0068] For each of the above examples and comparative examples of silicon-carbon anode materials, the silicon element content and the carbon nanotube content were measured. The silicon content in the silicon-carbon anode material was measured using ICP (Inductively Coupled Plasma Atomic Emission Spectrometer) characterization.

[0069] The specific method for measuring the carbon nanotube content was as follows. First, the yield in the RF+HMT mixing / curing / carbonization / and activation process was calculated from the total amount of RF and HMT added in Comparative Example 1 and the weight of the resulting porous carbon skeleton, and was set to d%. Then, the yield in the RF+HMT+CNTs mixing / curing / carbonization / and activation process in other examples was recorded and set to e%. In this case, the proportion of CNTs in the porous carbon skeleton of the example was f%=(ed) / e×100%, and the remaining porous carbon skeleton component was g%=1-f%, so the CNTs content in the silicon carbon anode material of the example was h%=f / (f+g+b)×100%, as shown in Table 2, and the content of the remaining porous carbon skeleton component was i%=1-h%-b%.

[0070] The electrical conductivity of the silicon-carbon anode material in each example and comparative example was measured, and the specific measurement method was as follows: The electrical conductivity of the silicon-carbon anode material powder was measured using a powder electrical conductivity meter (model: FT-8100), and the standard GB / T1552-1995 was referenced based on the four-point probe detection principle. A known amount of silicon-carbon anode material powder was used, and the volume was compressed to a set pressure value or pressure intensity under hydraulic power. The electrical conductivity of the silicon-carbon anode material powder was measured online, and the data was recorded in Table 2.

[0071] The particle strength of the silicon-carbon anode material in each example and comparative example was measured, and the specific measurement method was as follows: The hardness and elastic modulus of a single particle of the silicon-carbon anode material were measured using a nanoindenter (model: Hysitron TI 950), and the measurement standard was JB / T 12721-2016. Before measurement, the silicon-carbon anode material powder was dispersed in epoxy resin and cured. The cured resin was cut by ion polishing, and a single particle was pressed using a nanoprobe. The depth of the indentation on the particle surface was monitored and converted to obtain the elastic modulus of the particle. For the same sample, the elastic modulus of five particles was measured in parallel, and the average value was taken to obtain the elastic modulus of the silicon-carbon anode material particles, which is listed in Table 2.

[0072] For each example and comparative example of soft-pack lithium-ion batteries, cycle characteristics and the full-charge expansion rate of the batteries were measured.

[0073] The specific method for measuring the cycle characteristics is as follows. Charge at a constant current up to 4.4 V at 0.7 C, charge at a constant voltage up to 0.025 C, let it stand for 5 minutes, and then discharge at 0.5 C down to 3.0 V. The capacity obtained in this step was taken as the initial capacity, and 0.7 C charging / 0.5 C discharging was performed to conduct a cycle test. The ratio of the capacity of each step to the initial capacity was calculated to obtain the capacity decay curve. The number of cycles until the capacity retention rate reached 90% at 25 °C (described in Table 2) was taken as the cycle characteristics of the battery at room temperature, and the number of cycles until the capacity retention rate reached 80% at 45 °C (described in Table 2) was taken as the cycle characteristics of the battery at high temperature. By comparing the number of cycles in the above two cases, the cycle characteristics of the material were compared.

[0074] The specific method for measuring the full charge expansion rate of the battery is as follows. Measure the thickness of a new soft-pack lithium-ion battery at half charge (50% state of charge (SOC)) with a spiral micrometer. When it has been cycled up to 400 times, the battery is in the full charge (100% SOC) state. At this time, measure the thickness of the battery again with a spiral micrometer and compare it with the thickness of the new battery at the initial half charge (50% SOC) to obtain the expansion rate of the full charge (100% SOC) battery at this time and record it in Table 2.

[0075] Measurement of particle size: Add about 0.02 g of a sample of silicon-carbon anode material powder to a 50 ml clean beaker, add about 20 ml of deionized water, and further drop 3 drops of 1% surfactant to completely disperse the silicon-carbon anode material powder in water. Perform ultrasonic treatment with a 120 W ultrasonic cleaner for 5 minutes and measure the particle size with a MasterSizer 2000.

[0076] Raman test Place the silicon-carbon anode material on a flat slide glass and perform a Raman test with a test range of 100 cm -1 ~1200 cm -1 and after the test, 450 cm -1 ~550 cm -1Focusing on characteristic peaks within the range, the point of maximum peak value was defined as the peak position of this peak, and half of the difference in the horizontal coordinates at half the peak value was defined as the full width at half maximum of this peak.

[0077] XRD measurement The silicon-carbon anode material was placed on a sample stand, and XRD measurements were performed on the powder. The measurement range was set to 10° to 90°, and the scanning speed was set to 5° / min. After the measurement was completed, a characteristic peak in the 15° to 35° range was noted, and the point of maximum peak value was defined as the peak position of this peak. Half of the difference in the horizontal coordinate at half the peak value was defined as the full width at half maximum of this peak.

[0078] Methods for measuring / testing pore volume The pore volume of the silicon-carbon anode material was measured using the N2 gas adsorption method. After obtaining adsorption / desorption data, the NRDFT model was fitted to the pore structure to obtain pore volume data for sizes smaller than 2 nm and pore volume data for sizes larger than 2 nm, respectively.

[0079] From the data in Tables 1 and 2, particularly when comparing Comparative Example 1 with the other examples, it can be seen that adding carbon nanotubes improves the strength of the silicon-carbon anode material, thereby improving its ability to buffer the volume expansion of the battery and further improving the battery's cycle characteristics. In addition, adding carbon nanotubes also improves the electrical conductivity of the silicon-carbon anode material, improving its electron diffusion ability and reducing the probability of lithium ions being trapped inside the silicon-carbon, thereby improving its initial efficiency and energy density. If the carbon nanotube content is too high, it tends to affect the initial charging efficiency. Specifically, if the carbon nanotube content is too high, it forms pores when forming the porous carbon skeleton, resulting in an excessively high proportion of micropores, making it difficult for silicon to deposit thereafter. This increases the specific surface area of ​​the final product, which increases SEI film formation in the electrochemical apparatus and further affects the initial charging efficiency. Similarly, the low silicon content in Comparative Example 2 resulted in insufficient pore filling in the porous carbon framework, increasing the specific surface area of ​​the final product, which in turn increased SEI film formation in the electrochemical apparatus and further affected the initial charge efficiency. Comparing Comparative Example 3 with the other examples, it can be seen that if the silicon content in the silicon-carbon anode material is too high, the binding ability of carbon nanotubes is insufficient, leading to severe battery swelling and poor cycle characteristics.

[0080] The embodiments disclosed herein are merely preferred embodiments and, of course, do not limit the invention thereto. Accordingly, equivalent modifications made based on the invention still fall within the scope of the invention.

Claims

1. A silicon-carbon anode material comprising a core and a shell, The core comprises a porous carbon skeleton and silicon dispersed within the pores of the porous carbon skeleton, wherein carbon nanotubes are encapsulated and dispersed within the porous carbon skeleton, and in the silicon-carbon anode material, the silicon element content in the silicon-carbon anode material is 25 wt% to 55 wt%. The aforementioned shell contains carbon material, A silicon-carbon anode material wherein, in the energy spectrum of the cross-section of the silicon-carbon anode material obtained by a linear scanning electron microscope, the standard deviation of the change in the silicon element content along the direction from the center to the edge of the cross-section is 200 or less.

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

3. The silicon-carbon anode material according to claim 1 or 2, wherein the carbon nanotube content is 0.2 wt% to 7 wt%.

4. The silicon-carbon anode material is (1) The X-ray diffraction pattern of the silicon-carbon anode material has one characteristic peak in the range of 20° to 30°, and the full width at half maximum of the characteristic peak is greater than 2°, (2) In the Raman spectrum of the silicon-carbon anode material, at 450 cm⁻¹ －１ ~500cm －１ It has one characteristic peak within the range, (3) The particle size Dv50 of the silicon-carbon anode material is 3 μm to 20 μm, and the particle size Dv99 of the silicon-carbon anode material is 3 μm to 20 μm. A silicon-carbon anode material according to any one of claims 1 to 3, satisfying at least one of the following conditions.

5. The silicon-carbon anode material according to any one of claims 1 to 4, wherein the pore volume of pores with a pore diameter greater than 2 nm in the silicon-carbon anode material is greater than the pore volume of pores with a pore diameter of 2 nm or less.

6. An electrochemical apparatus comprising a positive electrode piece, a negative electrode piece, and a separator, The negative electrode piece includes a negative electrode current collector and a negative electrode active layer. The negative electrode active layer contains a negative electrode active material, An electrochemical apparatus comprising the anode active material described in any one of claims 1 to 5, wherein the anode active material includes the silicon-carbon anode material described in any one of claims 1 to 5.

7. The negative electrode active material further contains graphite, The electrochemical apparatus according to claim 6, wherein the content of the silicon-carbon anode material in the anode active material is 5 wt% to 40 wt%, and the content of the graphite is 95 wt% to 60 wt%.

8. A method for preparing a silicon-carbon anode material according to claim 1, A step of mixing resin and carbon nanotubes to form a mixture, and curing the mixture, The steps include carbonizing the hardened mixture described above, activating it to obtain a porous carbon skeleton, and A method for preparing a silicon-carbon anode material, comprising the steps of depositing silane onto the porous carbon skeleton to form a core, and then forming a shell with an alkane.

9. The carbonization conditions described above involve raising the temperature to 700°C to 1100°C and maintaining the temperature for 1 to 5 hours. The method for preparing a silicon-carbon anode material according to claim 8, wherein the activation is performed after cooling down following carbonization using carbon dioxide, water vapor, sodium hydroxide, potassium hydroxide, or phosphoric acid.

10. The step of "depositing silane onto the porous carbon skeleton to form a core, and then forming a shell with an alkane" is as follows: The porous carbon skeleton is gradually heated to 400°C to 600°C under an inert atmosphere and then maintained at that temperature. After that, the atmosphere is switched to a silane mixed gas, and the skeleton is deposited for 1 to 20 hours. The silane mixed gas contains 2% to 20% silane and 80% to 98% inert gas by mass percentage. Furthermore, the atmosphere is switched to an alkane mixed gas at a temperature of 500°C to 1000°C and held for 10 hours to form a shell. After that, the atmosphere is switched to an inert atmosphere and cooled to room temperature. The alkane mixed gas contains 5% to 100% alkanes and 0% to 95% inert gas by mass percentage. A method for preparing a silicon-carbon anode material according to claim 8 or 9.

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