Silicon-carbon negative electrode material, and preparation method therefor and use thereof

Abstract

A silicon-carbon negative electrode material, which comprises a core and a shell, wherein the core comprises a porous carbon skeleton and silicon, which is dispersed in pores of the porous carbon skeleton. Carbon nanotubes are wrapped in and dispersed in the porous carbon skeleton; the content of silicon element in the silicon-carbon negative electrode material is 25 to 55 wt%; and the shell comprises a carbon material. In a linear scanning electron microscope energy spectrum of a cross section of the silicon-carbon negative electrode material, the standard deviation of the content change of silicon element does not exceed 200 in the direction from the center to the edge of the cross section. Provided are an electrochemical device and a preparation method for the silicon-carbon negative electrode material.

Description

Silicon-carbon negative electrode material and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 29, 2023, with application number 202310786717.2 and invention name “Silicon-carbon negative electrode material, preparation method and application thereof”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of electrochemical energy storage, and in particular to a silicon-carbon negative electrode material and a preparation method thereof, a negative electrode plate using the silicon-carbon negative electrode material, and an electrochemical device using the negative electrode plate. Background Art

[0003] Lithium-ion batteries, with their advantages of high volumetric and mass energy density, environmental friendliness, high operating voltage, compact size, light weight, and long cycle life, are widely used in portable consumer electronics. With the rapid development of electric vehicles and mobile electronic devices in recent years, people are increasingly demanding higher standards for battery energy density, safety, and cycle performance. They are eagerly awaiting the emergence of new lithium-ion batteries with comprehensively improved performance. Energy density and cycle performance have become key technical challenges that need to be addressed urgently, and improving the active materials in electrodes is one research direction to address these issues.

[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 commercial graphite has been nearly fully exploited, and its low capacity and safety concerns associated with lithium dendrites hinder its further application. Compared to graphite as an anode material, elemental silicon is considered the most promising lithium battery anode material to replace graphite due to its ultra-high theoretical specific capacity and suitable operating voltage. However, its low electrical conductivity and significant volume expansion during the alloying / de-alloying process severely restrict its large-scale application in lithium-ion batteries.

[0005] Summary of the Invention

[0006] The present application provides a silicon-carbon negative electrode material capable of improving conductivity and reducing expansion.

[0007] In addition, the present application also provides a negative electrode sheet using the silicon-carbon negative electrode material and an electrochemical device using the negative electrode sheet. The present application also provides a method for preparing the above-mentioned silicon-carbon negative electrode material.

[0008] In a first aspect, the present application provides a silicon-carbon negative electrode 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-carbon negative electrode material comprises a silicon content of 25 wt% to 55 wt%, and the shell comprises a carbon material. In a linear scanning electron microscope energy spectrum of a cross-section of the carbon-silicon negative electrode material, the standard deviation of the variation in silicon content along the direction from the center to the edge of the cross-section does not exceed 200.

[0009] The silicon-carbon negative electrode material of the present application has good electrical conductivity and mechanical properties due to the carbon nanotubes therein. Therefore, the specific content of carbon nanotubes dispersed in the porous carbon skeleton can, on the one hand, improve the electrical conductivity of the silicon-carbon negative electrode material, and on the other hand, improve the mechanical properties of the silicon-carbon negative electrode material and restrain the expansion of the silicon-carbon negative electrode material, which is beneficial to improving the stability of the structure. According to the linear scanning electron microscope energy spectrum of the cross section of the carbon-silicon negative electrode material, it can be seen that the silicon element is uniformly distributed in the silicon-carbon negative electrode material, which is beneficial to further improve the electrical conductivity and mechanical properties of the silicon-carbon negative electrode material. When the above-mentioned silicon-carbon negative electrode material is used in the negative electrode sheet of the electrochemical device, the improvement of the electrical conductivity of the above-mentioned silicon-carbon negative electrode material 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 improving the initial charge and discharge performance. At the same time, the carbon nanotubes can also alleviate the volume expansion of silicon-carbon during the lithium insertion process, improve the expansion pulverization and structural stability of silicon-carbon during charging and discharging, thereby improving its cycle performance.

[0010] Based on the first aspect, in some possible embodiments, the electrical conductivity of the silicon-carbon negative electrode material is 9 S / cm to 30 S / cm. In the above possible embodiments, the electrical conductivity within the specific range is conducive to ensuring the electrical conductivity of the silicon-carbon negative electrode material and further improving the cycle performance of the electrochemical device using the silicon-carbon negative electrode material.

[0011] Based on the first aspect, in some possible embodiments, the particle elastic modulus of the silicon-carbon negative electrode material is 4 GPa to 10 GPa. The particle elastic modulus within a specific range is beneficial to improving the stability of the silicon-carbon negative electrode material structure, and is beneficial to further improving the expansion and cycle performance of the electrochemical device using the silicon-carbon negative electrode material.

[0012] Based on the first aspect, in the carbon-silicon negative electrode material, the content of the carbon nanotubes is 0.2wt% to 7.0wt%. The carbon nanotube content within this range can further alleviate the volume expansion of silicon-carbon during the lithium insertion process, and can control the formation of pores formed during the preparation of the porous carbon skeleton, reduce the proportion of macropores, improve the deposition uniformity of silicon, further improve the cycle performance of the electrochemical device and reduce the cycle expansion rate of the electrochemical device.

[0013] Based on the first aspect, in some possible embodiments, the silicon-carbon negative electrode material satisfies at least one of the following conditions: (1) the X-ray diffraction spectrum of the silicon-carbon negative electrode material has a characteristic peak in the range of 20° to 30°, and the half-maximum width of the characteristic peak is greater than 2°; (2) the Raman spectrum of the silicon-carbon negative electrode material has a characteristic peak at 450 cm -1 Up to 500cm -1 (3) the particle size Dv50 of the silicon-carbon negative electrode material is 3 μm to 20 μm, and the particle size Dv99 of the silicon-carbon negative electrode material is 3 μm to 20 μm.

[0014] In the above possible implementation, the characteristics of the X-ray diffraction spectrum of the silicon-carbon negative electrode material show that the pores in the porous carbon skeleton are micropores, that is, 90% of the pores in the porous carbon skeleton have a pore size of less than 2 nanometers, that is, it is beneficial to make the size of the silicon embedded in the porous carbon skeleton in the silicon-carbon negative electrode material mostly less than 2 nanometers, thereby helping to further reduce the expansion of the silicon-carbon negative electrode material. The characteristics of the Raman spectrum of the silicon-carbon negative electrode material show that the size of the silicon in the silicon-carbon negative electrode material is small and amorphous, which is beneficial to further reduce the expansion of the silicon-carbon negative electrode material. The silicon-carbon negative electrode material in a specific particle size range is smoother in the subsequent stirring and coating process for preparing the negative electrode sheet, and is convenient for matching with graphite.

[0015] Based on the first aspect, in some possible embodiments, the pore volume of pores with a pore diameter exceeding 2 nm in the silicon-carbon negative electrode material is greater than the pore volume of pores with a pore diameter not exceeding 2 nm. In some possible embodiments, the pore volume of pores with a pore diameter exceeding 2 nm ranges from 0.04 to 0.20. In the above possible embodiments, since the adsorption effect of micropores is better, silicon is preferentially adsorbed and deposited in the micropores. Combined with the premise that 90% of the pores in the porous carbon skeleton have a pore diameter of less than 2 nanometers, it can be seen that the size of silicon in the silicon-carbon negative electrode material is mostly less than 2 nanometers, which is conducive to further reducing the expansion of the silicon-carbon negative electrode material.

[0016] A second aspect of the present application provides a negative electrode plate, comprising a current collector and a negative electrode active layer, wherein the negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon negative electrode material described above.

[0017] The negative electrode plate of the present 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 lithium removal capacity of the electrochemical device and improving the initial charge and discharge performance. At the same time, the carbon nanotubes can also alleviate the volume expansion of silicon-carbon during the lithium insertion process, combined with the uniform distribution of silicon elements to improve the expansion and pulverization and structural stability of silicon-carbon during charging and discharging, thereby improving its cycle performance.

[0018] Based on the second aspect, in some possible implementations, the negative electrode active material further includes graphite. In the negative electrode active material, 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 %.

[0019] In the above possible implementation manner, the silicon-carbon negative electrode material with the above specific content can effectively reduce the effect of silicon expansion on the negative electrode plate while fully utilizing the characteristics of silicon such as ultra-high theoretical specific capacity and suitable operating voltage, which is beneficial to improving the first coulombic efficiency, energy density and cycle performance of the negative electrode plate. Specifically, if the silicon-carbon negative electrode material is too high, the volume expansion of the negative electrode active layer will be obvious, and the cycle performance will deteriorate; if the silicon-carbon negative electrode material is too low, it will not be conducive to improving the first coulombic efficiency. Since graphite has a certain degree of flexibility, its combination with the silicon-carbon negative electrode material can alleviate the volume expansion of the negative electrode active layer. In addition, the negative electrode plate can also fully utilize the advantages of both silicon-carbon negative electrode material and graphite to achieve better electrochemical performance.

[0020] A third aspect of the present application provides an electrochemical device comprising the negative electrode sheet as described above.

[0021] The electrochemical device of the present application improves the conductivity of the silicon-carbon negative electrode material by uniformly distributing carbon nanotubes and silicon elements in the negative electrode sheet, 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 initial charge and discharge performance. At the same time, the carbon nanotubes can also alleviate the volume expansion of silicon-carbon during the lithium insertion process, and combined with the uniform distribution of silicon elements, improve the expansion and pulverization and structural stability of silicon-carbon during charging and discharging, thereby improving its cycle performance.

[0022] The fifth aspect of the present application provides a method for preparing the silicon-carbon negative electrode material as described above, comprising: mixing a resin and carbon nanotubes to form a mixture and curing the mixture; carbonizing the cured mixture and 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 through alkanes.

[0023] The preparation method of the above-mentioned silicon-carbon negative electrode material of the present application is to first mix the resin and carbon nanotubes to form a mixture, solidify it, and then carbonize it to form a porous skeleton, which is convenient for the dispersion of the carbon nanotubes in the porous skeleton, thereby helping to enhance the binding of the carbon nanotubes on the prepared silicon-carbon negative electrode material, and then alleviate the volume expansion of the silicon-carbon negative electrode material to improve the expansion pulverization and structural stability of silicon-carbon during charging and discharging, and improve its cycle performance; at the same time, it is also beneficial to improve the overall conductivity of the silicon-carbon negative electrode material, and then improve the probability of lithium being captured inside the silicon-carbon negative electrode material to improve the delithiation capacity of the electrochemical device and improve the initial charge and discharge performance.

[0024] Based on the second aspect, in some possible embodiments, the carbonization conditions are heating to 700°C to 1100°C and keeping warm for 1 to 5 hours, and the activation is specifically activated by carbon dioxide, water vapor, sodium hydroxide, potassium hydroxide or phosphoric acid after carbonization cooling.

[0025] Based on the second aspect, in some possible embodiments, the step of "depositing silane on the porous carbon skeleton to form a core and then forming a shell through alkanes" is specifically: slowly heating the porous carbon skeleton to 400°C to 600°C under an inert atmosphere and then keeping it warm, and then switching the atmosphere to a silane mixture for deposition for 1 to 20 hours, wherein the silane mixture contains 2% to 20% silane by mass and 80% to 98% inert gas; at a temperature of 500°C to 1000°C, the atmosphere is switched to an alkane mixture for 10 hours to form a shell, and then the atmosphere is switched to an inert atmosphere and cooled to room temperature, wherein the alkane mixture contains 5% to 100% alkanes by mass and 95% to 0% inert gas. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application are described clearly and in detail below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present application. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0027] Below, embodiments of the present application will be described in detail. However, the present application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments illustrated herein. Rather, these exemplary embodiments are provided to make the present application thorough and detailed for those skilled in the art.

[0028] Further, when describing embodiments of the present application, the use of “may” refers to “one or more embodiments of the present application.”

[0029] The technical terms used herein are for the purpose of describing specific embodiments and are not intended to limit this application. As used herein, the singular form is intended to also include the plural form, unless the context clearly indicates otherwise. It should be further understood that the term "comprising", when used in this specification, refers to 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 example, 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.

[0030] In this application, the relationship between parameter values ​​that is greater than, less than, or not equal to the design relationship needs to exclude the reasonable error of the measuring equipment.

[0031] One embodiment of the present application provides an electrochemical device comprising a positive electrode sheet, a negative electrode sheet, and a separator. The separator is disposed between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet, the separator, and the negative electrode sheet may be alternately stacked in sequence to form a laminated electrode assembly, or the positive electrode sheet, the separator, and the negative electrode sheet may be stacked in sequence and then wound to form a wound electrode assembly.

[0032] The electrochemical device further includes a housing and an electrolyte. The positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte are housed within the housing. The housing can be a packaging bag formed using a packaging film, such as, but not limited to, an aluminum-plastic film. That is, the electrochemical device can be a soft-pack battery. The housing can also be, but is not limited to, those disclosed in the prior art, such as steel-cased batteries and aluminum-cased batteries.

[0033] The positive electrode plate includes a positive electrode current collector and a positive electrode active layer provided on the positive electrode current collector. The positive electrode current collector can be made of aluminum foil or nickel foil, etc., or it can be any composite current collector disclosed in the prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive electrode active layer contains a positive electrode active material, and the positive electrode active material includes a compound that can reversibly embed and deintercalate lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese and nickel. In some embodiments, the positive electrode active material may include but is not limited to 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 O4) or at least one of lithium iron phosphate (LiFePO4).

[0034] The positive electrode active layer further comprises a binder for bonding the positive electrode active material particles to facilitate the formation of a film layer, while also being able to improve the binding force between the positive electrode active layer and the positive electrode current collector.

[0035] In some embodiments, the binder 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, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon.

[0036] The positive electrode active layer may further comprise a conductive material, including but not limited to a carbon-based material, a metal-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material 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 fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0037] The negative electrode plate includes a negative electrode current collector and a negative electrode active layer disposed on the negative electrode current collector. The negative electrode current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector, 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 negative electrode active layer includes a negative electrode active material, which includes a silicon-carbon negative electrode material.

[0038] The silicon-carbon negative electrode material includes a core and a shell, wherein the core includes a porous carbon skeleton and silicon dispersed in the pores of the porous carbon skeleton. Carbon nanotubes are wrapped and dispersed in the porous carbon skeleton. In the carbon-silicon negative electrode material, the content of silicon in the silicon-carbon negative electrode material is 25wt% to 55wt%, and the shell includes a carbon material. In the linear scanning electron microscope energy spectrum of the cross section of the carbon-silicon negative electrode material, the standard deviation of the content change of the silicon element along the direction from the center to the edge of the cross section does not exceed 200. Specifically, the silicon-carbon negative electrode material particles are cut out of the cross section as a test surface by ion polishing. The cut silicon-carbon negative electrode material is transferred to a field emission scanning electron microscope and the test surface is scanned from the center to the outside to record the content distribution of each element. For example, 100 data points are taken at equal distances from the silicon element content data obtained by the line scan, and the 100 data points are subjected to mathematical statistical standard deviation analysis to obtain the standard deviation of the silicon element content change.

[0039] The above-mentioned silicon-carbon negative electrode material has good electrical conductivity and mechanical properties due to the carbon nanotubes therein. Therefore, the specific content of carbon nanotubes dispersed in the porous carbon skeleton can, on the one hand, improve the electrical conductivity of the silicon-carbon negative electrode material, and on the other hand, improve the mechanical properties of the silicon-carbon negative electrode material and restrain the expansion of the silicon-carbon negative electrode material, which is beneficial to improving the stability of the structure. According to the linear scanning electron microscope energy spectrum of the cross-section of the carbon-silicon negative electrode material, it can be seen that the silicon element is uniformly distributed in the silicon-carbon negative electrode material. The dispersion of carbon nanotubes and the uniform distribution of silicon elements can improve the conductivity and strength of the silicon-carbon negative electrode material, reduce the capture of lithium in the silicon-carbon negative electrode material, alleviate the expansion of the silicon-carbon negative electrode material, and improve the first coulomb efficiency of the electrochemical device using the negative electrode plate, thereby improving the energy density and cycle performance of the electrochemical device.

[0040] In some embodiments, the silicon content is 20% to 45%, which can further reduce the expansion of the silicon-carbon negative electrode material and improve the cycle performance of the electrochemical device.

[0041] In some embodiments, the electrical conductivity of the silicon-carbon negative electrode material is 9 S / cm to 30 S / cm. In some embodiments, the electrical conductivity of the silicon-carbon negative electrode material is 11 S / cm to 25 S / cm. In some embodiments, the electrical conductivity of the silicon-carbon negative electrode material is 9 S / cm, 11 S / cm, 14 S / cm, 20 S / cm, 25 S / cm, 30 S / cm, or a value within a range formed by any two of the foregoing values. When the electrical conductivity of the silicon-carbon negative electrode material is within the foregoing range, the electrical conductivity of the silicon-carbon negative electrode material can be ensured, further improving the cycling performance of the electrochemical device.

[0042] In some embodiments, the particle elastic modulus of the silicon-carbon negative electrode material is 4 GPa to 10 GPa, which is beneficial to ensure the mechanical strength of the silicon-carbon negative electrode material to relieve internal stress, further reduce the expansion of the silicon-carbon negative electrode material and improve the cycle performance of the electrochemical device.

[0043] In some embodiments, in the carbon-silicon negative electrode material, the content of the carbon nanotubes is 0.2wt% to 7.0wt%. In some embodiments, the content of the carbon nanotubes is 1.0wt% to 6.0wt%. In some embodiments, the content of the carbon nanotubes is 0.2wt%, 1.0wt%, 1.5wt%, 2.5wt%, 4.5wt%, 6.0wt%, 6.5wt%, 7.0wt% or a value within the range formed by any two of the above values. The carbon nanotube content within the above range helps to further alleviate the volume expansion of silicon-carbon during the lithium insertion process, and can control the formation of pores formed during the preparation of the porous carbon skeleton, reduce the proportion of macropores, improve the deposition uniformity of silicon, ensure the first efficiency of the electrochemical device (i.e., the first coulombic efficiency), further improve the cycle performance of the electrochemical device and reduce the cycle expansion rate of the electrochemical device.

[0044] In some embodiments, the X-ray diffraction pattern of the silicon-carbon anode material has a characteristic peak within the range of 20° to 30°, and the half-maximum width of the characteristic peak is greater than 2°, indicating that the pores in the porous carbon skeleton are micropores, and 90% of the pores in the porous carbon skeleton have a pore diameter of less than 2 nanometers. In other words, the size of the silicon embedded in the porous carbon skeleton in the silicon-carbon anode material is mostly less than 2 nanometers, thereby further reducing the expansion of the silicon-carbon anode material.

[0045] Furthermore, the pore volume of pores with a pore diameter exceeding 2 nm in the silicon-carbon negative electrode material is greater than the pore volume of pores with a pore diameter not exceeding 2 nm. Since micropores have a better adsorption effect, silicon is preferentially adsorbed and deposited in micropores. Therefore, the size of silicon in the silicon-carbon negative electrode material is mostly less than 2 nanometers, which helps further reduce the expansion of the silicon-carbon negative electrode material. In some embodiments, the pore volume of pores with a pore diameter exceeding 2 nm ranges from 0.04 to 0.20.

[0046] In some embodiments, the Raman spectrum of the silicon-carbon negative electrode material is at 450 cm -1 Up to 500cm -1 There is a characteristic peak within the range of , that is, the size of silicon in the silicon-carbon negative electrode material is small and amorphous, which is beneficial to further reduce the expansion of the silicon-carbon negative electrode material.

[0047] The particle size Dv50 of the silicon-carbon negative electrode material can be 3μm to 20μm, and the particle size Dv99 of the silicon-carbon negative electrode material can be 3μm to 20μm, so that it can be smoother when subsequently mixed, stirred and coated with other materials (such as binders) to form a negative electrode active layer, and it is easy to mix and match other materials.

[0048] The negative electrode active material may further include graphite. Due to its flexibility, graphite, when combined with the silicon-carbon negative electrode material, can mitigate volume expansion of the negative electrode active layer. Simultaneously, using both graphite and the silicon-carbon negative electrode material as active materials helps reduce overall expansion of the negative electrode active layer and leverages the advantages of both the silicon-carbon negative electrode material and graphite to achieve superior electrochemical performance.

[0049] In the negative electrode active material, the content of the silicon-carbon negative electrode material may be 5 wt % to 40 wt %. Furthermore, the content of the graphite may be 95 wt % to 60 wt %.

[0050] The negative electrode active layer further comprises a binder for binding the positive electrode active material particles to facilitate the formation of a film layer, while also being able to improve the binding force between the negative electrode active layer and the negative electrode current collector.

[0051] In some embodiments, the binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon, etc.

[0052] The negative electrode active layer may further include a conductive material, including but 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 fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0053] The separator includes a porous membrane layer, 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 can be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane.

[0054] The electrolyte may be in one or more of a gel state, a solid state, and a liquid state. In some embodiments, the liquid electrolyte comprises a lithium salt and an organic solvent. The lithium salt can 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 bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethylsulfonyl)imide (LiN(SO2CF3)2, tris(trifluoromethylsulfonyl)methyl lithium (LiC(SO2CF3)3), lithium bisoxalatoborate (LiBOB) and lithium difluorophosphate (LiPO2F2). For example, the lithium salt is selected from LiPF6 because it can give high ionic conductivity and improve cycle characteristics. The organic solvent can be a carbonate compound, a carboxylate compound, an ether compound, 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), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene 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.

[0055] The electrochemical device is applied to an electronic device to power other electronic components in the electronic device. Since the silicon-carbon negative electrode material in the electrochemical device is conducive to improving cycle performance and energy density, it is conducive to improving the service life of the electronic device. The electronic device may include but is not limited to a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium-ion capacitor, etc.

[0056] The present application also provides a method for preparing a silicon-carbon negative electrode material, which comprises the following steps:

[0057] Step S1: mixing resin and carbon nanotubes to form a mixture and curing the mixture.

[0058] Methods for forming the mixture include, but are not limited to, ball milling. An organic solvent may also be added during mixing to facilitate dispersion of the carbon nanotubes. The aforementioned curing step involves drying and shaping the mixture. Common curing conditions in the prior art can be employed and are not detailed here.

[0059] Step S2, carbonizing and activating the solidified mixture to obtain a porous carbon skeleton.

[0060] Specifically, the solidified mixture is heated to 700° C. to 1100° C. under an inert atmosphere (such as, but not limited to, a nitrogen atmosphere) for carbonization for 1 to 5 hours, and then activated to form a porous carbon skeleton after cooling to room temperature. The activation can be performed by carbonizing the structure with carbon dioxide or water vapor, or by alkali etching or acid etching. The alkali etching can typically be performed with sodium hydroxide or potassium hydroxide, and the acid etching can typically be performed with phosphoric acid.

[0061] Step S3, performing silane deposition on the porous carbon skeleton to form a core body, and then coating the core body with a shell formed by forming a carbon coating layer using an alkane.

[0062] Specifically, the porous carbon framework is slowly heated to 400°C to 600°C under an inert atmosphere (such as, but not limited to, argon), and then the atmosphere is switched to a silane gas mixture for deposition for 1 to 20 hours to form a nucleus. The heating rate of the slow heating may be, but is not limited to, 0.5°C / min to 5°C / min, and the silane gas mixture comprises 2% to 20% by weight of silane and 80% to 98% by weight of an inert gas (such as, but not limited to, argon).

[0063] The atmosphere is then switched to an alkane gas mixture at a temperature of 500° C. to 1000° C. and maintained for 2 to 20 hours to form a shell of the carbon coating layer covering the core body. The atmosphere is then switched to an inert atmosphere (such as, but not limited to, a nitrogen atmosphere) and cooled to room temperature to finally obtain the silicon-carbon negative electrode material. The alkane gas mixture comprises 5% to 100% by weight of an alkane (such as, but not limited to, acetylene) and 0% to 95% by weight of an inert gas (such as, but not limited to, argon).

[0064] Example 1

[0065] Preparation of negative electrode sheet:

[0066] 1) Preparation of silicon-carbon negative electrode material: Linear phenolic resin (RF): hexamethylenetetramine (HMT): carbon nanotubes (CNTs) were mixed in a weight ratio (described in Table 1) to form a mixture. The mixing method was ball milling, and the ball milling speed was 500r / min and the ball milling time was 6h. The dark black ball-milled mixture was heated to 130°C and kept warm for 10h for curing. The cured mixture was transferred to a box furnace and heated to 900°C under a nitrogen atmosphere for carbonization for 2h. After cooling, it was transferred to a rotary kiln and activated under a carbon dioxide atmosphere for 9h to obtain a porous carbon skeleton. The porous carbon skeleton continued to be heated to 500°C at a rate of 2°C / min under an argon atmosphere, and then the atmosphere was 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 continued to be deposited at 500°C for 10 hours. After that, it was switched to nitrogen and then cooled to room temperature of 25°C to obtain a silicon-carbon negative electrode material.

[0067] 2) A mixture of graphite and the aforementioned silicon-carbon negative electrode material in a weight ratio of 80:20 was used as the negative electrode active material. The negative electrode active material, styrene-butadiene rubber (SBP), and sodium carboxymethyl cellulose (CMC) were thoroughly stirred and mixed in a weight ratio of 97:2:1 in an appropriate amount of deionized water to form a uniform negative electrode slurry having a solid content of 40 wt%. This slurry was coated onto a negative electrode current collector copper foil, dried at 85°C, cold pressed, cut, and slit, and then dried under vacuum at 120°C for 12 hours to obtain a negative electrode sheet.

[0068] Preparation of the positive electrode sheet: The positive electrode active material, lithium cobalt oxide (LiCoO2), conductive carbon black Super P, and polyvinylidene fluoride (PVDF), were thoroughly stirred 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 the positive electrode current collector aluminum foil, dried at 85°C, and then cold pressed, cut, and slit, and then dried under vacuum at 85°C for 4 hours to obtain the positive electrode sheet.

[0069] Preparation of electrolyte: In a dry argon atmosphere glove box, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:EMC:DEC = 30:50:20, and then lithium salt LiPF6 was added and mixed to obtain an electrolyte. In the electrolyte, the mass percentage of LiPF6 was 12.5%.

[0070] Preparation of isolation membrane: A 7 μm thick polyethylene (PE) porous polymer film was used as the isolation membrane.

[0071] Preparation of a lithium-ion battery: The positive electrode sheet, separator, and negative electrode sheet are stacked, wound, and welded to the tabs before being placed in an aluminum-plastic film packaging bag. Electrolyte is then injected and the battery undergoes vacuum packaging, resting, formation, shaping, and capacity testing to produce a soft-pack lithium-ion battery.

[0072] The specific steps for preparing the lithium-ion batteries of Examples 2-12 and Comparative Examples 1-3 refer to Example 1, and the differences can be seen in Table 1.

[0073] Table 1

[0074] [Corrected 26.02.2025 according to Rule 26] The button cells formed by the negative electrode sheets of each of the above embodiments and comparative examples were subjected to the first efficiency test of lithium-ion batteries. 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 cm 2 The working electrode is then used as the counter electrode, and the porous polyethylene film is used as the separator. After injecting the electrolyte, a button battery is assembled; the button battery is first discharged to 0V at a three-stage low current of 0.05C / 50μA / 20μA, and the first discharge capacity of the button battery is recorded; then it is charged to 2.0V at a constant current of 0.1C, and the first charge capacity of the button battery is recorded. First efficiency = first charge capacity / first discharge capacity × 100%; the first reversible gram capacity of the negative electrode active material from 0V to 2.0V = the first charge capacity of the button battery / the mass of the negative electrode active material. The electrolyte contains 12.5% ​​by mass of lithium salt LiPF6, and the solvent is obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a mass ratio of 1:1. The first efficiency of the button batteries corresponding to each embodiment and comparative example obtained by testing is recorded in Table 2.

[0075] Scanning electron microscopy (SEM) line scan elemental analysis was performed on the silicon-carbon anode materials of each of the aforementioned Examples and Comparative Examples. The specific method was as follows: The negative electrode sheets prepared in the corresponding Examples or Comparative Examples were cut into cross sections using ion polishing. The cut cross sections of the carbon-silicon anode materials were then transferred to a field emission scanning electron microscope (FESEM) and focused for analysis. During the test, a line scan was performed from the center of the particle outward to measure the silicon content in the carbon-silicon anode materials. The silicon content distribution was recorded, and the scanned values ​​were then subjected to variance analysis to obtain the standard deviation, which is reported in Table 2.

[0076] The silicon-carbon negative electrode materials of the above-mentioned embodiments and comparative examples were tested for the content of silicon elements and the content of carbon nanotubes.

[0077] Among them, the silicon content in the silicon-carbon negative electrode material is characterized and tested by ICP (inductively coupled plasma spectrometer).

[0078] The specific method for testing the 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 skeleton, calculate the yield of RF+HMT in the mixing / curing / carbonization / and activation process, recorded as d%; then record the yield of RF+HMT+CNTs in the mixing / curing / carbonization / and activation process in other examples, recorded as e%; then the proportion of CNTs in the porous carbon skeleton of the example is f% = (ed) / e×100%, and the remaining porous carbon skeleton components are g% = 1-f%; then the CNTs content in the silicon-carbon negative electrode material of the example is h% = f / (f+g+b)×100%, which is recorded in Table 2, and the content of the remaining porous carbon skeleton components is i% = 1-h%-b%.

[0079] Conductivity testing was performed on the silicon-carbon anode materials used in each example and comparative example. The specific testing method is as follows: The conductivity of the silicon-carbon anode material powder was measured using a powder conductivity meter (model FT-8100) based on the four-probe test principle and in accordance with standard GB / T1552-1995. A known amount of silicon-carbon anode material powder was compressed under hydraulic pressure to a set pressure. The conductivity of the silicon-carbon anode material powder was then measured online, and the data is recorded in Table 2.

[0080] The particle strength test of the silicon-carbon negative electrode material of each embodiment and comparative example is carried out, and the specific test method is as follows: a nanoindenter (model: Hysitron TI 950) is used to test the hardness and elastic modulus of a single particle of the silicon-carbon negative electrode material, and the test standard is JB / T 12721-2016. Before the test, the silicon-carbon negative electrode material powder is dispersed in an epoxy resin and cured. The cured resin is cut by ion polishing, and a nanoprobe is used to apply pressure to a single particle. The indentation depth on the particle surface is monitored and the elastic modulus of the particle is converted. The elastic modulus of five particles of the same sample is tested in parallel and the average value is taken to obtain the particle elastic modulus of the silicon-carbon negative electrode material, which is recorded in Table 2.

[0081] The soft-pack lithium-ion batteries of the embodiments and comparative examples were subjected to cycle performance tests and battery full-charge expansion rate tests.

[0082] Among them, the specific method of the cycle performance test is as follows: charge to 4.4V at a constant current of 0.7C, charge to 0.025C at a constant voltage, and discharge to 3.0V at 0.5C after standing for 5 minutes. The capacity obtained in this step is taken as the initial capacity, and a cycle test is performed with 0.7C charging / 0.5C discharging. The capacity of each step is compared with the initial capacity to obtain a capacity decay curve. The number of cycles at 25°C until the capacity retention rate is 90% (recorded in Table 2) is recorded as the room temperature cycle performance of the battery, and the number of cycles at 45°C until the capacity retention rate is 80% (recorded in Table 2) is recorded as the high temperature cycle performance of the battery. The cycle performance of the material is compared by comparing the number of cycles under the above two conditions.

[0083] The specific method for testing the battery full-charge expansion rate is as follows: Use a screw micrometer to measure the thickness of a fresh soft-pack lithium-ion battery at half charge (50% state of charge (SOC)). When the battery is fully charged (100% SOC) after 400 cycles, the screw micrometer is used to measure the thickness of the battery at this time. Compare it with the thickness of the fresh battery at the initial half charge (50% SOC) to obtain the expansion rate of the fully charged (100% SOC) battery at this time and record it in Table 2.

[0084] Particle size testing:

[0085] Add about 0.02 g of silicon-carbon negative electrode material powder sample to a 50 ml clean beaker, add about 20 ml of deionized water, and then add 3 drops of 1% surfactant to completely disperse the silicon-carbon negative electrode material powder in the water. Use a 120 W ultrasonic cleaner to ultrasonicate for 5 minutes, and use MasterSizer 2000 to test its particle size.

[0086] Raman testing

[0087] The silicon-carbon anode material was loaded onto a flat glass slide for Raman testing with a test range of 100 cm -1 Up to 1200cm -1 ;After the test is completed, pay attention to 450cm -1 Up to 550cm -1 For characteristic peaks within the range, the maximum point of the peak is taken as the peak position of the peak, and 1 / 2 of the difference of the horizontal coordinates at half the peak is taken as the half-peak width of the peak.

[0088] XRD test

[0089] The silicon-carbon negative electrode material was loaded onto a sample carrier for powder XRD testing. The test range was 10° to 90° and the scanning speed was 5° / min. After the test was completed, attention was paid to the characteristic peaks in the range of 15° to 35°, and the maximum point of the peak was taken as the peak position of the peak, and 1 / 2 of the difference in the horizontal coordinates at half the peak was taken as the half-peak width of the peak.

[0090] Determination / testing method of pore volume

[0091] The pore volume of the silicon-carbon negative electrode 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 of <2nm and >2nm, respectively.

[0092] It can be seen from the data recorded in Table 1 and Table 2, especially in comparison with Comparative Example 1 and other embodiments, the strength of the silicon-carbon negative electrode material is improved after the addition of carbon nanotubes, so the ability of the buffer battery volume expansion is improved, which is beneficial to improving the cycle performance of the battery. In addition, the electrical conductivity of the silicon-carbon negative electrode material is also improved after the addition of carbon nanotubes, so that the electron diffusion ability of the silicon-carbon negative electrode material is improved, and the probability of lithium ions being trapped (trapping) inside the silicon-carbon is reduced, so that its first efficiency is improved and the energy density is improved. When the carbon nanotube content is too high, it is easy to affect the first charge efficiency. Specifically, when the carbon nanotube content is too high, the formation of pores when forming the porous carbon skeleton is easy to make the proportion of macropores too high, making it difficult for subsequent silicon to deposit, resulting in an increase in the specific surface area of ​​the final product, thereby increasing the SEI film formation in the electrochemical device, and thus affecting the first charge efficiency. Similarly, in Comparative Example 2, the silicon content is too low, so that the pores in the porous carbon skeleton are insufficiently filled, resulting in an increase in the specific surface area of ​​the final product, thereby increasing the SEI film formation in the electrochemical device, and thus affecting the first charge efficiency. However, from the comparison between Comparative Example 3 and other embodiments, it can be seen that the silicon content in the silicon-carbon negative electrode material is too high, the binding capacity of the carbon nanotubes is limited, the battery expands severely, and the cycle performance deteriorates.

[0093] The above disclosure is only a preferred embodiment of the present application, and certainly cannot be used to limit the present application. Therefore, equivalent changes made based on the present application are still within the scope covered by the present application.

Claims

1. A silicon-carbon negative electrode material, wherein: It comprises a core body and a shell, wherein the core body comprises a porous carbon skeleton and silicon dispersed in the pores of the porous carbon skeleton, carbon nanotubes are wrapped and dispersed in the porous carbon skeleton, in the carbon-silicon negative electrode material, the content of silicon in the silicon-carbon negative electrode material is 25wt% to 55wt%, and the shell comprises a carbon material; In the linear scanning electron microscope energy spectrum of the cross section of the carbon silicon negative electrode material, the standard deviation of the change in the content of the silicon element does not exceed 200 along the direction from the center to the edge of the cross section.

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

3. The silicon-carbon negative electrode material according to claim 1 or 2, wherein: The content of the carbon nanotubes is 0.2 wt % to 7 wt %.

4. The silicon-carbon negative electrode material according to any one of claims 1 to 3, wherein The silicon-carbon negative electrode material satisfies at least one of the following conditions: (1) The X-ray diffraction spectrum of the silicon-carbon negative electrode material has a characteristic peak in the range of 20° to 30°, and the half-peak width of the characteristic peak is greater than 2°; (2) In the Raman spectrum of the silicon-carbon negative electrode material, at 450 cm -1 Up to 500cm -1 There is a characteristic peak in the range of (3) The particle size Dv50 of the silicon-carbon negative electrode material is 3 μm to 20 μm, and the particle size Dv99 of the silicon-carbon negative electrode material is 3 μm to 20 μm.

5. The silicon-carbon negative electrode material according to any one of claims 1 to 4, wherein The pore volume of pores with a pore diameter exceeding 2 nm in the silicon-carbon negative electrode material is greater than the pore volume of pores with a pore diameter not exceeding 2 nm.

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

7. The electrochemical device according to claim 6, wherein: The negative electrode active material further comprises graphite. In the negative electrode active material, the content of the silicon-carbon negative electrode material is 5wt% to 40wt%, and the content of the graphite is 95wt% to 60wt%.

8. A method for preparing the silicon-carbon negative electrode material according to claim 1, comprising: mixing the resin and the carbon nanotubes to form a mixture and curing it; Carbonizing and activating the solidified mixture to obtain a porous carbon skeleton; as well as The porous carbon skeleton is subjected to silane deposition to form a core body and then an alkane is used to form a shell body.

9. The method for preparing the silicon-carbon negative electrode material according to claim 8, wherein: The carbonization conditions are to raise the temperature to 700° C. to 1100° C. and keep the temperature for 1 to 5 hours. The activation is specifically to activate by carbon dioxide, water vapor, sodium hydroxide, potassium hydroxide or phosphoric acid after the carbonization temperature is lowered.

10. The method for preparing the silicon-carbon negative electrode material according to claim 8 or 9, wherein: The step of "forming a core body by silane deposition on the porous carbon skeleton and then forming a shell by alkane deposition" is specifically as follows: The porous carbon skeleton is slowly heated to 400°C to 600°C under an inert atmosphere and then kept warm. The atmosphere is then switched to a silane mixed gas for deposition for 1 to 20 hours, wherein the silane mixed gas contains 2% to 20% silane by mass and 80% to 98% inert gas by mass. The atmosphere is then switched to an alkane mixed gas at a temperature of 500°C to 1000°C and maintained for 10 hours to form a shell, and then the atmosphere is switched to an inert atmosphere and cooled to room temperature, wherein the alkane mixed gas contains 5% to 100% alkane by mass and 0% to 95% inert gas by mass.

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