Silicon-carbon composite material, and preparation method therefor and use thereof

Abstract

The present application relates to the technical field of batteries, and specifically relates to a silicon-carbon composite material, and a preparation method therefor and the use thereof. The silicon-carbon composite material of the present application comprises a silicon nanosheet and a composite carbon structure located on the surface of the silicon nanosheet, wherein the composite carbon structure comprises a carbon framework and S, and the carbon framework and S form an S-C bond. The silicon-carbon composite material of the present application has great structural stability, can improve the ion and electron conductivity of a silicon negative electrode, and forms a three-dimensional ion- and electron-conducting path inside the silicon negative electrode, thereby ameliorating the problem of capacity attenuation of a pure silicon negative electrode during a cycling process, so as to achieve stable long-cycling performance and obtain a high capacity retention rate.

Description

Silicon-carbon composite material, preparation method and application thereof

[0001] Cross-reference to related applications

[0002] The present application claims priority to the Chinese patent application No. CN2024112298068, filed on September 03, 2024, and entitled "Silicon-carbon composite material, preparation method and application thereof", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present application relates to the technical field of batteries, in particular to a silicon-carbon composite material, a preparation method and application thereof. BACKGROUND

[0004] Lithium ion batteries are used as energy storage systems in electric vehicles and portable electronic devices, and need to meet the requirements of large capacity, long cycle life and good safety performance. However, the traditional negative electrode materials of lithium ion batteries have some problems that are difficult to solve, including low theoretical specific capacity of carbon-based materials, volume expansion of silicon-based materials, low initial efficiency and poor conductivity.

[0005] In the current production route of carbon-based negative electrodes, most material manufacturers on the market composite artificial graphite through uniform coating, structural design and other production processes. However, the theoretical specific capacity of 372mAh / g of graphite materials significantly limits their application in the next generation of lithium ion batteries. The negative electrode material is crucial for the cycle life and capacity of the battery. Silicon is abundant in the earth's crust and has a wide range of sources. It is a negative electrode material with the highest theoretical specific capacity (4200mAh / g), and is less likely to form lithium dendrites compared to lithium metal, playing an important role in improving the energy density and safety of the battery.

[0006] However, the current silicon-carbon composite material still has the problems of fast capacity decay rate and unsatisfactory cycle performance.

[0007] Therefore, the present application is proposed.

[0008] SUMMARY

[0009] Therefore, the present application aims to provide a silicon-carbon composite material to solve the technical problems of fast capacity decay rate and unsatisfactory cycle performance of the silicon-carbon composite material in the prior art.

[0010] In a first aspect, the present application provides a silicon-carbon composite material, comprising silicon nanosheets and a composite carbon structure on the surface of the silicon nanosheets, wherein the composite carbon structure comprises a carbon framework and an element S, and the carbon framework and the element S form S-C bonds.

[0011] Beneficial effects: the silicon-carbon composite material has excellent structural stability, can improve the ion and electron conductivity of the silicon negative electrode, forms a three-dimensional ion and electron conducting path in the silicon negative electrode, thereby relieving the capacity decay problem of the pure silicon negative electrode in the cycle process, to realize stable long cycle performance, and obtain high capacity retention rate.

[0012] In an alternative embodiment, the composite carbon structure comprises C-SO x -C and H-S x -C.

[0013] In an alternative embodiment, the mass ratio of the composite carbon structure to the mass of the silicon-carbon composite material is 0.7-0.9.

[0014] In an alternative embodiment, the mass ratio of the element S to the carbon framework is 1:(1-4).

[0015] In an alternative embodiment, the thickness of the silicon nanosheet is 5-10 nm.

[0016] In an alternative embodiment, the first coulombic efficiency of the battery prepared by the silicon-carbon composite material is greater than 81%.

[0017] In an alternative embodiment, the capacity retention rate of the battery prepared by the silicon-carbon composite material is greater than 71% after 250 cycles.

[0018] In a second aspect, the application provides a preparation method of the silicon-carbon composite material, comprising the following steps:

[0019] Mixing and processing the carbon source, amorphous sulfur, silicon nanosheet and organic solvent to obtain a mixed slurry, and heating and carbonizing the mixed slurry.

[0020] Beneficial effects: the method is simple and easy to operate, and through the cooperation of each step, a silicon-carbon composite material with high capacity and stable structure is obtained.

[0021] In an alternative embodiment, the mass ratio of the silicon nanosheet, the carbon source and the amorphous sulfur is 2:(0.7-1):(0.6-0.8).

[0022] In an alternative embodiment, the mass ratio of the total mass of the silicon nanosheet, the carbon source and the amorphous sulfur to the mass of the organic solvent is (170-200) mg:(1.8-3) mL.

[0023] In an alternative embodiment, the carbon source comprises at least one of resin and pitch.

[0024] In an alternative embodiment, the thickness of the silicon nanosheet is 5-10 nm.

[0025] In an alternative embodiment, the method for preparing the silicon nanosheet comprises: washing calcium disilicide with an alkaline solution, and then performing chemical exfoliation with concentrated hydrochloric acid; or performing thermal treatment on a silicon tetrachloride liquid to obtain silicon atoms and deposit them on the surface of heated silicon dioxide under a hydrogen atmosphere.

[0026] In an alternative embodiment, the mixing treatment comprises ultrasonic treatment and stirring treatment.

[0027] In an alternative embodiment, the temperature of the heating treatment is 75-85℃.

[0028] In an alternative embodiment, the carbonization treatment comprises first carbonization and second carbonization; the temperature of the first carbonization is 350-450℃, and the time of the first carbonization is 5-7 h; the temperature of the second carbonization is 750-850℃, and the time of the second carbonization is 2-4 h.

[0029] In an alternative embodiment, the carbonization treatment is performed under a protective gas condition.

[0030] In a third aspect, the present application provides a negative electrode sheet comprising the silicon-carbon composite material.

[0031] In a fourth aspect, the present application provides a battery comprising the negative electrode sheet.

[0032] Compared with the prior art, the present application has the following beneficial effects:

[0033] (1) The silicon-carbon composite material of the present application can improve the ionic and electronic conductivity of the silicon negative electrode by virtue of the sulfur-modified porous carbon structure, and form a three-dimensional ion-conducting and electron-conducting path inside the silicon negative electrode, thereby alleviating the capacity decay problem of the pure silicon negative electrode during the cycle process, achieving stable long cycle performance, and obtaining a high capacity retention rate; the carbon framework doped with sulfur contains sulfur functional groups that can form S-C bonds, increase the crosslinking of the carbon layer, and thus increase the mechanical strength of the carbon matrix to adapt to the volume change of Si and maintain the integrity of the matrix under stress; the sulfur-containing functional groups with a large size can distort the carbon layer structure, form defects, increase the interlayer spacing, and thus provide more diffusion paths, reduce the surface activation energy, and improve the reaction kinetics of lithium ions; and the large carbon layer spacing and defects caused by sulfur doping can promote Li diffusion and electrode reactions.

[0034] (2) The preparation method of the silicon-carbon composite material of the present application, the carbon source contains rigid polycyclic aromatic hydrocarbons with defects, in the process of converting the carbon source into carbonaceous material, sulfur reacts with the defects in the polycyclic aromatic hydrocarbons to form sulfur-containing functional groups between different carbon layers, and the excess sulfur is converted into gas and forms pore structures. The larger interlayer spacing of the carbon layers and the defects generated by the doping of sulfur can promote Li diffusion and electrode reactions; the two-dimensional silicon nanosheet can effectively alleviate the problem of volume expansion, and the high specific surface area provides a large number of active sites for lithium ion reactions. After carbonization treatment, the sulfur functional groups remaining between different carbon layers can form S-C bonds to increase the cross-linking of the carbon layers, thereby increasing the mechanical strength of the carbon matrix to adapt to the volume change of Si and maintain the integrity of the matrix under stress, thereby improving the reaction kinetics of lithium ions.

[0035] (3) The battery of the present application has excellent high initial efficiency, high capacity, good cycle stability and good safety. DETAILED DESCRIPTION

[0036] The embodiments of the present application will be described in detail below with reference to examples, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. If no specific conditions are specified in the examples, the conventional conditions or the conditions recommended by the manufacturer are used. If no manufacturer of the reagent or instrument is specified, it is a conventional product that can be obtained by purchase on the market.

[0037] According to one aspect of the present application, the present application relates to a silicon-carbon composite material, comprising silicon nanosheets and a composite carbon structure on the surface of the silicon nanosheets, the composite carbon structure comprising a carbon framework and an element S, the carbon framework and the element S forming S-C bonds.

[0038] The silicon-carbon composite material of the present application has a core layer of silicon nanosheets and a cladding layer of a composite carbon structure, in which the carbon framework has a porous structure, and the sulfur-modified porous carbon structure can improve the ionic and electronic conductivity of the silicon negative electrode, forming a three-dimensional ion-conducting and electron-conducting path inside the silicon negative electrode, thereby alleviating the capacity decay problem of the pure silicon negative electrode during the cycle process, achieving stable long cycle performance, and obtaining a high capacity retention rate; the sulfur-doped carbon framework contains sulfur functional groups that can form S-C bonds to increase the cross-linking of the carbon layers, thereby increasing the mechanical strength of the carbon matrix to adapt to the volume change of Si and maintain the integrity of the matrix under stress; in addition, the larger sulfur-containing functional groups will distort the carbon layer structure, form defects, increase the interlayer spacing, thereby providing more diffusion paths, reducing the surface activation energy, and improving the reaction kinetics of lithium ions. The larger interlayer spacing of the carbon layers and the defects generated by the doping of sulfur can promote Li diffusion and electrode reactions.

[0039] In some embodiments, the composite carbon structure comprises C-SO x -C and H-Sx -C, such as C-SO-C and H-S 0.5 -C.

[0040] In some embodiments, the ratio of the mass of the composite carbon structure to the mass of the silicon-carbon composite material is 0.7-0.9. The ratio of the mass of the composite carbon structure to the mass of the silicon-carbon composite material includes, but is not limited to, 0.7, 0.72, 0.75, 0.78, 0.8, 0.82, 0.85, 0.88, or 0.9, or any range between any two of these values. In some embodiments, the mass ratio of the element S to the carbon framework is 1:(1-4), including but not limited to 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, or any range between any two of these values. The ratio of the mass of the composite carbon structure to the mass of the silicon-carbon composite material and the mass ratio of the element S to the carbon framework in the present application within the above range can make the silicon-carbon composite material have more excellent electrochemical performance. The structural damage caused by the volume expansion / contraction of the silicon anode during the lithium intercalation / deintercalation process is greatly alleviated by the sulfur-modified porous carbon structure, which can ensure the necessary ion and electron transport network inside the silicon anode and the structural stability of the electrode during the cycle process, so that the battery has the characteristics of high capacity and high cycle stability. If the ratio of the mass of the composite carbon structure to the mass of the silicon-carbon composite material is too high or too low, the capacity of the battery decreases and the stability becomes poor.

[0041] In some embodiments, the thickness of the silicon nanosheet is 5-10 nm. This includes, but is not limited to, 5 nm, 6 nm, 7 nm, 8 nm, 10 nm, or any range between any two of these values.

[0042] In some embodiments, the battery prepared from the silicon-carbon composite material has a first coulombic efficiency greater than 81%, including but not limited to 81%, 81.5%, 85%, 88%, 90%, 92%, or any range between any two of these values. The silicon-carbon anode material of the present application can obtain a battery with excellent first coulombic efficiency.

[0043] In some embodiments, the battery prepared from the silicon-carbon composite material has a capacity retention rate of greater than 71% after 250 cycles, including but not limited to 71%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, or any range between any two of these values. The silicon-carbon composite material of the present application has excellent cycle stability.

[0044] According to another aspect of the present application, the present application also relates to a method for preparing the silicon-carbon composite material as described above, comprising the following steps:

[0045] The carbon source, amorphous sulfur, silicon nanosheet, and organic solvent are mixed to obtain a mixed slurry, and the mixed slurry is subjected to heating treatment and carbonization treatment.

[0046] In the preparation method of the silicon-carbon composite material of the present application, since the carbon source contains a rigid polycyclic aromatic hydrocarbon with defects, in the process of converting the carbon source into carbonaceous material, sulfur reacts with the defects in the polycyclic aromatic hydrocarbon to form sulfur-containing functional groups between different carbon layers, and the excess sulfur is converted into gas and forms a pore structure. The larger interlayer distance and the defects generated by sulfur doping can promote Li diffusion and electrode reaction; the two-dimensional silicon nanosheet itself has a layered structure, which can effectively alleviate the problem of volume expansion, and the higher specific surface area provides a large number of active sites for lithium ion reaction, so that this material has more excellent reaction kinetics. The materials are uniformly mixed to form a uniform mixed slurry, and further heat treatment is performed to remove the solvent. After carbonization treatment, the sulfur functional groups remaining between different carbon layers can form S-C bonds to increase the crosslinking of the carbon layers, thereby increasing the mechanical strength of the carbon matrix to adapt to the volume change of Si and maintain the integrity of the matrix under stress, thereby improving the reaction kinetics of lithium ions.

[0047] In some embodiments, the mass ratio of the silicon nanosheet, the carbon source and the amorphous sulfur is 2:(0.7-1):(0.6-0.8). In some embodiments, the mass ratio of the silicon nanosheet, the carbon source and the amorphous sulfur includes but is not limited to 2:0.7:0.6, 2:0.8:0.8, 2:0.9:0.6, 2:1:0.6, 2:1:0.7, 2:1:0.8, etc. The silicon nanosheet, the carbon source and the amorphous sulfur of the present application adopt a suitable mass ratio, thereby ensuring that the obtained carbon layers are better crosslinked, ensuring the mechanical strength of the composite carbon structure to adapt to the volume change of Si; at the same time, it can ensure that the larger size sulfur-containing functional groups will distort the carbon layer structure, form defects, increase the interlayer distance, thereby providing more diffusion paths, reducing the surface activation energy, and improving the reaction kinetics of lithium ions.

[0048] In some embodiments, the mass ratio of the total mass of the silicon nanosheet, the carbon source and the amorphous sulfur to the organic solvent is (170-200) mg:(1.8-3) mL.

[0049] In some embodiments, the carbon source contains at least one of a resin and pitch. The resin includes phenolic resin, (C6H6O·CH2) x .

[0050] In some embodiments, the thickness of the silicon nanosheet is 5-10 nm. In some embodiments, the thickness of the silicon nanosheet includes but is not limited to 5 nm, 5.5 nm, 6 nm, 7 nm, 8 nm, 9 nm or 10 nm, or any range between any two of them. The silicon nanosheet of the present application has a suitable thickness to ensure that the obtained silicon-carbon composite material has better electrochemical performance.

[0051] In some embodiments, the method for preparing the silicon nanosheets comprises: washing calcium disilicide with an alkali solution, and then performing chemical exfoliation with concentrated hydrochloric acid. The alkali solution comprises a sodium hydroxide solution. The concentration of the concentrated hydrochloric acid is 10-14 mol / L.

[0052] In some embodiments, the method for preparing the silicon nanosheets comprises: performing thermal treatment on a silicon tetrachloride liquid to obtain silicon atoms and deposit the silicon atoms on the surface of heated silicon dioxide under a hydrogen atmosphere. The temperature of the thermal treatment is 55-75 °C.

[0053] The two-dimensional silicon nanosheets obtained by the method for preparing the silicon nanosheets described above have a layered structure, which can effectively alleviate the problem of volume expansion. In addition, the high specific surface area can provide a large number of active sites for the reaction of lithium ions, thereby making the material have more excellent reaction kinetics.

[0054] In some embodiments, the mixing treatment comprises ultrasonic treatment and stirring treatment. The ultrasonic treatment and the stirring treatment are performed to fully mix the materials.

[0055] In some embodiments, the temperature of the heating treatment is 75-85 °C, including but not limited to 75 °C, 80 °C, 82 °C, 85 °C, or a range between any two of them. An appropriate heating treatment temperature is used to remove the organic solvent.

[0056] In some embodiments, the carbonization treatment comprises first carbonization and second carbonization. The temperature of the first carbonization is 350-450 °C, including but not limited to 350 °C, 380 °C, 400 °C, 450 °C, or a range between any two of them. The time of the first carbonization is 5-7 h, including but not limited to 5 h, 5.5 h, 6 h, 6.5 h, or 7 h, or a range between any two of them. The temperature of the second carbonization is 750-850 °C, including but not limited to 750 °C, 780 °C, 800 °C, 820 °C, or 850 °C, or a range between any two of them. The time of the second carbonization is 2-4 h, including but not limited to 2 h, 2.5 h, 3 h, 3.5 h, or 4 h, or a range between any two of them. In some embodiments, the carbonization treatment is performed under a protective gas, such as argon or helium. An appropriate carbonization treatment condition is used to control the amount of sulfur-containing functional groups, so as to ensure the structural stability of the silicon-carbon composite material obtained finally, and make the silicon-carbon composite material have a high capacity.

[0057] In a preferred embodiment, the method for preparing the silicon-carbon composite material comprises the following steps:

[0058] (a) Preparation of silicon nanosheets: calcium disilicide is washed with lye, and then chemically exfoliated with 10-14 mol / L concentrated hydrochloric acid; or, silicon tetrachloride liquid is subjected to heat treatment at 55-75°C, and silicon atoms are obtained and deposited on the surface of heated silicon dioxide under a hydrogen atmosphere; silicon nanosheets with a thickness of 5-10 nm are obtained.

[0059] (b) The mixed system of carbon source, amorphous sulfur, silicon nanosheets of step (a) above, and organic solvent is subjected to ultrasonic treatment and stirring treatment to obtain a mixed slurry.

[0060] (c) The mixed slurry is subjected to heat treatment at a temperature of 75-85°C, and then subjected to carbonization treatment under a protective gas atmosphere, which includes first carbonization and second carbonization; the temperature of the first carbonization is 350-450°C, and the time of the first carbonization is 5-7 h; the temperature of the second carbonization is 750-850°C, and the time of the second carbonization is 2-4 h.

[0061] According to another aspect of the present application, the present application also relates to a negative electrode sheet comprising the silicon-carbon composite material.

[0062] The negative electrode sheet of the present application comprises a negative electrode current collector and a negative electrode layer on at least one side surface of the negative electrode current collector, and the negative electrode layer comprises the above-mentioned silicon-carbon composite material, a conductive agent, and a binder. The conductive agent includes conductive carbon black (Super P). The binder includes styrene-butadiene rubber (SBR). The negative electrode current collector includes copper foil.

[0063] In one embodiment, the preparation method of the negative electrode sheet comprises: mixing the above-mentioned silicon-carbon composite material, the conductive agent, and the binder to form a negative electrode slurry, and coating the negative electrode slurry onto the surface of the negative electrode current collector by drop coating or doctor blade coating to obtain a negative electrode sheet with a loading of 0.15-0.3 mg / cm 2 , including but not limited to 0.15 mg / cm 2 , 0.18 mg / cm 2 , 0.2 mg / cm 2 , 0.25 mg / cm 2 , or 0.3 mg / cm 2 , etc.

[0064] According to another aspect of the present application, the present application also relates to a battery comprising the negative electrode sheet.

[0065] The battery of the present application comprises the negative electrode sheet, a positive electrode sheet, a separator, and an electrolyte.

[0066] The battery of the present application has excellent structural stability, high capacity, high initial efficiency, and good cycle stability.

[0067] The application will be further explained in connection with specific examples and comparative examples.

[0068] Example 1

[0069] A method for preparing a silicon-carbon composite material, comprising the following steps:

[0070] (1) Commercial calcium disilicide is rinsed with sodium hydroxide, and then chemically stripped with 12 mol / L concentrated hydrochloric acid to obtain two-dimensional silicon nanosheets with a thickness of 8 nm.

[0071] (2) According to the mass ratio of two-dimensional silicon nanosheets: resin: amorphous sulfur = 2:1:0.6, 112 mg of silicon nanosheets, 56 mg of resin and 33.6 mg of amorphous sulfur are weighed and added to 2 mL of DMF, and a uniformly mixed slurry is obtained by ultrasonic treatment and stirring, and the resin is a phenolic resin.

[0072] (3) The mixed slurry is heated to 80°C to remove DMF, and then the obtained sample is carbonized at 400°C for 6h and carbonized at 800°C in an Ar atmosphere for 3h.

[0073] A method for preparing a negative electrode sheet, comprising the following steps:

[0074] The above-mentioned silicon-carbon negative electrode material, carbon black and butadiene-styrene rubber in the example are mixed and uniformly slurried according to a mass ratio of 85:10:5. A clean copper foil is taken and the surface is cleaned with anhydrous ethanol, and after the surface is dried, the above-mentioned slurry is applied to the surface of the copper foil by scraping and coating (silicon loading = 0.20 mg / cm 2 ). After drying, the above-mentioned electrode sheet is punched into a negative electrode sheet with a diameter of 12 mm by using a punching machine.

[0075] Example 2

[0076] A method for preparing a silicon-carbon composite material, comprising the following steps:

[0077] (1) Silicon tetrachloride liquid is heated to 60°C, and the obtained gas is reacted to generate silicon atoms and deposited on a heated silicon dioxide substrate to form two-dimensional silicon nanosheets with a thickness of 7 nm.

[0078] (2) According to the mass ratio of two-dimensional silicon nanosheets: resin: amorphous sulfur = 2:1:0.6, 112 mg of silicon nanosheets, 56 mg of resin and 33.6 mg of amorphous sulfur are weighed and added to 2 mL of DMF, and a uniformly mixed slurry is obtained by ultrasonic treatment and stirring, and the resin is a phenolic resin.

[0079] (3) The mixed slurry is heated to 80°C to remove DMF, and then the obtained sample is carbonized at 400°C for 6h and carbonized at 800°C in an Ar atmosphere for 3h.

[0080] The preparation method of the negative electrode sheet of the present embodiment is the same as that of Example 1 except that the silicon-carbon composite material of the present embodiment is used.

[0081] Example 3

[0082] The preparation method of the silicon-carbon composite material is different from that of Example 1 in that:

[0083] In step (2), 112 mg of silicon nanosheets, 56 mg of resin, and 33.6 mg of amorphous sulfur were weighed according to the mass ratio of two-dimensional silicon nanosheets:resin:amorphous sulfur=2:1:0.7, and added to 2 mL of DMF. A uniformly mixed slurry was obtained by ultrasonic treatment and stirring, and the resin was a phenolic resin.

[0084] The preparation method of the negative electrode sheet of the present embodiment is the same as that of Example 1 except that the silicon-carbon composite material of the present embodiment is used.

[0085] Example 4

[0086] The preparation method of the silicon-carbon composite material is different from that of Example 1 in that:

[0087] In step (2), 112 mg of silicon nanosheets, 56 mg of resin, and 44.8 mg of amorphous sulfur were weighed according to the mass ratio of two-dimensional silicon nanosheets:resin:amorphous sulfur=2:1:0.8, and added to 2 mL of DMF. A uniformly mixed slurry was obtained by ultrasonic treatment and stirring, and the resin was a phenolic resin.

[0088] The preparation method of the negative electrode sheet of the present embodiment is the same as that of Example 1 except that the silicon-carbon composite material of the present embodiment is used.

[0089] Example 5

[0090] The preparation method of the silicon-carbon composite material is different from that of Example 1 in that:

[0091] In step (2), 112 mg of silicon nanosheets, 44.8 mg of resin, and 33.6 mg of amorphous sulfur were weighed according to the mass ratio of two-dimensional silicon nanosheets:resin:amorphous sulfur=2:0.8:0.6, and added to 2 mL of DMF. A uniformly mixed slurry was obtained by ultrasonic treatment and stirring, and the resin was a phenolic resin.

[0092] The preparation method of the negative electrode sheet of the present embodiment is the same as that of Example 1 except that the silicon-carbon composite material of the present embodiment is used.

[0093] Example 6

[0094] A preparation method of a silicon-carbon composite material, which is different from that of Example 1 in that:

[0095] In step (2), 112 mg of silicon nanosheets, 44.8 mg of resin and 39.2 mg of amorphous sulfur were weighed according to the mass ratio of two-dimensional silicon nanosheets:resin:amorphous sulfur = 2:0.8:0.7, and added to 2 mL of DMF to obtain a uniformly mixed slurry by ultrasonic treatment and stirring, and the resin was a phenolic resin.

[0096] The preparation method of the negative electrode sheet of the present example is the same as that of Example 1 except that the silicon-carbon composite material of the present example is used.

[0097] Example 7

[0098] A preparation method of a silicon-carbon composite material, which is different from that of Example 1 in that:

[0099] In step (2), 112 mg of silicon nanosheets, 44.8 mg of resin and 44.8 mg of amorphous sulfur were weighed according to the mass ratio of two-dimensional silicon nanosheets:resin:amorphous sulfur = 2:0.8:0.8, and added to 2 mL of DMF to obtain a uniformly mixed slurry by ultrasonic treatment and stirring, and the resin was a phenolic resin.

[0100] The preparation method of the negative electrode sheet of the present example is the same as that of Example 1 except that the silicon-carbon composite material of the present example is used.

[0101] Example 8

[0102] A preparation method of a silicon-carbon composite material, which is different from that of Example 1 in that:

[0103] In step (3), the obtained sample was carbonized at 350℃ for 7h and at 750℃ for 4h in an Ar atmosphere.

[0104] The preparation method of the negative electrode sheet of the present example is the same as that of Example 1 except that the silicon-carbon composite material of the present example is used.

[0105] Example 9

[0106] A preparation method of a silicon-carbon composite material, which is different from that of Example 1 in that:

[0107] In step (3), the obtained sample was carbonized at 450℃ for 5h and at 850℃ for 2h in an Ar atmosphere.

[0108] The preparation method of the negative electrode sheet of the present example is the same as that of Example 1 except that the silicon-carbon composite material of the present example is used.

[0109] Example 10

[0110] A method for preparing a silicon-carbon composite material, which is different from example 1 in that:

[0111] The resin is replaced with a pitch material with a softening point of 200°C.

[0112] A method for preparing a negative electrode sheet of the present embodiment, except that the silicon-carbon composite material of the present embodiment is used, and other conditions are the same as in example 1.

[0113] Example 11

[0114] A method for preparing a silicon-carbon composite material, which is different from example 1 in that:

[0115] The resin is replaced with a pitch material with a softening point of 250°C.

[0116] A method for preparing a negative electrode sheet of the present embodiment, except that the silicon-carbon composite material of the present embodiment is used, and other conditions are the same as in example 1.

[0117] Example 12

[0118] A method for preparing a silicon-carbon composite material, which is different from example 1 in that:

[0119] The resin is replaced with a pitch material with a softening point of 300°C.

[0120] A method for preparing a negative electrode sheet of the present embodiment, except that the silicon-carbon composite material of the present embodiment is used, and other conditions are the same as in example 1.

[0121] Example 13

[0122] A method for preparing a negative electrode sheet of the present embodiment, which is different from example 1 in that:

[0123] First group: PAA is used to replace carbon black.

[0124] Second group: SBR is used to replace carbon black.

[0125] Third group: CMC is used to replace carbon black.

[0126] Comparative example 1

[0127] A method for preparing a silicon-carbon composite material, which is different from example 1 in that:

[0128] Micron silicon powder is used to replace two-dimensional silicon nanosheets.

[0129] A method for preparing a negative electrode sheet of the present embodiment, except that the silicon-carbon composite material of the present embodiment is used, and other conditions are the same as in example 1.

[0130] Comparative example 2

[0131] A method for preparing a silicon-carbon composite material, which is different from example 1 in that:

[0132] The two-dimensional silicon nanosheet is replaced by a silicon nanowire.

[0133] The negative electrode sheet of the present comparative example was prepared in the same manner as in Example 1, except that the silicon-carbon composite material of the present example was used.

[0134] Comparative Example 3

[0135] A method for preparing a silicon-carbon composite material, which is different from Example 1 in that:

[0136] In step (2), no amorphous sulfur is added.

[0137] The negative electrode sheet of the present comparative example was prepared in the same manner as in Example 1, except that the silicon-carbon composite material of the present example was used.

[0138] Experimental Example

[0139] A battery was prepared using the negative electrode sheet of each example and comparative example, including the following steps:

[0140] (1) Preparation of the LMFP positive electrode sheet: positive electrode powder lithium manganese iron phosphate, conductive agent carbon nanotube, super P and binder PVDF were uniformly dispersed in solvent NMP at a mass ratio of 90:2.5:2.5:5. The solid content of the positive electrode slurry after uniform dispersion was controlled at 60%, and the positive electrode slurry was uniformly coated on an aluminum foil, dried, rolled, and die-cut for use.

[0141] (2) Preparation of the electrolyte: the mass percentage of each component in the electrolyte was as follows: lithium hexafluorophosphate (LiPF6) 12.8%, lithium difluoro(oxalato)borate (LiDFOB) 1.5%, ethylene carbonate (EC) 25%, methyl ethyl carbonate (EMC) 43.2%, diethyl carbonate (DEC) 16%, and vinylene carbonate (VC) 1.5%.

[0142] (3) Separator: a polyethylene (PE) base film 9 μm with double-sided aluminum oxide ceramic separator (Al2O3) was used.

[0143] (4) Negative electrode sheet: the negative electrode sheet used was the composite silicon negative electrode sheet prepared in Examples 1-11 and Comparative Examples 1-4.

[0144] (5) Preparation of the battery: the above positive and negative electrode sheets, separator and electrolyte were assembled into a full battery for testing.

[0145] The electrochemical performance test method of the full battery included: standing for five minutes; 0.05C constant current discharge to 0.005V; 50 microampere constant current discharge to 0.005V; standing for five minutes; 0.1C constant current charging to 2V.

[0146] The test results are shown in Table 1.

[0147] Table 1: Electrochemical performance test results of the battery

[0148] As shown in Table 1, the silicon-carbon composite material prepared by the preparation method of the application can limit the volume expansion of silicon, maintain the stability of the electrode structure, and improve the cycle performance of the electrode. The first coulomb efficiency of the battery prepared by the silicon-carbon composite material of the application is greater than 81%, and the capacity retention rate after 250 cycles is greater than 71%.

[0149] The electrochemical performance of the silicon-carbon composite material prepared by the method of Comparative Examples 1-3 is poor, and the first coulomb efficiency of the battery prepared is relatively low, and the capacity retention rate is relatively low.

[0150] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the application, and not to limit them; although the application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the application. Industrial applicability

[0151] The silicon-carbon composite material of the application has excellent structural stability, can improve the ionic and electronic conductivity of the silicon negative electrode, forms a three-dimensional ion-conducting and electron-conducting path in the silicon negative electrode, thereby alleviating the capacity decay problem of the pure silicon negative electrode during the cycle process, to realize stable long cycle performance, and obtain high capacity retention rate.

Claims

1. A silicon-carbon composite material, characterized by, The silicon nanosheet and a composite carbon structure on the surface of the silicon nanosheet, the composite carbon structure comprising a carbon framework and an element S, the carbon framework and the element S forming an S-C bond.

2. The silicon-carbon composite material of claim 1, wherein, At least one of the following features (1) to (4) is included: (1) the complex carbon structure comprises C-SO x -C and H-S x -C; (2) The ratio of the mass of the composite carbon structure to the mass of the silicon-carbon composite material is 0.7 to 0.9; (3) The mass ratio of the element S to the carbon framework is 1:(1 to 4); (4) The thickness of the silicon nanosheet is 5 to 10 nm.

3. The silicon-carbon composite of claim 1, wherein, The first coulombic efficiency of a battery prepared from the silicon-carbon composite material is greater than 81%; The capacity retention rate of a battery prepared from the silicon-carbon composite material after 250 cycles is greater than 71%.

4. The method for producing a silicon-carbon composite material according to any one of claims 1 to 3, wherein The method comprises the following steps: Mixing a carbon source, amorphous sulfur, a silicon nanosheet, and an organic solvent to obtain a mixed slurry, and then heating and carbonizing the mixed slurry.

5. The method of claim 4, wherein the silicon-carbon composite material is prepared by a process comprising: At least one of the following features (1) to (3) is included: (1) The mass ratio of the silicon nanosheet, the carbon source, and the amorphous sulfur is 2:(0.7 to 1):(0.6 to 0.8); (2) The mass ratio of the total mass of the silicon nanosheet, the carbon source, and the amorphous sulfur to the mass of the organic solvent is (170 to 200) mg:(1.8 to 3) mL; (3) The carbon source comprises at least one of a resin and pitch.

6. The method of making a silicon-carbon composite of claim 4, wherein, At least one of the following features (1) to (2) is included: (1) The thickness of the silicon nanosheet is 5 to 10 nm; (2) The preparation method of the silicon nanosheet comprises the following steps: washing calcium disilicide with an alkali solution, and then chemically exfoliating with concentrated hydrochloric acid; Or, performing thermal treatment on a silicon tetrachloride liquid to obtain silicon atoms and deposit them on the surface of heated silicon dioxide in a hydrogen atmosphere.

7. The method of making a silicon-carbon composite of claim 4, wherein, At least one of the following features (1) to (2) is included: (1) The mixing process comprises ultrasonic treatment and stirring treatment; (2) The temperature of the heating process is 75 to 85°C.

8. The method of making a silicon-carbon composite of claim 4, wherein, At least one of the following features (1) to (2) is included: (1) The carbonization process comprises a first carbonization and a second carbonization; the temperature of the first carbonization is 350 to 450°C, and the time of the first carbonization is 5 to 7 hours; the temperature of the second carbonization is 750 to 850°C, and the time of the second carbonization is 2 to 4 hours; (2) The carbonization process is performed in a protective gas atmosphere.

9. A negative electrode sheet characterized by comprising: The silicon-carbon composite material according to any one of claims 1 to 3 is included.

10. A battery, characterized by The negative electrode sheet according to claim 9 is included.

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