Silicon carbon material, preparation method thereof and secondary battery

Abstract

The invention relates to the technical field of secondary battery negative electrode materials, in particular to a silicon carbon material, a preparation method thereof and a secondary battery. The silicon-carbon material comprises a silicon-carbon inner core and a hard carbon shell coating the silicon-carbon inner core, and a gap is formed between the silicon-carbon inner core and the hard carbon shell; the silicon-carbon inner core comprises nano silicon and a carboxylated conductive carbon material coated on the surface of the nano silicon; wherein a hydrogen bond is formed by carboxyl in the carboxylated conductive carbon material and hydroxyl in the nano silicon. Wherein the carboxylated conductive carbon material coats the surfaces of the nano silicon particles through hydrogen bond self-assembly, and the carboxylated conductive carbon material and the nano silicon particles are tightly connected, so that the conductivity of the silicon-carbon material is effectively improved, a smooth ion pathway is formed, and the resistivity of a pole piece of a negative electrode prepared from the silicon-carbon material is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary battery negative electrode materials, and in particular to a silicon-carbon material, a preparation method thereof, and a secondary battery. Background Art

[0002] The silicon-carbon anode, through the synergistic effect of silicon and carbon, effectively solves the volume expansion problem of silicon materials while fully leveraging the advantages of high capacity. This represents a key development direction for future high-energy-density batteries. Currently, this material has been used in high-end electronic products, and with continued technological breakthroughs, its penetration in the electric vehicle sector is expected to gradually increase.

[0003] However, the existing method of vapor deposition of nano-silicon on porous carbon will cause gaps between the nano-silicon particles and the porous carbon, resulting in a large electrode resistance and affecting battery performance.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The first objective of the present invention is to provide a silicon-carbon material in which a carboxylated conductive carbon material is coated on the surface of nano-silicon particles through hydrogen bonding, creating a close connection between the two. This effectively improves the conductivity of the silicon-carbon material, forms a smooth ion pathway, and reduces the resistivity of the negative electrode sheet made from the silicon-carbon material. This solves the problem that conventional vapor-deposited nano-silicon on porous carbon results in gaps between the nano-silicon particles and the porous carbon, which in turn increases the electrode sheet resistance.

[0006] The second object of the present invention is to provide a method for preparing silicon-carbon material, which is simple to operate and can achieve batch production.

[0007] The third object of the present invention is to provide a secondary battery with excellent cycle performance, small thickness rebound of the electrode when fully charged, and low internal resistance of the battery.

[0008] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted:

[0009] The present invention first provides a silicon-carbon material, comprising a silicon-carbon core and a hard carbon shell coating the silicon-carbon core, with a gap between the silicon-carbon core and the hard carbon shell; the silicon-carbon core comprises nano-silicon and a carboxylated conductive carbon material coated on the surface of the nano-silicon; wherein the carboxyl group in the carboxylated conductive carbon material forms a hydrogen bond with the hydroxyl group in the nano-silicon.

[0010] Furthermore, the carboxylated conductive carbon material includes at least one of carboxylated carbon nanotubes, carboxylated graphene and carboxylated graphene nanoribbons.

[0011] Furthermore, the distance between the silicon carbon core and the hard carbon shell is 5 to 200 nm.

[0012] Furthermore, the thickness of the hard carbon shell is 10 to 500 nm.

[0013] The present invention further provides a method for preparing the above-mentioned silicon-carbon material, comprising the following steps: adding an organic silicon source to a mixture of nano-silica sol, carboxylated conductive carbon material, alkali solution and solvent and performing self-assembly coating and hydrolysis coating reactions, solid-liquid separation and then calcining to obtain an intermediate material with a SiO2-coated silicon-carbon core, wherein the silicon-carbon core includes nano-silicon and carboxylated conductive carbon material that is self-assembled and coated on the surface of the nano-silicon; mixing the intermediate material, organic carbon source and organic solvent, then performing solid-liquid separation and sintering to obtain a composite material of the hard carbon-coated intermediate material; mixing the composite material with hydrofluoric acid and performing an etching reaction to obtain the silicon-carbon material.

[0014] Furthermore, the mass ratio of the nano-silicon in the nano-silica sol to the carboxylated conductive carbon material is 1 to 10:1.

[0015] Furthermore, the mass of the organic silicon source is 1% to 100% of the mass of the nano-silicon in the nano-silica sol.

[0016] Furthermore, the preparation method of the carboxylated conductive carbon material includes: mixing the conductive carbon material with a dilute acid solution and ultrasonically obtaining a decontaminated conductive carbon material; and mixing the decontaminated conductive carbon material with concentrated acid and then performing a carboxylation reaction to obtain the carboxylated conductive carbon material.

[0017] Furthermore, the conductive carbon material includes at least one of carbon nanotubes, graphene and graphene nanoribbons.

[0018] Furthermore, the dilute acid solution includes at least one of a dilute hydrochloric acid solution and a dilute nitric acid solution.

[0019] Furthermore, the molar concentration of the dilute acid solution is 0.5 to 1 mol / L.

[0020] Furthermore, the concentrated acid includes a mixed acid of concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 2 to 7:1.

[0021] Furthermore, the mass ratio of the conductive carbon material after impurity removal to the concentrated acid is 1:5-10.

[0022] Furthermore, the impurity-removed conductive carbon material and the concentrated acid are ultrasonically mixed during the mixing process.

[0023] Furthermore, the temperature of the carboxylation reaction is 50 to 80° C., and the time of the carboxylation reaction is 2 to 12 hours.

[0024] Furthermore, the preparation method of the nano-silica sol includes: mixing nano-silica with hydrofluoric acid and ultrasonicating, washing after solid-liquid separation, and then adding ethanol and ultrasonicating to obtain the nano-silica sol.

[0025] Furthermore, the median particle size of the nano-silicon is 20 to 500 nm.

[0026] Furthermore, the alkali solution includes an ammonia solution.

[0027] Furthermore, the solvent includes a mixed solvent of ethanol and deionized water.

[0028] Furthermore, the temperature of the self-assembly coating and hydrolysis coating reaction is 60-80° C., and the time of the self-assembly coating and hydrolysis coating reaction is 1-10 hours.

[0029] Furthermore, the organosilicon source is added by dropwise addition at a rate of 1 to 7 mL / h.

[0030] Furthermore, the organic silicon source includes tetraethoxysilane.

[0031] Furthermore, the calcination temperature is 300-400° C., and the calcination holding time is 1-10 hours.

[0032] Furthermore, the mass ratio of the intermediate material to the organic carbon source is 1:0.1-10.

[0033] Furthermore, the organic carbon source includes at least one of phytic acid, citric acid, polyacrylic acid, ascorbic acid, chitosan, cellulose, polyvinyl alcohol, glucose and sucrose.

[0034] Furthermore, the sintering temperature is 700-1000° C., and the holding time is 2-10 hours.

[0035] The present invention also provides a secondary battery comprising the above silicon-carbon material.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) The silicon-carbon material provided by the present invention comprises a carboxylated conductive carbon material coated on the surface of nano-silicon particles through hydrogen bond self-assembly. The carboxylated conductive carbon material and the nano-silicon are tightly bonded, which effectively improves the conductivity of the silicon-carbon material and forms a smooth ion path, thereby reducing the resistivity of the negative electrode made from the silicon-carbon material. This solves the problem that the existing vapor-phase deposition of nano-silicon on porous carbon leads to gaps between the nano-silicon particles and the porous carbon, thereby increasing the electrode sheet resistance.

[0038] (2) The silicon-carbon material provided by the present invention provides space for silicon to expand, solving the problem of silicon volume expansion. Furthermore, the outermost hard carbon shell can restrain the expansion of the silicon anode.

[0039] (3) The silicon-carbon material provided by the present invention is used to make negative electrode sheets and batteries, which can improve the cycle performance of the battery, reduce the rebound of the electrode sheet when it is fully charged, and reduce the internal resistance of the battery. DETAILED DESCRIPTION

[0040] The technical scheme of the present invention will be clearly and completely described below in conjunction with specific embodiments, but it will be understood by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work premise belong to the scope of protection of the present invention. Those who do not specify specific conditions in the embodiments are carried out according to normal conditions or the conditions recommended by the manufacturer. Those whose reagents or instruments are not specified by the manufacturer are conventional products that can be purchased commercially.

[0041] Unless otherwise specified, in the present invention, terms such as "first aspect," "second aspect," "third aspect," and "fourth aspect" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor as implicitly indicating the importance or quantity of the technical features indicated. Furthermore, terms such as "first," "second," "third," and "fourth" serve only as non-exhaustive enumeration and description, and should not constitute closed-ended limitations on quantity.

[0042] Unless otherwise specified, the terms "include" and "comprising" used in the present invention may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0043] Unless otherwise specified, in the present invention, "one or more" or "at least one" refers to any one, any two, or any two or more of the listed items. Among them, "several" refers to any two or any two or more.

[0044] In a first aspect, the present invention provides a silicon-carbon material for a battery negative electrode, comprising a silicon-carbon core and a hard carbon shell covering the silicon-carbon core, with a gap between the silicon-carbon core and the hard carbon shell. It is understood that the hard carbon shell is made of hard carbon.

[0045] The silicon-carbon core comprises nano-silicon and a carboxylated conductive carbon material coated on the surface of the nano-silicon; wherein the carboxyl groups in the carboxylated conductive carbon material form hydrogen bonds with the hydroxyl groups in the nano-silicon.

[0046] The carboxyl groups in the carboxylated conductive carbon material form hydrogen bonds with the hydroxyl groups in the nano-silicon. The carboxylated conductive carbon material self-assembles and coats the surface of the nano-silicon particles through hydrogen bonding. The carboxylated conductive carbon material and the nano-silicon are tightly bonded, effectively improving the conductivity of the silicon-carbon material and forming a smooth ion pathway, thereby reducing the resistivity of the negative electrode made from the silicon-carbon material. This solves the problem of existing vapor-phase deposition of nano-silicon on porous carbon, which results in gaps between the nano-silicon particles and the porous carbon, thereby increasing the electrode sheet resistance.

[0047] In addition, the gap between the silicon-carbon core and the hard carbon shell provides space for silicon to expand, solving the problem of volume expansion of silicon materials.

[0048] In addition, the hard carbon shell located on the outermost layer can restrain the expansion of the silicon anode.

[0049] Therefore, the silicon-carbon material provided by the present invention has the advantages of high capacity and low internal resistance.

[0050] In some specific embodiments, the carboxylated conductive carbon material includes at least one of carboxylated carbon nanotubes, carboxylated graphene, and carboxylated graphene nanoribbons. This helps improve the conductivity of the silicon-carbon material and reduce the electrode sheet resistance.

[0051] In some specific embodiments, the distance between the silicon-carbon core and the hard carbon shell is 5 to 200 nm, including but not limited to any one of 5 nm, 10 nm, 20 nm, 30 nm, 50 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm, and 200 nm, or any range therebetween. This facilitates expansion of the silicon. It should be understood that the distance between the silicon-carbon core and the hard carbon shell described above refers to the average distance.

[0052] In some specific embodiments, the hard carbon shell has a thickness of 10 to 500 nm, including but not limited to any one of 10 nm, 20 nm, 30 nm, 50 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, and 500 nm, or any range therebetween. It should be understood that the aforementioned thickness of the hard carbon shell refers to an average thickness.

[0053] In some specific embodiments, the median particle size of the nano-silicon in the silicon-carbon core is 20 to 500 nm, including but not limited to any point value of 20 nm, 30 nm, 50 nm, 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm or a range value between any two of them.

[0054] In a second aspect, the present invention provides a method for preparing a silicon-carbon material, comprising the following steps:

[0055] First, in an inert environment (such as a nitrogen atmosphere or an argon atmosphere), an organic silicon source is added to a mixed material of nano-silica sol, a carboxylated conductive carbon material, an alkali solution and a solvent, and a self-assembly coating and a hydrolysis coating reaction are carried out, that is, a reaction in which the carboxylated conductive carbon material self-assembles and coats the nano-silicon and the organic silicon source is hydrolyzed to generate silicon dioxide and then coated on the surface of the silicon-carbon core. During the reaction, magnetic stirring is performed at a speed of 400 to 600 rpm. The product is then collected by centrifugation, washed with ethanol, and vacuum-dried at 50 to 70°C for 12 to 24 hours. It is then calcined under an inert atmosphere (such as nitrogen or argon) to obtain an intermediate material of a SiO2-coated silicon-carbon core, wherein the silicon-carbon core includes nano-silicon and a carboxylated conductive carbon material that is self-assembled and coated on the surface of the nano-silicon.

[0056] Then, the intermediate material, an organic carbon source, and an organic solvent (such as ethanol) are mixed to coat the intermediate material with the organic carbon source, followed by centrifugal drying and sintering under an inert atmosphere (such as nitrogen or argon) to obtain a composite material of the intermediate material coated with hard carbon. The hard carbon is formed by sintering the organic carbon source.

[0057] Finally, the composite material is mixed with hydrofluoric acid and subjected to an etching reaction to remove the SiO2 in the composite material. The material is then centrifuged and washed multiple times to obtain the silicon-carbon material. This material comprises a silicon-carbon core coated with hard carbon, with a gap between the hard carbon and the silicon-carbon core. The silicon-carbon core comprises nanosilicon and a carboxylated conductive carbon material coated on the surface of the nanosilicon.

[0058] The carboxyl groups on the surface of the carboxylated conductive carbon material form hydrogen bonds with the hydroxyl groups on the surface of the nano-silicon, firmly bonding the carboxylated conductive carbon material to the surface of the nano-silicon particles. This preparation method can produce a silicon-carbon material with a specific structure, significantly improving its conductivity and reducing the resistivity of the negative electrode sheet made from it.

[0059] Moreover, the preparation method is simple to operate and can realize mass production.

[0060] In some specific embodiments, the mass ratio of the nano-silicon in the nano-silica sol to the carboxylated conductive carbon material is 1 to 10:1, for example, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 or 10:1.

[0061] In some specific embodiments, the mass of the organosilicon source is 1% to 100% of the mass of the nano-silicon in the nano-silica sol / the mass of the carboxylated conductive carbon material / the sum of the mass of the nano-silicon in the nano-silica sol and the mass of the carboxylated conductive carbon material, including but not limited to any one of 1%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%, or any range therebetween. By controlling the mass of the organosilicon source, the thickness of the SiO2 coating formed by its hydrolysis and the size of the voids subsequently formed can be controlled.

[0062] In some specific embodiments, the preparation method of the carboxylated conductive carbon material, i.e., the carboxylation treatment method of the conductive carbon material, comprises: ultrasonically mixing the conductive carbon material with a dilute acid solution, ultrasonically dispersing the conductive carbon material, and then centrifuging and washing, repeating the process several times to obtain a decontaminated conductive carbon material. The decontaminated conductive carbon material is mixed with concentrated acid and ultrasonically dispersed for 10 to 30 minutes (to ensure uniform mixing), followed by a carboxylation reaction. The liquid obtained after the carboxylation reaction is cooled and diluted with deionized water, and then dialyzed and washed multiple times until the pH of the dispersion is 6 to 7. Finally, the resulting dispersion is freeze-dried to obtain the carboxylated conductive carbon material.

[0063] Among them, the dilute acid solution can remove impurities in the conductive carbon material.

[0064] Dilution and washing can remove acid impurities in the carboxylated conductive carbon material.

[0065] In some specific embodiments, the conductive carbon material includes at least one of carbon nanotubes, graphene, and graphene nanoribbons.

[0066] In some specific embodiments, the dilute acid solution includes at least one of a dilute hydrochloric acid solution and a dilute nitric acid solution.

[0067] In some specific embodiments, the molar concentration of the dilute acid solution is 0.5 to 1 mol / L, including but not limited to any one of 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, and 1 mol / L, or a range between any two of them.

[0068] In some specific embodiments, the concentrated acid comprises a mixed acid of concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 2 to 7:1 (e.g., 2:1, 3:1, 4:1, 5:1, 6:1, or 7:1).

[0069] In some specific embodiments, the mass ratio of the conductive carbon material after impurity removal to the concentrated acid is 1:5 to 10, for example, 1:5, 1:6, 1:7, 1:8, 1:9 or 1:10.

[0070] In some specific embodiments, the impurity-removed conductive carbon material and the concentrated acid are ultrasonically mixed during the mixing process to ensure uniform dispersion.

[0071] In some specific embodiments, the temperature of the carboxylation reaction of the conductive carbon material after impurity removal and concentrated acid is 50-80°C, including but not limited to any one of 50°C, 60°C, 70°C, and 80°C, or a range of values ​​between any two of them; the time of the carboxylation reaction of the conductive carbon material after impurity removal and concentrated acid is 2-12h, including but not limited to any one of 2h, 3h, 4h, 5h, 6h, 8h, 10h, and 12h, or a range of values ​​between any two of them.

[0072] In some specific embodiments, the reaction further comprises washing and drying steps after completion.

[0073] In some specific embodiments, the preparation method of the nano-silica sol includes: mixing nano-silica with hydrofluoric acid and ultrasonicating for 5 to 15 minutes, followed by immediate centrifugation, washing three times with deionized water to obtain a fresh silicon surface, adding ethanol and ultrasonically dispersing for 30 to 60 minutes to obtain the nano-silica sol. This step can remove the native SiO2 layer.

[0074] In some specific embodiments, the median particle size of the nano-silicon is 20 to 500 nm, including but not limited to any point value of 20 nm, 30 nm, 50 nm, 80 nm, 100 nm, 150 nm, 200 nm, 25 nm, 300 nm, 350 nm, 400 nm, 450 nm, and 500 nm, or a range value between any two of them.

[0075] In some specific embodiments, the alkaline solution includes an ammonia solution, which can provide a weak alkaline environment to promote the hydrolysis of the organosilicon source.

[0076] In some specific embodiments, the solvent includes a mixed solvent of ethanol and deionized water.

[0077] In some specific embodiments, the temperature of the self-assembly coating and hydrolysis coating reaction is 60-80° C., for example, 60° C., 65° C., 70° C., 75° C., or 80° C. The time of the self-assembly coating and hydrolysis coating reaction is 1-10 h, for example, 1 h, 2 h, 4 h, 5 h, 6 h, 8 h, or 10 h.

[0078] In some specific embodiments, the organosilicon source is slowly added dropwise through a constant pressure dropping funnel at a rate of 1 to 7 mL / h, including but not limited to any one of 1 mL / h, 2 mL / h, 3 mL / h, 4 mL / h, 5 mL / h, 6 mL / h, and 7 mL / h, or any range therebetween. Excessively fast rates may result in a porous / discontinuous coating layer.

[0079] In some specific embodiments, the organosilicon source includes tetraethoxysilane (TEOS).

[0080] In some specific embodiments, the calcination temperature is 300-400° C., for example, 300° C., 320° C., 350° C., 360° C., 380° C., or 400° C. The calcination holding time is 1-10 h, for example, 1 h, 2 h, 4 h, 5 h, 6 h, 8 h, or 10 h.

[0081] In some specific embodiments, the mass ratio of the intermediate material to the organic carbon source is 1:0.1-10, for example, 1:0.1, 1:0.5, 1:1, 1:2, 1:3, 1:5, 1:8 or 1:10.

[0082] In some specific embodiments, the organic carbon source includes at least one of phytic acid, citric acid, polyacrylic acid, ascorbic acid, chitosan, cellulose, polyvinyl alcohol, glucose and sucrose.

[0083] In some specific embodiments, the sintering temperature is 700-1000° C., including but not limited to any one of 700° C., 750° C., 800° C., 850° C., 900° C., 950° C., and 1000° C., or any range therebetween. The sintering holding time is 2-10 h, including but not limited to any one of 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, and 10 h, or any range therebetween.

[0084] In a third aspect, the present invention provides a secondary battery comprising a silicon-carbon material.

[0085] The secondary batteries include lithium-ion batteries and / or sodium-ion batteries.

[0086] It is understood that silicon-carbon materials are used for the negative electrode of secondary batteries. Specifically, the secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the negative electrode sheet includes a current collector and a negative electrode active layer, and the negative electrode active layer contains the above-mentioned silicon-carbon material.

[0087] The use of the above silicon-carbon material can improve the electrochemical performance of the secondary battery.

[0088] The embodiments of the present invention will be described in detail below with reference to the examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present invention and should not be construed as limiting the scope of the invention. Where specific conditions are not specified in the examples, the methods were performed according to conventional conditions or the conditions recommended by the manufacturer. Where the manufacturers of the reagents or instruments are not specified, they are all commercially available conventional products.

[0089] Example 1

[0090] The method for preparing the silicon-carbon material provided in this embodiment comprises the following steps:

[0091] (1) Carboxylation treatment of conductive carbon materials: First, carbon nanotubes are mixed with 0.8 mol / L dilute hydrochloric acid solution and ultrasonically removed to remove impurities in the carbon nanotubes. After ultrasonic dispersion, centrifugal washing is repeated several times to obtain the impurity-removed conductive carbon material. Concentrated nitric acid and concentrated sulfuric acid are mixed in a volume ratio of 3:1, and the above-mentioned impurity-removed conductive carbon material is added thereto. Ultrasonic dispersion is carried out for 20 minutes to ensure uniform mixing. The mass ratio of the impurity-removed conductive carbon material to the concentrated acid is 1:5. The mixture is then stirred at 60°C and reacted for 8 hours. The mixed liquid obtained after the reaction is then cooled and diluted with deionized water. The mixture is then dialyzed and washed several times until the pH of the dispersion is 6.5. Finally, the obtained dispersion is freeze-dried to obtain carboxylated carbon nanotubes.

[0092] (2) Method for self-assembly and coating nano-silicon with carboxylated conductive carbon materials: First, 1 g of nano-silicon powder (median particle size of 200 nm) was added to 50 mL of hydrofluoric acid (mass fraction of 2 wt%) and ultrasonicated for 10 min, followed by immediate centrifugation and washing three times with deionized water to obtain a fresh silicon surface; then 50 mL of ethanol was added thereto and ultrasonically dispersed for 30 min to obtain a stable nano-silicon ethanol sol. The above-mentioned nanosilicon ethanol sol (containing 200 mg of nanosilicon), 200 mL of ethanol and 50 mL of deionized water, 5 mL of ammonia solution and 100 mg of carboxylated carbon nanotubes (i.e., the mass ratio of nanosilicon to carboxylated conductive carbon material in the nanosilica sol is 2:1) are added to a three-necked flask in a nitrogen environment, heated to 60°C, and magnetically stirred at a speed of 500 rpm; then TEOS (tetraethoxysilane) is slowly added thereto through a constant pressure dropping funnel, wherein the mass of TEOS is 30% of the mass of nanosilicon in the nanosilica sol, and the dropping rate is controlled to be 3 mL / h, and self-assembly coating and hydrolysis coating reactions are carried out. After the reaction is continued for 4 hours, the product is collected by centrifugation, washed with ethanol, and vacuum dried at 60°C for 12 hours to obtain the reacted material. Finally, the reacted material was calcined at 300°C for 2h in an argon environment to obtain an intermediate material with a SiO2-coated silicon-carbon core (denoted as SiO2@CNT / Si), in which the silicon-carbon core included nanosilicon and a carboxylated conductive carbon material self-assembled and coated on the surface of the nanosilicon.

[0093] (3) Preparation of silicon-carbon materials: First, the SiO2@CNT / Si was added to ethanol, and phytic acid was added thereto, wherein the mass ratio of SiO2@CNT / Si to phytic acid was 1:5. The mixed material was then centrifuged and dried to obtain a silicon-carbon material precursor. The silicon-carbon material precursor was then sintered at 800°C in an argon atmosphere for 8 hours to obtain a composite material of a hard carbon-coated intermediate material. Finally, hydrofluoric acid was added to the composite material to perform an etching reaction. After multiple centrifugal washings, the silicon-carbon material was obtained.

[0094] This silicon-carbon material comprises a silicon-carbon core and a hard carbon shell coating the core, with a gap between the core and the shell. The core comprises nano-silicon and a carboxylated conductive carbon material that self-assembles and coats the nano-silicon. The carboxyl groups in the carboxylated conductive carbon material form hydrogen bonds with the hydroxyl groups in the nano-silicon. The distance between the core and the hard carbon shell is 10 nm, and the hard carbon shell is 20 nm thick.

[0095] Example 2

[0096] The preparation method of the silicon-carbon material provided in this embodiment is substantially the same as that in embodiment 1, except that in step (1), the carbon nanotubes are replaced with graphene to obtain carboxylated graphene.

[0097] Example 3

[0098] The preparation method of the silicon-carbon material provided in this embodiment is substantially the same as that in embodiment 1, except that in step (1), the carbon nanotubes are replaced with graphene nanoribbons to obtain carboxylated graphene nanoribbons.

[0099] Example 4

[0100] The preparation method of the silicon-carbon material provided in this embodiment is substantially the same as that in embodiment 1, except that in step (1), the mass ratio / molar ratio of the conductive carbon material after impurity removal to the concentrated acid is replaced with 1:7.

[0101] Example 5

[0102] The preparation method of the silicon-carbon material provided in this embodiment is substantially the same as that in embodiment 1, except that in step (2), the mass ratio of nano-silicon to carboxylated conductive carbon material in the nano-silica sol is replaced with 7:1.

[0103] Example 6

[0104] The preparation method of the silicon-carbon material provided in this embodiment is substantially the same as that in embodiment 1, except that in step (2), the mass of TEOS is replaced by 70% of the mass of nano-silicon in the nano-silica sol.

[0105] In the silicon-carbon material prepared in this embodiment, the distance between the silicon-carbon core and the hard carbon shell is 60 nm.

[0106] Example 7

[0107] The preparation method of the silicon-carbon material provided in this embodiment is basically the same as that in Example 1, except that in step (3), the mass ratio of SiO2@CNT / Si to phytic acid is replaced with 1:2.

[0108] Example 8

[0109] The preparation method of the silicon-carbon material provided in this embodiment is basically the same as that in Example 1, except that in step (3), phytic acid is replaced by an equal mass of citric acid.

[0110] In the silicon-carbon material prepared in this embodiment, the thickness of the hard carbon shell is 50 nm.

[0111] Example 9

[0112] The preparation method of the silicon-carbon material provided in this embodiment is basically the same as that in Example 1, except that in step (3), phytic acid is replaced by an equal mass of glucose.

[0113] In the silicon-carbon material prepared in this embodiment, the thickness of the hard carbon shell is 70 nm.

[0114] Example 10

[0115] The preparation method of the silicon-carbon material provided in this embodiment is basically the same as that in Example 1, except that in step (2), the median particle size of the nano-silicon powder is replaced with 400 nm.

[0116] Comparative Example 1

[0117] The preparation method of the silicon-carbon material provided in this comparative example is the vapor deposition of nano-silicon on porous carbon, which specifically includes the following steps: using SiH4 as the silicon source, depositing nano-silicon on ordered mesoporous carbon: placing the mesoporous carbon in a tubular furnace, passing Ar gas to purge for 30 minutes, heating to 600°C, passing 5% SiH4 / Ar mixed gas (flow rate 100sccm), depositing for 30 minutes, and cooling to obtain a composite material in which silicon particles are uniformly distributed in the pores.

[0118] Comparative Example 2

[0119] The preparation method of the silicon-carbon material provided in this comparative example is basically the same as that in Example 1, except that TEOS is not added in step (2) and there is no step (3).

[0120] That is, the silicon-carbon material prepared in this comparative example includes nano-silicon and the carboxylated conductive carbon material (without a hard carbon shell) self-assembled and coated on the surface of the nano-silicon.

[0121] Comparative Example 3

[0122] The preparation method of the silicon-carbon material provided in this comparative example is basically the same as that in Example 1, except that TEOS is not added in step (2) and hydrofluoric acid is not added in step (3).

[0123] That is, the silicon-carbon material prepared in this comparative example comprises a silicon-carbon core and a hard carbon shell covering the silicon-carbon core, wherein there is no gap between the silicon-carbon core and the hard carbon shell.

[0124] Experimental example

[0125] The silicon-carbon materials prepared in each embodiment and each comparative example were respectively used to make negative electrode sheets, and then the sheet resistivity was tested. The results are shown in Table 1.

[0126] The preparation method of the negative electrode sheet includes: mixing the above silicon-carbon material with carbon black, sodium carboxymethyl cellulose, and polyacrylic acid binder in a mass ratio of 95:1:1:3, adding deionized water to homogenize the slurry with a solid content of 50%, coating the resulting slurry on a 10μm copper foil, and drying it to a density of 5mg / cm 2 .

[0127] The test method for the resistivity of the negative electrode sheet includes: (1) Sample preparation: The electrode sheet must be flat and wrinkle-free, with a size of ≥10mm×10mm and a thickness of ≤100μm (if too thick, it needs to be thinned); remove burrs from the edge of the electrode sheet to avoid short circuits. (2) Equipment setup: Use a four-probe resistivity tester (such as the RTS-9 model), with four probes arranged in a straight line, usually with a spacing of 1mm; apply a constant current (1-10mA) to ensure that the current passes evenly through the electrode sheet. (3) Data acquisition: Press the probe lightly on the surface of the electrode sheet (pressure of about 0.1-0.5N) to avoid piercing the electrode sheet; record the voltage value, repeat the measurement 3-5 times, and take the average value. (4) Calculation method: For thin electrode sheets (thickness d<<probe spacing), the resistivity formula is: ρ=I / V×C×d; where V is voltage (V), I is current (A), d is electrode thickness (cm), and C is the probe correction factor (related to the probe spacing, usually calibrated by the equipment).

[0128] Table 1 Negative electrode sheet resistivity test results

[0129]

[0130]

[0131] It can be seen from Table 1 that, compared with Comparative Example 1, the silicon-carbon materials prepared in each embodiment have a lower negative electrode sheet resistivity.

[0132] It can be seen that the carboxylated conductive carbon material in the silicon-carbon material provided by the present invention is coated on the surface of the nano-silicon particles through hydrogen bond self-assembly, and the two are tightly connected, which effectively improves the conductive properties of the silicon-carbon material, forms a smooth ion path, and reduces the electrode sheet resistivity of the negative electrode made of the silicon-carbon material.

[0133] Furthermore, button batteries were made using the silicon-carbon materials prepared in each embodiment and each comparative example, and electrochemical tests were performed. The results are shown in Table 2.

[0134] The button cell manufacturing method includes: mixing silicon carbon material, acetylene black and PVDF in a mass ratio of 8:1:1, adding NMP to the mixture to obtain a slurry with a solid content of 50%, and then coating the prepared slurry on a 12μm copper foil and drying it to obtain a coating surface density of 10mg / cm 2 , obtaining a negative electrode sheet; assembling the negative electrode sheet and lithium metal into a CR2032 button battery, and selecting 1M LiPF6 in EC+EMC (v:v=1:1) with 3% FEC as the electrolyte.

[0135] The electrochemical testing method of the button cell includes: first charging and discharging the battery at a low current of 0.1C for 3 cycles to form the battery, and then cycling the battery at 0.5C charge and 1C discharge (1C = 2000 mA / g).

[0136] Table 2 Electrochemical performance test results

[0137]

[0138]

[0139] It can be seen from Table 2 that, compared with Comparative Examples 1 to 3, the batteries made of silicon-carbon materials in each embodiment have more excellent electrochemical performance.

[0140] Among them, in Comparative Example 2, the thickness of the electrode rebounded greatly when fully charged because the silicon-carbon material obtained had no hard carbon shell and silicon expansion space.

[0141] In Comparative Example 3, the capacity rapidly decays due to the lack of silicon expansion space.

[0142] It can be seen that the use of the silicon-carbon material provided by the present invention to prepare negative electrode sheets and batteries can improve the cycle performance of the battery and reduce the full-charge rebound of the electrode sheet and the internal resistance of the battery.

[0143] Although the present invention has been illustrated and described using specific embodiments, it should be appreciated that the above embodiments are merely intended to illustrate the technical solutions of the present invention rather than to limit them. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and scope of the present invention. However, these modifications or replacements do not deviate from the essence of the corresponding technical solutions within the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A silicon-carbon material, characterized in that: The silicon carbon core comprises a silicon carbon core and a hard carbon shell covering the silicon carbon core, wherein a gap is formed between the silicon carbon core and the hard carbon shell; The silicon-carbon core comprises nano-silicon and a carboxylated conductive carbon material coated on the surface of the nano-silicon; wherein the carboxyl groups in the carboxylated conductive carbon material form hydrogen bonds with the hydroxyl groups in the nano-silicon.

2. The silicon-carbon material according to claim 1, characterized in that: The carboxylated conductive carbon material includes at least one of carboxylated carbon nanotubes, carboxylated graphene and carboxylated graphene nanoribbons.

3. The silicon-carbon material according to claim 1, characterized in that: The distance between the silicon carbon core and the hard carbon shell is 5 to 200 nm; And / or, the thickness of the hard carbon shell is 10 to 500 nm.

4. The method for preparing the silicon-carbon material according to any one of claims 1 to 3, wherein: The steps include: Adding an organic silicon source to a mixture of nano-silica sol, carboxylated conductive carbon material, alkali solution and solvent and performing self-assembly coating and hydrolysis coating reactions, followed by solid-liquid separation and calcination to obtain an intermediate material having a SiO2-coated silicon-carbon core, wherein the silicon-carbon core comprises nano-silicon and the carboxylated conductive carbon material self-assembled and coated on the surface of the nano-silicon; The intermediate material, an organic carbon source and an organic solvent are mixed, followed by solid-liquid separation and sintering to obtain a composite material of the hard carbon-coated intermediate material; The composite material is mixed with hydrofluoric acid and subjected to etching reaction to obtain the silicon-carbon material.

5. The method for preparing the silicon-carbon material according to claim 4, wherein: The mass ratio of the nano-silicon in the nano-silica sol to the carboxylated conductive carbon material is 1 to 10:1; And / or, the mass of the organic silicon source is 1% to 100% of the mass of the nano-silicon in the nano-silica sol.

6. The method for preparing the silicon-carbon material according to claim 4, wherein: The preparation method of the carboxylated conductive carbon material comprises: mixing the conductive carbon material with a dilute acid solution and ultrasonically obtaining a conductive carbon material after impurities are removed; mixing the conductive carbon material after impurities are removed with concentrated acid and then performing a carboxylation reaction to obtain the carboxylated conductive carbon material; Preferably, the conductive carbon material comprises at least one of carbon nanotubes, graphene and graphene nanoribbons; Preferably, the dilute acid solution comprises at least one of a dilute hydrochloric acid solution and a dilute nitric acid solution; Preferably, the molar concentration of the dilute acid solution is 0.5 to 1 mol / L; Preferably, the concentrated acid comprises a mixed acid of concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 2 to 7:1; Preferably, the mass ratio of the impurity-removed conductive carbon material to the concentrated acid is 1:5-10; Preferably, the impurity-removed conductive carbon material and the concentrated acid are ultrasonically mixed during the mixing process; Preferably, the temperature of the carboxylation reaction is 50 to 80° C., and the time of the carboxylation reaction is 2 to 12 hours.

7. The method for preparing the silicon-carbon material according to claim 4, wherein: The preparation method of the nano-silica sol comprises: mixing nano-silica with hydrofluoric acid and ultrasonicating, washing after solid-liquid separation, and then adding ethanol and ultrasonicating to obtain the nano-silica sol; Preferably, the median particle size of the nano-silicon is 20 to 500 nm.

8. The method for preparing the silicon-carbon material according to claim 4, wherein: At least one of the following conditions is met: (1) The alkali solution includes an ammonia solution; (2) The solvent includes a mixed solvent of ethanol and deionized water; (3) The temperature of the self-assembly coating and hydrolysis coating reaction is 60-80° C., and the time of the self-assembly coating and hydrolysis coating reaction is 1-10 hours; (4) The organosilicon source is added dropwise at a rate of 1 to 7 mL / h; (5) The organosilicon source includes tetraethoxysilane; (6) The calcination temperature is 300-400°C, and the calcination holding time is 1-10 hours.

9. The method for preparing the silicon-carbon material according to claim 4, wherein: At least one of the following conditions is met: (1) The mass ratio of the intermediate material to the organic carbon source is 1:0.1-10; (2) the organic carbon source comprises at least one of phytic acid, citric acid, polyacrylic acid, ascorbic acid, chitosan, cellulose, polyvinyl alcohol, glucose and sucrose; (3) The sintering temperature is 700-1000°C, and the holding time is 2-10 hours.

10. A secondary battery, characterized in that: The method comprises the silicon-carbon material according to any one of claims 1 to 3.

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