Silicon-carbon composite active material and preparation method thereof, negative electrode sheet and secondary battery

Abstract

The present invention belongs to the technical field of secondary batteries, and in particular, relates to a silicon-carbon composite active material and a preparation method thereof, a negative electrode sheet and a secondary battery. The method comprises the following steps: step S1, embedding a silicon-oxygen material into a graphite material to obtain a silicon-carbon mixture, screening, and the material on the screen is a silicon-embedded mixture; step S2, mixing the silicon-embedded mixture with a sealing agent, sealing, crushing, and screening to obtain a silicon-carbon composite active material; wherein, the particle size D50 of the silicon-oxygen material is 1 to 60 μm, and the particle size D50 of the graphite material is 0.1 to 10 μm. The preparation method of a silicon-carbon composite active material of the present invention provides more contact sites between the silicon-oxygen material and the graphite material by setting silicon-oxygen material and graphite material of a certain particle size, so that the silicon-oxygen material is more easily embedded in the graphite material to form a silicon-embedded mixture, and then the silicon-carbon composite active material is obtained by sealing. The preparation method is simple, easy to operate, and can be mass-produced.

Description

Technical Field

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

[0002] In the past two decades, in order to meet consumers' demand for the energy storage and use capabilities of lithium-ion batteries for communication equipment and electric vehicles, people have been committed to the research and development of high-performance lithium-ion battery technology. Silicon-based materials (such as Si, SiOx) are considered to be one of the most promising alternative materials to replace carbon negative electrode materials due to their ultra-high specific capacity, abundant reserves in nature, and relatively low lithium absorption voltage. However, when pure silicon negative electrodes are fully lithiated, they can produce a huge volume expansion rate of 200% to 300%, which can easily cause the electrode to pulverize or even rupture and the active material to stratify, causing the SEI film on the negative electrode surface to rupture and form repeatedly. The continuous growth of new SEI films will continue to consume electrolytes, resulting in rapid battery capacity decay. How to solve this type of problem is a research hotspot for silicon-based negative electrode materials.

[0003] At present, the most common solution is to introduce a matrix material with small volume effect and good electrical properties into silicon-based materials to prepare silicon-based composite materials. On the one hand, the mechanical flexibility of the matrix material is used as the base material for the spatial expansion of the silicon-based negative electrode material, thereby optimizing its cycle performance. On the other hand, the good conductivity of the matrix material itself is used to overcome the low conductivity of the silicon-based negative electrode material. Graphite has good mechanical properties, but how to overcome the side effects of the volume expansion of SiOx particles in graphite materials is still a thorny problem. Summary of the invention

[0004] One of the purposes of the present invention is to provide a method for preparing a silicon-carbon composite active material in view of the shortcomings of the prior art. By limiting the particle size of the graphite material and the silicon-oxygen material, the silicon-oxygen material is more easily embedded in the graphite material to form a silicon-embedded mixture. After sealing treatment, the silicon-oxygen composite active material is obtained, which has good electrochemical properties and less volume expansion.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing a silicon-carbon composite active material comprises the following steps:

[0007] Step S1, embedding silicon-oxygen material into graphite material to obtain a silicon-carbon mixture, screening, and the material on the screen is the silicon-embedded mixture;

[0008] Step S2, mixing the silicon-embedded mixture with a sealing agent, sealing, crushing, and screening to obtain a silicon-carbon composite active material;

[0009] The particle size D50 of the silicon oxide material is 1 to 60 μm, and the particle size D50 of the graphite material is 0.1 to 10 μm.

[0010] Preferably, in step S1, the weight ratio of the silicon oxide material to the graphite material is 0.1-40:100-120.

[0011] Preferably, the embedding method in step S1 includes at least one of mechanical stirring, ball milling, ultrasonic vibration, mechanical vibration, spraying, sedimentation, deposition, atomization, infiltration, gasification, polymerization, soaking, and liquefaction.

[0012] Preferably, the graphite material is at least one of artificial porous graphite, natural porous graphite, modified porous graphite, porous soft carbon, and porous hard carbon; the silicon-oxygen material is SiOx, where 0≤x≤2, and SiOx includes but is not limited to at least one of silicon oxide, silicon dioxide, and metal-containing composite SiOx.

[0013] Preferably, in step S2, the weight ratio of the silicon-embedded mixture to the sealing agent is 100-120:20-40.

[0014] Preferably, the sealing treatment in step S2 is specifically: mixing the sealing agent with the silicon-embedded mixture at a temperature of 15-400°C.

[0015] Preferably, the sealing agent in step S2 is at least one of lithium borate, polyvinylidene fluoride, polyvinyl fluoride, urea, lithium silicofluoride, and lithium oxalate difluoroborate.

[0016] The second object of the present invention is to provide a silicon-carbon composite active material with good electrochemical performance and low volume expansion rate in view of the deficiencies of the prior art.

[0017] In order to achieve the above object, the present invention adopts the following technical solutions:

[0018] A silicon-carbon composite active material is prepared by the above-mentioned method for preparing the silicon-carbon composite active material.

[0019] The third object of the present invention is to provide a negative electrode sheet with good electrochemical performance and low volume expansion rate in view of the shortcomings of the prior art.

[0020] In order to achieve the above object, the present invention adopts the following technical solutions:

[0021] A negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises the above-mentioned silicon-carbon composite active material.

[0022] A fourth object of the present invention is to provide a secondary battery with good electrochemical performance and low volume expansion rate in view of the deficiencies of the prior art.

[0023] In order to achieve the above object, the present invention adopts the following technical solutions:

[0024] A secondary battery comprises the above-mentioned negative electrode sheet.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: a method for preparing a silicon-carbon composite active material of the present invention, by setting silicon-oxygen material and graphite material of a certain particle size, allows the silicon-oxygen material to have more contact sites with the graphite material, makes it easier for the silicon-oxygen material to be embedded in the graphite material to form a silicon-embedded mixture, and then seals the mixture to obtain the silicon-carbon composite active material. The preparation method is simple, easy to operate, and can be mass-produced. DETAILED DESCRIPTION

[0026] 1. A method for preparing a silicon-carbon composite active material, comprising the following steps:

[0027] Step S1, embedding silicon-oxygen material into graphite material to obtain a silicon-carbon mixture, screening, and the material on the screen is the silicon-embedded mixture;

[0028] Step S2, mixing the silicon-embedded mixture with a sealing agent, sealing, crushing, and screening to obtain a silicon-carbon composite active material;

[0029] The particle size D50 of the silicon oxide material is 1 to 60 μm, preferably 4 to 30 μm, and the particle size D50 of the graphite material is 0.1 to 10 μm, preferably 3 to 6 μm.

[0030] The present invention discloses a method for preparing a silicon-carbon composite active material. By setting silicon-oxygen material and graphite material of a certain particle size, the silicon-oxygen material and the graphite material have more contact sites, so that the silicon-oxygen material is more easily embedded in the graphite material to form a silicon-embedded mixture, and then the silicon-carbon composite active material is obtained by sealing treatment. The preparation method is simple, easy to operate, and can be mass-produced. The sealing treatment can seal the embedded silicon, ensure that the silicon and the porous graphite are firmly combined, and avoid the silicon-oxygen material from expanding and overflowing from the pores or sheets due to volume expansion, while a certain buffer space will still be left inside the pores or sheets of the graphite material, avoiding the low volume of the pores or sheets to reduce the pulverization of Si particles, ensuring the physical integrity of the pole piece, and preventing de-embedding; after the sealing treatment, the material is dried at a temperature of 60 to 150°C, and the sieve material with a particle size ≥Lμm is screened out, which is the silicon-carbon composite active material; further, the L range is 1 to 5.

[0031] The ratio of GR-D50 / SiOx-D50 is 1 to 8, preferably 1.2 to 5. A certain ratio is set to make it easier for the silicon-oxygen material to be embedded in the graphite material to form a silicon-embedded mixture. Due to the embedding of the silicon-oxygen material, the particle size of the silicon-embedded mixture formed is slightly larger than that of the graphite material. After screening, the silicon-embedded mixture with a particle size larger than SiOx-D50 is selected as a silicon-carbon mixture.

[0032] Preferably, in step S1, the weight ratio of the silicon-oxygen material to the graphite material is 0.1-40:100-120. The particle size of the graphite material is set to be larger than that of the silicon-oxygen material, so that the silicon-oxygen material is more easily embedded in the graphite material, and a certain weight ratio is set so that the silicon-oxygen material is fully embedded in the graphite material, so as to obtain a silicon-embedded mixture with good electrochemical performance and low volume expansion rate.

[0033] Further preferably, the weight ratio of the silicon oxide material to the graphite material in step S1 is 0.1-2: 100-105, 2-5: 100-105, 5-8: 100-105, 8-10: 100-105, 10-15: 100-105, 15-20: 100-105, 20-25: 100-105, 25-30: 100-105, 30-35: 1 00~105、35~40:100~105、0.1~2:105~110、2~5:105~110、5~8:105~110、8~10:105~110、10~15:105~110、15~20:105~110、20~25:105~110、25~30:105~110、30~35:105~110、35~ 40: 105-110, 0.1-2: 110-115, 2-5: 110-115, 5-8: 110-115, 8-10: 110-115, 10-15: 110-115, 15-20: 110-115, 20-25: 110-115, 25-30: 110-115, 30-35: 110-115, 35-40: 110-11 5. Any proportion relationship among 0.1-2: 115-120, 2-5: 115-120, 5-8: 115-120, 8-10: 115-120, 10-15: 115-120, 15-20: 115-120, 20-25: 115-120, 25-30: 115-120, 30-35: 115-120, 35-40: 115-120.

[0034] Preferably, the embedding method in step S1 includes at least one of mechanical stirring, ball milling, ultrasonic vibration, mechanical vibration, spraying, sedimentation, deposition, atomization, infiltration, gasification, polymerization, immersion, and liquefaction. The silicon-oxygen material and the graphite material are mixed and embedded by physical means to obtain a silicon-embedded mixture with good electrochemical properties.

[0035] Preferably, the graphite material is at least one of artificial porous graphite, natural porous graphite, modified porous graphite, porous soft carbon, and porous hard carbon; the silicon-oxygen material is SiOx, wherein 0≤x≤2, and SiOx includes at least one of silicon oxide, silicon dioxide, and metal-containing composite SiOx. Silicon-oxygen materials include but are not limited to nano silicon element (Si), micro silicon element (Si), nano silicon oxide (SiO), micro silicon oxide (SiO), nano silicon dioxide (SiO2), micro silicon dioxide (SiO2), metal-containing composite nano SiOx, and metal-containing composite micro SiOx. When x=1, SiOx is silicon oxide SiO. Preferably, SiOx can be doped with at least one element of carbon, sulfur, hydrogen, nitrogen, phosphorus, boron, tin, selenium, lithium, magnesium, sodium, potassium, calcium, beryllium, strontium, zirconium, vanadium, titanium, boron, zinc, aluminum, silver, and fluorine.

[0036] Preferably, in step S2, the weight ratio of the silicon-embedded mixture to the sealing agent is 100-120:20-40. The sealing agent mainly seals the pores in the graphite material to prevent the embedded silicon-oxygen material from coming out and improve the bonding degree.

[0037] Further preferably, the weight ratio of the silicon-embedded mixture to the sealant in step S2 is any one of 100:20, 100:25, 100:28, 100:30, 100:32, 100:35, 100:40, 110:20, 110:25, 110:28, 110:30, 110:32, 110:35, 110:40, 120:20, 120:25, 120:28, 120:30, 120:32, 120:35, and 120:40.

[0038] Preferably, the sealing treatment in step S2 is specifically: mixing the sealing agent with the silicon-embedded mixture at a temperature of 15-400° C. The mixing is performed by one of mechanical stirring / ball milling / ultrasonic vibration and the like.

[0039] Preferably, the sealing agent in step S2 is at least one of lithium borate, polyvinylidene fluoride, polyvinyl fluoride, urea, lithium silicofluoride, and lithium oxalate difluoroborate.

[0040] 2. A silicon-carbon composite active material with good electrochemical properties and low volume expansion rate.

[0041] A silicon-carbon composite active material is prepared by the above-mentioned method for preparing the silicon-carbon composite active material.

[0042] 3. A negative electrode sheet with good electrochemical performance and low volume expansion rate.

[0043] A negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises the above-mentioned silicon-carbon composite active material.

[0044] 4. A secondary battery with good electrochemical performance and low volume expansion rate.

[0045] A secondary battery comprises the above-mentioned negative electrode sheet.

[0046] The secondary battery can be a lithium-ion battery, a magnesium-ion battery, a calcium-ion battery, or a sodium-ion battery. The following is an example of a lithium-ion battery. A lithium-ion battery comprises a positive electrode sheet, a separator, a negative electrode sheet, an electrolyte, and a shell, wherein the separator separates the positive electrode sheet and the negative electrode sheet, and the shell encapsulates and wraps the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte. The negative electrode sheet is the above-mentioned negative electrode sheet.

[0047] A method for preparing a lithium ion battery comprises the following steps:

[0048] Step A1, mixing the above-mentioned silicon-carbon composite active material, conductive additive, and binder in a mass ratio of 80-98:0.5-12:0.5-5 to prepare a slurry to obtain a silicon-carbon composite slurry;

[0049] Step A2, coating the silicon-carbon composite slurry on the negative electrode current collector, drying into a pole piece, pressing, cutting, and vacuum drying to obtain a silicon-carbon composite negative electrode sheet;

[0050] Step A3, stacking the silicon-carbon composite negative electrode sheet, the separator, and the positive electrode sheet in sequence, winding them to obtain a battery cell, casing the battery cell, injecting the electrolyte into the battery shell, encapsulating, standing, forming, and dividing the capacity to obtain a lithium-ion battery.

[0051] In the mixing and slurrying step in the above step A1, the amount of the silicon-carbon composite active material added accounts for 50-99% of the mass of the silicon-carbon composite slurry, preferably 90-98%.

[0052] In the above step A1, the conductive additive is at least one of copper powder, nickel powder, conductive carbon black, acetylene black, graphite, graphene, fibrous carbon conductive agent, carbon nanotubes, tin oxide, iron oxide, zinc oxide, copper oxide, aluminum oxide, metal-fiber composite conductive agent, and metal-carbon composite powder conductive agent.

[0053] In the above step A1, the binder is at least one of the single-group classification, multi-group classification, and polymer classification of guar gum, sodium alginate, acrylic acid, vinyl alcohol, polyaniline, benzimidazole, styrene-butadiene rubber, gum arabic, xanthan gum, karaya gum, sodium / lithium carboxymethyl cellulose, PVDF, etc.

[0054] In the above-mentioned negative electrode sheet, the mass of silicon element in the silicon-carbon composite negative electrode sheet accounts for 0.1% to 80% of the mass of the film on the negative electrode current collector, preferably 2% to 35%.

[0055] The negative electrode current collector is at least one of copper foil, porous copper foil, foam nickel / copper foil, tin-zinc-plated copper foil, carbon-coated copper foil, alloy copper foil, nickel foil, steel foil, and titanium foil.

[0056] The expansion rate of the single-layer membrane on the negative electrode sheet in the above-mentioned negative electrode current collector after the first filling is 21-50%, preferably 20-30%; when the membrane on the silicon-carbon composite negative electrode sheet is n layers, the expansion rate (%) is calculated as = (thickness of the silicon-carbon composite negative electrode sheet after filling-thickness of the silicon-carbon composite negative electrode sheet before not filling) / (thickness of the silicon-carbon composite negative electrode sheet before not filling-thickness of the negative electrode current collector)*100%*(1 / n), n=1, 2, 3, 4, 5, 6, n is a positive integer greater than or equal to 1.

[0057] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material may include but is not limited to a chemical formula such as Li a Ni x Co y M z O 2-b N b (wherein 0.95≤a≤1.2, x>0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from a combination of one or more of Mn and Al, and N is selected from a combination of one or more of F, P, and S) The positive electrode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc., one or more combinations thereof, the positive electrode active material specifically can be at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese phosphate, and lithium iron phosphate. The positive electrode active material can also be modified, and the method for modifying the positive electrode active material should be known to those skilled in the art. For example, the positive electrode active material can be modified by coating, doping, etc. The material used for the modification can include but is not limited to Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc., one or more combinations thereof. The positive electrode current collector is usually a structure or part for collecting current, and the positive electrode current collector can be various materials suitable for use as a positive electrode current collector for lithium ion batteries in the art. For example, the positive electrode current collector can include but is not limited to metal foil, etc., and more specifically can include but is not limited to aluminum foil, etc.

[0058] The separator can be any material suitable for lithium-ion battery separators in the art, for example, it can be a combination of one or more materials including but not limited to polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fiber.

[0059] The lithium-ion battery also includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt and an additive. Among them, the electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in anti-overcharge electrolytes; it can also be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; it can also be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additives include but are not limited to at least one of film-forming additives, conductive additives, flame retardant additives, anti-overcharge additives, additives for controlling the content of H2O and HF in the electrolyte, additives for improving low-temperature performance, and multifunctional additives. Lithium salts include, but are not limited to, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl imide), lithium bis(trifluoromethanesulfonyl imide), lithium trifluoromethanesulfonate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorophosphate, lithium difluorobis(oxalatophosphate) and lithium tetrafluorooxalatophosphate.

[0060] The shell is made of at least one of an aluminum-plastic film soft shell, an aluminum shell, a steel shell, and a plastic hard shell.

[0061] The present invention will be described in further detail below in conjunction with specific implementation modes, but the implementation modes of the present invention are not limited thereto.

[0062] Example 1

[0063] A lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet and a separator separating the positive electrode sheet and the negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet described above.

[0064] Preparation of negative electrode:

[0065] A negative electrode material, comprising:

[0066] 0.5 kg of silicon dioxide SiO2 (containing 0.10 kg of lithium carbonate) with a D50 of 3.7 μm was mixed by mechanical stirring for 2 h at a stirring speed of 300 r / min, and embedded into the pores of 4.5 g of porous graphite with a D50 of 7.4 m to obtain a silicon-carbon mixture, and 4.97 kg of the oversize with a particle size of ≥2 μm was sieved out, which was the silicon-embedded mixture;

[0067] 4.0 kg of silicon-embedded mixture and 0.25 kg of molten polyvinyl fluoride sealing agent were mechanically stirred and mixed for 2 hours at a machine speed of 300 r / min. The mixture was sealed, crushed, dried at 65°C, and 4.11 kg of the sieve material with a particle size ≥ 2 μm was sieved to obtain the silicon-carbon composite active material.

[0068] A negative electrode sheet, the preparation method of the negative electrode sheet is: silicon-carbon composite active material, Super-P, conductive carbon tube CNT, sodium carboxymethyl cellulose CMC, and binder styrene-butadiene rubber SBR are mixed and slurried according to mass ratio to obtain silicon-carbon composite slurry, deionized water is added and negative electrode slurry is obtained under vacuum stirring, the slurry is evenly coated on a current collector copper foil, dried into electrode sheets, pressed, cut, vacuum dried, and dried to obtain SiO / C composite negative electrode sheets.

[0069] Preparation of positive electrode:

[0070] The positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2, NCM811), conductive carbon Super-P, and binder polyvinylidene fluoride PVDF are mixed in a mass ratio to prepare a slurry, and a solvent N-methylpyrrolidone NMP is added to obtain a uniform slurry after vacuum stirring. The slurry is evenly coated on an aluminum foil and dried to obtain a positive electrode sheet.

[0071] Preparation of lithium-ion secondary batteries:

[0072] The negative electrode sheet, separator and positive electrode sheet prepared as above are wound to obtain a battery cell, the battery cell is put into a shell, the electrolyte is injected into the battery shell, packaged, formed and divided into volumes, so as to obtain a lithium ion secondary battery.

[0073] The difference between Example 2-4 and Example 1 lies in the usage of silicon-oxygen composite active material and Super-P. The specific usage of the positive electrode raw material ratio and the negative electrode raw material ratio of Examples 1-4 is recorded in Table 1.

[0074] Example 5

[0075] A lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet and a separator separating the positive electrode sheet and the negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet described above.

[0076] Preparation of negative electrode:

[0077] A negative electrode material, comprising:

[0078] 0.5 kg of silicon dioxide SiO2 (containing 0.15 kg of lithium carbonate) with a D50 of 3.7 μm was mixed by mechanical stirring for 2 h at a stirring speed of 300 r / min, and embedded into 4.5 kg of porous graphite with a D50 of 7.4 m to obtain a silicon-carbon mixture, and 4.99 kg of the oversize with a particle size of ≥ 2 μm was sieved out, which was the silicon-embedded mixture;

[0079] 4.0 kg of silicon-embedded mixture and 0.5 kg of molten urea sealing agent were mechanically stirred and mixed for 2 h at a machine speed of 300 r / min. The mixture was sealed, crushed, dried at 65°C, and 4.37 kg of the sieve material with a particle size ≥ 2 μm was sieved out, which was the silicon-carbon composite active material.

[0080] A negative electrode sheet, the preparation method of the negative electrode sheet is: silicon-carbon composite active material, Super-P, conductive carbon tube CNT, sodium carboxymethyl cellulose CMC, binder styrene butadiene rubber SBR are mixed according to mass ratio, deionized water is added and then a negative electrode slurry is obtained under vacuum stirring, the slurry is evenly coated on copper foil, pressed, and dried to obtain a composite negative electrode sheet.

[0081] Preparation of positive electrode:

[0082] The positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2), conductive carbon Super-P, and binder polyvinylidene fluoride PVDF are mixed according to a mass ratio, and a solvent N-methylpyrrolidone NMP is added to obtain a uniform slurry after vacuum stirring. The slurry is evenly coated on an aluminum foil and dried to obtain a positive electrode sheet.

[0083] Preparation of lithium-ion secondary batteries:

[0084] The composite negative electrode sheet, the separator and the positive electrode sheet are wound to obtain a battery cell, the battery cell is put into a shell, the electrolyte is injected into the battery shell, packaged, formed and divided into different volumes, and thus a lithium-ion secondary battery is obtained.

[0085] The difference between Example 6-8 and Example 5 lies in the usage of silicon-oxygen composite active material and Super-P. The specific usage of positive electrode raw material ratio and negative electrode raw material ratio of Examples 5-8 is recorded in Table 1.

[0086] Comparative Example 1

[0087] A lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet and a separator separating the positive electrode sheet and the negative electrode sheet.

[0088] Preparation of negative electrode:

[0089] A negative electrode material comprises: 0.5 kg of silicon dioxide SiO2 (containing 0.15 kg of lithium carbonate) and 4.5 kg of graphite are mechanically stirred and mixed for 2 hours; wherein the stirring speed is 300 r / min, and the SiO2 / C negative electrode material is prepared.

[0090] A negative electrode sheet, the preparation method of the negative electrode sheet is: SiO / C negative electrode material, Super-P, conductive carbon tube CNT, sodium carboxymethyl cellulose CMC, and binder styrene butadiene rubber SBR are mixed according to mass ratio, deionized water is added, and a uniform slurry is obtained under vacuum stirring, the slurry is evenly coated on copper foil, pressed into sheets, and dried to obtain the SiO / C negative electrode sheet.

[0091] Preparation of positive electrode:

[0092] The positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2), conductive carbon Super-P, and binder polyvinylidene fluoride PVDF are mixed according to a mass ratio, and a solvent N-methylpyrrolidone NMP is added to obtain a uniform slurry after vacuum stirring. The slurry is evenly coated on an aluminum foil and dried to obtain a positive electrode sheet.

[0093] Preparation of lithium-ion secondary batteries:

[0094] The negative electrode sheet, the separator and the positive electrode sheet are wound to obtain a battery cell, the battery cell is put into a shell, the electrolyte is injected into the battery shell, packaged, formed and divided into volumes, and a lithium-ion secondary battery is obtained.

[0095] The difference between Comparative Examples 2-4 and Comparative Example 1 lies in the usage of the silicon-oxygen composite active material and Super-P. The specific usage of the positive electrode raw material ratio and the negative electrode raw material ratio of Comparative Examples 1-4 is recorded in Table 1.

[0096] Table 1

[0097]

[0098] The secondary batteries prepared in the above-mentioned Examples 1-8 and Comparative Examples 1-4 were subjected to initial efficiency, capacity retention rate test, membrane thickness, expansion rate test and membrane surface appearance record, and the results are recorded in Table 2.

[0099] The cycle battery performance test is as follows: at room temperature 25°C, with the starting and ending voltages of 2.8V and 4.35V, 1C charge to 2.8V, then 1C charge to 4.35V, then 4.35V constant voltage charge until the current drops to 0.05C, and 0.2C discharge to 2.8V;

[0100] The thickness of the diaphragm was measured by a spiral micrometer; the expansion rate (%) of the diaphragm was calculated as follows: (the thickness of the silicon-carbon composite negative electrode sheet after being filled - the thickness of the silicon-carbon composite negative electrode sheet before being uncharged) / (the thickness of the silicon-carbon composite negative electrode sheet before being uncharged - the thickness of the negative electrode current collector) * 100% * (1 / n), n = 2; the appearance of the diaphragm was observed after the 400th cycle of the battery using a scanning electron microscope.

[0101] Table 2 Test data of Examples 1 to 8 and Comparative Examples 1 to 4.

[0102]

[0103]

[0104] In Examples 1 to 8, silicon oxide SiO2 and modified porous graphite are mixed to obtain a silicon-embedded mixture, and then molten polyvinyl fluoride or molten urea is added to the silicon-embedded mixture to obtain a silicon-carbon composite active material, while Comparative Examples 1 to 4 use conventional silicon oxide and graphite to obtain SiO2 / C negative electrode materials. From Table 2, the battery performance of the two materials finally prepared shows that the thickness of the negative electrode film in Examples 1 to 8 and Comparative Examples 1 to 4 is 102 to 126 mm, and Examples 1 to 8 have better cycle performance, especially the first coulomb efficiency at 0.2C is higher, and the 100th cycle capacity retention rate and the 400th cycle capacity retention rate are better than those of the comparative examples. The first coulombic efficiency at 0.2C, the 100th cycle capacity retention rate, and the 400th cycle capacity retention rate of Examples 1 to 4, and the expansion rate of the two-layer membrane after the 400th cycle of Examples 1 to 8 of 26.17 to 30.35% are all lower than the expansion rate of the two-layer membrane of Comparative Examples 1 to 4 of 35.32 to 39.10%; in Comparative Examples 1 to 4, there are many cracks on the surface of the membrane after the 400th cycle of cyclic charge and discharge, and the rupture is obvious, and the structural stability of the material is poor; after the 400th cycle of cyclic charge and discharge of Examples 1 to 8, there are very few or no fine cracks or ruptures on the surface of the membrane, and the structural stability of the material is good, and the side effects caused by volume expansion are significantly improved and improved. By comparing Examples 1-4, it can be concluded that as the content of silicon-oxygen negative electrode active material in the negative electrode material increases, the performance of the prepared secondary battery first increases and then decreases. When the ratio of silicon-oxygen negative electrode active material to conductive carbon is set to 92.5:2.5, the performance of the prepared lithium-ion battery is better. From the comparison of Examples 5-8, it can be concluded that as the content of silicon-oxygen negative electrode active material in the negative electrode material increases, the performance of the prepared lithium-ion battery first increases and then decreases. When the ratio of silicon-oxygen negative electrode active material to conductive carbon is set to 90.5:3.5, the performance of the prepared lithium-ion battery is better. In conclusion, when silicon dioxide SiO (containing 0.15kg lithium carbonate) is set for mixing, the weight of the mixed silicon-embedded mixture is 4.0kg, and the weight of molten urea is 0.5kg, the performance of the prepared lithium-ion battery is better.

[0105] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs can also change and modify the above embodiment. Therefore, the present invention is not limited to the above specific embodiment, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention belongs to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for preparing a silicon-carbon composite active material, It is characterized in that The following steps are involved: Step S1, embedding silicon-oxygen material into graphite material to obtain a silicon-carbon mixture, screening, and the material on the screen is the silicon-embedded mixture; Step S2, mixing the silicon-embedded mixture with a sealing agent, sealing, crushing, and screening to obtain a silicon-carbon composite active material; The particle size D50 of the silicon oxide material is 1 to 60 μm, and the particle size D50 of the graphite material is 0.1 to 10 μm; The graphite material is at least one of artificial porous graphite, natural porous graphite, modified porous graphite, porous soft carbon, and porous hard carbon; the silicon-oxygen material is SiOx, wherein 0≤x≤2, and SiOx includes at least one of silicon oxide, silicon dioxide, and metal-containing composite SiOx; Among them, the sealing agent in step S2 is at least one of lithium borate, polyvinylidene fluoride, polyvinyl fluoride, urea, lithium silicofluoride, and lithium oxalate difluoroborate, and the sealing agent is used to seal the holes in the graphite material to prevent the embedded silicon oxide material from escaping.

2. The method for preparing the silicon-carbon composite active material according to claim 1, It is characterized in that In the step S1, the weight ratio of the silicon oxide material to the graphite material is 0.1-40:100-120.

3. The method for preparing the silicon-carbon composite active material according to claim 1, It is characterized in that The embedding method in step S1 includes at least one of mechanical stirring, ball milling, ultrasonic vibration, mechanical vibration, spraying, sedimentation, deposition, atomization, infiltration, gasification, polymerization, soaking, and liquefaction.

4. The method for preparing the silicon-carbon composite active material according to claim 1, It is characterized in that In the step S2, the weight ratio of the silicon-embedded mixture to the sealing agent is 100-120:20-40.

5. The method for preparing the silicon-carbon composite active material according to claim 1, It is characterized in that The sealing treatment in step S2 is specifically as follows: mixing the sealing agent with the silicon-embedded mixture at a temperature of 15-400°C.

6. A silicon-carbon composite active material, It is characterized in that The silicon-carbon composite active material is prepared by the preparation method of any one of claims 1 to 5.

7. A negative electrode sheet, It is characterized in that It comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises the silicon-carbon composite active material according to claim 6.

8. A secondary battery, It is characterized in that A negative electrode sheet comprising the negative electrode sheet as claimed in claim 7.

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