Nano-silicon-carbon composite materials, their preparation methods, and lithium-ion battery anode sheets

Abstract

This invention relates to a nano-silicon-carbon composite material, its preparation method, and a lithium-ion battery anode sheet. The nano-silicon-carbon composite material and nano-silicon particles are described, wherein the fluorinated porous hard carbon is hard carbon with multiple pore structures, and the exposed surface of the hard carbon is coated with a fluorinated carbon layer, and the nano-silicon particles are embedded in some of the pore structures of the fluorinated porous hard carbon. The nano-silicon-carbon composite material of this invention exhibits high capacity, low expansion, and excellent rate performance, enabling the lithium-ion battery anode sheet to possess high initial coulombic efficiency and good cycle stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material technology, and in particular to a nano-silicon-carbon composite material, its preparation method, and a lithium-ion battery anode sheet. Background Technology

[0002] With the development and widespread application of new energy vehicles, graphite-based materials, due to their limited theoretical specific capacity, cannot meet the ever-growing demand of long-range electric vehicles. Among emerging alternative anode materials, silicon and its oxides have attracted widespread attention due to their ultra-high theoretical capacity (3579 mAh g), suitable electrochemical potential (approximately 0.2 V-0.4 V), and abundant reserves in the Earth's crust. However, the massive volume expansion during the alloying and dealloying processes of silicon particles and lithium, exceeding 300%, causes surface cracks and fractures in the silicon particles due to significant tensile stress, ultimately leading to their pulverization and making direct utilization difficult.

[0003] Furthermore, the continuous fracture of silicon particles leads to the ongoing disruption and remodeling of the solid electrolyte interphase (SEI) layer, rapidly consuming electrolyte and lithium ions, resulting in a sustained decrease in battery performance. Therefore, the mechanical degradation of the silicon particle surface and the unstable SEI film result in lower silicon particle capacity and reduced coulombic efficiency. These problems severely limit the application of silicon particles in lithium-ion batteries.

[0004] To address the issues of volume expansion and internal stress in silicon particles, silicon particles are combined with carbon materials. Carbon materials, acting as a matrix, can increase electrode conductivity, buffer volume expansion, and achieve higher mass and volumetric capacity. Traditional technologies typically use graphite-silicon particle composites to improve energy density and fast-charging performance. However, both silicon particles and graphite materials have relatively high expansion rates, making the structural stability of the cell easily compromised during charge-discharge cycles, resulting in rapid cycle degradation and low initial coulombic efficiency. While adding lithium powder, lithium foil, or other lithium additives can alleviate the low initial coulombic efficiency to some extent, the inherent safety risks of lithium metal pose uncontrollable risks to battery production and use.

[0005] In traditional technologies, designing multi-level silicon-carbon composite materials—either with silicon nanoparticles as the core, amorphous carbon as the intermediate coating layer, and fluorinated carbon as the outer shell, or with porous carbon embedded with silicon nanoparticles as the core and fluorinated carbon as the outer shell—can improve the first-order coulombic efficiency to some extent. However, in multi-level structures, the presence of silicon nanoparticles makes it difficult to maximize the fluorination surface of the fluorinated carbon shell, leading to defects in the SEI film. Furthermore, the complete coating of silicon nanoparticles by the carbon layer and / or fluorinated carbon layer restricts lithium migration to solid-phase diffusion, reducing the migration rate and thus limiting rate performance. Summary of the Invention

[0006] Therefore, it is necessary to provide a nano-silicon-carbon composite material, its preparation method, and a lithium-ion battery anode sheet to address the above problems. The nano-silicon-carbon composite material has high capacity, low expansion, and excellent rate performance, enabling the lithium-ion battery anode sheet to have high initial coulombic efficiency and good cycle stability.

[0007] A nano-silicon-carbon composite material includes fluorinated porous hard carbon and nano-silicon particles, wherein the fluorinated porous hard carbon is hard carbon with multiple pore structures, and the exposed surface of the hard carbon is coated with a fluorinated carbon layer, and the nano-silicon particles are embedded in part of the pore structure of the fluorinated porous hard carbon.

[0008] In one embodiment, the thickness of the fluorinated carbon layer is 1 nm-3 nm, and the mass fraction of the fluorinated carbon layer in the fluorinated porous hard carbon is 0.2%-5%.

[0009] In one embodiment, the particle size of the fluorinated porous hard carbon is 1 μm-100 μm;

[0010] And / or, the pore size of the pore structure is 50nm-200nm;

[0011] And / or, the particle size of the nano-silicon particles is 1nm-150nm.

[0012] In one embodiment, the porosity of the fluorinated porous hard carbon is 50%-80%;

[0013] And / or, the embedding rate of the silicon nanoparticles is 10%-30%.

[0014] In the nano-silicon-carbon composite material of the present invention, fluorinated porous hard carbon is used as the framework structure. On the one hand, the outer surface of the hard carbon, which is difficult to graphitize, and the surface of the porous structure both have dense and uniform fluorine.

[0015] The fluorinated carbon layer, with its large fluorinated area, not only reduces the side reactions caused by direct contact between hard carbon and the electrolyte, but also allows it to react with lithium to form a dense and uniform artificial SEI film of lithium fluoride. This suppresses side reactions and the formation of lithium dendrites, which is beneficial for improving the first coulombic efficiency of lithium-ion batteries. On the other hand, the irregular carbon layer structure of the fluorinated porous hard carbon provides stronger binding force on the expansion of the nano-silicon particles, enabling the embedded nano-silicon particles and the fluorinated porous hard carbon to synergistically improve specific capacity and rate performance, thereby increasing energy density.

[0016] Meanwhile, since the fluorinated carbon layer is coated on the exposed surface of hard carbon and does not close the pore structure of hard carbon, the nano-silicon particles embedded in the pore structure will not be closed in the pore structure, which is beneficial to improving the lithium migration rate and further improving the rate performance.

[0017] A method for preparing the nano-silicon-carbon composite material as described above includes the following steps:

[0018] A biological matrix is ​​provided as a carbon source. The carbon source is pretreated and then subjected to a hydrothermal reaction to obtain a porous hard carbon precursor. The porous hard carbon precursor is then calcined to obtain porous hard carbon.

[0019] 5. Provide a fluorine source, and perform a fluorination reaction between the porous hard carbon and the fluorine source to obtain fluorinated porous hard carbon;

[0020] The fluorinated porous hard carbon and nano-silicon particles were mixed, ball-milled, and then calcined to obtain a nano-silicon-carbon composite material.

[0021] In one embodiment, the mass ratio of the porous hard carbon to the fluorine source is 200:1-20:1;

[0022] And / or, the mass ratio of the fluorinated porous hard carbon to the nano-silicon particles is 100:1-100:20.

[0023] 0 In one embodiment, the bio-matrix is ​​selected from at least one of rice husks, sucrose, grapefruit peel, corn stalks, and peanut shells;

[0024] And / or, the fluorine source is selected from at least one of fluorine gas and fluoropolymers.

[0025] In one embodiment, the hydrothermal reaction is carried out at a temperature of 150°C-220°C for a duration of 12h-24h.

[0026] And / or, in the step of calcining the porous hard carbon precursor, the temperature is 300℃-900℃ and the time is 1.5h-4h;

[0027] And / or, the fluorination reaction is carried out at a temperature of 300℃-600℃ for a time of 1h-10h;

[0028] And / or, in the step of ball milling the fluorinated porous hard carbon with nano-silicon particles and then calcining it, the temperature is 300℃-500℃ and the time is 2h-4h.

[0029] A lithium-ion battery negative electrode sheet comprising, as described above, a nano-silicon composite material.

[0030] In one embodiment, the lithium-ion battery negative electrode sheet comprises the following components: 90wt%-98wt% of nano-silicon composite material, 0.5wt%-5wt% of conductive agent, and 0.5wt%-5wt% of binder.

[0031] The preparation method described in this invention utilizes the fluorination reaction between porous hard carbon and a fluorine source. On one hand, it can form a CF structure on the surface and internal crystal structure of the porous hard carbon, which can enhance the structural stability of the porous hard carbon and reduce the resistance during lithium-ion diffusion, thereby improving the specific capacity and the charge-discharge performance of the nano-silicon-carbon composite material. On the other hand, it can precisely control the thickness and density of the fluorinated carbon layer to ensure that the pore structures on both the outer and inner surfaces of the porous hard carbon are coated with a fluorinated carbon layer. Furthermore, ball milling is used to embed nano-silicon particles into part of the pore structure, coordinating and controlling the embedding rate of nano-silicon particles, and a single calcination is used to fix the nano-silicon particles in the pore structure, thereby constituting a high-performance nano-silicon-carbon composite material.

[0032] Therefore, the nano-silicon-carbon composite material of the present invention has high capacity, low expansion and excellent rate performance, and the lithium-ion battery anode sheet prepared with it has high initial coulombic efficiency and good cycle stability. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of a nano-silicon-carbon composite material in one embodiment of the present invention.

[0034] Among them, 10 is fluorinated porous hard carbon; 101 is pore structure; 102 is fluorinated carbon layer; and 20 is nano-silicon particles. Detailed Implementation

[0035] To facilitate understanding of the present invention, it will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to make the disclosure of the present invention more thorough and complete.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments or examples only and is not intended to be limiting of the invention.

[0037] like Figure 1 As shown, the present invention provides a nano-silicon-carbon composite material, which includes fluorinated porous hard carbon 10 and nano-silicon particles 20. The fluorinated porous hard carbon 10 is a hard carbon with multiple pore structures 101, and the exposed surface of the hard carbon is covered with a fluorinated carbon layer 102. The nano-silicon particles 20 are embedded in part of the pore structures 101 of the fluorinated porous hard carbon 10.

[0038] It should be noted that hard carbon refers to carbon that is difficult to graphitize and can be obtained through the pyrolysis of polymers, petrochemical products, biomass, etc.

[0039] In the nano-silicon-carbon composite material of the present invention, fluorinated porous hard carbon 10 is used as the skeleton structure. On the one hand, the outer surface of the hard carbon, which is difficult to graphitize, and the surface of the pore structure 101 have a dense and uniform fluorinated carbon layer 102. The fluorination area is large, which can not only reduce the side reaction caused by direct contact between hard carbon and electrolyte, but also react with lithium to form a dense and uniform artificial SEI film lithium fluoride, thereby suppressing the side reaction and the formation of lithium dendrites, which is beneficial to improving the first coulombic efficiency of lithium-ion batteries.

[0040] On the other hand, compared with graphite and soft carbon materials with regular layered structures, fluorinated porous hard carbon 10 has an irregular carbon layer structure, which has a stronger binding force on the expansion of nano-silicon particles 20. This allows the embedded nano-silicon particles 20 and fluorinated porous hard carbon 10 to synergistically improve specific capacity and rate performance, thereby increasing energy density.

[0041] Meanwhile, since the fluorinated carbon layer 102 is coated on the exposed surface of hard carbon and does not close the pore structure 101 of hard carbon, the nano-silicon particles 20 embedded in the pore structure 101 will not be closed in the pore structure 101, which is beneficial to improve the lithium migration rate and further improve the rate performance.

[0042] Therefore, the nano-silicon-carbon composite material of the present invention has high capacity, low expansion and excellent rate performance.

[0043] In order to ensure that the porous hard carbon has a uniform fluorinated carbon layer 102 on the outer and inner surfaces of the pore structure, and to ensure that the fluorinated carbon layer 102 does not block the pore structure, so that the pore structure 101 has a certain pore size and porosity, the thickness of the fluorinated carbon layer 102 is preferably 1nm-3nm.

[0044] In order for the fluorinated carbon layer 102 to react with lithium to form a dense and uniform SEI film, the mass fraction of the fluorinated carbon layer 102 in the fluorinated porous hard carbon 10 is 0.2%-5%, preferably 0.5%-2%.

[0045] In one embodiment, the particle size of the fluorinated porous hard carbon 10 is 1μm-100μm, preferably 50μm-100μm; and / or, the pore size of the pore structure 101 is 50nm-200nm, preferably 100nm-200nm.

[0046] Furthermore, in order to ensure that the nano-silicon particles 20 can be embedded in the porous structure 101, the particle size of the nano-silicon particles 20 needs to be smaller than the pore size of the porous structure 101. The particle size of the nano-silicon particles 20 is preferably 1nm-150nm, and more preferably 40nm-120nm.

[0047] Considering that the porosity of fluorinated porous hard carbon 10 will affect the embedding effect of nano-silicon particles 20, the porosity of the fluorinated porous hard carbon 10 is preferably 50%-80%, and more preferably 60%-75%, which is the volume ratio of the pore structure 101 in the fluorinated porous hard carbon 10.

[0048] In order to maximize the absorption of the expansion of the nano-silicon particles 20 by the fluorinated porous hard carbon 10, and at the same time to achieve a high specific capacity of the nano-silicon carbon composite material, the embedding rate of the nano-silicon particles 20 is preferably 10%-30%, more preferably 10%-20%. This embedding rate is the volume percentage of the nano-silicon particles 20 in the pore structure 101, that is, 70%-90% of the pore structure 101 does not contain embedded nano-silicon particles 20.

[0049] This invention also proposes a method for preparing nano-silicon-carbon composite materials, comprising the following steps:

[0050] S11, a biological matrix is ​​provided as a carbon source, the carbon source is pretreated and then subjected to a hydrothermal reaction to obtain a porous hard carbon precursor, and the porous hard carbon precursor is calcined to obtain porous hard carbon.

[0051] S12, providing a fluorine source, and reacting the porous hard carbon with the fluorine source to obtain fluorinated porous hard carbon;

[0052] S13, the fluorinated porous hard carbon and nano-silicon particles are mixed, ball-milled, and then calcined to obtain a nano-silicon carbon composite material.

[0053] In step S11, the pretreatment includes pulverization and high-temperature pre-carbonization. By pre-carbonizing the pulverized biological matrix at 300℃-500℃ in a protective gas for 1h-3h, it is beneficial for the biological matrix to form an irregularly structured pre-carbonized carbon source.

[0054] Specifically, the biological substrate is selected from at least one of rice husks, sucrose, grapefruit peel, corn stalks, and peanut shells.

[0055] Hydrothermal reactions can be used to form a uniform porous structure on the surface and inside of the pre-carbonized carbon source, while further forming hard carbon.

[0056] Specifically, the hydrothermal reaction includes the following steps: adding a pre-carbonized carbon source to an alkaline solution with a concentration of 1 mol / L-3 mol / L, mixing, and reacting at 150℃-220℃ for 12h-24h, followed by washing, drying, and sieving to obtain a porous hard carbon precursor. The alkaline solution is preferably a potassium hydroxide solution (KOH).

[0057] The porous hard carbon precursor can be further carbonized by calcination in a protective gas to obtain porous hard carbon with an irregular carbon layer structure. Specifically, the calcination step of the porous hard carbon precursor is carried out at a temperature of 300℃-900℃ for 1.5h-4h.

[0058] Specifically, the calcination of the porous hard carbon precursor can be completed in one step or in stages, preferably in stages. Optionally, the calcination includes a first-stage calcination and a second-stage calcination. The first-stage calcination is carried out at a temperature of 300℃-500℃ for 0.5h-2h, and the second-stage calcination is carried out at a temperature of 600℃-900℃ for 1h-2h.

[0059] In step S12, the fluorination reaction between porous hard carbon and a fluorine source can, on the one hand, form a CF structure on the surface and internal crystal structure of the porous hard carbon, which can enhance the structural stability of the porous hard carbon and reduce the resistance during lithium-ion diffusion, thereby improving the specific capacity and the charge-discharge performance of the nano-silicon-carbon composite material; on the other hand, the thickness and density of the fluorinated carbon layer can be precisely controlled to ensure that the pore structure on both the outer and inner surfaces of the porous hard carbon is coated with a fluorinated carbon layer.

[0060] To further precisely control the thickness and density of the fluorinated carbon layer, the mass ratio of the porous hard carbon to the fluorine source is 200:1-20:1.

[0061] The fluorine source is selected from at least one of fluorine gas and fluoropolymers, and the fluoropolymer is preferably a perfluorinated resin.

[0062] Furthermore, coordinating and controlling the fluorination reaction temperature and time is beneficial for obtaining a fluorinated carbon layer of a certain thickness. The fluorination reaction temperature is 300℃-600℃ and the time is 1h-10h.

[0063] Specifically, the fluorination reaction includes: placing a certain amount of fluorine source separately from porous hard carbon, and then carrying out a fluorination reaction under a protective gas, so that the fluorine source reacts with the carbon layer on the surface and inside the porous hard carbon to generate a fluorinated carbon layer, thereby obtaining fluorinated porous hard carbon.

[0064] In step S13, ball milling allows the nano-silicon particles to be embedded in a portion of the porous structure, and the embedding rate of the nano-silicon particles can be controlled in a coordinated manner.

[0065] To further coordinate and control the embedding rate of silicon nanoparticles, the preferred mass ratio of fluorinated porous hard carbon to silicon nanoparticles is 100:1-100:20.

[0066] Calcination can fix nano-silicon particles in a porous structure, thereby forming a high-performance nano-silicon-carbon composite material.

[0067] Specifically, in the step of mixing and ball-milling the fluorinated porous hard carbon with nano-silicon particles and then calcining them, the temperature of the first calcination is 300℃-500℃ and the time is 2h-4h.

[0068] The present invention also provides a lithium-ion battery negative electrode sheet, which comprises the nano-silicon composite material as described above.

[0069] In one embodiment, the lithium-ion battery negative electrode sheet comprises the following components: 90wt%-98wt% of nano-silicon composite material, 0.5wt%-5wt% of conductive agent, and 0.5wt%-5wt% of binder.

[0070] Preferably, the lithium-ion battery negative electrode sheet comprises the following components: 95wt%-98wt% of nano-silicon composite material, 1wt%-3wt% of conductive agent, and 1wt%-3wt% of binder.

[0071] Specifically, the conductive agent is selected from at least one of conductive carbon black, Ketjen black, acetylene black, carbon fiber, carbon nanotubes, conductive graphite, and graphene.

[0072] The adhesive is selected from at least one of polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), sodium carboxymethyl cellulose (CMC-Na), potassium carboxymethyl cellulose (CMC-K), styrene-butadiene rubber (SBR), gelatin (GEL), sodium alginate (SA), polyacrylates (PAA), polyolefins (PO), polyacrylates (PEA), polyimide (PI), polyamide (PA), polyamide-imide (PAI), and polyacrylonitrile (PAN).

[0073] In one embodiment, the method for preparing a lithium-ion battery negative electrode sheet includes the following steps:

[0074] S21, Mix the solvent and adhesive evenly to obtain a colloid;

[0075] S22, the colloid is mixed and stirred with a conductive agent, nano-silicon composite material and solvent to obtain a negative electrode slurry;

[0076] S23, the negative electrode slurry is coated onto the current collector and dried to obtain an unrolled negative electrode sheet; and

[0077] S24, the unrolled negative electrode sheet is rolled to obtain a lithium-ion battery negative electrode sheet.

[0078] Specifically, in step S21, the solid content of the colloid is 0.5%-2%, preferably 0.8%-1.8%.

[0079] In step S22, the solid content of the negative electrode slurry is 40%-60%, and the viscosity is 2000mPa·s-10000mPa·s.

[0080] In step S23, the areal density of the unrolled negative electrode sheet is 5 mg / cm³. 2 -12mg / cm 2 .

[0081] In step S24, the density of the lithium-ion battery negative electrode sheet is 0.8 g / cm³. 3 -1.8g / cm 3 .

[0082] The following specific embodiments will further illustrate the nano-silicon-carbon composite material, its preparation method, and the lithium-ion battery negative electrode sheet.

[0083] Example 1

[0084] Rice husks were crushed and pre-carbonized at 300℃ for 3 hours under a nitrogen atmosphere to obtain a pre-carbonized carbon source. The pre-carbonized carbon source was added to a 1.2 mol / L KOH solution, stirred and mixed evenly, and then subjected to a hydrothermal reaction at 150℃ for 24 hours. After washing, drying and sieving, a porous hard carbon precursor was obtained. The porous hard carbon precursor was then subjected to multi-stage calcination under a nitrogen atmosphere to obtain porous hard carbon. The first stage calcination temperature was 300℃ and the calcination time was 2 hours, and the second stage calcination temperature was 600℃ and the calcination time was 2 hours.

[0085] 200g of porous hard carbon and 5g of perfluorinated resin were placed separately in quartz boats, separated by a porous separator. The quartz boats were then placed in a heat treatment tube furnace. Argon gas was introduced to purge air, and the gas valve was closed. The furnace was heated to 400℃ at a rate of 2℃ / min and held for 1 hour. Then, it was cooled to 300℃ at the same rate and held for 1 hour. After cooling to room temperature, the fluorinated porous hard carbon was obtained. The fluorinated porous hard carbon had a particle size of 50μm, a pore size of 150nm, and a porosity of 75%.

[0086] 100g of fluorinated porous hard carbon and 10g of 70nm nano-silicon particles were mixed and ball-milled, and then calcined at 300℃ for 4h to obtain nano-silicon-carbon composite material.

[0087] Example 2

[0088] After pulverizing sucrose, it was pre-carbonized at 350℃ for 2.5h under a nitrogen atmosphere to obtain a pre-carbonized carbon source. The pre-carbonized carbon source was added to a 1.5mol / L KOH solution, stirred and mixed evenly, and then subjected to a hydrothermal reaction at 165℃ for 22h. After washing, drying and sieving, a porous hard carbon precursor was obtained. The porous hard carbon precursor was calcined in multiple stages under a nitrogen atmosphere to obtain porous hard carbon. The first stage calcination temperature was 350℃ and the calcination time was 1.5h, and the second stage calcination temperature was 700℃ and the calcination time was 1.5h.

[0089] 100g of porous hard carbon was placed in a quartz boat, which was then placed in a heat treatment tube furnace. Argon gas was introduced to purge air, followed by 0.5L of fluorine gas for 20 minutes. The furnace was heated to 400℃ at a rate of 2℃ / min and held for 1 hour. The temperature was then lowered to 300℃ at the same rate and held for 1 hour. After cooling to room temperature, the fluorinated porous hard carbon was obtained. The fluorinated porous hard carbon had a particle size of 50μm, a pore size of 150nm, and a porosity of 70%.

[0090] 100g of fluorinated porous hard carbon and 5g of 70nm nano-silicon particles were mixed and ball-milled, and then calcined at 300℃ for 4h to obtain nano-silicon carbon composite material.

[0091] Example 3

[0092] Peanut shells were crushed and pre-carbonized at 400℃ for 2 hours under a nitrogen atmosphere to obtain a pre-carbonized carbon source. The pre-carbonized carbon source was added to a 2 mol / L KOH solution, stirred and mixed evenly, and then subjected to a hydrothermal reaction at 180℃ for 20 hours. After washing, drying and sieving, a porous hard carbon precursor was obtained. The porous hard carbon precursor was then subjected to multi-stage calcination under a nitrogen atmosphere to obtain porous hard carbon. The first stage calcination temperature was 400℃ and the calcination time was 1.5 hours, and the second stage calcination temperature was 750℃ and the calcination time was 1.5 hours.

[0093] 200g of porous hard carbon and 5g of perfluorinated resin were placed separately in quartz boats, separated by a porous separator. The quartz boats were then placed in a heat treatment tube furnace. Argon gas was introduced to purge air, and the gas valve was closed. The furnace was heated to 500℃ at a rate of 2℃ / min and held for 1 hour. Then, it was cooled to 400℃ at the same rate and held for 2 hours. After cooling to room temperature, the fluorinated porous hard carbon was obtained. The fluorinated porous hard carbon had a particle size of 50μm, a pore size of 200nm, and a porosity of 70%.

[0094] 100g of fluorinated porous hard carbon was mixed with 8g of 70nm nano-silicon particles and ball-milled, then calcined at 400℃ for 3h to obtain nano-silicon-carbon composite material.

[0095] Example 4

[0096] Corn stalks were crushed and pre-carbonized at 450℃ for 1.5 h under a nitrogen atmosphere to obtain a pre-carbonized carbon source. The pre-carbonized carbon source was added to a 2.5 mol / L KOH solution, stirred and mixed evenly, and then subjected to a hydrothermal reaction at 200℃ for 15 h. After washing, drying and sieving, a porous hard carbon precursor was obtained. The porous hard carbon precursor was then subjected to multi-stage calcination under a nitrogen atmosphere to obtain porous hard carbon. The first stage calcination temperature was 450℃ and the calcination time was 1 h, and the second stage calcination temperature was 800℃ and the calcination time was 1.2 h.

[0097] 200g of porous hard carbon and 8g of perfluorinated resin were placed separately in quartz boats, separated by a porous separator. The quartz boats were then placed in a heat treatment tube furnace. Argon gas was introduced to purge air, and the gas valve was closed. The furnace was heated to 400℃ at a rate of 2℃ / min and held for 1 hour. Then, it was cooled to 300℃ at the same rate and held for 3 hours. After cooling to room temperature, the fluorinated porous hard carbon was obtained. The fluorinated porous hard carbon had a particle size of 80μm, a pore size of 150nm, and a porosity of 75%.

[0098] 100g of fluorinated porous hard carbon and 15g of 100nm nano-silicon particles were mixed and ball-milled, and then calcined at 300℃ for 4h to obtain nano-silicon-carbon composite material.

[0099] Example 5

[0100] Grapefruit peel was crushed and pre-carbonized at 500℃ for 1 hour under a nitrogen atmosphere to obtain a pre-carbonized carbon source. The pre-carbonized carbon source was added to a 3 mol / L KOH solution, stirred and mixed evenly, and then subjected to a hydrothermal reaction at 220℃ for 12 hours. After washing, drying and sieving, a porous hard carbon precursor was obtained. The porous hard carbon precursor was then subjected to multi-stage calcination under a nitrogen atmosphere to obtain porous hard carbon. The first stage calcination temperature was 500℃ and the calcination time was 0.5 hours, and the second stage calcination temperature was 900℃ and the calcination time was 1 hour.

[0101] 200g of porous hard carbon and 10g of perfluorinated resin were placed separately in quartz boats, separated by a porous separator. The quartz boats were then placed in a heat treatment tube furnace. Argon gas was introduced to purge air, and the gas valve was closed. The furnace was heated to 400℃ at a rate of 2℃ / min and held for 2 hours. Then, it was cooled to 300℃ at the same rate and held for 1 hour. After cooling to room temperature, the fluorinated porous hard carbon was obtained. The fluorinated porous hard carbon had a particle size of 100μm, a pore size of 150nm, and a porosity of 65%.

[0102] 100g of fluorinated porous hard carbon and 12g of 70nm nano-silicon particles were mixed and ball-milled, and then calcined at 300℃ for 4h to obtain nano-silicon-carbon composite material.

[0103] Comparative Example 1

[0104] The difference between Comparative Example 1 and Example 1 is that porous graphite is used instead of porous hard carbon.

[0105] Comparative Example 2

[0106] The difference between Comparative Example 2 and Example 1 is that the porous hard carbon and nano-silicon particles were first mixed and ball-milled before being subjected to fluorination treatment. The nano-silicon-carbon composite material obtained in this comparative example has a porous hard carbon core with embedded silicon nanoparticles as the core and fluorinated carbon as the outer shell.

[0107] Comparative Example 3

[0108] The difference between Comparative Example 3 and Example 1 is that the porous hard carbon and nano-silicon particles were first mixed and ball-milled.

[0109] The mixture was then ball-milled with fluorinated graphite. The resulting nano-silicon-carbon composite material had a porous hard carbon core with embedded silicon nanoparticles and a fluorinated graphite shell.

[0110] The structures of the nano-silicon-carbon composite materials prepared in Examples 1-5 and Comparative Examples 1-3 were characterized and tested, and the results are shown in Table 1.

[0111] Table 1

[0112]

[0113] As shown in Table 1, the mass fraction of the fluorinated carbon layer per unit thickness in Examples 1-5 is higher than that in Comparative Examples 1-3. Therefore, in the nano-silicon-carbon composite material of the present invention, the fluorinated carbon layer has a better densification effect.

[0114] Application Example 1

[0115] Mix 1000g of water with 15g of CMC-K to obtain a colloid with a solid content of 1.5%.

[0116] Take 80g of the nano-silicon-carbon composite material prepared in Examples 1-5, and mix it with 70g of colloid, 0.6g of acetylene black and 1g of water to obtain negative electrode slurry samples 1-5 with a solid content of 50% and a viscosity of 5000mPa·s.

[0117] The negative electrode slurry samples 1-5 were uniformly coated onto the current collector, dried, and then rolled to obtain a compaction density of 1 g / cm³. 3 Lithium-ion battery negative electrode samples 1-5.

[0118] Application Comparative Example 1

[0119] Using the same preparation method, the nano-silicon-carbon composite materials prepared in Comparative Examples 1-3 were respectively prepared into lithium-ion battery anode samples 6-8.

[0120] Lithium-ion battery negative electrode samples 1-8 were assembled with commercial ternary lithium-ion battery positive electrode samples into full cells and their electrochemical performance was tested on a blue battery cabinet. Under normal temperature conditions, the charge and discharge voltage was limited to 2.75-4.2V. The results are shown in Table 2. The specific capacity was tested as follows: 0.1C capacity / mass of active material; the rate capability was tested as: 2C capacity / 1C capacity; the initial coulombic efficiency was tested as: 0.33C initial discharge capacity / 0.33C initial charge capacity; the cycle stability was tested as follows: 200 constant current charge and discharge cycles at 0.5C rate, with cycle retention as: discharge capacity at the 200th cycle / discharge capacity at the 1st cycle; the expansion rate was calculated as: cell thickness at the 200th cycle / cell thickness at the 1st cycle.

[0121] Table 2

[0122]

[0123] As shown in Table 2, the lithium-ion battery anode sheet prepared by the nano-silicon-carbon composite material of the present invention has a lower thickness expansion rate, high specific capacity, good rate performance, high initial coulombic efficiency, and good cycle stability.

[0124] Example 6

[0125] The difference between Example 6 and Example 1 is that 200g of porous hard carbon and 15g of perfluorinated resin were used. The tube furnace was heated to 500°C at a rate of 2°C / min and held for 3 hours. After being cooled directly to room temperature, the carbon was removed to obtain fluorinated porous hard carbon.

[0126] Example 7

[0127] The difference between Example 7 and Example 1 is that 200g of porous hard carbon and 10g of perfluorinated resin were used.

[0128] Example 8

[0129] The difference between Example 8 and Example 1 is that 100g of porous hard carbon and 7g of perfluorinated resin were used.

[0130] The structures of the nano-silicon-carbon composite materials prepared in Examples 6-8 were characterized and tested, and the results are shown in Table 3.

[0131] Table 3

[0132]

[0133] Application Example 2

[0134] The nano-silicon-carbon composite materials prepared in Examples 6-8 were used to prepare lithium-ion battery anode samples 9-11, and their performance was tested. The results are shown in Table 4.

[0135] Table 4

[0136]

[0137] Comparing Examples 1 and 6-8, it can be seen that when the thickness of the fluorinated carbon layer is 1nm-3nm and the mass fraction is in the range of 0.5%-2%, the performance of the lithium-ion battery anode sheet is better.

[0138] Example 9

[0139] The difference between Example 9 and Example 1 is that 100g of fluorinated porous hard carbon and 45g of nano-silicon particles with a particle size of 70nm are used.

[0140] Example 10

[0141] The difference between Example 10 and Example 1 is that 100g of fluorinated porous hard carbon and 20g of nano-silicon particles with a particle size of 70nm were used.

[0142] The structures of the nano-silicon-carbon composite materials prepared in Examples 9-10 were characterized and tested, and the results are shown in Table 5.

[0143] Table 5

[0144]

[0145] Application Example 3

[0146] The nano-silicon-carbon composite materials prepared in Examples 9-10 were used to prepare lithium-ion battery anode samples 12-13, and their performance was tested. The results are shown in Table 6.

[0147] Table 6

[0148]

[0149] Comparing Examples 1 and 9-10, it can be seen that when the embedding rate of the nano-silicon particles is in the range of 10%-20%, the performance of the lithium-ion battery anode sheet is better.

[0150] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0151] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A nano-silicon-carbon composite material for use as a negative electrode in lithium-ion batteries, characterized in that, The nano-silicon-carbon composite material comprises fluorinated porous hard carbon and nano-silicon particles. The fluorinated porous hard carbon is a hard carbon with multiple pore structures, and the exposed surface of the hard carbon is coated with a fluorinated carbon layer. The thickness of the fluorinated carbon layer is 1 nm-3 nm, and the mass fraction of the fluorinated carbon layer in the fluorinated porous hard carbon is 0.2%-5%. The nano-silicon particles are embedded in part of the pore structure of the fluorinated porous hard carbon, with an embedding rate of 10%-20%. The pore size of the pore structure is 50 nm-200 nm, and the porosity of the fluorinated porous hard carbon is 50%-80%. The preparation method of the nano-silicon-carbon composite material includes the following steps: A biological matrix is ​​provided as a carbon source. The carbon source is pretreated and then subjected to a hydrothermal reaction to obtain a porous hard carbon precursor. The porous hard carbon precursor is then calcined to obtain porous hard carbon. A fluorine source is provided, and the porous hard carbon is subjected to a fluorination reaction with the fluorine source to obtain fluorinated porous hard carbon. The fluorinated porous hard carbon and nano-silicon particles were mixed, ball-milled, and then calcined to obtain a nano-silicon-carbon composite material.

2. The nano-silicon-carbon composite material according to claim 1, characterized in that, The particle size of the fluorinated porous hard carbon is 1μm-100μm; And / or, the particle size of the nano-silicon particles is 1nm-150nm.

3. A method for preparing the nano-silicon-carbon composite material as described in claim 1 or 2, characterized in that, The preparation method includes the following steps: A biological matrix is ​​provided as a carbon source. The carbon source is pretreated and then subjected to a hydrothermal reaction to obtain a porous hard carbon precursor. The porous hard carbon precursor is then calcined to obtain porous hard carbon. A fluorine source is provided, and the porous hard carbon is subjected to a fluorination reaction with the fluorine source to obtain fluorinated porous hard carbon. The fluorinated porous hard carbon and nano-silicon particles were mixed, ball-milled, and then calcined to obtain a nano-silicon-carbon composite material.

4. The method for preparing the nano-silicon-carbon composite material according to claim 3, characterized in that, The mass ratio of the porous hard carbon to the fluorine source is 200:1-20:1; And / or, the mass ratio of the fluorinated porous hard carbon to the nano-silicon particles is 100:1-100:

20.

5. The method for preparing the nano-silicon-carbon composite material according to claim 3, characterized in that, The biological substrate is selected from at least one of rice husks, sucrose, grapefruit peel, corn stalks, and peanut shells; And / or, the fluorine source is selected from at least one of fluorine gas and fluoropolymers.

6. The method for preparing the nano-silicon-carbon composite material according to claim 3, characterized in that, The hydrothermal reaction is carried out at a temperature of 150℃-220℃ for a duration of 12h-24h. And / or, in the step of calcining the porous hard carbon precursor, the temperature is 300℃-900℃ and the time is 1.5h-4h; And / or, the fluorination reaction is carried out at a temperature of 300℃-600℃ for a time of 1h-10h; And / or, in the step of ball milling the fluorinated porous hard carbon with nano-silicon particles and then calcining it, the temperature is 300℃-500℃ and the time is 2h-4h.

7. A lithium-ion battery negative electrode sheet, characterized in that, The lithium-ion battery negative electrode sheet comprises the nano-silicon-carbon composite material as described in claim 1 or 2.

8. The lithium-ion battery negative electrode sheet according to claim 7, characterized in that, The lithium-ion battery negative electrode sheet comprises the following components: 90wt%-98wt% of nano-silicon-carbon composite material, 0.5wt%-5wt% of conductive agent, and 0.5wt%-5wt% of binder.

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