CN107681125B - Negative electrode material for lithium ion battery, preparation method of negative electrode material and lithium ion secondary battery - Google Patents

Negative electrode material for lithium ion battery, preparation method of negative electrode material and lithium ion secondary battery Download PDF

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CN107681125B
CN107681125B CN201610623533.4A CN201610623533A CN107681125B CN 107681125 B CN107681125 B CN 107681125B CN 201610623533 A CN201610623533 A CN 201610623533A CN 107681125 B CN107681125 B CN 107681125B
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negative electrode
electrode material
lithium ion
ion battery
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CN107681125A (en
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文娟·刘·麦蒂斯
魏冠杰
王小绘
裴卫兵
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Weihong Advanced Materials Co
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Microvast Power Systems Huzhou Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/386—Silicon or alloys based on silicon
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/387—Tin or alloys based on tin
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/483—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides for non-aqueous cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

The invention relates to the field of lithium ion secondary battery manufacturing and energy storage, in particular to a lithium ion battery cathode material, a preparation method thereof and a lithium ion secondary battery. The invention provides a negative electrode material for a lithium ion battery, which comprises a nano high-capacity active material, a hard carbon layer coated on the outer surface of the nano high-capacity active material and a soft carbon layer coated on the outer surface of the hard carbon layer. The cathode material for the lithium ion battery provided by the invention is coated with the asphalt soft carbon for the second time, so that the specific surface area of the composite material can be reduced, and the first efficiency and the cycle performance of the material are improved.

Description

Negative electrode material for lithium ion battery, preparation method of negative electrode material and lithium ion secondary battery
Technical Field
The invention relates to the field of lithium ion secondary battery manufacturing and energy storage, in particular to a lithium ion battery cathode material, a preparation method thereof and a lithium ion secondary battery.
Background
With the increasing pressure of the demand of high energy density lithium ion batteries, more and more attention is paid to the development of battery cathode materials with high capacity, long service life, high safety, high energy density and high power density. Among a plurality of negative electrode materials, the theoretical specific capacity of the negative electrode such as Si, Ge, Sn, Al, Sb, Bi and the like is extremely high (for example, Si forms Li15Si43590mAh g-1Formation of Li22Si4When it is about 4200mAh g-1About ten times as much as graphite (372mAh g)-1) Therefore, it is very popular with researchers. In addition, the reversible deintercalation lithium potential of Si negative electrode (0.5V vs. Li/Li) is+) Between graphite (0.05V vs. Li/Li)+) And Li4Ti5O12(1.5V vs.Li/Li+) In the meantime, it is not as good as the formerBecause the potential is too low, the potential causes potential safety hazard, and energy loss caused by too high potential like the latter can be avoided. However, high capacity negative electrodes undergo volume expansion during lithium intercalation (e.g., Si forms Li)15Si4The volume expansion rate at this time is about 370%), which not only makes the material crush ineffective, but also makes the active material out of contact with the current collecting plate, resulting in a great drop in battery capacity. The commercial application of high capacity negative electrodes has been greatly hindered because of the above problems.
Related studies have shown that there are several factors that cause failure of high capacity cathodes. First, lithium intercalation can cause the high capacity negative electrode to expand, inducing extreme stress; secondly, the volume expansion can cause the high-capacity cathode to generate violent deformation, and the integrity of the whole electrode is damaged; thirdly, an SEI film is generated between the high-capacity negative electrode and the electrolyte, and the stability of the SEI film has great influence on the cycle life and the coulombic efficiency of the battery; finally, the volume expansion of the high capacity negative electrode may crack SEI, causing fresh surfaces of the negative electrode material to be exposed to the electrolyte, and the SEI film to be continuously thickened, so that active particles of the high capacity negative electrode are separated by the insulated SEI film, resulting in a decrease in electrochemical activity. Therefore, in order to develop a battery negative electrode material with high energy density and high power density, the above technical problems need to be solved first.
Disclosure of Invention
In order to solve the above problems, the present invention provides an anode material for a lithium ion battery, including a nano high capacity active material, a hard carbon layer coated on an outer surface of the nano high capacity active material, and a soft carbon layer coated on an outer surface of the hard carbon layer.
The nano high-capacity active material is coated with the asphalt soft carbon for the second time, so that the specific surface area of the composite material can be reduced, and the first efficiency and the cycle performance of the material are improved. The high-capacity active material is in a nanometer level, and the nanometer active material can effectively relieve volume expansion and improve cycle performance. The theoretical capacity of the high-capacity active material is 600-4000 mAh/g. The inner layer close to the active material is hard carbon, the outer layer is soft carbon, and the porosity of the hard carbon layer close to the active material in the structure is high, so that the volume expansion of the active material can be relieved, and the cycle performance can be improved; however, since the specific surface area of the hard carbon is large, the first efficiency is reduced, and therefore, the present invention improves the first efficiency by coating the outer surface of the hard carbon layer with soft carbon. The structure can improve the comprehensive electrical property of the cathode material.
In one embodiment, the nano high capacity active material has a mass of 2% to 50% of the mass of the negative electrode material. More preferably, the mass of the nano high capacity active material is 5% to 40% of the mass of the negative electrode material. More preferably, the mass of the nano high capacity active material is 10% to 30% of the mass of the negative electrode material. The proportion of the nano high-capacity active material is selected to be beneficial to gram capacity exertion and cycle performance of the negative electrode material.
In one embodiment, the nano high capacity active material has an average particle size of 5 to 200 nm. More preferably, the average particle size of the nano high-capacity active material is 10 to 150 nm. More preferably, the nano high-capacity active material has an average particle size of 20 to 80 nm. The active material with smaller particle size can effectively relieve volume expansion and improve cycle performance. However, the smaller the particle size of the active material, the higher the preparation difficulty and cost. Therefore, the optimized particle size can not only ensure the electrical property of the active material, but also meet the industrial production requirement.
In one embodiment, the nano high capacity active material is at least one selected from the group consisting of silicon, tin, silicon-tin alloy, silicon oxide, and tin oxide. Preferably, the silicon oxide is silicon monoxide, and the tin oxide is stannous oxide.
The invention also aims to provide a preparation method of the anode material for the lithium ion battery, which comprises the following steps: a) adding the nano high-capacity active material into a polyacrylonitrile solution to obtain a suspension; b) adding the suspension obtained in the step a) into a coagulating bath to form a precursor of the polyacrylonitrile-coated nano high-capacity active material; c) pre-oxidizing the precursor obtained in the step b), mixing the precursor with asphalt, and carbonizing the mixture after mixing to obtain the cathode material.
The invention firstly mixes the nanometer level high-capacity active material with the polyacrylonitrile solution to obtain the suspension with the nanometer active material highly dispersed. And then adding the suspension into a coagulating bath to quickly solidify the suspension to form a precursor of the polyacrylonitrile uniformly coated with the nano active material. And then pre-oxidizing the precursor, and mixing the precursor with a pitch carbon source. And finally, carbonizing the mixture (performing pyrolysis) to obtain the high-capacity composite material of the asphalt soft carbon and polyacrylonitrile hard carbon double-coated nano active material. The material is prepared by rapidly solidifying the highly dispersed nano active material in a coagulating bath, so that the active material can be prevented from being re-aggregated in the solvent removing process, and the nano active material can be dispersed in the composite material. In addition, the specific surface area of the composite material can be reduced through secondary coating of the asphalt soft carbon, and the primary efficiency and the cycle performance of the material are improved. The experimental materials (such as polyacrylonitrile, nano active materials, asphalt and the like) used in the invention are all sold in the market, and subsequent purification treatment is not needed.
In one embodiment, the polyacrylonitrile has a weight average molecular weight of 50000-200000. More preferably, the polyacrylonitrile has a weight average molecular weight of 80000 to 150000. More preferably, the polyacrylonitrile has a weight average molecular weight of 100000 to 120000. The molecular weight of the invention is selected to better exert the performance of the polyacrylonitrile hard carbon.
As an embodiment, the preparation method of the polyacrylonitrile solution in the step a) includes adding polyacrylonitrile to dimethylformamide or dimethylacetamide to form the polyacrylonitrile solution.
In one embodiment, the mass ratio of polyacrylonitrile to dimethylformamide is (5: 95) - (20: 80); the mass ratio of polyacrylonitrile to dimethylacetamide is (5: 95) - (20: 80). Preferably, the mass ratio of the polyacrylonitrile to the dimethylformamide is (8: 92) - (17: 83); the mass ratio of the polyacrylonitrile to the dimethylacetamide is (8: 92) - (17: 83). More preferably, the mass ratio of polyacrylonitrile to dimethylformamide is (10: 90) to (15: 85); the mass ratio of the polyacrylonitrile to the dimethylacetamide is (10: 90) - (15: 85). If the solubility of polyacrylonitrile is too low, the solvent is too much, which is not beneficial to material preparation; if too high, the viscosity is too high, which is not good for dispersing silicon powder, so it needs to be selected moderately.
As an embodiment, the preparation method of the polyacrylonitrile solution in the step a) includes adding polyacrylonitrile into dimethylformamide or dimethylacetamide, and stirring by a water bath to form the polyacrylonitrile solution. The water bath stirring is that in the process of preparing the polyacrylonitrile solution, prepared raw materials of dimethylformamide or dimethylacetamide and polyacrylonitrile are placed in a water bath and stirred simultaneously.
In one embodiment, the temperature of the water bath stirring is controlled to be 40-80 ℃. Preferably, the temperature of the water bath stirring is controlled to be 60-80 ℃.
In one embodiment, the stirring time in the water bath is controlled to be 0.5-24 hours. Preferably, the time for stirring in the water bath is controlled to be 2-10 hours. Preferably, the stirring time of the water bath is controlled to be 5-8 hours.
In one embodiment, in the step b), the mixed solution obtained in the step a) is added into a coagulation bath, and is dispersed by at least one of sanding, stirring and ultrasound to form a precursor of the polyacrylonitrile-coated nano high-capacity active material.
In one embodiment, the stirring time is controlled to be 1-24 hours; the ultrasonic time is controlled to be 0.5-10 hours; and the sanding time is controlled to be 2-10 hours.
In one embodiment, in step b), the coagulant in the coagulation bath is at least one of water, methanol, ethanol, ethylene glycol, n-butanol, formic acid, and glycerol.
In one embodiment, in the step b), the time in the coagulation bath is controlled to be 0.5 to 5 hours. Preferably, in the step b), the time in the coagulation bath is controlled to be 0.5 to 2 hours. The preferred time frame ensures sufficient interdiffusion of the solvent and coagulant.
As an embodiment, in the step a), the mass ratio of the nano high capacity active material to polyacrylonitrile is (1: 100) to (10: 1). Preferably, in the step a), the mass ratio of the nanometer high-capacity active material to the polyacrylonitrile is (1: 40) to (1: 2). As a further preference, in the step a), the mass ratio of the nano high capacity active material to polyacrylonitrile is (1: 20) to (1: 4). If the proportion of the active material is too low, the capacity of the anode material is not high; if it is too high, polyacrylonitrile cannot uniformly disperse and completely coat the active material, and both should be balanced, so the ratio should be strictly controlled.
In one embodiment, the temperature of the pre-oxidation treatment in the step c) is controlled to 200 to 300 ℃, and the time of the pre-oxidation treatment is controlled to 0.5 to 4 hours. Preferably, the temperature of the pre-oxidation treatment in the step c) is controlled to 240 to 280 ℃, and the time of the pre-oxidation treatment is controlled to 1 to 3 hours. The selection of the pre-oxidation temperature and time can ensure the complete crosslinking of polyacrylonitrile and the yield and performance of carbonized hard carbon.
In one embodiment, the temperature increase rate of the pre-oxidation treatment in step c) is controlled to be 2 ℃/min to 25 ℃/min. Preferably, the temperature increase rate of the pre-oxidation treatment in step c) is controlled to 10 to 25 ℃/min. The selection of the pre-oxidation heating rate aims to ensure the complete crosslinking of polyacrylonitrile and ensure the yield and performance of carbonized hard carbon.
As an embodiment, the atmosphere of the pre-oxidation treatment in step c) is air.
As an embodiment, the mixing in step c) is ball milling and/or stirring.
In one embodiment, the temperature of the carbonization treatment in step c) is controlled to 600 ℃ to 1500 ℃, and the time of the carbonization treatment is controlled to 0.5 hour to 10 hours. Preferably, the temperature of the carbonization treatment in the step c) is controlled to be 800 to 1200 ℃, and the time of the carbonization treatment is controlled to be 2 to 6 hours. The carbonization temperature and time not only ensure that the cathode material has higher conductivity, but also inhibit the generation of silicon carbide.
In one embodiment, the temperature increase rate of the carbonization treatment in step c) is controlled to be 2 ℃/min to 25 ℃/min. Preferably, the temperature increase rate of the carbonization treatment in the step c) is controlled to 10 to 20 ℃/min.
As an embodiment, the carbonation treatment in step c) is carried out in an inert atmosphere. Preferably, the inert atmosphere is nitrogen and/or argon.
As an embodiment, the gas flow rate of the inert atmosphere in step c) is controlled to be 20 ml/min to 100 ml/min. Preferably, the gas flow rate of the inert atmosphere in step c) is controlled to be 40 ml/min to 80 ml/min.
In one embodiment, the negative electrode material obtained in step c) is sieved, and the sieving mesh is 200-400 meshes.
The invention also provides a lithium ion secondary battery which comprises a negative electrode, wherein the negative electrode is prepared by using the negative electrode material for the lithium ion battery.
The invention can achieve the following technical effects:
1. the preparation of the high-capacity composite material provided by the invention is to rapidly solidify the highly dispersed nano active material in a coagulating bath, so that the nano active material is dispersed in the composite material in a nano scale, and the nano active material is prevented from electrochemical fusion in the charging and discharging processes, thereby improving the cycle performance of the material.
2. The method has low cost and easy operation, and can be used for preparing the high-capacity composite material for the lithium ion battery in a large scale.
3. The preparation method provided by the invention can isolate the nano active material from the electrolyte, and form a stable SEI film between the soft carbon coating layer and the electrolyte, thereby improving the cycle performance of the material.
4. The preparation method provided by the invention can effectively reduce the specific surface area of the composite material and improve the first efficiency and the cycle performance of the material.
Detailed Description
The following specific examples are intended to describe the present invention in detail, but the present invention is not limited to the following examples.
Example 1
1) Adding 14% polyacrylonitrile by mass into dimethylformamide under stirring, and stirring at 60 deg.C for 4 hr to obtain solution with weight average molecular weight of 50000.
2) Adding silicon powder with the particle size of 50nm into the solution obtained in the step 1), wherein the mass ratio of the added silicon powder to polyacrylonitrile in the solution is 1: 10, stirring for 18 hours, and then carrying out ultrasonic treatment for 2 hours to obtain a suspension with highly dispersed silicon powder.
3) Adding the nano silicon powder suspension obtained in the step 2) into a water bath, and curing for 0.5 hour to form a precursor of polyacrylonitrile uniformly coated with nano silicon powder.
4) Pre-oxidizing the precursor obtained in the step 3) in an atmosphere furnace, wherein the pre-oxidation temperature is 270 ℃, the heating rate is 20 ℃/min, the heat preservation time is 3 hours, and the atmosphere is air.
5) And (4) ball-milling and uniformly mixing the material obtained in the step (4) and the asphalt carbon source by a roller.
6) Carbonizing the mixture obtained in the step 5) in an atmosphere furnace, wherein the carbonizing temperature is 900 ℃, the heat preservation time is 4 hours, the inert protective atmosphere is nitrogen, the temperature rising speed of the carbonizing treatment is 5 ℃/min, and the gas flow in the inert protective atmosphere is 100 ml/min.
7) Screening the material obtained in the step 6) by a 200-mesh sieve to obtain the high-capacity composite material of the nano silicon powder doubly coated by the asphalt soft carbon and the polyacrylonitrile hard carbon.
Battery manufacturing and electrical performance testing
And (3) carrying out button cell test on the composite material, wherein the specific operation flow is as follows: uniformly mixing the composite material, the conductive carbon black and polyvinylidene fluoride (PVDF) according to a mass ratio of 80:10:10, adding a proper amount of N-methyl pyrrolidone (NMP), stirring for 10min, and uniformly coating the mixture on a copper foil by using an automatic film coating machine, wherein the areal density is about 3.5mg/cm2. After air blast drying, rolling on a roller press to prepare a pole piece with the diameter of 14mm, and putting the pole piece into a vacuum drying oven to dry for 12 hours at the temperature of 80 ℃. Assembling a battery in a glove box, taking a composite material pole piece as a positive electrode, a metal lithium piece as a counter electrode and 1mol/L LiPF6-Ethylene Carbonate (EC) -diethyl carbonate (DEC) (volume ratio 3: 7) as an electrolyte. Charging and discharging instrument for XinweiThe test cell was described above. And (3) testing conditions are as follows: the current density is 0.12mA/cm2The voltage is 0.01-1.5V, and the test result of constant current charge and discharge is the average result of 3 batteries.
In this example, the content of the nano-silica powder in the composite material is about 20 wt.%. The gram capacity of the composite material is 640mAh/g, and the first efficiency can reach 80%. The capacity retention rate of the button cell is 92% after 100 cycles.
Example 2
1) Adding 10% polyacrylonitrile by mass into dimethylacetamide under stirring, and stirring at 80 deg.C for 4 hr to obtain solution with weight average molecular weight of 100000.
2) Adding tin powder with the particle size of 80nm into the solution obtained in the step 1), wherein the mass ratio of the added tin powder to polyacrylonitrile in the solution is 1: 10, sanding for 4 hours, and then performing ultrasonic treatment for 3 hours to obtain a suspension with highly dispersed tin powder.
3) Adding the nano tin powder suspension obtained in the step 2) into an ethanol bath, and curing for 1 hour to form a precursor of polyacrylonitrile uniformly coated with the nano tin powder.
4) Pre-oxidizing the precursor obtained in the step 3) in an atmosphere furnace, wherein the pre-oxidation temperature is 280 ℃, the heating rate is 10 ℃/min, the heat preservation time is 4 hours, and the atmosphere is air.
5) And (4) uniformly mixing the material obtained in the step (4) with the pitch carbon source through a V-shaped mixer.
6) Carbonizing the mixture obtained in the step 5) in an atmosphere furnace, wherein the carbonization temperature is 1000 ℃, the heat preservation time is 3 hours, the inert protective atmosphere is argon, the temperature rising speed of carbonization is 10 ℃/min, and the gas flow in the inert protective atmosphere is 100 ml/min.
7) And (4) screening the material obtained in the step (6) by a 400-mesh sieve to obtain the high-capacity composite material of the nano tin powder doubly coated by the asphalt soft carbon and the polyacrylonitrile hard carbon.
Battery manufacturing and electrical performance testing
And (3) carrying out button cell test on the composite material, wherein the specific operation flow is as follows: mixing a composite material, conductive carbon black and polyvinylidene fluoride (PVDF) according to a mass ratio of 80:10:10, adding a proper amount of N-methyl pyrrolidone (NMP), stirring for 10min, and uniformly coating on a copper foil by an automatic film coating machine with the surface density of about 3.5mg/cm2. After air blast drying, rolling on a roller press to prepare a pole piece with the diameter of 14mm, and putting the pole piece into a vacuum drying oven to dry for 12 hours at the temperature of 80 ℃. Assembling a battery in a glove box, taking a composite material pole piece as a positive electrode, a metal lithium piece as a counter electrode and 1mol/L LiPF6Ethylene Carbonate (EC) -diethyl carbonate (DEC) (volume ratio 3: 7) was used as an electrolyte the cells were tested on a novice charge-discharge meter. And (3) testing conditions are as follows: the current density is 0.12mA/cm2The voltage is 0.01-1.5V, and the test result of constant current charge and discharge is the average result of 3 batteries.
In this example, the content of the nano tin powder in the composite material is about 20 wt.%. The gram capacity of the composite material is 530mAh/g, and the first efficiency can reach 81%. The capacity retention rate of the button cell is 90% after 100 cycles.
Example 3
1) Adding 8% polyacrylonitrile by mass into dimethylformamide under stirring, and stirring at 80 deg.C for 4 hr to obtain solution with weight average molecular weight of 150000.
2) Adding silicon monoxide with the particle size of 50nm into the solution obtained in the step 1), wherein the mass ratio of the added silicon monoxide to polyacrylonitrile in the solution is 1: 7.5, sanding for 2 hours, and then performing ultrasonic treatment for 4 hours to obtain a highly dispersed SiO suspension.
3) Adding the SiO suspension obtained in step 2) into ethylene glycol bath, and curing for 1.5 hours to form a precursor of polyacrylonitrile uniformly coated with nano-SiO.
4) Pre-oxidizing the precursor obtained in the step 3) in an atmosphere furnace, wherein the pre-oxidation temperature is 250 ℃, the heating rate is 10 ℃/min, the heat preservation time is 3 hours, and the atmosphere is air.
5) And (4) uniformly mixing the material obtained in the step (4) with the pitch carbon source through a V-shaped mixer.
6) Carbonizing the mixture obtained in the step 5) in an atmosphere furnace, wherein the carbonization temperature is 800 ℃, the heat preservation time is 2 hours, the inert protective atmosphere is argon, the temperature rising speed of carbonization is 10 ℃/min, and the gas flow in the inert protective atmosphere is 100 ml/min.
7) Screening the material obtained in the step 6) by a 250-mesh sieve to obtain the high-capacity composite material of the nano-silicon monoxide doubly coated by the asphalt soft carbon and the polyacrylonitrile hard carbon.
And (3) carrying out button cell test on the composite material, wherein the specific operation flow is as follows: uniformly mixing the composite material, the conductive carbon black and polyvinylidene fluoride (PVDF) according to a mass ratio of 80:10:10, adding a proper amount of N-methyl pyrrolidone (NMP), stirring for 10min, and uniformly coating the mixture on a copper foil by using an automatic film coating machine, wherein the areal density is about 3.5mg/cm2. After air blast drying, rolling on a roller press to prepare a pole piece with the diameter of 14mm, and putting the pole piece into a vacuum drying oven to dry for 12 hours at the temperature of 80 ℃. Assembling a battery in a glove box, taking a composite material pole piece as a positive electrode, a metal lithium piece as a counter electrode and 1mol/L LiPF6Ethylene Carbonate (EC) -diethyl carbonate (DEC) (volume ratio 3: 7) was used as an electrolyte the cells were tested on a novice charge-discharge meter. And (3) testing conditions are as follows: the current density is 0.12mA/cm2The voltage is 0.01-1.5V, and the test result of constant current charge and discharge is the average result of 3 batteries.
In this example, the amount of nano-SiO in the composite is about 25 wt.%. The gram capacity of the composite material is 660mAh/g, and the first efficiency can reach 80%. The capacity retention rate of the button cell is 93 percent after 100 cycles.

Claims (23)

1. A preparation method of a negative electrode material for a lithium ion battery comprises the following steps: a) adding the nano high-capacity active material into a polyacrylonitrile solution to obtain a suspension; b) adding the suspension obtained in the step a) into a coagulating bath to form a precursor of the polyacrylonitrile-coated nano high-capacity active material; c) pre-oxidizing the precursor obtained in the step b), mixing the precursor with asphalt, and carbonizing the mixture to obtain the negative electrode material; the negative electrode material comprises a nano high-capacity active material, a hard carbon layer coated on the outer surface of the nano high-capacity active material and a soft carbon layer coated on the outer surface of the hard carbon layer; the preparation method of the polyacrylonitrile solution in the step a) comprises the steps of adding polyacrylonitrile into dimethylformamide or dimethylacetamide to form a polyacrylonitrile solution; the mass ratio of the polyacrylonitrile to the dimethylformamide is (17: 83) - (20: 80); the mass ratio of the polyacrylonitrile to the dimethylacetamide is (17: 83) - (20: 80); in the step a), the mass ratio of the nano high-capacity active material to polyacrylonitrile is (1: 20) to (1: 4).
2. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: the weight average molecular weight of the polyacrylonitrile is 50000-200000.
3. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: the preparation method of the polyacrylonitrile solution in the step a) comprises the steps of adding polyacrylonitrile into dimethylformamide or dimethylacetamide, and stirring through water bath to form the polyacrylonitrile solution.
4. The method for preparing the negative electrode material for a lithium ion battery according to claim 3, characterized in that: the temperature of the water bath stirring is controlled to be 40-80 ℃.
5. The method for preparing the negative electrode material for a lithium ion battery according to claim 3, characterized in that: the stirring time of the water bath is controlled to be 0.5 to 24 hours.
6. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: in the step b), the suspension obtained in the step a) is added into a coagulating bath, and is dispersed by at least one of sanding, stirring and ultrasound to form a precursor of the polyacrylonitrile-coated nano high-capacity active material.
7. The method for preparing the negative electrode material for a lithium ion battery according to claim 6, wherein: the stirring time is controlled to be 1-24 hours; the ultrasonic time is controlled to be 0.5 to 10 hours; the sanding time is controlled to be 2-10 hours.
8. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: in the step b), the coagulant in the coagulating bath is at least one of water, methanol, ethanol, ethylene glycol, n-butanol, formic acid and glycerol.
9. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: in the step b), the time in the coagulation bath is controlled to be 0.5 to 5 hours.
10. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: the temperature of the pre-oxidation treatment in the step c) is controlled to be 200-300 ℃, and the time of the pre-oxidation treatment is controlled to be 0.5-4 hours.
11. The method for preparing the negative electrode material for a lithium ion battery according to claim 10, wherein: the temperature of the pre-oxidation treatment in the step c) is controlled to be 240-280 ℃, and the time of the pre-oxidation treatment is controlled to be 1-3 hours.
12. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: the temperature rising speed of the pre-oxidation treatment in the step c) is controlled to be 2-25 ℃/min.
13. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: the mixing mode in the step c) is ball milling mixing and/or stirring mixing.
14. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: the temperature of the carbonization treatment in the step c) is controlled to be 600-1500 ℃, and the time of the carbonization treatment is controlled to be 0.5-10 hours.
15. The method for preparing the negative electrode material for a lithium ion battery according to claim 14, wherein: the temperature of the carbonization treatment in the step c) is controlled to be 800-1200 ℃, and the time of the carbonization treatment is controlled to be 2-6 hours.
16. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: the heating rate of the carbonization treatment in the step c) is controlled to be 2-25 ℃/min.
17. The method for preparing the negative electrode material for a lithium ion battery according to claim 1, wherein: the carbonation treatment in step c) is carried out in an inert atmosphere.
18. The method for preparing the negative electrode material for a lithium ion battery according to claim 17, wherein: the inert atmosphere is nitrogen and/or argon.
19. An anode material, characterized in that: the lithium ion battery negative electrode material is prepared by the preparation method of the lithium ion battery negative electrode material according to claim 1.
20. The negative electrode material of claim 19, wherein: the mass of the nanometer high-capacity active material is 2-50% of that of the negative electrode material.
21. The negative electrode material of claim 19, wherein: the average particle size of the nano high-capacity active material is 5-200 nm.
22. The negative electrode material of claim 19, wherein: the nano high-capacity active material is selected from at least one of silicon, tin, silicon-tin alloy, silicon oxide and tin oxide.
23. A lithium ion secondary battery comprising a negative electrode, characterized in that: the negative electrode is prepared using a negative electrode material according to any one of claims 19 to 22.
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