A low-expansion and low-resistivity silicon-carbon anode material, its preparation method and application

Abstract

The present invention relates to a material, in particular to a low-expansion and low-resistivity silicon-carbon anode material and its preparation method and application. It comprises the following steps: (1) preparing a silicon-iron composite material; (2) preparing a silicon-iron @ carbon nanotube composite material: placing the silicon-iron composite material in a fluidized bed reactor, protecting it with an inert gas, introducing a gaseous carbon source, and filling a certain amount of carbon nanotubes in the silicon-iron composite material through the cracking of the gaseous carbon source; (3) purification; (4) preparing a hard carbon-modified silicon @ carbon nanotube composite material: dispersing a certain amount of modifier in a solvent, and subjecting the purified silicon @ carbon nanotube composite material to oxidation treatment in the solvent under certain temperature conditions; (5) carbonization treatment; (6) powder making treatment; (7) surface modification. The silicon-carbon material prepared by the present invention has obvious low resistivity of particles and overall structural stability, achieving the effects of low expansion and low resistivity of the silicon-carbon composite material.

Description

Technical Field

[0001] The invention relates to a material, in particular to a low expansion and low resistivity silicon-carbon negative electrode material and a preparation method and application thereof. Background Art

[0002] The mileage of electric vehicles is gradually increasing, and the further development of lithium batteries with long driving range and good safety has always been a favorable support for the healthy development of the electric vehicle industry. As the negative electrode material of lithium-ion batteries, graphite material has low cost, environmental friendliness, low charge and discharge voltage, and good stability, making it the most commercialized negative electrode material. However, the limited specific capacity of graphite materials cannot meet the high energy density battery cell requirements for long driving range.

[0003] In addition to graphite negative electrode materials, silicon-based materials have become the preferred negative electrode material for current high-energy-density lithium-ion batteries due to their higher gram capacity (4200 mAh / g), which is about 10 times that of graphite. Silicon has the advantages of high electrochemical lithium insertion potential (about 0.4 V vs. Li / Li+), high safety, and abundant reserves, but it also has serious disadvantages: severe volume changes during charging and discharging (about 300%), and rapid capacity decay; silicon has poor conductivity, and its performance is not ideal when charging and discharging with large currents. Therefore, starting from the material structure itself, the rational design of silicon-based negative electrode materials to achieve materials with low expansion and low resistivity is still a technical issue that needs to be studied in depth.

[0004] Therefore, the field urgently needs to develop a lithium-ion battery graphite negative electrode material with low expansion and low resistivity and simple preparation process. It is of practical significance to prepare a silicon-carbon negative electrode material with low expansion and low resistivity in an industrially easy manner. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the disadvantages of the existing technology that the silicon-based materials have serious volume changes (about 300%) during charging and discharging, rapid capacity decay, poor conductivity, and unsatisfactory performance during large current charging and discharging, and provide a low-expansion and low-resistivity silicon-carbon negative electrode material, preparation method and application. The silicon-carbon negative electrode material prepared by the present invention has low expansion and low resistivity, simple preparation process, easy industrialization, abundant raw material reserves and guaranteed source, which can meet the characteristic requirements of high-performance negative electrode materials for lithium-ion batteries.

[0006] The present invention provides a method for preparing a low expansion and low resistivity silicon-carbon negative electrode material, which is characterized by comprising the following steps:

[0007] (1) Preparing a ferrosilicon composite material: taking a dispersant, nano silicon powder, and nano iron powder, and adding them to an ethanol solvent in a stirring state, stirring and dispersing them sufficiently, and removing the ethanol solvent to obtain a ferrosilicon composite material;

[0008] (2) Ferrosilicon@carbon nanotube composite material: Place the ferrosilicon composite material in a fluidized bed reactor, protect it with an inert gas, and at a certain temperature, introduce a gaseous carbon source at a certain flow rate and maintain for a period of time. The gaseous carbon source is cracked to fill a certain amount of carbon nanotubes in the ferrosilicon composite material, obtaining the ferrosilicon@carbon nanotube composite material;

[0009] (3) Purification: Add the ferrosilicon@carbon nanotube composite material to concentrated nitric acid, reflux and oxidize for a certain time, filter and wash until neutral, and dry to obtain the purified silicon@carbon nanotube composite material;

[0010] (4) Hard carbon modified silicon@carbon nanotube composite material: Take a certain amount of modifier and disperse it in a solvent. Under certain temperature conditions, place the purified silicon@carbon nanotube composite material in the solvent for oxidation treatment to obtain the hard carbon modified silicon@carbon composite material;

[0011] (5) Carbonization treatment: Perform high-temperature heat treatment on the hard carbon modified silicon@carbon composite material to obtain the silicon@carbon composite material;

[0012] (6) Powder making treatment: Crush the silicon@carbon composite material after high-temperature heat treatment by using the common mechanical physical pulverization method to obtain silicon@carbon composite material powder of a certain specification;

[0013] (7) Surface modification: Perform surface gas-phase coating modification on the silicon@carbon composite material powder to obtain amorphous carbon-coated silicon@carbon composite material powder, and perform screening processing to obtain the low-expansion and low-resistivity silicon-carbon anode material.

[0014] In step (1), the dispersant is commercially available chemically pure polyvinylpyrrolidone (PVP, K30), and the addition amount of the dispersant is 1%-10% by mass (the sum of the masses of PVP, nano-silicon powder, and nano-iron powder)

[0015] In step (1), the median particle size D50 of the nano-silicon powder is 30-50 nm, which is spherical or flaky. If it is spherical particles, D99 < 200 nm. The median particle size D50 of the nano-iron powder is 50-100 nm, which is spherical particles, D99 < 500 nm;

[0016] In step (1), the mass ratio of the nano-silicon powder to the iron powder is 9.5:0.5 - 8:2;

[0017] In step (1), the stirring equipment is any mixer that can disperse and stir the materials, with a rotation speed of 10-50 r / min and a stirring time of 10-30 min;

[0018] In step (1), to remove the ethanol solvent, a rotary evaporator is used until the ethanol is evaporated and removed;

[0019] In step (2), the fluidized bed reactor is a reactor with an inner diameter of 50 - 100 mm;

[0020] In step (2), the inert gas is nitrogen with a purity of 99.9%; the carbon source gas is any mixture of CH4, C2H2, C2H4, etc. and nitrogen, and the ratio of the mixture is 1:1 - 1:3, with a flow rate of 1 L / min - 3 L / min;

[0021] In step (2), the certain temperature is 700°C - 900°C; the period of time is 30 min - 60 min;

[0022] In step (3), the concentrated nitric acid is commercially available chemically pure, the reflux oxidation time is 3 h - 6 h, and the reflux temperature is 80°C - 90°C;

[0023] In step (4), the modifier is various softening point modified pitches, which can be coal - based modified pitch or petroleum - based modified pitch;

[0024] In step (4), the solvent is one or a mixture of light oil, wash oil, phenol oil, and naphthalene oil;

[0025] In step (4), the dosage of the modifier is 3% - 8% of the mass of the purified silicon @ carbon nanotube composite material;

[0026] In step (4), the oxidation operation is to introduce air for oxidation, and the air flow rate is 0.5 L / min - 3 L / min;

[0027] In step (4), the oxidation temperature is 300°C - 500°C, and the oxidation time is 0.5 h - 3 h;

[0028] In step (5), the heat treatment conditions are: heating from room temperature to 500 - 700°C at a rate of 3.0 - 5.0°C / min, holding for 0.5 h, then heating to 900 - 1200°C at a rate of 3.0 - 5.0°C / min, holding for 1 - 2 h, and after the holding ends, naturally cooling to room temperature.

[0029] Preferably, the temperature measuring device for the high - temperature carbonization furnace in step (5) includes a carbonization furnace;

[0030] It further includes a pull-out type temperature measuring component. The pull-out type temperature measuring component includes a bracket, an electric telescopic rod, a sliding sleeve, a temperature sensor and a cover plate. The bracket is installed at the left end of the carbonization furnace. The left end of the electric telescopic rod is connected to the right end of the left part of the bracket. A temperature measuring hole is provided at the left end of the carbonization furnace. The sliding sleeve is slidably connected to the temperature measuring hole. A sliding hole is provided at the left end of the sliding sleeve. The temperature sensor is slidably connected to the sliding hole. The output end of the electric telescopic rod is connected to the left end of the temperature sensor. The left end of the cover plate is connected to the right end of the temperature sensor.

[0031] More preferably, a cooling cavity is provided inside the sliding sleeve, and a water outlet and a water inlet are respectively provided at the top and bottom of the left end of the cooling cavity.

[0032] More preferably, a chamfer is provided at the left end of the temperature measuring hole.

[0033] In step (6), the pulverizing device can be any one of mechanical pulverization and airflow pulverization, and the material can be any one of stainless steel materials.

[0034] In step (6), the pulverized material needs to pass through an 80-mesh sieve, and the material passing through the sieve is taken.

[0035] In step (7), the gaseous carbon source for gas-phase coating can be any mixture of CH4, C2H2 and nitrogen; the ratio of the mixture is 1:1 - 1:3, and the flow rate is 1 L / min - 3 L / min;

[0036] In step (7), the gas-phase coating conditions are as follows: under room temperature conditions, nitrogen protection is passed, the flow rate of the nitrogen atmosphere is 0.1 - 0.8 mL / min, the temperature is raised from room temperature to 700 - 1000 °C at 3.0 - 5.0 °C / min, the mixture gas is switched, and it is kept at a constant temperature for 1.5 - 3.0 h. After the constant temperature ends, it is naturally cooled to room temperature.

[0037] In step (7), the sieving treatment is carried out using a 300 - 350-mesh standard sieve, and the material passing through the sieve is taken. The median particle size of the material passing through the sieve: D50 = 7 - 15 μm.

[0038] On the basis of conforming to the common knowledge in the art, the above conditions can be combined arbitrarily to obtain various preferred examples of the present invention.

[0039] The reagents and raw materials used in the present invention are all commercially available.

[0040] Compared with the prior art, in the present invention, the nano-silicon particles are uniformly coated with in-situ grown carbon nanotubes, supplemented by rigid hard carbon materials and amorphous carbon coating, achieving the effects of low expansion and low resistivity of the silicon-carbon composite material.

[0041] The positive and progressive effects of the present invention are as follows:

[0042] The present invention uses nanoscale silicon powder as the main raw material, and forms a flexible low-resistivity carbon nanotube-confined nanosilicon composite material by means of in-situ generation of carbon nanotube coating; utilizes the rigid characteristics of hard carbon materials to further stabilize the flexible silicon@carbon nanotube skeleton; and finally performs surface modification treatment on the powder to form a low-expansion and low-resistivity silicon-carbon anode material with uniform surface coating.

[0043] When using the temperature measuring device of the high-temperature carbonization furnace, the electric telescopic rod can be operated to output and drive the temperature sensor to slide to the right, insert the temperature sensor into the carbonization furnace for temperature detection at the corresponding position, and when the temperature measurement is completed, the electric telescopic rod can be operated to retract and drive the temperature sensor to slide to the left until the right cover plate seals the temperature measuring hole to protect the temperature sensor; through this device, the temperature inside the furnace can be automatically detected and good self-protection can be achieved when not measuring temperature, the service life of the temperature sensor can be improved, thereby enhancing the practicability, and it is beneficial to the precise temperature control of the high-temperature carbonization furnace, thus improving the carbonization heat treatment effect, and thus being beneficial to the preparation of a low-expansion and low-resistivity silicon-carbon anode material with uniform surface coating.

[0044] The silicon-carbon material prepared by the present invention has a simple process, strong operability, and at the same time has obvious low resistivity of particles and overall structural stability, achieving the effects of low expansion and low resistivity of the silicon-carbon composite material. Description of the Drawings

[0045] Figure 1 It is a schematic structural diagram of a low-expansion and low-resistivity silicon-carbon composite anode material prepared in Example 1.

[0046] Figure 2 It is an SEM diagram of a low-expansion and low-resistivity silicon-carbon composite anode material prepared in Example 1.

[0047] Figure 3 It is a schematic structural diagram of the temperature measuring device for the high-temperature carbonization furnace during the carbonization heat treatment of the present invention.

[0048] Figure 4 It is an enlarged structural diagram of A of the temperature measuring device for the high-temperature carbonization furnace during the carbonization heat treatment of the present invention.

[0049] Figure 5 It is a left view structural diagram of the sliding sleeve of the temperature measuring device for the high-temperature carbonization furnace during the carbonization heat treatment of the present invention.

[0050] Figure 6 It is a left view structural diagram of the maintenance bracket of the temperature measuring device for the high-temperature carbonization furnace during the carbonization heat treatment of the present invention.

[0051] Reference numerals in the drawings: 1, carbonization furnace; 2, support; 3, electric telescopic rod; 4, sliding sleeve; 5, temperature sensor; 6, cover plate; 7, temperature measuring hole; 8, cooling cavity; 9, heat insulation cotton; 10, chamfer; 11, maintenance support; 12, arc-shaped groove; 13, rubber pad; 14, screw. Detailed implementation manners

[0052] The present invention will be further described below by way of examples, but the present invention is not limited to the scope of the described examples. For the experimental methods without specific conditions noted in the following examples, they are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0053] Example 1

[0054] (1) Take 500 mL of anhydrous ethanol solution, add 24 g of polyvinylpyrrolidone (K30), stir at a rotation speed of 10 r / min for 20 min, and successively add 114 g of nano-spherical silicon powder with a median particle size D50 = 30 nm and 6 g of nano-iron powder with a median particle size D50 = 50 nm, and stir and disperse for 60 min; transfer the dispersed slurry to a rotary evaporator, and keep the rotary evaporator at 80 °C until ethanol is removed to obtain a dry powder;

[0055] (2) Take 100 g of nano-silicon-iron composite powder and place it in a fluidized bed reactor with an inner diameter of 50 mm. Under a nitrogen atmosphere, the nitrogen flow rate is 0.1 mL / min, and it is heated from room temperature to 700 °C at a rate of 3.0 °C / min. Nitrogen is switched to a mixed gas of CH4 and nitrogen, the mixing ratio of the mixed gas is 1:1, and the flow rate of the mixed gas is 1 L / min and kept at a constant temperature for 30 min. After the constant temperature ends, it is naturally cooled to room temperature.

[0056] (3) Add 190 g of the material prepared by the fluidized bed reactor to 500 mL of concentrated nitric acid, reflux and oxidize at a reflux temperature of 80 °C for 3 h. After cooling to room temperature, the material is filtered and washed until neutral. Under nitrogen protection, the filtered and washed filter cake is dried at a drying temperature of 100 °C for 24 h. A purified silicon@carbon nanotube composite material is obtained.

[0057] (4) Disperse 5.4 g of low-temperature petroleum-based modified asphalt with a softening point of 150 °C in 300 mL of light oil, and then add 180 g of the purified silicon@carbon nanotube composite material. Heat from room temperature to 300 °C at a rate of 3.0 °C / min, and carry out air oxidation treatment with an air flow rate of 0.5 L / min and an oxidation time of 0.5 h. After natural cooling to room temperature, a silicon@carbon composite material modified with hard carbon is obtained;

[0058] (5) Carbonization: Place the silicon@carbon composite material modified with hard carbon in a carbonization furnace. Under a nitrogen atmosphere, with a nitrogen flow rate of 0.1 mL / min, heat it from room temperature to 500 °C at a rate of 3.0 °C / min, hold for 0.5 h, then heat it to 900 °C at a rate of 3.0 °C / min, hold for 1 h. After the holding is completed, cool it naturally to room temperature. Obtain the silicon@carbon composite material;

[0059] Among them, a temperature measuring device for the high-temperature carbonization furnace during carbonization heat treatment, such as Figure 3-6 shown, includes a carbonization furnace 1; it also includes a pull-out type temperature measuring assembly. The pull-out type temperature measuring assembly includes a bracket 2, an electric telescopic rod 3, a sliding sleeve 4, a temperature sensor 5 and a cover plate 6. The bracket 2 is installed at the left end of the carbonization furnace 1. The left end of the electric telescopic rod 3 is connected to the right end of the left part of the bracket 2. A temperature measuring hole 7 is provided at the left end of the carbonization furnace 1. The sliding sleeve 4 is slidably connected to the temperature measuring hole 7. A sliding hole is provided at the left end of the sliding sleeve 4. The temperature sensor 5 is slidably connected to the sliding hole. The output end of the electric telescopic rod 3 is connected to the left end of the temperature sensor 5. The left end of the cover plate 6 is connected to the right end of the temperature sensor 5; when using the temperature measuring device of the high-temperature carbonization furnace, the electric telescopic rod can be operated to drive the temperature sensor to slide to the right, extend the temperature sensor into the carbonization furnace for temperature detection at the corresponding position. When the temperature measurement is completed, the electric telescopic rod can be operated to retract to drive the temperature sensor to slide to the left until the right cover plate seals the temperature measuring hole to protect the temperature sensor; through this device, the temperature inside the furnace can be automatically detected and it can provide good protection for itself when not measuring temperature, improve the service life of the temperature sensor, and thus enhance the practicability.

[0060] A temperature measuring device for a high-temperature carbonization furnace of the present invention, such as Figure 3-6As shown, a cooling cavity 8 is provided inside the sliding sleeve 4, and a water outlet and a water inlet are respectively provided at the top and bottom of the left end of the cooling cavity 8; when the high-temperature carbonization furnace is in use, cooling water can be introduced into the cooling cavity to cool the sliding sleeve, further reducing the ambient temperature when the temperature sensor is not in use, thereby enhancing the practicability. The temperature measuring device further includes a heat insulation cotton 9, and the heat insulation cotton 9 is installed at the right end of the cover plate 6; the heat insulation cotton can further reduce the heat transfer in the carbonization furnace, thereby enhancing the practicability. A chamfer 10 is provided at the left end of the temperature measuring hole 7; the chamfer can make it more convenient to insert the sliding sleeve into the temperature measuring hole when replacing the sliding sleeve and the temperature sensor, thereby enhancing the practicability. The temperature measuring device further includes a maintenance bracket 11, and the maintenance bracket 11 is installed at the inner bottom end of the bracket 2; during maintenance, the temperature sensor can be pulled out of the temperature measuring hole and supported by the maintenance bracket, thereby enhancing the practicability. At the top end of the maintenance bracket 11 of the temperature measuring device, an arc-shaped groove 12 is provided; the arc-shaped groove can make the maintenance bracket support the temperature sensor more fittingly, thereby enhancing the practicability. The temperature measuring device further includes a rubber pad 13, and the rubber pad 13 is installed on the arc-shaped groove 12; the rubber pad can buffer the temperature sensor during maintenance, thereby enhancing the practicability. At the right end of the temperature sensor 5 of the temperature measuring device, a screw 14 is provided, and a threaded hole is provided in the middle of the left end of the cover plate 6, and the screw 14 is threadedly connected to the threaded hole; when disassembling and assembling the sliding sleeve and the temperature sensor, the cover plate can be rotated and disassembled more conveniently, thereby enhancing the practicability.

[0061] For a temperature measuring device for a high-temperature carbonization furnace according to the present invention, during its operation, when using the temperature measuring device of the high-temperature carbonization furnace, the electric telescopic rod can be operated to output and drive the temperature sensor to slide to the right, and the temperature sensor is extended into the carbonization furnace to detect the temperature at the corresponding position. After the temperature measurement is completed, the electric telescopic rod can be operated to retract and drive the temperature sensor to slide to the left until the right cover plate seals the temperature measuring hole to protect the temperature sensor.

[0062] (6) Powder making: The above-mentioned carbonized material is subjected to air flow pulverization to obtain a pulverized material, and the pulverized material is passed through a 80-mesh standard sieve to obtain silicon @ carbon composite material powder.

[0063] (7) Surface modification: Take 150 g of silicon @ carbon composite material powder and place it in a tube furnace. Under room temperature conditions, nitrogen is passed for protection, the nitrogen flow rate is 0.1 mL / min, and it is heated from room temperature to 700 °C at a rate of 3.0 °C / min, and then the mixed gas is switched. The mixed gas is composed of CH4 and N2, the ratio is 1:1, and the flow rate is 1 L / min; keep the temperature constant for 1.5 h, and after the temperature constant ends, naturally cool down to room temperature. Amorphous carbon-coated silicon @ carbon composite material powder is obtained, and it is screened with a 350-mesh standard sieve, and the sieved material is taken. The median particle size of the sieved material: D50 = 10 um. A low-expansion and low-resistivity silicon-carbon anode material is obtained.

[0064] AsFigure 2 Shown are SEM photos of the prepared silicon-carbon anode material with low expansion and low resistivity, which were obtained by observing with a field emission electron scanning microscope JSM-IT800 produced by JEOL Ltd.

[0065] Electrochemical performance test:

[0066] A CR2032 coin cell was used, with a lithium sheet as the counter electrode, a Celgard 2300 PP / PE / PP three-layer microporous composite membrane as the separator, and a 1M LiPF6 / EC+EMC+DMC solution as the supporting electrolyte. The sample after passing through a 350-mesh standard sieve above was formulated into a slurry in the ratio of SP∶CMC∶SBR = 95.5∶1.5∶1.5∶1.5, then coated on a conductive copper foil, dried at 120°C for 2 h, and roll-pressed into shape under a pressure of 10 MPa using a rolling press. After assembling the positive and negative electrode sheets, the separator, and the electrolyte, it was stamped and sealed. All assembly processes were carried out in a dry glove box filled with argon.

[0067] The lithium-ion battery with the above structure was allowed to keep warm overnight at room temperature. The charge-discharge performance of the battery was tested using an Arbin charge / discharge tester. The test charge-discharge current density was 0.6 mA / cm 2 , and the cut-off charge-discharge voltage was 0.005 - 1.500 V. The initial capacity of the lithium-ion battery was measured, and the pole piece rebound of the lithium-ion battery at 1.5 weeks (i.e., full charge state, voltage 1.5 V) was measured.

[0068] Powder resistivity test: The powder resistivity of the silicon-carbon anode material was measured using an FT-341A four-probe powder resistivity tester.

[0069] The data of the initial delithiation capacity, the number of pole piece rebounds at 1.5 weeks in the full charge state, and the powder resistivity are shown in Table 1.

[0070] Example 2

[0071] (1) Take 2000 mL of anhydrous ethanol solution, add 40 g of polyvinylpyrrolidone (K30), stir at a rotation speed of 50 r / min for 30 min, and sequentially add 350 g of nano-spherical silicon powder with a median particle size D50 = 50 nm and 50 g of nano-iron powder with a median particle size D50 = 100 nm, and stir and disperse for 60 min; transfer the dispersed slurry to a rotary evaporator, and keep the rotary evaporator at 80°C until ethanol is removed to obtain a dry powder.

[0072] (2) Take 400 g of nano-silicon-iron composite powder and place it in a fluidized bed reactor with an inner diameter of 100 mm. Under a nitrogen atmosphere, with a nitrogen flow rate of 0.1 mL / min, heat it from room temperature to 900 °C at a rate of 3.0 °C / min. Then switch the nitrogen to a mixture of C2H2 and nitrogen with a mixing ratio of 1:3 and a mixed gas flow rate of 3 L / min. Keep it at a constant temperature for 60 min, and after the constant temperature ends, let it cool naturally to room temperature.

[0073] (3) Add 500 g of the material prepared in the fluidized bed reactor to 1000 mL of concentrated nitric acid, and carry out reflux oxidation at a reflux temperature of 80 °C for 3 h. After cooling to room temperature, filter and wash the material until it is neutral. Under nitrogen protection, dry the filter cake obtained from filtration and washing at a drying temperature of 100 °C for 24 h. The purified silicon@carbon nanotube composite material is obtained.

[0074] (4) Disperse 32 g of low-temperature petroleum-based modified asphalt with a softening point of 200 °C in 700 mL of light oil, and then add 400 g of the purified silicon@carbon nanotube composite material. Heat it from room temperature to 400 °C at a rate of 3.0 °C / min, and carry out air oxidation treatment with an air flow rate of 3 L / min for 3 h. Let it cool naturally to room temperature to obtain a hard carbon-modified silicon@carbon composite material;

[0075] (5) Carbonization: Place the hard carbon-modified silicon@carbon composite material in a carbonization furnace. Under a nitrogen atmosphere, with a nitrogen flow rate of 0.1 mL / min, heat it from room temperature to 600 °C at a rate of 3.0 °C / min, keep it at a constant temperature for 0.5 h, then heat it to 1100 °C at a rate of 3.0 °C / min, and keep it at a constant temperature for 1 h. After the constant temperature ends, let it cool naturally to room temperature. The silicon@carbon composite material is obtained;

[0076] (6) Powder making: Carry out air flow pulverization on the above carbonized material to obtain a pulverized material, and pass the pulverized material through an 80-mesh standard sieve to obtain the silicon@carbon composite material powder.

[0077] (7) Surface modification: Take 150 g of the silicon@carbon composite material powder and place it in a tubular furnace. Under nitrogen protection at room temperature, with a nitrogen flow rate of 0.1 mL / min, heat it from room temperature to 700 °C at a rate of 3.0 °C / min, and then switch to a mixed gas. The mixed gas is composed of CH4 and N2 with a ratio of 1:1 and a flow rate of 1 L / min; keep it at a constant temperature for 1.5 h. After the constant temperature ends, let it cool naturally to room temperature. The amorphous carbon-coated silicon@carbon composite material powder is obtained, and it is screened with a 350-mesh standard sieve. Take the undersize material, and the median particle size of the undersize material: D50 = 15 um. The low-expansion and low-resistivity silicon-carbon anode material is obtained.

[0078] The remaining steps and electrochemical evaluation are the same as those in Example 1.

[0079] Example 3

[0080] Steps (1)-(6) are the same as in Example 1,

[0081] (7) Surface modification: Take 150 g of silicon@carbon composite powder and place it in a tube furnace. Under room temperature conditions, purge with nitrogen protection, nitrogen flow rate 0.1 mL / min, heat from room temperature to 1000 °C at 3.0 °C / min, switch to the mixed gas, which is composed of CH4 and N2, with a ratio of 1:2 and a flow rate of 2 L / min; keep at a constant temperature for 3 h, and after the constant temperature ends, naturally cool down to room temperature. Obtain amorphous carbon-coated silicon@carbon composite powder, perform screening treatment with a 350-mesh standard sieve, take the undersize material, and the median particle size of the undersize material: D50 = 12 μm. Obtain a low-expansion and low-resistivity silicon-carbon anode material.

[0082] The remaining steps and electrochemical evaluation are the same as in Example 1.

[0083] Comparative Example 1

[0084] (1) Disperse 5.4 g of low-temperature petroleum-based modified asphalt with a softening point of 150 °C in 300 mL of light oil, and then add 114 g of nano-spherical silicon powder with a median particle size D50 = 30 nm. Heat from room temperature to 300 °C at 3.0 °C / min, conduct air oxidation treatment, air flow rate 0.5 L / min, oxidation time 0.5 h, and naturally cool down to room temperature to obtain a hard carbon-modified silicon@carbon composite material;

[0085] (2) Carbonization: Place the hard carbon-modified silicon@carbon composite material in a carbonization furnace. Under a nitrogen atmosphere, nitrogen flow rate 0.1 mL / min, heat from room temperature to 500 °C at 3.0 °C / min, keep at a constant temperature for 0.5 h, then heat to 900 °C at 3.0 °C / min, keep at a constant temperature for 1 h, and after the constant temperature ends, naturally cool down to room temperature. Obtain a silicon@carbon composite material;

[0086] (3) Powder making: Subject the above carbonized material to air jet milling to obtain a milled material, and pass the milled material through an 80-mesh standard sieve to obtain silicon@carbon composite powder.

[0087] (4) Surface modification: Take 150 g of silicon@carbon composite powder and place it in a tube furnace. Under room temperature conditions, purge with nitrogen protection, nitrogen flow rate 0.1 mL / min, heat from room temperature to 700 °C at 3.0 °C / min, switch to the mixed gas, which is composed of CH4 and N2, with a ratio of 1:1 and a flow rate of 1 L / min; keep at a constant temperature for 1.5 h, and after the constant temperature ends, naturally cool down to room temperature. Obtain amorphous carbon-coated silicon@carbon composite powder, perform screening treatment with a 350-mesh standard sieve, take the undersize material, and the median particle size of the undersize material: D50 = 10 μm. Obtain a silicon-carbon composite anode material.

[0088] Table 1 Performance tests of examples and comparative examples

[0089]

[0090] Note: "Full charge bounce %" reflects the degree of volume change of the electrode material. The lower the bounce rate, the smaller the volume expansion during charging and discharging of the electrode sheet, and the more stable the electrode material. "Resistivity" reflects the resistance of the electrode material. Materials with low resistance have high conductivity and good electrical conductivity, and the electrode material will have better fast charging performance.

[0091] Data description of examples and comparative examples:

[0092] When the silicon composite granulation preparation technology is different, the electrochemical performance of the prepared silicon-carbon negative electrode material varies greatly. The silicon-carbon material prepared by the method of the present invention has the characteristics of low expansion and low resistivity. The bounce and resistivity of the examples are both better than those of the comparative examples.

[0093] This specific embodiment is only an explanation of the present invention, and it is not a limitation of the present invention. Those skilled in the art can make modifications to this embodiment without creative contributions according to needs after reading this specification, but as long as it is within the scope of the claims of the present invention, it is protected by the patent law.

Claims

1. A preparation method of a low-expansion and low-resistivity silicon-carbon anode material, characterized in that, The steps include: (1) Preparing a ferrosilicon composite material: taking a dispersant, nano silicon powder, and nano iron powder, and adding them to an ethanol solvent in a stirring state, stirring and dispersing them sufficiently, and removing the ethanol solvent to obtain a ferrosilicon composite material; (2) Preparation of ferrosilicon@carbon nanotube composite material: placing the ferrosilicon composite material in a fluidized bed reactor, passing an inert gas as protection, introducing a gaseous carbon source at a certain temperature and flow rate for a period of time; the gaseous carbon source is cracked to fill a certain amount of carbon nanotubes in the ferrosilicon composite material, thereby obtaining a ferrosilicon@carbon nanotube composite material; (3) Purification: adding the silicon iron@carbon nanotube composite material to concentrated nitric acid, reflux oxidation for a certain period of time, filtering and washing until neutral, and drying to obtain a purified silicon@carbon nanotube composite material; (4) Preparation of hard carbon modified silicon@carbon nanotube composite material: taking a certain amount of modifier and dispersing it in a solvent, placing the purified silicon@carbon nanotube composite material in the solvent for oxidation treatment under certain temperature conditions to obtain a hard carbon modified silicon@carbon nanotube composite material; In step (4), the modifier is coal-based modified asphalt or petroleum-based modified asphalt; The solvent is one or more mixtures of light oil, washing oil, phenol oil and naphthalene oil; The amount of the modifier is 3%-8% of the mass of the purified silicon@carbon nanotube composite material; The oxidation treatment is carried out by introducing air for oxidation, and the air flow rate is 0.5L / min-3L / min; The oxidation temperature is: 300°C-500°C, and the oxidation time is 0.5h-3h; (5) Carbonization treatment: subjecting the hard carbon-modified silicon@carbon nanotube composite material to high-temperature heat treatment to obtain a silicon@carbon composite material; in step (5), the heat treatment conditions are: heating from room temperature to 500-700°C at a rate of 3.0-5.0°C / min, maintaining the temperature for 0.5h, then heating to 900-1200°C at a rate of 3.0-5.0°C / min, maintaining the temperature for 1-2h, and naturally cooling to room temperature after the temperature is maintained; (6) Powdering treatment: Pulverizing the high-temperature heat-treated silicon@carbon composite material to obtain silicon@carbon composite material powder of a certain specification; (7) Surface modification: The surface of the silicon@carbon composite material powder is modified by vapor phase coating to obtain an amorphous carbon-coated silicon@carbon composite material powder, which is then screened to obtain a low expansion and low resistivity silicon-carbon negative electrode material.

2. The preparation method of a low-expansion and low-resistivity silicon-carbon anode material according to claim 1, wherein, In step (1), the ethanol solvent is removed by using a rotary evaporator to evaporate and remove the ethanol.

3. The preparation method of a low-expansion and low-resistivity silicon-carbon anode material according to claim 2, characterized in that In step (2), the fluidized bed reactor is a reactor with an inner diameter of 50-100 mm; The inert gas is nitrogen with a purity of 99.9%; the gas carbon source is a mixture of any one of CH4, C2H2, C2H4 and nitrogen, the ratio of the mixture is 1:1-1:3, and the flow rate is 1L / min-3L / min; The certain temperature is 700° C.-900° C.; the period of time is 30 min-60 min.

4. A method for preparing a low-expansion and low-resistivity silicon-carbon anode material according to claim 3, characterized in that, In step (3), the concentrated nitric acid is commercially available chemically pure, the reflux oxidation time is 3h-6h, and the reflux temperature is 80°C-90°C.

5. A method for preparing a low-expansion and low-resistivity silicon-carbon anode material according to claim 4, characterized in that, In step (6), the comminution equipment is any one of mechanical comminution and jet milling, and the material of the equipment is any one of stainless steel materials; The comminuted material needs to pass through an 80-mesh sieve, and the undersize material is taken.

6. The preparation method of a low-expansion and low-resistivity silicon-carbon anode material according to claim 5, characterized in that, In step (7), the gaseous carbon source for gas-phase coating is a mixture of any one of CH4 and C2H2 and nitrogen; the ratio of the mixture is 1:1 - 1:3, and the flow rate is 1 L / min - 3 L / min.

7. A method for preparing a low-expansion and low-resistivity silicon-carbon anode material according to claim 6, characterized in that, The gas-phase coating conditions are as follows: under room temperature conditions, nitrogen protection is passed, the flow rate of the nitrogen atmosphere is 0.1 - 0.8 mL / min, the temperature is raised from room temperature to 700 - 1000 °C at 3.0 - 5.0 °C / min, the mixture gas is switched, and it is kept at a constant temperature for 1.5 - 3.0 h. After the constant temperature ends, it is naturally cooled to room temperature; the screening process uses a 300 - 350-mesh standard sieve, and the undersize material is taken. The median particle size of the undersize material: D50 = 7 - 15 μm.

8. An application of a negative electrode material obtained by the method for preparing a low-expansion and low-resistivity silicon-carbon negative electrode material according to claim 1, characterized in that: It is used for preparing lithium-ion batteries.

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