An expanded graphite-silicon composite negative electrode material, a negative electrode and a lithium ion battery

Abstract

The present application relates to a kind of expanded graphite silicon composite negative electrode material, negative electrode and lithium ion battery;The preparation method of negative electrode material includes the following steps: S1, take expanded graphite in the reaction chamber of PEALD, set the temperature of reaction chamber to 200-300 DEG C, set plasma source as radio frequency plasma, and radio frequency power is 50W-150W;First, silicon precursor is introduced, then plasma is introduced to carry out reaction;Reaction is completed, and one cycle is ended;Repeat the cycle, obtain expanded graphite / silicon composite material;S2, the surface of the expanded graphite / silicon composite material is coated with conductive agent, i.e.get.The preparation method of the present application can effectively increase the specific surface area of negative electrode material, and the conductive agent coating is more uniform, the bonding force is enhanced, can effectively improve the interface stability of lithium ion battery, reduce electrolyte side reaction, effectively improve the first coulomb efficiency and long-term cycle stability of lithium ion battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and relates to a lithium ion battery, in particular to an expanded graphite-silicon composite negative electrode material, a negative electrode and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries are currently the mainstream energy storage technology and are widely used in electric vehicles, consumer electronics and renewable energy storage fields. The negative electrode material is one of the key components that determine the performance (such as capacity, cycle life and safety) of the battery. The traditional commercial negative electrode material is mainly graphite, which has the advantages of high conductivity, stable structure, low cost, good processing performance and environmental friendliness, but its theoretical capacity is limited, it is prone to volume expansion during lithium intercalation, it is prone to reaction with electrolyte at high temperature, and it is prone to lithium precipitation during fast charging, which greatly limits the improvement of the performance of the graphite negative electrode lithium battery. Expanded graphite (EG) not only has a larger interlayer spacing and abundant pore structure, but also retains the excellent conductivity of ordinary graphite and has good flexibility. These characteristics make expanded graphite a fast-charging negative electrode material, but the first efficiency of the expanded graphite negative electrode material is low and the long-term cycle stability is insufficient. A layer of nano-silicon is coated on the surface of the expanded graphite to improve the specific capacity of the material. Currently, chemical vapor deposition, physical vapor deposition, electrochemical deposition and ball milling are commonly used for Si coating.

[0003] The high-temperature furnace used in the traditional high-temperature expansion method for preparing expanded graphite has high energy consumption and low efficiency. Among the commonly used Si coating methods, the chemical vapor deposition method has high energy consumption and may damage the graphite structure, and it does not have shape retention, so it cannot uniformly coat Si on the recessed parts of the graphite; the physical vapor deposition method has low production efficiency and weak bonding force between the silicon layer and the graphite; the electrochemical deposition method may introduce impurities, resulting in insufficient purity of the deposited Si and poor deposition uniformity; and the ball milling method has weak bonding force between silicon and graphite and uneven distribution, and may also damage the crystal structure of the graphite. SUMMARY

[0004] Therefore, the purpose of the present application is to provide an expanded graphite-silicon composite negative electrode material, a negative electrode and a lithium ion battery. The specific surface area of the negative electrode material is large, the Si and conductive agent are uniformly and stably coated, and the first coulombic efficiency and long-term cycle stability of the lithium ion battery can be effectively improved.

[0005] The first aspect of the present application is to provide a preparation method of a negative electrode material, comprising the following steps:

[0006] S1, the expandable graphite is taken into the reaction chamber of PEALD, the temperature of the reaction chamber is set to 200-300℃, the plasma source is set to radio frequency plasma, and the radio frequency power is 50-150W; the silicon precursor is first introduced, then the plasma and the silicon precursor adsorbed on the surface of the expandable graphite are reacted; the reaction is completed, one cycle is ended; the cycle is repeated to obtain the expandable graphite / silicon composite material;

[0007] S2, the surface of the expandable graphite / silicon composite material is coated with a conductive agent to obtain the negative electrode material.

[0008] In some embodiments, the temperature of the reaction chamber is set to 200-250℃, and the radio frequency power of the radio frequency plasma is set to 80-150W.

[0009] In some embodiments, the number of cycles is 20-150, preferably 80-120, and more preferably 90-110.

[0010] In some embodiments, the pulse time of the radio frequency plasma is 0.1-1s.

[0011] In some embodiments, the mass of the conductive agent accounts for 1-5% of the mass of the expandable graphite / silicon composite material, preferably 1-3%.

[0012] In some embodiments, the expandable graphite is prepared by the following method: (1) the flake graphite is subjected to an oxidation reaction with a mixed acid containing sulfuric acid and nitric acid, washed to neutral, dried to obtain expandable graphite; (2) the expandable graphite is subjected to microwave treatment and residual acid is removed to obtain the expandable graphite.

[0013] In some embodiments, the volume ratio of sulfuric acid to nitric acid in the mixed acid is (2-5):1, preferably (2-4):1.

[0014] In some embodiments, the temperature of the oxidation reaction is 20-25℃, and the time is 1-4h; preferably, the temperature of the oxidation reaction is 20-23℃, and the time is 1.5-2.5h.

[0015] In some embodiments, the power of the microwave treatment is 500-800W, and the time is 10-60s; preferably, the power of the microwave treatment is 500-600W, and the time is 20-40s.

[0016] In some embodiments, the residual acid is removed by annealing at 250-350℃ for 1-3h; preferably, the residual acid is removed by annealing at 280-320℃ for 1.5-3.5h.

[0017] In some embodiments, the silicon precursor is selected from at least one of monosilane (SiH4), disilane (Si2H6) and trichlorosilane (SiHCl3).

[0018] In some embodiments, the plasma gas source is H2.

[0019] In some embodiments, the conductive agent is selected from at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0020] In some embodiments, a gas flow coating instrument is used to coat the conductive agent on the surface of the expanded graphite / silicon composite material.

[0021] In some embodiments, the carrier gas pressure of the gas flow coating instrument is 0.1-0.3 MPa, and the atomization gas pressure is 0.2-0.7 MPa; preferably, the carrier gas pressure of the gas flow coating instrument is 0.15-0.25 MPa, and the atomization gas pressure is 0.4-0.6 MPa.

[0022] A second aspect of the present application provides a negative electrode material obtained by the method as described above.

[0023] A third aspect of the present application provides a lithium ion battery negative electrode, which comprises the negative electrode material as described above.

[0024] A fourth aspect of the present application provides a lithium ion battery, which comprises a positive electrode, a negative electrode and a non-aqueous electrolyte, and the negative electrode comprises the negative electrode material as described above.

[0025] The preparation method of the negative electrode material of the application is as follows: firstly, Si is coated on the expanded graphite by PEALD to obtain an expanded graphite / silicon composite material, and then a conductive agent is coated on the surface of the expanded graphite / silicon composite material. The inventors find that, by using the PEALD method to coat Si on the surface of the expanded graphite and jointly controlling the temperature of the reaction chamber and the radio frequency frequency of the plasma source, the purity of Si can be higher and the distribution of Si can be more uniform, and meanwhile the dispersibility and the binding force of the conductive agent coating layer can be greatly improved. On the one hand, after the Si is coated on the surface of the expanded graphite, the Si is oxidized to generate silicon oxide, the hydrophilicity of the material is improved, and the binding force between the material and the conductive agent can be enhanced; on the other hand, under the condition of suitable reaction temperature and radio frequency frequency, the physical etching effect of the radio frequency plasma on the expanded graphite can not only improve the specific surface area and the surface roughness of the expanded graphite, but also can expose more hydrophilic active sites of the expanded graphite under the condition of ensuring the stability of the crystal structure of the expanded graphite, and these active sites can react with the hydrophilic groups of the conductive agent. In this way, the dispersibility of the conductive agent coating layer can be greatly improved, the binding force between the conductive agent and the expanded graphite / silicon composite material can be enhanced, the interface stability of the lithium ion battery can be significantly improved by using the negative electrode material of the application, the side reaction of the electrolyte can be reduced, and the first coulombic efficiency and the long-term cycle stability of the lithium ion battery can be effectively improved. DETAILED DESCRIPTION

[0026] In order to facilitate the understanding of the present application, the present application will be described more fully below. The present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0027] The experimental methods in the following examples not specifically noted are generally carried out according to the conventional conditions or according to the conditions suggested by the manufacturers. The various common chemical reagents used in the examples are all commercially available products.

[0028] Unless otherwise defined, all technical and scientific terms used in the present application have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing the specific embodiments and are not used to limit the present application. The term "and / or" used in the present application includes any and all combinations of one or more related listed items.

[0029] The present application will be further described in detail below in combination with specific embodiments.

[0030] PEALD: Plasma-Enhanced Atomic Layer Deposition, plasma-enhanced atomic layer deposition.

[0031] The expanded graphite used in the following examples is prepared by the following method:

[0032] (1) The scale graphite is oxidized with mixed acid (sulfuric acid: nitric acid = 3:1) at 20°C and 300 RPM stirring rate for 2h, then washed with deionized water until neutral, remove residual acid, and then vacuum dried at 70°C to obtain expandable graphite;

[0033] (2) The expandable graphite is laid in a quartz crucible, then placed in an industrial microwave oven, treated at 500W microwave power for 30s, and then annealed at 300°C for 2h in Ar atmosphere to remove residual acid, and the obtained graphite is the expanded graphite.

[0034] Example 1

[0035] The example provides a negative electrode material, which is prepared by the following method:

[0036] (1) 1000g of expanded graphite is weighed in the reaction chamber of PEALD and vacuum sealed, then the temperature of the reaction chamber is set to 200°C and heated to the set temperature, while the radio frequency power of the plasma source is set to 80W, the pulse time is 0.5s, and then Ar is introduced into the system for 10min. After purging, the precursor silane (SiH4) is introduced to be adsorbed on the surface of the expanded graphite, and after complete adsorption (whether the precursor is contained in the tail gas can be monitored by online mass spectrometry or gas chromatography, etc. to judge when the reaction with the expanded graphite is complete) the precursor silane is stopped and the excess precursor silane and by-products are removed by Ar purging system, after purging, H2 plasma is introduced to react with the precursor silane adsorbed on the surface of the expanded graphite, and after the reaction is completed, the excess H2 plasma and by-products are removed by Ar purging system, and one cycle is completed. After 80 cycles, the expanded graphite / silicon composite material is obtained.

[0037] (2) The expanded graphite / silicon composite material is coated with 2% single-walled carbon nanotubes on the surface by using a gas flow coating instrument (carrier gas pressure is 0.2MPa, atomizing gas pressure is 0.5MPa) to obtain the final product.

[0038] Example 2

[0039] The example provides a negative electrode material, which is prepared by the following method:

[0040] (1) 1000 g of expanded graphite is weighed into the reaction chamber of the PEALD and vacuum sealed, then the temperature of the reaction chamber is set to 200°C and heated to the set temperature, while the radio frequency power of the plasma source is set to 80 W, the pulse time is 0.5 s, then Ar is introduced into the system for 10 min. After purging, the precursor silane (SiH4) is introduced to be adsorbed on the surface of the expanded graphite, after complete adsorption, the precursor silane is stopped and the excess precursor silane and by-products are removed by Ar purging, after purging, H2 plasma is introduced to react with the precursor silane adsorbed on the surface of the expanded graphite, after the reaction is completed, the excess H2 plasma and by-products are removed by Ar purging, to complete one cycle. After 100 cycles, the expanded graphite / silicon composite material is obtained.

[0041] (2) The expanded graphite / silicon composite material is coated with 2% single-walled carbon nanotubes on the surface by using a gas flow coating instrument (carrier gas pressure is 0.2 MPa, atomizing gas pressure is 0.5 MPa) to obtain the final product.

[0042] Example 3

[0043] The negative electrode material is prepared by the following method:

[0044] (1) 1000 g of expanded graphite is weighed into the reaction chamber of the PEALD and vacuum sealed, then the temperature of the reaction chamber is set to 200°C and heated to the set temperature, while the radio frequency power of the plasma source is set to 80 W, the pulse time is 0.5 s, then Ar is introduced into the system for 10 min. After purging, the precursor silane (SiH4) is introduced to be adsorbed on the surface of the expanded graphite, after complete adsorption, the precursor silane is stopped and the excess precursor silane and by-products are removed by Ar purging, after purging, H2 plasma is introduced to react with the precursor silane adsorbed on the surface of the expanded graphite, after the reaction is completed, the excess H2 plasma and by-products are removed by Ar purging, to complete one cycle. After 100 cycles, the expanded graphite / silicon composite material is obtained.

[0045] (2) The expanded graphite / silicon composite material is coated with 2% single-walled carbon nanotubes on the surface by using a gas flow coating instrument (carrier gas pressure is 0.2 MPa, atomizing gas pressure is 0.5 MPa) to obtain the final product.

[0046] Example 4

[0047] The negative electrode material is prepared by the following method:

[0048] (1) Take 1000g of expanded graphite into the reaction chamber of PEALD and vacuum seal, then set the temperature of the reaction chamber to 200°C and heat to the set temperature, while setting the radio frequency power of the plasma source to 80W, the pulse time to 0.5s, then introduce Ar to purge the system for 10min. After purging, introduce the precursor disilane (Si2H6) to adsorb on the surface of the expanded graphite, stop introducing the precursor disilane after complete adsorption and remove the excess precursor disilane and byproducts by purging with Ar, after purging, introduce H2 plasma to react with the precursor disilane adsorbed on the surface of the expanded graphite, after the reaction is complete, introduce Ar to remove the excess H2 plasma and byproducts, complete one cycle. Repeat the cycle for 100 times to obtain the expanded graphite / silicon composite material.

[0049] (2) Use the airflow coating instrument (carrier gas pressure is 0.2MPa, atomizing gas pressure is 0.5MPa) to coat 2% of single-walled carbon nanotubes on the surface of the expanded graphite / silicon composite material to obtain the final product.

[0050] Example 5

[0051] This example provides a negative electrode material, which is prepared by the following method:

[0052] (1) Take 1000g of expanded graphite into the reaction chamber of PEALD and vacuum seal, then set the temperature of the reaction chamber to 250°C and heat to the set temperature, while setting the radio frequency power of the plasma source to 150W, the pulse time to 0.5s, then introduce Ar to purge the system for 10min. After purging, introduce the precursor monosilane to adsorb on the surface of the expanded graphite, stop introducing the precursor monosilane after complete adsorption and remove the excess precursor monosilane and byproducts by purging with Ar, after purging, introduce H2 plasma to react with the precursor monosilane adsorbed on the surface of the expanded graphite, after the reaction is complete, introduce Ar to remove the excess H2 plasma and byproducts, complete one cycle. Repeat the cycle for 100 times to obtain the expanded graphite / silicon composite material.

[0053] (2) Use the airflow coating instrument (carrier gas pressure is 0.2MPa, atomizing gas pressure is 0.5MPa) to coat 2% of single-walled carbon nanotubes on the surface of the expanded graphite / silicon composite material to obtain the final product.

[0054] Example 6

[0055] This example provides a negative electrode material, which is prepared by the following method:

[0056] (1) 1000 g of expanded graphite was weighed into the reaction chamber of the PEALD and vacuum sealed, and then the temperature of the reaction chamber was set to 300°C and heated to the set temperature, while the radio frequency power of the plasma source was set to 50 W, the pulse time was 0.5 s, and then Ar was introduced into the system for 10 min. After purging, the precursor silane was introduced to be adsorbed on the surface of the expanded graphite, and after complete adsorption, the precursor silane was stopped and the excess precursor silane and by-products were removed by Ar purging. After purging, H2 plasma was introduced to react with the precursor silane adsorbed on the surface of the expanded graphite, and after the reaction was completed, the excess H2 plasma and by-products were removed by Ar purging, and one cycle was completed. After 100 cycles, an expanded graphite / silicon composite material was obtained.

[0057] (2) The surface of the expanded graphite / silicon composite material was coated with 2% single-walled carbon nanotubes using a gas flow coating instrument (carrier gas pressure 0.2 MPa, atomizing gas pressure 0.5 MPa) to obtain the final product.

[0058] Comparative Example 1

[0059] This comparative example provides a negative electrode material, which is prepared by the following method:

[0060] (1) The surface of the expanded graphite was coated with 2% single-walled carbon nanotubes using a gas flow coating instrument (carrier gas pressure 0.2 MPa, atomizing gas pressure 0.5 MPa) to obtain the final product.

[0061] Comparative Example 2

[0062] This comparative example provides a negative electrode material, which is prepared by the following method:

[0063] (1) 1000 g of expanded graphite was weighed into the reaction chamber of the PEALD and vacuum sealed, and then the temperature of the reaction chamber was set to 300°C and heated to the set temperature, while the radio frequency power of the plasma source was set to 50 W, the pulse time was 0.5 s, and then Ar was introduced into the system for 10 min. After purging, the precursor silane was introduced to be adsorbed on the surface of the expanded graphite, and after complete adsorption, the precursor silane was stopped and the excess precursor silane and by-products were removed by Ar purging. After purging, H2 plasma was introduced to react with the precursor silane adsorbed on the surface of the expanded graphite, and after the reaction was completed, the excess H2 plasma and by-products were removed by Ar purging, and one cycle was completed. After 100 cycles, an expanded graphite / silicon composite material was obtained.

[0064] Comparative Example 3

[0065] (1) The expanded graphite was soaked in 5% nitric acid solution for 1 hour, then treated with 10% HF solution for 10 minutes, and then vacuum dried at 80°C for 12 hours. The pretreatment of the expanded graphite was completed. Then the pretreated expanded graphite was evenly spread in a quartz boat (thickness 1.5 mm), and then the quartz boat was placed in the center of the CVD furnace constant temperature zone. Finally, the reaction tube was sealed, vacuumed to 10 -3 Pa, and then 30 min of Ar / H2 mixed gas (Ar: 200 sccm, H2: 50 sccm) was introduced. Then the temperature was increased to 600°C at a rate of 5°C / min. Finally, 2.5 vol% silane (SiH4: Ar = 20 sccm: 400 sccm) was introduced, and 50 sccm of H2 was introduced at the same time, and the expanded graphite / silicon composite material was obtained after 10 min of deposition.

[0066] (2) The expanded graphite / silicon composite material was coated with 2% single-walled carbon nanotubes using a gas flow coating instrument (carrier gas pressure 0.2 MPa, atomizing gas pressure 0.5 MPa) to obtain the final product.

[0067] Comparative Example 4

[0068] This comparative example provides a negative electrode material, which is prepared by the following method:

[0069] (1) 1000 g of expanded graphite was weighed into the reaction chamber of the PEALD and vacuum sealed. Then the temperature of the reaction chamber was set to 200°C and heated to the set temperature, and the radio frequency power of the plasma source was set to 200 W with a pulse time of 0.5 s. Then Ar was introduced to purge the system for 10 min. After purging, the precursor silane was introduced to adsorb on the surface of the expanded graphite. After complete adsorption, the precursor silane was stopped and the excess precursor silane and by-products were removed by Ar purging. After purging, H2 plasma was introduced to react with the precursor silane adsorbed on the surface of the expanded graphite. After the reaction was completed, excess H2 plasma and by-products were removed by Ar purging to complete one cycle. After 100 cycles, the expanded graphite / silicon composite material was obtained.

[0070] (2) The expanded graphite / silicon composite material was coated with 2% single-walled carbon nanotubes using a gas flow coating instrument (carrier gas pressure 0.2 MPa, atomizing gas pressure 0.5 MPa) to obtain the final product.

[0071] Comparative Example 5

[0072] This comparative example provides a negative electrode material, which is prepared by the following method:

[0073] (1) Take 1000g of expanded graphite into the reaction chamber of PEALD and vacuum seal, then set the temperature of the reaction chamber to 400°C and heat to the set temperature, while setting the radio frequency power of the plasma source to 80W, the pulse time to 0.5s, then introduce Ar into the system for 10min. After purging, introduce the precursor silane to be adsorbed on the surface of the expanded graphite, stop introducing the precursor silane after complete adsorption and remove the excess precursor silane and byproducts by Ar purging, after purging, introduce H2 plasma to react with the precursor silane adsorbed on the surface of the expanded graphite, after the reaction is complete, introduce Ar into the system to remove excess H2 plasma and byproducts. After 100 cycles, the expanded graphite / silicon composite material is obtained.

[0074] (2) Use the gas flow coating instrument (carrier gas pressure is 0.2MPa, atomizing gas pressure is 0.5MPa) to coat 2% single-walled carbon nanotubes on the surface of the expanded graphite / silicon composite material to obtain the final product.

[0075] Comparative Example 6

[0076] This comparative example provides a negative electrode material, which is prepared by the following method:

[0077] (1) Take 1000g of expanded graphite into the reaction chamber of PEALD and vacuum seal, then set the temperature of the reaction chamber to 150°C and heat to the set temperature, while setting the radio frequency power of the plasma source to 80W, the pulse time to 0.5s, then introduce Ar into the system for 10min. After purging, introduce the precursor silane to be adsorbed on the surface of the expanded graphite, stop introducing the precursor silane after complete adsorption and remove the excess precursor silane and byproducts by Ar purging, after purging, introduce H2 plasma to react with the precursor silane adsorbed on the surface of the expanded graphite, after the reaction is complete, introduce Ar into the system to remove excess H2 plasma and byproducts, complete one cycle. Repeat the cycle 100 times to obtain the expanded graphite / silicon composite material.

[0078] (2) Use the gas flow coating instrument (carrier gas pressure is 0.2MPa, atomizing gas pressure is 0.5MPa) to coat 2% single-walled carbon nanotubes on the surface of the expanded graphite / silicon composite material to obtain the final product.

[0079] Comparative Example 7

[0080] This comparative example provides a negative electrode material, which is prepared by the following method:

[0081] (1) Take 1000g of expanded graphite into the reaction chamber of PEALD and vacuum seal, then set the temperature of the reaction chamber to 200°C and heat to the set temperature, while setting the radio frequency power of the plasma source to 30W, the pulse time to 0.5s, then introduce Ar to purge the system for 10min. After purging, introduce the precursor methylsilane to adsorb on the surface of the expanded graphite, stop introducing the precursor methylsilane after complete adsorption and remove the excess precursor methylsilane and by-products by purging with Ar, after purging, introduce H2 plasma to react with the precursor methylsilane adsorbed on the surface of the expanded graphite, after the reaction is completed, introduce Ar to remove the excess H2 plasma and by-products, complete one cycle. Repeat the cycle for 100 times to obtain the expanded graphite / silicon composite material.

[0082] (2) Use the airflow coating instrument (carrier gas pressure is 0.2MPa, atomizing gas pressure is 0.5MPa) to coat 2% single-walled carbon nanotubes on the surface of the expanded graphite / silicon composite material to obtain the final product.

[0083] The preparation methods of the above examples and comparative examples are shown in Table 1 below.

[0084] Table 1

[0085]

[0086] Performance data

[0087] The specific surface area of the final product of the expanded graphite / silicon / conductive agent negative electrode material prepared by the above examples and comparative examples is tested.

[0088] Meanwhile, lithium iron phosphate is used as the positive electrode active material, and the final product of the expanded graphite / silicon composite material prepared by the above examples and comparative examples is used as the negative electrode material to prepare button lithium ion batteries, and the preparation method is as follows:

[0089] 1. Preparation of positive electrode slurry

[0090] Formulation: LiFePO4: conductive carbon black: PVDF = 80:10:10 (mass ratio), solvent is NMP.

[0091] Preparation steps:

[0092] a) Slowly add PVDF to NMP, 60°C magnetic stirring until completely dissolved;

[0093] b) Add conductive carbon black and ultrasonic dispersion for 10min;

[0094] c) Add LiFePO4 in portions, homogenizer stirring for 2 hours (to avoid bubbles);

[0095] d) The viscosity of the slurry is controlled at 3000-5000 mPa-s.

[0096] 2. Preparation of negative electrode slurry

[0097] Formulation: negative electrode material: conductive carbon black: PVDF = 90:5:5 (mass ratio), and the solvent is NMP.

[0098] Preparation steps:

[0099] a) PVDF is slowly added to NMP, and magnetic stirring is performed at 60°C until complete dissolution;

[0100] b) Add conductive carbon black and ultrasonically disperse for 10 minutes;

[0101] c) Add graphite in portions and stir with a homogenizer for 2 hours (to avoid air bubbles);

[0102] d) The viscosity of the slurry is controlled at 3000-5000 mPa-s.

[0103] 3. Preparation of non-aqueous electrolyte

[0104] a) Dehydrate the EC, EMC solvent with 3A molecular sieves to make the water content of the solvent ≤10 ppm (soak for ≥24 h);

[0105] b) Configure the lithium salt solution in the glove box: dissolve 4.89 g of LiPF6 in 100 ml of a mixed solution of EC and EMC (EC: EMC = 3:7);

[0106] c) Respectively weigh 0.1 g of additive VC and 0.1 g of FEC into the above lithium salt solution, and mix at low speed for 30 min to make the additives completely dissolved;

[0107] d) Filter out particulate matter with a 0.22 μm PTFE filter membrane, then transfer to a brown glass bottle, seal with argon, and store in a freezer at -20°C in the dark.

[0108] 4. Electrode coating and drying

[0109] a) Coating: use a coating machine to uniformly coat the negative electrode slurry and the positive electrode slurry on copper foil (negative electrode) and aluminum foil (positive electrode) respectively, with a wet film thickness of about 100-150 μm;

[0110] b) Drying: pre-dry at 80°C for 30 minutes, then transfer to vacuum drying at 120°C for 12 hours.

[0111] 5. Electrode sheet punching

[0112] Use a sheet punching machine to punch the dried electrodes in step 4 into round sheets with a diameter of 12 mm to obtain the corresponding negative electrode sheets and positive electrode sheets.

[0113] 6. Assemble the button cell in the glove box

[0114] Assemble the battery in the glove box with high purity argon atmosphere in the order of negative electrode shell, negative electrode sheet, non-aqueous electrolyte, polypropylene diaphragm, non-aqueous electrolyte, positive electrode sheet, gasket, spring, positive electrode shell, put into a sealed bag, use a manual button cell sealing machine to package the battery, get a button cell for subsequent test.

[0115] Electrochemical test is carried out on the button lithium ion battery prepared, and the test method is as follows:

[0116] 0.5C discharge capacity: charge to the upper limit voltage 3.65V at 0.5C rate, then constant voltage charge to current ≤0.05C, stop charging, then stand for 5-10 minutes, then discharge to the cut-off voltage 2.5V at 0.5C rate.

[0117] First coulombic efficiency: first charge-discharge test is carried out at 0.1C current density in the range of 2.50V-3.65V.

[0118] 1C cycle 100 times capacity retention rate: 100 cycle charge-discharge test is carried out at 1C current density in the range of 2.50V-3.65V.

[0119] The test results are as follows in Table 2:

[0120] Table 2 Test results table

[0121]

[0122] From the above Table 2, it can be seen that after the Si coating of the expanded graphite using the PEALD method, the specific surface area of the negative electrode material prepared is larger. This is because the PEALD method has higher purity and more uniform distribution of Si; at the same time, the plasma is introduced during the Si coating, and the plasma not only reacts with the precursor adsorbed on the surface of the expanded graphite, but also has etching effect on the expanded graphite, so as to thin the expanded graphite and improve its surface roughness. In this way, more hydrophilic active sites of the expanded graphite can be exposed, which is more conducive to the subsequent coating of the conductive agent and better improves the battery related performance.

[0123] The buckle type lithium ion batteries prepared by the present application all have high 0.5C discharge gram capacity, first coulomb efficiency and 1C cycle 100 cycle capacity retention rate (Examples 1-6). This is because the Si purity is higher and the distribution is more uniform when Si is coated on the expanded graphite surface using the PEALD method under suitable reaction temperature and radio frequency conditions, which is more conducive to improving the specific capacity of the battery. Moreover, when Si is coated on the expanded graphite surface under the PEALD conditions, the physical etching effect of the radio frequency plasma on the expanded graphite not only improves the specific surface area and surface roughness of the expanded graphite, but also exposes more hydrophilic active sites of the expanded graphite under the condition of ensuring the stability of the crystal structure of the expanded graphite. These active sites can react with the hydrophilic groups of the conductive agent, thereby greatly enhancing the uniformity and binding force of the conductive agent and the expanded graphite / silicon composite material, avoiding the peeling of the coating layer. The uniform coating and enhanced binding force of the conductive agent make the battery current density uniform, the impedance reduced, the interface stability of the battery improved, the electrolyte side reaction reduced, the first coulomb efficiency and long-term cycle stability of the battery improved.

[0124] Compared with Example 1, the cycle number of PEALD in Example 2 is increased from 80 to 100, and the etching effect of the plasma on the expanded graphite is greater than that in Example 1, so the uniformity and binding force of the conductive agent in Example 2 are better than those in Example 1, and finally the electrochemical performance is slightly better than that in Example 1.

[0125] Compared with Example 2, the cycle number of PEALD in Example 3 is changed from 100 to 120, and the 0.5C discharge gram capacity, first coulomb efficiency and capacity retention rate of Example 3 are all slightly less than those of Example 2. This may be because the increase in cycle number makes the Si content too high, resulting in a large battery impedance that is not conducive to the transmission of lithium ions.

[0126] Compared with Example 2, the precursor used in PEALD in Example 4 is ethylsilane, and the performance of the final product anode material prepared is basically the same, indicating that the precursor has no obvious effect on the final product.

[0127] Compared with Example 2, Comparative Example 1 does not coat Si on the expanded graphite before directly coating the conductive agent, and the specific surface area of the anode material prepared is reduced. Moreover, in terms of electrical performance, not only the 0.5C discharge gram capacity is significantly reduced, but also the first coulomb efficiency and capacity retention rate are significantly deteriorated. This is because the dispersibility of the conductive agent in the expanded graphite in the anode material of Comparative Example 1 is very poor, which makes the local accumulation of the conductive agent, the battery impedance becomes large, the current density is uneven, the binding force of the conductive agent coating layer is significantly reduced, the interface stability is reduced, and thus the battery electrical performance is very poor.

[0128] Compared with Example 2, the PEALD method in the preparation method of Comparative Example 2 does not coat the expanded graphite with a conductive agent after coating the Si on the expanded graphite, and the oxidation of the coated Si on the surface of the expanded graphite to form silicon oxide significantly improves the hydrophilicity of the material, but since no further conductive agent layer is coated, this leads to the weakening of the wetting effect of the non-aqueous electrolyte on the negative electrode material, resulting in an increase in the battery interface contact resistance, uneven current distribution, and thus exacerbation of the local aging of the interface, which is not conducive to the formation of a uniform SEI film. The electrochemical performance of the battery prepared using the negative electrode material of Comparative Example 2 is significantly deteriorated.

[0129] Compared with Example 2, Comparative Example 3 uses the CVD method to coat the expanded graphite with Si. Since the CVD method does not etch the expanded graphite matrix when coating, it is not conducive to increasing the specific surface area of the expanded graphite and exposing more hydrophilic active sites, which is not conducive to the dispersion of the conductive agent in the graphite, and the bonding force between the conductive agent and the expanded graphite / silicon material surface is also significantly reduced, which is not conducive to the formation of a stable SEI film. Therefore, the electrical performance of Comparative Example 3 is worse than that of Example 2.

[0130] Compared with Example 2, in Comparative Example 4, the radio frequency power of the plasma is too high when the expanded graphite is coated with Si using the PEALD method; in Comparative Example 5, the temperature in the reaction chamber is too high when the expanded graphite is coated with Si using the PEALD method. In both cases, the crystal structure of the expanded graphite is destroyed, which not only reduces the conductivity due to the poor transmission of lithium ions, but also significantly reduces the bonding force between the conductive agent and the expanded graphite / silicon material, thereby reducing the stability of the SEI film and leading to the deterioration of the electrical performance of the battery.

[0131] Compared with Example 2, in Comparative Example 6, the temperature is reduced from 200°C to 150°C when the expanded graphite is coated with Si using the PEALD method. The reduction in temperature leads to a decrease in the deposition rate of Si, which is not conducive to improving the coating efficiency; and since the deposition time is prolonged, the material is etched for a longer time, which may damage the crystal structure of the material and reduce its conductivity; in addition, the reduction in temperature reduces the reactivity and reduces the exposure of the hydrophilic sites on the surface of the expanded graphite, which affects the uniform coating and bonding force of the conductive agent, ultimately leading to the deterioration of the electrical performance of the battery.

[0132] Compared with Example 2, in Comparative Example 7, the radio frequency power is reduced from 80W to 30W when the expanded graphite is coated with Si using the PEALD method. The reduction in radio frequency power reduces the concentration of the plasma, which not only hinders the improvement of the coating efficiency, but also reduces the etching effect on the expanded graphite, which is not conducive to increasing the roughness of the graphite surface and exposing the active sites, making it difficult for the conductive agent to be uniformly coated, and the bonding force between the conductive agent coating layer and the expanded graphite / silicon composite material is weakened, leading to the deterioration of the electrical performance of the battery.

[0133] The above embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are within the scope of protection of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for producing a negative electrode material, characterized by, The method comprises the following steps: S1, taking the expanded graphite into a reaction chamber of PEALD, setting the temperature of the reaction chamber to 200-300℃; setting the plasma gas source as radio frequency plasma, and the radio frequency power to 50-150W, the pulse time to 0.1-1s; first introducing the silicon precursor, then introducing the plasma to react with the silicon precursor adsorbed on the surface of the expanded graphite; after the reaction is completed, one cycle is ended; repeating the cycle for 20-150 times to obtain the expanded graphite / silicon composite material; S2, coating the surface of the expanded graphite / silicon composite material with a conductive agent to obtain the negative electrode material; The plasma gas source is H2. The conductive agent is selected from at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

2. The production method according to claim 1, wherein The mass of the conductive agent accounts for 1-5% of the mass of the expanded graphite / silicon composite material.

3. The production method according to claim 1, wherein The expanded graphite is prepared by the following method: (1) oxidizing flake graphite with mixed acid containing sulfuric acid and nitric acid, washing to neutral, drying to obtain expandable graphite; (2) microwave treatment of the expandable graphite and removal of residual acid to obtain the expanded graphite.

4. The production method according to claim 3, wherein The volume ratio of sulfuric acid to nitric acid in the mixed acid is (2-5):1; and / or, The temperature of the oxidation reaction is 20-25℃, and the time is 1-4h; and / or, The power of the microwave treatment is 500-800W, and the time is 10-60s; and / or, Annealing at 250-350℃ for 1-3h to remove residual acid.

5. The production method according to claim 1, wherein The silicon precursor is selected from at least one of monosilane, disilane and trichlorosilane.

6. The negative electrode material prepared by the method of any one of claims 1-5.

7. A lithium-ion battery anode, characterized in that, The negative electrode comprises the negative electrode material of claim 6.

8. A lithium ion battery comprising a positive electrode, a negative electrode and a nonaqueous electrolyte, characterized in that, The negative electrode comprises the negative electrode material of claim 6.

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