Silicon-carbon negative electrode material and preparation method and application thereof

Abstract

The invention provides a silicon-carbon negative electrode material and a preparation method and application thereof, and belongs to the technical field of lithium ion battery negative electrode materials. The preparation method comprises the following steps: adding silicon tetrachloride and calcium carbide into a carbon tetrachloride solution, uniformly stirring, and transferring into a high-pressure reaction kettle for reaction to obtain a Si-C precursor; adding the obtained Si-coated C precursor into absolute ethyl alcohol and carrying out ball milling treatment; and carrying out suction filtration and drying on the material obtained by ball milling, calcining and carbonizing in inert gas, and cleaning the obtained product to obtain the silicon-carbon negative electrode material. The silicon-carbon negative electrode material prepared by the invention is used for preparing a negative electrode material of a lithium ion battery, has good cycle performance, has relatively high reversible specific capacity and first coulombic efficiency, and is excellent in overall electrochemical performance.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium ion battery negative electrode materials, in particular to a silicon-carbon negative electrode material prepared in situ by a one-step method and application thereof. BACKGROUND

[0002] The silicon-carbon negative electrode material is a research hotspot in the field of lithium ion batteries. The theoretical specific capacity of a traditional graphite negative electrode is only 372 mAh / g, which is difficult to meet the increasing energy storage demand. Silicon is considered as a potential next-generation high-capacity negative electrode material due to its theoretical specific capacity of 4200 mAh / g (about 10 times that of graphite) and relatively high working voltage. However, the significant volume expansion of silicon during charging and discharging will cause electrode structure damage, electrical contact failure, and seriously damage the cycle performance, capacity retention rate and safety of the battery.

[0003] Current research strategies are mostly focused on growing silicon on the surface of porous carbon materials to construct a silicon-carbon negative electrode. Although increasing the proportion of porous carbon helps to buffer the volume expansion of silicon and prolong the cycle life, the overall current density is reduced, and the method is more complicated, requires higher equipment, and the production cost is also increased, which is not conducive to large-scale production. To break through these limitations, the project is committed to developing a one-step in-situ generated silicon-carbon composite material, which directly coats the carbon chain on the surface of silicon through the nucleophilic reaction of silicon source and carbon source. The silicon-carbon negative electrode material prepared by this new and efficient method has the characteristics of high capacity and high stability, and can well improve the electrochemical properties. SUMMARY

[0004] To solve the above problems, the applicant first provides a preparation method for realizing one-step in-situ generation of “silicon-carbon”.

[0005] Specifically, the technical scheme adopted by the present application is as follows:

[0006] A preparation method of a silicon-carbon negative electrode material, the steps are as follows:

[0007] S1, SiCl4 and CaC2 are added to a CCl4 solution, stirred uniformly, and then transferred into a high-pressure reaction kettle for high-temperature reaction to obtain a Si@C precursor;

[0008] S2, the Si@C precursor is subjected to ball milling treatment;

[0009] S3, the material obtained by ball milling treatment is suction filtered and dried, and then high-temperature calcination is carried out in an inert gas (such as Ar);

[0010] S4, the calcination product is washed to obtain a silicon-carbon negative electrode material.

[0011] The preparation method of the silicon-carbon negative electrode material provided by the application is characterized in that SiCl4 is used as a silicon source, SiCl4 is dispersed in a CCl4 solvent, the CCl4 solvent is a good solvent for SiCl4 and can well disperse SiCl4. SiCl4 has a low boiling point (57.6 DEG C) and is extremely volatile at room temperature, and direct feeding has a large loss, a high vapor pressure, and a risk of leakage and corrosion. CCl4 has a boiling point of 76.7 DEG C, is completely miscible with SiCl4, forms a low-vapor-pressure mixed liquid, significantly reduces the system pressure, is convenient for closed conveying and metering, and is suitable for continuous processes. CaC2 is a solid powder, and SiCl4 is a highly volatile liquid, and direct solid-liquid contact has poor mass transfer and is prone to local overheating. CCl4, as an inert solvent, wets the surface of CaC2 and forms a liquid film, so that SiCl4 is more uniformly diffused to the surface of CaC2 particles, improves the uniformity of the reaction, and reduces sintering and agglomeration. CCl4 can be pyrolyzed to generate carbon at high temperatures, and the generated carbon can be deposited on the surface of newly generated nanosilicon to form an additional carbon coating layer. The reaction of SiCl4 and CaC2 is highly exothermic, and the high specific heat capacity and latent heat of vaporization of CCl4 can absorb part of the heat, reducing the risk of rapid temperature rise. At the same time, CCl4 vapor can dilute the concentration of SiCl4, inhibit the instantaneous and violent reaction of SiCl4 and CaC2, and avoid local sintering. Cl2 generated by the pyrolysis of CCl4 can react with the byproduct Ca of CaC2 to generate CaCl2, and no additional impurities are introduced, and the system is clean. CaC2 and the mixture are placed in a closed high-temperature and high-pressure environment, CaC2 is decomposed into C2 2- and Ca 2+ , C2 2- has a reducing property, SiCl4 is reduced to silicon and Cl - , Cl - can react with the decomposed Ca 2+ to generate CaCl2, and C2 2- is oxidized to C to become a carbon source.

[0012] Further, as preferred:

[0013] In S1,

[0014] The molar ratio of SiCl4 to CaC2 is 1:1 to 4.

[0015] The high-pressure reaction kettle adopts a polytetrafluoroethylene lining.

[0016] The reaction temperature is 200 to 240 DEG C, and the reaction time is 6 to 24 hours.

[0017] In S2,

[0018] Anhydrous ethanol is added as a ball milling aid during the ball milling process.

[0019] The ball milling speed is 300 to 1000 r / min, and the ball milling time is 1 to 6 hours.

[0020] In S3,

[0021] The calcination treatment temperature is 500-850 DEG C, and the calcination treatment time is 2-8h.

[0022] In S4,

[0023] The cleaning includes ethanol cleaning, hydrochloric acid cleaning and deionized water cleaning in sequence; more preferably, the hydrochloric acid is 1M hydrochloric acid.

[0024] The second aspect of the application is to provide a silicon-carbon negative electrode material prepared by the above method, which is a core-shell structure of carbon-coated silicon and has a loose and dispersed layered structure in the cross section, which is beneficial to the penetration and shuttle of electrolyte.

[0025] The third aspect of the application is to provide the application of the silicon-carbon negative electrode material prepared by the above method in the preparation of lithium ion battery negative electrode material, which endows the lithium ion battery with excellent electrochemical performance, good cycle performance, and high reversible specific capacity and first coulomb efficiency.

[0026] The beneficial effects of the application are:

[0027] (1) In the above preparation scheme proposed in the application, a "silicon-carbon-calcium chloride" reaction system is generated in situ in one step, the nano-silicon generated at high temperature is coated by the carbon layer synchronously precipitated, forming a natural "core-shell" structure, which saves the additional carbon source and coating process, simplifies the process and reduces energy consumption.

[0028] (2) The by-product calcium chloride can be recycled as fused salt electrolyte or industrial raw material, reducing the comprehensive cost.

[0029] (3) The material has exothermic reaction, wide temperature window, and is easy to scale up to continuous equipment such as rotary kiln and fluidized bed, solving the problems of low yield of traditional silane CVD and high cost of silane storage and transportation.

[0030] (4) The raw material SiCl4 comes from the by-product of polysilicon industry, and CaC2 is a large-tonnage product in the calcium carbide industry, with mature supply chain and ton cost controlled in the range of 800-10000 yuan, which is reduced by more than 30% compared with the silane CVD route. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 It is a preparation flowchart of the application;

[0032] Figure 2 It is the X-ray photoelectron spectroscopy data analysis of the product prepared in Example 1 of the application;

[0033] Figure 3Scanning electron microscope image (SEM) of the product prepared for Example 1 of the present application;

[0034] Figure 4 Scanning electron microscope image (SEM) of the product prepared for Comparative Example 1 of the present application;

[0035] Figure 5 CV cycle curve of the product prepared for Example 1 of the present application at a scan rate of 1mV / s for the first 3 cycles;

[0036] Figure 6 Specific capacity-voltage first cycle graph of the product prepared for Example 1 of the present application at a current density of 200mA / g;

[0037] Figure 7 Cycle curve graph of the products prepared for Example 1, Example 2, Comparative Example 1, Comparative Example 2 of the present application at a current density of 200mA / g for 100 cycles;

[0038] Figure 8 Rate performance test graph of the products prepared for Example 1, Example 2, Comparative Example 1, Comparative Example 2 of the present application at different current densities. DETAILED DESCRIPTION

[0039] The technical solutions of the present application will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0040] In order to facilitate the description of the preparation method of the present application and the silicon-carbon negative electrode material prepared by the present application, the present application provides the following specific embodiments. It should be noted that the present application is not limited to the following embodiments. The following embodiments do not exhaust the technical solutions of the present application.

[0041] Example 1

[0042] The preparation process of the silicon-carbon negative electrode material of the present embodiment is shown as follows: Figure 1

[0043] Step one, weigh 10ml of silicon tetrachloride (SiCl4) liquid and place it in a polytetrafluoroethylene bottle. Add 40ml of carbon tetrachloride (CCl4) solvent to the polytetrafluoroethylene bottle, and weigh 10.26g of calcium carbide (CaC2) powder and place it in the bottle.

[0044] The molar ratio of raw materials silicon tetrachloride and calcium carbide is 1:2.

[0045] ​Step 2: Place the polytetrafluoroethylene bottle into a high-pressure reactor and react at 220°C for 12 hours to obtain the Si@C precursor.

[0046] Step 3: After cooling down, filter and dry the liquid and solid in the PTFE bottle, add 1M hydrochloric acid to wash, and then wash repeatedly with ethanol and deionized water.

[0047] Step 4: Dry the product and place it in a ball mill. Add an appropriate amount of ethanol and ball mill it using a wet method at 300 r / min for 6 hours.

[0048] Step 5: Wash the product with ethanol and deionized water, filter and dry it, and calcine it in an inert gas Ar.

[0049] Step six: Wash the product obtained in step five with anhydrous ethanol, 1M HCl and deionized water to obtain silicon-carbon anode material.

[0050] Elemental analysis of the obtained product yielded the following results: Figure 2 As shown: When the molar ratio of the reaction raw materials is 1:2, the contents of C, Cl, O and Si elements in the product of Example 1 are 63.2%, 0.8%, 11.8% and 24.2%, respectively.

[0051] Figure 3 The scanning electron microscope (SEM) image of the product prepared in this embodiment shows that the product exhibits a loose and dispersed layered structure, which is highly conducive to the penetration and shuttle of the electrolyte.

[0052] Example 2

[0053] This embodiment has the same setup as Embodiment 1, except that the molar ratio of the raw materials is changed from 1:2 to 1:1.

[0054] The C, Cl, O, and Si content of the product corresponding to Example 2 is 61.5%, 0.7%, 11.3%, and 26.5%, respectively.

[0055] Figure 4 The image shown is a scanning electron microscope (SEM) image of the product prepared in this embodiment, which is consistent with that in Example 1: the product exhibits a loose and dispersed layered structure.

[0056] Example 3

[0057] This embodiment has the same setup as Embodiment 1, except that the molar ratio of the raw materials is changed from 1:2 to 1:4.

[0058] The C, Cl, O, and Si content of the product corresponding to Example 3 was 65.5%, 0.7%, 11.3%, and 22.5%, respectively.

[0059] Example 4

[0060] The present example is identical to the setup of Example 1, except that the reaction temperature is replaced from 220°C to 200°C.

[0061] The C, CI, O, Si element contents of the product of Example 4 are 67.1%, 0.9%, 11.9%, and 20.1%, respectively.

[0062] Example 5

[0063] The present example is identical to the setup of Example 1, except that the reaction temperature is replaced from 220°C to 240°C.

[0064] The C, CI, O, Si element contents of the product of Example 5 are 62.4%, 0.6%, 11.4%, and 25.6%, respectively.

[0065] Comparative Example 1

[0066] The present comparative example is identical to the setup of Example 1, except that the calcium carbide powder is replaced by graphite, and the molar ratio of silicon-carbon raw materials is still 1:2.

[0067] Comparative Example 2

[0068] The present comparative example is identical to the setup of Example 1, except that the calcium carbide powder is replaced by activated carbon, and the molar ratio of silicon-carbon raw materials is still 1:2.

[0069] The products obtained in the above examples and comparative examples are applied as negative electrode materials in button-type supercapacitors, and the assembly process is as follows:

[0070] 1) Preparation of working electrode: the Si@C silicon-carbon negative electrode material obtained in the present example is mixed with conductive agent acetylene black and binder polyvinylidene fluoride (PVDF) in a ratio of 8:1:1 in a mortar, and an appropriate amount of N-methyl pyrrolidone (NMP) is added as a solvent. After grinding for about 40 min, a uniform and shiny slurry is formed. The slurry is coated on a copper foil current collector using a four-sided coater, and dried in a 60°C oven. The obtained electrode sheet is cut into small round sheets of 15 mm in diameter using a sheet press.

[0071] 2) Assembly of button-type lithium battery: the small round sheets obtained in step 1) are used as the negative electrode, 1.0M LiPF6, EC and DMC mixed solvent (EC: DMC = 1:1) is used as the electrolyte, glass fiber membrane is used as the separator, and lithium sheet is used as the positive electrode. The assembly is carried out in an anhydrous and anaerobic glove box.

[0072] The button-type supercapacitors obtained are subjected to electrochemical performance test in a new battery test system, and the results are shown in Table 1.

[0073] Table 1: Charge-discharge cycle test results of negative electrode materials obtained in Examples 1-5 and Comparative Examples 1 and 2

[0074] Reversible specific capacity (mAh / g) First coulombic efficiency (%) Retention rate of reversible cycle for 100 weeks (%) Example 1 1645 94.8 93.4 Example 2 1563 87.5 84.8 Example 3 1182 90.2 92.6 Example 4 1276 88.7 85.5 Example 5 1290 90.3 86.7 Comparative Example 1 894 68.8 62.7 Comparative Example 2 855 62.6 57.5 .

[0075] Summary:

[0076] As can be seen from the half-cell test data in Table 1:

[0077] The silicon-carbon negative electrode material provided in Example 1 performs well in terms of capacity, initial efficiency and cycle stability: the voltage-current curves of the first, second and third cycles have a high degree of overlap (see Figure 5 ), indicating that the silicon-carbon negative electrode material of the application has good cycle stability when applied in batteries; the specific capacity of the first charge can reach 1645 mAh / g, and the specific capacity of the first discharge is 1735 mAh / g (see Figure 6 ), the coulombic efficiency is 94.8%, and the reversible cycle retention rate after 100 cycles is as high as 93.4%.

[0078] In Example 2, the proportion of silicon increases after increasing the content of silicon tetrachloride, and relatively, the capacity increases, but the side reaction is relatively more, and the volume expansion of silicon is more difficult to limit, and its cycle performance is worse. In terms of parameters, the reversible specific capacity of Example 2 is comparable to that of Example 1, but the first coulombic efficiency and the reversible cycle retention rate after 100 cycles are lower than those of Example 1; in Example 3, the increase in the content of calcium carbide leads to an excessively thick carbon coating layer, resulting in a decrease in capacity (1182 mAh / g); in Example 4, the decrease in reaction temperature may hinder the reaction kinetics at low temperature, leading to incomplete reaction, and the unreacted CaC2 may wrap the product (such as Si or C), forming a dense layer that hinders further reaction, so both the specific capacity and the coulombic efficiency cycle retention rate are lower than those of Example 1; in Example 5, due to the excessively high reaction temperature, side reactions such as decomposition of calcium carbide and re-chlorination of Si may occur.

[0079] In summary, the overall performance of Example 1 is better than that of the silicon-carbon negative electrode materials provided in Examples 2-5, which is the best embodiment.

[0080] In addition, the application also provides two groups of comparative examples, combined with Figure 7 , Figure 8 (0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, 20C, 0.1C current density, respectively, to obtain the rate performance test graph) can be seen:

[0081] In Comparative Example 1, graphite is used as the carbon source, but the activity of C2 2-The activity of the active carbon is low, and the required reaction temperature for reducing Si element is higher, so under the same conditions, the example 1 can obtain good electrochemical performance, while the comparative example 1 can not well reduce the silicon at the temperature, and the specific capacity is significantly reduced, and the specific capacity of the comparative example 1 is about half of that of the example 1.

[0082] In the comparative example 2, the active carbon is used as the carbon source, and the activity of the active carbon is low, and the required reaction temperature for reducing Si element is higher, so under the same conditions, the example 1 can obtain good electrochemical performance, while the comparative example 2 can not well reduce the silicon at the temperature, and the specific capacity is significantly reduced. At the same time, the active carbon can produce more side reactions during pyrolysis, and the side reaction products are wrapped in the composite material, which further promotes the reduction of the specific capacity and the cycle stability, and the specific capacity of the comparative example 2 is about half of that of the example 1, and the reversible cycle retention rate is only 57.5% after 100 cycles.

[0083] The carbon-coated silicon silicon-carbon negative electrode material provided by the application has good cycle performance, high reversible specific capacity and high initial coulombic efficiency, and excellent overall electrochemical performance when used for preparing a negative electrode material of a lithium ion battery.

[0084] The above-described examples only express several feasible implementation manners of the application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the patent scope of the application, and the examples are not used to limit the protection scope in the claims of the application. For ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the application, and any equivalent implementation or change made without departing from the application should be included in the technology of the application.

Claims

1. A method for preparing a silicon-carbon negative electrode material, characterized by, The steps are as follows: Step one, silicon tetrachloride and calcium carbide are added into carbon tetrachloride solution, stirred uniformly, and then transferred into a high-pressure reaction kettle for reaction to obtain Si@C precursor; Step two, the obtained Si@C precursor is added into anhydrous ethanol for ball milling treatment; Step three, the material obtained by ball milling treatment is dried by suction filtration, calcined and carbonized in an inert gas, and the obtained product is washed to obtain a silicon-carbon negative electrode material.

2. The method for preparing a silicon-carbon anode material according to claim 1, characterized in that: The molar ratio of silicon tetrachloride to calcium carbide is 1:1-4.

3. The method for preparing a silicon-carbon anode material according to claim 1, characterized in that: The high-pressure reaction kettle is lined with polytetrafluoroethylene.

4. The method for preparing a silicon-carbon anode material according to claim 1, characterized in that: In step one, the reaction temperature is 200-240 DEG C, and the reaction time is 6-24 h.

5. The method for preparing a silicon-carbon anode material according to claim 4, characterized in that: The reaction temperature is 220-240 DEG C.

6. The method for preparing a silicon-carbon anode material according to claim 1, characterized in that: The molar ratio of silicon tetrachloride to calcium carbide is 1:1, the reaction temperature is 220-240 DEG C, and the reaction time is 12-24 h.

7. The method for preparing a silicon-carbon anode material according to claim 1, characterized in that: In step two, the rotation speed of the ball milling treatment is 300-1000 r / min, and the time is 1-6 h. 8.The method of claim 1, wherein: the silicon-carbon negative electrode material is prepared by the following steps: mixing a silicon source and a carbon source to obtain a mixture; and heating the mixture to obtain the silicon-carbon negative electrode material. In step three, the calcination and carbonization temperature is 500-850 DEG C, and the time is 2-8 h.

9. The silicon-carbon anode material prepared by the method of claim 1, characterized in that: The silicon-carbon negative electrode material is a core-shell structure of carbon-coated silicon and has a loose and dispersed layered structure on the cross section.

10. Use of the silicon-carbon anode material produced by the method of claim 1, characterized in that: The silicon-carbon negative electrode material is used for preparing a lithium ion battery negative electrode material.

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