Lithium-sulfur battery electrode material and applications thereof

Abstract

The application relates to a lithium-sulfur battery electrode material and application thereof, and belongs to the technical field of surface chemical materials. The application discloses a lithium-sulfur battery electrode material, wherein the electrode material is a double-layer structure, a carbon-sulfur composite is used as a base material, and a silicon carbide nano layer is coated on the surface of the carbon-sulfur composite. The application further discloses a high-specific-energy lithium-sulfur battery, wherein the positive electrode of the high-specific-energy lithium-sulfur battery comprises a binder, a dispersion medium, a conductive agent and the electrode material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of surface chemical materials, and relates to a lithium-sulfur battery electrode material and application thereof. BACKGROUND

[0002] Lithium-sulfur battery (Li-S) has a high theoretical energy density (2600 Whg -1 ) and is considered as a new generation of high-energy-density lithium-ion battery with great potential. The theoretical specific capacity of a sulfur-based positive electrode is 1672 mAhg -1 , which is higher than that of the currently used positive electrode materials (<200 mAhg -1 ). In addition, sulfur-based materials have the advantages of low cost, no pollution, and rich reserves, but they have not been widely commercialized. The main reason is that the sulfur-based materials are inherently insulating (resistivity 1*1015 mΩ), have poor cycle performance, low coulombic efficiency, and shuttle effect of polysulfides, which hinder the practical application of Li-S.

[0003] At present, the electrical conductivity of sulfur can be increased and the formation of polysulfides can be reduced by constructing carbon-sulfur composites, or the electrochemical performance of the substrate can be improved by coating a carbon layer on the surface of the porous carbon substrate and the sulfur substrate. Other methods include using high-concentration electrolyte, carbon interlayer, etc. However, these methods have the problems of low sulfur loading, sulfur shuttle in the composite electrode, and increased weight and internal resistance of the battery due to the use of the interlayer. SUMMARY

[0004] The purpose of the present application is to solve the above-mentioned problems existing in the prior art, and a lithium-sulfur battery electrode material is provided, which is coated with a silicon carbide layer on the surface of a carbon-sulfur composite to improve the cycle performance and rate performance of the lithium-sulfur battery.

[0005] The present application adopts the following technical solutions to achieve the above-mentioned purposes:

[0006] A lithium-sulfur battery electrode material, characterized in that the electrode material has a double-layer structure, and a carbon-sulfur composite is used as a substrate, and a silicon carbide nanolayer is coated on the surface of the carbon-sulfur composite.

[0007] Preferably, the electrode material is formed by a low-temperature in-situ method under the induction of fluorine radicals to form a silicon carbide nanolayer by bonding silicon with carbon atoms in the graphene layer.

[0008] Preferably, the low-temperature in-situ method comprises the following steps:

[0009] S1. Graphene and sulfide are sequentially added to dilute hydrochloric acid for ultrasonic reaction, and then the sulfur-loaded graphene is obtained by washing and drying;

[0010] S2. Nanosilicon powder is dispersed in a mixed solution of ammonium fluoride and NMP for fluorination, and the fluorinated silicon powder is obtained by drying after the reaction is completed.

[0011] S3, adding the fluorinated silicon powder prepared in step S2 into the sulfur-loaded graphene prepared in step S1 to prepare a mixed powder, and performing an emulsification reaction in a mixed solution of water and isopropyl alcohol, and then cleaning, filtering and drying to obtain an electrode material with a silicon carbide nanolayer coated on the surface of the carbon-sulfur composite.

[0012] The present application forms a silicon carbide layer on the surface of the carbon-sulfur composite by a low-temperature in-situ method, and under the induction of fluorine radicals, the carbon atoms in the graphene layer form a silicon carbide nanolayer with high conductivity by bonding with silicon, and the dense coating can inhibit the entry and exit of polysulfide, thereby improving the cycle performance and rate performance of lithium-sulfur.

[0013] Preferably, the mass ratio of graphene to sulfide in step S1 is 1:(12-20).

[0014] Further preferably, the mass ratio of graphene to sulfide is 1:(15-18).

[0015] Preferably, the total mass of graphene and sulfide in step S1 is added in an amount of 5-12 g / 100 ml in a dilute hydrochloric acid solution.

[0016] Further preferably, the concentration of the dilute hydrochloric acid solution is 0.01-0.2 mol / L.

[0017] In step S1, the reaction of hydrochloric acid and sulfide loads elemental sulfur on the surface of graphene.

[0018] Further preferably, the sulfide is sodium thiosulfate.

[0019] Preferably, the ultrasonic process in step S1 is performed for 20-40 min.

[0020] Preferably, the amount of nano-silicon powder added in the mixed solution of ammonium fluoride and NMP in step S2 is 0.01-0.3 g / 100 ml.

[0021] The amount of nano-silicon powder added in the mixed solution of ammonium fluoride and NMP is kept within the above range, which can make the coating thickness reasonable, facilitate the subsequent reaction, and the lithium-sulfur battery prepared also has good performance.

[0022] Further preferably, the concentration of the mixed solution of ammonium fluoride and NMP is 0.01-0.08 mol / L.

[0023] Further preferably, the particle size of the nano-silicon powder is 40-60 nm.

[0024] As preferred, the temperature of the fluorination reaction in step S2 is 70-85℃, and the time is 60-90min.

[0025] As preferred, the volume ratio of water to isopropyl alcohol in step S3 is 1:(1-2).

[0026] As preferred, the temperature of the emulsification reaction in step S3 is 60-75℃, and the time is 1-3h.

[0027] The application also discloses a high specific energy lithium-sulfur battery, and the positive electrode of the high specific energy lithium-sulfur battery comprises a binder, a dispersion medium, a conductive agent and a lithium-sulfur battery electrode material.

[0028] As preferred, the preparation method of the positive electrode of the high specific energy lithium-sulfur battery comprises the following steps: mixing the electrode material, the dispersion medium, the binder and the conductive agent to obtain a slurry, coating the slurry on the surface of a current collector and drying.

[0029] Further preferably, the loading amount of the slurry on the surface of the current collector is 2-4g / cm 2 .

[0030] Further preferably, the current collector is an aluminum sheet.

[0031] As preferred, in the slurry, the mass fraction of the electrode material is 70-85%, the mass fraction of the dispersion medium is 5-20%, the mass fraction of the binder is 5-15%, and the mass fraction of the conductive agent is 5-15%.

[0032] Further preferably, the dispersion medium comprises one or more of water, ethanol and NMP.

[0033] Further preferably, the binder is PVDF.

[0034] Further preferably, the concentration of the PVDF is 0.01-0.07mol / L.

[0035] Further preferably, the conductive agent comprises one or more of acetylene black, carbon black and conductive graphite.

[0036] Compared with the prior art, the application has the following beneficial effects:

[0037] 1. By controlling the addition amount of the nano-silicon powder in the mixed solution of ammonium fluoride and NMP, the application can obtain a suitable coating thickness, and facilitate the occurrence of subsequent electrochemical reactions.

[0038] 2. In the application, the silicon fluoride and the sulfur-loaded graphene are mixed, and a low-temperature in-situ method is adopted. Under the induction of fluorine radicals, the carbon atoms in the graphene layer form a high-conductivity silicon carbide nano layer by bonding with silicon, and the dense coating can hinder the entry and exit of polysulfur, thereby improving the cycle performance and rate performance of the lithium-sulfur battery.

[0039] 3、The application makes graphene and sulfide into carbon-sulfur composite by the method of ultrasonic, makes the graphene matrix load sulfur element, and the graphene itself has better electronic conductivity, which improves the rate performance and coulomb efficiency to a certain extent after combination.

[0040] 4、The prepared positive electrode material can improve the overall conductivity and also can hinder the shuttle of sulfide.

[0041] 5、The method is simple and easy to operate, the byproduct is water, the preparation process is environmentally friendly, and can be popularized and used in industry.

[0042] DRAWINGS

[0043] Figure 1 The cycle performance diagram of the battery prepared in Example 2 of the application.

[0044] Figure 2 The rate performance diagram of the battery prepared in Examples 1-3 and Comparative Examples 1-3 of the application. DETAILED DESCRIPTION

[0045] The following are specific embodiments of the application, which further describe the technical solutions of the application, but the application is not limited to these embodiments.

[0046] Example 1:

[0047] A 200 mL dilute hydrochloric acid solution of 1 mol / L is configured, 1 g of graphene (brand: Aladdin, CAS number: 7782-42-5) is weighed and dispersed in the dilute hydrochloric acid solution, and 15.8 g of sodium thiosulfate is weighed and slowly added to the above mixed solution to form sulfur element under ultrasonic assistance; the graphene carrying the sulfur monomer is washed and dried for later use; 0.01 mol of ammonium fluoride is dissolved in 200 ml of NMP to configure a 0.05 mol / L solution, and 0.05 g of nano silicon powder is dispersed in the above solution, and the silicon powder is fluorinated at 80°C for 80 min, so that the silicon powder is fully fluorinated, and then dried in a 100°C oven for 24 h to obtain fluorinated silicon powder. The fluorinated silicon powder is placed in the above sulfur-loaded graphene sheet, added to a mixed solution of water and isopropyl alcohol in a volume ratio of 1:1.5, and reacted at 70°C for 2 h, and then washed and filtered by centrifugation with deionized water and anhydrous ethanol, and vacuum dried at 60°C to obtain an electrode material; NMP is used as a dispersion medium, 10% of a binder is added and stirred into a colorless transparent liquid, 10% of an electrically conductive agent, acetylene black, is added, and then the remaining powder is added and stirred uniformly to form a slurry; the slurry is coated on the surface of an aluminum electrode, and the loading amount is 2.5 g / cm 2The positive electrode is prepared after drying, and is assembled into a button cell together with a negative electrode and a separator. The button cell prepared is subjected to performance testing, and the initial specific discharge capacity is 1305 mAh / g, and the initial discharge efficiency at 0.1 C is 92.9%.

[0048] Example 2

[0049] Compared with Example 1, the difference lies in that the addition amount of the nano-silicon powder is 0.1 g. The button cell prepared is subjected to performance testing, and the cycle performance graph is shown in Figure 1 , and the specific discharge capacity is still 846 mAh / g after 200 cycles; the rate performance graph is shown in Figure 2 , the rate performance is good, the initial specific discharge capacity is 1315.9 mAh / g, and after 2C rate, the specific capacity is still 1108 mAh / g at 0.1 C, and is stable; the initial discharge efficiency at 0.1 C is 94.7%.

[0050] Example 3

[0051] Compared with Example 1, the difference lies in that the addition amount of the nano-silicon powder is 0.3 g. The button cell prepared is subjected to performance testing, and the rate performance graph is shown in Figure 2 , the initial specific discharge capacity is 1279.3 mAh / g, and the initial discharge efficiency at 0.1 C is 93.1%.

[0052] Comparative Example 1

[0053] Compared with Example 1, the difference lies in that the silicon fluoride coating is not performed. The button cell prepared is subjected to performance testing, and the rate performance graph is shown in Figure 2 , the initial specific discharge capacity is 1265.4 mAh / g, and the initial discharge efficiency at 0.1 C is 91.6%.

[0054] Comparative Example 2

[0055] Compared with Example 1, the difference lies in that the addition amount of the graphene is 0.7 g. The button cell prepared is subjected to performance testing, and the rate performance graph is shown in Figure 2 , the initial specific discharge capacity is 1195.8 mAh / g, and the initial discharge efficiency at 0.1 C is 87.3%.

[0056] Comparative Example 3

[0057] Compared with Example 1, the difference lies in that the addition amount of the graphene is 1.4 g. The button cell prepared is subjected to performance testing, and the rate performance graph is shown in Figure 2 , the initial specific discharge capacity is 1346 mAh / g, and the initial discharge efficiency at 0.1 C is 86.9%.

[0058] In Examples 1-3, the addition amount of the nano-silicon powder is changed to control the coating thickness. Too much addition amount of the nano-silicon powder (i.e. too little addition amount of the graphene) will result in too much content of the silicon fluoride, too thick coating layer, increased internal resistance, poor conductivity, too long ion transmission time, poor cycle performance and rate performance. Too little addition amount of the nano-silicon powder will result in uneven coating or no coating in some parts. In Comparative Example 1, no silicon fluoride is added, i.e. the performance of the sulfur-loaded graphene alone. The electrode prepared has poor binding capacity for elemental sulfur, and the elemental sulfur is easily dissolved out, so the performance is poor.

[0059] In summary, the present application controls the addition amount of each raw material, and uses a low-temperature in-situ method to uniformly coat a silicon carbide layer on the surface of the carbon-sulfur composite, to improve the conductivity of the material and hinder the shuttling of sulfides. The battery composed of the positive electrode material prepared has good rate performance and cycle performance.

[0060] The specific embodiments described herein are merely illustrative of the spirit of the present application. Those skilled in the art of the present application can make various modifications or supplements to the described specific embodiments or use similar ways to replace them, without departing from the spirit of the present application or exceeding the scope defined by the appended claims.

Claims

1. A lithium-sulfur battery electrode material, characterized in that, The electrode material has a double-layer structure, with a carbon-sulfur composite as the substrate and a silicon carbide nanolayer coated on the surface of the carbon-sulfur composite. The electrode material forms a silicon carbide nanolayer by bonding silicon with carbon atoms in the graphene layer under the induction of fluorine free radicals through a low-temperature in-situ method. The aforementioned low-temperature in-situ method includes the following steps: S1. Graphene and sulfide are added to dilute hydrochloric acid in sequence for ultrasonic reaction, and then sulfur-loaded graphene is obtained after cleaning and drying. S2. Disperse the nano-silicon powder in a mixed solution of ammonium fluoride and NMP for fluorination. After the reaction is complete, dry the powder to obtain silicon fluoride powder. S3. The silicon fluoride powder obtained in step S2 is added to the sulfur-loaded graphene obtained in step S1 to form a mixed powder. After emulsification in a mixed solution of water and isopropanol, the powder is washed, filtered, and dried to obtain an electrode material with a silicon carbide nanolayer coated on the surface of the carbon-sulfur composite.

2. The lithium-sulfur battery electrode material according to claim 1, characterized in that, The mass ratio of graphene to sulfide in step S1 is 1:(12-20).

3. The lithium-sulfur battery electrode material according to claim 1, characterized in that, The total mass of graphene and sulfide mentioned in step S1 is 5-12 g / 100 ml in dilute hydrochloric acid solution.

4. The lithium-sulfur battery electrode material according to claim 1, characterized in that, In step S2, the amount of nano-silicon powder added to the mixed solution of ammonium fluoride and NMP is 0.01-0.3 g / 100 ml.

5. The lithium-sulfur battery electrode material according to claim 1, characterized in that, The fluorination reaction in step S2 is carried out at a temperature of 70-85°C for 60-90 minutes.

6. The lithium-sulfur battery electrode material according to claim 1, characterized in that, The volume ratio of water to isopropanol in step S3 is 1:(1-2).

7. The lithium-sulfur battery electrode material according to claim 1, characterized in that, The emulsification reaction in step S3 is carried out at a temperature of 60-75°C for 1-3 hours.

8. A high-energy-density lithium-sulfur battery, characterized in that, The positive electrode of the high-energy-density lithium-sulfur battery includes a binder, a dispersion medium, a conductive agent, and the lithium-sulfur battery electrode material as described in claim 1.

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