An ultrathin carbon sheet and a preparation method thereof, and a preparation method of a carbon / sulfur positive electrode

Abstract

The application discloses an ultrathin carbon sheet and a preparation method thereof, and a preparation method of a carbon / sulfur positive electrode. The preparation method of the ultrathin carbon sheet comprises the following steps: (1) wrapping physic nuts in gauze, immersing the physic nuts in deionized water, soaking and kneading to obtain a glue solution A; (2) mixing the glue solution A with an alkali metal salt to obtain a modified glue solution B, and freeze-drying to obtain a glue A; (3) performing high-temperature treatment on the glue A in an inert atmosphere, washing with water, and drying to obtain the ultrathin carbon sheet material. The prepared ultrathin carbon sheet is applied to a lithium-sulfur battery, can greatly improve the sulfur loading capacity, improve the conductivity, and effectively improve the positive electrode capacity, energy density and capacity retention rate of the lithium-sulfur battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrode material preparation, and particularly relates to preparation of an ultrathin carbon sheet and application thereof in a lithium-sulfur battery. BACKGROUND

[0002] As a global mainstream energy storage means, lithium secondary batteries have been widely used in daily life since being commercially promoted in the 1990s. From large fixed energy storage to portable intelligent 3C products, lithium ion batteries can be seen everywhere in life. Nowadays, due to the rapid growth of mobile energy storage demand, the requirement for energy density in modern society is getting higher and higher. Limited by the theoretical capacity of traditional metal oxide positive electrode materials, lithium-sulfur batteries have attracted people's attention due to their environmental friendliness and low price, and have become a new type of lithium secondary battery that is highly concerned.

[0003] Lithium-sulfur batteries have attracted attention due to their extremely high theoretical specific capacity (1672 mAh g -1 ) and energy density (2500 Wh kg -1 ). However, the practical application of lithium-sulfur batteries faces the following problems: unlike traditional intercalation cathodes, sulfur cathodes provide electrons by reacting with lithium ions to form polysulfides. This process is a multi-stage reaction in which the eight-membered ring of elemental sulfur is opened, gradually forming long-chain polysulfides to short-chain lithium sulfide. In the entire reaction process, long-chain polysulfides such as Li2S x (6≤x<8) can be dissolved in electrolyte. Therefore, lithium-sulfur batteries face the following limitations: (1) elemental sulfur has low conductivity, and its actual utilization rate is low, resulting in extremely low actual energy density; (2) polysulfide dissolution causes "shuttle effect", and active material loss is extremely fast, resulting in poor cycle performance; (3) during charging and discharging, the electrode volume changes greatly and the structure is unstable.

[0004] In view of the intrinsic defects of lithium-sulfur batteries, scholars have proposed many solutions, such as binder modification, electrolyte modification, acceleration of electrode reaction by catalyzing sulfur conversion reaction, change of sulfur structure, etc. One of the most effective solutions is to introduce high-performance conductive materials as a framework to improve the conductivity of the electrode and control the dissolution of polysulfides through surface modification and physical restriction. Generally, high-conductivity carbon materials are widely studied as a common sulfur carrier material due to their good plasticity and conductivity, but the sulfur loading capacity in the carbon-based conductive framework is usually low, and too low sulfur loading capacity will lead to a decrease in the energy density of the entire electrode, while the cost of super-high-conductivity, high-specific-surface-area materials such as graphene is usually high. The patent with publication number CN109437146A discloses a scheme of mixing graphite oxide, calcium salt and sodium citrate, preparing a colloid, and then further high-temperature calcining and acid washing to form a porous carbon, the obtained product has rich pores and can be used as a carrier for elemental sulfur to improve the polysulfide problem; the patent with publication number CN109301252A discloses a method of surface modification of porous carbon with a chelating agent to increase the chemical adsorption capacity of the carrier material, but in these schemes, the proportion of carbon material in the positive electrode is still high (> 25%), so the overall energy density of the positive electrode is low.

[0005] False acid melon seeds are a kind of natural plant seeds, the surface of which is composed of a large number of polysaccharides with pyranoid sugar ring as the basic structural unit, and the seeds can easily form a colloid in water and are usually used as raw materials in the food industry or as thickening agents. SUMMARY

[0006] The purpose of the present application is to provide an ultrathin carbon sheet and a preparation method thereof, and a preparation method of a carbon / sulfur positive electrode. The present application uses cheap natural plant material false acid melon seeds to extract and modify the surface colloid of the seeds to synthesize an ultrathin nanocarbon sheet. The nanocarbon sheet has high conductivity and ultrahigh specific surface area, and can effectively provide high sulfur loading capacity when applied in lithium-sulfur batteries, improve the conductivity of the electrode, improve the cycle problem and volume expansion problem, and solve some of the difficulties currently faced by lithium-sulfur battery positive electrode materials.

[0007] To achieve the above technical effects, the technical scheme of the present application is as follows:

[0008] A preparation method of an ultrathin carbon sheet, comprising the following steps:

[0009] (1) wrapping the false acid melon seeds in gauze, immersing them in deionized water, soaking and kneading to obtain a glue solution A;

[0010] (2) mixing the glue solution A with an alkali metal salt to obtain a modified glue solution B, and freeze-drying to obtain a colloid A;

[0011] (3) high temperature treatment of the gel A under inert atmosphere, water washing, drying, to obtain the ultra-thin carbon sheet material.

[0012] In the method, the gauze in step (1) is 40-100 mesh.

[0013] In the method, the mass ratio of the fructus pruni neoheterophyllae to deionized water in step (1) is 1:3-10.

[0014] In the method, the alkali metal salt in step (2) includes sodium chloride and / or potassium chloride.

[0015] In the method, the concentration of the alkali metal salt in the glue A in step (2) is 1wt%-10wt% in water.

[0016] In the method, the freezing drying time in step (2) is 12h-48h.

[0017] In the method, the high temperature treatment condition in step (3) is 600-1100℃, the time is 2-6h, the heating rate is 5℃ / min-10℃ / min, and the protective atmosphere is nitrogen atmosphere or argon atmosphere.

[0018] In the method, the loose bulk density of the ultra-thin carbon sheet is 0.01-0.5g / m 3 , can be applied as a sulfur carrier in a lithium-sulfur battery positive electrode material, has a very high sulfur loading capacity, and can improve the specific capacity, energy density and cycle life of the positive electrode material.

[0019] The present application utilizes the space cross-linking structure formed by the gel polysaccharide with pyran ring as the basic structure on the surface of natural plant material in water. The substance can be removed from the surface by simple mechanical rolling, dissolved in water without complex extraction process, and the hydrophobicity of the sol structure is enhanced by adding alkali metal salt. A large amount of water is filled into the network structure, and the process of freezing and sublimation can maintain this structure with voids. At the same time, the chloride ion can destroy part of the hydrogen bond, so that the originally spherical gel structure is opened, and the nano carbon sheet structure with ultra-high specific surface area is formed in the further carbonization process. The present application has the characteristics of high conductivity and high specific surface area, and can be applied to lithium-sulfur battery positive electrode as a sulfur carrier with ultra-high sulfur loading capacity. The ultra-thin property can significantly improve the sulfur loading capacity of the positive electrode, further improve the energy density, and the high specific surface area can effectively adsorb polysulfides, improve the specific capacity, efficiency and cycle performance of lithium-sulfur battery. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a 20000 times magnification scanning electron microscope image of the material obtained in Example 1;

[0021] Figure 2 is a 50000 times magnification scanning electron microscope image of the material obtained in Example 1;

[0022] Figure 3 is a capacity graph of the button cells prepared in Example 1 and the comparative example, which were cycled at 0.5C;

[0023] Figure 4 is a rate performance graph of the button cells prepared in Example 1 and the comparative example;

[0024] Figure 5 is an impedance performance graph of the button cells prepared in Example 1 and the comparative example. DETAILED DESCRIPTION

[0025] Test method:

[0026] (1) Electrochemical test: The above material was cut into a 12mm diameter disc, lithium metal was used as the counter electrode, a button cell was assembled with a separator, an electrolyte and a CR2032 button cell shell in an argon-filled glove box, and the battery was tested for charge and discharge by a new battery device.

[0027] (2) Calculation of the sulfur loading of the positive electrode material:

[0028] p1 = (m2-m1) ÷ m2 x 100%

[0029] Wherein, p1 is the sulfur loading of the positive electrode material (%), m2 is the mass of the sulfur-mixed positive electrode material (g), and m1 is the mass of the carbon nanosheet material before mixing with sulfur (g).

[0030] (3) Calculation of the sulfur loading of the electrode:

[0031] p2 = (m4-m3) x p1 x 0.8 ÷ 0.0113;

[0032] Wherein, p2 is the sulfur loading of the electrode (mg / cm 2 ), m4 is the mass of the electrode sheet (mg), m3 is the mass of the uncoated 12mm diameter aluminum foil (mg), p1 is the sulfur loading of the positive electrode material (%), 0.8 is the concentration of the positive electrode material in the slurry, and 0.0113 is the area of the electrode sheet (cm 2 ).

[0033] Example 1

[0034] (1) 100g of fructus physali tenuiflorae seeds were wrapped in 100 mesh gauze and immersed in 500mL of deionized water. After soaking and kneading for 30min, the gel was obtained;

[0035] (2) 10g of sodium chloride was added to the gel, and the modified gel was obtained after stirring evenly;

[0036] (3) The modified glue solution is frozen, placed in a freeze dryer, dried for 20 hours, and then taken out to obtain dried glue;

[0037] (4) The glue is placed in a tube furnace and calcined at 800°C for 2 hours in an argon atmosphere at a heating rate of 5°C / min. After cooling, the obtained product is washed with deionized water and filtered, and dried in an oven for 12 hours to obtain an ultrathin carbon nanosheet material.

[0038] The loose bulk density of the material is 0.32 g / cm 3 The specific surface area of the material is tested by a Belsorp 3H-2000A full-automatic specific surface area tester, the desorption temperature is 200°C, the desorption time is 2 hours, and the tested specific surface area is 72.32 m 2 / g.

[0039] To test the performance of the lithium ion battery, a carbon / sulfur electrode is prepared according to the following method for electrochemical testing:

[0040] (1) Preparation of a carbon / sulfur positive electrode: The prepared ultrathin carbon nanosheet and elemental sulfur are fully ground at a mass ratio of 0.5:9.5, and then placed in a tube furnace and heated at 155°C for 12 hours in an argon atmosphere. The obtained powder material, Super P and PvDF are fully ground at a mass ratio of 8:1:1 in NMP, and then coated on the surface of an aluminum foil and dried to obtain a positive electrode sheet.

[0041] (2) Electrochemical testing: The voltage range is 1.7-2.3V.

[0042] The sulfur loading of the obtained carbon / sulfur positive electrode material is 92%, the sulfur loading of the obtained carbon / sulfur electrode is 2.2 mg / cm 2 , the initial discharge specific capacity of the material is 1079 mAh / g at a current density of 0.1C, the specific capacity is 391 mAh / g at a rate of 2C, and the capacity retention rate is 93.14% after 300 cycles at a current density of 0.5C.

[0043] Example 2

[0044] (1) 100g of fake acid melon seeds are wrapped in 40 mesh gauze, immersed in 1000mL of deionized water, soaked and rubbed for 30 minutes, and then taken out to obtain a glue solution;

[0045] (2) 11g of sodium chloride is added to the glue solution, stirred uniformly to obtain a modified glue solution;

[0046] (3) The modified glue solution is frozen, placed in a freeze dryer, dried for 12 hours, and then taken out to obtain dried glue;

[0047] (4) The gel was placed in a tube furnace and calcined at 600°C for 4 hours in an argon atmosphere at a heating rate of 5°C / min. After cooling, the product was washed and filtered in deionized water and dried in an oven for 12 hours to obtain the desired ultrathin carbon nanosheet material.

[0048] The loose bulk density of the material, as measured by the Koster loose bulk density meter, was 0.24 g / cm³. 3 The specific surface area of ​​the material was tested using a Bestech 3H-2000A fully automatic specific surface area analyzer. The desorption temperature was 200℃, the desorption time was 2 hours, and the tested specific surface area was 90.15 m². 2 / g.

[0049] To verify the performance of this invention in lithium-ion batteries, a carbon / sulfur electrode was prepared according to the following method for electrochemical testing:

[0050] (1) Preparation of carbon / sulfur cathode: The prepared ultrathin carbon nanosheets and elemental sulfur were thoroughly ground at a mass ratio of 0.5:9.5 and placed in a tube furnace, and kept at 155℃ for 12h under an argon atmosphere. The resulting powder material was then thoroughly ground with Super P and PvDF at a mass ratio of 8:1:1 in NMP, coated onto the surface of aluminum foil and dried to obtain the cathode sheet.

[0051] (2) Electrochemical test: voltage range is 1.7-2.3V.

[0052] The obtained carbon / sulfur cathode material has a sulfur loading of 93%, and the obtained carbon / sulfur electrode has a sulfur loading of 2.5 mg / cm³. 2 The material exhibits an initial discharge specific capacity of 869 mAh / g at a current density of 0.1C, a specific capacity of 382 mAh / g at a 2C rate, and a capacity retention of 74.34% after 300 cycles at a current density of 0.5C.

[0053] Example 3

[0054] (1) Wrap 100g of false groundcherry seeds in 60-mesh gauze, immerse in 300mL of deionized water, soak and rub for 30 minutes, and then take out to obtain the gel.

[0055] (2) Add 30g of potassium chloride to the adhesive solution and stir evenly to obtain the modified adhesive solution;

[0056] (3) After freezing the modified adhesive solution, place it in a freeze dryer and dry for 12 hours to obtain the dried adhesive.

[0057] (4) The gel was placed in a tube furnace and calcined at 1100℃ for 6 hours in a nitrogen atmosphere with a heating rate of 10℃ / min. After cooling, the product was washed and filtered in deionized water and dried in an oven for 48 hours to obtain the desired ultrathin carbon nanosheet material.

[0058] The loose bulk density of the material, as measured by the Koster loose bulk density meter, was 0.47 g / cm³. 3 The specific surface area of ​​the material was tested using a Bestech 3H-2000A fully automatic specific surface area analyzer. The desorption temperature was 200℃, the desorption time was 2 hours, and the tested specific surface area was 72.13 m². 2 / g.

[0059] To verify the performance of this invention in lithium-ion batteries, a carbon / sulfur electrode was prepared according to the following method for electrochemical testing:

[0060] (1) Preparation of carbon / sulfur cathode: The prepared ultrathin carbon nanosheets and elemental sulfur were thoroughly ground at a mass ratio of 0.5:9.5 and placed in a tube furnace, and kept at 155℃ for 12h under an argon atmosphere. The resulting powder material was then thoroughly ground with Super P and PvDF at a mass ratio of 8:1:1 in NMP, coated onto the surface of aluminum foil and dried to obtain the cathode sheet.

[0061] (2) Electrochemical test: voltage range is 1.7-2.3V.

[0062] The resulting carbon / sulfur cathode material had a sulfur loading of 85%, and the resulting carbon / sulfur electrode had a sulfur loading of 1.92 mg / cm³. 2 The material exhibits an initial discharge specific capacity of 923 mAh / g at a current density of 0.1C, a specific capacity of 305 mAh / g at a 2C rate, and a capacity retention of 83.15% after 300 cycles at a current density of 0.5C.

[0063] Comparative Example 1

[0064] (1) Wrap 100g of false groundcherry seeds in 100-mesh gauze, immerse in 500mL of deionized water, soak and rub for 30 minutes, and then take out to obtain the gel.

[0065] (2) After freezing the adhesive solution, place it in a freeze dryer and dry for 20 hours to obtain the dried adhesive.

[0066] (3) The gel was placed in a tube furnace and calcined at 800°C for 2 hours in an argon atmosphere at a heating rate of 5°C / min. After cooling, the product was washed and filtered in deionized water and dried in an oven for 12 hours to obtain ultrathin carbon nanosheets.

[0067] To verify the performance of this invention in lithium-ion batteries, a carbon / sulfur electrode was prepared according to the following method for electrochemical testing:

[0068] (1) Preparation of carbon / sulfur cathode: The prepared ultrathin carbon nanosheets and elemental sulfur were thoroughly ground at a mass ratio of 0.5:9.5 and placed in a tube furnace, and kept at 155℃ for 12h under an argon atmosphere. The resulting powder material was then thoroughly ground with Super P and PvDF at a mass ratio of 8:1:1 in NMP, coated onto the surface of aluminum foil and dried to obtain the cathode sheet.

[0069] (2) Electrochemical test: voltage range is 1.7-2.3V.

[0070] The loose bulk density of the material, as measured by the Koster loose bulk density meter, was 1.04 g / cm³. 3 The specific surface area of ​​the material was tested using a Bestech 3H-2000A fully automatic specific surface area analyzer. The desorption temperature was 200℃, the desorption time was 2 hours, and the tested specific surface area was 12.37 m². 2 / g.

[0071] The obtained carbon / sulfur cathode material has a sulfur loading of 67%, and the obtained carbon / sulfur electrode has a sulfur loading of 1.23 mg / cm³. 2 The material exhibits an initial discharge specific capacity of 705 mAh / g at a current density of 0.1C, a specific capacity of 17 mAh / g at a 2C rate, and a capacity retention of 28.38% after 300 cycles at a current density of 0.5C.

[0072] Comparative Example 2

[0073] (1) Dissolve 20g of citric acid in 500mL of deionized water and stir until homogeneous to obtain a citric acid solution;

[0074] (2) Add 10g of sodium chloride to the citric acid solution and stir until a mixed solution is obtained;

[0075] (3) After freezing the mixed solution, place it in a freeze dryer and dry for 20 hours to obtain the dried gel.

[0076] (4) The colloid was placed in a tube furnace and calcined at 800°C for 2 hours in an argon atmosphere at a heating rate of 5°C / min. After cooling, the product was washed and filtered in deionized water and dried in an oven for 12 hours to obtain porous carbon material.

[0077] The loose bulk density of the material, as measured by the Koster loose bulk density meter, was 0.95 g / cm³. 3 The specific surface area of ​​the material was tested using a Bestech 3H-2000A fully automatic specific surface area analyzer. The desorption temperature was 200℃, the desorption time was 2 hours, and the tested specific surface area was 23.37 m². 2 / g.

[0078] To verify the performance of this invention in lithium-ion batteries, a carbon / sulfur electrode was prepared according to the following method for electrochemical testing:

[0079] (1) Preparation of carbon / sulfur cathode: The prepared porous carbon material and elemental sulfur were thoroughly ground at a mass ratio of 0.5:9.5 and placed in a tube furnace, and kept at 155℃ for 12h under an argon atmosphere. The resulting powder material was then thoroughly ground with Super P and PvDF at a mass ratio of 8:1:1 in NMP, coated onto the surface of aluminum foil and dried to obtain the cathode sheet.

[0080] (2) Electrochemical test: voltage range is 1.7-2.3V.

[0081] The resulting carbon / sulfur cathode material had a sulfur loading of 75%, and the resulting carbon / sulfur electrode had a sulfur loading of 1.5 mg / cm³. 2 The material exhibits an initial discharge specific capacity of 876 mAh / g at a current density of 0.1C, a specific capacity of 304 mAh / g at a 2C rate, and a capacity retention of 67.03% after 300 cycles at a current density of 0.5C.

Claims

1. A method for preparing an ultrathin carbon sheet, comprising the following steps: (1) Wrap the false groundcherry seeds in gauze, immerse them in deionized water, soak and rub them to obtain gelatinous solution A; (2) Modified adhesive B is obtained by mixing adhesive A with alkali metal salt, and then freeze-drying to obtain adhesive A; (3) The adhesive A is subjected to high temperature treatment under an inert atmosphere, washed with water and dried to obtain the ultrathin carbon sheet material. The alkali metal salts mentioned in step (2) include sodium chloride and / or potassium chloride.

2. The method according to claim 1, characterized in that: The gauze used in step (1) is 40-100 mesh.

3. The method according to claim 1, characterized in that, The mass ratio of pseudophyte seeds to deionized water in step (1) is 1:3-10.

4. The method according to claim 1, characterized in that: The concentration of the alkali metal salt in water in step (2) is 1 wt%-10 wt%.

5. The method according to claim 1, characterized in that: The high-temperature treatment conditions described in step (3) are 600-1100℃, duration of 2-6h, heating rate of 5℃ / min-10℃ / min, and protective atmosphere of nitrogen or argon.

6. The ultrathin carbon sheet prepared by the method according to any one of claims 1-5, characterized in that, The loose density is 0.01-0.5 g / m3.

7. A method for preparing a carbon / sulfur cathode, comprising the following steps: The ultrathin carbon sheet described in claim 6 is ground with elemental sulfur at a mass ratio of 0.5:9.5-10.5 and then kept at 155°C for 12 hours under an argon atmosphere. The resulting powder material is ground with SuperP and PvDF at a mass ratio of 8:1:1 in NMP, then coated onto the surface of aluminum foil and dried to obtain a positive electrode sheet.

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