Intercalation composite film, preparation method thereof, and lithium-sulfur battery

Abstract

The present invention discloses an intercalation composite film, a preparation method thereof, and a lithium-sulfur battery. The intercalation composite film comprises a conductive substrate with zinc oxide loaded both internally and on its surface. The intercalation composite film can be applied to lithium-sulfur batteries, specifically sandwiched between the positive electrode and separator of the battery. During battery charging, the zinc oxide absorbs polysulfides dissolved in the electrolyte, suppressing the "shuttle effect" of the lithium-sulfur battery. The conductive substrate, while serving as a support, provides good conductivity, utilising the adsorbed polysulfides, increasing the utilization rate of active materials and thereby improving the performance of the lithium-sulfur battery.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to an intercalation composite film, a preparation method thereof, and a lithium-sulfur battery. Background Art

[0002] Lithium-sulfur batteries have extremely high theoretical specific capacity (1675mAh / g) and theoretical energy density (2600Wh kg -1 ), and its low cost and environmental friendliness have attracted widespread attention and research from the scientific and industrial communities. Lithium-sulfur batteries are secondary battery systems with lithium as the negative electrode and sulfur or sulfur-based composite materials as the positive electrode. During the discharge process of the battery, the positive electrode sulfur-based material will form a variety of chain polysulfides during the redox reaction. These polysulfides are soluble in the electrolyte and will diffuse through the diaphragm to the negative electrode side due to the concentration difference. They are then reduced to insulating and insoluble Li2S2 and Li2S and then cover the surface of the negative electrode, causing a "shuttle effect". This part of the material cannot enable the system to obtain current, thereby reducing the utilization rate of the active materials negative electrode lithium and positive electrode sulfur, thereby affecting battery performance. To this end, there is an urgent need to find a solution to the above problems. Summary of the Invention

[0003] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides an intercalation composite film, a preparation method thereof, and a lithium-sulfur battery.

[0004] In a first aspect of the present invention, an intercalation composite film is provided, comprising a conductive base layer, wherein the interior and surface of the conductive base layer are loaded with zinc oxide.

[0005] According to the embodiment of the present invention, the intercalation composite film has at least the following beneficial effects: the intercalation composite film includes a conductive base layer and zinc oxide loaded inside and on the surface of the conductive base layer. The intercalation composite film can be applied to lithium-sulfur batteries, specifically, it can be sandwiched between the positive electrode sheet and the separator of the lithium-sulfur battery. During the battery charging process, the zinc oxide can adsorb polysulfides dissolved in the electrolyte, reducing or even preventing the dissolved polysulfides from diffusing and migrating to the negative electrode side to cause side reactions to generate insulating and insoluble Li2S2 and Li2S, which then cover the negative electrode surface, thereby inhibiting the "shuttle effect" of the lithium-sulfur battery; and the conductive base layer can serve as a support and provide good conductivity at the same time, so that the adsorbed polysulfides can be utilized to improve the utilization rate of the active material, thereby improving the performance of the lithium-sulfur battery, including improving the electrochemical reversibility, rate cycling performance and cycle stability, and reducing capacity attenuation.

[0006] The conductive substrate has a porous structure, wherein the pores may be interlayer pores of a layered conductive substrate or internal pores of another porous conductive substrate; the zinc oxide is specifically loaded in the internal pores and on the surface of the conductive substrate. In some embodiments of the present invention, the conductive substrate is selected from at least one of graphite paper, carbon fiber paper, and carbon cloth. Preferably, the conductive substrate is selected from graphite paper, which has a layered structure, and the zinc oxide is loaded between the layers (i.e., between the graphite sheets) and on the surface of the graphite paper.

[0007] In some embodiments of the present invention, the zinc oxide is nano-zinc oxide. Preferably, the nano-zinc oxide is a flower-like zinc oxide nanomaterial. This nano-zinc oxide has a larger specific surface area, thereby improving the adsorption efficiency of polysulfides. For conductive graphite paper with a layered structure, a zinc oxide layer can be formed between and on the graphite sheets of the nano-zinc oxide-loaded graphite paper, with a thickness of 200 to 500 nm.

[0008] In some embodiments of the present invention, the thickness of the intercalated composite film is 15 to 30 μm, generally about 20 μm.

[0009] The second aspect of the present invention provides a method for preparing any one of the intercalated composite films provided in the first aspect of the present invention, comprising the following steps:

[0010] S1, using the conductive substrate as the working electrode and cooperating with the first pair of electrodes to perform electrolysis in the first electrolyte;

[0011] S2, electrochemically depositing metallic zinc inside and on the surface of the conductive base layer obtained in step S1;

[0012] S3. calcining the film material obtained in step S2 to convert the metallic zinc into zinc oxide, thereby obtaining an intercalated composite film.

[0013] The preparation method of the intercalated composite film according to the embodiment of the present invention has at least the following beneficial effects: the preparation method first electrolyzes the conductive base layer to open its structure (if it is a conductive base layer with a layered structure, its layers can be opened by electrolysis) to provide more sites for the attachment of zinc oxide, which is conducive to the uniform deposition of zinc ions; then, metallic zinc is uniformly deposited on the interior and surface of the electrolyzed conductive base layer by electrochemical deposition, and then the metallic zinc is converted into zinc oxide by calcination, thereby achieving uniform and stable loading of zinc oxide on the surface and interior of the conductive base layer, and the obtained zinc oxide has a neatly arranged flower cluster structure with a large specific surface area; the above process is simple and convenient The production operation is suitable for large-scale production; and through the above method, the obtained product, the intercalated composite film, can maintain the complete structure of the conductive base layer (such as the complete layered structure of graphite paper), so that the film has an independent self-supporting structure; the intercalated composite film can be applied to lithium-sulfur batteries, specifically, it can be sandwiched between the positive electrode sheet and the separator. During the discharge process of the battery, the zinc oxide on the intercalated composite film can effectively adsorb dissolved polysulfides, thereby suppressing the "shuttle effect" of the lithium-sulfur battery; and, while serving as a support, the conductive base layer can provide good conductivity, utilize the adsorbed polysulfides, improve the utilization rate of the active material, and improve the performance of the lithium-sulfur battery.

[0014] In some embodiments of the present invention, in step S1, the first electrolyte is selected from at least one of a sulfuric acid solution, a hydrochloric acid solution, and a nitric acid solution. The thickness of the conductive substrate can be controlled to be between 10 and 20 μm, typically approximately 18 μm. The first pair of electrodes can be platinum electrodes. After electrolysis, the conductive substrate can be further cleaned, specifically by rinsing with deionized water.

[0015] In some embodiments of the present invention, in step S2, a zinc salt solution is used as the second electrolyte, and the conductive base layer is used as the second working electrode, in conjunction with a second pair of electrodes for electrochemical deposition. Electrochemical deposition can be used to prepare nanoscale materials, and is simple to operate, low-cost, and highly efficient. It can also achieve uniform loading of metallic zinc and maintain the integrity and smoothness of the surface of the conductive base layer. The second pair of electrodes can be platinum electrodes. Specifically, a nano zinc oxide layer with a thickness of about 200 to 500 nm can be prepared by electrochemical deposition. If graphite paper is used as the conductive base layer, the thickness can be increased by 2 to 4 μm after electrolysis and electrochemical deposition in steps S1 and S2.

[0016] In some embodiments of the present invention, the zinc salt solution is selected from at least one of zinc sulfate solution, zinc chloride solution, and zinc nitrate solution.

[0017] In some embodiments of the present invention, the calcination is performed by burning the film material in air using an alcohol spray gun.

[0018] In a third aspect of the present invention, a lithium-sulfur battery is provided, comprising a positive electrode sheet, a separator and a negative electrode sheet stacked in sequence; and further comprising any one of the above intercalation composite films or an intercalation composite film prepared by any one of the above intercalation composite film preparation methods, wherein the intercalation composite film is arranged between the positive electrode sheet and the separator.

[0019] Among them, the positive electrode sheet may include a positive electrode current collector and a positive electrode active material layer coated on the surface of the positive electrode current collector. The material of the positive electrode active material layer may include a sulfur-based positive electrode active substance, a conductive agent and a binder. The sulfur-based positive electrode active substance may be a sulfur element or a sulfur-based composite material. Sulfur, as a positive electrode active substance, has a low conductivity, which will lead to a decrease in the utilization rate of the active substance sulfur and a decrease in the battery cycle performance. However, by adding a conductive agent to fully contact the active substance, the utilization efficiency of the active substance can be improved. The conductive agent can specifically be conductive carbon black, carbon nanotubes, etc. In addition, the addition of a binder can better combine the active substance with the conductive agent, enhance the conductivity, and maintain the stability of the internal structure during the charge and discharge of the battery.

[0020] The diaphragm may be one or more composite films of polyethylene, polypropylene, and polyvinylidene fluoride, but is not limited thereto and may also be other diaphragms. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0022] Figure 1 Schematic diagram of the preparation process of the intercalated composite film in Example 1;

[0023] Figure 2 Schematic diagram of the assembly structure of the lithium-sulfur battery in Example 2;

[0024] Figure 3 This is a surface SEM image of the intercalated composite film prepared in Example 1;

[0025] Figure 4 Graph showing the contact angle test results between the graphite paper obtained in step S1 of Example 1 and the finally prepared intercalation composite film and the electrolyte;

[0026] Figure 5 This is a graph showing the test results of polysulfide adsorption on the intercalated composite film prepared in Example 1;

[0027] Figure 6 Graph showing the charge-discharge cycle performance test results of the lithium-sulfur batteries of Example 2 and Comparative Examples 1 and 2;

[0028] Figure 7 Graph showing the test results of different rate cycle performance of lithium-sulfur batteries of Example 2 and Comparative Example 1. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0030] Example 1

[0031] This embodiment prepares an intercalation composite film, such as Figure 1 As shown, its preparation method is as follows:

[0032] S1. Prepare a 0.1 mol / L H2SO4 solution, then extract 30 mL of the H2SO4 solution and place it in a 50 mL electrolysis tank A as the electrolyte; clamp a 1 cm × 2 cm graphite paper at one end as the working electrode, and a platinum electrode of the same area at the other end as the counter electrode; immerse both the working electrode graphite paper and the counter electrode platinum electrode in the electrolyte in the electrolysis tank A, apply a 5 V direct current for electrolysis, and electrolyze the front and back sides of the graphite paper for 40 seconds each; then rinse the electrolyzed graphite paper three times with deionized water, soak it in deionized water and store it for later use;

[0033] S2. Prepare a 0.1 mol / L ZnSO4 solution, and then extract 30 mL of the ZnSO4 solution and place it in a 50 mL electrolysis tank B as the electrolyte; clamp the graphite paper obtained by electrolysis in step S1 at one end of the electrode as the working electrode, and use a platinum electrode of the same area as the counter electrode at the other end; immerse the working electrode graphite paper and the counter electrode platinum electrode in the electrolyte in the electrolysis tank B, apply a 5 V direct current for electrochemical deposition, and deposit on both sides of the graphite paper for 1 min; deposit neatly arranged flower clusters of metallic zinc on the graphite paper, rinse repeatedly with deionized water three times, and then soak in deionized water for 10 min to remove excess impurity ions, such as free zinc ions and sulfate ions.

[0034] S3. The graphite paper deposited with metallic zinc obtained in step S2 is placed in a crucible and burned in air with an alcohol spray gun for 30 seconds to convert the metallic zinc into zinc oxide, thereby obtaining an intercalated composite film.

[0035] Example 2

[0036] This embodiment prepares a lithium-sulfur battery, and its preparation method is as follows:

[0037] S1. Preparation of positive electrode sheet, including:

[0038] (1) Weigh 150 mg of battery-grade polyvinylidene fluoride (PVDF) powder into a vial. Use a 10 mL syringe to add 9.5 mL of N-methyl-2-pyrrolidone (NMP) as solvent. Add a stirring bar and stir at room temperature for 4 h to fully dissolve. Set aside.

[0039] (2) Weigh 700 mg of elemental sulfur (positive electrode material) and 200 mg of conductive agent Super-P, then use a syringe to draw 7 mL of PVDF solution with NMP as solvent from a vial and add them to a ball mill. Mill at a ball-to-material ratio of 50:1 and a speed of 1032 r / min for 3 h. Then collect the milled positive electrode slurry and seal it.

[0040] (3) Take a 20×20 cm glass plate, drop 0.5 mL of alcohol on it, place aluminum foil on the glass plate, and then drop about 0.5 mL of alcohol on the aluminum foil. Use paper to wipe the aluminum foil until it is flat; use a 150 mm scraper to evenly apply about 2 mL of the above-prepared positive electrode slurry on the aluminum foil; dry it on a 60 ° C hot plate for 12 hours, and after drying, use a roller press to press the sheet at a compression ratio of 1:1.5; slice the obtained positive electrode sheet with a 12 mm diameter cutter, weigh it, and select the active material mass of 1.1-1.2 mg / cm 2 The positive electrode discs on the left and right are used as positive electrode sheets and are collected and placed in an argon glove box for later use. The mass of the active material is obtained by the following method: weigh the cut positive electrode sheet and record the mass as m1. Then, weigh a copper foil of the same size and record the mass as m2. The mass of the active material is equal to (m1-m2)*active material ratio.

[0041] S2. A lithium sheet was used as the negative electrode sheet, a PP separator was used as the diaphragm, and the electrolyte was a mixed solvent of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) (1:1, v / v) containing 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.2M lithium nitrate (LiNO3). The intercalated composite film prepared in Example 1 was used to assemble a button cell. Figure 2 As shown, a lithium sheet 12 is placed in the positive electrode shell 11, and an appropriate amount of electrolyte is added. Then, a separator 13 and an intercalation composite film 14 are placed in sequence, and the electrolyte is added dropwise. Then, the positive electrode sheet 15, a gasket 16, and a spring 17 are placed. Finally, the negative electrode shell 18 is used to press the positive electrode shell 18, and the button battery is assembled, which is the product lithium-sulfur battery.

[0042] Comparative Example 1

[0043] In this comparative example, a lithium-sulfur battery was prepared. The difference between this comparative example and Example 2 is that in step S2, the intercalation composite film between the separator and the positive electrode sheet was eliminated. Other operations were the same as in Example 2.

[0044] Comparative Example 2

[0045] In this comparative example, a lithium-sulfur battery was prepared. The difference between this comparative example and Example 2 is that in step S2, the intercalation composite film between the diaphragm and the positive electrode sheet is eliminated; in addition, a graphite zinc oxide coating is provided on one surface of the PP diaphragm, and the PP diaphragm provided with the graphite zinc oxide coating is used as the diaphragm. During the battery assembly process, the side of the diaphragm provided with the graphite zinc oxide coating is oriented toward the positive electrode sheet; other operations are the same as in Example 2.

[0046] The preparation of the above diaphragm in this comparative example includes: mixing graphite paper and zinc oxide in a mortar and grinding them, and then mixing them with a binder and a solvent to form a slurry, wherein the mass ratio of the binder, graphite paper and zinc oxide is 1:8:1, and then coating the slurry on the surface of the PP diaphragm with a coating thickness of about 20 μm, and then drying to form a graphite tin oxide coating on the surface of the PP diaphragm.

[0047] Performance Testing

[0048] The surface of the intercalated composite film prepared in Example 1 was observed using a scanning electron microscope (SEM). Figure 3 As shown. Figure 3 As shown, the surface zinc oxide layer of the intercalated composite film prepared by the preparation method in Example 1 has a neatly arranged flower cluster structure.

[0049] Take the graphite paper obtained by step S1 in Example 1 and the intercalation composite film finally obtained, add a drop of lithium-sulfur electrolyte on the surface of each of them, and then measure the contact angle between the electrolyte and the electrolyzed graphite paper and the intercalation composite film. The results are as follows: Figure 4 shown. Figure 4 (a) is the test result of the contact angle between the electrolyte and the graphite paper after electrolysis, and the contact angle is 34.6°; (B) is the test result of the contact angle between the electrolyte and the intercalation composite film, and the contact angle is 23.1°. It can be seen that the contact angle between the electrolyte and the intercalation composite film is smaller than the contact angle with the graphite paper after electrolysis, which further indicates that lithium ions can pass through more quickly, which is more conducive to the electrochemical reaction.

[0050] In order to investigate the adsorption performance of the intercalation composite film prepared in this application on polysulfide, the intercalation composite film prepared in Example 1 was placed in a light yellow polysulfide solution (prepared by reacting elemental sulfur and lithium sulfide with a molar ratio of 1:1 in 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) (1:1, v / v) solvent) and allowed to stand for 1 hour for observation. The results are as follows: Figure 5 shown. Figure 5(a) shows the sample before adding the intercalation composite film to the polysulfide solution; (b) shows the sample after adding the intercalation composite film and allowing the solution to rest for 1 hour. The test results show that the color of the polysulfide solution changes from its initial pale yellow to clear and transparent after adding the intercalation composite film. This is because the zinc oxide in the intercalation composite film absorbs the polysulfide dissolved in the electrolyte.

[0051] In addition, at a rate of 0.2C, the voltage window was set to 0-2.8V, and the charge-discharge cycle performance of the lithium-sulfur batteries prepared in Example 2 and Comparative Examples 1-2 was tested. The results are shown in FIG. Figure 6 As shown. Figure 6 It can be seen that after 350 cycles of charge and discharge at a rate of 0.2C, the lithium-sulfur battery in Comparative Example 1 did not have an intercalated composite film between the PP separator and the positive electrode sheet, and its capacity was only 762.0 mAh / g; the lithium-sulfur battery in Example 2 had an intercalated composite film of Example 1 sandwiched between the PP separator and the positive electrode sheet, and the capacity of the battery after 350 cycles of charge and discharge at a rate of 0.2C was still 1100.2 mAh / g, and the capacity decay was significantly reduced; the lithium-sulfur battery in Comparative Example 2 had a graphite zinc oxide coating on the side of the PP separator facing the positive electrode sheet, and the capacity of the battery after 350 cycles of charge and discharge at a rate of 0.2C was 877.0 mAh / g, which was higher than that of Comparative Example 1, but still significantly lower than that of Example 2. From the above, the lithium-sulfur battery of Example 2 sandwiches the intercalated composite film of Example 1 between the PP separator and the positive electrode sheet, wherein zinc oxide is loaded between the layers and on the surface of the graphite paper. Zinc oxide can adsorb polysulfides generated by the sulfur-based positive electrode active material during the redox process and dissolve in the electrolyte. The adsorbed polysulfides can be utilized through the good conductivity of the graphite paper, thereby improving the utilization rate of the active material, improving the cycle performance of the battery, and reducing capacity attenuation.

[0052] In addition, the rate cycling performance of the lithium-sulfur batteries of Example 2 and Comparative Example 1 was tested at current densities of 0.2C, 0.5C, 1C, 1.5C, and 2C, respectively. The results are shown in FIG. Figure 7 As shown. Figure 7As shown, the initial discharge capacity of the lithium-sulfur battery in Comparative Example 1 at a current density of 0.2C is 1110.6 mAh / g, and the capacity decreases significantly after 5 cycles. When the current density increases to 0.5C, 1.0C, 1.5C, and 2.0C, the discharge capacities are 864.3 mAh / g, 647.7 mAh / g, 519.2 mAh / g, and 438.9 mAh / g, respectively. The reversible capacity of the lithium-sulfur battery in Example 2 at rates of 0.2C, 0.5C, 1C, 1.5C, and 2C are 1238.1 mAh / g, 1027.9 mAh / g, 914.6 mAh / g, 836.9 mAh / g, and 745.1 mAh / g, respectively. By comparison, it can be seen that the lithium-sulfur battery in Comparative Example 1 does not have an intercalated composite film between the PP separator and the positive electrode sheet, and the rate performance of the battery is significantly worse than that of the lithium-sulfur battery in Example 2. This indicates that the electrode electrochemical reaction kinetics is slow at higher current density, and the shuttling of lithium polysulfide during the test leads to serious loss of active materials. Therefore, its reversible specific capacity is low at a higher current density.

[0053] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A method for preparing an intercalated composite film, characterized in that: The intercalation composite film is configured for a lithium-sulfur battery and is sandwiched between a positive electrode sheet and a separator of the lithium-sulfur battery. The preparation method of the intercalation composite film comprises the following steps: S1. Using a conductive substrate having a layered structure as a working electrode, and cooperating with a first pair of electrodes to perform electrolysis in a first electrolyte to open the layered structure of the conductive substrate; the first electrolyte is selected from at least one of a sulfuric acid solution, a hydrochloric acid solution, and a nitric acid solution; S2, electrochemically depositing metallic zinc inside and on the surface of the conductive base layer obtained in step S1; S3. calcining the film material obtained in step S2 to convert the metallic zinc into zinc oxide, thereby obtaining an intercalated composite film.

2. The method for preparing the intercalated composite film according to claim 1, wherein: In step S2, a zinc salt solution is used as the second electrolyte, and the conductive base layer is used as the second working electrode to cooperate with the second pair of electrodes to perform electrochemical deposition.

3. The method for preparing an intercalated composite film according to claim 2, wherein the zinc salt solution is selected from at least one of a zinc sulfate solution, a zinc chloride solution, and a zinc nitrate solution.

4. The method for preparing the intercalated composite film according to claim 1, wherein: In step S3, the calcination is performed by burning the film material in air using an alcohol spray gun.

5. An intercalated composite film, characterized in that: The intercalation composite film is prepared by the preparation method of any one of claims 1 to 4; the intercalation composite film comprises a first conductive base layer, wherein the interior and surface of the first conductive base layer are loaded with zinc oxide.

6. The intercalation composite film according to claim 5, characterized in that The first conductive base layer is selected from at least one of graphite paper, carbon fiber paper, and carbon cloth.

7. The intercalation composite film according to claim 5, characterized in that The zinc oxide is nano zinc oxide.

8. The intercalation composite film according to claim 5, characterized in that The thickness of the intercalated composite film is 15-30 μm.

9. A lithium-sulfur battery comprising a stacked positive electrode sheet, a separator, and a negative electrode sheet, characterized in that: It also includes an intercalation composite film prepared by the method for preparing an intercalation composite film according to any one of claims 5 to 8, wherein the intercalation composite film is sandwiched between the positive electrode sheet and the separator.

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