Lithium-sulfur battery diaphragm and preparation method thereof

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a lithium-sulfur battery diaphragm and a preparation method thereof, and the preparation method comprises the following steps: (1) dispersing a conductive agent into a solvent, and stirring to obtain a mixture 1; (2) adding an adhesive into the mixture 1, and stirring to obtain coating slurry; and (3) coating the surface of a diaphragm substrate with the coating slurry, and performing vacuum drying to obtain the lithium-sulfur battery diaphragm, according to the lithium-sulfur battery diaphragm obtained by the invention, the shuttle effect of polysulfide in the battery can be effectively inhibited, and the cycle performance and the rate capability of the lithium-sulfur battery are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur batteries, and particularly relates to a separator for lithium-sulfur batteries and a preparation method thereof. Background Art

[0002] With the continuous development of the economic society, energy problems and environmental problems are becoming increasingly severe. Energy conservation and emission reduction, development and utilization of new energy and renewable energy, and development of efficient and clean energy conversion and storage technologies are important topics and challenges in today's society, scientific and technological circles, and industrial circles. Lithium-sulfur batteries have attracted the attention of researchers due to their incomparable high specific energy and other properties. In recent years, relevant research work at home and abroad has been quite active, and it is currently in the critical stage of technological breakthrough.

[0003] The lithium-sulfur battery uses high-specific-capacity elemental sulfur as the positive electrode material, and its theoretical energy density can reach 2600 Wh·kg -1 . At the same time, elemental sulfur also has the advantages of rich reserves, low price, and environmental friendliness. Therefore, lithium-sulfur batteries are considered to be a new battery system with great research value and development potential. However, the polysulfides (Li2S x (x = 4 - 8)) generated during the cycling of the sulfur positive electrode in the lithium-sulfur battery are extremely soluble in the electrolyte. The dissolved high-valence polysulfides will migrate to the lithium metal negative electrode under the action of the concentration gradient and react with it to form short-chain polysulfides; the short-chain polysulfides then diffuse back to the positive electrode and are oxidized into long-chain polysulfides again, resulting in a "shuttle effect" inside the battery. This shuttle effect will cause continuous consumption of the lithium negative electrode and the active material, resulting in a low Coulomb efficiency, and will seriously affect the high-voltage plateau region in the charge-discharge curve.

[0004] Regarding the "shuttle" effect caused by the dissolution and migration of polysulfides, the current solutions are very limited, and people mostly start from the perspective of the electrolyte. One method is to add lithium nitrate as an additive to the electrolyte. The addition of lithium nitrate can form a protective layer on the surface of the lithium anode, but it will also have an impact on the battery cathode. The generated lithium sulfide deposits on the cathode, hindering mass transfer and electron conduction. Another method is to add toluene, methyl acetate, etc. to the electrolyte to inhibit the dissolution of polysulfides, but this method is prone to cause a decrease in conductivity. The third method is to use a composite polymer gel electrolyte diaphragm. The gel electrolyte is a gel polymer network with a suitable microporous structure formed by a polymer, a plasticizer (lithium salt solvent, ionic liquid, etc.), and a lithium salt. The ion conduction is achieved by the liquid electrolyte molecules fixed in the microstructure in the polymer network system swollen by the solvent. Its unique network structure makes the gel have both the cohesion of a solid and the dispersion and conductivity of a liquid. Since the electrolyte solution is "coated" within the polymer network, the dissolution of polysulfides is inhibited, thereby possibly solving the problem of the loss of sulfur active substances to a certain extent; however, the conductivity and strength of the gel electrolyte diaphragm are both relatively low.

[0005] Therefore, it is necessary to develop a lithium-sulfur battery diaphragm with high battery efficiency and stability and its preparation method. Summary of the Invention

[0006] Based on the deficiencies in the prior art, the present invention aims to provide a lithium-sulfur battery diaphragm with high battery efficiency and stability and its preparation method.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] On the one hand, the present invention provides a preparation method of a lithium-sulfur battery diaphragm, comprising the following steps:

[0009] (1) Disperse the conductive agent into the solvent and stir to obtain mixture 1;

[0010] (2) Add the binder to mixture 1 and stir to obtain the coating slurry;

[0011] (3) Coat the coating slurry on the surface of the diaphragm substrate and dry it under vacuum to obtain the lithium-sulfur battery diaphragm.

[0012] The conductive agent described in the above step (1) is a mixture of modified carbon nanotubes and carbon black; the mass ratio of the two is 1:2 - 3; preferably 1:2.

[0013] In some embodiments, the modified carbon nanotubes are carbon nanotubes modified with a silane coupling agent.

[0014] In some embodiments, the preparation method of the carbon nanotubes modified with a silane coupling agent is:

[0015] The carbon nanotubes are acidified, then a silane coupling agent and water are added and reacted under the condition of 70 - 80 °C. After the reaction is completed, filtration, washing, and drying are carried out to obtain silane coupling agent - modified carbon nanotubes.

[0016] In some embodiments, the acidification step is as follows: The carbon nanotubes and a strong oxidizing acid are mixed evenly by ultrasonic oscillation, and an oxidation reaction is carried out under the condition of 70 - 80 °C. After the oxidation reaction is completed, filtration, washing, and vacuum drying are carried out to obtain acidified carbon nanotubes.

[0017] In some embodiments, the silane coupling agent is 3 - aminopropyltriethoxysilane.

[0018] In some embodiments, the mass fraction of the aqueous solution of the silane coupling agent is 50 - 60%.

[0019] In some embodiments, the mass ratio of the silane coupling agent to the acidified carbon nanotubes is 2 - 3:1;

[0020] In some embodiments, the mass ratio of the silane coupling agent to the acidified carbon nanotubes is 2.5 - 3:1;

[0021] In some embodiments, the mass ratio of the silane coupling agent to the acidified carbon nanotubes is 2.5:1.

[0022] In some embodiments, the mass - to - volume ratio of the acidified carbon nanotubes to water is 1 g:20 - 30 mL.

[0023] In some embodiments, the strong oxidizing acid is aqua regia.

[0024] In some embodiments, the mass - to - volume ratio of the carbon nanotubes to aqua regia is 1 g:20 - 30 mL.

[0025] In some embodiments, the drying temperature is drying at 80 - 100 °C for 10 - 15 h.

[0026] The carbon nanotubes are single - wall carbon nanotubes or multi - wall carbon nanotubes; the length of the carbon nanotubes is 30 - 50 microns, and the diameter is 15 - 25 nm.

[0027] In the implementation process of the present invention, using a silane coupling agent to modify carbon nanotubes significantly improves the dispersion performance of carbon nanotubes, enabling them to better mix with other components during the preparation of the slurry, making the coating uniformity better, and thus achieving a better conductive effect.

[0028] During the implementation of the present invention, it was unexpectedly found that using a mixture of modified carbon nanotubes and carbon black as a conductive agent, a separator prepared by controlling the mass ratio of the two to be 1:2 - 3 can not only endow the lithium-sulfur battery with good electrochemical performance, but also significantly improve the cycling stability of the lithium-sulfur battery at high rates.

[0029] In addition, using a mixture of carbon nanotubes and carbon black as a conductive agent in the present invention, by controlling the mass ratio of the two to be 1:2 - 3, it can be relatively fully dissolved in the solvent without the need to additionally add a dispersant during the dispersion process, saving time and materials and being easy to process.

[0030] The solvent described in the above step (1) is ethanol.

[0031] The binder described in the above step (2) is a mixture of bisphenol A epoxy resin and sodium carboxymethyl cellulose, and the mass ratio of the two is 3 - 5:1; preferably 4:1.

[0032] Using a mixture of bisphenol A epoxy resin and sodium carboxymethyl cellulose as a binder in the present invention and controlling the mass ratio of the two to be 3 - 5:1 can not only improve the fluidity and uniformity of the coating material, making the coating thickness more uniform; but also can adjust the porosity of the coating, enabling lithium ions to be smoothly transmitted and improving the ion conduction performance of the battery.

[0033] The separator substrate described in the above step (3) is one or more of polypropylene, polyethylene, polyimide, polyvinylidene fluoride, polytetrafluoroethylene, and glass ceramics.

[0034] The thickness of the separator substrate is 20 - 30 μm; the thickness of the coating is 10 - 15 μm.

[0035] The temperature of the vacuum drying described in the above step (3) is 50 - 60 °C, and the time is 10 - 20 hours.

[0036] On the other hand, the present invention also provides a lithium-sulfur battery separator prepared by the above method.

[0037] On yet another aspect, the present invention also provides a lithium-sulfur battery having the above lithium-sulfur battery separator.

[0038] The lithium-sulfur battery separator prepared by the above method of the present invention can effectively inhibit the "shuttle effect" of polysulfides in the battery and significantly improve the cycling performance and rate performance of the lithium-sulfur battery.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] (1) In the implementation process of the present invention, the modification of carbon nanotubes with silane coupling agent significantly improves the dispersion performance of carbon nanotubes, enabling them to mix better with other components during the preparation of the slurry, resulting in better coating uniformity and thus achieving a better conductive effect.

[0041] Using a mixture of modified carbon nanotubes and carbon black as the conductive agent, the separator prepared by controlling the mass ratio of the two to be 1:2 - 3 can not only endow the lithium-sulfur battery with good electrochemical performance, but also significantly improve the cycling stability of the lithium-sulfur battery at high rates. In addition, using a mixture of carbon nanotubes and carbon black as the conductive agent in the present invention, by controlling the mass ratio of the two to be 1:2 - 3, it can be relatively fully dissolved in the solvent without the need to additionally add a dispersant during the dispersion process, saving time and materials and being easy to process.

[0042] (2) The present invention uses a mixture of bisphenol A epoxy resin and sodium carboxymethyl cellulose as the binder, and by controlling the mass ratio of the two to be 3 - 5:1, it can not only improve the fluidity and uniformity of the coating material, making the coating thickness more uniform, but also adjust the porosity of the coating, enabling lithium ions to be smoothly transmitted and improving the ion conduction performance of the battery.

[0043] (3) The lithium-sulfur battery separator prepared by the above method of the present invention can effectively inhibit the "shuttle effect" of polysulfides in the battery, and significantly improve the cycling performance and rate performance of the lithium-sulfur battery. Detailed implementation method

[0044] Next, the examples and comparative examples of the present invention are shown to illustrate the composition of the present invention in more detail, but the present invention is not limited thereto.

[0045] The present invention does not limit the source of the raw materials used. Unless otherwise specified, the raw materials used in the present invention are all ordinary commercially available products in the technical field.

[0046] Basic Example 1: Preparation method of modified carbon nanotubes:

[0047] The preparation method of the modified carbon nanotubes, which are carbon nanotubes modified with silane coupling agent, is as follows:

[0048] (1) Mix carbon nanotubes and aqua regia (mass-to-volume ratio of 1 g:25 mL) evenly by ultrasonic oscillation, carry out an oxidation reaction at 75 °C. After the oxidation reaction is completed, filter and wash until neutral, and then dry at 90 °C for 12 h to obtain acidified carbon nanotubes;

[0049] (2) Add the acidified carbon nanotubes and 3-aminopropyltriethoxysilane to water (the mass ratio of 3-aminopropyltriethoxysilane to acidified carbon nanotubes is 2.5:1, and the mass-volume ratio of acidified carbon nanotubes to water is 1 g:25 mL), and react under the condition of 80 °C. After the reaction, filter, wash, and dry at 90 °C for 12 h to obtain the carbon nanotubes modified with silane coupling agent.

[0050] The carbon nanotubes described are single-walled carbon nanotubes or multi-walled carbon nanotubes; the length of the carbon nanotubes is 30 - 50 microns, and the diameter is 15 - 25 nm.

[0051] Example 1: A preparation method of a lithium-sulfur battery separator

[0052] It includes the following steps:

[0053] (1) Disperse the modified carbon nanotubes prepared in Basic Example 1 and carbon black (mass ratio 1:2) into ethanol (the material-liquid ratio is 1 g:25 mL), stir to obtain Mixture 1;

[0054] (2) Add bisphenol A epoxy resin and sodium carboxymethyl cellulose with a mass ratio of 3:1 to Mixture 1, and stir to obtain the coating slurry;

[0055] (3) Coat the coating slurry on the surface of the separator substrate, and dry it in vacuum at 55 °C for 10 hours to obtain the lithium-sulfur battery separator.

[0056] The thickness of the separator substrate described is 20 μm; the thickness of the coating is 10 μm.

[0057] Basic Example 2: A preparation method of modified carbon nanotubes:

[0058] The preparation method of the modified carbon nanotubes, which are carbon nanotubes modified with silane coupling agent, is as follows:

[0059] (1) Mix the carbon nanotubes and aqua regia (mass-volume ratio 1 g:30 mL) evenly by ultrasonic oscillation, carry out the oxidation reaction under the condition of 75 °C. After the oxidation reaction is completed, filter, wash until neutral, and dry at 90 °C for 12 h to obtain the acidified carbon nanotubes;

[0060] (2) Add the acidified carbon nanotubes and 3-aminopropyltriethoxysilane to water (the mass ratio of 3-aminopropyltriethoxysilane to acidified carbon nanotubes is 2:1, and the mass-volume ratio of acidified carbon nanotubes to water is 1 g:30 mL), and react under the condition of 80 °C. After the reaction, filter, wash, and dry at 90 °C for 12 h to obtain the carbon nanotubes modified with silane coupling agent.

[0061] The carbon nanotubes described are single-walled carbon nanotubes or multi-walled carbon nanotubes; the length of the carbon nanotubes is 30-50 microns, and the diameter is 15-25 nm.

[0062] Example 2: A method for preparing a lithium-sulfur battery separator

[0063] It includes the following steps:

[0064] (1) Disperse the modified carbon nanotubes prepared in Basic Example 1 and carbon black (mass ratio 1:3) into ethanol (material-liquid ratio 1 g: 30 mL), and stir to obtain Mixture 1;

[0065] (2) Add bisphenol A epoxy resin and sodium carboxymethyl cellulose with a mass ratio of 4:1 to Mixture 1, and stir to obtain the coating slurry;

[0066] (3) Coat the coating slurry on the surface of the separator matrix, and vacuum dry at 55 °C for 15 hours to obtain the lithium-sulfur battery separator.

[0067] The thickness of the separator matrix described is 20 μm; the thickness of the coating is 10 μm.

[0068] Comparative Example 1

[0069] The difference from Example 1 is that the carbon nanotubes are unmodified carbon nanotubes, and the others are the same as in Example 1.

[0070] Comparative Example 2

[0071] The difference from Example 1 is that the conductive agent is only modified carbon nanotubes, and the others are the same as in Example 1.

[0072] Comparative Example 3

[0073] The difference from Example 1 is that the conductive agent is only carbon black, and the others are the same as in Example 1.

[0074] Comparative Example 4

[0075] The difference from Example 1 is that the binder is only bisphenol A epoxy resin, and the others are the same as in Example 1.

[0076] Application Example:

[0077] Assembly of the lithium-sulfur battery: Prepare a sulfur positive electrode with a carbon-sulfur composite material, use a lithium metal sheet as the negative electrode, and assemble a button battery with 1 layer of the separators prepared in Examples 1-2 and Comparative Examples 1-3, and fill it with an electrolyte. The electrolyte of the electrolyte of the lithium-sulfur battery is LiTFSI, and the solvent of the electrolyte is a mixture of ethylene glycol dimethyl ether and 1,3-dioxolane with a volume ratio of 1:1.

[0078] Effect data: Detection of the battery cycle performance at different rates (cycle 500 times)

[0079] Electrochemical performance test of the battery: The battery was charged and discharged at room temperature (25°C) using a charge and discharge instrument. The charge and discharge voltage range was 1.5V - 2.8V. The test results are shown in Table 1 below.

[0080] Table 1

[0081]

[0082]

[0083] From the test results in Table 1 above, it can be seen that the battery separators prepared in Example 1 and Example 2 can significantly improve the battery cycle performance of lithium-sulfur batteries when used in lithium-sulfur batteries. Especially in Example 1, by controlling the mass ratio of the conductive agent-modified carbon nanotubes and carbon black to be 1:2, the conductive effect of the separator can be significantly improved. And by controlling the mass ratio of the binder bisphenol A epoxy resin and sodium carboxymethyl cellulose to be 3:1, not only can the fluidity and uniformity of the coating material be improved, making the coating thickness more uniform; but also the porosity of the coating can be adjusted to enable the smooth transmission of lithium ions, significantly improving the cycle stability of the battery. Especially when cycling 500 times at a high rate, its stability remains above 80%.

[0084] In Comparative Example 1, unmodified carbon nanotubes were used. Due to the large aspect ratio of carbon nanotubes, they are prone to entanglement and bonding together to form large aggregates, which will seriously hinder the dispersion of carbon nanotubes, thus affecting the conductive performance of the separator and significantly reducing the cycle stability of lithium-sulfur batteries.

[0085] In Comparative Example 2, only modified carbon nanotubes were used as the conductive agent. Its cycle stability can reach over 90% at a rate of 0.5C, but when the rate increases to 3C or 5C, its cycle stability significantly decreases. The predicted reason may be that although the dispersibility of the modified carbon nanotubes has been improved to a certain extent, it cannot really improve the entanglement and bonding to form aggregates. Therefore, when the charging rate increases, it will affect the stability of the separator and significantly reduce the cycle stability.

[0086] In Comparative Example 3, only carbon black was used as the conductive agent. Due to the excessive addition of carbon black, it may cause the instability of the separator structure and cracks to appear during the cycle, thus reducing the cycle performance.

[0087] In Comparative Example 4, when the binder was only bisphenol A epoxy resin, the cycle stability of the lithium-sulfur battery significantly decreased. The predicted reason is that: bisphenol A epoxy resin has a relatively high viscosity. Using only bisphenol A epoxy resin will affect the fluidity and uniformity of the separator raw materials, making the porosity of the coating uneven, thus affecting the transmission of lithium ions and reducing the cycle performance of the lithium-sulfur battery.

[0088] In summary, when using a mixture of modified carbon nanotubes and carbon black as the conductive agent and controlling the mass ratio of the two to be 1:2-3, and using a mixture of bisphenol A epoxy resin and sodium carboxymethyl cellulose as the binder and controlling the mass ratio of the two to be 3-5:1, the obtained separator has better cycle stability.

[0089] The above embodiments are merely examples clearly described and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or variations derived therefrom are still within the protection scope of the present invention.

Claims

1. A preparation method of a lithium-sulfur battery separator, comprising the following steps: (1) Disperse a conductive agent into a solvent and stir to obtain mixture 1; (2) Add an adhesive to mixture 1 and stir to obtain a coating slurry; (3) Coat the coating slurry on the surface of a separator substrate and dry it under vacuum to obtain the lithium-sulfur battery separator; Among them, In step (1), the conductive agent is a mixture of modified carbon nanotubes and carbon black, and the mass ratio of the two is 1:2 - 3.

2. The preparation method according to claim 1, characterized in that: The modified carbon nanotubes are carbon nanotubes modified with a silane coupling agent.

3. The preparation method according to claim 2, characterized in that: The preparation method of the carbon nanotubes modified with a silane coupling agent is as follows: Acidify the carbon nanotubes, then add a silane coupling agent and water and react under the condition of 70 - 80 °C. After the reaction ends, filter, wash, and dry to obtain the carbon nanotubes modified with a silane coupling agent.

4. The preparation method according to claim 3, characterized in that: The acidification step is as follows: Mix the carbon nanotubes and a strong oxidizing acid evenly by ultrasonic oscillation, carry out an oxidation reaction under the condition of 70 - 80 °C. After the oxidation reaction is completed, filter, wash, and dry under vacuum to obtain the acidified carbon nanotubes.

5. The preparation method according to claim 3, characterized in that: The silane coupling agent is 3-aminopropyltriethoxysilane; the mass ratio of the silane coupling agent to the acidified carbon nanotubes is 2 - 3:1; the mass-volume ratio of the acidified carbon nanotubes to water is 1 g:20 - 30 mL.

6. The preparation method according to claim 4, characterized in that: The strong oxidizing acid is aqua regia; the mass-volume ratio of the carbon nanotubes to aqua regia is 1 g:20 - 30 mL.

7. The preparation method according to claim 1, characterized in that: In step (2), the adhesive is a mixture of bisphenol A epoxy resin and sodium carboxymethyl cellulose, and the mass ratio of the two is 3 - 5:

1.

8. The preparation method according to claim 1, characterized in that: In step (3), the separator substrate is one or more of polypropylene, polyethylene, polyimide, polyvinylidene fluoride, polytetrafluoroethylene, and glass ceramics; the thickness of the separator substrate is 20 - 30 μm; the thickness of the coating is 10 - 15 μm.

9. A lithium-sulfur battery separator prepared by the preparation method according to any one of claims 1 - 8.

10. Use of the lithium-sulfur battery separator obtained by the preparation method according to any one of claims 1 - 8 in the preparation of a lithium-sulfur battery.

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