Composite diaphragm, preparation method thereof and lithium-sulfur battery comprising composite diaphragm

Abstract

The invention provides a composite diaphragm, a preparation method thereof and a lithium-sulfur battery comprising the composite diaphragm. The composite diaphragm provided by the invention comprises a base membrane and a coating arranged on the surface of the base membrane, wherein the coating comprises layered nickel-cobalt-aluminum-lithium oxide, a conductive agent and a binder. The composite diaphragm provided by the invention can effectively improve the capacity and cycle performance of the lithium-sulfur battery.

Description

Technical Field

[0001] The present invention relates to the field of battery separators, and particularly to a composite separator, a preparation method thereof, and a lithium-sulfur battery comprising the composite separator. Background Art

[0002] With the increasing energy demand, it is crucial to explore alternative energy storage systems with higher specific capacities to alleviate the over-reliance on lithium-ion batteries. Due to its high theoretical specific capacity (1675 mAh / g) and theoretical energy density (2600 Wh / kg), the lithium-sulfur battery is highly competitive in the next generation of high-energy secondary batteries. In addition, the positive electrode active material sulfur has the advantages of rich natural resources, low price, and environmental friendliness.

[0003] However, there are still many problems in the commercial application of current lithium-sulfur batteries. The side reaction "shuttle effect" inside the battery is the biggest obstacle restricting its application. Specifically, the shuttle effect means that a series of sulfur-containing intermediate products are generated during the electrochemical charge and discharge process of lithium-sulfur batteries. Long-chain polysulfides (Li2S x , 2 < x ≤ 8) are soluble in the electrolyte, and then under the action of the concentration gradient and electric field, they penetrate through the conventional polyolefin separator and finally react chemically with the lithium negative electrode, resulting in the loss of active substances and an increase in the internal resistance of the battery, thereby leading to a decrease in battery capacity and cycle performance. In addition, the charge and discharge process of lithium-sulfur batteries involves multiple solid-liquid conversions, and the electronic conductivities of sulfur and the discharge end product lithium sulfide are poor. Therefore, the conversion reaction kinetics of Li2S x is slow, and the battery rate performance is poor.

[0004] At present, in order to improve the electrochemical performance of lithium-sulfur batteries, some promising strategies have been explored and adopted. Among them, coating a modification layer on the surface of the separator is considered to be the most convenient and economical strategy for restricting Li2S x shuttle and accelerating Li2S x conversion because it has little impact on the energy density of the battery and is compatible with the existing battery manufacturing process. Commonly used separator modification materials include porous carbon, inorganic compounds, metal-organic framework materials, etc., but they have problems such as limited effects, complex synthesis processes, and high costs.

[0005] In order to improve the electrochemical performance of lithium-sulfur batteries to meet the requirements of new energy electric vehicles and large-capacity energy storage systems for higher energy density batteries, there is still a need to develop high-performance battery separators. Summary of the Invention

[0006] In view of the above problems of the prior art, the present invention provides a composite separator that can improve the capacity and cycle performance of lithium-sulfur batteries.

[0007] A first aspect of the present invention provides a composite diaphragm, comprising a base film and a coating disposed on the surface of the base film, wherein the coating comprises layered nickel-cobalt-aluminum-lithium oxide, a conductive agent and a binder.

[0008] The second aspect of the present invention provides a method for preparing the composite diaphragm according to the first aspect, comprising the following steps:

[0009] S1: mixing layered nickel-cobalt-aluminum lithium oxide with a conductive agent to obtain a mixture;

[0010] S2: dispersing the mixture and the binder in a solvent to obtain a coating slurry;

[0011] S3: coating the coating slurry on the surface of the base film, and then removing the solvent to obtain the composite diaphragm.

[0012] The third aspect of the present invention provides the use of the composite diaphragm described in the first aspect or the composite diaphragm prepared by the preparation method described in the second aspect in a secondary battery.

[0013] A fourth aspect of the present invention provides a lithium-sulfur battery, comprising a positive electrode, a negative electrode and a separator disposed between the positive electrode and the negative electrode, wherein the separator comprises the composite separator described in the first aspect or the composite separator prepared by the preparation method described in the second aspect.

[0014] The present invention has the following advantages:

[0015] The composite coating of the composite diaphragm of the present invention contains layered nickel-cobalt-aluminum lithium oxide, which can chemically adsorb polysulfides generated inside the lithium-sulfur battery and catalytically accelerate the conversion of polysulfides during the charge and discharge period, thereby improving the capacity and cycle performance of the lithium-sulfur battery. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with embodiments. The specific embodiments described herein are only used to explain the present invention and are not intended to constitute any limitation of the present invention. In addition, in the following description, the description of known structures and technologies is omitted to avoid unnecessary confusion of the concepts of the present disclosure. Such structures and technologies are also described in many publications.

[0017] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0018] The invention provides a composite diaphragm, which comprises a base film and a coating arranged on the surface of the base film, wherein the coating comprises a layered nickel-cobalt-aluminum-lithium oxide, a conductive agent and a binder.

[0019] The present invention adds layered nickel-cobalt-aluminum lithium oxide to the composite coating. On the one hand, the layered nickel-cobalt-aluminum lithium oxide can chemically adsorb polysulfides generated inside the lithium-sulfur battery and catalytically accelerate the conversion of polysulfides during the charge and discharge period. On the other hand, compared with layered nickel-cobalt-manganese lithium oxide, the layered nickel-cobalt-aluminum lithium oxide has better affinity with the base film and can form a more uniform and dense coating, thereby effectively improving the capacity and cycle performance of the lithium-sulfur battery.

[0020] In some embodiments, the coating has a thickness of 1 μm-30 μm. In some embodiments, the coating has a thickness of 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 23 μm, 25 μm, 27 μm, or any value therebetween. In some embodiments, the coating has a thickness of 2 μm-10 μm.

[0021] In the present invention, the coating layer is located on one side or on both sides of the base film, and the thickness of the coating layer in the present invention refers to the thickness of the coating layer located on one side of the base film.

[0022] In some embodiments, in the coating, the mass ratio of the conductive agent to the layered nickel cobalt aluminum lithium oxide is (0.01-1):1. In some embodiments, the mass ratio of the conductive agent to the layered nickel cobalt aluminum lithium oxide is 0.05:1, 0.08:1, 0.10:1, 0.13:1, 0.15:1, 0.17:1, 0.20:1, 0.23:1, 0.25:1, 0.27:1, 0.30:1, 0.33:1, 0.35:1, 0.37:1, 0.40:1, 0.43:1, 0.45:1, 0.47:1, 0.50:1, 0.55:1, 0.60:1, 0.65:1, 0.70:1, 0.75:1, 0.80:1, 0.85:1, 0.90:1, 0.95:1 or any value therebetween. In some embodiments, in the coating, the mass ratio of the conductive agent to the layered nickel cobalt aluminum lithium oxide is (0.05-0.50): 1. In some embodiments, in the coating, the mass ratio of the conductive agent to the layered nickel cobalt aluminum lithium oxide is (0.05-0.30): 1, for example, 0.05: 1, 0.06: 1, 0.07: 1, 0.08: 1, 0.09: 1, 0.10: 1, 0.20: 1, 0.30: 1 or any value therebetween.

[0023] In some embodiments, in the coating, the mass ratio of the binder to the layered nickel cobalt aluminum lithium oxide is (0.01-0.5): 1. In some embodiments, in the coating, the mass ratio of the binder to the layered nickel cobalt aluminum lithium oxide is (0.05-0.15): 1, for example, 0.05: 1, 0.06: 1, 0.07: 1, 0.08: 1, 0.09: 1, 0.1: 1, 0.12: 1, 0.13: 1, 0.14: 1, 0.15: 1 or any value therebetween.

[0024] In some embodiments, the layered nickel-cobalt-aluminum lithium oxide is selected from LiNi 1-m-n Co m Al n O2, 0<m≤1, 0<n≤1. In some embodiments, the layered nickel cobalt aluminum lithium oxide is selected from LiNi 0.8 Co 0.15 Al 0.05 O2.

[0025] In the present invention, there is no special requirement for the base film, and any commonly used base film can be used in the present invention. According to some embodiments of the present invention, the material of the base film is selected from one or more of polyethylene, polypropylene, polyimide, polyacrylonitrile, polyethylene terephthalate, polytetrafluoroethylene and polyvinylidene fluoride. In some embodiments, the material of the base film is polyethylene and / or polypropylene.

[0026] In the present invention, there is no special requirement for the conductive agent, and commonly used conductive agents can be used in the present invention. According to some embodiments of the present invention, the conductive agent includes but is not limited to at least one of graphene, graphene oxide, reduced graphene oxide, carbon nanotubes, carbon nanofibers, acetylene black, Super P and Ketjen black.

[0027] In the present invention, there is no special requirement for the binder, and commonly used binders can be used in the present invention. According to some embodiments of the present invention, the binder includes but is not limited to at least one of polyvinyl alcohol, carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl pyrrolidone, styrene-butadiene rubber and polyacrylate.

[0028] The present invention provides a method for preparing a composite diaphragm, which comprises the following steps:

[0029] S1: mixing layered nickel-cobalt-aluminum lithium oxide with a conductive agent to obtain a mixture;

[0030] S2: dispersing the mixture and the binder in a solvent to obtain a coating slurry;

[0031] S3: coating the coating slurry on the surface of the base film, and then removing the solvent to obtain the composite diaphragm.

[0032] According to a preferred embodiment of the present invention, the optional range of the types of the solvent is relatively wide. According to a preferred embodiment of the present invention, the solvent includes but is not limited to at least one of deionized water, anhydrous ethanol, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide and N-methylpyrrolidone.

[0033] In the present invention, the coating method in S3 can be a conventional method in the art. According to a preferred embodiment of the present invention, the coating in S3 is achieved by at least one of casting, blade coating, spraying, spin coating, gravure roller coating, and slot extrusion.

[0034] The present invention provides application of the composite diaphragm of the present invention in a secondary battery such as a sulfur-lithium battery.

[0035] The present invention provides a lithium-sulfur battery, which comprises a positive electrode, a negative electrode and a diaphragm arranged between the positive electrode and the negative electrode, wherein the diaphragm comprises the composite diaphragm described in the present invention.

[0036] The present invention will be described in detail below through examples.

[0037] In the present invention, the assembly of the lithium-sulfur battery and the testing process of the battery performance are as follows:

[0038] 1. Assembly of lithium-sulfur battery samples:

[0039] Prepare the positive electrode: mix the active material sublimated sulfur, the conductive agent Ketjen black, and the binder polyvinylidene fluoride in a weight ratio of 6:3:1, add N-methylpyrrolidone to form a positive electrode slurry; coat the positive electrode slurry on an aluminum foil, and dry it to obtain a positive electrode.

[0040] Assemble 2025 button cells in an argon glove box with a water and oxygen content of less than 0.1 ppm: assemble in the order of positive electrode shell, positive electrode, separator, lithium negative electrode, nickel foam, and negative electrode shell, and add 100 μL of electrolyte.

[0041] The electrolyte used was a 1,3-dioxolane / ethylene glycol dimethyl ether (DOL / DME, volume ratio of 1:1) mixed solution containing 1 mol / L lithium bis(trifluoromethylsulfonyl)imide and 0.2 mol / L lithium nitrate.

[0042] 2. The rate performance and cycle performance of the lithium-sulfur battery samples prepared as described above were tested by constant current charge and discharge tests.

[0043] (1) Rate performance test:

[0044] The lithium-sulfur battery sample was charged and discharged for 5 cycles at 1C (1C = 1675 mA / g) in the voltage range of 1.7-2.7 V. The charge specific capacity of the corresponding charge and discharge cycle was recorded, and the average charge specific capacity of the 5 cycles was calculated:

[0045] Average charge capacity of 5 cycles at 1C = (sum of charge capacities from 1st to 5th cycle) / 5

[0046] Similarly, the charge-discharge cycle was repeated for 5 cycles at 2C and 3C, and the average charge specific capacity of the 5 cycles at 2C and 3C was measured.

[0047] (2) Cyclic performance test:

[0048] The lithium-sulfur battery sample was charged and discharged for 2 cycles at 0.1C and 0.2C in the voltage range of 1.7-2.7V, and then the lithium-sulfur battery sample was cycled for 100 cycles at 0.5C. The charge specific capacity after the first cycle at 0.5C and the 100th cycle was recorded. The capacity retention rate after 100 cycles at 0.5C was calculated according to the following formula to characterize the cycle performance.

[0049] Capacity retention rate after 100 cycles = charge specific capacity at the 100th cycle / charge specific capacity at the 1st cycle × 100%.

[0050] 3. LiNi in the present invention 0.8 Co 0.15 Al 0.05 O2 was purchased from a commercial product.

[0051] Example 1

[0052] Preparation of composite diaphragm:

[0053] (1) 0.40 g LiNi 0.8 Co 0.15 Al 0.05 O2 and 0.05 g Super P were ground and mixed in a mortar to obtain a mixture in which Super P and LiNi 0.8 Co 0.15 Al 0.05 The mass ratio of O2 is 0.125:1;

[0054] (2) dispersing the mixture and 0.05 g of polyvinylidene fluoride in N-methylpyrrolidone, stirring and mixing evenly to obtain a coating slurry;

[0055] (3) The coating slurry is evenly coated on one side of the polyethylene / polypropylene base film by a doctor blade method, and then dried to remove the solvent, thereby obtaining a composite diaphragm having a composite coating evenly distributed on one side of the polymer base film, wherein the thickness of the composite coating is 6 μm.

[0056] Performance test of composite diaphragm:

[0057] According to the description of the test method above, the prepared composite separator was used to assemble lithium-sulfur battery samples and perform performance tests. The specific test results are shown in Table 1.

[0058] Example 2

[0059] The difference between Example 2 and Example 1 is that 0.44 g LiNi 0.8 Co 0.15 Al 0.05 O2 and 0.01gSuperP, that is, Super P and LiNi 0.8 Co 0.15 Al 0.05 The mass ratio of O2 is 0.02:1. The prepared composite membrane is assembled into lithium-sulfur battery samples and the performance is tested. The specific test results are shown in Table 1.

[0060] Example 3

[0061] The difference between Example 3 and Example 1 is that 0.42 g LiNi 0.8 Co 0.15 Al 0.05 O2 and 0.03gSuperP, that is, Super P and LiNi 0.8 Co 0.15 Al 0.05The mass ratio of O2 is 0.07:1. The prepared composite membrane is assembled into lithium-sulfur battery samples and the performance is tested. The specific test results are shown in Table 1.

[0062] Example 4

[0063] The difference between Example 4 and Example 1 is that 0.35 g LiNi 0.8 Co 0.15 Al 0.05 O2 and 0.1g Super P, that is, Super P and LiNi 0.8 Co 0.15 Al 0.05 The mass ratio of O2 is 0.29:1. The prepared composite membrane is assembled into lithium-sulfur battery samples and the performance is tested. The specific test results are shown in Table 1.

[0064] Example 5

[0065] The difference between Example 5 and Example 1 is that 0.3 g LiNi 0.8 Co 0.15 Al 0.05 O2 and 0.15g Super P, that is, Super P and LiNi 0.8 Co 0.15 Al 0.05 The mass ratio of O2 is 0.5:1. The prepared composite membrane is assembled into lithium-sulfur battery samples and the performance is tested. The specific test results are shown in Table 1.

[0066] Example 6

[0067] The difference between Example 6 and Example 1 is that the thickness of the coating is 2 μm. The prepared composite membrane is assembled into a lithium-sulfur battery sample and the performance is tested. The specific test results are shown in Table 1.

[0068] Example 7

[0069] The difference between Example 7 and Example 1 is that the thickness of the coating is 10 μm. The prepared composite diaphragm is assembled into a lithium-sulfur battery sample and the performance is tested. The specific test results are shown in Table 1.

[0070] Example 8

[0071] The difference between Example 8 and Example 1 is that the thickness of the coating is 15 μm. The prepared composite membrane is assembled into lithium-sulfur battery samples and the performance is tested. The specific test results are shown in Table 1.

[0072] Comparative Example 1

[0073] Example 1 was repeated except that 0.40 g LiNi 0.8 Co 0.15 Al 0.05O2 was replaced with 0.40g SuperP.

[0074] Table 1

[0075]

[0076] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A composite diaphragm, comprising a base film and a coating disposed on the surface of the base film, wherein the coating comprises a layered nickel-cobalt-aluminum-lithium oxide, a conductive agent and a binder.

2. The composite diaphragm according to claim 1, characterized in that: The thickness of the coating is 1 μm-30 μm, preferably 2 μm-10 μm.

3. The composite diaphragm according to claim 1 or 2, characterized in that: In the coating, the mass ratio of the conductive agent to the layered nickel-cobalt-aluminum-lithium oxide is (0.01-1):1, preferably (0.05-0.50):1, and more preferably (0.05-0.30):

1.

4. The composite diaphragm according to any one of claims 1 to 3, characterized in that: In the coating, the mass ratio of the binder to the layered nickel-cobalt-aluminum lithium oxide is (0.01-0.50):1, preferably (0.05-0.15):

1.

5. The composite diaphragm according to any one of claims 1 to 4, characterized in that: The layered nickel-cobalt-aluminum lithium oxide is selected from LiNi 1-m-n Co m Al n One or more of O2, 0<m≤1, 0<n≤1; preferably selected from LiNi 0.8 Co 0.15 Al 0.05 O2.

6. The composite diaphragm according to any one of claims 1 to 5, characterized in that: The material of the base film is selected from one or more of polyethylene, polypropylene, polyimide, polyacrylonitrile, polyethylene terephthalate, polytetrafluoroethylene and polyvinylidene fluoride; preferably polyethylene and / or polypropylene.

7. The composite diaphragm according to any one of claims 1 to 6, characterized in that: The conductive agent is selected from one or more of graphene, graphene oxide, reduced graphene oxide, carbon nanotubes, carbon nanofibers, acetylene black, Super P and Ketjen black; and / or The binder is selected from one or more of polyvinyl alcohol, carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl pyrrolidone, styrene-butadiene rubber and polyacrylate.

8. A method for preparing the composite diaphragm according to any one of claims 1 to 7, comprising the following steps: S1: mixing layered nickel-cobalt-aluminum lithium oxide with a conductive agent to obtain a mixture; S2: dispersing the mixture and the binder in a solvent to obtain a coating slurry; S3: coating the coating slurry on the surface of the base film, and then removing the solvent to obtain the composite diaphragm; Preferably, in S2, the solvent is selected from one or more of deionized water, anhydrous ethanol, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide and N-methylpyrrolidone; Preferably, in S3, the coating is achieved by at least one of a casting method, a doctor blade coating method, a spray coating method, a spin coating method, a gravure roller coating method and a narrow slot extrusion method.

9. Use of the composite separator according to any one of claims 1 to 7 in a secondary battery.

10. A sulfur-lithium battery comprising a positive electrode, a negative electrode and a separator disposed between the positive electrode and the negative electrode, wherein the separator comprises the composite separator according to any one of claims 1 to 7.

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