Lithium ion battery diaphragm and preparation method and application thereof

Abstract

The application provides a lithium ion battery diaphragm and a preparation method and application thereof, and relates to the technical field of lithium batteries. Specifically, the diaphragm comprises a modified fluoropolyolefin polymer, and the polymer is obtained through polymerization of a fluoropolyolefin and a modified monomer; wherein the modified monomer comprises at least one of imidazole and derivatives thereof, pyridine olefin substitutes and derivatives thereof. The modified polymer raw material used in the diaphragm of the application comprises a fluoropolyolefin main chain and a side chain with a scavenging function, and is prepared into a film through electrospinning, and has the effect of absorbing and scavenging hydrogen fluoride. The lithium ion battery comprising the diaphragm has a lower transition metal dissolution amount and better cycle performance, and has a broad application prospect.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a lithium-ion battery separator, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, as highly efficient electrochemical energy storage devices, have attracted widespread attention from academia and industry due to their high energy density and low redox potential. To date, lithium-ion battery electrolytes have primarily consisted of lithium hexafluorophosphate (LiPF6) and alkyl carbonate solvents; however, lithium hexafluorophosphate decomposes under high temperature or aqueous conditions to produce hydrofluoric acid (HF) and hexafluorophosphate (PF5). - This leads to the dissolution of transition metals (such as nickel, cobalt, and manganese) in the positive electrode material; and the dissolution, migration, and deposition of transition metal elements in the electrolyte to the negative electrode of the battery leads to severe gas generation and increased internal resistance; in addition, metal deposition promotes the continuous destruction and regeneration of the SEI film of the negative electrode, consumes a large amount of active lithium, and leads to a decline in battery cycle performance.

[0003] Existing technologies offer various solutions or preventative measures against the adverse consequences of lithium hexafluorophosphate decomposition. For example, coating or doping the cathode material is an effective way to prevent transition metal dissolution, but this method cannot reduce the impact of hydrofluoric acid on other battery structures or components. Furthermore, replacing lithium hexafluorophosphate with other lithium salts can fundamentally reduce the generation and diffusion of hydrofluoric acid, but this presents problems such as poor solvent solubility and low ionic conductivity. Existing literature has demonstrated that the application of materials with hydrofluoric acid scavenging capabilities can effectively improve battery cycle performance; introducing electrolyte additives with hydrofluoric acid absorption functions is a common approach. However, the use of additives may lead to adverse consequences such as decreased electrolyte conductivity and increased battery internal resistance.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The primary objective of this invention is to provide a lithium-ion battery separator that addresses the potential negative impacts of lithium hexafluorophosphate electrolyte decomposition. The separator of this invention contains hydrofluoric acid absorption groups in its molecular structure, thereby removing hydrofluoric acid generated in the battery electrolyte, reducing the dissolution of transition metal ions, and improving battery cycle stability.

[0006] The second objective of this invention is to provide a method for preparing the lithium-ion battery separator described above, which uses electrospinning to obtain a separator with small pore size, high porosity, and strong affinity for electrolyte.

[0007] A third objective of this invention is to provide an application of the aforementioned lithium-ion battery separator in the field of lithium batteries.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0009] A lithium-ion battery separator includes a modified fluorinated polyolefin polymer, wherein the polymer is obtained by polymerization of a fluorinated polyolefin and a modified monomer; wherein the modified monomer includes at least one of imidazole and its derivatives containing olefin substitution, and pyridine and its derivatives containing olefin substitution.

[0010] Preferably, the fluorinated polyolefin includes at least one of polyvinylidene fluoride or polyvinylidene fluoride-hexafluoropropylene, polyvinyl fluoride, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer and tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether;

[0011] More preferably, the number-average molecular weight of the fluorinated polyolefin is 300,000 to 700,000.

[0012] Preferably, the modified monomer includes at least one of vinylimidazole, 2-methylvinylimidazole, allylimidazole, 2-methylallylimidazole, 2-vinylpyridine, 3-vinylpyridine, 2-methylvinylpyridine, allylpyridine, or 4-vinylpyridine.

[0013] Preferably, the mass ratio of the fluorinated polyolefin to the modified monomer is (2-5):1.

[0014] The preparation method of the lithium-ion battery separator includes the following steps: Step 1, mixing a solution of fluorinated polyolefin, a modified monomer, a catalyst and a ligand compound, and then heating and polymerizing the mixture to obtain a modified fluorinated polyolefin polymer; Step 2, preparing the separator by electrospinning the modified fluorinated polyolefin polymer.

[0015] Preferably, in step one, the solvent of the fluorinated polyolefin solution includes at least one of toluene, o-xylene, chloroform, dichloromethane, carbon tetrachloride, N-methylpyrrolidone, dimethyl sulfoxide, or N,N-dimethylformamide; the catalyst includes at least one of cuprous chloride, cuprous bromide, ruthenium chloride, ruthenium bromide, or ferrous chloride; and the ligand compound includes at least one of 1,1,4,7,10,10-hexamethyltrimethyltetramine, tris(N,N-dimethylaminoethyl)amine, or 4,4-dimethyl-2,2-bipyridine, 2,2,2-bipyridine, 4,4-di-tert-butyl-2,2-bipyridine, or 4,4-di(5-nonyl)-2,2-bipyridine.

[0016] Preferably, in step one, the mass fraction of the fluorinated polyolefin solution is 5% to 20%; the mass of the catalyst is 0.5% to 3% of the fluorinated polyolefin; and the mass ratio of the ligand compound to the catalyst is (4 to 6): 1.

[0017] Preferably, in step one, the polymerization reaction temperature is 60℃~120℃, and the polymerization reaction time is 24h~72h.

[0018] Preferably, in step two, after the modified fluorinated polyolefin polymer is thoroughly mixed with the second solvent, a spinning solution is obtained and used for electrospinning; wherein, the second solvent includes at least one selected from toluene, p-xylene, o-xylene, tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, dichloromethane, trichloromethane, carbon tetrachloride, or cyclohexane;

[0019] More preferably, the mass concentration of the modified fluorinated polyolefin polymer in the spinning solution is 2% to 10%.

[0020] Preferably, in step two, when electrospinning is performed, the temperature of the spinning solution is 30°C to 60°C, and the voltage of the electrospinning is 15kV to 30kV.

[0021] The applications of the lithium-ion battery separator in the field of lithium-ion batteries include, but are not limited to, lithium-ion batteries containing the lithium-ion battery separator and electrical appliances containing the lithium-ion battery separator.

[0022] Preferably, the application can be a lithium-ion battery, including a positive electrode, a negative electrode, a separator, and an electrolyte; wherein the active material in the positive electrode includes at least one of lithium manganese oxide, lithium nickel manganese oxide, lithium manganese iron phosphate, or lithium nickel cobalt manganese oxide, and the material of the negative electrode includes at least one of graphite, silicon carbon, or silicon oxide.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention provides an electrospun separator with hydrofluoric acid absorption and removal function, its preparation method and application, wherein the polymer used in the separator contains a fluorinated polyolefin main chain and a side chain with removal function; the lithium-ion battery containing the separator has a lower amount of transition metal dissolution and better cycle performance. Attached Figure Description

[0024] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0025] Figure 1 A comparison chart of the cycle performance in Experiment Example 2 is provided. Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0027] The present invention is carried out through the following specific embodiments: a lithium-ion battery separator, comprising a modified fluorinated polyolefin polymer, wherein the polymer is obtained by polymerization reaction of fluorinated polyolefin and modified monomer; wherein the modified monomer comprises at least one of imidazole and its derivatives, pyridine olefin substitutes and their derivatives.

[0028] It should be noted that "imidazolium and its derivatives containing olefin substitution" in this invention refers to imidazolium and its derivatives containing at least one olefin substituent, which can be directly purchased by those skilled in the art as needed, or synthesized by substitution using imidazolium and its derivatives as raw materials; similarly, "pyridine and its derivatives containing olefin substitution" refers to pyridine and its derivatives containing at least one olefin substituent, which can be directly purchased by those skilled in the art as needed, or synthesized by substitution using pyridine and its derivatives as raw materials.

[0029] The preparation method of the lithium-ion battery separator includes the following steps: Step 1, mixing the solution of the fluorinated polyolefin, the modified monomer, the catalyst and the ligand compound, and then heating and polymerizing to obtain the modified fluorinated polyolefin polymer; Step 2, preparing the lithium-ion battery separator by electrospinning the modified fluorinated polyolefin polymer.

[0030] As a preferred embodiment, step one specifically includes: 1) mixing the fluorinated polyolefin and solvent, stirring thoroughly at 30°C to 80°C until completely dissolved, and then cooling to room temperature to obtain a solution of the fluorinated polyolefin; 2) thoroughly mixing the solution of the fluorinated polyolefin, the modified monomer, and the catalyst under an inert environment, then adding the ligand compound and performing a heating polymerization reaction; 3) purifying the reaction mixture to obtain the modified fluorinated polyolefin polymer. As a more preferred embodiment, the purification process includes: pouring the mixture into methanol, allowing it to settle, filtering, and then redissolving the filtered product in NMP solution; repeating the above steps three times, and then sequentially settling, filtering, and drying the final filtered product in methanol to obtain the modified fluorinated polyolefin polymer.

[0031] In a preferred embodiment, the number-average molecular weight of the fluorinated polyolefin includes, but is not limited to, 30, 35, 40, 45, 50, 55, 60, 65, and 70 (unit: ten thousand).

[0032] In one preferred embodiment, the mass ratio of the fluorinated polyolefin to the modified monomer includes, but is not limited to, 2:1, 3:1, 4:1, and 5:1.

[0033] In a preferred embodiment, the mass fraction (based on solute) of the fluorinated polyolefin solution includes, but is not limited to, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%.

[0034] In one preferred embodiment, the mass of the catalyst is 0.5%, 1%, 1.5%, 2%, 2.5%, or 3% of the mass of the fluorinated polyolefin; in another preferred embodiment, the mass of the catalyst is 1% of the mass of the fluorinated polyolefin.

[0035] In a preferred embodiment, the mass ratio of the ligand compound to the catalyst includes, but is not limited to, 4:1, 4.5:1, 5:1, 5.5:1, and 6:1.

[0036] In a preferred embodiment, the temperature of the polymerization reaction includes, but is not limited to, 60, 70, 80, 90, 100, 110, and 120 (unit: °C), and the time of the polymerization reaction includes, but is not limited to, 24, 30, 36, 48, 60, and 72 (unit: h).

[0037] In a preferred embodiment, when performing electrospinning, the temperature of the spinning solution is including but not limited to 30, 35, 40, 45, 50, 55, 60 (unit: °C), and the voltage of the electrospinning is including but not limited to 15, 20, 25, 30 (unit: kV).

[0038] Example 1

[0039] (1) Grafting modification of PVDF

[0040] Add 5g PVDF and 50mL NMP to a three-necked flask and dissolve at 80℃. After the PVDF is completely dissolved, cool to room temperature. Add 2.5g 1-ethyleneimidazole and 0.05g CuCl to the three-necked flask, introduce argon gas into the apparatus and continue stirring for 30min. Add 0.25g 1,1,4,7,10,10-hexamethyltrimethyltetramine, then heat to 120℃ and react for 48h to terminate the reaction. Pour the reaction solution into methanol to settle, filter, and dry to obtain the grafted product of this example.

[0041] (2) Electrospinning for film production

[0042] 1 g of grafted product was dissolved in 10 g of a mixed solvent containing tetrahydrofuran, N-methylpyrrolidone, and N,N-dimethylacetamide, wherein the mass ratio of the three solvents was 5:3:2; the solution was stirred at 45 °C for 2 h to obtain an electrospinning solution; electrospinning was performed under the conditions of a spinning voltage of 20 kV, a solution feed rate of 1.0 mL / min, and a receiving distance of 20 cm to obtain the diaphragm of this embodiment.

[0043] Example 2

[0044] The example is basically the same as Example 1, except that PVDF in step (1) is replaced with PVDF-HFP, and 1-vinylimidazole is replaced with 2-methylallylimidazole.

[0045] Example 3

[0046] It is basically the same as Example 1, except that the 2.5g of 1-vinylimidazole in step (1) is replaced with 1g of 2-vinylpyridine.

[0047] Example 4

[0048] The example is basically the same as Example 1, except that NMP in step (1) is replaced with DMF and 1,1,4,7,10,10-hexamethyltrimethyltetramine is replaced with 4,4-dimethyl-2,2-bipyridine.

[0049] Example 5

[0050] The process is basically the same as in Example 1, except that "heating to 120°C and reacting for 48 hours" in step (1) is replaced with "heating to 60°C and reacting for 72 hours".

[0051] Example 6

[0052] The process is basically the same as in Example 1, except that the 10g mixed solvent of tetrahydrofuran, N-methylpyrrolidone and N,N-dimethylacetamide in step (2) is replaced with N-methylpyrrolidone, dimethyl sulfoxide and cyclohexane.

[0053] Example 7

[0054] It is basically the same as Example 1, except that "stirring at 45°C for 2 hours" in step (2) is replaced with "stirring at 60°C for 1.5 hours".

[0055] Example 8

[0056] It is basically the same as Example 1, except that the 20kV spinning voltage in step (2) is replaced with 30kV.

[0057] Comparative Example

[0058] 1 g of PVDF was dissolved in 10 g of a mixed solvent of tetrahydrofuran, N-methylpyrrolidone, and N,N-dimethylacetamide in a mass ratio of 5:3:2. The solution was stirred at 45 °C for 2 h to obtain an electrospinning solution. Electrospinning was performed under the conditions of a spinning voltage of 20 kV, a solution feed rate of 1.0 mL / min, and a receiving distance of 20 cm to obtain a PVDF membrane.

[0059] Experimental Example 1

[0060] The diaphragms from each example and the comparative example were placed in sample vials containing 30 ml of 1 mol / L LiPF6 / EC electrolyte, with an initial acidity of 353 ppm. Three portions of the diaphragm prepared for each example (comparative example) were taken and tested independently three times. After storing the sample vials at 45°C for 7 days, the acidity of each electrolyte was tested, and the results are shown in Table 1 below. Compared to the comparative example, the diaphragm system obtained according to this example can remove HF generated in the electrolyte.

[0061] Table 1

[0062]

[0063]

[0064] Experimental Example 2

[0065] Single-cell batteries were assembled using the separators from Example 1 and the comparative example. The positive electrode ratio was lithium nickel manganese oxide: PVDF: SP = 96:2:2; the negative electrode ratio was graphite: SP: CMC: SBR = 96.5:1.5:1.5:1.5; the electrolyte was a 1 mol / L LiPF6 solution, and the solvent was EC + EMC (volume ratio 3:7).

[0066] Cyclic testing was performed on the two types of assembled single-cell batteries. The specific testing method was as follows: 0.5C / 1C charge-discharge cycle testing was conducted in a constant temperature chamber at 25°C, with a voltage range of 3.5-5V. The obtained cycle data is compared to... Figure 1 As shown.

[0067] Meanwhile, ICP testing was performed on the negative electrode and separator of the two types of assembled single-cell batteries. The specific testing method was as follows: the two types of single-cell batteries were soaked in aqua regia and diluted before the content of each element was tested. The test data are shown in Table 2 below.

[0068] pass Figure 1 As can be seen from Table 2, compared with the ungrafted diaphragm system in the comparative example, the cell of the grafted diaphragm system in this embodiment has better cycle performance and lower transition metal dissolution, demonstrating excellent technical performance.

[0069] Table 2

[0070]

[0071] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A lithium-ion battery separator, characterized by, The diaphragm comprises a modified fluoropolyolefin polymer, and the polymer is obtained by polymerization of a fluoropolyolefin and a modified monomer; The modified monomer comprises at least one of imidazole and derivatives thereof substituted with an olefin; the modified monomer comprises at least one of vinyl imidazole, 2-methyl vinyl imidazole, allyl imidazole, and 2-methyl allyl imidazole; The preparation method of the diaphragm comprises the following steps: Step one: mixing a solution of a fluoropolyolefin, a modified monomer, a catalyst, and a ligand compound, and obtaining a modified fluoropolyolefin polymer by heating polymerization; Step two: obtaining the diaphragm by electrospinning the modified fluoropolyolefin polymer; The polymerization temperature is 60-120°C, and the polymerization time is 24-72 hours.

2. The lithium-ion battery separator of claim 1, wherein, The fluoropolyolefin comprises at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyfluoroethylene, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether.

3. The lithium-ion battery separator of claim 1, wherein, The number average molecular weight of the fluoropolyolefin is 300-700 thousand.

4. The lithium-ion battery separator of claim 1, wherein, The mass ratio of the fluoropolyolefin to the modified monomer is (2-5):

1.

5. The lithium-ion battery separator of claim 1, wherein, In step one, the solvent of the solution of the fluoropolyolefin comprises at least one of toluene, o-xylene, chloroform, dichloromethane, carbon tetrachloride, N-methyl pyrrolidone, dimethyl sulfoxide, or N,N-dimethylformamide; The catalyst comprises at least one of cuprous chloride, cuprous bromide, ruthenium chloride, ruthenium bromide, or ferrous chloride; The ligand compound comprises at least one of 1,1,4,7,10,10-hexamethyl triazacyclononane, tris(N,N-dimethylaminoethyl)amine, 4,4-dimethyl-2,2-bipyridine, 2,2,2-bipyridine, 4,4-di-tert-butyl-2,2-bipyridine, or 4,4-bis(5-nonyl)-2,2-bipyridine.

6. The lithium-ion battery separator of claim 1, wherein, The mass fraction of the solution of the fluoropolyolefin is 5-20%.

7. The lithium-ion battery separator of claim 1, wherein, The mass of the catalyst is 0.5-3% of the fluoropolyolefin.

8. The lithium-ion battery separator of claim 1, wherein, The mass ratio of the ligand compound to the catalyst is (4-6):

1.

9. The lithium-ion battery separator of claim 1, wherein, In step two, after the modified fluoropolyolefin polymer is fully mixed with a second solvent, a spinning solution is obtained and used for the electrospinning; the second solvent comprises at least one of toluene, p-xylene, o-xylene, tetrahydrofuran, N-methyl pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, dichloromethane, chloroform, carbon tetrachloride, or cyclohexane.

10. The lithium-ion battery separator of claim 9, wherein, The mass concentration of the modified fluoropolyolefin polymer in the spinning solution is 2-10%.

11. The lithium-ion battery separator of claim 9, wherein, In step two, when the electrospinning is performed, the temperature of the spinning solution is 30-60°C, and the voltage of the electrospinning is 15-30 kV.

12. Use of the lithium ion battery diaphragm according to any one of claims 1-11 in the field of lithium ion batteries.

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