Pre-lithiated diaphragm as well as preparation method and application thereof

Abstract

The invention provides a pre-lithiation diaphragm as well as a preparation method and application thereof, and relates to the technical field of lithium ion batteries. The preparation method of the pre-lithiated diaphragm comprises the following steps: S1, soaking a diaphragm in a solvent to obtain an infiltrated diaphragm; s2, under the protective gas atmosphere, soaking the infiltrated diaphragm in an oxidizing agent solution, and carrying out heating reaction to obtain a hydroxylated diaphragm; s3, soaking the hydroxylated diaphragm in a solution containing a vinyl amide compound in a protective gas atmosphere, and carrying out a heating reaction to obtain an amidated diaphragm; s4, under the protective gas atmosphere, soaking the amidated diaphragm in an alkaline solution to obtain a carboxylated diaphragm; and S5, soaking the carboxylated diaphragm in the lithium-containing compound solution to obtain the pre-lithiated diaphragm. The wettability of the pre-lithiated diaphragm prepared by the preparation method to electrolyte is obviously improved, and the rate capability of the lithium ion battery can be effectively improved.

Description

Technical Field

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

[0002] Battery separators, as a key material in lithium-ion batteries, prevent direct contact between the positive and negative electrodes, thus isolating electron flow while allowing lithium ions to pass freely. Currently, commercially available lithium-ion battery separators are primarily made of polyolefin materials such as polyethylene or polypropylene. These materials are non-polar and have very poor wettability to polar electrolytes, resulting in low electrolyte retention. This reduces the rate of lithium-ion transport, increases electrochemical impedance, and significantly impacts the rate performance of lithium-ion batteries.

[0003] High ionic conductivity is a crucial indicator of the electrical performance of the separator. Improving ionic conductivity can effectively enhance the high-rate fast-charging performance and long-term cycle stability of lithium-ion battery systems, reduce energy loss due to high internal resistance, and further improve battery performance. Therefore, improving ionic conductivity is currently an important means to enhance the electrical performance of lithium-ion batteries. Simultaneously, during the early stages of battery formation and capacity testing, as well as later cycling processes, the formation of the SEI film and side reactions continuously consume lithium in the battery, leading to a continuous decrease in capacity. Therefore, replenishing lithium can increase battery capacity and extend its lifespan. Summary of the Invention

[0004] Based on the technical problems existing in the background technology, the present invention proposes a pre-lithiation separator, its preparation method and application.

[0005] The present invention proposes a method for preparing a pre-lithiated separator, comprising the following steps:

[0006] S1. Immerse the diaphragm in a solvent to obtain a wetted diaphragm;

[0007] S2. Under a protective atmosphere, the wetted diaphragm is immersed in an oxidizing agent solution and heated to react, thus obtaining a hydroxylated diaphragm.

[0008] S3. Under a protective atmosphere, the hydroxylated membrane is immersed in a solution containing vinylamide compounds and heated to react, thereby obtaining an amidated membrane.

[0009] S4. Under a protective atmosphere, the amidated membrane is immersed in an alkaline solution to obtain a carboxylate-treated membrane.

[0010] S5. Immerse the carboxylate-treated membrane in a lithium-containing compound solution to obtain a pre-lithiated membrane.

[0011] The present invention sequentially performs hydroxylation, amidation, carboxylation and lithiation on the separator to obtain a pre-lithiated separator, thereby improving the rate performance of the assembled lithium-ion battery.

[0012] Preferably, in S1, the diaphragm is selected from one or more of polyethylene diaphragms and polypropylene diaphragms.

[0013] Preferably, in S1, the solvent is selected from one or more of methanol and water.

[0014] More preferably, the volume ratio of methanol to water is (20-80):(20-80).

[0015] Preferably, in step S1, the soaking time is 2-5 hours.

[0016] Immersing the diaphragm in a solvent to obtain a wetted diaphragm facilitates the next step of hydroxylation treatment.

[0017] Preferably, in S2, the mass concentration of the oxidant in the oxidant solution is 3wt%-30wt%.

[0018] Within a certain range, the mass concentration of the oxidant in the oxidant solution is conducive to surface hydroxylation treatment. Too low a concentration may lead to insufficient modification effect; too high a concentration may lead to over-oxidation, destroying the membrane matrix structure and reducing mechanical strength.

[0019] Preferably, in S2, the oxidant is selected from one or more of potassium persulfate, hydrogen peroxide, and potassium permanganate.

[0020] Preferably, in step S2, the heating temperature is 40-90℃ and the heating time is 0.5-3h.

[0021] Hydroxylation treatment can be performed on the surface of the impregnated diaphragm by immersing it in an oxidant solution and controlling the heating temperature and time.

[0022] Preferably, after S2, the hydroxylated membrane is further cleaned and dried.

[0023] More preferably, the cleaning solvent is deionized water, the drying temperature is 40-80℃, and the drying time is 2-8 hours.

[0024] The purpose of cleaning and drying the hydroxylation membrane is to effectively remove reaction residues and prevent them from interfering with subsequent modification processes and affecting the modification effect.

[0025] Preferably, in S2, S3, and S4, the protective gas is selected from one or more of nitrogen, argon, and helium.

[0026] Preferably, in S3, the mass concentration of the vinylamide compound in the solution containing the vinylamide compound is 1wt%-5wt%.

[0027] Preferably, in S3, the solution containing the vinylamide compound further includes nitric acid and cerium ammonium nitrate.

[0028] More preferably, the molar ratio of nitric acid to cerium ammonium nitrate is (10-40):1.

[0029] Maintaining a suitable molar ratio of nitric acid to ceric ammonium nitrate within a certain range helps optimize grafting efficiency and balance reactivity and side reactions. Excessive ceric ammonium nitrate concentration may trigger over-oxidation or chain termination, while insufficient concentration leads to incomplete grafting.

[0030] Preferably, in S3, the acetamide compound is selected from one or more of acrylamide, but-3-enamide, pent-4-enamide, 2-methylbut-3-enamide, 2,2-dimethylbut-3-enamide, 2-methylpent-4-enamide, 3-methylpent-4-enamide, 2,3-dimethylpent-4-enamide, 2-trifluoromethylbut-3-enamide, 2,2-bis(trifluoromethyl)but-3-enamide, 2-trifluoromethylpent-4-enamide, 3-trifluoromethylpent-4-enamide, and 2,3-bis(trifluoromethyl)pent-4-enamide.

[0031] By grafting vinylamide compounds onto the surface of polyolefins, the separator can achieve improvements in various aspects. This invention uses 2-trifluoromethylbut-3-eneamide monomers for grafting, which not only effectively improves the ionic conductivity of the polyolefin separator but also significantly enhances its wettability to the electrolyte due to the introduction of the trifluoromethyl polar group. The rate performance of lithium-ion batteries prepared using this separator is improved, thus enhancing the overall electrical performance of the lithium-ion battery.

[0032] Preferably, after step S3, the process further includes cleaning the amidated membrane.

[0033] Preferably, in step S4, the alkaline solution is selected from one or more of sodium hydroxide solution, potassium hydroxide solution, and sodium carbonate solution.

[0034] Preferably, in step S4, the alkaline solution is a heated alkaline solution with a temperature of 40-70°C and a concentration of 0.5-2 mol / L.

[0035] The role of the heated alkaline solution is to promote the hydrolysis of amide groups, which is difficult to carry out at room temperature.

[0036] Preferably, in step S4, the soaking time is 0.5-3 hours.

[0037] Immersing the amidated membrane in an alkaline solution helps to hydrolyze the amide groups to form carboxylates.

[0038] If S5 is carried out directly from S3, the lithiation reaction cannot be completed.

[0039] Preferably, in S5, the lithium-containing compound is selected from one or more of lithium hydroxide, lithium nitrate, and lithium carbonate.

[0040] Among lithium-containing compounds, lithium hydroxide has the mildest reaction conditions and is more suitable for pre-lithiation.

[0041] Preferably, in step S5, the concentration of the lithium compound solution is 0.5-2 mol / L.

[0042] The role of lithium-containing compounds is to react with carboxylate groups on the membrane surface to form lithium carboxylate, thus obtaining a pre-lithiated membrane.

[0043] Preferably, in step S5, the soaking time is 0.5-3 hours.

[0044] A pre-lithiated polyolefin separator is prepared by the above-described method.

[0045] Application of the above-described pre-lithiated polyolefin separator or the pre-lithiated polyolefin separator prepared by the above-described preparation method in lithium-ion batteries.

[0046] Preferably, the lithium-ion battery further includes a positive electrode, a negative electrode, and an electrolyte.

[0047] More preferably, the positive electrode active material in the positive electrode sheet is selected from lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, and LiMn. x Co 1-x O2, LiNi x Co 1-x O2, LiMn x Ni 1-x O2, LiNi x Co y Mn 1-x-y One or more of O2, wherein 0 < x < 1, 0 < y < 1, 0 < x + y < 1.

[0048] More preferably, the negative electrode active material in the negative electrode sheet is selected from one or more of graphite and SiC.

[0049] More preferably, the lithium salt in the electrolyte is selected from one or more of LiPF6, LiBF4, LiAsF6, and LiSbF6.

[0050] The beneficial effects of this invention are as follows:

[0051] The high lithium-ion conductivity separator prepared by the method of this invention, using a polyolefin separator as the base membrane, involves hydroxylating the surface of the polyolefin separator, grafting a monomer containing vinyl amides onto the surface of the polyolefin, followed by a hydrolysis reaction and carboxylation to obtain a base membrane with high lithium-ion conductivity. Finally, the surface of the base membrane is further lithiated with lithium hydroxide to obtain a pre-lithiated base membrane with high lithium-ion conductivity. Different grafted monomers can provide different improvements to the separator. Introducing a grafted monomer containing trifluoromethyl polar groups significantly improves the wettability of the separator to the electrolyte, effectively enhancing the rate performance of the lithium-ion battery. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of the surface pre-lithiation modification of the pre-lithiation polyolefin separator proposed in Example 1 of the present invention. Detailed Implementation

[0053] The technical solution of the present invention will be described in detail through specific embodiments.

[0054] In the following examples and comparative examples, the specific information regarding the raw materials used is as follows:

[0055] PE diaphragm: Manufacturer: Hebei Jinli New Energy Technology Co., Ltd., Model: SG12S;

[0056] PP diaphragm: Manufacturer: Shenzhen ZTE New Material Technology Co., Ltd., Grade: ZS14421;

[0057] 2-Trifluoromethylbut-3-enamide: Manufacturer: Shaanxi Xihua Chemical Industry Co., Ltd.;

[0058] Acrylamide: Manufacturer: Condis Chemical Co., Ltd.;

[0059] But-3-enamide: Manufacturer: Shanghai Branch of Wuhan Yitai Technology Co., Ltd.

[0060] Unless otherwise specified, all materials and reagents used in the following examples and comparative examples are commercially available.

[0061] Example 1

[0062] A method for preparing a pre-lithiated separator includes the following steps:

[0063] S1. Immerse a 12μm PE membrane in a mixed solution of methanol and deionized water in a volume ratio of 50:50 for 3 hours to obtain the impregnated PE membrane.

[0064] S2. Under an argon atmosphere, the impregnated PE membrane is placed in a 10% potassium persulfate solution and reacted at 80°C for 2 hours to obtain a hydroxylated PE membrane. The hydroxylated PE membrane is washed with deionized water and dried at 60°C for 8 hours.

[0065] S3. Under an argon atmosphere and at 50°C, the hydroxylated PE membrane was immersed in a mixed solution containing nitric acid, ceric ammonium nitrate, and excess 2-trifluoromethylbut-3-enamide (3 wt%) for 1 hour. The molar ratio of nitric acid to ceric ammonium nitrate was 20:1. After the reaction was completed, an amidated PE membrane was obtained. The amidated PE membrane was then subjected to ultrasonic treatment in hot water at 60°C for 2 hours to remove unreacted 2-trifluoromethylbut-3-enamide.

[0066] S4. Immerse the amidated PE membrane in a 1mol / L sodium hydroxide solution at 50℃ for 1h to carry out a hydrolysis reaction, and obtain a carboxylate-treated PE membrane.

[0067] S5. The carboxylated PE membrane is immersed in a 0.5 mol / L lithium hydroxide solution for 2 hours to carry out the lithiation reaction, and a pre-lithiated PE membrane is obtained.

[0068] Example 2

[0069] A method for preparing a pre-lithiated separator includes the following steps:

[0070] S1. Immerse a 12μm PE membrane in a mixed solution of methanol and deionized water in a volume ratio of 50:50 for 3 hours to obtain the impregnated PE membrane.

[0071] S2. Under an argon atmosphere, the impregnated PE membrane is placed in a 10% potassium persulfate solution and reacted at 80°C for 2 hours to obtain a hydroxylated PE membrane. The hydroxylated PE membrane is washed with deionized water and dried at 60°C for 8 hours.

[0072] S3. Under an argon atmosphere and at 50°C, the hydroxylated PE membrane was immersed in a mixed solution containing nitric acid, ceric ammonium nitrate, and excess acrylamide (3 wt%) for 1 hour. The molar ratio of nitric acid to ceric ammonium nitrate was 20:1. After the reaction was completed, an amidated PE membrane was obtained. The amidated PE membrane was then subjected to ultrasonic treatment in hot water at 60°C for 2 hours to remove unreacted acrylamide.

[0073] S4. Immerse the amidated PE membrane in a 1mol / L sodium hydroxide solution at 50℃ for 1h to carry out a hydrolysis reaction, and obtain a carboxylate-treated PE membrane.

[0074] S5. The carboxylated PE membrane is immersed in a 0.5 mol / L lithium hydroxide solution for 2 hours to carry out the lithiation reaction, and a pre-lithiated PE membrane is obtained.

[0075] Example 3

[0076] A method for preparing a pre-lithiated separator includes the following steps:

[0077] S1. Immerse a 12μm PE membrane in a mixed solution of methanol and deionized water in a volume ratio of 50:50 for 3 hours to obtain the impregnated PE membrane.

[0078] S2. Under an argon atmosphere, the impregnated PE membrane is placed in a 10% potassium persulfate solution and reacted at 80°C for 2 hours to obtain a hydroxylated PE membrane. The hydroxylated PE membrane is washed with deionized water and dried at 60°C for 8 hours.

[0079] S3. Under an argon atmosphere and at 50°C, the hydroxylated PE membrane was immersed in a mixed solution containing nitric acid, ceric ammonium nitrate, and excess butyronamide (3 wt%) for 1 hour. The molar ratio of nitric acid to ceric ammonium nitrate was 20:1. After the reaction, an amidated PE membrane was obtained. The amidated PE membrane was then subjected to ultrasonic treatment in hot water at 60°C for 2 hours to remove unreacted butyronamide.

[0080] S4. Immerse the amidated PE membrane in a 1mol / L sodium hydroxide solution at 50℃ for 1h to carry out a hydrolysis reaction, and obtain a carboxylate-treated PE membrane.

[0081] S5. The carboxylated PE membrane is immersed in a 0.5 mol / L lithium hydroxide solution for 2 hours to carry out the lithiation reaction, and a pre-lithiated PE membrane is obtained.

[0082] Example 4

[0083] A method for preparing a pre-lithiated separator includes the following steps:

[0084] S1. Soak a 14μm PP membrane in a mixed solution of methanol and deionized water in a volume ratio of 50:50 for 3 hours to obtain the soaked PP membrane.

[0085] S2. Under an argon atmosphere, the impregnated PP membrane is placed in a 10% potassium persulfate solution and reacted at 80°C for 2 hours to obtain a hydroxylated PP membrane. The hydroxylated PP membrane is washed with deionized water and dried at 60°C for 8 hours.

[0086] S3. Under an argon atmosphere and at 50°C, the dried hydroxylated PP membrane is immersed in a mixed solution containing nitric acid, cerium ammonium nitrate, and excess 2-trifluoromethylbut-3-enamide (3wt%) and reacted for 1 hour. The molar ratio of nitric acid to cerium ammonium nitrate is 20:1. After the reaction is completed, an amidated PP membrane is obtained. The amidated PP membrane is then placed in hot water at 60°C and sonicated for 2 hours to remove unreacted 2-trifluoromethylbut-3-enamide.

[0087] S4. Immerse the amidated PP membrane in a 1mol / L sodium hydroxide solution at 50℃ for 1h to carry out a hydrolysis reaction, and obtain a carboxylate-treated PP membrane.

[0088] S5. The carboxylated PE membrane is immersed in a 0.5 mol / L lithium hydroxide solution for 2 hours to carry out the lithiation reaction, and a pre-lithiated PE membrane is obtained.

[0089] Comparative Example 1

[0090] Untreated PE membranes were used as Comparative Example 1.

[0091] Comparative Example 2

[0092] Untreated PP membranes were used as Comparative Example 2.

[0093] Comparative Example 3

[0094] The only difference between Comparative Example 3 and Example 1 is that step S4 was not performed; the remaining steps are the same as in Example 1.

[0095] The prepared separator was cut into 67mm wide sections and wound into batteries to obtain a pouch cell. The positive electrode material was lithium manganese iron phosphate, the negative electrode material was graphite, and the electrolyte was LiPF6. The separator was tested for thickness, permeability, areal density, contact angle, ionic conductivity, and porosity. The test results are shown in Table 1. The pouch cell assembled with the separator was then subjected to rate performance testing to determine its charge-discharge capability. The test results are shown in Table 2.

[0096] Table 1

[0097]

[0098] Table 1 shows the physicochemical properties of the membrane. After carboxylation, there were no significant changes in the membrane's thickness, permeability, or areal density. Electrolyte contact angle testing revealed that lithium carboxylation significantly improved the membrane's wettability to the electrolyte. Conductivity testing showed that the lithium carboxylation-treated membrane exhibited significantly higher ionic conductivity compared to the untreated polyolefin membrane.

[0099] Table 2

[0100]

[0101] As can be seen from the results in Table 2, the 2C and 3C constant current charge ratios of the membranes obtained in Examples 1-4 of this invention are significantly better than those of the membranes in Comparative Examples 1-3. Furthermore, Examples 1 and 4, which are grafted with 2-trifluoromethylbut-3-enamide, have better wettability due to the introduction of the trifluoromethyl group, and their double charge performance is better than that of Examples 2 and 3.

[0102] In summary, pre-lithiation treatment of the polyolefin separator surface can effectively improve the lithium-ion conductivity of the separator and enhance its wettability with the electrolyte, while having no significant impact on the separator's thickness, air permeability, or areal density. Furthermore, the soft-pack test results show that the embodiment significantly improves the battery's rate performance.

[0103] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a pre-lithiated separator, characterized in that, Includes the following steps: S1. Immerse the diaphragm in a solvent to obtain a wetted diaphragm; S2. Under a protective atmosphere, the wetted diaphragm is immersed in an oxidizing agent solution and heated to react, thus obtaining a hydroxylated diaphragm. S3. Under a protective atmosphere, the hydroxylated membrane is immersed in a solution containing vinylamide compounds and heated to react, thereby obtaining an amidated membrane. S4. Under a protective atmosphere, the amidated membrane is immersed in an alkaline solution to obtain a carboxylate-treated membrane. S5. Immerse the carboxylate-treated membrane in a lithium-containing compound solution to obtain a pre-lithiated membrane.

2. The preparation method according to claim 1, characterized in that, In S1, the diaphragm is selected from one or more of polyethylene diaphragm and polypropylene diaphragm; the solvent is selected from one or more of methanol and water; and the soaking time is 2-5 hours.

3. The preparation method according to claim 1, characterized in that, In S2, the mass concentration of the oxidant in the oxidant solution is 3wt%-30wt%; the oxidant is selected from one or more of potassium persulfate, hydrogen peroxide, and potassium permanganate; the heating temperature is 40-90℃, and the heating time is 0.5-3h.

4. The preparation method according to claim 1, characterized in that, In the S3, the solution containing the vinylamide compound also includes nitric acid and cerium ammonium nitrate; the molar ratio of the nitric acid and cerium ammonium nitrate is (10-40):

1.

5. The preparation method according to claim 1, characterized in that, In S3, the mass concentration of the vinylamide compound in the solution containing the vinylamide compound is 1wt%-5wt%; the vinylamide compound is selected from one or more of acrylamide, but-3-enamide, pent-4-enamide, 2-methylbut-3-enamide, 2,2-dimethylbut-3-enamide, 2-methylpent-4-enamide, 3-methylpent-4-enamide, 2,3-dimethylpent-4-enamide, 2-trifluoromethylbut-3-enamide, 2,2-bis(trifluoromethyl)but-3-enamide, 2-trifluoromethylpent-4-enamide, 3-trifluoromethylpent-4-enamide, and 2,3-bis(trifluoromethyl)pent-4-enamide.

6. The preparation method according to claim 1, characterized in that, In step S4, the alkaline solution is selected from one or more of sodium hydroxide solution, potassium hydroxide solution, and sodium carbonate solution; the alkaline solution is a heated alkaline solution with a temperature of 40-70℃ and a concentration of 0.5-2 mol / L; the soaking time is 0.5-3 h.

7. The preparation method according to claim 1, characterized in that, In step S5, the lithium-containing compound is selected from one or more of lithium hydroxide, lithium nitrate, and lithium carbonate; the concentration of the lithium-containing compound solution is 0.5-2 mol / L; and the soaking time is 0.5-3 h.

8. The preparation method according to claim 1, characterized in that, In S2, S3, and S4, the protective gas is selected from one or more of nitrogen, argon, and helium.

9. A pre-lithiated polyolefin separator, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.

10. The application of a pre-lithiated polyolefin separator according to claim 9 or a pre-lithiated polyolefin separator prepared by any one of claims 1-8 in a lithium-ion battery.

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