Composite lithium battery diaphragm with lithium-containing polymer and preparation method of composite lithium battery diaphragm

Abstract

The preparation method comprises the following steps: putting polyacrylic acid into water, adding lithium carbonate into a reaction system, stirring at room temperature for complete reaction, and drying a solvent to prepare lithium polyacrylate; the preparation method comprises the following steps: dissolving polyvinylidene fluoride and lithium polyacrylate in an organic solvent, heating and stirring, and completely dissolving to obtain a membrane casting solution; and pouring the membrane casting solution on a commercial polyethylene or polypropylene diaphragm, and evaporating the solvent to prepare the composite lithium battery diaphragm containing the lithium-containing polymer. The composite lithium battery diaphragm needle prepared by the invention shows more excellent ionic conductivity, and the prepared lithium ion battery shows good cycle performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery separators, in particular to a composite lithium battery separator with lithium-containing polymer and a preparation method thereof. BACKGROUND

[0002] Lithium battery separator is one of the key components of lithium battery, known as the "third electrode" of the battery. It is located between the positive and negative electrodes of the battery, playing an important role in isolating the positive and negative electrodes, preventing short circuit, and allowing lithium ions to pass through. The performance of the separator directly affects the safety, energy density, cycle life and rate performance of the lithium battery.

[0003] Currently, the commercialized lithium battery separator is mainly based on polyethylene (PE) and polypropylene (PP) and other polyolefin materials. These materials, although low in price and mature in technology, have some obvious limitations.

[0004] Therefore, polyvinylidene fluoride (PVDF) as a high-performance fluorine-containing polymer has shown its unique advantages in the field of lithium battery separators and has become a hot material for research and application.

[0005] However, there are still some problems with polyvinylidene fluoride (PVDF) lithium battery separators; insufficient hydrophilicity, poor electrolyte wettability, difficulty in balancing size stability and flexibility, and low ion conductivity hinder its development and application in the field of lithium battery separators.

[0006] In order to solve these problems, researchers have proposed various methods to improve polyvinylidene fluoride (PVDF) lithium battery separators. For example, in a method for preparing a lithium ion battery separator (CN105118946A), a composite separator is prepared by blending polyvinylidene fluoride hexafluoropropylene, high-density polyethylene particles and polyvinylpyrrolidone, which greatly improves the lithium ion conductivity; in a method for preparing a composite polymer electrolyte with different forms for lithium secondary batteries (CN1610169A), a composite separator is prepared by blending polyethylene, polypropylene, polyimide, polysulfone, polyurethane, polyvinyl chloride, cellulose, nylon, polyacrylonitrile, polyvinylidene fluoride and polytetrafluoroethylene, which has better mechanical properties and higher lithium ion conductivity. SUMMARY

[0007] The present application relates to the field of lithium ion battery separators, in particular to a composite lithium battery separator with lithium-containing polymer and a preparation method thereof.

[0008] To achieve the above purpose, the present application adopts the following technical solutions: A method for preparing a composite lithium battery separator with lithium-containing polymer, comprising the following specific steps: (1) Preparation of lithium polyacrylate: Put polyacrylic acid in water, add lithium carbonate to the reaction system, stir until the reaction is complete, evaporate the solvent, and obtain lithium polyacrylate; (2) Preparation of polyvinylidene fluoride and lithium polyacrylate blended casting solution: Dissolve polyvinylidene fluoride and lithium polyacrylate in an organic solvent, heat and stir, and obtain a casting solution after complete dissolution; (3) Preparation of composite separator: Pour the above casting solution on a commercial polyethylene or polypropylene separator, evaporate the solvent, and obtain a composite lithium battery separator with lithium-containing polymer.

[0009] Further, in step (1), the molar ratio of monomers in polyacrylic acid to lithium carbonate is (1-2):2, and the molecular weight of the prepared lithium polyacrylate is 8000-30000.

[0010] Further, in step (1), the reaction temperature is -10-10°C, and the reaction time is 30-120 min.

[0011] Further, in step (2), the organic solvent is any one or a mixture of two or more of N-methyl pyrrolidone (NMP), dimethyl sulfoxide (DMSO), and acetone.

[0012] Further, in step (2), the mass fraction of lithium polyacrylate in polyvinylidene fluoride and lithium polyacrylate is 1-10%, and the mass concentration of polyvinylidene fluoride in the casting solution is 5-11%.

[0013] Further, in step (2), the heating and stirring temperature is 50-150°C.

[0014] Further, in step (3), the evaporation temperature of the solvent is 70-100°C.

[0015] Further, in step (3), after evaporation of the solvent, the thickness of the lithium-containing polymer layer is 10-20µm.

[0016] Preferably, in step (2), the mass ratio of polyvinylidene fluoride to lithium polyacrylate is 5-10%.

[0017] The composite lithium battery separator with lithium-containing polymer prepared by the above preparation method.

[0018] The composite lithium battery separator prepared by the present application optimizes the problems of insufficient hydrophilicity and low ion mobility of PVDF, and after being combined with a commercial separator, it exhibits excellent ion conductivity, and the prepared lithium ion battery exhibits good cycle performance. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1is a graph of the cycle characteristics of unit cells using the composite separator of the present application and a commercial polypropylene separator (Celgard 2500). DETAILED DESCRIPTION

[0020] The technical solutions of the present application are further described in detail below in combination with the drawings and examples.

[0021] Example 1 A method for preparing a composite lithium battery separator with lithium-containing polymers, the process is as follows: (1) Polyacrylic acid (10.8 g) was placed in water (100 mL), lithium carbonate (7.4 g, 0.1 mol) was slowly added to the reaction system, and the reaction was carried out at 0°C for 60 min. After complete dissolution, the solvent was evaporated in an oven at 70-80°C to obtain lithium polyacrylate (10.92 g, 0.13 mol, molecular weight 24362); (2) Polyvinylidene fluoride (5.76 g) and prepared lithium polyacrylate (0.64 g) were placed in a mixed solution of N-methylpyrrolidone and acetone (mass ratio 3:1, 50 g), and the temperature was raised to 60°C. After stirring until complete dissolution, a blended solution was obtained (mass ratio of polyvinylidene fluoride and lithium polyacrylate 90:10, i.e. lithium polyacrylate accounted for 10wt%); (3) A commercial polypropylene separator was laid flat on a clean glass plate, and the above prepared blended solution (10 g) was evenly coated on the commercial polypropylene separator (Celgard 2500, 25µm). Then it was placed in an oven and the solvent was evaporated at 70°C to obtain a composite separator (thickness 35µm, ionic conductivity 0.53 mS / cm).

[0022] Example 2 A method for preparing a composite lithium battery separator with lithium-containing polymers, the process is as follows: (1) Polyacrylic acid (10.8 g, 0.15 mol) was placed in water (100 mL), lithium carbonate (7.4 g, 0.1 mol) was slowly added to the reaction system, and the reaction was carried out at 0°C for 60 min. After complete dissolution, the solvent was evaporated in an oven at 70-80°C to obtain lithium polyacrylate (10.92 g, 0.13 mol, molecular weight 25185); (2) Polyvinylidene fluoride (6.08 g) and prepared lithium polyacrylate (0.30 g) were placed in a mixed solution of N-methylpyrrolidone and acetone (mass ratio 3:1, 50 g), and the temperature was raised to 60°C. After stirring until complete dissolution, a blended solution was obtained; (mass ratio of polyvinylidene fluoride and lithium polyacrylate 95:5, i.e. lithium polyacrylate accounted for 5wt%); (3) The commercial polypropylene membrane was laid flat on a clean glass plate. The blend solution (10 g) prepared above was uniformly coated on the commercial polypropylene membrane (Celgard 2500, 25µm). Then it was placed in an oven and the solvent was evaporated at 70°C to obtain a composite membrane (thickness 35µm, ionic conductivity 0.46mS / cm).

[0023] Cyclic performance test The positive electrode (LiFePO4, 1.77cm) 2 Loading capacity 7.3 mg / cm³ 2 Place the positive electrode in the center of the positive electrode shell, ensuring the battery material side faces upwards. Add an appropriate amount of electrolyte (10µL) (1M LiPF6 + 2wt% VC + 0.5wt% biphenyl, EC:DMC=3:7), and let it stand for 1-2 minutes for initial wetting. Cover the positive electrode with the wetting separator (Examples 1, 2, and Comparative Example 1, commercially available Celgard 2500), and add another appropriate amount of electrolyte (10µL) to ensure the separator fully absorbs the liquid. Place the negative electrode sheet (graphite, 1.96cm) on top. 2 The loading capacity is 2 mg / cm³ 2 The negative electrode shell is laid flat on the separator, and after ensuring alignment, it is snapped together with the positive electrode shell to obtain a semi-finished battery. The semi-finished battery is then fed into a battery press to press it into a finished battery. After being activated in a glove box for 6-24 hours, a single-cell cycle test is performed at a 1C rate (temperature 25±2℃, humidity <30%) to test its capacity retention rate after 1250 cycles.

[0024] Regarding the capacity retention rate of single cells assembled from Examples 1, 2, and Comparative Example 1 after 1250 cycles, as shown in... Figure 1 As shown; by Figure 1 It can be seen that after the first charge and discharge, the initial capacity of the prepared single cell was approximately 15 mAh. The capacity retention rate of the single cell reached its maximum value at the 50th cycle. Among them, the single cell assembled in Example 2 had the highest capacity retention rate, reaching 103.8%. In the 100-400 cycles, the capacity of the single cell assembled in Example 2 decreased more rapidly than that of the single cell assembled in Comparative Example 1. The capacity of the single cell assembled in Comparative Example 1 decreased rapidly thereafter. After the 1250th cycle, the capacity retention rates of the single cell assembled in Example 2 and Example 2 reached approximately the same level, namely 82.3% (Example 2) and 82.1% (Comparative Example 1), respectively. In the 1250 cycles, the cycle characteristic curve of the single cell assembled in Example 2 was flatter, and no accelerated decay phenomenon was observed. For the single cell composed of the example 1, compared with the example 2 and the comparative example 1, the capacity retention rate is at a higher level throughout the cycle process, only mutations occur at 820th, 1200th and 1230th, the overall cycle curve is smooth, no accelerated attenuation phenomenon occurs, and the capacity retention rate still maintains at about 90% after 1250 cycles, which proves that the addition of lithium polyacrylate can effectively improve the capacity retention rate after cycle and inhibit the accelerated attenuation.

[0025] Although the preferred embodiments of the present application have been shown and described, it is to be understood that the described embodiments are only part of the embodiments of the present application, and are not all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

Claims

1. A method for preparing a composite lithium battery separator containing a lithium polymer, characterized in that, The specific steps are as follows: (1) Place polyacrylic acid in water, add lithium carbonate to the reaction system, stir until the reaction is complete, evaporate the solvent, and obtain lithium polyacrylate; (2) Dissolve polyvinylidene fluoride and lithium polyacrylate in an organic solvent, heat and stir until completely dissolved to obtain a casting solution; (3) Casting solution is poured onto commercial polyethylene or polypropylene separator, solvent is evaporated, and a composite lithium battery separator containing lithium polymer is obtained.

2. The method for preparing a composite lithium battery separator containing a lithium polymer according to claim 1, characterized in that, In step (1), the molar ratio of monomer to lithium carbonate in polyacrylic acid is (1~2):2, and the molecular weight of the resulting lithium polyacrylic acid is 8000~30000.

3. The method for preparing a composite lithium battery separator containing a lithium polymer according to claim 1, characterized in that, In step (1), the reaction temperature is -10~10℃ and the reaction time is 30~120min.

4. The method for preparing a composite lithium battery separator containing a lithium polymer according to claim 1, characterized in that, In step (2), the organic solvent is any one or a mixture of two or more of N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and acetone in any proportion.

5. The method for preparing a composite lithium battery separator containing a lithium polymer according to claim 1, characterized in that, In step (2), the mass ratio of polyvinylidene fluoride to lithium polyacrylate is 1~10%, and the mass concentration of polyvinylidene fluoride in the casting solution is 5~11%.

6. The method for preparing a composite lithium battery separator containing a lithium polymer according to claim 1, characterized in that, In step (2), the heating and stirring temperature is 50~150℃.

7. The method for preparing a composite lithium battery separator containing a lithium polymer according to claim 1, characterized in that, In step (3), the temperature of the solvent evaporation is 70~100℃.

8. The method for preparing a composite lithium battery separator containing a lithium polymer according to claim 1, characterized in that, In step (3), after evaporating the solvent, the thickness of the lithium-containing polymer layer is 10~20µm.

9. The method for preparing a composite lithium battery separator containing a lithium polymer according to claim 4, characterized in that, The organic solvent is a mixture of N-methylpyrrolidone (NMP) and acetone in a mass ratio of (1~4):

1.

10. A composite lithium battery separator containing a lithium polymer prepared by the preparation method according to any one of claims 1 to 9.

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