Fluoropolymer porous lithium-ion battery separator and method of making the same

Abstract

This invention discloses a fluorinated polymer porous lithium-ion battery separator and its preparation method. The method involves dissolving or dispersing a polymer material in a solvent to obtain a polymer solution, which includes single polymer solutions and mixed polymer solutions. Fluorinated acrylate monomers are introduced into the polymer solution to obtain a functionalized polymer solution. The functionalized polymer solution is poured into a mold and subjected to low-temperature freezing to crystallize the solvent, obtaining a precursor membrane. The precursor membrane is then freeze-dried under vacuum to remove the solvent, yielding a lithium-ion battery separator with a porous structure. This invention, by controlling the polymer material system, introducing fluorinated functional monomers, and combining a freeze-drying process to construct a porous lithium-ion battery separator, meets the requirements of lithium-ion batteries for separator structural stability, interface compatibility, and application reliability.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and in particular to a fluorine-containing polymer porous lithium-ion battery separator and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used in electric vehicles, portable electronic devices, and energy storage systems due to their high energy density and good cycle performance. In lithium-ion batteries, the separator, as a key component located between the positive and negative electrodes, primarily functions to prevent short circuits caused by direct electrode contact, provide a transport channel for lithium ions, and maintain the stability of the battery's internal structure. Its performance directly affects the battery's safety and lifespan.

[0003] Currently, commercial lithium-ion battery separators are mostly made of polyolefin materials such as polyethylene or polypropylene. These separators possess certain mechanical strength and chemical stability, but their surface polarity is low, resulting in limited wettability with commonly used liquid electrolytes. This can easily lead to insufficient electrolyte wetting during battery assembly and operation, thus affecting lithium-ion transport efficiency. Furthermore, polyolefin separators are prone to thermal shrinkage under high-temperature conditions, posing certain safety hazards and failing to meet the demands for high safety and high stability in lithium-ion batteries.

[0004] To improve membrane performance, existing research has attempted to prepare functionalized membranes using polymer materials. This involves introducing hydrophilic or polar groups to enhance the membrane's wettability with the electrolyte, or constructing porous structures to optimize lithium-ion transport channels. However, the preparation of existing polymer membranes often requires organic solvent systems or complex chemical cross-linking reactions, resulting in demanding processing conditions, insufficient environmental friendliness, and significant challenges in controlling pore structure. Furthermore, some polymer membranes still fall short in balancing mechanical strength and ion conductivity, limiting their further application in lithium-ion batteries.

[0005] On the other hand, fluorinated functional monomers, due to their unique chemical structures, show potential advantages in improving the interfacial stability and electrochemical compatibility of polymer materials. Existing technologies have attempted to introduce fluorinated structures into battery-related material systems to enhance their stability under electrochemical conditions. However, current research on introducing fluorinated functional monomers into polymer membrane systems and combining them with freeze-drying processes to construct porous membranes suitable for lithium-ion batteries is still relatively limited. A relatively simple, structurally tunable, and suitable method for preparing polymer membranes for lithium-ion batteries is still lacking. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a fluorine-containing polymer porous lithium-ion battery separator and its preparation method, so as to meet the requirements of lithium-ion batteries for separator in terms of structural stability, interface adaptability and application reliability.

[0007] To solve the above technical problems, according to one aspect of the present invention, a method for preparing a fluorine-containing polymer porous lithium-ion battery separator is provided, comprising: Step 1: Dissolve or disperse the polymer material in a solvent to obtain a polymer solution, wherein the polymer solution includes a single polymer solution and a mixed polymer solution; The polymer material is selected from one or more combinations of polyacrylic acid (PAA), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG) and polyacrylamide (PAM); Step 2: Introduce fluorinated acrylate monomers into the polymer solution to obtain a functionalized polymer solution; Step 3: Pour the functional polymer solution into a mold and freeze it at low temperature to crystallize the solvent and obtain the precursor film. Step four: The precursor membrane is freeze-dried under vacuum conditions to remove the solvent and obtain a lithium-ion battery separator with a porous structure.

[0008] In a preferred embodiment, in step one, the solvent is selected from deionized water or an organic solvent, wherein the organic solvent is selected from one or more combinations of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), γ-butyrolactone (GBL), acetonitrile (ACN), and tetrahydrofuran (THF).

[0009] In a preferred embodiment, in step one, the polymer mixture solution is obtained by dissolving or dispersing two polymer materials in a solvent.

[0010] As a preferred embodiment, the two polymer materials are selected from any combination of the following: Polyacrylic acid (PAA) and polyvinyl alcohol (PVA); Polyacrylic acid (PAA) and polymethyl methacrylate (PMMA); Polyethylene glycol (PEG) and polyvinyl alcohol (PVA); Polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA); Polyethylene glycol (PEG) and polyacrylamide (PAM).

[0011] As a preferred embodiment, the ratio of the two polymer materials is 1:5 to 5:1 by mass.

[0012] In a preferred embodiment, in step one, a homogeneous and stable polymer solution is obtained by adjusting the dissolution temperature of the polymer material.

[0013] In a preferred embodiment, in step two, the fluorinated acrylate monomer is selected from one or more of 2,2,2-trifluoroethyl acrylate (TFEA), hexafluorobutyl acrylate (HFBA), and perfluoroalkyl acrylate (PFAA) in any combination.

[0014] As a preferred embodiment, the mass of the fluorinated acrylate monomer introduced is 5%-20% of the mass of the polymer material, by mass fraction.

[0015] In a preferred embodiment, step three includes pre-freezing and continued freezing. The pre-freezing temperature is -20℃ to -40℃ and the pre-freezing time is 1h to 4h. The continued freezing temperature is -50℃ to -60℃ and the continued freezing time is 2h to 8h.

[0016] According to another aspect of the present invention, a fluorine-containing polymer porous lithium-ion battery separator obtained by any of the above methods is provided.

[0017] This invention constructs a porous lithium-ion battery separator by regulating the polymer material system, introducing fluorine-containing functional monomers, and combining it with a freeze-drying process.

[0018] The lithium-ion battery separator provided by this invention is mainly composed of a polymer network structure, wherein the polymer materials form a stable continuous network structure through physical entanglement, hydrogen bonding, or a combination thereof; at the same time, by introducing fluorinated acrylate functional monomers, the interfacial compatibility between the separator and the electrolyte is further improved.

[0019] This invention constructs a porous structure in the membrane through a freezing process. This porous structure facilitates the wetting and distribution of electrolyte in the membrane and provides a continuous channel for lithium ion transport, thereby contributing to the effective migration of ions in the membrane.

[0020] In summary, the lithium-ion battery separator and its preparation method provided by this invention can meet the requirements of lithium-ion batteries for separators in terms of structural stability, interface compatibility and application reliability, and are suitable for use as separators in lithium-ion batteries. Attached Figure Description

[0021] Figure 1 The graph shows the 0.2C stable cycle performance of the CR2032 coin cell assembled with the freeze-dried porous membrane prepared in Example 1. Figure 2 EIS curve of the freeze-dried porous membrane prepared in Example 2; Figure 3 The graph shows the stable cycling performance at 0.2C for the CR2032 coin cell prepared in Example 5. Figure 4 Linear sweep voltammetry curves of steel-steel symmetric cells assembled with freeze-dried porous membranes prepared in Example 5; Figure 5 Linear sweep voltammetry curves of steel-steel symmetric cells assembled with PP separators of the commercially available Celgard 2400 for Comparative Example 1; Figure 6 0.2C stable cycle performance of CR2032 coin cells assembled with PP separator of commercial Celgard 2400 for Comparative Example 1; Figure 7 The graph shows the 0.2C stable cycle performance of the CR2032 coin cell assembled with a separator as shown in Comparative Example 2. Figure 8 The graph shows the 0.2C stable cycle performance of the CR2032 coin cell assembled with a separator as shown in Comparative Example 3. Detailed Implementation

[0022] The basic concept of this embodiment is to use a solvent system of soluble or dispersible polymeric materials as the basis for film formation. By controlling the types and combinations of polymeric materials, dissolution conditions and functional components, and introducing fluorine-containing functional monomers into the polymeric membrane system, a porous membrane structure is constructed by combining freeze-drying process, thereby obtaining a polymeric membrane material suitable for lithium-ion batteries.

[0023] Based on this, a typical embodiment of the present invention provides a method for preparing a porous lithium-ion battery separator containing fluorine polymers, comprising the following steps one to four.

[0024] Step 1: Prepare the polymer solution.

[0025] Polymer materials are dissolved or dispersed in a solvent to obtain a polymer solution. The polymer material can be a single polymer material or a combination of multiple polymer materials, and the obtained polymer solutions include single polymer solutions and mixed polymer solutions. Different polymer materials can form stable continuous network structures through physical entanglement, hydrogen bonding, or a combination thereof.

[0026] In the process of preparing a polymer solution, a uniform and stable polymer solution is obtained by controlling the dissolution temperature of the polymer material. The dissolution temperature is 20 to 100 ℃, for example, 20 ℃, 30 ℃, 50 ℃, 80 ℃, 100 ℃, etc.

[0027] The polymer material is selected from one or more combinations of polyacrylic acid (PAA), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and polyacrylamide (PAM).

[0028] A preferred embodiment involves preparing a polymeric mixed solution from a composite system consisting of two polymeric materials. The two polymeric materials are selected from any combination of the following: Polyacrylic acid (PAA) and polymethyl methacrylate (PMMA); Polyethylene glycol (PEG) and polyvinyl alcohol (PVA); Polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA); Polyethylene glycol (PEG) and polyacrylamide (PAM).

[0029] The ratio of the two polymer materials by mass is 1:5 to 5:1. Taking the composite system of polyacrylic acid (PAA) and polyvinyl alcohol (PVA) as an example, the mass ratio of polyacrylic acid (PAA) to polyvinyl alcohol (PVA) can be 1:5, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, or 5:1.

[0030] The solvent can be selected based on the solubility characteristics of the polymer material. In a preferred embodiment, the solvent is selected from deionized water or an organic solvent; wherein the organic solvent is selected from one or more combinations of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), γ-butyrolactone (GBL), acetonitrile (ACN), and tetrahydrofuran (THF).

[0031] Step 2: Introduce fluorine-containing functionalized components.

[0032] Fluorinated acrylate monomers are introduced into the polymer solution obtained in step one to obtain a functionalized polymer solution.

[0033] The introduced fluorine-containing structure can enhance the stability of the membrane system under electrochemical conditions and help improve the interfacial contact between the membrane, electrolyte, and electrode materials.

[0034] The fluorinated acrylate monomers are selected from one or more of 2,2,2-trifluoroethyl acrylate (TFEA), hexafluorobutyl acrylate (HFBA), and perfluoroalkyl acrylate (PFAA) or any combination thereof.

[0035] The mass fraction of fluorinated acrylate monomers is 5%-10% of the total mass of the polymer material, such as 5%, 6%, 6.5%, 7%, 8%, 8.6%, 9%, 10%, etc.

[0036] Step 3: Freezing.

[0037] The functional polymer solution is poured into a mold and frozen at low temperature to crystallize the solvent, providing a template for the subsequent formation of porous structures. The precursor membrane is obtained after the freezing treatment.

[0038] Preferably, the freezing process employs a distributed freezing procedure, including pre-freezing and continued freezing. That is, it is first pre-frozen at a relatively high low temperature for a period of time, and then continued to be frozen at a relatively low low temperature for a period of time.

[0039] The pre-freezing temperature is -20℃ to -40℃, and the pre-freezing time is 1h to 4h; the continued freezing temperature is -50℃ to -60℃, and the continued freezing time is 2h to 8h.

[0040] The step-by-step freezing process described above allows the solvent in the precursor membrane to gradually crystallize, which is beneficial for the formation of a porous structure inside the membrane.

[0041] Step 4: Vacuum freeze drying.

[0042] The precursor membrane is freeze-dried under vacuum conditions of 10-20 Pa to remove the solvent, resulting in a lithium-ion battery separator with a porous structure. The freeze-drying time is typically set to 10-40 h.

[0043] After the separator is prepared, it can be used as an isolation layer in a lithium-ion battery and assembled with the positive electrode, electrolyte, and negative electrode. For example, a liquid lithium-ion battery can be prepared based on the porous lithium-ion battery separator obtained in step four, lithium hexafluorophosphate (LiPF6) electrolyte, lithium negative electrode, and lithium iron phosphate positive electrode.

[0044] The technical solutions claimed in this invention will be further described below through some embodiments. However, the embodiments and comparative examples are for explaining the implementation schemes of this invention and do not exceed the scope of the subject matter of this invention. The scope of protection of this invention is not limited by the embodiments. Unless otherwise specified, the materials and reagents used in this invention can be obtained from commercially available products in the art.

[0045] Example 1 Weigh 0.15 g of polyacrylic acid (PAA) and add it to 10 mL of deionized water. Stir and dissolve at 25 °C for 2 h to obtain an aqueous PAA solution. Weigh 0.35 g of polyvinyl alcohol (PVA) and add it to 5 mL of deionized water. Stir and dissolve at 90 °C for 30 min to obtain an aqueous PVA solution. Mix the above PAA and PVA aqueous solutions and stir at room temperature for 12 h to obtain a homogeneous polymer mixture. Add 0.025 g of 2,2,2-trifluoroethyl acrylate (TFEA) to the obtained polymer mixture and continue stirring for 1 h to ensure uniform dispersion, obtaining a functionalized polymer solution. Pour the above functionalized polymer solution into a polytetrafluoroethylene mold and freeze it directly at -50 °C for 6 h. Freeze-dry the frozen precursor membrane under vacuum for 24 h to obtain a porous polymer membrane, which is then stored for later use.

[0046] Figure 1 The graph shows the 0.2C stable cycle performance of the CR2032 coin cell assembled with the freeze-dried porous membrane prepared in Example 1.

[0047] Example 2 Weigh 0.20 g of polyacrylic acid (PAA) and add it to 12 mL of N,N-dimethylformamide (DMF). Stir and dissolve at 60 °C for 4 h, then continue stirring at room temperature for 8 h to obtain a single polymer solution. Add 0.04 g of hexafluorobutyl acrylate (HFBA) to this single polymer solution and stir for 1 h to obtain a functionalized polymer solution. Pour this functionalized polymer solution into a mold and pre-cool at -20 °C for 3 h, then freeze at -60 °C for 4 h. Freeze-dry the frozen precursor membrane for 30 h to obtain a porous membrane, which is then stored for later use.

[0048] Figure 2 The EIS curve of the freeze-dried porous membrane prepared in Example 2 is shown below. Figure 2 As shown, the ionic conductivity of the battery is 1.10 × 10⁻⁶. -4 S·cm -1 .

[0049] Example 3 0.30 g of polyethylene glycol (PEG) was weighed and added to 8 mL of deionized water, and dissolved by stirring at 40 °C. 0.25 g of polyvinyl alcohol (PVA) was weighed and added to 4 mL of dimethyl sulfoxide (DMSO), and dissolved by stirring at 90 °C. The two solutions were mixed and stirred at room temperature for 10 h to obtain a homogeneous polymer mixture. 0.03 g of perfluoroalkyl acrylate (PFAA) was added to this polymer mixture, and stirring was continued for 1 h to obtain a functionalized polymer solution. This functionalized polymer solution was poured into a mold, pre-frozen at -30 °C for 2 h, and then transferred to -55 °C for 4 h to obtain a precursor membrane. The precursor membrane was then freeze-dried under vacuum for 24 h to obtain a porous polymer membrane, which was stored for later use.

[0050] Example 4 Weigh 0.25 g of polyethylene glycol (PEG) and 0.25 g of polyacrylamide (PAM), add them to 15 mL of N-methylpyrrolidone (NMP), and stir to dissolve at 70 °C for 5 h. Continue stirring at room temperature for 6 h to obtain a homogeneous polymer mixture. Add 0.05 g of 2,2,2-trifluoroethyl acrylate (TFEA) to the polymer mixture and stir until homogeneous to obtain a functionalized polymer solution. Pour the functionalized polymer solution into a mold, freeze at -50 °C for 2 h, freeze-dry for 15 h, and store for later use.

[0051] Example 5 Weigh 0.30 g of polyvinyl alcohol (PVA) and add it to 8 mL of deionized water. Stir and dissolve at 90 °C for 30 min to obtain an aqueous PVA solution. Weigh 0.20 g of polyvinylpyrrolidone (PVP) and add it to 8 mL of deionized water. Stir and dissolve at 30 °C for 1 h to obtain an aqueous PVP solution. Mix the above aqueous PVA solution and aqueous PVP solution and stir at room temperature for 12 h to obtain a homogeneous polymer mixture solution. Add 0.025 g of 2,2,2-trifluoroethyl acrylate (TFEA) and continue stirring for 1 h to uniformly disperse the fluorinated acrylate monomers in the polymer system to obtain a functionalized polymer solution. Pour the above functionalized polymer solution into a polytetrafluoroethylene mold and pre-freeze at -20 °C for 3 h. Then place it at -60 °C and continue freezing for 4 h to allow the solvent to crystallize. Freeze-dry the frozen precursor membrane under vacuum for 30 h to remove the solvent by sublimation, and obtain a polymer membrane with a porous structure for later use.

[0052] Figure 3 The graph shows the stable cycling performance at 0.2C for the CR2032 coin cell prepared in Example 5. Figure 4 Linear sweep voltammetry curves of steel-steel symmetric cells assembled with the freeze-dried porous membrane prepared in Example 5.

[0053] Comparative Example 1 Liquid lithium-ion batteries were prepared using PP separators selected from commercially available Celgard 2400, lithium hexafluorophosphate (LiPF6) electrolyte, lithium anode, and lithium iron phosphate cathode. Figure 5 Linear sweep voltammetry curves of steel-steel symmetric cells assembled with PP separators of commercially available Celgard 2400 for Comparative Example 1. Figure 6 The graph shows the 0.2C stable cycle performance of a CR2032 coin cell assembled with a PP separator from the commercially available Celgard 2400 as Comparative Example 1.

[0054] Comparative Example 2 Except for the absence of fluorinated acrylate monomers, the remaining steps were the same as in Example 5. Specifically, without the addition of 2,2,2-trifluoroethyl acrylate (TFEA), the resulting polymer mixture was directly poured into a mold to form a film. Figure 7 The graph shows the 0.2C stable cycle performance of the CR2032 coin cell assembled with a separator, which is a comparative example 2.

[0055] Comparative Example 3 Except for the absence of freezing and freeze-drying treatments, the remaining steps were the same as in Example 5. Specifically, the obtained functionalized polymer solution was poured into a polytetrafluoroethylene mold to form a film, and then directly vacuum-dried at 60 °C for 24 h to obtain a diaphragm, which was then stored for later use.

[0056] Figure 8 The graph shows the 0.2C stable cycle performance of the CR2032 coin cell assembled with a separator as shown in Comparative Example 3.

[0057] At room temperature, the cells were activated at 0.1C, charged and discharged at 0.2C, and then stabilized for 50 cycles. The discharge capacity of the first cycle is denoted as C1, and the discharge capacity of the 50th cycle is denoted as C2. 50 Capacity retention is denoted by CR, where CR = C. 50 / C1*100%. Table 1 shows a comparison of the electrochemical performance of Example 5 and Comparative Example 1.

[0058] Table 1

[0059] By comparing Example 5 with Comparative Examples 1, 2, and 3, the freeze-dried fluorinated polymer porous membrane exhibits higher discharge capacity and capacity retention.

[0060] The scope of protection claimed by this invention is not limited to the specific embodiments described above. For those skilled in the art, this invention can have various modifications and alterations. Any modifications, improvements, and equivalent substitutions made within the concept and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for preparing a fluorine-containing polymer porous lithium-ion battery separator, characterized in that, include: Step 1: Dissolve or disperse the polymer material in a solvent to obtain a polymer solution, wherein the polymer solution includes a single polymer solution and a mixed polymer solution; The polymer material is selected from one or more combinations of polyacrylic acid (PAA), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG) and polyacrylamide (PAM); Step 2: Introduce fluorinated acrylate monomers into the polymer solution to obtain a functionalized polymer solution; Step 3: Pour the functional polymer solution into a mold and freeze it at low temperature to crystallize the solvent and obtain the precursor film. Step four: The precursor membrane is freeze-dried under vacuum conditions to remove the solvent and obtain a lithium-ion battery separator with a porous structure.

2. The method for preparing the fluorine-containing polymer porous lithium-ion battery separator according to claim 1, characterized in that: In step one, the solvent is selected from deionized water or an organic solvent, wherein the organic solvent is selected from one or more combinations of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), γ-butyrolactone (GBL), acetonitrile (ACN), and tetrahydrofuran (THF).

3. The method for preparing the fluorine-containing polymer porous lithium-ion battery separator according to claim 1 or 2, characterized in that: In step one, the polymer mixture solution is obtained by dissolving or dispersing two polymer materials in a solvent.

4. The method for preparing the fluorine-containing polymer porous lithium-ion battery separator according to claim 3, characterized in that: The two polymer materials are selected from any combination of the following: Polyacrylic acid (PAA) and polyvinyl alcohol (PVA); Polyacrylic acid (PAA) and polymethyl methacrylate (PMMA); Polyethylene glycol (PEG) and polyvinyl alcohol (PVA); Polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA); Polyethylene glycol (PEG) and polyacrylamide (PAM).

5. The method for preparing the fluorine-containing polymer porous lithium-ion battery separator according to claim 4, characterized in that: The ratio of the two polymer materials by mass is 1:5 to 5:

1.

6. The method for preparing a fluorine-containing polymer porous lithium-ion battery separator according to claim 1, 2, 4 or 5, characterized in that: In step one, a homogeneous and stable polymer solution is obtained by adjusting the dissolution temperature of the polymer material.

7. The method for preparing the fluorine-containing polymer porous lithium-ion battery separator according to claim 6, characterized in that: In step two, the fluorinated acrylate monomers are selected from one or more of 2,2,2-trifluoroethyl acrylate (TFEA), hexafluorobutyl acrylate (HFBA), and perfluoroalkyl acrylate (PFAA) in any combination.

8. The method for preparing the fluorine-containing polymer porous lithium-ion battery separator according to claim 7, characterized in that: The mass fraction of fluorinated acrylate monomers is 5%-20% of the mass of the polymer material.

9. The method for preparing a fluorine-containing polymer porous lithium-ion battery separator according to claim 8, characterized in that: In step three, the freezing process includes pre-freezing and continued freezing. The pre-freezing temperature is -20℃ to -40℃ and the pre-freezing time is 1h to 4h. The continued freezing temperature is -50℃ to -60℃ and the continued freezing time is 2h to 8h.

10. A fluorine-containing polymer porous lithium-ion battery separator obtained by any one of claims 1-9.

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