Preparation method of a polymer electrolyte with an improved phase separation structure

Abstract

The present invention discloses a preparation method of a polymer electrolyte with an improved phase separation structure, and the method is as follows: First, epoxy resin, a curing agent and a lithium salt are stirred and dissolved at room temperature to form a transparent and uniform prepolymer solution A; Second, PVDF, a lithium salt and a solvent are stirred and dissolved to form a transparent and uniform solution B; Third, the prepolymer solution A and the solution B are mixed evenly, defoamed by ultrasonic treatment and then transferred to a mold, and dried and cured in a vacuum oven to obtain a polymer electrolyte with an improved phase separation structure. Through the ring-opening polymerization reaction of epoxy groups and amino groups, the present invention incorporates polyetheramine with amorphous long flexible chain segments into the polymer molecular system, and the obtained polymer network system has the advantages of high flexibility, low crystallinity and high affinity with lithium salts. Using such a polymer system as a bridge, the phase separation problem of PVDF-based solid electrolytes is solved, and thus a PVDF-based polymer electrolyte with a homogeneous structure is prepared.

Description

Technical Field

[0001] The present invention relates to a method for preparing a polymer electrolyte, and particularly to a method for preparing a polymer electrolyte based on phase separation structure adjustment. Background Art

[0002] In the past decade, the development of mobile electronic devices, new energy vehicles, and large-scale energy storage power stations has stimulated the wide application of lithium-ion batteries. Currently, the electrolyte used in lithium batteries is a liquid electrolyte, but it has inherent defects such as electrolyte leakage and explosion, which have caused widespread safety problems. To solve the safety problems and meet the demand for higher energy density, solid-state lithium batteries with solid electrolytes have emerged. Compared with liquid lithium battery systems, solid-state lithium batteries are safer and have a higher theoretical capacity density (3860 Ah kg -1 ).

[0003] Currently, the electrolytes used in solid-state lithium batteries are divided into three categories: polymer electrolytes, inorganic solid electrolytes, and composite electrolytes. Sulfide solid electrolytes (Li6PS5Cl, Li4SnS4), oxide solid electrolytes (LLZTO, LLZO, LATP), and halide solid electrolytes (LiNTiCl6, Li 1.75 ZrCl 4.75 O 0.5 ) have been widely studied due to their high room-temperature ionic conductivity. However, due to the inherent brittleness and rigidity of inorganic solid electrolytes, their interfacial contact performance with electrode materials is poor. Compared with inorganic solid electrolytes, polymer electrolytes and polymer-based composite electrolytes have more flexible mechanical properties and simpler forming processes. Therefore, polymer-based electrolytes have been widely studied in the past few decades and have become one of the mainstream directions in the research of solid electrolytes, with great development potential.

[0004] The commonly used polymer matrices for polymer electrolytes are polyethylene oxide (PEO) and polyvinylidene fluoride (PVDF). As a commonly used polymer electrolyte matrix, PEO has good lithium salt affinity. However, PEO has the disadvantages of semi-crystallinity, low ionic conductivity, and poor electrochemical stability, which cannot meet the actual needs of lithium batteries. Although the inherent defects of PEO can be partially solved by molecular modification, material composite, and other methods. However, from the current development status, there is still a large gap from actual applications. Compared with PEO, PVDF has excellent mechanical properties (Young's modulus is about 1.2 GPa) and electrochemical stability (electrochemical window is about 5.0 V). Therefore, PVDF electrolytes have broad development prospects and are expected to meet the actual needs of lithium batteries for the electrochemical performance of polymer electrolytes. By constructing polymers in salt electrolytes, the ionic conductivity of PVDF-based polymer electrolytes can reach a very high level (≥10-3 S / cm). However, PVDF has a semi-crystalline structure (mainly α and β crystal structures), with poor affinity for traditional lithium salts, resulting in frequent phase separation in PVDF-based electrolytes, leading to uneven distribution of Li + transport channels, which affects the improvement of the ionic conductivity of PVDF-based electrolytes. However, the phase structure adjustment of PVDF-based electrolytes has not received enough attention. Therefore, improving the electrochemical performance of PVDF-based polymer electrolytes by adjusting the phase structure is a promising research direction. Therefore, it is necessary to develop a polymer electrolyte system with high ionic conductivity and convenient processing technology to meet the development needs of solid-state lithium batteries. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a preparation method of a polymer electrolyte with an improved phase separation structure. The present invention adopts a polymer-mediated phase separation structure adjustment strategy to improve the phase separation defects of PVDF-based polymer electrolytes. Using a designed flexible polymer system as an inducer, the α and β crystal phases of PVDF are transformed into amorphous phases, and the LiTFSI and PVDF molecular chains are connected to achieve uniform distribution of the polymer matrix and LiTFSI. Due to the uniform distribution of the components of the entire electrolyte system, the Li + transition network in the polymer electrolyte is wider, resulting in + significantly improved Li + conductivity and uniform Li

[0006] deposition. With the designed electrolyte, the Li||LiFePO4 battery exhibits excellent rate performance and cycling stability. This polymer-mediated phase separation regulation strategy can solve the distribution problem of Li+ transport channels in polymer electrolytes and further guide the design of high-performance PVDF-based polymer electrolytes.

[0007] A preparation method of a polymer electrolyte with an improved phase separation structure, comprising the following steps:

[0008] Step 1: Stir and dissolve epoxy resin, curing agent and lithium salt at room temperature for 10-15 hours until a transparent and uniform prepolymer solution A is formed, where:

[0009] The mass ratio of the epoxy resin to the curing agent is 1:1-10;

[0010] The lithium salt is not limited to one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluoro(oxalato)borate, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium perchlorate, and the mass of the lithium salt is 10-50% of the total mass of the prepolymer solution A;

[0011] The epoxy resin is one or a complex of epoxy resin A and epoxy resin B. When the epoxy resin is a complex of epoxy resin A and epoxy resin B, the mass ratio of epoxy resin A to epoxy resin B is 1:1 to 1:5;

[0012] The epoxy resin A has the following molecular structural characteristics:

[0013]

[0014] In the formula, R1 and R2 are not limited to one or several of alkyl, amino, heterocycle, heteroaromatic ring, hydrogen, and fluorine;

[0015] The epoxy resin B has the following molecular structural characteristics:

[0016]

[0017] In the formula, R3 and R4 are not limited to one or several of alkyl, amino, heterocycle, heteroaromatic ring, hydrogen, and fluorine;

[0018] The curing agent is a long-chain flexible curing agent and has the following molecular structural characteristics:

[0019]

[0020] In the formula, R5 and R6 are not limited to one or several of mercapto, amino, amide, hydroxyl, and carboxyl; R7 is not limited to one or several of hydrogen, alkyl, long-chain alkyl, hydroxymethyl, and carboxymethyl; n is any number between 1 and 100;

[0021] Step 2: Stir and dissolve PVDF, lithium salt, and solvent under certain temperature conditions until a transparent and uniform solution B is formed, where:

[0022] The temperature of the stirring is 0 to 100 °C, and the time is 1 to 2 hours;

[0023] The solvent is not limited to at least one of isopentane, n-pentane, petroleum ether, hexane, cyclohexane, cyclopentane, heptane, carbon tetrachloride, benzene, toluene, xylene, chlorobenzene, o-dichlorobenzene, dichloromethane, carbon tetrachloride, methanol, ethanol, ethylene glycol, propanol, isopropanol, butanol, isobutanol, tert-butanol, pentanol, benzyl alcohol, ethyl acetate, ether, petroleum ether, isopropyl ether, tetrahydrofuran, chloroform, dioxane, pyridine, acetone, acetonitrile, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide;

[0024] The dosage ratio of PVDF, lithium salt, and solvent is 1:0.5 to 3:5 to 10;

[0025] The PVDF has the following molecular structural characteristics:

[0026]

[0027] Wherein, n is any number between 1 and 20,000;

[0028] Step 3: Mix the prepolymer solution A and the solution B evenly, remove the bubbles by ultrasonic treatment, and then transfer them to a mold, and dry and cure them in a vacuum oven to obtain a polymer electrolyte with an improved phase separation structure, where:

[0029] The mass ratio of the prepolymer solution A to the solution B is 1:10 to 50;

[0030] The temperature for drying and curing is 50 to 120 °C, and the time is 1 to 12 hours.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] 1. Through the ring-opening polymerization reaction of epoxy groups and amino groups, the present invention incorporates polyetheramine with amorphous long flexible chain segments into the polymer molecular system. The obtained polymer network system has the advantages of high flexibility, low crystallinity, and high affinity for lithium salts. Using such a polymer system as a bridge, the phase separation problem of PVDF-based solid electrolytes is solved, and a PVDF-based polymer electrolyte with a homogeneous structure is thus prepared.

[0033] 2. The present invention uses a polymer system with excellent lithium salt affinity as a medium to regulate the phase separation structure between polyvinylidene fluoride (PVDF) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). In the system, PVDF exists as a polymer matrix, while the polymer network (BPE) formed by cross-linking epoxy resin and polyetheramine acts as a mediator between PVDF and LiTFSI. The polymer network designed in the present invention has excellent LiTFSI affinity, which not only promotes the dissociation of LiTFSI but also acts as a bridge between PVDF and LiTFSI, forming a uniform phase structure between the hybrid polymer molecular network and LiTFS1, thereby enhancing the + transport ability of Li + in the polymer and homogenizing the transport channels of Li. Therefore, the polymer electrolyte (BPLE) prepared by the present invention has excellent ionic conductivity and excellent cycle stability.

[0034] 3. The method for preparing the polymer electrolyte proposed by the present invention is simple, the raw materials are easily available, and it has the prospect of industrial production and application, which has an inspiring effect on the development of high-performance polymer-based solid-state batteries. Detailed implementation manners

[0035] The technical solution of the present invention will be further described below in conjunction with embodiments, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be covered by the protection scope of the present invention.

[0036] Example 1: Preparation of prepolymer solution A

[0037] Take 5 g of curing agent and 3 g of lithium bis(trifluoromethanesulfonyl)imide, stir at room temperature for 12 hours until the lithium salt is completely dissolved, then add 1.2 g of epoxy resin A, stir for 5 minutes until evenly mixed to form prepolymer solution A, and then remove bubbles by ultrasound.

[0038] In this example, the molecular structural formula of the epoxy resin is:

[0039]

[0040] In this example, the molecular structural formula of the curing agent is:

[0041]

[0042] Example 2: Preparation of polymer electrolyte A using prepolymer solution A as a component

[0043] Take 1 g of PVDF-A and 1 g of lithium bis(trifluoromethanesulfonyl)imide, stir at room temperature for 12 hours until the lithium salt is completely dissolved, then add 1.0 g of prepolymer solution A, stir for 5 minutes until evenly mixed, and then remove bubbles by ultrasound. Then transfer it to a circular mold and dry at 120 °C for 4 hours to obtain polymer electrolyte A, and its physical properties and electrochemical properties are shown in Table 1.

[0044] In this example, the molecular structural formula of PVDF-A is:

[0045]

[0046] Table 1 Physical properties and electrochemical properties of polymer electrolyte A

[0047]

[0048]

[0049] Example 3: Preparation of prepolymer solution B

[0050] Take 6 g of curing agent and 4.8 g of lithium bis(trifluoromethanesulfonyl)imide, stir at room temperature for 12 hours until the lithium salt is completely dissolved, then add 1.0 g of epoxy resin B, stir for 5 minutes until evenly mixed to form prepolymer solution B, and then remove bubbles by ultrasound.

[0051] In this example, the molecular structural formula of the epoxy resin is:

[0052]

[0053] In this embodiment, the molecular structural formula of the curing agent is as follows:

[0054]

[0055] Example 4: Preparation of Polymer Electrolyte B using prepolymer solution B as a component

[0056] Take 1 g of PVDF-B and 1.75 g of lithium bis(trifluoromethanesulfonyl)imide and stir at room temperature for 12 hours until the lithium salt is completely dissolved. Then add 0.75 g of prepolymer solution B, stir for 5 minutes until evenly mixed, and then remove bubbles by ultrasonic treatment. After that, transfer it to a circular mold and dry at 120 °C for 2 hours to obtain Polymer Electrolyte B. Its physical properties and electrochemical properties are shown in Table 2.

[0057] In this embodiment, the molecular structural formula of PVDF-B is as follows:

[0058]

[0059] Table 2 Physical properties and electrochemical properties of Polymer Electrolyte B

[0060] Project Performance Tensile strength (MPa) 2.4±0.5 Young's modulus (MPa) 710.0±0.5 Tensile strain (%) 18.0±0.5 Tg (°C) -35.0±0.5 <![CDATA[Lithium ion conductivity (10 -3 S / cm)]]> 1.4±0.05 Electrochemical window (V) 5.1±0.05 Lithium ion transference number 0.63±0.05

[0061] Example 5: Preparation of prepolymer solution C

[0062] Take 6 g of the curing agent and 3.0 g of lithium trifluoromethanesulfonate and stir at room temperature for 12 hours until the lithium salt is completely dissolved. Then add 1.2 g of epoxy resin C, stir for 5 minutes until evenly mixed to form prepolymer solution C, and then remove bubbles by ultrasonic treatment.

[0063] In this embodiment, the molecular structural formula of the epoxy resin is as follows:

[0064]

[0065] In this embodiment, the molecular structural formula of the curing agent is as follows:

[0066]

[0067] Example 6: Preparation of Polymer Electrolyte C using prepolymer solution C as a component

[0068] Take 1 g of PVDF-C and 1.5 g of lithium bis(trifluoromethanesulfonyl)imide and stir at room temperature for 12 hours until the lithium salt is completely dissolved. Then add 0.75 g of prepolymer solution C and stir for 5 minutes until evenly mixed. Then remove the bubbles by ultrasonic treatment. After that, transfer it to a circular mold and dry at 120 °C for 2 hours to obtain polymer electrolyte B. Its physical properties and electrochemical properties are shown in Table 2.

[0069] In this example, the molecular structural formula of PVDF-C is:

[0070]

[0071] Table 2 Physical properties and electrochemical properties of polymer electrolyte B

[0072] Project Performance Tensile strength (MPa) 2.2±0.5 Young's modulus (MPa) 1100.0±0.5 Tensile strain (%) 10.0±0.5 Tg (°C) -39.0±0.5 <![CDATA[Lithium ion conductivity (10 -3 S / cm)]]> 1.6±0.05 Electrochemical window (V) 5.0±0.05 Lithium ion transference number 0.65±0.05

Claims

1. A method for preparing a polymer electrolyte with an improved phase separation structure, characterized in that The method comprises the following steps: Step 1: Stir and dissolve epoxy resin, curing agent and lithium salt until a transparent and uniform prepolymer solution A is formed, where: The mass ratio of the epoxy resin to the curing agent is 1:1 - 10; The mass of the lithium salt is 10 - 50% of the total mass of the prepolymer solution A; The epoxy resin is one or a combination of epoxy resin A and epoxy resin B; The epoxy resin A has the following molecular structural characteristics: In the formula, R1 and R2 are one or several of alkyl, amino, heterocycle, heteroaromatic ring, hydrogen, and fluorine; The epoxy resin B has the following molecular structural characteristics: In the formula, R3 and R4 are one or several of alkyl, amino, heterocycle, heteroaromatic ring, hydrogen, and fluorine; The curing agent has the following molecular structural characteristics: In the formula, R5 and R6 are one or several of mercapto, amino, amide group, hydroxyl group, and carboxyl group; R7 is one or several of hydrogen, alkyl, long-chain alkyl, hydroxymethyl, and carboxymethyl; n is any number between 1 and 100; Step 2: Stir and dissolve PVDF, lithium salt and solvent until a transparent and uniform solution B is formed, where: The dosage ratio of the PVDF, lithium salt and solvent is 1:0.5 - 3:5 - 10; Step 3: Mix the prepolymer solution A and the solution B evenly, remove bubbles by ultrasonic treatment, and then transfer them to a mold, and dry and cure in a vacuum oven to obtain a polymer electrolyte with an improved phase separation structure, where: The mass ratio of the prepolymer solution A to the solution B is 1:10 - 50.

2. The preparation method of the polymer electrolyte with an improved phase separation structure according to claim 1, characterized in that The lithium salt is one or several of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium difluoro(oxalato)borate, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, and lithium perchlorate.

3. The preparation method of the polymer electrolyte with an improved phase separation structure according to claim 1, wherein When the epoxy resin is a combination of epoxy resin A and epoxy resin B, the mass ratio of epoxy resin A to epoxy resin B is 1:1 - 1:

5.

4. The preparation method of the polymer electrolyte with an improved phase separation structure according to claim 1, characterized in that In Step 1, the stirring and dissolving time is 10 - 15 hours.

5. The preparation method of the polymer electrolyte with an improved phase separation structure according to claim 1, characterized in that In Step 2, the stirring temperature is 0 - 100 °C and the time is 1 - 2 hours.

6. The preparation method of the polymer electrolyte with an improved phase separation structure according to claim 1, characterized in that The solvent is at least one of isopentane, n-pentane, petroleum ether, hexane, cyclohexane, cyclopentane, heptane, carbon tetrachloride, benzene, toluene, xylene, chlorobenzene, o-dichlorobenzene, dichloromethane, carbon tetrachloride, methanol, ethanol, ethylene glycol, propanol, isopropanol, butanol, isobutanol, tert-butanol, pentanol, benzyl alcohol, ethyl acetate, ether, petroleum ether, isopropyl ether, tetrahydrofuran, chloroform, dioxane, pyridine, acetone, acetonitrile, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

7. The method for preparing a polymer electrolyte having an improved phase separation structure according to claim 1, characterized in that The PVDF has the following molecular structural characteristics: In the formula, n is any number between 1 and 20000.

8. The preparation method of the polymer electrolyte with an improved phase separation structure according to claim 1, characterized in that The drying and curing temperature is 50 - 120 °C and the time is 1 - 12 hours.

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