Modified pvdf separator and method for preparing the same

Abstract

The application discloses an alkali treatment modified polyvinylidene fluoride (PVDF) nanofiber membrane and a preparation method thereof. The application takes PVDF as raw material, first carries out alkali treatment on the PVDF, and then utilizes electrostatic spinning to prepare the modified PVDF nanofiber membrane, so that the modified PVDF nanofiber membrane has suitable hydrophilicity, porosity and excellent mechanical properties. The modified PVDF nanofiber membrane obtained by the application has a porosity of 45-55%, a water absorption rate of 250-350%, a contact angle of 55-60 DEG, a breaking strength of 8-9 MPa and an elongation rate of 7-8%. When used in a water-based zinc ion battery, the modified PVDF nanofiber diaphragm prepared by the application shows more excellent electrochemical performance than a traditional glass fiber diaphragm, and has a discharge specific capacity of 128 mAh / g at a current density of 1.0 A / g, which is 1.88 times of that of the glass fiber diaphragm battery, and the cycle number is more than 1000 times.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrochemistry, in particular to a separator for aqueous zinc ion battery and a preparation technology thereof. BACKGROUND

[0002] Energy crisis is a major problem faced by mankind. Secondary batteries with efficient electrical energy / chemical energy conversion are an effective green energy solution. Secondary batteries, also known as rechargeable batteries, have basic components including negative electrode, positive electrode, separator and electrolyte. Lithium ion batteries are currently the dominant secondary batteries in the market, but the scarcity of lithium resources, high cost, poor safety, and potential safety hazards such as fire and explosion limit their large-scale promotion and application. Aqueous zinc ion battery has the advantages of abundant zinc reserves, low cost, safe operation, no environmental pollution and high ionic conductivity, and is an ideal green battery energy storage system. The disadvantages of aqueous zinc ion battery are narrow window voltage, easy side reaction of zinc negative electrode, and easy dissolution of positive active material, which leads to poor cycle stability of the battery.

[0003] The separator is an important component of the secondary battery and has an important influence on the performance of the battery. The separator can provide a physical barrier between the positive and negative electrodes of the battery to prevent short circuiting of the battery. During the charge and discharge cycle of the battery, the separator also has the function of allowing electrolyte ions to pass through. The composition, porosity, ionic conductivity and mechanical strength of the separator will affect the transport behavior of ions inside the separator and at the electrolyte-negatve electrode interface, thereby affecting the cycle life of the negative electrode. The separator of zinc ion battery is generally a few hundred microns thick glass fiber membrane with high hydrophilicity. However, the glass fiber separator has high brittleness, and the zinc dendrites generated during the charge and discharge process of the zinc ion battery can easily penetrate the separator, inducing short circuit of the battery. Therefore, it is urgent to develop high-performance separators to further improve the electrochemical performance of the zinc negative electrode and the entire battery system.

[0004] Weijun et al. [Energy Storage Mater. 2022, 44: 57-65] used cotton cellulose as the separator of aqueous zinc ion battery, which has a dense and uniform nanopore, good electrolyte wettability, excellent mechanical properties and high ionic conductivity. The disadvantage is that the cellulose separator will swell after being soaked in the electrolyte solution, which will reduce the strength of the separator and make it easy to be penetrated by the growing zinc dendrites, leading to short circuit of the battery. Meena et al. [Energy Technol. 2019: 1900442] used Nafion as the separator of zinc-vanadium battery, which can attract zinc ions in the electrolyte directionally, making zinc ions pass through the separator uniformly, reducing the irregular deposition of zinc ions and the large growth of zinc dendrites. The number of zinc ions transferred by the sulfonic acid group is 0.52, the discharge specific capacity at 0.25 A / g is 510 mAh / g, close to the theoretical capacity of 589 mAh / g, and the capacity retention rate after 1800 cycles is 88%. However, the cost of Nafion is too high and it is heavily dependent on imports, which limits its large-scale promotion and application. SUMMARY

[0005] To solve the above problems, the present application provides an alkali-treated modified PVDF separator and a preparation method thereof. Specifically, PVDF is treated with alkali to contain a certain amount of hydrophilic functional groups, exhibit suitable hydrophilic properties, and be processed into a nanofiber membrane by electrospinning to further improve the water absorption and mechanical strength of the separator.

[0006] The technical solution adopted by the present application is: PVDF is treated with alkali to obtain hydrophilically modified PVDF, and then electrospinning is used to obtain alkali-treated modified PVDF nanofiber separator.

[0007] In a first aspect of the application, there is provided:

[0008] An alkali-treated modified PVDF nanofiber separator, characterized by a porosity of 45-55%, a water absorption rate of 250-350%, a contact angle of 55-60°, a breaking strength of 8-9 MPa, and an elongation of 7-8%.

[0009] In a second aspect of the application, there is provided:

[0010] A preparation method of an alkali-treated modified PVDF nanofiber separator, characterized by:

[0011] (1) 5 g of PVDF is added to 50 mL of 0.3-0.7 mol / L alkali / ethanol solution, 0.1-0.3 g of tetrabutylammonium bromide (TBAB) is added, stirring at 60°C for 0.5-0.8 h, filtering, washing the solid to neutral, and drying to obtain modified PVDF solid.

[0012] (2) The modified PVDF solid obtained in step (1) is dissolved in any one of N, N-dimethylformamide, N, N-dimethylacetamide, N-methylpyrrolidone to obtain a spinning solution with a concentration of 17wt%, and electrospinning is performed to obtain an alkali treatment modified PVDF nanofiber membrane.

[0013] The alkali in step (1) is NaOH or KOH, and the preferred concentration is 0.5mol / L.

[0014] In step (1), the preferred stirring time is 0.5h, and the amount of TBAB is 0.15g.

[0015] The alkali treatment modified PVDF nanofiber separator prepared by the application has better mechanical properties than traditional glass fiber separators. When applied to water-based zinc ion batteries, the modified PVDF nanofiber separator prepared by the application shows more excellent electrochemical performance, and the discharge specific capacity at a current density of 1.0A / g is 128mAh / g, which is 1.88 times that of a glass fiber separator battery, and the cycle number is more than 1000 times, which is much larger than the 400 times of a glass fiber separator battery.

[0016] The beneficial effects of the application are:

[0017] (1) The application performs alkali treatment on PVDF to obtain alkali treatment modified PVDF, so that PVDF has suitable hydrophilicity and can be used in water-based batteries.

[0018] (2) The application uses electrospinning to prepare a modified PVDF nanofiber separator, which further improves the mechanical properties, porosity and water absorption of the modified PVDF separator.

[0019] (3) The alkali treatment modified PVDF nanofiber separator prepared by the application has better specific capacity and cycle service life than traditional glass fiber separators when applied to water-based zinc ion batteries. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The cycle service performance of the alkali treatment modified PVDF nanofiber separator obtained in Example 1 and the glass fiber separator in the zinc ion battery at 1A / g. DETAILED DESCRIPTION

[0021] In order to further illustrate the technical means adopted by the application, the following will be specifically described in combination with the embodiments of the application and the accompanying drawings.

[0022] The specific embodiments of the application are as follows:

[0023] Example 1

[0024] (1) Alkali treatment of PVDF. 5 g PVDF powder was added into 50 mL 0.5 mol / L NaOH ethanol solution and stirred at 60 °C for 0.5 h, and the amount of TBAB added was 3 mg / mL. Filtration was performed, and the collected PVDF powder was repeatedly washed with deionized water until neutral, and dried at 105 °C for standby use.

[0025] (2) Preparation of modified PVDF spinning solution. 1.6 g of modified PVDF powder was added into 10 mL DMF, and magnetic stirring was performed at 60 °C for 8 h to completely dissolve the PVDF powder, to obtain a modified PVDF spinning solution with a concentration of 17 wt%.

[0026] (3) Preparation of alkali-treated modified PVDF nanofiber. The modified PVDF solution was spun by using an electrospinning technology to obtain a modified PVDF nanofiber. The spinning parameters were as follows: spinning voltage 18 KV, injection pump speed 1.0 mL / h, receiving distance 15 cm, and drum rotation speed 60 r / min.

[0027] Comparative Example 1

[0028] (1) Alkali treatment of PVDF. 5 g PVDF powder was added into 50 mL 1 mol / L NaOH ethanol solution and stirred at 60 °C for 0.5 h, and the amount of TBAB added was 3 mg / mL. Filtration was performed, and the collected PVDF powder was repeatedly washed with deionized water until neutral, and dried at 105 °C for standby use.

[0029] (2) Preparation of modified PVDF spinning solution. 1.6 g of modified PVDF powder was added into 10 mL DMF, and magnetic stirring was performed at 60 °C for 8 h to completely dissolve the PVDF powder, to obtain a modified PVDF spinning solution with a concentration of 17 wt%.

[0030] (3) Preparation of modified PVDF nanofiber. The modified PVDF solution was spun by using an electrospinning technology to obtain a modified PVDF nanofiber. The spinning parameters were as follows: spinning voltage 18 KV, injection pump speed 1.0 mL / h, receiving distance 15 cm, and drum rotation speed 60 r / min.

[0031] Comparative Example 2

[0032] (1) Alkali treatment of PVDF. 5 g PVDF powder was added into 50 mL 2 mol / L NaOH ethanol solution and stirred at 60 °C for 0.5 h, and the amount of TBAB added was 3 mg / mL. Filtration was performed, and the collected PVDF powder was repeatedly washed with deionized water until neutral, and dried at 105 °C for standby use.

[0033] (2) Preparation of modified PVDF spinning solution. 1.6 g of modified PVDF powder was added to 10 mL of DMF, and the PVDF powder was completely dissolved by magnetic stirring at 60°C for 8 h to obtain a modified PVDF spinning solution with a concentration of 17 wt%.

[0034] (3) Preparation of modified PVDF nanofiber. The modified PVDF solution was spun using an electrospinning technique to obtain modified PVDF nanofiber. The spinning parameters were: spinning voltage 18 KV, injection pump speed 1.0 mL / h, receiving distance 15 cm, and drum rotation speed 60 r / min.

[0035] Comparative Example 3

[0036] (1) Alkaline treatment of PVDF. 5 g of PVDF powder was added to 50 mL of 4 mol / L NaOH ethanol solution and stirred at 60°C for 0.5 h, with a TBAB addition amount of 3 mg / mL. The collected PVDF powder was repeatedly washed with deionized water until neutral, and dried at 105°C for standby use.

[0037] (2) Preparation of modified PVDF spinning solution. 1.6 g of modified PVDF powder was added to 10 mL of DMF, and the PVDF powder was completely dissolved by magnetic stirring at 60°C for 8 h to obtain a modified PVDF spinning solution with a concentration of 17 wt%.

[0038] (3) Preparation of modified PVDF nanofiber. The modified PVDF solution was spun using an electrospinning technique to obtain modified PVDF nanofiber. The spinning parameters were: spinning voltage 18 KV, injection pump speed 1.0 mL / h, receiving distance 15 cm, and drum rotation speed 60 r / min.

[0039] The modified PVDF nanofiber separator prepared in Example 1 above and the glass fiber separators of Comparative Examples 1-3 were tested for porosity, water absorption, contact angle, and mechanical properties, and were respectively assembled into Zn / / Zn and Zn / / MnO2 CR2032 type button cells. The impedance and cyclic use performance of the batteries were tested using an electrochemical workstation, and the test data are shown in Table 1. The positive and negative electrodes of the Zn / / Zn battery each used a zinc disc with a diameter of 15 mm, and the electrolyte was 3 mol / L ZnSO4 aqueous solution. The negative electrode of the Zn / / MnO2 battery was a zinc disc with a diameter of 15 mm, and the positive electrode was MnO2 with a loading of about 2.0 mg / cm 2 , and the electrolyte was an aqueous solution of 2 mol / L ZnSO4 and 0.2 mol / L MnSO4.

[0040] Table 1 Properties of different alkaline-treated modified PVDF nanofiber separators and glass fiber separators and their use performance in zinc ion batteries

[0041]

[0042] Table 1 lists the properties of Example 1, Comparative Examples 1-3 and glass fiber separator and their performance data when used as zinc ion battery separator. The contact angle of Example 1 is 55°, which is hydrophilic, the breaking strength is 8.4 MPa, which is better than Comparative Examples 1-3 and glass fiber separator. When used in Zn / / Zn symmetric battery, the charge transfer resistance of Example 1 battery is 45.51 Ω, the conductivity is 0.80 mS / cm, and the battery service life is 92 h, which is significantly greater than that of glass fiber separator battery, while the battery service life of Comparative Examples 1-3 is less than that of glass fiber separator battery. When used in Zn / / MnO2 battery, the battery of Example 1 has a charge-discharge cycle number greater than 1000 times, a charge transfer resistance of 175.4 Ω, a conductivity of 2.39 mS / cm, and a maximum discharge specific capacity of 128 mAh / g, which is significantly better than that of the glass fiber separator battery.

[0043] The above only lists specific embodiments of the present application. Obviously, the present application is not limited to the above embodiments. All modifications that can be directly derived or conceived by those of ordinary skill in the art from the technical content disclosed in the present application should be considered as falling within the protection scope of the present application.

Claims

1. A method for preparing a modified PVDF nanofiber separator, characterized by: The preparation method comprises the following steps: (1) PVDF alkali treatment: 5 g of PVDF powder is added to 50 mL of 0.5 mol / L NaOH ethanol solution, stirred at 60 ℃ for 0.5 h, and tetrabutylammonium bromide TBAB is added at a dosage of 3 mg / mL; filtration, repeatedly washing the collected PVDF powder with deionized water until neutral, drying at 105 ℃ for standby, and obtaining modified PVDF powder; (2) Preparation of modified PVDF spinning solution: 1.6 g of modified PVDF powder is added to 10 mL of N,N-dimethylacetamide DMF, and 60 ℃ magnetic stirring is carried out for 8 h to completely dissolve the PVDF powder, and a modified PVDF spinning solution with a concentration of 17 wt% is obtained; (3) Preparation of alkali treatment modified PVDF nanofiber: the modified PVDF solution is spun by using electrospinning technology to obtain a modified PVDF nanofiber separator; the spinning parameters are as follows: spinning voltage 18 KV, injection pump speed 1.0 mL / h, receiving distance 15 cm, and drum rotating speed 60 r / min; The porosity of the nanofiber separator is 50.4%, the water absorption rate is 303.2%, the contact angle is 55°, the breaking strength is 8.4 MPa, and the elongation rate is 7.3%; When the nanofiber separator is used in a Zn / / MnO2 battery, the battery charge-discharge times are greater than 1000 times, the charge transfer impedance is 175.4 Ω, the conductivity is 2.39 mS / cm, and the maximum discharge specific capacity is 128 mAh / g.

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