Polyvinyl alcohol-based composite electrolyte, and preparation method and application thereof

Abstract

The application discloses a polyvinyl alcohol-based composite electrolyte, which comprises 70-90 parts of polyvinyl alcohol, 8-25 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt and 2-5 parts of lithium sulfide, and the lithium sulfide and the 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt are uniformly dispersed in the polyvinyl alcohol matrix; and the application further discloses a preparation method of the polyvinyl alcohol-based composite electrolyte and the application of the polyvinyl alcohol-based composite electrolyte to lithium batteries; the crystallinity of the polymer electrolyte is adjusted by adjusting the use proportion of polyvinyl alcohol with different molecular weights, the interface compatibility, the ionic conductivity and the thermal stability are enhanced by adding an appropriate amount of ionic liquid, and the ionic conductivity and the electrochemical performance are further improved by adding an appropriate amount of lithium sulfide as a lithium ion source.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of electrochemical energy storage, and particularly relates to a polyvinyl alcohol-based composite electrolyte, a preparation method thereof and a lithium battery. BACKGROUND

[0002] With the rapid expansion of the consumer electronics and electric vehicle markets, the continuous growth of energy demand requires us to develop high energy density, high safety energy storage devices with sustainability. Among the many energy storage solutions, lithium batteries have become the focus of extensive research due to their excellent reversible capacity characteristics. However, the liquid electrolyte used in traditional lithium batteries often has serious side reactions when interacting with the electrode, which not only limits the capacity of the battery, but also reduces its cycle stability. In addition, the potential risk of liquid electrolyte leakage also greatly weakens its safety performance.

[0003] In the pursuit of higher performance lithium batteries, polymer solid-state electrolytes have attracted much attention due to their unique advantages. Among them, polymers such as PVDF (polyvinylidene fluoride) and polyvinyl alcohol as the base material of solid-state electrolyte, due to its low density, easy processing, flexible shape and high safety factor, it shows broad application prospects. However, the high crystallinity of these polymers at room temperature seriously restricts their ionic conductivity, thereby affecting the overall performance of solid-state lithium batteries.

[0004] Therefore, how to effectively reduce the crystallinity of PVDF, PVA and other polymer electrolytes, and then improve their ionic conductivity, has become the key to the development of current lithium battery technology. Researchers are trying to improve its performance by changing the structure of polyvinyl alcohol and adding new additives. In addition, there is usually a large interface impedance between the polymer electrolyte and the positive and negative electrode materials, mainly because the compatibility of the interface between the polymer electrolyte and the positive and negative electrode materials is relatively poor. SUMMARY

[0005] In order to solve the above-mentioned problems, one of the purposes of the present application is to provide a polyvinyl alcohol-based composite electrolyte, by adjusting the amount of polyvinyl alcohol with different molecular weights to adjust the crystallinity of the polymer electrolyte, adding an appropriate amount of ionic liquid to enhance the interface compatibility and ionic conductivity and thermal stability, and adding an appropriate amount of lithium sulfide as a lithium ion source to further improve the ion conductivity and electrochemical performance.

[0006] The technical scheme adopted by the present application to solve its technical problems is: a polyvinyl alcohol-based composite electrolyte, comprising, by weight fraction, 70-90 parts of polyvinyl alcohol, 8-25 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt and 2-5 parts of lithium sulfide, the lithium sulfide and the 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt being uniformly dispersed in the polyvinyl alcohol matrix.

[0007] Further, the polyvinyl alcohol is selected as a mixture of medium molecular weight polyvinyl alcohol and high molecular weight polyvinyl alcohol with a weight ratio of 1:5-9, the molecular weight of the medium molecular weight polyvinyl alcohol is 10000-100000, and the molecular weight of the high molecular weight polyvinyl alcohol is 100000-2000000.

[0008] Further, the weight ratio of the medium molecular weight polyvinyl alcohol and the high molecular weight polyvinyl alcohol is 1:7.

[0009] Further, the polyvinyl alcohol is 80 parts, the 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is 17 parts, and the lithium sulfide is 3 parts. The content of the polyvinyl alcohol, the ionic liquid, and the lithium sulfide has a great influence on the ionic conductivity and the comprehensive cycle performance, and the above composition preparation can obtain good ionic conductivity.

[0010] The second object of the present application is to provide a preparation method of the polyvinyl alcohol-based composite electrolyte, comprising the following steps:

[0011] (1) 70-90 parts of polyvinyl alcohol is dissolved in an organic solvent to obtain a polyvinyl alcohol organic solution, and then 8-25 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is added to the polyvinyl alcohol organic solution to obtain a composite solution after stirring uniformly; the stirring temperature is controlled at 40-70℃, and the stirring time is 7-10h, and appropriate stirring temperature and stirring time can promote the uniform dispersion of the ionic liquid in the crosslinked network of the polyvinyl alcohol;

[0012] (2) Lithium sulfide and sulfur powder are dissolved in a solvent according to a mass ratio of 1:2-5 to obtain a lithium sulfide organic solution, wherein the solvent is a mixed solvent of dioxolane and dimethyl ether with a volume ratio of 1:1;

[0013] (3) The lithium sulfide organic solution obtained in step (2) is added to the composite solution of step (1) and stirred uniformly to obtain a composite electrolyte slurry, the composite electrolyte slurry is coated on a glass slide and solidified, and then defoaming, drying, and stirring are carried out, the stirring temperature is 40-60℃, the drying temperature is 50-70℃, the appropriate stirring temperature can promote the uniform dispersion of the lithium sulfide in the composite electrolyte, and the appropriate drying temperature can maintain the morphology of the polymer composite material and avoid the generation of voids, and finally the polyvinyl alcohol-based composite electrolyte is obtained.

[0014] The third object of the present application is to provide the above-mentioned polyvinyl alcohol-based composite electrolyte for lithium batteries.

[0015] The present application has the following beneficial technical effects:

[0016] The application adjusts the crystallinity of the polymer electrolyte by adjusting the dosage ratio of polyvinyl alcohol with different molecular weights, further improves the ion conductivity and electrochemical performance by adding appropriate amount of lithium sulfide as a lithium ion source, and enhances the interface compatibility, ion conductivity and thermal stability by adding appropriate amount of ionic liquid.

[0017] The application finds that the medium molecular weight PVA generally has better solubility, can be more easily dissolved in the solvent to form a uniform solution, which helps to prepare a uniform electrolyte film and improve the processability, the molecular chain of the medium molecular weight PVA is shorter, and the chain segment movement is more flexible, which is beneficial to ion transmission, and the medium molecular weight PVA electrolyte can have higher ion conductivity. The molecular chain of the high molecular weight PVA is longer, and the intermolecular force is stronger, which has higher mechanical strength, helps to improve the mechanical stability and durability of the electrolyte film; the molecular chain of the high molecular weight PVA is long, and the chain segment movement is relatively slow, which helps to reduce the size change of the electrolyte film in the charging and discharging process and improve the cycle stability of the battery. Therefore, the medium molecular weight PVA and the high molecular weight PVA have complementarity in solubility, ion transmission speed and mechanical strength, etc., and by using the two together, the advantages of the two can be comprehensively utilized to prepare an electrolyte film with good solubility and processability, high mechanical strength and stability, and high ion conductivity. However, the dosage of the two will affect the synergistic effect, and the application systematically studies the dosage of the two to select a suitable configuration ratio.

[0018] The application uses lithium sulfide to provide sufficient lithium source for the polymer solid-state battery, which can further improve the ion conductivity, and the addition of lithium sulfide can well interact with polyvinyl alcohol, the PVA can adsorb and retain sulfur ions, and there is interaction between the hydroxyl (-OH) in the PVA and the lithium ion (Li+), which can form a lithium ion transmission channel, and the interaction helps the rapid migration of lithium ions and improves the ion conductivity of the electrolyte.

[0019] The application first uses 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt to be added to polyvinyl alcohol, the ion structure of the 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt interacts with the polar group of the PVA to form a more stable complex or cross-linking structure, thereby further improving the performance of the PVA and the interface performance, and the 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt has good low volatility and thermal stability, which promotes the PVA to have good stability under high temperature or vacuum conditions, and the addition of the ionic liquid further improves the ion conductivity of the polyvinyl alcohol polymer electrolyte. DETAILED DESCRIPTION

[0020] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with examples.

[0021] Additional aspects and advantages of the present application will be described in the following description, will become apparent from the following description, or will be learned by practice of the present application. It is understood that the following description is merely to explain the present application, and is not intended to limit the present application.

[0022] The raw materials and equipment used in the present application, unless specified, can be purchased from the market or commonly used in the art. The methods in the examples, unless specified, are conventional methods in the art.

[0023] The present application will be further described below in combination with specific embodiments. Example 1

[0024] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 10 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 70 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetic acid, then 17 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt is added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 3 parts of lithium sulfide and 6 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry; the composite electrolyte slurry is coated on a glass slide and solidified, and then left to stand to remove bubbles and dried to obtain the polyvinyl alcohol-based composite electrolyte.

[0025] The polyvinyl alcohol of the present application is used as the main substrate in the polymer solid-state electrolyte, which determines the mechanical strength and film-forming performance of the electrolyte. An appropriate amount has an important influence on the mechanical strength and film-forming performance, and the content also affects the crystallization performance of the polymer electrolyte. The polar functional groups in the polyvinyl alcohol and the trifluoromethanesulfonylimide of the ionic liquid have good compatibility, which improves the ionic conductivity.

[0026] The 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt of the present application is used as an ionic liquid additive, which contains a cation (1-ethyl-3-methyl imidazole) and an anion (bis-trifluoromethanesulfonyl imide), has excellent electrochemical performance and thermal stability, and can improve the ionic conductivity and interfacial compatibility of the polymer composite electrolyte.

[0027] The lithium sulfide of the present application provides sufficient lithium source for the polymer solid-state battery, can further improve the ion conductivity, and the addition of the lithium sulfide can well interact with the polyvinyl alcohol, wherein the PVA can adsorb and retain the sulfur ions, and there is interaction between the hydroxyl (-OH) in the PVA and the lithium ion (Li+), which can form a lithium ion transmission channel. Such interaction helps the rapid migration of lithium ions and improves the ionic conductivity of the electrolyte.

[0028] Ion conductivity test: the above prepared polyvinyl alcohol-based composite electrolyte is tested with two stainless steel electrodes (SS) to form a SS / CSEs / SS simulation battery. The assembly process of the battery is carried out in a glove box with a water and oxygen content of less than 0.05 ppm. The alternating current impedance spectrum (EIS) test adopts a frequency range of 0.01 Hz to 106 Hz, and the electrochemical impedance of the composite electrolyte is determined at a constant temperature of 25°C. The impedance data obtained by the EIS test are used to calculate the ion conductivity of the prepared composite solid-state electrolyte.

[0029] Lithium ion battery performance test: the above prepared polyvinyl alcohol-based composite electrolyte is combined with lithium iron phosphate (LiFePO4) positive electrode and lithium metal negative electrode to form a button cell. The battery is prepared in a glove box. The battery is subjected to charge-discharge cycle test at a rate of 0.5C, and the attenuation of the discharge capacity is detected.

[0030] Further, the following examples and comparative examples are characterized by using the same test method, and the specific corresponding parameters are shown in the following table. Example 2

[0031] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 13 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 67 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetic acid, then 17 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide salt is added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 3 parts of lithium sulfide and 6 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry. The composite electrolyte slurry is coated on a glass slide and solidified, and then placed to remove bubbles and dried to obtain a polyvinyl alcohol-based composite electrolyte. Example 3

[0032] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 8 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 62 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetonitrile, 17 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is then added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 3 parts of lithium sulfide and 6 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry; the composite electrolyte slurry is coated on a glass slide, solidified, left to remove bubbles, and dried to obtain the polyvinyl alcohol-based composite electrolyte. Example 4

[0033] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 8 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 62 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetonitrile, 17 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is then added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 3 parts of lithium sulfide and 6 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry; the composite electrolyte slurry is coated on a glass slide, solidified, left to remove bubbles, and dried to obtain the polyvinyl alcohol-based composite electrolyte. Example 5

[0034] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 8 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 62 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetonitrile, 17 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is then added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 3 parts of lithium sulfide and 6 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry; the composite electrolyte slurry is coated on a glass slide, solidified, left to remove bubbles, and dried to obtain the polyvinyl alcohol-based composite electrolyte. Example 6

[0035] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 8 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 64 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetic acid, then 25 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 5 parts of lithium sulfide and 10 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry; the composite electrolyte slurry is coated on a glass slide and solidified, left to remove bubbles, and dried to obtain the polyvinyl alcohol-based composite electrolyte. Example 7

[0036] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 8 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 64 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetic acid, then 25 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 5 parts of lithium sulfide and 10 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry; the composite electrolyte slurry is coated on a glass slide and solidified, left to remove bubbles, and dried to obtain the polyvinyl alcohol-based composite electrolyte.

[0037] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 8 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 64 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetic acid, then 25 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 5 parts of lithium sulfide and 10 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry; the composite electrolyte slurry is coated on a glass slide and solidified, left to remove bubbles, and dried to obtain the polyvinyl alcohol-based composite electrolyte.

[0038] The polyvinyl alcohol-based composite electrolyte disclosed in this embodiment has the following preparation steps: 8 parts of medium molecular weight polyvinyl alcohol PVOH-13 and 64 parts of high molecular weight polyvinyl alcohol PVOH-1788 are dissolved in acetic acid, then 25 parts of 1-ethyl-3-methyl imidazole bis(trifluoromethyl sulfonyl) imide salt is added to the polyvinyl alcohol solution and stirred uniformly to obtain a composite solution; 5 parts of lithium sulfide and 10 parts of sulfur powder are dissolved in a mixed solvent of dioxolane and dimethyl ether, and then added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry; the composite electrolyte slurry is coated on a glass slide and solidified, left to remove bubbles, and dried to obtain the polyvinyl alcohol-based composite electrolyte.

[0039] Comparative Example 3: 85 parts of medium molecular weight polyvinyl alcohol PVOH-13 was weighed and dissolved in acetonitrile, and 15 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt was added to the polyvinyl alcohol solution, stirred uniformly to obtain a composite solution; the composite solution was coated on a glass slide and cured, air bubbles were removed by standing, and dried to obtain a polyvinyl alcohol-based composite electrolyte.

[0040] The physical performance parameters of each example and comparative example are shown in the following table.

[0041] .

[0042] From the data in the above table, it can be seen that Examples 1-3 compare the effects of different molecular weight polyvinyl alcohol ratio and dosage on the ionic conductivity and cycle stability of the electrolyte. Different ratios have different effects on ionic conductivity and cycle discharge performance.

[0043] Comparing the data of Examples 4, 6-7 and Comparative Examples 1-3, it can be found that the content and composition of each component of polyvinyl alcohol have important effects on the ionic conductivity and electrochemical performance of the electrolyte.

[0044] It should be noted that the present application is not limited to the above embodiments. The above embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solutions of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part of the components of the embodiments, are also included in the scope of the present application.

Claims

1. A polyvinyl alcohol-based composite electrolyte, characterized by: The polyvinyl alcohol-based composite electrolyte comprises 70-90 parts by weight of polyvinyl alcohol, 8-25 parts by weight of 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt and 2-5 parts by weight of lithium sulfide, and the lithium sulfide and the 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt are uniformly dispersed in the polyvinyl alcohol matrix; the polyvinyl alcohol is a mixture of medium molecular weight polyvinyl alcohol and high molecular weight polyvinyl alcohol at a weight ratio of 1:5-9, the molecular weight of the medium molecular weight polyvinyl alcohol is 10,000-100,000, and the molecular weight of the high molecular weight polyvinyl alcohol is 1,000,000-2,000,000.

2. A polyvinyl alcohol-based composite electrolyte according to claim 1, characterized in that, The weight ratio of the medium molecular weight polyvinyl alcohol to the high molecular weight polyvinyl alcohol is 1:

7.

3. A polyvinyl alcohol-based composite electrolyte according to claim 1 or 2, characterized in that, The polyvinyl alcohol is 80 parts, the 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt is 17 parts, and the lithium sulfide is 3 parts.

4. A method of preparing a polyvinyl alcohol-based composite electrolyte as claimed in claim 1, characterized by, The method comprises the following steps: (1) polyvinyl alcohol is weighed and dissolved in an organic solvent to obtain a polyvinyl alcohol organic solution, and then 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt is added and stirred uniformly to obtain a composite solution; (2) lithium sulfide and sulfur powder are weighed according to a mass ratio of 1:2-5 and dissolved in a solvent to obtain a lithium sulfide organic solution, wherein the solvent is a mixed solvent of dioxolane and dimethyl ether at a volume ratio of 1:1; (3) the lithium sulfide organic solution is added to the composite solution and stirred uniformly to obtain a composite electrolyte slurry, the composite electrolyte slurry is coated on a glass slide and solidified, and then defoaming, drying and other processes are performed to obtain a polyvinyl alcohol-based composite electrolyte.

5. The polyvinyl alcohol-based composite electrolyte of claim 1 is used for lithium batteries.