Lithium ion battery electrolyte, preparation method thereof and lithium ion battery

Abstract

The application discloses a lithium ion battery electrolyte, which comprises a lithium salt, a cyclic ether and an additive; the additive is a soluble polyimide in the cyclic ether. The application also discloses a lithium ion battery, which comprises a lithium metal battery negative electrode and the lithium ion battery electrolyte; the polyimide in the lithium ion battery electrolyte forms a protective layer on the surface of the lithium metal battery negative electrode. The application also discloses a preparation method of the lithium ion battery electrolyte. The lithium ion battery electrolyte can form a protective polymer coating on the surface of the lithium metal battery negative electrode, has good interface compatibility with the lithium ion negative electrode, plays a role in inhibiting destructive lithium dendrites and side reactions, has excellent coulomb efficiency and cycle stability, and can be applied to the field of new advanced lithium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and particularly to a lithium-ion battery electrolyte, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium metal is a popular research topic as the anode material for lithium-ion batteries. It boasts the highest energy density and lowest redox potential, making it one of the most promising candidate materials. Xin-Bing Cheng, in *CHEMICAL REVIEWS*, points out that the application of lithium metal anodes is limited by the following reasons: 1) lithium metal has strong reducing properties; 2) the SEI (Sediment Ion) formed by the reaction of lithium metal with the electrolyte has insufficient performance; 3) lithium ions are unevenly deposited on the anode surface, forming lithium dendrites, which can puncture the separator and cause short circuits. Polymer coatings are currently a promising approach to address these issues. Yi Cui, in *JACS*, suggests that a good polymer coating can improve the performance of lithium metal and mitigate these problems. However, most artificial SEI films are currently applied manually to lithium metal, leading to uneven thickness control and excessive thickness, resulting in high interfacial impedance and performance degradation. Summary of the Invention

[0003] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the present invention aims to provide a lithium-ion battery electrolyte that can form a protective polymer coating on the surface of the negative electrode of a lithium metal battery, thereby inhibiting destructive lithium dendrites and side reactions.

[0004] Another object of the present invention is to provide a lithium-ion battery comprising the above-mentioned lithium-ion battery electrolyte.

[0005] Another object of the present invention is to provide a method for preparing the above-mentioned lithium-ion battery electrolyte.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A lithium-ion battery electrolyte includes a lithium salt, a cyclic ether, and an additive; the additive is a polyimide soluble in the cyclic ether.

[0008] Preferably, the polyimide has the following structure:

[0009]

[0010] The structure of R1 is one of the following:

[0011]

[0012] Among them, the structure of R2 is one of the following:

[0013]

[0014] Preferably, the cyclic ether is one of dioxolane, tetrahydrofuran, tetrahydropyran, and dioxane.

[0015] Preferably, the solid content of the additive in the electrolyte is 0.1 wt% to 8 wt%.

[0016] More preferably, the solid content of the additive in the electrolyte is 4wt% to 6wt%.

[0017] Preferably, the lithium salt is any one of LiPF6, LiBF4, LiClO4, LiAsF6, LiTFSI, LiCF3SO3, or LiN(CF3SO2)2.

[0018] A lithium-ion battery includes a lithium metal battery negative electrode and a lithium-ion battery electrolyte; the polyimide in the lithium-ion battery electrolyte forms a protective layer on the surface of the lithium metal battery negative electrode.

[0019] Preferably, the thickness of the protective layer is 2 to 25 micrometers.

[0020] More preferably, the thickness of the protective layer is 5 to 10 micrometers.

[0021] The method for preparing the lithium-ion battery electrolyte includes the following steps:

[0022] Polyimide is dissolved in cyclic ether to prepare a solution with a concentration range of 1wt% to 10wt%. Lithium salt is then added and stirred until fully dissolved to obtain a lithium-ion battery electrolyte.

[0023] Preferably, the concentration of lithium salt in the lithium-ion battery electrolyte is 1–5 mol / L.

[0024] Preferably, the polyimide has a molecular weight of 1,000 to 100,000.

[0025] Preferably, the polyimide is a fluorinated polyimide.

[0026] Preferably, the preparation process of the fluorinated polyimide is as follows:

[0027] Under high temperature conditions, diamine and dianhydride containing trifluoromethyl are dissolved in an organic solvent, polymerized and dehydrated and cyclized, and then poured into ethanol to precipitate, yielding fluorinated polyimide.

[0028] Preferably, the organic solvent includes any one of DMAc, DMF, DMSO, m-cresol, or xylene.

[0029] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0030] (1) The lithium-ion battery electrolyte of the present invention can generate a polymer protective layer in situ on the surface of the negative electrode of the lithium metal battery, which can suppress destructive lithium dendrites and side reactions.

[0031] (2) The lithium-ion battery electrolyte of the present invention generates a polymer protective layer that has good interfacial compatibility with lithium metal and can provide necessary mechanical strength and protection.

[0032] (3) The lithium-ion battery electrolyte of the present invention has excellent coulombic efficiency and cycle stability.

[0033] (4) The lithium-ion battery electrolyte of the present invention has high stability and simple storage, low storage requirements, and excellent safety performance.

[0034] (5) The lithium-ion battery electrolyte of the present invention is simple to prepare and inexpensive, and is suitable for large-scale preparation.

[0035] (6) The electrolyte of the present invention is easy to use and suitable for large-scale industrialization.

[0036] (7) The lithium-ion battery of the present invention has good ionic conductivity and electrochemical stability, which can meet the practical application requirements of the next generation of high energy density lithium-ion batteries. Attached Figure Description

[0037] Figure 1 The ionic conductivity of 5 mol / L LiTFSI DOL and 5 mol / L LiTFSI DOL FPI electrolytes in the embodiments of the present invention was measured using an electrochemical workstation, model Chenhua CHI006D.

[0038] Figure 2 The specific capacity and coulombic efficiency of the battery assembled with 5 mol / L LiTFSI DOL and 5 mol / L LiTFSI DOL FPI electrolyte in Example 1 of this invention were tested at rates of 0.1C, 0.2C, 0.5C, 1C, and 2C. The testing instrument used was a battery workstation, model Blue Electric 2001A.

[0039] Figure 3 The specific capacity and coulombic efficiency of the battery assembled with 5 mol / L LiTFSI DOL and 5 mol / L LiTFSI DOL FPI electrolyte in Example 1 of this invention were tested at a 0.5C rate. The test instrument used was a battery workstation, model Blue Electric 2001A.

[0040] Figure 4This is a scanning electron microscope (SEM) image of the negative electrode lithium metal of the battery assembled with 5 mol / L LiTFSI DOL electrolyte in Example 1 of the present invention after 500 cycles at 0.5C. The testing instrument used was a Zeiss EVO18 scanning electron microscope.

[0041] Figure 5 This is a scanning electron microscope (SEM) image of the negative electrode lithium metal of the battery assembled with 5 mol / L LiTFSI DOL-FPI electrolyte in Example 1 of the present invention after 500 cycles at 0.5C. The testing instrument used was a Zeiss EVO18 scanning electron microscope.

[0042] Figure 6 To measure the specific capacity and coulombic efficiency at 0.5C rate of the lithium-ion battery electrolytes with polyimide solid content of 2wt% and 5wt% in Example 1 of the present invention, the testing instrument used was a battery workstation, model Blue Electric 2001A. Detailed Implementation

[0043] The present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited thereto.

[0044] Example 1

[0045] The lithium-ion battery electrolyte of this embodiment includes a lithium salt, a cyclic ether, and an additive; the additive is a polyimide soluble in the cyclic ether, wherein the polyimide has the following structure:

[0046]

[0047] The lithium-ion battery electrolyte of this embodiment is prepared by the following method:

[0048] 4,4'-diaminodiphenyl ether and 4,4'-(hexafluoroisopropene)phthalic anhydride were dissolved in m-cresol in a 1:1 molar ratio. Nitrogen gas was introduced, and the mixture was mechanically stirred at room temperature to obtain a polyamic acid solution with a solid content of 30%. The polyamic acid solution was defoamed and then dehydrated and cyclized at 180°C using a heating device to prepare a polyimide solution. The polyimide solution was poured into ethanol, and polyimide began to precipitate, yielding a white solid. The white solid was dried at 200°C. A lithium salt was dissolved in dioxolane to prepare a 5 mol / L solution, which was stirred until completely dissolved. The solution and polyimide were mixed and stirred to obtain a dioxolane / lithium salt electrolyte (LiTFSI DOL FPI) with added polyimide, wherein the polyimide solid content was 5 wt%.

[0049] To better illustrate the performance of the lithium-ion battery electrolyte in this embodiment, a comparative sample dioxolane / lithium salt electrolyte (LiTFSI DOL) was also prepared. Except for the absence of polyimide, its preparation process was the same as that of LiTFSI DOL FPI.

[0050] Figure 1 The figure shows the lithium-ion conductivity of dioxolane / lithium salt electrolyte and dioxolane / lithium salt electrolyte with added polyimide at different temperatures. As can be seen from the figure, the ionic conductivity of the electrolyte with added polyimide increases significantly with increasing temperature, indicating that the polymer also plays a positive role in lithium-ion migration within the electrolyte system. Higher ionic conductivity can effectively suppress lithium dendrite puncture of the separator and improve specific capacity.

[0051] Figure 2 The figure shows the coulombic efficiency of the specific capacity of the dioxolane / lithium salt electrolyte and the dioxolane / lithium salt electrolyte with added polyimide at different charge / discharge rates at room temperature. As can be seen from the figure, the electrolyte with added polyimide has a higher specific capacity coulombic efficiency, thus improving battery performance.

[0052] Figure 3 The tests involved long-range cycling of the dioxolane / lithium salt electrolyte and the dioxolane / lithium salt electrolyte with added polyimide at room temperature. At 1000 cycles at a 2C rate, the electrolyte with added polyimide consistently exhibited higher coulombic efficiency and specific capacity.

[0053] Figure 4 This is a scanning electron micrograph of the lithium metal anode in a lithium metal / dioxolane-lithium salt / lithium iron phosphate battery after 500 cycles. The lithium metal reacts with the electrolyte to form a loose, weakly protective solid electrolyte interphase (SEI). The SEI film is 42 micrometers thick. An excessively thick SEI film would hinder lithium-ion migration, leading to performance degradation.

[0054] Figure 5 This is an electron scanning micrograph of the lithium metal anode of a lithium metal / dioxolane-lithium salt-polyimide / lithium iron phosphate battery after 500 cycles. A distinct polymer protective layer with a thickness of 7 micrometers can be clearly seen on the lithium metal, which plays a crucial role in improving battery performance.

[0055] Example 2

[0056] To better illustrate the effect of polyimide solid content on the performance of the lithium-ion battery electrolyte in this embodiment, the following comparative experiment was conducted: lithium-ion battery electrolytes with polyimide solid content of 2wt% and 5wt% were prepared respectively (except for the polyimide solid content, the reaction raw materials and preparation steps were the same as in Example 1), and their coulombic efficiency was tested. The results are as follows: Figure 6 As shown, the lithium-ion battery electrolyte with a polyimide solid content of 5 wt% has a higher coulombic efficiency, indicating that the SEI film formed by it also has relatively good performance.

[0057] In the above embodiments, the polyimide may also be any of the following structures:

[0058]

[0059] The structure of R1 is one of the following:

[0060]

[0061] Among them, the structure of R2 is one of the following:

[0062]

[0063] In the above embodiments, the cyclic ether can be dioxolane, tetrahydrofuran, tetrahydropyran, or dioxane.

[0064] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the embodiments described above. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A lithium-ion battery electrolyte, characterized in that, It includes lithium salt, cyclic ether, and additives; the additive is a polyimide soluble in the cyclic ether; the polyimide has the following structure: ； The polyimide forms a protective layer on the surface of the negative electrode of a lithium metal battery; the thickness of the protective layer is 2-25 micrometers. The electrolyte contains additives with a solid content of 0.1 wt% to 8 wt%.

2. The lithium-ion battery electrolyte according to claim 1, characterized in that, The cyclic ether is one of dioxolane, tetrahydrofuran, tetrahydropyran, or dioxane.

3. The lithium-ion battery electrolyte according to claim 1, characterized in that, The electrolyte contains 4wt% to 6wt% of additives.

4. The lithium-ion battery electrolyte according to claim 1, characterized in that, The lithium salt is any one of LiPF6, LiBF4, LiClO4, LiAsF6, LiTFSI, LiCF3SO3, or LiN(CF3SO2)2.

5. A lithium-ion battery, characterized in that, It includes a lithium metal battery anode and a lithium-ion battery electrolyte as described in any one of claims 1 to 4.

6. The method for preparing the lithium-ion battery electrolyte according to any one of claims 1 to 4, characterized in that, Includes the following steps: Polyimide is dissolved in cyclic ether to prepare a solution with a concentration range of 1wt% to 10wt%, and then lithium salt is added and stirred until fully dissolved to obtain a lithium-ion battery electrolyte.

7. The preparation method according to claim 6, characterized in that, The concentration of lithium salt in the lithium-ion battery electrolyte is 1~5 mol / L.

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