Ionic liquid and application thereof in high-voltage lithium ion battery electrolyte and solid electrolyte

Abstract

The invention belongs to the field of synthesis and application of energy storage directional functional ionic liquid, and discloses ionic liquid and application thereof in high-voltage lithium ion battery electrolyte and solid electrolyte. The ionic liquid comprises cations and anions, the cations comprise 1-substituted imidazole groups and 2-substituent structures, the anions are obtained through ion exchange according to the requirements of an electrolyte system, and the ionic liquid is synthesized and prepared through a two-step method. And the 1-substituent imidazole is one of 1-methylimidazole, 1-ethylimidazole, 1-propylimidazole, 1-hydroxymethyl imidazole and 1-hydroxyethyl imidazole. The 1, 2-substituent group structure is one of methyl tetrahydrofuran, methyl dioxo cyclopentane, dimethoxymethane and 1, 2-dimethoxyethane. The ionic liquid can form a positive electrode interface passivation film, reduce side reaction, improve interface performance, combine thermal / electrochemical stability, match a high-voltage positive electrode and improve lithium ion battery performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of synthesis and application of energy storage directional functional ionic liquid, and particularly relates to an ionic liquid and application thereof in high-voltage lithium ion battery electrolyte and solid-state electrolyte. BACKGROUND

[0002] Lithium ion batteries are widely favored due to their advantages of high energy density, high working voltage, no memory effect, long cycle life and low self-discharge. After years of development, small-capacity lithium ion batteries have been widely used in portable electronic devices such as mobile phones, notebook computers and digital cameras. With the popularization of electric vehicles and other equipment, the demand for high-energy-density lithium ion batteries is becoming more and more urgent. The high reactivity between lithium metal and traditional carbonate electrolyte is a major obstacle to achieving long-term cycling capability of lithium metal batteries. Ether electrolyte has good stability to lithium metal anode. Whether it is cyclic ether or chain ether, it is a solvent with low viscosity, low melting point, high dielectric constant and high lithium salt solubility, which is widely used in battery electrolyte research. However, under high voltage, due to the increase in the reactivity of the cathode material, the ether electrolyte is prone to oxidation, which destroys the charge-discharge chemical system in the battery, thereby causing the lithium ion battery matched with the ether electrolyte to have obvious overcharge and rapid capacity decay under high voltage, which seriously shortens the battery life. In the development of solid-state lithium batteries, polymer electrolytes have better flexibility, processability, designability and interface compatibility, so they have great advantages in the future development of solid-state batteries. However, the commonly used ion-conducting polymers represented by PEO and PVDF-HFP all exhibit significant crystalline morphology at room temperature. The highly oriented crystal regions in the polymer molecular chains severely limit the movement of the polymer molecules, making it difficult to achieve continuous conduction of lithium ions. Therefore, polymer electrolytes generally have the serious problems of low room temperature ionic conductivity and difficulty in stable ion conduction. The common method to improve the ionic conductivity of polymers is to blend, graft or add plasticizers, etc. to form a composite solid-state electrolyte. As a new type of green solvent, ionic liquid has intrinsic thermal / electrochemical stability, which makes it possible to replace organic electrolyte as a new type of electrolyte. Therefore, by redesigning the ether electrolyte to prepare a new type of ionic liquid and applying it in high-voltage lithium ion battery electrolyte, the direct contact between the easily oxidized part of the electrolyte and the high-voltage positive electrode can be prevented, the oxidation and decomposition of the electrolyte under high voltage can be effectively inhibited, and the high-voltage performance and energy density of the battery can be improved. In the PEO solid-state electrolyte, the compatibility problem is solved, the oxidation resistance of the polymer electrolyte system is improved, the possibility of phase separation during charge-discharge cycling is reduced, and the stability of the high-voltage positive electrode is optimized.

[0003] Therefore, in order to meet the needs of battery energy density, it is necessary to develop a new type of ionic liquid and apply it in high-voltage lithium ion battery electrolyte and solid-state electrolyte to meet the use conditions of lithium ion batteries under high voltage. SUMMARY

[0004] In view of this, the purpose of the present application is to provide an ionic liquid and its application in high-voltage lithium-ion battery electrolyte and solid-state electrolyte to solve the problems pointed out in the background art.

[0005] In order to achieve the above-mentioned purpose of the application, the technical solutions adopted are as follows: An ionic liquid, comprising a cation and an anion, the cation comprising a 1-substituted imidazole group and a 2-substituted group structure, and the anion being obtained by ion exchange according to the requirements of the electrolyte system, the ionic liquid being prepared by a two-step method.

[0006] As a further improvement of the present application, the 1-substituted imidazole is one of 1-methyl imidazole, 1-ethyl imidazole, 1-propyl imidazole, 1-hydroxymethyl imidazole and 1-hydroxyethyl imidazole.

[0007] As a further improvement of the present application, the 2-substituted group structure is one of methyl tetrahydrofuran, methyldioxolane, dimethoxymethane and 1,2-dimethoxyethane.

[0008] As a further improvement of the present application, the synthesis method of the cation part of the ionic liquid is one-step substitution reaction, and the structural formula is as follows: .

[0009] As a further improvement of the present application, the synthesis method of the anion part of the ionic liquid is one-step ion exchange reaction, and the corresponding product composition comprises one of the following anions: perchlorate, hexafluorophosphate, tetrafluoroborate, bistrifluoromethanesulfonylimide, hexafluoroarsenate, bisoxalate borate, difluorooxalate borate and trifluoromethylsulfonyl anion.

[0010] A high-voltage lithium-ion battery liquid electrolyte, which can be applied to a lithium-ion battery with a maximum working voltage of 4.2V-5.0V, comprises a main solvent, a lithium salt and a cosolvent, the main solvent being the ionic liquid according to any one of the above-mentioned embodiments, accounting for 50%-100% of the total mass of the electrolyte, and the concentration of the lithium salt being 0.5-2mol / L; wherein the lithium salt is at least one of lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bistrifluoromethanesulfonylimide, lithium hexafluoroarsenate, lithium bisoxalate borate, lithium difluorooxalate borate and lithium trifluoromethylsulfonyl; and the cosolvent is at least one of tetrahydrofuran (THF), 1,3-dioxolane, dimethoxymethane and 1,2-dimethoxyethane.

[0011] A preparation method of a high-voltage lithium-ion battery liquid electrolyte, comprising the following steps: mixing an ionic liquid, a lithium salt and a cosolvent to inhibit the oxidative decomposition of the cosolvent and match the high-voltage positive electrode material.

[0012] A high-pressure-resistant solid-state polymer electrolyte film, comprising: Polyethylene oxide (PEO); An ionic liquid, accounting for 1%-10% of the total mass of the solid-state electrolyte; A lithium salt, wherein the molar ratio of lithium ions to ether oxygen units in polyethylene oxide is 1:(10-20).

[0013] A preparation method of a high-pressure-resistant solid-state polymer electrolyte film, comprising the following steps: S1, polyethylene oxide, ionic liquid and lithium salt are added to a solvent to dissolve and configure a homogeneous polymer concentrated solution; wherein the solvent is at least one of anhydrous acetonitrile, anhydrous N-methyl pyrrolidone, anhydrous N,N dimethylformamide, anhydrous acetone, and anhydrous isopropyl alcohol; S2, the homogeneous polymer concentrated solution is slowly poured into a polytetrafluoroethylene mold by a casting method, a polymer electrolyte wet film is prepared by natural volatilization in an inert gas, and a polymer electrolyte dry film is prepared by desolventizing in a vacuum drying box; S3, the polymer electrolyte dry film is heat-pressed, and finally a high-pressure-resistant solid-state polymer electrolyte film is prepared.

[0014] A polymer solid-state lithium ion battery, comprising a positive electrode, a negative electrode and an electrolyte layer, wherein the positive electrode is any one of lithium iron phosphate, lithium cobaltate and NCM ternary positive electrode material, the negative electrode is metal lithium, and the electrolyte layer is the high-pressure-resistant solid-state polymer electrolyte film described above.

[0015] The beneficial effects of the present application are: The present application has the following advantages and effects compared with the prior art: the present application provides an ionic liquid and its application in high-voltage lithium ion battery electrolyte and solid-state electrolyte. The synthesis of the ionic liquid only needs a simple two-step method; the prepared ionic liquid-based electrolyte in the first charging process of the battery, the ionic liquid participates in the formation of the positive electrode interface passivation film, blocks the direct contact of the easily oxidized part in the electrolyte with the positive electrode surface, reduces the occurrence of side reactions between the cathode and the electrolyte, plays a protective role for the cathode, improves the electrolyte-positive electrode material interface performance, can effectively play the advantages of low viscosity electrolyte and polyethylene oxide, and combined with the thermal / electrochemical stability of the ionic liquid, it can match the high-voltage positive electrode, realize high-performance lithium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0016] The accompanying drawings, which form a part of this application, are intended to provide further understanding of the application and are incorporated herein for a purpose of explanation and are not intended to limit the application. In the drawings: Figure 1 The rate performance comparison chart of Examples 1-4 and Comparative Example 1. Figure 2 Long cycle test chart for some embodiments. DETAILED DESCRIPTION

[0017] The application will be further described in conjunction with specific embodiments. It should be understood that the following embodiments are merely exemplary and explanatory of the application and should not be interpreted as limiting the scope of the application. Any technology realized based on the above description of the application is included in the scope intended to be protected by the application.

[0018] The experimental methods used in the following examples are conventional methods unless otherwise specified; the reagents, materials, etc. used in the following examples are commercially available unless otherwise specified.

[0019] Example 1 Preparation of ionic liquid THF-im: 24.1 g of 2-chloromethyltetrahydrofuran and 19.7 g of N-methylimidazole were weighed into a 250 mL three-necked flask, the system was vacuumed and pure nitrogen was introduced, and the system was stirred in an oil bath at 90°C for 24 h under inert gas. After the reaction was completed, the mixture was cooled to room temperature and diluted with 50 mL of ethyl acetate. The diluted lower mixture was then transferred to a rotary evaporator to remove volatile components. The viscous material obtained after rotary evaporation was washed with ethyl acetate three times, and then the washed viscous material was further treated with a rotary evaporator at 80°C for 1 h, and then placed in a vacuum drying oven at 90°C for 24 h. The final product was a yellow oily onium salt intermediate (THF-im + (Cl - ). 10.91 g of (THF-im + (Cl - ) and 4.56 g of lithium bis-trifluoromethanesulfonimide (LiTFSI) were respectively dissolved in 100 mL of deionized water, and after the two were completely dispersed and dissolved and the reaction heat was completely dissipated, they were mixed in a 500 mL conical flask and stirred at room temperature for 24 h of displacement reaction. After the reaction was completed and cooled to room temperature, the supernatant was slowly poured out, and then 50 mL of deionized water was added to stir and wash the precipitate three times. Then the precipitate was dissolved with ethyl acetate and transferred to a rotary evaporator, and the solution was concentrated at 80°C until the mass remained unchanged to remove the solvent. The obtained viscous material was further treated with a rotary evaporator for 1 h, and then placed in a vacuum drying oven at 90°C for 24 h. The final product was a light yellow oily product (THF-im + (TFSI - ), abbreviated as THF-im.

[0020] Preparation of electrolyte: In an argon-filled glove box (moisture <1 ppm, oxygen <1 ppm), the main solvent (THF-im), the cosolvent tetrahydrofuran (THF) and the lithium salt (LiTFSI) were mixed to prepare a 1 mol / L LiTFSI solution, wherein the mass fraction of the main solvent (THF-im) was 72%, and the lithium ion battery electrolyte of Example 1 was obtained after uniform stirring.

[0021] Preparation of positive electrode sheet: The positive electrode active material nickel-cobalt-manganese-based material (NCM811), the conductive agent Super-P and the binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in an N-methylpyrrolidone system at a mass ratio of 80:10:10, coated on the positive electrode current collector Al foil, and then vacuum dried and cut to obtain the positive electrode sheet.

[0022] Preparation of separator: GF / C glass fiber was used as the separator.

[0023] Preparation of lithium ion battery: The obtained positive electrode sheet was placed in an oven and dried at 80°C, and then moved into a glove box. Then, the treated positive electrode sheet, the separator and the lithium sheet were sequentially placed, assembled into a CR2032 button cell, and 100 μl of the prepared electrolyte was injected.

[0024] Example 2 Preparation of electrolyte: In an argon-filled glove box (moisture <1 ppm, oxygen <1 ppm), the main solvent (THF-im), the cosolvent tetrahydrofuran (THF) and the lithium salt (LiTFSI) were mixed to prepare a 1 mol / L LiTFSI solution, wherein the mass fraction of the main solvent (THF-im) was 75%, and the lithium ion battery electrolyte of Example 2 was obtained after uniform stirring.

[0025] The remaining steps were the same as in Example 1.

[0026] Example 3 Preparation of electrolyte: In an argon-filled glove box (moisture <1 ppm, oxygen <1 ppm), the main solvent (THF-im), the cosolvent tetrahydrofuran (THF) and the lithium salt (LiTFSI) were mixed to prepare a 1 mol / L LiTFSI solution, wherein the mass fraction of the main solvent (THF-im) was 80%, and the lithium ion battery electrolyte of Example 3 was obtained after uniform stirring.

[0027] The remaining steps were the same as in Example 1.

[0028] Example 4 Preparation of electrolyte: In an argon-filled glove box (moisture <1 ppm, oxygen <1 ppm), the main solvent (THF-im), the co-solvent tetrahydrofuran (THF) and the lithium salt (LiTFSI) were mixed to form a 1 mol / L LiTFSI solution, in which the mass fraction of the main solvent (THF-im) was 86%, and the lithium ion battery electrolyte of Example 4 was obtained after uniform stirring.

[0029] The remaining steps were the same as in Example 1.

[0030] Example 5 The solid-state electrolyte film in this example included PEO, ionic liquid THF-im and lithium bis(trifluoromethylsulfonyl)imide. The number average molecular weight of PEO was 1 million; the mass fraction of THF-im was 3%; and the molar ratio of lithium bis(trifluoromethylsulfonyl)imide to ether oxygen units in PEO was 1:18.

[0031] The preparation of the positive electrode sheet was the same as in Example 1.

[0032] Preparation of lithium ion battery: The obtained positive electrode sheet was placed in an oven and dried at 80°C, and then moved into a glove box. Then the treated positive electrode sheet, the solid-state electrolyte film and the lithium sheet were sequentially placed, and assembled into a CR2032 button cell.

[0033] Example 6 The solid-state electrolyte film in this example included PEO, ionic liquid THF-im and lithium bis(trifluoromethylsulfonyl)imide. The number average molecular weight of PEO was 1 million; the mass fraction of THF-im was 6%; and the molar ratio of lithium bis(trifluoromethylsulfonyl)imide to ether oxygen units in PEO was 1:18.

[0034] The preparation of the positive electrode sheet was the same as in Example 1.

[0035] The preparation of the lithium ion battery was the same as in Example 5. Comparative Example 1 Preparation of electrolyte: In an argon-filled glove box (moisture <1 ppm, oxygen <1 ppm), the solvent (THF-im) and the lithium salt (LiTFSI) were mixed to form a 1 mol / L LiTFSI solution, and the lithium ion battery electrolyte of Comparative Example 1 was obtained after uniform stirring.

[0036] The preparation of the positive electrode sheet, the preparation of the separator film and the preparation of the lithium ion battery were the same as in Example 1.

[0037] The batteries prepared in Examples 1-6 and Comparative Example 1 were allowed to stand for one day, and then electrochemical performance tests were performed.

[0038] Test conditions: rate test at 0.1C-1C in 3.0-4.2V, long cycle test at 0.2C in 3.0-4.2V.

[0039] Figure 1 For the rate performance comparison chart of examples 1-4 and comparative example 1, from Figure 1 It can be seen that the rate performance of the battery assembled by the electrolyte of examples 1-4 is better than that of comparative example 1, indicating that the synergistic effect of the cosolvent and the ionic liquid can improve the rate performance of the battery; Figure 2 For the long cycle test chart of examples 1 and 2, the battery using the ionic liquid described in the application shows more excellent cycle stability in long-term cycling, proving that the positive electrode interface passivation film formed by the ionic liquid in the first charging process can effectively block the direct contact between the easily oxidized part of the electrolyte and the positive electrode surface, significantly reduce the side reaction between the cathode and the electrolyte, thereby protecting the cathode and improving the electrolyte-cathode material interface performance. At the same time, the synergistic effect of the cosolvent and the ionic liquid further guarantees the performance stability of the battery in long-term cycling, realizing the technical effect of high-performance lithium ion battery.

[0040] Table 1 Cycle performance test of NCM811 / Li battery assembled by solid-state electrolyte film As can be seen from table 1, the solid-state electrolyte containing the ionic liquid described in the application can significantly improve the cycle stability of the battery. The batteries of examples 5 (ionic liquid mass fraction 3%) and example 6 (ionic liquid mass fraction 6%) both maintain a high discharge specific capacity (173 mAh / g and 158 mAh / g, respectively) after 100 cycles, indicating that the solid-state electrolyte can effectively maintain the capacity output of the battery and has slow cycle decay, verifying that the synergistic effect of the ionic liquid and the polyethylene oxide (PEO) matrix can improve the stability of the solid-state electrolyte. In addition, the coulombic efficiency of the two is 98% and 99%, respectively, close to 100%, indicating that the side reaction during the charging and discharging process of the battery is very small, and the charge utilization rate is high. It is proved that the positive electrode interface film formed by the ionic liquid can effectively block the contact between the easily oxidized part of the electrolyte and the positive electrode, reduce the cathode-electrolyte side reaction, and thus prolong the cycle life of the battery.

[0041] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, component disassembly or combination made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An ionic liquid characterized in that: The cation comprises a 1-substituted imidazole group and a 2-substituted group structure, and the anion is obtained by ion exchange according to the electrolyte system requirement, and the ionic liquid is prepared by a two-step method.

2. The ionic liquid of claim 1, wherein: The 1-substituted imidazole is one of 1-methyl imidazole, 1-ethyl imidazole, 1-propyl imidazole, 1-hydroxymethyl imidazole and 1-hydroxyethyl imidazole.

3. The ionic liquid of claim 1, wherein: The 2-substituted group structure is one of methyl tetrahydrofuran, methyldioxolane, dimethoxymethane and 1,2-dimethoxyethane.

4. The ionic liquid of claim 1, wherein: The synthesis method of the cation part of the ionic liquid is a one-step substitution reaction, and the structure is as follows: 。 5. The ionic liquid of claim 1, wherein: The synthesis method of the anion part of the ionic liquid is a one-step ion exchange reaction, and the corresponding product composition comprises one of the following anions: perchlorate, hexafluorophosphate, tetrafluoroborate, bistrifluoromethylsulfonylimide, hexafluoroarsenate, bisoxalate borate, difluorooxalate borate and trifluoromethylsulfonyl anion.

6. A high-voltage lithium-ion battery liquid electrolyte, which can be applied to a lithium-ion battery with a maximum working voltage of 4.2V-5.0V, characterized in that: The electrolyte comprises a main solvent, a lithium salt and a cosolvent, the ionic liquid of any one of claims 1-5 accounts for 50%-100% of the total mass of the electrolyte, the concentration of the lithium salt is 0.5-2 mol / L; wherein the lithium salt is at least one of lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bistrifluoromethylsulfonylimide, lithium hexafluoroarsenate, lithium bisoxalate borate, lithium difluorooxalate borate and lithium trifluoromethylsulfonate; and the cosolvent is at least one of tetrahydrofuran, 1,3-dioxolane, dimethoxymethane and 1,2-dimethoxyethane.

7. A process for the preparation of a liquid electrolyte for high voltage lithium-ion batteries according to claim 6, characterized in that, The method comprises the following steps: The ionic liquid, the lithium salt and the cosolvent are mixed to inhibit the oxidative decomposition of the cosolvent and to match the high-voltage positive electrode material.

8. A high-pressure resistant solid-state polymer electrolyte membrane, characterized by, The method comprises the following steps: Polyethylene oxide; The ionic liquid of any one of claims 1-2 accounts for 1%-10% of the total mass of the solid-state electrolyte; The lithium salt, wherein the molar ratio of lithium ions to ether oxygen units in the polyethylene oxide is 1:(10-20).

9. A method for producing a high-pressure-resistant solid-state polymer electrolyte film, characterized by, The method comprises the following steps: S1, polyethylene oxide, ionic liquid and lithium salt are added to a solvent for dissolution to configure a homogeneous polymer concentrated solution; wherein the solvent is at least one of anhydrous acetonitrile, anhydrous N-methyl pyrrolidone, anhydrous N,N dimethylformamide, anhydrous acetone and anhydrous isopropyl alcohol; S2, the homogeneous polymer concentrated solution is slowly poured into a polytetrafluoroethylene mold by a casting method, a polymer electrolyte wet film is prepared by natural volatilization in an inert gas, and a polymer electrolyte dry film is prepared by solvent removal through a vacuum drying box; S3, the polymer electrolyte dry film is hot-pressed to finally prepare a high-pressure-resistant solid-state polymer electrolyte film.

10. A polymer solid-state lithium-ion battery, characterized by: The method comprises the following steps: The positive electrode is any one of lithium iron phosphate, lithium cobaltate and NCM ternary positive electrode material, the negative electrode is metal lithium, and the electrolyte layer is the high-pressure-resistant solid-state polymer electrolyte film of claim 8 or 9.

Images