Non-flammable local high-concentration ionic liquid electrolyte containing chlorinated hydrocarbon diluent and application

Abstract

The application belongs to the technical field of lithium batteries, and particularly relates to a non-flammable local high-concentration ionic liquid electrolyte containing a chlorinated hydrocarbon diluent and application. The application successfully finds that the non-flammable chlorinated hydrocarbon with lower cost can be used as a diluent of the local high-concentration ionic liquid electrolyte, so as to improve the problems of high viscosity, poor impregnation with a separator and low ionic conductivity of the ionic liquid electrolyte, and can effectively improve the cycle performance, rate performance and safety performance of the lithium battery, and improve the safety of the electrolyte. The chlorinated hydrocarbon can replace the higher-cost hydrofluoroether compound, and then reduce the production cost of the local high-concentration ionic liquid electrolyte. Moreover, the local high-concentration ionic liquid electrolyte using the chlorinated hydrocarbon compound has an electrochemical stability voltage of more than 4.5V, is suitable for a high-voltage positive electrode material, fully develops the battery capacity, and improves the energy density of the battery. Meanwhile, the local high-concentration ionic liquid electrolyte of the application can meet high-temperature operation, and has a wide application scenario.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of lithium batteries, and particularly relates to a non-flammable local high-concentration ionic liquid electrolyte containing a chlorinated hydrocarbon diluent and application thereof. BACKGROUND

[0002] High safety is the primary condition for the practical application of lithium batteries. However, most of the electrolytes currently used in lithium batteries are composed of organic solvents, lithium salts and battery additives. However, such electrolytes have the disadvantages of being volatile and flammable, which leads to accidents such as leakage, fire and explosion of the battery when the battery is subjected to improper operations such as collision, overcharging, overheating and internal short circuit. Ionic liquids have the characteristics of being non-volatile, non-flammable and environmentally friendly, and have excellent chemical and electrochemical stability, which helps to improve the safety of lithium batteries and increase the working temperature. However, ionic liquids also have obvious disadvantages: when ionic liquids are used as electrolytes, high viscosity, low lithium ion concentration and low electrolyte conductivity occur. In order to overcome the above-mentioned shortcomings, researchers have proposed methods such as adding organic solvents or using local high-concentration electrolytes to reduce the viscosity of ionic liquids and improve the conductivity of ionic liquids.

[0003] Local high-concentration electrolyte (LHCE) usually contains three main components: ion-conducting salt, soluble salt solvent and diluent. In some cases, other additives are introduced to optimize the electrolyte for specific battery chemistry. When formulating LHCE, the lithium salt is required to have good / high solubility in the selected solvent to form HCE (usually with a salt concentration of > 3 M). Compared with conventional dilute electrolytes and HCE, the key feature of LHCE is its unique solvation structure: high-concentration salt-solvent clusters are dispersed in the diluent. In HCE and LHCE electrolytes, almost all the solvent molecules of the soluble salt are coordinated with the cation, and there are few or even no free solvent molecules, so Li+ is forced to share the solvent molecules, and a large part of the anions also participate in the coordination of Li+, resulting in the formation of contact ion pairs (CIP) and cation-anion aggregates. The introduction of diluent in HCE reduces the total salt concentration in LHCE, while retaining the local coordination environment of high-concentration salt-solvent clusters in HCE. The lithium salt has little or no solubility in the diluent used, which is miscible with the salt-dissolving solvent in HCE to form a clear and uniform solution, avoiding phase separation; in addition, the diluent should have a low viscosity to reduce the overall viscosity of the electrolyte; and it should have sufficient stability.

[0004] The local high-concentration electrolyte currently used has good compatibility and high coulombic efficiency, but the high-voltage stability of such electrolyte is poor, and it can only be well matched with a lithium iron phosphate positive electrode with a low working voltage (~3.2-3.3 V).

[0005] The selection of the diluent needs to be able to maintain high concentration of Li + - Solvent - TESI - The coordination structure, in addition, needs to maintain the advantages of high electrochemical stability of high concentration electrolyte and overcome the disadvantages of high viscosity and high cost of high concentration electrolyte. Hydrofluoroether meets the above requirements and is a suitable diluent. Currently, the hydrofluoroether compounds widely used are HFE, 2, 2, 2-trifluoroethyl-1, 1, 2, 2-tetrafluoroethyl ether (TTE) and BTFE. However, such compounds have high cost, which has a significant impact on the practical application of lithium batteries. Moreover, the lithium battery prepared by the current local high-concentration electrolyte cannot operate at high temperature (> 45℃), which seriously limits its application range, such as mobile phones, electric vehicles, etc. SUMMARY

[0006] The purpose of the present application is to overcome the shortcomings and deficiencies of the prior art and provide a non-flammable local high-concentration ionic liquid electrolyte containing chlorinated hydrocarbon diluent and application.

[0007] The technical scheme adopted by the present application is as follows: a non-flammable local high-concentration ionic liquid electrolyte containing chlorinated hydrocarbon diluent, the electrolyte comprising lithium salt, ionic liquid dissolving lithium salt, diluent, the diluent being one or more mixtures of chloroform, tetrachloromethane and chlorinated hydrocarbon compounds with molecular formula C n H 2n+2-m Cl m , n is an integer ≥ 2, and m is an integer ≥ 1.

[0008] Preferably, the ionic liquid is selected from at least one of imidazole ionic liquid and pyrrolidine ionic liquid.

[0009] Preferably, the anion of the ionic liquid is a fluorine-containing group, such as hexafluorophosphate PF6 - , bis (trifluoromethylsulfonyl) imide TFSI - , bisfluorosulfonylimide FSI - , and difluoro oxalate borate DFOB - .

[0010] Preferably, the lithium salt is at least one of lithium bis (trifluoromethylsulfonyl) imide and lithium bisfluorosulfonylimide.

[0011] Preferably, the electrolyte comprises an additive, and the additive is at least one of fluoroethylene carbonate and vinylene carbonate.

[0012] Preferably, the molar concentration of lithium salt in the ionic liquid is 0.5-6.0 mol / L; the mass fraction of the additive is 0.1-5.0%, and the volume fraction of the chlorinated hydrocarbon diluent is 20-80%.

[0013] Use of the non-flammable local high-concentration ionic liquid electrolyte containing the chlorinated hydrocarbon diluent as described above to prepare a lithium metal battery.

[0014] A lithium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, the electrolyte being a non-flammable local high-concentration ionic liquid electrolyte containing the chlorinated hydrocarbon diluent as described above; the material of the negative electrode being a lithium metal foil, a carbon-based material, or a silicon-based material; the material of the positive electrode being lithium cobaltate, ternary nickel-cobalt-manganese, or ternary nickel-cobalt-aluminum; the material of the separator being polypropylene, polyethylene or a composite of polypropylene and polyethylene.

[0015] The beneficial effects of the present application are as follows: the present application successfully finds a non-flammable, lower-cost chlorinated hydrocarbon as a diluent for a local high-concentration ionic liquid electrolyte, to improve the problems of high viscosity, poor wettability with the separator, and low ionic conductivity of the ionic liquid electrolyte, which can effectively improve the cycle performance, rate performance and safety performance of the lithium battery, and at the same time improve the safety of the electrolyte, which can replace the higher-cost hydrofluoroether compound, thereby reducing the production cost of the local high-concentration ionic liquid electrolyte. Moreover, the local high-concentration ionic liquid electrolyte using the chlorinated hydrocarbon compound has an electrochemical stability voltage exceeding 4.5V, which is suitable for high-voltage positive electrode materials, fully utilizes the battery capacity, and improves the energy density of the battery. At the same time, the local high-concentration ionic liquid electrolyte of the present application can meet the high-temperature operation, and has a relatively wide application scenario. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained from these drawings without creative labor.

[0017] Figure 1 is the flame retardant performance test diagram of the electrolyte of the present application embodiment 1;

[0018] Figure 2 is the electrolyte of the present application embodiment 1, embodiment 2 and embodiment 3 and polypropylene separator wettability contact angle diagram;

[0019] Figure 3 is the electrochemical stability window test diagram of the lithium ion battery of the electrolyte of the present application embodiment 2;

[0020] Figure 4 is the rate performance diagram of the assembled lithium battery of the present application embodiment 3;

[0021] Figure 5 is the charge-discharge curve of the assembled lithium battery of embodiment 4 of the present application at 25 °C and 55 °C. DETAILED DESCRIPTION

[0022] 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 reference to the drawings.

[0023] The present application provides a non-flammable local high-concentration ionic liquid electrolyte containing chlorinated hydrocarbon diluents, which comprises a lithium salt, an ionic liquid dissolving the lithium salt, and a diluent, wherein the diluent is one or more mixtures of chloroform, tetrachloromethane, and chlorinated hydrocarbon compounds with the molecular formula C n H 2n+2-m Cl m , n is an integer ≥ 2, and m is an integer ≥ 1. In some embodiments of the present application, various conventional commercially available liquid chlorinated hydrocarbon compounds at room temperature, such as dichloromethane, chloroform, dichloroethane, chlorohexane, chlorocyclohexane, etc., are used to prepare a local high-concentration ionic liquid electrolyte, and assembled into a lithium battery for electrochemical testing at room temperature, and the electrochemical performance thereof is not weaker than that of a local high-concentration ionic liquid electrolyte using a hydrofluoroether compound. Then, the lithium battery is further placed in a 55 °C temperature environment for full battery testing, wherein the chloroform, tetrachloromethane, and chlorinated hydrocarbon compounds with the molecular formula C n H 2n+2-m Cl m still operate normally at high temperature, and the battery life is not significantly reduced, but the lithium battery using dichloromethane as a diluent fails to operate normally after a period of high-temperature operation.

[0024] Further, in some embodiments of the present application, the ionic liquid is selected from at least one of imidazole ionic liquid and pyrrolidine ionic liquid.

[0025] Further, in some embodiments of the present application, the anion of the ionic liquid is a fluorine-containing group.

[0026] Further, in some embodiments of the present application, the lithium salt is at least one of lithium bis(trifluoromethylsulfonyl)imide and lithium bis(fluorosulfonyl)imide.

[0027] Further, in some embodiments of the present application, the electrolyte comprises an additive, which is at least one of fluoroethylene carbonate and vinylene carbonate. The addition of the additive can improve the uniformity and stability of the solid-solid interface film, and significantly improve the cycle stability and safety performance of the battery.

[0028] Further, in some embodiments of the present application, the molar concentration of lithium salt in the ionic liquid is 0.5-6.0 mol / L; the mass fraction of the additive is 0.1-5.0%, and the volume fraction of the chlorinated hydrocarbon diluent is 20-80%.

[0029] To make the purpose, technical solutions and advantages of the present application more clear, some embodiments and comparative examples of the present application will be provided below.

[0030] Example 1

[0031] A local high-concentration ionic liquid electrolyte, the preparation method is: mixing bis (trifluoromethyl sulfone) imidazole salt and chloroform according to the volume ratio 1:1, then adding lithium salt bis trifluoromethyl sulfone imide lithium (LiTFSI), so that the concentration reaches 1 mol / L. Then add 0.1% of the additive fluoroethylene carbonate by mass fraction. The electrolyte system is used for full battery test with metal lithium sheet as negative electrode, LiNi 0.8 Co 0.1 Mn 0.1 02 as the positive electrode, and the charge and discharge is carried out at 0.2 C rate in the voltage range of 4.3-2.8 V to the capacity drop to 80% of the initial capacity as the test termination.

[0032] Example 2

[0033] A local high-concentration ionic liquid electrolyte, the preparation method is: mixing bis (trifluoromethyl sulfone) imidazole salt and chloroform according to the volume ratio 1:1, then adding lithium salt bis trifluoromethyl sulfone imide lithium (LiTFSI), so that the concentration reaches 1 mol / L. Then add 0.1% of the additive fluoroethylene carbonate by mass fraction. The electrolyte system is used for full battery test with metal lithium sheet as negative electrode, LiNi 0.8 Co 0.1 Mn 0.1 02 as the positive electrode, and the charge and discharge is carried out at 0.2 C rate in the voltage range of 4.3-2.8 V to the capacity drop to 80% of the initial capacity as the test termination.

[0034] Example 3

[0035] A local high-concentration ionic liquid electrolyte, the preparation method is: mixing bis (trifluoromethyl sulfone) imidazole salt and chloroform according to the volume ratio 1:1, then adding lithium salt bis trifluoromethyl sulfone imide lithium (LiTFSI), so that the concentration reaches 1 mol / L. Then add 0.1% of the additive fluoroethylene carbonate by mass fraction. The electrolyte system is used for full battery test with metal lithium sheet as negative electrode, LiNi 0.8 Co 0.1 Mn 0.102Full cell test as cathode, charge-discharge in the voltage range of 4.3-2.8 V at 0.2 C rate to the capacity drop to 80% of the initial capacity as the test termination.

[0036] Example 4

[0037] A local high-concentration ionic liquid electrolyte, its preparation method is: the mixed volume ratio of bisfluorosulfonylimide pyrrole salt and chlorocyclohexane is 1:2, then lithium salt bis-trifluoromethyl sulfonimide lithium (LiTFSI) is added, and the concentration reaches 2 mol / L. Add 0.2% of the additive fluoroethylene carbonate by mass fraction. The electrolyte system is used for lithium metal sheet as negative electrode, LiNi 0.8 Co 0.1 Mn 0.1 02Full cell test as cathode, charge-discharge in the voltage range of 4.3-2.8 V at 0.2 C rate to the capacity drop to 80% of the initial capacity as the test termination.

[0038] Example 5

[0039] A local high-concentration ionic liquid electrolyte, its preparation method is: the mixed volume ratio of bis-trifluoromethyl sulfonimide imidazole salt and chlorocyclohexane is 1:2, then lithium salt bis-fluoromethyl sulfonimide lithium (LiFSI) is added, and the concentration reaches 3 mol / L. Add 0.2% of the additive fluoroethylene carbonate by mass fraction. The electrolyte system is used for lithium metal sheet as negative electrode, LiNi 0.8 Co 0.1 Mn 0.1 02Full cell test as cathode, charge-discharge in the voltage range of 4.3-2.8 V at 0.2 C rate to the capacity drop to 80% of the initial capacity as the test termination.

[0040] Example 6

[0041] A local high-concentration ionic liquid electrolyte, its preparation method is: the mixed volume ratio of bis-trifluoromethyl sulfonimide imidazole salt and chlorocyclohexane is 1:2, then lithium salt bis-fluoromethyl sulfonimide lithium (LiFSI) is added, and the concentration reaches 3 mol / L. Add 0.2% of the additive fluoroethylene carbonate by mass fraction. The electrolyte system is used for lithium metal sheet as negative electrode, LiNi 0.8 Co 0.1 Mn 0.1 02Full cell test as cathode, charge-discharge in the voltage range of 4.3-2.8 V at 0.2 C rate to the capacity drop to 80% of the initial capacity as the test termination.

[0042] Comparative Example 1

[0043] The electrolyte does not contain diluent chloroform, and the other conditions are the same as in Example 1.

[0044] Comparative Example 2

[0045] The electrolyte does not contain diluent chloroform, and the other conditions are the same as in Example 2.

[0046] Comparative Example 3

[0047] The electrolyte does not contain additive vinylene carbonate, and the other conditions are the same as in Example 3.

[0048] Comparative Example 4

[0049] The electrolyte does not contain additive vinylene carbonate, and the other conditions are the same as in Example 3.

[0050] Comparative Example 5

[0051] The electrolyte does not contain diluent chloroform, and the other conditions are the same as in Example 1.

[0052] Comparative Example 6

[0053] The electrolyte does not contain diluent chloroform, and the other conditions are the same as in Example 4.

[0054]

[0055] As shown in Table 1, the local high-concentration ionic liquid electrolyte containing chlorohydrocarbon diluent and film-forming additive significantly improves the cycle life of the battery composed of metal oxide positive electrode and lithium metal negative electrode.

[0056]

[0057] As shown in Table 2, the local high-concentration ionic liquid electrolyte containing chlorohydrocarbon diluent and film-forming additive improves the cycle life of the battery operating at high temperature. More importantly, using Comparative Example 5 and Comparative Example 6 with dichloromethane as diluent, the battery fails to operate normally, and therefore, cannot be used as diluent for the battery that may operate in high temperature environment.

[0058] In addition, the non-flammable local high-concentration ionic liquid electrolyte containing chlorohydrocarbon diluent in the present application has the advantages of low viscosity and good membrane wetting. The electrolyte has a small contact angle on the membrane, indicating that it has good wetting (Figure). The electrolyte prepared in Example 1 was tested by linear sweep voltammetry on an electrochemical workstation, and the test voltage range was 3.0-5.5 V, and the scan rate was 0.1 mV / s, from which Figure 2It can be seen that the obtained local high-concentration ionic liquid electrolyte electrochemical stability window is greater than 4.5 V.

[0059] The above disclosure is only the preferred embodiment of the present application, and of course cannot be used to limit the scope of the rights of the present application, so the equivalent changes made by the claims of the present application still fall within the scope of the present application.

Claims

1. A non-flammable, locally high-concentration ionic liquid electrolyte containing a chlorinated hydrocarbon diluent, the electrolyte comprising a lithium salt, an ionic liquid dissolving the lithium salt, and a diluent, characterized in that: The diluent is chloroform, tetrachloroform, or a substance with the molecular formula [missing information]. One or more chlorinated hydrocarbon compounds, where n is an integer ≥2 and m is an integer ≥1; The electrolyte includes an additive, which is at least one of fluoroethylene carbonate and vinylene carbonate. The molar concentration of lithium salt in the ionic liquid is 0.5–6.0 mol / L; the mass fraction of additives is 0.1–5.0%, and the volume fraction of chlorinated hydrocarbon diluent is 20–80%. The ionic liquid is selected from at least one of imidazole ionic liquids and pyrrolidine ionic liquids.

2. The non-flammable, locally high-concentration ionic liquid electrolyte containing chlorinated hydrocarbon diluent according to claim 1, characterized in that: The anion of the ionic liquid is a fluorine-containing group.

3. The non-flammable, locally high-concentration ionic liquid electrolyte containing chlorinated hydrocarbon diluent according to claim 1, characterized in that: The lithium salt is at least one of lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide.

4. The application of the non-flammable, locally high-concentration ionic liquid electrolyte containing chlorinated hydrocarbon diluent as described in any one of claims 1-3 in the preparation of lithium metal batteries.

5. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The electrolyte is a non-flammable, locally high-concentration ionic liquid electrolyte containing chlorinated hydrocarbon diluents as described in any one of claims 1-3; the negative electrode material is lithium metal foil, carbon-based material, or silicon-based material; the positive electrode material is lithium cobalt oxide, ternary nickel-cobalt-manganese, or ternary nickel-cobalt-aluminum; and the separator material is polypropylene, polyethylene, or a composite of polypropylene and polyethylene.

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