An electrolyte, its preparation method and application

Abstract

This invention provides an electrolyte, its preparation method, and its application. The electrolyte comprises a lithium salt, an ether-functionalized ionic liquid solvent, a diluent, and additives. This invention reduces the viscosity and melting point of the ionic liquid by introducing ether functional groups into the ionic liquid cation, while also providing a high electrochemical window suitable for high-voltage systems. The electrolyte is stable for lithium metal and enables uniform lithium deposition at the negative electrode, effectively suppressing lithium dendrite growth. It is applicable to high-energy-density lithium metal secondary battery systems with lithium metal as the negative electrode, as well as novel lithium battery systems with N / P < 1 (positive electrode excess). This type of lithium battery not only utilizes the lithium intercalation capacity of the negative electrode carbon material but also the capacity of the lithium metal deposited on the negative electrode surface, which can greatly improve the energy density of the battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and relates to an electrolyte and a preparation method and application thereof. BACKGROUND

[0002] In recent years, lithium ion batteries have been widely applied to various 3C electronic products and electric tools in people's life, and are rapidly developing towards special fields such as electric vehicles, power grid storage, space exploration, national defense application and submarine operation. With the continuous growth of application demand in these special fields, people have put forward higher and higher requirements for the energy density and working temperature of the energy storage batteries. However, the traditional commercial electrolyte generally uses carbonate organic solvents, which has problems such as flammability, high melting point, poor electrochemical stability and the like, seriously affecting the safety and high and low temperature performance of the battery, and greatly limiting their application in these special fields.

[0003] From the perspective of safety, ionic liquids have low saturated vapor pressure, wide electrochemical window, excellent thermal stability and flame retardancy, and have broad application prospects in safe electrolytes. However, in practical application, ionic liquids generally have problems such as high viscosity, low ionic conductivity, poor diaphragm wettability, high melting point and high cost, which seriously affect their performance under low temperature conditions, and they can only be applied to room temperature and high temperature lithium battery systems, greatly limiting the practical application of ionic liquids. Therefore, at the present stage, it is urgent to develop a new type of wide temperature safe electrolyte, which not only has the wide electrochemical window, excellent flame retardancy and thermal stability of ionic liquids, but also has low viscosity, high ionic conductivity and wettability, low melting point and cost, so as to better meet the actual application requirements of people.

[0004] CN109830752B discloses a non-flammable high-voltage lithium ion battery electrolyte and a preparation method and application thereof. The electrolyte is composed of an electrolyte organic solvent and a lithium salt, wherein the electrolyte organic solvent is composed of two or more fluorocarbonates, but at least one cyclic fluorocarbonate and one chain fluorocarbonate are contained in the fluorocarbonates, and the low temperature performance of the electrolyte is poor.

[0005] CN102324566B discloses a preparation method of an ionic liquid electrolyte, which uses lithium salt, carbonate, ionic liquid, 12-crown-4 and its derivatives, long-chain alkyl polymethacrylate, long-chain alkyl polyacrylate and flame retardant as raw materials, and mixes the lithium salt, carbonate, ionic liquid, 12-crown-4 and its derivatives, long-chain alkyl polymethacrylate, long-chain alkyl polyacrylate and flame retardant in a mass percentage ratio of (0.001%~95%):(0.001%~95%):(0.001%~95%):(0.001%~95%):(0.001%~15%):(0.001%~15%):(0.001%~15%), and stirs until all materials are completely dissolved, to obtain the ionic liquid electrolyte. The electrolyte disclosed in CN102324566B has high viscosity, low ionic conductivity and poor membrane wettability.

[0006] The above-mentioned scheme has the problems of poor low-temperature performance, or the electrolyte prepared by using ionic liquid has high viscosity, low ionic conductivity and poor membrane wettability, and therefore it is necessary to develop an electrolyte with good low-temperature performance and avoiding the problems of high viscosity, low ionic conductivity and poor membrane wettability of the electrolyte prepared by using ionic liquid. SUMMARY

[0007] The electrolyte provided by the present application has high electrochemical window, is stable to lithium metal, can make lithium uniformly deposit on the negative electrode and effectively inhibit the growth of lithium dendrites, can be applied to a high-energy-density lithium metal secondary battery system using lithium metal as the negative electrode, and can be applied to a new lithium battery system with N / P<1 (excess positive electrode), which not only utilizes the lithium intercalation capacity of the negative electrode carbon material, but also utilizes the capacity of the lithium metal deposited on the surface of the negative electrode, so that the energy density of the battery can be greatly improved.

[0008] To achieve the object of the present application, the present application adopts the following technical scheme:

[0009] In a first aspect, the present application provides an electrolyte, which comprises a lithium salt, an ether group functionalized ionic liquid solvent, a diluent and an additive, the ether group functionalized ionic liquid solvent has a general structure of [A] + [B] - , the cation [A] + in the general structure has any one of the following structures (1)-(10) or any combination of two or more thereof:

[0010]

[0011] wherein the substituents R1, R2, R3or R4are the same or different, and at least one is an ether group, the number of carbon atoms of R1, R2, R3or R4is 1-20, and the anion [B] - is selected from FSI - , TFSI - , BF4 - , PF6 - , BOB - , DFOB - , or DFP - .

[0012] The present application is advantageous in reducing the melting point and viscosity of electrolyte by introducing ether group functional group into the cation of ionic liquid, while having high electrochemical window, suitable for high voltage system. By adding low concentration of lithium salt and hydrofluoroether wetting agent, it is advantageous to reduce the viscosity and cost of electrolyte, improve ionic conductivity and separator wettability. At the same time, such electrolyte is completely non-flammable, which is advantageous to inhibit lithium dendrite growth, can effectively improve the safety and energy density of battery, suitable for wide temperature range lithium ion battery and high energy density lithium metal secondary battery system.

[0013] Preferably, the ether group functional ionic liquid solvent includes any one or combination of at least two of N-methyl, methoxyethyl pyrrolidine bis-trifluoromethanesulfonimide salt ionic liquid, N-ethyl, methoxypropyl pyrrolidine bis-trifluoromethanesulfonimide salt ionic liquid, N-methyl, ethoxyethyl piperidine alkyl bis-fluorosulfonimide salt ionic liquid, N-ethyl, methoxybutyl tetrafluoroborate salt ionic liquid, N-methyl, methoxyethyl diethyl ammonium tetrafluoroborate salt ionic liquid, 1-ethyl-3-methoxyethyl imidazole bis-trifluoromethanesulfonimide salt ionic liquid, or N-ethyl, methoxyethyl piperazine bis-trifluoromethanesulfonimide salt ionic liquid.

[0014] Preferably, the volume fraction of the ether group functional ionic liquid solvent is 20-70%, for example 20%, 30%, 50%, 70%, etc., based on 100% of the volume of the electrolyte.

[0015] Preferably, the lithium salt includes any one or combination of at least two of lithium bis-fluorosulfonimide, lithium bis-trifluoromethanesulfonimide, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium bis-oxalate borate, lithium hexafluoroarsenate, lithium difluoro-oxalate borate, lithium difluorophosphate, or lithium 4,5-dicyano-2-trifluoromethyl imidazole.

[0016] Preferably, the concentration of the lithium salt in the ionic liquid is 0.01-0.45 mol / L, for example 0.01 mol / L, 0.05 mol / L, 0.1 mol / L, 0.3 mol / L, or 0.45 mol / L, etc., preferably 0.1-0.3 mol / L.

[0017] Preferably, the diluent is a fluoroether compound.

[0018] Preferably, the fluoroether compound has a general structure of R5-O-R6, wherein R5, R6 is an alkyl group or a fluorine-containing alkyl group with a carbon atom number of 1-10, and at least one substituent is a fluoroalkyl group.

[0019] Preferably, the diluent includes any one or a combination of at least two of 1,1,1,3,3,3-hexafluoroisopropyl methyl ether, 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 2,2,2-trifluoroethyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl nonafluorobutyl ether, or ethyl nonafluorobutyl ether.

[0020] Preferably, the volume fraction of the diluent is 30-80%, for example, 30%, 50%, 70%, or 80%, etc., based on 100% of the volume of the electrolyte.

[0021] Preferably, the additive includes any one or a combination of at least two of ethylene carbonate, vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, propylene sulfate, vinyl sulfite, propylene sulfite, 1,3-propane sultone, and 1,4-butane sultone.

[0022] Preferably, the volume fraction of the additive is 0.1-10%, for example, 0.1%, 0.5%, 1%, 3%, 5%, or 10%, etc., based on 100% of the volume of the electrolyte.

[0023] In a second aspect, the present application provides a preparation method of the electrolyte according to the first aspect, the preparation method comprising the following steps:

[0024] (1) mixing equal molar amounts of a halide of a cation containing an ether group [A] + and a salt of an anion [B] - with an organic solvent, allowing the mixture to stand and separate after reaction, taking the organic phase for washing and drying to obtain an ether group functionalized ionic liquid solvent;

[0025] (2) mixing the ether group functionalized ionic liquid solvent obtained in step (1) with a diluent and an additive, adding a lithium salt and stirring to obtain the electrolyte.

[0026] Preferably, the organic solvent in step (1) includes any one or a combination of at least two of dichloromethane, trichloromethane, tetrachloromethane, toluene, xylene, or chlorobenzene.

[0027] Preferably, the washing agent of the washing treatment comprises deionized water.

[0028] Preferably, the end point of the washing treatment is that no precipitate is generated after adding silver nitrate solution into the aqueous phase.

[0029] In a third aspect, the present application provides a lithium battery comprising the electrolyte as described in the first aspect.

[0030] Preferably, the lithium battery further comprises a positive electrode, a negative electrode and a separator.

[0031] Preferably, the active material of the positive electrode comprises a composite metal oxide of lithium.

[0032] Preferably, the active material of the negative electrode comprises any one or a combination of at least two of lithium metal, lithium alloy, crystalline carbon, amorphous carbon, carbon fiber, hard carbon or soft carbon.

[0033] Preferably, the separator comprises any one or a combination of at least two of glass cellulose membrane, cellulose membrane or porous polyolefin compound membrane.

[0034] Compared with the prior art, the present application has the following beneficial effects:

[0035] (1) The present application reduces the viscosity and melting point of the ionic liquid by introducing an ether group functional group into the cation of the ionic liquid, and at the same time, the cost and viscosity of the electrolyte are reduced by using a low concentration of lithium salt and adding a low viscosity and low melting point hydrofluoroether solvent, the ionic conductivity of the electrolyte is improved, and the wettability of the electrolyte to the separator and the electrode sheet is improved. The prepared electrolyte is not only suitable for high temperature lithium ion batteries, but also can effectively improve the low temperature performance of the electrolyte.

[0036] (2) The electrolyte of the present application is stable to lithium metal, can make lithium uniformly deposit on the negative electrode, and effectively inhibit the growth of lithium dendrites. It can be applied to high energy density lithium metal secondary battery system with lithium metal as negative electrode, and new structure lithium battery system with N / P<1 (positive electrode excess). This structure of lithium battery not only utilizes the lithium intercalation capacity of the negative electrode carbon material, but also utilizes the capacity of the lithium metal deposited on the negative electrode surface, which can greatly improve the energy density of the battery.

[0037] (3) The discharge specific capacity of the NCM622 / Li lithium metal battery prepared using the electrolyte of the present application can reach 187 mAh·g -1 The above, the coulombic efficiency can reach more than 81%, the capacity retention rate of 50 cycles can reach more than 90%, and the capacity retention rate at-20℃ can reach more than 55%. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a scanning electron microscope image of the surface lithium plating morphology of the electrolyte described in Example 1.

[0039] Figure 2 This is a scanning electron microscope (SEM) magnification image of the lithium plating morphology on the electrolyte surface described in Example 1.

[0040] Figure 3 This is a cyclic voltammetry (CV) curve of the battery prepared with the electrolyte described in Example 1 at 25°C and a scan rate of 0.1 mV / s.

[0041] Figure 4 This is a cyclic voltammetry (CV) curve of the battery prepared with the electrolyte described in Example 2 at 70°C and a scan rate of 0.1 mV / s.

[0042] Figure 5 This is a coulombic efficiency graph of the Cu / Li battery assembled with the electrolyte described in Example 1 over 100 cycles. Detailed Implementation

[0043] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0044] Example 1

[0045] This embodiment provides an electrolyte, which is prepared by the following method:

[0046] (1) In a 250 mL single-necked round-bottom flask, first add 11 g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 30 mL of deionized water, and stir continuously until completely dissolved. In a 100 mL beaker, add 10 g of N-methyl,methoxyethyl pyrrolidine bromide and 45 mL of dichloromethane, stir until completely dissolved, and then add dropwise to the round-bottom flask. Stir at room temperature for 24 h. After the reaction is complete, separate the liquids using a separatory funnel. Take the organic phase solution and repeatedly extract and wash it with deionized water until no obvious white precipitate is formed when silver nitrate aqueous solution is added to the washed aqueous phase solution. The organic phase solution is then placed in a vacuum dryer at 110 °C for 3 days to obtain the product N-methyl,methoxyethyl pyrrolidine bis(trifluoromethanesulfonyl)imide salt ionic liquid.

[0047] (2) Under a protective atmosphere (H2O < 1 ppm), 68% by volume of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 30% by volume of N-methyl,methoxyethylpyrrolidine bis(trifluoromethanesulfonyl)imide salt ionic liquid and 2% by volume of fluoroethylene carbonate are mixed evenly, and 0.1 M of lithium difluorooxalate borate is added. The mixture is stirred thoroughly and dissolved evenly until the electrolyte is clear and transparent to obtain the electrolyte.

[0048] The scanning electron microscope image of the lithium plating morphology on the electrolyte surface is shown below. Figures 1-2 As shown, fromFigures 1-2 It can be seen that in this type of low-concentration electrolyte, lithium metal will be densely packed on the surface in the form of large, flat particles, indicating that the electrolyte can effectively suppress the formation of lithium dendrites and reduce the occurrence of side reactions between lithium metal and electrolyte.

[0049] Example 2

[0050] This embodiment provides an electrolyte, which is prepared by the following method:

[0051] (1) In a 250 mL single-necked round-bottom flask, first add 11 g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 30 mL of deionized water, and stir continuously until completely dissolved. In a 100 mL beaker, add 10 g of N-methyl,methoxyethyl pyrrolidine bromide and 45 mL of dichloromethane, stir until completely dissolved, and then add dropwise to the round-bottom flask. Stir at room temperature for 24 h. After the reaction is complete, separate the liquids using a separatory funnel. Take the organic phase solution and repeatedly extract and wash it with deionized water until no obvious white precipitate is formed when silver nitrate aqueous solution is added to the washed aqueous phase solution. The organic phase solution is then placed in a vacuum dryer at 110 °C for 3 days to obtain the product N-methyl,methoxyethyl pyrrolidine bis(trifluoromethanesulfonyl)imide salt ionic liquid.

[0052] (2) Under a protective atmosphere (H2O < 1 ppm), 68% by volume of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 30% by volume of N-methyl,methoxyethylpyrrolidine bis(trifluoromethanesulfonyl)imide salt ionic liquid and 2% by volume of fluoroethylene carbonate are mixed evenly, and 0.2 M of bis(trifluoromethanesulfonyl)imide is added. The mixture is stirred thoroughly and dissolved evenly until the electrolyte is clear and transparent to obtain the electrolyte.

[0053] Example 3

[0054] The only difference between this embodiment and Embodiment 1 is that the concentration of lithium salt in step (2) is 0.4M, while the other conditions and parameters are exactly the same as in Embodiment 1.

[0055] Example 4

[0056] The only difference between this embodiment and Embodiment 1 is that the concentration of lithium salt in step (2) is 0.05M, while the other conditions and parameters are exactly the same as in Embodiment 1.

[0057] Example 5

[0058] The only difference between this embodiment and embodiment 1 is that the volume ratio of the ionic liquid in step (2) is 80% and the diluent ratio is 28%, while the other conditions and parameters are exactly the same as in embodiment 1.

[0059] Example 6

[0060] The only difference between this embodiment and embodiment 1 is that the volume ratio of the ionic liquid in step (2) is 10%, the diluent ratio is 78%, and the other conditions and parameters are exactly the same as in embodiment 1.

[0061] Comparative Example 1

[0062] The only difference between this comparative example and Example 1 is that no diluent is added; all other conditions and parameters are exactly the same as in Example 1.

[0063] Comparative Example 2

[0064] The only difference between this comparative example and Example 1 is that the ionic liquid is replaced with N-methyl,propylpyrrolidine bis(trifluoromethanesulfonyl)imide salt ionic liquid, and no diluent is added. All other conditions and parameters are exactly the same as in Example 1.

[0065] Batteries were made using the electrolytes obtained in Examples 1-6 and Comparative Examples 1-2, and their performance was tested. The test results are shown in Table 1.

[0066] Table 1

[0067]

[0068] As shown in Table 1, and based on Examples 1-6, the NCM622 / Li lithium metal battery prepared using the electrolyte described in this invention can achieve a discharge specific capacity of 187 mAh·g. -1 The above results show that the coulombic efficiency can reach over 81%, the capacity retention rate after 50 cycles can reach over 90%, and the capacity retention rate at -20℃ can reach over 55%.

[0069] A comparison of Examples 1 and 3-4 shows that the concentration of lithium salt in the electrolyte of the present invention affects the performance of the electrolyte. When the concentration of lithium salt is controlled at 0.1-0.3M, the performance of the electrolyte is excellent. If the concentration of lithium salt is too high, the viscosity of the electrolyte will increase, the lithium salt will not dissolve completely, resulting in less battery capacity and affecting the battery cycle performance. If the concentration of lithium salt is too low, it will lead to slow lithium ion kinetics and affect the long-term cycle stability of the battery.

[0070] A comparison of Examples 1 and 5-6 shows that the volume percentage of the ionic liquid in the electrolyte affects the performance of the electrolyte. Controlling the volume percentage of the ionic liquid in the electrolyte to 20-70% results in an electrolyte with excellent performance. If the volume percentage of the ionic liquid in the electrolyte is too high, the electrolyte viscosity will be too high, affecting the battery's capacity performance under normal and low temperature conditions. If the volume percentage of the ionic liquid in the electrolyte is too low, the lithium salt will not dissolve completely, affecting the battery's cycle performance.

[0071] As can be seen from the comparison between Example 1 and Comparative Example 1, the present invention, by adding a diluent, helps to reduce the viscosity of the electrolyte, improve the ionic conductivity and membrane wettability, thereby improving the battery capacity and cycle performance.

[0072] As can be seen from the comparison between Example 1 and Comparative Example 2, the present invention, by introducing ether functional groups into the ionic liquid cation, is beneficial to reducing the melting point and viscosity of the electrolyte, increasing the battery capacity, and improving low-temperature performance.

[0073] The cyclic voltammetry (CV) curves of the battery prepared with the electrolyte described in Example 1 at 25°C and a scan rate of 0.1 mV / s are shown below. Figure 3 As shown, from Figure 3 It can be seen that the battery has symmetrical and consistent redox peaks during the 2.8–4.5V cycling process, indicating that the electrolyte has excellent reversibility and cycling performance in the battery system.

[0074] The cyclic voltammetry (CV) curves of the battery prepared with the electrolyte described in Example 2 at 70°C and a scan rate of 0.1 mV / s are shown below. Figure 4 As shown, through Figure 4 This demonstrates that the electrolyte maintains excellent stability at high temperatures and can still exhibit good cycle performance in high-temperature lithium metal battery applications.

[0075] The electrolyte described in Example 1 was used to assemble a Cu / Li battery. The coulombic efficiency of the Cu / Li battery over 100 cycles was as follows: Figure 5 As shown, through Figure 5 This indicates that the battery has high lithium insertion / extraction coulombic efficiency and good cycle stability, suggesting that the electrolyte can effectively suppress the formation of lithium dendrites and dead lithium, reduce the occurrence of side reactions, and thus improve the battery coulombic efficiency.

[0076] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing an electrolyte, characterized by, The preparation method comprises the following steps: (1) mixing equal molar amounts of N-methyl, methoxyethyl pyrrolidine bromide, lithium bis-trifluoromethanesulfonimide and an organic solvent, standing and separating after reaction, taking the organic phase, washing and drying to obtain an ether functionalized ionic liquid solvent; (2) mixing the ether functionalized ionic liquid solvent obtained in step (1), a diluent, an additive, adding lithium salt and stirring to obtain the electrolyte; The electrolyte comprises a lithium salt, an ether functionalized ionic liquid solvent, a diluent and an additive. The ether functionalized ionic liquid solvent is N-methyl, methoxyethyl pyrrolidine bis-trifluoromethanesulfonimide salt ionic liquid. The concentration of the lithium salt in the ionic liquid is 0.1-0.3 mol / L. The diluent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether. The volume fraction of the diluent is 30-70% based on 100% of the volume of the electrolyte.

2. The production method according to claim 1, wherein The volume fraction of the ether functionalized ionic liquid solvent is 20-70% based on 100% of the volume of the electrolyte.

3. The production method according to claim 1, wherein The lithium salt comprises any one or a combination of at least two of lithium bisfluorosulfonimide, lithium bis-trifluoromethanesulfonimide, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium hexafluoroarsenate, lithium difluoro(oxalato)borate, lithium difluorophosphate or lithium 4,5-dicyano-2-trifluoromethylimidazole.

4. The production method according to claim 1, wherein The additive comprises any one or a combination of at least two of vinyl carbonate, vinylene carbonate, fluoro-vinyl carbonate, vinyl sulfate, propylene sulfate, vinyl sulfite, propylene sulfite, 1,3-propane sultone and 1,4-butane sultone.

5. The production method according to claim 1, wherein The volume fraction of the additive is 0.1-10% based on 100% of the volume of the electrolyte.

6. The production method according to claim 1, wherein The organic solvent in step (1) comprises any one or a combination of at least two of dichloromethane, trichloromethane, tetrachloromethane, toluene, xylene or chlorobenzene.

7. The production method according to claim 1, wherein The washing agent for the washing treatment comprises deionized water.

8. The production method according to claim 1, wherein The end point of the washing treatment is that no precipitate is generated after adding silver nitrate solution in the aqueous phase.

9. A lithium battery, characterized by The lithium battery comprises the electrolyte prepared by the preparation method in any one of claims 1-8.

10. The lithium battery of claim 9, wherein the lithium metal anode is coated with a layer of lithium phosphorus oxynitride. The lithium battery further comprises a positive electrode, a negative electrode and a separator.

11. The lithium battery of claim 10, wherein the lithium metal anode is a lithium foil anode. The active material of the positive electrode comprises a lithium composite metal oxide.

12. The lithium battery of claim 10, wherein the lithium metal anode is coated with a layer of lithium phosphorus oxynitride. The active material of the negative electrode comprises any one or a combination of at least two of lithium metal, lithium alloy, crystalline carbon, amorphous carbon or carbon fiber.

13. The lithium battery of claim 10, wherein the lithium metal anode is coated with a layer of lithium phosphorus oxynitride. The separator comprises any one or a combination of at least two of glass cellulose membrane or porous polyolefin compound membrane.

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