Multifunctional electrolyte for lithium metal battery and application thereof

Abstract

The application discloses a multifunctional electrolyte for a lithium metal battery and application thereof, which comprises a lithium salt and an organic solvent, wherein the organic solvent is composed of tetrahydro-pyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,3-dioxolane in a volume ratio of (2.5-7):(2.5-7):(1-3); and the lithium salt is composed of lithium bis(fluorosulfonyl)imide and lithium perchlorate in a molar ratio of (0.5-4):(0.5-4). The electrolyte has the characteristics of ultralow melting point, low impedance, high temperature resistance, high pressure resistance and the like, significantly improves the performance of ether electrolyte, and is applied to the lithium metal battery, which widens the voltage window and working temperature range, solves the problem of low-temperature capacity attenuation, and enhances the high-temperature cycle stability.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of high and low temperature electrolyte of lithium metal battery, and relates to a multifunctional electrolyte for lithium metal battery and application thereof, in particular to a multifunctional electrolyte for high and low temperature lithium metal battery and application thereof. BACKGROUND

[0002] Since the commercialization and popularization of secondary batteries, the demand for high energy density batteries has continued to rise, among which lithium metal batteries have shown great potential to lead the innovation trend of battery technology due to their unparalleled high energy density characteristics, and are expected to drive the leapfrog progress of electric vehicles, consumer electronics and even energy storage industry. However, the current commercial carbonate electrolyte system dominated by ethylene carbonate (EC) has exposed significant technical bottlenecks when facing high and low temperature application scenarios, especially under extreme low temperature and high temperature conditions. Under low temperature environment, the conductivity of the electrolyte system decreases sharply, even solidifies, directly leading to significant capacity attenuation of the battery; while under high temperature conditions, the electrolyte decomposition is accelerated, and the interfacial reaction is more intense, thereby causing safety hazards, which seriously restricts the performance and application of lithium metal batteries under these extreme conditions.

[0003] At present, exploring and developing high-performance electrolyte systems suitable for high and low temperature environments has become the key to further development of lithium metal battery technology. Under extreme low temperature or high temperature environment, the electrolyte will have a series of problems such as viscosity increase, conductivity decrease, electrolyte evaporation, electrolyte decomposition, which affect the performance and safety of the battery. Ether-based electrolyte has the advantages of low viscosity, high conductivity, good fast charging and low temperature application, which can effectively improve the application of lithium metal battery in high and low temperature environments, but ether-based electrolyte also has the problems of easy evaporation, instability, strong reactivity with lithium and low electrochemical stability. Especially under high voltage conditions, the ether molecules in the electrolyte may be electrolytically decomposed, further affecting the performance of the battery.

[0004] Therefore, it is of great significance to develop a lithium metal battery electrolyte that can not only adapt to high-voltage positive materials but also ensure stable performance in high and low temperature environments for the in-depth development of lithium metal battery electrolyte technology. SUMMARY

[0005] In view of the deficiencies in the prior art, the first object of the present application is to provide a multifunctional electrolyte for lithium metal battery. The electrolyte has small viscosity, low freezing point and good thermal stability by the synergistic use of three specific ether reagents and double lithium salt. At the same time, the electrolyte not only has good compatibility with the negative electrode of lithium metal battery, but also can adapt to high-voltage positive materials.

[0006] A second objective of the present invention is to provide a multifunctional electrolyte for lithium metal batteries. This electrolyte significantly improves the performance of lithium metal batteries under extreme temperature conditions. In particular, it improves the stability of the electrolyte / electrode interface in high and low temperature environments, broadens the operating temperature range of the battery, and enables lithium metal batteries to maintain excellent long-cycle performance at both high and low temperatures. Furthermore, the electrolyte's perfect compatibility with high-voltage cathode materials significantly expands the application scope of lithium metal batteries.

[0007] In order to achieve the above technical objectives, the present invention provides a multifunctional electrolyte for a lithium metal battery, comprising a lithium salt and an organic solvent, wherein the organic solvent is composed of tetrahydropyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,3-dioxolane in a volume ratio of (2.5-7):(2.5-7):(1-3); the lithium salt is composed of lithium bis(fluorosulfonyl)imide and lithium perchlorate in a molar ratio of (0.5-4):(0.5-4).

[0008] The reason why the electrolyte of the present invention can operate efficiently in high and low temperature environments is that the core is that it uses ether solvents instead of traditional carbonate solvents (such as EC, DMC). Ether solvents, with their shorter molecular chains and excellent fluidity, ensure that high conductivity can be maintained even under cold conditions, while showing significant advantages such as low melting point, low viscosity and high cost-effectiveness. However, for lithium metal batteries, when a single ether solvent is used, the surge in lithium ion coordination sites can easily cause solvent decomposition, hindering the discharge process, especially when paired with high-voltage positive electrode materials. To solve this problem, the present invention cleverly uses three ether solvents for use in combination, among which tetrahydropyran and 1,3-dioxolane are cyclic structure ether compounds, while 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is a linear ether solvent. By adopting a cyclic ether solvent, its weak solvation characteristics can be utilized, which can greatly promote Li +The interaction between the 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and the anions facilitates their decomposition at the electrode interface. By introducing a linear ether such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether into the cyclic ether solvent system, not only an appropriate number of lithium ion coordination sites is ensured to maintain efficient ion conduction, but the melting point and viscosity of the electrolyte are further reduced, thereby perfectly adapting to high-voltage cathode materials and exhibiting excellent electrochemical performance in high and low temperature environments. Furthermore, the 1,3-dioxolane used in the electrolyte system of the present invention undergoes a ring-opening polymerization reaction induced by the lithium salt, rapidly polymerizing on the inorganic two-dimensional solid electrolyte interface (SEI) to form an upper organic-lower inorganic composite SEI film. The lower inorganic SEI layer has high mechanical strength, which can prevent the SEI film from rupture and failure, while the upper organic SEI film is insoluble in organic solvents, effectively preventing the co-intercalation of solvent molecules, greatly improving the stability of the electrode material and thereby enhancing the capacity and cycle life of the battery.

[0009] The inventors discovered that compared to other common cyclic ether solvents, tetrahydropyran has a six-membered ring structure, which gives it higher chemical stability and reduces the byproducts produced by solvent decomposition. When it is used in combination with 1,3-dioxolane and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, it can extend the cycle life of the battery and improve safety.

[0010] The key to the technical solution of the present invention lies in controlling the volume ratio of tetrahydropyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,3-dioxolane to be (2.5-7):(2.5-7):(1-3). In the present invention, tetrahydropyran and 1,3-dioxolane are both cyclic ethers. By controlling the amount of these two and the amount of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, when applied to lithium metal batteries (especially lithium cobalt oxide batteries), the problem of excessive viscosity of the electrolyte at low temperatures can be effectively improved, and the weak solvation characteristics of the solvent can be further reduced, which can greatly reduce the Li +The interaction between the solvent and the electrolyte further reduces the occurrence of side reactions of the solvent at the interface; if the amount of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is too small (that is, the amount of cyclic ether solvent in the electrolyte is too high), due to the poor oxidation resistance of the cyclic ether itself, oxidative decomposition will occur on the positive electrode side under high voltage, thereby adversely affecting the performance of the lithium metal battery; and when the amount of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is too large, the surge in lithium ion coordination sites can easily trigger solvent decomposition, hindering the discharge process. In addition, if the amount of 1,3-dioxolane is too high, the 1,3-dioxolane monomer will be triggered by the lithium salt to undergo a ring-opening polymerization reaction, rapidly polymerize and prevent it from further dissolving, resulting in limited dissolution of the lithium salt, seriously affecting and reducing the ionic conductivity of the electrolyte, and the uneven electrolyte formed by the polymerization will further deteriorate the electrode interface; when the amount of 1,3-dioxolane is too low, it will cause the stability of the lithium metal battery to deteriorate during high and low temperature cycles; and when the amount of tetrahydropyran is too low, it is not conducive to the decomposition of anions at the electrode interface, forming a strong inorganic SEI layer, resulting in a decrease in electrode strength and stability; and when the amount of tetrahydropyran is too high, due to the problem of excessive viscosity of tetrahydropyran at low temperatures, it will hinder the migration of lithium ions in the electrolyte, resulting in a slower ion conduction rate, and at the same time it will also cause the polarization phenomenon of the battery to be aggravated during the charge and discharge process, further aggravating the attenuation of battery performance. More preferably, the volume ratio of tetrahydropyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,3-dioxolane is (6-7):(2.5-3):(2-3).

[0011] As a preferred embodiment, the lithium salt comprises lithium bis(fluorosulfonyl)imide and lithium perchlorate in a molar ratio of (0.5-4):(0.5-4). The present invention utilizes lithium bis(fluorosulfonyl)imide and lithium perchlorate in synergistic fashion to form a solid electrolyte layer rich in LiF and LiCl on the positive electrode side of the battery, effectively preventing the ether organic solvent of the present invention from contacting the positive electrode material, reducing the occurrence of side reactions under high and low temperature operation, thereby protecting the positive electrode material and improving the battery's cycle stability. Within the molar ratio range selected by the present invention, the effect of reducing side reactions is optimal. Further preferably, the lithium salt comprises lithium bis(fluorosulfonyl)imide and lithium perchlorate in a molar ratio of (3-4):(0.5-1).

[0012] As a preferred solution, the total concentration of the lithium salt in the electrolyte is 0.1 to 4 mol / L. When the lithium salt concentration is too low, the lithium ion conductivity will be low, resulting in poor low-temperature performance of the lithium-ion battery. When the concentration is too high, the viscosity of the electrolyte will be too high, affecting the migration speed of lithium ions at low temperatures. It is further preferably 0.1 to 1.5 mol / L, and even more preferably 0.8 to 1.5 mol / L.

[0013] As a preferred solution, the electrolyte further comprises at least one of phosphate compounds, fluorocarbonate compounds, borate compounds, sulfite compounds, sultone compounds and ether compounds.

[0014] The present invention also provides an application of a multifunctional electrolyte, which is applied to high and low temperature lithium metal batteries. The present invention mainly combines the weak solvation properties of the main solvent tetrahydropyran, which can greatly promote the + The interaction between the electrolyte and anions facilitates the decomposition of anions at the electrode interface, forming a robust inorganic SEI layer. The cosolvent 1,3-dioxolane facilitates the formation of an organic SEI coating on top of the inorganic SEI, forming an organic-inorganic composite SEI layer at the electrode interface. The cosolvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether significantly enhances the electrolyte's antioxidant properties. Its extremely low freezing point (-94.27°C) and high boiling point (93.17°C) allow the electrolyte to remain liquid over a wide temperature range, while exhibiting excellent low-temperature, thermal, and chemical stability. The synergistic effect of these three factors improves the electrolyte / electrode interface stability in both high and low-temperature environments, enabling lithium metal batteries to maintain excellent long-cycle performance at both high and low temperatures and stable charge and discharge over a wide operating temperature range exceeding 100°C. Furthermore, the electrolyte's perfect compatibility with high-voltage cathode materials greatly expands the application scope of lithium metal batteries.

[0015] As a preferred solution, the lithium metal battery includes a positive electrode sheet, a negative electrode sheet and a separator.

[0016] As a preferred solution, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector; the active material of the positive electrode active material layer is lithium cobalt oxide or lithium nickel cobalt manganese oxide; it is further preferred that the active material of the positive electrode active material layer is lithium cobalt oxide. As a low-temperature positive electrode material, due to its unique layered structure, it has the advantages of high operating voltage, low self-discharge, superior electrochemical performance, and stable structure compared to other positive electrode materials. Compared with other electrolytes, the electrolyte of the present invention can be more adapted to the high voltage window of lithium cobalt oxide.

[0017] As a preferred solution, the active material of the negative electrode plate is lithium metal or lithium alloy.

[0018] As a preferred solution, the lithium alloy includes at least one of Li-Zn alloy, Li-Zn-O alloy, Li-Sn alloy, Li-Sn-O alloy, Li-Mg alloy and Li-Al alloy.

[0019] The present invention provides a method for preparing a multifunctional electrolyte for a lithium metal battery, comprising: mixing a lithium salt, tetrahydropyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and 1,3-dioxolane in an inert gas atmosphere, and then stirring the mixture with a magnetic stirrer for 30 to 45 minutes.

[0020] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0021] (1) The electrolyte of the present invention effectively improves the solvation structure of the electrolyte through the synergistic effect of tetrahydropyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,3-dioxolane, greatly reduces the viscosity of the electrolyte and improves the conductivity of the electrolyte at high and low temperatures, so that lithium ions can be quickly transported in the electrolyte and have a faster desolvation process. At the same time, in the electrolyte system of the present invention, 1,3-dioxolane and tetrahydropyran can also construct an organic-inorganic composite SEI film layer, wherein the inorganic SEI of the lower layer has high mechanical strength, which can avoid the rupture and failure of the SEI film, while the organic SEI film of the upper layer is insoluble in organic solvents, which can effectively prevent the co-embedding of solvent molecules, greatly improving the stability of the electrode material, thereby enhancing the capacity and cycle life of the battery.

[0022] (2) The electrolyte provided by the present invention is applied to high and low temperature lithium metal batteries and can stably charge and discharge at a wide operating temperature range of over 100°C. In addition, in low temperature environments, lithium metal batteries can maintain a high discharge capacity and cycle stability, significantly improving the low temperature performance of lithium metal batteries. In normal temperature environments, lithium metal batteries can achieve stable cycling at a high voltage of 4.45V and can also achieve 10C ultra-high rate charge and discharge, significantly improving the high voltage performance and rate performance of lithium metal batteries.

[0023] (3) The present invention uses two specific organic-inorganic lithium salts in conjunction with an organic solvent to form a solid electrolyte layer rich in LiF and LiCl on the positive electrode side of the battery, which can effectively prevent the ether electrolyte of the present invention from contacting the positive electrode material, reduce the occurrence of side reactions under high and low temperature operation, thereby protecting the positive electrode material and improving the cycle stability of the battery.

[0024] (4) The electrolyte provided by the present invention can adapt to high-voltage positive electrode materials.

[0025] (5) The raw materials of the electrolyte of the present invention are simple and easy to obtain, the production cost is low, and it has the potential for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1The cycle performance diagram of the lithium cobalt oxide full battery assembled in Example 2 is charged to 4.45V at a constant current of 0.5C at 30°C and then discharged to 2.5V at a constant current of 0.5C.

[0027] Figure 2 Rate performance diagram of the lithium cobalt oxide full batteries assembled in Comparative Examples 1-2 and Example 2 at 30°C with a voltage range of 2.5-4.35V.

[0028] Figure 3 The cycling performance diagram of the lithium cobalt oxide full battery assembled in Example 2 at 6C ultra-high rate with a voltage range of 2.5~4.35V at 30°C.

[0029] Figure 4 Long cycle performance diagram of the lithium cobalt oxide full battery assembled in Comparative Examples 1-2 and Example 2 at a low temperature of -40°C, charged to 4.45V at a constant current of 0.05C and discharged to 2.5V at a constant current of 0.1C, where the data of Comparative Examples 1 and 2 overlap.

[0030] Figure 5 This is a long cycle performance diagram of the lithium cobalt oxide full battery assembled in Comparative Examples 1-2 and Example 2 at a high temperature of 55°C, charged to 4.45V at a constant current of 0.5C, and discharged to 2.5V at a constant current of 0.5C.

[0031] Figure 6 This is a discharge curve diagram of the lithium cobalt oxide full battery assembled in Example 2 working at over 100°C. DETAILED DESCRIPTION

[0032] The following description sets forth numerous specific details to facilitate a thorough understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art may make similar modifications without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific implementations disclosed below.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in this specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0034] In the embodiments of the present invention, unless otherwise specified, the chemical reagents used can be purchased or prepared by existing preparation methods, and the instruments and equipment used are conventional equipment in the prior art.

[0035] Example 1

[0036] The electrolyte of this embodiment is composed of a lithium salt and an organic solvent. The lithium salts are lithium bis(fluorosulfonyl)imide and lithium perchlorate (LiClO4). The total lithium salt concentration is 1 mol / L, and the molar ratio of the two lithium salts is 4:1. The organic solvent is composed of tetrahydropyran, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, with a volume ratio of 2.5:3:7. Under an inert gas atmosphere, the lithium salt and organic solvent are mixed and stirred for 30 minutes to obtain an electrolyte.

[0037] Example 2

[0038] The electrolyte of this embodiment is composed of a lithium salt and an organic solvent. The lithium salts are lithium bis(fluorosulfonyl)imide and lithium perchlorate. The total lithium salt concentration is 1 mol / L, and the molar ratio of the two lithium salts is 4:1. The organic solvent is composed of tetrahydropyran, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, with a volume ratio of 7:3:2.5. Under an inert gas atmosphere, the lithium salt and organic solvent are mixed and stirred for 30 minutes to obtain an electrolyte.

[0039] Example 3

[0040] The electrolyte of this embodiment is composed of a lithium salt and an organic solvent. The lithium salts are lithium bis(fluorosulfonyl)imide and lithium perchlorate. The lithium salt concentration is 1 mol / L, and the molar ratio of the two lithium salts is 4:1. The organic solvent is composed of tetrahydropyran, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, with a volume ratio of 4.75:3:4.75. Under an inert gas atmosphere, the lithium salt and organic solvent are mixed and stirred for 30 minutes to obtain an electrolyte.

[0041] Example 4

[0042] The electrolyte of this embodiment is composed of a lithium salt and an organic solvent. The lithium salts are lithium bis(fluorosulfonyl)imide and lithium perchlorate. The total lithium salt concentration is 1 mol / L, and the molar ratio of the two lithium salts is 4:1. The organic solvent is composed of tetrahydropyran, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. The volume ratio of the three solvents is 4.375:1.875:6.25. Under an inert gas atmosphere, the lithium salt and organic solvent are mixed and stirred for 30 minutes to obtain an electrolyte.

[0043] Comparative Example 1

[0044] The electrolyte used in this comparative example is a commercially available conventional electrolyte (LB-515) consisting of a lithium salt and an organic solvent. The lithium salt is lithium hexafluorophosphate, with a lithium salt concentration of 1 mol / L. The organic solvent is composed of ethylene carbonate, ethyl methyl carbonate, and fluoroethylene carbonate, with a volume ratio of 3:7:1. Under an inert gas atmosphere, the lithium salt and organic solvent are mixed and stirred for 30 minutes until uniformly mixed to obtain an electrolyte.

[0045] Comparative Example 2

[0046] The electrolyte in this comparative example is composed of a lithium salt and an organic solvent. The lithium salts are lithium bis(fluorosulfonyl)imide and lithium perchlorate. The total lithium salt concentration is 1 mol / L, and the molar ratio of the two lithium salts is 4:1. The organic solvent is tetrahydropyran. Under an inert gas atmosphere, the lithium salt and the organic solvent are mixed and stirred for 30 minutes until uniformly mixed to obtain an electrolyte.

[0047] Comparative Example 3

[0048] The only difference between this comparative example and Example 2 is that 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is not added to the organic solvent, and the other steps and conditions are the same.

[0049] Comparative Example 4

[0050] The difference between this comparative example and Example 2 is that the organic solvent is changed to tetrahydropyran and 1,3-dioxolane in a volume ratio of 1:1, and the other steps and conditions are the same.

[0051] Comparative Example 5

[0052] The difference between this comparative example and Example 2 is that the organic solvent is changed to tetrahydropyran and 1,3-dioxolane in a volume ratio of 3:7, and the other steps and conditions are the same.

[0053] Comparative Example 6

[0054] The only difference between this comparative example and Example 2 is that the organic solvent is changed to 1,3-dioxolane, and the other steps and conditions are the same.

[0055] Comparative Example 7

[0056] The only difference between this comparative example and Example 2 is that the organic solvent is replaced by tetrahydropyran, 1,3-dioxolane and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in a volume ratio of 1.75:0.75:10, and the other steps and conditions are the same.

[0057] Comparative Example 8

[0058] The only difference between this comparative example and Example 2 is that tetrahydropyran is replaced by an equal volume of linear ether ethylene glycol dimethyl ether, and the remaining steps and conditions are the same.

[0059] Performance testing:

[0060] Raman spectra of the electrolyte were collected using a Raman microscope under 532nm excitation laser and the FSI in the Raman spectrum was recorded. - Anion peak (Raman shift 775cm -1 The Raman shift offset values ​​of the position are shown in Table 1.

[0061] Nuclear magnetic resonance (NMR) was used to examine the solvation structure in the electrolyte. Further testing was performed by dissolving 5 mg of samples of different solutions and electrolytes in CDCl₃₄₀ in glass tubes to detect the presence of aggregates in the solvent.

[0062]

[0063] From the results in Table 1, it can be seen that when the volume of tetrahydropyran solvent is appropriately reduced, no polymer will appear in the electrolyte, and FSI - The absolute value of the peak shift value gradually decreases, indicating that FSI - Anions and Li + The binding ability of the solvent and Li is enhanced, making it easier to form a stable and strong SEI film at the interface. + The weakening of the binding of tetrahydropyran reduces the decomposition of the solvent at the interface, improves the cycle performance and rate performance of the battery, and enhances the safety of the battery. However, if the volume of tetrahydropyran in the organic solvent is too small, polymers will appear.

[0064] Using a conductivity meter, the electrolyte was placed in a constant temperature box at 30°C for 1 hour before the conductivity test was performed. The conductivity values ​​were recorded. The results are shown in Table 2.

[0065]

[0066] From the results in Table 2, it can be seen that when the co-solvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is gradually added, since it is a non-polar solvent and does not dissolve lithium salts, the lithium ion concentration in the electrolyte decreases, and the ionic conductivity decreases. Therefore, only by adding an appropriate amount of linear ether can the conductivity be guaranteed to be higher than that of conventional commercial electrolytes.

[0067] The electrochemical performance of the LCO / Li half-cell prepared using the electrolyte of the above embodiment was tested using a Xinwei battery testing system.

[0068] High-voltage charge-discharge experiment: The battery obtained in Example 1 was placed in a 30°C constant temperature oven for 12 hours, then charged at 0.1C to 4.45V. It was then discharged at 0.1C constant current to 2.5V. The charge and discharge capacities under high voltage were recorded. The results are shown in Table 3.

[0069]

[0070] As shown in Table 3, it can be seen from Comparative Examples 2 to 6 and 8 that when the tetrahydropyran volume ratio is small, since 1,3-dioxolane undergoes ring-opening polymerization under the addition of bis(fluorosulfonyl)imide lithium, it is impossible to achieve a high cut-off voltage charge-discharge test. As shown in Comparative Example 8 and Example 2, when replaced with an isovolumetric linear ether glycol dimethyl ether, the charge-discharge performance test cannot be achieved at a high cut-off voltage. This is because tetrahydropyran has a more stable six-membered ring structure than glycol dimethyl ether, and its antioxidant capacity is stronger. At the same time, tetrahydropyran can provide lower weak solvation energy and low interfacial impedance. As shown in Example 2 and Comparative Example 2, when the solvent is completely replaced with tetrahydropyran, it will also have an adverse effect on high-voltage performance. This is because the oxidative stability of tetrahydropyran itself is relatively poor and it is easy to decompose on the positive electrode side, thereby affecting the high-voltage performance of the battery.

[0071] Figure 1 This is the long cycle performance diagram of the LCO|Li full battery assembled in Example 2 at 30°C and 4.45V high voltage. After 200 cycles, the capacity retention rate reaches 90.7%.

[0072] High rate performance experiment: The battery obtained in the example was placed in a constant temperature box at 30°C for 12 hours and then subjected to constant current charge and discharge test. The test voltage range was 2.5~4.35V.

[0073] Figure 2 This graph shows the rate performance of LCO|Li full cells assembled from Comparative Examples 1-2 and Example 2 at 30°C. At an ultra-high rate of 20C, the capacity remains at 93.4 mAh / g, with a capacity retention rate of 64.3% at 0.1C charge / discharge. However, the commercial electrolytes in Comparative Examples 1 and 2 exhibit significantly lower capacities than Example 2 at an ultra-high rate of 20C.

[0074] Figure 3 The specific capacity-voltage diagram of the LCO|Li full battery assembled in Example 2 at 30°C and 6C rate. After 400 cycles, the capacity is still 100.4 mAh / g.

[0075] Low-temperature performance test: The batteries obtained in Comparative Examples 1-2 and Example 2 were activated at room temperature, placed in a low-temperature box, and allowed to stand at -40°C for 2 hours. They were then charged at a constant current of 0.05C to 4.45V, and then discharged at a constant current of 0.1C to 2.5V for a cycle test.

[0076] Figure 4 Figure 2 shows the cycling performance of LCO|Li full batteries assembled in Comparative Examples 1-2 and Example 2 at -40°C. After 100 cycles, the full battery made with the electrolyte of Example 2 of the present invention still has a capacity of 76.8 mAh / g, with a capacity retention rate of 80%. However, the full batteries made with the commercial electrolytes of Comparative Examples 1 and 2 cannot operate at low temperatures.

[0077] High-temperature performance test: The batteries obtained in Comparative Examples 1-2 and Example 2 were activated at room temperature, placed in a high-temperature box, and allowed to stand at 55°C for 2 hours. They were then charged at a constant current of 0.5C to 4.3V, and then discharged at a constant current of 0.5C to 2.5V for a cycle test.

[0078] Figure 5 Figure 2 shows the cycling performance of LCO|Li full cells assembled from Comparative Examples 1-2 and Example 2 at 55°C. After 100 cycles, the full cell made with the electrolyte from Example 2 still maintained a capacity of 153.5 mAh / g, with a capacity retention rate of 90%, significantly higher than that of Comparative Examples 1 and 2.

[0079] High and low temperature discharge performance experiment: The battery obtained in Example 2 was activated at room temperature and tested for room temperature capacity. It was then placed in a thermostat at different temperatures. After standing for 2 hours at the tested temperature, it was discharged at a constant current of 0.1C to 2.5V. The discharge capacity and voltage at different temperatures were recorded.

[0080] Figure 6 This is the discharge capacity-voltage diagram of the LCO|Li full battery assembled in Example 2 at -50~55°C. The battery can operate normally in a wide temperature range exceeding 100°C.

[0081] From the above electrochemical test results, it can be seen that the battery prepared with the electrolyte of the present invention has better high-voltage performance, room-temperature rate, high and low-temperature performance and more stable cycle performance. The coordinated use of two lithium salts to form a solid electrolyte layer rich in LiF and LiCl on the positive electrode side of the battery can effectively prevent the electrolyte from contacting the positive electrode material, reduce the occurrence of side reactions under high and low temperature operation, thereby protecting the positive electrode material and improving the cycle stability of the battery. The addition of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,3-dioxolane effectively improves the solvation structure of the electrolyte, greatly reduces the viscosity of the electrolyte and improves the conductivity of the electrolyte at high and low temperatures, so that lithium ions can be quickly transported in the electrolyte and have a faster desolvation process. In summary, the electrolyte combination of the present invention improves the voltage window of ether solvents and the transmission rate of lithium ions at high and low temperatures, improves the electrode interface compatibility, and enables the battery to achieve stable charge and discharge at a wide operating temperature range of over 100°C.

[0082] In summary, the present invention effectively improves the ionic conductivity of the electrolyte, enhances the high and low temperature performance and fast charging performance of the lithium metal battery, and realizes the role of a multifunctional electrolyte through the synergistic effect between cyclic ether organic solvents, linear ether organic solvents and lithium salts.

[0083] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A multifunctional electrolyte for a lithium metal battery, comprising a lithium salt and an organic solvent, characterized in that: The organic solvent is composed of tetrahydropyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,3-dioxolane in a volume ratio of (2.5-7): (2.5-7): (1-3); The lithium salt is composed of lithium bis(fluorosulfonyl)imide and lithium perchlorate in a molar ratio of (0.5-4): (0.5-4); The active material of the positive electrode active material layer on the positive electrode plate of the lithium metal battery is lithium cobalt oxide or lithium nickel cobalt manganese oxide.

2. The multifunctional electrolyte for lithium metal batteries according to claim 1, characterized in that: The total concentration of the lithium salt in the electrolyte is 0.1-4 mol / L.

3. A multifunctional electrolyte for lithium metal batteries according to claim 1 or 2, characterized in that: The electrolyte further comprises at least one of a phosphate compound, a fluorocarbonate compound, a borate compound, a sulfite compound, a sultone compound, and an ether compound.

4. Use of a multifunctional electrolyte for a lithium metal battery according to any one of claims 1 to 3, characterized in that: The electrolyte is applied to high-temperature or low-temperature lithium metal batteries.

5. The use of a multifunctional electrolyte for lithium metal batteries according to claim 4, characterized in that: The lithium metal battery includes a positive electrode sheet, a negative electrode sheet and a separator; The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; The active material of the positive electrode active material layer is lithium cobalt oxide or lithium nickel cobalt manganese oxide; The active material of the negative electrode plate is lithium metal or lithium alloy.

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