High-voltage-resistant flame-retardant electrolyte and lithium metal battery

Abstract

The invention discloses a high-voltage-resistant flame-retardant electrolyte and a lithium metal battery, the high-voltage-resistant flame-retardant electrolyte comprises a lithium salt, a nitrile-based plastic crystal solvent, a diluent and an additive, the diluent is a fluorinated ether solvent and / or a fluorinated phosphazene flame retardant, and the additive comprises a fluorocarbonate solvent. According to the invention, the fluorinated / or phosphazene diluent is introduced into the nitrile electrolyte to form a solvation structure rich in anions, so that the stability of the nitrile electrolyte to a lithium metal negative electrode is improved, and meanwhile, the intrinsic characteristic of high voltage resistance and the characteristic of high safety are maintained; the problems of high viscosity and poor wettability of a high-concentration plastic nitrile electrolyte are effectively solved, the advantage of high energy density of a lithium metal battery can be effectively exerted, and meanwhile, the safety of the lithium metal battery is improved. The fluorinated solvent is used as the lithium protection additive, so that the cycle life of the lithium metal battery can be prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and more particularly, to a high-voltage resistant and flame-retardant electrolyte and a lithium metal battery. Background Art

[0002] Lithium metal batteries have become a research hotspot in the field of energy storage due to their high theoretical energy density and low electrochemical potential. However, lithium metal batteries face challenges such as uncontrollable dendritic growth and electrode / electrolyte side reactions in practical applications. Among them: the electrolyte, as a key component in lithium metal batteries, its performance directly affects the safety, cycle stability and energy density of the battery.

[0003] Although traditional organic liquid electrolytes have good electrochemical properties, they are prone to thermal runaway and combustion. To solve such problems, researchers have begun to explore new electrolyte materials: nitrile-based electrolytes exhibit a relatively wide potential window and thermal stability, and are expected to become substitutes for carbonate-based electrolytes. Among all nitrile-based electrolytes, succinonitrile-based electrolytes (also known as succinonitrile, SN) have attracted a large amount of research due to their low cost, pollution-free and high room-temperature ionic conductivity. Succinonitrile-based electrolytes have a high decomposition voltage and good conductivity, which is beneficial to realizing lithium metal batteries with higher energy density. In addition, succinonitrile-based electrolytes also have good mechanical properties and chemical stability, and can effectively resist volume changes and chemical corrosion during the charge and discharge process of the battery. However, the interfacial compatibility between succinonitrile and the lithium metal electrode is poor, and side reactions are prone to occur, which will deteriorate the electrode / electrolyte interface, resulting in a decline in the performance of lithium metal batteries and potential safety hazards.

[0004] To improve the interfacial stability between succinonitrile and lithium metal electrodes, the research team led by Professor Yin Geping at Harbin Institute of Technology proposed to configure a high-concentration electrolyte based on succinonitrile by changing the concentration of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in succinonitrile (SN), increasing the coordination number of TFSI−, thereby forming a SEI film rich in LiF, enhancing the interfacial stability and improving the battery cycle performance. However, the high-concentration electrolyte strategy has problems such as high viscosity, poor wettability, and high cost, seriously hindering the development of practical high-energy-density lithium metal batteries. The research team led by Professor Zhang Jiaheng at Harbin Institute of Technology proposed a eutectic electrolyte: by using the strong intermolecular force in the eutectic molecule to increase the solubility of lithium nitrate in the eutectic electrolyte, an incombustible and safe electrolyte is formed. At the same time, nitrate anions participate in the solvation structure of lithium ions, which helps to form a stable solid electrolyte interface film derived from rich anions, effectively inhibiting the growth of lithium dendrites. When applied to lithium metal batteries composed of high-energy-density ternary cathodes, it shows long-term stable cycle performance. However, by increasing the lithium salt concentration or changing the type of lithium salt through the above strategies, the principle is to improve the interfacial compatibility by changing the solvation structure of lithium ions, which cannot solve the difficulties in lithium ion diffusion and transmission caused by increased viscosity. At the same time, it will also increase the risk of poor wetting between the electrolyte and the electrode or separator, affecting the battery rate performance.

[0005] Therefore, there is an urgent need to provide an electrolyte to solve the problems of high viscosity, low ionic conductivity, and poor stability to lithium negative electrodes in nitrile-based high-concentration electrolytes. Summary of the Invention

[0006] In view of this, the present invention provides a high-voltage resistant and flame-retardant electrolyte and a lithium metal battery to reduce viscosity, improve ion conduction, and improve the stability to lithium.

[0007] On the one hand, the present invention provides a high-voltage resistant and flame-retardant electrolyte, comprising: a lithium salt, a nitrile-based plastic crystal solvent, a diluent, and an additive, wherein the diluent is a fluorinated ether solvent and / or a fluorinated phosphazene flame retardant, and the additive includes a fluorinated carbonate solvent.

[0008] Optionally, the nitrile-based plastic crystal solvent is at least one of succinonitrile, ethyl succinonitrile, adiponitrile, glutaronitrile, suberonitrile, sebaconitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 2-fluorohexanedinitrile / 2,2-difluorosuccinonitrile.

[0009] Optionally, the fluorinated phosphazene flame retardant is at least one of hexafluorocyclotriphosphazene, ethoxypentafluorocyclotriphosphazene, trifluoroethoxypentafluorocyclotriphosphazene, (phenoxy)pentafluorocyclotriphosphazene.

[0010] Optionally, the fluorinated ether solvent is at least one of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, tris(trifluoroethoxy)methane, and bis(2,2,2-trifluoroethyl) ether.

[0011] Optionally, the fluorinated carbonate solvent is at least one of fluorinated ethylene carbonate and difluoroethylene carbonate.

[0012] Optionally, in the high-voltage resistant and flame-retardant electrolyte, the mass fraction of the lithium salt is 16 wt% - 24 wt%, the mass fraction of the plastic crystal solvent is 20 wt% - 30 wt%, the mass fraction of the diluent is 40 wt% - 60 wt%, and the mass fraction of the additive is 4 wt% - 6 wt%.

[0013] Optionally, the volume fraction of the nitrile-based plastic crystal solvent in the electrolyte is 20% - 40%; the volume fraction of the diluent in the electrolyte is 60% - 80%, and the volume fraction of the fluorinated carbonate solvent in the electrolyte is 4% - 8%.

[0014] Optionally, the lithium salt is at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, and lithium tetrafluoroborate.

[0015] On the other hand, the present invention also provides a lithium metal battery, including a negative electrode sheet, a positive electrode sheet, a separator, and an electrolyte, and the electrolyte is the above-mentioned high-voltage resistant and flame-retardant electrolyte.

[0016] Optionally, the positive electrode sheet includes a high-nickel ternary positive electrode material or a lithium-rich manganese-based positive electrode material.

[0017] Compared with the prior art, the high-voltage resistant and flame-retardant electrolyte and lithium metal battery provided by the present invention have at least achieved the following beneficial effects:

[0018] By introducing a fluorinated / or phosphazene-based diluent into the nitrile-based electrolyte, the present invention forms a solventized structure rich in anions, improves the stability of the nitrile-based electrolyte towards the lithium metal negative electrode, and at the same time maintains the intrinsic high-voltage resistance and high safety characteristics;

[0019] The nitrile-based locally high-concentration electrolyte provided by the present invention effectively solves the problems of high viscosity and poor wettability existing in the plastic nitrile-based high-concentration electrolyte, can effectively exert the high energy density advantage of the lithium metal battery, and at the same time improves its safety;

[0020] The present invention uses a fluorinated solvent as a lithium protection additive, which can extend the cycle life of a lithium metal battery. From the perspective of molecular dynamics simulation (MD), the present invention confirms that the fluorinated solvent is in the second solvation sheath or in a free state, and this microstructure can promote more anions to enter the first sheath to coordinate with Li+ and a rapid desolvation process; at the same time, part of the free fluoroethylene carbonate (FEC) preferentially decomposes on the surface of the lithium negative electrode, forming lithium fluoride (LiF) in the solid electrolyte interface (SEI), which is beneficial to interfacial ion transport and the stability of the interfacial film. This synergistic effect is beneficial to enhancing the cycle life of the lithium metal battery.

[0021] Of course, any product implementing the present invention does not necessarily need to achieve all the above-mentioned technical effects simultaneously.

[0022] Other features and advantages of the present invention will become clear from the following detailed description of the exemplary embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings incorporated in and constituting a part of this specification illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

[0024] Figure 1 is a schematic diagram of the solvation structure of the locally high-concentration electrolyte based on succinonitrile in Example 1;

[0025] Figure 2 is the radial distribution function and coordination number of the locally high-concentration electrolyte based on succinonitrile in Example 1;

[0026] Figure 3 is the radial distribution function and coordination number of the locally high-concentration electrolyte based on succinonitrile in Example 2;

[0027] Figure 4 is a schematic diagram of the solvation structure of the high-concentration electrolyte based on succinonitrile in Comparative Example 2;

[0028] Figure 5 is the radial distribution function and coordination number of the high-concentration electrolyte based on succinonitrile in Comparative Example 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] Now, various exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that: unless otherwise specifically stated, the relative arrangements of components and steps, numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present invention.

[0030] The following description of at least one exemplary embodiment is merely illustrative in nature and in no way serves as a limitation to the present invention or its application or use.

[0031] Technologies, methods, and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, such technologies, methods, and devices should be considered as part of the specification.

[0032] In all examples shown and discussed herein, any specific values should be construed as merely exemplary and not as a limitation. Thus, other examples of the exemplary embodiments may have different values.

[0033] It should be noted that like reference numerals and letters refer to like items in the following figures, and thus, once an item is defined in one figure, further discussion thereof in subsequent figures is not necessary.

[0034] The high-voltage resistant and flame-retardant electrolyte provided by the embodiments of the present invention includes: a lithium salt, a nitrile-based plastic crystal solvent, a diluent, and an additive, wherein the diluent is a fluorinated ether solvent and / or a fluorinated phosphazene flame retardant, and the additive includes a fluorinated carbonate solvent.

[0035] The lithium salt dissociates into lithium ions in the electrolyte (a form of existence of the electrolyte), and these ions migrate between the positive and negative electrodes during the charge and discharge process of the battery to transfer electrical energy.

[0036] In the embodiments of the present invention, the electrolyte includes a plastic crystal solvent, which can dissolve the lithium salt, conduct lithium ions, improve the high-voltage resistance performance, and improve the flame retardant performance.

[0037] In the embodiments of the present invention, the diluent can reduce the viscosity, improve the wetting, and increase the ionic conductivity, so that the final electrolyte forms an anion-dominated solvation structure, which is beneficial to improving the deposition stability of the lithium metal negative electrode.

[0038] The embodiments of the present invention introduce a fluorinated ether solvent and / or a fluorinated phosphazene flame retardant into the nitrile-based high-concentration electrolyte. While reducing the viscosity, improving the ionic conduction and the stability to lithium, it maintains the anion-dominated solvation structure, enabling the high-energy density lithium metal battery to operate safely and durably.

[0039] Optionally, the nitrile-based plastic crystal solvent is at least one of succinonitrile, ethyl succinonitrile, adiponitrile, glutaronitrile, suberonitrile, sebaconitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 2-fluorohexanedinitrile / 2,2-difluorosuccinonitrile.

[0040] Specifically, succinonitrile, ethyl succinonitrile, adiponitrile, glutaronitrile, suberonitrile, sebaconitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 2-fluorohexanedinitrile / 2,2-difluorosuccinonitrile have a high decomposition voltage and good conductivity, and also have good mechanical properties and chemical stability, and can effectively resist volume changes and chemical corrosion during the charge and discharge process of the battery.

[0041] Optionally, the phosphazene fluoride flame retardant is at least one of hexafluorocyclotriphosphazene (HFPN), ethoxypentafluorocyclotriphosphazene (PFPN), trifluoroethoxypentafluorocyclotriphosphazene (TFPN), and (phenoxy)pentafluorocyclotriphosphazene (FPPN).

[0042] On the one hand, the phosphazene fluoride flame retardant can significantly improve the flame retardancy of the electrolyte, thus ensuring the safe use of the battery. On the other hand, introducing the phosphazene fluoride flame retardant can form a solvation structure rich in anions, improve the stability of nitrile electrolytes towards lithium metal anodes, while maintaining the intrinsic high-voltage resistance and high safety characteristics.

[0043] Optionally, the fluorinated ether solvent is at least one of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, tris(trifluoroethoxy)methane, and bis(2,2,2-trifluoroethyl) ether.

[0044] On the one hand, the fluorinated ether solvent can significantly improve the flame retardancy of the electrolyte, thus ensuring the safe use of the battery. On the other hand, introducing the fluorinated ether solvent can form a solvation structure rich in anions, improve the stability of nitrile electrolytes towards lithium metal anodes, while maintaining the intrinsic high-voltage resistance and high safety characteristics.

[0045] Optionally, the fluorinated solvent is at least one of fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC).

[0046] Both fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC) have excellent chemical and thermal stabilities. As an additive for lithium-ion battery electrolytes, fluoroethylene carbonate (FEC) can form a better-performing SEI film (solid electrolyte interface film). The SEI film has a dense structural layer but does not increase the impedance, and can effectively prevent further decomposition of the electrolyte, thereby improving the low-temperature performance of the electrolyte. Fluoroethylene carbonate (FEC) can also increase the battery capacity and, due to its fluorine-containing structure, has good oxidation resistance and is used in the high-voltage system of the present invention.

[0047] Optionally, in the high-voltage resistant and flame retardant electrolyte, the mass fraction of the lithium salt is 16 wt% - 24 wt%, the mass fraction of the plastic crystal solvent is 20 wt% - 30 wt%, the mass fraction of the diluent is 40 wt% - 60 wt%, and the mass fraction of the additive is 4 wt% - 6 wt%.

[0048] Optionally, the mass fraction of the lithium salt can be 16 wt%, 17 wt%, 18 wt%, 10 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt% or 24 wt%, or can be any value between 16 wt% and 24 wt%, and no specific limitation is made here. When the mass fraction of the lithium salt is 16 wt% - 24 wt%, it can ensure that the electrolyte has appropriate viscosity, ionic conductivity and chemical stability, which helps to improve the energy density, cycle life and safety performance of the battery.

[0049] Optionally, the mass fraction of the plastic crystal solvent can be 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt% or 30 wt%, or can be any value between 20 wt% and 30 wt%. The content of the plastic crystal solvent cannot be too high or too low. If the introduced amount is too high, it will mainly affect the viscosity of the electrolyte too much, and there is a risk of poor wetting of the electrolyte with the separator and the electrode. If the introduced amount is too low, the lithium ion concentration in the electrolyte will be low and the conductivity will decrease. In the present invention, when the mass fraction of the plastic crystal solvent is between 20 wt% and 30 wt%, it can not only reduce the viscosity of the electrolyte but also increase the conductivity.

[0050] Optionally, the mass fraction of the diluent can be 40 wt%, 45 wt%, 50 wt%, 55 wt% or 60 wt%, or can be any value between 40 wt% and 60 wt%. The content of the diluent cannot be too high or too low. If the introduced amount is too low or too high, it will affect the lithium ion transport and the solvation structure, and further affect the performance of the lithium metal battery. In the present invention, when the mass fraction of the diluent is between 40 wt% and 60 wt%, it can ensure the lithium ion transport and the solvation structure.

[0051] Optionally, the mass fraction of the additive fluorinated carbonate solvent can be 4 wt%, 4.5 wt%, 5 wt%, 5.5 wt% or 6 wt%, or can be any value between 4 wt% and 6 wt%. When the mass fraction of the additive is 4 wt% - 6 wt%, it can ensure that the electrolyte has excellent physical and chemical properties and electrochemical properties, which helps to improve the energy density, cycle life, rate performance and low temperature performance of the battery.

[0052] Optionally, the volume fraction of the nitrile-based plastic crystal solvent in the electrolyte is 20% - 40%; the volume fraction of the diluent in the electrolyte is 60% - 80%, and the volume fraction of the fluorinated carbonate solvent in the electrolyte is 4% - 8%.

[0053] Optionally, the volume fraction of the nitrile plastic crystal solvent in the electrolyte can be 20%, 25%, 30%, 35% or 40%, or any value between 20% and 40%; the volume fraction of the diluent in the electrolyte can be 60%, 65%, 70%, 75% or 80%, or any value between 60% and 80%. The volume ratio of the fluorinated carbonate solvent in the electrolyte can be 4%, 5%, 6%, 7% or 8%, or any value between 4% and 8%, which can ensure that the electrolyte has excellent physical and chemical properties and electrochemical properties, and helps to improve the energy density, cycle life, rate performance and low-temperature performance of the battery.

[0054] Optionally, the lithium salt is at least one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium tetrafluoroborate;

[0055] The molar concentration of the lithium salt in the electrolyte is 1.2 M to 1.5 M. Optionally, the molar concentration of the lithium salt in the electrolyte is 1.2 M, 1.35 M, 1.4 M, 1.5 M, and no specific limitation is made here.

[0056] On the other hand, the present invention also provides a lithium metal battery, including a negative electrode sheet, a positive electrode sheet, a separator and an electrolyte, and the electrolyte is the above-mentioned high-voltage resistant and flame-retardant electrolyte.

[0057] Optionally, the positive electrode sheet includes a high-nickel ternary positive electrode material or a lithium-rich manganese-based positive electrode material.

[0058] The high-nickel ternary positive electrode material can be LiNi x Co y Mn z O2, where x + y + z = 1 and 0.6 ≤ x < 1. For example, the high-nickel ternary positive electrode material is LiNi 0.8 Co 0.1 Mn 0.1 O2 or LiNi 0.88 Co 0.06 Mn 0.06 O2.

[0059] The lithium-rich manganese-based positive electrode material can be xLi2MnO3· (1-x) LiMeO2, where Me can be Mn, Ni or Co.

[0060] Taking succinonitrile, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, and fluoroethylene carbonate (FEC) as examples, the electrolyte preparation process is described as follows: (1) Mix 40% (by volume) of succinonitrile with 60% (by volume) of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether evenly; (2) Add 6% (by volume) of fluoroethylene carbonate (FEC) to the above solution and continue stirring until it becomes clear; (3) Add 1.35 M (molar concentration) of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to the above solution and continue stirring until the lithium salt is completely dissolved for later use;

[0061] The lithium metal electrode uses a copper-lithium composite tape, purchased from Tianjin Zhongneng Lithium Industry, and is used after being cut into pieces by a die-cutting device. The positive electrode uses high-nickel ternary, with NMP as the solvent, and 5 wt% PVDF, 5 wt% conductive carbon black (SP), and 95 wt% high-nickel ternary are used as raw materials for slurry coating to prepare a positive electrode sheet with a surface capacity of 5 mAh / cm2. The separator uses a commercial Celgard separator.

[0062] Example 1:

[0063] Dissolve 6 vol% of fluoroethylene carbonate (FEC) in succinonitrile (SN) first to form a homogeneous and transparent solution. Then, dissolve 1.5 M of LiFSI in the above solution. After complete dissolution, add 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) as a diluent to form the final succinonitrile-based locally high-concentration electrolyte. Among them, the volume fraction of SN is 40%, and the volume fraction of the HFE diluent is 60%.

[0064] Using a stainless steel sheet as the positive electrode, a lithium sheet as the negative electrode, a Celgard separator, and the above electrolyte, assemble a SS|Li half-cell and perform linear voltammetry testing (LSV). Its oxidation onset potential is about 4.96 V, which can meet the usage requirements of NCM ternary and lithium-rich manganese positive electrode (LRMO). Using lithium sheets as the positive and negative electrodes, drop the above electrolyte between the electrodes, assemble a Li|Li symmetric cell. At a current density of 1 mA / cm 2 It can stably cycle for more than 300 h. The excellent cycling performance is mainly attributed to (1) the introduction of the diluent is beneficial to the formation of a solvation structure rich in anions. Refer to Figure 1 , Figure 1 where A is an anion, S is succinonitrile, and D is a diluent; (2) there are a large number of free FECs in the solvation structure, which is beneficial to the formation of an SEI dominated by LiF. Refer to Figure 2 , Figure 2 where a is Li-FSI; b is Li-FEC, Figure 2The red line is the radial distribution function, and the blue line is the coordination number. Secondly, through MD simulation, the diffusion coefficients of Li+ and FSI- can be obtained, and it is deduced that the increase in the transference number of effective Li+ helps to improve the battery performance.

[0065] In the embodiment of the present invention, a fluorinated solvent is introduced as a lithium protection additive into the above-mentioned nitrile-based local electrolyte, and molecular dynamics simulation (MD) is carried out through the open-source software Gromacs2019, which proves that the fluorinated solvent is in a free state in the nitrile-based local high-concentration electrolyte. The free FEC preferentially decomposes on the surface of the lithium negative electrode and forms LiF in the SEI, which is beneficial to interfacial ion transport and the stability of the SEI, and further enhances the cycling performance of the high-voltage lithium metal battery.

[0066] Example 2:

[0067] 6 vol% of fluoroethylene carbonate (FEC) is first dissolved in succinonitrile (SN) to form a homogeneous and transparent solution. Then, 1.5 M of LiFSI is dissolved in the above solution. After complete dissolution, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) is added as a diluent to form the final succinonitrile-based local high-concentration electrolyte. Among them, the volume fraction of SN is 30%, and the volume fraction of the HFE diluent is 70%.

[0068] Using a stainless steel sheet as the positive electrode, a lithium sheet as the negative electrode, a Celgard separator, and the above electrolyte, a SS|Li half-cell is assembled and linear sweep voltammetry (LSV) is performed. Its oxidation onset potential is about 4.98 V, which can meet the usage requirements of NCM ternary and lithium-rich manganese positive electrodes (LRMO). Using lithium sheets as the positive and negative electrodes, dropping the above electrolyte between the electrodes, a Li|Li symmetric battery is assembled. At a current density of 1 mA / cm 2 It can stably cycle for more than 360 h. The reason for the improvement in cycling performance is the reduction of free SN and the increase in free FEC, which improves the cycling stability of lithium. Refer to Figure 3 , Figure 3 where a is Li-FSI; b is Li-FEC, Figure 2 The red line is the radial distribution function, and the blue line is the coordination number.

[0069] Example 3:

[0070] 5.5 vol% of difluoroethylene carbonate (DFEC) was first dissolved in ethyl butyronitrile (ESN) to form a homogeneous and transparent solution. Then, 1.35 M of LiTFSI was dissolved in the above solution. After complete dissolution, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TTE) was added as a diluent to form the final ethyl butyronitrile-based locally concentrated electrolyte. Among them, the volume fraction of ESN was 30%, and the volume fraction of the TTE diluent was 70%.

[0071] Using a stainless steel sheet as the positive electrode, a lithium sheet as the negative electrode, a Celgard separator, and the above electrolyte, an SS|Li half-cell was assembled and linear sweep voltammetry (LSV) was performed. Its oxidation onset potential was about 5.02 V, which could meet the usage requirements of NCM ternary and lithium-rich manganese positive electrodes (LRMO). Using lithium sheets as both the positive and negative electrodes, the above electrolyte was dropped between the electrodes to assemble a Li|Li symmetric cell. At a current density of 1 mA / cm 2 The cell could stably cycle for more than 380 h. It should be noted that one of the reasons for the improved cycling performance here is that DFEC has a high fluorination ratio, resulting in a higher LiF content in the SEI film formed.

[0072] Example 4:

[0073] 5 vol% of difluoroethylene carbonate (DFEC) was first dissolved in ethyl butyronitrile (ESN) to form a homogeneous and transparent solution. Then, 1.3 M of LiTFSI was dissolved in the above solution. After complete dissolution, ethoxy pentafluorocyclotriphosphazene (PFPN) was added as a diluent to form the final ethyl butyronitrile-based locally concentrated electrolyte. Among them, the volume fraction of ESN was 30%, and the volume fraction of the PFPN diluent was 70%.

[0074] Using a stainless steel sheet as the positive electrode, a lithium sheet as the negative electrode, a Celgard separator, and the above electrolyte, an SS|Li half-cell was assembled and linear sweep voltammetry (LSV) was performed. Its oxidation onset potential was about 5.2 V, which could meet the usage requirements of NCM ternary and lithium-rich manganese positive electrodes (LRMO). Using lithium sheets as both the positive and negative electrodes, the above electrolyte was dropped between the electrodes to assemble a Li|Li symmetric cell. At a current density of 1 mA / cm 2 The cell could stably cycle for more than 320 h. The highlight of this example was that the flame retardant phosphazene PFPN was introduced as a diluent to further improve the flame retardant performance of the electrolyte and improve the wettability with the electrodes and separator.

[0075] Comparative Example 1:

[0076] Dissolve 3M of LiFSI in succinonitrile (SN) to form a homogeneous and transparent solution, thus forming the final high-concentration succinonitrile-based electrolyte. Using a stainless steel sheet as the positive electrode, a lithium sheet as the negative electrode, a Celgard separator, and the above electrolyte, assemble an SS|Li half-cell and conduct a linear voltammetry test (LSV). Its oxidation onset potential is approximately 4.93V, meeting the usage requirements of NCM ternary and lithium-rich manganese positive electrodes (LRMO). Using lithium sheets as both the positive and negative electrodes, drop the above electrolyte between the electrodes, assemble a Li|Li symmetric cell. At a current density of 0.3 mA / cm 2 current density, it can stably cycle for more than 120h. The decrease in the critical current density is mainly due to the high viscosity of the high-concentration succinonitrile-based electrolyte and poor ionic conduction performance.

[0077] Comparative Example 2:

[0078] First dissolve 6 vol% of fluoroethylene carbonate (FEC) in succinonitrile (SN) to form a homogeneous and transparent solution. Then, dissolve 3M LiFSI in the above solution to form the final high-concentration succinonitrile-based electrolyte. Using a stainless steel sheet as the positive electrode, a lithium sheet as the negative electrode, a Celgard separator, and the above electrolyte, assemble an SS|Li half-cell and conduct a linear voltammetry test (LSV). Its oxidation onset potential is approximately 4.95V, which can meet the usage requirements of NCM ternary and lithium-rich manganese positive electrodes (LRMO). Using lithium sheets as both the positive and negative electrodes, drop the above electrolyte between the electrodes, assemble a Li|Li symmetric cell. At a current density of 0.3 mA / cm2, it can stably cycle for more than 180h. The improvement in the cycling performance is mainly attributed to (1) the formation of an anion-dominated solvation structure, referring to Figure 4 , Figure 4 where A is an anion and S is succinonitrile; (2) there are a large number of free FECs in the solvation structure, referring to Figure 5 , Figure 5 where a is Li-FSI; b is Li-FEC, Figure 2 where the red line in

[0079] Comparative Example 3:

[0080] First dissolve 5 vol% of difluoroethylene carbonate (DFEC) in ethylsuccinonitrile (ESN), and then dissolve 3M of LiTFSI in the above solution to form the final high-concentration ethylsuccinonitrile-based electrolyte.

[0081] Using a stainless steel sheet as the positive electrode, a lithium sheet as the negative electrode, a Celgard separator, and the above electrolyte, a half-cell of SS|Li was assembled and linear sweep voltammetry (LSV) was performed. Its oxidation onset potential was approximately 4.98 V, which could meet the usage requirements of NCM ternary and lithium-rich manganese positive electrodes (LRMO). Using lithium sheets as both the positive and negative electrodes, the above electrolyte was dropped between the electrodes to assemble a Li|Li symmetric cell. At a current density of 0.3 mA / cm2, it could stably cycle for more than 220 h. The improvement in cycling performance was mainly attributed to the presence of a large amount of free DFEC in the solvation structure, which was beneficial to the formation of a LiF-dominated SEI.

[0082] Result comparison:

[0083] For the electrolytes prepared in Examples 1-3 and Comparative Examples 1-3, MD simulations and full-cell performance tests were carried out. The specific test method was as follows: Ni88 high-nickel ternary was selected to make a positive electrode sheet with a areal capacity of 5 mAh / cm 2 The copper-lithium composite tape with a single-sided thickness of 30 μm was selected as the negative electrode sheet, and a 1AH soft-pack battery with three positive electrodes and four negative electrodes was assembled for the first-cycle Coulombic efficiency and cycling performance tests. The test method for the first-cycle Coulombic efficiency was the discharge capacity divided by the charge capacity at a rate of 0.1C; the cycling test rate was 0.2C charge and 0.5C discharge, and the voltage cut-off condition was 2.8V - 4.3V. The results are shown in Table 1:

[0084] Table 1 Test results of different electrolytes in Examples and Comparative Examples

[0085] Group <![CDATA[Li+ diffusion coefficient (1e-5 cm 2 / s)]]> Li+ Migration Number First Efficiency Retention Rate after 100 Cycles Example 1 0.008573 0.44 89.3％ 92.5％ Example 2 0.0115 0.39 88.5％ 93.4％ Example 3 0.0103 0.38 88.6％ 94.5％ Comparative Example 1 0.003056 0.34 80.6％ 60.3％ Comparative Example 2 0.002774 0.36 87.6％ 75.6％ Comparative Example 3 0.002854 0.35 87.4％ 78.9％

[0086] As can be seen from the above examples and comparative examples, the present invention has obtained a lithium metal battery electrolyte with high voltage resistance (>5V), high safety (non-flammable), and stability to lithium;

[0087] Based on the high-concentration salt formula of the present invention, the viscosity of the nitrile-based electrolyte is reduced, and the wettability with the electrode / electrolyte is improved, which is expected to be applied to positive electrodes with a large loading capacity to achieve a lithium metal battery with a high energy density;

[0088] By introducing a diluent, the present invention improves the ionic conductivity of the locally high-concentration nitrile-based electrolyte and increases the design of free FEC / DFEC, thereby enhancing the rate and cycling performance of the lithium metal battery.

[0089] Although some specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are only for illustration and not for limiting the scope of the present invention. Those skilled in the art should understand that the above embodiments can be modified without departing from the scope and spirit of the present invention. The scope of the present invention is defined by the appended claims.

Claims

1. A high-voltage resistant and flame-retardant electrolyte, characterized in that, Comprising: A lithium salt, a nitrile-based plastic crystal solvent, a diluent, and an additive. Among them, the diluent is a fluorinated ether solvent and / or a fluorinated phosphazene flame retardant, and the additive includes a fluorinated carbonate solvent.

2. The high-voltage resistant and flame-retardant electrolyte according to claim 1, wherein, The nitrile-based plastic crystal solvent is at least one of succinonitrile, ethyl succinonitrile, adiponitrile, glutaronitrile, suberonitrile, sebaconitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 2-fluorohexanedinitrile / 2,2-difluorosuccinonitrile.

3. The high-voltage resistant and flame-retardant electrolyte according to claim 1, wherein The fluorinated phosphazene flame retardant is at least one of hexafluorocyclotriphosphazene, ethoxypentafluorocyclotriphosphazene, trifluoroethoxypentafluorocyclotriphosphazene, (phenoxy)pentafluorocyclotriphosphazene.

4. The high-voltage resistant and flame-retardant electrolyte according to claim 1, wherein The fluorinated ether solvent is at least one of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, tris(trifluoroethoxy)methane, bis(2,2,2-trifluoroethyl) ether.

5. The high-voltage resistant and flame-retardant electrolyte according to claim 1, wherein The fluorinated carbonate solvent is at least one of fluoroethylene carbonate, difluoroethylene carbonate.

6. The high-voltage resistant and flame-retardant electrolyte according to claim 1, wherein In the high-voltage resistant flame-retardant electrolyte, the mass fraction of the lithium salt is 16wt% - 24wt%, the mass fraction of the plastic crystal solvent is 20wt% - 30wt%, the mass fraction of the diluent is 40wt% - 60wt%, and the mass fraction of the additive is 4wt% - 6wt%.

7. The high-voltage resistant and flame-retardant electrolyte according to claim 1, wherein The volume fraction of the nitrile-based plastic crystal solvent in the electrolyte is 20% - 40%; the volume fraction of the diluent in the electrolyte is 60% - 80%, and the volume fraction of the fluorinated carbonate solvent in the electrolyte is 4% - 8%.

8. The high-voltage resistant and flame-retardant electrolyte according to claim 1, wherein The lithium salt is at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate.

9. A lithium metal battery, characterized in that, Comprising a negative electrode plate, a positive electrode plate, a separator, and an electrolyte, and the electrolyte is the high-voltage resistant flame-retardant electrolyte according to any one of claims 1 to 8.

10. The lithium metal battery according to claim 9, characterized in that, The positive electrode plate includes a high-nickel ternary positive electrode material or a lithium-rich manganese-based positive electrode material.

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