Highly fluorinated electrolyte for high-specific-energy lithium metal battery as well as preparation method and application of highly fluorinated electrolyte

Abstract

The invention relates to the technical field of electrochemical energy and new materials, in particular to a highly fluorinated electrolyte for a high-specific-energy lithium metal battery as well as a preparation method and application of the highly fluorinated electrolyte. The electrolyte comprises an organic solvent and an electrolyte, the organic solvent comprises a first organic solvent and a second organic solvent, the first organic solvent is fluoroethylene carbonate (FEC) or 3, 3, 3-trifluoropropylene carbonate (TFPC), and the second organic solvent is dimethyl trifluorocarbonate (MFDMC) or (2, 2, 2-trifluoroethyl) methyl carbonate (FEMC). The preparation method of the electrolyte is simple, the solvation structure of lithium ions can be directly adjusted, meanwhile, the interface properties of the positive electrode and the negative electrode of the lithium metal battery are improved, the stability of the electrolyte is good, and the electrochemical window reaches up to 4.6 V. The high-voltage lithium metal battery prepared from the highly fluorinated electrolyte has the advantages of high energy density, excellent cycle life and excellent rate capability.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical energy and new material technology, and in particular to a highly fluorinated electrolyte for lithium metal batteries and a preparation method and use thereof. Background Art

[0002] Lithium metal batteries are widely used in various electronic devices, electric vehicles, and energy storage systems due to their high energy density and long cycle life. However, with the increasing demand for applications, especially the increasingly prominent requirements for stable operation at high voltage and high temperature, traditional electrolyte systems face many challenges. For example, ester-based electrolytes such as conventional ethylene carbonate are easily affected by temperature. As the temperature rises (40–60°C), significant oxidative decomposition will occur, which will generate a large amount of gas and heat, and there are safety hazards.

[0003] As we all know, the energy density of lithium-ion batteries has gradually approached its limit. Lithium metal batteries with lithium metal as the negative electrode material (ternary high nickel LiNi x Co y Mn 1-x-y O 2 The positive electrode (>4.0 V) has become a hot topic of research in the world in recent years due to its high energy density. It is a next-generation battery system with great application prospects. However, there is a strong side reaction between metallic lithium and the commercial carbonate electrolyte that currently dominates. This not only causes the continuous and rapid consumption of metallic lithium and electrolyte, but also easily leads to the generation and rapid growth of lithium dendrites, causing safety accidents. In order to further improve the specific energy of lithium metal batteries and reduce their costs, ultra-high nickel LiNi with higher specific capacity is needed. x Co y Mn 1-x-y O 2 (x>0.8) is the inevitable choice for low-cost and high-energy-density lithium metal battery positive electrode materials. Therefore, it is necessary to find a suitable material for lithium metal negative electrode and ultra-high nickel LiNi x Co y Mn 1-x-y O 2 (x>0.8) Functional electrolytes with high stability at high voltages (>4.6 V) are the key to achieving low-cost, high-energy-density lithium metal batteries with long-term stable cycling. Summary of the invention

[0004] The purpose of the present invention is to provide a highly fluorinated electrolyte for a high-energy-density lithium metal battery, a preparation method and a use thereof in view of the deficiencies in the prior art. The highly fluorinated electrolyte for a high-energy-density lithium metal battery improves the high voltage and high temperature performance of the lithium metal battery while ensuring the electrochemical performance of the lithium metal battery, and solves the potential safety hazard of oxidative decomposition of the electrolyte solvent caused by high temperature to generate a large amount of gas and heat.

[0005] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme: In a first aspect, the present invention provides a highly fluorinated electrolyte for a high specific energy lithium metal battery, comprising an organic solvent and an electrolyte, wherein the organic solvent comprises a first organic solvent and a second organic solvent, the first organic solvent is FEC or TFPC, and the second organic solvent is MFDMC or FEMC methyl carbonate.

[0006] Based on the highly fluorinated electrolyte for high specific energy lithium metal battery, the volume of the first organic solvent accounts for 20-40% of the total volume of the organic solvent, and the volume of the second organic solvent accounts for 60-80% of the total volume of the organic solvent.

[0007] Furthermore, the electrolyte is a lithium salt.

[0008] Furthermore, the concentration of the lithium salt in the high-voltage electrolyte is 0.1-5 mol / L.

[0009] Preferably, the lithium salt is one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, lithium bis(oxalatoborate), lithium difluorooxalatoborate and lithium nitrate.

[0010] More preferably, the lithium salt is lithium hexafluorophosphate, and the concentration of lithium hexafluorophosphate in the electrolyte is 0.5-1.5 mol / L.

[0011] In a second aspect, the present invention provides a method for preparing a highly fluorinated electrolyte for a high energy density lithium metal battery, comprising the following steps: mixing and stirring an organic solvent in a sealed box filled with argon, then adding a formulated amount of electrolyte thereto and stirring until completely dissolved, stirring evenly to obtain the above-mentioned electrolyte.

[0012] Furthermore, the moisture content in the sealed box is less than 0.01 ppm, and the oxygen content is less than 0.01 ppm.

[0013] In a third aspect, the present invention provides the use of the highly fluorinated electrolyte for high energy density lithium metal battery in lithium metal batteries.

[0014] Furthermore, the present invention provides a high-energy-density lithium metal battery, which comprises a highly fluorinated electrolyte for a lithium metal battery.

[0015] The highly fluorinated electrolyte for high-energy-density lithium metal batteries prepared by the present invention helps to form a high-quality solid electrolyte interface film, which can effectively inhibit the side reactions between lithium and the electrolyte, reduce product accumulation and the formation of "dead lithium", and the good cycle stability provides the battery with reliable performance and provides strong support for its long-term stable operation.

[0016] Beneficial effects of the present invention: The preparation method of the highly fluorinated electrolyte for high specific energy lithium metal battery of the present invention is simple, the raw materials are cheap and easy to obtain, and the solvation structure of lithium ions can be directly adjusted, while improving the ultra-high nickel positive electrode material LiNi 0.9 Co 0.05 Mn 0.05 O 2 The interface properties of the positive and negative electrodes of the (NCM90) battery, the highly fluorinated electrolyte for the lithium metal battery has good stability, and the electrochemical window is as high as 4.6V, which is much higher than the 4.3 V electrochemical window of the ordinary ester electrolyte. The lithium metal battery prepared by the highly fluorinated electrolyte for the high specific energy lithium metal battery of the present invention has high energy density, excellent cycle life and high rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0018] Figure 1 This is a comparison chart of the Coulombic efficiency certification performance test of lithium-copper half-cells.

[0019] Figure 2 To further verify the stability diagram of solid electrolyte interfaces in different dielectrics through lithium-lithium symmetric cells.

[0020] Figure 3 This is a graph of cyclic voltammetry test results when the voltage range is 0-6V.

[0021] Figure 4 This is the cyclic voltammetry test result diagram when the voltage range is 4-7V.

[0022] Figure 5 The top view and cross-sectional view of lithium deposition morphology of batteries with different electrolyte compositions; ad and ef are the lithium deposition morphology of batteries with different electrolyte compositions at 1 mA·cm -2 Current density and 1 mAh·cm -2 Top view and cross-sectional view of lithium deposition morphology at planar capacity.

[0023] Figure 6 Figure 2 is the carbon-carbon single bond (CC) / carbon-hydrogen single bond (CH) state diagram in the carbon (C 1s) spectra of the three electrolytes.

[0024] Figure 7State diagram of carbon-oxygen single bond (CO) and carbon-oxygen double bond (C=O) in the carbon (C 1s) spectra of the three electrolytes.

[0025] Figure 8 Figure 2 is the state diagram of carbon-fluorine single bond (CF) (organic F) in the carbon (C 1s) spectra of the three electrolytes.

[0026] Fig. 9 This is a transmission electron microscopy image of the comparative example 2 after 200 cycles in the electrolyte.

[0027] Fig.10 This is a transmission electron microscopy image of the comparative example 1 after 100 cycles in the electrolyte.

[0028] Fig.11 This is a transmission electron microscopy image of the electrolyte in Example 1 after 200 cycles.

[0029] Fig.12 This is a diagram of the electrochemical performance test of a lithium metal battery assembled using the electrolyte of Comparative Example 2.

[0030] Fig.13 This is a diagram of the electrochemical performance test of a lithium metal battery assembled using the electrolyte of Comparative Example 1.

[0031] Fig.14 This is a diagram of the electrochemical performance test of a lithium metal battery assembled using the electrolyte of Example 1.

[0032] Fig.15 Specific capacity diagram of batteries with different electrolyte groups under different temperature conditions.

[0033] Fig.16 This is a graph showing the electrochemical performance test of a lithium metal battery assembled using the electrolyte of Comparative Example 2 at high temperature (50°C) and a voltage range of 2.8-4.6V.

[0034] Fig.17 This is a graph showing the electrochemical performance test of a lithium metal battery assembled using the electrolyte of Comparative Example 1 at high temperature (50°C) and a voltage range of 2.8-4.6V.

[0035] Fig.18 This is a graph showing the electrochemical performance test of a lithium metal battery assembled using the electrolyte of Example 1 at high temperature (50°C) and a voltage range of 2.8-4.6V. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in combination with specific embodiments and comparative examples.

[0038] Example 1 In a glove box filled with argon gas, with a moisture content of less than 0.01 ppm and an oxygen content of less than 0.01 ppm, TFPC and FEMC were mixed in a mass ratio of 3:7 to form an electrolyte solvent system, and then lithium hexafluorophosphate (LiPF 6 ) is stirred until it is completely dissolved, and the concentration of lithium hexafluorophosphate in the electrolyte is 1 mol / L, thus obtaining a highly fluorinated electrolyte (TF).

[0039] Example 2 In a glove box filled with argon gas, with a moisture content of less than 0.01ppm and an oxygen content of less than 0.01ppm, FEC and MFDMC in a mass ratio of 4:6 were mixed to form an electrolyte solvent system, and then lithium bis(fluorosulfonyl)imide was added thereto and stirred until completely dissolved. The concentration of lithium bis(fluorosulfonyl)imide in the electrolyte was 1 mol / L, thus obtaining a highly fluorinated electrolyte (TF).

[0040] Example 3 In a glove box filled with argon gas, with a moisture content of less than 0.01ppm and an oxygen content of less than 0.01ppm, TFPC and FEMC in a mass ratio of 2:8 were mixed to form an electrolyte solvent system, and then lithium bis(trifluorosulfonyl)imide was added thereto and stirred until completely dissolved. The concentration of lithium trifluorosulfonyl imide in the electrolyte was 1 mol / L, thus obtaining a highly fluorinated electrolyte (TF).

[0041] Example 4 In a glove box filled with argon gas, with a moisture content of less than 0.01ppm and an oxygen content of less than 0.01ppm, TFPC and FEMC in a mass ratio of 4:6 are mixed to form an electrolyte solvent system, and then the electrolyte is added thereto and stirred until completely dissolved. The electrolyte is lithium dioxalatoborate and lithium difluorooxalatoborate, and the mixing mass ratio is 1:2:1. The concentration of the electrolyte in the electrolyte is 1 mol / L, and a highly fluorinated electrolyte (TF) is obtained.

[0042] Comparative Example 1 In a glove box filled with argon gas, with a moisture content of less than 0.01 ppm and an oxygen content of less than 0.01 ppm, TFPC and ethyl methyl carbonate (EMC) in a mass ratio of 3:7 are mixed to form an electrolyte solvent system, and then lithium hexafluorophosphate is added thereto and stirred until completely dissolved. The concentration of lithium hexafluorophosphate in the electrolyte is 1 mol / L, thereby obtaining a fluoride-rich electrolyte (TE).

[0043] Comparative Example 2 In a glove box filled with argon gas, with a moisture content of less than 0.01 ppm and an oxygen content of less than 0.01 ppm, propylene carbonate (PC) and EMC in a mass ratio of 3:7 are mixed to form an electrolyte solvent system, and then lithium hexafluorophosphate is added thereto and stirred until completely dissolved. The concentration of lithium hexafluorophosphate in the electrolyte is 1 mol / L, thereby obtaining a fluoride-free electrolyte (PE). Test example In the following test examples, TE is the fluorinated electrolyte prepared in Comparative Example 1, PE is the fluorinated electrolyte prepared in Comparative Example 2, and TF is the highly fluorinated electrolyte prepared in Example 1; 1. Battery Coulombic efficiency certification performance test: Coulombic efficiency performance is related to the solid electrolyte interface. The reduction of Coulombic efficiency indicates the cracking of the solid electrolyte interface, resulting in irreversible consumption of Li+. Li-Cu batteries were assembled for testing. The three electrolytes were tested at a current density of 0.5 mA·cm -2 , with a fixed capacity of 1 mAh·cm -2 The metal lithium deposition dissolution efficiency at .

[0044] like Figure 1 As shown, the battery using the PE electrolyte of Comparative Example 2 has a -2 The average Coulombic efficiency of the first 30 cycles at a current density of about 79% was then sharply reduced.

[0045] Compared with the PE group in comparative example 2, the lithium-copper battery using the TE electrolyte group has a higher coulombic efficiency (85%, 0.5 mA cm -2 ) and stably cycled for more than 60 weeks, after which the battery degradation rate accelerated. Although the battery performance has been greatly improved, it still cannot achieve the expected performance.

[0046] Compared with the previous two, the coulombic efficiency of the lithium-copper battery using the TF electrolyte group of Example 1 of the present invention is significantly improved, at 0.5 mA·cm -2 The coulombic efficiency reached an astonishing 95% at a current density of 1.5 % and was stably cycled for more than 150 cycles. It can be seen that with the increase in the degree of fluorination, the lithium metal anode forms a more stable solid electrolyte interface.

[0047] 2. Battery life test like Figure 2 The stability of the solid electrolyte interface in the non-conductive medium was further verified by a lithium-lithium symmetric battery. The results showed that the fixed capacity and current density were 0.5 mAh cm -2 and 1.0 mA·cm -2 . The batteries composed of electrolyte PE in comparative example 2, electrolyte TE in comparative example 1, and TF electrolyte in example 1 exhibited cycle lives of approximately 200 h, 1100 h, and 1390 h, respectively. As the cycle time increases, the overvoltage of the battery gradually increases. The battery composed of electrolyte PE began to show obvious voltage fluctuations after 160 h, which put the battery at risk of serious short circuit. In contrast, the battery composed of electrolyte TE also showed similar voltage fluctuations after 800 h and quickly failed within 1100 h. However, after all traditional carbonate-based solvents (such as PC and EMC) were replaced with fluorinated carbonate-based solvents (such as TFPC and FEMC), the battery composed of TF electrolyte exhibited excellent performance, with a cycle life of more than 1390 h. In addition, the excellent performance of highly fluorinated electrolytes has also been verified in the rate performance of lithium metal negative electrodes. At various current densities, the batteries of the PE electrolyte group and the TE electrolyte group showed significantly higher polarization potentials, and at a current density of 5 mA·cm -2 However, the battery using TF electrolyte group can basically meet the rate test and has a longer cycle life. This shows that highly fluorinated electrolyte helps to form a high-quality solid electrolyte interface, effectively inhibits the side reaction between lithium and electrolyte, reduces product accumulation and the formation of "dead lithium", and good cycle stability provides the battery with reliable performance and provides strong support for its long-term stable operation.

[0048] 3. Cyclic voltammetry test like Figure 3 and Figure 4 In order to further determine the actual effect of the degree of fluorination in the electrolyte system on the oxidation stability, cyclic voltammetry tests were performed on electrolytes with different degrees of fluorination. As can be seen from the figure, the PE electrolyte showed an obvious oxidation peak at 5.1 V, while the oxidation behavior of the TE and TF electrolytes was significantly suppressed, and the oxidation stability of the TF electrolyte was better than that of the TE electrolyte. This further shows that the combination of TFPC and FEMC is competent and compatible with high voltage.

[0049] 4. Analysis of positive electrode materials circulating in different electrolytes In PE electrolyte, after cycling, the surface of the copper foil is loose and covered with a large number of relatively slender lithium dendrites ( Figure 5-a). This indicates that serious side reactions occurred between lithium and the electrolyte, resulting in the formation of a large amount of "inactivated" lithium. In the TE electrolyte, the deposited lithium presents a blocky morphology, but there are also some irregular pits and a small amount of lithium dendrites ( Figure 5 -b). It is noteworthy that the solid electrolyte interface formed in TF electrolyte becomes more dense with uniform Li deposition ( Figure 5 -c), almost no lithium dendrites were observed, which helps to improve the coulombic efficiency of lithium metal batteries. Then, the cross-section of lithium deposited in batteries with different electrolyte compositions was observed using a solid electrolyte interface ( Figure 5 df), the results are shown in the top view ( Figure 5 ac), more dendrites are produced on the surface of the PE system, forming a thicker porous lithium layer. Although the TE system is improved compared with the PE system, it can be found from the cross-section that there is still a large range of pore structures in the middle of the deposited lithium layer surface, which may have a series of negative effects on the electrochemical performance. The main reasons are: on the one hand, the porous lithium layer with a high surface area is in direct contact with the electrolyte, causing electrolyte degradation and loss of active lithium, thereby reducing the Coulomb efficiency; on the other hand, the thicker solid electrolyte interface layer may also have an adverse effect on lithium ion migration, inducing electrical isolation and the generation of "inactivated" lithium, thereby weakening the Coulomb efficiency and the cycle life of metallic lithium. However, in the TF electrolyte, lithium ion deposition has good compactness characteristics, with almost no cracks and pores. According to the morphological changes in lithium layer deposition observed in PE, TE and TF electrolytes, it can be seen that the high-quality solid electrolyte interface layer comes directly from the fast Li + , thus leading to better Li cycling performance.

[0050] Transmission electron microscopy image of the NCM90 battery cathode after cycling. After 200 cycles in PE electrolyte, the NCM90 cathode shows a thick and rough solid electrolyte interface layer with a thickness of about 14.2 nm ( Fig. 9 After 100 cycles in TE electrolyte, the NCM90 cathode exhibited a thick but uniform solid electrolyte interface layer with a thickness of approximately 7.9 nm ( Fig.10 After 200 cycles in TF electrolyte, the NCM90 cathode exhibited a thin and uniform solid electrolyte interface layer with a thickness of about 5.2 nm ( Fig.11 ).

[0051] 5. Electrochemical testing In order to evaluate the stability of the ester-based electrolyte with BBAE (1,2-bis(bromoacetoxy)ethane) as an additive under high pressure, Li||NMC90 batteries were assembled for electrochemical testing. The lithium metal batteries assembled using the electrolytes of Example 1, Comparative Example 1 and Comparative Example 2 were subjected to electrochemical performance tests to measure the cycle and rate performance at room temperature. The results are as follows: Fig.12 , Fig.13 , Fig.14 and Fig.15 As shown ( Fig.12 , Fig.13 , Fig.14 and Fig.15 The arrows in the figure indicate that the solid-line box is an enlarged view of the dotted-line box); Fig.12 It can be seen that during the 100 cycles, the specific capacity of the PE electrolyte battery increased from 228 mAh g -1 Decayed to 154mAh·g -1 , the capacity retention rate is 67.5%; Fig.13 It can be seen that the battery capacity and cycle stability of the TE electrolyte group battery are improved compared with the PE group battery, which is consistent with the results of the previous analysis. Fig.14 It can be seen that compared with them, the electrochemical performance of the battery using the TF electrolyte of Example 1 is the best, with a specific capacity of 226.5 mAh·g -1 Decay to 186.4 mAh g -1 , the capacity retention rate is 82.3%. Fig.15 It can be seen that under the conditions of 0.2, 0.5, 1, 2, 5 and 0.2 C, the specific capacities of the PE electrolyte group batteries are 219.5, 186.1, 157.2, 136, 118.1 and 196.5 mAh·g, respectively. -1 The specific capacities of the batteries with TE electrolyte are 219.9, 208.7, 192.7, 178.3, 157.9 and 203.6 mAh·g, respectively. -1 , while the specific capacities of the TF group batteries are 223.8, 221.6, 211.8, 200.1, 180.8 and 223.2 mAh·g -1. It can be found that the rate performance of the battery using the TF electrolyte group is even better. As the current gradually increases, the capacity advantage of the battery in the highly fluorinated group is more obvious. Especially at 5 C, the specific capacity of the battery in the TF electrolyte group is nearly 1.53 times that of the battery in the TE electrolyte group. This may be because under high current, the negative / positive electrode solid electrolyte interface generated in the PE electrolyte is thicker and looser, and the ion conductivity is smaller, so the resistance is greater, making the battery polarization become very large and unable to charge and discharge normally. The TF electrolyte is conducive to the uniform deposition of lithium ions and generates a more stable, thinner, and more uniform high-quality negative / positive electrode solid electrolyte interface. The modified battery exhibits excellent rate performance.

[0052] Subsequently, in order to explore the effects of electrolytes with different fluorination degrees on the electrochemical performance of Li||NCM90 batteries at high temperatures, Li||NCM90 batteries were assembled using the three electrolytes of Example 1, Comparative Example 1 and Comparative Example 2. In this experiment, a current of 0.1 C was cycled five times at high temperature (50°C) and a voltage range of 2.8–4.6 V, and the battery cycle performance was studied at a charge and discharge rate of 1 C. The results are shown in Figure 2. Fig.16 , Fig.17 and Fig.18 As shown in Figure 2, during the 100 cycles, the specific capacity of the PE electrolyte group battery increased from 240.8 mAh·g -1 Decayed to 138.4 mAh g -1 The capacity retention rate was 57.5%, and the specific capacity of the battery was greatly attenuated after the current density increased from 0.1 C to 1 C, and the specific capacity was attenuated from 223.1 to 189.5 mAh g -1 ; The TE electrolyte group battery has improved both the battery specific capacity and cycle stability compared with the PE group; Compared with Comparative Examples 1 and 2, the TF electrolyte group battery has the best electrochemical performance, with the specific capacity decaying from 243 to 200.9 mAh·g -1 , the capacity retention rate is 82.6%, and after the current density is increased from 0.1 to 1 C, the specific capacity of the battery has almost no obvious attenuation. In general, fluorinated carbonate-based electrolytes are usually enhanced by introducing fluorine atoms into carbonate solvents. Fluorinated carbonate-based electrolytes have more advantages in high temperature performance than ordinary carbonate-based electrolytes, and can provide better thermal stability and service life.

[0053] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0054] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A highly fluorinated electrolyte for a high specific energy lithium metal battery, characterized in that: The electrolyte includes an organic solvent and an electrolyte, wherein the organic solvent includes a first organic solvent and a second organic solvent, the first organic solvent is FEC or TFPC, and the second organic solvent is MFDMC or FEMC.

2. The highly fluorinated electrolyte for high specific energy lithium metal battery according to claim 1, characterized in that: The volume of the first organic solvent accounts for 20-40% of the total volume of the organic solvent, and the volume of the second organic solvent accounts for 60-80% of the total volume of the organic solvent.

3. The highly fluorinated electrolyte for high specific energy lithium metal battery according to claim 1, characterized in that: The electrolyte is a lithium salt.

4. The highly fluorinated electrolyte for high specific energy lithium metal battery according to claim 3, characterized in that: The lithium salt is one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, lithium bis(oxalatoborate), lithium difluorooxalatoborate and lithium nitrate.

5. The highly fluorinated electrolyte for high specific energy lithium metal battery according to claim 4, characterized in that: When the lithium salt is lithium hexafluorophosphate, the concentration of lithium hexafluorophosphate in the electrolyte is 0.5-1.5 mol / L.

6. The highly fluorinated electrolyte for high specific energy lithium metal battery according to claim 1, characterized in that: The concentration of the lithium salt in the electrolyte is 0.1-5 mol / L.

7. The method for preparing the highly fluorinated electrolyte for high specific energy lithium metal battery according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: mixing and stirring an organic solvent in a sealed box filled with argon gas, and then adding a formulated amount of electrolyte therein to mix, so as to obtain the electrolyte.

8. The preparation method according to claim 7, characterized in that: The moisture content in the sealed box is less than 0.01ppm, and the oxygen content is less than 0.01ppm.

9. Use of the highly fluorinated electrolyte for high energy density lithium metal battery according to any one of claims 1 to 6 in lithium metal batteries.

10. A high specific energy lithium metal battery, characterized in that: The lithium metal battery comprises the highly fluorinated electrolyte for a lithium metal battery according to any one of claims 1 to 6.

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