CN113871698B

CN113871698B - Electrolyte and lithium battery containing same

Info

Publication number: CN113871698B
Application number: CN202111027264.2A
Authority: CN
Inventors: 白岩; 刘心同; 周世波; 郑军华
Original assignee: Svolt Energy Technology Co Ltd
Current assignee: Svolt Energy Technology Co Ltd
Priority date: 2021-09-02
Filing date: 2021-09-02
Publication date: 2023-05-26
Anticipated expiration: 2041-09-02
Also published as: CN113871698A

Abstract

The invention relates to the technical field of lithium ion batteries, and particularly provides an electrolyte and a lithium battery containing the electrolyte.

Description

Electrolyte and lithium battery containing same

Technical Field

The invention relates to the technical field of lithium ion batteries, in particular to an electrolyte and a lithium battery containing the same.

Background

Development of high voltage positive electrode materials is one of the important paths for developing high energy density batteries. However, the conventional electrolyte is easy to generate side reaction on the surface of the positive electrode, and the high-voltage positive electrode material is influenced. The high-voltage electrolyte is required to meet a wide electrochemical stability window, has high ionic conductivity, is nonflammable, has good compatibility with a negative electrode, and the like. At present, the main method for realizing the high-voltage of the high-voltage electrolyte has two ways: (1) improving the oxidation resistance of the solvent: through design, the HOMO value of solvent molecules is reduced by adopting substitution, grafting and other methods, and the electrochemical stability window of the solvent molecules is widened; (2) electrolyte additives are used: film-forming additive: the additive can be better than carbonic ester to lose electrons to oxidize, a protective film is formed on the surface of the positive electrode, dissolution of transition metal ions is inhibited, and the electrolyte is prevented from reacting with the electrode; adsorption type additive: the additive is complexed with metal ions on the surface of the electrode to ensure the charge balance on the surface of the electrode and passivate the catalytic active sites, so that the system is stable.

The development of high-voltage and high-capacity positive electrode materials leads the upper limit of charging voltage of the materials to exceed the limit of electrochemical stability of carbonic ester and organic electrolyte (more than 4.5V), and the carbonic ester solvent is easy to volatilize and burn, thereby reducing the service temperature range of the battery and bringing potential safety hazard to the battery.

The room temperature ionic liquid is a novel solvent which is formed by anions and cations and is in a liquid state at room temperature. Ionic liquids are of great interest due to their low vapor pressure, low melting point, high boiling point, high specific heat capacity, incombustibility, high ionic conductivity, high chemical stability, and the like. However, ionic liquids have high anion reduction potentials, and the corresponding cations may undergo reversible deintercalation between graphite layers to destroy the stability of the graphite structure; the defects of high viscosity, low conductivity and the like can lead to poor cycle stability and rate capability; the ionic liquid has higher price and restricts the commercial application thereof.

Therefore, how to improve the cycle stability of lithium batteries is a technical problem that needs to be solved by current research and development personnel.

Disclosure of Invention

Therefore, the technical problem to be solved by the invention is to overcome the defect of poor cycle stability of the lithium battery in the prior art, thereby providing an electrolyte and the lithium battery containing the electrolyte.

The invention provides an electrolyte comprising a fluorine-containing lithium salt and an additive composition comprising a dinitrile compound, a pentafluoro (phenoxy) cyclotriphosphazene (PFPN) and lithium bisoxalato borate (LiBOB).

Further, the fluorine-containing lithium salt is selected from lithium hexafluorophosphate (LiPF) ₆ ) And/or lithium tetrafluoroborate.

Further, the dinitrile compound is at least one selected from the group consisting of octadinitrile (SUN), succinonitrile, adiponitrile.

Further, the molar concentration of the lithium bisoxalato borate in the electrolyte is 1.0-1.5mol/L.

Further, the molar concentration of the fluorine-containing lithium salt in the electrolyte is 1.0-1.1mol/L.

Further, the volume ratio of the dinitrile compound to the pentafluoro (phenoxy) cyclotriphosphazene is 1:1-1:5.

Further, the electrolyte also comprises 5-20vt percent of ethylene carbonate and/or 5-20vt percent of propylene carbonate by volume percent.

The invention also provides a preparation method of the electrolyte, which comprises the following steps:

mixing fluorine-containing lithium salt, dinitrile compound, pentafluoro (phenoxy) cyclotriphosphazene and lithium bisoxalato borate to obtain the compound. Mixing is carried out according to conventional methods, for example stirring and mixing at room temperature, at a speed of 200-300rpm.

The invention also provides a lithium battery comprising the electrolyte or the electrolyte prepared by the preparation method.

Further, the positive electrode material of the lithium battery is at least one selected from a lithium nickel manganese oxide material, a lithium nickel oxide material, a lithium cobalt oxide material, a lithium nickel cobalt oxide material and a lithium nickel manganese cobalt oxide material.

Further, the membrane is characterized in that the membrane is at least one selected from a polyacrylonitrile membrane, a polyvinylidene fluoride membrane and an ethylene terephthalate membrane.

Generally, substituents with strong electron withdrawing property, such as fluorine substituent, sulfone functional group (-SO 2-), and nitrile functional group (-CN), are introduced into the molecule, SO that the electron cloud density of the substituted molecule can be effectively reduced, the dipole moment, anode stability and dielectric constant of the molecule are enhanced, and the valence electrons in the molecule are difficult to be taken away by the anode, SO that the oxidation resistance potential of the molecule is improved. Common fluorinated solvents are fluorinated carbonate solvents such as fluoroethylene carbonate (FEC) and trifluoropropylene carbonate (TFPC). The XPS technology finds that the high concentration C-F group in the CEI film in the fluorinated solvent system is beneficial to the stability of the positive electrode interface, and meanwhile, the interface impedance can be reduced by reducing the LiF content. A small amount of fluorinated solvent molecules are reduced and decomposed at the interface of the carbon negative electrode, so that the SEI film structure of the negative electrode can be optimized, and the compatibility of the electrolyte and the negative electrode material is improved. The fluoro-substituted solvent has a certain flame-retardant effect, and can effectively improve the thermal stability and safety of the electrolyte. However, fluorine has a strong electron withdrawing effect, decreasing DN (donor number) value of the solvent, thereby decreasing the dissolving ability of lithium salt. The sulfone solvent based on sulfone as functional group has electrochemical window up to 5.0-5.9V (vs Li+/Li). Most sulfones have the problems of high melting point, high viscosity, poor compatibility with graphite and the like, and the use of the sulfones as a high-voltage electrolyte solvent is limited. Proper substituent groups are required to be introduced to reduce the symmetry of the molecular structure, and the characteristics of the molecule are optimized; improving the alkyl functional group of sulfones can also improve the compatibility with graphite negative electrodes.

Cyano compounds generally have adsorption phenomena on the surface of transition metals, and the stability of different adsorption configurations is greatly different. A surface complex can be formed between the cyano functional group and the cobalt atom on the electrode surface, so that the thermal stability of the material can be significantly improved. The nitriles have a certain inhibition effect on the corrosion of the aluminum current collector under high potential. However, the nitrile solvent has poor compatibility with graphite, metallic lithium or the like at a low lithium removal potential, and is extremely easy to generate polymerization reaction on the surface of the negative electrode, and the raw polymer can organize Li+ to be removed and is limited to be used as a single solvent.

The use of electrolyte additives is an economical and efficient way to improve the high pressure stability of the electrolyte. Among the additives, the lithium bisoxalato borate does not contain fluorine or phosphorus and has higher stability; particularly, the SEI film can be directly formed on the surface of the graphite cathode, and even in pure PC electrolyte, PC can be effectively inhibited from being co-embedded in a graphite layer, so that graphite stripping is inhibited. The lithium bisoxalato borate can also participate in the formation of the positive electrode interface film, and can be oxidized and decomposed into boric acid or oxalic acid functional group compounds under high pressure to participate in the positive electrode interface film. However, lithium bisoxalato borate has a low solubility in carbonate solvents, and its low concentration and conductivity do not meet the requirements of industrialization and increasing energy density.

According to the invention, the research discovers that the performance of the electrolyte can be greatly improved by using the dinitrile compound, the pentafluoro (phenoxy) cyclotriphosphazene and the lithium bisoxalato borate in a matched mode.

The technical scheme of the invention has the following advantages:

1. the electrolyte provided by the invention has the advantages that the additive composition comprising the dinitrile compound, the pentafluoro (phenoxy) cyclotriphosphazene and the lithium bisoxalato borate is matched with fluorine-containing lithium salt for use, so that a synergistic effect is exerted, the side reaction of a battery can be effectively inhibited, the electrochemical polarization rate is reduced, an effective CEI interface film is formed, the stability of the material structure of the positive electrode material in the high-voltage circulation process is greatly improved, and the circulation performance of the battery is improved.

2. According to the electrolyte provided by the invention, the ionic conductivity of the electrolyte can be further increased and the rate performance of a battery can be improved by controlling the molar concentration of the lithium bisoxalato borate in the electrolyte to be 1.0-1.5mol/L.

3. According to the electrolyte provided by the invention, the dinitrile compound is at least one selected from the group consisting of octadinitrile, succinonitrile and adiponitrile, and is preferably matched with other additives, so that the ionic conductivity of the electrolyte can be further increased, and the rate performance of a battery can be improved.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are needed in the description of the embodiments or the prior art will be briefly described, and it is obvious that the drawings in the description below are some embodiments of the present invention, and other drawings can be obtained according to the drawings without inventive effort for a person skilled in the art.

FIG. 1 is a molecular structure diagram of octadinitrile and lithium bis (oxalato) borate;

FIG. 2 is a molecular structure diagram of a pentafluoro (phenoxy) cyclotriphosphazene;

FIG. 3 is a graph showing the change in conductivity with temperature of the electrolyte of example 1 of the present invention;

fig. 4 is a first charge-discharge curve of the positive electrode material in experimental example 1 of the present invention in the electrolyte of example 1;

FIG. 5 is a cycle curve of the positive electrode material in experimental example 1 of the present invention in the electrolyte of example 1;

FIG. 6 is a graph showing the charge and discharge curves of the positive electrode material of experimental example 1 according to the present invention after various cycles of the electrolyte of example 1;

FIG. 7 is a charge-discharge curve of 4.5-3.9V in experimental example 2 of the present invention;

FIG. 8 is a charge-discharge curve of 4.5-3.9V in experimental example 3 of the present invention;

FIG. 9 is the median voltage after various cycles in Experimental example 4 of the present invention;

FIG. 10 is an SEM image of the lithium ion battery of example 4 of experimental example 5 of the present invention after 500 cycles of charge-discharge experimental cycles at a current density of 140 mAh/g;

FIG. 11 is an SEM image of comparative example 1 of experimental example 5 of the present invention after 500 cycles of charge and discharge test at a current density of 140 mAh/g;

FIG. 12 is an SEM image of comparative example 3 of experimental example 5 of the present invention after 500 cycles of charge and discharge test at a current density of 140 mAh/g;

FIG. 13 is a TEM image of the lithium ion battery of example 4 of experimental example 5 of the present invention after 500 cycles of charge-discharge experimental cycle at a current density of 140 mAh/g;

FIG. 14 is a TEM image of the lithium ion battery of comparative example 1 of experimental example 5 of the present invention after 500 cycles of charge-discharge experimental cycle at a current density of 140 mAh/g;

FIG. 15 is a TEM image of the lithium ion battery of comparative example 3 in Experimental example 5 of the present invention after 500 cycles of charge-discharge experimental cycle at a current density of 140 mAh/g;

fig. 16 is a charge-discharge curve of the lithium battery of example 4 in experimental example 6 of the present invention;

fig. 17 is a cycle curve of the lithium battery of example 4 in experimental example 6 of the present invention at 0.5C rate.

Detailed Description

The following examples are provided for a better understanding of the present invention and are not limited to the preferred embodiments described herein, but are not intended to limit the scope of the invention, any product which is the same or similar to the present invention, whether in light of the present teachings or in combination with other prior art features, falls within the scope of the present invention.

The specific experimental procedures or conditions are not noted in the examples and may be followed by the operations or conditions of conventional experimental procedures described in the literature in this field. The reagents or apparatus used were conventional reagent products commercially available without the manufacturer's knowledge.

Example 1

This example provides an electrolyte comprising 1.0mol/L lithium hexafluorophosphate, 1.0mol/L lithium bisoxalato borate, 25vt% octadinitrile and 75vt% pentafluoro (phenoxy) cyclotriphosphazene.

The preparation method of the electrolyte comprises the following steps: 500mL of octanedionitrile (4A molecular sieve for water removal) and 1500mL of pentafluoro (phenoxy) cyclotriphosphazene are mixed to obtain a mixed solution 1, 2mol of lithium bisoxalato borate is dissolved in the mixed solution 1 to obtain a mixed solution 2, and 2mol of lithium hexafluorophosphate is dissolved in the mixed solution 2 to prepare an electrolyte.

The conductivity of the electrolyte at different temperatures was measured and the results are shown in fig. 3, with the conductivity being increased as the temperature was increased.

Example 2

This example provides an electrolyte comprising 1mol/L lithium hexafluorophosphate, 1.0mol/L lithium bisoxalato borate, 25vt% succinonitrile and 75vt% pentafluoro (phenoxy) cyclotriphosphazene.

The preparation method of the electrolyte comprises the following steps: 500mL of succinonitrile (4A molecular sieve for water removal) and 1500mL of pentafluoro (phenoxy) cyclotriphosphazene are mixed to obtain a mixed solution 1, 2mol of lithium bisoxalato borate is dissolved in the mixed solution 1 to obtain a mixed solution 2, and 2mol of lithium hexafluorophosphate is dissolved in the mixed solution 2 to prepare an electrolyte.

Example 3

This example provides an electrolyte comprising 1.0mol/L lithium tetrafluoroborate, 1.5mol/L lithium bisoxalato borate, 45vt% octanedionitrile, 45vt% pentafluoro (phenoxy) cyclotriphosphazene and 10vt% ethylene carbonate.

The preparation method of the electrolyte comprises the following steps: 900mL of octanedionitrile (4A molecular sieve for water removal), 900mL of pentafluoro (phenoxy) cyclotriphosphazene and 200mL of ethylene carbonate are mixed to obtain a mixed solution 1, 3mol of lithium bisoxalato borate is dissolved in the mixed solution 1 to obtain a mixed solution 2, and 2mol of lithium tetrafluoroborate is dissolved in the mixed solution 2 to prepare an electrolyte.

Example 4

The present example provides a battery, in which positive electrodes were prepared by mixing a lithium cobalt oxide positive electrode material (commercially available from Xiamen tungsten, model: CC-01, nominal voltage: 4.2-4.5V), carbon black, and NMP solution having a solid content of 20% PVDF in a mass ratio of 1:1:2, coating, baking, rolling, and cutting. Then, respectively assembling button half batteries, wherein the negative electrode is lithium metal, the diaphragm is PE diaphragm, and the electrolyte adopts the electrolyte in the embodiment 1 to obtain the lithium ion battery.

Example 5 and example 6

Examples 5 and 6 each provided a battery having a raw material composition substantially the same as that of example 4 except that the electrolyte was used in the battery of example 5 and the electrolyte of example 2 was used in the battery of example 6 and the electrolyte of example 3 was used.

Comparative example 1

This example provides an electrolyte containing 1.0mol/L of lithium hexafluorophosphate in ethylene carbonate.

The preparation method of the electrolyte comprises the following steps: 1mol of lithium hexafluorophosphate was dissolved in 1L of ethylene carbonate to prepare an electrolyte. A battery was produced in the same manner as in example 4 using the electrolyte of this comparative example.

Comparative example 2

This example provides an electrolyte containing 1.0mol/L lithium hexafluorophosphate, 45vt% octadinitrile and 55vt% ethylene carbonate.

The preparation method of the electrolyte comprises the following steps: 450mL of octadinitrile (4A molecular sieve water) and 550mL of ethylene carbonate were mixed to obtain a mixed solution 1, and 1mol of lithium hexafluorophosphate was dissolved in the mixed solution 1 to prepare an electrolyte. A battery was produced in the same manner as in example 4 using the electrolyte of this comparative example.

Comparative example 3

This example provides an electrolyte solution of ethylene carbonate containing 1.0mol/L lithium hexafluorophosphate and 1.5mol/L lithium bisoxalato borate.

The preparation method of the electrolyte comprises the following steps: 1.5mol of lithium bisoxalato borate and 1mol of lithium hexafluorophosphate were dissolved in 1L of ethylene carbonate to prepare an electrolyte. A battery was produced in the same manner as in example 4 using the electrolyte of this comparative example.

Comparative example 4

This example provides an electrolyte of 45vt% pentafluoro (phenoxy) cyclotriphosphazene and 55vt% ethylene carbonate containing 1.0mol/L lithium hexafluorophosphate.

The preparation method of the electrolyte comprises the following steps: 450mL of pentafluoro (phenoxy) cyclotriphosphazene and 550mL of ethylene carbonate were mixed to obtain a mixed solution 1, and 1mol of lithium hexafluorophosphate was dissolved in 1000mL of the mixed solution 1 to prepare an electrolyte. A battery was produced in the same manner as in example 4 using the electrolyte of this comparative example.

Experimental example 1

The lithium ion batteries of examples 4 to 6 and comparative examples 1 to 4 were subjected to charge and discharge experiments at a current density of 14mAh/g at normal temperature, and were tested for the first charge and discharge capacity, coulombic efficiency and 500-cycle charge and discharge retention rate. The results are shown in Table 1.

Table 1 charge and discharge test results table

As can be seen from fig. 4-6 and the above table, the lithium cobaltate material increases with the cycle and the discharge curve changes in the low voltage direction, which indicates that the operating voltage gradually declines as the lithium cobaltate material circulates and the battery energy density decreases. When lithium bisoxalato borate, octanedonitrile and pentafluoro (phenoxy) cyclotriphosphazene are used as combined additives, the material has smaller polarization rate and optimal electrochemical stability. Therefore, the composition additive of the lithium bisoxalato borate, the octanedonitrile and the pentafluoro (phenoxy) cyclotriphosphazene is beneficial to the maintenance of a voltage platform of the lithium cobaltate material, so that the cycle stability is greatly improved, the addition of the composition additive can be reflected, and the stability of the material structure of the lithium cobaltate material in the high-voltage cycle process can be improved.

Experimental example 2

Mixing high-potential lithium cobalt oxide anode material (commercially available from Xiamen tungsten industry, model: CB-06), carbon black and NMP solution with 20% PVDF solid content according to the mass ratio of 1:1:2, coating, baking, rolling, and cutting to obtain the positive electrode. Then, button half batteries were assembled respectively, the negative electrode was lithium metal, the separator was a PE separator, and the electrolyte of example 1 was used to obtain a lithium cobaltate/Li button battery (nominal voltage of 4.5V). And charging and discharging the lithium cobaltate/Li button cell at a current density of 14mAh/g for two weeks in a voltage interval of 3.9-4.5V, charging to a current density of 4.5V at 140mAh/g, charging for 4h at 4.5V, and taking out the positive electrode material in a glove box after the charging is finished.

The charge-discharge curve is shown in fig. 7, and it can be seen from fig. 7 that the charge-discharge is good in the voltage interval of 3.9-4.5V, which means that the cycle stability is good.

Experimental example 3

The positive electrode is prepared by mixing a lithium cobaltate positive electrode material (purchased from Xiamen tungsten industry, model: CD-05), carbon black and NMP solution with 20% PVDF solid content according to the mass ratio of 1:1:2, coating, baking, rolling and cutting. Then, button half batteries were assembled respectively, the negative electrode was lithium metal, the separator was a PE separator, and the electrolyte of example 1 was used to obtain a lithium cobaltate/Li button battery (nominal voltage: 3.9V). After the lithium cobaltate/Li button cell is charged and discharged for two circles at the current density of 14mAh/g, the 500 th circle is charged and discharged at the current density of 140mAh/g, the discharging is stopped after the discharging reaches 3.9V, and the anode material is taken out.

The charge-discharge curve is shown in fig. 8, and it can be seen from fig. 8 that the charge-discharge is good in the voltage interval of 3.9-4.5V, indicating that the cycle stability is good.

Experimental example 4

The positive electrode is prepared by mixing high-potential lithium cobalt oxide positive electrode material (purchased from Xiamen tungsten industry, model: SC-02), carbon black and NMP solution with 20% PVDF solid content according to the mass ratio of 1:1:2, coating, baking, rolling and cutting. Then, button half batteries were assembled respectively, the negative electrode was lithium metal, the separator was a PE separator, and the electrolyte of example 1 was used to obtain a lithium cobaltate/Li button battery (nominal voltage of 4.5V). And charging and discharging the lithium cobaltate/Li button cell at a current density of 14mAh/g for two weeks in a voltage interval of 3.0-4.5V, charging to a current density of 4.5V at 140mAh/g, charging for 4h at 4.5V, and taking out the positive electrode material in a glove box after the charging is finished.

As can be seen from fig. 9, the median voltage was 3.8V or higher, which indicates good cycle stability.

Experimental example 5

Mixing high-potential lithium cobalt oxide anode material (commercially available from Xiamen tungsten industry, model: CB-06), carbon black and NMP solution with 20% PVDF solid content according to the mass ratio of 1:1:2, coating, baking, rolling, and cutting to obtain the positive electrode. Then, button half batteries were assembled respectively, the negative electrode was lithium metal, the separator was a PE separator, and the electrolytes of example 1 and comparative examples 1 and 3 were used respectively to obtain three sets of lithium cobaltate/Li button batteries (nominal voltage of 4.5V). Three groups of lithium cobaltate/Li button cells are respectively charged and discharged for two weeks at a current density of 14mAh/g and a voltage interval of 3.9-4.5V, then are charged to 4.5V at a current density of 140mAh/g, and are charged for 4h at 4.5V, and after 500 circles of charge and discharge experiment, the positive electrode material is taken for scanning electron microscope shooting and transmission electron microscope shooting, as shown in figures 10-15.

In the reference electrolysis, since the electrolyte is continuously oxidized and decomposed, the electrode surface is covered with a uniform and thick layer of decomposition product, and the electrode surface has agglomerated particles. In a lithium bisoxalato borate single additive system, a uniform and compact surface film is formed on the surface of the electrode, and particularly in a combined solvent system of lithium bisoxalato borate, octadinitrile and pentafluoro (phenoxy) cyclotriphosphazene, the surface film almost covers the whole surface of the electrode. It can be seen that the composition of lithium bisoxalato borate, octanedionitrile and pentafluoro (phenoxy) cyclotriphosphazene is more favorable for forming a uniform and compact surface film, thereby effectively inhibiting the oxidative decomposition of the electrolyte. Further contributes to the stability of the material structure and the improvement of the circulation stability.

The positive electrode which is not stained with the electrolyte is a smooth and clean interface, and a layer of interface film covers after the conventional electrolyte is circulated, and the circulated interface film is more compact and reliable in the mixed additive of lithium bisoxalato borate, octadinitrile and pentafluoro (phenoxy) cyclotriphosphazene.

Experimental example 6

High-voltage Lithium Nickel Manganese Oxide (LNMO) (purchased from Homehundred technologies, model XC-86), a conductive agent and a binder (PVDF) are mixed according to a mass ratio of 8:1:1, adding NMP 2 times the mass of the mixture, stirring for 8 hours, coating on aluminum foil, drying at 80 ℃ for 12 hours in a vacuum drying oven, preparing a positive electrode, taking a lithium sheet as a negative electrode, taking Celgard K2045 (PE) as a diaphragm, and assembling a battery (nominal voltage 4.7V) by adopting the electrolyte prepared in the example 1. After the battery is assembled, the battery is firstly kept stand for 12 hours to fully infiltrate electrolyte, and then the electrochemical performance of the battery is tested. The constant current discharge experiment of the battery is carried out on a test cabinet. The voltage range is 3.5-5.0V (vs Li+/Li). The cyclic voltammogram of the cell was performed on an electrochemical workstation with a scan voltage in the range of 3.5-5.0V and a scan speed of 0.1mv/s. EIS testing was also performed on an electrochemical workstation with a frequency range of 0.01HZ to 100KHZ.

As shown in FIG. 16, the gram capacity of the material reaches 140mAh/g under the action of the high-voltage electrolyte, the voltage platform is stable, and the capacity is excellent. At a rate of 0.5C, the capacity remained at 136mAh/g after 100 weeks of cycling.

It is apparent that the above examples are given by way of illustration only and are not limiting of the embodiments. Other variations or modifications of the above teachings will be apparent to those of ordinary skill in the art. It is not necessary here nor is it exhaustive of all embodiments. While still being apparent from variations or modifications that may be made by those skilled in the art are within the scope of the invention.

Claims

1. An electrolyte comprising 1.0mol/L lithium tetrafluoroborate, 1.5mol/L lithium bisoxalato borate, 45vt% octadinitrile, 45vt% pentafluoro (phenoxy) cyclotriphosphazene and 10vt% ethylene carbonate.

2. A method for preparing the electrolyte according to claim 1, which is characterized by comprising the step of mixing lithium tetrafluoroborate, octanedionitrile, ethylene carbonate, pentafluoro (phenoxy) cyclotriphosphazene and lithium bisoxalato borate.

3. A lithium battery comprising the electrolyte of claim 1 or the electrolyte prepared by the preparation method of claim 2.

4. The lithium battery according to claim 3, wherein the positive electrode material of the lithium battery is at least one selected from the group consisting of a lithium nickel manganese oxide material, a lithium nickel oxide material, a lithium cobalt oxide material, a lithium nickel cobalt oxide material, and a lithium nickel manganese cobalt oxide material.

5. The lithium battery according to claim 3 or 4, wherein the separator of the lithium battery is at least one selected from the group consisting of a polyacrylonitrile separator, a polyvinylidene fluoride separator, and an ethylene terephthalate separator.