Electrolyte of lithium battery, preparation method of electrolyte and lithium battery

Abstract

The invention provides an electrolyte of a lithium battery, a preparation method of the electrolyte and the lithium battery. The electrolyte of the lithium battery comprises a solvent, an electrolyte additive and a lithium salt dissolved in the solvent, the electrolyte additive comprises lithium perfluorooctanoate, and the lithium perfluorooctanoate comprises perfluorooctanoic acid radicals with oxidation sites and reduction sites, so that the lithium perfluorooctanoate has relatively high reaction activity; the surfaces of the positive electrode and the negative electrode of the lithium battery can be preferentially oxidized or reduced and decomposed to form CEI and SEI rich in lithium fluoride, so that the structures of the positive electrode and the negative electrode of the lithium battery can be stabilized at high voltage.

Description

Technical Field

[0001] This application relates to the technical field of lithium batteries. Specifically, this application relates to an electrolyte for a lithium battery, a preparation method thereof, and a lithium battery. Background Art

[0002] Lithium cobalt oxide (LiCoO2, LCO) is the earliest cathode material applied to commercial lithium batteries, and lithium cobalt oxide batteries are widely used in the field of consumer electronics. In recent years, the demand for the energy density of lithium batteries in high-end portable electronic products has been increasing; currently, the charging cut-off voltage of lithium batteries is usually less than 4.5V (volts). Therefore, in order to improve the energy density of lithium batteries, how to increase the charging cut-off voltage of lithium batteries has become the focus of research. Summary of the Invention

[0003] In view of the shortcomings of the existing methods, this application proposes an electrolyte for a lithium battery, a preparation method thereof, and a lithium battery, so as to solve the technical problem of the relatively low charging cut-off voltage of lithium batteries in the related art.

[0004] In a first aspect, an embodiment of this application provides an electrolyte for a lithium battery, including: a solvent, a lithium salt, and an electrolyte additive, and the electrolyte additive includes lithium perfluorooctanoate.

[0005] Optionally, the lithium perfluorooctanoate includes a perfluorooctanoate group having an oxidation site and a reduction site; The perfluorooctanoate group is prepared by fluorination and hydroxylation of octane.

[0006] Optionally, the lithium salt includes at least one of LiPF6, LiNO3, LiN(SO2CF3)2, LiCF3SO3, LiN(FSO2)3, and LiClO4.

[0007] Optionally, the solvent includes at least one of a fluorinated carbonate solvent, a fluorinated carboxylate solvent, a fluorinated ether solvent, a carbonate solvent, a carboxylate solvent, and an ether solvent; The carbonate includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; The carboxylate includes at least one of ethyl acetate, methyl acetate, and methyl propionate; The ether includes at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,4-dioxolane.

[0008] Optionally, the concentration of the lithium perfluorooctanoate includes: 0.001 - 3 mol / L; The concentration of the lithium salt includes: 0.001 - 3 mol / L.

[0009] In a second aspect, an embodiment of the present application provides a method for preparing an electrolyte of a lithium battery according to any one of the above first aspects, including: Under the protection of an inert atmosphere, a lithium salt is dissolved in an organic solvent, and an electrolyte additive including lithium perfluorooctanoate is added, and stirred to obtain a uniformly mixed electrolyte.

[0010] Optionally, before dissolving the lithium salt in the organic solvent and stirring, it further includes: Octane is fluorinated and hydroxylated to form a perfluorooctanoate radical with a reduction site and an oxidation site.

[0011] In a third aspect, an embodiment of the present application provides a lithium battery, including: a positive electrode, a negative electrode, and the electrolyte according to any one of the above first aspects; The lithium perfluorooctanoate in the electrolyte is used to form a positive electrode electrolyte film containing LiF on the surface of the positive electrode; the lithium perfluorooctanoate in the electrolyte is used to form a solid electrolyte film containing LiF on the surface of the negative electrode.

[0012] Optionally, the material of the positive electrode includes at least one of lithium cobaltate, lithium nickel cobalt manganate, and lithium-rich manganese; The material of the negative electrode includes at least one of metallic lithium, silicon carbon, and graphite.

[0013] Optionally, the material of the positive electrode is the same as the material of the negative electrode; The materials of the positive electrode and the negative electrode include at least one of metallic lithium and lithium-rich manganese.

[0014] The beneficial technical effects brought by the technical solution provided by the embodiment of the present application include: The electrolyte of the lithium battery provided by the embodiment of the present application includes: a solvent, an electrolyte additive, and a lithium salt dissolved in the solvent. The electrolyte additive includes lithium perfluorooctanoate. The lithium perfluorooctanoate includes a perfluorooctanoate group having an oxidation site and a reduction site, so that the lithium perfluorooctanoate has a high reactivity and can preferentially oxidize or reductively decompose on the surfaces of the positive and negative electrodes of the lithium battery to form a cathode electrolyte interface (CEI) rich in lithium fluoride and a solid electrolyte interface (SEI). During the process of generating the CEI and SEI rich in lithium fluoride, basically no other organic components are generated, so that the CEI and SEI have high electrochemical stability and modulus, can maintain structural integrity and avoid electrolyte penetration, thereby can stabilize the structures of the positive and negative electrodes of the lithium battery, and make the SEI have a high lithium deposition Coulombic efficiency, so that the positive and negative electrodes of the lithium battery protected by the CEI and SEI rich in lithium fluoride can withstand a high charging cut-off voltage, such as 4.8 V (volts), can improve the energy density of the lithium battery; and make the lithium battery have high cycle stability.

[0015] For example, by adding lithium perfluorooctanoate to the electrolyte, the discharge capacity of the lithium cobalt oxide battery can be increased from 185 mAh / g (milliamperes per gram) at 4.5 V to 244 mAh / g at 4.8 V, so that the energy density of the lithium cobalt oxide battery under the same volume is increased by 36%, greatly improving the battery life of the electronic device.

[0016] Additional aspects and advantages of the present application will be given in part in the following description, and these will become obvious from the following description, or can be understood through the practice of the present application. Brief Description of the Drawings

[0017] The above-mentioned and / or additional aspects and advantages of the present application will become obvious and easy to understand from the following description of the embodiments in conjunction with the drawings, where: Figure 1 It is a schematic diagram of the chemical formula of octane fluorination and carboxylation in the electrolyte of a lithium battery provided by an embodiment of the present application; Figure 2a It is a schematic diagram of the 0.2C discharge specific capacity of a lithium metal-lithium cobalt oxide battery at different charging cut-off voltages provided by an embodiment of the present application; Figure 2b It is a schematic diagram of the energy density corresponding to a lithium metal-lithium cobalt oxide battery at different charging cut-off voltages provided by an embodiment of the present application; Figures 3a - 3b It is a schematic diagram of the charge-discharge curves of a lithium metal-lithium cobalt oxide battery electrolyte containing different contents of lithium perfluorooctanoate provided by an embodiment of the present application; Figure 4aSchematic diagram of the charge-discharge curve when the electrolyte of the lithium metal-lithium cobalt oxide battery in the embodiment of the present application contains 0.5 wt% of lithium perfluorooctanoate at a charging voltage of 4.8 V; Figure 4b Schematic diagram of the charge-discharge curve when the electrolyte of the lithium metal-lithium cobalt oxide battery in the related art does not contain lithium perfluorooctanoate at a charging voltage of 4.8 V; Figure 5 Schematic diagram of the cycle performance of the lithium-lithium symmetric battery. Detailed implementation manners The embodiments of the present application will be described below with reference to the accompanying drawings in the present application. It should be understood that the implementation manners described below with reference to the accompanying drawings are exemplary descriptions for explaining the technical solutions of the embodiments of the present application, and do not constitute limitations on the technical solutions of the embodiments of the present application.

[0018] Those skilled in the art of the present technology can understand that, unless specifically stated otherwise, the "the" and "this" used here may also include plural forms. It should be further understood that the term "including" used in the specification of the present application means the presence of the described features, integers, steps, operations, elements, and / or components, but does not exclude the implementation of other features, information, data, steps, operations, elements, components, and / or combinations thereof supported by the art of the present technology. The term "and / or" used here means at least one of the items defined by the term, for example, "A and / or B" can be implemented as "A", or implemented as "B", or implemented as "A and B".

[0019] To make the objectives, technical solutions, and advantages of the present application clearer, the embodiments of the present application will be further described in detail below with reference to the accompanying drawings.

[0020] The technical solutions of the present application and how the technical solutions of the present application solve the above technical problems will be described in detail below with specific embodiments. It should be noted that the following implementation manners can refer to, draw on, or combine with each other. For the same terms, similar features, and similar implementation steps in different implementation manners, they will not be described repeatedly.

[0021] The embodiment of the present application provides an electrolyte for a lithium battery, and the electrolyte includes: a solvent, a lithium salt, and an electrolyte additive, and the electrolyte additive includes lithium perfluorooctanoate.

[0022] The electrolyte of the lithium battery provided by the embodiment of the present application includes: a solvent, an electrolyte additive, and a lithium salt dissolved in the solvent. The electrolyte additive includes lithium perfluorooctanoate. The lithium perfluorooctanoate includes a perfluorooctanoate radical having an oxidation site and a reduction site, so that the lithium perfluorooctanoate has a high reactivity and can preferentially oxidize or reductively decompose on the surfaces of the positive and negative electrodes of the lithium battery to form a cathode electrolyte interface (CEI) rich in lithium fluoride and a solid electrolyte interface (SEI). During the process of generating the CEI and SEI rich in lithium fluoride, basically no other organic components are generated, so that the CEI and SEI have high electrochemical stability and modulus, can maintain structural integrity and avoid electrolyte penetration, thereby can stabilize the structures of the positive and negative electrodes of the lithium battery, and make the SEI have a high lithium deposition Coulomb efficiency, so that the positive and negative electrodes of the lithium battery protected by the CEI and SEI rich in lithium fluoride can withstand a high charging cut-off voltage, such as 4.8V (volts), and can improve the energy density of the lithium battery; and make the lithium battery have high cycle stability.

[0023] Optionally, in an embodiment of the present application, as Figure 1 shown, the lithium perfluorooctanoate includes a perfluorooctanoate radical having an oxidation site and a reduction site.

[0024] The perfluorooctanoate radical is prepared by fluorination and hydroxylation of octane.

[0025] In the embodiment of the present application, the chemical formula of lithium perfluorooctanoate (LiPFOA) is C8F 15 CO2Li. The precursor of lithium perfluorooctanoate is octane, and the chemical formula of octane is C8H 18 , fluorinating octane to generate perfluorooctane. The molecular structure of perfluorooctane exhibits an electron-deficient characteristic and is prone to gaining electrons and being reduced. The chemical formula of perfluorooctane is C8F 18 . After carboxylating octane, an electron-rich carboxyl group can be introduced to generate an octanoate radical, making the octanoate radical easier to be reduced than octane. The chemical formula of the octanoate radical is C7H 15 COO - . Carboxylating and fluorinating octane simultaneously generates a perfluorooctanoate radical, and the chemical formula of the perfluorooctanoate radical is C7F 15 COO -, since perfluorooctanoate has both oxidation sites and reduction sites, the electrolyte containing lithium perfluorooctanoate is more likely to be reduced on the surface of the negative electrode of the lithium battery to form a SEI rich in LiF, and oxidized on the surface of the positive electrode to form a CEI rich in LiF. LiF has high electrochemical stability and high modulus. LiF can maintain structural integrity while avoiding electrolyte penetration, enabling the positive electrode electrolyte film on the surface of the positive electrode to ensure the stability of the positive electrode when the charging cut-off voltage is greater than 4.5V. At the same time, the stable solid electrolyte interface formed on the surface of the negative electrode can improve the Coulombic efficiency of lithium deposition and cycle stability, thereby improving the performance of the battery.

[0026] Optionally, in an embodiment of the present application, the lithium salt includes at least one of LiPF6, LiNO3, LiN(SO2CF3)2, LiCF3SO3, LiN(FSO2)3, and LiClO4.

[0027] Optionally, in an embodiment of the present application, the solvent includes at least one of a fluorinated carbonate solvent, a fluorinated carboxylate solvent, a fluorinated ether solvent, a carbonate solvent, a carboxylate solvent, and an ether solvent.

[0028] The carbonate includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0029] The carboxylate includes at least one of ethyl acetate, methyl acetate, and methyl propionate.

[0030] The ether includes at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,4-dioxolane.

[0031] In the embodiment of the present application, the preferred solvent is one or a mixture of a fluorinated carbonate solvent, a fluorinated carboxylate solvent, a fluorinated ether solvent, a carbonate solvent, a carboxylate solvent, and an ether solvent.

[0032] Optionally, in an embodiment of the present application, the concentration of lithium perfluorooctanoate includes 0.001 - 3 mol / L (moles per liter); the concentration of the lithium salt includes 0.001 - 3 mol / L.

[0033] In the embodiment of the present application, the preferred concentration of the lithium salt includes 1 mol / L. The preferred content of lithium perfluorooctanoate in the electrolyte additive includes 0.5 wt% (weight percentage).

[0034] Based on the same inventive concept, the embodiment of the present application provides a method for preparing an electrolyte of a lithium battery, including: Under the protection of an inert atmosphere, a lithium salt is dissolved in an organic solvent, and an electrolyte additive including lithium perfluorooctanoate is added, and the mixture is stirred to obtain a uniformly mixed electrolyte.

[0035] Optionally, in one embodiment of the present application, before dissolving the lithium salt in the organic solvent and stirring, it further includes: Fluorinating and hydroxylation octane to form perfluorooctanoate with a reduction site and an oxidation site.

[0036] The electrolyte provided by the embodiment of the present application includes an electrolyte additive containing lithium perfluorooctanoate. Lithium perfluorooctanoate has both an oxidation site and a reduction site, so that a positive electrode electrolyte film can be preferentially formed on the surface of the positive electrode of the lithium battery and a solid electrolyte interface can be formed on the surface of the negative electrode, thereby stabilizing the structures of the positive and negative electrodes. Moreover, the electrolyte can dynamically generate a protective film on the surfaces of the positive and negative electrodes to protect the stability of the structures of the positive and negative electrodes.

[0037] Based on the same inventive concept, the embodiment of the present application provides a lithium battery, including: a positive electrode, a negative electrode, and the electrolyte of any one of the above embodiments.

[0038] Lithium perfluorooctanoate in the electrolyte is used to form a positive electrode electrolyte film containing LiF on the surface of the positive electrode; lithium perfluorooctanoate in the electrolyte is used to form a solid electrolyte film containing LiF on the surface of the negative electrode.

[0039] In the embodiment of the present application, a preferred electrolyte formulation includes: dissolving lithium hexafluorophosphate in fluoroethylene carbonate and ethyl methyl carbonate to prepare a base electrolyte with a lithium salt concentration of 1 M (mol / L, mole per liter), wherein the volume ratio of fluoroethylene carbonate (FEC) to ethyl methyl carbonate (EMC) in the base electrolyte is 3:7, and adding an electrolyte additive to the base electrolyte, and stirring to obtain a uniformly mixed electrolyte. The electrolyte additive contains 0.5 wt% (mass percentage) of lithium perfluorooctanoate. In a specific embodiment of the present application, the electrolyte is prepared according to the preferred electrolyte formulation of the present application, using a lithium cobaltate electrode with a loading of 10 mg cm -2 as the positive electrode and a 450 mm lithium foil as the negative electrode, assembling into a CR2025 button cell for performance evaluation. The charge cut-off voltage is set to 4.5V, 4.6V, 4.7V, 4.8V respectively, and the discharge cut-off voltage is 3.0V. Discharge is carried out at 0.2C (referring to the charge and discharge rate of the battery), and the obtained results are as Figures 2a - 2bAs shown, when the charging voltage of the lithium metal-lithium cobalt oxide battery is increased from 4.5 V to 4.7 V and 4.8 V, the discharge specific capacity at 0.2 C increases from 185 mAh / g to 230 mAh / g and 244 mAh / g respectively, and the energy density increases by 36%. It can be seen that increasing the charging voltage is beneficial to improving the battery energy density, thereby increasing the battery life of electronic devices. Since the electrolyte in the embodiment of the present application contains an electrolyte additive of lithium perfluorooctanoate, lithium perfluorooctanoate can preferentially form a cathode electrolyte film rich in lithium fluoride on the surface of the cathode of the lithium metal-lithium cobalt oxide battery, and basically no other organic components are generated during the formation of the cathode electrolyte film rich in lithium fluoride, so that the cathode electrolyte film has high electrochemical stability and modulus, can maintain structural integrity and avoid electrolyte penetration, and at the same time can inhibit the parasitic reaction between O n-2 and Co 4+ and prevent oxygen from escaping from the internal crystal, so that the lithium metal-lithium cobalt oxide battery with a cathode protected by a CEI rich in lithium fluoride can withstand a higher charging cut-off voltage.

[0040] Optionally, in an embodiment of the present application, the material of the positive electrode of the lithium battery includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, and lithium-rich manganese.

[0041] The material of the negative electrode of the lithium battery includes at least one of metallic lithium, silicon carbon, and graphite.

[0042] Optionally, in an embodiment of the present application, the material of the positive electrode of the lithium battery is the same as the material of the negative electrode.

[0043] The materials of the positive electrode and the negative electrode of the lithium battery include at least one of metallic lithium and lithium-rich manganese.

[0044] Taking lithium hexafluorophosphate as the lithium salt, fluoroethylene carbonate and ethyl methyl carbonate as the solvents, lithium cobalt oxide as the positive electrode material of the lithium battery, and metallic lithium as the negative electrode material of the lithium battery as an example, a specific description is given below.

[0045] Example 1 Under the protection of an inert atmosphere, 152 g (grams) of lithium hexafluorophosphate was dissolved in 280 mL (milliliters) of fluoroethylene carbonate and 650 mL of ethyl methyl carbonate to prepare a basic electrolyte with a lithium salt concentration of 1 M (mol / L, mole / liter), and an electrolyte additive was added to the basic electrolyte, and stirred to obtain a uniformly mixed electrolyte. The electrolyte additive contains 0.5 wt% (mass percentage) of lithium perfluorooctanoate. With a loading of 10 mg cm -2The lithium cobalt oxide electrode sheet with a loading of Figures 3a - 3b and Figure 4a mg / cm² was used as the positive electrode, and a 450-mm lithium foil was used as the negative electrode to assemble a CR2025 coin cell for performance evaluation. The electrolyte volume in the coin cell was 40 μL. Each test was conducted on four cells simultaneously. The results of the four-cell tests were averaged, and the cell data closest to the average were compared with the average value. If the cell data deviated significantly, the test was repeated. The charge-discharge test was carried out under a constant current mode. The charge cut-off voltage was 4.8 V, and the discharge cut-off voltage was 3.0 V. The first two cycles were activated at 0.1 C (the charge-discharge rate of the battery). After the activation was completed, charging was carried out at 0.5 C and discharging was carried out at 1 C. The test results are shown in

[0046] Comparative Example 1 Under an inert atmosphere, 152 g of lithium hexafluorophosphate was dissolved in 280 mL of fluoroethylene carbonate and 650 mL of ethyl methyl carbonate to prepare an electrolyte with a lithium salt concentration of 1 M. A lithium cobalt oxide electrode sheet with a loading of 10 mg / cm -2 was used as the positive electrode, and a 450-mm lithium foil was used as the negative electrode to assemble a CR2025 coin cell for performance evaluation. The electrolyte volume in the coin cell was 40 μL. Each test was conducted on four cells simultaneously. The results of the four-cell tests were averaged, and the cell data closest to the average were compared with the average value. If the cell data deviated significantly, the test was repeated. The charge-discharge test was carried out under a constant current mode. The charge cut-off voltage was 4.8 V, and the discharge cut-off voltage was 3.0 V. The first two cycles were activated at 0.1 C. After the activation was completed, charging was carried out at 0.5 C and discharging was carried out at 1 C. The test results are shown in Figures 3a - 3b and Figure 4b shown.

[0047] Comparative Example 2 152 g of lithium hexafluorophosphate was dissolved in 280 mL of fluoroethylene carbonate and 650 mL of ethyl methyl carbonate to prepare a basic electrolyte with a lithium salt concentration of 1 M. An electrolyte additive was added to the basic electrolyte and stirred to obtain a uniformly mixed electrolyte. The electrolyte additive contained 0.1 wt% of lithium perfluorooctanoate. A lithium cobalt oxide electrode sheet with a loading of 10 mg / cm -2The lithium cobalt oxide electrode was used as the positive electrode, and a 450 mm lithium foil was used as the negative electrode to assemble a CR2025 coin cell for performance evaluation. The electrolyte volume in the coin cell was 40 μL. Each test was conducted on four cells simultaneously. The results of the four cells were averaged, and the cell data closest to the average was compared with the number of the average. If the cell data deviated too much, the test was repeated. The charge-discharge test was carried out in a constant current mode. The charge cut-off voltage was 4.8 V, and the discharge cut-off voltage was 3.0 V. The first two cycles were activated at 0.1 C. After activation, charging was carried out at 0.5 C and discharging was carried out at 1 C. The test results are as Figures 3a - 3b shown.

[0048] Comparative Example 3 152 g of lithium hexafluorophosphate was dissolved in 280 mL of fluoroethylene carbonate and 650 mL of ethyl methyl carbonate to prepare a basic electrolyte with a lithium salt concentration of 1 M. An electrolyte additive was added to the basic electrolyte and stirred to obtain a uniformly mixed electrolyte. The electrolyte additive contained 0.2 wt% of lithium perfluorooctanoate. Using a lithium cobalt oxide electrode with a loading of 10 mg cm -2 as the positive electrode and a 450 mm lithium foil as the negative electrode, a CR2025 coin cell was assembled for performance evaluation. The electrolyte volume in the coin cell was 40 μL. Each test was conducted on four cells simultaneously. The results of the four cells were averaged, and the cell data closest to the average was compared with the number of the average. If the cell data deviated too much, the test was repeated. The charge-discharge test was carried out in a constant current mode. The charge cut-off voltage was 4.8 V, and the discharge cut-off voltage was 3.0 V. The first two cycles were activated at 0.1 C. After activation, charging was carried out at 0.5 C and discharging was carried out at 1 C. The test results are as Figures 3a - 3b shown.

[0049] Comparative Example 4 152 g of lithium hexafluorophosphate was dissolved in 280 mL of fluoroethylene carbonate and 650 mL of ethyl methyl carbonate to prepare a basic electrolyte with a lithium salt concentration of 1 M. An electrolyte additive was added to the basic electrolyte and stirred to obtain a uniformly mixed electrolyte. The electrolyte additive contained 1 wt% of lithium perfluorooctanoate. Using a lithium cobalt oxide electrode with a loading of 10 mg cm -2The lithium cobalt oxide electrode was used as the positive electrode, and a 450 mm lithium foil was used as the negative electrode to assemble a button cell of model CR2025 for performance evaluation. The electrolyte volume in the button cell was 40 μL. Each test was carried out on four cells simultaneously. The results of the four cells were averaged, and the cell data closest to the average were compared with the number of the average. If the cell data deviated too much, the test was repeated. The charge and discharge tests were carried out in a constant current mode. The charge cut-off voltage was 4.8 V, and the discharge cut-off voltage was 3.0 V. The first two cycles were activated at 0.1 C. After the activation, charging was carried out at 0.5 C and discharging was carried out at 1 C. The test results are as Figure 3a shown.

[0050] Comparative Example 5 152 g of lithium hexafluorophosphate was dissolved in 280 mL of fluoroethylene carbonate and 650 mL of ethyl methyl carbonate to prepare a basic electrolyte with a lithium salt concentration of 1 M. An electrolyte additive was added to the basic electrolyte and stirred to obtain a uniformly mixed electrolyte. The electrolyte additive contained 1.5 wt% of lithium perfluorooctanoate. Using a lithium cobalt oxide electrode with a loading of 10 mg cm -2 as the positive electrode and a 450 mm lithium foil as the negative electrode, a button cell of model CR2025 was assembled for performance evaluation. The electrolyte volume in the button cell was 40 μL. Each test was carried out on four cells simultaneously. The results of the four cells were averaged, and the cell data closest to the average were compared with the number of the average. If the cell data deviated too much, the test was repeated. The charge and discharge tests were carried out in a constant current mode. The charge cut-off voltage was 4.8 V, and the discharge cut-off voltage was 3.0 V. The first two cycles were activated at 0.1 C. After the activation, charging was carried out at 0.5 C and discharging was carried out at 1 C. The test results are as Figure 3a shown.

[0051] Figures 3a - 3b are the charge-discharge curves of the lithium metal-lithium cobalt oxide battery when the electrolyte contains different mass percentages of lithium perfluorooctanoate. As Figure 3a shown, when the content of lithium perfluorooctanoate in the electrolyte additive is greater than 0.5 wt%, the charge-discharge polarization of the battery is very large, while when the content of lithium perfluorooctanoate in the electrolyte additive is less than 0.5 wt%, the discharge capacity is almost close; as Figure 3b shown, when the concentration of lithium perfluorooctanoate in the electrolyte additive is 0.5 wt%, the battery has better cycle stability. Therefore, the preferred concentration of lithium perfluorooctanoate in the electrolyte additive is 0.5 wt%.

[0052] Figure 4a are the charge-discharge curves of the lithium metal-lithium cobalt oxide battery when the electrolyte contains 0.5 wt% of lithium perfluorooctanoate and the charging voltage is 4.8 V, Figure 4bThe charge-discharge curve when the electrolyte of the lithium metal-lithium cobalt oxide battery does not contain lithium perfluorooctanoate at a charging voltage of 4.8V. As Figure 4a shown, the lithium metal-lithium cobalt oxide battery containing 0.5 wt% of lithium perfluorooctanoate can stably cycle nearly 200 times; however, the lithium metal-lithium cobalt oxide battery without lithium perfluorooctanoate can only stably cycle less than 30 times, as Figure 4b shown. The electrolyte of the embodiment of the present application includes an electrolyte additive containing lithium perfluorooctanoate. Lithium perfluorooctanoate has bifunctional reaction sites and is rich in fluorine atoms, so that a positive electrode electrolyte film rich in lithium fluoride can be preferentially formed by oxidation on the surface of the positive electrode, and at the same time, a solid electrolyte interface can be preferentially formed by reduction on the surface of the negative electrode, enabling the lithium metal-lithium cobalt oxide battery to still have stable cycling at a relatively high voltage, such as when the charging voltage is 4.8V, thereby improving the performance of the battery.

[0053] Hereinafter, taking lithium hexafluorophosphate as the lithium salt, fluoroethylene carbonate and ethyl methyl carbonate as the solvents, and metallic lithium as the positive and negative electrode materials of the lithium battery as examples, specific descriptions will be made.

[0054] Example 2 152 g of lithium hexafluorophosphate was dissolved in 280 mL of fluoroethylene carbonate and 650 mL of ethyl methyl carbonate to prepare a base electrolyte with a lithium salt concentration of 1 M, and an electrolyte additive was added to the base electrolyte and stirred to obtain a uniformly mixed electrolyte. The electrolyte additive contained 0.5 wt% of lithium perfluorooctanoate. Using a 450 mm lithium foil as the negative electrode, a lithium-lithium symmetric battery of model CR2025 was assembled to evaluate the performance of the lithium metal. The amount of electrolyte in the battery was 40 μL. Each test was carried out on four batteries simultaneously. The results of the four battery tests were averaged, and the battery data closest to the average value was selected for comparison with the number of average values. If the battery data deviated too much, the test was repeated. The test current density was 2 mA cm -2 (milliamperes per square centimeter), the lithium deposition time was 1 hour, and the test ended when the battery short-circuited or micro-short-circuited. The test results are as Figure 5 shown.

[0055] Comparative Example 6 152 g of lithium hexafluorophosphate was dissolved in 280 mL of fluoroethylene carbonate and 650 mL of ethyl methyl carbonate to prepare an electrolyte with a lithium salt concentration of 1 M. Using a 450 mm lithium foil as the negative electrode, a lithium-lithium symmetric battery of model CR2025 was assembled to evaluate the performance of the lithium metal. The amount of electrolyte was 40 μL. Each test was carried out on four batteries simultaneously. The results of the four battery tests were averaged, and the battery data closest to the average value was selected for comparison with the number of average values. If the battery deviated too much, the test was repeated. The test current density was 2 mA cm -2, the lithium deposition time is 1 hour. When the battery undergoes short circuit or micro short circuit, the test ends, and the test results are as Figure 5 shown.

[0056] Figure 5 is the cycling performance of the lithium-lithium symmetric battery. As can be seen from Figure 5 , after adding the electrolyte additive containing lithium perfluorooctanoate to the electrolyte, the lithium-lithium symmetric battery can stably cycle for more than 700 hours at 2 mA cm -2 . However, without adding the electrolyte additive containing lithium perfluorooctanoate to the electrolyte, the lithium-lithium symmetric battery shows a short circuit phenomenon after cycling for less than 110 hours at 2 mA cm -2 .

[0057] When the electrolyte additive containing lithium perfluorooctanoate is not added to the electrolyte, due to the extremely active lithium metal, during the deposition process of lithium metal, part of the lithium metal reacts with the electrolyte and loses its activity, resulting in a lower cycling efficiency. And during the repeated charge and discharge process, the continuous reaction between the lithium metal negative electrode and the organic solvent in the electrolyte will lead to the gradual consumption and exhaustion of the electrolyte, thus severely limiting the cycle life of the battery. Moreover, during the charging and recovery process of lithium metal, it does not deposit evenly on the surface of the negative electrode, but grows excessively in the form of dendrites at some higher potentials, which is likely to pierce the separator during the battery cycling process and cause internal short circuit of the lithium battery. In contrast, after adding the electrolyte additive containing lithium perfluorooctanoate to the electrolyte, lithium perfluorooctanoate in the electrolyte is preferentially reduced on the surface of the lithium metal negative electrode to form a solid electrolyte interface. The solid electrolyte interface prevents the reaction between the lithium metal negative electrode and the organic solvent in the electrolyte. At the same time, the electrolyte of the present application has a very good effect of suppressing lithium dendrites. In addition, the electrolyte of the present application can dynamically generate a solid electrolyte interface on the surface of the lithium metal negative electrode to protect the stability of the negative electrode structure.

[0058] In summary, in the embodiments of the present application, adding an electrolyte additive including lithium perfluorooctanoate to the base electrolyte to make an electrolyte; lithium perfluorooctanoate with bifunctional reaction sites can stabilize the lithium metal negative electrode and at the same time ensure the stability of the lithium cobaltate positive electrode structure under high voltage, so that the charging cut-off voltage of the lithium metal-lithium cobaltate battery can reach 4.8V, thereby improving the energy density of the battery.

[0059] Applying the embodiments of the present application can at least achieve the following beneficial effects: 1. The electrolyte of the lithium battery provided by the embodiment of the present application includes: a solvent, an electrolyte additive, and a lithium salt dissolved in the solvent. The electrolyte additive includes lithium perfluorooctanoate, and the lithium perfluorooctanoate includes a perfluorooctanoate group with an oxidation site and a reduction site, so that the lithium perfluorooctanoate has high reactivity and can preferentially oxidize or reductively decompose on the surfaces of the positive and negative electrodes of the lithium battery to form a cathode electrolyte interface (CEI) rich in lithium fluoride and a solid electrolyte interface (SEI). During the process of generating the CEI and SEI rich in lithium fluoride, basically no other organic components are generated, so that the CEI and SEI have high electrochemical stability and modulus, can maintain structural integrity and avoid electrolyte penetration, thereby can stabilize the structures of the positive and negative electrodes of the lithium battery, and make the SEI have a high lithium deposition Coulomb efficiency, so that the positive and negative electrodes of the lithium battery protected by the CEI and SEI rich in lithium fluoride can withstand a high charging cut-off voltage, such as 4.8V (volts), can improve the energy density of the lithium battery; and make the lithium battery have high cycle stability.

[0060] For example, by adding lithium perfluorooctanoate to the electrolyte, the discharge capacity of the lithium cobalt oxide battery can be increased from 185 mAh / g (milliampere-hour per gram) at 4.5V to 244 mAh / g at 4.8V, so that the energy density of the lithium cobalt oxide battery in the same volume is increased by 36%, greatly improving the battery life of the electronic device.

[0061] Those skilled in the art of the present technology can understand that the steps, measures, and solutions in various operations, methods, and processes discussed in the present application can be alternated, changed, combined, or deleted. Further, the other steps, measures, and solutions in the various operations, methods, and processes discussed in the present application can also be alternated, changed, rearranged, decomposed, combined, or deleted. Further, the steps, measures, and solutions in the related technologies that are the same as those disclosed in the present application can also be alternated, changed, rearranged, decomposed, combined, or deleted.

[0062] The terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present application, unless otherwise specified, the meaning of "a plurality" is two or more.

[0063] In the description of this specification, specific features, structures, materials, or characteristics may be combined in a suitable manner in any one or more embodiments or examples.

[0064] The above are only some embodiments of the present application. It should be noted that for those of ordinary skill in the art, without departing from the technical concept of the present application, other similar implementation means based on the technical idea of the present application also fall within the protection scope of the embodiments of the present application.

Claims

1. An electrolyte for a lithium battery, characterized in that, Comprising: A solvent, a lithium salt, and an electrolyte additive, wherein the electrolyte additive comprises lithium perfluorooctanoate.

2. The electrolyte of the lithium battery according to claim 1, characterized in that, The lithium perfluorooctanoate comprises a perfluorooctanoate radical having an oxidation site and a reduction site; The perfluorooctanoate radical is prepared by fluorination and hydroxylation of octane.

3. The electrolyte of the lithium battery according to claim 1, wherein The lithium salt comprises at least one of LiPF6, LiNO3, LiN(SO2CF3)2, LiCF3SO3, LiN(FSO2)3, and LiClO4.

4. The electrolyte of the lithium battery according to claim 1, wherein The solvent comprises at least one of a fluorinated carbonate solvent, a fluorinated carboxylate solvent, a fluorinated ether solvent, a carbonate solvent, a carboxylate solvent, and an ether solvent; The carbonate comprises at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; The carboxylate comprises at least one of ethyl acetate, methyl acetate, and methyl propionate; The ether comprises at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,4-dioxolane.

5. The electrolyte of the lithium battery according to claim 3, wherein The concentration of the lithium perfluorooctanoate comprises: 0.001 - 3 mol / L; The concentration of the lithium salt comprises: 0.001 - 3 mol / L.

6. A method for preparing an electrolyte of a lithium battery according to any one of the above claims 1-5, characterized in that, Comprising: Under the protection of an inert atmosphere, the lithium salt is dissolved in an organic solvent, and an electrolyte additive comprising lithium perfluorooctanoate is added, and stirred to obtain a uniformly mixed electrolyte.

7. The preparation method according to claim 6, characterized in that, Before dissolving the lithium salt in the organic solvent and stirring, it further comprises: Fluorinating and hydroxylating octane to form a perfluorooctanoate radical having a reduction site and an oxidation site.

8. A lithium battery, characterized in that, Comprising: A positive electrode, a negative electrode, and the electrolyte according to any one of claims 1 - 5 above; The lithium perfluorooctanoate in the electrolyte is used to form a positive electrode electrolyte film containing LiF on the surface of the positive electrode; the lithium perfluorooctanoate in the electrolyte is used to form a solid electrolyte film containing LiF on the surface of the negative electrode.

9. The lithium battery according to claim 8, characterized in that, The material of the positive electrode comprises at least one of lithium cobaltate, lithium nickel cobalt manganate, and lithium-rich manganese; The material of the negative electrode comprises at least one of metallic lithium, silicon carbon, and graphite.

10. The lithium battery according to claim 8, characterized in that, The material of the positive electrode is the same as the material of the negative electrode; The material of the positive electrode and the negative electrode comprises at least one of metallic lithium and lithium-rich manganese.

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