Electrolyte for improving cycle performance of lithium metal battery, preparation method and lithium metal battery

Abstract

The invention provides an electrolyte for improving the cycle performance of a lithium metal battery, a preparation method and the lithium metal battery. The electrolyte comprises an organic solvent, a lithium salt and an additive, and the molar concentration of the lithium salt in the electrolyte is 2-3 mol / L; the mass ratio of the additive to the lithium salt is (1-7): (93-99), and the additive at least comprises one of fluoroethylene carbonate and tris (pentafluorophenyl) borane. According to the invention, by optimizing the components of the electrolyte, particularly by taking lithium bis (fluorosulfonyl) imide and lithium bis (oxalato) borate as additives, the stability of a solid electrolyte interface film is synergistically improved, the compatibility of the electrolyte and an electrode is improved, the growth of lithium dendrites is inhibited, and the stability of the electrolyte in a circulating process is improved, so that the performance of the lithium metal battery is improved. Particularly, the cycle performance of a nickel cobalt lithium manganate ternary lithium metal battery and a nickel cobalt lithium aluminate ternary lithium metal battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium metal battery, and particularly to an electrolyte for improving the cycle performance of lithium metal battery, a preparation method and the lithium metal battery. BACKGROUND

[0002] Metal lithium battery is considered as one of the most potential technologies in the future energy storage field due to its high energy density and high theoretical specific capacity. However, the compatibility of electrolyte and electrode and the stability of electrolyte itself seriously restrict the cycle life and performance of the battery during the cycle process of the metal lithium battery.

[0003] During the charging and discharging process of the metal lithium battery, the deposition of lithium metal and glass will cause a dramatic change in the interface volume, destroy the stability of the interface (SEI), cause the continuous consumption of active lithium and electrolyte, and significantly reduce the cycle performance of the battery; at the same time, lithium metal is easy to form dendrites, which may pierce the separator and cause short circuit, further reducing the cycle stability and safety of the battery. Therefore, it is of important practical significance to develop an electrolyte which can effectively improve the cycle performance of the metal lithium battery. SUMMARY

[0004] The purpose of the present application is to provide an electrolyte for improving the cycle performance of lithium metal battery, a preparation method and the lithium metal battery, so as to solve the problems in the background art.

[0005] The technical scheme adopted by the present application comprises: an electrolyte for improving the cycle performance of lithium metal battery, which comprises:

[0006] an organic solvent;

[0007] a lithium salt, the molar concentration of the lithium salt in the electrolyte is 2-3 mol / L;

[0008] an additive, the mass ratio of the additive to the lithium salt is (1-7):(93-99), and the additive at least comprises one of fluoroethylene carbonate and tris(pentafluorophenyl)borane.

[0009] Preferably, the lithium salt at least comprises one of lithium bisfluorosulfonylimide and lithium bis(oxalato)borate.

[0010] Preferably, the mass ratio of lithium bisfluorosulfonylimide to lithium bis(oxalato)borate in the lithium salt is (10-20):1.

[0011] Preferably, the organic solvent at least comprises one of methyl ethyl carbonate, ethylene carbonate and ethylene glycol dimethyl ether.

[0012] Preferably, in the organic solvent, the volume fraction of methyl ethyl carbonate is greater than or equal to 50%.

[0013] Preferably, in the organic solvent, the volume ratio of ethylene carbonate to dimethyl ether of ethylene glycol is 1:1.

[0014] The technical scheme of the present application also includes a method for preparing the electrolyte for improving the cycle performance of a lithium metal battery, which comprises the step of uniformly mixing the lithium salt, the additive and the organic solvent to obtain the electrolyte for improving the cycle performance of the lithium metal battery.

[0015] The technical scheme of the present application also includes a lithium metal battery, which comprises:

[0016] a positive electrode containing a positive electrode active material, the positive electrode active material at least including one of a lithium nickel cobalt manganese oxide ternary material and a lithium nickel cobalt aluminum oxide ternary material;

[0017] a lithium metal negative electrode;

[0018] and the electrolyte for improving the cycle performance of the lithium metal battery.

[0019] Preferably, the content of nickel in the positive electrode active material is greater than or equal to 70%.

[0020] The present application has the beneficial effect that by optimizing the composition of the electrolyte, especially using lithium bisfluorosulfonylimide and lithium bisoxalate borate as additives, the stability of the solid electrolyte interface film is improved, the compatibility of the electrolyte with the electrode is improved, the growth of lithium dendrites is inhibited, the stability of the electrolyte in the cycle process is improved, and thus the cycle performance of the lithium metal battery, especially the lithium nickel cobalt manganese oxide ternary lithium metal battery and the lithium nickel cobalt aluminum oxide ternary lithium metal battery, is improved. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is the cycle performance diagram of the lithium metal battery prepared in Example 1 and Comparative Example 1 of the present application;

[0022] Figure 2 is the cycle performance diagram of the lithium metal battery prepared in Example 2 and Comparative Example 2 of the present application;

[0023] Figure 3 is the cycle performance diagram of the lithium metal battery prepared in Example 3 and Comparative Example 3 of the present application. DETAILED DESCRIPTION

[0024] The embodiments of the present application will be described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as a limitation of the present application.

[0025] This invention provides an electrolyte for improving the cycle performance of lithium metal batteries, a method for preparing the electrolyte, and a lithium metal battery containing the electrolyte. By optimizing the electrolyte composition, the compatibility between the electrolyte and the electrode is improved, lithium dendrite growth is suppressed, and the stability of the electrolyte during cycling is enhanced, thereby improving the cycle performance of the lithium metal battery.

[0026] The electrolyte for improving the cycle performance of lithium metal batteries provided in this embodiment includes an organic solvent, a lithium salt, and an additive. The molar concentration of the lithium salt in the electrolyte is 2-3 mol / L, and the mass ratio of the additive to the lithium salt is (1-7):(93-99). The additive includes at least one of fluoroethylene carbonate and tris(pentafluorophenyl)borane, preferably a mixture of the two.

[0027] In lithium metal batteries containing the above electrolyte, during charge-discharge cycles, fluoroethylene carbonate preferentially undergoes a reduction decomposition reaction on the negative electrode surface, generating a solid electrolyte interphase (SEI) film rich in lithium fluoride. Meanwhile, the high fluoride content of tris(pentafluorophenyl)borane and the B2O2 produced by the decomposition of tris(pentafluorophenyl)borane further contribute to the formation of a lithium-ion-rich solid electrolyte interphase (SEI) film. x O y The BOB bond network in the electrolyte bridges LiF nanoparticles (particle size <5nm) to form a composite structure, which contributes to the formation of the solid electrolyte interfacial film. This solid electrolyte interfacial film is uniform and structurally highly stable, effectively suppressing the growth of lithium dendrites during the charging and discharging process of the lithium metal electrode and improving the stability of the lithium metal anode. Furthermore, tris(pentafluorophenyl)borane can significantly improve the ion conductivity and toughness of the interfacial film. When used in combination with fluoroethylene carbonate, it can simultaneously construct a dense surface film on both the positive and negative electrodes, maintaining the stability of the battery during cycling. In addition, through the optimized design of the addition amounts of fluoroethylene carbonate and tris(pentafluorophenyl)borane, the solvation structure of metal ions can be improved, thereby enhancing the cycle performance of the lithium metal battery.

[0028] In this embodiment, the lithium salt includes at least one of lithium bis(fluorosulfonyl)imide and lithium di(oxalato)borate, preferably a mixture of the two, wherein the mass ratio of lithium bis(fluorosulfonyl)imide to lithium di(oxalato)borate is preferably (10-20):1.

[0029] The above-mentioned method for improving the cycle performance of lithium metal batteries is as follows: In a glove box, lithium salt, additives and organic solvents are mixed in proportion, wherein: the molar concentration of lithium salt in the electrolyte is 2-3 mol / L, the lithium salt includes lithium bis(fluorosulfonyl)imide and lithium dioxalate borate, the mass ratio of additives to lithium salt is (1-7):(93-99), and the additives include fluoroethylene carbonate and tris(pentafluorophenyl)borane.

[0030] The embodiment also provides a lithium metal battery, which comprises a positive electrode, a lithium metal negative electrode and the above-mentioned electrolyte for improving the cycle performance of a lithium metal battery, and the positive electrode comprises a positive electrode active material, and the positive electrode active material at least comprises one of lithium nickel cobalt manganese oxide ternary material and lithium nickel cobalt aluminum oxide ternary material.

[0031] Preferably, the content of nickel in the positive electrode active material is greater than or equal to 70%.

[0032] The lithium metal battery containing the above-mentioned electrolyte, especially the lithium metal battery of lithium nickel cobalt manganese oxide ternary, has improved cycle performance, and the capacity retention rate after a long cycle is effectively improved, and has better cycle life and safety than the lithium metal battery using a conventional electrolyte.

[0033] The following is a specific embodiment of the present application.

[0034] Embodiment 1

[0035] (1) Preparation of a to-be-liquid-injected battery

[0036] Seven lithium nickel cobalt manganese oxide ternary materials (the content of nickel is 70% to 80%), conductive agents (carbon black, carbon nanotubes) and binders (PVDF) are uniformly mixed in a mass ratio of 95:2:1:2, an appropriate amount of N-methyl pyrrolidone is added, mixing is performed through a homogenizing tank, a positive electrode slurry is prepared, the positive electrode slurry is uniformly coated on an aluminum current collector by using a transfer coating machine to prepare a double-sided electrode sheet, and the positive electrode sheet is punched by a positive electrode die-cutting machine to obtain a positive electrode sheet; the lithium metal negative electrode sheet is punched by a negative electrode die-cutting machine; the positive electrode sheet and the lithium metal negative electrode sheet are laminated by a laminating machine, and after welding and packaging, a to-be-liquid-injected battery is obtained.

[0037] (2) Preparation of an electrolyte

[0038] The organic solvent component is methyl ethyl carbonate: ethylene carbonate: ethylene glycol dimethyl ether = 1:0.5:0.5 by volume.

[0039] The lithium bisfluorosulfonylimide: lithium bisoxalate borate: fluorinated ethylene carbonate: tris(pentafluorophenyl)borane = 1:0.1:0.03:0.02 by mass.

[0040] The above-mentioned substances are taken and mixed according to the above-mentioned proportion to prepare an electrolyte with a lithium salt concentration of 2 mol / L.

[0041] (3) The to-be-liquid-injected battery obtained in (1) and the electrolyte obtained in (2) are used to perform liquid injection and complete packaging to obtain a liquid-injected battery.

[0042] (4) The liquid-injected battery obtained in (3) is subjected to formation and capacity test, and then subjected to 0.2C constant current charging and 1C constant current discharging.

[0043] Embodiment 2

[0044] (1) Preparation of the to-be-liquid-injected battery cell

[0045] Mix the eight-system (nickel content in 80%~90%) lithium nickel cobalt manganese oxide ternary material, conductive agent (carbon black, carbon nanotube) and binder (PVDF) in a mass ratio of 95:2:1:2, add an appropriate amount of N-methyl pyrrolidone, mix through the homogenizing tank, prepare the positive electrode slurry, uniformly coat the positive electrode slurry on the aluminum current collector by using the transfer coating machine to prepare the double-sided electrode sheet, punch the positive electrode sheet by using the positive electrode die-cutting machine to obtain the positive electrode sheet; punch the lithium metal negative electrode sheet by using the negative electrode die-cutting machine to obtain the lithium metal negative electrode sheet; stack the positive electrode sheet and the lithium metal negative electrode sheet by using the stacking machine, and after welding and packaging, obtain the to-be-liquid-injected battery cell.

[0046] (2) Preparation of the electrolyte

[0047] The organic solvent component is methyl ethyl carbonate: ethylene carbonate: ethylene glycol dimethyl ether = 1:0.5:0.5 by volume;

[0048] The lithium difluorosulfonylimide: lithium bisoxalate borate: fluoroethylene carbonate: tris(pentafluorophenyl)borane = 1:0.1:0.05:0.02 by mass;

[0049] Take and mix the above substances according to the proportion to prepare the electrolyte with a lithium salt concentration of 2.5 mol / L.

[0050] (3) Take the to-be-liquid-injected battery cell obtained in (1) and the electrolyte obtained in (2), perform liquid injection and complete packaging to obtain the liquid-injected battery cell.

[0051] (4) After formation and capacity test of the liquid-injected battery cell obtained in (3), perform 0.2C constant current charging and 1C constant current discharging to perform cycle test.

[0052] Example 3

[0053] (1) Preparation of the to-be-liquid-injected battery cell

[0054] Mix the eight-system (nickel content in 80%~90%) lithium nickel cobalt aluminum oxide ternary material, conductive agent (carbon black, carbon nanotube) and binder (PVDF) in a mass ratio of 95:2:1:2, add an appropriate amount of N-methyl pyrrolidone, mix through the homogenizing tank, prepare the positive electrode slurry, uniformly coat the positive electrode slurry on the aluminum current collector by using the transfer coating machine to prepare the double-sided electrode sheet, punch the positive electrode sheet by using the positive electrode die-cutting machine to obtain the positive electrode sheet; punch the lithium metal negative electrode sheet by using the negative electrode die-cutting machine to obtain the lithium metal negative electrode sheet; stack the positive electrode sheet and the lithium metal negative electrode sheet by using the stacking machine, and after welding and packaging, obtain the to-be-liquid-injected battery cell.

[0055] (2) Preparation of the electrolyte

[0056] The organic solvent component is methyl ethyl carbonate: ethylene carbonate: ethylene glycol dimethyl ether = 1:0.5:0.5 by volume;

[0057] Lithium difluorosulfimide: lithium difluorooxalate borate: fluoroethylene carbonate: tris(pentafluorophenyl)borane = 1:0.08:0.03:0.02 by mass;

[0058] The electrolyte with a lithium salt concentration of 2.5 mol / L was prepared by taking and mixing the above substances in the above proportions.

[0059] (3) The liquid injection battery cell obtained in (1) and the electrolyte obtained in (2) were subjected to liquid injection and packaging to obtain a liquid injection battery cell.

[0060] (4) The liquid injection battery cell obtained in (3) was subjected to formation and capacity test, and then subjected to 0.2C constant current charging and 1C constant current discharging for cycle test.

[0061] Comparative Example 1

[0062] Compared with Example 1, the only difference is that Comparative Example 1 uses a conventional electrolyte, and the preparation scheme is as follows:

[0063] The organic solvent component is ethylene glycol dimethyl ether: 1,2-dimethoxyethane = 5:3 by volume;

[0064] Lithium difluorosulfimide: lithium difluorooxalate borate: fluoroethylene carbonate: tris(pentafluorophenyl)borane = 1:0.08:0.03:0.02 by mass;

[0065] The electrolyte with a lithium salt concentration of 1.5 mol / L was prepared by taking and mixing the above substances in the above proportions.

[0066] Comparative Example 2

[0067] Compared with Example 2, the only difference is that Comparative Example 2 uses a conventional electrolyte, and the preparation scheme is as follows:

[0068] The organic solvent component is ethylene glycol dimethyl ether: 1,2-dimethoxyethane = 5:3 by volume;

[0069] Lithium difluorosulfimide: lithium difluorooxalate borate: fluoroethylene carbonate: tris(pentafluorophenyl)borane = 1:0.08:0.03:0.02 by mass;

[0070] The electrolyte with a lithium salt concentration of 1.5 mol / L was prepared by taking and mixing the above substances in the above proportions.

[0071] Comparative Example 3

[0072] Compared with Example 3, the only difference is that Comparative Example 3 uses a conventional electrolyte, and the preparation scheme is as follows:

[0073] The organic solvent component is ethylene glycol dimethyl ether: 1,2-dimethoxyethane = 5:3 by volume;

[0074] Lithium difluorosulfimide: Lithium difluoro oxalate borate: Vinylene carbonate: Vinyl sulfate = 5: 1: 0.15: 0.1 by mass;

[0075] The electrolyte with a lithium salt concentration of 1.5 mol / L was prepared by taking and mixing the above substances in the above proportions.

[0076] The cycle test results of Examples 1-3 and Comparative Examples 1-3 are shown in Table 1 and FIGS. 1-3. Figures 1-3 FIG. 1 is a graph showing the cycle performance of lithium metal batteries prepared in Example 1 and Comparative Example 1. Figure 1 FIG. 2 is a graph showing the cycle performance of lithium metal batteries prepared in Example 1 and Comparative Example 2. Figure 2 FIG. 3 is a graph showing the cycle performance of lithium metal batteries prepared in Example 3 and Comparative Example 3. Figure 3 FIG. 3 is a graph showing the cycle performance of lithium metal batteries prepared in Example 3 and Comparative Example 3.

[0077] Table 1

[0078]

[0079]

[0080] Analysis Figure 1 The test results show that the overall cycle capacity retention rate of Example 1 is higher than that of Comparative Example 1, and the cycle life is increased by 54 cycles.

[0081] Analysis Figure 2 The test results show that the overall cycle capacity retention rate of Example 2 is higher than that of Comparative Example 2, although the overall capacity of Example 2 decays rapidly in the last few dozen cycles, the cycle life of the battery is improved, and Example 2 cycles 39 more cycles than Comparative Example 2.

[0082] Analysis Figure 3 The test results show that the overall cycle capacity retention rate of Example 3 is higher than that of Comparative Example 3, although the overall capacity of Example 3 decays rapidly in the last few dozen cycles, the cycle life of the battery is improved, and Example 3 cycles 83 more cycles than Comparative Example 3.

[0083] It should be understood that the above specific embodiments of the present application are merely used for illustrative or explanatory purposes, and do not constitute a limitation on the present application. Therefore, any modifications, equivalent replacements, improvements, etc. made without departing from the spirit and scope of the present application shall be included in the protection scope of the present application. In addition, the appended claims of the present application are intended to cover all variations and modifications falling within the scope and boundary of the claims, or the equivalent forms of such scope and boundary.

Claims

1. An electrolyte for improving the cycle performance of lithium metal batteries, characterized in that, include: Organic solvents; Lithium salt, wherein the molar concentration of the lithium salt in the electrolyte is 2-3 mol / L; An additive, wherein the mass ratio of the additive to the lithium salt is (1-7):(93-99), and the additive comprises at least one of fluoroethylene carbonate and tris(pentafluorophenyl)borane.

2. The electrolyte for improving the cycle performance of lithium metal batteries according to claim 1, characterized in that, The lithium salt includes at least one of lithium bis(fluorosulfonyl)imide and lithium dioxaborate.

3. The electrolyte for improving the cycle performance of lithium metal batteries according to claim 2, characterized in that, The mass ratio of lithium difluorosulfonylimide to lithium dioxazoborate in the lithium salt is (10-20):

1.

4. The electrolyte for improving the cycle performance of lithium metal batteries according to any one of claims 1-3, characterized in that, The organic solvent includes at least one of ethyl methyl carbonate, ethylene carbonate, and ethylene glycol dimethyl ether.

5. The electrolyte for improving the cycle performance of lithium metal batteries according to claim 4, characterized in that, In the organic solvent, the volume percentage of methyl ethyl carbonate is greater than or equal to 50%.

6. The electrolyte for improving the cycle performance of lithium metal batteries according to claim 5, characterized in that, In the organic solvent, the volume ratio of ethylene carbonate to ethylene glycol dimethyl ether is 1:

1.

7. A method for preparing the electrolyte according to any one of claims 1-6 for improving the cycle performance of lithium metal batteries, characterized in that, The process includes the following steps: mixing the lithium salt, the additive, and the organic solvent in a specific ratio to obtain an electrolyte that improves the cycle performance of lithium metal batteries.

8. A lithium metal battery, characterized in that, include: A positive electrode containing a positive electrode active material, wherein the positive electrode active material includes at least one of lithium nickel cobalt manganese oxide ternary material and lithium nickel cobalt aluminum oxide ternary material; Lithium metal anode; And the electrolyte for improving the cycle performance of lithium metal batteries according to any one of claims 1-6.

9. The lithium metal battery according to claim 8, characterized in that, The nickel content in the positive electrode active material is greater than or equal to 70%.

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