A medium salt concentration electrolyte for lithium-sulfur batteries

Abstract

The present invention discloses a medium salt concentration electrolyte for lithium-sulfur batteries, the electrolyte containing a mixed lithium salt, a first and second ether solvents and a double ether-fluorinated ether tri-solvent system formed by at least one fluorinated ether solvent; the mixed lithium salt contains a lithium salt that can form a passivation film on a lithium metal negative electrode; the oxygen coordination steric hindrance of the first ether solvent is small (the number of carbon atoms / oxygen atoms in the molecule is ≤4), and the lithium salt can be dissociated; the oxygen coordination steric hindrance of the second ether solvent is large (the number of carbon atoms / oxygen atoms in the molecule is ≥5), and the salt concentration and the viscosity of the electrolyte can be reduced; the fluorinated ether solvent can further reduce the lithium salt concentration and improve the miscibility of the electrolyte, and its fluorinated group can promote the formation of a stable lithium metal negative electrode passivation layer, thereby improving the deposition and stripping efficiency of the negative electrode metal. The electrolyte provided by the present invention can be used to assemble a lithium battery, and the assembled lithium battery has a long cycle life, a weak self-discharge effect, high conductivity, low viscosity, good wettability, and has a high commercial application value.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to a medium salt concentration electrolyte for lithium-sulfur batteries. Background Art

[0002] Since lithium metal has a very high theoretical specific capacity, lithium metal batteries using lithium metal as the negative electrode have outstanding theoretical energy density, so they have great potential as the next generation of secondary batteries. During the charge and discharge process, the electrolyte will continuously react with metallic lithium and be accompanied by the growth of lithium dendrites, thus causing battery capacity decay or short circuit. NCM positive electrode materials and sulfur-containing materials will also cause serious self-discharge and capacity decay of the battery accompanied by the dissolution of components. Elemental sulfur is abundant and inexpensive, with a very high theoretical specific capacity (1670mAh / g). However, traditional lithium-sulfur batteries face the problem of active material loss caused by the shuttle effect of the discharge intermediate product lithium polysulfide; and these problems seriously limit the large-scale commercial application of lithium-sulfur batteries.

[0003] In response to the above problems, researchers have proposed a series of different solutions, such as using transition metal ion-doped nanocarbon with adsorption capacity for lithium polysulfide as the support and coating structure of the positive electrode material, or using a surface-modified diaphragm, and using an ether electrolyte containing lithium nitrate additives that has a passivating effect on the negative electrode metal surface. Traditional ester electrolytes cannot effectively passivate the surface of the metal lithium negative electrode, resulting in low lithium stripping and deposition efficiency and short battery cycle life. In addition, the existing technical solution represented by the ethylene glycol dimethyl ether-1,3-dioxolane mixed solvent system still causes the lithium polysulfide shuttle effect, and the lithium deposition and stripping efficiency is generally less than 99%, and the battery cycle life is short. Although the previously reported local high-difficulty electrolyte based on fluoroether as a single diluent can inhibit the dissolution of the positive electrode active material, the fluoroether solvents are highly volatile and costly, and cannot promote the dissolution of lithium salts, resulting in this type of electrolyte being unable to meet commercial requirements. In addition, fluorobenzene and its homologues are potential low-cost alternatives to fluoroether solvents, but the solvents have low fluorine content and high toxicity. Therefore, the corresponding electrolyte cannot form an excellent passivation layer on the surface of lithium metal and is harmful to the environment. In addition, since the dissociation of lithium ions is greatly restricted, the electrolyte using a single low-viscosity inert solvent (e.g., n-butyl ether) exhibits very low lithium ion conductivity. Summary of the invention

[0004] In view of the above technical problems, the present invention provides a medium salt concentration electrolyte for lithium-sulfur batteries to solve the problems of rapid capacity decay under low-rate cycle conditions, severe self-discharge during storage, and severe side reactions between lithium negative electrodes and electrolytes in the prior art. The lithium metal battery electrolyte provided by the present invention contains a mixed lithium salt and a ternary mixed solvent composed of a diether-fluorinated ether, which can be used to assemble lithium metal batteries. The assembled lithium batteries can simultaneously meet the practical requirements of long cycle life and weak self-discharge effect, and the electrolyte has sufficiently high conductivity, low viscosity and excellent wettability, so it has high commercial value.

[0005] The technical solution provided by the present invention is as follows:

[0006] A medium salt concentration electrolyte for lithium-sulfur batteries: comprising a mixed lithium salt, a first ether solvent, a second ether solvent and at least one fluorinated ether solvent to form a diether-fluorinated ether tri-solvent system;

[0007] in,

[0008] The mixed lithium salt contains a lithium salt capable of forming a passivation film at a lithium metal negative electrode;

[0009] The first ether solvent has a small oxygen coordination steric hindrance, the number of carbon atoms / oxygen atoms in the molecule is ≤4, and can dissociate lithium salt;

[0010] The second ether solvent has a large oxygen coordination steric hindrance, and the number of carbon atoms / oxygen atoms in the molecule is ≥5, which can reduce the salt concentration and the viscosity of the electrolyte;

[0011] The fluorinated ether solvent can further reduce the lithium salt concentration and improve the miscibility of the electrolyte, and its fluorinated group can promote the formation of a stable lithium metal negative electrode passivation layer.

[0012] Furthermore, the first ether solvent is selected from one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether and tetrahydrofuran. Preferably, the first ether solvent is tetrahydrofuran.

[0013] Further, the second ether solvent is selected from tetrahydropyran, n-propyl ether, isopropyl ether, ethyl butyl ether, ethyl isobutyl ether, n-butyl ether, isobutyl ether, n-pentyl ether, and isopentyl ether. Preferably, the second ether solvent is isopropyl ether.

[0014] Furthermore, the fluorinated ether solvent is selected from one or more of 1-(2,2,2-trifluoroethoxy)-1,1,2,2-tetrafluoroethane, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, and di(2,2,2-trifluoroethyl) ether. Preferably, the fluorinated ether solvent is 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether isopropyl ether.

[0015] Furthermore, the first ether solvent is tetrahydrofuran, and the fluorinated ether solvent is 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether isopropyl ether, and the molar ratio of the two is 1:(0.1-3). The preferred molar ratio is 8:9.

[0016] Furthermore, the mixed lithium salt is selected from two or more of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium bis(fluorooxalatoborate), lithium hexafluoroarsenate or lithium perchlorate.

[0017] Furthermore, the mixed lithium salt is a mixture of lithium bis(trifluoromethylsulfonyl)imide and lithium bis(fluorosulfonyl)imide, with a molar ratio of 1:(0.1-3). The preferred molar ratio is 1:3.

[0018] Furthermore, the molar ratio of the first ether solvent to the second ether solvent is (0.1-10):1.

[0019] Furthermore, the molar ratio of the first ether solvent to the fluorinated ether solvent is 1:(0.1-3).

[0020] Furthermore, the total concentration of the mixed lithium salt is 1.0 mol / L to 2.0 mol / L, preferably 1.5 to 2.0 mol / L.

[0021] The beneficial effects of the present invention are as follows:

[0022] (1) The present invention uses a ternary mixed solvent composed of a diether-fluorinated ether solvent as a diluent for lithium-sulfur battery electrolyte. The chemically inert weak polar ether solvent (first and second ether solvents) effectively reduces the use ratio of fluorinated ether to reduce costs, improves the lithium salt dissociation ability and reduces the salt concentration and electrolyte viscosity; the use of fluorinated ether helps the electrolyte to form a robust passivation layer on the lithium metal negative electrode, improves the deposition and stripping efficiency of the negative electrode metal, and effectively inhibits side reactions. In addition, the introduction of fluorinated ether improves the miscibility of weak polar ether solvents and expands the operating temperature range of the electrolyte.

[0023] (2) The concentration of the mixed lithium salt in the electrolyte of the present invention is 2.0 mol / L to 1.0 mol / L. Under the premise of ensuring that the lithium negative electrode has a high deposition and stripping efficiency, the electrolyte within this salt concentration range has a low viscosity and synergistically inhibits the dissolution of the positive electrode active material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Lithium-lithium symmetric cycling tests for solution assembly of different embodiments.

[0025] Figure 2The lithium polysulfide solubility test of the solutions of different embodiments.

[0026] Figure 3 To test the stripping efficiency of lithium deposition by introducing different proportions of fluorinated ether solution.

[0027] Figure 4 This is the capacity retention test of the solution-assembled battery in control group 1 after long-term storage.

[0028] Figure 5 This is a test of the capacity retention rate of the solution-assembled battery of Example 2 after long-term storage. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the embodiments and the accompanying drawings. The specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0030] Example 1

[0031] A solution was prepared by mixing lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, tetrahydrofuran, isopropyl ether and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether in a molar ratio of 1:3:8:2:5.

[0032] Example 2

[0033] A solution was prepared by mixing lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, tetrahydrofuran, isopropyl ether and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether in a molar ratio of 1:3:8:2:9.

[0034] Example 3

[0035] A solution was prepared by mixing lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, tetrahydrofuran, isopropyl ether and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether in a molar ratio of 1:3:8:2:18.

[0036] Control group 1

[0037] The control group electrolyte was prepared by dissolving lithium bis(trifluoromethylsulfonyl)imide in a mixed solvent of ethylene glycol dimethyl ether and 1,3-dioxolane in a volume ratio of 1:1. The lithium salt concentration was 1 mol / L. This electrolyte is the most widely used electrolyte in current lithium-sulfur batteries.

[0038] Control group 2

[0039] A solution was prepared by mixing lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, tetrahydrofuran and isopropyl ether in a molar ratio of 1:3:8:2.

[0040] The performance test results of the electrolyte prepared in the embodiment are as follows:

[0041] The electrolytes of control group 2, example 1, example 2, and example 3 were respectively used together with lithium sheets, separators, and steel meshes to assemble lithium-lithium symmetrical batteries. -2 Long cycle test was carried out at a current density of 1 h each time. Figure 1 As shown, the control group 2, Example 1, and Example 2 can all achieve stable cycling for a relatively long time, and Examples 1 and 2 maintain relatively low polarization voltages. However, Example 3 exhibits a larger polarization voltage due to the lower lithium salt concentration, and short circuit occurs as the cycle time increases.

[0042] Add lithium polysulfide to the corresponding solutions of control group 1, control group 2, example 1, example 2, and example 3 and stir for 30 minutes to form a saturated solution. After standing for 1 week, observe the color of the solution. Figure 2 As shown, the control group 1 exhibits an opaque dark brown color, indicating that a large amount of lithium polysulfide has been dissolved, and Examples 1, 2, and 3 show lighter colors than the control group 2, which indicates that the use of a mixed diluent can further inhibit the dissolution of lithium polysulfide.

[0043] The solutions described in Control Group 2, Example 1, Example 2, and Example 3 were used together with lithium sheets, copper sheets, and separators to assemble lithium-copper half-cells. -2 The current density and 1 mAh cm -2 The deposition-stripping coulombic efficiency test was carried out at the surface capacity density. Figure 3 As shown, in Control Group 2 and Example 2, that is, when the molar ratio of tetrahydrofuran to fluorinated ether is 8:0 and 8:9, the corresponding lithium deposition and stripping efficiencies are 99.3% and 99.6%, respectively, and both can be stably cycled for not less than 300 times, indicating that the lithium deposition and stripping efficiency can be significantly improved by introducing an appropriate proportion of fluorinated ether.

[0044] The solution of control group 1 and example 2 was used together with lithium foil, separator, and sulfur positive electrode to assemble a button-like lithium-sulfur battery. After several charge and discharge cycles in the voltage range of 1.0V to 3.0V, the battery was discharged to 1.9V and then placed on standby for 10 days before continuing the cycle. Figure 4 As shown in the figure, the battery using the control group electrolyte showed obvious self-discharge after being left alone. Figure 5 As shown, the battery using the experimental group electrolyte of the present invention does not show capacity decay caused by self-discharge after being left alone.

[0045] The above description is only a preferred specific implementation manner of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention should be included in the scope of protection of the invention.

Claims

1. A medium salt concentration electrolyte for lithium-sulfur batteries, characterized in that: The electrolyte contains a mixed lithium salt, a first ether solvent, a second ether solvent and at least one fluorinated ether solvent to form a double ether-fluorinated ether three-solvent system; in, The mixed lithium salt contains a lithium salt capable of forming a passivation film on a lithium metal negative electrode; the mixed lithium salt is a mixture of lithium bis(trifluoromethylsulfonyl)imide and lithium bis(fluorosulfonyl)imide; The oxygen coordination steric hindrance of the first ether solvent is small, the number of carbon atoms / the number of oxygen atoms in the molecule is ≤4, and the lithium salt can be dissociated; the first ether solvent is tetrahydrofuran; The oxygen coordination steric hindrance of the second ether solvent is relatively large, and the number of carbon atoms / oxygen atoms in the molecule is ≥5, which can reduce the salt concentration and the viscosity of the electrolyte; the second ether solvent is isopropyl ether; The fluorinated ether solvent can further reduce the lithium salt concentration and improve the miscibility of the electrolyte, and its fluorinated group can promote the formation of a stable lithium metal negative electrode passivation layer; the fluorinated ether solvent is selected from one or more of 1-(2,2,2-trifluoroethoxy)-1,1,2,2-tetrafluoroethane, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, and di(2,2,2-trifluoroethyl) ether; A solution was prepared by mixing lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, tetrahydrofuran, isopropyl ether and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether in a molar ratio of 1:3:8:2:

9.

2. The electrolyte according to claim 1, characterized in that: The total concentration of the mixed lithium salt is 1.0 mol / L to 2.0 mol / L.

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