Liquid-liquid two-phase electrolyte and its use in the preparation of lithium-oxygen batteries

Abstract

The application discloses a liquid-liquid two-phase electrolyte and application thereof in preparation of a lithium-oxygen battery, wherein two phases of the electrolyte are dimethyl sulfoxide solvent and octyltrimethoxysilane solvent respectively; the electrolyte comprises an electrolyte lithium salt; a redox medium is dissolved in the dimethyl sulfoxide solvent, and the redox medium is insoluble in the octyltrimethoxysilane solvent. Compared with the prior art, the application has the following advantages: (1) without introducing other physical structures, the application can block the shuttle of the redox medium in the lithium-oxygen battery by designing two-phase electrolyte of solvents with different polarities, thereby obviously reducing the overpotential in the lithium-oxygen battery and improving the cycle stability of the battery; and (2) the electrolyte solves the shuttle problem and simplifies the assembly process of the lithium-oxygen battery, thereby reducing the failure problem of the fragile film-shaped material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium-oxygen batteries, and relates to an electrolyte in a lithium-oxygen battery, in particular to a liquid-liquid two-phase electrolyte and application thereof in preparation of a lithium-oxygen battery. BACKGROUND

[0002] The theoretical energy density of a lithium-oxygen battery is as high as 3500 Wh / kg, and development of a high-efficiency and stable lithium-oxygen battery is of great significance to energy storage. However, due to problems such as slow oxidation kinetics of lithium peroxide as a discharge product, electrolyte decomposition, and positive electrode pore blockage, the lithium-oxygen battery has the disadvantages of large charging overpotential, poor cycle stability, and low actual capacity. Use of a redox mediator (RM) can effectively solve the above problems.

[0003] The RM in the charging process converts the limited solid-solid contact between the positive electrode and lithium peroxide into a more extensive solid-liquid contact, reduces the high charging potential of lithium peroxide to the low potential of the RM, and improves the cycle performance of the battery. However, the RM moves to the negative electrode and reacts with lithium metal during the charging and discharging process, resulting in a shuttle effect, which leads to inactivation of the RM and reduces the cycle life of the battery.

[0004] To solve the problem of RM shuttling, researchers use polymer membranes, solid-state electrolytes, or composite layers to physically hinder the RM between the positive and negative electrodes, which blocks the RM shuttling but also causes the problems of complexity in battery assembly and failure of the fragile film-shaped material. SUMMARY

[0005] The technical problem solved by the application is that, in order to overcome the deficiencies of the prior art, the application forms a liquid-liquid two-phase interface that is immiscible by using dimethyl sulfoxide solvents and octyltrimethoxysilane solvents of different polarities, so that the redox mediator can only be dissolved in the dimethyl sulfoxide solvent and not in the octyltrimethoxysilane solvent, thereby blocking the shuttling of the redox mediator in the lithium-oxygen battery. In addition, the wettability of the octyltrimethoxysilane solvent to lithium metal is better than that of the dimethyl sulfoxide solvent, which can directly block the contact between the dimethyl sulfoxide solvent and lithium metal, thereby inhibiting the side reaction between the dimethyl sulfoxide solvent and lithium metal and ensuring excellent cycle stability of the lithium-oxygen battery.

[0006] The technical solution is a liquid-liquid two-phase electrolyte, the two phases of the electrolyte are dimethyl sulfoxide solvent and octyltrimethoxysilane solvent; the electrolyte includes an electrolyte lithium salt, the concentration of which is 0.5-1 mol / L; the redox mediator is dissolved in the dimethyl sulfoxide solvent, the concentration of which is 0.02-0.1 mol / L, and the redox mediator is insoluble in the octyltrimethoxysilane solvent.

[0007] Preferably, the electrolyte lithium salt is lithium bistrifluoromethanesulfonylimide or lithium hexafluorophosphate.

[0008] Preferably, the redox mediator is N-methyl phenothiazine or lithium bromide.

[0009] Preferably, the concentration of the electrolyte lithium salt in the dimethyl sulfoxide solvent and the octyl trimethoxysilane solvent is 1 mol / L.

[0010] Preferably, the concentration of the redox mediator in the dimethyl sulfoxide solvent is 0.1 mol / L.

[0011] The application of any of the above-mentioned liquid-liquid two-phase electrolyte in the preparation of a lithium-oxygen battery.

[0012] Preferably, the positive electrode side electrolyte of the lithium-oxygen battery is a dimethyl sulfoxide solvent phase, and the negative electrode side electrolyte is an octyl trimethoxysilane solvent phase. Dimethyl sulfoxide (DMSO) and octyl trimethoxysilane (TOOS) are mutually insoluble and naturally form a liquid-liquid two-phase interface under the action of gravity; the contact angle of TOOS with lithium metal is 0°, and the contact angle of DMSO with lithium metal is 47.1°, so the wettability of TOOS with lithium metal is better than that of DMSO, which can directly block the contact between DMSO and lithium metal.

[0013] Preferably, the dimethyl sulfoxide solvent and the octyl trimethoxysilane solvent are respectively dropped onto the liquid-absorbing film, and assembled in the order of negative electrode, liquid-absorbing film with dropped octyl trimethoxysilane solvent, liquid-absorbing film with dropped dimethyl sulfoxide solvent, and positive electrode.

[0014] Preferably, the lithium-oxygen battery is a button cell or a Swagelok cell.

[0015] Preferably, the negative electrode of the lithium-oxygen battery uses lithium metal; the positive electrode includes a porous carbon positive electrode material and a current collector, wherein the porous carbon positive electrode material is pressed on the current collector by rolling film or coated on the current collector by stirring slurry; the porous carbon positive electrode material is at least one of conductive carbon black, Ketjen black, single-walled carbon nanotubes, and graphene.

[0016] The design idea and principle of the liquid-liquid two-phase electrolyte of the application are as follows: from the design idea, the application forms a liquid-liquid two-phase interface that is not mutually soluble by using dimethyl sulfoxide solvents and octyltrimethoxysilane solvents with different polarities, so that the redox medium can only be dissolved in the dimethyl sulfoxide solvent and cannot be dissolved in the octyltrimethoxysilane solvent, thereby blocking the shuttle of the redox medium in the lithium-oxygen battery. In addition, the wettability of the octyltrimethoxysilane solvent to lithium metal is better than that of the dimethyl sulfoxide solvent, which can directly block the contact between the dimethyl sulfoxide solvent and the lithium metal, thereby inhibiting the side reaction between the dimethyl sulfoxide solvent and the lithium metal and ensuring that the lithium-oxygen battery has excellent cycle stability. From the working principle, the liquid-liquid two-phase electrolyte prepared by the application is used in a lithium-oxygen battery, which is assembled in the order of a negative electrode, a liquid-absorbing film with the octyltrimethoxysilane solvent dropped, a liquid-absorbing film with the dimethyl sulfoxide solvent dropped, and a positive electrode. Therefore, a two-phase that is not mutually soluble is formed at the contact surface of the two liquid-absorbing films, N-methyl phenothiazine is locked on one side of the liquid-absorbing film with the dimethyl sulfoxide solvent dropped, the shuttle is blocked, the metal lithium negative electrode is protected, and the cycle stability of the lithium-oxygen battery is improved.

[0017] In view of the above design idea and working principle, the application has the following advantages:

[0018] (1) Without introducing other physical structures, the application can block the shuttle of the redox medium in the lithium-oxygen battery by designing a two-phase electrolyte of solvents with different polarities, thereby significantly reducing the overpotential in the lithium-oxygen battery and improving the cycle stability of the battery.

[0019] (2) The electrolyte of the application solves the problem of shuttle while simplifying the assembly process of the lithium-oxygen battery and reducing the failure problem of the fragile film-shaped material. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a schematic diagram of the TOOS / DMSO two-phase electrolyte of the application blocking the RM shuttle;

[0021] Figure 2 is an optical picture of the TOOS / DMSO two-phase electrolyte of the application blocking the RM shuttle;

[0022] Figure 3 is an electrochemical curve of the TOOS / DMSO two-phase electrolyte of the application blocking the RM shuttle;

[0023] Figure 4 is a full charge-discharge curve of the lithium-oxygen battery assembled based on the TOOS / DMSO two-phase electrolyte of the application;

[0024] Figure 5is the constant volume cycle curve of a lithium-oxygen battery assembled based on the TOOS / DMSO two-phase electrolyte described in the application;

[0025] Figure 6 is the SEM characterization result of the positive electrode sheet of a lithium-oxygen battery prepared based on Super P as the carbon positive electrode in the application after initial condition, discharge and charge;

[0026] Figure 7 is the XRD and Raman characterization result of the positive electrode sheet of a lithium-oxygen battery prepared based on Super P as the carbon positive electrode in the application after initial condition, discharge and charge;

[0027] Figure 8 is the voltage curve of a Li-Li symmetric battery assembled based on the TOOS / DMSO two-phase electrolyte and the DMSO single-phase electrolyte described in the application. DETAILED DESCRIPTION

[0028] The following examples further illustrate the content of the application, but should not be understood as limiting the application. Modifications and replacements of the method, steps or conditions of the application, without departing from the spirit and essence of the application, all belong to the scope of the application. If not specifically indicated, the technical means used in the examples is the conventional means familiar to those skilled in the art.

[0029] Example 1

[0030] The present embodiment provides a preparation method of a liquid-liquid two-phase electrolyte based on mutually immiscible dimethyl sulfoxide (DMSO) and octyl trimethoxysilane (TOOS) solvents, comprising the following steps:

[0031] Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is dried in a vacuum oven at 80℃ for 12 hours, and then the dried LiTFSI is dissolved in the DMSO solvent and stirred uniformly, N-methyl phenothiazine (MPT) is added to the obtained DMSO / LiTFSI solution and stirred uniformly to obtain the positive electrode side electrolyte, i.e. the DMSO single-phase electrolyte; LiTFSI is dissolved in the TOOS solvent and stirred uniformly to obtain the negative electrode side electrolyte. The two kinds of electrolytes are mixed and can spontaneously separate into two layers, i.e. the TOOS / DMSO two-phase electrolyte described in the application is obtained. The concentration of LiTFSI is 1 mol / L -1 , and the concentration of MPT is 0.1 mol / L -1 .

[0032] Example 2

[0033] The present embodiment provides a preparation method of a liquid-liquid two-phase electrolyte based on mutually immiscible dimethyl sulfoxide (DMSO) and octyl trimethoxysilane (TOOS) solvents, comprising the following steps:

[0034] Lithium hexafluorophosphate (LiPF6) was dried in a vacuum oven at 80°C for 12 hours, and then the dried LiPF6 was dissolved in DMSO solvent and stirred evenly. N-methylphenothiazine (MPT) was added to the obtained DMSO / LiPF6 solution and stirred evenly to obtain the positive electrode side electrolyte, i.e., DMSO single-phase electrolyte; LiPF6 was dissolved in TOOS solvent and stirred evenly to obtain the negative electrode side electrolyte. The two electrolytes were mixed and spontaneously separated to obtain the TOOS / DMSO two-phase electrolyte of the present invention. The concentration of LiPF6 was 0.8 mol L -1 The concentration of MPT was 0.05 mol L -1 .

[0035] Example 3

[0036] This embodiment provides a TOOS / DMSO two-phase electrolyte to block RM + The shuttle proof method includes the following steps:

[0037] According to the method of Example 2, two-phase and single-phase electrolytes were prepared respectively, placed in a transparent electrochemical cell, and current was applied to oxidize MPT to MPT. + , in situ observation of MPT + The diffusion behavior of Figure 2 As shown, in a single-phase electrolyte, after 5 minutes of oxidation, the dark brown MPT + diffuses to all areas of the electrolyte; in a two-phase electrolyte, MPT + is restricted to the positive side.

[0038] Charging curve Figure 3 As shown in the figure, the voltage of the battery using two-phase electrolyte increases and charging stops after all MPT is oxidized, while the battery using single-phase electrolyte produces obvious overcharging due to the shuttle effect.

[0039] Example 4

[0040] This embodiment provides an assembly operation of a lithium-oxygen battery, wherein the electrolyte described in this embodiment is prepared by Example 2.

[0041] A certain mass of Super P positive electrode carbon material (90wt%) was mixed with polyvinylidene fluoride (PVDF, 10wt%) binder, and then N-methylpyrrolidone solvent was added and stirred continuously for 2 hours to form a uniformly mixed slurry. The prepared slurry was evenly coated on a carbon paper disc with a diameter of 10mm and placed in a vacuum drying oven to dry for 12 hours to obtain a positive electrode sheet.

[0042] The lithium-oxygen battery was assembled according to the order of negative electrode sheet, liquid-absorbing film (dropping negative electrode TOOS electrolyte), liquid-absorbing film (dropping positive electrode DMSO electrolyte), and positive electrode. The negative electrode was a lithium sheet with a diameter of 14 mm.

[0043] The assembled battery was connected to a LAND battery test system for constant current charge and discharge test after being introduced into a certain amount of oxygen. The current rate was 100 mA g -1 , the charge cut-off voltage was 4.5 V, and the discharge cut-off voltage was 2 V. The full charge and discharge curve of the battery is shown in Figure 4 , and the discharge specific capacity of the battery can reach 2500 mAh g -1 .

[0044] Example 5

[0045] This example provides the assembly operation of a lithium-oxygen battery, wherein the electrolyte described in this example is prepared by Example 1.

[0046] A certain mass of Super P positive electrode carbon material (90 wt%) was mixed with a polyvinylidene fluoride (PVDF, 10 wt%) binder, and then N-methyl pyrrolidone solvent was added. After continuous stirring for 2 hours, a uniformly mixed slurry was formed. The prepared slurry was uniformly coated on a carbon paper disc with a diameter of 10 mm, and then placed in a vacuum drying box for drying for 12 hours to obtain a positive electrode sheet.

[0047] The lithium-oxygen battery was assembled according to the order of negative electrode sheet, liquid-absorbing film (dropping negative electrode TOOS electrolyte), liquid-absorbing film (dropping positive electrode DMSO electrolyte), and positive electrode. The negative electrode was a lithium sheet with a diameter of 14 mm.

[0048] A lithium-oxygen battery was assembled according to the above method using a DMSO single-phase electrolyte as a control.

[0049] The assembled battery was connected to a LAND battery test system for constant current charge and discharge test after being introduced into a certain amount of oxygen. The current rate was 500 mA g -1 , the limited capacity was 1000 mAh g -1 , and the obtained battery constant-volume cycle curve is Figure 5 . The battery can be stably cycled for 200 times at a charge platform of 3.6 V, while the cycle life of the battery without using a two-phase electrolyte is less than 20 cycles.

[0050] Figure 6 SEM characterization results of the positive electrode sheet of the lithium-oxygen battery under initial conditions, after discharge, and after charging. It can be seen that circular ring-shaped discharge products are generated on the sheet after discharge, and the products are completely decomposed after charging.

[0051] Figure 7are the XRD and Raman characterization results of the positive electrode after initial condition, discharge and charge. It can be seen that the discharge products of lithium-oxygen batteries using two-phase electrolyte and single-phase electrolyte are both lithium peroxide, and the products are completely decomposed after charging.

[0052] Example 6

[0053] This example provides the assembly and testing method of Li-Li symmetric battery using the TOOS / DMSO two-phase electrolyte and DMSO single-phase electrolyte prepared in Example 1, the specific steps are as follows:

[0054] A 2032 button cell was assembled in the order of lithium sheet, liquid-absorbing film (dropping negative electrode TOOS electrolyte), liquid-absorbing film (dropping positive electrode DMSO electrolyte), lithium sheet, wherein the diameter of lithium sheet was 12 mm.

[0055] Li-Li symmetric battery was assembled according to the above method using DMSO single-phase electrolyte as a control.

[0056] The assembled battery was connected to a LAND battery test system for testing, and the current density was 0.1 mA cm -2 , and the battery voltage distribution is shown in Figure 8 . The battery using two-phase electrolyte can maintain stable polarization for 800 h, while the battery using single-phase electrolyte has a sharp increase in polarization after 100 h.

Claims

1. A liquid-liquid two-phase electrolyte for preparing a lithium-oxygen battery, characterized in that: The two phases of the electrolyte are dimethyl sulfoxide solvent and octyltrimethoxysilane solvent respectively; the electrolyte includes an electrolyte lithium salt with a concentration of 0.5-1 mol / L; a redox mediator is dissolved in the dimethyl sulfoxide solvent with a concentration of 0.02-0.1 mol / L, and the redox mediator is insoluble in the octyltrimethoxysilane solvent; Wherein, the redox mediator is N-methylphenothiazine or lithium bromide; The electrolyte on the positive electrode side of the lithium oxygen battery is a dimethyl sulfoxide solvent phase, and the electrolyte on the negative electrode side is an octyltrimethoxysilane solvent phase.

2. The liquid-liquid two-phase electrolyte for preparing a lithium oxygen battery according to claim 1, characterized in that: The electrolyte lithium salt is lithium bis(trifluoromethanesulfonyl)imide or lithium hexafluorophosphate.

3. The liquid-liquid two-phase electrolyte for preparing a lithium oxygen battery according to claim 1, characterized in that: The concentration of the electrolyte lithium salt in the dimethyl sulfoxide solvent and the octyltrimethoxysilane solvent is 1 mol / L.

4. The liquid-liquid two-phase electrolyte for preparing a lithium oxygen battery according to claim 1, characterized in that The concentration of the redox mediator in the dimethyl sulfoxide solvent was 0.1 mol / L.

5. The liquid-liquid two-phase electrolyte for preparing a lithium oxygen battery according to claim 1, characterized in that: The dimethyl sulfoxide solvent and octyltrimethoxysilane solvent are respectively added dropwise to the liquid-absorbing membrane, and assembled in the order of negative electrode, liquid-absorbing membrane with octyltrimethoxysilane solvent added dropwise, liquid-absorbing membrane with dimethyl sulfoxide solvent added dropwise, and positive electrode.

6. The liquid-liquid two-phase electrolyte for preparing a lithium-oxygen battery according to claim 1, characterized in that: The lithium oxygen battery is a button cell or a Swagelok mold cell.

7. The liquid-liquid two-phase electrolyte for preparing a lithium oxygen battery according to claim 1, characterized in that: The negative electrode of the lithium-oxygen battery uses lithium metal; the positive electrode includes a porous carbon positive electrode material and a current collector, wherein the porous carbon positive electrode material is pressed on the current collector by rolling a film or slurrying and coating the current collector; the porous carbon positive electrode material is at least one of conductive carbon black, Ketjen black, single-walled carbon nanotubes and graphene.

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