Electrolyte for an alkali-sulfur battery, alkali-sulfur battery containing the electrolyte, and uses of the electrolyte

Abstract

The invention relates to an electrolyte, which is provided for an alkali-sulfur battery (e.g. for a Li—S battery). The electrolyte contains a non-polar, acyclic and non-fluorinated ether, a polar aprotic organic solvent, and a conducting salt for an alkali-sulfur battery. It has been found that, when such an electrolyte is used in an alkali-sulfur battery, a high-capacity, a low overvoltage, a high cycle stability, and a high Coulomb efficiency can be achieved in the alkali-sulfur battery and, in addition, as compared with an alkali-sulfur battery which contains a fluorinated ether in the electrolyte, a considerably improved gravimetric energy density is obtained. The invention further relates to a battery comprising the electrolyte according to the invention and to uses of the electrolyte according to the invention.

Claims

1. An alkali-sulfur battery containing an electrolyte for an alkali-sulfur battery, the electrolyte comprising (a) a nonpolar, acyclic ether with the chemical formula X—O—Y, wherein O is an oxygen atom; and X and Y are each a saturated or unsaturated linear or cyclic hydrocarbon residue, wherein the carbon residues of X and Y together have at least five carbon atoms; (b) a polar aprotic organic solvent, wherein the polar aprotic organic solvent is selected from an acyclic ether having the chemical formula A-(O—B)n-O—C, where O is an oxygen atom and A, B, and C each represent a saturated or unsaturated linear or cyclic hydrocarbon residue, where B includes at least 2 carbon atoms, and n ≥1, 1,3-dioxolane, acetonitrile, pyridine, dimethyl sulfoxide, sulfone, and mixtures thereof; and (c) a conductive salt for an alkali-sulfur battery, wherein the nonpolar acyclic ether is a non-fluorinated ether.

2. The alkali-sulfur battery in accordance with claim 1, wherein the carbon residues of X and Y together have (i) at least six carbon atoms; and/or (ii) have a maximum of 20 carbon atoms.

3. The alkali-sulfur battery in accordance with claim 1, wherein the carbon residue of X or Y has (i) at least two carbon atoms; and/or (ii) the carbon residue of X and/or Y respectively has a maximum of ten carbon atoms.

4. The alkali-sulfur battery in accordance with claim 1, wherein the nonpolar, acyclic, non-fluorinated ether has a density of 1.4 g/cm.sup.3.

5. The alkali-sulfur battery in accordance with claim 1, wherein the nonpolar, acyclic, non-fluorinated ether is selected from the group consisting of ethyl propyl ether, dipropyl ether, diisopropyl ether, di-(1,2-dimethyl propyl) ether, methyl butyl ether, ethyl butyl ether, propyl butyl ether, dibutyl ether, diisobutyl ether, methyl pentyl ether, ethyl pentyl ether, propyl pentyl ether, butyl pentyl ether, dipentyl ether, 1-(2,2-dimethyl propoxy)-2,2-dimethyl propane, isopentyl ether, methyl hexyl ether, ethyl hexyl ether, propyl hexyl ether, butyl hexyl ether, pentyl hexyl ether, dihexyl ether, methoxy cylohexane, phenetol, 2-methoxy propane, 2-methoxy butane, 2-methoxy pentane, 2-methoxy hexane, 2-ethoxy propane, 2-ethoxy butane, 2-ethoxy pentane, 2-ethoxy hexane, and mixtures thereof.

6. The alkali-sulfur battery in accordance with claim 1, wherein the polar aprotic, organic solvent has (i) a Gutmann donor number of ≥14; and/or (ii) a Gutmann acceptor number of 8.

7. The alkali-sulfur battery in accordance with claim 1, wherein the polar aprotic organic solvent is selected from the group consisting of 1,2-dimethoxyethane, 1,3-dioxolane, 2-methoxyethyl ether, bis(2-(2-methoxy ethoxy)ethyl) ether and mixtures thereof; and/or wherein B of the ether has a maximum of four carbon atoms and/or n is ≥4.

8. The alkali-sulfur battery in accordance with claim 1, wherein the volume ratio of the nonpolar, acyclic, non-fluorinated ether to the polar aprotic, organic solvent amounts to ≥1:1 (v/v).

9. The alkali-sulfur battery in accordance with claim 1, wherein the conductive salt (i) is selected from the group consisting of LiPF.sub.6, LiBF.sub.4, LiClO.sub.4, LiSO.sub.3CF.sub.3, LiN(SO.sub.2F).sub.2, LiN(SO.sub.2CF.sub.3).sub.2, LiN(SO.sub.2F)(SO.sub.2CF.sub.3), LiN(SO.sub.2F)(SO.sub.2C.sub.4F.sub.9), LiN(SO.sub.2CF.sub.2CF.sub.3).sub.2, LiC(SO.sub.2CF.sub.2CF.sub.3).sub.3, LiC(SO.sub.2CF.sub.3).sub.3, LiI, LiCl, LiF, LiPF.sub.5(SO.sub.2CF.sub.3), LiPF.sub.4(SO.sub.2CF.sub.3).sub.2, LiTFSI, and mixtures thereof; and/or (ii) is contained in a concentration of a maximum of 4 mol/l.

10. The alkali-sulfur battery in accordance with claim 1, wherein the electrolyte (i) does not contain any LiNO.sub.3, and/or (ii) is liquid, of gel form, or solid.

11. The alkali-sulfur battery in accordance with claim 1, which has a cathode that contains sulfur particles and a carbon material.

12. A method of preparing an electrolyte for an alkali-sulfur battery comprising providing a composition comprising hexylmethyl ether and 1,2-dimethoxyethane as the electrolyte.

13. The alkali-sulfur battery in accordance with claim 1, wherein the alkali-sulfur battery contains a cathode, an anode, and a separator.

14. The alkali-sulfur battery in accordance with claim 13, wherein the cathode comprises sulfur and the anode comprises an alkali metal.

15. The alkali-sulfur battery in accordance with claim 14, wherein the cathode comprises sulfur and carbon and the anode comprises lithium.

Description

(1) The subject matter in accordance with the invention will be explained in more detail with reference to the following Figures and examples without intending to restrict it to the specific embodiments shown here.

(2) FIG. 1 shows by way of example a plurality of non-fluorinated, nonpolar, acyclic ethers usable in the electrolyte in accordance with the invention and their properties with respect to the solution capability of the conductive salt LiTFSI, its density, and its melting point and boiling point;

(3) FIG. 2 shows by way of example a plurality of polar aprotic, organic solvents usable in the electrolyte in accordance with the invention and their Gutmann donor numbers and acceptor numbers; and

(4) FIG. 3 shows charge curves and discharge curves for the cycles 1, 2, 5, and 10 of a lithium sulfur battery that contains an electrolyte in accordance with the invention and has been produced as disclosed in Example 3.

1ST EXAMPLE—ELECTROLYTE IN ACCORDANCE WITH THE INVENTION FOR AN ALKALI-SULFUR BATTERY

(5) A preferred electrolyte in accordance with the invention for an alkali-sulfur battery (e.g. for an Li—S battery) contains:

(6) 80% v/v hexyl methyl ether

(7) 20% v/v 1,2-dimethoxyethane

(8) 2 M LiTFSI

2ND EXAMPLE—SPECIFIC EXAMPLES FOR NONPOLAR, ACYCLIC, NON-FLUORINATED ETHERS

(9) Some non-fluorinated, nonpolar, acyclic ethers that can be used in the electrolyte in accordance with the invention will be named by way of example in the following: methoxy cyclohexane (density 0.88 g/mL); ethyl propyl ether (density 0.88 g/mL); butyl ethyl ether (density 0.88 g/mL); hexyl methyl ether (density 0.88 g/mL); dihexyl ether (density 0.88 g/mL); dibutyl ether (density 0.88 g/mL); diisopropyl ether (density 0.88 g/mL); phenetol (density 0.88 g/mL).

(10) All of the above-named non-fluorinated, nonpolar, acyclic ethers have a density of ≤0.97 g/mL that is considerably below the density of fluorinated, acyclic ethers used in the prior art whose density is typically below 1.4 g/mL.

(11) An overview of further properties of these ethers can be found in FIG. 1.

3RD EXAMPLE—ALKALI-SULFUR BATTERY WITH AN ELECTROLYTE IN ACCORDANCE WITH THE INVENTION

(12) The cathode was first provided for a Li—S battery as an alkali-sulfur battery in that a C/S composite (1:2, m:m) was manufactured by melting carbon (C) and sulfur (S) in a drying cabinet at 155° C. for 30 min. The C/S composite (97 wt %) was ground with PTFE (3 wt %) and was pressed at an elevated temperature in a milling process to form a cathode film. A lamination onto an aluminum film primed with carbon subsequently takes place. The sulfur load of the cathode amounted to approximately 2.1-2.2 mg-S/cm.sup.2, it density to approximately 0.5 g/cm.sup.3.

(13) The provided cathode was subsequently installed in a button cell (CR2016). A button cell (CR2016) having the following composition was assembled for this purpose: 1×cathode (see above), diameter 15 mm; 1×separator (Toray F.sub.12BMS), diameter 19 mm; 1×250 μm lithium film as the anode (China Energy Lithium), diameter 16.5 mm; 1×1 mm V2A spacer (16 mm diameter).

(14) An electrolyte comprising 2 M LiTFSI (BASF, 99.9%) in 80% v/v hexyl methyl ether (HME, TCl, min. 98%), and 20% v/v 1,2-dimethoxyethane (DME, BASF 99.98%) was subsequently added to the button cell for the manufacture of the Li—S battery. The electrolyte-to-sulfur ratio amounted to 5 μl electrolyte/mg sulfur.

(15) The Li—S battery was galvanostatically measured after the manufacture. The cycling (charging/discharging) took place between 1.5 V and 2.8 V vs. Li/li+ at a rate of C/10. In the first discharge step, a forming of the cell took place with a discharge at a rate of C/20.

(16) The result is shown in FIG. 3. The charge/discharge curves shown illustrate the cycles 1, 2, 5, and 10.

(17) On discharging, on a use of the electrolyte 2 M LiTFSI in the nonpolar, acyclic, non-fluorinated ether hexyl methyl ether (HME) and the ether 1,2-dimethoxyethane (DME) 8:2, v/v) a discharge voltage was reached of a constant approximately 2.1 V vs. Li/Li.sup.+. A pronounced first plateau at 2.1 V to 2.3 V vs. Li/Li.sup.+ is not obtained with this electrolyte.

(18) In the charge process, a high overpotential was first observed that is caused by an Li.sub.2S activation at the start of the charge process. A pronounced plateau was also displayed at approximately 2.3 V vs. Li/Li.sup.+ on charging.

(19) The suppression of a second plateau on charging and discharging indicates a suppression of long chain polysulfides, whereby it is shown that the polysulfide shuttle mechanism can also be successfully suppressed by the electrolyte in accordance with the invention without the addition of further additives.