Lithium electrochemical accumulator of the lithium-sulfur type comprising a specific negative-electrode material

Abstract

A lithium-sulfur accumulator comprising at least one electrochemical cell comprising a positive electrode comprising, as active material, at least one sulfur-containing material, a negative electrode and an electrolyte conducting lithium ions disposed between the negative electrode and the positive electrode, wherein the negative electrode comprises, as active material, a lithium and calcium alloy, wherein the calcium is present in the alloy to the extent of 2% to 34% atomic.

Claims

1. A lithium-sulfur accumulator comprising at least one electrochemical cell comprising a positive electrode comprising, as active material, at least one sulfur-containing material, a negative electrode and an electrolyte conducting lithium ions disposed between said negative electrode and said positive electrode, characterised in that the negative electrode consists solely of an active material, which is a lithium and calcium alloy, wherein the calcium is present in the alloy to the extent of 2% to 34% atomic.

2. The lithium-sulfur accumulator according to claim 1, wherein the negative electrode is in the form of a sheet or plate having a thickness ranging from 5 to 200 μm.

3. The lithium-sulfur accumulator according to claim 1, wherein the lithium and calcium alloy comprises solely lithium and calcium.

4. The lithium-sulfur accumulator according to claim 1, wherein the lithium and calcium alloy is a lithium alloy comprising calcium to the extent of 2% to 15% atomic.

5. The lithium-sulfur accumulator according to claim 1, wherein the calcium in the lithium and calcium alloy is wholly or partly in the form of CaLi.sub.2.

6. The lithium-sulfur accumulator according to claim 1, wherein the negative electrode is self-supporting.

7. The lithium-sulfur accumulator according to claim 1, wherein the sulfur-containing active material is elementary sulfur (S.sub.8), lithium disulfide (Li.sub.2S) or a compound comprising at least one disulfide group —S—S—.

8. The lithium-sulfur accumulator according to claim 1, wherein the sulfur-containing active material is elementary sulfur.

9. The lithium-sulfur accumulator according to claim 1, wherein the positive electrode further comprises an electrically conductive additive and optionally an organic binder.

10. The lithium-sulfur accumulator according to claim 9, wherein the electrically conductive additive is a carbon black powder, carbon nanotubes, carbon fibres, metal particles, conductive polymers or mixtures thereof.

11. The lithium-sulfur accumulator according to claim 9, wherein the organic binder is a polymeric binder.

12. The lithium-sulfur accumulator according to claim 1, wherein the electrolyte conducting lithium ions is a liquid electrolyte comprising at least one organic solvent and at least one lithium salt.

13. The lithium-sulfur accumulator according to claim 12, wherein the organic solvent or solvents are chosen from solvents comprising one or more ether, nitrile, sulfone and/or carbonate functions.

14. Lithium-sulfur accumulator according to claim 12, wherein the organic solvent is an ether solvent or a mixture of ether solvents.

15. The lithium-sulfur accumulator according to claim 12, wherein the lithium salt is chosen from the group consisting of LiPF.sub.6, LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, LiI, LiNO.sub.3, LiOH, LiRfSO.sub.3, with R.sub.f corresponding to a perfluoroalkyl group comprising 1 to 8 carbon atoms, LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiN(SO.sub.2F).sub.2, LiCH.sub.3SO.sub.3 or LiB(C.sub.2O.sub.4).sub.2.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a graph illustrating the change in the potential E (in V) as a function of the capacity C (in an arbitrary unit, u.a) for a conventional lithium-sulfur accumulator.

(2) FIG. 2 is a graph illustrating the change in the potential E (in V) as a function of the time t (in s), curve a) corresponding to that obtained for the first accumulator, curve b) corresponding to that obtained for the second accumulator and curve c) corresponding to that obtained for the third accumulator of the example disclosed below.

DETAILED DISCLOSURE OF A PARTICULAR EMBODIMENT

Example

(3) The following example illustrates two lithium-sulfur accumulators according to the invention, each in the form of a button battery, comprising: a negative electrode consisting of a disc with a diameter of 16 mm and a thickness of 100 μm and composed of a lithium and calcium alloy with calcium to the extent of 2% atomic (for the first accumulator) or calcium at 25% atomic (for the second accumulator); a positive electrode consisting of a 16 mm diameter disc composed of a composite material comprising polyvinylidene fluoride to the extent of 10% by mass and a sulfur/carbon black mixture to the extent of 90% by mass (the sulfur representing 66% by mass of the mixture), said positive electrode being deposited on an aluminium current collector 20 μm thick; between the negative electrode and the positive electrode, a Celgard® 2325 separator (25 μm) coupled to a non-woven fabric of polyolefins of the Viledon® type (140 μm), the assembly forming a 16 mm diameter disc, said assembly being impregnated with an electrolyte consisting of a mixture of TEGDME/DIOX solvents in 50/50 proportions and a 1 M lithium salt LiTFSI.

(4) By way of comparison, another lithium-sulfur accumulator not in accordance with the invention (referred to as the third accumulator) was produced in this example, this accumulator meeting the same specificities as those of the invention except that the negative electrode is made from pure lithium.

(5) These three accumulators were subjected to cycling tests at a rating of C/20-D/20 with a capacity calculated according to the quantity of sulfur introduced into the positive electrode.

(6) The results are set out in FIG. 2, which illustrates the change in potential E (in V) as a function of time t (in s), curve a) corresponding to that obtained for the first accumulator, curve b) corresponding to that obtained for the second accumulator and curve c) corresponding to that obtained for the third accumulator.

(7) These three curves have a trend characteristic of a lithium-sulfur accumulator in a charging-discharging system, each having two discharging and charging levels. The duration of discharge for the accumulators according to the invention is 6 to 8 hours, which attests to the possibility of effectively using a lithium and calcium alloy for forming negative electrodes.

(8) Furthermore, the presence of calcium in the lithium alloy constituting the negative electrode helps to make the alloy easier to roll than pure lithium, which makes it possible to envisage the use of such an alloy for forming thinner negative electrodes and thus larger developed surfaces, which may prove a major asset in accumulators intended for power applications.