Electrochemical cell for lithium accumulator comprising a specific negative electrode made of metallic lithium and a positive electrode on aluminium collector

Abstract

An electrochemical cell for a lithium accumulator comprising: a negative electrode comprising metallic lithium as active material; a positive electrode associated with an aluminium current collector; and an electrolyte placed between the negative electrode and the positive electrode, wherein the negative electrode is provided with a layer comprising a compound containing aluminium at its face in contact with the electrolyte, and in that the electrolyte comprises at least one lithium salt chosen from among lithium imide, lithium triflate, lithium perchlorate salts and mixtures thereof.

Claims

1. Electrochemical cell for lithium accumulator comprising: a negative electrode consisting of an active material, the active material consisting of metallic lithium; a positive electrode associated with an aluminium current collector; and a liquid or gel electrolyte placed between said negative electrode and said positive electrode, characterised in that the negative electrode is provided at a face in contact with the electrolyte with a layer comprising a compound containing aluminium, said compound being an aluminium salt, and in that the electrolyte comprises at least one lithium salt chosen from among lithium imide, lithium triflate, lithium perchlorate salts and mixtures thereof.

2. Cell according to claim 1, wherein the positive electrode comprises an active material, the positive electrode active material chosen from among lithiated phosphates comprising at least one metallic transition element, lithiated oxides comprising at least one metallic element, lithium based metal sulfides, vanadium oxides, disulfides based on at least one metallic transition element, elementary sulfur and mixtures thereof.

3. Cell according to claim 2 wherein, when the positive electrode active material is a lithiated phosphate comprising at least one metallic transition element, it corresponds to a lithiated phosphate with formula LiM.sup.1PO.sub.4, wherein M.sup.1 is chosen from among Fe, Mn, Ni, Co and mixtures thereof.

4. Cell according to claim 2 wherein, when the positive electrode active material is a lithiated oxide comprising at least one metallic transition element, it corresponds to a lithiated oxide comprising nickel, cobalt, manganese and/or aluminium.

5. Cell according to claim 4, wherein the lithiated oxide comprising nickel, cobalt, manganese and/or aluminium satisfies the following formula:
Li.sub.1+xM.sup.2O.sub.2, wherein M.sup.2 is an element chosen from among Ni, Co, Mn, Al and mixtures thereof and x is greater than or equal to 0.

6. Cell according to claim 2 wherein, when the positive electrode active material is a lithium based metal sulfide, it corresponds to a lithium based metal sulfide of the “rock-salt” type.

7. Cell according to claim 2 wherein, when the positive electrode active material is a vanadium oxide, it corresponds to vanadium oxide V.sub.2O.sub.5.

8. Cell according to claim 2 wherein, when the positive electrode active material is a disulfide based on at least one metallic transition element, it corresponds to a crystallising disulfide in a pyrite type crystalline system.

9. Cell according to claim 1, wherein the liquid electrolyte comprises one or several organic solvents in the carbonate solvents family and/or the ether solvents family.

10. Cell according to claim 1, wherein the gel electrolyte is a liquid electrolyte impregnating a polymer matrix that gelifies in contact with the liquid electrolyte.

11. Cell according to claim 10, wherein the liquid electrolyte impregnating the polymer matrix is a liquid electrolyte comprising one or several organic solvents in the carbonate solvents family and/or the ether solvents family.

12. Cell according to claim 1 wherein, when the lithium salts are lithium imide salts, they are chosen from among lithium bis(trifluoromethane)sulfonylimide with formula Li[N(SO.sub.2CF.sub.3).sub.2), lithium bis(fluorosulfonyl)imidide with formula Li[N(SO.sub.2F).sub.2] and lithium bis(pentafluoroethane)sulfonylimide with formula Li[N(SO.sub.2CF.sub.2CF.sub.3).sub.2].

13. Cell according to claim 1, wherein: when the lithium salt is lithium bis(trifluoromethane)sulfonylimide, the positive electrode comprises, as active material, a lithiated phosphate comprising at least one metallic transition element, such as LiFePO.sub.4 and/or a disulfide based on at least one metallic transition element, such as TiS.sub.2 or FeS.sub.2 and/or a vanadium oxide, such as V.sub.2O.sub.5 and/or a metal sulfide based on lithium of the “rock-salt” type, such as Li.sub.2TiS.sub.3 or Li.sub.3NbS.sub.4; when the lithium salt is lithium perchlorate, the positive electrode comprises, as active material, elementary sulfur; when the lithium salt is lithium bis(fluorosulfonyl)imide, the positive electrode comprises, as active material, a lithiated phosphate comprising at least one metallic transition element, and/or a disulfide based on at least one metallic transition element, a vanadium oxide, and/or a metal sulfide based on lithium of the “rock-salt” type, and/or a lithiated oxide comprising at least one metal element; when the lithium salt is lithium triflate, the positive electrode comprises, as active material, elementary sulfur and/or a disulfide based on at least one metallic transition element, and/or a metal sulfide based on lithium of the “rock-salt” type; when the lithium salt is LiBETI, the positive electrode comprises, as active material, a lithiated phosphate comprising at least one metallic transition element, and/or elementary sulfur and/or a disulfide based on at least one metallic transition element, and/or a vanadium oxide and/or a metal sulfide based on lithium of the “rock-salt” type, and/or a lithiated oxide comprising at least one metallic element or an overlithiated oxide comprising manganese and nickel.

14. Cell according to claim 1, wherein the aluminium salt comprises, as anions, anions comprising one or more several halogen atoms.

15. A method of preparing or regenerating a cell as defined in claim 1, comprising a step consisting of subjecting: a cell comprising a negative electrode consisting of an active material, the active material consisting of metallic lithium; a positive electrode associated with an aluminium current collector; and an electrolyte located between said negative electrode and said positive electrode comprising at least one lithium salt chosen from among lithium imide, lithium triflate, lithium perchlorate salts and mixtures thereof; to a corrosion potential of aluminium for a sufficiently long time to obtain partial corrosion of the aluminium collector and the concomitant formation of a layer comprising a compound containing aluminium on the face of the negative electrode in contact with the electrolyte.

16. A method of preparing and electrochemical cell for a lithium accumulator, the method comprising depositing a layer comprising a compound containing an aluminium salt, on a face of a negative electrode, wherein the negative electrode consists of an active material, the active material consisting of metallic lithium wherein the face of the negative electrode is in contact with a liquid or gel electrolyte chosen from among lithium imide, lithium triflate, lithium perchlorate salts and mixtures thereof: and wherein the liquid or gel electrolyte is arranged between said face of the negative electrode and a positive electrode associated with an aluminium current collector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents a sectional view of a specific cell according to the invention;

(2) FIG. 2 is a graph illustrating the variation of the potential U (in V) (left ordinate) as a function of time (in h) (curve a) and the simultaneous variation of the current I (in A) (right ordinate) as a function of time (in h) (curve b) for a specific cell conforming with the invention.

(3) FIG. 3 is a graph illustrating the variation of the potential U (in V) (left ordinate) as a function of time (in h) (curve a) and the simultaneous variation of the current I (in A) (right ordinate) as a function of time (in h) (curve b) for a specific cell not conforming with the invention.

(4) FIG. 4 is a graph illustrating the variation of the discharge capacity C (in Ah) as a function of the number N of cycles, (curve a) for a specific cell conforming with the invention and curve b) for a specific cell not conforming with the invention-.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

EXAMPLE

(5) This example illustrates an electrochemical cell conforming with the invention and, for comparison, a cell not conforming with the invention (called first cell not conforming with the invention) so as to demonstrate the benefit of the layer comprising a compound containing aluminium on the lithium electrode, on the cyclability properties of this cell.

(6) In this example, before the formation cycle, said electrochemical cells are in the form of a button battery comprising a stack of layers as illustrated in appendix FIG. 1 comprising: a negative electrode 1 composed of metallic lithium in the form of a 16 mm diameter 150 μm thick disk deposited on stainless steel packing (not shown) that acts as a current collector; a positive electrode 3 comprising, as active material, LiFePO.sub.4, as conducting electricity material, carbon black and PVDF as polymeric binder in proportions by mass of 90.5/5/4.5 respectively, said positive electrode being deposited on an aluminium current collector 5 composed of a 20 μm thick disk with a surface area of 154 mm.sup.2; and a porous separator 7 composed of a 16.5 mm diameter and 25 μm thick polyolefin disk, said separator being soaked with a liquid electrolyte consisting, for the cell conforming with the invention, of a mixture of carbonate solvents (ethylene carbonate/dimethyl carbonate) and 1M LiTFSI and, for the cell not conforming with the invention, of a mixture of carbonate solvents (ethylene carbonate/dimethyl carbonate) and 1M LiPF.sub.6.

(7) Each of these cells is subjected to a formation cycle, particularly with the application of a fixed potential level of 4.5V as common characteristic (vs. Li.sup.+/Li) for a duration of 10 hours.

(8) The characteristics of the formation cycle and the current generated during this cycle are shown in FIG. 2 for the cell conforming with the invention, and on FIG. 3 for the cell not conforming with the invention, these figures illustrating firstly the variation of the potential U (in V) (left ordinate) as a function of the duration t (in h) (curves a) on FIG. 2 and FIG. 3) and the simultaneous variation of the current I (in A) (right ordinate) as a function of the duration in t (curves b) on FIG. 2 and FIG. 3).

(9) For the cell conforming with the invention, during the 4.5 V plateau, the first observation was a reduction in the current, followed by an increase representing a corrosion current indicating a partial corrosion phenomenon of the aluminium collector.

(10) More specifically, during the plateau at 4.5V, a capacity of 0.94 mAh is charged. Assuming the Al.fwdarw.Al.sup.3++3 e.sup.− corrosion reaction (3×96500 C per mol of corroded aluminium, namely 80.4 Ah/mol), 1.17×10.sup.−5 mol of aluminium was corroded. This corresponds to 0.3 mg of aluminium, or 0.1 mm.sup.3 of Al. The surface area of the collector in the button battery is 154 mm.sup.2. It is deduced that about 0.7 μm of collector was corroded out of a total of 20 μm, which does not jeopardise its electrical conduction phenomenon.

(11) For the cell not conforming with the invention, during the 4.5 V plateau, the first observation was a reduction in the current, followed by stabilisation of the current, which shows that there is no corrosion, this no corrosion continuing until at least 5 V.

(12) After this formation cycle, each cell is subjected to galvanostatic cycling in a range of potentials varying from 2.5 to 3.7 V and a current of 0.85 mA, the results being shown on the appended FIG. 4 illustrating the variation of the discharge capacity C (in Ah) as a function of the number of cycles N (curve a) for the cell conforming with the invention and curve b) for the cell not conforming with the invention).

(13) A significant improvement in cyclability of the cell according to the invention is found (with the capacity stabilising at at least 60 cycles) compared with the cell not conforming with the invention using LiPF.sub.6 as lithium salt not having been subjected to a partial corrosion phenomenon of the aluminium collector.

(14) The authors of this invention performed this galvanostatic cycling with a second series of cells (one cell conforming with the invention and one cell not conforming with the invention) satisfying the same specific features as mentioned above and having been subjected to a formation cycle identical to that defined above, galvanostatic cycling being stopped in this case at 6 cycles, with the objective of finding an explanation for this phenomenon.

(15) The cells were then opened and the lithium negative electrode was analysed by EDX spectroscopy.

(16) For the cell conforming with the invention, the presence of a layer comprising a compound containing aluminium was found on the lithium electrolyte, the aluminium originating from partial corrosion of the current collector.

(17) There is no such layer on the surface of the lithium electrode of the cell not conforming with the invention, and in particular there is no aluminium on the surface of the lithium electrode.

(18) It is thus deduced that the presence of this layer on the surface of the lithium electrode in contact with the liquid electrolyte containing LiTFSI contributes to improving cyclability of the cell conforming with the invention.

(19) Finally, to clearly demonstrate that the improvement in cyclability is not simply due to the lithium salt used, cells not conforming with the invention (more specifically a second cell not conforming with the invention using as electrolyte, LiPF.sub.6 in a mixture of EC/DMC carbonate solvents and a third cell not conforming with the invention using, as electrolyte, LiTSI in a mixture of EC/DMC carbonate solvents, these non-conforming cells being similar in other respects to the first cell not conforming with the invention relating to positive and negative electrodes) were manufactured and cycled between 2.5 and 3.7 V without application of a plateau at 4.5 V (namely a plateau possibly enabling formation of the layer comprising aluminium on the surface of the negative electrode). In both cases, cyclability is similar is similar to cyclability of the first cell not conforming with the invention.