LITHIUM ELECTRODES FOR LITHIUM-SULPHUR BATTERIES

Abstract

The present invention pertains to a process for manufacturing a film, said process comprising: (i) providing a composition [composition (C)] comprising, preferably consisting of: —at least one fluoropolymer [polymer (F)] comprising recurring units derived from at least one fluorinated monomer comprising a —SO 3 M functional group, wherein M is an alkaline metal [monomer (FM)] and—a liquid medium [medium (L)] comprising at least 50% by weight, based on the total weight of said medium (L), of at least one alkyl carbonate; (ii) processing the composition (C) provided in step (i) into a film; and (iii) drying the film provided in step (ii). The present invention further pertains to use of said film in a process for manufacturing a lithium electrode and to use of said lithium electrode in a process for manufacturing a lithium-sulphur battery.

Claims

1. A process for manufacturing a film, said process comprising: processing a composition (C) into a film, said composition (C) comprising: at least one fluoropolymer [polymer (F)] comprising recurring units derived from at least one fluorinated monomer comprising a —SO.sub.3M functional group, wherein M is an alkaline metal [monomer (FM)] and a liquid medium (L) comprising at least 50% by weight, based on the total weight of said medium (L), of at least one alkyl carbonate; and drying the film.

2. The process according to claim 1, wherein the composition (C) is in the form of a solution.

3. The process according to claim 1, wherein the polymer (F) comprises recurring units derived from: at least one monomer (FM) selected from the group consisting of fluorovinylethers of formula CF.sub.2═CF—O—(CF.sub.2).sub.m—SO.sub.3Li, wherein m is an integer comprised between 1 and 6, and tetrafluoroethylene.

4. The process according to claim 1, wherein the alkyl carbonate is selected from the group consisting of linear alkyl carbonates of formula (I) and cyclic alkylene carbonates of formula (II): ##STR00003## wherein: R.sub.a and R.sub.b, equal to or different from each other, are independently C.sub.1-C.sub.6 alkyl groups, and R.sub.c is a hydrogen atom or a C.sub.1-C.sub.6 alkyl group.

5. The process according to claim 1, wherein medium (L) further comprises at least one alkyl ether.

6. A film obtained by the process according to claim 1.

7. The film according to claim 6, said film comprising at least one layer comprising at least one fluoropolymer [polymer (F)] comprising recurring units derived from at least one fluorinated monomer comprising a —SO.sub.3M functional group, wherein M is an alkaline metal [monomer (FM)].

8. An electrode comprising a current collector, said current collector comprising: at least one lithium layer, and adhered to said at least one lithium layer, the film according to claim 6.

9. A process for manufacturing the electrode according to claim 8, said process comprising: applying a composition (C) onto the at least one lithium layer of a current collector comprising at least one lithium layer, said composition (C) comprising: at least one fluoropolymer [polymer (F)] comprising recurring units derived from at least one fluorinated monomer comprising a —SO.sub.3M functional group, wherein M is an alkaline metal [monomer (FM)] and a liquid medium (L) comprising at least 50% by weight, based on the total weight of said medium (L), of at least one alkyl carbonate; and drying the film.

10. A process for manufacturing the electrode according to claim 8, said process comprising: applying a film onto the at least one lithium layer of a current collector comprising at least one lithium layer, said film being obtained by a process comprising: processing a composition (C) into a film, said composition (C) comprising: at least one fluoropolymer [polymer (F)] comprising recurring units derived from at least one fluorinated monomer comprising a —SO.sub.3M functional group, wherein M is an alkaline metal [monomer (FM)] and a liquid medium (L) comprising at least 50% by weight, based on the total weight of said medium (L), of at least one alkyl carbonate; drying the film.

11. A process for manufacturing the electrode according to claim 8, said process comprising: depositing at least one lithium layer onto a film, said film being obtainable by a process comprising: processing a composition (C) into a film, said composition (C) comprising: at least one fluoropolymer [polymer (F)] comprising recurring units derived from at least one fluorinated monomer comprising a —SO.sub.3M functional group, wherein M is an alkaline metal [monomer (FM)] and a liquid medium (L) comprising at least 50% by weight, based on the total weight of said medium (L), of at least one alkyl carbonate; and drying the film said film having an inner surface and an outer surface; and optionally, applying at least one metal layer onto the at least one lithium layer.

12. A secondary battery comprising: (a) an electrode comprising a current collector, said current collector comprising: at least one lithium layer, and adhered to said at least one lithium layer, the film according to claim 6, (b) a positive electrode, and (c) a separator.

13. The secondary battery according to claim 12, wherein the separator (c) is adhered between the film of the electrode (a) and the positive electrode (b).

14. The secondary battery according to claim 12, said secondary battery being a lithium-sulphur (Li—S) battery wherein the positive electrode (b) comprises a current collector, said current collector comprising at least one sulphur layer.

15. The Li—S battery according to claim 14, wherein the positive electrode (b) comprises a current collector, said current collector comprising: at least one carbon layer, and adhered to said at least one carbon layer, at least one sulphur layer.

16. The process according to claim 3, wherein m is an integer comprised between 2 and 4.

17. The process according to claim 4, wherein: R.sub.a and R.sub.b, equal to or different from each other, are independently C.sub.1-C.sub.4 alkyl groups, and R.sub.c is a hydrogen atom or a C.sub.1-C.sub.4 alkyl group.

18. The process according to claim 17, wherein: R.sub.a and R.sub.b, equal to or different from each other, are independently C.sub.1-C.sub.2 alkyl groups, and R.sub.c is a hydrogen atom or a C.sub.1-C.sub.2 alkyl group.

Description

EXAMPLE 1

[0201] A solution containing 5% by weight of polymer (F-1) having an equivalent weight of 660 g/eq in propylene carbonate was prepared after 4 hours under stirring at 80° C. The solution so obtained was clear and homogeneous.

EXAMPLE 2

[0202] A solution containing 10% by weight of polymer (F-1) having an equivalent weight of 870 g/eq in propylene carbonate was prepared after 4 hours under stirring at 80° C. The solution so obtained was clear and homogeneous.

COMPARATIVE EXAMPLE 1

[0203] The same procedure as detailed under Example 1 was followed but using dimethyl sulphoxide.

COMPARATIVE EXAMPLE 2

[0204] The same procedure as detailed under Example 1 was followed but using N-methyl-2-pyrrolidone.

EXAMPLE 3

Manufacture of a Film

[0205] A film having a thickness of 20 μm was manufactured using the solution prepared according to Example 1 by tape casting and drying (48 hours under vacuum at 120° C.).

EXAMPLE 4

Manufacture of a Negative Electrode

[0206] A lithium electrode was prepared using a current collector comprising a lithium foil which was cut in the desired dimensions. The solution prepared according to Example 1 was then coated by doctor blade technique onto the lithium foil of the current collector and then dried at 60° C. (firstly under argon, then under vacuum) thereby providing a protective layer having a final thickness of about 30 μm. The assembly so obtained was cut thereby providing a negative electrode comprising a lithium layer having a diameter of 14 mm and, adhered to said lithium layer, a protective film having a diameter of 16 mm.

EXAMPLE 5

Manufacture of a Negative Electrode

[0207] The film prepared according to Example 3 was dried under vacuum to remove water traces. Lithium metal deposition with thicknesses up to about 1 μm was performed on the film by vacuum evaporation technique. The lithium/protective film stack was then cut into a lithium electrode.

COMPARATIVE EXAMPLE 3

[0208] The same procedure as detailed under Example 4 was followed but using the solution prepared according to Comparative Example 1.

COMPARATIVE EXAMPLE 4

[0209] The same procedure as detailed under Example 4 was followed but using the solution prepared according to Comparative Example 2.

[0210] Manufacture of a Sulphur Positive Electrode

[0211] A sulphur cathode was prepared by mixing carbon black powder (10% by weight), sulphur powder (80% by weight) and a polyvinylidene fluoride binder (10% by weight) in N-methyl-2-pyrrolidone. The slurry was then coated onto an aluminium foil of 20 μm to a thickness of 100 μm. After drying at 55° C., the thickness of the electrode was about 15 μm, with a loading of sulphur of about 1.8 mg/cm.sup.2.

EXAMPLE 6

Manufacture of a Li—S Battery

[0212] A coin cell was assembled under controlled atmosphere in a glove box. The lithium electrode prepared according to Example 4 was cut into a 14 mm disk and then dried under vacuum. An assembly was prepared using a CR2032 coin cell casing, said assembly comprising, in succession, a sulphur positive electrode having a diameter of 14 mm, a porous separator made of polypropylene having a diameter of 16.5 mm and the negative electrode prepared according to Example 4. An electrolyte medium containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) 1M in tetraethylene glycol dimethyl ether (TEGDME)/1,3-dioxolane (DIOX) (50/50 by volume) was impregnated into the cell so obtained. The cell was then sealed in the glove box and then cycled between 1.5 V and 3 V vs. Li+/Li at C/10.

EXAMPLE 7

Manufacture of a Li—S Battery

[0213] A coin cell was assembled under controlled atmosphere in a glove box. The film prepared according to Example 3 was cut into a 16.5 mm disk and then dried under vacuum. An assembly was prepared using a CR2032 coin cell casing, said assembly comprising, in succession, a sulphur positive electrode having a diameter of 14 mm, a porous separator made of polypropylene having a diameter of 16.5 mm, the film prepared according to Example 3 having a diameter of 16.5 mm and a lithium electrode having a diameter of 16 mm. An electrolyte medium containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) 1M in tetraethylene glycol dimethyl ether (TEGDME)/1,3-dioxolane (DIOX) (50/50 by volume) was impregnated into the cell so obtained. The cell was then sealed in the glove box and then cycled between 1.5 V and 3 V vs. Li+/Li at C/10.

COMPARATIVE EXAMPLE 5

[0214] A mixture comprising 10.4% by weight of polymer (F-1) having an equivalent weight of 790, 75.0% by weight of water and 14.6% by weight of n-propanol was prepared and subsequently dropped onto a circular sample of porous separator made of polypropylene (weight: 170 mg, area: 95 cm.sup.2, thickness: 30 μm) at room temperature. The wet separator so obtained was then dried in an oven using the following temperature program: 1.5 hours at 65° C., 1.5 hours at 90° C. and 15 minutes at 160° C. After drying, the weight increase and the SEM analysis confirmed the presence of a dense and homogeneous polymer film covering the pores initially present on the polypropylene support (0.25 mg/cm.sup.2 on each side). An assembly was prepared using a CR2032 coin cell casing, said assembly comprising, in succession, a sulphur positive electrode having a diameter of 14 mm, the separator so obtained having a diameter of 16.5 mm and a lithium electrode having a diameter of 16 mm. An electrolyte medium containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) 1M in tetraethylene glycol dimethyl ether (TEGDME)/1,3-dioxolane (DIOX) (50/50 by volume) was impregnated into the cell so obtained. The cell was then sealed in the glove box and then cycled between 1.5 V and 3 V vs. Li+/Li at C/10.

COMPARATIVE EXAMPLE 6

[0215] A coin cell was assembled under controlled atmosphere in a glove box. An assembly was prepared using a CR2032 coin cell casing, said assembly comprising, in succession, a sulphur positive electrode having a diameter of 14 mm, a porous separator made of polypropylene having a diameter of 16.5 mm and a lithium electrode having a diameter of 16 mm. An electrolyte medium containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) 1M in tetraethylene glycol dimethyl ether (TEGDME)/1,3-dioxolane (DIOX) (50/50 by volume) was impregnated into the cell so obtained. The cell was then sealed in the glove box and then cycled between 1.5 V and 3 V vs. Li+/Li at C/10.

COMPARATIVE EXAMPLE 7

[0216] A Li/S cell was prepared following the same procedure as detailed under Example 6 but using the lithium electrode prepared according to Comparative Example 3.

COMPARATIVE EXAMPLE 8

[0217] A Li/S cell was prepared following the same procedure as detailed under Example 6 but using the lithium electrode prepared according to Comparative Example 4.

[0218] Electrochemical Measurements

[0219] Electrochemical measurements were performed in CR2032 coin cells at room temperature and C/100 between 1.5V and 3V.

[0220] The results are set forth in Table 1 here below.

[0221] Data reported in Table 1 represent average values of two cell test measurements carried out in parallel.

[0222] The specific discharge capacity values [mAh/g of S] are representative of the percentage of sulphur utilization in the Li—S coin cells.

[0223] The capacity retention values [%] are representative of the retention of the initial specific discharge capacity values upon charge/discharge cycles of the Li—S coin cells. The higher the capacity retention values, the better the cycle life of the cell.

[0224] The columbic efficiency values [%] are representative of the fraction of the electrical charge stored during charging that is recoverable during discharge.

TABLE-US-00001 TABLE 1 C. C. Run Ex. 7 C. Ex. 5 C. Ex. 6 Ex. 7 Ex. 8 Specific Cycle 1 1050 806 780 715 790 Discharge Capacity at C/100 [mAh/g] Capacity Cycle 2 64  66 — — — retention at 20° C. Cycle 25 51 — — — — and C/100 [%] Cycle 50 40 — — — — Columbic Cycle 2 96 <50 — — — Efficiency at 20° C. Cycle 25 88 — — — — and C/100 [%] Cycle 50 89 — — — —

[0225] The runs corresponding to the electrochemical measurements of the Li—S coin cells of Comparative Examples 6, 7 and 8 were stopped after the first cycle due to polysulphide shuttle mechanism. Also, the run corresponding to the electrochemical measurements of the Li—S coin cell of Comparative Example 5 was stopped after the second cycle due to polysulphide shuttle mechanism.

[0226] As shown by the charge/discharge curves of the Li—S coin cells of Comparative Examples 5, 6, 7 and 8, an infinite charging threshold was registered leading to reduced columbic efficiency of the cell.

[0227] In contrast, good capacity retention and columbic efficiency (after at least up to 50 cycles) were observed during the electrochemical measurements of the Li—S batteries of the present invention as notably embodied by the Li—S coin cells of Examples 6 and 7 according to the invention. Without wishing to be bound by theory, this indicates an absent or very reduced polysulphide shuttle mechanism in the cells according to the invention.

[0228] Moreover, as shown in Table 1 here above, the Li—S coin cells of Example 7 according to the invention successfully exhibited both higher specific discharge capacity values and higher columbic efficiency values as compared to conventional Li—S batteries as notably embodied by any of the Li—S coin cells of Comparative Examples 5, 6, 7 and 8.

[0229] Further, as shown in Table 1 here above, the Li—S coin cells of Example 7 according to the invention successfully exhibited good or higher capacity retention values as compared to conventional Li—S batteries as notably embodied by the Li—S coin cell of Comparative Example 5.

[0230] In view of all the above, it has been thus found that the Li—S battery of the invention successfully exhibited absent or reduced polysulphide shuttle mechanism, while maintaining good or increased capacity values, as compared to conventional Li—S batteries.