Positive electrode for lithium-sulfur electrochemical accumulator having a specific structure

Abstract

The invention relates to a positive electrode for a lithium-sulfur electrochemical accumulator comprising an electrically conductive substrate selectively coated, over at least one of its faces, with carbon nanotubes so as to create a zone coated with carbon nanotubes within which a plurality of separate zones without carbon nanotubes are arranged, these separate zones being qualified as empty.

Claims

1. An electrochemical accumulator of the lithium-sulfur type comprising at least one electrochemical cell that comprises: a positive electrode comprising an electrically conductive substrate selectively coated, over at least one of its faces, with carbon nanotubes so as to create a zone coated with carbon nanotubes, which is a carpet of carbon nanotubes, within which a plurality of separate zones without carbon nanotubes are arranged, these separate zones being qualified as empty; a negative electrode; and an electrolyte conducting lithium ions arranged between said positive electrode and said negative electrode.

2. The electrochemical accumulator according to claim 1, wherein the electrically conductive substrate is selectively coated, over at least two of its faces, with carbon nanotubes so as to create, on each of said faces, a zone coated with said carbon nanotubes, which is a carpet of carbon nanotubes, within which a plurality of separate zones without carbon nanotubes are arranged, these separate zones being qualified as empty.

3. The electrochemical accumulator according to claim 1, wherein the carbon nanotubes are perpendicular to the surface of the substrate.

4. The electrochemical accumulator according to claim 1, wherein the empty spaces are circular empty spaces.

5. The electrochemical accumulator according to claim 1, wherein the empty spaces have a larger distance greater than 10 nm, the larger distance corresponding to the maximum separation between two points defining the contour of the empty space considered in the plane of the substrate, the larger distance corresponding to the diameters of these empty spaces, when these empty spaces are circular.

6. The electrochemical accumulator according to claim 5, wherein the larger distance ranges from 10 nm to 100 m or from 50 nm to 100 m, or more specifically, from 1 m to 20 m.

7. The electrochemical accumulator according to claim 1, wherein the minimum distance between two adjacent empty spaces ranges from 20 nm to 200 m, more specifically from 100 nm to 200 m, still more specifically from 20 m to 200 m, and even more specifically from 50 m to 100 m, the two adjacent empty spaces corresponding to the minimum separation between two points defining the contour of each of the empty spaces.

8. The electrochemical accumulator according to claim 1, wherein the electrically conductive substrate comprises a metal material, such as aluminum.

9. The electrochemical accumulator according to claim 1, which includes a sulfurated active material deposited on the face coated with carbon nanotubes, the active material being able to be elementary sulfur, lithium disulfide Li.sub.2S or lithium polysulfides Li.sub.2S.sub.n, with n being an integer from 2 to 8.

10. The electrochemical accumulator according to claim 1, wherein the negative electrode comprises a current collecting substrate on which at least the active material of the negative electrode is placed, said active material being metal lithium.

11. The electrochemical 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.

12. The electrochemical accumulator according to claim 11, wherein the organic solvent(s) are chosen from among ether solvents.

13. The electrochemical accumulator according to claim 11, wherein the lithium salt is chosen from the group made up of LiPF.sub.6, LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, LiI, LiNO.sub.3 LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2 (also called lithium bis[(trifluoromethyl)sulfonyl]imide, LiTFSI), LiN(C.sub.2F.sub.5SO.sub.2).sub.2 (also called lithium bis[perfluoroethyl)sulfonyl]imide, LiBETI), LiCH.sub.3SO.sub.3, LiB(C.sub.2O.sub.4).sub.2 (also called lithium bis(oxalato)borate, LiBOB) and mixtures thereof.

14. The electrochemical accumulator according to claim 1, wherein the electrolyte further comprises at least one lithium polysulfides compound with formula Li.sub.2S.sub.n with n being an integer from 2 to 8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph illustrating the evolution of the potential E (in V) as a function of the capacity C (in u.a.).

(2) FIG. 2 is a top perspective view of a positive electrode according to the invention having a particular configuration.

(3) FIG. 3 is a photograph from above of the positive electrode obtained in example 1.

(4) FIG. 4 is a graph illustrating the evolution of the specific capacity of the accumulator C (in mAh/g) as a function of the height of the carbon nanotubes H (in m).

DETAILED DESCRIPTION OF THE INVENTION

EXAMPLE 1

(5) The present example illustrates the preparation of a positive electrode according to the invention and an accumulator comprising such an electrode. To that end, two steps are carried out: a step for preparing the positive electrode (step a) below); a step for preparing the accumulator (step b below).

(6) a) Preparation of the Positive Electrode

(7) An aluminum sheet with a thickness of approximately 20 m and a diameter of 14 mm is first carefully cleaned using an O.sub.2 plasma cleaning method.

(8) The sheet thus cleaned is next coated with a layer of iron (0.5 nm thick) by physical vapor deposition (PVD), this layer of iron being intended to form the catalyst for the carbon nanotube growth.

(9) This layer of iron is partially removed by laser ablation on circular zones measuring 30 m in diameter, the centers of which are spaced apart by 40 m.

(10) The sheet is next inserted into a chemical vapor deposition (CVD) chamber for the carbon nanotube growth. It is understood that only the zone coated with a layer of iron will host the growth of the carbon nanotubes (in other words, the circular zones will have no carbon nanotubes).

(11) The CVD growth is done after cleaning via an air plasma (O.sub.2:N.sub.2 20:80) under the following conditions: Increase of the temperature of the reactor up to 600 C. in 15 min. under an atmosphere made up of C.sub.2H.sub.2 (5 sccm), H.sub.2 (90 sccm) and He (110 sccm) for a total pressure of 0.9 Torr; Maintenance of the aforementioned atmosphere at 600 C. for 1 hour; and Cooling under helium.

(12) This results in a carpet of carbon nanotubes having a height from 50 to 60 m having circular empty spaces (with dimensions corresponding to the dimensions of the aforementioned circular zones) distributed periodically on the surface of the sheet (as illustrated in FIG. 3, appended), thus forming the positive electrode.

(13) As an alternative to the plasma-assisted CVD growth, the CVD growth may be done using tungsten filaments brought to a high temperature in the reactor.

(14) The implementation conditions are then as follows: the setpoint temperature of the reactor is lowered to 450 C.; the composition of the gases is slightly modified: C.sub.2H.sub.2:H.sub.2:He (20:50:110); the power of the filaments is set at 500 W; a growth time of 30 minutes is sufficient.

(15) This results in a denser carpet of carbon nanotubes having a height of 140 m.

(16) b) Preparation of the Accumulator

(17) The sulfurated active material of the positive electrode is brought in the form of a catholyte, which comprises the following ingredients: 1,3-dioxolane solvent (supplied by Aldrich) at a rate of 50 L; tetraethyleneglycol dimethylether solvent (supplied by Aldrich) at a rate of 50 L; lithium polysulfide Li.sub.2S.sub.6 at 0.25 M; lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) 1M; and lithium nitrate salt LiNO.sub.3 0.1 M.

(18) The accumulator is mounted in the form of a button cell (CR 2032), which respectively includes: a circular positive electrode with a diameter of 14 mm obtained by cutting the sheet obtained in step a) explained above; a metal lithium negative electrode, which consists of a disc 130 m thick and 16 mm in diameter, this disc being deposited on a stainless steel disc serving as a current collector; and a separator arranged between the positive electrode and the negative electrode imbibed with the catholyte defined above.

EXAMPLE 2

(19) This example is similar to example 1, except that the accumulator is prepared differently, as described below.

(20) To that end, a positive electrode is prepared by cutting out a disc with a diameter of 14 mm from a sheet partially covered with carbon nanotubes prepared according to the conditions of step a) of example 1 (obtained by plasma CVD).

(21) The electrode thus obtained is pressed on a heating plate. The quantity of sulfur that one wishes to introduce into the accumulator (from 1 to 3 mg) is deposited in powder form on the electrode. The temperature of the heating plate is increased gradually, so as to melt the solid sulfur (the melting temperature being approximately 115 C.). Thus melted, the sulfur penetrates the pores of the carpet of carbon nanotubes by capillarity.

(22) The positive electrode thus sulfurated is mounted in a button cell according to the same terms as stated in example 1, using an electrolyte comprising the following ingredients: 1,3-dioxolane solvent (supplied by Aldrich) at a rate of 50 L; tetraethyleneglycol dimethylether solvent (supplied by Aldrich) at a rate of 50 L; lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) 1M; and lithium nitrate salt LiNO.sub.3 0.1 M.

EXAMPLE 3

(23) The present example illustrates the preparation of a positive electrode according to the invention and an accumulator comprising such an assembly. To that end, two steps are carried out: a step for preparing the aforementioned assembly (step a) below); a step for preparing the accumulator (step b below).

(24) a) Preparation of the Positive Electrode

(25) A sheet of aluminum according to that used in example 1 is cleaned according to the same conditions as defined in that example.

(26) Next, a positive photoresist is spread on the aluminum sheet thus cleaned, then insulated using a mask with appropriate patterns, such that the insulation is effective over the entire surface of the resin, with the exception of circular zones measuring 20 to 30 m in diameter, spaced apart by 40 m. After development, a fine layer of iron (0.5 nm in diameter) is deposited by PVD on the entire surface of the sheet. A lift-off is done, in order to remove the rest of the resin and the iron deposited on that resin.

(27) The growth of the carbon nanotubes is done in the same way as in example 1, with or without hot filaments depending on the desired carpet height.

(28) b) Preparation of the Accumulator

(29) The accumulator of this example is prepared according to the same conditions as in example 1.

EXAMPLE 4

(30) The present example illustrates the preparation of a positive electrode according to the invention and an accumulator comprising such an assembly. To that end, two steps are carried out: a step for preparing the aforementioned positive electrode (step a) below); a step for preparing the accumulator (step b below).

(31) a) Preparation of the Positive Electrode

(32) A sheet of aluminum according to that used in example 1 is cleaned according to the same conditions as defined in that example.

(33) A lithography technique using a copolymer as photoresist is implemented to produce a nanometric structuring of the carpet of carbon nanotubes.

(34) To that end, a photosensitive di-block copolymer, polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA, 104 kg.Math.mol.sup.1) is deposited on the sheet by centrifugal coating to form a layer with a thickness of approximately 60 nm. The layer is annealed at 190 C. for 7 days, which makes it possible to obtain a phase separation between the PS and PMMA blocks of the copolymer, this phase separation taking the form of the formation of PS and PMMA domains consisting of blades approximately 50 nm wide oriented perpendicular to the sheet.

(35) Next, the entire surface of the assembly is insulated. The PMMA is photosensitive and may then be removed by submerging the sample in an acetic acid bath. The cleaning may be completed by a short RIE (reactive ion etching) treatment, such that only the PS domains remain on the specimen. A fine layer of iron (0.5 nm thick) is deposited by PVD on the entire surface of the substrate. A lift-off is done, in order to remove the PS part of the resin and the iron deposited on that resin.

(36) This technique makes it possible to create an array of discontinuous carbon nanotubes, having pores with the shape and size of the PS domains present during the deposition of the catalyst. These micro-porosities may be associated with meso-porosities through partial laser ablation of the catalyst, as explained in example 1.

(37) The growth of the carbon nanotubes is done in the same way as in example 1, with or without hot filaments depending on the desired carpet height.

(38) b) Preparation of the Accumulator

(39) The accumulator of this example is prepared according to the same conditions as in example 1.

COMPARATIVE EXAMPLE 1

(40) The present example illustrates the preparation of a positive electrode not according to the invention and an accumulator comprising such an electrode. To that end, two steps are carried out: a step for preparing the positive electrode (step a) below); a step for preparing the accumulator (step b below).

(41) a) Preparation of the Positive Electrode

(42) First, an ink is produced from the following ingredients: elementary sulfur (supplied by Aldrich) (80 wt %); carbon black (Super P, supplied by Timcal) (10 wt %); a binder (polyvinylidene difluoride, supplied by Solvay) (10 wt %) placed in solution in N-methylpyrrolidone.

(43) The ink is deposited by coating using a micrometric scraper on an aluminum sheet approximately 20 m thick, 50 cm long and 20 cm wide.

(44) The layer thus obtained is dried at 55 C. for 24 hours under air.

(45) b) Preparation of the Accumulator

(46) The accumulator is mounted in the form of a button cell (CR 2032), which respectively includes: a circular positive electrode with a diameter of 14 mm obtained by cutting the sheet obtained in step a) explained above; a metal lithium negative electrode, which consists of a disc 130 m thick and 16 mm in diameter, this disc being deposited on a stainless steel disc serving as a current collector; and a separator arranged between the positive electrode and the negative electrode imbibed with an electrolyte comprising LiTFSI (1 mol.Math.L.sup.1)+LiNO.sub.3 (0.1 M) in a 50/50 mixture by volume of tetraethyleneglycol dimethyl ether and dioxolane.

COMPARATIVE EXAMPLE 2

(47) The present example illustrates the preparation of a positive electrode not according to the invention and an accumulator comprising such an assembly. To that end, two steps are carried out: a step for preparing the positive electrode (step a) below); and a step for preparing the accumulator (step b below).

(48) a) Preparation of the Positive Electrode

(49) An aluminum sheet with a thickness of approximately 20 m and a diameter of 14 mm is first carefully cleaned using an O.sub.2 plasma cleaning method.

(50) The sheet thus cleaned is next coated with a layer of iron (0.5 nm thick) by physical vapor deposition (PVD), this layer of iron being intended to form the catalyst for the carbon nanotube growth.

(51) The sheet is next inserted into a chemical vapor deposition (CVD) chamber for the carbon nanotube growth.

(52) The CVD growth is done using an air plasma (O.sub.2:N.sub.2 20:80) under the following conditions: Increase of the temperature of the reactor up to 600 C. in 15 min. under an atmosphere made up of C.sub.2H.sub.5 (5 sccm), H.sub.2 (90 sccm) and He (110 sccm) for a total pressure of 0.9 Torr; Maintenance of the aforementioned atmosphere at 600 C. for 1 hour; and Cooling under helium.

(53) This results in a carpet of carbon nanotubes having a height from 50 to 60 m and without empty spaces.

(54) b) Preparation of the Accumulator

(55) The sulfurated active material of the positive electrode is brought in the form of a catholyte, which comprises the following ingredients: 1,3-dioxolane solvent (supplied by Aldrich) at a rate of 50 L; tetraethyleneglycol dimethylether solvent (supplied by Aldrich) at a rate of 50 L; lithium polysulfide Li.sub.2S.sub.6 at 0.25 M; lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) 1M; lithium nitrate salt LiNO.sub.3 0.1 M.

(56) The accumulator is mounted in the form of a button cell (CR 2032), which respectively includes: a circular positive electrode with a diameter of 14 mm obtained by cutting the sheet obtained in step a) explained above; a metal lithium negative electrode consists of a disc 130 m thick and 16 mm in diameter, this disc being deposited on a stainless steel disc serving as a current collector; and a separator arranged between the positive electrode and the negative electrode imbibed with the catholyte defined above.

(57) This example was reiterated several times, modifying the height of the carbon nanotubes and measuring the specific capacity of the accumulator.

(58) FIG. 4 is a graph illustrating the evolution of the specific capacity of the accumulator C (in mAh/g) as a function of the height of the carbon nanotubes H (in m).

(59) It is possible to see that the specific capacity increases with the height of the carpet of carbon nanotubes. Thus, a larger specific surface makes it possible to improve the use of the active material to a certain extent. Furthermore, the specific capacity of the accumulator is capped from a certain carpet height. Yet the achieved capacity (approximately 600 mAh/g) is still far from the theoretical capacity of 1675 mAh/g, which shows the problem of the accessibility of the surface offered by the carpet of carbon nanotubes, due to the absence of circulation zones with no carbon nanotubes.