Lithium accumulator comprising a positive electrode material based on a specific carbon material functionalized by specific organic compounds

Abstract

The invention relates to a lithium accumulator comprising at least one electrochemical cell comprising an electrolyte positioned between a positive electrode and a negative electrode, said positive electrode comprising a positive electrode material comprising a carbonaceous material selected from carbon nanotubes, graphene or derivatives of graphene selected from graphene oxides, reduced graphene oxides, said carbonaceous material is covalently functionalized by at least one organic compound comprising at least one electron attractor group.

Claims

1. A lithium accumulator comprising at least one electrochemical cell comprising an electrolyte positioned between a positive electrode and a negative electrode, said positive electrode comprising a positive electrode material comprising a carbonaceous material selected from carbon nanotubes, graphene, and derivatives of graphene comprising graphene oxides, or reduced graphene oxides, wherein said carbonaceous material is covalently functionalized by at least one organic compound comprising at least one electron attractor group.

2. The lithium accumulator according to claim 1, wherein the organic compound comprising at least one electron attractor group is a compound comprising one or several cyclic groups, for which at least one of these groups bears at least one electron attractor group.

3. The lithium accumulator according to claim 1, wherein the electron attractor group is selected from carbonyl groups, disulfide groups, and thiocarbonyl groups.

4. The lithium accumulator according to claim 1, wherein, when the electron attractor group is a carbonyl group, the electron attractor group is conjugate with a double bond.

5. The lithium accumulator according to claim 1, wherein, when the electron attractor group is a carbonyl group, the organic compound comprising such an electron attractor group is a quinone compound.

6. The lithium accumulator according to claim 1, wherein, when the electron attractor group is a carbonyl group, the organic compound comprising such an electron attractor group is selected from: benzoquinone compounds of the following formulae (I) and (II): ##STR00026## a naphthoquinone compound of the following formula (III): ##STR00027## an anthraquinone compound of the following formula (IV): ##STR00028## a phenanthrenequinone compound of the following formula (V): ##STR00029## wherein the bonds located at the middle of the carbon-carbon bonds indicate that the attachment to the carbonaceous material of the relevant compound is ensured by any of the carbon atoms making up the benzene ring(s).

7. The lithium accumulator according to claim 6, wherein the organic compound comprising at least one electron attractor group is a compound of the following formula (VI): ##STR00030## the bond intercepted with a bracket indicating that the attachment of the relevant compound by covalence to the carbonaceous material is carried out via this bond.

8. The lithium accumulator according to claim 1, wherein, when the electron attractor group is a carbonyl group, the compound comprising such electron attractor group is a polymer comprising at least one recurrent unit, said recurrent unit comprises one or several rings, for which one of these rings is a ring comprising at least one carbonyl group.

9. The lithium accumulator according to claim 8, wherein the compound comprises at least one recurrent unit from the family of quinones.

10. The lithium accumulator according to claim 1, wherein, when the electron attractor group is a disulfide group, the compound comprising the electron attractor group is a cyclic compound including one or several rings, wherein at least one ring includes a disulfide group.

11. The lithium accumulator according to claim 10, wherein the compound is a cyclic compound including one or several rings, wherein at least one ring includes a disulfide group and includes 4 atoms and two of these atoms are sulfur atoms.

12. The lithium accumulator according to claim 1, wherein, when the electron attractor group is a disulfide group, the compound comprising the electron attractor group is a cyclic compound comprising at least one aromatic ring beside at least one ring comprising a disulfide group.

13. The lithium accumulator according to claim 1, wherein, when the electron attractor group is a disulfide group, the compound comprising the electron attractor group is a compound fitting one of the following formulae (VII) to (X): ##STR00031## the bond intercepting the carbon-carbon bond indicating that the bond between the benzene ring(s) and the carbonaceous material is ensured through one of the carbon atoms of this or these ring(s).

14. The lithium accumulator according to claim 1, wherein, when the electron attractor group is a disulfide group, the compound comprising the electron attractor group is a polymer comprising at least one recurrent unit, which recurrent unit comprises one or several rings, for which one of these rings is a ring comprising a disulfide group.

15. The lithium accumulator according to claim 14, wherein the compound comprises at least one recurrent unit fitting one of the following formulae (XI) to (XV): ##STR00032## wherein said recurrent units may be bound to the carbonaceous material via an organic group forming a bridge between the relevant recurrent unit and the carbonaceous material.

16. A positive electrode material for a lithium accumulator comprising a carbonaceous material selected from carbon nanotubes, graphene, and derivatives of graphene comprising graphene oxides, or reduced graphene oxides, wherein said carbonaceous material is covalently functionalized by at least one organic compound comprising at least one electron attractor group, which is a disulfide group.

17. The positive electrode material according to claim 16, wherein the compound comprising a disulfide group is a cyclic compound including one or several rings, wherein at least one ring includes a disulfide group.

18. The positive electrode material according to claim 16, wherein the compound comprising a disulfide group is a cyclic compound including one or several rings, wherein at least one ring includes a disulfide group and includes 4 atoms and-two of these atoms are sulfur atoms.

19. The positive electrode material according to claim 16, wherein the compound comprising a disulfide group is a cyclic compound comprising at least one aromatic ring beside at least one ring comprising a disulfide group.

20. The positive electrode material according to claim 16, wherein the compound comprising a disulfide group is a compound fitting one of the following formulae (VII) to (X): ##STR00033## wherein the bond intercepting the carbon-carbon bond indicates that the bond between the benzene ring(s) and the carbonaceous material is ensured by one of the carbon atoms of this or these ring(s).

21. The positive electrode material according to claim 16, wherein the compound comprising a disulfide group is a polymer comprising at least one recurrent unit, and said recurrent unit comprises one or several rings, for which one of these rings is a ring comprising a disulfide group.

22. The positive electrode material according to claim 21, wherein the compound comprises at least one recurrent unit fitting one of the following formulae (XI) to (XV): ##STR00034## wherein said recurrent units may be bound to the carbonaceous material via an organic group forming a bridge between the relevant recurrent unit and the carbonaceous material.

23. A positive electrode comprising a positive electrode material comprising a material as defined according to claim 16.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an infrared spectrum obtained for the product of Example 1 (part b).

(2) FIG. 2 is a diagram illustrating the obtained cycling curves, by cyclic voltammetry, with the material obtained in Example 1 (part d).

(3) FIG. 3 is an illustration in an exploded view, of the lithium accumulator prepared according to Example 2.

(4) FIG. 4 is a diagram illustrating the obtained cycling curves by cyclic voltammetry, with the accumulator prepared in Example 2.

(5) FIG. 5 is a diagram illustrating the obtained cycling curves by cyclic voltammetry, with the system of test A of Example 2.

(6) FIG. 6 is a diagram illustrating the obtained cycling curves, by cyclic voltammetry, with the system of test B of Example 2.

(7) FIG. 7 is a diagram illustrating obtained charging-discharging curves, at constant intensity (10 A i.e. 5 mg.g.sup.1 for the tested samples), with the accumulator prepared according to Example 2.

(8) FIG. 8 is a diagram illustrating the time-dependent change of the potential E (V vs. Li.sup.+/Li) according to the discharge capacity C (in electrode mAh.g.sup.1) with accumulators prepared according to Example 2.

(9) FIG. 9 is a diagram illustrating the time-dependent change in the discharge capacity C (in electrode mAh.g.sup.1) according to the number of cycles N for three button batteries prepared according to Example 2.

(10) FIG. 10 is a diagram illustrating cyclic curves obtained from systems described in Example 4.

(11) FIG. 11 is a diagram illustrating the time-dependent change of the potential E (V vs. Li.sup.+/Li) according to the discharge capacity (in electrode mAh.g.sup.1) for button batteries described in Example 4.

(12) FIG. 12 is a diagram illustrating the time-dependent change in the discharge capacity C (in electrode mAh.g .sup.1) according to the number of cycles for three button batteries prepared according to Example 4.

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

EXAMPLE 1

(13) This example illustrates the preparation of a carbonaceous material according to the invention consisting in covalently functionalized multi-walled carbon nanotubes, by polymers comprising as a recurrent unit, a recurrent unit of the following formula:

(14) ##STR00020##

(15) Both bonds intercepting the carbon-carbon bonds indicating that the latter are bound to one of the carbon atoms of the benzene ring, for which they intercept the carbon-carbon bond,

(16) the polymer resulting from the recurrence of said recurrent unit being bound to the carbonaceous material via an organic group forming a bridge between the relevant recurrent unit and the carbonaceous material, this organic group forming a bridge fitting the following formula:

(17) ##STR00021##

(18) The bond intercepted by a bracket indicating that the group is bound to the wall of the carbon nanotubes through a carbon atom of the benzene ring, the other bond intercepting the benzene ring indicating that the group is bound to another recurrent unit.

(19) To do this, it is proceeded with the application of the following steps: a step for purifying the carbon nanotubes (step a); a step for preparing a diazoanthraquinone compound (step b); a step for grafting the diazoanthraquinone compound via a chemical route (step c) or via an electrochemical route (step d).

(20) a) Purification of the Carbon Nanotubes

(21) The carbon nanotubes used are multi-walled carbon nanotubes (NC-3100), obtained from Nanocyl.

(22) In order to purify them, they are dispersed in 65% nitric acid by sonication for 30 minutes. The mixture is then refluxed to 110 C. for 8 hours. This mixture is then filtered on a polytetrafluoroethylene membrane (having a pore size of 0.45 m) for recovering the carbon nanotubes. The latter are then re-dispersed in a 0.5 M soda NaOH solution by sonication for 30 minutes, and they are again recovered by filtration, washed with water and then with a 1 M hydrochloric acid solution. Finally, they are again rinsed with water, with acetone and then with diethyl ether.

(23) b) Preparation of a Diazoanthraquinone Compound

(24) The diazoanthraquinone compound is prepared from 2-aminoanthraquinone according to the following reaction scheme:

(25) ##STR00022##

(26) The diazoanthraquinone compound is thus prepared by reducing 2-aminoanthraquinone with an excess of nitrosonium tetrafluoroborate NOBF.sub.4 in the dichloromethane at 0 C. for 2 hours.

(27) The solvent is then evaporated and the resulting product is dried in vacuo.

(28) The obtained product is analysed by infrared spectroscopy, the obtained spectrum being illustrated in FIG. 1 illustrating the time-dependent change in the transmittance (in %) versus the wave number (in cm.sup.1).

(29) The band at 2,300 cm.sup.1 is the signature of the presence of diazo N.sub.2.sup.+ functions.

(30) c) Chemical Grafting of the Diazoanthraquinone Compound

(31) The grafting of the diazoanthraquinone compound prepared in step b) is carried out by reducing the diazonium function, in return for which there is removal of N.sub.2 and concomitant formation of an anthraquinone polymer around carbon nanotubes as explained at the beginning of Example 1.

(32) To do this, the carbon nanotubes purified in step a) are dispersed in N-methyl-2-pyrrolidone by sonication for 30 minutes.

(33) The diazoanthraquinone compound prepared according to step b) is then added according to a content of 1 equivalent per carbon (i.e. one molecule per carbon atom) followed by a spatula touch of iron powder.

(34) The dispersion is stirred for 12 hours.

(35) The nanotubes are then recovered by filtration on a polytetrafluoroethylene membrane (having a pore size of 0.45 m) and then washed with 1 M hydrochloric acid and then with water.

(36) The procedure is repeated once again entirely.

(37) The thereby grafted carbon nanotubes are finally dried in vacuo.

(38) d) Electrochemical Grafting of the Diazoanthraquinone Compound

(39) In order to graft the diazoanthraquinone compound via an electrochemical route, a three electrode system is used, comprising: a working electrode consisting of carbon nanotubess prepared in step a), this electrode appearing as a disc in carbon nanotubes, which disc has a diameter of 16 mm and a thickness of 10-20 micrometres; a platinum counter-electrode appearing as a platinum wire; a reference electrode consisting in a massive silver wire put into contact with an AgNO.sub.3 solution (10.sup.3 M) in acetonitrile; an electrolyte consisting in acetonitrile containing lithium perchlorate LiClO.sub.4(0.1 M) and the saturated diazoanthraquinone compound.

(40) After degassing the electrolyte by N.sub.2 bubbling, the system is subject to cyclic voltammetry, consisting of carrying out cycling between 1.25 to +1 V/AgAg.sup.+ (10.sup.3 M in acetonitrile) at a sweep rate of 100 mV.s.sup.1, the number of performed cycles being 10, the cycling curves being illustrated in FIG. 2 (I expressed in mA versus the potential E expressed in V). In this figure, a reduction peak at 0.45 V may be observed which corresponds to the peak for reducing the diazonium function.

EXAMPLE 2

(41) In this example, is illustrated the preparation of a lithium accumulator as a button battery comprising, as a positive electrode, the electrode formed with a material obtained in Example 1.

(42) The accumulator was assembled in a glove box under an inert argon atmosphere.

(43) The accumulator was made as illustrated in the appended FIG. 3, by beginning with the positive electrode and finishing with the negative electrode, the different elements of the accumulator being the following in this order: a casing bottom 3; a seal gasket 5; a positive electrode 7 appearing as a disc of carbon nanotubes as described in Example 1; a disc of Viledon 9 (which is a membrane in non-woven fibres of polyolefins (polypropylene/polyethylene)) and a disc of Celgard 11 (which is a polypropylene membrane); a negative electrode 13 in lithium metal as a disc having a diameter of 16 mm; a shim (not shown) and a spring 17; and a lid 19.

(44) The electrolyte impregnates both aforementioned separator discs as well as the porosity of the positive electrode. It consists in an organic electrolyte comprising a mixture of solvents, tetraethylene glycol dimethylether (TEGDME)/1,3-dioxolane (DIOX) 50/50 comprising a lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) 1 M.

(45) The aforementioned inert atmosphere is used in order to avoid any reaction of the lithium metal making up the negative electrode and of the electrolyte with water and the oxygen of ambient air.

(46) The resulting accumulator is subject to a cyclic voltammetry test, consisting of carrying out cycling between +1.5 to +3.5 V/LiLi.sup.+ at a sweep rate of 10 mV.s.sup.1, the number of performed cycles being at most 50, an example of cycling curves being illustrated in FIG. 4 (I expressed in mA versus the potential E expressed in V)). In this figure a reduction peak at 2.10 V vs. Li.sup.+/Li may be observed, which corresponds to the reduction peak of the carbonyl functions of the anthraquinone group.

(47) Indeed, it is possible to confirm with certainty that the peak at 2.10 V vs. Li.sup.+/Li corresponds to the peak of the carbonyl functions of the anthraquinone group with the following tests: a cyclic voltammetry test with non-grafted carbon nanotubes as a working electrode (said to be test A below); and a cyclic voltammetry test with glassy carbon as a working electrode in order to test the electrochemical signature of non-grafted anthraquinone (said to be test B, below).

(48) For test A, the experimental conditions are the following.

(49) A three electrode system is used, comprising: a working electrode consisting of purified carbon nanotubes according to step a) of Example 1, said electrode appearing as a disc with a diameter of 16 mm and a thickness of 10-20 micrometers; a platinum counter-electrode consisting in platinum wire; a reference electrode consisting in a massive silver wire put into contact with an AgNO.sub.3 solution (10.sup.3 M) in acetonitrile; an electrolyte consisting in acetonitrile containing lithium perchlorate LiClO.sub.4(0.1 M).

(50) After degassing the electrolyte by N.sub.2 bubbling, the system is subject to cyclic voltammetry, consisting of carrying out cycling between 1.5 to +0.25 V/AgAg.sup.+ (10.sup.3 M in acetonitrile) at a sweeping rate of 100 mV.s.sup.1, as the number of cycles carried out is 10, the cycling curves are illustrated in FIG. 5 (I is expressed in mA versus the potential E expressed in V).

(51) No peak is observed on these curves.

(52) For test B, the experimental conditions are the following.

(53) A three electrode system is used comprising: a working electrode consisting of glassy carbon appearing as a disc with a diameter of 3 mm; a platinum counter-electrode consisting in platinum wire; a reference electrode consisting in a massive silver wire put into contact with an AgNO.sub.3 solution (10.sup.3M) in acetonitrile; an electrolyte consisting in acetonitrile containing lithium perchlorate LiClO.sub.4(0.1 M) and anthraquinone 10.sup.3 M.

(54) After degassing the electrolyte by N.sub.2 bubbling, the system is subject to cyclic voltammetry, consisting of carrying out cycling between 1.0 to +1.02 V/AgAg.sup.+ (10.sup.3 M in acetonitrile) at a sweeping rate of 100 mV.s.sup.1, the number of performed cycles being 5, the cycling curves are illustrated in FIG. 6 (I is expressed in mA versus the potential E expressed in V).

(55) Conventional curves of a reversible electrochemical system are observed and having a reduction peak at 1.10 V vs. Ag/Ag.sup.+ ascribable to carbonyl functions (reduction wave of 2e.sup.) of the anthraquinone compound, this peak, reduced to the Li/Li.sup.+ system corresponding to a peak at +2.10 V.

(56) Thus, it may be inferred without any ambiguity that, with the button battery according to the invention, the peak at +2.10 V is actually due to the anthraquinone compounds grafted to the surface of the carbon nanotubes.

(57) In parallel, charging/discharging profiles were recorded with the accumulator according to the invention prepared according to this example, upon applying a current of 10 A, these profiles being copied in FIG. 7. The charging/discharging curves have a quite similar profile over time, which confirms the excellent resistance to cycling.

(58) As a comparison, charging/discharging profiles were also carried out under conditions similar to those listed above, except that non-grafted carbon nanotubes were used as an electrode material, which gave the possibility of demonstrating the significant contribution of the grafted anthraquinone compound. Indeed, specific capacities up to 20 times greater were able to be obtained for the accumulator according to the invention.

(59) Finally, tests by applying a 10 A current (i.e. 5 mAh.g.sup.1 of electrode capacity) and by measuring the time-dependent change of the potential E (V vs. Li.sup.+/Li) versus the discharging capacity (in electrode mAh.g.sup.1) (curve a) for non-grafted nanotube batteries and curve b) for grafted nanotube batteries (cf. FIG. 8) gave the possibility of determining that specific capacities up to 20 times greater were able to be obtained for button batteries using grafted nanotubes. Thus, values of the order of 100 mAh.g.sup.1 of electrode capacity for batteries using grafted nanotubes were able to be observed, versus 5 mAh.g.sup.1 of electrode capacity for batteries using non-grafted nanotubes. As the operating potential is located around 2.2 V vs. Li.sup.+/Li, a mass energy density of about 230 Wh.g.sup.1 may be extrapolated.

(60) Finally, other tests were conducted with both aforementioned button batteries and with a third button battery, the positive electrode of which comprises a mixture comprising non-grafted nanotubes and anthraquinone molecules. These tests consisted in measuring the time-dependent change of the discharging capacity C (in electrode mAh.g.sup.1) versus the number of cycles for these three button batteries and the results are copied in FIG. 9 (curve a respectively) for batteries with non-grafted nanotubes, curve b) for batteries with grafted nanotubes and curve c) for batteries with a mixture (non-grafted nanotubes+anthraquinone molecules).

(61) Curve c) indicates the obtaining of a clearly smaller specific capacity (around 25 mAh.g.sup.1 of electrode capacity) for identical experimental conditions. Indeed, the active material not grafted to the nanotubes is partly dissolved in the electrolyte and causes a significant loss of capacity.

(62) On the contrary, for samples of covalently functionalized nanotubes with the anthraquinone molecule (curve b), excellent stability is observed after 50 cycles. Other tests gave the possibility of demonstrating that this capacity may be maintained at 80% of its initial value even after 800 cycles.

(63) Post-mortem tests were also conducted with the three aforementioned button batteries, consisting of disassociating them for analyzing the color of the electrolyte.

(64) For the battery with grafted nanotubes and the battery with non-grafted nanotubes, no coloration of the electrolyte is observed.

(65) For the battery with a mixture (non-grafted nanotubes+sulfur-containing molecules), the electrolyte assumed a brown coloration, a sign of the dissolution of the anthraquinone molecule in the solvent.

(66) These tests confirm that there is no dissolution of the active material in the case of covalent grafting of the anthraquinone molecule to the carbon nanotubes.

(67) The proposed new cathode material therefore actually preserves the system from loss of capacity during cycling by immobilizing the active material at the positive electrode.

(68) As a conclusion, the accumulator according to the invention has excellent resistance to cycling.

(69) The covalent grafting preserves the system from loss of capacity during cycling by immobilizing the active material at the positive electrode, which means, in other words, that there is no dissolution of the active material into the electrolyte, unlike the other systems of the prior art.

EXAMPLE 3

(70) This example illustrates the preparation of a compound comprising a precursor group of a disulfide electron attractor group and comprising a diazonium group, which compound is able to be grafted covalently to a carbonaceous material, such as carbon nanotubes.

(71) This compound fits the following formula:

(72) ##STR00023##

(73) The preparation reaction scheme is the following:

(74) ##STR00024##

(75) After dissolution of 1,2-dimethyl-4-nitrobenzene (compound 1; 5 g) in a water/dichloromethane mixture (50/50), two equivalents of Br.sub.2 (3.4 ml) are added. The solution is stirred for 48 hours. After extraction, the organic phase is dried with Na.sub.2SO.sub.4. The organic solvent is evaporated and the product is dried in vacuo. The compound noted as 2 on the reaction scheme above is obtained as pale yellow crystals.

(76) The compound 2 (4 g) is then dissolved in methanol (100 ml). To the resulting mixture is added an excess of KSCOCH.sub.3 (4.4 g; 3 equivalents). The whole is stirred for 4 hours. The organic solvent is then evaporated. The resulting product is again dissolved in dichloromethane (100 ml) and washed with water. After extraction, the organic phase is dried with Na.sub.2SO.sub.4. The organic solvent is evaporated and the product is purified on a silica column (cyclohexane/ethyl acetate 10:1). The compound noted as 3 on the reaction scheme above is thereby obtained.

(77) The compound 3 (1.9 g) is then dissolved in a water/ethanol mixture 50/50. To the resulting mixture is added an excess of Na.sub.2S.sub.2O.sub.4 (5 g). The whole is stirred for 12 hours at 50 C. After extraction, the organic phase is dried with Na.sub.2SO.sub.4. The organic solvent is evaporated and the product is purified on a silica column (dichloromethane/methanol 99:1). The compound noted as 4 on the reaction scheme above is thereby obtained.

(78) The compound 5 is obtained by reaction of the compound 4 (2 g) with NOBF.sub.4 (1. 3g) in dichloromethane at 0 C. for 2 hours. The solvent is evaporated and the resulting product is dried in vacuo.

(79) The compound 6 may be obtained, before grafting, by reaction with a solution of methanol hydroxide in air.

(80) Next, carbon nanotubes were functionalized with a chemical grafting method similar to the one discussed in Example 1 for anthraquinone, two functionalization routes having been explored: a functionalization route by grafting the compound 5 on carbon nanotubes, the grafting being followed by a transformation of the S(COCH.sub.3) groups into disulfide bridges by reaction with a solution of methanol hydroxide in air (a so-called route I); a functionalization route by grafting the compound 6 directly on carbon nanotubes (a so-called route II).

(81) For both of these routes, tracking by spectrometry of photoelectrons induced by x-rays (so-called XPS spectrometry) was carried out before and after grafting. While sulfur is not detected on non-functionalized nanotubes, a substantial amount is detected after grafting (5%), notably with the majority presence of a signal ascribable to the sulfur atoms bound to the carbon atoms in the grafted molecule.

(82) After functionalization of the nanotubes with the sulfur-containing molecule, for both of these routes, the grafted and non-grafted samples were observed with a scanning electron microscope, showing the formation of a grafted polymer around the carbon nanotubes for the grafted samples, this grafted polymer for the carbon nanotubes obtained via route II fitting the following formula:

(83) ##STR00025##

(84) A single grafted compound has been represented for reasons of simplification, on the above formula, being aware that it is assumed that several compounds of this type are grafted covalently to the carbon nanotubes.

EXAMPLE 4

(85) In this example, the electrochemical signature of the carbon nanotubes in a first phase was studied before and after grafting by cyclic voltammetry, the tested grafted nanotubes being those of the route II mentioned in Example 3 above.

(86) To do this, a three electrode system was used, which system comprises: a working electrode consisting of grafted or non-grafted carbon nanotubes, said electrode appearing as a disc with a diameter of 16 mm and a thickness of 10-20 micrometers; a platinum counter-electrode consisting in a platinum wire; a reference electrode consisting in a massive silver wire put into contact with an AgNO.sub.3 solution (10.sup.3 M) in acetonitrile; an electrolyte consisting in acetonitrile containing lithium perchlorate LiClO.sub.4(0.1 M).

(87) After degassing the electrolyte by N.sub.2 bubbling, the system is subject to cyclic voltammetry, consisting of carrying out cycling between 1.5 to +0.25 V/AgAg.sup.+ (10.sup.3 M in acetonitrile) at a sweeping rate of 100 mV.s.sup.1, the cycling curves (I expressed in mA versus E expressed in V) being illustrated in FIG. 10 (curve a) for with the system with non-grafted nanotubes and curve b) with the system with grafted nanotubes.

(88) For the system with non-grafted nanotubes, no activity is observed (curve a).

(89) For the system with grafted carbon nanotubes, two reduction peaks and two re-oxidation peaks are observed (curve b), corresponding to the reduction and to the re-oxidation of the disulfide bridge at the sulfur-containing molecule, in other words upon opening and again closing this bridge. These results confirm the efficiency of the grafting method as well as the electrochemical activity of the grafted sulfur-containing molecule.

(90) In a second phase, tests were conducted with two button batteries, a button battery comprising, as a positive electrode, an electrode formed with grafted carbon nanotubes and a button battery comprising, as a positive electrode, an electrode formed with non-grafted carbon nanotubes, respectively, the button batteries moreover fitting the same specificities as those described in Example 2.

(91) In the tests, a current of 10 A is imposed. Specific capacities up to 20 times greater were able to be obtained for button batteries using grafted nanotubes, as confirmed by FIG. 11, which illustrates the time-dependent change of the potential E (V vs. Li.sup.+/Li) according to the discharging capacity (in mAh.g.sup.1 of electrode capacity) (curve a) for the batteries with non-grafted nanotubes and curve b) for the batteries with grafted nanotubes). Thus, values of the order of 100 mAh.g.sup.1 of electrode capacity for batteries using grafted nanotubes were able to be observed, versus 5 mAh.g.sup.1 of electrode capacity for batteries using non-grafted nanotubes. As the operating potential is located around 2.3 V vs. Li.sup.+/Li, a mass energy density of about 230 Wh.g.sup.1 may be extrapolated. The coupled analyses of these results and XPS results gave the possibility of establishing that the whole of the grafted disulfide bridges is involved in the redox process during cycling.

(92) In a third phase, other tests were conducted with both aforementioned button batteries and with a third button battery for which the positive electrode comprises a mixture between non-grafted nanotubes and the sulfur-containing molecules of formula (6) as defined in Example 3. These tests consisted of measuring the time-dependent change of the discharging capacity C (in mAh.g.sup.1 of electrode capacity) versus the number of cycles for these three button batteries and the results are copied in FIG. 12 (curve a) for batteries with non-grafted nanotubes, curve b) for batteries with grafted nanotubes and curve c) for batteries with a mixture (non-grafted nanotubes+sulfur-containing molecules), respectively.

(93) For curve c), a very significant reduction in the specific capacity is observed during the first 5 cycles, the latter passing from 100 to about 40 mAh.g.sup.1 of electrode capacity. This decrease is due to the dissolution of a large portion of the active material in the electrolyte. The decrease, although clearly not as strong, continues during the following cycles. On the contrary, for the samples with covalently functionalized nanotubes with the sulfur-containing molecule, excellent stability is observed, with 95% of the initial capacity maintained after 50 cycles.

(94) Post-mortem tests were also conducted with the three aforementioned button batteries, consisting of dissociating them in order to analyze the color of the electrolyte.

(95) For the battery with grafted nanotubes and the battery with non-grafted nanotubes, no coloration of the electrolyte is observed.

(96) For the battery with a mixture (non-grafted nanotubes+sulfur-containing molecules), the electrolyte assumed a brown-green coloration, a sign of the dissolution of the sulfur-containing molecule in the solvent.

(97) These tests confirm that there is no dissolution of the active material in the case of covalent grafting of the sulfur-containing molecule to the carbon nanotubes.

(98) The proposed new cathode material therefore actually preserves the system from the loss of capacity during cycling by immobilizing the active material at the positive electrode.