Electrochemical supercapacitor device made from an electrolyte comprising, as a conductive salt, at least one salt made from an alkali element other than lithium

Abstract

The invention relates to a device of the hybrid supercapacitor type comprising at least one cell comprising: a porous positive electrode comprising activated carbon; a negative electrode comprising a carbonaceous material capable of inserting an alkaline element other than lithium, this carbonaceous material being different from the activated carbon used at the positive electrode; and a non-aqueous electrolyte comprising a salt selected from salts of an alkaline metal other than lithium.

Claims

1. A hybrid supercapacitor device comprising at least one cell comprising: a porous positive electrode comprising activated carbon; a negative electrode comprising, as an active material, a carbonaceous material capable of inserting an alkaline element other than lithium, this carbonaceous material being different from the activated carbon used at the positive electrode; and a non-aqueous electrolyte comprising a salt selected from among salts of an alkaline metal other than lithium.

2. The device according to claim 1, wherein the salts of alkaline metal other than lithium are selected from among sodium salts, potassium salts, rubidium salts, cesium salts and mixtures thereof.

3. The device according to claim 1, wherein the salts of alkaline metal other than lithium are selected from among sodium salts, potassium salts and mixtures thereof.

4. The device according to claim 1, wherein the activated carbon is present, in the positive electrode, at a content of at least 60% by mass based on the total mass of the electrode.

5. The device according to claim 1, wherein the carbonaceous material capable of inserting an alkaline element other than lithium is a carbonaceous material comprising graphite.

6. The device according to claim 1, wherein the positive electrode and the negative electrode comprise at least one organic binder selected from among polymeric binders.

7. The device according to claim 6, wherein the polymeric binders comprise one or several polymers selected from fluorinated polymers, polyimides, polyacrylonitriles and mixtures thereof.

8. The device according to claim 1, wherein the positive electrode further comprises an electrically conductive carbonaceous additive other than activated carbon, selected from among carbon blacks, acetylene blacks, graphite, carbon nanotubes, carbon fibers and mixtures thereof.

9. The device according to claim 1, wherein the negative electrode further comprises an electrically conductive carbonaceous additive other than activated carbon, capable of inserting as defined in claim 1, selected from among carbon blacks, acetylene blacks, graphite, carbon nanotubes, carbon fibers and mixtures thereof.

10. The device according to claim 8, wherein said electrically conductive carbonaceous additive is present in a content ranging up to 15% by mass based on the total mass of the positive or negative electrode.

11. The device according to claim 1, wherein the salt is a sodium salt selected from among NaClO.sub.4, NaBF.sub.4, NaPF.sub.6, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium bis(oxalato)borate, NaSCN, NaSbF.sub.6, NaAsF.sub.6, NaAlCl.sub.4, NaSiF.sub.6, NaSO.sub.3CF.sub.3 and mixtures thereof.

12. The device according to claim 1, wherein the salt is a potassium salt selected from among KClO.sub.4, KBF.sub.4, KPF.sub.6, potassium bis(trifluoromethanesulfonyl)imide, potassium bis(fluorosulfonyl)imide, potassium bis(oxalato)borate, KSCN, KSbF.sub.6, KAsF.sub.6, KAlCl.sub.4, KSiF.sub.6, KSO.sub.3CF.sub.3 and mixtures thereof.

13. The device according to claim 1, wherein the salt(s) present in the electrolyte is(are) in solution in at least one organic solvent.

14. The device according to claim 13, wherein the organic solvent(s) is(are) selected from among nitrile solvents, carbonate solvents, lactones solvents, sulfone solvents, lactam solvents, amide solvents, ketone solvents, nitroalkane solvents, amine solvents, sulfoxide solvents, ester solvents, linear ether solvents, cyclic ether solvents, oxazolidone solvents and mixtures thereof.

15. The device according to claim 1, wherein, when the electrolyte comprises at least one sodium salt, the electrolyte comprises a salt selected from among NaClO.sub.4, NaPF.sub.6, NaBF.sub.4 and mixtures thereof, in solution in at least one solvent selected from among carbonate solvents, linear ether solvents, nitrile solvents, lactones solvents, amide solvents and mixtures thereof.

16. The device according to claim 15, wherein, when the electrolyte comprises at least one sodium salt, the electrolyte comprises at least one sodium salt selected from among NaClO.sub.4, NaPF.sub.6, NaBF.sub.4 and mixtures thereof, in solution in at least one solvent selected from among propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, dimethoxyethane, acetonitrile, γ-butyrolactone, dimethylformamide and mixtures thereof.

17. The device according to claim 15, wherein, when the electrolyte comprises at least one sodium salt, the electrolyte comprises, as a sodium salt, NaClO.sub.4, NaPF.sub.6 or NaBF.sub.4 in solution in a solvent or a mixture of solvents consisting of: propylene carbonate alone; a ethylene carbonate/diethyl carbonate (1:1) mixture; a ethylene carbonate/dimethyl carbonate (1:1) mixture; a dimethoxyethane/propylene carbonate (1:2) mixture; acetonitrile alone; γ-butyrolactone alone; or dimethylformamide alone.

18. The device according to claim 1, wherein, when the electrolyte comprises at least one potassium salt, the electrolyte comprises at least one potassium salt selected from among KClO.sub.4, KPF.sub.6, KBF.sub.4 and mixtures thereof, in solution in at least one solvent selected from among carbonate solvents, linear ether solvents, nitrile solvents, lactone solvents, amide solvents and mixtures thereof.

19. The device according to claim 18, wherein, when the electrolyte comprises at least one potassium salt, the electrolyte comprises at least one potassium salt selected from among KClO.sub.4, KPF.sub.6, KBF.sub.4 and mixtures thereof, in solution in at least one solvent selected from among propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, dimethoxyethane, acetonitrile, γ-butyrolactone, dimethylformamide and mixtures thereof.

20. The device according to claim 18, wherein, when the electrolyte comprises at least one potassium salt, the electrolyte comprises, as a potassium salt, KClO.sub.4, KPF.sub.6 or KBF.sub.4 in solution in a solvent or a mixture of solvents consisting of: propylene carbonate alone; a ethylene carbonate/diethyl carbonate (1:1) mixture; a ethylene carbonate/dimethyl carbonate (1:1) mixture; a dimethoxyethane/propylene carbonate (1:2) mixture; acetonitrile alone; γ-butyrolactone alone; or dimethylformamide alone.

21. The device according to claim 13, wherein the organic solvent is acetonitrile.

22. The device according to claim 1, wherein said negative electrode and said positive electrode are each associated with an electrically conductive current collector.

23. The device according to claim 22, wherein the electrically conductive current collector comprises aluminium.

24. The device according to claim 22, wherein, when the electrically conductive current collector comprises copper, the electrolyte does not contain any acetonitrile.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph illustrating the time-dependent change in the capacity mass density D (expressed in mAh/g) depending on the ratio R.sub.e+/e− for various supercapacitors discussed in paragraph g) of Example 1.

(2) FIG. 2 is a graph illustrating the time-dependent change in the capacity mass density D (expressed in mAh/g) depending on the ratio R.sub.e+/e− for various supercapacitors discussed in paragraph g) of Example 2.

(3) FIG. 3 is a graph illustrating the time-dependent change in the voltage U (in V) depending on the duration T (in s) for a supercapacitor in accordance with the invention and a supercapacitor non-compliant with the invention subject to charging/discharging conditions according to what is discussed in paragraph g) of Example 1.

(4) FIG. 4 is a graph illustrating the time-dependent change in the voltage U (in V) depending on the duration T (in s) for a supercapacitor in accordance with the invention and a supercapacitor non-compliant with the invention subject to charging/discharging conditions according to what is discussed in paragraph g) of Example 2.

(5) FIG. 5 is a graph illustrating the time-dependent change in the intensity I (in mA) depending on the potential E (in V) for a supercapacitor in accordance with the invention with an electrolyte comprising acetonitrile and a potassium salt compliant with part a) of Example 3.

(6) FIG. 6 is a graph illustrating the time-dependent change of the intensity I (in mA) depending on the potential E (in V) for a supercapacitor in accordance with the invention with an electrolyte comprising acetonitrile and a potassium salt compliant with part b) of Example 3.

(7) FIG. 7 is a graph illustrating a voltammogram obtained with a conventional hybrid supercapacitor.

(8) FIGS. 8 to 11 are graphs illustrating voltammograms (I (in mA) versus the potential E (in V)) for different cells of Example 4 (part a).

(9) FIG. 12 is a graph illustrating a voltammogram obtained by cycling between 1.5 and 3.2 V (3 cycles) and by cycling between 1.5 and 3.7 V (3 cycles) with the second cell of Example 4 (part b).

(10) FIG. 13 is a graph illustrating a voltammogram obtained within the scope of the test conducted in Example 5 (part a).

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

COMPARATIVE EXAMPLE 1

(11) This example illustrates the preparation of various supercapacitors non-compliant with the invention, the preparation methods of which appear below.

(12) a) Producing a First Supercapacitor Non-Compliant with the Invention

(13) The first supercapacitor non-compliant with the invention is prepared with an identical positive electrode and negative electrode, i.e. of the same nature and of the same basis weight, said electrodes being prepared by coating on an etched aluminium collector of a thickness of 30 μm with a composition comprising: 84% of activated carbon of reference YP50F (obtained from Kuraray Chemicals Co., Japan); 4% of a styrene-butadiene rubber (obtained from BASF, LD417); 8% of carbon black of reference superC65 (obtained from Timcal Co., Switzerland); 4% of carboxymethylcellulose with a mass average molecular mass of 300,000 (obtained from Aldrich);

(14) the % being expressed mass percentages based on the total mass of the electrode excluding the current collector,

(15) said electrodes have a thickness of 158 μm (collector included) and an active material mass of 21.4 mg.

(16) The aforementioned electrodes with a diameter of 14 mm are assembled in a button cell. The electrolyte used is NaPF.sub.6 (1M) in acetonitrile and is used with a sufficient amount in order to impregnate the whole of the button cell. The separator used is PDA25® (which corresponds to polypropylene) (obtained from Treofan GmbH, Germany) with a thickness of 25 μm.

(17) The system is tested via galvanostatic cycling. The volume and gravimetric capacities are measured, after 10 cycles between 0 V and 2.5 V under conditions of 0.3 A/g (gram of electrodes), between 2.43 V and 1.35 V for applying a linear regression on the discharge curve.

(18) b) Producing a Second Supercapacitor Non-Compliant with the Invention

(19) In this embodiment, the electrolyte of mode a) is replaced with 1M NaClO.sub.4 in acetonitrile. The electrodes have a thickness of 135 μm (collector included) corresponding to 15 mg of active material per electrode.

(20) c) Producing a Third Supercapacitor Non-Compliant with the Invention

(21) In this embodiment, the electrolyte of mode a) is replaced with LiPF.sub.6 1M in an EC/PC/DMC (1/1/1) mixture. The electrodes have a thickness of 160 μm (collector included) corresponding to 21.1 mg of active material per electrode.

(22) d) Results

(23) The table below shows the results obtained with the embodiments a) to c) in terms of mass capacities and of mass densities of the electrodes.

(24) TABLE-US-00001 Modes Mass capacity (in F/g) Mass energy density (Wh/kg) a 32 27.1 b 34.4 25.2 c 31.1 26.1

(25) These results show that replacing a lithiated salt with a salt containing sodium does not have any effect on the mass capacity and the mass energy density of tested supercapacitors, which may suggest that the use of a salt containing sodium is equivalent to that of a lithiated salt in terms of results.

EXAMPLE 1

(26) This example illustrates the preparation of various power supercapacitors with high energy density in accordance with the invention and, as a comparison, of supercapacitors non-compliant with the invention.

(27) a) Producing a First Supercapacitor in Accordance with the Invention

(28) In a first phase, it is proceeded with the preparation of a positive electrode and of a negative electrode.

(29) The positive electrode is prepared by coating on an etched aluminium collector with a thickness of 30 μm a composition comprising: 84% of activated carbon of reference YP50F (obtained from Kuraray Chemicals Co., Japan); 4% of a styrene-butadiene rubber (obtained from BASF, LD417); 8% of carbon black with reference superC65 (obtained from Timcal Co., Switzerland); 4% of carboxymethylcellulose with a mass average molecular mass of 300,000 (obtained from Aldrich);

(30) the % being mass percentages expressed based on the total mass of the electrode excluding the current collector,

(31) this electrode has a thickness of 156 μm (collector included) and has an active material mass of 15.8 mg.

(32) The negative electrode is prepared by coating on an etched aluminium collector with a thickness of 30 μm a composition comprising: 91.7% of graphite with reference KS6 (obtained from Timcal Co., Switzerland); 3.15% of a styrene-butadiene rubber (obtained from BASF, LD417); 3.15% of carbon black with reference superC65 (obtained from Timcal Co., Switzerland); 2% of carboxymethylcellulose with a mass molecular mass (M.sub.w) of 300,000 (obtained from Aldrich);

(33) the % being mass percentages expressed based on the total mass of the electrode excluding the current collector,

(34) this electrode having a thickness of 59 μm (collector excluded) and having an active material mass of 19 mg.

(35) The aforementioned electrodes with a diameter of 14 mm are assembled in a button cell. The electrolyte used is NaPF.sub.6 (1M) in acetonitrile and is used in a sufficient amount in order to impregnate the whole of the button cell. The separator used is PDA25® (which corresponds to polypropylene) (obtained from Treofan GmbH, Germany) with a thickness of 25 μm.

(36) The system is tested via galvanostatic cycling. The energy density is measured after 10 cycles between 0 V and 2.5 V under conditions of 0.1 A/g (gram of electrodes).

(37) b) Producing a Second Supercapacitor in Accordance with the Invention

(38) This second supercapacitor is produced according to a procedure similar to the one discussed in paragraph a) above, except that the electrolyte is replaced with NaClO.sub.4 1M in acetonitrile.

(39) c) Producing a Third Supercapacitor in Accordance with the Invention

(40) This third supercapacitor is produced according to a procedure similar to the one discussed in paragraph a) above, except that the electrolyte is replaced with NaClO.sub.4 1M in an ethylene carbonate/dimethyl carbonate mixture.

(41) d) Producing a Fourth Supercapacitor Non-Compliant with the Invention

(42) This fourth supercapacitor is produced according to a procedure similar to the one discussed in paragraph a) above, except that the activated carbon is placed at the negative electrode and the graphite at the positive electrode.

(43) e) Producing a Fifth Supercapacitor Non-Compliant with the Invention

(44) This fifth supercapacitor is produced according to a procedure similar to the one discussed in paragraph a) except that the electrolyte is replaced with LiPF.sub.6 1M in acetonitrile.

(45) f) Results

(46) The cyclings are carried out between 0 and 2.5 V and for each of the supercapacitors made, it was proceeded with measurement of the mass energy E (expressed in Wh/kg), of the mass power P (expressed in W/kg) at 72 s.

(47) The obtained results appear in the table below.

(48) TABLE-US-00002 E (in Wh/kg) P (in W/kg) at 72 s First supercapacitor 6.4 180 Second supercapacitor 4.6 120 Third supercapacitor 1.03 70 Fourth supercapacitor 0 0 Fifth supercapacitor Not measurable be- Not measurable be- cause of corrosion cause of corrosion

(49) It is seen that for the supercapacitors in accordance with the invention, high values are obtained both in terms of mass energy and of mass power.

(50) As regards the fourth supercapacitor non-compliant with the invention, no capacity was able to be measured, which is explained by the fact that the sodium can only be inserted at the negative electrode and that the activated carbon present at this negative electrode is not suitable for allowing insertion of sodium.

(51) As regards the fifth supercapacitor non-compliant with the invention, the formation of a lithium-aluminium alloy with acetonitrile is observed, causing total degradation of the negative electrode. This therefore excludes the combined use of lithium, aluminium and acetonitrile.

(52) g) Comparison of Voltage Profiles Between a Supercapacitor of the Invention and a Supercapacitor Non-Compliant with the Invention.

(53) A supercapacitor in accordance with the first aforementioned supercapacitor is subject to several charging/discharging conditions (respectively, to 0.6 A/g of active material and 1.25 A/g of active material), the time-dependent change in the voltage U (in V) depending on the duration T (in s) being illustrated by curves a) and b) of FIG. 3 enclosed as an annex.

(54) A supercapacitor non-compliant with the invention corresponds to the supercapacitor of mode a) of comparative example 1 except that the electrolyte is replaced with TEABF.sub.4 1M in acetonitrile, this supercapacitor being subject to charging/discharging conditions at 0.6 A/g, the time-dependent change in the voltage U (in V) depending on the duration T (in s) being illustrated by curve c) of FIG. 2 enclosed as an annex.

(55) For identical charging conditions, it was seen that the mass energy is 20% higher than in the case of the supercapacitor in accordance with the invention.

(56) h) Proposal for Maximizing the Energy Density of the Supercapacitors of the Invention

(57) One way of maximizing the energy density emitted by a supercapacitor is to balance the energy density of both of these electrodes. To do this, it is necessary to determine the capacity of the constitutive electrode materials of the supercapacitors of the invention.

(58) A plan of experiments was elaborated in order to test the effects of the various thickness ratios for the positive electrode and the negative electrode of the supercapacitors according to the invention and for notably finding the optimum ratio for obtaining maximum energy density. This plan was carried out with supercapacitors similar to the one described in a), i.e. notably with an electrolyte NaPF.sub.6 1M in acetonitrile, except that the thickness ratios between the positive and negative electrodes were respectively varied, as well as the active material contents and the active material masses used (the active materials respectively being the activated carbon, designated below as “active material+”, for the positive electrodes and graphite, designated below as “active material −”, for the negative electrodes).

(59) The table below groups the characteristics of the tested supercapacitors, the characteristics being the following: the ratio of the masses of active materials of the positive electrode over those of the negative electrode, entitled as R e.sup.+/e.sup.−; the mass percentage of the active material+, entitled % mat+; the mass percentage of the active material−, entitled % mat−; the total mass of the positive electrode, entitled as mtot+, expressed in mg; the total mass of the negative electrode, entitled mtot−, expressed in mg; the total mass used of positive active material, entitled as mact+, is expressed in mg; the total mass used of negative active material, entitled as mact−, is expressed in mg; the thickness of the positive electrode, entitled as e+, is expressed in μm; the thickness of the negative electrode, entitled as e−, is expressed in μm.

(60) TABLE-US-00003 R e.sup.+/e.sup.− % mat+ % mat− mtot+ mtot− mact+ mact− e+ e− 0.39 81 92.8 15.8 31.1 10.0 25.6 76 80 0.41 81 91.8 18.8 36.1 12.4 30.0 106 121 0.61 81 91.7 15.8 21.3 10.0 16.3 76 49 0.625 81 91.7 15.9 21.0 10.0 16.0 78 45 0.79 81 91.7 18.9 20.7 12.5 15.8 106 44 0.83 84 91.7 22.3 24.3 15.8 19.0 156 69 1.41 81 91.7 36.9 24.3 27.1 19.1 305 68 1.51 81 91.7 32.9 21.1 23.0 15.2 269 48

(61) In a similar way like for the supercapacitor elaborated in a), the supercapacitors, for which the characteristics are shown in the table above, are elaborated in a glove box and tested in cycling. The first formation step consists of achieving galvanostatic cycling with a low current (here, 100 μA, i.e. 3 to 6 mA/g, which corresponds to a charging/discharging process at C/2 approximately for the negative electrode).

(62) The capacities of the cells were measured (Q.sub.cell) and, with the obtained values, the capacity mass densities of the positive electrode and of the negative electrode (Q.sup.+ and Q.sup.− respectively), expressed in mAh/g were determined by means of the following formulae:
Q.sup.+=(Q.sub.cell/mtot.sup.+) Q.sup.−=(Q.sub.cell/mtot.sup.−)

(63) As this emerges from the graph of FIG. 1 (respectively curve a) for Q.sup.+ and curve b) for Q.sup.− for a test at 2 mA), the Re.sup.+.sub./e.sup.− ratio has little influence on Q.sup.+. As for Q.sup.−, the curve breaks down into a linear range, wherein Q.sup.− increases linearly versus Re.sup.+.sub./e.sup.−.

(64) This is compliant with the theory: the negative electrode is much more capacitive than the positive electrode. By not completely inserting the negative electrode, it is possible to access a high power level. For an Re.sup.+/e.sup.− ratio=1, the capacities of both electrodes are identical.

COMPARATIVE EXAMPLE 2

(65) This example illustrates the preparation of different supercapacitors non-compliant with the invention, the preparation methods of which appear below.

(66) a) Producing a First Supercapacitor Non-Compliant with the Invention

(67) The first supercapacitor non-compliant with the invention is prepared with identical positive electrode and negative electrode, i.e. of the same nature and of the same basis weight, said electrodes being prepared by coating on an etched aluminium collector with a thickness of 30 μm a composition comprising: 84% of activated carbon of reference YP50F (obtained from Kuraray Chemicals Co., Japan); 4% of a styrene-butadiene rubber (obtained from BASF, LD417); 8% of carbon black of reference superC65 (obtained from Timcal Co., Switzerland); 4% of carboxymethylcellulose with a mass average molecular mass of 300,000 (obtained from Aldrich);

(68) the % being mass percentages expressed based on the total mass of the electrode excluding the current collector,

(69) said electrodes having a thickness of 106 μm (collector included) and an active material mass of 15.9 mg.

(70) The aforementioned electrodes with a diameter of 14 mm are assembled in a button cell. The electrolyte used is KPF.sub.6 (1M) in acetonitrile and is used in a sufficient amount for impregnating the whole of the button cell. The separator used is PDA25® (which corresponds to polypropylene) (obtained from Treofan GmbH, Germany) with a thickness of 25 μm.

(71) The system is tested via galvanostatic cycling. The volume and gravimetric capacities are measured, after 10 cycles between 0 V and 2.5 V under conditions of 0.3 A/g (gram of electrodes), between 2.43 V and 1.35 V by applying a linear regression on the discharge curve.

(72) b) Producing a Second Supercapacitor Non-Compliant with the Invention

(73) In this embodiment, the electrolyte of mode a) is replaced with KClO.sub.4 1M in acetonitrile. The electrodes have a thickness of 150 μm (collector included) corresponding to 21 mg of active material per electrode.

(74) c) Producing a Third Supercapacitor Non-Compliant with the Invention

(75) In this embodiment, the electrolyte of mode a) is replaced with LiPF.sub.6 1M in an EC/PC/DMC (1/1/1) mixture. The electrodes have a thickness of 160 μm (collector included) corresponding to 21.1 mg of active material per electrode.

(76) d) Results

(77) The table below shows the results obtained with the embodiments a) to c) in terms of mass capacities and of mass densities of the electrodes.

(78) TABLE-US-00004 Modes Mass capacity (in F/g) Mass energy density (Wh/kg) a 26.6 20.3 b 35.3 28.4 c 31.1 26.1

(79) These results show that replacing a lithiated salt with a salt containing potassium has no effect on the mass capacity and the mass energy density of the tested supercapacitors, which may suggest that the use of a salt containing sodium is equivalent to that of a lithiated salt in terms of results.

EXAMPLE 2

(80) This example illustrates the preparation of various power supercapacitors with high energy density in accordance with the invention and, as a comparison, of supercapacitors non-compliant with the invention.

(81) a) Producing a First Supercapacitor in Accordance with the Invention

(82) In a first phase, it is proceeded with the preparation of a positive electrode and of a negative electrode.

(83) The positive electrode is prepared by coating on an etched aluminium collector with a thickness of 30 μm a composition comprising: 84% of activated carbon of reference YP50F (obtained from Kuraray Chemicals Co., Japan); 4% of a styrene-butadiene rubber (obtained from BASF, LD417); 8% of carbon black of reference superC65 (obtained from Timcal Co., Switzerland); 4% of carboxymethylcellulose with a mass average molecular mass of 300,000 (obtained from Aldrich);

(84) the % being mass percentages expressed on the basis of the total mass of the electrode excluding the current collector,

(85) this electrode having a thickness of 168 μm (collector included) and having an active material mass of 17.8 mg.

(86) The negative electrode is prepared by coating on an etched aluminium collector with a thickness of 30 μm a composition comprising: 91.7% of graphite of reference KS6 (obtained from Timcal Co., Switzerland); 3.15% of a styrene-butadiene rubber (obtained from BASF, LD417); 3.15% of carbon black of reference superC65 (obtained from Timcal Co., Switzerland); 2% of carboxymethylcellulose of a mass average molecular mass (M.sub.w) of 300,000 (obtained from Aldrich);

(87) the % being mass percentages expressed on the basis of total mass of the electrode excluding the current collector,

(88) this electrode having a thickness of 66 μm (collector excluded) and having an active mass of 18.9 mg.

(89) The aforementioned electrodes with a diameter of 14 mm are assembled in a button cell. The electrolyte used is KPF.sub.6 (1M) in acetonitrile and is used in a sufficient amount for impregnating the whole of the button cell. The separator used is PDA25® (which is polypropylene) (obtained from Treofan GmbH, Germany) with a thickness of 25 μm.

(90) The system is tested via galvanostatic cycling. The energy density is measured after 10 cycles between 0 V and 2.5 V under conditions of 0.1 A/g (gram of electrodes).

(91) b) Producing a Second Supercapacitor in Accordance with the Invention

(92) This second supercapacitor is produced according to a procedure similar to the one discussed in paragraph a) above, except that the electrolyte is replaced with KPF.sub.6 1M in an ethylene carbonate/dimethyl carbonate mixture.

(93) c) Producing a Third Supercapacitor in Accordance with the Invention

(94) This third supercapacitor is produced according to a procedure similar to the one discussed in paragraph a) above, except that the electrolyte is replaced with KClO.sub.4 1M in an ethylene carbonate/dimethyl carbonate mixture.

(95) d) Producing a Fourth Supercapacitor Non-Compliant with the Invention

(96) This fourth supercapacitor is produced according to a procedure similar to the one discussed in paragraph a) above, except that the activated carbon is placed at the negative electrode and the graphite at the positive electrode.

(97) e) Producing a Fifth Supercapacitor Non-Compliant with the Invention

(98) This fifth supercapacitor is produced according to a procedure similar to the one discussed in paragraph a) above, except that the electrolyte is replaced with LiPF.sub.6 1M in acetonitrile.

(99) f) Results

(100) The cyclings were carried out between 0 and 2.5 V and for each of the produced supercapacitors, it was proceeded with the measurement of the mass energy E (expressed in Wh/kg) and of the mass power P (expressed in W/kg) at 72 s.

(101) The obtained results appear in the table below.

(102) TABLE-US-00005 E (in Wh/kg) P (in W/kg) at 72 s First supercapacitor 5.0 250 Second supercapacitor 1.2 60 Third supercapacitor 1.4 65 Fourth supercapacitor 0 0 Fifth supercapacitor Not measurable be- Not measurable be- cause of corrosion cause of corrosion

(103) It is seen that, for the supercapacitors in accordance with the invention, higher values are obtained in terms of mass energy and of mass power.

(104) As regards the fourth supercapacitor non-compliant with the invention, no capacity was able to be measured, which is explained by the fact that sodium can only be inserted at the negative electrode and that the activated carbon present at this negative electrode is not suitable for allowing insertion of sodium.

(105) As regards the fifth supercapacitor non-compliant with the invention, the formation of a lithium-aluminium alloy is observed.

(106) This therefore excludes the combined use of lithium, aluminium and acetonitrile.

(107) g) Comparison of Voltage Profiles Between a Supercapacitor of the Invention and a Supercapacitor Non-Compliant with the Invention

(108) A supercapacitor compliant with the first aforementioned supercapacitor is subject to several charging/discharging conditions (respectively, at 0.6 A/g of active material, 1.25 A/g of active material, 1.85 A/g of active material and 3.7 A/g of active material), the time-dependent change in the voltage U (in V) depending on the duration T (in s) being illustrated by curves a) to d) of FIG. 3 enclosed as an annex.

(109) A supercapacitor non-compliant with the invention corresponds to the supercapacitor of mode a) of Comparative Example 1 except that the electrolyte is replaced with TEABF.sub.4 1M in acetonitrile, this supercapacitor being subject to charging/discharging conditions at 0.6 A/g, the time-dependent change in the voltage U (in V) depending on the duration T (in s) being illustrated by curve e) of FIG. 4 enclosed as an annex.

(110) For identical charging conditions, it was seen that the mass energy is 1.9 times higher in the case of the supercapacitor in accordance with the invention.

(111) h) Proposal for Maximizing the Energy Density of the Supercapacitors of the Invention

(112) One way of maximizing the energy density emitted by a supercapacitor is to balance the energy density of both of these electrodes. To do this, it is necessary to determine the capacity of the constitutive electrode materials of the supercapacitors of the invention.

(113) A plan of experiments was elaborated in order to test the effects of the different thickness ratios for the positive electrode and the negative electrode of the supercapacitors according to the invention and notably finding the optimum ratio for obtaining maximum energy density. This plan was carried out with supercapacitors like the one described in a), i.e. notably with an electrolyte KPF.sub.6 1M in acetonitrile, except that the thickness ratios between the electrodes, the active material contents and the active material masses used (the active materials respectively being the activated carbon, designated below as “active material+”, for the positive electrodes and graphite, designated below as “active material−”, for the negative electrodes) were varied respectively.

(114) The table below groups the characteristics of the tested supercapacitors, the characteristics being the following: the ratio of the masses of active materials of the positive electrode over those of the negative electrode, entitled as R e.sup.+/e.sup.−; the mass percentage of the active material+, entitled as % mat+; the mass percentage of the active material−, entitled as % mat−; the total mass of the positive electrode, entitled as mtot+, expressed in mg; the total mass of the negative electrode, entitled as mtot−, expressed in mg; the total mass used of positive active material, entitled as mact+, expressed in mg; the total mass used of negative active material, entitled as mact−, expressed in mg; the thickness of the positive electrode, entitled as e+, expressed in μm; the thickness of the negative electrode, entitled as e−, expressed in μm.

(115) TABLE-US-00006 R e.sup.+/e.sup.− % mat+ % mat− mtot+ mtot− mact+ mact− e+ e− 0.54 81 90.8 18.6 28.5 12.2 22.7 108 84 0.78 81 91.7 23.5 27.5 17.2 22.0 157 70 0.94 86 91.7 24.2 24.1 17.8 18.9 168 66 1.02 81 91.7 27.3 24.3 19.3 19.0 125 68 1.41 81 91.7 31.2 20.8 22.4 15.9 250 47 1.68 81 91.7 36.9 21.1 27.1 16.1 305 46

(116) In the same way as for the supercapacitor elaborated in a), the supercapacitors, for which the characteristics are stated in the table above, are elaborated in a glove box and tested in cycling. The first formation step consists of producing galvanostatic cycling with a low current (here, 100 μA, i.e. 3 to 6 mA/g, which corresponds to a charging/discharging process at about C/2 for the negative electrode).

(117) The capacities of the cells were measured (Q.sub.cell) and, with the obtained values, the capacity mass densities of the positive electrode and of the negative electrode (Q.sup.+ and Q.sup.− respectively), expressed in mAh/g were determined via the following formulae:
Q.sup.+=(Q.sub.cell/mtot.sup.+) Q.sup.−=(Q.sub.cell/mtot.sup.−)

(118) As apparent from FIG. 2 (respectively curve a) for Q.sup.+ and curve b) for Q.sup.− for a test at 2 mA), the Re.sup.+.sub./e.sup.− ratio has little influence on Q.sup.+. As regards Q.sup.−, the curve is broken down into a linear domain, wherein Q.sup.− linearly increases versus Re.sup.+.sub./e.sup.−. This is compliant with theory: the negative electrode is much more capacitive than the positive electrode. By not completely inserting the negative electrode, it is possible to access a high power level. For a ratio Re.sup.+/e.sup.−=1, the capacities of both electrodes are identical.

EXAMPLE 3

(119) This example aims at demonstrating the benefit of using, as a solvent, acetonitrile in the devices of the invention with view to giving it good performances, whether the electrolyte is based on a potassium salt (part a) of this example) or on a sodium salt (part b) of this example).

(120) a) Test Conducted with a Device According to the Invention with an Electrolyte Based on a Potassium Salt

(121) In a first phase, it is proceeded with the preparation of a positive electrode and of a negative electrode.

(122) The positive electrode is prepared by coating on an etched aluminium collector with a thickness of 30 μm a composition comprising: 84% of activated carbon of reference YP50F (obtained from Kuraray Chemicals Co., Japan); 4% of a styrene-butadiene rubber (obtained from BASF, LD417); 8% of carbon black of reference superC65 (obtained from Timcal Co., Switzerland); 4% of carboxymethylcellulose with a mass average molecular mass of 300,000 (obtained from Aldrich);

(123) the % being mass percentages expressed based on the total mass of the electrode excluding the current collector.

(124) The negative electrode is prepared by coating on an etched aluminium collector with a thickness of 30 μm a composition comprising: 94% of graphite of reference SLP30 (obtained from Timcal Co., Switzerland); 2% of conductive carbon VGCF (acronym corresponding to “vapour grown carbon fibre”); 2% of carboxymethylcellulose at 2% (reference 7HXF at Aqualon); 2% of a dispersion with 51% of styrene-butadiene rubber (obtained from BASF under the brand LD417®),

(125) the % being mass percentages expressed based on the total mass of the electrode except for the current collector.

(126) The aforementioned positive and negative electrodes with a surface area equal to 10.24 cm.sup.2 (i.e. dimensions of 3.2×3.2 cm) are assembled in a cell of the “pouch cell” type.

(127) The electrolyte used is a solution comprising acetonitrile comprising a potassium salt (KPF.sub.6 1M) and is used in a sufficient amount in order to impregnate the whole of the cell. The separator used is a separator in PDA25® (which corresponds to polypropylene) (obtained from Treofan GmbH, Germany) with a thickness of 25 μm.

(128) The cell is studied by cyclic voltammetry, the results being transferred onto FIG. 5.

(129) The area under the voltammogram is particularly large, which confirms a significant capacity of the supercapacitor, being aware that the area under the voltammogram is proportional to the capacity of the supercapacitor.

(130) b) Test Conducted with a Device in Accordance with the Invention with an Electrolyte Based on a Sodium Salt

(131) The tested device in this part is similar to the one exemplified in part a), except for the electrolyte which is in this case a solution comprising acetonitrile and a sodium salt NaPF.sub.6 (1M).

(132) The device is also studied by cyclic voltammetry, the results being copied onto FIG. 6.

(133) The results show a similar trend to the one observed with the exemplified device in part a), except that the area under the voltammogram obtained with the device of part a) is larger than the one obtained with the device of part b).

EXAMPLE 4

(134) This example aims at demonstrating the influence of balancing of the electrodes of the devices of the invention, on the performances of the latter (part a) of the example) and on the stability of the latter (part b) of the example).

(135) a) Influence of the Balancing of the Electrodes on the Performances of the Device

(136) As a preliminary remark, it is recalled, with reference to FIG. 7, which illustrates a voltammogram obtained with a hybrid supercapacitor, the conventional electrochemical behavior expected with such a supercapacitor.

(137) In this figure, the presence of two parts may be seen (designated as part a and part b in the figure respectively), part a corresponding to the so-called “supercapacitive” part and the part b corresponding to the so-called “battery” part of the device.

(138) Now, as the final energy of the device is proportional to the capacity and to the square of the imposed potential, it may therefore be interesting to manage to displace the operation of the device towards the “battery” part (which gives the possibility of accessing a maximum capacity) towards potentials as high as possible.

(139) To do this, tests were carried out with devices similar to those described in paragraph a) of Example 3 (for which the salt of the electrolyte is a potassium salt KPF.sub.6) comprising negative electrodes having different base weights (or surface masses), and more specifically with the following devices: a cell, a so-called first cell, equipped with a negative electrode at 4 mg/cm.sup.3 and a positive electrode at 8 mg/cm.sup.3, which corresponds to a (positive electrode/negative electrode) ratio of 2; a cell, a so-called second cell, equipped with a negative electrode at 6 mg/cm.sup.3 and of a positive electrode at 8 mg/cm.sup.3, which corresponds to a (positive electrode/negative electrode) ratio of 1.3; a cell, a so-called third cell, equipped with a negative electrode at 9 mg/cm.sup.3 and with a positive electrode at 8 mg/cm.sup.3, which corresponds to a (positive electrode/negative electrode) ratio of 0.88; and a cell, a so-called fourth cell, equipped with a negative electrode at 13 mg/cm.sup.3 and with a positive electrode at 8 mg/cm.sup.3, which corresponds to a (positive electrode/negative electrode) ratio of 0.61.

(140) These different cells are subject to cyclic voltammetry tests (from 0.5.fwdarw.3.2 to 0.5.fwdarw.4 V), the results being respectively transferred onto FIGS. 8 to 11 for the first to the fourth cell.

(141) For the first cell (FIG. 8), as soon as the first cycle (from 0.5 to 2.2 V) occurs, the presence of a peak may be noticed showing that we are already in the “battery” part.

(142) Conversely, for the fourth cell (FIG. 11), the first cycle (from 0.5 to 2.2 V) has a rectangular shape specific to the “supercapacitive” part, the “battery” part, as for it, only appearing during the cycle from 0.5 to 3 V, i.e. a gain of about 1V with respect to the first cell.

(143) The second and third cells follow the same trend.

(144) These tests show, that it is easily possible, with the devices in accordance with the invention, of moving towards the “battery” part by means of an excessive basis weight of the negative electrode with respect to the positive electrode.

(145) b) Influence of the Balancing and of the Cycling Limits on the Stability of the System

(146) In this part, the influence of the balancing of the electrodes on the devices of the invention on the stability of the latter is studied.

(147) To do this, tests were carried out with devices similar to those described in paragraph a) of Example 3 (for which the salt of the electrolyte is a potassium salt KPF.sub.6) comprising negative electrodes having various basis weights (or surface masses), and more specifically with the following devices: a cell, a so-called first cell, equipped with a negative electrode at 4 mg/cm.sup.3 and with a positive electrode at 8 mg/cm.sup.3, which corresponds to a (positive electrode/negative electrode) ratio of 2; and a cell, a so-called second cell, equipped with a negative electrode at 13 mg/cm.sup.3 and with a positive electrode at 8 mg/cm.sup.3, which corresponds to a (positive electrode/negative electrode) ratio of 0.61.

(148) The aforementioned cells were tested by galvanostatic cycling from 1.5 to 3.7 V during 1,000 cycles at 20 mA.

(149) It is seen that increasing the basis weight has an influence on the stability of the device (notably materialized by a loss of capacities at cycle 1,000 as compared with cycle 1 of more than 40% for the second cell, while it is greater than 60% for the first cell).

(150) In order to further improve the stability, reduction in the upper operating limit of the device, by passing from 3.7 V to 3.2 V, was tested.

(151) It was ascertained that, for a negative electrode with a basis weight of 13 mg/cm.sup.2, if it is cycled up to 3.2 V, there is no current loss at the end of the three cycles (while a loss is already ascertained when cycled up to 3.7 V), as confirmed by FIG. 12 illustrating a voltammogram obtained by cycling between 1.5 and 3.2 V (3 cycles) and by cycling between 1.5 and 3.7 V (3 cycles) with the second cell.

(152) By operating galvanostatic cycling over 1,000 cycles between 1.5 and 3.2 V, a loss of capacities of less than 10% was ascertained, which may prove to be interesting for many applications.

EXAMPLE 5

(153) This example aims at demonstrating the safety aspect of the devices of the invention as compared with storage systems based on lithium, notably related to the following facts: the possibility of completely discharging the devices of the invention (part a); the possibility of doing without a passivation layer (part b); the possibility of doing without over-dimensioning of the negative electrode with respect to the positive electrode (part c); the possibility of dissolving solid potassium in acetonitrile.

(154) The tested device of the invention is the one of part a) of Example 3.

(155) a) Possibility of Completely Discharging the Devices According to the Invention

(156) In the case of a storage system based on lithium, it is not possible to completely discharge the system, which implies that there exists energy loaded into the system and it therefore is a potential risk related to safety.

(157) In the case of the tested device of the invention, in order to make sure that it is possible to completely discharge the device, it was proceeded with three cycles from 0.5 to 3.2 V and then with three cycles from 0 to 3.2 V.

(158) As confirmed in FIG. 13, illustrating the voltammogram of the test mentioned above, discharging the device completely has no influence on the performances of the latter, in the sense that the device has the same capacity during the following cycles.

(159) Further, a “post-mortem” study of the device, i.e. after opening the latter and visually inspecting these different elements, gave the possibility of asserting that none of the elements was subject to degradation subsequent to complete discharge of the latter.

(160) As a conclusion, the possibility of completely discharging the system represents a non-negligible advantage, for example, in the case when it is absolutely necessary to directly intervene on the device.

(161) b) Possibility of Omitting a Passivation Layer

(162) In the case of a system based on lithium, one of the safety problems is related to the requirement of forming a passivation layer at the negative electrode. The generally applied solution consists of applying an additional lithium sheet for saturating the system with lithium ions, the majority of which is consumed during the first cycles. This application poses safety problems and significant overcost.

(163) In the case of the device of the invention, it is not necessary to require the formation of a passivation layer, which has an advantage from the point of view of safety.

(164) c) Possibility of Doing without Over-Dimensioning of the Negative Electrode with Respect to the Positive Electrode

(165) In the case of a system based on lithium, with view to doing without the optional deposition of lithium metal at the surface of the negative electrode, it is customary to over-dimension the negative electrode.

(166) In the case of the devices of the invention, a test was carried out by over-dimensioning the negative electrode (35*35 mm instead of 32*32 mm for the positive electrode). This test did not show any difference in terms of performances, which further is another advantage of the devices of the invention.

(167) d) Possibility of Dissolving Solid Potassium in Acetonitrile

(168) The use of acetonitrile certainly has an advantage, in the sense that it gives the possibility, because of the nature of the potassium in the latter, of getting rid of the risk related to the deposition of solid potassium at the surface of the electrode.