Porous electrolyte membrane, manufacturing process thereof and electrochemical devices comprising same

Abstract

A porous electrolyte membrane including a first main surface and a second main surface that are separated by a thickness, where carbon nanotubes, defining through-pores or through-channels that are open at their two ends, have a diameter smaller than or equal to 100 nm, are oriented in the direction of the thickness of the membrane, and are all substantially parallel over the entire thickness of the membrane, connect the first main surface and the second main surface; the carbon nanotubes are separated by a space, and the space between the carbon nanotubes is completely filled with at least one solid material, and an electrolyte is confined inside the carbon nanotubes. A method for preparing the membrane and an electrochemical device, such as a lithium accumulator or battery, including the electrolyte membrane.

Claims

1. A battery, comprising: a positive electrode; a negative electrode; and a porous membrane with an electrolyte between the positive and negative electrodes, the membrane having a first main surface and a second main surface separated by a thickness, wherein the membrane comprises: carbon nanotubes connecting the first main surface and the second main surface, defining through-pores or through-channels open at both of their ends, having a diameter of less than or equal to 100 nm, oriented in the direction of the thickness of the membrane and all substantially parallel and separated by a space, on the totality of the thickness of the membrane; at least one solid material totally filling the space between the carbon nanotubes; and an electrolyte confined inside the carbon nanotubes such that a chemical composition of the confined electrolyte is the same in all the inside of the carbon nanotubes.

2. The battery according to claim 1, wherein the first and the second main surfaces are planar and parallel, the membrane is a planar membrane and the nanotubes, the pores or channels, are substantially aligned, or aligned, perpendicularly to said surface.

3. The battery according to claim 1, wherein the carbon nanotubes are functionalized on their outer wall in order to make them electronically insulating, or else the carbon nanotubes are functionalized on their outer wall with redox species and/or electroactive species.

4. The battery according to claim 1, wherein the carbon nanotubes have an internal diameter of from 1 to 100 nm.

5. The battery according to claim 1, wherein the carbon nanotubes and the pores or channels have a length of from 10 microns to 100 mm.

6. The battery according to claim 1, wherein the solid material is selected from the group consisting of electronically insulating materials and electronically conducting materials for which the outer surface, in contact with the outside of the membrane, has been made electronically insulating.

7. The battery according to claim 1, wherein the solid material is selected from the group consisting of organic polymers, metals and metal oxides.

8. The battery according to claim 1, wherein the electrolyte is at least one selected from the group consisting of a proton carrier or proton conductor, a zwitterion ionic liquid, an acid dissolved in an organic polymer, an ionic liquid, an ionic liquid comprising an ionic conducting salt, a liquid organic solvent or an organic polymer comprising an ionic conducting salt, an ionic liquid in an organic polymer, a mixture of an organic polymer and of an organic solvent, a mixture of an ionic liquid and of an organic solvent, a mixture of an ionic liquid, of an organic solvent and of a salt of an alkaline or earth-alkaline metal, a mixture of an organic polymer, of an organic solvent and of a salt of an alkaline or earth-alkaline metal, and a mixture of a salt of an alkaline or earth-alkaline metal in a protonic ionic liquid.

9. The battery according to claim 8, wherein the organic polymer is a polymer selected from the group consisting of homopolymers and copolymers of ethylene oxide, and their derivatives.

10. The battery according to claim 8, wherein the organic polymer has a molar mass of less than 10.sup.6 g/mol.

11. The battery according to claim 8, wherein the ionic conducting salt is a salt of an alkaline metal or a salt of an earth-alkaline metal.

12. The battery according to claim 11, wherein the ionic conducting salt is a lithium salt, selected from the group consisting of LiAsF.sub.6, LiClO.sub.4, LiBF.sub.4, LiPF.sub.6, lithium bis(oxalato)borate (LiBOB), LiODBF, LiB(C.sub.6H.sub.5), LiRFSO.sub.3, LiCH.sub.3SO.sub.3, LiN(R.sub.FSO.sub.2).sub.2, LiC(R.sub.FSO.sub.2).sub.3, wherein R.sub.F is selected from the group consisting of a fluorine atom and a perfluoroalkyl group comprising from 1 to 9 carbon atoms, or a sodium salt analogous to the lithium salts thereof but comprising a sodium ion instead of a lithium ion.

13. The battery according to claim 8, wherein the concentration of ionic conducting salt in the electrolyte is from 1 to 50% by mass based on the mass of the electrolyte.

14. The battery according to claim 1, wherein the electrolyte totally fills the carbon nanotubes.

15. A method for preparing the battery according to claim 1, comprising sequentially: a) growing carbon nanotubes, all substantially parallel, and separated by a space, on a surface of a substrate provided with a growth catalyst of carbon nanotubes; b) totally filling said space between the carbon nanotubes with a solid material; or else the following a1) is carried out: a1) growing carbon nanotubes, all substantially parallel, and separated, on a surface of a substrate and inside the pores of a porous solid material with oriented pores; and then, at the end of b) or of a1), the following c) is carried out: c) removing the substrate and, any possible solid material in excess, and opening both ends of the carbon nanotubes; and then, at the end of c), the following d) is carried out to obtain the porous membrane with an electrolyte: d) filling the inside of the nanotubes with an electrolyte; and then, at the end of d), providing positive and negative electrodes such that the membrane is positioned between the positive and negative electrodes.

16. The method according to claim 15, wherein the growth substrate is a silicon wafer, or a stainless steel or aluminium sheet on which is deposited an alumina layer, and the growth catalyst of the carbon nanotubes is deposited on the alumina layer.

17. The method according to claim 15, wherein the growth catalyst of the carbon nanotubes is selected from the group consisting of iron, nickel, cobalt, and alloys thereof.

18. The method according to claim 15, wherein the carbon nanotubes are grown by a chemical vapor deposition method CVD.

19. The method according to claim 15, wherein the solid material is an organic polymer and b) is carried out: either by dissolving the organic polymer in a solvent in order to form a solution of the organic polymer, by totally filling the space between the carbon nanotubes with the solution of the organic polymer and by evaporating the solvent; or by heating the organic polymer in the absence of any solvent above its glass transition temperature (Tg) or above its melting point for making it fluid, and by leaving the fluid polymer be absorbed in the space between the carbon nanotubes; or by filling the space between the carbon nanotubes with a mixture comprising organic monomers, or organic oligomers modified by reactive functions, or organic copolymers, and further one or several photosensitive and/or thermo-sensitive free radicals initiator(s); and then by cross-linking the mixture thermally or by means of photon radiation.

20. The method according to claim 15, wherein the solid material is a metal, and then b) is carried out by depositing said metal by an electrochemical deposition method in the space between the carbon nanotubes, or else the solid material is a metal oxide and then b) is carried out by depositing said metal oxide by an electrochemical deposition method, or by a sol-gel method, in the space between the carbon nanotubes.

21. The method according to claim 15, wherein b) is carried out by projecting said solid material in the space between the carbon nanotubes.

22. The method according to claim 15, wherein c) is carried out by mechanical polishing and/or plasma etching.

23. The battery according to claim 3, wherein the carbon nanotubes are functionalized on their outer wall in order to make them electronically insulating by fluorination or by an organic compound.

24. The battery according to claim 8, wherein the electrolyte comprises a proton carrier or a proton conductor, and the proton carrier or the proton conductor is a protonic ionic liquid or a protonic conducting polymer.

25. The battery according to claim 8, wherein the ionic conducting salt is a salt of an alkaline metal or a salt of an earth-alkaline metal.

26. The battery according to claim 8, wherein the electrolyte comprises a mixture of a salt of an alkaline or earth-alkaline metal in a protonic ionic liquid, where the alkaline or earth-alkaline metal is lithium.

27. The battery according to claim 12, wherein LiR.sub.FSO.sub.3 is LiCF.sub.3SO.sub.3, LiN(R.sub.FSO.sub.2).sub.2 is LiN(CF.sub.3SO.sub.2).sub.2 (LiTFSI) or LiN(C.sub.2F.sub.5SO.sub.2).sub.2 (LiBETI), and LiC(R.sub.FSO.sub.2).sub.3 is LiC(CF.sub.3SO.sub.2).sub.3 (LiTFSM).

28. The battery according to claim 12, wherein R.sub.F is selected from the group consisting of a fluorine atom and a perfluoroalkyl group comprising from 1 to 8 carbon atoms.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of the steps a) (FIG. 1A), b) (FIG. 1B) and c) (FIG. 1C) of the method according to the invention during which a carpet or a forest of carbon nanotubes (FIG. 1A) is transformed into a membrane of carbon nanotubes (FIG. 1C).

(2) FIG. 2 is a schematic view of a battery, accumulator, such as a lithium battery, accumulator comprising the membrane with an electrolyte according to the invention.

(3) FIG. 3 is a schematic view of a particular embodiment of a lithium battery, accumulator comprising the membrane with an electrolyte according to the invention.

(4) FIG. 4 is a schematic view of another particular embodiment of a lithium battery, accumulator, a so called Full 1D lithium battery, accumulator, comprising the membrane with an electrolyte according to the invention.

(5) FIG. 5 is a photograph taken with a scanning electron microscope of the carpet or forest of carbon nanotubes obtained at the end of step 1 of Example 1.

(6) The scale plotted in FIG. 5 represents 10 m.

(7) FIG. 6 is a photograph taken with a scanning electron microscope of the membrane obtained at the end of step 3 of Example 1.

(8) The scale plotted in FIG. 6 represents 100 m.

(9) FIG. 7 is a graph which gives, at room temperature, the self-diffusion coefficient measured by .sup.19F NMR with a field gradient, of the ionic liquid confined in the pores of the CNTs membrane of Example 1, i.e. 1-octyl-3-methylimidazolium tetrafluoroborate, (OMIMBF.sub.4) ( points); and the self-diffusion coefficient of this same ionic liquid, i.e. 1-octyl-3-methylimidazolium tetrafluoroborate, (OMIMBF.sub.4), but not confined in the pores of the membrane (o points). This non-confined ionic liquid is also called a volume ionic liquid or a bulk ionic liquid.

(10) In abscissas, is plotted the diffusion function (G.sup.2.sup.2.sup.2(/3)) in s/cm.sup.2 (second/centimeter.sup.2), wherein G is the pulsed field gradient, is the gyromagnetic ratio of the studied nucleushere fluorine-19- the duration of the gradient pulse, and A the diffusion time (in this experiment, we used a sequence of the stimulated gradient type with =3 ms, A=50 ms and G varying from 5 G/cm to 700 G/cm).

(11) In ordinates is plotted the relative change of the NMR signal (without any unit).

(12) FIG. 8 is a schematic layout of the device which gave the possibility of measuring by impedance spectroscopy, at room temperature, the conductivity of the ionic liquid confined in the pores of the CNTs membrane of Example 1, i.e. 1-octyl-3-methylimidazolium tetrafluoroborate, (OMIMBF.sub.4); and the conductivity of this same ionic liquid, i.e. 1-octyl-3-methylimidazolium tetrafluoroborate, (OMIMBF.sub.4), but not confined in the pores of the membrane.

(13) FIG. 9 is a simplified schematic layout of the device of FIG. 8.

(14) FIG. 10 is a graph which gives the electric impedance of the ionic liquid confined in the pores of the CNTs membrane of Example 1, i.e. 1-octyl-3-methylimidazolium tetrafluoroborate, (OMIMBF.sub.4) (OmimBF.sub.4 Bulk+CNT); and the impedance of this same ionic liquid, i.e. 1-octyl-3-methylimidazolium tetrafluoroborate, (OMIMBF.sub.4), but not confined in the pores of the membrane (OmimBF.sub.4 Bulk).

(15) In ordinates, is plotted the imaginary part of the electric impedance (in Ohms), and in abscissas is plotted the real part of the electric impedance (in Ohms).

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

(16) This description more particularly refers to an embodiment in which the porous membrane with an electrolyte according to the invention is a membrane with an ionic liquid electrolyte, notably the membrane with an electrolyte of a lithium battery, accumulator, but it is quite obvious that the following description may easily be extended, if required, to any membrane with an electrolyte which may be applied in any electrochemical device or system, regardless of the electrolyte.

(17) Further, the description which follows is rather accomplished by convenience in connection with the method for preparing the membrane according to the invention but it also contains teachings which relate to the membrane prepared by this method.

(18) In order to prepare the membrane according to the invention, it is possible to begin by preparing, synthesizing, a carpet of carbon nanotubes, which may also be described as a forest of carbon nanotubes, on a surface (3) of a solid substrate (4) (FIG. 1A).

(19) From this carpet, or forest, a membrane is then obtained by bridging, filling the inter-tube space and by opening the carbon nanotubes on each side of the membrane (see FIG. 1B and FIG. 1C).

(20) The carpet of CNTs (1) may be synthesized with a chemical vapor deposition method CVD.

(21) According to a first embodiment, a carpet (1) of multi-walled nanotubes (2) may be synthesized on a surface (3) of a solid substrate (4).

(22) In this case, the growth substrate is a silicon wafer on which is deposited by an ALD (Atomic Layer Deposition) method an alumina layer generally with a thickness of 20 nm.

(23) According to an alternative, this alumina layer may be deposited on the substrate by cathode sputtering, or by an Ion Beam Sputtering or IBS method.

(24) On this substrate, a catalyst is deposited, this catalyst may for example be iron, nickel, cobalt, or an alloy of these metals.

(25) This catalyst generally appears as a layer for example with a thickness from 0.2 to 2 nm.

(26) In this embodiment where a carpet of small multi-walled nanotubes (i.e. with a diameter from 3 to 5 nm) is synthesized, an iron layer with a thickness of 1 nm is deposited by evaporation with an electron gun.

(27) This substrate just before the growth of the nanotubes may optionally be treated with a plasma.

(28) However, this treatment with a plasma is not mandatory and may be omitted.

(29) For example, this substrate may be treated by means of a succession of two air plasmas generally at the pressure of 0.3 mbar. This treatment has the goal of suppressing the parasitic carbon of the sample and of oxidizing the catalyst.

(30) The first plasma may be a plasma with a period of 20 minutes with a power of 80 W, the second plasma may be a plasma with a period of 20 minutes with a power of 30 W.

(31) The substrate is then introduced into a chemical vapor deposition CVD enclosure.

(32) This deposition enclosure may, in this embodiment where a carpet of small (i.e. with a diameter from 3 to 5 nm) multi-wall nanotubes is synthesized, including a network of 10 filaments mounted in parallel.

(33) The filaments are for example located at 1 cm from the sample holder (sole) and spaced apart by 1 cm.

(34) The gas mixture, consisting for example of 20 sccm of acetylene, 50 sccm of hydrogen and 110 sccm of helium, is introduced under cold conditions at a pressure for example of 0.9 mbar.

(35) The sole is brought to the temperature of 400 C. in 10 minutes and then a plateau is observed at this temperature. The filaments are heated by the joule effect with a power for example of 800 W.

(36) Under these conditions after a plateau of 20 minutes at the temperature of 400 C., a carpet of nanotubes is obtained for which the average diameter is 4.5 nm and the height is 200 m.

(37) If the plateau is brought to 45 minutes, carpets with a height of 400 m are obtained.

(38) The density of the nanotubes on the substrate is generally greater than 10.sup.11 cm.sup.2. According to a second embodiment, a carpet (1) of single-walled carbon nanotubes (2) may be synthesized on a surface (3) of a solid substrate (4).

(39) In this case relatively to the first embodiment of the synthesis of the carpet of nanotubes: the thickness of the catalyst layer like iron is reduced to 0.25 nm, the temperature of the heating sole is brought to 500 C., the number of filaments is reduced to 6, the gas mixture consists of 5 sccm of acetylene, 200 sccm of hydrogen, and 200 sccm of helium.

(40) The other conditions of the synthesis of the carpet of nanotubes are identical with those of the first embodiment of this synthesis.

(41) During this step for synthesizing the carpet of carbon nanotubes CNTs, the man skilled in the art may easily adapt the conditions of the method so as to obtain single-walled or multi-walled carbon nanotubes having the diameter, the grafting density of the CNTs, and the desired length of the CNTs, within wide ranges.

(42) Thus: Diameter of the pores: CNTs may be obtained for which the diameter is found in the range from 1 to 100 nm, it is believed that the increase in the diffusion coefficient and/or in the ionic conductivity is inversely proportional to the diameter of the CNTs. Preferably it is sought to obtain CNTs for which the diameter is in the range from 1 to 3 nm. Grafting density of the CNTs: It is possible to obtain a grafting density from 10.sup.9 to 10.sup.13 cm.sup.2. It will generally be sought to optimize the grafting density of the CNTs, so that it is as high as possible, for example in the range from 10.sup.11 cm.sup.2 to 10.sup.13 cm.sup.2. Length of the CNTs: It is possible to obtain CNTs with a length in the range from 10 microns to 100 mm, preferably from 50 microns to 500 microns, for example 150 microns.

(43) After having synthesized the carpet (1) of carbon nanotubes (2), on a surface (3) of a solid substrate (4), this carpet is transformed into a membrane by bridging/filling/filling up the space between the carbon nanotubes, CNTs, with a solid material such as an organic polymer also called a matrix material (5) (FIG. 1B).

(44) In the case when the solid material is an organic polymer, this polymer may be dissolved in an adequate solvent for obtaining a solution of the organic polymer in the solvent.

(45) The polymer such as the polystyrene of 350,000 g/mol may for example be dissolved in toluene for obtaining a solution at 20% by mass.

(46) The solution of the organic polymer is poured on the carpet of CNTs, in return for which it fills the space between the carbon nanotubes, and then the solvent is left to evaporate.

(47) Or else, it is possible to use an organic polymer without any solvent and to heat it, preferably in vacuo, above its glass transition temperature Tg, or above its melting point, for thus obtaining a fluid or molten polymer. This fluid or molten polymer may be left to be absorbed in the intertube space by simple capillarity.

(48) If the solid material is a metal or metal oxide, said metal or said metal oxide may then be deposited by one of the methods already mentioned above, such as an electrochemical deposition method or a sol-gel method, in the space between the carbon nanotubes.

(49) Regardless of the solid material, the space between the carbon nanotubes may be filled by projecting said solid material in the space between the carbon nanotubes.

(50) According to another embodiment, instead of growing carbon nanotubes, all substantially parallel and separated by a space, on a surface of a substrate provided with a growth catalyst of the carbon nanotubes, and then totally filling said space between the carbon nanotubes with a solid material, it is possible in a single step to grow carbon nanotubes, all substantially parallel and separated, on a surface of a substrate and inside the pores of a porous solid material with oriented pores.

(51) Such a porous solid material with oriented pores may for example be selected from among porous aluminas, and the growth may be achieved with a chemical vapor phase deposition method CVD.

(52) Next, the substrate is removed, any optional solid material in excess is removed, and both ends (6, 7) of the carbon nanotubes (2) (FIG. 1C) are opened.

(53) The optional solid material in excess is essentially the solid material in excess which covers the end (6) of the carbon nanotubes on the side opposite to the substrate.

(54) The removal of the substrate, of the optional solid material in excess, and the opening of both ends of the carbon nanotubes may be achieved with any adequate technique for example by mechanical polishing and/or by plasma etching.

(55) Next, in a final step (not illustrated in FIG. 1), the inside of the nanotubes is filled with an electrolyte.

(56) The electrolyte has already been described above.

(57) As this was already indicated above, in the case when the electrolyte is a polymer which contains a conductive salt, this is then referred to as an electrolyte polymer or a polymeric electrolyte.

(58) Any type of electrolyte polymer may be used for example a solution of an alkaline salt in poly(oxyethylene) (POE). The mass of the poly(oxyethylene)(POE) may be in the range between 44 and 10.sup.6 g/mol.

(59) Ionic salts and ionic liquids have already been listed above.

(60) The filling may be carried out by simple imbibition, either spontaneous or in vacuo, of the core, the inside of the CNTs in contact with the electrolyte, for example the ionic liquid.

(61) In the case of a polymeric electrolyte, the latter may be confined in the pores by immersing it in an excess of molten or liquid polymeric electrolyte, preferably in vacuo and in hot conditions above the melting point of the electrolyte.

(62) It may be stated that the liquid polymeric electrolyte penetrates the porous structure by simple capillarity.

(63) The membrane with an electrolyte, for example with an ionic liquid or with a polymer, according to the invention such as has been described above may be used in any electrochemical system applying a polymeric electrolyte (FIG. 2).

(64) The membrane with an electrolyte comprises a first main surface (21) and a second main surface (22) separated by a thickness (23).

(65) Carbon nanotubes define through-pores or channels (24) open at both of their ends (25, 26), with a diameter of less than or equal to 100 nm, oriented in the sense of the thickness (23) of the membrane and all substantially parallel, on the totality of the thickness (23) of the membrane. These pores or channels connect the first main surface (21) and the second main surface (22); and an electrolyte is confined in the pores (24) of the membrane.

(66) The electrochemical system may notably be a rechargeable electrochemical accumulator such as a lithium accumulator or battery, which in addition to the membrane with an electrolyte, as defined above comprises a positive electrode; a negative electrode; generally current collectors (27,28), generally made of copper for the negative electrode, or made of aluminium for the positive electrode, which allows circulation of the electrons, and therefore electron conduction, in the outer circuit (29); and generally a separator giving the possibility of preventing the contact between the electrodes and therefore the short-circuits, these separators may be microporous polymeric membranes. The negative electrode may consist of lithium metal as an electrochemically active material, in the case of lithium-metal accumulators, batteries, otherwise the negative electrode may comprise as an electrochemically active material, insertion materials such as graphite carbon (C.sub.gr), or lithiated titanium oxide (Li.sub.4Ti.sub.5O.sub.12) in the case of accumulators, batteries, based on the lithium-ion technology.

(67) The positive electrode generally comprises, as an electrochemically active material, lithium insertion such as lamellar oxides of lithiated transition metals, olivins or lithiated iron phosphates (LiFePO.sub.4) or spinels (for example, the spinel LiNi.sub.0.5Mn.sub.1.5O.sub.4).

(68) More specifically, the electrodes, in the case when they do not consist of lithium metal, comprise a binder which is generally an organic polymer, an electrochemically active material of a positive or negative electrode, optionally one or electron conductive additive(s), and a current collector.

(69) In the positive electrode, the electrochemically active material may be selected from among the compounds already mentioned above in the present description; and from among LiCoO.sub.2; compounds derived from LiCoO.sub.2 obtained by substitution preferably with Al, Ti, Mg, Ni and Mn, for example LiAl.sub.xNi.sub.yCo.sub.(1-x-y)O.sub.2 wherein x<0.5 and y<1, LiNi.sub.xMn.sub.xCo.sub.1-2xO.sub.2; LiMn.sub.2O.sub.4; LiNiO.sub.2; compounds derived from LiMn.sub.2O.sub.4 obtained by substitution, preferably with Al, Ni and Co; LiMnO.sub.2; compounds derived from LiMnO.sub.2 obtained by substitution preferably with Al, Ni, Co, Fe, Cr and Cu, for example LiNi.sub.0.5O.sub.2; olivins LiFePO.sub.4, Li.sub.2FeSiO.sub.4, LiMnPO.sub.4, LiCoPO.sub.4; iron phosphates and sulfates either hydrated or not; LiFe.sub.2(PO.sub.4).sub.3; vanadyl phosphates and sulfates either hydrated or not, for example VOSO.sub.4, nH.sub.2O and Li.sub.xVOPO.sub.4, nH.sub.2O (0<x<3, 0<n<2); Li.sub.(1+x)V.sub.3O.sub.8, 0<x<4, Li.sub.xV.sub.2O.sub.5, nN.sub.2O, with 0<x<3 and 0<n<2; and mixtures thereof.

(70) In the negative electrode, the electrochemically active material may be selected from among the compounds already mentioned above in the present description; and from among the carbonaceous compounds such as natural or synthetic graphites and disordered carbons; the lithium alloys of the Li.sub.xM type with M=Sn, Sb, Si; the compounds Li.sub.xCu.sub.6Sn.sub.5 with 0<x<13; iron borates; simple oxides with reversible decomposition, for example CoO, CO.sub.2O.sub.3, Fe.sub.2O.sub.3; pnicures, for example Li.sub.(3-x-y)Co.sub.yN, Li.sub.(3-x-y)Fe.sub.yN, Li.sub.xMnP.sub.4, Li.sub.xFeP.sub.2; Li.sub.xFeSb.sub.2; and insertion oxides such as titanates, for example TiO.sub.2, Li.sub.4Ti.sub.5O.sub.12, Li.sub.xNiP.sub.2, Li.sub.xNiP.sub.3, MoO.sub.3 and WO.sub.3 and mixtures thereof, or of any material known to the man skilled in the art in this technical field.

(71) The optional electron conducting additive may be selected from among metal particles such as Ag particles, graphite, carbon black, carbon fibers, carbon nanowires, carbon nanotubes and the electron conducting polymers, and mixtures thereof.

(72) The current collectors are generally made of copper for the negative electrode and made of aluminium for the positive electrode.

(73) Between the positive electrode, more exactly the current collector of the positive electrode (28) and the first main surface (21) of the membrane with an electrolyte is generally found the same electrolyte as the one confined in the pores of the membrane, inside the nanotubes. However, this electrolyte which is found between the positive electrode, more exactly the current collector of the positive electrode (28) and the first main surface (21) of the membrane with an electrolyte is a bulk electrolyte not confined to the difference of the electrolyte which is inside the nanotubes (see also FIG. 8).

(74) This bulk electrolyte, not confined, is generally in contact with the positive electrode, more exactly the current collector of the positive electrode (28) and the first main surface (21) of the membrane with an electrolyte, and is generally in fluidic communication with the electrolyte confined inside the pores (see also FIG. 8).

(75) In the same way, between the negative electrode, more exactly the current collector of the negative electrode (27) and the second main surface (22) of the membrane with an electrolyte is generally found the same electrolyte as the one confined in the pores of the membrane, inside the nanotubes. However, this electrolyte which is found between the negative electrode, more exactly the current collector of the negative electrode (27) and the second main surface (22) of the membrane with an electrolyte is a bulk electrolyte not confined to the difference of the electrolyte which is inside the nanotubes (see also FIG. 8).

(76) This bulk electrolyte, not confined is generally in contact with the negative electrode more exactly the current collector of the negative electrode (27) and the second main surface (22) of the membrane with an electrolyte, and is generally in fluidic communication with the electrolyte confined inside the pores (see also FIG. 8).

(77) FIG. 3 shows a particular embodiment of an accumulator, battery, such as a lithium accumulator, battery according to the invention.

(78) This battery comprises a negative electrode (31) for example a negative lithium metal electrode, a membrane with an electrolyte according to the invention (32), and a positive electrode (33).

(79) The membrane with an electrolyte (32) comprises an electrolyte, for example an ionic liquid containing a lithium salt, confined in pores defined by carbon nanotubes, for example with a diameter from 2 to 8 nm.

(80) The membrane with an electrolyte according to the invention (32), comprising CNTs which are electron conductors, the operation of the device optionally requires the insertion of a medium which is both a porous and good electric insulating medium (34) between the membrane comprising CNTs (32) and one of the two electrodes.

(81) The insulating porous medium may for example be a porous membrane or an assembly of the sol-gel type. It is desirable, but not necessary that the pores of this insulating porous medium be macroscopically oriented. The diameter of the pores of this insulating porous medium should be greater than the diameters of the CNTs.

(82) As a porous membrane (34), a porous alumina membrane, such as a membrane made of Anodic Aluminum Oxide or AAO, may be used. These are ceramic membranes (very good electrical insulator) with a side of a few centimeters, for example from 0.1 to 100 and of a few hundred of microns in thickness, for example from 1 to 500.

(83) In FIG. 3, such a membrane made of a porous alumina (34) is inserted between the negative electrode (31) and the membrane with an electrolyte (32) according to the invention.

(84) FIG. 4 shows another particular embodiment of an accumulator, battery, such as a lithium battery according to the invention which may be called a lithium Full 1D battery.

(85) This battery comprises a negative electrode (41) for example a negative lithium metal electrode, a membrane with an electrolyte according to the invention (42), and a positive electrode (43).

(86) The membrane with an electrolyte according to the invention (42) comprises an electrolyte, for example an ionic liquid containing a lithium salt, confined in pores defined by carbon nanotubes CNTs, for example with a diameter from 2 to 8 nm.

(87) But in this embodiment, during the preparation of the membrane with an electrolyte and before conversion of the carpets of CNTs to a membrane, the hybridization of the carbon atoms of the CNTs was modified by grafting a polymer.

(88) From this fact, the CNTs then become electronically insulating and the porous medium to be inserted between the membrane with an electrolyte according to the invention, and one of the electrodes becomes superfluous (see FIG. 4).

(89) The positive electrode (43) of the accumulator, battery, according to this embodiment may be any known positive electrode, however in FIG. 4, the illustrated positive electrode (43) is an electrode obtained by functionalization of the CNT carpets with electro-active species, redox species like for example Anthraquinone AAQ.

(90) The accumulators, batteries which comprise the membrane with an electrolyte, for example with a polymeric electrolyte, according to the invention may notably be used for automobile propulsion like batteries in electric or hybrid vehicles, like batteries for supplying power to portable electronic apparatuses, such as computers, telephones, watches and portable game consoles; more generally like batteries for supplying power to electronic apparatuses such as computers, video players, MP3, MP4 players etc.; like batteries for supplying power to electronic apparatuses loaded on board for example of aircrafts; like batteries for storing the energy produced by intermittent electricity generating devices, like wind turbines and solar panels.

(91) The invention will now be described with reference to the following examples, given as an illustration and not as a limitation.

EXAMPLES

Example 1

(92) In this example, a membrane with an electrolyte according to the invention is prepared.

(93) The method for preparing this membrane with an electrolyte according to the invention comprises four successive steps.

(94) Step 1.

(95) During this step (FIG. 1A) a carpet or a forest of multi-walled carbon nanotubes is prepared, synthesized on a substrate, by a chemical vapor deposition method (CVD).

(96) The growth substrate is a silicon wafer on which is deposited by a ALD (Atomic Layer Deposition) method an aluminium layer with a thickness of 20 nm.

(97) On this alumina layer, an iron layer with a thickness of 1 nm playing the role of a catalyst is deposited by evaporation with an electron gun.

(98) Just before proceeding with the growth of the nanotubes, the substrate provided with the iron layer undergoes two successive treatments with air plasmas at the pressure of 0.3 mbars.

(99) The first treatment is a treatment with a period of 20 minutes with a power of 80 W, and the second is a treatment with a period of 20 minutes at a power of 30 W.

(100) The substrate is then introduced into a chemical vapor deposition enclosure CVD including a network of 10 filaments mounted in parallel.

(101) The filaments are located at 1 cm from the sample holder, consisting of a sole, and they are spaced apart by 1 cm.

(102) The filaments are heated by Joule effect with a power of 800 W.

(103) The gas mixture consisting of 20 sccm of acetylene, 50 sccm of hydrogen and 110 sccm of helium is introduced in cold conditions in the CVD enclosure at a pressure of 0.9 mbars.

(104) The sole is brought to a temperature of 400 C. in 10 minutes, and then a plateau is observed at the temperature of 400 C. for a period of 20 minutes or 45 minutes.

(105) After having observed a plateau at 400 C. for a period of 20 minutes, a carpet of carbon nanotubes is obtained for which the average diameter is 4.5 nm and the height, length is of 200 m.

(106) If the plateau is increased to 45 minutes, carpets of carbon nanotubes are also obtained with an average diameter of 4.5 nm, but for which the height, length is 400 m.

(107) The density of the nanotubes over the surface of the substrate is greater than 10.sup.11 cm.sup.2.

(108) FIG. 5 is a photograph taken with a scanning electron microscope of the carpet or forest of carbon nanotubes obtained at the end of step 1.

(109) Step 2.

(110) During this step the carpet of carbon nanotubes is transformed into a membrane by bridging, filling in the empty space between the carbon nanotubes CNTs of the carpet of carbon nanotubes, of the forest of carbon nanotubes with a polymer (FIG. 1B).

(111) The polymer is polystyrene for which the molecular mass is 350,000 g/mol.

(112) This polymer is dissolved in toluene for obtaining a solution at 20% by mass.

(113) This solution is poured onto the carpet, the forest, of carbon nanotubes, and then the solvent is left to evaporate.

(114) At the end of this step, a membrane is obtained wherein the carbon nanotubes are surrounded by a polymer matrix.

(115) Generally the end of the carbon nanotubes opposite to the substrate is covered with polymer (FIG. 1B) and excess polymer is therefore present on the nanotubes.

(116) Step 3.

(117) During this step, mechanical polishing is achieved of both faces of the membrane obtained in step 2 in order to remove the excess polymer, to remove the substrate and to open the carbon nanotubes at both of their ends (FIG. 1C).

(118) FIG. 6 is a photograph taken with the scanning electron microscope of the membrane obtained at the end of step 3.

(119) Step 4.

(120) During this step, the inside, the core of the carbon nanotubes is filled with an electrolyte.

(121) This electrolyte may consist of the ionic liquid 1-octyl-3-methylimidazolium tetrafluoroborate (OMIMBF.sub.4), or else of the ionic liquid BMIMTFSI 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl imide) or else of POE-LiTFSI (M.sub.POE=500 g/mol, 29% by mass of LiTFSI).

(122) The filling is achieved by simple spontaneous, imbibition, impregnation or in vacuo of the core of the CNTs in contact with the electrolyte, for example with the ionic liquid.

(123) In the following examples 2 and 3, the properties of the membrane with an electrolyte according to the invention, prepared in Example 1, were measured.

(124) In Example 2, the diffusion coefficient or more exactly the self-diffusion coefficient (self-diffusion coefficient) of the ionic liquid OMIMBF.sub.4 confined in the membrane of carbon nanotubes prepared in Example 1 was measured at room temperature.

(125) The measurement was made by Nuclear Magnetic Resonance (NMR) with a pulsed field gradient (Pulsed Field Gradient Nuclear Magnetic Resonance or PFG-NMR) of Fluorine 19 (FIG. 7).

(126) In Example 2, additional measurements by PFG-NMR of hydrogen, of Fluorine 19 or of Lithium 7 were also carried out, of the self-diffusion coefficients (D.sub.S) of the different electrolytes, mentioned above, confined in the membrane of carbon nanotubes CNTs prepared in Example 1.

(127) In Example 3, the conductivity of the ionic liquid (OMIMBF.sub.4) confined in the membrane of carbon nanotubes was measured at room temperature.

(128) The measurement was made by impedance spectroscopy (FIGS. 8 and 9).

Example 2

(129) In this example, first of all the diffusion coefficient of the ionic liquid OMIMBF.sub.4 confined in the membrane of carbon nanotubes CNTs prepared in Example 1 at room temperature, is measured.

(130) The measurement is made by Nuclear Magnetic Resonance (NMR) with a pulsed field gradient (Pulsed Field Gradient Nuclear Magnetic Resonance or PFG-NMR) of Fluorine 19 (FIG. 7).

(131) For comparison purposes, the diffusion coefficient of the same ionic liquid as the one confined in the pores of the membrane of CNTs of Example 1, i.e. 1-octyl-3-methylimidazolium tetrafluoroborate, (OMIMBF.sub.4), but not confined in the pores, is also measured. This non-confined ionic liquid is also called a volumic or bulk ionic liquid.

(132) The results of these measurements are plotted in FIG. 7.

(133) This figure shows that at room temperature, the self-diffusion coefficient of the ionic liquid (1-octyl-3-methylimidazolium tetrafluoroborate, OMIMBF.sub.4) confined according to the invention in a membrane of carbon nanotubes for which the average internal diameter of the pores is 4 nm, is about 3 times greater than the self-diffusion coefficient of the same non-confined volumic, bulk ionic liquid.

(134) In other words, an increase by a factor of about three is obtained of the self-diffusion coefficient because of the confinement, since this self-diffusion coefficient is 4.4 (+/0.3) 10.sup.8 cm.sup.2 s.sup.1 for the volumic, bulk ionic liquid, and 1.3 (+/0.2) 10.sup.7 cm.sup.2 s.sup.1 for the same ionic liquid confined in the membrane.

(135) Next, additional measurements are carried out by PFG-NMR of hydrogen, of Fluorine 19 or of Lithium 7, of the self-diffusion coefficients (D.sub.S) of different electrolytes confined in the membrane of carbon nanotubes CNTs prepared in Example 1.

(136) These electrolytes are the following: an electrolyte consisting of OMIMBF.sub.4; or an electrolyte consisting of BMIMTFSI 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl imide); or an electrolyte consisting of POE-LiTFSI (M.sub.POE=500 g/mol, 29% by mass of LiTFSI).

(137) The measurements carried out are the following OMIMBF.sub.4: .sup.1H-PFG-NMR measurements (cation dynamics) and .sup.19F (anion dynamics) at room temperature (25 C.) and at 55 C.; BMIMTFSI: .sup.1H-PFG-NMR measurements (cation dynamics) and .sup.19F (anion dynamics) at room temperature; PEO-LiTFSI (M.sub.POE=500 g/mol, 29% by mass of LiTFSI): .sup.1H-PFG-NMR measurements (PEO dynamics).sup.19F (anion dynamics), .sup.7Li (Li dynamics) at room temperature.

(138) For comparison purposes, the self-diffusion coefficients (D.sub.S) of these same electrolytes but not confined in the pores of the membrane of CNTs are also measured at room temperature.

(139) These non-confined electrolytes are also called volumic or bulk electrolytes.

(140) The results of the additional measurements carried out are grouped in the Table 1 below.

(141) TABLE-US-00001 TABLE 1 Self-diffusion coefficients (D.sub.s) of the electrolytes OMIMBF.sub.4, BMIMTFSI and PEO-LiTFSI measured by PFG-NMR (.sup.1H, .sup.19F, and .sup.7Li) confined in the CNT membrane (@CNT) and not confined in the membrane (Bulk). The ratio or D.sub.S-Confined/D.sub.S-bulk ratio is also indicated. .sup.1H-25 C. .sup.19F-25 C. .sup.1H-55 C. .sup.1Li-25 C. Sample (10.sup.8 cm.sup.2/s) (10.sup.8 cm.sup.2/s) (10.sup.8 cm.sup.2/s) (10.sup.8 cm.sup.2/s) OMIMBF.sub.4 3.7 0.2 4.7 0.3 14.5 1.0 OMIMBF.sub.4 8.2 0.8 11.5 1.4 24 3 @ NTC Ratio 2.2 0.3 2.6 0.4 1.7 0.3 BMIMTFSI 27 2 21.5 2 BMIMTFSI 31.0 3 22 2 @ NTC Ratio 1.1 0.2 1.0 0.1 PEO-LiTFSI 86 0.6 10.1 1 6.9 1.1 @ NTC PEO-LiTFSI .sup.6 0.4 10 0.5 5.2 0.4 bulk Ratio 1.4 0.2 .sup.1 0.3 1.3 0.2

(142) Confined in the CNTs, the mobility of BMIMTFSI is not altered relatively to the non-confined electrolyte (table 1: ratio1).

(143) However, this result shows that there is no (or very little) interactions with the internal surface of the CNTs.

(144) At 55 C., the mobility of OMIMBF.sub.4 increases under confinement by a factor 1.7.

(145) When the temperature decreases, this effect is further increased. Indeed an increase in the mobility (of the cation and of the anion) is observed by a factor 2-3.

(146) A possible interpretation is the modification of the self-organization at the nanometric scale of OMIMBF.sub.4. Under confinement, this nano-organization decreases and facilitates the mobility at the micrometric scale (measurement scale of the PFG-NMR).

(147) The results obtained for OMIMBF.sub.4 at 55 C. and BMIMTFSI correspond with this explanation: in both cases, the nanostructuration of the ionic liquid LI is less pronounced and the D.sub.S-confined/D.sub.S-bulk ratio is consequently less important.

(148) The results obtained with POE-LiTFSI show that the principle of the measurement also operates for this type of electrolyte, although the gain in mobility is low.

(149) However by reducing the diameter of the CNTS, a clearly greater gain is expected.

Example 3

(150) In this example, the conductivity of the ionic liquid confined in the membrane of carbon nanotubes CNTs prepared in Example 1 is measured at room temperature.

(151) The measurement is conducted by impedance spectroscopy (FIG. 8).

(152) For comparison purposes, the conductivity of the same ionic liquid as the one confined in the pores of the membrane of CNTS of Example 1, i.e. 1-octyl-3-methylimidazolium ttrafluoroborate, (OMIMBF.sub.4), but non-confined in the pores, is also measured at room temperature. This non-confined ionic liquid is also called a volumic or bulk ionic liquid.

(153) This schematic layout of the device which gave the possibility of measuring by impedance spectroscopy at room temperature, the conductivity of the volumic, bulk, non-confined ionic liquid OMIMBF.sub.4, and in a membrane of carbon nanotubes is shown in FIG. 8.

(154) This device comprises an upper electrode (71), and a lower electrode (72) separated by a distance L. Between both of these electrodes (71, 72) is placed the membrane according to the invention, prepared in Example 1 (73), which comprises carbon nanotubes (74) (conveniently only a single carbon nanotube has been illustrated) inside which is confined the ionic liquid OMIMBF4 (75) in a polystyrene matrix (76).

(155) For ensuring perfect electric contact between the membrane (73) confining the ionic liquid (75) (the confined ionic liquid is also noted as IL@CNT and its impedance is Z.sub.IL@CNT) and the electrodes (71, 72), an excess of volumic ionic liquid also called bulk liquid with a known thickness, i.e. of the order of a few millimeters is maintained on each side of the membrane.

(156) Thus between the lower surface of the electrode (71) and the membrane is found some volumic, bulk ionic liquid (77) of known thickness E1 (79), and between the m embrane (73) and the lower surface of the electrode (72) is found volumic, bulk ionic liquid (78) of a same known thickness E2 equal to E1 (710). The total impedance of the volumic, bulk ionic liquid with a total thickness of E1+E2 is therefore Z.sub.bulk.

(157) The active surface (711) of the ionic liquid (77) or (78) may be designated by S.

(158) The impedance of the volumic, bulk ionic liquid (77) of known thickness E1 (79), and the impedance of the volumic, bulk ionic liquid of a known thickness E2 (710) is therefore Z.sub.bulk/2.

(159) The total impedance Z.sub.tot of the system consisting of the volumic, bulk ionic liquid, and of the confined ionic liquid is therefore the following: Z.sub.tot=Z.sub.bulk+Z.sub.IL@CNT

(160) The results of the measurements made in this example are plotted in FIG. 10 which gives the Cole-Cole representation of the electric impedance of the volumic, bulk electrolyte (bulk electrolyte) and of the electrolyte confined in the nanotubes, of a membrane comprising carbon nanotubes in a polystyrene matrix.

(161) In order to give an estimation of the uncertainty of the measurement, in each case, two successive measurements are illustrated.

(162) The electric impedance of the volumic, bulk electrolyte (Z.sub.bulk) and the total impedance of the system (Z.sub.tot), are indicated in FIG. 10.

(163) The resistance of the confined ionic liquid in the nanotubes CNTs of the membrane is Z.sub.IL@CNT-PS=Z.sub.totZ.sub.bulk1500-1000=500.

(164) R.sub.CNT=1.5 nm, .sub.CNT=3.010.sup.11 NTC/cm.sup.2, e.sub.CNT=125 m, and S=0.5 cm.sup.2, are respectively the internal radius, the surface density, the length of the CNTs (or equivalently the thickness of the membrane), and the useful contact surface between the electrodes and the membrane comprising carbon nanotubes in a polystyrene matrix.

(165) The conductivity of the electrolyte confined in the membrane comprising carbon nanotubes is in a polystyrene matrix is .sub.IL@CNT-PS=e.sub.CNT/(Z.sub.IL@CNT-PSR.sub.CNT.sup.2.sub.CNT/S=0.236 S/m.

(166) The conductivity of the volumic, bulk electrolyte under the same conditions is .sub.IL bulk=0.07 S/m.

(167) Under confinement, the gain in conductivity is therefore 3.41.

(168) Examples 3 and 4 show that the confinement of the electrolyte gives the possibility of obtaining a greater self-diffusion coefficient and consequently a greater ionic conductivity of the electrolyte (a factor of 3.4 is shown here), than those of the volumic, bulk electrolyte.