ELECTRODE FOR PHOTOBATTERY

Abstract

An electrode comprising an electrode material of the same type as electrode materials used in Li-ion batteries and a dye is provided. The electrode may further comprise a semiconductor material. The electrode is used in the manufacture of a battery that is rechargeable using light.

Claims

1. Electrode comprising an electrode material of the same type as electrode materials used in Li-ion batteries and a photosensitive dye.

2. Electrode according to claim 1, further comprising a semiconductor material.

3. Electrode comprising a cathode material and/or an anode material, and a photosensitive dye, wherein the cathode and anode materials are of the same type as cathode and anode materials used in Li-ion batteries.

4-6. (canceled)

7. Electrode according to claim 3, wherein the cathode material is an olivine type material selected from LiFePO.sub.4 and LiCoO.sub.2, or FeS.sub.2, or V.sub.2O.sub.5.

8. (canceled)

9. Electrode according to claim 3, wherein the anode material is metallic lithium, graphite, silicon, or a metal oxide.

10. Electrode according to claim 1, wherein the electrode material is coated with carbon.

11. Electrode according to claim 1, wherein the electrode material comprises particles having a size below 1 m or a size below 0.1 m.

12. Electrode according to claim 2, wherein the semiconductor material is TiO.sub.2, ZnO, SnO.sub.2, a core-shell, or a combination thereof.

13. Electrode according to claim 2, wherein the semiconductor material comprises particles having a size below 100 nm or a size below 30 nm.

14. Electrode according to claim 2, wherein the semiconductor material is pre-calcined.

15. Electrode according to claim 1, wherein the photosensitive dye is N3, black dye, SJW-E1, N719, an organic photosensitive dye, or a combination thereof.

16. Electrode according to claim 1, further comprising a solvent, a dispersant, a binder, or a combination thereof.

17. Electrode according to claim 16, wherein: the solvent is N-methyl-2-pyrrolidine (NMP), water, acetone, an alcohol, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or a combination thereof; and/or the dispersant is polvvinvlidene difluoride (PVDF), a tension-active agent that does not react with electrode materials, an alkyl bromide ammonium salt, an alkylbenzyldimethylammonium halide, a glycol ester, a glycerol ester, or a combination thereof; and/or the binder is polyethylene glycol (PEG), polvvinylidene difluoride (PVDF), polyvinyl acetate (PVA) or a combination thereof.

18-21. (canceled)

22. Electrode material according to claim 1, wherein the photosensitive dye is anchored to the surface of the electrode material particles.

23. Electrode material according to claim 2, wherein the photosensitive dye is anchored to the surface of the electrode material particles and the semiconductor material particles.

24. Solid substrate having deposited thereon a material as defined in claim 1; optionally the solid subtract is a fluorine-doped tin oxide glass (FTO glass).

25. (canceled)

26. (canceled)

27. Method of manufacturing an electrode, comprising the following steps: (a) preparing a film comprising an electrode material of the same type as electrode materials used in Li-ion batteries; and (b) bringing into contact the film and a solution comprising a photosensitive dye.

28. Method according to claim 27, further comprising a preliminary step of (a1) mixing the electrode material with a semiconductor material prior to conducting step (a).

29. Method according to claim 27, wherein step (a) comprises depositing the electrode material on a solid substrate, optionally the solid substrate is a fluorine-doped tin oxide glass (FTO glass).

30. Method according to claim 28, wherein step (a) comprises depositing the mixture electrode material and semiconductor material on a solid substrate; optionally the solid substrate is a fluorine-doped tin oxide glass (FTO glass).

31. Method according to claim 29, wherein depositing of the material is carried out by the Doctor Blade method, by the immersion withdrawal or dipping withdrawing method, by a serigraphy method, by the spin-coating method, or a combination thereof.

32. Method according to claim 27, wherein step (a) comprises using a solvent; and the method further comprises, between steps (a) and (b), a drying step followed by a cooling step; optionally the drying step is carried out at a temperature of about 400 C. and under inert atmosphere, and cooling is carried out naturally until room temperature is reached.

33. (canceled)

34. Method according to claim 27, wherein step (b) comprises dipping the film into the solution comprising a photosensitive dye.

35. Method according to claim 27, further comprising a drying step after step (b) followed a cooling step; optionally the drying step is carried out at a temperature between room temperature and 120 C. and under inert atmosphere; and cooling is carried out naturally until room temperature is reached.

36. (canceled)

37. Battery using an electrode as defined in claim 1.

38. Battery according to claim 37, which is rechargeable using light.

39-42. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In the appended drawings:

[0057] Scheme 1 succinctly describes the operation of a Li-ion battery.

[0058] Scheme 2 succinctly outlines a dye-sensitized solar cell (DSSC).

[0059] FIG. 1 succinctly outlines the operation of a DSSC.

[0060] Scheme 3 illustrates an assembly according to the invention.

[0061] FIG. 2 is a scanning electron micrograph (SEM) of titanium oxide powder pre-calcined at 400 C. (Example 1).

[0062] FIG. 3 is a scanning electron micrograph (SEM) of the surface of the electroactive electrode (Example 1).

[0063] FIG. 4 is an X-ray diffractogram (XRD) of a raw electrode assembly of Example 1.

[0064] FIG. 5 is a cyclic voltammetry (CV) of an electrode according to Example 1, measured at 0.1 mV/s, 116 A.h capacity for a surface of 2.7 cm.sup.2.

[0065] FIG. 6 is an open circuit voltage (OCV) evolution during light exposure of the photo battery according to the invention (Example 1).

[0066] FIG. 7 is an X-ray diffractogram (XRD) of an electrode according to Example 1 after OCV.

[0067] FIG. 8 is a high resolution scanning electron micrograph (HRSEM) of the surface of the film deposited on a fluorine-doped tin oxide glass (FTO glass) after annealing for 1 hour at 400 C. under a flux of nitrogen (Example 2).

[0068] FIG. 9 outlines the electrochemical characterizations for a three-electrode cell of the photoactive electrode according to Example 2 in an electrolyte containing 0.3M LiTFSI in a EC-DEC (30-70 mass %) solution vs. Li, and with a Li.sup.+/Li reference: part A of the figure is an OCV evolution during light exposure; and part B is the CV of the film, the potential is achieved with a scanning speed of 0.1 mv/s yielding a capacity of 74 A.h/cm.sup.2 (Example 2).

[0069] FIG. 10 is a scanning electron micrograph (SEM) of hydrothermally obtained LiFePO.sub.4 crystals (Example 3).

[0070] FIG. 11 illustrates the OCV evolution during light exposure of the film assembled as a three-electrode cell using an electrolyte consisting of 1M LiPF.sub.6 dissolved in EC-DEC (30-70 mass %) vs. Li, and with a Li.sup.+/Li reference, total duration 11 days (Example 3).

[0071] FIG. 12 illustrates measurement of the OCV evolution during light exposure of a film consisting of submicron particles of LFP (lithium ferrophosphate or lithium iron phosphate (LiFePO.sub.4)) and N719 dye. The film was assembled as a three-electrode cell in LiPF.sub.6 (1M) dissolved in EC-DEC (30-70 mass %) vs. Li, and using a Li.sup.+/Li reference (Example 4).

[0072] FIG. 13 is an X-ray diffractogram by grazing incident angle of Example 4, before OCV.

[0073] FIG. 14 is a high resolution transmission electron micrograph (HRTEM) of Example 4 before OCV. The inclusion is the Fourrier transformation of the image which proves that LiFePO.sub.4 (LFP) of tryphilite structure was initially present.

[0074] FIG. 15 is an X-ray microgram by grazing incident angle of Example 4, after OCV.

[0075] FIG. 16 is a high resolution transmission electron micrograph (HRTEM) of Example 4, after OCV. The inclusion is the Fourrier transformation of the image which proves that FePO.sub.4 (LFP) of heterosite structure is formed after light exposure.

[0076] FIG. 17 is an X-ray photoelectron spectroscopy (XPS) spectrum of Example 5, before OCV (part A of the figure) and after OCV (part B of the figure).

[0077] FIG. 18 is a measurement of the OCV evolution of a film consisting of submicron LFP particles and N719 dye. The film was kept in the dark and assembled as a three-electrode cell in LiPF.sub.6 (1M) dissolved in EC-DEC (30-70 mass %) vs. Li, and using a Li.sup.+/Li reference (Example 5).

[0078] FIG. 19 is a measurement of the OCV evolution of a film consisting of TiO.sub.2-LFP and N719 dye. The film was kept in the dark and assembled as a three electrode cell in LiPF.sub.6 (1M) dissolved in EC-DEC (30-70 massv%) vs. Li, and using a Li.sup.+/Li reference (Example 6).

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0079] As used herein, the expression electrode material of the same type as electrode materials used in Li-ion batteries refers to materials having characteristics that are similar to those of anode and cathode materials used in Li-ion batteries (i.e., a material that reacts as an anode or a cathode material in a Li-ion battery). More specifically, the expression refers to a cathode material of the same type as cathode materials used in Li-ion batteries or an anode material of the same type as anode materials used in Li-ion batteries.

[0080] As used herein, the expression light sensitive dye of the same type as light sensitive dyes used in dye-sensitized solar cells (DSSC) or the expression photosensitive dye refers to dyes having characteristics that are similar to those of dyes used in photo batteries (i.e., a dye that reacts as a dye in a photo battery). More specifically, one or the other of the two expressions refers to a material wherein molecules absorb photons. As such at least one electron of the molecule moves from the highest occupied molecular energy (HOMO) to the lowest unoccupied molecular orbital (LUMO).

[0081] As used herein, the expression semiconductor material refers to a material having a bandgap below about 4 eV.

[0082] The inventors have designed and built an electrode which, when used in a battery, allows for the battery to be charged using light. The electrode according to the invention comprises, in combination, an electrode material of the same type as electrode materials used in Li-ion batteries and a photosensitive dye of the same type as photosensitive dyes used in dye-sensitized solar cells (DSSC). In an embodiment of the invention, the electrode may further comprise a semiconductor material. A battery comprising the electrode according to the invention may be charged using light, instead of an external current.

[0083] In an embodiment of the invention, the cathode is obtained by combining a cathode material of the same type as cathode materials used in Li-ion batteries and a light sensitive dye of the same type as light sensitive dyes used in DSSC. In a preferred embodiment, the combination may comprise a semiconductor material.

[0084] The cathode material of the same type as cathode materials used in Li-ion batteries may be an olivine type material. In an embodiment of the invention, the cathode material may be LiFePO.sub.4, LiCoO.sub.2, FeS.sub.2 or V.sub.2O.sub.5. In a preferred embodiment, the cathode material is LiFePO.sub.4. The cathode material of the invention may be coated with carbon.

[0085] In an embodiment of the invention, the anode is obtained by combining an anode material of the same type as anode materials used in Li-ion batteries and a light sensitive dye of the same type as light sensitive dyes used in DSSC. In a preferred embodiment, the combination may comprise a semiconductor material.

[0086] The anode material of the same type as anode materials used in Li-ion batteries may be metallic lithium, graphite, silicon, or a metal oxide such as Fe.sub.2O.sub.3, TiO.sub.2, Li.sub.4Ti.sub.5O.sub.12, etc. In a preferred embodiment, the anode material may be metallic lithium or graphite. The anode material according to the invention may be coated with carbon.

[0087] The light sensitive dye of the same type as light sensitive dyes used in DSSC may be N3, black dye, SJW-E1 or N719. In a preferred embodiment of the invention, the light sensitive dye is N719.

[0088] The semiconductor material may be TiO.sub.2, ZnO, SnO.sub.2 or core-shell. In a preferred embodiment of the invention, the semiconductor material is TiO.sub.2.

[0089] The three components of the cathode according to the invention, namely, cathode material of the same type as cathode materials used in Li-ion batteries, light sensitive dye of the same type as light sensitive dyes used in DSSC and semiconductor material, are in close contact. In a preferred embodiment, the cathode material and the semiconductor material may first be mixed together then the dye added.

[0090] The cathode material used is in the form of submicron particles or in the form of hydrothermally obtained particles. The size of the particles may be below 1 m. In a preferred embodiment of the invention, the size of the particles is below 0.1 m.

[0091] The semiconductor material used is in the form of nanometric powder. The size of the particles may be below 100 nm. In a preferred embodiment of the invention, the size of the particles is below 30 nm. The semiconductor material may be in the form of a pre-calcined powder.

[0092] The dye used may be in liquid form, and in a preferred embodiment, addition of the dye may be performed by dipping into the liquid the film comprising the cathode material and the semiconductor material. In an embodiment of the invention, the film comprises the cathode material but not the semiconductor material.

[0093] The mixture, cathode material semiconductor material may further comprise other agents such as solvents (for example water, N-methyl-2-pyrrolidone (NMP), etc.), binders (for example PEG, etc.), dispersants (for example Triton-X100, etc.).

[0094] The electrode material (Scheme 3, arrow 2) is deposited on a substrate that is transparent and conductive (Scheme 3, arrow 1). The substrate is both transparent to light and electronically conductive, allowing movement of electrons from or to an external circuit. In this embodiment, the electrolyte (Scheme 3, arrow 3) is lithiated similarly to electrolytes used in Li-ion batteries. Also, the electrolyte contains no sacrificial redox couple. The anode (Scheme 3, arrow 4) is of the same type as anodes used in Li-ion batteries.

[0095] Similarly to DSSC, during light exposure, the photosensitive dye is excited and may finally be oxidized, since at least one electron of the molecule moves to the LUMO which is of higher energy than the conduction band of the semiconductor. The semiconductor thus sends electrons towards the external circuit via the current collector. The cathode material of the battery or battery cathode material (BCM) which has been reduced, acts as sacrificial redox couple similarly to sacrificial redox couples in DSSC. Oxidation of the reduced BCM, Li(BCM) is performed according to the following reaction:

Li(BCM)+S.sup.+.fwdarw.Li.sup.++(BCM)+S

[0096] For the reaction to proceed spontaneously, it is necessary that the standard potential of the redox couple of the dye, S.sup.+/S be superior to the standard potential of sacrificial redox couple, Li.sup.+/Li(BCM). Typically in DSSC, the redox couple used is I.sub.3.sup./I.sup. which has a standard potential of 0.53V vs. ESH and which corresponds to 3.57V vs. Li.sup.+/Li. Accordingly, any cathode material used in a battery having a standard potential below 3.6V vs. Li.sup.+/Li may be oxidized when the material is combined with a photosensitive dye of the same type as dyes used in DSSC. In this case, the battery cathode material in reduced state, Li(BCM) may be oxidized into BCM replacing the sacrificial redox couple I.sup./I.sub.3.sup. of DSSC. In a case where a cathode material is used in a battery having a standard potential higher than 3.6V vs. Li.sup.+/Li, synthesis and/or use of specific dyes, similarly to DSSC using the redox couple Co.sup.3+/Co.sup.2+, may be contemplated.

[0097] Lithium ions released following the reaction at the cathode are reduced at the anode by electrons from the external circuit. Accordingly, it is possible to obtain oxidation of the reduced cathode material with no external current.

EXAMPLE 1

[0098] Composition of the Cathode Material (Paste): [0099] TiO.sub.2 in nanomeric powder form and pre-calcined at 400 C./1 h (FIG. 2): 0.3 g [0100] Submicron LiFePO.sub.4 particles: 0.3 g [0101] PVDF: 0.06 g [0102] NMP: 3 mL

[0103] A wet film of the above paste was deposited on a FTO glass substrate (Cytodiagnostics TEC 7, 6-8) by the Doctor Blade method (3 mils). The film was annealed under a nitrogen flux according to the following procedure: the temperature is brought to 400 C. within 1 hour and maintained at 400 C. during 1 hour followed by natural cooling until room temperature is reached.

[0104] The film obtained was dipped into an aqueous solution containing photosensitive dye N719 (410.sup.4M) during 24 hours. Dye was anchored to the surface of the cathode material particles. Samples of the film were dried at 50 C. under vacuum during 48 hours (FIG. 3).

[0105] X-ray diffraction (XRD) diagram of the photoactive raw electrode assembly (FIG. 4) shows the presence of tin oxide from the FTO glass, anatase and brookite phases from titanium oxide, and tryphilite phase from LiFePO.sub.4. Peak distribution indicates that no heterosite FePO.sub.4 phase is present.

[0106] The photoactive electrode was assembled in a three-electrode configuration, in LiPF.sub.6 solution (1M) dissolved in EC-DEC-VC as electrolyte vs. Li with a Li.sup.+/Li reference.

[0107] Electrochemical characteristics of these electrodes were measured by cyclic voltammetry (CV) with a scanning speed of 0.1 mV/s and a voltage range from 2.5V to 4V with regard to lithium (FIG. 5). The CV was completed at a potential of 2.5V. The CV obtained is representative of a film consisting of submicron LFP, given its oxidation potential at 3.46V and its reduction potential at 3.4V.

[0108] The open circuit voltage (OCV) of the sample kept in the three-electrode cell and under the light hood was registered (FIG. 6) once the CV was completed at a 2.5V potential. As expected, a rapid increase of the OCV from 2.5V to 3.4V within 1 hour was obtained. Thereafter, the OCV remained stable during 24 hours at a potential between 3.43V and 3.44V. This stability allowed for the battery to be charged. At the end of the OCV, the voltage of the battery was increased from 3.44V to 3.65V. This increase is due to the charging of the battery or photo battery as expected.

[0109] After the OCV, the sample was again analyzed by X-ray diffraction (XRD) (FIG. 7). The main peaks correspond to phases observed in FIG. 4, namely, casserite (tin oxide), brookite and anatase. However, some peak changes can be seen in the lower angle region (2<30). These changes are probably due to the fact that part of phosphate lithium iron has been oxidized to phosphate iron. It is not possible to assess from the diffractogram whether all triphilite LiFePO.sub.4 was converted to heterosite FePO.sub.4, however, the main part was converted.

[0110] An estimated mass ratio Fe/Ti at various locations of the film was determined. The ratios were all between 1.49 and 2.02. It should be noted that this ratio is around 0.59 for a homogenous mixture comprising 50-50 mass % LiFePO.sub.4-TiO.sub.2. The difference is probably due to the fact that TiO.sub.2 particles were not well dispersed in the NMP (part of the titanium oxide remained at the bottom of the pot). A lack of titanium oxide may have an impact on the performance of the photo battery. Accordingly, NMP may be replaced by other solvents such as water for example.

EXAMPLE 2

[0111] Composition of the Cathode Material (Paste): [0112] TiO.sub.2 in nanomeric powder form and pre-calcined at 400 C./1 h: 0.4 g [0113] Submicron LiFePO.sub.4 particles: 0.3 g [0114] PEG as binder: 0.1 g [0115] Water: 2 mL

[0116] A wet film of the above paste was deposited on a FTO glass substrate (Cytodiagnostics TEC 7, 6-8) by the Doctor Blade method (3 mils). The film was annealed under a nitrogen flux according to the following procedure: the temperature is brought to 400 C. within 1 hour and maintained at 400 C. during 1 hour followed by natural cooling until room temperature is reached.

[0117] The film obtained was dipped into an aqueous solution containing photosensitive dye N719 (410.sup.4M) during 24 hours. Dye was anchored to the surface of the cathode material particles. Samples of the film were dried at 50 C. under vacuum during 24 hours.

[0118] High resolution scanning electron microscopy (HRSEM) and energy dispersive spectrometry (EDS) analyses were conducted in order to assess the particles morphology and the mass ratio Fe/Ti. This sample contains no agglomerate having a needle form, contrary to Example 1 (FIG. 8). A mass ratio Fe/Ti of about 0.28 was measured by EDS. It should be noted that the expected mass ratio for this paste was 0.44. This means that the amount of LiFePO.sub.4was lower. Agglomerates having a needle form in Example 1 are probably due to an aggregation of nanomeric LiFePO.sub.4 particles.

[0119] The photo oxidation process was monitored by OCV (FIG. 9) and measured during light exposure. In this case, the photo oxidation is faster than in Example 1, since a 4.2V voltage in less than 1 hour was measured. This improvement is probably due to a better mixture between LiFePO.sub.4 and TiO.sub.2. In addition, a CV test of the film shows that the film is reduced at a potential between 2.8-3.0V and is oxidized at a potential between 3-3.2V. This is in conformity with the submicron size of the crystals.

EXAMPLE 3

[0120] Composition of the Cathode Material (Paste): [0121] TiO.sub.2 in nanomeric powder form and pre-calcined at 400 C./1 h: 5 g [0122] LiFePO.sub.4 particles hydrothermally obtained (FIG. 10): 5 g [0123] Triton-X100 as binder: 0.3 mL [0124] Water: 112 mL

[0125] This paste was deposited on a FTO glass substrate (Cytodiagnostics TEC 7, 6-8) by the method called immersion withdrawal or dipping withdrawing. The film was annealed under a nitrogen flux according to the following procedure: the temperature is brought to 400 C. within 1 hour and maintained at 400 C. during 1 hour followed by natural cooling until room temperature is reached. The film obtained was dipped into an aqueous solution containing photosensitive dye N719 (410.sup.4M) during 24 hours. Dye was anchored to the surface of the cathode material particles. Samples of the film were dried at 50 C. under vacuum during 24 hours.

[0126] OCV of the film during light exposure was then conducted and monitored: an increase of the potential from 3.45V to 3.65V in 11 days was observed. Comparing to Example 2, this photo-oxidation was slower. This is probably due to the fact that LiFePO.sub.4 particles of the film are far bigger than the particles in Example 2.

EXAMPLE 4

[0127] Composition of the Cathode Material (Paste): [0128] Submicron LiFePO.sub.4 particles: 5 g [0129] Triton-X100 as binder: 0.15 mL [0130] Water: 50 mL

[0131] This paste was deposited on a FTO glass substrate (Cytodiagnostics TEC 7, 6-8) by the method called immersion withdrawal or dipping withdrawing. The film was annealed under a nitrogen flux according to the following procedure: the temperature is brought to 400 C. within 1 hour and maintained at 400 C. during 1 hour, followed by natural cooling until room temperature is reached. The film obtained was dipped into an aqueous solution containing photosensitive dye N719 (410.sup.4M) during 24 hours. Dye was anchored to the surface of the cathode material particles. Samples of the film were dried at 50 C. under vacuum during 24 hours.

[0132] OCV of the film under light exposure was then conducted and monitored (FIG. 12): an increase of the potential from 3.4V to 3.75V in 21 days was observed.

[0133] FIG. 13 and FIG. 14 are, respectively, X-ray diffractogram (XRD) and high resolution transmission electron micrographs (HRTEM) performed prior to photo-oxidation of the film. It can be seen from these figures that before photo-oxidation, the electrode material consists exclusively of LiFePO.sub.4 crystals having triphilite structure. Conversion of triphilite LiFePO.sub.4 to heterosite FePO.sub.4 obtained by photo-oxidation of the film due to the presence of the photosensitive dye can be seen on the XRD diffractogram and the HRTEM micrographs in FIG. 15 and FIG. 16.

[0134] FIG. 17 represents XPS measurements performed before (A) and after (B) photo-oxidation. A peak increase for Fe.sup.3+ species is observed after photo-oxidation. This is in conformity with the conversion of the LiFePO.sub.4 triphilite phase to heterosite FePO.sub.4. This transformation of Fe.sup.2+ from LiFePO.sub.4 into Fe.sup.3+ of FePO.sub.4 may be monitored by the strong reduction of peaks located at 709 and 723 eV both due to Fe.sup.2+ species and the increase of peaks located at 712 and 723 eV due to Fe.sup.3+ species.

[0135] The characterization methods in this example show that photo-oxidation of LiFePO.sub.4 to FePO.sub.4 is possible with no addition of a semiconductor such as TiO.sub.2. However, it appears that use of a semiconductor allows for an increase of the rate of the photo-oxidation.

EXAMPLE 5

[0136] Composition of the Cathode Material (Paste): [0137] Submicron LiFePO.sub.4 particles: 5 g [0138] Triton-X100 as binder: 0.15 mL [0139] Water: 50 mL

[0140] This paste was deposited on a FTO glass substrate (Cytodiagnostics TEC 7, 6-8) by the method called immersion withdrawal or dipping withdrawing. The film was annealed under a nitrogen flux according to the following procedure: the temperature is brought to 400 C. within 1 hour and maintained at 400 C. during 1 hour followed by natural cooling until room temperature is reached. The film obtained was dipped into an aqueous solution containing photosensitive dye N719 (410.sup.4M) during 24 hours. Dye was anchored to the surface of the cathode material particles. Samples of the film were dried at 50 C. under vacuum during 24 hours.

[0141] The film was kept in the dark during OCV measurement. After 23 days, the potential reached a plateau at 3.4V vs. Li.sup.+/Li (FIG. 18). Contrary to Example 4, no increase of the potential was observed. This example shows the requirement of photosensitive dye to be present in order to remove lithium from LiFePO.sub.4. Also, it appears that light is required.

EXAMPLE 6

[0142] Composition of the Cathode Material (Paste): [0143] TiO.sub.2 in nanomeric powder form and pre-calcined at 400 C./1 h: 5 g [0144] Submicron LiFePO.sub.4 particles: 5 g [0145] Triton-X100 as binder: 0.3 mL [0146] Water: 112 mL

[0147] This paste was deposited on a FTO glass substrate (Cytodiagnostics TEC 7, 6-8) by the method called immersion withdrawal or dipping withdrawing. The film was annealed under a nitrogen flux according to the following procedure: the temperature is brought to 400 C. within 1 hour and maintained at 400 C. during 1 hour followed by natural cooling until room temperature is reached. The film obtained was dipped into an aqueous solution containing photosensitive dye N719 (410.sup.4M) during 24 hours. Dye was anchored to the surface of the cathode material particles. Samples of the film were dried at 50 C. under vacuum during 24 hours.

[0148] The film was kept in the dark during OCV measurement. Contrary to Example 2 wherein the system reached a potential of 4.2V in less than 1 hour, the potential reached a plateau of 3.4V with no increase for 14 days (FIG. 19). This example shows that despite the presence of a semiconductor used to improve the removal of lithium in LiFePO.sub.4, light was required for the reaction to take place.

[0149] Although the present invention has been described hereinabove by way of specific embodiments thereof, it may be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims.

[0150] The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.

REFERENCES

[0151] B. O'Reagan and M. Grtzel, Nature (1991) 353, 737-740.