Patent Application
20070292760
Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention, given for non-restrictive example purposes only and represented in the accompanying drawings, in which:
The state of charge of a Li-Ion storage battery comprising:
a positive electrode active material presenting a constant lithium insertion/extraction potential over most of the capacity operating range and selected among LiNi0.5Mn1.5O4 and derivatives thereof,
and titanium oxide TiO2 of bronze type structure, also called TiO2-B, as negative electrode active material,
can easily be checked by a simple reading of the operating voltage.
Indeed, unlike the lithium storage battery comprising Li4Ti5O12 as negative electrode material and a positive electrode active material presenting a constant lithium insertion/extraction potential over most of the capacity operating range (for example LiFePO4 or LiNi0.5Mn1.5O4 and derivatives thereof), a Li-Ion storage battery with TiO2-B as negative electrode active material presents an operating potential continually varying according to the state of charge (or discharge). As the operating potential of the positive electrode is constant over most of the capacity operating range, the voltage delivered by the storage battery also varies continuously according to the state of charge or discharge of said storage battery and measuring it enables said state to be checked.
Moreover, bronze type titanium oxide presents at least equal electrochemical performances to those obtained with a Li4Ti5O12-based negative electrode. Indeed, among the numerous structural varieties of titanium oxide (rutile, anatase, . . . ), the bronze type structure presents the advantage of having an open three-dimensional structure forming channels, as reported in the article “TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17” (Material Research Bulletin Vol. 15, p 1129-1133, 1980) by Rene Marchand et al., or the international patent application WO2006/033069 which also mentions the possibility of forming an electrochemical cell with TiO2-B and LiFePO4 respectively as negative and positive electrode materials. Such channels are propitious for insertion and extraction of lithium. Thus, when a TiO2-B-based lithium storage battery is operating, the lithium insertion reaction in TiO2-B leads to composition of LixTiO2-B in which at least 0.6 mole of lithium ions can be inserted and then extracted. In a general manner, the lithium insertion/extraction reaction in TiO2-B is written:
Li-A+TiO2Li1−xA+LixTiO2
where Li-A corresponds to the active material of the positive electrode.
The active material of the positive electrode is a material presenting a constant lithium insertion/extraction potential over most of the capacity operating range. Thus, the active material of the positive electrode is selected among LiNi0.5Mn1.5O4 and derivatives of LiNi0.5Mn1.5O4. Among the derivatives of LiNi0.5Mn1.5O4, the active material can for example be in accordance with the following formula: Li1−aNi0.5−bMn1.5−cO4−d, with a, b, c and d comprised between −0.1 and +0.1. What is meant by a, b, c and d comprised between −0.1 and +0.1 is that each of the parameters a, b, c and d is greater than or equal to −0.1 and smaller than or equal to +0.1. More particularly, the active material can be a derivative of LiNi0.5Mn1.5O4 in accordance with the following general formula:
LiNi0.5−xMn1.5+xO4−d, with −0.1≦x≦0.1 and d≦+0.1
LiNi0.5Mn1.5O4 and its derivatives, like LiFePO4, present a constant lithium insertion/extraction potential over most of the capacity operating range. The materials presenting this characteristic are also called dual phase materials. LiNi0.5Mn1.5O4 on the other hand presents the advantage, compared with LiFePO4, of having a higher lithium insertion/extraction potential. In
The lithium insertion/extraction reaction in TiO2-B takes place at a mean potential of about 1.6 V vs. Li+/Li and it is generally perfectly reversible. The corresponding experimental specific capacity is about 200 mAh/g. Thus, for the theoretical value x=1 corresponding to the total reaction of reduction of Ti4+ into Ti3+, the theoretical specific capacity of a TiO2-B-based storage battery is 335 mAh/g whereas, for a Li4Ti5O12-based storage battery, it is 175 mAh/g.
The gain in intrinsic capacity obtained by replacing Li4Ti5O12 by TiO2-B makes it possible for example to use thinner electrodes, which therefore give better performances power-wise, while maintaining an equivalent global capacity of the storage battery. In addition, unlike Li4Ti5O12, the bipolar technology described in the patent application WO03/047021 is perfectly applicable to TiO2-B on account of its higher operating potential at 1 V vs. Li+/Li.
Synthesis of TiO2-B can be performed by any type of known synthesizing methods. Certain synthesizing methods enable for example a TiO2-B to be achieved in the form of grains of micrometric or nanometric size. In addition, the synthesis can also be chosen according to a predetermined type of grain morphology. It may in fact be advantageous to choose a particular TiO2-B grain morphology as the electrochemical properties of said material vary substantially with the morphology of the grains, in particular in terms of practical specific capacity and more or less pronounced variation of the operating potential, in the course of the lithium insertion/extraction reaction. More particularly, it is possible to synthesize TiO2-B particles without any particular shape or in the form of nanowires or nanotubes as reported in the article “Lithium-Ion Intercalation into TiO2-B nanowires” (Advanced Materials, 2005, 17, No. 7, p 862-865) by A. Robert Armstrong et al. and in the article “Nanotubes with the TiO2-B structure” (Chem. Commun., 2005, p 2454-2456) by Graham Armstrong et al.
Preferably, the different synthesizing methods employed are chosen to enable Li-Ion storage batteries with good performances to be achieved, with a mean operating voltage of about 1.6 V vs. Li+/Li and varying in more or less pronounced manner according to the state of charge of said storage battery. More particularly, the operating voltage of the storage battery varies in increasing manner with respect to the state of charge.
In practical manner, the positive and negative electrodes of the Li-Ion storage battery according to the invention can be fabricated by any type of known means. For example, the active material of each electrode can be put in the form of an intimate dispersion, in aqueous or organic solution, with an electronic conducting additive such as carbon and a binder designed to provide a good ionic conduction and a satisfactory mechanical strength. The binder can be an organic binder, such as polyethers, polyester, a methyl methacrylate-base polymer, acrylonitrile, or vinylidene fluoride. The binder can also be a component soluble in water such as natural or synthetic rubber. The dispersion, when it is aqueous, can also comprise a thickener, for example of carboxymethyl cellulose, hydroxypropyl, or methyl cellulose type, and/or a surface active agent and/or a salt (LiOH for example). The dispersion, also called “ink”, is then deposited on a metal foil sheet, for example made of aluminium and acting as current collector. The electronic conducting additive can be carbon.
The fact that TiO2-B presents an operating potential higher than 1 V vs Li+/Li presents the advantage of limiting and even preventing degradation of the electrolyte at the interface between the TiO2-B and the electrolyte. The choice of electrolyte can therefore be of any known type. It can for example be formed by a salt comprising at least the Li+ cation. The salt is for example selected among LiClO4, LiAsF6, LiPF6, LiBF4, LiRFSO3, LiCH3SO3, LiN(RFSO2)2, LiC(RFSO2)3, LiTFSI, LiBOB, and LiBETI, RF being selected among a fluorine atom and a perfluoroalkyl group comprising between one and eight carbon atoms. LiTFSI is the acronym for lithium trifluoromethanesulfonylimide, LiBOB that of lithium bis(oxalato)borate, and LiBETI that of lithium bis(perfluoroethylsulfonyl)imide. The electrolyte salt is preferably dissolved in an aprotic polar solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, etc. The electrolyte can be supported by a separating element arranged between the two electrodes of the storage battery, the separating element then being imbibed with electrolyte. Preferably, the electrolyte is chosen such as to present a good thermal stability, the highest possible ionic conductivity, the lowest toxicity and the least cost. In all cases, the electrolyte must be stable at the operating potentials of the two electrodes or it must develop a relatively stable passivation layer at the electrode/electrolyte interface, in the course of the first charge/discharge cycle, which layer is not insulating from an ionic point of view. Likewise, the electrolyte must be chemically stable with respect to the electrode materials with which it is in contact.
For comparative example purposes, two Li-Ion storage batteries, noted batteries A and B, were produced and tested. Batteries A and B each comprise a negative electrode having a particular active material as base.
For battery A, the active material of the negative electrode is Li4Ti5O12. It is for example prepared by mixing 201.05 grams of TiO2 of anatase variety (Huntsman) with 76.11 grams of Li2CO3 (Aldrich) for two hours in a planetary mill in the presence of heptane. After drying, the homogenate is heated to 500° C. for 15 hours, and then to 680° C. for 15 hours and finally to 900° C. for 5 hours. It is then homogenized in a planetary mill for one hour, and then heated again to 900° C. for 5 hours. Final milling is then performed for 24 hours before the powder obtained is heated directly to 500° C. for 15 minutes in a sealed quartz tube under argon and is then rapidly cooled to ambient temperature. The X-ray diffraction diagram performed on said powder enables the presence of the pure and well crystallized Li4Ti5O12 compound to be confirmed.
For battery B, the active material of the negative electrode is a TiO2-B compound synthesized by hydrolysis of potassium tetratitanate, as described in the article “TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17” (Material Research Bulletin Vol. 15, p 1129-1133, 1980) by Rene Marchand et al. More particularly, 14.81 grams of potassium nitrate (KNO3; Merck) are mixed in a mill with 23.17 grams of anatase variety titanium oxide (TiO2-anatase; Huntsman). After milling, the mixture is heated to 1000° C. for 24 hours so as to obtain the compound K2Ti4O9. This compound is then placed in an acidified aqueous solution (for example HNO3 at 3 mol/L) and the whole mixture is mechanically stirred for 12 hours at ambient temperature. The powder obtained is then washed several times in demineralized water and then heated to 400° C. for 3 hours to obtain a titanium oxide TiO2 of “Bronze” type structural form. The size of the TiO2-B particles is micrometric.
In order to determine the electrochemical performances of the TiO2-B synthesized in this way, a Lithium-Metal storage battery of the “button cell” type is produced with:
a lithium negative electrode in the form of a disk with a diameter of 16 mm and a thickness of 130 μm deposited on a nickel disk acting as current collector,
a positive electrode formed by a disk with a diameter of 14 mm taken from a composite film with a thickness of 25 μm comprising 80% in weight of TiO2-B compound as produced above, 10% in weight of carbon black and 10% in weight of polyvinylidene hexafluoride, the disk being deposited on an aluminium foil strip with a thickness of 20 micrometers acting as current collector,
a separator imbibed with the LiPF6 salt-base liquid electrolyte (1 mol/L) in solution in a mixture of ethylene carbonate and dimethyl carbonate. As represented in
The two storage batteries A and B also each comprise a LiNi0.5Mn1.5O4-base positive electrode and a separating element marketed under the name of Celgard 2400 and imbibed with liquid electrolyte.
The liquid electrolyte is formed by 1 mol/L of LiPF6 in solution in a mixture of propylene carbonate, dimethyl carbonate and ethylene carbonate.
Furthermore, the active material of the positive electrode, LiNi0.5Mn1.5O4, is prepared by intimate blending of 10.176 g of nickel carbonate, 6.066 g of lithium carbonate and 29.065 g of manganese carbonate under stoichiometric conditions, with an excess of 3% molar of Li. The intimate blending is performed in a Retsch planetary mill comprising a 250 ml bowl with 13 to 15 balls 20 mm in diameter and each weighing 10.8 g, for 20 hours at 500 rpm, in the presence of hexane (submerged powder). The mixture is then dried overnight at 55° C. before being subjected to thermal treatment at 600° C. for 10 hours, and then at 900° C. for 15 hours. Cooling to ambient temperature at the rate of 0.1°/min is then performed. X-ray diffraction analysis enables the formation of the LiNi0.5Mn1.5O4−d compound to be observed, with d close to 0, the unit cell parameter of said compound being 8.167 Angstroms.
The electrodes of the storage batteries A and B are each produced by mixing 80% in weight of active material, 10% in weight of carbon black acting as electronic conducting material, and 10% in weight of polyvinylidene hexafluoride acting as binder. The mixture is then deposited on an aluminium current collector.
The two storage batteries A and B were tested at 20° C., with a five-hour charge and discharge cycle (C/5 conditions).
Battery A enables lithium ions to be exchanged at a fixed potential of 3.2 V over most of its capacity operating range. Thus, as represented in
On the contrary, battery B enables lithium ions to be exchanged in the potential range of about 1.5V-4V (
It is possible, due to an intrinsically higher specific capacity of the TiO2-B-base negative electrode compared with the Li4Ti5O12-base one, to use a smaller weight of negative electrode for a storage battery whose base is formed by the LiNi0.5Mn1.5O4/TiO2-B couple than that in the case of a storage battery whose base is formed by the LiNi0.5Mn1.5O4/Li4Ti5O12 couple with the same overall capacity.
According to an alternative embodiment, titanium oxide TiO2 of “Bronze” type structural form can be produced by means of another synthesizing method than that used in the case of battery B. For example, TiO2-B can be synthesized by hydrothermal means as reported in the above-mentioned article “Nanotubes with the TiO2-B” by G. Armstrong et al. More particularly, 5 g of TiO2 in anatase form (Huntsman) are placed in 84 mL of soda at 15 mol/L. The mixture is stirred for 1 hour and is then placed in a teflon autoclave cell (PARR vessel—125 mL), which is then placed in an oven at 170° C. for 68 hours. The mixture is then removed, washed twice in distilled water and centrifuged. The isolated powder is then placed in 1 L of distilled water containing 0.05 mol/L of hydrochloric acid (stirred for 2 h). After decanting, the recovered powder is again washed twice and centrifuged. Finally, a titanium oxide TiO2 of “Bronze” type structural form is obtained, after drying in a vacuum at 80° C. for 24 hours.
This synthesizing method enables a bronze type titanium oxide to be achieved presenting different morphological specificities from the oxide produced for battery B. The TiO2-B particles are partially in the form of nanowires and partially in the form of agglomerated amorphous particles of different sizes.
In order to determine the electrochemical performances of TiO2-B, a Lithium-Metal storage battery of the “button cell” type is produced in the same way as in the previously described Lithium-Metal storage battery, the bronze type titanium oxide obtained by hydrolysis of the potassium tetratitanate being replaced by that obtained by hydrothermal means.
As represented in
| Number | Date | Country | Kind |
|---|---|---|---|
| 06 05467 | Jun 2006 | FR | national |
