This invention relates to a process for preparation of a lithiated or overlithiated transition metal oxide, this lithiated or overlithiated oxide beneficially being usable as an active electrode material and more particularly for a positive electrode.
The invention also relates to the electrode, and particularly the positive electrode containing this material.
Finally, the invention relates to lithium batteries with a metallic or composite negative electrode using the said positive electrode.
The technical domain of the invention may generally be considered as being rechargeable Secondary Lithium Cells or Secondary Lithium Batteries.
A historical overview of the development of rechargeable secondary lithium batteries is given in the document by K. BRANDT xe2x80x9cHistorical Development of Secondary Lithium Batteriesxe2x80x9d, Solid State Ionics 69 (1994), 173-183.
The operating principle of all lithium battery systems is the same: each time that the battery is charged or discharged, lithium in ionic form (Li+) is exchanged between the positive and negative electrodes. The quantity of energy exchanged during each charge or discharge (supplied by the battery during discharge or supplied to the battery during charge) is exactly proportional to the quantity of lithium that can be exchanged during the electrochemical reaction.
This xe2x80x9cexchangeablexe2x80x9d lithium must be supplied by a lithium xe2x80x9csourcexe2x80x9d. This source is the negative electrode in the case of systems using a lithium metal negative electrode. For systems using a carbon based negative electrode which in principle does not contain any lithium by construction, the lithium source must be contained in the positive electrode. In this case, the active material in the positive electrode acts as the lithium source. Therefore, it can be seen that it becomes necessary to include the largest possible quantity of lithium in the active material of the positive electrode during its synthesis, in order to provide a sufficient reserve of lithium to obtain interesting electrochemical performances.
A cell is characterized by its operating voltage, which is determined by the potential difference between the negative electrode and the positive electrode. The absolute potential (non-measurable) of the negative electrode made of lithium metal is constant, since it is a pure metal.
Therefore, the voltage of a cell with a lithium metal negative electrode depends entirely on the potential of the positive electrode, which depends on the crystallographic structure of the active material in the positive electrode, and which changes as a function of the quantity of the lithium contained in it. As the cell discharges, the lithium enters the crystalline structure of this active material in which the potential drops regularly. The cell voltage drops. This is the reverse of what occurs during charging.
Active materials all have a different variation of their potential (with respect to Li/Li+) with time depending on the quantity of lithium contained in them; thus each active material has a characteristic xe2x80x9celectrochemical signaturexe2x80x9d. In some, lithium is inserted at between 3.5 and 4.5 Volts, for example as in the case of cobalt oxides for which the potential (with respect to Li/Li+) varies between 3.5 V (for LiCoO2) and 4.5 Volts (for Li1xe2x88x92xCoO2, where x≈0.7 after the cell has been charged).
As another example, the potential (with respect to Li/Li+) of manganese oxides with a composition similar to Li0.3MnO2 used by TADIRAN Batteries Ltd. for batteries made using the technology described in patent U.S. Pat. No. 5,506,068 at which lithium is inserted is between 3.4 Volts (the composition of the active material in the positive electrode is then close to Li0.3MnO2) and 2 Volts (the composition of the active material in the positive electrode is then close to LiMnO2). This is the xe2x80x9c3 Volts Lithium-metal liquid electrolytexe2x80x9d system.
Other materials based on manganese oxides are more versatile; thus manganese oxides with a spinel structure usually have two operating potential xe2x80x9cplateausxe2x80x9d. For example for the compound with a spinel structure and formula LiMn2O4, most of the lithium is extracted from this structure at between about 3.2 Volts and 4.4 Volts (with respect to Li/Li+) (the composition of the active material in the positive electrode after the charge to 4.4 Volts is then close to Mn2O4), whereas lithium can be inserted between about 3.2 Volts and 1.8 Volts in the LiMn2O4 structure (the composition of the active material in the positive electrode at the end of the discharge of the cell to 1.8 Volts is then close to Li2Mn2O4).
Therefore, it can be seen that it is possible and even necessary to choose the active compound in the positive electrode to optimize the global performances of the system.
Lithium cells may be classified in different categories or systems, the first of these systems being the xe2x80x9c3 Voltsxe2x80x9d lithium metal liquid electrolyte system.
Chronologically, the first lithium cells developed about 20 years ago used a lithium metal negative electrode.
Although these batteries provide high energy densities due to the large reserve of lithium contained in the negative electrode, this system was abandoned by most battery manufacturers due to the poor reconstitution of the metal surface at the negative electrode/electrolyte interface during charging and discharging cycles, resulting in inadequate lives (xcx9c200 cycles). Experience showed that dendritic growth phenomena (in the form of needles) appeared gradually during reconstitution of the lithium metal, during successive charging/discharging cycles. These needles eventually filled in the space between the negative electrode and the positive electrode after about 200 cycles, which caused internal short circuits.
However, some battery manufacturers have successfully limited this phenomenon. For example, the document by E. MENGERITSKY, P. DAN, I. WEISSMAN, A. ZABAN; D. AURBACH, xe2x80x9cSafety and Performances of TADIRAN TLR-7103 Rechargeable Batteriesxe2x80x9d, J. Electrochem, Soc, Vol. 143, No. 7, July 1996 describes a battery operating at between 2 and 3.4 volts with a lithium metal negative electrode and with a liquid electrolyte with an interesting life due to a new electrolyte formulation, but the life is nevertheless limited to about 500 charging/discharging cycles.
An additional improvement may be achieved by the use of a positive electrode material containing a larger quantity of lithium.
A different system called the 4 Volt xe2x80x9cLithium-ionxe2x80x9d system was suggested at the beginning of the 1980s in order to overcome the difficulty caused by dendritic growth.
This system consists of substituting a carbon based lithium insertion compound to replace the lithium metal negative electrode.
In this case, the lithium metal negative electrode is replaced by an electrode containing a carbon based lithium insertion compound in which lithium is reversibly inserted during successive cycles, in exactly the same way as it does in the positive electrode insertion compound. This is the xe2x80x9c4 Volt lithium-IONxe2x80x9d system.
However, due to this choice:
the negative electrode is no longer capable of acting as a reservoir for the lithium necessary for the electrochemical reaction, which makes it essential to use a positive electrode compound containing lithium by construction.
part of the lithium originating from the positive electrode is irreversibly consumed by the carbon negative electrode the first time that the cell is charged (corresponding to the first time that lithium is inserted in the carbonated negative electrode) which results in an equivalent loss of capacity of the cell.
These limitations confirm that it would be useful to be able to synthesize a positive electrode active material containing the largest possible amount of lithium.
For example, up to now, the compounds based on manganese oxide with the best electrochemical characteristics in the xe2x80x9c4 Volt lithium-IONxe2x80x9d system described above, are products with a spinel structure and a composition similar to LiMn2O4. They enable electrochemical cycling between 3.2 and 4.4 Volts (with respect to Li/Li+).
It can be seen that in this case some of the lithium contained in LiMn2O4 is irreversibly consumed by the negative electrode during the first charge. The usefulness of including an additional quantity of lithium by construction in the spinel structure of the compound LiMn2O4 can be seen, to provide an additional reserve of lithium.
Despite many attempts throughout the world, for example as described in the document by TARASCON J. M., GUYOMARD D., J. Electrochem.Soc, Vol 138, No. 10, 2804-2868, 1991, it has not been possible up to now to synthesize a compound with a spinel structure with formulation Li1xe2x88x92xMnO2O4, where 0 less than xxe2x89xa61 using an economic and easily industrializable process.
This is why the use of materials based on manganese oxide as the active materials for positive electrodes has been severely limited in xe2x80x9c4 Volt lithium-IONxe2x80x9d cells.
This also explains that compounds selected for this xe2x80x9c4-Volt Li-IONxe2x80x9d system were essentially mixed oxides of lithium and cobalt (LiCoO2) or nickel (LiNiO2).
These compounds do have the advantage that they can easily be synthesized by heat treatment of appropriate precursors while maintaining an acceptable energy density exceeding 100 Wh/kg. In this case, the relatively low quantity of lithium stored in the positive electrode (proportional to the I.t capacity) is compensated by the high operating voltage U close to 4 Volts (energy=U.I.t).
Since the end of the 1980s, most cell manufacturers have been developing this four Volt Lithium Ion Cell (4-Volts Li-ION) which associates a positive electrode compound of the cobalt oxide LiCoO2 or nickel oxide LiNio2 type operating between 3.5 and 4.5 Volts (with respect to Li/Li+) and special carbonated negative electrode materials limiting the loss of capacity in the first cycle. These developments are described in the document by K.BRANDT mentioned above.
These systems confirm the energy densities of about 110 to 120 Wh/kg corresponding to an endurance of 170 to 200 km for an electrical vehicle, and with a life of close to 800 cycles.
The disadvantages of this type of battery are the high cost of cobalt and nickel oxides, and their energy density that, despite everything, remains low (compared with systems using a lithium metal negative electrode).
Therefore, in all cases (systems with a lithium metal or carbon based negative electrode), it appears that the use of a highly lithiated positive electrode active compound would be a significant factor towards improving the performances of the cell.
The existence of highly lithiated transition metal oxides has been demonstrated since in discharging cells, this type of compound is naturally introduced by the electrochemical insertion of the lithium ion in the positive electrode material.
The search for xe2x80x9cartificialxe2x80x9d synthesis processes has already explored several options.
The first of these options is synthesis in the solid phase making use of the reaction at different temperatures between salts or powders of transition metal oxides and lithium salts, but products with the required stocheometry and structure have never been obtained using this process.
The second of these options is synthesis by reaction using a reduction agent in solution. For example, this reduction agent could be lithium n-butyl or lithium iodide.
Reduction by lithium n-butyl was described by David W. I. F., Thackeray M. M., Picciotto L. A., Goodenough J. B., J. of Solid State Chemistry, 67, 316-323, 1987.
In this case, the lithiation reaction of manganese oxides by lithium n-butyl is very slow. It takes several days and uses a reagent, namely lithium n-butyl that is very expensive and dangerous since it is unstable in air.
The stoichiometry of the compounds obtained is difficult to control. The quantity of lithium n-butyl used must be close to the required stoichiometry. An excess of this reagent will result in excessive reduction of the transition metal oxide. Therefore, the reaction cannot be accelerated in this way.
The compounds obtained have a crystallographic structure approximately identical to the structure of compounds formed when lithium is inserted in the positive electrode compound when a cell is discharging, but they are not stable in air.
The reduction by lithium iodide was described by Tarascon J. M., Guyomard D., J. Of Electrochem. Soc, vol. 138, No. 10, 2864-2868, 1991.
The lithiation reaction of manganese oxides by lithium iodide is also fairly slow, since it takes about 24 hours and requires good control of the stocheometry of the reagents. However, the reaction can result in compounds stable in air with the required crystallographic structure, in other words similar to the structure obtained in a cell during discharge. Furthermore, lithium iodide is very expensive.
Document U.S. Pat. No. 5,549,880 describes the production of lithiated vanadium oxides starting from a lithium alkoxide prepared by reaction between lithine and a light alcohol such as ethanol or methanol. The final lithiated product is then difficult or even impossible to obtain since it becomes delithiated very quickly. The use of heavier alcohols results in extremely long reaction times.
Therefore, there is a need for a process for the preparation of a xe2x80x9clithiated or overlithiatedxe2x80x9d transition metal oxide that does not have the disadvantages and limitations of processes according to prior art, particularly concerning the reaction rate and the cost.
The purpose of the invention is to provide a process for the preparation of xe2x80x9clithiated or overlithiatedxe2x80x9d transition metal oxides that satisfies these and other requirements, and which overcomes the problems that occur with processes according to prior art.
This and other purposes are achieved according to the invention by a process for the preparation of a xe2x80x9clithiated or overlithiatedxe2x80x9d transition metal oxide comprising the following three steps, carried out successively or in a simultaneous manner:
Preparation of a solution of lithium alkoxide (alcoolate) by dissolving lithium metal in the corresponding alcohol, the said alcohol being chosen among the alcohols originating from linear or ramified alkanes comprising at least three carbon atoms, the alcohols originating from unsaturated aliphatic hydrocarbides, and mixtures of them;
Addition of a transition metal oxide powder to the said lithium alkoxide solution to obtain a dispersion;
Controlled reduction of the said transition metal oxide by the said alkoxide to obtain a xe2x80x9clithiated or overlithiatedxe2x80x9d transition metal oxide with a defined (required) Li:Metal stoichiometry; this stoichiometry is dependent of the composition and structure of the initial transition metal oxide;
Evaporation of the residual alcohol,
Rinsing of the powder thus obtained,
Drying of the powder.
xe2x80x9cOverlithiationxe2x80x9d means insertion of lithium in the structure of a commercial transition metal oxide before it is used in the positive electrode; the characteristics (chemical, crystallographic, electrochemical) of the xe2x80x9coverlithiatedxe2x80x9d compound thus formed are similar to the characteristics of the product obtained by inserting lithium in the initial commercial oxide as it is generated during an infinitely slow discharge of the cell down to a voltage between the voltage (with respect to Li/Li+) at which this initial oxide is abandoned and 1.0 Volt (with respect to Li/Li+).
Lithiation, or a lithiated product, means any intermediate step between the initial product and the overlithiation corresponding to the overlithiated product defined above.
The process according to the invention is fundamentally different from processes according to prior art for the preparation of xe2x80x9clithiated or overlithiatedxe2x80x9d transition metal oxides in that it uses lithium alkoxides as reactional intermediaries, that according to an essential characteristic of the invention are obtained by dissolving lithium metal in a corresponding alcohol, this alcohol being derived from a linear or ramified alkane with at least three carbon atoms or an alcohol originating from an unsatruated aliphatic hydrocarbide. Obviously, any mixture of these alcohols in any proportions could be used.
The use of lithium alkoxides in the preparation of positive electrode compounds for lithium cells has certainly been mentioned before, but only for the preparation of lithiated vanadium oxides (Patent U.S. Pat. No. 5,549,880).
However, the process used in this document uses lithine LiOH and an alcohol as alkoxide preparation precursors, such that due to constants of the chemical equilibrium between LIOH and alcohols, the solution obtained cannot contain appreciable quantities of alkoxide unless the alcohol used is a light alcohol (essentially methanol or ethanol).
This document discourages the use of heavier alcohols and specifically recommends that methanol or ethanol should be used, as is the case in the single example given in this document.
The use of a heavier alcohol such as pentanol-1 in particular, under the conditions of the process according to this document starting from lithine, leads to the formation of very low concentrations of alkoxides in the solution, and therefore very low reaction rates.
The process according to this application demonstrates the need to use an alkoxide derived from the reaction between lithium and an alcohol produced by a linear or ramified alkane containing at least three carbon atoms or an alcohol derived from an unsaturated aliphatic hydrocarbide.
In general, it has been demonstrated that lithiated or overlithiated transition metal oxides are delithiated in ethanol, thus for example, we were able to demonstrate that an LiMn2O4 spinel lithiated by the process according to the invention in pentanol-1 becomes quickly delithiated during its residence in pure ethanol.
This confirms the predominance of the stability of lithium ethoxide compared with transition metal oxides lithiated or overlithiated by our process, and therefore conversely the impossibility of obtaining these same lithiated or overlithiated compounds in dispersion in a light alcohol solution such as ethanol or methanol.
Surprisingly, the preparation of alkoxide from lithium metal according to the first step of the process according to the invention, can result in concentrated solutions of lithium alkoxide even with alcohols containing three or more atoms of carbon, contrary to what is described in document U.S. Pat. No. 5,549,880.
Consequently, the rate of lithiation of transition metal oxide is faster, or is even instantaneous, and the efficiency is close to 1 due to the stronger reduction power of heavy alkoxides (originating from alcohols derived from linear or ramified alkanes with at least three atoms of carbon or alcohols derived from unsaturated aliphatic hydrocarbons), compared with light alkoxides.
The process according to the invention does not form part of the same domain as xe2x80x9cmildxe2x80x9d chemistry performed in solution and at low temperature.
The process according to the invention provides a solution to the problems that arise with processes according to prior art.
It only uses standard, inexpensive and easily available reagents that do not introduce any particular risks.
Unlike processes according to prior art, the process according to the invention also takes place quickly and there are few intermediate steps since the essential step in the process is the controlled reduction of the transition metal oxide.
The process according to the invention is sufficiently xe2x80x9cmildxe2x80x9d and precise so that the required stoichiometry can be achieved.
The reaction stops at a Li:metal stoichiometry that is perfectly defined by the choice of the alcohol, even in the presence of a large excess of alkoxide, and that depends on the composition and crystallographic structure of the initial transition metal oxide.
A perfectly defined stocheometry usually means an Li:metal ratio greater than or equal to 0.5, and preferably between 0.5 and 2.
The process according to the invention also has the advantage that it is significantly less expensive than the first other two processes according to prior art described above.
Thus, based on costs estimated by comparing prices per mole of lithiation reagents (mole of lithium), it was estimated that the cost of the process according to the invention would be about 18 times less than the cost of the process using lithium iodide and the reaction time would be about 20 times less, and the cost would be about 3 times less than the process using lithium n-butyl and the reaction time would be divided by at least 100.
The process according to the invention has the advantage that it can be carried out at low temperature; a low temperature usually means that the various steps can be carried out at ambient temperature (usually about 20xc2x0 C.), as is the case for the preparation of the lithium alkoxide solution, the addition of metal oxide powder to the lithium alkoxide solution or rinsing of the powder obtained, or at a low temperature as in the case of controlled reduction which is usually carried out at a temperature of 50 or 70 to 260xc2x0 C., which is approximately equal to the boiling temperature of the alcohol used (heated to reflux) or drying temperature which is usually 80 to 150xc2x0 C.
Obviously, these temperature ranges may be lowered to ambient temperature or even lower, if the operations are carried out at low pressure instead of at atmospheric pressure.
The use of low temperatures makes it easy to obtain an electrochemically reversible compound with a perfectly defined Li:metal stoichiometry.