Chemistry of inorganic compounds – Phosphorus or compound thereof – Oxygen containing
Reexamination Certificate
2002-05-17
2004-05-04
Maples, John S. (Department: 1745)
Chemistry of inorganic compounds
Phosphorus or compound thereof
Oxygen containing
C423S593100, C423S594150, C423S594200, C423S659000
Reexamination Certificate
active
06730281
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for producing electrode active materials which can be used to formulate electrodes for use in electrochemical cells in batteries. More particularly, the present invention relates to methods that involve reduction of a transition metal to form the active material.
BACKGROUND OF THE INVENTION
Lithium batteries have become a useful and desirable energy source in recent years. Generally speaking lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include a negative electrode, a positive electrode, and an electrolyte interposed between spaced apart positive and negative electrodes. By convention, the negative electrode is the electrode that acts as an anode (where oxidation occurs) on discharge, while the positive electrode is the one that acts as a cathode (where reduction occurs) on discharge.
Batteries with anodes of metallic lithium and containing metal chalcogenides cathode active material have received acceptance in industry and commerce.
So-called lithium ion batteries are well known. Lithium ion batteries have an insertion anode, such as a lithium metal chalcogenide, lithium metal oxide, coke or graphite. These types of electrodes are typically used with lithium-containing insertion cathodes to form an electroactive couple in a cell. The resulting cells are not charged in an initial condition. Before this type of cell can be used to deliver electrochemical energy, it must be charged. In the charging operation, lithium is transferred from the lithium-containing electrode cathode (the positive electrode) to the negative electrode. During discharge the lithium is transferred from the negative electrode back to the positive electrode. During a subsequent recharge, the lithium is transferred back to the negative electrode where it reinserts. Thus with each charge/discharge cycle, the lithium ions (Li+) are transported between the electrodes. Such rechargeable batteries having no free metallic species, are called rechargeable ion batteries or rocking chair batteries.
Known positive electrode active materials include LiCoO
2
, LiMn
2
O
4
, and LiNiO
2
. Lithium compounds containing cobalt are relatively expensive to synthesize due to the intermediates required, while successful synthesis of lithium-nickel compounds is relatively complex and difficult. Lithium-manganese compounds, such as LiMn
2
O
4
, are generally more economical to synthesize than the preceding material and result in a relatively economical positive electrode.
Unfortunately all of the foregoing materials have drawbacks as electroactive materials in electrochemical cells. Cells employing the foregoing materials in the cathode experience significant loss of charge capacity over repeated charge/discharge cycles, commonly referred to as cycle fading. Furthermore, the initial capacity available (amp hours/gram) from the materials is less than the theoretical capacity because significantly less than 1 atomic unit of lithium engages in the electrochemical reaction. This initial capacity value is significantly diminished during the first cycle of operation and diminishes even further on every successive cycle of operation. For LiNiO
2
only about 0.5 atomic units of lithium is reversibly cycled during cell operation.
Many attempts have been made to reduce capacity fading, for example, as described in U.S. Pat. No. 4,828,834 by Niagara et al. However, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing electrode material having acceptable capacity without the disadvantage of significant capacity loss when used in a cell.
Alternative active materials for lithium ion applications are constantly being sought. In addition, there remains a need for providing an economical and reproducible synthesis method for such materials that will provide good quality material in suitable yields.
SUMMARY OF THE INVENTION
A method for carrying out solid state reactions under reducing conditions is provided. Solid state reactants include at least one inorganic metal compound and a source of reducing carbon. The reaction may be carried out in a reducing atmosphere in the presence of reducing carbon. Reducing carbon may be supplied by elemental carbon, by an organic material, or by mixtures. The organic material is one that can form decomposition products containing carbon in a form capable of acting as a reductant. The reaction proceeds without significant covalent incorporation of organic material into the reaction product. In a preferred embodiment, the solid state reactants also include an alkali metal compound.
The products of the method find use in lithium ion batteries as cathode active materials. Preferred active materials include lithium-transition metal phosphates and lithium-transition metal oxides. In a preferred embodiment, the reaction product contains carbon particles intimately mixed among crystals of the active materials. Such products may be produced by heating a metal compound with a source of carbon.
In a preferred embodiment, reaction is carried out in a stoichiometric excess of carbon. The resulting reaction product contains a mixture of a metal compound with a carbonaceous material having a high atom percent of carbon. The organic material or carbonaceous material is not significantly covalently incorporated into the reaction product, but rather the carbonaceous material is intimately mixed with the reduced metal compound.
In another aspect, a reductive reaction of an alkali metal compound and a transition metal compound in the presence of reducing carbon is carried out in a reducing atmosphere. The reducing atmosphere may contain a reducing gas such as hydrogen, methane, ammonia, or carbon monoxide.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
Reductive methods are provided for synthesizing transition metal compounds and other compounds. In one aspect, the reaction products find use as battery active materials or as precursors for the synthesis of battery active materials.
Active materials of the invention contain at least one alkali metal and at least one other metal capable of being oxidized to a higher oxidation state. Preferred other metals are accordingly selected from the group consisting of transition metals (defined as Groups 4-11 of the periodic table), as well as certain other non-transition metals such as tin, bismuth, and lead. The active materials may be synthesized in single step reactions or in multi-step reactions. In at least one of the steps of the synthesis reaction, reducing carbon is used as a starting material. During the reductive step at least one metal is reduced in oxidation state.
In a preferred embodiment, the invention provides a method for synthesizing an inorganic metal compound, preferably a transition metal compound, comprising the steps of:
providing starting materials comprising at least one particulate metal compound and at least one organic material;
combining the starting materials to form a mixture; and
heating the mixture at a temperature and for a time sufficient to form a reaction product In a preferred embodiment, at least one metal of the starting material is reduced in oxidation state during heating to form the metal compound. In one aspect, the metal compound includes a transition metal; in another aspect, the metal compound includes a non-transition element such as tin.
The reductant in the synthetic steps involving reduction of a metal is supplied by a reducing carbon. In one aspect, the reducing carbon is provided by elemental carbon, preferably in particulate form such as graphites, amorphous carbon, carbon blacks and the like. In another aspect, reducing carbon may also be provided by an organic precursor material, or by a mixture of elemental carbon and organic precursor material. The organic precursor material will also be referred to in this application a
Barker Jeremy
Dong Ming
Saidi M. Yazid
Swoyer Jeffrey L.
Kovacevic Cynthia S.
Maples John S.
Valence Technology Inc.
Williams Roger A.
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