Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-11-19
2002-09-10
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231100
Reexamination Certificate
active
06447951
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to improved materials usable as electrode active materials, method for making such improved materials, and electrodes formed from it for electrochemical cells in batteries.
BACKGROUND OF THE INVENTION
Lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between spaced apart positive and negative electrodes. Batteries with anodes of metallic lithium and containing metal chalcogenide cathode active material are known. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. Other electrolytes are solid electrolytes typically called polymeric matrixes that contain an ionic conductive medium, typically a metallic powder or salt, in combination with a polymer that itself may be ionically conductive which is electrically insulating. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. Cells having a metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemical active (electroactive) material of the cathode whereupon they release electrical energy to an external circuit.
It has recently been suggested to replace the lithium metal anode with an intercalation anode, such as a lithium metal chalcogenide or lithium metal oxide. Carbon anodes, such as coke and graphite, are also intercalation materials. Such negative electrodes are used with lithium-containing intercalation cathodes, in order to form an electroactive couple in a cell. Such cells, in an initial condition, are not charged. In order to be used to deliver electrochemical energy, such cells must be charged in order to transfer lithium to the anode from the lithium-containing cathode. During discharge the lithium is transferred from the anode back to the cathode. During a subsequent recharge, the lithium is transferred back to the anode where it reintercalates. Upon subsequent charge and discharge, 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. See U.S. Pat. Nos. 5,418,090; 4,464,447; 4,194,062; and 5,130,211.
Preferred positive electrode active materials include LiCoO
2
, LiMn
2
O
4
, and LiNiO
2
. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn
2
O
4
, for which methods of synthesis are known, and involve reacting generally stoichiometric quantities of a lithium-containing compound and a manganese containing compound. The lithium cobalt oxide (LiCoO
2
), the lithium manganese oxide (LiMn
2
O
4
), and the lithium nickel oxide (LiNiO
2
) all have a common disadvantage in that the charge capacity of a cell comprising such cathodes suffers a significant loss in capacity. That is, the initial capacity available (amp hours/gram) from LiMn
2
O
4
, LiNiO
2
, and LiCoO
2
is less than the theoretical capacity because less than 1 atomic unit of lithium engages in the electrochemical reaction. Such an initial capacity value is significantly diminished during the first cycle operation and such capacity further diminishes on every successive cycle of operation. The specific capacity for LiMn
2
O
4
is at best 148 milliamp hours per gram. As described by those skilled in the field, the best that one might hope for is a reversible capacity of the order of 110 to 120 milliamp hours per gram. Obviously, there is a tremendous difference between the theoretical capacity (assuming all lithium is extracted from LiMn
2
O
4
) and the actual capacity when only 0.8 atomic units of lithium are extracted as observed during operation of a cell. For LiNiO
2
and LiCoO
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 Nagaura 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 chalcogenide electrode material having acceptable capacity without disadvantage of significant capacity loss when used in a cell.
SUMMARY OF THE INVENTION
The invention provides novel lithium-containing phosphate materials having a high proportion of lithium per formula unit of the material. Upon electrochemical interaction, such material deintercalates lithium ions, and is capable of reversibly cycling lithium ions. The invention provides a rechargeable lithium battery which comprises an electrode formed from the novel lithium-containing phosphates, preferably lithium-metal-phosphates. Methods for making the novel phosphates and methods for using such phosphates in electrochemical cells are also provided. Accordingly, the invention provides a rechargeable lithium battery which comprises an electrolyte; a first electrode having a compatible active material; and a second electrode comprising the novel phosphate materials. The novel materials, preferably used as a positive electrode active material, reversibly cycle lithium ions with the compatible negative electrode active material. Desirably, the phosphate has a proportion in excess of 2 atomic units of lithium per formula unit of the phosphate, and upon electrochemical interaction the proportion of lithium ions per formula unit become less. Desirably, the lithium-containing phosphate is represented by the nominal general formula Li
a
E′
b
E″
c
(PO
4
)
3
where in an initial condition “a” is about 3, and during cycling varies as 0≦a≦3; b and c are both greater than 0, and b plus c is about 2. In one embodiment, elements E′ and E″ are the same. In another embodiment, E′ and E″ are different from one another. At least one of E′ and E″ is an element capable of an oxidation state higher than that initially present in the lithium phosphate compound. Correspondingly, at least one of E′ and E″ has more than one oxidation state. Both E′ and E′ may have more than one oxidation state and both may be oxidizable from the state initially present in the phosphate compound. Desirably, at least one of E′ and E″ is a metal or semi-metal. Preferably, at least one of E′ and E″ is a metal. Preferably, the phosphate is represented by the nominal general formula Li
3
M′
b
M″
c
(PO
4
)
3
, where M′ and M″ are each metalloids or metals, b plus c is about 2, and M′ and M″ satisfy the conditions of oxidizability and oxidation state given for E′ and E″. Many combinations satisfying the above conditions are possible. For example, in one embodiment M′ and M″ are each transition metals. In still another embodiment of the formulation Li
3
M′
y
M″
2−y
(PO
4
)
3
, M′ may be selected from non-transition metals and semi-metals (metalloids). In another embodiment, such non-transition metal has only one oxidation state and is nonoxidizable from its oxidation state in the final compound Li
3
M′
y
M″
2−y
(PO
4
)
3
. In this case, M′ may be selected from metals, such as aluminum, magnesium, calcium, potassium, and other Groups I and II metals. In this case, M″ has more than one oxidation state, and is oxidizable from its oxidation state in the end product, and M″ is preferably a transition metal. In another embodiment, the non-transition-metal element has more than one oxidation state. In one preferred embodiment, M″ is a transition metal, and M′ is a non-transition-metal met
Barker Jeremy
Saidi M. Yazid
Chaney Carol
Harness & Dickey & Pierce P.L.C.
Valence Technology Inc.
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