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.
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 LiCoO2, LiMn2O4, and LiNiO2. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn2O4, 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 (LiCoO2), the lithium manganese oxide (LiMn2O4), and the lithium nickel oxide (LiNiO2) 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 LiMn2O4, LiNiO2, and LiCoO2 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 LiMn2O4 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 LiMn2O4) and the actual capacity when only 0.8 atomic units of lithium are extracted as observed during operation of a cell. For LiNiO2 and LiCoO2 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.
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 LiaExe2x80x2bExe2x80x3c(PO4)3 where in an initial condition xe2x80x9caxe2x80x9d is about 3, and during cycling varies as 0xe2x89xa6axe2x89xa63; b and c are both greater than 0, and b plus c is about 2. In one embodiment, elements Exe2x80x2 and Exe2x80x3 are the same. In another embodiment, Exe2x80x2 and Exe2x80x3 are different from one another. At least one of Exe2x80x2 and Exe2x80x3 is an element capable of an oxidation state higher than that initially present in the lithium phosphate compound. Correspondingly, at least one of Exe2x80x2 and Exe2x80x3 has more than one oxidation state. Both Exe2x80x2 and Exe2x80x3 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 Exe2x80x2 and Exe2x80x3 is a metal or semi-metal. Preferably, at least one of Exe2x80x2 and Exe2x80x3 is a metal. Preferably, the metal phosphate is represented by the nominal general formula Li3Mxe2x80x2bMxe2x80x3c(PO4)3, where Mxe2x80x2 and Mxe2x80x3 are each metals, b plus c is about 2, and Mxe2x80x2 and Mxe2x80x3 satisfy the conditions of oxidizability and oxidation state given for Exe2x80x2 and Exe2x80x3. Many combinations satisfying the above conditions are possible. For example, in one embodiment Mxe2x80x2 and Mxe2x80x3 are each transition metals. In still another embodiment where the formulation Li3Mxe2x80x2Mxe2x80x3 (PO4)3 comprises two different metals, Mxe2x80x2 and Mxe2x80x3, one metal Mxe2x80x2 may be selected from non-transition metals and semi-metals. In another embodiment, such non-transition metal has only one oxidation state and is nonoxidizable from its oxidation state in the final compound Li3Mxe2x80x2Mxe2x80x3(PO4) 3. In this case, Mxe2x80x2 may be selected from metals, such as aluminum, magnesium, calcium, potassium, and other Groups I and II metals. In this case, Mxe2x80x3 is a metal having more than one oxidation state, and is oxidizable from its oxidation state in the end product, and Mxe2x80x3 is preferably a transition metal. In another embodiment, the non-transition metal has more than one oxidation state. Examples of semi-metals having more than one oxidation state are selenium and tellurium; other non-transition metals with more than one oxidation state are tin and lead. Metallic elements include metals and semi-metals, such as semi-conductors, including silicon (Si), tellurium (Te), selenium (Se), antimony (Sb), and arsenic (As). The metal phosphates are alternatively represented by the nominal general formula Li3xe2x88x92xMxe2x80x2Mxe2x80x3(PO4)3 (0xe2x89xa6xxe2x89xa63), signifying capability to deintercalate and reinsert lithium. Li3xe2x88x92xMxe2x80x2yMxe2x80x32xe2x88x92y(PO4)3 signifies that the relative amount of Mxe2x80x2 and Mxe2x80x3 may vary, with o less than y less than 2, some Mxe2x80x2 and Mxe2x80x3 are each present. The same criteria as to the values of x and y apply to Li3xe2x88x92xExe2x80x2yExe2x80x32xe2x88x92y(PO4)3. The active material of the counter electrode is any material compatible with the lithium-metal-phosphate of the invention. Where the lithium-metal-phosphate is used as a positive electrode active material, metallic lithium may be used as the negative electrode active material where lithium is removed and added to the metallic negative electrode during use of the cell. The negative electrode is desirably a nonmetallic intercalation compound. Desirably, the negative electrode comprises an active material from the group consisting of metal oxide, particularly transition metal oxide, metal chalcogenide, carbon, graphite, and mixtures thereof. It is preferred that the anode active material comprises graphite. The lithium-metal-phosphate of the invention may also be used as a negative electrode material.
The present invention resolves the capacity problem posed by widely used cathode active material. It has been found that the capacity of cells having the preferred Li3Mxe2x80x2Mxe2x80x3(PO4)3 active material of the invention are greatly improved, for example, over LiMn2O4. Optimized cells containing lithium-metal-phosphates of the invention potentially have performance greatly improved over all of the presently used lithium metal oxide compounds. Advantageously, the novel lithium-metal-phosphate compounds of the invention are relatively easy to make, and readily adaptable to commercial production, are relatively low in cost, and have very good specific capacity.
Objects, features, and advantages of the invention include an improved electrochemical cell or battery based on lithium which has improved charging and discharging characteristics, a large discharge capacity, and which maintains its integrity during cycling. Another object is to provide a cathode active material which combines the advantages of large discharge capacity and with relatively lesser capacity fading. It is also an object of the present invention to provide positive electrodes which can be manufactured more economically and relatively more conveniently, rapidly, and safely than present positive electrodes which react readily with air and moisture. Another object is to provide a method for forming cathode active material which lends itself to commercial scale production providing for ease of preparing large quantities.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.