An experimental and computational study of the electrode material olivine-LiCoAsO4

A novel olivine LiCoAsO4 has been successfully synthetized and characterized. This novel compound is electrochemically active at an average voltage of 4.7 V versus a lithium electrode; this is about 0.1 V below the lithium deinsertion voltage of LiCoPO4, the general electrochemical characteristic being similar in both materials. Calculations using the local density approximation with the Hubbard U parameter correction method (LDA+U) allow a correct prediction of the insertion voltage confirming that the insertion voltage in CoAsO4 is some milivolts lower than in CoPO4. Calculated results evidence that replacing the (PO4)(-3) group for the less covalent (AsO4)(-3) group increases the covalency of the Co-O bond and raises up the energy of the Co d states. As a result, the energy of the Co+3/Co+2 couple gets closer to that of the Li+/Li redox couple and the lithium insertion takes place at a lower voltage. This subtle voltage shift is very beneficial for lithium batteries operating near the limit of the electrolyte decomposition.