LIMN2-XCUXO4 SPINELS LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.5) - A NEW CLASS OF 5 V CATHODE MATERIALS FOR LI BATTERIES - I - ELECTROCHEMICAL, STRUCTURAL, AND SPECTROSCOPIC STUDIES

A series of electroactive spinel compounds, LiMn2-xCuxO4 (0.1 less tha n or equal to x less than or equal to 0.5), has been studied by crysta llographic, spectroscopic, and electrochemical methods and by electron microscopy. These LiMn2-xCuxO4, spinels are nearly identical in struc ture to cubic LiMn2O4 and successfully undergo reversible Li intercala tion. The electrochemical data show a remarkable reversible electroche mical process at 4.9 V which is attributed to the oxidation of Cu2+ to Cu3+. The inclusion of Cu in the spinel structure enhances the electr ochemical stability of these materials upon cycling. The initial capac ity of LiMn2-xCuxO4 spinels decreases with increasing x from 130 mAh/g in LiMn2O4 (x=0) to 70 mAh/g in ''LiMn1.5Cu0.5O4'' (x=0.5). The data also show slight shifts to higher voltage for the delithiation reactio n that normally occurs at 4.1 V in standard Li1-xMn2O4 electrodes (1 g reater than or equal to x greater than or equal to 0) corresponding to the oxidation of Mn3+ to Mn4+. Although the powder X-ray diffraction pattern of ''LiMn1.5Cu0.5O4'' shows a single-phase spinel product, neu tron diffraction data show a small but significant quantity of an impu rity phase, the composition and structure of which could not be identi fied. X-ray absorption spectroscopy was used to gather information abo ut the oxidation states of the manganese and copper ions. The composit ion of the spinel component in the LiMn1.5Cu0.5O4 was determined from X-ray diffraction and X-ray absorption near-edge spectroscopy to be Li 1.01Mn1.67Cu0.32O4, suggesting to a best approximation that the impuri ty in the sample was a lithium-copper-oxide phase. The substitution of manganese by copper enhances the reactivity of the spinel structure t oward hydrogen: the compounds are more easily reduced at moderate temp erature (similar to 200 degrees C) than LiMn2O4.