LiMn2-xCuxO4 spinels (0.1≤x≤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 than or equal to x less than or equal to 0.5), has been studied by crystallographic, spectroscopic, and electrochemical methods and by electron microscopy. These LiMn2-xCuxO4, spinels are nearly identical in structure to cubic LiMn2O4 and successfully undergo reversible Li intercalation. The electrochemical data show a remarkable reversible electrochemical 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 electrochemical stability of these materials upon cycling. The initial capacity 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 reaction that normally occurs at 4.1 V in standard Li1-xMn2O4 electrodes (1 greater 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, neutron diffraction data show a small but significant quantity of an impurity phase, the composition and structure of which could not be identified. X-ray absorption spectroscopy was used to gather information about the oxidation states of the manganese and copper ions. The composition of the spinel component in the LiMn1.5Cu0.5O4 was determined from X-ray diffraction and X-ray absorption near-edge spectroscopy to be Li1.01Mn1.67Cu0.32O4, suggesting to a best approximation that the impurity in the sample was a lithium-copper-oxide phase. The substitution of manganese by copper enhances the reactivity of the spinel structure toward hydrogen: the compounds are more easily reduced at moderate temperature (similar to 200 degrees C) than LiMn2O4.