LiMn2 − x Cu x O 4 Spinels (0.1 ⩽ x ⩽ 0.5): A new Class of 5 V Cathode Materials for Li Batteries: I. Electrochemical, Structural, and Spectroscopic Studies

Abstract

A series of electroactive spinel compounds, (0.1 ⩽ x ⩽ 0.5), has been studied by crystallographic, spectroscopic, and electrochemical methods and by electron microscopy. These spinels are nearly identical in structure to cubic 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 to . The inclusion of Cu in the spinel structure enhances the electrochemical stability of these materials upon cycling. The initial capacity of spinels decreases with increasing x from 130 mAh/g in (x = 0) to 70 mAh/g in “ ” (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 electrodes (1 ⩾ x ⩾ 0) corresponding to the oxidation of to . Although the powder X‐ray diffraction pattern of “ ” 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 was determined from X‐ray diffraction and X‐ray absorption near‐edge spectroscopy to be , 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 (∼200°C) than .