Electrochemical Studies of Substituted Spinel Thin Films
Thin lithium manganese oxide spinel films, prepared with pulsed laser deposition have been used as a model system for the study of oxide electrochemical properties and performance degradation mechanisms in the absence of carbon or binder materials. Films (0.3 mm) of LixMn2-yMeyO4, where Me = Ni, Co and y = 0, 0.1, 0.25, were crystalline as-prepared. The cyclic voltammetric response as a function of oxide composition was measured in liquid electrolyte over the range of 2 to 5.8 V vs. Li/Li+. Quantitative analysis of the two 4 V peaks, for x > 0.5 and x < 0.5, correlated well with predicted film stoichiometry. The capacity of the 4.6 V redox peaks in the Ni-substituted films were consistent with the oxidation of Ni2+ to Ni4+. No significant capacity was observed in LiMn2O4 above 4.5 V. The shape of the voltammetric peaks in the 3 V region suggested that intercalation kinetics are slowed by the Jahn-Teller distortion, while all compositions in the 4 V region showed reversible behavior, except for the LiNi0.25Mn1.75O4 film which showed lower electronic conductivity. The LiMn1.90Ni0.10O4 films showed no loss in discharge capacity after being charged up to 5.7 V vs. Li/Li+ with window-opening cyclic voltammetry. LiMn2O4 and LiMn1.75Co0.25O4 films were stable to 5.6 and 5.4 V vs. Li/Li+, respectively. Explanations for the superior stability of the films relative to powder electrodes are examined.