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 Li_xMn_2-yMe_yO₄, 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 LiNi_0.25Mn_1.75O₄ film which showed lower electronic conductivity. The LiMn_1.90Ni_0.10O₄ 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 LiMn_1.75Co_0.25O₄ 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.