Electrochemical Studies of Substituted Spinel Thin Films

Abstract

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 {micro}m) of Li{sub x}Mn{sub 2{minus}y}Me{sub y}O{sub 4}, 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{sup +}. 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 Ni{sup 2+} to Ni{sup 4+}. No significant capacity was observed in LiMn{sub 2}O{sub 4} 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{sub 0.25}Mn{sub 1.75}O{sub 4} film which showed lower electronic conductivity. The LiMn{sub 1.90}Ni{sub 0.10}O{sub 4} films showed no loss in discharge capacitymore » after being charged up to 5.7 V vs. Li/Li{sup +} with window-opening cyclic voltammetry. LiMn{sub 2}O{sub 4} and LiMn{sub 1.75}Co{sub 0.25}O{sub 4} films were stable to 5.6 and 5.4 V vs. Li/Li{sup +}, respectively. Explanations for the superior stability of the films relative to powder electrodes are examined.« less