Electrochemical properties of LiNi0.5Mn1.5O4 films prepared by spin-coating deposition

Highly uniform films of pure LiNi0.5Mn1.5O4 spinel were obtained by spin-coating stoichiometric amounts of Mn(CH3COO)2 and Ni(CH3COO)2 dissolved in de-ionized water, and Li(i-OC3H7) dissolved in a 1:2 CH3-COOH/i-C3H7OH mixture, onto an Au substrate. The resulting gel was transformed into a spinel by calcining at variable temperatures from 400 to 800 °C. The deposits, thus, obtained were characterized by scanning electron microscopy (SEM), and X-ray diffraction and X-ray photoelectron spectroscopy (XPS), and their electrochemical properties determined from potentiostatic and galvanostatic measurements. The spinel formed at 400 °C, with an earthworm-like shape, the origin of which was ascribed to the formation of a layer of undecomposed organic components. At 800 °C, the film exhibited markedly improved in crystallinity and the spinel particles adopted a polyhedral shape of submicron size with well-defined edges and faces, thus, reducing the interparticle connectivity. Nickel atoms were found to occur in two chemical environments, namely: as Ni2+ and Ni3+, the former being the major component. The potentiostatic curves obtained were consistent with the spinel formula as they exhibited two well-defined peaks in the 4.6–4.8 V region associated to Ni2+ → Ni3+ → Ni4+ processes. Also, both potentiostatic curves and galvanostatic curves were consistent with a reversible Li insertion/extraction reaction; however, the cells exhibited significant overcharge that was probably due to electrolyte decomposition catalyzed by gold. The thin deposits obtained were found to deliver capacities around 148 mAh g−1 and to efficiently retain them upon extended cycling. However, the coulombic efficiency of the deposits was rather low (around 50% at the 50th cycle). Increasing the film thickness, raised the coulombic efficiency to 85% but decreased the delivered capacity, probably through increased resistance of the electrode; however, capacity retention on cycling was maintained.