Structural and electrochemical evolution with post-annealing temperature of solution-based LiNi0.5Mn1.5O4 thin-film cathodes for microbatteries with cyclic stability

Spinel LiNi0.5Mn1.5O4 (LNMO) cathodes were prepared as polycrystalline thin films on Pt substrates via a sol-gel procedure at elevated post-annealing temperature (TA) for high-potential (∼4.7 V) Li-ion microbattery operation. X-ray diffraction and Raman spectral patterns revealed that the LNMO cathodes obtained at TA = 500 and 600 °C have ordered P4332 crystal structure while those at 800 °C have disordered Fd3¯m structure. The two phases were found to coexist in the LNMO cathodes with TA = 700 °C. Scanning electron microscopy images revealed that the P4332-structured samples contain small-sized grains down to ∼30 nm while the Fd3¯m-structured ones (TA = 800 °C) contain large-sized grains at least ∼200 nm. The electrochemical properties of the LNMO cathodes near 4.7 V were characterized by charge-discharge (C-D) cycling performance, cyclic voltammetry, and rate capability. The results indicate that the grain size is most important for the electrochemical performance: the LNMO cathodes with P4332 structure (TA = 600 and 700 °C) exhibited capacity retention of more than 95% after 400 C-D cycles, while those with Fd3¯m structure (TA = 800 °C) exhibited no C-D activity near 4.7 V