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 (∼4.7V) 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 Fd 3 ¯ 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 ∼30nm while the Fd 3 ¯ m-structured ones (TA =800°C) contain large-sized grains at least ∼200nm. The electrochemical properties of the LNMO cathodes near 4.7V 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 Fd 3 ¯ m structure (TA =800°C) exhibited no C-D activity near 4.7V