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 sal-gel procedure at elevated post-annealing temperature (T-A) for high-potential (similar to 4.7V) Li-ion microbattery operation. X-ray diffraction and Raman spectral patterns revealed that the LNMO cathodes obtained at T-A = 500 and 600 degrees C have ordered P4(3)32 crystal structure while those at 800 degrees C have disordered Fd (3) over barm structure. The two phases were, found to coexist in the LNMO cathodes with T-A = 700 degrees C. Scanning electron microscopy images revealed that the P4(3)32-structured samples contain small-sized grains down to similar to 30nm while the Fd (3) over barm-structured ones (T-A = 800 degrees C) contain large-sized grains at least similar to 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 P4(3)32 structure (T-A = 600 and 700 degrees C) exhibited capacity retention of more than 95% after 400 C-D cycles, while those with Fd (3) over barm structure (T-A =800 degrees C) exhibited no C-D activity near 4.7V (C) 2014 Elsevier Ltd. All rights reserved.