Influence of annealing temperature on microstructural and electrochemical properties of rf-sputtered LiMn2O4 film cathodes

Abstract

The microstructural and electrochemical properties of rf-sputtered LiMn2O4 films were investigated as a function of post-deposition process. The degree of crystallization in the films gradually increased with the increase of annealing temperature (Ta). The films annealed at Ta = 973 K exhibited characteristic peaks with predominant (111) orientation representing the cubic spinel structure of Fd3m symmetry. The estimated Mn–Mn and Mn–O distances obtained from the X-ray diffraction data were observed to be increased slightly with Ta. Characteristic changes in surface morphological features were observed as a function of Ta as evidenced from scanning electron microscopy. The estimated root mean square (RMS) roughness of the films increased from 97 to 161 nm with augmentation of Ta. The electrochemical studies, viz. cyclic voltammetry (CV), specific discharge capacity and Li ion diffusion coefficient were carried out for annealed LiMn2O4 films in saturated aqueous electrolyte (Li2SO4) in the potential window of 0–1.2 V and correlated with surface morphology and grain size. The LiMn2O4 films annealed at Ta = 973 K exhibited better electrochemical performance and demonstrated a discharge capacity of about 53.5 μA h cm−2 μm−1 with diffusion coefficient of 1.2 × 10−13 cm2 s−1.