Stability improvement of LiMn2O4 thin-film cathodes under high rate and over-discharge cycling

Thin films of spinel lithium manganese oxides were prepared by sol-gel method. The annealing time and temperatures were optimized to obtain crystalline LiMn2O4 thin films. The as-synthesized films were coated with a thin layer of alumina or lithium phosphorus oxynitride (LiPON) by radio frequency sputtering. The coatings were amorphous as characterized by X-ray diffraction. The electrochemical properties of the coated and uncoated thin-film cathodes were characterized and compared. The cells made of the lithium manganese oxide cathodes were discharged at different rates. At higher discharge rates, both the alumina and the LiPON-coated samples exhibited higher discharge potential plateaus than the uncoated spinel. As the cells were cycled in a wide voltage window of 4.0-1.5 V, the coated cathodes exhibited less capacity fading. For samples immersed in acid solution before cycling, the surface-coated samples showed greater stability. (c) 2006 The Electrochemical Society.