Modification of sputter-deposited nanocrystalline LixMn2-yO4 thin-film cathodes by in situ substrate bias and postanneal

Thin films of lithium manganese oxides were deposited by radio frequency magnetron sputtering. Negative electric bias was applied on the substrates during deposition. The deposited films exhibited a defected spinel structure, LixMn2-yO4, as characterized by X-ray diffraction and micro-Raman spectroscopy. As the substrate bias increased, the films developed stronger (111) orientation. At high substrate bias, the films tended to lose lithium and a second phase of manganese oxide developed. Postanneal was also carried out to fabricate well-crystallized LiMn2O4 spinel thin films for comparison. The film morphology also varied with substrate bias as observed by scanning electron microscope. By substrate bias and anneal, well-crystallized films with different morphologies can be obtained. Charge-discharge and cyclic voltammetry curves of these thin films were measured and compared. The different electrochemical characteristics of these films were attributed to the modified crystallography, morphology, and film stress. The films deposited with in situ substrate bias exhibited larger capacity without well-defined plateau. The postannealed films without substrate bias showed well-defined 4.1 and 3.9 V plateaus with slightly lower capacity. The biased and annealed films had low capacity but showed only 3.5% of capacity loss after 30 cycles. (c) 2005 The Electrochemical Society.