Polycrystalline LiMn2O4 thin films are deposited by radio frequency (rf) magnetron sputtering followed by air annealing at 600 and 400 degrees C. The films are then treated with an rf driven oxygen plasma. The crystallographic structures and surface morphologies of the LiMn2O4 thin films are studied. Plasma treatments induce different trends of microstructural evolution in 600 and 400 degrees C annealed films. The microstructural evolutions are induced by the combination effects of plasma treatments and air annealing. The results can be interpreted by the atomic relaxation and resputtering models. For both high and low temperature annealed films, a well-tuned plasma treatment can result in films with a dense subsurface microstructures and a grain size of similar to 10 nm, which shows a greatly enhanced electrochemical performances. Under optimal treatments conditions, the electrochemical performances of the LiMn2O4 films can be greatly improved. The treated samples are tested under harsh conditions, including deep discharge to 1.5 V and cycling at elevated temperature of 60 degrees C, and such subsurface structure is found to suppress the development of Li1+xMn2O4 rock-salt phase. (C) 2011 The Electrochemical Society. [DOI:10.1149/1.3531988] All rights reserved.