Polycrystalline LiMn2O4 thin films were deposited by radio-frequency (rf) magnetron sputtering followed by annealing at 600 degrees C in air. The films were then treated with an rf-driven oxygen plasma. The crystallization and surface morphology of LiMn2O4 thin films were correlated with rf powers. The treated samples were tested under harsh conditions such as deep discharge to 1.5 V and cycling at an elevated temperature of 60 degrees C to verify the electrochemical performances of the LiMn2O4 cathodes. The oxygen plasma treatments significantly improved the electrochemical properties of LiMn2O4 thin films. The results showed that the plasma treatments not only resulted in surface densification but also induced finer grains in the subsurface region (10-100 nm), which were believed to enhance the stability of LiMn2O4 cathodes.