Electrochemical STM observation of Li1+xMn2-xO4 thin films prepared by pulsed laser deposition

Spinel Li1+xMn2-xO4 (x = 0.01 and 0.03) thin films were prepared by pulsed laser deposition (PLD). The Li/Mn ratio of Li1+xMn2-xO4 thin films were controlled by the energy fluence of the laser beam in the PLD method. Changes in surface morphology of the thin films after potential cycling in different potential ranges were observed in 1 mol dm(-3) LiClO4/propylene carbonate by electrochemical scanning tunnel microscopy (STM). The discharge capacities of the nearly stoichiometric Li1.01Mn1.99O4 electrode faded rapidly upon charge/discharge cycling in a higher potential region (4.04-4.25V). After repeated cycling in the higher potential region, smaller round-shaped particles of around 100 nm in diameter were newly formed on the electrode, and the entire surface was covered with them. These small particles were likely to be newly formed from the electrolyte solution through dissolution/precipitation reactions. Capacity-fading with repeated cycling was suppressed for the lithium-ion excess Li1.03Mn1.97O4 thin film electrode compared to Li1.01Mn1.99O4. In addition, no small, round-shaped particles appeared on Li1.03Mn1.97O4 even after the 75th cycle. Based on these results, it was concluded that the observed changes in morphology in the higher potential region are closely related to the observed capacity-fading of the nearly stoichiometric Li1.01Mn1.99O4. (c) 2007 The Electrochemical Society.