Electrochemical AFM study of LiMn2O4 thin film electrodes exposed to elevated temperatures

Surface morphology changes of LiMn₂O₄ thin film positive electrodes in lithium-ion batteries after repeated potential cycling or storage at elevated temperatures were observed by in situ atomic force microscopy (AFM) to elucidate the origin of capacity fading of LiMn₂O₄. After repeated potential cycling in the overall potential range from 3.50 to 4.30 V at elevated temperatures, the entire thin film surface was covered with small round-shaped particles accompanied by capacity fading of the electrode, while no significant changes were observed at 25 °C. The discharge capacity decreased more significantly when cycled in the lower potential range (3.81–4.07 V) than when cycled in the higher potential range (4.04–4.30 V). After storage at elevated temperatures at a depth of discharge (DOD) of 75%, which is located in the lower potential range, similar surface morphology changes were observed. In addition, discharge capacity markedly decreased, and the crystallinity of the LiMn₂O₄ thin film was lowered after storage. Hence, the observed changes in morphology at elevated temperatures are closely related to capacity fading of the LiMn₂O₄ thin film. The loss of crystallinity was caused by the formation of small particles on the LiMn₂O₄ surface, which would be accelerated on the LiMn₂O₄ surface in contact with an electrolyte solution through some kind of dissolution/precipitation reaction.