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Surface chemistry of the capacity fading of the Li x Mn2O4 cathode was investigated using atomic force microscopy (AFM), energy-dispersive X-ray analysis (EDAX), and X-ray photoelectron spectroscopy (XPS). Measurements show a decrease in the cathode capacity from 124 mA h g-1 before storage to 102 mA h g-1 after storage in an electrolyte of 1 M LiPF6/EC + DMC + DEC at 70 °C for 5 days. Surface morphological changes of the Li x Mn2O4 cathode were monitored using contact and tapping AFM and lateral force microscopy in air. Nanoscale changes of the charged cathode before and after storage at 70 °C were observed. Before storage, homogeneous grains of approximately 100−200 nm are seen. After storage, fine and nearly round shaped structures of 10−30 nm in size are observed covering the larger grains on the surface of the cathode. This change in morphology suggests film deposition on the cathode's surface, which increases the resistance for Li+ ion transport in and out of the cathode. Results from EDAX show that compounds containing phosphorus and fluorine are also deposited on the surface of the cathode. Surface analysis of the cathode with XPS suggests the presence of MnF2. The conversion of the oxidation state of manganese on the surface of the cathode from MnO2 to MnO during storage at the elevated temperature was observed with XPS.

Surface reactions are responsible for the cycling performance of Li-ion rechargeable batteries based on LiMn2O4. We report structural and electrochemical studies of the LiMn2O4 cathode at room temperature in LiPF6 electrolyte using in situ electrochemical atomic force microscopy (EC-AFM) and lateral force microscopy (LFM). Surface dynamics were monitored in situ in organic electrolyte under potentiostatic conditions. After the first charge/discharge cycle, surface features were clearly visible. During the second charging process at 4.3 V vs Li/Li+, dissolution of surface particles was observed. The surface topography was quantitatively analyzed by height distribution functions. The dissolution rate (calculated from the change of particle volume with time) was found to follow the square root of time. During the second discharge process at 3.8 V vs Li/Li+, new particles were formed on the surface. However, there were no further changes in the surface topography with time during polarization at 3.8 V vs Li/Li+. The surface dynamics monitored for various charge/discharge conditions showed that a complex dissolution/precipitation reaction of manganese and lithium compounds is involved in the charge/discharge process of spinel LiMn2O4-based cathode material.