Surface Characterization of the Spinel Li_xMn₂O₄ Cathode before and after Storage at Elevated Temperatures

Surface chemistry of the capacity fading of the Li x Mn 2 O 4 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 LiPF 6 /EC + DMC + DEC at 70 °C for 5 days. Surface morphological changes of the Li x Mn 2 O 4 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 MnF 2 . The conversion of the oxidation state of manganese on the surface of the cathode from MnO 2 to MnO during storage at the elevated temperature was observed with XPS.