Surface characterization of the spinel LixMn2O4 cathode before and after storage at elevated temperatures

Surface chemistry of the capacity fading of the LixMn2O4 cathode was invest igated using atomic force microscopy (AFM), energy-dispersive X-ray analysi s (EDAX), and X-ray photoelectron spectroscopy (XPS). Measurements show a d ecrease in the cathode capacity from 124 mA h g(-1) before storage to 102 m A h g(-1) after storage in an electrolyte of 1 M LiPF6/EC + DMC + DEC at 70 degreesC for 5 days. Surface morphological changes of the LixMn2O4 cathode were monitored using contact and tapping AFM and lateral force microscopy in air. Nanoscale changes of the charged cathode before and after storage a t 70 C were observed. Before storage, homogeneous grains of approximately 1 00-200 nxn are seen. After storage, fine and nearly round shaped structures of 10-30 um 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 ph osphorus and fluorine are also deposited on the surface of the cathode. Sur face analysis of the cathode with XPS suggests the presence of MnF2. The co nversion of the oxidation state of manganese on the surface of the cathode from MnO2 to MnO during storage at the elevated temperature was observed wi th XPS.