Delithiation of olivine - structured LiFexMn1-xPO4 cathode materials. Mössbauer studies

This paper presents the results of applying the 57Fe Mössbauer effect technique to studies of the delithiation mechanism of LixMn0.55Fe0.45PO4 olivine samples, and also investigations of the origin of the widely discussed, astonishing high electronic conductivity of tungsten-doped LiFePO4 samples, providing evidence of the presence of a residual, iron-containing and highly conductive phase. The delithiation process is perceived by iron ions as a change of their valence and symmetry of the local surroundings upon lithium extraction. The LixMn0.55Fe0.45PO4 compound, which belongs to a novel group of cathode materials for Li-ion batteries, exhibits a single-phase deintercalation region, in contrast to LiFePO4 exhibiting two-phase mechanism of electrochemical lithium extraction/insertion in the entire lithium concentration range, as well as to LiMnPO4, for which the deintercalation process is practically irreversible. The range of deintercalation mechanism in LixMn0.55Fe0.45PO4 was found to be exactly related to the content of Fe2+ ions in the cathode material. A surface sensitive technique, Conversion Electron Mössbauer Spectroscopy (CEMS), was used to prove the presence of traces of iron phosphides on the grain surfaces of tungsten-doped LiFePO4 samples, pointing to the minor phase as being responsible for the high electronic conductivity of these samples.