Comparison of the electrochemical lithiation/delitiation mechanisms of FeP_x (x = 1, 2, 4) based electrodes in Li-ion batteries

The electrochemical reaction mechanisms of FeP_x (x = 1, 2, 4) as electrode materials in Li batteries were analyzed by ⁵⁷Fe Mössbauer spectroscopy, magnetic and NMR measurements. Depending on their stoichiometry, iron phosphides react with lithium along different pathways. FeP and FeP₂ are fully or partially converted to a composite electrode of Li₃P and Fe. For both, the same intermediate phases, namely FeP and Li_xFeP, were identified in cycling and FeP is partially regenerated on charge. Surprisingly, FeP₄ reacts with Li through an insertion type process previously identified for earlier transition metal phosphides (Ti, V, and Mn), and no conversion reaction to Li₃P/Fe is observed. The long-term stability of the composite electrode Li₃P/Fe⁰ and of the intermediate phase Li_xFeP is analyzed.