Identification of an Intermediate Phase, Li1/2FeSO4F, Formed during Electrochemical Cycling of Tavorite LiFeSO4F

Many compounds adopting the tavorite -type crystal structure have attracted considerable attention as cathode materials for lithium ion batteries due to the favorable structural characteristics, facilitating promising electrochemical performance. Recent reports have highlighted the complex mechanism of lithium insertion/extraction in some of these compounds, such as the stabilization of intermediate phases in the LiFeSO 4 OH and LiVPO 4 F systems. In the case of tavorite LiFeSO 4 F, reported density functional theory (DFT) calculations have suggested the possibility of a similar behavior, but thus far, no experimental verification of such a process has, to the best of our knowledge, been successfully demonstrated. In this work, we investigate the structural evolution of LiFeSO 4 F upon extraction/insertion of lithium ions from/into the host framework. By thorough ex situ characterizations of chemically and electrochemically prepared Li x FeSO 4 F-samples (0 ≤ x ≤ 1), we demonstrate the stabilization of an intermediate phase, Li 1/2 FeSO 4 F, for which one possible structural model is proposed. However, results indicating charge ordering on the iron-sites, suggesting the formation of a super structure with a larger unit cell, are also highlighted. Moreover, the degree of formation of Li 1/2 FeSO 4 F is shown to be highly dependent on the rate of lithium extraction as a result of an exceptionally small potential separation (∼15 mV during charging) of the two subsequently occurring biphasic processes, LiFeSO 4 F/Li 1/2 FeSO 4 F and Li 1/2 FeSO 4 F/FeSO 4 F. Finally, the intermediate phase is shown to be formed both on charge and discharge during battery cycling, even though an apparent asymmetrical electrochemical trace suggests the contrary.