Multiphase Electrochemical Li-Ion Intercalation in Iron Selenite: Evidence of Fe^3+/4+ Redox and Single-Crystal-to-Single-Crystal Topochemical Transformation during Chemical Lithiation

An iron selenite-based cathode compound, LiFe(SeO3)2, is synthesized via a simple hydrothermal route. The bulk purity of the compound is confirmed by powder X-ray diffraction (PXRD), Mössbauer spectroscopy, and Fourier transform infrared spectroscopy (FTIR). The crystal structure determined from the single-crystal X-ray diffraction and Rietveld refinement of synchrotron PXRD data perfectly matches the previously reported structure in the I4̅2d space group. The crystal structure is built up of edge-sharing of FeO6 octahedra and LiO4 tetrahedra interconnected by SeO3 trigonal pyramidal units, forming a three-dimensional open framework network with channels through all three axes. The Li atoms do not occupy channels that can be viewed along the a-, b-, and c-axis, leaving them empty. The compound is capable of inserting Li in the structure through both chemical and electrochemical reduction. Mössbauer spectroscopy confirms a reduction of 61% of the Fe3+ sites through chemical reduction. When tested as a Li-ion battery cathode using galvanostatic charge–discharge, the compound achieves a maximum capacity of ∼77 mAh/g, which corresponds to reversible intercalation of 0.9 mol of Li by accessing the Fe2+/Fe3+ redox. The electrochemical intercalation of Li occurs via multiple phase transitions, resulting in a staircase-like voltage–composition profile, which is also explained by theoretical structural optimization. The differential capacity curve shows very low polarization upon Li intercalation. When subjected to electrochemical oxidation first, it shows 0.2 mol of Li extraction from LiFe(SeO3)2, suggesting a partial Fe3+ to Fe4+ redox in a comparatively lower potential (<4.2 V) than commonly seen. The presence of Fe4+ in the charged state is confirmed by Mössbauer spectroscopy. Additionally, we successfully determined the single-crystal structure of the partially lithiated phase Li1.5Fe(SeO3)2. This was achieved through chemical reductive insertion on single crystals of LiFe(SeO3)2 in solution, allowing us to pinpoint the location of the new lithium atom.