Reversible Fe3+/Fe2+ and Ti4+/Ti3+ redox couple in Fe-substituted LiTi2O4 ramsdellite and its electrochemical properties as electrode material in lithium ion batteries

Abstract

Ti/Fe substitution in ramsdellite LiTi2O4, an attractive insertion material for lithium-ion battery applications, has been performed by the ceramic method yielding electrode materials with general stoichiometry LiFexTi2􀀀 xO4. Samples with 0 ≤ x ≤ 0.5 resulted in well crystallized orthorhombic ramsdellite phases, space group Pbnm, while samples with x > 0.5 formed spinel, space group Fd-3m, as revealed by powder X-ray diffraction. The ramsdellites were further investigated with X-EDS microanalysis, 57Fe M¨ossbauer spectroscopy and electrochemical discharge-charge cycling in lithium cells. This provided a comprehensive insight into the Ti/Fe substitution mechanism, which turned out to be rather more complex than the predicted simple M3+ isovalent substitution, with participation of Fe3+/Fe2+ and Ti4+/Ti3+. Upon testing against lithium in the low voltage range (ocv-1 V), ramsdellite with low Ti/Fe substitution, namely LiFe0.125Ti1.875O4, outperformed undoped LiTi2O4 electrochemically, delivering a 1st cycle capacity of 180 mAh g􀀀 1 at C/30 (5.3 mA g􀀀 1) stabilized at 140 mAh g􀀀 1 upon cycling, compared to 120 and 80 mAh g􀀀 1, respectively, in LiTi2O4. Two active redox pairs, Ti4+/Fe3+ and Fe3+/ Fe2+, combined with better electrical properties due to the presence of metallic iron, boosting farther electronic conductivity in LiFe0.125Ti1.875O4, allow a noticeable capacity of 71 mAh g􀀀 1 still to be held at a high current of 2 C (320 mA g􀀀 1). In the high voltage range (ocv-4 V), LiFe0.125Ti1.875O4 electrochemically outmatched the higher Fe-substituted ramsdellites, which were characterized by a high irreversible capacity ascribed to the unavailability of Fe3+/Fe4+ redox couple and electrolyte decomposition.