Conversion of Li₂FeSbO₅ to the Fe(III)/Fe(V) Phase LiFeSbO₅ via Topochemical Lithium Extraction

Reaction between Na2FeSbO5 and LiNO3 at 300 °C yields the metastable phase Li2FeSbO5 which is isostructural with the sodium “parent” phase (space group Pbna, a = 15.138(1) Å, b = 5.1440(3) Å, c = 10.0936(6) Å) consisting of an alternating stack of Li2Fe2O5 and Li2Sb2O5 sheets containing tetrahedral coordinated Fe3+ and octahedrally coordinated Sb5+, respectively. Further reaction between Li2FeSbO5 with NO2BF4 in acetonitrile at room temperature yields LiFeSbO5, which adopts an orthorhombic structure (space group Pbn21, a = 14.2943(4) Å, b = 5.2771(1) Å, c = 9.5610(3) Å) in which the LiFeO5 layers have shifted on lithium extraction, resulting in an octahedral coordination for the iron cations. 57Fe Mössbauer data indicate that the nominal Fe4+ cations present in LiFeSbO5 have disproportionated into a 1:1 combination of Fe3+ and Fe5+ centers which are ordered within the LiFeSbO5 structural framework. It is widely observed that Fe4+ centers tend to be unstable in delithiated Li–Fe–X–O phases currently proposed as lithium-ion battery cathode materials, so the apparent stability of highly oxidized Fe5+ centers in LiFeSbO5 is notable, suggesting cathode materials based on oxidizing Fe3+ could be possible. However, in this instance, the structural change which occurs on delithiation of Li2FeSbO5 prevents electrochemical cycling of this material.