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

Reaction between Na2FeSbO5and LiNO3at 300 °C yields the metastable phase Li2FeSbO5which 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 Li2Fe2O5and Li2Sb2O5sheets containing tetrahedral coordinated Fe3+and octahedrally coordinated Sb5+, respectively. Further reaction between Li2FeSbO5with NO2BF4in 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 LiFeO5layers 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 LiFeSbO5have disproportionated into a 1:1 combination of Fe3+and Fe5+centers which are ordered within the LiFeSbO5structural 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 LiFeSbO5is notable, suggesting cathode materials based on oxidizing Fe3+could be possible. However, in this instance, the structural change which occurs on delithiation of Li2FeSbO5prevents electrochemical cycling of this material.