Influence of Cation Substitution on Cycling Stability and Fe-Cation Migration in Li₃Fe_3-MTe₂O₁₂ (M = Al, In) Cathode...

Li₃Fe₃Te₂O₁₂ adopts a crystal structure, described in space group Pnnm, related to that of LiSbO₃, in which Te⁶⁺, Fe³⁺, and Li⁺ cations reside in a partially ordered configuration within an hcp array of oxide ions. Chemical or electrochemical insertion of lithium is accompanied by a fully reversible migration of some of the Fe cations with an initial capacity of 120 mA h g^-1 (2.85 Li per formula unit). Long-term cycling stability is limited by the facile reduction of Te⁶⁺ to elemental Te, which leads to cathode decomposition. Partial substitution of Fe by In suppresses Te⁶⁺ reduction, such that Li₃Fe₂InTe₂O₁₂ shows no sign of this cathode decomposition pathway, even after 100 cycles. In contrast, Al-for-Fe substitution is chemically limited to Li₃Fe_2.6Al_0.4Te₂O₁₂ and appears to have almost no influence on cathode longevity. These features of the Li₃Fe_3-xMTe₂O₁₂ system are discussed on the basis of a detailed structural analysis performed using neutron and synchrotron X-ray diffraction.