Influence of Cation Substitution on Cycling Stability and Fe-Cation Migration in Li 3 Fe 3- x M x Te 2 O 12 (M = Al, In) Cathode Materials.

Li ₃ Fe ₃ Te ₂ O ₁₂ adopts a crystal structure, described in space group Pnnm , related to that of LiSbO ₃ , in which Te ⁶⁺ , Fe ³⁺ , 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 Te ⁶⁺ to elemental Te, which leads to cathode decomposition. Partial substitution of Fe by In suppresses Te ⁶⁺ reduction, such that Li ₃ Fe ₂ InTe ₂ O ₁₂ shows no sign of this cathode decomposition pathway, even after 100 cycles. In contrast, Al-for-Fe substitution is chemically limited to Li ₃ Fe _2.6 Al _0.4 Te ₂ O ₁₂ and appears to have almost no influence on cathode longevity. These features of the Li ₃ Fe _3-x M _x Te ₂ O ₁₂ system are discussed on the basis of a detailed structural analysis performed using neutron and synchrotron X-ray diffraction.