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.