Formation of a Complete Solid Solution between the Triphylite and Fayalite Olivine Structures

The recent infatuation for LiFePO 4 as positive electrode material in Li-ion batteries has prompted a renewed interest in olivine-type structures, with a view to enhance their conduction properties. We show that the dual substitution of Li for Fe and of P for Si in the olivine LiFePO 4 phase leads to a complete solid solution Li 1− x Fe 1+ x P 1− x Si x O 4 as deduced from combined X-ray diffraction, Mössbauer, and NMR experiments. Our findings challenge the common belief that the anionic network cannot be substituted. Moreover, it is found that such a substitution promotes Li intersite mixing between the olivine M1 and M2 sites. Such mixing, together with the worsening of the conducting properties of the dually substituted samples, is believed to be responsible for the poor electrochemical performances of the member’s series. Beyond x = 0.20, the samples were electrochemically inactive. While the current materials are disappointing application-wise, such a study provides clues to the rich chemistry remaining to be unveiled with olivine-type structures in particular and polyanionic compounds in general.