Synthesis and electrochemical study of antifluorite-type phases in the Li-M-N-O (M = Ti, V) systems

Antifluorite-type phases have been prepared in the Li-MN-O (M = Ti, V) systems by solid state reaction under nitrogen starting from mixtures of lithium nitride, different amounts of lithium oxide and the transition metal nitride. The two series of samples obtained can be formulated as Li5TiN3 (.) xLi(2)O and Li7VN4 (.) xLi(2)O solid solutions. X-ray diffraction data indicate that, as the amount of oxygen increases, the superstructure peaks decrease in intensity without changing their position, whereas the peaks corresponding to the simple anti-fluorite structure increase. Thus, samples with low oxygen content are a mixture of two phases: the corresponding ternary nitride (either Li5TiN3 or Li7VN4) and the quaternary oxinitride with a disordered antifluorite structure and hence structural disorder does not seem to be induced progressively, as previously believed. In agreement with previous studies, a decrease of the cell parameter with oxygen content is observed in both solid solutions. Even though no deintercalation/intercalation reactions were expected (no cationic vacancies exist in the sample and the transition metals are present in their highest oxidation states) an evaluation of the electrochemical performances of the samples vs. metallic lithium was done to test the possibility of conversion reactions taking place. However, none of them showed any electroactivity.