Antifluorite-type lithium chromium oxide nitrides:: Synthesis, structure, order, and electrochemical properties

Antifluorite-type lithium chromium oxide nitrides were prepared by solid-state reaction of Li3N, Li2O, and Cr2N. Depending on the reaction time and starting Li/Cr and O/Cr ratios, either an ordered or a disordered phase (or mixtures of both) is obtained. The formation of the former is favored by short reaction times and low Cr/O ratios whereas the formation of the latter is favored by higher Cr/O ratios and longer reaction times. The two phases were characterized, and the first one was confirmed to be the already reported Li14Cr2N8O phase, whereas the stoichiometry of the second is Li10CrN4O2. Interestingly, even if both contain cationic vacancies in the structure, electrochemical lithium intercalation could only be achieved for Li10CrN4O2. This phase exhibits a reversible capacity of 160 mAh/g very stable upon cycling. Bond valence and first-principles DFT calculations were carried out to understand the absence of lithium insertion in Li14Cr2N8O. Li-Li repulsion and destabilization of the tetrahedral CrN4 units induced by occupation of the potential sites, as well as the absence of energetically favorable pathways for transport of the ions to these sites, are suggested to be the reasons.