A review on the problems of the solid state ions diffusion in cathodes for rechargeable Mg batteries

This work summarizes the results of our studies devoted to Mg ions mobility in Chevrel phases (CPs), MxMo6T8 (M - metal, T = S, Se) and presents our vision of the problem of multivalent ions' diffusion in intercalation compounds. A simplified analysis of the main factors, which affect the activation energy barriers, as well as experimental data of Mg ions insertion into different hosts, show that low Mg ions mobility in common transition metal oxides or sulfides cannot be explained only by strong ionic interactions, but it is rather caused by a hard redistribution of the bivalent cations charge in inorganic materials. In contrast to these hosts, CPs allow for a high mobility of multivalent cations, because their unusual crystal structure with octahedral Mo-6-clusters enables a fast and efficient attainment of local electro-neutrality upon insertion of cations of high charge density. Analysis of diffusion pathways based on the detailed structural determinations sheds light on important aspects of the electrochemical behavior of CPs, such as partial Mg ions trapping in the course of reversible Mg insertion and the ways to avoid it.