Reversible electrochemical insertion/extraction of Mg and Li ions for orthorhombic Mo₉Se₁₁ with cluster structure

The reversible electrochemical insertion/extraction of Mg²⁺ and Li⁺ into/from a crystalline has been found in orthorhombic Mo₉Se₁₁ (o-Mo₉Se₁₁), the crystal structure of which is composed of molybdenum cluster units. The insertion/extraction reversibility of bivalent ion, Mg²⁺, could be ascribed to improvement of slow diffusion in the host lattice through the delocalization effect of electrons induced by cluster structure as supposed in Chevrel phase. The characteristic of Li/Mg-ion half-cell with o-Mo₉Se₁₁ cathode, such as discharge curve and theoretical capacity, are discussed based on the electronic structure. A part of discharge curve in the insertion process of Li⁺ was likely ascribed to the psuedogap structure in the density of state for the electronic band. The reversible capacity of o-Mg_xMo₉Se₁₁ was below 40% of the theoretical capacity deduced from the molecular orbital model, whereas over 80% of that was observed in o-Li_xMo₉Se₁₁. The smaller reversible capacity for Mg²⁺ could be ascribed to the Coulomb repulsion between bivalent Mg-ions confined in the one-dimensional channel of o-Mo₉Se₁₁, which highly prevents ion-insertion for bivalent Mg-ions compared with that for monovalent Li-ions. We suggest that both delocalized electronic structure and high dimensional ion-channel are necessary to realize reversible cathodes of Mg-ion battery with high capacity.