Rechargeable ion-batteries, in which ions such as Li+ carry charges between electrodes, have been contributing to the improvement of power-source performance in a wide variety of mobile electronic devices. Among them, Mg-ion batteries are recently attracting attention due to possible low cost and safety, which are realized by abundant natural resources and stability of Mg in the atmosphere. However, only a few materials have been known to work as rechargeable cathodes for Mg-ion batteries, owing to strong electrostatic interaction between Mg2+ and the host lattice. Here we demonstrate rechargeable performance of Mg-ion batteries at ambient temperature by selecting TiSe2 as a model cathode by focusing on electronic structure. Charge delocalization of electrons in a metal-ligand unit through d-p orbital hybridization is suggested as a possible key factor to realize reversible intercalation of Mg2+ into TiSe2. The viewpoint from the electronic structure proposed in this study might pave a new way to design electrode materials for multivalent-ion batteries.

Rechargeable performance is realized in Mg batteries using a TiS3 cathode without the nanometer-scale downsizing of electrode particles. The specific capacity is about 80 mAh/g for the first 50 cycles at room temperature. This observed specific capacity is comparable to that of the prototype cathode for Mg batteries. First-principles calculation indicates that TiS3 is a semiconductor with d–p orbital hybridized electronic structures around the Fermi level. The reversible electrode performance is likely assisted by the delocalized electronic distribution over metal–ligand units through d–p orbital hybridization.

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The reversible electrochemical insertion/extraction of Mg 2+ and Li + into/from a crystalline has been found in orthorhombic Mo 9 Se 11 (o-Mo 9 Se 11 ), the crystal structure of which is composed of molybdenum cluster units. The insertion/extraction reversibility of bivalent ion, Mg 2+ , 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-Mo 9 Se 11 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 x Mo 9 Se 11 was below 40% of the theoretical capacity deduced from the molecular orbital model, whereas over 80% of that was observed in o-Li x Mo 9 Se 11 . The smaller reversible capacity for Mg 2+ could be ascribed to the Coulomb repulsion between bivalent Mg-ions confined in the one-dimensional channel of o-Mo 9 Se 11 , 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. Reversible electrochemical insertion/extraction of cations into/from materials is an important phenomenon from the view point of today's science and technology. In particular, reversible insertion/extraction of Li-ion is applied to secondary battery systems (Li-ion batteries) and has enabled realization of widely used portable devices such as cellular phones and lap-top computers.1-4 In recent years, an effort for developing new energy storage systems, which use ion-insertion mechanism, is being invested also in nonlithium-ion battery systems, such as sodium-ion, 5,6 potassium-ion 7,8 and multivalent-ion batteries. 9-16 Among them, Mg-ion battery is one of the fascinating battery systems due to its possible low cost and safety, which could be realized by rich resource and moderate reactivity of magnesium. [9][10][11][12] However, in most materials, the strong electrostatic interaction between introduced Mg 2+ and host lattice, which is due to bivalence of Mg 2+ , induces the slow solid state diffusion of Mg-ions within the crystal lattice and hampers reversible electrochemical insertion/extraction of Mg 2+ . 9,11,12 At present, the only family of insertion systems, into/from which reversible insertion/extraction of Mg-ions has been rigorously established, is Chevrel phases, Mo 6 X 8 (X = S, Se). These systems have demonstrated reversible insertion/extraction of Mg-ions over 1000 cycles and the capacity over 70 mAh/g, which is higher than those of other transition metal sulphides, as a cathode of Mg-ion battery at ambient temperature. 17,18 The successful performance of Chevrel phases as Mg-insertion materials is suggested...

For reliable use of lithium ion batteries, their cyclic property must be further improved. Several phase separating materials called zero strain electrodes are very promising because they possess negligible lattice mismatches at the phase interface and significantly reduce the mechanically induced deterioration. However, the structures and chemistry of actual phase interfaces are still not well understood in this class of materials. In this study, the phase interfaces of Li1+x Rh2O4 cubic spinel, one of the stable zero strain cathodes, are analyzed in an atomic scale using high-angle annular dark-field (HAADF)/annular bright-field (ABF) scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). Electrochemically induced phase interfaces can be clearly visualized in the ABF STEM images, where they tend to lie on {111} crystallographic lattice planes. The zero strain nature near the observed interface can be confirmed by real space strain analysis, which is in good agreement with the previous X-ray diffraction experiments. By performing the canonical Monte Carlo simulations with effective cluster interaction energies, it is concluded that the stability of {111} phase interfaces will be attributed to their small number density of close lithium ionic pairs formed near the interface plane.