Reversible Electrochemical Insertion/Extraction of Mg and Li Ions for Orthorhombic Mo₉Se₁₁with Cluster Structure

Abstract

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...

Beyond Li-ion: electrode materials for sodium- and magnesium-ion batteries

“…Further, the ability of Mo 6 to easily accommodate four electrons per cluster helps maintain local electroneutrality, which in turn enables rapid intercalation kinetics [100,102]. Using alternative cluster compounds has been reported in the case of Mo 9 Se 11 [151], and clusters such as Fe 6 , Co 6 , and Cr 6 have been suggested as an interesting potential research direction for kinetically fast MIB cathode materials with higher energy densities [103], although they have not been demonstrated to date.…”

Rechargeable magnesium-ion battery based on a TiSe2-cathode with d-p orbital hybridized electronic structure

“…As a consequence of charge distribution over multiple atoms, the charge imbalance during the insertion of bivalent ions is supposed to be easily relieved in the Chevrel phase. In the recent studies, other compounds with clustered structures, such as o-Mo 9 Se 11 and C 60 , are also reported to display reversible Mg-ion insertion reactions 10 30 . Thus, systems with delocalized electrons over multiple atoms might be expected as good candidates for rechargeable cathodes of Mg-ion batteries.…”

“…The strong electrostatic interaction between bivalent Mg-ions and host lattices often causes slow solid state diffusion of Mg 2+ within the local crystal structure and prevents reversible insertion/extraction of Mg 2+ . In the past years, various kinds of materials have been proposed as candidates for cathodes of Mg-ion batteries, such as Chevrel phases Mo 6 X 8 ( X = S, Se) 8 9 , o-Mo 9 Se 11 10 , TiS 2 nanotube 11 , graphene-like MoS 2 12 , WSe 2 nanowire 13 , Mg M SiO 4 ( M = Fe, Mn, Co) 14 15 16 17 18 19 20 , MgFePO 4 F 21 , FePO 4 22 , MnO 2 23 24 , V 2 O 5 xerogel/thin film 25 26 27 , organic compounds 28 29 30 , Prusian Blue analogues 31 32 . However, in most cases, Mg-ion batteries could only function by assistance of nanoscale morphology of electrode materials 11 12 13 14 15 16 17 18 19 20 21 23 24 25 or screening of the electrostatic interaction by solvent species such as aqueous ions 31 32 .…”

Status and challenge of Mg battery cathode

“…29 Similar to CP, its structure contains Mo 9 clusters. The electrochemical test was carried out using Mo 9 Se 11 cathodes and magnesium metal anodes in 0.25 M Mg(AlCl 2 EtBu) 2 /THF electrolyte solution.…”