Electrochemical Performance of Solid State Derived Chevrel Phase Mo6T8 (T = S, Se) Cathodes for Rechargeable Sodium and Magnesium-Ion Batteries
Authors: Jampani Hanumantha Prashanth , Saha Partha , Datta Moni Kanchan , Hong Daeho , Okoli Christopher U , Manivannan A. , Kumta Prashant N 
Published: February 2020

The need to transition from the ubiquitous lithium-ion chemistry to more abundant raw-material based systems is precipitated by the additional advantages of such chemistries including lower cost and improved safety. Sodium (SIB) and magnesium (MIB) ion based systems are of greatest relevance on account of the theoretical energy density of both systems and the relative abundance of both elements in the earth’s crust. In order for Na-ion and Mg-ion batteries to compete with the much researched Li-ion systems, considerable improvements in energy density, cyclability and rate capability is required prior to their consideration for practical use as electrical energy storage (EES) devices. Identification of suitable cathodes and anodes which can exhibit high specific capacity, low irreversible loss, high coulombic efficiency and long cycle/calendar life will be a paradigm shift in the development of high energy density SIBs.
Layered cathodes of sodium NaxMnO2 (0 < x ≤ 1; M = Mn, Ni, Co) have received considerable interest due to their structural similarity with well-known Li-ion battery electrodes LiMO2 (M = Mn, Ni, Co). Ternary molybdenum compounds of MxMo6T8 (M = metal, transition element, rare-earths; T = chalcogen) are known as ‘Chevrel phase (CP)’ since 1971 (1). The crystal structure of Chevrel phase consists of Mo6- octahedron clusters surrounded by eight chalcogens (S, Se) atoms at the corners of a distorted cube (2). The Mo6S8 units are linked with each other and form a three-dimensional framework with open cavities/channels that can be filled with a wide-variety of guest atoms giving rise to ternary Chevrel phase compounds MxMo6S8 (0 < x < 4). Among the three different Chevrel phase families (Mo6T8, T = S, Se, Te), sulfide CPs have received significant attention due to their high ionic mobility at room temperature allowing the transport of monovalent (Li+, Na+), and bivalent (Mg2+) cations serving as cathodes for rechargeable batteries (3-5). Out interest in the present work stems from the rapid synthesis of Cu2Mo6S8 and Cu2Mo6Se8 Chevrel phases in a time-efficient manner and correspondingly use of de-cuprated Mo6S8 and Mo6Se8 as promising cathodes for rechargeable magnesium batteries (6).
High energy mechanical milling (HEMM) of stoichiometric mixtures of molybdenum and copper chalcogenide (CuT and CuT2) followed by short thermal treatments at elevated temperature resulted in Chevrel phases (Cu2Mo6T8; T = S, Se), serving as cathodes for Na and Mg ion batteries. Electrochemical performances of the Mo6S8 and Mo6Se8phases were evaluated by cyclic voltammetry (CV), galvanostatic cycling, electrochemical impedance spectroscopy (EIS). Using EIS (Fig. 1), a comparison is made between the nature of the ion trapping mechanisms occurring during the respective sodium/magnesium intercalation and deintercalation processes.

References

1. R. Chevrel, M. Sergent and J. Prigent, Journal of Solid State Chemistry, 3, 515 (1971).
2. Ø. Fischer, Appl. Phys., 16, 1 (1978).
3. D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi, Nature, 407, 724 (2000).
4. W. R. McKinnon and J. R. Dahn, Physical Review B, 31, 3084 (1985).
5. E. Gocke, W. Schramm, P. Dolscheid and R. Scho¨llhorn, Journal of Solid State Chemistry, 70, 71 (1987).
6. P. Saha, M. K. Datta, O. I. Velikokhatnyi, A. Manivannan, D. Alman and P. N. Kumta, Progress in Materials Science,

Figure 1