Electrochemical studies and diffusion kinetics in the Chevrel phase (Mo6S8) for rechargeable Mg batteries

Abstract

Based on its availability, cost and stability, rechargeable Mg batteries (RMBs) are potential candidates to fulfill the futuristic demand for high energy density storage devices. However, they are minimally explored due to sluggish Mg ion diffusion in cathode materials. Literature suggests that the Chevrel phase (CP) (Mo6S8) holds promise as a cathode (positive electrode) for RMBs due to its open structure and self-healing properties during cycling. This study reports the electrochemical performance of Mo6S8 (synthesized using cost effective precursors (Cu–Mo–MoS2)) as a cathode for RMBs for the first time. The development of Mo6S8 is a two-step process: (i) synthesis of Cu2Mo6S8 via high energy milling method using Cu, Mo and MoS2 as precursors and (ii) leaching of Cu from Cu2Mo6S8. The morphological and structural characteristics of the developed materials are recorded using x-ray diffraction and field emission scanning electron microscopy. The developed Mo6S8 is cuboid-shaped with a rhombohedral unit cell. To record the electrochemical performance of Mo6S8 as a positive electrode for rechargeable Mg batteries (RMBs), CR2016 type half-cells are fabricated. It is observed that the initial discharge capacity was 89 mA h g−1 at a current density of 25 mA g−1 (1C = 128 mA g−1). Interestingly, the capacity increases from 89 to ≈100 mA h g−1 during 50 cycles which is higher than reported in the literature. The coulombic efficiency (CE) of ≈90% is observed for 100 cycles. Additionally, the over-potential decreases with an increase in cycle number. Importantly, the authors explained the diffusion behavior of Mg ions in Mo6S8 with 0.4 M 2(PhMgCl)-AlCl3/THF (APC) electrolyte via cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic intermittent titration (GITT) technique. The diffusion coefficients have been calculated and fall in the range of 10−8−10−14 cm2 s−1. Also, the authors explain the effect of outer site activation during cycling on the diffusion kinetics of the materials using the GITT technique. This investigation of diffusion kinetics of Mg ions in Mo6S8 may pave the way for evaluating various CPs as electrode materials for future rechargeable magnesium battery systems.