Thermodynamic Origin of Irreversible Magnesium Trapping in Chevrel Phase Mo6S8: Importance of Magnesium and Vacancy Ordering

Abstract

The rechargeable magnesium battery is an alternative to current Li-ion technologies, potentially offering a higher energy density by employing metal magnesium as an anode material. The realization of a magnesium battery, however, is hampered by the barrier to finding a cathode material that reversibly stores and releases Mg ions through electrochemical intercalation. Understanding the underlying mechanism that prevents successful Mg intercalation is now crucial for the design and exploration of new cathode candidates. Our work reports the critical effect of a thermodynamic factor, the ordering of Mg and interstitial vacancies, on the activity of a Chevrel phase Mo6S8 cathode. Specifically, we demonstrate that the irreversible trapping in the Chevrel phase at low Mg concentrations is thermodynamically driven by the ordering of Mg and vacancies instead of being kinetic in origin. Through a combination of nudged elastic band, geometry-confined static calculation, and ab initio molecular dynamics methods, we ...