Thermodynamic Origin of Irreversible Magnesium Trapping in Chevrel Phase Mo<sub>6</sub>S<sub>8</sub>: Importance of Magnesium and Vacancy Ordering

2017-04-04T00:00:00Z (GMT) by Chen Ling Koji Suto
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 Mo<sub>6</sub>S<sub>8</sub> 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 show that the kinetic diffusion of Mg in a Chevrel phase cathode depends little on the composition. On the other hand, the formation of ordered Mg and vacancy structure along the ⟨100⟩ direction at low Mg concentrations greatly decreases the population of mobile ions, reducing the chemical diffusivity by 3–5 orders of magnitude and consequently causing the irreversible trapping. At high concentrations, the ordering is disrupted by the repulsion between neighboring Mg–Mg pairs, which enhances Mg mobility and leads to fully reversible intercalation. Our results not only provide mechanistic knowledge about the irreversibility of a prototype Mg battery cathode but also highlight the apparent importance of the ordering effect in Mg intercalation for the future exploration of cathode materials.