Selective Metathesis Synthesis of MgCr2S4 by Control of Thermodynamic Driving Forces

MgCr<sub>2</sub>S<sub>4</sub> thiospinel is predicted to be a compelling Mg-cathode material, but its preparation via traditional solid-state synthesis methods has proven challenging. Wustrow et al. [Inorg. Chem. 57, 14 (2018)] found that the formation of MgCr<sub>2</sub>S<sub>4</sub> from MgS + Cr<sub>2</sub>S<sub>3</sub> binaries requires weeks of annealing at 800 ℃ with numerous intermediate regrinds. The slow reaction kinetics of MgS + Cr<sub>2</sub>S<sub>3 </sub>--> MgCr<sub>2</sub>S<sub>4</sub> can be attributed to a miniscule thermodynamic driving force of ΔH = –2 kJ/mol. Here, we demonstrate that the double ion-exchange metathesis reaction, MgCl<sub>2</sub> + 2 NaCrS<sub>2</sub> --> MgCr<sub>2</sub>S<sub>4</sub> + 2 NaCl, has a reaction enthalpy of ΔH = –47 kJ/mol, which is thermodynamically driven by the large exothermicity of NaCl formation. Using this metathesis reaction, we successfully synthesized MgCr<sub>2</sub>S<sub>4</sub> nanoparticles (< 200 nm) from MgCl<sub>2</sub> and NaCrS<sub>2</sub> precursors in a KCl flux at 500 °C in only 30 minutes. NaCl and other metathesis byproducts are then easily washed away by water. We rationalize the selectivity of MgCr<sub>2</sub>S<sub>4</sub> in the metathesis reaction from the topology of the DFT-calculated pseudo-ternary MgCl<sub>2</sub>-CrCl<sub>3</sub>-Na<sub>2</sub>S phase diagram. Our work helps to establish metathesis reactions as a powerful alternative synthesis route to inorganic materials that have otherwise small reaction energies from conventional precursors.<br>