Patrick Bonnick

A high capacity thiospinel cathode for Mg batteries

Magnesium batteries are energy storage systems that potentially offer high energy density owing to their ability to employ magnesium metal as a negative electrode. Their development, however, has been thwarted by a paucity of functional positive electrode materials after the seminal discovery of the Mo6S8 Chevrel phase over 15 years ago. Herein, we report the second such material – a thiospinel – and demonstrate fully reversible Mg2+ electrochemical cycling vs. a Mg anode, which is complemented by diffraction and first principles calculations. The capacity approaches 80% of the theoretical value at a practical rate (C/5) at 60 °C, and yields a specific energy of 230 Wh kg−1, twice that of the Chevrel benchmark. Our results emphasize the advantage in employing “soft” anions to achieve practical divalent cation mobility.

Monovalent versus Divalent Cation Diffusion in Thiospinel Ti2S4

Diffusion coefficients (D) for both Li+ and Mg2+ in Ti2S4 were measured using the galvanic intermittent titration technique (GITT) as a function of both ion concentration (x) and temperature. During discharge at 60 °C, DLi descends gradually from 2 × 10-8 cm2/s at xLi ≈ 0 to 2 × 10-9 cm2/s at xLi ≈ 1.9. In contrast, DMg decreases sharply from 2 × 10-8 to 1 × 10-12 cm2/s by xMg ≈ 0.8. This kinetic factor limits the maximum practical discharge capacity of MgxTi2S4. The difference in behavior vis a vis Li+ implies that either increasing Mg2+ occupation of the tetrahedral site at xMg > 0.6 and/or interactions between diffusing cations play a larger role in mediating the diffusion of divalent compared to monovalent cations. Diffusion activation energies (Ea) extracted from the temperature-dependent data revealed that Ea,Mg (540 ± 80 meV) is about twice that of Ea,Li (260 ± 50 meV), explaining the poorer electrochemical performance of MgxTi2S4 at room temperature.