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.