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, D_Li descends gradually from 2 × 10^-8 cm²/s at x_Li ≈ 0 to 2 × 10^-9 cm²/s at x_Li ≈ 1.9. In contrast, D_Mg decreases sharply from 2 × 10^-8 to 1 × 10^-12 cm²/s by x_Mg ≈ 0.8. This kinetic factor limits the maximum practical discharge capacity of Mg_xTi₂S₄. The difference in behavior vis a vis Li⁺ implies that either increasing Mg²⁺ occupation of the tetrahedral site at x_Mg > 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 (E_a) extracted from the temperature-dependent data revealed that E_a,Mg (540 ± 80 meV) is about twice that of E_a,Li (260 ± 50 meV), explaining the poorer electrochemical performance of Mg_xTi₂S₄ at room temperature.