Insights into Mg²⁺ Intercalation in a Zero-Strain Material: Thiospinel Mg_xZr₂S₄

The Mg battery cathode material, thiospinel MgxZr2S4 (0 ≤ x ≤ 1), exhibits negligible volume change (ca. 0.05%) during electrochemical cycling, providing valuable insight into the limiting factors in divalent cation intercalation. Rietveld refinement of XRD data for MgxZr2S4 electrodes at various states of charge, , coupled with EDX analysis, demonstrates that Mg2+ can be inserted into Zr2S4 at 60 °C up to x = 0.7 at a C/10 rate (up to x = 0.9 at very slow rates) and cycled with a high Coulombic efficiency of 99.75%. HAADF-STEM studies provide clear visual evidence of Mg-ion occupation in the lattice, whereas XAS studies show that Zr4+ was reduced upon Mg2+ intercalation. Operando and synchrotron XRD studies reveal the creation of two phases during the latter stages of discharge (x > 0.5) as the lattice fills and Mg2+ ions begin occupying tetrahedral (8a) sites in addition to octahedral (16c) interstitial sites. Compared to the isostructural Ti2S4 thiospinel, Zr2S4 presents a slightly larger cell volume and hence an almost ideal zero-strain lattice on Mg2+ insertion. Nonetheless, its 4-fold lower electronic conductivity results in a diffusion coefficient for Mg2+ ions (DMg; 1 × 10–10 to 1 × 10–9 cm2/s) that is more than a factor of 10 lower than in Ti2S4. This shows that delocalization of the electron charge carriers in the framework is a very important factor in governing multivalent ion diffusivity in the thiospinel framework and, by extension, in other materials.