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 Mg 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 Mgion occupation in the lattice, whereas XAS studies show that Zr was reduced upon Mg 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 Mg 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 Mg insertion. Nonetheless, its 4-fold lower electronic conductivity results in a diffusion coefficient for Mg ions (DMg; 1 × 10 −10 to 1 × 10−9 cm/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.