Electrochemical Intercalation of Calcium and Magnesium in TiS2: Fundamental Studies Related to Multivalent Battery Applications

A comparative study of the electrochemical intercalation of Ca2+ and Mg2+ in layered TiS2 using alkylcarbonate-based electrolytes is reported, and for the first time, reversible electrochemical Ca2+ insertion is proved in this compound using both X-ray diffraction and differential absorption X-ray tomography at the Ca L2 edge. Different new phases are formed upon M2+ insertion that are structurally characterized, their amount and composition being dependent on M2+ and the experimental conditions. The first phase formed upon reduction is found to be the result of an ion-solvated intercalation mechanism, with solvent molecule(s) being cointercalated with the M2+ cation. Upon further reduction, new non-cointercalated calcium-containing phases seem to form at the expense of unreacted TiS2. The calculated activation energy barriers for Ca2+ migration in TiS2 (0.75 eV) are lower than those previously reported for Mg (1.14 eV) at the dilute limit and within the CdI2 structural type. DFT results indicate that the expansion of the interlayer space lowers the energy barrier and favors a different pathway for Ca2+ migration.