Mechanism-Informed Interfacial Chemistry and Structural Evolution of TiS2 During Ca2+ Intercalation in Concentrated Aqueous CaCl2 Electrolytes.

This study examines the interfacial and structural evolution of titanium disulfide (TiS2) during Ca2+ intercalation/deintercalation in concentrated aqueous CaCl2. Electrochemical measurements were combined with ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy to characterize the solvation structure, potential window, and reversibility in concentrated CaCl2 electrolytes. Increasing the CaCl2 concentration from 1.0 to 8.0 M was accompanied by reduced gas evolution and an expanded practical operating window. Stepwise analysis identified the potential range -1.00 to 0.10 V (vs. the saturated calomel electrode) as a practical window that minimized TiO2/S8 formation while preserving reversible Ca2+ intercalation. Ex situ XRD showed reversible (001) shifts, consistent with interlayer expansion and contraction, and peak broadening was indicative of partial amorphization and defects. XPS revealed CaS and polysulfides (Sz2-, 2 ≤ z ≤ 8) to be the prevalent surface species with limited Ca(OH)2 and CaSO4; within the detection limits, no chlorine-containing reduction products were observed after charging. The electrochemical and spectroscopic results indicate that intercalation is accompanied by partial sulfur-centered reduction and defect signatures, with associated changes in the interfacial charge-transfer characteristics and reversibility. These findings link the potential, interfacial chemistry, and lattice response, and suggest design considerations for stable aqueous multivalent-ion storage.