Molecular Insights into Anion-Specific Freezing Point Depression in Lithium Salt Solutions

The depression of freezing points in electrolyte aqueous solutions, a well-known colligative property, is traditionally attributed to entropy increases arising from ion-induced disruption of the hydrogen-bonding networks. However, the microscopic mechanisms governing this phenomenon remain poorly understood, particularly at concentrated salt concentrations where ion-specific effects emerge. In this study, we combined Raman spectroscopy, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations to investigate the hydrogen-bonding structures of water in lithium salt solutions containing typical anions. MD simulations reveal that diffusion barriers of water are influenced by the anion identity, while DFT calculations indicate that anions with lower surface electrostatic potentials weaken the disruption of the hydrogen-bonding network caused by the cation. By systematically evaluating five lithium salts─LiClO₄, LiNO₃, LiBF₄, LiCl, and LiTFSI─we show that freezing point depression in lithium salt solutions arises from a complex interplay of anion-water, cation-anion, and cation-water interactions. Notably, the freezing point trends deviate from the Hofmeister series, suggesting the critical role of ion-pairing and aggregate formation in determining solution behavior. Our results further indicate that rather than the intrinsic structure─disrupting ability of Hofmeister anions, the mobility of water molecules within the ions' hydration shells is a primary determinant of freezing behavior, challenging the conventional view and revealing the critical influence of local water dynamics on solid/liquid transitions. These findings provide molecular-level insights into the ion-specific effects governing freezing point depression in electrolyte solutions, with implications for lithium-ion battery electrolytes and other concentrated ionic systems.