Molecular Dynamics Simulation Investigation of Freezing Point Depression in NaClO₄ Electrolyte Solution by CaCl₂

The development of inorganic antifreeze electrolytes is of paramount importance for the application of sodium-ion batteries under low-temperature conditions. However, there is little reported about their molecular mechanism for lowering the freezing point of electrolytes. Therefore, this study explores the mechanism by which CaCl₂ lowers the freezing point of the NaClO₄ electrolyte. Hexagonal ice (ice I_h) was used as the ice seed, and CaCl₂ was selected as the antifreeze agent. The coexistence system of ice and solution was constructed to simulate the freezing process. It was found that there is ion rejection at the ice layer, with ions predominantly distributed in the solution. Over time, ions form an ion adsorption layer at the ice-solution interface. The radial distribution function (RDF) and spatial distribution function (SDF) of Na⁺, ClO₄^-, Ca²⁺, and Cl^- revealed that ions form the first solvation shells with water molecules. The interaction energy between ions and water molecules is greater than that between ice nuclei and water. Therefore, ions are better able to maintain the stability of their solvation shells and inhibit the growth of ice I_h through a mechanism of competition for water molecules. Furthermore, the dissolution free energy of Na⁺ and Ca²⁺ in the aqueous phase was studied. The results indicated that Ca²⁺ has a stronger affinity for water molecules than Na⁺, making it more competitive in competing for water with ice I_h. Therefore, CaCl₂ in NaClO₄ solution can reduce the freezing point. This work provides a molecular-level understanding of how CaCl₂ reduces the freezing point of NaClO₄ solution, which is beneficial for designing strategies for low-temperature electrolytes.