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 Ih) 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 Ih 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 Ih. 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.