Methanol Antisolvent Engineering in High-Concentration Electrolytes: Reshaping Solvation Shells for Efficient Sodium-Ion Transport

In sodium-ion batteries (SIBs), antisolvent addition enhances battery performance at high salt concentrations while potentially degrading it at low salt concentrations. Nevertheless, the combined effect of electrolyte concentration and antisolvent on the Na+ solvation structure and dynamics is not fully understood. Here, we investigate methanol-containing aqueous NaTFSI electrolytes across concentrations using steady-state infrared (IR) spectroscopy combined with computational methods. We employ IR modes of anions and water as structural probes. With increasing NaTFSI concentration or antisolvent ratio, blue-shifted O–H stretching of water and red-shifted C–F asymmetric stretching are observed in linear IR spectra. Both shifts reflect reduced water hydrogen bonding interactions. Molecular dynamics simulations show a decrease in coordination number, which demonstrates a loosened first solvation shell around Na+ as the electrolyte concentration increases, and a loosened second solvation shell as the antisolvent ratio increases. Both effects enhance the Na+ transport efficiency. Frontier orbital calculations further reveal that the elevated electrolyte concentration and the antisolvent ratio improve water stability and suppress hydrogen/oxygen reactions at electrodes in different ways. As the concentration increases, the HOMO energy level located on the water molecules is more negative and less likely to react. In the methanol-containing electrolyte, the HOMO energy level is localized to the antisolvent, which can better protect the aqueous components of the electrolyte. These findings elucidate how electrolyte composition and antisolvent addition modulate the solvation structure and dynamics, providing potential design principles for high-performance SIBs.