Solvation structure and interfacial chemistry in low-temperature sodium-ion batteries

Sodium-ion batteries (SIBs) offer an attractive platform for large-scale electrochemical energy storage, yet reliable operation at subzero temperatures remains limited by coupled transport, interfacial, and electrode-kinetic constraints. As temperature decreases, electrolyte phase instability and reduced molecular mobility restrict bulk Na + transport, while changes in solvation structure, slower interfacial charge transfer, hindered Na + migration through the SEI/CEI, and reduved solid-state diffusion increase polarization and can accelerate interphase evolution and Na plating. This review develops a mechanism-oriented framework linking electrolyte composition and Na + solvation chemistry to these low-temperature failure processes. We examine how salt identity, solvent chemistry, concentration, and additives regulate ion association, bulk transport, solvation-shell reorganization, interphase formation, and electrode compatibility. Recent electrolyte-engineering strategies are critically compared, including carbonate, ester, ether, and fluorinated solvent design, single and multi-salt chemistries, multifunctional additives, high and low-concentration formulations, localized high-concentration electrolytes, and high-entropy or temperature-adaptive solvation. Particular attention is given to electrode–electrolyte matching and to the distinction between practical full-cell evidence and mechanistic Na-counter-electrode studies. By connecting composition, solvation, transport, interphase evolution, and electrode-specific failure, this review provides design principles for developing safe and kinetically accessible SIB electrolytes across subzero and wide-temperature operating conditions.