This study investigates the role of glyoxylic acetal-based electrolytes in enhancing the stability of the solid electrolyte interphase (SEI) formed on hard carbon (HC) anodes in sodium-ion batteries (SIBs). It examines the effects of sodium bis(fluorosulfonyl)imide (NaFSI) and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in 1,1,2,2-tetraethoxyglyoxal (TEG):propylene carbonate (PC) as alternatives to sodium hexafluorophosphate (NaPF₆) in ethylene carbonate (EC):PC and evaluates how variations in electrolyte composition and C-rate influence SEI formation, electrochemical performance, and Na⁺ transport.

This study systematically investigates the feasibility of replacing conventional sodium hexafluorophosphate (NaPF₆) in carbonate-based electrolytes with sodium bis(fluorosulfonyl)imide (NaFSI) and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in a 1,1,2,2-tetraethoxyglyoxal (TEG):propylene carbonate (PC) solvent system tested with hard carbon (HC) anode materials for sodium-ion batteries (SIBs). The influence of electrolyte composition and cycling conditions on the evolution of the solid electrolyte interphase (SEI) and overall electrochemical performance of the HC is comprehensively evaluated by means of electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The SEI chemical composition, transport properties, and stability are thoroughly characterized. The results demonstrate that the HC tested in NaFSI/TEG:PC electrolyte exhibits superior performance compared to both the conventional NaPF₆/ethylene carbonate (EC):PC system and the NaTFSI/TEG:PC-based alternative, achieving higher initial coulombic efficiencies (ICEs), lower interfacial resistance, and enhanced Na⁺ transport properties. The improved electrochemical stability of the HC in NaFSI/TEG:PC electrolyte is attributed to the formation of a bilayered SEI, comprising an inorganic-rich inner layer and an organic-rich outer layer. These findings underscore the pivotal role of electrolyte formulation in enhancing the HC SEI characteristics and cycling performance, thereby positioning NaFSI in TEG:PC chemistry as a promising electrolyte candidate for next-generation SIBs.