We report a systematic investigation of Na-based electrolytes that comprise various NaX [X=hexafluorophosphate (PF6), perchlorate (ClO4), bis(trifluoromethanesulfonyl)imide (TFSI), fluorosulfonyl-( trifluoromethanesulfonyl)imide (FTFSI), and bis- (fluorosulfonyl)imide (FSI)] salts and solvent mixtures [ethylene carbonate (EC)/dimethyl carbonate (DMC), EC/diethyl carbonate (DEC), and EC/propylene carbonate (PC)] with respect to the Al current collector stability, formation of soluble degradation compounds, reactivity towards sodiated hard carbon (Nax- HC), and solid–electrolyte interphase (SEI) layer formation. Cyclic voltammetry demonstrates that the stability of Al is highly influenced by the nature of the anions, solvents, and additives. GC–MS analysis reveals that the formation of SEI telltales depends on the nature of the linear alkyl carbonates and the battery chemistry (Li+ vs. Na+). FTIR spectroscopy shows that double alkyl carbonates are the main components of the SEI layer on Nax-HC. In the presence of Na salts, EC/DMC and EC/DEC presented a higher reactivity towards Nax-HC than EC/ PC. For a fixed solvent mixture, the onset temperature follows the sequence NaClO4