Comprehensive Insights into the Reactivity of Electrolytes Based on Sodium Ions

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 (Na-x-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 Na-x-HC. In the presence of Na salts, EC/DMC and EC/DEC presented a higher reactivity towards Na-x-HC than EC/PC. For a fixed solvent mixture, the onset temperature follows the sequence NaClO4<NaFTFSI<NaPF6<NaTFSI<NaFSI, and the total heat generated increases in the order NaFSI<NaTFSI<NaClO4<NaPF6<NaFTFSI.