A 45.6 m NaCTFSI/NaFSI hybrid electrolyte for high-voltage aqueous sodium-ion batteries operable at subzero...

Aqueous sodium-ion batteries (SIBs) offer a safe and sustainable energy storage option, but their performance is restricted by the narrow electrochemical stability window (ESW) of water and poor low-temperature operability of conventional water-in-salt electrolytes (WISEs). Here, we show that sodium cyano(trifluoromethanesulfonyl)imide (NaCTFSI) addresses these issues. A 17.6 m NaCTFSI solution forms a true WISE, providing an ESW of 2.78 volts and high-voltage compatibility, but crystallizes below room temperature, severely degrading performance. To overcome this, we systematically explore a eutectic-like hybrid electrolyte by mixing NaCTFSI (14.1 m) with sodium bis(fluorosulfonyl)imide (NaFSI; 31.5 m), representing the most concentrated Na⁺-based WISE to date, which remains liquid down to -20°C and extends the ESW to 3.07 volts. Incorporating CTFSI^- anions mitigates FSI^--induced anodic corrosion, ensuring long-term cyclability. Electrochemical tests confirm that KMnC | hybrid | NaMnF full cells sustain over 500 charge-discharge cycles across a wide temperature range, maintaining high capacity even at -20°C, demonstrating a promising platform for safe and wide-climate aqueous SIBs.