Ultraconcentrated Sodium Bis(fluorosulfonyl)imide-Based Electrolytes for High-Performance Sodium Metal Batteries

We present an ultraconcentrated electrolyte composed of 5 M sodium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane for Na metal anodes coupled with high-voltage cathodes. Using this electrolyte, a very high Coulombic efficiency of 99.3% at the 120th cycle for Na plating/stripping is obtained in Na/stainless steel (SS) cells with highly reduced corrosivity toward Na metal and high oxidation durability (over 4.9 V versus Na/Na + ) without corrosion of the aluminum cathode current collector. Importantly, the use of this ultraconcentrated electrolyte results in substantially improved rate capability in Na/SS cells and excellent cycling performance in Na/Na symmetric cells without the increase of polarization. Moreover, this ultraconcentrated electrolyte exhibits good compatibility with high-voltage Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) and Na 0.7 (Fe 0.5 Mn 0.5 )O 2 cathodes charged to high voltage…