Towards stable sodium metal battery with high voltage output through dual electrolyte design

The application of sodium metal batteries (SMBs) is hindered by the notable challenges of side reactions between electrolyte and Na as well as the growth of Na dendrites. Herein, we report that a unique design of dual electrolytes within double separators, namely lean diglyme-based electrolyte hosted by the polypropylene separator together with sulfolane-based electrolyte hosted by the glass fiber separator (denoted as D-e@PP/S-e@GF), can effectively prevent Na dendrite growth and suppress side reactions. D-e@PP/S-e@GF ingeniously combines the advantages of D-e and S-e by using their wettability difference on separators. Finite element method (FEM) simulations and in-situ optical microscopy reveal that the D-e@PP layer promotes uniform deposition and significantly reduces side reactions. The S-e@GF layer can be paired with high voltage cathodes. D-e@PP/S-e@GF enables a high CE of Na plating/stripping as high as 97.22% over 560 cycles at 0.5 mAh cm−2 and ultra-long cycle life of more than 1900 h at 1 mAh cm−2 in Na||Na symmetric cell. Furthermore, this design also demonstrates excellent cycling stability of full cells using Na3V2(PO4) (NVP) and Prussian Blue (PB) as cathodes. The unique design of dual electrolytes within double separators provides a new and promising avenue to develop high-performance SMBs.