Fig. 5: Electrochemical performance of Na metal batteries at “three high” conditions.

Cyling performance of Na| |Cu cells at (a) 5 mA cm⁻² and 5 mAh cm⁻², and (b) 8 mA cm⁻² and 4 mAh cm⁻², with insets showing the surface morphology of Na metal after cycling. EIS of Na| |Cu cell cycled at 3 mA cm⁻² and 3 mAh cm⁻² in (c) NG2 and (d) NGMPE electrolytes, indicating the evolution of surface integrity. Symmetric Na| |Na cell at (e) 1 mA cm⁻² and 1 mAh cm⁻², DoD 50%, and (f) 5 mA cm⁻² and 5 mAh cm⁻², DoD 75%, (g) rate test from 0.1 mA cm⁻² to 15 mA cm⁻². h Comparison of half-cell performance in this work with those in other studies related to electrolyte engineering of SMBs (specification and corresponding reference are shown in Supplementary Table 5)^{28,36,41,57,58,78,79,80,81,82,83,84,85,86,87,88,89}. Micro-CT 3D reconstruction of sodium metal after 50 cycles in (i) NG2 electrolyte and (j) NGMPE electrolyte at 8 mA cm⁻² and 4 mAh cm⁻², with the images of 2D horizontal slices at the same depth showing distribution and density of pores. k SEM images of surface morphology of sodium metal in NG2 and NGMPE electrolytes after 50 cycles at 5 mA cm⁻² and 5 mAh cm⁻².