Extended Data Fig. 5: Na/Cl₂ battery performances when 2 wt% FEC and 2 wt% NaPF₆ were used as the electrolyte additives and XPS of Na metal immersing in electrolytes with different additives (2 wt% NaFSI + 2 wt% NaTFSI, 2 wt% NaPF₆, and 2 wt% FEC) and after battery cycling.

a, Na/Cl₂ battery cycling performance at 500 mAh g⁻¹, 150 mA g⁻¹ when 4 M AlCl₃ in SOCl₂ + 2 wt% FEC was used as the electrolyte. The battery behaved poorly and died after cycle 9. b, Na/Cl₂ battery cycling performance at 1,200 mAh g⁻¹, 100 mA g⁻¹ when 4 M AlCl₃ in SOCl₂ + 2 wt% NaPF₆ was used as the electrolyte. The battery showed worse cycling performance than when 2 wt% NaFSI + 2 wt% NaTFSI was used as the electrolyte additive. c, Atomic percentage of different elements, calculated from XPS survey spectrum, on the Na metal after immersing in 4 M AlCl₃ in SOCl₂ with different additives (2 wt% NaFSI + 2 wt% NaTFSI, 2 wt% NaPF₆ and 2 wt% FEC). d, Cl 2p spectrum of Na metal after immersing in 4 M AlCl₃ in SOCl₂ with different additives (2 wt% NaPF₆ and 2 wt% FEC). e, F 1s spectrum of Na metal after immersing in 4 M AlCl₃ in SOCl₂ with different additives (2 wt% NaPF₆ and 2 wt% FEC). f, S 2p spectrum of Na metal after immersing in 4 M AlCl₃ in SOCl₂ with different additives (2 wt% NaPF₆ and 2 wt% FEC). g, Atomic percentage of different elements, calculated from XPS survey spectrum, on the Na electrode after cycling in batteries using 4 M AlCl₃ in SOCl₂ with different additives (2 wt% NaFSI + 2 wt% NaTFSI, 2 wt% NaPF₆ and 2 wt% FEC) as the electrolyte. h, F 1s spectrum of Na electrode after cycling in batteries using 4 M AlCl₃ in SOCl₂ with different additives (2 wt% NaPF₆ and 2 wt% FEC) as the electrolyte. The batteries using 2 wt% NaFSI + 2 wt% NaTFSI and 2 wt% NaPF₆ as the electrolyte additives in g, h were stopped at cycle 21. The battery using 2 wt% FEC as the electrolyte additive was stopped at cycle 9 when the battery died.