Fig. 5: Electrochemical performance of K-Na/S batteries with 4 M and 8 M sulfur in catholytes.

a Charge/discharge voltage profiles of cells with 4 M sulfur in CPL/acetamide eutectic solvent (ES) and TEGDME catholyte. The current densities are 0.5 mA cm⁻² (0.09 C). b Cycling performance of cells with sulfur concentrations of 4 M and 8 M [S] in the cathode. The current densities are 0.5 mA cm⁻². c Rate performance of cells with 4 M sulfur in ES electrolyte and TEGDME electrolyte. d Cycling performance of cells with 4 M sulfur in the cathode at 2 mA cm⁻². The first 5 cycles are at 0.5 mA cm⁻². e Charge/discharge voltage profiles of cells with various C-rates. f Electrochemical impedance spectroscopy of cells with 4 M sulfur in the cathode at the fully charged state (C) and at a discharge capacity of 460 mAh g⁻¹ sulfur (DC). The electrolytes are ES and TEGDME, respectively. g The discharge time dependence of material cost for K-Na/S battery with 1 M [S] /4 M [S] /8 M [S] at 2 mA cm⁻². h Eutectic solvent with a separator (ES, left) and conventional Li-ion battery electrolyte with a separator (LP40, right) on fire. Samples with polysulfides inside can be found in Supplementary Video 5. All electrochemical tests for a–f are done in a 75 °C oven.