Extended Data Fig. 8: Improved Ca²⁺ de-solvation and Ca plating/stripping.

a, Raman spectra of the electrolytes with different volume ratio of DMSO, suggesting the coordination of DMSO and Ca²⁺. b, Percentage of Ca²⁺-coordinated BF₄⁻/TFSI⁻ as determined by the ratio between the peak area of coordinated species and the total peak area from the Raman spectra. DMSO effectively reduces Ca²⁺-coordinated BF₄⁻/TFSI⁻. c, ⁴³Ca NMR spectra of electrolytes with DMSO (EMIM-BF₄/DMSO, 50:50 vol%) and without DMSO (only EMIM-BF₄) have different chemical shifts and shapes, suggesting that DMSO relieves the Ca²⁺ coordination by BF₄⁻/TFSI⁻ anions. d, e, Temperature-dependent Nyquist plots of symmetric batteries using the electrolytes with DMSO (EMIM-BF₄/DMSO, 50:50 vol%, in d) and without DMSO (only EMIM-BF₄, in e). f, Arrhenius plots of the resistance corresponding to Ca²⁺ de-solvation in electrolytes with DMSO (red) and without DMSO (blue), suggesting that the introduced DMSO can reduce the Ca²⁺ de-solvation energy barrier. g, Voltage curves of Ca | |Ca symmetric cells using electrolytes with DMSO (EMIM-BF₄/DMSO, 50:50 vol%) and without DMSO (only EMIM-BF₄) at a current density of 0.2 mA/cm² and an areal capacity of 0.2 mAh/cm². Inset, the enlarged voltage curves.