Samples were prepared in an Ar-filled glovebox (O 2 <0.1 ppm, H 2 O <0.1 ppm). The Na metal (Sigma-Aldrich, 99.9 % trace metals basis) which was delivered in mineral oil was first washed in hexane and then placed and rolled out using a protective PE (polyethylene) sheet to reduce sticking to the rolling pin and contamination's on a stainless-steel spacer. The samples were placed in high-density polyethylene vials with 1 mL of the three electrolytes mention above and stored in the electrolytes for 2 minutes, 4 minutes and 2 hours, respectively. After storage, the Na electrodes were washed twice by immersion for 2 minutes in a glass vial containing 3 mL of DEC or PC and dried under vacuum. These conditions are based on a pre-study investigating the washing conditions and it was found that this procedure removes almost all residual conductive salt.Furthermore, reference samples of NaCl and pristine Na were measured, where NaCl powder was mounted to the sample holder by pressing the powder into conductive double sided carbon tape.