The coin cells (CR2032-type) were assembled using the HC electrode as a working electrode and metallic sodium (Alfa Aesar 99.8%) as a counter electrode. The separator used was a glass microfiber filter (Whatman, GF/B, 0.68 mm thickness). The electrolyte was 1 M NaPF6 (Alfa Aesar ≥99%) in ethylene carbonate (Sigma-Aldrich ≥99%, acid <10 ppm, H2O<10 ppm) and dimethyl carbonate (Sigma-Aldrich ≥99%, acid <10 ppm, H2O<10 ppm) (1:1 vol%). The cells were galvanostatically charged and discharged in a voltage range of 0-2 V versus Na+/Na using a potentiostat (Biologic VSP-300) at a current density of 15 μA mg -1 at room temperature. At the end of the discharge the voltage was held constant at 0 V until the current decayed to 1.5 μA mg -1 .