Aprotic electrolytes are much less feasible for producing cost-effective Na metal-based CO₂ batteries due to high cost and flammability. As an alternative electrolyte, super-concentrated electrolytes, referred to as “water-in-salt (WiS) electrolytes”, have been received attention due to their wide electrochemical stability window, cost-effectiveness, and non-flammability. However, the highly reactive Na metal prevents the direct use of WiS electrolytes because the unsolvated water molecules would react with the Na Metal.

In this study, we demonstrated the ability of a WiS and NASICON electrolyte-based Na-CO₂ battery for utilizing CO₂ gas and serving as an energy storage cell. The NASICON separator allowed us to fabricate a hybrid Na-CO₂ battery comprising Na metal as the anode material and WiS as the cathode electrolyte without damage of Na metal. Additionally, linear sweep voltammetry (LSV) with corresponding differential electrochemical mass spectroscopy (DEMS) measurements rendered the direct observation of H₂ evolution retardation with increasing WiS concentration. Furthermore, we introduced a nano-sized Ru catalyst onto the current collector using Joule heating method for reducing the overpotential gap. Consequently, the Na-CO₂ batteries with Ru@carbon current collector reduced the overpotential gap and exhibit a cycling endurance of over 75 cycles.