“Water-in-salt ”and NASICON Electrolyte-Based Na–CO 2 Battery

Abstract

Super concentrated electrolytes, referred to as "water-in-salt (WiS) electrolytes", are being increasingly employed because of their wide electrochemical stability window, cost-effectiveness, and non-flammability. However, the free water molecules present in WiS electrolytes prevent the use of highly abundant, low-cost Na metal as the anode for various Na-gas batteries. In this study, we develop a WiS-based hybrid Na-CO2 battery that utilizes CO2 and serves as an energy storage cell, where a Na super-ionic conductor enables us to directly use Na metal as the anode component and a WiS electrolyte for the cathode electrolyte. In particular, linear sweep voltammetry with corresponding differential electrochemical mass spectrometry ensures an expanded electrochemical stability window, which guarantees Na-CO2 operation without electrolyte degradation during the charge process. Furthermore, we introduce a nano-sized Ru catalyst to the current collector using the Joule-heating method for lowering the discharge-charge gap. Consequently, the Na-CO2 batteries with these Ru@carbon current collectors reduce the overpotential gap and exhibit a cycling endurance of over 75 cycles (50 days) without significant alteration. These promising results demonstrate the potential of cost-effective, WiS-based Na-CO2 batteries that utilize CO2 and can be employed as energy storage cells.