“Water-in-salt” and NASICON Electrolyte-Based Na–CO2 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.