Free-Radical Catalysis and Enhancement of the Redox Kinetics for Room-Temperature Sodium-Sulfur Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries offer the potential for inexpensive stationary energy storage at the grid and local level. However, their practical performance remains far from theoretical due to sluggish reaction kinetics, which limits both their energy and their rate characteristics. To overcome this, a conceptually new mechanism is demonstrated on the basis of the catalysis by stabilized free-radical species, as indicated by electron spin resonance measurements, generated on the surface of a Na2S6 catholyte-infiltrated activated carbon cloth cathode. X-ray photoelectron spectroscopy characterizations reveal that free-radical catalysis promotes reduction to end-discharged products, via a surface-bound intermediate state, ACC-S3-. Due to this free-radical catalytic activity, our RT Na-S cell achieves a high nominal cell potential of 1.85 V. At a rate of 0.5 C, the Na-S cell delivers a high specific capacity of 866 mA h g(S)-1 and retains 678 mA h g(S)-1 after 700 cycles. The concept of a free-radical mechanism, as described herein, could be adapted to enhance the electrochemical kinetics of other energy storage devices that involve radical intermediate species.