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

Abstract

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.

Isotropous Sulfurized Polyacrylonitrile Interlayer with Homogeneous Na+ Flux Dynamics for Solid‐State Na Metal Batteries

“…In contrast, the cycled pPAN‐S pellet only shows the peaks of polythionate and thiosulfate (Figure 5h), demonstrating the poor kinetics and undesired irreversibility of Na + insertion among pPAN‐S interlayer. [ 50 ] These results are also consistent well with the CV and GITT curves.…”

“…The EPR spectra provide clear evidence on the existence of radical species on SPAN layer, and the structure and quantity of the radical species remain similar with the extent of cycling time. [ 49,50 ] Considering the stable cycling performance of integrated batteries, it can be deduced that the delocalized radicals of conjugated polymer backbones can activate the formed short‐chain sulfides to reconnect to the backbones, thus demonstrating superior structure and component stability of SPAN. Similar results have also been verified by Archer et al.…”

Room‐Temperature Sodium–Sulfur Batteries and Beyond: Realizing Practical High Energy Systems through Anode, Cathode, and Electrolyte Engineering

“…constructed CS bonds by activating sulfiphilic sites on the surface of carbon matrix. [ 125 ] They activated carbon cloths through alkali treatment at elevated temperature which generates unpaired electrons (i.e., dangling bonds) on its surface. These unsaturated carbon atoms undergo free‐radical coupling with the S 3 •− radical monoanions from the dissolution of polysulfides, and then instantly form sturdy CS bonds and anchor polysulfides on the surface of carbon matrix.…”

Recent Advances in Emerging Non‐Lithium Metal–Sulfur Batteries: A Review

“…Other Na 2 S x , such as Na 3 S 4 and Na 2 S 6 , have also been employed to fabricate sulfur cathodes. [117][118][119][120] For example, Mitra et al prepared a freestanding cathode using manganese dioxide-decorated carbon cloth (CC@MnO 2 ) as a polysulfide reservoir with Na 2 S 6 solution as the active material. [119] Benefiting from the highly conductive electron pathways and efficient polysulfide adsorption capability, the as-obtained sulfur cathode showed an impressive specific capacity of 938 mAh g −1 with a remarkable capacity retention of ≈67% after 500 cycles (Figure 8b).…”