High performance potassium-sulfur batteries and their reaction mechanism

Benefiting from the high natural abundance and high theoretical specific capacities of potassium and sulfur, potassium-sulfur (K-S) batteries are deemed to be a promising energy storage system for large-scale energy storage applications. However, their application is hindered by poor kinetics due to the large radius of K+ ions and the notorious shuttling effect of soluble polysulfides as well as the lack of understanding of the underlying electrochemical reaction mechanism of sulfur with potassium. Here, a free-standing microporous carbon nanofiber/small-molecule sulfur composite is designed and fabricated, which provides fast kinetics, prevents the generation of soluble polysulfides, and creates strong chemical and physical confinement to sulfur. Unprecedented electrochemical performance is demonstrated with a high reversible capacity of 1392 mA h g(-1) and ultra-stable cycling performance with 88% capacity retention after 2000 cycles. More importantly, the storage chemistry is systematically explored by multiple characterization techniques, including high-resolution transmission electron microscopy, sulfur K-edge X-ray absorption near-edge structure spectroscopy and ex situ X-ray photoelectron spectroscopy, and K2S is verified as the final potassiation product. The findings provide a fundamental understanding of the reaction mechanism and will guide the material design for high performance K-S batteries.