The chemical compound known as potassium sulfur (K2SO4) is a natural mineral and fertilizer. It is produced by treating potassium chloride with raw sulfuric acid and it is dominantly used as an alternative to conventional potash for crops that are sensitive to the chloride ion found in most commercial fertilizers such as coffee, tea, tobacco, some vegetables and fruits.

Benefiting from the high natural abundance and high theoretical specific capacity, the potassium-sulfur battery (K-S) has been identified as a potential candidate for large-scale energy storage applications. However, the performance of K-S batteries remains plagued by parasitic reactions caused by soluble polysulfide species which migrate across the electrolyte-electrode interface during cell operation and deplete the cathode active material.

To overcome these challenges and achieve the high performance required for practical applications, we report here an efficient strategy to reduce parasitic reactions by providing a functional separator that prevents soluble polysulfide species from being re-oxidized. To do this, we designed and fabricated a microporous carbon nanofiber/sulfur composite that provides fast kinetics, restricts the generation of soluble polysulfides, and creates strong chemical and physical confinement to sulfur.

This unique and innovative approach demonstrates the high potential of K-S batteries for large-scale energy storage applications. The results also open the door for more detailed mechanistic investigations to accelerate progress toward achieving high energy densities in K-S batteries by understanding how soluble polysulfide intermediates are generated and reacted during discharge. This knowledge will be useful for designing functional separators and optimizing the electrode, electrolyte, and anode formulations.