Understanding potassium ion storage mechanism in pitch-derived soft carbon and the consequence on cyclic stability

Carbon materials are considered the most promising anodes for emerging potassium ion batteries. While hard carbon has shown attractive capacities, soft carbon possesses advantages in the tap density for obtaining high volumetric energy density. Systematic studies are conducted in this work to explore the potential active sites for K ion storage and the associated stability upon repeated K ion insertion/extraction. Pitch-derived soft carbon is utilized as a model material due to its high carbon purity so that the interference of heteroatoms could be minimized. Stepwise carbonization is performed to gradually tune the degree of order, allowing the establishment of the correlation between the charge storage mechanism and microstructure by in-situ Raman spectroscopy. Ex-situ transmission electron microscope (TEM) images of the electrodes after cycles are collected to examine the likely structural deterioration upon the insertion of relative large K ions. The kinetics of charge transfer in various active sites are exploited to achieve a holistic performance in both the energy and power densities. This work unravels the structure-dependent potassium storage behaviors in soft carbon and would benefit the optimization of microstructure for designing advanced anodes.