Crystalline-topologies engineering of bio-spore-derived hard carbon for efficient low-potential potassium ion storage

Abstract

Clarifying the relationship between crystalline topologies of hard carbon and electrochemical K+ storage behaviors is essential to develop low-cost anode for K+ battery. Herein, bio-mass derived hard carbons with various crystalline topologies were synthesized for the first time by one-step pyrolysis of spores of sorghum smut fungus as new precursors. The crystalline-structures of spore-derived hard carbons including graphitic-like layer number, spacing and curvature can be effectively modulated by pyrolysis temperatures, leading to significant difference between correspondingly as-synthesized hard carbons for K+ storage. The hard carbon obtained at 1400 °C presents a unique crystalline structure with fewer graphite stacking layers down to 2, achieving the excellent K+ storage performance, in which, it presents a high capacity of 495.0 mAh/g with a significant plateau capacity of 368.9 mAh/g (< 0.5 V), a good rate performance of 175.0 mAh g-1 at 500 mA g-1 and stable cycling performance (125.0 mAh g-1 at 500 mA g-1 over 600 cycles), outperforming the most of reported hard carbons derived from various precursors. Meanwhile, the DFT calculation further clarifies that the crystalline topologies of hard carbon play a critical role for efficient K+ storage. In particular, there is an interesting discovery that the number of graphite stacking layers can significantly affect the migration and diffusion of K+, in which, the fewer the number of graphite stacking layers, the lower the energy barrier of K+ diffusion between graphite layers. These results in the work provide new insights for the understanding of K+ storage behavior in hard carbon.