High-rate soft carbon anode in potassium ion batteries: The role of chemical structures of pitches

Soft carbons (SCs) have become a promising anode for potassium ion batteries (PIBs). However, it is challenging to maintain high capacity and long cycle life at rapid charge/discharge rate due to the sluggish insertion/deinsertion of K+. To solve this problem, the morphology of the SCs was regulated by using pitches with different chemical structures. The results showed that the chemical structure of the pitch characterized by less long aliphatic chains led to the largest average distance between polycyclic aromatic nuclei, which caused mild condensation of polycyclic aromatic nuclei and thus the formation of lamellar structures. Abundant short chains of the pitch ignited violent cyclization and aromatization and the shortened average distance, inducing violent condensation to generate the bulk structures. The cyclopenta-fused rings and aromatic methyl groups of the pitch minimized the average distance, which also built the bulk structures. More exposed defect-edge sites and larger active surface of the lamellar SC contributed to the adsorption of K+ and improved reaction kinetics. When the current density increased to 3C,bulk structures did not work while the lamellar still remained high reversible capacity of 160.9 mAh g−1 after 1000 cycles. These findings paved a new way for the design of SCs at the molecular level to satisfy high electrochemical performance at high charge/discharge rate.