Semi-ionic C-F bond inducing fast ion storage and electron transfer in carbon anode for potassium-ion batteries

Fluorine (F)-heteroatom-functionalized carbon anodes can effectively increase the potassium (K) storage capacity by forming more defect sites; however, the mechanism behind the improvement in electrochemical performance remains unclear, and the fundamental understanding of which kind of C-F bond profoundly determines K storage properties is still lacking. Hence, we report a series of F-doped carbon and demonstrate that it is a semi-ionic C-F bond rather than an ionic C-F bond, and carbonization temperature has a substantial impact on the defect level. Moreover, numerous defects induced by the high percentage of semi-ionic C-F bonds can function as active sites to adsorb many K-ions associated with capacitive behavior, which not only lengthens the cycle lifespan, but is also positively correlated with rate capacity at a high current density. Density functional theory calculations confirm that the existence of a semi-ionic C-F bond can improve the K-ion adsorption capability of carbon and simultaneously increase electronic conductivity, leading to a high capacity and rate. Furthermore, both K adsorption energy and conductivity are optimized by coupling semi-ionic C-F and pyridinic N bonds, resulting in superior capacity (245.2 mA h g−1) and exceptional rate capacity in a K-half battery and high energy density (143.9 W h kg−1) in a K-full battery.