Non-negligible Influence of Oxygen in Hard Carbon as an Anode Material for Potassium-Ion Batteries

The development of potassium-ion batteries (PIBs) has been impeded by the lack of an appropriate carbon anode material that could accommodate K⁺ with a large ionic radius. Hard carbon with low cost and larger interlayer spacing is a promising anode material for PIBs. However, the impact of oxygen-containing functional groups in hard carbon (HC) is less reported. Herein, a hypercrosslinked polymer (HCLP) is prepared and used for the synthesis of microporous hard carbons with superior structural stability and abundant oxygen-containing functional groups and defects, in which the crosslinking agent provided copious oxygen atoms. It is found that a large number of C═O groups and micropores provide more storage sites for K⁺. The surface-controlled process is dominated by the reversible reaction of C═O + K⁺ + e^- ↔ C-O-K, which directly increases the capacity contribution. The HCs obtained at 600 °C exhibit good cycling and rate performance with an initial specific capacity of about 254.3 mAh g^-1 and the capacity retention of 83.2% after 200 cycles at 50 mA g^-1. The capacity reached up to 121 mAh g^-1 at 2 A g^-1. A possible capacitive-adsorption mechanism is proposed by kinetic analysis. The redox reaction mechanism between C═O and K⁺ at the HC is clearly also revealed.