Enhanced Capacity and Rate Capability of Nitrogen/Oxygen Dual-Doped Hard Carbon in Capacitive Potassium-Ion Storage

The intercalation of potassium ions into graphite is demonstrated to be feasible, while the electrochemical performance of potassium-ion batteries (KIBs) remains unsatisfying. More effort is needed to improve the specific capacity while maintaining a superior rate capability. As an attempt, nitrogen/oxygen dual-doped hierarchical porous hard carbon (NOHPHC) is introduced as the anode in KIBs by carbonizing and acidizing the NH2 -MIL-101(Al) precursor. Specifically, the NOHPHC electrode delivers high reversible capacities of 365 and 118 mA h g-1 at 25 and 3000 mA g-1 , respectively. The capacity retention reaches 69.5% at 1050 mA g-1 for 1100 cycles. The reasons for the enhanced electrochemical performance, such as the high capacity, good cycling stability, and superior rate capability, are analyzed qualitatively and quantitatively. Quantitative analysis reveals that mixed mechanisms, including capacitance and diffusion, account for the K-ion storage, in which the capacitance plays a more important role. Specifically, the enhanced interlayer spacing (0.39 nm) enables the intercalation of large K ions, while the high specific surface area of ≈1030 m2 g-1 and the dual-heteroatom doping (N and O) are conducive to the reversible adsorption of K ions.