Engineering of edge nitrogen dopant in carbon nanosheet framework for fast and stable potassium-ion storage

Amorphous carbons are promising candidates as the anode materials for potassium-ion hybrid capacitors (PIHCs). The insufficient storage sites and inferior diffusion kinetics limit their potassium-ion storage capability. Edge nitrogen and morphology engineering are effective pathways to construct accessible active sites and enhanced diffusion kinetics. However, the organic integration of both pathways in amorphous carbon is still challenging. Herein, a “twice-cooking” strategy, including two-step carbonization processes at 700 °C, is designed to synthesize edge-nitrogen-rich lignin-derived carbon nanosheet framework (EN-LCNF). In the first-step carbonization process, the staged gas releases of CO and CO 2 from CaC 2 O 4 decomposition exfoliate the carbon matrix into a carbon nanosheet framework. In the second-step carbonization process, the generated CaO reacts with the cyanamide units of graphitic carbon nitride (g-C 3 N 4 ) to form an edge-nitrogen-rich framework, which is then integrated into the meso-/macropores of carbon nanosheet framework through sp 3 -hybridized C–N bonds. EN-LCNF with a high edge-nitrogen level of 7.0 at.% delivers an excellent capacity of 310.3 mAh g −1 at 50 mA g −1 , a robust rate capability of 126.4 mAh g −1 at 5000 mA g −1 , and long cycle life. The as-assembled PIHCs based on EN-LCNF anode and commercial activated carbon cathode show a high energy density of 110.8 Wh kg −1 at a power density of 100 W kg −1 and excellent capacitance retention of 98.7% after 6000 cycles. This work provides a general strategy for the synthesis of edge-nitrogen-rich lignin-derived carbon materials for advanced potassium-ion storage. Graphical Abstract