Nitrogen-rich carbon-onion-constructed nanosheets: an ultrafast and ultrastable dual anode material for sodium and potassium storage

The development of anode materials for both sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) with promising performance is of fundamental and technological significance. In this work, N-rich hollow carbon-onion-constructed nanosheets (HCONs) are developed from Co–hexamine coordination frameworks. Impressively, as anode materials for SIBs, the HCONs demonstrate an ultrastable Na-ion storage capability of 151 mA h g−1 at 5 A g−1 even after 10 000 cycles, with a capacity retention of 96%. For KIBs, the HCONs also deliver a high-rate capability of 105 mA h g−1 at 10 A g−1 and an excellent long-term cycling performance of 132 mA h g−1 at 2 A g−1 after 5000 cycles. The outstanding electrochemical performance can be ascribed to the interconnected carbon onions constructing a unique 2D nanosheet structure. On the one hand, the small graphitic layer size of carbon onions together with the abundance of N atoms can greatly enhance the ion diffusion and capacitive effects as supported by kinetic analysis. On the other hand, the hollow carbon onions possess highly stable structures that can mitigate the volume strain induced by the Na- and K-ion intercalation process. The results shed light on the further design of graphitic carbon towards favourable Na- and K-ion storage.