Freestanding film made by necklace-like N-doped hollow carbon with hierarchical pores for high-performance potassium-ion storage

Potassium-ion batteries (PIBs) have drawn much attention as a replacement for lithium-ion batteries (LIBs) owing to their low cost and high safety. However, it is still an open challenge to design stable and high-capacity nanocarbon for efficient intercalation/deintercalation of large K ions. Herein, we report a class of ultra-high pyrrolic/pyridinic-N-doped necklace-like hollow carbon (NHC) material as a novel free-standing anode for enhancing PIBs in terms of their capacity, rate ability and durability. The as-made NHC film features abundant hierarchical micro/meso/macro-pores, a necklace-like hollow structure, ultra-high pyrrolic/pyridinic-N doping and a high specific surface area, which could finally promote the intercalation/deintercalation of K ions, reduce the volume expansion and improve the stability of PIBs. These new characteristics allow the NHC to deliver a high reversible specific capacity of 293.5 mA h g^-1 at 100 mA g^-1, outstanding rate property (204.8 mA h g^-1 at 2000 mA g^-1) and cycling performance (161.3 mA h g^-1 at 1000 mA g^-1 after 1600 cycles), which represent the best performance for carbon-based non-metal materials for PIB anode. Density functional theory (DFT) calculations demonstrate that pyrrolic and pyridinic-N doping can efficiently change the charge density distribution of carbon and promote the adsorption of K⁺ on the NHC electrode, which promotes K ion storage.