B/N co-doped hard carbon with expanded interlayers and closed subnanometer pores for high-rate sodium storage

The interdependence of interlayer spacing, defect chemistry, and pore architecture makes it hard to simultaneously improve the key electrochemical properties of hard carbon anodes. Herein, we develop a cross-linking strategy to synthesize B/N co-doped hard carbon using sucrose and ammonium pentaborate tetrahydrate. This dual-function precursor treatment widens the interlayer spacing, forming curved graphitic domains and numerous closed subnanometer pores, and results in a depth-dependent distribution of heteroatom doping. This design balances the dopant distribution and the carbon microstructure, enhancing the rapid Na + transport. Mechanistic studies reveal that Na storage evolves from surface/defect adsorption and pseudocapacitive adsorption within disordered carbon in the sloping region to interlayer Na insertion in the middle plateau, and finally to the coexistence of interlayer Na insertion and cluster-like Na storage within closed subnanometer pores at plateau end. Benefiting from these merits, the hard carbon achieves a high capacity retention of 81.57% over 3000 cycles. When coupled with a Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 cathode, the resulting full cell sustains stable cycling and exhibits outstanding rate capability, affording 72.94 mAh g −1 at 20 C. This work correlates co-doping-induced micro-deformation with sodium storage, providing a strategy to optimize structural parameters in hard carbon anodes.