Probing the active sites of 2D nanosheets with Fe-N-C carbon shell encapsulated Fe_xC/Fe species for boosting sodium-ion storage performances

Developing stable but high active metal-nitrogen-carbon (M-N-C)-based hard carbon anode is a promising way to be the alternatives to graphene and blank hard carbon for sodium-ion batteries (SIBs), requiring the precise tailoring of the electronic structure for optimizing the Na⁺ intercalation behavior, yet is greatly challenging. Herein, Fe-N-C graphitic layer-encapsulating Fe₃C species within hard carbon nanosheets (Fe-N-C/Fe₃C@HCNs) are rationally engineered by pyrolysis of self-assembled polymer. Impressively, the Fe-N-C/Fe₃C@HCNs exhibit outstanding rate capacity (242 mAh·g⁻¹ at 2,000 mA·g⁻¹), which is 2.1 and 4.2 times higher than that of Fe-N-C and N-doped carbon (N-C), respectively, and prolonged cycling stability (176 mAh·g⁻¹ at 2,000 mA·g⁻¹ after 2,000 cycles). Theoretical calculations unveil that the Fe₃C species enhance the electronic transfer from Na to Fe-N-C, resulting in the charge redistribution between the interfaces of Fe₃C and Fe-N-C. Thus, the optimized adsorption behavior towards Na⁺ reduces the thermodynamic energy barriers. The synergistic effect of Fe₃C and Fe-N-C species maintains the structural integrity of electrode materials during the sodiation/desodiation process. The in-depth insight into the advanced Na⁺ storage mechanisms of Fe₃C@Fe-N-C offers precise guidance for the rational establishment of confinement heterostructures in SIBs.