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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.
This work was financially supported by the National Key R&D Program of China (Nos. 2016YFA0204100 and 2016YFA0200200), the National Natural Science Foundation of China (Nos. 21890753, 21988101, 22162026, and 21875221), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB36030200), the Danish company Haldor Topsøe A/S, the Youth Talent Support Program of High-Level Talents Special Support Plan in Henan Province (No. ZYQR201810148), Creative talents in the Education Department of Henan Province (No. 19HASTIT039). We thank the staff at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facilities for assistance with the EXAFS and XANES measurements.