Hierarchically porous closed-pore hard carbon as a plateau-dominated high-performance anode for sodium-ion batteries

Micro-spherical hard carbons (MSHCs) with distinct porosity features have been synthesized from an easy microwave-assisted solvothermal pre-treatment of sucrose, followed by carbonization, as anodes for sodium-ion batteries. The MSHC exhibits large interlayer spacing of turbostratic graphene nanosheets with more defective graphene planes, hierarchical pore structures, and closed pores. The MSHC anode delivered a high reversible capacity of 422 mA h g^-1 at 0.1C rate with a low-potential battery-like plateau contribution of 57%, which is the best reported reversible sodium storage performance to date for an unmodified HC for SIBs. The MSHC shows 251 and 140 mA h g^-1 high-rate capacities at 1C and 5C, respectively, with excellent capacity retention of 84% after 500 cycles at 1C. GITT and EPR measurements confirm the storage mechanism shift from intercalation to the quasi-metallic sodium clusters in the closed pores at low potentials. The full cell with the MSHC anode and a P2-Na_0.67Ni_0.33Mn_0.67O₂ (NNMO) cathode delivered a high energy density of 292 W h kg^-1 at a working potential of 3.2 V.