Ultrathin porous carbon nanosheets with enhanced surface energy storage for high-performance sodium-ion batteries

Carbon materials have long been the primary electrode materials for a series of electrochemical devices, but their applications for sodium-ion batteries (SIBs) are still restricted by limited embedding pathways between narrow graphene layers owing to relatively large size of Na⁺. Here, the narrow interlayer issue is circumvented by enlarging the surface active sites of carbon materials, and the SIB performance is significantly promoted owing to enhanced surface energy storage. To provide more active areas and reduced ion diffusion distance, carbon nanosheets (CNSs) with ultrathin overall structure and highly porous microstructure were prepared by direct pyrolysis of potassium citrate. Potassium species serve as templates and activation agent for organic species carbonization and activation. The CNS exhibits an ultrahigh-specific surface area of 2062.7 m² g⁻¹ due to its rich porous structure and two-dimensional nanosheet structure. These characteristics make our CNSs have numerous defects and active sites, as well as the reduced diffusion path for sodium-ion diffusion. CNS-700 also shows excellent energy storage performance as electrode material of SIBs. When used as an anode material, the CNS-700 exhibits a reversible capacity of 230 mA h g⁻¹ when cycled at 0.3 A g⁻¹ for 550 cycles. Furthermore, the CNS-700 displays a capacity of 175 mA h g^‒1 when the current increases to 2 A g^‒1.