Dual-Functionalized Ca Enables High Sodiation Kinetics for Hard Carbon in Sodium-Ion Batteries

Hard carbons (HCs), while a leading candidate for sodium-ion battery (SIB) anode materials, face challenges in their unfavorable sodiation kinetics since the intricate microstructure of HCs complicates the Na⁺ diffusion channel. Herein, a Hovenia dulcis-derived HC realizes a markedly enhanced high-rate performance in virtue of dual-functionalized Ca. The interlayer doped Ca²⁺ effectively enlarges the interlayer spacing, while the in situ-formed CaSe templates induce the formation of hierarchical pore structures and intrinsic defects, significantly providing fast Na⁺ diffusion channels and abundant active sites and thus enhancing the sodium storage kinetics. Achieved by the synergistic effect of regulation of intrinsic microcrystalline and pore structures, the optimized HC shows remarkable performance enhancements, including a high reversible capacity of 350.3 mA h g^-1 after 50 cycles at 50 mA g^-1, a high-capacity retention rate of 95.3% after 1000 cycles, and excellent rate performance (108.4 mA h g^-1 at 2 A g^-1). This work sheds light on valuable insight into the structural adjustment of high-rate HCs, facilitating the widespread utilization of SIBs.