Lotus Seedpod-Derived Hard Carbon with Hierarchical Porous Structure as Stable Anode for Sodium-Ion Batteries

Hard carbon material is one of the candidates with great promise as anode-active material for sodium-ion batteries (SIBs). Here, new types of biomass-derived hard carbons were obtained via one-step carbonization of lotus seedpods at 1000-1400 °C, respectively. The control of carbonization temperature proved to be significant in controlling the lattice characterization of lotus seedpod-derived hard carbon. Higher temperature generally promoted the lattice graphitization and thus generated a more narrowed d-interlayer space with limited pore volume. The hard carbon pyrolyzed at 1200 °C achieved an optimized reversible capacity of 328.8 mAh g^-1 and exhibited a remarkable capacity retention of 90% after 200 cycles. In addition, such a biomass-derived hard carbon presented improved cyclic stability and rate performance, revealing capacity of 330.6, 288.9, 216.9, 116.5, and 78.3 mAh g^-1 at 50, 100, 200, 500, and 1000 mA g^-1, respectively. Intrinsically, high pyrolysis temperature (1400 °C) gave rise to more narrowed carbon lattice and reduced pore volume and, thus, failed to accommodate sodium ions either from the intercalation into lattice or the ion adsorption onto the pore surface. Such combined advantages of lotus seedpod-derived hard carbon, including the abundance, sufficiently adequate reversible capacity, and prominent cycling and rate property allowed for its large-scale application as promising anode material for SIBs.