Pore-Structure Modulation of Macadamia Shell-Derived Hard Carbon for High-Performance Sodium-Ion Battery Anodes

Hard carbon is widely recognized as a viable anode candidate for sodium-ion batteries (SIBs) owing to its electrochemical advantages, yet simultaneously enhancing specific capacity and rate capability, arising from insufficient plateau capacity, remains a long-standing challenge. Herein, we present a strategy for fabricating ZnCl2-modified hard carbon (HCMZ-X) using waste macadamia shells and ZnCl2 as a multifunctional structural modifier through a facile high-temperature carbonization. This approach effectively expands the graphite interlayer spacing to 0.394 nm, reduces microcrystalline size, and induces abundant closed pores, synergistically improving sodium-ion storage kinetics within the hard carbon framework. Mechanistic investigations confirm an “adsorption-intercalation-filling” storage mechanism. Hence, the optimized HCMZ-3 delivers a high reversible capacity of 382.05 mAh g−1 at 0.05 A g−1, with the plateau region contributing approximately 70%, significantly outperforming that of unmodified hard carbon (262.64 mAh g−1). Remarkably, it achieves stable rate performance, delivering 190 mAh g−1 at 1 A g−1, along with excellent cycling stability, retaining over 90% after 500 cycles. By rational pore-structure modulation rather than excessive surface activation, this cost-effective method utilizing agricultural waste and ZnCl2 dual-functional modification partially alleviates the intrinsic energy-density limitation of hard carbon anodes, advancing the development of high-performance, eco-friendly anodes for scalable energy storage systems.