Alkali-tailored walnut shell hard carbon anodes with synergistic pore-kinetics optimization for sodium-ion batteries

Sodium-ion batteries (SIBs) hold promise for large-scale storage due to sodium abundance and low cost, but hard carbon (HC) anode limitations, low initial Coulombic efficiency (ICE) and sluggish kinetics stemming from unoptimized pore structure, hinder deployment. We address this via an innovative alkaline hydroxide (KOH, NaOH, Mg(OH)₂) activation strategy for walnut shell-derived HC. This approach leverages the distinct hydrated radii and reactivities of K⁺, Na⁺ and Mg²⁺ to differentially regulate the surface structure of the material during synthesis. Crucially, KOH fosters a microporous-dominated framework (59% micropores) with expanded interlayer spacing (0.3751 nm) and selectively enriched electroactive C=O groups, while suppressing detrimental oxygen functionalities. This synergistic pore-kinetics optimization yields a high reversible capacity of 360 mAh g^-1 at 30 mA g^-1, dominated by a high plateau capacity of 273.3 mAh g^-1 (76% of total), alongside robust cycling stability. NaOH activation yields defect-rich carbon with balanced porosity, enabling a high reversible capacity of 215 mAh g^-1 even at an ultrahigh current density of 1500 mA g^-1. Mg(OH)₂ treatment achieves favorable initial capacity (350 mAh g^-1) but induces structural instability leading to severe decay. Our work clarifies how alkali-specific structural features (porosity, microcrystal ordering, defects, surface chemistry) influence electrochemical performance, offering structural insights for the design of biomass-derived HC anodes in next-generation SIBs.