Surface-functionalized walnut-derived hard carbon as a high-rate anode material for sodium-ion batteries

Abstract

As the foremost candidate anode material for sodium-ion batteries, hard carbon (HC) exhibits exceptional promise, yet its electrochemical performance is constrained by suboptimal initial coulombic efficiency (ICE) and limited rate capability. These limitations are intrinsically linked to the microstructure and surface chemistry of HC. To enhance electrochemical performance, this work introduces surface functionalization of walnut shell-derived HC using anhydrous sodium acetate. This process yields HC with optimized structural ordering, expanded interlayer spacing (0.3723 nm), and enhanced functional group compatibility, which synergistically enhance sodium-ion storage performance. The optimized sample (WHC@Ac-5) exhibits an ordered structure, an expanded interlayer spacing of 0.3723 nm, and effective incorporation of CC and CO functional groups, which delivers a high reversible specific capacity of 346.09 mA h g⁻¹ while maintaining a favorable ICE of 83.09%. At a high current density of 1500 mA g⁻¹, sodium acetate-treated samples (WHC@Ac-4, WHC@Ac-5, and WHC@Ac-10) show significant reversible capacity enhancements of 26.17%, 16.12%, and 28.38%, respectively, compared to the pristine sample (WHC). This surface engineering strategy not only elevates the specific capacity but also dramatically improves the rate performance, offering a viable approach to advance sodium-ion battery development.