Ultrathin Al ₂ O ₃ ‐Coated Biomass Carbon for Sodium‐Ion Batteries via a Synergistic Storage Mechanism

ABSTRACT Hard carbon (HC) is a promising anode candidate for sodium‐ion batteries (SIBs), yet its application is plagued by unstable interfaces and poor long‐term cyclability. Herein, we develop a facile solvent evaporation strategy to synthesize ultrathin Al 2 O 3 ‐coated biomass‐derived HC (GSC‐Al 2 O 3 ‐3%). The conformal Al 2 O 3 layer passivates defects and micropores, suppresses side reactions, and promotes the formation of a robust organic–inorganic hybrid solid electrolyte interphase. Comprehensive characterizations, including in situ X‐ray diffraction, ex situ Raman spectra, X‐ray photoelectron spectroscopy, time of flight secondary ion mass spectrometry, solid‐state 27 Al nuclear magnetic resonance, and atomic force microscope modulus mapping, demonstrate that Al 2 O 3 actively participates in SEI reconstruction, enhancing the chemical and mechanical stability. Electrochemical tests reveal that the optimized GSC‐Al 2 O 3 ‐3% anode delivers 91% capacity retention after 1000 cycles at 1.0 A g −1 , and possesses excellent wide‐temperature tolerance (149.3 mAh g⁻¹ at −30°C and 286.8 mAh g −1 at 60°C). Mechanistic studies confirm a synergistic Na + storage process involving “adsorption–intercalation–pore filling,” while density functional theory calculations and electrostatic potential mapping reveal that Al 2 O 3 coating regulates interfacial charge distribution and reduces Na + migration barriers. A full cell paired with a NaNi 0.5 Fe 0.5 MnO 4 cathode exhibits a high initial capacity of 395.7 mAh g −1 and outstanding cycling stability (200 cycles). This work provides fundamental mechanistic insights into interfacial engineering of HC and establishes a cost‐effective, scalable route for the next generation high‐performance SIBs.