Endogenous Functional Group Engineering Enables High-Efficiency Hard Carbon Anodes for Sodium-ion Batteries

Hard carbon is a leading anode for sodium-ion batteries, but its widespread use is still prevented from low initial Coulombic efficiency (ICE) and unstable electrode-electrolyte interfaces. In this work, we show that engineering endogenous functional groups can directly encode solid-electrolyte interphase (SEI) chemistry and sodium storage performance of hard carbon through molecular design of the carbon precursors. The introduction of distinct functional groups (-COOH, -CHO, and -OH) into the precursor affords systematic control over the microstructure and surface groups (C═O, C─O, and O─C═O) of the hard carbon. For example, hard carbon incorporating -COOH groups promotes the formation of a thin NaF-rich SEI inner layer, whereas that with -OH groups yields a thick organic SEI layer. The optimally engineered interface enables outstanding performance in hard carbon anodes, delivering an 87.9% ICE with a reversible capacity of 353 mAh g^-1 at 20 mA g^-1 and a 95.7% capacity retention rate after 200 cycles at 50 mA g^-1. This work highlights a molecular strategy to elaborately direct interfacial chemistry, providing a generalizable principle for integrating precursor engineering with interphase control in next-generation sodium-ion batteries (SIBs).