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

ABSTRACT 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).