Revealing Non-Steady-State Structure–Performance Relationship in Plateau-Capacity-Dominated Hard Carbon Anodes for High-Rate Reversible Sodium Storage

Abstract The sodium storage structure–performance relationship of hard carbon anodes has long been regarded as an intrinsic material property and forms the basis for electrode design. This common view, however, does not hold with varying operating temperatures. At room temperature, optimally structured hard carbon with enhanced plateau capacity still exhibits poor rate performance due to kinetic limitations. In contrast, the same hard carbon delivers drastically improved high-rate capability at elevated temperatures from 36.89 mAh g–1 to 253.97 mAh g–1 at 1.0 A g–1, with the plateau capacity further increasing from 12.75 mAh g–1 to 203.42 mAh g–1, which cannot be explained by the conventional structure–performance relationship. Our mechanistic work shows that the accelerated ion/electron transport and a thin, inorganic-rich solid electrolyte interphase synergistically unlock pore-filling sodium storage, dominating the performance enhancement. These results go against the conventional steady view and establish a non-steady-state structure–performance relationship, where hard carbon’s sodium storage behavior is dynamically regulated by testing conditions. This concept is validated in Ah-level pouch cells (28.2% vs 85.0% high capacity retention at 0.75 A g–1) and commercial cylindrical cells (60.1% vs 84.5% at 6.0 A g–1), providing a new design guideline for sodium ion batteries under practical temperature-varying conditions.