Curvature as the missing descriptor of sodium storage in hard carbon

Abstract

The charge storage mechanism of hard carbon anode materials in sodium-ion batteries remains a highly debated topic. Although multiple models have been proposed, their accuracy is still limited, underscoring the need for a deeper investigation. Our study explores the processes behind charge storage using operando powder X-ray diffraction, ex situ small-angle and wide-angle X-ray scattering, operando Raman spectroscopy, and ex situ scanning transmission electron microscopy with integrated differential phase contrast imaging. We propose that the exceptional electrochemical performance of hard carbon compared to graphite can be attributed to its curvature—a unique microstructural feature that promotes a high density of defects with electron-withdrawing properties. To demonstrate the significant role of curvature in hard carbon performance, we computationally analyze sodium adsorption using density functional theory. Our experimental and computational findings provide empirical evidence of changes in curvature and the interlayer spacing during the charge–discharge cycle. By revisiting previous findings, we aim to address long-standing issues in understanding charge storage mechanism in hard carbon materials.