Bridging Microstructure and Sodium-Ion Storage Mechanism in Hard Carbon for Sodium Ion Batteries

Hard carbon (HC) has emerged as a strong anode candidate for sodium-ion batteries due to its high theoretical capacity and cost-effectiveness. However, its sodium storage mechanism remains contentious, and the influence of the microstructure on sodium storage performance is not yet fully understood. This study successfully correlates structural attributes with electrochemical performance, shedding light on what makes HC effective for sodium-ion storage. It is found that HC featuring larger interlayer spacing and smaller and thinner pseudographite domains is beneficial for facile Na+ intercalation. Conversely, the presence of a long-range ordered graphite structure should be avoided, which may result in the reduction of reversible capacity. Through detailed analysis of three commercial HC products, including in situ X-ray diffraction and Raman measurements, the "adsorption-intercalation-filling" mechanism is validated as a convincing explanation for the varying sodium storage behaviors. Consequently, this work is expected to deepen our understanding of the sodium storage mechanism and provide insightful criteria for the further development of advanced HC materials.