Pushing slope- to plateau-type behavior in hard carbon for sodium-ion batteries via local structure rearrangement

Elucidating the microstructure of hard carbon is essential for uncovering the sodium storage mechanism and constructing state-of-the-art hard carbon anodes for sodium-ion batteries. Guided by an understanding of the crystallization process and inverse materials design principles, we design hard carbon anodes with different local fragments to understand the correlation between the microstructure of hard carbon and sodium storage behavior from the commercialization perspective. The sodiation transformation of hard carbon from slope- to plateau-type is realized via a series of local structure rearrangements, including tuning of the interlayer distance, average crystallite width of graphitic domains, and defect density. We found that the increase in plateau capacity is mainly related to the transition from the critical interlayer distance to the average crystallite width of graphitic domain control, and is limited by the closed pore volume of hard carbon. During sodiation, the formation of NaF and Na₂O in the slope region, as well as Na₂O₂ and NaO₂ in the plateau region, is always accompanied by the production of Na₂CO₃. This work provides insights into understanding the sodium storage behavior in hard carbon anodes and defines general structural design principles for transitioning from slope-type to plateau-type hard carbon.