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

Abstract

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 understanding the crystallization process and inverse materials design principles, we design hard carbon anodes with different local fragments to understand the correlating microstructure of hard carbon and sodium storage behaviors 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 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 critical interlayer distance to average crystallite width of graphitic domains 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, are 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 from the slope- to plateau type of hard carbon.