Microstructural Investigation into Na-Ion Storage Behaviors of Cellulose-Based Hard Carbons for Na-Ion Batteries

The physicochemical properties of hard carbon can vary significantly depending on the precursor used during its synthesis. Among various precursors for hard carbon, cellulose has been getting significant attention because of the excellent electrochemical performances of the hard carbon obtained from its decomposition. However, the correlation between the microstructure or crystal structure of the resultant hard carbons from cellulose and their electrochemical properties looks still elusive. Hence, this study compares the sodium-ion storage behaviors of fiber cellulose-based hard carbon composed of crystalline and amorphous grains with those of microcrystalline cellulose (MCC)-based hard carbon without any amorphous grains. The presence or absence of amorphous regions affects the pore size and distribution of hard carbons significantly. Particularly, the hard carbon obtained from a thermal decomposition of MCC exhibited a high capacity reaching 300 mA h g-1 thanks to plenty of open pores, whose size is primarily less than 2 nm. Additionally, 23Na MAS NMR analysis revealed that the characteristic pore structure from MCC played a key role in extending the reversible sodium storage of hard carbon. This study provides a meaningful insight into understanding the effect of the microstructure and crystal structure on the sodium storage mechanism of hard carbons with different cellulose precursors.