Controlling intercalation sites of hard carbon for enhancing Na and K storage performance

Hard carbon is considered as a promising anode material for sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) owing to its high low-voltage plateau capacity and excellent long-term stability over several charging cycles. Based on the adsorption–intercalation mechanism, the low-voltage plateau capacity originates from the intercalation of carrier ions between misaligned graphene layers. In this study, the intercalation sites in cellulose-derived hard carbon were controlled by varying the degree of crystallinity of cellulose. The hard carbon synthesized from cellulose with an optimum degree of crystallinity delivered high specific capacities of 322 and 281 mAh g−1 in NIB and KIB, respectively. The increased total capacity emerges predominantly from the low-voltage plateau capacities (207 and 175 mAh g−1 for NIB and KIB, respectively) rather than the high-voltage sloping capacities. Based on the correlation analysis between the type of intercalation sites and low-voltage plateau capacity, pseudo-graphitic domains with interlayer spacings between 0.36 and 0.40 nm are responsible for the low-voltage plateau capacity in NIBs and KIBs. The new insight into the increased percentage of pseudo-graphitic regions in hard carbon could provide a rational guide for designing high-performance anodes in NIBs and KIBs for practical use.