Rational Route for Increasing Intercalation Capacity of Hard Carbons as Sodium‐Ion Battery Anodes

Hard carbon (HC) is the most promising candidate for sodium‐ion battery anode materials. Several material properties such as intensity ratio of the Raman spectrum, lateral size of HC crystallite ( L a ), and interlayer distance ( d 002 ) have been discussed as factors affecting anode performance. However, these factors do not reflect the bulk property of the Na + intercalation reaction directly, since Raman analysis has high surface sensitivity and L a and d 002 provide only one‐dimensional crystalline information. Herein, it was proposed that the crystallite interlayer area ( A i ) defined using L a , d 002 , and stacking height ( L c ) governs Na + intercalation behavior of various HCs. It was revealed that various wood‐derived HCs exhibited the similar total capacity of approximately 250 mAh g −1 , whereas the Na + intercalation capacity ( C i ) was proportional to A i with the correlation coefficient of R 2 =0.94. The evaluation factor of A i was also adaptable to previous reports and strongly correlated with their C i , indicating that A i is more widely adaptable than the conventional evaluation methods.