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 andL(a)andd(002)provide only one-dimensional crystalline information. Herein, it was proposed that the crystallite interlayer area (A(i)) defined usingL(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 toA(i)with the correlation coefficient ofR(2)=0.94. The evaluation factor ofA(i)was also adaptable to previous reports and strongly correlated with theirC(i), indicating thatA(i)is more widely adaptable than the conventional evaluation methods.