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 (d₀₀₂) 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 d₀₀₂ provide only one-dimensional crystalline information. Herein, it was proposed that the crystallite interlayer area (A_i) defined using L_a, d₀₀₂, 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 R²=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.