Functionalized fullerenes for highly efficient lithium ion storage: Structure-property-performance correlation with energy implications

Here, we report that spherical C₆₀ derivatives with well-defined molecular structures hold great promise to be advanced anode materials for lithium-ion batteries (LIBs). We studied four C₆₀ molecules with various functional groups, including pristine, carboxyl, ester, and piperazine C₆₀. The comparison of these C₆₀s elucidated a strong correlation between functional group, overall packing (crystallinity), and the anode performance in LIBs. Specifically, carboxyl C₆₀ and neutral ester C₆₀ showed higher charge capacities than pristine C₆₀, whereas positively-charged piperazine C₆₀ exhibited lower capacity. The highest charge capacity was achieved on the carboxyl C₆₀ (861 mAh g⁻¹ at 100th cycle), which is five times higher than that of pristine C₆₀ (170 mAh g⁻¹), more than double the theoretical capacity of commercial graphite (372 mAh g⁻¹), and even higher than the theoretical capacity of graphene (744 mAh g⁻¹). Carboxyl C₆₀ also showed a high capacity at a fast discharge-charge rate (370 mAh g⁻¹ at 5 C). The exceptional performance of carboxyl C₆₀ can be attributed to multiple key factors. They include the complex formation between lithium ions and oxygen atoms on the carboxyl group, the improved lithium-binding capability of C₆₀ cage due to electron donating from carboxylate groups, the electrostatic attraction between carboxylate groups and lithium ions, and the large lattice void space and high specific area due to carboxyl functionalization. This study indicates that, while maintaining the basic C₆₀ electronic and geometric properties, functionalization with desired groups can achieve remarkably enhanced capacity and rate performance for lithium storage.