A First-Principles Investigation of Lithium and Sodium Ion Diffusion in C₆₀ Molecular Solids

C₆₀-based molecular solids have shown promise as important constituents in electrode materials as well as for providing interfacial stability for solid state electrolytes in alkali ion batteries. At room temperature, the solid state C₆₀ crystal is characterized by a face-centered cubic (FCC) structure, with large interstitial voids, which can accommodate and promote ion transport. In this regard, using density functional theory (DFT), we examined the diffusion of lithium and sodium ions within the FCC C₆₀ lattice. The underlying diffusion mechanism for both ions consisted of motion between interstitial tetrahedral and octahedral voids within the C₆₀ lattice. Multiple energy minimum sites were located within each interstitial void, and the diffusion of the ions involved jumps within voids as well as between voids. The rate-limiting step for ion diffusion corresponded to the motion between the tetrahedral and octahedral voids, and the respective activation barriers were determined to be 0.34 eV for lithium and 0.28 eV for sodium. Importantly, the evaluated activation barriers compare favorably with those of currently used solid state electrolytes and electrode materials in alkali ion batteries. These calculations provide insights into the diffusion mechanisms of alkali ions in C₆₀ lattices and should enable utilizing C₆₀ as important components for alkali-ion batteries.