Abnormally High-Lithium Storage in Pure Crystalline C₆₀ Nanoparticles.

Abstract

Li⁺ intercalates into a pure face-centered-cubic (fcc) C₆₀ structure instead of being adsorbed on a single C₆₀ molecule. This hinders the excess storage of Li ions in Li-ion batteries, thereby limiting their applications. However, the associated electrochemical processes and mechanisms have not been investigated owing to the low electrochemical reactivity and poor crystallinity of the C₆₀ powder. Herein, a facile method for synthesizing pure fcc C₆₀ nanoparticles with uniform morphology and superior electrochemical performance in both half- and full-cells is demonstrated using a 1 m LiPF₆ solution in ethylene carbonate/diethyl carbonate (1:1 vol%) with 10% fluoroethylene carbonate. The specific capacity of the C₆₀ nanoparticles during the second discharge reaches ≈750 mAh g^-1 at 0.1 A g^-1 , approximately twice that of graphite. Moreover, by applying in situ X-ray diffraction, high-resolution transmission electron microscopy, and first-principles calculations, an abnormally high Li storage in a crystalline C₆₀ structure is proposed based on the vacant sites among the C₆₀ molecules, Li clusters at different sites, and structural changes during the discharge/charge process. The fcc of C₆₀ transforms tetragonal via orthorhombic Li_x C₆₀ and back to the cubic phase during discharge. The presented results will facilitate the development of novel fullerene-based anode materials for Li-ion batteries.