Abnormally High-Lithium Storage in Pure Crystalline C-60 Nanoparticles

Abstract

Li+ intercalates into a pure face-centered-cubic (fcc) C-60 structure instead of being adsorbed on a single C-60 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 C-60 powder. Herein, a facile method for synthesizing pure fcc C-60 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 C-60 nanoparticles during the second discharge reaches approximate to 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 C-60 structure is proposed based on the vacant sites among the C-60 molecules, Li clusters at different sites, and structural changes during the discharge/charge process. The fcc of C-60 transforms tetragonal via orthorhombic LixC60 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.