Low-dimensional Li+ ion dynamics in the lithiated Buckminster fullerene Li4C60

Abstract

Lithiated Buckminster fullerene (Li₄C₆₀) has recently been identified as a fast Li⁺ ion conductor. Here, we present a comprehensive NMR-based analysis of ⁷Li dynamics in Li₄C₆₀. Our findings indicate that long-range lithium hopping is to be characterized by an activation energy of 0.26 eV. At 378 K, the Li⁺ jump rate turned out to be in the order of 10⁹ s⁻¹, which translates into Einstein-Smoluchowski diffusion coefficients D ranging from ca. 2 to 5 × 10⁻⁷ cm² s⁻¹, depending on the jump distance chosen (3.6–5 Å) to convert the jump rate into D. The corresponding Arrhenius pre-factor reaches 3 × 10¹² s⁻¹ and lies in the range of typical phonon frequencies. Comparing our relaxation rates with those presented in the literature earlier suggests low-dimensional Li⁺ diffusion in Li₄C₆₀. For short-range or localized Li⁺ jump processes, presumably governed by motional correlation effects, we find much lower activation energies ranging from 0.08 eV to 0.17 eV.