Microscopic Access to Long-Range Diffusion Parameters of the Fast Lithium Ion Conductor Li7BiO6 by Solid State 7Li Stimulated Echo NMR

Abstract

Li self-diffusion in rhombohedral Li₇BiO₆, being a promising basic material for cathodes of rechargeable ion batteries, is studied by means of ⁷Li stimulated echo NMR. Using the pulse sequence introduced by Jeener and Broekaert, a spin-alignment echo is created whose amplitude decay is recorded as a function of mixing time. The so-obtained two-time correlation functions follow stretched exponential behavior and lead to decay rates which can be identified directly with microscopic Li motional correlation rates (τ^-1). Using a jump distance of about 0.2 nm, this results in a diffusion coefficient (D) of about 0.5 × 10^-16 m² s^-1 at 294 K. The activation energy turned out to be 0.53(3) eV which is in very good agreement with recently obtained results by means of dc-conductivity measurements probing long-range diffusion parameters. This shows that stimulated echo NMR, due to its inherent time scale, gives microscopic access to long-range transport. The prefactor τ₀^-1 of the corresponding Arrhenius law lies in the typical range of phonon frequencies, τ₀^-1 = 3 × 10¹² s^-1.