Ceramic-glass pellet thickness and Li diffusion in NASICON-type LAGP (Li1.5Al0.5Ge1.5(PO4)3) studied by pulsed field gradient NMR spectroscopy

Abstract

LAGP (Li1.5Al0.5Ge1.5(PO4)3) is one of inorganic solid electrolytes (ISEs) and is relatively stable against atmospheric CO2 and H2O. Five LAGP pellet samples with thicknesses ranging from 0.5 to 4 mm (10−3 m) were investigated by pulsed field gradient (PFG) 7Li NMR spectroscopy. PFG was applied in the thickness direction and the Li diffusion in the same direction was observed. The apparent diffusion coefficients (Dapparent) of Li were determined over the observation time (Δ) ranging from 5 to 100 × 10−3 s and the distances from 0.1 to 1 × 10−6 m. The Dapparent of LAGP samples was 10−11 to 10−13 m2s−1 at 28 °C. The results showed that the Li diffusion phenomenon was thickness dependent. When Δ was short, diffusion of Li was slower for thinner samples, suggesting that the Li pathway was made to avoid surface boundaries. The Li ions were found to diffuse through preferential and longer paths of the pellet samples. As Δ became longer, the diffusion process of Li averaged out and the diffusion coefficient converged to a constant value regardless of the thickness. The PFG-NMR observes a diffusion coefficient of 7Li species which are marked by the first PFG and detected by the second PFG after Δ. Impedance spectroscopy, on the other hand, measures the ionic conductivity for continuous charge transfer between electrodes under electric field, and higher ionic conductivities have been reported for thinner samples.