Lithium-Ion Trapping from Local Structural Distortions in Sodium Super Ionic Conductor (NASICON) Electrolytes

Herein, we report a study on the structural and thermodynamic effects that cation size disparity may have in NASICON-type solid solutions. A sol–gel procedure was used to synthesize two new NASICON-type lithium-ion conductors with nominal compositions LiGe2–ySny(PO4)3 and Li1+xAlxGe2–y–(1/2)xSny–(1/2)x(PO4)3. The effect of tin substitution on structure and lithium-ion conductivity was studied with powder X-ray diffraction, Raman spectroscopy, and dielectric spectroscopy. It is found that, although increased unit-cell dimensions derived from X-ray data suggest that tin incorporation should open the conduction bottleneck regions and improve conductivity, a decrease in conductivity is observed. Analysis of the electrical data shows that the conduction activation energy is comprised of contributions from carrier motion and generation, the latter accounting for up to 20% of the total activation energy. This result, currently unreported for NASICON-type materials, is correlated with local structural distortions observed in Raman spectra. It is deduced that the bottleneck regions suffer distortions due to the large ionic radius disparity among cationic constituents, which results in the "trapping" of charge carriers. Data estimated for the entropy of motion are also presented and discussed, considering the most probable thermodynamic equilibrium states.