Structural changes and their effect on Li-ion conductivity upon quenching of La_(1-x)/3Li_xNbO₃ solid electrolytes

La_(1-x)/3Li_xNbO₃ (LLNbO) solid solutions constitute a promising family of electrolyte materials for use in all-solid-state lithium-ion batteries (ASSLIBs) because of their good electrochemical stability in contact with Li metal. Even in monocrystalline form, however, their Li-ion conductivities are insufficient for practical use. Post-synthesis heat treatment is a commonly applied technique for modifying nano- and microstructures, and hence properties of materials, although the effect of thermal treatment on A-site deficient layered perovskites is not well understood. Here we combine high temperature in situ X-ray diffraction, atomic-resolution scanning transmission electron microscopy, and molecular dynamics simulations to examine the effect of quenching on LLNbO with Li contents x ≈ 0.05 (Li-poor) and x ≈ 0.1 (Li-rich). Quenching results in a number of nanostructural changes, including weakening of the modulated structure by disordering of La atoms and vacancies within A1 layers, and movement of some La and Li atoms from A1 layers into A2 layers. Rumpling of Nb-O-Nb layers in the [001]_p direction also becomes less pronounced and domain boundaries disappear as a result of suppression of NbO₆ octahedral tilting. In the case of the Li-poor sample, the Li-ion conductivity decreased by about 66% after quenching, while that of the Li-rich sample increased by about 20%. Thus, despite success in displacing some cations from A1 to A2 layers, the combined effects of quenching failed to increase the Li-ion conductivities to useful levels. More effective means of increasing charge carrier concentrations while decreasing migration barrier energies in LLNbO need to be found if it is to be competitive as a solid electrolyte in ASSLIBs.