Lithium Atom and A-Site Vacancy Distributions in Lanthanum Lithium Titanate

Abstract

Lanthanum lithium titanate (LLTO) is one of the most promising electrolyte materials for all-solid-state lithium-ion batteries. Despite numerous studies, the detailed crystal structure is still open to conjecture because of the difficulty of identifying precisely the positions of Li atoms and the distribution of intrinsic cation vacancies. Here we use subangstrom resolution scanning transmission electron microscopy (STEM) imaging methods and spatially resolved electron energy loss spectroscopy (EELS) analysis to examine the local atomic structure of LLTO. Direct annular bright-field (ABF) observations show Li locations on O4 window positions in Li-poor phase La0.62Li0.16TiO3 and near to A-site positions in Li-rich phase La0.56Li0.33TiO3. Local clustering of A-site vacancies results in aggregation of Li atoms, enhanced octahedral tilting and distortion, formation of O vacancies, and partial Ti4+ reduction. The results suggest local LLTO structures depend on a balance between the distribution of A-site vacancies and the need to maintain interlayer charge neutrality. The associated local clustering of A-site vacancies and aggregation of Li atoms is expected to affect the Li-ion migration pathways, which change from two-dimensional in Li-poor LLTO to three-dimensional in Li-rich LLTO. This study demonstrates how a combination of advanced STEM and EELS analysis can provide critical insights into the atomic structure and crystal chemistry of solid ionic conductors.

Advanced analytical electron microscopy for lithium-ion batteries

“…45 Furthermore, the Li content, the valence state of Ti and the geometry variation of the oxygen octahedra in the alternating La-rich and La-poor layers were revealed via the associated EELS analysis. 55 The same authors also reported the local structural and chemical variation across the boundaries between the La-rich/poor ordering domains, and they concluded that the domain boundaries serve as obstacles for the Li transport. 49 The precise analysis of the local features provided invaluable insight for interpreting the ionic conduction mechanism.…”

“…42,46,47 As a result, the atomic-resolution (S)TEM studies on solid electrolyte materials are quite rare. 44,[47][48][49][50][51][52][53][54][55] Regardless, the limited number of papers have provided unique insight into their ionic conduction behaviors and paved the way for the design and discovery of high-performance solid electrolytes.…”

“…53 In addition to the overall crystal structure, AEM is also very effective for probing the local chemical variation, such as the Li distribution, element valence state change, alteration of bond angles and so on. Gao et al 55 took advantage of the light-element sensitivity of annular BF-STEM and precisely determined the Li position in (Li 3x La 2/3-x )TiO 3 , a system showing the highest bulk conductivity among all of the oxide solid electrolytes (Figures 4a and b). 45 Furthermore, the Li content, the valence state of Ti and the geometry variation of the oxygen octahedra in the alternating La-rich and La-poor layers were revealed via the associated EELS analysis.…”

“…Its high spatial resolution offers a unique advantage for studying localized features such as grain boundaries and interfaces. However, because the electron radiation 55 (c) HAADF images of a grain boundary in LLTO. 54 EELS data of (d) Li-K and (e) Ti-L 2,3 edges for the grain boundary and the bulk of LLTO.…”

Novel Solid Electrolytes for Li-Ion Batteries: A Perspective from Electron Microscopy Studies

“…The (S)TEM studies made important contributions toward this cause. Taking advantage of the sensitivity of annularbright-field (ABF) STEM imaging to light elements such as Li, Gao et al (2013) directly visualized the variation of Li positions in different LLTO polymorphs. Li was observed to reside at the O4 window for the Li-poor composition La0.62Li0.16TiO3, but near the A-site position for the Li-rich composition La0.56Li0.33TiO3.…”