Solid Electrolyte Battery Material

Schematic diagram of conventional lithium ion battery vs. all-solid-state lithium ion battery
(a) HAADF-STEM image of a Li3xLa2/3-xTiO3 grain boundary, exhibiting both dark- and normal-contrast regions, labelled as Type I and Type II, respectively. Within the grains, a row of atomic columns for a La-poor layer and one for a La-rich layer were indicated by green and red arrows on the left hand side of the image, respectively. (b) Further magnified Type I GB feature. (c) Further magnified Type II GB feature. Reproduced from Energy Environ. Sci., 2014, 7, 1638.
Unit cell of tetragonal Li3xLa2/3-xTiO3. The unit cells are formed by La1 site, B-site, and La2 site layers in repetition. The La1 site layer is referred to as La-rich layer, consisting of predominantly La ions with few Li ions. The La2 site layer composes approximately all vacant A-sites, quantitively increased Li ions and decreased La ions, and is referred to as La-poor layer.

Secondary lithium ion battery (LIB) has been extensively developed and used for the applications of portable electronic devices, hybrid and electric vehicles for the past three decades. It has been regarded as the most dominant and promising energy storage device because of its high energy density, high power density, long cycle life, and low self-discharge. Liquid electrolyte, as the media for ion transportation for current commercial LIBs, still suffers from a few drawbacks, including the safety concerns of their thermal stability and the leakage. On the other hand, solid-state electrolyte is intrinsically safe and free from leakage compared with liquid electrolyte. Solid oxide electrolytes, including garnet (e.g., Li₇La₃Zr₂O₁₂), NASICON (e.g., Li_1.3Al_0.3Ti_1.7(PO₄)₃ and LiZr₂(PO₄)₃), and perovskite (e.g., Li_0.3La_0.5TiO₃), have a high Li⁺ conductivity from 10⁻⁵ to 10⁻³ S/ cm at 25 °C. Among these solid electrolytes, perovskite electrolyte Li_3xLa_2/3-xTiO₃ (LLTO, 0 < x < 0.16) exhibits the highest bulk Li⁺ conductivity of 10⁻³ S/ cm at 25 °C. However, the large grain-boundary resistance because of the Li-deficiency on the grain surface and the reduction of Ti⁴⁺ to Ti³⁺ at voltages below 2.0 V limit the application of LLTO.

Several studies synthesized perovskite Li-Sr-Ta-Zr structure with some A-site vacancies for Li⁺ transport, and the optimized composition Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃ (LSTZ) exhibited high total Li⁺ conductivity on the order of 10⁻⁴ S/ cm at 25 °C, which makes it possible for the application in all-solid-state Li-ion batteries. Furthermore, LSTZ was found to be stable at least above 1 V vs Li/ Li⁺, so that several anode materials operating at high potential for Li⁺ storage such as Li₄Ti₅O₁₂, TiO₂ and Nb₂O₅ can be potentially used for constituting all-solid-state batteries. However, all of these studies have mainly focused on the material synthesis. The origin of high total Li⁺ conductivity of LSTZ remains an unanswered question. In order to further improve LSTZ and other similar materials, a thorough understanding of the ionic transport mechanism in these materials is critical for a revolutionary development of next-generation Li ion batteries.

Battery research at Pan Group focuses on fundamental study of the structural property relationship of LSTZ solid electrolyte. Our goal is to examine its microstructure using Scanning Transmission Electron Microscopy (STEM) and use our finding to provide an explanation for the origin of high total Li⁺ conductivity. The result of this study will give insights to designing better perovskite solid oxide electrolytes.