Atomic-scale origin of the large grain-boundary resistance in perovskite Li-ion-conducting solid electrolytes

Li-ion-conducting solid electrolytes are the potential solution to the severe safety issues that occur with conventional batteries based on solvent-based electrolytes. The ionic conductivity of solid electrolytes is in general too low, however, due to a high grain-boundary (GB) resistance. A thorough understanding of the ionic transport mechanism at GBs in these materials is critical for a revolutionary development of next-generation Li batteries. Herein we present the first atomic-scale study to reveal the origin of the large GB resistance; (Li3xLa2/3-x)TiO3 was chosen as a prototype material to demonstrate the concept. A strikingly severe structural and chemical deviation of about 2-3 unit cells thick was revealed at the grain boundaries. Instead of preserving the ABO(3) perovskite framework, such GBs were shown to consist of a binary Ti-O compound, which prohibits the abundance and transport of the charge carrier Li+. This observation has led to a potential strategy for tailoring the grain boundary structures. This study points out, for the first time, the importance of the atomic-scale grain-boundary modification to the macroscopic Li+ conductivity. Such a discovery paves the way for the search and design of solid electrolytes with superior performance.