• Energy Research

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 ABO3 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.

Abstract Li 0.35 La 0.55 TiO 3 (LLTO) powder was mixed with Li 7 La 3 Zr 2 O 12 (LLZO) sol and finally sintered into ceramic pellets. In the low-temperature calcined precursor powder, Zr exists in the amorphous phase around LLTO aggregated particles. After high-temperature sintering, the LLZO phase does not exist, whereas Zr is doped into the LLTO grain interior but is richer in the grain boundary. The introduction of LLZO could modify the grain boundary region of the ceramics and thus change its electrical properties. The LLTO-based ceramics sintered at 1350 °C with 5 wt.% LLZO sol introduced in the precursor powder exhibit a high total conductivity of about 1.2 × 10 − 4 S/cm at room temperature.