UCI and UCSD Scientists Unravel an Atomic-scale Origin of the Low Grain-boundary Resistance in Perovskite Solid Electrolyte
Lithium-ion batteries are considered promising energy storage solutions as they have already penetrated our daily life, but there still lies the safety concern especially with flammable liquid electrolyte. Despite high anticipation towards all-solid-state battery (ASSB) that uses chemically more stable solid electrolyte, numerous challenges still stand in the way of employing solid electrolytes, preventing its realization as a practical alternative to the commercial batteries.
One widely known is the large resistance coming from grain boundaries, found in most of poly-crystalline oxide solid electrolytes, which apparently is the main culprit that prevents electrolyte from achieving high ionic conductivity, thus limiting the performance of ASSBs.
Recently, researchers at the University of California, Irvine and the University of California, San Diego discovered anomalously small grain boundary resistance, even smaller than that of the grain bulk, in perovskite solid electrolyte Li0.375Sr0.4375Ta0.75Zr0.25O3 (LSTZ). The team successfully unraveled the origin at an atomic level using aberration-corrected scanning transmission electron microscopy and spectroscopy, in conjunction with an active learning moment tensor potential. Their work has recently been published in Nature Communications.
“Our study showcases the importance of understanding bulk structural ordering and grain boundary structures in improving the macroscopic properties of polycrystalline materials”, said co-PI Xiaoqing Pan, UCI professor of Materials Science and Engineering and Physics. “Specifically, Li depletion, which causes large grain boundary resistance in oxide electrolytes, is absent for the grain boundaries of LSTZ. Instead, LSTZ benefits from the formation of a defective perovskite interfacial nanostructure with abundant vacancies.” Basically, the large number of cation vacancies at grain boundaries provides more Li percolation pathways, which facilitate Li ion migration and decrease grain boundary resistance.
To prove this, the researchers had to characterize Li ion distribution at the atomic scale but soon realized this is rather challenging that simply performing conventional core-loss electron energy loss spectroscopy will not give the desired result. “In terms of the amount of Li per volume, LSTZ is much lower than other Li-ion conductors”, said co-author Tom Lee, a graduate student in Pan’s group at UCI. “This and the interference from another signal that is close to the Li signal made it difficult for us to identify the Li-K edge from core-loss electron energy loss spectroscopy. As a result, we had to look to an alternative technique.”
The researchers then turned their attention to a more advanced technique – vibrational electron energy loss spectroscopy (VibEELS). “We exploited the fact that heavier elements Sr, Ta, and Zr contribute to lower energy vibrations with O while the lighter Li vibrates with O at a higher frequency”, said another co-author Chaitanya Gadre. “Given that core-loss EELS data showed the constant concentration of O throughout the region of interest, energy loss signals from Li-O vibrations now represent the elemental distribution of Li.” Based on the constant Li-O vibrational signal across the grain boundary, the team concluded that there is no decrease in Li concentration at the grain boundary, thus confirming low grain boundary resistance in LSTZ.
“This work will open up a new avenue in the study of interfacial science of complex concentrated material systems in correlation with their macroscopic properties”, said Pan.
Funded primarily by the UCI Materials Research Science and Engineering Centers, Center for Complex and Active Materials, the research was done in collaboration with Prof. Xiaoqing Pan at UCI and Prof. Shyue Ping Ong and Prof. Jian Luo at UCSD.