Designing High Entropy Amorphous Oxides for Li-Battery Electrolytes

Abstract

Amorphous Li-oxides such as LiPON and amorphous Li garnet are considered as promising solid-state electrolytes for use in hybrid or all-solid-state oxide- and sulfide-based batteries as separators or protective layers.1-3 These materials possess several appealing characteristics, such as their intrinsic grain-boundary-free nature and relatively low manufacturing temperature (ranging from room temperature to 600 °C), which facilitates co-synthesis with Co-substituted or even Co-free cathodes that are unstable at standard electrolyte sintering temperatures. Alternatively, they can be applied as protective coatings toward Li anodes and other Li-free anode concepts, bridging the electrochemical stability voltage gap with liquid electrolytes or catholytes and preventing uneven interfacial reactions. The amorphous Li-oxides can be classified based on their structural entropy, i.e., the number and types of local bonding units (LBUs). As of today, ‘low entropy’ amorphous LiPON with only one type of LBU achieves the highest cycle number and battery lifetime.4 ‘High entropy’ amorphous Li-ion conductors, such as Li perovskites or Li garnets, exhibit an unusually high number of LBUs. local ordering and the Li-ion dynamics remain poorly understood, in part owing to the difficulty in characterizing their disordered states. This study employed a novel synthesis protocol to stabilize amorphous Al-doped Li garnets, representing so far the highest number of LBUs (≥ 4) in an amorphous Li-ion conductor.5 We resolved their phase evolution and local structures by a combination of spectroscopy, microscopy, and calorimetry techniques. A much wider (<680 °C) but processing-friendly temperature range was identified to stabilize various amorphous phases with edge- and face-sharing Zr, La, and Li LBUs that do not conform to the formation rules for Zachariasen’s glasses. These amorphous Li-ion conductors reveal an unusual setting in which Li and Zr act as network formers and La acts as a network modifier, with the highest Li-dynamics observed for smaller Li–O and Zr–O coordination among the amorphous phases. Our insight provides fundamental guidelines for the phase, local structure, and Li-transport modulation for amorphous Li garnets and pave the way for their integration in next-generation solid-state or hybrid battery designs with enhanced safety and lifetime. Acknowledgments Y.Z. acknowledges financial support provided by the MIT Energy Initiative fellowship offered by ExxonMobil. This research was supported by Samsung Electronics. This research was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which was supported by the National Science Foundation under NSF award no. 1541959. CNS is a part of Harvard University. This research used resources of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, and was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357, and the Canadian Light Source and its funding partners.