2, U.S. Army Research Laboratory, Adelphi, Maryland, United States
While there have been recent advances in solid ion conductors exhibiting conductivities comparable to liquid electrolytes, how to best capitalize on these materials discoveries to enable new energy storage technology is currently not known. Of particular interest is the integration solid-state electrolyte to allow for the safe and stable use of metallic Li anodes. Because Li offers a ~4X increase in volumetric energy density compared to state-of-the-art graphite anodes, significant gains in cell energy density are possible.
The solid electrolyte based on garnet-type oxide, of nominal composition Li7La3Zr2O12 (LLZO), simultaneously exhibits fast-ion conductivity and stability against metallic Li. Typically, LLZO is studied in the polycrystalline form and made using conventional ceramic processing techniques such as solid-state powder synthesis, calcination, and densification. These process introduce defects such as pores, grain boundaries, impurities, surface contamination, etc.. This paper discusses the major outcomes of a systematic study to characterize the effect of each microstructural defect and its impact on the maximum tolerable current density at and above which Li metal propagates through LLZO.
Aspects such as grain size, grain boundary orientation, mechanical properties, surface chemistry, and external variables such as cell temperature and stack pressure will be described. EIS, DC, SEM, Raman, XPS, acoustic emission, and indentation analysis was used as an integral part of the systematic study. Altogether, the results of this study define a path towards the stable plating of Li through LLZO in the mA/cm2 range. These findings could facilitate the development of viable, high energy density solid-state batteries.