Experimental and Theoretical Studies on the Effect of Microstructure and Mechanical Properties of LLZO

All-solid-state lithium batteries are considered an ultimate choice for energy storage systems due to their high energy density and safety. Unlike conventional lithium-ion batteries, solid-state batteries do not rely on the flammable organic electrolyte and are expected to prevent lithium dendrite penetrations, which lead to the use of lithium metal anode further enhancing the energy density. However, the practical implications of solid-state batteries are limited due to poor mechanical stability and complex electro-chemo-mechanical reactions at the interface between solid electrolyte and metal anode. Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is one of the promising solid electrolytes for all-solid-state lithium batteries due to its low activation energy, high ionic conductivity, and high chemical stability against metal lithium enabling safe cycling of Li anodes. Although LLZO has a much higher modulus than lithium metal, penetrations of lithium are still observed in LLZO electrolytes leading to premature failures of the batteries. This is particularly evident at the grain boundaries involving nano-scale events of grain-boundary fracture, lithium deposition/penetration, and crack nucleation/propagation. Therefore, it is important to understand the fundamental mechanical properties of LLZO at the nanoscale and the influence of microstructures and loading conditions. In this work, nano-indentation and micro-pillar compression tests were performed to evaluate the mechanical properties of LLZO and clarify the effect of microstructures. In addition, <i>ab initio</i> simulations were conducted to evaluate the change in the mechanical strength of LLZO at different lithium concentrations which may occur during the charge/discharge process and at or near the grain boundaries. The insights obtained in this work provide a fundamental understanding of the mechanical properties and nano-mechanics of LLZO solid electrolytes, contributing to the development of all-solid-state lithium battery technology.