Effect of Asymmetric Loading and Fracture on Polymorphism and Transport Properties in La₃Li₇Zr₂O₁₂ (LLZO)

Solid state electrolytes comprise a class of materials that is critical for enabling higher energy density Li metal battery cells. The solid electrolyte, La_{<span style="font-size:10.8333px">3</span>}Li_{<span style="font-size:10.8333px">7</span>}Zr₂O₁₂ (LLZO) is a promising candidate due to its high ionic conductivity and electrochemical stability. However, this material has a propensity for cracking under large electrochemical loads. In our work, we used molecular dynamics simulations to probe the interplay between crack propagation and phase transformation. Our results indicate that under high uniaxial loads, LLZO transforms from the cubic (Ia-4d) polymorph to a tetragonal polymorph (I41/acd). These two morphologies have radically different Li⁺ self-diffusion rates, with the tetragonal polymorph being severely disadvantageous in engineering applications due to its prohibitively low conductivity. Furthermore, we find that during intergranular fracture, the phase transformation leads to temperature-dependent Li clustering. Taken together, these results add shades of complexity and probe the fundamental mechanisms surrounding fracture in this material.