Compositionally Complex Perovskite Oxides: Discovering a New Class of Solid Electrolytes with Interface-Enabled Conductivity Improvements

Compositionally complex ceramics (CCCs), including high-entropy ceramics, offer a vast, unexplored compositional space for materials discovery. Herein, we propose and demonstrate strategies for tailoring CCCs via a combination of non-equimolar compositional designs and control of grain boundaries (GBs) and microstructures. Using oxide solid electrolytes for all-solid-state batteries as an example, we have discovered a class of compositionally complex perovskite oxides (CCPOs) with improved lithium ionic conductivities beyond the limit of conventional doping. For example, we demonstrate that the ionic conductivity can be improved by {$>$}60\% in (Li0.375Sr0.4375)(Ta0.375Nb0.375Zr0.125Hf0.125)O3-{$\delta$} compared with the (Li0.375Sr0.4375)(Ta0.75Zr0.25)O3-{$\delta$} (LSTZ) baseline. Furthermore, the ionic conductivity can be improved by another {$>$}70\% via quenching, achieving {$>$}270\% of the LSTZ. Notably, we demonstrate GB-enabled conductivity improvements via both promoting grain growth and altering GB structures through compositional designs and processing. In a broader perspective, this work suggests new routes for discovering and tailoring CCCs for energy storage and many other applications.