Design of High-Performance Solid Electrolytes Inspired by Advanced Characterizations

Solid electrolytes (SEs) are crucial components that significantly impact the performance of all-solid-state batteries. In recent years, many Li⁺ and Na⁺ solid-state ionic conductors, primarily oxide- and sulfide-based, have been extensively studied. Halides, such as those in the Li₃MX₆ family (where M can be Y, In, or Sc, and X can be Cl or Br), are an emerging group of SEs offering several advantages over oxides and sulfides. However, the mechanisms of ionic diffusion in halides are not fully understood. Experimentally and theoretically, a type-II superionic transition has been observed in several halide SEs, but the structural changes causing this transition remain unclear.<br/>In this study, we conducted in-depth synchrotron and neutron characterizations to understand the superionic transition in Li₃YCl₆. Variable temperature diffraction refinements revealed significant changes in critical bond lengths and diffusion pathway bottlenecks around the transition temperature (Tc). These changes result in markedly different diffusion energy barriers in both the ab-plane and c-direction. Further analysis indicates that these changes are due to variations in the vibration modes of anions.<br/>Based on these findings, we propose a strategy to lower Tc to maintain the low activation energy barrier above Tc, thereby achieving high room-temperature conductivity. We designed a series of compounds by tuning anion compositions, successfully lowering Tc to 70 °C. Another compound was designed to further reduce Tc, achieving a very low Tc of -10 °C and resulting in an ultra-high room-temperature ionic conductivity of 12 mS/cm.<br/>This work provides insights into the type-II superionic transition in halide SEs and presents successful examples of materials design guided by these insights. It demonstrates the critical role that crystal structure characterization plays in materials design and development.