Electrochemically stable lithium-ion and electron insulators (LEIs) for solid-state batteries

Rechargeable solid-state Li metal batteries demand ordered flows of Li-ions and electrons in and out of solid structures, with repeated waxing and waning of Li_BCC phase near contact interfaces which gives rise to various electro-chemo-mechanical challenges. There have been approaches that adopt three-dimensional (3D) nanoporous architectures consisting of mixed ion-electron conductors (MIECs) to combat these challenges. However, there has remained an issue of Li_BCC nucleation at the interfaces between different solid components (e.g., solid electrolyte/MIEC interface), which could undermine the interfacial bonding, thereby leading to the evolution of mechanical instability and the loss of ionic/electronic percolation. In this regard, the present work shows that the Li-ion and electron insulators (LEIs) that are thermodynamically stable against Li_BCC could combat such challenges by blocking transportation of charge carriers on the interfaces, analogous to dielectric layers in transistors. We searched the ab initio database and have identified 48 crystalline compounds to be LEI candidates (46 experimentally reported compounds and 2 hypothetical compounds predicted to be stable) with a band gap greater than 3 eV and vanishing Li solubility. Among these compounds, those with good adhesion to solid electrolyte and mixed ion-electron conductor of interest, but are lithiophobic, are expected to be the most useful. We also extended the search to Na or K metal compatible alkali-ion and electron insulators, and identified some crystalline compounds with a property to resist corresponding alkali-ions and electrons.