Materials Design for Super-Ionic Conductors for Solid State Li, Na, and Li-S Batteries

All-solid state Li-ion and Li-S batteries (ASSBs) have emerged as very attractive alternatives to conventional liquid electrolyte cells for e-mobility, owing to their enhanced safety and higher energy densities. Similarly, low-cost solid state Na-ion batteries may prove an promising alternative for stationary applications.<br/>ASSBs are founded on high performance fast-ion conducting electrolytes, and in the important search for new materials, alkali thiophosphates (sulfides) and (oxy)halides are particularly promising classes of materials owing to their excellent mechanical properties. Sulfide-based solid catholytes are especially promising materials for high-performance solid-state sulfur cells that operate on the reversible conversion of S ↔ Li2S. However, their electrochemical decomposition above 2.5 V vs Li+/Li causes progressive degradation in the operating window of the cell, because the free S2- ions are oxidized to sulfur at a similar potential as Li2S. Oxyhalides show particular advantage owing to their good interface stability with high voltage NMC-type cathodes. <br/>The talk will cover an overview of the state-of-the art in the field, followed by a focus on recent findings in our laboratory and the understanding of superionic conductivity in these materials using a combination of structural elucidation via single crystal X-ray/powder neutron diffraction, ion conductivity mechanisms, and <i>ab initio</i> molecular dynamics simulations. We correlate crystal structure with ionic conductivity in a range of our newly developed fast ion Li and Na conductors. These considerations lead to an overarching understanding of the relative importance of interstitial alkali ion migration, defect concentration, and the factors that govern thermodynamic (meta)stability. The talk will also highlight their application in solid state batteries and factors that govern the cathode-electrolyte interface.