Balancing fracture toughness and ionic conductivity in lithium thiosilicate glassy electrolytes

The use of solid instead of liquid electrolytes can lead to increased battery capacity and safety. However, solid-state electrolytes bring a number of challenges, especially the lower ionic conductivity and the risk of material cracking, ultimately causing battery failure. This work addresses these challenges by probing the archetypical electrolyte family of lithium thiosilicate glasses (xLi₂S-(100-x)SiS₂) to study the influence of lithium sulfide content on the mechanical and ionic transport properties. Interestingly, we find a decreasing fracture toughness and increasing ionic conductivity with increasing Li₂S content. We ascribe this to the depolymerization of the glassy network with increasing Li₂S content and a decoupled activation mechanism of thermal diffusion and movement under mechanical strain. Ultimately, the investigated glasses offer insights into battery operation where the electrolyte is continuously cycled through high- and low-lithium content states. In turn, this highlights the need to consider the material properties across a wide range of compositions when engineering future solid-state electrolytes.