Abstract

Rechargeable solid-state batteries (SSBs) continue to gain prominence due to their increased safety. However, a number of outstanding challenges still prevent their adoption in mainstream technology. This study reveals one of the origins of electronic conductivity, σ_e, in solid electrolytes (SEs), which is deemed responsible for SSB degradation, as well as more drastic short-circuit and failure mechanisms. Using first-principles defect calculations and physics-based models, we predict σe in three topical SEs: Li₆PS₅Cl and Li₆PS₅I argyrodites, and Na₃PS₄ for post-Li batteries. We treat SEs as materials with finite band gaps and apply the defect theory of semiconductors to calculate the native defect concentrations and associated electronic conductivities. Li₆PS₅Cl, Li₆PS₅I and Na₃PS₄ were synthesized and characterized with UV-vis spectroscopy, which validates our computational approach confirming the occurrence of defects within the band gap of these SEs. The quantitative agreement of the predicted σ_e in these SEs and those measured experimentally strongly suggests that doping by native defects is a major source of electronic conductivity in SEs even without considering purposefully introduced dopants and/or grain boundaries. We find that Li₆PS₅Cl and Li₆PS₅I are n-type (electrons are the majority carriers), while Na₃PS₄ is p-type (holes). We suggest general defect engineering strategies pertaining to synthesis protocols to reduce σ_ein SEs, and thereby, curtailing the degradation mechanism. The methodology presented here can be extended to estimate σ_e in solid-electrolyte interphases. Our methodology also provides a quantitative measure of the native defects in SEs at different synthesis conditions, which is paramount to understand the effects of defects on the ionic conductivity.