Revealing and excluding the root cause of the electronic conductivity in Mg-ion MgSc2Se4 solid electrolyte

Abstract

MgSc2Se4 is a promising magnesium-ion solid electrolyte possessing a high room temperature Mg+2 conductivity (~ 10−4 S/cm). MgSc2Se4 also has demonstrated considerably high electronic conductivity (10−5÷10−8 S/cm). Such high electronic conductivity substantially restricts the applications of the material in all solid-state Mg-ion batteries. The efforts to lower the electronic conductivity include the enrichment of the material with selenium aliovalent doping, albeit without any tangible success. This suggests that the electronic conductivity may follow a model, which is different from a common model of charge transfer by electrons in a conduction band of the wide band gap semiconductor MgSc2Se4. Here, we provide an evidence that the electronic transport in the Mg-Sc-Se ceramic is facilitated via the Berthelot-type conductivity mechanism, suggesting an electronic transport through a low-conducting matrix stuffed with nano-scaled free electron-containing regions. These inclusions are well-distributed across the Mg-Sc-Se, and the electronic transport takes place via electron tunneling between these conductive regions, through low-conducting matrix spacers. It is shown that these conductive zones comprised of elemental metallic Sc and ScSe mix being a metallic-type electronic conductor. The nature of these conductive inclusions and the synthetic parameters controlling the appearance of such conductive areas are discussed, as well as routes to mitigate their formation.