Mixed ion-electron conductive materials: A path to higher energy density all-solid-state lithium-ion batteries

The cathode of conventional lithium-ion batteries(LIBs) consist of three components including active materials, binders, and electron conductive agents. Binders and conductive agents, which are not directly involved in Faradaic reactions, should be minimized to incorporate more active materials into the electrode. Unlike conventional LIBs, in all-solid-state batteries(ASSBs) - considered as the next-generation batteries-, there is no liquid electrolyte to impart ionic conductivity through wetting the electrodes. Therefore, cathodes for ASSBs require both ion and electron-conducting additives to facilitate charge transport which complicates the preparation of the cathode with intimate triple contact between active material, ion conductive agent and electron conductive agent. In this perspective, mixed ion-electron conductive(MIEC) materials can be regarded as intrinsic ion-electron conductors for electrodes to ease the cathode preparation, boost gravimetric/volumetric energy density, and dig the path to monocomponent electrodes -solely active materials-. This review covers the potential of MIEC materials to be utilized as active material, binder, interlayer, and conductive scaffold to boost the electrochemical performance of solid-state LIBs. Besides, the potential of ASSBs with monocomponent electrodes was evaluated from the perspective of MIEC materials. Moreover, the feasibility of 2D structures were evaluated as MIEC materials for the ASSB electrodes. The concept of MIEC was not be confined to intrinsic MIEC materials but the materials that turned into MIEC by compositing, doping or heat-treatment were considered as MIEC materials in this study.