Interfacial engineering with mixed conductor for lithium-ion and lithium-metal batteries

In state-of-the-art lithium-ion and lithium-metal batteries based on liquid electrolytes, interfacial protective layers with low electronic conductivity or even insulating characteristics are frequently used. These layers are supposed to suppress side reactions by limiting electron supply and reducing chemical reactivity at the electrode/electrolyte interface. Emerging findings now challenge this conventional wisdom, showing that appropriately high formation current and surface chemical reactivity can instead promote the rapid formation of a stable electrode/electrolyte interphase. Moreover, electron and lithium-ion transport are inherently coupled in electrochemical systems. Therefore, restricting electron supply fundamentally undermines electrochemical performance and diminishes the intended benefits of side reaction suppression. Recent studies increasingly underscore the effectiveness of mixed ionic-electronic conductors (MIECs) in interfacial modulations for these systems. In this review, these findings are summarized and the currently developed MIEC strategies are surveyed, covering both single-phase and heterogeneous composite MIECs and encompassing conventional methods alongside emerging strategies. This review aims to promote the applications of MIEC-based interfacial modifications to enhance battery performances, and to offer methodological insights into achieving mixed conductivity in interfacial engineering materials. Finally, the review concludes with a forward-looking perspective on key research directions for advancing MIEC-based interfacial design in next-generation battery systems.