Garnet/polymer hybrid ion-conducting protective layer for stable lithium metal anode

Rechargeable Li metal batteries using Li metal anodes have attracted worldwide interest because of their high energy density. The critical barriers limiting their commercial application include uncontrolled dendritic Li growth and the unstable Li–electrolyte interface. Considerable efforts have been directed towards solving these problems, e.g., modifying the electrolyte, creating artificial interfacial layers for the Li metal, and constructing three-dimensional structures for the Li metal. However, stabilizing the Li metal interface remains challenging because of the highly reactive nature of the Li metal. In this study, we utilize a Li-ion conducting hybrid film comprising a garnet-type ion conductor and a poly(ethylene oxide)-based polymer electrolyte as a protective layer to stabilize the Li–electrolyte interface and mitigate the growth of Li dendrites. The hybrid ion-conducting layer can block Li dendrites from proliferating and accommodate Li volume expansion because of its robust mechanical properties. Moreover, the ion-conducting layer allows Li deposition only underneath it, rather than on the surface, functioning as a permanent protective layer to ensure the stability of the Li metal over a long cycling life. The dendrite-inhibiting effect of the ion-conducting protective layer is visually evidenced by in situ microscopy using planar batteries. The protective Li metal anode exhibits excellent cycling stability and low voltage hysteresis (~15 mV at 0.2 mA·cm^–2) for a cycle life as long as 1, 000 h. It also shows a high Coulombic efficiency (~99.5%) in a full cell against a LiFePO₄ cathode, exhibiting promise for application in Li metal batteries. Our results imply that the ion-conducting protective layer markedly improves the metal anode, yielding safe, long-life, and high-energy-density batteries.