LAUSR

Solid‐state electrolytes (SSEs) hold a critical role in enabling high‐energy‐density and safe rechargeable batteries with Li metal anode. Unfortunately, nonuniform lithium deposition and dendrite penetration due to poor interfacial solid–solid contact are hindering their practical applications. Here, solid‐state lithium naphthalenide (Li‐Naph(s)) is introduced as a plastic monolithic mixed‐conducting interlayer (PMMCI) between the garnet electrolyte and the Li anode via a facile cold process. The thin PMMCI shows a well‐ordered layered crystalline structure with excellent mixed‐conducting capability for both Li+ (4.38 × 10–3 S cm–1) and delocalized electrons (1.01 × 10–3 S cm–1). In contrast to previous composite interlayers, this monolithic material enables an intrinsically homogenous electric field and Li+ transport at the Li/garnet interface, thus significantly reducing the interfacial resistance and achieving uniform and dendrite‐free Li anode plating/stripping. As a result, Li symmetric cells with the PMMCI‐modified garnet electrolyte show highly stable cycling for 1200 h at 0.2 mA cm–2 and 500 h at a high current density of 1 mA cm–2. The findings provide a new interface design strategy for solid‐state batteries using monolithic mixed‐conducting interlayers.