Interfacial Engineering with PEO-Based Ultrathin Layers for Stable All-Solid-State Lithium Batteries: From Passive Protection to Active Regulation

Unstable solid-solid interfaces present a major challenge for all-solid-state lithium batteries. Poly (ethylene oxide)-based electrolytes offer good processability and intrinsic compatibility with electrodes. However, their narrow electrochemical stability window and low room-temperature ionic conductivity limit practical application. Recent work reveals a transition in interfacial engineering. The field is moving from passive physical barriers to ultrathin, actively regulated multifunctional interfacial layers. This review summarizes recent advances in PEO-based ultrathin interfacial modification layers. These layers are fabricated by in situ polymerization, atomic layer deposition, and self-assembly. The discussion focuses on three core interfacial regulation mechanisms. The first mechanism is the controlled formation and compositional tuning of solid electrolyte interphase and cathode electrolyte interphase layers. The second mechanism is viscoelastic mechanical adaptation. This adaptation suppresses stress accumulation and lithium dendrite penetration. The third mechanism is homogenized lithium-ion transport. It enhances interfacial charge-transfer kinetics. The synergistic integration of these functions effectively stabilizes the lithium metal anode and high-voltage cathode interfaces. Emerging strategies involving smart-responsive interfaces and bio-inspired multilayer architectures are also discussed. This review provides a concise and mechanism-oriented framework. It guides the rational design of PEO-based interfacial layers and supports the development of high-energy-density, high-safety all-solid-state lithium batteries.