Multiscale Design of Poly(ethylene oxide)-Based Composite Electrolyte: Enabling High Ionic Conductivity and Interfacial Stability for Lithium Metal Batteries

Abstract PEO-based solid-state electrolytes (PSEs) suffer from insufficient mechanical strength, low ionic conductivity, and poor interfacial stability, which severely undermines their feasibility for practical applications. The conventional approach of adding plasticizers to enhance ionic conductivity is often accompanied by a loss of mechanical strength and carries the risk of side reactions at the lithium metal interface. To address this contradiction, a multiscale engineering strategy is developed by incorporating coumarin into the PEO/LiTFSI/SN system and subsequently embedding the mixtures within a PTFE fibrous membrane to afford a structurally integrated electrolyte. At the macroscale, the PTFE rigid scaffold interpenetrates the electrolyte and reinforces it mechanically. At the microscale, the co-plasticization of PEO by SN and coumarin significantly reduces its crystallinity, thereby creating fast ion transport channels. At the nanoscale, coumarin acts as a sacrificial agent that induces the formation of a thin and uniform solid electrolyte interphase (SEI) on the lithium metal anode, thereby enhancing the interfacial stability. The resulting polymer electrolyte exhibits a maximum tensile strength of 9.7 MPa and a puncture resistance of 5.9 N, and delivers an optimized ionic conductivity of 0.79 × 10–3 S cm–1 at 60 °C. The Li∥Li symmetric cells based on this electrolyte demonstrate stable cycling for over 3200 cycles at current densities of 0.1 mA cm–2. Furthermore, the corresponding LiFePO4∥Li full cells achieve a high-capacity retention of 84.1% after 700 cycles at 0.5C. This study provides insights into multicomponent SSEs and an avenue to address the bottlenecks of solid-state Li-metal batteries.