Interfacial functionalization with cyclic ethers enables subzero temperature operation for safer quasi-solid lithium-ion electrolytes

Despite the continual surge in demand for energy storage in a variety of application spaces and increasingly extreme conditions, Li-ion batteries (LIBs) and particularly solid-state electrolytes (SSEs) have long been limited to room temperature operation. Substantial improvements to SSE performance at low temperature (<0 °C) are required. In this work, we propose a highly facile method of cyclic ether functionalization of a composite solid polymer electrolyte to prepare a quasi-solid-state electrolyte that enables low-temperature operation. The functionalization enhances bulk ionic conductivity by over an order of magnitude, creates highly conformal interfacial contact points, and enables fast Li⁺ transport through weak ion complexation at the interface. Consequently, the cyclic-ether functionalized quasi-solid-state electrolyte demonstrates excellent electrochemical performance in a Li || LiFePO₄ cell from 0 to −25 °C and provides >91 % capacity retention after 100 cycles at −25 °C. This study demonstrates a widely generalizable strategy of interfacial modification to advance the development of SSE systems for low-temperature operation.