In Situ Formation of LiF‐Rich SEI in Quasi‐Solid Electrolytes Enables Long‐Life Electrochromic Devices

Abstract Electrochromic devices (ECDs) offer significant energy‐saving potential for applications such as smart windows and displays by modulating optical properties in response to electrical stimuli. However, their widespread adoption is limited by challenges associated with electrolyte stability and the formation of a robust solid‐electrolyte interphase (SEI). In this study, a novel quasi‐solid polymer electrolyte (QSPE) based on a UV‐curable matrix of poly(ethylene glycol) diacrylate (PEGDA) incorporated with poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) [P(VDF‐TrFE‐CFE), abbreviated as PVTC] is presented. The high dielectric constant of PVTC facilitates lithium‐ion transport, while electrochemical cycling triggers partial dehydrofluorination, thereby promoting in situ formation of a LiF‐rich SEI layer on WO 3 surface. The optimized electrolyte exhibits excellent properties, including high optical transparency (88.7%), ionic conductivity (1.76 mS cm −1 ), and mechanical robustness. When applied in ECDs, PVTC enables outstanding performance, achieving 86.29% optical retention from the 5000th to the 40 000th cycle and 98.78% charge retention after 50 000 charge–discharge cycles. Furthermore, prototype demonstrations in smart windows and electrochromic sunglasses validate the scalability and flexibility of the proposed system, highlighting a promising strategy for advancing durable, high‐performance ECDs through innovative electrolyte design.