A robust in-situ solid electrolyte interphase for long-term stable electrochromic WO3 thin films

Tungsten trioxide (WO 3 ) is regarded as one of the most promising electrochromic materials due to its relatively low material cost, dual-band tunability and high optical modulation. Currently, the key interface of WO 3 /electrolyte suffers from severe cycling degradation, which limits its further practical applications. Herein, we report the design of a robust in-situ solid electrolyte interphase (SEI) via electrolyte engineering with lithium difluoro(oxalato)borate (LiDFOB) and fluoroethylene carbonate (FEC) as additives. The resulting SEI is a thin, compact organic–inorganic composite comprising LiF, Li 2 O, Li 2 CO 3 , and LiCl, which collectively enhance interfacial mechanical strength, facilitate Li⁺ transport, and suppress ion trapping in WO 3 . As a result, the WO 3 electrodes exhibit outstanding cycling stability, retaining 90.87% of the initial optical contrast after 1500 electrochromic cycles and maintaining 71.97% capacity after 5000 charge–discharge cycles. This work provides the first direct evidence of in-situ SEI formation in WO 3 -based electrochromic systems and offers a scalable interfacial design strategy. It contributes fundamentally to surface and interface science by enabling stable ion transport and interfacial regulation, paving the way for next-generation electrochemical devices.