A Squaraine-Linked Zwitterionic Covalent Organic Framework Nanosheets Enhanced Poly(ethylene oxide) Composite Polymer Electrolyte for Quasi-Solid-State Li–S Batteries

A squaric acid-based zwitterionic covalent organic framework nanosheet (HUT4) was successfully prepared and incorporated as a filler into a poly(ethylene oxide) (PEO) matrix for excellent-performance solid-state polymer electrolytes (SPEs). Introduction of a HUT4 nanosheet will have a positive impact on the migration number of lithium ions and also improve the stability of the lithium metal negative electrode, which can increase the overall cycle capacity of quasi-solid-state lithium sulfur (Li–S) batteries. Among the obtained membranes with different proportions, the PEO–10%HUT4 electrolyte exhibits the highest lithium ion migration number, up to 0.62 at 60 °C. The Li–S battery composed of PEO–10%HUT4 electrolyte displays outstanding electrochemical performance, with a superior cycling stability of 1057 mAh g–1 at 0.2 C and an outstanding capacity retention, reaching 93.1% after 500 cycles.