Zwitterionic metal covalent organic frameworks constructed from lithium salts to reinforce poly(ethylene oxide)/poly(propylene carbonate) composite polymer electrolytes

Abstract

The active Li⁺ was coordinated with the zwitterionic part (squaric acid) of HUT4 by mechanical grinding to prepare a metal covalent organic framework HUT4Li, which was used as the filler of PEO and PPC composite polymer electrolytes (CPEs). The Li⁺ on the surface of HUT4Li can bind to oxygen atoms in PEO and PPC substrates as Lewis acid sites to promote Li⁺ transport. In addition, metal covalent organic frameworks with zwitterion ions (HUT4Li) can balance the uneven charge distribution of CPEs, cations capture anions in CPEs to accelerate the transport of Li⁺, and anions promote the dissolution of lithium salts and better distribute charge. Among them, the PEO-10%HUT4Li composite  polymer electrolyte exhibited the best ion conductivity (3.4 × 10⁻³ S cm⁻¹, 90 °C) and the highest lithium ion mobility (0.68, 60 °C). The PPC-10%HUT4Li composite polymer electrolyte has better ionic conductivity and a wider electrochemical window. The capacity retention rate of quasi-solid-state Li–S batteries with the PPC-10%HUT4Li electrolyte was 88.9% after 100 cycles at 0.2C.