Facile One Pot Polycondensation Method to Synthesize the Crosslinked Polyethylene glycol-Based Copolymer Electrolytes

Comb-like polyethylene glycol (PEG)-based copolymer electrolytes with given crosslinking density are synthesized using PEG as the main chain segments (denoted as MC-PEG), methoxy polyethylene glycol (MPEG) as side chain segments (denoted as SC-PEG), and hexamethylene-1,6-diisocyanate homopolymer (HDI trimer) as bridging and/or crosslinking agent. In this study, the effects of molecular weight of MC-PEG and SC-PEG on Li ion conductivities of the copolymer have been examined. If MC-PEG and/or SC-PEG do not crystallize, the ion conductivities of graft copolymer electrolyte increase with either decreasing MC-PEG or increasing SC-PEG segment length. The highest ion conductivity of the copolymer electrolyte at room temperature reaches up to 7.7 × 10⁻⁴ S cm⁻¹ when the molecular weight of MC-PEG is 400 g mol⁻¹ and SC-PEG is 1000 g mol⁻¹ (denoted as GC400-1000). However, when the MC-PEG and/or SC-PEG segments begin to crystallize (i.e., GC400-2000), where the flexibility of the related PEG molecular chains is largely reduced, the ion conductivity drops by almost two orders comparing to that of GC400-1000 copolymer electrolyte. Meanwhile, crosslinked GC400-1000 electrolyte has good thermal and electrochemical stability as well as high mechanical properties, making it suitable for commercial applications in Li ion secondary batteries.