Novel single-ion conducting electrolytes based on vinylidene fluoride copolymer for lithium metal batteries

Abstract

A single-ion polymer electrolyte (SIPE) based on PVDF-based random copolymer (Li-PVDF) was synthesized in water from the radical copolymerization of vinylidene fluoride (VDF) with perfluoro-2-methyl-3-oxa-5-sulfonimido[-3-oxa-5-sulfonyl fluoride] vinyl ether (VEPFSIS) initiated by potassium persulfate. The purified polymer, characterized by 1H and 19F NMR spectroscopy, ICP-MS, and size-exclusion chromatography, contains 96 mol% VDF with an average molar mass of 128 kg mol−1. The Li-PVDF was cast into self-standing membranes and then swollen with a 50/50 v/v mixture of ethylene carbonate and propylene carbonate. The membrane was analyzed using DSC, TGA, pulse field gradient (PFG)-NMR and electrochemical methods. The solvent-doped membrane exhibited a lithium ion transport number close to 1 with lithium ion conductivities higher than 10−4 S cm−1 at 30 °C, an electrochemical stability up to 4.3 V vs. Li+/Li and an excellent reversibility upon continuous lithium plating/striping cycles. In addition, prototype Li/NMC battery cells using Li-PVDF as both electrolyte and cathode binder in the NMC based composite cathode showed reversible cycling and good performance at 60 °C. The results demonstrated the promising potential of this new single-ion polymer family in intrinsically safe all solid-state lithium metal based batteries.