The Influences of DMF Content in Composite Polymer Electrolytes on Li+‐Conductivity and Interfacial Stability with Li‐Metal

Trace N, N‐dimethylformamide(DMF) containing composite polymer electrolytes (CPEs) has attracted much attention owing to the dramatically increased Li+‐conductivity. But the amount of DMF is critical and needs to be clarified for the interfacial stability, since DMF is easily reduced by Li‐metal. Herein, the influences of DMF in poly(ethylene oxide) (PEO) and poly(vinylidene fluoride) (PVDF) based CPEs are studied on the Li+‐conductivity and interfacial stability. In PEO‐based CPEs, owing to a stronger interaction of lithium bis(trifluoromethanesulfon)imide (LiTFSI) with PEO than DMF, DMF can not be confined and be easily evaporated off. Only ≈0.25wt.% DMF is absorbed on ceramic electrolyte fillers, giving two times increased Li+‐conductivity compared with the DMF‐free counterparts and generating stable interface with Li‐metal; but over much DMF (≥2.2 wt.%) leads to serious interfacial reactions with Li‐metal. While in PVDF‐based CPEs, ≈8wt.% DMF is confined by LiTFSI owing to a stronger interaction of LiTFSI with DMF than with PVDF. Short‐term stable interface with Li‐metal can be obtained, but longer‐term cycling or higher current density leads to the gradually aggravated reactions with Li‐metal. Thanks to the high‐voltage stability of PVDF based CPEs, better cycling performance is obtained when they are used as catholytes to match high‐voltage cathodes. In PEO‐LiTFSI electrolyte, LiTFSI shows stronger interaction with PEO than DMF, making DMF free and can be removed easily; in PVDF‐LiTFSI electrolyte, LiTFSI owns stronger interaction with DMF than PVDF, immobilizing DMF well in Li(DMF)TFSI complex. Although DMF is helpful to improve the Li+‐conductivity, it brings challenge for the stability with Li‐metal, with the exception of trace amount of DMF (0.2–0.3 wt.%) absorbed on ceramic electrolyte particles.