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.