N-n-Butyl-N-methylpyrrolidinium hexafluorophosphate-added electrolyte solutions and membranes for lithium-secondary batteries

Polymer electrolytes were prepared using 1 M LiPF6 in ethylene carbonate–dimethyl carbonate 1:1 wt/wt (LP 30 SelectiLyte™: LP 30 henceforth), N-n-butyl-N-methylpyrrolidinium hexafluorophosphate ([Py14]PF6), and poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP). To determine adequate composition of liquid electrolyte, we investigated the effect of the ratio of [Py14]PF6 to LP 30, in terms of thermal stability, ionic conductivity and electrochemical stability. The solution, LP 30 with 30 wt% [Py14]PF6, gave better properties compared to pure LP 30 and other mixtures examined in this study. Then this was used for galvanostatic charge–discharge tests of a lithium cell. The cell, with the chosen electrolyte solution, demonstrated reversible capacity reaching the theoretical values, when Li metal was combined with LiFePO4 or Li4Ti5O12 electrode. A polymer membrane was prepared by combining the chosen solution with PVdF-HFP matrix. The ionic conductivity of this membrane was 4 × 10−4 S cm−1 at room temperature. Furthermore, the polymer electrolyte displayed reasonable thermal stability, reflecting the properties of [Py14]PF6 in improving the performance of the polymer electrolyte.

► Safety working range of LiPF6–EC–DMC was improved after mixing it with ionic liquid. ► The electrolyte solution retained specific charge capacity over 164 mAh g−1 at C/3. ► The electrolyte solution was mixed with PVdF-HFP to form a polymer electrolyte film. ► The ionic conductivity of the film was 4 × 10−4 S cm−1 at room temperature.