Safe gel polymer electrolytes for high voltage Li-batteries

Gel polymer electrolytes (GPEs), based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC), so called LP30, and 1‑butyl‑1-methylpyrrolidinium hexafluorophosphate ([Py14]PF6), have been developed to improve inherent safety of the electrolyte for high voltage lithium batteries. Dry PVdF-HFP membranes, prepared with EC/DMC, were soaked in LP30 in addition to its mixtures with 30 wt% and 50 wt% [Py14]PF6 to obtain GPEs. Non-flammable character and mechanical stability enhanced via physical crosslinks of the GPEs are demonstrated with flammability tests and rheological measurements, respectively. Lithium ion conduction property in the GPEs is evaluated by means of both electrochemical impedance spectroscopy and NMR spectroscopy (Pulsed Field Gradient-PFG and relaxation time). Galvanostatic charge and discharge cycles of lithium metal batteries having LiNi0.5Mn1.5O4 as a high voltage cathode demonstrate improved electrochemical performance with coulombic efficiency above 99% and high specific capacity above 110 mAh g −1 at C/5 throughout 150 cycles, when the GPE containing LP30 with 30 wt% [Py14]PF6 is used. The capacity retention is sufficiently high compared to the GPE without [Py14]PF6, confirming that the IL is a highly promising additive for improving battery life.