Electrospun polymer membrane activated with room temperature ionic liquid: Novel polymer electrolytes for lithium batteries

Abstract: A new class of polymer electrolytes (PEs) based on an electrospun polymer membrane incorporating a room-temperature ionic liquid (RTIL) has been prepared and evaluated for suitability in lithium cells. The electrospun poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) membrane is activated with a 0.5M solution of LiTFSI in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) or a 0.5M solution of LiBF4 in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4). The resulting PEs have an ionic conductivity of 2.3×10−3 Scm−1 at 25°C and anodic stability at >4.5V versus Li+/Li, making them suitable for practical applications in lithium cells. A Li/LiFePO4 cell with a PE based on BMITFSI delivers high discharge capacities when evaluated at 25°C at the 0.1C rate (149mAhg−1) and the 0.5C rate (132mAhg−1). A very stable cycle performance is also exhibited at these low current densities. The properties decrease at the higher, 1C rate, when operated at 25°C. Nevertheless, improved properties are obtained at a moderately elevated temperature of operation, i.e. 40°C. This is attributed to enhanced conductivity of the electrolyte and faster reaction kinetics at higher temperatures. At 40°C, a reversible capacity of 140mAhg−1 is obtained at the 1C rate. [Copyright &y& Elsevier]