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

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 0.5 M solution of LiTFSl in 1-butyl-3-methylimidazolium bis(tiifluoromethanesulfonyl)imide (BMITFSI) or a 0.5 M solution of LiBF4 in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4). The resulting PEs have an ionic conductivity of 2.3 x 10(-3) S CM-1 at 25 degrees C and anodic stability at >4.5 V 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 degrees C at the 0.1C rate (149 mAh g(-1)) and the 0.5C rate (132 mAh g(-1)). A very stable cycle performance is also exhibited at these low current densities. The properties decrease at the higher, I C rate, when operated at 25 degrees C. Nevertheless, improved properties are obtained at a moderately elevated temperature of operation, i.e. 40 degrees C. This is attributed to enhanced conductivity of the electrolyte and faster reaction kinetics at higher temperatures. At 40 degrees C, a reversible capacity of 140 mAh g-1 is obtained at the I C rate. (c) 2007 Elsevier B.V. All rights reserved.