The electrochemical performance of LiMn0.4Fe0.6PO4, prepared by modified mechanical activation, as the cathode active material for lithium batteries was tested with an ionic liquid-based electrolyte using an electrospun polyimide matrix. The ionic liquid electrolyte of 0.5M lithium bis(trifluoromethanesulfonylimide) (LiTFSI) in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylimide) (BMImTFSI) was incorporated in an electrospun polyimide (PI) film to prepare the ionic liquid-based polymer electrolyte (ILPE). This PI-based ILPE exhibited good mechanical strength and high thermal stability. Impedance spectroscopy was applied to investigate the electrolyte/electrode resistance over time. The impedance resistance of a Li/ILPE/Li cell was found to increase over time to 7 days, decreasing thereafter. Impedance experiments further indicated good cycling stability for the cell when tested at room temperature. The cell based on Li/ILPE/LiMn0.4Fe0.6PO4 exhibited an initial discharge capacity of 163.6mAhg−1 at 0.1 C-rate at room temperature. Finally, a high capacity and good cycling performance could be obtained even using the high current density of 2C.