Electrochemical performance of polymer electrolytes based on Poly(vinyl alcohol)/Poly(acrylic acid) blend and Pyrrolidinium ionic liquid for lithium rechargeable batteries

Polymer electrolytes offer the most promising solution to address the all-solid-state battery requirements such as flexibility, leak-proof packing and easy processing. In this study, a polymer blend of 25mol% poly(acrylic acid) (PAA) and 75mol% poly(vinyl alcohol) (PVA) was optimized based on its thermal, mechanical and structural properties. The ionic liquid (IL) electrolyte, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYRTFSI) with 0.2m lithium bis(trifluoromethansulfonyl)imide (LiTFSI), was added to the polymer blend in different molar ratios. A maximum ionic conductivity of 1mScm−1 is observed at 90°C in the membrane with 70mol% IL. Cyclic voltammetry of the polymer electrolytes shows peaks corresponding to lithium stripping (+0.25V vs. Li+/Li) and deposition (−0.3V vs. Li+/Li) processes indicating the occurrence of a highly reversible redox process. The electrochemical stability window of these polymer electrolytes, as determined by linear sweep voltammetry, extends up to 5V, suggesting that these electrolytes could be suitable for batteries that use high voltage cathode materials. A lithium transference number (tLi+) of 0.4 was determined for the polymer electrolytes by using chronoamperometry and impedance measurements. Galvanostatic charge-discharge studies of the polymer electrolytes in a lithium half-cell with LiCoO2 (LCO) as cathode delivers a capacity of about 100mAhg−1 at 60°C. Coin-type half-cell with LiFePO4 (LFP) cathode and the polymer electrolyte containing 70mol% IL delivered a capacity of 172mAhg−1. Interestingly, the LFP/polymer composite cathode (LFP-C) delivers a higher capacity (215mAhg−1 at 60°C) than the pristine LiFePO4 cathode.