Solid polymer electrolytes with poly(vinyl alcohol) and piperidinium based ionic liquid for Li-ion batteries

Abstract

We report the preparation of a polymer electrolyte using poly(vinyl alcohol) (PVA) as the polymer network, piperidinium cation based ionic liquid (IL) as a plasticizer and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the lithium source. Various concentrations of the LiTFSI salt were added to the IL to prepare lithium-IL solutions. Polymer membranes were prepared by adding these lithium IL solutions to PVA by solvent casting technique. The melting temperature (Tm) of PVA membrane decreases with the addition of IL confirming that the IL acts as a plasticizer leading to an increase in ionic conductivity. Tan delta (from dynamic mechanical analysis) of the polymer-IL samples show broader α relaxation peaks than pure PVA confirming the increase in segmental motion of the polymer chain. The maximum room temperature conductivity was observed for the membrane with 0.6 m PMPTFSI-LiTFSI (1.2 mS cm−1) and could be correlated to the low viscosity, large number of mobile lithium ions and mobility of lithium ions. Cyclic voltammetry of the polymer electrolytes confirms the reversible redox process (lithium stripping and deposition) and linear sweep voltammetry shows that these electrolytes are stable up to 4.7 V. Galvanostatic charge-discharge studies of the polymer electrolytes in coin cell with LiFePO4 (LFP) cathode delivered a capacity of about 180 mAh g−1 at 60 °C. The cell has an excellent capacity retention property which is observed in retaining the capacity value (170 mAh g−1) equivalent to 99% columbic efficiency after 50 cycles.