Sulfonium cation based ionic liquid incorporated polymer electrolyte for lithium ion battery

Abstract

Polymer electrolyte (PE) composed of poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-co-HFP) and triethylsulfonium bis(trifluoromethylsulfonyl)imide (SEt3TFSI) ionic liquid (IL) had been evaluated in lithium ion battery for the first time in order to improve its performance and cycle life. X-ray diffraction analysis (XRD) reveals that incorporation of the IL (20 and 25 wt%) into the polymer matrix results in the change of state of the material from semi-crystalline to amorphous nature. Thermo-gravimetric and differential thermal analysis (TG/DTA) of the PE sample with 25 wt% of the IL shows high thermal stability. The nature of functional groups present in the PE was investigated by Raman spectrum. Surface morphological characteristics indicate that increase in the loading of the IL into the polymer matrix leads to maximum number of pores with good interconnected network. Polymer/IL electrolyte (wt. ratio of 75:25) having a maximum ionic conductivity of 6.93 × 10−5 S/cm at 303 K with an activation energy of 0.23 eV shows excellent electrochemical potential stability of 4.4 V vs Li, as revealed by cyclic voltammetry (CV). Charge–discharge characteristics of the coin cell containing the above optimized ratio of PE with LiFePO4 cathode and Li anode shows a discharge capacity of 133 mAh/g, which is stable up to ten cycles.