Effect of phosphonium based Ionic liquid on structural, electrochemical and thermal behaviour of polymer poly(ethylene oxide) containing salt lithium bis(trifluoromethylsulfonyl)imide Himani Gupta a , Shalu a , Liton Balo a , Varun Kumar Singh a , Sujeet Kumar Chaurasia b and Rajendra Kumar Singh* a a. Ionic liquid and solid state ionic lab, Department of physics, Banaras Hindu University, Varansi-221005, India, Email: rksingh_17@rediffmail.com and rajendrasingh.bhu@gmail.com, Fax: +915422368390, Tel. No. +915422307308 b. Department of Physics and Astrophysics, University of Delhi, Delhi-110007, India. Supporting information: FTIR analysis : FTIR spectroscopy has been used to investigate the interaction of IL cation and salt (LiTFSI) with polymer backbone and to observe the conformational change in PEO polymer matrix due to incorporation of IL. The FTIR spectra of PEO, IL and PEO + 20wt% LiTFSI + Xwt% IL (for X = 0, 10) are given in region 3200-400 cm -1 as shown in Fig.S1. For present discussion, two regions (1000-1200 cm -1 and 2800-3000 cm -1 ) of FTIR spectra are of particular interest. As we found some significant changes only in these regions. The expanded region 1000-1200 cm -1 is given in Fig.S2. In this region, the peak related to C-O-C stretching of PEO was found at 1060 cm -1 and 1146 cm -1 . [R(1)-R(2)] When 20wt% salt LiTFSI was mixed in polymer (PEO) to obtain polymer electrolyte film (PEO + 20wt% LiTFSI), the interaction of LiTFSI salt cation (Li + ) occurred with five ether oxygen of PEO chain as depicted in Fig.S3. [R(3)] Li + moves in polymer matrix by two processes (1) by hopping mechanism, (2) by diffusion mechanism. In hopping mechanism, Li + interacts with five ether oxygen of PEO and moves in polymer matrix by hopping at ether oxygen as shown in Fig. Electronic Supplementary Material (ESI) for RSC Advances. This journal is © The Royal Society of Chemistry 2016