Novel poly(ionic liquid)s reinforced with three-dimensional electrospun fiber network as electrolyte for all solid state lithium batteries

Abstract

Solid polymer electrolytes have been identified lately as the most pragmatic research area towards the development of safe and reliable lithium batteries with zero electrolyte leakage. In this work, ionene oligomers based on 1,4-Diazabicyclo [2.2.2] octane (DABCO) bearing oxyethylene spacers with varying –CH2CH2O- units and having bis (trifluoromethanesulfonyl) imide (TFSI−) counter anion were synthesized. The ionene oligomers exhibited excellent thermal stability up to 300 °C with glass transition below room temperature. Interestingly, the presence of oxyethylene linkers between the cationic moieties in the oligomer backbone significantly affected the Li+ ion coordination and ionic conductivity as explained by Raman spectroscopy and electrochemical impedance spectroscopy (EIS) studies carried out on binary mixtures of POnTFSI/LiTFSI (where ‘P’ indicates the oligomer and ‘On’ denotes the number ‘n’ of oxyethylene units in the spacer). The oxygen atoms present in the spacer backbone of the ionene can interact with the Li+ ions and prevent the formation of aggregates with the anions while the mobility of the TFSI−anions is restricted by the polar nature of the ionene backbone. Such binary mixtures were incorporated in an electrospun nanofibrous framework, to form a free-standing solid polymer electrolyte and the electrochemical performance was evaluated in a Li/LiFePO4 (LFP) battery.