Here, two ionic liquids, N-ethoxyethyl-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide (M TFSI) and N-ethoxyethyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide (P TFSI) were synthesized and compared. Fundamental relevant properties, such as thermal and electrochemical stability, density, and ionic conductivity were analyzed to evaluate the effects caused by the presence of the ether bond in the side chain and/or in the organic cation ring. Upon lithium salt addition, two electrolytes suitable for lithium batteries applications were found. Higher conducting properties of the piperidinium-based electrolyte resulted in enhanced cycling performances when tested with LiFePO (LFP) cathode in lithium cells. When mixing the P TFSI/LiTFSI electrolyte with a tailored alkyl carbonate mixture, the cycling performance of both Li and Li-ion cells greatly improved, with prolonged cyclability delivering very stable capacity values, as high as the theoretical one in the case of Li/LFP cell configurations.

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We investigated the miscibility of 12 different ionic liquids (ILs) with poly(ethylene oxide) (PEO) under excess salt conditions, using hard and soft, acids and bases (HSAB) theory. The hardness of the component ions of a series of ILs was estimated from the energy calculation of their HOMO and LUMO. ILs composed of relatively hard acids, such as imidazolium cation, and soft bases such as bis(trifluoromethanesulfonyl)imide anion, displayed excellent miscibility with PEO regardless of mixing ratio. Good miscibility implies that the ions strongly interact with PEO chains, and accordingly the ionic conductivity decreased with increasing PEO content. On the other hand, the same ionic conductivity was found regardless of mixing ratio when ILs composed of soft acid and soft base such as N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide was mixed with PEO. These data suggest that high ionic conductivity should be supported by the micro-phase separation of polymers and ionic liquids.

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