Evidence for ion pairs and/or triple ions from transport measurements in mixed-alkali polyether electrolytes

Abstract

This work concerns amorphous poly(ethylene oxide) (PEO) rubbery electrolytes with new lithium salts, LiN(CF 3CO) 2 and Li[CF 3SO 2N(CH 2) 3OCH 3] (LiMPSA), which contain carbonyl or ether groups in their anions. Although these salts are markedly less conductive than LiN(CF 3SO 2) 2 or LiClO 4, one of them (LiMPSA) yields a large fraction of a species that promotes lithium transport through back diffusion under polarization in Li/PEO–LiX/Li symmetrical cells. Polarization experiments carried out on similar cells filled with PEO–LiX–KX, mixed-alkali electrolytes show that this species is a LiX − 2 complex of lower mobility than Li + in PEO. In turn, these experiments reveal that LiN(CF 3CO) 2 yields a large fraction of LiX ion pairs more mobile than Li + in PEO. These features, which are founded on theoretical models for polarization of PEO–LiX–KX electrolytes between lithium electrodes, agree with other properties (T g elevation and conductivity) of the present systems.