Ionic conductivity polyether/salt hybrids

Ohno, H., Nakai, Y., and Ito, K., Ionic conductivity of molten salts formed by polyether/salt hybrids, Chem. Lett., 15,1998. 

Ambient temperature molten salt can be obtained by several methods. One effective way to obtain a room-temperature molten salt is by the introduction of polyether chains to ions. The term polyether/salt hybrid is used in this chapter as a common name for polyether oligomers having anionic or cationic charge(s) on the chain (Figure 22.1). Polyethers, such as poly-(ethylene oxide) (PEO), are known as representative ion conductive polymers [1]. Polyether/salt hybrids have been studied as a kind of room-temperature molten salt apart from the development of onium-type ionic liquids [2]. The preparation of ionic liquids consisting of metal ions has been one of the important goals in this research field. Polyether/salt hybrid derivatives give one such solution for this task. 

Although a number of salts having organic chains have been synthesized to date, only a few salts were obtained as liquid or glass. In the case of the polyether/salt hybrid, the crystalline phase of both the salt and polyether disappear under appropriate conditions (Figure 22.2). The Polyether moiety in the polyether/salt hybrid is assumed to inhibit the crystallization of the salts by its flexible segmental motion. Since polyethers help dissociate the bound salt through their ion-dipole interactions, the polyether/salt hybrid inherently shows ionic conductivity. 

On the other hand, we have reported the preparation of gel-type polymer electrolytes using polyether/salt hybrid oligomers [12]. A network-type PPO film was swollen with the PPO/triiluoromethylsulfonamide Uthium salt hybrid. The obtained gel containing 50 wt% of PPO/salt hybrid showed a low Tg of —62°C. The PPO/salt hybrid behaved as not only an added salt but also a plasticizer. The ionic conductivity was over 10 S cm at room temperature. Furthermore this gel electrolyte showed a favorable lithium-ion transference number of 0.7-0.8. 

---