Effect of crown ether on the ionic conductivity of the poly(ethylene oxide) lithium salt electrolyte

The addition of small amounts of crown ethers was observed to enhance the conductivity of the system PEO(20)LiCF(3)SO(3) (PEO: poly(ethylene oxide)). Optimum conductivity enhancement was observed for the system PEO(20)LiCF(3)SO(3)(crown ether)(0.003) when 12-crown-4 and benzo-12-crown-4 were used and for the system PEO(20)LiCF(3)SO(3)(crown ether)(0.03) containing the corresponding azacrown ether (cyclen) and thiacrown ether. Maximum conductivity enhancement was observed for PEO(20)LiCF(3)SO(3) (cyclen)(0.03). Over the temperature range studied, the conductivity of this system was 1 - 1.5 orders of magnitude higher than for the system PEO(20)LiCF(3)SO(3). The Li+ diffusion coefficient for the system PEO(20)LiCF(3)SO(3)(cyclen)(0.03) was determined to be 1.5 . 10(-9) cm(2) . s(-1) at 90 degrees C. This is lower than the values obtained by other workers in the absence of cyclen using pulsed field gradient NMR techniques. The presence of cyclen thus appeared to have the effect of suppressing the Li+ motion. The presence of small quantities of 18-crown-6 in PEO(8)Ca(CF3SO3)(2) electrolytes was also associated with a conductivity enhancement. This showed that this effect was not limited to systems containing monovalent cations.