Study of Diffusion of Lithium Salt in Block Copolymer

In this study, the diffusion coefficient of lithium salt through a polystyrene-poly(ethylene oxide) block copolymer (SEO) was investigated using time-resolved Fourier Transform infrared - attenuated total reflectance (FTIR-ATR) spectroscopy. FTIR-ATR directly measures change of concentration in response to a concentration gradient, which is relevant for battery operation. FTIR-ATR is simpler and faster than electrochemical measurements, such as restricted diffusion, that require calibration and assumption about how cell potential relates to concentration gradient. Since the measurements are made in the absence of other driving forces, such as applied potential, the diffusion coefficient is independent of ionic conductivity and transference number. We studied the effect of salt concentration on the diffusion coefficient above the melting temperature of the electrolyte. Our results indicate that the diffusion coefficient is concentration-dependent, but not monotonic.