Fluorescence spectroscopy for local viscosity measurements in polyacrylonitrile (pan)-based polymer gel electrolytes

Abstract

A fluorescence probe sensitive to local viscosity was used to study conduction property of the polymer gel electrolytes, polyacrylonitrile (PAN)/ propylene carbonate (PC) + ethylene carbonate (EC) in 1 : 3 volume ratio/LiClO4. The conductivity of the gel electrolytes was in the range of 10-2-10-3 S cm-1, which is much higher than values for the conventional polymer electrolytes such as PEO/Li salts. The conductivity profile of the polymer gel electrolytes, according to the LiClO4 concentration ([LiClO4]/[PC + EC] = 0.05–0.11), looked similar to that of the liquid electrolytes of the same composition. Also, the Arrhenius activation energies of the gel electrolytes (10–15 kJ mol−1) were rather closer to those (5 KJ Mol−1) of the liquid electrolytes than those for the unplasticized polymer electrolytes (a few tens to hundreds kj mol−1). However, the observed local viscosities of the gel electrolytes were higher than those of the liquid electrolytes by a factor of 20, while the conductivity was smaller only by a factor of 2–9, suggesting that the inverse relationship between local viscosity and conductivity does not hold in this system. FTIR spectra of the gel electrolytes showed that the −CN groups in PAN interact with Li+ ions, causing an increase in the local viscosity, probably due to a contraction of the local free volumes. In short, the conduction property of the gel electrolytes looked similar to that of the liquid electrolytes in several ways. However, the Li+ ion-polar group interactions in the gel electrolytes render the inverse relationship invalid and consequently the observed conductivity and activation energy values are somewhat different from those for liquid electrolytes of the same composition.