Ionic Conductivity of a Solid Polymer Electrolyte Confined in Nanopores
Chien‐Hua Tu, Lothar Veith, Hans‐Jürgen Butt, George Floudas
Abstract
Using in situ nanodielectric spectroscopy, we investigate if and how the ionic conductivity of the archetypal polymer electrolyte poly(ethylene oxide)/lithium bis(trifluoromethane sulfone)imide (PEO/LiTFSI) is affected during and after imbibition in nanopores. We identify two distinct stages of imbibition. In the first stage, up to the complete pore filling, ion conductivity increased above the bulk value. In the second stage, after imbibition, ion conductivity decreased following a stretched exponential dependence. Time-of-flight secondary ion mass spectroscopy revealed a uniform distribution of Li+ and TFSI– ions in the templates. The timescale of conductivity reduction was very long. For a given molar mass, the characteristic times strongly depend on the ratio 2Rg/D where Rg is the radius of gyration and D is the pore diameter. For a given pore diameter, the characteristic times were some 9 orders of magnitude slower than the PEO terminal relaxation and more than 11 orders of magnitude slower than the segmental relaxation. The reduced ionic conductivity is explained by the adsorption of polymer segments on the pore walls. Polymer adsorption inevitably affects ion dynamics by (i) increasing the glass temperature and (ii) reducing the number of mobile ions. The molar mass dependence of the characteristic adsorption times (τads ≈ N2) was in agreement with a scaling theory proposed by de Gennes. Possible consequences of the current study to energy conversion are discussed.