Salt Diffusion Coefficients in Block Copolymer Electrolytes

Abstract

The salt diffusion coefficient in a series of nanostructured block copolymer electrolytes was measured in a symmetric lithium/polymer electrolyte/lithium cell using the restricted diffusion technique. The decay of the open-circuit potential as measured by this technique was analyzed by a Laplace inversion algorithm to give the distribution of relaxation processes characteristic of the electrolytes. The distribution function was characterized by two parameters, an average diffusion coefficient, Davg, and a polydispersity index, PDIdiffusion, which is a measure of the width of the distribution. We compare these parameters obtained from a series of nearly symmetric poly(styrene)-block-poly(ethylene oxide) (SEO) block copolymer electrolytes containing lithium bis(trifluoromethanesulfone)imide salt (LiTFSI) with those obtained from a homogeneous poly(ethylene oxide) (PEO)/LiTFSI mixture. Davg of the SEO/LiTFSI mixtures increases with increasing molecular weight of the PEO block, MPEO, and reaches a plateau of 2/3DPEO when MPEO exceeds 50 kg/mol (DPEO is the average salt diffusion coefficient in PEO homopolymer). The PDIdiffusion values obtained for SEO copolymers are significantly higher than those obtained in PEO homopolymer.