Effect of Molecular Weight and Salt Concentration on Conductivity of Block Copolymer Electrolytes

The ionic conductivity, σ, of mixtures of nearly symmetric polystyrene-block-poly(ethylene oxide) copolymers and LiN(SO2CF3)2 (LiTFSI) salt was measured as a function of molecular weight, salt concentration, and temperature. The molecular weight of the poly(ethylene oxide) block, MPEO, was varied from 7 to 98 kg/mol. The molar ratio of lithium to ethylene oxide, r, was varied from 0.02 to 0.10. In general, σ increases with increasing MPEO for all values of r. The data can be summarized by plots of normalized conductivity, σn, versus MPEO, where σn = σ/(fϕPEOσPEO), ϕPEO is the PEO volume fraction in the copolymer, σPEO is the conductivity of PEO homopolymer, and f is a morphology-dependent factor set equal to 2/3 for our lamellar samples. The temperature-dependent conductivity data at a given salt concentration collapse onto a single curve when plotted in this format. At r = 0.085, σn values reach a plateau in the vicinity of unity in the high MPEO limit. At other values of r, σn continues to increase with MPEO within the experimental range and reaches a value of around 0.5 in the high MPEO limit.