High ambient dc and ac conductivities in solvent-free, low-dimensional polymer electrolyte blends with lithium salts

Abstract

Measurements of dc conductivity on solvent-free, low dimensional polymer electrolyte complexes with lithium salts using Li metal electrodes and giving σ=10 −3 S cm −1 at 25–40°C are reported. The materials are blends of the amphiphilic helical polyethers poly[2,5,8,11,14-pentaoxapentadecamethylene(5-alkyloxy-1,3-phenylene)], coded CmO5 or ( I) (where m=16, 18 or equimolar 12/16 mixture is the number of carbon atoms in the n-alkyl side chains), and a polytetrahydrofuran copolymer ( II) with various Li salts. Heptamer segments of II are in equimolar proportion to the repeating units of I. In Li ∣ I: II:Li salt ∣ Li cells, dc conductivities of 1×10 −3 to 3×10 −3 S cm −1 are achieved by an apparent ‘self-tracking’ process from a low level (10 −7–10 −6 S cm −1) over 12 to 24 h. The dc results are complemented by ac impedance spectroscopy measurements with ITO electrodes that show a ‘transformation’ from the low level up to 6×10 −4 S cm −1 at 20°C after a heating excursion to 100°C. The ac data also demonstrate temperature-independent conductivity, with σ=8×10 −5 S cm −1 at −5°C. X-ray diffraction, thermal analysis and molecular dynamics modelling suggest a structure that would allow Li + to be mobile in the polyether helices of I whilst anions are mobile in the unimpeded spaces between them. Ions transfer between the pathways of I via a matrix of II when the polymers are intimately blended.