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

Measurements of de conductivity on solvent-free, low dimensional polymer electrolyte complexes with lithium salts using Li metal electrodes and giving sigma =10(-3) S cm(-1) at 25-40 degreesC 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, de conductivities of 1 x 10(-3) to 3 x 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 24h. The de results are complemented by ac impedance spectroscopy measurements with ITO electrodes that show a 'transformation' from the low level up to 6 x 10(-4) S cm(-1) at 20 degreesC after a heating excursion to 100 degreesC. The ac data also demonstrate temperature-independent conductivity, with sigma = 8 x 10(-5) S cm(-1) at - 5 degreesC. 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. (C) 2001 Elsevier Science Ltd. All rights reserved.