Glass transition and free volume behaviour of poly(acrylonitrile)/LiCF3SO3polymer-in-salt electrolytes compared to poly(ether urethane)/LiClO4 solidpolymer electrolytes

Measurements of the glass transition temperature (T-g) and free volume beha viour of poly(acrylonitrile) (PAN) and PAN/lithium triflate (LiTf), with va rying salt composition from 10 to 66 wt% LiTf, were made by positron annihi lation lifetime spectroscopy (PALS). Addition of salt from 10 to 45 wt% LiT f resulted in an increase in the mean free volume cavity size at room tempe rature (r.t.) as measured by the orthoPositronium (oPs) pickoff lifetime, t au(3), with little change in relative concentration of free volume sites as measured by oPs pickoff intensity, I-3. The region from 45 to 66 wt% salt displayed no variation in relative fi ee volume cavity size and concentrati on. This salt concentration range (45 wt% < [LiTf] < 66 wt%) corresponds to a region of high ionic conductivity of order 10(-5) to 10(-6) S cm(-1) at T-g as measured by PALS. A percolation phenomenon is postulated to describe conduction in this composition region. Salt addition was shown to lower th e T-g as measured by PALS; T-g was 115 degrees C for PAN and 85 degrees C f or PAN/66 wt% LiTf. The T-g and free volume behaviour of this polymer-in-sa lt electrolyte (PISE) was compared to a poly(ether urethane)/LiClO4 where t he polymer is the major component, i.e. traditional solid polymer electroly te (SPE). In contrast to the PISE, the T-g of the SPE was shown to increase with increasing salt concentration from 5.3 to 15.9 wt%. The relative free volume cavity size and concentration at r.t. were shown to decrease with i ncreasing salt concentration. Ionic conductivity in this SPE was of order 1 0-5 S cm(-1) at r.t., which is over 60 degrees C above T-g, 10(-8) S cm(-1) at 25 degrees C above T-g, and conductivity was not measurable at T-g. (C) 2000 Elsevier Science Ltd. All rights reserved.