Glass transition and free volume behaviour of poly(acrylonitrile)/LiCF3SO3 polymer-in-salt electrolytes compared to poly(ether urethane)/LiClO4 solid polymer electrolytes

Measurements of the glass transition temperature (<i>T</i>_g) and free volume behaviour of poly(acrylonitrile) (PAN) and PAN/lithium triflate (LiTf), with varying salt composition from 10 to 66 wt% LiTf, were made by positron annihilation lifetime spectroscopy (PALS). Addition of salt from 10 to 45 wt% LiTf resulted in an increase in the mean free volume cavity size at room temperature (r.t.) as measured by the orthoPositronium (oPs) pickoff lifetime, <i>τ</i>₃, with little change in relative concentration of free volume sites as measured by oPs pickoff intensity, <i>I</i>₃. The region from 45 to 66 wt% salt displayed no variation in relative free volume cavity size and concentration. 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 <i>T</i>_g as measured by PALS. A percolation phenomenon is postulated to describe conduction in this composition region. Salt addition was shown to lower the <i>T</i>_g as measured by PALS; <i>T</i>_g was 115°C for PAN and 85°C for PAN/66 wt% LiTf. The <i>T</i>_g and free volume behaviour of this polymer-in-salt electrolyte (PISE) was compared to a poly(ether urethane)/LiClO₄ where the polymer is the major component, i.e. traditional solid polymer electrolyte (SPE). In contrast to the PISE, the <i>T</i>_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 increasing salt concentration. Ionic conductivity in this SPE was of order 10⁻⁵ S cm⁻¹ at r.t., which is over 60°C above <i>T</i>_g, 10⁻⁸ S cm⁻¹ at 25°C above <i>T</i>_g, and conductivity was not measurable at <i>T</i>_g.<br />