Concentration dependent salt-polymer-dynamic bond interactions dictate non-monotonic conductivity and viscoelasticity in vitrimer electrolytes

Vitrimer electrolytes (VEs), polymer networks with dynamic covalent bond crosslinks and added salt, were investigated with ethylene oxide (EO) chains and dynamic imine crosslinks to determine how LiTFSI salt concentration modulate viscoelasticity and ionic conductivity. Oscillatory shear rheology was used to measure crossover times and rubber plateau moduli, and impedance spectroscopy measured the ionic conductivity. Variations in salt concentration at fixed crosslink density resulted in doubly non-monotonic trends in modulus and relaxation times which increase, decrease, and then increase again with added salt. Ionic conductivity shows the inverse of these trends, attributed to the complex interplay between segmental dynamics, catalyzed bond exchange and ion aggregation. Wide-angle X-ray scattering revealed ionic aggregate peaks which support the picture of fewer free Li ions leading to lower conductivity and longer relaxation times. In contrast, the glass transition temperature (Tg) continuously increased with salt addition. This work furthers the understanding of how dynamic bond exchange and salt content contribute to the viscoelasticity and conductivity which are important for battery electrolytes.