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

Abstract

Vitrimer electrolytes (VEs) are an emerging class of solid polymer electrolytes that couple dynamic covalent bond exchange with high ionic conductivity to mitigate lithium dendrite formation while enabling self-healing and recyclability. Here, VEs with ethylene oxide (EO) chains and dynamic imine crosslinks were synthesized to systematically determine how crosslink density and LiTFSI salt concentration dictate viscoelasticity and ionic conductivity. The VEs exhibit a non-monotonic dependence of relaxation times and ionic conductivity on crosslink density at fixed salt. Variations in salt concentration at fixed crosslink density resulted in doubly non-monotonic trends with three distinct regimes in modulus and relaxation times which increase, decrease, and then increase again with added salt (r = 0.1 to 0.5). Ionic conductivity showed 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, longer relaxation times, and higher modulus. In contrast, the glass transition temperature (T_g) continuously increased with salt. All VEs were amorphous with a plateau modulus above 10⁵ Pa and ionic conductivities exceeding that of linear PEO by an order of magnitude. This work establishes key design principles linking dynamic bond exchange and salt content to the viscoelasticity and conductivity for next-generation vitrimer electrolytes.