Salt-Induced Nanoparticle Aggregation Decouples Mechanical Reinforcement and Ionic Transport in Polymer Nanocomposite Electrolytes

We report a salt-responsive strategy for mechanically reinforcing polymer nanocomposite (PNC) electrolytes while retaining ionic conductivity (σdc) of neat polymers. In poly(ethylene glycol) (PEG) electrolytes containing negatively charged silica nanoparticles, dissociated Li+ or Na+ cations screen the silica surfaces and induce particle aggregation. Polymer-ion-silica interactions convert the resulting aggregates into stress-bearing structures, increasing the shear modulus by up to 106-fold depending on PEG molecular weight and particle size. In contrast, σdc remains comparatively insensitive to silica incorporation. Temperature-dependent σdc is instead strongly influenced by PEG-salt crystallization and segmental relaxation, indicating strong coupling to polymer dynamics. Vogel–Tammann–Fulcher analysis yields pseudo-activation energies of approximately 10–11 kJ/mol, while the Vogel temperature varies more strongly with PEG molecular weight and cation chemistry than with silica incorporation. These results identify salt-induced particle organization as a practical route to decouple mechanical reinforcement from ionic transport in PNC electrolytes.