Dielectric spectroscopy and electrochemical modulation of polymorphic Ag+ migration in PVP-coated AgI nanoparticles

Polyvinylpyrrolidone (PVP)-coated silver iodide nanoparticles (12.3–15.6 nm) were synthesized to investigate Ag+ transport relevant to halide solid electrolytes for all-solid-state batteries. Broadband dielectric spectroscopy and electrochemical modulation reveal a strongly suppressed α → β/γ-phase transition (hysteresis ≈ 110 °C) due to surface energy and PVP coordination. In the β/γ regime, two relaxations correspond to interstitial Ag+ hopping and electrode polarization. In the α-phase, two distinct processes emerge: a low-frequency fragile relaxation (Pα1) following the Vogel–Fulcher–Tammann law, attributed to glassy surface dynamics, and a high-frequency Arrhenius process (Pα2) representing local hopping within the crystalline core. The activation energy of Pα2 increases with decreasing size but decreases with electrochemically enriched Ag+ content that enhances α-phase conductivity. These results establish a spatially layered model of Ag+ migration, under nanoscale confinement and suggest a viable strategy for designing high-performance halide solid-state electrolytes.