Size-controlled stabilization of the superionic phase to room temperature in polymer-coated AgI nanoparticles

Solid-state ionic conductors are actively studied for their large application potential in batteries and sensors. From the view of future nanodevices, nanoscaled ionic conductors are attracting much interest. Silver iodide (AgI) is a well-known ionic conductor for which the high-temperature alpha-phase shows a superionic conductivity greater than 1 Omega(-1) cm(-1). Below 147 degrees C, alpha-AgI undergoes a phase transition into the poorly conducting beta- and gamma-polymorphs, thereby limiting its applications. Here, we report the facile synthesis of variable-size AgI nanoparticles coated with poly-N-vinyl-2-pyrrolidone (PVP) and the controllable tuning of the alpha- to beta-/gamma-phase transition temperature (Tc). Tc shifts considerably to lower temperatures with decreasing nanoparticle size, leading to a progressively enlarged thermal hysteresis. Specifically, when the size approaches 10-11 nm, the alpha-phase survives down to 30 degrees C--the lowest temperature for any AgI family material. We attribute the suppression of the phase transition not only to the increase of the surface energy, but also to the presence of defects and the accompanying charge imbalance induced by PVP. Moreover, the conductivity of as-prepared 11 nm beta-/gamma-AgI nanoparticles at 24 degrees C is approximately 1.5 x 10(-2) Omega(-1) cm(-1)--the highest ionic conductivity for a binary solid at room temperature. The stabilized superionic phase and the remarkable transport properties at a practical temperature reported here suggest promising applications in silver-ion-based electrochemical devices.