Coupled Ion-Conduction Mechanism for Enhanced Ionic Conductivity in Mg2B2O5 Nanowires Enabled Poly(ethylene oxide)-Based Quasi Solid State Electrolytes

Abstract

Achieving room temperature high ionic conductivity of polymer solid state electrolytes equivalent to organic liquid electrolytes currently used in lithium ion batteries is a challenging task. The glass transition, crystallinity, free volume structure, salt dissociation, and ionic diffusion affect the ionic conductivity of polymer-based solid state electrolytes. In the present study, we report high-ionic conductivity (4.1 × 10–4 Scm–1) at room temperature for Mg2B2O5 nanowire-enabled solid state electrolytes of fully amorphous poly(ethylene oxide) (PEO). The Mg2B2O5 nanowires are synthesized using a single-step solid state reaction from inexpensive materials. The detailed investigation of the ion conduction mechanism using broadband dielectric spectroscopy confirms that the ionic conduction and segmental relaxations are a coupled phenomenon in these electrolytes, and segmental relaxation-mediated ionic diffusion is the primary ionic conduction mechanism. The enhancement in ionic conductivity on loading of Mg2B2O5 nanowires is attributed to the enhancement in the dissociated ionic concentration, ionic diffusion, and faster segmental relaxation due to interactions between the polymer, Li salt, and the nanowires. The smaller free volume size and enhanced free volume density of PEO in the electrolytes determined using positron annihilation lifetime spectroscopy and reduction in glass transition temperature determined from differential scanning calorimetry further confirm the role of modified chain packing and segmental relaxations in enhancement of ionic conductivity.