Interplay of surface chemistry and ion content in nanoparticle-filled solid polymer electrolytes

We investigate the influence of nanofiller surface chemistry and ion content on the conductivity of nanofilled PEO + LiClO₄ solid polymer electrolytes (SPEs) using dielectric spectroscopy, differential scanning calorimetry (DSC), FESEM, and quasi-elastic neutron scattering (QENS). We consider the concentration series EO/Li = 8:1, 10:1 (eutectic composition), 14:1, with both acidic α-Al₂O₃ and neutral γ-Al₂O₃ nanoparticles. The acidic filler is more effective at increasing conductivity at the non- eutectic compositions. In contrast, the two surface chemistries provide comparable increases at the eutectic composition. This composition maximizes the influence of nanofillers, regardless of surface chemistry. We find no significant changes in crystallinity, glass transition temperature, nanoparticle dispersion, or PEO segmental dynamics as a function of surface chemistry. In the absence of salt, acidic particles slow PEO dynamics more than neutral particles, suggesting that the PEO chains and the acidic surface sites share a favorable interaction. A rotation consistent with the crystal structure PEO₆:LiClO₄ is observed up to 50 °C (8:1) and up to 75 °C (eutectic composition). PEO₆ rotation is hindered by nanoparticles, with the degree of restriction as a function of surface chemistry. We propose a mechanism where nanofillers stabilize the PEO₆ structure at their surface, and differences in surface chemistry and composition dictate the extent of stabilized PEO₆ and whether Li movement in this structure is facile or restricted.