Correlations between free volume structure and ionic conductivity of poly (ethylene oxide) and dendritic fibrous nanosilica composite based electrolyte: An investigation using positron annihilation and broadband dielectric spectroscopy

Passive and active fillers loaded poly(ethylene oxide), PEO, based solid state polymer electrolytes (SPEs) are considered promising alternatives for currently used flammable liquid electrolytes in lithium metal batteries. The enhancement in ionic conductivity of PEO based composite electrolytes is attributed to the additional ion-conduction pathways available at the interphase region. Considering this aspect, in the present study, we have prepared dendritic fibrous nanosilica (DFNS) loaded PEO based polymer composite (PEO-DNFS) and electrolytes (PEO-Li, PEO-Li-DFNS) having (EO:Li = 20:1). DFNS has been chosen as filler due to its unique dendritic fibrous structure which is expected to create large interphase region in the composites. These composites and electrolytes were characterized using various techniques viz. powder X-ray diffraction (PXRD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC) for determining the modifications in chemical bonding and thermal properties. Positron annihilation lifetime spectroscopy (PALS) and broadband dielectric spectroscopy (BDS) were employed to determine the modifications in the free volume structure/chain packing of PEO and ion conduction mechanism, respectively. The relative free volume was observed to increase with DFNS loading due to modifications in the chain packing. The higher number of free volume holes provide additional pathways for ionic diffusion leading to an enhancement in ionic conductivity. The role of relative free volume in ionic conductivity enhancement is further established through the strong coupling observed between ionic conduction and segmental relaxations of PEO electrolytes investigated using BDS.