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

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