The Effect of Nanoparticle Shape on the Electrical and Thermal Properties of Solid Polymer Electrolytes

The influence of nanoparticle shape (aspect ratio) on the ionic conductivity and thermal properties of solid polymer electrolytes (SPEs) was investigated in this study. Electrolytes of polyethylene oxide (PEO) and LiClO4 filled with TiO2 ellipse and TiO2 nanorods (NR) in the concentration range of 1-10 wt.% and 1-2.5 wt.% respectively, were measured at an ether oxygen to Li ratio of 10:1. When considered with previously published results for Fe2O3 nanoparticle (NP) and NR with similar surface chemistry, the results show that the filler concentration that gives the maximum conductivity boost scales inversely with aspect ratio (AR): 10 wt.% for NP (AR = 1), 5 wt.% for ellipses (AR ≈ 4.4), and 1-2 wt.% for NRs (AR ≈ 6.6). Because the results are similar for two different types of metal oxides, the relationship between the conductivity improvement and filler aspect ratio may be a universal property, and not unique to one type of filler chemistry. While the filler concentration for maximum conductivity improvement is correlated to the nanoparticle shape, the magnitude of the improvement is correlated to the surface-to volume ( S V ) ratio. Specifically, the larger the S V , the larger the conductivity boost, suggesting that ion transport is faster at the interface between the electrolyte and the filler compared to the bulk electrolyte. Thermal properties, measured by differential scanning calorimetry (DSC), show a decoupling between increased crystallinity and decreased conductivity. Specifically, the conductivity of the SPE filled with 2 wt.% TiO2 NRs is one order of magnitude higher at room temperature than the sample filled with 5 wt.% elliptical TiO2, even though the TiO2 NR sample is semi-crystalline and the elliptical sample is amorphous. It is possible that the high aspect ratio NRs induce the formation of the conductive (PEO)6:LiClO4 crystalline phase.