Magnetic Alignment of Gamma (Core)–Alpha (Shell) Fe₂O₃ Nanorods in a Solid Polymer Electrolyte for Li-Ion Batteries

The temperature-dependent ionic conductivity and thermal properties are characterized for a solid polymer electrolyte of poly­(ethylene oxide) (PEO) and LiClO₄ filled with 1 wt % γ-phase core (maghemite) and α-phase shell (hematite) Fe₂O₃ nanorods. Samples are solvent-cast in the absence and presence of a 0.5 T magnetic field, dried at room temperature under vacuum for 72 h, and measured under nitrogen. Vibrating sample magnetometry indicates that the magnetic treatment aligns the nanorods to some extent in the desired orientation normal to the electrode surface. For samples with an ether oxygen to lithium ratio (EO/Li) of 10:1, the nanorods induce sample-to-sample variability in the ionic conductivity. The magnetic treatment eliminates this variability, and differential scanning calorimetry data support the observation that the magnetic treatment increases the structural homogeneity of the electrolyte. For samples with an EO/Li of 3:1, the ionic conductivity is 3 orders of magnitude larger for samples containing 5 times more of the crystal structure, (PEO)₆/LiClO₄. This result is surprising because an inverse relationship between crystallinity and conductivity is normally observed for semicrystalline, solid polymer electrolytes. When the crystal fraction is increased by a factor of 8 via the combination of nanorods and magnetic treatment, the conductivity does not continue to increase, showing that the effect does not persist beyond a critical fraction of (PEO)₆/LiClO₄. The results demonstrate that field-effect alignment of magnetic nanorods increases the crystal fraction and homogeneity of PEO/LiClO₄, but does not affect the ionic conductivity in the range of salt and nanorod concentrations investigated

Influence of Fe₂O₃ Nanofiller Shape on the Conductivity and Thermal Properties of Solid Polymer Electrolytes: Nanorods versus Nanospheres

The influence of nanofiller shape on the ionic conductivity and thermal properties of solid polymer electrolytes is investigated. Electrolytes of polyethylene oxide (PEO) and LiClO₄ filled with 1–20 wt % spherical Fe₂O₃ nanoparticles and 0.5–10 wt % Fe₂O₃ nanorods are measured at an ether oxygen to Li ratio of 10:1. Nanorods improve the ionic conductivity to a similar extent as spherical nanoparticles, except at concentrations 10–20 times lower. The maximum conductivity improvement occurs at a spherical metal oxide nanoparticle loading of 10 wt %; however, an equivalent nanorod loading decreases the conductivity below that of the unfilled electrolyte. This result suggests that the long-range morphology of the two nanocomposites differs widely, where high concentrations of nanorods will inhibit instead of enhance Li transport. The shape of the nanofiller also affects the crystallization rate and resulting crystal structure. Differential scanning calorimetry measurements show that samples containing nanorods crystallize faster than those containing spherical nanoparticles, and nanorods favor formation of the (PEO)₆:LiClO₄ crystal phase. Previous studies have shown that this channel-like structure is more conductive than the amorphous phase. If nanorods could be used to induce the formation and alignment of this conductive structure normal to the electrode surface, perhaps ionic conductivity could be further enhanced in nanofilled solid polymer electrolytes where the nanoscale structure is precisely controlled