Magnetic Alignment of Gamma (core)/Alpha (shell) Fe2O3 Nanorods in a Solid Polymer Electrolyte

One method to improve the ionic conductivity of solid polymer electrolytes (SPEs) is to add metal oxide nanofillers. [1,2]  This modification avoids the use of plasticizers and retains the solid state property of the electrolyte; however, the conductivity remains low (~10^-6 S/cm) at room temperature.[1,2]  Because the interface between the nanofiller surface and the SPE is more conductive than the bulk, a nanofiller with a large aspect ratio aligned normal to the electrode surface may further improve conductivity.  This arrangement gives the ions the fastest and most direct path between electrodes.  In this work, we add gamma (core) –alpha (shell) Fe₂O₃ nanorods to a SPE of polyethylene oxide (PEO)₃:LiClO₄, align the rods in a 0.5 T magnetic field and evaluate the relationship between polymer structure and ionic conductivity. The gamma core permits alignment of the rods normal to the electrode surface, while the alpha shell promotes ion conduction at the filler/SPE interface. 

Vibrating sample magnetometry indicates nanorod alignment along the long axis normal to the electrode surface (Fig. 1).  Although the fraction of aligned rods cannot be quantified by this measurement, it confirms that some degree of alignment is achieved compared to the untreated sample where rods are arranged isotropically.

Both the presence of the nanorods and the magnetic treatment strongly affect the structural properties of the electrolyte. In the absence of a magnetic field, the nanorods increase the fraction of (PEO)₆:LiClO₄by a factor of 2, and the magnetic treatment in combination with the rods increases the fraction by a factor of 9 (Fig. 2).

Because the ionic conductivity is coupled to polymer mobility, crystallinity typically decreases conductivity. However, impedance measurements show that the ionic conductivity remains unchanged after magnetic treatment of the nanofilled samples, despite the large crystal fractions that are present.  Moreover, when the fraction of these crystals decrease by a factor of 4 – 5 after the first heat (Table 1), the conductivity decreases by more than three orders of magnitude at room temperature (Fig.3).

Gadjourova et al., showed that the (PEO)₆ crystal structure conducts ions better than the amorphous equivalent,[3] but the structure must be aligned normal to the electrode surface or transport will be blocked.[4]  Our results show that when large crystal fractions are present (H_f > 20 J/g for (PEO)₆), the conductivity is several orders of magnitude larger than for samples where H_f< 20 J/g. However, increasing the crystal fraction by up to a factor of 9 via nanorods and magnetic treatment does not further improve the conductivity. 

This study challenges the idea that crystal structure degrades the performance of SPEs, providing direct evidence that the conductivity is significantly higher in samples with larger fractions of (PEO)₆.  This study also shows that once a critical fraction of (PEO)₆ forms, additional (PEO)₆has no effect on the conductivity in the lithium concentration range investigated.

Acknowledgements:This material is based upon work supported by the U.S. Army TARDEC under Contract No. W56HZV-08-C-0236, through a subcontract with Mississippi State University, and was performed for the Simulation Based Reliability and Safety (SimBRS) research program. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. Army TARDEC. The National Science Foundation supported the VSM measurements under Grant No. ECCS-0923243.

[1] W. Krawiec et al., J. Power Sources 54, 310–315 (1995).

[2] F. Croce, et al., Nature 394, 456 (1998).

[3] Z. Gadjourova, et al., Nature, 412, 520 (2001)

[4] E. Staunton et al., Faraday Discuss. 2007, 134, 143–156.