Electrospun Sodium Conducting PEO Membranes
Authors: Weindl Daniel Fabian , Nilges Tom 
Published: December 2024

The transition of the energy grid to renewable energy is a significant challenge to overcome global warming and the climate change it induces. Vast amounts of energy need to be stored, particularly for decentralized energy sources such as wind and solar. The rise in the popularity of private solar energy infrastructure has especially come with increasing demand for energy storage in battery systems [1]. To face the challenge of both grid-scale and household storage, developing a cheap, sustainable, and safe option is paramount. A switch from Lithium to Sodium could reduce the resource strain such high demands in capacity pose. Changing from a liquid to a solid electrolyte both facilitates the use of high energy density cathodes and anodes as well as a dramatic increase in safety. In addition, it lowers the potential of short circuits [2]. One favorable contender in this all-solid-state battery system is a polyethylene oxide (PEO) based solid polymer electrolyte. Being a non-toxic, cheap system, the main detriments are its low conductivity and limited stability [3].

In this work, we use the technique of electrospinning to create fully flexible, free-standing polymer electrolyte membranes consisting of PEO and Sodium trifluoromethanesulfonimide (NaTFSI), as well as successfully incorporating plasticizer Succinonitrile (SN) and ceramic additive NaAlO2. In this study we show by using potentiostatic electrochemical impedance spectroscopy (PEIS) and X-ray diffraction (XRD) that highly amorphous membranes can be successfully prepared and utilized as conductive separators. Electospun systems with and without plasticizer were fabricated and tested in relation to solution casted systems of the same composition. Electrospun and plasticizer free systems show promising conductivities of up to 0.01 mS/cm at room temperature and 0.26 mS/cm at 55°C. The transport processes are investigated via solid-state NMR spectroscopy. Membranes containing additional SN were produced in order to analyze the effect of the plasticizer on the conductivity. The influence of plasticizers is much lower compared to PEO-Li systems where the SN additive increases the conductivity up to two orders of magnitude [4] which is of benefit for their application in battery systems due to the low electrochemical stability of SN.

Other aspects like the morphology and electrospinning parameters were investigated to further improve the electrochemical performance. Different fiber morphologies were prepared by varying electrospinning parameters and quantified by scanning electron microscopy (SEM). These studies revealed that lowering the thickness of threads is not an important factor as long as a substantial fiber network is formed and densified before testing. Further, the parameters utilized for electrospinning were evaluated. SEM characterization reveals that parameters like a long distance between the nozzle and collector and high humidity lead to excessive fiber welding, vastly decreasing the conductivity. One important aspect for applications is the low operation cell pressure of 0.5 MPa for the conductive membranes in batteries. Adding NaAlO2 improved the mechanical stability of the membranes. Cycling in symmetrical Na|polymer electrolyte|Na cells showed reversible Na plating and stripping capability. Stable cycling occurs over 50 cycles with no visible signs of degradation

[1] IEA. World Energy Outlook 2023. https:// www.iea.org / reports/ world- energy- outlook- 2023 (accessed 24-03-21).

[2] Hwang, J.-Y.; Myung, S.-T.; Sun, Y.-K. Chem. Soc. Rev.
2017, 46, 3529–3614.

[3] Reinoso, D. M.; Frechero, M. A. Energy Storage Materials
2022, 52, 430–464.

[4] Freitag, K.M.; Kirchhain, H.; van Wüllen, L.; Nilges, T. Inorg. Chem. 2017, 56, 2100–2107.