Tuning the Properties of PEO/PAN-Based Electrospun Membranes for Solid-State Li- and Na-Ion Batteries

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Long-term independence from fossil and nuclear energy sources is essential to achieve the 1.5-degree target of the Paris Agreement and reduce dependence on energy imports [1]. Expanding electromobility is crucial in the transport sector to reduce CO2 emissions. Lithium-ion batteries (LIBs) are the key technology for the mobility transition and storage of renewable energy from wind, water, and sun. Thermal, electrical, or mechanical mishandling of lithium-ion batteries can cause thermal runaways and battery failure. Replacing the liquid electrolyte with a solid one in a so-called all-solid-state battery system can minimize the risk of these LIBs malfunctions [2]. Solid polymer electrolytes, standing out for their mechanical flexibility and electrochemical stability, are among the most promising candidates for replacing liquid-based electrolytes in LIBs [3]. Here, we present a solid polymer electrolyte based on a polyacrylonitrile (PAN) and polyethylene oxide (PEO) blend, whose mechanical flexibility and suppressed crystallinity are ensured by the production technique electrospinning.Adding the structure-giving polymer PAN to the ion-conducting polymer PEO leads to a free-standing membrane with enhanced thermal stability up to 100 °C, above the melting point of PEO, which is checked using scanning electron microscopy. The arising beneficial phase-separated fiber structure and its resulting properties, such as morphology and conductivity, can be tuned and optimized by the production parameters of electrospinning. If the composition of polymer to plasticizer and conducting salt is kept constant, low humidity during the electrospinning process and a short period between the production of the polymer electrolyte and its drying in vacuo, as well as an enhanced drying procedure, lower the porosity and increase the ionic conductivity by up to two orders of magnitude to 0.4 mScm-1 at 328 K. We ensured the capability of reversible lithium transport through the membrane in a symmetric Li-metal|polymer-electrolye|Li-metal cell. Furthermore, we compared our lithium-based system with a sodium system, following a beyond-lithium approach, both with the bis(trifluoromethanesulfonyl)imide-anion. All polymer membranes’ crystallinity was analyzed by X-ray diffraction, and their conductivity and activation energy were determined by impedance spectroscopy and using the Arrhenius equation. Swapping lithium with more abundant sodium leads to a decrease in conductivity of one order of magnitude. The elastic, free-standing membranes with a dense fiber structure show the great potential of electrospun solid polymer electrolytes in all-solid-state batteries.