Use of Al2O3/Nafion Membranes for Room-Temperature Sodium-Sulfur (Na-S) Batteries
Authors: Cengiz Elif Ceylan , Erdol Zeynep , Aslan Ayse , Ata Ali , Demir-Cakan Rezan 
Published: February 2020

In recent years, Lithium-Sulfur (Li-S) batteries have attracted much attention due to their high theoretical capacity (1675 mAh/g), environmental benignancy, abundancy and cost-effectivity of sulfur. However, due to the increase in the price of lithium, an alternative material to be used as anode is highly needed. Sodium is a promising candidate because of its natural abundancy and low cost. Thus, conventional Sodium-sulfur (Na-S) batteries were already demonstrated which operates at high temperatures (>300 °C) bringing safety problems because of the molten state of electrodes at high temperature range. Park et al recently showed that Na-S batteries could be used at room-temperatures1. As expected, room temperature Na-S batteries have the same problems as of Li-S batteries; insulating nature of sulfur, dissolution of polysulfide species formed upon discharging of the battery, shuttle effect of these polysulfide species, huge volume change during the formation of the end of discharge product, Na2S, and dendrite formation on sodium anode. These issues result in rapid capacity fading of the battery and limit their usage in practice. To mitigate these drawbacks, encapsulating sulfur into meso-/micro-porous structures were suggested2-4. However, the leakage of polysulfide species from the porous matrixes is inevitable, thus recent studies of the room-temperature Na-S batteries mostly focus on the development of protective barrier between anode and cathode electrodes to prevent polysulfide migration 5-11. To do so, ß-alumina5,6 was firstly employed as polysulfide barrier followed by nafion dispersion7, carbon nanotubes or carbon fibers 8,9,10,11.
Herein, in order to prevent the poisoning of sodium anode from the dissolved polysulfides, we developed Al2O3/Nafion membranes12. Nafion has an ability of cation selectivity and anion nonpermeability which play perfect roles on preventing migration of polysulfides towards anode side. On the other hand, Al2O3, is an electrochemically inactive material, has porous structure which provides lots of channels to pass Na+ ions and it can absorb polysulfides13. By combining their constructive properties, enhancement of the battery performance was aimed. Within this context, porous separator were coated by Nafion solutions together with different amount of Al2O3nanoparticles by using a serigraphy (silk screen) printing machine. Nice uniform coating of the protective barriers were obtained which were confirmed by microstructural analysis. Electrochemical cycling performances of those membranes resulted better capacity retention and stability in comparison with the bare separator. Moreover, shuttle effect of the Na-S cells were highly eliminated resulting higher Coulombic efficiencies.

ACKNOWLEDGEMENTS

This work is partially supported by National Young Researchers Career Development Grant of TUBITAK (contract no: 213M374).

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