The development of a high cycle life rechargeable lithium-sulfur battery is of substantial interest due to its high theoretical specific capacity of 1675 mAh/g and energy density of 2600 Wh/kg. These properties combined with the low cost and abundance of sulfur are attractive for practical and high performance lithium-sulfur batteries.  However, the realization of these lithium-sulfur batteries have been limited by performance issues relating to their poor cycle life and low energy efficiency. The typical lithium-sulfur cell loses 20-50% of its initial capacity within 100 cycles due to a parasitic reaction known as the polysulfide shuttle.

The polysulfide shuttle originates from the inherent solubility of the higher order polysulfides, S₈^2-, S₆^2-, and S₄^2-, at the positive electrode. These polysulfides are capable of shuttling between the positive sulfur electrode and negative lithium electrode. When a polysulfide species contacts the negative electrode, it will be reduced to a lower order polysulfide species. If reduced to another soluble species such as S₆^2- or S₄^2-, it will shuttle back to the positive electrode where it must be reoxidized, resulting in charging inefficiency. If reduced to an insoluble species such as S₂^2- or S^2-, it will precipitate, resulting in irreversible capacity loss. Therefore, inhibiting the polysulfide shuttle is necessary to improving the energy efficiency and cycle life of lithium-sulfur batteries.

We herein present a method of inhibiting the polysulfide shuttle through use of a thin barrier membrane. The thin barrier membrane is non-porous and capable of conducting lithium ions. The non-porosity of the membrane allows for physical exclusion of polysulfide species from the lithium electrode while the ionic conductivity allows for transport of lithium ions necessary for charge and discharge processes. In a cell configuration, the thin barrier membrane is sandwiched between two porous separators to maintain electronic isolation and inserted between the sulfur and lithium electrode.

We have studied the effect of the thin barrier membrane in 2032-type coin cells consisting of sulfur-carbon composite positive electrodes, lithium foil negative electrodes, and 1 M LiTFSI electrolyte. The performance of the membrane was compared to cells of similar construction without the membrane and also to cells with 0.25 M lithium nitrate, the state-of-the-art polysulfide shuttle inhibiting additive. We have analyzed the performance of all cells using galvanostatic cycling, electrochemical impedance spectroscopy, and direct shuttle current measurements.¹ We have found that the thin barrier membrane improves the energy efficiency and cycle life of lithium-sulfur cells by suppressing the polysulfide shuttle.

References:

1. D. Moy, A. Manivannan, and S. R. Narayanan, J. Electrochem. Soc., 2015, 162, A1-A7