Conventional lithium-ion (Li-ion) batteries dominate development efforts in electrical vehicles due to their high open circuit voltage, desirable power density, and long cycle life. Despite these advantages, state-of-the-art Li-ion batteries barely meet the energy requirement of EV-based power trains due low energy density (180-200 Wh/kg cell) and high cost. Lithium sulfur (Li-S) batteries are promising for EV applications because they address both these barriers. Li-S batteries exploit a redox couple described by the reaction below, which has an open circuit potential of 2.2 V with respect to Li⁺/Li:

16 Li⁺ + S₈ + 16e^-→ 8 Li₂S

Assuming full conversion, the theoretical energy density and specific capacity of Li-S batteries is 2500 Wh/kg S and 1675 mAh/g S, respectively.¹ In addition, sulfur, the key active cathode material is significantly less expensive than conventional transition metal oxide cathodes used in Li-ion batteries. However, Li-S batteries have not gained acceptance for practical applications because of their poor cycle life. This limitation originates from polysulfide diffusion from the cathode through the electrolyte to the anode which causes parasitic side-reactions and limits capacity and cycle retention.

Our work aims to overcome the challenges of polysulfide diffusion by incorporating lithium-ion exchange (LIE) ionomers as polysulfide blocking barriers in Li-S cells. When exposed to electrolyte solvent, these LIE membranes provide a physical barrier that limits polysulfide diffusion, while also delivering high Li⁺ ion conductivity through swelling in electrolyte solvent. The LIE ionomers can be applied in a variety of ways including as: binders in the electrode formulation, coatings applied directly to the electrode surface, and as freestanding polymer electrolyte membranes. We have also developed a porous, dimensionally stable membrane (DSM) which can be impregnated with LIE ionomers to improve mechanically stability by limiting in-plane (X-Y direction) swelling. This general approach of incorporating LIE ionomers into Li-S cells has achieved improvements in discharge capacity, cycle life, coulombic efficiency, and energy density.

References:

1. Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M., Li-O2 and Li-S batteries with high energy storage. Nat Mater 2012, 11 (1), 19-29.