Progress in the Development of Sulfur Cathodes for Solid State Batteries

Solid-state lithium-sulfur (Li-S) batteries promise high energy density, long cycle life, and low cost. They naturally address the polysulfide shuttle problem that has plagued that stability of Li-S batteries with liquid electrolytes. However, realizing long cycle life and high utilization for the sulfur cathode is extremely challenging due to low conductivity, low reactivity, and volume change induced microstructural failure.

We have focused on improving both sulfur-based cathodes and lithium sulfide-based cathodes. To improve the conductivity of sulfur, we have shown that introducing halide dopants can form new compounds with appreciable electronic conductivities. Sulfur iodide, for example, also appear to stabilize the polysulfide intermediates, which are known to be unstable in the solid state. The large volume change of the cathode, however, still induces electrode swelling and degradation, particularly under modest stack pressure. Heating of the cathode can help restore the degraded interfaces.

Recently, we have also revisited the approach of activated Li2S as the cathode. One potential advantage is that the electrode will be under compressive, rather than tensile stress during operation. To improve the reactivity, we have introduced several dopants (both metal and non-metal). Selected dopants have shown the ability to improve electronic and ionic conductivities and to enhance reactivity. These approaches are promising in pushing the energy density and cycle life of solid state Li-S batteries.