A Perspective on Energy Densities of Rechargeable Li-S Batteries and Alternative Sulfur-Based Cathode Materials

Lithium-ion battery has reached its capacity and energy density limits. In the past decade, significant efforts have been taken to explore new electrode materials that have the potential to enable high-energy-density battery systems. Among them, elemental sulfur is one of the high-capacity cathode candidates and has been studied intensively over the past decade. The formation of lithium polysulfides in ethereal liquid electrolyte upon cycling results in several challenges such as active material dissolution, shuttle effect, and limited cycle life. Although some approaches have been developed to overcome these issues, the attainable energy densities of lithium-sulfur (Li-S) batteries seem to be low. The main reason is largely due to the high electrolyte/sulfur (E/S) ratios used in the sulfur cathode. This perspective provides new insights on the energy density analysis of sulfur cathode. The average mass density of sulfur cathode is found to be a useful parameter for this purpose. Some emerging alternative sulfur-based cathode materials such as organopolysulfides and metal polysulfides which possess unique properties and performances are presented. They are promising to overcome the intrinsic issues associated with elemental sulfur cathode and enable truly high-energy-density Li-S battery systems.