Inorganic Polysulfide Chemistries for Better Energy Storage Systems

ConspectusSulfur-based cathode materials have become a research hot spot as one of the most promising candidates for next-generation, high-energy lithium batteries. However, the insulating nature of elemental sulfur or organosulfides has become the biggest challenge that leads to dramatic degradation and hinders their practical application. The disadvantage is more obvious for all-solid-state battery systems, which require both high electronic and ionic migration at the same sites to realize a complete electrochemical reaction. In addition to adding conductive components into the cathode composites, another effective way to realize high-reversibility sulfur-based cathodes is by optimizing the inherent nature of sulfur-based materials to make them "conductive". Inorganic polysulfide materials including polysulfide molecules, selenium-sulfur solid solutions, and (lithium) metal polysulfides are promising, as they have different structures that can make them intrinsically conductive or becoming conductive during lithiation. They all contain at least one −S–S– bridged bond, which is the intrinsic structural characteristic and the source of the chemical properties of these polysulfide compounds. For example, by balancing the conductivity and reversible capacity, researchers in the US National Aeronautics and Space Administration (NASA) have shown that 500 Wh/kg solid-state Li–Se/S batteries can power cars and even electric aircraft.We have long been focusing on the inorganic polysulfide component, reported the selenium-sulfur solid solutions, the first sulfur-rich phosphorus polysulfide molecules, and followed the research of metal polysulfide components. The proposed Account summarizes our current knowledge of the fundamental aspects of inorganic polysulfides in energy storage systems based on state-of-the-art publications on this topic. Both fast electron and ion migrations within the electrode materials are vital to achieving high-energy batteries. We begin by illustrating effective approaches to enhance the electronic/ionic conductivity of sulfur-based electrode materials. We then present some basic observations and properties (especially the intrinsic high conductivities) of the inorganic polysulfide electrode materials. The key chemical and structural factors dictating their conductive and electrochemical behaviors will be discussed. Finally, we show the advantages and broad applications of inorganic polysulfides in energy storage areas. The proposed Account will provide an insightful perspective on the current knowledge of inorganic polysulfide materials, as well as their future research directions and development potential, serving as a keynote reference for researchers in the field of energy storage.