Anion Redox in Sulfides

Conventional battery cathodes store charge using electronic states dominated primarily by transition metal d electrons leaving the structural anions largely redox inactive. Accessing the sp electrons of the anions represents an exciting frontier in charge storage mechanisms that has a potential to increase energy density. Though much of the field is focused on exploring high voltage anion redox in oxides, here we will discuss the benefits of evaluating sulfides. Not only can we obtain clear mechanistic information, but the material stoichiometry can be tuned to reach relevant energy densities despite the inherently low voltage of sulfur redox. We will discuss materials whose redox chemistry has allowed us to uncover the structural implications of anion redox in sulfides – namely the implications of forming a S-S bond – and the evolution of the electronic structure as a result of anion oxidation. We will show experimental data that confirms sulfur oxidation forms persulfide bonds, the formation of the persulfide bonds require the presence of cation vacancies, and sulfide oxidation occurs from nonbonding sulfur bands. Using this understanding of anion redox in sulfides, we have been able to develop a material that reaches over 1000 Wh/kg using only industrial elements.