The Effect of Metal d Band Position on Anion Redox in Alkali-Rich Sulfides

New energy storage methods are emerging to increase the energy density of the state-of-the-art battery systems beyond the conventional intercalation electrode materials. For instance, employing anion redox can yield higher capacities compared to transition metal redox alone. Anion redox in sulfides has been recognized since the early days of rechargeable battery research. however, now we aim to understand the charge compensation mechanisms and how to control them. Here, we study the effect of d-p overlap in controlling anion redox by shifting the metal d band position relative to the S p band. We aim to determine the effect of shifting the d band position on the electronic structure and ultimately on charge compensation. Two isostructural sulfides LiNaFeS2 and LiNaCoS2 are directly compared with the hypothesis that the Co material should yield more covalent metal-anion bonds. The newly reported material LiNaCoS2 exhibits multielectron capacity of >/=1.7 electrons per formula unit, but despite the lowered Co d band, the voltage of anion redox is close to that of LiNaFeS2. Interestingly, the material suffers from rapid capacity fade. Through a combination of solid-state nuclear magnetic resonance spectroscopy, Co and S X-ray absorption spectroscopy, X-ray diffraction, and partial density of states calculations, we demonstrate that S oxidation to [S2]2- occurs in early states of charge which leads to an irreversible phase transition into pyrite CoS2 and lithiated cobalt spinel phases such as LixCo3S4 and LixCo9S8. Thus, we conclude that anion oxidation occurs from S nonbonding p orbitals and the Co d bands are too low in energy to prevent a phase transition to more thermodynamically stable persulfide-containing phases. Further, the higher crystal field stabilization energy for octahedral coordination over tetrahedral coordination leads to phase transition in LiNaCoS2.