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

Abstract

New energy storage methods are emerging to increase the energy density of state-of-the-art battery systems beyond conventional intercalation electrode materials. For instance, employing anion redox can yield higher capacities compared with transition metal redox alone. Anion redox in sulfides has been recognized since the early days of rechargeable battery research. 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 LiNaFeS 2 and LiNaCoS 2 are directly compared to the hypothesis that the Co material should yield more covalent metal−anion bonds. LiNaCoS 2 exhibits a 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 LiNaFeS 2 . 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 oxidation of S nonbonding p states to S 2 2− occurs in early states of charge, which leads to an irreversible phase transition. We conclude that the lower energy of Co d bands increases their overlap with S p bands while maintaining S nonbonding p states at the same higher energy level, thus causing no alteration in the oxidation potential. Further, the higher crystal field stabilization energy for octahedral coordination over tetrahedral coordination is proposed to cause the irreversible phase transition in LiNaCoS 2 .

Cation Vacancies Enable Anion Redox in Li Cathodes

“…In fact, by evaluating two isostructural sulfides with two metals of very different d band positions, Fe and Co, we find that the anion oxidation plateau voltage is independent of the metal, suggesting that S nonbonding p orbitals are the operative electronic state from which oxidation occurs. 53 Furthermore, we note that similar electronic states have been associated with the origin of anion redox activity in oxides. Li–O–Li configurations have been suggested to release O p orbitals from metal–anion bonding to yield “orphaned,” unhybridized O 2p orbitals from which anion oxidation is suggested to occur.…”

“…Additionally, it is likely that vacancies release S p orbitals from M–anion bonding orbitals, allowing for rehybridization to form the new S–S bond. In fact, by evaluating two isostructural sulfides with two metals of very different d band positions, Fe and Co, we find that the anion oxidation plateau voltage is independent of the metal, suggesting that S nonbonding p orbitals are the operative electronic state from which oxidation occurs . Furthermore, we note that similar electronic states have been associated with the origin of anion redox activity in oxides.…”