Anionic Redox Regulated via Metal–Ligand Combinations in Layered Sulfides

Abstract

breakthroughs revealed by both experimental and theoretical works. In recent reports, anionic redox can be activated in layered cathodes by A-L-A configuration, [1] where L stands for ligands and A stands for alkali metals, alkaline-earth metals, vacancies, or other elements with little hybridization with ligand orbitals. Ligands, mostly oxygen, are designed with nonbonding p electrons under A-L-A configuration to participate in charge compensation processes. However, from band structure intuition, cationic and anionic redox processes can be arranged by selecting different metal-ligand combinations without the existence of A-L-A configuration. Within the framework of Zaanen-Sawatzky-Allen model, [2] defining the Mott-Hubbard U and charge-transfer energy Δ, the metal-ligand combinations in layered cathode can be classified into three categories: U/2 < Δ, U/2 > Δ, and U/2 ≈ Δ. Mott-Hubbard U denotes the d-d Coulomb and exchange interactions of electrons, while Δ denotes the charge-transfer energy from the top of anion p valence band to Fermi level. In following parts, for convenience, we use transition metal (TM) d bands to represent TM(nd)-ligand(np) hybrid orbital mainly contributed by TM (nd), and vice versa.When U/2 < Δ, the energy level of occupied TM d band is higher than ligand valence p band, electrons will be firstThe increasing demand for energy storage is calling for improvements in cathode performance. In traditional layered cathodes, the higher energy of the metal 3d over the O 2p orbital results in one-band cationic redox; capacity solely from cations cannot meet the needs for higher energy density. Emerging anionic redox chemistry is promising to access higher capacity. In recent studies, the low-lying O nonbonding 2p orbital was designed to activate one-band oxygen redox, but they are still accompanied by reversibility problems like oxygen loss, irreversible cation migration, and voltage decay. Herein, by regulating the metal-ligand energy level, both extra capacities provided by anionic redox and highly reversible anionic redox process are realized in NaCr 1−y V y S 2 system. The simultaneous cationic and anionic redox of Cr/V and S is observed by in situ X-ray absorption near edge structure (XANES). Under high d-p hybridization, the strong covalent interaction stabilizes the holes on the anions, prevents irreversible dimerization and cation migration, and restrains voltage hysteresis and voltage decay. The work provides a fundamental understanding of highly reversible anionic redox in layered compounds, and demonstrates the feasibility of anionic redox chemistry based on hybridized bands with d-p covalence.

From Oxygen Redox to Sulfur Redox: A Paradigm for Li-Rich Layered Cathodes

“…This might lead to the common view that the TM-ligand covalency could be further enhanced by replacing oxygen with more covalent sulfur. This is supported by the sulfur-based layered structural framework during cycling in Li 2 FeS 2 , Li 1.33–2 y /3 Ti 0.67– y /3 Fe y S 2 , and NaCr 1– y V y S 2 . ,, Unlike the oxygen-based host framework in which the TM–O bond is easily broken to form an overoxidized O–O dimer or O 2 , the strong covalent TM–S bond can withstand a certain degree of structural distortion even in highly charged states. That is, the TM–S bond could provide a stable host framework for oxidized sulfur and exhibit a persistent honeycomb-ordered structure in the bulk during long cycling (Figure a,b).…”

“…Furthermore, previous studies suggest that the Ti 4+ in sulfides is inactive owing to the empty 3d orbital configuration. ,, The evolution of redox couples was closely investigated via ex situ XANES during the initial cycle with marked charged and discharged states (Figure e–i and Figure S5). In principle, the energy shifts of the Ni and Ti K-edges can be determined from the location of the white line peaks, while the valence state of S can be indirectly determined from the relative area of the pre-edge peaks, which has been suggested to be a reliable method for determining the valence state of TMs and sulfur. , There were almost no energy shifts of the Ti K-edge in the charged and discharged states, further indicating that Ti was an inactive component of the structural frameworks (Figure S5). Therefore, only Ni and S participated in charge compensation in the LLS.…”

“…As a homologue of oxygen, the degree of TM-ligand hybridization is greater for sulfur, which could enhance the stability of the host framework. This should be ascribed to the highly delocalized S electron orbital that allows the formation of stronger π-type interactions with 3d TMs to endure structural distortion upon cycling (Figure b). ,− Additionally, density functional theory (DFT) calculations were performed to distinguish the differences in the electron structures between Li 2 MnO 3 and Li 2 TiS 3 (Supporting Information Note 1). The electron localization function (ELF) distributions around the TM-ligand in Figure c revealed that the pair electrons in Li 2 TiS 3 were more delocalized than those in Li 2 MnO 3 , further confirming that covalent bonds were dominant in Li 2 TiS 3 .…”

Enhanced sodium ion storage in MnO2 through asymmetric orbital hybridization induced by spin-paired ion doping

“…4e and f). 21 The pCOHP curves are grounded in the findings of plane-wave electronic structure computations, allowing for the observation of bonding, non-bonding, and anti-bonding energy domains between atoms. Fig.…”

“…20 Moreover, Wang et al ingeniously manipulated the metal–ligand energy level to amplify Cr (3d)–S (2p) orbital hybridization, thereby preventing irreversible dimerization and cation migration and limiting voltage hysteresis and decay. 21 Notably, the fundamental Mn–O–Mn unit in MnO 2 encompasses two identical orbital hybridizations between O 2p and the two neighbouring Mn 3d atoms. Regulation of this Mn (3d)–O (2p) orbital hybridization is postulated to yield remarkable performance in both energy storage and electrocatalysis.…”