Hole Localization Induces Anionic Redox in Li-Ion Cathodes
Authors: Ramachandran Hari , Mu Edward Weian , Lomeli Eder , Braun Augustin , LIU Jue , Solomon Edward I. , Yang Wanli , Devereaux Thomas P. , Chueh William C. 
Published: December 2024

Anionic redox is defined as the depopulation of electronic states consisting of unhybridized or minimally hybridized O 2p orbitals. In recent years, anionic redox has been investigated in great detail due its potential to increase the high-voltage capacity of Na-ion and Li-ion intercalation positive electrodes (cathodes) for batteries. However, anionic redox almost always manifests alongside a gamut of undesirable effects, from > 1 V of hysteresis, to voltage and capacity fade over hundreds of cycles. These effects have hindered the widespread utilization and commercialization of anionic redox-active cathodes. Understanding the origin and role of anionic redox is vital if we are to leverage it in high-voltage cathodes and mitigate its negative effects.Anionic redox occurs across many bulk structural motifs - from Na and Li-layered oxides to cation-disordered rocksalt-type (DRX) structures. While it was initially thought to only occur in systems with alkali metal-excess environments (such as the Li-O-Li type local configurations), it has hence been observed even in conventional stoichiometric cathodes at high states-of-charge. The crystal structure origins of anionic redox in terms of metal-oxygen decoordination and metal migrations have been well explored in the literature, a similar robust origin theory stemming from electronic structure arguments is lacking.

To address this, we note that there are two manifestations of anionic redox observed in the literature - the kind that involves O-O dimerization (observed in several systems such as DRX compounds, Li2RuO3, Li- and Mn-rich NMCs, etc.) and the kind that involves localized polarons without dimers (observed in Na2Mn3O7 and Na0.6Li0.2Mn0.8O2). In attempting to reconcile both observed types of behaviour, we observe that the chief difference stems from the stabilization mechanism for adding holes to the ligand orbitals. In both cases, holes are added to ligand or ligand-dominated orbitals, with minimal to no hybridization with the adjacent transition metals. In the former kind of anionic redox, the ligand holes are stabilized by forming a sigma-like O-O bond, while in the latter, it is a weak pi-type hybridization that stabilizes the addition of ligand holes. We hypothesize that the localization of the ligand holes governs the manifestation of anionic redox in a system.

In this work, we investigate this relationship between localized ligand holes and anionic redox by tuning the cation ordering in Li4FeSbO6 (LFSO). This honeycomb-ordered Li-excess material is the perfect model system to use for this study, as the electrochemical behaviour of this system features a single high-voltage plateau at 4.2 V vs. Li+/Li which is charge compensated by a novel Fe(III)/(V) redox couple. The Fe redox couple involves an electronic transition from a 3d5
majority ground state to a negative charge transfer 3d5
L
2
ground state, involving highly hybridized and localized ligand holes on Fe-O orbitals. Despite the presence of hole density on O orbitals and the presence of Li-O-Li local environments, the system cannot be classified as being anion redox active due to the contribution of the metal states to the charge compensation.

However, by systematically tuning the extent of cation disorder in LFSO, we are able to convert highly metal-hybridized holes into unhybridized ligand holes, inducing anionic redox. We use a combination of soft X-ray spectroscopy, synchrotron diffraction, neutron diffraction, charge-transfer atomic multiplet calculations and density functional theory to unravel the mechanistic relationship between cation disorder, ligand hole localization and anionic redox, giving us a rational design rule for synthesizing materials with low hysteresis at high potentials. Our findings reveal the crucial role of cation disorder in promoting anionic redox.