Mechanism of Exact Transition between Cationic and Anionic Redox Activities in Cathode Material Li2FeSiO4

Multiple Electron Transfers Enable High-Capacity Cathode Through Stable Anionic Redox

Single-electron transfer, low alkali metal contents, and large-molecular masses limit the capacity of cathodes. This study uses a cost-effective and light-molecular-mass orthosilicate material, K2FeSiO4, with a high initial potassium content, as a cathode for potassium-ion batteries to enable the transfer of more than one electron. Despite the limited valence change of Fe ions during cycling, K2FeSiO4 can undergo multiple electron transfers via successive oxygen anionic redox reactions to generate a high reversible capacity. Although the formation of O‒O dimers in K2FeSiO4 occur upon removing large amounts of potassium, the strong binding effect of Si on O mitigates irreversible oxygen release and voltage degradation during cycling. K2FeSiO4 achieves 236 mAh g-1 at 50 mA g-1, with an energy density of 520 Wh kg-1, which can be comparable with commercial LiFePO4 materials. Moreover, it also exhibits 1400 stable cycles under high-current conditions. These findings enhance the potential commercialization prospects for potassium-ion batteries.

Operando Synchrotron X-Ray Absorption Spectroscopy: A Key Tool for Cathode Material Studies in Next-Generation Batteries

Rechargeable batteries are central to modern energy storage systems, from portable electronics to electric vehicles. The cathode material, a critical component, largely dictates a battery's energy density, capacity, and overall performance. This review focuses on the application of operando X-ray absorption spectroscopy (XAS) to study cathode materials in Li-ion, Na-ion, Li-S, and Na-S batteries. Operando XAS provides real-time insights into the local electronic structure, oxidation states, and coordination environments, which are crucial for understanding complex electrochemical processes, such as redox reactions, phase transitions, and structural degradation. The review highlights the strengths of hard and soft XAS techniques in probing transition metal (TM) and anionic redox processes, particularly in layered oxide cathodes, where reversible oxygen redox and TM behavior are pivotal. The role of operando XAS is also explored in analyzing conversion-type S-based cathodes, where it helps unravel the intricate reaction mechanisms. Furthermore, the review addresses the challenges of in situ cell design for operando XAS, especially under ultrahigh vacuum conditions for soft XAS. By discussing recent advancements and future directions, this review underscores the critical role of operando XAS in driving innovation and improving the design and performance of next-generation battery technologies.

The Analysis of Electron Densities: From Basics to Emergent Applications

The electron density determines all properties of a system of nuclei and electrons. It is both computable and observable. Its topology allows gaining insight into the mechanisms of bonding and other phenomena in a way that is complementary to and beyond that available from the molecular orbital picture and the formal oxidation state (FOS) formalism. The ability to derive mechanistic insight from electron density is also important with methods where orbitals are not available, such as orbital-free density functional theory (OF-DFT). While density topology-based analyses such as QTAIM (quantum theory of atoms-in-molecules) have been widely used, novel, vector-based techniques recently emerged such as next-generation (NG) QTAIM. Density-dependent quantities are also actively used in machine learning (ML)-based methods, in particular, for ML DFT functional development, including machine-learnt kinetic energy functionals. We review QTAIM and its recent extensions such as NG-QTAIM and localization-delocalization matrices (LDM) and their uses in the analysis of bonding, conformations, mechanisms of redox reactions excitations, as well as ultrafast phenomena. We review recent research showing that direct density analysis can circumvent certain pitfalls of the FOS formalism, in particular in the description of anionic redox, and of the widely used (spherically) projected density of states analysis. We discuss uses of density-based quantities for the construction of DFT functionals and prospects of applications of analyses of density topology to get mechanistic insight with OF-DFT and recently developed time-dependent OF-DFT.

Unexpectedly Large Contribution of Oxygen to Charge Compensation Triggered by Structural Disordering: Detailed Experimental and Theoretical Study on a Li3NbO4-NiO Binary System

Dependence on lithium-ion batteries for automobile applications is rapidly increasing. The emerging use of anionic redox can boost the energy density of batteries, but the fundamental origin of anionic redox is still under debate. Moreover, to realize anionic redox, many reported electrode materials rely on manganese ions through π-type interactions with oxygen. Here, through a systematic experimental and theoretical study on a binary system of Li₃NbO₄-NiO, we demonstrate for the first time the unexpectedly large contribution of oxygen to charge compensation for electrochemical oxidation in Ni-based materials. In general, for Ni-based materials, e.g., LiNiO₂, charge compensation is achieved mainly by Ni oxidation, with a lower contribution from oxygen. In contrast, for Li₃NbO₄-NiO, oxygen-based charge compensation is triggered by structural disordering and σ-type interactions with nickel ions, which are associated with a unique environment for oxygen, i.e., a linear Ni-O-Ni configuration in the disordered system. Reversible anionic redox with a small hysteretic behavior was achieved for LiNi_2/3Nb_1/3O₂ with a cation-disordered Li/Ni arrangement. Further Li enrichment in the structure destabilizes anionic redox and leads to irreversible oxygen loss due to the disappearance of the linear Ni-O-Ni configuration and the formation of unstable Ni ions with high oxidation states. On the basis of these results, we discuss the possibility of using σ-type interactions for anionic redox to design advanced electrode materials for high-energy lithium-ion batteries.

Direct Identification of Antisite Cation Intermixing and Correlation with Electronic Conduction in CuBi2O4 for Photocathodes

Cu-based p-type semiconducting oxides have been sought for water-reduction photocathodes to enhance the energy-conversion efficiency in photoelectrochemical cells. CuBi₂O₄ has recently attracted notable attention as a new family of p-type oxides, based on its adequate band gap. Although the identification of a major defect structure should be the first step toward understanding the electronic conduction behavior, no direct experimental analysis has been carried out yet. Using atomic-scale scanning transmission electron microscopy together with chemical probing, we identify a substantial amount of Bi_Cu-Cu_Bi antisite intermixing as a major point-defect type. Our density functional theory calculations also show that antisite Bi_Cu can seriously hinder the hole-polaron hopping between Cu, in agreement with lower conductivity and a larger thermal activation barrier under a higher degree of intermixing. These findings highlight the value of the direct identification of point defects for a better understanding of electronic properties in complex oxides.

Oxygen-Based Anion Redox for Lithium Batteries

ConspectusThe importance of current Li-ion batteries (LIBs) in modern society cannot be overstated. While the energy demands of devices increase, the corresponding enhancements in energy density of battery technologies are highly sought after. Currently, many different battery concepts, such as Li-S and metal-air among many others, have been investigated. However, their practical implementation has mostly been restricted to the prototyping stage. In fact, most of these technologies require rework of existing Li-ion battery manufacturing facilities and will naturally incur resistance to change from industry. For this reason, one specifically attractive technology, anionic redox in transition metal oxides, has gained much attention in the recent years. Its ability to be directly used in already established processes and higher energy density with similar electrolyte formulation make it a key materials research direction for next generation Li-ion batteries. In regular LIBs, the redox active centers are the transition metal cation. In anion redox, both the anion (typically O) and the transition metal cation are utilized as redox centers with enormous implications for increasing energy density. This new material can be highly competitive for replacing the current LIB technologies. However, much is still unknown about its cycling mechanism. Upon activating the O redox couples, most cationic and anionic redox active materials will either evolve O₂ or undergo irreversible structural degradation with associated severe decreases in electrochemical performance. By understanding the transition from full anion redox to partial cationic and anionic redox, we hope readers can gain a deeper understanding of the topic.This Account will focus mainly on the work that was conducted by our group at Argonne National Laboratory. The phenomenon of cationic and anionic redox in a lithium-ion battery cathode will first be discussed. Our work in resonant inelastic X-ray scattering to investigate the spectroscopic features of O after delithiation has found potential "fingerprint" signals that could likely be used to identify and confirm reversible O redox if corroborated with other techniques. To follow, we will examine our work on Li-O₂ batteries. While our group and the research community have had many significant contributions and improvements to the field of Li-O₂ (such as decreasing overpotential and achieving cyclability in air environment), its practical application is still far from realization. Perhaps our most important contribution to this area is the discovery that Ir deposited on reduced graphene oxide can be used to halt the reduction of O₂ at the LiO₂ oxidation state. This not only significantly decreases the charge overpotential but also presents the important concept of oxidation-state controlled discharge. Subsequently, we will focus on our oxidation state-controlled redox-based charging of oxygen in a pure oxygen redox Li-ion battery. Future implications of this technology will be emphasized.

Cationic and anionic redox in lithium-ion based batteries

This review will present the current understanding, experimental evidence and future direction of anionic and cationic redox for Li-ion batteries.

Discovering unusual structures from exception using big data and machine learning techniques

Recently, machine learning (ML) has become a widely used technique in materials science study. Most work focuses on predicting the rule and overall trend by building a machine learning model. However, new insights are often learnt from exceptions against the overall trend. In this work, we demonstrate that how unusual structures are discovered from exceptions when machine learning is used to get the relationship between atomic and electronic structures based on big data from high-throughput calculation database. For example, after training an ML model for the relationship between atomic and electronic structures of crystals, we find AgO₂F, an unusual structure with both Ag³⁺ and O₂^2-, from structures whose band gap deviates much from the prediction made by our model. A further investigation on this structure might shed light into the research on anionic redox in transition metal oxides of Li-ion batteries.

Antidoping in Insulators and Semiconductors Having Intermediate Bands with Trapped Carriers

Ordinary doping by electrons (holes) generally means that the Fermi level shifts towards the conduction band (valence band) and that the conductivity of free carriers increases. Recently, however, some peculiar doping characteristics were sporadically recorded in different materials without noting the mechanism: electron doping was observed to cause a portion of the lowest unoccupied band to merge into the valance band, leading to a decrease in conductivity. This behavior, that we dub as "antidoping," was seen in rare-earth nickel oxides SmNiO_{3}, cobalt oxides SrCoO_{2.5}, Li-ion battery materials, and even MgO with metal vacancies. We describe the physical origin of antidoping as well as its inverse problem-the "design principles" that would enable an intelligent search of materials. We find that electron antidoping is expected in materials having preexisting trapped holes and is caused by the annihilation of such "hole polarons" via electron doping. This may offer an unconventional way of controlling conductivity.

Tuning polaronic redox behavior in olivine phosphate

In order to understand and improve the conductivity of LiFePO₄, lots of attempts have been made both experimentally and theoretically.