A multi-edge study: investigating Co oxidation states of pristine LiNixMnyCo1-x-yO2 cathode materials by high energy-resolution X-ray spectrometry

The investigation of Co oxidation states in pristine LiNixMnyCo1-x-yO2 (NMC) cathodes (NMC111, NMC622, NMC811) has been a subject of ongoing debate, with conflicting findings in the literature. In this study, we present a novel and comprehensive approach to address and clarify this issue using a variety of high energy-resolution X-ray spectroscopy techniques. To shed light on the Co oxidation states in NMC cathodes, we employed independent measurements including X-ray absorption spectrometry in both soft and hard X-ray ranges, as well as resonant X-ray emission spectrometry in the soft X-ray range. The investigation centered on the transition metal (TM) K and L edges, providing a thorough exploration of the electronic structure transitions. The study identified minor shifts in Co oxidation states, and theoretical calculations quantified the ratio of Co atoms undergoing oxidation state changes, which were approximately 2.05% (NMC111 to NMC622) and 3.75% (NMC111 to NMC811). Independent measurements that targeted electronic structure transitions using K-edge and L-edge absorption and emission spectrometry were strategically combined to enhance the reliability of the results. The diverse methodological approach aimed to contribute to a comprehensive understanding of Co oxidation states in NMC cathodes. This study highlights the importance of combining complementary techniques to address intricate scientific debates effectively.

Understanding anion-redox reactions in cathode materials of lithium-ion batteries throughin situcharacterization techniques: a review

As the demand for rechargeable lithium-ion batteries (LIBs) with higher energy density increases, the interest in lithium-rich oxide (LRO) with extraordinarily high capacities is surging. The capacity of LRO cathodes exceeds that of conventional layered oxides. This has been attributed to the redox contribution from both cations and anions, either sequentially or simultaneously. However, LROs with notable anion redox suffer from capacity loss and voltage decay during cycling. Therefore, a fundamental understanding of their electrochemical behaviors and related structural evolution is a prerequisite for the successful development of high-capacity LRO cathodes with anion redox activity. However, there is still controversy over their electrochemical behavior and principles of operation. In addition, complicated redox mechanisms and the lack of sufficient analytical tools render the basic study difficult. In this review, we aim to introduce theoretical insights into the anion redox mechanism andin situanalytical instruments that can be used to prove the mechanism and behavior of cathodes with anion redox activity. We summarized the anion redox phenomenon, suggested mechanisms, and discussed the history of development for anion redox in cathode materials of LIBs. Finally, we review the recent progress in identification of reaction mechanisms in LROs and validation of engineering strategies to improve cathode performance based on anion redox through various analytical tools, particularly,in situcharacterization techniques. Because unexpected phenomena may occur during cycling, it is crucial to study the kinetic properties of materialsin situunder operating conditions, especially for this newly investigated anion redox phenomenon. This review provides a comprehensive perspective on the future direction of studies on materials with anion redox activity.

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.

A green process for phosphorus recovery from spent LiFePO4 batteries by transformation of delithiated LiFePO4 crystal into NaFeS2

Recovery of high-content and valuable elements including phosphorus (P) is critical for recycling of spent LiFePO₄ battery, but P recovery is challengeable due to the poor solubility of lithium phosphate and iron phosphate. This study compared two strategies to recover P by adopting sulfide salt to induce P dissolution, i.e., recovery of P directly from LiFePO₄, and step-by-step recovery of Li then P. The results revealed that the second strategy was more efficient because of the higher recovering efficiency and selectivity. Accordingly, an acid-free process to recover P was successfully demonstrated. Li-recovery efficiency of 97.5 % was reached at a leaching time of 65 min, and nearly 100 % P-recovery efficiency was reached at 5 h. Mechanism analysis revealed that the transforming of delithiated LiFePO₄ crystal to NaFeS₂ was mainly responsible for P dissolution. Thermodynamic analysis and density functional theory calculation further proved the transformation reaction, and a stepwise-transformation mechanism was proposed. In addition, P was reclaimed in the form of soluble phosphate salts. The process is especially appealing due to its environmental and economic benefits for recycling spent LiFePO₄ batteries.

Reversible Anionic Redox Activities in Conventional LiNi1/3 Co1/3 Mn1/3 O2 Cathodes

Redox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium-excessive layered-oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi_1/3 Co_1/3 Mn_1/3 O₂ . The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high-capacity lithium-ion batteries.

Tuning Oxygen Redox Reaction through the Inductive Effect with Proton Insertion in Li-Rich Oxides

As a parent compound of Li-rich electrodes, Li₂MnO₃ exhibits high capacity during the initial charge; however, it suffers notoriously low Coulombic efficiency due to oxygen and surface activities. Here, we successfully optimize the oxygen activities toward reversible oxygen redox reactions by intentionally introducing protons into lithium octahedral vacancies in the Li₂MnO₃ system with its original structural integrity maintained. Combining structural probes, theoretical calculations, and resonant inelastic X-ray scattering results, a moderate coupling between the introduced protons and lattice oxygen at the oxidized state is revealed, which stabilizes the oxygen activities during charging. Such a coupling leads to an unprecedented initial Coulombic efficiency (99.2%) with a greatly improved discharge capacity of 302 mAh g^-1 in the protonated Li₂MnO₃ electrodes. These findings directly demonstrate an effective concept for controlling oxygen activities in Li-rich systems, which is critical for developing high-energy cathodes in batteries.

Advances in soft X-ray RIXS for studying redox reaction states in batteries

High-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) for detecting and quantifying both cationic and anionic redox states in batteries.

Harnessing the surface structure to enable high-performance cathode materials for lithium-ion batteries

The impact of surface structure and interface reconstruction on the electrochemical performances of lithium-ion battery cathode materials is summarized.

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.

First-Principles Study of the Impact of Grain Boundary Formation in the Cathode Material LiFePO4

Motivated by the need to understand the role of internal interfaces in Li migration occurring in lithium-ion batteries, a first-principles study of a coincident site lattice grain boundary in LiFePO4 cathode material and in its delithiated counterpart FPO4 is performed. The structure of the investigated grain boundary is obtained, and the corresponding interface energy is calculated. Other properties, such as ionic charges, magnetic moments, excess free volume, and the lifetime of positrons trapped at the interfaces are determined and discussed. The results show that while the grain boundary in LiFePO4 has desired structural and bonding characteristics, the analogous boundary in FePO4 needs to be yet optimized to allow for an efficient Li diffusion study.

Charge Distribution on S and Intercluster Bond Evolution in Mo6S8 during the Electrochemical Insertion of Small Cations Studied by X-ray Absorption Spectroscopy

Mo₆S₈ is regarded as a promising cathode material in rechargeable Mg batteries. Despite extensive studies, some fundamental questions are still unclarified, including the origination of the chemical stability, key factors inducing the structural evolution, and the factors determining the electrochemical reversibility. Herein Mo L_2,3 and S K-edge X-ray absorption spectroscopy are utilized to uncover the underlying mechanism. Two kinds of S with different effective charge are found, indicating the nonuniform charge distribution. With one cation inserted, the charge distribution becomes homogeneous, relevant to the chemical stability and electrochemical reversibility. The structural evolution is attributed to the change of bond length induced by the delocalization of inserted cations. Moreover, the evolution of intercluster Mo-Mo bond length can be revealed by the drastic change of the S K pre-edge and is closely related to the electrochemical reversibility. This study can shed light on the aforementioned questions and guide the development of Mg cathode material.

Metal-oxygen decoordination stabilizes anion redox in Li-rich oxides

Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li⁺ in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li_2-xIr_1-ySn_yO₃ model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure-redox coupling arises from the local stabilization of short approximately 1.8 Å metal-oxygen π bonds and approximately 1.4 Å O-O dimers during oxygen redox, which occurs in Li_2-xIr_1-ySn_yO₃ through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighbouring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry.

Fingerprint Oxygen Redox Reactions in Batteries through High-Efficiency Mapping of Resonant Inelastic X-ray Scattering

Realizing reversible reduction-oxidation (redox) reactions of lattice oxygen in batteries is a promising way to improve the energy and power density. However, conventional oxygen absorption spectroscopy fails to distinguish the critical oxygen chemistry in oxide-based battery electrodes. Therefore, high-efficiency full-range mapping of resonant inelastic X-ray scattering (mRIXS) has been developed as a reliable probe of oxygen redox reactions. Here, based on mRIXS results collected from a series of Li1.17Ni0.21Co0.08Mn0.54O2 electrodes at different electrochemical states and its comparison with peroxides, we provide a comprehensive analysis of five components observed in the mRIXS results. While all the five components evolve upon electrochemical cycling, only two of them correspond to the critical states associated with oxygen redox reactions. One is a specific feature at 531.0 eV excitation and 523.7 eV emission energy, the other is a low-energy loss feature. We show that both features evolve with electrochemical cycling of Li1.17Ni0.21Co0.08Mn0.54O2 electrodes, and could be used for characterizing oxidized oxygen states in the lattice of battery electrodes. This work provides an important benchmark for a complete assignment of all mRIXS features collected from battery materials, which sets a general foundation for future studies in characterization, analysis, and theoretical calculation for probing and understanding oxygen redox reactions.

Re-evaluation of experimental measurements for the validation of electronic band structure calculations for LiFePO4 and FePO4

Experimental measurements used to validate previous electronic band structure calculations for olivine LiFePO₄ and its delithiated phase, FePO₄, have been re-investigated in this study. Experimental band gaps of LiFePO₄ and FePO₄ have been determined to be 6.34 eV and 3.2 eV by electron energy loss spectroscopy (EELS) and UV-Vis-NIR diffusion reflectance spectroscopy, respectively. X-ray photoemission (XPS) and Raman spectroscopy show that the surfaces of very carefully synthesized LiFePO₄ display Li-depletion, which affects optical reflectance determinations. Based on these experimental measurements, functionals for density functional theory (DFT) calculations of the electronic properties have been revisited. Overall, electronic structures of LiFePO₄ and FePO₄ calculated using sX-LDA show the best self-consistent match to combined experimentally determined parameters. Furthermore, the open-circuit voltages of the LiFePO₄ half-cell have been interpreted in terms of both Fermi levels and Gibbs free energies, which provides additional support for the electronic band structures determined by this research.

The surface Li depletion affects the determination of optical gap for LiFePO₄, which was previously used for validation of DFT calculations.

Large Charge-Transfer Energy in LiFePO4 Revealed by Full-Multiplet Calculation for the Fe L3 -edge Soft X-ray Emission Spectra

We analyzed the Fe 3d electronic structure in LiFePO₄ /FePO₄ (LFP/FP) nanowire with a high cyclability by using soft X-ray emission spectroscopy (XES) combined with configuration-interaction full-multiplet (CIFM) calculation. The ex situ Fe L_2,3 -edge resonant XES (RXES) spectra for LFP and FP are ascribed to oxidation states of Fe²⁺ and Fe³⁺ , respectively. CIFM calculations for Fe²⁺ and Fe³⁺ states reproduced the Fe L₃ RXES spectra for LFP and FP, respectively. In the calculations for both states, the charge-transfer energy was considerably larger than those for typical iron oxides, indicating very little electron transfer from the O 2p to Fe 3d orbitals and a weak hybridization on the Fe-O bond during the charge-discharge reactions.

Elemental-sensitive Detection of the Chemistry in Batteries through Soft X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering

Energy storage has become more and more a limiting factor of today's sustainable energy applications, including electric vehicles and green electric grid based on volatile solar and wind sources. The pressing demand of developing high-performance electrochemical energy storage solutions, i.e., batteries, relies on both fundamental understanding and practical developments from both the academy and industry. The formidable challenge of developing successful battery technology stems from the different requirements for different energy-storage applications. Energy density, power, stability, safety, and cost parameters all have to be balanced in batteries to meet the requirements of different applications. Therefore, multiple battery technologies based on different materials and mechanisms need to be developed and optimized. Incisive tools that could directly probe the chemical reactions in various battery materials are becoming critical to advance the field beyond its conventional trial-and-error approach. Here, we present detailed protocols for soft X-ray absorption spectroscopy (sXAS), soft X-ray emission spectroscopy (sXES), and resonant inelastic X-ray scattering (RIXS) experiments, which are inherently elemental-sensitive probes of the transition-metal 3d and anion 2p states in battery compounds. We provide the details on the experimental techniques and demonstrations revealing the key chemical states in battery materials through these soft X-ray spectroscopy techniques.

Elucidating anionic oxygen activity in lithium-rich layered oxides

Recent research has explored combining conventional transition-metal redox with anionic lattice oxygen redox as a new and exciting direction to search for high-capacity lithium-ion cathodes. Here, we probe the poorly understood electrochemical activity of anionic oxygen from a material perspective by elucidating the effect of the transition metal on oxygen redox activity. We study two lithium-rich layered oxides, specifically lithium nickel metal oxides where metal is either manganese or ruthenium, which possess a similar structure and discharge characteristics, but exhibit distinctly different charge profiles. By combining X-ray spectroscopy with operando differential electrochemical mass spectrometry, we reveal completely different oxygen redox activity in each material, likely resulting from the different interaction between the lattice oxygen and transition metals. This work provides additional insights into the complex mechanism of oxygen redox and development of advanced high-capacity lithium-ion cathodes.

A reversible oxygen redox process contributes extra capacity and understanding this behavior is of high importance. Here, aided by resonant inelastic X-ray scattering, the authors reveal the distinctive anionic oxygen activity of battery electrodes with different transition metals.

Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li1.17-x Ni0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.

Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials

Recent progress in the understanding of percolation theory points to cation-disordered lithium-excess transition metal oxides as high-capacity lithium-ion cathode materials. Nevertheless, the oxygen redox processes required for these materials to deliver high capacity can trigger oxygen loss, which leads to the formation of resistive surface layers on the cathode particles. We demonstrate here that, somewhat surprisingly, fluorine can be incorporated into the bulk of disordered lithium nickel titanium molybdenum oxides using a standard solid-state method to increase the nickel content, and that this compositional modification is very effective in reducing oxygen loss, improving energy density, average voltage, and rate performance. We argue that the valence reduction on the anion site, offered by fluorine incorporation, opens up significant opportunities for the design of high-capacity cation-disordered cathode materials.The performance of lithium-excess cation-disordered oxides as cathode materials relies on the extent to which the oxygen loss during cycling is mitigated. Here, the authors show that incorporating fluorine is an effective strategy which substantially improves the cycling stability of such a material.

Visualizing redox orbitals and their potentials in advanced lithium-ion battery materials using high-resolution x-ray Compton scattering

Reduction-oxidation (redox) reactions are the key processes that underlie the batteries powering smartphones, laptops, and electric cars. A redox process involves transfer of electrons between two species. For example, in a lithium-ion battery, current is generated when conduction electrons from the lithium anode are transferred to the redox orbitals of the cathode material. The ability to visualize or image the redox orbitals and how these orbitals evolve under lithiation and delithiation processes is thus of great fundamental and practical interest for understanding the workings of battery materials. We show that inelastic scattering spectroscopy using high-energy x-ray photons (Compton scattering) can yield faithful momentum space images of the redox orbitals by considering lithium iron phosphate (LiFePO₄ or LFP) as an exemplar cathode battery material. Our analysis reveals a new link between voltage and the localization of transition metal 3d orbitals and provides insight into the puzzling mechanism of potential shift and how it is connected to the modification of the bond between the transition metal and oxygen atoms. Our study thus opens a novel spectroscopic pathway for improving the performance of battery materials.

Electric-field control of tri-state phase transformation with a selective dual-ion switch

Materials can be transformed from one crystalline phase to another by using an electric field to control ion transfer, in a process that can be harnessed in applications such as batteries, smart windows and fuel cells. Increasing the number of transferrable ion species and of accessible crystalline phases could in principle greatly enrich material functionality. However, studies have so far focused mainly on the evolution and control of single ionic species (for example, oxygen, hydrogen or lithium ions). Here we describe the reversible and non-volatile electric-field control of dual-ion (oxygen and hydrogen) phase transformations, with associated electrochromic and magnetoelectric effects. We show that controlling the insertion and extraction of oxygen and hydrogen ions independently of each other can direct reversible phase transformations among three different material phases: the perovskite SrCoO_3-δ (ref. 12), the brownmillerite SrCoO_2.5 (ref. 13), and a hitherto-unexplored phase, HSrCoO_2.5. By analysing the distinct optical absorption properties of these phases, we demonstrate selective manipulation of spectral transparency in the visible-light and infrared regions, revealing a dual-band electrochromic effect that could see application in smart windows. Moreover, the starkly different magnetic and electric properties of the three phases-HSrCoO_2.5 is a weakly ferromagnetic insulator, SrCoO_3-δ is a ferromagnetic metal, and SrCoO_2.5 is an antiferromagnetic insulator-enable an unusual form of magnetoelectric coupling, allowing electric-field control of three different magnetic ground states. These findings open up opportunities for the electric-field control of multistate phase transformations with rich functionalities.

High resolution x-ray absorption and emission spectroscopy of Li x CoO2 single crystals as a function delithiation

The effect of delithiation in Li _x CoO₂ is studied by high resolution Co K-edge x-ray absorption and x-ray emission spectroscopy. Polarization dependence of the x-ray absorption spectra on single crystal samples is exploited to reveal information on the anisotropic electronic structure. We find that the electronic structure of Li _x CoO₂ is significantly affected by delithiation in which the Co ions oxidation state tending to change from 3+  to 4+. The Co intersite (intrasite) 4p-3d hybridization suffers a decrease (increase) by delithiation. The unoccupied 3d t _2g orbitals with a _1g symmetry, containing substantial O 2p character, hybridize isotropically with Co 4p orbitals and likely to have itinerant character unlike anisotropically hybridized 3d e _g orbitals. Such a peculiar electronic structure could have significant effect on the mobility of Li in Li _x CoO₂ cathode and hence the battery characteristics.

Improving the electrochemical properties of LiNi0.5Co0.2Mn0.3O2 at 4.6 V cutoff potential by surface coating with Li2TiO3 for lithium-ion batteries

The Li₂TiO₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ (LTO@NCM) cathode materials are synthesized via an in situ coprecipitation method to improve the electrochemical performance of NCM.