Anionic Redox Processes Research Articles

Strong Covalent Metal-Ligand Interaction Enables a Fast Kinetic and Structurally Stable Na-Ion Layered Cathode.

Anionic redox chemistry has attracted increasing attention for the improvement in the reversible capacity and energy density of cathode materials in Li/Na-ion batteries. However, adverse electrochemical behaviors, such as voltage hysteresis and sluggish kinetics resulting from weak metal-ligand interactions, commonly occur with anionic redox reactions. Currently, the mechanistic investigation driving these issues still remains foggy. Here, we chemically designed Na0.8Fe0.4Ti0.6S2 and Na0.8Fe0.4Ti0.6O2 as model cathodes to explore the covalency effects on metal-ligand interactions during anionic redox process. Na0.8Fe0.4Ti0.6S2 with strengthened covalent interaction of metal-ligand bonds exhibits smaller voltage hysteresis and faster kinetics than Na0.8Fe0.4Ti0.6O2 during (de)sodiation process. Theoretical calculations suggest that Fe is the dominant redox-active center in Na0.8Fe0.4Ti0.6S2, whereas the redox-active center moves from Fe to O with the removal of Na+ in Na0.8Fe0.4Ti0.6O2. We attribute the above different redox behaviors between Na0.8Fe0.4Ti0.6S2 and Na0.8Fe0.4Ti0.6O2 to the charge transfer kinetics from ligand to metal. Moreover, the structural stability of Na0.8Fe0.4Ti0.6S2 is enhanced by increasing the cation migration barriers through strong metal-ligand bonds during desodiation. These insights into the originality of metal-ligand interactions provide guidance for the design of high-capacity and structurally stable cathode materials for Li/Na-ion batteries.

Towards the Direct Observation of the Local and Electronic Structure Evolution of Positive Electrode Materials by X-Ray Raman Scattering and X-Ray Emission Spectroscopy

The emergence of high brilliance synchrotron sources and the availability of more sophisticated infrastructure have opened doors to advanced material characterization, enabling a profound understanding of new processes and mechanisms in battery research. [1] More specifically, non-resonant Valence-to-Core X-ray emission spectroscopy (VtC-XES) and X-ray Raman scattering (XRS) stand out as complementary photon-in/photon-out X-ray spectroscopy techniques. The unique capability of these techniques lies in their access to soft X-ray edges using a hard X-ray beam, providing comprehensive information on both the electronic and local structure, due to their high sensitivity to local structure and coordination. [2,3] In this work, two different families of materials with different compositions and crystal structures are studied using XES and XRS.The first part focuses on the detailed study of oxygen substituted KVPO4F1-xOx (x = 0, 0.25, 0.5, 0.75, 1) using VtC-XES combined to ab initio theoretical calculations. The average working potential increases with increasing oxygen content by activating the V3+/4+ and V4+/5+ redox couples and replacing the VO4F2 “ionic” entity by {V=O}O5 unit with a highly “covalent” vanadyl-type bond. [4] VtC-XES allowed us to distinguish the contributions of V-F, V-O, and V=O bonds in the local environment, with spectral intensities highly correlated to the F- and O2- anionic composition. In addition, specific spectral evolutions are also associated to the presence of highly covalent V=O bonds, strongly influencing the electrode potential of the material.The second part examines a series of electrochemically obtained LiyNiO2 layered oxide materials (y = 0.02, 0.25, 0.33, 0.5, 0.67, 1.0). VtC-XES performed at the Ni Kβ emission line allows to study the local structure evolution around Ni, whereas XRS spectroscopy at both the O K-edge and Ni L2,3 L-edges gives an access to the interplay between Ni and O through the hybridized states of Ni 3d and O 2p. [5] This work provides a survey on both the cationic and anionic redox processes, and unveils the underlying phenomena related to the charge compensation processes occurring in this material.Overall, we present a comprehensive study of different polyanionic and layered oxide positive electrode materials using complementary X-ray spectroscopic techniques to probe both the local and electronic structures. We demonstrate that they allow to follow and understand evolution and changes in the local structure in materials of intricate crystal structures.AcknowledgementThis work was supported by the DESTINY Marie Skłodowska-Curie Actions COFUND PhD Programme (Grant Agreement #945357) co-funded by the European Union's Horizon2020 research and innovation program and the Synchrotron SOLEIL. ANR is also acknowledged for funding the RS2E network through the STORE-EX Labex Project ANR-10-LABX-76-01. Alistore-ERI network is also acknowledged.

Band Structure Engineering Promotes Anionic Redox Reversibility of Cobalt-Free Li-Rich Layered Oxides Cathodes.

Li-rich layered oxides cathodes (LLOs) have prevailed as the promising high-energy-density cathode materials due to their distinctive anionic redox chemistry. However, uncontrollable anionic redox process usually leads to structural deterioration and electrochemical degradation. Herein, a Mo/Cl co-doping strategy is proposed to regulate the relative position of energy band for modulating the anionic redox chemistry and strengthening the structural stability of Co-free Li1.16Mn0.56Ni0.28O2 cathodes. The incorporation of Mo with high d state orbit and Cl with low electronegativity can narrow the band energy gap between bonding and antibonding bands via increasing the filled lower-Hubbard band (LHB) and decreasing the non-bonding O 2p energy bands, promoting the anionic redox reversibility. In addition, strong covalent Mo─O and Mn─Cl bonding further increases the covalency of Mn─O band to further stabilize the O2 n- species and enhance the reversible distortion of MnO6 octahedron. The strengthening electronic conductivity, together with the epitaxial structure Li2MoO4 facilitates the fast Li+ kinetics. As a result, the dual doping material exhibits enhanced anionic redox reversibility and suppressed oxygen release with increased cyclic stability and excellent rate performance. This strategy provides some guidance to design high-energy-density LLOs with desirable anionic redox reversibility and stable crystal structure via band structure engineering.

Understanding Lattice Oxygen Redox Behavior in Lithium‐Rich Manganese‐Based Layered Oxides for Lithium‐Ion and Lithium‐Metal Batteries from Reaction Mechanisms to Regulation Strategies

AbstractLithium‐rich manganese‐based layered oxides (LMLOs) are considered to be one type of the most promising materials for next‐generation cathodes of lithium batteries due to their distinctive anionic redox processes contributing ultrahigh capacity and energy density. Unfortunately, their practical applications are still plagued by several challenges such as undesirable interfacial reactions and structural evolution, as well as voltage hysteresis/recession, in which irreversible anionic redox behavior bears the brunt as the primacy factor. Undoubtedly, a deep understanding of anionic redox reaction mechanisms and irreversible behavior of oxygen species is crucial in order to provide essential guidance for modification strategies for LMLOs. In this paper, the fundamental understanding of intricate anionic redox reaction mechanisms from thermodynamics models to kinetic anionic redox reaction pathways is comprehensively reviewed, and the existing challenges of LMLOs related with irreversible oxygen reaction behavior are analyzed. Furthermore, numerous representative modification strategies for overcoming these challenges, coupled with their underlying mechanisms for regulating anionic redox reversibility are summarized. In addition, the aspects of multi‐scale structural modifications, integration of interdisciplinary technologies, and application in quasi‐/all‐solid‐state battery systems are given some emphasis in terms of further improvement of LMLOs‐based cathode materials for advanced lithium batteries‐based energy storage systems.

Effect of the Nature of Both Cation and Anion Substitution on the Structural Symmetry of Li‐Rich 3d‐Metal Chalcogenide Electrodes

AbstractLi‐rich layered chalcogenides have recently led to better understanding of the anionic redox process and its associated high capacity while providing ways to overcome its practical limitations of voltage fade and irreversibility. This study reports on the feasibility of triggering anionic activity in Li2TiS3, through anionic substitution (Se for S) or cationic substitution (Fe for Ti). Herein, the chalcogenide chemical space is further explored to prepare mono‐substituted Li1.7Ti0.85Mn0.45Ch3 (Ch = S/Se) and doubly substituted cationic and anionic phases (Li1.7Ti0.85Fe0.45S3‐zSez) which crystallize either in the O3‐ or O1‐type structures depending upon substituents. All series show a bell‐shape capacity variation as function of the transition metal (TM) substitution degree with values up to 240 mAh g−1. For specific compositions, a structural O3 to O1 phase transition is observed upon Li removal, which is not reversible upon Li re‐insertion due to kinetic limitations and negatively affects long‐term cycling performance. Density functional theory (DFT) calculations confirm the O3/O1 relative stability along the different series and point subtle electronic differences in the TM‐doping, rationalizing the structural and electrochemical behaviors of these phases upon cycling. These findings provide further insights into the link between structural and electronic stability, which is of key importance for designing chalcogenide‐based anionic redox compounds.

Cationic–Anionic Redox Chemistry in Multivalent Metal‐Ion Batteries: Recent Advances, Reaction Mechanism, Advanced Characterization Techniques, and Prospects

AbstractRechargeable multivalent metal‐ion batteries (MMIBs) have garnered a surge of attention as competitive candidates for large‐scale energy storage applications owing to their high capacity, abundant resources, and good security. However, their practical implementation is still stuck at the prototype stage, mainly plagued with the lack of suitable cathode materials capable of reversible insertion/extraction of multivalent ions and the intrinsically complicated redox reaction mechanism. Recently, anionic redox chemistry has shown to be an effective strategy to increase energy density, providing a new research direction for the next generation of high‐energy rechargeable batteries. Unfortunately, anion redox chemistry has not received sufficient attention in MMIBs so far. Here, the fundamental principle and mechanism of anionic redox reactions in MMIBs are discussed and the recent advances regarding cathode materials based on cooperative cationic–anionic redox (CCAR) mechanism are summarized. Additionally, various advanced characterization techniques for studying the anionic redox process are highlighted, aiming to effectively illustrate the underlying reaction mechanism. Finally, challenges and perspectives for the future research on cationic–anionic redox chemistry in MMIBs are proposed. Insight into the significance of CCAR chemistry is provided here in MMIBs, presenting a new avenue for the development of high‐energy‐density cathode materials for MMIBs.

Facilitating structural stability of the Li-rich layered cathode materials by cation rearrangements and Li vacancies

The anomalously high capacities of Li-rich materials are a result of contributions from both cationic and anionic redox processes, however, anionic redox also leads to capacity loss and structural degradation. In this study, the content of Li-O-Li configuration and the types of Li-O-TM configuration are regulated by introducing the Li deficiencies and changing the ratio of Ni/Co/Mn. The results of XRD, Raman spectra, XPS, HR-TEM and in-situ XRD characterizations indicate that the generation of the spinel structure and crystal defects strengthen the structural stability in the modified Li-rich materials. Besides, the density functional theory (DFT) calculation result prove that increasing the ratio of Ni/(Co+Mn) further enhance the electron contribution of the Ni ions and the electronic conductivity of as-prepared samples. The LR1.8–811 sample presents outstanding cycling performance and energy density, that is, the capacity retention is 98.64% and energy density is 764.7 Wh kg−1 after 100 cycles at a rate of 0.2C, respectively. These improvements are ascribed to the less content of Li-O-Li configuration and the more content of Li-O-Ni configuration in the LR1.8–811 material, which mitigated the oxygen redox and enhanced the structural stability.

Structural insights into lithium-deficient type Li-rich layered oxide for high-performance cathode

As one of the promising candidate cathode materials for the high-performance lithium-ion batteries, Li-rich layered oxides still suffer from a series of critical drawbacks, such as voltage decay, oxygen release, irreversible migration of transition metal ions, etc. In this work, Li-deficient method has been confirmed as an effective approach to improve the overall electrochemical performances of Li-rich cathode. The optimized lithium-deficient Li-rich layered cathode exhibits splendid discharge capacity of ∼297 mAh/g at 0.1 C and prominent rate performance of ∼143 mAh/g at 5 C. Subsequently, neutron diffraction in combination with Raman spectroscopy is applied to explore and clarify the underlying mechanism for improved performances. It was found that the lithium-deficient induced nickel migration and oxygen vacancy play an significant role in improving electrochemical performances, because migration of nickel into Li layer is able to expand the Li layer spacing and reduce the Li/Ni antisite, leading to facilitated diffusion of lithium ions. Moreover, the formation of oxygen vacancy is able to promote anionic redox processes and suppress the gas release, thus leading to higher capacity. The results present valuable structural insights into the influence of lithium deficiency and provide guidance for the development of Li-rich cathode materials.

Modulation of Local Charge Distribution Stabilized the Anionic Redox Process in Mn-Based P2-Type Layered Oxides.

An anionic redox reaction is an extraordinary method for obtaining high-energy-density cathode materials for sodium-ion batteries (SIBs). The commonly used inactive-element-doped strategies can effectively trigger the O redox activity in several layered cathode materials. However, the anionic redox reaction process is usually accompanied by unfavorable structural changes, large voltage hysteresis, and irreversible O2 loss, which hinders its practical application to a large extent. In the present work, we take the doping of Li elements into Mn-based oxide as an example and reveal the local charge trap around the Li dopant will severely impede O charge transfer upon cycling. To overcome this obstacle, additional Zn2+ codoping is introduced into the system. Theoretical and experimental studies show that Zn2+ doping can effectively release the charge around Li+ and homogeneously distribute it on Mn and O atoms, thus reducing the overoxidation of O and improving the stability of the structure. Furthermore, this change in the microstructure makes the phase transition more reversible. This study aimed to provide a theoretical framework for further improve the electrochemical performance of similar anionic redox systems and provide insights into the activation mechanism of the anionic redox reaction.

Phosphorylation of Li-Rich Mn-Based Layered Oxides for Anion Redox and Structural Stability.

Li-rich Mn-based layered oxides are proposed to be candidates for high-energy Li-ion batteries. However, their large-scale production is still hampered by poor rate capability and severe voltage decay. It was mainly attributed to the irreversible oxygen loss, which induces transition metal ion migration, electrolyte consumption, and structural evolution. Herein, we propose an effective strategy of phosphorylation, in which the phosphate ion is induced to remove the surface labile oxygen. It urges the Li2MnO3 component to transform to the spinel-like structure and promotes the anionic redox process, thus facilitating lithium-ion diffusion and improving structural stability. As a result, the Li2MnO3 component is more prone to be activated, with the capacity increased by 18% in comparison with the pristine one. It also exhibits a superior capacity retention of 86.1% after 150 extended cycles and better rate performance delivering a capacity of 148.1 mA h g-1 even at 10 C. The effective phosphorylation opens a new way to tune anion redox chemistry and obtain structurally stable materials.

Facilitating both anionic and cationic redox processes in Na-rich layered cathode materials by heteroatomic doping

Low energy density was regarded as stumbling block for the commercialization process of sodium-ion batteries (SIBs). In addition to cationic redox process, anionic redox process is considered as supplementary process to contribute extra capacity in cathode materials of SIBs. Nevertheless, to develop high-performance cathodes with joint cationic and anionic redox activities are still facing great challenges, such as sluggish kinetics, irreversible evolution and poor cyclic property, etc. To explore the potential benefits of utilizing both cationic and anionic redox processes, heteroatomic doping strategy with cobalt was adopted to develop high-capacity Na-rich Na2RuO3 cathode. As a result, cobalt-doped layered cathode can exhibit not only high capacity of 177 mAh g−1 at 0.2C within 1.5–4.3 V, but also excellent rate performance with initial capacity of 127 mAh g−1 at high rate of 5C. Moreover, the fabulous capacity retention of 74% is also obtained after 300 cycles, representing prominent electrochemical reversibility as well as enhanced ionic redox of Na2RuO3 system. It is evident that cobalt serves as redox mediator to accelerate anionic kinetics and consequently facilitating anionic redox activity. This work would be of significance for the design of high-capacity cathodes with cationic/anionic redox activity for SIBs.

Cooperative Cationic and Anionic Redox Reactions in Ultrathin Polyvalent Metal Selenide Nanoribbons for High-Performance Electrochemical Magnesium-Ion Storage.

Rechargeable magnesium batteries (RMBs) are considered as potential energy storage devices due to their high volumetric specific capacity, good safety, as well as source abundance. Despite extensive efforts devoted to constructing an efficient magnesium battery system, the sluggish Mg2+ diffusion in conventional cathode materials often leads to slow rate kinetics, low capacity, and poor cycling lifespan. Although transition metal selenides with soft anion frameworks have attracted extensive attention, their Mg2+ storage mechanism still needs to be clarified. Herein, we demonstrate that the ultrathin CoSe2 nanoribbons can be used as a robust cathode material for RMBs and reveal a novel Mg2+ storage mechanism based on cooperative cationic (Co) and anionic (Se) redox processes via systematic ex-situ characterizations. Compared to other metal selenide cathodes based on conversion reactions of solely metal cations, the cooperative cationic-anionic redox reactions of the CoSe2 cathode contribute to obtaining an enhanced specific capacity and boosted electrochemical kinetics. Moreover, on one hand, the ultrathin nanoribbon structure enables effective contact between the electrode material and electrolyte and on the other hand significantly reduces the length and time consumption of Mg2+ diffusion, leading to dominated surface-driven capacitance-controlled Mg2+ storage behavior and rapid Mg2+ storage kinetics. As a result, the ultrathin CoSe2 nanoribbon cathode exhibits a reversible discharge capacity of ∼130 mAh g-1 at 100 mA g-1, good rate capability (116 mAh g-1 at 300 mA g-1), and long cyclability over 600 cycles. This finding confirms the development potentiality of polyvalent metal selenide cathode materials based on a cooperative cationic-anionic redox mechanism for the construction of next-generation multivalent secondary batteries.

Stabilization of Lattice Oxygen in Li‐Rich Mn‐Based Oxides via Swing‐like Non‐Isothermal Sintering

AbstractRedox from the holes at the O2p orbitals is a well‐known phenomenon in Li‐rich Mn‐based batteries. However, such an anionic redox process results in the formation of O2, leading to structural instability owing to unstable O2p holes. Herein, a swing‐like non‐isothermal sintering technique is used to stabilize the lattice oxygen by suppressing the formation of O2 during charging. It reduces both the number of intrinsic oxygen vacancies of the Li‐rich Mn‐based oxides and the formation of O2 during charging as compared with traditional constant high‐temperature sintering. Consequently, the number of holes generated during charging in the O2p orbitals increases, whereas the number of unstable O2p holes forming O2 decreases. Therefore, the sample prepared via swing‐like non‐isothermal sintering exhibited considerably slower voltage fading and better cycling stability. This study provides valuable guidelines for stabilizing the lattice oxygen and improving the structural stability of the oxide cathodes for electrochemical energy storage.

Stabilizing anionic redox processes in electrospun NiS2–based cathode towards durable aluminum-ion batteries

Rechargeable aluminum-ion batteries (AIBs) are receiving considerable attention as a desirable device for large-scale energy storage owing to high theoretical capacity and abundance of aluminum. However, due to strong charge of Al3+, the state-of-the-art AIBs often show sluggish electrode reaction kinetics and rapid capacity fading and the available cathode materials always demonstrate poor structural stability, thereby greatly hindering their practical use. NiS2 with anion redox species (S22– dimers) and favorable electronic conductivity is a promising cathode to boost the performance of AIBs in terms of reversible capacity, rate capability and cycling stability. Here, we report a systematic investigation of the Al storage behavior and mechanism of NiS2/S-doped carbon (NiS2/SC) cathode based on a series of electrochemical tests and ex situ measurements. We further develop electrospun NiS2/S-doped carbon@S-doped carbon nanofiber (NiS2/SC@SCNF) structure as the cathode of AIBs. The as-fabricated AIB delivers an unprecedented Al3+ storage performance with a stable capacity of 76 mAh/g at 0.5 A/g CV 500 cycles and a superior cycling Coulombic efficiency of 97 %. This study reveals that NiS2/SC@SCNF undergoes a reversible evolution of initial Al3+ insertion followed by anionic redox between S22– and S2–, paving the road for the futher development of NiS2–based cathodes for AIBs.

Impact of Ni Content on the Structure and Electrochemical Performance of the Co-Free, Li/Mn-Rich Layered Cathode Materials

Since early 1990s lithium-ion batteries (LIBs) have become the most important secondary/rechargeable battery technology for portable electronic devices; and more recently for battery powered vehicles. Extensive research has been conducted on high-capacity electrode materials for LIBs due to the emerging demands for high energy density batteries for electric vehicles. Compared with the conventional cationic redox-based cathode materials, such as NMC, LMO and LFP, Li-excess, Manganese rich layered cathode (LLC) materials containing cationic and anionic redox process, show great potential as the next generation cathode materials because of their higher theoretical specific capacity and lower raw material cost.Our work has shown that Ni content of the Co-free, LLC materials, Li1.2NixMn0.8-xO2 (x = 0.12, 0.24, 0.30 and 0.36) prepared by a citric acid method, has a significant impact on the material structure, electrochemical performance, and thermal stabilities. Generally, the primary particle size increased as the Ni content increased with a primary particle size distribution in the range of 200–600 nm for the samples investigated. Monoclinic phase (C2/m) and spinel (Fd3m) phase depending on the content of Ni have been identified by XRD. With increasing Ni content, the fraction of the spinel phase decreased and eventually the pure layered structure was obtained on the samples at x = 0.30 and 0.36. Our work demonstrates that the charging and discharging capacities are determined by the Ni or Mn content. The highest charging and discharging capacities were achieved when x = 0.24. As Ni content increased, the first cycle coulombic efficiency decreased, and the thermal stability deteriorated. But, an increase in Ni content results in an increase in average discharge voltage, higher electrochemical stability of the layered structure, fast Li ions diffusivity. Li1.2Ni0.12Mn0.68O2 sample exhibited the best capacity retention and thermal stability among the tested samples. However, its specific energy density was limited due to the lowest average discharge voltage. The Li1.2Ni0.36Mn0.44O2 sample which contained the highest Ni content exhibited the worst thermal stability, but relatively higher specific energy density due to the highest average discharge voltage.

High-Capacity Cathodes for All-Solid-State Thin-Film Batteries

Lithium-ion batteries (LIBs) are critically important rechargeable energy storage devices, finding widespread application in consumer electronics and electric vehicles. However, safety issues associated with their flammability impede their development for many applications. As a fully solid-state device, the thin film battery (TFB) is a compelling candidate for advanced microscale applications, including wearable devices, smart ID cards, and microelectromechanical systems (MEMS). A typical TFB design consists of solid thin films of crystalline cathode, amorphous LiPON electrolyte, Li metal anode and current collectors supported on a rigid substrate. Polycrystalline cathodes are often employed, such as LiCoO2, Li2Mn2O4, LiNiO2, V2O5, etc., because they provide high specific capacity and good reversibility. However, these cathodes require thermal treatment to generate the crystalline microstructure for optimal electrochemical performance, demanding substrates that are stable to several hundreds of degrees Celsius. The industry is interested in lower-cost, flexible all-solid-state thin-film batteries that can be readily integrated into a wide range of applications, including wearable devices. Thus, there is strong demand for thin film cathodes that can be processed at low temperatures.Herein, we report a new amorphous inorganic thin-film cathode material, which is a solid state composite of Li2RuO3 and LiCoO2. (LRCO). Without any post-deposition annealing steps, as-deposited LRCO thin films possess a high discharge capacity of over 110 µAh cm-2 µm-1 at 0.3 C and capacity retention over 92% after 150 cycles owing to both cationic and anionic (oxygen oxidation) redox processes, which is competitive with conventional polycrystalline thin-film cathodes. The surface of LRCO cathode is smooth and isotropic, leading to uniform Li ion transport and thereby reducing the possibility of short-circuiting. More importantly, the entire preparation process is conducted at room temperature, enabling wider substrate options for TFBs, such as Kapton® films, polyethylene terephthalate (PET) films, or even paper sheets. Using PET plastic sheets to support LRCO TFBs; a specific capacity of 101 µAh cm-2 µm-1 at 0.3 C and capacity retention of 97.5% after 120 cycles was achieved, even when the substrate was mechanically bent (flexed).

Anionic Redox Regulated via Metal-Ligand Combinations in Layered Sulfides.

The 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 wasdesigned 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 NaCr1- y Vy S2 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.

On the Origin of Reversible and Irreversible Reactions in LiNixCo(1−x)/2Mn(1−x)/2O2

Bond formation and breakage is crucial upon energy storage in lithium transition metal oxides (LiMeO2, Me = Ni, Co, Mn), i.e., the conventional cathode materials in Li ion batteries. Near-edge X-ray absorption finestructure spectroscopy (NEXAFS) of the Me L and O K edge performed upon the first discharge of LiNixCo(1−x)/2Mn(1−x)/2O2 (x = 0.33: NCM111, x = 0.6: NCM622, x = 0.8: NCM811) in combination with charge transfer multiplet (CTM) calculations provide unambiguous evidence that redox reactions in NCMs proceed via a reversible oxidation of Ni associated with the formation of covalent bonds to O neighbors, and not, as widely assumed, via pure cationic or more recently discussed, pure anionic redox processes. Correlating these electronic changes with crystallographic data using operando synchrotron X-ray powder diffraction (SXPD) shows that the amount of ionic Ni limits the reversible capacity— at states of charge where all ionic Ni is oxidized (above 155 mAh g−1), the lattice parameters collapse, and irreversible reactions are observed. Yet the covalence of the Ni–O bonds also triggers the electronic structure and thus the operation potential of the cathodes.

(Battery Division Student Research Award Address Sponsored by Mercedes-Benz Research & Development) Towards Understanding and Controlling Bulk Anionic Redox in Li-ion Positive Electrode Materials Through Electronic Structure Tuning

Positive electrode materials such as layered lithium nickel, manganese and cobalt oxides (NMC)1 ,2 in Li-ion batteries typically store charge by relying on the redox activity of transition metal species, which is accompanied by the intercalation and deintercalation of Li ions into and out the host structure. Anionic redox in positive electrode materials can provide additional charge storage beyond the conventional metal redox. However, the physical origin of observed anion redox as well as the requirements for the reversible anionic redox activity remain under debate, hindering rational design of new electrode materials leveraging reversible anionic redox.In this talk, we first focus on understanding the cationic and anionic redox process in the positive electrode materials upon lithium deintercalation using X-ray absorption and emission spectroscopy (XAS and XES), X-ray photoelectron spectroscopy (XPS), coupled with density functional theory (DFT) calculations. We show electronic signatures of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O2-/(O2)n-), in charged Li2-xRuO3 after Ru oxidation (Ru4+/Ru5+) upon first-electron removal with lithium de-intercalation. This lattice oxygen redox of Li2-xRuO3 was accompanied by bulk Ru reduction.3 This observed redox trend is in stark contrast of the observations in charged Ni-rich NMC upon charging. In Ni-rich NMC positive electrodes, nickel oxidation is primarily responsible for the charge capacity up to removing ~0.7 Li, beyond which is followed by Ni reduction near the surface (up to 100 nm) due to oxygen release, where there is no significant bulk metal reduction observed.4 The uniqueness of Ru-based system lies in the highly covalent nature of Ru-O bond, which stabilizes the O2-/(O2)n- intermediates, forbidding further oxygen release.We further discuss that a strong metal-oxygen covalency is needed to enhance a reversible anionic redox.5 Differential electrochemical mass spectrometry (DEMS) was employed to monitor the oxygen release and quantify the reversibility of anionic redox of Li2Ru0.75 M 0.25O3 (M= Ti, Cr, Mn, Fe, Ru, Sn, Pt, Ir) upon first charge. We show through X-ray absorption spectroscopy that more ionic substituents and reduced metal-oxygen covalency introduce irreversible oxygen redox, accompanied with easier distortion of M-O octahedron and smaller barrier for forming oxygen dimer within the octahedron. We propose a universal electronic structure descriptor that could control bulk anionic redox, leading to a rational design strategy to enhance the cycling stability of Ni-rich NMC6 as well as Li-rich positive electrodes. Our study has laid a solid foundation for future high-throughput screening of novel and affordable metal oxides for battery and electrocatalysis applications. Reference (1) Noh, H.-J.; Youn, S.; Yoon, C. S.; Sun, Y.-K. Comparison of the Structural and Electrochemical Properties of Layered Li [NixCoyMnz] O2 (X= 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) Cathode Material for Lithium-Ion Batteries. Journal of power sources 2013, 233, 121–130.(2) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNixMnyCozO2 (NMC) Cathode Materials for Li-Ion Batteries. Journal of The Electrochemical Society 2017, 164 (7), A1361–A1377.(3) Yu, Y.; Karayaylali, P.; Nowak, S. H.; Giordano, L.; Gauthier, M.; Hong, W.; Kou, R.; Li, Q.; Vinson, J.; Kroll, T. Revealing Electronic Signatures of Lattice Oxygen Redox in Lithium Ruthenates and Implications for High-Energy Li-Ion Battery Material Designs. Chemistry of Materials 2019, 31 (19), 7864–7876.(4) Yu, Y.; Karayaylali, P.; Giordano, L.; Corchado-García, J.; Hwang, J.; Sokaras, D.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-Horn, Y. Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries. ACS Applied Materials & Interfaces 2020.(5) Yu, Y.; Karayaylali, P.; Sokaras, D.; Giordano, L.; Kou, R.; Sun, C.-J.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-Horn, Y. Towards Controlling the Reversibility of Anionic Redox in Transition Metal Oxides for High-Energy Li-Ion Positive Electrodes. Energy & Environmental Science 2021, 14 (4), 2322–2334.(6) Yu, Y.; Zhang, Y.; Giordano, L.; Zhu, Y. G.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-horn, Y. Enhanced Cycling of Ni-Rich Positive Electrodes by Fluorine Modification. Journal of The Electrochemical Society 2021.

Stabilizing Anionic Redox Reactions By Regulating Frontier Orbitals in Cation-Disordered Rock-Salt Oxides

Cation-disordered rock-salt oxides (DRXs) with both cationic and O2-/O2 n- redox reactions, such as Li1.2Mn0.4Ti0.4O2, Li1.25Nb0.25Fe0.5O2, and Li1.3Ta0.3Mn0.4O2 are promising materials for designing a new generation of lithium-ion batteries with high energy density. [1, 2] However, the irreversible loss of lattice oxygen during charge/discharge cycling, fast capacity fading, and severe voltage decay are three major challenges, which impede the practical application of those materials. [3, 4] Therefore, developing strategies to stabilize the anionic redox processes in DRXs cathodes is currently a major focus of the battery research community.Here, we combine experiment and theory to demonstrate that regulating frontier orbitals via the redox-active transition-metal ratio is an effective strategy to enhance the oxygen reaction stability to enhance the cycling stability of DRXs [5]. We investigated a series of xLi2TiO3-(1 - x)LiMnO2 (0 ≤ x ≤1) materials and discovered that only Li1.2Mn0.4Ti0.4O2 (x=0.4) and Li1.1Mn0.7Ti0.2O2 (x=0.2) can form phase-pure DRXs. The newly discovered Li1.1Mn0.7Ti0.2O2 DRX exhibits a remarkable capacity retention of 84.4% after 20 cycles compared to only 60.8% for Li1.2Mn0.4Ti0.4O2. Based on the dQ/dV curves and X-ray absorption near edge structure (XANES) results, we show that the reversibility of the lattice oxygen redox of the materials improves while the cationic Mn redox reaction dominates. Therefore, adjusting the TM and Li content in the oxides is an important strategy to balance the high capacity enabled by the oxygen redox and the cyclic stability of the DRXs. Density functional theory (DFT) calculations reveal that the Mn-to-Ti ratio, which also determines the lithium content by charge balance, intrinsically regulates redox-active frontier orbitals close to the Fermi level, i.e., (Mn-O)* antibonding orbitals in Li1.1Mn0.7Ti0.2O2 and Li-O-Li unhybridized orbitals in Li1.2Mn0.4Ti0.4O2. The former results in low charge transfer to anionic O and a high aggregation barrier, promoting high reversibility of the oxygen redox reaction. Whereas, the latter generates high charge transfer to anionic O and a low O···O aggregation barrier, resulting in low cycling stability and O2 release.