The Role of Metal Substitution in Tuning Anion Redox in Sodium Metal Layered Oxides Revealed by X-Ray Spectroscopy and Theory

Local Structure Regulation for Oxygen Redox and Structure Stability of P2-Type Cathodes

The local structure plays a crucial role in oxygen redox reactions, which boosts the capacity of layered oxide cathodes for sodium-ion batteries. While studies on local structural ordering have primarily focused on the intra-layer ordering, there has been limited research on the inter-layer stacking for the layered cathode materials for sodium-ion batteries. In this work, the impact of the intra-layer and inter-layer local structural regulation on anionic kinetics and the structure stability are explored through experimental analysis and theoretical calculations. Cu2+ substitution is introduced to adjust the transition metal inter-layer structure of P2-Na0.67Mg0.28Mn0.72O2, obtaining a zig-zag stacked honeycomb superlattice structure in P2- Na0.67Cu0.14Mg0.14Mn0.72O2. The local structure regulation mitigates the cation migration, improves the structure reversibility even at a deeply desodiation state of Na0.05, and the reductive coupling between cationic and anionic redox processes facilitates electron transfer from oxygen to copper ions and governs the properties of electrochemical kinetics and hysteresis. A full cell with hard carbon anode shows commendable energy density at high power density. This study paves an optional path for enhancing the structure stability and dynamics of oxygen redox chemistry in P2-type cathode materials for sodium-ion battery systems.

K-Doping Suppresses Oxygen Redox in P2-Na0.67Ni0.11Cu0.22Mn0.67O2 Cathode Materials for Sodium-Ion Batteries

In P2-type layered oxide cathodes, Na site-regulation strategies are proposed to modulate the Na+ distribution and structural stability. However, their impact on the oxygen redox reactions remains poorly understood. Herein, the incorporation of K+ in the Na layer of Na0.67Ni0.11Cu0.22Mn0.67O2 is successfully applied. The effects of partial substitution of Na+ with K+ on electrochemical properties, structural stability, and oxygen redox reactions have been extensively studied. Improved Na+ diffusion kinetics of the cathode is observed from galvanostatic intermittent titration technique (GITT) and rate performance. The valence states and local structural environment of the transition metals (TMs) are elucidated via operando synchrotron X-ray absorption spectroscopy (XAS). It is revealed that the TMO2 slabs tend to be strengthened by K-doping, which efficiently facilitates reversible local structural change. Operando X-ray diffraction (XRD) further confirms more reversible phase changes during the charge/discharge for the cathode after K-doping. Density functional theory (DFT) calculations suggest that oxygen redox reaction in Na0.62K0.03Ni0.11Cu0.22Mn0.67O2 cathode has been remarkably suppressed as the nonbonding O 2p states shift down in the energy. This is further corroborated experimentally by resonant inelastic X-ray scattering (RIXS) spectroscopy, ultimately proving the role of K+ incorporated in the Na layer.

Unexpected Elevated Working Voltage by Na+/Vacancy Ordering and Stabilized Sodium-Ion Storage by Transition-Metal Honeycomb Ordering

Na+/vacancy ordering in sodium-ion layered oxide cathodes is widely believed to deteriorate the structural stability and retard the Na+ diffusion kinetics, but its unexplored potential advantages remain elusive. Herein, we prepared a P2-Na0.8Cu0.22Li0.08Mn0.67O2 (NCLMO-12 h) material featuring moderate Na+/vacancy and transition-metal (TM) honeycomb orderings. The appropriate Na+/vacancy ordering significantly enhances the operating voltage and the TM honeycomb ordering effectively strengthens the layered framework. Compared with the disordered material, the well-balanced dual-ordering NCLMO-12 h cathode affords a boosted working voltage from 2.85 to 3.51 V, a remarkable ~20 % enhancement in energy density, and a superior cycling stability (capacity retention of 86.5 % after 500 cycles). The solid-solution reaction with a nearly "zero-strain" character, the charge compensation mechanisms, and the reversible inter-layer Li migration upon sodiation/desodiation are unraveled by systematic in situ/ex situ characterizations. This study breaks the stereotype surrounding Na+/vacancy ordering and provides a new avenue for developing high-energy and long-durability sodium layered oxide cathodes.

Sodium layered oxide cathodes: properties, practicality and prospects

Rechargeable sodium-ion batteries (SIBs) have emerged as an advanced electrochemical energy storage technology with potential to alleviate the dependence on lithium resources. Similar to Li-ion batteries, the cathode materials play a decisive role in the cost and energy output of SIBs. Among various cathode materials, Na layered transition-metal (TM) oxides have become an appealing choice owing to their facile synthesis, high Na storage capacity/voltage that are suitable for use in high-energy SIBs, and high adaptivity to the large-scale manufacture of Li layered oxide analogues. However, going from the lab to the market, the practical use of Na layered oxide cathodes is limited by the ambiguous understanding of the fundamental structure-performance correlation of cathode materials and lack of customized material design strategies to meet the diverse demands in practical storage applications. In this review, we attempt to clarify the fundamental misunderstandings by elaborating the correlations between the electron configuration of the critical capacity-contributing elements (e.g., TM cations and oxygen anion) in oxides and their influence on the Na (de)intercalation (electro)chemistry and storage properties of the cathode. Subsequently, we discuss the issues that hinder the practical use of layered oxide cathodes, their origins and the corresponding strategies to address their issues and accelerate the target-oriented research and development of cathode materials. Finally, we discuss several new Na layered cathode materials that show prospects for next-generation SIBs, including layered oxides with anion redox and high entropy and highlight the use of layered oxides as cathodes for solid-state SIBs with higher energy and safety. In summary, we aim to offer insights into the rational design of high-performance Na layered oxide cathode materials towards the practical realization of sustainable electrochemical energy storage at a low cost.

Modulating Valence Electrons and Na Occupancy in Layered Cathodes for High-Performance Na-Ion Batteries

Na-ion batteries (NIBs) hold promise as a leading option for large-scale energy storage. However, their development faces challenges due to the lack of high-performance cathode materials. P2-type layered oxides are seen as potential cathode materials for NIBs due to higher structure stability, yet their commercialization is hindered by limited capacity and subpar phase transitions during Na extraction and insertion at high voltages. In this study, we introduce a new P2-type cathode material, Na0.76Ni0.23Li0.1Ti0.02Mn0.65O1.998F0.02 (NLTMOF), synthesized with ternary Li/Ti/F substitution. This modification of ternary Li/Ti/F substitution significantly tailors the electronic structures, increasing the number of valence electrons near the Fermi energy level. This facilitates the electronic conductivity and their involvement in charge compensation, thereby enhancing reversible capacity. Additionally, ternary doping synergistically adjusts the Na occupancy at the Na layer for favorable Na extraction without P2-O2 phase transitions even under a high voltage of 4.4 V, boosting cycling stability. As a result, NLTMOF demonstrates a reversible capacity of 110.0 and 132.2 mAh g-1 at 2-4.2 and 2-4.4 V, respectively, and maintains greatly enhanced cycling stability over long cycles. This study sheds light on the design of transition metal oxides for advanced cathode materials through the modulation of electronic structure and Na occupancy in cathode materials, thus promoting the development of NIBs.

High Voltage Ga-Doped P2-Type Na2/3 Ni0.2 Mn0.8 O2 Cathode for Sodium-Ion Batteries

Ni/Mn-based oxide cathode materials have drawn great attention due to their high discharge voltage and large capacity, but structural instability at high potential causes rapid capacity decay. How to moderate the capacity loss while maintaining the advantages of high discharge voltage remains challenging. Herein, the replacement of Mn ions by Ga ions is proposed in the P2-Na2/3 Ni0.2 Mn0.8 O2 cathode for improving their cycling performances without sacrificing the high discharge voltage. With the introduction of Ga ions, the relative movement between the transition metal ions is restricted and more Na ions are retained in the lattice at high voltage, leading to an enhanced redox activity of Ni ions, validated by ex situ synchrotron X-ray absorption spectrum and X-ray photoelectron spectroscopy. Additionally, the P2-O2 phase transition is replaced by a P2-OP4 phase transition with a smaller volume change, reducing the lattice strain in the c-axis direction, as detected by operando/ex situ X-ray diffraction. Consequently, the Na2/3 Ni0.21 Mn0.74 Ga0.05 O2 electrode exhibits a high discharge voltage close to that of the undoped materials, while increasing voltage retention from 79% to 93% after 50 cycles. This work offers a new avenue for designing high-energy density Ni/Mn-based oxide cathodes for sodium-ion batteries.

Boosting the Reversibility and Kinetics of Anionic Redox Chemistry in Sodium-Ion Oxide Cathodes via Reductive Coupling Mechanism

Activating anionic redox chemistry in layered oxide cathodes is a paradigmatic approach to devise high-energy sodium-ion batteries. Unfortunately, excessive oxygen redox usually induces irreversible lattice oxygen loss and cation migration, resulting in rapid capacity and voltage fading and sluggish reaction kinetics. Herein, the reductive coupling mechanism (RCM) of uncommon electron transfer from oxygen to copper ions is unraveled in a novel P2-Na0.8Cu0.22Li0.08Mn0.67O2 cathode for boosting the reversibility and kinetics of anionic redox reactions. The resultant strong covalent Cu-(O-O) bonding can efficaciously suppress excessive oxygen oxidation and irreversible cation migration. Consequently, the P2-Na0.8Cu0.22Li0.08Mn0.67O2 cathode delivers a marvelous rate capability (134.1 and 63.2 mAh g-1 at 0.1C and 100C, respectively) and outstanding long-term cycling stability (82% capacity retention after 500 cycles at 10C). The intrinsic functioning mechanisms of RCM are fully understood through systematic in situ/ex situ characterizations and theoretical computations. This study opens a new avenue toward enhancing the stability and dynamics of oxygen redox chemistry.

Revealing the structural chemistry in Na6-2xFex(SO4)3 (1.5 ≤ x ≤ 2.0) for low-cost and high-performance sodium-ion batteries

Fe-based polyanionic sulfate materials are one of the most promising candidates for large-scale applications in sodium-ion batteries due to their low cost and excellent electrochemical performance. Although great achievements have been gained on a series of Na6-2xFex(SO4)3 (NFSO-x, 1.5 ≤ x ≤ 2.0) materials such as Na2Fe2(SO4)3, Na2Fe1.5(SO4)3, and Na2.4Fe1.8(SO4)3 for sodium storage, the phase and structure characteristics on these NFSO-x are still controversial, making it difficult to achieve phase-pure materials with optimal electrochemical properties. Herein, six NFSO-x samples with varied x are investigated via both experimental methods and density functional theory calculations to analyze the phase and structure properties. It reveals that a pure phase exists in the 1.6 ≤ x ≤ 1.7 region of the NFSO-x, and part of Na ions tend to occupy Fe sites to form more stable frameworks. The NFSO-1.7 exhibits the best electrochemical performance among the NFSO-x samples, delivering a high discharge capacity (104.5 mAh g-1 at 0.1 C, close to its theoretical capacity of 105 mAh g-1), excellent rate performance (81.5 mAh g-1 at 30 C), and remarkable cycle stability over 10,000 cycles with high-capacity retention of 72.4%. We believe that the results are useful to clarify the phase and structure characteristics of polyanionic materials to promote their application for large-scale energy storage.

Boosting the electrochemical performance of Li-rich Mn-based cathode materials via oxygen vacancy and spinel phase integration

Li-rich Mn-based oxide cathodes (LMOs) are regarded as one of the most prospective high energy density cathodes due to the reversible anion redox reaction, which gives them a very high capacity. However, LMOs materials usually have problems like low initial coulombic efficiency (ICE) and poor cycling performance during cycling, which are associated with irreversible surface O₂ release and unfavourable electrode/electrolyte interface side reactions. Herein, an innovative and scalable NH₄Cl-assisted gas-solid interfacial reaction treatment technique is employed to construct oxygen vacancies and spinel/layered heterostructures simultaneously on the surface of LMOs. The synergistic effect of the oxygen vacancy and the surface spinel phase can not only effectively enhance the redox properties of the oxygen anion and inhibit irreversible oxygen release, but also effectively mitigate the side reactions at the electrode/electrolyte interface, inhibit the formation of CEI films and stabilize the layered structure. The electrochemical performance of the treated NC-10 sample improved significantly, showing an increase in ICE from 77.4 % to 94.3 % and excellent rate capability and cycling stability, with a capacity retention of 77.9 % after 400 cycles at 1 C. This oxygen vacancy and spinel phase integration strategy offers an exciting prospect and avenue for improving the integrated electrochemical performance of LMOs.

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.

Realizing High-Performance Cathodes with Cationic and Anionic Redox Reactions in High-Sodium-Content P2-Type Oxides for Sodium-Ion Batteries

P2-type layered transition-metal oxides with anionic redox reactions are promising cathodes for sodium-ion batteries. In this work, a high-sodium-content P2-type Na7/9Li1/9Mg1/9Cu1/9Mn2/3O2 (NLMC) cathode material is prepared by substituting Li/Mg/Cu for Mn sites in Na2/3MnO2. The Li/Mg ions trigger the anionic redox reaction, while the Cu ions enhance the structure stability during electrochemical cycling. As a result, the oxide has a high reversible capacity of 225 mAh g-1 originating from both cationic and anionic redox activities with a capacity retention of 77% after 100 cycles. The migration energy barrier and Na ion diffusion kinetics are studied using density functional theory (DFT) calculations and the galvanostatic intermittent titration technique. Furthermore, X-ray diffraction, DFT, scanning electron microscopy, and transmission electron microscopy are applied to reveal the structural evolution and charge compensation of NLMC, providing a thorough understanding of the structural and morphology evolution of Na-deficient oxides during cycling. The results are inspiring for the design of a high-Na content P2-type layered oxide cathode for sodium-ion batteries.

Study of Synergistic Effects of Cu and Fe on P2-Type Na0.67MnO2 for High Performance Na-Ion Batteries

P2-type Na_0.67MnO₂ with a stable structure and an open framework can provide numerous channels for fast Na⁺ de/intercalation, for which it is considered to be advantageous in application of the cathode material for Na-ion batteries. However, the complex phase transition occurring during cycling and the lattice distortion triggered by the Jahn-Teller effect severely restrict its development. Herein, the modified Na_0.67MnO₂ with Cu or Fe single-element doping as well as Cu and Fe double-element doping was synthesized by the sol-gel method, and the effects of doping on the crystal structure and electrochemical performances of Na_0.67MnO₂ were studied. It was demonstrated that the phase of the material did not change after the introduction of Cu and Fe elements, and the cycling stability and rate performance were greatly improved by Cu and Fe double-doping owing to their synergistic effect. The Na_0.67Mn_0.92Fe_0.04Cu_0.04O₂ (NMFCO) cathode delivers discharge specific capacities of 110.5 mA h g^-1 at 5 C and 91.8 mA h g^-1 at 10 C and exhibits the high-capacity retention of 94.35% at 1 C and 90.68% at 5 C after 100 cycles. Overall, this study offers a guiding direction for accelerating the modification of P2-type Na_0.67MnO₂ as a cathode active material for high performance Na-ion batteries.

Anomalous Redox Features Induced by Strong Covalency in Layered NaTi1-y Vy S2 Cathodes for Na-Ion Batteries

The rising demand for energy density of cathodes means the need to raise the voltage or capacity of cathodes. Transition metal (TM) doping has been employed to enhance the electrochemical properties in multiple aspects. The redox voltage of doped cathodes usually falls in between the voltage of undoped layered cathodes. However, we found anomalous redox features in NaTi_1-y V_y S₂ . The first discharge platform potential (2.4 V) is significantly higher than that of undoped NaTiS₂ and NaVS₂ (both around 2.2 V), and the energy density is raised by 15 %. We speculate that the anomalous voltage is mainly attributed to the strong hybridization in the Ti-V-S system. Ti³⁺ and V³⁺ undergo charge transfer and form a more stable Ti (t_2g ⁰ e_g ⁰ ) and V (t_2g ³ e_g ⁰ ) electronic configuration. Our results indicate that higher voltage of cathode materials could be achieved by strong TM-ligand covalency, and this conclusion provides possible opportunities to explore high voltage materials for future layered cathodes.

Tailoring Anionic Redox Activity in a P2-Type Sodium Layered Oxide Cathode via Cu Substitution

Na-ion cathode materials cycling at high voltages with long cycling life and high capacity are of imminent need for developing future high-energy Na-ion batteries. However, the irreversible anionic redox activity of Na-ion layered cathode materials results in structural distortion and poor capacity retention upon cycling. Herein, we develop a facile doping strategy by incorporating copper into the layered cathode material lattice to relieve the irreversible oxygen oxidation at high voltages. On the basis of a comprehensive comparison with the Cu-free material, both the over-oxidation of O^2- to trapped molecular O₂ and Mn-related Jahn-Teller distortion have been effectively inhibited by restraining both the oxygen activity and participation of Mn⁴⁺/Mn³⁺ redox activity. Not limited to discovering stable cycling behavior at high voltages after Cu substitution, our findings also highlight an effective strategy to stabilize the anionic redox activity and elucidate the stabilization mechanism of Cu substitution, thus paving the way for further improvement of layered oxide cathode materials for high-energy Na-ion batteries.

Uncommon Behavior of Li Doping Suppresses Oxygen Redox in P2-Type Manganese-Rich Sodium Cathodes

Utilizing both cationic and anionic oxygen redox reactions is regarded as an important approach to exploit high-capacity layered cathode materials with earth abundant elements. It has been popular strategies to effectively elevate the oxygen redox activities by Li-doping to introduce unhybridized O 2p orbitals in Na_x MnO₂ -based chemistries or enabling high covalency transition metals in P2-Na_0.66 Mn_x TM_{1- x} O₂ (TM = Fe, Cu, Ni) materials. Here, the effect of Li doping on regulating the oxygen redox activities P2-structured Na_0.66 Ni_0.25 Mn_0.75 O₂ materials is investigated. Systematic X-ray characterizations and ab initio simulations have shown that the doped Li has uncommon behavior in modulating the density of states of the neighboring Ni, Mn, and O, leading to the suppression of the existing oxygen and Mn redox reactivities and the promotion of the Ni redox. The findings provide a complementary scenario to current oxygen redox mechanisms and shed lights on developing new routes for high-performance cathodes.

Tailoring Co3d and O2p Band Centers to Inhibit Oxygen Escape for Stable 4.6 V LiCoO2 Cathodes

High-voltage LiCoO₂ delivers a high capacity but sharp fading is a critical issue, and the capacity decay mechanism is also poorly understood. Herein, we clarify that the escape of surface oxygen and Li-insulator Co₃ O₄ formation are the main causes for the capacity fading of 4.6 V LiCoO₂ . We propose the inhibition of the oxygen escape for achieving stable 4.6 V LiCoO₂ by tailoring the Co3d and O2p band center and enlarging their band gap with MgF₂ doping. This enhances the ionicity of the Co-O bond and the redox activity of Co and improves cation migration reversibility. The inhibition of oxygen escape suppresses the formation of Li-insulator Co₃ O₄ and maintains the surface structure integrity. Mg acts as a pillar, providing a stable and enlarged channel for fast Li⁺ intercalation/extraction. The modulated LiCoO₂ shows almost zero strain and achieves a record capacity retention at 4.6 V: 92 % after 100 cycles at 1C and 86.4 % after 1000 cycles at 5C.

Stabilizing Transition Metal Vacancy Induced Oxygen Redox by Co2+ /Co3+ Redox and Sodium-Site Doping for Layered Cathode Materials

Anionic redox is an effective way to boost the energy density of layer-structured metal-oxide cathodes for rechargeable batteries. However, inherent rigid nature of the TMO₆ (TM: transition metals) subunits in the layered materials makes it hardly tolerate the inner strains induced by lattice glide, especially at high voltage. Herein, P2-Na_0.8 Mg_0.13 [Mn_0.6 Co_0.2 Mg_0.07 □_0.13 ]O₂ (□: TM vacancy) is designed that contains vacancies in TM sites, and Mg ions in both TM and sodium sites. Vacancies make the rigid TMO₆ octahedron become more asymmetric and flexible. Low valence Co²⁺ /Co³⁺ redox couple stabilizes the electronic structure, especially at the charged state. Mg²⁺ in sodium sites can tune the interlayer spacing against O-O electrostatic repulsion. Time-resolved in situ X-ray diffraction confirms that irreversible structure evolution is effectively suppressed during deep desodiation benefiting from the specific configuration. X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations demonstrate that, deriving from the intrinsic vacancies, multiple local configurations of "□-O-□", "Na-O-□", "Mg-O-□" are superior in facilitating the oxygen redox for charge compensation than previously reported "Na-O-Mg". The resulted material delivers promising cycle stability and rate capability, with a long voltage plateau at 4.2 V contributed by oxygen, and can be well maintained even at high rates. The strategy will inspire new ideas in designing highly stable cathode materials with reversible anionic redox for sodium-ion batteries.

Coulombically-stabilized oxygen hole polarons enable fully reversible oxygen redox

Stabilizing high-valent redox couples and exotic electronic states necessitate an understanding of the stabilization mechanism. In oxides, whether they are being considered for energy storage or computing, highly oxidized oxide-anion species rehybridize to form short covalent bonds and are related to significant local structural distortions. In intercalation oxide electrodes for batteries, while such reorganization partially stabilizes oxygen redox, it also gives rise to substantial hysteresis. In this work, we investigate oxygen redox in layered Na2-XMn3O7, a positive electrode material with ordered Mn vacancies. We prove that coulombic interactions between oxidized oxideanions and the interlayer Na vacancies can disfavor rehybridization and stabilize hole polarons on oxygen (O-) at 4.2 V vs. Na/Na+. These coulombic interactions provide thermodynamic energy saving as large as O-O covalent bonding and enable ~ 40 mV voltage hysteresis over multiple electrochemical cycles with negligible voltage fade. Our results establish a complete picture of redox energetics by highlighting the role of coulombic interactions across several atomic distances and suggest avenues to stabilize highly oxidized oxygen for applications in energy storage and beyond.