Enhancing the Reversibility of Lattice Oxygen Redox Through Modulated Transition Metal-Oxygen Covalency for Layered Battery Electrodes

Superexchange interaction regulates Ni/Mn spin states triggering Ni-t2g/O-2p reductive coupling enabling stable lithium-rich cathode

Lithium-rich layer oxides are expected to be high-capacity cathodes for next-generation lithium-ion batteries, but their performance is hindered by irreversible anionic redox, leading to voltage decay, lag, and slow kinetics. In order to solve these problems, we regulate the Ni/Mn spin state in Li1.2Mn0.6Ni0.2O2 by Be doping, which generates the superexchange interaction and activates Ni-t2g orbitals. The activation of Ni-t2g orbitals triggers the reductive coupling mechanism between Ni/O, which improves the reversibility and kinetics of anionic redox. The strong π-type Ni-t2g/O-2p interaction forms a stable Ni-(O-O) configuration, suppressing excessive anion oxidation. In this work, the Be modified cathodes have good cycle stability, 0.04 mAh/g and 0.5 mV decay per cycle over 400 cycles at 1 C (60 min, 250 mA g-1), with a rate performance of 187 mAh/g at 10 C (6 min, 2500 mA g-1), providing a strategy for stabilising oxygen redox chemistry and designing high performance lithium-rich cathodes.

A Bifunctional Fibrous Scaffold Implanted with Amorphous Co2P as both Cathodic and Anodic Stabilizer for High-Performance Li─S Batteries

The shuttling of lithium polysulfides (LiPSs) and the formation of lithium dendrites have substantially impeded the practical application of lithium-sulfur (Li─S) batteries. To simultaneously solve these issues, a porous carbon fibrous scaffold embedded with amorphous Co2P (A─Co2P) is designed as both a cathodic and anodic stabilizer to construct high-rate and long-life Li─S batteries. The meticulously designed self-supporting membrane with an integrated carbon network and porous structure offers superior conductivity and copious spaces for uniform Li2S precipitation in the cathode and Li deposition in the anode. Moreover, the incorporated A─Co2P provides abundant unsaturated sites, which can not only facilitate the exposure of active sites but also modulate the electronic configuration for enhanced LiPSs adsorption and catalysis capability. Concurrently, the presence of lithiophilic A─Co2P sites also reinforces the stability of Li anode with the suppressed formation of dendrites. The constructed full Li─S batteries deliver a high areal capacity of 6.6 mAh cm-2 with a sulfur loading of 8.5 mg cm-2 and a low capacity decay rate of 0.047% per cycle after 800 cycles. This work provides a simple yet effective strategy to construct practical Li─S batteries by simultaneously addressing LiPSs shuttling and Li dendrite growth.

Utilizing Oxygen-Vacancy-Rich Violet Tungsten Oxide Enabling Ultralong Cycling of Nickel-Rich Cathodes at High Voltage

Nickel-rich cathodes have become widely used recently but tend to become unstable under high voltage, resulting in metal atom rearrangement and oxygen lattice distortions. During high-voltage charging, nickel oxidation weakens oxygen-transition metal bonds, causing oxygen release and increasing risks like thermal runaway. To address these stability issues, we developed an oxygen vacancy-rich WO2.72 coating that promotes oxygen vacancy formation on the particle surface, regulating lattice oxygen release within the bulk phase. Additionally, we conducted a detailed analysis of the inhibition mechanism of lattice oxygen release in electrochemical processes, using methods like EPR, DEMS, XPS, and RIXS. Due to its effective inhibition of oxygen release and enhanced particle mechanical properties, the coating achieved an 80.3% retention rate after 600 cycles at 4.5 V, highlighting its potential for stabilizing nickel-rich cathodes in lithium-ion batteries. This method can also be directly applied to designing other high-energy cathode materials facing stability and capacity challenges.

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.

Crystal Modulation of Mn-Based Layered Oxide toward Long-Enduring Anionic Redox with Fast Kinetics for Sodium-Ion Batteries

Mn-based layered oxide cathodes for sodium-ion batteries with anionic redox reactions hold great potential for energy storage applications due to their ultra-high capacity and cost effectiveness. However, achieving high capacity requires overcoming challenges such as oxygen-redox failure, sluggish kinetics, and structural degradation. Herein, we employ an innovative crystal modulation strategy, using Mn-based Na0.72Li0.24Mn0.76O2 as a representative cathode material, which shows that the highly exposed {010} active facets enable an enhanced rate capability (119.6 mAh g-1 at 10 C) with fast kinetics. Meanwhile, the reinforced Mn-O bond inhibits excessive oxidation of lattice oxygen and O-O cohesion loss, stabilizing and maintaining a long-enduring reversible oxygen-redox activity (100 % high capacity retention after 100 cycles at 0.5 C and 84.28 % retention after 300 cycles at 5 C). Time-resolved operando two-dimensional X-ray diffraction reveals the robust structural stability, zero-strain behavior, and suppressed phase transition with ultra-low volume variation during cycling at different rates (0.1 C: 1.75 %, 1 C: 0.31 %, 5 C: 0.04 %). Additionally, the full cell coupled with hard carbon achieves a high energy density of approximately 211 Wh kg-1 with superior performance. This work highlights the significance of crystal modulation and presents a universal approach in developing Mn-based oxide cathodes with stable anionic redox for high-performance sodium-ion batteries.

The Heisenberg-RIXS instrument at the European XFEL

Resonant inelastic X-ray scattering (RIXS) is an ideal X-ray spectroscopy method to push the combination of energy and time resolutions to the Fourier transform ultimate limit, because it is unaffected by the core-hole lifetime energy broadening. Also, in pump-probe experiments the interaction time is made very short by the same core-hole lifetime. RIXS is very photon hungry so it takes great advantage from high-repetition-rate pulsed X-ray sources like the European XFEL. The Heisenberg RIXS instrument is designed for RIXS experiments in the soft X-ray range with energy resolution approaching the Fourier and the Heisenberg limits. It is based on a spherical grating with variable line spacing and a position-sensitive 2D detector. Initially, two gratings were installed to adequately cover the whole photon energy range. With optimized spot size on the sample and small pixel detector the energy resolution can be better than 40 meV (90 meV) at any photon energy below 1000 eV with the high-resolution (high-transmission) grating. At the SCS instrument of the European XFEL the spectrometer can be easily positioned thanks to air pads on a high-quality floor, allowing the scattering angle to be continuously adjusted over the 65-145° range. It can be coupled to two different sample interaction chambers, one for liquid jets and one for solids, each state-of-the-art equipped and compatible for optical laser pumping in collinear geometry. The measured performances, in terms of energy resolution and count rate on the detector, closely match design expectations. The Heisenberg RIXS instrument has been open to public users since the summer of 2022.

Stabilized Oxygen Vacancy Chemistry toward High-Performance Layered Oxide Cathodes for Sodium-Ion Batteries

Anionic redox has emerged as a transformative paradigm for high-energy layered transition-metal (TM) oxide cathodes, but it is usually accompanied by the formation of anionic redox-mediated oxygen vacancies (OVs) due to irreversible oxygen release. Additionally, external factor-induced OVs (defined as intrinsic OVs) also play a pivotal role in the physicochemical properties of layered TM oxides. However, an in-depth understanding of the interplay between intrinsic and anionic redox-mediated OVs and the corresponding regulation mechanism of the dynamic evolution of OVs is still missing. Herein, we disclose the strong interrelationship between these OVs and demonstrate that the presence of intrinsic OVs in the TMO2 layers could induce weak integrity of the TM-O frameworks and unlock additional diffusion paths to trigger the generation and migration of anionic redox-mediated OVs. Accordingly, an OV stabilization strategy is proposed by deliberately introducing high-valence Nb5+, which could serve as an important building block in anchoring the oxygen sublattice and preventing the formation of a percolating OV migration network, thereby suppressing the formation/diffusion of anionic redox-mediated OVs. Consequently, superb structural integrity and improved electrochemical performance with reversible anionic redox chemistry are achieved. This work advances our understanding of the role of OVs for developing high-performance energy storage systems utilizing anionic redox.

Mn─O Covalency as a Lever for Na⁺ Intercalation Kinetics: The Role of Oxygen Edge-Sharing Co Octahedral Sites in MnO₂

The strategic enhancement of manganese-oxygen (Mn─O) covalency is a promising approach to improve the intercalation kinetics of sodium ions (Na⁺) in manganese dioxide (MnO2). In this study, an augmenting Mn─O covalency in MnO2 by strategically incorporating cobalt at oxygen edge-sharing Co octahedral sites is focused on. Both experimental results and density functional theory (DFT) calculations reveal an increased electron polarization from oxygen to manganese, surpassing that directed toward cobalt, thereby facilitating enhanced electron transfer and strengthening covalency. The synthesized Co-MnO2 material exhibits outstanding electrochemical performance, demonstrating a superior specific capacitance of 388 F g-1 at 1 A g-1 and maintaining 97.21% capacity retention after 12000 cycles. Additionally, an asymmetric supercapacitor constructed using Co-MnO2 achieved a high energy density of 35 Wh kg-1 at a power density of 1000 W kg-1, underscoring the efficacy of this material in practical applications. This work highlights the critical role of transition metal-oxygen interactions in optimizing electrode materials and introduces a robust approach to enhance the functional properties of manganese oxides, thereby advancing high-performance energy storage technologies.

Reversible Oxygen Redox With Enhanced Structural Stability Through Covalency Modulation for Layered Oxide Cathodes

P2-type Mn-based layered oxides have emerged as one of the most promising cathode materials for sodium-ion batteries owing to their advantages of facile preparation and high theoretical capacity. However, challenges such as phase transition and irreversible oxygen release during cycling often lead to rapid structural distortion and the formation of oxygen vacancies, ultimately resulting in rapid capacity decay. Herein, a covalency modulation strategy is adopted to address these challenges and successfully achieved a stable P2-type Mn-based layered oxide by introducing strong covalent Ni─O bonds. The robust Ni─O motif plays a crucial role in maintaining the rigidity of transition metal (TM) layered frameworks, which efficiently alleviates the structural distortion and degradation of the coordination environments of local TM sites, thereby achieving durable structural stiffness over extended cycles. In addition, the strong covalent Ni─O bonds can also stabilize the local oxygen environment, effectively suppressing the irreversible oxygen release. Benefiting from these advancements, the as-designed Na0.6Mg0.15Mn0.7Ni0.15O2 cathode displays a full solid-solution behavior with a low volume change of only 0.9% and an enhanced reversibility of lattice oxygen redox (OR) reaction. This investigation emphasizes the crucial role of covalency modulation in regulating OR chemistry and structural integrity to achieve high-energy-density Mn-based layered oxides.

Decoding the entropy-stabilized matrix of high-entropy layered double hydroxides: Harnessing strain dynamics for peroxymonosulfate activation and tetracycline degradation

The current understanding of the mechanism of high-entropy layered double hydroxide (LDH) on enhancing the efficiency of activating peroxymonosulfate (PMS) remains limited. This work reveals that a strong strain effect, driven by high entropy, modulates the structure of FeCoNiCuZn-LDH (HE-LDH) as evidenced by geometric phase analysis (GPA) and density functional theory (DFT) calculations. Compared to FeCoNiZn-LDH and FeCoNi-LDH with weaker strain effects, the high entropy-driven strain effect in HE-LDH shortens metal-oxygen-hydrogen (MOH) bond lengths, allows system to be in a constant steady state during catalysis, reduces the leaching of active M-OH sites, and enhances the adsorption capacity of these sites and the excess strain strength of the interfacial stretches the IO-O of the PMS, facilitates reactive oxygen species (·OH, SO4·-, 1O2 and O2·-) generation, and thereby improving the efficiency of PMS in degrading tetracycline (TC). Consequently, HE-LDH demonstrated a 90% TC degradation within 3 min, maintained over 92% TC removal across a wide pH range (3-11), and achieved over 90% degradation performance after 6 cycles. This study reports the first use of high-entropy LDH material as a non-homogeneous catalyst and provides insights into the extremely different catalytic behaviors of high entropy mechanisms for the activation of PMS.

Gradient and De-Clustered Anionic Redox Enabled Undetectable O2 Formation in 4.5 V Sodium Manganese Oxide Cathodes

Anionic redox chemistry presents a promising approach to enhancing the energy density of oxide cathode materials. However, anionic redox reactions invariably lead to O2 formation, either as free gaseous O2 or trapped molecular O2, which destabilizes the material's structure. Here, this critical challenge is addressed by constructing a crystal structure with both gradient redox activity and de-clustered redox-active oxygen. This design strategy is directly validated by operando differential electrochemical mass spectrometry and ex situ 50 K electron paramagnetic resonance, revealing no release of O2 or trapped O2 in the 4.5 V P2-type sodium manganese-based layered oxide. Notably, the material exhibits a highly reversible capacity of 247 mA h g-1 at 20 mA g-1 and exceptional capacity retention of 91.4% after 300 cycles at 300 mA g-1. In situ X-ray diffraction further suggests that the absence of O2 formation suppresses the typical P2-O2 phase transition, resulting in a minimal lattice volume change of only 0.5%. Ex situ neutron diffraction studies and theoretical calculations further elucidate that the locally ordered lattice is well-preserved, attributable to reduced cationic migrations during cycling.

A Unique Wide-Spacing Fence-Type Superstructure for Robust High-Voltage O3-Type Sodium Layered Cathode

Enhancing the energy density of layered oxide cathode materials is of great significance for realizing high-performance sodium-ion batteries and promoting their commercial application. Lattice oxygen redox at high voltage usually enables a high capacity and energy density. But the structural degradation, severe voltage decay, and the resultant poor cycling performance caused by irreversible oxygen release seriously restrict the practical application. Herein we introduce a novel fence-type superstructure (2a×3a type supercell) into O3-type layered cathode material Na0.9Li0.1Ni0.3Mn0.3Ti0.3O2 and achieve a stable cycling performance at a high voltage of 4.4 V. The fence-type superstructure effectively inhibits the formation of the vacancy clusters resulting from out-of-plane Li migration and in-plane transition metal migration at high voltage due to the wide d-spacing, thereby significantly reducing the irreversible release of lattice oxygen and greatly stabilizing the crystal structure. The cathode exhibits a high energy density of 545 Wh kg-1, a high rate capability (112.8 mAh g-1 at 5 C) and a high cycling stability (85.8 %@200 cycles with a high initial capacity of 148.6 mAh g-1 at 1 C) accompanied by negligible voltage attenuation (98.5 %@200 cycles). This strategy provides a distinct spacing effect of superstructure to design stable high-voltage layered cathode materials for Na-ion batteries.

Lithium Orbital Hybridization Chemistry to Stimulate Oxygen Redox with Reversible Phase Evolution in Sodium-Layered Oxide Cathodes

Searching for high energy-density electrode materials for sodium ion batteries has revealed Na-deficient intercalation compounds with lattice oxygen redox as promising high-capacity cathodes. However, anionic redox reactions commonly encountered poor electrochemical reversibility and unfavorable structural transformations during dynamic (de)sodiation processes. To address this issue, we employed lithium orbital hybridization chemistry to create Na-O-Li configuration in a prototype P2-layered Na43/60Li1/20Mg7/60Cu1/6Mn2/3O2 (P2-NaLMCM') cathode material. That Li+ ions, having low electronegativity, reside in the transition metal slabs serves to stimulate unhybridized O 2p orbitals to facilitate the stable capacity contribution of oxygen redox at high state of charge. The prismatic-type structure evolving to an intergrowth structure of the Z phase at high charging state could be simultaneously alleviated by reducing the electrostatic repulsion of O-O layers. As a consequence, P2-NaLMCM' delivers a high specific capacity of 183.8 mAh g-1 at 0.05 C and good cycling stability with a capacity retention of 80.2% over 200 cycles within the voltage range of 2.0-4.5 V. Our findings provide new insights into both tailoring oxygen redox chemistry and stabilizing dynamic structural evolution for high-energy battery cathode materials.

Fast Na+ Kinetics and Suppressed Voltage Hysteresis Enabled by a High-Entropy Strategy for Sodium Oxide Cathodes

O3-type layered transition metal cathodes are promising energy storage materials due to their sufficient sodium reservoir. However, sluggish sodium ions kinetics and large voltage hysteresis, which are generally associated with Na+ diffusion properties and electrochemical phase transition reversibility, drastically minimize energy density, reduce energy efficiency, and hinder further commercialization of sodium-ion batteries (SIBs). Here, this work proposes a high-entropy tailoring strategy through manipulating the electronic local environment within transition metal slabs to circumvent these issues. Experimental analysis combined with theoretical calculations verify that high-entropy metal ion mixing contributes to the improved reversibility of redox reaction and O3-P3-O3 phase transition behaviors as well as the enhanced Na+ diffusivity. Consequently, the designed O3-Na0.9Ni0.2Fe0.2Co0.2Mn0.2Ti0.15Cu0.05O2 material with high-entropy characteristic could display a negligible voltage hysteresis (<0.09 V), impressive rate capability (98.6 mAh g-1 at 10 C) and long-term cycling stability (79.4% capacity retention over 2000 cycles at 5 C). This work provides insightful guidance in mitigating the voltage hysteresis and facilitating Na+ diffusion of layered oxide cathode materials to realize high-rate and high-energy SIBs.

Gaining More Insights from Synchrotron-Based X-ray Spectroscopy for Alkali Ion Rechargeable Batteries

Alkali ion rechargeable batteries play a significant part in portable electronic devices and electronic vehicles. The rapid development of renewable energy technology nowadays demands batteries with even higher energy density for grid storage. To fulfill such demand, extensive research efforts have been devoted to optimizing electrochemical properties as well as developing novel energy storage schemes and designing new systems. In the investigation process, synchrotron-based X-ray spectroscopy plays a vital role in investigating the detailed degradation mechanism and developing novel energy storage schemes. Herein, we critically review the applications of synchrotron-based X-ray spectroscopy in battery research in recent years. This review begins with a discussion of the different scientific issues in alkali ion rechargeable batteries within various time and space scales. Subsequently, the principle of synchrotron-based X-ray spectroscopy is introduced, and the characteristics of various characterization techniques are summarized and compared. Typical application cases of synchrotron-based X-ray spectroscopy are then introduced into battery investigations. The final part presents perspectives in the development direction of both alkali ion rechargeable battery systems and synchrotron-based X-ray spectroscopy in the future.

Interfacial Engineering of P2-Type Ni/Mn-Based Layered Oxides by a Facile Water-Washing Method for Superior Sodium-Ion Batteries

Owing to the strong basicity and reactivity, residual sodium compounds (RSCs) on the surface of Na-based layered oxides for sodium-ion batteries (SIBs) cause the deterioration of the electrochemical performance and processability of the oxide cathode materials. Herein, considering P2-type Na0.66Ni0.26Zn0.07Mn0.67O2 as the model material, the water-washing treatment is proven to be a facile, economic, and highly efficient method to improve the electrochemical performance of P2-type Ni/Mn-based layered oxides. Experimental results show that RSCs on material surfaces can be effectively removed by water washing without causing severe damage to the bulk structure. Notably, water washing triggers the formation of an ultrathin (2-3 nm thick) Na-poor disordered interfacial layer on the surface of Na0.66Ni0.26Zn0.07Mn0.67O2. This layer plays a passivating role in further enhancing the material's resistance to water and reduces the reactivity of the material surface with the electrolyte. These compositional and structural optimizations for P2-type Na0.66Ni0.26Zn0.07Mn0.67O2 effectively suppress the release of gaseous CO2, formation of thick cathode-electrolyte interphase films, and consumption of active Na+, enabling good Na+ transport kinetics during cycling. The water-washed Na0.66Ni0.26Zn0.07Mn0.67O2 exhibits significantly improved cycling stability with a capacity retention of 89.1% at 100 mA g-1 after 100 cycles and rate capability with a discharge capacity of 76.3 mA g-1 at 2000 mA g-1; these values are higher than those of the unwashed Na0.66Ni0.26Zn0.07Mn0.67O2 (83.3%, 71.4 mA h g-1). This work provides fundamental insights into the detrimental effect of RSCs on the electrochemical performance of layered oxides and highlights the importance of regulating interfacial compositions for developing high-performance layered-oxide cathode materials for SIBs.

Correlating concerted cations with oxygen redox in rechargeable batteries

Rechargeable batteries currently power much of our world, but with the increased demand for electric vehicles (EVs) capable of traveling hundreds of miles on a single charge, new paradigms are necessary for overcoming the limits of energy density, particularly in rechargeable batteries. The emergence of reversible anionic redox reactions presents a promising direction toward achieving this goal; however this process has both positive and negative effects on battery performance. While it often leads to higher capacity, anionic redox also causes several unfavorable effects such as voltage fade, voltage hysteresis, sluggish kinetics, and oxygen loss. However, the introduction of cations with topological chemistry tendencies has created an efficient pathway for achieving long-term oxygen redox with improved kinetics. The cations serve as pillars in the crystal structure and meanwhile can interact with oxygen in ways that affect the oxygen redox process through their impact on the electronic structure. This review delves into a detailed examination of the fundamental physical and chemical characteristics of oxygen redox and elucidates the crucial role that cations play in this process at the atomic and electronic scales. Furthermore, we present a systematic summary of polycationic systems, with an emphasis on their electrochemical performance, in order to provide perspectives on the development of next-generation cathode materials.

Stabilizing the Deep Sodiation Process in Layered Sodium Manganese Cathodes by Anchoring Boron Ions

Advanced high-energy-density sodium-ion batteries (SIBs) are inseparable from cathode materials with high specific capacities. Layered manganese-rich oxides (NaxMnO2, 0.6 ≤ x ≤1) are promising cathode materials owing to their ease of intercalation and extraction of a considerable amount of sodium ions. However, lattice interactions, especially electrostatic repulsive forces and anisotropic stresses, are usually caused by deep desodiatin/sodiation process, resulting in intragranular cracks and capacity degradation in SIBs. Here, boron ions are introduced into the layered structure to build up B─O─Mn bonds. The regulated electronic structure in Na0.637B0.038MnO2 (B-NMO) materials inhibits the deformation of MnO6 octahedra, which finally achieves a gentle structural transition during the deep sodiation process. B-NMO electrode exhibits a high capacity (141 mAh g-1) at 1 C with a capacity retention of 81% after 100 cycles. Therefore, anchoring boron to manganese-rich materials inhibits the detrimental structural evolution of deep sodiation and can be used to obtain excellent cathode materials for SIBs.

Facilitating Layered Oxide Cathodes Based on Orbital Hybridization for Sodium-Ion Batteries: Marvelous Air Stability, Controllable High Voltage, and Anion Redox Chemistry

Layered oxides have become the research focus of cathode materials for sodium-ion batteries (SIBs) due to the low cost, simple synthesis process, and high specific capacity. However, the poor air stability, unstable phase structure under high voltage, and slow anionic redox kinetics hinder their commercial application. In recent years, the concept of manipulating orbital hybridization has been proposed to simultaneously regulate the microelectronic structure and modify the surface chemistry environment intrinsically. In this review, the hybridization modes between atoms in 3d/4d transition metal (TM) orbitals and O 2p orbitals near the region of the Fermi energy level (EF) are summarized based on orbital hybridization theory and first-principles calculations as well as various sophisticated characterizations. Furthermore, the underlying mechanisms are explored from macro-scale to micro-scale, including enhancing air stability, modulating high working voltage, and stabilizing anionic redox chemistry. Meanwhile, the origin, formation conditions, and different types of orbital hybridization, as well as its application in layered oxide cathodes are presented, which provide insights into the design and preparation of cathode materials. Ultimately, the main challenges in the development of orbital hybridization and its potential for the production application are also discussed, pointing out the route for high-performance practical sodium layered oxide cathodes.

Regulating Oxygen Redox Chemistry through the Synergistic Effect of Transition-Metal Vacancy and Substitution Element for Layered Oxide Cathodes

Reversible oxygen redox (OR) is considered as a paradigmatic avenue to boost the energy densities of layered oxide cathodes. However, its activation is largely coupled with the local coordination environment around oxygen, which is usually accompanied with irreversible oxygen release and unfavorable structure distortion. Herein, it is revealed that the synergistic effect of transition-metal (TM) vacancy and substitution element for modulating the OR activity and reversibility of layered Na0.67 MnO2 through multimodal operando synchrotron characterizations and electrochemical investigations. It is disclosed that TM vacancy can not only suppress the complicated phase transition but also stimulate the OR activity by creating nonbonding O 2p states via the Na─O─vacancy configurations. Notably, the substitution element plays a decisive role for regulating the reversibility of vacancy-boosted OR activity: the presence of strong Al─O bonds stabilizes the Mn-O motifs by sharing O with Al in the rigid Mn─O─Al frameworks, which mitigates TM migration and oxygen release induced by TM vacancy, leading to enhanced OR reversibility; while the introduction of weak Zn─O bonds exacerbates TM migration and irreversible oxygen release. This work clarifies the critical role of both TM vacancy and substitution element for regulating the OR chemistry, providing an effective avenue for designing high-performance cathodes employing anionic redox.

Slow-Released Cationic Redox Activity Promoted Stable Anionic Redox and Suppressed Jahn-Teller Distortion in Layered Sodium Manganese Oxides

Manganese-based layered oxides are considered promising cathodes for sodium ion batteries due to their high capacity and low-cost manganese and sodium resources. Triggering the anionic redox reaction (ARR) can exceed the capacity limitation determined by conventional cationic redox. However, the unstable ARR charge compensation and Jahn-Teller distortion of Mn3+ ions readily result in structural degradation and rapid capacity fade. Here, we report a P2-type Na0.8Li0.2Mn0.7Cu0.1O2 cathode that shows a capacity retention of 84.5% at 200 mA/g after 200 cycles. Combining in situ X-ray diffraction and multi other ex situ characterizations, we reveal that the enhanced cycling stability is ascribed to a slow release of cationic redox activity which can well suppress the Jahn-Teller distortion and favor the ARR reversibility. Furthermore, density-functional theory calculations demonstrate that the inhibited interlayer migration and reduced band gap facilitate the stability and kinetic behavior of ARR. These findings provide a perspective for designing high-energy-density cathode materials with ARR activity.

Lattice Engineering on Li2 CO3 -Based Sacrificial Cathode Prelithiation Agent for Improving the Energy Density of Li-Ion Battery Full-Cell

Developing sacrificial cathode prelithiation technology to compensate for active lithium loss is vital for improving the energy density of lithium-ion battery full-cells. Li2 CO3 owns high theoretical specific capacity, superior air stability, but poor conductivity as an insulator, acting as a promising but challenging prelithiation agent candidate. Herein, extracting a trace amount of Co from LiCoO2 (LCO), a lattice engineering is developed through substituting Li sites with Co and inducing Li defects to obtain a composite structure consisting of (Li0.906 Co0.043 ▫0.051 )2 CO2.934 and ball milled LiCoO2 (Co-Li2 CO3 @LCO). Notably, both the bandgap and Li─O bond strength have essentially declined in this structure. Benefiting from the synergistic effect of Li defects and bulk phase catalytic regulation of Co, the potential of Li2 CO3 deep decomposition significantly decreases from typical >4.7 to ≈4.25 V versus Li/Li+ , presenting >600 mAh g-1 compensation capacity. Impressively, coupling 5 wt% Co-Li2 CO3 @LCO within NCM-811 cathode, 235 Wh kg-1 pouch-type full-cell is achieved, performing 88% capacity retention after 1000 cycles.

Using Highly Electronegative Zn to Regulate the Superlattice Structure for the Na-Ion Layered Oxide Cathode with Superior Electrochemical Performance

The introduction of a superlattice structure into layered oxide cathode materials is a novel strategy to improve the structural stability of sodium-ion batteries (SIBs). However, the superlattice structure gradually disappears during cycling, which shortens the long life of SIBs. Here, the highly electronegative Zn is introduced into a P2-type layered oxide to regulate the superlattice structure. The obtained P2-Na0.80Li0.13Ni0.20Zn0.03Mn0.64O2 exhibits excellent cycling performance (the capacity retention is 96.7% after 100 cycles at 0.5C) and rate capability (95.8 mAh g-1 at 5C). Zn effectively inhibits the Li migration and the Mn dissolution, which ensures the integrity of the Li/Mn superlattice structure during long cycling, thus achieving an ultralong cycling life of SIBs. The introduction of Zn dramatically increases the length of the c-axis, leading to a faster de-embedding rate of Na+ and a better diffusion kinetics. Meanwhile, the larger pristine volume can withstand more stress/strain due to the sharp increase in the level of O-O repulsion during the desodiation process. In addition, Raman test results show that Zn can inhibit the Na+/vacancy ordering transition and improve the structural stability. This study confirms the feasibility of a Zn-regulated superlattice structure. It provides inspiration for the construction of stable layered oxide cathode materials for SIBs.

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.

Achieving Long-Enduring High-Voltage Oxygen Redox in P2-Structured Layered Oxide Cathodes by Eliminating Nonlattice Oxygen Redox

Triggering reversible lattice oxygen redox (LOR) in oxide cathodes is a paradigmatic approach to overcome the capacity ceiling determined by orthodox transition-metal (TM) redox. However, the LOR reactions in P2-structured Na-layered oxides are commonly accompanied by irreversible nonlattice oxygen redox (non-LOR) and large local structural rearrangements, bringing about capacity/voltage fading and constantly evolving charge/discharge voltage curves. Herein, a novel Na0.615 Mg0.154 Ti0.154 Mn0.615 ◻0.077 O2 (◻ = TM vacancies) cathode with both NaOMg and NaO◻ local configurations is deliberately designed. Intriguingly, the activating of oxygen redox at middle-voltage region (2.5-4.1 V) via NaO◻ configuration helps in maintaining the high-voltage plateau from LOR (≈4.38 V) and stable charge/discharge voltage curves even after 100 cycles. Hard X-ray absorption spectroscopy (hXAS), solid-state NMR, and electron paramagnetic resonance studies demonstrate that both the involvement of non-LOR at high-voltage and the structural distortions originating from Jahn-Teller distorted Mn3+ O6 at low-voltage are effectively restrained in Na0.615 Mg0.154 Ti0.154 Mn0.615 ◻0.077 O2 . Resultantly, the P2 phase is well retained in a wide electrochemical window of 1.5-4.5 V (vs Na+ /Na), resulting in an extraordinary capacity retention of 95.2% after 100 cycles. This work defines an effective approach to upgrade the lifespan of Na-ion battery with reversible high-voltage capacity provided by LOR.

Injecting Excess Na into a P2-Type Layered Oxide Cathode to Achieve Presodiation in a Na-Ion Full Cell

The initial Na loss limits the theoretical specific capacity of cathodes in Na-ion full cell applications, especially for Na-deficient P2-type cathodes. In this study, we propose a presodiation strategy for cathodes to compensate for the initial Na loss in Na-ion full cells, resulting in a higher specific capacity and a higher energy density. By employing an electrochemical presodiation approach, we inject 0.32 excess active Na into P2-type Na_0.67Li_0.1Fe_0.37Mn_0.53O₂ (NLFMO), aiming to compensate for the initial Na loss in hard carbon (HC) and the inherent Na deficiency of NLFMO. The structure of the NLFMO cathode converts from P2 to P'2 upon active Na injection, without affecting subsequent cycles. As a result, the HC||NLFMO_preNa full cell exhibits a specific capacity of 125 mAh/g, surpassing the value of 61 mAh/g of the HC||NLFMO full cell without presodiation due to the injected active Na. Moreover, the presodiation effect can be achieved through other engineering approaches (e.g., Na-metal contact), suggesting the scalability of this methodology.

Boosting CO2 Electroreduction to C2H4 via Unconventional Hybridization: High-Order Ce4+ 4f and O 2p Interaction in Ce-Cu2O for Stabilizing Cu. ACS NANO 2023. [PMID: 37410800 DOI: 10.1021/acsnano.3c03952]

Efficient conversion of carbon dioxide (CO₂) into value-added materials and feedstocks, powered by renewable electricity, presents a promising strategy to reduce greenhouse gas emissions and close the anthropogenic carbon loop. Recently, there has been intense interest in Cu₂O-based catalysts for the CO₂ reduction reaction (CO₂RR), owing to their capabilities in enhancing C-C coupling. However, the electrochemical instability of Cu⁺ in Cu₂O leads to its inevitable reduction to Cu⁰, resulting in poor selectivity for C₂₊ products. Herein, we propose an unconventional and feasible strategy for stabilizing Cu⁺ through the construction of a Ce⁴⁺ 4f-O 2p-Cu⁺ 3d network structure in Ce-Cu₂O. Experimental results and theoretical calculations confirm that the unconventional orbital hybridization near E_f based on the high-order Ce⁴⁺ 4f and 2p can more effectively inhibit the leaching of lattice oxygen, thereby stabilizing Cu⁺ in Ce-Cu₂O, compared with traditional d-p hybridization. Compared to pure Cu₂O, the Ce-Cu₂O catalyst increased the ratio of C₂H₄/CO by 1.69-fold during the CO₂RR at -1.3 V. Furthermore, in situ and ex situ spectroscopic techniques were utilized to track the oxidation valency of copper under CO₂RR conditions with time resolution, identifying the well-maintained Cu⁺ species in the Ce-Cu₂O catalyst. This work not only presents an avenue to CO₂RR catalyst design involving the high-order 4f and 2p orbital hybridization but also provides deep insights into the metal-oxidation-state-dependent selectivity of catalysts.

Quantifying Effects of Ligand-Metal Bond Covalency on Oxygen-Redox Electrochemistry in Layered Oxide Cathodes

Oxygen-redox electrochemistry is attracting tremendous attention due to its enhanced energy density for layered oxide cathodes. However, quantified effects of ligand-metal bond covalency on the oxygen-redox behaviors are not fully understood, limiting a rational structure design for enhancing the oxygen redox reversibility. Here, using Li₂Ru_1-xMn_xO₃ (0 ≤ x ≤ 0.8) which includes both 3d- and 4d-based cations as model compounds, we provide a quantified relation between the ligand-metal bond covalency and oxygen-redox electrochemistry. Supported by theoretical calculations, we reveal a linear positive correlation between the transition metal (TM)-O bond covalency and the overlap area of TM nd and O 2p orbitals. Furthermore, based on the electrochemical tests on the Li₂Ru_1-xMn_xO₃ systems, we found that the enhanced TM-O bond covalency can increase the reversibility of oxygen-redox electrochemistry. Due to the strong Ru-O bond covalency, the thus designed Ru-doped Li-rich Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode shows an enhanced initial coulombic efficiency, increased capacity retention, and suppressed voltage decay during cycling. This systematic study provides a rational structure design principle for the development of oxygen-redox-based layered oxide cathodes.

Enabling an Excellent Ordering-Enhanced Electrochemistry and a Highly Reversible Whole-Voltage-Range Oxygen Anionic Chemistry for Sodium-Ion Batteries

Though considerable Mg-doped layered cathodes have been exploited, some new differences relative to previous reports can be concluded by doping a heavy dose of Mg via two rational strategies. Unlike the common unit cell of the P6₃/mmc group by X-ray diffraction, neutron diffraction reveals a large supercell of the P6₃ group and enhanced ordering for Na_11/18Mg_1/18[Ni_1/4Mg_1/9Mn_11/18]O₂ with Mg occupying both the Na and Mn sites. Compared with only one obvious voltage plateau of Na_0.5[Ni_0.25Mn_0.75]O₂ (NNM), Na_11/18Mg_1/18[Ni_1/4Mg_1/9Mn_11/18]O₂ (NMNMM) shows more severe voltage plateaus but with excellent electrochemical performance. Na_0.5[Mg_0.25Mn_0.75]O₂ (NMM) with Mg only occupying the Ni site displays a highly reversible whole-voltage-range oxygen redox chemistry and smooth voltage curves without any voltage hysteresis. Cationic Ni²⁺/Ni⁴⁺ couples are responsible for the charge compensations of NNM and NMNMM, while only the oxygen anionic reaction accounts for the capacity of NMM between 2.5 and 4.3 V. Interestingly, the Mn³⁺/Mn⁴⁺ pair contributes all capacity for all cathodes between 1.5 and 2.5 V. All cathodes undergo a double-phase mechanism: an irreversible P2-O2 phase transition for NNM, an enhanced reversible P2-O2 phase transition for NMNMM, and a highly reversible P2-OP4 phase transition for NMM. In addition, the designed cathodes display excellent rate capability and long-term cycling stability but with a large difference in the various voltage ranges of 2.5-4.3 and 1.5-2.5 V, respectively. This work provides a good understanding of ion doping and some new insights into exploiting high-performance 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.

Oxygen Anion Redox Chemistry Correlated with Spin State in Ni-Rich Layered Cathodes

Despite the low cost and high capacity of Ni-rich layered oxides (NRLOs), their widespread implementation in electric vehicles is hindered by capacity decay and O release. These issues originate from chemo-mechanical heterogeneity, which is mainly related to oxygen anion redox (OAR). However, what to tune regarding OAR in NRLOs and how to tune it remains unknown. In this study, a close correlation between the OAR chemistry and Li/Ni antisite defects is revealed. Experiments and calculations show the opposite effects of aggregative and dispersive Li/Ni antisite defects on the NiO₆ configuration and Ni spin state in NRLOs. The resulting broad or narrow spans for the energy bands caused by spin states lead to different OAR chemistries. By tuning the Li/Ni antisite defects to be dispersive rather than aggregative, the threshold voltage for triggering OAR is obviously elevated, and the generation of bulk-O₂ -like species and O₂ release at phase transition nodes is fundamentally restrained. The OAR is regulated from irreversible to reversible, fundamentally addressing structural degradation and heterogeneity. This study reveals the interaction of the Li/Ni antisite defect/OAR chemistry/chemo-mechanical heterogeneity and presents some insights into the design of high-performance NRLO cathodes.

Prolonged lifespan of initial-anode-free lithium-metal battery by pre-lithiation in Li-rich Li2Ni0.5Mn1.5O4 spinel cathode

Anode-free lithium metal batteries (AF-LMBs) can deliver the maximum energy density. However, achieving AF-LMBs with a long lifespan remains challenging because of the poor reversibility of Li⁺ plating/stripping on the anode. Here, coupled with a fluorine-containing electrolyte, we introduce a cathode pre-lithiation strategy to extend the lifespan of AF-LMBs. The AF-LMB is constructed with Li-rich Li₂Ni_0.5Mn_1.5O₄ cathodes as a Li-ion extender; the Li₂Ni_0.5Mn_1.5O₄ can deliver a large amount of Li⁺ in the initial charging process to offset the continuous Li⁺ consumption, which benefits the cycling performance without sacrificing energy density. Moreover, the cathode pre-lithiation design has been practically and precisely regulated using engineering methods (Li-metal contact and pre-lithiation Li-biphenyl immersion). Benefiting from the highly reversible Li metal on the Cu anode and Li₂Ni_0.5Mn_1.5O₄ cathode, the further fabricated anode-free pouch cells achieve 350 W h kg^-1 energy density and 97% capacity retention after 50 cycles.

Suppressing the Dynamic Oxygen Evolution of Sodium Layered Cathodes through Synergistic Surface Dielectric Polarization and Bulk Site-Selective Co-Doping

Utilizing anionic redox activity within layered oxide cathode materials represents a transformational avenue for enabling high-energy-density rechargeable batteries. However, the anionic oxygen redox reaction is often accompanied with irreversible dynamic oxygen evolution, leading to unfavorable structural distortion and thus severe voltage decay and rapid capacity fading. Herein, it is proposed and validated that the dynamic oxygen evolution can be effectively suppressed through the synergistic surface CaTiO₃ dielectric coating and bulk site-selective Ca/Ti co-doping for layered Na_2/3 Ni_1/3 Mn_2/3 O₂ . The surface dielectric coating layer not only suppresses the surface oxygen release but more importantly inhibits the bulk oxygen migration by creating a reverse electric field through dielectric polarization. Meanwhile, the site-selective doping of oxygen-affinity Ca into Na layers and Ti into transition metal layers effectively stabilizes the bulk oxygen through modulating the O 2p band center and the oxygen migration barrier. Such a strategy also leads to a reversible structural evolution with a low volume change because of the enhanced structural integrality and improved oxygen rigidity. Because of these synergistic advantages, the designed electrode exhibits greatly suppressed voltage decay and capacity fading upon long-term cycling. This study affords a promising strategy for regulating the dynamic oxygen evolution to achieve high-capacity layered cathode materials.

Rational design of intergrowth P2/O3 biphasic layered structure with reversible anionic redox chemistry and structural evolution for Na-ions batteries

Layered oxides have attracted unprecedented attention for their outstanding performance in sodium-ion battery cathodes. Among them, the two typical candidates P2 and O3 type materials generally demonstrate large diversities in specific capacity and cycling endurance with their advantages. Thus, composite materials that contain both P2 and O3 have been widely designed and constructed. Nevertheless, the anionic/cationic ions' behavior and structural evolution in such complex structures remain unclear. In this study, a deep analysis of an advanced Na_0.732Ni_0.273Mg_0.096Mn_0.63O₂ material that contains 78.39 wt% P2 phase and 21.61 wt% O3 phase is performed based on two typical cathodes P2 Na_0.67Ni_0.33Mn_0.67O2 and O3 NaNi_0.5Mn_0.5O₂ that have the same elemental constitution but different crystal structures. Structural analysis and density functional theory (DFT) calculations suggest that the composite is preferred to form a symbiotic structure at the atomic level, and the complex lattice texture of the biphase structure can block unfavorable ion and oxygen migration in the electrode process. Consequently, the biphase structure has significantly improved the electrochemical performance and kept preferable anionic oxygen redox reversibility. Furthermore, the hetero-epitaxy-like structure of the intergrowth of P2 and O3 structures share multi-phase boundaries, where the inconsistency in electrochemical behavior between P2 and O3 phases leads to an interlocking effect to prevent severe structural collapse and relieves the lattice strain from Na⁺ de/intercalation. Hence, the symbiotic P2/O3 composite materials exhibited a preferable capacity and cyclability (∼130 mAh g^-1 at 0.1 C, 73.1% capacity retention after 200 cycles at 1 C), as well as reversible structural evolution. These findings confirmed the advantages of using the bi/multi-phase cathode for high-energy Na-ion batteries.