Unraveling the Anionic Redox Chemistry in Aqueous Zinc-MnO2 Batteries.

Activating anionic redox reaction (ARR) has attracted a great interest in Li/Na-ion batteries owing to the fascinating extra-capacity at high operating voltages. However, ARR has rarely been reported in aqueous zinc-ion batteries (AZIBs) and its possibility in the popular MnO2-based cathodes has not been explored. Herein, the novel manganese deficient MnO2 micro-nano spheres with interlayer "Ca2+-pillars" (CaMnO-140) are prepared via a low-temperature (140 °C) hydrothermal method, where the Mn vacancies can trigger ARR by creating non-bonding O 2p states, the pre-intercalated Ca2+ can reinforce the layered structure and suppress the lattice oxygen release by forming Ca-O configurations. The tailored CaMnO-140 cathode demonstrates an unprecedentedly high rate capability (485.4 mAh g-1 at 0.1 A g-1 with 154.5 mAh g-1 at 10 A g-1) and a marvelous long-term cycling durability (90.6% capacity retention over 5000 cycles) in AZIBs. The reversible oxygen redox chemistry accompanied by CF3SO3- (from the electrolyte) uptake/release, and the manganese redox accompanied by H+/Zn2+ co-insertion/extraction, are elucidated by advanced synchrotron characterizations and theoretical computations. Finally, pouch-type CaMnO-140//Zn batteries manifest bright application prospects with high energy, long life, wide-temperature adaptability, and high operating safety. This study provides new perspectives for developing high-energy cathodes for AZIBs by initiating anionic redox chemistry.

Enhancing Reversibility and Kinetics of Anionic Redox in O3-NaLi1/3Mn2/3O2 through Controlled P2 Intergrowth.

Anionic redox chemistry can surpass theoretical limits of conventional layered oxide cathodes in energy density. A recent model system of sodium-ion batteries, O3-NaLi1/3Mn2/3O2, demonstrated full anionic redox capacity but is limited in reversibility and kinetics due to irreversible structural rearrangement and oxygen loss. Solutions to these issues are missing due to the challenging synthesis. Here, we harness the unique structural richness of sodium layered oxides and realize a controlled ratio of P2 structural intergrowth in this model compound with the overall composition maintained. The resulted O3 with 27% P2 intergrowth structure delivers an excellent initial Coulombic efficiency of 87%, comparable to the state-of-the-art Li-rich NMCs. This improvement is attributed to the effective suppression of irreversible oxygen release and structural changes, evidenced by operando Differential Electrochemical Mass Spectroscopy and X-ray Diffraction. The as-prepared intergrowth material, based on the environmentally benign Mn, exhibits a reversible capacity of 226 mAh g-1 at C/20 rate with excellent cycling stability stemming from the redox reactions of oxygen and manganese. Our work isolates the role of P2 structural intergrowth and thereby introduces a novel strategy to enhance the reversibility and kinetics of anionic redox reactions in sodium layered cathodes without compromising capacity.

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.

Interplay between Intermediate Anionic and Cationic Species and the Reflection to Voltage Hysteresis of Oxygen-Redox Battery Electrodes: A Holistic Picture.

Ligand-to-metal charge transfer (LMCT) is conceived as a universal theory to account for voltage hysteresis in oxygen-redox battery electrodes. However, the influence of oxygen anionic species on mediating LMCT and its reflection to voltage hysteresis remain poorly understood. Herein, we demonstrate a close interplay between the chemical states of oxidized oxygen species, the cationic species, and the kinetics of LMCT and forcefully identify their influence on the magnitude of voltage hysteresis. Combining electrochemical/spectroscopic evidence and first-principles calculations, we clarify two distinct kinds of dynamic LMCT processes─that is, the formation of trapped molecular O2 accompanied by the reduction of Ni4+/Ni3+ to Ni2+ (fast LMCT) during relaxation in Li-rich cation-disordered rock-salt (DRX) Li1.3Ni0.27Ta0.43O2 with extremely large voltage hysteresis, the formation of O-O dimers, and the partial reduction of Mn4+ to Mn3+ (slow LMCT) in DRX-Li1.3Mn0.4Ta0.3O2 with medium hysteresis. We further validate the maintenance of both cationic (Mn4+) and anionic (O-•) species during relaxation in Na2Mn3O7, reconciling its nonhysteretic behavior to the absence of LMCT. This study highlights the critical role of intermediate anionic species in mediating LMCT and provides a causal explanation of various voltage hysteresis signatures of oxygen-redox materials.

Reversible Oxygen Redox Chemistry in High-Entropy P2-Type Manganese-Based Cathodes via Self-Regulating Mechanism.

The irreversible oxygen-redox reactions in the high-voltage region of sodium-layered cathode materials lead to poor capacity retention and structural instability during cycling, presenting a significant challenge in the development of high-energy-density sodium-ion batteries. This work introduces a high-entropy design for layered Na0.67Li0.1Co0.1Cu0.1Ni0.1Ti0.1Mn0.5O2 (Mn-HEO) cathode with a self-regulating mechanism to extend specific capacity and energy density. The oxygen redox reaction was activated during the initial charging process, accompanied by the self-regulation of active elements, enhancing the ionic bonds to form a vacancy wall near the TM vacancies and thus preventing the migration of transition metal elements. Systematic in situ/ex situ characterizations and theoretical calculations comprehensively support the understanding of the self-regulation mechanism of Mn-HEO. As a result, the Mn-HEO cathode exhibits a stable structure during cycling. It demonstrates almost zero strain within a wide voltage range of 2.0-4.5 V with a remarkable specific capacity (177 mAh g-1 at 0.05 C) and excellent long-term cycling stability (87.6% capacity retention after 200 cycles at 2 C). This work opens a new pathway for enhancing the stability of oxygen-redox chemistry and revealing a mechanism of crystal structure evolution for high-energy-density layered oxides.

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.

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.

Perspective on High-Stability Single-Crystal Li-Rich Cathode Materials for Li-Ion Batteries.

Benefiting from anionic and cationic redox reactions, Li-rich materials have been regarded as next-generation cathodes to overcome the bottleneck of energy density. However, they always suffer from cracking of polycrystalline (PC) secondary particles and lattice oxygen release, resulting in severe structural deterioration and capacity decay upon cycling. Single-crystal (SC) design has been proven as an effective strategy to relieve these issues in traditional Li-rich cathodes with PC morphology. Herein, we first reviewed the main synthesis routes of SC Li-rich materials including solid-state reaction, molten salt-assisted, and hydrothermal/solvothermal methods, in which the differences in grain morphology, electrochemical behaviors, and other properties induced by various routes were analyzed and discussed. Furthermore, the distinct characteristics were compared between SC and PC cathodes from the aspects of irreversible capacity, structural stability, capacity/voltage degradation, and gas release. Besides, recent advances in layered SC Li-rich oxide cathodes were summarized in detail, where the unique structural designs and modification strategies could greatly promote their structural/electrochemical stability. At last, challenges and perspectives for the emerging SC Li-rich cathodes were proposed, which provided an exceptional opportunity to achieve high-energy-density and high-stability Li-ion/metal batteries.

Achieving a Deeply Desodiated Stabilized Cathode Material by the High Entropy Strategy for Sodium-ion Batteries.

Manganese-based layered oxides are currently of significant interest as cathode materials for sodium-ion batteries due to their low toxicity and high specific capacity. However, the practical applications are impeded by sluggish intrinsic Na+ migration and poor structure stability as a result of Jahn-Teller distortion and complicated phase transition. In this study, a high-entropy strategy is proposed to enhance the high-voltage capacity and cycling stability. The designed P2-Na0.67Mn0.6Cu0.08Ni0.09Fe0.18Ti0.05O2 achieves a deeply desodiation and delivers charging capacity of 158.1 mAh g-1 corresponding to 0.61 Na with a high initial Coulombic efficiency of 98.2 %. The charge compensation is attributed to the cationic and anionic redox reactions conjunctively. Moreover, the crystal structure is effectively stabilized, leading to a slight variation of lattice parameters. This research carries implications for the expedited development of low-cost, high-energy-density cathode materials for sodium-ion batteries.

Dual-Site Doping in Transition Metal Oxide Cathode Enables High-Voltage Stability of Na-Ion Batteries.

Designing cathode materials that effectively enhancing structural stability under high voltage is paramount for rationally enhancing energy density and safety of Na-ion batteries. This study introduces a novel P2-Na0.73K0.03Ni0.23Li0.1Mn0.67O2 (KLi-NaNMO) cathode through dual-site synergistic doping of K and Li in Na and transition metal (TM) layers. Combining theoretical and experimental studies, this study discovers that Li doping significantly strengthens the orbital overlap of Ni (3d) and O (2p) near the Fermi level, thereby regulates the phase transition and charge compensation processes with synchronized Ni and O redox. The introduction of K further adjusts the ratio of Nae and Naf sites at Na layer with enhanced structural stability and extended lattice space distance, enabling the suppression of TM dissolution, achieving a single-phase transition reaction even at a high voltage of 4.4 V, and improving reaction kinetics. Consequently, KLi-NaNMO exhibits a high capacity (105 and 120 mAh g-1 in the voltage of 2-4.2 V and 2-4.4 V at 0.1 C, respectively) and outstanding cycling performance over 300 cycles under 4.2 and 4.4 V. This work provides a dual-site doping strategy to employ synchronized TM and O redox with improved capacity and high structural stability via electronic and crystal structure modulation.

Engineering Reversible Lattice Structure for High-Capacity Co-Free Li-Rich Cathodes with Negligible Capacity Degradation.

Co-free Li-rich Mn-based cathode materials are garnering great interest because of high capacity and low cost. However, their practical application is seriously hampered by the irreversible oxygen escape and the poor cycling stability. Herein, a reversible lattice adjustment strategy is proposed by integrating O vacancies and B doping. B incorporation increases TM─O (TM: transition metal) bonding orbitals whereas decreases the antibonding orbitals. Moreover, B doping and O vacancies synergistically increase the crystal orbital bond index values enhancing the overall covalent bonding strength, which makes TM─O octahedron more resistant to damage and enables the lattice to better accommodate the deformation and reaction without irreversible fracture. Furthermore, Mott-Hubbard splitting energy is decreased due to O vacancies, facilitating electron leaps, and enhancing the lattice reactivity and capacity. Such a reversible lattice, more amenable to deformation and forestalling fracturing, markedly improves the reversibility of lattice reactions and mitigates TM migration and the irreversible oxygen redox which enables the high cycling stability and high rate capability. The modified cathode demonstrates a specific capacity of 200 mAh g-1 at 1C, amazingly sustaining the capacity for 200 cycles without capacity degradation. This finding presents a promising avenue for solving the long-term cycling issue of Li-rich cathode.

A Cable-Stayed Honeycomb Superstructure to Improve the Stability of Li-Rich Materials via Inhibiting Interlaminar Lattice Strain.

In layered Li-rich materials, over stoichiometric Li forms an ordered occupation of LiTM6 in transition metal (TM) layer, showing a honeycomb superstructure along [001] direction. At the atomic scale, the instability of the superstructure at high voltage is the root cause of problems such as capacity/voltage decay of Li-rich materials. Here a Li-rich material with a high Li/Ni disorder is reported, these interlayer Ni atoms locate above the honeycomb superstructure and share adjacent O coordination with honeycomb TM. These Ni─O bonds act as cable-stayed bridge to the honeycomb plane, and improve the high-voltage stability. The cable-stayed honeycomb superstructure is confirmed by in situ X-ray diffraction to have a unique cell evolution mechanism that it can alleviate interlaminar lattice strain by promoting in-plane expansion along a-axis and inhibiting c-axis stretching. Electrochemical tests also demonstrate significantly improved long cycle performance after 500 cycles (86% for Li-rich/Li half cell and 82% for Li-rich/Si-C full cell) and reduced irreversible oxygen release. This work proves the feasibility of achieving outstanding stability of lithium-rich materials through superstructure regulation and provides new insights for the development of the next-generation high-energy-density cathodes.

Hidden Negative Issues and Possible Solutions for Advancing the Development of High-Energy-Density in Lithium Batteries: A Review.

This review article discusses the hidden or often overlooked negative issues of large-capacity cathodes, high-voltage systems, concentrated electrolytes, and reversible lithium metal electrodes in high-energy-density lithium batteries and provides some feasible solutions that can realize the construction of realistic rechargeable batteries with higher energy densities. Similar objective discussion of the negative aspects of lithium-air batteries, multi-valent shuttles, anion shuttles, sulfur cathode systems, and all-solid ceramic batteries can help fabricate more realistic batteries.

Unlocking Double Redox Reaction of Metal-Organic Framework for Aqueous Zinc-Ion Battery.

Metal-organic frameworks (MOFs) show wide application as the cathode of aqueous zinc-ion batteries (AZIBs) in the future owning to their high porosity, diverse structures, abundant species, and controllable morphology. However, the low energy density and poor cycling stability hinder the feasibility in practical application. Herein, an innovative strategy of organic/inorganic double electroactive sites is proposed and demonstrated to obtain extra capacity and enhance the energy density in a manganese-based metal-organic framework (Mn-MOF-74). Simultaneously, its energy storage mechanism is systematically investigated. Moreover, profiting from the coordination effect, the Mn-MOF-74 features with stable structure in ZnSO4 electrolyte. Therefore, the Zn/Mn-MOF-74 batteries exhibit a high energy density and superior cycling stability. This work aids in the future development of MOFs in AZIBs.

Competing Mechanisms Determine Oxygen Redox in Doped Ni-Mn Based Layered Oxides for Na-Ion Batteries.

Cation doping is an effective strategy for improving the cyclability of layered oxide cathode materials through suppression of phase transitions in the high voltage region. In this study, Mg and Sc are chosen as dopants in P2-Na0.67Ni0.33Mn0.67O2, and both have found to positively impact the cycling stability, but influence the high voltage regime in different ways. Through a combination of synchrotron-based methods and theoretical calculations it is shown that it is more than just suppression of the P2 to O2 phase transition that is critical for promoting the favorable properties, and that the interplay between Ni and O activity is also a critical aspect that dictates the performance. With Mg doping, the Ni activity can be enhanced while simultaneously suppressing the O activity. This is surprising because it is in contrast to what has been reported in other Mn-based layered oxides where Mg is known to trigger oxygen redox. This contradiction is addressed by proposing a competing mechanism between Ni and Mg that impacts differences in O activity in Na0.67MgxNi0.33- xMn0.67O2 (x < 0 < 0.33). These findings provide a new direction in understanding the effects of cation doping on the electrochemical behavior of layered oxides.

Suppressing Surface Ligand-to-Metal Charge Transfer toward Stable High-Voltage LiCoO2.

Increasing the charging cutoff voltage is a viable approach to push the energy density limits of LiCoO2 and meet the requirements of the rapid development of 3C electronics. However, an irreversible oxygen redox is readily triggered by the high charging voltage, which severely restricts practical applications of high-voltage LiCoO2. In this study, we propose a modification strategy via suppressing surface ligand-to-metal charge transfer to inhibit the oxygen redox-induced structure instability. A d0 electronic structure Zr4+ is selected as the charge transfer insulator and successfully doped into the surface lattice of LiCoO2. Using a combination of theoretical calculations, ex situ X-ray absorption spectra, and in situ differential electrochemical mass spectrometry analysis, our results show that the modified LiCoO2 exhibits suppressed oxygen redox activity and stable redox electrochemistry. As a result, it demonstrates a robust long-cycle lattice structure with a practically eliminated voltage decay (0.17 mV/cycle) and an excellent capacity retention of 89.4% after 100 cycles at 4.6 V. More broadly, this work provides a new perspective on suppressing the oxygen redox activity through modulating surface ligand-to-metal charge transfer for achieving a stable high-voltage ion storage structure.