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

Multifunctional role of gallium-doping in O3-type layered-oxide cathodes for sodium-ion batteries: Enhancing bulk-to-surface stability

Charging O3-type layered-oxide cathodes to a high cutoff voltage of 4.3 V (vs. Na+/Na) can enhance the energy density of sodium-ion batteries (SIBs). However, the irreversible oxygen redox reaction at high voltages often leads accelerated capacity degradation. Herein, a series of Ga3+-doped O3-type Na0.9Zn0.07Ni0.38-0.5xGaxMn0.45-0.5xTi0.1O2 cathode materials are prepared, and the impact of Ga3+ doping on their bulk/interface properties and electrochemical performance is systematically examined. Ga3+ incorporation enhances the structural ordering of the layered framework and widens Na+ transport pathways, thereby reducing Na+ transport barrier. The Ga3+-doped material demonstrates superior structural reversibility and mechanical stability compared to the pristine counterpart during cycling. As evidenced by the density functional theory calculations, Ga3+ doping modulates the O 2p state near the Fermi level, mitigating the charge compensation mechanism of lattice oxygen, oxygen vacancy formation, and electrolyte decomposition at high voltages. Consequently, within the voltage range of 2.2-4.3 V, Na0.9Zn0.07Ni0.35Ga0.06Mn0.42Ti0.1O2 exhibits a higher capacity retention after 100 cycles at 100 mA g-1 (86.4 % vs. 68.1 %) and better rate capability at 2000 mA g-1 (94.1 mAh g-1 vs. 80.0 mAh g-1) than Na0.9Zn0.07Ni0.38Mn0.45Ti0.1O2. This work provides valuable insights into the role of Ga3+ in high-voltage O3-type layered oxides and offers guidance for the design of high-entropy cathode materials for SIBs.

High-energy and long-life O3-type layered cathode material for sodium-ion batteries

O3-type layered oxide for sodium-ion batteries have attracted significant attention owing to their low cost and high energy density. However, their applications are restricted by rapid capacity decay during long-term cycling, with uneven Na+ distribution and microcrack formation being key contributing factors. In this study, a customized reconstruction layer integrating a fast ion conductor NaCaPO4 coating with gradient Ca2+ doping is developed to enhance the surface chemical and mechanical stability of the layered cathodes. The gradient Ca2+ doped interphase facilitates uniform phase transformation within the particles, minimizes lattice mismatch, ensures even Na+ distribution, and mitigates microcrack formation through a pinning effect. Consequently, the optimized sample exhibits improved electrochemical performance and robust reliability under high-voltage conditions and a broad temperature range (-10 to 50 °C). The practical feasibility of a pouch-type full cell paired with a hard carbon anode is demonstrated by a high capacity retention of 82.9% after 300 cycles at 0.5 C. This scalable interface modification strategy provides valuable insights into the development of advanced oxide cathode materials for sodium-ion batteries.

Intrinsic Distortion against Jahn-Teller Distortion: A New Paradigm for High-Stability Na-Ion Layered Mn-Rich Oxide Cathodes

Layered manganese-rich oxides (LMROs) are widely recognized as the leading cathode candidates for grid-scale sodium-ion batteries (SIBs) owing to their high specific capacities and cost benefits, but the notorious Jahn-Teller (J-T) distortion of Mn3+ always induces severe structural degradation and consequent rapid cathode failure, impeding the practical implementation of such materials. Herein, we unveil the "intrinsic distortion against J-T distortion" mechanism to effectively stabilize the layered frameworks of LMRO cathodes. The intrinsic distortion simply constructed by introducing bulk oxygen vacancies is systematically confirmed by advanced synchrotron X-ray techniques, atomic-scale imaging characterizations, and theoretical computations, which can counteract the J-T distortion during cycling due to their opposite deformation orientations. This greatly decreases and uniformizes the lattice strain within the ab plane and along the c axis of the material, thereby alleviating the P2-P'2 phase transition as well as suppressing the edge dislocation and intragranular crack formation upon repeated cycles. As a result, the tailored P2-Na0.72Mg0.1Mn0.9O2 cathode featuring intrinsic distortion delivers a considerably enhanced cycling durability (91.9 % capacity retention after 500 cycles) without sacrificing the Mn3+/Mn4+ redox capacity (186.5 mAh g-1 at 0.3 C). This intrinsic distortion engineering paves a brand-new and prospective avenue toward achieving high-performance LMRO cathodes for SIBs.

Negative Enthalpy Doping Stabilizes P2-Type Oxides Cathode for High-Performance Sodium-Ion Batteries

P2-type Na0.67Ni0.33Mn0.67O2 (NNMO) as cathode material for sodium-ion batteries (SIBs) largely suffers from continuous accumulation of local stress caused by destructive structural evolution and irreversible oxygen loss upon cycling, leading to rapid capacity degradation. Herein, a strategy of negative enthalpy doping (NED), wherein transition metal (TM) sites are substituted with 0.01 mol each Sn, Sb, Cu, Ti, Mg, and Zn to increase the stability of the TM layers, is proposed. The robust structure of NED-NNMO significantly suppresses the P2 to O2 phase transition and improves the Na+ kinetics upon long-term cycling. Consequently, the NED-NNMO exhibits much smoothened voltage platforms and improved oxygen redox reversibility, thus considerably extended lifetime as compared with the pristine NNMO sample. The NED-NNMO delivers a high capacity of 138.9 mAh g-1 with an operation voltage of 3.51 V under 0.1 C and prominent capacity retention of 94.6% after 100 cycles under 1 C, and 90.0% over 3000 cycles under ultra-high rate of 30 C, which is among the best over previous reports. Moreover, an ampere-hour scale pouch cell based on the NED-NNMO demonstrates an energy density of 139 Wh kg-1. This work sheds light on a route of negative enthalpy doping to design high-performance sodium-ion batteries.

Halting Oxygen Evolution to Achieve Long Cycle Life in Sodium Layered Cathodes

Oxygen redox chemistries at high voltage have materialized as a revolutionary paradigm for cathodes with high-energy density; however, they are plagued by the challenges of labile oxygen loss and rapid degradations upon cycling, even after concerted endeavors from the research community. Here we propose a multi-concentration stratagem propelled by entropy reinforcement to enhance the electronic structure disorder (ESD) at high desodiation states for impeding undesired oxygen mobility and ensuring controlled oxygen activity, elucidated by density functional theory calculations. The increased disorder strengthens the reversible electrochemistry of lattice oxygen redox, leading to effectively suppressed P-O structural evolution and highly stable localized TMO6 octahedral environments, as demonstrated by soft/hard X-ray absorption spectroscopy. Furthermore, through a comparative analysis of sodium-layered cathodes with different configuration entropy, we reveal that a high-entropy state induced by cationic disordering has the capacity to perturb cationic redox boundaries, significantly restraining the formation of detrimental O'3 phases. As a consequence, the high-voltage cycling stability has been greatly upgraded, up to 4.4 V versus Na+/Na, with an impressive 90.1 % capacity retention at 1 C over 100 cycles and 76.1 % capacity retention at 2 C over 300 cycles. The resilient oxygen redox, enabled through the control of ESD, broadens the horizons for entropy engineering and lays the foundation for advancements in high-energy, long-cycling, and safe batteries.

Conductive MOF-Derived Coating for Suppressing the Mn Dissolution in LiMn2O4 toward Long-Life Lithium-Ion Batteries

Spinel lithium manganese oxide (LiMn2O4, LMO) is a promising cathode material with nontoxicity, high operating voltage, and low cost. However, structural collapse during battery cycling ─ caused by Mn dissolution and the Jahn-Teller effect ─ is a critical disadvantage, reducing cycle retention, particularly at high temperatures. In this study, to solve these critical issues, we introduce Cu3(HITP)2 (CuHITP; HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a conductive two-dimensional (2D) metal-organic framework (MOF) as a surface coating material. The CuHITP-derived coating increases the electrical conductivity and suppresses Mn dissolution by enriching the LMO surface with Mn4+. By suppressing Mn dissolution, structural stability also improves, offsetting the inherent problems. As a result, at 60 °C, CuHITP-LMO exhibits an initial capacity of 95.8 mAh g-1 at 100 mA g-1 and achieves a capacity of 42.4 mAh g-1 after 300 cycles. This research highlights the potential of conductive 2D MOFs to improve the electrochemical performances of LMO.

Building Robust Manganese Hexacyanoferrate Cathode for Long-Cycle-Life Sodium-Ion Batteries

Manganese Hexacyanoferrate (Mn─HCF) is a preferred cathode material for sodium-ion batteries used in large-scale energy storage. However, the inherent vacancies and the presence of H2O within the imperfect crystal structure of Mn─HCF lead to material failure and interface failure when used as a cathode. Addressing the challenge of constructing a stable cathode is an urgent scientific problem that needs to be solved to enhance the performance and lifespan of these batteries. In this review, the crystal structure of Mn─HCF is first introduced, explaining the formation mechanism of vacancies and exploring the various ways in which H2O molecules can be present within the crystal structure. Then comprehensively summarize the mechanisms of material and interfacial failure in Mn─HCF, highlighting the key factors contributing to these issues. Additionally, eight modification strategies designed to address these failure mechanisms are encapsulated, including vacancy regulation, transition metal substitution, high entropy, the pillar effect, interstitial H2O removal, surface coating, surface vacancy repair, and cathode electrolyte interphase reinforcement. This comprehensive review of the current research advances on Mn─HCF aims to provide valuable guidance and direction for addressing the existing challenges in their application within SIBs.

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.

Cation migration in layered oxide cathodes for sodium-ion batteries: fundamental failure mechanisms and practical modulation strategies

Sodium-ion batteries (SIBs) are regarded as competitive candidates for the next generation of electrochemical energy storage (EES) systems due to their low cost and abundant sodium resources. Layered oxide cathodes have attracted much interest owing to their simple preparation process, high specific capacity and environmental friendliness. However, undesired cation migration during electrochemical reactions can lead to irreversible phase transitions and structural degradation of layered oxide cathode materials, resulting in a sharp decrease in specific capacity and energy density. Therefore, in order to find effective strategies to suppress cation migration, the fundamental failure mechanism of layered oxides and the practical approaches to solve this key scientific issue are thoroughly investigated, and herein the history and current status of developments in this field are also reviewed. Elemental doping and structural design can directionally modify the electronic structure, energy band structure and electronic density of states in layered oxides and enhance cation migration barriers, which benefits the improvement of electrochemical performance and structural stability during the whole sodiation/desodiation process. The summary and prospects of inhibiting cation migration in layered oxides provide insights into the development of advanced cathode materials with high energy density and excellent structural stability for the commercialization of SIBs.

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.

Surface Gradient Desodiation Chemistry in Layered Oxide Cathode Materials

Sodium-ion batteries (SIBs) as a promising technology for large-scale energy storage have received unprecedented attention. However, the cathodes in SIBs generally suffer from detrimental cathode-electrolyte interfacial side reactions and structural degradation during cycling, which leads to severe capacity fade and voltage decay. Here, we have developed an ultra-stable Na0.72Ni0.20Co0.21Mn0.55Mg0.036O2 (NCM-CS-GMg) cathode material in which a Mg-free core is encapsulated by a shell with gradient distribution of Mg using coprecipitation method with Mg-hysteretic cascade feedstock followed by calcination. From the interior to outer surface of the shell, as the content of electrochemically inactive Mg gradually increases, the Na+ deintercalation amount gradually decreases after charged. Benefiting from this surface gradient desodiation, the surface transition metal (TM) ion migration from TM layers to Na layers is effectively inhibited, thus suppressing the layered-to-rock-salt phase transition and the resultant microcracks. Besides, the less formation of high-valence TM ions on the surface contributes to a stable cathode-electrolyte interface. The as-prepared NCM-CS-GMg exhibits remarkable cycling life over 3000 cycles with a negligible voltage drop (0.127 mV per cycle). Our findings highlight an effective way to developing sustainable cathode materials without compromising on the initial specific capacity for SIBs.

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.

Annealing Modulation Defect Chemistry toward High-Performance Sodium-Layered Cathodes

Layered sodium transition-metal oxides generally encounter severe capacity decay and inferior rate performance during cycling, especially at a high state of charge. Herein, defect concentration is rationally modulated to explore the impact on electrochemical behavior in NaNi1/3Fe1/3Mn1/3O2 layered oxides. Bulk vacancies are increased through annealing in an oxygen-rich atmosphere, demonstrated by electron paramagnetic resonance measurement. It is found that the cathode with enriched oxygen vacancies exhibits significantly enhanced reversibility of redox reactions with a higher initial Coulombic efficiency of 90.0%. Furthermore, the reduced volume variations during the initial charge/discharge process are also confirmed by in situ X-ray diffraction. As a result, the oxygen-vacancy-rich cathode shows great cycling stability and superior rate performances. Also, full cells deliver a specific capacity of approximately 145.2 mAh g-1 at 0.5 C, with a high capacity retention of 78.3% after 100 cycles. This work presents a viable strategy for designing Na+ intercalated cathodes with a high-energy density.

Promoting threshold voltage of P2-Na0.67Ni0.33Mn0.67O2 with Cu2+ cation doping toward high-stability cathode for sodium-ion battery

P2-type Na0.67Ni0.33Mn0.67O2 has attracted considerable attraction as a cathode material for sodium-ion batteries owing to its high operating voltage and theoretical specific capacity. However, when the charging voltage is higher than 4.2 V, the Na0.67Ni0.33Mn0.67O2 cathode undergoes a detrimental irreversible phase transition of P2-O2, leading to a drastic decrease in specific capacity. To address this challenge, we implemented a Cu-doping strategy (Na0.67Ni0.23Cu0.1Mn0.67O2) in this work to stabilize the structure of the transition metal layer. The stabilization strategy involved reinforcing the transition metal-oxygen (TMO) bonds, particularly the MnO bond and inhibiting interlayer slip during deep desodiation. As a result, the irreversible phase transition voltage is delayed, with the threshold voltage increasing from 4.2 to 4.4 V. Ex-situ X-ray diffraction measurements revealed that the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode maintains the P2 phase within the voltage window of 2.5-4.3 V, whereas the P2-Na0.67Ni0.33Mn0.67O2 cathode transforms entirely into O2-type Na0.67Ni0.33Mn0.67O2 when the voltage exceeds 4.3 V. Furthermore, absolute P2-O2 phase transition of the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode occurred at 4.6 V, indicating that Cu2+ doping enhances the stability of the layer structure and increases the threshold voltage. The resulting Na0.67Ni0.23Cu0.1Mn0.67O2 cathode exhibited superior electrochemical properties, demonstrating an initial reversible specific capacity of 89.1 mAh/g at a rate of 2C (360 mA g-1) and retaining more than 78 % of its capacity after 500 cycles.

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.

Electronic States Tailoring and Pinning Effect Boost High-Power Sodium-Ion Storage of Oriented Hollow P2-Type Cathode Materials

Fierce phase transformation and limited sodium ion diffusion dynamics are critical obstacles that hinder the practical energy storage applications of P2-type layered sodium transition metal oxides (NaxTMO2). Herein, a synergistic strategy of electronic state tailoring and pillar effect was carefully implemented by substituting divalent Mg2+ into Na0.67Ni0.33Mn0.67O2 material with unique oriented hollow rodlike structures. Mg2+substitution can not only facilitate the anionic oxygen redox reactions and electronic conductivity through increasing the electronic states at Femi energy but also act as pillars within TMO2 layers to alleviate the severe phase transformation to improve structure stability. Moreover, the oriented hollow structure incorporating sufficient buffer spaces and rationally exposed electrochemically active facets effectively alleviates the stresses induced by low volume changes of 8% and provides more open channels for Na+ ion diffusion without crossing multiple grain boundaries. Hence, the Na0.67Mg0.08Ni0.25Mn0.67O2 cathode showed a superior rate capability with high energy density and cycling stability for sodium-ion storage. The underlying mechanisms of these achievements were deciphered through diversified dynamic analysis and the first principle calculations, providing new insights into P2-type NaxTMO2 cathodes for the infinite prospect as an alternative to lithium-ion batteries.

Cooperative Catalysis of Polysulfides in Lithium-Sulfur Batteries through Adsorption Competition by Tuning Cationic Geometric Configuration of Dual-active Sites in Spinel Oxides

Fundamentally understanding the structure-property relationship is critical to design advanced electrocatalysts for lithium-sulfur (Li-S) batteries, which remains a formidable challenge. Herein, by manipulating the regulable cations in spinel oxides, their geometrical-site-dependent catalytic activity for sulfur redox is investigated. Experimental and theoretical analyses validate that the modulation essence of cooperative catalysis of lithium polysulfides (LiPSs) is dominated by LiPSs adsorption competition between Co³⁺ tetrahedral (Td) and Mn³⁺ octahedral (Oh) sites on Mn³⁺ _Oh -O-Co³⁺ _Td backbones. Specifically, high-spin Co³⁺ _Td with stronger Co-S covalency anchors LiPSs persistently, while electron delocalized Mn³⁺ _Oh with adsorptive orbital (d_z ² ) functions better in catalyzing specialized LiPSs conversion. This work inaugurates a universal strategy for sculpting geometrical configuration to achieve charge, spin, and orbital topological regulation in electrocatalysts for Li-S batteries.