Restraining Oxygen Loss and Boosting Reversible Oxygen Redox in a P2-Type Oxide Cathode by Trace Anion Substitution

Stabilizing the Layer-Structured Oxide Cathode by Modulating the Oxygen Redox Activity for Sodium Ion Batteries

The layer-structured oxide cathode for sodium-ion batteries has attracted a widespread attention due to the unique redox properties and the anionic redox activity providing additional capacity. Nevertheless, such excessive oxygen redox reactions will lead to irreversible oxygen release, resulting in a rapid deterioration of the cycling stability. Herein, sulfur ion is successfully introduced to the O3-NaNi0.3Mn0.5Cu0.1Ti0.05W0.05O2 material through high-temperature quenching, thereby developing a novel Na2S-modified O3/P2-NaNi0.3Mn0.5Cu0.1Ti0.05W0.05O2 composite with extended cycling life. The S2- is analyzed for the ability to enhance the reversibility of oxidation-reduction reactions under high voltage and suppress the loss of lattice oxygen during cycling. The stable S─O covalent bonds are found to inhibit the oxygen generation and release within the structure. Benefiting from these improvements, the Na₂S-modified O3/P2-NaNi0.3Mn0.5Cu0.1Ti0.05W0.05O2 exhibited a high reversible capacity of 173.1 mA h g-1 over a wide voltage range of 1.5-4.3 V under test conditions at 0.1 C and 81.5% capacity retention after 120 cycles at 1 C. The Na₂S-modified O3/P2-NaNi0.3Mn0.5Cu0.1Ti0.05W0.05O2 demonstrates the excellent rate capability with the reversible capacities of 173.1,137.0,114.7,96.7, and 80.1 mA h g-1 at 0.1, 0.2, 0.5, 1, and 2 C.

Challenges and Modification Strategies on High-Voltage Layered Oxide Cathode for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted great attention due to their advantages on resource abundance, cost and safety. Layered oxide cathodes (LOCs) of SIBs possess high theoretical capacity, facile synthesis and low cost, and are promising candidates for large scale energy storage application. Increasing operating voltage is an effective strategy to achieve higher specific capacity and also high energy density of SIBs. However, at high operating voltages, LOCs will undergo a series of phase transitions in bulk phase, leading to huge change of volume and layer spacings accompanied by severe lattice stress and cracking formation. Degeneration of surface also occurs between LOCs and electrolytes, resulting in sustained growth of cathode electrolyte interphase (CEI) and release of O2 and CO2. These induce structural destruction and electrochemical performance degradation in high voltage regions. Recently, many strategies have been proposed to improve electrochemical performance of LOCs under high voltages, including bulk element doping, structural design, surface coating and gradient doping. This review describes pivotal challenges and occurrence mechanisms at high voltages, and summarizes strategies to improve stability of bulk and surface. Viewpoints will be provided to promote development of high energy density SIBs.

Stabilizing Interlayer Repulsion in Layered Sodium-Ion Oxide Cathodes via Hierarchical Layer Modification

Layered sodium-ion oxides hold considerable promise in achieving high-performance sodium-ion batteries. However, the notorious phase transformation during charging, attributed to increased O2-─O2- repulsion, results in substantial performance decay. Here, a hierarchical layer modification strategy is proposed to stabilize interlayer repulsion. During desodiation, migrated Li+ from the transition metal layer and anchored Ca2+ in sodium sites maintain the cationic content within the sodium layer. Meanwhile, partial oxygen substitution by fluorine and the involvement of oxygen in redox reactions increase the average valence of the oxygen layer. This sustained cation presence and elevated anion valence collectively mitigate increasing O2-─O2- repulsion during sodium extraction, enabling the Na0.61Ca0.05[Li0.1Ni0.23Mn0.67]O1.95F0.05 (NCLNMOF) cathode to retain a pure P2-type structure across a wide voltage range. Unexpected insights reveal the interplay between different doping elements: the robust Li─F bonds and Ca2+ steric effects suppressing Li+ loss. The NCLNMOF electrode exhibits 82.5% capacity retention after 1000 cycles and a high-rate capability of 94 mAh g-1 at 1600 mA g-1, demonstrating the efficacy of hierarchical layer modification for high-performance layered oxide cathodes.

Toward the High-Voltage Stability of Layered Oxide Cathodes for Sodium-Ion Batteries: Challenges, Progress, and Perspectives

Sodium-ion batteries (SIBs) have garnered significant attention as ideal candidates for large-scale energy storage due to their notable advantages in terms of resource availability and cost-effectiveness. However, there remains a substantial energy density gap between SIBs and commercially available lithium-ion batteries (LIBs), posing challenges to meeting the requirements of practical applications. The fabrication of high-energy cathodes has emerged as an efficient approach to enhancing the energy density of SIBs, which commonly requires cathodes operating in high-voltage regions. Layered oxide cathodes (LOCs), with low cost, facile synthesis, and high theoretical specific capacity, have emerged as one of the most promising candidates for commercial applications. However, LOCs encounter significant challenges when operated in high-voltage regions such as irreversible phase transitions, migration and dissolution of metal cations, loss of reactive oxygen, and the occurrence of serious interfacial parasitic reactions. These issues ultimately result in severe degradation in battery performance. This review aims to shed light on the key challenges and failure mechanisms encountered by LOCs when operated in high-voltage regions. Additionally, the corresponding strategies for improving the high-voltage stability of LOCs are comprehensively summarized. By providing fundamental insights and valuable perspectives, this review aims to contribute to the advancement of high-energy SIBs.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

Sodium-ion batteries (SIBs) are experiencing a large-scale renaissance to supplement or replace expensive lithium-ion batteries (LIBs) and low energy density lead-acid batteries in electrical energy storage systems and other applications. In this case, layered oxide materials have become one of the most popular cathode candidates for SIBs because of their low cost and comparatively facile synthesis method. However, the intrinsic shortcomings of layered oxide cathodes, which severely limit their commercialization process, urgently need to be addressed. In this review, inherent challenges associated with layered oxide cathodes for SIBs, such as their irreversible multiphase transition, poor air stability, and low energy density, are systematically summarized and discussed, together with strategies to overcome these dilemmas through bulk phase modulation, surface/interface modification, functional structure manipulation, and cationic and anionic redox optimization. Emphasis is placed on investigating variations in the chemical composition and structural configuration of layered oxide cathodes and how they affect the electrochemical behavior of the cathodes to illustrate how these issues can be addressed. The summary of failure mechanisms and corresponding modification strategies of layered oxide cathodes presented herein provides a valuable reference for scientific and practical issues related to the development of SIBs.

Highly Reversible Local Structural Transformation Enabled by Native Vacancies in O2-Type Li-Rich Layered Oxides with Anion Redox Activity

A novel O2-phase Li_1.033Ni_0.2[□_0.1Mn_0.5]O₂ cathode with native vacancies (denoted as "□") was delicately designed. By a combination of noninvasive ⁷Li pj-MATPASS NMR and electron paramagnetic resonance measurements, it is unequivocally shown that the reservation of native vacancies enables the fully reversible local structural transformation without the formation of Li in the Li layer (Li_tet) in Li_1.033Ni_0.2[□_0.1Mn_0.5]O₂ during the initial and subsequent cycling. In addition, the pernicious in-plane Mn migration that would result in the generation of trapped molecular O₂ is effectively mitigated in Li_1.033Ni_0.2[□_0.1Mn_0.5]O₂. As a result, the cycle stability of Li_1.033Ni_0.2[□_0.1Mn_0.5]O₂ is significantly enhanced compared to that of the vacancy-free Li_1.033Ni_0.2Mn_0.6O₂, showing an extraordinary capacity retention of 102.31% after 50 cycles at a rate of 0.1C (1C = 100 mA g^-1). This study defines an efficacious strategy for upgrading the structural stability of O2-type Li-rich layered oxide cathodes with reversible high-voltage anion redox activity.

Oxygen Redox Activation at Initial Cycle to Improve Cycling Stability for the Na0.83Li0.12Ni0.22Mn0.66O2 System

Oxygen reactions are commonly used to increase the specific capacities of Na-ion batteries, especially for the Na_xLi_yTMO₂ systems. Previous research focused on improving the stabilities of oxygen reactions to enhance cycling stability. However, the effects of oxygen reactions on the distribution of Li ions in the transition metal (TM) and alkali metal (AM) layers for the Na-ion battery are relatively unexplored and rarely employed. In this study, we employ a layered P2-Na_0.83Li_0.12Ni_0.22Mn_0.66O₂ cathode to control the effects of the oxygen reactions on the distributions of Li ions in two layers. With oxygen-redox-activation-at-first-cycle (ORAFIC)-cycling, which cycled first within 2.0-4.6 V to activate oxygen redox and then cycled within 2.0-4.2 V, this cathode exhibited better cycling stability compared to low-voltage (LV)-cycling of 2.0-4.2 V and high-voltage (HV)-cycling of 2.0-4.6 V. Using nuclear magnetic resonance spectroscopy, electron paramagnetic resonance, inductively coupled plasma experiments, and X-ray diffraction, it is confirmed that ORAFIC-cycling stabilizes the crystal structure and distributions of Li ions in the TM and AM layers and reduces Li-ion loss, thus improving the cycling stability.

Fluorine Substitution Promotes Air-Stability of P'2-Type Layered Cathodes for Sodium-Ion Batteries

As one of the most promising cathode materials in sodium-ion batteries, manganese-based layered oxides have aroused wide attention due to their high specific capacity and plentiful reserves. However, they are plagued by poor air stability rooting in water/Na⁺ exchange and adverse structural reconstruction, hindering their practical applications. Herein, it is demonstrated that utilizing fluorine to substitute oxygen atoms can narrow the interlayer spacing of novel P'2-Na_0.67 MnO_1.97 F_0.03 (NMOF) cathode material, which resists the attack of water molecules, significantly prolonging exposure time in air. Density functional theory (DFT) calculation results indicate that fluorine substitution alleviates the insertion of water molecules and spontaneous extraction of Na⁺ effectively. Benefiting from the structural modulation, NMOF can deliver a high specific capacity of 227.1 mAh g^-1 at 20 mA g^-1 and a promising capacity retention of 84.0% after 100 cycles at 200 mA g^-1 . This facile and available strategy provides a feasible way to strengthen the air-stability and expands the scope of practical applications of layered oxide cathodes.

Ion Substitution Strategy of Manganese-Based Layered Oxide Cathodes for Advanced and Low-Cost Sodium Ion Batteries

Sodium ion batteries (SIBs) have recently been promising in the large-scale electric energy storage system, due to the low cost, abundant sodium resources. Mn-based layered oxide cathode materials have been widely investigated, because of the high theoretical specific capacity, low cost, and abundant reserves. However, their development is limited by the problems of Jahn-Teller distortion, Na⁺ /vacancy ordering, complex phase transitions, and irreversible anionic redox during cycling. Ion substitution strategy is one simple and effective way to regulate the crystal structure and boost sodium-storage performances of Mn-based cathode materials. In this review, we summarize the progress and mechanism of ion-substituted Mn-based oxides, establish a composition-crystal structure-electrochemical performance relationship, and also offer perspectives for guiding the design of high-performance Mn-based oxides for SIBs.

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

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

Unraveling Anionic Redox for Sodium Layered Oxide Cathodes: Breakthroughs and Perspectives

Sodium-ion batteries (SIBs) as the next generation of sustainable energy technologies have received widespread investigations for large-scale energy storage systems (EESs) and smart grids due to the huge natural abundance and low cost of sodium. Although the great efforts are made in exploring layered transition metal oxide cathode for SIBs, their performances have reached the bottleneck for further practical application. Nowadays, anionic redox in layered transition metal oxides has emerged as a new paradigm to increase the energy density of rechargeable batteries. Based on this point, in this review, the development history of anionic redox reaction is attempted to systematically summarize and provide an in-depth discussion on the anionic redox mechanism. Particularly, the major challenges of anionic redox and the corresponding available strategies toward triggering and stabilizing anionic redox are proposed. Subsequently, several types of sodium layered oxide cathodes are classified and comparatively discussed according to Na-rich or Na-deficient materials. A large amount of progressive characterization techniques of anionic oxygen redox is also summarized. Finally, an overview of the existing prospective and the future development directions of sodium layered transition oxide with anionic redox reaction are analyzed and suggested.

Coexistence of (O2)n- and Trapped Molecular O2 as the Oxidized Species in P2-Type Sodium 3d Layered Oxide and Stable Interface Enabled by Highly Fluorinated Electrolyte

The interface stability of cathode/electrolyte for Na-ion layered oxides is tightly related to the oxidized species formed during the electrochemical process. Herein, we for the first time decipher the coexistence of (O₂)^n- and trapped molecular O₂ in the (de)sodiation process of P2-Na_0.66[Li_0.22Mn_0.78]O₂ by using advanced electron paramagnetic resonance (EPR) spectroscopy. An unstable interface of cathode/electrolyte can thus be envisaged with conventional carbonate electrolyte due to the high reactivity of the oxidized O species. We therefore introduce a highly fluorinated electrolyte to tentatively construct a stable and protective interface between P2-Na_0.66[Li_0.22Mn_0.78]O₂ and the electrolyte. As expected, an even and robust NaF-rich cathode-electrolyte interphase (CEI) film is formed in the highly fluorinated electrolyte, in sharp contrast to the nonuniform and friable organic-rich CEI formed in the conventional lowly fluorinated electrolyte. The in situ formed fluorinated CEI film can significantly mitigate the local structural degeneration of P2-Na_0.66[Li_0.22Mn_0.78]O₂ by refraining the irreversible Li/Mn dissolutions and O₂ release, endowing a highly reversible oxygen redox reaction. Resultantly, P2-Na_0.66[Li_0.22Mn_0.78]O₂ in highly fluorinated electrolyte achieves a high Coulombic efficiency (CE) of >99% and an impressive cycling stability in the voltage range of 2.0-4.5 V (vs Na⁺/Na) under room temperature (147.6 mAh g^-1, 100 cycles) and at 45 °C (142.5 mAh g^-1, 100 cycles). This study highlights the profound impact of oxidized oxygen species on the interfacial stability of cathode/electrolyte and carves a new path for building stable interface and enabling highly stable oxygen redox reaction.

What Triggers the Voltage Hysteresis Variation beyond the First Cycle in Li-Rich 3d Layered Oxides with Reversible Cation Migration?

Herein, the structure-electrochemistry relationship of O2-Li_5/6(Li_0.2Ni_0.2Mn_0.6)O₂ is deliberately studied by local-structure probes including site-sensitive ⁷Li pj-MATPASS NMR, quantitative ⁶Li magic-angle spinning NMR, and electron paramagnetic resonance (EPR). The extraction and reinsertion of Li_TM (Li in the transition metal layer) during the first cycle are only partially reversible, bringing about the formation of tetrahedral Li_Li (Li in the Li layer) that can be reversibly (de)intercalated after the activation cycle. The high-voltage oxygen redox process is preserved beyond the first cycle, further manifesting the structural superiority of O2 stacking over O3 stacking in bolstering oxygen redox. Moreover, the (de)lithiation process is highly reversible without pronounced structural hysteresis after the rearrangement of Li and transition metal upon the activation cycle, which can explain well the variation of voltage hysteresis from the first cycle to second cycle. These insights elucidate the imperfect structural stability of O2-type Li-rich layered oxides, which could be further improved by streamlining the returning path of Li_TM.