Dopant Segregation Boosting High-Voltage Cyclability of Layered Cathode for Sodium Ion Batteries

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.

Sodium layered oxide cathodes: properties, practicality and prospects

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

Emerging Chemistry for Wide-Temperature Sodium-Ion Batteries

The shortage of resources such as lithium and cobalt has promoted the development of novel battery systems with low cost, abundance, high performance, and efficient environmental adaptability. Due to the abundance and low cost of sodium, sodium-ion battery chemistry has drawn worldwide attention in energy storage systems. It is widely considered that wide-temperature tolerance sodium-ion batteries (WT-SIBs) can be rapidly developed due to their unique electrochemical and chemical properties. However, WT-SIBs, especially for their electrode materials and electrolyte systems, still face various challenges in harsh-temperature conditions. In this review, we focus on the achievements, failure mechanisms, fundamental chemistry, and scientific challenges of WT-SIBs. The insights of their design principles, current research, and safety issues are presented. Moreover, the possible future research directions on the battery materials for WT-SIBs are deeply discussed. Progress toward a comprehensive understanding of the emerging chemistry for WT-SIBs comprehensively discussed in this review will accelerate the practical applications of wide-temperature tolerance rechargeable batteries.

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.

Regulating Anionic Redox via Mg Substitution in Mn-Rich Layered Oxide Cathodes Enabling High Electrochemical Stability for Sodium-Ion Batteries

With the limited resources and high cost of lithium-ion batteries (LIBs) and the ever-increasing market demands, sodium-ion batteries (SIBs) gain much interest due to their economical sustainability, and similar chemistry and manufacturing processes to LIBs. As cathodes play a vital role in determining the energy density of SIBs, Mn-based layered oxides are promising cathodes due to their low cost, environmental friendliness, and high theoretical capacity. However, the main challenge is structural instability upon cycling at high voltage. Herein, Mg is introduced into the P2-type Na0.62 Ni0.25 Mn0.75 O2 cathode to enhance electrochemical stability. By combining electrochemical testing and material characterizations, it is found that substituting 10 mol% Mg can effectively alleviate the P2-O2 phase transition, Jahn-Teller distortion, and irreversible oxygen redox. Moreover, structural integrity is greatly improved. These lead to enhanced electrochemical performances. With the optimized sample, a remarkable capacity retention of 92% in the half cell after 100 cycles and 95% in the full cell after 170 cycles can be achieved. Altogether, this work provides an alternative way to stabilize P2-type Mn-based layer oxide cathodes, which in turn, put forward the development of this material for the next-generation SIBs.

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

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

Magnesium Mitigation Behavior in P2-Layered Sodium-Ion Battery Cathode

Heteroatom incorporation can effectively suppress the phase transition of layered sodium-ion battery cathode, but heteroatom behaviors during operating conditions are not completely understood at the atomic scale. Here, density functional theory calculations are combined with experiments to explore the mitigation behavior of Mg dopant and its mechanisms under operating conditions in P2-Na0.67Ni0.33Mn0.67O2. The void formed by Na extraction will pump some Mg dopants into Na layers from TM layers, and the collective diffusion of more than one Mg ion most likely occurs when the Mg content is relatively high in the TM layer, finally aggregating to form Mg-enrich regions (i.e., Mg segregation) apart from Ni vacancies. The void-pump-effect-induced Mg segregation effectively suppresses the P2-O2 phase transition owing to the stronger Mg-O electrostatic attraction that enhances the integrate of two adjacent oxygen layers and prevents the crack growth by mitigating the lattice volume variation under high-voltage cycling. Our work provides a fundamental understanding of heteroatom mitigation behavior in layered cathodes at the atomic level for next-generation energy storage technologies.

Sodium Layered/Tunnel Intergrowth Oxide Cathodes: Formation Process, Interlocking Chemistry, and Electrochemical Performance

Manganese-based layered oxides are prospective cathode materials for sodium-ion batteries (SIBs) due to their low cost and high theoretical capacities. The biphasic intergrowth structure of layered cathode materials is essential for improving the sodium storage performance, which is attributed to the synergistic effect between the two phases. However, the in-depth formation mechanism of biphasic intergrowth materials remains unclear. Herein, the layered/tunnel intergrowth Na0.6MnO2 (LT-NaMO) as a model material was successfully prepared, and their formation processes and electrochemical performance were systematically investigated. In situ high-temperature X-ray diffraction displays the detailed evolution process and excellent thermal stability of the layered/tunnel intergrowth structure. Furthermore, severe structural strain and large lattice volume changes are significantly mitigated by the interlocking effect between the phase interfaces, which further enhances the structural stability of the cathode materials during the charging/discharging process. Consequently, the LT-NaMO cathode displays fast Na+ transport kinetics with a remarkable capacity retention of ∼70.5% over 300 cycles at 5C, and its assembled full cell with hard carbon also exhibits high energy density. These findings highlight the superior electrochemical performance of intergrowth materials due to interlocking effects between layered and tunnel structures and also provide unique insights into the construction of intergrowth cathode materials for SIBs.

Boosting Cycling Stability of Polymer Sodium Battery by "Rigid-Flexible" Coupled Interfacial Stress Modulation

The discontinuous interfacial contact of solid-state polymer metal batteries is due to the stress changes in the electrode structure during cycling, resulting in poor ion transport. Herein, a rigid-flexible coupled interface stress modulation strategy is developed to solve the above issues, which is to design a rigid cathode with enhanced solid-solution behavior to guide the uniform distribution of ions and electric field. Meanwhile, the polymer components are optimized to build an organic-inorganic blended flexible interfacial film to relieve the change of interfacial stress and ensure rapid ion transmission. The fabricated battery comprising a Co-modulated P2-type layered cathode (Na_0.67Mn_2/3Co_1/3O₂) and a high ion conductive polymer could deliver good cycling stability without distinct capacity fading (72.8 mAh g^-1 over 350 cycles at 1 C), outperforming those without Co modulation or interfacial film construction. This work demonstrates a promising rigid-flexible coupled interfacial stress modulation strategy for polymer-metal batteries with excellent cycling stability.

Impact of Ti and Zn Dual-Substitution in P2 Type Na2/3 Ni1/3 Mn2/3 O2 on Ni-Mn and Na-Vacancy Ordering and Electrochemical Properties

High-entropy layered oxide materials containing various metals that exhibit smooth voltage curves and excellent electrochemical performances have attracted attention in the development of positive electrode materials for sodium-ion batteries. However, a smooth voltage curve can be obtained by suppression of the Na⁺ -vacancy ordering, and therefore, transition metal slabs do not need to be more multi-element than necessary. Here, the Na⁺ -vacancy ordering is found to be disturbed by dual substitution of Ti^IV for Mn^IV and Zn^II for Ni^II in P2-Na_2/3 [Ni_1/3 Mn_2/3 ]O₂ . Dual-substituted Na_2/3 [Ni_1/4 Mn_1/2 Ti_1/6 Zn_1/12 ]O₂ demonstrates almost non-step voltage curves with a reversible capacity of 114 mAh g^-1 and less structural changes with a high crystalline structure maintained during charging and discharging. Synchrotron X-ray, neutron, and electron diffraction measurements reveal that dual-substitution with Ti^IV and Zn^II uniquely promotes in-plane Ni^II -Mn^IV ordering, which is quite different from the disordered mixing in conventional multiple metal substitution.

Facile Synthesis of O3-Type NaNi0.5Mn0.5O2 Single Crystals with Improved Performance in Sodium-Ion Batteries

Sodium-ion batteries can be a practical alternative to lithium-ion batteries due to the relatively high abundance of sodium and the projected scarcity of lithium. Both of these factors are critical considerations for grid-scale energy storage, but the central challenge to implementing sodium layered oxides in sodium-ion batteries is their relatively poor cycle life. Single-crystal particles with micrometer size can mitigate several failure mechanisms related to sodium layered oxides and can improve performance when compared to the commonly used polycrystalline particles. This work demonstrates a novel two-step molten-salt synthesis method using sodium chloride and metal oxides to form "single crystals" of a mixed-phase, spinel/rock-salt intermediate that crystallizes as micron-sized truncated octahedra. The mixed-phase spinel/rock-salt material is effectively used as a precursor to form O3-type NaNi_0.5Mn_0.5O₂ with large primary particles and substantially improved cycle life. This synthesis route offers the added benefit of using simple metal oxides instead of hydroxide precursors, eliminating the need for coprecipitation. Particle morphology is found to be a critical factor in mitigating the structural damages incurred during phase transformations and maintaining the electrochemical performance.

Atomistic Simulation Informs Interface Engineering of Nanoscale LiCoO2

Lithium-ion batteries continue to be a critical part of the search for enhanced energy storage solutions. Understanding the stability of interfaces (surfaces and grain boundaries) is one of the most crucial aspects of cathode design to improve the capacity and cyclability of batteries. Interfacial engineering through chemical modification offers the opportunity to create metastable states in the cathodes to inhibit common degradation mechanisms. Here, we demonstrate how atomistic simulations can effectively evaluate dopant interfacial segregation trends and be an effective predictive tool for cathode design despite the intrinsic approximations. We computationally studied two surfaces, {001} and {104}, and grain boundaries, Σ3 and Σ5, of LiCoO₂ to investigate the segregation potential and stabilization effect of dopants. Isovalent and aliovalent dopants (Mg²⁺, Ca²⁺, Sr²⁺, Sc³⁺, Y³⁺, Gd³⁺, La³⁺, Al³⁺, Ti⁴⁺, Sn⁴⁺, Zr⁴⁺, V⁵⁺) were studied by replacing the Co³⁺ sites in all four of the constructed interfaces. The segregation energies of the dopants increased with the ionic radius of the dopant. They exhibited a linear dependence on the ionic size for divalent, trivalent, and quadrivalent dopants for surfaces and grain boundaries. The magnitude of the segregation potential also depended on the surface chemistry and grain boundary structure, showing higher segregation energies for the Σ5 grain boundary compared with the lower energy Σ3 boundary and higher for the {104} surface compared to the {001}. Lanthanum-doped nanoparticles were synthesized and imaged with scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) to validate the computational results, revealing the predicted lanthanum enrichment at grain boundaries and both the {001} and the {104} surfaces.

Transition metal oxides as a cathode for indispensable Na-ion batteries

The essential requirement to harness well-known renewable energy sources like wind energy, solar energy, etc. as a component of an overall plan to guarantee global power sustainability will require highly efficient, high power and energy density batteries to collect the derived electrical power and balance out variations in both supply and demand. Owing to the continuous exhaustion of fossil fuels, and ever increasing ecological problems associated with global warming, there is a critical requirement for searching for an alternative energy storage technology for a better and sustainable future. Electrochemical energy storage technology could be a solution for a sustainable source of clean energy. Sodium-ion battery (SIB) technology having a complementary energy storage mechanism to the lithium-ion battery (LIB) has been attracting significant attention from the scientific community due to its abundant resources, low cost, and high energy densities. Layered transition metal oxide (TMO) based materials for SIBs could be a potential candidate for SIBs among all other cathode materials. In this paper, we discussed the latest improvement in the various structures of the layered oxide materials for SIBs. Moreover, their synthesis, overall electrochemical performance, and several challenges associated with SIBs are comprehensively discussed with a stance on future possibilities. Many articles discussed the improvement of cathode materials for SIBs, and most of them have pondered the use of Na x MO2 (a class of TMOs) as a possible positive electrode material for SIBs. The different phases of layered TMOs (Na x MO2; TM = Co, Mn, Ti, Ni, Fe, Cr, Al, V, and a combination of multiple elements) show good cycling capacity, structural stability, and Na+ ion conductivity, which make them promising cathode material for SIBs. This review discusses and summarizes the electrochemical redox reaction, structural transformations, significant challenges, and future prospects to improve for Na x MO2. Moreover, this review highlights the recent advancement of several layered TMO cathode materials for SIBs. It is expected that this review will encourage further development of layered TMOs for SIBs.

Formulating High-Rate and Long-Cycle Heterostructured Layered Oxide Cathodes by Local Chemistry and Orbital Hybridization Modulation for Sodium-Ion Batteries

It is still very urgent and challenging to simultaneously develop high-rate and long-cycle oxide cathodes for sodium-ion batteries (SIBs) because of the sluggish kinetics and complex multiphase evolution during cycling. Here, the concept of accurately manipulating structural evolution and formulating high-performance heterostructured biphasic layered oxide cathodes by local chemistry and orbital hybridization modulation is reported. The P2-structure stoichiometric composition of the cathode material shows a layered P2- and O3-type heterostructure that is explicitly evidenced by various macroscale and atomic-scale techniques. Surprisingly, the heterostructured cathode displays excellent rate performance, remarkable cycling stability (capacity retention of 82.16% after 600 cycles at 2 C), and outstanding compatibility with hard carbon anode because of the integrated advantages of intergrowth structure and local environment regulation. Meanwhile, the formation process from precursors during calcination and the highly reversible dynamic structural evolution during the Na⁺ intercalation/deintercalation process are clearly articulated by a series of in situ characterization techniques. Also, the intrinsic structural properties and corresponding electrochemical behavior are further elucidated by the density of states and electron localization function of density functional theory calculations. Overall, this strategy, which finely tunes the local chemistry and orbitals hybridization for high-performance SIBs, will open up a new field for other materials.

Recent Advances in Layered Metal-Oxide Cathodes for Application in Potassium-Ion Batteries

To meet future energy demands, currently, dominant lithium-ion batteries (LIBs) must be supported by abundant and cost-effective alternative battery materials. Potassium-ion batteries (KIBs) are promising alternatives to LIBs because KIB materials are abundant and because KIBs exhibit intercalation chemistry like LIBs and comparable energy densities. In pursuit of superior batteries, designing and developing highly efficient electrode materials are indispensable for meeting the requirements of large-scale energy storage applications. Despite using graphite anodes in KIBs instead of in sodium-ion batteries (NIBs), developing suitable KIB cathodes is extremely challenging and has attracted considerable research attention. Among the various cathode materials, layered metal oxides have attracted considerable interest owing to their tunable stoichiometry, high specific capacity, and structural stability. Therefore, the recent progress in layered metal-oxide cathodes is comprehensively reviewed for application to KIBs and the fundamental material design, classification, phase transitions, preparation techniques, and corresponding electrochemical performance of KIBs are presented. Furthermore, the challenges and opportunities associated with developing layered oxide cathode materials are presented for practical application to KIBs.

koLayered Oxide Cathode-Electrolyte Interface towards Na-Ion Batteries: Advances and Perspectives

With the ever increasing demand for low-cost and economic sustainable energy storage, Na-ion batteries have received much attention for the application on large-scale energy storage for electric grids because of the worldwide distribution and natural abundance of sodium element, low solvation energy of Na⁺ ion in the electrolyte and the low cost of Al as current collectors. Starting from a brief comparison with Li-ion batteries, this review summarizes the current understanding of layered oxide cathode/electrolyte interphase in NIBs, and discusses the related degradation mechanisms, such as surface reconstruction and transition metal dissolution. Recent advances in constructing stable cathode electrolyte interface (CEI) on layered oxide cathode are systematically summarized, including surface modification of layered oxide cathode materials and formulation of electrolyte. Urgent challenges are detailed in order to provide insight into the imminent developments of NIBs.

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

Utilizing reversible lattice oxygen redox (OR) in battery electrodes is an essential strategy to overcome the capacity limitation set by conventional transition metal redox. However, lattice OR reactions are often accompanied with irreversible oxygen oxidation, leading to local structural transformations and voltage/capacity fading. Herein, it is proposed that the reversibility of lattice OR can be remarkably improved through modulating transition metal-oxygen covalency for layered electrode of Na-ion batteries. By developing a novel layered P2-Na_0.6 Mg_0.15 Mn_0.7 Cu_0.15 O₂ electrode, it is demonstrated that the highly electronegative Cu dopants can improve the lattice OR reversibility to 95% compared to 73% for Cu-free counterpart, as directly quantified through high-efficiency mapping of resonant inelastic X-ray scattering. Crucially, the large energetic overlap between Cu 3d and O 2p states dictates the rigidity of oxygen framework, which effectively mitigates the structural distortion of local oxygen environment upon (de)sodiation and leads to the enhanced lattice OR reversibility. The electrode also exhibits a completely solid-solution reaction with an ultralow volume change of only 0.45% and a reversible metal migration upon cycling, which together ensure the improved electrochemical performance. These results emphasize the critical role of transition metal-oxygen covalency for enhancing the reversibility of lattice OR toward high-capacity electrodes employing OR chemistry.

Regulation of Surface Defect Chemistry toward Stable Ni-Rich Cathodes

Surface reconstruction of Ni-rich layered oxides (NLO) degrades the cycling stability and safety of high-energy-density lithium-ion batteries (LIBs), which challenges typical surface-modification approaches to build a robust interface with electrochemical activity. Here, a strategy of leveraging the low-strain analogues of Li- and Mn-rich layered oxides (LMR) to reconstruct a stable surface on the Ni-rich layered cathodes is proposed. The new surface structure not only consists of a gradient chemical composition but also contains a defect-rich structure regarding the formation of oxygen vacancies and cationic ordering, which can simultaneously facilitate lithium diffusion and stabilize the crystal structure during the (de)lithiation. These features in the NLO lead to a dramatic improvement in electrochemical properties, especially the cyclability under high voltage cycling, exhibiting the 30% increase in capacity retention after 200 cycles at the current density of 1 C (3.0-4.6 V). The findings offer a facile and effective way to regulate defect chemistry and surface structure in parallel on Ni-rich layered structure cathodes to achieve high-energy density LIBs.

Mg2+-Doping Constructed a Continuous and Homogeneous Cathode-Electrolyte Interphase Film on Na3.12Fe2.44(P2O7)2 with Superior and Stable High-Temperature Performance for Sodium-Ion Storage

Sodium-ion batteries (SIBs) are on the verge of achieving practical applications, and the key is to find suitable electrode materials. The polyanionic iron-based material Na_3.12Fe_2.44(P₂O₇)₂ (NFPO) possesses an open three-dimensional framework structure with good thermal stability and is regarded as an outstanding cathode material for SIBs. Nevertheless, its poor electrical conductivity, problems with erosion of electrolytes, and structural deterioration during cycling still need to be urgently addressed. Here, we first design a Mg²⁺-doped NFPO (NFPO-Mg) material with a dual-action effect. On the one hand, Mg²⁺ improves the intrinsic conductivity of the NFPO material, and on the other hand, Mg²⁺ promotes the formation of a homogeneous and stable cathode-electrolyte interphase film during the cycling process, which results in a superior rate performance and cycling stability. A capacity of 68.6 mAh g^-1 was achieved at 50C (1C = 117.4 mAh g^-1), and a capacity retention of 79.1% was maintained after 3000 cycles at 20C. More impressively, NFPO-Mg exhibits outstanding high-temperature electrochemical performance, with a capacity retention of 95.3% after 400 cycles at 10C at 60 °C (much higher than the 54.2% for the NFPO). This paper explores an effective method for improving the electrochemical performance of cathode materials, which may prove instrumental in guiding the design of more high-performance cathode materials in the future.

Challenges of layer-structured cathodes for sodium-ion batteries

As the most promising alternative to lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) still face many issues that hinder their large-scale commercialization. Layered transition metal oxide cathodes have attracted widespread attention owing to their large specific capacity, high ionic conductivity, and feasible preparation conditions. However, their electrochemical properties are usually limited by the irreversible phase transition and harsh storage conditions caused by humidity sensitivity. Recently, tremendous efforts have been devoted to solving these issues toward advanced high-performance layered oxide cathodes. Herein, we summarize these remaining challenges of layered oxide cathodes and the corresponding modification strategies such as the variations in chemical compositions, the architecture of (nano)micro-structures, surface engineering, and the regulation of phase compositions. We hope that the understanding presented in this review can provide useful guidance to developing high-performance layer-structured cathode materials for advanced SIBs.

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.

Unusual Site-Selective Doping in Layered Cathode Strengthens Electrostatic Cohesion of Alkali-Metal Layer for Practicable Sodium-Ion Full Cell

P2-type Na_0.67 Ni_0.33 Mn_0.67 O₂ is a dominant cathode material for sodium-ion batteries due to its high theoretical capacity and energy density. However, charging P2-type Na_0.67 Ni_0.33 Mn_0.67 O₂ to voltages higher than 4.2 V (vs. Na⁺ /Na) can induce detrimental structural transformation and severe capacity fading. Herein, stable cycling and moisture resistancy of Na_0.67 Ni_0.33 Mn_0.67 O₂ at 4.35 V (vs. Na⁺ /Na) are achieved through dual-site doping with Cu ion at transition metal site (2a) and unusual Zn ion at Na site (2d) for the first time. The Cu ion doping in 2a site stabilizes the metal layer, while more importantly, the unusual alkali-metal site doping by Zn ion serves as O^2- Zn²⁺ O^2- "pillar" for enhancing electrostatic cohesion between two adjacent transition metal layers, preventing the crack of active material along the a-b-plane and restraining the generation of O2 phase upon deep desodiation. This unique dual-site-doped [Na_0.67 Zn_0.05 ]Ni_0.18 Cu_0.1 Mn_0.67 O₂ cathode exhibits a prominent cyclability with 80.6% capacity retention over 2000 cycles at an ultrahigh rate of 10C, demonstrating its great potential for practical applications. Impressively, the full cell devices with [Na_0.67 Zn_0.05 ]Ni_0.18 Cu_0.1 Mn_0.67 O₂ and commercial hard carbon as cathode and anode, respectively, can deliver a high energy density of 217.9 Wh kg^-1 and excellent cycle life over 1000 cycles.

Synergetic stability enhancement with magnesium and calcium ion substitution for Ni/Mn-based P2-type sodium-ion battery cathodes

The conventional P2-type cathode material Na0.67Ni0.33Mn0.67O2 suffers from an irreversible P2-O2 phase transition and serious capacity fading during cycling. Here, we successfully carry out magnesium and calcium ion doping into the transition-metal layers (TM layers) and the alkali-metal layers (AM layers), respectively, of Na0.67Ni0.33Mn0.67O2. Both Mg and Ca doping can reduce O-type stacking in the high-voltage region, leading to enhanced cycling endurance, however, this is associated with a decrease in capacity. The results of density functional theory (DFT) studies reveal that the introduction of Mg2+ and Ca2+ make high-voltage reactions (oxygen redox and Ni4+/Ni3+ redox reactions) less accessible. Thanks to the synergetic effect of co-doping with Mg2+ and Ca2+ ions, the adverse effects on high-voltage reactions involving Ni-O bonding are limited, and the structural stability is further enhanced. The finally obtained P2-type Na0.62Ca0.025Ni0.28Mg0.05Mn0.67O2 exhibits a satisfactory initial energy density of 468.2 W h kg-1 and good capacity retention of 83% after 100 cycles at 50 mA g-1 within the voltage range of 2.2-4.35 V. This work deepens our understanding of the specific effects of Mg2+ and Ca2+ dopants and provides a stability-enhancing strategy utilizing abundant alkaline earth elements.

Advanced cobalt-free cathode materials for sodium-ion batteries

Attempts to utilize lithium-ion batteries (LIBs) in large-scale electrochemical energy storage systems have achieved initial success, and solid-state LIBs using metallic lithium as the anode have also been well developed. However, the sharply increased demands/costs and the limited reserves of the two most important metal elements (Li & Co) for LIBs have raised concerns about future development. Sodium-ion batteries (SIBs) equipped with advanced cobalt-free cathodes show great potential in solving both "lithium panic" and "cobalt panic", and have made remarkable progress in recent years. In this review, we comprehensively summarize the recent advances of high-performance cobalt-free cathode materials for advanced SIBs, systematically analyze the conflicts of structural/electrochemical stability with intrinsic insufficiencies of cobalt-free cathode materials, and extensively discuss the strategies of constructing stable cobalt-free cathode materials by making full use of non-cobalt transition-metal elements and suitable crystal structures, all of which aim to provide insights into the key factors (e.g., phase transformation, particle cracks, crystal defects, lattice distortion, lattice oxygen oxidation, morphology, transition-metal migration/dissolution, and the synergistic effects of composite structures) that can determine the stability of cobalt-free cathode materials, provide guidelines for future research, and stimulate more interest on constructing high-performance cobalt-free cathode materials.

Clarifying the Roles of Cobalt and Nickel in the Structural Evolution of Layered Cathodes for Sodium-Ion Batteries

Layered sodium manganese-based oxides are appealing cathode candidates due to their high capacity and cost-effectiveness, yet performance degradation related with unwanted structural evolution still remains a disturbing disadvantage. Herein, atomic resolution STEM (scanning transmission electron microscopy) images of Na-extracted Na_2/3Ni_xCo_1/3-xMn_2/3O₂ (x = 0, 1/6, 1/3) are collected and analyzed, to decipher the effect of cobalt and nickel substitution on the structural integrity of layered manganese-based oxides. Cobalt substitution is demonstrated to alleviate the lattice stress and retain the layered structure after sodium removal, and only a local P2-to-O2 phase transition could be identified. By contrast, various types of defects and phase transformation, including rarely reported P2-to-O3, are discovered in the Ni-substituted oxides. The generation of spinel and rock-salt phases is the critical evidence of cation mixing that leads to unrecoverable capacity loss. The interplay of different transition metals is complex, and compositional optimization is encouraged to minimize the effect of the concomitant phase transition.

Pinning Effect Enhanced Structural Stability toward a Zero-Strain Layered Cathode for Sodium-Ion Batteries

Layered oxides as the cathode materials of sodium-ion batteries are receiving extensive attention due to their high capacity and flexible composition. However, the layered cathode tends to be thermodynamically and electrochemically unstable during (de)sodiation. Herein, we propose the pinning effect and controllable pinning point in sodium storage layered cathodes to enhance the structural stability and achieve optimal electrochemical performance. 0 %, 2.5 % and 7.3 % transition-metal occupancies in Na-site as pinning points are obtained in Na_0.67 Mn_0.5 Co_0.5-x Fe_x O₂ . 2.5 % Na-site pinned by Fe³⁺ is beneficial to restrain the potential slab sliding and enhance the structural stability, resulting in an ultra-low volume variation of 0.6 % and maintaining the smooth two-dimensional channel for Na-ion transfer. The Na_0.67 Mn_0.5 Co_0.4 Fe_0.1 O₂ cathode with the optimal Fe³⁺ pinning delivers outstanding cycle performance of over 1000 cycles and superior rate capability up to 10 C.

P3/O3 Integrated Layered Oxide as High-Power and Long-Life Cathode toward Na-Ion Batteries

Low-cost and stable sodium-layered oxides (such as P2- and O3-phases) are suggested as highly promising cathode materials for Na-ion batteries (NIBs). Biphasic hybridization, mainly involving P2/O3 and P2/P3 biphases, is typically used to boost their electrochemical performances. Herein, a P3/O3 intergrown layered oxide (Na_2/3 Ni_1/3 Mn_1/3 Ti_1/3 O₂ ) as high-rate and long-life cathode for NIBs via tuning the amounts of Ti substitution in Na_2/3 Ni_1/3 Mn_{2/3- x} Ti_x O₂ (x = 0, 1/6, 1/3, 2/3) is demonstrated. The X-ray diffraction (XRD) Rietveld refinement and aberration-corrected scanning transmission electron microscopy show the co-existence of P3 and O3 phases, and density functional theory calculation corroborates the appearance of the anomalous O3 phase at the Ti substitution amount of 1/3. The P3/O3 biphasic cathode delivers an unexpected rate capability (≈88.7% of the initial capacity at a high rate of 5 C) and cycling stability (≈68.7% capacity retention after 2000 cycles at 1 C), superior to those of the sing phases P3-Na_2/3 Ni_1/3 Mn_2/3 O₂ , P3-Na_2/3 Ni_1/3 Mn_1/2 Ti_1/6 O₂ , and O3-Na_2/3 Ni_1/3 Ti_2/3 O₂ . The highly reversible structural evolution of the P3/O3 integrated cathode observed by ex situ XRD, ex situ X-ray absorption spectra, and the rapid Na⁺ diffusion kinetics, underpin the enhancement. These results show the important role of P3/O3 biphasic hybridization in designing and engineering layered oxide cathodes for NIBs.

Facilitating Phase Evolution for a High-Energy-Efficiency, Low-Cost O3-Type NaxCu0.18Fe0.3Mn0.52O2 Sodium Ion Battery Cathode

The phase transition and lattice parameter evolution of O3 structure commonly occurs in O3-type sodium ion battery (SIB) cathodes, which might enlarge the voltage hysteresis and lower the energy efficiency. Given that the cost is one of the issues discouraging the application of SIBs in large-scale energy storage, here we focus on Co/Ni-free Na_xCu_0.18Fe_0.3Mn_0.52O₂ (x = 0.8, 0.85, 0.9) and propose a convenient strategy to reduce the voltage hysteresis. It is found that when the Na content is 0.8, the highest energy efficiency of 95.4% after activation is achieved (2.5-4.0 V, 50 mA g^-1, the 50th cycle), in addition to a satisfactory capacity retention (about 0.056% decay per cycle). The further characterizations reveal that Na_0.8Cu_0.18Fe_0.3Mn_0.52O₂ owns a gentle O3-P3 phase transition process and does not undergo O3 phase lattice parameter evolution. The key point lies in the attainability of the O3/P3 composite of the material. This work will provide a simple strategy for the rational design of O3-type cathodes with a high energy efficiency and might offer inspiration to search for layered oxides with a higher O3/P3 critical Na content through element adjustments.

Deciphering the Origin of High Electrochemical Performance in a Novel Ti-Substituted P2/O3 Biphasic Cathode for Sodium-Ion Batteries

The layered Mn-based oxides (Na_xMnO₂), which is one of the most promising cathode families for rechargeable sodium-ion batteries, have received considerable attention because of their tunable electrochemical performances and low costs. Herein, a novel P2/O3 intergrown Li-containing Na_0.8Li_0.27Mn_0.68Ti_0.05O₂ cathode material prepared by Ti-substitution into Mn-site is reported. Benefiting from the synergistic effects of the biphasic composite structure and inactive d⁰ element substitution, this P2/O3 electrode exhibits high initial charge/discharge capacity and excellent cycling performance. The combination of different characterization techniques including solid-state NMR, electron paramagnetic resonance, X-ray adsorption spectroscopy, and high-resolution transmission electron microscopy gives insights into the local electronic environment, the redox chemistry, and also the microstructure rigidity of these cathode materials upon cycling. On the basis of comprehensive comparison with the Ti-free P2/O3-Na_0.8Li_0.27Mn_0.73O₂, the observed improvement on the electrochemical performance is primarily attributed to the mitigation of notorious Mn³⁺/Mn⁴⁺ redox and the enhanced stability of the oxygen charge compensation behavior. From the viewpoint of structure evolution, Ti-substitution restrains the Li⁺ loss and irreversible structural degradation during cycling. This study provides an in-depth understanding of the electronic and crystal structure evolutions after inactive d⁰ element substitution and may shed light on the rational design of high-performance P2/O3 biphasic Mn-based layered cathodes.