Cu2+ Dual-Doped Layer-Tunnel Hybrid Na0.6Mn1- xCu xO2 as a Cathode of Sodium-Ion Battery with Enhanced Structure Stability, Electrochemical Property, and Air Stability

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.

Sodium Stoichiometry Tuning of the Biphasic-Nax MnO2 Cathode for High-Performance Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are promising alternatives for large-scale energy storage owing to the rich resource and cost effectiveness. However, there are limitations of suitable low-cost, high-rate cathode materials for fast charging and high-power delivery in grid systems. Herein, a biphasic tunnel/layered 0.80Na0.44 MnO2 /0.20Na0.70 MnO2 (80T/20L) cathode delivering exceptional rate performance through subtly regulating the sodium and manganese stoichiometry is reported. It delivers a reversible capacity of 87 mAh g-1 at 4 A g-1 (33 C), much higher than that of tunnel Na0.44 MnO2 (72 mAh g-1 ) and layered Na0.70 MnO2 (36 mAh g-1 ). It proves that the one-pot synthesized 80T/20L is able to suppress the deactivation of L-Na0.70 MnO2 under air-exposure, which improves the specific capacity and cycling stability. Based on electrochemical kinetics analysis, the electrochemical storage of 80T/20L is mainly based on pseudocapacitive surface-controlled process. The thick film of 80T/20L cathode (a single-side mass loading over 10 mg cm-2 ) also has superior properties of pseudocapacitive response (over 83.5% at a low sweep rate of 1 mV s-1 ) and excellent rate performance. In this sense, the 80T/20L cathode with outstanding comprehensive performance could meet the requirements of high-performance SIBs.

Sustainable Enhanced Sodium-Ion Storage at Subzero Temperature with LiF Integration

Though layered sodium oxide materials are identified as promising cathodes in sodium-ion batteries, biphasic P3/O3 depicts improved electrochemical performance and structural stability. Herein, a coexistent P3/O3 biphasic cathode material was synthesized with "LiF" integration, verified with X-ray diffraction and Rietveld refinement analysis. Furthermore, the presence of Li and F was deduced by inductively coupled plasma-optical emission spectrometry (ICP-OES) and energy dispersive X-ray spectroscopy (EDS). The biphasic P3/O3 cathode displayed an excellent capacity retention of 85% after 100 cycles (0.2C/30 mA g^-1) at room temperature and 94% at -20 °C after 100 cycles (0.1C/15 mA g^-1) with superior rate capability as compared to the pristine cathode. Furthermore, a full cell comprising a hard carbon anode and a biphasic cathode with 1 M NaPF₆ electrolyte displayed excellent cyclic stabilities at a wider temperature range of -20 to 50 °C (with the energy density of 151.48 Wh kg^-1) due to the enhanced structural stability, alleviated Jahn-Teller distortions, and rapid Na⁺ kinetics facilitating Na⁺ motion at various temperatures in sodium-ion batteries. The detailed post-characterization studies revealed that the incorporation of LiF accounts for facile Na⁺ kinetics, boosting the overall Na storage.

Reaching the initial coulombic efficiency and structural stability limit of P2/O3 biphasic layered cathode for sodium-ion batteries

The P2/O3 biphasic layered oxide (Na_xMn_1-yM_yO₂, M: doping elements) is a cathode family with great promise for sodium-ion batteries (SIBs) because of their tunable electrochemical performance and low cost. However, the ultrahigh initial coulombic efficiency (ICE) and inferior cycling performance of P2/O3-Na_xMn_1-yM_yO₂ need to be improved for practical application. Herein, Ni/Cu co-doped P2/O3-Na_0.75Mn_1-yNi_y-zCu_zO₂ materials are well-designed. The ultrahigh ICE can be restrained by altering the ratio of P2/O3 via adjusting Ni content, and the structural stability can be improved by Cu doping via enlarging parameter c of O3 phase and suppressing irreversible P2-O2 phase transformation. The optimal P2/O3-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ delivers a capacity of 142.4 with ICE of 107.8%, superior capacity retention in the temperature range of -40 ∼ 30 °C, and rate performance of 95.9 mAh g^-1 at 1.2 A g^-1. The overall storage mechanism of P2/O3-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ is revealed by the combination of electrochemical profiles, in situ X-ray diffraction, and first-principles calculations. The Na-ion full battery based on P2/O3-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ cathode can achieve a remarkable energy density of 306.9 Wh kg^-1 with a power density of 695.5 W kg^-1 at 200 mA g^-1. This work may shed light on the rational design of high-performance P2/O3 biphasic layered cathode for SIBs.

Recent Advances on High-Capacity Sodium Manganese-Based Oxide Cathodes for Sodium-ion Batteries

Sodium manganese-based oxides (NMO) are attracting huge attention as safe and cost-effective cathode materials for sodium-ion batteries (SIBs). To date, one of the most important challenges of NMO-based cathodes is the relatively low capacity. Therefore, it is of great significance to develop high-capacity NMO-based cathodes. Great efforts have been made to enhance the reversible capacity of NMO-based cathodes, achieving considerable progress not only on electrochemical performance, but also the mechanism of massive sodium ion storage. In this paper, the structure and sodium storage mechanism for typical phases of NMO are reviewed, including P2, P3, O3, tunnel-type, and spinel-type NMO-based cathodes. Strategies for high-capacity NMO-based cathodes, such as cationic substitution, anion redox activation, etc are introduced in detail. Last but not least, the future opportunities and challenges for high-capacity NMO-based cathode are prospected.

Understanding the Aging Mechanism of Na-Based Layered Oxide Cathodes with Different Stacking Structures

Manganese-based layered oxides are one of the most promising cathodes for Na-ion batteries, but the prospect of their practical application is challenged by high sensitivity to ambient air. The stacking structure of materials is critical to the aging mechanism between layered oxides and air, but there remains a lack of systematic study. Herein, comprehensive research on model materials P-type Na0.50MnO2 and O-type Na0.85MnO2 reveals that the O-phase displays a much higher dynamic affinity toward moisture air compared to P-type compounds. For air-exposed O-type material, Na+ ions are extracted from the crystal lattice to form alkaline species at the surface in contact with air, accompanying by the increase of the valence state of transition metals. The series of undesired reactions result in an increase of interfacial resistance and huge capacity loss. Comparatively, the insertion of H2O into the Na layer is the main reaction during air-exposure of P-type material, and the inserted H2O can be extracted by high-temperature treatment. The H2O de/insertion process not only causes no performance degradation but also can enlarge the interlayer distance. With these understandings, we further propose a washing-resintering strategy to recover the performance of aged O-type materials and an aging strategy to build high-performance P-type materials.

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.

Copper Substitution in P2-Type Sodium Layered Oxide To Mitigate Phase Transition and Enhance Cyclability of Sodium-Ion Batteries

Development of high-performance cathode materials is one of the key challenges in the practical application of sodium-ion batteries. Among all the cathode materials, layered sodium transition-metal oxides are particularly attractive. However, undesired phase transitions are often reported and have detrimental effects on the structure stability and electrochemical performance. Cu substitution of zinc in the P2-type Na_0.6Mn_0.7Ni_0.15Zn_0.15-xCu_xO₂ (x = 0, 0.075, and 0.15) composites was investigated in this study for mitigating the biphase transition and enhancing the electrochemical performance of sodium-ion batteries. The coupling effect of Zn and Cu enables an excellent capacity retention of 96.4% of the initial discharge capacity after 150 cycles at 0.1 C in the Na/Na_0.6Mn_0.7Ni_0.15Zn_0.075Cu_0.075O₂ cell. The biphase transition that occurred in the high voltage range has been significantly suppressed after the incorporation of Cu in Na_0.6Mn_0.7Ni_0.15Zn_0.15O₂, which was confirmed by in situ X-ray diffraction studies. Moreover, the substitution of the inert element Zn with electrochemically active Cu leads to the suppression of anionic redox and the occurrence of Cu^2+/3+ redox reaction, and the electrolyte decomposition is impeded after the introduction of electrochemically active Cu in the Na_0.6Mn_0.7Ni_0.15Zn_0.15-xCu_xO₂ composite cathode. The enhanced electrochemical performance in the Na_0.6Mn_0.7Ni_0.15Zn_0.075Cu_0.075O₂ electrode can be ascribed to the coexistence of Zn and Cu and alleviated volumetric change as well as suppressed electrode/electrolyte side reaction after Cu substitution.

High-Voltage Stabilization of O3-Type Layered Oxide for Sodium-Ion Batteries by Simultaneous Tin Dual Modification

O3-type layered oxide materials are considered to be a highly suitable cathode for sodium-ion batteries (NIBs) due to their appreciable specific capacity and energy density. However, rapid capacity fading caused by serious structural changes and interfacial degradation hampers their use. A novel Sn-modified O3-type layered NaNi_1/3Fe_1/3Mn_1/3O₂ cathode is presented, with improved high-voltage stability through simultaneous bulk Sn doping and surface coating in a scalable one-step process. The bulk substitution of Sn⁴⁺ stabilizes the crystal structure by alleviating the irreversible phase transition and lattice structure degradation and increases the observed average voltage. In the meantime, the nanolayer Sn/Na/O composite on the surface effectively inhibits surface parasitic reactions and improves the interfacial stability during cycling. A series of Sn-modified materials are reported. An 8%-Sn-modified NaNi_1/3Fe_1/3Mn_1/3O₂ cathode exhibits a doubling in capacity retention increase after 150 cycles in the wide voltage range of 2.0-4.1 V vs Na/Na⁺ compared to none, and 81% capacity retention is observed after 200 cycles in a full cell vs hard carbon. This work offers a facile process to simultaneously stabilize the bulk structure and interface for the O3-type layered cathodes for sodium-ion batteries and raises the possibility of similar effective strategies to be employed for other energy storage materials.

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.

O3-NaFe(1/3-x)Ni1/3Mn1/3AlxO2 Cathodes with Improved Air Stability for Na-Ion Batteries

Na-ion batteries (NIBs) have been considered as potential candidates for large-scale energy storage, where O3-type Na-based layered oxide cathodes have attracted great attention due to their high capacity and low cost. However, O3-Na_xTMO₂ materials still suffer from insufficient air stability, which could lead to deteriorative electrochemical properties and thus hinder their practical application. In this work, a series of Al-doped O3-NaFe_(1/3-x)Ni_1/3Mn_1/3Al_xO₂ cathodes prepared by a co-precipitation method were investigated to enhance their electrochemical performance and air stability through stabilizing their structural and interface chemical properties. The Al-doped O3-NaFe_(1/3-0.01)Ni_1/3Mn_1/3Al_0.01O₂ (NFNMA_0.01) cathode delivers a comparable capacity of 138 mAh g^-1 and keeps a capacity retention of 85.88% after 50 cycles at 0.2 C, while the undoped O3-NaFe_1/3Ni_1/3Mn_1/3O₂ (NFNM) can only keep a capacity retention of 71.02%, although with an initial capacity of 141 mAh g^-1 at 0.2 C. For the air stability, the capacity decay rates are 58.87 and 5.07% for the undoped NFNM and Al-doped NFNMA_0.01 after the air exposure for 30 days, respectively. The greatly decaying electrochemical performance could be due to the formation of carbonates during air exposure, which can be efficiently suppressed by Al doping. The doped Al³⁺ has been confirmed to be inserted into the NFNM crystal lattice, inducing the reduced values of lattice parameters a and c due to the smaller ionic radius of Al³⁺ (53.5 pm) vs Fe³⁺ (55.0 pm). This study demonstrates that Al doping plays an important role in the air stability and cycling stability for layered cathode materials, which offers an efficient strategy to optimize the material design for their practical application in NIBs.

Flake (NH4)6Mo7O24/ Polydopamine as a High Performance Anode for Lithium Ion Batteries

Ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄) (AMT) is commonly used as the precursor to synthesize Mo-based oxides or sulfides for lithium ion batteries (LIBs). However, the electrochemical lithium storage ability of AMT itself is unclear so far. In the present work, AMT is directly examined as a promising anode material for Li-ion batteries with good capacity and cycling stability. To further improve the electrochemical performance of AMT, AMT/polydopamine (PDA) composite was simply synthesized via recrystallization and freeze drying methods. Unlike with block shape for AMT, the as-prepared AMT/PDA composite shows flake morphology. The initial discharge capacity of AMT/PDA is reached up to 1471 mAh g⁻¹. It delivers a reversible discharge capacity of 702 mAh g⁻¹ at a current density of 300 mA g⁻¹, and a stable reversible capacity of 383.6 mA h g⁻¹ is retained at a current density of 0.5 A g⁻¹ after 400 cycles. Moreover, the lithium storage mechanism is fully investigated. The results of this work could potentially expand the application of AMT and Mo-based anode for LIBs.

Exploring the Stability Effect of the Co-Substituted P2-Na0.67[Mn0.67Ni0.33]O2 Cathode for Liquid- and Solid-State Sodium-Ion Batteries

Pursuing high-performance cathode materials for sodium-ion batteries (SIBs) has great significance in the modern green energy world. The P2-type sodium-based layered oxide Na_0.67[Mn_0.67Ni_0.33]O₂ with high operating potential upon 4.3 V and high theoretical capacity has emerged as the most promising cathode. However, the material suffers from severe capacity decay during the electrochemical reaction process. Herein, the P2-Na_0.67[Mn_0.67Ni_0.21Li_0.06Zn_0.06]O₂ cathode is gained by moderately substituting lithium/zinc for the nickel sites. The inactive Li/Zn co-substitution is endowed with the ability to stabilize the crystal structure, resulting in enhanced electrochemical kinetics and remarkable long cyclic performance in liquid- and solid-state electrolytes. Thus, the Li/Zn co-substituted cathode presents a specific capacity of 154 mAh g^-1 at the first discharge process, excellent rate capability with 77 mAh g^-1 at a high current density of 5 C, and long cyclic stability in liquid-state batteries. Excitingly, it is also endowed with a high capacity retention of 85% after 500 cycles in solid-state batteries. Furthermore, ex situ XRD, TEM, and ex situ XPS are applied to reveal the structural evolution and charge compensation mechanism of P2-Na_0.67[Mn_0.67Ni_0.21Li_0.06Zn_0.06]O₂, allowing a deep insight into the great significance of structural stability.

Cu-Doped P2-Na0.7Mn0.9Cu0.1O2 Sodium-Ion Battery Cathode with Enhanced Electrochemical Performance: Insight from Water Sensitivity and Surface Mn(II) Formation Studies

Sodium-ion batteries (SIBs) show great application prospects in large-scale energy storage. P2-type manganese-based layered oxides have received special attention by virtue of their high theoretical capacity, low cost, and environmental friendliness. However, water sensitivity and limited cycling stability hinder their application, especially since the underlying mechanisms for the above two issues are still unclear. In this work, copper substitution is used to suppress the Jahn-Teller effect of Mn³⁺ and affect the corresponding lattice structure. The water sensitivity and charge compensation mechanism were carefully investigated. Results demonstrate that water sensitivity of the electrode is related to the order of Na⁺/vacancy in the Na interlayers since water molecules are more easily inserted into the charged state electrodes, but the tendency for the water uptake does not increase with Na⁺ extraction. Furthermore, Mn²⁺ forms on the surface of electrodes in the initial discharge process, and the redox reaction in the bulk is predominantly between Mn³⁺ and Mn⁴⁺. Cu-substituted in TM layer affects the arrangement of Na⁺/vacancy and suppresses the Mn²⁺ formation on the Na_0.7Mn_0.9Cu_0.1O₂ electrode that results in superior air stability and better storage properties.

The stability of P2-layered sodium transition metal oxides in ambient atmospheres

Air-stability is one of the most important considerations for the practical application of electrode materials in energy-harvesting/storage devices, ranging from solar cells to rechargeable batteries. The promising P2-layered sodium transition metal oxides (P2-Na_xTmO₂) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-Na_xTmO₂ have not yet been clearly elucidated. Herein, taking P2-Na_0.67MnO₂ and P2-Na_0.67Ni_0.33Mn_0.67O₂ as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-Na_xTmO₂ in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-Na_xTmO₂ is determined by the amount of extracted Na⁺, which is mainly compensated by Na⁺/H⁺ exchange. By expanding our study to a series of Mn-based oxides, we reveal that the air-stability of P2-Na_xTmO₂ is highly related to their oxidation features in the first charge process and further propose a practical evaluating rule associated with redox couples for air-stable Na_xTmO₂ cathodes.

Air-stability is a critical challenge faced by layered sodium transition metal oxide cathodes. Here, the authors depict a general and in-depth model of the structural/chemical evolution of P2-type layered oxides in air and propose an evaluation rule for the air-stability of layered sodium cathodes.

Low-Strain Reticular Sodium Manganese Oxide as an Ultrastable Cathode for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are recognized as attractive alternatives for grid-scale electrochemical energy storage applications. Transition metal oxide cathodes represent one of the most dynamic materials for industrialization among the various cathodes for SIBs. Here, a cation-doped cathode Na_0.44Mn_0.89Ti_0.11O₂ with a tunnel structure is introduced, which undergoes a lowered volume change of only 5.26% during the Na⁺ insertion/extraction process. Moreover, the average Na⁺ diffusion coefficients are enhanced by more than 3-fold upon the doping of the Ti cation. The obtained cathode delivers a practically usable capacity of 119 mAh g^-1 at 0.1 C as well as an enhanced discharge capacity of 96 mAh g^-1 at 5 C. Durability is demonstrated by the retained 71 mAh g^-1 after 1000 cycles, corresponding to a capacity retention of 74%. This work demonstrates that the reticular Na_0.44Mn_0.89Ti_0.11O₂ is a promising ultrastable cathode material for the development of long-life sodium-ion batteries.

Deciphering the Structure-Property Relationship of Na-Mn-Co-Mg-O as a Novel High-Capacity Layered-Tunnel Hybrid Cathode and Its Application in Sodium-Ion Capacitors

Developing novel cathode materials with a high energy density and long cycling stability is necessary for Na-ion batteries and Na-ion hybrid capacitors (NICs). Despite their high energy density, structural flexibility, and ease of synthesis, P-type Na layered oxides cannot be utilized in energy-storage applications owing to their severe capacity fading. In this regard, we report a novel composite layered-tunnel Na_0.5Mn_0.5Co_0.48Mg_0.02O₂ cathode whose binary structure was confirmed via scanning electron microscopy and high-resolution transmission electron microscopy. Combination of the two-dimensional (2D) layered oxides with the three-dimensional tunnel structure, as well as the presence of Mg²⁺ ions, resulted in a high capacity of 145 mAh g^-1 at a current density of 85 mA g^-1, along with a high stability and rate capability. An NIC was fabricated with composite layered-tunnel structure as a battery-type electrode and commercial activated carbon as a counter electrode. The NIC exhibited a maximum energy density of 35 Wh kg^-1 and good stability retaining 72% of its initial energy density after 3000 cycles. This integrated approach provides a new method for designing high-energy and high-power cathodes for NICs and NIBs.

A novel Mn-based P2/tunnel/O3′ tri-phase composite cathode with enhanced sodium storage properties

A novel P2/tunnel/O3′ tri-phase composite Na_0.7Bi_0.01MnO₂ is developed for the first time by the Na⁺-site modification of Bi³⁺ for a high-performance cathode in SIBs.

Mo-Modified P2-type Manganese Oxide Nanoplates with an Oriented Stacking Structure and Exposed {010} Active Facets as a Long-Life Sodium-Ion Battery Cathode

Layered manganese-based cathode materials are of great interest because of their high specific capacities for sodium-ion batteries. However, the Jahn-Teller effect and the inevitable phase transition are detrimental for achieving considerable cycling stability and rate capability. Herein, a P2-type manganese oxide nanoplate cathode material modified by Mo-substitution with an oriented stacking structure and exposed {010} active facets is reported. The manganese oxide nanoplate cathode yields remarkable capacity retention of 86% after 1200 cycles at 10 C (2000 mA g^-1). The specific power density is estimated to be as high as 530 W kg^-1 with a specific discharge capacity 143.9 mA h g^-1 at 1 C and 89.6% capacity retention up to 100 cycles. The superior electrochemical performances can be attributed to the efficient chemical modification and the unique structural features of the present manganese oxide nanoplate. Mo-modification can endow the manganese oxide cathode with enlarged lattice space and average oxidation state and thus favorable Na⁺ diffusion to inhibit the Jahn-Teller effect and improve the structure stability, thereby achieving an extremely long cycling life. A multilayer oriented stacking nanoplate structure with exposed {010} active facets is also beneficial for providing more surface active sites and shortening the Na⁺ diffusion path, leading to better rate capability.

Ion-Doping-Site-Variation-Induced Composite Cathode Adjustment: A Case Study of Layer-Tunnel Na0.6MnO2 with Mg2+ Doping at Na/Mn Site

Composite cathodes have attracted great attention due to the integrated advantages of each pure structure. Also, the component ratio deserves a careful modulation to further improve the corresponding electrochemical performance. Mn-based layer-tunnel hybrid composite became a focus in sodium-ion batteries due to the superiority in terms of high performance, low cost, and nontoxicity. In the previous reports, the structure modulation was carried out via changing the synthesis condition, varying the transition-metal-element ratio, and different ion doping. Also, it is still challenging to explore a more feasible method to simplify the adjustment process. Herein, we introduced Mg²⁺ into Na sites or transition-metal sites in Na_0.6MnO₂ and first discovered the doping-site-variation-induced structural adjustment phenomenon. Specifically, Mg doping in transition-metal sites could be beneficial for the growth of the P2-type structure, while layer/tunnel component ratio decreased when locating Mg²⁺ in Na sites. The P2-O2 phase transformations could be effectively suppressed by locating Mg²⁺ in both sites in high-voltage regions and thus improve the cycling performance. The designed material, Na_0.6Mn_0.99Mg_0.01O₂, could attain a decent capacity of 100 mA h g^-1 at 1000 mA g^-1 and a satisfied retention of 76.6% after 500 cycles. Additionally, ex situ X-ray diffraction analysis experiments verify the excellent structural stability of Mg-substituted samples during charge-discharge processes. Moreover, the Na_0.6Mn_0.99Mg_0.01O₂ also displays superior sodium-ion full-cell properties when merged with hard carbon anode. Thus, this research may indicate a proper novel thread for designing high-performance composite electrodes.

Suppression of Monoclinic Phase Transitions of O3-Type Cathodes Based on Electronic Delocalization for Na-Ion Batteries

As high capacity cathodes, O3-type Na-based oxides always suffer from a series of monoclinic transitions upon sodiation/desodiation, mainly caused by Na⁺/vacancy ordering and Jahn-Teller (J-T) distortion, leading to rapid structural degradation and serious performance fading. Herein, a simple modulation strategy is proposed to address this issue based on refrainment of electron localization in expectation to alleviate the charge ordering and change of electronic structure, which always lead to Na⁺/vacancy ordering and J-T distortion, respectively. According to density functional theory calculations, Fe³⁺ with slightly larger radius is introduced into NaNi_0.5Mn_0.5O₂ with the intention of enlarging transition metal layers and facilitating electronic delocalization. The obtained NaFe_0.3Ni_0.35Mn_0.35O₂ exhibits a reversible phase transition of O3_hex-P3_hex without any monoclinic transitions in striking contrast with the complicated phase transitions (O3_hex-O'3_mon-P3_hex-P'3_mon-P3'_hex) of NaNi_0.5Mn_0.5O₂, thus excellently improving the capacity retention with a high rate kinetic. In addition, the strategy is also effective to enhance the air stability, proved by direct observation of atomic-scale ABF-STEM for the first time.

Air-Stable Na x TMO2 Cathodes for Sodium Storage

Sodium-ion batteries are considered to be the most promising alternative to lithium-ion batteries for large-scale stationary energy storage applications due to the abundant sodium resource in the Earth' crust and as a result, relatively low cost. Sodium layered transition metal oxides (Na_xTMO₂) are proper Na-ion cathode materials because of low cost and high theoretical capacity. Currently most researchers focus on the improvement of electrochemical performance such as high rate capability and long cycling stability. However, for Na_xTMO₂, the structure stability against humid atmosphere is essentially important since most of them are instable in air, which is not favorable for practical application. Here we provide a comprehensive review of recent progresses on air-stable Na_xTMO₂ oxides. Several effective strategies are discussed, and further investigations on the air-stable cathodes are prospected.

A Hydrostable Cathode Material Based on the Layered P2@P3 Composite that Shows Redox Behavior for Copper in High-Rate and Long-Cycling Sodium-Ion Batteries

Low-cost layered oxides free of Ni and Co are considered to be the most promising cathode materials for future sodium-ion batteries. Biphasic Na_0.78 Cu_0.27 Zn_0.06 Mn_0.67 O₂ obtained via superficial atomic-scale P3 intergrowth with P2 phase induced by Zn doping, consisting of inexpensive transition metals, is a promising cathode for sodium-ion batteries. The P3 phase as a covering layer in this composite shows not only in excellent electrochemical performance but also its tolerance to moisture. The results indicate that partial Zn substitutes can effectively control biphase formation for improving the structural/electrochemical stability as well as the ionic diffusion coefficient. Based on in situ synchrotron X-ray diffraction coupled with electron-energy-loss spectroscopy, a possible Cu^2+/3+ redox reaction mechanism has now been revealed.