Rational design of intergrowth P2/O3 biphasic layered structure with reversible anionic redox chemistry and structural evolution for Na-ions batteries

Regulation of P2/O3 layered-oxide cathode by cation potential and dual-site doping provides excellent electrochemical performance

Enhancing the cycle stability of sodium-ion battery cathode materials at high current rates remains a critical challenge. Although layered oxides exhibit high capacity, their long-term stability requires improvement. In this study, we present a low-Ni, Co-free P2/O3-Na0.8K0.05Ca0.05Ni0.2Fe0.2Mn0.55Mg0.05O2 layered oxide, engineered through dual-site doping and cation potential to create a stable two-phase structure The synergistic effects of K-Ca-Mg co-doping and the P2/O3 hybrid structure effectively suppress detrimental phase transitions and Na+/vacancy ordering at high voltage, enhancing both rate capability and cycle stability. The material exhibits a high reversible discharge capacity of 143 mAh g-1 at 0.1C, and maintains over 80 % capacity retention after 250 cycles at 1C and excellent rate performance (96 mA h g-1 at 5C and 82 mA h g-1 at 10C. Even after 600 cycles at 10C, the capacity retention remains 80 %). Galvanostatic intermittent titration technique (GITT) analysis also confirms superior Na+ diffusivity compared to conventional Ni-Fe-Mn layered oxides and density functional theory (DFT) calculations further validate the feasibility of the dual-position doping strategy, demonstrating its effectiveness in enhancing electrochemical performance through the synergistic effect of K-Ca-Mg. In conclusion, these findings highlight the potential of P/O-KCNFMM as a high-performance cathode material, leveraging the combined advantages of dual-site doping and the P2/O3 hybrid structure, thus providing new insights into the design of sodium-ion battery cathodes.

Progress of anion redox in Na-rich layered transition metal oxides (Na2MO3) as cathode materials for sodium-ion batteries

Under the background of surging global demand for batteries and scarcity of Li resources, sodium-ion batteries (SIBs) are attracting attention as a potential alternative with their unique advantages, and the layered transition metal (TM) oxides therein are considered to be one of the most promising cathode materials. In this paper, firstly, the diversity of cathode materials for SIBs is systematically introduced, as well as the layered oxide structures among them are categorized, and then it focuses on the O3-type sodium-rich Na2MO3, which is promising for large-scale commercial applications, illustrating the development and mechanism of anion redox. Excess Na transforms the TM layer into the mixed Na1/3M2/3O2layer, leading to the formation of localized configuration Na-O-Na. Thereby, isolated nonhybridized O 2p states are introduced, which participate in the charge compensation process (O2-/On-) under high-voltage conditions and provide the battery with additional capacity beyond the cation redox reaction. Therefore, the Na2MO3formed by its TM element located in different periods are classified, discussed and summarized in terms of structural change characteristics, electrochemical properties and anion-redox mechanism. However, this particular redox mechanism is also accompanied by the challenges such as voltage hysteresis, irreversible oxygen loss, TM migration, capacity decay and poor air stability. Therefore, to address these challenges, various improvement strategies have been proposed, including doping of large radius metal ions, light metal ions, TM ions with high covalency with O, nonmetal ions, formation of mixed phases, and surface modification. This work is expected to provide new ways to find and design novel high-capacity Na-rich layered oxide cathode materials.

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

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

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.

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

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

Architecting O3/P2 layered oxides by gradient Mn doping for sodium-ion batteries

O3 phase layered oxides are highly attractive cathode materials for sodium-ion batteries because of their high capacity and decent initial Coulombic efficiency. However, their rate capability and long cycling life are unsatisfactory due to the narrow Na+ transfer channel and irreversible phase transitions of O3 phase during sodiation/desodiation process. Constructing O3/P2 multiphase structures has been proven to be an effective strategy to overcome these challenges. In this study, we synthesized bi-phasic structured O3/P2 Na(Ni2/9Fe1/3Cu1/9Mn1/3)1-xMnxO2 (x = 0.01, 0.02, 0.03, 0.04, 0.05) materials through Mn doping during sodiation process. Benefiting from surface P2 phase layer with the enhanced Na+ transfer dynamics and high structural stability, the Na(Ni2/9Fe1/3Cu1/9Mn1/3)0.98Mn0.02O2 (NFCM-M2) cathode delivers a reversible capacity of 139.1 mA h g-1 at 0.1 C, and retains 71.4 % of its original capacity after 300 cycles at 1 C. Our work provides useful guidance for designing multiphase cathodes and offers new insights into the structure-performance correlation for sodium-ion cathode materials.

Manipulating Local Chemistry and Coherent Structures for High-Rate and Long-Life Sodium-Ion Battery Cathodes

Layered sodium transition-metal (TM) oxides generally suffer from severe capacity decay and poor rate performance during cycling, especially at a high state of charge (SoC). Herein, an insight into failure mechanisms within high-voltage layered cathodes is unveiled, while a two-in-one tactic of charge localization and coherent structures is devised to improve structural integrity and Na+ transport kinetics, elucidated by density functional theory calculations. Elevated Jahn-Teller [Mn3+O6] concentration on the particle surface during sodiation, coupled with intense interlayer repulsion and adverse oxygen instability, leads to irreversible damage to the near-surface structure, as demonstrated by X-ray absorption spectroscopy and in situ characterization techniques. It is further validated that the structural skeleton is substantially strengthened through the electronic structure modulation surrounding oxygen. Furthermore, optimized Na+ diffusion is effectively attainable via regulating intergrown structures, successfully achieved by the Zn2+ inducer. Greatly, good redox reversibility with an initial Coulombic efficiency of 92.6%, impressive rate capability (86.5 mAh g-1 with 70.4% retention at 10C), and enhanced cycling stability (71.6% retention after 300 cycles at 5C) are exhibited in the P2/O3 biphasic cathode. It is believed that a profound comprehension of layered oxides will herald fresh perspectives to develop high-voltage cathode materials for sodium-ion batteries.

Rational Regulation of High-Voltage Stability in Potassium Layered Oxide Cathodes

Layered oxide cathode materials may undergo irreversible oxygen loss and severe phase transitions during high voltage cycling and may be susceptible to transition metal dissolution, adversely affecting their electrochemical performance. Here, to address these challenges, we propose synergistic doping of nonmetallic elements and in situ electrochemical diffusion as potential solution strategies. Among them, the distribution of the nonmetallic element fluorine within the material can be regulated by doping boron, thereby suppressing manganese dissolution through surface enrichment of fluorine. Furthermore, in situ electrochemical diffusion of fluorine from the surface into the bulk of the materials after charging reduces the energy barrier of potassium ion diffusion while effectively inhibiting irreversible oxygen loss under high voltage. The modified K0.5Mn0.83Mg0.1Ti0.05B0.02F0.1O1.9 layered oxide cathode exhibits a high capacity of 147 mAh g-1 at 50 mA g-1 and a long cycle life of 2200 cycles at 500 mA g-1. This work demonstrates the efficacy of synergistic doping and in situ electrochemical diffusion of nonmetallic elements and provides valuable insights for optimizing rechargeable battery materials.

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.

The innovative design of high-performance layered transition metal oxides for sodium-ion batteries from a commercial perspective

Sodium-ion batteries (SIBs), which have ample reserves and low production costs, are receiving more and more attention. As promising cathode candidates, layered transition metal oxides (LTMOs) have attracted intensive interest for their nontoxicity, high theoretical capacities, ease of manufacture, suitable voltage, abundant resources, and potential low cost. However, the commercial implementation of LTMOs is still hampered by their low rate capability, low energy density, insufficient cycling stability, and air instability. Therefore, this review comprehensively summarizes the research progress and modification strategies for LTMOs to enhance the stability of SIBs from microscopic heterostructure regulation to macroscale interface engineering modification. With the aim of realizing commercial applications of SIBs, more attention and research for improving the coulombic efficiency of LTMOS and close communication between academic and industrial organizations are also needed. It is expected that we will be able to provide unique perspectives for the design of powerful LTMOs in SIBs and guide the development of commercial application.

Aluminum ion chemistry of Na4Fe3(PO4)2(P2O7) for all-climate full Na-ion battery

Na4Fe3(PO4)2(P2O7) (NFPP) is currently drawing increased attention as a sodium-ion batteries (SIBs) cathode due to the cost-effective and NASICON-type structure features. Owing to the sluggish electron and Na+ conductivities, however, its real implementation is impeded by the grievous capacity decay and inferior rate capability. Herein, multivalent cation substituted microporous Na3.9Fe2.9Al0.1(PO4)2(P2O7) (NFAPP) with wide operation-temperature is elaborately designed through regulating structure/interface coupled electron/ion transport. Greatly, the derived Na vacancy and charge rearrangement induced by trivalent Al3+ substitution lower the ions diffusion barriers, thereby endowing faster electron transport and Na+ mobility. More importantly, the existing Al-O-P bonds strengthen the local environment and alleviate the volume vibration during (de)sodiation, enabling highly reversible valence variation and structural evolution. As a result, remarkable cyclability (over 10,000 loops), ultrafast rate capability (200 C), and exceptional all-climate stability (-40-60 °C) in half/full cells are demonstrated. Given this, the rational work might provide an actionable strategy to promote the electrochemical property of NFPP, thus unveiling the great application prospect of sodium iron mixed phosphate materials.

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-performance heterostructure Na0.7MnO2.05-Na0.91MnO2 as a lithium-free cathode for lithium-ion batteries

In this investigation, a lithium-free cathode material, Na0.7MnO2.05-Na0.91MnO2, was synthesized by the solid phase method. The intercalation mechanism and partial phase transformation mechanism of NMO600 were clarified by in situ X-ray diffraction and impedance. The design of the heterostructure is favourable for improving the lithium ion storage of NMO600, which can deliver a discharge capacity of 83.12 mA h g-1 at 1 A g-1 and keep 61.71 mA h g-1 after 700 cycles.

Structural and electrochemical progress of O3-type layered oxide cathodes for Na-ion batteries

Sodium-ion batteries (SIBs) have attracted great attention being the most promising sustainable energy technology owing to their competitive energy density, great safety and considerable low-cost merits. Nevertheless, the commercialization process of SIBs is still sluggish because of the difficulty in developing high-performance battery materials, especially the cathode materials. The discovery of layered transition metal oxides as the cathode materials of SIBs brings infinite possibilities for practical battery production. Thereinto, the O3-type layered transition metal oxides exhibit attractive advantages in terms of energy density benefiting from their higher sodium content compared to other kinds of layered transition metal oxides. Enormous research studies have largely put forward their progress and explored a wide range of performance improvement approaches from the morphology, coating, doping, phase structure and redox aspects. However, the progress is scattered and has not logically evolved, which is not beneficial for the further development of more advanced cathode materials. Therefore, our work aims to comprehensively review, classify and highlight the most recent advances in O3-type layered transition metal oxides for SIBs, so as to scientifically cognize their progress and remaining challenges and provide reasonable improvement ideas and routes for next-generation high-performance cathode materials.

Research development on electrolytes for magnesium-ion batteries

Magnesium-ion batteries (MIBs) are considered strong candidates for next-generation energy-storage systems owing to their high theoretical capacity, divalent nature and the natural abundancy of magnesium (Mg) resources on Earth. However, the development of MIBs has been mainly limited by the incompatibility of Mg anodes with several Mg salts and conventional organic-liquid electrolytes. Therefore, one major challenge faced by MIBs technology lies on developing safe electrolytes, which demonstrate appropriate electrochemical voltage window and compatibility with Mg anode. This review discusses the development of MIBs from the point-of-view of the electrolyte syntheses. A systematic assessment of promising electrolyte design strategies is proposed including liquid and solid-state electrolytes. Liquid-based electrolytes have been largely explored and can be categorized by solvent-type: organic solvent, aqueous solvent, and ionic-liquids. Organic-liquid electrolytes usually present high electrochemical and chemical stability but are rather dangerous, while aqueous electrolytes present high ionic conductivity and eco-friendliness but narrow electrochemical stability window. Some ionic-liquid electrolytes have proved outstanding performance but are fairly expensive. As alternative to liquid electrolytes, solid-state electrolytes are increasingly attractive to increase energy density and safety. However, improving the ionic conductivity of Mg ions in these types of electrolytes is extremely challenging. We believe that this comprehensive review will enable researchers to rapidly grasp the problems faced by electrolytes for MIBs and the electrolyte design strategies proposed to this date.