Progress in High‐Voltage Cathode Materials for Rechargeable Sodium‐Ion Batteries

Room‐temperature rechargeable sodium‐ion batteries are considered as a promising alternative technology for grid and other storage applications due to their competitive cost benefit and sustainable resource supply, triumphing other battery systems on the market. To facilitate the practical realization of the sodium‐ion technology, the energy density of sodium‐ion batteries needs to be boosted to the level of current commercial Li‐ion batteries. An effective approach would be to elevate the operating voltage of the battery, which requires the use of electrochemically stable cathode materials with high voltage versus Na⁺/Na. This review summarizes the recent progress with the emerging high‐voltage cathode materials for room‐temperature sodium‐ion batteries, which include layered transitional‐metal oxides, Na‐rich materials, and polyanion compounds. The key challenges and corresponding strategies for these materials are also discussed, with an emphasis placed on the intrinsic structural properties, Na storage electrochemistry, and the voltage variation tendency with respect to the redox reactions. The insights presented in this article can serve as a guide for improving the energy densities of room‐temperature Na‐ion batteries.     

High Performance Na0.5[Ni0.23Fe0.13Mn0.63]O2 Cathode for Sodium‐Ion Batteries

The synthesis of a new layered cathode material, Na_0.5[Ni_0.23Fe_0.13Mn_0.63]O₂, and its characterization in terms of crystalline structure and electrochemical performance in a sodium cell is reported. X‐ray diffraction studies and high resolution scanning electron microscopy images reveal a well‐defined P2‐type layered structure, while the electrochemical tests demonstrate excellent characteristics in terms of high capacity and cycle life. This performance, the low cost, and the environmental compatibility of its component poses Na_0.5[Ni_0.23Fe_0.13Mn_0.63]O₂ to be among the most promising materials for the next generation of sodium‐ion batteries.     

High‐Abundance and Low‐Cost Metal‐Based Cathode Materials for Sodium‐Ion Batteries: Problems,Progress, and Key Technologies

Recently, room‐temperature stationary sodium‐ion batteries (SIBs) have received extensive investigations for large‐scale energy storage systems (EESs) and smart grids due to the huge natural abundance and low cost of sodium. The SIBs share a similar “rocking‐chair” sodium storage mechanism with lithium‐ion batteries; thus, selecting appropriate electrodes with a low cost, satisfactory electrochemical performance, and high reliability is the key point for the development for SIBs. On the other hand, the carefully chosen elements in the electrodes also largely determine the cost of SIBs. Therefore, earth‐abundant‐metal‐based compounds are ideal candidates for reducing the cost of electrodes. Among all the high‐abundance and low‐cost metal elements, cathodes containing iron and/or manganese are the most representative ones that have attracted numerous studies up till now. Herein, recent advances on both iron‐ and manganese‐based cathodes of various types, such as polyanionic, layered oxide, MXene, and spinel, are highlighted. The structure–function property for the iron‐ and manganese‐based compounds is summarized and analyzed in detail. With the participation of iron and manganese in sodium‐based cathode materials, real applications of room‐temperature SIBs in large‐scale EESs will be greatly promoted and accelerated in the near future.     

Exploration of Advanced Electrode Materials for Rechargeable Sodium‐Ion Batteries

As the rapid growth of the lithium‐ion battery (LIB) market raises concerns about limited lithium resources, rechargeable sodium‐ion batteries (SIBs) are attracting growing attention in the field of electrical energy storage due to the large abundance of sodium. Compared with the well‐developed commercial LIBs, all components of the SIB system, such as the electrode, electrolyte, binder, and separator, need further exploration before reaching a practical industrial application level. Drawing lessons from the LIB research, the SIB electrode materials are being extensively investigated, resulting in tremendous progress in recent years. In this article, the progress of the research on the development of electrode materials for SIBs is summarized. A variety of new electrode materials for SIBs, including transition‐metal oxides with a layered or tunnel structure, polyanionic compounds, and organic molecules, have been proposed and systematically investigated. Several promising materials with moderate energy density and ultra‐long cycling performance are demonstrated. Appropriate doping and/or surface treatment methodologies are developed to effectively promote the electrochemical properties. The challenges of and opportunities for exploiting satisfactory SIB electrode materials for practical applications are outlined.     

A New P2‐Type Layered Oxide Cathode with Extremely High Energy Density for Sodium‐Ion Batteries

Herein, a new P2‐type layered oxide is proposed as an outstanding intercalation cathode material for high energy density sodium‐ion batteries (SIBs). On the basis of the stoichiometry of sodium and transition metals, the P2‐type Na_0.55[Ni_0.1Fe_0.1Mn_0.8]O₂ cathode is synthesized without impurities phase by partially substituting Ni and Fe into the Mn sites. The partial substitution results in a smoothing of the electrochemical charge/discharge profiles and thus greatly improves the battery performance. The P2‐type Na_0.55[Ni_0.1Fe_0.1Mn_0.8]O₂ cathode delivers an extremely high discharge capacity of 221.5 mAh g^?1 with a high average potential of ≈2.9 V (vs Na/Na⁺) for SIBs. In addition, the fast Na‐ion transport in the P2‐type Na_0.55[Ni_0.1Fe_0.1Mn_0.8]O₂ cathode structure enables good power capability with an extremely high current density of 2400 mA g^?1 (full charge/discharge in 12 min) and long‐term cycling stability with ≈80% capacity retention after 500 cycles at 600 mA g^?1. A combination of electrochemical profiles, in operando synchrotron X‐ray diffraction analysis, and first‐principles calculations are used to understand the overall Na storage mechanism of P2‐type Na_0.55[Ni_0.1Fe_0.1Mn_0.8]O₂.     

Controlled Oxygen Redox for Excellent Power Capability in Layered Sodium‐Based Compounds

A high‐rate of oxygen redox assisted by cobalt in layered sodium‐based compounds is achieved. The rationally designed Na_0.6[Mg_0.2Mn_0.6Co_0.2]O₂ exhibits outstanding electrode performance, delivering a discharge capacity of 214 mAh g^?1 (26 mA g^?1) with capacity retention of 87% after 100 cycles. High rate performance is also achieved at 7C (1.82 A g^?1) with a capacity of 107 mAh g^?1. Surprisingly, the Na_0.6[Mg_0.2Mn_0.6Co_0.2]O₂ compound is able to deliver capacity for 1000 cycles at 5C (at 1.3 A g^?1), retaining 72% of its initial capacity of 108 mAh g^?1. X‐ray absorption spectroscopy analysis of the O K‐edge indicates the oxygen‐redox species (O^2?/1?) is active during cycling. First‐principles calculations show that the addition of Co reduces the bandgap energy from ≈2.65 to ≈0.61 eV and that overlapping of the Co 3d and O 2p orbitals facilitates facile electron transfer, enabling the long‐term reversibility of the oxygen redox, even at high rates. To the best of the authors' knowledge, this is the first report on high‐rate oxygen redox in sodium‐based cathode materials, and it is believed that the findings will open a new pathway for the use of oxygen‐redox‐based materials for sodium‐ion batteries.     

Nanoarchitectured Array Electrodes for Rechargeable Lithium‐ and Sodium‐Ion Batteries

Rechargeable ion batteries have contributed immensely to shaping the modern world and been seriously considered for the efficient storage and utilization of intermittent renewable energies. To fulfill their potential in the future market, superior battery performance of high capacity, great rate capability, and long lifespan is undoubtedly required. In the past decade, along with discovering new electrode materials, the focus has been shifting more and more toward rational electrode designs because the performance is intimately connected to the electrode architectures, particularly their designs at the nanoscale that can alleviate the reliance on the materials' intrinsic nature. The utilization of nanoarchitectured arrays in the design of electrodes has been proven to significantly improve the battery performance. A comprehensive summary of the structural features and fabrications of the nanoarchitectured array electrodes is provided, and some of the latest achievements in the area of both lithium‐ and sodium‐ion batteries are highlighted. Finally, future challenges and opportunities that would allow further development of such advanced electrode configuration are discussed.     

A Layered–Tunnel Intergrowth Structure for High‐Performance Sodium‐Ion Oxide Cathode

Delivery of high‐energy density with long cycle life is facing a severe challenge in developing cathode materials for rechargeable sodium‐ion batteries (SIBs). Here a composite Na_0.6MnO₂ with layered–tunnel structure combining intergrowth morphology of nanoplates and nanorods for SIBs, which is clearly confirmed by micro scanning electron microscopy, high‐resolution transmission electron microscopy as well as scanning transmission electron microscopy with atomic resolution is presented. Owing to the integrated advantages of P2 layered structure with high capacity and that of the tunnel structure with excellent cycling stability and superior rate performance, the composite electrode delivers a reversible discharge capacity of 198.2 mAh g^?1 at 0.2C rate, leading to a high‐energy density of 520.4 Wh kg^?1. This intergrowth integration engineering strategy may modulate the physical and chemical properties in oxide cathodes and provide new perspectives on the optimal design of high‐energy density and high‐stable materials for SIBs.     

Anodes and Sodium‐Free Cathodes in Sodium Ion Batteries

The increase in electricity generation poses growing demands on energy storage systems, thus offering a chance for the success of the reliable and cost‐effective energy storage technologies. Sodium ion batteries are emerging as such a technology, which is however not yet mature enough to enter the market. At the crux of building practical sodium ion batteries is the development of electrode materials that promise sufficient cost‐ and performance‐competitiveness. As such, herein, all typical sodium storage materials are discussed, considering their fabrication methods and sodiation mechanisms in detail. A comprehensive cross‐literature and cross‐material comparison, which also includes the related thermodynamic analysis of their sodiation products, is also provided. The review focusses particularly on anodes and sodium‐free cathodes, as they both play the role of the acceptor rather than the donor of sodium ions in their operation in batteries; their difference lies in the (de‐)sodiation voltage. In the discussion, special attention is paid to contradictory observations and interpretations in contemporary sodium ion battery research, since debates on these controversies are likely to fuel future sodium battery research.     

Advanced Organic Electrode Materials for Rechargeable Sodium‐Ion Batteries

Benefiting from the high abundance and low cost of sodium resource, rechargeable sodium‐ion batteries (SIBs) are regarded as promising candidates for large‐scale electrochemical energy storage and conversion. Due to the heavier mass and larger radius of Na⁺ than that of Li⁺, SIBs with inorganic electrode materials are currently plagued with low capacity and insufficient cycling life. In comparison, organic electrode materials display the advantages of structure designability, high capacity and low limitation of cationic radius. However, organic electrode materials also encounter issues such as high‐solubility in electrolyte and low conductivity. Here, recently reported organic electrode materials, which mainly include the reactions based on either carbon‐oxygen double bond or carbon‐nitrogen double bond, and doping reactions, are systematically reviewed. Furthermore, the design strategies of organic electrodes are comprehensively summarized. The working voltage is regulated through controlling the lowest unoccupied molecular orbital energies. The theoretical capacity can be enhanced by increasing the active groups. The dissolution is inhibited with elevating the intermolecular forces with proper molecular weight. The conductivity can be improved with extending conjugated structures. Future research into organic electrodes should focus on the development of full SIBs with aqueous/aprotic electrolytes and long cycling stability.     

Bioinspired Surface Layer for the Cathode Material of High‐Energy‐Density Sodium‐Ion Batteries

Cathode materials are usually active in the range of 2–4.3 V, but the decomposition of the electrolytic salt above 4 V versus Na⁺/Na is common. Arguably, the greatest concern is the formation of HF after the reaction of the salts with water molecules, which are present as an impurity in the electrolyte. This HF ceaselessly attacks the active materials and gradually causes the failure of the electrode via electric isolation of the active materials. In this study, a bioinspired β‐NaCaPO₄ nanolayer is reported on a P2‐type layered Na_2/3[Ni_1/3Mn_2/3]O₂ cathode material. The coating layers successfully scavenge HF and H₂O, and excellent capacity retention is achieved with the β‐NaCaPO₄‐coated Na_2/3[Ni_1/3Mn_2/3]O₂ electrode. This retention is possible because a less acidic environment is produced in the Na cells during prolonged cycling. The intrinsic stability of the coating layer also assists in delaying the exothermic decomposition reaction of the desodiated electrodes. Formation and reaction mechanisms are suggested for the coating layers responsible for the excellent electrode performance. The suggested technology is promising for use with cathode materials in rechargeable sodium batteries to mitigate the effects of acidic conditions in Na cells.     

A New Strategy for High‐Voltage Cathodes for K‐Ion Batteries: Stoichiometric KVPO4F

The exploration of high‐energy‐density cathode materials is vital to the practical use of K‐ion batteries. Layered K‐metal oxides have too high a voltage slope due to their large K⁺–K⁺ interaction, resulting in low specific capacity and average voltage. In contrast, the 3D arrangement of K⁺, with polyanions separating them, reduces the strength of the effective K⁺‐K⁺ repulsion, which in turn increases specific capacity and voltage. Here, stoichiometric KVPO₄F for use as a high‐energy‐density K‐ion cathode is developed. The KVPO₄F cathode delivers a reversible capacity of ≈105 mAh g^?1 with an average voltage of ≈4.3 V (vs K/K⁺), resulting in a gravimetric energy density of ≈450 Wh kg^?1. During electrochemical cycling, the K_xVPO₄F cathode goes through various intermediate phases at x = 0.75, 0.625, and 0.5 upon K extraction and reinsertion, as determined by ex situ X‐ray diffraction characterization and ab initio calculations. This work further explains the role of oxygen substitution in KVPO_4+xF_1?x: the oxygenation of KVPO₄F leads to an anion‐disordered structure which prevents the formation of K⁺/vacancy orderings without electrochemical plateaus and hence to a smoother voltage profile.     

Soft Carbon as Anode for High‐Performance Sodium‐Based Dual Ion Full Battery

Sodium‐based dual ion full batteries (NDIBs) are reported with soft carbon as anode and graphite as cathode for the first time. The NDIBs operate at high discharge voltage plateau of 3.58 V, with superior discharge capacity of 103 mA h g^?1, excellent rate performance, and long‐term cycling stability over 800 cycles with capacity retention of 81.8%. The mechanism of Na⁺ and PF₆^? insertion/desertion during the charging/discharging processes is proposed and discussed in detail, with the support of various spectroscopies.     

A Stable Layered Oxide Cathode Material for High‐Performance Sodium‐Ion Battery

As one of the most promising cathode candidates for room‐temperature sodium‐ion batteries (SIBs), P2‐type layered oxides face the challenge of simultaneously realizing high‐rate performance while achieving long cycle life. Here, a stable Na_2/3Ni_1/6Mn_2/3Cu_1/9Mg_1/18O₂ cathode material is proposed that consists of multiple‐layer oriented stacking nanoflakes, in which the nickel sites are partially substituted by copper and magnesium, a characteristic of the material that is confirmed by multiscale scanning transmission electron microscopy and electron energy loss spectroscopy techniques. Owing to the optimal morphology structure modulation and chemical element substitution strategy, the electrode displays remarkable rate performance (73% capacity retention at 30C compared to 0.5C) and outstanding cycling stability in Na half‐cell system couple with unprecedented full battery performance. The underlying thermal stability, phase stability, and Na⁺ storage mechanisms are clearly elucidated through the systematical characterizations of electrochemical behaviors, in situ X‐ray diffraction at different temperatures, and operando X‐ray diffraction upon Na⁺ deintercalation/intercalation. Surprisingly, a quasi‐solid‐solution reaction is switched to an absolute solid‐solution reaction and a capacitive Na⁺ storage mechanism is demonstrated via quantitative electrochemical kinetics calculation during charge/discharge process. Such a simple and effective strategy might reveal a new avenue into the rational design of excellent rate capability and long cycle stability cathode materials for practical SIBs.     

Stabilizing Reversible Oxygen Redox Chemistry in Layered Oxides for Sodium‐Ion Batteries

Triggering oxygen‐related activity is demonstrated as a promising strategy to effectively boost energy density of layered cathodes for sodium‐ion batteries. However, irreversible lattice oxygen loss will induce detrimental structure distortion, resulting in voltage decay and cycle degradation. Herein, a layered structure P2‐type Na_0.66Li_0.22Ru_0.78O₂ cathode is designed, delivering reversible oxygen‐related and Ru‐based redox chemistry simultaneously. Benefiting from the combination of strong Ru 4d‐O 2p covalency and stable Li location within the transition metal layer, reversible anionic/cationic redox chemistry is achieved successfully, which is proved by systematic bulk/surface analysis by in/ex situ spectroscopy (operando Raman and hard X‐ray absorption spectroscopy, etc.). Moreover, the robust structure and reversible phase transition evolution revealed by operando X‐ray diffraction further establish a high degree reversible (de)intercalation processes (≈150 mAh g^?1, reversible capacity) and long‐term cycling (average capacity drop of 0.018%, 500 cycles).