Recent Progress of Layered Transition Metal Oxide Cathodes for Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) are attracting increasing attention and considered to be a low‐cost complement or an alternative to lithium‐ion batteries (LIBs), especially for large‐scale energy storage. Their application, however, is limited because of the lack of suitable host materials to reversibly intercalate Na⁺ ions. Layered transition metal oxides (Na_xMO₂, M = Fe, Mn, Ni, Co, Cr, Ti, V, and their combinations) appear to be promising cathode candidates for SIBs due to their simple structure, ease of synthesis, high operating potential, and feasibility for commercial production. In the present work, the structural evolution, electrochemical performance, and recent progress of Na_xMO₂ as cathode materials for SIBs are reviewed and summarized. Moreover, the existing drawbacks are discussed and several strategies are proposed to help alleviate these issues. In addition, the exploration of full cells based on Na_xMO₂ cathodes and future perspectives are discussed to provide guidance for the future commercialization of such systems.     

High‐Energy/Power and Low‐Temperature Cathode for Sodium‐Ion Batteries: In Situ XRD Study and Superior Full‐Cell Performance

Sodium‐ion batteries (SIBs) are still confronted with several major challenges, including low energy and power densities, short‐term cycle life, and poor low‐temperature performance, which severely hinder their practical applications. Here, a high‐voltage cathode composed of Na₃V₂(PO₄)₂O₂F nano‐tetraprisms (NVPF‐NTP) is proposed to enhance the energy density of SIBs. The prepared NVPF‐NTP exhibits two high working plateaux at about 4.01 and 3.60 V versus the Na⁺/Na with a specific capacity of 127.8 mA h g^?1. The energy density of NVPF‐NTP reaches up to 486 W h kg^?1, which is higher than the majority of other cathode materials previously reported for SIBs. Moreover, due to the low strain (≈2.56% volumetric variation) and superior Na transport kinetics in Na intercalation/extraction processes, as demonstrated by in situ X‐ray diffraction, galvanostatic intermittent titration technique, and cyclic voltammetry at varied scan rates, the NVPF‐NTP shows long‐term cycle life, superior low‐temperature performance, and outstanding high‐rate capabilities. The comparison of Ragone plots further discloses that NVPF‐NTP presents the best power performance among the state‐of‐the‐art cathode materials for SIBs. More importantly, when coupled with an Sb‐based anode, the fabricated sodium‐ion full‐cells also exhibit excellent rate and cycling performances, thus providing a preview of their practical application.     

Ti‐Substituted NaNi0.5Mn0.5‐xTixO2 Cathodes with Reversible O3−P3 Phase Transition for High‐Performance Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) have been considered as potential candidates for stationary energy storage because of the low cost and wide availability of Na sources. O3‐type layered oxides have been considered as one of the most promising cathodes for SIBs. However, they commonly show inevitable complicated phase transitions and sluggish kinetics, incurring rapid capacity decline and poor rate capability. Here, a series of sodium‐sufficient O3‐type NaNi_0.5Mn_{0.5‐ x} Ti _x O₂ (0 ≤ x ≤ 0.5) cathodes for SIBs is reported and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective end‐members. The combined analysis of in situ X‐ray diffraction, ex situ X‐ray absorption spectroscopy, and scanning transmission electron microscopy for NaNi_0.5Mn_0.2Ti_0.3O₂ reveals that the O3‐type phase transforms reversibly into a P3‐type phase upon Na+ deintercalation/intercalation. The substitution of Ti for Mn enlarges interslab distance and could restrain the unfavorable and irreversible multiphase transformation in the high voltage regions that is usually observed in O3‐type NaNi_0.5Mn_0.5O₂, resulting in improved Na cell performance. This integration of macroscale and atomicscale engineering strategy might open up the modulation of the chemical and physical properties in layered oxides and grasp new insight into the optimal design of high‐performance cathode materials for SIBs.     

Regulating Surface and Grain‐Boundary Structures of Ni‐Rich Layered Cathodes for Ultrahigh Cycle Stability

The wide applications of Ni‐rich LiNi_1‐x‐yCo_xMn_yO₂ cathodes are severely limited by capacity fading and voltage fading during the cycling process resulting from the pulverization of particles, interfacial side reactions, and phase transformation. The canonical surface modification approach can improve the stability to a certain extent; however, it fails to resolve the key bottlenecks. The preparation of Li(Ni_0.4Co_0.2Mn_0.4)_1‐xTi_xO₂ on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ particles with a coprecipitation method is reported. After sintering, Ti diffuses into the interior and mainly distributes along surface and grain boundaries. A strong surface and grain boundary strengthening are simultaneously achieved. The pristine particles are fully pulverized into first particles due to mechanical instability and high strains, which results in serious capacity fading. In contrast, the strong surface and the grain boundary strengthening can maintain the structural integrity, and therefore significantly improve the cycle stability. A general and simple strategy for the design of high‐performance Ni‐rich LiNi_1‐x‐yCo_xMn_yO₂ cathode is provided and is applicable to surface modification and grain‐boundary regulation of other advanced cathodes for batteries.     

Insight into Ca‐Substitution Effects on O3‐Type NaNi1/3Fe1/3Mn1/3O2 Cathode Materials for Sodium‐Ion Batteries Application

O3‐type NaNi_1/3Fe_1/3Mn_1/3O₂ (NaNFM) is well investigated as a promising cathode material for sodium‐ion batteries (SIBs), but the cycling stability of NaNFM still needs to be improved by using novel electrolytes or optimizing their structure with the substitution of different elements sites. To enlarge the alkali‐layer distance inside the layer structure of NaNFM may benefit Na⁺ diffusion. Herein, the effect of Ca‐substitution is reported in Na sites on the structural and electrochemical properties of Na_1?xCa_x/2NFM (x = 0, 0.05, 0.1). X‐ray diffraction (XRD) patterns of the prepared Na_1?xCa_x/2NFM samples show single α‐NaFeO₂ type phase with slightly increased alkali‐layer distance as Ca content increases. The cycling stabilities of Ca‐substituted samples are remarkably improved. The Na_0.9Ca_0.05Ni_1/3Fe_1/3Mn_1/3O₂ (Na_0.9Ca_0.05NFM) cathode delivers a capacity of 116.3 mAh g^?1 with capacity retention of 92% after 200 cycles at 1C rate. In operando XRD indicates a reversible structural evolution through an O3‐P3‐P3‐O3 sequence of Na_0.9Ca_0.05NFM cathode during cycling. Compared to NaNMF, the Na_0.9Ca_0.05NFM cathode shows a wider voltage range in pure P3 phase state during the charge/discharge process and exhibits better structure recoverability after cycling. The superior cycling stability of Na_0.9Ca_0.05NFM makes it a promising material for practical applications in sodium‐ion batteries.     

Few Atomic Layered Lithium Cathode Materials to Achieve Ultrahigh Rate Capability in Lithium‐Ion Batteries

The most promising cathode materials, including LiCoO₂ (layered), LiMn₂O₄ (spinel), and LiFePO₄ (olivine), have been the focus of intense research to develop rechargeable lithium‐ion batteries (LIBs) for portable electronic devices. Sluggish lithium diffusion, however, and unsatisfactory long‐term cycling performance still limit the development of present LIBs for several applications, such as plug‐in/hybrid electric vehicles. Motivated by the success of graphene and novel 2D materials with unique physical and chemical properties, herein, a simple shear‐assisted mechanical exfoliation method to synthesize few‐layered nanosheets of LiCoO₂, LiMn₂O₄, and LiFePO₄ is used. Importantly, these as‐prepared nanosheets with preferred orientations and optimized stable structures exhibit excellent C‐rate capability and long‐term cycling performance with much reduced volume expansion during cycling. In particular, the zero‐strain insertion phenomenon could be achieved in 2–3 such layers of LiCoO₂ electrode materials, which could open up a new way to the further development of next‐generation long‐life and high‐rate batteries.     

Layered Oxide Cathode for Potassium‐Ion Battery: Recent Progress and Prospective

Rechargeable potassium‐ion batteries (KIBs) have demonstrated great potential as alternative technologies to the currently used lithium‐ion batteries on account of the competitive price and low redox potential of potassium which is advantageous to applications in the smart grid. As the critical component determining the energy density, appropriate cathode materials are of vital need for the realization of KIBs. Layered oxide cathodes are promising candidates for KIBs due to high reversible capacity, appropriate operating potential, and most importantly, facile and easily scalable synthesis. In light of this trend, the recent advancements and progress in layered oxides research for KIBs cathodes, covering material design, structural evolution, and electrochemical performance are comprehensively reviewed. The structure–performance correlation and some effective optimization strategies are also discussed. Furthermore, challenges and prospects of these layered cathodes are included, with the purpose of providing fresh impetus for future development of these materials for advanced energy storage systems.     

High‐Safety Nonaqueous Electrolytes and Interphases for Sodium‐Ion Batteries

Rapidly developed Na‐ion batteries are highly attractive for grid energy storage. Nevertheless, the safety issues of Na‐ion batteries are still a bottleneck for large‐scale applications. Similar to Li‐ion batteries (LIBs), the safety of Na‐ion batteries is considered to be tightly associated with the electrolyte and electrode/electrolyte interphase. Although the knowledge obtained from LIBs is helpful, designing safe electrolytes and obtaining stable interphases in Na‐ion batteries is still a huge challenge. Therefore, it is of significance to investigate the key factors and develop new strategies for the development of high‐safety Na‐ion batteries. This comprehensive review introduces the recent efforts from nonaqueous electrolytes and interphase aspects of Na‐ion batteries, proposes their design strategies and requirements for improving safety characteristics, and discusses the potential issues for practical applications. The insight to formulate safe electrolytes and design the stable interphase for Na‐ion batteries with high safety is intended to be provided herein.     

Nitrogen Doped γ‐Graphyne: A Novel Anode for High‐Capacity Rechargeable Alkali‐Ion Batteries

High energy density is the major demand for next‐generation rechargeable batteries, while the intrinsic low alkali metal adsorption of traditional carbon–based electrode remains the main challenge. Here, the mechanochemical route is proposed to prepare nitrogen doped γ‐graphyne (NGY) and its high capacity is demonstrated in lithium (LIBs)/sodium (SIBs) ion batteries. The sample delivers large reversible Li (1037 mAh g^?1) and Na (570.4 mAh g^?1) storage capacities at 100 mA g^?1 and presents excellent rate capabilities (526 mAh g^?1 for LIBs and 180.2 mAh g^?1 for SIBs) at 5 A g^?1. The superior Li/Na storage mechanisms of NGY are revealed by its 2D morphology evolution, quantitative kinetics, and theoretical calculations. The effects on the diffusion barriers (E_b) and adsorption energies (E_ad) of Li/Na atoms in NGY are also studied and imine‐N is demonstrated to be the ideal doping format to enhance the Li/Na storage performance. Besides, the Li/Na adsorption routes in NGY are optimized according to the experimental and the first‐principles calculation results. This work provides a facile way to fabricate high capacity electrodes in LIBs/SIBs, which is also instructive for the design of other heteroatomic doped electrodes.     

Manganese‐Based Na‐Rich Materials Boost Anionic Redox in High‐Performance Layered Cathodes for Sodium‐Ion Batteries

To improve the energy and power density of Na‐ion batteries, an increasing number of researchers have focused their attention on activation of the anionic redox process. Although several materials have been proposed, few studies have focused on the Na‐rich materials compared with Li‐rich materials. A key aspect is sufficient utilization of anionic species. Herein, a comprehensive study of Mn‐based Na_1.2Mn_0.4Ir_0.4O₂ (NMI) O3‐type Na‐rich materials is presented, which involves both cationic and anionic contributions during the redox process. The single‐cation redox step relies on the Mn³⁺/Mn⁴⁺, whereas Ir atoms build a strong covalent bond with O and effectively suppress the O₂ release. In situ Raman, ex situ X‐ray photoelectron spectroscopy, and soft‐X‐ray absorption spectroscopy are employed to unequivocally confirm the reversibility of O₂^2? species formation and suggest a high degree of anionic reaction in this NMI Na‐rich material. In operando X‐ray diffraction study discloses the asymmetric structure evolution between the initial and subsequent cycles, which also explains the effect of the charge compensation mechanism on the electrochemical performance. The research provides a novel insight on Na‐rich materials and a new perspective in materials design towards future applications.     

A Heterostructure Coupling of Bioinspired,Adhesive Polydopamine,and Porous Prussian Blue Nanocubics as Cathode for High‐Performance Sodium‐Ion Battery

Prussian blue (PB) and its analogues are recognized as promising cathodes for rechargeable batteries intended for application in low‐cost and large‐scale electric energy storage. With respect to PB cathodes, however, their intrinsic crystal regularity, vacancies, and coordinated water will lead to low specific capacity and poor rate performance, impeding their application. Herein, nanocubic porous Na_xFeFe(CN)₆ coated with polydopamine (PDA) as a coupling layer to improve its electrochemical performance is reported, inspired by the excellent adhesive property of PDA. As a cathode for sodium‐ion batteries, the Na_xFeFe(CN)₆ electrode coupled with PDA delivers a reversible capacity of 93.8 mA h g^?1 after 500 cycles at 0.2 A g^?1, and a discharge capacity of 72.6 mA h g^?1 at 5.0 A g^?1. The sodium storage mechanism of this Na_xFeFe(CN)₆ coupled with PDA is revealed via in situ Raman spectroscopy. The first‐principles computational results indicate that Fe^II sites in PB prefer to couple with the robust PDA layer to stabilize the PB structure. Moreover, the sodium‐ion migration in the PB structure is enhanced after coating with PDA, thus improving the sodium storage properties. Both experiments and computational simulations present guidelines for the rational design of nanomaterials as electrodes for energy storage devices.