Improved Electrochemical Performance of Fe-Substituted NaNi0.5Mn0.5O2 Cathode Materials for Sodium-Ion Batteries

High-Entropy Configuration Strategy to Build High Performance Na-Ion Layered Oxide Cathodes Derived from Simple Techniques

Layered transition metal oxides are commonly used as the cathode materials in sodium-ion batteries due to their low cost and easy manufacturing. However, the application is hindered by poor rate performance and complex phase transitions. To address these challenges, a new seven-component high-entropy layered oxide cathode material, O3-NaNi0.25Fe0.15Mn0.3Ti0.1Sn0.05Co0.05Li0.1O2 (HEO) has been developed. The entropy stabilization effect plays a crucial role in improving the performance of electrochemical systems and the stability of structures. The HEO exhibits a specific discharge capacity of 154.1 mA h g-1 at 0.1 C and 94.5 mA h g-1 at 7 C. In-situ and ex-situ XRD results demonstrate that the HEO effectively retards complex phase transitions. This work provides a high-entropy design for the storage materials with a high energy density. Meanwhile, it eliminates industry doubts about the performance of sodium ion layered oxide cathode materials.

Understanding the Correlation between Electrochemical Performance and Operating Mechanism of a Co-free Layered-Spinel Composite Cathode for Na-Ion Batteries

Compositing different crystal structures of layered transition metal oxides (LTMOs) is an emerging strategy to improve the electrochemical performance of LTMOs in sodium-ion batteries. Herein, a cobalt-free P2/P3-layered spinel composite, P2/P3-LS-Na1/2Mn2/3Ni1/6Fe1/6O2 (LS-NMNF), is synthesized, and the synergistic effects from the P2/P3 and spinel phases were investigated. The material delivers an initial discharge capacity of 143 mAh g-1 in the voltage range of 1.5-4.0 V and displays a capacity retention of 73% at the 50th cycle. The material shows a discharge capacity of 72 mAh g-1 at 5C. This superior rate performance by the material could be by virtue of the increased electronic conductivity contribution of the incorporated spinel phase. The charge compensation mechanism of the material is investigated by in operando X-ray absorption spectroscopy (in a voltage range of 1.5-4.5 V vs Na+/Na), which revealed the contribution of all transition metals toward the generated capacity. The crystal structure evolution of each phase during electrochemical cycling was analyzed by in operando X-ray diffraction. Unlike in the case of many reported P2/P3 composite cathode materials and spinel-incorporated cobalt-containing P2/P3 composites, the formation of a P'2 phase at the end of discharge is absent here.

A surface-modified Na3V2(PO4)2F3 cathode with high rate capability and cycling stability for sodium ion batteries

High voltage, high rate, and cycling-stable cathodes are urgently needed for development of commercially viable sodium ion batteries (SIBs). Herein, we report a facial ball-milling to synthesize a carbon-coated Na3V2(PO4)2F3 composite (C-NVPF). Benefiting from the highly conductive carbon layer, the C-NVPF material exhibits a high reversible capacity (110.6 mA h g-1 at 0.1C), long-term cycle life (54% of capacity retention up to 2000 cycles at 5C), and excellent rate performance (35.1 mA h g-1 at 30C). The present results suggest promising applications of the C-NVPF material as a high-performance cathode for sodium ion batteries.

Toward the High-Voltage Stability of Layered Oxide Cathodes for Sodium-Ion Batteries: Challenges, Progress, and Perspectives

Sodium-ion batteries (SIBs) have garnered significant attention as ideal candidates for large-scale energy storage due to their notable advantages in terms of resource availability and cost-effectiveness. However, there remains a substantial energy density gap between SIBs and commercially available lithium-ion batteries (LIBs), posing challenges to meeting the requirements of practical applications. The fabrication of high-energy cathodes has emerged as an efficient approach to enhancing the energy density of SIBs, which commonly requires cathodes operating in high-voltage regions. Layered oxide cathodes (LOCs), with low cost, facile synthesis, and high theoretical specific capacity, have emerged as one of the most promising candidates for commercial applications. However, LOCs encounter significant challenges when operated in high-voltage regions such as irreversible phase transitions, migration and dissolution of metal cations, loss of reactive oxygen, and the occurrence of serious interfacial parasitic reactions. These issues ultimately result in severe degradation in battery performance. This review aims to shed light on the key challenges and failure mechanisms encountered by LOCs when operated in high-voltage regions. Additionally, the corresponding strategies for improving the high-voltage stability of LOCs are comprehensively summarized. By providing fundamental insights and valuable perspectives, this review aims to contribute to the advancement of high-energy SIBs.

Amorphous Aluminum Oxide-Coated NaFe0.33Ni0.33Mn0.33O2 Cathode Materials: Enhancing Interface Charge Transfer for High-Performance Sodium-Ion Batteries

Layered cathode materials for sodium-ion batteries (SIBs) have gained considerable attention as promising candidates owing to their high capacity and potential for industrial scalability. Nonetheless, challenges arise from stress and structural degradation resulting from the deposition of larger ion radius species, leading to diminished cyclic stability and rate performance. In this study, we present a novel and straightforward strategy that combines the synergistic effects of an amorphous aluminum oxide coating and aluminum ion doping. This approach effectively addresses the issues of grain cracking and expands the interlayer spacing of alkali metal ions in SIB materials, thereby enhancing their overall performance. Consequently, it optimizes the diffusion of charge carriers and facilitates interfacial charge transfer, leading to remarkable improvements in the performance of the NaNi0.33Mn0.33Fe0.33O2 material with 0.4 wt % amorphous aluminum oxide coating (NNMF-0.4A), which exhibits reversible capacities of 135.7, 114.3, 106.8, 99.9, 89.5, and 77.1 mAh g-1 at 0.1, 0.5, 1, 2, 5, and 10 C, respectively. Furthermore, the NNMF-0.4A material maintains a capacity of 76.7 mA g-1 after 500 cycles at a current density of 800 mA g-1 (10 C), with a capacity retention rate of 98.2%. Our findings present a groundbreaking pathway for modifying high-power sodium-ion battery cathode materials, contributing to the advancement of sustainable energy storage technologies.

NASICON-Type NaTi2(PO4)3 Surface Modified O3-Type NaNi0.3Fe0.2Mn0.5O2 for High-Performance Cathode Material for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have shown great potential as energy storage devices due to their low price and abundant sodium content. Among them, O3-type layered oxides are a promising cathode material for sodium-ion batteries; however, most of them suffer from slow kinetics and unfavorable structural stability, which seriously hinder their practical application. O3-NaNi0.3Fe0.2Mn0.5O2 surface modification is performed by a simple wet chemical method of coating NaTi2(PO4)3 on the surface. The NASICON-type NaTi2(PO4)3 coating layer has a special three-dimensional channel, which facilitates the rapid migration of Na+, and the NaTi2(PO4)3 coating layer also prevents direct contact between the electrode and the electrolyte, ensuring the stability of the interface. In addition, the NaTi2(PO4)3 coating layer induces part of the Ti4+ doping into the transition metal layer of NaNi0.3Fe0.2Mn0.5O2, which increases the stability of the transition metal layer and reduces the resistance of Na+ diffusion. More importantly, the NaTi2(PO4)3 coating layer can suppress the O3-P3 phase transition and reduce the volume change of the materials throughout the charge/discharge process. Thus, the NaTi2(PO4)3 coating layer can effectively improve the electrochemical performance of the cathode materials. The NFM@NTP3 has a capacity retention of 86% (2.0-4.0 V vs Na+/Na, 300 cycles) and 85% (2.0-4.2 V vs Na+/Na, 100 cycles) at 1C and a discharge capacity of 107 mAh g-1 (2.0-4.0 V vs Na+/Na) and 125 mAh g-1 (2.0-4.2 V vs Na+/Na) at 10C, respectively. Therefore, this surface modification strategy provides a simple and effective way to design and develop high-performance layered oxide cathode materials for sodium-ion batteries.

Conversion of Surface Residual Alkali to Solid Electrolyte to Enable Na-Ion Full Cells with Robust Interfaces

The deposition of volatilized Na+ on the surface of the cathode during sintering results in the formation of surface residual alkali (NaOH/Na2 CO3 NaHCO3 ) in layered cathode materials, leading to serious interfacial reactions and performance degradation. This phenomenon is particularly evident in O3-NaNi0.4 Cu0.1 Mn0.4 Ti0.1 O2 (NCMT). In this study, a strategy is proposed to transform waste into treasure by converting residual alkali into a solid electrolyte. Mg(CH3 COO)2 and H3 PO4 are reacted with surface residual alkali to generate the solid electrolyte NaMgPO4 on the surface of NCMT, which can be labeled as NaMgPO4@NaNi0.4 Cu0.1 Mn0.4 Ti0.1 O2 -X (NMP@NCMT-X, where X indicates the different amounts of Mg2+ and PO4 3- ). NaMgPO4 acts as a special ionic conductivity channel on the surface to improve the kinetics of the electrode reactions, remarkably improving the rate capability of the modified cathode at a high current density in the half-cell. Additionally, NMP@NCMT-2 enables a reversible phase transition from the P3 to OP2 phase in the charge-discharge process above 4.2 V and achieves a high specific capacity of 157.3 mAh g-1 and outstanding capacity retention in the full cell. The strategy can effectively and reliably stabilize the interface and improve the performance of layered cathodes for Na-ion batteries (NIBs).

Multifunctional Effect of Fe Substitution in Na Layered Cathode Materials for Enhanced Storage Stability

Developing stable cathode materials that are resistant to storage degradation is essential for practical development and industrial processing of Na-ion batteries as many sodium layered oxide materials are susceptible to hygroscopicity and instability upon exposure to ambient air. Among the various layered compounds, Fe-substituted O3-type Na(Ni1/2Mn1/2)1-xFexO2 materials have emerged as a promising option for high-performance and low-cost cathodes. While previous reports have noted the decent air-storage stability of these materials, the role and origin of Fe substitution in improving storage stability remain unclear. In this study, we investigate the air-resistant effect of Fe substitution in O3-Na(Ni1/2Mn1/2)1-xFexO2 cathode materials by performing systematic surface and structural characterizations. We find that the improved storage stability can be attributed to the multifunctional effect of Fe substitution, which forms a surface protective layer containing an Fe-incorporated spinel phase and decreases the thermodynamical driving force for bulk chemical sodium extraction. With these mechanisms, Fe-containing cathodes can suppress the cascades of cathode degradation processes and better retain the electrochemical performance after air storage.

Recent Progress in Rechargeable Sodium Metal Batteries: A Review

Sodium metal batteries (SMBs) have been widely studied owing to their relatively high energy density and abundant resources. However, they still need systematic improvement to fulfill the harsh operating conditions for their commercialization. In this review, we summarize the recent progress in SMBs in terms of sodium anode modification, electrolyte exploration, and cathode design. Firstly, we give an overview of the current challenges facing Na metal anodes and the corresponding solutions. Then, the traditional liquid electrolytes and the prospective solid electrolytes for SMBs are summarized. In addition, insertion- and conversion-type cathode materials are introduced. Finally, an outlook for the future of practical SMBs is provided.

A High-Rate, Durable Cathode for Sodium-Ion Batteries: Sb-Doped O3-Type Ni/Mn-Based Layered Oxides

O3-Type layered oxides are widely studied as cathodes for sodium-ion batteries (SIBs) due to their high theoretical capacities. However, their rate capability and durability are limited by tortuous Na⁺ diffusion channels and complicated phase evolution during Na⁺ extraction/insertion. Here we report our findings in unravelling the mechanism for dramatically enhancing the stability and rate capability of O3-NaNi_0.5Mn_0.5-xSb_xO₂ (NaNMS) by substitutional Sb doping, which can alter the coordination environment and chemical bonds of the transition metal (TM) ions in the structure, resulting in a more stable structure with wider Na⁺ transport channels. Furthermore, NaNMS nanoparticles are obtained by surface energy regulation during grain growth. The synergistic effect of Sb doping and nanostructuring greatly reduces the ionic migration energy barrier while increasing the reversibility of the structural evolution during repeated Na⁺ extraction/insertion. An optimized NaNMS-1 electrode delivers a reversible capacity of 212.3 mAh g^-1 at 0.2 C and 74.5 mAh g^-1 at 50 C with minimal capacity loss after 100 cycles at a low temperature of -20 °C. Such electrochemical performance is superior to most of the reported layered oxide cathodes used in rechargeable SIBs.

Low-Cost Al-Doped Layered Cathodes with Improved Electrochemical Performance for Rechargeable Sodium-Ion Batteries

O3-NaNi0.25Fe0.5Mn0.25O2 layered oxide is considered one of the most promising cathode candidates for sodium-ion batteries because of its advantages, such as its large capacity and low cost. However, the practical application of this material is limited by its poor cyclic stability and insufficient rate capability. Here, a strategy to substitute the Fe3+ in NaNi0.25Fe0.5Mn0.25O2 with Al3+ is adopted to address these issues. The substitution of Fe3+ with Al3+ enhances the framework stability and phase transition reversibility of the parent NaNi0.25Fe0.5Mn0.25O2 material by forming a stronger TM-O bond, which improves the cycling stability. Moreover, partial Al3+ substitution increases the interslab distance, providing a spacious path for Na+ diffusion and resulting in fast diffusion kinetics, which lead to improved rate capability. Consequently, the target NaNi0.25Fe0.5-xAlxMn0.25O2 sample with optimal x = 0.045 exhibits a remarkable electrochemical performance in a Na-ion cell with a large reversible capacity of 131.7 mA h g-1, a stable retention of approximately 81.6% after cycling at 1C for 100 cycles, and a rate performance of 81.3 mA h g-1 at 10C. This method might pave the way for novel means of improving the electrochemical properties of layered transitional-metal oxides and provide insightful guidance for the design of low-cost cathode materials.

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.

Using High-Entropy Configuration Strategy to Design Na-Ion Layered Oxide Cathodes with Superior Electrochemical Performance and Thermal Stability

Na-ion layered oxide cathodes (Na_xTMO₂, TM = transition metal ion(s)), as an analogue of lithium layered oxide cathodes (such as LiCoO₂, LiNi_xCo_yMn_1-x-yO₂), have received growing attention with the development of Na-ion batteries. However, due to the larger Na⁺ radius and stronger Na⁺-Na⁺ electrostatic repulsion in NaO₂ slabs, some undesired phase transitions are observed in Na_xTMO₂. Herein, we report a high-entropy configuration strategy for Na_xTMO₂ cathode materials, in which multicomponent TMO₂ slabs with enlarged interlayer spacing help strengthen the whole skeleton structure of layered oxides through mitigating Jahn-Teller distortion, Na⁺/vacancy ordering, and lattice parameter changes. The strengthened skeleton structure with a modulated particle morphology dramatically improves the Na⁺ transport kinetics and suppresses intragranular fatigue cracks and TM dissolution, thus leading to highly improved performances. Furthermore, the elaborate high-entropy TMO₂ slabs enhance the TM-O bonding energy to restrain oxygen release and thermal runaway, benefiting for the improvement of thermal safety.

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.

Boron-doped sodium layered oxide for reversible oxygen redox reaction in Na-ion battery cathodes

Na-ion cathode materials operating at high voltage with a stable cycling behavior are needed to develop future high-energy Na-ion cells. However, the irreversible oxygen redox reaction at the high-voltage region in sodium layered cathode materials generates structural instability and poor capacity retention upon cycling. Here, we report a doping strategy by incorporating light-weight boron into the cathode active material lattice to decrease the irreversible oxygen oxidation at high voltages (i.e., >4.0 V vs. Na⁺/Na). The presence of covalent B-O bonds and the negative charges of the oxygen atoms ensures a robust ligand framework for the NaLi_1/9Ni_2/9Fe_2/9Mn_4/9O₂ cathode material while mitigating the excessive oxidation of oxygen for charge compensation and avoiding irreversible structural changes during cell operation. The B-doped cathode material promotes reversible transition metal redox reaction enabling a room-temperature capacity of 160.5 mAh g^-1 at 25 mA g^-1 and capacity retention of 82.8% after 200 cycles at 250 mA g^-1. A 71.28 mAh single-coated lab-scale Na-ion pouch cell comprising a pre-sodiated hard carbon-based anode and B-doped cathode material is also reported as proof of concept.

Role of Lithium Doping in P2-Na0.67Ni0.33Mn0.67O2 for Sodium-Ion Batteries

P2-structured Na_0.67Ni_0.33Mn_0.67O₂ (PNNMO) is a promising Na-ion battery cathode material, but its rapid capacity decay during cycling remains a hurdle. Li doping in layered transition-metal oxide (TMO) cathode materials is known to enhance their electrochemical properties. Nevertheless, the influence of Li at different locations in the structure has not been investigated. Here, the crystallographic role and electrochemical impact of lithium on different sites in PNNMO is investigated in Li _x Na_0.67-y Ni_0.33Mn_0.67O_2+δ (0.00 ≤ x ≤ 0.2, y = 0, 0.1). Lithium occupancy on prismatic Na sites is promoted in Na-deficient (Na < 0.67) PNNMO, evidenced by ex situ and operando synchrotron X-ray diffraction, X-ray absorption spectroscopy, and ⁷Li solid-state nuclear magnetic resonance. Partial substitution of Na with Li leads to enhanced stability and slightly increased specific capacity compared to PNNMO. In contrast, when lithium is located primarily on octahedral TM sites, capacity is increased but at the cost of stability.

Synergistic Effect of Polymorphs in Doped NaNi0.5Mn0.5O2 Cathode Material for Improving Electrochemical Performances in Na-Batteries

Layered NaNi0.5Mn0.5O2, employed as cathode materials in sodium ion batteries, is attracting interest due to its high working potential and high-capacity values, thanks to the big sodium amount hosted in the lattice. Many issues are, however, related to their use, particularly, the complex phase transitions occurring during sodium intercalation/deintercalation, detrimental for the structure stability, and the possible Mn dissolution into the electrolyte. In this paper, the doping with Ti, V, and Cu ions (10% atoms with respect to Ni/Mn amount) was used to stabilize different polymorphs or mixtures of them with the aim to improve the capacity values and cells cyclability. The phases were identified and quantified by means of X-ray powder diffraction with Rietveld structural refinements. Complex voltammograms with broad peaks, due to multiple structural transitions, were disclosed for most of the samples. Ti-doped sample has, in general, the best performances with the highest capacity values (120 mAh/g at C/10), however, at higher currents (1C), Cu-substituted sample also has stable and comparable capacity values.

Electrochemical Insight into the Sodium-Ion Storage Mechanism on a Hard Carbon Anode

Hard carbon (HC) has been actively investigated as a high-capacity and low-cost anode material for sodium-ion batteries (SIBs); however, its sodium-storage mechanism has remained controversial, which imposes great difficulties in the design and construction of better microstructured HC materials. To obtain a deeper understanding of the Na-storage mechanism, we comparatively investigated electrochemical behaviors of HC and graphite for Na- and Li-storage reactions. The experimental results reveal that the Na-storage reaction on HC at a low-potential plateau proceeds in a manner similar to the Li⁺-insertion reaction on graphite but very differently from the Li⁺-storage process on HC, suggesting that the Na-storage mechanism of HC at a low-voltage plateau operates through the Na⁺ intercalation into the graphitic layers for the formation of sodium-graphite intercalation compounds (Na-GICs) and is consistent with the "adsorption-intercalation" mechanism. Our work might provide new insight for designing better HC materials of high-energy density SIBs.

Intrinsic enhancement of the rate capability and suppression of the phase transition via p-type doping in Fe-Mn based P2-type cathodes used for sodium ion batteries

In this study, we present improved power characteristics and suppressed phase transition by incorporating elemental doping into a P2-type cathode of sodium ion batteries. A Cu-doped Fe-Mn based P2-type Na0.67Cu0.125Fe0.375Mn0.5O2 cathode was designed based on the calculations of the electronic structure and then examined experimentally. Using first principles, we introduced instrinsic p-type conductivity by elemental doping with Cu. Introduction of Cu generated electron holes above the Fermi level in the electronic structure, which is typical of p-type semiconductors. Charge analyses suggested that the hole generation was driven primarily by the greater reduced characteristics of Cu as compared with those of Fe and Mn. In addition, introduction of Cu retaining high reduced property also suppressed phase transition from the P2 to Z phase by Fe migration to empty Na layers mainly. Electrochemical experiments revealed improved power characteristics upon the introduction of p-type conductivity. This could be attributed to the increase in the electronic conductivity by hole generation in the valence band. This study suggests that the introduction of p-type conductivity could be a rational tactic for the development of promising cathode materials for high performance sodium ion batteries.

Nano Polymorphism-Enabled Redox Electrodes for Rechargeable Batteries

Nano polymorphism (NPM), as an emerging research area in the field of energy storage, and rechargeable batteries, have attracted much attention recently. In this review, the recent progress on the composition and formation of polymorphs, and the evolution processes of different redox electrodes in rechargeable metal-ion, metal-air, and metal-sulfur batteries are highlighted. First, NPM and its significance for rechargeable batteries are discussed. Subsequently, the current NPM modulation strategies of different types of representative electrodes for their corresponding rechargeable battery applications are summarized. The goal is to demonstrate how NPM could tune the intrinsic material properties, and hence, improve their electrochemical activities for each battery type. It is expected that the analysis of polymorphism and electrochemical properties of materials could help identify some "processing-structure-properties" relationships for material design and performance enhancement. Lastly, the current research challenges and potential research directions are discussed to offer guidance and perspectives for future research on NPM engineering.

Revealing the role of dopants in mitigating degradation phenomena in sodium-ion layered cathodes

Prevention of the degradation of sodium-based layered cathode materials is the key to developing high-performance and high-stability sodium-ion batteries. In this study, the working mechanism of Mg and Ti dopants in mitigating degradation was investigated through the use of first-principles calculations. More specifically, the effects of each dopant in suppressing the phase transition, lattice expansion and shrinkage, and possible oxygen generation during repeated charging and discharging processes were validated. The results showed that the pristine structure exhibits irreversible O3-P3 phase transition after 75% desodiation, while doping with Mg or Ti effectively delays this transition. In addition, the change in lattice parameters as well as in the volume during desodiation was investigated. It was found that both dopants reduce the magnitude of structural change, which potentially improves the structural stability. Furthermore, introducing the dopants increases the thickness of the Na diffusion channel, possibly leading to an enhanced rate capability. Finally, the oxygen atomic charge variation during charging indicated that doping can enhance the oxygen stability by reducing the initial charge of oxygen as well as its increase during desodiation.

CNT-modified two-phase manganese hexacyanoferrate as a superior cathode for sodium-ion batteries

The two-phase KNa-MnFe(CN)₆@CNT material was synthesized via a facile concentration-gradient coprecipitation method. The outstanding electrochemical performance was achieved for KNa-MnFe(CN)₆@CNT material with the addition of CNT.

Origins of Irreversibility in Layered NaNixFeyMnzO2 Cathode Materials for Sodium Ion Batteries

Layered NaNi_xFe_yMn_zO₂ cathode (NFM) is of great interest in sodium ion batteries because of its high theoretical capacity and utilization of abundant, low-cost, environmentally friendly raw materials. Nevertheless, there remains insufficient understanding on the concurrent local environment evolution in each transition metal (TM) that largely influences the reversibility of the cathode materials upon cycling. In this work, we investigate the reversibility of TM ions in layered NFMs with varying Fe contents and potential windows. Utilizing ex situ synchrotron X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure of precycled samples, the valence and bonding evolution of the TMs are elucidated. It is found that Mn is electrochemically inactive, as indicated by the insignificant change of Mn valence and the Mn-O bonding distance. Fe is electrochemically inactive after the first five cycles. The Ni redox couple contributes most of the charge compensation for NFMs. Ni redox is quite reversible in the cathodes with less Fe content. However, the Ni redox couple shows significant irreversibility with a high Fe content of 0.8. The electrochemical reversibility of the NFM cathode becomes increasingly enhanced with the decrease of either Fe content or with lower upper charge cutoff potential.

State-of-the-Art Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) were investigated as recently as in the seventies. However, they have been overshadowed for decades, due to the success of lithium-ion batteries that demonstrated higher energy densities and longer cycle lives. Since then, the witness a re-emergence of the SIBs and renewed interest evidenced by an exponential increase of the publications devoted to them (about 9000 publications in 2019, more than 6000 in the first six months this year). This huge effort in research has led and is leading to an important and constant progress in the performance of the SIBs, which have conquered an industrial market and are now commercialized. This progress concerns all the elements of the batteries. We have already recently reviewed the salts and electrolytes, including solid electrolytes to build all-solid-state SIBs. The present review is then devoted to the electrode materials. For anodes, they include carbons, metal chalcogenide-based materials, intercalation-based and conversion reaction compounds (transition metal oxides and sulfides), intermetallic compounds serving as functional alloying elements. For cathodes, layered oxide materials, polyionic compounds, sulfates, pyrophosphates and Prussian blue analogs are reviewed. The electrode structuring is also discussed, as it impacts, importantly, the electrochemical performance. Attention is focused on the progress made in the last five years to report the state-of-the-art in the performance of the SIBs and justify the efforts of research.

Heterotrimetallic Precursor with 2:2:1 Metal Ratio Requiring at Least a Pentanuclear Molecular Assembly

This work represents an important step in the quest to make heteromultimetallic molecules featuring specific metal types and complicated metal ratios. The rational design, synthesis, and characterization of a complex heterotrimetallic single-source molecular precursor for the next generation sodium-ion battery cathode material, Na₂Mn₂FeO₆, is described. A unique pentametallic platform [Mn^II(ptac)₃-Na-Mn^III(acac)₃-Na-Mn^II(ptac)₃] (1) was derived from the known polymeric structure of [NaMn^II(acac)₃]_∞, through a series of elaborate design procedures, such as mixed-ligand, unsymmetric ligand, and mixed-valent approaches. Importantly, the application of those techniques results in a molecule with distinctively different transition metal positions in terms of ligand environment and oxidation states. An isovalent substitution of Fe^III for the central Mn^III ion forms the target heterotrimetallic precursor [Mn^II(ptac)₃-Na-Fe^III(acac)₃-Na-Mn^II(ptac)₃] (3) with an appropriate metal ratio of Na:Mn:Fe = 2:2:1. The arrangement of metal ions and ligands in this pentametallic assembly was confirmed by single crystal X-ray investigation. The unambiguous assignment of the positions and oxidation states of the Periodic Table neighbors Fe and Mn in 3 has been achieved by a combination of investigative techniques that include synchrotron resonant diffraction, X-ray multiwavelength anomalous diffraction, X-ray fluorescence spectroscopy, Mössbauer spectroscopy, and gas-phase DART mass spectrometry. The heterotrimetallic single-source precursor 3 was shown to exhibit a clean decomposition pattern yielding the phase-pure P2-Na₂Mn₂FeO₆ quaternary oxide with high uniformity of metal ion distribution as confirmed by electron microscopy.

Insights into P2-Type Layered Positive Electrodes for Sodium Batteries: From Long- to Short-Range Order

P2-type Fe- and Mn-based layered sodium transition metal oxides are promising positive electrode materials for sodium batteries due to their high energy density and low costs of the constituting transition metals. However, poor structural reversibility and fast capacity decay have prevented their breakthrough so far. Herein, the real-time dynamic phase transitions and capacity fading mechanism of the P2 Na_0.67Fe_0.5Mn_0.5O₂ positive electrode are revealed by operando X-ray diffraction, operando/ex situ X-ray absorption spectroscopy, neutron powder diffraction, and neutron pair distribution functions. Upon the desodiation process, a layered OP4 phase with long-range order is found as an intermediate state. With further deep desodiation, the formation of a Na-depleted ramsdellite phase with a short coherent length of 30 Å is observed for the first time. However, the transition from OP4 to ramsdellite is considered to be irreversible due to the breakdown of the layered structural characteristics, resulting in poor cycling performance in a variety of Fe-based layered sodium transition metal oxides. This work suggests that stabilizing the crystal structure by substitution or chemical modification can be a favorable strategy to avoid the degradation of positive electrodes and thus to improve the cycling performance.

A NaNi0.5Mn0.5SnxO2 cathode with anti-structural deformation enhancing long lifespan and super power for a sodium ion battery

Introduction of Sn⁴⁺ enlarges the interlayer spacing and builds a heteroatomic skeleton preventing multiphase transition of O3-NaNi_0.5Mn_0.5O₂ for a sodium ion battery.

Critical Role of Titanium in O3-Type Layered Cathode Materials for Sodium-Ion Batteries

Recently, the substitution of inactive elements has been reported as a promising strategy for improving the structural stability and electrochemical performance of layered cathode materials for sodium-ion batteries (SIBs). In this regard, we investigated the positive effects of inactive Ti substitution into O3-type NaFe_0.25Ni_0.25Mn_0.5O₂ based on first-principles calculations and electrochemical experiments. After Ti substitution, Na[Ti_0.03(Fe_0.25Ni_0.25Mn_0.5)_0.97]O₂ exhibits improved capacity retention and rate capability compared with Ti-free NaFe_0.25Ni_0.25Mn_0.5O₂. Such an improvement is primarily attributed to the enhanced structural stability and lowered activation energy for Na⁺ migration, which is induced by Ti substitution in the host structure. Based on first-principles calculations of the average net charges and partial densities of states, we suggest that Ti substitution effectively enhances the binding between transition metals and oxygen by increasing the oxygen electron density, which in turn lowers the energy barrier of Na⁺ migration, leading to a notable enhancement in the rate capability of Na[Ti_0.03(Fe_0.25Ni_0.25Mn_0.5)_0.97]O₂. Compared with other inactive elements (e.g., Al and Mg), Ti is a more suitable substituent for improving the electrochemical properties of layered cathode materials because of its large total charge variation contributing to capacity. The results of this study provide practical guidelines for developing highly reliable layered cathode materials for SIBs.

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_x MO₂ , 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_x MO₂ 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_x MO₂ cathodes and future perspectives are discussed to provide guidance for the future commercialization of such systems.

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.

Co-Substitution Enhances the Rate Capability and Stabilizes the Cyclic Performance of O3-Type Cathode NaNi0.45- xMn0.25Ti0.3Co xO2 for Sodium-Ion Storage at High Voltage

O3-type NaNiO₂-based cathode materials suffer irreversible phase transition when they are charged to above 4.0 V in sodium-ion batteries. To solve this problem, we partially substitute Ni²⁺ in O3-type NaNi_0.45Mn_0.25Ti_0.3O₂ by Co³⁺. NaNi_0.45Mn_0.25Ti_0.3O₂ with co-substitution possesses an expanded interlayer and exhibits higher rate capability, as well as cyclic stability, compared with the pristine cathode in 2.0-4.4 V. The optimal NaNi_0.4Mn_0.25Ti_0.3Co_0.05O₂ delivers discharge capacities of 180 and 80 mA  h  g^-1 at 10 and 1000 mA g^-1. At 100 mA g^-1, NaNi_0.4Mn_0.25Ti_0.3Co_0.05O₂ exhibits 152 mA h g^-1 in the initial cycle and maintains 91.4 mA h g^-1 after 180 cycles. Through ex situ X-ray diffraction, co-substitution is demonstrated to be effective in enhancing the reversibility of P3-P3″ phase transition from 4.0 to 4.4 V. Electrochemical impedance spectroscopy indicates that higher electronic conductivity is achieved by co-substitution. Moreover, cyclic voltammetry and the galvanostatic intermittent titration technique demonstrate faster kinetics for Na⁺ diffusion due to the co-substitution. This study provides a reference for further improvement of electrochemical performance of cathode materials for high-voltage sodium-ion batteries.

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.

Advanced P2-Na2/3Ni1/3Mn7/12Fe1/12O2 Cathode Material with Suppressed P2-O2 Phase Transition toward High-Performance Sodium-Ion Battery

As a promising cathode material of sodium-ion battery, P2-type Na_2/3Ni_1/3Mn_2/3O₂ (NNMO) possesses a theoretically high capacity and working voltage to realize high energy storage density. However, it still suffers from poor cycling stability mainly incurred by the undesirable P2-O2 phase transition. Herein, the electrochemically active Fe³⁺ ions are introduced into the lattice of NNMO, forming Na_2/3Ni_1/3Mn_{2/3- x}Fe _xO₂ ( x = 0, 1/24, 1/12, 1/8, 1/6) to effectively stabilize the P2-type crystalline structure. In such Fe-substituted materials, both Ni²⁺/Ni⁴⁺ and Fe³⁺/Fe⁴⁺ couples take part in the redox reactions, and the P2-O2 phase transition is well restrained during cycling, as verified by ex situ X-ray diffraction. As a result, the optimized Na_2/3Ni_1/3Mn_7/12Fe_1/12O₂ (1/12-NNMF) has a long-term cycling stability with the fading rate of 0.05% per cycle over 300 cycles at 5 C. Furthermore, the 1/12-NNMF delivers excellent rate capabilities (65 mA h g^-1 at 25 C) and superior low-temperature performance (the capacity retention of 94% at -25 °C after 80 cycles) owing to the enhanced Na diffusion upon Fe doping, which is deduced by the studies of electrode kinetics. More significantly, the 1/12-NNMF also displays remarkable sodium-ion full-cell properties when merged with an LS-Sb@G anode, thus implying the possibility of their practical application.

Electrochemical Property-Structure Correlation for Ni-Based Layered Na-Ion Cathodes

A class of Ni-based layered Na _xNi_0.5Mn_0.3Co_0.2O₂ oxide composites is prepared via a solid-state process. Mixed P-, O-, and P-/O-type phases can be obtained by tuning the Na content and annealing temperature, as demonstrated by structural and chemical characterization. Among these materials, the triphase P2/P'3/O'3-type composite exhibits the best overall electrochemical performance. Specifically, this triphase composite delivers a high specific capacity of 126 mA h g^-1 in the potential range of 1.5-4.2 V, high rate capability (∼72% of its initial capacity at a rate of 5 C), and good capacity retention after 100 cycles at 0.5 C. The structural transition mechanism for each phase upon electrochemical cycling is investigated, providing insights into the correlation between electrochemical properties and the crystal structure of Ni-rich layered Na _xNi_0.5Mn_0.3Co_0.2O₂ oxide composites.

Exposing {010} Active Facets by Multiple-Layer Oriented Stacking Nanosheets for High-Performance Capacitive Sodium-Ion Oxide Cathode

As one of the most promising cathodes for rechargeable sodium-ion batteries (SIBs), O3-type layered transition metal oxides commonly suffer from inevitably complicated phase transitions and sluggish kinetics. Here, a Na[Li0.05 Ni0.3 Mn0.5 Cu0.1 Mg0.05 ]O2 cathode material with the exposed {010} active facets by multiple-layer oriented stacking nanosheets is presented. Owing to reasonable geometrical structure design and chemical substitution, the electrode delivers outstanding rate performance (71.8 mAh g-1 and 16.9 kW kg-1 at 50C), remarkable cycling stability (91.9% capacity retention after 600 cycles at 5C), and excellent compatibility with hard carbon anode. Based on the combined analyses of cyclic voltammograms, ex situ X-ray absorption spectroscopy, and operando X-ray diffraction, the reaction mechanisms behind the superior electrochemical performance are clearly articulated. Surprisingly, Ni2+ /Ni3+ and Cu2+ /Cu3+ redox couples are simultaneously involved in the charge compensation with a highly reversible O3-P3 phase transition during charge/discharge process and the Na+ storage is governed by a capacitive mechanism via quantitative kinetics analysis. This optimal bifunctional regulation strategy may offer new insights into the rational design of high-performance cathode materials for SIBs.