Fe2O3-reduced graphene oxide composites synthesized via microwave-assisted method for sodium ion batteries

Microwaves in clean energy technologies

Energy in the microwave spectrum is increasingly applied in clean energy technologies. This review discusses recent innovations using microwave fields in hydrogen production and synthesis of new battery materials, highlighting the unique properties of microwave heating. Key innovations include microwave-assisted hydrogen generation from water, hydrocarbons and ammonia and the synthesis of high-performance anode and cathode materials. Microwave-assisted catalytic water splitting using Gd-doped ceria achieves efficient hydrogen production below 250°C. For hydrocarbons, advanced microwave-active catalysts Fe-Ni alloys and ruthenium nanoparticles enable high conversion rates and hydrogen yields. In ammonia synthesis, microwaves reduce the energy demands of the Haber-Bosch process and enhance hydrogen production efficiency using catalysts such as ruthenium and Co2Mo3N. In battery technology, microwave-assisted synthesis of cathode materials like LiFePO4 and LiNi0.5Mn1.5O4 yields high-purity materials with superior electrochemical performance. Developing nanostructured and composite materials, including graphene-based anodes, significantly improves battery capacities and cycling stability. The ability of microwave technology to provide rapid, selective heating and enhance reaction rates offers significant advancements in clean energy technologies. Ongoing research continues to bridge theoretical understanding and practical applications, driving further innovations in this field. This review aims to highlight recent advances in clean energy technologies based upon the novel use of microwave energy. The potential impact of these emerging applications is now being fully understood in areas that are critical to achieving net zero and can contribute to the decarbonization of key sectors. Notable in this landscape are the sectors of hydrogen fuel and battery technologies. This review examines the role of microwaves in these areas.This article is part of the discussion meeting issue 'Microwave science in sustainability'.

Molten salt synthesis of disordered spinel CoFe2O4 with improved electrochemical performance for sodium-ion batteries

Sodium-ion (Na-ion) batteries are currently being investigated as an attractive substitute for lithium-ion (Li-ion) batteries in large energy storage systems because of the more abundant and less expensive supply of Na than Li. However, the reversible capacity of Na-ions is limited because Na possesses a large ionic radius and has a higher standard electrode potential than that of Li, making it challenging to obtain electrode materials that are capable of storing large quantities of Na-ions. This study investigates the potential of CoFe2O4 synthesised via the molten salt method as an anode for Na-ion batteries. The obtained phase structure, morphology and charge and discharge properties of CoFe2O4 are thoroughly assessed. The synthesised CoFe2O4 has an octahedron morphology, with a particle size in the range of 1.1-3.6 μm and a crystallite size of ∼26 nm. Moreover, the CoFe2O4 (M800) electrodes can deliver a high discharge capacity of 839 mA h g-1 in the first cycle at a current density of 0.1 A g-1, reasonable cyclability of 98 mA h g-1 after 100 cycles and coulombic efficiency of ∼99%. The improved electrochemical performances of CoFe2O4 can be due to Na-ion-pathway shortening, wherein the homogeneity and small size of CoFe2O4 particles may enhance the Na-ion transportation. Therefore, this simple synthetic approach using molten salt favours the Na-ion diffusion and electron transport to a great extent and maximises the utilisation of CoFe2O4 as a potential anode material for Na-ion batteries.

Realizing Ultrafast and Robust Sodium-Ion Storage of Iron Sulfide Enabled by Heteroatomic Doping and Regulable Interface Engineering

Fe-based sulfides are a promising type of anode material for sodium-ion batteries (SIBs) due to their high theoretical capacities and affordability. However, these materials often suffer from issues such as capacity deterioration and poor conductivity during practical application. To address these challenges, an N-doped Fe₇S₈ anode with an N, S co-doped porous carbon framework (PPF-800) was synthesized using a template-assisted method. When serving as an anode for SIBs, it delivers a robust and ultrafast sodium storage performance, with a discharge capacity of 489 mAh g^-1 after 500 cycles at 5 A g^-1 and 371 mAh g^-1 after 1000 cycles at 30 A g^-1 in the ether-based electrolyte. This impressive performance is attributed to the combined influence of heteroatomic doping and adjustable interface engineering. The N, S co-doped carbon framework embedded with Fe₇S₈ nanoparticles effectively addresses the issues of volumetric expansion, reduces the impact of sodium polysulfides, improves intrinsic conductivity, and stimulates the dominant pseudocapacitive contribution (90.3% at 2 mV s^-1). Moreover, the formation of a stable solid electrolyte interface (SEI) film by the effect of uniform pore structure in ether-based electrolyte produces a lower transfer resistance during the charge-discharge process, thereby boosting the rate performance of the electrode material. This work expands a facile strategy to optimize the electrochemical performance of other metal sulfides.

Enhanced Electrochemical Performances of Mn3O4/Heteroatom-Doped Reduced Graphene Oxide Aerogels as an Anode for Sodium-Ion Batteries

Owing to their high theoretical capacity, transition-metal oxides have received a considerable amount of attention as potential anode materials in sodium-ion (Na-ion) batteries. Among them, Mn₃O₄ has gained interest due to the low cost of raw materials and the environmental compatibility. However, during the insertion/de-insertion process, Mn₃O₄ suffers from particle aggregation, poor conductivity, and low-rate capability, which, in turn, limits its practical application. To overcome these obstacles, we have successfully prepared Mn₃O₄ nanoparticles distributed on the nitrogen (N)-doped and nitrogen, sulphur (N,S)-doped reduced graphene oxide (rGO) aerogels, respectively. The highly crystalline Mn₃O₄ nanoparticles, with an average size of 15-20 nm, are homogeneously dispersed on both sides of the N-rGO and N,S-rGO aerogels. The results indicate that the N-rGO and N,S-rGO aerogels could provide an efficient ion transport channel for electrolyte ion stability in the Mn₃O₄ electrode. The Mn₃O₄/N- and Mn₃O₄/N,S-doped rGO aerogels exhibit outstanding electrochemical performances, with a reversible specific capacity of 374 and 281 mAh g^-1, respectively, after 100 cycles, with Coulombic efficiency of almost 99%. The interconnected structure of heteroatom-doped rGO with Mn₃O₄ nanoparticles is believed to facilitate fast ion diffusion and electron transfer by lowering the energy barrier, which favours the complete utilisation of the active material and improvement of the structure's stability.

Ultra-Fine Ruthenium Oxide Quantum Dots/Reduced Graphene Oxide Composite as Electrodes for High-Performance Supercapacitors

This study synthesized ultra-fine nanometer-scaled ruthenium oxide (RuO2) quantum dots (QDs) on reduced graphene oxide (rGO) surface by a facile and rapid microwave-assisted hydrothermal approach. Benefiting from the synergistic effect of RuO2 and rGO, RuO2/rGO nanocomposite electrodes showed ultra-high capacitive performance. The impact of different RuO2 loadings in RuO2/rGO nanocomposite on their electrochemical performance was investigated by various characterizations. The composite RG-2 with 38 wt.% RuO2 loadings exhibited a specific capacitance of 1120 F g-1 at 1 A g-1. In addition, it has an excellent capacity retention rate of 84 % from 1A g-1 to 10 A g-1, and excellent cycling stability of 89% retention after 10,000 cycles, indicating fast ion-involved redox reactions on the nanocomposite surfaces. These results illustrate that RuO2/rGO composites prepared by this facile process can be an ideal candidate electrode for high-performance supercapacitors.

Microwave-Enabled Size Control of Iron Oxide Nanoparticles on Reduced Graphene Oxide

Nanoparticle-functionalized 2D material networks are promising for a wide range of applications, but in situ formation of nanoparticles is commonly challenged by rapid growth. Here, we demonstrate controlled synthesis of small and dispersed iron oxide nanoparticles on reduced graphene oxide (rGO) networks through rapid localized heating with microwaves with low-cost iron nitrate as the precursor. The strong coupling of the microwave radiation with the rGO network rapidly heats the network locally to decompose the iron nitrate and generate iron oxide nanoparticles, while cessation of microwaves leads to rapid cooling to minimize crystal growth. Small changes in the microwave reaction time (<1 min) led to very large changes in the iron oxide morphology. The solid-state microwave syntheses produced narrower nanoparticle size distribution than conventional heating. These results illustrate the potential of solid-state microwave syntheses to control the nanoparticle size on 2D materials through rapid localized heating under the microwave process conditions, which should be extendable to a variety of transition metal oxide-rGO systems.

A review of the microwave-assisted synthesis of carbon nanomaterials, metal oxides/hydroxides and their composites for energy storage applications

Currently, nanomaterials are considered to be the backbone of modern civilization. Especially in the energy sector, nanomaterials (mainly, carbon- and metal oxide/hydroxide-based nanomaterials) have contributed significantly. Among the various green approaches for the synthesis of these nanomaterials, the microwave-assisted approach has attracted significant research interest worldwide. In this context, it is noteworthy to mention that because of their enhanced surface area, high conducting nature, and excellent electrical and electrochemical properties, carbon nanomaterials are being extensively utilized as efficient electrode materials for both supercapacitors and secondary batteries. In this review article, we briefly demonstrate the characteristics of microwave-synthesized nanomaterials for next-generation energy storage devices. Starting with the basics of microwave heating, herein, we illustrate the past and present status of microwave chemistry for energy-related applications, and finally present a brief outlook and concluding remarks. We hope that this review article will positively convey new insights for the microwave synthesis of nanomaterials for energy storage applications.

High-Voltage Cathode α-Fe2O3 Nanoceramics for Rechargeable Sodium-Ion Batteries

Previously, α-Fe2O3 nanocrystals are recognized as anode materials owing to their high capacity and multiple properties. Now, this work provides high-voltage α-Fe2O3 nanoceramics cathodes fabricated by the solvothermal and calcination processes for sodium-ion batteries (SIBs). Then, their structure and electrical conductivity were investigated by the first-principles calculations. Also, the SIB with the α-Fe2O3 nanoceramics cathode exhibits a high initial charge-specific capacity of 692.5 mA h g-1 from 2.0 to 4.5 V at a current density of 25 mA g-1. After 800 cycles, the discharge capacity is still 201.8 mA h g-1, well exceeding the one associated with the present-state high-voltage SIB. Furthermore, the effect of the porous structure of the α-Fe2O3 nanoceramics on sodium ion transport and cyclability is investigated. This reveals that α-Fe2O3 nanoceramics will be a remarkably promising low-cost and pollution-free high-voltage cathode candidate for high-voltage SIBs.

Engineered two-dimensional nanomaterials: an emerging paradigm for water purification and monitoring

Water scarcity has become an increasingly complex challenge with the growth of the global population, economic expansion, and climate change, highlighting the demand for advanced water treatment technologies that can provide clean water in a scalable, reliable, affordable, and sustainable manner. Recent advancements on 2D nanomaterials (2DM) open a new pathway for addressing the grand challenge of water treatment owing to their unique structures and superior properties. Emerging 2D nanostructures such as graphene, MoS₂, MXene, h-BN, g-C₃N₄, and black phosphorus have demonstrated an unprecedented surface-to-volume ratio, which promises ultralow material use, ultrafast processing time, and ultrahigh treatment efficiency for water cleaning/monitoring. In this review, we provide a state-of-the-art account on engineered 2D nanomaterials and their applications in emerging water technologies, involving separation, adsorption, photocatalysis, and pollutant detection. The fundamental design strategies of 2DM are discussed with emphasis on their physicochemical properties, underlying mechanism and targeted applications in different scenarios. This review concludes with a perspective on the pressing challenges and emerging opportunities in 2DM-enabled wastewater treatment and water-quality monitoring. This review can help to elaborate the structure-processing-property relationship of 2DM, and aims to guide the design of next-generation 2DM systems for the development of selective, multifunctional, programmable, and even intelligent water technologies. The global significance of clean water for future generations sheds new light and much inspiration in this rising field to enhance the efficiency and affordability of water treatment and secure a global water supply in a growing portion of the world.

Functionalized M2TiC2Tx MXenes (M = Cr and Mo; T = F, O, and OH) as high performance electrode materials for sodium ion batteries

First-principles calculations were performed to study the electrochemical performance of M2TiC2 (M = Cr or Mo) and M2TiC2Tx (T = O, F or OH) used as anode materials for sodium ion batteries (SIBs). The O functionalized MXenes (Cr2TiC2O2 and Mo2TiC2O2) are found to be more stable than F and OH terminated systems. The diffusion performance of sodium in MXene materials is mainly affected by the functional groups. The lowest diffusion barrier of functionalized MXenes is about one order larger in magnitude than that of bare MXenes. Although the introduction of O-groups hinders the diffusion of sodium, it can greatly improve the theoretical storage capacities. Meanwhile, the diffusion paths and diffusion energy barriers of Na are affected by Na concentration effects, while the interactions between terminations have little effect. Furthermore, multiple layers of sodium atoms are found to be adsorbed between the layers of M2TiC2O2, thus significantly increasing the theoretical capacities. The theoretical sodium storage capacities of M2TiC2O2 monolayers reach 515.70 mA h g-1 (M = Cr) and 362.46 mA h g-1 (M = Mo) and the OCVs can approach 0.034 V (M = Cr) and 0.042 V (M = Mo). Therefore, Cr2TiC2O2 and Mo2TiC2O2 are expected to be promising anode materials for SIBs due to their excellent properties, such as good electronic conductivity, low sodium diffusion barrier, and high theoretical sodium storage capacity.

A carbon-coated shuttle-like Fe2O3/Fe1-x S heterostructure derived from metal-organic frameworks with high pseudocapacitance for ultrafast lithium storage

Pursuing active, low-cost, and stable electrode materials with superior rate capability and long-life cycling performances for lithium-ion batteries remains a big challenge. In this study, a carbon-coated shuttle-like Fe2O3/Fe1-x S heterostructure is synthesized by simply annealing Fe-based metal-organic frameworks (MIL-88(Fe)) as precursors and sublimed sulfur. Carbon-coated Fe2O3/Fe1-x S displays a unique structure with ultrafine Fe2O3/Fe1-x S nanoparticles distributed in the hollow and porous carbon matrix, which offers a large specific surface area and fast charge transfer ability, and alleviates the volume change upon cycling. When evaluated as an anode material for lithium-ion batteries, it exhibits an ultra-high specific capacity of 1200 mA h g-1 at 0.1 A g-1, and superior high rate capability with a capacity of 345 mA h g-1 at a very high current density of 5.0 A g-1 owing to its high electrical conductivity and enhanced pseudocapacitive contribution from surface effects. The current strategy is promising to synthesize the carbon-coated porous structure from metal-organic frameworks for next-generation energy-storage applications.

Effect of Hematite Doping with Aliovalent Impurities on the Electrochemical Performance of α-Fe2O3@rGO-Based Anodes in Sodium-Ion Batteries

The effect of the type of dopant (titanium and manganese) and of the reduced graphene oxide content (rGO, 30 or 50 wt %) of the α-Fe₂O₃@rGO nanocomposites on their microstructural properties and electrochemical performance was investigated. Nanostructured composites were synthesized by a simple one-step solvothermal method and evaluated as anode materials for sodium ion batteries. The doping does not influence the crystalline phase and morphology of the iron oxide nanoparticles, but remarkably increases stability and Coulombic efficiency with respect to the anode based on the composite α-Fe₂O₃@rGO. For fixed rGO content, Ti-doping improves the rate capability at lower rates, whereas Mn-doping enhances the electrode stability at higher rates, retaining a specific capacity of 56 mAhg^-1 at a rate of 2C. Nanocomposites with higher rGO content exhibit better electrochemical performance.

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.

Plasma Enabled Fe2O3/Fe3O4 Nano-aggregates Anchored on Nitrogen-doped Graphene as Anode for Sodium-Ion Batteries

Low electrical conductivity severely limits the application of Fe₂O₃ in lithium- and sodium-ion batteries. In respect of this, we design and fabricate Fe₂O₃/Fe₃O₄ nano-aggregates anchored on nitrogen-doped graphene as an anode for sodium-ion batteries with the assistance of microwave plasma. The highly conductive Fe₃O₄ in the composite can function as a highway of electron transport, and the voids and phase boundaries in the Fe₂O₃/Fe₃O₄ heterostructure facilitate Na⁺ ion diffusion into the nano-aggregates. Furthermore, the Fe–O–C bonds between the nano-aggregates and graphene not only stabilize the structural integrity, but also enhance the charge transfer. Consequently, the Fe₂O₃/Fe₃O₄/NG anode exhibits specific capacity up to 362 mAh g⁻¹ at 100 mA g⁻¹, excellent rate capability, and stable long-term cycling performance. This multi-component-based heterostructure design can be used in anode materials for lithium- and sodium-ion batteries, and potential opens a new path for energy storage electrodes.

Three-dimensional mesoporous γ-Fe2O3@carbon nanofiber network as high performance anode material for lithium- and sodium-ion batteries

Electrode materials that can function well in both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) are desirable for electrochemical energy storage applications, especially under high rate. In this work, a three-dimensional (3D) mesoporous γ-Fe₂O₃@carbon nanofiber (γ-Fe₂O₃@CNF) mat has been successfully synthesized by sol-gel based electrospinning and carbonization. It delivers a specific capacity of 820 mAh g^-1 at 0.5 C after 250 cycles, 430 mAh g^-1 at 6 C after 1000 cycles, and 222 mAh g^-1 at ultrahigh rate of 60 C for LIBs, while for SIBs it delivers a specific capacity of 360 mAh g^-1 at 1 C after 1000 cycles and 130 mAh g^-1 at 60 C. Besides, the result of ex situ microstructure examination shows the polycrystalline nature of γ-Fe₂O₃ nanoparticle still exists in LIB even after 1000 cycles, while it vanishes in SIB, suggesting that the relatively larger volume expansion occurred during Na⁺ insertion/deinsertion, resulting in pulverization of the particles. The CNFs maintained their pristine 3D network structure after the charge/discharge, which demonstrated the critical role of a robust conductive electrode in promoting fast Li⁺/Na⁺ transportation. More importantly, they act as an electrical bridge between Li⁺/Na⁺ and γ-Fe₂O₃ nanoparticles, therefore suppressing the cell impedance growth and γ-Fe₂O₃ volume expansion, resulting in the enhancement in both cyclic and rate capability.

Facile and scalable synthesis of α-Fe2O3/γ-Fe2O3/Fe/C nanocomposite as advanced anode materials for lithium/sodium ion batteries

To develop low-cost advanced anode materials for lithium/sodium ion batteries, the chemical reaction equilibrium of Fe(NO₃)₃ and glucose in hot aqueous solution is creatively used to fabricate a new α-Fe₂O₃/γ-Fe₂O₃/Fe/C nanocomposite with the primary particle sizes concentrated at 25-80 nm. As anodes for lithium ion batteries, it exhibits a discharge capacity of ∼878 mAh g^-1 after 200 cycles at a current density of 200 mA g^-1. Moreover, even after 1000 cycles at a current density of 3200 mA g^-1, the discharge capacity is as high as ∼532 mAh g^-1, with a capacity retention of over than 100% against that of the second cycle. As anodes for sodium ion batteries, the nanocomposite displays a stable discharge capacity of ∼400 mAh g^-1 at a current density of 100 mA g^-1 and no obvious capacity degradation happens after 200 cycles. During cycling, the α-Fe₂O₃/γ-Fe₂O₃/Fe/C nanocomposite electrodes shows high structural stability and relatively faster reaction kinetics, which should be responsible for its excellent electrochemical performance. This work provides a facile and scalable route to synthesize high-performance and low-cost Fe₂O₃-based nanocomposite for the secondary batteries.

Engineering of a bowl-like Si@rGO architecture for an improved lithium ion battery via a synergistic effect

In this work we propose a facile template-sacrificing method to prepare bowl-like silicon@reduced graphene oxide (Si@rGO) hybrids as a high-performance anode for lithium ion batteries (LIBs). Uniform SiO₂ spheres were initially synthesized and wrapped by GO, forming a three-dimensional (3D) skeleton. After reduction and etching, Si nanoparticles were obtained and evenly distributed on the flexible rGO layer, resulting in a bowl-like nanoarchitecture. A benefit of this novel structure is that the volume change of Si can be confined during the charge-discharge process. As a result, the Si@rGO anode exhibited a high first discharge capacity of ∼1890 mAh  g^-1 with a Coulombic efficiency of 90.79% at a current density of 0.1 A  g^-1. After 100 cycles, a stable specific capacity of 450 mAh  g^-1 was achieved, which is twice that of pure Si nanospheres (208 mAh  g^-1) and rGO (260 mAh  g^-1). Moreover, when the current density increased to 1 A g^-1, the specific capacity of Si@rGO was 100 mAh  g^-1, whereas it was 34 mAh  g^-1 for Si nanospheres, demonstrating the advantage of Si@rGO. By analyzing the electrochemical behavior, it is found that the outstanding LIB performance of Si@rGO can be ascribed to the involvement of rGO which constructs the 3D nanoarchitecture that acts as a buffer layer to stabilize the Si and promotes Li⁺ diffusion as well as the conductivity of the electrodes. This work highlights the significance of the microstructure for lithium ion storage performance of Si-based nanocomposites.

Nanohybrids of shuttle-like α-Fe2O3 nanoparticles and nitrogen-doped graphene for simultaneous voltammetric detection of dopamine and uric acid

Shuttle-like α-Fe₂O₃ nanoparticles and nitrogen-doped graphene nanocomposites as a low cost and efficient electrocatalyst for detecting dopamine and uric acid.

N-Doped Modified Graphene/Fe2O3 Nanocomposites as High-Performance Anode Material for Sodium Ion Storage

Sodium-ion storage devices have received widespread attention because of their abundant sodium resources, low cost and high energy density, which verges on lithium-ion storage devices. Electrochemical redox reactions of metal oxides offer a new approach to construct high-capacity anodes for sodium-ion storage devices. However, the poor rate performance, low Coulombic efficiency, and undesirable cycle stability of the redox conversion anodes remain a huge challenge for the practical application of sodium ion energy storage devices due to sluggish kinetics and irreversible structural change of most conversion anodes during cycling. Herein, a nitrogen-doping graphene/Fe₂O₃ (N-GF-300) composite material was successfully prepared as a sodium-ion storage anode for sodium ion batteries and sodium ion supercapacitors through a water bath and an annealing process, where Fe₂O₃ nanoparticles with a homogenous size of about 30 nm were uniformly anchored on the graphene nanosheets. The nitrogen-doping graphene structure enhanced the connection between Fe₂O₃ nanoparticles with graphene nanosheets to improve electrical conductivity and buffer the volume change of the material for high capacity and stable cycle performance. The N-GF-300 anode material delivered a high reversible discharge capacity of 638 mAh g⁻¹ at a current density of 0.1 A g⁻¹ and retained 428.3 mAh g⁻¹ at 0.5 A g⁻¹ after 100 cycles, indicating a strong cyclability of the SIBs. The asymmetrical N-GF-300//graphene SIC exhibited a high energy density and power density with 58 Wh kg⁻¹ at 1365 W kg⁻¹ in organic solution. The experimental results from this work clearly illustrate that the nitrogen-doping graphene/Fe₂O₃ composite material N-GF-300 is a potential feasibility for sodium-ion storage devices, which further reveals that the nitrogen doping approach is an effective technique for modifying carbon matrix composites for high reaction kinetics during cycles in sodium-ion storage devices and even other electrochemical storage devices.

Rational Design and Controllable Synthesis of Multishelled Fe2O3@SnO2@C Nanotubes as Advanced Anode Material for Lithium-/Sodium-Ion Batteries

Hierarchical Fe₂O₃ and SnO₂ nanostructures have shown great potential for applications in high-performance ion batteries because of their superiority, including wide resources, facile preparation, environmental friendliness, and high energy density. However, some severe challenges, such as rapid capacity decay due to volume expansion upon cycling and poor conductivity, limit their rate performance. To address this issue, multishelled Fe₂O₃@SnO₂@C (FSC) nanotubes were designed and synthesized by using a template method and Ostwald interaction. The as-prepared FSC nanotubes can deliver a high capacity of 1659 mA h g^-1 at a current density of 200 mA g^-1 and a high reversible capacity of 818 mA h g^-1 at 2000 mA g^-1 for lithium-ion batteries. Particularly, a high specific capacity of 1024 mA h g^-1 is still maintained after 100 charging/discharging cycles at 200 mA g^-1. Applied in sodium-ion batteries, the multishelled FSC nanotubes manifest a high specific capacity of 449 mA h g^-1 after 180 cycles at 50 mA g^-1. Such excellent performances of the as-fabricated FSC nanotubes may be due to the unique multishelled tubular structure, porous characteristics, and high specific surface area. Therefore, the present work provides an outstanding method to improve the energy storage performance of metal oxide composites and other types of nanocomposites.

Microwave-Assisted Synthesis of SPION-Reduced Graphene Oxide Hybrids for Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) is a useful tool for disease diagnosis and treatment monitoring. Superparamagnetic iron oxide nanoparticles (SPION) show good performance as transverse relaxation (T2) contrast agents, thus facilitating the interpretation of the acquired images. Attachment of SPION onto nanocarriers prevents their agglomeration, improving the circulation time and efficiency. Graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), are appealing nanocarriers since they have both high surface area and functional moieties that make them ideal substrates for the attachment of nanoparticles. We have employed a fast, simple and environmentally friendly microwave-assisted approach for the synthesis of SPION-RGO hybrids. Different iron precursor/GO ratios were used leading to SPION, with a median diameter of 7.1 nm, homogeneously distributed along the RGO surface. Good relaxivity (r2*) values were obtained in MRI studies and no significant toxicity was detected within in vitro tests following GL261 glioma and J774 macrophage-like cells for 24 h with SPION-RGO, demonstrating the applicability of the hybrids as T2-weighted MRI contrast agents.

Recent Progress of Electrochemical Energy Devices: Metal Oxide–Carbon Nanocomposites as Materials for Next-Generation Chemical Storage for Renewable Energy

With the importance of sustainable energy, resources, and environmental issues, interest in metal oxides increased significantly during the past several years owing to their high theoretical capacity and promising use as electrode materials for electrochemical energy devices. However, the low electrical conductivity of metal oxides and their structural instability during cycling can degrade the battery performance. To solve this problem, studies on carbon/metal-oxide composites were carried out. In this review, we comprehensively discuss the characteristics (chemical, physical, electrical, and structural properties) of such composites by categorizing the structure of carbon in different dimensions and discuss their application toward electrochemical energy devices. In particular, one-, two-, and three-dimensional (1D, 2D, and 3D) carbon bring about numerous advantages to a carbon/metal-oxide composite owing to the unique characteristics of each dimension.

Morphology-Dependent Electrochemical Sensing Properties of Iron Oxide-Graphene Oxide Nanohybrids for Dopamine and Uric Acid

Various morphologies of iron oxide nanoparticles (Fe2O3 NPs), including cubic, thorhombic and discal shapes were synthesized by a facile meta-ion mediated hydrothermal route. To further improve the electrochemical sensing properties, discal Fe2O3 NPs with the highest electrocatalytic activity were coupled with graphene oxide (GO) nanosheets. The surface morphology, microstructures and electrochemical properties of the obtained Fe2O3 NPs and Fe2O3/GO nanohybrids were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. As expected, the electrochemical performances were found to be highly related to morphology. The discal Fe2O3 NPs coupled with GO showed remarkable electrocatalytic activity toward the oxidation of dopamine (DA) and uric acid (UA), due to their excellent synergistic effect. The electrochemical responses of both DA and UA were linear to their concentrations in the ranges of 0.02-10 μM and 10-100 μM, with very low limits of detection (LOD) of 3.2 nM and 2.5 nM for DA and UA, respectively. Moreover, the d-Fe2O3/GO nanohybrids showed good selectivity and reproducibility. The proposed d-Fe2O3/GO/GCE realized the simultaneous detection of DA and UA in human serum and urine samples with satisfactory recoveries.

Unique Double-Interstitialcy Mechanism and Interfacial Storage Mechanism in the Graphene/Metal Oxide as the Anode for Sodium-Ion Batteries

Graphene/metal oxides (G/MO) composite materials have attracted much attention as the anode of sodium ion batteries (SIBs), because of the high theoretical capacity. However, most metal oxides operate based on the conversion mechanism and the alloying mechanism has changed to Na₂O after the first cycle. The influence of G/Na₂O (G/N) on the subsequent sodiation process has never been clearly elucidated. In this work, we report a systematic investigation on the G/N interface from both aspects of theoretical simulation and experiment characterization. By applied first-principles simulations, we find that the sluggish kinetics in the G/MO materials is mainly caused by the high diffusion barrier (0.51 eV) inside the Na₂O bulk, while the G/N interface shows a much faster transport kinetics (0.25 eV) via unique double-interstitialcy mechanism. G/N interface possesses an interfacial storage of Na atom through the charge separation mechanism. The experimental evidence confirms that high interfacial ratio structure of G/N greatly improves the rate performance and endows G/MO materials the interfacial storage. Furthermore, the experimental investigation finds that the high interfacial ratio structure of G/N also benefits from the reversible reaction between SnO₂ and Sn during cycling. Lastly, the effects of (N, O, S) doping in graphene systems at the G/N interface were also explored. This work provides a fundamental comprehension on the G/MO interface structure during the sodiation process, which is helpful to design energy storage materials with high rate performance and large capacity.

Precise control of the interlayer spacing between graphene sheets by hydrated cations

Based on DFT computations, we show that different hydrated cations can precisely control the interlayer spacings between graphene sheets, which are smaller than that between graphene oxide sheets, indicating an ion sieving.

FeO x -Based Materials for Electrochemical Energy Storage

Iron oxides (FeO x ), such as Fe2O3 and Fe3O4 materials, have attracted much attention because of their rich abundance, low cost, and environmental friendliness. However, FeO x , which is similar to most transition metal oxides, possesses a poor rate capability and cycling life. Thus, FeO x -based materials consisting of FeO x , carbon, and metal-based materials have been widely explored. This article mainly discusses FeO x -based materials (Fe2O3 and Fe3O4) for electrochemical energy storage applications, including supercapacitors and rechargeable batteries (e.g., lithium-ion batteries and sodium-ion batteries). Furthermore, future perspectives and challenges of FeO x -based materials for electrochemical energy storage are briefly discussed.

The State and Challenges of Anode Materials Based on Conversion Reactions for Sodium Storage

Sodium-ion batteries (SIBs) have huge potential for applications in large-scale energy storage systems due to their low cost and abundant sources. It is essential to develop new electrode materials for SIBs with high performance in terms of energy density, cycle life, and cost. Metal binary compounds that operate through conversion reactions hold promise as advanced anode materials for sodium storage. This Review highlights the storage mechanisms and advantages of conversion-type anode materials and summarizes their recent development. Although conversion-type anode materials have high theoretical capacities and abundant varieties, they suffer from multiple challenging obstacles to realize commercial applications, such as low reversible capacity, large voltage hysteresis, low initial coulombic efficiency, large volume changes, and low cycling stability. These key challenges are analyzed in this Review, together with emerging strategies to overcome them, including nanostructure and surface engineering, electrolyte optimization, and battery configuration designs. This Review provides pertinent insights into the prospects and challenges for conversion-type anode materials, and will inspire their further study.

Recent Progress in Iron-Based Electrode Materials for Grid-Scale Sodium-Ion Batteries

Grid-scale energy storage batteries with electrode materials made from low-cost, earth-abundant elements are needed to meet the requirements of sustainable energy systems. Sodium-ion batteries (SIBs) with iron-based electrodes offer an attractive combination of low cost, plentiful structural diversity and high stability, making them ideal candidates for grid-scale energy storage systems. Although various iron-based cathode and anode materials have been synthesized and evaluated for sodium storage, further improvements are still required in terms of energy/power density and long cyclic stability for commercialization. In this Review, progress in iron-based electrode materials for SIBs, including oxides, polyanions, ferrocyanides, and sulfides, is briefly summarized. In addition, the reaction mechanisms, electrochemical performance enhancements, structure-composition-performance relationships, merits and drawbacks of iron-based electrode materials for SIBs are discussed. Such iron-based electrode materials will be competitive and attractive electrodes for next-generation energy storage devices.

Sodium-ion batteries: present and future

Energy production and storage technologies have attracted a great deal of attention for day-to-day applications. In recent decades, advances in lithium-ion battery (LIB) technology have improved living conditions around the globe. LIBs are used in most mobile electronic devices as well as in zero-emission electronic vehicles. However, there are increasing concerns regarding load leveling of renewable energy sources and the smart grid as well as the sustainability of lithium sources due to their limited availability and consequent expected price increase. Therefore, whether LIBs alone can satisfy the rising demand for small- and/or mid-to-large-format energy storage applications remains unclear. To mitigate these issues, recent research has focused on alternative energy storage systems. Sodium-ion batteries (SIBs) are considered as the best candidate power sources because sodium is widely available and exhibits similar chemistry to that of LIBs; therefore, SIBs are promising next-generation alternatives. Recently, sodiated layer transition metal oxides, phosphates and organic compounds have been introduced as cathode materials for SIBs. Simultaneously, recent developments have been facilitated by the use of select carbonaceous materials, transition metal oxides (or sulfides), and intermetallic and organic compounds as anodes for SIBs. Apart from electrode materials, suitable electrolytes, additives, and binders are equally important for the development of practical SIBs. Despite developments in electrode materials and other components, there remain several challenges, including cell design and electrode balancing, in the application of sodium ion cells. In this article, we summarize and discuss current research on materials and propose future directions for SIBs. This will provide important insights into scientific and practical issues in the development of SIBs.

Nitrogen-Doped TiO2-C Composite Nanofibers with High-Capacity and Long-Cycle Life as Anode Materials for Sodium-Ion Batteries

Nitrogen-doped TiO₂-C composite nanofibers (TiO₂/N-C NFs) were manufactured by a convenient and green electrospinning technique in which urea acted as both the nitrogen source and a pore-forming agent. The TiO₂/N-C NFs exhibit a large specific surface area (213.04 m² g^-1) and a suitable nitrogen content (5.37 wt%). The large specific surface area can increase the contribution of the extrinsic pseudocapacitance, which greatly enhances the rate capability. Further, the diffusion coefficient of sodium ions (D _Na+) could be greatly improved by the incorporation of nitrogen atoms. Thus, the TiO₂/N-C NFs display excellent electrochemical properties in Na-ion batteries. A TiO₂/N-C NF anode delivers a high reversible discharge capacity of 265.8 mAh g^-1 at 0.05 A g^-1 and an outstanding long cycling performance even at a high current density (118.1 mAh g^-1) with almost no capacity decay at 5 A g^-1 over 2000 cycles. Therefore, this work sheds light on the application of TiO₂-based materials in sodium-ion batteries.

Carbon-Encapsulated Sn@N-Doped Carbon Nanotubes as Anode Materials for Application in SIBs

Carbon-encapsulated Sn@N-doped carbon tubes with submicron diameters were obtained via the simple reduction of C@SnO₂@N-doped carbon composites that were fabricated by a hydrothermal approach. Sn nanoparticles encapsulated in carbon layers were distributed uniformly on the surfaces of the N-doped carbon nanotubes. The electrochemical performances of the composites were systematically investigated as anode materials in sodium-ion batteries (SIBs). The composite electrode could attain a good reversible capacity of 398.4 mAh g^-1 when discharging at 100 mA g^-1, with capacity retention of 67.3% and very high Coulombic efficiency of 99.7% over 150 cycles. This good cycling performance, when compared to only 17.5 mAh g^-1 delivered by bare Sn particles prepared via the same method without the presence of N-doped carbon, could be mainly ascribed to the uniform distribution of the precursor SnO₂ on the substrate of N-doped carbon tubes with three-dimensional structure, which provides more reaction sites to reduce the diffusion distance of Na⁺, further facilitating Na⁺-ion diffusion and relieves the huge volume expansion during charging/discharging. These outcomes imply that such a Sn/C composite would provide more options as an anode candidate for SIBs.

Interconnected CoFe2O4-Polypyrrole Nanotubes as Anode Materials for High Performance Sodium Ion Batteries

CoFe₂O₄-coated polypyrrole (PPy) nanotubes (CFO-PPy-NTs) with three-dimensional (3-D) interconnected networks have been prepared through a simple hydrothermal method. The application has been also studied for sodium ion batteries (SIBs). The finely crystallized CoFe₂O₄ nanoparticles (around 5 nm in size) are uniformly grown on the PPy nanotubes. When tested as anode materials for SIBs, the CFO-PPy-NT electrode maintains a discharge capacity of 400 mA h g^-1 and a stable Coulombic efficiency of 98% after 200 cycles at 100 mA g^-1. Even at a higher current density of 1000 mA g^-1, the composite can still retain a discharge capacity of 220 mA h g^-1 after 2000 cycles. The superior electrochemical performance could be mainly ascribed to the uniform distribution of CoFe₂O₄ on the 3-D matrix of PPy interconnected nanotubes, which favors the diffusion of sodium ions and electronic transportation and also buffers the large volumetric expansion during charge/discharge. Thereby our study suggests that such CFO-PPy-NTs have great potential as an anode material for SIBs.

Iso-Oriented NaTi2(PO4)3 Mesocrystals as Anode Material for High-Energy and Long-Durability Sodium-Ion Capacitor

Sodium-ion capacitors (SIC) combine the merits of both high-energy batteries and high-power electrochemical capacitors as well as the low cost and high safety. However, they are also known to suffer from the severe deficiency of suitable electrode materials with high initial Coulombic efficiency (ICE) and kinetic balance between both electrodes. Herein, we report a facile solvothermal synthesis of NaTi₂(PO₄)₃ nanocages constructed by iso-oriented tiny nanocrystals with a mesoporous architecture. It is notable that the NaTi₂(PO₄)₃ mesocrystals exhibit a large ICE of 94%, outstanding rate capability (98 mA h g^-1 at 10 C), and long cycling life (over 77% capacity retention after 10 000 cycles) in half cells, all of which are in favor to be utilized into a full cell. When assembled with commercial activated carbon to an SIC, the system delivers an energy density of 56 Wh kg^-1 at a power density of 39 W kg^-1. Even at a high current rate of 5 A g^-1 (corresponds to finish a full charge/discharge process in 2 min), the SIC still works well after 20 000 cycles without obvious capacity degradation. With the merits of impressive energy/power densities and longevity, the obtained hybrid capacitor should be a promising device for highly efficient energy storage systems.

In Situ Grown Fe2O3 Single Crystallites on Reduced Graphene Oxide Nanosheets as High Performance Conversion Anode for Sodium-Ion Batteries

Electrochemical conversion reactions of metal oxides provide a new avenue to build high capacity anodes for sodium-ion batteries. However, the poor rate performance and cyclability of these conversion anodes remain a significant challenge for Na-ion battery applications because most of the conversion anodes suffer from sluggish kinetics and irreversible structural change during cycles. In this paper, we report an Fe₂O₃ single crystallites/reduced graphene oxide composite (Fe₂O₃/rGO), where the Fe₂O₃ single crystallites with a particle size of ∼300 nm were uniformly anchored on the rGO nanosheets, which provide a highly conductive framework to facilitate electron transport and a flexible matrix to buffer the volume change of the material during cycling. This Fe₂O₃/rGO composite anode shows a very high reversible capacity of 610 mAh g^-1 at 50 mA g^-1, a high Coulombic efficiency of 71% at the first cycle, and a strong cyclability with 82% capacity retention after 100 cycles, suggesting a potential feasibility for sodium-ion batteries. More significantly, the present work clearly illustrates that an electrochemical conversion anode can be made with high capacity utilization, strong rate capability, and stable cyclability through appropriately tailoring the lattice structure, particle size, and electronic conduction channels for a simple transition-metal oxide, thus offering abundant selections for development of low-cost and high-performance Na-storage electrodes.