Co3O4 Nanomaterials in Lithium‐Ion Batteries and Gas Sensors

Co₃O₄ nanotubes, nanorods, and nanoparticles are used as the anode materials of lithium‐ion batteries. The results show that the Co₃O₄ nanotubes prepared by a porous‐alumina‐template method display high discharge capacity and superior cycling reversibility. Furthermore, Co₃O₄ nanotubes exhibit excellent sensitivity to hydrogen and alcohol, owing to their hollow, nanostructured character.     

Growth and Electrochemical Characterization versus Lithium of Fe3O4 Electrodes Made by Electrodeposition

Interest in binary oxides has been renewed because of their novel reactivity towards Li at low potential, which leads to large capacity gains. Here, the structural, morphological, and electrochemical properties of copper‐supported Fe₃O₄ deposits prepared by cathodic reduction of a Fe^III chelate in alkaline solution are reported. By tuning the deposit growth parameters, namely the deposition time and the temperature of the electrolytic bath, it is possible to prepare deposits with various morphologies. Thick deposits with well‐faceted particles of Fe₃O₄ are produced when long deposition times are used; thin and shapeless deposits are formed after shorter electrolysis times. A screening study shows Fe₃O₄ films prepared at 50 °C under –5 mA cm^–2 for 40 s give the best electrochemical performance towards Li, namely with a sustained reversible capacity for over 50 cycles and outstanding rate capability even after 50 repeated charge–discharge sequences. Electrodeposition techniques provide an alternative efficient way to configure high‐performance conversion electrodes based on carbon‐free binary oxides.     

Nanophase ZnCo2O4 as a High Performance Anode Material for Li‐Ion Batteries

ZnCo₂O₄ has been synthesized by the low‐temperature and cost‐effective urea combustion method. X‐ray diffraction (XRD), HR‐TEM and selected area electron diffraction (SAED) studies confirmed its formation in pure and nano‐phase form with particle size ～ 15–20 nm. Galvanostatic cycling of nano‐ZnCo₂O₄ in the voltage range 0.005–3.0 V versus Li at 60 mA g^–1 gave reversible capacities of 900 and 960 mA h g^–1, when cycled at 25 °C and 55 °C, respectively. These values correspond to ～ 8.3 and ～ 8.8 mol of recyclable Li per mole of ZnCo₂O₄. Almost stable cycling performance was exhibited in the range 5–60 cycles at 60 mA g^–1 and at 25 °C with ～ 98 % coulombic efficiency. A similar cycling stability at 55 °C, and good rate‐capability both at 25 and 55 °C were found. The average discharge‐ and charge‐potentials were ～ 1.2 V and ～ 1.9 V, respectively. The ex‐situ‐XRD, ‐HRTEM, ‐SAED and galvanostatic cycling data are consistent with a reaction mechanism for Li‐recyclability involving both de‐alloying‐alloying of Zn and displacement reactions, viz., LiZn ? Zn ? ZnO and Co ? CoO ? Co₃O₄. For the first time we have shown that both Zn‐ and Co‐ions act as mutual beneficial matrices and reversible capacity contribution of Zn through both alloy formation and displacement reaction takes place to yield stable and high capacities. Thus, nano‐ZnCo₂O₄ ranks among the best oxide materials with regard to Li‐recyclability.     

Rambutan‐Like Hybrid Hollow Spheres of Carbon Confined Co3O4 Nanoparticles as Advanced Anode Materials for Sodium‐Ion Batteries

Transition metal oxides, possessing high theoretical specific capacities, are promising anode materials for sodium‐ion batteries. However, the sluggish sodiation/desodiation kinetics and poor structural stability restrict their electrochemical performance. To achieve high and fast Na storage capability, in this work, rambutan‐like hybrid hollow spheres of carbon confined Co₃O₄ nanoparticles are synthesized by a facile one‐pot hydrothermal treatment with postannealing. The hierarchy hollow structure with ultrafine Co₃O₄ nanoparticles embedded in the continuous carbon matrix enables greatly enhanced structural stability and fast electrode kinetics. When tested in sodium‐ion batteries, the hollow structured composite electrode exhibits an outstandingly high reversible specific capacity of 712 mAh g^?1 at a current density of 0.1 A g^?1, and retains a capacity of 223 mAh g^?1 even at a large current density of 5 A g^?1. Besides the superior Na storage capability, good cycle performance is demonstrated for the composite electrode with 74.5% capacity retention after 500 cycles, suggesting promising application in advanced sodium‐ion batteries.     

Template‐Assisted Formation of Rattle‐type V2O5 Hollow Microspheres with Enhanced Lithium Storage Properties

In this work, rattle‐type ball‐in‐ball V₂O₅ hollow microspheres are controllably synthesized with the assistance of carbon colloidal spheres as hard templates. Carbon spheres@vanadium‐precursor (CS@V) core–shell composite microspheres are first prepared through a one‐step solvothermal method. The composition of solvent for the solvothermal synthesis has great influence on the morphology and structure of the vanadium‐precursor shells. V₂O₅ hollow microspheres with various shell architectures can be obtained after removing the carbon microspheres by calcination in air. Moreover, the interior hollow shell can be tailored by varying the temperature ramping rate and calcination temperature. The rattle‐type V₂O₅ hollow microspheres are evaluated as a cathode material for lithium‐ion batteries, which manifest high specific discharge capacity, good cycling stability and rate capability.     

Non‐Close‐Packed Crystals from Self‐Assembled Polystyrene Spheres by Isotropic Plasma Etching: Adding Flexibility to Colloid Lithography

Hexagonally ordered arrays of non‐close‐packed nanoscaled spherical polystyrene (PS) particles are prepared exhibiting precisely controlled diameters and interparticle distances. For this purpose, a newly developed isotropic plasma etching process is applied to extended monolayers of PS colloids (starting diameters <300 nm) deposited onto hydrophilic silicon. Accurate size, shape, and smoothness control of such particles is accomplished by etching at low temperatures (?150 °C) with small rates not usually available in standard reactive ion etching equipment. The applicability of such PS arrays as masks for subsequent pattern transfer is demonstrated by fabricating arrays of cylindrical nanopores into Si.     

Engineering Hierarchical Hollow Nickel Sulfide Spheres for High‐Performance Sodium Storage

Sodium‐ion batteries (SIBs) are considered as promising alternatives to lithium‐ion batteries (LIBs) for energy storage due to the abundance of sodium, especially for grid distribution systems. The practical implementation of SIBs, however, is severely hindered by their low energy density and poor cycling stability due to the poor electrochemical performance of the existing electrodes. Here, to achieve high‐capacity and durable sodium storage with good rate capability, hierarchical hollow NiS spheres with porous shells composed of nanoparticles are designed and synthesized by tuning the reaction parameters. The formation mechanism of this unique structure is systematically investigated, which is clearly revealed to be Ostwald ripening mechanism on the basis of the time‐dependent morphology evolution. The hierarchical hollow structure provides sufficient electrode/electrolyte contact, shortened Na⁺ diffusion pathways, and high strain‐tolerance capability. The hollow NiS spheres deliver high reversible capacity (683.8 mAh g^?1 at 0.1 A g^?1), excellent rate capability (337.4 mAh g^?1 at 5 A g^?1), and good cycling stability (499.9 mAh g^?1 with 73% retention after 50 cycles at 0.1 A g^?1).     

Local Built‐In Electric Field Enabled in Carbon‐Doped Co3O4 Nanocrystals for Superior Lithium‐Ion Storage

In this work, a novel concept of introducing a local built‐in electric field to facilitate lithium‐ion transport and storage within interstitial carbon (C‐) doped nanoarchitectured Co₃O₄ electrodes for greatly improved lithium‐ion storage properties is demonstrated. The imbalanced charge distribution emerging from the C‐dopant can induce a local electric field, to greatly facilitate charge transfer. Via the mechanism of “surface locking” effect and in situ topotactic conversion, unique sub‐10 nm nanocrystal‐assembled Co₃O₄ hollow nanotubes (HNTs) are formed, exhibiting excellent structural stability. The resulting C‐doped Co₃O₄ HNT‐based electrodes demonstrate an excellent reversible capacity ≈950 mA h g^?1 after 300 cycles at 0.5 A g^?1 and superior rate performance with ≈853 mA h g^?1 at 10 A g^?1.     

Crystallinity Control of a Nanostructured LiNi0.5Mn1.5O4 Spinel via Polymer‐Assisted Synthesis: A Method for Improving Its Rate Capability and Performance in 5 V Lithium Batteries

Li–Ni–Mn spinels of nominal composition LiNi_0.5Mn_1.5O₄, which are functional materials for electrodes in high‐voltage lithium batteries, are prepared by thermal decomposition of mixed nanocrystalline oxalates obtained by grinding hydrated salts and oxalic acid in the presence of polyethyleneglycol 400. Their structure, microstructure, and texture are established from combined X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction, transmission electron microscopy (TEM), IR spectroscopy, and N₂ absorption measurements. The polymer tailors the shape of particles, which adopt a nanorodlike morphology at low temperatures (400 °C). In fact, the nanorods consist of highly distorted oriented nanocrystals connected by a polymer‐based film as inferred from IR and XPS spectra. The electrochemical properties of spinels in this peculiar form are quite poor, mainly as a result of the high microstrain content of their nanocrystals. Raising the temperature up to 800 °C partially destroys the nanorods, which become highly crystalline nanoparticles approximately 80 nm in size. At this temperature, the polymer facilitates crystal growth; this leads to highly crystalline polyhedral nanoparticles as revealed from TEM images and microstrain data. Following functionalization as a cathode in lithium cells, this material exhibits a very good rate capability, coulombic efficiency, and capacity retention even upon cycling at voltages as high as 5 V. Moreover, it withstands fast‐charge–slow‐discharge processes, which is an important cycle‐life‐related property for commercial batteries.     

Co3O4 Nanostructures with Different Morphologies and their Field‐Emission Properties

We report an efficient method to synthesize vertically aligned Co₃O₄ nanostructures on the surface of cobalt foils. This synthesis is accomplished by simply heating the cobalt foils in the presence of oxygen gas. The resultant morphologies of the nanostructures can be tailored to be either one‐dimensional nanowires or two‐dimensional nanowalls by controlling the reactivity and the diffusion rate of the oxygen species during the growth process. A possible growth mechanism governing the formation of such nanostructures is discussed. The field‐emission properties of the as‐synthesized nanostructures are investigated in detail. The turn‐on field was determined to be 6.4 and 7.7 V μm^–1 for nanowires and nanowalls, respectively. The nanowire samples show superior field‐emission characteristics with a lower turn‐on field and higher current density because of their sharp tip geometry and high aspect ratio.     

Ultrathin‐Nanosheet‐Induced Synthesis of 3D Transition Metal Oxides Networks for Lithium Ion Battery Anodes

A general ultrathin‐nanosheet‐induced strategy for producing a 3D mesoporous network of Co₃O₄ is reported. The fabrication process introduces a 3D N‐doped carbon network to adsorb metal cobalt ions via dipping process. Then, this carbon matrix serves as the sacrificed template, whose N‐doping effect and ultrathin nanosheet features play critical roles for controlling the formation of Co₃O₄ networks. The obtained material exhibits a 3D interconnected architecture with large specific surface area and abundant mesopores, which is constructed by nanoparticles. Merited by the optimized structure in three length scales of nanoparticles–mesopores–networks, this Co₃O₄ nanostructure possesses superior performance as a LIB anode: high capacity (1033 mAh g^?1 at 0.1 A g^?1) and long‐life stability (700 cycles at 5 A g^?1). Moreover, this strategy is verified to be effective for producing other transition metal oxides, including Fe₂O₃, ZnO, Mn₃O₄, NiCo₂O₄, and CoFe₂O₄.     

High‐Rate LiFePO4 Electrode Material Synthesized by a Novel Route from FePO4 · 4H2O

A LiFePO₄ material with ordered olivine structure is synthesized from amorphous FePO₄ · 4H₂O through a solid–liquid phase reaction using (NH₄)₂SO₃ as the reducing agent, followed by thermal conversion of the intermediate NH₄FePO₄ in the presence of LiCOOCH₃ · 2H₂O. Simultaneous thermogravimetric–differential thermal analysis indicates that the crystallization temperature of LiFePO₄ is about 437 °C. Ellipsoidal particle morphology of the resulting LiFePO₄ powder with a particle size mainly in the range 100–300 nm is observed by using scanning electron microscopy and transmission electron microscopy. As an electrode material for rechargeable lithium batteries, the LiFePO₄ sample delivers a discharge capacity of 167 mA h g^–1 at a constant current of 17 mA g^–1 (0.1 C rate; throughout this study n C rate means that rated capacity of LiFePO₄ (170 mA h g^–1) is charged or discharged completely in 1/n hours), approaching the theoretical value of 170 mA h g^–1. Moreover, the electrode shows excellent high‐rate charge and discharge capability and high electrochemical reversibility. No capacity loss can be observed up to 50 cycles under 5 C and 10 C rate conditions. With a conventional charge mode, that is, 5 C rate charging to 4.2 V and then keeping this voltage until the charge current is decreased to 0.1 C rate, a discharge capacity of ca. 134 mA h g^–1 and cycling efficiency of 99.2–99.6 % can be obtained at 5 C rate.     

Template‐Based Engineering of Carbon‐Doped Co3O4 Hollow Nanofibers as Anode Materials for Lithium‐Ion Batteries

Co₃O₄ anode materials exhibit poor conductivity and a large volume change, rendering controlling of their nanostructure essential to optimize their lithium storage performance. Carbon‐doped Co₃O₄ hollow nanofibers (C‐doped Co₃O₄ HNFs), for the first time are synthesized using bifunctional polymeric nanofibers as template and carbon source. Compared with undoped Co₃O₄ HNFs and solid Co₃O₄ NFs, C‐doped Co₃O₄ HNFs feature a remarkably high specific capacity, excellent cycling stability, and superior rate capacity as anode materials for lithium‐ion batteries. The superior performance of C‐doped Co₃O₄ HNFs electrodes can be attributed to their structural features, which confer enhanced electron transportation and Li⁺ ion diffusion due to C‐doping, and tolerance for volume change due to the 1D hollow structure. Density functional theory calculations provide a good explanation of the observed enhanced conductivity in C‐doped Co₃O₄ HNFs.     

In Situ Generated Gas Bubble‐Directed Self‐Assembly: Synthesis,and Peculiar Magnetic and Electrochemical Properties of Vertically Aligned Arrays of High‐Density Co3O4 Nanotubes

A novel and versatile gas bubble induced self‐assembly technique is developed for the one‐step fabrication of vertically aligned polycrystalline Co₃O₄ nanotube arrays (NTAs) by the rapid thermal decomposition of Co(NO₃)₂·6H₂O on a flat substrate. In this protocol, the in situ generation and release of gas bubbles, which can be regulated by elaborately adjusting the kinetic factors such as reaction time, decomposition temperature and pressure as well as the content of the chemically adsorbed water, play a vital role in the formation of the Co₃O₄ NTAs. Due to the shape anisotropy, ordered hierarchically porous structure and high surface area, the as‐obtained Co₃O₄ NTAs show unique magnetic properties of a low Néel temperature and a large exchange bias field, as well as an initial discharge capacity up to 1293 mAh·g^?1 at 35 mA·g^?1 and the retention of a charge capacity as high as 895.4 mAh·g^?1 after 10 cycles. This endows them with important potential use in magnetic shielding, magnetic recording media, and lithium ion batteries, etc. Due to the simplicity of the self‐assembly method, this process is applicable to the large‐scale production of the Co₃O₄ NTAs, and may be extended to other materials.     

Synthesis of Uniquely Structured SnO2 Hollow Nanoplates and Their Electrochemical Properties for Li‐Ion Storage

A new mechanism for the transformation of nanostructured metal selenides into uniquely structured metal oxides via the Kirkendall effect, which results from the different diffusion rates of metal and Se ions and O₂ gas, is proposed. SnSe nanoplates are selected as the first target material and transformed into SnO₂ hollow nanoplates by the Kirkendall effect. SnSe‐C composite powder, in which SnSe nanoplates are attached or stuck to amorphous carbon microspheres, transforms into several tens of SnO₂ hollow nanoplates by a thermal oxidation process under an air atmosphere. Core–shell‐structured SnSe‐SnSe₂@SnO₂, SnSe₂@SnO₂, Se‐SnSe₂@SnO₂, and Se@SnO₂ and yolk–shell‐structured Se@void@SnO₂ intermediates are formed step‐by‐step during the oxidation of the SnSe nanoplates. The uniquely structured SnO₂ hollow nanoplates have superior cycling and rate performance for Li‐ion storage. Additionally, their discharge capacities at the 2nd and 600th cycles are 598 and 500 mA h g^‐1, respectively, and the corresponding capacity retention measured from the 2nd cycle is as high as 84%.     

Dual Electrostatic Assembly of Graphene Encapsulated Nanosheet‐Assembled ZnO‐Mn‐C Hollow Microspheres as a Lithium Ion Battery Anode

Graphene encapsulated nanosheet‐assembled ZnO‐Mn‐C hierarchical hollow microspheres are produced through a simple yet effective dual electrostatic assembly strategy, followed by a heating treatment in inert atmosphere. The modification of graphene sheets, metal Mn, and in situ carbon leads to form 3D interconnected conductive framework as electron highways. The hollow structure and the open configuration of hierarchical microspheres guarantee good structural stability and rapid ionic transport. More importantly, according to the density functional theory calculations, the oxygen vacancies in the hierarchical microspheres would cause an imbalanced charge distribution and thus the formation of local in‐plane electric fields around oxygen vacancy sites, which is beneficial for the ionic/electronic transport during cycling. Due to this multiscale coordinated design, the as‐fabricated graphene encapsulated nanosheet‐assembled ZnO‐Mn‐C hierarchical hollow microspheres demonstrate good lithium storage properties in terms of high reversible capacity (1094 mA h g^?1 at 100 mA g^?1), outstanding high‐rate long‐term cycling stability (843 mA h g^?1 after 1000 cycles at 2000 mA g^?1), and remarkable rate capability (422 mA h g^?1 after total 1600 cycles at 5000 mA g^?1).     

Three‐Dimensional Co3O4@MnO2 Hierarchical Nanoneedle Arrays: Morphology Control and Electrochemical Energy Storage

In this paper, a highly ordered three‐dimensional Co₃O₄@MnO₂ hierarchical porous nanoneedle array on nickel foam is fabricated by a facile, stepwise hydrothermal approach. The morphologies evolution of Co₃O₄ and Co₃O₄@MnO₂ nanostructures upon reaction times and growth temperature are investigated in detail. Moreover, the as‐prepared Co₃O₄@MnO₂ hierarchical structures are investigated as anodes for both supercapacitors and Li‐ion batteries. When used for supercapacitors, excellent electrochemical performances such as high specific capacitances of 932.8 F g^?1 at a scan rate of 10 mV s^?1 and 1693.2 F g^?1 at a current density of 1 A g^?1 as well as long‐term cycling stability and high energy density (66.2 W h kg^?1 at a power density of 0.25 kW kg^?1), which are better than that of the individual component of Co₃O₄ nanoneedles and MnO₂ nanosheets, are obtained. The Co₃O₄@MnO₂ NAs are also tested as anode material for LIBs for the first time, which presents an improved performance with high reversible capacity of 1060 mA h g^?1 at a rate of 120 mA g^?1, good cycling stability, and rate capability.     

Graphene‐Mimicking 2D Porous Co3O4 Nanofoils for Lithium Battery Applications

2D nanoscale oxides have attracted a large amount of research interest due to their unique properties. Here, a facile synthetic approach to prepare graphene‐mimicking, porous 2D Co₃O₄ nanofoils using graphene oxide (GO) as a sacrificial template is reported. The thermal instability of graphene, as well as the catalytic ability of Co₃O₄ particles to degrade carbon backbones, allow the fabrication of porous 2D Co₃O₄ nanofoils without the loss of the 2D nature of GO. Based on these results, a graphene mimicking as a route for large‐area 2D transition metal oxides for applications in electrochemical energy storage devices is proposed. As a proof of concept, it is demonstrated that graphene‐like, porous 2D Co₃O₄ nanofoils exhibit a high reversible capacity (1279.2 mAh g^?1), even after 50 cycles. This capacity is far beyond the theoretical capacity of Co₃O₄ based on the conversion mechanism from Co₃O₄ to Li₂O and metallic Co.