Rapid Amorphization in Metastable CoSeO3·H2O Nanosheets for Ultrafast Lithiation Kinetics

Achieving high-capacity aqueous supercapacitors via anion-doped construction of dual redox centers in NixCo1-xSeO3

In asymmetric supercapacitors, transition metal selenates are promising electrodes, but their capacity are limited by a single redox center. To further enhance the performance of transition metal selenates, NixCo1-xSeO3 (NCSeO) doped with N and Cl was prepared on nickel-plated carbon cloth (NCSeO-NCl-NiCC). During electrochemical reactions, NCSeO can be converted to M(OH)2 (M = Ni/Co) and OH- is replaced by N and Cl. Two redox centers, M(OH)2/MOOH and M(OH)xN2-x/NO3-, are formed during charging and discharging, which is attributed to the increased capacity of the NCSeO-NCl-NiCC electrode. On NCSeO, the substitution of Cl facilitates the regulation of the electronic structure and enhances the stability of N-doping. The optimised electrode exhibits a high capacity of 417 mA h g-1 at 1 A g-1 and an impressive rate capability of 235 mA h g-1 at 50 A g-1. Asymmetric supercapacitors with this design have an ultra-high energy density of 73.6 W h kg-1, as well as an excellent rate and cycling performance with a capacitance retention of 97.8% after 20 000 cycles at a current density of 20 A g-1.

Emerging two dimensional metastable-phase oxides: insights and prospects in synthesis and catalysis

Since the discovery of graphene, the development of new two-dimensional (2D) materials has received considerable interest. Recently, as a newly emerging member of the 2D family, 2D metastable-phase oxides that combine the unique advantages of metal oxides, 2D structures, and metastable-phase materials have shown enormous potential in various catalytic reactions. In this review, the potential of various 2D materials to form a metastable-phase is predicted. The advantages of 2D metastable-phase oxides for advanced applications, reliable methods of synthesizing 2D metastable-phase oxides, and the application of these oxides in different catalytic reactions are presented. Finally, the challenges associated with 2D metastable-phase oxides and future perspectives are discussed.

Single-Phase Ternary Compounds with a Disordered Lattice and Liquid Metal Phase for High-Performance Li-Ion Battery Anodes

Si is considered as the promising anode materials for lithium-ion batteries (LIBs) owing to their high capacities of 4200 mAh g-1 and natural abundancy. However, severe electrode pulverization and poor electronic and Li-ionic conductivities hinder their practical applications. To resolve the afore-mentioned problems, we first demonstrate a cation-mixed disordered lattice and unique Li storage mechanism of single-phase ternary GaSiP2 compound, where the liquid metallic Ga and highly reactive P are incorporated into Si through a ball milling method. As confirmed by experimental and theoretical analyses, the introduced Ga and P enables to achieve the stronger resistance against volume variation and metallic conductivity, respectively, while the cation-mixed lattice provides the faster Li-ionic diffusion capability than those of the parent GaP and Si phases. The resulting GaSiP2 electrodes delivered the high specific capacity of 1615 mAh g-1 and high initial Coulombic efficiency of 91%, while the graphite-modified GaSiP2 (GaSiP2@C) achieved 83% of capacity retention after 900 cycles and high-rate capacity of 800 at 10,000 mA g-1. Furthermore, the LiNi0.8Co0.1Mn0.1O2//GaSiP2@C full cells achieved the high specific capacity of 1049 mAh g-1 after 100 cycles, paving a way for the rational design of high-performance LIB anode materials.

Rapid Amorphization in MOF/Metal Selenite Nanocomposites for Enhanced Capacity in Supercapacitors

MOF/inorganic nanocomposites combine the advantages of each component. Herein, two MOF/metal selenite nanocomposites, Co-NH₂-BDC/CoSeO₃·H₂O and Co-BDC/CoSeO₃·H₂O, are prepared on nickel foam through a facile two-step hydrothermal method, which inherit the 2D morphology and porosity properties of their MOF precursors. Furthermore, during the electrochemical activation process, the crystallized nanocomposites can easily transform into amorphous structures in a short time of 20 min in the presence of an electric field, similar to CoSeO₃·H₂O. Due to amorphization, the electrochemical performance of the two nanocomposites is much enhanced relative to that of their MOF precursors. Specifically, the areal capacitances of Co-NH₂-BDC/CoSeO₃·H₂O and Co-BDC/CoSeO₃·H₂O are 5.35 and 10.65 F·cm^-2 at 2 mA·cm^-2, respectively. The assembled asymmetric supercapacitor (ASC) using Co-NH₂-BDC/CoSeO₃·H₂O as positive electrodes delivers an energy density of 0.207 mWh·cm^-2 at a power density of 0.799 mW·cm^-2 with outstanding cycling stability (93% capacity retention after 5000 cycles). Using Co-BDC/CoSeO₃·H₂O as positive electrodes, the ASC can reach a high energy density of 0.483 mWh·cm^-2 at a power density of 0.741 mW·cm^-2 and 84% capacity retention after 5000 cycles. This work provides an efficient strategy for constructing MOF/metal selenite nanocomposites for energy storage and conversion.

High-Entropy Sn0.8(Co0.2Mg0.2Mn0.2Ni0.2Zn0.2)2.2O4 Conversion-Alloying Anode Material for Li-Ion Cells: Altered Lithium Storage Mechanism, Activation of Mg, and Origins of the Improved Cycling Stability

Benefits emerging from applying high-entropy ceramics in Li-ion technology are already well-documented in a growing number of papers. However, an intriguing question may be formulated: how can the multicomponent solid solution-type material ensure stable electrochemical performance? Utilizing an example of nonequimolar Sn-based Sn_0.8(Co_0.2Mg_0.2Mn_0.2Ni_0.2Zn_0.2)_2.2O₄ high-entropy spinel oxide, we provide a comprehensive model explaining the observed very good cyclability. The material exhibits a high specific capacity above 600 mAh g^-1 under a specific current of 50 mA g^-1 and excellent capacity retention near 100% after 500 cycles under 200 mA g^-1. The stability originates from the conversion-alloying reversible reactivity of the amorphous matrix, which forms during the first lithiation from the initial high-entropy structure, and preserves the high level of cation disorder at the atomic scale. In the altered Li-storage mechanism in relation to the simple oxides, the unwanted aggregated metallic grains are not exsolved from the anode and therefore do not form highly lithiated phases characterized by large volumetric changes. Also, the electrochemical activity of Mg from the oxide matrix can be clearly observed. Because the studied compound was prepared by a conventional solid-state route, implementation of the presented approach is facile and appears usable for any oxide anode material containing a high-entropy mixture of elements.

Facile Synthesis of Hierarchical CoSeO3‧2H2O Nanoflowers Assembled by Nanosheets as a Novel Anode Material for High-Performance Lithium-Ion Batteries

As novel anodic materials for lithium-ion batteries (LIBs), transitional metal selenites can transform into metal oxide/selenide heterostructures in the first cycle, which helps to enhance the Li⁺ storage performance, especially in terms of high discharge capacity. Herein, well-defined hierarchical CoSeO₃‧2H₂O nanoflowers assembled using 10 nm-thick nanosheets are successfully synthesized via a facile one-step hydrothermal method. When used as anodic materials for LIBs, the CoSeO₃‧2H₂O nanoflowers exhibit a considerably high discharge capacity of 1064.1 mAh g⁻¹ at a current density of 0.1 A g⁻¹. In addition, the obtained anode possesses good rate capability and cycling stability. Owing to the superior electrochemical properties, the CoSeO₃‧2H₂O nanoflowers would serve as promising anodic materials for high-performance LIBs.

Nanoengineering of Advanced Carbon Materials for Sodium-Ion Batteries

Recent research shows that the continuing importance of carbon anode materials plays an important role in the development of sodium-ion batteries. Nevertheless, the practical deployment of sodium-ion batteries still faces many challenges such as mediocre sodium storage capability and short cycle life. Therefore, it is imperative to explore improvement methods to boost their competitiveness. Herein, various nanoengineering strategies, including nanostructure design, defect and heteroatom doping, and nanocomposite optimization, are proposed as reliable and effective approaches to improve electrochemical performances and structural stability of carbon-based anode materials for sodium-ion batteries (SIBs). The feasibility of nanoengineering is highlighted as a promising approach to develop next-generation carbon materials for sodium-ion batteries.

Manipulation on Two-Dimensional Amorphous Nanomaterials for Enhanced Electrochemical Energy Storage and Conversion

Low-carbon society is calling for advanced electrochemical energy storage and conversion systems and techniques, in which functional electrode materials are a core factor. As a new member of the material family, two-dimensional amorphous nanomaterials (2D ANMs) are booming gradually and show promising application prospects in electrochemical fields for extended specific surface area, abundant active sites, tunable electron states, and faster ion transport capacity. Specifically, their flexible structures provide significant adjustment room that allows readily and desirable modification. Recent advances have witnessed omnifarious manipulation means on 2D ANMs for enhanced electrochemical performance. Here, this review is devoted to collecting and summarizing the manipulation strategies of 2D ANMs in terms of component interaction and geometric configuration design, expecting to promote the controllable development of such a new class of nanomaterial. Our view covers the 2D ANMs applied in electrochemical fields, including battery, supercapacitor, and electrocatalysis, meanwhile we also clarify the relationship between manipulation manner and beneficial effect on electrochemical properties. Finally, we conclude the review with our personal insights and provide an outlook for more effective manipulation ways on functional and practical 2D ANMs.

Kinetic 2D Crystals via Topochemical Approach

The designing of novel materials is a fascinating and innovative pathway in materials science. Particularly, novel layered compounds have tremendous influence in various research fields. Advanced fundamental studies covering various aspects, including reactants and synthetic methods, are required to obtain novel layered materials with unique physical and chemical properties. Among the promising synthetic techniques, topochemical approaches have afforded the platform for widening the extent of novel 2D materials. Notably, the synthesis of binary layered materials is considered as a major scientific breakthrough after the synthesis of graphene as they exhibit a wide spectrum of material properties with varied potential applicability. In this review, a comprehensive overview of the progress in the development of metastable layered compounds is presented. The various metastable layered compounds synthesized from layered ternary bulk materials through topochemical approaches are listed, followed by the descriptions of their mechanisms, structural analyses, characterizations, and potential applications. Finally, an essential research direction concerning the synthesis of new materials is indicated, wherein the possible application of topochemical approaches in unprecedented areas is explored.

Atomic Rearrangement and Amorphization Induced by Carbon Dioxide in Two-Dimensional MoO3-x Nanomaterials

Supercritical carbon dioxide (SC CO₂) has shown great potential in fabrication of two-dimensional (2D) amorphous nanomaterials with excellent electric and optical properties, while the amorphization mechanism led by SC CO₂ is still unclear. In this work, by investigating the amorphization kinetics of MoO_3-x nanomaterials in SC CO₂, we find two amorphization mechanisms dependent on the SC CO₂ pressure. At lower pressure, forming oxygen vacancies is the dominant effect, while at higher pressure, atomic rearrangement is the controlling factor. Furthermore, we demonstrate that amorphization directly affects the optical performance of MoO_3-x nanosheets because of the change in coordination, which further indicates the atomic rearrangement during the amorphization process. Therefore, this work reveals the amorphization mechanism led by SC CO₂ and builds a link between amorphization and optical performance; it also provides new inspiration for fabrication of amorphous nanomaterials with tunable optical and photocatalytic performance.

Selenic Acid Etching Assisted Vacancy Engineering for Designing Highly Active Electrocatalysts toward the Oxygen Evolution Reaction

Oxygen evolution electrocatalysts are central to overall water splitting, and they should meet the requirements of low cost, high activity, high conductivity, and stable performance. Herein, a general, selenic-acid-assisted etching strategy is designed from a metal-organic framework as a precursor to realize carbon-coated 3d metal selenides M_m Se_n (Co_0.85 Se_{1- x} , NiSe_{2- x} , FeSe_{2- x} ) with rich Se vacancies as high-performance precious metal-free oxygen evolution reaction (OER) electrocatalysts. Specifically, the as-prepared Co_0.85 Se_{1- x} @C nanocages deliver an overpotential of only 231 mV at a current density of 10 mA cm^-2 for the OER and the corresponding full water-splitting electrolyzer requires only a cell voltage of 1.49 V at 10 mA cm^-2 in alkaline media. Density functional theory calculation reveals the important role of abundant Se vacancies for improving the catalytic activity through improving the conductivity and reducing reaction barriers for the formation of intermediates. Although phase change after long-term operation is observed with the formation of metal hydroxides, catalytic activity is not obviously affected, which strengthens the important role of the carbon network in the operating stability. This study provides a new opportunity to realize high-performance OER electrocatalysts by a general strategy on selenic acid etching assisted vacancy engineering.

Crystal Phase-Controlled Synthesis of the CoP@Co2P Heterostructure with 3D Nanowire Networks for High-Performance Li-Ion Capacitor Applications

The paramount focus in the construction of lithium-ion capacitors (LICs) is the development of anode materials with high reversible capacity and fast kinetics to overcome the mismatch of kinetics and capacity between the anode and cathode. Herein, a strategy is presented for the controllable synthesis of cobalt-based phosphides with various morphologies by adjusting the time of the phosphidation process, including 3D hierarchical needle-stacked diabolo-shaped CoP nanorods, 3D hierarchical stick-stacked diabolo-shaped Co₂P nanorods, and 3D hierarchical heterostructure CoP@Co₂P nanorods. 3D hierarchical nanostructures and a highly conductive project to accommodate volume changes are rational designs to achieve a robust construction, effective electron-ion transportation, and rapid kinetics characteristics, thus leading to excellent cycling stability and rate performance. Owing to these merits, the 3D hierarchical CoP, Co₂P, and CoP@Co₂P nanorods demonstrate prominent specific capacities of 573, 609, and 621 mA h g^-1 at 0.1 A g^-1 over 300 cycles, respectively. In addition, a high-performance CoP@Co₂P//AC LIC is successfully constructed, which can achieve high energy densities of 166.2 and 36 W h kg^-1 at power densities of 175 and 17524 W kg^-1 (83.7% capacity retention after 12000 cycles). Therefore, the controllable synthesis of various simultaneously constructed crystalline phases and morphologies can be used to fabricate other advanced energy storage devices.

Enhanced lithium storage performance of porous Si/C composite anodes using a recrystallized NaCl template

Silicon (Si) has recently aroused great interest as a promising anode material for lithium-ion batteries with high energy density due to its high theoretical capacity. However, the application of Si remains a great challenge owing to its extremely large volume change during cycling, thus resulting in dramatic capacity fading. Herein, a novel structure design of the porous Si/C composite with Si nanoparticles embedded in the carbon nanosheets has been successfully achieved by using a recrystallized NaCl template with appropriate particle size. The outermost sheet-like carbon coating can improve the electronic conductivity and contribute to the formation of a more stable solid-electrolyte interphase layer, while the inner void space effectively buffers the volume expansion of Si during the lithiation process. In addition, only a structure with Si particles anchored on the surface of carbon nanosheets has been obtained by using a commercial NaCl template with large particle size, confirming the effective regulation of the NaCl template in the microstructure and thus the electrochemical properties of the Si/C composites. As expected, benefiting from the combination of the outermost carbon coating and recrystallized NaCl-derived porous structure, the as-obtained Si/C composite demonstrates attractive cycling stability and rate performance as an anode material for lithium-ion batteries.

Boosting electrocatalytic oxygen evolution activity of bimetallic CoFe selenite by exposing specific crystal facets

It was of great significance to develop efficient and stable electrocatalysts for oxygen evolution reactions via simple and environmentally-friendly methods.

Design of double-shell TiO2@SnO2 nanotubes via atomic layer deposition for improved lithium storage

TiO₂@Void@SnO₂ nanotubes synthesized by atomic layer deposition (ALD) display high capacity for lithium storage.

A 1D Honeycomb-Like Amorphous Zincic Vanadate for Stable and Fast Sodium-Ion Storage

Developing nanomaterials with synergistic effects of various structural merits is considered to be an effective strategy to improve the sluggish ion kinetics and severe structural degradation of sodium-ion battery (SIB) anodes. Herein, honeycomb-like amorphous Zn₂ V₂ O₇ (ZVO-AH) nanofibers as SIBs anode material with plentiful defective sites, complex cavities, and good mechanical flexibility are reported. The fabrication strategy relies on the expansive and volatile nature of the organic vanadium source, based on a simple electrospinning with subsequent calcination. Originating from the synergies of amorphous nature and honeycomb-like cavities, ZVO-AH shows increased electrochemical activity, accelerated Na-ion diffusion, and robust structure. Impressively, the ZVO-AH anode delivers superior cycle stability (112% retention at 5 A g^-1 after 5000 cycles) and high rate capability (150 mAh g^-1 at 10 A g^-1 ). The synthetic versatility is able to synergistically promote the practical application of more potential materials in sodium-ion storage.

Two-dimensional bimetallic CoFe selenite via metal-ion assisted self-assembly for enhanced oxygen evolution reaction

A 2D CoFe selenite crystals was used as an efficient OER catalyst, which was obtained in a high yield via a simple metal-ion self-assembly strategy under hydrothermal condition.

Investigation of Binary Metal (Ni, Co) Selenite as Li-Ion Battery Anode Materials and Their Conversion Reaction Mechanism with Li Ions

Highly efficient anode materials with novel compositions for Li-ion batteries are actively being researched. Multicomponent metal selenite is a promising candidate, capable of improving their electrochemical performance through the formation of metal oxide and selenide heterostructure nanocrystals during the first cycle. Here, the binary nickel-cobalt selenite derived from Ni-Co Prussian blue analogs (PBA) is chosen as the first target material: the Ni-Co PBA are selenized and partially oxidized in sequence, yielding (NiCo)SeO₃ phase with a small amount of metal selenate. The conversion mechanism of (NiCo)SeO₃ for Li-ion storage is studied by cyclic voltammetry, in situ X-ray diffraction, ex situ X-ray photoelectron spectroscopy, in situ electrochemical impedance spectroscopy, and ex situ transmission electron microscopy. The reversible reaction mechanism of (NiCo)SeO₃ with the Li ions is described by the reaction: NiO + CoO + xSeO₂ + (1 - x)Se + (4x + 6)Li⁺ + (4x + 6)e^- ↔ Ni + Co + (2x + 2)Li₂ O + Li₂ Se. To enhance electrochemical properties, polydopamine-derived carbon is uniformly coated on (NiCo)SeO₃ , resulting in excellent cycling and rate performances for Li-ion storage. The discharge capacity of C-coated (NiCo)SeO₃ is 680 mAh g^-1 for the 1500th cycle when cycled at a current density of 5 A g^-1 .

Two-Dimensional Bismuth Oxide Heterostructured Nanosheets for Lithium- and Sodium-Ion Storages

Two-dimensional (2D) bismuth oxide (Bi₂O₃) heterostructured nanosheets (BOHNs) were first fabricated by a solution-based molecular self-assembly approach. The synthesized BOHNs nanosheets feature mixed α- and β-phases and rich surface/edge-active sites. When utilized as anode materials for rechargeable batteries, dual-phase BOHNs deliver an initial discharge capacity as high as 647.6 mAh g^-1 and an increased capacity of over 200 mAh g^-1 remained after 260 cycles for lithium-ion batteries (LIBs), and a stable cycling capacity at ∼50 mAh g^-1 after 500 cycles for sodium-ion batteries (SIBs). A novel flexible 2D/1D/2D structure is further developed by implanting 2D BOHNs into conductive 1D carbon nanotubes and 2D graphene to form composite (BOHNCG) paper as free-standing anodes for both LIBs and SIBs. The capacity of 2D/1D/2D BOHNCG as a LIB anode reaches 823.5 mAh g^-1, corresponding to an enhancement of ∼27%, and remains at >110 mAh g^-1 after 80 cycles as a SIB anode with greatly improved cycling stability. This work verifies the promising potential of 2D BOHNs for practical energy-related devices and enriches the current research on emerging 2D nanomaterials.

Synthesis Process of CoSeO3 Microspheres for Unordinary Li-ion Storage Performances and Mechanism of Their Conversion Reaction with Li ions

Multicomponent materials with various double cations have been studied as anode materials of lithium-ion batteries (LIBs). Heterostructures formed by coupling different-bandgap nanocrystals enhance the surface reaction kinetics and facilitate charge transport because of the internal electric field at the heterointerface. Accordingly, metal selenites can be considered efficient anode materials of LIBs because they transform into metal selenide and oxide nanocrystals in the first cycle. However, few studies have reported synthesis of uniquely structured metal selenite microspheres. Herein, synthesis of high-porosity CoSeO₃ microspheres is reported. Through one-pot oxidation at 400 °C, CoSe_x -C microspheres formed by spray pyrolysis transform into CoSeO₃ microspheres showing unordinary cycling and rate performances. The conversion mechanism of CoSeO₃ microspheres for lithium-ion storage is systematically studied by cyclic voltammetry, in situ X-ray diffraction and electrochemical impedance spectroscopy, and transmission electron microscopy. The reversible reaction mechanism of the CoSeO₃ phase from the second cycle onward is evaluated as CoO + xSeO₂ + (1 - x)Se + 4(x + 1)Li⁺ + 4( x + 1)e^- ↔ Co + (2x + 1)Li₂ O + Li₂ Se. The CoSeO₃ microspheres show a high reversible capacity of 709 mA h g^-1 for the 1400th cycle at a current density of 3 A g^-1 and a high reversible capacity of 526 mA h g^-1 even at an extremely high current density of 30 A g^-1 .

Detecting Li Dendrites in a Two-Electrode Battery System

The use of high-energy-density Li metal anodes in rechargeable batteries is not possible because of dendrite formation that can potentially result in a battery fire. Although so-called dendrite-free Li metal anodes have been reported in many recent publications, Li dendrite growth is still kinetically favorable and it remains a severe safety concern in mass production. Here, a detection system capable of alerting for Li dendrite formation in a two-electrode battery with no additional electrodes required is reported. When dendrites contact a red phosphorous-coated separator, dendrite growth is revealed by a significant voltage change. This can activate a signal through the battery management system, warning of the presence of Li dendrites and leading to shutdown of the battery before the dendrites become dangerous.

Controllable fabrication of LiMnPO4 microspheres assembled by radially arranged nanoplates with highly exposed (010) facets for an enhanced electrochemical performance

LiMnPO₄ hollow microspheres assembled by radially aligned nanoplates with a high percentage of exposed (010) facets are prepared to improve the rate capability.

Freestanding CoSeO3·H2O nanoribbon/carbon nanotube composite paper for 2.4 V high-voltage, flexible, solid-state supercapacitors

The integration of high flexibility, high energy density and wide voltage window for solid-state supercapacitors remains a big challenge to date. Herein, ultrathin CoSeO3·H2O nanoribbons (thickness: ∼14 nm) with typical pseudocapacitive behavior were synthesized in a high yield by a solution-based refluxing process. Freestanding CoSeO3·H2O ribbon/hydroxylated multi-walled carbon nanotube (HWCNT) paper could be fabricated through a vacuum-assisted filtration strategy owing to its ultrathin nature, ribbon-like morphology and inherent flexibility. Unexpectedly, an asymmetric supercapacitor constructed from this as-prepared CoSeO3·H2O/HWCNT hybrid paper exhibits a high 2.4 V voltage window as well as excellent rate capability and cycle performance. The energy density of this device is 132.3 W h kg-1 at 960 W kg-1 with a stable cycling ability of up to 10 000 cycles, which is superior to those of almost all previously reported asymmetric supercapacitors based on freestanding paper. Furthermore, this supercapacitor shows outstanding bendability and mechanical stability at different bending degrees from 0° to 180° with no changes in capacitive behavior. Our work provides new opportunities for developing high-performance asymmetric supercapacitors with high energy density, wide voltage window, and high flexibility in a novel CoSeO3·H2O system for potential applications including flexible displays, collapsible mobile phones, and wearable equipment.