NbS2 Nanosheets with M/Se (M = Fe, Co, Ni) Codopants for Li+ and Na+ Storage

In Situ Lattice-Resolution Revelation of the Origins of Unexplored Anisotropic Sodiation Kinetics and Phase Transition in the Niobium Sulfide Anode

Layered transition metal dichalcogenides (TMDs) have exhibited huge potential as anode materials for sodium-ion batteries. Most of them usually store sodium via an intercalation-conversion mechanism, but niobium sulfide (NbS2) may be an exception. Herein, through in situ transmission electron microscopy, we carefully investigated the insertion behaviors of Na ions in NbS2 and directly visualized anisotropic sodiation kinetics. Lattice-resolution imaging coupled with density functional theory calculations reveals the preferential diffusion of Na ions within layers of NbS2, accompanied by observable interlayer lattice expansion. Impressively, the Na-inserted layers can still withstand in situ mechanical testing. Further in situ observation vertical to the a/b plane of NbS2 tracked the illusive conversion reaction, which could result from interlayer gliding or wrinkling associated with stress accumulation. In situ electron diffraction measurements ruled out the possibility of such a conversion mechanism and identified a phase transition from pristine 3R-NbS2 to 2H-NaNbS2. Therefore, the NbS2 anode stores Na ions via only the intercalation mechanism, which conceptually differs from the well-known intercalation-conversion mechanism of typical TMDs. These findings not only decipher the whole sodiation process of the NbS2 anode but also provide valuable reference for unraveling the precise sodium storage mechanism in other TMDs.

Designing a Se-intercalated MOF/MXene-derived nanoarchitecture for advancing the performance and durability of lithium-selenium batteries

Lithium-selenium batteries have emerged as a promising alternative to lithium-sulfur batteries due to their high electrical conductivity and comparable volume capacity. However, challenges such as the shuttle effect of polyselenides and high-volume fluctuations hinder their practical implementation. To address these issues, we propose synthesizing Fe-CNT/TiO2 catalyst through high-temperature sintering of an amalgamated nanoarchitecture of carbon nanotubes decorated metal-organic framework (MOF) and MXene, optimized for efficient selenium hosting, leveraging the distinctive physicochemical properties. The catalytic features inherent in the porous Se@Fe-CNT/TiO2 nanoarchitecture were instrumental in promoting efficient ion and electron transport, and lithium-polyselenide kinetics, while its inherent porosity could play a crucial role in inhibiting electrode stress during cycling. This nanoarchitecture exhibits remarkable battery performance, retaining 99.7% of theoretical capacity after 425 cycles at 0.5 C rate and demonstrating 95.8% capacity retention after 2000 cycles at 1 C rate, with ∼100% Coulombic efficiency. Additionally, the Se@Fe-CNT/TiO2 electrode exhibited an impressive recovery of 297.5 mAh/g (97.9%) capacity after undergoing 450 cycles at a charging rate of 10 C and a discharging rate of 1 C. This synergistic integration of MOF- and MXene-derived materials unveils new possibilities for high-performance and durable LSeBs, thus advancing electrochemical energy storage systems.

Elucidating Local Structure and Positional Effect of Dopants in Colloidal Transition Metal Dichalcogenide Nanosheets for Catalytic Hydrogenolysis

Tailoring nanoscale catalysts to targeted applications is a vital component in reducing the carbon footprint of industrial processes; however, understanding and controlling the nanostructure influence on catalysts is challenging. Molybdenum disulfide (MoS2), a transition metal dichalcogenide (TMD) material, is a popular example of a nonplatinum-group-metal catalyst with tunable nanoscale properties. Doping with transition metal atoms, such as cobalt, is one method of enhancing its catalytic properties. However, the location and influence of dopant atoms on catalyst behavior are poorly understood. To investigate this knowledge gap, we studied the influence of Co dopants in MoS2 nanosheets on catalytic hydrodesulfurization (HDS) through a well-controlled, ligand-directed, tunable colloidal doping approach. X-ray absorption spectroscopy and density functional theory calculations revealed the nonmonotonous relationship between dopant concentration, location, and activity in HDS. Catalyst activity peaked at 21% Co:Mo as Co saturates the edge sites and begins basal plane doping. While Co prefers to dope the edges over basal sites, basal Co atoms are demonstrably more catalytically active than edge Co. These findings provide insight into the hydrogenolysis behavior of doped TMDs and can be extended to other TMD materials.

3D Magnetic Metal-Organic Frameworks Current Collectors Accelerate the Lithium-Ion Diffusion Rate for Superlong Cyclic Lithium Metal Anode

Lithium, is the most ideal anode material for lithium-based batteries. However, the overgrowth of lithium dendrites and the low lithium-ion diffusion rate at low temperatures limit the further application of lithium metal anodes. Here, the applied magnetic field is introduced inside the lithium metal anode by using a novel magnetic metal-organic framework as a current collector. The magnetic field can improve the conductivity of this novel current collector, thus accelerating the diffusion of lithium ions in the battery, an advantage that is particularly prominent at low temperatures. In addition, the current collector can stabilize the solid electrolyte interface and inhibit the growth of lithium dendrites, resulting in excellent electrochemical performance. The symmetrical cell at room temperature can exceed 4600 h with a hysteresis voltage of only 9 mV. After 300 cycles at room temperature, the capacity of full cell is still 142 mA h g-1 , and it remains stable for 380 cycles at 5 °C (capacity above 120 mA h g-1 ). The strategy of constructing novel current collector with magnetic field can promote the further application of lithium batteries in extreme conditions such as low temperatures.

Molecular modulation strategies for two-dimensional transition metal dichalcogenide-based high-performance electrodes for metal-ion batteries

In the past few decades, great efforts have been made to develop advanced transition metal dichalcogenide (TMD) materials as metal-ion battery electrodes. However, due to existing conversion reactions, they still suffer from structural aggregation and restacking, unsatisfactory cycling reversibility, and limited ion storage dynamics during electrochemical cycling. To address these issues, extensive research has focused on molecular modulation strategies to optimize the physical and chemical properties of TMDs, including phase engineering, defect engineering, interlayer spacing expansion, heteroatom doping, alloy engineering, and bond modulation. A timely summary of these strategies can help deepen the understanding of their basic mechanisms and serve as a reference for future research. This review provides a comprehensive summary of recent advances in molecular modulation strategies for TMDs. A series of challenges and opportunities in the research field are also outlined. The basic mechanisms of different modulation strategies and their specific influences on the electrochemical performance of TMDs are highlighted.

WS2-Graphene van der Waals Heterostructure as Promising Anode Material for Lithium-Ion Batteries: A First-Principles Approach

In this work, we report the results of density functional theory (DFT) calculations on a van der Waals (VdW) heterostructure formed by vertically stacking single-layers of tungsten disulfide and graphene (WS2/graphene) for use as an anode material in lithium-ion batteries (LIBs). The electronic properties of the heterostructure reveal that the graphene layer improves the electronic conductivity of this hybrid system. Phonon calculations demonstrate that the WS2/graphene heterostructure is dynamically stable. Charge transfer from Li to the WS2/graphene heterostructure further enhances its metallic character. Moreover, the Li binding energy in this heterostructure is higher than that of the Li metal's cohesive energy, significantly reducing the possibility of Li-dendrite formation in this WS2/graphene electrode. Ab initio molecular dynamics (AIMD) simulations of the lithiated WS2/graphene heterostructure show the system's thermal stability. Additionally, we explore the effect of heteroatom doping (boron (B) and nitrogen (N)) on the graphene layer of the heterostructure and its impact on Li-adsorption ability. The results suggest that B-doping strengthens the Li-adsorption energy. Notably, the calculated open-circuit voltage (OCV) and Li-diffusion energy barrier further support the potential of this heterostructure as a promising anode material for LIBs.

The Coexistence of Superconductivity and Topological Order in Van der Waals InNbS2

The research on systems with coexistence of superconductivity and nontrivial band topology has attracted widespread attention. However, the limited availability of material platforms severely hinders the research progress. Here, it reports the first experimental synthesis and measurement of high-quality single crystal van der Waals transition-metal dichalcogenide InNbS2 , revealing it as a topological nodal line semimetal with coexisting superconductivity. The temperature-dependent measurements of magnetization susceptibility and electrical transport show that InNbS2 is a type-II superconductor with a transition temperature Tc of 6 K. First-principles calculations predict multiple topological nodal ring states close to the Fermi level in the presence of spin-orbit coupling. Similar features are also observed in the as-synthesized BiNbS2 and PbNbS2 samples. This work provides new material platforms ANbS2 (A = In, Bi, and Pb) and uncovers their intriguing potential for exploring the interplay between superconductivity and band topology.

Carbon-supported T-Nb2O5 nanospheres and MoS2 composites with a mosaic structure for insertion-conversion anode materials

Reasonably combining the strengths of insertion and conversion anode materials to create an advanced anode material remains a formidable challenge for rechargeable lithium-ion batteries (LIBs). In this work, bulk MoS2 embedded with T-Nb2O5 nanospheres was synthesized via a simple hydrothermal process and a polydopamine carbon source was introduced by heat treatment. The design strategy can effectively accelerate the charge transfer and reduce the volume expansion during electrochemical cycling, leading to an improvement in lithium storage performance. As a consequence, the coexistence of T-Nb2O5, MoS2 and C can achieve the best synergistic effect when the molar ratio of Nb and Mo sources was 1 : 1. Notably, the T-Nb2O5@MoS2@C-1-1 electrode not only delivered an excellent reversible capacity of 518 mA h g-1 at a current density of 0.1 A g-1 but also exhibited superb cycling stability. The specific capacity of this electrode maintained 187 mA h g-1 at 2 A g-1 after 1000 cycles with a negligible capacity fading rate of only 0.015% per cycle.

Anion Defects Engineering of Ternary Nb-Based Chalcogenide Anodes Toward High-Performance Sodium-Based Dual-Ion Batteries

Highlights

We developed an efficient and extensible strategy to produce the single-phase ternary NbSSe nanohybrids with defect-enrich microstructure. The anionic-Se doping play a key role in effectively modulating the electronic structure and surface chemistry of NbS2 phase, including the increased interlayers distance (0.65 nm), the enhanced intrinsic electrical conductivity (3.23 × 103 S m-1) and extra electroactive defect sites. The NbSSe/NC composite as anode exhibits rapid Na+ diffusion kinetics and increased capacitance behavior for Na+ storage, resulting in high reversible capacity and excellent cycling stability.

Abstract

Sodium-based dual-ion batteries (SDIBs) have gained tremendous attention due to their virtues of high operating voltage and low cost, yet it remains a tough challenge for the development of ideal anode material of SDIBs featuring with high kinetics and long durability. Herein, we report the design and fabrication of N-doped carbon film-modified niobium sulfur–selenium (NbSSe/NC) nanosheets architecture, which holds favorable merits for Na+ storage of enlarged interlayer space, improved electrical conductivity, as well as enhanced reaction reversibility, endowing it with high capacity, high-rate capability and high cycling stability. The combined electrochemical studies with density functional theory calculation reveal that the enriched defects in such nanosheets architecture can benefit for facilitating charge transfer and Na+ adsorption to speed the electrochemical kinetics. The NbSSe/NC composites are studied as the anode of a full SDIBs by pairing the expanded graphite as cathode, which shows an impressively cyclic durability with negligible capacity attenuation over 1000 cycles at 0.5 A g−1, as well as an outstanding energy density of 230.6 Wh kg−1 based on the total mass of anode and cathode.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40820-023-01070-0.

Rational design of NiO/NiSe2@C heterostructure as high-performance anode for Li-ion battery

Biphasic or multiphase heterostructures have promising futures in advanced electrode materials for energy-related applications because of their desirable synergistic effects. Here we prepared a rational NiO/NiSe₂@C heterostructure microsphere through carbonization, selenization, and oxidation using Ni-MOF as a precursor. Electrochemical studies were conducted to examine the Li⁺ storage characteristics, and density functional theory (DFT) was utilized to comprehend the underlying mechanism. When employed as the anode for LIBs, the NiO/NiSe₂@C showed a high specific capacity and long-term cyclic stability, with a specific capacity of 992 mAh g^-1 for 600 cycles at a current density of 0.2 A g^-1. The NiO/NiSe₂@C exhibits a significantly enhanced lithium-ion diffusion coefficient ( [Formula: see text] ) value. The DFT results show that an electron-rich area forms at the NiO/NiSe₂ heterointerface, where the metalloid selenium transfers electrons to the oxygen atoms. The lithiation reactions were improved dramatically by redistributing interfacial charges, which can trigger a built-in electric field that dramatically promotes the capacitance contribution of electrode materials, enhances the lithium storage capacity, and accelerates the ion/electron transmission. The rational synthesis of NiO/NiSe₂@C heterostructure can provide an idea for designing novel heterostructure anode materials.

Anion-Vacancy Modified WSSe Nanosheets on 3D Cross-Networked Porous Carbon Skeleton for Non-Aqueous Sodium-Based Dual-Ion Storage

Sodium-based dual-ion batteries (SDIBs) have become a new type of energy storage device with great application value because of their high operating voltage, high energy density, and low cost. However, transition-metal dichalcogenide (TMD) anodes show unsatisfactory Na⁺ electrochemical performance owing to the low intrinsic conductivity and inferior ion transport kinetics. Here, an elaborate design is developed to prepare a composite of WSSe nanosheets supported on a 3D cross-networked porous carbon skeleton (WSSe@CPCS), which possesses en-rich anion vacancies and WSSe with expanded inter-layer spacing, as well as an interconnected porous structure. As a result, the WSSe@CPCS anode for sodium-ion batteries (SIBs) exhibits preeminent reversible capacities, excellent cycle stability, and superior rate capability. The systematic electrochemical kinetic analysis and density functional theory results further show that the effect of anion vacancies and CPCS synergistically enhances the conductivity and reduces charge transfer resistance, thus making a great contribution to fast reaction kinetics. Finally, the implementations of the WSSe@CPCS anode in progressive SIB and DIB full-cell configurations exhibit satisfactory performance, which reveals their widely practical application. This research will provide an exciting approach to designing advanced defect-structured tungsten-based TMD materials for SIBs, DIBs, and even a broad range of energy storage.

Constructing a Micrometer-Sized Structure through an Initial Electrochemical Process for Ultrahigh-Performance Li+ Storage

Orthorhombic niobium pentoxide (T-Nb₂O₅) is a promising anode to fulfill the requirements for high-rate Li-ion batteries (LIBs). However, its low electric conductivity and indistinct electrochemical mechanism hinder further applications. Herein, we develop a novel method to obtain a micrometer-sized layer structure of S-doped Nb₂O₅ on an S-doped graphene (SG) surface (the composite is denoted S-Nb₂O₅/SG) after the initial cycle, which we call "in situ electrochemically induced aggregation". In situ and ex situ characterizations and theoretical calculations were carried out to reveal the aggregation process and Li⁺ storage process. The unique merits of the composite with a micrometer-sized layer structure increased the reaction degree, structural stability, and electrochemical kinetics. As a result, the electrode exhibited a large capacity (∼598 mAh g^-1 at 0.1 A g^-1), outstanding cycling stability (∼313 mAh g^-1 at 5 A g^-1 and remains at ∼313 mAh g^-1 after 1000 cycles), and a high Coulombic efficiency and has a high fast-charging performance and excellent cycling stability.

Carbon-coated MoS1.5Te0.5 nanocables for efficient sodium-ion storage in non-aqueous dual-ion batteries

Sodium-based dual-ion batteries have received increased attention owing to their appealing cell voltage (i.e., >3 V) and cost-effective features. However, the development of high-performance anode materials is one of the key elements for exploiting this electrochemical energy storage system at practical levels. Here, we report a source-template synthetic strategy for fabricating a variety of nanowire-in-nanotube MS_xTe_y@C (M = Mo, W, Re) structures with an in situ-grown carbon film coating, termed as nanocables. Among the various materials prepared, the MoS_1.5Te_0.5@C nanocables are investigated as negative electrode active material in combination with expanded graphite at the positive electrode and NaPF₆-based non-aqueous electrolyte solutions for dual-ion storage in coin cell configuration. As a result, the dual-ion lab-scale cells demonstrate a prolonged cycling lifespan with 97% capacity retention over 1500 cycles and a reversible capacity of about 101 mAh g⁻¹ at specific capacities (based on the mass of the anode) of 1.0 A g⁻¹ and 5.0 A g⁻¹, respectively.

Sodium-based dual-ion batteries are promising electrochemical energy storage devices. Here, the authors report a source-template synthetic strategy to prepare carbon-coated MoS_1.5Te_0.5 nanocables and their use as anode active materials in Na-based dual ion cells.

Hydrangea-like NiMoO4-Ag/rGO as Battery-type electrode for hybrid supercapacitors with superior stability

It is a great challenge to design electrode materials with good stability and high specific capacitance for supercapacitors. Herein, a three-dimensional (3D) hydrangea-like NiMoO₄ micro-architecture with Ag nanoparticles anchored on the surface has been designed by adding EDTA-2Na, which was assembled with reduced graphene oxide (rGO) and named as NiMoO₄-Ag/rGO composite. Benefiting from the synergetic contributions of structural and componential properties, NiMoO₄-Ag/rGO composite exhibits a high specific capacitance of 566.4 C g^-1 at 1 A g^-1, and great cycling performance with 90.5% capacitance retention after 1000 cycles at 10 A g^-1. The NiMoO₄-Ag/rGO electrode shows an enhanced cycling stability due to the two-dimensional towards two-dimensional (2D-2D) interface coupling between rGO and NiMoO₄ nanosheets, and the stable 3D hydrangea-like micro-architecture. Moreover, NiMoO₄-Ag/rGO with 5-15 nm pore structure and enhanced conductivity exhibits improved charge transfer and ions diffusion. Besides, NiMoO₄-Ag/rGO//AC capacitor displays an outstanding energy density of 40.98 Wh kg^-1 at 800 kW kg^-1, and an excellent cycling performance with 73.3% capacitance retention at 10 A g^-1 after 8000 cycles. The synthesis of NiMoO₄-Ag/rGO composite can provide an effective strategy to solve the poor electrochemical stability and slow electron/ion transfer of NiMoO₄ material as supercapacitors electrode.

Exploring the physicochemical role of Pd dopant in promoting Li-ion diffusion dynamics and storage performance of NbS2 at the atomic scale

The two-dimensional layered niobium disulfide (NbS2), as a kind of anode material for Li-ion batteries, has received great attention because of its excellent electronic conductivity and structural stability.

Solid Solution Metal Chalcogenides for Sodium-Ion Batteries: The Recent Advances as Anodes

The sodium-ion battery (SIB) has attracted ever growing attention as a promising alternative of the lithium-ion battery (LIB). Constructing appropriate anode materials is critical for speeding up the application of SIB. This review aims at guiding anode design from the material's perspective, and specifically focusing on solid solution metal chalcogenide anode. The sodium ion storage mechanisms of a solid solution metal chalcogenide anode is overviewed on basis of the elements it is composed of, and discusses how the solid solution character alters the electrochemical performances through diffusion and surface-controlled processes. In addition, by classifying solid solution metal chalcogenide as cation and anion, their recent applications are updated, and understanding the roles of guest elements in improving the electrochemical behaviors of a solid solution metal chalcogenide is carried out. After that, discussion of possible strategies to further optimize these anode materials in the future, flowing from crystal structure design to morphology control and finally to the intimacy improvement between conductive matrix and solid solution metal chalcogenide are also provided.

Doping regulation in transition metal compounds for electrocatalysis

In electrocatalysis, doping regulation has been considered as an effective method to modulate the active sites of catalysts, providing a powerful means for creating a large variety of highly efficient catalysts for various reactions. Of particular interest, there has been growing research concerning the doping of two-dimensional transition-metal compounds (TMCs) to optimize their electrocatalytic performance. Despite the previous achievements, mechanistic insights of doping regulation in TMCs for electrocatalysis are still lacking. Herein, we provide a systematic overview of doping regulation in TMCs in terms of background, preparation, impacts on physicochemical properties, and typical applications including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, and N₂ reduction reaction. Notably, we bridge the understanding between the doping regulation of catalysts and their catalytic activities via focusing on the physicochemical properties of catalysts from the aspects of vacancy concentrations, phase transformation, surface wettability, electrical conductivity, electronic band structure, local charge distribution, tunable adsorption strength, and multiple adsorption configurations. We also discuss the existing challenges and future perspectives in this promising field.

Thermodynamic Insights into Polymorphism-Driven Lithium-Ion Storage in Monoelemental 2D Materials

Monoelemental two-dimensional materials (borophene, silicene, etc.) are exciting candidates for electrodes in lithium-ion batteries because of their ultralight molar mass. However, these materials' lithium-ion binding mechanism can be complex as the inherited polymorphism may induce phase changes during the charge-discharge cycles. Here, we combine genetic-algorithm-based bottom-up and stochastic top-down structure searching techniques to conduct thermodynamic scrutiny of the lithiated compounds of 2D allotropes of four elements: B, Al, Si, and P. Our first-principles-based high-throughput computations unveil polymorphism-driven lithium-ion binding process and other nonidealities (e.g., bond cleavage, adsorbent phase change, and electroplating), which lacks mention in earlier works. While monolayer B (2479 mAh/g), Al (993 mAh/g), and Si (954 mAh/g) have been demonstrated here as excellent candidates for Li-ion storage, P falls short of the expectation. Our well-designed computational framework, which always searches for lithiated structures at global minima, provides convincing thermodynamical insights and realistic reversible specific-capacity values. This will expectedly open up future experimental efforts to design monoelemental two-dimensional material-based anodes with specific polymorphic structures.

Designing vertically aligned porous NiCo2O4@MnMoO4 Core@Shell nanostructures for high-performance asymmetric supercapacitors

NiCo₂O₄@MnMoO₄ core@shell nanostructures are synthesized as electrode material using hydrothermal method for the fabrication of asymmetric supercapacitor (ASC) device. The NiCo₂O₄@MnMoO₄ electrode shows better electrochemical performance with specific capacitance (SC) of 1821 F/g at current density of 5 A/g and cycling stability of 94%. The NiCo₂O₄@MnMoO₄ core@shell electrode shows better SC compared to pure NiCo₂O₄ and MnMoO₄ electrodes. An ASC device is fabricated using NiCo₂O₄@MnMoO₄ as a positive and rGO/Fe₂O₃ as negative electrode materials. Remarkably, the fabricated device shows a SC of 294 F/g at current density 4 A/g, with an energy density of 91.87 Wh/kg at a power density of 374.15 W/kg. The device shows good reversibility with cycling stability of 68% after 2,000 cycles. The ASC device is used to illuminate nine green color LEDs for 35 min. Therefore, the present report provides a simple method to fabricate efficient and stable energy storage devices for industrial applications.

Natural Soft/Rigid Superlattices as Anodes for High-Performance Lithium-Ion Batteries

Volume expansion and poor conductivity are two major obstacles that hinder the pursuit of the lithium-ion batteries with long cycling life and high power density. Herein, we highlight a misfit compound PbNbS₃ with a soft/rigid superlattice structure, confirmed by scanning tunneling microscopy and electrochemical characterization, as a promising anode material for high performance lithium-ion batteries with optimized capacity, stability, and conductivity. The soft PbS sublayers primarily react with lithium, endowing capacity and preventing decomposition of the superlattice structure, while the rigid NbS₂ sublayers support the skeleton and enhance the migration of electrons and lithium ions, as a result leading to a specific capacity of 710 mAh g^-1 at 100 mA g^-1 , which is 1.6 times of NbS₂ and 3.9 times of PbS. Our finding reveals the competitive strategy of soft/rigid structure in lithium-ion batteries and broadens the horizons of single-phase anode material design.

Colloidal Synthesis of NbS2 Nanosheets: From Large-Area Ultrathin Nanosheets to Hierarchical Structures

Layered NbS₂, a member of group-V transition metal dichalcogenides, was synthesized via a colloidal synthesis method and employed as a negative material for a supercapacitor. The morphologies of NbS₂ can be tuned from ultrathin nanosheets to hierarchical structures through dynamics controls based on growth mechanisms. Electrochemical energy storage measurements present that the ultrathin NbS₂ electrode exhibits the highest rate capability due to having the largest electrochemical surface area and its efficient ion diffusion. Meanwhile, the hierarchical NbS₂ shows the highest specific capacitance at low current densities for small charge transfer resistance, displays 221.4 F g⁻¹ at 1 A g⁻¹ and 117.1 F g⁻¹ at 10 A g⁻¹, and cycling stability with 78.9% of the initial specific capacitance after 10,000 cycles. The aggregate or stacking of nanosheets can be suppressed effectively by constructing hierarchical structure NbS₂ nanosheets.

Two-Dimensional Transition Metal Chalcogenides for Alkali Metal Ions Storage

On the heels of exacerbating environmental concerns and ever-growing global energy demand, development of high-performance renewable energy-storage and -conversion devices has aroused great interest. The electrode materials, which are the critical components in electrochemical energy storage (EES) devices, largely determine the energy-storage properties, and the development of suitable active electrode materials is crucial to achieve efficient and environmentally friendly EES technologies albeit the challenges. Two-dimensional transition-metal chalcogenides (2D TMDs) are promising electrode materials in alkali metal ion batteries and supercapacitors because of ample interlayer space, large specific surface areas, fast ion-transfer kinetics, and large theoretical capacities achieved through intercalation and conversion reactions. However, they generally suffer from low electronic conductivities as well as substantial volume change and irreversible side reactions during the charge/discharge process, which result in poor cycling stability, poor rate performance, and low round-trip efficiency. In this Review, recent advances of 2D TMDs-based electrode materials for alkali metal-ion energy-storage devices with the focus on lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), potassium-ion batteries (PIBs), high-energy lithium-sulfur (Li-S), and lithium-air (Li-O₂ ) batteries are described. The challenges and future directions of 2D TMDs-based electrode materials for high-performance LIBs, SIBs, PIBs, Li-S, and Li-O₂ batteries as well as emerging alkali metal-ion capacitors are also discussed.

Metallic-State SnS2 Nanosheets with Expanded Lattice Spacing for High-Performance Sodium-Ion Batteries

Metallic-state 2D SnS₂ nanosheets with expanded lattice spacing and a defect-rich structure were synthesized by the intercalation of Ni into the van der Waals gap of SnS₂ . The expanded lattice spacing efficiently enhanced the electrochemical performance of the SnS₂ for sodium-ion batteries owing to the change electron state density and energy band structure. In operando synchrotron XRD and theoretical calculations were used to gain insight into the influence of foreign metal-ion doping and its location. The optimized architecture obtained by in situ uniform growth of nanosheets on carbon fibers significantly enhanced the electrochemical performance. The inherent advantages of this architecture are shorter paths for ion insertion and extraction, larger contact area for more sodium diffusion pathways, and superior electrolyte penetration. Benefiting from the Ni intercalated SnS₂ bilayer, the internal adjustment of the electronic state and the enlarged interlayer spacing significantly enhanced the electron transport kinetics, which can be explained by the metallic-state properties. The integrated electrode exhibited an initial high reversible capacity of 795 mAh g^-1 at 0.1 A g^-1 , with a stable capacity retention of 666 mAh g^-1 after 100 cycles. Good rate capability was also exhibited with specific capacities of 691, 564, 437 mAh g^-1 at current densities of 200, 500, and 1000 mA g^-1 , respectively.

Decreasing Ion-Diffusion Barrier Enables Superior Na-Ion Storage by Synergizing Hierarchical Architecture and Lattice Distortion

Compared with nanosized materials, the long-pathway isolation of the interior part from the electrolyte for bulk electrode materials may result in high ionic diffusion barrier, leading to the poor rate behavior. Either the modification of lattice or the construction of a porous structure is generally effective to decrease the ion-diffusion barrier; however, achieving these multiscaled modulations simultaneously via a facile approach is still a challenge. Herein, we manipulate a bifunctional dopant to prepare micron-sized Na₃V₂(PO₄)₃ with extraordinary synergy of hierarchical architecture and lattice distortion. The cations Zn²⁺ not only substitute partial V³⁺ to enhance the solid-phase ion diffusivity but also stabilize the lattice structure due to the pillar effect. Additionally, the anions CH₃COO^- also participate in the reaction to modulate the porous architecture. The analysis results of galvanostatic intermittent titration technique, cyclic voltammetry, and electrochemical impedance spectroscopy demonstrate that the rational design of morphology and structure compounding lowers the ion-diffusion barrier and strengthens the Na⁺ migration kinetics. When evaluated as the cathode electrode, the optimal composite exhibits improved Na⁺ ion transport kinetics and superior rate behaviors of 72.2 and 58.7 mAh g^-1 at 100 and 200C, respectively.

Hollow-Carbon-Templated Few-Layered V5S8 Nanosheets Enabling Ultrafast Potassium Storage and Long-Term Cycling

Due to the abundant potassium resource on the Earth's crust, researchers now have become interested in exploring high-performance potassium-ion batteries (KIBs). However, the large size of K⁺ would hinder the diffusion of K ions into electrode materials, thus leading to poor energy/power density and cycling performance during the depotassiation/potassiation process. So, few-layered V₅S₈ nanosheets wrapping a hollow carbon sphere fabricated via a facile hollow carbon template induced method could reversibly accommodate K storage and maintain the structure stability. Hence, the as-obtained V₅S₈@C electrode enables rapid and reversible storage of K⁺ with a high specific capacity of 645 mAh/g at 50 mA/g, a high rate capability, and long cycling stability, with 360 and 190 mAh/g achieved after 500 and 1000 cycles at 500 and 2000 mA/g, respectively. The excellent electrochemical performance is superior to the most existing electrode materials. The DFT calculations reveal that V₅S₈ nanosheets have high electrical conductivity and low energy barriers for K⁺ intercalation. Furthermore, the reaction mechanism of the V₅S₈@C electrode in KIBs is probed via the in operando synchrotron X-ray diffraction technique, and it indicates that the V₅S₈@C electrode undergoes a sequential intercalation (KV₅S₈) and conversion reactions (K₂S₃) reversibly during the potassiation process.

Transition-Metal Oxynitride: A Facile Strategy for Improving Electrochemical Capacitor Storage

The use of transition-metal oxide (TMO) as an extended-life electrochemical energy storage material remains challenging because TMO undergoes volume expansion during energy storage. In this work, a transition-metal oxynitride layer (TMON, M: Fe, Co, Ni, and V) was synthesized on TMO nanowires to address the crucial issue of volume expansion. The unique oxynitride layer possesses numerous active sites, excellent conductivity, and outstanding stability. These characteristics enhance specific capacitance and alleviate volume expansion effectively. Specifically, the specific capacity of the TMON electrode is enhanced by approximately twofold relative to that of its corresponding oxide. Notably, the capacitance of the TMON remains above 94% even after 10 000 cycles. This result indicates that the cycling performance of the TMON electrode is superior to that of its corresponding oxide. First-principles and quantitative kinetics analyses are performed to investigate the mechanism underlying the improved electrochemical performances of the TMON layers. Results demonstrate that the proposed TMON layer has attractive applications in the fields of energy storage, conversion, and beyond.

Formation of CoNi2S4 nanofibers with 3D hierarchical pompom-like structure for high-rate electrochemical capacitors

Pompom-like 3D hierarchical structured CoNi₂S₄ nanofibers are obtained via a two-step anion exchange route, exhibiting good high-rate performance.

Construction of Sb2Se3 nanocrystals on Cu2−xSe@C nanosheets for high performance lithium storage

Cu_2−xSe@C@Sb₂Se₃ with enhanced electrochemical performance was designed and fabricated, where Sb₂Se₃ nanoparticles were anchored on Cu_2−xSe@C nanosheets.

Scalable synthesis of a foam-like FeS2 nanostructure by a solution combustion-sulfurization process for high-capacity sodium-ion batteries

Pyrite-type FeS2 is regarded as a promising anode material for sodium ion batteries. The synthesis of FeS2 in large quantities accompanied by an improved cycling stability, as well as retaining high theoretical capacity, is highly desirable for its commercialization. Herein, we present a scalable and simple strategy to prepare a foam-like FeS2 (F-FeS2) nanostructure by combining solution combustion synthesis and solid-state sulfurization. The obtained F-FeS2 product is highly uniform and built from interconnected FeS2 nanoparticles (∼50 nm). The interconnected feature, small particle sizes and porous structure endow the product with high electrical conductivity, good ion diffusion kinetics, and high inhibition capacity of volume expansion. As a result, high capacity (823 mA h g-1 at 0.1 A g-1, very close to the theoretical capacity of FeS2, 894 mA h g-1), good rate capability (581 mA h g-1 at 5.0 A g-1) and cyclability (754 mA h g-1 at 0.2 A g-1 with 97% retention after 80 cycles) can be achieved. The sodium storage mechanism has been proved to be a combination of intercalation and conversion reactions based on in situ XRD. Furthermore, high pseudocapacitive contribution (i.e. ∼87.5% at 5.0 mV s-1) accounts for the outstanding electrochemical performance of F-FeS2 at high rates.

CoSe2-Decorated NbSe2 Nanosheets Fabricated via Cation Exchange for Li Storage

Though 2D transition metal dichalcogenides have attracted a lot of attention in energy-storage applications, the applications of NbSe₂ for Li storage are still limited by the unsatisfactory theoretical capacity and uncontrollable synthetic approaches. Herein, a controllable oil-phase synthetic route for preparation of NbSe₂ nanoflowers consisted of nanosheets with a thickness of ∼10 nm is presented. Significantly, a part of NbSe₂ can be further replaced by orthorhombic CoSe₂ nanoparticles via a post cation exchange process, and the predominantly 2D nanosheet-like morphology can be well-maintained, resulting in the formation of CoSe₂-decorated NbSe₂ (denoted as CDN) nanosheets. More interestingly, the CDN nanosheets exhibit excellent lithium-ion battery performance. For example, it achieves a highly reversible capacity of 280 mAh g^-1 at 10 A g^-1 and long cyclic stability with specific capacity of 364.7 mAh g^-1 at 5 A g^-1 after 1500 cycles, which are significantly higher than those of reported pure NbSe₂.

Lithium- and Magnesium-Storage Mechanisms of Novel Hexagonal NbSe2

As a novel and potential transition metal dichalcogenide (TMDC), NbSe₂ has low ion diffusion barrier when applied in energy-storage systems, such as traditional lithium-ion batteries and novel magnesium-ion batteries (MIBs). In this work, we have developed a novel hexagonal NbSe₂ material with a nanosized surface via a facile microwave-hydrothermal method. The Li⁺-storage mechanism of NbSe₂ with surface conversion and internal intercalation is thoroughly revealed by in situ X-ray diffraction (XRD), ex situ high-resolution transmission electron microscopy, and ex situ scanning electron microscopy. Besides, Mg²⁺ intercalation mechanism is confirmed via ex situ XRD and ex situ X-ray photoelectron spectroscopy for the first time. In addition, as the cathode for MIBs, NbSe₂ with a nanosized surface exhibits a high rate capacity of 101 mA h g^-1 at 200 mA g^-1 with a high discharge plateau at 1.30 V. Our work builds a deep understanding of ion-storage mechanisms in TMDCs and provides guidance for designing new electrode materials with high electrochemical performances.

Encapsulation of CoS x Nanocrystals into N/S Co-Doped Honeycomb-Like 3D Porous Carbon for High-Performance Lithium Storage

A honeycomb-like 3D N/S co-doped porous carbon-coated cobalt sulfide (CoS, Co9S8, and Co1-x S) composite (CS@PC) is successfully prepared using polyacrylonitrile (PAN) as the nitrogen-containing carbon source through a facile solvothermal method and subsequent in situ conversion. As an anode for lithium-ion batteries (LIBs), the CS@PC composite exhibits excellent electrochemical performance, including high reversible capacity, good rate capability, and cyclic stability. The composite electrode delivers specific capacities of 781.2 and 466.0 mAh g-1 at 0.1 and 5 A g-1, respectively. When cycled at a current density of 1 A g-1, it displays a high reversible capacity of 717.0 mAh g-1 after 500 cycles. The ability to provide this level of performance is attributed to the unique 3D multi-level porous architecture with large electrode-electrolyte contact area, bicontinuous electron/ion transport pathways, and attractive structure stability. Such micro-/nanoscale design and engineering strategies may also be used to explore other nanocomposites to boost their energy storage performance.

MoO3 nanosheet arrays as superior anode materials for Li- and Na-ion batteries

MoO3 is a promising anode material for energy storage, but its electrochemical performance is still unsatisfactory. Herein, α-MoO3 nanosheets vertically grown on activated carbon fiber cloth as superior anode materials for Li- and Na-ion batteries were achieved by the method of controlled preparation. For Li-ion batteries, the resulting MoO3 array electrodes exhibit a high discharge capacity of 4.48 mA h cm-2 (1780 mA h g-1) at 0.1 mA cm-2 and outstanding cycling stability at 0.5 mA cm-2 (94% capacity retention after 200 cycles). Moreover, for Na-ion batteries, they also show an excellent electrochemical performance with a discharge capacity of 2.5 mA h cm-2 (1621 mA h g-1) at 0.1 mA cm-2 and capacity retention of 90% after 200 cycles at 0.2 mA cm-2. This study suggests that MoO3 array electrodes can be hopefully used as promising anode materials for energy storage applications.

Recent Progress on Two-Dimensional Nanoflake Ensembles for Energy Storage Applications

The rational design and synthesis of two-dimensional (2D) nanoflake ensemble-based materials have garnered great attention owing to the properties of the components of these materials, such as high mechanical flexibility, high specific surface area, numerous active sites, chemical stability, and superior electrical and thermal conductivity. These properties render the 2D ensembles great choices as alternative electrode materials for electrochemical energy storage systems. More recently, recognition of the numerous advantages of these 2D ensemble structures has led to the realization that the performance of certain devices could be significantly enhanced by utilizing three-dimensional (3D) architectures that can furnish an increased number of active sites. The present review summarizes the recent progress in 2D ensemble-based materials for energy storage applications, including supercapacitors, lithium-ion batteries, and sodium-ion batteries. Further, perspectives relating to the challenges and opportunities in this promising research area are discussed.