A Review on the Current Progress and Challenges of 2D Layered Transition Metal Dichalcogenides as Li/Na‐ion Battery Anodes

A first-principles study of the BC3N2 monolayer and a BC3N2/graphene heterostructure as promising anode materials for sodium-ion batteries

High-performance sodium-ion batteries (SIBs) require anode materials with high capacity and fast kinetics. Based on first-principles calculations, we propose BC3N2 and BC3N2/graphene (B/G) heterostructure as potential SIB anode materials. The BC3N2 monolayer exhibits intrinsic metallic behavior. In addition, BC3N2 possesses a low Na+ diffusion barrier (0.15 eV), a high storage capacity (777 mA h g-1), a low open-circuit voltage (0.72 V), and a tiny axial expansion (0.36%). Compared with the BC3N2 monolayer, the B/G heterostructure exhibits a lower diffusion barrier of 0.027 eV, suggesting a much faster diffusion. More importantly, although the B/G heterostructure possesses heavier molar weight, its theoretical capacity (689 mA h g-1) is comparable to that of the BC3N2 monolayer. Based on the above-mentioned properties, we hope both the BC3N2 monolayer and the B/G heterostructure would be promising anodes for SIBs.

Addressing Irreversibility and Structural Distortion in WS2 Inorganic Fullerene-Like Nanoparticles: Effects of Voltage Cutoff Experiments in Beyond Li+-Ion Storage Applications

Large interlayer spacing beneficially allows Na+- and K+-ion storage in transition-metal dichalcogenide (TMD)-based electrodes, but side reactions and volume change, which pulverize the TMD crystalline structure, are persistent challenges for the utilization of these materials in next-generation devices. This study first determines whether irreversibility due to structural distortion, which results in poor cycling stability, is also apparent in the case of inorganic fullerene-like (IF) tungsten disulfide (WS2) nanocages (WS2IF). To address these problems, this study proposes upper and lower voltage cutoff experiments to limit specific reactions in Na+/WS2IF and K+/WS2IF half-cells. Three-dimensional (3D) differential capacity curves and derived surface plots highlight the continuation of reversible reactions when a high upper cutoff technique is applied, thereby indirectly suggesting restricted structural dissolution. This resulted in improved capacity retention with stable performance and a higher Coulombic efficiency, laying the ground for the use of TMD-based materials beyond Li+-ion storage devices.

CrXY (X/Y = S, Se, Te) monolayers as efficient anode materials for Li and Na-ion batteries: a first-principles study

Over the last decade, two-dimensional (2D) materials have been of great interest in the energy storage field. Large-scale electrochemical energy storage is based on the intercalation of metal ions in layered materials having van der Waals gaps. In this work, by means of first-principles calculations, we explored the use of 2D Janus transition metal dichalcogenides (TMDs) CrSSe, CrSTe and CrSeTe as anode materials for lithium and sodium-ion batteries. To examine the electronic properties and electrochemical performance, density functional theory (DFT) calculation was used. Our research shows that lithium diffuses easily with short diffusion distances and prefers to bind effectively to the monolayer. These structures are metallic in their bare phases. The highest adsorption energy shown by CrSSe, CrSTe, and CrSeTe is -1.86 eV, -1.66 eV, -2.15 eV with a low diffusion barrier of 0.3 eV, 0.6 eV, and 0.1 eV for the Li atoms and 0.54 eV, 0.32 eV and 0.15 eV for the Na atoms, respectively. At different chemical stoichiometries, we discovered negligible average open-circuit voltages of 1.0 V, 0.52 V, 0.6 V for lithium and 0.1 V, 0.49 V, and 0.51 V for sodium atoms respectively. The storage capacities shown by CrSSe, CrSTe, and CrSeTe are 348 mA h g-1, 254 mA h g-1, 208 mA h g-1 for the Li atoms and 260 mA h g-1, 198 mA h g-1, 177 mA h g-1 for the Na atoms respectively.

Biotene: Earth-Abundant 2D Material as Sustainable Anode for Li/Na-Ion Battery

Natural ores are abundant, cost-effective, and environmentally friendly. Ultrathin (2D) layers of a naturally abundant van der Waals mineral, Biotite, have been prepared in bulk via exfoliation. We report here that this 2D Biotene material has shown extraordinary Li-Na-ion battery anode properties with ultralong cycling stability. Biotene shows 302 and 141 mAh g-1 first cycle-specific charge capacity for Li- and Na-ion battery applications with ∼90% initial Coulombic efficiency. The electrode exhibits significantly extended cycling stability with ∼75% capacity retention after 4000 cycles even at higher current densities (500-2000 mA g-1). Further, density functional theory studies show the possible Li intercalation mechanism between the 2D Biotene layers. Our work brings new directions toward designing the next generation of metal-ion battery anodes.

Colloidal Synthesis of Ultrathin and Se-Rich V2Se9 Nanobelts as High-Performance Anode Materials for Li-Ion Batteries

In this study, the one-dimensional (1D) material V2Se9 was successfully synthesized using a colloidal method with VO(acac)2 and Se powder as precursors in a 1-octadecene solvent. The obtained colloidally synthesized V2Se9 (C-V2Se9) has an ultrathin nanobelt shape and a 4.5 times higher surface area compared with the bulk V2Se9, which is synthesized in a solid-state reaction as previously reported. In addition, all surfaces of C-V2Se9 are exposed to Se atoms, which is advantageous for storing Li through the conversion reaction into the Li2Se phase. Herein, the electrochemical performance of the C-V2Se9 anode material is evaluated; thus, the novelty of C-V2Se9 as a Se-rich 1D anode material is verified. The C-V2Se9 electrode exhibits a reversible capacity of 893.21 mA h g-1 and a Coulombic efficiency of 97.82% at the 100th cycle and excellent structural stability. Compared with the bulk V2Se9 electrode, the outstanding electrochemical performance of C-V2Se9 is attributed to its ultrathin nanobelt shape, high surface area, shorter Li diffusion length, and more electrochemically active sites. This work indicates the great potential of the Se-rich 1D material, C-V2Se9, as a post-transition metal dichalcogenide material for high-performance LIBs.

Multi-Pleated Alkalized Ti3 C2 Tx MXene-Based Sandwich-Like Structure Composite Nanofibers for High-Performance Sodium/Lithium Storage

The volume expansion of CoFe2 O4 anode poses a significant challenge in the commercial application of lithium/sodium-ion batteries (LIBs/SIBs). However, metal-organic-frameworks (MOF) offer superior construction of heterostructures with refined interfacial interactions and lower ion diffusion barriers in Li/Na storage. In this study, the CoFe2 O4 @carbon nanofibers derived from MOF are produced through electrospinning, in situ growth followed by calcination, which are then confined within an MXene-confined MOF-derived porous CoFe2 O4 @carbon composite architecture under alkali treatment. The CoFe2 O4 nanofibers anchor on the alkalized MXene that is decorated with the NaOH solution to form a multi-pleated structure. The sandwich-like structure of the composite effectively alleviates the volume expansion and shortens the Li/Na-ion diffusion path, which displays high capacity and outstanding rate performance as anode materials for LIBs/SIBs. As a consequence, the obtained CoFe2 O4 @carbon@alkalized MXene composite anode shows satisfied rate performance at current density of 10 A g-1 for LIBs (318 mAh·g-1 ) and 5 A g-1 for SIBs (149 mAh g-1 ). The excellent cycling performance is further demonstrated at a high current density, where it maintains a discharge capacity of 807 mAh g-1 at 2 A g-1 after 400 cycles for LIBs and 130 mAh g-1 at 1 A g-1 even after 1000 cycles for SIBs.

Aluminene as a Low-Cost Anode Material for Li- and Na-Ion Batteries

Two-dimensional (2D) materials are promising candidates for next-generation battery technologies owing to their high surface area, excellent electrical conductivity, and lower diffusion energy barriers. In this work, we use first-principles density functional theory to explore the potential for using a 2D honeycomb lattice of aluminum, referred to as aluminene, as an anode material for metal-ion batteries. The metallic monolayer shows strong adsorption for a range of metal atoms, i.e., Li, Na, K, and Ca. We observe surface diffusion barriers as low as 0.03 eV, which correlate with the size of the adatom. The relatively low average open-circuit voltages of 0.27 V for Li and 0.42 V for Na are beneficial to the overall voltage of the cell. The estimated theoretical specific capacity has been found to be 994 mA h/g for Li and 870 mA h/g for Na. Our research highlights the promise of aluminene sheets in the development of low-cost, high-capacity, and lightweight advanced rechargeable ion batteries.

2D Heterolayer-Structured MoSe2-Carbon with Fast Kinetics for Sodium-Ion Capacitors

Two-dimensional (2D) layered MoSe₂ has been demonstrated to be a promising electrode material for new energy storage systems. However, its nature of poor conductivity and the undesirable interlayer spacing hinder its further application. In this paper, a general and simple plasma-enhanced chemical vapor deposition method is proposed to produce 2D heterolayer-structured MoSe₂-carbon (MoSe₂/C) with carbon atoms inserted in the MoSe₂ layers. After morphology optimization, when applying flat-type MoSe₂/C-200 nanosheets with an enlarged interlayer spacing of 0.79 nm as the anode and activated carbon as the cathode, the assembled sodium-ion hybrid capacitors can reach a maximum energy/power density of 116.5 W h kg^-1/107.5 W kg^-1 and exhibit superior cycling durability (91.3% capacitance retention after 4000 cycles at 1 A g^-1). The good electrochemical property can be ascribed to the enlarged interlayer spacing that can offer fast diffusion channels for Na ions, and the carbon layer sandwiched in the MoSe₂ layer can not only enhance the electron transfer, accelerating the reaction kinetics, but also alleviate the volume change of MoSe₂, ensuring the good stability of the electrode. The proposed approach can also be extended to other 2D transition metal chalcogenide (TMC) materials for constructing the TMC/C heterostructures for the application in energy storage systems.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

2D PdTe2 Thin-Film-Coated Current Collectors for Long-Cycling Anode-Free Rechargeable Batteries

The practical implementation of anode-free batteries is limited by factors such as lithium dendrite growth and low cycling Coulombic efficiency (CE). In this study, the improvement in the electrochemical performance of anode-free rechargeable lithium batteries bearing a Cu current collector (CC) coated with PdTe₂ thin films is reported. The optimized thickness and sputtering heating conditions of the PdTe₂ layer are 15 nm and 473.15 K, respectively. Upon deposition on a CC, PdTe₂ works as a seed layer that considerably improves the CE in half-cells, owing to its unique 2D structure that reduces the nucleation overpotential. A further contribution to the high performance is brought about by a CuTe interphase between the coating layer and Cu CC formed during heating. Such an interphase contributes to the high CE by improving the uniformity of the current density distribution on the CC that suppresses lithium dendrite growth. A low nucleation overpotential and uniform current density distribution, in turn, result in a smooth morphology of the plated Li. The full cell obtained with the PdTe₂-coated CC exhibits a capacity retention of 70.7% after the 100th cycle, with an average CE of 99.65% at a 0.2C rate─an outstanding result in view of the rapid development of lithium-ion batteries.

Weyl semimetal orthorhombic Td-WTe2as an electrode material for sodium- and potassium-ion batteries

Alkali metals such as sodium and potassium have become promising candidates for the next generation of monovalent-ion batteries. However, a challenge for these battery technologies lies in the development of electrode materials that deliver high capacity and stable performance even at high cycling currents. Here we study orthorhombic tungsten ditelluride or Td-WTe₂as an electrode material for sodium- (SIB) and potassium-ion batteries (KIB) in propylene carbonate (PC) based electrolyte. Results show that despite larger Shannon's radius of potassium-ions and their sluggish diffusion in Td-WTe₂due to higher overpotential, at 100 mA.g^-1KIB-half cells showed higher cycling stability and low capacity decay of 4% versus 16% compared to SIB-half cells. Likewise, in a rate capability test at 61^stcycle (at 50 mA.g^-1), the KIB-half cells yielded charge capacity of 172 mAh.g^-1versus 137 mAh.g^-1of SIB-half cells. The superior electrochemical performance of Td-WTe₂electrode material in KIB-half cells is explained based on the concept of Stokes' radius-smaller desolvation activation energy resulted in higher mobility of potassium-ions in PC-based electrolyte. In addition, the likely mechanisms of electrochemical insertion and extraction of Na- and K-ions in Td-WTe₂are also discussed.