A first-principles study of Janus monolayer TiSSe and VSSe as anode materials in alkali metal ion batteries

Metallic 2D Janus SNbSe layers driven by a structural phase change

The discovery of two-dimensional (2D) Janus materials has ignited significant research interest, particularly for their distinct properties diverging from their classical 2D transition metal dichalcogenide (TMD) counterparts. While semiconducting 2D Janus TMDs have been demonstrated, examples of metallic Janus layers are still rather limited. Here, we address this gap by experimentally synthesizing and characterizing metallic Janus layers, focusing on SNbSe and SeNbS, derived from monolayer NbS2 and NbSe2 using a plasma-assisted technique. Our results show that Nb-based 2D Janus layers form after 1H-to-1T phase transition, marking a phase transition-induced formation of Janus layers. Our comprehensive spectroscopy and microscopy studies, including Z-contrast high angle annular dark field scanning transmission electron microscopy, reveal the phononic and structural properties during Janus SeNbS formation and establish their energetic stability. Density functional theory (DFT) simulations provide insights into the phononic and electronic properties of these materials, shedding light on their potential for diverse applications. Overall, our results demonstrate the realization of niobium-based Janus metals and expand the library of metallic Janus layers.

First principle calculations of Janus 2D-TiSSe as an anodic electrode in batteries of lithium, sodium, and magnesium ions

CONTEXT

In recent years, rechargeable batteries have received considerable attention as a way to improve energy storage efficiency. Anodic (negative) electrodes based on Janus two-dimensional (2D) monolayers are among the most promising candidates. In this effort, the adsorption and diffusion of these Li, Na, and Mg ions on and through Janus 2D-TiSSe as anodic material was investigated by means of periodic DFT-D3 calculations. Electrochemical parameters were computed and compared. The diffusion barrier energies for migration of ions in monolayer TiSSe in three different potential directions were determined. Furthermore, the electronic properties and Mulliken charge analysis and plots of CDD were employed to investigate the interaction between ions and their surrounding surface. Our results show that the adsorption ability of TiSSe surface up to 32 metal ions falls in the following order: Li+  > Na+  > Mg2+. The maximum storage capacity is 337.37 mAh/g for Li/Na ion and 674.75 mAh/g for Mg ion. The average open-circuit voltage is 1.39, 0.93, and 0.73 V for Li, Na, and Mg ions, respectively. Lastly, the minimum diffusion barriers follow the order Li+  < Na+  < Mg2+. The structural, energetic, and thermal stability of clean Janus surface and its saturated adsorbed systems was proved by MD simulations. In addition, we compared the obtained electrochemical parameters to those reported by other researchers. This comprehensive approach demonstrates valuable insights, furthering our understanding of TiSSe's behavior and its suitability for use in MIBs.

METHODS

DFT calculations were applied with projector augmented plane waves (PAWs)/PBE functional. A two-dimensional (2D) monolayer TiSSe surface was built with a 4 × 4 supercell. The energy cutoff of plane waves was set to 400 eV. The DFT-D3 model has been used to incorporate van der Waals interactions. A geometric relaxation process was conducted using Monhkorst-Pack 8 × 8 × 1 in reciprocal space. The relaxation and electronic calculations were carried out using the Vienna Ab initio Simulation Package (VASP). Using the transition state (TS) search algorithm implemented in the Dmol3 module, linear synchronous transition and quadratic synchronous transit tools were utilized to find the minimum energy paths.

Ab Initio Prediction of Two-Dimensional GeSiBi2 Monolayer as Potential Anode Materials for Sodium-Ion Batteries

The conceptualization and deployment of electrode materials for rechargeable sodium-ion batteries are key concerns for next-generation energy storage systems. In this contribution, the configuration stability of single-layer GeSiBi2 is systematically discussed based on first-principles calculations, and its potential as an anode material is further investigated. It is demonstrated that the phonon spectrum confirms the dynamic stability and the adsorption energy identifies a strong interaction between Na atoms and the substrate material. The electronic bands indicative of inherent metallicity contribute to the enhancement of electronic conductivity after Na adsorption. The multilayer adsorption of Na provides a theoretical capacity of 361.7 mAh/g, which is comparable to that of other representative two-dimensional anode materials. Moreover, the low diffusion barriers of 0.19 and 0.15 eV further guarantee the fast diffusion kinetics. These contributions signal that GeSiBi2 can be a compatible candidate for sodium-ion batteries anodes.

Metallic CoSb and Janus Co2AsSb monolayers as promising anode materials for metal-ion batteries

Structural symmetry breaking plays a pivotal role in fine-tuning the properties of nano-layered materials. Here, based on the first-principles approaches we propose a Janus monolayer of metallic CoSb by breaking the out-of-plane structural symmetry. Specifically, within the CoSb monolayer by replacing the top-layer 'Sb' with 'As' atoms entirely, the Janus Co2AsSb monolayer can be formed, whose structure is confirmed via structural optimization and ab initio molecular dynamics simulations. Notably, the Janus Co2AsSb monolayer demonstrates stability at an elevated temperature of 1200 K, surpassing the stability of the CoSb monolayer, which remains stable only up to 900 K. We propose that both the CoSb and Janus Co2AsSb monolayers could serve as capable anode materials for power-driven metal-ion batteries, owing to their substantial theoretical capacity and robust binding strength. The theoretical specific capacities for Li/Na reach up to 1038.28/1186.60 mA h g-1 for CoSb, while Janus Co2AsSb demonstrates a marked improvement in electrochemical storage capacity of 3578.69/2215.38 mA h g-1 for Li/Na, representing a significant leap forward in this domain. The symmetry-breaking effect upgrades the CoSb monolayer, as a more viable contender for power-driven metal-ion batteries. Furthermore, electronic structure calculations indicate a notable charge transfer that augments the metallic nature, which would boost electrical conductivity. These simulations demonstrate that the CoSb and Janus Co2AsSb monolayers have immense potential for application in the design of metal-ion battery technologies.

Remarkable enhancement of the adsorption and diffusion performance of alkali ions in two-dimensional (2D) transition metal oxide monolayers via Ru-doping

Transition metal oxides (TMO) are the preferred materials for metal ion battery cathodes because of their high redox potentials and good metal-ion intercalation capacity, which serve as an outstanding replacement for layered sulphide. In this work, using first-principles calculations based on Density functional theory approach, we explored the structural and electronic properties which comprise of adsorption and diffusion behaviour along with the analysis of voltage profile and storage capacity of Ru doped two-dimensional transition metal oxide [Formula: see text], [Formula: see text], and [Formula: see text] monolayers. The adsorption of alkali ions (Li, Na) to the surface of TMOs is strengthened by Ru-atom doping. Ru doping enhanced the adsorption energy of Li/Na-ion by 25%/11% for [Formula: see text], 8%/13% for [Formula: see text], and 10%/11% [Formula: see text] respectively. The open circuit voltage (OCV) also increases due to the high adsorption capacity of doped Monolayers. Ru doping makes the semiconducting TMOs conduct, which is suitable for battery application. As alkali ion moves closer to the dopant site, the adsorption energy increases. When alkali ions are close to the vicinity of doping site, their diffusion barrier decrease and rises as they go further away. Our current findings will be useful in finding ways to improve the storage performance of 2D oxide materials for application in energy harvesting and green energy architecture.

Polarity reversal and strain modulation of Janus MoSSe/GaN polar semiconductor heterostructures

Beyond three-dimensional (3D) architectures, polar semiconductor heterostructures are developing in the direction of two-dimensional (2D) scale with mix-dimensional integration for novel properties and multifunctional applications. Herein, we stacked 2D Janus MoSSe and 3D wurtzite GaN polar semiconductors to construct MoSSe/GaN polar heterostructures by polarity configurations. The structural stability was enhanced as binding energy changed from -0.08 eV/-0.17 eV in the N polarity to -0.24 eV/-0.42 eV in the Ga polarity. In particular, the polarity reversal of GaN in contact with Janus MoSSe not only determined the charge transfer direction but also significantly increased the electrostatic potential difference from 0.71 eV/0.78 eV in the N polarity to 3.13 eV/2.24 eV in the Ga polarity. In addition, strain modulation was further utilized to enhance interfacial polarization and tune the electronic energy band profiles of Janus MoSSe/GaN polar heterostructures. By applying in-plane biaxial strains, the AA and AA' polarity configurations induced band alignment transition from type I (tensile) to type II (compressive). As a result, both the polarity reversal and strain modulation provide effective ways for the multifunctional manipulation and facile design of Janus MoSSe/III-nitrides polar heterostructures, which broaden the Janus 2D/3D polar semiconducting devices in advanced electronics, optoelectronics, and energy harvesting applications.

Monolayer TiSi2P4as a high-performance anode for Na-ion batteries

Exploring anode materials with overall excellent performance remains a great challenge for rechargeable Na-ion battery technologies. Herein, we have identified that monolayer TiSi2P4is just such a prospective anode candidate via first-principles calculations. It is showed to be dynamically, thermally, mechanically, and energetically stable, which provides feasibility for experimental realization. The Na diffusion on the its surface is proved to be ultrafast, with a migration energy barrier as low as 73 meV. Electronic structure confirms that the pristine system undergoes a transition from the semiconductor to metal during the whole sodiation process, which is a significant advantage to the electrode conductivity. More excitingly, monolayer TiSi2P4can accommodate up to double-sided five-layer adatoms, resulting in an ultrahigh theoretical capacity of 1176 mA h g-1and a low average open-circuit voltage of 0.195 V. Moreover, the maximally sodiated electrode monolayer yields rather small in-plane lattice expansion of only 1.40%, which ensures reversible deformation and excellent cycling stability as further corroborated by structural relaxation andab initiomolecular dynamics simulation. Overall, all of these results point to the potential that monolayer TiSi2P4can serve as a promising anode candidate for application in high-performance low-cost Na-ion batteries.

Exploring a novel class of Janus MXenes by first principles calculations: structural, electronic and magnetic properties of Sc2CXT, X = O, F, OH; T = C, S, N

The already intriguing electronic and optical properties of the MXene Sc₂C family can be further tuned through a wide range of possible functionalizations. Here, by means of density functional theory, we show that the 36 possible elements of the Janus MXT (M: Sc₂C, X: O, F, OH, T: C, N, S) family, built by considering the four possible structural models (i) FCC, (ii) HCP, (iii) FCC + HCP, and (iv) HCP + FCC, are all potentially stable. The analysis of their mechanical properties shows the excellent mechanical flexibility of functionalized MXenes (f-MXenes) under large strain, making them more suitable for applications where stress could be an issue. Interestingly, while Sc₂C presents a metallic character, Sc₂COS, Sc₂CFN and Sc₂COHN are found to be semiconductors with bandgaps of 2.5 eV (indirect), 1.67 eV (indirect) and 1.1 eV (direct), respectively, which presents promising applications for nano- and optoelectronics. Moreover, Sc₂CFC presents a ferromagnetic ground state with the 2 × 2 × 1 supercell magnetic moment of 3.99 μ_B, while the ground state of Sc₂COHC might be antiferromagnetic with a magnetic moment of 3.98 μ_B, depending on the environment. Remarkably, the band structures of Sc₂CFC and Sc₂COHC present a half-metallic character with an HSE06 fundamental band gap of 0.60 eV and 0.48 eV, respectively. Our results confirm the extraordinary potential of the Janus MXT (M: Sc₂C, X: O, F, OH, T: C, N, S) family for novel applications in 2D nano-,opto- and spintronics.

Morphotaxy of Layered van der Waals Materials

Layered van der Waals (vdW) materials have attracted significant attention due to their materials properties that can enhance diverse applications including next-generation computing, biomedical devices, and energy conversion and storage technologies. This class of materials is typically studied in the two-dimensional (2D) limit by growing them directly on bulk substrates or exfoliating them from parent layered crystals to obtain single or few layers that preserve the original bonding. However, these vdW materials can also function as a platform for obtaining additional phases of matter at the nanoscale. Here, we introduce and review a synthesis paradigm, morphotaxy, where low-dimensional materials are realized by using the shape of an initial nanoscale precursor to template growth or chemical conversion. Using morphotaxy, diverse non-vdW materials such as HfO₂ or InF₃ can be synthesized in ultrathin form by changing the composition but preserving the shape of the original 2D layered material. Morphotaxy can also enable diverse atomically precise heterojunctions and other exotic structures such as Janus materials. Using this morphotaxial approach, the family of low-dimensional materials can be substantially expanded, thus creating vast possibilities for future fundamental studies and applied technologies.

Nb2N monolayer as a promising anode material for Li/Na/K/Ca-ion batteries: a DFT calculation

Developing ranking anode materials with sufficient electrical conductivity, ultrafast ion diffusion ability and considerable storage capacity is of great importance for rechargeable ion batteries but still challenging. Herein, using first-principles calculations, the potential of monolayer Nb2N as an anode material for alkali metal (e.g., Li, Na, K and Ca) ion batteries (LIBs, SIBs, PIBs and CIBs) has been explored. The calculated results indicate that the Nb2N monolayer is dynamically and thermally stable with excellent electronic conductivity. To be specific, the Li, Na, K and Ca atoms can be steadily adsorbed on the Nb2N monolayer with a low adsorption energy of -0.996, -1.263, -1.568, and -1.401 eV, respectively. Impressively, the calculated low diffusion barriers for Li, Na, K and Ca on the Nb2N monolayer are 0.047, 0.029, 0.015 and 0.051 eV, respectively, implying its high performance for the ultrafast charge and discharge processes. More importantly, the maximum storage capacities are 536 mA h g-1 for LIBs and 1072 mA h g-1 for CIBs, which are much larger than those of common anode materials. This work not only demonstrates that the Nb2N monolayer can be used as a promising anode material but also inspires the future rational design of other nitride MXenes in energy conversion and storage devices.