Metallic CrP2 monolayer: potential applications in energy storage and conversion

Phosphorus-rich compounds have emerged as a promising class of energy storage and conversion materials due to their interesting structures and electrochemical properties. Herein, we propose that a metallic CrP2 monolayer, isomorphic to 1H-phase MoS2, is a good prospect as an anode for K-ion batteries and a catalyst for hydrogen evolution through first-principles calculations. The CrP2 monolayer demonstrates not only a desirable high K storage capacity (940 mA h g-1) but also a low K-ion diffusion barrier (0.10 eV) and average open circuit voltage (0.40 V). On the other hand, its Gibbs free energy (0.02 eV)/active site density is superior/comparable to that of commercial Pt, resulting from the contribution of the lone pair electrons of the P atom. Its high structural stability and intrinsic metallicity can ensure high safety and performance during the cyclic process. These interesting properties make the CrP2 monolayer a promising multifunctional material for energy storage and conversion devices.

Theoretical Studies on the Quantum Capacitance of Two-Dimensional Electrode Materials for Supercapacitors

In recent years, supercapacitors have been widely used in the fields of energy, transportation, and industry. Among them, electrical double-layer capacitors (EDLCs) have attracted attention because of their dramatically high power density. With the rapid development of computational methods, theoretical studies on the physical and chemical properties of electrode materials have provided important support for the preparation of EDLCs with higher performance. Besides the widely studied double-layer capacitance (C_D), quantum capacitance (C_Q), which has long been ignored, is another important factor to improve the total capacitance (C_T) of an electrode. In this paper, we survey the recent theoretical progress on the C_Q of two-dimensional (2D) electrode materials in EDLCs and classify the electrode materials mainly into graphene-like 2D main group elements and compounds, transition metal carbides/nitrides (MXenes), and transition metal dichalcogenides (TMDs). In addition, we summarize the influence of different modification routes (including doping, metal-adsorption, vacancy, and surface functionalization) on the C_Q characteristics in the voltage range of ±0.6 V. Finally, we discuss the current difficulties in the theoretical study of supercapacitor electrode materials and provide our outlook on the future development of EDLCs in the field of energy storage.

A Novel Two-Dimensional ZnSiP2 Monolayer as an Anode Material for K-Ion Batteries and NO2 Gas Sensing

Using the crystal-structure search technique and first-principles calculation, we report a new two-dimensional semiconductor, ZnSiP₂, which was found to be stable by phonon, molecular-dynamic, and elastic-moduli simulations. ZnSiP₂ has an indirect band gap of 1.79 eV and exhibits an anisotropic character mechanically. Here, we investigated the ZnSiP₂ monolayer as an anode material for K-ion batteries and gas sensing for the adsorption of CO, CO₂, SO₂, NO, NO₂, and NH₃ gas molecules. Our calculations show that the ZnSiP₂ monolayer possesses a theoretical capacity of 517 mAh/g for K ions and an ultralow diffusion barrier of 0.12 eV. Importantly, the ZnSiP₂ monolayer exhibits metallic behavior after the adsorption of the K-atom layer, which provides better conductivity in a period of the battery cycle. In addition, the results show that the ZnSiP₂ monolayer is highly sensitive and selective to NO₂ gas molecules.

A two-dimensional metallic SnB monolayer as an anode material for non-lithium-ion batteries

Na-, K- and Mg-ion batteries (NIBs, KIBs and MIBs) have drawn considerable interest due to their high abundance and excellent safety. However, the lack of high-performance anode materials is a major obstacle to its development. A metallic SnB planar monolayer is predicted by using the two-dimensional global minimum structure search method of swarm intelligence. Based on first-principles calculations, we proved that the metal SnB monolayer has high binding energy and excellent dynamical, thermal and mechanical stability. It is worth noting that the SnB monolayer has several stable adsorption sites for Na-, K- and Mg-ions, so it has a high theoretical capacity of 620.93, 517.44 and 620.93 mA h g^-1, respectively. For Na-, K- and Mg-ion batteries, the low diffusion barriers of the SnB monolayer are 0.22, 0.07 and 0.68 eV, and the low average open circuit voltages are 0.42, 0.49 and 0.23 V, which ensure long service life and fast charging in practical applications. In addition, it is proved that the SnB monolayer maintains excellent conductivity and stability during the charge-discharge process. The results show that the SnB monolayer offers innovative advantages for the development of new two-dimensional planar structures that further advance the development of anode materials for metal ion batteries.

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.

Two-Dimensional TeB Structures with Anisotropic Carrier Mobility and Tunable Bandgap

Two-dimensional (2D) semiconductors with desirable bandgaps and high carrier mobility have great potential in electronic and optoelectronic applications. In this work, we proposed α-TeB and β-TeB monolayers using density functional theory (DFT) combined with the particle swarm-intelligent global structure search method. The high dynamical and thermal stabilities of two TeB structures indicate high feasibility for experimental synthesis. The electronic structure calculations show that the two structures are indirect bandgap semiconductors with bandgaps of 2.3 and 2.1 eV, respectively. The hole mobility of the β-TeB sheet is up to 6.90 × 10² cm² V^-1 s^-1. By reconstructing the two structures, we identified two new horizontal and lateral heterostructures, and the lateral heterostructure presents a direct band gap, indicating more probable applications could be further explored for TeB sheets.

Semiconducting MnB5monolayer as a potential photovoltaic material

Exploring new two-dimensional (2D) materials is of great significance for both basic research and practical applications. Although boron can form various 3D and 2D allotropes due to its ease of forming multi-center bonds, the coexistence of honeycomb and kagome boron structures has never been observed in any 2D material yet. In this article we apply first-principle swarm structural searches to predict the existence of a stable MnB₅structure, consisting of a sandwich of honeycomb and kagome borophenes. More interestingly, a MnB₅nanosheet is a semiconductor with a band gap of 1.07 eV and a high optical absorption in a broad band, which satisfies the requirements of a very good photovoltaic material. Upon moderate strain, MnB₅undergoes a conversion from an indirect to a direct band gap semiconductor. The power conversion efficiency of a heterostructure solar cell made of MnB₅is up to 18%. The MnB₅nanosheet shows a robust dynamical and thermal stability, stemming from the presence of intra- and interlayer multi-center σ and π bonds. These characteristics make MnB₅a promising photovoltaic material.

Modelling high performance potassium-ion battery anode materials with two-dimensional vanadium carbide MXene: the role of surface O- and S-terminations

Due to the low cost, high element abundance and intrinsic safety, potassium-ion batteries (KIBs) have attracted a surge of interest in recent years. Currently, the key challenge and obstacle to the development of KIBs is to find suitable anode materials with large capacity, high rate capability and small lattice changes during the charge/discharge process. MXenes with excellent energy storage properties are promising anode materials for KIBs and their energy performance largely depends on the surface termination. Here, two-dimensional O- and S-terminated V2C MXene anode materials are designed to model high performance potassium-ion batteries. Using first-principles calculations, the structural properties and potential battery performance in KIBs of V2CO2 and V2CS2 are systematically investigated. The inherent metallic nature, a small diffusion barrier, a low average open circuit voltage, and a high theoretical specific capacity (489.93 mA h g-1 of V2CO2 and 200.24 mA h g-1 of V2CS2) demonstrate that both of them are ideal anode materials for KIBs. Meanwhile, we also investigated the mechanism of the difference in energy performance between V2CO2 and V2CS2 at atomic and electronic levels, in other words, the energy performance difference introduced by surface O- and S-terminations.