First-Principles Study of Discharge Products and Their Stability for Lithium-Nitrogen Batteries.

Li-N2 batteries present a relatively novel approach to N2 immobilization, and an advanced N2/Li3N cycling method is introduced in this study. The low operating overpotential of metal-air batteries is quite favorable to their stable cycling performance, providing a prospect for the development of a new type of battery with extreme voltage. The battery system of Li-N2 uses N2 as the positive electrode, lithium metal as the negative electrode, and a conductive medium containing soluble lithium salts as the electrolyte. In accordance with its voltage-distribution trend, a variety of lithium-nitrogen molecule intermediates are produced during the discharge process. There is a lack of theoretical description of material changes at the microscopic level during the discharge process. In this paper, the first-principles approach is used to simulate and analyze possible material changes during the discharge process of Li-N2 batteries. The discharge process is simulated on a 4N-graphene anode substrate model, and simulations of its electrostatic potential, Density of States (DOS), HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) aspects confirm that the experimentally found Li3N becomes the final stabilized product of the Li-N2 battery. It can also be seen in the density of states that graphene with adsorption of 4N transforms from semiconducting to metallic properties. In addition, the differential charge also indicates that the Li-N2 material has a strong adsorption effect on the substrate, which can play the dual role of electricity storage and nitrogen fixation.

Electronic Properties of CrB/Co2CO2 Superlattices by Multiple Descriptor-Based Machine Learning Combined with First-Principles.

In recent times, newly unveiled 2D materials exhibiting exceptional characteristics, such as MBenes and MXenes, have gained widespread application across diverse domains, encompassing electronic devices, catalysis, energy storage, sensors, and various others. Nonetheless, numerous technical bottlenecks persist in the development of high-performance, structurally flexible, and adjustable electronic device materials. Research investigations have demonstrated that 2D van der Waals superlattices (vdW SLs) structures comprising materials exhibit exceptional electrical, mechanical, and optical properties. In this work, the advantages of both materials are combined and compose the vdW SLs structure of MBenes and MXenes, thus obtaining materials with excellent electronic properties. Furthermore, it integrates machine learning (ML) with first-principles methods to forecast the electrical properties of MBene/MXene superlattice materials. Initially, various configurations of MBene/MXene superlattice materials are explored, revealing that distinct stacking methods exert significant influence on the electronic structure of MBene/MXene materials. Specifically, the BABA-type stacking of CrB (layer A) and Co2CO2 MXene (layer B) is most stable configureation. Subsequently, multiple descriptors of the structure are constructed to predict the density of states  of vdW SLs through the employment of ML techniques. The best model achieves a mean absolute error (MAE) as low as 0.147 eV.

First-principles study of the discharge electrochemical and catalytic performance of the sulfur cathode host Fe0.875M0.125S2 (M = Ti, V).

Lithium-sulfur batteries (LSBs) are one of the most promising energy storage devices with high energy density. However, their application and commercialization are hampered by the slow Li-S redox chemistry. Fe0.875M0.125S2 (M = Ti, V), as the sulfur cathode host, enhances the Li-S redox chemistry. FeS2 with Pa3̄ is transformed into Li2FeS2 with P3̄m1 after discharge. The structure changes and physicochemical properties during Fe0.875M0.125S2 discharge process are further investigated to screen out the sulfur cathode host materials with the best comprehensive properties. The discharge structure of Fe0.875M0.125S2 is verified by the thermodynamic stability of Li-deficient phases, voltage and capacity based on Monte Carlo methods. Fe0.875M0.125S2 with Pa3̄ is transformed into Li2Fe0.875M0.125S2 with P3̄m1 after discharge. Using the first-principles calculations, the physicochemical properties of Li2Fe0.875M0.125S2 are systematically investigated, including the formation energy, voltage, theoretical capacity, electrical conductivity, Li+ diffusion, catalytic performance and Li2S oxidation decomposition. The average redox voltage of Li2Fe0.875V0.125S2 is higher than that of Li2Fe0.875Ti0.125S2. Li2Fe0.875M0.125S2 shows metallic properties. Li2Fe0.875V0.125S2 is more beneficial to the reduction reaction of Li2S2 and Li2S oxidation decomposition. Fe0.875V0.125S2 has more potential as the sulfur cathode host than Fe0.875Ti0.125S2 in LSBs. A new strategy for the selection of the sulfur cathode host material for LSBs is provided by this work.

Selecting non-metal doped FeS2 as efficient sulfur cathode host for enhanced Li-S redox chemistry.

Lithium-sulfur batteries (LSBs) are considered as the development direction of the new generation energy storage system due to their high energy density and low cost. The slow redox kinetics of sulfur and the shuttle effect of lithium polysulfide (LiPS) are considered to be the main obstacles to the practical application of LSBs. Transition-metal sulfide as the cathode host can improve the Li-S redox chemistry. However, there has been no investigation of the application of FeS2 host in Li-S redox chemistry. Applying the first-principles calculations, we investigated the formation energy, band gap, Li+ diffusion, adsorption energy, catalytic performance and Li2 S decomposition barrier of FeAx S2-x (A=N, P, O, Se; x=0, 0.125, 0.25, 0.375) to explore the Li-S redox chemistry and finally select excellent host material. FeA0.25 S1.75 (A=P, Se) has a low Li+ diffusion barrier and superior electronic conductivity. FeO0.25 S1.75 is more favorable for LiPS adsorption, followed by FeP0.25 S1.75 . FeP0.25 S1.75 (001) shows a low overpotential for the Li-S redox chemistry. In summary, FeP0.25 S1.75 has more application potential in LSBs due to its physical and chemical properties, followed by FeSe0.25 S1.75 . This work provides theoretical guidance for the design and selection of the sulfur cathode host materials in LSBs.

Double strengthening induced by grain boundary segregation of solute elements in gradient nano Ni-Co alloys.

Gradient induced unusual strain hardening achieves the equilibrium of the strength and plasticity of alloys, and is an important strategy for the optimization of the mechanical properties of metals and alloys. The segregation of solute elements can greatly improve the grain boundary stability, inhibit grain coarsening and promote the mechanical strength of the alloy. In our efforts, the segregation structure of the solute element Co was designed and added to the gradient nano Ni-Co alloy, and the two strengthening strategies were applied simultaneously in one structure. The mechanical strength of the alloy achieved a second increase based on the unique combination of gradient induced strain hardening and high plasticity, especially the yield strength of alloy increase amplitude reach to 42%. This provides a positive direction for the alloy strengthening strategy. In the process of secondary strengthening, the micro-mechanism is divided into two stages: in the first stage, the gradient strain provides the alloy with geometrically necessary dislocations and a multi-axial stress state, and the existence of large numbers of geometrically necessary dislocations creates good conditions for the second stage strengthening. In the second stage, the solute segregation induced stable grain boundaries produce a strong pinning effect on the geometrically necessary dislocation, which realizes the coupling of grain boundary strengthening and dislocation strengthening. This provides a new strengthening strategy and positive theoretical guidance for the experimental preparation of advanced alloys with excellent properties.

Insights into the electrochemical properties of Li2FeS2 after FeS2 discharging.

All-solid-state lithium-sulfur batteries (ASSLSBs) have high reversible characteristics owing to the high redox potential, high theoretical capacity, high electronic conductivity, and low Li+ diffusion energy barrier in the cathode. Monte Carlo simulations with cluster expansion, based on the first-principles high-throughput calculations, predicted a phase structure change from Li2FeS2 (P3̄M1) to FeS2 (PA3̄) during the charging process. LiFeS2 is the most stable phase structure. The structure of Li2FeS2 after charging was FeS2 (P3̄M1). By applying the first-principles calculations, we explored the electrochemical properties of Li2FeS2 after charging. The redox reaction potential of Li2FeS2 was 1.64 to 2.90 V, implying a high output voltage of ASSLSBs. Flatter voltage step plateaus are important for improving the electrochemical performance of the cathode. The charge voltage plateau was the highest from Li0.25FeS2 to FeS2 and followed from Li0.375FeS2 to Li0.25FeS2. The electrical properties of LixFeS2 remained metallic during the Li2FeS2 charging process. The intrinsic Li Frenkel defect of Li2FeS2 was more conducive to Li+ diffusion than that of the Li2S Schottky defect and had the largest Li+ diffusion coefficient. The good electronic conductivity and Li+ diffusion coefficient of the cathode implied a better charging/discharging rate performance of ASSLSBs. This work theoretically verified the FeS2 structure after Li2FeS2 charging and explored the electrochemical properties of Li2FeS2.

Insight into the Electronic Properties of Semiconductor Heterostructure Based on Machine Learning and First-Principles.

A first-principles approach is a powerful means of gaining insight into the intrinsic structure and properties of materials. However, with the implementation of material genetic engineering, it is still a challenging road to discover materials with high satisfaction. One alternative is to employ machine-learning techniques to mine data and predict performance. In this present contribution, the method is taken to predict the band gap opening value of graphene in a heterostructure. First, the data of 2076 binary compounds in the Materials Project library are used to achieve visual dimensionality reduction of the data set through a t-distributed stochastic neighbor embedding (t-SNE) algorithm in unsupervised learning. Then, a series of semiconductor components are screened out and form heterostructures with graphene. Second, by means of the ensemble learning EXtreme Gradient Boost (XGBoost) algorithm and support vector machine (SVM) technology, two prediction frameworks are built to predict the band gap opening value of the graphene in the system. Finally, density functional theory (DFT) is used to calculate the energy band and density of states for comparison. Analysis shows that the prediction model has an accuracy rate of 88.3%, and there is little difference between prediction results and calculation results. We anticipate that this framework model would have fascinating applications in predicting the electronic properties of various multiphase materials.

Cluster expansion Monte Carlo study of indium-aluminum segregation and homogenization in CuInSe2-CuAlSe2 pseudobinary alloys.

The influence of temperature and Al content on the segregation and homogenization behaviour of In-Al atoms in CuIn1-xAlxSe2 (CIAS) pseudobinary alloys is studied using a combination of cluster expansion Monte Carlo simulations and first-principles calculations. Such alloys are promising materials for a number of solar-energy-related applications. We found that the segregation of In-Al atoms in CIAS alloys with different Al contents occurs at relatively low temperatures. The cluster morphology of Al(In) atoms in CIAS alloys at 73 K appears in an ellipsoidal, rod-like or lamellar form, depending on the Al(In) content. The spatial distribution of In-Al atoms becomes homogeneous as the temperature increases. By determining the inhomogeneity degree σ of In-Al distributions in CIAS alloys at a series of temperatures, we found that the variation of σ with temperature (T) for all the considered CIAS alloys are sigmoidal in general and the sharp decrease in σ within a certain temperature range implies the occurrence of inhomogeneous-to-homogeneous phase transition. The inhomogeneity degree σ of CIAS alloys before or after the phase transition (phase segregation) increases as the content of Al(x) and In(1 - x) atoms gets closer. The σ(T) data points obtained by us can be well fitted with the Boltzmann function, which can give several physically meaningful parameters such as the phase transition temperature T0, temperature range of phase transition ΔT and so on. The fitted T0 and ΔT values for CIAS alloys with different Al content were proved to be reliable. The novel method for predicting the T0 and ΔT may be applied to many other binary or pseudobinary material systems with positive formation energy.

Manganese doping mechanism in a CsPbI2Br photovoltaic material: a first-principles study.

As a light absorbing material of perovskite solar cells, Mn-doped CsPbI2Br has a better phase stability than the undoped one. In order to deeply understand the doping mechanism of Mn, the effect of substitutional and interstitial Mn doping on the structural, electronic and optical properties of CsPbI2Br has been investigated by first-principles calculations based on density functional theory. It is found that the binding energy of both the substitutional and the interstitial Mn-doped CsPbI2Br is negative and the binding energy difference between them is only 2.8 meV, which indicates that both the substitutional and the interstitial doping structures should be stable for Mn-doped CsPbI2Br and the latter is slightly preferred over the former due to the lower binding energy. The lattice parameters of CsPbI2Br change oppositely for two Mn-doping cases. Based on the comparative analysis of the electronic structures for CsPbI2Br and Mn-doped CsPbI2Br, we found that the substitutional doping of Mn introduces intermediate bands near the Fermi level, making CsPbI2Br an intermediate band semiconductor; for the interstitial Mn-doped CsPbI2Br the Fermi level enters conduction bands, making it an n-type semiconductor material with enhanced conductivity. The complex dielectric function and the absorption spectrum of Mn-doped and undoped CsPbI2Br were calculated and are basically consistent with the experimental results.

Linear-scaling density functional simulations of the effect of crystallographic structure on the electronic and optical properties of fullerene solvates.

In this work, the crystal properties, HOMO and LUMO energies, band gaps, density of states, as well as the optical absorption spectra of fullerene C60 and its derivative phenyl-C61-butyric-acid-methyl-ester (PCBM) co-crystallised with various solvents such as benzene, biphenyl, cyclohexane, and chlorobenzene were investigated computationally using linear-scaling density functional theory with plane waves as implemented in the ONETEP program. Such solvates are useful materials as electron acceptors for organic photovoltaic (OPV) devices. We found that the fullerene parts contained in the solvates are unstable without solvents, and the interactions between fullerene and solvent molecules in C60 and PCBM solvates make a significant contribution to the cohesive energies of solvates, indicating that solvent molecules are essential to keep C60 and PCBM solvates stable. Both the band gap (Eg) and the HOMO and LUMO states of C60 and PCBM solvates are mainly determined by the fullerene parts contained in solvates. Chlorobenzene- and ortho-dichlorobenzene-solvated PCBM are the most promising electron-accepting materials among these solvates for increasing the driving force for charge separation in OPVs due to their relatively high LUMO energies. The UV-Vis absorption spectra of solvent-free C60 and PCBM crystals in the present work are similar to those of C60 and PCBM thin films shown in the literature. Changes in the absorption spectra of C60 solvates relative to the solvent-free C60 crystal are more significant than those of PCBM solvates due to the weaker effect of solvents on the π-stacking interactions between fullerene molecules in the latter solvates. The main absorptions for all C60 and PCBM crystals are located in the ultraviolet (UV) region.