Insight into the structural and electrochemical properties of the interface between a Na6SOI2 solid electrolyte and a metallic Na anode.

Solid electrolytes (SE) have attracted a great deal of interest as they can not only mitigate the safety issues related to currently used liquid organic electrolytes but also enable the introduction of a metallic Na anode with extreme energy density in sodium-ion batteries. For such application, SE should exhibit high interfacial stability against metallic Na as well as high ionic conductivity, and Na6SOI2 with a Na-rich double anti-perovskite structure was recently identified as a promising SE candidate. In this work, we performed first principles calculations to investigate the structural and electrochemical properties of the interface between Na6SOI2 and a metallic Na anode. Our calculations revealed that interfaces could be formed safely, keeping the ultra-fast ionic conductivity of the bulk phase near the interface. Through the electronic structure analysis of the interface models, we found the change of upward valence band bending at the surface to downward band bending at the interface, being accompanied by electronic charge transfer from a metallic Na anode to Na6SOI2 SE at the interface. This work provides valuable atomistic insight into the formation and properties of the interface between SE and alkali metal for enhancing battery performance.

Defect formation and ambivalent effects on electrochemical performance in layered sodium titanate Na2Ti3O7.

Point defects can be formed readily in layered transition metal oxides used as electrode materials for alkali-ion batteries but their influence on the electrode performance is yet obscure. In this work, we report a systematic first-principles study of intrinsic point defects and defect complexes in sodium titanate Na2Ti3O7, a low-voltage anode material for sodium-ion batteries. Within the density functional theory framework, we calculate the defect formation energies with a set of atomic chemical potentials, which define the synthesis conditions for the stable Na2Ti3O7 compound. Given the atomic chemical potential landscape and defect formation energies, we find that Na interstitials (Nai+), Na antisites (NaTi3-), and Na vacancies (VNa-) are dominant defects depending on the synthesis conditions. Furthermore, our calculations reveal that O vacancies (VO) and Ti antisites (TiNa) lower the electrode potential compared with the perfect system, whereas Ti vacancies (VTi) and NaTi increase the voltage. Finally, we evaluate the activation barriers for vacancy-mediated Na diffusion in the defective systems, finding that the intrinsic point defects improve the Na ion conduction. Our results provide a profound understanding of defect formation and influences on electrode performance, paving a way to designing high-performance anode materials.

Point defects and their impact on electrochemical performance in Na0.44MnO2 for sodium-ion battery cathode application.

Sodium manganese oxide Na0.44MnO2 (NMO) in an open structure with large tunnels is of great interest for sodium-ion battery cathode materials due to its high electrode voltage and capacity. However, its practical application is limited by poor rate performance, which can be tuned and improved by controlling point defects. We herein present a comprehensive study of intrinsic point defects in NMO using density functional theory (DFT) calculations in combination with thermodynamics. Using the DFT+U approach, we determine the formation energies of elementary defects and defect complexes depending on the sets of atomic chemical potentials, corresponding to a certain thermodynamic condition for the synthesis of stable NMO. Sodium interstitials are found to have the lowest formation energies in the relevant ranges of temperature and pressure. Other intrinsic point defects such as oxygen vacancies, sodium vacancies and manganese antisites can also be formed with proper formation energies and have an impact on the cathode performance. Compared to the perfect system, oxygen vacancies lower the electrode voltage, whereas manganese vacancies and antisites increase the voltage. We find that most point defects and defect complexes improve the sodium ion diffusivity, highlighting a proper control of defect formation for enhancing the performance of sodium-ion batteries.

Superior thermoelectric properties of ternary chalcogenides CsAg5Q3 (Q = Te, Se) predicted using first-principles calculations.

Tailoring novel thermoelectric materials (TEMs) with a high efficiency is challenging due to the difficulty in realizing both low thermal conductivity and high thermopower factor. In this work, we propose ternary chalcogenides CsAg5Q3 (Q = Te, Se) as promising TEMs based on first-principles calculations of their thermoelectric properties. Using lattice dynamics calculations within self-consistent phonon theory, we predict their ultralow lattice thermal conductivities below 0.27 W m-1 K-1, revealing the strong lattice anharmonicity and rattling vibrations of Ag atoms as the main origination. By using the mBJ exchange-correlation functional, we calculate the electronic structures with the direct band gaps in good agreement with experiments, and evaluate the charge carrier lifetime as a function of temperature within the deformation potential theory. Our calculations to solve Boltzmann transport equations demonstrate high thermopower factors of 2.5 mW m-1 K-2 upon p-type doping at 300 K, comparable to the conventional dichalcogenide thermoelectric GeTe. With these ultralow thermal conductivities and high thermopower factors, we determine a relatively high thermoelectric figure of merit ZT along the z-axis, finding the maximum value of ZTz to be 2.5 at 700 K for CsAg5Se3 by optimizing the hole concentration. Our computational results highlight the great potentiality of CsAg5Q3 (Q = Te, Se) for high-performance thermoelectric devices operating at room temperature.

First-principles study of the structural and electrochemical properties of NaxTi2O4 (0 ≤x≤ 1) with tunnel structure for anode applications in alkali-ion batteries.

Due to their low cost and easy synthesis method, several kinds of sodium titanates have been explored as anode materials for sodium ion batteries (SIBs). However, some of them have not yet been considered as electrode materials for SIBs, and here we have carried out a first-principles study on NaxTi2O4 compounds with two different tunnel structures, denoted as single and double phases, to demonstrate their structural and electrochemical properties upon Na or Li insertion. Our calculation results reveal that these compounds exhibit structural stability during sodiation/desodiation and a moderate electrode voltage of ∼0.82 V vs. Na+/Na with a specific capacity of ∼150 mA h g-1. In particular, the activation energy of Na+ ion migration in the double phase is estimated to be as low as 0.28 eV, which is the lowest value among the SIB electrodes developed so far, and this can be attributed to the wide tunnel structure. In addition, we verify their potentiality for use as anode materials in lithium ion batteries (LIBs) by exploring their properties upon Li insertion. Since these compounds are predicted to be promising anode materials for SIBs or LIBs by our calculations, we believe that our findings will promote further experimental studies.

Two-Dimensional Hybrid Composites of SnS2 with Graphene and Graphene Oxide for Improving Sodium Storage: A First-Principles Study.

Among the recent achievements of sodium-ion battery (SIB) electrode materials, hybridization of two-dimensional (2D) materials is one of the most interesting appointments. In this work, we propose to use the 2D hybrid composites of SnS2 with graphene or graphene oxide (GO) layers as the SIB anode, based on the first-principles calculations of their atomic structures, sodium intercalation energetics, and electronic properties. The calculations reveal that a graphene or GO film can effectively support not only the stable formation of a heterointerface with the SnS2 layer but also the easy intercalation of a sodium atom with low migration energy and acceptable low volume change. The electronic charge-density differences and the local density of states indicate that the electrons are transferred from the graphene or GO layer to the SnS2 layer, facilitating the formation of a heterointerface and improving the electronic conductance of the semiconducting SnS2 layer. These 2D hybrid composites of SnS2/G or GO are concluded to be more promising candidates for SIB anodes compared with the individual monolayers.

First-Principles Study on Structural, Electronic, and Optical Properties of Inorganic Ge-Based Halide Perovskites.

Using density functional theory calculations, we explore the structural, electronic, and optical properties of the inorganic Ge-based halide perovskites AGeX3 (A = Cs, Rb; X = I, Br, Cl) that can possibly be used as light absorbers. We calculate the lattice parameters of the rhombohedral unit cell with an R3 m space group, frequency-dependent dielectric constants, photoabsorption coefficients, effective masses of charge carriers, exciton binding energies, and electronic band structures by use of PBEsol and HSE06 functionals with and without SOC effect. We also predict the absolute electronic energy levels with respect to the external vacuum level by using the (001) surfaces with AX and GeX2 terminations, demonstrating their strong dependence on the surface terminations. The calculated results are found to be in reasonable agreement with the available experimental data for the cases of CsGeX3, while for the cases of RbGeX3 they are predicted for the first time in this work. We reveal that replacement of Cs with Rb can offer reasonable flexibility in optoelectronic properties matching for solar cell design and optimization, while X anion exchange gives rise to large changes.

First-principles study of NaxTiO2 with trigonal bipyramid structures: an insight into sodium-ion battery anode applications.

Developing efficient anode materials with low electrode voltage, high specific capacity and superior rate capability is urgently required on the road to commercially viable sodium-ion batteries (SIBs). Aiming at finding a new SIB anode material, we investigate the electrochemical properties of NaxTiO2 compounds with unprecedented penta-oxygen-coordinated trigonal bipyramid (TB) structures by using first-principles calculations. Identifying the four different TB phases, we perform the optimization of their crystal structures and calculate their energetics such as sodium binding energy, formation energy, electrode potential and activation energy for Na ion migration. The computations reveal that the TB-I phase is the best choice among the four TB phases for a SIB anode material due to a relatively low volume change of under 4% upon Na insertion, low electrode voltage under 1.0 V with a possibility of realizing the highest specific capacity of ∼335 mA h g-1 from full sodiation at x = 1, and reasonably low activation barriers under 0.35 eV at the Na content from x = 0.125 to x = 0.5. Through the analysis of electronic density of states and charge density difference upon sodiation, we find that the NaxTiO2 compounds in TB phases change from electron insulating to electron conducting materials due to the electron transfer from Na atoms to Ti ions, offering the Ti4+/Ti3+ redox couple for SIB operation.

Revealing the formation and electrochemical properties of bis(trifluoromethanesulfonyl)imide intercalated graphite with first-principles calculations.

Graphite has been reported to have anion, as well as cation, intercalation capacities as both a cathode and an anode host material for dual ion batteries. In this work, we study the intercalation of bis(trifluoromethanesulfonyl)imide (TFSI) anions from an ionic liquid electrolyte into graphite with first-principles calculations. We build models for TFSI-Cn compounds with systematically increasing graphene sheet unit cell sizes and investigate their stabilities by calculating the formation energy, resulting in the linear decrease of and arrival at the limit of stability. With unit cell sizes identified for stable compound formation, we reveal that the interlayer distance and relative volume expansion ratio of TFSI-Cn increases as we increase the concentration of the TFSI intercalate during the charge process. The electrode voltage is determined to range from 3.8 V to 3.0 V at a specific capacity ranging from 30 mA h g-1 to 54 mA h g-1, in agreement with experiment. Moreover, a very low activation barrier of under 50 meV for TFSI migration, as well as a good electronic conductivity, provide evidence for using these compounds as a promising cathode. Through the analysis of the charge transfer, we clarify the mechanism of TFSI-Cn formation, and reveal new prospects for developing graphite based cathodes.

Ab initio thermodynamic study of the SnO2(110) surface in an O2 and NO environment: a fundamental understanding of the gas sensing mechanism for NO and NO2.

For the purpose of elucidating the gas sensing mechanism of SnO2 for NO and NO2 gases, we determine the phase diagram of the SnO2(110) surface in contact with an O2 and NO gas environment by means of an ab initio thermodynamic method. Firstly we build a range of surface slab models of oxygen pre-adsorbed SnO2(110) surfaces using (1 × 1) and (2 × 1) surface unit cells and calculate their Gibbs free energies considering only oxygen chemical potential. The fully reduced surface containing the bridging and in-plane oxygen vacancies under oxygen-poor conditions, while the fully oxidized surface containing the bridging oxygen atom and the oxygen dimer under oxygen-rich conditions, and the stoichiometric surface in between, was proved to be most stable. Using the selected plausible NO-adsorbed surfaces, we then determine the surface phase diagram of SnO2(110) surfaces in (ΔµO, ΔµNO) space. Under NO-rich conditions, the most stable surfaces were those formed by NO adsorption on the most stable surfaces in contact with only oxygen gas. Through the analysis of electronic charge transfer and density of states during NOx adsorption on the surface, we provide a meaningful understanding about the gas sensing mechanism.

First-principles study of molecular hydrogen binding to heme in competition with O2, NO and CO.

Molecular hydrogen shows antioxidant activity and distinct efficacy towards vascular diseases, but the understanding of this is not yet satisfactory at the atomic level. In this work, we study the binding properties of H2 to the heme group in relation with other diatomic molecules (DMs), including O2, NO and CO, and their displacement reactions, using first-principles calculations. We carry out molecular modeling of the heme group, using iron-porphyrin with the imidazole ligand, i.e., FePIm, and smaller models of Fe(CnHn+2N2)2NH3 with n = 3 and 1, and of molecular complexes of heme-DM and -H. Through analysis of optimized geometries and energetics, it is found that the order of binding strength of DMs or H to the Fe of heme is NO > O2 > CO > H > H2 for FePIm-based systems, while it is H > O2 > NO > CO > H2 for model-based systems. We calculate the activation energies for displacement reactions of H2 and H by other DMs, revealing that the H2 displacements occur spontaneously while the H displacements require a large amount of energy. Finally, our calculations corroborate that the rate constants increase with increasing temperature according to the Arrhenius relation.

First-principles study of sodium adsorption on defective graphene under propylene carbonate electrolyte conditions.

Hard carbon (HC) has been predominantly used as a typical anode material of sodium-ion batteries (SIBs) but its sodiation mechanism has been debated. In this work, we investigate the adsorption of Na atoms on defective graphene under propylene carbonate (PC) and water solvent as well as vacuum conditions to clarify the sodiation mechanism of HC. Within the joint density functional theory framework, we use the nonlinear polarizable continuum model for PC and the charge-asymmetric nonlocally-determined local electric solvation model for water. Our calculations reveal that the centre of each point defect such as mono-vacancy (MV), di-vacancy (DV) and Stone-Wales is a preferable adsorption site and the electrolyte enhances the Na adsorption through implicit interaction. Furthermore, we calculate the formation energies of multiple Na atom arrangements on the defective graphene and estimate the electrode potential versus Na/Na+, verifying that the multiple Na adsorption on the MV and DV defective graphene under the PC electrolyte conditions is related to the slope region of the discharge curve in HC. This reveals new prospects for optimizing anodes and electrolytes for high performance SIBs.

First-principles study of mercaptoundecanoic acid molecule adsorption and gas molecule penetration onto silver surface: an insight for corrosion protection.

Recently, 11-mercaptoundecanoic acid (MUA) molecule has attracted attention as a promising passivation agent of Ag nanowire (NW) network electrode for corrosion inhibition, but the underneath mechanism has not been elaborated. In this work, we investigate adsorption of MUA molecule on Ag(1 0 0) and Ag(1 1 1) surface, adsorption of air gas molecules of H2O, H2S and O2 on MUA molecular end surface, and their penetrations into the Ag surface using the first-principles calculations. Our calculations reveal that the MUA molecule is strongly bound to the Ag surface with the binding energies ranging from -0.47 to -2.06 eV and the Ag-S bond lengths of 2.68-2.97 Å by Lewis acid-base reaction. Furthermore, we find attractive interactions between the gas molecules and the MUA@Ag complexes upon their adsorptions and calculate activation barriers for their migrations from the outermost end of the complexes to the top of Ag surface. It is found that the penetrations of H2O and H2S are more difficult than the O2 penetration due to their higher activation barriers, while the O2 penetration is still difficult, confirming the corrosion protection of Ag NW network by adsorbing the uniform monolayer of MUA. With these findings, this work can contribute to finding a better passivation agent in the strategy of corrosion protection of Ag NW network electrode.

Revealing the effect of Nb or V doping on anode performance in Na2Ti3O7 for sodium-ion batteries: a first-principles study.

Sodium titanate Na2Ti3O7 (NTO) has superior electrochemical properties as an anode material in sodium-ion batteries (SIBs), and Nb or V doping is suggested to enhance the electrode performance. In this work, we carry out systematic first-principles calculations of the structural, electronic and electrochemical properties of NTO and Na2Ti2.75M0.25O7 (M = Nb, V), using supercells to reveal the effect of Nb or V NTO-doping on its anode performance. It is found that Nb doping gives rise to the expansion of cell volume but V doping induces the shrinkage of cell volume due to the larger and smaller ionic radius of the Nb and V ions, respectively, compared to that of the Ti ion. We perform structural optimization of the intermediate phases of Na2+xM3O7 with increasing Na content x from 0 to 2, revealing that the overall relative volume expansion rate is slightly increased by Nb and V doping but remains lower than 3%. Our calculations demonstrate that the electrode potential of NTO is slightly raised and the specific capacity is reduced, but the electronic and ionic conductivities are improved by Nb or V doping. With the revealed understanding and mechanisms, our work will contribute to the search for advanced electrode materials for SIBs.

First-principles study on structural, electronic and optical properties of halide double perovskite Cs2AgBX6 (B = In, Sb; X = F, Cl, Br, I).

All-inorganic halide double perovskites (HDPs) attract significant attention in the field of perovskite solar cells (PSCs) and light-emitting diodes. In this work, we present a first-principles study on structural, elastic, electronic and optical properties of all-inorganic HDPs Cs2AgBX6 (B = In, Sb; X = F, Cl, Br, I), aiming at finding the possibility of using them as photoabsorbers for PSCs. Confirming that the cubic perovskite structure can be formed safely thanks to the proper geometric factors, we find that the lattice constants are gradually increased on increasing the atomic number of the halogen atom from F to I, indicating the weakening of Ag-X and B-X interactions. Our calculations reveal that all the perovskite compounds are mechanically stable due to their elastic constants satisfying the stability criteria, whereas only the Cl-based compounds are dynamically stable in the cubic phase by observing their phonon dispersions without soft modes. The electronic band structures are calculated with the Heyd-Scuseria-Ernzerhof hybrid functional, demonstrating that the In (Sb)-based HDPs show direct (indirect) transition of electrons and the band gaps are decreased from 4.94 to 0.06 eV on going from X = F to I. Finally, we investigate the macroscopic dielectric functions, photo-absorption coefficients, reflectivity and exciton properties, predicting that the exciton binding strength becomes weaker on going from F to I.

Improving the stability of hybrid perovskite FAPbI3 by forming 3D/2D interfaces with organic spacers.

Interfaces composed of three-dimensional (3D) and 2D organic-inorganic hybrid formamidinium lead iodide (FAPbI3) linked by organic spacers (OSs) are studied using first-principles calculations. The OS cations with aromatic rings, like phenylethylammonium and anilinium (AN), are found to be more favourable for enhancing the stability of the 3D/2D interface than butylammonium with aliphatic chains. The AN-based interface shows the highest resistance to penetration of water molecules.

High thermoelectric performance in metal phosphides MP2 (M = Co, Rh and Ir): a theoretical prediction from first-principles calculations.

Although metal phosphides have good electronic properties and high stabilities, they have been overlooked in general as thermoelectrics based on expectation of high thermal conductivity. Here we propose the metal phosphides MP2 (M = Co, Rh and Ir) as promising thermoelectrics through first-principles calculations of their thermoelectric properties. By using lattice dynamics calculations within unified theory of thermal transport in crystal and glass, we obtain the lattice thermal conductivities κ l of MP2 as 0.63, 1.21 and 1.81 W m-1 K at 700 K for M = Co, Rh and Ir, respio ectively. Our calculations for crystalline structure, phonon dispersion, Grüneisen parameters and cumulative κ l reveal that such low κ l originates from strong rattling vibrations of M atoms and lattice anharmonicity, which significantly suppress heat-carrying acoustic phonon modes coupled with low-lying optical modes. Using mBJ exchange-correlation functional, we further calculate the electronic structures and transport properties, which are in good agreement with available experimental data, evaluating the relaxation time of charge carrier within deformation potential theory. We predict ultrahigh thermopower factors as 10.2, 7.1 and 6.4 mW m-1 K2 at 700 K for M = Co, Rh and Ir, being superior to the conventional thermoelectrics GeTe. Finally, we estimate their thermoelectric performance by computing figure of merit ZT, finding that upon n-type doping ZT can reach ∼1.7 at 700 K specially for CoP2. We believe that our work offers a novel materials platform to search for high-performance thermoelectrics using metal phosphides.

Antioxidation activity of molecular hydrogen via protoheme catalysis in vivo: an insight from ab initio calculations.

Recently, molecular hydrogen has been found to exhibit antioxidation activity through many clinical experiments, but the mechanism has not been fully understandable at atomic level. In this work, we perform systematic ab initio calculations of protoheme-hydrogen complexes to clarify the antioxidation mechanism of molecular hydrogen. We make molecular modeling of iron-protoporphyrin coordinated by imidazole, FeP(Im), and its hydrogen as well as dihydrogen complexes, together with reactive oxygen/nitrogen species (RONS). We carry out structural optimization and Mulliken charge analysis, revealing the two kinds of bonding characteristics between FeP(Im) and H[Formula: see text]: dihydrogen bonding in the end-on asymmetric configuration and Kubas bonding in the side-on symmetric configuration of H[Formula: see text] molecule. The activation barriers for adsorption and dissociation of H[Formula: see text] on and further desorption of H atom from FeP(Im) are found to be below 2.78 eV at most, which is remarkably lower than the H-H bond breaking energy of 4.64 eV in free H[Formula: see text] molecule. We find that the hydrogen bond dissociation energies of FeP(Im)-H[Formula: see text] and -H complexes are lower than those of RONS-H complexes, indicating the decisive role of protoheme as an effective catalyst in RONS antioxidation by molecular hydrogen in vivo.

Enhancing the Photocatalytic Hydrogen Evolution Performance of the CsPbI3/MoS2 Heterostructure with Interfacial Defect Engineering.

Developing highly efficient photocatalysts for the hydrogen evolution reaction (HER) by solar-driven water splitting is a great challenge. Here, we study the atomistic origin of interface properties and the HER performance of all-inorganic iodide perovskite ß-CsPbI3/2H-MoS2 heterostructures with interfacial vacancy defects using first-principles calculations. Both CsI/MoS2 and PbI2/MoS2 heterostructures have strong binding and dipole moment, which are enhanced by interfacial iodine vacancies (VI). Because of the nature of type II heterojunctions, photogenerated electrons on the CsPbI3 side are promptly transferred to the MoS2 side where HER occurs, and sulfur vacancies (VS) spoil this process, acting as surface traps. The formation energies of various defects are calculated by applying atomistic thermodynamics, identifying the growth conditions for promoting VI and suppressing VS formation. The HER performance is enhanced by forming interfaces with lower ΔGH values for hydrogen adsorption on the MoS2 side, suggesting PbI2/MoS2 with VI to be the most promising photocatalyst.

First-principles study on structural, electronic, magnetic and thermodynamic properties of lithium ferrite LiFe5O8.

Lithium ferrite, LiFe5O8 (LFO), has attracted great attention for various applications, and there has been extensive experimental studies on its material properties and applications. However, no systematic theoretical study has yet been reported, so understanding of its material properties at the atomic scale is still required. In this work, we present a comprehensive investigation into the structural, electronic, magnetic and thermodynamic properties of LFO using first-principles calculations. We demonstrate that the ordered α-phase with ferrimagnetic spin configuration is energetically favourable among various crystalline phases with different magnetic configurations. By applying the DFT + U approach with U = 4 eV, we reproduce the lattice constant, band gap energy, and total magnetization in good agreement with experiments, emphasizing the importance of considering strong correlation and spin-polarization effects originating from the 3d states of Fe atoms. We calculated the phonon dispersions of LFO with ferrimagnetic and non-magnetic states, and subsequently evaluated the Gibbs free energy differences between the two states, plotting the P-T diagram for thermodynamic stability of the ferrimagnetic against non-magnetic state. From the P-T diagram, the Curie temperature is found to be ∼925 K at the normal condition and gradually increase with increasing pressure. Our calculations explain the experimental observations for material properties of LFO, providing a comprehensive understanding of the underlying mechanism and useful guidance for enhancing performance of LFO-based devices.