Origins of High Air Sensitivity and Treatment Strategies in O3-Type NaMn1/3 Fe1/3Ni1/3O2.

Sodium-ion layered oxides are one of the most highly regarded sodium-ion cathode materials and are expected to be used in electric vehicles and large-scale grid-level energy storage systems. However, highly air-sensitive issues limit sodium-ion layered oxide cathode materials to maximize cost advantages. Industrial and scientific researchers have been developing cost-effective air sensitivity treatment strategies with little success because the impurity formation mechanism is still unclear. Using density functional theory calculations and ab initio molecular dynamics simulations, this work shows that the poor air stability of O3-type NaMn1/3Fe1/3Ni1/3O2 (NMFNO) may be as follows: (1) low percentage of nonreactive (003) surface; (2) strong surface adsorption capacity and high surface reactivity; and (3) instability of the surface sodium ions. Our physical images point out that the high reactivity of the NMFNO surface originates from the increase in electron loss and unpaired electrons (magnetic moments) of the surface oxygen active site as well as the enhanced metal coactivation effect due to the large radius of the sodium ion. We also found that the hydrolysis reaction requires a higher reactivity of the surface oxygen active site, while the carbon hybridization mode transformation in carbonate formation depends mainly on metal activation and does not even require the involvement of surface oxygen active sites. Based on the calculation results and our proposed physical images, we discuss the feasibility of these treatment strategies (including surface morphology modulation, cation/anion substitution, and surface configuration design) for air-sensitive issues.

TiB4 and SrB8 monolayers: high capacity and zero strain-like anode materials for Li/Na/K/Ca ion batteries.

Storage capacity, average open circuit voltage (OCV), diffusion barrier, lattice parameter changes, etc. are key indicators of whether a material would be suitable for use as a Li-ion or non-Li-ion battery (LIB or NLIB) anode. The rapid development of 2D materials over the past few decades has opened up new possibilities for these metrics. Using first-principles calculations, we prove that two 2D materials, TiB4 and SrB8, show excellent performance in terms of the above metrics when used as anodes for LIBs or NLIBs. As detailed, TiB4 has an Li\Na\K\Ca storage capacity of 588 mA h g-1, 588 mA h g-1, 588 mA h g-1, and 1176 mA h g-1, respectively, and SrB8 has an Li\Na\K\Ca storage capacity of 308 mA h g-1, 308 mA h g-1, 462 mA h g-1, and 616 mA h g-1, respectively, and they show good electrical conductivity whether existing Li, Na, K or Ca is adsorbed or not. The diffusion barriers on both surfaces are low, indicating good rate performance. The average OCV is also very low. In particular, the lattice parameters of the two materials change very little during the embedding of Li\Na\K\Ca. For Ti9B36 the corresponding values are about 0.37% (Li), 0.33% (Na), 0.64% (K) and 0.03% (Ca), and for Sr8B64 the corresponding values are about 0.70% (Li), 0.16% (Na), 0.13% (K) and 0.004% (Ca), which imply zero strain-like character and great cycling performance. All the above results show that TiB4 and SrB8 monolayers are very promising Li\Na\K\Ca ion battery anodes.

Structural and Na-ion diffusion behavior of O3/P3/P2-type NaNi1/3Mn1/3Fe1/3O2 cathode for Na-ion batteries from first-principles study.

A layered sodium-ion battery cathode, O3/P3/P2-type NaNi1/3Mn1/3Fe1/3O2, has been systematically investigated by first-principles density functional theory to explore the detailed structural and Na-ion diffusion behavior during desodiation. Our results suggest that the (NaO6) spacing is greatest in the P3 phase and lowest in the O3 phase, with the P2 phase exhibiting intermediate spacing. During desodiation, the intermediate stages have a greater (NaO6) spacing than the initial and final stages. The great (NaO6) spacing facilitates the formation of the P3 phase, resulting in the structural evolution of NaxNi1/3Mn1/3Fe1/3O2 from the O3 to the P3 phase at x ≈ 0.59, finally reaching the O3 structure again at x ≈ 0.12. The electronic structure clearly proves that both Ni and Fe are active in O3/P3/P2-type NaxNi1/3Mn1/3Fe1/3O2. Ni2+ is oxidized to Ni3+ as Na content decreases from x = 1 to x = 0.66, then further oxidized to Ni4+ at x = 0.33, and finally, Fe3+ → Fe4+ oxidation occurs at x = 0. In the Na ion diffusion behavior, the order of the barrier is O3 (0.82 eV) > P2 (0.53 eV) > P3 (0.35 eV) at the initial stage, whereas it is O3 (0.53 eV) > P3 (0.21 eV) > P2 (0.16 eV) at a highly desodiated stage. The former can be traced back to the (NaO6) spacing, but the latter is related to the different Na sites. Our results thus provide a factor of the structural evolution and Na ion diffusion barrier by considering (NaO6) width and Na site changes during desodiation.

A Metal-Organic Frameworks Derived 1T-MoS2 with Expanded Layer Spacing for Enhanced Electrocatalytic Hydrogen Evolution.

Metal phase molybdenum disulfide (1T-MoS2 ) is considered a promising electrocatalyst for hydrogen evolution reaction (HER) due to its activated basal and superior electrical conductivity. Here, a one-step solvothermal route is developed to prepare 1T-MoS2 with expanded layer spacing through the derivatization of a Mo-based organic framework (Mo-MOFs). Benefiting from N,N-dimethylformamide oxide as external stress, the interplanar spacing of (002) of the MoS2 catalyst is extended to 10.87 Å, which represents the largest one for the 1T-MoS2 catalyst prepared by the bottom-up approach. Theoretical calculations reveal that the expanded crystal planes alter the electronic structure of 1T-MoS2 , lower the adsorption-desorption potentials of protons, and thus, trigger efficient catalytic activity for HER. The optimal 1T-MoS2 catalyst exhibits an overpotential of 98 mV at 10 mA cm-2 for HER, corresponding to a Tafel slope of 52 mV dec-1 . This Mo-MOFs-derived strategy provides a potential way to design high-performance catalysts by adjusting the layer spacing of 2D materials.

The Mechanism of Li Deposition on the Cu Substrates in the Anode-Free Li Metal Batteries.

Due to the rapid growth in the demand for high-energy-density Lithium (Li) batteries and insufficient global Li reserves, the anode-free Li metal batteries are receiving increasing attention. Various strategies, such as surface modification and structural design of copper (Cu) current collectors, have been proposed to stabilize the anode-free Li metal batteries. Unfortunately, the mechanism of Li deposition on the Cu surfaces with the different Miller indices is poorly understood, especially on the atomic scale. Here, the large-scale molecular dynamics simulations of Li deposition on the Cu substrates are performed in the anode-free Li metal batteries. The results show that the surface properties of the Li panel can be altered through the different Cu substrate surfaces. Through surface similarity analysis, potential energy distributions,and inhomogeneous deposition simulations, it is found that the Li atoms exhibit different potential energy variances and kinetic characteristics on the different Cu surfaces. Furthermore, a proposal to reduce the fraction of the (110) facet in commercial Cu foils is made to improve the reversibility and stability of Li plating/stripping in the anode-free Li metal batteries.

Tuning a small electron polaron in FePO4 by P-site or O-site doping based on DFT+U and KMC simulation.

Due to the existence of a small polaron, the intrinsic electronic conductivity of olivine-structured LiFePO4 is quite low, limiting its performance as a cathode material for lithium-ion batteries (LIBs). Previous studies have mainly focused on improving intrinsic conductivity through Fe-site doping while P-site or O-site doping has rarely been reported. Herein, we studied the formation and dynamics of the small electron polaron in FeP1-αXαO4 and FePO4-ßZß by employing the density functional theory with the on-site Hubbard correction terms (DFT+U) and Kinetic Monte Carlo (KMC) simulation, where X and Z indicate the doping elements (X = S, Se, As, Si, V; Z = S, F, Cl), and α and ß indicate the light doping at the P position (α = 0.0625) and O position (ß = 0.015625), respectively. We confirmed the small electron polaron formation in pristine FePO4 and its doped systems, and the polaron hopping rates for all systems were calculated according to the Marcus-Emin-Holstein-Austin-Mott (MEHAM) theory. We found that the hopping process is adiabatic for most cases with the defects breaking the original symmetry. Based on the KMC simulation results, we found that the doping of S at the P site changes the polaron's motion mode, which is expected to increase the mobility and intrinsic electronic conductivity. This study attempts to provide theoretical guidance to improve the electronic conductivity of LiFePO4-like cathode materials with better rate performance.

Theoretical study on the magnetic properties of cathode materials in the lithium-ion battery.

The layered LiMO2 (M = Co, Ni, and Mn) materials are commonly used as the cathode materials in the lithium-ion battery due to the distinctive layer structure for lithium extraction and insertion. Although their electrochemical properties have been extensively studied, the structural and magnetic properties of LiNiO2 are still under considerable debate, and the magnetic properties of monoclinic LiMnO2 are seldom reported. In this work, a detailed study of LiNiO2, LiMnO2, and a half-doped material LiNi0.5Mn0.5O2 is performed via both first-principles calculations and Monte Carlo simulations based on the effective spin Hamiltonian model. Through considering different structures, it is verified that a structure with a zigzag-type pattern is the most stable one of LiNiO2. Moreover, in order to figure out the magnetic properties, the spin exchange interactions are calculated, and then magnetic ground states are predicted in these three systems. The results show that LiNiO2 forms a spiral order that is caused by the competition from both the short-range and long-range spin exchange interactions, whereas the magnetic ground state of LiMnO2 is collinearly antiferromagnetic due to its nearest and next-nearest neighbor antiferromagnetic spin exchange interactions. However, LiNi0.5Mn0.5O2 is collinearly ferrimagnetic because of the ferromagnetic nearest neighbor Ni-Ni and Mn-Mn exchange interactions. Our work demonstrates the competition between the different exchange interactions in these cathode materials, which may be relevant to the performance of the lithium-ion battery.

High Cycling Stability of the LiNi0.8 Co0.1 Mn0.1 O2 Cathode via Surface Modification with Polyimide/Multi-Walled Carbon Nanotubes Composite Coating.

The Ni-rich LiNi0.8 Co0.10 Mn0.1 O2 (NCM811) cathode coated by combining with multi-walled carbon nanotubes (MWCNTs) and polyimide (PI) produces a PI3-NCM811 cathode, which markedly improves cycling stability and suppresses secondary crystal cracking. The initial discharge capacity of the PI3-NCM811 cathode is 199.6 mAh g-1 between 2.8 and 4.3 V at 0.1 C @ 25 °C, which is slightly lower than that of NCM811 (201.1 mAh g-1 ). The PI3-NCM811 and NCM811 cathodes keep 90.6% and 64.8% of their initial discharge capacity at 1 C between 2.8 and 4.3 V after 500 cycles, respectively. Furthermore, the difference (21.1%) in capacity retention rate between PI3-NCM811 and NCM811 under the condition of 2.8-4.5 V became smaller compared with the difference (25.8%) under the condition of 2.8-4.3 V. This better cyclic stability is mainly attributed to the toughness and elasticity of PI, which inhibits the secondary cracking, maintains the structural integrity of the cathode particles, and protects the particles from electrolyte damage during long-term cycling.

The thermodynamics and electronic structure analysis of P-doped spinel Co3O4.

The thermodynamics of phosphorus (P) doping to spinel Co3O4, for both bulk cases and (100) and (110) surface cases, is studied using first principles calculations. The doping energies of the P atom at different doping sites are carefully calculated and compared. It is shown that P doping at Co sites, at either tetrahedral or octahedral sites, is energetically favorable, while P doping and replacing O atoms are energetically unfavorable. The doping energy difference is large enough to conclude that P doping has a very strong preference to take the Co sites, rather than the O sites in spinel Co3O4. Even when O-vacancy is available, P doping and taking the O-vacancy site is thermodynamically unfavorable. The physical/chemical mechanism behind this phenomenon is carefully analyzed. Electronic structure analysis shows that P doping and replacing the Co atom brings excess electrons to the Co3O4 system, which is beneficial to enhance the electrochemical and catalytic performance of the spinel Co3O4. Our results clarified the misleading results of P doping and replacing O atoms in spinel Co3O4 reported in the literature.

First-principles study of χ3-borophene for charge-modulated switchable CO2 capture.

A first-principles calculation was performed to investigate the switchable CO2 capture on χ3-borophene by injecting/removing the extra electrons. The results show that the CO2 adsorption energy on the neutral χ3-borophene is 0.150 eV. After extra 2.5 e are injected, the adsorption energy is raised up to 0.802 eV, showing a significant enhancement with the change from the physical adsorption to chemical adsorption. Furthermore, both the CO2 capture and release processes are exothermic reactions involving injecting/removing extra electrons. χ3-borophene possesses a metallic electronic structure, which is conducive to the injection of extra electrons. The minimum charge density for CO2 capture on the negatively charged χ3-borophene is 1.6 × 1014 e cm-2. The CO2 capture capacity of χ3-borophene is 4.09 × 1014 cm-2. Finally, we study the selectivity of negatively charged χ3-borophene. The results show that the negatively-charged χ3-borophene possesses a high selectivity for CO2 from its mixtures with CO, CH4, NH3, N2, H2S, and H2. χ3-borophene is a new promising charge-modulated switchable CO2 capture material with good stability, high CO2 capture capacity, high selectivity, and excellent electrical conductivity.

Hydrogen solution in tungsten (W) under different temperatures and strains: a first principles calculation study.

In this paper, H solution behaviors are systematically studied under varied external tensile/compressive strains in bcc W using first-principles calculations. The results show that the H solution energy is not only dependent on the ground state energy of the W lattice, but also strongly dependent on the entropy effect. The entropy effect includes not only the contribution from lattice vibrations, but also the configurational entropy of the H distribution in the interstitial sites. As the entropy effect is directly associated with the temperature, the H solubility in W is strongly dependent on the temperature and the magnitude of the H solubility is increased from 3 × 10-18 at 300 K to 1.1 × 10-3 at 1800 K under strain free conditions. The results also show that external strain can also play an important role in changing the H solution behavior in W. Tensile strain promotes the H solubility while compressive strain suppresses the solution of H.

Dissociation of (Li2O2)0,+ on graphene and boron-doped graphene: insights from first-principles calculations.

Reducing charge overpotential is of great significance to enhance the efficiency and cyclability of Li-O2 batteries. Here, a dramatically reduced charge overpotential via boron-doped graphene as a catalytic substrate is successfully predicted. By first-principles calculations, from the perspective of reaction thermodynamics and kinetics, the results show that the electrochemical oxidation of the Li2O2+ cation is easier than the chemical oxidation of the neutral Li2O2 molecule, and the oxidation of (Li2O2)0,+ is facilitated by boron-doping in pristine graphene. More importantly, the results reveal the oxidation mechanism of (Li2O2)0,+: two-step dissociation with the LiO2 molecule as a reactive intermediate has advantages over one-step dissociation; the rate-determining step for the dissociation of (Li2O2+)G is the oxygen evolution process, while the lithium removal process is the rate-determining step for the dissociation of (Li2O20)G, (Li2O20)BG, and (Li2O2+)BG.

Theoretical prediction of T-graphene as a promising alkali-ion battery anode offering ultrahigh capacity.

The pursuit of high capacity is one of the key challenges for the development of alkaline ion batteries (AIBs, namely Li/Na/K-ion batteries; LIBs, NIBs, or KIBs). Carbon-based anode materials represented by graphite have been widely used in secondary batteries; however, their storage capacity is generally not high. Graphene was once considered a promising candidate, but it has proven to be incapable of interacting strongly with alkali ions. Here, by first-principles calculations, we predict an allotrope of graphene that may soon be experimentally synthesized, called T-graphene, which is a promising anode material for AIBs. We find that when it is used as the anode for NIBs, its theoretical capacity can be as high as 2232 mA h g-1 (Na6C6), which is six times that of graphite. For LIB and KIB anodes, the specific capacities are 744 mA h g-1 and 1116 mA h g-1, corresponding to the Li2C6 and K3C6 chemical formula, respectively. We first demonstrate that the material is mechanically stable. We further show that the material has good electrical conductivity whether it is before or after adsorption of Li(Na or K). We also studied the diffusion of Li(Na or K) on its surface and found that their corresponding diffusion barriers are very low (Li, Na and K are about 0.37 eV, 0.35 eV and 0.25 eV, respectively), which means good rate performance. It is calculated that the average open circuit voltage of the corresponding three half-cells at full charge is also low (LIBs is about 0.20 V, NIBs is about 0.12 V, and KIBs are about 0.37 V), which is beneficial for increasing the operating voltage of the full battery. In addition, during the adsorption of lithium, sodium and potassium, the lattice change of the material is very small (about 1.0% for lithium, about 1.4% for sodium, and about 1.9% for potassium), which means good cycling performance. These results indicate that T-graphene is expected to replace graphene and become a very attractive anode material for AIBs.

Phosphorus-Mediated MoS2 Nanowires as a High-Performance Electrode Material for Quasi-Solid-State Sodium-Ion Intercalation Supercapacitors.

Molybdenum disulfide (MoS2 ) is a promising electrode material for electrochemical energy storage owing to its high theoretical specific capacity and fascinating 2D layered structure. However, its sluggish kinetics for ionic diffusion and charge transfer limits its practical applications. Here, a promising strategy is reported for enhancing the Na+ -ion charge storage kinetics of MoS2 for supercapacitors. In this strategy, electrical conductivity is enhanced and the diffusion barrier of Na+ ion is lowered by a facile phosphorus-doping treatment. Density functional theory results reveal that the lowest energy barrier of dilute Na-vacancy diffusion on P-doped MoS2 (0.11 eV) is considerably lower than that on pure MoS2 (0.19 eV), thereby signifying a prominent rate performance at high Na intercalation stages upon P-doping. Moreover, the Na-vacancy diffusion coefficient of the P-doped MoS2 at room temperatures can be enhanced substantially by approximately two orders of magnitude (10-6 -10-4 cm2 s-1 ) compared with pure MoS2 . Finally, the quasi-solid-state asymmetrical supercapacitor assembled with P-doped MoS2 and MnO2 , as the positive and negative electrode materials, respectively, exhibits an ultrahigh energy density of 67.4 W h kg-1 at 850 W kg-1 and excellent cycling stability with 93.4% capacitance retention after 5000 cycles at 8 A g-1 .

Scandium Molybdate Microstructures with Tunable Phase and Morphology: Microwave Synthesis, Theoretical Calculations, and Photoluminescence Properties.

In this paper, scandium molybdate microstructures have been prepared from solution via a microwave heating method. By controlling the experimental parameters such as molar ratio of reagent and reaction time, scandium molybdates with tunable phase and diverse morphologies including snowflakes, microflowers, microsheets, and branched spindles were obtained. The density of states and surface energies of Sc2Mo3O12 were primarily studied from first-principles calculations. An indirect band gap of 3.56 eV was observed for crystalline Sc2Mo3O12, and the surface energies of various facets were determined to be 0.27-0.91 J/m2. The influence of n(Sc3+): n(Mo7O246-) (short for Sc/Mo) molar ratio was systematically investigated and well-characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and UV-vis absorption spectroscopy (UV-vis). Results indicate that the Sc/Mo molar ratio has a great effect on the phase and morphology. Diffuse reflection spectra (DRS) revealed the Egap can be readily tuned from 3.69 to 4.16 eV, which is in accordance with the theoretical result. The photoluminescence (PL) properties of Eu3+-doped Sc2Mo3O12 were discussed. This facile synthesis strategy could be extended to the synthesis of other molybdates.

Interpenetrating graphene network bct-C40: a promising anode material for Li ion batteries.

The stable sp2-C atoms in graphite enable its excellent structural and electrochemical stability as an anode material for Li-ion battery applications, while the limited Li-storage capacity of graphite also originates from the sp2 hybridization. Herein, from first-principles calculations, we show that a synergistic effect of sp2 and sp3 hybridized C atoms can substantially enhance the Li-storage performance in carbon-based anodes, using bct-C40 as an example, which is constructed with interconnected graphene layers (sp2 hybridized C atoms) and the connecting points are composed of sp3-C atoms. Charge transfer from sp2-C atoms to sp3-C atoms has been found, leading to unoccupied electronic states forming around the Fermi level. Furthermore, we found that the unoccupied electronic states are contributed by the pz orbital of the sp2-C atoms, resulting in stronger interactions between C atoms and intercalated Li atoms. As a result, the Li intercalation concentration in bct-C40 can reach as high as LiC2.5 (corresponding to a capacity of 893 mA h g-1), much higher than that of LiC6 in graphite (372 mA h g-1). Furthermore, bct-C40 inherits good structural and electrochemical stability, a metallic electronic structure, and low Li-ion migration energy barriers (0.067-0.112 eV) from the sp2 hybridized graphene structures, therefore very good Li-storage performance is expected, indicating that bct-C40 can be used as a high-performance anode material for lithium ion batteries. Our study provides new insights into the functionality of sp2- and sp3-C atoms in carbon-based anode materials and is helpful for the designing of new carbon-based anodes.

Structural and electronic properties of small lithium peroxide clusters in view of the charge process in Li-O2 batteries.

The Li-O2 battery is an ideal energy storage device due to its highest theoretical energy density; however, its high charge overpotential limits its practical application. Herein, through ab initio calculations, we systematically investigated the structural and electronic properties of small (Li2O2)nm+ (n = 1, m = 0, 1 and n = 2, m = 0, 1, and 2) clusters and calculated the reaction energies of various decomposition reactions. Results show that the (Li2O2)1 monomer has a low spin, whereas the (Li2O2)2 dimer has a high spin. The analysis of bond length, molecular orbitals, and projected density of states reveals that the interaction of O-O is stronger in the cationic cluster than in the neutral one, whereas the interaction of O-Li is weaker in the cationic cluster than in the neutral one; this facilitates the decomposition of cationic lithium peroxide cluster. Furthermore, the calculated reaction energies indicate that the peroxide lithium decomposition preferentially favors two-step reaction over one-step reaction. Finally, the lowest-energy reaction pathway for the decomposition of (Li2O2)2 dimer was predicted to be (Li2O2)2 → Li2O2 → (Li2O2)+ → LiO2 → O2, and the rate-determining step was predicted to be the first step.

The effect of protons on the Mg2+ migration in an α-V2O5 cathode for magnesium batteries: a first-principles investigation.

The scarce inventory of cathode materials with reasonable diffusion of Mg ions is the main obstacle in the development of rechargeable magnesium batteries. In this regard, vanadium pentoxide (V2O5) has been reported to be a candidate cathode material for Mg batteries. In this study, via first-principles calculations, we showed that the Mg-ion diffusion energy barrier in α-V2O5 could be substantially decreased through hydrogenation. It is found that the Mg-ion migration energy barrier in HxV2O5 is gradually decreased with an increase in H concentration. When the H concentration x reaches 2, the migration barrier is decreased to 0.56 eV from that in α-V2O5 without hydrogenation (1.28 eV). This indicates that the Mg diffusion kinetics can be substantially improved through hydrogenation, and the resultant energy barrier makes Mg diffusion acceptable even at room temperature. The mechanism of the H-enhanced Mg-diffusion has also been studied, and it has been found that H atoms not only can expand the Mg-diffusion pathway, but also have a screening effect on the interactions between Mg ions and the α-V2O5 lattice.

Density functional theory prediction of Mg3N2 as a high-performance anode material for Li-ion batteries.

Two dimensional (2D) materials have great potential for application in energy storage due to their unique structural characteristics. Here we propose for the first time a density functional theory study into the scientific feasibility of using g-Mg3N2, which is a novel graphene-like 2D material, as a high-capacity anode material for Li-ion batteries (LIBs). The favorable Li-adsorption geometries and the Li adsorption thermodynamics are explored in detail. It is found that monolayer g-Mg3N2 can be lithiated up to Li7Mg3N2 that offers a super high theoretical capacity of 1858 mA h g-1 and the average intercalation potential ranging from 0.2 to 0.7 V is suitable for anode applications. The metallic electronic structures of LixMg3N2, in combination with the low Li-ion diffusion energy barriers on the honeycomb structure, promote high electron and Li-ion conductivity to ensure fast charge/discharge cycling. The excellent structural stability of Mg3N2 is good for the cycling performance. These results predict that g-Mg3N2 can serve as a high-performance anode material for LIBs.

Bandgap tuning in MoSSe bilayers: synergistic effects of dipole moment and interlayer distance.

From first principles calculations, the lowest energy structures of bilayer MoSSe with different combination patterns (S-S, Se-Se, and S-Se bilayers) have been confirmed. The results demonstrate that the band gap of bilayer MoSSe can be tuned by interlayer distance and dipole moment. A larger interlayer distance increases the band gap, while a larger dipole moment reduces the band gap. That is, the gap is affected by the synergistic effects of dipole moment and interlayer distance in a MoSSe bilayer. Our results provide a new way to realize band gap modulation by changing the dipole moment or the interlayer distance. Impressively, another important finding is that the S-Se bilayer has a type-II band alignment, which makes it a good candidate for applications in optoelectronics.