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MnSb2S4 Monolayer as an Anode Material for Metal-Ion Batteries Zizhong Zhang, Yongfan Zhang, Yi Li, Jing Lin, Donald G. Truhlar, and Shuping Huang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05311 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Chemistry of Materials

MnSb2S4 Monolayer as an Anode Material for Metal-Ion Batteries Zizhong Zhang,a Yongfan Zhang,a Yi Li,a Jing Lin,a Donald G. Truhlar,b and Shuping Huang a,b,c* a College of Chemistry, b Department

Fuzhou University, Fuzhou, Fujian 350108, P. R. China

of Chemistry, Chemical Theory Center, and Minnesota Supercomputing Institute,

207 Pleasant Street SE, University of Minnesota, Minneapolis, MN 55455-0431 c Fujian

Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen, Fujian,

361005, China

ABSTRACT. We present density functional calculations showing that monolayer MnSb2S4 is promising as an anode material for Li-, Na-, and Mg-ion batteries, and that the adsorption of Zn or Al atoms on the surface of MnSb2S4 monolayer is not energetically favorable. The calculations show electron transfer from Li, Na, or Mg to the empty orbitals of nearby Sb and S atoms. The calculations indicate that an adsorption mechanism is followed by a conversion mechanism during charging, and the storage capacities can reach as high as 879 mAh/g for Li, Na, and Mg. The most favorable diffusion path for Li, Na, and Mg on the surface of MnSb2S4 monolayer is along the b direction; the lowest diffusion barriers for one Li, Na, and Mg are 0.18, 0.10, 0.32 eV, respectively. Good charge–discharge rates can be expected for the MnSb2S4 monolayer when it is used as an electrode for Li, Na, and Mg ion batteries.

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INTRODUCTION The discovery of graphene1 opened up possibilities for exploring new low-dimensional monolayer materials, such as transition-metal dichalcogenide, 2-5 phosphorene, 6-8 silicene,9,10 hBN,11,12 and heterometallic chalcogenide monolayers. 13 These low-dimensional materials have attracted great interest because of their distinctive electronic, optical, and catalytic properties. We recently demonstrated excellent photocatalytic properties for MnSb2S4 (MSS) monolayer nanosheets.13 This material has an advantage over graphene because its band gap is appropriate for solar applications and for possible use as a photocatalyst. Free-standing MSS twodimensional monolayer nanosheets display high efficiency and stable activity for photocatalytic water-splitting under visible light irradiation due to efficient separation of electrons from holes. In addition to their application as photocatalysts, low-dimensional nanomaterials have advantages over bulk materials as electrode materials for secondary batteries due to their high surface-to-volume ratios and short diffusion paths.14-22Among secondary batteries, lithium-ion batteries (LIBs) have achieved great commercial success, and Na, Mg, Al, and Zn ion batteries are viable alternatives to LIBs. Sodium and magnesium have the advantage of being less expensive and more abundant than Li, and divalent Mg and Zn and trivalent Al have the advantage that they can in principle store two or three times as much charge per atom, thereby potentially leading to high specific capacity. The charge storage mechanisms of Na, Mg, Al, and Zn ion batteries are similar to that of LIBs. During charging, the metal ions are removed from the positive electrode (cathode) and reduced and inserted into the negative electrode (anode), while electrons move externally from the positive electrode to the negative electrode. 23 The metal ions can be incorporated into and removed from the negative electrode in several possible ways, such as intercalation (e.g., into


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graphene), alloying (with Group IV and V elements), conversion reactions, or adsorption on the surfaces of nanosheets. Anode materials operating by alloying reactions have high specific capacity, but their volume changes are too large for long cycle life. Motivated by their high surface-to-volume ratio and excellent electrochemical properties, many two-dimensional (2D) material, including graphene, metal nitrides, metal oxides, transition-metal dichalcogenides, MXenes, and black phosphorus, have been investigated as electrode materials of secondary batteries. Recently, large-scale syntheses of MSS 2D monolayer nanosheets have been achieved by calcination of layered MSS chalcogenides with interlayer hydrazine bridges. The resulting MSS monolayer has good thermal stability; in particular it is stable even at the high temperature of 773 K.13 Although the photocatalytic properties of MSS have been studied, it has not been explored how MSS behaves as a battery electrode. The present work uses density functional electronic structure calculations to explore the structural and electronic properties of monoclinic MSS bulk material and monolayer MSS nanosheets and the possibility of using the monolayer material as an electrode material in Li, Na, Mg, Al, or Zn ion batteries.

COMPUTATIONAL DETAILS Quantum mechanical calculations were carried out employing three choices of exchangecorrelation density functional: the Perdew-Burke-Ernzerhof24 (PBE) generalized gradient approximation, the PBE functional together with a Hubbard correction25 (abbreviated PBE+U), and the Heyd-Scuseria-Ernzerhof26 screened hybrid approximation (abbreviated HSE06). It is well known that the PBE functional underestimates band gaps, and the other two functionals correct this in two different ways. In particular, the Hubbard U correction adjusts the strength of



Coulomb and exchange interactions in selected subshells, and the screened hybrid approximation replaces some of the density-dependent exchange at small interelectronic separations by HartreeFock exchange. Both corrections introduce new parameters, as explained next. The Hubbard correction used here is the version of Dudarev et al.,25 in which the adjustment to the Coulomb and exchange integrals involves only a single parameter, sometimes called U − J and sometimes called just U; here we just call it U. In the present work the Hubbard correction is applied only to the d subshell of Mn, and the value of U is parameterized (as explained in more detail below) on the basis of calculated lattice constants. The version of the screened exchange functional used here is the 2006 parameterization called HSE06, in which nonlocal Hartree-Fock exchange decreases from 25% at small interelectronic separation to zero at large interelectronic separation. Functionals with HartreeFock exchange are expected to give more accurate band structures, but Hartree-Fock exchange also brings in static correlation error.27 The projector augmented wave (PAW) potentials 28 , 29 provided in the Vienna Ab Initio Simulation package30,31 (VASP) were used to describe the core electrons. There are 12 electrons in the core for Mn, 46 in the core for Sb, 10 in the core for S, and 0, 4, 10, 10, and 18 respectively for Li, Na, Mg, Al, and Zn. The PAW potentials used for Mn, Sb, S, Li, Na, Mg, Zn, and Al are Mn_pv, Sb, S, Li_sv, Na_pv, Mg, Zn, and Al, respectively, in the notation of VASP. Proper inclusion of damped dispersion interactions is important to describe the structures and properties of layered materials, but the functionals used here do not describe long-range electron correlations responsible for dispersion interactions 32 and they are quantitatively inaccurate for short-range damped dispersion. This can be remedied to some extent by the DFTD approach, 33 which consists in adding a semi-empirical potential energy term in a post-selfconsistent-field step to the conventional Kohn-Sham DFT energy; the added term accounts for 4 ACS Paragon Plus Environment

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dispersion energy at long distances between atoms and for damped dispersion at van der Waals distances. The DFT-D2 method of Grimme34 is employed in this work. Adding this molecular mechanics potential to PBE and PBE+U yields respectively PBE-D2 and PBE+U-D2. For the bulk calculations, a 1×2×1 supercell was used, and there are 8 Mn atoms (40 d electrons) in the 1×2×1 supercell. The 1×2×2 supercell of fully relaxed bulk MnSb2S4 (Figure 1a) was used to build the initial monolayer of MnSb2S4 (Figures 1b - 1c) by cutting to provide more than 10 Å of vacuum space to ensure no spurious interaction between the periodically repeated layers or adsorbates in the direction normal to the layer. There are four Mn atoms (20 d electrons) in each cell of monolayer MSS. A 4×7×4 Monkhorst-Pack k point mesh was used for the calculations on the 1×2×1 supercell of bulk monoclinic MnSb2S4, and a 4×7×1 one was used for the monolayer. The geometry optimizations employed a 400 eV cutoff energy for the plane-wave basis, and the single-point electronic structure calculations used a 600 eV cutoff energy. All calculations were spin polarized. The oxidation states of Mn, Sb, and S are respectively +2, +3, and -2. The electron configuration is d5 for Mn2+; in the high-spin states, there are five spin-up electrons for each Mn2+, and in the low-spin states there are three spin-up electrons and two spin-down electrons for each Mn2+. For the high spin-states, there may be ferromagnetic (FM) or antiferromagnetic (AFM) coupling between the Mn2+ ions, and we calculated both possibilities. In addition, all the low-spin FM and AFM states are considered; the optimizations of low-spin states relaxed to their corresponding high spin states. Ab initio molecular dynamics (MD) simulations were carried out to explore the most stable metalated structures with a low precision. The simulation length was 10 ps with a time step of 1 fs. We carried out the MD simulations at a constant temperature of 300 K by using the Nosé thermostat to control the temperature of the metalated structures. Then we sampled



configurations from the MD simulations. Further structural optimizations using accurate settings were performed to determine the most energetically favorable configurations. After metal binding there are no unpaired electrons on Li, Na, or Mg; their magnetic moments are 0. The energy criterion for self-consistency was set to less than 0.0001 eV/(unit cell), and the force criterion for convergence of structure relaxation was set to less than 0.001eV/Å. The climbing-image nudged elastic band (CI-NEB)35 method was used to determine the energy barriers for Li-ion diffusion. Five images were employed between two end points.

Figure 1. (a) The 1×2×2 supercell of monoclinic bulk MnSb2S4 (mC28), (b) the side view of the monolayer sheets, and (c) the top view of monolayer MSS. The A, B, C, D, E, F, G, H and I sites are possible adsorption positions on the surface of monolayer MSS. The purple, orange, and yellow balls represent Mn, Sb, and S, respectively.


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RESULTS AND DISCUSSION Structures and Energies There are two different crystal structures of MSS, namely monoclinic and orthorhombic. We are only concerned with the monoclinic, which has been found experimentally to be a semiconducting antiferromagnet with an electronic band gap of 0.77 eV.36 In monoclinic MSS, the chains of MnS6 octahedra are linked by [SbS3]3- units to form layers; between the layers there are three kinds of Sb-S coordination with three Sb-S bonds of about 2.6 Å, two Sb-S bonds of about 3.0 Å, and one or two nonbonding Sb-S interactions of about 3–4 Å. The short Sb-S bonds link edge-sharing MnS6-octahedra within a chain along b direction, and the longer bonds link the octahedra of different chains to form a layered structure. Nonbonding Sb-S distances are along the c axis, which is normal to the layers. The [SbS3]3- units cause structural distortion of MnS6 octahedra. The Mn-Mn distance along the Mn chain is about 3.8 Å, whereas the shortest Mn-Mn distances along the a and c directions are larger than 6 Å. There are two nonequivalent Mn positions in a primitive cell of monoclinic MSS. In the monolayer MSS, the Sb-S bonds of about 3.0 Å are 0.09 Å longer than the ones in bulk, and the Sb-S bonds that are nearly parallel to the c direction are a little shorter than the ones in bulk, which might be ascribed to the absence of the interlayer van der Waals interactions. Both the high-spin configuration of Mn2+ with five unpaired spins and the low-spin configuration with one unpaired spin are considered. The high spin states in both ferromagnetic (FM) and antiferromagnetic (AFM) states (three possible AFM states in bulk and two possible AFM states in monolayer) are found to be energetically more favorable than the corresponding low spin states, so only the high-spin state is considered in the rest of the paper.



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Table 1 compares the calculated lattice constants and energy gaps obtained with and without the D2 damped dispersion terms. The D2 terms do not affect the calculated SCF energies and orbitals for a given structure, but they affect the calculated structures, and hence they affect the band gaps indirectly through their effect on the structures. The calculated c value when damped dispersion is included is about 0.7 Å shorter than the one calculated without it, and the calculated gap is ~ 0.2 eV lower when calculated for the structures obtained with damped dispersion. The results are only mildly sensitive to U, and we will use U = 6 eV in future tables.

a

Table 1. The calculated lattice constants and indirect energy gaps in a 1×2×1 supercell (Mn8Sb16S32) of bulk MSS for the high-spin antiferromagnetic state with the Mn+2 spins aligned b oppositely in the chains along the b direction

PBE PBE-D2 PBE+U PBE+U-D2 PBE+U-D2 PBE+U-D2 Expt.

U (eV) 0 0 6 4 6 8

a (Å) 12.616 12.319 12.915 12.567 12.635 12.715 12.747

b (Å) 7.669 7.678 7.702 7.693 7.701 7.707 7.598

c (Å) 15.353 14.663 15.598 14.816 14.870 14.892 15.106

Gap (eV) 0.82 0.61 1.49 1.18 1.25 1.29

a

The lattice parameters are reported here for the 1×2×1 supercell.

b

Note that Mn+2 denotes an oxidation state of +2, in contrast to Mn , which would denote a

2+

physical charge of +2.

Table 2 shows the energies of the bulk material for the FM state and the three AFM states. All the calculations show that the energy of the FM state is higher than that of the AFM state, and the AFM configuration with the Mn2+ spins aligned oppositely along the c direction (AFM3) is higher than the one with Mn2+ spins aligned oppositely along the a (AFM1) or b (AFM2)


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directions. The PBE, PBE-D2, and PBE+U-D2 calculations predict that AFM2 is more stable than AFM1, which is consistent with the previous result of Matar et al.37

Table 2. The energies (eV) of the 1×2×1 supercell of the MSS bulk in high-spin FM and AFM states. Energies PBE PBE-D2 PBE+U PBE+U-D2

FM -289.672 -302.817 -276.836 -289.415

AFM1a -289.873 -303.053 -276.860 -289.433

AFM2b -289.953 -303.208 -276.848 -289.434

AFM3c -289.709 -302.896 -276.841 -289.424

a

The AFM1 configuration with the spins aligned oppositely in Mn2+ along the a direction. The AFM2 configuration with the spins aligned oppositely in Mn2+ along the b direction. c The AFM3 configuration with the spins aligned oppositely in Mn2+ along the c direction. b

Table 3. The energies (eV) of the monolayer (Mn4Sb8S16) in high-spin FM and AFM states Energies PBE PBE-D2 PBE+U PBE+U-D2 a b

FM -144.273 -148.957 -137.926 -142.517

AFM1a -144.352 -149.044 -137.932 -142.524

AFM2b -144.397 -149.094 -137.928 -142.522

The AFM1 configuration with the spins aligned oppositely in Mn2+ along the a direction. The AFM2 configuration with the spins aligned oppositely in Mn2+ along the b direction.

Table 3 lists the energies of the monolayer for the high-spin FM state and for the AFM states aligned along different directions. The trend of the relative energies of the various electronic states in the monolayer is similar to that in the bulk. The PBE+U-D2 calculations, which should be the most reliable result in Tables 2 and 3, predict that the energies of AFM1 and AFM2 are very close in both the monolayer and the bulk. In the following, the adsorption energies, electronic structures, and diffusion calculations are based on the AFM2 state. To study the nature of interactions of Li, Na, Mg, Zn, and Al to MSS bulk or monolayer, we first determined the most favorable binding site for a single metal atom by comparing their binding energies. The energy change per atom upon binding is defined as 9 ACS Paragon Plus Environment


=

( − − )

where is the energy change upon binding, and are the calculated total energies of the MSS supercell before and after the metal binding, respectively, and represents the calculated energy per atom in bulk metal. A variety of intercalation positions are considered in the bulk MSS. We found that intercalations of these metals do not change the electronic ground state of bulk MSS. The most favorable intercalation configuration of Li, Na, Mg, Zn, and Al in bulk MSS is shown in Figure S1. The Li, Na, and Mg are four-coordinated to S, while Zn and Al are three-coordinated to S. The intercalation of Li, Na, Mg, Zn, or Al weakens the Sb-S bonds in which the S is bonded to the guest metal. For example, after Na intercalation, two Sb-S bond lengths change from 2.96 Å to 3.09 Å. The calculated energy change upon binding (Tables S1-S3) is negative for Li and Na, close to zero for Mg, but positive for Zn and Al. The average energy change upon binding is more negative (i.e., more favorable) when more Li and Na atoms are intercalated; similarly the intercalation is energetically more favorable when more Zn and Mg are intercalated. The volume change is smaller than 5%, when four Li, Na, Mg, Zn, or Al are intercalated in the 1×2×1 bulk. A single metal atom is initially put on reach of the labeled positions (A, B, C, D, E, F, G, H and I in Figure 1c) of the surface of monolayer MSS and then relaxed. The Li at the E site relaxes to the C site, and the Li at the H or I site relaxes to the G site. For Li and Na, the most favorable adsorption site is C. For Mg, the Li at the C site relaxes to the E site, which is the most favorable one. The Mg occupies the position of one of the surface Sb atoms, and the Sb is squeezed out from its initial position to the top of surface with three Sb-S bonds breaking up (Figure S4). But for Zn and Al, site J is the most favorable adsorption site.


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With volume and entropy effects both neglected, the average voltage is calculated by the following equation:

=

+ −

where y represents the electronic charge of the guest metal ions (y = 1 for Li and Na, y = 2 for Mg and Zn, y = 3 for Al.) The specific capacity is calculated by the following equation = / where x represents the number of electrons involved in the electrochemical process, F is the Faraday constant (whose value is 26.8 Ah mol-1), and is the mass of MnSb2S4. Because the mass of MnSb2S4 is high, the specific capacity of Li, Na, Mg, Zn, or Al in bulk MSS is low.

Table 4 The energy changes per atom (eV) upon of binding, the average voltages, and the capacities for addition of 1 Li, 8 Li, 1 Na, 8 Na, or 1Mg to a cell (Mn4Sb8S16) of monolayer MSS. 1 Li

8 Li

1 Na

8 Na

1 Mg

Eb(eV/atom)

-0.56

-0.85

-0.35

-0.79

-0.51

(V)

0.56

0.85

0.35

0.79

0.26

Capacity (mAh/g)

15.7

125.6 15.7

125.6 31.4

The energy changes per atom (eV) upon binding, the average voltages, and the specific capacities for adding various numbers of Li, Na, Mg, Zn, and Al for monolayer MSS are shown in Tables 4 and Tables S4–S5. The energy changes upon binding Li, Na, or Mg on monolayer MSS are all negative, which means that these adsorption processes are exoergic. The energy



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changes upon binding Zn (1–8 Zn) or Al (1-8 Al) are positive, indicating the that the adsorption of Zn or Al atoms on the surface of MSS monolayer is not energetically favorable. The MD simulations show that when 1–8 Li or 1–8 Na are adsorbed on the surface, Li and Na stay adsorbed on the surface during the entire 10 ps MD simulations at 300 K, and thus none of the Mn-S or Sb-S bonds are broken. We also found that when the number of Li or Na is greater than 8, Li and Na diffuse into interstitial sites in the MSS monolayer and some of Mn-S and Sb-S bonds are broken during the MD simulations (Figures S2–S3), indicating that a phase transition may occur in the anode material. Therefore, we consider the possibility of a conversion mechanism for MSS monolayer after the capacities reach 125.6 mAh/g for Li and Na. The reaction energies ∆E of possible conversion reactions, their corresponding average voltages V, and theoretical capacities C are as follows: MnSb2S4 + 14Li = 4Li2S + 2Li3Sb + Mn Bulk ΔE = -20.38 eV V = 1.45 V Monolayer ΔE = -20.86 eV V = 1.49 V

C = 879 mAh/g C = 879 mAh/g

(1)

MnSb2S4 + 12Li = 3Li2S + 2Li3Sb + MnS Bulk ΔE = -17.40 eV V = 1.45 V Monolayer ΔE = -17.88 eV V = 1.49 V

C = 753 mAh/g C = 753 mAh/g

(2)

MnSb2S4 + 14Na = 4Na2S + 2Na3Sb + Mn Bulk ΔE = -15.14 eV V = 1.08 V Monolayer ΔE = -15.69 eV V = 1.12 V

C = 879 mAh/g C = 879 mAh/g

(3)

MnSb2S4 + 12Na = 3Na2S + 2Na3Sb + MnS Bulk ΔE = -12.96 eV V = 1.08 V Monolayer ΔE = -13.45 eV V = 1.12 V

C = 753 mAh/g C = 753 mAh/g

(4)

The voltages for the two possible different paths are the same for Li and Na. The voltage for Na is smaller than that for Li. When the capacities are smaller than 126 mAh/g for the MSS monolayer, it follows an adsorption mechanism. Between 127 ~ 879 mAh/g, it may follow a conversion mechanism. 12 ACS Paragon Plus Environment

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The energy change upon adsorption of a single Mg on the surface of MSS monolayer without bond breaking is 0.67 eV, which is more positive than the energy change for Mg intercalation in MSS bulk. In the most favorable structure for one Mg binding to the MSS monolayer with three Sb-S bonds broken (Figure S4), the energy change upon binding is -0.51 eV. When covered with more (2~7) Mg atoms, geometry optimization leads to structures with Mg adsorption on the surface, but the energy change upon adsorption is positive. The MD simulation at 300 K show that some of the Mg atoms fill interstitial sites of the MSS monolayer with some of Sb-S bonds broken, and further geometry optimizations show that the magnesiation is energetically favorable. Therefore, we also consider the possibility of a conversion mechanism. The reaction energies of possible conversion reactions ∆E, their corresponding average voltage V, and capacities C are as follows: MnSb2S4 + 7Mg = 4MgS + Mg3Sb2 + Mn Bulk ΔE = -8.80 eV V = 0.63 V Monolayer ΔE = -9.35 eV V = 0.67 V

C = 879 mAh/g C = 879 mAh/g

(5)

MnSb2S4 + 6Mg = 3MgS + Mg3Sb2 + MnS Bulk ΔE = -7.33 eV V = 0.61 V Monolayer ΔE = -7.88 eV V = 0.66 V

C = 753 mAh/g C = 753 mAh/g

(6)

Volume expansion or contraction upon discharging or charging is a key parameter controlling whether electrode reactions are reversible. Too much expansion can damage the electrode material to a great enough extent that the reaction is irreversible. In order to validate the reversibility of the conversion reactions, we calculate the volume ratio

!"#$%&'(

)

*

)

, where the

!"#$%&'(

is the total volume of the products, and

is the volume of MSS. The volume ratios for reactions (1), (2), (3), (4), (5), and (6)

are respectively1.96, 1.79, 3.12, 2.80, 1.75, and 1.64. If one charges the battery to a specific 13 ACS Paragon Plus Environment


capacity of 400 MAh/g, these ratios correspond to volume changes in the range 44–51%. (Actually this calculation is an overestimate since it does not account for the Li that stay on the surface, and those Li are not expected to affect the volume very much, although they do contribute to the capacity.) Since some electrode reactions are reversible even with volume changes as large as 100%,38 the reactions might be reversible. It is hard to estimate this theoretically because reversibility is a strong function of material preparation (which affects particle size and experimental conditions. Table 5 lists the binding energies (eV, where a more negative number indicates more favorable binding) for adsorption of one Li, Na, or Mg on the (001) surface, for adsorption on the surface of a monolayer, bilayer, or trilayer, and for intercalation between the layers of bilayer, trilayer, or bulk. The intercalation energy of a single Li, Na, or Mg atom between the layers of bilayer, or trilayer is more negative than the one for a single Li, Na, or Mg adsorbed on the (001) surface or on the monolayer, bilayer, or trilayer MSS surface; this is due to the increased metal coordination between the layers. The same trend holds for Li and Na intercalated between the layers of bulk but not for Mg; the intercalation of a Mg in the bulk is unfavorable; in the bulk, the Mg intercalation does not cause a large movement of Sb, while in the other intercalation cases Mg moves toward one of the layers, and one Sb moves far away from its previous position (Figures S1 and S4).

Table 5 The energy changes (eV) upon adsorption of one Li, Na, or Mg.

monolayer bilayer trilayer (001) surface bulk

On the surface a Li, Na, Mg -0.56, -0.35,-0.51 -0.50, -0.28,-0.50 -0.57,-0.33,-0.57 -0.57,-0.33,-0.57

Between the layersb Li, Na, Mg -0.78, -0.50,-0.71 -0.75,-0.45,-0.69 -0.75,-0.42,-0.67 a -0.76,-0.39, 0.04 14 ACS Paragon Plus Environment

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a

Adsorption on the surface of a monolayer, bilayer, or trilayer and on the (001) surface of bulk material. b

Intercalation between the top layer and the second layer of a bilayer or a trilayer or the (001) surface of bulk material and intercalation between two layers of bulk material. Electronic Structures The electronic structure calculations by PBE+U-D2 show that the band gaps of bulk and

monolayer MSS are 1.25 and 1.73 eV, respectively. Both of these band gaps are indirect, which differs from the situation for MoS2; a monolayer MoS2 has a direct band gap while the bulk material is an indirect-band semiconductor. 39 As the dimensionality of MSS decreases from bulk to nanosheets, the energy gap increases due to the quantum confinement effect. The calculated gap 1.73 eV for the monolayer by PBE+U-D2 (Table S6) is consistent with the experimental one obtained from optical absorption spectra.13 Figure 2 shows the band structure of MSS monolayer in AFM state by PBE+U-D2. The bottom of the conduction band is at the Γ point, while the top of valence band is at the Y point. The calculated indirect band gap is 1.73 eV, and the direct band gap at the Γ point is 1.79 eV. The partial density of states (PDOS) for MSS monolayer by PBE+U-D2 (Figure S5) is similar to our HSE06 result.13 The top of the valence band originates from d orbitals of Mn and p orbitals of S, with a non-negligible contribution from p orbitals of Sb. The bottom of the conduction band is mainly due to p states of Sb.



Figure 2. The band structure of MSS monolayer in AFM state by PBE+U-D2 (U = 6 for the d orbitals of Mn2+). Γ (0.0 0.0 0.0), Y (0.0 0.5 0.0), A (0.5 -0.5 0.0), B (0.5 0.0 0.0).

Figure 3 and Figures S6-S7 show the total and partial density of states of Li, Na, and Mg adsorbed on an MSS monolayer. The adsorption of one Li or Na makes the MSS monolayer metallic, and the adsorption of one Mg decreases the gap from 1.73 eV to 0.54 eV, indicating the adsorptions of these metals increase the electronic conductivity. There is significant overlap between the metal s orbital and the sulfur p orbital in the valence bands. The calculated gaps of MSS monolayers with adsorbed Li, Na, and Mg are shown in Tables S7 – S9. All the metal adsorptions or intra-layer absorptions decrease the gaps. It is interesting to find that when the number of Li or Na adsorbed is odd, the material is metallic. This indicates that the doping of an odd number of Li or Na atoms into the MSS monolayer can increase the electronic conductivity of the MSS monolayer as an anode material.


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Figure 3. The partial and total density of states for one Li atom on Mn4Sb8S16 monolayer in the AFM state by PBE+U-D2 (U = 6 for the d orbitals of Mn2+). To obtain further insight into the adsorption process, we characterized the charge transfer by performing Hirshfeld charge analysis. In Table S10, the Hirshfeld charges for one Li, Na, Mg, Zn, or Al adsorbed on an MSS monolayer are shown. The charge of the Sb closest to the Li, Na, or Mg decreases by more than 0.1 e, and the charges of the S atoms which are bonded to Li, Na, or Mg become more negative. This means that after the adsorption of a single metal atom, the charge transfer is from the metal to the nearby Sb and S. The charges for Li, Na, Mg, Zn, and Al are 0.13, 0.28, 0.18, 0.08, 0.10 e, respectively. The charge transfer between Zn or Al and the MSS monolayer is smaller than that for Li, Na, or Mg, which is consistent with the trend of adsorption energies. When the coverage of the metal is higher, more Sb and S are reduced, and the oxidations of Mn are evident (Tables S11). For eight Na atoms adsorbed on an MSS monolayer, the Hirshfeld charge of Mn is increased from 0.13 e to 0.18 e.



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Diffusion The mobility of the metal ions is an important factor in the rate performance of a battery electrode. Therefore we investigate the diffusion of metal atoms on the surface of an MSS monolayer by the CI-NEB method. We consider several possible diffusion pathways between two neighboring favorable adsorption sites (Figure 1). The most favorable diffusion pathway for Li, Na, and Mg is along the b direction. The calculated barriers for Li moving from A to C and from C to J are 0.18 and 0.42 eV, respectively, which are comparable to the diffusion barriers of Li atoms in graphite (0.2 - 0.5 eV) 40-43 and MoS2 (0.25 eV) 44 and are smaller than those in silicon (0.57 eV) 45,46 and TiO2-based polymorphs (~0.65 eV) 47,48. The calculated barriers for Na transporting from A to C and from C to J are 0.10 and 0.13 eV, respectively. The calculated barrier for Mg from A to J is 0.32 eV.

Table 6. The lowest diffusion barrier of M at the surface of monolayer (parallel to the layers while being between the layers in the bulk). Barrier (eV)

Li 0.19 (0.28)

Na 0.10 (0.26)

Mg 0.32 (1.08)

Al 0.39 (0.76)

Zn 0.19 (0.34)

Table 6 shows the lowest diffusion barrier of M at the surface of monolayer and parallel to the layers while being between the layers in the bulk. The diffusion barriers of Mg and Al are larger than that of Na, which may be due to larger electrostatic forces. The diffusion barriers of Na, Mg, Al, and Zn on the surfaces of monolayer MSS are smaller than in the bulk; this indicates that metal ions can diffuse faster in the monolayer nanosheets than in the bulk. The metal ion diffusion along the b direction is more favorable than the diffusion along the a direction for both bulk and monolayer. The diffusion barrier of Na along the b direction in the surface of MSS 18 ACS Paragon Plus Environment

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monolayer is the lowest, only 0.10 eV, which is significantly lower than the calculated barrier 0.56 eV for Na diffusion on MoN2 monolayer.15 Furthermore, the barriers for Mg/Al/Zn diffusion along the b direction are all lower than 0.5 eV. Excellent charge-discharge rates can be expected for MSS monolayer when it is used as electrode for these metal ion batteries. CONCLUSIONS Monolayer MnSb2S4 has good thermal stability, and in this work we used density functional calculations to investigate the possible usefulness of this material as an electrode material for metal-ion batteries. Our results showed that the adsorption of Li, Na, or Mg is energetically favorable, but the adsorption of Zn or Al atoms is not. Hirshfeld charge analysis indicates that during charging electrons are transferred from the metal (Li, Na, and Mg) to the empty orbitals of Sb and S atoms that are close to the metal. The calculations indicate that an adsorption mechanism is followed by a conversion mechanism during charging and the storage capacities can reach as high as 879 mAh/g for Li, Na, and Mg, although, as usual, one might be limited to lower specific capacities to achieve good reversibility. The calculated voltages for Li, Na, and Mg are 0.56–1.49 V, 0.35–1.12 V, 0.26 –0.66 V, respectively, which are between those of commercial anode materials, 0.11 V for graphite and 1.5-1.8 V for TiO2.14 Metal ions diffuse faster in the MSS monolayer than in bulk, and the MSS monolayer is expected to have good charge-discharge rates when it is used as electrode for metal ion batteries.

ASSOCIATED CONTENT Supporting Information. Tables and figures showing structures, PDOS, binding energies, voltages, capacities, energy gaps, and charges. The Supporting Information is available free of



charge.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgments S. H. and Z. Z. acknowledge financial support from the National Natural Science Foundation of China (No. 21703036 and No. 21673042, respectively). Y. L. acknowledges financial support from the Natural Science Foundation of Fujian Province (2017J01409). Computations were performed using resources of (1) the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory of Pacific Northwest National Laboratory sponsored by the U. S. Department of Energy, (2) Minnesota Supercomputing Institute, and (3) the National Energy Research Scientific Computing Center. D. G. T. acknowledges Air Force Office of Scientific Research grant no. FA9550-16-1-0134.

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