Density Functional Theory Modeling the Interfacial Chemistry of theLiNO3 Additive for Lithium−Sulfur Batteries by Means of SimulatedPhotoelectron SpectroscopyMahsa Ebadi,† Matthew J. Lacey,† Daniel Brandell,† and C. Moyses Araujo*,‡

†Department of ChemistryÅngstrom Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden‡Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

*S Supporting Information

ABSTRACT: Lithium−sulfur (Li−S) batteries are considered candidates for next-generationenergy storage systems due to their high theoretical specific energy. There exist, however,some shortcomings of these batteries, not least the solubility of intermediate polysulfides intothe electrolyte generating a so-called “redox shuttle”, which gives rise to self-discharge. LiNO3is therefore frequently used as an electrolyte additive to help suppress this mechanism, but theexact nature of the LiNO3 functionality is still unclear. Here, density functional theorycalculations are used to investigate the electronic structure of LiNO3 and a number of likelyspecies (N2, N2O, LiNO2, Li3N, and Li2N2O2) resulting from the reduction of this additive onthe surface of Li metal anode. The N 1s X-ray photoelectron spectroscopy core level bindingenergies of these molecules on the surface are calculated in order to compare the results withexperimentally reported values. The core level shifts (CLS) of the binding energies are studiedto identify possible factors responsible for the position of the peaks. Moreover, solid phases of(cubic) c-Li3N and (hexagonal) α-Li3N on the surface of Li metal are considered. The N 1sbinding energies for the bulk phases of Li3N and at the Li3N/Li interfaces display higher valuesas compared to the Li3N molecule, indicating a clear correlation between the coordination number and the CLS of the solidphases of Li3N.

1. INTRODUCTION

Rechargeable batteries based on lithium metal negativeelectrodes are potential candidates for electrical energy storagesystems due to the high theoretical capacity and the highnegative electrochemical potential of the Li metal electrode.However, these rechargeable batteries also suffer from seriousproblems such as dendrite growth during cycling, safety risks,and low coulombic efficiency. Therefore, today’s lithium-basedstorage technologies in the form of Li-ion batteries (LIBs) areinstead utilizing graphite as the negative electrode.1 Duringrecent years, however, the industrial development of LIBs haveled them to approach their theoretical energy density limits. Tofurther improve the energy density of Li-based battery systems,application of Li metal electrodes has regained significantinterest for a number of different cell chemistries.2

Lithium−sulfur (Li−S) and Li−air batteries have in thiscontext been introduced as possible candidates for enhancingthe driving ranges of electric vehicles, although this is likely farinto the future. Li−S batteries have a theoretical specific energyaround five times greater than that of LIBs.3 In these batteries,traditional LIB cathode materials (such as LiCoO2 or LiFePO4)are replaced by a low-cost sulfur cathode, which possesses thesignificant advantages of being inexpensive and nontoxic, whileLi metal constitutes the negative electrode material.3

Several important challenges exist for implementation of Li−S batteries, including addressing the poor electronic and ionic

conductivities of sulfur, the solubility of polysulfides in theliquid electrolyte, and a high level of self-discharge. Thesolubility of polysulfides in the electrolyte causes migration ofthese species back and forth between the positive and negativeelectrodes. This constitutes a “redox shuttle mechanism”, whichconsequently leads to corrosion of the lithium metal andprecipitation of insoluble Li2S and Li2S2 on the surface of the Lielectrode.3

To passivate and better protect the Li metal surface fromthese parasitic reactions, different additives are used in theliquid electrolyte. Lithium nitrate (LiNO3) is nowadays acommon such electrolyte additive in Li−S batteries, often usedin concentrations of ca. 0.1−0.5 M, for the protection of the Limetal anode.4 The exact functionality of the nitrate additive isstill not clear, but it is often argued that this oxidizing additive isreduced to LixNOy compounds while oxidizing the sulfides toLixSOy, thereby forming a passivation layer on the Li metalsurface.5 There have been a number of experimental studies onthe role of LiNO3 additive in Li−S batteries.6−10 By means ofscanning electron microscopy (SEM) and X-ray photoelectronspectroscopy (XPS) techniques, it has been shown that bycontrolling the concentration of LiNO3 and additives in the

Received: August 7, 2017Revised: October 4, 2017Published: October 6, 2017

Article

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© 2017 American Chemical Society 23324 DOI: 10.1021/acs.jpcc.7b07847J. Phys. Chem. C 2017, 121, 23324−23332

Cite This: J. Phys. Chem. C 2017, 121, 23324-23332

electrolyte, a stable and uniform solid slectrolyte interphase(SEI) layer on the surface of the Li metal electrode can beformed.9 It has also been reported that LiNO3 as lithium salt oradditive can suppress the shuttle mechanism in Li−S batteries.8The evolution of N2 and N2O gases in Li−S cells with LiNO3additive has also recently been investigated by means ofpressure measurements and gas analysis.11

There exists a limited number of molecular-level computa-tional studies of the Li−S systems,12−14 mainly focusing onpolysulfides. Moore et al.15 reported a comprehensive set ofpossible reactions in Li−S batteries using high-level quantumchemical methods. The authors considered the reversibleformation of Li2S from lithium and a sulfur (S8) cathode in anonaqueous solvent. In another study, first-principle methodshave been applied to predict thermodynamic and electronicproperties of Li, Li2S2, Li2S, α-sulfur, and β-sulfur.

16 Balbuena etal.17 have studied surface reactions and reduction mechanismsof various electrolyte components at the Li electrode, includingpolysulfide compounds, using density functional theory (DFT)and ab initio molecular dynamics (AIMD) simulations. A seriesof heteroatom-doped (B, N, O, F, S, P, and Cl) graphenenanoribbons has been modeled, and their interactions withboth polar lithium polysulfide and nonpolar elemental sulfurhave been studied by first-principle calculations. It is shown thatN or O dopant can effectively prevent shuttle of polysulfides bysignificantly higher interaction between the carbon hosts andthe polysulfide guests.18

Li3N has been reported as one of the SEI components in thepresence of LiNO3 in the electrolyte.5 This component hasbeen shown to have a very high Li ion conductivity and be apromising candidate in order to passivate the surface of the Limetal.19 A recent combined theoretical−experimental study20

has been performed on nitride materials chemistry to stabilizeLi metal anode. The authors showed that oxides, sulfides, andhalides are reduced by the Li metal and therefore have a poorstability against the metal. On the other hand, nitride materialsincluding Li3N are thermodynamically stable against Li metal.So far, there has been no computational study on the LiNO3

additive in Li−S batteries. In the present work, we have usedperiodic DFT to study the interactions of LiNO3 itself, andexperimentally reported species originating from this additive,on the surface of a Li metal negative electrode. After analysis ofthe stability of these compounds on the Li metal surface, corelevel binding energies have been calculated to gain a betterinsight into the XPS properties of the nitrogen-containingcompounds in order to achieve a direct comparison withexperimental data. In addition to the molecules on the surfaceof Li metal, Li3N/Li interface has also been studied in thiswork. To our knowledge, this is the first computational studyon the electronic structure and core level binding energies ofsuch interface.

2. SIMULATION METHODSPlane wave DFT has been used employing the Vienna ab initiosimulation package.21 The electronic states have been describedby the projector augmented wave (PAW) method22,23 withinthe generalized gradient approximation (GGA) of Perdew,Burke, and Ernzerhof (PBE) to the exchange-correlationalfunctional.24 PAW potentials with valence states 1s22s1 for Li,2s22p3 for N, and 2s22p4 for O have been applied. The energycutoff for the plane wave was 550 eV, and a 3 × 3 × 1Monkhorst−Pack k-point mesh has been used for thesupercells.

The Methfessel−Paxton approximation of the first order with0.2 eV smearing width was chosen for the Li surface, whereasGaussian smearing with a width of 0.1 eV was used for the slabapproach calculations. Geometry optimizations have beencarried out within the convergence criteria of 10−6 and 10−5

eV for electronic self-consistent iteration and ionic relaxation,respectively.To study the Li metal surface, in analogy with our previous

study,25 a (4 × 4) slab with five layers was built up along the(100) crystal orientation. The overall supercell used toconstruct the slab has 12 × 12 × 27 Å3 dimensions, with avacuum region of 15 Å. Also following previous work,25 DFT-D326 has been considered for the dispersive van der Waals(vdW) interactions for more accurate descriptions of theadsorbed molecules on the Li metal surface.Adsorption energies (Eads.) for the molecules on the Li metal

surface have been calculated as

= − +E E E E( )ads. Li slab/mol. Li slab mol. (1)

where ELi slab/mol., ELi slab, and Emol. are the total energies of theLi slab containing the interacting molecules, the clean Li metalslab, and the isolated molecule in the vacuum after relaxation,respectively.Bader charge analyses were computed on the total charge

density using the code developed by the Henkelman group.27

Projected density of states (PDOS) was computed by Gaussiansmearing with a width of 0.1 and a 7 × 7 × 1 Monkhorst−Packk-point mesh for better accuracy.

2.1. Li/Li3N Interfaces. Three steps have been taken toconstruct the Li/Li3N interface: first, a full relaxation of bulkcrystals of Li and alpha and cubic phase of Li3N was performed;second, the most stable surface orientation for each phase ofLi3N was targeted; and third, a minimal mismatch between thetwo surfaces (less than 2%) was found within a reasonable sizefor the supercell. A vacuum region around 15 Å was consideredon the top of the interface to avoid periodic image interactions.The interface builder in Virtual Nanolab (VNL)28 has beenused for building the interfaces.

2.2. Core Level Shifts. Core level shifts (CLS) have beenobtained as29

= −CLS BE (sys. ) BE (ref. )N1s N1s N1s (2)

where the core binding energies (BEs) of the systems arecompared with the core binding energies of the referencemolecule to avoid the corresponding errors in the descriptionof core electrons with PAW pseudopotentials. Core level BEs ofthe corresponding systems are calculated as

= −E EBEN1s Fermi N1s (3)

where EN1s is the Kohn−Sham core state energy of the 1sorbital of the nitrogen atom and EFermi is the Fermi energy ofthe system. Half core hole occupancy has been used in thesecalculations following the Janak−Slater (JS) transition statemethod.30 This is a way to account for the full core hole effect,which could otherwise be obtained from the total energydifferences between the neutral and photoexcited systems. Thismethodology is consistent with a very recent study onapproaches implemented in VASP for estimating core levelbinding energy shifts.31

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3. RESULTS AND DISCUSSION

3.1. The LiNO3 Additive on the Li Metal Surface. As afirst step, different molecules which have been detected asreaction products of the LiNO3 additive in Li−S batteries werestudied on a Li metal surface.5,8,9,11 These chosen molecules areN2, N2O, LiNO2, Li3N, and Li2N2O2, since the main reductionproducts from LiNO3 in the SEI film have been identified asLiNxOy and Li2N2O2.

5,7 N2 and N2O molecules have beenselected based on a recent study on gas evolution in LiNO3-containing Li−S batteries.11

To model the interaction of these molecules with the Limetal surface, a number of initial adsorption sites andadsorption configurations for each molecule on the Li metalsurface were considered (see Figure S1). All adsorptionenergies, after relaxation, are reported in Table S1. The moststable configuration of each molecule, i.e., with the lowest

adsorption energy, was selected for further investigation and isshown in Figure 1. For N2O and Li3N on the surface, two initialstructures and all initial structures lead to quite similaradsorption energies, respectively (see Table S1). Therefore,any of the optimized structures from these initial supercells canbe selected. In some cases, the adsorbed molecules aredecomposed after relaxation, and some atoms originatingfrom the nitrogen-containing species can be observed to diffuseinto the surface of Li metal. A similar observation for otherdecomposed electrolyte products on Li metal was made byBalbuena et al.17

The N2 molecule diffused into the Li surface, causing itsbond length to elongate from 1.10 to 1.26 Å after adsorption.The N−N and N−O bond distances of the gas phase N2Omolecule were calculated to be 1.14 and 1.19 Å, respectively.However, after adsorption on the Li surface, the N2O moleculedecomposed into N2 and O, and the N−N bond was elongated

Figure 1. Projected densities of state (PDOS) for the gas phase and the adsorbed molecules on the Li(100) slab with the side and top views of theoptimized supercells of (a) N2, (b) N2O, (c) LiNO2, (d) Li3N, (e) Li2N2O2, and (f) LiNO3 at the Li(100) slab. The Fermi level is set as the origin ofthe energy x-axis which is shifted to zero. The DOS are scaled up five times for visualization.

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to 1.30 Å. The Li3N molecule, in turn, formed a Li6Noctahedral complex after adsorption on the Li metal surface(see Figure 1d). This correlates well with other recenttheoretical studies, where the cubic Li3N phase consisting ofcorner-sharing Li6N octahedral complexes has been proposedto be the most stable phase of Li3N under ambientconditions.32,33 The Li−N bond distances before and afteradsorption on the Li surface are 1.73 and 1.90 Å, respectively.For the adsorbed Li2N2O2 molecule, the N−O and N−N bonddistances are also slightly increased by 0.03 and 0.05 Å,respectively. The LiNO2 and LiNO3 molecules decomposedinto LiNO and O fragments after adsorption, where after the Oatoms diffused into the Li surface (see Figure 1c and 1f). Thebond distances of N−O fragments are elongated to 1.5 Å,which is due to electrons transfer from Li atoms to the π*orbital of the NO molecule.3.2. Core Level Binding Energy. The N 1s core level BEs

have been calculated for the molecules adsorbed on the Lisurface, shown in Figure 1, and also their corresponding gasphase molecules as reference. The BE values are presented inTable 1. The calculated XPS plots for the gas phase andadsorbed molecules on the Li(100) surface are shown in Figure2a and 2b, respectively. For the gas phase N2O molecule(Figure 2a), two different BEs can be calculated for the Nt andNc atoms (Nt and Nc refer to the N atoms in the terminal andcenter positions of the N2O molecule, respectively), due to thedifferent chemical environment for these atoms in the molecule(see Table 1). The N 1s spectrum for Nt is close to that of theN atoms in the N2 molecule, with a chemical shift of around 1eV (see Figure 2a and Table 1). The BE of the Nc atom, on theother hand, shows a shift of +4 eV compared to the Nt atom,which is due to the depletion of electron density on the Nc in

the vicinity of the O atom, which consequently leads to a shiftto higher BE.34,35 The calculated BE shifts for the N atoms inthe N2O molecule are in very good agreement with thereported values in previous works.36,37 Furthermore, for the gasphase molecules, the core level BE for the N atom in the Li3Nmolecule is obtained as 402.05 eV. This value is subsequentiallyraised to 407.78, 409.11, and 412.05 eV for Li2N2O2, LiNO2,and LiNO3 molecules, respectively, following a trend of oxygenricher environments around the N atom. The shift towardhigher BE for the LiNO3 with respect to the BE of LiNO2 (2.94eV) is expected, since the presence of an extra O atom in theLiNO3 decreases the charge density on the N atom.34 This shiftis also in good agreement with the reported experimental valueof 3.7 eV for the separation between the peaks of LiNO3 andNO2

− species.38 Although there are two N atoms in Li2N2O2,both N atoms have the same contribution to the XPS spectradue to molecular symmetry.As seen in Figure 2b, introducing the Li metal surface alters

the position of the N 1s BE peaks for the adsorbed molecules.This is also highly correlated with the decomposition of someof the molecules on the surface; see Figure 1. Generally, thereare fewer differences between the N 1s signals in the XPSspectra for the adsorbed molecules (in the range 403.5−410eV) as compared to those of the gas phase molecules (in therange 401.5−412.5 eV), which indicates a more uniformchemical environment for N atoms on the Li metal surface. TheN2O molecule, for example, which has two distinct peaks in thegas phase XPS spectrum (Figure 2a), shows only one peak inthe spectrum of the adsorbed molecule (Figure 2b), close tothat of the N2 molecule, indicating that both N atomsexperience similar chemical environments after decompositionof the N2O molecule into N2. The BEs for the N atoms in the

Table 1. Calculated N 1s Core Level Binding Energies (BEs) in eV for the Gas Phase and Adsorbed Molecules on the Li(100)Slab, N 1s Core Level Shift (CLSN1s in eV), and Net Amount of Electron Transfer to the N Atom upon Adsorption of theMolecules on the Li(100) Surface (Charge Transfer: CT)

ref. molecule atom BE mol/Li (100) atom BE CLSN1sb CT

N2 N 403.36 N2 N 406.50 3.13 −1.20N2O Nt

a 404.37 N2O Nt 406.20 − −1.44Nca 408.68 Nc 406.17 − −1.63

LiNO2 N 409.11 LiNO2 N 405.84 − −1.93Li3N N 402.05 Li3N N 404.20 2.15 −0.48LiNO3 N 412.05 LiNO3 N 406.37 − −2.34Li2N2O2 N 407.78 Li2N2O2 N 408.90 1.00 −0.10

aNc and Nt refer to the central and terminal N atom in N2O molecule, respectively. bFor the molecules which decomposed on the surface, the CLSwith respect to the corresponding molecules in vacuum are not reported in the table.

Figure 2. N 1s XPS spectra for (a) the gas phase molecules and (b) the adsorbed molecules on the Li(100) surface. For (b), the legend boxrepresents the starting structures on the surface, not the resulting compounds formed.

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adsorbed N2 molecule are shifted to higher values, which can bedue to the observed N−N bond elongation and a subsequentdecrease of the electron density on the N atoms. The CLSpeaks for the adsorbed Li3N and Li2N2O2 molecules are alsoshifted to higher BEs, while the XPS peaks of the LiNO2 andLiNO3 molecules are located at lower BEs (close to the N2 andN2O peaks) than their corresponding gas phase molecules,which is most likely due to the decomposition of thesemolecules into LiNO and O fragments on the Li surface. Allinitial structures investigated of LiNO3 on the Li metal surfacedecomposed to either LiNO2 + O or LiNO + 2O, with thelatter being the energetically most stable.The experimental data in literature regarding the N 1s XPS

peak on the Li metal surface in LiNO3 containing Li−Sbatteries is not fully consistent, perhaps due to the fact thatdifferent cycling schemes, concentrations, and electrolytesolvents are used in these studies. Aurbach et al.5 for exampleobserved 3 major peaks at 401, 403, and 405 eV, which wereassigned to N−H/N−C/N−O, NO2

−, and NO3−, respectively.

Similarly, Li et al.9 also assigned peaks to NO2− and NO3

−, butat 404 and 408 eV, respectively, while a third peak at 400 eVwas assigned to N−S. By contrast, Xiong et al.8 did not assignany peaks to NO2

− and NO3−. Moreover, the observed four N

1s peaks located at comparatively lower BEs. The peaks foundwere assigned to Li2N2O2 (401 eV), Li3N (399 eV), LiNxOy at397 eV, and RCH2NO2 at 395 eV.The XPS results simulated here for the most stable systems

do not support any of the assignments to NO2− and NO3

−,since these species obviously decompose on the Li surface, andare thus more consistent with the work of Xiong et al. It shouldbe mentioned, however, that although the most stablesimulated system for LiNO2 is the decomposed molecule(generating LiNO and O), several other initial structuresconsidered for this molecule (see Figure S1) did notdecompose on the Li metal surface. For the second moststable structure, Figure S1C, the calculated BE is 409 eV. This

results in a trend in BE that is LiNO2 > Li2N2O2 > Li3N, whichcompares well with the experimental studies. Also the Li2N2O2and Li3N products identified by these authors are obviouslystable in the calculations here, and the order of the BEs(Li2N2O2 > Li3N) are also similar. On the other hand, theabsolute values of the BEs are significantly different betweenthe simulated data and the experiments by Xiong et al. and aremore similar to those obtained by Aurbach et al. or Li et al.Moreover, it is likely so that when the Li metal surface is gettingcovered by a thicker SEI layer of decomposition products, itchanges the surface reactivity, which in turn can render NO2

−

and NO3− more stable in the outer region of the SEI, and thus

be detected by XPS. These issues cannot be resolved by thecalculations in the present study. The different conditions inthe computational study and the experimental system, wherethe adsorbed species are surrounded by a matrix of otherdecomposition products, are also likely to affect the absoluteBE values. Another reason for any observed discrepancy can beerrors in the employed theory level to calculate core level BEs,which in turn implies that a reference system is necessary forstudying the core level shifts.29 It should be mentioned that twoLi2N2O2 molecules have been adsorbed on the surface toinvestigate the effect of higher surface coverage on the BEs.However, no significant changes are observed in the N 1s BEvalues. This molecule has been selected since it was stable afterthe adsorption on the Li metal.Different mechanisms have been proposed to describe the

CLS, such as environmental charge density, hybridization, andscreening effects.35,39 Although it is not straightforward tointerpret the chemical shifts of the peaks and the mechanismsbehind them, some possible reasons can be addressed. Chargetransfer to or from an atom will change the electrostaticpotential, which a core electron experiences due to an electronin the valence orbital, and therefore leads to shifts of the corelevel BEs of that atom.35 Hence, a Bader charge analysis wasperformed, and the difference between atomic charge on the N

Figure 3. Projected DOS of α-Li3N, bulklike and interfacial regions of α-Li3N(100)/Li(100) (a); the crystal structure of α-Li3N (b); the α-Li3N(100)/Li(100) interface (c). Projected DOS of c-Li3N, bulklike and interfacial regions of c-Li3N(100)/Li(100) (d); crystal structure of c-Li3N(e); the c-Li3N(100)/Li(100) interface (f). The origin of the energy axis is set at the Fermi level. The density of states in (a) and (d) are scaled uptwo and four times, respectively.

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atoms before and after the adoption of molecules on the Lisurface was calculated (charge transfer; CT). Despite the highercharge density observed on the N atoms of the molecules, thecalculated core level BEs shift to higher values. This indicatesthat it might be other contributions such as hybridization thatmay affect the CLSs.39

To further investigate the different behavior of CLSs, thePDOS projected on the N, Li, and O atoms of these molecules,before and after adsorption on the Li metal surface, are plottedin Figure 1. Any changes in the electronic structure of thesemolecules are thus due to the presence of the Li surface. A clearoverlap (hybridization) between the s states of Li slab and the sand p states of N (or O) atom can be seen in all cases, whichredistributes the valence electron density between theadsorbents and the Li metal surface. It has indeed beendemonstrated that the core level BEs are sensitive to variationsin the local electron density distribution.39

3.3. Li3N/Li Interface. Different stable phases of Li3N existdepending on pressure: α-Li3N with the space group of P6/mmm, which appears under ambient pressures, transforms athigher pressures into β-Li3N and γ-Li3N with the space groupsP63/mmc and Fm3m, respectively.

32,40 The cubic Li3N (c-Li3N)with the space group Pm3m is another stable phase underambient pressure, which has been reported recently.32,33 In thisstudy, we have considered α-Li3N and c-Li3N to simulate thesolid phase of the Li3N structure on the surface of the Li metal.The crystal structures of these unit cells are shown in Figures3b and 3e, respectively.It should be mentioned that the crystal structure of the

hexagonal α-Li3N converts to an orthorhombic structure with a= ahex, b = √3bhex, and c = chex after full relaxation.41 TheLi(1)−N bond distance in α-Li3N is calculated to 1.89 Å, whilethe Li(2)−N bond length is around 2.06 Å, which is in goodagreement with experimental results where the Li(1)−N andLi(2)−N are reported to 1.93 and 2.01 Å, respectively.42 In c-Li3N, the Li−N bond distance is calculated to 1.91 Å, which isconsistent with the previously reported theoretical value of 1.93Å.33

The core level BEs of the bulk Li3N phases have beencalculated using the same approach as in the previous section,and the calculated N 1s XPS spectra for these bulk crystals arepresented in Figure 4. The peaks for α-Li3N and c-Li3N arefound at 403.9 and 402.3 eV, respectively. Indeed, this shift inthe peaks can be attributed to the different chemicalenvironment in these bulk phases. The coordination number(CN) of the N atom in the α-Li3N structure is 8, while it is 6 inc-Li3N. The results show that there is a relation between theCN and the BE shifts, so that an increasing CN leads to higher

BE values (more positive CLS). The BE shifts can however notbe attributed only to the environmental charge density, sincethe calculated charges on the N atoms of the isolated Li3Nmolecule in c-Li3N and in α-Li3N show negligible difference(−2.57, −2.49, and −2.47 e, respectively). If comparing toexperimental data, finally, the calculated BEs for c-Li3N arecloser to the XPS peaks assigned to Li3N on Li metalelectrodes,7,8 which indicates that this is the phase preferentiallyformed in the Li−S battery system. The characters of Li3Nbonds have generally been considered as ionic, but alsocovalent characteristic of these bonds have been discussed inthe literature.43 The PDOS plots, presented in Figures 3a and3d, show a higher degree of overlap between the N(s), N(p),and Li(s) states in the α-Li3N than in the c-Li3N phase, leadingto a higher BE for the α-Li3N phase.To build the α-Li3N and c-Li3N surfaces, different

orientations have been considered by cleavage of their bulkstructures. The surface energies for the different orientations ofLi3N slabs with different number of layers have been calculatedto obtain the minimum number of layers required and the moststable orientation for the Li3N slab. The results are plotted inFigure S2. The most stable surface orientation for both the α-Li3N and c-Li3N surfaces is (100), which displays surfaceenergies almost constant with increasing number of layers (seeFigure S2). Therefore, four layers were selected for these slabsto generate sufficient thickness. This number of layers for theLi3N slab has also been reported to be sufficient in a previouscomputational study.44

To build the Li3N(100)/Li(100) interface, two differentphases of Li3N(100) surfaces have been used, generating amismatch of 0.67% and 1.7% for the α-Li3N(100)/Li(100) andc-Li3N(100)/Li(100), respectively. The “interfacial” and “bulk-like” regions of these structures are also illustrated in Figures 3cand 3f.The N 1s core level BEs for the N atoms in three different

regions, viz., interfacial, bulklike, and surface (the top layer ofLi3N in contact with the vacuum layer) of the Li3N(100)/Li(100) slabs have been calculated, and their correspondingXPS spectra are presented in Figure 5. As for the bulk crystal of

Li3N, the core level BE peaks for N atoms in the α-Li3N(100)/Li(100) interface generally displayed higher BEs than those inthe c-Li3N(100)/Li (100) interface, which may be due to thelarger CN for the N atoms in the α-Li3N(100)/Li(100)interface. The N 1s XPS spectrum for the interfacial region ofthe α-Li3N(100)/Li(100) is observed at lower BEs than that forthe bulklike region. It is interesting, however, to observe thatFigure 4. N 1s XPS spectra for bulk α-Li3N and c-Li3N.

Figure 5. N 1s XPS for (a) α-Li3N (100)/Li (100) and (b) c-Li3N(100)/Li (100) interfaces.

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the BE shifts in the c-Li3N(100)/Li(100) interface are in thereverse order with respect to the α-Li3N(100)/Li(100)interface for the interfacial and bulklike regions, indicatingthat the N atoms become more electron rich when approachingthe surface in the α-Li3N(100)/Li(100) structure but are in aless electron rich environment for the case of the c-Li3N(100)/Li (100) interface. There is no significant difference for the BEof the surface region, in the α-phase, from the interfacial andbulk regions. However, the difference among these parts ismore pronounced in the c-phase. It can be seen that the BEs ofthe surface region are smaller than the BEs of bulk in bothphases indicating the relation between the smaller coordinationnumber of atoms on the surface region than that of the bulkand consequently the smaller BE values in the surface part.To further investigate the observed trends between the

interface and bulklike regions, CT and PDOS have beenstudied. The average amounts of CTs to the N atoms of the α-Li3N(100)/Li(100) and c-Li3N(100)/Li(100) interfaces, com-pared to their corresponding bulk phases, are −0.1 and −0.004,respectively, which is not considerable. However, the PDOS(Figure 3a and 3d) are more helpful for interpretation of theseCLSs (shown in Figure 6). First of all, the PDOS plots show a

higher degree of overlap between the N and Li(s) orbitals forthe α-Li3N(100)/Li(100) interface than the c-Li3N(100)/Li(100), which causes a reduction of the electron densities onthe N atoms in the α-Li3N(100)/Li(100) interface. This mightbe the reason for the increasing core level BEs in the α-Li3N(100)/Li(100) interface. Furthermore, the delocalizationof electron density is greater for the interfacial region than thebulklike region in the α-Li3N(100)/Li(100) interface, whichcould be expected to lead to higher BEs, but the oppositepattern is actually observed. A higher degree of delocalizationcan be seen in the interfacial region of the c-Li3N(100)/Li(100), which may result in a higher BE than in the bulklikeregion.

4. CONCLUSIONSThe electronic structure and spectroscopy properties of LiNO3,and its decomposition products N2, N2O, LiNO2, Li3N, andLi2N2O2 on the interaction with the Li metal surface, have beeninvestigated within DFT framework. In all cases, noticeablechanges in bond distances or dissociations occurred in thestructure upon relaxation. These structures have then been used

to calculate the N 1s XPS BEs of the supercells, andcomparisons are made to isolated molecules in vacuum.It is not straightforward to describe the electronic structure

background to the chemical shifts of the XPS peaks; however,the redistribution of the valence electron density due to thecharge transfer and hybridization between the s states of Li slaband the s and p states of N (or O) atom might have a majoreffect on these chemical shifts. It is also interesting to note thatsome of the experimentally assigned N-containing compounds(NO3

− and NO2−) decompose on Li metal, although the

stability is likely higher for LiNO2 than LiNO3. There is on theother hand support for the stability of NO, Li2N2O2, and Li3N.The differences between the observed species in this study andthose assigned in experimental work can perhaps be related tothe limited thickness for the interphase on the Li metal surfacein this study.The CLS for α-Li3N and c-Li3N demonstrate a clear relation

between the CN and BEs. By increasing CN from 3 inmolecular Li3N to 6 and 8 in the c-Li3N and α-Li3N,respectively, BEs shift to higher energies. The BE shifts inthe α-Li3N(100)/Li(100) and c-Li3N(100)/Li(100) interfacesexhibit opposite trends for the interfacial and bulklike regions.The PDOS analyses show higher overlaps between the N(p)and Li(s) orbitals in the α-Li3N(100)/Li(100) interface thanthe c-Li3N(100)/Li(100), which might be the reason forincreasing the core level BEs in the α-Li3N(100)/Li(100)interface. The calculated BEs for c-Li3N bulk and interfaces inthis study are in somewhat better agreement with the XPSpeaks experimentally assigned to Li3N in similar systems.Electrochemical data and SEM images have shown that both

LiNO3 and polysulfides play an important role for suppressingthe redox shuttle mechanism in Li−S systems.45 In futurestudies, it is of high interest to also consider polysulfidestogether with the nitrogen-containing species on the surface ofLi metal in order to simulate a more realistic system.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.7b07847.

Initial adsorption sites/configurations for differentmolecules in the study on a Li metal surface; adsorptionenergies (eV) for relaxed supercells from different initialsites/configurations of molecules on the Li(100) surface;surface energies convergence for Li3N surfaces vsdifferent number of layers in two different phases ofLi3N (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: moyses.araujo@physics.uu.se.ORCIDMahsa Ebadi: 0000-0001-8525-7339Daniel Brandell: 0000-0002-8019-2801C. Moyses Araujo: 0000-0001-5192-0016NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis project was supported by the Swedish Energy Agencygrant number 39036-1, STandUP for Energy, the Carl Tryggers

Figure 6. Core level binding energy shifts for α-Li3N, c-Li3N, α-Li3N(100)/Li(100), and c-Li3N(100)/Li(100) interface, surface, andbulklike regions (The reference is Li3N molecule).

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Foundation and the Swedish Research Council (VR) grant no.2014-5984 and 2015-05754. The computations were performedon resources provided by the Swedish National Infrastructurefor Computing (SNIC) at the PDC Center for HighPerformance Computing.

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