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Density Functional Theory Modelling the Interfacial Chemistry of the LiNO Additive for Lithium-Sulfur Batteries by Means of Simulated Photoelectron Spectroscopy 3

Mahsa Ebadi, Matthew James Lacey, Daniel Brandell, and C. Moyses Graça Araujo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07847 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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The Journal of Physical Chemistry

Density Functional Theory Modelling the Interfacial Chemistry of the LiNO3 Additive for Lithium-Sulfur Batteries by Means of Simulated Photoelectron Spectroscopy Mahsa Ebadi1, Matthew J. Lacey1, Daniel Brandell1, C. Moyses Araujo2 * 1 2

Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

ABSTRACT: Lithium-sulfur (Li-S) batteries are considered candidates for next-generation energy storage systems due to their high theoretical specific energy. There exists, however, some shortcomings of these batteries, not least the solubility of intermediate polysulfides into the electrolyte generating a so-called ‘redox shuttle’, which gives rise to self-discharge. LiNO3 is therefore frequently used as an electrolyte additive to help suppressing this mechanism, but the exact nature of the LiNO3 functionality is still unclear. Here, density functional theory calculations are used to investigate the electronic structure of LiNO3 and a number of likely species (N2, N2O, LiNO2, Li3N, and Li2N2O2) resulting from the reduction of this additive on the surface of Li-metal anode. The N 1s X-ray photoelectron spectroscopy (XPS) core level binding energies of these molecules on the surface are calculated in order to compare the results with experimentally reported values. The core level shifts (CLS) of the binding energies are studied to 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 1s binding energies for the bulk phases of Li3N and at the Li3N/Li interfaces display higher values as compared to the Li3N molecule, indicating a clear correlation between the coordination number and the CLS of the solid phases of Li3N.

1. INTRODUCTION Rechargeable batteries based on lithium metal negative electrodes are potential candidates for electrical energy storage systems due to the high theoretical capacity and the high negative electrochemical potential of the Li metal electrode. However, these rechargeable batteries also suffer from serious problems such as dendrite growth during cycling, safety risks and low columbic efficiency. Therefore, today’s lithium-based storage technologies in the form of Li-ion batteries (LIBs) are instead utilizing graphite as the negative electrode.1 During recent years, however, the industrial development of LIBs have led them to approach their theoretical energy density limits. To further improve the energy density of Li-based battery systems, application of Li metal electrodes has regained significant interests for a number of different cell chemistries.2 Lithium-sulfur (Li–S) and Li–air batteries have in this context been introduced as possible candidates for enhancing the driving ranges of electric vehicles, although this is likely far into the future. Li–S batteries have a theoretical specific energy around 5 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 the significant advantages of being inexpensive and non-toxic, while Li 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 the liquid electrolyte, and a high level of self-discharge. The solubility of polysulfides in the electrolyte causes migration of these species back and forth between the positive and negative electrodes. This constitutes a ‘redox shuttle mechanism’, which consequently leads to corrosion of the lithium metal and precipitation of insoluble Li2S and Li2S2 on the surface of the Li electrode.3 To passivate and better protect the Li metal surface from these parasitic reactions, different additives are used in the liquid electrolyte. Lithium nitrate (LiNO3) is nowadays a common such electrolyte additive in Li-S batteries, often used in concentrations of ca. 0.10.5 M, for the protection of the Li metal anode.4 The exact functionality of the nitrate additive is still not clear, but it is often argued that this oxidizing additive is reduced to LixNOy compounds while oxidizing the sulfides to LixSOy, thereby forming a passivation layer on the Li metal surface.5 There have been a number of experimental studies on the role of LiNO3 additive in Li-S batteries.6–10 By means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques, it has been shown that by controlling the concentration of LiNO3 and PS additives in the electrolyte, a stable and uniform solid slectrolyte interphase (SEI) layer on the surface of the 1

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Li metal electrode can be formed.9 It has also been reported that LiNO3 as lithium salt or additive can suppress the shuttle mechanism in Li-S batteries.8 The evolution of N2 and N2O gases in Li-S cells with LiNO3 additive has also recently been investigated by means of pressure measurements and gas analysis.11 There exists a limited number of molecular-level computational studies of the Li-S systems, 12–14 mainly focusing on polysulfides. Moore et al.15 reported a comprehensive set of possible reactions in Li−S batteries using high-level quantum chemical methods. The authors considered the reversible formation of Li2S from lithium and a sulfur (S8) cathode in a nonaqueous solvent. In another study, first principle methods have been applied to predict thermodynamic and electronic properties of Li, Li2S2, Li2S, α-sulfur, and β–sulfur.16 Balbuena et al.17 have studied surface reactions and reduction mechanisms of various electrolyte components at the Li electrode, including polysulfide compounds, using density functional theory (DFT) and ab-initio molecular dynamics (AIMD) simulations. A series of heteroatom-doped (B, N, O, F, S, P and Cl) graphene nanoribbons has been modeled and their interactions with both polar lithium polysulfide and nonpolar elemental sulfur have been studied by first principle calculations. It is shown that N or O dopant can effectively prevent shuttle of polysulfides by significantly higher interaction between the carbon hosts and the polysulfide guests.18 Li3N has been reported as one of the SEI components in the presence of LiNO3 in the electrolyte.5 This component has been shown to have a very high Li ion conductivity and be a promising candidate in order to passivate the surface of the Li metal.19 A recent combined theoretical-experimental study,20 has been performed on nitride materials chemistry to stabilize Li metal anode. The authors showed that oxides, sulfide and halides are reduced by the Li metal and therefore have a poor stability against the metal. On the other hand, nitride materials including 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 used periodic DFT to study the interactions of LiNO3 itself, and experimentally reported species originating from this additive, on the surface of a Li metal negative electrode. After analyzing the stability of these compounds on the Li metal surface, core level binding energies have been calculated to gain a better insight into the XPS properties of the nitrogencontaining compounds in order to achieve a direct comparison with experimental data. In addition to the molecules on the surface of Li metal, Li3N/Li interface have also been studied in this work. To our

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knowledge, this is the first computational study on the electronic structure and core level binding energies of such interface.

2. SIMULATION METHODS Plane wave DFT has been used employing the Vienna ab initio simulation package.21 The electronic states have been described by the projector augmented wave (PAW) method22,23 within the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) to the exchange-correlational functional.24 PAW potentials with valence states 1s22s1 for Li, 2s22p3 for N, 2s22p4 for O, have been applied. The energy cutoff for the plane wave was 550 eV and a 3×3×1 Monkhorst−Pack k-point mesh has been used for the supercells. The Methfessel−Paxton approximation of the first order with 0.2 eV smearing width was chosen for the Li surface, whereas Gaussian smearing with a width of 0.1 eV was used for the slab approach calculations. Geometry optimizations have been carried 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 (4x4) slab with five layers was built up along (100) crystal orientation. The overall supercell used to construct the slab has 12×12×27 Å3 dimensions, with a vacuum 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 the adsorbed molecules on the Li metal surface. Adsorption energies (Eads.) for the molecules on the Li metal surface have been calculated as: (1) E. = E /. − (E + E. ) where E /. , E , and E. are the total energies of the Li slab containing the interacting molecules, the clean Li metal slab 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 Gaussian smearing with a width of 0.1 and a 7×7×1 Monkhorst-Pack k-point mesh for better accuracy. 2.1. Li/Li3N Interfaces Three steps have been taken to construct the Li/Li3N interface: first, a full relaxation of bulk crystals of Li, alpha and cubic phase of Li3N was performed; second, the most stable surface orientation for each phase of Li3N was targeted; and third, a minimal mismatch between the two surfaces (less than 2%) 2


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3. RESULTS AND DISCUSSION

initial supercells can be selected. In some cases, the adsorbed molecules are decomposed after relaxation, and some atoms originating from the nitrogen containing species can be observed to diffuse into the surface of Li metal. A similar observation for other decomposed electrolyte products on Li-metal was made by Balbuena et al. 17 The N2 molecule diffused into the Li surface, causing its bond length to elongate from 1.10 to 1.26 Å after adsorption. The N-N and N-O bond distances of the gas phase N2O molecule were calculated to be 1.14 and 1.19 Å, respectively. However, after adsorption on the Li surface, the N2O molecule decomposed into N2 and O, and the N-N bond was elongated to 1.30 Å. The Li3N molecule, in turn, formed a Li6N octahedral complex after adsorption on the Li metal surface (see Fig. 1d). This correlates well with other recent theoretical studies, where the cubic Li3N phase consisting of corner sharing Li6N octahedral complexes has been proposed to be the most stable phase of Li3N under ambient conditions.32,33 The Li-N bond distances before and after adsorption on the Li surface are 1.73 and 1.90 Å, respectively. For the adsorbed Li2N2O2 molecule, the N-O and N-N bond distances are also slightly increased by 0.03 and 0.05 Å, respectively. The LiNO2 and LiNO3 molecules decomposed into LiNO and O fragments after adsorption, where after the O atoms diffused into the Li surface (see Fig. 1c and 1f). The bond 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.

3.1 The LiNO3 Additive on the Li Metal Surface

3.2 Core Level Binding Energy

As a first step, different molecules which have been detected as reaction products of the LiNO3 additive in Li-S batteries were studied on a Li metal surface.5,8,9,11 These chosen molecules are N2, N2O, LiNO2, Li3N and Li2N2O2, since the main reduction products from LiNO3 in the SEI film have been identified as LiNxOy and Li2N2O2.5,7 N2 and N2O molecules have been selected based on a recent study on gas evolution in LiNO3-containing Li-S batteries.11 To model the interaction of these molecules with the Li metal surface, a number of initial adsorption sites and adsorption configurations for each molecule on the Li metal surface were considered (see Fig. S1). All adsorption energies, after relaxation, are reported in Table S1. The most stable configuration of each molecule, i.e. with the lowest adsorption energy, were selected for further investigation and are shown in Fig. 1. For N2O and Li3N on the surface, two initial structures and all initial structures lead to quite similar adsorption energies, respectively (see Table S1). Therefore, any of the optimized structures from these

The N 1s core level BEs have been calculated for the molecules adsorbed on the Li surface, shown in Fig. 1, and also their corresponding gas phase molecules as reference. The BE values are presented in Table 1. The calculated XPS plots for the gas phase and adsorbed molecules on the Li(100) surface are shown in Fig. 2a and 2b, respectively. For the gas phase N2O molecule (Fig. 2a), two different BEs can be calculated for the Nt and Nc atoms (Nt and Nc refers to the N atoms in the terminal and center positions of N2O molecule, respectively), due to the different chemical environment for these atoms in the molecule (see Table 1). The N 1s spectra for Nt is close to that of the N atoms in the N2 molecule, with a chemical shift of around 1 eV (see Fig. 2a and Table 1). The BE of the Nc atom, on the other 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 shift to higher BE.34,35 The calculated BE shifts for the N atoms in the N2O molecule is in very good agreement with the

was found within a reasonable size for the supercell. A vacuum region around 15 Å was considered on the top of the interface to avoid periodic image interactions. The interface builder in Virtual Nanolab (VNL)28 have been used for building the interfaces. 2.2. Core-Level Shifts Core level shifts (CLS) have been obtained as:29 CLS = BE (sys. ) − BE (ref. )

(2)

where the core binding energies (BEs) of the systems are compared with the core binding energies of the reference molecule to avoid the corresponding errors in the description of core electrons with PAW pseudopotentials. Core level BEs of the corresponding systems are calculated as BE = E ! − E (3) where E is the Kohn-Sham core state energy of the 1s orbital of the nitrogen atom and E ! is the Fermi energy of the system. Half core hole occupancy has been used in this calculations following the Janak–Slater (JS) transition state method.30 This is a way to account for the full core hole effect, which could otherwise be obtained from the total energy differences between the neutral and photo-excited systems. This methodology is consistent with a very recent study on approaches implemented in VASP for estimating core-level binding energy shifts.31

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reported values in previous works.36,37 Furthermore for the gas phase molecules, the core level BE for the N atom in the Li3N molecule is obtained as 402.05

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eV. This value is subsequentially raised to 407.78, 409.11, and

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 the optimized supercells of (a) N2 (b) N2O (c) LiNO2 (d) Li3N (e) Li2N2O2 (f) LiNO3 at Li(100) slab. The Fermi level is set as the origin of the energy x-axis which is shifted to zero. The DOS are scaled up 5 times for visualization.

412.05 eV for Li2N2O2, LiNO2, and LiNO3 molecules, respectively, following a trend of oxygen richer environments around the N atom. The shift toward higher BE for the LiNO3 with respect to the BE of LiNO2 (2.94 eV) is expected, since the presence of an extra O atom in the LiNO3 decreases the charge density on the N atom.34 This shift is also in good agreement with the reported experimental val-

ue of 3.7 eV for the separation between the peak of LiNO3 and NO2- species.38 Although there are two N atoms in Li2N2O2, both N atoms have the same contribution to the XPS spectra due to molecular symmetry. As seen in Fig. 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 4


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the decomposition of some of the molecules on the surface; see Fig. 1. Generally, there are fewer differences between the N 1s signals in the XPS spectra

for the adsorbed molecules (in the range 403.5-410 eV) as compared to those of the gas phase molecules (in the range 401.5-412.5 eV), which indicates a

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 (CLSN 1s in eV), and net amount of electron transfer to the N atom upon adsorption of the molecules on the Li(100) surface (Charge transfer; CT). CT Atom BE Mol/Li (100) Atom BE CLSN 1s ** N 403.36 N2 N 406.50 3.13 -1.20 Nt * 404.37 N 2O Nt 406.20 -1.44 Nc * 408.68 Nc 406.17 -1.63 LiNO2 N 409.11 LiNO2 N 405.84 -1.93 Li3N N 402.05 Li3N N 404.20 2.15 -0.48 LiNO3 N 412.05 LiNO3 N 406.37 - 2.34 Li2N2O2 N 407.78 Li2N2O2 N 408.90 1.00 -0.10 * Nc and Nt refer to the central and terminal N atom in N2O molecule, respectively. ** For the molecules which decomposed on the surface, the CLS with respect to the corresponding molecules in vacuum are not reported in the table. Ref. Molecule N2 N 2O

more uniform chemical environment for N atoms on the Li metal surface. The N2O molecule for example, which has two distinct peaks in the gas phase XPS spectrum (Fig. 2a), shows only one peak in the spectrum of the adsorbed molecule (Fig. 2b), close to that of the N2 molecule, indicating that both N atoms experience similar chemical environments after decomposition of the N2O molecule into N2. The BEs for the N atoms in the adsorbed N2 molecule are shifted to higher values, which can be due to the observed N-N bond elongation and a subsequent decrease of the electron density on the N atoms. The CLS peaks for the adsorbed Li3N and Li2N2O2 molecules are also shifted to higher BEs, while the XPS peaks of the LiNO2 and LiNO3 molecules are located at lower BEs (close to the N2 and N2O peaks) than their corresponding gas phase molecules, which is most likely due to the decomposition of these molecules into LiNO and O fragments on the Li surface. All initial structures investigated of LiNO3 on the Li-metal surface decomposed to either LiNO2 + O or LiNO + 2O, with the latter 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-S batteries is not fully consistent, perhaps due to that different cycling schemes, concentrations and electrolyte solvents used in these studies. Aurbach et al.5 for example observed 3 major peaks at 401, 403 and 405 eV, which were assigned to N-H/N-C/N-O,

NO2- and NO3-, respectively. Similarly, Li et al.9 also assigned peaks to NO2- and NO3-, but at 404 and 408 eV, respectively, while a third peak at 400 eV was assigned to N-S. By contrast, Xiong et al.8 did not assign any peaks to NO2- and NO3-. Moreover, the observed four N 1s peaks located at comparatively lower BEs. The peaks found were assigned to Li2N2O2 (401 eV), Li3N (399 eV), LiNxOy at 397 eV and RCH2NO2 at 395 eV. The XPS results simulated here for the most stable systems does not support any of the assignments to NO2- and NO3-, since these species obviously decomposes on the Li surface, and are thus more consistent with the work of Xiong et al. It should be mentioned, however, that although the most stable simulated system for LiNO2 is the decomposed molecule (generating LiNO and O), several other initial structures considered for this molecule (see Fig. S1) did not decompose on the Li metal surface. For the second most stable structure, Fig. S1C, the calculated BE is 409 eV. This result in a trend in BE that is LiNO2 > Li2N2O2 >Li3N, which compares well with the experimental studies. Also the Li2N2O2 and Li3N products identified by these authors are obviously stable in the calculations here, and the order of the BEs (Li2N2O2 > Li3N) are also similar. On the other hand, the absolute values of the BEs are significantly different between the simulated data and the experiments by Xiong et al., and are more similar to those obtained by Aurbach 5



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et al. or Li et al. Moreover, it is likely so that when the Li-metal surface is getting covered by a thicker SEI layer of decomposition products, it changes the surface reactivity, which in turn can render NO2- and NO3- more stable in the outer region of the SEI, and thus being detected by XPS. These issues cannot be resolved by the calculations in the present study. The different conditions in the computational study and the experimental system, where the adsorbed species are surrounded by a matrix of other decomposition products, are also likely to affect the absolute BE values. Another reason for any observed discrepancy can be errors in the employed theory level to calculate core level BEs, which in turn implies that a reference system is necessary for studying the core level shifts.29 It should be mentioned that two Li2N2O2 molecules have been adsorbed on the surface to investigate the effect of higher surface coverage on the BEs. However, no significant changes are observed on the N 1s BE values. This molecule has been selected since it was stable after the adsorption on the Li metal. Different mechanisms have been proposed to describe the CLS, such as environmental charge density, hybridization, and screening effects.35,39 Although it is not straightforward to interpret the chemical shifts of the peaks and the mechanisms behind them, some possible reasons can be addressed. Charge transfer to

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or from an atom will change the electrostatic potential, which a core electron experiences due to an electron in the valence orbital, and therefore leads to shifts of the core-level BEs of that atom.35 Hence, a Bader charge analysis was performed and the difference between atomic charge on the N atoms before and after the adoption of molecules on the Li surface was calculated (Charge Transfer; CT). Despite the higher charge density observed on N atoms of the molecules, the calculated core level BEs shift to higher values. This indicates that it might be other contributions such as hybridization that may affect the CLSs.39 To further investigate the different behavior of CLSs, the PDOS projected on the N, Li, and O atoms of these molecules, before and after adsorption on the Li metal surface are plotted in Fig. 1. Any change in the electronic structure of these molecules are thus due to the presence of the Li surface. A clear overlap (hybridization) between the s states of Li slab and the s and p states of N (or O) atom can be seen in all cases, which redistributes the valence electron density between the adsorbents and the Li metal surface. It has indeed been demonstrated that the core level BEs are sensitive to variations in the local electron density distribution.39

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 box represents the starting structures on the surface, not the resulting compounds formed.

3.3 Li3N/Li Interface Different stable phases of Li3N exist depending on pressure: α-Li3N with the space group of P6/mmm, which appear under ambient pressures, transform at 6


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higher pressures into β-Li3N and γ-Li3N with the space groups P6) /mmc and Fm3-m, respectively.32,40 The cubic Li3N (c-Li3N) with the space group Pm3m is another stable phase under ambient pressure, which has been reported recently.32,33 In this study, we have considered α-Li3N and c-Li3N to simulate the solid phase of the Li3N structure on the surface of the Li metal. The crystal structures of these unit cells are shown in Figs. 3b and 3e, respectively. It should be mentioned that the crystal structure of the hexagonal α-Li3N converts to an orthorhombic structure with a = a/ 0, b = √3 b/ 0 and c = c/ 0 after full relaxation.41 The Li(1)-N bond distance in α-Li3N is calculated to 1.89 Å, while the Li(2)-N bond length

is around 2.06 Å, which is in good agreement with experimental results where the Li(1)-N and Li(2)-N are reported to 1.93 and 2.01 Å, respectively.42 In cLi3N, the Li-N bond distance is calculated to 1.91 Å, which is consistent with the previously reported theoretical value of 1.93 Å.33 The core level BEs of the bulk Li3N phases have been calculated using the same approach as in the previous section, and the calculated N 1s XPS spectra for these bulk crystals are presented in Fig. 4. The peaks for αLi3N and c-Li3N are found at 403.9 and 402.3 eV, respectively. Indeed, this shift in the peaks can be attributed to the different chemical environment in these bulk phases. The coordination number (CN) of

Figure 3. Projected DOS of 3-Li3N, bulk-like and interfacial regions of 3-Li3N(100)/Li(100) (a); the crystal structure of 3Li3N (b); the 3-Li3N(100)/Li(100) interface (c); Projected DOS of 4-Li3N, bulk-like and interfacial regions of 4Li3N(100)/Li(100) (d); crystal structure of 4-Li3N (e); the 4-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 up 2 times and 4 times, respectively.

the N atom in the α-Li3N structure is 8, while it is 6 in c-Li3N. The results show that there is a relation between the CN and the BE shifts, so that an increasing CN leads to higher BE values (more positive CLS). The BE shifts can however not be attributed only to the environmental charge density, since the calculated 7



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charge on the N atoms of the isolated Li3N molecule in c-Li3N and in α-Li3N show negligible difference (2.57, -2.49 and -2.47 e, respectively). If comparing to experimental data, finally, the calculated BEs for cLi3N are closer to the XPS peaks assigned to Li3N on Li-metal electrodes,7,8 which indicates that this is the phase preferentially formed in the Li-S battery system. The character of Li3N bonds have generally been considered as ionic, but also covalent characteristic of these bonds have been discussed in the literature.43 The PDOS plots, presented in Figs. 3a and 3d, show a higher degree of overlap between the N (s), (p) and Li(s) states in the α-Li3N than in the c-Li3N phase, leading to 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 bulk structures. The surface energies for the different orientations of Li3N slabs with different number of layers have been calculated to obtain the minimum number of layers required and the most stable orientation for the Li3N slab. The results are plotted in Fig. S2. The most stable surface orientation for both the αLi3N and c-Li3N surfaces is (100), which display surface energies almost constant with increasing number of layers (see Fig. S2). Therefore, four layers were selected for these slabs to generate sufficient thickness. This number of layers for the Li3N slab has also been reported to be sufficient in a previous computational study.44

Figure 4. N 1s XPS spectra for bulk 3-Li3N and c-Li3N.

To build the Li3N(100)/Li(100) interface, two different phases of Li3N(100) surfaces have been used, generating a mismatch of 0.67 % and 1.7 % for the α-Li3N(100)/Li(100) and c-Li3N(100)/Li(100), respectively. The ‘interfacial’ and ‘bulk-like’ regions of these structures are also illustrated in Figs. 3c and 3f.

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Figure 5. N 1s XPS for (a) 3-Li3N (100) /Li (100) and (b) 4-Li3N (100) /Li (100) interfaces.

The N 1s core level BEs for the N atoms in three different regions, viz. interfacial, bulk-like and surface (the top layer of Li3N in contact with the vacuum layer) of the Li3N(100)/Li(100) slabs have been calculated and their corresponding XPS spectra are presented in Fig. 5. As for the bulk crystal of Li3N, the core level BE peaks for N atoms in the αLi3N(100)/Li(100) interface generally displayed higher BEs than those in the c-Li3N(100)/Li (100) interface, which may be due to the larger CN for the N atoms in the α-Li3N(100)/Li(100) interface. The N 1s XPS spectrum for the interfacial region of the αLi3N(100) /Li(100) is observed at lower BEs than that for the bulk-like region. It is interesting, however, to observe that the BE shifts in the c-Li3N(100)/Li(100) interface are in the reverse order with respect to the α-Li3N(100)/Li(100) interface for the interfacial and bulk-like regions, indicating that the N atoms become more electron rich when approaching the surface in the α-Li3N(100)/Li(100) structure but are in a less electron rich environment for the case of the cLi3N(100)/Li (100) interface. There is no significant difference for the BE of the surface region, in the α −phase, from the interfacial and bulk regions. However, the difference among these parts is more pronounced in the 4 −phase. It can be seen that the BEs of the surface region is smaller than the BEs of bulk in both phases indicating the relation between the smaller coordination number of atoms on the surface region than that of the bulk and consequently the smaller BE values in the surface part. To further investigate the observed trends between the interface and bulk like regions, CT and PDOS have been studied. The average amount of CTs to the N atoms of the α-Li3N(100)/Li(100) and c8


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Li3N(100)/Li(100) interfaces, comparing to their corresponding bulk phases, are -0.1 and -0.004, respectively, which is not considerable. However, the PDOS (Fig. 3a and 3d) are more

Figure 6. Core level binding energy shifts for 3-Li3N, cLi3N, 3-Li3N(100)/Li(100), and 4-Li3N(100)/Li(100) interface, surface and bulk like regions (The reference is Li3N molecule).

helpful for interpretation of these CLSs (shown in Fig. 6). First of all, the PDOS plots show a higher degree of overlap between the N and Li(s) orbitals for the α-Li3N(100)/Li(100) interface than the c-Li3N (100)/Li(100), which causes a reduction of the electron densities on the N atoms in the αLi3N(100)/Li(100) interface. This might be the reason for the increasing core level BEs in the αLi3N(100)/Li(100) interface. Furthermore, the delocalization of electron density is greater for the interfacial region than the bulk like region in the αLi3N(100)/Li(100) interface, which could be expected to lead to higher BEs, but the opposite pattern is actually observed. A higher degree of delocalization can be seen in the interfacial region of the cLi3N(100)/Li(100), which may result in a higher BE than in the bulk-like region.

4. CONCLUSIONS The electronic structure and spectroscopy properties of LiNO3, and its decomposition products N2, N2O, LiNO2, Li3N and Li2N2O2 on the interaction with the Li-metal surface, have been investigated within DFT framework. In all cases, noticeable changes in bond

distances or dissociations occurred in the structure upon relaxation. These structures have then been used to calculate the N 1s XPS BEs of the supercells and comparisons 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 the charge transfer and hybridization between the s states of Li slab and the s and p states of N (or O) atom might have a major effect on these chemical shifts. It is also interesting to note that some 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 the other hand support for the stability of NO, Li2N2O2 and Li3N. The differences between the observed species in this study and those assigned in experimental work can perhaps be related to the limited thickness for the interphase on the Li metal surface in this study. The CLS for α-Li3N and c-Li3N demonstrate a clear relation between the CN and BEs. By increasing CN from 3 in molecular Li3N to 6 and 8 in the c-Li3N and α-Li3N, respectively, BEs shifts to higher energies. The BE shifts in the α-Li3N(100)/Li(100) and cLi3N(100)/Li(100) interfaces exhibit opposite trends for the interfacial and bulk-like regions. The PDOS analyses show higher overlaps between the N(p) and Li(s) orbitals in the α-Li3N(100)/Li(100) interface than the c-Li3N(100)/Li(100), which might be the reason for increasing the core level BEs in the αLi3N(100)/Li(100) interface. The calculated BEs for c-Li3N bulk and interfaces in this study are in somewhat better agreement with the XPS peaks experimentally assigned to Li3N in similar systems. As electrochemical data and SEM images have shown that both LiNO3 and polysulfides play an important role for suppressing the redox shuttle mechanism in Li-S systems.45 In future studies, it is of high interest to also consider polysulfides together with the nitrogen containing species on the surface of Li metal in order to simulate a more realistic system.

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AUTHOR INFORMATION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Corresponding Author: [email protected]

ACKNOWLEDGMENT

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This project was supported by the Swedish Energy Agency grant number 39036-1, STandUP for Energy and the Swedish Research Council (VR) grant no. 2014-5984 and 2015-05754. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the PDC Center for High Performance Computing.

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ASSOCIATED CONTENT

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: Initial adsorption sites/configurations for different molecules in the study on a Li metal surface; Adsorption energies (eV) for relaxed supercells from different initial sites/configurations of molecules on the Li (100) surface; Surface energies convergence for Li3N surfaces vs. different number of layers in two different phases of Li3N.

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