Li2S Film Formation on Lithium Anode Surface of LiâS batteries - ACS

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LiS Film Formation on Lithium Anode Surface of Li-S batteries Zhixiao Liu, Samuel Bertolini, Perla B. Balbuena, and Partha P. Mukherjee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11803 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 7, 2016

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ACS Applied Materials & Interfaces

Li2S Film Formation on Lithium Anode Surface of Li-S batteries Zhixiao Liu,1 Samuel Bertolini,3 Perla B. Balbuena,2,3* and Partha P. Mukherjee1* 1

Department of Mechanical Engineering, 2Department of Chemical Engineering, 3Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843

*

Correspondence: [email protected] (P. B. Balbuena), [email protected] (P. P. Mukherjee)

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Abstract The precipitation of lithium sulfide (Li2S) on the Li metal anode surface adversely impacts the performance of lithium-sulfur (Li-S) batteries. In this study, a first-principles approach including density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations is employed to theoretically elucidate the Li2S/Li metal surface interactions and the nucleation and growth of a Li2S film on the anode surface due to long-chain polysulfide decomposition during battery operation. DFT analyses of the energetic properties and electronic structures demonstrate that a single molecule adsorption on Li surface releases energy forming chemical bonds between the S atoms and Li atoms from the anode surface. Reaction pathways of the Li2S film formation on Li metal surfaces are investigated based on DFT calculations. It is found that a distorted Li2S (111) plane forms on a Li (110) surface and a perfect Li2S (111) plane forms on a Li (111) surface. The total energy of the system decreases along the reaction pathway; hence Li2S film formation on the Li anode surface is thermodynamically favorable. The calculated difference charge density of Li2S film/Li surface suggests that the precipitated film would interact with the Li anode via strong chemical bonds. AIMD simulations reveal the role of the anode surface structure and the origin of the Li2S formation via decomposition of Li2S8 polysulfide species formed at the cathode side and dissolved in the electrolyte medium in which they travel to the anode side during battery cycling. Keywords: lithium-sulfur battery; Li metal anode; Li2S precipitation; first-principles approach; density functional theory; ab initio molecular dynamics; polysulfide decomposition


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Introduction Our planet currently suffers energy and pollution crises caused by traditional fuel combustion. An important goal of modern society is to reduce the consumption of fossil fuels via uses of sustainable and green energy resources. In this regard, great progress has been witnessed in the development of alternative energy sources and technologies including fuel cells,1 photovoltaic cells2 and wind turbines.3 However, the bottleneck for new energy developments is the availability of efficient energy storage technology for sustainable energy applications, especially for vehicle electrification. In recent years, lithium-ion batteries (LIBs) have successfully powered portable devices and even driven electric vehicles.4-8 However, the current commercialized LIBs can only deliver a specific energy density of 150 Wh⋅kg-1, which is too low to match the demands from vehicle electrification in a long term.9,10 To go beyond LIBs, the Li-S electrochemical system is an appealing candidate in the next generation of energy storage technologies.11-14 Sulfur as active cathode material may achieve a specific energy density of 2567 Wh ⋅ kg-1 when -sulfur is completely reduced to S2- by electrochemical reduction without forming any other polysulfides (PSs) S (x = 2~8). Sulfur is abundant in the Earth crust, hence it is more economical as cathode active material compared to transition metal oxides used in LIBs.15 Additionally, sulfur and its discharge products are nontoxic and environmentally friendly. Although the Li-S electrochemical system has enormous potential in energy storage, Li-S batteries are far away from commercialization due to critical limitations. In the cathode side, the 3 ACS Paragon Plus Environment


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precipitation of insoluble discharge product Li2S blocks pores for Li+ ion transportation.16,17 Li2S also acts as the insulator18 which can cut off pathways for electrons migrating from carbon matrix to long-chain PSs. The growth of the insoluble and insulating product can cause a sudden death before the battery arrives to its theoretical discharge capacity.19 Another key challenge for Li-S batteries is the high solubility of intermediate discharge products: long-chain PSs, in the electrolyte. During discharge, dissolved long-chain PSs from the cathode side diffuse to the anode side due to the potential and concentration gradient.20 Long-chain PSs are reduced to insoluble and insulating products precipitating on the Li anode surface, which consume active materials and increase the resistance for Li+ diffusion.21 Keeping soluble PSs in the cathode framework is considered as an effective strategy to improve the performance of Li-S battery. A large number of novel micro/nanostructures have been synthesized to serve as cathode frameworks to trap PSs.22-26 Developing electrolyte additives to protect the Li metal anode surface is another way to improve specific capacity.27 The most popular additive, LiNO3,28,29 is reduced on the Li metal surface to insoluble LixNOy and oxidizes the PSs to insoluble LixSOy, all of which passivate Li anode surface and prevent electron transfer from the Li metal to PSs.30 However, Zhang reported that the passivation film grows endlessly with the consumption of LiNO3,31 and in addition LiNO3 can also be irreversibly reduced on the carbon cathode surface, with the products adversely affecting the reversibility and capacity of the battery. Here we focus on the understanding of Li2S precipitation on the Li anode surface. We employ a first-principles approach including density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations to investigate the interaction mechanisms between the insoluble Li2S molecule and the Li metal surface, the atomic structure evolution during the 4 ACS Paragon Plus Environment

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formation of a Li2S film on the Li surface, and the dynamics of Li2S formation due to polysulfide decomposition on the Li metal surface. Computational Methods and Models First-principles calculations are performed using the Vienna ab initio Simulation Package (VASP)

32,33

based on DFT34,35 within the plane wave basis set approach.36,37 The electron-ion

interactions are described by the projector augmented wave (PAW) method,38,39 and the electronelectron exchange correlations are described by the Perdew-Burke-Ernzerhof (PBE) functional.40 The Monkhorst-Pack (MP) technique41 is employed to generate k-point grids for the Brillouin zone (BZ) sampling. A 400 eV energy cut-off for the plane-wave basis set is used to achieve both computational accuracy and efficiency. The Hellman-Feynman forces are less than 0.02 eV/Å when optimizing the atomic positions. For the evaluation of Li2S adsorption and film formation on the anode surface, slab models with Li (110)-(2×2) surface unit cell (SUC) and Li (111)-(2×2) SUC are employed to represent the Li anode surface. The (110) surface is the close-packed plane of Li crystal with body centered cubic (bcc) structure. The (111) surface has a two dimensional hexagonal structure which is similar to the structure of crystalline Li2S (111) surface. To avoid interactions between consecutive slabs, two adjacent slabs are separated by 16 Å of vacuum. The Li (110) surface model consists of 5 atom layers and the (111) surface model consists of 7 atom layers. The upper three layers are relaxed and the bottom layers are fixed as in the bulk-like positions. Li2S molecules are placed on the relaxed side of the Li slab where the effect of the induced dipole moment is taken into account by applying a dipole correction.42



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To evaluate the interaction strength between an adsorbed Li2S molecule and the Li surface, the surface energy is calculated as = @ − − ,

(1)

where @ is the total energy of the Li2S on Li surface, is the energy of an isolated Li2S molecule calculated in a 20×20×20 Å3 box, and is the energy of the clean surface. A negative represents an exothermic reaction and attractive interaction between the Li2S molecule and the Li metal surface. For the AIMD simulations carried out using the VASP program, an electrolyte solution is built in contact with a lithium metal surface represented by a metal slab. The electrolyte solvent is 1,3 Dioxolane (DOL) with Bis(trifluoromethane)sulfonimide Li salt (LiTFSI) mixed with a long chain PS, Li2S8, at concentrations of 1M and 3M, respectively. Higher concentrations of PS species near the anode have been reported from experiments.43 A Li metal (110) surface slab in contact with ethylene carbonate (EC) instead of DOL as described in our recent report44 was used for comparison purposes. Electron-ion interactions were described by the PAW pseudopotentials

38,45

as provided in the VASP databases. A conjugate-gradient algorithm was

employed to relax the ions into their instantaneous ground state. A Gaussian smearing with a width of 0.05 eV was also utilized. For the surface Brillouin zone integration, a 2×2×1 Monkhorst-Pack k-point mesh was used. The convergence criteria for electronic self-consistent iteration and ionic relaxation were set to 10-4 eV and 10-3 eV, respectively. The PS species was optimized in the presence of DOL and van der Waals (vdW) dispersion corrections were included using the DFT-D3 approximation by Becke-Johnson46. The LiTFSI salt was optimized using the Gaussian 09 (G09) package 47 with a hybrid functional B3PW91 and the 6-311++G(p,d)


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basis set

48,49

. For AIMD simulations the same parameters were set, including vdW dispersion

corrections. Simulations were carried out in the NVT ensemble at 330 K using a time step of 1 femtosecond. The Nose thermostat50 was used to control the temperature oscillations during the simulation with a Nose-mass parameter of 0.5. The density of the liquid-phase solvent was estimated by placing randomly 6 DOL molecules (density = 1.06 g/cm3) in contact with the model metal anode surface, respectively. Subsequently, the solvent molecules (liquid-phase) were allowed to relax using a classical molecular mechanics minimization. For the minimization, the consistent valence force field (CVFF)51 with a conjugate gradient algorithm as implemented in the Materials Studio software was used52. The maximum force among all the atoms in the system required for convergence was set to 0.005 kcal mol-1 Å-1. Afterwards, the minimized systems were allowed to run for 20 ps of AIMD simulation. Charge transfer was calculated by using the Bader charge analysis. In this method, the total electronic charge of an atom is approximated by the charge enclosed in the Bader volume defined by zero flux surfaces.53-55 Results and Discussion Li2S molecular adsorption on Li metal surfaces Figure 1 shows the stable atomic configuration of a Li2S molecule adsorbed on the Li (110)(2×2) surface. The atomic structure is visualized using the software Visualization for Electronic and Structural Analysis (VESTA).56 Figure 1(a) clearly demonstrates that the Li2S molecule adsorbs almost parallel to the Li (110) plane. Due to the strong interaction between the Li2S molecule and the Li substrate, an obvious relaxation of the topmost atom layer of Li (110) surface is observed and the originally flat Li (110) plane is bent. Figure 1(b) shows the atomic positions projected onto the Li (110) surface. It is found that the S atom is located at the bridge site of two adjacent Li atoms in the topmost layer. The bond length between S and Li in the 7 ACS Paragon Plus Environment


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substrate is 2.47 Å (Table 1), which agrees well with the 2.48 Å Li-S bond length in crystalline Li2S.57 Table 1 also shows the geometric parameters of adsorbed Li2S on Li (110) surface. It is found that the Li-S bond length of the molecule is stretched to 2.33 Å, and the Li-S-Li bond angle decreases to 86.4°. The adsorption energy of Li2S on Li (110) surface is -3.22 eV, which is more negative than the adsorption energy of single Li2S on crystalline Li2S surface. It can be inferred that the attractive interaction between the Li2S molecule and the Li (110) surface is stronger than that between the Li2S molecule and a pre-deposited Li2S film. This strong attraction between the adsorbate and substrate weakens intramolecular interactions; hence the LiS bond is stretched.

Figure 1. (a) Side view and (b) top view of a single Li2S molecule adsorbed on the Li (110) surface. Yellow spheres represent S atoms. Violet spheres and blue spheres represent Li atoms in the Li metal substrate and adsorbed Li2S molecules, respectively.

The atomic structure of a Li2S molecule adsorbed on the Li (111) surface is shown in Figure 2. From the side view of the structure it can be seen that the molecule adsorbs parallel to 8 ACS Paragon Plus Environment

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the substrate (Figure 2(a)). Due to the open structure of Li (111) plane, the S atom cannot only interact with Li atoms from the topmost layer but also with a Li atom from the second layer. Figure 2(b) clearly shows the positions of atoms in Li2S projected to Li (111) surface. The S atom is located at the hcp hollow site and Li atoms in Li2S are located at the fcc hollow sites. Similarly with Li2S adsorption on the Li (110) surface, the interaction from the Li (111) surface weakens the intramolecular Li-S bond and stretches the bond to 2.38 Å. The adsorption energy of Li2S on the Li (111) surface is -3.56 eV, which indicates that the attraction between the Li2S molecule and the Li (111) surface is stronger than that on the Li (110) surface. The reason is that the Li (111) plane is not the close-packed plane; hence the Li (111) surface has more dangling bonds which can accept the Li2S molecule. It is interesting to compare the adsorption of Li2S to that of H2O. Michaelides et al. systematically studied single water molecule adsorption on transition and noble metal surfaces, and found that parallel H2O is the most stable configuration for adsorption on metal surfaces.58 Since both Li and H are in the first group of the periodic table, and S as well as O belong to the chalcogen group, Li2S and H2O should follow the same mechanism when interacting with metal surfaces. It was found that the molecular orbitals of the adsorbate optimize their mixing with the substrate and become greatly stabilized when the adsorbate lies flat on the metal surface.58



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Figure 2. (a) Side view and (b) top view of a single Li2S molecule adsorbed on the Li (111) surface. Yellow spheres represent S atoms. Color code as in Figure 1. Table 1. Energetic and geometric properties of Li2S adsorption on Li (110) and (111) surfaces. is the adsorption energy. is the LiSLi bond angle, is the Li-S bond length in the molecule and is the distance between S and Li atoms of the anode substrate. Configuration Li2S@Li(110) Li2S@Li(111) Li2S@Li2S(110)57 Li2S@Li2S(111)57 Isolated Li2S57

(eV) -3.22 -3.57 -2.88 -1.78 --

(°) 85.4 134.0 107.2 97.6 115.7

(Å) 2.33 2.38 2.22 2.18 2.11

(Å) 2.47 2.47 2.38 2.31 --

Further analysis of the electronic structures of Li2S molecule adsorption on the Li metal surface allows better understanding of the interaction mechanisms. Figure 3 depicts the charge density difference of Li2S adsorption on Li metal surfaces. The charge density difference is calculated by Δ = @ − − .

(2)

Here @ is the total charge density of the entire system, is the charge density of the substrate and is the charge density of the substrate. When calculating the 10 ACS Paragon Plus Environment

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charge density of the substrate or adsorbate, atoms are in the same positions as in the complete system. The charge density difference clearly demonstrates that electron accumulation regions appear between S atoms and Li atoms from the substrate. Electron depletion regions also appear between S atom and Li atoms in the adsorbed molecule, which indicate that the intramolecular Li-S bonds are weakened by the Li metal substrate. Electron accumulation indicating strong chemical interactions is observed between substrate Li atoms and adsorbate Li atoms. According to what we discussed above, it is obvious that both Li and S atoms in the adsorbed Li2S molecule make contributions to interact with the Li anode surface. In contrast, for the single Li2S adsorption on graphene, Li2S only interacts with the substrate via S-C bond, and electron redistribution between Li and C cannot be observed.59 Bader charge analysis is performed to calculate the net charge of the adsorbate. It is found that the Li2S molecule acts as the electron donor and the Li metal substrate as the acceptor. For Li2S adsorption on the Li (110) surface, the adsorbate is negatively charged with 0.39 |e|. On the Li (111) surface, a significant electron migration from the substrate to the adsorbate is observed. The 2s orbitals of Li atoms in the adsorbate are fully occupied. This is in contrast with the Li2S molecule acting as an electron donor when adsorbing on graphene.59 The different behavior is attributed to the activity of the substrate. The Pauling electronegativity of Li is 0.98 and C is 2.55. Hence the Li metal is more active to give electrons to the adsorbate than graphene.



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Figure 3. Charge density difference of the Li2S molecule adsorption on (a) Li(110) surface and (b) Li(111) surface. The red isosurface (3.5 × 10% &/Å% ) represents electron accumulation and the green isosurface (3.5 × 10% &/Å% ) represents electron depletion. Color code as in Figure 1.

Li2S film formation on Li metal surfaces The atomic structure evolution of Li2S film formation on the Li (110)-(2×2) SUC (shown in Figure 4) and on the Li (111)-(2×2) SUC (shown in Figure 5) are studied by DFT simulations. The different states during the formation of Li2S film are represented by co-adsorption of Li2S molecules. The formation of a Li2S film on Li (110) surface is discussed first. The stable atomic structure of two Li2S molecules co-adsorption on the (2×2) SUC is shown in Figure 4(a). It can be seen that Li2S columns appear along the [001] orientation. By periodically extending the atomic structure along the [001] and [11)0] orientations, we can see that the arrangement of Li and S atoms in the adsorbates is similar to that of a typical Li2S (110) plane. The Li-S bond length in the Li2S film is 2.37 Å, and the S-S distance is 3.44 Å, which are close to the corresponding values in the crystalline Li2S (110) plane. However, the distance between two Li2S columns in Figure 4(a) is 9.73 Å, which is 1.7 times of that in a typical Li2S (110) plane. This significant difference is attributed to the lattice mismatch between the Li (110) and Li2S (110) planes. 12 ACS Paragon Plus Environment

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Based on the atomic structure shown in Figure 4(a), one more Li2S molecule is placed on the surface, which means that three Li2S molecules co-adsorb on the Li (110)-(2×2) SUC. The atomic positions after structure optimization are depicted in Figure 4(b). It is interesting that the hexagon consisting of 6 S atoms linked by green lines shown in Figure 4(b) appears in the deposited Li2S film. An S hexagon with an S atom at the center is the feature of the typical crystalline Li2S (111) plane. The atomic structure shown in Figure 4(b) can be an intermediate state during the formation of the Li2S (111) film. In this intermediate state, the distance between two adjacent S atoms of the hexagon varies from 3.91 Å to 5.20 Å, and the S-S distance in a perfect crystalline Li2S (111) plane is 4.05 Å.59 Figure 4(c) depicts the top view of the stable atomic structure in which four Li2S molecules are placed on the Li (110)-(2×2) SUC. It is obvious that the S positions projected on to the substrate follow the pattern of S arrangement in the crystalline Li2S (111) plane as discussed above. In the atomic structure shown in Figure 4(c), each S atom is surrounded by six Li atoms and the Li-S distance varies from 2.53 Å to 4.25 Å. The side view of this fully covered Li (110) surface (Figure 4(c)) is shown in Figure 5(a). The arrangement of atoms along the normal direction in the deposited Li2S film is different from crystalline Li2S (111) plane. In the perfect Li2S (111) plane, all S atoms are in one layer. However, in the Li2S film on Li (110) surface, S atoms are distributed into two layers. This Li2S film can be treated as a Li2S (111) plane distorted along the normal direction. Previous theoretical and experimental studies demonstrated that the facets of solid Li2S are dominated by the (111) surface which has the lowest Gibbs free energy.57,60-62 Hence, the distorted Li2S (111) film formed on the Li (110) surface can be the base for the precipitation of solid Li2S.



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Figure 4. Top view of (a) two , (b) three, and (c) four Li2S molecules adsorption on Li2S(110)(2×2) surface unit cell which is marked by a black dash square. Color code as in Figure 1.

Figure 5. Difference charge density of Li2S film adsorption on (a) Li(110) surface and (b) Li(111) surface. The red isosurface (3.5 × 10% &/Å% ) represents electron accumulation and the green isosurface (3.5 × 10% &/Å% ) represents electron depletion. Color code as in Figure 1.

Snapshots in Figure 6 demonstrate the mechanism of Li2S film formation on the Li (111)(2×2) SUC. Figure 6(a) depicts the stable atomic structure of two Li2S molecules co-adsorption on the surface. It is found that a (Li2S)2 cluster forms on the Li (111) surface. In the cluster, each Li2S unit shares one Li atom with its partner, hence each S atom coordinates with 3 Li atoms. The Li-S bond length in the adsorbed (Li2S)2 varies from 2.30 Å to 2.51Å, which is longer than the Li-S bond length of free Li2Sx (x = 1, 2) molecule. Figure 6(b) depicts the optimized 14 ACS Paragon Plus Environment

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configuration of three Li2S molecules co-adsorption on Li (111)-(2×2) SUC. In this case, a (Li2S)3 cluster forms on the anode surface. There are two kinds of S atoms in the cluster: the one coordinated with three Li atoms is named S3Li, and the one coordinated with four Li atoms is named S4Li. Li2S3Li molecule shares both of its Li atoms with partners, and Li2S4Li molecules share only one Li with partners. The length of Li-S3Li bonds varies from 2.38 Å to 2.48 Å, and the length of Li-S4Li bonds varies from 2.31 Å to 2.41 Å. Figure 6(c) depicts the atomic structure of a fully covered Li (111) surface, which is represented by four Li2S molecules co-adsorption on the Li (111)-(2×2) SUC. It is clearly shown that the atom positions projected to the surface exactly follow the atomic arrangement in the crystalline Li2S (111) plane. As evident from Figure 6(c), the hexagon consisting of six S atoms can be identified, and the center of the hexagon is occupied by another S atom. Each S atom is surrounded by six Li atoms and the averaged Li-S bond length is around 2.93 Å, which is 0.45 Å longer than the Li-S bond in the Li2S crystal. The averaged distance between two adjacent S atoms is around 4.86 Å, which is also longer than the S-S distance in the Li2S crystal by 0.8 Å. These slight differences are attributed to the lattice mismatch between the Li (111) surface and the Li2S (111) surface. The atomic positions in the Li2S film along the normal direction are shown in Figure 5(b), which is the side view of Li2S film/Li (111) interface. It is obvious that the S atoms are in the same layer and the coordinating Li atoms are above and below the S layer alternatively.



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Figure 6. Top view of (a) two Li2S molecules, (b) three molecules, and (c) four Li2S molecules adsorption on Li2S(111)-(2×2) surface unit cell which is marked by a black dash parallelogram. Color code as in Figure 1. The energy profile of the Li2S film formation on Li anode is calculated to confirm that the mechanisms shown in Figures 4 and 6 are thermodynamically favorable. Here the clean surface is preset as the reference state, and the energy difference Δ induced by Li2S adsorption is estimated by Δ* = * − +, + . /.

(3)

In Eqn. (3), , is the energy of the clean surface, * is the energy of Li surface with Li2S adsorbate, and n is the number of adsorbed Li2S molecules. Here n = 4 represents that the surface is fully covered by the Li2S film. Figure 7 demonstrates that the energy decreases as the number of adsorbed Li2S molecules increases until the Li surface is fully covered. This trend indicates that the formation of Li2S film on Li metal is an exothermic and thermodynamically favorable process. The probability of a Li2S molecule detachment from the substrate can be estimated by Arrhenius equation 0 = exp 4−

567 56789 :;

* = *?@ − , − ,

(5)

A

where is the energy of the Li2S film. The interfacial binding energy of the distorted Li2S (111) film on Li (110)-(2×2) SUC is -5.22 eV and that of the Li2S (111) film on Li (111)-(2×2) SUC is -4.06 eV. These binding energies indicate strong chemical interactions between the Li2S film and the anode surface. To verify this argument, the difference charge density of Li2S film/Li anode interface is generated as shown in Figure 5. Apparently, electron accumulation regions (red isosurface) appear between Li surface and Li2S film, and the bonds formed by S atoms and Li atoms in the substrate (violet sphere) penetrate electron accumulation regions. The electronic structures demonstrate that the Li2S film interacts with the Li anode surface via strong chemical bonds.



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Figure 7. Energy profile of Li2S film formation on Li (110)-(2×2) SUC and Li (111)-(2×2) SUC. The clean surface is set as the reference state with ΔE = 0 eV.

Li2S formation from decomposition of PS species In the previous sections we analyzed the adsorption and deposition of Li2S films on Li metal surfaces. In this section we incorporate the effect of the electrolyte medium where S atoms are generated via decomposition of long-chain PS species on the Li metal surface. AIMD simulations demonstrate rapid decomposition of the dissolved Li2S8 species on the Li metal surface. Figure 8 shows the dynamic evolution of Li and S atoms over the first 6.5 ps of simulation time for the Li (111) and Li (110) surface slabs. In order to analyze the results, the PS species were followed in two different groups, molecules closest of the surface (around 5Å) and those farther from it (around 10Å). Similarly, different colors were employed to characterize the trajectories of the Li atoms belonging to the anode (colored purple) and those of the electrolyte (green). On both surfaces the PS species closest to the surface react very rapidly with the Li metal, tending to form an amorphous Li2S layer over the outermost layers of the anode, while the farthest PS species stay in the electrolyte phase for longer time and form clusters with other intact or fragmented PS species stabilized by Li ions from LiTFSI and from the original Li2S8 molecules.


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Figure 8. Snapshots of the dynamic evolution PS decomposition in contact with Li (111) surface slab (A) and Li (110) surface slab (B). Here, purple, green and yellow spheres represent Li from anode, Li from electrolyte and S atoms, respectively. The DOL solvent is represented in a line display style where O (red) and C (gray) atoms are shown. Green lines indicate the respective crystallography plane; red dashed lines show the orientation where S atoms tend to accommodate; the blue circle shows where the cluster is localized.

However the structure of the exposed Li metal facet affects significantly the initial stages of the PS decomposition. On the Li (110) surface slab (Figure 8B), although initially very fast, the total decomposition of PS species closest to the anode is slower than that on the (111) facet



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(Figure 8A) occurring within the initial 4.5 ps of simulation time. This is mainly attributed to a higher amount of S anions and radicals produced by the initial fast decomposition on the (110) facet that accumulate and start Li2S nucleation on the surface leaving a lower number of exposed active sites available for reaction. In contrast, on the more open (111) facet the reaction is slower initially and the decomposed S atoms migrate easily into the subsurface where they start forming Li2S. Thus, after the closest PS molecules were reduced, Li atoms from the surface were observed to diffuse in the direction of the electrolyte phase reducing other PS molecules and stabilizing the fragments located farther from the surface. On the (110) facet, after the group of molecules closest to the surface become completely reduced, PS clusters (blue circle in Figure 8 bottom) become stabilized in the electrolyte phase for longer times. Cluster formation is observed mainly after short PS chains (S < 4) have been formed as a result of PS decomposition. In these clusters, short S chains share Li atoms, as observed in previous studies.63-65 During the initial Li2S8 decomposition step, the PS chain reacts with two Li atoms and in most cases it was observed to break into Li2S3 and Li2S5. However, this bond scission can also occur simultaneously with another S-S bond of the PS. The subsequent steps depend on the surface charge transfer, availability of Li atoms and the position of the PS species.44 Additionally, some reactions may allow a chain to increase (e.g. Figure 9, S5-S9 bond).


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Figure 9: Time evolution of a PS decomposition on the Li (111) surface slab. A common shared characteristic is the first bond to break (S3-S4 or S5-S6), while the others depend on the initial position of the polysulfide and surface structure. Reconstruction of the chain can occur, which can be intra- or inter PS (e.g S5-S9).

Bader charge analysis was performed on individual S atoms that belong to a PS species (labeled as in Figure 9) that completely decompose over the (111) facet. Initial charge accumulates on the S atoms that are located at the ends of the chain, while the other S atoms are almost neutral, with the end S atoms bearing an average charge of -0.67|e| and the remaining S atoms having a charge of -0.08|e|, this difference is due to the direct bond between edge S (S1 and S8) atoms with Li atoms, while the other S atoms (from S2 to S7) are bonded only with S atoms. The Bader charges of S atoms converge to an average value of -1.75|e| for the (111) surface (Figure 10), as reported earlier on the (110) surface

44

. It is interesting to note that on

both facets, the charge on the surface Li atoms converge faster to a value of approximately +0.80|e|, indicating the formation of Li2S. The main difference on the Li2S films that cover each surface is the alignment of S atoms relative to the surface, as seen in Figure 8. S atoms tend to take the same orientation (red and dashed lines on Figure 8) of the crystallographic planes, represented by the green lines in Figure 8. It is important to remark that an amorphous structure



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is observed, given the short time that does not allow the surface to organize. For this reason a direct comparison with the structures shown in Figures 4 and 5 is not possible. We note that the surface with a higher surface energy is always more active to interact with adsorbates. In previous work44 we calculated the surface energies of the Li(110) surface as 0.031 eV/Å2 and 0.033 eV/ Å2 for the (111). Hence the (110) surface is slightly more stable than the (111) surface. The more active (111) surface interacts more strongly with Li2S (adsorption energy of -3.57 eV) than the (110) surface (-3.22 eV). Additionally, the more active (111) surface also facilitates the decomposition of Li2S8 to Li2S as shown by the AIMD simulations. Overall, there are not significant differences in the time for PS decomposition in different solvents. The high reactivity of the Li metal overcomes any other possible interaction for example solvation effects of the Li ions in the proximity of the metal surface.

Figure 10: Time evolution of the Bader charge of individual sulfur species initially present on the same PS, which completely dissociate by 4.5 ps of simulation time on the (111) surface. S9* represents one S atom from another PS.

Conclusions


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A combined first-principles approach based on DFT and AIMD simulations reveals new insights regarding the formation of a Li2S film on Li anode surfaces of Li-S batteries. DFT analyses shows details of Li2S molecular adsorption on Li (110) and Li (111) surfaces with energies of 3.22 eV and -3.57 eV respectively, which denote the strong interaction between adsorbate and substrate also confirmed by the difference charge density that shows chemical bonds formation between S atoms and Li atoms from the anode surface. For the Li (110) surface, a Li2S film with a Li2S (110)-like structure is predicted to form first and then the structure of the Li2S film is converted to a distorted Li2S (111) plane until the Li (110) surface is fully covered. For the Li (111) surface, (Li2S)n clusters form on the surface first and a perfect Li2S (111) plane appears finally. Both the interaction energy analysis and electronic structure analysis suggest that the Li2S film interact with the Li anode surface via strong chemical bonds, and the decomposition of Li2S film is difficult. The effect of the electrolyte and finite temperature are incorporated via AIMD simulations. Details about the decomposition of Li2S8 are followed by analyses of S-S bond distances and charges on the S atoms, both indicating the formation of the Li2S film. The structure of the Li surface is shown to affect the way the molecules decompose and the rate of Li2S formation: on the (110) surface although a very fast decomposition is detected initially, the large amount of S atoms interacting with the Li surface atoms impedes the access of new molecules to exposed active surface sites; in contrast the (111) surface let S atoms to go into the subsurface where Li2S is formed. The conclusions of this study suggest that inhibiting Li2S formation strategies should come from various fronts: a) based on new electrolyte formulations that equivalently to the role of LiNO3 would be able to generate a passivation layer that avoids or at least moderates Li2S decomposition; b) actual modifications to the Li metal surface including physical barriers for



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diffusion of the long-chain polysulfide species; c) retention of PS species at the cathode via composite electrodes providing physical or chemical barriers to mass transport and/or electrolyte formulations. Successfully implementing these strategies requires a thorough understanding of the chemical, mechanical, and electrochemical implications occurring in the various parts of the Li/S battery, which is the main consequence of this much interconnected chemistry. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-EE0006832

under

the

Advanced

Battery Materials

Research

(BMR)

Program.

Supercomputer resources from Texas A&M University High Performance Computer Center and Texas Advanced Computing Center (TACC) are gratefully acknowledged. S. B. acknowledges CAPES Foundation, Ministry of Education of Brazil, for support as Bolsista da CAPES - Proc BEX 11941/13-8. References (1) Zhou, L. Progress and Problems in Hydrogen Storage Methods. Renewable and Sustainable Energy Rev. 2005, 9, 395-408. (2) Tachan, Z.; Rühle, S.; Zaban, A. Dye-Sensitized Solar Tubes: A New Solar Cell Design for Efficient Current Collection and Improved Cell Sealing. Sol. Energy Mater. Sol. Cells 2010, 94, 317-322. (3) Lu, L.; Yang, H.; Burnett, J. Investigation on Wind Power Potential on Hong Kong Islands—An Analysis of Wind Power and Wind Turbine Characteristics. Renewable Energy 2002, 27, 1-12. (4) Tarascon, J. M. Key Challenges in Future Li-Battery Research. Philos. Trans. R Soc. A 2010, 368, 3227-3241. (5) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359-367. (6) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243-3262. (7) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652657. 24 ACS Paragon Plus Environment

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