Adsorption Mechanisms of Lithium Polysulfides on Graphene-Based

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Adsorption Mechanisms of Lithium Polysulfides on Graphene-Based Interlayers in Lithium Sulfur Batteries Chi-You Liu, and Elise Y. Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00096 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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ACS Applied Energy Materials

Adsorption Mechanisms of Lithium Polysulfides on Graphene-Based Interlayers in Lithium Sulfur Batteries

Chi-You Liu and Elise Y. Li* Department of Chemistry, National Taiwan Normal University No. 88, Section 4, Tingchow Road, Taipei 116, Taiwan

* Corresponding author E-mail: [email protected] Phone: (886)-2-77346219 Fax: (886)-2-29324249 1

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Abstract One of the most critical problems in lithium-sulfur (Li-S) batteries is the shuttle effect. The transfer of soluble lithium polysulfides (LiPSs) from the sulfur cathode to the lithium anode leads to a degradation in Li-S battery capacity and life cycles. Recent studies reveal that the carbon-based interlayer materials introduced between the cathode and anode can effectively improve the shuttle effect problem and increase the battery life cycles. In this work, different types of the N-doped, S-doped, and N, S co-doped graphene surfaces are investigated by theoretical calculations. We find that a strong interaction may exist between some of the heteroatom-doped graphene surfaces and lithium ions, and that the adsorption of LiPSs may proceed via one of the three mechanisms, the dissociative, the destructive, and the intact adsorptions. Detailed structural and electronic analyses indicate that the Li-trapped N, S co-doped graphene interlayers (NSG1 and NSG2) could efficiently reduce the shuttle effect through the intact adsorption mechanism. Our results provide a plausible explanation on the observed better performance of the N, S co-doped graphene interlayers in Li-S batteries.

Keywords: Li-S battery; Heteroatom-doped graphene; Lithium polysulfides; Shuttle effect; DFT calculation; VASP 2


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I. Introduction Lithium-sulfur (Li-S) battery is a promising candidate for future energy storage system due to its high theoretical energy density (2600 Wh/kg), high theoretical capacity (1675 mAh/g), as well as the abundance and non-toxicity of the sulfur element.1-3 A typical Li-S battery consists of a lithium metal anode, a sulfur (S8) cathode, and electrolytes. The overall discharge reaction is S8 + 16Li+ + 16e- → 8Li2S. During the discharge process, the Li ions, oxidized from the Li anode, diffuse to the cathode and combine with sulfur to form various intermediates, including the soluble (Li2Sn, n=3~8) and insoluble (Li2Sn, n=1~2) lithium polysulfides (LiPSs).1-8 Despite of the many advantages of Li-S batteries, some persisting problems need to be solved, such as poor cathode conductivity, cathode volume expansion, and lithium dendrite formation at the anode.1-19 One of the most difficult problems is the shuttle effect, which directly leads to low capacity, short life cycles, and an overall poor battery performance.1-3 During the battery operation, the soluble LiPS intermediates diffuse from the cathode to the anode and create an electrochemical inactive layer on the anode surface.1 Some literatures reported setting up a polymer separator between the cathode and the anode, in an attempt to reduce the shuttle effect by physically blocking the LiPSs diffusion, but the effect is limited.20-21 A better improvement may be achieved by applying an extra coating on the separator, such as 3



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metal oxides (TiO2, SiO2, and ZrO2),21-24 to enhance the adsorptivity of the LiPSs through the polar metal-oxygen bonds,21 or 2D metal carbides (MXene phases).20, 25 A different strategy to resolve the shuttle effect problem is to add an interlayer between the separator and the sulfur cathode to restrict the soluble LiPSs,7-8, 26-30 as shown in Scheme 1. Both the interlayer and the coated separator in the Li-S battery attract the LiPSs via the chemical interaction between LiPSs and the surfaces. Recent studies reveal that the carbon-based interlayer materials, such as heteroatom-doped graphenes,26, 30-32 g-C3N4,33-34 and graphene oxides (GO),35 can effectively improve the shuttle effect problem and increase battery life cycles. In particular, the N, S co-doped graphene has been reported to show the best performance among different heteroatom-doped graphenes.26, 30 Although the interaction between LiPSs and graphene-based interlayers has been addressed by several theoretical studies,26, 30-34 most of the studies focus mainly on the relative magnitude of the adsorption energies of LiPSs.

Nevertheless, the

adsorption of LiPS on different heteroatom-doped graphene surfaces may involve more complicated phenomena, e.g. Li-trapping by some N-doped graphene surfaces.32, 36

Yi et al. recently reported the theoretical simulation on the adsorption of LiPS on

Li-trapped N-doped graphene.36 However, only one type of N-doped graphene surface consisted of three pyridinic N’s and one trapped Li atom was considered. It is evident 4


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that a comprehensive investigation on different configurations of the N-doped, S-doped, or N, S co-doped graphenes has not been performed. A thorough computational study remains indispensable in order to elucidate the adsorption mechanism between LiPSs and heteroatom-doped graphene interlayer surfaces. In this work, we construct different types of the N-doped, S-doped, and N, S co-doped graphene interlayer surfaces, and investigate the adsorption phenomena of soluble LiPSs (Li2Sn, n=3~8). Our Bader charge analyses indicate that some of the surfaces may induce a strong interaction with Li ions, forming the Li-trapped surfaces. Through a detailed structural and electronic examination, we propose three adsorption mechanisms between LiPSs and different surfaces. Our results show that the Li-trapped N, S co-doped graphene interlayer could reduce the shuttle effect most effectively via the intact adsorption mechanism, which is consistent with previous experimental observations.

II. Computational Details The density-functional theory (DFT) and the plane-wave method, as implemented in the Vienna ab initio simulation package (VASP),37-40 are employed to calculate the energies and structures of adsorbates and surfaces. The projector-augmented-wave method (PAW)41-42 is used in conjunction with the generalized gradient-approximation 5



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(GGA) and Perdew-Burke-Ernzerhof (PBE)43 exchange-correlation functional, with a cutoff energy of 400 eV. The dispersion correction is included by DFT-D3 method.44-47 The energy convergence criteria for the electronic and the ionic steps are 10-4 and 10-3 eV, respectively. The supercell for the graphene-based surfaces include 6 x 6 repeated unit cells and a vacuum space of over 15 Å to ensure negligible interactions between surfaces. The Monkhorst-Pack k-point grids48 are set as 5 x 5 x 1 for all supercells. All atoms are fully relaxed during the optimization. The Bader charge analyses is used in our calculations.49-51 For systems containing an unpaired electron, the spin polarization is considered and the total magnetization is constrained (ISPIN = 2 and NUPDOWN = 1). The formation energies (Ef) of surfaces are calculated as Ef = E(CxNySz) - nxμC nyμN - nzμS, where E(CxNySz) represents the energy of the heteroatom-doped graphene surfaces; nx, ny, and nz represent the number of C, N, and S atoms, respectively; μC, μN, and μS represent the chemical potential per C (as in graphene), per N (as in gas-phase N2), and per S (as in bulk S8), respectively. The adsorption energies (Eads), relaxation energies (Erel), and distortion energies (Edis) are calculated by the following (eq 1~3): Eads = E(sur + adsorbate) - (Esur + Eadsorbate)

(1)

Erel = E(distorted sur) - Esur

(2) 6


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Edis = E(distorted adsorbate) - Eadsorbate

(3)

where E(sur + adsorbate), Esur, Eadsorbate, are the computed electronic energies of the surface with adsorbed molecule, the un-adsorbed surface, the free gas phase molecule, respectively. The E(distorted sur) and E(distorted adsorbate) represent the computed electronic energies of the distorted surfaces and the distorted molecule due to the adsorption, respectively.

III. Results and discussions Structures of heteroatom-doped graphene surfaces and LiPSs Previous studies have reported several possible doping configurations of heteroatom-doped graphenes, including the doping type of the pyridinic, the pyrrolic, and the quaternary nitrogens, as well as the thiophene and the “C-S-C” sulfurs.52-54 The same doping configurations have also been observed on the N, S co-doped graphenes.52-54 Figure 1 shows the optimized structures as well as the formation energies (Ef) of the pristine (G) and different types of heteroatom-doped graphenes considered in this study. The N-doped graphenes involve either one quaternary N (NG1), two pyridinic and one pyrrolic N’s (NG2), three pyridinic N’s (NG3), or four pyridinic N’s (NG4); the S-doped graphenes involve either one quaternary S (SG1) or one thiophenic S (SG2); the N, S co-doped graphenes involve either one quaternary N 7



and one quaternary S co-doped in ortho-, meta-, or para-positions (o-NSG, m-NSG, or p-NSG), two pyridinic N’s and one thiopyran-like S (NSG1), two pyridinic N’s and one thiophenic S (NSG2), or four pyridinic N’s and one thiadiazole-like S (NSG3). Surfaces with an Ef over 5 eV (NG2, o-NSG, m-NSG, and p-NSG) may not exist in an actual experimental system and are excluded from later investigations. For the LiPSs, the soluble Li2S8, Li2S6, Li2S4, and Li2S3 are considered. Their geometries and the electrostatic potential (ESP) are shown in Figure 2. As can be expected, the strong positive potential is located at two lithium atoms, and the negative potential is concentrated at the terminal sulfur atoms (Terminal S) with respect to the internal sulfurs (Internal S), as indicated by the ESP results. The Bader charge population analyses of LiPSs are consistent with the ESP results. The charges of Li atoms are almost +1 |e| and the negative charges are mainly distributed at the Terminal S in LiPSs, as shown in Table S1. Adsorption of LiPSs on heteroatom-doped graphene surfaces The adsorption energies of LiPSs on different surfaces (Eads) are shown in Table 1. For the first few heteroatom-doped surfaces (NG1, SG1 and SG2), the calculated Eads’s are close to the values of pristine graphene (-0.5 > Eads > -1.0 eV). A general trend that the Eads decreases with the length of the polysulfide-chain can be observed. Other surfaces show stronger LiPSs adsorption energies, e.g. NSG1 and NSG2 (about 8


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-1.5 eV) and NG3 and NG4 (over -2 eV). The optimized adsorbed structures are shown in Figure S1. It is noted that, for the long-chain species (the Li2S6 and the Li2S8), the two Li atoms in the LiPS always approach the surface in a roughly perpendicular orientation with only one attached Li atom. This is not always the case for the short-chain species (the Li2S3 and the Li2S4), which may also prefer to approach the surface in a more horizontal orientation with two attached Li atoms. Also note that for surfaces with stronger adsorption (NSG1, NSG2, NG3, and NG4), the distances between the two Li atoms (d) in the perpendicularly-attached LiPSs generally increase upon adsorption, as shown by examples of Li2S6 and Li2S8 in Figure S2. The extent of Li-Li bond lengthening for the adsorbed LiPSs, with respect to gas-phase species, is in the order of NG4 (Δd ~ 1.65) > NG3 (Δd ~ 0.76) > NSG1 (Δd ~ 0.40) ~ NSG2 (Δd ~ 0.35). This phenomenon indicates a pre-dissociative (Li2Sn → Li…LiSn) or a dissociative (Li2Sn → Li + LiSn) adsorption of LiPSs, as evidenced by the large distortion energy (Edis) of LiPSs, as shown in Table S2. The strong distortion of LiPSs in turn induces a slight structural change for the surfaces, as reflected by the non-zero relaxation energy (Erel) for these surfaces. In other words, for surfaces exhibiting pre-dissociative or dissociative adsorptions, both the relaxation and distortion energies (Table S2) are positively correlated with the adsorption energies (Table 1). For example, the strongly adsorbed Li2S4 on NG4 (Eads = -2.85 eV) 9



shows a Erel and a Edis as large as 0.18 and 3.89 eV, respectively. The Bader charge population of the polysulfide-chains in LiPSs (q(Sn)), as shown in Table 2, clearly reveals different extent of charge transfer upon physical, pre-dissociative and dissociative adsorption. In a physical adsorption, the charge transfer is almost negligible with a q(Sn) close to -2 |e|. In a pre-dissociative or dissociative adsorption, the charge transfer from the LiPS to surface is more pronounced, as shown by the q(Sn) in NSG1 and NSG2 (-1.27 to -1.87 |e|) or in NG3 and NG4 (-0.6 to -1.5 |e|), respectively. The q(Sn) values show a positive correlation with the |Eads|, as presented in Figure 3. The extent of charge transfer from the polysulfides to the surfaces also reflects the adsorption orientation of the LiPSs (horizontal or perpendicular) and the number of dissociated Li atoms onto the surfaces, as shown in Table 2 and Figure S1. For example, the horizontal adsorption orientation of the short-chain LiPSs on NG3 or NG4 leads to a dissociation of almost two Li atoms (Li2Sn → xLi + Li2-xSn, 1 < x < 2) from the LiPS, which results in a larger charge transfer from LiPS to the surface, and thus a less anionic polysulfides. The long-chain LiPSs on NG3 or NG4, on the contrary, induces a dissociation of less than one Li atom, (Li2Sn → xLi + Li2-xSn, x < 1) and a more anionic polysulfides. This also explains why the charge of the Li2S3 species (q(S3)), mostly in the horizontal orientation, are often distinctive from those of 10


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other polysulfides on the same surface. Adsorption of LiPSs on Li-trapped heteroatom-doped graphene surfaces The observed pre-dissociative and dissociative adsorptions on NSG1, NSG2, NG3 and NG4 imply a possible formation of Li-trapped surfaces32 with indefinite number of Li. The calculated structures and Eads of different numbers of adsorbed Li on the four surfaces are considered and presented in Figure S3 and Table S3, respectively. The successive adsorption energies in Table S3 show a general decreasing trend as Li atoms are gradually attached to the surfaces. For all four surfaces, the first two Li ions are adsorbed at opposite side of the surfaces at the active center of the heteroatoms with moderate to large Eads. The third Li, when attached, becomes less strongly adsorbed at a position relatively away from the active site for the NSG1, NSG2, and NG3 surfaces. For the NG4 surface (Figure S3(d)), however, the third Li remains relatively close to the center of the four nitrogen atoms, and it is not until the fourth adsorbed Li that starts to drift away. Based on the successive Eads of Li ions on surfaces, the structural analyses, as well as the previously suggested Li-trapped N-doped surfaces,32 we propose the formation of Li–trapped surfaces with either two (2Li@NSG1, 2Li@NSG2, and 2Li@NG3) or three (3Li@NG4) Li’s, as summarized in Figure 4(a)~(d). We then consider the adsorption of LiPSs on these Li-trapped surfaces. The 11



adsorption energies and Bader charge analyses of the adsorbed LiPSs are shown in Table 1 and Table 2, respectively, with their correlations shown in Figure 5. The Li-trapped surfaces show similar or smaller Eads than their non-Li-trapped counterparts, but still larger Eads than the physisorption cases, (i.e. G, NG1, SG1, SG2, and NSG3 in Figure 3), especially for short-chain polysulfides. The 2Li@NSG1, 2Li@NSG2, and 2Li@NG3 surfaces show similar Eads (-0.9 ~ -1.5 eV) but largely varying q(Sn), while the 3Li@NG4 exhibits the highest adsorption energies (-2.8 ~ -3.3 eV) and unexpectedly highly anionic polysulfide species (q(Sn) over -3 |e|). A detailed structural examination reveals that the adsorption of LiPSs on 3Li@NG4 leads to a destructive splitting into two smaller PSs (2Lisur +Li2Sn → Li2Sm + Li2Sm’), where Lisur represents the originally trapped Li on the surface, as shown in Figure S4. Similar destructive adsorptions also occur when the long-chain LiPSs are attached to the 2Li@NG3 and 2Li@NSG2 surfaces. Thus the reported q(Sn), actually the sum of q(Sm) and q(Sm’), becomes more negative for these adsorbed species. Both the dissociative and destructive adsorptions cause irreversible damage to LiPSs and may decrease the capacity and life cycles of Li-S batteries. An ideal interlayer materials for Li-S battery must exhibit an intact adsorption with no LiPS splitting. The Li-trapped surfaces fulfilling this criterion are the 2Li@NSG1 and 2Li@NSG2 (for most cases), as shown in Figure S4(a), with its q(Sn) values close to 12


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-2 |e| for all LiPS species. In order to elucidate the origin of different adsorption behaviors, we compute the electronic localization function (ELF) and the Bader charge (Figure 6) for selected Li-trapped surfaces. The ELF results indicate a strongly localized distribution for electrons at the N atoms on all surfaces, and a more dispersive distribution for electrons at the sulfur region on the N, S co-doped surfaces, as indicated by the black circle in Figure 6(a) and (b), which can be rationalized by the similar electronegativities of C and S (2.55 and 2.58). The Bader charge analyses are consistent with the ELF diagram results. The carbon atoms are found to be highly positively-charged (+0.92 ~ +1.28 |e|) when bonded with N, and slightly negatively-charged (-0.16 ~ -0.30 |e|) when located near S. The difference in the electron distribution on surfaces plays an important role in the adsorption types. It governs the direction and extent of charge transfer upon LiPS adsorption, as shown by the electron density difference in Figure 7. For example, in the intact adsorption, when Li2S6 is adsorbed on the 2Li@NSG1 surface (Figure 7(a)), a charge transfer from the Li on the surface to the polysulfide is compensated by a charge back-transfer from the Li in Li2S6 to the surface. On the other hand, in the destructive adsorption, when Li2S6 is adsorbed on the 2Li@NG3 surface (Figure 7(b)), this charge back-transfer does not occur. In this case, the excessive electron 13



density from the forward electron transfer from the surface to LiPS accumulates at the polysulfide in Li2S6 molecule results to a more anionic polysulfide (q(S6) = -2.69 |e| as in Table 2), which in turn induces a fragmentation of the S6 into two S3 species separated by over 3 Å , as shown by Figure 7(b). The fragmentation of the polysulfides may also be demonstrated by the partial density of states (PDOS) analyses for the p-band states of the S6 chain in adsorbed Li2S6 on 2Li@NSG1, 2Li@NSG2, and 2Li@NG3 surfaces, as shown in Figure S5. The intact S6 on 2Li@NSG1 shows a distinct PDOS distribution from the fragmented [S3-S3] on 2Li@NSG2, and 2Li@NG3 surfaces. Note that the Eads of the three adsorption events are very close (-1.05 ~ -1.27 eV), although they belong to different adsorption types. This example emphasizes the importance of detailed electronic analyses since the information of Eads may not be sufficient to predict all possible adsorption phenomena of LiPS on the Li-S battery interlayers.

VI. Conclusion Through a comprehensive DFT investigation, we find the adsorption of LiPSs on different graphene surfaces may involve three mechanisms, as summarized in Scheme 2. In Mechanism A (the dissociative adsorption), one or two Li atoms almost fully dissociate from the LiPS upon adsorption on some of the N-doped surfaces (e.g., NG3 14


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and NG4), leading to a strong electron transfer from the adsorbate to the surface. Even when Li-trapping is considered, the adsorption of LiPSs may still be followed by an irreducible damage of polysulfide splitting on the Li-trapped N-doped surfaces, (e.g., 2Li@NG3 and 3Li@NG4), as illustrated in Mechanism B (the destructive adsorption). In Mechanism C (the intact adsorption), the LiPSs are adsorbed without any dissociation or destruction on some of the Li-trapped N, S co-doped surfaces (e.g., 2Li@NSG1 and 2Li@NSG2) and shows a higher adsorption energy than on the pristine graphene or other non-Li-trapped surfaces. In conclusion, our computational results confirm the observed better performance of the Li-trapped N, S co-doped graphenes for LiPS adsorption. Our study indicates that such material systems may be an ideal candidate for Li-S battery interlayers, which could reduce the shuttle effect and increase the life cycles in a efficiency way. We hope that this mechanistic study could inspire future material designs for better Li-S battery interlayers and related applications.

Supporting Information Detailed structural and electronic calculation data of LiPSs and adsorbed surfaces.

Acknowledgments 15



This research is supported by the Ministry of Science and Technology (MOST) in Taiwan (MOST 106-2113-M-003-010-MY3). We thank the National Center for High-performance Computing (NCHC) of Taiwan for the help on computational resources.

ORCID Elise Y. Li: 0000-0003-1206-1110

16


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Synergistic

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the

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Scheme 1. The composition of an improved Li-S battery including an interlayer and a separator.

26


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Figure 1. Optimized structures of different N-doped, S-doped, and N, S co-doped graphene-based surfaces. The formation energies (in eV) are also shown. The colors of elements C, N, and S are gray, blue, and yellow, respectively.

27



Figure 2. Electrostastic potential (ESP) diagrams and structures of optimized (a) Li2S8, (b) Li2S6, (c) Li2S4, and (d) Li2S3 molecules (electron density isosurface = 0.001 |e|/Bohr3). The colored regions form blue to red represent the positive and negative potential distribution (in eV) of atoms, respectively. The colors of elements Li and S are purple and yellow, respectively.

28


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Figure 3. The magnitude of the adsorption energies (|Eads|, in eV, shown as bars) and the Bader charges of polysulfide-chain (q(Sn), in |e|, shown as curves) in LiPSs on nine different surfaces. The adsorptions are classified into three types, physisorption, pre-dissociative adsorption, and dissociative adsorptions, according to the magnitude of the adsorption energy and the extent of charge transfer.

29



Figure 4. The optimized structures of Li-trapped surfaces involving the most possible number of Li (a) 2Li@NSG1, (b) 2Li@NSG2, (c) 2Li@NG3, and (d) 3Li@NG4. The colors of elements C, N, Li, and S are gray, blue, purple, and yellow, respectively.

30


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Figure 5. The magnitude of the adsorption energies (|Eads|, in eV, shown as bars) and the Bader charges of polysulfide-chain (q(Sn), in |e|, shown as curves) in LiPSs on Li-trapped surfaces.

31



Figure 6. The electronic localization function (ELF) diagrams of (a) 2Li@NSG1, (b) 2Li@NSG2, and (c) 2Li@NG3 surfaces. The Bader charges (in |e|) are labeled on the elements. The sliced region where ELF is plotted is shown by the red bars in the side view. The colors of elements C, N, Li, and S are gray, blue, purple, and yellow, respectively.

32


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Figure 7. Calculated structures and electron density difference diagrams of the Li2S6 adsorption event on (a) 2Li@NSG1 and (b) 2Li@NSG3 surfaces. The different distances between selected sulfur atoms (in Å ) are shown in figure. Yellow and blue represent charge accumulation and depletion regions, respectively. The colors of elements C, N, Li, and S are gray, blue, purple, and yellow, respectively.

33



Scheme 2. Proposed LiPSs adsorption mechanisms on different graphene-based interlayers.

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Table 1. The calculated adsorption energies (in eV) of lithium polysulfides (LiPSs) on non-Li-trapped and Li-trapped surfaces. Non-Li-trapped surfaces Eads

Li2S8

Li2S6

Li2S4

Li2S3

G

-0.99

-0.74

-0.58

-0.58

NG1

-1.01

-0.75

-0.60

-0.54

SG1

-1.12

-0.89

-0.73

-0.73

SG2

-0.96

-0.76

-0.62

-0.65

NSG3

-1.28

-1.08

-0.88

-0.86

NSG2

-1.64

-1.59

-1.27

-1.50

NSG1

-1.64

-1.59

-1.33

-1.48

NG3

-2.29

-2.09

-2.21

-2.58

NG4

-2.53

-2.33

-2.85

-3.35

Li-trapped surfaces 2Li@NSG1

-1.27

-1.11

-1.09

-1.13

2Li@NSG2

-1.05

-1.27

-1.15

-1.18

2Li@NG3

-1.44

-1.05

-0.95

-1.06

3Li@NG4

-3.08

-3.22

-3.02

-2.81

35



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Table 2. The calculated Bader charge populations (q, in |e|) of polysulfide-chain in lithium polysulfides (LiPSs) on non-Li-trapped and Li-trapped surfaces. The orientation of the two adsorbed Li ions, perpendicular (labeled as p) or horizontal (labeled as h), are also shown. Non-Li-trapped surfaces q

S8n- (Li2S8)

S6n- (Li2S6)

S4n- (Li2S4)

S3n- (Li2S3)

G

-1.91 (p)

-1.91 (p)

-1.91 (p)

-1.74 (h)

NG1

-1.91 (p)

-1.91 (p)

-1.91 (p)

-1.87 (h)

SG1

-1.92 (p)

-1.91 (p)

-1.91 (p)

-1.78 (h)

SG2

-1.91 (p)

-1.91 (p)

-1.91 (h)

-1.75 (h)

NSG3

-1.90 (p)

-1.90 (p)

-1.91 (p)

-1.90 (p)

NSG2

-1.87 (p)

-1.85 (p)

-1.57 (p)

-1.40 (h)

NSG1

-1.87 (p)

-1.83 (p)

-1.78 (p)

-1.27 (h)

NG3

-1.48 (p)

-1.49 (p)

-0.95 (h)

-1.01 (h)

NG4

-1.44 (p)

-1.39 (p)

-0.60 (h)

-0.79 (h)

Li-trapped surfaces 2Li@NSG1

-2.00

-1.94

-1.92

-1.93

2Li@NSG2

-2.15

-2.92

-1.91

-1.87

2Li@NG3

-2.46

-2.69

-1.92

-1.92

3Li@NG4

-3.44

-3.78

-3.83

-3.35

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Table of contents Stability of lithium polysulfides adsorbed on different graphene-based surfaces.

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Adsorption Mechanisms of Lithium Polysulfides on Graphene-Based

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