A New Insight into the Anchoring Mechanism of Polysulfides inside

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Functional Nanostructured Materials (including low-D carbon)

A New Insight into the Anchoring Mechanism of Polysulfides inside Nanoporous Covalent Organic Frameworks for Lithium-Sulfur Batteries Xuedan Song, Mengru Zhang, Man Yao, Ce Hao, and Jieshan Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16172 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on November 28, 2018

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ACS Applied Materials & Interfaces

A New Insight into the Anchoring Mechanism of Polysulfides inside Nanoporous Covalent Organic Frameworks for Lithium-Sulfur Batteries Xuedan Song,*,† Mengru Zhang,† Man Yao,‡ Ce Hao† and Jieshan Qiu† †State

Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, Liaoning

116024, China ‡School

of Materials Science and Engineering, Dalian University of Technology, Dalian,

Liaoning 116024, China

KEYWORDS: lithium-sulfur batteries, shuttle effect, nanoporous materials, covalent organic frameworks, Li2Sx species, first-principles simulation

ACS Paragon Plus Environment

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ABSTRACT: The application prospects of lithium-sulfur (Li-S) batteries are constrained by many challenges, especially the shuttle effect of lithium polysulfides (Li2Sx). Recently, microporous covalent organic framework (COF) materials have been used to anchor-electrodes in lithium-sulfur (Li-S) batteries, because of its preferable characteristics, such as self-design ability, suitable pore size and various active groups. In order to identify the ideal anchoring materials that can effectively restrain the shuttle of Li2Sx species, the anchoring mechanism between COF materials and Li2Sx species should be investigated in depth. Therefore, we systematically investigated the anchoring mechanism between specific COF nanomaterials (consist of Boron, Oxygen atoms and Benzene group) and Li2Sx (x=1, 2, 4, 6, 8) species on the surface and inside the pore using the density functional theory methods with van der Waals interactions. The detailed analysis of the adsorption energy, difference charge density, charge transfer and atomic density of states can be used to determine that the COF nanomaterials, with the structure of boroxine connecting to benzene groups and boroxine groups not constructed at the corner of the structure, can effectively anchor Li2Sx series. Accordingly, this study provides the theoretical basis for the molecular-scale design of ideal anchoring materials, which can be useful to improve the performance of the Li-S batteries.


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INTRODUCTION

With the increasing energy demands of modern society, lithium-sulfur (Li-S) batteries have attracted considerable attention due to their exceptional theoretical capacity (1,672 mA h g-1) and high theoretical energy density (2,600 W h kg-1).1-5 Moreover, due to the many advantages of sulfur, such as lightweight, high natural abundance, low cost and environmental friendliness, LiS batteries have been recognized as the potential energy storage systems for commercial applications in the future.6-7 However, as noted above Li-S batteries have numerous advantages, Li-S batteries face many challenges, including low Coulomb efficiency, poor cycle life, etc.8-12 One of the reasons for some of these challenges is the shuttle effect caused by the intermediate products polysulfides dissolved in the electrolyte during the charge and discharge processes, which leads to the loss of reactant sulfur.13-14 In recent years, many researchers have put forward various solutions to solve this problem, from the perspectives of electrolytes15-16, SEI films17-18 and cathode materials19-31. Some of them adopt nanoporous materials as anchoring material, such as carbon nanotubes,19-20 metal oxides,21-23 metal organic framework (MOF) materials,24-26 covalent organic frameworks (COFs) materials,27-31 etc. COFs have received great attention for their application in anchoring cathode materials of Li-S batteries due to their preferable characteristics in various active sites, suitable pore size, selfdesign ability, large specific surface area and small density. From 2014 to 2016, N-doped COFs have been used to sulfur cathode materials, like CTF-1, Por-COF, Py-COF and Azo-COF. 27-29 Then Yoo et al. adopted the composite structure of microporous COF-1(B, O-doped) grown on mesoporous carbon nanotube as new interlayer cathode materials in Li-S batteries experimentally. Meanwhile in this study, they mainly analyzed adsorption phenomenon of Li2S on the COF-1 using DFT and grand canonical Monte Carlo calculations and emphasized the


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chemical affinity for chemical trapping of Li2S is caused by boron atom of COF-1.31 After that, Ghazi et al. directly adopted COF-1 as cathode materials in Li-S batteries experimentally, which showed relatively stable cycle life in the charge/discharge processes.30 Till now, there is an urgent but little-reported problem: the systematic anchoring mechanism for restraining the shuttle effect of polysulfides by COF nanomaterials is ambiguous. The COF composites generally contain nitrogen (N), boron (B) and oxygen (O) atoms and benzene groups. Due to the influence of the electronegativity, the activity of the B-O polar bond generated by the interaction of the positive B atom with the negative O atom is higher than that of the N-N non-polar bond. The benzene group, with large electron cloud density resulting from the conjugate π orbital electron that may interact with lithium polysulfides, could be an important active site to restrain the loss of polysulfides. Accordingly, we concentrated on two common active groups as research objects, consisting of Boron and Oxygen atoms, namely the active boroxine group and active pentacyclic group (consisting of the boronate ester bond O-B-O bonded to an aromatic ring). Thus, we constructed two COFs, one consisting of boroxine and benzene groups, termed COF-PA, and the other consisting of an active pentacyclic group and a benzene group, termed COF-PB, as shown in Figure 1(a, b). COF-PA and COF-PB are modeled by referring to the construction of COF-1 and COF-5 synthesized by Yaghi et al.32 And the theoretical XRD patterns of COF-PA and COF-PB in bulk state after optimization are simulated and shown in Figure S1, which fits well with experimental XRD data.32 It should be note that COF-PA is somewhat different from COF-1: the porsize of COF-1 is 0.7 nm because of its ABstaggered stacking arrangement, but the constructed COF-PA is AA stacking arrangement with the porsize of 1.5 nm, as shown in Figure S2. Considering the key factor of pore effect in most


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COFs, and making the universal but useful conclusion of the adsorption function of the Boron, Oxygen and benzene active sites in normal COFs, we have constructed the COF-PA structure. In our studies, systematic calculations of the interaction mechanism between Li2Sx (x=1, 2, 4, 6, 8) species (as shown in Figure 1(c)) and the modeled COFs on both the surface and inside the pore, were performed by the DFT method. Based on the comparison of the adsorption energy, difference charge density, Bader charge and atomic density of states, we found that boroxine and benzene groups of COF-PA (Eads= -3.8 ~ -0.7 eV) is easier to interact with the Li2Sx species during the lithiation process than COF-PB (Eads= -3.6 ~ -0.3 eV), and the COF nanomaterials, with relatively little steric hindrance and structure of boroxine connecting to benzene groups, can effectively anchor Li2Sx series, which provides the theoretical basis for the design of ideal cathode materials to restrain the shuttle effect of Li2Sx species.

(a)

(b)

(c) Li1

S2 S1

Li2 Li2 Li1

Figure 1. Constructions of (a) COF-PA and (b) COF-PB are used as anchoring materials, (c) molecule configurations for Li2Sx species from Li2S to unlithiated S8 at various lithiation stages, which are all optimized with optPBE-vdW method. Color scheme: O, red; C, grey; B, pink; H, white; S, yellow; Li, purple.


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METHOD

Our calculations are carried out using density functional theory (DFT) with the Vienna ab initio simulation package (VASP)33 under the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.34 The frozencore projector augmented wave (PAW) method is adopted to describe the ion-electron interactions.35 Particularly, we choose the optPBE-vdW functional36-37 to account for the van der Waals (vdW) interactions after making the functional tests in Table S1. Self-consistent field (SCF) calculations are performed with a convergence criterion of 1×10-4 eV atom-1. The cutoff energy is set to 520 eV and the magnitude of the force on atoms is minimized to be less than 0.05 eV Å-1. To simulate both surface and pore-inside adsorption between Li2Sx species and the modeled COFs, the 6-layers constructions of COF-PA and COF-PB are constructed through the total energy and electronic computations, and the vacuum is sufficiently set to 20 Å in the z-direction. The optimized structure of COF-PA, COF-PB and Li2Sx (x=1, 2, 4, 6, 8) species are shown in Figure 1(a-c), respectively. To describe the interaction between Li2Sx species and the modeled COFs, the adsorption energy (𝐸𝑎𝑑𝑠) can be calculated as: 𝐸𝑎𝑑𝑠 = 𝐸𝐶𝑂𝐹𝑠 ― 𝐿𝑖2𝑆𝑥 ― 𝐸𝐶𝑂𝐹𝑠 ― 𝐸𝐿𝑖2𝑆𝑥 (1) Where 𝐸𝐶𝑂𝐹𝑠 ― 𝐿𝑖2𝑆𝑥, 𝐸𝐶𝑂𝐹𝑠 and 𝐸𝐿𝑖2𝑆𝑥 represent the total energy for adsorption system of Li2Sx species with the modeled COFs, COFs (COF-PA or COF-PB) and Li2Sx species, respectively. To have an evident view of charge transfer between Li2Sx and the modeled COFs, the charge density difference (Δ𝜌) can be defined as: ∆𝜌 = 𝜌𝐶𝑂𝐹𝑠 ― 𝐿𝑖2𝑆𝑥 ― 𝜌𝐶𝑂𝐹𝑠 ― 𝜌𝐿𝑖2𝑆𝑥 (2) Where 𝜌𝐶𝑂𝐹𝑠 ― 𝐿𝑖2𝑆𝑥, 𝜌𝐶𝑂𝐹𝑠 and 𝜌𝐿𝑖2𝑆𝑥 represent the charge density for adsorption system of Li2Sx species with the modeled COFs, COFs and Li2Sx species, respectively.


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RESULTS AND DISCUSSION

To obtain the relatively stable active sites for surface adsorption, we selected the surface adsorption between Li2S2 and COF-PA as research objects. All possible adsorption sites of COFPA were considered, including B, O and benzene active sites, as shown in Figure 2(a). Adsorption patterns consisted of specific atoms of Li2S2 molecule anchored on specific adsorption sites of COF-PA, and seven possible cases were considered, as shown in Table 1. After comparing the adsorption energy, the energetically more favorable partial adsorption configurations were confirmed as Ap-2 and Ap-3 patterns (Figure 2(b, c)), whose adsorption energy is -2.97 eV and -2.95 eV, respectively. Accordingly, we considered the adsorption patterns of Ap-2 and Ap-3 separately to compare the Li2Sx adsorption properties on the surface of COF-PA and COF-PB. The Li2Sx species, including Li2S8, Li2S6, Li2S4, Li2S2 and Li2S, were considered, because they represent the lithiation/delithiation processes.


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(a)

B-b O-a

B-a

O-b

Ph-m

(b)

(c)

Figure 2. (a) Illustration of different adsorption sites in COF-PA. The energetically more favorable partial configurations of Li2S2 molecule on the surface of COF-PA (b) in Ap-2 pattern and (c) in Ap-3 pattern. Color scheme: O, red; C, grey; B, pink; H, white; S, yellow; Li, purple.

Table 1. The adsorption energy or final Ads. pattern between Li2S2 and COF-PA of 7 different kinds of adsorption pattern.

Ads. pattern

Sites O-a O-b

Ap-1

Li1

Ap-2

Li1 Li2

Ap-3

Li1

B-a B-b Ben-m

𝐸𝑎𝑑𝑠/eV or final Ads. pattern -0.71

S1

-2.97 Li2

Ap-4

Li1

Ap-5

Li1

Ap-6

S1

Ap-7

S1

-2.95 Ap-2

Li2

Ap-3 -0.82

S2

Ap-2


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First, the stable Ap-2 and Ap-3 adsorption configurations of the Li2Sx species on the surface of the modeled COFs were calculated and are shown in Figure S3-S4, and the partial adsorption configurations around adsorption sites are shown in Figure 3(a-d), respectively. In addition, their simulated adsorption energies are presented in Figure 3(e). From a general perspective, the adsorption strength of the Li2Sx species and COF-PA (-3.8 ~ -0.7 eV) are clearly lower than that of the Li2Sx species and COF-PB (-3.6 ~ -0.3 eV). This indicates that regardless of the Ap-2 or Ap-3 adsorption style, COF-PA has stronger interactions with the Li2Sx species during the lithiation process. A comparison of the adsorption energy curves of the Ap-2 and Ap-3 patterns reveals that the adsorption energy values in the Ap-3 pattern are generally lower than that in the Ap-2 pattern whether in COF-PA or in COF-PB. Noteworthy, the adsorption energy values of Li2S4 and Li2S6 with COF-PB in the Ap-3 pattern decreased by 0.5 eV and 0.4 eV, respectively, compared with that in the Ap-2 pattern. This means that in the Ap-3 pattern the combination of O and benzene active sites plays an important role in increasing the adsorption interaction of the Li2Sx species. By means of Bader charge method,38-39 the Bader charge curves of the Li2Sx species at various lithiation stages in the Ap-2 and Ap-3 patterns are shown in Figure 3(f). The curves reveal that the tendency of electron transfer from the Li2Sx species to the COFs is quite consistent with the adsorption energy curves, which is the reason why the adsorption energy of the Ap-2 and Ap-3 patterns have different value.


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(a)

Li1

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O1

Li2 O2

(b)

Li1 O1 Li2 O2

(c)

(d)

Li2S

(e)

Li2S2

Li2S4

Li2S6

Li2S8

(f)

Figure 3. The illustrations of stable partial adsorption configurations of Li2Sx (x= 1, 2, 4, 6, 8) series adsorbing on the surface of COF-PA (a) in Ap-2 pattern and (b) in Ap-3 pattern, and of COF-PB (c) in Ap-2 pattern and (d) in Ap-3 pattern after optimization. (e) The adsorption energy curves and (f) the Bader charge curves of Li2Sx species and the modeled COFs in Ap-2 and Ap-3 adsorption patterns. The negative value of Bader charge means lose electron for Li2Sx species. Color scheme: O, red; C, grey; B, pink; H, white; S, yellow; Li, purple.


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Based on the stable adsorption configurations described above, the visualized difference in the charge transfer could be obtained by calculating the charge density difference. Since the adsorption energy values of Li2S4 and Li2S6 are relatively small, we take the adsorption system of Li2S4 and Li2S6 on the modeled COFs as examples, and their charge density different images in the Ap-2 and Ap-3 patterns are shown in Figure S5, and the charge density difference pictures around adsorption sites are shown in Figure 4(a-d), respectively. First, in the Ap-2 pattern, the charge transfer between COF-PA and Li2S4 (or Li2S6) is larger than that between COF-PB and Li2S4 (or Li2S6), which indicates that it is easier for COF-PA to generate chemical bonds between COF-PA and Li2Sx species. Meanwhile, charge is lost along the Li − S bond in the Li2Sx (blue part) to COF-PA, which means that charge was transferred from Li2Sx to COF-PA. Second, in the Ap-3 pattern, the charge exchange between COF-PB and Li2S4 (or Li2S6) significantly is increased compared with that in the Ap-2 pattern, almost comparable to the charge exchange between COF-PA and Li2S4 (or Li2S6). Moreover, it is obvious that the charge exchange between Li2Sx species and the active benzene group plays an important role in chemical bonds generation.


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(a)

(b)

(c)

(d)

Li2S4

(e)

Li2S6

Li2S4

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Li2S6

(f)

Figure 4. The illustrations of charge density difference of Li2Sx (x=4, 6) absorbing on the COFPA (a) in Ap-2 pattern and (b) in Ap-3 pattern, and on the COF-PB (c) in Ap-2 pattern and (d) in Ap-3 pattern. Here, blue regions mean losing charge, red regions mean gaining charge. Atomic PDOS near the Fermi energy region for Li2S4 adsorbing on the surface of COF-PA (e) in Ap-2 pattern and (f) in Ap-3 pattern. Among them, Ben means active benzene group. To further investigate the charge transfer quantitatively, we selected Li2S4 and Li2S6 as examples, using the Bader charge method to calculate the number of valence electrons of the atoms in Li2Sx and the COFs in the Ap-2 and Ap-3 patterns, as shown in Table 2. The positive (negative) value of the transfer charge means gain (loss) charge; the Δe(Li) values are very small, whereas the Δe(S) values are much larger, which suggests that the chemical interaction derives


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from the electron transfer between the S atoms in Li2Sx into the COFs. A detailed analysis shows that in the Ap-2 pattern the electron is dominantly captured by the O atom of COFs, and the ability to capture electrons of the O atom in COF-PA is better than that of the O atom in COF-PB, which increased electrostatic interactions. For the Ap-3 pattern, the analysis reveals that the electron is dominantly captured by the benzene group of COFs, and, whether in the COF-PA or in COF-PB, the ability of the benzene group to capture the charge is strong. In general, COF-PA, with O-B hexatomic rings and active benzene groups, can effectively capture the electron, and thus enhances the anchoring effect.

Table 2. Bader charge differences of Li2Sx (x=4, 6) molecule adsorbing on different atoms of the COFs in Ap-2 and Ap-3 pattern. Δe(Li2Sx)

COF-PA-Ap-2 COF-PB-Ap-2 COF-PA-Ap-3 COF-PB-Ap-3

Δe(COFs)

Δe(Li)

Δe(S)

Δe(O)

Δe(B)

Δe(C)

Δe (Li2Sx)= - Δe(COFs)

Li2S4

-0.004

-0.090

0.081

-0.064

-

-0.094

Li2S6

0.004

-0.118

0.091

-0.008

-

-0.114

Li2S4

-0.003

-0.021

0.012

0.003

-

-0.024

Li2S6

-0.001

-0.006

-0.044

0.000

-

-0.007

Li2S4

-0.004

-0.185

0.066

0.002

0.262

-0.189

Li2S6

0.012

-0.103

0.030

-0.029

0.373

-0.091

Li2S4

-0.005

-0.140

0.060

0.006

0.378

-0.145

Li2S6

0.005

-0.022

-0.026

-0.010

0.393

-0.017

In addition, in order to understand the nature of the chemical bonding between Li2Sx species and the COFs, analysis of the atomic partial density of states (PDOS) near the Fermi energy region was performed. The Li2S4 with the COF-PA was chosen to elucidate the difference in electronic structure in the Ap-2 and Ap-3 patterns, as shown in Figure 4(e, f). The PDOS of the Li2S4 and COF-PA system in the Ap-2 pattern shown in Figure 4(e) indicates that the main


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orbital interactions of the Li2S4 and COF-PA system are between the Li1-2s and O1-2p orbitals and between the Li2-2s and O2-2p orbitals around the Fermi energy level. Additionally, no obvious overlapping is observed between the Li-2s and Ben-pz orbitals. The PDOS of the Li2S4 and COF-PA system in the Ap-3 pattern shown in Figure 4(f) reveals that, besides the hybridization between the Li1-2s and the O1-2p orbitals, there is an even stronger hybridization between the Li2-2s and Ben-pz orbitals. Similar interpretations can be given for the orbital interactions of the Li2S4 and COF-PB system in the Ap-2 and Ap-3 patterns, as shown in Figure S6. To sum up, the analyses of the electronic structure and charge transfer further explain and agree well with the difference in adsorption energy between Li2Sx species and the modeled COFs.

(a)

(c)

(b)

COF-PA-Li2S4-Ap-a

COF-PA-Li2S4-Ap-b

(d)

COF-PA-Li2S6-Ap-a

COF-PA-Li2S6-Ap-b

Figure 5. The illustrations of the stable adsorption configurations of the Li2S4 anchored in the pore of the COF-PA (a) in the Ap-a pattern and (b) in Ap-b pattern, and the Li2S6 anchored in the pore of the COF-PA (c) in the Ap-a pattern and (d) in Ap-b pattern.


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Since there are many regular pores in COF structures, we also considered the adsorption inside the pore, which was performed by active sites across the layers. Based on the electronegativity, we considered two possible adsorption patterns inside the pore, namely the Ap-a and Ap-b patterns, as shown in Figure 5. The Ap-a pattern represents the interaction pattern between the Li atoms in Li2Sx and the active O atoms in the COFs. The Ap-b represents the interaction pattern between the S atoms in Li2Sx and the active benzene groups in the COFs. The stable adsorption configurations of the Li2Sx (x=4, 6) anchored in the pore of the COF-PA and COF-PB in the Apa and Ap-b patterns were calculated and are shown in Figure 5 and Figure S7, respectively. In addition, their simulated adsorption energy and Bader charge are listed in Table 3. The adsorption energies of Li2Sx (x=4, 6) and COF-PA range from -0.6 to -0.8 eV, and the adsorption energies of Li2S4 and COF-PB range from -0.3 to -0.5 eV, which are comparable to the magnitude of the surface adsorption energies. In particular, the adsorption energy between Li2S6 and COF-PB in the Ap-b pattern is lower (about 0.5 eV) than that on the surface, and even lower (about 0.45 eV) than that between Li2S6 and COF-PA in the Ap-b pattern. The Bader charge analyses clearly establish that the Bader charges are consistent with the adsorption energies inside the pore.

Table 3. The adsorption energy and the Bader charge of Li2Sx (x=4, 6) and the modeled COFs in Ap-a and Ap-b adsorption patterns. 𝐸𝑎𝑑𝑠 (eV)

Bader charge (e)

Li2S4

Li2S4

Li2S6

Li2S6

Ap-a

Ap-b

Ap-a

Ap-b

Ap-a

Ap-b

Ap-a

Ap-b

COF-PA

-0.61

-0.72

-0.63

-0.77

-0.043

-0.059

-0.030

-0.058

COF-PB

-0.30

-0.45

-0.33

-1.23

-0.019

-0.024

-0.022

-0.063

*The

negative value of Bader charge means lose electron for Li2Sx species.


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To further understand the distinct adsorption energies of the Li2S6 anchored in the pore of the COF-PA and COF-PB in the Ap-b patterns, the charge density difference were simulated and are shown in Figure S8-S9, and the charge density difference pictures of which around adsorption sites are shown in Figure 6(a, b). The simulations reveal that the change density difference can be visually illustrated by the relatively little steric hindrance of the COF-PB compared with that of COF-PA in Ap-b pattern. This increases the interaction between Li2S6 and the active sites (O and benzene) in two adjacent layers of COF-PB, thus augmenting the charge exchange between Li2S6 and COF-PB from the charge density difference analysis in Figure 6(b). To summarize, the adsorption caused by the active sites across the layers in the pore, whether in COF-PA or in COF-PB, can contribute to anchoring Li2Sx species. In addition, we also considered the solvent effect on adsorption interaction of Li2Sx and COFs inside the pore as shown in Figure S10. It indicates that with the presence of the DME solvent the adsorption interaction of Li2S4 and COFs is enhanced, which means the shuttle effect of polysulfide dissolution is well suppressed.

(a)

(b)


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Figure 6. The illustrations of charge density difference of Li2S6 absorbing in the pore of (a) the COF-PA and (b) the COF-PB in Ap-b pattern. Here, blue regions mean losing charge, red regions mean gaining charge. In addition, the reaction potential at the active site of COFs was calculated, and free energy diagram for reaction coordination in Li-S battery is shown as Figure 7. After the calculation of reaction Gibbs free energy, the main reactions at the different active site of COFs (COF-PA and COF-PB) for sulfur reduction can be summarized as follows: S8 →2(Li2S4)→4(Li2S2)→8(Li2S). Through the analysis of free energy diagram, it is concluded that the reaction free energy of S8 converting to Li2S4 on the COF-PA is larger than that on the COF-PB, which means S8 is easier to convert to Li2S4 on the COF-PA than on the COF-PB during the charge and discharge processes. Meanwhile, Ap-3 adsorption style can make S8 easier to convert to Li2S4. This means a lot, because the shuttle effect is caused by Li2Sx (x=8, 6, 4) dissolved in the electrolyte during the discharge processes. Hence, it further confirms the conclusion above. 𝟏 𝑺 + 𝟖𝑳𝒊 𝟐 𝟖 𝑳𝒊𝟐 𝑺𝟒 + 𝟔𝑳𝒊

𝟐𝑳𝒊𝟐 𝑺𝟐 + 𝟒𝑳𝒊

𝟒𝑳𝒊𝟐 𝑺

Figure 7. The illustration of the free energy diagram for reaction coordination at different active site of COF-PA and COF-PB.


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CONCLUSIONS

In summary，we systematically investigated the mechanism of constraining the shuttle effect of Li2Sx (x=1, 2, 4, 6, 8) species anchored by COF-PA (consisting of active boroxine and benzene groups) and COF-PB (consisting of active pentacyclic and active benzene groups), by means of the DFT calculations with van der Waals interactions. From the surface adsorption perspective, we filtered out two relatively stable adsorption patterns, AP-2 (two Li atoms interacting with two O atoms active sites in the COFs, separately) and AP-3 (two Li atoms interacting with O, benzene active sites in the COFs, separately). The adsorption energy of anchoring Li2Sx species shows that, in general, COF-PA is better than COF-PB, and the AP-3 adsorption pattern plays an important role in increasing the adsorption interaction of the Li2Sx species. Through the thorough analyses of the charge density difference and Bader charge, it is determined that the active O atoms in the boroxine and benzene groups are the main contributors to the adsorption of Li2Sx (x=4, 6) species. In addition, the PDOS confirms the above conclusion. From the perspective of the adsorption inside the pore, the adsorption energy of COF-PB with Li2S6 inside the pore (𝐸𝑎𝑑𝑠= -1.02 eV) is lower than that on the surface (𝐸𝑎𝑑𝑠= -0.03 eV) because of the relatively small steric hindrance of the COF-PB resulted in the exposure of active sites. Accordingly, the COF nanomaterials, with the structure of boroxine connecting to benzene groups and boroxine groups not constructed at the corner of the structure, can effectively adsorb Li2Sx species. Therefore, our study provides the theoretical basis for the molecular-scale design of ideal anchoring materials for restraining the shuttle effect of Li2Sx species, which can be useful for improving the performance of Li-S batteries.

ASSOCIATED CONTENT Supporting Information


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Figure S1: The theoretical XRD patterns of COFs in bulk state after optimizaiton; Figure S2: The illustrations of stable configurations of COF-1 in AA stacking arrangement and AB stacking arrangement (COF-PA); Figure S3-S5, S7-S10: The optimized construction and the charge density difference of Li2Sx species adsorbing on the modeled COFs in detail; Figure S6: the illustrations of Atomic PDOS for Li2S4 adsorbing on the surface of COF-PB; Table S1: the functional correction of van der Waals (vdW) interactions tests. (PDF) AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xuedan Song: 0000-0001-7531-4344 Man Yao: 0000-0002-7322-9258 Ce Hao: 0000-0002-4379-0474 Jieshan Qiu: 0000-0002-6291-3791 Notes

The authors declare no competing financial interest. ACKNOWLEDGMENT

This work has been supported by the National Natural Science Foundation of China (Grant Nos. 21606040 and 21677029); the Fundamental Research Funds for the Central Universities (DUT18LK26); the Supercomputing Center of Dalian University of Technology; the National Supercomputing Center in LvLiang of China. REFERENCES

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A New Insight into the Anchoring Mechanism of Polysulfides inside

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