Physisorption Mechanism of Solvated Polysulfide Chains on

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Physisorption Mechanism of Solvated Polysulfide Chains on Graphene Oxides with Varied Functional Groups Aniruddha M Dive, Min-Kyu Song, and Soumik Banerjee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12468 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 21, 2017

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The Journal of Physical Chemistry

Physisorption Mechanism of Solvated Polysulfide Chains on Graphene Oxides with Varied Functional Groups Aniruddha M. Dive, Min-Kyu Song and Soumik Banerjee1 School of Mechanical and Materials Engineering Washington State University, Pullman, Washington 99164-2920, U.S.A. ABSTRACT Despite the high theoretical capacity (~1675 mAh/g) and energy density, commercialization of the lithium – sulfur (Li-S) batteries has been hindered primarily due the loss of active material at the cathode through the “polysulfide shuttle” effect during repeated charge – discharge cycles. Graphene and graphene oxide (GO) are being explored as effective cathode supports to alleviate these problems. However, there is a lack in fundamental understanding of the physical interactions between polysulfide and graphene/GO substrates. In order to determine the dominant mechanisms for physisorption of polysulfides on GO, we employed molecular dynamics (MD) to simulate polysulfides (S82-) solvated in standard dimethoxy ethane (DME), dioxalane (DOL) (1:1 v/v) solvent near a range of graphene and GO structures. The results indicate that the extent of physisorption of polysulfide is governed by Coulombic interactions with GO, solvent orientation near substrates and steric factors that depend on the nature and surface density of functional groups on the GO. Based on physisorption, GO with hydroxyl functional groups is the most effective in anchoring polysulfides. These results can potentially pave the way for design of molecularly-tailored cathode supports to mitigate polysulfide shuttle and therefore improve performance of Li-S batteries.

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Corresponding author, Tel: +1 509 3350294, E-mail: [email protected]

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Introduction Electrochemical energy storage devices for next generation transportation and power grid applications require significantly high energy and power densities. Commercially available lithium ion batteries lack the required energy and power densities for such applications, paving a way to explore novel chemistries1. Rechargeable Li – S batteries2, with a theoretical capacity of ~ 1675 mAh/g of sulfur, hold excellent promise for such energy intensive applications. A high theoretical capacity and energy density, coupled with abundance of sulfur deposits globally, make Li – S batteries economically viable. Additionally, the environmental compliance of sulfur makes it a suitable candidate for automobile and power grid applications. Despite these advantages, commercialization of Li – S batteries has been marred by some critical limitations. Poor electrical conductivity/low utilization of pure sulfur cathode, loss of active material and poor Coulombic efficiency are the major limitations hindering the commercialization of these batteries2. “Polysulfide shuttle”3-7 of intermediate high-order polysulfides (Li2Sx, 4 ≤ x ≤8) is responsible for the latter two issues. Pure sulfur is reduced to S82- during the first step of discharge to form polysulfides. These polysulfides react with lithium ions to form lithium polysulfide Li2S8. The polysulfides undergo further reduction to S62- and S42- and subsequently form lower order polysulfides Li2Sx (1≤x70 degrees) with the direction normal to the substrate is greater for GO – E1 as compared to that of GO – E2. This indicates that larger number of DOL molecules tend to align parallel to the GO – E1 substrate as compared to GO – E2 substrate, which has a lower concentration of epoxy functional groups. Lower number of DOL molecules aligned parallel to the substrate makes it easier for polysulfides to approach the substrate as the steric hindrance due to parallel alignment of DOL molecule is reduced. This can lead to improved polysulfide adsorption onto the GO – E2 substrate as compared to GO – E1 that was observed in Figure 5.

(a)

(b)

Figure 14. Comparison of orientation (a) C1 – C1 vectors and (b) C1 – O2 vectors for GO – E1 and GO – H substrates is shown. The data is presented in terms of percentage of total DOL molecules. Navg is the time-averaged number of DOL molecules.


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Overall, results in Figure 13 suggest that the orientation of solvent molecules plays an important role in the deposition of polysulfides on the GO – E1 and GO – E2 substrates. While the snapshot in Figure 6 suggests minimal solvent layer formation in GO – H, for the sake of comparison, we analyzed the orientation of DOL molecules near GO – H. Figures 14 (a) and (b) show a comparison of solvent orientation in GO – E1 in terms of the alignment of DOL molecules (expressed as percentage of total DOL molecules) near the substrates. The analysis was performed on DOL molecules within a specified cut off distance from the GO substrate. The cut off distance was selected based on the location of the peaks in Figure 5. The percentage of C1 – C1 vectors aligned at relatively high angles (θ ≥ 60 degrees) with direction normal to the substrate is similar for both GO – H and GO – E1, whereas the percentage of C1 – O2 vectors aligned at high angles (θ ≥ 60 degrees) is 10 – 15% greater in case of GO – E1 than for GO – H. This shows that the DOL molecules have a much greater tendency to align parallel to substrate in GO – E1 than in GO – H, which, in conjunction with minimal layering on substrate, leads to less steric hindrance for polysulfides to approach the GO – H substrate. This effect, coupled with the strong Coulombic attractions between terminal sulfur atoms of polysulfides and the hydrogen atoms of hydroxyl groups, leads to successful polysulfide anchoring onto the GO – H substrate.

Conclusion We performed MD simulations of polysulfides (S82-) in standard solvents near different graphene/GO substrates at room temperature. Overall, the results from the simulations demonstrate that GO holds promise in reducing polysulfide dissolution by stabilizing polysulfides either onto or in close proximity of the substrate by way of physisorption. The mechanisms by which the polysulfides are stabilized depend on the type as well as the concentration of functional groups on the GO substrate, which in turn indicates varying abilities 23 ACS Paragon Plus Environment


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of GO to anchor and immobilize polysulfides. Of the simulated structures, GO – H system is the most effective in stabilizing the polysulfides on the GO substrate, whereas GO – E1 and GO – E2 systems tend to stabilize the polysulfides at a certain distance from the substrate. Stabilization of polysulfides in both cases has the potential to lower the “polysulfide shuttle” and thereby improve the performance of the lithium sulfur batteries. All the analysis done in the present work accounts for purely physical interactions between the polysulfides (S82-) and the functional groups of different GO substrates. We have not taken into account any chemical bonding and reactions between the polysulfide and GO based cathode supports that might also play an important role to reduce the polysulfide shuttle. Modeling efforts in the future that investigate possible chemical reactions between the polysulfides (S82-) and the functional groups on GO will provide comprehensive information and help identify the most favorable GO structures to reduce polysulfide shuttle. AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT. The authors acknowledge the use of Washington State University’s high performance computing cluster for carrying out the simulations.


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