Computational Criteria for Evaluating Polysulfide Cohesion, Solvation

Subscriber access provided by University of Colorado Boulder

Article

Computational Criteria for Evaluating Polysulfide Cohesion, Solvation and Stabilization: Approach for Screening Effective Anchoring Substrates Chenggang Zhou, Zhuan Ji, Bo Han, Qiyang Li, Qiang Gao, Kaisheng Xia, and Jinping Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09577 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17

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

The Journal of Physical Chemistry

Computational Criteria for Evaluating Polysulfide Cohesion, Solvation and Stabilization: Approach for Screening Effective Anchoring Substrates Chenggang Zhou, Zhuan Ji, Bo Han*, Qiyang Li, Qiang Gao, Kaisheng Xia, Jinping Wu* Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, Hubei, China P.R. E-mail: [email protected] (B. Han)

[email protected] (J. Wu)

Abstract Utilizing the functional groups of carbon substrate or third-party additives to adhere lithium polysulfides for suppressing their dissolution has been demonstrated to be capable of improving the sulfur cathode stability of lithium-sulfur batteries. In the present first-principles study, we systematically investigated the competitions between polysulfide self-cohesion, solvation and its anchoring strengths on substrates. The dissolution probability of polysulfides in ether-based electrolytes is evaluated by a defined solvation potential ∆PS-C , which confirms that Li2S8 is the most soluble species; the competition of Li2S8 anchoring strength on different substrates and its solvation energy is described by a stabilizing potential ∆PS-A , which can be used to verify if a certain substrate can effectively stabilizing polysulfides in cathodes. Two properties for a feasible substrate, containing affinitive sites with high electron density for anchoring polysulfides and containing sufficient affinitive sites to provide multiple interactions for enhancing the stabilization, are necessarily proposed. Accordingly, phosphorylated chitosan (PCS), among several substrates, is predicted to be a promising third-party substrate to preserve polysulfides in cathode and prevent them from being dissolved. Our computational scheme may provide a reliable procedure for rapid screening most appropriate candidates for designing novel architecture of sulfur cathodes. Introduction Lithium sulfur (Li/S) battery, due to the high theoretical energy density of 2600 Wh kg-1 of sulfur cathode, has been accepted as one of the most promising candidates for the next-generation energy storage systems particularly for electric vehicles. However, suffering from the severe “shuttle effect” led by polysulfides dissolution, which causes rapid capacity decay, low Coulombic efficiency as well as anode corrosion, a practical realization of Li-S system has been hurdled1-3.

ACS Paragon Plus Environment


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

In recent years, numerous successful strategies focusing on cathode assemblies and modifications have been proposed to tackle these issues4-6. The majority of the efforts aim to prevent the polysulfide intermediates from being dissolved into ether-based electrolytes. In general, the insulating nature of sulfur requires it to be incorporated with conductive substrates such as advanced carbons, where the nano/meso/micro pores of carbons serving both as sulfur container and electronic conductor7-14. Modification of the C/S materials is always required to constrain lithium polysulfides in cathode. Incorporating third-party agents15-22, including metal oxides (MnO2)20, polymers (PEG)22 and biomolecules (DNA)15, have been demonstrated to be effective approaches to suppress polysulfide dissolution for improving cell sustainability. Theoretical simulations23-28 approve that the interactions between the lone-pair electrons of additives or carbon substrates and polysulfides are the dominative contributor to anchor polysulfides. Upon these, Cui and coworkers24 suggested PVP (poly(vinylpyrrolidone)) as a bi-functional binder which enables considerably enhanced cyclic performance of Li2S cathode in contrast with conventional PVDF (polyvinylidene fluoride) binder. Two benignant modes of interactions, as identified by our previous theoretical work28, the electrostatic attractions between Li+ and electron lone-pairs, and the hydrogen bonding between polysulfide anions with the H atoms of substrate, are positive to increase the anchoring strength. The formal one is the governing factor where the charge environments of the adsorption sites are determinative. However, the third mode, structural distortion or rearrangement of the substrates, is detrimental to polysulfide anchoring. Apparently, without any additives, due to the solvation of long-chain polysulfides in ether-based solvents29, which should be more preferential than the self-cohesion of polysulfide, the dissolution is consequently inevitable. With the finite existence of additives at the electrolyte/electrode interfaces, a new competition between the anchoring strength of polysulfides on substrate and their solvation strength emerges, which should be the critical criteria to determine whether polysulfides dissolve or not at the local circumstances. Therefore, to screen an appropriate third-party substrate for stabilizing sulfur cathodes prior to experimental trial, a detailed theoretical understanding towards the anchoring mechanism and energetics as well as their competition with polysulfide solvation is necessarily demanded. In light of this, a systematic density functional theory (DFT) investigation on the self-cohesion,


Page 2 of 17

Page 3 of 17

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


solvation and anchoring behaviors on various substrates of lithium polysulfide were conducted in the present work. Here, the typical dimethoxyethane (DME) and 1,3-dioxolane (DOL) mixed solvent (v:v 1:1) was selected to model the solvation, where a solvation potential ∆PS-C was suggested to evaluate the dissolution probability. Using PVP and PVDF as anchoring substrates, another stabilizing potential (∆PS-A) was also suggested as the criteria to evaluate the dissolution trend of polysulfides with the presence of additives. A novel additive, phosphorylated chitosan (PCS), was predicted. Computational details DFT calculations were carried out using the Vienna Ab Initio Simulation package (VASP). The spin-polarized generalized gradient approximation (GGA) coupled with the Perdew−Burke−Ernzerhof functional (PBE) functional was utilized to deal with the exchange-correlation effects30. The electrons-ion interactions were described by the projector augmented wave (PAW) method. The wave functions of the valence electrons were expanded in terms of a plane-wave basis set with a kinetic energy cutoff of 450 eV. The Brillouin zone integration was sampled using the Monkhorst-Pack scheme with 2×2×1 k-points mesh. A vacuum space of 15 Å was set to avoid interactions between polymer and its neighboring image. Population analysis was performed on the basis of the Bader charge division scheme31. The ionic degrees of freedom were relaxed using the conjugate-gradient (CG) algorithm, and the convergence of energy minimization was achieved when the maximum force component became smaller than 0.05 eV/Å. The cohesive energies of polysulfides are evaluated by calculating the clustering strength as defined by

∆ECE = [n * E (Li 2Sx ) - E ((Li 2Sx ) n )] / n ( x = 2, 3, 4, 6 and 8; n = 2 ~ 6)

(1)

Here, E(Li2Sx) and E((Li2Sx)n) represent the calculated electronic energies of the Li2Sx and (Li2Sx)n aggregates (n stands for the number of lithium polysulfides), respectively. The solvation energies in DME:DOL (v:v=1:1) medium are defined as follows

∆ESE = [ E (S1) + E (S2) + E (Li 2Sx )] − E (S1-Li 2Sx -S2)

(x =2, 3, 4, 6 and 8)

(2)

where S1 and S2 represent as either DOL or DME, and E(S1-Li2Sx-S2) is the calculated energy of the polysulfide molecule with the two involved solvent molecules. The anchoring strength of Li2S8 on the substrates are defined by

∆EAS = [ E (Sub) + E (Li 2S8 )] − E (Sub -Li 2S8 )

(3)

where E(Sub) and E(Sub-Li2S8) stands for the calculated energies of bare substrates and the Li2S8 molecule upon substrates, respectively. Higher values of ∆ECE , ∆ESE



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

and ∆EAS indicate stronger cohesion, solvation and anchoring strengths, respectively, and vice versa. All the energetics are obtained from the most stable structures, which is selected from at least 10 different configurations, to grantee a global minimum. Results and Discussion As dissolution occurs, Li2Sx monomer departs from its aggregates, followed by simultaneous solvation and dissociation to Li+ and Sx2-. Therefore, the dissolution is highly correlated to two interactions: the cohesion strength between Li2Sx monomers ( ∆ECE ), which determines the molecular dissolution of Li2Sx monomer from the aggregates; and the binding interaction between Li+ and Sx2-, which governs the dissociation of Li2Sx molecule itself. We first optimized the structures of the possible polysulfides (Li2Sx, x=2, 3, 4, 6 and 8) and their cluster aggregates ((Li2Sx)n, x=2, 3, 4, 6 and 8; n=1~6). Figure 1a displays the optimized configurations of (Li2S2)n (n=1~6) (other Li2Sx aggregates are depicted in Fig. S1). As the cluster size increases, the coordination number of both Li and S atoms gradually increases, forming several new Li-S bonds to stabilize the aggregates. As a result, the calculated cohesion energies rise from 1.239 eV to 1.730 eV with respect to cluster sizes before tetramer, after which ∆ECE becomes insensitive to aggregate sizes (Fig. 1b). Similar phenomena are also observed for other Li2Sx, indicating that the cohesion energy of (Li2Sx)n could be rationally evaluated with n=6 to minimize the computational efforts. For convenience, we use ∆ E CE of (Li2Sx)6 as the threshold in the following discussions. It is noted that ∆ E CE decays rapidly from 1.730 eV of Li2S2 to 0.790 eV of Li2S8. To further understand the interactions between Li+ and Sx2-, the Li-S distance distributions of (Li2Sx)6 clusters are projected in Fig. 1c. As x increases, the Li-S distance significantly elongates from 2.37 Å to 2.53 Å, indicating gradually weakened interaction between Li+ and Sx2-, which implies more facile dissolution of long chain polysulfides, in line with experimental observations32. Since the cohesion strength is much weaker than the binding strength (3.3~3.8 eV28), we can speculate that polysulfides prefer molecular dissolution rather than dissociation to cations and anions. Consequently, we consider only the molecular dissolution in our following discussions.


Page 4 of 17

Page 5 of 17

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


Figure 1. (a) Optimized configurations of (Li2S2)n (n=1~6); (b) the calculated cohesion energies of (Li2Sx)n (x=2, 3, 4, 6, 8; n=2~6); (c) Li-S distance distribution of (Li2Sx)6 (x=2, 3, 4, 6, 8). Apparently, the attraction between DME or DOL solvent molecules and Li2Sx is the competitor to dig polysulfide molecule from their aggregates, which determines whether dissolution occurs or not. According to our previous theoretical predictions28, the two Li+ ions in Li2Sx should be the most affinitive sites for the O atoms of both DME and DOL. Since only one solvent molecule is accessible for each Li atom, in combination with the steric effect originated from the large molecular size of the solvents, only two solvent molecules with different combinations are considered to minimize computational consumption. In fact, adding two more solvent molecules would result in negligible increase on the adsorption energy (< 0.04 eV), suggesting that the solvation effect could be rationally simulated with our model. Figure 2 shows the optimized adsorption structures of Li2S2 on three typical solvation environments, including two DME, two DOL as well as one DME and one DOL, as denoted as DME-DME, DOL-DOL and DME-DOL, respectively (see Fig. S2 for other Li2Sx). Strong attractions between the positively charged Li+ and the electron lone-pair of O atoms are observed with the calculated Li-O distances ranging from 1.892 Å to 1.930



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

Page 6 of 17

Å. We note that the calculated attraction interactions are approximately identical in all the three solvent environments with the calculated attraction energies fluctuating in a very narrow range between 1.337 eV and 1.392 eV (Table 1). The degeneracy of the attraction energies indicates that the solvent-polysulfide interaction relies only to the local interaction environment, virtually independent to the structure of solvent molecules. Similar interaction configurations and energetics were also obtained for other Li2Sx (Fig. S2 and Table 1). As expected, the highly localized Li-O interaction gives rise to negligible fluctuations of the calculated adsorption energies for all Li2Sx molecules. Since the three solvent environments coexist in electrolytes, the adsorption energies of Li2Sx on DME-DME, DOL-DOL and DME-DOL are averaged to represents the solvation energy of polysulfides ( ∆ESE ). Again, the solvation energies fall into a narrow range between 1.312 eV to 1.395 eV, suggesting that the solvent effect could be considered as a constant in studying the dissolution behavior. Table 1. Solvation energies of Li2Sx in three solvent environments. ∆E SE (eV)

Li2S2

Li2S3

Li2S4

Li2S6

Li2S8

DME-DME

1.392

1.369

1.312

1.356

1.365

DME-DOL

1.367

1.362

1.328

1.385

1.419

DOL-DOL

1.337

1.325

1.295

1.356

1.402

∆ ESE

1.365

1.352

1.312

1.366

1.395

Figure 2. Optimized adsorption configurations of Li2S2 in three solvent environments.


Page 7 of 17

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


We naturally define a competitive solvation potential ( ∆PS-C ) by directly comparing the cohesion strength and the solvation energy of Li2Sx monomer ( ∆ PS-C = ∆ESE − ∆ECE ) to evaluate the dissolution probability of polysulfides. As shown in Fig. 3, the calculated ∆PS-C values exhibit a monotonically increasing trend with respect to the size of polysulfide monomers. For Li2S8 monomer, the highest solvation potential of 0.605 eV reveals its facile

dissolution. As a

consequence,

the

strong

solvent-polysulfide attraction associate with weak cohesions are the driving forces for the readily dissolution of Li2S8, which leads to a Li2S8 molecule departure from cathode into electrolyte. In contrast, the comparable strength on the cohesion and solvation interactions of Li2S3 leads to a much smaller ∆PS-C value of 0.137 eV, suggesting the dissolution of short chain polysulfides could be much slower than long chain discharge products. In particular, Li2S2 monomer shows a negative solvation potential of -0.365 eV, which indicates the insoluble nature of the deep discharge products, agrees well with experimental observations33-36.

Figure 3. The solvation potential ( ∆PS-C ) of Li2Sx (x=2, 3, 4, 6, 8). Since Li2S8 monomer is the most soluble specie, fastening Li2S8 molecule in fact means that all polysulfide intermediates could be reserved in cathode. When using a certain substrate to stabilize Li2S8 molecule via anchoring, it is necessary to figure out the prerequisite electronic and structural features of the substrate. Here, we selected PVDF and PVP, where the formal one being identified to be “inert” for anchoring polysulfides and the latter one is an effective polysulfide preserver, as the probes to unravel the interaction mechanism between polysulfide and substrate. The optimized adsorption configurations of Li2S8 molecule on PVDF and PVP are depicted in Fig. 4.



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

Page 8 of 17

When Li2S8 approaches to a PVDF chain, one Li+ is attracted by two F atoms, establishing two Li-F interactions with the average distance of 2.275 Å. However, this attraction is relatively weak with the calculated adsorption strength of 0.568 eV, which is significantly lower than the solvation energy of Li2S8 monomer (1.395 eV). For PVP, the adsorption configuration of Li2S8 molecule is quite different to that of PVDF. Here, both two Li+ ions involve in the anchoring, constructing two strong Li-O interactions with two adjacent carbonyl groups on the chain. The calculated average Li-O distance of 1.890 Å is much shorter than the Li-F distance in PVDF. As a result, a significantly enhanced anchoring strength of 1.563 eV is achieved on PVP. It is worth noting that the interaction between Li2S8 molecule and PVP is even stronger than the solvation energy of Li2S8 monomer, which should be the determinative origin that makes PVP an appropriate bi-functional binder, as suggested by Cui et al24. Herein,

we

sequentially

define

a

new

competitive

stabilizing

potential

( ∆ PS-A =∆ESE -∆EAS ) to describe the dissolution probability of polysulfide with the presence of affinitive substrates. The negative ∆ PS-A of PVP (-0.168 eV) indicates that the most soluble Li2S8 molecule can be tightly bound on PVP, while the positive ∆ PS-A of PVDF (0.827 eV) suggests that the substrate contributes negligibly for

preserving polysulfides.

Figure 4. Optimized adsorption configurations of Li2S8 on PVDF and PVP. To understand the different anchoring strength of Li2S8 molecule on the two substrates, we further conducted Bader charge analysis for the anchored structures, as shown in Table S1. Negligible electron transfer was observed between Li2S8 and the binders, indicating the driving force for Li2S8 adsorption should be electrostatic attraction between Li+ and electron lone-pair of F or O atom. Therefore, the anchoring strength is highly correlated to the charge densities of the participating atoms. Functional groups with high electron density in substrates would intensively benefit polysulfide


Page 9 of 17

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


anchoring. For O atoms of PVP, an evident electron enrichment was observed with the calculated Bader charge of 1.083 e, twice of the value of F atoms of PVDF (0.495 e). The differential charge densities of PVDF and PVP are projected in Fig. S3. The s-p orbital hybridization results in electron enrichment on certain directions, where Li+ preferentially resides. For PVP, the polarization direction of electrons on O atoms matches well with Li+ of Li2S8 molecule. In contrast, the electron polarization on F atoms of PVDF is almost parallel to the polymer chain, leading to poor electrostatic attraction. Based on this understanding, it is not surprise to observe a negative ∆ PS-A of PVP. In fact, Li2S8 molecule could adopt another adsorption pattern on PVP with only one Li atom participating in the adsorption, as shown in Fig. S4. In this case, one strong Li-O interaction is established with the Li-O distance of 1.849 Å, slightly shorter than the case of dual Li+ adsorption. However, due to the absence of another Li-O attraction, the anchoring strength is significantly reduced to 1.066 eV. Therefore, rather than single affinitive interaction, an additive with more affinitive groups should benefit polysulfide anchoring. Accordingly, we could rationally propose that an effective third-party substrate should host two necessary properties: 1) contains affinitive sites with high electron density for polysulfide anchoring; 2) contains sufficient affinitive groups to provide multiple interactions for enhancing the stabilization. In light of this, we further investigated the anchoring behavior of Li2S8 on several other substrates, including polyethylene glycol (PEG), polyvinyl alcohol (PVA), polypyrrole (PPy), polyaniline (PANI) and phosphorylated chitosan (PCS). The optimized adsorption configurations and calculated stabilizing potentials ( ∆ PS-A ) are displayed in Fig. 5, where the ∆ PS-A of PVDF and PVP are also included. For PPy and PANI, the active sites for Li2S8 molecule anchoring are located around N atoms, which shows relatively high electron density. However, these active N atoms are somewhat embedded due to the strong conjugation effect of the polymer, leading to intensive steric hindrance for Li2S8 molecule approaching. Moreover, the electron delocalization would further reduce the electron density on N atoms. As a result, the anchoring strengths are relatively weak with the calculated adsorption energies of 0.672 eV and 0.948 eV for PPy and PANI (Table S2), respectively. For PEG and PVA, the O atoms are preferential sites for Li2S8 molecule. Although there is no conjugation effect on these two polymers, the spatial configuration of the affinitive groups gives



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

poor accessibility to polysulfides. Indeed, structural optimizations for at least 10 adsorption configurations show that the most stable anchoring structure allows only one Li+ participating in the adsorption. Comparing with PVP, the Li-O distances on PEG and PVA increase to 1.991 Å and 1.939 Å, respectively. Therefore, the anchoring strengths of Li2S8 monomer on PEG and PVA are significantly lowered with adsorption energy less than 0.75 eV (Table S2). For PCS, Li2S8 molecule adopts a similar adsorption configuration with PVP, where both Li+ ions of Li2S8 molecule are engaged in interaction. As we have shown previously28, the –P=O group is recognized as the most active adsorption site for Li+ due to its high electron density with the calculated average Bader charge of 1.387 e (Table S2), which gives rise to formation of two strong Li-O attractions. The high adsorption energy of 1.581 eV indicates that polysulfide dissolution could be largely mitigated by PCS. Figure 5b shows that the stabilizing potential ∆ PS-A of PVDF, PPy, PANI, PEG and PVA are all beyond zero, suggesting that they could not effectively stabilize polysulfides. In contrast, the negative and comparable ∆ PS-A values of PCS and PVP suggesting that these two substrates could, thermodynamically, preserve polysulfides and prevent them from being dissolved.

Figure 5. (a) The optimized adsorption configurations and (b) the calculated stabilizing potential ∆ PS-A of Li2S8 on various substrates. Finally, we select PVP and PCS to evaluate their anchoring efficiency, which is directly related to the adding dosage of these substances in cathode, by calculating the stabilizing potential as the function of Li2S8 loading in per polymer unit. Figure 6 shows that, as the Li2S8 loading increases, ∆ PS-A of PVP gradually approaches zero, indicating that the competition between the solvation of Li2S8 molecule and its anchoring on PVP becomes balanced. As the loading increases to 1, ∆ PS-A increases


Page 10 of 17

Page 11 of 17

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


to be slightly positive, suggesting that excessive Li2S8 molecules nearby the PVP chain would prefer being dissolved rather than being stabilized in cathode. Figure S5 shows that, when each PVP unit accommodates one Li2S8 (loading equals 1), the excessive Li2S8 approaching to the chain would be strongly repulsed due to the limited redundant spaces. However, the stabilizing potential of PCS almost levels out in the negative zone up to a Li2S8 loadings of 1. The degenerated ∆ PS-A is attributed to the large unit size of PCS, which lowers the repulsion between adjacent Li2S8 molecules (Fig. S6). Moreover, the flexibility of phosphate groups on PCS could further facilitate their accessibility for polysulfides. As a result, a PCS unit is capable of accommodating at least one Li2S8 molecule, leading to better anchoring efficiency than that of PVP.

Figure 6. The solvation potentials at various Li2S8 loadings of PVP and PCS.

Conclusions Constraining lithium polysulfides in sulfur cathodes via the affinitive interaction between polysulfides and various functional groups has been experimentally demonstrated to be capable of preventing polysulfide dissolution, which could mitigate the shuttle effect of Li/S batteries. In the present density functional theory study, we first defined a solvation potential, ∆PS-C , to evaluate the competitions between polysulfides self-cohesion and their solvation energies. It was found that

∆PS-C increases rapidly with respect to the size of polysulfide monomer, revealing the



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

nature that long chain discharge products are more soluble than short chain polysulfides. To evaluate the anchoring effect of third-party substrates, the competition of Li2S8 anchoring strength on different substrates and its solvation energy is described by a stabilizing potential ∆PS-A . The negative ∆PS-A of PVP (-0.168 eV) indicates that the most soluble Li2S8 can be tightly bound on PVP, while the positive ∆PS-A of PVDF (0.827 eV) suggests that the substrate contributes negligibly for preserving polysulfides. Two properties for a feasible substrate, containing affinitive sites with high electron density for anchoring polysulfides and containing sufficient affinitive sites to provide multiple interactions for enhancing the stabilization, are necessarily proposed. Based on these criteria, a fast screening to a series of polymers, including PEG, PPy, PVA, PANI and PCS, are conducted to identify the most effective substrate. Among the substrates, PCS is predicted to be a promising third-party substrate capable of preserving polysulfides in cathode and preventing them from being dissolved. The predicted substrate would be carefully verified by first-principles calculations and, ultimately, validated by experiments.

Acknowledgements This work was supported by the Fundamental Research Founds for National University, China University of Geosciences Wuhan (Innovative Team, Grant CUG120115; “Yaolan” plan, Grant CUGL150414 and CUGL140413), and the Natural Science Foundation of Hubei (2013CFB413). Support from the National Natural Science Foundation of China (No. 21203169) is also gratefully acknowledged.

Supporting Information Electronic Supplementary Information (ESI) available: Optimized structures, adsorption structures, differential charge densities, average charges of adsorption sites and charge transfer amounts; the coordinates of (Li2Sx)6 clusters.


Page 12 of 17

Page 13 of 17

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


References (1) Kim, J.; Lee, D.-J.; Jung, H.-G.; Sun, Y.-K.; Hassoun, J.; Scrosati, B. An Advanced Lithium-Sulfur Battery. Adv. Funct. Mater. 2013, 23, 1076-1080. (2) Manthiram, A.; Fu, Y. Z.; Su, Y. S. Challenges and Prospects of Lithium-Sulfur Batteries. Acc. Chem. Res. 2013, 46, 1125-1134. (3) Song, M.-K.; Cairns, E. J.; Zhang, Y. G. Lithium/Sulfur Batteries with High Specific Energy: Old Challenges and New Opportunities. Nanoscale 2013, 5, 2186-2204. (4) Wang, D.-W.; Zeng, Q. C.; Zhou, G. M.; Yin, L. C.; Li, F.; Cheng, H. M.; Gentle, I. R.; Lu, G. Q. M. Carbon–Sulfur Composites for Li–S Batteries Status and Prospects. J. Mater. Chem. A 2013, 1, 9382-9394. (5) Yang, Y.; Zheng, G. Y.; Cui, Y. Nanostructured Sulfur Cathodes. Chem. Soc. Rev. 2013, 42, 3018-3032. (6) Fedorková, A.; Oriňáková, R.; Čech, O.; Sedlaříková, M. New Composite Cathode Materials for Li/S Batteries: A Review. Int. J. Electrochem. Sci. 2013, 8, 10308-10319. (7) Zhou, Y.; Zhou, C. G.; Li, Q. Y.; Yan, C. J.; Han, B.; Xia, K. S.; Gao, Q.; Wu, J. P. Enabling Prominent High-Rate and Cycle Performances in One Lithium-Sulfur Battery: Designing Permselective Gateways for Li+ Transportation in Holey-CNT/S Cathodes. Adv. Mater. 2015, 27, 3774-3781. (8) Zheng, G. Y.; Yang, Y.; Cha, J. J.; Hong, S. S.; Cui, Y. Hollow Carbon Nanofiber-Encapsulated Sulfur Cathodes for High Specific Capacity Rechargeable Lithium Batteries. Nano Lett. 2011, 11, 4462-4467. (9) Deng, Z. F.; Zhang, Z. A.; Lai, Y. Q.; Liu, J.; Liu, Y. X.; Li, J. A Sulfur-Carbon Composite for Lithium/Sulfur Battery based on Activated Vapor-Grown Carbon Fiber. Solid State Ionics 2013, 238, 44-49. (10) Elazari,

R.;

Salitra,

C.;

Carsuch,

A.;

Panchenko,

A.;

Aurbach,

D.

Sulfur-Impregnated Activated Carbon Fiber Cloth as a Binder-Free Cathode for Rechargeable Li-S Batteries. Adv. Mater. 2011, 23, 5641-5644.



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

(11) Zhang, B.; Qin, X.; Li, G. R.; Gao, X. P. Enhancement of Long Stability of Sulfur Cathode by Encapsulating Sulfur Into Micropores of Carbon Spheres. Energy Environ. Sci. 2010, 3, 1531-1537. (12) Shin, E. S.; Kim, M.-S.; Cho, W. I.; Oh, S. H. Sulfur-Graphitic Hollow Carbon Sphere Nano-Composite as a Cathode Material for High-Power Lithium-Sulfur Battery. Nanoscale Res. Lett. 2013, 8, 343-351. (13) Li, Z.; Jiang, Y.; Yuan, L. X.; Yi, Z. Q.; Wu, C.; Liu, Y.; Strasser, P.; Huang, Y. H. A Highly Ordered Meso@Microporous Carbon-Supported Sulfur@Smaller Sulfur Core-Shell Structured Cathode for Li-S Batteries. ACS Nano 2014, 8, 9295-9303. (14) Xin, S.; Gu, L.; Zhao, N. H.; Yin, Y. X.; Zhou, L. J.; Guo, Y. G.; Wan, L. J. Smaller Sulfur Molecules Promise Better Lithium-Sulfur Batteries. J. Am. Chem. Soc 2012, 134, 18510-18513. (15) Li, Q. Y.; Zhou, C. G.; Ji, Z.; Han, B.; Feng, L.; Wu, J. P. High-Performance Lithium/Sulfur Batteries by Decorating CMK-3/S Cathodes with DNA. J. Mater. Chem. A 2015, 3, 7241-7247. (16) Lee, K. T.; Black, R.; Yim, T.; Ji, X. L.; Nazar, L. F. Surface-Initiated Growth of Thin Oxide Coatings for Li-Sulfur Battery Cathodes. Adv. Energy Mater. 2012, 2, 1490-1496. (17) Zheng, G. Y.; Zhang, Q. F.; Cha, J. J.; Yang, Y.; Li, W. Y.; Seh, Z. W.; Cui, Y. Amphiphilic Surface Modification of Hollow Carbon Nanofibers for Improved Cycle Life of Lithium Sulfur Batteries. Nano Lett. 2013, 13, 1265-1270. (18) Yang, Y.; Yu, G. H.; Cha, J. J.; Wu, H.; Vosgueritchian, M.; Yao, Y.; Bao, Z. N.; Cui, Y. Improving the Performance of Lithium-Sulfur Batteries by Conductive Polymer Coating. ACS Nano 2011, 5, 9187-9193. (19) Wang, M. J.; Wang, W. K.; Wang, A. B.; Yuan, K. G.; Miao, L. X.; Zhang, X. L.; Huang, Y. Q.; Yu, Z. B.; Qiu, J. Y. A Multi-Core-Shell Structured Composite Cathode Material with a Conductive Polymer Network for Li-S Batteries. Chem. Commun. 2013, 49, 10263-10265. (20) Liang, X.; Hart, C.; Pang, Q.; Garsuch, A.; Weiss, T.; Nazar, L. F. A Highly Efficient Polysulfide Mediator for Lithium-Sulfur Batteries. Nat. Commun. 2015, 6,


Page 14 of 17

Page 15 of 17

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


5682-5688. (21) Ji, X. L.; Evers, S.; Black, R.; Nazar, L. F. Stabilizing Lithium-Sulphur Cathodes Using Polysulphide Reservoirs. Nat. Commun. 2011, 2, 325-332. (22) Ji, X. L.; Lee., K. T.; Nazar, L. F. A Highly Ordered Nanostructured Carbon– Sulphur Cathode for Lithium-Sulfur Batteries. Nat. Mater. 2009, 8, 500-506. (23) Li, W. Y.; Zhang, Q. F.; Zheng, G. Y.; Seh, Z. W.; Yao, H. B.; Cui, Y. Understanding the Role of Different Conductive Polymers in Improving the Nanostructured Sulfur Cathode Performance. Nano Lett. 2013, 13, 5534-5540. (24) Seh, Z. W.; Zhang, Q. F.; Li, W. Y.; Zheng, G. Y.; Yao, H. B.; Cui, Y. Stable Cycling of Lithium Sulfide Cathodes through Strong Affinity with a Bifunctional Binder. Chem. Sci. 2013, 4, 3673-3677. (25) Wang, Z. G.; Niu, X. Y.; Xiao, J.; Wang, C. M.; Liu, J.; Gao, F. First Principles Prediction of Nitrogen-Doped Carbon Nanotubes as a High-Performance Cathode for Li–S Batteries. RSC Adv. 2013, 3, 16775-16780. (26) Seh, Z. W.; Wang, H. T.; Hsu, P.-C.; Zhang, Q. F.; Li, W. Y.; Zheng, G. Y.; Yao, H. B.; Cui, Y. Facile Synthesis of Li2S–Polypyrrole Composite Structures for High-Performance Li2S Cathodes. Energy Environ. Sci. 2014, 7, 672-676. (27) Wang, B.; Alhassan, S. M.; Pantelides, S. T. Formation of Large Polysulfide Complexes during the Lithium-Sulfur Battery Discharge. Phys. Rev. Appl. 2014, 2, 0340041-0340047. (28) Ji, Z.; Han, B.; Li, Q. Y.; Zhou, C. G.; Gao, Q.; Xia, K. S.; Wu, J. P. Anchoring Lithium Polysulfides via Affinitive Interactions: Electrostatic Attraction, Hydrogen Bonding, or in Parallel? J. Phys. Chem. C 2015, 119, 20495-20502. (29) Vijayakumar, M.; Govind, N.; Walter, E.; Burton, S. D.; Shukla, A.; Devaraj, A.; Xiao, J.; Liu, J.; Wang, C. M.; Karim, A.; Thevuthasan, S. Molecular Structure and Stability of Dissolved Lithium Polysulfide Species. Phys. Chem. Chem. Phys. 2014, 16, 10923-10932. (30) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B



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

1992, 46, 6671-6687. (31) Tang, W.; Sanville, E.; Henkelman, G. A Grid-Based Bader Analysis Algorithm Without Lattice Bias. J. Phys.: Condens. Matter 2009, 21, 0842041-0842047. (32) Hagen, M.; Schiffels, P.; Hammer, M.; Dörfler, S.; Tübke, J.; Hoffmann, M. J.; Althues, H.; Kaskel, S. In-Situ Raman Investigation of Polysulfide Formation in Li-S Cells. J. Electrochem. Soc. 2013, 160, A1205-A1214. (33) Jung, Y. J.; Kim, S.; Kim, B.-S.; Han, D.-H.; Park, S.-M.; Kwak, J. Effect of Organic Solvents and Electrode Materials on Electrochemical Reduction of Sulfur. Int. J. Electrochem. Sci. 2008, 3, 566-577. (34) Levillain, E.; Gaillard, F.; Leghie, P.; Demortier, A.; Lelieur, J. P. On the Understanding of the Reduction of Sulfur (S8) in Dimethylformamide (DMF). J. Electroanal. Chem. 1997, 420, 167-177. (35) Leghié, P.; Lelieur, J.-P.; Levillain, E. Comments on the Mechanism of the Electrochemical Reduction of Sulphur in Dimethylformamide. Electrochem. Commun. 2002, 4, 406-411. (36)Gorlin, Y.; Patel, M. U. M.; Freiberg, A.; He, Q.; Piana, M.; Tromp, M.; Gasteiger, H. A. Understanding the Charging Mechanism of Lithium-Sulfur Batteries Using Spatially Resolved Operando X-Ray Absorption Spectroscopy. J. Electrochem. Soc. 2016, 163, A930-A939.


Page 16 of 17

Page 17 of 17

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


Table of Content


Computational Criteria for Evaluating Polysulfide Cohesion, Solvation

Recommend Documents