Lithium Bond Impact on Lithium Polysulfide Adsorption with

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C: Energy Conversion and Storage; Energy and Charge Transport

Lithium Bond Impact on Lithium Polysulfide Adsorption with Functionalized Carbon Fiber Paper Interlayer for Lithium-Sulfur Batteries Thana Maihom, Siriroong Kaewruang, Nutthaphon Phattharasupakun, Poramane Chiochan, Jumras Limtrakul, and Montree Sawangphruk J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09392 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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The Journal of Physical Chemistry

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Lithium Bond Impact on Lithium Polysulfide Adsorption with Functionalized Carbon Fiber Paper Interlayer for Lithium-Sulfur Batteries Thana Maihoma,b*, Siriroong Kaewruanga, Nutthaphon Phattharasupakuna, Poramane Chiochana, Jumras Limtrakulc and Montree Sawangphruka* a

Department of Chemical and Biomolecular Engineering,

School of Energy Science and

Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong, 21210, Thailand b

Department of Chemistry,

Faculty of Liberal Arts and Science,

Kasetsart University,

Kamphaengsaen Campus, Nakhon Pathom, 73140, Thailand c

Department of Materials Science and Engineering,

School of Molecular Science and

Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong, 21210, Thailand

*Corresponding author Tel : +66(0)33-01-4251 Fax : + 66(0)33-01-4445. E-mail address : [email protected] (T. Maihom) *Corresponding author. Tel : +66(0)33-01-4251 Fax : + 66(0)33-01-4445. E-mail address : [email protected] (M. Sawangphruk).

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ABSTRACT Lithium-sulfur batteries (LSBs) are of great interest as a promising energy storage device due to their high theoretical capacity and energy density. However, they exhibit poor discharge capacity and capacity retention during long-term cycling due to its inherent drawbacks including the poor conductivity of sulfur and lithium sulfide, the shuttle effect of lithium polysulfides (LiPSs), and the large volume expansion of sulfur to lithium sulfide. An effective approach that can solve these problems is to use an interlayer inserted between the separator and the cathode. Nevertheless, the underlying adsorption mechanism of LiPSs on the interlayer has not yet been widely investigated. Herein, the effect of lithium bond chemistry on the adsorption of LiPSs on the functionalized carbon fiber paper (CFP) interlayer containing hydroxyl, carboxyl, or amide functional groups is investigated by a density functional theory (DFT) approach. It is found that the functionalized-CFP exhibits strong lithium bond interaction between the Li electron-acceptor of LiPSs and the N or O electron-donor of the functionalized CFP interlayer. In addition, the correlation between the adsorption energy of LiPSs on the interlayer and the electrochemical performance of LSBs is investigated. The results provide the fundamental understanding of the structure-property relationship for the adsorption of LiPSs on the functional groups of the interlayer, which will be beneficial for the further development of advanced LSBs.


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1. INTRODUCTION Lithium-Sulfur batteries (LSBs) are one of the promising next generation energy storage devices due to their high theoretical capacity (1675 mAh g-1) and energy density (2600 Wh kg-1 and 2800 Wh L-1).1-3 LSBs are suitable for uses in many advanced energy applications such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and grid scale energy storage since the conventional lithium-ion batteries cannot meet these ever-increasing requirements due to their low

capacity

(ca. 300

mAh

g-1). 3 In addition,

environmentally friendly, and inexpensive.4 Although

the sulfur active material is abundant, the

LSBs

have

many

promising

advantages, their practical commercial applications are still hindered by several drawbacks. First, the insulating nature of sulfur (5x10-30 S cm-1 at 25oC) and its final product lithium sulfide, Li2S (10-13 S cm-1 at 25oC) leads to a low utilization of active materials.5-6 Second, the dissolution of intermediate lithium polysulfide species (LiPSs) into an organic electrolyte can cause an irreversible consumption of active materials leading to a poor discharge capacity over long cycling. During discharging, the highly soluble long-chain LiPSs can diffuse through the polymer separator to the Li anode, get reduced forming short-chain LiPSs, and then diffuse back to the cathode side getting oxidized and forming long-chain LiPSs. This cycle process is socalled the “shuttle mechanism effect”. Third, once the insulating Li2S film covers the entire cathode surface, it can eventually terminate the discharge reaction of LSBs.5 Lastly, the volume expansion of sulfur during cycling, which is ca. 80% upon full lithiation to Li2S, can cause the pulverization of material leading to the structural destruction and rapid capacity decay of LSBs.7 To overcome these issues, several strategies including the modification of electrolyte, separator, and cathode have been previously proposed.8-10 However, it cannot yet solve the drawbacks of LSBs. Among many approaches, encapsulating the sulfur active material with carbon-based materials such as carbon nanotubes,

nanofibers,

heteroatom-doped

carbons,

graphene, and activated carbon is one of the best approaches, which can increase the overall performance of LSBs because the carbon materials have high electrical conductivity, ability to accommodate the volume expansion of sulfur to lithium sulfide, and ability to suppress the shuttle mechanism effect of LiPSs via physisorption and chemisorption processes.11-13 However, the sulfur/carbon composites cannot effectively limit the dissolution of LiPSs during long-term cycling due to its open porous structure.



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Alternatively, a conductive interlayer inserted between the polymer separator and the cathode electrode has been proved to serve as the electron

conducting layer suppressing the

migrating LiPSs leading to the improvement of the electrochemical performance of LSBs.14-15 The carbon fiber paper (CFP) interlayer with surface modification has been recently found to be one of the promising interlayers since it has high flexibility, light weight, good electrochemical stability, good electrical conductivity, and tunable functional groups on its surface such as hydroxyl and carboxylic groups adsorbing the liquid LiPSs.14,

16

Recently, we experimentally

demonstrated the successful utilization of the functionalized CFP containing hydroxyl, carboxyl, and amide groups as one of the most effective interlayers.17 The LSBs using the functionalized CFP

interlayer exhibits an outstanding discharge capacity with excellent capacity retention and

low degradation rate over long-term cycling. However, the experiment investigation itself cannot fundamentally provide the full understanding on the excellent performance of the as-fabricated LSBs. As a result, to fully understand the chemical interaction of LiPSs on the functionalized CFP at the molecular level, theoretical calculations based on quantum chemistry are used in this work. The strong chemical interaction of LiPSs on the functionalized CFP via heteroatoms such as oxygen and nitrogen is theoretically interpreted as the electrostatic attraction of the “lithium bond” considered as a dipole-dipole interaction between LiPSs and the lone pair electrons of those heteroatoms.18-22 Note, the lithium bond chemistry in this work has been inspired by the concept of the hydrogen bond. To the best of our knowledge, there were a few previous reports relating to the LiPS adsorption on the interlayer materials17,23 but the lithium bond interaction of LiPSs on the CFP and the modified one has not yet been investigated. Herein, the quantum chemical calculation is used to study the lithium bond interaction of LiPSs on a few interlayers including the CFP, the treated-CFP (t-CFP), and the ethylenediaminemodified t-CFP (t-CFP-EDA). In addition, our calculation results can explore the correlation between the lithium bond effect and the interaction energy leading to a simple and practical concept for the development of LSBs. The concept is based on the correlation between the LiPS adsorption energy via the lithium bond interaction and the dipole moment of the CFP materials. It can affect the electrochemical performance of the LSB cells fabricated by using CFP, t-CFP, and t-CFP-EDA as the interlayer, which can provide the fundamental understanding of the LiPS adsorption.


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2. COMPUTATIONAL METHODS All calculations were performed using the M06-2X density functional and the 6-31G (d, p) basis set as implemented in Gaussian 09 code. 24 Previous studies of the adsorption and mechanism of several organic reactions have also

verified the M06-2X method usability. 25-26

During the geometry optimization, the LiPS molecules and the entire CFP model structures were allowed to relax. The calculated binding energy (∆E) of LiPSs (Li2Sn; n = 4, 6, and 8) on the CFP interlayer was defined as a following equation (1): ∆E = E(Li2Sn-CFP) - E(Li2Sn) - E(CFP)

(1)

where E(Li2Sn-CFP), E(Li2Sn), and E(CFP) are the total energies of the adsorbed Li2Sn on CFP, the isolated Li2Sn,

and the CFP interlayer,

respectively. Basis set superposition error (BSSE)

corrections were also estimated by the counterpoise (CP) method27 to obtain more reliable interaction energies. In addition, the vibrational frequencies of all structures were also calculated at the same level of theory to verify the local minimum nature of all stationary points. The natural bond orbital (NBO) method was also used to analyze the charge transfer between LiPSs and CFP interlayers. 28 The charge transfer from the donor-acceptor interaction was estimated as a following equation (2): ∆ρ = |ρCFP-LiPS - ρLiPS|

(2)

where ρCFP-LiPS and ρLiPS are the net charge of the adsorbed LiPSs on the CFP interlayer and bare LiPSs, respectively.

3. RESULTS AND DISCUSSION 3.1 Structure and energetics of t-CFP, t-CFP-EDA, and LiPSs The models of the CFP, t-CFP, and t-CFP-EDA are displayed in Figure 1. For the unmodified CFP model, a single graphene-like carbon layer composing of 24 C atoms and 12 H atoms is used as a basic structure as reported in a previous work.29 The average C-C bond length is 1.42 Å with the dihedral angle of either 0˚ or 180˚. A zero dipole moment is also observed for this model and the NBO analysis shows small negative charges accumulated on its surface. In case of the t-CFP models, the hydroxyl (t-CFP-OH) and the carboxyl (t-CFP-COOH) groups are functionalized at the edge site of the CFP layer as shown in Figure 1a and 1b, respectively. The bond length of C-O(H) and C-C(OOH) in the t-CFP-OH and t-CFP-COOH models are calculated to be 1.36 and 1.49 Å, respectively. The t-CFP-OH model provides the stretching vibration of O-



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H and C-O groups at the wavenumber of 3899 and 1313 cm-1, respectively. Whilst, the calculated stretching vibration of C=O group in the t-CFP-COOH is 1874 cm-1. These vibrational modes are comparable to the corresponding vibrational frequencies of the FTIR spectra of the functionalized carbon materials in other previous experimental studies.30-32 The obtained dipole moments for t-CFP-OH and t-CFP-COOH are 1.214 and 1.996 Debye, respectively. The natural population analysis (NPA) also shows that the negative charges on the O atom of -OH and COOH are -0.711e and -0.620e, respectively. The results indicate that the O sites on the t-CFPs can serve as the active site for adsorbing LiPSs via the lithium bond interaction. For the EDA functionalization, the diamine molecules can easily react with the -OH or COOH on the edge of CFP via the dehydration reaction leading to the formation of diaminefunctionalized CFP (t-CFP-EDA), including CFP-NHCH2CH2NH2 (t-CFP-EDA-1) and CFPCONHCH2CH2NH2 (t-CFP-EDA-2) as shown in Figures 1c and 1d, respectively.30 The calculated C-NH and CO-NH stretching modes of the t-CFP-EDA-1 and t-CFP-EDA-2 are at 1525 and 1567 cm-1, respectively. Three vibrational modes of bending, symmetrical and asymmetrical stretching for the terminal NH2 group are found at 1670, 3539, and 3638 cm-1 for tCFP-EDA-1 as well as at 1668, 3532, and 3584 cm-1 for t-CFP-EDA-2, respectively. For the tCFP-EDA-2, the C=O stretching vibration is at 1805 cm-1, which is red-shift by 69 cm-1 with respect to the corresponding vibration in the t-CFP-COOH model due to the substitution of ethylene diamine molecule. These results are in good agreement with previous experimental results investigated by FTIR measurement.30 The obtained dipole moments of t-CFP-EDA-1 and t-CFP-EDA-2 are 4.225 and 3.299 Debye, respectively. Furthermore, the charge distributions were examined using NPA analysis which was found that most of the negative charges are accumulated on the N and O atoms of the EDA chains. The N1 and N2 atoms of the t-CFP-EDA1 carry the negative charges of -0.694 and -0.927e, respectively. In case of the t-CFP-EDA-2, the negative charges of -0.637 and -0.925e are found for N and O atoms, respectively, indicating their lone-pair electrons, which can form Li bonds with LiPSs.


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(a)

(b)

(d)

(c)

(e)

Figure 1. Different model structures of the interlayers; (a) CFP, (b) t-CFP-OH, (c) t-CFPCOOH, (d) t-CFP-EDA-1, and (e) t-CFP-EDA-2. Distances are in Å. The soluble LiPSs i.e., Li2S4, Li2S6, and Li2S8 are considered in this study due to their highly soluble property in organic electrolyte leading to a large capacity fading during cycling. The optimized structures of these LiPSs are shown in Figure 2. Selected geometrical parameters are listed in Table S1 including bond lengths between Li-Li, Li-S, and S-S atoms in Li2Sn molecules. When the number of S atoms increases, the Li-Li bond length is shorter due to the increasing number of electron transfers from Li to sulfur atoms. From this reason, the charge summation on Li2 atoms is reduced from Li2S4 by 0.053 and 0.057e for Li2S6 and Li2S8, respectively. Note

that

the LiPSs structures and their evolutions are in good agreement with

previous theoretical calculations.33



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(a)

(b)

(c)

Figure 2. The optimized structures of (a) Li2S4, (b) Li2S6, and (c) Li2S8 molecules. 3.2 Lithium bond interaction of soluble LiPSs on the CFP interlayer The adsorption of LiPSs on the unmodified CFP interlayer was firstly investigated as shown in Figure S1. The LiPSs can interact with the CFP surface via Li+- π-electron interaction which is consistent with the previous report of the adsorption of LiPSs on graphene.33-34 In this interaction,

the electrons on the surface of the CFP are induced by the Li ion of LiPSs causing

the increase in their negative charges. Owing

to the small dipole moment of the CFP (0.122

Debye), the interaction between Li and the CFP is based on the dipole-induced dipole interaction which is not classified as the lithium bond interaction. The calculated binding energies for Li2S4, Li2S6, and Li2S8 on the CFP are -13.3, -13.7, and -16.8 kcal mol-1, respectively. The interaction complexes of LiPSs on both t-CFP-OH and t-CFP-COOH models are shown in Figures 3 and 4, respectively. The LiPSs adsorb on t-CFP via lithium bond between Li and lone pair electron of the O atom of the -OH and -COOH functional groups. For t-CFP-OH, the adsorption affects both the structures of the t-CFP-OH and LiPSs by increasing the C-O bond length of t-CFP-OH and the Li-Li and Li-S bond distances of LiPSs as compared to their original structures. The intermolecular distances of Li•••O are 1.94, 1.98, and 2.01 Å for Li2S4, Li2S6, and Li2S8, respectively. Figure 3d shows the overlap between the sp-orbital of Li atom in LiPSs and the corresponding orbital of O atom with mainly sp-character. The electron density is donated from O to LiPSs resulting in the increasing negative charge of LiPSs in the range of -0.060 to 0.089e. For the t-CFP-COOH,

the interaction is found to be the bridge-bi-dentate structure,

forming one lithium bond between Li and O and one hydrogen bond between S and H atom as shown in Figure 4. The increase in C=O and O-H bond lengths in t-CFP-COOH and the Li-Li and Li-S bond distances of LiPSs can also be observed. According to the NBO analysis, the first intermolecular bond originally forms between the sp-orbital of S atom of LiPSs and the s-orbital


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of H site. The second

bond origins from the sp-orbital of the Li atom of the LiPSs and the O

atom in the COOH group of the t-CFP-COOH ( Figure 4d) . The adsorbed LiPSs contain the negative charge in the range of -0.050 to -0.055e which is lower than that of the t-CFP-OH since there is the electron density back-donated mode from the

S atom of LiPSs into the H of the t-

CFP-COOH. The calculated interaction energies of the Li2S4, Li2S6, and Li2S8 are -16.4, -18.0, and -19.2 kcal mol-1 for the t-CFP-OH as well as -24.6, -23.0, and -24.5 kcal mol-1 for the t-CFPCOOH, respectively. Therefore, the -COOH group on the t-CFP is preferable to the -OH group for adsorbing LiPSs.

(a) Li2S4

(b) Li2S6

(c) Li2S8

(d) Interacting orbitals

Figure 3. The optimized structures of LiPS adsorption complexes on t-CFP-OH and its interacting orbitals (distances are in Å).



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(a) Li2S4

(b) Li2S6

(c) Li2S8


Figure 4. The optimized structures of LiPS adsorption complexes on t-CFP-COOH and its interacting orbitals (distances are in Å). For the t-CFP-EDA system, active sites of

the LiPS species are found to be able to interact with the

t-CFP-EDA via two lithium bonds,

bidentate chelating and bidentate bridging

conformations as shown in Figures 5-8. For the t-CFP-EDA-1,

the bidentate chelating mode

presents the interaction between one Li atom of LiPSs and both N atoms of the t-CFP-EDA-1 (see Figure 5). The distances for Li1•••N1 and Li1•••N2 are almost equal in all LiPSs. The NBO analysis in Figure 5d shows the overlapping orbital from the 2sp-orbital of N atoms (N1 and N2) and the sp-orbital of Li atom. The electron density is transferred from the t-CFP-EDA-1 to LiPSs leading to the contained negative charge on the LiPSs. The interaction energies are calculated to be -34.1, -34.5, and -37.6 kcal mol-1 for Li2S4, Li2S6, and Li2S8, respectively. In the bidentate bridging structure of the t-CFP-EDA-1 (Figure 6), the binuclear Li atoms of the LiPSs interact at both N sites with nearly identical intermolecular distances of the Li1•••N1 and Li2•••N2. The


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overlapping orbitals are between the sp-orbital of N atoms in t-CFP-EDA-1 and the matching orbital of Li with the sp-character. The calculated binding energies are -31.3,

-31.0, and -30.1

kcal mol-1 for Li2S4, Li2S6, and Li2S8, respectively, which are lower than that of the chelating mode. The results indicate that the bidentate chelating is preferable over the bidentate bridging mode for the LiPS interaction on the t-CFP-EDA-1.

(a) Li2S4

(b) Li2S6

(c) Li2S8


Figure 5. The optimized structures of Li2Sn adsorption complexes on t-CFP-EDA-1 in bidentate chelating and its interacting orbitals (distances are in Å).



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(a) Li2S4

(b) Li2S6

(c) Li2S8


Figure 6. The optimized structures of the Li2Sn adsorption complexes on t-CFP-EDA-1 in bidentate bridging and its interacting orbitals (distances are in Å). For the t-CFP-EDA-2, the adsorption structure of LiPSs in both bidentate chelating and bidentate bridging modes is very similar to that

of t-CFP-EDA-1 as discussed above. In the

bidentate chelating, the Li atom of LiPSs interacts via two lithium bonds with N and O of the tCFP-EDA-2 as shown in Figure 7. The C=O and C-N bonds of the t-CFP-EDA-2 as well as the Li-Li bond of the LiPSs are lengthened due to this interaction. The NBO analysis shows that the bonding between O•••Li and N•••Li occurs through the dominated orbital of the N, O, and Li spatomic orbitals ( Figure 7d) . The adsorbed LiPSs are contained with the negative charge by receiving the electron density from the adsorbates. The adsorption energies of this configuration are -33.2, -33.1, and -37.7 kcal mol-1 for Li2S4, Li2S6, and Li2S8, respectively. For the bridging configuration, two Li atoms of LiPSs interact with bridging linkage at N and O sites (Figure 8). According to the NBO analysis, the O•••Li and N•••Li interactions originally formed between the

sp-orbital of O and N of t-CFP-EDA-2 and the sp-orbital of Li atoms (Figure 8d). The

interaction energies

are -36.4, -34.7, and -32.2 kcal mol-1 for the Li2S4, Li2S6, and Li2S8


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adsorption, respectively. It can be clearly seen that the interaction energies of the Li2S4 and Li2S6 in the bridging configuration are more stable than the chelating one. In contrast, the chelating is more stable than bridging configuration for Li2S8. The reason might be due to the hydrogen bond stabilization between S of LiPSs and H of the t-CFP-EDA-2. In addition, the S atom of the LiPSs can slightly bind to the CFP sheet. The result implies that the Li2S4 and Li2S6 prefer to adsorb on the interlayer via the bridging configuration while the highest soluble Li2S8 interacts via the chelating one.

(a) Li2S4

(b) Li2S6

(c) Li2S8


Figure 7. The optimized structures of the Li2Sn adsorption complexes on t-CFP-EDA-2 in bidentate chelating and its interacting orbitals (distances are in Å).



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(a) Li2S4

(b) Li2S6

(c) Li2S8


Figure 8. The optimized structures of the Li2Sn adsorption complexes on t-CFP-EDA-2 in bidentate bridging and its interacting orbitals (distances are in Å). The calculated adsorption energies relative to the LiPSs and the t-CFP materials with M06-2X functional and counterpoise BSSE corrections are summarized in Figure 9. It can be clearly seen that the adsorption of all LiPSs on the t-CFP is more stable than on the unmodified CFP. This is due to the formation of one lithium bond found in the t-CFP cases. The results demonstrate the improvement of CFP for anchoring the LiPSs by the functional groups obtained from the surface modification of CFP. Moreover, with the presence of two lithium bonds for the LiPSs adsorbed on the t-CFP-EDA materials, the adsorption energies of LiPS species are higher than that of one lithium bond in the t-CFP and non-lithium bond in the unmodified CFP. These findings indicate that the lithium bond plays an important role in the adsorption of LiPSs on the carbon-based materials for which the more lithium bonds lead to the higher stable adsorption


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energies.In addition, because the adsorption of LiPSs is considered as the dipole-dipole interaction, we hypothesize that the adsorption energy would be related to the dipole moment of the CFP interlayer. Figure 10 shows the adsorption energy of LiPSs against the dipole moment of CFP materials. It was found that the adsorption energy correlates to the dipole moment of the CFP materials. The adsorption energy increases when the dipole moment increases. As a result, the dipole moment could therefore be considered as an important factor for selecting the effective interlayer of LSBs.

Figure 9. Comparison of the adsorption energies of the Li2Sn species on CFP, t-CFP, and t-CFPEDA. (a) and (b) denote as the bidentate chelating and bidentate bridging adsorption structures, respectively.



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(a)

(b)

(c) Figure 10. Correlation between LiPSs adsorption energy and dipole moment of CFP materials; (a) Li2S4, (b) Li2S6, and (c) Li2S8. To explain the effect of the lithium bond formation on the electrochemical performance, the LSBs were experimentally studied by fabricating the coin cells with different CFP interlayers by following our recent report elsewhere (see Figure 11).17 By using t-CFP-EDA, two lithium bonds formed lead to a high initial discharge capacity of ca. 1800 mAh g-1 at a current density of 0.1C, which is 8% and 16% higher than that of the t-CFP and CFP, respectively. Moreover, the cycling performance of the LSB

cell using

the t-CFP-EDA

interlayer exhibits a capacity

retention of 91.53% with a low cycling decay rate of 0.046% after 200 cycles at a current density of 1C. Whilst, the LSB cells using t-CFP and CFP interlayers provide a capacity retention of 75.62% and 65.05%, respectively. The charge/discharge voltage profiles of the LSBs with and without t-CFP-EDA interlayer at various current densities from 0.1C to 1C in the potential range of 1.6 to 3.0 V are


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shown in Figure S2 of the Supporting Information. It can be seen that two clearly discharge plateaus are observed even at a high current density of 1C corresponding to the transformation of cyclic sulfur (S8) to high-order lithium polysulfides (Li2Sx, 4 ≤ n ≤8) and the conversion to loworder Li2S2/Li2S. The reverse reaction presented in the charge curve shows two plateaus which can be attributed to the backward reaction from Li2S2/Li2S to Li2S4 and finally to Li2S8/S. The results are consistent with the CV curve described in our recent report elsewhere.17 The LSBs containing the t-CFP-EDA interlayer shows more stable at the upper discharge plateaus and relatively lower polarization (∆E) as compared to LSBs without interlayer at various current densities as shown in Figure S2. This result indicates that the t-CFP-EDA interlayer can promote the redox reaction kinetics and reduce polysulfide migration leading to high reversibility of LSBs. Furthermore, the rate capabilities of pure sulfur cathodes at the high sulfur loading content of ca. 6.5 mg cm-2 (10.01 mg per cell) with and without interlayer at different current densities from 0.1-1.0C show that the LSB with the t-CFP-EDA interlayer outperforms others (see Figure S3). However, some polysulfides could be still possible to diffuse through the interlayer. The further improvement is therefore still needed in order to make the LSB more stable. The correlation between the electrochemical performances with the quantum chemical calculation results described above is also shown in Figures S4 and S5. A good linear relationship between the initial discharge capacity and the adsorption energy of LiPSs and the dipole moment of CFP materials was also shown in Figure S4. The interlayer with greater LiPS adsorption energy and dipole moment leads

to higher initial discharge capacity. The same

correlations are also found for the capacity retention (Figure S5). The interlayer with higher adsorption energy and dipole moment exhibits higher capacity retention after long-term cycling. However, the dipole moment is actually problematic to control in the experimental study. It can be zero for adsorbent materials with a highly symmetric structures. Therefore, the adsorption energy of LiPSs contributed by the lithium bond formation of the CFP materials could be only considered as a good quantum chemical descriptor for advanced LSBs.



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Figure 11. Electrochemical performances of LSBs: (a) the rate capabilities of the sulfur-loaded activated carbon (S@AC) at the sulfur loading content of ca. 2.5 mg cm-2 (3.85 mg per cell) with and without interlayer at different current densities from 0.1-1.0C, (b) the cycling performance of LSBs with 100% Coulombic efficiency for 200 cycles. 4. CONCLUSIONS The lithium bond interaction between the LiPSs and the functionalized CFP interlayer on the electrochemical performance of LSBs has been investigated using the DFT approach. Strong lithium bonds between the Li atom of LiPSs and the heteroatoms (N and O) of the functional groups of t-CFP and t-CFP-EDA interlayers can be observed leading to the suppression of the shuttle mechanism effect. The binding energies of LPSs on t-CFP-OH, t-CFP-COOH and t-CFPEDA are much higher than those on the unmodified CFP. Among all functionalized samples, the


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EDA-modified CFP outperforms others. The t-CFP shows less lithium bond interaction and the unmodified CFP exhibits only the dipole-induced dipole interaction. In addition, we have also found that there is an approximately linear relationship between “the initial discharge capacity and the capacity retention” and “the adsorption energy”. The adsorption energy could therefore be considered as the quantum chemical descriptor for LSB technologies. ASSOCIATED CONTENT Supporting information Table S1 Bond lengths of Li-Li, Li-S, and S-S atoms in Li2Sn molecules (unit in Å). Figure S1 The optimized structures of the interaction between Li2Sn species and the CFP model. Figure S2 Charge/discharge voltage profiles of the sulfur-loaded activated carbon (S@AC) without (a) and with t-CFP-EDA interlayer (b) at different current densities within a potential range of 1.6 to 3.0 V vs Li+/Li. Figure S3 Electrochemical performances of LSBs: The rate capabilities of sulfur cathodes at the high sulfur loading content of ca. 6.5 mg cm-2 (10.01 mg per cell) with and without interlayers at different current densities from 0.1-1.0C.Figure S4 Correlation between the initial discharge capacity and (a) Li2S8 adsorption energy obtained from the preferred adsorption modes and (b) dipole moment of CFP materials. Figure S5 Correlation between the capacity retention after tested for 200 cycles at 1C and (a) Li2S8 adsorption energy obtained from the preferred adsorption modes and (b) dipole moment of CFP materials. This material is available free of charge via the Internet at http://pubs.acs.org.

Author information *Corresponding author Tel : +66(0)33-01-4251 Fax : + 66(0)33-01-4445. E-mail address : [email protected] (T. Maihom) *Corresponding author. Tel : +66(0)33-01-4251 Fax : + 66(0)33-01-4445. E-mail address : [email protected] (M. Sawangphruk).

Contributions



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M.S. directed the lithium sulfur battery research with interlayer, T. M. and M.S. conceived and designed this work and wrote the paper; S.K., N. P., and P.C. carried out the experiment, T. M. and J. L. performed computational calculation. All authors participated in the analysis and discussion of the results. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was financially supported by the Thailand Research Fund, Vidyasirimedhi Institute of Science and Technology (RSA5880043) and the Postdoctoral Fellowship from Vidyasirimedhi Institute of Science and Technology (VISTEC).

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Lithium Bond Impact on Lithium Polysulfide Adsorption with

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