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Mechanism on the Improved Performance of Lithium Sulfur Batteries with MXene-based Additives Dewei Rao, Lingyan Zhang, Yun-hui Wang, Zhaoshun Meng, Xinye Qian, Jiehua Liu, Xiangqian Shen, Guanjun Qiao, and Rui-Feng Lu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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The Journal of Physical Chemistry

Mechanism on the Improved Performance of Lithium Sulfur Batteries with MXene-Based Additives Dewei Rao,†,║ Lingyan Zhang,‡,║ Yunhui Wang,‡ Zhaoshun Meng,‡ Xinye Qian,† Jiehua Liu,§ Xiangqian Shen,*,† Guanjun Qiao,† and Ruifeng Lu*,‡ †

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013,

People’s Republic of China. ‡

Department of Applied Physics, Nanjing University of Science and Technology,

Nanjing 210094, People’s Republic of China. §

Future Energy Laboratory, School of Materials Science and Engineering, Hefei

University of Technology, Hefei 230009, People’s Republic of China.

ABSTRACT The loss of sulfur in cathode of lithium sulfur battery (LSB) severely hinders the practical application of LSB, and so do the insulativity of S and its lithiation end-products. The incorporation of MXene can significantly improve the performance of LSBs, however, the underlying mechanism at atomic scale has not been deeply explored. In present work, by using density functional theory calculations, we systemically studied the interactions of lithium (poly)sulfides (Li2Sm) on Ti-based bare MXenes (TinXn-1) and surface functionalized Ti2C with –F, –O and –OH groups. Through analyzing the geometric and electronic structures, binding energies, deformation charge densities of Li2Sm adsorbed MXenes, we found that the strong Ti-S bonds dominate the interactions between Li2Sm and MXenes. The strong Coulombic interactions help cathodes to confine S from dissolution. Besides, the conductivities of MXenes and Li2Sm@MXenes are beneficial for the overall performance of LSB. These will provide in-depth theoretical guidance support for the utilization of MXene in LSBs.

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Introduction For current and next-generation batteries, the cathode materials should have two important properties, i.e., excellent conductivity and high capacity. Lithium sulfur battery (LSB) has been regarded as one of the most promising next-generation batteries1, 2 for its active material of sulfur (S) in cathode has high specific capacity, which is high up to 1675 mAh/g in theory. However, the loss of sulfur caused by sulfides (Li2Sm, m=1,2,4,6,8) dissolution or deposition in electrolyte can heavily destroy the specific density and cycle property of LSB. Besides, the insulativity of S and its lithiation end-products (Li2S and Li2S2) also affect the overall performance of LSB, including rate and cycle properties. To overcome these obstacles, tremendous efforts have been devoted to seeking excellent additives for cathodes,3 exploring new electrolyte4, 5 and developing protection methods for anodes.6 The introduction of additives in cathodes should be one of the most popular methods to enhance the LSB performance. Carbon based materials1, 7–9 have been regarded as the potential additives, because they have large space and high surface area which could provide enough accommodation for lithium sulfides and the active sites for electrochemical reactions. Additionally, good conductivities of carbon materials can compensate for the drawbacks caused by insulativity of S cathode, resulting in high rate and cycling performances of batteries.10, 11 Nevertheless, the weak interaction between C and S cannot fully prevent the dissolution of S. Thus, more strategies are needed to further improve the performances of carbon materials in LSB, such as B or N doping, as well as vacancy.12–17 Meanwhile, development of new 2


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additives also can improve the performance of LSB. In recent years, many materials have been employed in LSB’s cathodes,18–21 such as metal oxides (MOs),3,

22–24

metal-organic frameworks (MOFs),25, 26 transition metal dichalcogenides,27 and so on. As reported3, 23 that the metal atoms or metal-oxide groups of MOs on surfaces can strongly bond with S atoms, resulting in an efficient confinement of S or Li2Sm, and other metal-incorporated materials also share the similar mechanism. However, most MOs and metal-incorporated materials are not conductors, which do not have satisfactory conductivities to greatly improve the cathode prosperities in LSBs and Li ion batteries. Recently, a series of novel two dimensional (2D) materials, named MXene,28, 29 have been successfully exfoliated through corroding the A constituent of MAX phases. Interestingly, previous studies reported that MXene sheets possess metallicity,30, 31 which allows for diffusing electron freely. For their good conductivities and 2D constructions, MXene sheets have been tested as the electrodes in batteries.32–36 Meanwhile, the bare metals on MXene surfaces are quite similar to metals on surfaces of MOs, where metals can bond S (Li2Sm) with strong interactions.3, 22–24 In addition, with merit of large surface area in 2D materials, it provides enough surfaces for electrochemical reactions and space for S accommodation. Additional, it has been demonstrated that the metalization materials37 can improve the performance of Li–S batteries. Therefore, as expected, MXene should be desirable additives in cathode to enhance the performances of LSBs. Very recently, Nazar and coworkers38, 39 have found that composited with Ti2C or Ti3C2 the S cathodes show high performance in 3



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LSB, and they explained that the chemisorption mechanism of polysulfides in form of Ti-S bonds could entrap the S and avoid them dissolution, and then the interface reactions between S and electrolyte would be largely reduced. Other work of Zhao et al.40 also demonstrated that Ti3C2 can improve specific density, rate and cycle performances of LSB. One should be noticed that, based on experimental findings, the bare MXene are difficult to obtained, and surface functionalized species with –OH, –O and –F groups are commonly observed.28,

29

Therefore, it is difficult to determine which one

dominates or more groups work together. Although, experimental study combined with theoretical calculation39 has evaluated the influence of –OH terminated MXene and its defective species, which are both beneficial for entrapment of S in LSB. However, more details on the mechanism are required to verify such an excellent achievement in experiments. Thus, we perform density functional theory (DFT) calculations to systemically investigate the interactions of Li2Sm on surfaces of bare MXene and functionalized Ti2C with –OH, –O and –F groups [Ti2C(OH)2, Ti2CO2, Ti2CF2]. We hope that this theoretical work will provide useful guidance for the application of MXene in LSBs. Computational Details The employed structure models are displayed in Figure 1. Vienna Ab initio Simulation Package (VASP)41 was used to optimize geometric structures and calculate the binding energies, charge densities, band structures and density of state (DOS). In all the calculations, ion-electron interactions were described by projected 4


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augmented

wave,42,

43

in

which

the

exchange-correlation

functional

was

Perdew-Burke-Ernzerhof within the generalized gradient approximation. Numerical convergence was achieved with thresholds of 10−5 eV (10−6 eV for DOS calculations) in energy and 10−2 eV/Å in force with cutoff energy of 500 eV. The employed k-mesh grids were 5×5×1 (15×15×1 for DOS calculations). A vacuum space of 20 Å was taken to avoid the interaction of periodic images. It is well known that the van der Waals interaction is key for the adsorption of small molecules, thus Grimme DFT-D2 dispersion correction method44 was chosen to describe such kind of weak interaction.

Figure 1. The models of MXene, functionalized Ti2C and Li2Sm. Ti: green balls; C: gray balls; O: red balls; H: white balls; F: orange balls; S: yellow balls; and Li: purple balls.

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The binding energies (Eb) for Li2Sm on MXene are defined by Eb = ELS@M – EM – ELS, where ELS@M, EM, and ELS represent the total energies of Li2Sm@MXene, MXene, and Li2Sm, respectively. The deformation charge density (ρd) describes the charge difference between atoms and compounds, and it was calculated by ρd = ρLS@M-scf – ρLS@M-atom, where ρLM-scf and ρLM-atom are the charge density of Li2Sm adsorbed on MXene and the charge densities of all atoms in whole system, respectively. Results and Discussion MXene,28, 29 as one kind of layered materials, has been prepared through removing the ‘A’ elements from MAX phases, and consists of early transition metals and C or N atoms with metal atoms exposing on surfaces. All the geometric structures of TinXn-1 (X=C, N; n=2, 3, 4) are optimized and displayed in Figure S1 in Supporting Information , in which the lattice parameters (a=b for all studied structures) and thicknesses (from the bottom Ti layer to the top Ti layer) of TinXn-1 agree well with previous experimental and theoretical values,34,

45–47

as listed in Table S1 in

Supporting Information. The Ti-C or Ti-N bond lengths in TinXn-1 are also collected in Table S2. Interestingly, the outer ones of Ti-C or Ti-N bonds are shorter than the inner ones, and most bond lengths are in consistent with published work48, which means that our selected calculation methods are reasonable for MXene materials. Moreover, the functionalized structures are then optimized as shown in Figure S2 in Supporting Information, in which the bond length of Ti-C, compared with bare MXene, are slightly stretched, indicating a strong attractive interaction between electronegative functional groups to electropositive Ti surface. Such a coulombic attraction leads to 6


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short O-Ti or F-Ti bond lengths (no more than 2.2 Å). We also find in Figure S1 (in Supporting Information) that the Ti-based MXene are metallic materials since their valence electrons are distributed on both side of Fermi level, which is in agreement with the reported studies.29,

48,

49

More computational works found that

chemically-functionalized MXenes are semiconductors, agreeing well with recent experimental findings.50 From Figure S3 and Figure S4, and also from Table S3 and Table S4, we found that the main properties of bare MXenes of different thickness change slightly, especially the binding energy between Li2Sm and bare MXenes. Therefore, only Ti2C is used to study the effect of surface functional group on its LSB application. To explore the interactions between MXene and sulfides, we optimize Li2Sm on surfaces of bare and functionalized MXenes, as displayed in Figure 2 (more details in Figure S3 in Supporting Information). Clearly, all S atoms of Li2Sm are dispersed on surfaces of bare MXenes, and the shortest distances from S to Ti are not larger than 2.5 Å (Table S3 in Supporting Information), which are in the same range of bond length of Ti-S in TiS2 crystal,51 implying strong interactions between S and Ti. More than that, all Li2Sm clusters are dissociated on bare MXenes, which means that the attractions of bare MXenes are strong enough to break the bonds of S-S and Li-S in Li2Sm. As elucidated in previous studies,52, 53 such a high reactivity is not suitable for Li-S batteries, and the strong Ti-S bonds can reduce the kinetic of sulfur, so it is difficult for S atoms to form Li2Sm with Li, then influencing the performance of batteries. An intermediate (neither too weak nor too strong) interaction would be the 7



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optimum. In contrast, Li2Sm on Ti2CO2 and Ti2CF2 can maintain their molecular configurations, which means that the interactions between Li2Sm and Ti2CO2 or Ti2CF2 are much weaker than on bare species. It is worth noting that the long-chain Li2Sm clusters on Ti2C(OH)2 are distorted without complete dissociation. This can be explained by that the attraction from Ti2C(OH)2 for Li2Sm should be more significant than that from Ti2CO2 or Ti2CF2 and weaker than from Ti2C. H atoms are dissociated from

Li2Sm@Ti2C(OH)2

compounds

and

combined

with

S,

such

as

in

Li2S@Ti2C(OH)2 and Li2S6@Ti2C(OH)2 structures, which can be used to explain the experimental result39 about the reduced fraction of OH groups and increased O group. Based on calculation data, it is demonstrated that the dissociation energy of –OH is 6.06 eV, larger than 2.92 eV for only H leaving from Ti2C(OH)2 surfaces, therefore, the H atoms are relatively easily replaced, which again provide evidence for the experimental phenomena39 that the O groups are increased and OH groups are decreased due to the introduction of long-chain Li2Sm. As for the role of Li in such interactions, we found that Li atoms are further away from the bare surface of Ti2C than S atoms, whereas they are closer to the functionalized species than S atoms, as seen in Figure 2 and Table S3. It is understandable because Li prefers the sites around negatively charged atoms, such as O and F, while it repels positively charged Ti atoms, which in fact affects the binding of Li2Sm on different surfaces.

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Figure 2. The optimized structures of Li2Sm on Ti2C, Ti2C(OH)2, Ti2CO2, Ti2CF2. Ti: green balls; C: gray balls; O: red balls; H: white balls; F: pink balls; S: yellow balls; Li: purple balls. To further quantify the interactions, the binding energies (Eb) of Li2Sm on functionalized MXene nanosheets are calculated, as indicated in Figure 3. In Table S4 in Supporting Information, the binding energies of Li2Sm are increased with increasing the amount of S on bare MXene, which can be elucidated by that the interactions among Li2Sm and bare MXene are partially dominated by Ti-S bonds. Interestingly, a monotonously linear relationship can be observed between Eb and amount of S, which means that the Ti-S interactions contribute to the total binding between Li2Sm and bare MXene since only Ti-S bonds are increased with the increase of S atoms (Figure S4). More than that, the binding energies of each Li2Sm on different bare MXenes are almost equal, which again demonstrated that the influence 9



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of different bare MXenes are not marked differences. For functionalized MXene samples, Li2Sm maintain their configurations on Ti2CO2 (Figure 2), because physisorption dominates the interactions. This can address the lower binding energies of Li2Sm on Ti2CO2 in Figure 3, especially for the highly soluble long-chain Li2Sm. The lowest binding energies for long-chain Li2Sm on Ti2CF2 are confirmed from Figure 3, which reasonably explain that in experiments the interactions between long-chain Li2Sm and –F group cannot be observed. By contrast, the firm binding of long-chain Li2Sm such as Li2S4, Li2S6, and Li2S8 on Ti2C(OH)2 should be attributed to the reduction of repulsive force caused by the negatively charged O and S atoms since O atoms are passivated with H atoms in Ti2C(OH)2, in-depth discussion will be described in next section. Specifically, the binding energy of Li2S4 is –3.43 eV, which is almost the same as that calculated in other work (–3.42 eV),39 The absolute values are high up to 4.79 eV and 4.90 eV, respectively, for Li2S6 and Li2S8 on Ti2C(OH)2, which are comparable to the binding energy between Li2Sm and vacancy graphene.54 These data indicate that the OH groups on MXene are effective for confining long-chain Li2Sm, especially verifying the conclusion of recent experiment39 combined

with

some

–OH

defects.

In addition,

we

also examine

the

quasi-symmetrically attached Li2S4 and Li2S8 on both sides of Ti2C(OH)2 and Ti2CO2. The optimized structures (symmetric slabs) are displayed in Figure S5. From Table S5, it is found that the average binding energies for two Li2S4 or Li2S8 on Ti2C(OH)2 and Ti2CO2 are almost equal to the corresponding values for only one Li2Sm cluster on these two functionalized MXenes, indicating that the asymmetric slabs are reasonable models. It has been reported that the dissolution of Li2Sm can be attributed to the stronger binding energy for Li2Sm interacting with electrolytes than that with host materials.54 Therefore, we further study the interaction between long-chain Li2Sm (Li2S4, Li2S6, Li2S8) and electrolytes such as 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) and lithium bis(trifluoromethane sulfonimide) (LiTFSI). From Table S6, we found that the interaction energies between Li2Sm and electrolytes are consistent with previous work.53 The corresponding values are much smaller than those between 10


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Li2Sm and bare or OH-terminated MXenes, which means that lithium (poly)sulfides prefer being adsorbed on the MXene surfaces instead of being dissolved in electrolytes. Furthermore, we tried to explore the influence of electrolytes on the binding between Li2Sm and MXene by using the method of Kamphaus et al.54 and by considering Li2S4 and Ti2C(OH)2 as the representative case. We note that the adsorbed structure of Li2S4 on Ti2C(OH)2 surface slightly changes with the introduction of electrolytes, particularly for some bond lengths (Ti-S, as shown in Table S7) by comparing Figure 2 and Figure S6. As a result, it is confirmed that the binding energies of Li2S4 on Ti2C(OH)2 with electrolytes (–7.66 eV, –7.58 eV and –8.16 eV respectively for DOL, DME and LiTFSI) show a significantly enhanced interaction than the cases without electrolyte. Therefore, the bare and OH-terminated MXenes can prevent the dissolution of Li2Sm, which will enhance the performance of LSBs.38,39

Figure 3. The binding energies (Eb, in eV) of Li2Sm on functionalized Ti2C without electrolytes. As expected, the differences in binding energy could be originated from the strong Coulomb interactions between Li2Sm and MXenes. Using a simple model in Figure 4, for Li2Sm on Ti2C, the strong attraction between S and Ti could dominate the binding energies, whereas for O or F terminated samples, the Ti-S interactions should be reduced rapidly for the increased repulsive force from O or F (pink layer 11



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atoms in schematic Figure 4) to S, that is why the binding energies of Li2Sm on functionalized MXene are much smaller than those on bare MXene (also see Figure 3 and Table S4 in Supporting Information). Besides, the repulsive forces will be slightly shielded by the added H layer (light blue layer), and the added positively charged H atoms could increase the attraction to S atoms, which reasonably explain that the binding energy for Li2Sm on Ti2C(OH)2 are larger than –O or –F terminated ones39 and smaller than bare MXene. As mentioned above, the presence of Li also influences the binding of Li2Sm on surfaces. The shorter distance between Li and O or F means the enhanced binding of Li2Sm from the interaction between Li and O or F atoms, however, it accompanies larger Ti-S distances leading to weak binding of Li2Sm on functionalized Ti2C. For OH-terminated surfaces, the interaction between H and S also contributes to their effective binding with Li2Sm. After careful comparison, we can conclude that the role of S atoms (Ti-S and H-S attraction) is more dominant than that of Li atoms (Ti-Li repulsion, O-Li or F-Li attraction) in binding of lithium sulfides on MXene materials.

Figure 4. The scheme of charged atoms in Li2Sm and MXenes. ( ‘+’ present the atoms electropositive, and ‘-’ present the atoms electronegative). Ti: green balls; S: yellow balls; Li purple balls; O or F: red balls in pink area; and H: white balls in light-blue area. 12


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For better understanding the Coulomb interactions between Li2Sm and various groups, the deformation charge density of Li2Sm attached to bare and functionalized MXenes are studied, which can describe the charge transfer. In Figure 5, there is no doubt that charge accumulations are mainly located around C, F, O and S atoms, indicating that they are electronegative, and Ti is electropositive due to the loss of electrons, in agreement with the proposed mechanism in Figure 4. Little electron transfers from Li atoms to MXene substrates since there is no charge around Li under the isosurface level of 0.015 e/Å3 in Figure 5, which means that the Coulomb interactions are very weak. While the great Coulombic forces of attraction between Ti-S dominate the binding between Li2Sm and bare MXene, for functionalized samples such strong Coulombic attractions between Ti and S are heavily weakened because of the introduction of electronegative groups. In general, O and F terminated MXenes have more electrons around their surfaces, which could induce strong repulsion to S atoms. Fortunately, the repulsion from O to S is hindered in OH-terminated surfaces with the help of blocking by positively charged H atoms. Meanwhile, the attractive interactions between S and H enhance the binding between Li2Sm and Ti2C(OH)2. Overall, the Coulomb interactions greatly influence the binding between Li2Sm and MXenes, and OH-terminated and bare MXenes could enhance the S entrapment in cathodes of LSBs.

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Figure 5. The deformation charge density of Li2Sm@MXenes with isosurface level of 0.015 e/Å3. Charge accumulation, blue areas, and charge depletion, red areas. Ti: green balls; C: gray balls; S: yellow balls; Li: purple balls; O: red balls; H: white balls and F: pink balls. Beyond the high binding to prevent the dissolution of Li2Sm, good electronic conductivity of MXene should remedy the drawback of insulativity for it can be used as electronic diffusion channels for the electrochemical reactions. One should be noticed that, in experiments, the mass of S in cathodes are often higher than 60%, which means that the additive atoms are often much less than S atoms. Therefore, in the process of charge/discharge cycles, an effective additive should have enough space for electrochemical reactions and supply more electrons to keep reaction 14


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running. For MXene-based samples, the first layer of S can be confined efficiently as we described above, and the subsequent S should be located above the first layer of S. Thus, the electronic conductivities of MXene and Li2Sm@MXene are important for the continuous electrochemical reactions. Unfortunately, recent works have demonstrated that functionalized MXene should be semiconductors,28, 55 which could reduce the property of electron diffusion and further affect the performance of LSBs. To estimate the conductivities of MXene and Li2Sm@MXene, the band structures and DOS are calculated, as depicted in Figure 6 and Figures S7-S8 in Supporting Information. Based on our computational results, Ti2C is conductor, and OH- and F-terminated Ti2C have narrow band gaps (

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