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. 2020 Nov 3;5(45):29272–29283. doi: 10.1021/acsomega.0c04043

Theoretical Insights into the Favorable Functionalized Ti2C-Based MXenes for Lithium–Sulfur Batteries

Qi Zhang †, Xiaofei Zhang †, Yuhong Xiao †, Cheng Li †, Hark Hoe Tan §, Jiaqin Liu †,*, Yucheng Wu ‡,*
PMCID: PMC7675968  PMID: 33225158

Abstract

graphic file with name ao0c04043_0012.jpg

Because of the high specific surface area, excellent electronic conductivity, facile Li diffusion, and rich functional groups, Ti2C-based MXenes have been widely used to improve the electrochemical property of lithium–sulfur batteries. The complex surface functionalization (such as −OH, −S, −F, and −O) of MXenes boosts the performance but also causes controversies about the favorable functionalized surface in the electrochemical reaction during the charge and discharge process. In the present work, a theoretical study based on density functional theory has been carried out to clarify the favorable functionalized surface by comparing pristine Ti2C and −OH-, −S-, −F-, and −O-functionalized Ti2C surfaces from the aspects of adsorption ability, electronic conductivity, and kinetic conversion ability. It is found that compared with severe polysulfide deformation on pristine Ti2C and Ti2C(OH)2 surfaces, Ti2CO2, Ti2CS2, and Ti2CF2 have effective polysulfide adsorption. Ti2CO2 has the largest surface adsorption energy, followed by Ti2CS2, and Ti2CF2 is the weakest. Meanwhile, the narrow-band gap semiconductor property of Ti2CO2 during adsorption indicates worse electronic conductivity than metallic Ti2CS2 and Ti2CF2. In addition, for the kinetic conversion ability, the Ti2CS2 surface has the fastest polysulfide conversion and Li diffusion, followed by Ti2CF2, and Ti2CO2 represents the slowest conversion and diffusion. Accordingly, because of the medium binding energy, good electronic conductivity, and fast polysulfide conversion and Li diffusion, Ti2CS2 is revealed to be the favorable functionalized surface. More importantly, the origin for the Ti2CS2 surface with medium adsorption ability represents the fastest polysulfide conversion, and Li diffusion is further clarified. The great affinity of the Ti2CS2 surface to the product Li2S leads to facile polysulfide conversion. The uniform charge distribution on the Ti2CS2 surface contributes to the fast Li diffusion.

1. Introduction

Because of the high theoretical capacity (1675 mA h g–1) and energy density (2600 W h kg–1), lithium–sulfur (Li–S) batteries have been considered as one of the most promising systems for the next generation high-energy rechargeable lithium batteries.1−7 However, the shuttle effect hinders the charge and discharge process. The soluble lithium polysulfide (LiPS) intermediates (Li2Sn, 3 ≤ n ≤ 8) dissolve in the electrolyte, migrate between the cathode and anode, and lead to serious side reactions on the lithium anode.8−10 Moreover, nonconductive sulfur and discharge products (Li2S2 and Li2S) hinder the electron transport during the charge and discharge process, resulting in low utilization of active materials.11,12 Two-dimensional materials such as graphene,13,14 transition metal disulfides,15,16 black phosphorus,17,18 and MXenes19−23 have been used in lithium–sulfur batteries to solve these problems.24,25 The rich active sites interact with polysulfides to inhibit the shuttle effect. The high electronic conductivity improves the electron transport of nonconductive active materials.26,27

Among these, the titanium carbide-based MXenes receive great attention because of the high specific surface area, excellent electronic conductivity, facile Li diffusion, and rich functional groups on the surface.19−23,28,29 In 2015, the Nazar group introduced Ti2C layers into the Li–S battery for the first time as the cathode host material.30 Recently, Wu et al. embedded sulfur-coated TiO2 hollow nanospheres into Ti2C layers.31 The S@TiO2/Ti2C composite electrode shows a high initial discharge specific capacity of 1408.6 mA h g–1 at 0.2 C and maintains a capacity of 464.0 and 227.3 mA h g–1 after 200 cycles at 2 C and 5 C, respectively. Chen et al. prepared a S@layered-Ti3C2 composite as the cathode material, showing a high initial discharge capacity of 1291 mA h g–1 at 200 mA g–1 and excellent capacity retention of 970 mA h g–1 after 100 cycles.32 Wang et al. prepared N-doped Ti3C2Tx nanosheets as the cathode with an area sulfur loading of up to 5.1 mg cm–2, demonstrating high reversible capacity (1144 mA h g–1 at 0.2 C rate) and extended cycling stability (610 mA h g–1 at 2 C after 1000 cycles).33 These outstanding studies illustrate the important effect of titanium carbide-based MXenes in lithium–sulfur batteries.

Note that the bare titanium carbide-based MXene surface does not exist. During the etching step of preparation, the surface is always terminated by functional groups such as −OH, −S, −F, and −O. The complex functionalization makes it difficult to reveal the role of the MXene surface in the electrochemical reaction during the charge and discharge process. Chung et al. investigated Ti2CF2 and Ti2CO2 and believed that Ti2CF2 restrains the shuttle effect via strong interactions with LiPSs.34 Ti2CO2 promotes the conversion of soluble high-order LiPSs to insoluble sulfur because of the improved conductivity after adsorption. However, Zhao, Y. and Zhao, J. et al. revealed different conclusions by investigating Ti2CO2, Ti2C(OH)2 and Ti2CF2 via theoretical calculations.35 It was found that the adsorption capacity of Ti2CO2 is significantly larger than that of Ti2CF2. Ti2CF2 and Ti2C(OH)2 cannot anchor polysulfides because of the weak adsorption capacity. Lu et al.36 investigated the interaction of LiPSs with −F-, −O-, and −OH-functionalized Ti2C and found that the adsorption capacity of Ti2CF2 is the smallest and thought that it could not significantly anchor LiPSs, which is consistent with the results of the Zhao group.35 However, the electronic structural results show that the adsorption of LiPSs does not change the electrical conductivity of Ti2CO2, which is not consistent with the conclusion of the Chung group.34 The S-functionalized Ti2CS2 was studied by the Zhao group; it was found that the adsorption capacity of Ti2CS2 is greater than that of Ti2CO2 and Ti2CF2.37 The maintained metallic properties during adsorption and low energy barrier of Li migration make Ti2CS2 a great material for inhibiting the shuttle effect.

Accordingly, the complex functionalization of the titanium carbide-based MXene surface leads to confusion and contradictions about the favorable functionalized surface for LiPS adsorption and conversion.38 Herein, we made the comprehensive comparison of −OH-, −S-, −F- and −O-functionalized Ti2C-based MXene surfaces based on density functional theory calculations. The adsorption configuration and binding energy (combined with charge transfer and charge density difference), conductivity change during adsorption, catalytic conversion of LiPSs, and Li migration on the surface were investigated to reveal the favorable functionalized surface. The results show that the Ti2CS2 surface with a medium binding energy, good electronic conductivity, and fast polysulfide conversion and Li diffusion is the most favorable functionalized surface. More importantly, the origin for the fast polysulfide conversion and Li migration on the Ti2CS2 surface is revealed. Compared with Ti2CO2 and Ti2CF2, the Ti2CS2 surface shows great affinity to the product Li2S, providing a driving force for polysulfide conversion. The uniform charge distribution on the Ti2CS2 surface contributes to facile Li diffusion.

2. Calculation Methods

All of the calculations were performed using the Dmol3 program in Materials Studio 7.0.39,40 The electronic exchange–correlation function is treated using a generalized gradient approximation, and the Perdew–Wang 91 (PW91) exchange functional is used as a nonlocal correction to calculate the electronic structure.41,42 The spin-unrestricted form is used. For geometric optimization and energy calculations, dispersion correction (DFT-D) with the OBS method is used to accurately describe long-range electrostatic interactions.43 The double-numerical basis set with orbital polarization d-function is selected to ensure accuracy and expand the valence electron function.44,45 For the converge criterion of geometry optimization, 1.0 × 10–5 Ha, 2.0 × 10–3 Ha Å–1, and 5.0 × 10–3 Å are set for the energy convergence, maximum force, and maximum distance, respectively. The convergence criterion of the self-consistent field is 1 × 10–6 Ha. In order to ensure high-quality results, the real-space global orbital cut-off is selected to be up to 5.4 Å in all calculations.

To avoid artificial interactions caused by periodicity between adjacent layers, all MXene models are set with 4 × 4 supercells and 20 Å vacuum layers. The Ti2C unit cell and the 4 × 4 supercell are composed of three atoms and 48 atoms, respectively. The unit cell of Ti2CS2, Ti2CF2, and Ti2CO2 is composed of five atoms, and the supercell is composed of 80 atoms. The unit cell of Ti2C(OH)2 is composed of seven atoms, and the supercell contains 112 atoms. Brillouin-zone integrations are performed using the 9 × 9 × 1 Monkhorst–Pack grid46 and a Methfessel–Paxton smearing47 of 0.005 Ha to calculate the supercell geometry optimizations, and the denser meshes of the 15 × 15 × 1 grid are used to calculate the electronic structure. All the atoms are processed in the core electronics of effective core potentials. For Li diffusion on the MXene surface, we use linear synchronous transit and quadratic synchronous transit (LST/QST) methods to confirm the migration path.48 Hirshfeld population analysis is used to calculate the charge transfer.

The anchoring strength between the Li2Sn molecule and the MXenes surface is calculated based on the binding energy (Eb), and the calculation equation is as follows

2.

where Eb is defined as the energy difference between the adsorbed Li2Sn species and the isolated MXene and Li2Sn molecules. Especies@MXene, Especies, and EMXene are total energies of the total adsorbed systems, isolated Li2Sn species, and MXene surface, respectively. According to the definition, larger positive binding energy indicates a more favorable binding effect between Li2Sn and MXenes.

3. Results and Discussion

3.1. Polysulfide Adsorption on the MXene Surface

Herein, we investigate the adsorption of polysulfides on the pristine Ti2C and functionalized Ti2CT2 (T = −OH, −S, −F, and −O). To reveal the adsorption mechanism, the adsorption configuration and binding energy (Section 3.1.1), charge transfer, and charge density difference (Section 3.1.2) were investigated. The band structure was also studied (Section 3.1.3) to reveal the electronic conductivity change during adsorption.

3.1.1. Adsorption Configurations and Energies

The adsorption of polysulfides on pristine Ti2C and Ti2C(OH)2 was first investigated, and the corresponding adsorption configurations and binding energies are shown in Figure 1. On the surface of pristine Ti2C (Figure 1a), S8 and all Li2Sn (n = 8, 6, 4, 2, and 1) clusters are severely deformed, and the S atoms are scattered on the Ti2C surface to form Ti–S bonds. The Ti–S bond length is in the range of 2.30–2.64 Å, which is close to 2.39 Å of the Ti–S bond in the Ti2CS2 surface (vide infra). It can be seen that the strong interaction exists between negatively charged S and positively charged Ti. The binding energies of S8, Li2S8, Li2S6, Li2S4, Li2S2, and Li2S on the surface of pristine Ti2C are 30.65, 32.33, 24.34, 16.96, 9.97, and 6.80 eV, respectively. The deformation of polysulfides also occurs on the surface of Ti2C(OH)2 (Figure 1b). For the S8 molecule, the S–S bonds break and S8 is decomposed into S5 and S3. The high-order polysulfides Li2S8 and Li2S6 are decomposed into the structure of S4–Li–S–Li–S3 and S3–Li–S–Li–S2, which are laid flat on the surface of Ti2C(OH)2. The shortest distances between the S atom and surface hydrogen atom are 1.38 and 1.39 Å, respectively. For mid-order polysulfide Li2S4, it decomposes into the structure of S2–Li–Li–S2. One Li atom occupies the hcp C site and forms Li–O bonds with three O atoms in the second layer. The shortest Li–O bond length is 2.11 Å. For the low-order polysulfides Li2S2 and Li2S, both Li atoms are bonded to O atoms, and the shortest bond lengths are 1.92 and 1.95 Å, respectively. The binding energies of S8, Li2S8, Li2S6, Li2S4, Li2S2, and Li2S on the surface of Ti2C(OH)2 are 7.59, 10.11, 9.29, 7.18, 7.81, and 5.00 eV, respectively. The deformation and great binding energies of polysulfides may be due to the strong interaction between the positively charged H and negatively charged S.

Figure 1.

Figure 1

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Stable adsorption configuration of polysulfides on (a) pristine Ti2C and (b) Ti2C(OH)2. Corresponding binding energies are also given.

Figure 2 shows the adsorption configurations and binding energies of polysulfides on the Ti2CS2, Ti2CF2, and Ti2CO2 surface. On these three surfaces, polysulfides are not decomposed, and the corresponding binding energies are significantly smaller than those of pristine Ti2C and Ti2C(OH)2 surfaces. On the surface of Ti2CS2 (Figure 2a), the binding energies of S8, Li2S8, Li2S6, Li2S4, Li2S2, and Li2S are 1.23, 3.55, 2.41, 3.06, 3.51, and 4.28 eV, respectively. Compared to the pristine Ti2C and Ti2C(OH)2 with positively charged Ti or H atoms on the surface, the Ti2CS2 surface is covered by negatively charged S atoms. The surface-functionalized atoms and S in LiPS are both negatively charged and cannot generate strong interactions. Therefore, the corresponding adsorption energy is significantly reduced. For S8@Ti2CS2, the S8 retains an eight-membered ring of crown shape. The shortest distance between S8 and the S atoms on the surface is 3.74 Å. The long distance is consistent with the low binding energy of 1.23 eV. For Li2S8@Ti2CS2, both Li atoms form Li–S bonds with the surface S atoms with the shortest Li–S bond length of 2.31 Å, which makes greater binding energy than S8@Ti2CS2 (3.55 vs 1.23 eV). For Li2S6@Ti2CS2, the Li–Li bond is perpendicular to the surface, and one Li atom is bonded to the surface S atom with the shortest Li–S bond length of 2.48 Å. For Li2S4@Ti2CS2, Li2S2@Ti2CS2, and Li2S@Ti2CS2, Li–Li is not bonded, and both Li atoms are bonded to surface S atoms. The shortest Li–S bond length is 2.55, 2.45, and 2.36 Å, respectively. In particular, for Li2S@Ti2CS2, the S atom of Li2S is also bonded to the surface S atom with a S–S bond length of 2.07 Å. The dual-bond formation is consistent with the largest binding energy.

Figure 2.

Figure 2

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Stable adsorption configuration of polysulfides on (a) Ti2CS2, (b) Ti2CF2, and (c) Ti2CO2. Corresponding binding energies are also given.

For the Ti2CF2 surface (Figure 2b), the binding energies of polysulfides are 1.29, 2.47, 2.14, 2.34, 2.80, and 2.74 eV, which are slightly lower than those of the Ti2CS2 surface. For S8@Ti2CF2, S8 is still far from the surface, and the shortest distance between S and the surface F atom is 3.39 Å. For Li2S8@Ti2CF2 and Li2S6@Ti2CF2, one Li atom interacts with the surface F atoms with the shortest Li–F bond lengths of 2.21 and 2.00 Å, respectively. For Li2S4@Ti2CF2, Li2S2@Ti2CF2, and Li2S@Ti2CF2, both Li atoms are bonded to the surface, and the shortest Li–F bond lengths are 1.90, 2.07, and 2.17 Å, respectively. Compared to the Li–S bond for the Ti2CS2 surface, the shorter Li–F bond length is consistent with the smaller atomic radius of F than that of S. Compared with S, F and Li have more matching radii, resulting in a better orbital overlap. Nevertheless, the electronegativity of F is much greater than that of S, and it does not tend to donate electrons to Li atoms. Therefore, the overall adsorption energies are smaller. On the Ti2CO2 surface (Figure 2c), the binding energies are 1.47, 5.18, 3.58, 4.07, 4.41, and 4.13 eV, which are larger than those of the Ti2CS2 and Ti2CF2 surface. For S8@Ti2CO2, the shortest distance between S and surface O is 3.37 Å. For Li2S8@Ti2CO2, Li2S6@Ti2CO2, Li2S4@Ti2CO2, Li2S2@Ti2CO2, and Li2S@Ti2CO2, both Li atoms are bonded to the surface O atoms and Ti atoms in the second layer, respectively. The shortest Li–O bond lengths are 2.03, 2.05, 2.10, 2.11, and 2.15 Å, respectively. The shortest Li–Ti bond lengths are 2.86, 2.90, 2.93, 2.91, and 2.91 Å, respectively. Compared with Ti2CS2 and Ti2CF2, the dual interaction of Li–O and Li–Ti for the Ti2CO2 surface results in the larger binding energies.

According to the abovementioned calculation results, for the pristine Ti2C and Ti2C(OH)2 surfaces, the binding energies are significantly higher than those of the Ti2CS2, Ti2CF2, and Ti2CO2 surfaces. In addition, the severe deformation of polysulfide occurs on the former two surfaces. The structure deformation leads to the separation of lithium and sulfur elements, which impairs the effect of sulfur and prevents the polysulfides from participating in the subsequent discharge process.24,49−51 Therefore, they are not favorable anchoring surfaces. By comparison, the polysulfides are not decomposed for the Ti2CS2, Ti2CF2, and Ti2CO2 surfaces. The binding energy order is Ti2CO2 > Ti2CS2 > Ti2CF2. In the following sections, we focus on the latter three surfaces.

3.1.2. Charge Transfer and Charge Density Difference

To clarify the mechanism of polysulfide adsorption, the charge transfer and charge density difference52−54 were investigated with Li2S6 adsorption on the surface of Ti2CT2 (T = −S, −F, and −O) as an example. As shown in Figure 3, the charge transfer Q of Li2S6 for the adsorption species Li2S6@Ti2CS2, Li2S6@Ti2CF2, and Li2S6@Ti2CO2 are 0.124 e, 0.112 e, and 0.772 e, respectively. The positive charge transfer value indicates that the charge transfers from the Li2S6 to Ti2CT2 surface during adsorption. Further, the charge density difference for the adsorption species was investigated. For all of these three adsorption species, the blue region exists between the Li and Ti2CT2 surface, the red region exists around S atoms. The blue and red regions represent charge accumulation and loss, respectively. Therefore, it is shown that Li atoms gained electrons and S atoms lost electrons during the adsorption process. At the 0.01 e/Å3 isosurface level, the size of the red and blue regions generally shows Li2S6@Ti2CO2 > Li2S6@Ti2CS2 > Li2S6@Ti2CF2, which is consistent with the trend of charge transfer. The order of both charge transfer and charge density difference is consistent with the binding energy order (the binding energies for Li2S6@Ti2CS2, Li2S6@Ti2CF2, and Li2S6@Ti2CO2 are 2.41, 2.14, and 3.58 eV, respectively). It is illustrated that the adsorption capacity is directly related to the charge transfer between polysulfides and the surfaces. As mentioned above, compared with S- and O-functionalization atoms, F atoms are more electronegative, so that they do not tend to transfer electrons to Li, which leads to less charge transfer. Comparing Ti2CS2 and Ti2CO2 surfaces, O atoms and Li atoms present closer radii than S and Li atoms, which leads to a better Li–O orbital overlap. The better Li–O interaction brings Li2S6 close to the surface, which in turn produces Li–Ti bonding. The dual bonding of Li–O and Li–Ti causes the largest charge transfer.

Figure 3.

Figure 3

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Charge transfer and charge density difference between Li2S6 and Ti2CT2 surfaces [(a–c) Ti2CS2, Ti2CF2, and Ti2CO2]. The isosurface level is set to be 0.01 e/Å3. Blue and red regions indicate charge accumulation and depletion, respectively.

3.1.3. Electronic Conductivity

Electronic conductivity of the electrode material has great effects on the performance of lithium–sulfur batteries. The polysulfide adsorption would affect the conductivity of the material.55,56 Herein, the band structure of the surface and the adsorption species (Li2S6@Ti2CT2 (T = −S, −F, and −O)) were investigated to demonstrate that the electronic conductivity changes during adsorption. Figure 4a–c shows the band structures of Ti2CS2, Ti2CF2, and Ti2CO2 surfaces. Ti2CS2 and Ti2CF2 show metallic electronic structures because of the electronic states across the Fermi level. The Ti2CO2 surface shows the direct band gap of 0.243 eV between the valence band maximum and the conduction band minimum, indicating slightly weaker conductivity than Ti2CS2 and Ti2CF2. Figure 4d–f shows the band structures of Li2S6@Ti2CS2, Li2S6@Ti2CF2, and Li2S6@Ti2CO2. The former two still show metallic properties. The adsorption of polysulfides does not change their conductivity. In addition, the curves before and after adsorption are similar, indicating that less charge transfer for Li2S6@Ti2CS2 and Li2S6@Ti2CF2 does not cause a great effect in the electronic state. Li2S6@Ti2CO2 shows the reduced band gap of 0.217 eV compared with the Ti2CO2 surface. This may be due to the great charge transfer from Li2S6 to Ti2CO2, leading to the shifted up Fermi level and the reduced semiconductor band gap. In summary, for Ti2CS2 and Ti2CF2, both of them display the metallic feature before and after polysulfide adsorption. In comparison, Ti2CO2 shows semiconductor properties with a small band gap and the band gap decreases after adsorption.

Figure 4.

Figure 4

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Band structures of (a) Ti2CS2, (b) Ti2CF2, and (c) Ti2CO2. Band structures of the adsorption species (d) Li2S6@Ti2CS2, (e) Li2S6@Ti2CF2, and (f) Li2S6@Ti2CO2. The Fermi level is set to zero.

3.2. Catalytic Mechanism on the MXene Surface

3.2.1. Polysulfide Conversion

In addition to the polysulfide adsorption, the catalytic conversion of polysulfide on the Ti2CT2 (T = −S, −F, and −O) surfaces was investigated by studying the deep lithiation step Li2S2 → Li2S (rate-determining step) in the discharge process.57,58Figure 5 shows the conversion mechanism and the corresponding energy profile on the Ti2CS2 surface. The Li2S2@Ti2CS2 is the starting point with both Li atoms bonded to the surface S atoms. Then, the species receives Li+ from the electrolyte and electron e– from the external circuit to obtain the intermediate Li2S2–Li@Ti2CS2. The Li atom is located at the hcp Ti site near Li2S2 and forms Li–S bonds with three S atoms on the surface. This is the lithiation step. The system energy is reduced to −4.11 eV after the step. Then, in Li2S2–Li@Ti2CS2, Li atom diffuses to bond with the S atoms of Li2S2 to form the intermediate Li3S2@Ti2CS2. This is the Li diffusion step. After the step, the energy of the system decreases to −4.33 eV. The intermediate Li3S2@Ti2CS2 then undergoes another lithiation step to obtain the intermediate Li3S2–Li@Ti2CS2 in which the Li atom occupies the hcp Ti site near Li3S2. The energy further decreases to −8.47 eV. Then, Li diffuses and bonds with the S atom of Li3S2 to form the intermediate Li4S2@Ti2CS2, and the system energy is further reduced to −8.75 eV. Finally, the intermediate Li4S2@Ti2CS2 undergoes a cleavage step to obtain the product Li2S@Ti2CS2 with an energy of −7.70 eV. Thus, the cleavage step is endoergic by 1.05 eV (Li4S2@Ti2CS2 → Li2S@Ti2CS2). Accordingly, the deep lithiation process of Li2S2@Ti2CS2 → Li2S@Ti2CS2 undergoes the lithiation–diffusion step twice and final cleavage step. Both the lithiation and diffusion steps are exergonic, whereas the cleavage step is endoergic. The overall energy decrease is −7.70 eV and the energy barrier is 1.05 eV.

Figure 5.

Figure 5

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Optimized configurations and energy profile of the Li2S2 → Li2S conversion mechanism on the Ti2CS2 surface (in eV).

Figures 6 and 7 show the conversion mechanism and corresponding energy profiles on Ti2CF2 and Ti2CO2 surfaces. As to the first lithiation–diffusion step on the Ti2CF2 surface (Figures 6a and 7a), Li2S2@Ti2CF2 first undergoes lithiation to obtain the intermediate Li2S2–Li@Ti2CF2, and the system energy decreases to −3.55 eV. Then, Li diffuses to the hcp C site near the Li2S2 molecule to bond with three F atoms on the surface and form the intermediate Li3S2@Ti2CF2. The system energy is reduced to −3.93 eV. The intermediate Li3S2@Ti2CF2 then undergoes the second lithiation–diffusion step to form the intermediate Li4S2@Ti2CF2 via the intermediate Li3S2–Li@Ti2CF2. The energy further decreases to −7.85 eV. Li4S2@Ti2CF2 finally undergoes the cleavage step to generate the discharge product Li2S@Ti2CF2 with an energy of −5.31 eV. Thus, the cleavage step is endoergic by 2.54 eV (Li4S2@Ti2CF2 → Li2S@Ti2CF2). As to the Ti2CO2 surface (Figures 6b and 7b), Li2S2@Ti2CO2 undergoes the first lithiation–diffusion step to obtain the intermediate Li3S2@Ti2CO2 with a decreased energy of −4.94 eV. Then, Li3S2@Ti2CO2 undergoes the second lithiation–diffusion to form Li4S2@Ti2CO2, and the system energy is −9.71 eV. Finally, Li4S2@Ti2CO2 undergoes the cleavage step to generate the discharge product Li2S@Ti2CO2, and the energy of the product is −6.48 eV. Thus, the cleavage step is endoergic by 3.23 eV (Li4S2@Ti2CO2 → Li2S@Ti2CO2).

Figure 6.

Figure 6

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Optimized configurations of the Li2S2 → Li2S conversion mechanism on (a) Ti2CF2 and (b) Ti2CO2 surface.

Figure 7.

Figure 7

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Energy profiles of the Li2S2 → Li2S conversion mechanism on (a) Ti2CF2 and (b) Ti2CO2 surface (in eV).

In summary, Li2S2 → Li2S conversion on Ti2CT2(T = −S, −F, −O) surfaces undergoes the lithiation–diffusion step twice and the final cleavage step. The lithiation and diffusion steps are exergonic, whereas the cleavage step is endoergic. Therefore, the cleavage step is a rate-determining step that determines the kinetics of the conversion. For the three surfaces, the energy demands of the cleavage step are 1.05 eV (Li4S2@Ti2CS2 → Li2S@Ti2CS2), 2.54 eV (Li4S2@Ti2CF2 → Li2S@Ti2CF2), and 3.23 eV (Li4S2@Ti2CO2 → Li2S@Ti2CO2). The energy demand of the Ti2CS2 surface is the lowest, indicating that the surface is conducive to the deep lithiation process.

3.2.2. Li Diffusion

Li diffusion plays a crucial role in the adsorption–diffusion–conversion process on the MXene surface.59−62 In accordance with previous studies, herein, we investigate the migration path of Li diffusion on Ti2CT2 (T = −S, −F, −O) surfaces and the corresponding diffusion energy barrier by the LST/QST method.63,64Figure 8 shows the migration path and energy profiles for the Ti2CS2 surface (Figure 8a,d), Ti2CF2 surface (Figure 8b,e), and Ti2CO2 surface (Figure 8c,f). On the Ti2CS2 surface, the stable adsorption site of Li, hcp Ti, is marked as the A site (the point is set as the zero energy point). The metastable adsorption site, hcp C, is marked as the B site with an energy of 0.06 eV. For the migration path, Li moves from the A site to the nearest B site and then to another A site (set as A′). As shown in Figure 8d, a transition state exists between A and B, and the corresponding energy barrier of the A → B → A′ path is 0.12 eV. On the Ti2CF2 surface, the stable adsorption site of Li is hcp C, namely, the B site (set as the zero energy point). The A site is a metastable adsorption site with an energy of 0.12 eV. For the migration path, Li diffuses from the B site to the nearest A site and then to another B site (set as B′). As shown in Figure 8e, a transition state exists between A and B, and the corresponding diffusion energy barrier of the B → A → B′ pathway is 0.20 eV. For Ti2CO2, the stable adsorption site of Li is hcp C, which is the B site (set as the zero energy point). The bridge position is a metastable adsorption site. The site is set as the C point, and the energy is 0.20 eV. Li migrates from the stable site B through the site C to another B site (set as B′). The energy gradually increases from B to C and there is no transition state between them. Thus, the point C is the highest energy point, and the corresponding diffusion energy barrier of the B → C → B′ pathway is 0.50 eV.

Figure 8.

Figure 8

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Migration paths of Li diffusion on the surfaces of (a) Ti2CS2, (b) Ti2CF2, and (c) Ti2CO2. (d–f) Corresponding diffusion energy barrier profiles.

Accordingly, the diffusion energy barriers of Li on Ti2CS2, Ti2CF2, and Ti2CO2 are 0.12, 0.20, and 0.50 eV, respectively. The order of the diffusion energy barrier is Ti2CO2 > Ti2CF2 > Ti2CS2. The Ti2CS2 monolayer has the best Li transport performance, effectively promoting the sulfur conversion kinetics.

4. Discussion

4.1. Overall Comparison of the MXene Surface

In Sections 3.1 and 3.2, the adsorption ability (binding energy, charge transfer, and charge density difference), electronic conductivity (band structure), and kinetic conversion ability (polysulfide conversion and Li diffusion) were investigated to measure the performance of the pristine Ti2C and four functionalized Ti2CT2 (T = −OH, −S, −F, and −O) in the lithium–sulfur batteries.

First, according to the calculation of the adsorption ability in Sections 3.1.1 and 3.1.2, the pristine Ti2C and Ti2C(OH)2 are not favorable anchoring materials because of excessive binding energies and severe structural deformation of polysulfides. For the surfaces of Ti2CO2, Ti2CS2, and Ti2CF2, the order of binding energy is Ti2CO2 > Ti2CS2 > Ti2CF2. Thus, the Ti2CO2 surface provides the largest adsorption ability, followed by the Ti2CS2 surface, and the Ti2CF2 surface shows the least adsorption ability. Therefore, from the perspective of the adsorption ability, Ti2CO2 and Ti2CS2 are more favorable than Ti2CF2. Second, according to the calculation of the electronic conductivity in Section 3.1.3, both Ti2CS2 and Ti2CF2 remain metallic during adsorption. For Ti2CO2, although the band gap is reduced after polysulfide adsorption, it shows the narrow-band gap semiconductor property during adsorption. Therefore, from the perspective of conductivity, Ti2CO2 is not favorable. Third, according to the calculation of the polysulfide conversion in Section 3.2.1, it is found that Li2S2@Ti2CT2 → Li2S@Ti2CT2 undergoes the lithiation–diffusion step twice and the final cleavage step. The cleavage step is the rate-determining step that determines the kinetics of the transformation. The energy demands of the cleavage steps for Ti2CS2, Ti2CF2, and Ti2CO2 surfaces (Li4S2@Ti2CT2 → Li2S@Ti2CT2) are 1.05, 2.54, and 3.23 eV, respectively. Thus, the order of the energy barrier is Ti2CO2 > Ti2CF2 > Ti2CS2. According to the calculation of the Li diffusion in Section 3.2.2, the diffusion energy barriers of Li on Ti2CS2, Ti2CF2, and Ti2CO2 are 0.12, 0.20, and 0.50 eV, with the order of Ti2CO2 > Ti2CF2 > Ti2CS2. Therefore, from the perspective of conversion kinetics (polysulfide conversion and Li diffusion), Ti2CS2 is more favorable than Ti2CO2 and Ti2CF2.

Comprehensively considering the abovementioned factors, we believe that the Ti2CS2 surface, with medium binding energy, good electronic conductivity, and fast polysulfide conversion and Li diffusion, is the most favorable functionalized surface. Interestingly, two inspiring phenomena are found: (1) The Ti2CS2 surface with medium binding energy (rather than the Ti2CF2 surface with the smallest binding energy) has the lowest polysulfide conversion energy barrier. (2) Compared with Ti2CO2 and Ti2CF2 surfaces, the Ti2CS2 surface has the lowest Li diffusion energy barrier. To clarify the interesting phenomena, detailed discussions were carried out in the following Sections 4.1.1 and 4.1.2.

4.1.1. Discussion on Polysulfide Conversion

As mentioned in Section 4.1, the Ti2CS2 surface with a medium binding energy has the lowest polysulfide conversion energy barrier. To clarify the intriguing phenomenon, the corresponding rate-determining cleavage step Li4S2@Ti2CT2 → Li2S@Ti2CT2 on Ti2CS2, Ti2CF2, and Ti2CO2 surfaces was compared. As shown in Figure 9, the three starting intermediates of the cleavage steps are Li4S2@Ti2CS2, Li4S2@Ti2CF2, and Li4S2@Ti2CO2 with energies of −8.75, −7.85, and −9.71 eV, respectively. The corresponding binding energies of Li4S2 on the three surfaces are 7.31, 5.70, and 9.16 eV, respectively. It can be seen that the greater the binding energy, the lower the corresponding intermediate energy. The charge transfer and Li–S bond length values are consistent with the relative order of the binding energy. The charge transfer of Li4S2 on three surfaces is 0.551 e, 0.451 e, and 0.800 e, respectively. The Ti2CO2 surface with the largest binding energy corresponds to the greatest Li4S2 charge transfer, followed by the Ti2CS2 surface, and the Ti2CF2 surface corresponds to the least Li4S2 charge transfer. The strong adsorption of polysulfide to the surface stretches the Li–S bond of the polysulfide. The corresponding Li–S bond lengths are 2.49, 2.42, and 2.58 Å, respectively. The order of the Li–S bond length is also consistent with the order of binding energy, which means that the greater the adsorption energy, the longer the bond length.

Figure 9.

Figure 9

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Comparison of the cleavage step for the surfaces of (a) Ti2CS2, (b) Ti2CF2, and (c) Ti2CO2.

The energies of the products Li2S@Ti2CS2, Li2S@Ti2CF2, and Li2S@Ti2CO2 are −7.70, −5.31 ,and −6.48 eV, respectively. The binding energies of Li2S on the three surfaces are 4.28, 2.74, and 4.31 eV, respectively. Li2S@Ti2CS2 showed the lowest energy and the largest binding energy. Therefore, the stable product Li2S@Ti2CS2 caused by the strong adsorption of Li2S on the surface of Ti2CS2 is the origin for the small energy barrier of the cleavage step. Compared with Li2S@Ti2CF2 and Li2S@Ti2CO2, there is not only the Li–S interaction but also the S–S interaction between the Ti2CS2 surface and Li2S. The S–S interaction leads to the increased charge transfer of Li2S (0.387 e of Li2S@Ti2CS2 vs 0.148 e and 0.298 e for Li2S@Ti2CF2 and Li2S@Ti2CO2) and the stretched Li–S bond (2.38 Å vs 2.27 and 2.36 Å). Accordingly, the affinity of the Ti2CS2 surface to Li2S leads to the dual Li–S and S–S interactions, thereby elevating the binding energy and resulting in the stable product. This finally decreases the energy barrier of the cleavage step and promotes the kinetic conversion process on the Ti2CS2 surface.

4.1.2. Discussion on Li Diffusion

As mentioned, compared with Ti2CO2 and Ti2CF2 surfaces, the Ti2CS2 surface shows the lowest Li diffusion energy barrier. Herein, the detailed analysis and discussion of this interesting phenomenon were carried out. Figure 10 shows the charge distribution and the side view of Li migration on the three surfaces. As shown in Figure 10a–c, the migration path of Li on Ti2CS2 is hcp Ti → hcp C → hcp Ti. The charge density of hcp Ti and hcp C is 0.169 e and −0.289 e with a difference of 0.458 e. The migration path on Ti2CF2 is hcp C → hcp Ti → hcp C. The charge density of hcp Ti and hcp C is 0.299 e and −0.350 e, with a difference of 0.649 e. The path on Ti2CO2 is hcp C → bridge → hcp C. The charge density of hcp Ti and hcp C is 0.389 e and −0.302 e, with a difference of 0.691 e. Compared with Ti2CF2 and Ti2CO2, the charge density difference between Ti and C atoms along the migration pathway on the Ti2CS2 surface is smaller, indicating the more uniform charge density of the surface. The uniform charge density of the surface contributes to the small migration resistance. Meanwhile, the uneven charge distribution on the surface of Ti2CF2 and Ti2CO2 leads to higher Li diffusion barriers. Figure 10d–f shows the side view of Li diffusion. The distances between Li and the surface during the Li diffusion process are also given. The distance span (Δd) is the difference between the longest distance (dmax) and the shortest distance (dmin). For Ti2CS2, the Δd is 0.11 Å, while for Ti2CF2 and Ti2CO2, the Δd is 0.16 and 0.32 Å, respectively. The distance change on the Ti2CS2 surface is the smallest, which is consistent with the uniform charge density of the surface. In summary, the uniform charge distribution on the surface of Ti2CS2 makes the small distance change between Li and the surface during the Li migration process, which ultimately leads to a small energy barrier. This guarantees the fast Li diffusion on the Ti2CS2 surface.

Figure 10.

Figure 10

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Charge distribution on (a) Ti2CS2, (b) Ti2CF2, and (c) Ti2CO2 surfaces. The side view of Li diffusion on (d) Ti2CS2, (e) Ti2CF2, and (f) Ti2CO2 surfaces.

5. Conclusions

The titanium carbide-based MXenes have received great attention in lithium–sulfur batteries because of the high specific surface area, excellent electronic conductivity, facile Li+ diffusion, and rich functional groups on the surface. However, the complex functionalization (such as −OH, −S, −F, and −O) of the MXene surface leads to many confusion and contradictions about the favorable functionalized surface for LiPS adsorption and conversion. In the present study, with pristine Ti2C and −OH-, −S-, −F-, and −O-functionalized Ti2C surfaces as objects, the DFT calculation of the adsorption ability, electronic conductivity, and kinetic conversion ability was carried out to clarify the favorable functionalized MXene surface. For the adsorption, the pristine Ti2C and Ti2C(OH)2 surfaces show severe polysulfide deformation. The binding energy order for Ti2CO2, Ti2CS2, and Ti2CF2 is Ti2CO2 > Ti2CS2 > Ti2CF2. For the electronic conductivity, Ti2CO2 shows the narrow-band gap semiconductor property, while both Ti2CS2 and Ti2CF2 remain metallic during adsorption. For the kinetic conversion ability, the energy barrier order for both polysulfide conversion and Li diffusion is Ti2CO2 > Ti2CF2 > Ti2CS2. Accordingly, the Ti2CS2 surface is revealed to be the favorable functionalized surface because of the medium binding energy, good electronic conductivity, and fast polysulfide conversion and Li diffusion. Interestingly, the Ti2CS2 surface with a medium binding energy has the fastest polysulfide conversion and Li diffusion. It is further revealed that compared to Ti2CO2 and Ti2CF2, the affinity of the Ti2CS2 surface to the product Li2S contributes to facile polysulfide conversion. The uniform charge distribution on the Ti2CS2 surface contributes to the fast Li diffusion.

Acknowledgments

We thank the NSFC (21702041, 51972093, U1910210, and U1810204), 111 Project “New Materials and Technology for Clean Energy” (B18018). This work is also supported by the Fundamental Research Funds for the Central Universities of China (JZ2020HGTB0062, PA2020GDJQ0026, JZ2018HGBZ0100, and PA2019GDPK0052), the Key Research and Development Plan of Anhui Province (202004b11020024), and the Nature Science Research Project of Anhui province (2008085ME129). The numerical calculations in this paper have been done on the supercomputing system in the National Supercomputing Centre in Shenzhen.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04043.

  • Calculation method of charge transfer, electronic energies, and Cartesian coordinates of related compounds (PDF)

Author Contributions

∥ Q.Z. and X.Z. contributed equally to this study.

The authors declare no competing financial interest.

Supplementary Material

ao0c04043_si_001.pdf (283.7KB, pdf)

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