First-Principle Study of Li-Ion Storage of Functionalized Ti2C

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First-principles study of Li-ion storage of functionalized TiC monolayer with vacancies 2

Qing Wan, Shunning Li, and Jian-Bo Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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ACS Applied Materials & Interfaces

First-principles study of Li-ion storage of functionalized Ti2C monolayer with vacancies Qing Wan, Shunning Li, and Jian-Bo Liu* Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Corresponding Authors * Jian-Bo Liu. Tel: +86-10-62772619. Fax: +86-10-62771160. E-mail: [email protected]. ORCID: Jian-Bo Liu: 0000-0001-6516-6966

Notes The authors declare no competing financial interest.

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ABSTRACT: Two-dimensional transition metals carbides are notable as promising anode materials for Li-Ion batteries (LIBs). Using first-principles calculations, we investigate the effect of vacancies on the Li adsorption and diffusion on Ti2C and Ti2CT2 (where T denotes surface terminations, F or OH) monolayers. Interestingly, we find that the carbon vacancies (VC) tend to enhance the adsorption of Li in Ti2C monolayer. While the titanium vacancies (VTi) play a similar role in Ti2CT2 when functional groups present. The presence of vacancies further leads to a change in the diffusion behavior of Li atoms. In this context, we propose an idea to mitigate the adverse effects on Li diffusion performance by regulating the functional groups. In the presence of VC, the surface of Ti2C monolayer is suggested to be modified with OH- groups due to its relatively low diffusion barrier in the range of 0.025~0.037 eV when Li diffuses around VC. While in the presence of VTi, the surface is suggested to remove the functional groups resulting in a decrease of energy barrier about 1 eV when Li atom diffuses around VTi. The present study may provide a guideline to improve the Li-ion storage performance of Ti2C monolayers as electrode materials in LIBs, with atomic vacancies being taken into consideration. KEYWORDS: Ti2C monolayer, Li-ion batteries, first-principles calculation, adsorption and diffusion, vacancy, functional groups.


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1. INTRODUCTION

Two-dimensional (2D) materials have attracted great interest due to their unique physical structures and chemical properties.1-3 Graphene, as the 2D honeycomb-like material with monolayer carbon atoms, exhibits good electrochemical performances in application for energy storage.4-5 Other free-standing 2D materials such as silicene,6 MoS27 monolayers have also been proved to be promising anode materials for Lithium-ion batteries (LIBs). Recently, a new family of 2D transition metal carbides and/or nitrides (MXene) was synthesized by extracting the “A” element from the MAX phases by hydrofluoric acid (HF) solutions.8 The MAX phases can be described with a general formula Mn+1AXn, where “M” stands for an early transition metal, “A” represents an A-group (mainly IIIA and IVA) element, “X” denotes C and/or N, and “n” can be 1, 2 or 3.9 Since MXene is usually etched in HF solution, it has a mixture of O-, F- and OH- terminations,8 which are definitely crucial for the distinctive properties.10-11 Non-terminated MXenes are yet to be synthesized. For the sake of brevity, this is usually denoted as Mn+1XnTx, where Tx stands for the surface terminations. As reported in the reviews,12 more than 15 different MXene compositions have subsequently been synthesized, like Ti2CTx, V2CTx, Nb2CTx, Mo2CTx, Ti3C2Tx, Ta4C3Tx, (Ti0.5Nb0.5)2CTx.13-15 There are many research about MXenes with specific terminations (O-, F- and OH-) and the mixed terminations.16-17 Via chemical treatment, thermal annealing and mechanical exfoliation processes, the carrier transport behavior of MXene can be tuned by modifying the surface groups.16

Due to the versatile chemistry of MXene, it is regarded as a potential material in a variety of fields including reinforcement for composites,18-19 water purification20-23 and energy storage12, 24. Many studies have shown that MXenes are promising anode materials for ion batteries due to their fast ion diffusion and good rate capability.25-27 Michael et.al28 predicted that the most lightweight members of MXene family (M = Sc, Ti, V, or Cr) used 3 ACS Paragon Plus Environment


in LIBs have gravimetric capacities above 400 mAh g-1, higher than graphite. Zhou et.al29 calculated the theoretical capacity of Mn2C sheet as 879 mAh g-1, which is greater than that of most 2D electrode materials of LIBs.

Defects are inevitably introduced during the etching process and can affect the physical and chemical properties of materials.30-31 Etching and delamination conditions during the preparation process of MXene can introduce different concentrations of defects. Sang et al.32 have observed different atomic defects in monolayer Ti3C2Tx through scanning transmission electron microscopy. They confirmed the defect structures using density function theory-based calculation and predicted that the surface morphology could be influenced by these defects. Li et al.33 have also discovered crystal defects, including vacancies and dislocations in the Ti2C monolayers. Zhao et.al34 have studied the Li adsorption and diffusion properties on pristine Ti2C monolayer, the influence of atomic defects on the performances of Ti2C monolayer as the electrode used in LIBs is still left unexplored. In our work, we choose the Ti2C monolayer, the lightest material in MXenes family, to assess the influence of point defects (including carbon vacancy Vc and titanium vacancy VTi) on the performances of MXene as an electrode used in LIBs. We determine the geometry and magnetic configuration of Ti2C monolayer structures. Since MXenes that are synthesized using acidic-fluoride-containing solutions would inevitably introduce the O-, F- and/or OH- terminations, non-terminated Ti2C monolayer are yet to be synthesized.12 In contrast, the synthesis of Ti2C monolayers terminated with O-, F- and/or OH- groups has been widely confirmed.33, 35 Based on these geometric structures, the formation energies of these atomic defects are calculated in the situation of different surface functional groups. Compared the calculated adsorption energies and diffusion barriers of Li around the atomic defects on Ti2C monolayer with or 4 ACS Paragon Plus Environment

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without the functional groups (F- and OH-), we demonstrate that the functional groups bring different impacts on the adsorption and diffusion around different vacancies. Therefore, we infer that surface modification of the Ti2C monolayer can be essential for the electrochemical performance of MXenes when atomic defects are taken into account. It may provide guidance for experiments. 2. COMPUTATIONAL DETAILS Our calculations are performed in the framework of density functional theory (DFT) by using the projected augmented wave (PAW) method,36-37 as implemented in the Vienna ab initio simulation package (VASP).38 The exchange-correlation function is described by the parametrization scheme of Perdew-Burke-Ernzerhof (PBE) of the generalized gradient approximation (GGA).39 A cutoff energy of 520 eV for the plane wave basis set is used in each case. A convergence threshold of 0.02 eV Å-1 in force is required to be reached for all configurations. A 5×5 supercell of monolayers Ti2C and Ti2CT2 (T = F, OH) is employed to model the point defects and the Li adsorption, and the k-point sampling employs a 3×3×1 mesh within the Monkhorst-Pack scheme.40 An atomic layer of Ti2C is put in the xy plane, and in the z-direction the thickness of vacuum layer is set to 20 Å. For the calculations of Li adsorbed on the surface of monolayers Ti2C and Ti2CT2, the van der Waals interaction is considered using a dispersion correction term with the DFT-D3 method.41 The defect formation energy is defined as: perfect

defect 𝐸fdefect = 𝐸tot − 𝐸tot

+ ∑𝑖 𝑛𝑖 𝜇𝑖

(1) perfect

defect where 𝐸tot is the total energy of the supercell containing vacancies. 𝐸tot

is the

total energy of the same supercell without defects. 𝑛𝑖 is the number of atoms that have been added to or removed from the supercell, and 𝜇𝑖 is the chemical potential of species i. 5 ACS Paragon Plus Environment


3. RESULTS AND DISCUSSION

Figure 1. (a) Top view of pristine monolayer Ti2C (5×5 supercell). (b) The considered adsorption sites on the surface of Ti2C and Ti2CT2 (T = F, OH) monolayers. (c-f) Side views of monolayers Ti2C, Ti2CO2, Ti2CF2 and Ti2C(OH)2, respectively. 3.1 Structures and Electronic Properties of Ti2C and Ti2CT2 (T= F or OH) Monolayers. Based on the result as reported in the literature,34 we begin with the examination of the optimized crystal structure of Ti2C monolayer as presented in Figure 1. The layer of C atom is between two Ti atom layers, referred as Ti(1) in the upper layer and Ti(2) in the bottom layer, respectively. Each Ti2C monolayer is built up of triple layers stacked in the sequence of Ti(1)-C-Ti(2). Because of the magnetism of Ti atom from partially filled atomic d-shell, three types of magnetic states, namely nonmagnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM), are taken into consideration during the 6 ACS Paragon Plus Environment

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structural optimization (Table S1). Based on the structural symmetry, seven different configurations of AFM states are investigated in our work. The energy of FM configuration per formula unit for Ti2C is 110 meV lower than that of NM configuration, which is in agreement with the result of 118 meV in the literature.42 And the energy of AFM1 configuration per formula unit for Ti2C is 27 meV lower than that of FM configuration. Therefore, our calculation suggests that Ti2C monolayer prefers to form the AFM1 arrangement, similar to Ti3C2 monolayer.43 For the sake of brevity, the AFM1 configuration is referred to AFM configuration hereinbelow. Table 1 compares our results and those reported in the literature. While Gao et.al42 predicted that the Ti2C monolayer with FM state is the most stable. The result are calculated by the first-principles fullpotential linearized augmented plane-wave method (Wien2k package)44. Focusing on the adsorption and diffusion properties, we verify the different magnetic states have a trivial effect on the adsorption energy of Li on Ti2C monolayer, as presented in Table S1. The total density of states (DOS) of Ti2C monolayer with FM and AFM configurations are presented in Supporting Information (Figure S4). And the DOS of Ti2C monolayer with FM configuration is similar to the results in the literature.34



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Table 1. Summary of the data for the three magnetic states, nonmagnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM). a (Å)

Thickness (Å)

dTi-C (Å)A

dTi-Ti (Å)B

ΔEtot (meV/cell)C

NM

3.040

2.310

2.101

2.901

0

FM

3.083

2.235

2.102

2.857

-110

AFM

3.058

2.285

2.103

2.888

-137

References

3.042a

2.291c

2.101d

2.899a

3.076b

2.900d

A

Inter-atomic distances between Ti atom and its nearest C atom.

B

Inter-atomic distances between Ti atom and its nearest Ti atom in the opposite layer.

C

Energy difference per formula unit for Ti2C respect to the nonmagnetic state.

a

Ref.45, WIEN2K.

b

Ref.46, VASP.

c

Ref.47, CASTEP.

d

Ref.43, VASP. After determining the geometry and magnetic configuration of Ti2C monolayer

structures, we investigate the structures of Ti2C monolayer with different functional groups, including O-, F- and OH- (denoted as Ti2CO2, Ti2CF2 and Ti2C(OH)2, respectively). Three possible configurations for each functional group are considered as shown in Figure S2. The most stable configurations are selected for investigation and their side views are presented in Figure 1(d-f). The functional groups are located above the hollow sites formed by three neighboring C atoms on both sides of Ti2C monolayer. Moreover, by fully relaxed spin-polarized calculations, we find that Ti2C monolayer with functional groups (O-, Fand OH-) are nonmagnetic. To ascertain the stability of the functionalized Ti2C monolayers, we calculate their formation energies, defined as: 𝐸f = 𝐸tot (Ti2 CX2 ) − 𝐸tot (Ti2 C) − (𝑛⁄2)𝐸tot (X2 ) − 𝑛𝜇𝑋 8 ACS Paragon Plus Environment

(2)

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where Etot(Ti2CX2) and Etot(Ti2C) are the total energy of monolayer Ti2CX2 (X = O, F and OH) and Ti2C, respectively. 𝐸tot (X2 ) is the total energy of O2, F2, or O2+H2 in their gaseous reference state corresponding to these functional groups (O-, F- and OH-). The number of chemical groups in the supercell is denoted by n, which is equal to 2 for the current form. And 𝜇𝑋 is the chemical potential of the chemisorbed atoms, which can vary as a function of temperature and pressure.48 𝐸f is calculated for O-, F- and OH- functional groups on the monolayer Ti2C surface using Eq 2 with 𝜇𝑋 = 0 eV.49 The formation energies are -9.78 eV, -13.14 eV and -9.61 eV for monolayer Ti2CX2 (X = O, F and OH), respectively. The values are consistent with the results reported in the literature.50 A negative value of 𝐸f is indicative of exothermic bonding. Therefore, the surface functionalized with O-, F- and OH- groups can enhance the stability of Ti2C monolayer in thermodynamics.



Figure 2. Optimized structures of (a) carbon vacancy VC and titanium vacancy VTi. (b) Vacancies formation energy of Ti2C and Ti2CT2 (T = F, OH) monolayers. 3.2 Vacancies in Ti2C and Ti2CT2 (T= F or OH) Monolayers. As mentioned above, the preparation of MXene flakes introduces not only many functional groups but also intrinsic defects, such as atomic vacancies (including carbon vacancy VC and titanium vacancy VTi). The centration of vacancies and surface functional groups can be changed by means of regulating the etching and delamination conditions. The equilibrium structures of two kinds of atomic vacancies including VC and VTi are taken into consideration in our work. The optimized structures of Ti2C and Ti2CT2 (T = F, OH) monolayers with vacancies 10 ACS Paragon Plus Environment

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are presented in Figure 2a, and the corresponding DOS are presented in Supporting Information (Figure S5~S7). The stability of the vacancies, as a key to describe their behavior, can be determined by calculating the formation energy. Although vacancy formation energy is regarded as a function of chemical potential μi ,51 over a range of condition from Ti-rich to C-rich, we are only interested in VC in C-rich condition and VTi in Ti-rich condition. When calculating the vacancy formation energy by Eq 1, μC = μbulk C is assigned for the carbon vacancy and μTi = μbulk is assigned for the titanium vacancy. Ti The superscript “bulk” denotes the corresponding bulk state, which is used to avoid precipitation of elemental solids51. The same method was used by Sang et al in their work.32 The formation energy of VTi in Ti2CO2 increases to 8.59 eV and is much higher than that for bare Ti2C monolayer, suggesting a strong interaction between Ti and the O- functional groups. It is worth mentioning that the VC formation energy of Ti2CO2 monolayer is negative, which indicates that the VC will be formed spontaneously, leading to a large concentration of defects. Therefore, in the following study, we exclude the case of Ofunctional groups, but focus on the three kinds of surfaces (bare, F- and OH-). The results of vacancies formation energy of Ti2C and Ti2CT2 (T = F, OH) monolayers are shown on Figure 2b. Among these three cases, the formation energy of VC is relatively low (2.42 eV) with OH- functional groups and that of titanium vacancy is relatively low (2.92 eV) with the bare surface. This is one of the important criteria to choose functional groups for different vacancies.



Figure 3. Color filled contour plots of adsorption energy of Li on the surface of pristine and defective Ti2C and Ti2CT2 (T = F, OH) monolayers. 3.3 Li Adsorption on Pristine Ti2C and Ti2CT2 (T= F or OH) Monolayers. In order to determine the favorable adsorption site, seven possible sites are considered for each atomic triangle, which is constituted by contiguous Ti(1) atom, C atom and Ti(2) atom, as indicated in Figure 1b. Site H, C and T are on the top of Ti atoms in the upper layer, C atoms in the middle layer and Ti atoms in the lower layer, respectively. Sites B1, B2 and B3 are on the top of the bridge sites and site O is above the center of the triangle. The full 12 ACS Paragon Plus Environment

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structures are relaxed except restraining the xy-direction of Li. Based on the adsorption energy of Li at these seven sites in each atomic triangle, a cubic interpolation is applied to generate the color filled contour plots with hexagonal areas, which are presented in Figure 3. The details of depicting the color filled contour plots are presented in Supporting Information (Figure S8). The adsorption energy per Li atom is defined as: 𝐸ad (Li) = 𝐸MXene+Li − 𝐸MXene − 𝐸Li

(3)

where 𝐸MXene+Li is the total energy of the pristine or defective Ti2C and Ti2CT2 (T= F or OH) monolayers adsorbed with one Li atom. 𝐸MXene is the total energy of corresponding monolayers Ti2C and Ti2CT2, and 𝐸Li is the total energy of one isolated Li atom. In the literatures, there are two types of definition for Li adsorption energy. One uses the energy of Li from the bulk sample with body-centered cubic structure,52 and the other uses the energy of an isolated Li atom.34, 53-59 Since the latter definition has already been widely accepted in the literature, we prefer to use the latter one. Under this definition, the higher the absolute value of 𝐸ad , the stronger the binding of Li to the monolayer. Hereafter, we use the terms “adsorption” and “binding” interchangeably.53 In the absence of defects, there are three typical sites on the monolayer Ti2C surface, H site, C site and T site. The corresponding adsorption energies are -2.291 eV, -2.262 eV and -2.149 eV, respectively. The calculated results show that the H site of Ti2C monolayer is the most stable site for Li adsorption, which is also consistent with the result reported in literature.34 However, due to the steric hindrance of the functional groups, the H site is not the best site but the C site in the Ti2CF2 monolayer, and the adsorption energy at the C site is -2.778 eV. In Ti2C(OH)2 monolayer, the adsorption energy is higher than that of Ti2C monolayer as a whole, and the adsorption energy ranges from -1.14 eV to -1.07 eV. From a thermodynamic point of view, this also means that Li is less readily adsorbed on the pristine monolayer Ti2C(OH)2 surface than pristine monolayer Ti2C and Ti2CF2. 13 ACS Paragon Plus Environment


3.4 Li Adsorption on Defective Ti2C and Ti2CT2 (T= F or OH) Monolayers. To examine the influence of atomic vacancy in Ti2C monolayer with different surfaces (bare, F- and OH-), the Li adsorption energy around the atomic vacancy are calculated. Compared to the case of the pristine Ti2C monolayer, the optimal adsorption site in the vicinity of VC changes from H site to T site with adsorption energy reduced by 0.265 eV. Furthermore, the Li adsorption energies in the vicinity of the VC are ranging from 0.026 eV to 0.408 eV lower than that of the corresponding sites in pristine Ti2C monolayer as a whole. Therefore, the presence of VC promotes the adsorption of Li atom in the Ti2C monolayer. However, the presence of functional groups seems to have a "buffering" effect on the change in adsorption energy with VC being introduced. On the surface of the monolayer Ti2CF2, the optimal adsorption site is still in C site with a little change in adsorption energy about 6 meV. On the surface of Ti2C(OH)2 monolayer, the influence of VC on the values of adsorption energy is trivial and the changes vary from -0.022 eV to 0.034 eV. Consequently, the presence of VC tends to enhance the Li adsorption on the Ti2C monolayer in the absence of functional groups. In contrast to VC, the influence of VTi in Li adsorption on Ti2C monolayer with different surfaces (bare, F- and OH-) is significant. For the Ti2C monolayer, the local minimum of adsorption energy in the vicinity of VTi is 0.569 eV higher than that of the sites away from the vacancy, indicating that the Li atom is not preferable to adsorb in the vicinity of the VTi. Since the adsorption energy is mostly contributed by the Li-Ti bonds, the presence of VTi can lead to weakening Li-Ti interaction in the case without functional groups, thus displaying a reduced preference of Li adsorption. While in the case of the Ti2CF2 monolayer, the site above the center of vacancy is the optimal adsorption site and the adsorption energy is 1.642 eV lower than that of pristine monolayer Ti2CF2. In the case of monolayer Ti2C(OH)2, the introduction of VTi leads to a decrease by 1.332 eV in Li adsorption energy in the vicinity of the vacancy. Consequently, the presence of VTi tends 14 ACS Paragon Plus Environment

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to enhance the Li adsorption on the Ti2C monolayer functionalized with F- or OH- groups. In addition, as presented in Figure 3, the diffusion of Li atom between the sites in the vicinity of VTi and the sites away from the vacancy requires a larger energy barrier. A detailed discussion of the energy barrier will be presented below.



Figure 4. Diffusion barriers of Li on pristine (a) Ti2C, (b) Ti2CF2 and (c) Ti2C(OH)2 monolayers. The minimum-energy pathways for Li diffusion are shown in the inset of each panel. 3.5 Li Diffusion on Pristine Ti2C and Ti2CT2 (T= F or OH) Monolayers. Since the rate capability of LIBs is closely linked with diffusion properties, we turn our attention to the mobility of Li atom on surfaces of monolayers Ti2C and Ti2CT2. Based on the calculated contour plot of adsorption energy, the energetically feasible diffusion paths are selected for further investigation. With optimizing the whole structures, the climbingimage nudged elastic band (CI-NEB) method60 is used to quantitatively determine the minimum-energy pathways and their corresponding diffusion energy barriers for Li on the surface of these monolayers. Since the diffusion process involves cooperative motion of several atoms simultaneously, including the Li atom and its neighboring atoms of the substrate, the relative diffusion coordinate is the cumulative sum of the trajectory length of all atoms in the structure and then normalized. We first focus on the situations of pristine monolayer Ti2C and Ti2CT2. Their corresponding migration pathways are depicted in Figure 4. The minimum-energy pathway of the Ti2C monolayer is from the H site through the C site to the nearest H site (H→C→H′). As Li atom diffused along the path H→C, it adsorbs on the C site to form a metastable configuration with an energy barrier of 0.034 eV. While the energy barrier of the second path C→H′ is only 0.004 eV. Zhao et.al34 calculated the energy barrier for Li diffused along the path H→C on pristine Ti2C monolayer as 0.02 eV. The difference of the energy barriers between the result of our calculation and the literature is because of the different supercell size used in the calculation. The 2×2 supercell of Ti2C monolayer is used in their calculation thus leading a high concentration of Li, while we use 5×5 supercell to get a more reasonable result. 16 ACS Paragon Plus Environment

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In the case of monolayer Ti2CF2, the configuration of Li atom adsorbed on the C site is used as the initial position. The minimum-energy pathway of the Ti2CF2 monolayer is from the C site through the T site to the nearest C site (C→T→C′) with an energy barrier of 0.207 eV, which is much higher than that of Ti2C monolayer. The minimum-energy pathway for monolayer Ti2C(OH)2 is completely different from the previous two. Li atom is migrating directly from H site to the adjacent H site in a linear path (H→H′) with an energy barrier of 0.043 eV. Overall, the diffusion of Li on the bare surface of Ti2C monolayer has a lower energy barrier than that of the surface with functional groups. In the absence of defects, the functional groups have a significant repulsive effect on the diffusion of Li atom due to the influence of steric hindrance.



Figure 5. Diffusion barriers of Li on (a) Ti2C monolayer with carbon vacancy VC, (b) Ti2CF2 monolayer with VC, (c) Ti2C(OH)2 monolayer with VC, (d) Ti2C monolayer with titanium vacancy VTi, (b) Ti2CF2 monolayer with VTi, (c) Ti2C(OH)2 monolayer with VTi. The energy barriers for Li diffusing to the vicinity of vacancy and away from the vacancy are colored by green and blue, respectively. Metastable sites on these paths are marked alphabetically in turn.


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3.6 Li Diffusion on Defective Ti2C and Ti2CT2 (T= F or OH) Monolayers. Next we examine the case of Li atom diffused on the surfaces of monolayers Ti2C and Ti2CT2, with atomic vacancies being taken into account. Based on the contour plots of adsorption energy in Figure 3, the minimum-energy pathways of Li diffused from the site away from the vacancy to the vicinity of the vacancy are selected for further investigation. Among the sites both distant and adjacent from vacancies, the sites in which the adsorption energies are the local minima are chosen as the initial and final points of the diffusion paths, respectively. A site with local minimum represents the metastable site for Li adsorption on the surface. The metastable sites on these paths are marked alphabetically in turn. The full structures are optimized and the energy barriers of these paths are calculated through CINEB method to study the influence of the vacancies on Li diffusion on different surfaces. After introducing the VC into the Ti2C monolayer, the nearest T site in the vicinity of VC is the optimal adsorption site as mentioned above and is chosen as the final point c. When Li atom diffuses to the c point, it requires a two-step transition (a→b→c), as presented in Figure 5a. The corresponding energy barriers are 0.085 eV and 0.068 eV, respectively. The first step a→b is consistent with the path C→H in pristine Ti2C monolayer, but the energy barrier is increased by 0.081 eV. In these steps, the diffusion barrier with the largest value is regarded as the energy barrier of the entire path. Therefore, the energy barrier when Li diffuses to the vicinity of VC is 0.085 eV. When Li diffuses away from the VC along the path c→b with an energy barrier of 0.163 eV, it will be arduous to migrate out once it adsorbs in the vicinity of vacancy. As the VC introduced into the Ti2CF2 monolayer, it required a four-step transition (a→b→c→d→e) for Li diffusing to the vicinity of VC, as presented in Figure 5b. The energy barrier of the path in this direction is 0.218 eV. When Li diffuses along the path, it will bypass the H site because these sites are adsorbed with F- functional groups, which 19 ACS Paragon Plus Environment


results in steric hindrance. The adsorption energy and the diffusion barrier are not significantly changed as compared with the pristine case. Thus, the presence of VC has a trivial influence on the diffusion of Li on the surface of Ti2CF2 monolayer. As the VC introduced into the Ti2C(OH)2 monolayer, there is an only one-step transition (a→b) with an energy barrier of 0.025 eV when Li diffuses to the vicinity of VC, as presented in Figure 5c. The sites in the vicinity of the vacancy are suitable for Li adsorption and can easily migrate out with a small energy barrier of 0.037 eV. Compared with the diffusion energy barriers on the three different surfaces (bare, Fand OH-) in the presence of VC, the energy barrier of Li on OH-terminated surface is relatively lower than that of the other two kinds of surface. Therefore, when VC is taken into account, the surface of Ti2C monolayer is suggested to be functionalized with the OHgroups, thus resulting in better rate performance in LIBs. After introducing the VTi into the Ti2C monolayer, Li is not preferable to adsorb in the vicinity of VTi as mentioned above. When Li diffuses to the vicinity of VTi, it is migrating directly from initial point a to the final point c in a linear path and requires a twostep transition (a→b→c) with an energy barrier of 0.473 eV. As the energy barrier for Li diffusing far away from VTi is only 0.036 eV (Figure S3), it is more preferable for Li to bypass the VTi. If Li is adsorbed near the center of VTi, it can rapidly escape from the vacancy due to the low energy barrier of 0.086 eV (c→b). When the surface is terminated with functional groups, the paths when Li diffuses to the vicinity of VTi are more complex than the case of Ti2C monolayer. As the VTi introduced into the Ti2CF2 monolayer, it required a three-step transition (a→b→c→d) with an energy barrier of 0.204 eV for Li diffusing to the vicinity of VTi, as presented in Figure 5e. When Li diffuses along the path, it still bypasses the H site because of the steric hindrance caused by the F-functional groups. However, Once Li adsorbs in the vicinity of the vacancy, it will 20 ACS Paragon Plus Environment

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be relatively difficult for Li to migrate away from these sites because of the high energy barrier of 1.192 eV (d→c). The energy barrier for Li diffusing away from the vacancy is much higher than the case for Li diffusing to the vicinity of the vacancy and the similar situation exists in the case of Ti2C(OH)2 monolayer. In the four-step transition (a→b→c→d→e) required for Li approaching the center of VTi on the surface of Ti2C(OH)2, the energy barrier is found to be 0.413 eV (d→e). Yet, it seems more comfortable for Li to stay at the d site due to the relatively high energy barrier from d site to e site. Therefore, the path d→e is left out of consideration in our work. Accordingly, the energy barrier for Li approaching VTi should be 0.242 eV (a→b→c→d), which is marked in the Figure 5f with green color. On the other hand, the diffusion barriers reach over 1 eV for Li diffusing away from the vacancy (b→a). Therefore, the introduction of VTi is not conducive to the diffusion of Li on the surface with F- or OH- functional groups. The details of the energy barriers in these paths as mentioned above are presented in Table S3 of the Supporting Information. Compared with the case of monolayer Ti2CT2, the diffusion barrier decreases by about 1 eV after removing the functional groups. Therefore, when VTi is taken into account, bare surface is recommended for Ti2C monolayer, which may provide more desirable performance than the functionalized ones. Considering the combinations of the two kinds of vacancies and three kinds of surfaces, the relatively lower energy barriers exist when Li diffuses in the case VC in Ti2C(OH)2, and VTi in Ti2C. The relatively higher energy barriers exist when Li diffuses in the case VC in Ti2C and Ti2CF2, and VTi in Ti2CF2 and Ti2C(OH)2.



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Table 2. The energy barriers that Li diffuses to the vicinity of vacancy on different twodimensional materials.

Pritine Defect

Ti2C

Ti2CF2

Ti2C(OH)2

0.034

0.207

0.043

VC

VTi

0.085

0.473

a

Ref.57

b

Ref.31

c

Ref.58

d

Ref.61

VC

VTi

0.218 0.204

Graphenea Siliceneb

MoS2c

Phosphorened

0.311

0.250

0.230

0.090

VC

VTi

VC

VSi

VS

VP

0.025

0.242

0.300

0.300

0.240

0.130

Defects are inevitably introduced during the synthesis process of 2D materials as an anode for LIBs, and the influence of defects on the adsorption and diffusion has been investigated in these 2D materials, such as graphene40, silicence31, MoS258 and phosphorene61. Furthermore, being comparable to the energy barriers in Table 2, there is a conspicuous superiority in energy barrier for pristine Ti2C monolayer among these 2D materials. However, after the introduction of defects, comparing with other 2D materials, the impact of defects for Ti2C monolayer is significant. In conjunction with the above discussion, the surface of Ti2C monolayer is suggested to be modified with OH- functional groups in the presence of VC. The energy barrier for Li diffusion to the vicinity of the vacancy is only 0.025 eV, which is much lower than that of graphene. In the presence of VTi, bare surface is recommended for Ti2C monolayer. Therefore, the minimum-energy pathway will bypass the vacancy with a small energy barrier of 0.036 eV. In this way, even the presence of vacancies is detrimental to Li diffusion, we can modify different surfaces of Ti2C monolayer to achieve better performance than other 2D materials. 22 ACS Paragon Plus Environment

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4. CONCLUSION In summary, using First-principles calculations, we choose functionalized Ti2C monolayer, the lightest materials in MXene family, to assess the influence of intrinsic vacancies (carbon vacancy and titanium vacancy) on the Li-ion storage performance of MXene as promising candidates for LIBs electrodes. Our calculations reveal that the carbon vacancies tend to enhance the adsorption of Li in Ti2C monolayer. While the titanium vacancies play a similar role in Ti2CT2 when functional groups present. The presence of vacancies further leads to a change in the diffusion behavior of lithium atoms. Based on our calculations of energy barriers for Li on Ti2C and Ti2CT2 monolayers, we propose an idea to mitigate the adverse effects on Li diffusion performance by regulating the surface functional groups. In the presence of VC, the surface of Ti2C monolayer is suggested to be modified with OH- functional groups due to its relatively low diffusion barrier in range of 0.025~0.037 eV when Li diffuses around VC. While in the presence of VTi, the surface of Ti2C monolayer is suggested to remove the functional groups resulting in a decrease of energy barrier about 1 eV when Li atom diffuses around VTi. Our study may provide a guideline to improve the rate performance of Ti2C monolayers as electrode materials in LIBs, with the atomic vacancies being taken into account.



ASSOCIATED CONTENT

Supporting Information. The Information is available free of charge via the Internet at http://pubs.acs.org. The details about different magnetic configurations of Ti2C monolayer, the considered types of Ti2CT2 (T = O, F and OH), the energy barriers for Li diffusion along the considered paths, the additional figure about Li diffusion on Ti2C monolayers with titanium vacancy and the DOS of the Ti2C and Ti2CT2 (T= F or OH) monolayers with or without atomic vacancies are given in the Supporting Information. 23 ACS Paragon Plus Environment




AUTHOR INFORMATION

Corresponding Authors * Jian-Bo Liu. Tel: +86-10-62772619. Fax: +86-10-62771160. E-mail: [email protected]. ORCID Jian-Bo Liu: 0000-0001-6516-6966 Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENT

The authors are grateful for the financial support from the Ministry of Science and Technology of China (2017YFB0702401, 2017YFB0702301, and 2017YFB0702201), the National Natural Science Foundation of China (51571129, 51631005), the Science Challenge Project (No. TZ2016004), and the Administration of Tsinghua University. The illustrations of crystal structures were drawn with VESTA software.62 

REFERENCES

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TOC graphic


First-Principle Study of Li-Ion Storage of Functionalized Ti2C

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