Role of Strain and Concentration on the Li Adsorption and Diffusion

Jun 20, 2014 - ... for High-Performance Supercapacitors. Zhenhua Li , Haohong Duan , Mingfei Shao , Jianbo Li , Dermot O'Hare , Min Wei , Zhong Lin Wa...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Role of Strain and Concentration on the Li Adsorption and Diffusion Properties on Ti2C Layer Shijun Zhao,*,†,‡ Wei Kang,†,‡ and Jianming Xue†,§ †

HEDPS, Center for Applied Physics and Technology, Peking University, Beijing 100871, China College of Engineering, Peking University, Beijing 100871, China § State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China ‡

S Supporting Information *

ABSTRACT: The performance of Li-ion batteries relies heavily on the capacity and stability of constituent electrodes. Recently synthesized 2D MXenes have demonstrated excellent Li-ion capacity with extremely high charging rates. In this work, first-principles calculations are employed to investigate the effects of external strain and Li concentration on the adsorption and diffusion of Li on Ti2C layer, a representative MXene. Our calculations demonstrate that the binding energy of Li atoms decreases monotonically with external strains, and the mechanical properties are not influenced by Li adsorption. For multiple Li atoms adsorption, their stable configurations show that the Li atoms tend to reside in one side first, in contrast with other 2D materials. We further show that the binding energy of Li is weakly dependent on the Li concentration. The diffusion barrier is calculated, and the results show that the strain and concentration have limited effects on the diffusion of Li atoms. Finally, the adsorption of Li atoms on two types of Ti2C double layer are considered. For all studied structures, their stabilities are examined by molecular dynamics simulations carried out at room temperature. The influence of Li adsorption on the electronic structures of Ti2C layer is also discussed. Our results suggest that Ti2C could be a promising electrode material for lithium ion batteries in terms of lithium storage capacity and stability at a high Li recycling rate.

■

represents an early transition metal, “A” is an A-group element (mostly groups IIIA and IVA), and “X” denotes carbon or nitrogen. The layered structure makes the exfoliation of MAX phase feasible. By removal of the “A” group layer from the MAX phase, 2D MXenes can be obtained which share the structural similarities with graphene.11,12 Soon after the discovery of MXenes, the feasibility and experimental demonstrations of the MXenes in energy storage applications such as LIBs and electrochemical capacitors (supercapacitors) are reported.13−15 The capacity of Ti3C2 for Li ions was found to be 320 mAh/g, which is comparable to the 372 mAh/g of graphite.11 Besides, the cycling rate is more superior due to the small diffusion barrier for Li on Ti3C2. Theoretically, the adsorption of Li ions on Ti3C2 monolayer was investigated with first-principles calculations,11,16 mainly concentrated on the stable adsorption site and diffusion barriers. Because the experimentally synthesized MXenes are always functionalized with various chemical radicals, the role of surface chemistry on the Li adsorption properties of MXenes are also studied.17 These results reveal that Tin+1Cn layers are promising candidates for LIBs, and surface functional groups

INTRODUCTION Because of the large surface-to-volume ratio and unique electronic properties distinguished from their bulk counterparts, 2D materials have attracted a great deal of attentions in the field of lithium ion battery (LIB).1,2 Since the first experimental realization of graphene, other free-standing 2D materials such as BN, dichalcogenides (such as MoS2, NbSe2), silicene, and germanene have also been predicted, and some of them have been successfully isolated either by mechanical and chemical cleavage or direct growth with chemical vapor deposition.3,4 The possibility of these 2D sheets used in LIB has also been explored in view of their unusual morphologies, which provide advantages for fast Li ion diffusion and large Li capacities.5,6 However, the weak binding of Li with graphene and other 2D sheet hinders its further applications in LIBs.5 Very recently, a family of 2D materials called as MAXenes were synthesized by exfoliation of the layered ternary transition-metal carbides, which are known as MAX phase.7,8 The MAX phases represent a large (more than 60 members) family of ternary layered machinable transition-metal carbides, nitrides, and carbonitrides, which possess unique properties such as remarkable machinability, high damage tolerance, excellent oxidation resistance, and high electrical as well as thermal conductivity.9,10 They belong to the space group of P63/mmc with the formula of Mn+1AXn (n = 1, 2, 3), where “M” © 2014 American Chemical Society

Received: May 7, 2014 Revised: June 18, 2014 Published: June 20, 2014 14983

dx.doi.org/10.1021/jp504493a | J. Phys. Chem. C 2014, 118, 14983−14990

The Journal of Physical Chemistry C

Article

tend to degrade the Li diffusions due to steric hindrance induced by the surface groups.16 Within this broad context, the interaction of Li with MXenes remains largely unexplored. An in-depth understanding of the performance of MXenes used for LIBs requires further exploration of the adsorption properties in response to external influences. For example, it has been shown that strain in nanomaterials has a great impact on the Li storages due to volume change.18,19 However, the characteristics of Li adsorption and diffusion on MXenes under external strain and different Li concentrations, which are especially important for battery performance, are still not investigated. Besides, the Li adsorption on multilayer MXenes has been investigated little up to now. To further assess the suitability of MXenes as a host material for LIBs, we employ first-principles calculations in this work to study the adsorption and diffusion of Li on Ti2C layer under external strains and different Li concentrations based on density functional theory (DFT). The free-standing single-layer and double-layer structures of Ti2C are considered, which is a prototype of MXenes. Our calculations indicate that a biaxial strain decreases the binding energy of Li atoms significantly due to the insufficient charge transfer under high strains, and the mechanical properties of Ti2C monolayer are not influenced by Li adsorption. The binding energy of Li is insensitive to the adsorption concentrations. We further show that multiple Li atoms tend to adsorb on one side of Ti2C layer rather than both sides, as found in other 2D materials. The stability of partially and fully lithiated structures is further evaluated by molecular dynamics (MD) simulations. The calculated diffusion barriers of Li are found to be little affected by strain and concentrations. These results suggest that the Ti2C layer is rather stable during lithiated and delithiated processes and provide explanations for the experimentally observed high cycling rate.

this definition, a higher value of Eb means stronger binding of Li to Ti2C layer. The calculated equilibrium lattice constant of Ti2C is increased gradually to model the biaxial strain to investigate the effects of strain on the adsorption behavior of Li atoms. Here the strain is defined as ε = (a − a0)/a0 = Δa/a0, where a0 and a is the lattice parameter of the unstrained and strained supercell, respectively. The Ti2C is represented by a 2 × 2 supercell with a vacuum space of 18 Å between two adjacent layers to avoid interactions between periodic images of slabs in the z direction. The Brillouin zone is sampled using a 8 × 8 × 1 Monkhorst−Pack grid. These parameters have been tested to ensure the convergence of energy to 8, the binding energy of Li starts to decrease remarkably, suggesting strong repulsion between adsorbed Li atoms. Hence, we take n = 8, corresponding to Li0.4[Ti2C]0.6 as the fully lithiated structure of monolayer Ti2C. The binding energies for all considered configurations as a function of Li concentration x are summarized in Figure 3. For one Li concentration, different binding energies obtained with different configurations are presented. It indicates that the binding energies of Li vary a little along with the lithiation content, and their values are all ∼2.05 eV, except for one Li atom adsorbed at the top of Ti(1) atom, which means that Li binds strongly with Ti2C monolayer, even for multi Li atoms. The small variation is also observed in other 2D materials such

Figure 3. Dependence of the binding energy of Li on its concentration x on Ti2C monolayer. For each Li concentration, different adsorption configurations are considered and thus different adsorption energies are obtained. The highest adsorption energies are almost all found when Li atoms are located at the top of C atoms, denoted by square red rectangles. The second highest adsorption energy is represented by blue circle, with some Li atoms adsorbed above Ti(2) atoms. The third and fourth highest adsorption energies are indicated by diamond and stars, respectively. The different adsorption configurations are all provided in the Figure S3 in the Supporting Information.

as silicene30 and MoS2.29 This result favors the lithiation process of monolayer Ti2C significantly because the lithiation can spontaneously occur if there is no kinetic transport resistance. It should be noted that these binding energies do not include the contribution of zero-point energy and entropic contributions. The relative stability of these adsorption configurations can be changed by these corrections, but the general trends are unlikely to be affected, as previously pointed out.6,28 The binding energy around 2.05 eV indicates Li atoms binding strongly with Ti2C monolayer. Here we have calculated the electronic properties including band structure and total density of states (TDOS) as well as projected density of states (PDOS) of pristine Ti2C monolayer and fully lithiated Ti2C monolayer. The results are demonstrated in Figure 4. It is shown from Figure 4c,d that upon complete lithiation, the magnetism of Ti2C monolayer totally disappears. The Ti2C monolayer has a magnetic moment of 1.87 μB/unit cell (Figure 4a,b), which is mainly attributed to Ti(3d) electrons in the surface. This magnetism has gone for fully lithiated Ti2C since Ti atoms are saturated. Actually, the magnetism is gradually decreased as the number of adsorbed Li atom increases until all surface Ti atoms are saturated, which finally results in a nonmagnetic transition. The metallic property of Ti2C is not influenced by lithiation. However, the gap ranging from −3 to −1.8 eV in the band of pristine Ti2C has been filled by the contribution of Li(s) and C(p) states due to their bonding interactions. To assess the stability of lithiated Ti2C, we have carried out MD simulations to analyze the structural evolution under room temperature. The results for four Li atoms adsorbed above four C atoms on one side are rather stable, while eight Li atoms adsorbed above four C atoms on two sides tend to transform to another configuration with four Li atoms below four C atoms, and another four Li atoms on the other side tend to occupy the hollow sites surrounding by C, Ti(1), and Ti(2) atoms, as shown in the Figure S4b in the Supporting Information. This configuration is unstable with 1.34 eV energy higher than the original configuration (Figure S4(a) in the Supporting 14986

dx.doi.org/10.1021/jp504493a | J. Phys. Chem. C 2014, 118, 14983−14990

The Journal of Physical Chemistry C

Article

Figure 5. Calculated energy profiles along the lowest energy diffusion paths for (a) one Li atom adsorbed on the pristine Ti2C layer, (b) one Li atom adsorbed on O-fractionalized Ti2C layer, (c) one Li atom adsorbed on the pristine Ti2C layer at 16% strains, and (d) seven Li atoms adsorbed on the pristine Ti2C layer. The diffusion path is provided in the insets. Figure 4. Band structures and projected density of states of pristine (a,b) and fully lithiated Ti2C monolayer (c,d).

Ti2C layer is 0.32 eV (Figure 5b), comparable to the value in previous calculations (0.27 eV).17 This result indicates that surface functionalization increases the barrier remarkably, although the binding energies are enhanced. Therefore, a compromise should be made to balance the capacity and diffusion for practical applications. The diffusion of Li atoms on Ti2C layer is almost unaffected by strain and concentration considered in this work. For example, the diffusion barrier for Li on Ti2C at 16% strain is 0.05 eV (Figure 5c). The diffusion of Li atoms at high concentrations is examined by the removal of an Li atom from the fully lithiated Ti2C layer to investigate the diffusion of one Li atom in the presence of other Li atoms. The initial and final configurations considered for Li diffusion are provided in the insets of Figure 5d. It is demonstrated that the Li atom initially located at the hollow site undergoes a arc diffusion path to its nearest neighboring hollow site, along with the exchange of adsorption sites for the other two Li atoms due to their repulsive interactions. The diffusion barrier is only 0.01 eV and comparable to that of a single Li atom on Ti2C layer. Therefore, the strain and concentration have a negligible effect on the diffusion of Li atoms on bare Ti2C layer, which should have great implications for practical applications. Li Adsorption on Double-Layer Ti2C. Because the synthesized MXenes are always multilayered, the Ti2C double layer with two monolayers stacked together is also considered in this work. The properties of Ti2C multilayer have not been fully explored so far. After fully optimization, the stable structures are found in either AA or AB arrangement, as shown in Figure 6a,b, respectively. By minimize the energy as a function of in-plane lattice constant, the lattice parameter is calculated as 3.095 and 3.093 Å for AA and AB stacking, respectively. The corresponding interlayer distance is 2.541 and 2.505 Å. In addition, the energy of AB arrangement is 0.56 eV lower than that of AA configuration, which means that AB stacking is more energy favorable. The obtained magnetic moment is 2.08 and 1.63 μB/ unit cell for AA and AB configurations, respectively. Their structural stabilities are reexamined by MD simulations. The

Information). Actually, the configuration (b) is converted to configuration (a) after structural optimization at 0 K. Therefore, our MD results demonstrate that the adsorption configurations of Li atoms on Ti2C monolayer can be influenced by the temperature effect. However, because the binding energies of low-energy structures are very similar, as indicated by Figure 3, these adsorption sites would switch back and forth due to temperature fluctuations. This observation also illustrates that the diffusion barrier of Li atoms is low on Ti2C monolayer. Because the experimentally synthesized MXenes are often functionalized with various radicals such as F, OH, and O, we also considered these functionalized Ti2C monolayers. The obtained results are consistent with recent report,17 which shows that the O-terminated Ti2C exhibits the strongest adsorption for Li atoms. The binding energy is 2.79 eV/atom for four Li atoms adsorption on one side and 2.94 eV/atom for eight Li atoms adsorption on both sides of O-functionalized Ti2C monolayer, while for F-functionalized Ti2C, these two binding energies are only 1.62 and 1.64 eV/atom, respectively. Nevertheless, the diffusion of Li atoms tends to degrade by these surface functionalizations, as will be discussed in detail later. Diffusion of Li Atoms on Ti2C Layer. The performance of MXenes used in LIBs relies heavily on the diffusion properties of Li atoms. It has been reported that the pristine MXenes have an rather low diffusion barrier for an isolated Li atom and the surface fictionalization has a significant influence on the diffusion of Li atoms.16 To evaluate the effect of the strain and concentration on the diffusion of Li atoms on Ti2C layer, we have calculated their diffusion barriers and demonstrate the results in Figure 5. It can be seen from Figure 5a that the diffusion barrier for a single Li atom on bare Ti2C layer is only 0.02 eV, which is in line with the diffusion behaviors previously observed in MD simulations. The diffusion barrier of Li on O-fractionalized 14987

dx.doi.org/10.1021/jp504493a | J. Phys. Chem. C 2014, 118, 14983−14990

The Journal of Physical Chemistry C

Article

that these configurations are unstable. The double-layer structure is still intact upon Li intercalation, indicating that the double-layer Ti2C layer will not exfoliate upon lithiation process. To assess the stability of fully lithiated Ti2C double layer, we also performed MD simulations at room temperature. The results demonstrate that the lithiated configuration of AA stacking undergoes a transformation into AB stacking, in accordance with pristine Ti2C double layer. Therefore, AB stacking is more energy favorable when temperature effects are included. The adsorption energies for Li on Ti2C double layer shown in Figure 7 vary in the range between 2.21 and 2.31 eV, which is ∼0.20 eV higher than those on the single-layer structure. These high binding energies indicate that Li interacts strongly with both Ti2C single layer and double layer. The TDOS of pristine double layer with AB stacking is shown in Figure 8a,

Figure 6. Structure of Ti2C double layer investigated in this work with two different stacking order: (a) AA and (b) AB.

simulation at 300 K reveals that the AA configurations only last ∼1 ps, after which the structure undergoes a transformation into AB configurations, in line with the total energy results. We have then investigated the interaction of Li atoms with these two types of double-layer Ti2C. The calculated stable structures as well as their binding energies are summarized in Figure 7. Here only the structures with the highest binding energies are shown for clarity.

Figure 7. Adsorption configurations with the highest binding energies for two types of Ti2C double layer: (a) AA stacking and (b) AB stacking. Figure 8. Total density of states of AB stacking pristine (a) and fully lithiated (b) Ti2C double layer. Projected density of states for Li atom (c), Ti atoms (d), and C atoms (e) are also provided.

It can be seen from Figure 7a,b that both the stable adsorption configurations of Li atoms on these two types of Ti2C double layer as well as the binding energies are very similar. For the case of two Li atoms, the lowest energy configuration contains a Li atom above one C atom and another on the top of a Ti(2) atom on one side for both AA and AB stacking. Four Li atoms prefer to adsorb on the top of four C atoms on one side. For more than four Li atoms, the stable structures are always composed of Li atoms on the top of C atoms on both sides of Ti2C double layer. Binding of a single Li atom between the two Ti2C layer is also considered, and their binding energies are found to be ∼2 eV smaller than those in the surface. Therefore, these configurations are less favorable. For the fully lithiated Ti2C double layer with a Li layer located in the interlayer, the adsorption energy is found to be 1.32 eV/atom for the AA stacking and 1.10 eV/atom for AB stacking, respectively. These adsorption energies are significantly lower than those adsorption sites on the top of the double layer, which suggests

and the TDOS and PDOS of fully lithiated AB configurations are displayed in Figure 8b−e, which exhibit the same characteristics as the Ti2C monolayer. Again, the magnetism is quenched by Li adsorption. The conduction band is mainly composed of d states of Ti, with a hybridization of p states of Li. The gap in the TDOS of pristine Ti2C double layer around −2 eV is now filled due to hybridization of states contributed by Li and C in this energy range, suggesting the strong interaction between Li and C as found in the Ti2C monolayer. The importance of strain considered in this work is mainly due to the volume change during lithiation/delithiation processes. Our results show that Ti2C layer possesses high elastic moduli and the adsorption of Li has little influence on its mechanical response. To further evaluate the volume change induced by lithiation/delithiation, we have calculated the equilibrium lattice constant of fully lithiated Ti2C monolayer and double layer. For monolayer, the lattice constant is 14988

dx.doi.org/10.1021/jp504493a | J. Phys. Chem. C 2014, 118, 14983−14990

The Journal of Physical Chemistry C

Article

(5) Pollak, E.; Geng, B.; Jeon, K.-J.; Lucas, I. T.; Richardson, T. J.; Wang, F.; Kostecki, R. The Interaction of Li+ with Single-Layer and Few-Layer Graphene. Nano Lett. 2010, 10, 3386−3388. (6) Tritsaris, G. A.; Kaxiras, E.; Meng, S.; Wang, E. Adsorption and Diffusion of Lithium on Layered Silicon for Li-Ion Storage. Nano Lett. 2013, 13, 2258−2263. (7) Barsoum, M. W. The MN+1AXN Phases: A New Class of Solids: Thermodynamically Stable Nanolaminates. Prog. Solid State Chem. 2000, 28, 201−281. (8) Eklund, P.; Beckers, M.; Jansson, U.; Hberg, H.; Hultman, L. The Mn+1AXn phases: Materials Science and Thin-Film Processing. Thin Solid Films 2010, 518, 1851−1878. (9) Barsoum, M. W.; Radovic, M. Elastic and Mechanical Properties of the MAX Phases. Annu. Rev. Mater. Res. 2011, 41, 195−227. (10) Wang, X.; Zhou, Y. Layered Machinable and Electrically Conductive Ti2AlC and Ti3AlC2 Ceramics: a Review. J. Mater. Sci. Technol. 2010, 26, 385−416. (11) Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248−4253. (12) Naguib, M.; Mashtalir, O.; Carle, J.; Presser, V.; Lu, J.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional Transition Metal Carbides. ACS Nano 2012, 6, 1322−1331. (13) Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall’Agnese, Y.; Rozier, P.; Taberna, P. L.; Naguib, M.; Simon, P.; Barsoum, M. W.; Gogotsi, Y. Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science 2013, 341, 1502−1505. (14) Naguib, M.; Come, J.; Dyatkin, B.; Presser, V.; Taberna, P.-L.; Simon, P.; Barsoum, M. W.; Gogotsi, Y. MXene: a Promising Transition Metal Carbide Anode for Lithium-Ion Batteries. Electrochem. Commun. 2012, 16, 61−64. (15) Come, J.; Naguib, M.; Rozier, P.; Barsoum, M. W.; Gogotsi, Y.; Taberna, P.-L.; Morcrette, M.; Simon, P. A Non-Aqueous Asymmetric Cell with a Ti2C-Based Two-Dimensional Negative Electrode. J. Electrochem. Soc. 2012, 159, A1368−A1373. (16) Tang, Q.; Zhou, Z.; Shen, P. Are MXenes Promising Anode Materials for Li Ion Batteries? Computational Studies on Electronic Properties and Li Storage Capability of Ti3C2 and Ti3C2X2 (X = F, OH) Monolayer. J. Am. Chem. Soc. 2012, 134, 16909−16916. (17) Xie, Y.; Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y.; Yu, X.; Nam, K.-W.; Yang, X.-Q.; Kolesnikov, A. I.; Kent, P. R. C. Role of Surface Structure on Li-Ion Energy Storage Capacity of TwoDimensional Transition-Metal Carbides. J. Am. Chem. Soc. 2014, 136, 6385−6394. (18) Lee, Y.; Kim, M. G.; Cho, J. Layered Li0.88[Li0.18Co0.33Mn0.49]O2 Nanowires for Fast and High Capacity Li-Ion Storage Material. Nano Lett. 2008, 8, 957−961. (19) Liu, J.; Liu, X.-W. Two-Dimensional Nanoarchitectures for Lithium Storage. Adv. Mater. 2012, 24, 4097−4111. (20) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. QUANTUM ESPRESSO: a Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (22) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (23) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (24) Gan, L.-Y.; Zhao, Y.-J.; Huang, D.; Schwingenschlögl, U. FirstPrinciples Analysis of MoS2/Ti2C and MoS2/Ti2CY2 (Y = F and OH) All-2D Semiconductor/Metal Contacts. Phys. Rev. B 2013, 87, 245307. (25) Kurtoglu, M.; Naguib, M.; Gogotsi, Y.; Barsoum, M. W. First Principles Study of Two-Dimensional Early Transition Metal Carbides. MRS Commun. 2012, 2, 133−137.

enlarged from 3.078 to 3.106 Å with a 0.91% expansion. The parameter for double layer of AB stacking is decreased from 3.093 to 3.088 Å with a 0.18% shrink. These results indicate that the lithiation/delithiation processes can only induce a small volume change of Ti2C layer, providing further evidence of the stability of Ti2C layer for practical applications in LIBs.

■

CONCLUSIONS First-principles calculations based DFT have been carried out to investigate the effect of strain and concentration on the Li storage and diffusion properties in Ti2C layer. Our results show that biaxial strain leads to the decrease in binding energy of Li atoms quickly due to reduced charge transfer between Li and Ti2C layer. The mechanical response is not influenced by the adsorption of Li atoms, suggesting that Ti2C monolayer is rather stable during Li-lithiated process. The binding of Li is insensitive to its concentrations, and multiple Li atoms tend to reside in one side first, indicating that Ti2C is very tough upon lithiation and delithiation process. Besides, the diffusion of Li atoms on Ti2C layer is almost unaffected by strain and concentrations. These results provide an explanation for the experimental observed high cycling rate of Li atoms. Finally, our results indicate that Li atoms can bind stronger on Ti2C double layer with binding energy 0.2 eV higher than those on monolayer. The electronic structure analysis indicates that the magnetism of pristine Ti2C layer is gradually quenched by Li adsorption. The stability of fully lithiated Ti2C layer is confirmed by MD simulations. Our results indicate that Ti2C layer has great potential as electrode material used in LIBs with high stability and fast cycling rate.

■

ASSOCIATED CONTENT

S Supporting Information *

Strain−stress relation of Ti2C. Spin density plot of Li adsorption at strained Ti2C. Adsorption configurations of Li on Ti2C. Configurations of fully lithiated Ti2C layer before and after MD simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-10-62760126. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is financially supported by NSFC (Grant No. 91226202 and No. 11274019), NSAF (Grant No. 538 U1230111), and the China Postdoctoral Science Foundation (Grant No. 2014M550561).

■

REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (2) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (3) Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; et al. TwoDimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568−571. (4) Tang, Q.; Zhou, Z. Graphene-Analogous Low-dimensional Materials. Prog. Mater. Sci. 2013, 58, 1244−1315. 14989

dx.doi.org/10.1021/jp504493a | J. Phys. Chem. C 2014, 118, 14983−14990

The Journal of Physical Chemistry C

Article

(26) Zhao, S.; Kang, W.; Xue, J. Manipulation of Electronic and Magnetic Properties of M2C (M = Hf, Nb, Sc, Ta, Ti, V, Zr) Monolayer by Applying Mechanical Strains. Appl. Phys. Lett. 2014, 104, 133106. (27) Yang, C.-K. A Metallic Graphene Layer Adsorbed with Lithium. Appl. Phys. Lett. 2009, 94, 163115. (28) Osborn, T. H.; Farajian, A. A. Stability of Lithiated Silicene from First Principles. J. Phys. Chem. C 2012, 116, 22916−22920. (29) Chen, H.; Huang, J.; Lei, X.; Wu, M.; Liu, G.; Ouyang, C.; Xu, B. Adsorption and Diffusion of Lithium on MoS2 Monolayer: The Role of Strain and Concentration. Int. J. Electrochem. Sci. 2013, 8, 2196. (30) Tritsaris, G. A.; Kaxiras, E.; Meng, S.; Wang, E. Adsorption and Diffusion of Lithium on Layered Silicon for Li-Ion Storage. Nano Lett. 2013, 13, 2258−2263.

14990

dx.doi.org/10.1021/jp504493a | J. Phys. Chem. C 2014, 118, 14983−14990