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Structural Transformation of MXene (V2C, Cr2C, and Ta2C) with O Groups during Lithiation: A First-Principles Investigation Dandan Sun,† Qianku Hu,*,† Jinfeng Chen,† Xinyu Zhang,‡ Libo Wang,† Qinghua Wu,† and Aiguo Zhou† †
School of Materials Science and Engineering, Henan Polytechnic University, 454000 Jiaozuo, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, 066004 Qinhuangdao, China
‡
S Supporting Information *
ABSTRACT: For high capacities and extremely fast charging rates, two-dimensional (2D) crystals exhibit a significant promising application on lithium-ion batteries. With density functional calculations, this paper systematically investigated the Li storage properties of eight 2D M2CO2 (M = V, Cr, Ta, Sc, Ti, Zr, Nb, and Hf), which are the recently synthesized transition-metal carbides (called MXenes) with O groups. According to whether the structural transformation occurs or not during the adsorption of the first Li layer, the adsorption of Li can be grouped into two types: V-type (V2CO2, Cr2CO2, and Ta2CO2) and Sc-type (Sc2CO2, Ti2CO2, Zr2CO2, Nb2CO2, and Hf2CO2). The structural transformation behaviors of V-type are reversible during lithiation/delithiation and are confirmed by ab initio molecular dynamic simulations. Except for Nb-MXene, the V-type prefers the sandwich H2H1T-M2CO2Li4 structure and the Sc-type prefers the TH1H2-M2CO2Li4 structure during the adsorption of the second Li layer. The H2H1T-M2CO2Li4 structure of O layer sandwiched by two Li layers preferred by V-type can prevent forming Li dendrite and therefore stabilize the lithiated system. The tendency of O bonding to Li rather than M in Vtype is bigger than that in Sc-type, which causes that the sandwich structure of H2H1T-M2CO2Li4 is more suitable for V-type than Sc-type. KEYWORDS: MXene, two-dimensional, V2CO2, Li ion batteries, structural transformation
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INTRODUCTION Lithium-ion batteries (LIBs) are one of the great successes of clean energy storage devices and have been extensively developed and commercialized in modern electrochemistry for decades.1,2 However, to meet the demand for next generation LIBs, with fast charging rate, high energy densities, and long cycle life, is still a huge challenge. The footsteps to search new energetic materials have never been stopped. Currently, the application of 2D electrode materials in LIBs, such as graphene and 2D MoS2, have attracted enormous attention because of their shortened paths for fast lithium ion diffusion and large surfaces for Li storage.3−5 Recently, a new kind of 2D material, MXene, were prepared by exfoliating the “A” element from the counterpart MAX phases.6,7 The latter are a group of 60+ layered ternary compounds with a general formula Mn+1AXn, where “M” represents an early transition metal, “A” represents IIIA and IVA group elements, “X” represents C, N, or both, and n = 1, 2, 3.8 Depending on the value of n, the M2AX, M3AX2, and M4AX3 phases are usually referred to 211, 312, and 413 phases, respectively. When soaked in hydrofluoric acid9 or fluoride salts,10 “A” element was removed from the hexagonal layered Mn+1AXn, and thus 2D Mn+1Xn sheets were left in the aqueous environment with the presence of both F− and OH−. As a result, Mn+1Xn was usually terminated with F and OH groups. Experimentally water has been also found to be intercalated between the MXene layers.11,12 Recent researches12,13 reported © XXXX American Chemical Society
that the intercalated water and the OH termination can be removed by high-temperature annealing of the MXenes. Ab initio molecular dynamics (MD) simulations show that F termination is unstable and will be replaced by O termination.13 Therefore, in this research, we chosen the MXenes with O group as the research system. Experiments have demonstrated that MXenes can be widely applied in energy devices such as hydrogen storage,14,15 supercapacitors10,16−18 and Li-ion batteries.13,19−26 Mashtalir et al.27 reported that the Li capacity of delaminated Ti3C2 is about 410 mAh g−1 at 1 C, corresponding three Li per Ti3C2 unit cell. However, by using density functional theory (DFT) calculations, Tang et al.21 predicted that bare Ti3C2 monolayers have a theoretical capacity of 320 mAh g−1 with Ti3C2Li2 stoichiometry, and upon covered by functional groups (F or OH), the Lithium capacity of Ti3C2 would rapidly fall off. On the other hand, V2C and Nb2C display a reversible Li capacities of 260 and 170 mAh g−1 at 1 C, but only 110 mAh g−1 is obtained for Ti2C at the same cycling rate even though the former two have relatively heavier gravimetric. To seek the origin of such discrepancy between experiments and calculations, Yu Xie et al.13 performed an extensive investigation on the relation between surface structure and Li-storage capacities Received: May 4, 2015 Accepted: December 15, 2015
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DOI: 10.1021/acsami.5b03863 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces of MXenes. They found that MXenes terminated with O groups exhibit the highest theoretical Li-ion storage capacities, which are able to adsorb two Li atomic layers on either of both sides. Such a distinct mechanism can explain the origin of measured more Li capacities in experiment27 than in calculation.21 Despite current research achievements on MXene LIBs electrodes, the reaction mechanism is still ambiguous. To the best of our knowledge, previous works were mostly focused on the role of surface structure on MXene’s Li-ion storage.13,21,22 However, in previous researches, a fact that the variations of Li concentration maybe also influence the surface structure of MXene host, has been ignored, which is especially important for understanding the mechanism of Li ion accommodated in MXene. Besides, adsorption energy of Li as a function of Li content for MXene has not been extensively investigated, therefore the details of lithiation process remain unclear. Furthermore, it should be clear whether all the MXene phases follow the same Li adsorption mechanism. For the reasons discussed above, we systematically explored the adsorption behaviors of Li ion on MXene and especially the change of surface structure as a function of Li content. We chose V2C as a representative of MXenes based on the following reasons: (1) V2C, a new member of MXene, is one promising electrode because of its good capability to handle high charge−discharge rates.11 (2) Because V is a relatively light atom among all transition metals, V2CO2 is likely to possess the highest Li capacity among MXene anodes.13 The calculated results predicted different Li reaction mechanisms for different MXene phases, which can be divided into two types: V- and Sctype. An occurrence of structural transformation for V-type MXene phases during lithiation was first observed for the 2D anode. The results are encouraging, and provide a promising conjecture that the migration of active groups in 2D materials induced by Li concentration can enhance the Li-ion storage and even non-Li-ion storage.
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where ELi presents the total energy of bulk BCC Li, EV2CO2 presents the total energy of the most stable V2CO2 configuration (H2-V2CO2), and EV2CO2Lix presents the total energy of 2D V2CO2 adsorbed by x (0 < x ≤ 2) Li atoms. For the second Li layer adsorption, the Eav is defined as Eave = [E V2CO2Lix − E V2CO2Li2 − xE Li]/x
where EV2CO2Li2 is the total energy of the most stable V2CO2Li2 (H1TV2CO2Li2) and x ranges from 2 to 4. Structural Models. Figure 1a, b shows the top and side views of V2C, respectively. On V2C surface, there exist three types of high
Figure 1. Schematic diagram shows the crystal structure of a 2D V2C monolayer with (a) top view and (b) side view. symmetry sites: Top (T) site directly above the V atoms on top surface; Hollow1 (H1) site directly above the C atoms (the center of yellow triangle in Figure 1a); Hollow2 (H2) site directly above the V atoms on bottom layer (the center of pink triangle in Figure 1a). Here according to the relative positions of the attached O termination groups, we calculated three configurations that are possible for the chemical terminations of a V2C system: Model 1 has O atoms located directly above the T sites (T-V2CO2); Model 2 has O atoms directly above the H1 sites (H1-V2CO2); Model 3 has O atoms directly above H2 sites (H2-V2CO2). For each type of model, they form a symmetric arrangement on the both sides of the V2C monolayer and after fully optimizing the atomic coordinates and the lattice parameters, the results show that the H2-V2CO2 is the most energetically stable configuration, consistent with the previous report.35
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RESULTS AND DISCUSSION Adsorption of Single-Layer Li on O-Terminated V2C Monolayer. For carefully seeking the potential lithiated phases, first, we systematically investigated all possible structures of V2CO2Li2 with the locations of O and Li atoms as variables. Table 1 shows the Eav of 9 possible lithiated
THEORETICAL METHOD
Computational Details. First-principles calculations were carried out based on DFT28 and all-electron projected augmented wave (PAW)29 method that is implemented in the Vienna ab initio simulation package (VASP).30 The exchange correlation energy is described by the generalized gradient approximation (GGA) in the scheme proposed by Perdew−Burke−Ernzerhof (PBE).31 The pseudopotentials generated utilized the valence state 1s12s12p1 for Li, 3p63d4s1 for V, 2s22p2 for C, and 2s22p4 for O. The kinetic energy cutoff 600 eV is used for the plane wave basis set and the Brillouin zone was integrated using a 7 × 7 × 2 Monkhorst−Pack32 (MP) kpoint grid during the relaxation and a denser 23 × 23 × 3 MP mesh for the density of states (DOS) calculations. To avoid any interaction due to the use of periodic boundary conditions, a vacuum separation between two neighbor MXene layers was set to more than 10 Å. The geometry optimizations were performed by using the conjugated gradient method, and the convergence threshold is set to be 1 × 10−5 eV/atom in energy and 1 × 10−3 eV/Å in force. The effects of van der Waals (vdW) interactions were checked. The energy difference calculated by plain DFT and DFT-D33 is
