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Exploring the possibilities of two-dimensional transition metal carbides as anode materials for sodium batteries† Eunjeong Yang,a Hyunjun Ji,a Jaehoon Kim,a Heejin Kimab and Yousung Jung*a Recently a group of two-dimensional materials called MXenes have been discovered and they have demonstrated their potential in Li rechargeable batteries. Herein, the Na storage and ion migration properties of M2C-type MXenes (M = Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo) were investigated using density functional theory (DFT) calculations, and were compared to the Li case. Based on the average voltage and migration barrier of surface ions, we suggest that M = Ti, V, Cr, Mn, and Mo are suitable for sodium ion battery (SIB) anodes. These screened M2C materials can provide a theoretical capacity of
Received 6th November 2014, Accepted 5th January 2015
190–288 mA h g
DOI: 10.1039/c4cp05140h
barrier of 0.1–0.2 eV for ionic motion, suggesting that the M2C materials are promising for high-power
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using electrostatic considerations.
1
by accommodating two alkali ions per formula unit. They also exhibit an activation
applications. The underlying aspects of the voltage differences between M2C materials are also discussed
Introduction Lithium ion batteries (LIBs) today have become an essential part of our daily lives, powering a variety of portable electronic devices. In recent years, the most remarkable growing area for batteries is the large-scale applications such as electric vehicles (EVs) or stationary energy storage systems (ESS) that can utilize the renewable energy sources.1–3 In this regard, sodium as an alternative to lithium for battery electrode materials has attracted great attention owing to its natural abundance and low cost that are advantageous when a large amount of alkali metal is required. While a number of materials including transition metal (TM) oxide and polyanionic compounds have been proposed as possible cathode materials for sodium ion batteries,4–6 only a few anode materials have been discovered. Since sodium can hardly be inserted into graphite to a meaningful extent, utilizing disordered carbons such as carbon microspheres,7 templated carbon with a hierarchical microstructure,8 hollow carbon nanospheres,9 and hollow carbon nanowires10 has been suggested. Nevertheless, the large surface area of these nanostructures tends to make them more susceptible to side reactions, deteriorating the cyclic performances. Among several Ti-based anodes investigated, nanosized a
Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon, Republic of Korea. E-mail: [email protected] b Suncheon Center, Korea Basic Science Institute, Suncheon 540-742, Republic of Korea † Electronic supplementary information (ESI) available: Additional computational data, Tables S1 and S2, Fig. S1. See DOI: 10.1039/c4cp05140h
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Na2Ti3O7 exhibits an initial discharge capacity of B220 mA h g 1 at B0.3 V vs. Na/Na+, but its reversible capacity reduces 59% only after 20 cycles.11 Various conversion and alloying compounds including NiCo2O4, FeS2, Sn/C, Sb/C and SnSb/C have also been studied and it was found that albeit their high specific capacity, such compounds suffer from significant volume change (typically B400% for individual metals) during cycling.12–16 Moreover, metallic Na as an anode is not suitable due to its high reactivity with organic electrolytes and low melting point. Therefore, identifying appropriate negative electrode materials is crucial for developing SIBs. In 2011, a new family of two-dimensional (2D) material, called MXene, was discovered by exfoliating MAX phases in hydrofluoric acid (HF).17 The MAX phases are layered ternary carbides or nitrides with chemical formula Mn+1AXn (n = 1, 2, or 3), where M is a transition metal, A is mostly groups 13 or 14 element, and X is C or N. Although only Ti2C, Ti3C2, Ta4C3, (Ti0.5Nb0.5)2C, (V0.5Cr0.5)3C2, V2C, Ti3CN, Nb4C3, and Nb2C phases have been reported to date,18–20 we expect that more diverse MXenes could be synthesized since more than 60 MAX phases are already known.21,22 Soon after these findings, the possibility of the MXenes in energy storage applications such as LIBs and electrochemical capacitors was investigated both experimentally and theoretically.18,23–29 As LIB anodes, Ti2C, V2C, Nb2C, and Ti3C2 showed reversible capacities of 110, 260, 170, and 100 mA h g 1 at 1 C rate, respectively. Especially, Nb2C and V2C exhibited 110 and 125 mA h g 1 even at a rate of 10 C, demonstrating that MXenes are capable of handling fast charge and discharge conditions. These results suggest that MXenes are promising electrode materials for rechargeable batteries.
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In this work, we performed first principles calculations to explore a variety of MXenes and their capabilities as anode materials for SIBs. The M2C-type MXenes with different transition metals (M = Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo) were considered and the adsorption and diffusion behaviours of Na ions on them were estimated.
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Computational details All computations were performed in the density functional theory (DFT) framework using the Vienna Ab-initio Simulation Package (VASP)30,31 with the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional32 and projector-augmented wave (PAW)33 pseudopotentials. Recently, it has been reported that PBE and Heyd–Scuseria–Ernzerhof (HSE06)34 functionals both give similar geometries and electronic structures for various functionalized MXenes containing Ti.35 In addition, surface adsorption behaviour of Li or Na ions has been properly described with the PBE functional as in previous studies.23,27 A kinetic energy cutoff of 500 eV was applied and the geometry optimizations were performed by a conjugated gradient method with a convergence threshold of 10 6 eV in energy and 0.02 eV Å 1 in force. Reciprocal space k-point meshes of 4 4 2 were generated by the Monkhorst–Pack scheme for the 3 3 supercell of MXene monolayers. The DFT-D3 method of Grimme36 was used for optimal performance. In order to determine the energy barriers and lowest energy pathways for ion diffusion, the nudged elastic band (NEB) method was used with the climbing image scheme for accurate activation energy evaluation. Visualization of the structures was done using VESTA software.37
Results and discussion Surface structure of M2C-type MXene The bare M2C-type MXenes are composed of an M–C–M trilayer, where each layer is stacked in a face-centered cubic (fcc) closepacking as marked by C–A–B in Fig. 1a. Since MXenes are exfoliated from MAX phases using a HF solution, their surfaces are likely to be covered with F, O, and/or OH. For all of these possible functionals, relative stabilities were identified in the case of homogeneous adsorption on MXene surfaces in previous studies.35,38 These DFT calculations showed that the stability of the M2C and M2N system increases in the order of F, O, and OH terminations. This result suggests that the surface F group adsorbed during the exfoliation step would likely be substituted by more favourable OH and O groups in the following steps, where the water is used for washing and storing. Consistently, a recent Ab initio molecular dynamics (AIMD) simulation27 showed that the surface of MXenes is terminated mostly by OH groups with a small amount of O and F groups when all three functional groups coexist in the ambient solution. Nevertheless, the authors of the latter suggested that the O termination would be dominant when MXenes are used as electrodes since the annealing process before the cell assembly converts the surface OH group into an O group. Indeed, the O terminated models explain the
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Fig. 1 Three different structures of oxygenated MXene: (a) side and top views of MXene with fcc-type O termination where every O atom is at the fcc hollow site; (b) side and top views of hcp-type O termination where all O atoms are at the hcp-hollow site; (c) the mixture of fcc-type and hcp-type surfaces on each side.
experimentally measured capacity and voltage properties better than OH terminated models for Li battery cycles. Therefore, we selected the O functionalized M2C layer as the model structure for searching the anode materials. After verifying that O atoms are stable at hollow sites of M2C rather than its bridge or top sites for all binding coverages, we built three different models for the O functionalized surface as shown in Fig. 1: (1) O atoms are located at the fcc hollow sites on each side of M2C up to one monolayer (1 ML) coverage, making O–M–C–M–O layers stacked in B–C–A–B–C order. We denote this surface structure as fcc-type O termination (Fig. 1a); (2) O atoms are positioned at the hexagonal close-packed (hcp) hollow sites on both sides of the M2C layer with 1 ML coverage, which makes the O–M–C–M–O layers stacked in the order of A–C–A–B–A. For each side of the surface, atoms are in hcp stacking and thereby we designate this structure as hcp-type O termination (Fig. 1b); (3) O atoms are located as fcc-type on one side and as hcp-type on the other side, making asymmetric surfaces (Fig. 1c). Hereafter, we denote these O terminated M2C structures as M2CO2 according to their chemical formula. For all considered O termination types and TMs, the MXene structures were stably maintained after the full geometry relaxations. Upon comparing the total energy of MXenes, the fcc-type O termination (Fig. 1a) was found to be the most stable configuration for all TMs except for Cr and Mo, where the hcptype O termination (Fig. 1b) was more stable than the others, which is consistent with previous reports.23,35,38 It has been noted that the relative stabilities of the fcc-type and hcp-type coordinations are associated with the number of d-electrons in TM and when the d-electron count is around 2, the relative stability of hcp-type coordination reaches its maximum.39
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Table 1 Calculated lattice parameter (a), thickness (L), M–C bond length (dM–C), M–O bond length (dM–O) of the M2CO2 sheet. Binding distances of each alkali ion are also presented in reference to the O layer (dion–O), and to M layer (dion–M). All distances are in angstrom unit
M in M2CO2
a
L
dM–C
dM–O
dion–O (Li/Na)
dion–M (Li/Na)
Ti V Cr Mn Fe Co Ni Nb Mo
3.03 2.89 2.69 2.92 2.81 2.77 2.90 3.15 2.87
4.45 4.41 4.82 4.34 4.42 4.46 4.12 4.69 5.20
2.19 2.05 2.02 2.09 2.03 2.06 2.00 2.21 2.16
1.98 1.96 1.92 1.92 1.91 1.85 1.93 2.13 2.06
1.92/2.30 1.89/2.23 1.87/2.27 1.83/2.21 1.83/2.20 1.82/2.15 1.82/2.15 1.97/1.97 1.91/2.34
2.16/2.68 2.16/2.67 2.27/2.79 2.03/2.57 1.99/2.54 1.98/2.49 1.93/2.42 2.29/2.29 2.36/2.92
Thus we believe that since only Cr and Mo satisfy this condition in our study, they prefer hcp-type O termination while the others prefer fcc-type O termination. The calculated total energy differences relative to the most stable configuration can be found in the ESI† (Table S1). The calculated lattice parameters (a), the thickness of the O terminated M2C sheet (L), the bond length between the metal and carbon (dM–C), and the binding distance of surface O from the metal center (dM–O) are summarized in Table 1. These geometric differences among MXenes directly affect the voltage properties as will be discussed in the next section. Alkali ion adsorption and voltage properties MXenes have been suggested as promising candidates for energy storage applications owing to their good electrical conductivity, large surface area, and high elastic moduli.17,19,40 To verify additional key properties that are necessary for the anode material, we first examined voltage properties by investigating the adsorption behaviours of alkali ions on aforementioned model structures of the M2CO2 sheet with varying TM elements. The voltage of an electrochemical cell reflects the interaction between binding ions and host structure. In terms of calculations, the open circuit voltage or equilibrium potential (VAM, where AM = Li or Na) can be approximated from the total energy difference between before and after ion adsorption as follows.30 VAM = [E(MXene) + xE(AM)
E(xAM + MXene)]/xF
(1)
In this expression, E(MXene), E(AM), and E(xAM + MXene) are the total energy of host MXene, alkali metal, and lithiated or sodiated MXene, respectively, where x is the composition of adsorbed Li or Na and F is the Faraday constant. In other words, the energy difference between reactants (the first two terms) and products (the last term) corresponds to the cell voltage. The average cell voltage can be used as a first-order criterion for selecting the anode material: typically a voltage of 0.0–1.0 V vs. alkali metal is desirable to prevent dendrite formation and to maximize the energy density. To evaluate the average voltage of MXenes (M2CO2 in the present study) from eqn (1), the surface structure of the final product, i.e. configuration of adsorbed alkali ions, should be determined first. For that, the most favourable adsorption site was examined on the 3 3 supercell and then the number of adsorbed atoms was systematically
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Fig. 2 Optimized geometries of the ion adsorbed MXenes: (a) side and top views of lithiated/sodiated MXenes with fcc-type O termination (M = Ti, V, Mn, Fe, Co, Ni, and Nb); (b) side and top views of M2CO2Li2/M2CO2Na2 with hcp-type O termination (M = Cr and Mo).
increased until alkali ions are not bound any more (negative binding energy). For the M2CO2 sheets with M = Ti, V, Mn, Fe, Co, Ni, and Nb, which have an oxygenated surface of fcc-type, alkali ions are again adsorbed at the fcc hollow site up to 1 ML coverage on each side, keeping the stacking order of fcc packing (Fig. 2a). On the other hand, for M = Cr and Mo, where the surface O termination is hcp-type, Li or Na atoms are located above the M of the other side, resulting in disordered stacking as shown in Fig. 2b (A–B–A–C stacking for C–M–O–Li or –Na). Since the M2CO2 sheets can accommodate alkali ions up to 1 ML coverage on each side, their theoretical capacities are 214–349 mA h g 1 and 190– 288 mA h g 1 for Li and Na, corresponding to the chemical stoichiometry of M2CO2Li2 or M2CO2Na2. Fig. 3 displays calculated average voltages of M2C-type MXenes with different TMs. Considering that the optimal Li or Na intercalation voltage of anode materials is in the range of 0–1.0 V vs. each alkali metal,4,6,41 M2C electrodes with M = Cr (1.00 V), Nb (0.84 V), and Mo (0.69 V) are suitable for LIB anodes, and those with M = Ti (0.40 V), V (0.52 V), Cr (0.26 V), Mn (0.80 V), and Mo (0.19 V) are suitable for SIB anodes. Among these candidates, M2C-type MXene phases with M = Cr, Mn, and Mo are yet to be reported; however, their MAX phases (Cr2AlC, Mn2GaC, and Mo2GaC) have already been synthesized.21,42,43 Therefore, we expect that all suggested MXenes, including Cr2CO2, Mn2CO2, and Mo2CO2, could be exfoliated from each MAX phases if appropriate etching conditions are reached. We note that, so far, MXenes have been experimentally tested only for the LIBs, where the measured average voltages are consistent with our predictions shown in Fig. 3. For example, estimated vs. measured VLi are 1.1 V vs. B1.2 V, 1.3 V vs. B1.4 V, and 0.8 V vs. B0.7 V for Ti2CO2, V2CO2, and Nb2CO2, respectively,18,25,27 supporting our screening results in Fig. 3 based on the predicted voltages. It is also noteworthy that for the same M2CO2 sheet, VNa was estimated to be lower than VLi overall by 0.5–1.2 V, similar to the common oxide-based electrodes. The fact that VNa is generally lower than VLi is advantageous when
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Fig. 3 Calculated voltages of MXenes containing different transition metals. Filled (open) squares represent V vs. Li/Li+ (V vs. Na/Na+) of M2CO2. M with * indicates that corresponding M2CO2 is synthesized and tested in electrochemical cells. Points inside the shaded region have voltages lower than 1.0 V.
the material is applied to negative electrodes since a low operating voltage would lead to a high energy density of the battery. In Fig. 3, it is seen that the voltage generally increases along the period except for group 6 elements (Cr and Mo), which have different stacking orders of O and alkali ion as described in Fig. 2. As expressed in eqn (1), the average cell voltage is directly related to the binding strength of alkali ions to the host M2CO2 sheet, implying that their binding distance could be a reliable descriptor for the cell voltage. While the adsorbed alkali ions are mainly interacting with the surface oxygen, such ion–O distances (dion–O) were similar for all TM carbides (Table 1). Instead, the difference in ion–TM distances (dion–M) was 2–3 times more significant than dion–O with varying TM elements. Since the electrostatic energy is proportional to the q1q2/d (where q1 and q2 are point charges and d is the distance between them), both larger charge (M3+ vs. O2 ) and significant variation in d for ion–TM interaction suggest that the repulsive interaction between alkali ion and TM is the primary reason for the voltage difference between TM carbides. Therefore, in order to understand the trend and deviations shown in voltages of Fig. 3, we measured the vertical distances from the alkali ion to the closest TM layer that can represent the difference in electrostatic interactions between the adsorbed ion and the M2CO2 host sheet. As shown in Fig. 4, the ion–TM interlayer distances for each alkali ion are highly correlated with voltages, supporting the above suggestion that the ion–TM distance is the primary descriptor for the cell voltage. The binding distance, however, is not the only factor that determines the voltage since the data points corresponding to VLi and VNa are on different trend lines. Note that the trend line for VNa is shifted upward compared to that for VLi. We suspect that this discordance in trend lines is caused by different charges of Li and Na ions. That is, a lower electronegativity of Na (0.9) than Li (1.0)44 makes the Na ions have more positive charge than Li, which enhances the electrostatic interactions. We performed the Bader charge analysis in order to see this trend. We note that, since the Bader population analysis can yield atomic charges quite different from other partitioning methods for some cases,45–47 we mainly use it here as an approximate qualitative measure of
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Fig. 4 Dependence of the calculated voltage (V vs. Li/Li+, filled squares, or V vs. Na/Na+, open squares) on the distance between the adsorbed ion layer and the transition metal layer. R2 is the coefficient of determination of the least square linear fit.
atomic charges. From the result presented in Table S2 (ESI†), we find that Na is indeed more ionized compared to Li, which supports our interpretation. Therefore, at the same distance, Na ions exhibit a stronger binding, i.e., higher voltage, than Li ions. Surface migration of alkali ions The ion mobility is another factor to be considered when evaluating the feasibility of the electrode for rechargeable batteries. Since fast charge and discharge rates require a high mobility of charge carriers, a low activation barrier for ionic motion is essential for applying electrode materials to the highpower devices. In 2D materials like MXenes, where the binding sites for alkali ions are largely exposed to the electrolyte, lithiation (or sodiation) and its reverse process can be proceeded in both surface ionic migration and direct adsorption–desorption of ions via the electrolyte. In both cases, the easiness of surface migration plays an important role in fast charging and discharging since the reordering of surface ions for the minimum energy configuration should occur, in general, through the surface migration process. Therefore, the surface ion migration paths and their energy barriers were identified on the previously screened M2CO2 (M = Ti, V, Cr, Mn, and Mo). Three kinds of nearest neighbour (NN) hopping pathways were investigated for each MXene as illustrated in Fig. 5: for pathway 1, ion moves from one adsorption site to its NN site via the hollow site; for pathway 2, ion passes through the site on top of oxygen so that the ion would move along the ion - top of O - ion route; for pathway 3, ion moves directly from one site to another in a linear way. After the path optimization using the NEB method, the straight route defined in pathway 3 relaxes to pathway 1 in all MXenes, which exhibits a lower activation barrier (Ea) than pathway 2 by 0.4–0.6 eV (Fig. 5), indicating that the ion prefers to migrate along pathway 1. The calculated activation barriers for Na ion migration on Ti2CO2, V2CO2, Cr2CO2, Mn2CO2, and Mo2CO2 were 0.18, 0.15, 0.09, 0.15, and 0.14 eV, respectively (Table 2). These energy barriers are lower than those of Li migration by 0.05–0.11 eV,
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Fig. 5 Considered ion migration pathways and corresponding energy profiles of Li and Na on (a) Ti2CO2 and (b) Cr2CO2. The energy profiles of ion migration on other MXenes can be found in the ESI.†
Table 2
Activation barrier for Li or Na migration on M2CO2 with different M
Migration barrier (eV)
Li Na
Ti
V
Cr
Mn
Mo
0.28 0.18
0.21 0.15
0.13 0.09
0.22 0.15
0.16 0.14
possibly due to the weaker interaction between MXenes and Na ions. These values are comparable to those of other anode materials such as Na2Ti2O7 (0.19 eV)11 and Na3Sb (0.21 eV).48 While the energy barriers of ionic motion on MXenes could be affected by diverse factors such as strain of the sheets, concentration of ions, and the number of stacked layers,26,27 these fairly low activation energies of Na diffusion suggest that MXene is a potential candidate for electrode materials with high rate capability.
Conclusions In summary, we investigated the Na adsorption and migration properties of two-dimensional M2CO2 sheets based on DFT calculations in order to explore their capabilities as anode materials. The main findings in this work are as follows. (1) Both alkali metal ions preferentially adsorb on the hollow site of M2CO2 sheets up to 1 ML coverage on each side, forming the sodiated and lithiated structures, respectively, corresponding to the chemical formula of M2CO2Li2 and M2CO2Na2. (2) The average voltage increases along the period of TMs (except for Cr and Mo), where the voltage is significantly correlated with the distance between the adsorbed ion and the TM layer since the ion–TM distance mainly affects the difference in interaction strength between them and therefore electrostatic energies of the system. (3) From the viewpoint of voltage, M = Ti, V, Cr, Mn and Mo are expected to be suitable for SIB anodes, and M = Cr, Nb and Mo are suitable for LIB anodes, where these M2CO2 sheets exhibit 0.2–0.9 V vs. each alkali metal.
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(4) The estimated average voltage for Na adsorption is lower than that of Li adsorption. The lower voltage of M2CO2 for Na adsorption is an advantageous point to be used as anode materials since the lower anode voltage results in a higher energy density of the cell. (5) For the screened M2CO2 sheets with M = Ti, V, Cr, Mn and Mo, Na ion migration barriers are 0.09–0.18 eV, suggesting that M2CO2 materials are capable of cycling at a fast rate in terms of ion mobility. (6) Although Cr2CO2, Mn2CO2, and Mo2CO2 have not been experimentally reported yet, we expect that these MXenes could be exfoliated in the near future since their corresponding MAX phases are known to exist. In contrast to the LIBs, where the carbons are used as an anode material in general, SIBs do not have attractive anode materials yet. Our results suggest that M2C-type MXenes are a new family of promising anode materials for SIBs indeed with desirable electrochemical properties.
Acknowledgements We acknowledge financial support from the National Research Foundation of Korea (NRF-2014R1A4A1003712 and NRF-2010C1AAA001-2010-0029031).
Notes and references 1 B. Dunn, H. Kamath and J. M. Tarascon, Science, 2011, 334, 928–935. 2 Z. G. Yang, J. L. Zhang, M. C. W. Kintner-Meyer, X. C. Lu, D. W. Choi, J. P. Lemmon and J. Liu, Chem. Rev., 2011, 111, 3577–3613. 3 C. Wadia, P. Albertus and V. Srinivasan, J. Power Sources, 2011, 196, 1593–1598. 4 S. W. Kim, D. H. Seo, X. H. Ma, G. Ceder and K. Kang, Adv. Energy Mater., 2012, 2, 710–721.
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5 V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-Gonzalez and T. Rojo, Energy Environ. Sci., 2012, 5, 5884–5901. 6 M. D. Slater, D. Kim, E. Lee and C. S. Johnson, Adv. Funct. Mater., 2013, 23, 947–958. 7 R. Alcantara, P. Lavela, G. F. Ortiz and J. L. Tirado, Electrochem. Solid-State Lett., 2005, 8, A222–A225. 8 S. Wenzel, T. Hara, J. Janek and P. Adelhelm, Energy Environ. Sci., 2011, 4, 3342–3345. 9 K. Tang, L. J. Fu, R. J. White, L. H. Yu, M. M. Titirici, M. Antonietti and J. Maier, Adv. Energy Mater., 2012, 2, 873–877. 10 Y. L. Cao, L. F. Xiao, M. L. Sushko, W. Wang, B. Schwenzer, J. Xiao, Z. M. Nie, L. V. Saraf, Z. G. Yang and J. Liu, Nano Lett., 2012, 12, 3783–3787. 11 H. L. Pan, X. Lu, X. Q. Yu, Y. S. Hu, H. Li, X. Q. Yang and L. Q. Chen, Adv. Energy Mater., 2013, 3, 1186–1194. 12 Y. H. Xu, Y. J. Zhu, Y. H. Liu and C. S. Wang, Adv. Energy Mater., 2013, 3, 128–133. 13 T. B. Kim, J. W. Choi, H. S. Ryu, G. B. Cho, K. W. Kim, J. H. Ahn, K. K. Cho and H. J. Ahn, J. Power Sources, 2007, 174, 1275–1278. 14 J. F. Qian, Y. Chen, L. Wu, Y. L. Cao, X. P. Ai and H. X. Yang, Chem. Commun., 2012, 48, 7070–7072. 15 L. Xiao, Y. Cao, J. Xiao, W. Wang, L. Kovarik, Z. Nie and J. Liu, Chem. Commun., 2012, 48, 3321–3323. 16 R. Alcantara, M. Jaraba, P. Lavela and J. L. Tirado, Chem. Mater., 2002, 14, 2847–2848. 17 M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. J. Niu, M. Heon, L. Hultman, Y. Gogotsi and M. W. Barsoum, Adv. Mater., 2011, 23, 4248–4253. 18 M. Naguib, J. Halim, J. Lu, K. M. Cook, L. Hultman, Y. Gogotsi and M. W. Barsoum, J. Am. Chem. Soc., 2013, 135, 15966–15969. 19 J. Come, M. Naguib, P. Rozier, M. W. Barsoum, Y. Gogotsi, P. L. Taberna, M. Morcrette and P. Simon, J. Electrochem. Soc., 2012, 159, A1368–A1373. 20 M. Ghidiu, M. Naguib, C. Shi, O. Mashtalir, L. M. Pan, B. Zhang, J. Yang, Y. Gogotsi, S. J. L. Billinge and M. W. Barsoum, Chem. Commun., 2014, 50, 9517–9520. 21 M. W. Barsoum, Prog. Solid State Chem., 2000, 28, 201–281. 22 Z. M. Sun, Int. Mater. Rev., 2011, 56, 143–166. 23 Q. Tang, Z. Zhou and P. W. Shen, J. Am. Chem. Soc., 2012, 134, 16909–16916. 24 M. R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall’Agnese, P. Rozier, P. L. Taberna, M. Naguib, P. Simon, M. W. Barsoum and Y. Gogotsi, Science, 2013, 341, 1502–1505. 25 M. Naguib, J. Come, B. Dyatkin, V. Presser, P. L. Taberna, P. Simon, M. W. Barsoum and Y. Gogotsi, Electrochem. Commun., 2012, 16, 61–64.
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26 S. J. Zhao, W. Kang and J. M. Xue, J. Phys. Chem. C, 2014, 118, 14983–14990. 27 Y. Xie, M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, X. Q. Yu, K. W. Nam, X. Q. Yang, A. I. Kolesnikov and P. R. C. Kent, J. Am. Chem. Soc., 2014, 136, 6385–6394. 28 M. D. Levi, M. R. Lukatskaya, S. Sigalov, M. Beidaghi, N. Shpigel, L. Daikhin, D. Aurbach, M. W. Barsoum and Y. Gogotsi, Adv. Energy Mater., 2014, DOI: 10.1002/aenm.201400815. 29 O. Mashtalir, M. Naguib, V. N. Mochalin, Y. Dall’Agnese, M. Heon, M. W. Barsoum and Y. Gogotsi, Nat. Commun., 2013, 4, 1716, DOI: 10.1038/ncomms2664. 30 M. K. Aydinol, A. F. Kohan, G. Ceder, K. Cho and J. Joannopoulos, Phys. Rev. B: Condens. Matter Mater. Phys., 1997, 56, 1354–1365. 31 G. Kresse and J. Furthmuller, Comput. Mater. Sci., 1996, 6, 15–50. 32 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868. 33 P. E. Blochl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953–17979. 34 J. Heyd, G. E. Scuseria and M. Ernzerhof, J. Chem. Phys., 2006, 124, 219906. 35 Y. Xie and P. R. C. Kent, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 235441. 36 S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104. 37 K. Momma and F. Izumi, J. Appl. Crystallogr., 2008, 41, 653–658. 38 M. Khazaei, M. Arai, T. Sasaki, C. Y. Chung, N. S. Venkataramanan, M. Estili, Y. Sakka and Y. Kawazoe, Adv. Funct. Mater., 2013, 23, 2185–2192. 39 M. Kertesz and R. Hoffmann, J. Am. Chem. Soc., 1984, 106, 3453–3460. 40 M. Kurtoglu, M. Naguib, Y. Gogotsi and M. W. Barsoum, MRS Commun., 2012, 2, 133–137. 41 M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa and S. Komaba, Phys. Chem. Chem. Phys., 2014, 16, 15007–15028. 42 P. Eklund, M. Beckers, U. Jansson, H. Hogberg and L. Hultman, Thin Solid Films, 2010, 518, 1851–1878. 43 A. S. Ingason, A. Petruhins, M. Dahlqvist, F. Magnus, A. Mockute, B. Alling, L. Hultman, I. A. Abrikosov, P. O. Å. Persson and J. Rosen, Mater. Res. Lett., 2013, 2, 89–93. 44 L. C. Allen, J. Am. Chem. Soc., 1989, 111, 9003–9014. 45 K. B. Wiberg and P. R. Rablen, J. Comput. Chem., 1993, 14, 1504–1518. 46 B. Wang, S. L. Li and D. G. Truhlar, J. Chem. Theory Comput., 2014, 10, 5640–5650. 47 C. Fonseca Guerra, J.-W. Handgraaf, E. J. Baerends and F. M. Bickelhaupt, J. Comput. Chem., 2004, 25, 189–210. 48 L. Baggetto, P. Ganesh, C. N. Sun, R. A. Meisner, T. A. Zawodzinski and G. M. Veith, J. Mater. Chem. A, 2013, 1, 7985–7994.
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Exploring the possibilities of two-dimensional transition metal carbides as anode materials for sodium batteries† Eunjeong Yang,a Hyunjun Ji,a Jaehoon Kim,a Heejin Kimab and Yousung Jung*a Recently a group of two-dimensional materials called MXenes have been discovered and they have demonstrated their potential in Li rechargeable batteries. Herein, the Na storage and ion migration properties of M2C-type MXenes (M = Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo) were investigated using density functional theory (DFT) calculations, and were compared to the Li case. Based on the average voltage and migration barrier of surface ions, we suggest that M = Ti, V, Cr, Mn, and Mo are suitable for sodium ion battery (SIB) anodes. These screened M2C materials can provide a theoretical capacity of
Received 6th November 2014, Accepted 5th January 2015
190–288 mA h g
DOI: 10.1039/c4cp05140h
barrier of 0.1–0.2 eV for ionic motion, suggesting that the M2C materials are promising for high-power
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using electrostatic considerations.
1
by accommodating two alkali ions per formula unit. They also exhibit an activation
applications. The underlying aspects of the voltage differences between M2C materials are also discussed
Introduction Lithium ion batteries (LIBs) today have become an essential part of our daily lives, powering a variety of portable electronic devices. In recent years, the most remarkable growing area for batteries is the large-scale applications such as electric vehicles (EVs) or stationary energy storage systems (ESS) that can utilize the renewable energy sources.1–3 In this regard, sodium as an alternative to lithium for battery electrode materials has attracted great attention owing to its natural abundance and low cost that are advantageous when a large amount of alkali metal is required. While a number of materials including transition metal (TM) oxide and polyanionic compounds have been proposed as possible cathode materials for sodium ion batteries,4–6 only a few anode materials have been discovered. Since sodium can hardly be inserted into graphite to a meaningful extent, utilizing disordered carbons such as carbon microspheres,7 templated carbon with a hierarchical microstructure,8 hollow carbon nanospheres,9 and hollow carbon nanowires10 has been suggested. Nevertheless, the large surface area of these nanostructures tends to make them more susceptible to side reactions, deteriorating the cyclic performances. Among several Ti-based anodes investigated, nanosized a
Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon, Republic of Korea. E-mail: [email protected] b Suncheon Center, Korea Basic Science Institute, Suncheon 540-742, Republic of Korea † Electronic supplementary information (ESI) available: Additional computational data, Tables S1 and S2, Fig. S1. See DOI: 10.1039/c4cp05140h
5000 | Phys. Chem. Chem. Phys., 2015, 17, 5000--5005
Na2Ti3O7 exhibits an initial discharge capacity of B220 mA h g 1 at B0.3 V vs. Na/Na+, but its reversible capacity reduces 59% only after 20 cycles.11 Various conversion and alloying compounds including NiCo2O4, FeS2, Sn/C, Sb/C and SnSb/C have also been studied and it was found that albeit their high specific capacity, such compounds suffer from significant volume change (typically B400% for individual metals) during cycling.12–16 Moreover, metallic Na as an anode is not suitable due to its high reactivity with organic electrolytes and low melting point. Therefore, identifying appropriate negative electrode materials is crucial for developing SIBs. In 2011, a new family of two-dimensional (2D) material, called MXene, was discovered by exfoliating MAX phases in hydrofluoric acid (HF).17 The MAX phases are layered ternary carbides or nitrides with chemical formula Mn+1AXn (n = 1, 2, or 3), where M is a transition metal, A is mostly groups 13 or 14 element, and X is C or N. Although only Ti2C, Ti3C2, Ta4C3, (Ti0.5Nb0.5)2C, (V0.5Cr0.5)3C2, V2C, Ti3CN, Nb4C3, and Nb2C phases have been reported to date,18–20 we expect that more diverse MXenes could be synthesized since more than 60 MAX phases are already known.21,22 Soon after these findings, the possibility of the MXenes in energy storage applications such as LIBs and electrochemical capacitors was investigated both experimentally and theoretically.18,23–29 As LIB anodes, Ti2C, V2C, Nb2C, and Ti3C2 showed reversible capacities of 110, 260, 170, and 100 mA h g 1 at 1 C rate, respectively. Especially, Nb2C and V2C exhibited 110 and 125 mA h g 1 even at a rate of 10 C, demonstrating that MXenes are capable of handling fast charge and discharge conditions. These results suggest that MXenes are promising electrode materials for rechargeable batteries.
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In this work, we performed first principles calculations to explore a variety of MXenes and their capabilities as anode materials for SIBs. The M2C-type MXenes with different transition metals (M = Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo) were considered and the adsorption and diffusion behaviours of Na ions on them were estimated.
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Computational details All computations were performed in the density functional theory (DFT) framework using the Vienna Ab-initio Simulation Package (VASP)30,31 with the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional32 and projector-augmented wave (PAW)33 pseudopotentials. Recently, it has been reported that PBE and Heyd–Scuseria–Ernzerhof (HSE06)34 functionals both give similar geometries and electronic structures for various functionalized MXenes containing Ti.35 In addition, surface adsorption behaviour of Li or Na ions has been properly described with the PBE functional as in previous studies.23,27 A kinetic energy cutoff of 500 eV was applied and the geometry optimizations were performed by a conjugated gradient method with a convergence threshold of 10 6 eV in energy and 0.02 eV Å 1 in force. Reciprocal space k-point meshes of 4 4 2 were generated by the Monkhorst–Pack scheme for the 3 3 supercell of MXene monolayers. The DFT-D3 method of Grimme36 was used for optimal performance. In order to determine the energy barriers and lowest energy pathways for ion diffusion, the nudged elastic band (NEB) method was used with the climbing image scheme for accurate activation energy evaluation. Visualization of the structures was done using VESTA software.37
Results and discussion Surface structure of M2C-type MXene The bare M2C-type MXenes are composed of an M–C–M trilayer, where each layer is stacked in a face-centered cubic (fcc) closepacking as marked by C–A–B in Fig. 1a. Since MXenes are exfoliated from MAX phases using a HF solution, their surfaces are likely to be covered with F, O, and/or OH. For all of these possible functionals, relative stabilities were identified in the case of homogeneous adsorption on MXene surfaces in previous studies.35,38 These DFT calculations showed that the stability of the M2C and M2N system increases in the order of F, O, and OH terminations. This result suggests that the surface F group adsorbed during the exfoliation step would likely be substituted by more favourable OH and O groups in the following steps, where the water is used for washing and storing. Consistently, a recent Ab initio molecular dynamics (AIMD) simulation27 showed that the surface of MXenes is terminated mostly by OH groups with a small amount of O and F groups when all three functional groups coexist in the ambient solution. Nevertheless, the authors of the latter suggested that the O termination would be dominant when MXenes are used as electrodes since the annealing process before the cell assembly converts the surface OH group into an O group. Indeed, the O terminated models explain the
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Fig. 1 Three different structures of oxygenated MXene: (a) side and top views of MXene with fcc-type O termination where every O atom is at the fcc hollow site; (b) side and top views of hcp-type O termination where all O atoms are at the hcp-hollow site; (c) the mixture of fcc-type and hcp-type surfaces on each side.
experimentally measured capacity and voltage properties better than OH terminated models for Li battery cycles. Therefore, we selected the O functionalized M2C layer as the model structure for searching the anode materials. After verifying that O atoms are stable at hollow sites of M2C rather than its bridge or top sites for all binding coverages, we built three different models for the O functionalized surface as shown in Fig. 1: (1) O atoms are located at the fcc hollow sites on each side of M2C up to one monolayer (1 ML) coverage, making O–M–C–M–O layers stacked in B–C–A–B–C order. We denote this surface structure as fcc-type O termination (Fig. 1a); (2) O atoms are positioned at the hexagonal close-packed (hcp) hollow sites on both sides of the M2C layer with 1 ML coverage, which makes the O–M–C–M–O layers stacked in the order of A–C–A–B–A. For each side of the surface, atoms are in hcp stacking and thereby we designate this structure as hcp-type O termination (Fig. 1b); (3) O atoms are located as fcc-type on one side and as hcp-type on the other side, making asymmetric surfaces (Fig. 1c). Hereafter, we denote these O terminated M2C structures as M2CO2 according to their chemical formula. For all considered O termination types and TMs, the MXene structures were stably maintained after the full geometry relaxations. Upon comparing the total energy of MXenes, the fcc-type O termination (Fig. 1a) was found to be the most stable configuration for all TMs except for Cr and Mo, where the hcptype O termination (Fig. 1b) was more stable than the others, which is consistent with previous reports.23,35,38 It has been noted that the relative stabilities of the fcc-type and hcp-type coordinations are associated with the number of d-electrons in TM and when the d-electron count is around 2, the relative stability of hcp-type coordination reaches its maximum.39
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Table 1 Calculated lattice parameter (a), thickness (L), M–C bond length (dM–C), M–O bond length (dM–O) of the M2CO2 sheet. Binding distances of each alkali ion are also presented in reference to the O layer (dion–O), and to M layer (dion–M). All distances are in angstrom unit
M in M2CO2
a
L
dM–C
dM–O
dion–O (Li/Na)
dion–M (Li/Na)
Ti V Cr Mn Fe Co Ni Nb Mo
3.03 2.89 2.69 2.92 2.81 2.77 2.90 3.15 2.87
4.45 4.41 4.82 4.34 4.42 4.46 4.12 4.69 5.20
2.19 2.05 2.02 2.09 2.03 2.06 2.00 2.21 2.16
1.98 1.96 1.92 1.92 1.91 1.85 1.93 2.13 2.06
1.92/2.30 1.89/2.23 1.87/2.27 1.83/2.21 1.83/2.20 1.82/2.15 1.82/2.15 1.97/1.97 1.91/2.34
2.16/2.68 2.16/2.67 2.27/2.79 2.03/2.57 1.99/2.54 1.98/2.49 1.93/2.42 2.29/2.29 2.36/2.92
Thus we believe that since only Cr and Mo satisfy this condition in our study, they prefer hcp-type O termination while the others prefer fcc-type O termination. The calculated total energy differences relative to the most stable configuration can be found in the ESI† (Table S1). The calculated lattice parameters (a), the thickness of the O terminated M2C sheet (L), the bond length between the metal and carbon (dM–C), and the binding distance of surface O from the metal center (dM–O) are summarized in Table 1. These geometric differences among MXenes directly affect the voltage properties as will be discussed in the next section. Alkali ion adsorption and voltage properties MXenes have been suggested as promising candidates for energy storage applications owing to their good electrical conductivity, large surface area, and high elastic moduli.17,19,40 To verify additional key properties that are necessary for the anode material, we first examined voltage properties by investigating the adsorption behaviours of alkali ions on aforementioned model structures of the M2CO2 sheet with varying TM elements. The voltage of an electrochemical cell reflects the interaction between binding ions and host structure. In terms of calculations, the open circuit voltage or equilibrium potential (VAM, where AM = Li or Na) can be approximated from the total energy difference between before and after ion adsorption as follows.30 VAM = [E(MXene) + xE(AM)
E(xAM + MXene)]/xF
(1)
In this expression, E(MXene), E(AM), and E(xAM + MXene) are the total energy of host MXene, alkali metal, and lithiated or sodiated MXene, respectively, where x is the composition of adsorbed Li or Na and F is the Faraday constant. In other words, the energy difference between reactants (the first two terms) and products (the last term) corresponds to the cell voltage. The average cell voltage can be used as a first-order criterion for selecting the anode material: typically a voltage of 0.0–1.0 V vs. alkali metal is desirable to prevent dendrite formation and to maximize the energy density. To evaluate the average voltage of MXenes (M2CO2 in the present study) from eqn (1), the surface structure of the final product, i.e. configuration of adsorbed alkali ions, should be determined first. For that, the most favourable adsorption site was examined on the 3 3 supercell and then the number of adsorbed atoms was systematically
5002 | Phys. Chem. Chem. Phys., 2015, 17, 5000--5005
Fig. 2 Optimized geometries of the ion adsorbed MXenes: (a) side and top views of lithiated/sodiated MXenes with fcc-type O termination (M = Ti, V, Mn, Fe, Co, Ni, and Nb); (b) side and top views of M2CO2Li2/M2CO2Na2 with hcp-type O termination (M = Cr and Mo).
increased until alkali ions are not bound any more (negative binding energy). For the M2CO2 sheets with M = Ti, V, Mn, Fe, Co, Ni, and Nb, which have an oxygenated surface of fcc-type, alkali ions are again adsorbed at the fcc hollow site up to 1 ML coverage on each side, keeping the stacking order of fcc packing (Fig. 2a). On the other hand, for M = Cr and Mo, where the surface O termination is hcp-type, Li or Na atoms are located above the M of the other side, resulting in disordered stacking as shown in Fig. 2b (A–B–A–C stacking for C–M–O–Li or –Na). Since the M2CO2 sheets can accommodate alkali ions up to 1 ML coverage on each side, their theoretical capacities are 214–349 mA h g 1 and 190– 288 mA h g 1 for Li and Na, corresponding to the chemical stoichiometry of M2CO2Li2 or M2CO2Na2. Fig. 3 displays calculated average voltages of M2C-type MXenes with different TMs. Considering that the optimal Li or Na intercalation voltage of anode materials is in the range of 0–1.0 V vs. each alkali metal,4,6,41 M2C electrodes with M = Cr (1.00 V), Nb (0.84 V), and Mo (0.69 V) are suitable for LIB anodes, and those with M = Ti (0.40 V), V (0.52 V), Cr (0.26 V), Mn (0.80 V), and Mo (0.19 V) are suitable for SIB anodes. Among these candidates, M2C-type MXene phases with M = Cr, Mn, and Mo are yet to be reported; however, their MAX phases (Cr2AlC, Mn2GaC, and Mo2GaC) have already been synthesized.21,42,43 Therefore, we expect that all suggested MXenes, including Cr2CO2, Mn2CO2, and Mo2CO2, could be exfoliated from each MAX phases if appropriate etching conditions are reached. We note that, so far, MXenes have been experimentally tested only for the LIBs, where the measured average voltages are consistent with our predictions shown in Fig. 3. For example, estimated vs. measured VLi are 1.1 V vs. B1.2 V, 1.3 V vs. B1.4 V, and 0.8 V vs. B0.7 V for Ti2CO2, V2CO2, and Nb2CO2, respectively,18,25,27 supporting our screening results in Fig. 3 based on the predicted voltages. It is also noteworthy that for the same M2CO2 sheet, VNa was estimated to be lower than VLi overall by 0.5–1.2 V, similar to the common oxide-based electrodes. The fact that VNa is generally lower than VLi is advantageous when
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Fig. 3 Calculated voltages of MXenes containing different transition metals. Filled (open) squares represent V vs. Li/Li+ (V vs. Na/Na+) of M2CO2. M with * indicates that corresponding M2CO2 is synthesized and tested in electrochemical cells. Points inside the shaded region have voltages lower than 1.0 V.
the material is applied to negative electrodes since a low operating voltage would lead to a high energy density of the battery. In Fig. 3, it is seen that the voltage generally increases along the period except for group 6 elements (Cr and Mo), which have different stacking orders of O and alkali ion as described in Fig. 2. As expressed in eqn (1), the average cell voltage is directly related to the binding strength of alkali ions to the host M2CO2 sheet, implying that their binding distance could be a reliable descriptor for the cell voltage. While the adsorbed alkali ions are mainly interacting with the surface oxygen, such ion–O distances (dion–O) were similar for all TM carbides (Table 1). Instead, the difference in ion–TM distances (dion–M) was 2–3 times more significant than dion–O with varying TM elements. Since the electrostatic energy is proportional to the q1q2/d (where q1 and q2 are point charges and d is the distance between them), both larger charge (M3+ vs. O2 ) and significant variation in d for ion–TM interaction suggest that the repulsive interaction between alkali ion and TM is the primary reason for the voltage difference between TM carbides. Therefore, in order to understand the trend and deviations shown in voltages of Fig. 3, we measured the vertical distances from the alkali ion to the closest TM layer that can represent the difference in electrostatic interactions between the adsorbed ion and the M2CO2 host sheet. As shown in Fig. 4, the ion–TM interlayer distances for each alkali ion are highly correlated with voltages, supporting the above suggestion that the ion–TM distance is the primary descriptor for the cell voltage. The binding distance, however, is not the only factor that determines the voltage since the data points corresponding to VLi and VNa are on different trend lines. Note that the trend line for VNa is shifted upward compared to that for VLi. We suspect that this discordance in trend lines is caused by different charges of Li and Na ions. That is, a lower electronegativity of Na (0.9) than Li (1.0)44 makes the Na ions have more positive charge than Li, which enhances the electrostatic interactions. We performed the Bader charge analysis in order to see this trend. We note that, since the Bader population analysis can yield atomic charges quite different from other partitioning methods for some cases,45–47 we mainly use it here as an approximate qualitative measure of
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Fig. 4 Dependence of the calculated voltage (V vs. Li/Li+, filled squares, or V vs. Na/Na+, open squares) on the distance between the adsorbed ion layer and the transition metal layer. R2 is the coefficient of determination of the least square linear fit.
atomic charges. From the result presented in Table S2 (ESI†), we find that Na is indeed more ionized compared to Li, which supports our interpretation. Therefore, at the same distance, Na ions exhibit a stronger binding, i.e., higher voltage, than Li ions. Surface migration of alkali ions The ion mobility is another factor to be considered when evaluating the feasibility of the electrode for rechargeable batteries. Since fast charge and discharge rates require a high mobility of charge carriers, a low activation barrier for ionic motion is essential for applying electrode materials to the highpower devices. In 2D materials like MXenes, where the binding sites for alkali ions are largely exposed to the electrolyte, lithiation (or sodiation) and its reverse process can be proceeded in both surface ionic migration and direct adsorption–desorption of ions via the electrolyte. In both cases, the easiness of surface migration plays an important role in fast charging and discharging since the reordering of surface ions for the minimum energy configuration should occur, in general, through the surface migration process. Therefore, the surface ion migration paths and their energy barriers were identified on the previously screened M2CO2 (M = Ti, V, Cr, Mn, and Mo). Three kinds of nearest neighbour (NN) hopping pathways were investigated for each MXene as illustrated in Fig. 5: for pathway 1, ion moves from one adsorption site to its NN site via the hollow site; for pathway 2, ion passes through the site on top of oxygen so that the ion would move along the ion - top of O - ion route; for pathway 3, ion moves directly from one site to another in a linear way. After the path optimization using the NEB method, the straight route defined in pathway 3 relaxes to pathway 1 in all MXenes, which exhibits a lower activation barrier (Ea) than pathway 2 by 0.4–0.6 eV (Fig. 5), indicating that the ion prefers to migrate along pathway 1. The calculated activation barriers for Na ion migration on Ti2CO2, V2CO2, Cr2CO2, Mn2CO2, and Mo2CO2 were 0.18, 0.15, 0.09, 0.15, and 0.14 eV, respectively (Table 2). These energy barriers are lower than those of Li migration by 0.05–0.11 eV,
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Fig. 5 Considered ion migration pathways and corresponding energy profiles of Li and Na on (a) Ti2CO2 and (b) Cr2CO2. The energy profiles of ion migration on other MXenes can be found in the ESI.†
Table 2
Activation barrier for Li or Na migration on M2CO2 with different M
Migration barrier (eV)
Li Na
Ti
V
Cr
Mn
Mo
0.28 0.18
0.21 0.15
0.13 0.09
0.22 0.15
0.16 0.14
possibly due to the weaker interaction between MXenes and Na ions. These values are comparable to those of other anode materials such as Na2Ti2O7 (0.19 eV)11 and Na3Sb (0.21 eV).48 While the energy barriers of ionic motion on MXenes could be affected by diverse factors such as strain of the sheets, concentration of ions, and the number of stacked layers,26,27 these fairly low activation energies of Na diffusion suggest that MXene is a potential candidate for electrode materials with high rate capability.
Conclusions In summary, we investigated the Na adsorption and migration properties of two-dimensional M2CO2 sheets based on DFT calculations in order to explore their capabilities as anode materials. The main findings in this work are as follows. (1) Both alkali metal ions preferentially adsorb on the hollow site of M2CO2 sheets up to 1 ML coverage on each side, forming the sodiated and lithiated structures, respectively, corresponding to the chemical formula of M2CO2Li2 and M2CO2Na2. (2) The average voltage increases along the period of TMs (except for Cr and Mo), where the voltage is significantly correlated with the distance between the adsorbed ion and the TM layer since the ion–TM distance mainly affects the difference in interaction strength between them and therefore electrostatic energies of the system. (3) From the viewpoint of voltage, M = Ti, V, Cr, Mn and Mo are expected to be suitable for SIB anodes, and M = Cr, Nb and Mo are suitable for LIB anodes, where these M2CO2 sheets exhibit 0.2–0.9 V vs. each alkali metal.
5004 | Phys. Chem. Chem. Phys., 2015, 17, 5000--5005
(4) The estimated average voltage for Na adsorption is lower than that of Li adsorption. The lower voltage of M2CO2 for Na adsorption is an advantageous point to be used as anode materials since the lower anode voltage results in a higher energy density of the cell. (5) For the screened M2CO2 sheets with M = Ti, V, Cr, Mn and Mo, Na ion migration barriers are 0.09–0.18 eV, suggesting that M2CO2 materials are capable of cycling at a fast rate in terms of ion mobility. (6) Although Cr2CO2, Mn2CO2, and Mo2CO2 have not been experimentally reported yet, we expect that these MXenes could be exfoliated in the near future since their corresponding MAX phases are known to exist. In contrast to the LIBs, where the carbons are used as an anode material in general, SIBs do not have attractive anode materials yet. Our results suggest that M2C-type MXenes are a new family of promising anode materials for SIBs indeed with desirable electrochemical properties.
Acknowledgements We acknowledge financial support from the National Research Foundation of Korea (NRF-2014R1A4A1003712 and NRF-2010C1AAA001-2010-0029031).
Notes and references 1 B. Dunn, H. Kamath and J. M. Tarascon, Science, 2011, 334, 928–935. 2 Z. G. Yang, J. L. Zhang, M. C. W. Kintner-Meyer, X. C. Lu, D. W. Choi, J. P. Lemmon and J. Liu, Chem. Rev., 2011, 111, 3577–3613. 3 C. Wadia, P. Albertus and V. Srinivasan, J. Power Sources, 2011, 196, 1593–1598. 4 S. W. Kim, D. H. Seo, X. H. Ma, G. Ceder and K. Kang, Adv. Energy Mater., 2012, 2, 710–721.
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5 V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-Gonzalez and T. Rojo, Energy Environ. Sci., 2012, 5, 5884–5901. 6 M. D. Slater, D. Kim, E. Lee and C. S. Johnson, Adv. Funct. Mater., 2013, 23, 947–958. 7 R. Alcantara, P. Lavela, G. F. Ortiz and J. L. Tirado, Electrochem. Solid-State Lett., 2005, 8, A222–A225. 8 S. Wenzel, T. Hara, J. Janek and P. Adelhelm, Energy Environ. Sci., 2011, 4, 3342–3345. 9 K. Tang, L. J. Fu, R. J. White, L. H. Yu, M. M. Titirici, M. Antonietti and J. Maier, Adv. Energy Mater., 2012, 2, 873–877. 10 Y. L. Cao, L. F. Xiao, M. L. Sushko, W. Wang, B. Schwenzer, J. Xiao, Z. M. Nie, L. V. Saraf, Z. G. Yang and J. Liu, Nano Lett., 2012, 12, 3783–3787. 11 H. L. Pan, X. Lu, X. Q. Yu, Y. S. Hu, H. Li, X. Q. Yang and L. Q. Chen, Adv. Energy Mater., 2013, 3, 1186–1194. 12 Y. H. Xu, Y. J. Zhu, Y. H. Liu and C. S. Wang, Adv. Energy Mater., 2013, 3, 128–133. 13 T. B. Kim, J. W. Choi, H. S. Ryu, G. B. Cho, K. W. Kim, J. H. Ahn, K. K. Cho and H. J. Ahn, J. Power Sources, 2007, 174, 1275–1278. 14 J. F. Qian, Y. Chen, L. Wu, Y. L. Cao, X. P. Ai and H. X. Yang, Chem. Commun., 2012, 48, 7070–7072. 15 L. Xiao, Y. Cao, J. Xiao, W. Wang, L. Kovarik, Z. Nie and J. Liu, Chem. Commun., 2012, 48, 3321–3323. 16 R. Alcantara, M. Jaraba, P. Lavela and J. L. Tirado, Chem. Mater., 2002, 14, 2847–2848. 17 M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. J. Niu, M. Heon, L. Hultman, Y. Gogotsi and M. W. Barsoum, Adv. Mater., 2011, 23, 4248–4253. 18 M. Naguib, J. Halim, J. Lu, K. M. Cook, L. Hultman, Y. Gogotsi and M. W. Barsoum, J. Am. Chem. Soc., 2013, 135, 15966–15969. 19 J. Come, M. Naguib, P. Rozier, M. W. Barsoum, Y. Gogotsi, P. L. Taberna, M. Morcrette and P. Simon, J. Electrochem. Soc., 2012, 159, A1368–A1373. 20 M. Ghidiu, M. Naguib, C. Shi, O. Mashtalir, L. M. Pan, B. Zhang, J. Yang, Y. Gogotsi, S. J. L. Billinge and M. W. Barsoum, Chem. Commun., 2014, 50, 9517–9520. 21 M. W. Barsoum, Prog. Solid State Chem., 2000, 28, 201–281. 22 Z. M. Sun, Int. Mater. Rev., 2011, 56, 143–166. 23 Q. Tang, Z. Zhou and P. W. Shen, J. Am. Chem. Soc., 2012, 134, 16909–16916. 24 M. R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall’Agnese, P. Rozier, P. L. Taberna, M. Naguib, P. Simon, M. W. Barsoum and Y. Gogotsi, Science, 2013, 341, 1502–1505. 25 M. Naguib, J. Come, B. Dyatkin, V. Presser, P. L. Taberna, P. Simon, M. W. Barsoum and Y. Gogotsi, Electrochem. Commun., 2012, 16, 61–64.
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26 S. J. Zhao, W. Kang and J. M. Xue, J. Phys. Chem. C, 2014, 118, 14983–14990. 27 Y. Xie, M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, X. Q. Yu, K. W. Nam, X. Q. Yang, A. I. Kolesnikov and P. R. C. Kent, J. Am. Chem. Soc., 2014, 136, 6385–6394. 28 M. D. Levi, M. R. Lukatskaya, S. Sigalov, M. Beidaghi, N. Shpigel, L. Daikhin, D. Aurbach, M. W. Barsoum and Y. Gogotsi, Adv. Energy Mater., 2014, DOI: 10.1002/aenm.201400815. 29 O. Mashtalir, M. Naguib, V. N. Mochalin, Y. Dall’Agnese, M. Heon, M. W. Barsoum and Y. Gogotsi, Nat. Commun., 2013, 4, 1716, DOI: 10.1038/ncomms2664. 30 M. K. Aydinol, A. F. Kohan, G. Ceder, K. Cho and J. Joannopoulos, Phys. Rev. B: Condens. Matter Mater. Phys., 1997, 56, 1354–1365. 31 G. Kresse and J. Furthmuller, Comput. Mater. Sci., 1996, 6, 15–50. 32 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868. 33 P. E. Blochl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953–17979. 34 J. Heyd, G. E. Scuseria and M. Ernzerhof, J. Chem. Phys., 2006, 124, 219906. 35 Y. Xie and P. R. C. Kent, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 235441. 36 S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104. 37 K. Momma and F. Izumi, J. Appl. Crystallogr., 2008, 41, 653–658. 38 M. Khazaei, M. Arai, T. Sasaki, C. Y. Chung, N. S. Venkataramanan, M. Estili, Y. Sakka and Y. Kawazoe, Adv. Funct. Mater., 2013, 23, 2185–2192. 39 M. Kertesz and R. Hoffmann, J. Am. Chem. Soc., 1984, 106, 3453–3460. 40 M. Kurtoglu, M. Naguib, Y. Gogotsi and M. W. Barsoum, MRS Commun., 2012, 2, 133–137. 41 M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa and S. Komaba, Phys. Chem. Chem. Phys., 2014, 16, 15007–15028. 42 P. Eklund, M. Beckers, U. Jansson, H. Hogberg and L. Hultman, Thin Solid Films, 2010, 518, 1851–1878. 43 A. S. Ingason, A. Petruhins, M. Dahlqvist, F. Magnus, A. Mockute, B. Alling, L. Hultman, I. A. Abrikosov, P. O. Å. Persson and J. Rosen, Mater. Res. Lett., 2013, 2, 89–93. 44 L. C. Allen, J. Am. Chem. Soc., 1989, 111, 9003–9014. 45 K. B. Wiberg and P. R. Rablen, J. Comput. Chem., 1993, 14, 1504–1518. 46 B. Wang, S. L. Li and D. G. Truhlar, J. Chem. Theory Comput., 2014, 10, 5640–5650. 47 C. Fonseca Guerra, J.-W. Handgraaf, E. J. Baerends and F. M. Bickelhaupt, J. Comput. Chem., 2004, 25, 189–210. 48 L. Baggetto, P. Ganesh, C. N. Sun, R. A. Meisner, T. A. Zawodzinski and G. M. Veith, J. Mater. Chem. A, 2013, 1, 7985–7994.
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