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sulfur decorated 2D MXene V2C for promising lithium ion battery anodes
Materials Today Communications 22 (2020) 100713
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Oxygen/sulfur decorated 2D MXene V2C for promising lithium ion battery anodes
T
Bingzhen Yan, Chengjie Lu, Peigen Zhang*, Jian Chen, Wei He, Wubian Tian, Wei Zhang, ZhengMing Sun* Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing, 211189, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: V2C MXene Li-ion battery First-principles Li storage capacity
Two-dimensional MXene V2C is a promising anode candidate for lithium ion battery. However, the fluorination or hydroxylation bonded to V2C hinders the fast Li ion transportation, and consequently decreases the lithium capacity significantly. In this work we investigate the possibility of V2CT2 (T]O, S) for LIB anodes by firstprinciples calculation. The calculation results indicate that compared with V2CF2 or V2C(OH)2, V2CT2 terminated with oxygen or sulfur possesses a superior electrochemical performance and retains metallic conductivity. In addition, a large absorption energy and a low diffusion barrier of Li atom are found in V2CT2 (T]O, S), which may render the MXenes excellent energy storage capacity and charge/discharge rate. Therefore, the oxygen/ sulfur decorated V2C would be a promising lithium ion battery anode material.
1. Introduction Two-dimensional (2D) materials like graphene, MoS2 and h-BN have gathered considerable attention due to their potential for a broad spectrum of applications [1–5]. The unique layered structures and high specific surface area endow them fast ion diffusion and ample tunnels for ion insertion, which just correspond to the demands for the energy storage materials. Recently, a novel series of 2D nano-laminated early transition metal carbides (MCs) and nitrides (MNs) called MXenes with the general formula of Mn+1XnTx have stood out by virtue of their promising properties. In the formula, M refers to early transition metal elements (Ti, V, Cr, Zr, etc.), X is carbon or nitrogen, and T represents the surface terminations (eO, eOH, eF) attached on their surfaces when prepared from their corresponding MAX precursors by etching in a concentrated HF solution [6–8]. Interestingly, MXenes also typify hydrophilic and tunable electronic properties by regulating surface functional groups or applying strain and electric field. For example, Khazaei et al. [9] found that Ti2CF2 and Ti2C(OH)2 are metallic while Ti2CO2 is semiconducting. Lee et al. [10] have predicted that Sc2CO2 has a tunable band gap by controlling the external strain, which favors its application for optical nanodevices. In addition to these fascinating properties, both experimental and theoretical investigations have proved that MXenes can serve as promising anode candidates for LiBs [11–14]. Tang et al. [11] predicted that the specific capacity of Ti3C2 (320 mA h /g) for LIBs is higher than that of TiO2 (< 200 mA h g−1)
⁎
along with a lower Li diffusion barrier using first-principles calculations, which contributes to an enhanced Li storage capability, faster Li transport ability and thus higher charge/discharge rates; however, the terminations of F/OH decrease Li storage capacity sharply for Ti3C2(130/67 mA h/g). And it is well established that functional groups on MXenes influence Li storage capacity obviously, which are unfavorable for MXene applications in energy storage field [11,12,15]. Nonetheless, by limits of present preparation methods, all the MXenes synthesized from aqueous solutions are inevitably covered with various function groups (eF/eO/eOH etc.). In addition, some other function groups like -N is possible to be introduced using hydrothermal method, which has been reported in our previous work [16]. Hence, to obtain a much more desirable electrochemical performance of MXenes, optimizing their terminations would be fruitful. It is reported that Nb2C (not a member of MXenes family) can be terminated with sulfur atoms by extracting Cu atoms from CuxNb2S2C, forming a unique structure similar to Nb2CTx MXene [17], which inspires us to explore the sulfur-terminated MXene (Mn+1XnSx). Yang et al. [18] investigated the stability and electronic properties of M2CS2 (M = Ta, V, Cr, Nb) via theoretical calculations, the results indicate that all the M2CS2 are thermodynamically stable and with metal conductivity (except Ta2CS2). In addition, Liang et al. [19] claimed that a part of the eOH groups can be replaced by sulfur atoms in Ti2C to form Ti2CS2, which results in an excellent specific capacity (∼1200 mA h/g) for lithium-sulfur (Li-S) batteries. Whereafter, an ab initio calculation
Corresponding authors. E-mail addresses: [email protected] (P. Zhang), [email protected] (Z. Sun).
https://doi.org/10.1016/j.mtcomm.2019.100713 Received 22 August 2019; Received in revised form 18 October 2019; Accepted 18 October 2019 Available online 21 November 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
Materials Today Communications 22 (2020) 100713
B. Yan, et al.
III). After structural optimizations, the most stable configuration of V2CT2 is shown in Fig. 1(b) and (c) with eS, eO terminated on Site II formed by three adjacent V atoms. Interestingly, the Type II of V2CT2 is the most energetically stable configuration for these two kinds of terminations (eS/eO) due to the unsaturated electronic structure around each neighboring V atom. The in-plane lattice parameter of V2CS2 is 3.06 Å, which agrees well with the reported result (3.06 Å) [18]. The bond length of VeS is calculated to be 2.37 Å, larger than that of VeO (1.96 Å) for V2CO2. It is the difference of atom radius and electronegativity that contributes to the change of bond length. The analysis of Bader charge shows that S atoms obtain less charge than O atoms from V2C due to its weaker electronegativity, as seen from Table 1. In order to evaluate the stability of V2CS2 (V2CO2 has been synthesized [29]), we calculated the formation energy of this novel MXene, defined as
targeted on Ti2CS2 proved that Ti2CS2 shows the highest binding energy to polysulfides compared with Ti2CO2, Ti2CF2 and Ti2C(OH)2 and a relatively low Li diffusion barrier which may facilitate its potential applications in Li-S batteries in future [20]. All the investigations mentioned above imply that S-decorated MXenes may possess favorable structural and electronic properties and hold a promise in energy storage. Ti2CS2 is evaluated to be unstable at 1000 K by molecular dynamic simulations, revealing the limited chance of experimental preparation [18], but V2CS2 is predicted to be even stable at 1000 K and has the highest stability among several M2CS2 (M = Ti, V, Cr, Nb) and it can be easily isolated into a single layer, which manifests the high feasibility of experimental synthesis. In addition, it has been verified that the Li storage performance of V2CTx is superior to that of Ti2CTx [15,21]. Consequently, it is reasonable to assume that V2CS2 may be suitable for LIB anode materials. It is noted that the potential of V2C and V2CTx(T]F, OH) as LIB anodes has been investigated, and the results indicate that the fluoridation or hydroxylation is the main cause of unsatisfactory electrochemical performance for V2C [12,15]. But further exploring the optimization of the electrochemical performance of V2C by tailoring functional groups (indispensable to MXene) has yet to be conducted. Meanwhile, it is noted that Ti2N with eO groups is proved to have an improved Li capacity compared with Ti2NF2 or Ti2N (OH)2, indicating that MXene with eO terminations may have good Li storage capacity [13]. Hence, in this work, we investigated the structural, electronic properties of V2CT2 (T]O, S) and their lithium storage capacity through first-principles calculations to better understand the effect of eS/eO groups on the electrochemical performance of V2C. Firstly, several possible anchoring sites of T atoms are investigated to confirm the most stable configuration of V2CT2. Then, the interaction between Li atoms and V2CT2 substrate is elucidated. Lastly, the Li storage capacity of these 2D materials is evaluated.
Ef = E[V2CS2] – E[V2C]-2E[S]
(1)
where E[V2CS2], E[V2C] and E[S] refer to the total energy of V2CS2, V2C and per atom of S8 molecule, respectively. The Ef is calculated to be −4.69 eV, and the negative value indicates its feasibility and stability to be terminated with S atoms. In addition, the stability of V2CS2 has been verified through phonon dispersions, which also indicates the stability of V2CS2 even at T = 1000 K [18]. More significantly, we measured the substitution formation energy of V2CS2 to shed more light on the potential transformation behavior for terminations from eO, eOH, and eF to eS in the V2CTx model. Fig. 1(d) and (e) illustrates the substitution process, and the formation energy Esub could be defined as the difference of the two models. The calculation results of Esub are shown in Table 2. It is indicated that the substitution of eS function for eF and eOH terminations are feasible, whose formation energies of Esub are negative, with the values of −0.83 eV and −1.93 eV, respectively. While in the case of V2C with eO terminations, a positive Esub (+2.37 eV) suggests that the substitution process is unfavorable in the energy aspect. Fig. 3(b) shows the total DOS of V2CS2, suggesting that the functional of S atoms for V2C retains metallic which favors its application for LIBs, while V2CT2 turns to be semiconductors when terminated with eF or eOH groups [12]. And when terminated with O atoms (V2CO2), it is the same case as V2CS2. To explore the potential of V2CT2 as lithium battery anodes, we first investigated single Li atom adsorption behavior for the 3 × 3×1 V2CT2 supercell. Three high symmetry surface sites are considered for Li: the site on top of the T atom (Type I), on top of the lower V atom (Type II) and the site on top of the C atom (Type III). Among these three configurations, V18C9S18Li with Type II is confirmed to be the most stable structure, as depicted in Fig. 2(a, b). The shortest distance between Li and adjacent S atoms is 2.28 Å. The adsorption energy (Eads) is defined as the difference between the total energy of the entire MXene with Li anchored on the surface and the sum of the total energy of an individual 3 × 3 V2CS2 monolayer and an isolated Li atom (originated from the Li bulk metal with body-centered cubic structure). The calculated adsorption energy is -1.66 eV, indicating an exothermic process and a strong adsorption interaction. For V2CO2 with Li adsorption, the most energetically stable site is Type III, with an adsorption energy of −1.80 eV. To gain a better insight into the interaction between the adsorbate and substrate, we computed the density of states (DOS) of V2CT2 monolayer and V2CT2 with one adsorbed Li atom (Fig. 3). Taking V2CS2 for example, it is clear that V2CS2 retains its metallic nature and the position of EF shifts insignificantly with a Li adsorption. An obvious overlap between the S 2p orbital and Li 2 s orbital is observed ranging from −3 eV to −2 eV, indicating a strong interaction. It is shown that the total DOS of V2CS2-Li moves towards the lower energy range compared with that of V2CS2, indicating a more stable configuration after lithiation. In addition, the charge redistribution of the whole system is also investigated through Bader charge and charge difference density
2. Methodology All the first-principles calculations in this work were performed by using the Vienna ab initio simulation package [22] (VASP), in which the interactions of core electrons were described by the projector augment wave (PAW) potentials [23,24] and the exchange and correlation functional were modelled by the Perdew–Burke–Ernzerhof (PBE) functionals derived from the generalized gradient approximation (GGA) [25]. A cut-off energy of 500 eV is used for the plane-wave basis set. A 12 × 12 × 1 Monkhorst-Pack (MP) k-point sampling is adopted for structural optimizations and a finer grid (24 × 24 × 1) for electronic structure calculations [26]. Geometry optimization is conducted using the conjugated gradient (CG) method with the convergence thresholds of 1 × 10−5 eV/atom for the total energy, 0.02 eV Å-1 for the maximum force, 0.02 GPa for the maximum stress and maximum ionic displacement for 5 × 10-4 Å. A 3 × 3 V2CT2 supercell, namely V18C9T18 is built to investigate the interactions between the single Li atom and MXene substrate. The van der Waals (vdW) interaction is considered by adopting the DFT-D3 method (an empirical dispersion correction scheme of Grimme) [27]. In order to avoid any possible artificial interactions between adjacent MXene sheets, a 20 Å vacuum space along the c direction is inserted. The minimum energy reaction pathway for Li diffusion and energy barrier were evaluated by using the climbingimage nudged elastic band (CI-NEB) method [28]. 3. Results and discussions The optimized model of bare V2C is derived from exfoliating Al elements from the corresponding MAX phase (V2AlC). When atoms (Tx, here we take S atoms for example) terminate the surfaces of V2C, there are three possible anchoring sites as shown in Fig. 1(a): The S atom is located above the top of the upper V atom (Site I), or placed at the hollow site of the lower V atom (Site II) or bonded with C atom (Site 2
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Fig. 1. The structure of V2C (a) and side view of 3 × 3 × 1 V2CS2(b), V2CO2(c); Schematic illustration of the formation of V2CS2 by substitution of termination atoms (d and e, in which T represents F,OH,O).
calculations. It shows that Li atom transfers 0.86 e, 0.89 e to V2CS2 and V2CO2, respectively, during the adsorption process. Taking V2CS2 for example, Fig. 4 depicts the charge difference density of V2CS2-Li, sky blue and yellow colors represent the regions of electron depletion and accumulation, respectively. It is clearly shown that a substantial charge distribution occurs between Li and MXene substrate. Hence, it proves that V2C with eS/eO terminations can adsorb Li atoms easily via strong chemical adsorption. The Li diffusion on the V2CT2 surface is studied by CI-NEB method in order to determine the diffusion barrier (Eb). A small Eb favors the fast charge/discharge rate of Li ions, which is beneficial to LIBs. The diffusion paths are chosen along the three high symmetry positions. The most stable configuration with Li adsorption (Fig. 2) is set to be the initial state, and the corresponding configuration at the most stable neighboring site is chosen as final state. For V2CS2 and V2CO2 (Fig. 5), the lowest migration barrier is measured to be 0.22 eV (Site 2→Site 3→ Site 2′) and 0.15 eV (Site 3→Site 2→Site 3′), respectively. It is noted that the diffusion barrier of V2CT2 (T]O, S) is lower than those of
Table 1 The in-plane lattice constant (Å) of V2CT2 (T]O, S), bond length of VeC and VeT (Å), thickness (d, Å) and charge transfers from V2C to T (Δq, e). T
a
V-C
V-T
d
Δq
V2CO2 V2CS2
2.90 3.06
2.05 2.08
1.96 2.37
4.41 5.34
0.99 0.62
Table 2 The substitution formation energy of V2CS2 for terminations from eO, eOH, and eF to eS for V2C.
-F -OH -O
E1 (eV)
E2 (eV)
Esub (eV)
−351.51 −460.89 −398.07
−352.34 −462.82 −395.70
−0.83 −1.93 +2.37
Fig. 2. The top view and side view of 3 × 3 × 1 V2CT2 supercell without and with Li adsorption. 3
Materials Today Communications 22 (2020) 100713
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Fig. 3. The PDOS and TDOS of 3 × 3 V2CT2 and V2CT2Li. (a) T = O, (b) T = S.
high adatom concentrations. A 2 × 2 × 1 V2CT2 supercell is built as the initial substrate model. The Li adsorption on both sides of MXene is taken into consideration to provide more practical guide for experiments. By varying the number (x) of Li atoms, it is calculated that the maximum numbers of Li adatom are both 8 for the V2CS2 and V2CO2 2 × 2 × 1 supercells, with the formula V2CT2Li2, corresponding to the values of theoretical capacity CT of 301.22 and 367.64 mA h/g, respectively, which are much higher than those of V2CF2 and V2C(OH)2 (116.78 and 181.33 mA h/g [11]). The theoretical capacity is given by equation (2): CT= (x × Z × F × 103)/M
(2)
where x is the maximum number of Li adatom for V2CT2 single cell, Z is the valence number (Z = 1 for Li), F is the Faraday constant (26.81 mA h/mol) and M is the total atomic mass of V2CT2. The Li capacity follows the order: V2CO2 > V2CS2 > V2C(OH)2 > V2CF2, indicating that V2C anchored with O or S atoms are more desirable for LIB anodes compared with those with eF or eOH terminations. Though surface terminations hinder the electrochemical performance of V2C, it is noted that the Li capacity of V2CT2 (T]S, O) is comparable to that of bare Ti3C2 monolayer (320 mA h/g) [11]. Therefore, to improve the performance of V2C, eF and eOH functional groups should be reduced or replaced by O and S atoms during the synthesis since surface terminations are inevitable. Experimental work towards this objective is highly possible since Yang et al. [30] recently reported a scalable fluoride-free MXene synthetic procedure by etching MAX phases in a binary aqueous electrolyte, turning the possibility of F-free MXene into
Fig. 4. Charge difference density of V2CS2Li. (Iso-surface value is 0.004 e/Å3).
V2CT2 (T]F, OH) (0.36 eV and 0.59 eV [12]), implying its superior electrochemical performance for LIBs. Furthermore, for V2CT2 (T]O, S), the value of Eb is also much lower than that of Ti3C2Tx (0.8–1.2 eV), the most widely studied MXene, suggesting that the former has much faster diffusion mobility and higher Li charge/discharge rate. To evaluate the theoretical Li storage capacity of V2CT2, it is of significance to investigate the Li adsorption behavior for V2CT2 with
Fig. 5. The diffusion path and energy profile of V2CS2 (a) and V2CO2(b). 4
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reality and opening a new path for the functional-tailoring synthesis of MXenes. More efforts on modifying the functional group of MXene are needed and this would be promising for the birth of more advanced MXene-based LIBs.
Mater. 24 (2012) 4097-111. [6] M.W. Barsoum, The MN+1AXNphases: a new class of solids: thermodynamically stable nanolaminates, Prog. Solid State Chem. 28 (1) (2000) 201–281. [7] M. Naguib, et al., Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2, Communication 23 (2011) 4248–4253. [8] M. Naguib, et al., Two-dimensional transition metal carbides, ACS Nano 6 (2012) 1322–1331. [9] M. Khazaei, et al., Novel electronic and magnetic properties of two-dimensional transition metal carbides and nitrides, Adv. Funct. Mater. 23 (17) (2013) 2185–2192. [10] Y. Lee, S.B. Cho, Y.-C. Chung, Tunable indirect to direct band gap transition of monolayer Sc2CO2 by the strain effect, ACS Appl. Mater. Interfaces 6 (16) (2014) 14724–14728. [11] Q. Tang, Z. Zhou, P. Shen, 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. 134 (40) (2012) 16909–16916. [12] J. Hu, et al., Investigations on V2C and V2CX2 (X = F, OH) monolayer as a promising anode material for Li ion batteries from first-principles calculations, J. Phys. Chem. C 118 (42) (2014) 24274–24281. [13] D. Wang, et al., First-principles calculations of Ti2N and Ti2NT2 (T = O, F, OH) monolayers as potential anode materials for lithium-ion batteries and beyond, J. Phys. Chem. C 121 (24) (2017) 13025–13034. [14] W. Zheng, et al., In situ synthesis of CNTs@Ti3C2 hybrid structures by microwave irradiation for high-performance anodes in lithium ion batteries, J. Mater. Chem. A 6 (8) (2018) 3543–3551. [15] M. Naguib, et al., New two-dimensional niobium and vanadium carbides as promising materials for Li-ion batteries, J. Am. Chem. Soc. 135 (43) (2013) 15966-9. [16] L. Yang, et al., Freestanding nitrogen-doped d-Ti3C2/reduced graphene oxide hybrid films for high performance supercapacitors, Electrochim. Acta 300 (2019) 349–356. [17] K. Sakamaki, et al., Topochemical formation of van der Waals type niobium carbosulfide 1T-Nb2S2C, J. Alloys Compd. 339 (1) (2002) 283–292. [18] J. Yang, et al., Stability and electronic properties of sulfur terminated two-dimensional early transition metal carbides and nitrides (MXene), Comput. Mater. Sci. 153 (2018) 303–308. [19] X. Liang, A. Garsuch, L.F. Nazar, Sulfur cathodes based on conductive MXene nanosheets for high-performance lithium-sulfur batteries, Angew. Chem.-Int. Ed. 54 (13) (2015) 3907–3911. [20] X. Liu, et al., Anchoring effects of S-terminated Ti2C MXene for lithium-sulfur batteries: a first-principles study, Appl. Surf. Sci. 455 (2018) 522–526. [21] J. Zhou, et al., Ti-enhanced exfoliation of V2AlC into V2C MXene for lithium-ion battery anodes, Ceram. Int. 43 (14) (2017) 11450–11454. [22] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47 (1) (1993) 558–561. [23] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (16) (1996) 11169–11186. [24] P.E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50 (24) (1994) 17953–17979. [25] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (18) (1996) 3865–3868. [26] H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (12) (1976) 5188–5192. [27] S. Grimme, et al., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu, J. Chem. Phys. 132 (15) (2010) 154104. [28] G. Henkelman, B.P. Uberuaga, H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, J. Chem. Phys. 113 (22) (2000) 9901–9904. [29] F. Liu, et al., Preparation of high-purity V2C MXene and electrochemical properties as Li-ion batteries, J. Electrochem. Soc. 164 (4) (2017) A709–A713. [30] S. Yang, et al., Fluoride-free synthesis of two-dimensional titanium carbide (MXene) using a binary aqueous system, Angew. Chem. Int. Ed. Engl. 57 (47) (2018) 15491–15495.
4. Summary In summary, we investigated the structural and electronic properties of V2CTx (T]O, S) as well as its lithium storage capacity by firstprinciples calculations. The most stable configuration of V2CT2 (T]O, S) is formed when T atoms anchor above the site of V hollow position. The calculated results of formation energy indicate the high feasibility of the synthesis of V2CS2. The DOS results show V2CO2 and V2CS2 become metal-conductive, while V2C terminated with F or OH groups are semiconductors (narrow-band gap). The potential of V2CT2 as LIB anodes is evaluated by the Li adsorption behaviors of these 2D materials. V2CO2 and V2CS2 after lithiation still remain metallic, which favors their applications in LIBs. Moreover, V2C terminated with eO/eS has much lower Li diffusion barriers and larger Li capacity compared with those of eF/eOH, showing their great potential as alternative anodes. The results show that it would be practical to synthesize V2CT2 (T]O, S) MXenes, and offer an improved understanding of tailoring functional groups to improve the performance of V2C as potential energy storage materials. Declaration of Competing Interest No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. Acknowledgements This work was supported by the Grant of National Natural Science Foundation of China (51731004), and Zhishan Youth Scholar Program of Southeast University. References [1] J. Xiao, et al., Exfoliated MoS2 nanocomposite as an anode material for lithium ion batteries, Chem. Mater. 22 (16) (2010) 4522–4524. [2] X. Wang, et al., A SnO2/graphene composite as a high stability electrode for lithium ion batteries, Carbon 49 (1) (2011) 133–139. [3] K. Bindumadhavan, S.K. Srivastava, S. Mahanty, MoS2-MWCNT hybrids as a superior anode in lithium-ion batteries, Chem. Commun. 49 (18) (2013) 1823–1825. [4] K. Yan, et al., Ultrathin two-dimensional atomic crystals as stable interfacial layer for improvement of lithium metal anode, Nano Lett. 14 (10) (2014) 6016–6022. [5] J. Liu, X.-W. Liu, Two-dimensional nanoarchitectures for lithium storage, Adv.
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Contents lists available at ScienceDirect
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Oxygen/sulfur decorated 2D MXene V2C for promising lithium ion battery anodes
T
Bingzhen Yan, Chengjie Lu, Peigen Zhang*, Jian Chen, Wei He, Wubian Tian, Wei Zhang, ZhengMing Sun* Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing, 211189, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: V2C MXene Li-ion battery First-principles Li storage capacity
Two-dimensional MXene V2C is a promising anode candidate for lithium ion battery. However, the fluorination or hydroxylation bonded to V2C hinders the fast Li ion transportation, and consequently decreases the lithium capacity significantly. In this work we investigate the possibility of V2CT2 (T]O, S) for LIB anodes by firstprinciples calculation. The calculation results indicate that compared with V2CF2 or V2C(OH)2, V2CT2 terminated with oxygen or sulfur possesses a superior electrochemical performance and retains metallic conductivity. In addition, a large absorption energy and a low diffusion barrier of Li atom are found in V2CT2 (T]O, S), which may render the MXenes excellent energy storage capacity and charge/discharge rate. Therefore, the oxygen/ sulfur decorated V2C would be a promising lithium ion battery anode material.
1. Introduction Two-dimensional (2D) materials like graphene, MoS2 and h-BN have gathered considerable attention due to their potential for a broad spectrum of applications [1–5]. The unique layered structures and high specific surface area endow them fast ion diffusion and ample tunnels for ion insertion, which just correspond to the demands for the energy storage materials. Recently, a novel series of 2D nano-laminated early transition metal carbides (MCs) and nitrides (MNs) called MXenes with the general formula of Mn+1XnTx have stood out by virtue of their promising properties. In the formula, M refers to early transition metal elements (Ti, V, Cr, Zr, etc.), X is carbon or nitrogen, and T represents the surface terminations (eO, eOH, eF) attached on their surfaces when prepared from their corresponding MAX precursors by etching in a concentrated HF solution [6–8]. Interestingly, MXenes also typify hydrophilic and tunable electronic properties by regulating surface functional groups or applying strain and electric field. For example, Khazaei et al. [9] found that Ti2CF2 and Ti2C(OH)2 are metallic while Ti2CO2 is semiconducting. Lee et al. [10] have predicted that Sc2CO2 has a tunable band gap by controlling the external strain, which favors its application for optical nanodevices. In addition to these fascinating properties, both experimental and theoretical investigations have proved that MXenes can serve as promising anode candidates for LiBs [11–14]. Tang et al. [11] predicted that the specific capacity of Ti3C2 (320 mA h /g) for LIBs is higher than that of TiO2 (< 200 mA h g−1)
⁎
along with a lower Li diffusion barrier using first-principles calculations, which contributes to an enhanced Li storage capability, faster Li transport ability and thus higher charge/discharge rates; however, the terminations of F/OH decrease Li storage capacity sharply for Ti3C2(130/67 mA h/g). And it is well established that functional groups on MXenes influence Li storage capacity obviously, which are unfavorable for MXene applications in energy storage field [11,12,15]. Nonetheless, by limits of present preparation methods, all the MXenes synthesized from aqueous solutions are inevitably covered with various function groups (eF/eO/eOH etc.). In addition, some other function groups like -N is possible to be introduced using hydrothermal method, which has been reported in our previous work [16]. Hence, to obtain a much more desirable electrochemical performance of MXenes, optimizing their terminations would be fruitful. It is reported that Nb2C (not a member of MXenes family) can be terminated with sulfur atoms by extracting Cu atoms from CuxNb2S2C, forming a unique structure similar to Nb2CTx MXene [17], which inspires us to explore the sulfur-terminated MXene (Mn+1XnSx). Yang et al. [18] investigated the stability and electronic properties of M2CS2 (M = Ta, V, Cr, Nb) via theoretical calculations, the results indicate that all the M2CS2 are thermodynamically stable and with metal conductivity (except Ta2CS2). In addition, Liang et al. [19] claimed that a part of the eOH groups can be replaced by sulfur atoms in Ti2C to form Ti2CS2, which results in an excellent specific capacity (∼1200 mA h/g) for lithium-sulfur (Li-S) batteries. Whereafter, an ab initio calculation
Corresponding authors. E-mail addresses: [email protected] (P. Zhang), [email protected] (Z. Sun).
https://doi.org/10.1016/j.mtcomm.2019.100713 Received 22 August 2019; Received in revised form 18 October 2019; Accepted 18 October 2019 Available online 21 November 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
Materials Today Communications 22 (2020) 100713
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III). After structural optimizations, the most stable configuration of V2CT2 is shown in Fig. 1(b) and (c) with eS, eO terminated on Site II formed by three adjacent V atoms. Interestingly, the Type II of V2CT2 is the most energetically stable configuration for these two kinds of terminations (eS/eO) due to the unsaturated electronic structure around each neighboring V atom. The in-plane lattice parameter of V2CS2 is 3.06 Å, which agrees well with the reported result (3.06 Å) [18]. The bond length of VeS is calculated to be 2.37 Å, larger than that of VeO (1.96 Å) for V2CO2. It is the difference of atom radius and electronegativity that contributes to the change of bond length. The analysis of Bader charge shows that S atoms obtain less charge than O atoms from V2C due to its weaker electronegativity, as seen from Table 1. In order to evaluate the stability of V2CS2 (V2CO2 has been synthesized [29]), we calculated the formation energy of this novel MXene, defined as
targeted on Ti2CS2 proved that Ti2CS2 shows the highest binding energy to polysulfides compared with Ti2CO2, Ti2CF2 and Ti2C(OH)2 and a relatively low Li diffusion barrier which may facilitate its potential applications in Li-S batteries in future [20]. All the investigations mentioned above imply that S-decorated MXenes may possess favorable structural and electronic properties and hold a promise in energy storage. Ti2CS2 is evaluated to be unstable at 1000 K by molecular dynamic simulations, revealing the limited chance of experimental preparation [18], but V2CS2 is predicted to be even stable at 1000 K and has the highest stability among several M2CS2 (M = Ti, V, Cr, Nb) and it can be easily isolated into a single layer, which manifests the high feasibility of experimental synthesis. In addition, it has been verified that the Li storage performance of V2CTx is superior to that of Ti2CTx [15,21]. Consequently, it is reasonable to assume that V2CS2 may be suitable for LIB anode materials. It is noted that the potential of V2C and V2CTx(T]F, OH) as LIB anodes has been investigated, and the results indicate that the fluoridation or hydroxylation is the main cause of unsatisfactory electrochemical performance for V2C [12,15]. But further exploring the optimization of the electrochemical performance of V2C by tailoring functional groups (indispensable to MXene) has yet to be conducted. Meanwhile, it is noted that Ti2N with eO groups is proved to have an improved Li capacity compared with Ti2NF2 or Ti2N (OH)2, indicating that MXene with eO terminations may have good Li storage capacity [13]. Hence, in this work, we investigated the structural, electronic properties of V2CT2 (T]O, S) and their lithium storage capacity through first-principles calculations to better understand the effect of eS/eO groups on the electrochemical performance of V2C. Firstly, several possible anchoring sites of T atoms are investigated to confirm the most stable configuration of V2CT2. Then, the interaction between Li atoms and V2CT2 substrate is elucidated. Lastly, the Li storage capacity of these 2D materials is evaluated.
Ef = E[V2CS2] – E[V2C]-2E[S]
(1)
where E[V2CS2], E[V2C] and E[S] refer to the total energy of V2CS2, V2C and per atom of S8 molecule, respectively. The Ef is calculated to be −4.69 eV, and the negative value indicates its feasibility and stability to be terminated with S atoms. In addition, the stability of V2CS2 has been verified through phonon dispersions, which also indicates the stability of V2CS2 even at T = 1000 K [18]. More significantly, we measured the substitution formation energy of V2CS2 to shed more light on the potential transformation behavior for terminations from eO, eOH, and eF to eS in the V2CTx model. Fig. 1(d) and (e) illustrates the substitution process, and the formation energy Esub could be defined as the difference of the two models. The calculation results of Esub are shown in Table 2. It is indicated that the substitution of eS function for eF and eOH terminations are feasible, whose formation energies of Esub are negative, with the values of −0.83 eV and −1.93 eV, respectively. While in the case of V2C with eO terminations, a positive Esub (+2.37 eV) suggests that the substitution process is unfavorable in the energy aspect. Fig. 3(b) shows the total DOS of V2CS2, suggesting that the functional of S atoms for V2C retains metallic which favors its application for LIBs, while V2CT2 turns to be semiconductors when terminated with eF or eOH groups [12]. And when terminated with O atoms (V2CO2), it is the same case as V2CS2. To explore the potential of V2CT2 as lithium battery anodes, we first investigated single Li atom adsorption behavior for the 3 × 3×1 V2CT2 supercell. Three high symmetry surface sites are considered for Li: the site on top of the T atom (Type I), on top of the lower V atom (Type II) and the site on top of the C atom (Type III). Among these three configurations, V18C9S18Li with Type II is confirmed to be the most stable structure, as depicted in Fig. 2(a, b). The shortest distance between Li and adjacent S atoms is 2.28 Å. The adsorption energy (Eads) is defined as the difference between the total energy of the entire MXene with Li anchored on the surface and the sum of the total energy of an individual 3 × 3 V2CS2 monolayer and an isolated Li atom (originated from the Li bulk metal with body-centered cubic structure). The calculated adsorption energy is -1.66 eV, indicating an exothermic process and a strong adsorption interaction. For V2CO2 with Li adsorption, the most energetically stable site is Type III, with an adsorption energy of −1.80 eV. To gain a better insight into the interaction between the adsorbate and substrate, we computed the density of states (DOS) of V2CT2 monolayer and V2CT2 with one adsorbed Li atom (Fig. 3). Taking V2CS2 for example, it is clear that V2CS2 retains its metallic nature and the position of EF shifts insignificantly with a Li adsorption. An obvious overlap between the S 2p orbital and Li 2 s orbital is observed ranging from −3 eV to −2 eV, indicating a strong interaction. It is shown that the total DOS of V2CS2-Li moves towards the lower energy range compared with that of V2CS2, indicating a more stable configuration after lithiation. In addition, the charge redistribution of the whole system is also investigated through Bader charge and charge difference density
2. Methodology All the first-principles calculations in this work were performed by using the Vienna ab initio simulation package [22] (VASP), in which the interactions of core electrons were described by the projector augment wave (PAW) potentials [23,24] and the exchange and correlation functional were modelled by the Perdew–Burke–Ernzerhof (PBE) functionals derived from the generalized gradient approximation (GGA) [25]. A cut-off energy of 500 eV is used for the plane-wave basis set. A 12 × 12 × 1 Monkhorst-Pack (MP) k-point sampling is adopted for structural optimizations and a finer grid (24 × 24 × 1) for electronic structure calculations [26]. Geometry optimization is conducted using the conjugated gradient (CG) method with the convergence thresholds of 1 × 10−5 eV/atom for the total energy, 0.02 eV Å-1 for the maximum force, 0.02 GPa for the maximum stress and maximum ionic displacement for 5 × 10-4 Å. A 3 × 3 V2CT2 supercell, namely V18C9T18 is built to investigate the interactions between the single Li atom and MXene substrate. The van der Waals (vdW) interaction is considered by adopting the DFT-D3 method (an empirical dispersion correction scheme of Grimme) [27]. In order to avoid any possible artificial interactions between adjacent MXene sheets, a 20 Å vacuum space along the c direction is inserted. The minimum energy reaction pathway for Li diffusion and energy barrier were evaluated by using the climbingimage nudged elastic band (CI-NEB) method [28]. 3. Results and discussions The optimized model of bare V2C is derived from exfoliating Al elements from the corresponding MAX phase (V2AlC). When atoms (Tx, here we take S atoms for example) terminate the surfaces of V2C, there are three possible anchoring sites as shown in Fig. 1(a): The S atom is located above the top of the upper V atom (Site I), or placed at the hollow site of the lower V atom (Site II) or bonded with C atom (Site 2
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Fig. 1. The structure of V2C (a) and side view of 3 × 3 × 1 V2CS2(b), V2CO2(c); Schematic illustration of the formation of V2CS2 by substitution of termination atoms (d and e, in which T represents F,OH,O).
calculations. It shows that Li atom transfers 0.86 e, 0.89 e to V2CS2 and V2CO2, respectively, during the adsorption process. Taking V2CS2 for example, Fig. 4 depicts the charge difference density of V2CS2-Li, sky blue and yellow colors represent the regions of electron depletion and accumulation, respectively. It is clearly shown that a substantial charge distribution occurs between Li and MXene substrate. Hence, it proves that V2C with eS/eO terminations can adsorb Li atoms easily via strong chemical adsorption. The Li diffusion on the V2CT2 surface is studied by CI-NEB method in order to determine the diffusion barrier (Eb). A small Eb favors the fast charge/discharge rate of Li ions, which is beneficial to LIBs. The diffusion paths are chosen along the three high symmetry positions. The most stable configuration with Li adsorption (Fig. 2) is set to be the initial state, and the corresponding configuration at the most stable neighboring site is chosen as final state. For V2CS2 and V2CO2 (Fig. 5), the lowest migration barrier is measured to be 0.22 eV (Site 2→Site 3→ Site 2′) and 0.15 eV (Site 3→Site 2→Site 3′), respectively. It is noted that the diffusion barrier of V2CT2 (T]O, S) is lower than those of
Table 1 The in-plane lattice constant (Å) of V2CT2 (T]O, S), bond length of VeC and VeT (Å), thickness (d, Å) and charge transfers from V2C to T (Δq, e). T
a
V-C
V-T
d
Δq
V2CO2 V2CS2
2.90 3.06
2.05 2.08
1.96 2.37
4.41 5.34
0.99 0.62
Table 2 The substitution formation energy of V2CS2 for terminations from eO, eOH, and eF to eS for V2C.
-F -OH -O
E1 (eV)
E2 (eV)
Esub (eV)
−351.51 −460.89 −398.07
−352.34 −462.82 −395.70
−0.83 −1.93 +2.37
Fig. 2. The top view and side view of 3 × 3 × 1 V2CT2 supercell without and with Li adsorption. 3
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Fig. 3. The PDOS and TDOS of 3 × 3 V2CT2 and V2CT2Li. (a) T = O, (b) T = S.
high adatom concentrations. A 2 × 2 × 1 V2CT2 supercell is built as the initial substrate model. The Li adsorption on both sides of MXene is taken into consideration to provide more practical guide for experiments. By varying the number (x) of Li atoms, it is calculated that the maximum numbers of Li adatom are both 8 for the V2CS2 and V2CO2 2 × 2 × 1 supercells, with the formula V2CT2Li2, corresponding to the values of theoretical capacity CT of 301.22 and 367.64 mA h/g, respectively, which are much higher than those of V2CF2 and V2C(OH)2 (116.78 and 181.33 mA h/g [11]). The theoretical capacity is given by equation (2): CT= (x × Z × F × 103)/M
(2)
where x is the maximum number of Li adatom for V2CT2 single cell, Z is the valence number (Z = 1 for Li), F is the Faraday constant (26.81 mA h/mol) and M is the total atomic mass of V2CT2. The Li capacity follows the order: V2CO2 > V2CS2 > V2C(OH)2 > V2CF2, indicating that V2C anchored with O or S atoms are more desirable for LIB anodes compared with those with eF or eOH terminations. Though surface terminations hinder the electrochemical performance of V2C, it is noted that the Li capacity of V2CT2 (T]S, O) is comparable to that of bare Ti3C2 monolayer (320 mA h/g) [11]. Therefore, to improve the performance of V2C, eF and eOH functional groups should be reduced or replaced by O and S atoms during the synthesis since surface terminations are inevitable. Experimental work towards this objective is highly possible since Yang et al. [30] recently reported a scalable fluoride-free MXene synthetic procedure by etching MAX phases in a binary aqueous electrolyte, turning the possibility of F-free MXene into
Fig. 4. Charge difference density of V2CS2Li. (Iso-surface value is 0.004 e/Å3).
V2CT2 (T]F, OH) (0.36 eV and 0.59 eV [12]), implying its superior electrochemical performance for LIBs. Furthermore, for V2CT2 (T]O, S), the value of Eb is also much lower than that of Ti3C2Tx (0.8–1.2 eV), the most widely studied MXene, suggesting that the former has much faster diffusion mobility and higher Li charge/discharge rate. To evaluate the theoretical Li storage capacity of V2CT2, it is of significance to investigate the Li adsorption behavior for V2CT2 with
Fig. 5. The diffusion path and energy profile of V2CS2 (a) and V2CO2(b). 4
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reality and opening a new path for the functional-tailoring synthesis of MXenes. More efforts on modifying the functional group of MXene are needed and this would be promising for the birth of more advanced MXene-based LIBs.
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4. Summary In summary, we investigated the structural and electronic properties of V2CTx (T]O, S) as well as its lithium storage capacity by firstprinciples calculations. The most stable configuration of V2CT2 (T]O, S) is formed when T atoms anchor above the site of V hollow position. The calculated results of formation energy indicate the high feasibility of the synthesis of V2CS2. The DOS results show V2CO2 and V2CS2 become metal-conductive, while V2C terminated with F or OH groups are semiconductors (narrow-band gap). The potential of V2CT2 as LIB anodes is evaluated by the Li adsorption behaviors of these 2D materials. V2CO2 and V2CS2 after lithiation still remain metallic, which favors their applications in LIBs. Moreover, V2C terminated with eO/eS has much lower Li diffusion barriers and larger Li capacity compared with those of eF/eOH, showing their great potential as alternative anodes. The results show that it would be practical to synthesize V2CT2 (T]O, S) MXenes, and offer an improved understanding of tailoring functional groups to improve the performance of V2C as potential energy storage materials. Declaration of Competing Interest No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. Acknowledgements This work was supported by the Grant of National Natural Science Foundation of China (51731004), and Zhishan Youth Scholar Program of Southeast University. References [1] J. Xiao, et al., Exfoliated MoS2 nanocomposite as an anode material for lithium ion batteries, Chem. Mater. 22 (16) (2010) 4522–4524. [2] X. Wang, et al., A SnO2/graphene composite as a high stability electrode for lithium ion batteries, Carbon 49 (1) (2011) 133–139. [3] K. Bindumadhavan, S.K. Srivastava, S. Mahanty, MoS2-MWCNT hybrids as a superior anode in lithium-ion batteries, Chem. Commun. 49 (18) (2013) 1823–1825. [4] K. Yan, et al., Ultrathin two-dimensional atomic crystals as stable interfacial layer for improvement of lithium metal anode, Nano Lett. 14 (10) (2014) 6016–6022. [5] J. Liu, X.-W. Liu, Two-dimensional nanoarchitectures for lithium storage, Adv.
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