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Applied Surface Science

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Monolayer Zr2B2: A promising two-dimensional anode material for Li-ionbatteries

Guanghui Yuana,1, Tao Boc,d,1, Xiang Qia,b, Peng-Fei Liuc,d, Zongyu Huanga,b,⁎,Bao-Tian Wangc,d,e,f,⁎⁎

a School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, PR ChinabHunan Key Laboratory for Micro-Nano Energy Materials and Devices, Xiangtan University, Hunan 411105, PR Chinac Institute of High Energy Physics, Chinese Academy of Sciences (CAS), Beijing 100049, ChinadDongguan Neutron Science Center, Dongguan 523803, Chinae Institute of Applied Physics and Computational Mathematics, Beijing 100088, Chinaf Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China

A R T I C L E I N F O

Keywords:First-principlesLi-ion batteryAnode materialMonolayer Zr2B2

A B S T R A C T

Aiming to find an excellent anode material for Li-ion batteries, we report the two-dimensional Zr2B2 monolayercombining the density functional theory and crystal structure prediction techniques. We find that the monolayerZr2B2 has initial metallicity and good stability, and keeps excellent electrical conductivity during the wholeprocess of lithiation. The adsorption energy of lithium is −0.628 eV, which is enough to make sure the stabilityof the processing of lithiation. Furthermore, the ultralow diffusion energy barrier of Li ion on the surface of themonolayer Zr2B2 (17meV) indicates an excellent charge-discharge rate. The theoretical specific capacity of526mA h g−1 is larger than that of the commercial graphite electrode. All these results propose the hexagonalmonolayer Zr2B2 as an excellent anode material for Li-ion batteries.

1. Introduction

With the increasing demand of mobile phones and environment-friendly vehicles [1,2], searching for batteries with outstanding capa-city and long cycling performance has stimulated great interests [3–5].Especially, recharge- able lithium-ion batteries (LIBs) have attractedgreat attention since their first commercialization in 1991, due to thecombination of remarkable capacity, great power density, superiorenergy efficiency, long cycle life and safe operation [1,6,7]. However,there still exists a great gap to satisfy the modern electronics market,because the specific capacity and safety of the commercialized LIBs arelimited [8–10]. The battery system consists of electrodes and electro-lyte. The function of the electrodes, including the anode and thecathode, is to store the chemical energy. The electrolyte is used to se-parate the electrons and ions produced during the process of chemicalreaction [11]. The capabilities of LIBs are greatly depended on theperformance of the electrode materials. Although graphite has beenstudied and used extensively as an anode material for LIBs, the quitelow theoretical specific capacity (372 mAh/g) and poor rate capability

hinder its further application [12,13]. In this regard, it is urgentlyneeded to search for new kinds of efficient anode materials.

In recent years, with the development of atomic technologies suchas atomic layer deposition [14] and molecular beam epitaxy [15], manylayered compounds have been successfully synthesized, some of whichhave been considered as anode materials for metal ions batteries[16,17]. Their large surface-to-volume ratio and unique electronicproperties make the fast ion diffusion and large ion capacity possible[18]. Researchers have studied the possibilities of using novel layeredmaterials as anode materials, such as graphite derivatives [19,20],MXenes [21–23], transition-metal sulfides [24], transition metal oxides[25] and transition metal borides (TMBs) [26–28]. Theoretically, ba-sing on the first-principles density functional calculations, the Ti3C2

MXene [21] with a higher capacity than graphite is proposed to besuitable for LIBs. The transition-metal sulfide VS2 [24] is also con-sidered to be anode material with higher diffusion rate for Li than thatof graphite. Guo et al. [29] reported the Pmmn phase of two-dimen-sional (2D) TMBs, including Mo2B2 and Fe2B2, which have similarstructure to the MXenes. They investigated them used as anode material

https://doi.org/10.1016/j.apsusc.2019.02.222Received 24 December 2018; Received in revised form 21 February 2019; Accepted 25 February 2019

⁎ Correspondence to: Z. Huang, School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, PR China.⁎⁎ Correspondence to: B.-T. Wang, Institute of High Energy Physics, Chinese Academy of Sciences (CAS), Beijing 100049, China.E-mail addresses: zyhuang@xtu.edu.cn (Z. Huang), wangbt@ihep.ac.cn (B.-T. Wang).

1 These authors contributed equally to this work

Applied Surface Science 480 (2019) 448–453

Available online 03 March 20190169-4332/ © 2019 Elsevier B.V. All rights reserved.

T

for LIBs. Motivated by that work, we had explored other TMBs in ourrecent work [26], and found that our obtained P6/mmm phase is morestable than the Pmmn phase. We investigated one of them, named Ti2B2,used as anode material for LIBs. We found that the Ti2B2 has ultralowdiffusion barrier. The above two works show that the TMBs have po-tential to be used as anode material for LIBs. However, the Li diffusionbarrier on the Pmmn phase of Mo2B2 is quite high and the P6/mmmphase of Ti2B2 has insufficient theoretical specific capacity of Li ion.Thus, it is necessary to find an anode material with ultralow diffusionbarrier and large theoretical specific capacity.

In the present study, we explore the potential of monolayer Zr2B2 asan anode material by performing density functional theory (DFT) andab-initio molecular dynamics (AIMD) calculations. Firstly, we in-vestigate the electronic structures and stability of the monolayer Zr2B2,which are crucial for the performance of anode materials. Secondly, wecalculate the adsorption energies and diffusion barriers of the Li ions onits surface. In the end, we discuss the average open-circuit voltages andtheoretical specific capacity. All these results show that the monolayerZr2B2 can be used as a promising anode material for LIBs.

2. Computational details

Our first-principles calculations were performed based on DFT [30]using Vienna ab-initio Simulation Package (VASP) [31]. In order tosearch global structures of the ZrB systems, we used the Crystal struc-ture AnaLYsis by Particle Swarm Optimization (CALYPSO) package[32,33], which has been successfully used in the prediction of the low-dimensional materials [34,35]. The exchange-correlation functionalwas described with the generalized gradient approximation (GGA) ofPerdew-Burke- Ernzerhof (PBE) type [36]. A plane wave cutoff energyof 550 eV was used for all the calculations. To simulate the adsorptionand diffusion of Li, a 2×2 supercell of the four-atoms Zr2B2 monolayerwas used and the Brillion zone was sampled using a 5×5 Monkhorst-Pack k -mesh [37]. To calculate the electronic density of state (DOS),one unit cell with the k -mesh of 18× 18 was used. A vacuum space of20 Å was set to avoid the interactions between adjacent layers. Atomicrelaxation was performed until the change of total energy was<0.01meV and all residual forces were< 0.01 eV/ Å. We used the climbingimage nudged elastic band method (CI-NEB) to determine the energybarriers and minimum energy paths of Li diffusion. We calculated theadsorption energies of Li ions according to the following equation,

= − −+E E E nE n( )/ads host Li host Li (1)

where Ehost+Li and Ehost represent the total energies of the monolayeradsorbed by Li ions and the isolated monolayer, respectively. ELi re-presents the total energy of per atom for the bulk Li and n is the numberof adsorbed Li ions. The theoretical capacity can be obtained from theequation,

=C czF/MZr B2 2 (2)

where c is the number of adsorbed Li ions, z is the valence number of Li,F is Faraday constant (26801mAh/mol) and MZr2B2

is the molar weightof Zr2B2. The potential V of the electrode can be obtained using a well-established approach according to the following equation,

= − − − − −V E E x x x x( ( ) E )/e( )x x 1 2 Li 1 22 1 (3)

where Ex1 and Ex2 are the total energies of the LixZr2B2 at two adjacentconcentration x1 and x2, both x1 and x2 are the stable concentration. ELirepresents the total energy of per atom for the bulk Li. To assess theadsorption stability of Li layer on the Zr2B2 monolayer, the averageadsorption energies for each Li layer are calculated according to thefollowing equation,

= − −−

E E E( mE )/mave Zr B Li Zr B Li Lin n2 2 2 2 ( 1) (4)

where the EZr2B2Lin and EZr2B2Li(n−1)are the total energies of 2D Zr2B2

monolayer with n and (n-1) adsorbed Li layers, ELi stands for the total

energy of per Li ion for the bulk. The number m represents the numberof adsorbed Li ions in each layer (for a 2× 2 supercell on both sides).

3. Results and discussions

3.1. Electronic structures and stability of Zr2B2

After structural prediction by the CALYPSO code and the total en-ergy DFT calculations from VASP, we find several 2D structures of theZr2B2 including the P4/nmm, P ¯3m1, Pmma and Pmmn phases shown inFig. S1(a)-(d) respectively in the electronic supplementary information(ESI) as well as the P6/mmm phase shown in Fig. 1. The phonon dis-persions along the high-symmetry k paths of these 2D structures arepresented in Fig. S1 in the ESI and in Fig. 2b. We also present thephonon density of state (PhDOS) in Fig. 2b for the P6/mmm phase. Onecan see that there is no imaginary frequency, indicating that thosestructures are all dynamically stable. The total energies of the men-tioned phases rank in the sequence of P6/mmm < Pmmn < Pmma <P ¯3m1 < P4/nmm with −25.362, −24.970, −24.969, −23.102,and− 22.958 eV per formula unit (f.u.), respectively. Thus, in the fol-lowing, we only focus on the structural and electronic properties of themost energetically stable phase of hexagonal P6/mmm. As shown inFig. 1, each Zr2B2 monolayer is consisted of three layers of atomsstacked in the order of Zr-B-Zr, which can be described as an inter-mediate boron honeycomb sheet sandwiched in between two hexagonalplanes of Zr atoms. The relaxed lattice constants are a= b=3.134 Å,which are comparable to those of the bulk ZrB2 (a=3.168 Å) [38]. Theoptimized thickness of 3.385 Å is also comparable to that of the bulkZrB2 (c=3.530 Å) [38]. Each boron atom bonds to three boron atomsand the length of the BeB bonds is 1.891 Å. Each Zr atom bonds to sixboron atoms and the length of the ZreB bonds is 2.664 Å.

In practical applications, the stability of the anode materials is quiteimportant because of the hundreds of charge-discharge cycles. Weconduct the AIMD [39] calculations for the monolayer Zr2B2 at tem-peratures of 300, 600, 1200 and 1500 K, respectively. As shown inFig. 2c, we find that the average values of the free energy keep stable at300 K, which means that the monolayer Zr2B2 is thermally stable atnormal atmosphere. We also present the AIMD results at some othertemperatures in Fig. S2 in the ESI. Although the stability of themonolayer Zr2B2 decreases upon heating, its thermal stability is still

Fig. 1. Top (a) and side (b) views of the stable structure (P6/mmm phase) of themonolayer Zr2B2. The brown and green balls represent the Zr and B atoms,respectively.

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enough to be used as an anode material. The electrical conductivity isalso an important aspect that can affect the rate capability of the anodematerials. Thus, we investigate the projected band structure and theelectronic DOS of the monolayer Zr2B2. As shown in Fig. 2a, the pristineZr2B2 is naturally metallic with many electronic bands crossing theFermi level. The Zr-d orbitals contribute dominantly near the Fermilevel while the contribution of the B-p orbitals is almost neglectable.Comparing with the semi- conducting or insulating anode materials, the

outstanding electronic conductivity of the monolayer Zr2B2 could be anintrinsic advantage for its application as an anode material for LIBs.

3.2. Li adsorption and diffusion on Zr2B2

Typically, the adsorption energy is an essential property to judgewhether a material is suitable to be an anode material or not [26,29]. Inorder to investigate the adsorption of Li on Zr2B2, we should know thefavorable sites for Li adsorption firstly. We have tested a lot of possiblepositions before we choose these three positions (labeled as A, B, and Cin Fig. 1a). To avoid unnecessary confusion, we have added all thesecomparison results of the length of the LieZr bonds and the adsorptionenergies in Fig. S3-S5 and Table S1 in ESI. We use a 2× 2 supercell toinvestigate the adsorption of one Li ion, which corresponds to a che-mical stoichiometry of Zr2B2Li0.25. The site A is above the B atom whilethe site B is above the Zr and the site C is on top of the ZreZr bridge. Wefind that the adsorption energy (−0.628 eV) on site A is the lowest,while the adsorption energies on sites B and C are −0.491 eV and−0.611 eV, respectively. The relative small values of the adsorptionenergies favor strong interactions between the Li ions and the Zr2B2

layer. This would prevent the formation of metallic Li and improve thesafety of LIBs [29]. The adsorption energy of Li ion on site C is quitesimilar to that of site A. Thus, it is easy for the Li ions to diffuse alongthe A-C-A direction. Besides, we calculate the difference charge den-sities and plot them in Fig. S6 in the ESI, with the yellow and blue areasrepresenting the electron gain and loss, respectively. We can see thatthe electrons tend to accumulate in between Li and its neighboring Zratoms. For the site A, a large charge transfer from Li ion to the Zr2B2

monolayer can be observed.Diffusion barrier, determining the ion mobility, is quite important

for the charge-discharge performance of the battery. Utilizing the CI-NEB method, we calculate the diffusion barrier of Li ion on themonolayer Zr2B2. As aforementioned, the Li ion prefers to diffuse alongthe A-C-A path. This is also reasonable according to the symmetry. Asshown in Fig. 3, we can see that the diffusion barrier for Li ion along theA-C-A pathway is 17meV, which is much lower than that of the Mo2B2

and Fe2B2 [29]. In order to assess the charge-discharge performance ofthe Zr2B2, we summarize the diffusion barriers of some available in-vestigated precedents in Table 1. Obviously, we can see that the theo-retical energy barrier of Zr2B2 is greatly lower than 220meV for VS2

Fig. 2. (a) Band structure and DOS of themonolayer Zr2B2 where the Fermi energy isset to 0 eV. The red circle and red line re-present the B-2p; the green circle and greenline stand for the Zr-5d; the black line re-presents the total density of states (TDOS).(b) Phonon spectrum and PhDOS. (c)Variation of the free energy in the AIMDsimulations at 300 K over the time scale of10 ps.

Fig. 3. (a) The diffusion path of the Li ions on the monolayer Zr2B2. The brown,green and blue ball represent the Zr, B and Li atoms, respectively. (b) Theenergy during the diffusion process.

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[24] and 68meV for Ti3C2 [21]. What's more, it is fundamentally lowerthan the energy barrier of 400meV for commercialized graphite[40,41]. Such a small value of the diffusion energy barrier means thatthe Li ions can diffuse very easily on its surface, which is beneficial forthe charge-discharge rate of anode material of LIBs.

3.3. Average open-circuit voltages and Li storage capacities of Zr2B2

The storage capacity is another essential factor for the efficiency ofthe LIBs. Firstly, we calculate the average adsorption energies of Li ionsadsorbed on the monolayer Zr2B2 according to Eq. (4). The adsorptionof one and two layers of Li ions, corresponding to Zr2B2Li2 and Zr2B2Li4,

respectively, is investigated and the optimized structures are shown inFig. S7 in the ESI. The Li ions of the first-layer are all adsorbed on thesite A and the average adsorption energy is −0.460 eV. The negativevalue of the average adsorption energy indicates that the Li ions of thefirst-layer can be adsorbed stable on the monolayer Zr2B2 withoutformation of clusters. The Li ions of the second layer are also adsorbedon the site A and the average adsorption energy is −0.012 eV, whichmeans the adsorption of two layers of Li ions is also stable. For thethird-layer adsorption, the average adsorption energy is 0.018 eV. Thepositive value means that it is unstable for the adsorption of three layersof Li ions.

Besides, we calculate the formation energy of the monolayer Zr2B2

adsorbed with Li ions, which is calculated according to the followingequation:

= − − +E E E( xE )/(x 1)f x Zr B Li Zr B( ) Lix2 2 2 2 (5)

where EZr2B2Lix and EZr2B2represent the energies of the Zr2B2Lix and Zr2B2

per f.u., respectively. The x is the concentration of Li ions adsorbed onZr2B2. ELi is the energy of per atom of bulk Li. We plot the convex hullfor the formation energies in Fig. 4, where the structures on the red linerepresent the thermodynamically stable states while the structures onthe black dot line are the metastable states. We can see that themonolayer Zr2B2 adsorbed with three layers of Li ions (Zr2B2Li6) isunstable, which is well consistent with the average adsorption energy.All these structures are presented in Fig. S7 in the ESI, we can see thatduring the process of lithiation the structure of the monolayer Zr2B2

doesn't change at all. (See Fig. 5.)

Table 1The theoretical specific capacity (mA h g−1) and diffusion barrier (meV) ofsome investigated promising anode materials for LIBs.

System Theoretical specific capacity Diffusion barrier Ref.

Graphite 372 400 [40,41]Phosphorene 433 80 [43]Borophene 1860 2.6 [44]Ti3C2 448 68 [21]VS2 466 220 [24]Zr2B2 175 300 This work

Fig. 4. Formation energies of LixZr2B2 systems with respect to 2D monolayerZr2B2 and Li bulk metal. Data points located on the convex hull (solid redsquares) represent stable adsorption against any type of decomposition. Themetastable phases are indicated by open squares.

Fig. 5. TDOS and partial density of states (PDOS) of the monolayer Zr2B2 adsorbed with (a) one layer and (b) two layers of Li ions, respectively. The green, red, andblue lines represent the PDOS of Zr, B, and Li atoms, respectively, while the black line represents the TDOS.

Fig. 6. Electrode potential of Li-intercalated Zr2B2 against Li/Li+.

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Furthermore, we calculate the open circuit voltage of the Zr2B2 forthe LIBs (see Fig. 6). There are four plateaus in the whole process oflithium insertion. When increasing the Li concentration from Zr2B2Li0to Zr2B2Li0.25, the voltage is 0.628 V. Comparing with the high voltageof 1.76 V for P-doped borophene [42], the initial low value of thevoltage makes the Zr2B2 monolayer more suitable to be an anode ma-terial. The open circuit voltages for Zr2B2Li0.25→ Zr2B2Li0.5,Zr2B2Li0.5→ Zr2B2Li2, and Zr2B2Li2→ Zr2B2Li4 are 0.510, 0.424 and0.012 V, respectively. The average potential of these four plateaus is0.236 V, adding one more merit to the monolayer Zr2B2 as an anodematerial.

According to Fig. 4, the maximum amount of Li ions is x=2. Theaverage adsorption energy for 2 < x < 3 is positive. Therefore, webelieve that the monolayer Zr2B2 can adsorb two layers of Li ions atmost. The two layers contain 16 Li ions in total. Thus, the theoreticalspecific capacity is 526mA h g−1 according to the Eq. (2). As shown inTable 1, the theoretical specific capacity of the Zr2B2 is larger than thatof 466mA h g−1 for VS2. We find that the capacity of Zr2B2 is larger by70mA h g−1 than that of Ti2B2 [26]. What's more, the capacity of themonolayer Zr2B2 is higher by 154mA h g−1 than that of the commercialgraphite anode [40].

4. Conclusion

We have performed systematic calculations to investigate the po-tential of the monolayer Zr2B2 being used as an anode material for theLIBs. The hexagonal P6/mmm phase was found to be the most en-ergetically stable structure. Our calculated phonon dispersions andAIMD results clearly indicate the dynamical and thermal stabilities ofthe monolayer Zr2B2. Great stability guarantees its safety duringthousands of charge-discharge processes. According to its electronicstructure, we found that it holds metallic in the whole process of thelithiation. Our calculated low diffusion barrier of 17meV and hightheoretical specific capacity of 526mA h g−1 make the 2D monolayerZr2B2 a promising anode material. Although such 2D TMBs have neverbeen reported in experiments, they may be synthesized through sub-stituting carbon/nitrogen in MXenes [22,29]. Our results provide a newchoice in the TMBs family and will stimulate further efforts in this field.

Acknowledgments

This research was supported by the National Natural ScienceFoundation of China (Grant No.11504312), National Basic ResearchProgram of China (No. 2015CB921103), China Postdoctoral ScienceFoundation (Grants No.2018M641477) and Science and TechnologyDepartment, Guangdong Province, China (No. 2018A0303100013).The calculations were performed at Supercomputer Centre in ChinaSpallation Neutron Source.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2019.02.222.

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