Borophene

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C: Energy Conversion and Storage; Energy and Charge Transport

Theoretical Prediction of Blue Phosphorene/Borophene Heterostructure as a Promising Anode Material for Lithium-Ion Batteries Qingfang Li, Juchuan Yang, and Lei Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05076 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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The Journal of Physical Chemistry

Theoretical Prediction of Blue phosphorene/Borophene Heterostructure as a Promising Anode Material for Lithium-Ion Batteries Qingfang Li,∗,†,‡ Juchuan Yang,† and Lei Zhang†,‡ † School of Physics & Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China ‡Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing 210044, China E-mail: [email protected](QingfangLi)

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Abstract The heterostructured electrodes assembled with various two-dimensional (2D) materials break the limitation of the restricted properties of individual building blocks and combine the advantages of single material systems. Here, we design a novel blue phosphorus/borophene (BP/borophene) heterostructure by combing BP and borophene monolayers together. We investigate the adsorption and diffusion of Li along the outside surfaces and the interlayer of BP/borophene to assess its suitability as the Li-ion battery anode material. It is revealed that the BP/borophene heterostructure possesses excellent structural stability and high mechanical stiffness. In contrast to the semiconductor character of pristine BP monolayer, BP/borophene is metallic. The adsorption energy of Li on the BP side of the BP/borophene heterostructure is higher than that on the BP monolayer, and Li adsorption on the borophene side of the BP/borophene system is also stronger than that on the borophene monolayer. In addition, the BP/borophene heterostructure possesses a specific capacity of 1019 mA h g−1 , which is larger than those of pristine BP monolayer and other BP-based heterostructures. Moreover, it is found that the energy barriers are relative low as Li diffuses in the BP/borophene. Given these advantages, that is, large adsorption energies, low diffusive energy barriers, high capacity and electrical conductivity, we conclude that the BP/borophene heterostructure can be an excellent candidate as anode material for Li-ion batteries.

Introduction Batteries become an essential component in various energy-efficient applications. The development speed of the modern portable electronic instruments and power electric vehicles mostly hinges on the progress of battery technologies. Presently, the rate capability and energy density of metal ion batteries are insufficient to satisfy the ever-increasing demand for large-scale stationary energy storage systems. Therefore, the rechargeable

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lithium-ion batteries (LIBs) with large reversible capacity, high energy storage density and good cycling stability are urgently demanded for new generation batteries. 1 Among all the explored candidates of anode materials, two-dimensional (2D) materials with single chemical constituent have attracted great attention due to their high surface-volume ratio as well as the fast metal ion diffusion along their surfaces. 2–6 Monocomponent layered nanosheets show some required properties of energy storage devices; however, none of them can offer all of the required properties. For example, graphene-based electrodes have high electrical conductivity, but exhibit only moderate energy capacity. 7 The 2D transition metal dichalcogenides (TMDCs) show high initial capacity, while demonstrate relatively poor electrical conductivity and capacity retention, and suffer from large volume change upon cycling. 2,8–10 To overcome these limitations, the researchers are shifting the main focus from individual 2D materials to the stacked 2D heterostructures by integrating different 2D materials. The 2D heterostructured electrodes combine the advantages of their individual building blocks and eliminate the associated shortcomings, which open new opportunity to improve the rate performance of current battery technology. 7 Recently, blue phosphorene (BP) nanosheet has been successfully synthesized using epitaxial growth methods. 11,12 The success of experiments inspired a flourish of investigations on the BP and BP-based heterostructures. 13–20 Especially, their electrochemical properties have been in the spotlight due to the potential applications as possible anode materials for metal-ion batteries. 4,17–19 The electrical conductivity, Li adsorption energies and mechanical properties of pristine BP monolayer are effectively improved by combing other 2D materials with BP. 17,18 However, the charge capacities of these BP-based heterostructures are lower than that of the BP monolayer. Therefore, it is highly desirable to design novel BP-based heterostructures with high charge capacity. 2D borophene has recently been realized in experiment 21 and found to exhibit promising electrochemical properties including ultrahigh capacity and very low Li diffusion energy barrier. 22–25 The electronic properties of borophene-based heterostructures have

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drew tremendous attention because the borophene can be used as a substitute for the traditional noble metal electrodes. 26,27 In addition, first principles studies indicate both boron phosphide monolayer and P-doped borophene are promising anode materials with charge capacities up to 1293 and 1732 mA h g−1 , 10,28 respectively. More importantly, the lattice mismatch between the borophene monolayer and BP monolayer is very small (< 2%). Therefore, it is expected the BP/borophene van der Waals(vdW) heterostructure composed of BP and borophene monolayers is the good candidate as the heterostructured electrodes with excellent electrochemical properties. In this paper, BP/borophene heterostructure was designed to find its potential application as an excellent anode material for LIBs. The structural stability, Li adsorption and diffusion properties of BP/borophene heterostructure were investigated systematically.

Computational Details All calculations were performed using Vienna ab-initio simulation package (VASP) within the framework of density functional theory. 29,30 For the exchange-correlation functional, the generalized gradient approximation with the form of Perdew-Burke-Ernzerhof (PBE) formalism was used. 31 The contribution of vdW interaction between BP and borophene layers was taken into account by adding the pairwise dispersive correction using the DFTD2 approach of Grimme. 32,33 The cutoff energy of a plane-wave basis was 500 eV. We used the Gaussian smearing method, 34 and the smearing width was chosen as 0.1 eV. To avoid ˚ was used. All the the self-interaction between the periodic layers, a large vacuum of 20 A structures are fully relaxed until the Hellman-Feynman force and energy are converged ˚ and 10−6 eV, respectively. within 0.01 eV/A We optimized all atomic positions and in-plane cells. The calculated lattice constants ˚ for the rectangle monolayer BP, while the lattice constants of the are a=3.28, b=5.69 A ˚ These results agree well with the previous monolayer borophene are a=1.61, b=2.87 A.

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reports. 4,35 In order to minimize the lattice mismatch effects between BP and borophene, the rectangle unit cell of BP/borophene heterostructure is formed by combining 1 × 1 rectangle BP unit cell and 2 × 2 borophene unit cells. The lattice constants of BP/borophene ˚ The lattice mismatch between the BP and borophene heterostructure are a=3.26, b=5.73 A. monolayer is less than 2%. To find out the Li diffusion pathways and migration energy barriers, the climbing image nudged elastic band (CI-NEB) method was employed. 36 The formation energy of BP/borophene heterostructure is calculated as

Ef =

EP + EB − EP/B n

(1)

where EP and EB represent the total energies of pristine BP and borophene monolayers. EP/B is the total relaxation energy of the BP/borophene heterostructure, and n is the total number of atoms in heterostructure. The adsorption energy for Li atoms is defined by the following formula:

Ead =

EP/B+mLi − mELi − EP/B m

(2)

where EP/B and EP/B+mLi represent the total energies of the pristine and Li adsorbed BP/borophene heterostructures, m is the number of Li adatoms, and E Li is the energy of a lithium atom.

RESULTS AND DISSCISION Adsorption of Li in the BP/borophene heterostructure To simulate lithium atom adsorption in the heterostructure, the vertical BP/borophene heterostructure with 48 atoms (P16 B32 ) is constructed by using 2 × 2 rectangle unit cells of BP and 4 × 4 unit cells of borophene as shown in Figure 1 (a). It indicates that the

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˚ which is shorter equilibrium interlayer distance between BP and borophene is 2.75 A, than those of the other BP-based heterostructures, demonstrating that the interface interactions between BP and borophene sheets are strong. To quantitatively characterize the interactions, we calculate the formation energy of this heterostructure. The formation energy of BP/borophene is 0.05 eV/atom, indicating an exothermic process in the formation of this heterostructure. Therefore, the BP/borophene heterostructure is suitable to be used as electrode material. The thermal stability of the BP/borophene heterostructure is examined by ab initio molecular dynamics (AIMD) simulations. The simulations are performed on a 4 × 4 BP/borophene supercell. The snapshots of the configuration are shown in Figure S1 of supporting information. After heating at 300K for 5 ps with a time step of 1fs, no broken bonds and geometric reconstructions occur in BP/borophene heterostructure, suggesting that the BP/borophene is thermally stable. During the lithiation process, Li concentration increases, which induces the structural and compositional change of the anode materials. Therefore, it is important to investigate the effects of Li coverage on BP/borophene. We investigate the effects of different Li concentrations (P0.5 BLix ) by gradually inserting Li adatoms into BP/borophene. For x=0.03 (P16 P32 Li), three typical places are considered: the outside surface of BP (Li/P/B), the outside surface of borophene (P/B/Li) and the interlayer of BP/borophene (P/Li/B). The structures with different adsorption sites of Li are fully relaxed. The most stable optimized configurations are displayed in Figure 1(b)-(d) and Table I lists the corresponding adsorption energies. Here, the more negative value of Ead means the stronger interaction between Li and BP/borophene. Three characteristics are found: (i) For Li/P/B and P/B/Li, the most stable adsorption sites of Li are directly above P (TP site) and B (TB site) atoms, respectively. Which is in accordance with the pristine BP and borophene situations. 4,22 The corresponding adsorption energies of Li are -2.28 and -2.87 eV, and decrease by 0.20 eV and 0.43 eV compared to the case of the pristine BP and borophene. The Li adsorption at the bridge site between two neighbor P atoms and above the center of the

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hexagonal phosphorus ring is weaker by about 0.1 eV and 0.2 eV than that on TP site, respectively. The adsorption energy above the middle-point of B-B bonds in the furrow is -2.83 eV which is slightly weaker than that of the most favorable adsorption site, suggesting it is very easy for lithium diffusion along the furrow direction. The adsorption energy above the middle-point of B-B bonds in the ridge is Ead =-2.39 eV. The results suggest that Li adsorption on the BP side and borophene side of the BP/borophene heterostructure is stronger than that on the monocomponent nanosheets, and the BP/borophene can effectively enhance the lithium bonding strength. (ii) The Li atom embedded in the interlayer of BP/borophene prefers to occupy the position between the top site of B and the bridge site of BP (B site) with an adsorption energy of -2.80 eV, which is the most stable occupied site on the monolayer borophene. (iii) The adsorption energy of lithium in P/Li/B is slightly weaker than that of P/B/Li, while both P/Li/B and P/B/Li are energetically more favorable than Li/P/B as well as the pristine monolayer (BP or borophene). These findings mean that Li adatoms may favorably occupy the outside surface of borophene and the interlayer of BP/borophene rather than insert into the outside surface of BP during lithiation. As the number of lithium atoms is increased, the BP/borophene heterostructure can well maintain the layered structure, but the surface structural change is pronounced. The highest adsorption concentration is x=1.0 with 32 Li adatoms. The most stable structures of P0.5 BLix (x=0.03, 0.06, 0.13, 0.25, 0.50, 0.75 and 1.0), corresponding to P16 P32 Li2 , P16 P32 Li4 , P16 P32 Li8 , P16 P32 Li16 , P16 P32 Li24 ,P16 P32 Li32 , are depicted in Figure S2 of supporting information. Compared to pristine BP/borophene, the BP monolayer shifts relative to the borophene monolayer along the x direction in P0.5 BLix with 0.03≤ x ≤0.25. As the Li coverage is x ≥0.25, the BP monolayer slightly moves towards the y direction. The corresponding adsorption energies of lithium are gathered in Figure 2(a). At the low lithium coverage (x ≤0.13), the adsorption energy reduces from -2.87 eV (x=0.03) to -3.14 eV (x=0.13), which is mainly associated with the surface reconstruction of lithiated

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BP/borophene heterostructures. At high lithium content, the change of the adsorption energy is attributed to the mutual competition of two effects. One is the electrostatic attraction between the reconstructed BP/borophene and the adsorbed lithium, and the other one is the Li-Li repulsive interaction. The large change of the area of 2D materials with varying Li is a challenging problem in the battery electrodes. 37–39 The percent change of the area of BP/borophene with the increase of Li content is defined as ∆S=[(SBP/borophene -SBP+borophene+nLi )×100%]/SBP/borophene , where SBP+borophene+nLi and SBP/borophene correspond to the area of the heterostructure with and without lithium adsorption. Figure 2(b) indicates the change in the area of BP/borophene is less than 1.5% during the charge process even if it displays an obvious surface reconstruction. In order to further examine the stability of BP/borophene during lithiation process, we calculate the stiffness of BP/borophene. The stiffness is defined as C=[∂2 E/∂2 ε]/A0 , where E is the total energy under strain, and A0 is the area of heterostructure at the equilibrium state. △ Li /Li0 is defined as the strain ε, in which Li0 and △ Li are the strain-free lattice constant and variation of the lattice constant under the compressive or tensile strain, respectively. The strain range (ε) is between -3% and 3% with an increment of 0.5% in X/Y directions. The stiffness of BP along X and Y directions is almost constant with C=79 N/m. The in-plane stiffness for borophene monolayer is calculated to be CXX =176 N/m and CYY =401 N/m. The obtained stiffness values of BP/borophene are 256 N/m (X direction) and 462 (Y direction). As for the BP/borophene, the stiffness of BP is greatly increased compared to that of bare BP monolayer. Such enhancement shows that the BP of BP/borophene can withstand large strains compared with the BP mononlayer. The high mechanical stiffness of BP/borophene can contribute to better cycle performance. In addition, the discrepancy of stiffness between X and Y direction in BP/borophene is smaller than that in borophene monolayer, suggesting that the anisotropic stiffness of borophene could be degenerated by combing with the isotropy stiffness of BP.

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For all the compositions, the interactions between Li and BP/borophene are similar. To clarify the interaction between Li and BP/borophene, we take one Li adsorption on the BP/borophene as an example, and estimate the charge transfers from the adsorbed Li to BP/borophene by Bader charge analysis. 40 The results are illustrated in Table I. As Li adsorption takes place on the outside surface of BP (Li/P/B), one lithium atom donates about 0.868 |e| to the heterostructure, and the BP and borophene obtain 0.528 |e| and 0.340 |e|, respectively. The calculated result reveals that the charge of lithium is mainly transferred to its adjacent P atoms. When one lithium atom is embedded in the interlayer of the BP/borophene (P/Li/B), the charges of Li atom transfer to BP and borophene with the value of -0.158 |e| and -0.673 |e|, respectively. This indicates the interaction between the adsorbed Li in the interlayer and borophene is stronger. Moreover, as one Li adsorption occurs on the outside surface of borophene (P/B/Li), the transferred charge of lithium is 0.868 |e|, which is the same as that of Li/P/B. Whereas the corresponding charges of BP and borophene are +0.209 |e| and -1.076 |e|. This result means both Li and BP donate charges to borophene. The phenomenon is also observed in the lithiated BP/NbS2 heterostructure. 17 There are obvious charge transfer characters in the lithiated BP/borophene, thus the interactions between the Li atom and BP/borophene are predominately ionic. To visualize the ionic bonds of lithium incorporation into BP/borophene and the charge transfer between Li and heterostructure, the charge density difference is calculated with the following equation 41 ∆ρ = ρ BP/borophene+ Li − ρ BP/borophene − ρ Li ,

(3)

where ρ BP/borophene+ Li , ρ BP/borophene , and ρ Li present the charge density of Li embedded in BP/borophene heterostructure, BP/borophene and isolated lithium atom, respectively. The calculated results are depicted in Figure 3. The blue and yellow regions indicate the electron depletion and accumulation, respectively. For the case of Li atom adsorption

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on the out surfaces of BP/borophene, it is found the net loss of charge above the lithium atoms and most of the charge of Li is transferred to its adjacent P or B atoms. The large losses and gains of charge demonstrate the strong ionic bonding between the Li and BP/borophene. As the Li atom is embedded in the interlayer of BP/borophene, a net gain of electron is found on the adjacent P and B atom of Li adatom, and the B atoms gain more electronic charges from Li. All these findings are consistent with the Bader analysis, which further indicates the strong ionic bond of the embedded lithium atom. The electronic conductivity can affect the rate capability of the anode materials. The borophene monolayer shows metallic characteristics 22 while the BP monolayer is semiconductor. 4 Because PBE method underestimates the band gap value of semiconductor, we testify the electronic properties of the BP monolayer and BP/borophene using the PBE and HSE06 methods, indicating in the Figure S3 of supporting information. Figure S3 demonstrates that both PBE and HSE06 methods present the similar electronic properties for the BP monolayer and BP/borophene. Unlike the pristine BP that exhibits the semiconducting character, the BP/borophene heterostructure is metallic. The BP/borophene is expected to have more superior electronic conductivity than the BP monolayer as the anode material. We will present only the calculated results using the PBE functional in the following discussion because the results adopting the PBE method are usable. The partial density of states (PDOS) of BP/borophene, Li/P/B, P/Li/B and P/B/Li are shown in Figure 4. The Fermi level of BP/borophene originates mostly from the p orbitals B and P atoms. After the Li atoms incorporation into the BP/borophene, the Li/P/B, P/Li/B and P/B/Li systems can preserve the metallic feature well, which ensures high electronic conductivity during lithiation process.

Open circuit voltage and capacity One of the important aspects for lithium batteries is the open-circuit voltage (OCV), relating to the total energy storage of lithium battery. The OCV can be determined by the 10


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average intercalation voltage. 42 The OCV of P0.5 BLix is obtained as 43

OCV ≈

EP0.5 BLix2 − EP0.5 BLix − ( x2 − x1 )µ Li 1

( x1 − x2 ) e

(4)

where EP0.5 BLix2 and EP0.5 BLix are the total energies of P0.5 BLix2 and P0.5 BLix1 , respectively. 1

µ Li is the chemical potential of lithium in bulk. The theoretical charge capacities of lithiated BP/borophene are expressed by the formula: 10 C=

x × F × 103 MP0.5 B

(5)

where F is the Faraday constant (26.801 Ah mol−1 ), x and MP0.5 B represent the concentration of Li and the mass of BP/borophene in the lithiated heterostructure (P0.5 BLix ). Figure 5 presents the voltage profiles as a function of charge capacity. It is noted that for all the cases, the OCV remains the positive value in the range of 0.41 - 1.97 V. The lack of negative voltage means the Li ions prefer to adsorb on BP/borophene instead of forming the metallic states. In addition, a slight initial increase in the voltage profile is observable which is associated with the structural reconstruction after lithiation. The feature is also found in the silicene/graphene heterostructure. 43 The OCV of BP/borophene is higher than that in BP and borophene monolayers. The BP/borophene can provide a charge capacity as high as 1019 mA h g−1 , which is much higher than that of the pristine BP monolayer 4 and the BP-based heterostructures(528 mA h g−1 ) 17 as well as the commercially available graphite for LIBs. 44

Diffusion properties of lithium The lithium diffusion characters are vital for the charge/discharge rate of the electrode materials, 39 thus we next investigate the diffusion properties of lithium in the BP/borophene. Also, three cases mentioned above are considered: (1) Li diffusion on the outside surface of BP in the BP/borophene (Li/P/B), (2) Li diffusion on the outside surface of borophene 11



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in the BP/borophene (Li/B/P), and (3) Li diffusion in the interlayer of BP/borophene. First, we investigate the possible diffusion paths and energy barriers of one lithium on the outside surface of BP and borophene in the heterostructure. For the Li diffusion on surface of BP, we consider two possible diffusion paths, which are the x (TP1 →BP1 →HP1 →BP2 →TP2 ) and y (TP2 →BP2 →HP2 →BP3 →TP3 ) directions, as shown in Figure 6(a). The positions above P atoms (TP1 , TP2 and TP3 in Figure 6a) correspond to the most stable adsorption sites of lithium. The calculated results indicate that one lithium migrates from one top site to another top site passing through the hollow site. As the Li at top site moves to the nearest neighboring top site along the x direction, the migration energy barrier is 0.19 eV (Figure 6c). These results are similar with those in the bare BP monolayer. 4 Different from the case of the pristine blue phosphorene, the Li atom feels the presence of borophene which induces the weak difference of diffusion barriers along y and x directions. This is caused by the surface reconstruction. As one Li atom diffuses on the outside surface of borophene, two diffusion pathways are studied according to the symmetric character of borophene, as indicated in Figure 6(b): one is along the x direction which is along the furrow (TB →Bx →TB ) and the other one is along the y direction (TB →By →TB ). The position above B atoms (TB ) represents the most stable occupied site of lithium. The diffusion energy barrier profiles of Li along the two paths are illustrated in Figure 6(d). For both pathways, the Li migrates from the TB site towards another TB site through the bridge site, while the diffusion barrier along the x direction (0.03 eV) is much lower than that along the y direction (0.46 eV). Compared to the bare BP and borophene monolayers, there is a slight enhancement in the energy barriers when one lithium diffuses on the outside surface of BP/borophene. Next, we discuss how lithium migrates in the interlayer of the BP/borophene. The most preferable adsorption sites are the positions between the top site of borophene and the bridge site of BP (B1 , B2 and B3 sites in Figure 7). The adsorption sites between the top site of borophene and the top site of BP (T1 and T2 in Figure 7) are metastable. Figure 7 (a)

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indicates the diffusion between two B sites along the x direction crosses one T site with an energy barrier of 0.28 eV, which is lower than the case of Li diffusion along the y direction. There are two possible diffusion pathways along the y direction. One is Li migration from one favored site (B1 ) to the metastable adsorption site (T2 ) overcoming a 0.47 eV energy barrier. The other is lithium diffusion between two B sites (B1 and B3 ) with the diffusion barrier of 0.41 eV. It can be seen Li prefers to diffuse along the x direction, which is the same as that of Li/B/P. For all considered situations, the migration energy barriers are less than 0.50 eV and the lowest diffusion energy barrier is only 0.03 eV, which ensures that the BP/borophene has good rate capability when it serves as the anode material for LIBs. At high coverage, the lithium diffusion is modeled by removing one Li adatom from the highest concentration as illustrated in Figure 8. For Li diffusion on the outside surface of BP, the lithium atom moves from one hollow side to the nearest neighboring one of the BP surface crossing the bridge side. The diffusion energy barrier is 0.11 eV which is lower than that of monolayer BP (0.24 eV). That is to say, for high coverage, the rate performance of BP/borophene is better than that of BP monolayer. Similar to the case of the low concentration in Li/B/P, the lithium atom prefers to diffuse along the x direction as Li adsorbs on the borophene surface of BP/borophene, which involves an energy barrier of 0.34 eV (Figure 8b). When Li moves between adjacent favored sites of the interlayer, the corresponding energy barrier is 0.20 eV as shown in Figure 8(c), which is smaller than that of the low coverage.

CONCLUSIONS In summary, we have investigated the adsorption and diffusion characters of lithium in the BP/borophene heterosturcture to explore the possibility of BP/borophene as the electrode material for LIBs using the first principles method based on vdW corrected density

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functional theory. Firstly, it is found that the BP monolayer shifts relative to the borophene monolayer during lithiation cycle; however, the BP/borophene can well maintain the layered structure and the largest area change is only 1.46%. Secondly, the adsorption energies of lithium on the BP (borophene) side of BP/borophene are stronger than that on the BP (borophene) monolayer. Thirdly, the diffusion energy barriers along the outside surfaces and the interlayer of heterostructure are low. Even if the Li coverage is at the highest value, the energy barrier is less than 0.30 eV. Finally, the BP/borophene has higher capacity and mechanical stiffness than those of the monolayer BP and other BP-based heterostructures. All these results suggest that the BP/borophene heterostructure is an ideal anode material of LIBs and the rate performance is significantly better than that of the BP monolayer.

Supporting Information Available The following files are available free of charge. • The snapshots from AIMD simulation for the BP/borophene at 300K. • The structures of lithiated BP/borophene. • DOS of pristine BP monolayer and BP/borophene based on PBE and HSE06.

Acknowledgement This work was financially supported by the National Natural Science Foundation of China under Grant No.11547030, 11704195 and 51702165.

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Table 1: Binding energy of one Li adsorption (Eb ). Charge transfer of Li, P and B atoms, ∆Q Li , ∆QP and ∆QB , respectively.

Li/P/B P/Li/B P/B/Li

Li sites TP B TB

∆Q Li +0.868 +0.831 +0.868

Eb (eV) -2.28 -2.80 -2.87

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∆QP -0.528 -0.158 +0.209


∆QB -0.340 -0.673 -1.076

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Figure 1: Top and side views of the BP/borophene without and with one Li adsorption, (a) pristine BP/borophene, (b) Li adsorption on the outside surface of BP, (c) Li embedded in the interlayer of BP/borophene and (d) Li adsorption on the outside surface of borophene. The blue, green and yellow balls represent the P, B and Li atoms, respectively.

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Figure 2: (a) Adsorption energies as a function of increasing Li content in P0.5 BLix . (b) Percent change in the area as the function of Li concentration x.

Figure 3: Top and side views of the charge density difference of one Li (a) adsorption on the out-surface of BP; (b) insertion into the interlayer of BP/borophene; (c) adsorption on the outside surface of borophene. The loss and gain of electrons are shown in blue and yellow, respectively.

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Figure 4: PDOS of B, P, and Li in (a) BP/borophene, (b) Li/P/B, (c) P/Li/B and (d) P/B/Li. The left axis is the PDOS of B and P; the right axis is the PDOS of Li.

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Figure 5: The voltage profile of BP/borophene heterostructure as a function of charge capacity.

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Figure 6: Diffusion path of one Li atom along the outside surface of (a) BP (Li/P/B) and (b) borophene (P/B/Li). Energy barriers of the Li atom diffusing along the outside surface of (c) BP (Li/P/B) and (d) borophene (P/B/Li).

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Figure 7: Energy barriers and diffusion paths of one Li atom in the interlay of BP/borophene (P/Li/B): (a) B1 →T1 →B2 , (b) B1 →H1 →T2 , and (c) B1 →T →B3 .

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Figure 8: Schematic representations and potential-energy curves of Li diffusion in P16 B32 Li31 : (a) Li diffusion on the outside surface of BP (Li/P/B), (b) Li diffusion on the outside surface of borophene (P/B/Li), (c) Li diffusion in the interlayer of BP/borophene (P/Li/B). The initial (i), intermediate (m), and final (f) configurations are depicted on the right side. The red circle highlights the diffusion of the Li atom.

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Borophene

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