Two-Dimensional Penta-BN2 with High Specific Capacity for Li-Ion

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Two-Dimensional Penta-BN2 with High Specific Capacity for Li-Ion Batteries Ting Zhang, Yandong Ma, Baibiao Huang, and Ying Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20566 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Applied Materials & Interfaces

Two-Dimensional Penta-BN2 with High Specific Capacity for Li-Ion Batteries Ting Zhang, Yandong Ma,* Baibiao Huang, and Ying Dai* School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Shandanan Street 27, Jinan 250100, China

Abstract: Searching for high-performance electrode materials is one of the most effective ways to improve the energy density of current lithium-ion batteries. Using first principles calculations, we reveal that pentagonal BN2 (Penta-BN2) can be served as a compelling anode material for lithiumion batteries. Penta-BN2 harbors intrinsic metallic nature before and after lithiation, showing an excellent electrical conductivity. Significantly, the fully lithium storage phase of Penta-BN2 is Li3BN2, corresponding to an ultra-high theoretical capacity of 2071 mAh∙g-1, superior to most of the previously reported 2D candidates. Meanwhile, the open-circuit voltages and diffusion barrier heights of Penta-BN2 are found to be fascinatingly low. Moreover, in light of its small Yang's modulus and robust lattice to the lithiation, Penta-BN2 can accommodate volume change during the charging-discharging processes, remarkably beneficial for fabricating flexible electrodes.

Keywords: lithium-ion batteries; anode; Penta-BN2; high-capacity; first-principles

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Introduction Electrochemical energy storage can manage intermittent renewable energy supplies, essential for future clean energy economy.1,2 As one of the most critical electrochemical energy storage technologies, lithium-ion batteries (LIBs) have attracted considerable attention in the past decade by virtue of their high energy density, wide voltage window and long cycle life.3 Nowadays, representative commercially used LIB electrode is graphite (372 mAh∙g-1), which exhibits good cycle stability.4 Nevertheless, it no longer provides satisfactory storage capacity and performance, as it operates close to the theoretical limit.5 To address this challenge, numerous efforts have since been focused on searching for a new LIB anode material with outstanding conductivity, high capacity, and low diffusion barrier height. Starting from the discovery of graphene, the area of two-dimensional (2D) materials have undergone rapid development in the last decade.6-11 Many 2D materials was proposed as host materials for LIBs, resulting from their diverse chemistries and morphologies. Graphene, the benchmark of 2D materials, has been tested for being a cathode and anode material for LIBs.12 Except for graphene, of particular interest are MXene and graphene analogies,13-15 which exhibit low barriers for superior rate performances and slightly high capacities. Nevertheless, these proposed 2D materials for LIBs suffer from many problems. For example, the MXene monolayers, such as Mo2C, Ti3C2, V2C, are not stable in oxygen and water environments, under which causes to the formation of metal oxide nanocrystals.16-18 While for Si-, Ge-, and Sn-based 2D materials, they all suffer from rapid capacities fading due to their low conductivity and substantial volume expansion (> 300%) in the processes of charging and discharging.19,20 Lately, hydrogenated counterpart of borophene, namely borophane, also exhibited its potential application as a highcapacity anode candidate in LIBs.21 However, full hydrogenation of 2D materials is still challenging experimentally, and the hydrogenated systems would be easily deformed at ambient conditions. Therefore, innovations towards promising 2D anode materials with excellent stability, high conductivity and high capacity are scientifically desirable. In this work, using first principles calculations, we identify the recently proposed 2D pentagonal BN2 (Penta-BN2) to be a competing anode material for LIBs.22 We find that Penta-BN2 not only exhibit excellent stability but also exhibit intrinsic metallic nature before and after Li adsorption, showing a high electrical conductivity. Most importantly, the maximum Li concentration is Li3BN2, giving rise to an extremely high specific Li capacity (2071 mAh∙g-1), even 5.5 times that of the graphite anode. Meanwhile, Penta-BN2 exhibits a relatively low surface diffusion barrier and open-circuit voltage. Furthermore, on account of its mechanical flexibility, Penta-BN2 can accommodate volume change upon lithiation/delithiation, while retaining rate capability, which is conducive to fabricating flexible LIBs. ACS Paragon Plus Environment 2

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Computational methods All calculations are performed using the Vienna ab initio simulation package (VASP).23 For the exchange and correlation functional, we use the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE).24 The DFT-D2 method with Grimme correction25 is adopted to describe the van der Waals interaction. A vacuum thickness of 23 Å along z direction is used to avoid interactions between adjacent layers. The cut-off energy is set to 600 eV. The convergence criteria of energy and force are set to10-5 eV and 0.01 eV/Å, respectively. The k-point meshes of 5 × 5 × 1 and 9 × 9 × 1 are adopted for geometry and electronic structure calculations, respectively. Ab initio molecular dynamics (AIMD) simulations are performed at 500 K and 300 K for 5 ps and 10 ps, respectively, with a time step of 1 fs. The phonon band is calculated using the PHONOPY code.26 To the migration pathways and diffusion barriers for Li on Penta-BN2 is calculated using NEB method.27 It should be noted that the GGA-PBE functional suffers from the “delocalization error” and overestimate polarizability and binding energy of the charge transfer complex. And Liu et al. proposed a more accurate method to avoid using DFT computation.28 Nevertheless, in this work, PBE functional is still employed to study the Li ion battery electrodes, as it has been widely used in the previous works13-15,29,32,38 and achieved good agreements with experiments30,31. Structure, Stability and Mechanical Properties

Figure 1. (a) Top and side views of Penta-BN2 with the dashed lines marking the unit cell. The Li adsorption sites on Penta-BN2 are marked in the right panel of (a). (b) 2D Brillouin zone. (c) Time evolution of free energy during the AIMD simulations. The insert in (c) is the snapshot taken from the end of AIMD simulation. (d) Phonon dispersion curves of Penta-BN2. ACS Paragon Plus Environment 3


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The crystal structure of Penta-BN2 is demonstrated in Figure 1a. Penta-BN2 has a buckled pentagonal structure with three atomic layers (N-B-N). B atoms are at the middle layer forming a square sublattice, while N atoms are at the upper and lower layers forming two stretched honeycomb sublattices. Each B atoms are bonded to four N atoms. The optimized lattice constants are a = b = 3.61 Å with a layer thickness of 1.27 Å. To assess the stability, we first perform AIMD simulations using a 4 × 4 supercell at a high temperature of 500 K. We can observe that there is neither bond broken nor structure reconstruction in Figure 1c. The planarity of Penta-BN2 monolayer is also maintained except for slight change in bond length, indicating that the Penta-BN2 is thermally stable. Then, we substantiate the dynamic stability of Penta-BN2 by carrying out phonon dispersion calculations. As illustrated in Figure 1d, the observed imaginary frequency at the Γ point is tiny,32 thus Penta-BN2 is a dynamically stable structure. The elastic constants of Penta-BN2 are C11 = C22 = 219 N/m, C12 = -2 N/m and C66 = 110 N/m, which meets the Born-Huang criteria (C11C22 - C122 > 0 and C66 > 0).33,8 This indicates that PentaBN2 is mechanically stable. Based on the elastic constants, we also calculate the in-plane Young’s moduli (Y) of Penta-BN2, which is defined as: Y=(C11C22 - C122)/C22. The Young’s modulus of Penta-BN2 is about 218 N/m. It is worth noting that this value is lower than that of hexagonal h-BN (271 N/m) and graphene (340 N/m).34,35 Therefore, the structure of Penta-BN2 possesses better mechanical flexibility, which is propitious to the manufacture of flexible battery materials. Interestingly enough, the unique pentagonal structure not only leads to high stability, but also produces an unusual negative Poisson's ratio due to the negative value of C12, which attracts Researchers’ in-depth study. We wish to stress that Penta-BN2 consists of B atoms and N2 dimers, and a similar form of Ti8C12 metallo-carbohedrene composed of Ti atoms and C2 dimers has been successfully synthesized.36 Moreover, Chen et al. suggested that penta-BN2 may be fabricated by introducing B atoms into the source of nitrogen molecules like the liquid nitrogen, indicating the experimental feasibility of Penta-BN2.37 Single Li Atom Adsorption and Diffusion in Penta-BN2 The cycle stability and rate capability of anode materials are strongly related to their electronic conductivity. Therefore, we study its electronic properties. As shown in Figure 2a, Penta-BN2 exhibits metallic features and high density of carriers, rendering it a desirable electrode material for high-performance LIBs. The metallicity of Penta-BN2 mainly arises from N-p states, while the hybridized states between N-p and B-p lie far away from the Fermi level, namely, larger than 3 eV. The metallic behavior as well as the intriguing crystal structure of Penta-BN2 inspires us to explore its potential as LIBs. Multiple adsorption sites with relatively low energies play a critical role as being a suitable anode material in LIBs. Then we first investigate the insertion of single Li onto Penta-BN2 to decide the most favorable adsorption site. A 2 × 2 suppercell is used to deposit ACS Paragon Plus Environment 4

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isolated Li on different sites, corresponding to a chemical formula of LiB8N16. As shown in Figure 1a, we consider six possible sites based on the geometric symmetry of Penta-BN2, referred as S1-S6 respectively. After full structure relaxation, four inequivalent stable adsorption sites (namely, S1, S2, S3 and S4) are obtained: S1 is above the bottom nitrogen atom; S2 is above the intermediate boron atom; S3 is above the midpoint of N-N bond in the bottom; S4 is above the midpoint of N-N bond in the top. While for S5 and S6, they are transformed into S3 and S4, respectively. The stability of a single Li adsorption on Penta-BN2 can be estimated by the adsorption energy (Ead), which is defined as

Ead  ELi / BN2  EBN2  ELi

(1)

Here, ELi / BN2 and EBN2 are the total energies of Li-inserted and pure Penta-BN2, respectively, and ELi is the energy per atom in bulk Li. The adsorption energies are plotted in Table 1. The negative values indicate that Li atoms prefer to be adsorbed separately on Penta-BN2 rather than forming metal cluster. S1 is the optimal site, while S3 is the second favorable site. Explanation of this phenomena can be sought into the bonding character of Penta-BN2. Both Li and B ions are cations, whereas N ions exhibit anionic nature. Therefore, Li ions tend to be closer to N ions as a result of Coulomb interactions. Moreover, as plotted in Table 1, the adsorption energies of the valley sites are lower than that of the peak sites, which is because of the fact that the adsorption heights of S1 and S3 are much smaller than S2 and S4. Interestingly, the adsorption energies and heights of S1 and S3 are very close, which may cause a low migration barrier along this direction. And it should be noted that these values are higher than the previously reported net W with high specific capacity (adsorption energy of Li is -0.91 eV).38 Moreover, the appreciate strong binding between Li atom and the host materials can effectively inhibit the formation of dendrites and facilitate rapid diffusion process under lower energy barriers.39 Table 1. Adsorption energies (Ead in eV), adsorption height (h in Å) and Bader charge of Li atom (eB in e) of a single Li-adsorbed on the Penta-BN2. Site S1 S2 S3 S4 S5 S6

Ead -1.37 -1.23 -1.34 -1.16 S3 S4

h 1.15 1.46 1.00 1.71 -

eB 0.897 0.891 0.892 0.879 -

The diffusion kinetic property affects the charging/discharging rate capability of rechargeable batteries, which is an critical factor to determine the performance of an anode material. In view of ACS Paragon Plus Environment 5


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high structural symmetry and favorable Li adsorption sites of Penta-BN2, three possible migration pathways are explored; see Figure 3a. When Li atoms diffuse along Path II and Path III, the calculated diffusion barrier is found to be 0.17eV and 0.24eV, respectively, while the diffusion barrier is as low as 0.05eV when Li atoms diffusing along Path I, as shown in Figure 3b. The results of the energy barriers are very consistent with our discussion of the adsorption energies above. It should be noted that Li ion will migrate along the entire surface of Penta-BN2 only by combining Path I and Path II, because Li ion cannot diffuse on the anode material continuously through Path I. Therefore, Path II is the rate-limiting step, and it is lower than that of some widelyknown anode materials such as TiO2 (0.35-0.65 eV)40, 41, bco-C26 (0.53 eV)42 and graphite (0.2180.4 eV)43-45, indicating an easier diffusion and an ultrafast charge-discharge rate for the Li ion on Penta-BN2.

Figure 2. The band structure and PDOS of (a) Penta-BN2 and (b) Li-adsorbed Penta-BN2 in S1 configuration. (c) Charge density difference of Li-adsorbed Penta-BN2 in S1 configuration. The band structure and PDOS of Penta-BN2 with Li adsorption are shown in Figure 2b. We can see that the Li adsorption do not distinctly change the electronic properties of Penta-BN2, and it maintains the metallic nature. In contrast to semiconducting transition-metal oxides and TMDs,46 the metallic properties for pristine and Li intercalated Penta-BN2 ensure its good electronic conductivity. And this is essential and favorable for its application as a battery electrode. To get a better insight into the adsorption behaviors of Li on Penta-BN2, we take the S1 configuration as an example to investigate the charge density difference between the pure and adsorbed phases. As illustrated in Figure 2c, the depletion of electrons around the Li ion (green region) and the accumulation of electrons on Penta-BN2 (yellow region) indicate the transfer of charge from the adatom to the substrate, similar to the case of Li adsorption on Mo2C.14 This result is in agreement with the fact that the electronegativity of nitrogen (3.04) is larger than that of lithium (0.98). Further Bader charge analysis demonstrates the amount of charge transfer from Li to Penta-BN2 is about 0.897 e, and Li atoms contribute almost all outer electrons to ions, resulting in strong Coulomb repulsion preventing the adsorbed Li ions from clustering. ACS Paragon Plus Environment 6

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Figure 3. (a) The diffusion pathways on Penta-BN2, and (b) the corresponding energy barriers. Storage Capacity of Li on Penta-BN2 One of the key performance parameters for anode materials is the storage capacity, which relies strongly on the number of Li atoms on Penta-BN2. To obtain the maximum capacity, we load Li layer by layer on both surfaces of Penta-BN2. We assume the following half-cell reaction during the charging-discharging process:

BN 2  xLi   xe   Lix BN 2

(2)

During charging processes charging, Li ions and electrons are deintercalated from the cathode material and inserted into Penta-BN2. The contribution of volume and entropy to the average adsorption energy can be ignored during this process.14 To evaluate the theoretical maximum capacity, the layer-by-layer average adsorption energy on Penta-BN2 (Eave) are calculated, which is described as

Eave 

ELin / BN2  ELi ( n1) / BN2   ELi



(3)

Here ELi / BN and ELi ( n1) / BN2 denote the total energies of Penta-BN2 adsorbed with n and (n - 1) n 2 layers of Li atoms, respectively. ELi is the energy per atom in bulk Li. λ represents the total amount of adsorbed Li in each layer (both sides). A negative value of Eave suggests this process is thermodynamically stable. The Li atoms in the initial planar layer are located at S1 sites, and totally 16 Li atoms are adsorbed on both sides of Penta-BN2. After structure relaxation, one-half of the Li atoms leave the layer of the other Li atoms, giving rise to one buckled Li layer on each side. Such a buckled layer can be regarded as two planar layers. To this end, we consider one planar Li layer containing 8 Li atoms on both sides as the first adsorption layer on Penta-BN2. The corresponding λ value is equal ACS Paragon Plus Environment 7


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to 8, and the crystal structure is illustrated in Figure 4a. The calculated average adsorption energy is -1.63 eV. When adsorbing the second Li layer, as shown in Figure 4b, the 8 Li atoms in the second layer also site at the S1 sites, forming two planar layers on both sides. The calculated average adsorption energy of the second layer is -0.02 eV. We wish to emphasize that, under relatively high adsorption concentration (λ > 8), the mutual repulsion will lead to that the Li atoms would distribute as evenly as possible on the surface. For the third adsorption layer, since the S1 sites being fully occupied, we test other three possible adsorption sites, i.e., S2, S3 and S4, which are named as Li4BN2-II, Li3BN2-III and Li3BN2-IV, respectively. For Li3BN2-IV, it spontaneously transforms into Li3BN2-III after structure relaxation. As listed in Table 2, Li3BN2-III is identified as the most stable configuration. Accordingly, the Li atoms in the third layer prefer to being adsorbed on S3 sites and the corresponding λ value is 8; see Figure 4c. When further increasing the number of adsorbed atoms, the average adsorption energy becomes positive, suggesting that the adsorption of the forth Li layer on Penta-BN2 is impossible. So, the average adsorption energy of the fully lithiated phase is -0.18 eV. Importantly, this value is between that of borophene (Li0.75B: -0.06 eV)47 and Cr2C (Li5Cr2C: -0.198 eV)48, ensuring good adsorption stability of Li atoms on Penta-BN2. Table 2. The average adsorption energy (Eave, in eV) and theoretical storage capacity (mAh∙g-1) for LixBN2. Species

Layer

Eave

Capacity

Li1BN2

1

-1.63

690

Li2BN2

2

-0.02

1380

Li4BN2-Ⅱ

3

0.02

-

Li3BN2-Ⅲ

3

-0.18

2071

Li3BN2-Ⅳ

3

-

-

The excellent charging-discharging reversibility of an anode material requires that the material should have no irreversible deformation during lithiation and delithiation processes. We then inspect the thermal stability of Li3BN2 via AIMD simulations. After running 10000 steps with a time step of 1 fs at 300K, the Penta-BN2 in Li3BN2 is still intact and 24 Li atoms are still adsorbed on the host material despite slight structural change; see Figure S1. And compared with the pure case, the change of the lattice constant after lithiation is only 3.3%, which also reflects good structural stability of Penta-BN2 as an anode material for LIBs. When removing all the adsorbed Li atoms and re-optimizing the structure for the Penta-BN2, the structure restores the pure one. In other words, its lattice constant, thickness and bond lengths are almost the same as the initial structure

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Two-Dimensional Penta-BN2 with High Specific Capacity for Li-Ion

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