Na-Ion Battery

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Penta-Graphene: A Promising Anode Material As the Li/Na-Ion Battery with Both Extremely High Theoretical Capacity and Fast Charge/Discharge Rate Bo Xiao, Yan-Chun Li, Xue-fang Yu, and Jian-Bo Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12727 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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ACS Applied Materials & Interfaces

Penta-Graphene: A Promising Anode Material As the Li/Na-Ion Battery with Both Extremely High Theoretical Capacity and Fast Charge/Discharge Rate Bo Xiao,a Yan-chun Li,b Xue-fang Yu,a,* Jian-bo Chenga a

The Laboratory of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, China b

Institute of Theoretical Chemistry, Jilin University, Changchun 130021, China

ABSTRACT Recently, a new two-dimensional (2D) carbon allotrope named penta-graphene, was theoretically proposed (Zhang et al. Proc. Natl. Acad. Sci. 2015, 112, 2372), and has been predicted to be the promising candidates for broad applications due to its intriguing properties. In this work, by using first-principles simulation, we have further extended the potential application of penta-graphene as the anode material for Li/Na-ion battery. Our results show that the theoretical capacity of Li/Na ions on penta-graphene reaches up to 1489 mAh·g-1, which is much higher than most of the previously reported 2D anode materials. Meanwhile, the calculated low open-circuit voltages (from 0.24 to 0.60 V), in combination with the low diffusion barriers (≤ 0.33 eV) and the high electronic conductivity during the whole Li/Na ions intercalation processes further show the advantages of penta-graphene as the anode material. Particularly, molecular dynamics simulation (300 K) reveals that Li ion could freely diffuse on the surface of pentagraphene, and thus the ultra-fast Li ion diffusivity is expected. Superior performance of penta-graphene is further confirmed by comparing with the other 2D anode materials. The light weight and unique atomic arrangement (with isotropic furrow paths on the surface) of penta-graphene are found to be mainly responsible for the high Li/Na ions

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storage capacity and fast diffusivity. In this regard, except penta-graphene, many other recently proposed 2D metal-free materials with pentagonal Cairo-tiled structures may be the potential candidates as the Li/Na-ion battery anodes. KEYWORDS:

first-principles

calculations,

penta-graphene,

Li/Na-ion

battery,

adsorption of Li/Na, diffusion of Li/Na

INTRODUCTION Large scale energy storage systems have become key issues in the daily life, and thus the development of the next-generation batteries has attracted intense attention from worldwide. More specifically, Li-ion batteries were extensively studied over the past decades and have been widely applied in the portable electronic devices.1-4 However, several issues such as safety, cost and capacity restrict the further improvement of Li-ion batteries.5,6 In this regard, the development of other metal-ion batteries (such as Na,7-10 Mg,11 Al,12 Ca13) have become another hot topic in the current research. In particular, Naion battery is one of the most competing candidates, which has gained increasing attention owing to the abundant availability of sodium on the earth and the associated low cost. Among all the electrode materials for metal-ion batteries, two-dimensional (2D) materials have been widely studied due to their unique atomic structures, which may enable the high capacity and the fast metal ion diffusivity. However, most of the currently proposed 2D materials exhibit either the high theoretical capacity or the fast metal ions diffusivity, thus finding the 2D materials with both above features is still an urgent task. Generally, to achieve the high theoretical capacity, the 2D materials with light weight are usually the promising candidates. For example, recently synthesized borophenes show


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ultra-high theoretical capacities as Li/Na ions anode (1984 and 1240 mAh·g-1 for β12- and χ3-borophene, respectively).14 However, the relatively high Li/Na ion diffusion barriers (0.66/0.33 and 0.60/0.34 eV for β12- and χ3-borophene, respectively) and quasi-onedimentional (1D) diffusion paths restrict the efficiency of Li/Na ion diffusion. On the other hand, the 2D materials with furrow paths on the surfaces usually exhibit fast Li/Na ions diffusivity due to the steric hindrance effect. For example, the Li or Na ion diffusion barriers along the furrow paths on the surface of MXene,15 phosphorene,16-18 Mo2C,19 SiS,20 and SiSe20 are generally lower than 0.1 eV, which are low enough to enable the ultra-fast Li or Na ion diffusion. However, the relatively low Li or Na ions theoretical capacities (≤ 526 mAh·g-1) are found in these materials partly because of their heavy weigh. Accordingly, to realize both high theoretical capacities and fast charge/discharge rates, the 2D materials with light weight and isotropic furrow paths on the surface are expected for the promising candidates as Li/Na-ion batteries. Recently, a 2D carbon allotrope named penta-graphene, which is entirely composed of carbon pentagons, was proposed from first principles simulation.21 Possible experimental routes were suggested to synthesize this material.21 So far, penta-graphene has been predicted to be the promising candidates for the broad applications due to its unique atomic and electronic structures, such as the nanoelectronics, nanomechanics and optical devices.22-27 More importantly, penta-graphene is a 2D material with light weight and shows the 2D furrow paths on the surface,21 which may enable both the high metal ion storage capacity and fast metal ion diffusivity. In this work, by combining the first-principles calculations and molecular dynamics simulations, we have studied the adsorption and diffusion behaviors of Li/Na ions on the


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penta-graphene to evaluate its potential application as an anode material in Li/Na-ion battery. Our results reveal that the theoretical capacity, the average open-circuit voltage (OCV) and the diffusion barrier of single Li/Na ion on penta-graphene are 1489/1489 mAh·g-1, 0.55/0.32 V and 0.17/0.28 eV, respectively. Moreover, the electronic conductivity of penta-graphene has been dramatically improved after the adsorption of Li/Na ions. All these features, including the extremely high theoretical Li/Na ion capacity, fast Li/Na ion diffusivity, low OCV, and excellent electronic conductivity, suggest that the penta-graphene could be a promising anode material as the Li/Na-ion battery.

THEORETICAL METHOD All calculations were performed using the Vienna ab initio simulation package (VASP).28,29 A plane wave basis with the cutoff energy of 500 eV is used. The electronion interaction was described by projector augmented-wave (PAW)30 pseudopotentials. For the exchange and correlation functionals, we use the Perdew–Burke–Ernzerhof (PBE) version of the generalized gradient approximation (GGA).31 For the penta-graphene with a 2 × 2 supercell, the Brillouin zone integration was sampled by 6 × 6 × 1 k-grid mesh for the structural relaxation, and 12 × 12 × 1 k-grid mesh was employed for the electronic properties calculations. The atomic structures were fully optimized until the force on each atom is less than 0.01 eV/Å. The vdW interaction was introduced, and described by a semi-empirical correction by the Grimme method.32 The periodic structure of pentagraphene has been decoupled by a vacuum thickness larger than 20 Å. To investigate the diffusion behavior of Li/Na ion on penta-graphene, first-principle molecular dynamics (MD) simulation was carried out at 300 K using the NVT ensemble,


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with the time step 2 fs. Here, the penta-graphene with a 3 × 3 supercell structure was employed during the MD simulation.

RESULTS AND DISCUSSION Prior to the calculation of lithiation and sodiation process, we have considered the relative stability of penta-graphene by comparing with other previously reported graphene allotropes. The total energies per C atom of various carbonous systems are listed in Figure 1, it is found that penta-graphene is metastable compared with graphene and some graphene allotropes,33-38 while it is more stable than some nanoporous carbon phases (such as α-graphyne,37 (3, 12)-carbon sheet,39 T-carbon40), C20 cage,41 and the smallest carbon nanotube.42 Note that both C20 cage and the smallest carbon nanotube have been identified in the experiment, which implies that the penta-graphene could also be synthesized. In addition, Zhang et al.’s theoretical study21 revealed that penta-graphene is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K; Very recently, Liu et al.’s experimental study43 showed that carefully designed phenylsubstituted polycyclic aromatic hydrocarbons can be used for the rational fabrication of five-membered rings in the bottom-up synthesis of carbon-based nanomaterials; Cerdá et al.44 have experimentally and theoretically identified the formation of Si atomic arrangement comprising perfect alternating pentagons structure on the Ag (110) surface, all these results give much confidence for the successful synthesis of penta-graphene in the near future. A. Single Li/Na atom adsorption on penta-graphene We first examine the adsorption structure of a single Li/Na atom on the surface of


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penta-graphene. All the possible adsorption sites (from A to I) are schematically shown in Figure 2a. In the calculation, the penta-graphene with a 3 × 3 supercell structure is employed to avoid the interaction between the adjacent Li/Na atoms. After the structure optimization, only A-, B- and C-site adsorption structures are obtained, while for other adsorption sites (from D to I), Li/Na atom will be relaxed to the above three sites during the optimization. The adsorption energy (Eads) of single Li/Na atom on penta-graphene is defined as Eads = ELi/Na-C ‒ EC ‒ ELi/Na, where ELi/Na-C and EC are the total energies of the penta-graphene with and without single Li/Na atom, ELi/Na is the energy per Li/Na atom in the bulk metal Li/Na. The negative value of Eads means the adsorption process is exothermic and energetically preferable. The calculated Eads of a single Li/Na atom on the A-, B- and C-sites of penta-graphene are -0.30/-0.41, -0.12/-0.13 and 0.16/0.13 eV, respectively. Accordingly, the A-site of penta-graphene (on the top of three-coordinated C atom downward the layer) is found to be the most stable position for the adsorption of a single Li/Na atom, the corresponding structure is shown in Figure 2b. The B-site adsorption (on the top of four-coordinated C atom) is energetically 0.18/0.28 eV less stable than A-site, the relatively small difference between the Eads of A-site and B-site suggests the low diffusion barrier of Li/Na atom along A ↔ B direction. The C-site adsorption is not stable since the positive value of Eads. In addition, the modulate value of Eads implies the low open-circuit voltage (OCV) for the Li/Na-ion battery application, which will be discussed in the later part. To better understand the bonding property between Li/Na atom and penta-graphene, the charge density difference of A-site adsorption structures are calculated as shown in Figure 3. It is found that the electrons mainly accumulated around the C atoms, which is due to the larger electronegativity of C


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atom than that of Li/Na atom. The Bader charge analysis reveals that the charge transfer from Li/Na atom to penta-graphene is 0.90/0.86 e. B. The theoretical capacity of Li/Na atoms on penta-graphene As we know, the high storage capacity is a key factor for a promising electrode material. Thus, the Li/Na adsorption at different concentrations is considered by increasing the numbers of Li/Na layers on both surfaces of penta-graphene with a 2 × 2 supercell. To do this, both A- and B-sites of penta-graphene are considered for the Li/Na layer adsorption since they are energetically preferable. It should be noted that B-site adsorption structures will be relaxed to A-site after the molecular dynamics (MD) simulation at room temperature, thus only the A-sites of penta-graphene are possible for the Li/Na layers adsorption. In order to estimate the maximum storage capacity, we calculate the average adsorption energies (Eadv) for the adsorption of nth Li/Na layer on penta-graphene as defined by

Eave =

( EM λn C − EM λ (n−1)C − λEM )

λ

(M = Li, Na)

where EM λn C and EM λ ( n-1)C are the total energies for the adsorption of n and (n-1) layers of Li/Na atoms on the penta-graphene. The λ value represents the number of the adsorbed Li/Na atoms in each layer of penta-graphene with a 2 × 2 supercell. Here, the “first Li/Na layer” means the adsorption of one Li/Na layer on single-side of penta-graphene; the “second Li/Na layer” means the adsorption of one Li/Na layer on another-side of pentagraphene after the first Li/Na layer adsorption; the “third and fourth Li/Na layer” is defined in the same manner. For the first Li layer adsorption on penta-graphene, Li atoms could occupy all the 8 Asites on single-side of of penta-graphene as shown in Figure 4a, and thus the λ value is 8.


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The calculated Eave values for the first (Figure 4a) and the second layers (Figure 4b) are 0.50 and -0.69 eV, respectively, while the Eave value becomes positive (0.08 eV) for the adsorption of additional Li layer. Accordingly, the maximum storage capacity of Li atoms on penta-graphene (2 × 2 supercell) is 16, with the chemical component of LiC1.5. The corresponding theoretical capacity of penta-graphene as Li-ion battery reaches up to 1489 mAh·g-1, which is obviously higher than most of the other 2D materials. Moreover, we estimate the OCV for the intercalation of Li ions on penta-graphene by using the following equation,

OCV =

( EC + xEM − EM xC ) xz

(M = Li, Na)

where x and z are the Li/Na ion content on penta-graphene and the electronic charge of Li/Na ion in the electrolyte (z = 1). Figure 5 shows the OCVs as a function of Li content x in LixC. Similar to the case of adsorption energies, the OCVs increase as the content of Li atoms increase. In detail, the calculated OCV values for the Li0.17C, Li0.34C, Li0.51C and Li0.68C systems are 0.50, 0.54, 0.56 and 0.60 V, respectively. The average OCV during the whole Li intercalation process is 0.55 V, which is moderate for constructing the full cell. As for Na-penta-graphene system, Na atoms are initially placed on all the 8 A-sites of penta-graphene with a 2 × 2 supercell, while one half of the Na atoms move out of the surface after the structure relaxation. This means the Na atoms could not occupy all the 8 adsorption sites on penta-graphene due to their large ionic diameters. Similar results also could be found in the adsorption of Na atoms on Mo2C monolayer.19 To clarify the maximum first-layer adsorption number of Na ions on penta-graphene, we have examined the numbers of 4 and 6 instead. It is found that only 4 Na atoms could adsorb on the penta-graphene with one-layer structure and the negative Eadv value (-0.36 eV) as


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shown in Figure 4c, and thus the λ value is 4. Subsequently, we have examined the adsorption of additional Na layers on penta-graphene. The calculated Eadv values are -0.43, -0.08 and -0.08 eV for the second (Figure 4d), the third (Figure 4e) and the fourth layer (Figure 4f), respectively, while the Eadv becomes positive (0.03 eV) for the adsorption of additional Na layer. Thus, the maximum adsorption numbers of Na ions on pentagraphene (2 × 2 supercell) is 16 (four Na layers), with the chemical component of NaC1.5, and the corresponding theoretical capacity is as high as 1489 mAh·g-1. As shown in Figure 5, the calculated OCV values for the Na0.17C, Na0.34C, Na0.51C and Na0.68C systems are 0.36, 0.40, 0.29 and 0.24 V, respectively. The average OCV is only 0.32 V to drive the Na ions intercalation on penta-graphene, which could effectively avoid the formation of Na dendrite. It should be noted that, in the real condition, the exfoliated monolayer nanosheets would re-stack easily due to the van der Waals forces when they are extracted from suspensions, which hinders the full utilization of their surfaces and consequently results in the capacity loss in the Metal-ion battery application. Under this consideration, there are only one Li/Na layer could exist between two layers of monolayer materials in the restacking structure, and thus the theoretically predicted capacity of monolayer materials were doubled in the real battery application. However, in the present work, we aim to propose a promising anode material (i.e., penta-graphene) as the Li/Na-ion battery by comparing its performance (such as theoretical capacity) with other previously reported 2D anode materials. In doing so, the same calculation method and model (single layer) as previous studies have been employed in our calculation. Thus, even if the theoretically predicted capacities of most of the reported monolayer materials were doubled, our


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current conclusion is still unchanged, that is the penta-graphene is a promising candidate as Li/Na-ion battery due to its higher theoretical capacity than most of previously reported monolayer anode materials. On the other hand, to suppress the re-stacking of monolayer materials, several facile strategies have been recently developed in experiment, which makes the fully utilization of the their surfaces, and thus dramatically enhance the storage capacity as compared with the stacking structure: for example, the monolayer WS2 has been homogeneously embedded in N-doped carbon nanofibers via electrospining method, which exhibits remarkably enhanced specific Li capacity;45 3D porous monolayer MoS2-graphene hybrid nanosheets have been successfully prepared by a facile hydrothermal method, which exhibits the excellent reversible capacity;46 A facile and scalable process has been developed for the synthesis of monolayer MoS2-graphene nanosheet based on the cetyltrimethylammonium bromide-assisted method, which delivered a large reversible capacity.47 All these results have provided the feasible strategies to fully utilize the surfaces of monolayer materials. Thus, such an obtained “doubled-capacity” on monolayer materials is meaningful even in the real battery application. To further understand the bonding feature and the charge distribution between Li/Na atomic layers and penta-graphene, the electron localization function (ELF) is calculated as shown in Figure 6. In this system, large ELF values between two atoms (sharedelectron interactions, red region) correspond to the covalent bonds, and the small ELF values between two atoms (unshared-electron interactions, blue region) correspond to the ionic bonds.48 In Figure 6, the shared-electron interactions could be found between the C atoms, and thus C-C bonds of penta-graphene show the covalent bonding property.


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Meanwhile, the zero electron localization in between Li/Na and C atoms indicates the ionic bonding property of Li-C and Na-C bonds. The different electron distribution behavior around Li/Na (blue region) and C (red region) atoms implies the charge transfer from Li/Na layer to the penta-graphene. Note that the electrons spreading out to the outer Li/Na layers (green region) exhibit the free electron gas behavior, and thus the excellent electronic conductivity is expected. Another important factor to take into account in Li/Na-ion battery is the possible volume change caused by lithiation and sodiation process. To associate an effective volume with penta-graphene, the thickness of penta-graphene is defined as the distance between the two external atomic planes augmented by twice the van der Waals radius of C, 1.7 Å. We can then conclude that upon lithiation and sodiation the overall change in the volume of the penta-graphene is smaller than 3.4% and 1.5%, respectively, indicating the good structural stability of penta-graphene as anode material for Li/Na-ion battery. C. The electronic properties of Li/Na adsorbed penta-graphene To explore the change of electronic conductivity of penta-graphene after the Li/Na ions adsorption, the density of states are calculated as shown in Figure 7. For the pure pentagraphene, the calculated band gap is about 2.33 eV, which agrees well with the previous study.27 After the adsorption of a single Li/Na atom (LiC54/NaC54) on A-site of pentagraphene, the Fermi level shifts upward and pins into the conduction band as shown in Figure 7, which implies that the electronic property of penta-graphene has become metallic. This feature is essential to its application as the Li/Na-ion battery electrode with good electronic conductivity, facilitating the fast charging and discharging. As the increase of the Li/Na content, the electronic conductivity of penta-graphene is further


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enhanced. D. The diffusion barrier of Li/Na ions on penta-graphene It is known that the fast charge/discharge capability is one of the most important factors for a promising Li/Na-ion battery electrode, which is partly related with the diffusion behavior of Li/Na ions in the material. Here the Li/Na ions diffusion processes are calculated by using the climbing-image nudged elastic band (CI-NEB) method.49 We first investigate the diffusion behavior of a single Li/Na ion on penta-graphene. In view of the highly symmetric structure of penta-graphene, two possible diffusion paths (Path-I and Path-II) between the neighboring lowest-energy adsorption sites (A-sites) are considered as shown in Figure 8a. In detail, Path-I is the Li/Na ion diffusion between two adjacent A-A sites, and Path-II is along the A-X-A sites. By combining the Path-I and Path-II, Li/Na ion could diffuse along the whole surface of penta-graphene. As shown in Figure 8b, the calculated diffusion barriers of Li/Na ion along the Path-I and Path-II are 0.04/0.01 eV and 0.17/0.28 eV, respectively, and thus the Path-II is the rate-limiting step. The low diffusion barriers of Li/Na ion on penta-graphene implies the fast diffusion processes, especially for the Li ion. It should be noted that the charge states on Li and Na atoms are positive 0.90 and 0.86 e, respectively, which means both Li and Na exist in the cationic state. In this regard, the diffusion of charged Li/Na atom on penta-graphene could be further enhanced by the external electric field during the operating processes. Thus, the ultra-fast charge/discharge rate is expected for the penta-graphene as the Li/Naion battery material. In addition, we also examine the Li/Na ion diffusion behavior on the penta-graphene with high Li/Na concentration. To do this, the diffusion barriers of single Li/Na vacancy


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on penta-graphene (3 × 3 supercell) covered by one-layer of Li/Na atoms are calculated by using CI-NEB method. For the single Li-vacancy diffusion, three diffusion paths (Path-I', Path-II' and Path-III') have been considered as schematically shown in Figure 9a. It is found that all the diffusion barriers are lower than 0.25 eV as shown in Figure 9c, indicating the fast diffusivity of Li ions on penta-graphene. As for the single Na-vacancy diffusion, two diffusion paths (Path-I'' and Path-II'') are considered as shown in Figure 9b. The results show that the diffusion barrier of Path-I'' (0.33 eV) is much lower than that of Path-II'' (0.83 eV) as shown in Figure 9c, and thus the Path-I'' is the most preferable path for the Na ions diffusion. In a brief summary, the diffusion barriers of Li/Na ions on penta-graphene are less than 0.33 eV even under high Li/Na concentration, which implies the fast diffusivity of Li/Na ions during the whole intercalation/deintercalation processes. Note that the barriers of Li ions diffusion are about 0.1 eV lower than that of Na ions, irrespective of the Li/Na ion concentrations, and thus faster diffusivity of Li ions on penta-graphene is expected. In addition, we calculate the theoretical capacity and diffusion barrier of Li/Na ions on penta-graphene without considering the vdw correction. Our results reveal that the vdw forces contribute about -0.35 eV to the adsorption energy of Li/Na atoms on pentagraphene, which is consistent with the previously reported Li-graphene system.50 Importantly, the Li/Na theoretical capacity on penta-graphene is still as high as 1489 mAh·g-1 even without considering the vdw correction. In the case of Li/Na ion diffusion, the calculated diffusion barrier of a single Li/Na ion on penta-graphene decreases about 0.13 eV after excluding the vdw correction as shown in Table 1. Accordingly, the DFT results with and without considering the vdw correction could give the same conclusion


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that the penta-graphene is a promising anode material for the Li/Na-ion battery with high theoretical storage capacity and fast charge/discharge rate. E. Comparison with other 2D anode materials At last, the performance of penta-graphene as the Li/Na-ion battery is compared with other previously reported 2D anode materials. The corresponding theoretical storage capacities and the single Li/Na ion diffusion barriers are listed in Table 1, where the data in β12- and χ3-borophenes are obtained by considering the vdw correction.14 It is found that Li/Na ions theoretical capacity on penta-graphene (1489/1489 mAh·g-1) is obviously higher than most of the other 2D anode materials, except the borophenes with β12 (1984/1984 mAh·g-1),14 χ3 (1240/1240 mAh·g-1)14 and ∆ (1860/1218 mAh·g-1)51,52 phases and the graphene with maximum divacancy content (1675/1450 mAh·g-1).53,54 Besides the high theoretical capacity, another key factor for a promising Li/Na-ion battery is the fast Li/Na ion diffusivity. Thus, we have compared the Li/Na ion diffusion behaviors among the penta-graphene, borophenes and defective graphene. As for ∆-borophene, the one-dimentional (1D) Li ion diffusion trajectory along the furrow path on ∆-borophene has been observed via the MD simulation at 300 K (without vdW correction).51 In view of the very closed diffusion barriers between Li and Na ion on ∆-borophene, similar 1D Na ion diffusion trajectory is expected.52 As a comparison, MD simulation (300K, without vdW correction) is performed in the Li/Na-penta-graphene system. Figure 10 shows the Li and Na ion diffusion trajectories on penta-graphene after 12 ps. It is found that Li ion is able to freely diffuse through almost all the A-sites on penta-graphene (Figure 10a), which further confirms the fast charge/discharge process for penta-graphene as anode material in the Li-ion battery. Most importantly, the 2D Li ion


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diffusion trajectory could be observed on penta-graphene, which is in great contrast to the case of Li-∆-borophene system (1D). This is mainly because of the existence of the 2D and 1D furrow paths on the surfaces of penta-graphene and ∆-borophene, respectively. Note that the diffusion paths of Li ion on penta-graphene are always along the Path I and II (Figure 8a), which is consistent with the above CI-NEB results. In the case of Na ion diffusion on penta-graphene, the diffusion trajectory is confined within a relatively small region as shown in Figure 10b, and thus the diffusion process of Na ion on pentagraphene is more difficult than that of Li ion. This feature is consistent with the above CINEB results (without vdw correction) that the diffusion barrier of Li ion on pentagraphene is 0.07 eV lower than that of Na ion. It should be noted that the diffusion barrier of Na ion on penta-graphene is only 0.12 eV (without vdw correction), which is already low enough to enable the fast Na ion diffusivity under the electric field. Besides, the 2D diffusion paths (Figure 8a) have been identified for Na ion diffusion on penta-graphene via CI-NEB calculation, which is in contrast to that of ∆-borophene (1D). As for β12- and χ3-borophenes, the Li/Na ion diffusion barriers (with vdw correction) are 0.66/0.33 and 0.60/0.34 eV, respectively,14 which are much higher than the one on penta-graphene (0.17/0.28 eV). Similar to the case of ∆-borophene, 1D Li/Na ion diffusion path along boron vacancies can be identified on β12- and χ3-borophenes,14 which is in contrast to that of penta-graphene (2D). In the case of graphene with maximum divacancy content, the Li/Na ion diffusion barrier (without vdw correction) is 0.37/0.30 eV,55,56 which is obviously higher than the one on penta-graphene (0.05/0.12 eV), and thus the relatively fast Li/Na ion diffusivity on penta-graphene is expected as compared with graphene with divacancy. It should be noted that the existence of defects on


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graphene could dramatically increase the Li/Na ion storage capacity due to the enhanced interaction strength between Li/Na ions and the defect sites of graphene. On the other hand, the structure of penta-graphene is found to be robust, despite the existence of various defects (such as the single vacancy, divacancy and Stone-Wales defect).21 Thus, to further increase the Li/Na ion storage capacity on penta-graphene, one possible strategy is to introduce the defects into penta-graphene. In this case, the diffusion barrier of Li/Na ions on defective penta-graphene is most likely to be increased as compared with pure penta-graphene, because of the enhanced adsorption strength of Li/Na ions on defect sites of penta-graphene. Based on the discussion above, penta-graphene could be a competing anode material as the Li/Na-ion battery with both extremely high Li/Na ions storage capacity and ultra-fast Li/Na ions diffusivity. F. Implications Ever since the proposal of penta-graphene, many other 2D materials with pentagonal Cairo-tiled structures (such as penta-silicene,60-62 penta-SiN2,24 penta-SiC2,24,63 pentaBxCy,64,65 penta-CxNy,66 penta-AlN2,67 penta-BxNy68) have been recently proposed, and have shown the potential applications in many fields (except in the Li/Na-ion batteries) due to their unique atomic arrangements. Even in confined 2D ice69 and experimentally realized silver azide (AgN3)70 systems, the pentagonal Cairo-tiled structures also have been identified. These findings imply that such unique atomic arrangement (pentagraphene-like Cairo-pentagon pattern) widely exists in the 2D materials, and thus the synthesis of these materials is highly possible. On the other hand, all the above mentioned B-, C-, N-, Si-, Al-containing 2D materials possess both light weight and isotropic furrow paths on the surface, which implies their potential applications as the Li/Na-ion battery


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materials with both high theoretical capacity and fast charge/discharge rate. Our finding may inspire intensive researches on exploring these derivatives of penta-graphene as the Li/Na-ion battery materials, and open up a new direction in material engineering to improve the performance of Li/Na-ion battery.

CONCLUSIONS We have systematically studied the structural and electronic properties of the adsorption of Li/Na ions on penta-graphene by using first-principle simulation. Our results show that the single Li/Na ion prefers to absorb on the A-site of penta-graphene with the adsorption energy of -0.30/-0.41 eV. By increasing the Li/Na ions concentration, it is found that the maximum adsorption number of Li/Na ions on penta-graphene (2 × 2 supercell) is 16, and the corresponding theoretical capacity reaches up to 1489 mAh·g-1. The average OCV to drive the Li/Na ions intercalation on penta-graphene is 0.55/0.32 V. Moreover, the calculated diffusion barriers of Li/Na ions on penta-graphene are lower than 0.33 eV, which suggests the fast diffusivity of Li/Na ions. In particular, MD simulations at 300 K show that the single Li ion could diffuse more freely than Na ion on the penta-graphene. At last, we compare our results with some recently proposed 2D anode materials (such as borophenes, phosphorene, and MXene et al.) based on the electrochemical performance as anode material, the results reveal the great potential of penta-graphene as the anode material for the Li/Na-ion battery and the advantage over most of the other 2D materials.

ACKNOWLEDGMENTS


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This work was supported by the Natural Science Foundation of Shandong Province (no. ZR2015BQ013 and ZR2016BQ06) and the National Natural Science Foundation of China (no. 21573188).

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Table 1. The theoretical capacities and single Li/Na ion diffusion barriers on the 2D materials. Here, “vdW” means the results obtained by considering the vdw correction, “1D” means the Li/Na ion diffusion path is along only one direction. The theoretical capacity is calculated by normalizing it with respect to the mass of pure electrode material. In the case of penta-graphene, the theoretical capacity value in bracket is obtained by normalizing it with respect to the mass of the sum of penta-graphene and Li/Na atom. 2D-Materials Penta-graphene (vdW) Penta-graphene

Li/Na Ion Storage Capacity (mAh·g-1) 1489/1489 (1072/654) 1489/1489 (1072/654)

Single Li/Na Ion Diffusion Barrier (eV)

References

0.17/0.28

This work

0.05/0.12

This work

β12-Borophene (vdW)

1984/1984

0.66/0.33 (1D)

14

∆-Borophene

1860/1218

0.003/0.002 (1D)

51,52

Graphene with Maximum Divacancy Content

1675/1450

0.37/0.30

53-56

χ3-Borophene (vdW)

1240/1240

0.60/0.34 (1D)

14

Silicene

954/none

0.23/none

57

BxCyNz

668/810

0.63/0.15

58

Mo2C

526/132

0.04/0.02

19

Ti3C2

448/352

0.07/0.10

15

VS2

466/466

0.20/0.11

56

Phosphorene

433/433

0.08/0.04 (1D)

16-18

Graphite

372/none

0.47/none

71,72


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Figure 1. Total energies per C atom for some allotropes of carbon. Here, the content of single and double vacancy in graphene is 3% and 6%, respectively, which means there is one single/double vacancy in every 32 C atoms of graphene.


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Penta-graphene

I

LiC54 or NaC54

A B F G E D HC

(a)

(b)

Figure 2. (a) A schematic illustration of all the possible Li/Na adsorption sites on pentagraphene, (b) the most stable adsorption structure of single Li/Na atom on penta-graphene.


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Li

(a)

Na

(b) Figure 3. The charge density difference for the adsorption of single (a) Li and (b) Na atom on penta-graphene. The cyan and yellow regions represent the electron loses and gains, respectively. Here, the charge density difference is defined as the charge density of the Li/Na adsorbed penta-graphene minus the charge density of the isolated Li/Na atom and penta-graphene.


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Li

Layer-1

Layer-2

(a) LiC3 (b) LiC1.5

Na

(e) NaC2

Layer-3

Layer-2 Layer-1

Layer-4

(c) NaC6 (d) NaC3

(f) NaC1.5 Figure 4. The adsorption structures of Li/Na atomic layers on penta-graphene.


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Figure 5. The open-circuit voltage (OCV) as a function of Li/Na content x in LixC or NaxC system.


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Li

1.0

0.8

0.6

Na

0.4

0.2

0.0

Figure 6. The electron localization function (ELF) for the adsorption of two (a) Li and (b) Na atomic layers on penta-graphene, respectively.


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Figure 7. The density of states of pure penta-graphene, LiC54/NaC54, LiC3/NaC3, and LiC1.5/NaC1.5.


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Path-I

Path-II

Path-I

Path-II

(a)

(b)

Figure 8. (a) Top and side views of single Li/Na ion diffusion along the Path-I and PathII on penta-graphene, and (b) the corresponding energy barriers.


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Path-III'

Path-I'

Path-II'

(a)

Path-II''

Path-I''

(b)

(c)

Figure 9. Top view of (a) single Li vacancy diffusion along the Path-I', Path-II' and Path III', (b) single Na vacancy diffusion along Path-I'' and Path-II'', and (c) the corresponding energy barriers.


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(a)

(b)

Figure 10. The diffusion trajectories of single (a) Li and (b) Na ion on penta-graphene.


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TOC


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Na-Ion Battery

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