- Email: [email protected]
Metallic two-dimensional Cu2Si monolayer as promising anode materials for lithium and sodium ion batteries, a first principles study
Accepted Manuscript Metallic two-dimensional Cu2Si monolayer as promising anode materials for lithium and sodium ion batteries, a first principles study Xiaoli Sun, Shiyun Wu, Khang Ngoc Dinh, Zhiguo Wang PII:
S0022-4596(19)30151-3
DOI:
https://doi.org/10.1016/j.jssc.2019.03.041
Reference:
YJSSC 20690
To appear in:
Journal of Solid State Chemistry
Received Date: 14 January 2019 Revised Date:
6 March 2019
Accepted Date: 23 March 2019
Please cite this article as: X. Sun, S. Wu, K.N. Dinh, Z. Wang, Metallic two-dimensional Cu2Si monolayer as promising anode materials for lithium and sodium ion batteries, a first principles study, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.03.041. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Metallic Two-Dimensional Cu2Si Monolayer as Promising Anode Materials for Lithium and Sodium Ion Batteries, a First Principles Study
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Xiaoli Sun,1 Shiyun Wu,2 Khang Ngoc Dinh3 and Zhiguo Wang1,* 1. School of Electronic Science and Engineering, Center for Public Security Technology, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China 2. School of Intelligent Manufacturing, Sichuan University of Arts and Science, Dazhou,
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635000, P.R. China
3. Energy Research Institute @ NTU (ERI@N), Interdisciplinary Graduate School, Nanyang
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Technological University, Singapore 637553, Singapore
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Density functional theory calculations show that the Cu2Si monolayer is metallic material, lithium and sodium show strong adsorption on the Cu2Si monolayer, but with small diffusion energy barrier. Thus, Cu2Si monolayer can be used as a promising anode material for lithium and sodium ion batteries.
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Metallic Two-Dimensional Cu2Si Monolayer as Promising Anode Materials for Lithium and Sodium Ion Batteries, a First Principles Study
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Xiaoli Sun,1 Shiyun Wu,2 Khang Ngoc Dinh3 and Zhiguo Wang1,* 1. School of Electronic Science and Engineering, Center for Public Security Technology, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China 2. School of Intelligent Manufacturing, Sichuan University of Arts and Science, Dazhou,
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635000, P.R. China
3. Energy Research Institute @ NTU (ERI@N), Interdisciplinary Graduate School, Nanyang
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Technological University, Singapore 637553, Singapore
* Correspondence: [email protected]; Tel.: +86-28-61830406 Abstract: Two-dimensional (2D) materials have attracted much attention as anode materials for lithium ion batteries. However, most of the 2D materials are semiconductors with limited electron transportation. In this paper, the electronic properties of Cu2Si monolayer and adsorption and diffusion of Li/Na on Cu2Si monolayer were investigated by the density
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functional theory. The Cu2Si monolayer shows metallic characteristics. The adsorption energies are around -2.5 eV, which is stronger than those of Li/Na absorbed on other 2D materials. In addition, the diffusion energy barriers are 0.15 and 0.11 eV for Li and Na diffuses
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on Cu2Si monolayer, respectively, indicating that Cu2Si monolayer can be fast charged/discharged. These results indicate that Cu2Si monolayer can be a promising anode
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material for lithium and sodium ion batteries. Keywords: Cu2Si monolayer; adsorption; diffusion; LIBs; SIBs; density functional theory
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ACCEPTED MANUSCRIPT 1. Introduction Lithium ion batteries (LIBs), as one of the most important energy storage technologies, plays an important role in promoting the rapid development of portable electronic devices as well as electric vehicles [1-3]. Currently, the energy density of commercial LIBs is around
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150~180 Wh·kg-1 with typical layered oxide or LiFePO4 as cathodes and graphite as anodes [3-6]. The low energy density of current widely used LIBs restricts their application in electric transportation and large-scale energy storage systems. Thus, searching for new electrode materials for LIBs with high energy density or new batteries beyond of LIBs is highly
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desirable. Sodium is located right below lithium in the periodic table; and hence, these two elements show similar chemical properties in many aspects, sodium ion batteries (SIBs) have
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recently received much interest to be used as a low-cost alternative to LIBs [7]. To meet the high performance demands, many groups are designing and developing new electrode materials for LIBs. Different types of materials and their derivatives are widely investigated. Among them, Si has been regarded as one of the most promising next-generation anodes due to its 10 times higher specific capacity than graphite anode [8]. Importantly, Si is
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the second most abundant element on the earth. However, before their commercialization, several critical drawbacks of Si should be overcome, such as poor cycling performance due to the large volume expansion (>300%) during the lithiation process [9-13], pulverization of
[15].
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reactant particles and crumbling of the electrode [14], and low electrical conductivity of Si
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Copper (Cu) is often used as the current collector for anode in LIBs [15], offering a longer cycle life with good capacity retention of LIBs. Cu has excellent conductivity and good solubility with Si [16]. Cu is a beneficial additive in various forms of anodes and plays a crucial role in holding the electrode together, minimizing the overall capacity loss and fastening electron transfer; hence, improving the electrochemical performances of the electrode [17-19]. The Cu coated amorphous Si particles exhibited significantly enhanced lithium storage capacity over pristine amorphous Si particles of about 7 times [20]. Cu-Si alloy mixture nanotubes demonstrated a high capacity (≈2000 mAh/g at 0.84 A/g) and a high capacity retention of 84% (>88%) at 3.2 A/g (or 20 A/g ) after 1000 cycles [21]. The Cu-Si 2
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that are anchored to the copper foil act as both current collectors and structural reinforcements for the Si shell [23]. In addition, Cu3Si intermetallic also can enhance the reversibility during galvanostatic cycling [24, 25].
To be an ideal anode for rechargeable batteries, the materials should have high gravimetric /volumetric capacity, low potential against cathode materials, long cycle life, environmental
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compatibility, low toxicity, and low cost [3, 10]. Two dimensional (2D) materials show higher charge/discharge rates and excellent charge capacity retention than their bulk counterparts due
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to the reduced diffusion path and increasing the surface area for the storage of lithium [26-30]. 2D materials as anode materials have been widely studied [31] [32-34] [35-37]. However, most of the 2D materials are semiconductors with low electron conductivity, such as MoS2 monolayer with a band gap of 1.67 eV. In this work, we studied the possible application of 2D Cu2Si monolayer as potential anodes for LIBs and SIBs using density functional theory (DFT). The Cu2Si monolayer, which has metallic character, large adsorption and low diffusion energy
2. Simulation Methods
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barriers for Li and Na, can be used as a potential anode for LIBs and SIBs.
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All the DFT calculations were performed with SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) codes [38] with norm-conserving Troullier-Martins
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pseudo-potentials [39] to describe the electron-ionic core interaction. The electron exchange-correlation was processed using the generalized gradient approximation (GGA) with the parametrization scheme of Perdew-Burke-Ernzerhof (PBE) [40]. The valence electron wave functions were expanded using double-ζ basis set plus polarization functions. Convergence of the total energy was tested by choosing an energy cutoff of 150 Ry to calculate the self-consistent Hamiltonian matrix elements. A 5×5 hexagonal supercell of Cu2Si monolayer was employed to model Li(Na) adsorption and diffusion. A large spacing of 35 Å between the Cu2Si monolayer was used to prevent interlayer coupling. The Brillouin zone integration was modeled using a special k-point 3
ACCEPTED MANUSCRIPT sampling of the Monkhorst-Pack scheme with a Г-centered grid. A 4×4×1 k-grid mesh was used for all the calculations. All atomic positions and lattice constants were optimized using the conjugate gradient method until the maximum Hellmann-Feynman force acting on each atom was less than 0.02 eV/Å.
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3. Results
Figure 1. (a) Cross and side views, (b) Phonon dispersion, (c) band structure, and (d) PDOS of
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Cu2Si monolayer. The Fermi energy is set to be 0. The Cu2Si monolayer is composed of Si and Cu atoms in the same plane with Si and Cu
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form triangular and honeycomb sub-lattice, respectively. As shown in Fig. 1a, the Cu2Si monolayer is with a 6-fold symmetric, each Si atom is coordinated with six Cu atoms and each Cu is coordinated with three Cu atoms and three Si atoms. The calculated lattice constant of Cu2Si monolayer is a=b= 4.183 Å with Cu-Si bond length of 2.415 Å. The dynamical stability of the Cu2Si monolayer was evaluated by the calculation of phonon dispersion along the high-symmetry k-point. As shown in Fig. 1b, there no imaginary frequency found in Cu2Si monolayer. The long range of the Coulomb interactions should lead to the frequencies of longitudinal optical (LO) modes above those of transversal optical (TO) modes [41-44]. Both the LO and TO branches have positive frequencies, indicating that both of them are reasonably stable. Thus, the Cu2Si monolayer shows good stability. The band structure of Cu2Si monolayer 4
ACCEPTED MANUSCRIPT along high-symmetry point and the projected density of states (PDOS) are shown in Fig. 1c-d. The Fermi energy (Ef) locates inside the band structure of Cu2Si monolayer, there is no band gap observed, indicating that the Cu2Si monolayer is metallic materials. PDOS shows that the states in the energy range between -6.0 and -2.0 eV below the Fermi energy is composed of the Cu states, whereas the states near the Fermi energy level are composed of Si and Cu states.
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To find out the storage mechanism of Cu2Si monolayer as the anode for LIBs (SIBs), the adsorption configurations of Li(Na) on Cu2Si monolayer were investigated. Considering the symmetry of Cu2Si monolayer, five possible adsorption sites on Cu2Si monolayer were considered (as shown in Fig. 2a): the top site above Cu (TCu), the top site above Si (TSi), the
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bridge site between Cu-Cu (BCu-Cu) and Si-Cu (BSi-Cu) bonds, and the center site of a triangle (TriC).
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The adsorption was evaluated by calculating the adsorption energy using equation (1): = / − − /
(1)
where / and are the total energies of Cu2Si monolayer with and without Li(Na) adsorption, respectively. / is the energy of a Li(Na) atom in their stable bulk state. According to equation (1), a more negative value of adsorption energy indicates a more favorable exothermic reaction between the Cu2Si monolayer and Li(Na).
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The calculated adsorption energies for Li(Na) on the possible adsorption sites are shown in Fig. 2b. The adsorption energies of Li on Cu2Si monolayer are in the range between -2.54 and -2.34 eV, and that for Na are between -2.66 and -2.48 eV. Both Li and Na are energy preferable
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to be absorbed at the top Cu. The PDOS of Cu2Si monolayer with Li(Na) absorbed at the TCu site was shown in Fig. 3. Li(Na) donates its electron to Cu2Si monolayer, which results in the strong adsorption. Cu2Si monolayer remains its metallic characteristics after the Li(Na)
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adsorption. The Li(Na) shows stronger absorption on Cu2Si monolayer than other 2D materials, such as the adsorption energy of Li on MoS2 monolayer is -1.24 eV [32], and the adsorption energies are for Li and Na are -1.02~-0.67 eV and -0.67~ -0.48 eV on graphene [45], respectively. The adsorption energy for Na on monolayer black phosphorus is -0.48~-0.02 eV [46]. The large negative value of adsorption energy indicates a more favorable exothermic reaction between the Li(Na) and Cu2Si monolayer than other 2D materials.
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Figure 2. (a) The lithium/sodium adsorption sites (b) the adsorption energies of lithium/sodium on Cu2Si. Diffusion energy barriers of the (c) neutral lithium, (d) neutral sodium, (e) Li+ ion and
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(f) Na+ ion along different diffusion paths.
Figure 3. The PDOS Cu2Si monolayer with (a) Li and (b) Na absorbed at the TCu site.
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As the charging/discharging rate of rechargeable ion batteries is mainly affected by the diffusivity of transporting ions on the electrode materials, the diffusion behavior of Li(Na) on Cu2Si monolayer was further investigated. As Li and Na are energy favorable to be absorbed at the top Cu (TCu), the diffusion one TCu site to another TCu site was examined, three possible diffusion paths as shown in Fig. 2a was investigated. The Li(Na) can diffuse to another TCu site through a BSi-Cu, BCu-Cu, or TriC sites, which were defined as path1, path2, and path3, respectively. As shown in Fig. 2c, the diffusion energy barriers are 0.29, 0.15, and 0.24 eV for Li diffusion along path1, path2 and path3, respectively. Both Li and Na prefers to diffusion along the path2 via the BCu-Cu site. The diffusion energy barrier is close to or even lower than 6
ACCEPTED MANUSCRIPT other potential anode 2D materials, namely, 0.36 [47], 0.21 [30], 0.26 [48], 0.21 [32], and 0.36 eV [49] for Li on boron phosphide, silicene, graphene, MoS2, and phosphorene, respectively. Furthermore, for Na diffusion, the diffusion energy barriers are 0.23, 0.11, and 0.22 eV (as shown in Fig. 2d), which is smaller than those of Na on graphene (0.19 eV) [45] and phosphorene (0.22) [49], and close to Na diffuses on a MoS2 monolayer (0.11 eV) [50]. The
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smaller diffusion energy barrier indicates that the Cu2Si monolayer can be used in high charging/discharging rates as an anode for LIBs (SIBs). The diffusion of Li+ and Na+ ions on Cu2Si were calculated to examine whether the Cu2Si monolayer is applicable for solid-state ion batteries. The diffusion energy profiles of Li+ and Na+ ions on Cu2Si were shown in Fig. 2e and 2f, respectively. The diffusion of ions shows the same characteristics as that of neutral Li/Na
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diffusion except for the small diffusion energy barriers. Both Li+ and Na+ ions prefer to diffusion along the path2 via the BCu-Cu site. The diffusion energy barriers are 0.24, 0.12, and
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0.20 eV for Li+ ion along path1, path2 and path3, respectively.
Figure 4. ab initio molecular dynamic simulation of the diffusion behavior of Li(Na) on the Cu2Si monolayer at a temperature of 500 K. The diffusion behavior of Li(Na) on Cu2Si monolayer was further studied with ab initio molecular dynamic (AIMD). A canonical NVT ensemble with a time step of 1.0 fs was used. A simulation time up to 10 ps was used to simulate diffusion. The diffusion paths for Li and Na staring from S site and finally diffuse to F site at a temperature of 500 K are shown in Fig. 4a and 4b, respectively. The enlarged parts around the Li and Na diffusion are shown in Fig. 4c 7
ACCEPTED MANUSCRIPT and 4d, respectively. The evolution of diffusion with time is denoted with different color balls. The Li(Na) prefer to diffuse to the adjacent site through the BCu-Cu site, which agrees well with the above discussion, the diffusion path is with low diffusion energy barrier. The Li(Na) does not diffuse to TSi site, since this site is with the small negative adsorption energy than other sites. Due to the smaller diffusion energy barrier of Na than Li on the Cu2Si monolayer, the
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diffusion path of Na is longer than that of Li.
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Figure 5. The voltage as a function of Li(Na) concentrations in Cu2Si monolayer. The open-circuit-voltage (OCV) of Li intercalated Cu2Si monolayer was calculated using equation (2) by calculating the energy difference of two neighboring Li Cu Si phase by neglecting the volume and entropy effects [51],
! " # ! " $ # % $ # %&
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(2)
where '()* +( and '( are the total energy of Li Cu Si and lithium metal, respectively. The OCV of sodium intercalated into Cu2Si monolayer was calculated in the same way. The
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calculated OCV was shown in Fig. 5. The OCV decreases with increase of Li(Na) content in Li Cu Si (Na Cu Si) as x is smaller than 0.75, then slightly increases. A voltage plateau can be clearly seen as x of Na Cu Si in the range between 0.25 and 0.75, which is much lower than
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Li Cu Si, indicating that Cu2Si monolayer could provide a lower charging voltage for SIBs. Li(Na) shows strong adsorption on the Cu2Si monolayer with low diffusion energy barrier than on other 2D anode materials; hence, the metallic Cu2Si monolayer could be used as a potential anode material for LIBs(SIBs). 5. Conclusions In summary, the structural, electronic properties and stability of Cu2Si monolayer were investigated using DFT calculation. The Cu2Si monolayer shows metallic behavior. The adsorption and diffusion of Li/Na on Cu2Si monolayer were further investigated. The 8
ACCEPTED MANUSCRIPT adsorption energies are ~-2.5 eV for Li/Na, which is much lower than those of lithium on the other 2D materials. The diffusion energy batteries depend on the diffusion paths, and the lowest ones are 0.15 and 0.11 eV for Li and Na, respectively. These results indicate that Cu2Si monolayer is a very promising anode material for LIBs/SIBs.
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Acknowledgments: This work was financially supported by the National Natural Science Foundation of China (NFSC 11474047) and the Fundamental Research Funds for the Central Universities (ZYGX2016J202). S.W. was financially supported by the Key Project of Sichuan
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Education Department (18ZA0420).
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Adsorption and diffusion of Li/Na on Cu2Si monolayer were investigated. Li and Na show strong adsorption on the Cu2Si monolayer. The diffusion barriers are relative small for Li and Na on Cu2Si monolayer. Cu2Si monolayer can be used as anode materials of LIBs and SIBs.
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S0022-4596(19)30151-3
DOI:
https://doi.org/10.1016/j.jssc.2019.03.041
Reference:
YJSSC 20690
To appear in:
Journal of Solid State Chemistry
Received Date: 14 January 2019 Revised Date:
6 March 2019
Accepted Date: 23 March 2019
Please cite this article as: X. Sun, S. Wu, K.N. Dinh, Z. Wang, Metallic two-dimensional Cu2Si monolayer as promising anode materials for lithium and sodium ion batteries, a first principles study, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.03.041. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Metallic Two-Dimensional Cu2Si Monolayer as Promising Anode Materials for Lithium and Sodium Ion Batteries, a First Principles Study
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Xiaoli Sun,1 Shiyun Wu,2 Khang Ngoc Dinh3 and Zhiguo Wang1,* 1. School of Electronic Science and Engineering, Center for Public Security Technology, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China 2. School of Intelligent Manufacturing, Sichuan University of Arts and Science, Dazhou,
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635000, P.R. China
3. Energy Research Institute @ NTU (ERI@N), Interdisciplinary Graduate School, Nanyang
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Technological University, Singapore 637553, Singapore
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Density functional theory calculations show that the Cu2Si monolayer is metallic material, lithium and sodium show strong adsorption on the Cu2Si monolayer, but with small diffusion energy barrier. Thus, Cu2Si monolayer can be used as a promising anode material for lithium and sodium ion batteries.
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Metallic Two-Dimensional Cu2Si Monolayer as Promising Anode Materials for Lithium and Sodium Ion Batteries, a First Principles Study
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Xiaoli Sun,1 Shiyun Wu,2 Khang Ngoc Dinh3 and Zhiguo Wang1,* 1. School of Electronic Science and Engineering, Center for Public Security Technology, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China 2. School of Intelligent Manufacturing, Sichuan University of Arts and Science, Dazhou,
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635000, P.R. China
3. Energy Research Institute @ NTU (ERI@N), Interdisciplinary Graduate School, Nanyang
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Technological University, Singapore 637553, Singapore
* Correspondence: [email protected]; Tel.: +86-28-61830406 Abstract: Two-dimensional (2D) materials have attracted much attention as anode materials for lithium ion batteries. However, most of the 2D materials are semiconductors with limited electron transportation. In this paper, the electronic properties of Cu2Si monolayer and adsorption and diffusion of Li/Na on Cu2Si monolayer were investigated by the density
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functional theory. The Cu2Si monolayer shows metallic characteristics. The adsorption energies are around -2.5 eV, which is stronger than those of Li/Na absorbed on other 2D materials. In addition, the diffusion energy barriers are 0.15 and 0.11 eV for Li and Na diffuses
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on Cu2Si monolayer, respectively, indicating that Cu2Si monolayer can be fast charged/discharged. These results indicate that Cu2Si monolayer can be a promising anode
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material for lithium and sodium ion batteries. Keywords: Cu2Si monolayer; adsorption; diffusion; LIBs; SIBs; density functional theory
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ACCEPTED MANUSCRIPT 1. Introduction Lithium ion batteries (LIBs), as one of the most important energy storage technologies, plays an important role in promoting the rapid development of portable electronic devices as well as electric vehicles [1-3]. Currently, the energy density of commercial LIBs is around
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150~180 Wh·kg-1 with typical layered oxide or LiFePO4 as cathodes and graphite as anodes [3-6]. The low energy density of current widely used LIBs restricts their application in electric transportation and large-scale energy storage systems. Thus, searching for new electrode materials for LIBs with high energy density or new batteries beyond of LIBs is highly
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desirable. Sodium is located right below lithium in the periodic table; and hence, these two elements show similar chemical properties in many aspects, sodium ion batteries (SIBs) have
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recently received much interest to be used as a low-cost alternative to LIBs [7]. To meet the high performance demands, many groups are designing and developing new electrode materials for LIBs. Different types of materials and their derivatives are widely investigated. Among them, Si has been regarded as one of the most promising next-generation anodes due to its 10 times higher specific capacity than graphite anode [8]. Importantly, Si is
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the second most abundant element on the earth. However, before their commercialization, several critical drawbacks of Si should be overcome, such as poor cycling performance due to the large volume expansion (>300%) during the lithiation process [9-13], pulverization of
[15].
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reactant particles and crumbling of the electrode [14], and low electrical conductivity of Si
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Copper (Cu) is often used as the current collector for anode in LIBs [15], offering a longer cycle life with good capacity retention of LIBs. Cu has excellent conductivity and good solubility with Si [16]. Cu is a beneficial additive in various forms of anodes and plays a crucial role in holding the electrode together, minimizing the overall capacity loss and fastening electron transfer; hence, improving the electrochemical performances of the electrode [17-19]. The Cu coated amorphous Si particles exhibited significantly enhanced lithium storage capacity over pristine amorphous Si particles of about 7 times [20]. Cu-Si alloy mixture nanotubes demonstrated a high capacity (≈2000 mAh/g at 0.84 A/g) and a high capacity retention of 84% (>88%) at 3.2 A/g (or 20 A/g ) after 1000 cycles [21]. The Cu-Si 2
ACCEPTED MANUSCRIPT foam also shows better electrochemical performance with Cu serves as a good conductive current collector and a rigid matrix that can suppress the volume variation of the active Si material during charge/discharge [22]. Cu-Si nanocables show excellent electrochemical performance including high specific capacity and cycling stability with conductive Cu cores
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that are anchored to the copper foil act as both current collectors and structural reinforcements for the Si shell [23]. In addition, Cu3Si intermetallic also can enhance the reversibility during galvanostatic cycling [24, 25].
To be an ideal anode for rechargeable batteries, the materials should have high gravimetric /volumetric capacity, low potential against cathode materials, long cycle life, environmental
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compatibility, low toxicity, and low cost [3, 10]. Two dimensional (2D) materials show higher charge/discharge rates and excellent charge capacity retention than their bulk counterparts due
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to the reduced diffusion path and increasing the surface area for the storage of lithium [26-30]. 2D materials as anode materials have been widely studied [31] [32-34] [35-37]. However, most of the 2D materials are semiconductors with low electron conductivity, such as MoS2 monolayer with a band gap of 1.67 eV. In this work, we studied the possible application of 2D Cu2Si monolayer as potential anodes for LIBs and SIBs using density functional theory (DFT). The Cu2Si monolayer, which has metallic character, large adsorption and low diffusion energy
2. Simulation Methods
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barriers for Li and Na, can be used as a potential anode for LIBs and SIBs.
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All the DFT calculations were performed with SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) codes [38] with norm-conserving Troullier-Martins
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pseudo-potentials [39] to describe the electron-ionic core interaction. The electron exchange-correlation was processed using the generalized gradient approximation (GGA) with the parametrization scheme of Perdew-Burke-Ernzerhof (PBE) [40]. The valence electron wave functions were expanded using double-ζ basis set plus polarization functions. Convergence of the total energy was tested by choosing an energy cutoff of 150 Ry to calculate the self-consistent Hamiltonian matrix elements. A 5×5 hexagonal supercell of Cu2Si monolayer was employed to model Li(Na) adsorption and diffusion. A large spacing of 35 Å between the Cu2Si monolayer was used to prevent interlayer coupling. The Brillouin zone integration was modeled using a special k-point 3
ACCEPTED MANUSCRIPT sampling of the Monkhorst-Pack scheme with a Г-centered grid. A 4×4×1 k-grid mesh was used for all the calculations. All atomic positions and lattice constants were optimized using the conjugate gradient method until the maximum Hellmann-Feynman force acting on each atom was less than 0.02 eV/Å.
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3. Results
Figure 1. (a) Cross and side views, (b) Phonon dispersion, (c) band structure, and (d) PDOS of
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Cu2Si monolayer. The Fermi energy is set to be 0. The Cu2Si monolayer is composed of Si and Cu atoms in the same plane with Si and Cu
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form triangular and honeycomb sub-lattice, respectively. As shown in Fig. 1a, the Cu2Si monolayer is with a 6-fold symmetric, each Si atom is coordinated with six Cu atoms and each Cu is coordinated with three Cu atoms and three Si atoms. The calculated lattice constant of Cu2Si monolayer is a=b= 4.183 Å with Cu-Si bond length of 2.415 Å. The dynamical stability of the Cu2Si monolayer was evaluated by the calculation of phonon dispersion along the high-symmetry k-point. As shown in Fig. 1b, there no imaginary frequency found in Cu2Si monolayer. The long range of the Coulomb interactions should lead to the frequencies of longitudinal optical (LO) modes above those of transversal optical (TO) modes [41-44]. Both the LO and TO branches have positive frequencies, indicating that both of them are reasonably stable. Thus, the Cu2Si monolayer shows good stability. The band structure of Cu2Si monolayer 4
ACCEPTED MANUSCRIPT along high-symmetry point and the projected density of states (PDOS) are shown in Fig. 1c-d. The Fermi energy (Ef) locates inside the band structure of Cu2Si monolayer, there is no band gap observed, indicating that the Cu2Si monolayer is metallic materials. PDOS shows that the states in the energy range between -6.0 and -2.0 eV below the Fermi energy is composed of the Cu states, whereas the states near the Fermi energy level are composed of Si and Cu states.
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To find out the storage mechanism of Cu2Si monolayer as the anode for LIBs (SIBs), the adsorption configurations of Li(Na) on Cu2Si monolayer were investigated. Considering the symmetry of Cu2Si monolayer, five possible adsorption sites on Cu2Si monolayer were considered (as shown in Fig. 2a): the top site above Cu (TCu), the top site above Si (TSi), the
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bridge site between Cu-Cu (BCu-Cu) and Si-Cu (BSi-Cu) bonds, and the center site of a triangle (TriC).
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The adsorption was evaluated by calculating the adsorption energy using equation (1): = / − − /
(1)
where / and are the total energies of Cu2Si monolayer with and without Li(Na) adsorption, respectively. / is the energy of a Li(Na) atom in their stable bulk state. According to equation (1), a more negative value of adsorption energy indicates a more favorable exothermic reaction between the Cu2Si monolayer and Li(Na).
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The calculated adsorption energies for Li(Na) on the possible adsorption sites are shown in Fig. 2b. The adsorption energies of Li on Cu2Si monolayer are in the range between -2.54 and -2.34 eV, and that for Na are between -2.66 and -2.48 eV. Both Li and Na are energy preferable
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to be absorbed at the top Cu. The PDOS of Cu2Si monolayer with Li(Na) absorbed at the TCu site was shown in Fig. 3. Li(Na) donates its electron to Cu2Si monolayer, which results in the strong adsorption. Cu2Si monolayer remains its metallic characteristics after the Li(Na)
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adsorption. The Li(Na) shows stronger absorption on Cu2Si monolayer than other 2D materials, such as the adsorption energy of Li on MoS2 monolayer is -1.24 eV [32], and the adsorption energies are for Li and Na are -1.02~-0.67 eV and -0.67~ -0.48 eV on graphene [45], respectively. The adsorption energy for Na on monolayer black phosphorus is -0.48~-0.02 eV [46]. The large negative value of adsorption energy indicates a more favorable exothermic reaction between the Li(Na) and Cu2Si monolayer than other 2D materials.
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Figure 2. (a) The lithium/sodium adsorption sites (b) the adsorption energies of lithium/sodium on Cu2Si. Diffusion energy barriers of the (c) neutral lithium, (d) neutral sodium, (e) Li+ ion and
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(f) Na+ ion along different diffusion paths.
Figure 3. The PDOS Cu2Si monolayer with (a) Li and (b) Na absorbed at the TCu site.
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As the charging/discharging rate of rechargeable ion batteries is mainly affected by the diffusivity of transporting ions on the electrode materials, the diffusion behavior of Li(Na) on Cu2Si monolayer was further investigated. As Li and Na are energy favorable to be absorbed at the top Cu (TCu), the diffusion one TCu site to another TCu site was examined, three possible diffusion paths as shown in Fig. 2a was investigated. The Li(Na) can diffuse to another TCu site through a BSi-Cu, BCu-Cu, or TriC sites, which were defined as path1, path2, and path3, respectively. As shown in Fig. 2c, the diffusion energy barriers are 0.29, 0.15, and 0.24 eV for Li diffusion along path1, path2 and path3, respectively. Both Li and Na prefers to diffusion along the path2 via the BCu-Cu site. The diffusion energy barrier is close to or even lower than 6
ACCEPTED MANUSCRIPT other potential anode 2D materials, namely, 0.36 [47], 0.21 [30], 0.26 [48], 0.21 [32], and 0.36 eV [49] for Li on boron phosphide, silicene, graphene, MoS2, and phosphorene, respectively. Furthermore, for Na diffusion, the diffusion energy barriers are 0.23, 0.11, and 0.22 eV (as shown in Fig. 2d), which is smaller than those of Na on graphene (0.19 eV) [45] and phosphorene (0.22) [49], and close to Na diffuses on a MoS2 monolayer (0.11 eV) [50]. The
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smaller diffusion energy barrier indicates that the Cu2Si monolayer can be used in high charging/discharging rates as an anode for LIBs (SIBs). The diffusion of Li+ and Na+ ions on Cu2Si were calculated to examine whether the Cu2Si monolayer is applicable for solid-state ion batteries. The diffusion energy profiles of Li+ and Na+ ions on Cu2Si were shown in Fig. 2e and 2f, respectively. The diffusion of ions shows the same characteristics as that of neutral Li/Na
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diffusion except for the small diffusion energy barriers. Both Li+ and Na+ ions prefer to diffusion along the path2 via the BCu-Cu site. The diffusion energy barriers are 0.24, 0.12, and
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0.20 eV for Li+ ion along path1, path2 and path3, respectively.
Figure 4. ab initio molecular dynamic simulation of the diffusion behavior of Li(Na) on the Cu2Si monolayer at a temperature of 500 K. The diffusion behavior of Li(Na) on Cu2Si monolayer was further studied with ab initio molecular dynamic (AIMD). A canonical NVT ensemble with a time step of 1.0 fs was used. A simulation time up to 10 ps was used to simulate diffusion. The diffusion paths for Li and Na staring from S site and finally diffuse to F site at a temperature of 500 K are shown in Fig. 4a and 4b, respectively. The enlarged parts around the Li and Na diffusion are shown in Fig. 4c 7
ACCEPTED MANUSCRIPT and 4d, respectively. The evolution of diffusion with time is denoted with different color balls. The Li(Na) prefer to diffuse to the adjacent site through the BCu-Cu site, which agrees well with the above discussion, the diffusion path is with low diffusion energy barrier. The Li(Na) does not diffuse to TSi site, since this site is with the small negative adsorption energy than other sites. Due to the smaller diffusion energy barrier of Na than Li on the Cu2Si monolayer, the
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diffusion path of Na is longer than that of Li.
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Figure 5. The voltage as a function of Li(Na) concentrations in Cu2Si monolayer. The open-circuit-voltage (OCV) of Li intercalated Cu2Si monolayer was calculated using equation (2) by calculating the energy difference of two neighboring Li Cu Si phase by neglecting the volume and entropy effects [51],
! " # ! " $ # % $ # %&
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(2)
where '()* +( and '( are the total energy of Li Cu Si and lithium metal, respectively. The OCV of sodium intercalated into Cu2Si monolayer was calculated in the same way. The
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calculated OCV was shown in Fig. 5. The OCV decreases with increase of Li(Na) content in Li Cu Si (Na Cu Si) as x is smaller than 0.75, then slightly increases. A voltage plateau can be clearly seen as x of Na Cu Si in the range between 0.25 and 0.75, which is much lower than
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Li Cu Si, indicating that Cu2Si monolayer could provide a lower charging voltage for SIBs. Li(Na) shows strong adsorption on the Cu2Si monolayer with low diffusion energy barrier than on other 2D anode materials; hence, the metallic Cu2Si monolayer could be used as a potential anode material for LIBs(SIBs). 5. Conclusions In summary, the structural, electronic properties and stability of Cu2Si monolayer were investigated using DFT calculation. The Cu2Si monolayer shows metallic behavior. The adsorption and diffusion of Li/Na on Cu2Si monolayer were further investigated. The 8
ACCEPTED MANUSCRIPT adsorption energies are ~-2.5 eV for Li/Na, which is much lower than those of lithium on the other 2D materials. The diffusion energy batteries depend on the diffusion paths, and the lowest ones are 0.15 and 0.11 eV for Li and Na, respectively. These results indicate that Cu2Si monolayer is a very promising anode material for LIBs/SIBs.
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Acknowledgments: This work was financially supported by the National Natural Science Foundation of China (NFSC 11474047) and the Fundamental Research Funds for the Central Universities (ZYGX2016J202). S.W. was financially supported by the Key Project of Sichuan
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Education Department (18ZA0420).
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Adsorption and diffusion of Li/Na on Cu2Si monolayer were investigated. Li and Na show strong adsorption on the Cu2Si monolayer. The diffusion barriers are relative small for Li and Na on Cu2Si monolayer. Cu2Si monolayer can be used as anode materials of LIBs and SIBs.
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