Porous graphene for high capacity lithium ion battery anode material

Accepted Manuscript Title: Porous Graphene for High Capacity Lithium Ion Battery Anode Material Author: Yusheng Wang Qiaoli Zhang Min Jia Dapeng Yang Jianjun Wang Meng Li Jing Zhang Qiang Sun Yu Jia PII: DOI: Reference:

S0169-4332(15)03015-9 http://dx.doi.org/doi:10.1016/j.apsusc.2015.11.264 APSUSC 32003

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APSUSC

Received date: Accepted date:

7-10-2015 30-11-2015

Please cite this article as: Y. Wang, Q. Zhang, M. Jia, D. Yang, J. Wang, M. Li, J. Zhang, Q. Sun, Y. Jia, Porous Graphene for High Capacity Lithium Ion Battery Anode Material, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.11.264 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.

*Highlights (for review)

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Porous graphene sheet as Li storage media. Excellent mobility both along in-plane and out-plane directions. The interactions can be easily tuned by an applied strain.

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*Graphical Abstract (for review)

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*Manuscript Click here to view linked References

Porous Graphene for High Capacity Lithium Ion Battery Anode Material Yusheng Wang,a,c* Qiaoli Zhang,a Min Jia,a Dapeng Yang,a Jianjun Wang,b Meng Li,b Jing Zhang,a Qiang Sun,c Yu Jiac* College of Mathematics and Information Science, North China University of Water Resources and Electric Power, Zhengzhou 450011, China

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College of Science, Zhongyuan University of Technology, Zhengzhou 450007, China

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School of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, China

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Abstract

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* Corresponding author, Tel.: 86-371-67739336; E-mail: [email protected] (Y. Jia)

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Based on density functional theory calculations, we studied the Li dispersed on porous graphene (PG) for its application as Li ion battery anode material. The hybridization of Li atoms and the carbon

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atoms enhanced the interaction between Li atoms and the PG. With holes of specific size, the PG can

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provide excellent mobility with moderate barriers of 0.37-0.39 eV. The highest Li storage composite can be LiC0.75H0.38 which corresponds to a specific capacity of 2857.7 mAh/g. Both specific capacity and binding energy are significantly larger than the corresponding value of graphite, this makes PG a

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promising candidate for the anode material in battery applications. The interactions between the Li atoms and PG can be easily tuned by an applied strain. Under biaxial strain of 16%, the binding energy of Li to PG is increased by 17% compared to its unstrained state.

Keywords: Graphene-like sheet; Li-ion battery anode; Li storage

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1. Introduction The development of portable and telecommunication electronic devices calls for efficient energy storage systems. Li-ion battery (LIB) is considered as one of the most promising energy storage

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applications owing to its higher efficiency towards energy conversion and storage[1, 2]. However, conventional LIBs based on graphite anodes with theoretical specific capacity of 372 mAh/g cannot

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satisfy the growing demand for high-energy storage systems[3-7]. Therefore, great effort is being

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expended on finding higher specific capacity anode materials. For LIBs anode materials, low oxidation-reduction potentials, high rate Li diffusivity, and stable mechanical structure are necessary.

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Moreover, the binding energy of Li atoms should be in the energy window of ~1.6-3.0 eV[8]. If the

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binding energy is less than ~1.6 eV, the cohesive energy of bulk Li, Li atoms may tend to cluster and also the specific capacity is low over extended cycling[9, 10]. If the binding energy is larger than ~3 eV, it

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does not work as an anode. Therefore, to achieve excellent anode materials, lots of efforts have been made to explore appropriate new LIB anode materials, such as fullerenes[11], carbon nanotubes[12-14],

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graphene[15-18], graphyne[7, 8, 19], silicene[20], and so on. Since the first experimental realization of graphene, other free-standing 2D materials have also been

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proposed and synthesized. Porous graphene (PG), as a collection graphene-related materials with repeat missed carbon ring that is terminated by hydrogen bonds, has been recently synthesized in experiment[21]. As well as the experimental investigations, several theoretical works have proposed PG as a hydrogen purification membrane and hydrogen storage material [22-26]. On the analogy of the Li-intercalated graphite, PG may be also a candidate for anode materials in LIBs due to its large surface area. The porous structure and semiconducting features may open up exciting opportunities for more efficient LIBs. Given

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that the specific surface area of PG is even larger than that of graphene, higher storage capacity may be achievable. More important, with single layer structure, excellent mobility can be offered by PG, this is crucial for the charging times of the LIB. In this paper, we have used density functional theory (DFT)

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method to study the adsorption and diffusion of Li atom on the PG. As shown below, the high storage capacity and high Li mobility indicate that PG may offer excellent performance as a promising candidate

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for the anode material of LIB.

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2. Computational Details

All calculations were carried out using density functional theory in the framework of generalized

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gradient approximation (GGA) in Perdew-Burke-Ernzerh of parametrization[27] with periodic boundary

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conditions using Vienna ab initio simulation package (VASP)[28, 29]. Since the van der Waals (vdW) interaction plays a crucial role in the adsorption of adatoms on the substrate, the vdW correction was

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taken into account in our calculations through employing DFT-D2 method of Grimme[30]. Two-dimensional periodic boundary conditions were applied to the PG, while a vacuum region of 17 Å

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was applied in the direction perpendicular to the PG to remove spurious interactions between image structures. An energy cutoff of 450 eV is adopted for the plan-wave expansion of the electronic wave

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function. The Brillouin zone has been sampled using the automatically generated 6 × 6 × 1 Monkhorst-Pack set of k-points[31]. The convergence criteria of total energy was chosen to be 10-4 eV between two consecutive steps, and the maximum Hellmann-Feynman force acting on each atom was less than 0.01 eV/Å upon ionic relaxation. To obtain diffusion pathways and the corresponding energy barriers, we performed standard nudged elastic band (NEB) calculations[32]. 3. Results and Discussion

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We first relaxed a unit cell of PG as shown in Fig. 1. The optimized lattice constant is 7.51 Å, in good agreement with other DFT calculations and experiment [21, 23]. We systematically decorated PG with Li atoms. As shown in Fig. 1, there are six different adsorption sites, which are the hollow center of

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the C hexagon (H1), the hollow center of C-H hexagon (H2), the hollow center of the H hexagon (H3), the bridge of C-C bond in the same C hexagon(B1), the bridge of C-C bond between two neighboring C

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hexagons(B2), and the top of the C atoms(T). After full relaxation, it is found that the Li atom over the

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H1 site is energetically favorable compared to other sites. Here the binding energy of Li atoms is defined

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as

(1)

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where n is the number of attached Li atoms per unit cell, the first, second and third term in the parenthesis of the right hand side represent the total energy of the PG with n adsorbed Li atoms, the energy of a free

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Li atom in a vacuum, and the energy of the pristine PG, respectively. In the case of single Li atom decorated PG, the binding energy is 1.62 eV/Li. The distance between the Li atom and the nearest C

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atoms is ~2.25 Å, and the distance between Li atom and the PG plane is 1.75 Å (see Fig. 2). To understand the interaction between the Li atom and the PG, we have calculated the partial density of

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states (PDOS) of Li/PG and PG and shown them in Fig. 3. From Fig. 3, we can see that the p orbitals of Li atom participate in the bonding and there is obvious hybridization between Li and C atoms. The Li atom donates its s electrons to the PG so that the p orbitals of C atom are partially filled, this is accord with the fact that the C p orbitals of the PG shift toward the lower energy region after the Li decoration as shown in Fig. 3b and 3c. Meanwhile, the empty p orbitals of the Li atom will split under the strong ligand field generated by the PG, and thus, the PG back donates some electrons to the low-lying Li p orbitals,

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resulting in s-p and p-p hybridization between Li and C. As a result, the Li adsorption on PG is energetically more preferable than that on graphene owing to the hybridization between Li and C atoms. This bonding mechanism can also be observed in the case of Li binding onto zigzag boron nanotubes and

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boron sheets[33, 34]. We also calculated the charge transfer character between Li and PG to illustrate the binding between the Li atom and PG. Fig. 2c and 2d shows the three-dimensional electron charge density

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denote the total electron densities of the relaxed PG system with and without

Li adatom, respectively, and

is the total electron density of the Li. The red and blue colors

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where

(2)

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differences for Li decorated PG, which were obtained from the formula

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represent charge accumulation and depletion. It is readily seen that the charges have transferred from Li to PG because the charges are mainly concentrated in the region between Li and PG whereas few charges

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are on top of Li. In addition, bader charge analyses have also been carried out and showed that Li atom denotes 0.874 electrons to the PG[35], indicating the ionic interaction between Li and C atoms of PG.

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Considering the importance of Li mobility in improving the performance of PG used in LIBs, we next shift our attention to the motion of a Li atom on PG. We investigate the diffusion of a single Li atom

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by calculating the Li diffusion barrier as Li moves between neighboring adsorption sites on PG. The diffusion paths and the corresponding energy profile are shown in Fig. 4. As has been shown before, H1 is the most stable adsorption site where Li resides at the hollow site with six nearest neighbor C atoms as shown in Fig. 2a. As Li moves between two neighboring H1 sties, it passes over two C atoms by overcoming an energy barrier of 0.40 eV (Fig. 4a). In the case of Li diffusion from H1 to H2, two diffusion paths are considered: (a) H1-T1-H2 and (b) H1-B1-H2, as shown in Fig. 4b. The diffusion

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barriers are 0.39 eV and 0.37 eV, respectively. The above diffusion barriers are smaller than or comparable to the energy barriers for Li diffusion in graphene and graphdiyne[36, 37]. However, the energy barrier for the Li diffusion along the H2→H3 direction is 0.92 eV (see Fig. 3c), due to the

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energetic favorability of the H2 site over the H3 site. In view of the energetic stability and kinetics, the in-plane Li diffusion on PG is mainly dominated by the paths of H1→H1 and H1→H2.

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The diameter of hydrogen hexagons, i.e., H3 sites, is 3.74 Å (measured from the two opposite

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hydrogen atoms in the same hydrogen hexagons). The size of the large hexagon is larger than that in graphene, which may facilitate the Li diffusion in PG. Therefore, we also explored the Li diffusion from

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one side of a PG layer to another side along the H3-H3 direction perpendicular to the PG plane. The

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results revealed that Li can easily pass through the hydrogen hexagon by overcoming a small energy barrier of only 0.19 eV. Therefore, it is reasonable to expect that the PG can provide excellent mobility

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for Li by in-plane diffusion. Once the Li atom moves to the H3 site, it can easily migrate to another opposite H3 site, i.e., out-of-plane diffusion.

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To examine the maximum Li loading capacity in PG, more Li atoms are introduced on PG. On the basis of the results of the single Li atom decorated PG, one can find that the H1 site is the most stable

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adsorption site. Therefore, we next study the structure of 4Li/PG with two Li atoms adsorbed above and two below the H1 sites of PG as shown in Fig. 5a. In the following discussions, we used the notation LiCxH0.5x to distinguish Li-decorated PG with different number of Li atoms. The structure of 4Li/PG corresponds to LiC3H1.5 with an average binding energy of Eb=1.67 eV/Li. We also calculated the specific capacities using following formula[19, 38]: (3)

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where m is the number of Li atoms, F is the Faraday constant (taken as 96500 mA h) and M is the mass of the molecular weight of PG unit in kg. The specific capacity of LiC3H1.5 is 714.8 mAh/g which is much larger than the conventional graphitic electrode capacity of ~372 mAh/g and falls in the acceptable range

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of gravimetric capacity for practical applications of battery electrode material. Besides the H1 site, H2 site is also possible adsorption site, because the binding energy of Li residing

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above the H2 site (1.45 eV/Li) is only slightly smaller than that above the H1 site (1.62 eV/Li). We next

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increase the number of Li atoms by placing the Li atoms on H2 sites. The full loaded case is the structure with 16 Li atoms per unit cell of 12 C and 6 H atoms where each H1 and H2 cavity can hold two Li atoms

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which are above and below PG plane respectively. The structures with 6, 8, 10, 12, 14, 16 Li atoms

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adsorbed on each unit cell are shown in Fig. 5b-5g respectively. The average binding energy and the corresponding specific capacity for different systems are listed in Table 1. It is clear that as the number of

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adsorbed Li atoms increases, the average binding energy of Li atoms is slightly increased. The specific capacities for the PG with 4-16 Li atoms per unit cell are all much larger than that of the conventional

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graphitic electrode. Especially, the composition LiC0.75H0.38 has a highest specific capacity of 2857.7 mAh/g. These results show that PG can be used to design an efficient anode material for LIBs.

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Previous reports have showed that external strain may influence the electronic, magnetic and hydrogen storage properties of various two dimensional nanomaterials [39-41]. Therefore, we have attempted to deal with the strain-induced stabilization of lithiated PG under the application of mechanical biaxial strain. Here the biaxial strain is defined as (a-a0)/a0, where a0 and a are the theoretical lattice constants of the unstrained and strained systems, respectively. Fig. 6 shows the calculated binding energy of 16Li/PG system under different biaxial strain ranging from -8% to 16%. When the biaxial strain is out

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of the range, the lithiated PG would crash. It is found that the binding energy can be modulated effectively by the biaxial strain. A stronger Li-PG binding is achieved by applying biaxial strain, whereas a weaker binding is achieved by applying compressive strain. Under biaxial strain of 16%, the binding

binding energy has the minimum 1.58 eV/Li at the compressive strain of -8%.

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4. Conclusion

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energy of Li to PG is increased by 17% (2.11 eV/Li) compared to its unstrained state (1.7 eV/Li). The

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In summary, by first-principles calculation we have shown that the porous graphene has good applications in high capacity LIBs anodes. We systematically investigated the adsorption and diffusion

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of Li on PG. It has been demonstrated that, due to the hybridization between Li and the carbon atoms,

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the binding energy of Li to PG is larger than the cohesive energy of Li, which has solved the problem of using single layer material as a Li storage material: the formation of the Li cluster. The highest Li

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storage capacity can reach up to 2857.7 mAh/g with an average binding energy of 1.80 eV/Li, which shows small variation with respect to Li content. The Li diffusion barriers are in the range 0.37-0.39 eV,

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which are relatively low. The binding energy of Li to PG can be easily tuned by an applied strain. Under biaxial strain, the interactions between the Li atoms and PG are enhanced. Once a compressive strain is

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applied, the interactions will become weaker.

Acknowledgements

The work was support by the NSF of China (Grant Nos. 11404112, 11104072, 11447155) and Research in Cutting-edge Technologies of Zhengzhou (Grant No. 141PRKXF622).

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Figure Captions: Fig. 1. The unit cell and optimized atomic structure corresponding to porous graphene. Fig. 2. Electron charge density differences with an isovalue of 0.002 e/Å3 for Li/PG: (a) Top view, (b)

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Side view. The red and blue iso-surface indicates space charge accumulation and depletion respextively.

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Fig. 3. (a) PDOS of the Li atom in Li/PG. (b) PDOS of the C atom in Li/PG. (c) PDOS of the C atom in

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PG. Fermi level is set to zero.

Fig. 4. Diffusion pathway for a single Li atom on PG: (a) H1-T1-T2-H1, (b) H1-T1-H2-H3 and

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H1-B1-H2, (c) H3-H3.

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Fig. 5. Schematics of PG with different number of Li atoms. Panels (a)~(g) corresponding 4, 6, 8, 10, 12, 14, 16 Li atoms adsorption cases, respectively.

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Fig. 6. The average binding energies as a function of the biaxial strain for the 16Li/PG system. The

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biaxial strain is described as an insert.

Table 1 The specific capacity and the corresponding binding energy for different systems.

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Table

Eb(eV/Li)

LiC12H6 LiC3H1.5 LiC2H1 LiC1.5H0.75 LiC1.2H0.6 LiC1H0.5 LiC0.86H0.43 LiC0.75H0.38

178.7 714.8 1072.2 1429.6 1787.0 2144.4 2493.5 2857.7

1.62 1.67 1.77 1.82 1.79 1.80 1.80 1.80

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specific capacity (mAh/g)

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system

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Number of Li atoms per unit cell 1 4 6 8 10 12 14 16

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Table 1 The specific capacity and the corresponding binding energy for different systems.

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