DOI: 10.1002/cphc.201402675
Articles
A First-Principles Study of Lithium Adsorption on a Graphene–Fullerene Nanohybrid System Wonsang Koh,[b] Hye Sook Moon,[a] Seung Geol Lee,*[a] Ji Ii Choi,[c] and Seung Soon Jang*[d] The mechanism of Li adsorption on a graphene–fullerene (graphene–C60) hybrid system has been investigated using density functional theory (DFT). The adsorption energy for Li atoms on the graphene–C60 hybrid system (2.285 eV) is found to be higher than that on bare graphene (1.375 eV), indicating that the Li adsorption on the former system is more stable than on the latter. This is attributed to the high affinity of Li atoms to C60 and the charge redistribution that occurs after graphene is
mixed with C60. The electronic properties of the graphene–C60 system such as band structure, density of states, and charge distribution have been characterized as a function of the number of Li atoms adsorbed in comparison to those of the pure graphene and C60. Li adsorption is found to preferentially occur on the C60 side due to the high adsorption energy of Li on C60, which imparts a metallic character to the C60 in the graphene–C60 hybrid system.
1. Introduction In the field of secondary rechargeable Li batteries, graphite has been widely adopted as the anode material and it exhibits a maximum specific insertion capacity of 372 mAh g1, corresponding to the formation of LiC6. However, it is expected that specific capacity values as high as 1100 mAh g1 can be achieved if graphene is used instead of graphite, since Li can be stored both on the surface as well as on the edges in the case of graphene.[1] In this context, the Li adsorption capacities of various forms of graphene such as graphene powder, nanoribbons, and nanosheets have been studied both theoretically[2] and experimentally[3] with the goal of employing graphene as electrode materials. Yoo et al.[3b] prepared various hybrid structures based on graphene nanosheets (GNSs), such as GNS/ carbon nanotubes (GNS/CNTs), and GNS/C60 systems, and measured their lithium insertion and extraction properties. Their results showed that the specific capacities of GNSs (540 mAh g1), GNS/CNTs (730 mAh g1), and GNS/C60 (784 mAh g1) were higher than that of graphite (372 mAh g1), which may be due to the increased d spacing (distance be-
tween the layers) in the case of GNS hybrid systems compared to graphite. In addition to its applications in the field of Li batteries, the graphene–C60 hybrid system can also be applied to other electrochemical fields such as thermoelectric power generation and photocurrent generation.[4] Zhang et al.[4a] synthesized and characterized a hierarchical graphene–C60 nanohybrid system for applications in high-performance organic thermoelectric materials, which play an important role in power generation and solid-state cooling and heating systems. They reported that graphene improves the electrical conductivity of C60, resulting in a synergistic effect on enhancing thermal/electric transport in the nanohybrid system. Umeyama et al.[4b] also reported that the incorporation of graphene–C60 hybrid systems in the fabrication of photoelectrochemical devices can improve the efficiency of charge transfer to the electrodes. These results indicate that the strategy of integrating graphene–C60 hybrid systems could be used as a new method for the preparation of high-performance materials. However, there have been only a few systematic studies focused on understanding the effect of integrating hybrid systems on the electronic properties of the resultant hybrid material at an atomistic level.[5] In this study, we have investigated the Li adsorption capabilities of the graphene–C60 hybrid system. The Li atoms are bound to the graphene–C60 hybrid system through charge transfer interaction. Owing to the higher electron affinity of C60 compared to graphene, C60 acts as the electron acceptor, whereas graphene acts as the charge transport channel throughout the electrode. Therefore, Li adsorption on the graphene–C60 electrode is expected to be more favorable than on a pure graphene-based electrode because of the higher electron affinity of C60. Herein, we have used first-principles computational methods to investigate the electrochemical characteristics, such as Li adsorption capabilities and charge transfer, of the graphene–C60 hybrid system. We calculated the Li ad-
[a] H. S. Moon, Prof. S. G. Lee Department of Organic Material Science and Engineering Pusan National University Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 609-735 (Korea) E-mail: [email protected] [b] Dr. W. Koh School of Physics Georgia Institute of Technology 837 State Street, Atlanta, GA 30332-0430 (USA) [c] Dr. J. I. Choi Graduate School of EEWS Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu, Daejeon 305-701 (Korea) [d] Prof. S. S. Jang School of Materials Science and Engineering Georgia Institute of Technology 771 Ferst Drive NW, Atlanta GA 30332-0245 (USA) E-mail: [email protected]
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Articles sorption energies on the graphene–C60 hybrid system and analyzed the accompanying changes in its electronic properties such as band structure, density of states (DOS), and charge distribution as a function of the number of Li atoms adsorbed using density functional theory (DFT). In addition, based on the Li adsorption energies in the various regions around the graphene–C60 hybrid system, we also discussed the mechanism of Li adsorption in comparison with the Li cluster formation.
Table 1. Adsorption energy and charge distribution (Mulliken charge) for single Li atom adsorption on the graphene–C60 hybrid system. System
graphene–C60 hybrid 1 Li on graphene (center of hexagon) 1 Li on C60 (pentagon) Pos1 of graphene side (region 1) Pos2 of graphene side (region 1) Pos1 of C60 side (region 2, hexa) Pos2 of C60 side (region 2, penta) Pos3 of C60 side (region 3, hexa) Pos4 of C60 side (region 3, penta) Pos5 of C60 side (region 4, hexa) Pos6 of C60 side (region 4, penta)
2. Results and Discussion The geometry of the graphene–C60 hybrid system was fully optimized without any fixed atoms on graphene or C60 to assess its capability for Li adsorption. Figure 1 shows the top, side,
Adsorption Energy C60 [eV]
Charges [e] Li
graphene
C60
0.720 (C60 Binding Energy) 1.375 (1.096)[2b]
N/A
0.095
0.095
0.813
0.813
N/A
1.838 (1.820)[7] 1.769
0.794
N/A
0.794
0.860
0.434
0.426
2.285
0.894
0.264
0.630
1.059
1.002
0.297
0.705
2.285
0.895
0.259
0.636
1.960
0.845
0.037
0.882
2.122
0.854
0.007
0.847
1.798
0.794
0.025
0.819
1.862
0.791
0.032
0.823
Figure 1. Unit cell structure of the graphene–C60 hybrid system: a) top view, b) side view, and c) expanded (2 2) view.
and the expanded view of the hybrid system, which was maintained by dispersion interactions between graphene and C60. The distance between the graphene and the center of the C60 was set to 6.35 in the hybrid structure, which was employed from a previously reported experimental value.[6] The binding energy and the charge transfer from graphene to C60 calculated through the Mulliken analysis is summarized in Table 1 and Figure 2. The Li adsorption energies were calculated using Equation (1): E binding ¼ ðE hybrid systemþLi E hybrid system n E Li Þ=n
ð1Þ
where Ehybrid system + Li is the total energy of the hybrid structure containing the adsorbed Li atoms, Ehybrid system is the total energy of the graphene–C60 hybrid system without Li atoms, and ELi is the energy of the Li atoms. As shown in the Table 1, it appears that the hybrid structure is more favorable in terms of the binding energy (0.720 eV) with the considerable charge transfer (j 0.095 j e). The charge distribution of the
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Figure 2. Structure of the graphene–C60 hybrid system with various regions for adsorption: a) front view, b) side view (regions 1, 2, 3, and 4 are in red, yellow, blue, and orange, respectively), c) the initial structure, and d) the optimized structure of a single Li atom adsorption at various positions around a graphene–C60 hybrid system.
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Articles phene system (adsorption energy of 1.375 eV). This enhanced adsorption can be due to the increased charge transfer from Li atom to the hybrid system. Some of the charges (j 0.095 j e) were already transferred from the graphene to the C60, rendering the graphene positively charged. As a result, additional charge transfer from the adsorbed Li may occur to a greater extent in the graphene region of the hybrid, thereby resulting in enhanced adsorption. At the same time, a Li atom in region 1 can still interact with C60 due to its proximity to C60 in the hybrid system. Referring to the positions shown in Figure 2, the Li atom at Pos2 in the graphene region moved to the same position as the Li atom in Pos2 of the C60 region after optimization, although the initial positions of these two atoms were different. Figure 3. Band structure (or energy level) of a) graphene, b) graphene–C60 hybrid, and c) the density of states (DOS) of graphene and the graphene–C60 hybrid. The Li adsorption energy shows lower values in the region between graphene and C60 representations, the Fermi levels are shifted to 0 eV. Since the (region 2; 2.285 eV) or between two C60s (region 3; graphene–C60 hybrid system is maintained through dispersion 2.122 eV). However, we found that the Li adsorption energy in region 4 is similar to that in pure C60 (1.798 eV and interactions, the hybrid system retains the characteristics of its components. In Figure 3 b, therefore, it is shown that the 1.862 eV, respectively), even though this value was still lower hybrid system keeps the graphene-like characteristics around than that (1.769 eV) of region 1. From these results, it apthe Fermi level, while it is thought that two other bands at pears that the Li adsorption is related to the charge transfer 0.5 eV are attributed to the t1u state of the neighboring C60 driven by the high electron affinity of C60 in the hybrid system. The corresponding band structures of the single Li adsorbed chain.[8] This is also observed in the density of states (DOS) of hybrid system are presented in Figure 4. While the band structhe hybrid system (Figure 3 c). The overall features in the DOS tures of the hybrid system did not change in comparison to of the graphene–C60 hybrid system seem to be a composite of the pure systems, the bands shifted downwards to Fermi level those of the components. when the Li atom was adsorbed on the C60 side of the hybrid Next, we studied the adsorption of single Li atoms at various system. positions on the graphene–C60 hybrid system. A Li atom was placed in the center of the hexagon site of graphene and the Next, we added a second Li atom, in order to investigate the pentagon or hexagon site of C60, since the most stable Li adadsorption mechanism of multiple Li atoms. Since the energy sorption on C60 can occur at these sites.[9] density is proportional to the number of Li atoms, it is imporIn order to systematically describe the Li adsorption mechatant to efficiently utilize the surface available on the granism on the graphene–C60 hybrid system, we defined four disphene–C60 electrode for adsorption rather than forming Li clustinct regions around the graphene–C60 hybrid system as ters by covering all the carbon rings of the graphene–C60 shown in Figure 2, namely, the graphene side (region 1, shown system with Li atoms. For this purpose, we calculated the adin red), the region between graphene and C60 (region 2, shown sorption of the second Li atom (blue or green) following the in yellow), the region between two C60s (region 3, shown in first Li atom (purple) in each region of the hybrid system as presented in Figure 5. The second Li atom was positioned at blue), and the C60 side (region 4, shown in orange). The Li either the nearest-neighboring (N.N.) site or the next-nearestatom can interact only with graphene in region 1 or only with neighboring (N.N.N.) site on the graphene side of the hybrid C60 in regions 3 and 4, whereas the Li atom can interact with system (Figure 5 a), while on the C60 side of the hybrid, the both graphene and C60 simultaneously in region 2. A single Li atom was placed at various positions in each second Li atom was adsorbed on the pentagonal or hexagonal region of the graphene–C60 hybrid system as shown in Figring in the radial or axial direction on the C60 to form the N.N. ure 2 c,d. The adsorption energy and the charge distribution of or the N.N.N. configuration (Figure 5 b). the single Li-adsorbed hybrid system are also summarized in The initial and optimized structures with two adsorbed Li Table 1. From Table 1, we observe that Li adsorption on the atoms in such various regions on the graphene–C60 system are graphene region of the hybrid system (adsorption energy of presented in Figure 6. Additionally, the Li adsorption energies 1.769 eV) was enhanced compared to that on a pure graof the two Li atoms are listed in Table 2. system calculated through the Mulliken population analysis showed a transfer of charge from graphene to C60 due to the relatively strong electron affinity of C60. The band structures of graphene and the graphene–C60 hybrid system are shown in Figure 3. In these band structure
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Figure 4. Band structures of the single-Li-adsorbed graphene–C60 hybrid system. The Li atom is adsorbed at various positions such as a) Pos1 on the graphene side, b) Pos2 on the graphene side, c) Pos1 on the C60 side, d) Pos2 on the C60 side, e) Pos3 on the C60 side, f) Pos4 on the C60 side, g) Pos5 on the C60 side, and h) Pos6 on the C60 side.
side (region 2) to another site in the C60 side or on the graphene side, forming a N.N.N. scheme. The adsorption energy on the C60 side (2.154 eV) was lower than that on the graphene side (1.888 eV) as expected, since C60 usually provides stronger adsorption sites owing to its strong electron affinity. Therefore, we confirm again that Li will cover the C60 surface first. The adsorption energy in region 3 (between the C60 regions, Figure 6 c) was also low, ranging from 1.891 eV to 2.066 eV due to the strong electron affinity of C60. It appears that the adsorption energy for the N.N.N. scheme is slightly low although the adsorption energy values are similar in the other directions. In region 4, the adsorption energy was calculated to be 1.971 eV and 1.899 eV for the pentagonal site (Figure 6 d, N.N.N. site) and the hexagonal site (N.N. site), respectively, which suggests that the Li adsorption will preferentially take
Table 2. Adsorption energy for the adsorption of two Li atoms on the graphene–C60 hybrid system.
Figure 5. Definition of the direction of adsorption of the second Li atom in the case of two Li atom adsorption on the hybrid system around a) graphene and b) C60. The first and second Li atoms are in purple and blue/ green, respectively.
In region 1 of the hybrid system (Figure 6 a), the adsorption energy is the same for both the N.N.N. and N.N. configurations since the second Li atoms in both the cases moved to the C60 side to have similar distance from C60. The adsorption energy was the lowest (Figure 6 b, 2.154 eV– 2.178 eV) in region 2 where Li atom can interact with both graphene and C60 together. The two Li atoms in region 2 had similar adsorption energy, regardless of the adsorption sites in region 2, whereas this was not the case in the other regions. We also examined the adsorption direction by comparing the adsorption energy of the system with two Li atoms, starting from Pos2 in the C60
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System
Adsorption Energy [eV]
2 Li on region 1: N.N.N. site 2 Li on region 1: N.N. site 2 Li on region 2: Axial (N.N.N. site) 2 Li on region 2: Axial (N.N. site) 2 Li on region 2: Radial (N.N.N. site) 2 Li on region 2 to region 1: N.N.N. site 2 Li on region 2 to region 1: N.N. site 2 Li on region 3: Axial (N.N.N. site) 2 Li on region 3: Axial (N.N. site) 2 Li on region 3 to region 2: N.N.N. site 2 Li on region 3 to region 2:N.N. site 2 Li on region 3 to region 4: N.N.N. site 2 Li on region 3 to region 4:N.N. site 2 Li on region 4: Radial (N.N.N. site) 2 Li on region 4: Radial (N.N. site) 8 Li atoms on graphene@hybrid 12 Li atoms on C60@hybrid 20 Li atoms on graphene–C60 hybrid LiLi
1.874 1.873 2.168 2.155 2.178 1.888 2.154 1.986 1.920 1.559 2.066 1.969 1.891 1.971 1.899 1.511 1.779 1.624 1.030[10]
place on the pentagonal (N.N.N.) sites of C60. Further, it appears that Li adsorption preferentially occurs on the N.N.N. scheme and the adsorption mechanism is strongly influenced by C60. 4
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Articles Finally, we investigated the adsorption of multiple Li atoms (such as 8, 12 and 20) on the hybrid system forming the N.N.N scheme. Pure graphene and C60 with the same number of Li atoms were also considered as references. The initial and optimized structures are presented in Figure 8, and the Li adsorption energies are listed in Table 2. From the optimized structures, we observed that while the Li atoms initially on the graphene of the hybrid system (Figure 8 a) move towards the C60 side, the Li atoms around C60 retained their positions during the geometry optimization (Figure 8 b and 8c). The adsorption energies for the adsorption of multiple Li atoms are 1.511 eV, 1.776 eV, and 1.621 eV for the graphene side, the C60 side, and for the overall hybrid system, respectively. These adsorption energies indicate that Li adsorption may predominantly take place on the C60 side first and then subsequently on the graphene side, similar to the observations in the case of two Li atom adsorption. Even though the Li adsorption energy Figure 6. Initial and optimized structures of the graphene–C60 hybrid system with two Li atoms adsorbed in different regions: a) region 1, b) region 2, c) region 3, and d) region 4. The first and second Li atoms are in purple and decreases upon increasing the blue/green, respectively. number of Li atoms, all the adsorption energies observed are lower than the LiLi binding energy (1.030 eV).[10] Please note Therefore, we think that Li adsorption will start taking place from C60 and proceed towards graphene. that, since this investigation is based on DFT calculation focusThe corresponding changes in the band structures of the systems with two adsorbed Li atoms were also analyzed in each region of the graphene–C60 hybrid system, as shown in Figure 7. A significant band shift was observed whenever an additional Li atom was added to the hybrid system compared to single Li atom adsorption. We think that the bands in close proximity to the additional Li atom were mainly affected and shifted down through increased Fermi levels as electrons were injected from the Li atoms into Figure 7. Band structures of the graphene–C60 hybrid system with two adsorbed Li atoms in various regions: the system. a) region 1, b) region 2, c) region 3, and d) region 4. ChemPhysChem 0000, 00, 0 – 0
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Figure 8. Initial and optimized structures of the graphene–C60 hybrid system with multiple Li atoms adsorbed: a) 8 Li atoms on the graphene side, b) 12 Li atoms on the C60 side, and c) 20 Li atoms adsorbed on the entire hybrid system.
ing on the ground state of the hybrid system, our study does not consider intermediate states kinetically generated during the operating process. In addition, the binding energy is still lower than that of the pure graphene (1.086 eV) and C60 (1.594 eV) systems having the same number of Li atoms, respectively. Therefore, from the point of view of Li adsorption, the graphene–C60 hybrid system appears to be promising for potential use as an electrode in Li batteries. Figures 9 a–c show the band structure of the graphene–C60 hybrid system with multiple Li atoms adsorbed. The band structure reveals that the number of available energy bands around the Fermi level increased significantly in the graphene– C60 system, indicating that the Li adsorption enhances the metallic characteristics of the system such as electrical conductivity. This result is confirmed by the DOS analysis as a function of the number of Li atoms adsorbed. In Figure 9 d, the hybrid system containing adsorbed Li atoms has more DOS around the Fermi level, especially after more than two Li atoms are adsorbed in comparison to the graphene–C60 hybrid system without Li adsorption. Such enhanced metallic character of the graphene–C60 system may also enhance the system’s electron
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Figure 9. Band structure corresponding to the adsorption of multiple Li atoms on the graphene–C60 hybrid system: a) 8 Li atoms on the graphene side, b) 12 Li atoms on the C60 side, c) 20 Li atoms adsorbed overall on the hybrid system, d) density of states for various number of Li atoms adsorbed on the graphene–C60 hybrid system, and e) density of states for multiple Li atoms adsorbed onto different sides of the hybrid system.
transport properties. Finally, we compared the DOS of the graphene–C60 hybrid system with that of pure graphene or C60 systems in order to examine the effect of Li adsorption on the electronic structure of the hybrid system. As shown in Figure 9 e, the DOS of the hybrid system retains more electrons around the Fermi level compared to pristine graphene and C60, inferring that the hybrid system with adsorbed Li atoms would have higher electron conductivity than the pure graphene or C60 systems have.
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Articles 3. Conclusions
Keywords: adsorption · anodes · density functional theory calculations · graphene-fullerene hybrids · lithium batteries
We investigated the adsorption of Li on the graphene–C60 hybrid system using the density functional theory (DFT). It was found that although the hybrid system retains the characteristics of its components in its electronic structure, the charges are transferred from the graphene to the C60 in the graphene– C60 hybrid system, leaving the graphene positively charged (+ 0.095 e) and the C60 negatively charged (0.095 e). It was also found that the adsorption of Li onto the hybrid system was enhanced compared to the pure graphene system, which would be due to the charge transfer from graphene to C60. Furthermore, by analyzing the Li adsorption in various regions of the graphene–C60 system, it was determined that the Li adsorption is likely to preferentially occur on the C60 side, particularly in the midway region between graphene and C60 or between two C60s. Although there is no significant change in the band structure of the graphene–C60 system after one Li adsorption, the adsorption of additional Li atoms cause the energy bands to shift downwards as a result of electron injection from the adsorbed Li atoms into the hybrid system. The DOS analysis in the hybrid system also indicates that the metallic character of the graphene–C60 system is increased with increasing the number of adsorbed Li atoms. Therefore, the graphene–C60 hybrid system is expected to demonstrate enhanced conductive properties in addition to excellent Li adsorption capabilities compared to the pure graphene system.
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Computational Details We used a generalized gradient approximation (GGA) Perdew– Burke–Ernzerhof (PBE) functional[11] with DFT-D3 correction[12] in DMol3[13] with DND basis set for all the DFT calculations. The GGAPBE functional has been used to accurately describe the interactions between an adsorbate and various surfaces[14] including sp2carbon-based materials.[5, 15] The dimensions of the unit cell used in this study were 12.3 12.3 35 , which was large enough to ensure that there is no direct interaction between the original structure and its self-image in the c-axis through the periodic boundary. The dimensions in the a- and b-axes were determined from the area of the graphene. The structure of this graphene–C60 hybrid system is presented in Figure 1. The k-point samplings for the Brillouin zone were performed using a 4 4 1 Monkhorst– Pack k-point mesh.[16]
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014R1A1A1004096). This research was supported by Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078881).
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Received: September 27, 2014 Revised: November 9, 2014 Published online on && &&, 2014
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ARTICLES W. Koh, H. S. Moon, S. G. Lee,* J. I. Choi, S. S. Jang*
The mechanism of Li adsorption on a graphene–fullerene nanohybrid system is investigated by density functional theory.
&& – && A First-Principles Study of Lithium Adsorption on a Graphene–Fullerene Nanohybrid System
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A First-Principles Study of Lithium Adsorption on a Graphene–Fullerene Nanohybrid System Wonsang Koh,[b] Hye Sook Moon,[a] Seung Geol Lee,*[a] Ji Ii Choi,[c] and Seung Soon Jang*[d] The mechanism of Li adsorption on a graphene–fullerene (graphene–C60) hybrid system has been investigated using density functional theory (DFT). The adsorption energy for Li atoms on the graphene–C60 hybrid system (2.285 eV) is found to be higher than that on bare graphene (1.375 eV), indicating that the Li adsorption on the former system is more stable than on the latter. This is attributed to the high affinity of Li atoms to C60 and the charge redistribution that occurs after graphene is
mixed with C60. The electronic properties of the graphene–C60 system such as band structure, density of states, and charge distribution have been characterized as a function of the number of Li atoms adsorbed in comparison to those of the pure graphene and C60. Li adsorption is found to preferentially occur on the C60 side due to the high adsorption energy of Li on C60, which imparts a metallic character to the C60 in the graphene–C60 hybrid system.
1. Introduction In the field of secondary rechargeable Li batteries, graphite has been widely adopted as the anode material and it exhibits a maximum specific insertion capacity of 372 mAh g1, corresponding to the formation of LiC6. However, it is expected that specific capacity values as high as 1100 mAh g1 can be achieved if graphene is used instead of graphite, since Li can be stored both on the surface as well as on the edges in the case of graphene.[1] In this context, the Li adsorption capacities of various forms of graphene such as graphene powder, nanoribbons, and nanosheets have been studied both theoretically[2] and experimentally[3] with the goal of employing graphene as electrode materials. Yoo et al.[3b] prepared various hybrid structures based on graphene nanosheets (GNSs), such as GNS/ carbon nanotubes (GNS/CNTs), and GNS/C60 systems, and measured their lithium insertion and extraction properties. Their results showed that the specific capacities of GNSs (540 mAh g1), GNS/CNTs (730 mAh g1), and GNS/C60 (784 mAh g1) were higher than that of graphite (372 mAh g1), which may be due to the increased d spacing (distance be-
tween the layers) in the case of GNS hybrid systems compared to graphite. In addition to its applications in the field of Li batteries, the graphene–C60 hybrid system can also be applied to other electrochemical fields such as thermoelectric power generation and photocurrent generation.[4] Zhang et al.[4a] synthesized and characterized a hierarchical graphene–C60 nanohybrid system for applications in high-performance organic thermoelectric materials, which play an important role in power generation and solid-state cooling and heating systems. They reported that graphene improves the electrical conductivity of C60, resulting in a synergistic effect on enhancing thermal/electric transport in the nanohybrid system. Umeyama et al.[4b] also reported that the incorporation of graphene–C60 hybrid systems in the fabrication of photoelectrochemical devices can improve the efficiency of charge transfer to the electrodes. These results indicate that the strategy of integrating graphene–C60 hybrid systems could be used as a new method for the preparation of high-performance materials. However, there have been only a few systematic studies focused on understanding the effect of integrating hybrid systems on the electronic properties of the resultant hybrid material at an atomistic level.[5] In this study, we have investigated the Li adsorption capabilities of the graphene–C60 hybrid system. The Li atoms are bound to the graphene–C60 hybrid system through charge transfer interaction. Owing to the higher electron affinity of C60 compared to graphene, C60 acts as the electron acceptor, whereas graphene acts as the charge transport channel throughout the electrode. Therefore, Li adsorption on the graphene–C60 electrode is expected to be more favorable than on a pure graphene-based electrode because of the higher electron affinity of C60. Herein, we have used first-principles computational methods to investigate the electrochemical characteristics, such as Li adsorption capabilities and charge transfer, of the graphene–C60 hybrid system. We calculated the Li ad-
[a] H. S. Moon, Prof. S. G. Lee Department of Organic Material Science and Engineering Pusan National University Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 609-735 (Korea) E-mail: [email protected] [b] Dr. W. Koh School of Physics Georgia Institute of Technology 837 State Street, Atlanta, GA 30332-0430 (USA) [c] Dr. J. I. Choi Graduate School of EEWS Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu, Daejeon 305-701 (Korea) [d] Prof. S. S. Jang School of Materials Science and Engineering Georgia Institute of Technology 771 Ferst Drive NW, Atlanta GA 30332-0245 (USA) E-mail: [email protected]
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Articles sorption energies on the graphene–C60 hybrid system and analyzed the accompanying changes in its electronic properties such as band structure, density of states (DOS), and charge distribution as a function of the number of Li atoms adsorbed using density functional theory (DFT). In addition, based on the Li adsorption energies in the various regions around the graphene–C60 hybrid system, we also discussed the mechanism of Li adsorption in comparison with the Li cluster formation.
Table 1. Adsorption energy and charge distribution (Mulliken charge) for single Li atom adsorption on the graphene–C60 hybrid system. System
graphene–C60 hybrid 1 Li on graphene (center of hexagon) 1 Li on C60 (pentagon) Pos1 of graphene side (region 1) Pos2 of graphene side (region 1) Pos1 of C60 side (region 2, hexa) Pos2 of C60 side (region 2, penta) Pos3 of C60 side (region 3, hexa) Pos4 of C60 side (region 3, penta) Pos5 of C60 side (region 4, hexa) Pos6 of C60 side (region 4, penta)
2. Results and Discussion The geometry of the graphene–C60 hybrid system was fully optimized without any fixed atoms on graphene or C60 to assess its capability for Li adsorption. Figure 1 shows the top, side,
Adsorption Energy C60 [eV]
Charges [e] Li
graphene
C60
0.720 (C60 Binding Energy) 1.375 (1.096)[2b]
N/A
0.095
0.095
0.813
0.813
N/A
1.838 (1.820)[7] 1.769
0.794
N/A
0.794
0.860
0.434
0.426
2.285
0.894
0.264
0.630
1.059
1.002
0.297
0.705
2.285
0.895
0.259
0.636
1.960
0.845
0.037
0.882
2.122
0.854
0.007
0.847
1.798
0.794
0.025
0.819
1.862
0.791
0.032
0.823
Figure 1. Unit cell structure of the graphene–C60 hybrid system: a) top view, b) side view, and c) expanded (2 2) view.
and the expanded view of the hybrid system, which was maintained by dispersion interactions between graphene and C60. The distance between the graphene and the center of the C60 was set to 6.35 in the hybrid structure, which was employed from a previously reported experimental value.[6] The binding energy and the charge transfer from graphene to C60 calculated through the Mulliken analysis is summarized in Table 1 and Figure 2. The Li adsorption energies were calculated using Equation (1): E binding ¼ ðE hybrid systemþLi E hybrid system n E Li Þ=n
ð1Þ
where Ehybrid system + Li is the total energy of the hybrid structure containing the adsorbed Li atoms, Ehybrid system is the total energy of the graphene–C60 hybrid system without Li atoms, and ELi is the energy of the Li atoms. As shown in the Table 1, it appears that the hybrid structure is more favorable in terms of the binding energy (0.720 eV) with the considerable charge transfer (j 0.095 j e). The charge distribution of the
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Figure 2. Structure of the graphene–C60 hybrid system with various regions for adsorption: a) front view, b) side view (regions 1, 2, 3, and 4 are in red, yellow, blue, and orange, respectively), c) the initial structure, and d) the optimized structure of a single Li atom adsorption at various positions around a graphene–C60 hybrid system.
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Articles phene system (adsorption energy of 1.375 eV). This enhanced adsorption can be due to the increased charge transfer from Li atom to the hybrid system. Some of the charges (j 0.095 j e) were already transferred from the graphene to the C60, rendering the graphene positively charged. As a result, additional charge transfer from the adsorbed Li may occur to a greater extent in the graphene region of the hybrid, thereby resulting in enhanced adsorption. At the same time, a Li atom in region 1 can still interact with C60 due to its proximity to C60 in the hybrid system. Referring to the positions shown in Figure 2, the Li atom at Pos2 in the graphene region moved to the same position as the Li atom in Pos2 of the C60 region after optimization, although the initial positions of these two atoms were different. Figure 3. Band structure (or energy level) of a) graphene, b) graphene–C60 hybrid, and c) the density of states (DOS) of graphene and the graphene–C60 hybrid. The Li adsorption energy shows lower values in the region between graphene and C60 representations, the Fermi levels are shifted to 0 eV. Since the (region 2; 2.285 eV) or between two C60s (region 3; graphene–C60 hybrid system is maintained through dispersion 2.122 eV). However, we found that the Li adsorption energy in region 4 is similar to that in pure C60 (1.798 eV and interactions, the hybrid system retains the characteristics of its components. In Figure 3 b, therefore, it is shown that the 1.862 eV, respectively), even though this value was still lower hybrid system keeps the graphene-like characteristics around than that (1.769 eV) of region 1. From these results, it apthe Fermi level, while it is thought that two other bands at pears that the Li adsorption is related to the charge transfer 0.5 eV are attributed to the t1u state of the neighboring C60 driven by the high electron affinity of C60 in the hybrid system. The corresponding band structures of the single Li adsorbed chain.[8] This is also observed in the density of states (DOS) of hybrid system are presented in Figure 4. While the band structhe hybrid system (Figure 3 c). The overall features in the DOS tures of the hybrid system did not change in comparison to of the graphene–C60 hybrid system seem to be a composite of the pure systems, the bands shifted downwards to Fermi level those of the components. when the Li atom was adsorbed on the C60 side of the hybrid Next, we studied the adsorption of single Li atoms at various system. positions on the graphene–C60 hybrid system. A Li atom was placed in the center of the hexagon site of graphene and the Next, we added a second Li atom, in order to investigate the pentagon or hexagon site of C60, since the most stable Li adadsorption mechanism of multiple Li atoms. Since the energy sorption on C60 can occur at these sites.[9] density is proportional to the number of Li atoms, it is imporIn order to systematically describe the Li adsorption mechatant to efficiently utilize the surface available on the granism on the graphene–C60 hybrid system, we defined four disphene–C60 electrode for adsorption rather than forming Li clustinct regions around the graphene–C60 hybrid system as ters by covering all the carbon rings of the graphene–C60 shown in Figure 2, namely, the graphene side (region 1, shown system with Li atoms. For this purpose, we calculated the adin red), the region between graphene and C60 (region 2, shown sorption of the second Li atom (blue or green) following the in yellow), the region between two C60s (region 3, shown in first Li atom (purple) in each region of the hybrid system as presented in Figure 5. The second Li atom was positioned at blue), and the C60 side (region 4, shown in orange). The Li either the nearest-neighboring (N.N.) site or the next-nearestatom can interact only with graphene in region 1 or only with neighboring (N.N.N.) site on the graphene side of the hybrid C60 in regions 3 and 4, whereas the Li atom can interact with system (Figure 5 a), while on the C60 side of the hybrid, the both graphene and C60 simultaneously in region 2. A single Li atom was placed at various positions in each second Li atom was adsorbed on the pentagonal or hexagonal region of the graphene–C60 hybrid system as shown in Figring in the radial or axial direction on the C60 to form the N.N. ure 2 c,d. The adsorption energy and the charge distribution of or the N.N.N. configuration (Figure 5 b). the single Li-adsorbed hybrid system are also summarized in The initial and optimized structures with two adsorbed Li Table 1. From Table 1, we observe that Li adsorption on the atoms in such various regions on the graphene–C60 system are graphene region of the hybrid system (adsorption energy of presented in Figure 6. Additionally, the Li adsorption energies 1.769 eV) was enhanced compared to that on a pure graof the two Li atoms are listed in Table 2. system calculated through the Mulliken population analysis showed a transfer of charge from graphene to C60 due to the relatively strong electron affinity of C60. The band structures of graphene and the graphene–C60 hybrid system are shown in Figure 3. In these band structure
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Figure 4. Band structures of the single-Li-adsorbed graphene–C60 hybrid system. The Li atom is adsorbed at various positions such as a) Pos1 on the graphene side, b) Pos2 on the graphene side, c) Pos1 on the C60 side, d) Pos2 on the C60 side, e) Pos3 on the C60 side, f) Pos4 on the C60 side, g) Pos5 on the C60 side, and h) Pos6 on the C60 side.
side (region 2) to another site in the C60 side or on the graphene side, forming a N.N.N. scheme. The adsorption energy on the C60 side (2.154 eV) was lower than that on the graphene side (1.888 eV) as expected, since C60 usually provides stronger adsorption sites owing to its strong electron affinity. Therefore, we confirm again that Li will cover the C60 surface first. The adsorption energy in region 3 (between the C60 regions, Figure 6 c) was also low, ranging from 1.891 eV to 2.066 eV due to the strong electron affinity of C60. It appears that the adsorption energy for the N.N.N. scheme is slightly low although the adsorption energy values are similar in the other directions. In region 4, the adsorption energy was calculated to be 1.971 eV and 1.899 eV for the pentagonal site (Figure 6 d, N.N.N. site) and the hexagonal site (N.N. site), respectively, which suggests that the Li adsorption will preferentially take
Table 2. Adsorption energy for the adsorption of two Li atoms on the graphene–C60 hybrid system.
Figure 5. Definition of the direction of adsorption of the second Li atom in the case of two Li atom adsorption on the hybrid system around a) graphene and b) C60. The first and second Li atoms are in purple and blue/ green, respectively.
In region 1 of the hybrid system (Figure 6 a), the adsorption energy is the same for both the N.N.N. and N.N. configurations since the second Li atoms in both the cases moved to the C60 side to have similar distance from C60. The adsorption energy was the lowest (Figure 6 b, 2.154 eV– 2.178 eV) in region 2 where Li atom can interact with both graphene and C60 together. The two Li atoms in region 2 had similar adsorption energy, regardless of the adsorption sites in region 2, whereas this was not the case in the other regions. We also examined the adsorption direction by comparing the adsorption energy of the system with two Li atoms, starting from Pos2 in the C60
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System
Adsorption Energy [eV]
2 Li on region 1: N.N.N. site 2 Li on region 1: N.N. site 2 Li on region 2: Axial (N.N.N. site) 2 Li on region 2: Axial (N.N. site) 2 Li on region 2: Radial (N.N.N. site) 2 Li on region 2 to region 1: N.N.N. site 2 Li on region 2 to region 1: N.N. site 2 Li on region 3: Axial (N.N.N. site) 2 Li on region 3: Axial (N.N. site) 2 Li on region 3 to region 2: N.N.N. site 2 Li on region 3 to region 2:N.N. site 2 Li on region 3 to region 4: N.N.N. site 2 Li on region 3 to region 4:N.N. site 2 Li on region 4: Radial (N.N.N. site) 2 Li on region 4: Radial (N.N. site) 8 Li atoms on graphene@hybrid 12 Li atoms on C60@hybrid 20 Li atoms on graphene–C60 hybrid LiLi
1.874 1.873 2.168 2.155 2.178 1.888 2.154 1.986 1.920 1.559 2.066 1.969 1.891 1.971 1.899 1.511 1.779 1.624 1.030[10]
place on the pentagonal (N.N.N.) sites of C60. Further, it appears that Li adsorption preferentially occurs on the N.N.N. scheme and the adsorption mechanism is strongly influenced by C60. 4
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Articles Finally, we investigated the adsorption of multiple Li atoms (such as 8, 12 and 20) on the hybrid system forming the N.N.N scheme. Pure graphene and C60 with the same number of Li atoms were also considered as references. The initial and optimized structures are presented in Figure 8, and the Li adsorption energies are listed in Table 2. From the optimized structures, we observed that while the Li atoms initially on the graphene of the hybrid system (Figure 8 a) move towards the C60 side, the Li atoms around C60 retained their positions during the geometry optimization (Figure 8 b and 8c). The adsorption energies for the adsorption of multiple Li atoms are 1.511 eV, 1.776 eV, and 1.621 eV for the graphene side, the C60 side, and for the overall hybrid system, respectively. These adsorption energies indicate that Li adsorption may predominantly take place on the C60 side first and then subsequently on the graphene side, similar to the observations in the case of two Li atom adsorption. Even though the Li adsorption energy Figure 6. Initial and optimized structures of the graphene–C60 hybrid system with two Li atoms adsorbed in different regions: a) region 1, b) region 2, c) region 3, and d) region 4. The first and second Li atoms are in purple and decreases upon increasing the blue/green, respectively. number of Li atoms, all the adsorption energies observed are lower than the LiLi binding energy (1.030 eV).[10] Please note Therefore, we think that Li adsorption will start taking place from C60 and proceed towards graphene. that, since this investigation is based on DFT calculation focusThe corresponding changes in the band structures of the systems with two adsorbed Li atoms were also analyzed in each region of the graphene–C60 hybrid system, as shown in Figure 7. A significant band shift was observed whenever an additional Li atom was added to the hybrid system compared to single Li atom adsorption. We think that the bands in close proximity to the additional Li atom were mainly affected and shifted down through increased Fermi levels as electrons were injected from the Li atoms into Figure 7. Band structures of the graphene–C60 hybrid system with two adsorbed Li atoms in various regions: the system. a) region 1, b) region 2, c) region 3, and d) region 4. ChemPhysChem 0000, 00, 0 – 0
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Figure 8. Initial and optimized structures of the graphene–C60 hybrid system with multiple Li atoms adsorbed: a) 8 Li atoms on the graphene side, b) 12 Li atoms on the C60 side, and c) 20 Li atoms adsorbed on the entire hybrid system.
ing on the ground state of the hybrid system, our study does not consider intermediate states kinetically generated during the operating process. In addition, the binding energy is still lower than that of the pure graphene (1.086 eV) and C60 (1.594 eV) systems having the same number of Li atoms, respectively. Therefore, from the point of view of Li adsorption, the graphene–C60 hybrid system appears to be promising for potential use as an electrode in Li batteries. Figures 9 a–c show the band structure of the graphene–C60 hybrid system with multiple Li atoms adsorbed. The band structure reveals that the number of available energy bands around the Fermi level increased significantly in the graphene– C60 system, indicating that the Li adsorption enhances the metallic characteristics of the system such as electrical conductivity. This result is confirmed by the DOS analysis as a function of the number of Li atoms adsorbed. In Figure 9 d, the hybrid system containing adsorbed Li atoms has more DOS around the Fermi level, especially after more than two Li atoms are adsorbed in comparison to the graphene–C60 hybrid system without Li adsorption. Such enhanced metallic character of the graphene–C60 system may also enhance the system’s electron
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Figure 9. Band structure corresponding to the adsorption of multiple Li atoms on the graphene–C60 hybrid system: a) 8 Li atoms on the graphene side, b) 12 Li atoms on the C60 side, c) 20 Li atoms adsorbed overall on the hybrid system, d) density of states for various number of Li atoms adsorbed on the graphene–C60 hybrid system, and e) density of states for multiple Li atoms adsorbed onto different sides of the hybrid system.
transport properties. Finally, we compared the DOS of the graphene–C60 hybrid system with that of pure graphene or C60 systems in order to examine the effect of Li adsorption on the electronic structure of the hybrid system. As shown in Figure 9 e, the DOS of the hybrid system retains more electrons around the Fermi level compared to pristine graphene and C60, inferring that the hybrid system with adsorbed Li atoms would have higher electron conductivity than the pure graphene or C60 systems have.
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Articles 3. Conclusions
Keywords: adsorption · anodes · density functional theory calculations · graphene-fullerene hybrids · lithium batteries
We investigated the adsorption of Li on the graphene–C60 hybrid system using the density functional theory (DFT). It was found that although the hybrid system retains the characteristics of its components in its electronic structure, the charges are transferred from the graphene to the C60 in the graphene– C60 hybrid system, leaving the graphene positively charged (+ 0.095 e) and the C60 negatively charged (0.095 e). It was also found that the adsorption of Li onto the hybrid system was enhanced compared to the pure graphene system, which would be due to the charge transfer from graphene to C60. Furthermore, by analyzing the Li adsorption in various regions of the graphene–C60 system, it was determined that the Li adsorption is likely to preferentially occur on the C60 side, particularly in the midway region between graphene and C60 or between two C60s. Although there is no significant change in the band structure of the graphene–C60 system after one Li adsorption, the adsorption of additional Li atoms cause the energy bands to shift downwards as a result of electron injection from the adsorbed Li atoms into the hybrid system. The DOS analysis in the hybrid system also indicates that the metallic character of the graphene–C60 system is increased with increasing the number of adsorbed Li atoms. Therefore, the graphene–C60 hybrid system is expected to demonstrate enhanced conductive properties in addition to excellent Li adsorption capabilities compared to the pure graphene system.
[1] a) Y. H. Liu, J. S. Xue, T. Zheng, J. R. Dahn, Carbon 1996, 34, 193 – 200; b) J. R. Dahn, T. Zheng, Y. Liu, J. S. Xue, Science 1995, 270, 590 – 593. [2] a) M. Khantha, N. A. Cordero, L. M. Molina, J. A. Alonso, L. A. Girifalco, Phys. Rev. B 2004, 70, 125422; b) K. T. Chan, J. B. Neaton, M. L. Cohen, Phys. Rev. B 2008, 77, 235430; c) W. Q. Deng, X. Xu, W. A. Goddard, Phys. Rev. Lett. 2004, 92, 166103; d) H. Tachikawa, Y. Nagoya, T. Fukuzumi, J. Power Sources 2010, 195, 6148 – 6152; e) K. Persson, Y. Hinuma, Y. S. Meng, A. Van der Ven, G. Ceder, Phys. Rev. B 2010, 82, 125416; f) G. Mpourmpakis, E. Tylianakis, G. E. Froudakis, Nano Lett. 2007, 7, 1893 – 1897; g) C. Ataca, E. Akturk, S. Ciraci, H. Ustunel, Appl. Phys. Lett. 2008, 93, 043123; h) F. Valencia, A. H. Romero, F. Ancilotto, P. L. Silvestrelli, J. Phys. Chem. B 2006, 110, 14832 – 14841; i) C. Uthaisar, V. Barone, J. E. Peralta, J. Appl. Phys. 2009, 106, 113715 – 113716. [3] a) G. X. Wang, X. P. Shen, J. Yao, J. Park, Carbon 2009, 47, 2049 – 2053; b) E. Yoo, J. Kim, E. Hosono, H. Zhou, T. Kudo, I. Honma, Nano Lett. 2008, 8, 2277 – 2282; c) S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen, R. S. Ruoff, Nature 2006, 442, 282 – 286; d) S. H. Lee, V. Sridhar, J. H. Jung, K. Karthikeyan, Y. S. Lee, R. Mukherjee, N. Koratkar, I. K. Oh, ACS Nano 2013, 7, 4242 – 4251; e) J. X. Zhang, Z. W. Xie, W. Li, S. Q. Dong, M. Z. Qu, Carbon 2014, 74, 153 – 162; f) W. Wang, I. Ruiz, S. R. Guo, Z. Favors, H. H. Bay, M. Ozkan, C. S. Ozkan, Nano Energy 2014, 3, 113 – 118. [4] a) K. Zhang, Y. Zhang, S. R. Wang, Sci. Rep. 2013, 3, 3448; b) T. Umeyama, J. Baek, N. Tezuka, K. Morita, H. Imahori, ECS J. Solid. State Sci. Technol. 2013, 2, M3001 – M3007. [5] a) W. Koh, J. I. Choi, K. Donaher, S. G. Lee, S. S. Jang, ACS Appl. Mater. Interfaces 2011, 3, 1186 – 1194; b) W. Koh, J. I. Choi, S. G. Lee, W. R. Lee, S. S. Jang, Carbon 2011, 49, 286 – 293. [6] V. Gupta, P. Scharff, K. Risch, H. Romanus, R. Muller, Solid State Commun. 2004, 131, 153 – 155. [7] Q. Sun, P. Jena, Q. Wang, M. Marquez, J. Am. Chem. Soc. 2006, 128, 9741 – 9745. [8] S. Saito, A. Oshiyama, Phys. Rev. Lett. 1991, 66, 2637 – 2640. [9] U. Zimmermann, N. Malinowski, A. Burkhardt, T. P. Martin, Carbon 1995, 33, 995 – 1006. [10] K. P. Huber, G. Herzberg, Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, NY. 1979. [11] a) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865 – 3868; b) J. P. Perdew, K. Burke, Y. Wang, Phys. Rev. B 1996, 54, 16533. [12] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010, 132, 154104. [13] a) B. Delley, J. Chem. Phys. 1990, 92, 508 – 517; b) B. Delley, J. Chem. Phys. 2000, 113, 7756 – 7764. [14] a) S. G. Lee, J. I. Choi, W. Koh, S. S. Jang, Appl. Clay Sci. 2013, 71, 73 – 81; b) S. Kwon, J. I. Choi, S. G. Lee, S. S. Jang, Comput. Mater. Sci. 2014, 95, 181 – 186. [15] a) D. P. Hashim, N. T. Narayanan, J. M. Romo-Herrera, D. A. Cullen, M. G. Hahm, P. Lezzi, J. R. Suttle, D. Kelkhoff, E. Munoz-Sandoval, S. Ganguli, A. K. Roy, D. J. Smith, R. Vajtai, B. G. Sumpter, V. Meunier, H. Terrones, M. Terrones, P. M. Ajayan, Sci. Rep. 2012, 2, 363; b) S. Z. Wen, W. Guan, Y. H. Kan, G. C. Yang, N. N. Ma, L. K. Yan, Z. M. Su, G. H. Chen, Phys. Chem. Chem. Phys. 2013, 15, 9177 – 9185; c) K. E. Kweon, G. S. Hwang, Y. H. Kim, ECS Solid State Lett. 2012, 1, M19 – M23; d) Y. Gao, X. J. Wu, X. C. Zeng, J. Mater. Chem. A 2014, 2, 5910 – 5914; e) W. Koh, J. I. Choi, E. Jeong, S. G. Lee, S. S. Jang, Curr. Appl. Phys. 2014, 14, 1748 – 1754. [16] H. J. Monkhorst, J. D. Pack, Phys. Rev. B 1976, 13, 5188 – 5192.
Computational Details We used a generalized gradient approximation (GGA) Perdew– Burke–Ernzerhof (PBE) functional[11] with DFT-D3 correction[12] in DMol3[13] with DND basis set for all the DFT calculations. The GGAPBE functional has been used to accurately describe the interactions between an adsorbate and various surfaces[14] including sp2carbon-based materials.[5, 15] The dimensions of the unit cell used in this study were 12.3 12.3 35 , which was large enough to ensure that there is no direct interaction between the original structure and its self-image in the c-axis through the periodic boundary. The dimensions in the a- and b-axes were determined from the area of the graphene. The structure of this graphene–C60 hybrid system is presented in Figure 1. The k-point samplings for the Brillouin zone were performed using a 4 4 1 Monkhorst– Pack k-point mesh.[16]
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014R1A1A1004096). This research was supported by Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078881).
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Received: September 27, 2014 Revised: November 9, 2014 Published online on && &&, 2014
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The mechanism of Li adsorption on a graphene–fullerene nanohybrid system is investigated by density functional theory.
&& – && A First-Principles Study of Lithium Adsorption on a Graphene–Fullerene Nanohybrid System
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