Interface and Doping Effects on Li Ion Storage Behavior of Graphene

Article pubs.acs.org/JPCC

Interface and Doping Effects on Li Ion Storage Behavior of Graphene/Li2O Tianshuai Wang,† Naiqin Zhao,†,‡ Chunsheng Shi,† Liying Ma,† Fang He,† Chunnian He,†,‡ Jiajun Li,† and Enzuo Liu*,†,‡ †

School of Materials Science and Engineering and Tianjin Key Laboratory of Composites and Functional Materials, Tianjin University, Tianjin 300072, China ‡ Collaborative Innovation Centre of Chemical Science and Engineering, Tianjin 300072, China S Supporting Information *

ABSTRACT: Graphene/metal oxide nanocomposites have been widely used as the anode materials for Li ion batteries, which exhibit much higher Li storage capacity beyond their theoretical capacity. In order to make clear the Li storage mechanism in graphene/metal oxide, we systematically investigated the interface and (B, N, O, S) doping effects on Li ion storage behavior in graphene/Li2O using first-principles total energy calculations. It is revealed that the doping elements increase the van der Waals interface interaction of graphene/Li2O by changing the electronic structure of graphene through different mechanisms. The Li storage at the graphene/Li2O interface exhibits the synergistic effect resulting from the enhanced interface interaction by the Li insertion at the interface. The p-type and n-type doping induced by B and N dopants in graphene enhance and reduce the Li storage capability of graphene/Li2O, respectively. O and S doping result in the localization of the electronic states in graphene which benefits the Li adsorption at the interface. The localization of electronic states combined with the appropriate dopant electronegativity can enhance the Li atoms adsorption and diffusion simultaneously. Thereby, the highest interfacial lithium storage (0.330 mhA/m2) is obtained for the O-doped system, while the S-doped system possesses the good balance between interfacial Li storage (0.220 mhA/m2) and diffusion energy barrier (0.27 eV). The results open a new insight for the design of graphene/metal oxide composites as energy storage materials.

1. INTRODUCTION Lithium ion batteries (LIBs) have been widely applied in the field of consumer electronics due to their advantages of long cycle life, high voltage, high energy density, and environmental friendliness.1,2 Since LIBs were introduced into the market in 1991, graphite has been employed as the anode material with the theoretical specific capacity of 372 mAh/g. However, the increasing applications such as electrical vehicles and hybrid electric vehicles require anode materials with much higher energy and power densities.3−5 Metal oxides, such as MnO2, Fe2O3, NiO, etc., have been investigated as promising anode materials with higher theoretical capacities than graphite.6,7 Nevertheless, these kinds of materials have poor electrical conductivity and large volume change during the charge−discharge process, resulting in poor rate performance and cycling stability.8 Graphene addition can improve the rate performance and the structure stability of the metal oxide, since the volume swing during the charging/discharging processes can be alleviated by the porous structure of the composites, and the electrical conductivity is expected to be enhanced by the graphene layer within the metal oxide nanomaterials.9−11 Moreover, graphene/transition metal oxide composite materials usually exhibit specific Li storage capacities exceeding the theoretical capacity limits anticipated on the basis of reaction stoichiometry.12−15 © XXXX American Chemical Society

On the other hand, one of the main lithiation mechanisms of metal oxides is the conversion reaction,16,17 whose final reaction products are metal nanoparticles distributed evenly in the Li2O matrix.18 Therefore, it will form the interfaces of the graphene/Li2O and metal/Li2O in the lithiation stage of graphene/metal oxide nanocomposites. Evidence in favor of interfacial Li storage within the electrode materials as the explanation for the excess capacity of metal oxides has been accumulating since the idea was first suggested by Balaya et al.,19 which was corroborated by the theoretical calculations on Ti/Li2O.20 It was also revealed that the additional capacity originating from the surface and interfacial Li storage via an electrostatic capacitive mechanism contributes significantly to the electrode capacity of graphene/TiO2.21 Therefore, Li storage behavior at the interface of graphene/Li2O is very important for the further understanding of the Li storage behavior in graphene/metal oxide. Furthermore, it was reported both experimentally and theoretically that B, N, O, and S doping adjust the electronic structure of graphene and affect the Li adsorption behavior on graphene.22 B doping enhances the Li adsorption on graphene, Received: May 15, 2017 Revised: August 15, 2017 Published: August 29, 2017 A

DOI: 10.1021/acs.jpcc.7b04642 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C while N doping weakens the Li adsorption and enhances the Li diffusion on graphene which benefits the rate performance.23,24 O dopant in graphene induces localized states in its vicinity, and the delocalization of the graphene π bond is destroyed.25 Therefore, heterogeneous elements doping can adjust the electronic structure of graphene,26 which affects the Li atoms adsorption and diffusion on graphene and further modifies the Li storage properties of graphene/metal oxide. Hence, in this work, we systematically investigated the Li storage behavior at the interfaces of the (B, N, O, S)-doped graphene/Li2O, as well as on the doped graphene and Li2O(111) surface, through first-principles total energy calculations. The electronic structures of the related systems were investigated to reveal the mechanism in microscale of the interface and doping effects on the Li atoms storage and diffusion at the interfaces.

Figure 1. Atomic structure of the doped graphene/Li2O(111) interface: side view (a) and top view (b); 1, 2, and 3 indicate the sites above which Li atoms are adsorbed at the interface, which are Li or O atoms in the first, second, and third atomic layer, respectively.

distances between Li2O(111) and graphene with different dopants are listed in Table 1.

2. COMPUTATIONAL METHODS The projector-augmented wave (PAW) formalism of density functional theory as implemented in the Vienna ab Initio Simulation Package (VASP) is used in the system energy and electronic structure calculations.27,28 In order to get a better picture of weak interactions,29 we choose a computationally cost-effective “optB86b-vdW” method for vdW interactions.30 The exchange-correlation functional is approximated with the local density approximation.31 The Gaussian smearing method32 was used, and the width of smearing was chosen as 0.05 eV. The energy cutoff for plane-wave expansion of the PAWs is 400 eV. The optimized lattice constants of graphene and Li2O are 2.45 and 4.53 Å, respectively, consistent with the experimental results (2.46 Å33 and 4.62 Å34). The slab model of the most stable Li2O(111)surface35 contains nine atomic layers in thickness with three bottom layers fixed, and a 3 × 3 supercell in the lateral plane is adopted, containing 54 Li and 27 O atoms. Graphene with a 4 × 4 supercell containing 32 C atoms contracts to fit the Li2O(111) surface. The lattice mismatch is about 1.7%. One doping atom was substituted for one C atom in the doped graphene, corresponding to the doping concentration of 3.1%. The doping concentration of B, N, O, and S are comparable to the values in the recent experimental reports.36−40 In the vertical direction, a vacuum layer of about 15 Å in thickness is introduced for all the surfaces and interfaces. The Brillouin zone41 is sampled using Monkhorst−Pack scheme with a k-point mesh of 3 × 3 × 1 in the Γ-centered grids for the structural relaxation, and 5 × 5 × 1 k-point mesh is used for the static calculations of all the systems. The structure relaxation is continued until the forces on all the atoms are converged to less than 0.01 eV/Å. The energy barrier for Li atoms diffusion is determined by the nudged elastic band (NEB) method42 with six images linearly generated between the initial and final states.

Table 1. Interface Interaction Energies E(r) (eV) and the Distance D (angstroms) between the Doped Graphene and Li2O(111), As Well As The Charge Changes (e) of the Three C Atoms around the Doping Atom (QC), the Doped Graphene (QG), and the Doping Atoms (QD) before and after the Doped Graphene Adsorbs on Li2O(111)a D Er QC QG QD

G/Li2O

BG/Li2O

NG/Li2O

OG/Li2O

SG/Li2O

2.80 1.57 N/A +0.27 N/A

2.54 1.96 +0.41 +0.64 −0.02

2.70 1.63 −0.22 +0.20 +0.01

2.62 1.64 −0.43 +0.21 +0.03

2.58 1.65 −0.81 +0.21 +0.02

a

The charge of one atom in one system is obtained by subtracting the electrons of the isolated atom from that owed to the atom in the system based on the Bader analysis. The positive value of QC, QG, and QD means that the electron density on the corresponding atoms increases. G/Li2O and XG/Li2O represent graphene/Li2O and Xdoped graphene/Li2O, respectively. N/A: not available.

As listed in Table 1, when the doping atom is introduced, the distance between the doped graphene and Li2O(111) is reduced compared with that between the pristine graphene and Li2O. As a result, E(r) of all the doped systems is bigger than that of the pristine graphene/Li2O. However, the electronic structure shown in Figure 2 and the charge changes before and after the doped graphene absorbs on Li2O listed in Table 1 indicate that different doping elements enhance the interface interaction through different mechanisms. As shown in Figure 2a, after the graphene is adsorbed on Li2O(111), charge densities around the graphene are redistributed. Electrons mainly accumulate below the graphene. Because the Li at Li2O(111) is electropositive, when the doped graphene achieves more electrons, the interface interaction will enhance. For the B-doped graphene, the three C atoms adjacent to the B atom are more likely to obtain electrons from the surface of Li2O(111) due to the p-type doping. Thus, the electrostatic interaction at the interface is enhanced, which results in the decrease of the interfacial distance and the increased interface interaction. For the N-, O-, S-doped graphene, the three C atoms adjacent to the doping atom lose electrons, but the C atoms away from the doping atom obtain more electrons than the C atoms in the undoped graphene. Therefore, the interface distance is reduced due to the local enhanced interface

3. RESULTS AND DISCUSSION 3.1. Interface Interaction between Li2O and the Doped Graphene. The atomic structures of the doped graphene/Li2O interfaces are depicted in Figure 1. The interface interaction energy is defined as E(r) = E(Gr) + E(Li2O) − E(Gr/Li2O), where E(Gr), E(Li2O), and E(Gr/ Li2O) represent the energies of the doped graphene, Li2O surface, and the corresponding doped graphene/Li2O interface, respectively. The interface interaction energies and the B


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Figure 2. Charge density difference of (a) G/Li2O, (b) BG/Li2O, (c) NG/Li2O, (d) OG/Li2O, and (e) SG/Li2O interfaces. Charge density difference is obtained by subtracting charge density of isolated doped graphene and the Li2O surface from that of the doped graphene/Li2O interfaces. Blue and yellow represent charge loss and charge accumulation, respectively. The isosurface value is set to 0.001 e/Å3.

Coulomb interaction between the doped graphene and Li2O(111), consistent with the charge density differences presented in Figure 2. However, due to the decrease of the total electrons obtained by N-, O-, and S-doped graphene, the interface interaction energy increases little compared with the case without dopants. Furthermore, as listed in Table 1, the electron loss of the three C atoms adjacent to the N, O, and S doping atoms increases gradually, and thus, the Coulomb repulsion effect between the C atoms adjacent to the doping atom and Li2O(111) increases. Considering that the total charge changes of the N-, O-, and S-doped graphene change little, the local attractive interaction between C atoms away from the doping atom and the surface also increases, which could result in the decrease of the interface distance. As a result, the interfacial interaction energies change little for N, O, and S dopants, although the interfacial distances for O and S doping reduce compared with the case of N doping. The results also indicate that the uniform distribution of the electrons transferred to the graphene is destroyed in different degrees due to the different dopants, consistent with the charge density differences shown in the Figure 2b−e. The electron localization closely relates to the Li adsorption ability of the doped graphene, which will be discussed in section 3.3. The electron localization function (ELF) was also calculated to determine the covalent bond interaction between the doped graphene and Li2O(111) qualitatively. From Figure S1, there is no covalent bond between the doped graphene and Li2O(111). Thus, the interaction between the doped graphene and Li2O(111) is mainly via van der Waals interaction. 3.2. Adsorption of Lithium Atoms on Li2O(111). For the adsorption of one Li atom on Li2O(111), three adsorption sites are considered as shown in Figure 1. The adsorption energy of Li atoms is defined as Ead = −(ET − E0 − nELi)/n, where E0 is the energy of the studied system, ET is the energy of the corresponding system with n Li atoms adsorbed, and ELi is the energy of a single Li atom. The adsorption energies at the sites 1, 2, and 3 are 0.13, 0.77, and 0.58 eV, respectively. To further understand the Li adsorption behavior on Li2O(111), the electronic structure of the most stable adsorption configuration is analyzed (Figure 3). The 2s electron of the adsorbed Li atom and 2p electrons of the nearest neighboring O atom at Li2O(111) are hybrid at the energy range from −2.5 to −4.6 eV, as well as near the Fermi energy (Figure 3a), indicating the strong interaction between the Li and O atoms.

Figure 3. (a) Partial DOS of the adsorbed Li atom and its nearest neighboring O atom at Li2O(111). The DOS of Li 2s is enhanced by 15 times to show clearly. The Fermi level is set as zero. (b) The charge density difference for the most stable adsorption configuration of one Li atom on Li2O(111), which is obtained by subtracting the charge densities of one isolated Li and Li2O(111) from that of the Li2O(111) with the Li atom adsorbed. Light blue and yellow represent charge loss and charge accumulation, respectively. The isosurface value is set to 0.001 e/Å3.

Figure 3b shows that the adsorbed Li atom loses electrons after being adsorbed on Li2O(111), while electrons accumulate between the O atom at the surface and the adsorbed Li atom, consistent with the density of states (DOS) analysis. Therefore, site 2 is the most stable adsorption site for Li adsorption on Li2O(111). However, due to the Coulomb repulsive effect between the Li atoms at Li2O(111) and the adsorbed Li atom, the Li adsorption energies are small. 3.3. Lithium Adsorption on the Pristine and Doped Graphene. The atomic structures of graphene and the doped graphene are shown in Figure 4. In order to make clear the

Figure 4. Top view of the atomic structure of (a) the pristine graphene and (b) the doped graphene. The numbers indicate the Li adsorption sites considered in the present study. C


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Figure 5. Panels a and f, b and g, c and h, d and i, and e and j are the DOS and charge density differences of G, BG, NG, OG, and SG, respectively. The partial density of states (PDOS) of the doping atoms are enhanced by 15 times to show clearly. The Fermi level is set as zero. C 2p is the PDOS of the sum of all the C atoms in the doped graphene. The charge density differences are obtained by subtracting the charge density of the corresponding isolated atoms from that of the doped graphene. The isosurface value is set to 0.0139 e/Å3. Panels k, l, m, n, and o are the charge density differences of the stable configurations with one Li atom adsorbed on G, BG, NG, OG, and SG, respectively, which are obtained by subtracting the charge density of the isolated graphene and the isolated Li atom from that of the doped graphene with one Li atom adsorbed. The isosurface value is set to 0.001 e/Å3. Light blue and yellow represent charge loss and accumulation, respectively.

effect of the dopants on the electronic structure of graphene, the DOS of the systems were investigated (Figure 5a−e). The pristine graphene is semi-metallic, in agreement with the previous reported results.43 After B doping, the Fermi level shifts left through the valence band, and the graphene becomes a p-type electron-deficient system. N doping leads to the right shift of the Fermi level through the conduction band, and the graphene becomes an n-type electron-rich system. Thus, the extrinsic conductivity of graphene is enhanced due to both B and N doping, in accordance with previous reports.44 However, the DOS of O- and S-doped graphene have sharp peaks near the Fermi level indicating the localization of the electrons. B and N dopants undergo sp2 hybridization forming three σ bonds with the three adjacent C atoms. There is almost no distortion to the graphene sheet.45 Since the planar honeycomb graphene structure is well-preserved after the B and N doping, the linear energy dispersion near the Dirac point is not entirely destroyed. When the O or S atom is doped in graphene, the

two unpaired electrons of the O and S dopants are shared with the three neighboring carbon atoms, and then it will form three relatively weaker σ bonds. Thus, the delocalization of the graphene π bond is destroyed by the O or S doping. The above discussion is also corroborated by the charge density differences analysis (Figure 5f−j). After B and N doping, the charge density differences with the hexatomic ring configuration encounter no apparent change, compared with that of the pristine graphene. However, for the O and S doping, electrons concentrate near the O atom while the S atom loses electrons. The integrated hexatomic ring character of the charge density differences has an obvious change, which increases the electron distribution discontinuity. The different effects of B, N, O, and S dopants on the electronic structure of graphene could result in the different Li adsorption behaviors on graphene. The Li adsorption behavior on the doped graphene was investigated. To find the most stable Li adsorption position on the doped graphene, different adsorption sites are considered, D


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the p-type and n-type doping resulting from B and N dopants, the Li adsorption energies are enhanced and reduced on B- and N-doped graphene, respectively. For the Li adsorption on the O-doped graphene, O 2p and Li 2s states are coupled below 0 eV with a few peaks superimposed (Figure 5d), indicating strong interactions between the O and Li atoms. Moreover, as shown in Figure 5i, the electrons of the C atom near the O atom decrease, which means that these C atoms become electron-deficient, and the ability of the related C atom getting the electrons from the Li atom enhances. For the Li adsorption on the S-doped graphene, due to the close electronegativity of S to that of C (Table S1), the adsorption of the Li atom results in the redistribution of electrons throughout the graphene (Figure 5o), different from the concentration on the particular hexagon ring in the systems with the other dopants, which will enhance the interaction between the Li and SG. On the basis of the above discussion, it can be concluded that B and N dopants do not change the shape of the DOS of graphene. Due to the p-type and n-type doping resulting from B and N dopants, the Li adsorption energies are enhanced and reduced on B- and N-doped graphene, respectively. However, O and S dopants both result in the localization of the electronic states in graphene, which results in the enhancement of Li adsorption due to the different Li adsorption mechanisms. 3.4. Lithium Adsorption and Diffusion at the Graphene/Li2O Interfaces. For one Li ion intercalation at the graphene/Li2O interface, different adsorption sites were studied (Figure S4a−d). It is revealed that Li tends to be at the “hollow” site of graphene and near the O atom of Li2O. The Li adsorption energy at the interface of graphene/Li2O is larger than the sum of that on the graphene and Li2O(111) (Table S2), exhibiting a synergistic effect for the interface Li storage. As discussed above, the outermost Li atoms of Li2O(111) surface are positively charged. Graphene adsorbed on Li2O(111) becomes negatively charged. When one Li atom is adsorbed at the interface of graphene/Li2O, each of the six C atoms adjacent to the adatom gains 0.08 e on average, while the O atoms adjacent to the adatom gain 0.02 e in total. This indicates that the interaction between the Li adatom and graphene is predominant in the adsorption process. After one Li atom is inserted at the interface, more electrons accumulate below the C atoms, compared with the case without Li atoms intercalation (Figure 2a and Figure S5a). Thus, the Coulomb interaction between the negatively charged graphene and the positively charged Li atoms at the outermost layer of the Li2O surface increases, which enhances the interface binding and stability of the graphene/Li2O interface. Charge density differences (Figure S5b−e) of all the doped graphene/Li2O systems with one Li atom adsorbed show that the adsorbed Li atom also interacts with the O atom at Li2O(111). Therefore,

as indicated in Figure 4. The adsorption energies are summarized in Table 2. On the pristine graphene, the hollow Table 2. Li Adsorption Energies (eV) at Difference Adsorption Sites on the Graphene with Different Dopants G BG NG OG SG a

1

2

0.86 2.17 0.61 0.98 2.94 (relax to 5)

0.65 1.93 0.54 1.56 2.14

3 relax relax relax relax 2.14

4 to to to to

1 1 2 2

N/Aa relax to 1 0.75 (relax to 5) relax to 2 2.94 (relax to 5)

N/A: not available.

site is the most stable adsorption position for Li atoms, in accordance with previous reports.46,47 The most stable adsorption positions for Li atoms on the B-, O-, N-, and Sdoped graphene are sites 1, 2, 5, and 5, respectively. According to Table 2, the adsorbed Li atom prefers to keep itself far away from the S doping atom when we put the Li atom in the hexagon ring which contains the S atom. In order to investigate this phenomenon, we analysis the Bader charge of the S-doped graphene. It shows that the three C atoms adjacent to the S atom gain 0.25 e after S doping, and the S atom loses 0.14 e. Thus, the hexagon ring containing the S atom becomes a negative atmosphere after S doping, which reduces the ability of the adjacent carbon atoms to obtain electrons from the adsorbed Li atom. We also calculate the adsorption behavior of Li atom at more sites (Figure S2). The results indicate the effect unfavorable, for the Li adsorption only concentrates on the hexagon ring containing the S atom. Li can adsorb stably above the center of the carbon hexagon ring without the S atom. To further understand the different Li adsorption behaviors on the doped graphene, charge density differences (Figure 5k− o) and DOS (Figure S3a−e) of the most stable adsorption configurations of the graphene with different dopants are investigated. As shown in Figure 5k, when one Li atom is adsorbed on graphene, there is charge loss region above the Li atom and charge accumulation in the intermediate region between the Li atom and graphene. Thus, electrons transfer from Li to graphene after adsorption. However, the gained electrons distributions near graphene with different dopants are different. For B and N doping, the graphene becomes electrondeficient and electron-rich systems, respectively. However, the shapes of the DOS of the B- and N-doped graphene are similar to that of pristine graphene except for the position of the Fermi level. As a result, the distributions of the charge density difference in Figure 5f−h and Figure 5k−m are similar for the pristine graphene and the B- and N-doped graphene. Due to

Figure 6. Top view of the atomic structures of the graphene/Li2O interfaces with 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), 6 (f), and 7 (g) Li atoms adsorbed. E


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Figure 7. (a) Relationship between the lithiation potential and the number of Li adatoms (capacity) at different doped graphene/Li2O interfaces. (b) The relationship between the Li adsorption energies and the number of Li adatom at different doped graphene/Li2O interfaces.

Figure 8. (a) The diffusion path and (b) the energy variations with the adsorbed Li atom diffusing along the path on Li2O, graphene, (doped) graphene/Li2O interfaces. Graphene is not shown in panel a.

affected by Li adsorption at other sites than that on pristine graphene. As shown in the Figure 7b, the effect makes the adsorption energy decline slowly with the amount increase of the adsorbed Li atoms, which allows more Li atoms insertion at the interface. In the graphene and B- or N-doped graphene, the delocalization of the conjugated π bonds is preserved. When one site of B- or N-doped graphene with delocalized π electrons adsorbs a Li atom, this adsorption markedly affects the Li adsorption at other sites, making the adsorption energy change fast as the amount of Li adatoms increases. Moreover, as shown in Figure 7a, the lithiation potential increases when the amount of the adsorbed Li atoms changes from 1 to 2 for the OG/Li2O and SG/Li2O systems. To further understand this phenomenon, the bond lengths of O or S dopant atom with three adjacent C atoms are listed in Table S3. As shown in Figure S6a−c, when one Li atom is adsorbed at the interface of SG/Li2O, the S−C bonds do not break. The C1−S bond breaks when two Li atoms are adsorbed at the interface of SG/ Li2O, forming two defective vacancies. Therefore, the adsorption energy of the two Li atoms adsorbed at the defective vacancies increases. With the number of Li adatoms increasing, no more defective vacancies forms, and thus, the average lithiation potential decreases. For the OG/Li2O system, as shown in Figure S6d−f, the C1−O bond breaks, and the adatom lies in defective vacancy when the Li atom is adsorbed at the interface of OG/Li2O. The adsorption energy is high due to the strong electronegativity of the O atom and the formation of the defective vacancy. When two Li atoms are adsorbed at the interface of OG/Li2O, the C1−O distance is further elongated. Considering that the two Li atoms exactly saturate

the interface Li storage exhibits the synergistic effect. Extra Li atoms are expected to be stored at the graphene/Li2O interface in the lithiation stage of the graphene/metal oxide nanocomposites. Thereby, the adsorption behavior of multiple Li atoms at the doped graphene/Li2O interfaces is studied (Figure 6). The lithiation potential is defined as EV = −(ET − E0 − nELi)/ne, where e is the electron charge, ELi is the energy of one Li atom in bulk Li, E0 is the energy of the studied system, and ET is the energy of the corresponding system with n Li atoms adsorbed. The positive lithiation potential indicates that the interfacial Li adsorption contributes to the storage capacity of the doped graphene/Li2O. When one Li atom is intercalated at the doped graphene/Li2O interfaces, the lithiation potential at all the doped graphene/Li2O are positive. In order to explore the maximum Li storage capacity of the doped graphene/Li2O interfaces, we gradually increase the number of intercalated Li atoms until the lithiation potential turns negative. Multiple Li atoms are all initially put at the most stable sites, and then fully relaxed. As shown in Figure 7a, the results show that NG/Li2O and BG/Li2O can adsorb one and three Li atoms, and the corresponding capacities are 0.055 and 0.165 mAh/m2, respectively. OG/Li2O and SG/Li2O can adsorb six and four Li atoms; the corresponding capacities are 0.330 and 0.220 mAh/m2, respectively. The different Li storage capabilities of the G/Li2O interfaces with different dopants relate to the change of the electronic structure of the graphene due to the dopants. As discussed in section 3.3, both O and S dopants induce localized states. The Li adsorption on O- or S-doped graphene at one site is less F


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Li2O(111). Compared with the Li adsorption on the pristine graphene and Li2O(111), the Li storage capability at the graphene/Li2O interface exhibits the synergetic effect, which contributes to the storage capacity of the graphene/metal oxide composites. Due to the p-type and n-type doping effects induced by B and N dopants in graphene, the Li storage capability of graphene/Li 2O is enhanced and reduced, respectively. For the O and S doping, the localization of electronic states induced by the dopants, as well as the dopant electronegativity, affects the Li atoms adsorption and diffusion at the interface. The O-doped graphene/Li2O exhibits the highest Li storage capacity of 0.330 mAh/m2 but shows the highest diffusion energy barrier of 0.74 eV due to the high electronegativity of O dopants. Because of the close electronegativity of S to that of C, the S-doped system possesses a good balance between the interfacial Li storage (0.220 mhA/ m2) and diffusion energy barrier (0.27 eV). The research illuminates the extra storage beyond the theoretical Li capacities of graphene/metal oxide reported by the experimental results and presents a comprehensive understanding in nanoscale of the doping effect on the Li storage capacity of graphene/metal oxide composites.

the two defective vacancies caused by the O doping, the adsorption energy for the two Li atoms is comparable to the adsorption energy for one Li atom, and the adsorption energy fluctuation is small. In order to study the doping effect on the rate performance of nanocomposites, we analyzed the electron structure and the energy barrier for Li atoms diffusion at the different interfaces. DOS of the Li2O surface is investigated to reveal the influence of graphene on the electronic conductivity of the systems. The Li2O is insulator, which has an indirect band gap of 5.8 eV.48,49 After the introduction of graphene, graphene/Li2O exhibits a semi-metallic character (Figure S7). Therefore, the existence of graphene improves the electronic conductivity of the hybrid structure. Fast Li ion diffusion in the crystal structure of the electrode materials is essential to the high-power capability of Li rechargeable batteries. In this section, the NEB method has been used to calculate the diffusion energy barriers of Li atoms at the interfaces. In the calculation of the diffusion energy barriers, a diffusion pathway from one stable site to the adjacent stable site is adopted (Figure 8a). The energy barriers for Li diffusion on the graphene, Li2O surface, and at the graphene/ Li2O interfaces are calculated and presented in Figure 8b. As shown in Table 3, although Li adsorption energies at the doped graphene/Li2O interfaces are enhanced, the diffusion

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04642. Electron localization function (ELF) of the doped graphene/Li2O, the different sites for Li adsorption on the S-doped graphene, and the atomic structure of graphene/Li2O with one Li adsorbed, DOS and the charge density difference of the doped graphene/Li2O with one Li atom adsorbed, the detailed data for the Li adsorption energies and lithiation potential at the doped graphene/Li2O interfaces, and the bond lengths in the SG/Li2O and OG/Li2O interfaces (PDF)

Table 3. Li Diffusion Energies Barriers (eV) in Different Systems Li2O

G

G/Li2O

BG/Li2O

OG/Li2O

NG/Li2O

SG/Li2O

0.32

0.24

0.28

0.45

0.74

0.40

0.27

ASSOCIATED CONTENT

energy barriers of Li atoms at the interface can still maintain relatively small values except for the OG/Li2O. OG/Li2O suffers from a high diffusion energy barrier (0.74 eV). According to the above results, the strong interaction between the O atom of OG and Li adatoms will increase the adsorption energy of Li. At the same time, the significant localization of electronic states due to the high electronegativity of O dopant results in the hard diffusion of Li atoms at the OG/Li2O. It is worth noting that the Li diffusion energy barrier at SG/Li2O (0.27 eV) is smaller than the diffusion energy barriers for Li atoms at other interfaces, even close to the diffusion barrier on graphene (0.24 eV), which can be understood based on the electronic structure analysis. As discussed in section 3.3, due to the close electronegativity of S to that of C, the adsorption of the Li atom results in the redistribution of electrons throughout the graphene, which results in a similar environment for the Li atom adsorption at the different sites in the interface, benefiting the Li diffusion. Therefore, the appropriate localization of electronic states combined with the appropriate dopant electronegativity can enhance the Li atoms adsorption and diffusion simultaneously.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone and Fax: +86 22 27891371. ORCID

Liying Ma: 0000-0002-9486-3522 Enzuo Liu: 0000-0002-3331-2532 Present Address

E.L.: School of Material Science and Engineering, Tianjin University, 300072 Tianjin, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 11474216). The work was carried out at the National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1(A).

4. CONCLUSIONS In conclusion, the adsorption and diffusion behaviors of Li atoms on Li2O(111), the doped graphene, and at the doped graphene/Li2O interfaces have been investigated by the firstprinciples calculations. The van der Waals interface interaction of graphene/Li2O is enhanced due to the electron deficiency in graphene caused by B dopant, while N, O, and S dopants enhance the interface interaction through changing the local distribution of the electrons in graphene transferred from the

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REFERENCES

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Interface and Doping Effects on Li Ion Storage Behavior of Graphene

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