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Study of Lithium Migration Pathways in the Organic Electrode Materials of Li-Battery by Dispersion-Corrected Density Functional Theory Yanhui Chen,† Shaorui Sun,*,† Xiayan Wang,† and Qinghua Shi‡ †

Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China ‡ Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China S Supporting Information *

ABSTRACT: Organic materials have been considered a promising alternative as electrodes for rechargeable lithium-ion batteries. However, there are some obvious shortcomings, especially poor dynamics performance. Approaches to understand the reason for the poor dynamic performance are the main point of the present work. In this paper, an organic electrode material,C12H4N4, is selected as a sample, and studied by dispersion-corrected density functional theory (DFT-D2). The calculation results show that the band gaps of delithiated and lithiated states are about 0.9 and 1.0 eV, respectively, which is consistent with the conventional conjugated organic materials implying the good electronic conductivity. The Li-ion migration pathway forms a complicated three-dimensional (3D) network. The migration energy barrier is higher than 0.53 eV, which is obviously higher than that of the inorganic electrode material, demonstrating the poor ionic conductivity. In organic materials, although the steric hindrance is lowered due to the large intermolecular space, the coulomb potential is significantly improved at the same time, which is the main reason for the high energy barrier of Li-ion migration. Effective ways to lower the lithium migration energy barrier and improve the ionic conductivity should be considered when synthesizing new organic electrode materials.

I. INTRODUCTION Li-organic batteries, originally developed in 1969, are as old as inorganic ones.1,2 The organic materials have been considered a promising alternative electrode for rechargeable lithium-ion batteries due to their low cost, structural diversity and environmental benignity.3−5 In recent years, several redoxactive organic materials as electrodes have been reported for Libased batteries, such as benzoquinone derivative, conjugated dicarboxylate, indigo dye, and purpurin.6−10 Most of the above organic compounds could be classified as the conjugated carbonyl compounds.11,12 The double bond between carbon and oxygen (CO) in the carbonyl group could serve as the redox center for Li storage, and the conjugated structure allows electron transferring easily during the reaction. Compared to inorganic materials, organic materials have many shortcomings, especially their poor dynamic performance which is often attributed to the low electronic conductivity. To improve electronic conductivity, a large ratio of carbon black was added when preparing the electrode which, however, cannot conquer the poor performance radically. In fact, the conjugated organic materials are extensively used in the electronic devices and optical devices as semiconductor materials which demonstrate that such materials should have good electron conductivity. For the organic electrode materials © 2015 American Chemical Society

which have been reported so far, their theoretical band gaps are all in the range of 0.5−2.5 eV,9−11 which implies they are all semiconductors with good electronic conductivity. Therefore, the electronic conductivity is no concern with the poor dynamic performance, and further theoretical investigation is needed. Density functional theory (DFT)13 has been widely applied to investigate the properties of the inorganic electrode materials in lithium batteries,14−17 including lithium intercalation voltage,18 electronic structure,19 and Li-ion diffusion mechanisms,20,21 and as to the organic electrode materials are completely different. For most of the DFT theoretical works, the redox reactions were simulated around a single organic molecule, and lithium ions were bounded to the oxygen atom in the carbonyl group (CO).22−27 In lithium batteries, most of the electrode materials are in the crystalline phases which have the periodic structures, and the single molecular model could not predict the charge/discharge potential, electronic structure, and lithium migration pathway. However, the DFT cannot describe long-rang van der Waals Received: August 16, 2015 Revised: October 15, 2015 Published: November 2, 2015 25719

DOI: 10.1021/acs.jpcc.5b07978 J. Phys. Chem. C 2015, 119, 25719−25725

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The Journal of Physical Chemistry C (vdW) interactions28−30 in the organic molecular crystal. Fortunately, a variety of different correction methods including the nonempirical vdW-DF method of Dion et al.,31 Grimme’s semiempirical approach (DFT-D),32−34 and the Tkatchnko and Scheffler’s approach (DFT-TS)35 have been developed for the vdW calculation, and applied to predict the structure of the organic molecular crystal.36−38 The DFT-D2 method has been successfully applied to investigate the charge/discharge potential and the electronic structure of a conjugated carbonyl material as cathode in lithium battery.39 In this present work, we theoretically study the lithium migration mechanism of an organic cathode material, tetracyanoquinodimethane (C12H4N4), which has a conjugated structure with four cyano groups (CN) and has been reported with two discharge voltages, 2.5 and 3.1 V.40 With the DFT-D2 method, the geometry structural parameters and intercalation potential are well consistent with the corresponding experimental values. The small band gap values from the theoretical calculation, 1.0/0.9 eV for the lithiated/delithiated state, demonstrate a good electronic conductivity. The lithium diffusion channels form a three-dimensional network (3D diffusion). While the migration energy barrier (0.53−0.63 eV) is obviously larger than that of the inorganic material, such as LiCoO2 (0.36 eV, 2D diffusion)41 and LiFePO4 (0.3 eV, 1D diffusion),42 which is due to the large coulomb energy between the Li-ion and the negative charge distributed on the neighboring molecule at the saddle point. The large barrier impedes the Li-ion diffusion and lowers the ionic conductivity. This may be the main reason for the poor dynamic performance of organic electrode materials.

Figure 1. Crystal structure of TCNQ.

Table 1. Comparison between the Calculated and Experimental Lattice Parameters of the TCNQ Unit Cell calcd exptl40

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

8.73 8.91

6.52 7.06

16.19 16.4

90 90

98.04 98.53

90 90

III. RESULTS AND DISCUSSION The average intercalation potentials (also called voltages) were obtained by the total energy difference between the delithiated and lithiated states.53 From a single molecule perspective, the lithiation from C12H4N4 to Li2C12H4N4 is illustrated in Figure

II. CALCULATION METHOD The calculations were based on the density functional theory (DFT) and performed with the Vienna Ab inito Simulation Package (VASP).13 The ion−electron interaction is described with the projector-augmented wave (PAW) method.43 The exchange-correlation energy is calculated with the generalized gradient approximation (GGA) with the functional parametrization of PBE.44 The plane-wave cutoff energy was set to 400 eV, and the Monkhorst−Pack scheme with 3 × 3 × 1 kpoints grid had been used for the integration of the first Brillouin zone.45 The semiempirical vdW corrected method, DFT-D2,46−49 was used for the long-range vdW interaction, and in order to verify the calculation, other vdW corrected methods, including DFT-D3,34 DFT-TS,35 and vdW-DF,31 are also considered. The nudged elastic band (NEB) method was used to search for the Li migration pathway.50,51 The crystal structure of the tetracyanoquinodimethane (TCNQ) is the monoclinic system with a space group of C2/c, and molecules are arranged into orderly stacked layers in the b-direction, as shown in Figure 1. The optimized lattice parameters of the unit cell are summarized in Table 1, which is consistent with the experimental results.52 After lithiation, the structure is completely optimized under different crystal symmetry, and that with C2/c space group is the most favorable one, which is the same as the delithation sate (Table S1 in the Supporting Information). Here, each unit cell contains four molecules and eight Li-ions at 8f Wyckoff positions.

Figure 2. Lithium intercalation and deintercalation around the single molecule.

2. In the unit cell, there are four C12H4N4 molecules, storing eight Li ions. So the average voltage can be calculated with V̅ =

E TCNQ + 8E Li − E Li−TCNQ

8F where F is the Faraday constant, E is the total energy, and 8 refers to the unit cell hosting 8 Li-ions. Here, five different theoretical methods, pure DFT, DFT-D2, DFT-D3, DFT-TS, and vdW-DF, are applied to calculate the potential. The calculated average potentials are listed in Table 2, and that of DFT-D2 is closest to the experimental value, 2.5−3.1 V.40 In the present work, the DFT-D2 method is applied to investigate the organic electrode material. The geometry structures of TCNQ and LiTCNQ (the lithiated TCNQ) are fully optimized, and Figure 3a and b shows the crystal structures of TCNQ and LiTCNQ (the lithiated TCNQ) viewed along the [100] direction, respec-

Table 2. Calculated Potentials in a Unit Cell with Different Theoretical Methods method average potential (V) 25720

DFT 2.13

DFT-D2 2.55

DFT-D3 2.11

DFT-TS 2.14

vdW-DF 1.82

DOI: 10.1021/acs.jpcc.5b07978 J. Phys. Chem. C 2015, 119, 25719−25725

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Figure 3. (a) Layered crystal structure of TCNQ viewed from the a direction. (b) Lithiated TCNQ viewed from the a direction. (c) Bird views of the crystal structure of TCNQ along the b direction. (d) Bird views of the crystal structure about the lithiated TCNQ along the b direction.

tively, and they are both layered structures with 3.5 Å between the two neighboring layers. The bird views of the two crystal structures along the b direction are presented in Figure 3c and d, respectively, and in the LiTCNQ each Li was coordinated with two N from two neighboring molecules. The ionic bond length of Li−N is about 1.99 Å, which connected the separated molecules into chains along the crystal direction [101]. The lithiated process regrouped the aromatic π-bond, it could be clearly verified by comparing the bond lengths of the delithiated and lithiated states, as shown in Figure 4a and b,

Figure 4. (a) Bond lengths of the delithiated state. (b) Bond lengths of the lithiated state.

respectively. After lithiation, two electrons were added to the LUMO (the lowest unoccupied molecular orbit) of the delithiated state, which changed the bond orders and the bond lengths of the molecule. The bond order is the number of chemical bonds between a pair of atoms. The bonds of C1−N1 (C3−N2, C12−N3, and C11−N4), C2−C4 (C7−C10), and C5−C6 (C8−C9) were lengthened, and their orders decreased at the same time; the bonds of C1−C2 (C2−C3, C10−C11, and C10−C12) and C4−C5 (C4−C9, C6−C7, and C7−C8) were shortened, and their orders increased at the same time. For the six-membered ring of the delithiated state, the two bonds C5−C6 and C8−C9 are shorter than the other four bonds; for that of the lithiated state, the six bonds have the same length. Then the molecular structure is significantly affected by the charge state. To further understand the structural characteristics, the charge density was studied which enabled us to visualize the chemical bonding nature between atoms accurately, and it revealed how the charge transfers between atoms. Charge transfer in electrode materials can be studied by subtracting the charge densities of the delithiated and the lithiated structures.53 Figure 5a shows the ±0.0068 e/Å3 isosurfaces for the chargedensity differences: (ρLiTCYQME − ρTCYQME), in which the yellow

Figure 5. Isourfaces of the charge density difference between the lithiated and delithiated states. Panels (a) and (b) show the same isosurface in different directions. Yellow and blue represent the positive and negative 0.0068 e/Å3 isosurfaces, respectively. Large green spheres are Li ions.

and blue surfaces represent +0.0068 and −0.0068 e/Å3 isosurfaces, respectively. After lithiation, the Li atom lost its 2s electron and formed monopositive ions; at the same time, C12H4N4 gained two electrons and is negatively charged. In Figure 5, the naked Li-ion suggested that its valence electron was completely deprived. Due to the coulomb interaction, the Li-ion attracted the electrons close to it, and as shown in Figure 5b, the charge density isosurfaces, including those around two nitrogen atoms and three carbon atoms, C1, C2, and C3 (C2 and C3 belong to the six-membered ring), expanded to the central Li-ion, which demonstrated that the ionic bonds were not only formed between the lithium and nitrogen, but also between the lithium and the carbon (in the six element circle). 25721

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Figure 6. DOS of TCNQ (a) and Li8TCNQ (b). Red dashed line marks the position of the Fermi energy.

Figure 7. (a) Eight lithium occupation sites. (b) Path 1−7. (c) R0 is the distance between the Li-ion (at the initial position) and the neighboring molecule level, and RS is the distance between the Li-ion (at the saddle point) and the neighboring molecule level.

The density of states (DOS) of TCNQ and LiTCNQ are shown in Figure 6 a and b, respectively, and three states around the Fermi level, α, β, and γ, are specified in Figure 6. For the DOS of TCNQ, the α state is the highest occupied state, that is, the valence band maximum (VBM), and the β state is the lowest unoccupied state, that is, the conductor band minimum (CBM). For the DOS of LiTCNQ, due to eight electrons accommodating into the β state (per unit cell), the β state becomes the highest occupied state (the VBM), and the γ state is the new lowest unoccupied state (the CBM). Here, in order to simplify the discussion, for each of the two DOS figures, the energy zero is set at the top of the α state. Before lithiation (as shown in Figure 6a), the Fermi energy is 0 eV, and the band gap, which is about 0.9 eV, is between the α and β state. After lithiation (as shown in Figure 6b), the Fermi level shifted up to 1.4 eV, and the band gap, about 1.0 eV, is between the β and γ state. It is well-known that lithium-ion diffusion limits the dynamic performance. Using the climbing-image nudged elastic band (NEB) method, we studied Li diffusion in the layered structure. As shown in Figure 7a, the TCNQ unit cell has 8f sites which are identical positions numbered from Li-1 to Li-8. Here, we assumed the Li-1 site is occupied by Li, and the other unoccupied seven positions are its migration destinations. There were seven possible pathways for Li migration, including the pathway from site Li-1 to site Li-2 (path 1−2), from site Li1 to site Li-3 (path 1−3), from site Li-1 to site Li-4 (path 1−4),

from site Li-1 to site Li-5 (path 1−5), and from site Li-1 to site Li-6 (path 1−6), from site Li-1 to site Li-7 (path 1−7), and from site Li-1 to site Li-8 (path 1−8). Site Li-7 was adjacent to site Li-1 most closely; path 1−7 is shown in Figure 7b, in which Li ion was moved between two neighboring layers. The fractional coordinate of site Li-1 and that of site Li-7 are (0.24, 0.25, 0.61) and (0.26, 0.25, 0.39), respectively. The initial structure is equivalent to the final structure, and the total energy on each side could be selected as the basis (zero) to identify the migration barrier energy. The migration barrier energy of each pathway is listed in Table 3. Table 3. Length of the Migration Pathway and the Corresponding Barrier Energy length (Å) barrier energy (eV)

1−2

1−3

1−4

1−5

1−6

1−7

1−8

5.36 0.623

9.87 0.536

5.50 0.6

6.02 0.96

8.83 0.532

3.77 0.631

5.87 0.626

Six of the migration energy barriers are located in the energy range from 0.53 to 0.63 eV. The migration routes form a reedimensional network, which is shown in Figure 8a (the bcplane), b (ac-plane) and c (ab-plane). In order to present the migration path clearly, only the Li-ion is displayed, the eight Li ions at 8f Wyckoff positions are shown as big green spheres, and the migration pathways are shown with small yellow 25722

DOI: 10.1021/acs.jpcc.5b07978 J. Phys. Chem. C 2015, 119, 25719−25725

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shorter than Rs, which demonstrates that coulomb potential of the saddle point is significantly larger than that at the initial point, and it may be the primary reason to its high migration energy barrier. Thus, for organic electrode materials, although the large intermolecular space evidently lowers the steric hindrance, the coulomb potential is significantly improved. An ideal Li-ion electrode material should have high electronic conductivity (the small band gap) and ionic conductivity (the low migration energy barrier), such as LiCoO2. For LiFePO4, the Li-ion migration energy barrier is small, but the large band gap leads to the low electronic conductivity. Then the second conductor phase, such as carbon black, is mixed with or coated on the LiFePO4 particle to improve the electronic conductivity. The organic electrode material, opposite from LiFePO4, has a higher migration barrier and smaller band gap, which leads to the low ionic conductivity. According to the calculation results, two possible approaches have been proposed to enhance the ionic conductivity: the first is to design novel organic electrode material with low Li-ion migration barrier, which is very challenging; the second is to shorten the path length of Li-ion diffusion through reducing the particle size of organic material to nanoscale and, at the same time, recombining with the second conductor phase, which has not only the high electronic conductivity, but also the high ionic conductivity.

Figure 8. Three-dimensional network structure shown within the bcplane (a), ac-plane (b), and ab-plane (c) in the 2 × 2 × 3 supercell.

IV. CONCLUSIONS In this paper, the dispersion-corrected density functional theory (DFT-D2) is applied to study an organic electrode material of Li-ion battery, tetracyanoquinodimethane. The material has a typical layered structure, and after Li-ion is inserted, the neighboring molecules are tightly connected with the ionic Li− N bond and form a chain structure along [101] direction. During the lithiated procss, the π-bond of the organic molecule is regrouped; the ionic bond forms between the inserted lithium ion and the organic molecule. The theoretical band gap of the material is around 1.0 eV, which implies a good electronic conductivity. The energy barrier of each lithium migration pathway is higher than 0.53 eV, which implies a poor ionic conductivity. The results also show that, in organic electrode materials, the Li-ion migration pathways form a complicated three-dimensional network. Although the steric hindrance is lowered due to the large intermolecular space, the coulomb potential is significantly improved at the same time, which is the main reason for the high energy barrier of Li-ion migration. The high energy barrier, which leads to low ionic conductivity, should be considered when designing and exploring new organic electrode materials.

spheres. Compared with that in the inorganic electrode material, the network in the present organic material is obviously more complicated, and as in the above discussion, a Li-ion could migrate from the initial position to seven neighboring positions. In the organic crystal, large space exists between the molecules, and lithium could move into it with small steric hindrance. The most important inorganic cathode material, LiCoO2, has a two-dimensional network for Li-ion diffusion, and the energy barrier is about 0.36 eV.41 The layered LiFeSO4OH also exhibits a two-dimensional lithium-ion diffusion with low activation energies of ∼0.2 eV.54 LiFePO4 is also an important inorganic cathode material with one-dimensional network, and the energy barrier is about 0.3 eV.42 The inorganic cathode materials, LiMn2O455 and LiTi2O4,56 have three-dimensional pathways with activation energies of 0.23 and 0.2 eV, respectively. Compared with one or two-dimensional migration network, in three-dimensional network, there are more ways to be selected for Li-ion migration, that is, if some way is blocked, Liion could migrate in other ways. Although TCNQ has a three-dimensional network and small steric hindrance for Li-ion migration, it has a migration barrier (0.53−0.63 eV) higher than any of the inorganic electrode materials listed before, which may be the leading cause for its poor dynamical performance. As shown in Figure 5, the inserted lithium is completely ionic, and the valence electron is stripped by the neighboring organic molecules. When Li-ion migrates in the organic material, such as along the path 1−7, the negative charge is redistributed at the same time. The coulomb potential between Li-ion and the negative charge distributed on the organic molecule also changes with Li-ion moving along the path. As shown in Figure 7c, the distance from the Li-ion (at the initial position) to the molecular plane is specified as R0, and that at the saddle point is specified as Rs. The length of R0 is obviously

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07978. Table showing the total energy of Li2TCNQ with different symmetry, and figure showing the XRD profile of (a) TCNQ and (b) Li2CNQ (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 8610-67396480. Notes

The authors declare no competing financial interest. 25723

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ACKNOWLEDGMENTS The research was supported by Key Program and General Projects of National Natural Science of China (Grant No. 21036009, 21276006), the Scientific Research Common Program of Beijing Municipal Commission of Education (KM201310005012), and the Funding Project for Academic Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality (Grant No. PHR201107104).

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DOI: 10.1021/acs.jpcc.5b07978 J. Phys. Chem. C 2015, 119, 25719−25725

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DOI: 10.1021/acs.jpcc.5b07978 J. Phys. Chem. C 2015, 119, 25719−25725