Article pubs.acs.org/cm
New 4V-Class and Zero-Strain Cathode Material for Na-Ion Batteries Jongsoon Kim,†,# Gabin Yoon,‡,§,∥,# Myeong Hwan Lee,‡,§ Hyungsub Kim,⊥ Seongsu Lee,⊥ and Kisuk Kang*,‡,§,∥ †
Department of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 05006, Republic of Korea ‡ Department of Materials Science and Engineering and §Research Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea ∥ Center for Nanoparticle Research at Institute for Basic Science (IBS), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea ⊥ Korea Atomic Energy Research Institute (KAERI), Daedeok-daero 989 Beon-Gil, Yuseong-gu, Daejeon 34057, Korea S Supporting Information *
ABSTRACT: Here, we introduce Na3V(PO3)3N as a novel 4V-class and zero-strain cathode material for Na-ion batteries. Structural analysis based on a combination of neutron and X-ray diffraction (XRD) reveals that the Na3V(PO3)3N crystal contains threedimensional channels that are suitable for facile Na diffusion. The Na (de)intercalation is observed to occur at ∼4 V vs Na/Na+ in the Na cell via the V3+/V4+ redox reaction with ∼67% retention of the initial capacity after over 3000 cycles. The remarkable cycle stability is attributed to the near-zero volume change (∼0.24%) and unique centrosymmetric distortion that occurs during a cycle despite the large ionic size of Na ions for (de)intercalation, as demonstrated by ex situ XRD analysis and first-principles calculations. We also demonstrate that the Na3V(PO3)3N electrode can display outstanding power capability with ∼84% of the theoretical capacity retained at 10C, even though the particle sizes are on the micrometer scale (>5 μm), which is attributed to its intrinsic three-dimensional open-crystal framework. The combination of this high power capability and extraordinary cycle stability makes Na3V(PO3)3N a new potential cathode material for Na-ion batteries.
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upshift the redox potential via the inductive effect.11,12,28−31 Specifically, the polyanion groups may share the O in the transition metal polyhedron, lowering the transition metal−O bond covalency. This reduces the gap between the bonding and the antibonding orbitals, which in turn lowers the energy level of antibonding orbitals where the redox reaction occur, thereby upshifting the redox potential of the transition metal. As Vbased compounds have generally shown higher voltages among reported electrode materials, various anion frameworks have been explored to further upshift the redox potential of V. For instance, the inclusion of the PO4 polyanion group leads to a notable increase of the redox potential. Although the Na insertion potential is ∼3 V vs Na/Na+ in V2O5 via the V4+/V5+ redox reaction, Na3V2(PO4)3 exhibits a redox potential of ∼3.4 V vs Na/Na+ even with the V3+/V4+ redox reaction.11,28,32−37 A further increase of the number of PO4 polyanion groups per V ion results in a higher operating voltage. For example, Na7V4(P2O7)4(PO4) with a V:P ratio of 4:9 exhibits a redox potential of 3.88 V vs Na/Na+, which is higher than that of
INTRODUCTION There are growing demands on large-scale energy storage systems to effectively utilize environmentally friendly energy resources.1−3 Li-ion batteries (LIBs) have been regarded as one of the important candidates for these large-scale applications; however, concerns regarding the high cost and availability of Li resources remain unresolved. As an alternative, Na-ion batteries (NIBs) have recently attracted considerable attention because of their similar electrochemistry to that of LIBs as well as the low cost and accessibility of the abundant Na resources.4−25 Nevertheless, the hurdles that must be overcome for the successful commercialization of NIBs include (1) the inherently low operating voltage resulting from the higher redox potential of Na/Na+ (−2.71 V vs the standard hydrogen electrode (SHE)) compared with that of Li/Li+ (−3.05 V vs SHE) and (2) the large volume change of electrode materials usually observed during charge/discharge due to the larger size of Na+ ions (∼1.02 Å) than Li+ ions (∼0.76 Å).4,5,14−27 To address these issues, extensive studies have focused on the search for new cathode materials for NIBs. To date, the strategies to overcome the first hurdle have included the utilization of transition metals with high redox potentials (such as Co and V) and the introduction of various anion groups to © 2017 American Chemical Society
Received: June 14, 2017 Revised: August 21, 2017 Published: August 21, 2017 7826
DOI: 10.1021/acs.chemmater.7b02477 Chem. Mater. 2017, 29, 7826−7832
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Chemistry of Materials Na3V2(PO4)3.30 These previous studies support the use of anion tuning as an effective strategy to upshift the redox potential of the electrode. Nonetheless, considering that anions are the building blocks of the crystal framework, it is believed that the use of anion engineering to tune the voltage should be considered in regards to structural robustness with the large Na-ion intercalation. Herein, we introduce a new V-based high-voltage cathode Na3V(PO3)3N, which utilizes the (PO3)3N polyanion group. The large portion of the anion group in the structure reduces the theoretical specific capacity; however, we find that the high ratio of V to P (1:3) substantially increases the voltage. Furthermore, the crystal framework consisting of the anion group opens up an ionic diffusion channel suitable for facile Na (de)intercalation. More importantly, the open structure is insensitive to the large Na-ion insertion and deinsertion, exhibiting near-zero strain during charge/discharge, similar to that of the well-known zero-strain spinel Li4Ti5O12 for Li insertion.38−40 These structural merits lead to a remarkable cycle stability of more than 3000 cycles and power capability up to 10C (retention of ∼84% of the theoretical capacity) despite the micrometer-scale (>5 μm) particle sizes without additional treatment.
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Computational Details. All density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP).41 We used projector-augmented wave (PAW) pseudopotentials42 with a plane-wave basis set as implemented in VASP. Perdew−Burke−Ernzerhof (PBE) parametrization of the generalized gradient approximation (GGA)43 was used for exchangecorrelation functional. The GGA+U method44 was adopted to address the localization of the d-orbital in V ions, with a U value of 4.2 eV, which is used in the study on Na3V2(PO4)3.45 All calculations were performed with an energy cutoff of 500 eV until the remaining force in the system converges to less than 0.02 eV/Å per unit cell. Nudged elastic band (NEB) calculations46 were conducted to determine the activation barrier of Na diffusion in the Na3V(PO3)3N structure. A unit cell made by four formula units of Na3V(PO3)3N was used, and one Na vacancy was generated to model the Na-ion diffusion. We considered seven intermediate states between the first and the final images of a single Na diffusion event. During the NEB calculation, all structures were allowed to relax within the fixed lattice parameters.
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RESULTS AND DISCUSSION Na3V(PO3)3N was successfully synthesized using a conventional solid-state synthesis route at 700 °C under anhydrous ammonia (flow rate = 30 mL min−1) condition. Scanning electron microscopy (SEM) analysis of the obtained Na3V(PO3)3N powder revealed that the size of the primary particles ranged from 1 to 10 μm with an average size of approximately ∼5 μm, as observed in Supporting Figure S1. The structure of the synthesized Na3V(PO3)3N was carefully analyzed using neutron diffraction (ND) and X-ray diffraction (XRD). No impurities or second phases were detected in the ND or XRD patterns, as shown in Figure 1a and Supporting Figure S2, respectively. Refinement of the ND crystal structure yielded a lattice parameter of a = 9.4490(1) Å with three crystallographically distinct Na sites within the P213 space group (cubic
EXPERIMENTAL SECTION
Synthesis of Na3V(PO3)3N. Na3V(PO3)3N powder was synthesized using the general solid-state method. (NaPO3)6 and V2O5 with molar ratios of 1:1 were used as precursors. They were thoroughly mixed and ground by planetary ball-milling with 10 wt % ascorbic acid. The mixed precursors were then fired at 700 °C under anhydrous ammonia (flow rate = 30 mL min−1) condition for 10 h. Materials Characterization. Neutron diffraction (ND) data of Na3V(PO3)3N were recorded using a High-Resolution Powder Diffractometer for Thermal Neutrons (HRPT) in the Paul Scherrer Institut (PSI). ND measurement was performed over a 2θ range of 5− 160° with a step size of 0.05° using a constant wavelength of λ = 1.1545 Å. The X-ray diffraction (XRD) data of Na3V(PO3)3N were analyzed using an X-ray diffractometer (PANalytical) equipped with Cu Kα radiation (λ = 1.5406 Å). XRD measurement was performed over a 2θ range of 10−90°, with a step size of 0.01°. Each step was exposed for 6 s. ND and XRD data were refined by the Rietveld method using Fullprof software. The particle size was investigated by field-emission scanning electron microscopy (FESEM). The ex situ XRD patterns of Na3−xV(PO3)3N (0 ≤ x ≤ 1) were analyzed using an X-ray diffractometer (PANalytical) equipped with Cu Kα radiation (λ = 1.5406 Å) over a 2θ range of 10−60°, with a step size of 0.01°. V Kedge X-ray absorption spectra (XAS) were taken on the 8C beamline at the Pohang Accelerlator Laboratory (PAL). V K-edge energy calibration was performed using V metal foil as reference. A reference spectrum was simultaneously recorded for the in situ spectrum using V metal foil. Electrochemistry. Electrochemical tests were performed in a CR2032-type coin cell assembled in an Ar-filled glovebox. The electrode was prepared as follows. 80 wt % of the active material and 20 wt % super P are homogeneously mixed using a ball-mill at 100 rpm for 10 h. Then, 90 wt % of this mixture is remixed with 10 wt % polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (99.5%, Aldrich) (NMP). The electrode was fabricated by pasting a slurry of the powder mixture onto Al foil using a doctor blade. NMP was evaporated in an oven at 80 °C for 12 h. The mass loading of the active material is 2 × 10−3 g cm−2. The cell was assembled using a Na counter electrode, a separator (GF/F glass fiber), and a 1 M solution of NaPF6 in ethyl carbonate/propylene carbonate (EC/PC, 1:1 v/v) in an Ar-filled glovebox. Galvanostatic charge/discharge tests were performed at various C rates (C/5, C/2, 1C, 2C, 4C, 6C, 8C, and 10C in the 2.5−4.25 V window, 1C corresponds to ∼73 mA g−1) for Na3V(PO3)3N using WBCS 3000 (WonA Tech).
Figure 1. Refined ND patterns of Na3V(PO3)3N (Rp = 5.35%, RI = 6.23%, RF = 7.63%, χ2 = 6.35%) and (b) crystal structure of Na3V(PO3)3N ((PO3)3N polyanion groups three-dimensionally connected to VO6 octahedra, as magnified in blue circle). 7827
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Chemistry of Materials symmetry), which is isostructural with the previously reported Na3M(PO3)3N [M = Ti, Al, etc.].47−50 The detailed structural information on the atomic positions from ND and XRD is presented in Supporting Table T1. A slight deficiency in the Na content (0.989) was observed for the Na1 site, whereas the Na2 and Na3 sites exhibited occupancies of close to unity. This deficiency is attributed to the high-temperature synthesis, which causes a loss of the volatile Na precursor, and the higher site energy of the Na1 site compared with those of the other Na sites, as will be discussed later. Figure 1b presents a schematic illustration of the crystal structure of Na3V(PO3)3N based on the structural information calculated from the refinement. The local structure denoted by the dotted circle in the figure indicates that the VO 6 octahedra are connected with [(PO3)3N]6− polyanion units, which are composed of three PO3 groups sharing a central N ion. Notably, all the O in the VO6 octahedron share corners with the PO3N tetrahedron, implying that a strong inductive effect is expected. The inductive effect by the indirect interaction between the V and P through each O corner of the octahedron would substantially upshift the redox potential of V in Na3V(PO3)3N compared with other V-based cathode materials for NIBs.51 Indeed, our first-principles calculations of the electronic structure of the material revealed that V has a Bader charge of +1.96 and +1.86 for Na3V(PO3)3N and NaVO2, respectively. Higher Bader charge implies the high ionicity (low covalency) of the V−O bond, leading to a higher redox potential. The units of the VO6 octahedra combined with [(PO3)3N]6− polyanions are arrayed to yield Na-ion channels that are connected three-dimensionally in space, as illustrated in Figure 1b. Figure 2a presents a representative charge/discharge profile of the Na3V(PO3)3N electrode in a Na half-cell. Within the voltage window of 2.5−4.25 V, approximately one Na ion could participate in the reversible intercalation, which corresponds to a specific capacity of ∼73 mAh g−1. Na ions could be deintercalated and reinserted with an average redox potential of ∼4.01 V vs Na+/Na, as denoted in the dQ/dV plot in the inset of Figure 2a. We attribute this exceptionally high redox potential of the V3+/V4+ redox couple to the maximized inductive effect around V ions resulting from the combination of nine PO3 polyanion units connected by three central N ions, as discussed earlier. An attempt to further utilize more than one Na ion in the structure (x > 1 in Na3−xV(PO3)3N) was unsuccessful because of the instability of the electrolyte at high voltage. As shown in Supporting Figure S3, we analyzed the electrochemical reaction in the extended voltage range from 2.0 to 4.5 V; however, no significant difference was found. The power capability of the Na3V(PO3)3N electrode was examined at various current rates (C/5, C/2, 1C, 2C, 4C, 6C, 8C, 10C), as shown in Figure 2b. At C/5 and C/2 rates, specific capacities close to the theoretical value corresponding to x = 1 in Na3−xV(PO3)3N were achievable. A slight reduction of the capacity was observed upon applying higher current rates; however, the discharge capacity of Na3V(PO3)3N remained ∼84% of the theoretical capacity even at 10C. It is noteworthy that the average particle size of the Na3V(PO3)3N is approximately 5 μm, as shown in Supporting Figure S1, with no additional conductive coating of the particles used to achieve this high power performance. To understand the observed power capability of the electrode, the intrinsic Na diffusion behavior in the Na3V(PO3)3N structure was investigated using first-principles calculations. Because there are three distinguishable Na sites
Figure 2. (a) Charge/discharge profile of Na3V(PO3)3N with the calculated voltage [inset: dQ/dV of Na3V(PO3)3N] and (b) various discharge capacities of Na3V(PO3)3N at different current rates (C/5, C/2, 1C, 2C, 4C, 6C, 8C, and 10C in the 2.5−4.25 V window, 1C = 73 mAh g−1).
in the Na3V(PO3)3N structure, as denoted in Figure 1a, there are apparently six different local hopping pathways for the Na ion, namely, Na1−Na1, Na2−Na2, Na3−Na3, Na1−Na2, Na1−Na3, and Na2−Na3 pathways. However, the direct Na1−Na1, Na2−Na2, and Na3−Na3 pathways are intrinsically blocked by other ions such as O and Na. Nevertheless, two distinguishable pathways exist between the Na1 and Na3 sites. Therefore, we considered four different Na-ion hopping pathways in our calculations, denoted as Na1−Na2, Na1− Na3 (short), Na1−Na3 (long), and Na2−Na3, as shown in Figure 3a−d, with the energy profiles calculated using nudged elastic band calculations (see the Supporting Information for computational details). Asymmetric activation barriers were observed for all the investigated pathways in the energy profile, which are attributed to the difference in site energies of Na ions in the Na3V(PO3)3N structure. The site energy for the Na1 site is particularly high compared with those of the Na2 and Na3 sites, which may be attributed to the high repulsion force between Na and V ions. Three first-neighbor V ions exist within 4 Å from the Na1 site, whereas the other Na sites have a repulsion with only one V ion. Along the Na1−Na2 path, it takes ∼800 meV for Na ions at the Na2 site to migrate to the Na1 site, whereas the reverse migration requires ∼450 meV. Except for the Na1 → Na2 forward pathway, the activation barriers were mostly under 550 eV, as summarized in Figure 3e, which are small enough for Na ions to migrate in the Na3V(PO3)3N structure. The Na-ion hopping through the Na2−Na3 pathway is expected to be particularly fast because of 7828
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Figure 3. (a−d) Na1−Na2, Na1−Na3 (long), Na1−Na3 (short), and Na2−Na3 diffusion pathways and their energy landscapes. (e) Summary of the calculated Na diffusion activation barriers in Na3V(PO3)3N. (f) Illustration of the three-dimensional Na diffusion pathways in Na3V(PO3)3N. Note that the Na1−Na2 pathways are excluded in this figure because of their high activation barriers (∼800 eV).
during the charge/discharge. Figure 4b presents the ex situ XRD patterns obtained for the Na3V(PO3)3N electrode during the electrochemical cycling along with the corresponding lattice parameters. In order to understand Na deintercalation and corresponding redox reaction of V, we probed the oxidation states of V in Na3−xV(PO3)3N samples at various charged states using X-ray absorption near edge spectroscopy (XANES) spectra. As observed in Supporting Figure S4, it is revealed that, when Na ions are deintercalated during charge, the growth of the V pre-edge and the shift of the K-edge toward the high energy level occur, which indicates the oxidation of V3+ to V4+. These results demonstrate that the volume difference between the pristine Na3V(PO3)3N and the fully desodiated Na2V(PO3)3N is as low as ∼0.24%, which indicates that virtually zero strain is involved during the charge/discharge of this material. To our best knowledge, this is one of the smallest values of volume change for Na-ion de/intercalation ever reported for Na insertion electrodes.4,5 Although the detailed atomistic structural analysis for the intermediate states using XRD was not trivial, first-principles calculations can provide the underlying reason for this unexpectedly low volume change despite the large ionic size of Na. Density functional theory (DFT) calculations confirmed that the volume change from the pristine Na3V(PO3)3N to the charged phase, Na2V(PO3)3N, is negligible, with a value of approximately ∼0.16% upon removal of 1 Na in Na3V(PO3)3N, which is consistent with the XRD results. The calculations reveal that Na in the Na1 sites (among the three different Na sites) was preferably desodiated because of the higher site energy compared with those of the Na2 and Na3 sites in Na3−xV(PO3)3N (0 ≤ x ≤ 1) (Supporting Figure S5). Interestingly, selective desodiation from the Na1 site results
its low activation barrier (∼287 meV), which is comparable to those reported for LiCoO2 and LiFePO4.52−54 Moreover, we observed that these fast Na hopping pathways are threedimensionally well interconnected without involving the highactivation-barrier Na1−Na2 pathway, which implies that fast Na-ion diffusion would be possible in this material. Figure 3f shows the three-dimensionally connected diffusion pathways only consisting of Na-ion diffusion paths with activation energies of less than 550 meV, where the energy barriers for each path are represented with color schemes. These welldefined three-dimensional Na diffusion pathways are critical to the power capability of Na3V(PO3)3N, as materials with limited interconnection of diffusion pathways (such as LiFePO4) often exhibit poor rate performance in the presence of even a small amount of site defects despite relatively low barriers for Li-ion diffusion.55 Although the individual activation barriers of Na diffusion are not extraordinarily low, these well-interconnected three-dimensional Na diffusion pathways are believed to have greatly contributed to the excellent power capability of Na3V(PO3)3N. The cycle stability of the Na3V(PO3)3N electrode in a Na half-cell is shown in Figure 4a. For the first 500 cycles, the capacity was well maintained without significant capacity degradation. In addition, approximately 67% of the initial capacity was retained even after 3000 cycles in the extended cycle test. Note that no treatment such as coating, doping, or the use of additives was performed when preparing the Na3V(PO3)3N electrode. The inset of Figure 4a compares the electrochemical profiles at every 500th cycle up to 3000 cycles, which reveals a negligible change in the profile except for the slight reduction of the capacity with cycles. We attribute this outstanding cyclability to the negligible structural change 7829
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Table T2). These two competing factors were almost comparable, resulting in the complete compensation of the overall volume change for the selective Na1 site desodiation. In addition, the crystallographic symmetry of Na3V(PO3)3N was shown to play an important role in the observed small volume change. As observed in the schematic in Figure 4c, the oxidized V ions tend to move away from the occupied Na2 site (Na+) and toward the unoccupied Na1 site (VaNa10) with the desodiation. This tendency is supported by the elongated distance between the Na2 site and V ions from 3.08 to 3.21 Å and decreased distance between the VaNa10 and V ions from 4.12 to 4.10 Å. This amount of V ion movement is sufficient to affect the local geometry and should cause a certain volume change. However, the centrosymmetry of Na3V(PO3)3N illustrated in Figure 4c results in the canceling out of the displacement vectors of all four V ions in a unit cell of Na3V(PO3)3N. Supporting Table T3 tabulates the Cartesian coordinates of four V ions in the unit cells of Na3V(PO3)3N and Na2V(PO3)3N. After the desodiation, the direction of V ion movements in the unit cell were approximately [11̅1̅], [1̅11̅], [11̅ 1̅ ], and [111], as depicted in Figure 4c. Therefore, the displacements of all the V ions cancel out, resulting in the negligible change of volume upon desodiation. If all the displacements of V ions were unidirectional and not canceled out, a significant change of the lattice parameter would be expected. This finding implies that the selective desodiation preferentially from the Na1 site, which induced the particular Na vacancy arrangement in its cubic symmetry, was pivotal to the canceling out of the displacement of V ions and the corresponding low volume change, even though large Na ions were extracted.
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CONCLUSION In summary, we demonstrated that Na3V(PO3)3N, a new cathode material for NIBs, exhibits a 4V-class redox potential and zero strain during Na (de)intercalation using a combination of experiments and first-principles calculations. Because of the maximized inductive effect caused by the three (PO3)3N polyanions surrounding every corner of the VO6 octahedron, Na3V(PO3)3N displayed an exceptionally high redox potential of ∼4.01 V (vs Na+/Na). Nearly no volume change was observed during charge/discharge, leading to the virtually zero strain, which resulted in a remarkable cycle stability for the Na3V(PO3)3N electrode. The unexpected zero strain is attributed to the unique local symmetry around V ions and the selective desodiation from the structure in the cubic symmetry. Furthermore, the three-dimensional open-crystal framework of Na3V(PO3)3N unveiled by the first-principles calculations enabled fast Na-ion diffusion in the structure, leading to its excellent power capability even without special treatment of the electrode. We believe that the novel Na3V(PO3)3N electrode introduced here can serve as a promising new candidate for Na-ion batteries with high power capability combined with extraordinary cycle stability.
Figure 4. (a) Cyclability of Na3V(PO3)3N at 1C during 3000 cycles and (b) volume change and ex situ XRD patterns of Na3−xV(PO3)3N (0 ≤ x ≤ 1). (c) Schematic of V-ion displacements upon desodiation of Na3V(PO3)3N, projected along the [111] direction of the unit cell. Detailed view of local structure of Na2V(PO3)3N showing that the desodiation of Na1 induces the displacement of V ions away from Na2, as indicated by the bond length changes. The movements of all the V ions are canceled out because of the cubic symmetry of Na3V(PO3)3N, resulting in the negligible total displacement of ∼0.02 Å.
in a volume change of ∼0.16%, whereas hypothetical desodiation from the other Na sites leads to much larger volume changes (∼4%), as demonstrated in Supporting Figure S5. Upon desodiation in Na3V(PO3)3N, V3+ is oxidized to V4+, generally accompanying the shrinkage of V−O bond lengths in all VO6 octahedra, which should contribute to the decrease of the crystal volume (Supporting Table T2). However, it was demonstrated that the extraction of Na ions also reduces the electrostatic attraction between the Na1 site and the neighboring six O ions, leading to the effectively larger Na1 vacancy site, which increases the crystal volume (Supporting
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02477. SEM image, structural information, and DFT calculation data of Na3V(PO3)3N (PDF) 7830
DOI: 10.1021/acs.chemmater.7b02477 Chem. Mater. 2017, 29, 7826−7832
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jongsoon Kim: 0000-0002-4122-4874 Kisuk Kang: 0000-0002-8696-1886 Author Contributions #
J.K. and G.Y. contributed equally to this work.
Funding
This work was supported by the faculty research fund of Sejong University in 2017, the National Research Foundation of Korea (NRF) under contract NRF-2012M2A2A6002461, and the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC-2015-C3-054). Notes
The authors declare no competing financial interest.
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