Superposed Redox Chemistry of Fused Carbon Rings in

Subscriber access provided by Washington University | Libraries

Article

Superposed Redox Chemistry of Fused Carbon Rings in Cyclooctatetraenebased Organic Molecules for High-voltage and High-capacity Cathodes Xiaolin Zhao, Wujie Qiu, Chao Ma, Yingqin Zhao, Kai-Xue Wang, Wenqing Zhang, Litao Kang, and Jianjun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15495 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Superposed Redox Chemistry of Fused Carbon Rings in Cyclooctatetraene-based Organic Molecules for High-voltage and High-capacity Cathodes Xiaolin Zhao1,2, Wujie, Qiu2, Chao Ma3, Yingqin Zhao2, Kaixue Wang3, Wenqing Zhang2, Litao Kang1,*, Jianjun Liu2,* 1

Department of Material Science and Engineering, Taiyuan University of Technology, 79

West Yingze Road, Taiyuan, Shanxi, 030024, P. R. China

2

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China

3

Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and

Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China

Corresponding Author * E-mail Address: [email protected]; [email protected] 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

ABSTRACT Even though many organic cathodes were developed and made a significant improvement in energy density and reversibility, some organic materials always generate relatively low voltage and limited discharge capacity because their energy storage mechanism are solely based on redox reactions of limited functional groups (N-O, C=X (X=O, N, S)) linking to aromatic rings. Here, a series of cyclooctatetraene-based (C8H8) organic molecules were demonstrated to have electrochemical activity of high-capacity and high-voltage from carbon rings by means of first-principles calculations and electronic structure analysis. Fused molecules of C8-C4-C8 (C16H12) and C8-C4-C8-C4-C8 (C24H16) contain respective four and eight electron-deficient carbons, generating high-capacity by their multiple redox reactions. Our sodiation calculations predict C16H12 and C24H16 exhibit discharge capacity of 525.3 and 357.2 mAh g-1 with the voltage change from 3.5-1.0 V and 3.7-1.3 V vs Na+/Na, respectively. Electronic structure analysis reveals that the high voltages are attributed to superposed electron stabilization mechanisms including double bond reformation and aromatization from carbon rings. High thermodynamic stability of these C24H16-based systems strongly suggests feasibility of experimental realization. The present work provides evidence that cyclooctatetraene-based organic molecules fused with C4 ring are promising in designing high-capacity and high-voltage organic rechargeable cathodes. KEYWORDS: DFT, COT-based Molecules, high-voltage, aromatization, double bond reformation, Na+-ion battery

2 ACS Paragon Plus Environment

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION The increasing demand for large-scale energy storage systems in electric vehicles (EVs), renewable energy and smart grids strongly push forwards advanced rechargeable battery techniques such as high-performance, low-cost and sustainable electrode materials.1-4 Although some inorganic materials have been developed to solve these issues,5-6 organic electrode materials attract more increasing research interest because they have larger structural flexibility, lower environmental footprints, and more eco-efficient production and disposal than inorganic materials.7-9 In these organic materials, well-designed functional groups (N-O,10 C=X (X=O,8-9,

11

N,12-14 S15-16)) play an important role in generating

electrochemical activity towards reversible Li+- and Na+-storage. However, such characteristic structure commonly results in relatively low discharge capacity and voltage due to limited electroactive sites and electrostatic repulsion between inserted ions. Another challenge faced by these organic electrodes is poor power capability because of their poor electric conductivity.9,

15

Therefore, to develop high-capacity and high-power-density

rechargeable organic electrode material, it is of vital importance to design novel organic molecules with multiple electroactive sites and efficient electron stabilization mechanism. The Na+-storage of carbon rings as electroactive sites is an intriguing question and is considered as an important strategy to increase specific capacity and improve power capability. Although fused C6 aromatic rings was determined to have Li+-storage electroactivity based on 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA),17 Na+-inserted in aromatic C6 rings has been precluded in theory and experiment due to thermodynamically weak binding and kinetically poor migration rate.18 Very recently, Peng et 3 ACS Paragon Plus Environment


Page 4 of 27

al. experimentally and theoretically studied multi-electron redox chemistry of π-conjugated quinoxaline-based heteroaromatic molecules (3Q)12, which made a significant breakthrough in electroactive site change from functional groups to heteroatom-containing rings. Carbon rings in tris N-salicylideneanthraquinoylamine (TSAQ) are not only found to have electrochemical activity for Na+-storage, but also generate a high capacity and good reversibility.19 In spite of facing a large challenge, these studies indicate that carbon rings are promising as important molecular building block to assembly electrode materials of Na+-ion battery. Designing an efficient molecular building block becomes significantly important for developing high-performance organic electrode materials. In nature, fusion of different carbon rings with different electron-efficiency is an important strategy to realize electrochemical reactions of carbon rings. Herein, we proposed cyclooctatetraene-based (C8H8) organic molecules as efficient building block to realize Na+-storage in carbon rings as electroactive centers. Although Stevenson et al. found different cyclooctatetraene-based fused molecules including C8-C6, C8-C4-C8, C8-C8, and C8-C4-C8 could be produced organic cathode materials, detailed electrochemical performance such as capacity and voltage have not yet been reported so far.20 At a fundamental level, electron stabilization capacity of molecular building block during multiple redox reactions determine amount of efficient charge storage during battery operation. These understandings offer a fundamental basis for assembling molecular building blocks into advanced electrode materials for improving electrochemical performance. By using first-principles computational methods, we screened high electroactive fused molecules as building block of electrode materials. The unit of C8-C4-C8 was found to have high Na+-storage capacity of 525.3 mAh 4 ACS Paragon Plus Environment

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


g-1 and voltage of 3.5 V. Electronic structure analysis revealed that redox reactions are mainly occurred in carbon rings through collaborative mechanism of aromatization and double bond reformation. COMPUTATIONAL METHODS All the calculations were performed using the density functional theory (DFT) with the Vienna Ab initio Simulation Package (VASP) and Gaussian 09 package. For the calculation of crystalline structure models, the projector augmented wave21 method as implemented was used in the VASP code.22-23 The generalized gradient approximation (GGA) of the exchange-correlation functional as formulated by Perdew, Burke, and Ernzerhof was assumed.24 For the organic materials, DFT shows proverbially poor ability to depict the long-range van der Waals (VDW) interaction between the molecules.25-27 So the empirical dispersions of Grimme (DFT-D2) was applied to account for the long-range van der Waals interactions.28 DFT-D2 was chosen as our computational functional based on the previous reports and ourselves tests in energy and structure. On one hand, the previous studies show that DFT-D1 has some shortcomings in systematic errors for molecular calculations containing

three-row

elements,

and

inconsistency

for

thermochemistry

due

to

double-counting problems. DFT-D2 made a significant improvement in these problems compared with DFT-D1.29 On the other hand, the previous study show that DFT-D2 method can more accurately calculate the structural change of Na-ions or Li-ions embedded in the organic electrode material compared to DFT-D3 method.30 Our test calculations also exhibit that DFT-D2-calculated structure and energy for C8H8 are more reasonable than those of DFT-D1 and DFT-D3. Based on these results, we took DFT-D2 as our computational method. 5 ACS Paragon Plus Environment


Page 6 of 27

The wave functions were expanded in plane-wave basis set up to a kinetic energy cutoff of 520 eV. Brillouin-zone integrations were performed by using the k-point sampling of the Monkhorst-Pack scheme with a 4×4×3 grid. The total energy was converged to within 10-7 eV and the force on each atom was converged to within 0.01 eV/ Å. The convergence of total energy with respect to the kinetic energy cutoff and the k-point sampling has been carefully examined. Minimization of the total energy was realized with a full relaxation of the atomic positions and cell parameters for each structure. The calculations of all molecular structures were carried out using the Gaussian 09 package.31 The geometrical optimization was performed to obtain more accurate geometrical and energetic information by the B3LYP32-35 functional with diffuse function basis set 6-311+G(d, p).36 The dispersion and basis set corrections to pure hybrid functional B3LYP were not included in our molecular calculations. As shown in Table S1, our test calculations including these corrections for C8H8 and NaC8H8 show B3LYP could generate accurate enough energetic and geometrical data by comparing with experimental report and other methods such as B3LYP-D3, M06 and BSSE. A spin-unrestricted scheme was used for odd number electron systems to allow for any possible bond cleavage during the geometry optimization. To ensure these structures were stabilized, vibrational frequency analyses were done with the same functional and basis set as for the geometry to check on all optimized structures don’t have imaginary frequencies for the minima. In terms of electronic structure, natural bond orbital (NBO)37 analysis was carried out with NBO program included in the Gaussian program package. In order to determine reliability of our computations, we compare the calculated data 6 ACS Paragon Plus Environment

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with experimental ones. The calculated lattice parameters of C8H8 (a=7.177 Å, b=7.193 Å, c=10.605 Å) by VASP are close to the experimental reports (a=7.664 Å, b=7.65 Å, c=10.688 Å).38 In addition, the C=C and C-C bond lengths in a C8H8 crystal are calculated as 1.345 Å, 1.469 Å, which are extremely consistent with experimental values (1.334 Å, 1.465 Å). The calculated discharge voltage of first Na+ is 1.69 V slightly lower than the experimented potential of 1.8 V.20 Furthermore, the calculated molecule of C8H8 by Gaussian is nonplanar tub-shaped geometry of D2d symmetry with alternating single (1.47 Å) and double (1.34 Å) bonds, which is similar to the previous works.39 All the little error margins in lattice parameters (-0.6%), C=O and C=C bond lengths (±0.01 Å) and voltage (-0.11 V) indicate that our computational methods are acceptable in the error range of DFT-calculations. RESULT AND DISCUSSION Determining Electrochemical Activity of Cyclooctatetraene. Hückel’s rule identifies that the annulenes with 4n+2 (n=0, 1, 2…) π-electrons have aromatic stability and planar shape.40-42 In contrast, those annulenes with 4n π-electrons are defined as nonaromatic compounds, resulting in non-planarity and thermodynamic instability. Commonly, the addition of one or two electrons to nonaromatic annulenes can lead to a significant aromatic stability and structural planarization by approaching electron count of 4n+2, which is called as aromatization mechanism. Therefore, nonaromatic molecules with distorted structure should be considered as high-activity electrochemical materials which can stabilize electrons by aromatization mechanism. The previous studies indicated that cyclooctatetraene and corresponding polymers could be made into cathode materials of Li-ion batteries.20 However, occupation sites and kinetic migration rates of ions, voltage, and lattice structure change are 7 ACS Paragon Plus Environment


Page 8 of 27

not discussed so far, leaving the Na+-storage mechanism quite elusive. To elucidate electrochemical activity of cyclooctatetraene, periodic DFT-calculations by using VASP program were performed to obtain Na+-occupied sites evolution, voltage change, ionic diffusion kinetics, and lattice change during the Na+-insertion, as shown in Figure 1. It is well-known that the neutral cyclooctatetraene moiety is distorted away from planarity, exhibiting eclipsed conformation. In crystal, these molecules are unsymmetrically stacked due to dipole interaction. Along (001) direction, the crystal structure exists a relatively large ion-migrated channel with a diameter of 4.07 Å. With Na-ions insert into these channels, C8H8 molecules become planar structure due to accepting additional one or two electrons in each C8H8. DFT-based molecular dynamic simulation for NaC8H8 was performed at room temperature to reveal the ion-migration channel described in Figure 1(b). We further estimated the diffusion coefficient of 4.23×10-9 cm2 s-1, showing a comparable diffusion rate with some inorganic cathode materials such as in P2-Nax[Ni1/3Mn2/3]O2 (1/3

Superposed Redox Chemistry of Fused Carbon Rings in

Recommend Documents