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Polycyclic Quinone Fused by a Sulfur-containing Ring as an Organic Positive-electrode Material for Use in Rechargeable Lithium Batteries
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 89 (2016) 222 – 230
CoE on Sustainable Energy System (Thai-Japan), Faculty of Engineering, Rajamangala University of Technology Thanyaburi (RMUTT), Thailand
Polycyclic quinone fused by a sulfur-containing ring as an organic positive-electrode material for use in rechargeable lithium batteries Masaru Yao*, Hisanori Ando, Tetsu Kiyobayashi Research Institute of Electrochemical Energy, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
Abstract As a minor-metal free positive-electrode active material for use in rechargeable lithium batteries, a polycyclic quinone fused by a sulfur-containing dithiin ring, dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT), was investigated. The prepared electrode in which the SPT was incorporated shows an initial discharge capacity of 231 mAh/g which corresponds to a four-electron transfer reaction of the SPT molecule. The average discharge potential of 2.6 V vs. Li+/Li for the SPT-electrode is 0.4 V higher than that of a previously reported cyclic quinone fused by a benzo-ring, i.e., 5,7,12,14-pentacenetetrone (PT). The effect of introducing saturated sulfur atoms into a polycyclic quinone skeleton is discussed along with the results of the DFT calculation, and a guide for designing an organic molecule which can satisfy both a high discharge potential and long cycle-life requirement is suggested. © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 12th EMSES 2015. Peer-review under responsibility of the organizing committee of the 12th EMSES 2015 Keywords: Lithium ion battery, Organic cathode, High capacity, High discharge potential.
* Corresponding author. Tel.: +81-72-751-9651; fax: +81-72-751-9629. E-mail address: [email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 12th EMSES 2015 doi:10.1016/j.egypro.2016.05.029
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Masaru Yao et al. / Energy Procedia 89 (2016) 222 – 230
1. Introduction In the current rechargeable lithium battery system, minor metal-based positive-electrodes are typically used; therefore, developing minor metal-free and low-polluting safe positive electrode materials is more desirable because of the increasing concern about their limited resources and environmental burden. The use of redox active organic compounds as a positive-electrode active material has recently attracted attention, since these organics contain no scarce metal resources, and they tend to show a high capacity due to their multi-electron transfer redox reactions [1– 3]. Several types of organic positive-electrode active materials have been proposed so far. Among them, we have focused our attention on a series of crystalline quinone-based materials [4–11]. Many quinone-type crystalline compounds show high utilization ratios during the initial charge/discharge cycles [4–12]; however, their discharge capacities often notably decrease upon cycling. One of the reasons for this capacity decay is considered to be the dissolution of the organic active materials into the electrolyte. We have previously reported that expanding the π-system in a quinone molecule can effectively suppress the dissolution level of the active materials in the electrolyte solution, which improves the cycle-stability [8,9]. For example, a large polycyclic quinone derivative, 5,7,12,14-pentacenetetrone (PT, C22H10O4) (Fig. 1a), which has a pentacene-based planar structure, shows a better cycle-stability than a medium-sized polycyclic quinone, 9,10anthraquinone (AQ, C14H8O2) containing an anthracene-type plane. However, enlarging the π-system generally lowers the discharge potential of each molecule, which results in reducing the energy density. To this point, we have investigated several polycyclic quinone derivatives, and discovered that fusing a quinone skeleton by a sulfur atom does not lower the discharge potential. In this study, dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT, C20H8O4S2) (Fig. 1b), a planar polycyclic quinone having a dithiin ring moiety [13], was synthesized as a model of a sulfur-fused large quinone, and its battery performance was compared to that of a similar-sized quinone, PT. The effect of introducing sulfur atoms into a quinone skeleton on the charge/discharge potential was discussed along with considering quantum chemistry calculations.
(a)
(b)
PT
SPT
Fig. 1. Chemical structures of (a) 5,7,12,14-pentacenetetrone (PT) and (b) dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT).
2. Experimental 2.1. Materials Dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT) was synthesized by the reaction between rubeanic acid and 2,3dichloro-1,4-naphthoquinone (Tokyo Kasei. Corp.) as described in the literature [14]. The purity was checked by a 1 H-NMR analysis. Its density was calculated to be 1.6 g/cm3 from the crystallographic data [13]. The other reagents including the components of the electrolyte solution were purchased and used without further purification.
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2.2. Preparation of electrodes and cells The powders of the organic active materials, polytetrafluoroethylene and acetylene black were mixed in the weight ratio of 4:1:5 to prepare a positive-electrode composite sheet. The sheet was then attached on an aluminum mesh current collector. The amount of the organic active material was adjusted to be about 3 mg per electrode. Cointype cells (IEC R2032) composed of the prepared positive-electrode, a lithium metal negative-electrode, and a glass filter separator were assembled with the solution of lithium bis(trifluoromethanesulfonyl)amide / triglyme (1 M, 0.2 mL) in a low dew point (< −70°C) atmosphere.
2.3. Battery test The prepared cells were initially discharged at the constant current density of 20 mA per gram of the organic active material with a cutoff potential of 1.5 V vs. Li +/Li. The following charge/discharge cycle was conducted at the same current density with a potential range of 1.5–3.5 or 1.5–4.0 V vs. Li+/Li. A galvanostat system (BLS series, Keisokuki Center Co., Ltd, or ABE system, Electrofield Co., Ltd) equipped with a thermostatic chamber at 30°C was used for this measurement. In this article, the capacities are expressed in terms of the mass of the organic active material in the electrode.
2.4. Quantum chemistry calculations Quantum chemistry calculations based on the density functional theory (DFT) were carried out using the GAUSSIAN 03W program [15] along with the Gauss View 3.0 software [16]. The geometries of the monomer states were optimized by using a hybrid functional of B3LYP [17,18] with a basis set of 6-31G(d). The electronic structures of the crystalline states were estimated using the crystallographic coordinates [13] at the same B3LYP/631G(d) level under the periodic boundary condition (PBC).
discussion 3. Results and 3.1. Initial discharge behavior Fig. 2 shows the initial discharge curves of the electrodes incorporating PT and SPT. The discharge curve of the PT-electrodes consists of three plateau potential regions at around 2.6, 2.4, and 2.0 V vs. Li +/Li. That of the SPTelectrode has two plateau potential regions at around 2.9 and 2.0 V vs. Li+/Li. The observed multiple-stage discharge behavior reflects the multi-electron-transfer cathodic reaction of these compounds in each electrode. The observed discharge capacities of 278 and 231 mAh/g for the electrodes using PT and SPT, respectively, are close to their theoretical values of 317 mAh/g(PT) and 285 mAh/g(SPT) based on assuming a four-electron transfer reaction per molecule as described in Fig. 3. As for the discharge potential, the SPT-electrode exhibits a higher value than the PT-electrode over a wide range of the discharge process, and the average potential of 2.57 V vs. Li+/Li for the SPTelectrode is 0.35 V higher than the value of 2.22 V vs. Li +/Li for the PT-electrode.
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3RWHQWLDO9YV/L/L
&DSDFLW\ P$KJ Fig. 2. Initial discharge curves of the SPT-electrode (―) and the PT-electrode (---).
(a) 'LVFKDUJH H
H &KDUJH
(b) 'LVFKDUJH H H &KDUJH
Fig. 3. Four-electron redox reactions of (a) PT and (b) SPT.
3.2. Cycle-life performance Fig. 4 shows the result of a cycle-life test for the SPT-electrode along with that of the PT-electrode. Whereas the capacity of the PT-electrode quickly drops during cycling as often observed for low-molecular-weight active materials [4–12], the SPT-electrode performs better. The SPT-electrode is relatively stable during the initial 14 cycles; 86% of the initial capacity at the 14th cycle. While it decreases thereafter, it maintains a capacity of 68 mAh/g (28%) after 50 cycles. As we previous reported, small quinone derivatives easily dissolve in the electrolyte solutions during cycling, which results in a poor cycle-life. Enlarging the molecular plane can indeed suppress the dissolution and effectively improves the cycle stability; however, it often lowers the potential as explained in the introduction [8,9]. In the
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present study, the electrode using SPT shows a relatively stable cycle performance compared to some smaller quinone derivatives [5,6,8,9], although the observed cycle performance of the SPT-electrode is still far from use in practical applications. What is noteworthy in this result is that the operating potential is not lowered by extending the molecular plane using a sulfur-containing ring. Such details are described in the discussion section.
&DSDFLW\ P$KJ
&\FOH QXPEHU Fig. 4. Cycle-life performance of the prepared electrodes (○: SPT, *: PT). Potential range: 1.5–4.0 V vs. Li+/Li for the SPT-electrode, 1.5–3.0 V vs. Li+/Li for the PT electrode.
3.3. DFT calculations for monomers As a theoretical approach to the electrochemical properties of SPT, a DFT calculation was carried out. First, the SPT monomer was calculated. Fig. 5 shows the optimized structure of the SPT. The DFT calculation suggested a planar structure (Fig. 5b) which is similar to the reported crystallographic analysis data [13]. O
(a)
O S
S O
O
(b)
Fig. 5. Optimized structure of the SPT monomer. (a) Top view and (b) side view.
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227
Next, the electronic state of the SPT monomer was compared to those of naphthoquinone (NQ) and PT. Fig. 6 shows the shapes of some obtained orbitals (LUMOs and NLUMOs) along with the energy diagrams. These molecular orbitals are delocalized over the π-system of the molecules and have π-characteristics. Especially, the shapes of the orbitals of SPT and PT seem similar; however, their energy levels differ; i.e., the LUMO and NLUMO of SPT are both lower than those of PT. The energy differences are 0.2 eV and 0.6 eV for the LUMOs and NLUMOs, respectively, which means that SPT should exhibit a 0.4 V higher potential than PT. This calculation is in agreement with the discharge potential difference of 0.35 V between the electrodes using PT and SPT. The LUMO of PT is 0.2 eV lower than that of NQ. The NLUMO of PT, in contrast, is 0.6 eV higher than the LUMO of NQ, which should result in a lower net redox potential for PT than that for NQ. On the other hand, the LUMO level difference between SPT and NQ is very small (< 0.02 eV), and the NLUMO of PT is 0.6 eV lower than the LUMO of NQ, which implies a higher redox potential for SPT than that for NQ.
NLUMO LUMO
LUMO
NLUMO LUMO
HOMO
HOMO ᨿᨿᨿ
ᨿᨿᨿ
HOMO
NQ
PT
ᨿᨿᨿ
(QHUJ\ /HYHO H9
SPT
Fig. 6. The calculated energy diagrams of NQ, PT, and SPT and their calculated LUMOs and NLUMOs.
3.4. Electronic structure of the crystalline state of SPT Fig. 7 shows the crystal structure of SPT [13]. In the crystal, the SPT molecules are one-dimensionally stacked by a π–π interaction to form a columnar structure, which is also prominent in the organic active materials previously reported by us. The mean intermolecular distance in the SPT column is 3.41 Å which is slightly larger than that of the PT column (3.38 Å). In either case, the intermolecular distances are close to the interlayer distance of graphite (3.35 Å).
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a c
b
Fig. 7. Crystal structure of SPT (Crystallographic parameters, Crystal system: triclinic, Space group: P-1, a=4.731(2) Å, b=7.763(3) Å, c=11.037(6) Å, α=97.09(4)°, β=95.01(4)°, γ= 98.60(3)°) [13].
'HQVLW\RI6WDWH '26
As the electronic state of the crystal, the density of state (DOS) was calculated along the π-stacked column of SPT (Fig. 8). The energy levels of some orbitals expand owing to the overlapping of the orbitals, forming electronic band structures. The band at around −4 eV originates from the LUMO of the monomer. During the discharging (reduction) process, electrons can flow into the interior of the crystals along the electronic band [4–11]. The mechanism of the electronic and ionic conductions in the crystals has not yet been experimentally revealed; however, the formation of this electronic band structure can partially explain the high utilization of the electrodes for the quinone-based low-molecular-weight materials having a stacked structure in their crystals.
(QHUJ\/HYHOH9 Fig. 8. Calculated density of state (DOS) along the π-stacked direction in the crystal of SPT.
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4. Discussion As explained in the introduction, π-conjugated large quinone derivatives tend to show low discharge potentials compared to the smaller quinones. In the DFT calculation section of monomers (§3.3), the energy levels of the orbitals of PT and NQ was first compared, because PT is considered to be a dimer of NQ fused by the benzo-ring. The calculation indicates that the energy level of the NLUMO of PT is higher than the LUMO of NQ, which is considered to be due to the effect of the introduced benzo-ring. On the other hand, the energy level of the NLUMO of SPT, which is a dimer of NQ fused by a sulfur-containing dithiin ring, is not significantly affected by the ringfusion. The incorporated sulfur atom has an sp 3-type saturated structure and it should have the ability to separate the conjugated π-electrons of the NQ moiety. This characteristic of the sulfur atom suppresses the energy level increase. As for the LUMO of SPT, the energy level is lower than that of NQ, probably due to the electron withdrawing effect of the sulfur atom via the σ-bond and/or the interaction between the orbitals of the NQ unit and the introduced dithiin unit, which is a favorable effect because it can increase the discharge potential of the molecule. To more clearly understand the sulfur-fusing effect, some benzoquinone oligomers fused by the dithiin ring were additionally calculated and compared to the benzo-fused ones. In this case, a similar tendency in the energy level change was again observed; therefore, introducing the dithiin ring is surely an effective way to maintain or increase the discharge potential of the quinone-based active materials.
5. Conclusions In this study, the applicability of the redox reaction of a sulfur-containing polycyclic quinone, dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT), was examined as a positive-electrode active material for rechargeable lithium batteries. The electrode incorporating SPT showed a more stable cycle life performance than the electrode using the similar sized benzo-fused quinone, 5,7,12,14-pentacenetetrone (PT). Furthermore, a favorable effect of the large sulfur-containing molecular plane on the operating potential was observed, i.e., the undesirable potential decrease due to the extension of the molecular plane, which was often observed for benzo-fused molecules, was suppressed by replacing the benzo-moiety with a sulfur-containing dithiin ring as the connector of the redox active moieties. A quantum calculation using the DFT method also theoretically supported the effect. This finding will be a guide for designing a new organic active material that can satisfy both the high energy density and long cycle-life requirements.
Acknowledgements We thank Ms. Miho Araki, AIST, for her technical support.
References [1] [2] [3] [4] [5] [6] [7] [8]
Armand M, Tarascon JM. Building better batteries. Nature 2008; 451: 652–657. Nishide H, Oyaizu K. Toward flexible batteries. Science 2008; 319: 737–738. Novák P, Müller K, Santhanam SV, Hass O. Electrochemically active polymers for rechargeable batteries. Chem Rev 1997; 97: 207–281. Yao M, Senoh H, Yamazaki S, Siroma Z, Sakai T, Yasuda K. High-capacity organic positive-electrode material based on a benzoquinone derivative for use in rechargeable lithium batteries. J Power Sources 2010; 195: 8336–8340. Yao M, Senoh H, Araki M, Sakai T, Yasuda K. Organic positive-electrode materials based on dialkoxybenzoquinone derivatives for use in rechargeable lithium batteries. ECS Trans 2010; 28(8): 3–10. Yao M, Senoh H, Kuratani K, Sakai T, Kiyobayashi T. Organic rechargeable lithium batteries using anthraquinone-based positive-electrode: the effect of a molecular additive on the cycle-life performance. ITE-IBA Lett 2011; 4: 52–56. Senoh H, Yao M, Sakaebe H, Yasuda, K, Siroma Z. A two-compartment cell for using soluble benzoquinone derivatives as active materials in lithium secondary batteries. Electrochim Acta 2011; 56: 10145–10150. Yao M, Senoh H, Sakai T, Kiyobayashi T. 5,7,12,14- Pentacenetetrone as a high-capacity organic positive-electrode material for use in rechargeable lithium batteries. Int J Electrochem Sci 2011; 6: 2905–2911.
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Masaru Yao et al. / Energy Procedia 89 (2016) 222 – 230 [9] Yao M, Yamazaki S, Senoh H, Sakai T, Kiyobayashi T. Crystalline polycyclic quinone derivatives as organic positive-electrode materials for use in rechargeable lithium batteries. Mater Sci Eng B 2012; 177: 483–487. [10] Yao M, Ando H, Kiyobayashi T. Dialkoxybenzoquinone-type active materials for rechargeable lithium batteries: the effect of the alkoxy group length on the cycle-stability, Energy Procedia 2013; 34: 880–887. [11] Yao M, Numoto T, Araki M, Ando H, Takeshita H. T, Kiyobayashi T. Long cycle-life organic electrode material based on an ionic naphthoquinone derivative for rechargeable batteries. Energy Procedia 2014; 56: 228–236. [12] Song Z, Zhan H, Zhou Y. Anthraquinone based polymer as high performance cathode material for rechargeable lithium batteries. Chem Commun 2009; 4: 448–450. [13] Martinez ES, Calleja RD, Behrenr J, Barges P, Kudnig J, Wölki N, Klar G. Dichalcogenins with electron-withdrawing substituents. Part 2. Structure, electrical conductivity and dielectric relaxation of dinaphtho[2,3-b;2’,3’-e][1,4]dithiine-5,7,12,14-tetraone. J Chem Res (M) 1991; 9: 2379–2396. [14] Fenical W, Jensen PR, La Clair JJ, Trzoss L, Fukuda T (The regents of the university of California). Seriniquinones, melanoma-specific anticancer agents; 2014, Patent: WO2014/18919 A1. [15] Frisch MJ. et al. Gaussian 03, Revision E.01, Gaussian, Inc., Wallingford, CT 2004. [16] Dennington R II, Todd K, Millam J, Eppinnett K, Hovell WL, Ray G. GaaussView, Version 4.1.2, Semichem, Inc., Shawnee Mission, KS, 2003. [17] Becke AD. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys Rev A 1988; 38: 3098–3100. [18] Lee C, Yang W, Parr RG. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys Rev B 1988; 37: 785–789.
ScienceDirect Energy Procedia 89 (2016) 222 – 230
CoE on Sustainable Energy System (Thai-Japan), Faculty of Engineering, Rajamangala University of Technology Thanyaburi (RMUTT), Thailand
Polycyclic quinone fused by a sulfur-containing ring as an organic positive-electrode material for use in rechargeable lithium batteries Masaru Yao*, Hisanori Ando, Tetsu Kiyobayashi Research Institute of Electrochemical Energy, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
Abstract As a minor-metal free positive-electrode active material for use in rechargeable lithium batteries, a polycyclic quinone fused by a sulfur-containing dithiin ring, dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT), was investigated. The prepared electrode in which the SPT was incorporated shows an initial discharge capacity of 231 mAh/g which corresponds to a four-electron transfer reaction of the SPT molecule. The average discharge potential of 2.6 V vs. Li+/Li for the SPT-electrode is 0.4 V higher than that of a previously reported cyclic quinone fused by a benzo-ring, i.e., 5,7,12,14-pentacenetetrone (PT). The effect of introducing saturated sulfur atoms into a polycyclic quinone skeleton is discussed along with the results of the DFT calculation, and a guide for designing an organic molecule which can satisfy both a high discharge potential and long cycle-life requirement is suggested. © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 12th EMSES 2015. Peer-review under responsibility of the organizing committee of the 12th EMSES 2015 Keywords: Lithium ion battery, Organic cathode, High capacity, High discharge potential.
* Corresponding author. Tel.: +81-72-751-9651; fax: +81-72-751-9629. E-mail address: [email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 12th EMSES 2015 doi:10.1016/j.egypro.2016.05.029
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Masaru Yao et al. / Energy Procedia 89 (2016) 222 – 230
1. Introduction In the current rechargeable lithium battery system, minor metal-based positive-electrodes are typically used; therefore, developing minor metal-free and low-polluting safe positive electrode materials is more desirable because of the increasing concern about their limited resources and environmental burden. The use of redox active organic compounds as a positive-electrode active material has recently attracted attention, since these organics contain no scarce metal resources, and they tend to show a high capacity due to their multi-electron transfer redox reactions [1– 3]. Several types of organic positive-electrode active materials have been proposed so far. Among them, we have focused our attention on a series of crystalline quinone-based materials [4–11]. Many quinone-type crystalline compounds show high utilization ratios during the initial charge/discharge cycles [4–12]; however, their discharge capacities often notably decrease upon cycling. One of the reasons for this capacity decay is considered to be the dissolution of the organic active materials into the electrolyte. We have previously reported that expanding the π-system in a quinone molecule can effectively suppress the dissolution level of the active materials in the electrolyte solution, which improves the cycle-stability [8,9]. For example, a large polycyclic quinone derivative, 5,7,12,14-pentacenetetrone (PT, C22H10O4) (Fig. 1a), which has a pentacene-based planar structure, shows a better cycle-stability than a medium-sized polycyclic quinone, 9,10anthraquinone (AQ, C14H8O2) containing an anthracene-type plane. However, enlarging the π-system generally lowers the discharge potential of each molecule, which results in reducing the energy density. To this point, we have investigated several polycyclic quinone derivatives, and discovered that fusing a quinone skeleton by a sulfur atom does not lower the discharge potential. In this study, dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT, C20H8O4S2) (Fig. 1b), a planar polycyclic quinone having a dithiin ring moiety [13], was synthesized as a model of a sulfur-fused large quinone, and its battery performance was compared to that of a similar-sized quinone, PT. The effect of introducing sulfur atoms into a quinone skeleton on the charge/discharge potential was discussed along with considering quantum chemistry calculations.
(a)
(b)
PT
SPT
Fig. 1. Chemical structures of (a) 5,7,12,14-pentacenetetrone (PT) and (b) dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT).
2. Experimental 2.1. Materials Dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT) was synthesized by the reaction between rubeanic acid and 2,3dichloro-1,4-naphthoquinone (Tokyo Kasei. Corp.) as described in the literature [14]. The purity was checked by a 1 H-NMR analysis. Its density was calculated to be 1.6 g/cm3 from the crystallographic data [13]. The other reagents including the components of the electrolyte solution were purchased and used without further purification.
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2.2. Preparation of electrodes and cells The powders of the organic active materials, polytetrafluoroethylene and acetylene black were mixed in the weight ratio of 4:1:5 to prepare a positive-electrode composite sheet. The sheet was then attached on an aluminum mesh current collector. The amount of the organic active material was adjusted to be about 3 mg per electrode. Cointype cells (IEC R2032) composed of the prepared positive-electrode, a lithium metal negative-electrode, and a glass filter separator were assembled with the solution of lithium bis(trifluoromethanesulfonyl)amide / triglyme (1 M, 0.2 mL) in a low dew point (< −70°C) atmosphere.
2.3. Battery test The prepared cells were initially discharged at the constant current density of 20 mA per gram of the organic active material with a cutoff potential of 1.5 V vs. Li +/Li. The following charge/discharge cycle was conducted at the same current density with a potential range of 1.5–3.5 or 1.5–4.0 V vs. Li+/Li. A galvanostat system (BLS series, Keisokuki Center Co., Ltd, or ABE system, Electrofield Co., Ltd) equipped with a thermostatic chamber at 30°C was used for this measurement. In this article, the capacities are expressed in terms of the mass of the organic active material in the electrode.
2.4. Quantum chemistry calculations Quantum chemistry calculations based on the density functional theory (DFT) were carried out using the GAUSSIAN 03W program [15] along with the Gauss View 3.0 software [16]. The geometries of the monomer states were optimized by using a hybrid functional of B3LYP [17,18] with a basis set of 6-31G(d). The electronic structures of the crystalline states were estimated using the crystallographic coordinates [13] at the same B3LYP/631G(d) level under the periodic boundary condition (PBC).
discussion 3. Results and 3.1. Initial discharge behavior Fig. 2 shows the initial discharge curves of the electrodes incorporating PT and SPT. The discharge curve of the PT-electrodes consists of three plateau potential regions at around 2.6, 2.4, and 2.0 V vs. Li +/Li. That of the SPTelectrode has two plateau potential regions at around 2.9 and 2.0 V vs. Li+/Li. The observed multiple-stage discharge behavior reflects the multi-electron-transfer cathodic reaction of these compounds in each electrode. The observed discharge capacities of 278 and 231 mAh/g for the electrodes using PT and SPT, respectively, are close to their theoretical values of 317 mAh/g(PT) and 285 mAh/g(SPT) based on assuming a four-electron transfer reaction per molecule as described in Fig. 3. As for the discharge potential, the SPT-electrode exhibits a higher value than the PT-electrode over a wide range of the discharge process, and the average potential of 2.57 V vs. Li+/Li for the SPTelectrode is 0.35 V higher than the value of 2.22 V vs. Li +/Li for the PT-electrode.
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3RWHQWLDO9YV/L/L
&DSDFLW\ P$KJ Fig. 2. Initial discharge curves of the SPT-electrode (―) and the PT-electrode (---).
(a) 'LVFKDUJH H
H &KDUJH
(b) 'LVFKDUJH H H &KDUJH
Fig. 3. Four-electron redox reactions of (a) PT and (b) SPT.
3.2. Cycle-life performance Fig. 4 shows the result of a cycle-life test for the SPT-electrode along with that of the PT-electrode. Whereas the capacity of the PT-electrode quickly drops during cycling as often observed for low-molecular-weight active materials [4–12], the SPT-electrode performs better. The SPT-electrode is relatively stable during the initial 14 cycles; 86% of the initial capacity at the 14th cycle. While it decreases thereafter, it maintains a capacity of 68 mAh/g (28%) after 50 cycles. As we previous reported, small quinone derivatives easily dissolve in the electrolyte solutions during cycling, which results in a poor cycle-life. Enlarging the molecular plane can indeed suppress the dissolution and effectively improves the cycle stability; however, it often lowers the potential as explained in the introduction [8,9]. In the
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present study, the electrode using SPT shows a relatively stable cycle performance compared to some smaller quinone derivatives [5,6,8,9], although the observed cycle performance of the SPT-electrode is still far from use in practical applications. What is noteworthy in this result is that the operating potential is not lowered by extending the molecular plane using a sulfur-containing ring. Such details are described in the discussion section.
&DSDFLW\ P$KJ
&\FOH QXPEHU Fig. 4. Cycle-life performance of the prepared electrodes (○: SPT, *: PT). Potential range: 1.5–4.0 V vs. Li+/Li for the SPT-electrode, 1.5–3.0 V vs. Li+/Li for the PT electrode.
3.3. DFT calculations for monomers As a theoretical approach to the electrochemical properties of SPT, a DFT calculation was carried out. First, the SPT monomer was calculated. Fig. 5 shows the optimized structure of the SPT. The DFT calculation suggested a planar structure (Fig. 5b) which is similar to the reported crystallographic analysis data [13]. O
(a)
O S
S O
O
(b)
Fig. 5. Optimized structure of the SPT monomer. (a) Top view and (b) side view.
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Next, the electronic state of the SPT monomer was compared to those of naphthoquinone (NQ) and PT. Fig. 6 shows the shapes of some obtained orbitals (LUMOs and NLUMOs) along with the energy diagrams. These molecular orbitals are delocalized over the π-system of the molecules and have π-characteristics. Especially, the shapes of the orbitals of SPT and PT seem similar; however, their energy levels differ; i.e., the LUMO and NLUMO of SPT are both lower than those of PT. The energy differences are 0.2 eV and 0.6 eV for the LUMOs and NLUMOs, respectively, which means that SPT should exhibit a 0.4 V higher potential than PT. This calculation is in agreement with the discharge potential difference of 0.35 V between the electrodes using PT and SPT. The LUMO of PT is 0.2 eV lower than that of NQ. The NLUMO of PT, in contrast, is 0.6 eV higher than the LUMO of NQ, which should result in a lower net redox potential for PT than that for NQ. On the other hand, the LUMO level difference between SPT and NQ is very small (< 0.02 eV), and the NLUMO of PT is 0.6 eV lower than the LUMO of NQ, which implies a higher redox potential for SPT than that for NQ.
NLUMO LUMO
LUMO
NLUMO LUMO
HOMO
HOMO ᨿᨿᨿ
ᨿᨿᨿ
HOMO
NQ
PT
ᨿᨿᨿ
(QHUJ\ /HYHO H9
SPT
Fig. 6. The calculated energy diagrams of NQ, PT, and SPT and their calculated LUMOs and NLUMOs.
3.4. Electronic structure of the crystalline state of SPT Fig. 7 shows the crystal structure of SPT [13]. In the crystal, the SPT molecules are one-dimensionally stacked by a π–π interaction to form a columnar structure, which is also prominent in the organic active materials previously reported by us. The mean intermolecular distance in the SPT column is 3.41 Å which is slightly larger than that of the PT column (3.38 Å). In either case, the intermolecular distances are close to the interlayer distance of graphite (3.35 Å).
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a c
b
Fig. 7. Crystal structure of SPT (Crystallographic parameters, Crystal system: triclinic, Space group: P-1, a=4.731(2) Å, b=7.763(3) Å, c=11.037(6) Å, α=97.09(4)°, β=95.01(4)°, γ= 98.60(3)°) [13].
'HQVLW\RI6WDWH '26
As the electronic state of the crystal, the density of state (DOS) was calculated along the π-stacked column of SPT (Fig. 8). The energy levels of some orbitals expand owing to the overlapping of the orbitals, forming electronic band structures. The band at around −4 eV originates from the LUMO of the monomer. During the discharging (reduction) process, electrons can flow into the interior of the crystals along the electronic band [4–11]. The mechanism of the electronic and ionic conductions in the crystals has not yet been experimentally revealed; however, the formation of this electronic band structure can partially explain the high utilization of the electrodes for the quinone-based low-molecular-weight materials having a stacked structure in their crystals.
(QHUJ\/HYHOH9 Fig. 8. Calculated density of state (DOS) along the π-stacked direction in the crystal of SPT.
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4. Discussion As explained in the introduction, π-conjugated large quinone derivatives tend to show low discharge potentials compared to the smaller quinones. In the DFT calculation section of monomers (§3.3), the energy levels of the orbitals of PT and NQ was first compared, because PT is considered to be a dimer of NQ fused by the benzo-ring. The calculation indicates that the energy level of the NLUMO of PT is higher than the LUMO of NQ, which is considered to be due to the effect of the introduced benzo-ring. On the other hand, the energy level of the NLUMO of SPT, which is a dimer of NQ fused by a sulfur-containing dithiin ring, is not significantly affected by the ringfusion. The incorporated sulfur atom has an sp 3-type saturated structure and it should have the ability to separate the conjugated π-electrons of the NQ moiety. This characteristic of the sulfur atom suppresses the energy level increase. As for the LUMO of SPT, the energy level is lower than that of NQ, probably due to the electron withdrawing effect of the sulfur atom via the σ-bond and/or the interaction between the orbitals of the NQ unit and the introduced dithiin unit, which is a favorable effect because it can increase the discharge potential of the molecule. To more clearly understand the sulfur-fusing effect, some benzoquinone oligomers fused by the dithiin ring were additionally calculated and compared to the benzo-fused ones. In this case, a similar tendency in the energy level change was again observed; therefore, introducing the dithiin ring is surely an effective way to maintain or increase the discharge potential of the quinone-based active materials.
5. Conclusions In this study, the applicability of the redox reaction of a sulfur-containing polycyclic quinone, dibenzo[b,i]thianthrene-5,7,12,14-tetrone (SPT), was examined as a positive-electrode active material for rechargeable lithium batteries. The electrode incorporating SPT showed a more stable cycle life performance than the electrode using the similar sized benzo-fused quinone, 5,7,12,14-pentacenetetrone (PT). Furthermore, a favorable effect of the large sulfur-containing molecular plane on the operating potential was observed, i.e., the undesirable potential decrease due to the extension of the molecular plane, which was often observed for benzo-fused molecules, was suppressed by replacing the benzo-moiety with a sulfur-containing dithiin ring as the connector of the redox active moieties. A quantum calculation using the DFT method also theoretically supported the effect. This finding will be a guide for designing a new organic active material that can satisfy both the high energy density and long cycle-life requirements.
Acknowledgements We thank Ms. Miho Araki, AIST, for her technical support.
References [1] [2] [3] [4] [5] [6] [7] [8]
Armand M, Tarascon JM. Building better batteries. Nature 2008; 451: 652–657. Nishide H, Oyaizu K. Toward flexible batteries. Science 2008; 319: 737–738. Novák P, Müller K, Santhanam SV, Hass O. Electrochemically active polymers for rechargeable batteries. Chem Rev 1997; 97: 207–281. Yao M, Senoh H, Yamazaki S, Siroma Z, Sakai T, Yasuda K. High-capacity organic positive-electrode material based on a benzoquinone derivative for use in rechargeable lithium batteries. J Power Sources 2010; 195: 8336–8340. Yao M, Senoh H, Araki M, Sakai T, Yasuda K. Organic positive-electrode materials based on dialkoxybenzoquinone derivatives for use in rechargeable lithium batteries. ECS Trans 2010; 28(8): 3–10. Yao M, Senoh H, Kuratani K, Sakai T, Kiyobayashi T. Organic rechargeable lithium batteries using anthraquinone-based positive-electrode: the effect of a molecular additive on the cycle-life performance. ITE-IBA Lett 2011; 4: 52–56. Senoh H, Yao M, Sakaebe H, Yasuda, K, Siroma Z. A two-compartment cell for using soluble benzoquinone derivatives as active materials in lithium secondary batteries. Electrochim Acta 2011; 56: 10145–10150. Yao M, Senoh H, Sakai T, Kiyobayashi T. 5,7,12,14- Pentacenetetrone as a high-capacity organic positive-electrode material for use in rechargeable lithium batteries. Int J Electrochem Sci 2011; 6: 2905–2911.
230
Masaru Yao et al. / Energy Procedia 89 (2016) 222 – 230 [9] Yao M, Yamazaki S, Senoh H, Sakai T, Kiyobayashi T. Crystalline polycyclic quinone derivatives as organic positive-electrode materials for use in rechargeable lithium batteries. Mater Sci Eng B 2012; 177: 483–487. [10] Yao M, Ando H, Kiyobayashi T. Dialkoxybenzoquinone-type active materials for rechargeable lithium batteries: the effect of the alkoxy group length on the cycle-stability, Energy Procedia 2013; 34: 880–887. [11] Yao M, Numoto T, Araki M, Ando H, Takeshita H. T, Kiyobayashi T. Long cycle-life organic electrode material based on an ionic naphthoquinone derivative for rechargeable batteries. Energy Procedia 2014; 56: 228–236. [12] Song Z, Zhan H, Zhou Y. Anthraquinone based polymer as high performance cathode material for rechargeable lithium batteries. Chem Commun 2009; 4: 448–450. [13] Martinez ES, Calleja RD, Behrenr J, Barges P, Kudnig J, Wölki N, Klar G. Dichalcogenins with electron-withdrawing substituents. Part 2. Structure, electrical conductivity and dielectric relaxation of dinaphtho[2,3-b;2’,3’-e][1,4]dithiine-5,7,12,14-tetraone. J Chem Res (M) 1991; 9: 2379–2396. [14] Fenical W, Jensen PR, La Clair JJ, Trzoss L, Fukuda T (The regents of the university of California). Seriniquinones, melanoma-specific anticancer agents; 2014, Patent: WO2014/18919 A1. [15] Frisch MJ. et al. Gaussian 03, Revision E.01, Gaussian, Inc., Wallingford, CT 2004. [16] Dennington R II, Todd K, Millam J, Eppinnett K, Hovell WL, Ray G. GaaussView, Version 4.1.2, Semichem, Inc., Shawnee Mission, KS, 2003. [17] Becke AD. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys Rev A 1988; 38: 3098–3100. [18] Lee C, Yang W, Parr RG. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys Rev B 1988; 37: 785–789.