Crystalline polycyclic quinone derivatives as organic positive-electrode materials for use in rechargeable lithium batteries

Materials Science and Engineering B 177 (2012) 483–487

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Crystalline polycyclic quinone derivatives as organic positive-electrode materials for use in rechargeable lithium batteries Masaru Yao ∗ , Shin-ichi Yamazaki, Hiroshi Senoh, Tetsuo Sakai, Tetsu Kiyobayashi Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

a r t i c l e

i n f o

Article history: Received 20 September 2011 Received in revised form 27 December 2011 Accepted 6 February 2012 Available online 18 February 2012 Keywords: Organic positive-electrode material Lithium ion battery Multi-electron redox

a b s t r a c t The performance of 9,10-anthraquinone (AQ), and 5,7,12,14-pentacenetetrone (PT) as active materials for rechargeable lithium batteries was investigated. Positive-electrodes in which AQ and PT were incorporated showed initial discharge capacities of greater than 200 mAh/g(AQ or PT) . The obtained discharge capacities suggest that a multi-electron redox reaction takes place in each derivative. The discharge capacity of the positive-electrode with AQ rapidly decreased during the charge/discharge cycles; however, the positive-electrode with PT showed a relatively good cycle-life performance; it maintained about 80% of the initial capacity even after 100 cycles. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Rechargeable lithium batteries have been widely used as the major power source for daily-use portable electric devices. Typical rechargeable lithium batteries are composed of a metal-oxide based positive-electrode and a graphite based negative-electrode, and various materials have been proposed to increase their energy densities. Furthermore, rare metal-free and low-polluting safe materials are recently becoming more desirable especially for the positive-electrode due to concern about their resource scarcity and environmental burden. One of the candidate categories is redox active organic materials that contain no scarce metal resources [1–13]. Several types of organic positive-electrode materials have already been proposed mainly based on polymers. As organic positive-electrode materials, we have focused our attention on the quinone-based materials, since the quinone skeleton undergoes a two-electron redox reaction which should lead to a high discharge capacity [14–17]. The simplest quinone compound, 1,4-benzoquinone (BQ) (Fig. 1a), has the possibility to have a high capacity of up to about 500 mAh/g; however, the use of BQ is difficult in practice because it is prone to sublimate and escape from the electrode. To immobilize the quinone skeleton in the electrode of batteries, polymerization is an approach, as suggested by several scientists [3,7,8]. However, the desired two-electron redox reaction per quinone skeleton has not been realized for these polymers. Recently, another approach

∗ Corresponding author. Tel.: +81 72 751 9651; fax: +81 72 751 9629. E-mail address: [email protected] (M. Yao). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.02.007

has been investigated, in which redox active low-molecular-weight crystalline organic compounds are used instead of polymerized materials [9–18]. The reported organic compounds tend to show high utilization ratios during the charge/discharge processes. In these cases, each molecule is immobilized by an intermolecular attraction such as the van der Waals’ force, ␲–␲ interaction and hydrogen bonding. Among the low-molecular-weight organic compounds reported so far, 9,10-anthraquinone (AQ, C14 H8 O2 ) (Fig. 1b), a polycyclic quinone derivative having an anthracene skeleton, also shows a discharge capacity close to its theoretical value; however, the AQbased electrode tends to degrade upon cycling because of the dissolution of the redox-related molecules into the electrolyte solution during cycling [10,16]. To suppress the dissolution of the quinone derivatives and to improve their cycle stability, we considered that enlargement of the ␲-system can be an effective way. As a larger polycyclic quinone derivative, we focused on 5,7,12,14pentacenetetrone (PT, C22 H10 O4 ) (Fig. 1c). PT is a planar molecule and has a highly developed ␲-system which can induce a strong ␲–␲ intermolecular interaction in the crystal. In our previous study [17], we demonstrated that an electrode containing PT had the potential to work as the positive electrode for lithium ion batteries at around 2.1 V vs. Li+ /Li with the initial discharge capacity of greater than 300 mAh/g. However, the cycle-life of the PT electrode was far too short for such an application; the capacity dropped to less than a sixth after 50 cycles when ␥-butyrolactone (GBL) was used as the electrolyte solvent. In the present study, we investigated the influence of the electrolyte solvent on the cycle-life of the PT electrode, and discovered that the PT electrode can be used much longer when an ether-based electrolyte solution,

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O

(a)

(b)

(c)

O

O

O

BQ

AQ

O

O

O

O

PT

Fig. 1. Chemical structures of: (a) 1,4-benzoquinone (BQ), (b) 9,10-anthraquinone (AQ), and (c) 5,7,12,14-pentacenetetrone (PT).

tetraglyme/lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), was used. Also described in the present study are the cyclic voltammetry of PT in the solution state, X-ray diffraction analysis of the electrode after several cycles and a rigorous quantum calculation considering the periodic boundary conditions (PBC) for which a primitive calculation was previously presented. These properties were compared to the smaller polycyclic quinone, AQ.

An ex situ X-ray diffraction (XRD) measurement was used to analyze the change in the crystal structure of the organic active materials upon charge/discharge cycling using an X’Pert PRO MPD (PANalytical B.V.) diffractometer. The electrodes removed from the cells after the charge/discharge cycling were washed with tetrahydrofuran to dissolve out the electrolyte salt before the measurement.

2. Experimental

2.4. Theoretical calculations

2.1. Materials

A quantum chemistry calculation based on the density functional theory (DFT) was performed using the GAUSSIAN 03 program package [22]. For an estimation of the electronic structure of the crystal state, a single point calculation considering the periodic boundary condition (PBC) was performed using the coordinates extracted from the X-ray analysis [19,20] at a B3LYP [23,24]/631G(d) level. The calculated molecular orbitals were visualized by Gauss View 3.0 [25].

2.2. Preparation of electrodes and cells Coin-type sealed cells for the battery tests were prepared as follows. A positive-electrode composite sheet was first prepared by mixing the AQ or PT powder, acetylene black as the conductive additive, and polytetrafluoroethylene as the binder in the weight ratio of 4:5:1. The sheet was then attached to an aluminum mesh current collector, and the resultant positive-electrode was dried. The amount of active material deposited was approximately 3 mg per electrode (AQ: 3.08 mg, PT: 3.10 mg) (electrode area: about 0.5 cm2 ). The prepared positive-electrode and a lithium metal negative-electrode were placed in an IEC R2032 coin-type cell case with a glass filter as the separator. After the electrolyte solution of tetraglyme/lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) equimolar mixture [21] (0.2 mL) was added, the cell case was sealed. 2.3. Measurements Apart from the battery tests, cyclic voltammetry (CV) was applied to a saturated solution of AQ or PT in an N,Ndimethylformamide (DMF)/lithium perchlorate system (0.1 mol/L) with an Ag+ /Ag reference electrode using an electrochemical analyzer (VersaSTAT-4, Princeton Applied Research). The working and counter electrodes were a glassy carbon disk and a platinum disk, respectively. The voltammograms were recorded at the scan speed of 50 mV/s in the potential range of −0.4 to −2.4 V vs. Ag+ /Ag at room temperature. The potential of the voltammograms is expressed as the value with respect to the potential of ferrocenium/ferrocene (Fc+ /Fc). During the charge/discharge cycle-life test, the prepared cointype cell was galvanostatically discharged at the current density of 20 mA/g(PT or AQ) with a cut-off voltage of 1.5 V vs. Li+ /Li, and charged at the same current density with a cut-off voltage of 3.0 V vs. Li+ /Li. The charge/discharge test was performed by a computercontrolled system (ABE System, Electrofield Co., Ltd.) equipped with a thermostatic chamber at 30 ◦ C. In this paper, the obtained capacities are expressed in terms of per mass of the active materials.

3. Results and discussion 3.1. Electrochemical property of the solution states To evaluate the preliminary electrochemical properties of AQ and PT, a CV measurement was first carried out for their DMF solutions. Fig. 2 shows the obtained cyclic voltammograms. As shown, AQ showed a reversible stepwise redox behavior, in which two redox pairs at around −1.3 and −1.6 V vs. Fc+ /Fc were observed, which reflects the two-electron transfer reaction per one AQ molecule. These potentials can be converted to 2.4 and 2.1 V vs. Li+ /Li. The pair at the higher potential corresponds to the redox reaction between the neutral states of AQ and the monoanion radical states and that at the lower potential to the redox reaction between the monoanion radical state and the dianion species. On the contrary, PT showed a complex redox behavior. Two reversible redox pairs at around −0.9 and −1.3 V vs. Fc+ /Fc, and one additional quasi-reversible asymmetric redox pair at around −1.9 V

AQ (a) Current, aubit.

9,10-Anthraquinone (AQ) (Kanto Chemical Co., Inc.), and 5,7,12,14-pentacenetetrone (PT) (Tokyo Kasei Corp.) were purchased and used without further purification. The densities calculated from the crystallographic data are 1.48 and 1.51 g/cm3 for AQ [19] and PT [20], respectively.

PT (b)

-3

-2

-1

Potential, V vs. Fc+/Fc Fig. 2. Cyclic voltammograms of DMF solutions of AQ and PT.

0

vs. Fc+ /Fc were observed. These potentials can be converted to 2.8, 2.4, and 1.8 V vs. Li+ /Li. Miller et al. reported a cyclic voltammogram of PT using a different electrolyte system, and inferred that a four-electron redox reaction was taking place in PT. In their report, they explained that the third redox wave at the lowest potential could be resolved into two closely spaced couples [26]. Fig. 2b is also consistent with a four-electron redox as the peak area of the redox pair at the lowest potential is about twice those of the pairs in the higher potential region. The observed redox behavior in a solution state indicates that more than three electrons can transfer during the redox reaction of PT, which implies the use of the PT moiety as a high-capacity positive-electrode material.

a

4

Potential, V vs. Li+/Li

M. Yao et al. / Materials Science and Engineering B 177 (2012) 483–487

3

485

AQ

1st-5th

2 1st-5th

1

0

0

3.2. Charge/discharge properties

50

100

150

200

250

Capacity, mAh/g

3.3. DFT calculations To obtain theoretical insight into the electronic conduction mechanism of the crystalline states of AQ and PT, theoretical calculations based on the density functional theory were carried out. First, the electronic states of the monomer states of AQ and PT

b 4

PT Potential, V vs. Li+/Li

Fig. 3 shows the first several charge/discharge curves of the positive-electrodes prepared using AQ and PT. The AQ electrode has a plateau at around 2.1 V vs. Li+ /Li during the discharge process. The observed discharge capacity of 217 mAh/g is 84% of the theoretical value of 257 mAh/g, which assumes the two-electron transfer reaction of AQ (Fig. 4a). On the other hand, PT showed a characteristic discharge curve consisting of a few plateau voltage regions at around 2.5, 2.3, and 1.8 V vs. Li+ /Li. This stepwise redox behavior is similar to the CV measurement results. The observed discharge capacity of 236 mAh/g is greater than that of AQ; while it is 74% of the theoretical value of 317 mAh/g, which assumes the full four-electron transfer reaction of PT (Fig. 4b). The capacity depends on the solvent (details are given below). In both cases, the obtained discharge capacities are greater than the practical capacity of the conventional LiCoO2 (∼140 mAh/g). In particular, the theoretical value of 317 mAh/g for PT is more than twice the capacity of LiCoO2 . From the viewpoint of energy density, the higher gravimetric capacities of these compounds may make up for the drawback that the average potentials of about 2.1 V vs. Li+ /Li are less positive than those of conventional positive-electrode materials, such as LiCoO2 (3.8 V vs. Li+ /Li) and LiFePO4 (3.4 V vs. Li+ /Li). Incidentally, the potentials of the present compounds are comparable to those of sulfide-based active materials (about 2 V vs. Li+ /Li).

1st-5th

3

2 2nd-5th 1st

1

0 0

50

100

were examined. Since both AQ and PT themselves initially undergo reduction reactions during the discharge processes, the electronic state of the unoccupied orbitals, such as the LUMOs, is important. The obtained LUMOs and NLUMOs of the monomer state of AQ and PT are shown in Fig. 5 along with the corresponding energy diagrams. Each molecular orbital has ␲-characteristics and is delocalized over the entire ␲-system of the molecules. The energy level of the LUMO of PT is 0.6 eV lower than that of AQ, which implies a higher redox potential for PT than that for AQ at least for the first two-electron redox. The calculation is in agreement with the redox

O

- 2eCharge

O

(b) O

O

Discharge

O

O

O

O

+ 4e- 4eO

O

250

Fig. 3. Charge/discharge curves of the electrodes using: (a) AQ and (b) PT during early cycles. (Current density: 20 mA/g, Potential range: 1.5 − 3.0 V vs. Li+ /Li.)

Discharge + 2e-

O

200

Capacity, mAh/g

(a) O

150

Charge

Fig. 4. Multi-electron redox reactions of: (a) AQ and (b) PT.

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a AQ Powder Initial electrode 1st cycle 2nd cycle

10

20

30

40

2θ, deg. (Cu Kα) b Fig. 5. The calculated energy diagrams of AQ and PT monomers, and some molecular orbitals. The geometries of these molecules are optimized at the B3LYP/6-31G(d) level.

potentials of AQ and PT observed in the CV measurement and the battery test; PT showed the higher redox potentials in the CV measurement (0.3–0.4 V) and the battery test (0.2–0.4 V) for the first two-electron transfer. The electronic states of the crystalline states of AQ and PT were calculated using the PBC technique. In the crystal, the molecules of AQ and PT are stacked one-dimensionally by ␲–␲ interactions to form column structures [19,20] as shown in Fig. 6a and b. In the columns, the mean intermolecular distances are 3.48 A˚ and 3.38 A˚ for AQ and PT, respectively, and these values are close to ˚ The densities of states the interlayer distance of graphite (3.35 A). (DOS) calculated for the ␲-stacked columns are shown in Fig. 6c. The energy levels of the many orbitals of both AQ and PT expand,

PT

Powder Initial electrode 1st cycle 2nd cycle

10

20

30

40

2θ, deg. (Cu Kα) Fig. 7. XRD patterns of: (a) AQ-based samples and (b) PT-based samples. All the patterns reflect the neutral (charged) states. (The peaks at 18◦ , 26◦ , and 38◦ are ascribed to PTFE, AB, and Al incorporated in the electrodes, respectively.)

forming electronic band structures along the ␲-stacked directions. The bands at around −3 eV that originated from the LUMOs of the isolated monomer also expand due to the overlapping of the ␲orbitals. The DOSs along the other directions were also calculated; however, they did not show band-like structures, which indicates less overlapping of the orbitals along the other directions. These electronic band structures can serve as electron-transfer pathways during the charge/discharge processes; thereby electrons can flow into the interior of the crystals, which is not directly attached to the current collector or a conductive additive [14–17]. While the mechanism of electronic conduction or ionic conduction in the crystals has not been experimentally revealed at present, the calculated characteristic electronic structure should contribute to the high utilization ratio of the molecules of AQ and PT in the crystals during the charge/discharge processes.

3.4. XRD analysis

Fig. 6. One-dimensionally stacked structures of: (a) AQ [19] and (b) PT [20] in the crystals. (c) Calculated density of states (DOS) for the stacked columns.

To examine the reversibility of the crystal structures of AQ and PT upon cycling, an XRD measurement was performed. Fig. 7 shows the comparisons of the XRD patterns of the charged (neutral) states of the electrodes upon cycling along with those of the crude powder samples. During the first discharge, the XRD peaks ascribed to the charged forms of AQ and PT become weak and broad. With subsequent charging, the XRD patterns of the electrodes were reversed to the original charged forms. This observation revealed that the crystalline states of AQ and PT in the electrodes are retained during cycling, suggesting that the changes in the crystal structure of AQ and PT via redox are reversible.

M. Yao et al. / Materials Science and Engineering B 177 (2012) 483–487

0.6 mmol/L). The presence of a salt in the solvent, LiTFSA in the present case, also influences the solubility of the active material. We confirmed that a higher concentration of LiTFSA led to a lower solubility of PT in the electrolyte solution. As proved by XRD, PT has a high reversibility of the bulk structure upon cycling. In addition, the low solubility of PT is considered to contribute to the better cycle-life performance. The expansion of the ␲-system effectively works to reduce the solubility. The obtained result implies a favorable effect of the large ␲-system on the cycle-life stability of the organic electrodes. We expect further improvements in the cycle-life performance using redox active molecules with larger ␲-systems.

350

Capacity, mAh/g

300 250 PT

200 150 100

487

AQ

50 0 0

20

40

60

80

100

Cycle Number Fig. 8. Cycle-life performance of the prepared electrodes (AQ: ♦, PT: 䊉) (Current density: 20 mA/g, Potential range: 1.5–3.0 V vs. Li+ /Li).

3.5. Cycle-life stabilities Finally, the results of a cycle-life test for the prepared positiveelectrodes are shown in Fig. 8. The capacity of the AQ electrode rapidly decreased to 49 mAh/g after 100 cycles. On the other hand, the capacity decay of the PT electrode upon cycling was low; it maintained 183 mAh/g even after 100 cycles. Although there are several reports on the battery performance of such low-molecular-weight compounds, their discharge capacities tend to significantly decay upon cycling. One of the reasons for this decrease in capacity is the dissolution of the redox active molecules into the electrolyte. The cycle-life performance of low-molecular weight compounds can be significantly affected by the composition of the electrolyte solution. For the AQ-based electrodes, Song et al. examined the influence of the electrolyte solutions on the cycle-stability [10] using 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME) and ethylene carbonate/dimethyl carbonate (EC/DMC). According to their report, the ester solvent (EC/DMC) gave a better cycle performance than the ether one (DOL/DME). On the other hand, in the present study, the ether-based electrolyte solution, tetraglyme-LiTFSA, performed better than the ester, GBL, with respect to the cycle-life of the AQ electrode. The cycle-life of the organic electrode differs from one electrolyte solution to another, and one cannot infer the influence of the electrolyte solutions on the cycle-life simply in terms of the generic classification of the solvent, ether or ester. The cycle-life performance of PT was also affected by the electrolyte solution systems. Previously, we reported the cycle performance of PT using GBL, containing the same lithium salt as in the present study, in which PT showed a high initial capacity of greater than 300 mAh/g; however, it dropped to less than 50 mAh/g after 50 cycles. As is the case for AQ, the cycle stability of PT in the tetraglyme-based system is better than that in the GBL-based system. In addition to the two solvents described above (GBL and tetraglyme), we tested other three ester-based solvents (ethylene carbonate/diethyl carbonate (EC/DEC), propylene carbonate (PC), butylene carbonate (BC)), and two ether-based solvents (triglyme, diglyme). As for the comparison between AQ and PT, in almost all the electrolyte solutions, PT showed a better cycle-life performance than AQ. One of the reasons for the dependence of cycle stability on the electrolyte solutions can be the solubility of the organic active materials in the solutions. PT shows a much lower solubility in the common organic solvents than AQ. For example, the solubility of PT in tetraglyme is less than one tenth of AQ (AQ: 7 mmol/L, PT:

4. Conclusions The potential capability of applying the redox properties of 9,10-anthraquinone (AQ) and 5,7,12,14-pentacenetetrone (PT) to rechargeable lithium batteries was investigated. These materials underwent multi-electron redox reactions which can be used as positive-electrode of rechargeable lithium batteries. PT showed higher initial discharge capacity (236 mAh/g(PT) ) than AQ (217 mAh/g(AQ) ). Furthermore, the cycle-stability of PT was better than that of AQ. One of the reasons for the capacity decay of the organic electrodes is the dissolution of the redox active organic materials in the electrolyte solutions, which can be suppressed by certain modifications in the organic molecule. The present study revealed that expanding the ␲-system in a molecule was an effective way to improve the cycle-life. References [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]

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