Alkali Metal Ion Storage of Quinone Molecules Grafted on Single

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Article Cite This: ACS Omega 2018, 3, 15598−15605

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Alkali Metal Ion Storage of Quinone Molecules Grafted on SingleWalled Carbon Nanotubes at Low Temperature Canghao Li,† Motoumi Nakamura,† Shunya Inayama,† Yosuke Ishii,*,† Shinji Kawasaki,*,† Ayar Al-zubaidi,‡ Kento Sagisaka,§ and Yoshiyuki Hattori§

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†

Department of Life Science and Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan ‡ Shenzhen Engineering Lab for Supercapacitor Materials, Shenzhen Key Laboratory for Advanced Materials, Department of Material Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Shenzhen 518055, China § Department of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan S Supporting Information *

ABSTRACT: 9,10-Anthraquinone and 9,10-phenanthrenequinone (PhQ) were grafted onto two kinds of single-walled carbon nanotube (SWCNT) samples having different mean tube diameters by diazo-coupling reactions. The structural details of PhQ-grafted SWCNT (PhQ/SWCNT) samples were analyzed by X-ray diffraction and Raman measurements. It was discussed that a few-nanometer-thick layer of polymerized PhQs covers the outside of SWCNT bundles. The obtained PhQ/SWCNT works very well as lithium-ion battery and sodium-ion battery electrodes, not only at room temperature but also at 0 °C. It should be noted that the cycle performance of the PhQ/SWCNT electrode is much better than that of PhQ encapsulated in SWCNT (PhQ@SWCNT). We also calculated molecular base reaction energies by density functional theory calculations to gain a qualitative insight into the observed discharge potentials of the PhQ/SWCNT electrode.

1. INTRODUCTION Owing to their high energy density, Li-ion batteries (LIBs) have become indispensable for energy storage in portable electronic devices such as cell phones and lap-top computers. However, to apply LIBs to larger applications such as electric vehicles, we have to improve LIBs to have higher safety, higher energy density, and lower cost.1−3 In current commercial LIBs, Li ions are stored using intercalation reactions in electrodes, including several kinds of precious rare metals such as Co and Ni.4 The intercalation reactions limit the rechargeable capacity of LIBs. The usage of rare metals increases the cost of LIBs. Therefore, eliminating the need for intercalation reactions, and using electrodes not made from rare-metals, are good development guidelines for next-generation LIBs.1,5,6 Some organic molecules consisting of only abundant elements can adsorb/desorb Li ions reversibly.5−10 As most of them have light molecular weight compared to the present inorganic cathode materials such as LiCoO2, they have high Liion storage capacity per unit weight. In addition to the high theoretical capacities, organic materials offer better electrode properties than inorganic materials. Organic materials are potentially low-cost because usually they consist of no expensive elements. They are also recyclable. Therefore, these organic molecules are fascinating materials as cathode © 2018 American Chemical Society

electrodes. Among all organic electrode materials, radical compounds and conducting polymers have long been investigated. On the other hand, nonradical and nonpolymeric organic molecules, recently, have been adapted not only to LIBs, but also to sodium-, magnesium-, zinc-ion batteries, and air batteries. However, it is not very easy to use the nonradical and nonpolymeric organic materials as electrode materials because most of them are easily dissoluble in organic electrolytes. Liang et al.5 and Zhao et al.11 wrote review articles on organic electrodes for Li- and Na-ion batteries (SIBs), respectively. As written in the articles, although carbonyl compounds, layered compounds having large planar structures, imide compounds, and so on have been intensively investigated, it is still difficult to replace the current inorganic electrodes by them. In such a situation, Liang proposed a new approach using quinone molecule electrodes in aqueous electrolyte batteries in 2017.12 Quinone molecules can work as electrode materials through a radical generation/recombination process. However, simple quinone compounds usually show capacity fading due to their dissolution in organic Received: October 17, 2018 Accepted: November 6, 2018 Published: November 16, 2018 15598

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Scheme 1. (a) Preparation of PhQ-Grafted SWCNTs Using the Diazo-Coupling Reaction. (b) Schematic Structure of the PhQ@SWCNT Electrode

carbon nanotubes (SWCNTs) was a good method to prevent the dissolution and improve the cycle performance of PhQ electrodes.15,16 SWCNTs have good electrical conductivity and work as a conductive additive for electrode materials. As the surface potential energy of the inner tube surface is integrated in the center of the tube from all diameter directions, a deep potential minimum is created in the tube center, if the tube diameter is small.17 That is the reason why the molecules encapsulated in SWCNTs are stabilized. We used this stabilization effect to reduce the dissolution of PhQ molecules and could achieve an improvement of the cycle performance.

electrolytes. In this paper, we describe the electrode properties of composites of quinone molecules and carbon nanotubes in organic electrolytes.8,10,13,14 9,10-Phenanthrenequinone (PhQ) consisting of only C, O, and H can store two Li ions per one molecule. The theoretical Li-ion storage capacity of PhQ can be calculated to be 258 mAh/g. Therefore, PhQ would be a good candidate as a highcapacity and low-cost electrode material. However, the storage capacity of PhQ bulk sample greatly decreases with charge− discharge cycles because of the dissolution of PhQ molecules in organic electrolytes. We have shown in a previous paper that the encapsulation treatment of PhQ molecules in single-walled 15599

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grafted phase in the high-resolution TEM (HRTEM) pictures of PhQ/SWCNT-2.5 (Figure S5). The amount of PhQ grafted on SWCNTs was evaluated by two approaches: thermogravimetric (TG) and X-ray photoelectron spectroscopy (XPS). The sublimation temperature of the PhQ molecule is about 200 °C, and a drastic weight loss from 200 °C was clearly observed in the TG curve in the case of PhQ crystals (Figure 1). On the other hand, the oxidation

However, unfortunately, we could not stop the capacity fading completely. In this work, we tried to graft PhQ molecules onto SWCNTs to obtain higher stability. As SWCNTs having a huge πconjugation system should be chemically very stable, it is not very easy to do chemical modification for SWCNTs. So, we performed a two-step process that involved the diazotization18 of PhQ molecules, followed by the reaction of PhQs with SWCNTs (see Scheme 1a). In the case of the encapsulation system (Scheme 1b), electrolyte ions should approach the encapsulated quinone molecules through open tube edges. On the other hand, the grafted quinone molecules (Scheme 1a) seem to be more easily accessible to the ions. We investigated the Li-ion and also the Na-ion storage properties of the synthesized sample of PhQ grafted on SWCNTs (PhQ/ SWCNT). If PhQ/SWCNT can store Na ion effectively, it can be used as a Na-ion battery (SIB) electrode material. As Na resource is abundant, SIB is expected to be a low-cost secondary battery.19−24 We also investigated the low-temperature performance of the PhQ/SWCNT electrode for Li and Na ions. Furthermore, we performed density functional theory (DFT) calculations to understand the detailed features of charge−discharge profiles of the PhQ/SWCNT electrode. On the basis of the calculation, we discussed the discharge potential difference for Li and Na ions.

Figure 1. TG curves of (a) bulk PhQ, (b) PhQ/SWCNT-2.5, and (c) SWCNT-2.5.

temperature of the untreated SWCNT-2.5 is about 590 °C, as shown in Figure 1. The rather high oxidation temperature also confirms the high crystallinity of SWCNT-2.5. As the temperature difference of the two TG threshold temperatures of PhQ and SWCNT is very large (∼400 °C), the amount of PhQ in the PhQ/SWCNT-2.5 sample should be evaluated by TG measurement. In fact, a two-step TG curve was observed for PhQ/SWCNT-2.5, as expected. However, the observed two steps are not very clear because PhQ molecules are stabilized by the connection with SWCNTs, and the crystallinity of SWCNTs should be lowered by the grafting. Therefore, it is very difficult to determine the critical temperature needed to evaluate the PhQ content in the sample. If we assume the SWCNT oxidation threshold temperature of PhQ/SWCNT-2.5 to be 590 °C, as that of the untreated SWCNT-2.5, the PhQ content can be determined as 33 wt %. Although this value overestimates the amount of grafted PhQ, because some of the SWCNTs of PhQ/SWCNT-2.5 should have lower oxidation temperature due to the defects formed by PhQ grafting, we can treat this value as the maximum PhQ content in the sample. We also performed XPS measurements to evaluate the PhQ content as we did for the SWCNT encapsulation sample (Figure 2). As we explained in our previous paper,16 the PhQ content can be estimated by the increase of the O/C atomic ratio. The O 1s peak on the XPS spectrum of SWCNT-2.5 drastically changes by PhQ grafting. For PhQ/SWCNT-2.5, the O 1s peak corresponds to oxygen atoms of PhQ, namely, oxygen atoms of CO. The signal coming from the CO group was observed at around 284.8 eV in the C 1s spectra. On the other hand, slightly included impurity oxygen atoms such as oxygen atom of the C−O−H group were observed for the untreated SWCNT-2.5 at around 286.7 eV. The PhQ amount of PhQ/SWCNT-2.5 evaluated by the increase in O/C ratio is 28.1%. Although this value is very close to the value determined by TG measurement, we should note that the value determined by XPS also overestimates the PhQ content for the following reason: as mentioned above, PhQ molecules are grafted only on the outer SWCNTs of the bundles having a

2. RESULTS AND DISCUSSION In this work, we used two kinds of SWCNT samples having mean tube diameters of 2.5 and 1.0 nm produced by e-Dips and HiPco methods, respectively. Hereinafter, and for the sake of convenience, the two samples will be denoted using their mean tube diameter values as SWCNT-2.5 and -1.0, respectively. Although we performed AQ and PhQ grafting treatments for the two samples, in this paper, we discuss mainly the electrode properties of the PhQ-grafted SWCNT2.5 sample (PhQ/SWCNT-2.5) in comparison with other combinations. As previously reported, since SWCNT-2.5 has less defective, narrow diameter distribution and well-bundled form, its physical characterization reveals a weak Raman D-band peak (Figure S1) and very clear X-ray diffraction (XRD) peaks (Figure S2).25 We can determine the mean tube diameter by performing XRD pattern simulation.26 The determined value 2.5 nm is in quite good agreement with the values estimated by the peak position of radial breathing modes,27,28 observed in the low-wavenumber region of the Raman spectrum. The high purity was also confirmed by transmission electron microscopy (TEM) observation (Figure S3). We analyzed the bundle diameter size using the TEM picture and estimated a mean bundle diameter of about 35 nm (Figure S4). It was observed that the intensity of the D-band peak of SWCNT-2.5 increases by the PhQ grafting treatment. This increase indicates the formation of sp3 carbon on the surface of SWCNTs by the reaction with diazo-PhQ molecules. On the other hand, we could not observe any significant changes in the XRD pattern of the quasi-two-dimensional hexagonal lattice of SWCNT-2.5 bundles after the grafting treatment, which means that the bundle form is maintained after the PhQ grafting treatment. Judging from these observations, we conclude that PhQ molecules were grafted on the outer SWCNTs of the bundle surface, and that the inner SWCNTs were kept as they were. We also performed TEM observation of the PhQ/ SWCNT-2.5 sample. However, we could not see the PhQ15600

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also between PhQ molecules themselves. Namely, polymerized PhQ molecules having an average thickness of 1.3 nm are attached to the outer surface of the SWCNT bundle. Unfortunately, the thickness of the PhQ polymer is too thin to be observed by HRTEM (Figure S5). From here we discuss the electrode properties of PhQ/ SWCNT-2.5. First, we found that PhQ/SWCNT-2.5 can adsorb/desorb Li ions reversibly, as shown in Figure 3. Two-

Figure 3. Room-temperature (RT) galvanostatic charge−discharge curves of PhQ/SWCNT-2.5 used for LIBs. The measurement was performed at a constant current density of 100 mA/g, where “g” means the total weight of the composite electrode (PhQ/SWCNT2.5).

step plateaus are clearly visible on the charge−discharge profiles at around 2.4 and 2.8 V, which are characteristic features of the PhQ electrode, indicating that the PhQ molecules grafted on SWCNTs can work as Li-ion battery electrode materials. Here, we note that the lithium storage capacity of SWCNTs in this voltage region is negligible because lithium insertion/extraction to/from carbon materials generally occurs in a lower potential region less than 1.0 V vs Li/Li+ (see Figure S7). In Figure 3, the charge−discharge capacity values per total amount of electrode and also per unit PhQ weight are shown. The capacity value per PhQ weight is calculated assuming 33 wt % PhQ content. Although the reversible capacity of 150 mAh/g, shown in Figure 3, was smaller than the theoretical capacity of PhQ molecules (258 mAh/g), it should be noted that the 33 wt % used for the Figure 3 is probably overestimated. Taking that into account, we can assume that most of the PhQ molecules, even in their polymerized form, could adsorb Li ions, and that electron transfer through the polymer to the SWCNT should be possible as well. As shown in Figure 4, the charge−discharge curves of PhQ/ SWCNT-2.5 observed for Na ions are almost similar to those observed for Li ions. Using the redox potentials of Li/Li+ (−3.05 V) and Na/Na+ (−2.73 V), we can compare the plateau potential of the discharge curves of PhQ/SWCNT-2.5 for Li and Na ions. It was found by this analysis that the discharge potential for a Na ion is about 700 mV lower than that for a Li ion, although the discharge profiles for Li and Na ions are quite similar. The discharge potential difference will be discussed later.

Figure 2. XPS spectra of (a) SWCNT-2.5 and (b) PhQ/SWCNT-2.5.

mean diameter of 35 nm. As Al Kα X-ray was used for the XPS measurement, we could not get any information from SWCNTs in the inner part of the bundle. Therefore, although we could not obtain the correct PhQ amount value by either TG or XPS measurements, we estimate that the PhQ content should be less than 33 wt %. Now we discuss how PhQ molecules are grafted on SWCNT-2.5. The mean tube diameter D and lattice constant a of the quasi-two-dimensional bundle hexagonal lattice were determined from XRD simulation, and had the values of 2.5 and 2.8 nm, respectively (Figure S2).16 The tube-to-tube distance, calculated from D and a, is 0.3 nm, and the value is very reasonable for a SWCNT bundle. Using these values, we calculated the bundle to contain one SWCNT per 6.79 nm2 of its cross section. Therefore, an average bundle having a diameter of 35 nm should consist of about 142 SWCNTs. Here, we think about a structural model of PhQ/SWCNT-2.5 such that a layer of PhQ x nm in thickness covers the SWCNT bundle of 35 nm diameter (Figure S6). Assuming the PhQ content to be 33 wt % and the density of the PhQ layer to be 1.4 g/cm3 as that of the PhQ crystal, the layer thickness x is calculated to be 1.3 nm. As this x value is much larger than the molecular size of PhQ, the PhQ layer should not be a monolayer. The grafting method used in this study can create bonds not only between PhQ molecules and SWCNTs but 15601

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probably increases due to the long-wound electron path through polymerized PhQs. As shown in Figure 5a, a small capacity drop was observed between 50 and 60 cycles. The reason why the capacity dropped is not clarified. However, PhQ/SWCNT showed higher capacity than PhQ@SWCNT after the capacity drop. Interestingly, the low-temperature performance of the PhQ/ SWCNT electrodes is very good. The present commercial LIB using intercalation reactions shows a reduction in its cell performance at low temperature. The redox reactions of quinone molecules would not be affected very much by decreasing temperature. Figure 6A,B show Li- and Na-ion

Figure 4. Room-temperature galvanostatic charge−discharge curves of PhQ/SWCNT-2.5 used for SIBs. The measurement was performed at a constant current density of 100 mA/g, where “g” means the total weight of the composite electrode (PhQ/SWCNT-2.5).

Figure 5 shows the 100 cycle charge−discharge data of PhQ/SWCNT-2.5 for Li-ion batteries. In the case of the bulk

Figure 5. Cycle performance of (a) PhQ/SWCNT-1.0, (b) PhQ@ SWCNT-2.5, and (c) a simple mixture of PhQ and carbon black electrode used for LIBs.

PhQ sample with carbon black conductive additive, severe capacity fading was observed in a few cycles because of the dissolution of PhQ molecules into the electrolyte. The dissolution of PhQ molecules was suppressed by the encapsulation treatment, as reported in our previous paper15,16 (see Figure 5b). The capacity loss of PhQ/ SWCNT-2.5 is much smaller than that of [email protected] because PhQs are covalently connected to SWCNTs. As shown in Figure S8, dissolution of PhQ did not appear even after the 100 charge−discharge cycling. In the previous paper, we reported that encapsulation of PhQ molecules in SWCNTs also suppresses the capacity fading greatly. However, and as shown in Figure 5, it was found that the PhQ grafting is much more effective for the improvement of cycle performance than the encapsulation treatment. Clear plateaus originating from the electrochemical behavior of grafted PhQ molecules were maintained even after 50 cycles, as shown in Figure S9. On the other hand, the rate performance of PhQ/SWCNT-2.5, shown in Figure S10, is not very good, although it is reported that the redox reactions of quinone molecules including PhQ are very fast.7,12,18,29 The internal resistance of PhQ/SWCNT-2.5

Figure 6. Galvanostatic charge−discharge curves of PhQ/SWCNT2.5 used for (A) LIBs and (B) SIBs. The measurements were performed at 0 °C and a constant current density of 100 mA/g, where “g” means the total weight of the composite electrode (PhQ/ SWCNT-2.5).

charge−discharge curves of PhQ/SWCNT-2.5 at 0 °C. As shown in Figure 6A,B, although the reversible capacities of PhQ/SWCNT-2.5 for both Li and Na ions are about 60% of the values observed at room temperature (RT), almost the same charge−discharge profiles are maintained at low temperature, except for slight increases in the charge− 15602

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can adsorb the second Li or Na ion very easily after capturing the first ion, whereas PhQ requires a relatively higher energy to catch the second ion, probably due to the repulsion between adsorbed alkaline ions (Figure S12). Next, we compare the discharge potentials of PhQ/ SWCNT-2.5 for Li and Na ions. As discussed in the previous paragraph, it was found that the discharge potential for a Na ion is about 700 mV lower than that for a Li ion. Of course, we should consider the kinetic parameters. However, it is well known that the Na ion has smaller solvation energy and larger ion mobility than the Li ion. So, if the discharge potential difference is caused by kinetic effects, the potential for the Li ion should be lower. Therefore, we should think about another reason. According to our calculations, the stabilization energies of the reaction products (LiPhQ and Li2PhQ) are larger than those of NaPhQ and Na2PhQ, probably because Li should have stronger covalent bonds with PhQ than Na. The stabilization energy difference would lead to the observed discharge potential difference. Finally, we discuss the discharge profiles of PhQ/SWCNT2.5. The discharge profiles of PhQ/SWCNT-2.5 show two-step plateaus similar to those of the PhQ bulk crystal and PhQ@ SWCNT samples. However, the plateaus of PhQ/SWCNT-2.5 are not very flat compared to those of other PhQ samples. In the cases of PhQ bulk crystal and PhQ@SWCNT samples, all PhQ molecules are equivalent, and the discharge potentials should be determined by reactions (3) or (4). However, PhQ molecules in PhQ/SWCNT-2.5 form a variety of polymers and are therefore not equivalent. Each molecule should have a slightly different redox potential from the values determined by (3) or (4). Therefore, PhQ/SWCNT-2.5 should have a nonflat plateau in the discharge profile.

discharge hysteresis. This indicates that ion-adsorbing sites are kept at low temperature, while the activation energy for ion adsorption depends on temperature. This profile change at low temperature looks quite like the change with the increase in charging rate. Both profile changes can be explained by the increase in the over-potential for ion adsorption due to iR drop. Of course, the increases in the current and internal resistance lead to the larger hysteresis with the increase in the charging rate and the decrease in temperature, respectively. Although the increase in the internal resistance at low temperature should be related to various factors, the change in ion mobility would be the main reason. In the case of the PhQ@SWCNT encapsulation electrode, ion diffusion into the inner tube space of SWCNTs is necessary for ion adsorption to occur. On the other hand, PhQ/SWCNT-2.5 does not require the diffusion process because PhQs are attached to the outer surface of the SWCNT bundle. That would be the reason for the better low-temperature performance. We also performed DFT calculations to discuss the electrode properties of PhQ/SWCNT-2.5. It is well known that the open-circuit voltage of a battery can be predicted by DFT calculations.29−34 This approach is applicable to the present quinone electrode batteries. We denote the quinone electrode by Qsolid. The total reaction of the quinone battery can be expressed as follows: Q solid + 2Li solid → Li 2Q solid

(1)

Therefore, the open-circuit voltage (electromotive force) E can be calculated by the following equation: E=−

G(Li 2Q solid) − G(Q solid) − 2G(Li solid) 2F

(2)

where G(M) expresses the Gibbs free energy of M, and F denotes the Faraday constant. However, it is very difficult to discuss Gibbs free energy of PhQ/SWCNT-2.5 because the exact structure of PhQ/ SWCNT-2.5 is not known. However, it would be meaningful to calculate the free energy difference ΔG of molecular entities in reactions (3) and (4) below, instead of calculating ΔG for the undefined solid-state structures in eq 2. PhQ + Li+ + e− → LiPhQ

(3)

LiPhQ + Li+ + e− → Li 2PhQ

(4)

3. CONCLUSIONS In conclusion, we prepared AQ and PhQ derivatives having amino groups and grafted quinone molecules onto SWCNTs by diazo-coupling reactions. The coupling reactions introduced bonds not only between quinone molecules and SWCNT but also between the quinone molecules themselves. Consequently, polymerized quinone molecules were connected to SWCNTs. The structural details of the PhQ/SWCNT-2.5 sample were analyzed by XRD and Raman measurements. It was discussed that a few-nanometer-thick layer of polymerized PhQs covered the outside of SWCNT bundles. The obtained PhQ/SWCNT-2.5 works very well as LIB and SIB electrodes not only at RT but also at 0 °C. It should be noted that the cycle performance of the PhQ/SWCNT-2.5 electrode is much better than that of the PhQ@SWCNT encapsulation electrode. We also calculated the molecular base reaction energies by DFT calculations to offer a qualitative insight into the factors influencing the observed discharge potentials of the PhQ/ SWCNT-2.5 electrode.

In this case, however, it is very hard to perform a quantitative comparison between the calculated values and experimentally observed data. Furthermore, it should be noted that eq 2 can predict the open-circuit voltage, whereas kinetic parameters should be considered when discussing discharge potentials. In this paper, we calculated molecular base reaction energies, such as (3) and (4), for several combinations of electrode molecules (AQ and PhQ) and electrolyte ions (Li and Na) to gain a qualitative insight into the observed discharge potentials. The values of Gibbs free energy obtained by the DFT calculation are summarized in Table S1. First, we discuss the very different charge−discharge profiles of AQ and PhQ, although these two molecules have the same chemical formula. AQ and PhQ show one-step and two-step plateau discharge curves, respectively (see Figures S11 and 3). For PhQ, the difference between the two ΔG values for reactions (3) and (4) is quite large for both Li and Na. On the other hand, the difference for AQ is very small. Namely, AQ

4. EXPERIMENTAL METHODS We purchased all chemicals used in this study: 9,10anthraquinone (AQ) (Wako pure chemical industries) and 9,10-PhQ (Sigma-Aldrich). Two kinds of SWCNT samples produced by HiPco35 and e-Dips36 methods were also purchased (HiPco: NanoIntegris Inc., e-Dips: Meijo Nanocarbon). For SWCNT samples, we performed purification treatment using acid and annealing treatment to obtain higher crystallinity. The detailed purification procedure is described in 15603

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our previous papers.15,16,25,37−39 Then, we grafted quinone molecules onto SWCNTs typically as follows. First, a PhQ powder sample was refluxed with nitric acid at 130 °C for 1 h to obtain nitrated PhQ. Then the obtained nitrated PhQ was dissolved in 2% NaOH aqueous solution, and an excess amount of sodium hydrosulfite was added to the solution to replace the nitro groups by amino groups. Subsequently, PhQs were grafted onto SWCNTs by diazo-coupling reactions as shown in Scheme 1. For comparison, we also prepared PhQencapsulated SWCNTs (PhQ@SWCNTs) using a process described in our previous publication.16 We performed Raman measurements to check the crystallinity, and synchrotron X-ray diffraction (XRD) measurements at the Tsukuba Photon Factory (KEK-PF) BL-18C to determine the mean tube diameter. PhQ derivatives were characterized by Fourier-transform infrared spectroscopy and NMR measurements. The final products (PhQ/SWCNT composites) were thoroughly investigated by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, Raman spectroscopy, XRD, thermogravimetric (TG) analysis, and X-ray photoelectron spectroscopy (XPS) measurements. The paper-form PhQ/SWCNT sample obtained by vacuum filtration was used for Li- and Na-ion charge−discharge experiments. Li and Na metal foils were used as counter electrodes. 1 mol/L LiClO4 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) solution (EC/ DEC = 1:1 in volume ratio) and NaClO4 dissolved in a mixture of EC and DEC solution (EC/DEC = 1:1 in volume ratio) were used as electrolytes for Li- and Na-ion charge− discharge experiments, respectively. The charge−discharge measurements using a conventional two-electrode-type cell (Hohsen, HS-Cell) were conducted using a galvanostat (Toyo System, TOSCAT-3200). The cell was assembled in an Arfilled glove box to avoid air exposure and contamination. Charge−discharge experiments were done inside a thermostatic chamber (Espec, SU-241) to control the temperature of the test cell. Density functional theory (DFT) calculations were performed using Gaussian 09 software.40 Here, we used the B3PYP exchange−correlation hybrid functional41 with Dunning’s double-zeta correlation consistent basis sets42 (ccpVDZ).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.I.). *E-mail: [email protected] (S.K.). ORCID

Yosuke Ishii: 0000-0003-0334-2228 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Naito Science and Engineering Foundation, Takahashi Industrial and Economic Research Foundation, Hitachi Global Foundation, Tanikawa Fund Promotion of Thermal Technology, and JSPS KAKENHI Grant Number 17K14543 for financial support to the present study. The synchrotron XRD measurements were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2018G088).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02844. Raman spectra (Figure S1), XRD pattern (Figure S2), TEM images of SWCNT-2.5 (Figure S3), diameter distribution data of SWCNT-2.5 (Figure S4), TEM images of PhQ/SWCNT-2.5 (Figure S5), schematic structure of quinone molecule grafted SWCNTs (Figure S6), charge−discharge data of SWCNT-2.5 (Figure S7), photograph of the electrolyte and separator after charge−discharge tests (Figure S8), charge−discharge data at room temperature (Figure S9), cycling data for the SIB electrode (Figure S10), charge−discharge data of AQ/SWCNT-2.5 (Figure S11), molecular structure obtained by DFT calculations (Figure S12), and energy data obtained by DFT calculations (Table S1) (PDF) 15604

DOI: 10.1021/acsomega.8b02844 ACS Omega 2018, 3, 15598−15605

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DOI: 10.1021/acsomega.8b02844 ACS Omega 2018, 3, 15598−15605