Alkali metal ion storage properties of sulphur and phosphorous molecules encapsulated in nanometer size carbon cylindrical pores

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1 Alkali metal ion storage properties of sulphur and phosphorous molecules encapsulated in nanometer size carbon cylindrical pores Yosuke Ishii, Yuki Sakamoto, Hayong Song, Kosuke Tashiro, Yoshiki Nishiwaki, Ayar Al-zubaidi, and Shinji Kawasaki Citation: AIP Advances 6, (2016); View online: View Table of Contents: Published by the American Institute of Physics Articles you may be interested in Iodine encapsulation in CNTs and its application for electrochemical capacitor AIP Conference Proceedings 1733, (2016); / Ion adsorption mechanism of bundled single-walled carbon nanotubes AIP Conference Proceedings 1733, (2016); / Thermodynamics and kinetics of defects in Li 2 S Applied Physics Letters 108, (2016); / Ultra-thin flexible GaAs photovoltaics in vertical forms printed on metal surfaces without interlayer adhesives Applied Physics Letters 108, (2016); / Monolayer borophene electrode for effective elimination of both the Schottky barrier and strong electric field effect Applied Physics Letters 109, (2016); / Wave propagation in nonlinear metamaterial multi-atomic chains based on homotopy method AIP Advances 6, (2016); /

2 AIP ADVANCES 6, (2016) Alkali metal ion storage properties of sulphur and phosphorous molecules encapsulated in nanometer size carbon cylindrical pores Yosuke Ishii, a Yuki Sakamoto, Hayong Song, Kosuke Tashiro, Yoshiki Nishiwaki, Ayar Al-zubaidi, and Shinji Kawasaki Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya , Japan (Received 5 February 2016; accepted 4 March 2016; published online 15 March 2016) We investigated the physical and chemical stabilities of sulfur and phosphorus molecules encapsulated in a mesoporous carbon (MPC) and two kinds of single-walled carbon nanotubes (SWCNTs) having different cylindrical pore diameters. The sublimation temperatures of sulfur molecules encapsulated in MPC and the two kinds of SWCNTs were measured by thermo-gravimetric measurements. It was found that the sublimation temperature of sulfur molecules encapsulated in SWCNTs having mean tube diameter of 1.5 nm is much higher than any other molecules encapsulated in larger pores. It was also found that the capacity fading of lithium-sulfur battery can be diminished by encapsulation of sulfur molecules in SWCNTs. We also investigated the electrochemical properties of phosphorus molecules encapsulated in SWCNTs (P@SWCNTs). It was shown that P@SWCNT can adsorb and desorb both Li and Na ions reversibly. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( [ I. INTRODUCTION Since the discovery of C 60 peapod (C 60 molecules are encapsulated in the hollow cores of single-walled carbon nanotubes) in 1998, 1 several kinds of molecules (e.g. coronene, β-carotene, 9,10-dichloroanthracene, water, iodine) have been encapsulated so far. 2 6 Some of the eager studies on structural and electronic properties of encapsulated molecules reported that some of the encapsulated molecules showed properties different from their bulk forms. 5,7 10 Due to such findings, it has been concluded that the tube interior provides a unique field to the encapsulated molecules to alter their properties. However, there are not that many reports describing the applications of encapsulated molecules. Lithium ion batteries (LIBs) have been widely used in portable electronic devices such as cell phones and lap-top computers, owing to their high energy density. However, new energy source with higher capacity and better rate performance is required because LIBs are not good enough for larger and higher power machines such as electric vehicles. Recently, metal-air, lithium-sulfur (LIS), sodium-ion batteries and others have been attracting much attention as post-lib energy devices. LIS battery is one alternative that has higher energy density than LIB has, and is expected to be used for large-scale devices. However, in order for the practical use of LIS battery to be realized, we have to solve the problem that polysulfide ions (S n ) are easily dissolved in electrolyte. This dissolution problem leads to capacity fading with charge-discharge cycles. Several investigations have been carried out to improve the cycle performance of LIS battery. In 2009, X. Ji et al. reported that sulfur molecules introduced in mesoporus carbon (MPC) work well as LIS battery electrodes, with very small capacity fading. 30 However, it is not understood very well how the sulfur molecules a ishii.yosuke@nitech.ac.jp /2016/6(3)/035112/9 6, Author(s) 2016.

3 Ishii et al. AIP Advances 6, (2016) are stabilized in MPC. On the other hand, K. Kaneko et al. reported the encapsulation of sulfur into the hollow core of single-walled carbon nanotube (SWCNT). 10 Another alternative is sodium-ion battery (SIB) that has received increasing attention as in the recent years, because SIB is very advantageous in regards to cost and resources compared to LIB. However, good anode materials having high capacity for SIBs have not been found yet. Graphite, which is used as a negative electrode of LIBs, does not adsorb Na ions. So far, SIB negative electrode properties of several kinds of carbon materials have been studied. Although some kinds of hard-carbons and Na alloys can adsorb and desorb sodium ions, 22,23 we should explore better anode materials to realize SIBs. Phosphorus is also expected as alternative electrode materials because of its high theoretical capacity (2596 mah/g) and relatively low volume expansion when adsorbing sodium ions. However, phosphorus electrode still suffers from instability problems, despite the numerous attempts that have been made with different phosphorus polymorphs and/or different kinds of electrode support materials In the present study, we use the unique environment provided by the hollow core inside SWCNTs to encapsulate both sulfur and phosphorous molecules, in order to address some of the unanswered questions and performance issues of both LIS and SIB. First, we compare the physical stability of sulfur molecules inserted in MPC and in SWCNTs having different tube diameters, then we report the LIS battery electrode properties of sulfur encapsulated SWCNTs (S@SWCNTs). We also report on the negative electrode properties of phosphorus encapsulated single-walled carbon nanotubes (P@SWCNTs), in which phosphorus atoms are inserted into the pores of SWCNT. Since SWCNTs have good electric conductivity, they should work as a conductive component for the highly resistive phosphorus electrode. II. EXPERIMENTAL A. Synthesis of mesoporus carbon MPC sample was prepared by soft template method using tri-block co-polymer (BASF, F-127) as a template. 34 Carbon source was resol, which was synthesized by polymerization of phenol molecules. Carbonization temperature of the MPC sample was set at 900 C. TEM observation was done on a JEOL JEM-z2500 transmission electron microscope operated at 200 kv. N 2 adsorptiondesorption isotherms at 77 K were measured using a Shimadzu Gemini 2375 instrument. The specific surface (S BET ) area was estimated using the Brunauer-Emmett-Teller (BET) method. 35 The pore-size distributions were calculated from the analysis of the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method. 36 Small-angle X-ray scattering (SAXS) measurements were carried out using a Rigaku NANO-Viewer diffractometer. B. Preparation of SWCNT papers Two types of SWCNT samples (SO type and e-dips 2.0 type) were purchased from Meijo NanoCarbon Co. Ltd., and were denoted as SO and EC for convenience. The SO and EC samples were synthesized by arc-discharge method and CVD method, and had mean tube diameters of about 1.5 and 2.0 nm, respectively. We fabricated a paper-form SWCNTs (buckypaper) by filtration of SWCNT dispersed water on a membrane filter and used it as a working electrode. C. Encapsulation of sulfur molecules into the pores of MPCs and SWCNTs Sulfur powder sample and MPC or SWCNTs (SO and EC) were heat-treated at 155 C in an evacuated glass tube. The sulfur deposited on the outer surface of MPC or SWCNTs was then removed using washing treatments with CS 2. The S/C values determined by thermogravimetric measurements (Shimadzu TGA-50H) were 1.56, 0.449, and for MPC, SO and EC, respectively. D. Encapsulation of phosphorus molecules into SWCNTs A buckypaper of SWCNTs and phosphorus (red) powder sample were heat-treated up to 560 C in an evacuated glass tube. When placed inside the glass tube, the phosphorus powder sample

4 Ishii et al. AIP Advances 6, (2016) was separated from SWCNTs by using an extra inner glass tube in order to avoid the direct contact between SWCNTs and phosphorus. The phosphorus sample deposited on the outer surface of SWCNTs was removed by subsequent heat-treatment at 200 C, where only the outside phosphorus sublimes. The success of encapsulation of sulfur and phosphorus was confirmed by TEM, SEM, Raman spectroscopy, and synchrotron XRD measurements preformed at Tsukuba Photon Factory, BL-18C. E. Electrochemical measurements electrode was prepared using poly-vinylidene difluoride (PVDF) as a binder (S@MPC: PVDF = 9:1), while both S@SWCNT and P@SWCNT samples were used for electrochemical measurements without any additives such as binders because the two samples maintained their paper form. In the present study, not only lithium but also sodium ion battery anode properties were investigated. We fabricated half-cell where lithium and sodium foils are used as the counter electrode. The cathode and anode are separated by porous polymer sheet (Celgard(R) 2500). 1.0 M LiClO 4 and 0.5 M NaClO 4 solutions (MClO 4 dissolved in 1:1 vol% ethylene carbonate and diethyl carbonate mixture) were used for lithium and sodium ion battery electrolyte, respectively. The electrochemical properties were investigated by charge/discharge measurements at a constant current. III. RESULTS AND DISCUSSION It was found by TEM and small angle XRD measurements that the obtained MPC has quasi two-dimensional lattice having tube center-to center distance of 11.1 nm. Figure S1 37 shows the observed N 2 adsorption isotherms of MPC before and after sulfur encapsulation treatment. The empty MPC shows typical type IV isotherm, indicating mesoporous structure. The BET surface area and mean pore size were determined to be 1236 m 2 /g and 7.1 nm, respectively. On the other hand, the sulfur isotherm decreases the BET surface area down to 13 m 2 /g. It indicates that sulfur molecules were inserted in the cylindrical pores of MPC. Similar decrease of BET surface area of SWCNTs was also observed for both sulfur and phosphorus encapsulations. Figure S2 37 shows the observed XRD patterns of bulk sulfur powder sample and S@MPC. No sharp diffraction line was observed for S@MPC. It indicates that the sulfur crystals deposited on the outer surface of MPC were completely removed by washing treatment and that amorphous form of sulfur was inserted in MPC pores. Similarly, we could not see any sharp crystal peaks in the XRD pattern of S@SWCNT(SO) (Fig. S3 37 ), albeit for a weak peak at around 14.8, which is not seen in pristine sample. That peak could be a diffraction of quasi one dimensional sulfur crystal formed in a SWCNT hollow core, although the detailed structure is not understood well. Sulfur contents of S@MPC and S@SWCNT(SO) were determined to be 61.0% and 31.0%, respectively. As mentioned above, the sulfur crystals deposited on the outer surface of MPC and SWCNTs were removed by washing treatment while the encapsulated sulfur molecules remained in the pores after washing. It indicates that the encapsulated sulfur molecules are somehow stabilized inside the tube. In order to confirm the stabilization, we measured the sublimation temperature of the encapsulated sulfur molecules by TG measurements. For bulk sample, monotonous weight decrease starts at about 200 C. (see Fig. 1(a)) In the case of S@SWCNT(EC) before washing, although the starting point of the weight decrease was almost the same as bulk sample, a two-step weight decrease was observed. (see Fig. 1(b)) The decrease rate changed at about 40% of weight loss and the weight decrease did not stop up to 350 C. The first and second steps of weight decrease correspond to the sublimations of outside sulfur and encapsulated sulfur, respectively. From the TG curve of unwashed S@SWCNT(EC), the weight ratio of SWCNT:outside sulfur:encapsulated sulfur was estimated to be 0.28:0.24:0.48. Taking only the encapsulated sulfur and SWCNT into account, it can be calculated that 63.2wt% sulfur to SWCNT was inserted in SWCNTs. Figure 1(c) shows the TG curve of the washed S@SWCNT. It is clearly seen that the sublimation temperature shifted toward higher temperature side. It means that the encapsulated sulfur molecules are stabilized in SWCNTs. On the other hand, the weight decrease stopped at the almost the same temperature (350 C) as the

5 Ishii et al. AIP Advances 6, (2016) FIG. 1. (A) TG curves of (a) bulk sulfur, and (b) before and (c) after washing treatment. (B) Comparison of sublimation temperatures of (a) bulk sulfur, (b) (c) and (d) samples. unwashed sample. The weight ratio of the encapsulated sulfur molecules (48.0wt%) derived from the TG curve of the washed nicely agrees with the value calculated using the second step weight loss of the unwashed Therefore, we can judge that the washing treatment removes only the outside sulfur molecules while the encapsulated molecules are not released from SWCNTs. Such stabilization in the pore strongly depends on the pore size. Figure 1(B) shows the change in the TG curve of sulfur sublimation with pore size. The sublimation temperature of is slightly higher than that of the bulk sulfur sample. For two kinds of SWCNTs having different mean tube diameters, although the starting points of the sublimation are almost the same as each other, the finishing point of the sublimation of is quite higher than that of These experimental results indicate that the physical stability of the encapsulated sulfur molecule increases with decreasing pore size. Since MPC and the two kinds of SWCNT samples consist of only carbon atoms, their surface potentials that stabilizes the adsorbed molecules should not be so different. However, in the case of small diameter SWCNTs,

6 Ishii et al. AIP Advances 6, (2016) FIG. 2. Charge-discharge curves of (A) bulk sulfur sample, (B) and (C) Sulfur contents (S/C weight ratio) of and were evaluated to be 1.63 and 0.47, respectively. Specific capacity values were calculated with S weights. the summation of surface potentials from round inner surface generate the potential minimum in the centre of tube and thereby not only the sulfur molecules adsorbed on the inner surface but also the sulfur molecules locating in the centre of the tube are stabilized. Furthermore, tube centre potential becomes deeper with decreasing tube diameter. Such potential features would be the reason why sublimation temperature increases with decreasing pore size. In this paragraph, we address the electrochemical properties of the encapsulated sulfur molecules. As mentioned in the introduction section, the capacity fading due to the dissolution of poly-sulfide ions is a serious problem for LIS battery to conquer. In fact, for the bulk sulfur sample, severe capacity fading was observed in Fig. 2(A). On the other hand, as shown in Fig. 2(B), the capacity fading is slightly diminished, although the capacity after 5 cycles was decreased down to 20% of the initial capacity. Cycle performance was improved by inserting sulfur molecules in SWCNTs (Fig. 2(C)). Figures 2(A)-2(C) seem to indicate that the electrochemical stability of the encapsulated sulfur molecules increases with decreasing pore size as physical stability does. However, it should be noted that the cycle performance of S@SWCNT(EC) is not very different from that of S@SWCNT(SO). Compared to sublimation properties, the electrochemical properties are rather complicated due to the involvement of several factors such as the electric conductive path and the interaction with electrolyte. Furthermore, the sulfur adsorbing lithium would overflow from the cylindrical pores because of the volume expansion during lithium insertion. If some part FIG. 3. Synchrotron XRD patterns of (a) pristine SWCNT(SO) and (b) P@SWCNT. In the measurement, wavelength of Xray was set to nm.

7 Ishii et al. AIP Advances 6, (2016) FIG. 4. (left) TEM photograph and (right) STEM-EDX images of of the expanded sulfur breaks the contact with MPC or SWCNTs, such parts would not work as electrode active materials. This process would be the main factor for capacity fading, although no direct experimental evidence was obtained. We would like to also discuss the difference of the charge-discharge curve profile of Plateaus at around 2.5 V were clearly observed in both first cycle discharge curves of bulk sample and S@MPC, while corresponding part of the discharge curve of S@SWCNT is not flat but decreasing FIG. 5. (A) Raman spectra of (a) pristine SWCNT(SO) and (b) P@SWCNT using 532 nm laser. (B) and (C) shows magnified view of the low-wavenumber region and G-band region, respectively.

8 Ishii et al. AIP Advances 6, (2016) gradually. This plateau is understood as reduction of S 8 ring. The diameter of SWCNT(SO) is about 1.5 nm and is too small to accept S 8 ring. Therefore, the molecular structure of the encapsulated sulfur molecule in S@SWCNT should be different form that of the S 8 ring. This should be the reason why the discharge curve profile of S@SWCNT is different from others. The ratio of the capacity in this plateau to the full capacity can be evaluated as 16.5, 13.2, and 4.9% for bulk sample, S@MPC and S@SWCNT, respectively. Since the ratio of S@MPC is almost the same as in the bulk sample, the molecular structure of the encapsulated sulfur of S@MPC is considered not to be different from the bulk form. On the other hand, S@SWCNT should be completely different system because the ratio is much smaller than others. Only in the case of S@SWCNT, the first-cycle Li extraction capacity is greater than the first-cycle Li insertion capacity. Although the reason for the overcharge is not very clear, the above-mentioned unique molecular structure of sulfurs encapsulated in SWCNTs and/or the charge transfer reactions between the sulfur molecules and the host SWCNTs might affect the overcharge phenomenon. Further experiments to observe the change in electronic structure of sulfur molecules during charge-discharge process would be required for more detailed discussion. In the case of P@SWCNT, we could not see any sharp diffraction lines on the XRD pattern in Fig. 3, while phosphorus atoms can be detected along with carbon atoms by elemental analysis of STEM observation (see Fig. 4). Therefore, phosphorus molecules are encapsulated in an amorphous form. Since no significant change in Raman spectrum after the encapsulation treatment was observed even in the D-band region (Fig. 5(A)), structural damage was not induced in the SWCNT FIG. 6. Charge-discharge curves of bulk phosphorus sample for (A) Li and (C) Na ions, and P@SWCNT for (B) Li and (D) Na ions, respectively. Phosphorus contents (P/C weight ratio) of P@SWCNT was evaluated to be 1.9. Specific capacity values were calculated with P weight.

9 Ishii et al. AIP Advances 6, (2016) framework structure by the treatment. As shown in the inset of Fig. 5(C), the G-band peak position was shifted toward higher wavenumber side by phosphorus encapsulation treatment. It indicates the charge transfer from SWCNT to the encapsulated phosphorus molecules. Figures 6(A) 6(D) show charge-discharge curves of bulk phosphorus sample and using Li and Na metals as counter electrodes. Cycle performance of the electrodes are shown in Fig. S4. 37 As shown in Fig. 6(A) and 6(C), bulk phosphorus sample does not work as anodes neither for LIB nor for SIB. On the other hand, can adsorb and desorb both Li and Na ions, although large charge-discharge hysteresis were observed. It is well known that very large irreversible capacity mainly due to the side reaction of electrolyte on the surface of SWCNT at around 1.0 V is observed for pristine SWCNT electrode. However, the coulombic efficiency of P@SWCNT is not very bad both for Li and Na ions. The discharge plateau potentials for Li and Na insertions were about 0.5 and 0.2 V, respectively. Since Fig. 6(B) and 6(D) were observed with Li + /Li and Na + /Na redox potentials as zero, respectively, the potentials recalculated for SHE standard are almost the same values. The potential is probably governed by reduction potential of phosphorus. Readers should find the reversible capacity of P@SWCNT for Li ion is much larger than that for Na ion. Li 3 P and Na 3 P are known as composites of phosphorus and Li or Na, and the volume expansion ratios are 300 and 491% for Li 3 P and Na 3 P, respectively. 31,38 Since the expanded composites may block the penetration of alkali ions and thereby decrease the usability of inner phosphorus, it would be the reason for the difference in reversible capacity. Furthermore, charge/discharge hysteresis of P@SWCNT for Li ion is much larger than that for Na ions. The hysteresis should be related to the kinetic nature of alkali ions in SWCNTs. In the case of Li ions, they are inserted deeper inside SWCNTs than Na ions. The diffusion of Li ions inserted in deeper part would be very slow, although further experiments (such as pulse-field gradient spin-echo NMR) are needed in order to confirm the kinetic properties. IV. CONCLUSIONS We investigated the sublimation temperatures of sulfur molecules encapsulated in different diameter cylindrical pores of MPC and two kinds of SWCNTs. It was found that the sublimation temperature of sulfur molecules encapsulated in SWCNTs having mean tube diameter of 1.5 nm is much higher than any other molecules encapsulated in larger pores. It was also found that the capacity fading of LIS battery can be diminished by encapsulation of sulfur molecules in SWCNTs. We also investigated the electrochemical properties of phosphorus molecules encapsulated in SWCNTs (P@SWCNTs), and showed that P@SWCNT can adsorb and desorb both Li and Na ions reversibly. ACKNOWLEDGMENTS The authors thank to Yashima Environment Technology Foundation, Tatematsu Foundation, Steel Foundation for Environmental Protection Technology, and JSPS KAKENHI (Grant number ) for their financial support to the present study. 1 B. W. Smith, M. Monthioux, and D. E. Luzzi, Nature 396, 323 (1998). 2 I. V. Anoshkin, A. V. Talyzin, A. G. Nasibulin, A. V. Krasheninnikov, H. Jiang, R. M. Nieminen, and E. I. Kauppinen, ChemPhysChem 15, 1660 (2014). 3 K. Yanagi, Y. Miyata, and H. Kataura, Adv. Mater. 18, 437 (2006). 4 Y. Iwai, M. Hirose, R. Kano, S. Kawasaki, Y. Hattori, and K. Takahashi, J. Phys. Chem. Solids 69, 1199 (2008). 5 Y. Maniwa, H. Kataura, M. Abe, A. Udaka, S. Suzuki, Y. Achiba, H. Kira, K. Matsuda, H. Kadowaki, and Y. Okabe, Chem. Phys. Lett. 401, 534 (2005). 6 H. Song, Y. Ishii, A. Al-zubaidi, T. Sakai, and S. Kawasaki, Phys. Chem. Chem. Phys. 15, 5767 (2013). 7 K. Koga, G. T. Gao, H. Tanaka, and X. C. Zeng, Nature 412, 802 (2001). 8 O. Byl, J.-C. Liu, Y. Wang, W.-L. Yim, J. K. Johnson, and J. T. Yates, J. Am. Chem. Soc. 128, (2006). 9 K. Urita, Y. Shiga, T. Fujimori, T. Iiyama, Y. Hattori, H. Kanoh, T. Ohba, H. Tanaka, M. Yudasaka, S. Iijima, I. Moriguchi, F. Okino, M. Endo, and K. Kaneko, J. Am. Chem. Soc. 133, (2011). 10 T. Fujimori, A. Morelos-Gómez, Z. Zhu, H. Muramatsu, R. Futamura, K. Urita, M. Terrones, T. Hayashi, M. Endo, S. Young Hong, Y. Chul Choi, D. Tománek, and K. Kaneko, Nat. Commun. 4 (2013). 11 G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, and W. Wilcke, J. Phys. Chem. Lett. 1, 2193 (2010).

10 Ishii et al. AIP Advances 6, (2016) 12 Y.-C. Lu, Z. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli, and Y. Shao-Horn, J. Am. Chem. Soc. 132, (2010). 13 W. Liu, Q. Sun, Y. Yang, J.-Y. Xie, and Z.-W. Fu, Chem. Commun. 49, 1951 (2013). 14 Z. Jiang, Y. Kato, A. Al-Zubaidi, K. Yamamoto, and S. Kawasaki, Mater. Express 4, 337 (2014). 15 J. Shim, K. A. Striebel, and E. J. Cairns, J. Electrochem. Soc. 149, A1321 (2002). 16 S.-R. Chen, Y.-P. Zhai, G.-L. Xu, Y.-X. Jiang, D.-Y. Zhao, J.-T. Li, L. Huang, and S.-G. Sun, Electrochim. Acta 56, 9549 (2011). 17 J. Guo, Y. Xu, and C. Wang, Nano Lett. 11, 4288 (2011). 18 S. Xin, L. Gu, N.-H. Zhao, Y.-X. Yin, L.-J. Zhou, Y.-G. Guo, and L.-J. Wan, J. Am. Chem. Soc. 134, (2012). 19 J. Schuster, G. He, B. Mandlmeier, T. Yim, K. T. Lee, T. Bein, and L. F. Nazar, Angew. Chem. Int. Ed. 51, 3591 (2012). 20 J.-j. Chen, Q. Zhang, Y.-n. Shi, L.-l. Qin, Y. Cao, M.-s. Zheng, and Q.-f. Dong, Phys. Chem. Chem. Phys. 14, 5376 (2012). 21 V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-Gonzalez, and T. Rojo, Energy Environ. Sci. 5, 5884 (2012). 22 S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama, A. Ogata, K. Gotoh, and K. Fujiwara, Adv. Funct. Mater. 21, 3859 (2011). 23 M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa, and S. Komaba, Phys. Chem. Chem. Phys. 16, (2014). 24 M. Hibino, R. Harimoto, Y. Ogasawara, R. Kido, A. Sugahara, T. Kudo, E. Tochigi, N. Shibata, Y. Ikuhara, and N. Mizuno, J. Am. Chem. Soc. 136, 488 (2014). 25 T. Matsushita, Y. Ishii, and S. Kawasaki, Mater. Express 3, 30 (2013). 26 T. Ichitsubo, T. Adachi, S. Yagi, and T. Doi, J. Mater. Chem. 21, (2011). 27 Y. Orikasa, T. Masese, Y. Koyama, T. Mori, M. Hattori, K. Yamamoto, T. Okado, Z.-D. Huang, T. Minato, C. Tassel, J. Kim, Y. Kobayashi, T. Abe, H. Kageyama, and Y. Uchimoto, Sci. Rep. 4, 5622 (2014). 28 T. Ishihara, M. Koga, H. Matsumoto, and M. Yoshio, Electrochem. Solid-State Lett. 10, A74 (2007). 29 T. Ishihara, Y. Yokoyama, F. Kozono, and H. Hayashi, J. Power Sources 196, 6956 (2011). 30 X. Ji, K. T. Lee, and L. F. Nazar, Nat. Mater. 8, 500 (2009). 31 J. Qian, X. Wu, Y. Cao, X. Ai, and H. Yang, Angew. Chem. Int. Ed. 52, 4633 (2013). 32 Y. Kim, Y. Park, A. Choi, N.-S. Choi, J. Kim, J. Lee, J. H. Ryu, S. M. Oh, and K. T. Lee, Adv. Mater. 25, 3045 (2013). 33 Y. Zhu, Y. Wen, X. Fan, T. Gao, F. Han, C. Luo, S.-C. Liou, and C. Wang, ACS Nano 9, 3254 (2015). 34 Y. Ishii, Y. Nishiwaki, A. Al-zubaidi, and S. Kawasaki, J. Phys. Chem. C 117, (2013). 35 S. Brunauer, P. H. Emmett, and E. Teller, J. Am. Chem. Soc. 60, 309 (1938). 36 E. P. Barrett, L. G. Joyner, and P. P. Halenda, J. Am. Chem. Soc. 73, 373 (1951). 37 See supplementary material at for Figures S1 S4. 38 J. Song, Z. Yu, M. L. Gordin, S. Hu, R. Yi, D. Tang, T. Walter, M. Regula, D. Choi, X. Li, A. Manivannan, and D. Wang, Nano Lett. 14, 6329 (2014).