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Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX-XXX
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Proton-Driven Intercalation and Ion Substitution Utilizing Solid-State Electrochemical Reaction Masaya Fujioka,*,† Chuanbao Wu,‡ Naoki Kubo,† Gaoyang Zhao,‡ Atsushi Inoishi,§ Shigeto Okada,§ Satoshi Demura,¶ Hideaki Sakata,¶ Manabu Ishimaru,∥ Hideo Kaiju,† and Junji Nishii† †
Research Institute for Electronic Science, Hokkaido University, Sapporo, Hokkaido 001-0020, Japan School of Material Science and Engineering, Xi’an University of Technology, Xi’an 710048, China § Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka, 816-8580, Japan ¶ Tokyo University of Science, Shinjuku, Tokyo 162-8601, Japan ∥ Department of Materials Science and Engineering, Kyushu Institute of Technology, Tobata, Kitakyushu, Fukuoka 804-8550, Japan ‡
S Supporting Information *
ABSTRACT: The development of an unconventional synthesis method has a large potential to drastically advance materials science. In this research, a new synthesis method based on a solid-state electrochemical reaction was demonstrated, which can be made available for intercalation and ion substitution. It was referred to as proton-driven ion introduction (PDII). The protons generated by the electrolytic dissociation of hydrogen drive other monovalent cations along a high electric field in the solid state. Utilizing this mechanism, Li+, Na+, K+, Cu+, and Ag+ were intercalated into a layered TaS2 single crystal while maintaining high crystallinity. This liquid-free process of ion introduction allows the application of high voltage around several kilovolts to the sample. Such a high electric field strongly accelerates ion substitution. Actually, compared to conventional solid-state reaction, PDII introduced 15 times the amount of K into Na super ionic conductor (NASICON)-structured Na3−xKxV2(PO4)3. The obtained materials exhibited a thermodynamically metastable phase, which has not been reported so far. This concept and idea for ion introduction is expected to form new functional compounds and/or phases.
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damage, such as that seen in ion irradiation,4,5 has not been witnessed. We focused on transition metal dichalcogenide TaS26−8 and Na superior ion conductor (NASICON)-structured Na3V2(PO4)39 for intercalation and ion substitution, respectively. TaS2 is well-known as a two-dimensional layered material, and various guest ions and molecules can be intercalated into the van der Waals gap of its interlayers.10,11 Liquid-phase processes are usually employed for intercalation as they are effective; however, it is well-known that guest ions and solution molecules can be simultaneously inserted into a host material.12−15 It can be difficult to achieve homogeneous intercalation while maintaining high crystallinity using liquidphase processes in general. As described above, since PDII is based on a solid-state electrochemical reaction without a liquidphase process, such simultaneous intercalation has never been observed in this method. Chemical vapor transport (CVT)16,17 and/or the two-bulb method wherein there is a direct reaction between guest vapor and host solid18−20 have also been used for intercalation. Such
INTRODUCTION
Due to the development and improvement in the synthesis apparatus and technique, materials science and design have advanced drastically. For the next breakthrough, a new idea in synthesis is desirable to create or evolve an unconventional synthesis method that can provide currently unobtainable materials. Herein, we report a new synthesis method that is available for ion introduction processes, such as intercalation and ion substitution. This method is performed under ambient hydrogen pressure, and it utilizes a phenomenon similar to that of “ion billiards” using protons generated by the electrolytic dissociation of hydrogen in a corona discharge and other monovalent cations. This method is referred to as protondriven ion introduction (PDII). The process of this method is thought to be similar to that of an electrochemical reaction.1−3 However, in PDII, a liquid-phase process is completely unnecessary; therefore, it is regarded as a solid-state electrochemical reaction. Furthermore, although the generated protons are accelerated by an applied high electric field, as comprehensively described later, PDII is also different from ion irradiation. Since the ion diffusion properties of the sample itself are utilized for this ion introduction, the structural © XXXX American Chemical Society
Received: September 1, 2017
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DOI: 10.1021/jacs.7b09328 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society
Figure 1. (a) Schematic of proton-driven ion introduction (PDII). Plate-like single crystal of TaS2, displayed in an optical image, is placed on the carbon cathode, and the ion-source material is placed on the single crystal. When protons are generated by the electrolytic dissociation of hydrogen, ion migration starts and current flows around the circuit. (b) Schematic of the electrochemical method for comparison with PDII. (c) X-ray diffraction (XRD) patterns for TaS2 and AxTaS2 (A = Li, Na, and K).
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INTERCALATION OF ALKALI METAL IONS (LI+, NA+, AND K+) Polycrystalline TaS2 was synthesized by a solid-state reaction from stoichiometric Ta and S powders. These were sealed into an evacuated quartz tube and sintered at 950 °C for 4 days. CVT was used for the synthesis of single crystals of TaS2. The obtained polycrystalline TaS2 and iodine of 5 mg/cm3 as a transporting agent were added in a closed quartz tube and sintered using a temperature gradient from 800 to 900 °C for 7 days. To intercalate alkali metal ions (e.g., K+, Na+, and Li+) into TaS2, phosphate glasses containing each alkali metal ion31 were prepared using a conventional melt-quenching technique as the ion-source materials, which must exhibit ion conducting properties at the treatment temperature of PDII. The raw materials A2CO3, La2O3, and H3PO4 (A = K, Na, and Li) were weighed and mixed. They were then melted in a platinum crucible for 30 min at 1200 °C in air. The molten materials were poured into a cylindrical carbon mold with an inner diameter of 10 mm, annealed at 380 °C for 1 h, and gradually cooled to room temperature in a furnace at a cooling rate of 30 °C/h. Disk-shaped glass plates with a diameter of 10 mm and a thickness of 1 mm were sliced from the cylindrical glass block, and both surfaces were polished. Figure 1a shows a schematic of the experimental apparatus for ion introduction. A single crystal of TaS2 with a plate-like shape was put on a carbon cathode in atmospheric hydrogen in a chamber. Also, the hydrogen concentration in PDII is one of the important parameters to promote ion introduction, as shown in Figure S1. In this research, 100% hydrogen atmosphere was used. The ion-source material was also placed on the single crystal of TaS2. When applying a high voltage between the needle-shaped anode and the carbon cathode, hydrogen is ionized to produce protons and electrons. The protons are accelerated to the ion-source material along the electric field. They then penetrate and push the alkali metal ions from the top side to the bottom side. Since charge neutrality should be retained in the ion-source material, the alkali metal ions must be released from the bottom side and arrive at TaS2. Simultaneously, electrons produced by the corona discharge move from the needle to TaS2 on the carbon
methods require the materials to be enclosed in a quartz or stainless-steel tube so that the sample size is always limited. Additionally, these methods require high temperature sintering to vaporize the guest materials or transport agents.16−20 However, both the limitations on the size of samples as well as high-temperature treatment are unnecessary in PDII. At only 100−350 °C, alkali metals, Cu and Ag ions can be intercalated into TaS2 via PDII. These findings suggest a potential for industrial applications utilizing these intercalation compounds, which has been difficult so far using conventional methods. Na3V2(PO4)3 (NVP) has also been investigated as a promising candidate for the cathode of a Na ion battery due to its high redox potential and excellent thermal stability, which originate from an inductive effect21,22 and strong (PO4)3− polyanion networks,23−25 respectively. Additionally, the substitution of K+ for Na+ was expected to improve the rate capability and cycling stability through various research studies.26−28 However, such a substitution is difficult due to the large difference in ion size between K+ and Na+. In previous research, X. Wang et al. reported completely substituted K3V2(PO4)3 (KVP) and presented a perfect electrochemical performance in sodium storage. However, the obtained structural phase was different from that of NASICONstructured NVP and is still unknown.29 On the other hand, S. J. Lim et al. also reported K substitution while maintaining the NASICON structure up to x = 0.12 in Na3−xKxV2(PO4)3.30 Although further K substitution is thought to be thermodynamically difficult by a solid-state reaction, we demonstrated 15 times higher K concentration of x = 1.8 in NASICONstructured Na3−xKxV2(PO4)3 by PDII. It is also found that this material shows an irreversible phase transition from a metastable phase to the reported unknown phase of KVP29 between 600 and 700 °C. Such a metastable phase would be induced by forcible ion substitution associated with a high applied electric field. This novel technique is expected to provide various new compounds and/or phases, which are difficult to synthesize using a conventional sintering process, resulting in a significant contribution to materials science. B
DOI: 10.1021/jacs.7b09328 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Figure 2. (a) Schematic for the process of Cu intercalation with expected chemical reactions shown at the indicated positions by arrows. (b) Optical images for the top and bottom surfaces of CuI after PDII. (c) Schematics for each different reaction process under different PDII treatment conditions.
tion to A1/3(H2O)yTaS2, superconductivity appeared in each sample, as shown in Figure S8. These results were consistent with those of previous studies on the liquid-phase process.14,15,32,35
cathode through the electrode. As a result, an electric current flows around this circuit and intercalation is electrochemically driven according to the following reaction: TaS2 + xA+ + xe− = AxTaS2. The observed time dependence of current is shown in Figure S2(a−c). Figure 1c shows X-ray diffraction (XRD; Rigaku: MiniFlex 300) patterns for TaS2 and AxTaS2. These were highly oriented to the c-axis because of the plate-like shape of the crystals, which have a two-dimensional layered structure. The (00l) indices gradually shift to a lower angle in the order K, Na, and Li. This means that the interlayer spaces between TaS2 layers are expanded according to the ionic sizes of the alkali metal ions. In principle, with this method, since guest ions are inserted from the top side of TaS2, a concentration gradient is likely to be observed. However, XRD measurements for both the top and bottom sides show the same patterns as shown in Figures S2(d−f). Their elemental compositions after intercalation were investigated by scanning electron microscopy− energy dispersive X-ray spectroscopy (SEM−EDS; JEOL: model 6000) measurements. Each alkali metal concentration was estimated to be x = 2/3 in both sides (Figures S2(g,h)). This x value is thought to be the solid solubility limit of these compounds. Actually, the total amount of electricity described in Figures S2(a−c) is sufficient to completely intercalate alkali metal ions in TaS2. Although A2/3TaS2 is stable in a dry atmosphere, it easily reacts with moisture in air to form hydrated compounds. These compounds show a much longer c lattice parameter in comparison with A2/3TaS2. Thus, the XRD patterns show a time dependence, as shown in Figure S3. It is also found that this gradual hydration expels the alkali metal ions from A2/3 TaS 2 to form A 1/3 (H 2O) y TaS 2 . The expelled ions precipitate and react with air on the surface of a single crystal. After peeling off the surface layers of samples, the alkali metal ion concentration decreases to x = 1/3, as shown in Figure S4. This means that a larger amount of alkali metal does not coexist with H2O molecules above x = 1/3 in TaS2 interlayers at atmospheric temperature and pressure. This also explains why the reported alkali metal ion concentration has always been below x < 1/3 for preparation by a liquid-phase process.11,14,15,32−34 Additionally, after hydration and decomposi-
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INTERCALATION OF THE TRANSITION METAL ION (CU+ AND AG+) Transition metal ions (Cu+ and Ag+) can also be intercalated into TaS2 by PDII. As ion supply materials, CuI and AgI were adopted due to their ion conducting properties.36 Figure 2a shows the schematic configuration for Cu intercalation with expected chemical reactions shown at the indicated positions by arrows. Phosphate glasses containing Na, CuI, and TaS2 were stacked on a carbon cathode in that order from the top. Without the phosphate glass, the hydrogen iodide (HI) gas, which is a poisonous and strong acidic substance, would be produced because copper ions were directly exchanged with protons during the PDII treatment. Figure 2b shows optical images of the top and bottom sides of CuI after PDII. At the top side, the color of CuI changed to red due to the replacement of Cu+ with Na+. The Na ions, which were driven by protons and released from the phosphide glass, were substituted for Cu ions on the top surface of CuI. The EDS measurement supports this process, as shown in Figure S5. On the other hand, at the bottom side, Cu ions were also extruded and intercalated into TaS2. Along with alkali metal ions, Cu and Ag ions were homogeneously intercalated into TaS2. The results of Ag intercalation are shown in Figure S6. Three black single crystals of CuxTaS2 are shown in Figure 2b. Red products composed of copper metal can also be observed around CuxTaS2. The estimated solid solubility limit of Cu+ was around two-thirds. Unlike with alkali metal ion intercalation, Cu2/3TaS2 showed no hydration or decomposition. To perform homogeneous intercalation, the supply rate of Cu+ from CuI and the diffusion coefficient of Cu+ in TaS2 are considered to be important parameters. As shown in Figure 2c, when the former is much smaller than the latter, Cu+ spread effectively throughout TaS2. However, when x increased to the solid solubility limit, Cu+ was precipitated as copper metal, C
DOI: 10.1021/jacs.7b09328 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Figure 3. (a) Time dependence of treatment conditions with a lower temperature and a higher applied voltage. (b) Time dependence of treatment conditions with a higher temperature and a lower applied voltage. (c) and (d) XRD patterns for TaS2 and both top and bottom surfaces of CuxTaS2 prepared under the conditions given in (a) and (b), respectively.
Table 1. Full Width at Half Maximum (fwhm) of Main Peak (002) and c Lattice Parameter for Each Single Crystal fwhm (deg.) Top (002) fwhm (deg.) Bottom (002) c lattice (Å)
TaS2
Li2/3TaS2
Na2/3TaS2
K2/3TaS2
Cu2/3TaS2
Ag2/3TaS2
0.057 0.057 12.080(6)
0.072 0.055 12.923(10)
0.064 0.060 14.248(7)
0.066 0.056 15.844(14)
0.051 0.091 13.189(3)
0.051 0.061 14.461(4)
using a low treatment temperature and a high voltage, the XRD pattern of the bottom side did not shift to a lower angle and the TaS2 phase still existed at the top side of the single crystal. Thus, homogeneous Cu-intercalated TaS2 could not be obtained under these conditions despite a long-time treatment of over 50 h. A cross section partially excised from this single crystal by a focused ion beam (FIB) is displayed in Figure S7. Although the top side surface shows the maximum Cu concentration (x = 2/3), this suddenly decreases to half at only 10 μm below the surface. However, the other conditions allow the formation of a homogeneous single crystal of Cu2/3TaS2, even within a short treatment time of around 12 h. In this way, by stacking a sample and ion-source materials, PDII enables the migration of ions beyond the interfaces of each material and the fabrication of a homogeneous single crystal by introducing guest ions up to a solid solubility limit. Table 1 summarizes the full width at half-maximum (fwhm) values of the main peak (002) and the c lattice parameters obtained from XRD for each material. Fwhm of intercalated compounds are nearly comparable to those of a TaS2 single crystal. This means that high crystallinity can be maintained after intercalation by PDII. In fact, the analysis employing
receiving electrons through Cu2/3TaS2. On the contrary, when the former is much larger than the latter, at the beginning stage, Cu+ was also inserted from the top side of TaS2. However, the Cu concentration immediately reached the solid solubility limit on the surface. Surplus Cu+ was then precipitated as copper metal. Such a precipitation separates CuI and TaS2 and prevents Cu+ from migrating to TaS2. Therefore, in general, the supply rate of guest ions should be smaller than their diffusion coefficient. Additionally, the ion supply rate corresponds to the electric current flowing around the circuit. This can be controlled by adjusting the applied voltage. On the other hand, since the diffusion coefficient (D) of the host material can be described by the formula: D = D0 exp (−Ea/KBT), where Ea and KB are the activation energy and the Boltzmann constant, respectively, and D exponentially increases with the treatment temperature. This means that the homogeneously intercalated compounds could be easily synthesized by adjusting the voltage and temperature. This suggestion is demonstrated via two different treatment conditions, as shown in Figure 3a,b, which exhibit the time dependence of the current, applied voltage, and temperature during PDII. Figure 3c,d show XRD patterns of Cu-intercalated TaS2 obtained under each condition. When D
DOI: 10.1021/jacs.7b09328 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Figure 4. (a) Schematic showing the process of K substitution into the Na site in Na3V2(PO4)3 with expected chemical reactions shown at the indicated positions by arrows. Optical images show the cross section of Na3−xKxV2(PO4)3 packed in an alumina cylinder. (b) XRD patterns for the samples obtained from the lower and upper regions after PDII, the annealed sample from the upper region at 600 °C, and a simulation pattern obtained from the Rietveld refinement. The described numbers are Miller indices (hkl).
Figure 5. (a) K 2p X-ray photoelectron spectroscopy (XPS) spectra for Na3V2(PO4)3 (NVP) and K-NVP. (b) Na 1s XPS spectra for NVP and KNVP. (c) V 2p XPS spectra for NVP and K-NVP. For charge calibration, the C 1s line at 284.6 eV BE is considered as a reference.
Cross-sectional optical images of NVP packed in the alumina cylinder are shown in Figure 4a. Obviously, the color of the powder changed to dark green in the upper region due to K substitution. In fact, from the EDS measurement, the elemental compositions in the upper and lower regions are estimated to be K1.69Na1.20V2.00P3.88O13.83 and Na3.18V2.00P3.02O10.86, respectively; note that the amount of V is normalized to 2.00. For the sake of simplicity, we denoted the powder in the upper region as K-NVP. The composition ratio of K to Na in K-NVP is estimated to be around 3:2. On the other hand, no K emission spectra were detected for the lower region. Additionally, XRD diffraction peaks from the lower region are consistent with the reported NVP pattern in Figure 4b. However, K-NVP shows different peak positions from NVP and peaks are wider than those obtained from the lower region. Therefore, to improve the crystallization, post-annealing was performed at various temperatures from 500 to 900 °C, as shown in Figure S10. The optimal annealing temperature is thought to be 600 °C. The primary peaks do not change and the color of the powder is still dark green at this temperature. Additionally, the diffraction peaks become sharper and some shoulder peaks, indicated by red arrows at 2θ values of 28.2° and 30.9°, disappeared in annealed K-NVP. We calculated the simulation pattern of KNVP by Rietveld analysis,37 assuming that the space group is the same as NVP (R3̅c) and the elemental composition ratio is K:Na = 3:2. The diffraction peaks of K-NVP after annealing are
transmission electron microscopy (TEM; JEOL: JEM-3000F) supported the high crystallinity of Cu2/3TaS2, as shown in Figure S9.
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ION SUBSTITUTION FOR NASICON-STRUCTURED NA3V2(PO4)3 Na3V2(PO4)3 (NVP) was synthesized using a one-step solidstate reaction. The stoichiometric NaH2PO4 and V2O3 powders were mixed and ground. Then, the mixture was heated at 900 °C for 24 h in 5% H2/Ar. Figure 4 shows the schematic configuration for K+ substitution into Na+ sites in Na3V2(PO4)3 (NVP) with a NASICON structure. In this case, a powdered sample rather than a single crystal was used for PDII. This NVP powder was put in a shallow alumina cylinder with inner and outer diameters of 4.5 and 10.0 mm, respectively, and placed on a carbon cathode stage. Then, a potassium-containing phosphate glass was also placed on the alumina cylinder as a K+ source material. When a voltage was applied, protons were replaced with K+ in the glass and drove them into NVP. Unlike the case of TaS2, K+ does not receive electrons between interfaces owing to electrical insulating properties of NVP. Therefore, even if high voltage is applied, K+ can continuously migrate without forming a precipitate. Conversely, discharged Na+ from the bottom side of NVP receives electrons and precipitates. These processes for ion migration are expressed by the equations in Figure 4a. E
DOI: 10.1021/jacs.7b09328 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Figure S14. Therefore, PDII is regarded as a highly versatile method for ion introduction. Furthermore, PDII is easily driven by just stacking the ionsource materials and samples, such as plate-like single crystals or aggregated powders. This means that ion migration can be controlled regardless of sample shapes without any cumbersome processes. Such a technique is thought to be applicable to the thin films deposited on the substrate. In addition, PDII can be driven at low temperature, as shown in Figures S15 and S16. Moreover, when a large number of evenly spaced anodes are located against a cathode, we can treat a large area at one time. These aspects suggest a potential for industrial applications. In PDII, there is significant room for the modification of this simple apparatus with respect to treatment temperature, atmosphere, and current control. When the apparatus is appropriately improved, there is a possibility to introduce multivalent cations and/or anions, such as hydroid (H−), by applying the reverse bias. In fact, since recently reported divalent ion conductors show high ion-conducting properties,39 they are interesting candidates for the ion-source materials of PDII. Such multivalent ion introduction is expected to form new functional compounds or modulate the electrical properties of host materials. Moreover, these new synthesis concepts and ideas based on solid-state electrochemistry may trigger other unconventional techniques and thus significantly contribute to materials science and design.
consistent with those of calculated patterns. The obtained lattice parameters are a = 8.6341 Å and c = 22.8663 Å. In comparison with the lattice parameter of NVP, the a lattice shrinks, but the c lattice expands with increasing K concentration. This tendency agrees with the previously reported results up to x = 0.12, as shown in Figure S11. The Rietveld analysis results are displayed in Figure S12. Figure 5 shows the X-ray photoelectron spectroscopy (XPS; JEOL: JPS-9200) results for NVP and K-NVP. K was clearly detected in K-NVP, but the peak intensity of Na located at 1071.6 eV drastically decreases after PDII, as shown in Figure 5a,b. This indicates that K+ replaced most of the Na+ in NVP. Furthermore, the V 2p1/2 and V 2p3/2 peaks at 523.4 and 516.4 eV, respectively, were assigned to the 3+ oxidation state of V in Figure 5c.38 The peak position of V 2p3/2 in K-NVP shows nearly the same position as that in NVP. However, an additional peak near the V 2p1/2 peak of K-NVP can be confirmed from peak-fit processing, which is probably derived from the partial reduction of V3+ to V2+ by Ar+ bombardment during XPS measurement.38 Moreover, there were no higher binding energy peaks near 519−520 eV (V4+ or V5+ oxidation peaks) in K-NVP. Thus, we can conclude that the valence state of V in K-NVP is maintained at 3+ after PDII. Furthermore, IR spectrometry (Thermo Fisher Scientific: Nicolet iS10 FT-IR spectrometer) and Raman spectroscopy (Renishaw: inVia Raman Microscope) indicated the NASICON structure after PDII in K-NVP, as shown in Figure S13. However, when K-NVP was annealed at 700 and 800 °C, a completely different phase appeared, as shown in Figure S10, and the obtained diffraction peaks are assigned to a previously reported unknown structure of KVP.29 Note that since the elemental compositions of K-NVP and KVP are different, the peak intensities are also different. This phase transition is irreversible, so the unknown KVP structure is thermodynamically more stable than the NASICON-structured K-NVP. In general, when a compound is prepared from raw materials by a solid-state reaction, the thermodynamically most stable phase at the given temperature should be formed. However, in the case of PDII, a complete structural framework, such as the NASICON structure, has already been formed before ion substitution. Guest ions are then forcibly inserted into a robust structural framework by applying a high electric field. This process probably does not impart large enough energy to destroy the structural framework. It is thought to be the reason why PDII can form a metastable structural phase. Consequently, using PDII, the substituted amount of K is 15 times as much as that by conventional solid-state reaction in NASICON-structured K-NVP.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b09328. Role of hydrogen in PDII; Intercalation of alkali-metal ions to TaS2; Intercalation of Cu and Ag ions to TaS2; TEM analysis for CuxTaS2; Superconductivity in ionintercalated TaS2; NASICON-structured Na3‑xKxV2(PO4)3; NbSe2 and graphite as host materials; Low temperature treatment (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Masaya Fujioka: 0000-0002-5829-6591 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research has been partially supported by a Grant-in-Aid for Young Scientists and Grant-in-Aid for Challenging Exploratory Research from the Japan Society for the Promotion of Science (JSPS; Grant No. 15K17711 and 86000149), Iketani Science and Technology Foundation, Nanotech Career-up Alliance (CUPAL), Environment and Materials and the Cooperative Research Program of the Network Joint Research Center for Materials and Devices from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and the Dynamic Alliance for Open Innovation Bridging Human from MEXT. The authors thank Prof. S. Noro (Hokkaido University) for the experimental help with IR spectroscopy measurements. The authors also thank N. Kawai (Hokkaido University) for the preparation of the sample for TEM analysis.
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ADVANTAGES OF PDII In this research, we focused on the Li+, Na+, K+, Cu+, and Ag+ ions. However, not only should they be available, but other monovalent cations as well. In fact, although we could not show any apparent evidence, H+ seems to be intercalated into TaS2 by PDII. On the other hand, various compounds with nanospaces can also serve as candidates of host materials, e.g., zeolites, metal organic frameworks (MOF), clathrate compounds, carbon-based materials, and various two-dimensional layered compounds, and they are thought to be preferable to have ion diffusion properties to some extent. At the current stage, although under investigation, NbSe2 and graphite have been confirmed as host materials, as shown in F
DOI: 10.1021/jacs.7b09328 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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DOI: 10.1021/jacs.7b09328 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
