Na(PO(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 effect (21, 22) and strong (POpolyanion networks, (23-25) respectively. Additionally, the substitution of Kfor Nawas 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 Kand Na. In previous research, X. Wang et al. reported completely substituted K(PO(KVP) and presented a perfect electrochemical performance in sodium storage. However, the obtained structural phase was different from that of NASICON-structured 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= 0.12 in Na(PO (30) Although further K substitution is thought to be thermodynamically difficult by a solid-state reaction, we demonstrated 15 times higher K concentration of= 1.8 in NASICON-structured Na(POby PDII. It is also found that this material shows an irreversible phase transition from a metastable phase to the reported unknown phase of KVP (29) 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.

Chemical vapor transport (CVT) (16, 17) and/or the two-bulb method wherein there is a direct reaction between guest vapor and host solid (18-20) have also been used for intercalation. Such 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 TaSvia PDII. These findings suggest a potential for industrial applications utilizing these intercalation compounds, which has been difficult so far using conventional methods.

We focused on transition metal dichalcogenide TaS (6-8) and Na superior ion conductor (NASICON)-structured Na(PO (9) for intercalation and ion substitution, respectively. TaSis 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 liquid-phase processes in general. As described above, since PDII is based on a solid-state electrochemical reaction without a liquid-phase process, such simultaneous intercalation has never been observed in this method.

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 proton-driven 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 damage, such as that seen in ion irradiation, (4, 5) has not been witnessed.

Although ATaSis stable in a dry atmosphere, it easily reacts with moisture in air to form hydrated compounds. These compounds show a much longerlattice parameter in comparison with ATaS. 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 ATaSto form A(HO)TaS. 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= 1/3, as shown in Figure S4 . This means that a larger amount of alkali metal does not coexist with HO molecules above= 1/3 in TaSinterlayers at atmospheric temperature and pressure. This also explains why the reported alkali metal ion concentration has always been below< 1/3 for preparation by a liquid-phase process. (11, 14, 15, 32-34) Additionally, after hydration and decomposition to A(HO)TaS, 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)

Figure 1. (a) Schematic of proton-driven ion introduction (PDII). Plate-like single crystal of TaS 2 , 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 TaS 2 and A x TaS 2 (A = Li, Na, and K).

Polycrystalline TaSwas 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 TaS. The obtained polycrystalline TaSand iodine of 5 mg/cmas 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 TaS, phosphate glasses containing each alkali metal ion (31) 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 ACO, La, and HPO(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 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 TaS 2 and both top and bottom surfaces of Cu x TaS 2 prepared under the conditions given in (a) and (b), respectively.

To perform homogeneous intercalation, the supply rate of Cufrom CuI and the diffusion coefficient of Cuin TaSare considered to be important parameters. As shown in Figure 2 c, when the former is much smaller than the latter, Cuspread effectively throughout TaS. However, whenincreased to the solid solubility limit, Cuwas precipitated as copper metal, receiving electrons through CuTaS. On the contrary, when the former is much larger than the latter, at the beginning stage, Cuwas also inserted from the top side of TaS. However, the Cu concentration immediately reached the solid solubility limit on the surface. Surplus Cuwas then precipitated as copper metal. Such a precipitation separates CuI and TaSand prevents Cufrom migrating to TaS. 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 () of the host material can be described by the formula:exp (−), whereandare the activation energy and the Boltzmann constant, respectively, andexponentially 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 3 a,b, which exhibit the time dependence of the current, applied voltage, and temperature during PDII. Figure 3 c,d show XRD patterns of Cu-intercalated TaSobtained under each condition. When 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 TaSphase still existed at the top side of the single crystal. Thus, homogeneous Cu-intercalated TaScould 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 (= 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 CuTaS, 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.

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.

Transition metal ions (Cuand Ag) can also be intercalated into TaSby PDII. As ion supply materials, CuI and AgI were adopted due to their ion conducting properties. (36) Figure 2 a shows the schematic configuration for Cu intercalation with expected chemical reactions shown at the indicated positions by arrows. Phosphate glasses containing Na, CuI, and TaSwere 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.

Ion Substitution for Nasicon-Structured Na 3 V 2 (PO 4 ) 3 ARTICLE SECTIONS Jump To

3 V 2 (PO 4 ) 3 (NVP) was synthesized using a one-step solid-state reaction. The stoichiometric NaH 2 PO 4 and V 2 O 3 powders were mixed and ground. Then, the mixture was heated at 900 °C for 24 h in 5% H 2 /Ar. + substitution into Na+ sites in Na 3 V 2 (PO 4 ) 3 (NVP) with a NASICON structure. Na(PO(NVP) was synthesized using a one-step solid-state reaction. The stoichiometric NaHPOand Vpowders were mixed and ground. Then, the mixture was heated at 900 °C for 24 h in 5% H/Ar. Figure 4 shows the schematic configuration for Ksubstitution into Nasites in Na(PO(NVP) with a NASICON structure.

Figure 4 Figure 4. (a) Schematic showing the process of K substitution into the Na site in Na 3 V 2 (PO 4 ) 3 with expected chemical reactions shown at the indicated positions by arrows. Optical images show the cross section of Na 3–x K x V 2 (PO 4 ) 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).

+ source material. When a voltage was applied, protons were replaced with K+ in the glass and drove them into NVP. Unlike the case of TaS 2 , 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 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 Ksource material. When a voltage was applied, protons were replaced with Kin the glass and drove them into NVP. Unlike the case of TaS, Kdoes not receive electrons between interfaces owing to electrical insulating properties of NVP. Therefore, even if high voltage is applied, Kcan continuously migrate without forming a precipitate. Conversely, discharged Nafrom the bottom side of NVP receives electrons and precipitates. These processes for ion migration are expressed by the equations in Figure 4 a.

1.69 Na 1.20 V 2.00 P 3.88 O 13.83 and Na 3.18 V 2.00 P 3.02 O 10.86 , 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 R3̅c) and the elemental composition ratio is K:Na = 3:2. The diffraction peaks of K-NVP after annealing are 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 Cross-sectional optical images of NVP packed in the alumina cylinder are shown in Figure 4 a. 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 KNaand Na, 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 4 b. 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 K-NVP by Rietveld analysis, (37) assuming that the space group is the same as NVP (3̅) and the elemental composition ratio is K:Na = 3:2. The diffraction peaks of K-NVP after annealing are consistent with those of calculated patterns. The obtained lattice parameters are= 8.6341 Å and= 22.8663 Å. In comparison with the lattice parameter of NVP, thelattice shrinks, but thelattice expands with increasing K concentration. This tendency agrees with the previously reported results up to= 0.12, as shown in Figure S11 . The Rietveld analysis results are displayed in Figure S12

+ replaced most of the Na+ in NVP. Furthermore, the V 2p 1/2 and V 2p 3/2 peaks at 523.4 and 516.4 eV, respectively, were assigned to the 3+ oxidation state of V in 3/2 in K-NVP shows nearly the same position as that in NVP. However, an additional peak near the V 2p 1/2 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.4+ 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. 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 5 a,b. This indicates that Kreplaced most of the Nain NVP. Furthermore, the V 2pand V 2ppeaks at 523.4 and 516.4 eV, respectively, were assigned to the 3oxidation state of V in Figure 5 c. (38) The peak position of V 2pin K-NVP shows nearly the same position as that in NVP. However, an additional peak near the V 2ppeak of K-NVP can be confirmed from peak-fit processing, which is probably derived from the partial reduction of Vto Vby Arbombardment during XPS measurement. (38) Moreover, there were no higher binding energy peaks near 519–520 eV (Vor Voxidation peaks) in K-NVP. Thus, we can conclude that the valence state of V in K-NVP is maintained at 3after PDII.

Figure 5 Figure 5. (a) K 2p X-ray photoelectron spectroscopy (XPS) spectra for Na 3 V 2 (PO 4 ) 3 (NVP) and K-NVP. (b) Na 1s XPS spectra for NVP and K-NVP. (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.

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.