Positive and Negative Aspects of Interfaces in Solid-State Batteries

Dec 1, 2017 - His work focuses on computational simulations of battery materials, especially for solid-state Li-ion batteries, to elucidate the electr...
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Positive and Negative Aspects of Interfaces in Solid-State Batteries Kazunori Takada,*,†,§ Takahisa Ohno,‡,§ Narumi Ohta,†,§ Tsuyoshi Ohnishi,†,§ and Yoshinori Tanaka§ †

Center for Green Research on Energy and Environmental Materials, ‡International Center for Materials Nanoarchitectonics, and Global Research Center for Environment and Energy Based on Nanomaterials Science, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan §

ABSTRACT: Solid-state lithium batteries are regarded as promising energy storage devices that meet the requirements for realizing a low-carbon society. Although solidstate batteries have been suffering from low power density, the power density has become comparable to or greater than that of liquid systems in a recently developed battery, which has been achieved not only by the high ionic conductivity of the used sulfide solid electrolyte. This Perspective presents anomalous transport properties appearing at the interfaces in solid-state batteries to highlight the importance of controlling the interface phenomena in achieving the high performance. The battery employs not only the highly conductive sulfide but also some oxides in spite of their low conductivity. LiNbO3 interposed to the cathode interface effectively reduces the cathode impedance by suppressing lithium depletion at the interface. Li4Ti5O12 used as the anode becomes a good conductor in its two-phase region because of enhanced transport properties at the phase boundaries.

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electrolytes in solid-state batteries. Because the side reactions are suppressed, solid-state batteries exhibit high durability, i.e., excellent cycling performance. In spite of the high reliability, solid-state batteries are suffering from low power densities. Because the low power densities are due to the low ionic conductivities of solid electrolytes, achieving fast ionic conduction in solids is the first task for realizing solid-state batteries. Fast ionic conduction is anticipated in chalcogenides because of the large size and high polarizability of chalcogenide ions. The large ionic radius provides wide channels for ionic conduction, and the high polarizability of the anionic frameworks weakens the interaction between the lithium ions and the framework, making the lithium ions mobile.4 In fact, the highest conductivities among sulfide reached 10−3 S cm−1 in the early 1980s5 and have exceeded 10−2 S cm−1 recently.6−8 This value is of the same order of that of nonaqueous liquid electrolytes used in current lithium-ion batteries. Moreover, lithium transport numbers are less than 0.5 in the liquid electrolyte,9 whereas they are unity in solids. Therefore, it can be concluded that solid electrolytes have become superior to liquid electrolytes in terms of lithium-ion conduction. Because the conductivities have become sufficient to generate practical

ithium-ion batteries have been contributing to the advancement of the “information society” as power sources in portable electronics, and they are expected to be energy storage devices in vehicles and smart grids for realizing a low-carbon society; however, some improvements are necessary to meet the requirements for such applications.1 Safety issues are an inevitable problem for lithium-ion batteries, because the organic-solvent electrolytes employed in the batteries are combustible. On the other hand, large batteries are required for energy storage, in which the safety issues become much more serious because of the increasing amount of combustible electrolytes and the decreasing heat radiation with increasing battery size. In addition, the batteries for energy storage should have much longer durability; for example, batteries installed in vehicles should have a useful life-span of 10 years or more. Solid-state batteries are anticipated to be an effective solution to these issues.2 First, inorganic solid electrolytes are nonflammable, which will provide a fundamental solution to the safety issues. In addition, side reactions, which lead to the deteriorations of batteries, are efficiently suppressed in solid electrolytes. Only lithium ions are mobile around room temperature, at which battery operation is expected, in solid electrolytes that make lithium-ion batteries all-solid-state. Because there are no other species that move to the electrode surface, solid electrolytes hardly undergo reductive or oxidative decomposition.3 Moreover, active materials never dissolve into © 2017 American Chemical Society

Received: November 9, 2017 Accepted: December 1, 2017 Published: December 1, 2017 98

DOI: 10.1021/acsenergylett.7b01105 ACS Energy Lett. 2018, 3, 98−103

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Cite This: ACS Energy Lett. 2018, 3, 98−103

ACS Energy Letters

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Because the high interfacial resistance originates from the lithium depletion induced by the high electrode potential of the cathode, one way to reduce the interfacial resistance is shielding the sulfide electrolyte from the high potential of the cathode by interposing an electronically insulating layer in between. The interposed layer must have lithium-ionic conduction, because lithium ions have to go through it during the battery operation. Another requirement for the interposed layer material is that its framework attracts lithium ions strongly enough not to make the high cathode potential deplete the lithium ions in the interposed layer. A typical material meeting these requirements is an oxide-based solid electrolyte, because it is electronically insulating and ionically conducting and its oxygen array attracts lithium ions strongly. A thin layer of oxide-based solid electrolyte interposed at the interface has successfully reduced the interfacial resistance as buffer layers against lithium depletion to increase the power density to a practical level (Figure 1b).15 It should be noted that the buffer layer can increase not only the power density but also the energy density because it allows us to use high-voltage cathodes. Because solid electrolytes that do not contain mobile anionic species will be free from oxidative decomposition, high-voltage cathodes have been expected to be available in solid-state batteries; however, lithium depletion in sulfide electrolytes is more severe at the interface to high-voltage cathodes. The buffer layer suppresses even the severe lithium depletion and makes the high-voltage cathodes work in sulfide electrolytes.16,17 The lithium depletion can be recognized experimentally as an additional oxidation current appearing as a potential slope in the very beginning of the first charging.18 Because lithium ions will not be deintercalated from LiCoO2 until μLi+ at the interface becomes low enough to make ϕ reach the redox potential of Co4+/Co3+, removal of lithium ions from the electrolyte side of the interface appears prior to the potential plateau originating from lithium deintercalation from LiCoO2 in the charging curves, as shown in Figure 2. Of course, the potential slope disappears when a buffer layer is introduced to the interface. The thickness of the lithium-depleted layer can be estimated from the capacity in the potential slope that corresponds to the number of removed lithium ions, the density of lithium ions in the electrolyte, and the interface area. The estimated thickness is ca. 10 nm,18 which agrees with a typical value for space-charge layers formed in a super ionic conductor.19 It should be noted that the removal of lithium ions from the interface is indeed accompanied by extraction of electrons to keep charge neutrality. Because the valence band maximum of the sulfide electrolyte is located above that of the oxide cathode,20 the electrons are extracted from the electrolyte side of the interface at the beginning to generate holes in the electrolyte. However, the extraction from the electrolyte side will not be consecutive, because the solid electrolyte does not provide electrons or anions to the interface. Although the lithium depletion has not been observed directly yet because of the difficulties in detecting lithium, some techniques have been proposed to be effective in observing the distribution of local electrostatic potential around the interface.21,22 In addition, the lithium-depleted layer is visualized by computation.20,23 Because the lithium ions on the electrolyte side are depleted, when the cathode exhibits high electrode potential, a sulfide electrolyte, Li3PS4, is in contact with FePO4, which is a fully charged state of LiFePO4, in the computation. Structural relaxation by a molecular dynamics simulation based on density functional theory in order to reveal the stable

power densities, interfaces are playing critical roles in battery performance. Ionic conductors sometimes exhibit anomalous ionic conduction at surfaces or interfaces. Such anomalous ionic conduction, categorized into “nanoionics”,10 has been usually reported as enhanced ionic conduction. The first paper on nanoionics is on enhancement of ionic conduction at the LiI/ Al2O3 interface.11 It reports that addition of Al2O3 nanoparticles to LiI increases the conductivity of LiI, although Al2O3 is a typical insulator. This anomalous phenomenon is explained by a space-charge layer model: vacancies or interstitials are formed at the interface to Al2O3 to contribute to the ionic conduction. Similar enhancement in ionic conduction has been observed in many composites, as summarized in ref 12, which are positive aspects of interfaces toward ionic transport. On the other hand, a negative aspect can be seen in solid-state batteries with sulfide electrolytes, which has a high interfacial resistance in the cathodes.13 Solid electrolytes undergo anodic polarization at the interface to high-voltage cathodes. Because electrochemical potential of lithium ion, μ̃ Li+ = μLi+ + eϕ (where μ̃ Li+ and μLi+ are the electrochemical and chemical potential of lithium ion, respectively, and e and ϕ are the elementary charge and the local electrostatic potential, respectively), should be constant across the interface, the anodic polarization increases the electrostatic energy, eϕ, and thus decreases μLi+ on the electrolyte side.10,14 On the other hand, lithium ions are weakly bonded to the anionic framework in sulfide electrolytes, suggesting relatively high μLi+ in the bulk. Therefore, decreasing μLi+ at the cathode interface is significant, leading to the intense decrease in lithium-ion concentration or depletion of the lithium ions. As a result, the cathode interfaces in sulfide electrolytes are highly resistive owing to the deficiency of mobile lithium ions in spite of the high ionic conductivities in their bulk, as illustrated in Figure 1a. That is, weak interaction between the framework and lithium ions provides fast ionic conduction to sulfide electrolytes; however, it causes high interfacial resistance in cathodes.

Figure 1. Schematic drawings of Li1−xCoO2/sulfide solid electrolyte interfaces without (a) and with (b) a buffer layer of an oxide solid electrolyte. μLi+ and μ̃Li+ are the chemical and electrochemical potential of lithium ion, respectively; e and ϕ are the elementary charge and the local electrostatic potential, respectively. 99

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Figure 3. Temperature dependence of the interfacial resistance with (triangles) and without (circles) a buffer layer. Ri and T are interfacial resistance and absolute temperature, respectively, and Ea’s are the activation energies estimated from the Arrhenius plots. The complex impedance for thin-film electrodes prepared by pulsed laser deposition in this measurement gives only one semicircle in the Nyquist plots, which is attributed to the interfacial resistance.

It should be noted that the negative effects of nanoionics are drastic. Although various kinds of oxides reduce the interfacial resistance,26 most of them are not good ionic conductors. For example, the ion-conducting oxide used in the first paper reporting the interface structure with a buffer layer is Li4Ti5O12, and its ionic conductivity is only of the order of 10−8 S cm−1.27 This implies that the interface effects decrease the high bulk conductivity of 10−3 S cm−1 of the sulfide solid electrolyte so dramatically that even such a poor ionic conductor improves the ion transport at the interface. The huge interfacial resistance appearing between high-voltage cathodes and sulfide solid electrolytes is a negative aspect of nanoionics. The buffer layer covers this negative aspect to reduce the highest resistance in solid-state batteries and make the power density of solid-state batteries exceed the conventional liquid electrolyte systems in combination with high ionic conductivity of the solid electrolyte in ref 8. On the other hand, one can see a positive aspect of interface in that paper.

Figure 2. First charge and following discharge curves of LiCoO2 in a sulfide solid electrolyte, Li3.25Ge0.25P0.75S4. The beginning of the charge curve in the upper panel is enlarged in the lower panel, and the insets in the lower panel are schematic drawings of the interface in the potential slope and plateau. The cathode is composed of 8.9 mg of LiCoO2 and 3.8 mg of Li3.25Ge0.25P0.75S4 and is cycled at a current density of 127 μA cm−2 at 25 °C.

interface structure transfers a considerable amount of lithium ions from the electrolyte to electrode and thus forms a lithiumdepleted layer on the electrolyte side of the Li1−xFePO4/Li3PS4 interface. Concurrently, electrons move in the same direction to generate holes on the electrolyte side. Of course, the FePO4 does not deplete the lithium ions when the solid electrolyte is an oxide.23 These results have revealed the formation of lithium-depleted layer at the interface; however, the kinetics has not been fully understood. For example, it is not clear whether the high interfacial resistance can be explained only by the lithium depletion or if mobility of lithium ions also decreases in the space-charge layer. Effects of a buffer layer on the temperature dependence of interfacial resistance shown in Figure 3 indicate that Li3PO4 interposed to the Li1−xCoO2/Li3.25Ge0.25P0.75S424 interface as the buffer layer reduces the electrode resistance by 2 orders of magnitude but does not change the activation energy,25 which suggests that the buffer layer increases the number of available lithium sites for the electrode reaction without changing the ion transport mechanism at the interface. Therefore, it can be concluded that the lithium depletion is the predominant factor in the high interfacial resistance observed in sulfide solid electrolytes. In addition, the activation energy being higher than that for bulk ionic conduction of the sulfide electrolyte (0.2 eV)24 suggests that the interface is still ratedetermining even after interposing the buffer layer.

The interface effects decrease the high bulk conductivity of 10−3 S cm−1 of the sulfide solid electrolyte so dramatically that even such a poor ionic conductor improves the ion transport at the interface. The paper presents three kinds of solid-state batteries: highvoltage, large current, and normal types. The cathode material used in these batteries is LiCoO2 coated with LiNbO328 in common. On the other hand, the high-voltage type employs graphite as the anode, while the other two use Li4Ti5O12. It is not surprising that buffer layers are not formed on the anode surface even in the large current type, because the abovementioned mechanism suggests that high electrode potential causes lithium depletion to increase the interfacial resistance, while low electrode potential of the anode will accumulate lithium ions,29 which may even promote ionic conduction at 100

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higher than that to the Li4Ti5O12, indicating that the lithium insertion preferentially takes place at the interface. That is, lithium insertion−extraction and the accompanying electronic conduction in the electrode reaction are promoted at the Li4Ti5O12/Li7Ti5O12 interface, which is an interface formed between two bad conductors. “Solid-state ionics” is the discipline concerned with mobile ions in solids,34,35 in which achieving high ionic conductivities has been one of the targets, and it has been expanded to cover ionic conduction at the interface in nanoionics. Although nanoionics might be regarded as an exotic way to explore new ionic conductors by taking advantage of enhanced ionic conduction at interfaces,36 interfaces are of growing importance in developing solid-state batteries because ion transport in the bulk of recently developed highly conductive solid electrolytes has ceased being rate-determining in solid-state batteries. It has been expected that solid-state batteries generate higher current drain than liquid systems, because single ion conduction in solid electrolytes minimizes the concentration gradient. Moreover, the desolvation process, which requires high activation energy in the intercalation reaction in liquid electrolytes,37 is absent. In fact, it was recently reported that LiCoO2 exhibits lower interfacial resistance to an oxide solid electrolyte than liquids.38 In spite of the high compatibility with cathodes, oxide solid electrolytes with high ionic conductivity show high grain boundary resistance,39 which is a significant hurdle to overcome. Understanding and controlling the interfacial ionic transport will pave the way to solid-state batteries superior to conventional liquid-electrolyte systems.

the interface. In fact, anodes always exhibit high rate capability in sulfide electrolytes.13,30,31 However, Li4Ti5O12 employed in the large current type is still surprising. The reason for the use of Li4Ti5O12 will be, of course, instability of the solid electrolyte against electrochemical reduction. It is quite natural to select Li9.54Si1.74P1.44S11.7Cl0.3 as the solid electrolyte in the large current type, because it exhibits the highest conductivity of 25 mS cm−1 among lithium-ion conductive solid electrolytes. In spite of the high ionic conductivity, it is not compatible with lithium metal or graphite anodes because of the instability;8 thus, Li4Ti5O12 is used as the anode instead. However, it seems strange that Li4Ti5O12 is available in the large current type battery in spite of its low ionic conductivity.27 A possible explanation is that electrode reactions in intercalation electrodes often give rise to nonstoichiometry by lithium intercalation or deintercalation, which may enhance ionic conduction due to the generation of vacancies or interstitials. However, this is not the case with Li4Ti5O12, because its electrode reactions proceed in a two-phase reaction between Li4Ti5O12 and Li7Ti5O12.32 In addition, both of them are insulating in terms of electronic conduction.33 That is, they are poor in the ionic and electronic transport in their bulk; therefore, their interface between Li4Ti5O12 and Li7Ti5O12 is considered to play an important role in generating the high power; otherwise, Li4Ti5O12 will not be available for the large current type.

Lithium insertion−extraction and the accompanying electronic conduction in the electrode reaction are promoted at the Li4Ti5O12/Li7Ti5O12 interface, which is an interface formed between two bad conductors.

Understanding and controlling the interfacial ionic transport will pave the way to solid-state batteries superior to conventional liquid-electrolyte systems.

A recent first-principles calculation33 has revealed that electronic conduction is induced at Li4Ti5O12/Li7Ti5O12 interface in spite of their insulating nature. The band structure calculated for the interface shows conduction electrons just below the Fermi level; moreover, they are located along the Li4Ti5O12/Li7Ti5O12 boundary, as illustrated in Figure 4. In addition, lithium insertion−extraction reactions are promoted at the interface. Formation energy for lithium vacancy, which corresponds to the energy needed for lithium extraction, at the interface is lower than that in the Li7Ti5O12 bulk, suggesting that lithium ions are removed from the interface more easily. In addition, energy gain for lithium insertion to the interface is

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kazunori Takada: 0000-0001-7568-1806 Notes

The authors declare no competing financial interest. Biographies Kazunori Takada is a Deputy Director-General of the Center for Green Research on Energy and Environmental Materials at National Institute for Materials Science (NIMS). His research interests are on ion-conducting ceramics, especially for solid-state batteries. Takahisa Ohno is the leader of the Computational Materials Science Group of Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN) at NIMS. His work focuses on computational simulations of battery materials, especially for solidstate Li-ion batteries, to elucidate the electronic and dynamic properties of bulk materials and their interfaces. Narumi Ohta is a Senior Researcher in the Rechargeable Battery Materials Group at NIMS. His research focuses on the design, synthesis, and evaluation of electrode/solid-electrolyte interfaces, to enhance the performance, such as the high-rate capabilities, capacities, and durability, of solid-state lithium batteries.

Figure 4. Density distribution of conduction electrons around a Li4Ti5O12/Li7Ti5O12 interface. Isosurfaces indicate the density distribution of conduction electrons located below and within 0.4 eV from the Fermi level, and the conduction electron density on the isosurfaces is 0.07 Å−3. 101

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(15) Ohta, N.; Takada, K.; Zhang, L. Q.; Ma, R. Z.; Osada, M.; Sasaki, T. Enhancement of the High-Rate Capability of Solid-State Lithium Batteries by Nanoscale Interfacial Modification. Adv. Mater. 2006, 18, 2226−2229. (16) Oh, G.; Hirayama, M.; Kwon, O.; Suzuki, K.; Kanno, R. BulkType All Solid-State Batteries with 5 V Class LiNi0.5Mn1.5O4 Cathode and Li10GeP2S12 Solid Electrolyte. Chem. Mater. 2016, 28, 2634−2640. (17) Yubuchi, S.; Ito, Y.; Matsuyama, T.; Hayashi, A.; Tatsumisago, M. 5 V Class LiNi0.5Mn1.5O4 Positive Electrode Coated with Li3PO4 thin film for All-Solid-State Batteries Using Sulfide Solid Electrolyte. Solid State Ionics 2016, 285, 79−82. (18) Takada, K.; Ohta, N.; Zhang, L. Q.; Xu, X. X.; Hang, B. T.; Ohnishi, T.; Osada, M.; Sasaki, T. Interfacial Phenomena in Solid-State Lithium Battery with Sulfide Solid Electrolyte. Solid State Ionics 2012, 225, 594−597. (19) Guo, X.; Maier, J. Comprehensive Modeling of Ion Conduction of Nanosized CaF2/BaF2 Multilayer Heterostructures. Adv. Funct. Mater. 2009, 19, 96−101. (20) Takada, K.; Ohno, T. Experimental and Computational Approaches to Interfacial Resistance in Solid-State Batteries. Front. Energy Res. 2016, 4, 10. (21) Yamamoto, K.; Iriyama, Y.; Asaka, T.; Hirayama, T.; Fujita, H.; Fisher, C. A. J.; Nonaka, K.; Sugita, Y.; Ogumi, Z. Dynamic Visualization of the Electric Potential in an All-Solid-State Rechargeable Lithium Battery. Angew. Chem., Int. Ed. 2010, 49, 4414−4417. (22) Masuda, H.; Ishida, N.; Ogata, Y.; Ito, D.; Fujita, D. Internal Potential Mapping of Charged Solid-State-Lithium Ion Batteries Using In Situ Kelvin Probe Force Microscopy. Nanoscale 2017, 9, 893−898. (23) Sumita, M.; Tanaka, Y.; Ikeda, M.; Ohno, T. Charged and Discharged States of Cathode/Sulfide Electrolyte Interfaces in AllSolid-State Lithium Ion Batteries. J. Phys. Chem. C 2016, 120, 13332− 13339. (24) Kanno, R.; Murayama, M. Lithium Ionic Conductor ThioLISICON. The Li2S-GeS2-P2S5 System. J. Electrochem. Soc. 2001, 148, A742−A746. (25) Xu, X. X.; Takada, K.; Fukuda, K.; Ohnishi, T.; Akatsuka, K.; Osada, M.; Hang, B. T.; Kumagai, K.; Sekiguchi, T.; Sasaki, T. Tantalum Oxide Nanomesh as Self-Standing One Nanometre Thick Electrolyte. Energy Environ. Sci. 2011, 4, 3509−3512. (26) Takada, K. Progress and Prospective of Solid-State Lithium Batteries. Acta Mater. 2013, 61, 759−770. (27) Wilkening, M.; Amade, R.; Iwaniak, W.; Heitjans, P. Ultraslow Li Diffusion in Spinel-Type Structured Li4Ti5O12A Comparison of Results from Solid State NMR and Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2007, 9, 1239−1246. (28) Ohta, N.; Takada, K.; Sakaguchi, I.; Zhang, L. Q.; Ma, R. Z.; Fukuda, K.; Osada, M.; Sasaki, T. LiNbO3-Coated LiCoO2 as Cathode Material for All Solid-State Lithium Secondary Batteries. Electrochem. Commun. 2007, 9, 1486−1490. (29) Landstorfer, M.; Funken, S.; Jacob, T. An Advanced Model Framework for Solid Electrolyte Intercalation Batteries. Phys. Chem. Chem. Phys. 2011, 13, 12817−12825. (30) Cervera, R. B.; Suzuki, N.; Ohnishi, T.; Osada, M.; Mitsuishi, K.; Kambara, T.; Takada, K. High Performance Silicon-Based Anodes in Solid-State Lithium Batteries. Energy Environ. Sci. 2014, 7, 662−666. (31) Miyazaki, R.; Ohta, N.; Ohnishi, T.; Sakaguchi, I.; Takada, K. An Amorphous Si Film Anode for All-Solid-State Lithium Batteries. J. Power Sources 2014, 272, 541−545. (32) Ohzuku, T.; Ueda, A.; Yamamoto, N. Zero-Strain Insertion Material of Li[Li1/3Ti5/3]O4 for Rechargeable Lithium Cells. J. Electrochem. Soc. 1995, 142, 1431−1435. (33) Tanaka, T.; Ikeda, M.; Sumita, M.; Ohno, T.; Takada, K. Firstprinciple Analysis on Role of Spinel (111) Phase Boundaries in Li4+3xTi5O12 Li-ion Battery Anodes. Phys. Chem. Chem. Phys. 2016, 18, 23383−23388. (34) Funke, K. Solid State Ionics; from Michael Faraday to Green Energy the European Dimension. Sci. Technol. Adv. Mater. 2013, 14, 043502.

Tsuyoshi Ohnishi is a Principal Researcher in the Center for Green Research on Energy and Environmental Materials (GREEN) at NIMS. His work focuses on thin-film synthesis of functional materials including that for Li-ion batteries. Yoshinori Tanaka is a NIMS Special Researcher in the All-Solid-State Battery Specially Promoted Research Team at GREEN. His work focuses on functional materials such as energy storage materials, which are predicted to contribute to realizing a sustainable society, on the basis of computational materials science.

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ACKNOWLEDGMENTS This study was partly supported by the MEXT Program for the “Development of Environmental Technology using Nanotechnology” from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The figures were drawn with the aid of computer program VESTA.40

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DOI: 10.1021/acsenergylett.7b01105 ACS Energy Lett. 2018, 3, 98−103