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[Kosuke Kawai](https://orcid.org/0000-0003-3840-2198), Seong‐Hoon Jang, Yuta Igarashi, Koji Yazawa, Kazuma Gotoh, [Jun Kikkawa](https://orcid.org/0000-0003-0659-1844), Atsuo Yamada, [Yoshitaka Tateyama](https://orcid.org/0000-0002-5532-6134), [Masashi Okubo](https://orcid.org/0000-0002-7741-5234)

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This is the peer reviewed version of the following article: K. Kawai, S.-H. Jang, Y. Igarashi, K. Yazawa, K. Gotoh, J. Kikkawa, A. Yamada, Y. Tateyama, M. Okubo, Angew. Chem. Int. Ed. 2025, 64, e202410971, which has been published in final form at https://doi.org/10.1002/anie.202410971. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Proton Intercalation into an Open‐Tunnel Bronze Phase with Near‐Zero Volume Change](https://mdr.nims.go.jp/datasets/fca436c4-f561-49dc-b382-7c8324c6693f)

## Fulltext

Proton Intercalation into an Open‐Tunnel Bronze Phase with Near‐Zero Volume ChangeAqueous Proton BatteriesProton Intercalation into an Open-Tunnel Bronze Phase with Near-Zero Volume ChangeKosuke Kawai, Seong-Hoon Jang, Yuta Igarashi, Koji Yazawa, Kazuma Gotoh,Jun Kikkawa, Atsuo Yamada, Yoshitaka Tateyama, and Masashi Okubo*Abstract: Managing safety and supply-chain risks asso-ciated with lithium-ion batteries (LIBs) is an urgent taskfor sustainable development. Aqueous proton batteriesare attractive alternatives to LIBs because using waterand protons addresses these two risks. However, mosthost materials undergo large volume changes upon H+intercalation, which induces intraparticle cracking toaccelerates parasitic reactions. Herein, we report thatMo3Nb2O14 bronze exhibits reversible H+ intercalation(200 mAhg� 1) with a Coulombic efficiency of 99.7%owing to near-zero volume change and solid-solution-type phase transition. Combination of experimental andtheoretical analyses clarifies that rotation and shrinkageof open tunnels, which consist of flexible corner-sharingMo/NbOn polyhedra, relieve local structural distortionsupon H+ intercalation to suppress intraparticle cracking.The prototype full cell of an aqueous proton batterywith a Mo3Nb2O14 anode operates stably over 1000charge/discharge cycles. This study reveals the impor-tance of implementing distortion-relieving voids in hostmaterials to reduce volume change upon charge/dis-charge.IntroductionElectrochemical energy storage (EES) is vital for building asustainable society because supplying intermittent powerfrom renewables to an electrical grid must be load-leveledusing EES systems. Currently, lithium-ion batteries (LIBs)dominate the EES market owing to their high energydensities, high energy efficiencies, and long cycle lives.[1]However, flammable organic electrolytes pose intrinsic risksof fire and explosion accidents. Therefore, replacing flam-mable organic electrolytes with non-flammable aqueouselectrolytes is highly desirable. Furthermore, the globalmaldistribution of lithium resources provides severe supply-chain risks that impede the mass production of large EESsystems. Aqueous proton batteries are regarded as promis-ing alternatives to LIBs for managing safety and supply-chain risks. Proton is the cation with the smallest ionicradius,[2] which enables host materials to be densely proto-nated with minimal lattice distortion during charging.Stable operation of an aqueous proton battery requiressuppressing parasitic side reactions (e.g., the hydrogen-evolution reactions (HER), oxygen-evolution reactions(OER), and electrode-material dissolution), which areinduced mainly by high water-molecule activity. Aqueouselectrolytes with less water-molecule activities have beendeveloped extensively. “Water-in-salt” electrolytes reducewater-molecule activity by forming coordination bondsbetween water molecules and excess cations.[3] Watermolecules in “water-in-sugar” electrolytes form hydrogen[*] K. Kawai, Y. Igarashi, M. OkuboDepartment of Electrical Engineering and Bioscience, School ofAdvanced Science and Engineering, Waseda University, 3-4-1Okubo, Shinjuku-ku, Tokyo 169-8555, JapanE-mail: m-okubo@waseda.jpS.-H. Jang, Y. TateyamaResearch Center for Energy and Environmental Materials (GREEN),National Institute for Materials Science (NIMS), 1-1 Namiki,Tsukuba, Ibaraki 305-0044, JapanS.-H. JangInstitute for Materials Research, Tohoku University, 2-1-1 Katahira,Aoba-ku, Sendai, Miyagi 980-8577, JapanK. YazawaJEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, JapanK. GotohCenter for Nano Materials and Technology (CNMT), JapanAdvanced Institute of Science and Technology (JAIST), 1-1 Asahidai,Nomi, Ishikawa, 923-1292, JapanJ. KikkawaCenter for Basic Research on Materials (CBRM), National Institutefor Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanA. YamadaDepartment of Chemical System Engineering, School of Engineer-ing, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JapanA. YamadaSungkyunkwan University Institute of Energy Science & Technology(SIEST), Sungkyunkwan University, Suwon 16419, KoreaY. TateyamaTokyo Institute of Technology (Tokyo Tech), 4259 Nagatsuta-cho,Midori-ku, Yokohama, 226-8501, JapanM. OkuboKagami Memorial Research Institute for Material Science andTechnology, Waseda University, Tokyo 169-0051, JapanAngewandteChemieForschungsartikelwww.angewandte.orgZitierweise: Angew. Chem. Int. Ed. 2024, e202410971doi.org/10.1002/anie.202410971Angew. Chem. 2024, e202410971 (1 of 8) © 2024 Wiley-VCH GmbHhttp://orcid.org/0000-0003-3840-2198http://orcid.org/0000-0002-7741-5234https://doi.org/10.1002/anie.202410971http://crossmark.crossref.org/dialog/?doi=10.1002%2Fange.202410971&domain=pdf&date_stamp=2024-10-26bonds with sugar additives, which significantly decreaseswater-molecule activity.[4] Another option involves the“molecular crowding” effect: molecular crowding agentssuch as polyethylene glycol (PEG) form hydrogen bondswith water molecules to reduce water-molecule activity.[5] Asa consequence, parasitic reactions are significantly sup-pressed at the electrode-electrolyte interface.Active materials should be highly durable againstmechanical stress and chemical changes upon charge/discharge to further suppress electrode performance degra-dation. In particular, volume expansion/shrinkage can crackactive materials, with the newly exposed surfaces accelerat-ing interfacial parasitic reactions. For example, layered α-MoO3 exhibits monoclinic distortion and undergoes avolume change (ΔV/V=5.5%) upon H+ intercalation,resulting in severe intraparticle cracking.[6] Other hostsmaterials (V2O3 and h-WO3 ·0.6H2O) also suffer fromvolume changes upon H+ intercalation, resulting in perform-ance degradation.[7] The microscopic origins of these struc-tural transformation involve either expansion/contraction ofthe interlayer distance between loosely stacked layers or thelocal distortion of MOn polyhedra upon H+ intercalation.H+ intercalation with near-zero volume change, which isrequired for stable operation of an aqueous proton battery,has rarely been reported.In this study, we report Mo3Nb2O14 as a H+-intercalationhost structure for aqueous rechargeable batteries (Fig-ure 1a). Mo3Nb2O14 is categorized as a bronze phase with ageneral MO3� x (M=Mo, Nb, W) formula. Edge- and corner-sharing Mo/NbOn (n=6 and 7) polyhedra form the flexibleframework with open tunnels that undergo rotation andshrinkage to relieve local structural distortion upon H+intercalation. Consequently, Mo3Nb2O14 exhibits a Coulom-bic efficiency of 99.7% without intraparticle cracking owingto near-zero volume change as well as solid-solution-typephase transition upon H+ intercalation.Results and DiscussionMo3Nb2O14 was synthesized using a conventional solid-statesynthesis method. The synchrotron powder X-ray diffraction(XRD) pattern of the synthesized sample is refined with P4/mbm space group. The refined lattice constants, namely a=23.161(5) Å and c=3.9984(8) Å, are in good agreement withthose reported previously (Figure S1).[8] Scanning electronmicroscopy shows an irregular block-like morphology owingto preferential growth along the c-axis, which is typical ofopen-tunnel bronze phases (Figure S2). Elemental mapsacquired by energy-dispersive X-ray spectroscopy revealFigure 1. Proton intercalation into Mo3Nb2O14 in molecular crowding electrolytes. (a) LSV curves of electrolytes on Ti foil (black) and CV curves ofMo3Nb2O14 (blue) acquired at 1 mVs� 1. The inset shows the crystal structure of Mo3Nb2O14. (b) Contact angles as functions of time after droppingelectrolytes onto Mo3Nb2O14 electrodes. The insets show optical images of electrolyte droplets on Mo3Nb2O14 electrodes; they were recorded 10 safter dropping each electrolyte. (c) Galvanostatic charge/discharge curves at a specific current of 58 mAg� 1 (0.3 C) with a cutoff potential between� 0.5 and 1.1 V vs. Ag/AgCl. (d) 1H MAS NMR spectra of HxMo3Nb2O14 before and after charging to � 0.5 V vs. Ag/AgCl.AngewandteChemieForschungsartikelAngew. Chem. 2024, e202410971 (2 of 8) © 2024 Wiley-VCH GmbH 15213757, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202410971 by Waseda University, Wiley Online Library on [14/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licenseuniformly distributed Mo and Nb with a Mo/Nb atomic ratioof 1.5 (Figure S3). These results confirm the successfulsynthesis of Mo3Nb2O14 bronze.Sulfuric-acid-based electrolytes were used for H+ inter-calation into Mo3Nb2O14 electrodes. Sulfuric acid was dilutedto 4.2 molL� 1 in a “molecular crowding” aqueous solution of(100� x)H2O� xPEG (x=0, 50, and 90), after which 2.5 or5.0 wt% 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (HFE) was added to improve electrolyte wettabilityon the Mo3Nb2O14 electrode. Flammability testing showsthat the resulting electrolytes do not ignite, confirming theirhigh safety (Figure S4). Linear sweep voltammetry (LSV) at1 mVs� 1 using a Ti current collector shows that 4.2 molL� 1H2SO4/100H2O (x=0) exhibits cathodic current flow at� 0.2 V vs. Ag/AgCl due to the HER (Figure 1a). Althoughslight reduction current, which may originate from decom-position of specifically adsorbed water molecules on a Ticurrent collector, is observed at � 0.2 V (vs Ag/AgCl) foreach electrolyte, increasing the PEG content reduces HER-current flow owing to the molecular crowding effect (i.e., areduction in water activity by the formation of hydrogenbonds between H2O and PEG). It should also be noted thatanodic stability is improved by increasing the PEG contentbecause anodic corrosion of the current collector and theOER are suppressed by reduced activities of H2O/H3O+ inmolecular crowding electrolytes (Figure S5).[9] Conse-quently, an electrolyte comprising 4.2 molL� 1 H2SO4/10H2O� 90PEG (x=90) successfully provides a stable cyclicvoltammogram for the Mo3Nb2O14 electrode over a widepotential range (� 0.45 to 0.4 V vs. Ag/AgCl). Dynamiccontact-angle measurement (Figure 1b) shows a contactangle of 118° for 4.2 molL� 1 H2SO4/100H2O (x=0), whichimplies highly hydrophobic electrode-electrolyte interface.This hydrophobicity is improved by adding PEG (lowercontact angles of 24° and 12° are recorded at x=50 and 90,respectively). Furthermore, adding HFE improves the wett-ability of the PEG-based electrolyte on the Mo3Nb2O14electrode; hence static contact angles are unable to bemeasured owing to the complete permeation of electrolytedroplets into the pores of the electrode. For 4.2 molL� 1H2SO4/10H2O� 90PEG+y wt% HFE, rates of dropletexpansion during the first ten seconds are � 0.55, � 0.77, and� 0.92 °/s for y=0, 2.5, and 5.0, respectively, indicatingimproved wettability of the electrolytes by adding HFEdiluent. Presumably, PEG and HFE are preferentiallyadsorb on the electrode surface to reduce activities of H2O/H3O+ at the solid-electrolyte interface, leading to thesuppression of parasitic reactions such as the HER andOER.[5b]Galvanostatic charge/discharge of the Mo3Nb2O14 elec-trode with the 4.2 molL� 1 H2SO4/10H2O� 90PEG (x=90)electrolyte (Figure 1c) affords a specific capacity of158 mAhg� 1 at a specific current of 58 mAg� 1 (0.3 C) duringthe second discharge. Here, “charge” and “discharge” referto the cathodic (H+ intercalation) and anodic (H+ dein-tercalation) processes, respectively. Meanwhile, an HFE-added electrolyte (4.2 molL� 1 H2SO4/10H2O� 90PEG (x=90)+5 wt% HFE) largely reduces polarization duringcharge/discharge to deliver a larger specific capacity of185 mAhg� 1, which is comparable to that observed for Li+(de)intercalation using a non-aqueous electrolyte (Fig-ure S6).Ex situ analyses were used to clarify the origin of thestable H+ (de)intercalation in Mo3Nb2O14. The1H solid-statemagic-angle-spinning nuclear magnetic resonance(MAS NMR) demonstrates that a sharp peak emerges at achemical shift of � 2.5 ppm, which originates from bare H+in oxide compounds, confirming H+ intercalation intoMo3Nb2O14 after charge (Figure 1d). Note that H3O+exhibits a chemical shift of 12 ppm in a solid.[10] The Mo K-edge X-ray absorption near edge structure (XANES)spectrum of pristine Mo3Nb2O14 is almost identical to that ofMoO3; hence, the oxidation state of Mo in pristineMo3Nb2O14 is +6 (Figure S7). The Mo K-edge XANESspectrum shifts reversibly upon charge/discharge, indicatingreversible Mo6+/Mo5+/Mo4+ redox reactions. The Nb K-edge XANES spectrum of pristine Mo3Nb2O14 is identical tothat of Nb2O5 (Nb5+); it shifts slightly and reversibly uponcharge/discharge, albeit with a much higher absorptionenergy than that observed for NbO2 (Nb4+). Although X-rayphotoelectron spectroscopy reveals the presence of Nb4+ atthe surface (Figure S8), the main charge-compensationmechanism for H+ intercalation in Mo3Nb2O14 involves theMo6+/Mo5+/Mo4+ redox reactions, with the minor Nb5+/Nb4+ contribution.Figure 2a displays charge/discharge curves at a specificcurrent of 58 mAg� 1 (0.3 C) using the optimal electrolyte(4.2 molL� 1 H2SO4/10H2O� 90PEG (x=90)+5 wt% HFE).The specific capacity increases slightly with repeated charge/discharge cycling in the early stage, presumably due to theformation of a superior electrode-electrolyte interface, andfinally reaches 200 mAhg� 1 with a Coulombic efficiency of99.7% at the 40th cycle. Irreversible capacity is recoveredduring the first cycle when a constant-current constant-voltage mode is applied (Figure S9), which implies sluggishH+ diffusion or the electronically insulating nature ofMo3Nb2O14 at the end of the discharge process. Indeed, thegalvanostatic intermittent titration technique reveals thatpolarization of H0.5Mo3Nb2O14 is ten-fold greater than thatof H4.5Mo3Nb2O14 (Figure S10). Transmission electron mi-croscopy images show a coherent lattice fringe and nointraparticle cracking over the entire surface of a Mo3Nb2O14particle, indicative of a highly stable electrode-electrolyteinterface (Figure S11). At a specific capacity of 192 mAg� 1(1 C), the Mo3Nb2O14 electrode delivers an average Coulom-bic efficiency of 99.99% over 500 cycles (Figure 2b). Notethat other H+-intercalation host materials, including α-MoO3, show lower Coulombic efficiency (~99%) in aqueouselectrolytes (Table S1).The specific capacity of the Mo3Nb2O14 electrodedecreases from 185 to 21 mAhg� 1 as a C-rate increases from0.3 to 30 C rate, with a cutoff potential between � 0.5 and1.1 V vs. Ag/AgCl in 4.2 molL� 1 H2SO4/10H2O� 90PEG (x=90)+5 wt% HFE (Figure 2c). Note that the 4.2 molL� 1H2SO4/50H2O� 50PEG (x=50) electrolyte, which possesseshigher ionic conductivity (126 mScm� 1) than its10H2O� 90PEG (x=90)+5 wt% HFE counterpart(2.2 mScm� 1), provides a superior rate capability, with aAngewandteChemieForschungsartikelAngew. Chem. 2024, e202410971 (3 of 8) © 2024 Wiley-VCH GmbH 15213757, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202410971 by Waseda University, Wiley Online Library on [14/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensecutoff potential between � 0.4 and 1.1 V vs. Ag/AgCl (Fig-ure S12). Hence, H+ transport in the electrolyte is the rate-determining step rather than H+ diffusion in Mo3Nb2O14.Synchrotron powder XRD patterns of the Mo3Nb2O14electrodes were recorded during the first cycle (Figure 3a).All the Bragg peaks are indexed using the single-phasemodel with the P4/mbm space group for each state-of-charge (Figure S13). The hk0 peaks shifts to lower 2θ whilethe 00 l peaks shift to higher 2θ upon H+ intercalation,which indicates the solid-solution-type phase transition withexpansion of the ab-plane and shrinkage of the c-axis uponH+ intercalation. All the Bragg peaks return to their original2θ positions upon H+ deintercalation, confirming thereversible structural transformation. Figure 3b shows latticeparameters as a function of x in HxMo3Nb2O14. Five-H+intercalation per formula unit leads to small changes in thevalues of the a and c lattice parameters (by +1.4% and� 2.3%, respectively). Importantly, an accumulated changein the unit-cell volume (ΔV/V) is only +0.4% even after thefull charge. The near-zero volume change as well as thesolid-solution-type phase transition minimize lattice strain inthe Mo3Nb2O14. Indeed, no apparent cracks are observed inthe Mo3Nb2O14 particles after H+ intercalation (Figure S11).It should be emphasized that Li+ intercalation in Mo3Nb2O14results in a large volume change (~8%),[11] and the cathodicpeak at 2.5 V vs. Li/Li+ in its dQ/dV plot fades rapidly withcharge/discharge cycling (Figure S14). In contrast, all dQ/dVpeaks for H+ (de)intercalation are retained even after 40cycles. The near-zero volume change of Mo3Nb2O14 uponH+ intercalation is most likely attained by (i) the smallerionic radius of H+ (8.3×10� 6 Å) compared to that of Li+(0.76 Å), (ii) the dense oxide-ion arrays of the bronze phase,which is favorable for H+ conduction, and (iii) open tunnelsthat relieve any local distortion induced by H+ intercalation.Figure 2. Charge/discharge performance of Mo3Nb2O14 electrodes in4.2 molL� 1 H2SO4/10H2O� 90PEG+5 wt% HFE. (a) Galvanostaticcharge/discharge curves at a specific current of 58 mAg� 1 (0.3 C). (b)Cycle performance at a specific current of 192 mAg� 1 (1 C). The cutoffvoltage is between � 0.5 and 1.1 V vs. Ag/AgCl. A low current density of58 mAg� 1 (0.3 C) was applied for the first cycle to activate electrodeperformance. (c) Rate performance of Mo3Nb2O14 electrodes in4.2 molL� 1 H2SO4/10H2O� 90PEG+5 wt% HFE with a cutoff voltagebetween � 0.5 and 1.1 V vs. Ag/AgCl, and 4.2 molL� 1 H2SO4/50H2O� 50PEG with a cutoff voltage between � 0.4 and 1.1 V vs. Ag/AgCl.Figure 3. Strain-free H+ intercalation of Mo3Nb2O14. (a) Ex situ powderXRD patterns of HxMo3Nb2O14. (b) Lattice-parameter and lattice-volume transitions for AxMo3Nb2O14 (A=H, red and blue symbols;A=Li, gray symbols). The data for LixMo3Nb2O14 are taken from Luoet al.[11]AngewandteChemieForschungsartikelAngew. Chem. 2024, e202410971 (4 of 8) © 2024 Wiley-VCH GmbH 15213757, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202410971 by Waseda University, Wiley Online Library on [14/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseThe local structural changes in Mo3Nb2O14 upon H+intercalation were analyzed using Rietveld refinement forthe ex situ XRD patterns (Figure S15). After H+ intercala-tion, the open tunnel in Mo3Nb2O14 shrinks by 4.4%(Figure 4a) and rotates 3.9° around the c-axis (Figure 4b)owing to the flexible corner-sharing Mo/NbOn polyhedra.These two local structural changes relieve expansion of theab-plane upon H+ intercalation. Meanwhile, transition-metal sites, which originally split along the c-axis owing tothe second Jahn-Teller distortion of Mo/Nb,[12] merge at thecenter of c-axis (z=0.5) upon H+ intercalation (Figure S16).Consequently, the shrinkage of c-axis compensates for theexpansion of a-axis, resulting in the near-zero volumechange upon H+ intercalation in Mo3Nb2O14. This micro-scopic insight suggests that a flexible substructure consistingof corner-sharing polyhedra and distortion-relieving voids isbeneficial to reduce volume change upon ion intercalation.For example, Wadsley-Roth and bronze phases with opentunnels (e.g., Mo3Nb2O14) may exhibit small volume changeupon H+ intercalation while layered structures, such as α-MoO3 and V2O5, undergo significant changes in the inter-layer distances between their loosely stacked layers and alarge volume change as a consequence.[6] On the other hand,perovskite family, which consist solely of corner-sharingoctahedra, are not favorable as H+-intercalation hosts owingto their low proton conductivities at room temperature;[13]the distance between oxide ions is too long for a proton toundergo anhydrous Grotthuss diffusion (consecutive H+rotation and hopping).[14] Note that near d0 electron config-urations of Mo and Nb contribute to the strain relief uponH+ intercalation because their weakly directional M� Obonds allow for bending and twisting of O� M� O bridges,and site displacements of Mo/Nb[15] as indicated by structuralanalysis (Figure S16). In this context, other early transitionmetals (e.g., Ti4+, V5+, and W6+) are likely to function asredox species with strain-relief effects when used in open-tunnel frameworks. Meanwhile, later transition metals (e.g.,Fe3+, Co3+, and Ni3+) may not be able to tolerate localFigure 4. Mechanism for strain-free H+ intercalation of Mo3Nb2O14. Local structural changes calculated using the Rietveld-refined data forHxMo3Nb2O14. (a) M� M distance and (b) rotational angle of the square open tunnel at the center of the Mo3Nb2O14 lattice viewed along the c-axis.HAADF- and ABF-STEM images of HxMo3Nb2O14 in its (c) pristine (x=0), (d) charged (x=5), and (e) discharged (x=0.5) states. The crystalstructures shown in the STEM images were obtained by Rietveld refinement. Yellow-dotted lines highlight the rotation angles of the squares at thecenters of the unit cells.AngewandteChemieForschungsartikelAngew. Chem. 2024, e202410971 (5 of 8) © 2024 Wiley-VCH GmbH 15213757, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202410971 by Waseda University, Wiley Online Library on [14/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensestructural distortion upon H+ intercalation because of theirdirectional M� O bonds.[16]Scanning transmission electron microscopy (STEM) is apowerful technique for visualizing the atomistic origin of thestructural change (Figure S17). High-angle annular darkfield (HAADF) images, where the image contrast isapproximately proportional to the square of the atomicnumber (/Z2), clearly show atomic columns of molybdenumand niobium, while annular bright-field (ABF) images (/Z1/3) visualize atomic columns composed of light atoms, i.e.,oxygen, in addition to those of molybdenum and niobium.The HAADF and ABF-STEM images of Mo3Nb2O14 alongthe c-axis clearly show triangular, square, pentagonal, andhexagonal open tunnels (Figure 4c). The square open tunnelclearly rotates without any appreciable changes in atomicarrangement and lattice constants after H+ intercalation(Figure 4d). Although the electron diffraction image of thecharged sample shows forbidden reflections arising frombroken symmetry (Figure S18), the volume change of Mo/NbOn (n=6 and 7) polyhedra in association with H+intercalation is cooperatively relieved by an open-tunnelbuffer, reducing expansion of the ab-plane. The atomicarrangement completely recovers to that of the pristine stateafter H+ deintercalation (Figure 4e).Density functional theory (DFT) calculations were usedto simulate the H+ intercalation in Mo3Nb2O14. Modelstructures of HxMo3Nb2O14 (0 �x� 5) with the most stableMo/Nb site arrangement and H+ distribution were preparedusing a Coulombic and total energy-based screening ap-proach, EwaldSolidSolution program to find optimal H+(forming O� H bond) configurations (Figure S19, see Meth-ods section for further details).[17] All H+ bond to oxide ions,but no specific oxide ions preferentially adsorbed H+(Figure S20). The Mo3Nb2O14 lattice expands to give ΔV/Vof 2.1% at x=5 (Figure 5a), in stark contrast to ΔV/V uponLi+ intercalation (ΔV/V=5.5%). The smaller ΔV/V ob-tained for HxMo3Nb2O14 compared to that for LixMo3Nb2O14reproduces the experimental trend. The operating voltagefor H+ intercalation was calculated based on the totalenergy of HxMo3Nb2O14 and a chemical potential of hydro-gen atom (Figure 5b and S21). H+ intercalation occurs from0.41 V for x=0 to � 0.32 V vs. Ag/AgCl for x=5. Further-more, density of state (DOS) calculations reveal that theempty electronic states of molybdenum (major contribution)and niobium (minor contribution) in Mo3Nb2O14 shift belowthe Fermi level upon H+ intercalation (Figure 5c and S22).Bader charge analysis also confirms decreases in the valencestates of both Mo (major) and Nb (minor) (Figure S23).DFT-based molecular dynamics (DFT-MD) simulationswere conducted under NVT ensemble conditions to clarifythe H+-diffusion mechanism in Mo3Nb2O14. The calculatedtrajectory clearly shows that H+ rotates around Mo/NbOn(n=6 and 7) and hops three-dimensionally between oxideions via the formation and cleavage of O� H bonds (Fig-Figure 5. DFT calculations for H+ intercalation into Mo3Nb2O14. (a) Lattice volume change of AxMo3Nb2O14 (A=H, blue; A=Li, black). (b)Calculated operation potential of Mo3Nb2O14 upon H+ intercalation. The dashed line corresponds to the experimental charge/discharge curves forMo3Nb2O14 in 4.2 molL� 1 H2SO4/10H2O� 90PEG+5 wt% HFE acquired during the second cycle. (c) Electronic densities of states of HxMo3Nb2O14(x=0 and 5). (d) DFT-MD simulation with proton trajectory density for H2.5Mo3Nb2O14 at T=700 K and (e) the Arrhenius plots of protonconductivities for HxMo3Nb2O14 (x=2.5 and 5.0).AngewandteChemieForschungsartikelAngew. Chem. 2024, e202410971 (6 of 8) © 2024 Wiley-VCH GmbH 15213757, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202410971 by Waseda University, Wiley Online Library on [14/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licenseure 5d). The mean-squared displacement (MSD) of H+ wascalculated at 300, 500, and 700 K for H2.5Mo3Nb2O14 (half-charged) and H5Mo3Nb2O14 (fully charged) (Figure S24);self-diffusion coefficients of 9.5×10� 8 and 3.8×10� 9 cm2 s� 1 arecalculated at 300 K for H2.5Mo3Nb2O14 and H5Mo3Nb2O14,respectively, while activation energies for H+ diffusion aredetermined to be 113 and 288 meV, respectively (Figure 5e),which are comparable to those of α-MoO3 and Brownmiller-ite Sr2Co2O5.[18] These results indicate that Mo3Nb2O14 maybecome capable of high power density after optimizing itsparticle size and improving ionic conductivity of electrolytes.Finally, to demonstrate the concept of aqueous protonrechargeable batteries, we fabricated a full cell consisting ofa Mo3Nb2O14 anode and a vanadium hexacyanoferrate(VHCF, Figure S25) cathode,[19] and 4.2 molL� 1 H2SO4/10H2O� 90PEG (x=90)+5 wt% HFE as the electrolyte(Figure 6a). The full cell delivers a specific capacity of115 mAhg� 1 (per the weight of Mo3Nb2O14) with an averagevoltage of 0.79 V at a specific current of 200 mAg� 1 (about1 C for Mo3Nb2O14) during early charge/discharge cycling(Figure 6b). The full cell retains 98.5% of its initial specificcapacity after 1000 cycles owing to the high electrochemicalstability of the molecular crowding electrolyte and thestructural integrity of Mo3Nb2O14 (Figure 6c). The full cellexhibits a superior capacity retention compared to mostrechargeable proton batteries reported in the literature(Table S2).ConclusionsThis study identifies H+ intercalation with near-zero volumechange in Mo3Nb2O14, an open-tunnel bronze phase. Amolecular crowding aqueous electrolyte (4.2 molL� 1 H2SO4/10H2O� 90PEG (x=90)+5 wt% HFE) and a Mo3Nb2O14electrode synergistically establish a highly stable electrolyte-electrode system, providing a capacity retention of approx-imately 100% and an average Coulombic efficiency of99.99% over 500 cycles. Detailed structural analyses revealthat the flexible open tunnel of Mo3Nb2O14 undergoesrotation and shrinkage to relieve local structural changeupon H+ intercalation, resulting in near-zero volumechange. Furthermore, Mo3Nb2O14 shows solid-solution-typephase transition upon H+ intercalation, which is alsobeneficial to reduce lattice mismatch. Owing to the strain-free properties, intraparticle cracking was suppressed aftercharge/discharge cycling. A full cell with the Mo3Nb2O14anode j4.2 molL� 1 H2SO4/10H2O� 90PEG+5 wt% HFE jVHCF cathode configuration operates stably with a capacityretention of 98.5% after the 1000th cycle. This study notonly highlights the importance of electrode materials withstructural integrity for the development of aqueous protonbatteries but also provides opportunities for the rationaldesign of electrode materials that exhibit near-zero volumechange.AcknowledgementsThis study was supported financially by the Core Researchfor Evolutional Science and Technology (CREST) Program(No. JPMJCR21O6) of JST, the Data Creation andUtilization Type Materials Research and DevelopmentProject (JPMXP1121467561) and Program for PromotingResearch on the Supercomputer Fugaku(JPMXP1020200301 and JPMXP1020230325) of the Ministryof Education, Culture, Sports, Science, and Technology(MEXT), and the Kazuchika Okura Memorial Foundation.This work was supported by JSPS KAKENHI Grant-in-Aidfor Transformative Research Areas (A) ’Ion Jamology’(Grant Number 24H02204). The authors are grateful to S.Figure 6. Electrochemical performance of a VHCF jMo3Nb2O14 full cellwith 4.2 molL� 1 H2SO4/10H2O� 90PEG+5 wt% HEF as the electrolyte.(a) Configuration of the full cell. (b) Galvanostatic charge/dischargecurves for Mo3Nb2O14 at a specific current of 200 mAg� 1 (about 1 C forMo3Nb2O14). (c) Cycle performance of Mo3Nb2O14 at a specific currentof 200 mAg� 1 (about 1 C for Mo3Nb2O14). A low current density of60 mAg� 1 (about 0.3 C for Mo3Nb2O14) was applied for during the firstcycle to activate cell performance. Note that specific capacities arecalculated based on Mo3Nb2O14.AngewandteChemieForschungsartikelAngew. Chem. 2024, e202410971 (7 of 8) © 2024 Wiley-VCH GmbH 15213757, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202410971 by Waseda University, Wiley Online Library on [14/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseMatsuoka of Waseda University for her support for theexperiments. XPS and TEM were performed using researchequipment (JPS-9010TR and JEM-2100: Material Character-ization Central Laboratory) shared by the MEXT Projectfor Promoting Public Utilization of Advanced ResearchInfrastructure (Program for Supporting the Construction ofCore Facilities) Grant Number JPMXS0440500023. TEMwas performed at the Joint Research Center for Environ-mentally Conscious Technologies in Materials Science(Grant No. JPMXP0621467974) of ZAIKEN Waseda Uni-versity. XRD and XAFS experiments were conducted at theBL5S2 and BL11S2 beamlines, respectively, of the AichiSynchrotron Radiation Center, Aichi Science & TechnologyFoundation, Aichi, Japan (Proposal No.2022D4023 and2023D3013). Specimen preparation for STEM observationwas supported by Advanced Research Infrastructure forMaterials and Nanotechnology (ARIM) of MEXT (ProposalNo. JPMXP1223NM0152). STEM was performed at NIMS.The calculations were performed on the supercomputers atNIMS (Numerical Materials Simulator).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.Keywords: aqueous battery · proton · bronze oxides ·molecular crowding electrolyte[1] F. Duffner, N. Kronemeyer, J. Tübke, J. Leker, M. Winter, R.Schmuch, Nat. Energy 2021, 6, 123–134.[2] a) N. Bezginov, T. Valdez, M. Horbatsch, A. Marsman, A. C.Vutha, E. A. Hessels, Science 2019, 365, 1007–1012; b) W.Xiong, A. Gasparian, H. Gao, D. Dutta, M. Khandaker, N.Liyanage, E. Pasyuk, C. Peng, X. Bai, L. Ye, K. Gnanvo, C.Gu, M. Levillain, X. Yan, D. W. Higinbotham, M. Meziane, Z.Ye, K. Adhikari, B. 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Kawai, S.-H. Jang, Y. Igarashi, K. Yazawa,K. Gotoh, J. Kikkawa, A. Yamada,Y. Tateyama, M. Okubo* e202410971Proton Intercalation into an Open-TunnelBronze Phase with Near-Zero VolumeChangeMo3Nb2O14 undergoes the near-zero vol-ume change upon H+ (de)intercalationin aqueous batteries owing to the opentunnels that buffer the local structurechange of Mo/NbOn polyhedra uponcharge/discharge. The optimal molecu-lar crowding electrolyte and the structur-al integrity of Mo3Nb2O14 electrodesynergistically establish a highly stableelectrolyte-electrode system, providing acapacity retention of approximately100% over 500 cycles.AngewandteChemieForschungsartikelAngew. Chem. 2024, e202410971 © 2024 Wiley-VCH GmbH 15213757, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ange.202410971 by Waseda University, Wiley Online Library on [14/01/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Proton Intercalation into an Open-Tunnel Bronze Phase with Near-Zero Volume Change Introduction Results and Discussion Conclusions Acknowledgements Conflict of Interest Data Availability Statement