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Masanao Ishijima, Yu Sakano, Kentaro Okada, Tamao Ishida, Kiyoshi Kanamura, [Toshihiko Mandai](https://orcid.org/0000-0002-2403-7794), Koichi Kajihara

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[Atmospheric pressure hydrothermal synthesis and characterization of hollandite-type α-Mn<sub>1−</sub><i>  <sub>x</sub></i>Ti<i>  <sub>x</sub></i>O<sub>2</sub> for rechargeable magnesium battery cathodes](https://mdr.nims.go.jp/datasets/b1b36d70-3d2e-4aa0-8f86-652047fb54b9)

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Atmospheric pressure hydrothermal synthesis and characterization of hollandite-type α-Mn1−xTixO2 for rechargeable magnesium battery cathodesFULL PAPERAtmospheric pressure hydrothermal synthesis and characterizationof hollandite-type ¡-Mn1¹xTixO2 for rechargeable magnesiumbattery cathodesMasanao Ishijima1,³, Yu Sakano1, Kentaro Okada1, Tamao Ishida2, Kiyoshi Kanamura1,Toshihiko Mandai3 and Koichi Kajihara1,‡1Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences,Tokyo Metropolitan University, 1–1 Minami-Osawa, Hachioji, Tokyo 192–0397, Japan2Research Center for Artificial Photosynthesis (ReCAP), Osaka Metropolitan University,3–3–138 Sugimoto, Sumiyoshi-ku, Osaka 558–8585, Japan3Research Center for Energy and Environmental Materials (GREEN), National Institute for Materials Science (NIMS),1–1 Namiki, Tsukuba, Ibaraki 305–0044, JapanHollandite-type ¡-MnO2 and its solid solutions with TiO2 for the cathode active materials of rechargeablemagnesium batteries were synthesized with a hydrothermal method under atmospheric pressure through theoxidation of Mn2+ ions with ammonium persulfate [(NH4)2S2O8] within 2 h. The solubility limit of Ti in ¡-Mn1¹xTixO2 was x Ä 0.2, and average particle size decreased with an increase in x. Despite the high surface area,¡-Mn1¹xTixO2 exhibited lower catalytic activity for oxidative electrolyte decomposition and better dischargecapacity retention than ¡-MnO2. Capacity retention was the highest and the increment of charge overpotentialduring cycles was the smallest at x = 0.2.Key-words : Hollandite-type ¡-Mn1−xTixO2, Atmospheric pressure hydrothermal synthesis, Rechargeablemagnesium battery[Received April 1, 2025; Accepted June 9, 2025]1. IntroductionRechargeable magnesium batteries (RMBs) with mag-nesium metal anodes have attracted attention as next gen-eration batteries because of the abundance of magnesium,high safety, and high theoretical capacity. Potential can-didates for high-voltage cathode active materials of RMBsinclude transition metal spinel oxides,1–6) hollandite-type ¡-MnO2,7–11) and amorphous oxide derived by thedelithiation of Li2Ti1/3Mo2/3O3.12) A crucial issue for thecathode materials of RMBs is to facilitate the migration ofMg2+ ions while mitigating their strong Coulombic inter-actions with anion sublattice. From this viewpoint, ¡-MnO2 is attractive because its one-dimensional channelsalong the c axis are large enough to accommodate Mg2+ions. The theoretical discharge capacity of ¡-MnO2 is 308mAhg¹1.13) However, the insertion of Mg2+ ions up to280mAhg¹1 leads to the destruction of the crystal struc-ture of ¡-MnO2.14) In addition, the high catalytic activityof Mn induces oxidative electrolyte decomposition dur-ing charging and disturbs the extraction of Mg2+ ions. Toovercome these problems, the partial cation substitutionsof the crystalline lattice of ¡-MnO2 have been inves-tigated. For example, V doping into ¡-KxMnO2 enhancedthe stability of the ¡-KxMnO2 phase and increased capac-ity retention upon cycling.15) Recent experimental evi-dence has also indicated that the co-insertion of alkali ionsmitigates the distortion of channels in ¡-MnO2 associatedwith the insertion of Mg2+ ions, leading to a decrease indischarge overpotential and an increase in discharge ca-pacity.16) Another candidate for the cation substitution of¡-MnO2 include Ti, because TiO2 has a polymorph withhollandite structure,17,18) and it may facilitate the formationof hollandite-type ¡-Mn1¹xTixO2 solid solutions.The conventional way of synthesizing hollandite-type¡-MnO2 is the hydrothermal oxidation of Mn2+ ions withpersulfate (S2O82¹) ions under the presence of structuraltemplate cations such as K+ and NH4+ ions. Typical reac-tion temperatures are ³120–180 °C15,19–26) and pressurevessels are routinely used. However, ¡-MnO2 has beenprepared by oxidizing Mn2+ ions with O2 in aqueous sul-furic acid solutions at 25 °C,27) treating Mn2O3 in aqueoussulfuric acid solutions at 95–100 °C,28) oxidizing Mn2+ions with MnO4¹ ions in water at 100 °C,29) and reducingMnO4¹ ions in ethanol at 78 °C,30) indicating that ¡-MnO2³ Corresponding author: M. Ishijima; E-mail: ishijima@tmu.ac.jp‡ Corresponding author: K. Kajihara; E-mail: kkaji@tmu.ac.jpJournal of the Ceramic Society of Japan 133 [9] 562-568 2025DOI https://doi.org/10.2109/jcersj2.25049 JCS-Japan©2025 The Ceramic Society of Japan562This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.https://doi.org/10.2109/jcersj2.25049https://creativecommons.org/licenses/by/4.0/can be formed as the primary phase bellow 100 °C. Inaddition, the decomposition temperature of S2O82¹ ions isalso lower than 100 °C (³85–90 °C).31) Indeed, the syn-thesis of ¡-MnO2 through the oxidation of Mn2+ ions withammonium persulfate [(NH4)2S2O8] or potassium persul-fate (K2S2O8) in a boiled water was reported, whereas theprecursor solution was acidified with H2SO4 and otheradditives were not used.32) These considerations promptedus to develop an easy way to hydrothermally synthesize¡-MnO2 at ³100 °C under atmospheric pressure throughthe oxidation of Mn2+ ions with S2O82¹ ions. Synthesisunder atmospheric pressure in conventional glass contain-ers offers a practical way for scalable synthesis, and theease of stirring is suitable for the improvement of uni-formity of samples.In this paper, we developed a facile and rapid at-mospheric pressure hydrothermal process to synthesizehollandite-type ¡-MnO2 for the cathode active materials ofRMBs, and examined the partial substitution of Ti into thecrystalline lattice of ¡-MnO2.2. Experimental procedure2.1 SynthesisManganese sulfate pentahydrate (MnSO4·5H2O, FujifilmWako Pure Chemical), titanyl sulfate (TiOSO4·nH2O,Kishida Chemical), ammonium sulfate [(NH4)2SO4,Fujifilm Wako Pure Chemical], and (NH4)2S2O8 (FujifilmWako Pure Chemical) were dissolved in distilled water ina MnSO4:TiOSO4:(NH4)2SO4:(NH4)2S2O8 molar ratio of1 ¹ x:x:y:1:100 with the total metal (Mn + Ti) content of15mmol in a glass screw cap vial of 50mL capacity.(NH4)2SO4 was added to promote the formation of thehollandite-type phases.21,22) The vial was sealed withoutbeing too tight and placed in an aluminum heating block ona hot stirring plate, and the solution was heated to 100 °Cand then maintained for t h while stirring. Caution: Thetemperature of reaction mixture should not be higherthan 100 °C to avoid the explosion of glass vials. Thetemperature of the reaction mixture was monitored byinserting a conventional stainless-sheathed K-type thermo-couple though the vial cap in several runs, and was used tocalibrate the temperature setting of the hot stirring plate.The resulting solid precipitates were centrifuged, washedwith water, dried, and heat treated at 300 °C for 5 h in a tubefurnace in air.2.2 CharacterizationThe resulting powder samples were evaluated by pow-der X-ray diffraction (XRD, RINT-TTR III, Rigaku),Fourier-transform infrared (FT-IR) spectroscopy (FT/IR-4600, JASCO) using an attenuated total reflection (ATR)unit with a diamond prism, scanning electron microscopywith energy dispersive X-ray spectroscopy (SEM-EDS,JSM-IT800, JEOL and PhenomPro, Thermo Fisher Sci-entific). Nitrogen adsorption–desorption isomers wererecorded using an automatic adsorption instrument(BELSORP MAX, MicrotracBEL) and specific surfacearea was determined by the Brunauer–Emmett–Teller(BET) method.2.3 Electrochemical analysisDry composite cathodes were prepared by mixing thepowder sample, acetylene black (AB, Denka; electricallyconductive support), and poly(tetrafluoroethylene) (PTFE,Du Pont-Mitsui Fluorochemicals; binder) in a mass ratioof 60:30:10, and pressing ³2mg of the composite ontoan Al mesh. Electrochemical measurements of the com-posite cathodes were conducted in an Ar-filled gloveboxwith a three-electrode cell using a Mg ribbon (99.9%,Yoneyama Yakuhin Kogyo) as the counter electrode, anda Ag wire immersed in a triglyme (G3, Kanto Chemical)solution of 0.01mol dm¹3 AgNO3 (Kanto Chemical) and0.1mol dm¹3 magnesium bis(trifluoromethanesulfonyl)amide (Mg[TFSA]2, Kishida Chemical) as the referenceelectrode. Two types of electrolytes, i.e., 0.3mol dm¹3[Mg(G4)][TFSA]2/[C3mPyr][TFSA],33,34) prepared fromtetraglyme (G4, Kishida Chemical), Mg[TFSA]2, and 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfo-nyl)amide ([C3mPyr][TFSA], Kanto Chemical), and 0.3mol dm¹3 G3 solution of magnesium tetrakis(hexafloro-isopropyloxy)borate (Mg[B(HFIP)4]2),35–37) were used.The water contents of these electrolytes measured usinga Karl Fischer titrator (MKC-710, Kyoto ElectronicsManufacturing) were ³45–65 ppm. Cyclic voltammetrywas performed at a scan rate of 0.1mV s¹1 in the potentialrange from ¹1.6 to 1.2V vs. Ag/Ag+ (from 1.0 to 3.8Vvs. Mg/Mg2+). Galvanostatic charge–discharge tests werecarried out using an electrochemical analyzer (HZ-Pro andHJ1020mSD8, Hokuto Denko) at 10mAg¹1 in the poten-tial range from ¹1.6 to 0.9V vs. Ag/Ag+ (from 1.0 to3.5V vs. Mg/Mg2+). They were initiated from the dis-charge step, and the charge capacity was restricted to 154mAhg¹1 (half of the theoretical discharge capacity of ¡-MnO2). The current densities of cyclic voltammetry andcharge–discharge measurements were normalized to themasses of cathode active materials.3. Results and discussionFigure 1 shows photographs of a precursor solutionprepared at (x, y) = (0, 2) and suspension obtained afterheat treatment at 100 °C for 2 h (t = 2). It took ³20minto heat up to 100 °C, during which the precursor solutionstarted to lose transparency, whereas the suspension wasFig. 1. Photograph of a precursor solution prepared at (x, y) =(0, 2) (left) and suspension obtained by the heat treatment of theprecursor solution at 100 °C for 2 h (t = 2) (right).Journal of the Ceramic Society of Japan 133 [9] 562-568 2025 JCS-Japan563subsequently kept stirring at 100 °C for t h. The suspen-sions did not boil probably because of their boiling pointelevation.Figure 2 shows powder XRD patterns of the samplesprepared at x = 0 and t = 2, before heat treatment at300 °C. Single-phase ¡-MnO238) was obtained at y = 2,whereas ramsdellite-type R-MnO239) were formed as animpurity phase at y = 0 and 1. Thus, (NH4)2SO4 was ne-cessary to synthesize single-phase ¡-MnO2 in this processas an extra source of the template NH4+ ions and hereaftery was fixed at 2.At (x, y) = (0, 2), single-phase ¡-MnO2 was obtained att = 0, 1, and 2. The yield of these powder samples wascalculated as the ratio of their mass after heat treatment at300 °C for 5 h in air to the ideal mass of MnO2 of 15mmol.The yields at t = 0, 1, and 2 were ³0.63, ³0.93 and³0.97, respectively. Thus, the reaction appears to be most-ly completed at t = 1, whereas all samples in subsequentexperiments were prepared at t = 2 to ensure the com-pletion of the hydrothermal reaction.Figure 3 shows the ATR-FT-IR spectra of the samplesprepared at (x, y) = (0, 2) before and after heat treatment at300 °C. In the as-prepared sample, absorption bands attrib-uted to the asymmetric stretching (¯3) and bending (¯4)modes of NH4+ ions, probably located in the channels ofhollandite-type ¡-MnO2, were observed at ³3200 and³1450 cm¹1, respectively.41) These absorption bands werealmost absent after heat treatment at 300 °C, indicating thatthis heat treatment is effective for eliminating interstitialNH4+ ions.Figure 4(a) shows powder XRD patterns of the samplesafter heat treatment at 300 °C for 5 h in air. The Rietveldrefinements of these patterns shown in Fig. S1 indicatedthat hollandite-type single-phase samples were obtained atx ¯ 0.2, whereas anatase-type TiO242) was formed as aminor impurity phase at x = 0.25 and 0.30. The refinedstructure parameters are listed in Tables S1–S5. Refine-ments with small electron density (O3) in the channelsgave better results, whereas the origin of the electrondensity remains uncertain. Figure 4(b) shows the depend-ence of lattice parameters on x of the hollandite phasederived by the Rietveld refinement. Lattice parameters aand c increased linearly with x at x ¯ 0.2. This increase inunit cell dimensions is consistent with the larger ionicradius of a Ti4+ ion (0.75¡43)) than a Mn4+ ion (0.67¡43))in an octahedral coordination. These observations con-firmed the formation of the solid solution ¡-Mn1¹xTixO2.The solubility limit of Ti with this process was x Ä 0.2.Figure 5 shows the FE-SEM images of the samples. Atx = 0, rod-shaped primary particles typical of hydro-thermally synthesized ¡-MnO211,21) were clearly seen. Theincorporation of Ti reduced both the length and diameterFig. 2. Powder XRD patterns of the samples prepared at x = 0and t = 2 before heat treatment at 300 °C, and simulated patternof ¡-MnO2 and R-MnO2 calculated using RIETAN-FP40) and therespective structure parameters reported in Refs. 38) and 39).Fig. 3. ATR-FT-IR spectra of the samples prepared at (x, y) =(0, 2) before and after heat treatment at 300 °C for 5 h in air.(a)(b)Fig. 4. (a) Powder XRD patterns of the samples prepared aty = 2 after heat treatment at 300 °C for 5 h in air, and simulatedpattern of ¡-MnO2 calculated using RIETAN-FP40) and thestructure parameters reported in Ref. 38). (b) Variation of latticeparameters with x.Ishijima et al.: Atmospheric pressure hydrothermal synthesis and characterization of hollandite-type ¡-Mn1−xTixO2 for rechargeablemagnesium battery cathodesJCS-Japan564of the primary particles. These results suggest the suppres-sion of crystal growth with the addition of Ti. The BETspecific surface areas of the samples prepared at x = 0,0.10, 0.20, and 0.25 were ³130, ³166, ³183, and ³199m2 g¹1, respectively. This increase in specific surface areawith x was consistent with the reduction of average pri-mary particle size.Figure 6 shows the cyclic voltammogram of the sam-ples. The cathodic peak observed at ³1.5V vs. Mg/Mg2+was attributed to the insertion of Mg2+ ions in ¡-Mn1¹xTixO2 and the resulting reduction of Mn4+ and/orMn3+ ions.10) In contrast, the anodic peak at ³3.5V vs.Mg/Mg2+ was due to the oxidation of Mn2+ and/or Mn3+ions associated with the extraction of Mg2+ ions. The(a) (b)(c) (d)Fig. 5. FE-SEM images of the samples prepared at y = 2 and (a) x = 0, (b) 0.10, (c) 0.20, and (d) 0.25.(a) (b)(c) (d)Fig. 6. Cyclic voltammograms of dry composite cathodes of the samples prepared at y = 2 and (a) x = 0,(b) 0.10, (c) 0.20, and (d) 0.25 in 0.3mol dm¹3 [Mg(G4)][TFSA]2/[C3mPyr][TFSA] at 80 °C.Journal of the Ceramic Society of Japan 133 [9] 562-568 2025 JCS-Japan565anodic current at & 3:6V vs. Mg/Mg2+, originated fromoxidative electrolyte decomposition on sample surfaces,was the largest in the Ti-free (x = 0) sample. However, theanodic current was suppressed in the samples containingTi (x ² 0.1), despite an increase in surface area with x,indicating that the incorporation of Ti is effective in reduc-ing catalytic activity for oxidative electrolyte decomposi-tion on ¡-MnO2.Figure 7 shows the galvanostatic charge–dischargecurves and capacity retention of dry composite cathodesof the samples in 0.3mol dm¹3 [Mg(G4)][TFSA]2/[C3mPyr][TFSA] at 80 °C. The sample prepared at x = 0exhibited a plateau at ³3.2V vs. Mg/Mg2+, attributed tooxidative electrolyte decomposition. In contrast, such aplateau was absent, and charge voltage increased monoton-ically in the samples containing Ti. These observations areconsistent with the cyclic voltammograms shown in Fig. 6.In all samples discharge capacity reached the capacitycutoff (154mAhg¹1) in the 1st and 2nd discharge, whereasit decreased with cycle number after that. The dischargecapacity at the 10th cycle was ³55mAhg¹1 in ¡-MnO2prepared at x = 0, whereas the sample prepared at x = 0.20exhibited a better discharge capacity of ³85mAhg¹1 afterthe 10th cycle. The capacity retention of the sample pre-pared at x = 0.25 was worse than that of the sample pre-pared at x = 0.20, probably because of the formation of thesecondary TiO2 phase.Figure 8 shows the galvanostatic charge–dischargecurves and discharge capacity retention of dry compositecathodes of the samples in Mg[B(HFIP)4]2/G3 at 30 °C. Inthe 1st cycle, discharge capacity reached 154mAhg¹1 inall samples. The sample prepared at x = 0 maintained thisdischarge capacity up to the 3rd cycle, whereas the capac-ity fading in the subsequent cycles was faster than that ofthe samples containing Ti. In addition, the incorporation ofTi reduced the increment of charge overvoltage duringcycles. Discharge capacity after the 10th cycle was betterfor the sample prepared at x = 0.20 (³75mAhg¹1) thanfor the one prepared at x = 0 (³40mAhg¹1). The partialcation substitution with Ti may stabilize the hollandite-type crystal structure, leading to an improvement of elec-trochemical properties.(a) (b)(c) (d)(e)Fig. 7. Galvanostatic charge–discharge curves of dry composite cathodes of the samples prepared at y = 2 and(a) x = 0, (b) 0.10, (c) 0.20, and (d) 0.25 in 0.3mol dm¹3 [Mg(G4)][TFSA]2/[C3mPyr][TFSA] at 80 °C.(e) Discharge capacity retention of the samples shown in panels (a)–(d).Ishijima et al.: Atmospheric pressure hydrothermal synthesis and characterization of hollandite-type ¡-Mn1−xTixO2 for rechargeablemagnesium battery cathodesJCS-Japan5664. ConclusionsA hydrothermal method to prepare hollandite-type ¡-MnO2 under atmospheric pressure was developed andsolid solutions ¡-Mn1¹xTixO2 were synthesized as newcathode materials for RMBs. The synthesis of ¡-Mn1¹xTixO2 was completed within 2 h and the solubilitylimit of Ti was x Ä 0.2. The partial cation substitution ofTi in ¡-MnO2 suppressed the growth of primary particlesand increased surface area, whereas it depressed anodiccurrent attributed to oxidative electrolyte decompositionduring charging. The partial Ti substitution decreasedoverpotential during charging and improved the elec-trochemical properties. These observations indicated that¡-Mn1¹xTixO2 with x = 0.2 is suitable for the cathodeactive materials of RMBs.Acknowledgments This work was supported by GteXProgram Japan Grant Number JPMJGX23S1. The authorsthank Dr. Yuma Shimbori for assistance with electrochemicalmeasurements.References1) S. Okamoto, T. Ichitsubo, T. Kawaguchi, Y. Kumagai, F.Oba, S. Yagi, K. Shimokawa, N. Goto, T. Doi and E.Matsubara, Adv. Sci. 2, 1500072 (2015).2) J. Han, S. Yagi and T. Ichitsubo, J. Power Sources 435,226822 (2019).3) H. Kobayashi, K. Samukawa, M. Nakayama, T. Mandaiand I. Honma, ACS Appl. Nano Mater. 4, 8328 (2021).4) K. Shimokawa, T. Atsumi, N. L. Okamoto, T.Kawaguchi, S. Imashuku, K. Wagatsuma, M.Nakayama, K. Kanamura and T. Ichitsubo, Adv. Mater.33, 2007539 (2021).5) K. Sone, Y. Hayashi, T. Mandai, S. Yagi, Y. Oaki and H.Imai, J. Mater. Chem. A 9, 6851 (2021).6) Y. Idemoto, M. 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