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[Masanao Ishijima](https://orcid.org/0000-0002-4230-0410), Arisa Omata, Kiyoshi Kanamura, [Toshihiko Mandai](https://orcid.org/0000-0002-2403-7794), Xiatong Ye, [Tetsu Ichitsubo](https://orcid.org/0000-0002-1127-3034), [Koichi Kajihara](https://orcid.org/0000-0002-0282-3890)

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[Oxalate-assisted Fe<sub>2</sub>O<sub>3</sub> surface functionalization of nanosized MgMn<sub>2</sub>O<sub>4</sub> and α-MnO<sub>2</sub> cathodes for rechargeable magnesium batteries](https://mdr.nims.go.jp/datasets/e46c24a4-d613-428d-909b-a364c1afeade)

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Oxalate-assisted Fe2O3 surface functionalization of nanosized MgMn2O4 and α-MnO2 cathodes for rechargeable magnesium batteriesRSCApplied InterfacesPAPERCite this: RSC Appl. Interfaces, 2025,2, 179Received 15th August 2024,Accepted 21st October 2024DOI: 10.1039/d4lf00290crsc.li/RSCApplInterOxalate-assisted Fe2O3 surface functionalizationof nanosized MgMn2O4 and α-MnO2 cathodes forrechargeable magnesium batteries†Masanao Ishijima, *a Arisa Omata,a Kiyoshi Kanamura,a Toshihiko Mandai, bXiatong Ye,c Tetsu Ichitsubo c and Koichi Kajihara *aMn-based transition metal oxide nanoparticles are promising candidates as cathode active materials forrechargeable magnesium batteries, but their high catalytic activity for oxidative electrolyte decompositionand large surface area deteriorate their cycle performance. A recent study [Yagi et al., J. Mater. Chem. A,2021, 9, 26401–26409] demonstrated that the catalytic activity was less prominent in Fe-based oxides thanin other transition metal oxides, containing Mn. Fe-based oxides show low catalytic activity for oxidativeelectrolyte decomposition compared with Mn-based congeners. The strong capability of oxalate ions forbridging transition metal ions was utilised to form thin, uniform Fe2O3 layers on nanoparticles of MgMn2O4and α-MnO2. The resulting Fe2O3 layers effectively suppressed side reactions during insertion andextraction of the Mg2+ ions and improved the capacity retention and cycle performance.IntroductionRechargeable magnesium batteries (RMBs) have attractedincreasing attention as advanced rechargeable batteries,because magnesium is an abundant element and magnesiummetal anodes have high theoretical capacities (volumetric:3833 mA h cm−3, gravimetric: 2205 mA h g−1). Promisingcathode active materials for 3 V-class RMBs includehollandite-type α-MnO2,1–4 spinel-type MgM2O4 (M: transitionmetal),5–7 and ZnMnO3.8–10 Electrolytes compatible with bothmagnesium metal anodes and transition metal oxidecathodes are presently limited to glyme-based electrolytes.9Despite the high catalytic activity of transition metal oxidecathodes containing Mn or Co, the oxidative decompositionof the glyme-based electrolytes on their surfaces during theextraction of Mg2+ ions is a crucial problem.11,12 Such sidereactions can be suppressed by passivating the surface activesites, where coating the surface of the cathode activematerials with inert oxides (e.g., V2O5 (ref. 13) or ZrO2 (ref.14)) or electroconductive polymers15,16 has been reported toimprove their electrochemical properties. On the other hand,recent studies have shown that oxidative electrolytedecomposition is less prominent on MgFe2O4 than onMgMn2O4 and MgCo2O4, indicating that the catalytic activityof Fe for oxidative electrolyte decomposition is lower thanthat of Mn and Co.11 The partial substitution of Fe into Mnand Co sites (e.g., Mg(Co0.4Fe0.6)O4 (ref. 12)) and coating ofthe surfaces of MgMn2O4 with Mg–Fe binary oxides17 alsoimproved the electrochemical properties.The diffusion of divalent Mg2+ ions in the transition metaloxides is sluggish because of the strong electrostaticinteractions between the Mg2+ ions and the oxide sublattice.To minimize the diffusion length of Mg2+ ions and facilitatetheir insertion and extraction, oxide-based RMB cathodematerials with nanosized dimensions and large surface areas,such as 3D-open channel nanostructures (structured MgMn2-O4 (ref. 18)) and ultrasmall (<2.5 nm) cubic MgMn2O4,5 havebeen utilized. Surface functionalization via self-organizationof reagents on the surface of transition metal oxides is anideal way to form thin, uniform, and dense layers ontransition metal oxides with large surface areas. As anexample of such a self-organizing process, we developed thephenyl phosphonate surface functionalization of structuredMgMn2O4 by utilizing the strong binding of thephenylphosphonate groups to the surface of transition metaloxides.19Herein, we present another self-organizing process to forma thin Fe2O3 layer on nanosized transition metal oxide-basedRMB cathode materials by employing oxalate ions, which aresmall polydentate ligands commonly used to form uniformRSC Appl. Interfaces, 2025, 2, 179–184 | 179© 2025 The Author(s). Published by the Royal Society of Chemistrya Department of Applied Chemistry for Environment, Graduate School of UrbanEnvironmental Sciences, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397,Japan. E-mail: ishijima@tmu.ac.jp, kkaji@tmu.ac.jpb Research Center for Energy and Environmental Materials (GREEN), NationalInstitute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044,Japanc Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan† Electronic supplementary information (ESI) available: Three electrode celldetails, capacity retention and Coulombic efficiency of the cathode materials.See DOI: https://doi.org/10.1039/d4lf00290cOpen Access Article. Published on 30 October 2024. Downloaded on 2/3/2025 1:14:33 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttp://crossmark.crossref.org/dialog/?doi=10.1039/d4lf00290c&domain=pdf&date_stamp=2025-01-14http://orcid.org/0000-0002-4230-0410http://orcid.org/0000-0002-2403-7794http://orcid.org/0000-0002-1127-3034http://orcid.org/0000-0002-0282-3890https://doi.org/10.1039/d4lf00290chttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4lf00290chttps://pubs.rsc.org/en/journals/journal/LFhttps://pubs.rsc.org/en/journals/journal/LF?issueid=LF002001180 | RSC Appl. Interfaces, 2025, 2, 179–184 © 2025 The Author(s). Published by the Royal Society of Chemistryprecipitates of multicomponent metal ions for the precursorsof multicomponent ceramics.20,21 The cathode activematerials functionalized with oxalate ions were formed bytreating in a solution of oxalate ions. They were converted toones functionalized with Fe2O3 through treatment in asolution of iron(III) ions and subsequent heat treatment. Theelectrochemical properties of the resulting Fe2O3-functionalized cathode materials are presented.ExperimentalSynthesis of MgMn2O4The structured MgMn2O4 powder was prepared by followinga reported procedure.18,19 Magnesium chloride hexahydrate(MgCl2·6H2O, 6 mmol, Fujifilm Wako Pure Chemical),manganese chloride tetrahydrate (MnCl2·4H2O, 12 mmol,Fujifilm Wako Pure Chemical), and citric acid (18 mmol,Fujifilm Wako Pure Chemical) were dissolved in 20 mL ofethanol, and propylene oxide (16 mL, Kanto Chemical) wasadded. The resulting metal–organic complex gel wasmaintained for 1 day at 25 °C, washed with ethanol andacetone to remove byproducts, and subjected to sequentialsolvent exchange with acetone for 1 day and cyclohexane 3times in 3 days. The resulting wet gel was dried at 60 °C andheat treated at 350 °C for 5 h in a tube furnace in air.Synthesis of α-MnO2The α-MnO2 powder was prepared by following a reportedprocedure.22–24 Manganese sulfate pentahydrate (MnSO4·5H2-O, 67 mmol, Fujifilm Wako Pure Chemical), ammoniumsulfate ((NH4)2SO4, 125 mmol, Fujifilm Wako Pure Chemical),and ammonium peroxydisulfate ((NH4)2S2O8, 67 mmol,Fujifilm Wako Pure Chemical) were dissolved in 100 mL ofdistilled water. The solution was then transferred to a Teflon-lined stainless-steel autoclave and heated at 140 °C for 12 hin an oven. The product was washed with an acetonitrilesolution of nitronium tetrafluoroborate (NO2BF4, FujifilmWako Pure Chemical).Fe2O3 functionalizationAmmonium oxalate monohydrate ((NH4)2ox·H2O, 0.25 mmol,Fujifilm Wako Pure Chemical) was dissolved in 3 g ofdistilled water. MgMn2O4 or α-MnO2 (0.625 mmol) was addedto the solution, and the suspension was stirred at roomtemperature for 3 h. The solid powder was separated bycentrifugation, washed with water, and dried at 60 °C for 12h. The dried solid powder was redispersed in a solutionprepared by dissolving iron(III) nitrate nonahydrate(Fe(NO3)3·9H2O, Fujifilm Wako Pure Chemical) in 3 g ofmethanol (Fujifilm Wako Pure Chemical) at a molar ratio ofMgMn2O4 or α-MnO2 : Fe(NO3)3 = 1 : x, and the suspensionwas stirred at room temperature for 3 h. The solid powderwas separated by centrifugation, washed with methanol,dried at 60 °C for 12 h, and heat treated at 350 °C for 5 h ina tube furnace in air.CharacterizationThe resulting powder samples were evaluated by powderX-ray diffraction (XRD, SmartLab, Rigaku), Fourier-transform infrared (FT-IR) spectrometry (FT/IR-4600, JASCO)using an attenuated total reflection (ATR) unit with adiamond prism, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS, PhenomPro,Thermo Fisher Scientific), and scanning transmissionelectron microscopy with EDS (STEM-EDS, JEM-ARM200FNEOARM, JEOL).Electrochemical analysisDry composite cathodes were prepared by mixing the powderof pristine or Fe2O3-functionalized MgMn2O4 or α-MnO2,acetylene black (AB, Denka; electrically conductive support),and poly(tetrafluoroethylene) (PTFE, Du Pont-MitsuiFluorochemicals; binder) in a mass ratio of 60 : 30 : 10, andpressing ∼2 mg of the composite with ∼1.2 mg of cathodeactive material onto an Al mesh. Electrochemicalmeasurements of the composite cathode were conducted inan Ar-filled glovebox with a three-electrode cell (Fig. S1†) usinga Mg ribbon (99.9%, Yoneyama Yakuhin Kogyo) as the counterelectrode, and a Ag wire immersed in a triglyme (G3, KantoChemical) solution of 0.01 mol dm−3 AgNO3 (Kanto Chemical)and 0.1 mol dm−3 magnesium bis(trifluoromethanesulfonyl)amide (Mg[TFSA]2, Kishida Chemical) as the referenceelectrode. The electrolytes used were 0.3 mol dm−3 [Mg(G4)][TFSA]2/[C3mPyr][TFSA],25,26 prepared from tetraglyme (G4,Kishida Chemical), Mg[TFSA]2, and 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C3-mPyr][TFSA], Tokyo Chemical Industry), and 0.3 mol dm−3 G3solution of magnesium tetrakis(hexafloroisopropyloxy)borate(Mg[B(HFIP)4]2).27–29 Galvanostatic charge–discharge andgalvanostatic intermittent titration technique (GITT) testswere carried out using electrochemical analyzers (HZ-Pro andHJ1020mSD8, Hokuto Denko) at 10 mA g−1 in the potentialrange from −1.6 to 0.9 V vs. Ag/Ag+ (from 1.0 to 3.5 V vs. Mg/Mg2+). The test was initiated from the discharge step, and thecharge capacity was restricted to 180 mA h g−1 for MgMn2O4and 200 mA h g−1 for α-MnO2 (approximately two-thirds of thetheoretical capacities of MgMn2O4 (270 mA h g−1)5 andα-MnO2 (308 mA h g−1)30). The rest period of the GITTmeasurements was 2 h.Results and discussionStructural characterizationFig. 1 shows a schematic illustration of the Fe2O3functionalization process. First, the surface of a cathodeactive material was modified with oxalate ions by suspendingthem in an aqueous ammonium oxalate solution. Fe3+ ionswere anchored to the surfaces of the resulting samples bytreatment with a methanol solution of iron(III) nitrate.Finally, oxalate ions and residual nitrate ions wereRSC Applied InterfacesPaperOpen Access Article. Published on 30 October 2024. Downloaded on 2/3/2025 1:14:33 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4lf00290cRSC Appl. Interfaces, 2025, 2, 179–184 | 181© 2025 The Author(s). Published by the Royal Society of Chemistrydecomposed by heat treatment to form thin uniform Fe2O3layers on the structured MgMn2O4.Fig. 2(a) shows the powder XRD patterns of the pristineand Fe2O3-functionalized MgMn2O4. The observed patternof the pristine sample was essentially identical to thosereported previously.18,19,31,32 The similarity of the patternsbefore and after Fe2O3 functionalization indicated that thegrowth of MgMn2O4 crystallites and other crystalline phasesduring heat treatment was insignificant. Fig. 2(b) shows theATR-FT-IR spectra of the MgMn2O4 powders during thecourse of Fe2O3 functionalization. The shoulder at ∼650cm−1 was attributed to the Mn–O stretching mode of theMgMn2O4.33 After treatment with the ammonium oxalatesolution (green line), absorption bands attributed to theoxalate ions were observed at ∼1320, ∼1375, and ∼1650cm−1,34–36 and the ammonium ions were observed at ∼1450cm−1.37 After treatment with the iron(III) nitrate solution(blue line), absorption bands attributed to the nitrate ionsappeared at ∼1040 and ∼1350 cm−1,37 where the broadabsorption band peaked at ∼3300 cm−1, originating fromthe O–H stretching mode of FeOH groups and adsorbed orcoordinated water molecules, became prominent. In thespectrum of the Fe2O3-functionalized sample after heattreatment (red line), the absorption bands attributed to theoxalate, nitrate, and OH groups disappeared almostcompletely. Fig. 2(c) shows the SEM-EDS spectra of thepristine and Fe2O3-functionalized MgMn2O4. The Fe Kβ linewas observed only in the spectrum of Fe2O3-functionalizedMgMn2O4, and intensified with increasing x. Fig. 2(d)shows the STEM-EDS elemental mapping images of the Fe-functionalized MgMn2O4, verifying the uniform distributionof Fe on the particles.To demonstrate the versatility of this method, Fe2O3functionalization was applied to α-MnO2. Fig. 3 shows thepowder XRD patterns and SEM-EDS spectra of α-MnO2 beforeand after Fe2O3 functionalization. Fe2O3 functionalizationpreserved the XRD patterns, whereas the Fe Kβ line appearedonly in the spectrum of the Fe2O3-functionalized sample,similar to the structured MgMn2O4 shown in Fig. 2.Electrochemical characterizationFig. 4 shows the galvanostatic charge–discharge curves of thedry composite cathodes of the pristine and Fe2O3-functionalized MgMn2O4 powders. The current density wasnormalized to the mass of MgMn2O4 powder, including thatof Fe2O3. For the pristine sample (Fig. 4(a)), the 1st chargecycle ended at ∼2.9 V vs. Mg/Mg2+, whereas the chargepotential notably increased with cycle number. This potentialincrease was probably due to oxidative electrolytedecomposition and the simultaneous increase in theoverpotential associated with the accumulation ofdecomposed products on the MgMn2O4 surface. Such anincrease in the charge overvoltage with the number of cycleswas significantly suppressed by surface functionalization withFe2O3.The initial discharge capacity was comparable for thepristine sample (∼230 mA h g−1) and those functionalizedwith Fe2O3 (∼210, ∼230, and ∼240 mA h g−1 at x = 0.1, 0.2,Fig. 1 Schematic illustration of the Fe2O3 functionalization process inthe oxalate-assisted method.Fig. 2 (a) Powder XRD patterns of the pristine and Fe2O3-functionalized structured MgMn2O4 powders. Simulated patterns werecalculated using RIETAN-FP38 and the structure parameters ofMgMn2O4 reported in ref. 31. (b) ATR-FT-IR spectra of the pristinestructured MgMn2O4 powder after treatment with ammonium oxalateor iron(III) nitrate solution, and after heat treatment at 350 °C for 5 h inair (Fe2O3-functionalized sample). (c) SEM-EDS spectra of pristine andFe2O3-functionalized structured MgMn2O4 powders. (d) STEM and EDSelemental mapping (Mg, Mn, and Fe) images of Fe2O3-functionalizedstructured MgMn2O4.Fig. 3 (a) Powder XRD patterns of pristine and Fe2O3-functionalizedα-MnO2 powders. Simulated pattern was calculated using RIETAN-FP,38 and the structure parameters of α-MnO2 reported in ref. 39. (b)SEM-EDS spectra of pristine and Fe2O3-functionalized α-MnO2powders.RSC Applied Interfaces PaperOpen Access Article. Published on 30 October 2024. Downloaded on 2/3/2025 1:14:33 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4lf00290c182 | RSC Appl. Interfaces, 2025, 2, 179–184 © 2025 The Author(s). Published by the Royal Society of Chemistryand 0.4, respectively). The decay in the discharge capacitybetween the 1st and 2nd cycles was mainly due to theincomplete extraction of Mg2+ ions during the 1st chargecycle. In the pristine sample, the discharge capacityrapidly deteriorated with cycle number. This dischargecapacity fading was also suppressed by the Fe2O3functionalization. Fig. 4(e) and S2† show the dischargecapacity retention and Coulombic efficiency. The dischargecapacity after the 10th cycle was ∼100 mA h g−1 for thesamples with x = 0.2 and 0.4, and was notably higherthan that of the pristine sample (∼25 mA h g−1).Coulombic efficiency derived as the ratio of the dischargecapacity to the charge capacity of the previous cycle, wasalso improved and exhibited ∼0.6 at the 10th cycle forsamples with x = 0.2 and 0.4.Fig. 5 and S3† show the galvanostatic charge–dischargecurves, discharge capacity retention, and Coulombicefficiency of the composite cathodes of pristine and Fe2O3-functionalized α-MnO2 (x = 0.2, and 0.4) in [Mg(G4)][TFSA]2/[C3mPyr][TFSA] at 100 °C. Similar to structured MgMn2O4,Fe2O3 functionalization suppressed the increase in the chargepotential with cycle number, and improved dischargecapacity retention. The GITT profiles of the same compositecathodes are shown in Fig. 6. The amplitude of transientpotential change was comparable in the 1st cycle, whereassmaller in the Fe2O3-functionalized sample in the 5th cycle.This observation indicates the reduction of overpotentialsduring charging and discharging by the Fe2O3functionalization. Fig. 7 and S4† show the galvanostaticcharge–discharge curves, discharge capacity retention, andCoulombic efficiency of the composite cathodes of pristineand Fe2O3-functionalized α-MnO2 in Mg[B(HFIP)4]2/G3 at 30°C. In the pristine sample, the potential reached to the upperbound (3.5 V vs. Mg/Mg2+) during charging, indicating a largeoverpotential. In the Fe2O3-functionalized samples, theoverpotential during charging was suppressed, and dischargecapacity for up to the 5–6 cycles was significantly improved.The rapid fading of the discharge capacity thereafter in theFe2O3-functionalized samples may be due to the reducedmobility of Mg2+ ions at 30 °C and the vulnerability oftransition metal oxides to reduction in solvent (glyme)-richelectrolytes, both of which favour the destruction of α-MnO2crystallites rather than the reversible insertion and extractionof Mg2+ ions. Coulombic efficiency at the 10th cycle of theFe2O3-functionalized samples with x = 0.4 was ∼0.8 both in[Mg(G4)][TFSA]2/[C3mPyr][TFSA] and Mg[B(HFIP)4]2/G3, andbetter than other two samples.Fig. 4 Galvanostatic charge–discharge curves of dry compositecathodes of pristine MgMn2O4 (a) and those functionalized with Fe2O3at x = 0.1 (b), 0.2 (c), and 0.4 (d) in 0.3 mol dm−3 [Mg(G4)][TFSA]2/[C3mPyr][TFSA] at 100 °C. (e) Discharge capacity retention of samplesshown in panels (a)–(d).Fig. 5 Galvanostatic charge–discharge curves of dry compositecathodes of the pristine α-MnO2 (a), and those functionalized withFe2O3 at x = 0.2 (b) and 0.4 (c) in 0.3 mol dm−3 [Mg(G4)][TFSA]2/[C3-mPyr][TFSA] at 100 °C. (d) Discharge capacity retention of samplesshown in panels (a)–(c).Fig. 6 GITT profiles of the pristine α-MnO2 (a), and thosefunctionalized with Fe2O3 at x = 0.2 (b) and 0.4 (c) in 0.3 mol dm−3[Mg(G4)][TFSA]2/[C3mPyr][TFSA] at 100 °C.RSC Applied InterfacesPaperOpen Access Article. Published on 30 October 2024. Downloaded on 2/3/2025 1:14:33 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4lf00290cRSC Appl. Interfaces, 2025, 2, 179–184 | 183© 2025 The Author(s). Published by the Royal Society of ChemistryConclusionsAn oxalate-assisted Fe2O3 functionalization technique wasdeveloped for the surface modification of nanosizedtransition metal oxides (MgMn2O4 and α-MnO2) with largesurface areas as cathode active materials for RMBs. Oxalateions worked as efficient bridging agents between iron(III) ionsand the surface of transition metal oxides. Fe2O3functionalization suppressed side reactions includingoxidative electrolyte decomposition, decreased overpotentialsduring charging and discharging, and improved dischargecapacity retention. These observations demonstrate that theoxalate-assisted Fe2O3 functionalization is one of thepowerful self-organizing surface functionalization techniquesfor the improvement of the electrochemical properties ofcathode active materials for RMBs.Data availabilityThe data of this study are available from the correspondingauthors upon reasonable request.Conflicts of interestThere are no conflicts to declare.AcknowledgementsThis work was supported by GteX Program Japan GrantNumber JPMJGX23S1. The authors thank Dr. Yuma Shimborifor assistance with electrochemical measurements, and Mr.Yuki Ono of Tokyo Metropolitan University for providingvaluable technical support during the STEM-EDSobservations.References1 X. Guo, S. Yang, D. Wang, A. Chen, Y. Wang, P. Li, G.Liang and C. Zhi, Curr. Opin. Electrochem., 2021, 30,100769.2 J. Shin, J. K. Seo, R. Yaylian, A. Huang and Y. S. Meng, Int.Mater. Rev., 2020, 65, 356–387.3 C. Yuan, Y. Zhang, Y. Pan, X. Liu, G. Wang and D. Cao,Electrochim. 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