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[Toshihiko Mandai](https://orcid.org/0000-0002-2403-7794), Mariko Watanabe

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[Oxygen – a fatal impurity for reversible magnesium deposition/dissolution](https://mdr.nims.go.jp/datasets/23d081a5-24cf-45fb-86c4-3d3424b3267d)

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Oxygen – a fatal impurity for reversible magnesium deposition/dissolutionrsc.li/materials-aAs featured in:See Toshihiko Mandai and Mariko Watanabe, J. Mater. Chem. A, 2023, 11, 9755.Showcasing research from Dr. Toshihiko Mandai’s group, Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), Japan.Oxygen – a fatal impurity for reversible magnesium deposition/dissolutionSystematic research revealed that O2, even in miniscule amounts, in the electrolyte solution is a fatal impurity for the reversible electrochemistry of magnesium metals. Complex interfacial processes induced by O2, magnesium, and electrolyte make fi rm insulating layer on the surface. The authors also demonstrated the feasibility of constructing an alloy barrier layer on the surface of magnesium to improve its air stability.Registered charity number: 207890Journal ofMaterials Chemistry ACOMMUNICATIONOxygen – a fatalaCenter for Green Research on Energy and Enfor Materials Science (NIMS), 1-1 Namiki, Tsmandai.toshihiko@nims.go.jp; Tel: +81-29-8bCenter for Advanced Battery Collaboration,Environmental Materials, National InstituteTsukuba, Ibaraki, 305-0044, Japan† Electronic supplementary information (supporting gures and tableshttps://doi.org/10.1039/d3ta01286gCite this: J. Mater. Chem. A, 2023, 11,9755Received 2nd March 2023Accepted 9th April 2023DOI: 10.1039/d3ta01286grsc.li/materials-aThis journal is © The Royal Society oimpurity for reversible magnesiumdeposition/dissolution†Toshihiko Mandai *ab and Mariko WatanabeaThe compatibility of rechargeable magnesium batteries (RMBs) withatmospheric components (except moisture) has not been studied. Weinvestigated the effects of atmospheric conditions on the electro-chemical dissolution–deposition behavior of magnesium in non-aqueous electrolytes. Oxygen, even in miniscule amounts in theelectrolyte was a fatal impurity for reversible magnesiumelectrochemistry.Lithium-ion battery (LIB) technology has revolutionized socialand industrial structures. To enhance our living environmentworldwide while maintaining symbiosis with nature, theperformance of energy storage technology must be improved.Among several potential candidate technologies, rechargeablebatteries based on magnesium metal negative electrodes arepromising due to the geometrical, economical, physicochem-ical, and electrochemical advantages of the metal. Theoreticalenergy densities of rechargeable magnesium batteries (RMBs)can reach values that are twice those of present LIBs bycombining two-electron electrochemistry at positive electrodesand high volumetric/specic capacities and sufficiently lowelectrode potential of magnesium at negative electrodes. Torealize such outperforming RMBs, research has been conductedon fundamental battery components as well as technologies forinterfacial engineering, and a variety of promising materialsand techniques have been developed.1–4 Understanding thefundamental characteristics of battery components is alsoessential for large-scale battery production and manufacturing.vironmental Materials, National Instituteukuba, Ibaraki, 305-0044, Japan. E-mail:60-4464Center for Green Research on Energy andfor Materials Science (NIMS), 1-1 Namiki,ESI) available: Experimental detail andare provided. See DOI:f Chemistry 2023Every battery component including electrode active mate-rials and electrolytes should be handled under a dry-air atmo-sphere during the mass production of batteries.Commercialized LIBs are assembled in a dry room with a dewpoint of −40 °C. From the industrial point of view, the samesystem should be adoptable for the manufacturing of post-LIBs.To determine their potentials for RMB fabrication, the intrinsicelectrochemical characteristics of electrodes and conductiveelectrolytes under a dry-air atmosphere must be known.Magnesium deposition/dissolution is the fundamentalelectrochemical reaction of the negative electrode of RMBs. It iswell known in the RMB research eld that the reaction activityof magnesium negative electrodes is sensitive to the watercontent of ethereal solutions.5 Simple salt-solvent solutions ofmagnesium bis(triuoromethanesulfonyl)amide (Mg[TFSA]2)and magnesium tetrakis(hexauoroisopropoxyl)borate (Mg[B(HFIP)4]2), which have a relatively high water content (>50ppm), are reported not to support reversible magnesiumdeposition/dissolution at negative electrodes, whereas it isknown that a small amount of water enhances the interfacialand bulk kinetics at positive electrodes.6 Water impuritiesfacilitate magnesium passivation associated with [TFSA]−decomposition,7 leading to an electrochemically retardedmagnesium negative electrode. Mg[Al(HFIP)4]2-based electro-lytes are rather insensitive to water impurities because theseelectrolytes allow reversible deposition/dissolution, even at highconcentrations of impurities (up to 1000 ppm).8 In contrast tothese extensive studies on the water impurities in RMB elec-trolytes, the compatibility of RMB components with dry air,especially O2 gas, remains unclear. This might be due to theuncertain preconception and misunderstanding of the passiv-ation characteristics of magnesium, namely, that a magnesiummetal can be easily oxidized to form insulative oxide on thesurface upon exposure to an ambient air atmosphere.9 Tounderstand the intrinsic electrochemistry of magnesium metalnegative electrodes, reversible magnesium deposition/dissolution were analyzed under different atmospheric condi-tions in this study. The results of combined electrochemicalJ. Mater. Chem. A, 2023, 11, 9755–9761 | 9755http://crossmark.crossref.org/dialog/?doi=10.1039/d3ta01286g&domain=pdf&date_stamp=2023-05-06http://orcid.org/0000-0002-2403-7794https://doi.org/10.1039/d3ta01286gJournal of Materials Chemistry A Communicationand spectroscopic investigations reveal that O2 is a fatal impu-rity in RMB (electro)chemistry. In addition, an effective surfacetreatment to sustain sufficient electrochemical activities, evenunder an O2 atmosphere, is provided.The compatibility of RMB components, especially magne-sium metal negative electrodes and electrolytes incorporatingrepresentative air-stable weakly-coordinated anions [B(HFIP)4]−or [Al(HFIP)4]− with dry air was investigated. The results ofa simple experimental survey demonstrated the sensitivity ofmagnesium metals to O2, due to reversible magnesiumdeposition/dissolution being possible when using magnesiummetals polished under dry air, where the dew point wascontrolled at approximately −70 °C (Fig. S1†). The Mg 2p and O1s spectra and depth proles of magnesium metals indicate thepresence of an oxide- and carbonate-based layer, the so-callednative solid electrolyte interface (SEI), on the outermostsurface, independent of the polishing environment (Fig. S2 andS3†). This suggests that the native SEI does not necessarilyimpede electrochemical magnesium deposition/dissolutionreactions. However, cells assembled under a dry-air atmo-sphere show no features of electrochemical magnesiumdeposition/dissolution (Fig. S1†).To identify what factor has a signicant detrimental effect onthe electrochemical characteristics of magnesium metal nega-tive electrodes and electrolytes, systematic cyclic voltammetry(CV) and open-circuit potential (OCP) measurements wereconducted under certain atmospheric conditions with Ar, N2,and O2 and an air-tight voltammetry cell equipped with gas-inlet lines. Aer 30 min of steady ow of each gas at 10mL min−1 in the voltammetry cell (volume of ∼30 mL)Fig. 1 CV andOCP profiles of Pt andMg electrodes under steady flowof dCV profiles using (a) Pt and (b) Mg working electrodes. Time profiles ofelectrolyte solutions. The time profiles of O2 concentrations under N2 a9756 | J. Mater. Chem. A, 2023, 11, 9755–9761assembled in an Ar-lled glovebox, the Pt and Mg workingelectrodes showed characteristic responses dependent on theinlet gases (Fig. 1). Similar to Ar gas, N2 gas also has minor orrather favorable effect on the electrochemical magnesiumdeposition/dissolution activity. In contrast, O2 gas is detri-mental to the electrochemical activities, independent of theworking electrodes. The time-prole of OCP during the steadyow of different gases supports the change in the nature of themagnesium electrodes, especially under an O2 atmosphere(Fig. 1c). The electrode potential of magnesiummetal increasedby ∼200 mV aer 30 min of steady ow of O2, accompanied bythe continuous increase of the O2 concentration in the elec-trolytes. CV measurements were conducted immediately aer 1and 5 min of steady ow of an O2 gas at the same rate. Theresults show that the introduction of O2 gas is fatal for revers-ible magnesium deposition/dissolution (Fig. S4†), because O2 inthe electrolyte solution, even at 17 ppm, inactivates themagnesium negative electrodes. Fig. S4b† shows that the inac-tivation of the magnesium electrodes by O2 is a dynamicprocess. The introduction of O2 in the electrolyte solution andresulting insulative layer formation on magnesium electrodesseem to be irreversible and catastrophic, because subsequent Arbubbling to deoxygenate the electrolytes did not recover theelectrochemical activities (Fig. S5†). The control measurementsusing the deoxygenated electrolyte and fresh magnesium piecesshowed reversible magnesium deposition/dissolution activity(Fig. S6†). It should be noted here that the introduction of O2can affect the water content of the electrolytes, possibly owing toour experimental setup. However, the detrimental effect ofwater impurity seems negligible as the electrolytes containingifferent gases recorded in 0.3mol dm−3 Mg[B(HFIP)4]2/G2 electrolytes.(c) OCV of Mg electrodes and (d) corresponding O2 concentration innd Ar were zero during measurements.This journal is © The Royal Society of Chemistry 2023Communication Journal of Materials Chemistry A>100 ppm of water represent favorable magnesium electro-chemistry (Fig. S7†). In contrast, the ionic conductivities andRaman spectra of the electrolytes are invariable, even aer30 min of O2 gas ow (Table S1 and Fig. S8†), implying that theelectrolytes are chemically stable against O2. These resultsstrongly suggest that soaking magnesium metal in an electro-lyte with O2 impurities retards the electrochemical activity ofmagnesium deposition/dissolution. Note that the potentialshi of the reference electrode (Ag+/Ag) is negligible because theequilibrium potential for the magnesium dissolution processremains unchanged, even aer exposure to dry air (see Fig. S4†).The sensitivity of electrochemical magnesium deposition/dissolution activities to O2 impurities seems to be electrolyte-independent. Electrolyte solutions of Mg[TFSA]2 did notsupport electrodeposition of magnesium under a dry-airatmosphere, irrespective of the presence/absence of Cl-basedactivation reagents (Fig. S9†), although the electrolyte compo-nents themselves are chemically stable against air andmoisture.10X-ray photoelectron spectroscopy (XPS) and electrochemicalimpedance spectroscopy (EIS) provide further insights into thesurface chemical states of magnesium metals soaked in anelectrolyte with and without O2 impurities. Fig. 2 shows the Mg2p, F 1s, and O 1s spectra of certainmagnesium strips. All theMgFig. 2 Surface Mg 2p, F 1s, and O 1s spectra of Mg strips soaked in elecspectra of symmetric [Mg‖Mg] cells. Schematic illustrations of the suratmosphere.This journal is © The Royal Society of Chemistry 20232p, F 1s and O 1s spectra of magnesium strips soaked in theelectrolyte under an Ar atmosphere have distinct peaks certainlyassignable to the products due to the adsorption and/or sidereaction of the electrolyte anion with magnesium metal, such asMgCO3, MgF2, Mg(OH)2, CFn, and B–O compounds (Fig. 2a–c).These results imply the strong reductive nature of magnesiumunder an inert atmosphere. In contrast, the F 1s and O 1s spectraof magnesium strips soaked in electrolytes with O2 impuritiesexhibit small MgF2 and negligible B–O compound peaks, sug-gesting the suppressed adsorption and subsequent decomposi-tion of the anions on the surface. The Mg 2p and O 1s spectrashow rather distinct contributions of MgO and Mg(OH)2 on thesurface of strips soaked in electrolytes with O2. The EIS spectra ofsymmetric [Mg‖Mg] cells also demonstrate the detrimental effectof O2 impurity. The extremely large interfacial resistance wasobserved for the cells assembled using the electrolytes with O2impurity (Fig. 2g). The cells using the electrolyte deoxygenated byAr bubbling showed a substantially low resistance, againemphasizing that the presence of O2 impurities in the electrolytessignicantly affects the electrochemical magnesium deposition/dissolution activity. The oxide- and hydroxide-based highly-insulative layer generated at the magnesium–electrolyte–O2three-phase boundary may represent a factor affecting thereversible electrochemistry (Fig. 2h and i).trolyte solutions under (a–c) Ar and (d–f) dry-air atmospheres. (g) EISface chemistry of magnesium electrodes under (h) Ar and (i) dry-airJ. Mater. Chem. A, 2023, 11, 9755–9761 | 9757Journal of Materials Chemistry A CommunicationThe above-mentioned characteristics of magnesium metalswill limit the mass production of RMBs. Therefore, techniquesto overcome such a limitation must be developed. The magne-sium electrochemistry under an O2 atmosphere has beenextensively studied in the magnesium–air battery eld. Similarto conventional aqueous zinc–air batteries, reactions at themagnesium negative electrode side in alkaline media could leadto the corrosion of the magnesium metal and magnesiumhydroxide formation upon discharge.11 However, due to thelarge overpotential, reverse reactions do not proceed. Thus,most aqueous magnesium–air batteries are primary and notsecondary batteries. Non-aqueous magnesium–oxygen batterieshave also been reported, and reversible cycling has been ach-ieved with a specic redox mediator, that is, an I2-DMSOcomplex, in the Mg(ClO4)2-dimethylsulfoxide electrolyte.12 Theabove-mentioned non-aqueous electrolyte does not supportmagnesium electrodeposition. The charge–discharge proles ofmagnesium–oxygen batteries indicate a poor reversibility,presumably due to the electrochemical inactivity at themagnesium negative electrode side.The surface functionalization by an articial interface is aneffective approach to modify the chemical nature of theFig. 3 CV profiles of modifiedMg electrodes recorded in 0.3mol dm−3 Melectrodes were modified using ethereal solutions of (a) metal chlorides aof zinc-modified Mg electrodes. (d) Zn 2p3/2 XPS spectrum of zinc-modifiviews of zinc-modified Mg electrodes after (e) preparation and (f) single dassembled under a dry-air atmosphere using zinc-modified and untrecoulombic efficiency including the results using bismuth- and tin-modifi9758 | J. Mater. Chem. A, 2023, 11, 9755–9761substrate surface. Ex/in situ-fabricated articial magnesiophilicinterfaces can mitigate undesired side reactions betweenmagnesium negative electrodes and electrolytes and thusenhance the interfacial magnesium deposition/dissolutionkinetics.4 Among various options, BiCl3-based surface func-tionalization yields a signicant charge–discharge reversibilityof hybrid magnesium–lithium–oxygen batteries in non-aqueousmedia.4 Although the detailed electrochemistry at the modiednegative electrodes in that system remains unclear, the resultsof our study demonstrate reversible magnesium deposition/dissolution on the BiCl3-treated magnesium metal undera dry-air atmosphere (Fig. 3a). As bismuth can formmagnesiumalloys, that is, Mg3Bi2, certain hybrid metal–alloy interfaces mayfunction as a magnesium ion-conductive barrier layer,4d whichsupports reversible electrochemistry, even in the presence of O2impurities. To optimize the interfacial chemical composition,screening surveys of alloying elements were conducted byadopting a simple dipping treatment methodology (detailedmethods are described in the ESI†). As the chemical reductionof alloying element ions by magnesium metal, known asa galvanic replacement reaction, is a driving force of sponta-neous alloying reactions, alloying elements that can be reducedg[B(HFIP)4]2/G2 electrolytes measured under a dry-air atmosphere. Mgnd (b) zinc-based non-chloride compounds. (c) Prolonged CV profilesed Mg. SEM images and corresponding EDX profiles of cross-sectionalissolution processes. (g) Galvanostatic cycling profiles of [Mg‖Cu] cellsated Mg electrodes measured at 1 mA cm−2 and (h) correspondinged Mg electrodes.This journal is © The Royal Society of Chemistry 2023Fig. 4 Discharge–charge profiles of (a) [Zn-modified Mg‖Mo6S8] and(b) [Zn-modified Mg‖a-MnO2] cells assembled under a dry-air atmo-sphere. The profiles using non-modified Mg negative electrodes areincluded as reference profiles.Communication Journal of Materials Chemistry Aby magnesium metal should be adoptable. Our results showthat magnesium electrodes treated with reductive lithium andcalcium-bearing solutions do not support electrochemicalmagnesium deposition/dissolution reactions (Fig. S10†).Based on the survey, chloride compounds of elementscapable of forming alloys with magnesium will lead to thefabrication of favorable interfaces. Magnesium metals treatedwith BiCl3, SnCl2, and ZnCl2 solutions facilitate reversiblemagnesium dissolution/deposition under a dry-air atmosphere(Fig. 3a). Except for zinc-based compounds, treatments usingneither compounds of non-alloying elements paired withchloride (e.g., NiCl2) nor organometallic compounds of alloyingelements (e.g., Bi(phenyl)3 and Sn(CH3)4) resulted in inactiveinterfaces (Fig. S11†). Chloride may solely impart effectiveinterfaces, because magnesium metals treated with a typicalGrignard reagent have slightly active CV proles (Fig. S12†).These results indicate that hybrid interfaces consisting ofalloying elements and chlorides may be particularly favorablefor reversible magnesium dissolution/deposition under a dry-air atmosphere. XPS and energy-dispersive X-ray spectroscopy(EDX) spectra indicate the formation of such interfaces usingsimple soaking treatments (Fig. S13 and S14†). Scanning elec-tron microscopy (SEM) images of cycled magnesium electrodesshow that such articial interfaces facilitate electrochemicalmagnesium dissolution/deposition, because preferential reac-tion sites are located in the vicinity of alloying compounds(Fig. S15†). Among the aforementioned alloying elements, zinccan impart favorable interfaces despite the absence of chloridespecies (Fig. 3b). As chloride species can lead to undesired cellfailure due to their highly corrosive nature,13 chloride-freearticial interfaces are preferred in practical battery applica-tions.5b Specically, magnesium metals pretreated with theZn(C2H5)2 solution supported the remarkable electrochemicalmagnesium dissolution/deposition activity. Zn(C2H5)2-modi-ed magnesium retained its favorable electrochemical activityfor more than 5 h, even at an equilibrium O2 concentrationunder standard atmospheric conditions (Fig. 3c). Theseoutstanding characteristics of zinc-modied interfaces mightbe due to the O2 gas barrier properties of Zn/ZnO.14 Zn 2p3/2spectra, SEM images, and corresponding EDX proles of thecross-sectional view of pristine and cycled zinc-modiedmagnesium electrodes indicate the formation of a magnesiumion-conductive Zn/ZnO layer upon the chemical reduction ofzinc species in pretreatment solutions by magnesium, becausemagnesium dissolution/deposition was evident thorough thatlayer (Fig. 3e and f). Closed Swagelok-type cells fabricated undera dry atmosphere facilitate reversible magnesium dissolution/deposition cycling for sufficiently long periods at a currentdensity of 1 mA cm−2 (Fig. 3g). However, the reactivity graduallydiminishes, possibly due to the inactivation of progressivelygenerated, untreated magnesium surfaces upon the exposure toO2 gas during CV measurements under open beaker-type cells.The surface modication by zinc seems particularly effective,because relatively stable cycling was achieved with zinc-modied magnesium electrodes compared with bismuth- andtin-modied counterparts (Fig. 3h and S16†). The smaller latticemismatch between both hcpmagnesium and zinc metals as wellThis journal is © The Royal Society of Chemistry 2023as good compatibility of MgO and ZnO4e,15 resulted in a well-integrated interface, and are likely the reasons for the stablemagnesium dissolution/deposition cycling. Note that thetypical magnesium–zinc solid solution is electrochemicallyinactive under a dry-air atmosphere (Fig. S17†). This infers thespecic function of the chemically generated zinc-based inter-face. Full cells assembled under a dry-air atmosphere weresuccessfully cycled using Mo6S8 and a-MnO2 positive electrodesand zinc-modied magnesium negative electrodes, as shown inFig. 4, while inferior cycling performances were observed for thecells using non-modied magnesium. These results furthercorroborate the favorable O2 barrier characteristics of the zinc-based interface. Recently, a successful cycling of RMB full cellsfabricated under a dry-air atmosphere was reported.16 However,the inferior battery performance of those cells, when comparedto the performance of the cells fabricated under an Ar atmo-sphere, implies a severe detrimental effect of O2 impurity onreversible magnesium electrochemistry.ConclusionsThe systematic study of magnesium negative electrodes underdifferent atmospheric conditions reveals that O2 in electrolyteJ. Mater. Chem. A, 2023, 11, 9755–9761 | 9759Journal of Materials Chemistry A Communicationsolutions is a fatal impurity for the reversible electrochemistryof magnesium metal. The results of our analyses indicate theinsulating/barrier characteristics of the rm oxide- andhydroxide-based interface with respect to magnesium ionconduction, which might be induced by complex O2, magne-sium, and electrolyte (solvent)-related interfacial processes.Appropriate surface modication of magnesium negative elec-trodes by zinc species can be used to prevent such an undesiredsituation. Although the formation and magnesium ion trans-port mechanisms must be further studied and insights intozinc-based interfaces must be provided to maximize thosebenets, our results provide guidance for the construction ofmanufacturing systems for practical RMBs based on the diver-sion of existing LIB systems.Author contributionsT. Mandai – data curation, funding acquisition, formal analysis,investigation, project administration, validation, writing oforiginal dra (lead). M. Watanabe – data curation (supporting).Conflicts of interestThere are no conicts to declare.AcknowledgementsThe authors thank Ms Makiko Oshida, Mr Keisuke Shinoda,and Mr Kazuo Yamaguchi for their support with SEM and TEMobservations and XPS measurements at the Battery ResearchPlatform of National Institute for Materials Science. This workwas nancially supported by the Advanced Low-CarbonTechnology-Specially Promoted Research for Innovative NextGeneration Batteries Program (ALCA-SPRING, Grant NumberJPMJAL1301), NEXT Center of Innovation Program (COI-NEXT,Grant Number JPMJPF2016) of the Japan Science and Tech-nology Agency, and KAKENHI (Grant No. 21K05263) of theJapan Society for the Promotion of Science.References1 (a) L. Li, Y. Lu, Q. Zhang, S. Zhao, Z. Hu and S.-L. Chou,Small, 2019, 17, 1902767; (b) Z. Zhang, S. Dong, Z. Cui,A. Du, G. 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See DOI: https://doi.org/10.1039/d3ta01286g Oxygen tnqh_x2013 a fatal impurity for reversible magnesium deposition/dissolutionElectronic supplementary information (ESI) available: Experimental detail and supporting figures and tables are provided. See DOI: https://doi.org/10.1039/d3ta01286g Oxygen tnqh_x2013 a fatal impurity for reversible magnesium deposition/dissolutionElectronic supplementary information (ESI) available: Experimental detail and supporting figures and tables are provided. See DOI: https://doi.org/10.1039/d3ta01286g Oxygen tnqh_x2013 a fatal impurity for reversible magnesium deposition/dissolutionElectronic supplementary information (ESI) available: Experimental detail and supporting figures and tables are provided. See DOI: https://doi.org/10.1039/d3ta01286g Oxygen tnqh_x2013 a fatal impurity for reversible magnesium deposition/dissolutionElectronic supplementary information (ESI) available: Experimental detail and supporting figures and tables are provided. See DOI: https://doi.org/10.1039/d3ta01286g