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

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[Enhanced Reversibility of Mg Plating/Stripping via Solvation Sheath Regulation by a Multidentate Linear Oligoether](https://mdr.nims.go.jp/datasets/e900901c-f60f-44f9-ac37-e1c9c04817c8)

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Enhanced Reversibility of Mg Plating/Stripping via Solvation Sheath Regulation by a Multidentate Linear Oligoetherwww.batteries-supercaps.orgEnhanced Reversibility of Mg Plating/Stripping viaSolvation Sheath Regulation by a Multidentate LinearOligoetherToshihiko MandaiMagnesium (Mg) is an abundant resource, and rechargeable Mgmetal batteries (RMMBs) could help to achieve a sustainable soci-ety. However, practical Mg batteries require electrolyte materialscompatible with both positive and negative Mg metal electrodes.Weakly coordinating anion (WCA)-based electrolytes meet theserequirements and have had a groundbreaking impact on this fieldof research. In this study, the effects of multidentate oligoetheradditives on the structural characteristics of WCA-based electro-lytes are examined. Integrating a linear oligoether of hexaglyme(G6) is found to be particularly effective at enhancing Mg plating/stripping performance, whereas the corresponding cyclic coun-terparts impart inferior performance. The combined electrochem-ical and spectroscopic analyses suggest that changes in thecoordination environments of Mg2þ in solution with a specificamount of G6 are responsible for the enhanced interfacialcharge-transfer kinetics. The results of this study will help guidethe design of fully ethereal RMMB electrolytes compatible withhighly reactive Mg metal-negative electrodes.1. IntroductionResource constraints require energy storage systems based onubiquitous elements. Rechargeable Mg metal batteries (RMMBs)are a potential energy-storage technology that meets many ofour current requirements.[1] The advantageous properties of Mgmetal mean that the energy density of RMMBs could be compara-ble to that of lithium-ion batteries, but their material costs wouldbe much lower.[2] Recent intensive research efforts to developfunctional materials and advanced analytical techniques in con-junction with computational science have paved the way for a fun-damental understanding of the working principles and bottleneckissues of RMMBs.[3–6] The results have contributed significantly tothe development of electrode and electrolyte materials withimproved electrochemical performance.[7–12]Among various potential candidates, electrolytes incorporat-ing a specific aluminate-based weakly coordinated anion (WCA),[Al(HFIP)4]�, with diglyme (G2) have exceptional compatibilitywith reversible Mg plating/stripping reactions.[13,14] The tenta-tive optimum electrolyte, namely, 0.3–0.4 M Mg[Al(HFIP)4]2/G2(M; mol dm�3), demonstrated near-unity efficiency for Mg plat-ing/stripping. However, its electrochemical performance withrespect to cycle life and cycling efficiency is short of the targetvalue, especially under practical experimental conditions.Specifically, the cycling efficiency is ≈98–99% at 1 mA cm�2(Figure S1, Supporting Information), while the value reaches>99% over 250 cycles at 0.5 mA cm�2.[13] Under relatively harshexperimental conditions, inferior cycling efficiency can beascribed to side reactions during Mg plating/stripping. Theintroduction of functional artificial interfaces is a common strat-egy for mitigating undesired side reactions. A suitable Mg2þ-conductive interface has been achieved by rationally designedartificial interfaces, which effectively suppress side reactions whilefacilitate interfacial charge-transfer reaction simultaneously.[15–18]However, the construction of long-term stable artificial interfacesonmetal electrodes remains challenging because of the successivemorphological changes of the electrodes associated with repeatedplating/stripping.[18] The associated mechanical stress induced bythe inhomogeneous morphological evolution also deteriorates theinterface, resulting in a limited lifespan.[4,19]Making an electrolyte stable against Mg plating/stripping isone of the most straightforward approaches for overcominginterfacial issues. A common strategy for solvation sheath regu-lation was adopted in this study. One of the potential workingprinciples behind the improved electrochemical performanceachieved by solvation sheath regulation is the suppression of sidereactions due to the elimination of associated anions in electro-lyte solutions.[20,21] The strong electric field of divalent Mg2þ polar-izes the electronic states of surrounding species (solvents andcounter anions). This polarization induces collective shifts inthe energy levels of the ionization potential and electron affinity,which makes the solvents and anions oxidatively stable butreductively unstable.[22] Certain electrolyte systems incorporatinghighly associative BH4� are not in the case due to its excellentreductive stability even in the association with divalentMg2þ.[23,24] However, the salt association state in solutions isT. MandaiFunctional Electrolyte Synthesis TeamResearch Center for Energy and Environmental Materials (GREEN)National Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: mandai.toshihiko@nims.go.jpSupporting information for this article is available on the WWW under https://doi.org/10.1002/batt.202500348© 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbH.This is an open access article under the terms of the Creative CommonsAttribution License, which permits use, distribution and reproduction inany medium, provided the original work is properly cited.Batteries & Supercaps 2025, 8, e202500348 (1 of 8) © 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsResearch Articledoi.org/10.1002/batt.202500348http://www.batteries-supercaps.orghttps://orcid.org/0000-0002-2403-7794mailto:mandai.toshihiko@nims.go.jphttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://doi.org/10.1002/batt.202500348http://crossmark.crossref.org/dialog/?doi=10.1002%2Fbatt.202500348&domain=pdf&date_stamp=2025-09-04critical for the systems incorporating species those reductive sta-bilities spanning the electrode potential of Mg, such as[N(CF3SO2)2] and [Z(HFIP)4] (Z= B or Al).[22] Although the associa-tion ability of WCAs is considerably weak, recent experimentalstudies have suggested the presence of associated species inelectrolyte solutions, even when incorporating WCAs.[25] Suchassociated species are potentially susceptible to reduction, result-ing in inferior efficiency. The introduction of electrophilic anionreceptors such as AlCl3 and B(OCH2CF3)3 is a potential approachto facilitate salt dissociation.[26,27] The structural bulkiness of mostWCAs, however, hinders the accommodation of anion receptorsin their environments.[14] The integration of nonethereal solvatingagents such as amines and phosphates also deteriorates or inac-tivates the electrochemical performance of representative WCA-based RMMB electrolytes.[22,28]Cyclic oligoethers, such as crown ethers, are the most repre-sentative solvating (complexing) agents.[29] The addition of crownethers to conventional Mg[N(SO2CF3)2]2-based ethereal electro-lyte solutions enhances Mg plating/stripping efficiency.[30]Strong solvation by the crown ether, arising from multidentatecharacteristics, weakens unfavorable Mg2þ–anion interactions,thereby suppressing anion decomposition. It is believed thatthe same approach can be adopted for WCA-based systemsbecause the partial association of WCAs in solutions is presumedto be the reason for their insufficient performance. In this study,multidentate oligoethers were used as ethereal solvating agentsto achieve favorable WCA-based fully ethereal electrolytes. A sys-tematic electrochemical and spectroscopic study of the oli-goether structural characteristics revealed that the interfacialcharge-transfer kinetics were enhanced by the integration ofcertain multidentate linear oligoethers into the WCA-based elec-trolyte because of the change in the coordination state of Mg2þand the consequent relaxation of Mg2þ solvation by the mainethereal solvent.2. Results and DiscussionThe galvanostatic cycling profiles of the 0.3 M Mg[Al(HFIP)4]2/G2electrolytes with and without 0.1 M oligoether additives areshown in Figure 1. The molecular structure formula of the addi-tives is displayed in Scheme 1. In contrast to conventional RMMBelectrolyte solutions, the efficacy of crown ether integration intoWCA-based electrolytes is limited. The cycling efficiencies for Mgplating/stripping were improved slightly. However, cycle livesand polarization deteriorated compared with the control experi-ment using the base electrolyte, irrespective of the crown ethercavity size (i.e., 2.0 Å for 15-crown-5 ether (15C5), while 2.9 Å for18-crown-6 ether (18C6)).[29] The polarization curves in the initialcycles (Figure 1c) and the plot of overpotential at an areal capac-ity of 0.5 mAh cm�2 (Figure S2, Supporting Information) clearlyindicate the negative effect of crown ether integration, especiallyin the case of 15C5. The size of the cavity and guest ions deter-mines the binding affinity, and the cavity size of 15C5 matcheswell with Mg2þ.[29,31] This is one reason why remarkably largeoverpotentials are required for plating and stripping with15C5. A similar result was reported for Mg[N(CF3SO2)2]2-basedelectrolytes, where the introduction of 15C5 resulted in no Mgplating activity.[30] In this study, 18C6 did not improve the Mgplating/stripping performance, which is inconsistent withFigure 1. a) Galvanostatic cycling profile and b) corresponding Coulombic efficiency of asymmetric [Mg || Cu] cells using 0.3 M Mg[Al(HFIP)4]2/G2 with andwithout 0.1 M oligoether measured at 1 mA cm�2 and 30 °C. The legend in (b) indicates the Coulombic efficiency of each cell. Polarization curves duringc) the 1st and d) 10th cycles. The arrow in (d) indicates the occurrence of a short-circuit.Batteries & Supercaps 2025, 8, e202500348 (2 of 8) © 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsResearch Articledoi.org/10.1002/batt.202500348 25666223, 2025, 10, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202500348 by National Institute For, Wiley Online Library on [10/10/2025]. 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 Licensehttp://doi.org/10.1002/batt.202500348previous studies. The polarization for Mg plating/stripping gradu-ally increased with cycling, and the cycle life of the cell wasreduced due to 18C6 integration compared with that of the controlexperiment. The larger polarization at the 10th plating/strippingcycle for 18C6 compared with that for 15C5 suggests the side reac-tions of the Mg metal with the electrolyte containing the 18C6.In contrast to the above multidentate cyclic oligoethers, theintegration of the linear oligoether G6 has a distinct impact onthe Mg plating/stripping performance. The cycle life increasedby almost three times without compromising the polarizationcharacteristics. The electrolyte–Mg interface was stabilized byG6, as the polarization behavior during the 10th cycle was almostidentical to that during the 1st cycle (Figure 1c,d). Comparativeexperiments using the shorter oligoethers G3 and G4 exhibitedminor impacts on electrochemical characteristics (Figure S3,Supporting Information). In contrast, G6 imparted exceptionalelectrochemical characteristics. The Coulombic efficiency forthe reversible Mg plating/stripping reached >99% over 30 cyclesfor the G6-integrated electrolyte, even at a relatively practicalareal capacity of 1 mAh cm�2 (Figure 1b), while the values ofthe other systems were ≈98%. The integration of G6 was also par-ticularly effective at improving the long-term cycling stability(Figure S4, Supporting Information).To provide in-depth insights into the interfacial behaviors ofthe different electrolytes, electrochemical impedance spectros-copy (EIS) was performed. Nyquist plots of the [Mg || Mg] sym-metric cells were fitted using a typical equivalent circuit, andthe fitting parameters are summarized in Table S1, SupportingInformation. The oligoether additive has a dominant effect, espe-cially on interfacial charge-transfer resistance (RCT; Figure 2 andTable S1, Supporting Information). The RCT values increased sig-nificantly with the integration of cyclic oligoethers (crown ethers).An exceptionally large RCT of the 15C5-based system demon-strated the strong binding of Mg2þ by 15C5 and hinderedcharge-transfer kinetics at the interface. The large RCT was respon-sible for the remarkably large polarization observed during Mgplating/stripping using this electrolyte (Figure 1c). In stark con-trast to cyclic oligoether-integrated systems, G6 integrationfacilitates interfacial charge-transfer kinetics. A similar positiveeffect was observed for the G4-integrated system, whereas G3resulted in inferior interfacial behavior (Figure S5, SupportingInformation). The preferential coordination number of Mg2þ inethereal solutions is 5–6.[32] Systematic Raman spectroscopicanalysis combined with X-ray crystallography and density func-tional theory calculations suggested that two orthogonallyoriented solvent molecules wrapped the Mg2þ ions in the G2and G3 solutions for stabilization.[33–35] However, a single, longeroligoether molecule can coordinate with several Mg2þ ionsbecause of its conformational flexibility and multiple coordina-tion sites. Such differences in coordination abilities can explainthe different functions of G3 and longer glymes as agents forsolvation sheath regulation.The surface morphology of the cycled Mg electrodes wasresponsible for the improved cycle lifetimes of the G6-integratedelectrolyte. In RMBs, uneven Mg plating/stripping reactions fol-lowed by the intrusion of three-dimensional (3D) Mg depositsinto the porous separator are recognized as the main reasonsfor short circuits.[4,18,19,36] Scanning electron microscopy (SEM)images of Mg electrodes cycled in the base electrolyte show typi-cal morphological evolution after stripping and plating reactions(Figure 3a). A number of pores generated during the electro-chemical stripping processes were discernible, and bulk Mg crys-tals were unevenly deposited on the surface. In contrast, thesurface appeared somewhat flat when Mg was cycled in anelectrolyte containing G6 (Figure 3b). Microcrystalline depositspreferentially fill their pores, and a relatively small portion ofMg crystals is deposited adjacent to the former deposits.Eventually, the protrusion of 3D-deposited Mg is effectively miti-gated by G6 integration, and this suppressed morphological evo-lution strongly contributes to the improved cycle life of Mgplating/stripping.The Raman spectra of the electrolytes show the different sol-vation characteristics of the cyclic and linear oligoethers. Thevibrational modes of the ethylene oxide units in oligoether mol-ecules are sensitive to their solvation state.[37] While bulk oli-goether solvents have broad peaks at ≈860–800 cm�1, adistinct peak is discernible at ≈900–870 cm�1 upon complexationwith metal ions, and this is the so-called ring breathing mode. Thepeak shift of the ring-breathing mode depends on the oligoether–metal ion interactions, shifting to higher wavenumbers withScheme 1. Molecular structure formula of G6, 15C5, and 18C6.Figure 2. Nyquist plots of symmetric [Mg || Mg] cells using 0.3 MMg[Al(HFIP)4]2/G2 with and without 0.1 M oligoether measured at 30 °C.Batteries & Supercaps 2025, 8, e202500348 (3 of 8) © 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsResearch Articledoi.org/10.1002/batt.202500348 25666223, 2025, 10, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202500348 by National Institute For, Wiley Online Library on [10/10/2025]. 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 Licensehttp://doi.org/10.1002/batt.202500348increasing interactions. Indeed, monovalent Liþ–glyme complexeshave the peak at ≈870 cm�1,[38–40] while the same peak appeared at≈890 cm�1 for divalent Mg2þ–glyme complexes.[41–43]The Raman spectra of the base electrolytes with and withoutoligoether additives are shown in Figure 4a. The spectrum of neatG2 was also used as a reference. The electrolytes exhibited char-acteristic spectral profiles. The distinct peak centered at 892 cm�1observed in the base electrolyte is a fingerprint of the Mg2þ–G2complex formation. The peak located at 765 cm�1 is assignable tothe vibrational mode of the counter anion. Unfortunately, thisband was less sensitive to the coordination state of the anion.[40]However, the addition of oligoethers to the base electrolyteaffected the coordination state of Mg2þ, as evidenced by thechange in the shift and shape of the corresponding peaks. Thecoordination state of Mg2þ drastically changed upon 15C5 inte-gration because the peak intensity of the original Mg2þ–G2complex decreased, and another peak appeared at higherwavenumbers. This observation indicates ligand exchangebetween G2 and 15C5 and two different coordination species,[Mg(G2)n]2þ and [Mg(15C5)]2þ, presented in the solution. Asdescribed above, the cavity size of 15C5 matches well with thatof Mg2þ, which induces strong binding between Mg2þ and 15C5.However, for 18C6, the peak position of the fingerprint wasalmost identical to that of the base electrolyte, suggesting thatG2 and 18C6 have comparable coordination ability. This maybe explained by the size mismatch between Mg2þ and the cavityof 18C6 and the relatively rigid framework of 18C6.Although the representative peak assignable to vibrationalmodes of anion is less sensitive to its coordination state, thetransport properties clearly indicated improved dissociation ofMg[Al(HFIP)4]2 in G2 upon multidentate oligoether integration.As summarized in Table S2, Supporting Information, the ionicconductivity of the base electrolyte was greatly improved bythe addition of the multidentate oligoethers irrespective of theirstructural characteristics. As the solution viscosity does notFigure 3. Outermost surface SEM images of the Mg electrodes after cycling.Cycled in the 0.3 M Mg[Al(HFIP)4]2/G2 electrolytes a) without and b) with0.1 M G6.Figure 4. a) Raman spectra of 0.3 M Mg[Al(HFIP)4]2/G2 with and without 0.1 M oligoether at ambient temperature. The spectrum of a neat G2 solventis included as a reference. The specific spectral range responsible for Mg2þ–ether coordination and the vibrational mode of the anion is depicted.b) Deconvolution of the fingerprint modes observed in 0.3 M Mg[Al(HFIP)4]2/G2 with and without 0.1 M G6.Batteries & Supercaps 2025, 8, e202500348 (4 of 8) © 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsResearch Articledoi.org/10.1002/batt.202500348 25666223, 2025, 10, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202500348 by National Institute For, Wiley Online Library on [10/10/2025]. 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 Licensehttp://doi.org/10.1002/batt.202500348change largely with and without the multidentate oligoethers,these oligoethers contributed to decrease associated Mg2þ–anion species. Among the studied oligoether, G6 integrationimparted the highest conductivity. This observation suggests adifferent coordination manner of linear G6 against cyclic 15C5and 18C6.A different spectral feature was indeed observed for the elec-trolyte with linear G6. The peak position of the fingerprint of com-plexation shifted from 892 cm�1 for the base electrolyte to888 cm�1 for the G6-integrated electrolyte, and the peak widthwas broadened. The spectral deconvolution provided clear evi-dence of change in coordination environment of Mg2þ in theG6-integrated electrolyte. The deconvolution results are shownin Figure 4b. The deconvolution and subsequent fitting of thefingerprint of the electrolytes with and without the G6 additiverevealed the presence of two components, located at 892 and887 cm�1, in the former system. This change indicates relaxationof the Mg2þ–G2 interaction and an increase in the population ofloosely bound Mg2þ-complexes in the solution. Because the des-olvation process at the Mg–electrolyte interface is the rate-determining step for Mg plating,[44] the weakened interactionfacilitates charge-transfer kinetics at the interface. This is alsoeffective in suppressing undesired side reactions, resulting inenhanced Mg plating/stripping performance with G6 integration.Revealing the detailed mechanism of the coordination change byG6 is beyond the scope of this study and will be investigated infuture work using structural analysis combined with theoreticalcalculations.To clarify the composition–performance relationship in thepresented system, the effect of the G6 concentration was evalu-ated. The G6 concentration has a dominant impact on electro-chemical performance (Figure 5). The optimal composition forMg plating/stripping should be a base electrolyte with 0.1M G6;however, the performance decreases with increasing G6 concen-trations. The cycle lives are almost the same among the electro-lytes, with the exception of 0.1 M G6. However, the Coulombicefficiency of Mg plating/stripping degraded with the increasingG6 concentration (Figure 5b). The considerably poor electrochem-ical performance of the electrolyte comprising the same conduc-tive salt and G6, 0.3M Mg[Al(HFIP)4]2/G6, compared to that of thebase electrolyte, implies insufficient compatibility of excess G6withMg plating/stripping reactions (Figure S6, Supporting Information).In addition to the cycle life and cycling efficiency, the polarizationbehavior of Mg plating/stripping is also dependent on the concen-tration of G6. A substantially large overpotential was observed forthe electrolytes containing certain amounts of G6. However, theplots of the stable plating/stripping polarization, e.g., the voltageat an areal capacity of 0.5 mAh cm�2, as a function of G6 concen-tration, represented the concave/convex-type profiles for plating/stripping, respectively (Figure S7, Supporting Information).The interfacial resistance exhibited a similar trend (Figure S8,Supporting Information). These observations strongly suggestchanges in the coordination state of Mg2þ at specific electrolytecompositions.Assuming that the associative nature of [Al(HFIP)4]� is com-parable to that of [B(HFIP)4]�, ≈20% of the anion should beFigure 5. a) Galvanostatic cycling profiles and b) corresponding Coulombic efficiencies of asymmetric [Mg || Cu] cells using (0.3 M Mg[Al(HFIP)4]2þ x MG6)/G2 electrolytes measured at 1 mA cm�2 and 30 °C. The legend in (b) represents the average Coulombic efficiency of each cell. Polarization curves forc) the 1st and d) 10th cycles.Batteries & Supercaps 2025, 8, e202500348 (5 of 8) © 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsResearch Articledoi.org/10.1002/batt.202500348 25666223, 2025, 10, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202500348 by National Institute For, Wiley Online Library on [10/10/2025]. 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 Licensehttp://doi.org/10.1002/batt.202500348loosely bound to Mg2þ in the solution, according to the precedingwork.[23] Based on the composition of the base electrolyte, theaddition of a certain amount of G6 with greater solvation abilitythan G2 can effectively facilitate the dissociation of the remainingassociated species. Elimination of associated species wouldenhance the electrochemical Mg plating/stripping performance.However, an excess of G6 causes a coordination exchangebetween G2 and G6, which leads to the formation of a dominantamount of [Mg(G6)]2þ. The desolvation energy of [Mg(G6)]2þ isdefinitely larger than that of [Mg(G2)n]2þ because of its well-known chelating effect. This is partly why large polarization isnecessary for Mg plating/stripping in an electrolyte containing0.3 M G6. However, the further addition of more G6 caused fur-ther changes in the coordination environment of Mg2þ but in adifferent manner because of the conformational flexibility andmultidentate nature of G6. Several G6 molecules participate inthe coordination of single Mg2þ, and this can reduce the bindingpower of each G6 molecule against Mg2þ. The post-analysis onthe Raman spectra also supports this hypothesis. The spectraldeconvolution suggested the formation of three different sol-vates depending on the G6 concentration. The deconvolutionresults of the selected spectra and fraction of each deconvolutedpeak are summarized in Figure S9, Supporting Information. Asshown in Figure 4b and S9, Supporting Information, while thebase G2-based electrolyte has a single peak located at892 cm�1, the addition of a small amount of G6 resulted in theformation of the G6-solvate, which was observed at 887 cm�1in their spectra, possibly accompanied with the dissociation ofthe remaining associated species. At the G6 concentration of0.3 M, all solvates presented in the solution should be replacedinto [Mg(G6)]2þ due to the strong chelating effect of G6, andthe single peak located at 887 cm�1 has indeed been found inthe corresponding spectrum. Notably, excess G6 resulted in thedistinct peak evolution at 893 cm�1 (Figure S9, SupportingInformation). This new peak can be assigned to the above differ-ent solvate species. Crystallographic studies on binary mixtures ofLi salt and oligoethers have indeed identified the different solvateformation depending on the ratio of salt and oligoethers: isolatedcolumn-like and polymeric chain-like structures.[45] Owing to thestructural similarity, a similar coordination capability is expectedfor G6. Although further detailed structural analysis is needed toidentify the exact coordination environment of Mg2þ in our sys-tem, such a change in the coordination environment due to theratio between Mg2þ and G6 may result in a convex/concave-typecomposition–polarization relationship.Finally, the compatibility of the developed electrolyte withrepresentative positive electrodes is assessed. Discharge–chargecycling performance of the [Mg || Mo6S8] cells using the base andoptimal electrolytes is shown in Figure 6. The deliverable capaci-ties were comparable, irrespective of the electrolyte formulation.This result is understandable because the polarization character-istics of these electrolytes were almost identical (Figure S7,Supporting Information). In contrast, the reproducibility of thebattery cycling results improved remarkably with the optimalelectrolyte. The profiles of the two independent measurementsoverlapped well, particularly for charging, whereas thoseobtained using the base electrolyte exhibited inferior reproduc-ibility. Although many independent and mutually correlatedfactors contribute to the conclusive battery cycling performance,the improved interfacial characteristics induced by G6 integrationmay minimize the negative contributions, which would impart afluctuating performance, and eventually lead to greater experi-mental reproducibility.3. ConclusionThis study investigated the effects of oligoether additives on theelectrochemical Mg plating/stripping performance of representa-tive WCA-based electrolytes. A systematic survey of the molecularstructure of oligoethers revealed that the introduction of cyclicoligoethers into the base electrolyte negatively affected the elec-trochemical performance, although its effectiveness has beenreported for other systems incorporating associative anions. Incontrast, the electrochemical performance was remarkablyimproved after integrating a specific linear oligoether of G6.EIS combined with Raman spectroscopy analysis indicated thatthe interfacial charge-transfer kinetics were enhanced by G6 inte-gration owing to changes in the coordination state of Mg2þ in theelectrolytes and consequent relaxation of the Mg2þ–G2 interac-tion. The flexible chain structure and the large number of coor-dination sites in a single G6 molecule are responsible for suchFigure 6. Discharge–charge cycling profiles of [Mg || Mo6S8] cells using a) optimal and b) base electrolytes.Batteries & Supercaps 2025, 8, e202500348 (6 of 8) © 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsResearch Articledoi.org/10.1002/batt.202500348 25666223, 2025, 10, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202500348 by National Institute For, Wiley Online Library on [10/10/2025]. 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 Licensehttp://doi.org/10.1002/batt.202500348favorable coordination environments. The optimum G6-integratedfully ethereal WCA electrolyte allowed stable Mg plating/stripping cycling with improved Coulombic efficiency. This elec-trolyte also improved the reproducibility of the battery cyclingperformance.The development of prospective electrolyte materials thatsatisfy various requirements for practical RMMB materializationhas been a long-standing challenge in this field of research.Regulation of the solvation sheath by solvating nonethereal sol-vents is an emerging concept for improving the overall perfor-mance of RMMB electrolytes. However, the selective compatibilityof Mg with organic solvents and the resulting large polarization forMg plating/stripping may limit the application of these nonethe-real solvents in practical batteries.[7] This work provides anotherpathway for designing rational electrolyte materials based on fullyethereal solvents that are ultimately compatible with reactive Mgmetal-negative electrodes.4. Experimental SectionMaterialsA conductive salt consisting of the representative WCA, Mg[Al(HFIP)4]2,was synthesized according to a previously reported procedure with aslight modification to the isolation step.[13] The crude product was puri-fied by washing with anhydrous diethyl ether (FUJIFILM WakoChemicals) until the solution phase became colorless, followed by vac-uum drying at 45 °C for 2 h, yielding microcrystalline colorless prod-ucts. The chemical structures and compositions were determinedby 1H and 19F NMR spectroscopy, and the spectra were identical tothose previously reported. Anhydrous G2 and G3 were obtained fromKanto Chemical Co., Inc., and used as received. G4 was purchased fromKishida Chemical Co., Ltd., and G6 was obtained from the NARDInstitute, Ltd. Both G4 and G6 were dehydrated over 4 A molecularsieves prior to use. We obtained 15-crown-5 ether (15C5) and18-crown-6 ether (18C6) from Tokyo Chemical Industry Co., Ltd., andthey were used without further purification. The electrolyte solutionswere prepared by mixing predetermined amounts of Mg[Al(HFIP)4]2and a specific oligoether in G2, followed by vigorous stirring at30 °C overnight in an Ar-filled glovebox (<1 ppmH2O and O2). For elec-trolytes containing 15C5 or 18C6, the resulting solutions were driedover 4 A molecular sieves for several days. The water content of theprepared electrolytes was determined to be 50–100 ppm using KarlFischer titration. Cu2Mo6S8, a precursor of the positive electrode activematerial, was purchased from Kojundo Chemical Laboratory Co., Ltd.,and oxidized to obtain Mo6S8 powder following a standard oxidationprotocol.[46]Electrochemical MeasurementsGalvanostatic Mg plating/stripping measurements were conductedon asymmetric and symmetric two-electrode cells. Asymmetric cellswere fabricated using Cu foil and Mg metal. To remove native oxidefilms, Cu foil was soaked in a 0.1 M HCl aqueous solution for 1 h,rinsed with deionized water, ethanol, and acetone, and then driedunder vacuum at 80 °C overnight. For the Mg (Rikazai; 99.94%,t= 0.04 mm) electrodes, the surface was polished mechanically withsandpaper, washed several times with anhydrous tetrahydrofuran(THF), and dried under vacuum at ambient temperature for 1 h,irrespective of the cell assemblies. The pretreatment of the Mg elec-trodes and all cell assemblies was performed in an Ar-filled glovebox.Mo6S8 composite electrodes were fabricated according to apreviously reported procedure.[18] The average loading of Mo6S8was fixed at ≈2mg cm�2. A glass fiber separator (GF/A, t= 0.260mm)was used as the standard separator for all electrochemical measure-ments, unless otherwise specified. Galvanostatic and discharge–chargemeasurements were conducted using an automatic charge–dischargeinstrument (HJ0610SD8C, Hokuto-Denko Co., Ltd.). For the galvano-static Mg plating/stripping cycling measurements, a fixed current den-sity of 1mA cm�2 was applied. The battery cycling test was conductedat 0.3–1.8 V and 30 °C with a current density of 12.2mA g�1, based onthe mass of Mo6S8. The EIS spectra of the [Mg || Mg] symmetric cellswere acquired using the complex impedance method with an imped-ance analyzer (VMP3, Biologic). The current frequency of the cells wasscanned from 1 to 50mHz with a sinusoidal alternating voltage ampli-tude of a 10 mV root mean square.CharacterizationThe surfaces of the cycled Mg electrodes were observed using SEM(JSM-7800 F, JEOL). The Mg electrodes retrieved from the cycled cellswere washed with anhydrous THF to remove the residual electrolyte,dried under high vacuum at ambient temperature, placed in an air-tight chamber, and transferred for SEM analysis without air exposure.The coordination states of Mg2þ in solution were evaluated usingRaman spectroscopy. The spectra were collected using a laserRaman spectrometer (NRS-4500, JASCO) equipped with a 785-nmlaser at a resolution of 1 cm�1. The sample solutions were hermeti-cally sealed in quartz glass tubes in an Ar-filled glovebox and weresubjected to spectrometry without any exposure to moisture. Theacquired spectra were calibrated using a Si standard. The spectraldeconvolution was carried out by the Gaussian–Lorentzian function.The ionic conductivities of a series of electrolyte solutions were mea-sured using the complex impedance method with an impedanceanalyzer (VMP3, Biologic). A commercial cell equipped with two plat-inized platinum electrodes (CT-57101B, TOA DKK Corporation) wasused for the measurements. The cell was placed in a temperature-controlled chamber and held at 30 °C for 1 h to equilibrate the tem-perature prior to the measurements. The viscosities were measuredat 30 °C using a kinematic viscometer (SVM3001, Anton Paar GmbH).All of the standard deviations in the experimental values were within�3% of the average.AcknowledgementsThis work was supported in part by the GteX Program (grantnumber JPMJGX23S1) and the NEXT Center of InnovationProgram (grant number JPMJPF2016) of the Japan Science andTechnology Agency. The authors acknowledge Ms. Watanabeand Ms. Oshida for their support in the electrochemical measure-ments and SEM observations. SEM images were acquired at theBattery-PF facilities of the National Institute for Materials Science.Conflict of InterestThe author declares no conflict of interest.Data Availability StatementThe data that support the findings of this study are available fromthe corresponding author upon reasonable request.Batteries & Supercaps 2025, 8, e202500348 (7 of 8) © 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsResearch Articledoi.org/10.1002/batt.202500348 25666223, 2025, 10, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202500348 by National Institute For, Wiley Online Library on [10/10/2025]. 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 Licensehttp://doi.org/10.1002/batt.202500348Keywords: electrolytes · ether · magnesium batteries ·multidentate · salvation[1] J. A. Blázquez, R. R. Maça, O. Leonet, E. Azaceta, A. Mukherjee,Z. Zhao-Karger, Z. Li, A. Kovalevsky, A. Fernandez-Barquín, A. R. Mainar,P. Jankowski, L. Rademacher, S. Dey, S. E. Dutton, C. P. Grey, J. Drews,J. Häcker, T. Danner, A. Latz, D. Sotta, M. R. Palacin, J.-F. Martin,J. M. G. Lastra, M. Fichtner, S. Kundu, A. Kraytsberg, Y. Ein-Eli, M. Noked,D. Aurbach, Energy Environ. Sci. 2023, 16, 1964.[2] I. D. Johnson, B. J. Ingram, J. Cabana, ACS Energy Lett. 2021, 6, 1892.[3] S. Okamoto, T. Ichitsubo, T. Kawaguchi, Y. Kumagai, F. Oba, S. Yagi,K. Shimokawa, N. Goto, T. Doi, E. Matsubara, Adv. Sci. 2015, 2, 1500072.[4] M. J. Park, H. Y. Asl, A. Manthiram, ACS Energy Lett. 2020, 5, 2367.[5] N. N. Rajput, X. Qu, N. Sa, A. K. Burrell, K. A. Persson, J. Am. Chem. Soc. 2015,137, 3411.[6] X. Liu, A. Du, Z. Guo, C. Wang, X. Zhou, J. Zhao, F. Sun, S. Dong, G. Cui, Adv.Mater. 2022, 34, 2201886.[7] K. Shimokawa, T. Atsumi, N. L. Okamoto, T. Kawaguchi, S. Imashuku,K. Wagatsuma, M. Nakayama, K. Kanamura, T. Ichitsubo, Adv. Mater.2021, 33, 2007539.[8] Y. Xiu, A. Mauri, S. Dinda, Y. Pramudya, Z. Ding, T. Diemant, A. Sarkar,L. Wang, Z. Li, W. Wenzel, M. Fichtner, Z. Zhao-Karger, Angew. Chem.,Int. Ed. 2023, 62, e202212339.[9] J. T. Herb, C. A. Nist-Lund, C. B. Arnold, ACS Energy Lett. 2016, 1, 1227.[10] Z. Zhao-Karger, M. E. G. Bardaji, O. Fuhr, M. Fichtner, J. Mater. Chem. A2017, 5, 10815.[11] T. Mandai, H. Somekawa, Chem. Commun. 2020, 56, 12122.[12] Z. Song, Z. Zhang, A. Du, S. Dong, G. Li, G. Cui, Adv. Mater. 2021, 33,2100224.[13] T. Mandai, Y. Youn, Y. Tateyama, Mater. Adv. 2021, 2, 6283.[14] T. Pavčnik, M. Lozinšek, K. Pirnat, A. Vizintin, T. Mandai, D. Aurbach,R. Dominko, J. Bitenc, ACS Appl. Mater. Interfaces 2022, 13, 26766.[15] K. Tang, A. Du, S. Dong, Z. Cui, X. Liu, C. Lu, J. Zhao, X. Zhou, G. Cui, Adv.Mater. 2020, 32, 1904987.[16] Z. Meng, Z. Li, L. Wang, T. Diemant, D. Bosubabu, Y. Tang, R. Berthelot,Z. Zhao-Karger, M. Fichtner, ACS Appl. Mater. Interfaces 2021, 13,37044.[17] Z. Li, T. Diemant, Z. Meng, Y. Xiu, A. Reupert, L. Wang, M. Fichtner,Z. Zhao-Karger, ACS Appl. Mater. Interfaces 2021, 13, 33123.[18] T. Mandai, U. Tanaka, M. Watanabe, Energy Storage Mater. 2024, 67,103302.[19] J. Eaves-Rathert, K. Moyer, M. Zohair, C. L. Pint, Joule 2020, 4, 1324.[20] S. Hou, X. Ji, K. Gaskell, P. Wang, L. Wang, J. Xu, R. Sun, O. Borodin,C. Wang, Science 2021, 374, 172.[21] M. Wang, W. Sun, K. Zhang, Z. Zhang, A. Du, S. Dong, J. Zhang, J. Liu,X. Chen, Z. Zhou, F. Li, Z. Li, G. Li, G. Cui, Energy Environ. Sci. 2024,17, 630.[22] T. Mandai, M. Yao, K. Sodeyama, A. Kagatsume, Y. Tateyama, H. Imai, J.Phys. Chem. C 2023, 127, 10419.[23] R. Mohtadi, M. Matsui, T. S. Arthur, S.-J. Hwang, Angew. Chem., Int. Ed.2012, 51, 9780.[24] F. Tuerxun, K. Yamamoto, M. Hattori, T. Mandai, K. Nakanishi,A. Choudhary, Y. Tateyama, K. Sodeyama, A. Nakao, T. Uchiyama,M. Matsui, K. Tsuruta, Y. Tamenori, K. Kanamura, Y. Uchimoto, ACSAppl. Mater. Interfaces 2020, 12, 25775.[25] J. Bitenc, K. Pirnat, O. Lužanin, R. Dominko, Chem. Mater. 2024, 36, 1025.[26] H. S. Kim, T. S. Arthur, G. D. Allred, J. Zajicek, J. G. Newman,A. E. Rodnyansky, A. G. Oliver, C. Boggess, J. Muldoon, Nat. Commun.2011, 2, 427.[27] C. Chen, J. Chen, S. Tan, X. Huang, Y. Du, B. Shang, B. Qu, G. Huang,X. Zhou, J. Wang, L. Li, F. Pan, Energy Storage Mater. 2023, 59, 102792.[28] T. Mandai, ChemSusChem 2025, in press, https://doi.org/10.1002/cssc.202500418.[29] C. J. Pedersen, H. K. Frensdorff, Angew. Chem., Int. Ed. 1972, 11, 16.[30] I. Weber, J. Ingenmey, J. Schnaidt, B. Kirchner, R. J. Behm,ChemElectroChem 2021, 8, 390.[31] R. D. Shannon, Acta Cryst. 1976, A32, 751.[32] S. Tsuzuki, T. Mandai, S. Suzuki, W. Shinoda, T. Nakamura, T. Morishita,K. Ueno, S. Seki, Y. Umebayashi, K. Dokko, M. Watanabe, Phys. Chem.Chem. Phys. 2017, 19, 18262.[33] T. Kimura, K. Fujii, Y. Sato, M. Morita, N. Yoshimoto, J. Phys. Chem. C 2015,119, 18911.[34] K. Fujii, M. Sogawa, N. Yoshimoto, M. Morita, J. Phys. Chem. B 2018, 122,8712.[35] O. Tutusaus, R. Mohtadi, T. S. Arthur, F. Mizuno, E. G. Nelson,Y. V. Sevryugina, Angew. Chem., Int. Ed. 2015, 54, 7900.[36] T. Mandai, ACS Appl. Mater. Interfaces 2020, 12, 39135.[37] D. Brouillette, D. E. Irish, N. J. Taylor, G. Perron, M. Odziemkowski,J. E. Desnoyers, Phys. Chem. Chem. Phys. 2002, 4, 6063.[38] K. Ueno, R. Tatara, S. Tsuzuki, S. Saito, H. Doi, K. Yoshida, T. Mandai,M. Matsugami, Y. Umebayashi, K. Dokko, M. Watanabe, Phys. Chem.Chem. Phys. 2015, 17, 8248.[39] T. Mandai, K. Yoshida, S. Tsuzuki, R. Nozawa, H. Masu, K. Ueno, K. Dokko,M. Watanabe, J. Phys. Chem. C 2015, 119, 1523.[40] T. Mandai, H. Naya, H. Masu, J. Phys. Chem. C 2023, 127, 7987.[41] S. Terada, T. Mandai, S. Suzuki, S. Tsuzuki, K. Watanabe, Y. Kamei, K. Ueno,K. Dokko, M. Watanabe, J. Phys. Chem. C 2016, 120, 1353.[42] K. Hashimoto, S. Suzuki, M. L. Thomas, T. Mandai, S. Tsuzuki, K. Dokko,M. Watanabe, Phys. Chem. Chem. Phys. 2018, 20, 7998.[43] T. Mandai, K. Tatesaka, K. Soh, H. Masu, A. Choudhary, Y. Tateyama, R. Ise,H. Imai, T. Takeguchi, K. Kanamura, Phys. Chem. Chem. Phys. 2019, 21,12100.[44] J. Drews, P. Jankowski, J. Häcker, Z. Li, T. Danner, J. M. G. Lastra, T. Vegge,N. Wagner, K. A. Friedrich, Z. Zhao-Karger, M. Fichtner, A. Latz,ChemSusChem 2021, 14, 4820.[45] C. Zhang, S. J. Lilley, D. Ainsworth, E. Staunton, Y. G. Andreev,A. M. Z. Slawin, P. G. Bruce, Chem. Mater. 2008, 20, 4039.[46] M. D. Levi, E. Lancry, H. Gizbar, Z. Lu, E. Levi, Y. Gofer, D. Aurbach, J.Electrochem. Soc. 2004, 151, A1044.Manuscript received: May 6, 2025Revised manuscript received: June 9, 2025Version of record online: September 4, 2025Batteries & Supercaps 2025, 8, e202500348 (8 of 8) © 2025 The Author(s). Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsResearch Articledoi.org/10.1002/batt.202500348 25666223, 2025, 10, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202500348 by National Institute For, Wiley Online Library on [10/10/2025]. 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 Licensehttps://doi.org/10.1002/cssc.202500418https://doi.org/10.1002/cssc.202500418http://doi.org/10.1002/batt.202500348 Enhanced Reversibility of Mg Plating/Stripping via Solvation Sheath Regulation by a Multidentate Linear Oligoether 1. Introduction 2. Results and Discussion 3. Conclusion 4. Experimental Section Outline placeholder Materials Electrochemical Measurements Characterization