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[Toshihiko Mandai](https://orcid.org/0000-0002-2403-7794), [Antoine Barthélemy](https://orcid.org/0000-0003-1522-3146), [Hendrik Koger](https://orcid.org/0009-0001-9485-9785), Harald Scherer, [Ingo Krossing](https://orcid.org/0000-0002-7182-4387)

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[Unsuccessful Coupling of Weakly Coordinating Anion–Based Mg Salts and Alkoxyalkylamine Solvents for Rechargeable Mg Battery Application: Trace Water‐Driven Interfacial Failure](https://mdr.nims.go.jp/datasets/8f582246-9e03-4753-bb42-29f8dca36171)

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Advanced Science www.advancedscience.comRESEARCH ARTICLEUnsuccessful Coupling of Weakly Coordinating Anion–Based Mg Salts and Alkoxyalkylamine Solvents for Rechargeable Mg Battery Application: Trace Water-Driven Interfacial Failure Toshihiko Mandai1 Antoine Barthélemy2 Hendrik Koger2 Harald Scherer2 Ingo Krossing2 1 Research Center for Energy and Environmental Materials (GREEN), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan 2 Institute For Inorganic Chemistry and Analytic Chemistry as Well as Freiburg Materials Research Center FMF, University of Freiburg, Freiburg, Germany Correspondence: Toshihiko Mandai ( mandai.toshihiko@nims.go.jp) Received: 30 March 2026 Revised: 9 July 2026 Accepted: 22 July 2026 Keywords: anion | electrolyte | interface | magnesium battery | solvation ABSTRACT Rechargeable magnesium metal batteries (RMMBs) are promising candidates for large-scale energy storage owing to the advantageous properties of magnesium metal negative electrodes. Recent advances, particularly in electrolyte chemistry, have accelerated progress toward practical RMMBs. Here, we combined two emerging concepts—weakly coordinating anion (WCA)- based Mg salts and alkoxyalkylamine solvents—in electrolyte design. Unexpectedly, the combination of a representative WCA-based Mg salt, Mg[Al(Ohfip)4 ]2 , and a conventional alkoxyalkylamine, 2-methoxyethylamine (MOEA), resulted in inferior electrochemical performance, whereas MOEA favored strongly coordinating anion-based Mg salts such as Mg[TfO]2 . Comprehensive analyses revealed distinct anion-dependent solvation structures and interfacial chemistries. In Mg[Al(Ohfip)4 ]2 systems, the anion and the solvent complexes are only minimally involved in initial Mg-interface formation. Yet, the presence of trace water in MOEA induces the MOEA-mediated partial hydroxylation of the anion [Al(Ohfip)4 ]− , which leads to the formation of unstable oxygen-rich interfaces upon cycling. On the contrary, Mg2 + , [TfO]− , and MOEA form associative ion- paired complexes that jointly contribute to the formation of fluorine- and nitrogen-rich hybrid interfaces that suppress parasitic reactions and enhance performance. Despite these benefits, the associative Mg[TfO]2 –MOEA electrolyte still exhibits limitations for practical RMMB application. The complex dilemma in developing rational electrolyte materials for RMMB applications will also be discussed. 1T  t  y  a  s  t          To©Ah Introduction he global demand for electricity continues to rise exponen-ially with advancements in science and technology. In recentears, the rapid expansion of artificial intelligence has furtherccelerated electricity consumption worldwide. This trend neces-itates continuous innovation in energy storage technologieshat harness renewable power sources such as wind and solarhis is an open access article under the terms of the Creative Commons Attribution Licenriginal work is properly cited. 2026 The Author(s). Advanced Science published by Wiley-VCH GmbH dvanced Science , 2026; 13:e76897 ttps://doi.org/10.1002/advs.76897energy. According to market forecasts, global battery demandis expected to exceed 5000 GWh by 2040—far surpassing theavailable lithium capacity [ 1 ]. Therefore, the development ofrechargeable batteries based on non-lithium chemistries hasbecome increasingly imperative. Among these, rechargeablemagnesium metal batteries (RMMBs) are particularly promisingfor large-scale energy storage applications, owing to the high2.33% natural abundance of magnesium in the earth crustse, which permits use, distribution and reproduction in any medium, provided the 1 of 16http://www.advancedscience.comhttps://doi.org/10.1002/advs.76897https://orcid.org/0000-0002-2403-7794https://orcid.org/0000-0003-1522-3146https://orcid.org/0009-0001-9485-9785https://orcid.org/0000-0002-7182-4387mailto:mandai.toshihiko@nims.go.jphttp://creativecommons.org/licenses/by/4.0/https://doi.org/10.1002/advs.76897http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadvs.76897&domain=pdf&date_stamp=2026-07-29(  c  f  d  p  d  a  sT  n  r  b  s  a  a  c  g  [  O  a  R  e  b  a  a  pC  s  g  l  p  h  e  2  s  s  o  a  d  t  a  a  i  R  s  y  p  i  c  a  o  m  s  o  d  p  e  i  c                                                  2 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Creacf. Li: 0.006%) [ 2, 3 ]. The theoretical energy density of RMMBsan rival that of current Lithium Ion Batteries (LIBs), benefitingrom the two-electron chemistry enabled by the transport ofivalent Mg2 + ions. Moreover, the electrochemical and chemicalroperties of Mg metal—its low tendency to form dendritesuring electroplating and the inherent stability of bulk Mg againstmbient conditions—make it an attractive choice for developingafe, practical batteries. he development and successful application of weakly coordi-ating anion (WCA)-based electrolytes have driven significantecent progress in the field of RMMBs [ 4, 5 ]. Certain WCA-ased electrolytes have demonstrated reversible Mg plating andtripping behavior with excellent cycling efficiency, even in thebsence of conventional double-edged reagents such as [BH4 ]−nd chloride species [ 6–8 ]. A wide range of WCAs, primarilyomposed of group XIII element complexes, have been investi-ated to date, including [HCB11 H11 ]− [ 6, 9 ], [HCB11 H10 F]− [ 10 ],B(Otfe)4 ]− (Otfe = OCH2 CF3 ) [ 11, 12 ], [B(Ohfip)4 )]− (Ohfip =CH(CF3 )2 ) [ 8, 13–16 ], [B(Otfe)3 (TfO)]− (TfO = OSO2 CF3 ) [ 17 ],nd [Al(Ohfip)4 ]− [ 7, 14, 18 ]. A recent comprehensive review byiedel et al. systematically compared various WCA-based RMMBlectrolytes in terms of their electrochemical performance, sta-ility, and synthetic feasibility [ 19 ]. Among them, [B(Ohfip)4 ]−nd [Al(Ohfip)4 ]− -based electrolytes are regarded as state-of-the-rt systems owing to their well-balanced electrochemical andractical characteristics. oncurrently, extensive efforts have been devoted to exploringolvents suitable for high-energy RMMBs. Ethereal solvents areenerally considered the first choice because of their excel-ent compatibility with Mg electrodes. To further enhance theerformance of ethereal electrolyte solutions, several strategiesave been investigated, including structural modification ofther molecules to tune their physicochemical properties [ 20,1 ], incorporation of additives and co-solvents in dual-solventystems [ 22–28 ], and substitution of ethers with alternativeolvent classes [ 29 ]. However, due to the limited compatibilityf non-ethereal solvents with Mg electrodes [ 30 ], the co-solventpproach remains a feasible and promising route to meet diverseesign requirements without compromising the inherent advan-ages of ethereal electrolytes. Among non-ethereal candidates,mines and their derivatives exhibit both high reductive stabilitynd strong solvating ability. Although the beneficial effects ofncorporating amine-based solvents into conventional etherealMMB electrolytes were first reported in the 1930s [ 31 ], thistrategy has attracted renewed and growing attention in recentears [ 22, 25, 32–34 ]. Enhanced electrochemical performance,articularly at the Mg electrode interface, has been achieved byncorporating certain alkoxyalkylamines and alkyldiamines intoonventional ethereal electrolytes. Comprehensive experimentalnd computational studies have revealed the critical influencef reactive nitrogen-bound hydrogen atoms within the amineolecular structure [ 35–37 ]. The formation of MgH2 -containingpecies, originating from parasitic reactions between primaryr secondary amines and the Mg metal surface, promotes theevelopment of compositionally graded inorganic–organic inter-hases accompanied by anion decomposition. These interphasesffectively suppress further electrolyte decomposition while facil-tating interfacial charge transfer, functioning analogously toonventional solid–electrolyte interphases (SEIs) [ 22, 25, 34 ].of 16Furthermore, Mg2 + coordination by amine molecules in solutionmarkedly affects the rate-determining steps of Mg plating andstripping, including charge-transfer kinetics and the desolvationprocess [ 35 ]. Given the synergy demonstrated by recent advances in WCAsand amine co-solvents for RMMB electrolytes, the developmentof promising new electrolyte systems can be anticipated bycombining these two groundbreaking concepts. In this work, wesystematically examined the electrochemical characteristics ofWCA-based ether–amine dual-solvent electrolytes and compre-hensively investigated the influence of the paired anion specieson their performance. Diglyme (G2) and 2-methoxyethylamine(MOEA) were adopted as benchmark solvents owing to theirproven applicability in RMMB electrolytes and their favorablesolubility for the studied salts (ESI; §3) [ 14, 18, 25, 34 ]. Two anionswith distinctly different coordinating abilities—[Al(Ohfip)4 ]− and [TfO]− —representing weakly coordinating and stronglycoordinating counterions, respectively, were selected to elucidatethe role of anions in electrochemical and interfacial behaviors. Unexpectedly, no beneficial synergistic effect was observed for theWCA–amine combination; rather, the system exhibited inferiorelectrochemical performance. Note that, due to its chemicalnature as a base and a primary amine, MOEA typically containshigher amounts of hydrogen-bonded trace water that is difficultto remove or even quantify. In addition, this trace water mayact as an acid toward the MOEA base and hence may formhydroxide ions and protonated MOEA in equilibrium. Bothmay possibly influence the electrolyte performance, as indicatedby comprehensive interfacial analyses, combined with detailedinsights into the coordination states of Mg2 + in solution. Ourinvestigations revealed an MOEA-mediated trace-water-drivenhydroxylation-based failure mechanism within the WCA–aminesystem, which is particularly critical for regulating the Mgelectrode–electrolyte interface. Finally, the complex dilemmaassociated with developing next-generation electrolyte materialsfor RMMB applications is discussed with new facets. 2 Results and Discussion 2.1 Mg Plating/Stripping Performance and Morphological Evolution Based on the preliminary compositional survey with respect tothe salt solubility and primary electrochemical performance, a 1:1(v/v) mixture of G2 and MOEA was employed as a benchmarksolvent system. The chemistry behind the salt solubility and basicelectrochemical properties is detailed in the electronic supportinginformation (ESI; §3 and Figures S1–S4 ). The electrochemicalMg plating/stripping performance of the electrolytes containingMg[Al(Ohfip)4 ]2 and Mg[TfO]2 salts were evaluated by system-atic galvanostatic cycling and impedance measurements. Thegalvanostatic cycling results revealed a pronounced dependenceof electrochemical performance on the nature of the anion.Unexpectedly, an electrolyte incorporating [Al(Ohfip)4 ]− exhib-ited unstable Mg plating/stripping behavior, irrespective of thecell configuration, and these cells experienced short-circuitingat relatively early stages of cycling (Figure 1a ). A similarly poorelectrochemical performance was also observed for the electrolyteAdvanced Science, 2026tive Commons LicenseFIGURE 1 (a, b) Galvanostatic Mg plating/stripping cycling profiles of symmetric [Mg || Mg] and asymmetric [Mg || Cu] cells using G2–MOEA- based electrolytes. Polarization curves of (c) symmetric and (e) asymmetric cells. (d) Electrochemical impedance spectra of symmetric cells before cycling. (f) Coulombic efficiencies. Rate capabilities of Mg[TfO]2 /G2-MOEA electrolytes with (g) symmetric and (h) asymmetric cells and (i) corresponding Coulombic efficiencies. Double arrows and single arrows in the cycling profiles indicate microshorts and definite shorts, respectively. c  S  t  a  b  p  r  w  c  c  s  bA  t  a  a  f  a  o  a  m  t  i  o  t  c  n                           A 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Creaontaining the structurally analogous [B(Ohfip)4 ]− anion (Figure5 ), indicating the limited compatibility of dual-solvent elec-rolyte systems with G2–MOEA and fluorinated alkoxyaluminatend alkoxyborate WCA-salts. In contrast, the coordinating-anion-ased Mg[TfO]2 /G2-MOEA electrolyte displayed excellent Mglating/stripping performance in both symmetric and asymmet-ic cell configurations. These cells exhibited highly stable cyclingith minimal polarization for more than 600 h, even at a practicalurrent density of 1 mA cm− 2 (Figure 1b ). The galvanostaticycling-hold measurements further corroborate the greater statictability of the Mg[TfO]2 -based against the Mg[Al(Ohfip)4 ]2 -ased electrolyte (Figure S6 ).  closer examination of the polarization behavior revealed thathe electrolytes containing weakly coordinating [Al(Ohfip)4 ]−nd strongly coordinating [TfO]− anions exhibited similar char-cteristics, regardless of the cell configuration. The overpotentialor Mg plating and stripping stabilized at approximately 80 mVfter the initial nucleation stage. The difference in the depthf the nucleation overpotential between the Mg[Al(Ohfip)4 ]2 -nd Mg[TfO]2 -based electrolytes—more pronounced in the sym-etric cell configuration (Figure 1c )—is most likely attributableo their distinctly different interfacial impedances, as shownn Figure 1d . The smaller interfacial charge-transfer resistancebserved for the Mg[Al(Ohfip)4 ]2 -based electrolyte correspondso the shallowest nucleation overpotential. The Coulombic effi-iency for Mg plating/stripping during the first cycle was alsoearly identical between these two electrolytes (around 90%);dvanced Science, 2026however, their long-term cycling stability differed significantly.The Coulombic efficiency of the Mg[Al(Ohfip)4 ]2 /G2-MOEAelectrolyte fluctuated throughout the first 13 cycles (Figure 1f ),and micro-short-circuiting events, possibly arising from inho-mogeneous Mg plating/stripping reactions (vide infra), wereobserved. In contrast, the efficiency of the Mg[TfO]2 /G2-MOEAelectrolyte stabilized rapidly after the initial formation and acti-vation processes, approaching 99.9% in subsequent cycles. Thiselectrolyte also demonstrated outstanding Mg plating/strippingperformance even at higher current densities, up to 5 mA cm− 2 ,regardless of the cell configuration examined (Figure 1g,i ). The morphological characteristics of the deposits reflect the dif-ferences in electrochemical performance among the electrolytes.As shown in Figure S7 , crystalline granular deposits with anaverage size of less than 10 µm were formed on the substratesregardless of the electrolyte type; however, their surface mor-phology differed significantly. The magnified scanning electronmicroscopy (SEM) images revealed that, in the Mg[Al(Ohfip)4 ]2 -based electrolyte, highly crystalline angular primary particlescoalesced (Figure S7b ), whereas in the Mg[TfO]2 -based, smallerprimary crystals fused together to form rounded secondaryparticles (Figure S7d ). Subsequent powder X-ray diffraction(pXRD) analysis confirmed successful plating of hcp-Mg withhigh crystallinity and almost identical crystal orientation on thesubstrates in both electrolytes (Figure S7e ). Despite exhibitingsimilar electrochemical Mg plating behavior, the weakly coordi-nating Mg[Al(Ohfip)4 ]2 and the strongly coordinating Mg[TfO]2 3 of 16tive Commons LicenseFIGURE 2 SEM and STEM analyses of Mg metals after electrochemical stripping–plating reactions conducted in (a–g) Mg[Al(Ohfip)4 ]2 /G2-MOEA and (h–n) Mg[TfO]2 /G2-MOEA electrolytes. Surface SEM images of Mg metals after (a, h) a single stripping process and (b, i) one stripping–plating cycle at a current density of 1 mA cm− 2 and a geometrically fixed areal capacity of 1 mAh cm− 2 . Cross-sectional (c, j) SEM images and the corresponding (d, k) STEM images, (e, l) EDX mappings, (f, m) position-resolved electron diffraction patterns, and (g, n) EELS spectra of the deposits and substrates are shown. The positions of STEM observations, diffraction measurements, and EELS analyses are indicated in (c) and (j). e  p  s  t  s  i  hR  c  c  i  b  s  a  t  T  c  t  p  p  o  a  a  pp  f                          4 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Crealectrolytes showed a pronounced difference in their long-termlating/stripping performance (Figure 1a,b ). These observationstrongly suggest that interfacial stability largely depends onhe coordinating characteristics of the anions in the electrolyteolution and potentially also the capacities of the system tonteract with the trace water present, e.g., by deprotonation andydroxide formation (vide infra.). epeated stripping and plating cycles inevitably induce suc-essive morphological changes in the metal electrodes. Theharacteristic plating behavior of Mg on substrates containing pitss widely recognized as a critical issue in the field of RMMBs,ecause such localized plating results in a markedly roughurface [ 13, 38–41 ]. This roughness, in turn, severely limits thective surface utilization and increases the risk of short-circuitinghrough the intrusion of metallic Mg into porous separators.he surface morphology of Mg after a single stripping–platingycle in the Mg[Al(Ohfip)4 ]2 -based electrolyte clearly exhibitedhis characteristic feature, as shown in Figure 2a,b . The depositsreferentially accumulated around the pits formed during therevious stripping process, while granular deposits were alsobserved both inside the pits and on the unreacted surfacereas. As Mg tends to deposit preferentially on less resistivereas [ 42 ], this observation suggests spatially inhomogeneousassivation. The surface image obtained after a single stripping–lating cycle also indicated partial detachment of the depositsrom the substrate (Figure 2b ). A similar low-adhesive behaviorof 16has also been reported for Mg deposited after a single platingprocess—without a preceding stripping step—in the MOEA-freeethereal Mg[Al(Ohfip)4 ]2 /G2 electrolyte [ 40 ]. The adhesion ofthe deposits to the substrate is particularly important for long-term cycling stability, as weakly adherent deposits can easily losephysical contact with the substrate, forming electrically isolated“dead” Mg, which in turn leads to poor Coulombic efficiency andan increased risk of short-circuiting upon percolation. Fractured“dead”Mgwas directly observed in the [Mg || Mg] symmetric cellswith the Mg[Al(Ohfip)4 ]2 /G2-MOEA electrolyte during repeatedplating and stripping cycles by in situ electrochemical confocalmicroscopy (vide infra). The cross-sectional view of the depositson the substrate provided additional insights into the stripping–plating behavior. Figure 2c shows the cross-sectional SEM imageof the Mg electrode after a single stripping–plating cycle, obtainedusing a focused ion beam technique. Semicircular, granular Mgparticles were deposited on the underlying Mg substrate. Thecorresponding high-resolution scanning transmission electronmicroscopy (STEM) image, together with position-resolved elec-tron diffraction, energy-dispersive X-ray spectroscopy (EDX), andelectron energy loss spectroscopy (EELS), clearly revealed theformation of an oxygen- and fluorine-containing amorphousphase at the interface between the deposits and the substrate(Figure 2d–g ). In the case of the Mg[TfO]2 -based electrolyte, granular Mgdeposits were observed uniformly across the substrate after aAdvanced Science, 2026tive Commons Licenses  b  M  c  h  t  p  t  d  s  MV  t  p  i  e  o  w  r  m  s  i  i  b  d  m  s  t  s  a  a  o  c  i  n  g  s  t  s  s  t  e  r  a  s  v  e  T  a  c  w  r  o  r  a  s  t  c  b  f                                                   A 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Creaingle stripping–plating cycle (Figure 2i ). Such distinct depositionehavior suggests the distinct interfacial chemistries between theg[Al(Ohfip)4 ]2 - and Mg[TfO]2 -based electrolytes. The detailedross-sectional STEM image, combined with subsequent compre-ensive analyses, demonstrated favorable Mg plating behavior inhe Mg[TfO]2 -based electrolyte. Only a very limited amorphoushase was detected at the interface between the deposits andhe substrate, as confirmed by EDX, position-resolved electroniffraction, and EELS analyses (Figure 2l–n ). These observationstrongly suggest the formation of a stable interfacial layer in theg[TfO]2 -based electrolyte. isual observation is a powerful learning tool, especially inhe field of battery research. Directly monitoring the dynamiclating and stripping processes provides crucial insights intonterfacial behavior and morphological evolution across differentlectrolytes. To visualize the Mg plating/stripping behavior underperating conditions, a specially designed electrochemical cellas fabricated in a symmetric [Mg || Mg] configuration, incorpo-ating a conventional glass fiber separator. Galvanostatic cyclingeasurements were then performed under a confocal micro-cope. As shown in Videos 1 and 2 deposited with the supportingnformation, the dynamic behavior varied significantly depend-ng on the electrolyte used. In the case of the Mg[Al(Ohfip)4 ]2 -ased electrolyte (Video 1), the electrode surfaces were alreadyamaged by the electrolyte prior to electrochemical measure-ents, and fractured Mg particles had penetrated into the porouseparator. The exposed electrode surface gradually respondedo the applied current, and localized plating and subsequenttripping of Mg metal were observed at the facing Mg electrodes,ccompanied by minimal gas evolution. The snapshots capturedt specific reaction intervals further supported these dynamicbservations (Figure S8 ). The cell experienced an undesired shortircuit during the third plating/stripping cycle, as evidenced byrregular voltage fluctuations, although this event was unfortu-ately not visible within the observed field. This observation is inood agreement with the unstable electrochemical performancehown in Figure 1a . The percolation of isolated Mg particleshrough localized, uneven plating/stripping likely accelerated thehort-circuit event. In contrast, surface reactions accompanied byignificant gas evolution (bubbling) initiated immediately afterhe current was applied in the cell employing the Mg[TfO]2 -basedlectrolyte (Video 2). These observations suggest that surfaceeactions are likely accelerated by freshly exposed, thus highlyctive Mg0 in this electrolyte. Spatially homogeneous plating andtripping reactions were also evident from the operando obser-ations. Notably, plating and stripping in the Mg[TfO]2 -basedlectrolyte were always accompanied by continuous bubbling.his finding implies a strong correlation between gas evolutionnd the uniformity of Mg plating and stripping, because theells employing the Mg[Al(Ohfip)4 ]2 -based electrolytes with andithout MOEA—where little or no bubbling was observed duringepeated plating/stripping cycles—exhibited uneven utilizationf the Mg electrodes (Video 1 and Video S1 ). The critical role ofeactive amine protons in alkylamines in influencing Mg platingnd stripping has been experimentally supported in previoustudies [ 22, 23, 36 ]. The observed results indicate that eitherhe amine protons of MOEA or the potentially present Mg-oordinated trace water exhibit greater reactivity in the Mg[TfO]2 -ased electrolyte, and this enhanced reactivity promotes theormation of a functional interfacial layer. In contrast, suchdvanced Science, 2026reactivity appears to be suppressed in the Mg[Al(Ohfip)4 ]2 -basedelectrolyte. It has been reported that replacing the moderately coordinating[TFSA]− into strongly coordinating [TfO]− anions in amine-based electrolytes can weaken the H2 evolution through -NH2 reduction, and this effectively enhances the stability of the Mg-electrolyte interface [ 25 ]. The recent combined theoretical andspectroscopic studies on the Mg salt-ether-amine systems, how-ever, suggested the dominant role of free or loosely-bound aminesolvents for inducing H2 gas evolution and associated MgH2 formation to stabilize the interface [ 34, 37 ]. A competition incoordination to Mg2 + among amine, ether, and counterions seemsimportant to stabilize the interface by amine decomposition, asthe single amine solvent systems, where the dominant speciesin electrolyte solutions should be Mg-amine solvates, exhibitedsubstantially poor performance compared to the correspondingdual amine-ether solvent systems. The present study also pointsout that the combined presence of water and amine with thestrongly coordinating [TfO]− anions would drive gas evolution(vide infra). As the present and preceding works draw somewhatdifferent conclusions with respect to the mechanisms of H2 gasevolution and stabilization of the Mg-electrolyte interface, andmore importantly, water contents of respective electrolyte solu-tions are unclear, further detailed compositional and mechanisticstudies with an aid of computational approaches combined withadvanced operando characterization techniques are necessary tofully understand the interfacial chemistry in Mg-amine systems. The importance of reactive amine protons or the trace-water-based reactivity in the presence of amines—rather than merelythe interfacial composition—toward electrochemical Mg platingand stripping was also supported experimentally. Indeed, nosignificant improvement in plating/stripping performance wasobserved for the MOEA-free Mg[Al(Ohfip)4 ]2 /G2 electrolyte,even when Mg electrodes previously cycled in the Mg[TfO]2 -based electrolyte were used for the subsequent measurements(Figure S9 ). As the amine-trace water proton-driven surface reac-tion consumes active amine molecules in electrolyte solutions[ 43 ], the cycle life of the cells is dominated by the amount of elec-trolyte. To achieve the sufficiently stable cycling of Mg electrodes,a statically stable interface is necessary to be developed. 2.2 Solvation State and Interfacial Chemistry The distinct solvation environments of G2 and MOEAin Mg[Al(Ohfip)4 ]2 /G2-MOEA and Mg[TfO]2 /G2-MOEAelectrolytes were investigated by 1 H NMR spectroscopy (FigureS10 ). The amine proton ( ─NH2 ) resonance at 1.2 ppm showedonly slight upfield shift upon mixing with G2, indicatingweak G2–MOEA interactions. Downfield shifts of the ─NH2 ,methoxide, and methylene groups confirm MOEA–Mg2 + interactions that differ by salt. In the Mg[Al(Ohfip)4 ]2 -basedsystem, ether oxygen (methoxide) atoms of MOEA bind strongerto Mg2 + ( ─NH2 downfield shift: 0.6 ppm), while in Mg[TfO]2 ,the larger 1.3 ppm ─NH2 shift indicates ─N(H)- H ⋯X H-bondingpresumably involving [TfO]− oxygen atoms, confirming MOEAparticipation in Mg2 + solvation via anion interaction, consistentwith earlier MD simulations [ 34 ]. Single-solvent MOEA spectra(Figure S11 ) show analogous trends. The small G2 chemical shift5 of 16tive Commons Licensec  G  e  sI  b  S  c  s  f  N  G  W  F  t  N  [  c  bR  S  s  y  7  c  T  [  b  [  H  iH  w  e  s  s  M  c  [  l  8  M  e  t  a  C  t  i  e  a  tT  c  t  S  E  t                                                           6 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Creahanges in the Mg[Al(Ohfip)4 ]2 -based electrolyte indicate minor2 participation in Mg2 + complexation within fast exchangequilibria, consistent with the absence of non-coalescent solventignals on the time scale of NMR measurements. n the WCA-based electrolytes, Mg2 + is preferentially solvatedy solvent molecules due to the low associativity of WCAs.mall (C ─)H chemical shift changes for both G2 and MOEAonfirm dual-solvent participation in Mg2 + solvation. Withouttrong H-bonding to [Al(Ohfip)4 ]− , the 0.6 ppm ─NH2 low-ield shift identifies MOEA as the preferred Mg2 + ligand viaH2 coordination, but still being in rapid equilibrium with2. By contrast, strong Mg2 + –[TfO]− association (confirmed byalden plots from conductivity and viscosity measurements,igure S12 ) shields Mg2 + and likely excludes G2 coordination inhe Mg[TfO]2 -based electrolyte, consistent with unchanged 1 HMR signals of G2. As a known H-bond acceptor (HBA) [ 44 ],TfO]− attracts the positively polarized amine protons of Mg2 + -oordinated MOEA through H-bonding, as further investigatedy Raman spectroscopy. aman analysis of the Mg[TfO]2 /G2-MOEA electrolyte (Figure13 ) focused on the CF3 symmetric bending mode δs (CF3 ),ensitive to ion-pair formation [ 45–47 ]. Voigt deconvolutionielded two modes at 767 and 761 cm− 1 , with no signal in the57–740 cm− 1 range, confirming the absence of any “free”, non-oordinated [TfO]− (reference: 752 cm− 1 for [N(Bu)4 ]+ salt [ 48 ]).he blueshifted 767 cm− 1 mode corresponds to strongly associatedTfO]− , and the 761 cm− 1 mode—consistent with hydrogen-onded [TfO]− near 762 cm− 1 in dialkylimidazolium systems 47 ]—supports HBA action of [TfO]− and corroborates (H)N- ⋯O-S H-bonding between [TfO]− and the NH2 group of MOEAn the [Mg(MOEA)n ]2 + complex. F-DFT calculations at the RI-B3LYP(D3BJ)/def2-TZVPP levelith COSMO implicit solvation ( εr = 7.23 for G2) [ 49 ] andxtra explicit solvent molecules were performed to support thepectroscopic findings. Thermodynamic values are reported inolution ( Δr G◦ at 298 K; all deposited in §2.1 ESI). For theg[Al(Ohfip)4 ]2 /G2-MOEA system (evaluated to always exhibitoordination number 6), the global minimum is [Mg(MOEA)3 ]2 + ;Mg(G2)2 ]2 + lies + 11 kJ mol− 1 higher. With Δr G◦(solution) = –RTn K, the equilibrium constant K is 10− 1.93 , implying only 1 out of5 Mg2 + ions takes up G2 at equilibrium. For the Mg[TfO]2 /G2-OEA system, the solvation structures of which were previouslylucidated [ 25, 34 ], our calculations agree with prior work: theight ion pair [Mg(MOEA)2 (TfO)2 ] is the dominant species, andll G2-containing structures are much higher in energy (Table S2 ).onversion to the free [Mg(MOEA)3 ]2 + ion (which dominates inhe Mg[Al(Ohfip)4 ]2 /G2-MOEA electrolyte) costs ca. 170 kJ mol− 1n Gibbs energy, confirming that [TfO]− ions persist in the Mg2 +nvironment, primarily as the tight ion pair [Mg(MOEA)2 (TfO)2 ]nd, as a very minor component, H-bonded [Mg(MOEA)3 (TfO)2 ],hat is ca. 40 kJ mol− 1 higher in energy. o elucidate the impact of the solvation state on the interfacialharacteristics of the Mg[Al(Ohfip)4 ]2 and Mg[TfO]2 -based elec-rolytes, comparative studies were conducted. Figure 3 presentsEM images of Mg metal soaked in these two electrolytes for 48 h.vidently, the Mg surface was damaged due to reactions withhe alkoxyalkylamine-integrated electrolyte solutions, whereasof 16no significant change in surface morphology was observedfor Mg metal soaked in the MOEA-free Mg[Al(Ohfip)4 ]2 /G2electrolyte (Figure 3 and Figure S14 ). Subsequent X-ray pho-toelectron spectroscopy (XPS) analysis of the soaked Mg met-als, however, revealed distinct reaction mechanisms betweenthe Mg[Al(Ohfip)4 ]2 - and Mg[TfO]2 -based electrolytes. For theMg[Al(Ohfip)4 ]2 -based electrolyte, the metallic Mg surface iswell exposed, and the residual surface composition was domi-nated by oxygen-based compounds such as MgO and Mg(OH)2 [ 50 ]. The larger relative content of Mg(0) against Mg-oxygencompounds and the negligible contribution of nitrogen- andfluorine-containing species to the surface composition stronglysuggests that the native oxygen-based surface layer was removedthrough an etching effect, while the MOEA solvent and the[Al(Ohfip)4 ]− anion do not likely decompose during this etchingprocess (vide infra for its mechanism). The etching behavior ofthe Mg[Al(Ohfip)4 ]2 -based electrolyte is further supported bythe exceptionally small interfacial resistance (Figure 1d ). In con-trast, for the Mg[TfO]2 -based system, various elements includingfluorine, nitrogen, and oxygen significantly contributed to theformation of a more complex surface layer. Notably, this layerconsisted of both organic (C-NH2 and CF3 ) and inorganic (MgF2 ,Mg(NOx )2 , and Mg(OH)2 ) species [ 13, 50 ]. These observationsclearly indicate that both the MOEA solvent and the [TfO]− anionparticipated in the surface reactions. The formation of a hybridorganic–inorganic interface through preferential decompositionof the MOEA-coordinated Mg2 + complex, accompanied by [TfO]− decomposition, has also been reported in previous studies [ 25, 34 ].The resulting hybrid interface, enriched with relatively insulatingcomponents, is responsible for the substantially larger interfacialresistance observed for Mg metal in the Mg[TfO]2 /G2-MOEAelectrolyte (Figure 1d ). In addition to these conventional interfacial components, metalhydrides originating from parasitic interfacial reactions haverecently been identified as functional agents in Mg plating andstripping processes [ 22, 23, 36, 37 ]. Because hydrogen ions aredifficult to detect by XPS, time-of-flight secondary ion massspectrometry (TOF-SIMS) analysis was conducted on the sameMg metal samples used for the XPS measurements. Figure 3e,fpresents the depth profiles of TOF-SIMS spectra obtained inthe positive and negative ion modes, respectively. The positive-mode profiles indicate a minimal contribution of metal hydridesto the surface layer composition. Hydride species were detectedeven in pristine Mg metal, and their relative intensities remainedalmost unchanged across the Mg samples soaked in differentelectrolytes. Taking a closer look at the SIMS profiles, partic-ularly at the outermost surface (sputter time = 0 s), the masssignals assignable to MgH+ appear slightly higher for the samplesoaked in Mg[Al(Ohfip)4 ]2 /G2-MOEA than for those soakedin Mg[TfO]2 /G2-MOEA. However, such a minor difference isinsufficient to account for the substantially larger disparity inthe interfacial characteristics of these two electrolytes. In contrastto the positive-ion mode, the negative-ion mode profiles clearlyrevealed compositionally distinct interlayer formations on theMg surfaces soaked in the Mg[Al(Ohfip)4 ]2 - and Mg[TfO]2 -based electrolytes. The surface of the Mg metal soaked in theMg[Al(Ohfip)4 ]2 -based electrolyte was enriched with oxygen-containing compounds, whereas significant contributions fromfluorine- and nitrogen-containing species were observed in thesample derived from the Mg[TfO]2 -based electrolyte. Notably,Advanced Science, 2026tive Commons LicenseFIGURE 3 Surface SEM images of (a) pristine Mg and Mg metals soaked in (b) Mg[Al(Ohfip)4 ]2 /G2-MOEA and (c) Mg[TfO]2 /G2-MOEA. (d) Mg 2p, N 1s, O 1s, and F 1s (left to right) XPS spectra of Mg electrodes soaked in (upper) Mg[Al(Ohfip)4 ]2 /G2-MOEA and (lower) Mg[TfO]2 /G2-MOEA. Depth-profiled SIMS spectra of pristine Mg and Mg metals soaked in the two electrolytes were acquired in (e) positive and (f) negative ion modes. The samples used for these characterizations were prepared by soaking mechanically polished Mg metals in the respective electrolytes for 48 h at 30◦C. Advanced Science, 2026 7 of 16 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Licenses  f  i  (  r  f  a  i  pT  M  c  s  t  r  t  s  c  U  c  c  s  d  s  a  5  d  p  a  B  s  s  i  f  t  r  t  M  o  (  s  b  S  d  s  n2[FD  y  d  f  a  o  b  t                                                          8 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Creaubstantially larger contributions of CN− and S− were discernibleor the Mg[TfO]2 /G2-MOEA electrolyte (Figure 3f ). These find-ngs are in good agreement with the results of the XPS analysisFigure 3d ). Considering the electrochemical and analyticalesults together, it can be concluded that the nitrogen- andluorine-containing interfacial components derived from MOEAnd [TfO]− play a crucial role in stabilizing the Mg/electrolytenterface by suppressing further parasitic reactions during Mglating and stripping cycles. he distinct compositions of the Mg–electrolyte interfaces ing[Al(Ohfip)4 ]2 /G2-MOEA and Mg[TfO]2 /G2-MOEA systemsan thus be rationalized in terms of their respective solutiontates. It is well recognized, particularly in RMMB research,hat species coordinated to Mg2 + become highly susceptible toeduction because of the strong electric field exerted by Mg2 + andhe resulting large shift in the electron affinity of the coordinatingpecies [ 20, 51, 52 ]. In Mg[Al(Ohfip)4 ]2 /G2-MOEA, the theoreticalalculations suggest [Mg(MOEA)3 ]2 + to be the dominant species.pon electronation, the Löwdin population analysis of the verti-al radical cation emphasizes that the [Mg(MOEA)3 ]⋅+ ion has aoefficient directly at the Mg atom (see §2.2 ESI). As the higherpin density on Mg and the NH2 groups stabilize the electronensity through effective spin-delocalization – similar to the hightability of even free solvated electrons in solutions of alkaline orlkaline earth metals in ethylene diamine H2 N-CH2 CH2 -NH2 [ 53,4 ], a [Mg(MOEA)3 ]⋅+ radical cation yields Mg-metal presumablyirectly plated on the interface with almost no decompositionroducts upon reduction at the Mg interface. This agrees with thebsence of any nitrogen signature in the XPS analysis (Figure 3d ).ecause [Al(Ohfip)4 ]− remains outside the primary solvationheath of Mg2 + , it retains its intrinsic reductive stability inolution. Yet, if the highly polarized coordinated G2 moleculesn the [Mg(G2)2 ]2 + ion available in equilibrium accept electronsrom the reductive Mg metal surface at the interface, the electronsaken up by the system are much higher in energy, thus moreeactive and likely induce ligand-degradation. Consequently,he surface of Mg metal in contact with Mg[Al(Ohfip)4 ]2 /G2-OEA slowly and over time becomes enriched with G2-derivedxygen-containing compounds, as evidenced by the XPS spectraFigure 3d ). In contrast, [TfO]− participates strongly in theolvation of Mg2 + and the -NH2 group of MOEA is furtheround to [TfO]− via H-bonding in the Mg[TfO]2 -based electrolyte.uch a multi-compositional solvation structure promotes theecomposition of both [TfO]− and MOEA at the Mg metalurface, leading to the formation of the observed fluorine- anditrogen-rich functional interface. .3 MOEA-Mediated Partial Hydroxylation of Al(Ohfip)4 ]− and Understanding of Dynamic ailure Mechanism in Mg[Al(Ohfip)4 ]2 /G2-MOEA uring the NMR investigations, a peculiar water-induced hydrox-lation process of the [Al(Ohfip)4 ]− ion was detected. Note that,espite adopting an intensive dehydration protocol to MOEAor this study – i.e., by distillation over CaH2 and storage withctivated molecular sieves for several days – the water content inur MOEA solvent batches was indirectly determined to spreadetween several hundred to several thousand ppm based onhe NMR analysis, where all products formed were assumed toof 16originate from the reaction of [Al(Ohfip)4 ]− with water remnantsin MOEA. To further support this point, we have investigated thewater content in the MOEA batch used by a titration protocol inthe presence of benzoic acid, that is reported bind all the basicMOEA amine, and only leaves the less basic water unaffectedand to be determined [ 55 ]. This protocol gave a water contentof 156 ppm in MOEA. As the MOEA was mixed with G2 in 1:1volume ratio in our electrolyte formulations, the water content inMg[Al(Ohfip)4 ]2 /G2-MOEA is assumed to be less than 150 ppm.However, these values are not very trustworthy in our opinion, asthey depend much on the titration criteria applied (details of thetitration protocol and criteria are given in the ESI). By contrast,water contamination of the Mg[Al(Ohfip)4 ]2 /G2 electrolyte doesnot significantly affect Mg plating/stripping performance andthe [Al(Ohfip)4 ]− anion tolerates even 1000 ppm water contentwith no visible byproduct as shown by NMR [ 18 ]. Since thiswork showed the somewhat moist Mg[Al(Ohfip)4 ]2 /G2-MOEAelectrolyte to support Mg plating and stripping (Figures 1 and 2 ),its water content seems to lie still below the direct hydrolysisthreshold of the [Al(Ohfip)4 ]− anion [ 18, 27 ]. Therefore, MOEAapparently plays a crucial role in enabling the hydroxylation of[Al(Ohfip)4 ]− . With this background, the stability of the [Al(Ohfip)4 ]− ion inthe presence of water and amine was investigated in solution.As shown in Figure 4a , Mg[Al(Ohfip)4 ]2 in G2 exhibits a char-acteristic septet 1 H resonance at 4.5 ppm, which is assignable tothe α-proton of the O-C( H )(CF3 )2 -group. However, an additionaldownfield resonance appears in the G2–MOEA solution that isassigned to a reaction product with [Al(Ohfip)4 ]− . The corre-sponding 19 F NMR spectrum confirms the formation of anotherOhfip-compound and the sharp 27 Al resonance arising fromthe tetracoordinated environment (AlCN4 ) in Mg[Al(Ohfip)4 ]2 /G2is now flanked by an additional broader signal at approxi-mately 30 ppm in the presence of MOEA/trace water. The shiftposition at 30 ppm agrees with a penta-coordinated structure[ 56 ]. For example, the trigonal bipyramidal aluminate anion[F2 Al(Ohfip)3 ]2 − , has a distinct 27 Al NMR signal at 33.7 ppm[ 57 ] (replace fluoride by isosteric hydroxide). Also a dimeric[(hfipO)3 Al-( µ-F)2 -Al(Ohfip)3 ]2 − is known [ 57 ], unfortunatelywithout NMR shift. If one again replaces F for OH, this could sug-gest dimerization of an initial tetracoordinate [Al(Ohfip)3 (OH)]− (cf. the known [ 58 ], bulkier [Al(OC(CF3 )3 )3 (OH)]− ) to give thepentacoordinate (AlCN5 ) [(hfipO)3 Al-( µ-OH)2 -Al(Ohfip)3 ]2 − dian-ion. From diffusion-ordered NMR spectroscopy measurements(data not shown), the formation of a large complex anion,diffusing slower than monomeric [Al(Ohfip)4 ]− , agrees with theassignment of dimeric [(hfipO)3 Al-( µ-OH)2 -Al(Ohfip)3 ]2 − , (as itsMg2 + complex?), as the pentacoordinate aluminate in solution.Also partial hydrolysis of the parent Mg[Al(Ohfip)4 ]2 ∙3G1 adductsis known to yield a binuclear aluminate anion [ 59 ]. In addition,solvent coordination may induce formation of penta-coordinatedAl species [ 60 ]. The relative intensities of the reaction products – the Ohfip-compound and the penta-coordinate Al species – are higher inthe single MOEA-solvent system than in the mixed G2–MOEAsolvent system (Figure 4 ), confirming that the reaction with[Al(Ohfip)4 ]− is significantly promoted by MOEA. Nevertheless,quantitative analysis of the relevant signal areas indicates thatonly about 20% of [Al(Ohfip)4 ]− undergoes reaction. Even anAdvanced Science, 2026tive Commons LicenseFIGURE 4 1 H, 19 F, and 27 Al NMR (25◦C; either 400.17, 376.54, and 104.27 MHz or 300.18, 282.45, and 78.22 MHz, respectively) spectra of Mg[Al(Ohfip)4 ]2 -based electrolytes. Selected spectral regions corresponding to [Al(Ohfip)4 ]− are highlighted. AlCN4 and AlCN5 denote Al species with a coordination number of 4 and 5, respectively. The intensive broad resonance between 0 and 100 ppm in the 27 Al-NMR spectrum is a background signal from the probehead, whereas the small broad shoulder at 30 ppm is a signal from the sample. i  w  i  u  M  i  e  p  s  r  aF  (  s  H  T  H  MT  r  o  a  p  a  w  h  A                 A 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Crententionally added far excess of water (ca. 0.5 mol dm− 3 !),hich would have been enough to react with all [Al(Ohfip)4 ]−n the electrolyte, shows that only about 20% of [Al(Ohfip)4 ]−ndergoes reaction (Figure S15 ) in the Mg[Al(Ohfip)4 ]2 /G2-OEA solutions. Moreover, the reaction reaches completionmmediately after sample preparation, as aging has no discernibleffect on the relative composition of [Al(Ohfip)4 ]− and its reactionroducts (cf. identical NMR spectra of as-prepared and agedamples; Figure S16 ). These observations suggest a peculiareaction mechanism driven by the water-contaminated MOEAnd [Al(Ohfip)4 ]− . urther Fourier transform attenuated total reflection infraredFT-ATR-IR) spectroscopy measurements of the electrolytehown in Figure S17 verify a collective large redshift of the N- vibrational modes in the Mg[Al(Ohfip)4 ]2 /G2-MOEA system.his strongly suggests protonation of MOEA resulting in strong-bonding visible by IR. Hence, also protonated MOEA ([H-OEA]+ ) appears to be relevant as part of this reaction. o identify the underlying cause of the undesired water-inducedeaction of [Al(Ohfip)4 ]− , the Gibbs free reaction energies ( Δr G◦)f possible reaction products were DFT calculated in G2 solutions described above [ 49 ]. Since MOEA is chemically a base (cf. Ka of its protonated form in water: 9.40 [ 61 ]), it may react in ancid-base equilibrium with water to yield hydroxide (reaction (1),ith hydration of the cation being considered). The nucleophilicydroxide ion formed in (1) could then attack the electrophilicl-atom of the [Al(Ohfip)4 ]− anion (2): MOEA (solv . ) + 2H2 O(solv . ) ⇄ [H2 O ⋯ H −MOEA ]+ (solv . ) +HO − (solv . ) Δ𝑟 𝐺◦(1) = + 6 kJ mol − 1 (1)HO − (solv . ) + [Al (Ohf ip ) 4 ] − (solv . ) → [Al (Ohf ip ) 4 OH ] 2 − (solv . ) Δ𝑟 𝐺◦(2) = − 62 kJ mol − 1 (2)dvanced Science, 2026Δr G◦(1) implies equilibrium, and at standard conditions onlyone out of 10 molecules on the left reacts to the right. Thesolvated hydroxide ions thereby formed, immediately react withthe [Al(Ohfip)4 ]− anion according to (2) and give the dianion[Al(Ohfip)4 OH]2 − . Yet, this structure is not stable and easily losessolvated [Ohfip]− according to (3) as the more stable anion (cf.p Ka values: HO-hfip is the stronger acid than water [ 62 ]). [Al (Ohf ip ) 4 OH ] 2 − (solv . ) → [Al (Ohf ip ) 3 OH] − (solv . ) +[Ohf ip ] − (solv . )Δ𝑟 𝐺◦(3) = −46 kJ mo l− 1 (3)With 0.3 m solvated Mg2 + ions in the liquid electrolyte close by,the solvated [Ohfip]− anion will immediately undergo ion-pairingas in (4) to give the known [ 63 ] compound Mg(Ohfip)2 : [Mg (MOEA ) 3 ] 2 + (solv . ) + 2 [Ohf ip] − (solv . ) → [Mg (MOEA ) 2 (Ohf ip ) 2 ] (solv . ) +MOEA Δ𝑟 𝐺◦(4) = − 269 kJ mo l− 1 (4)The solvated [Al(Ohfip)3 OH]− ion formed in Equation ( 3 ) willlikely be in equilibrium with dimeric forms (5), e.g., mediated byion-pairing with the Mg2 + ion in (6): [Al (Ohf ip ) 3 OH ] − (solv . ) ⇄ [(hf ipO ) 3 Al ( 𝜇 − OH ) 2 Al (Ohf ip ) 3 ] 2 − (solv . ) Δ𝑟 𝐺◦(5) = + 44 kJ mo l− 1 (5)[(hf ipO ) 3 Al ( 𝜇 − OH ) 2 Al (Ohf ip ) 3 ] 2 − (solv . ) +[Mg (MOEA ) 3 ] 2 + (solv . ) → [(hf ipO ) 6 Al2 (OH ) 2 Mg (MOEA )] (solv . ) + 2MOE A(solv . ) Δ𝑟 𝐺◦(6) = − 127 kJ mo l− 1 (6)9 of 16ative Commons LicenseSCHEME 1 Top, solution part: the four ions shown to be relevant for the solution chemistry; all others were too high in energy and hence not present in equilibrium. Bottom: Reaction at the Mg oxide-hydroxide interphase. The rather reactive [Mg(MOEA)3 (OH2 )]2 + ion contains an (acidified, positively polarized) water molecule that may protonate a surface Mg-OH and thereby induce dissolution and etching of surface Mg(OH)2 in the subsequent reaction sequence. The square brackets for the two middle species at the interphase indicate short-lived intermediates. Note that the entire scheme is stoichiometrically balanced. T  t  O  n  M  a  MN  s  p  w  K  I  0  2  W  s i  d  t  a  w  i  t  i  hT  t  i  a  w  c  t  M  e                                  1 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Creahermodynamically, the very exergonic reaction (6) will removehe dianion from the left side of the endergonic reaction (5).verall, the sequence (1) to (6) implies that the pentacoordi-ate aluminate (AlCN5 ) observed by NMR-spectroscopy is theg2 + complex of the dianion [(hfipO)3 Al( µ-OH)2 Al(Ohfip)3 ]2 −nd the second Ohfip-containing compound is the long-knowng(Ohfip)2 . ow we need to address the question as to why the hydroxylationtops at about 20% conversion of [Al(Ohfip)4 ]− to the Mg2 + com-lex of [(hfipO)3 Al( µ-OH)2 Al(Ohfip)3 ]2 − . Hence, if we start in (1)ith 2280 ppm water ( = 0.12 mol dm− 3 ), this should generate with (1) = 0.1 only a small amount of hydroxide (0.006 mol dm− 3 ).n a 0.3 mol dm− 3 electrolyte, the [Al(Ohfip)4 ]− concentration is.6 mol dm− 3 . Experimentally, we have seen the hydroxylation of0% of [Al(Ohfip)4 ]− , that would correspond to 0.12 mol dm− 3 .ith the exergonic reactions (2) ( − 62 kJ mol− 1 ) and the [Ohfip]−cavenging by Mg2 + in (4) ( − 269 kJ mol− 1 ), the solvated HO− (solv.)s withdrawn from (1) and yields the hydroxylated aluminateianion. Apparently, and from the experiments described above,he formation of the hydroxylated aluminate dianion levels offt 0.12 mol dm− 3 concentration. This may occur, when either allater is consumed or the reaction levels off due to the strongncrease of the concentration of the cation [H2 O ∙∙∙H-MEOA]+hat has to form concomitantly with hydroxide in (1) and whichncreases the chemical potential in the system and stops furtherydroxylation. he analysis of the hydroxylation of the [Al(Ohfip)4 ]− anion inhe G2-MOEA system revealed that hydroxide ions are presentn the electrolyte system. A broad evaluation of the structurend energetics of solvated [Mg(OH)]+ or [Mg(OH)2 ] speciesas performed, that also did include water complexes of Mgompounds. Out of the investigation, the four ions shown onhe top of Scheme 1 below were found to be relevant in theg[Al(Ohfip)4 ]2 /MOEA solution; all others were too high innergy and hence not present in equilibrium. In the subsequent0 of 16reactions, shown below at the Mg oxide-hydroxide interphase,now a surface Mg(OH)2 may be solubilized by one reactive[Mg(MOEA)3 (OH2 )]2 + ion in the form of two solvated [Mg(OH)]+ complexes via the ammonium-assisted solubilization mechanism[ 64 ] – one emerging from the solution and one emerging fromthe surface-species. Hence, the high available concentrationof [Mg(MOEA)3 ]2 + ions quickly removes the surface Mg(OH)2 interphase as observed in the experiment. A substantial stabilityof amines against an electron also renders absence of MOEAdecomposition during etching process very likely (cf. Scheme S5 )[ 54, 65, 66 ]. The main difference between the [Al(Ohfip)4 ]− and the [TfO]− electrolytes is the majority species in solution. While in the[Al(Ohfip)4 ]− -based, the reactive [Mg(MOEA)3 ]2 + ion is themajority species, in the [TfO]− electrolyte the ion-paired and H-bonded solvated [Mg(TfO)2 ] is the majority ion. As mentionedearlier, removal of the coordinated [TfO]− ions to give the[Mg(MOEA)3 ]2 + ion as found in the [Al(Ohfip)4 ]− electrolyte,is a highly endergonic process. Hence, one expects the solvated[Mg(TfO)2 ] to be too unreactive, to activate and to etch theMg oxide-hydroxide interphase. Moreover, [TfO]− is well knownfor its stability against hydrolysis [ 67 ]. The 19 F NMR spectra ofthe Mg[TfO]2 -based electrolytes clearly exclude the possibility of[TfO]− decomposition induced by trace water (Figure S18 ). Allinvestigations into the structure and energetics of this process,the formation of water or hydroxide complexes, gave high energyspecies with the exception of one water adduct, but absolutely nohydroxide complex. These are at least 128 kJ mol− 1 endergonicand not available for any surface modification. Overall, the Mg-coordination of the [TfO]− ion in the electrolytes lowers thereactivity so strongly that it can no longer serve to clean thesurface. For full details, see the deposited sequence of reactionsassessed in the §2.3 ESI. The presence of an appreciable amountof hydroxide ions in moist amine solvents also accounts forthe relatively inferior Mg plating/stripping performance of theMg[TFSA]2 -based amine-integrated systems compared to theAdvanced Science, 2026tive Commons LicenseM  uO  i  d  n  t  s  M  y  i  m  t  a  e  f  p  s  s  A  p  M  a  b  g  s  s  o  t  a  a  M  t  a  e  i  d  c  c  M  w  b  b  1 o  S  c  iI  c  i  d  f  c  b  i  c  l  c                                                       A 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Creag[TfO]2 -based [ 22, 25 ], due to the limited stability of [TFSA]−nder highly basic conditions [ 68 ]. verall, the dynamic failure mechanism of the WCA-systems most likely driven by the MOEA-mediated and trace-water-riven hydroxylation of [Al(Ohfip)4 ]− , followed by inhomoge-eous passivation and eventual localized utilization. Observa-ion of negligible signals of fluorine- and nitrogen-containingpecies in the XPS spectra of the Mg metal soaked in theg[Al(Ohfip)4 ]2 /G2-MOEA (Figure 3d ) implies that the hydrox-lation reaction byproducts, the Mg2 + complex with the dian-on [(hfipO)6 Al2 (OH)2 Mg(MOEA)] and [Mg(MOEA)2 (Ohfip)2 ],ight be stable against initial, etched Mg metal. The elec-rochemical processes, however, promote decomposition of theccumulated byproducts on fresh Mg surfaces exposed throughlectrochemistry, eventually leading to the resistive interlayerormation. The suggestion of electrochemistry-promoted decom-osition of byproducts is in good agreement with the XPSpectra of the cycled Mg electrodes, where the surface compo-ition was rich in nitrogen and fluorine species (Figure S19 ). relatively thick layer of highly amorphous decompositionroducts for the cycled Mg electrodes in Mg[Al(Ohfip)4 ]2 /G2-OEA also supports the instability of the byproducts against fresh Mg surface (Figure 2c–g ). Moreover, as the stripping ofulk metal preferentially takes place at the dislocation and/orrain boundary [ 69–71 ], the resulting resistive layer would bepatially inhomogeneous. The spatial inhomogeneity in theurface resistance will cause the uneven, localized utilizationf Mg metal, ultimately leading to short-circuiting events athe early stage of cycling. These sequential interfacial processesre likely responsible to the complicated surface chemistrynd particularly inferior electrochemical performance in theg[Al(Ohfip)4 ]2 /G2-MOEA electrolyte (Figure 1a ). Note thathe equilibrium products between MOEA and H2 O initiate thebove dynamic failure in composition, interface, and consequentlectrochemistry. As the dynamic desolvation process at thenterface does not directly correlate to the above interfacialecomposition processes, current densities for the galvanostaticycling measurements have a minor impact on the long-termycling stability (Figure S20 ). On the other hand, the failure ofg plating/stripping in Mg[B(Ohfip)4 ]2 /G2-MOEA (Figure S5 )ould likely be induced by different mechanisms as the sp 3oron center is not capable to accept additional coordinationy OH− due to its elemental characteristics. The 1 H, 11 B, and9 F NMR spectra indeed corroborated the substantial stabilityf [B(Ohfip)4 ]− even in the presence of hydroxide ions (Figure21 ). As [B(Ohfip)4 ]− gradually undergoes hydrolysis by waterontamination [ 72 ], the decomposition of [B(Ohfip)4 ]− causesmpedance growth at a later stage and a short eventually. t should be emphasized here that the above scenario, espe-ially in the Mg[Al(Ohfip)4 ]2 /G2-MOEA electrolyte, are evidentlynduced by water-contaminated MOEA; hence, a completelyifferent picture in solution and at the interface can be anticipatedor the water-free system. Nevertheless, due to the intrinsicharacteristics of primary and secondary amines to form H-onding with water molecules, complete dehydration of MOEAs unfeasible in practical as shown also by our rigorous dryingonditions that could not remove the trace water to the desiredow ppm level. Moreover, water contamination from other batteryomponents, such as composite positive electrodes, separators,dvanced Science, 2026negative electrodes, tub, and casing, is inevitable in practicalbattery fabrication from a process perspective. Furthermore, side-reactions during battery operation can also potentially result inthe generation of water molecules in the end [ 73, 74 ]. Sincehydroxylation of [Al(Ohfip)4 ]− is thermodynamically favorable,the following fluctuation of the Mg-electrolyte interface isalso unavoidable once the electrolyte gets water contaminationthrough various undesired events. Therefore, although investi-gation of the water-free systems will give distinct and valuableinsights into the compositional and interfacial chemistries, elec-trolytes based on hydroxide-accepting WCAs with amine solventsare somehow inappropriate to practical battery applications andmay reveal surprises with the capacity to donate hydroxide ionsalong the acid-base equilibrium. Hence, such failure mechanism– reported here for the first time for battery electrolytes – need tobe considered in RMMB and all other battery systems, given thatBrønsted basic solvents with amine functions are used. 2.4 Full-Cell Performance and Complex Dilemma As demonstrated above, no synergistic effect was observedfrom combining the two concepts—WCA-based Mg salts andalkoxyalkylamines—in RMMB electrolytes. Alkoxyalkylaminespreferentially coordinate with coordinating and H-bondinganion-based Mg salts such as Mg[TfO]2 , leading to excel-lent electrochemical Mg plating/stripping performance in theMg[TfO]2 /G2-MOEA electrolyte. Although these outstandingcharacteristics appear promising for practical applications, thereality is less favorable because this electrolyte exhibits seri-ous drawbacks at the positive electrode side. Preliminarydischarge–charge tests employing Chevrel phase Mo6 S8 weresuccessful while those employing high-voltage layered ironvanadate FeV3 O9 ∙1.1H2 O (FeVO) failed to exhibit reversiblecycling with the Mg[TfO]2 /G2-MOEA electrolyte (Figure 5 ). Incontrast, the MOEA-free simple ethereal electrolytes based onMg[Al(Ohfip)4 ]2 exhibit good compatibility even with FeVOelectrodes. The linear sweep voltammograms clearly revealthe inferior anodic stability of amine-containing electrolytes,regardless of the conductive salt used (Figure S22 ). A noticeablecurrent increase due to electrolyte oxidation was observed atapproximately 2.0 V vs. Mg, which is out of the voltage windowsfor operating high-voltage positive electrode materials. Moreover,[TfO]− is known to be highly corrosive toward Al current collec-tors, even at relatively low potentials [ 75 ]. This characteristic wasalso confirmed in the present study through chronoamperometricmeasurements and subsequent SEM observations (Figure S22 ).The intrinsically poor oxidation stability of amine solvents andstrong corrosiveness of [TfO]− anions jointly contributed tothe failure of cycling with FeVO. These background knowl-edge of the intrinsic physicochemical characteristics of everyMg[TfO]2 -amine system are particularly important in designinghigh-performance electrolytes that are compatible with bothMg negative and high-voltage positive electrodes for the futuredevelopment. Considering all experimental and calculational results collec-tively, the relationship between compositional and interfacialchemistry is summarized in Figure 6 . The mechanistic inves-tigation based on NMR spectroscopy, combined with quantum11 of 16tive Commons LicenseFIGURE 5 Discharge-charge profiles of (a, b) [Mg || Mo6 S8 ] and (d, e) [Mg || FeVO] cells using (a, d) Mg[TfO]2 /G2-MOEA and (b, e) Mg[Al(Ohfip)4 ]2 /G2 as electrolytes. Discharge capacity retention of (c) [Mg || Mo6 S8 ] and (f) [Mg || FeVO] cells. FIGURE 6 Schematic illustration of the relationship between compositional and interfacial chemistries in (a) Mg[Al(Ohfip)4 ]2 /G2-MOEA and (b) Mg[TfO]2 /G2-MOEA electrolytes. 12 of 16 Advanced Science, 2026 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Licensec  M  [  a  f  a  h  j  w  t  t  i  t  M  a  o  M  t  iw  c  r  M  s  i  T  a  e  p  g  c3T  m  l  b  (  t  c  c  e  r  M  i  a  I  r  i  p  a  f  s  i  a  w  s  p  o                                                     A 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Creahemical calculations and vibrational analysis, revealed theOEA-mediated and trace-water-driven partial hydroxylation ofAl(Ohfip)4 ]− (Figure 6a ). The rather reactive [Mg(MOEA)3 ]2 +nd [Mg(MOEA)3 (OH2 )]2 + ions present would dissolve the sur-ace Mg(OH)2 via the ammonium-assisted solubilization mech-nism [ 64 ]. Consumption of water impurity by the partialydroxylation and [Mg(MOEA)3 ]2 + -mediated etching effect mayointly contribute to forming less insulative oxygen-rich interface,hich likely accounts for the exceptionally low interfacial resis-ance (Figure 1d ). However, the hydroxylation of the anion andhe resulting complicated cation-anion complex formation wouldn turn result at later stages in an unstable interface for the elec-rochemical Mg plating/stripping reactions (Figure 1a ). For theg[TfO]2 -based electrolyte, Mg2 + is coordinated by the stronglyssociative [TfO]− ion and MOEA in the solutions. The reactivityf trace water molecules is suppressed through H-bonding withOEA. The strongly solvated [TfO]− and MOEA species inhe primary solvation sheath are prone to reduction at thenterface. The mutual cation-anion, solvent–anion, and solvent–ater interactions and the resulting functional interphase mayollectively contribute to the stabilization of the interface andemarkable electrochemical performance for the Mg[TfO]2 /G2-OEA electrolyte (Figure 1b ). Unfortunately, its limited anodictability and strong corrosiveness toward the Al current collectorn turn hinder its application in practical RMMBs (Figure 6b ).herefore, the development of electrolyte systems still facesn unresolved fundamental dilemma: materials that providexcellent compatibility at one electrode often compromise theerformance at the other. Rational, multilateral design strate-ies are urgently needed to develop high-performance RMMBsapable of overcoming these complex trade-offs.  Conclusions he development of electrolyte materials that meet the require-ents of both negative and positive electrode materials remains aong-standing challenge in RMMB research. Two recent ground-reaking concepts—the adoption of weakly coordinating anionsWCAs) and the use of alkoxyalkylamine-based solvents in elec-rolyte formulations—have individually demonstrated excellentompatibility, particularly with Mg negative electrodes. However,ombining these two approaches has led to unexpectedly poorlectrochemical performance. The electrolyte formulation withather associative anions, in turn, exhibits remarkably favorableg plating/stripping performance. Comprehensive electrochem-cal, compositional, physical, spectroscopic, and computationalnalyses revealed a counter-anion-dependent solvation structure.n the Mg[Al(Ohfip)4 ]2 /G2-MOEA system, Mg2 + is primarily sur-ounded solely by chelating MOEA solvent molecules, whereasn the Mg[TfO]2 -based counterpart, both [TfO]− and MOEAarticipate in coordination of Mg2 + , and MOEA moleculesre also bound to coordinated [TfO]− via H-bonding, therebyorming a multi-compositional solvation sheath. Such distinctolvation environments are likely responsible for the observednterfacial compositions and consequent electrochemical char-cteristics. Moreover, detailed spectroscopic analyses combinedith quantum chemical calculations revealed an unprecedentedusceptibility of [Al(Ohfip)4 ]− to trace amounts of water in theresence of MOEA. The MOEA-mediated partial hydroxylationf [Al(Ohfip)4 ]− combined with a peculiar etching effect effec-dvanced Science, 2026tively scavenges detrimental trace water and removes insulativeMg(OH)2 from the initial Mg-interface, thereby leading to theformation of a less-insulative interphase. Yet, the hydroxylationalso results in the formation of large cation-anion aggregatecomplexes, which in turn causes undesirable electrochemicalperformance. In contrast, the hydrophobic and highly associativenature of Mg[TfO]2 prevents water molecules from approachingthe inner solvation sheath. The passivation of Mg metal by tracewater is likely suppressed through the formation of a fluorine-and nitrogen-rich functional interphase derived from [TfO]− and MOEA, as well as by the restriction of water reactivity viaH-bonding with MOEA and [TfO]− . Despite the excellent electrochemical Mg plating/strippingbehavior of the Mg[TfO]2 /G2-MOEA electrolyte, its limitedanodic stability and strong corrosiveness toward the Al currentcollector hinder its application in practical RMMBs. Regardingcompatibility with positive electrodes, conventional etherealelectrolytes based on WCAs exhibit clear advantages over [TfO]− –amine systems, although their Mg plating/stripping performanceremains insufficient for practical battery implementation. Withrespect to compositional engineering, regulating the interfacialstability of the WCA-salt ether systems by rational halide-freeadditives would be the most straightforward approaches to satisfythe requirements from both positive and negative electrodesides. Ishfaq et al., recently demonstrated significantly enhancedperformance of the benchmarking Mg[Al(Ohfip)4 ]2 /G2 elec-trolyte upon the integration of a well-controlled amount offluorinated cyclic ether [ 76 ]. Our group also developed the mixed-coordination electrolyte combined with a fluorinated molecularadditive which promotes anion- and additive-derived favorableinterphase formations and enhances cycling stability in high-voltage RMBs [ 77 ]. As the integration of large quantity ofnon-coordinating diluents like fluorinated ether in the etherealelectrolytes is detrimental for Mg plating/stripping performance[ 20, 21 ], the dosage needs to be adjusted appropriately. In additionto the above compositional approaches, Mg2 + -conductive alloy-derived artif icial interphases have attracted significant attentionowing to their favorable characteristics [ 41, 78, 79 ]. Certainartificial interphase effectively regulates Mg2 + -ion flux at the elec-trode surface and promote interfacial kinetics, thereby improvingoverall battery performance. Since the development of elec-trolyte materials through the adoption of non-ethereal solventsremains challenging due to the limited compatibility of highlyreductive Mg metal with organic compounds [ 30 ], combiningadvanced ethereal electrolyte solutions with functional inter-phases could pave the way toward realizing high-energy-densityRMMBs. Author Contributions Toshihiko Mandai: Conceptualization, Data curation, Formal analysis,Funding acquisition, Investigation, Project administration, Resources,Validation, Writing – original draft, reviewing, and editing. AntoineBarthélemy: Quantum chemical calculations, Formal analysis, Valida-tion, Writing – reviewing and editing. Hendrik Koger: Karl-Fischertitration, FT-IR measurements, Formal analysis, Resources, Validation,Writing – reviewing and editing. Harald Scherer: NMR measurements,Formal analysis, Validation, Writing – reviewing and editing. IngoKrossing: Formal analysis, Validation, Writing – reviewing and editing. 13 of 16tive Commons LicenseAT  P  a  o  w  s  F  (  4  a  I  M  m  T  c  oCTDT  cR o  B2  o  S3  i  D4  C  W  55  I6  Y  R  E7  A  L8  C  M9  MA1  C  T  (1 E  M                                           1 21983844, 2026, 59, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.76897 by Toshihiko Mandai - National Institute For , Wiley Online Library on [23/10/2026]. 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 Ccknowledgements his work was supported in part by the NEXT Center of Innovationrogram (COI-NEXT; grant number JPMJPF2016) of the Japan Sciencend Technology Agency and KAKENHI (grant number JP26K01623)f the Japan Society for the Promotion of Science. Additionally, thisork was supported by the Albert-Ludwigs-Universität Freiburg, by thetate of Baden-Württemberg through bwHPC and the German Researchoundation (DFG) through grant number INST 40/467-1 and 575-1 FUGGJUSTUS 1 and 2 cluster), the DFG project AMPERE (project number41215516), the EU project HighMag (Grant agreement ID: 101202882),s well as the European Research Council’s ERC Advanced GrantnnoChem (Grant agreement ID: 101052935). The authors acknowledges. Watanabe, Ms. Oshida, and Mr. Shinoda for their assistance withicroscopic observations. The XRD profiles, SEM images, XPS spectra,OF-SIMS spectra, STEM images, EELS spectra, and electrochemicalonfocal microscopic analyses of Mg plating/stripping reactions werebtained using the facilities of Battery-PF at NIMS, Japan. onflicts of Interest he authors declare no conflicts of interest. ata Availability Statement he data that support the findings of this study are available from theorresponding author upon reasonable request. eferences 1 . F. Degen, M. Winter, D. Bendig, and J. Tübke, “Energy Consumptionf Current and Future Production of Lithium-Ion and Post Lithium-Ionattery Cells,” Nature Energy 8 (2023): 1284.  . J. A. Blázquez, R. R. Maça, O. Leonet, et al., “A Practical Perspectiven the Potential of Rechargeable Mg Batteries,” Energy & Environmentalcience 16 (2023): 1964.  . D. Wang, Z. Zhang, Y. Hao, et al., “Challenges and Pr ogr essn Rechargeable Magnesium-Ion Batteries: Materials, Interfaces, andevices,” Advanced Functional Materials 34 (2024): 2410406.  . I. M. 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Wu, et al., “Bismuth/Tin-Based Artificial Interfaceor Overcoming the Anode Passivation of Rechargeable Magnesiumatteries,” Chemical Communications 61 (2025): 12546. upporting Information dditional supporting information can be found online in the Supportingnformation section. upporting File 1: advs76897-sup-0001-SuppMat.docx. upporting File 2: advs76897-sup-0002-SuppVideos.zip. 6 of 16 Advanced Science, 2026ley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttps://doi.org/10.1002/advs.75749 Unsuccessful Coupling of Weakly Coordinating Anion-Based Mg Salts and Alkoxyalkylamine Solvents for Rechargeable Mg Battery Application: Trace Water-Driven Interfacial Failure 1 | Introduction 2 | Results and Discussion 2.1 | Mg Plating/Stripping Performance and Morphological Evolution 2.2 | Solvation State and Interfacial Chemistry 2.3 | MOEA-Mediated Partial Hydroxylation of [Al(Ohfip)4]− and Understanding of Dynamic Failure Mechanism in Mg[Al(Ohfip)4]2/G2-MOEA 2.4 | Full-Cell Performance and Complex Dilemma 3 | Conclusions Author Contributions Acknowledgements Conflicts of Interest Data Availability Statement References Supporting Information