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[Dedy Setiawan](https://orcid.org/0000-0003-3560-0869), [Omar Falyouna](https://orcid.org/0000-0003-4236-6433), [Toshihiko Mandai](https://orcid.org/0000-0002-2403-7794)

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[Beyond Half‐Cell Success: Cathode‐Electrolyte Reactivity Driving Magnesium Battery Full‐Cell Degradation at Elevated Temperature](https://mdr.nims.go.jp/datasets/e8fbc8d9-fc5b-4f96-b04a-26610f13685e)

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Beyond Half‐Cell Success: Cathode‐Electrolyte Reactivity Driving Magnesium Battery Full‐Cell Degradation at Elevated TemperatureRESEARCH ARTICLEwww.advancedscience.comBeyond Half-Cell Success: Cathode-Electrolyte ReactivityDriving Magnesium Battery Full-Cell Degradation atElevated TemperatureDedy Setiawan,* Omar Falyouna, and Toshihiko Mandai*Rechargeable magnesium battery (RMB) is gaining attention as a promisingalternative to lithium-ion batteries, offering advantages such as low cost andhigh theoretical capacity of magnesium metal anodes. Yet, realizing stable,high-voltage RMB full cells remains a considerable challenge. In this study, afull-cell configuration is explored combining a vanadium oxide (VO2) cathodewith a weakly coordinating anion-based electrolyte. While encouragingperformance is observed in half-cell setups, translating it into full-celloperation proves complex, particularly at elevated temperatures. At 60 °C, theinitial discharge capacity of 77 mAh g−1 decreases notably to 28 mAh g−1 inthe second cycle, whereas performance at 30 °C remains more stable≈25 mAh g−1. Three-electrode measurement suggests increasingoverpotentials at the Mg anode as a key factor in the capacity degradation.Further analysis points to issues such as uneven Mg plating/stripping,surface pitting, and minor vanadium dissolution, contributing to impedancegrowth and cross-over effects. These are linked to cathode–electrolyte sidereactions, particularly under high-voltage. Overall, the results emphasize theimportance of developing stable interphases to enhance the long-termperformance of RMB full cells, especially at elevated temperatures.1. IntroductionGrowing concerns over climate change and the increasing re-liance on intermittent renewable energy sources such as so-lar and wind have intensified the global demand for reliable,high-capacity electrochemical energy storage systems.[1–4] Whilelithium-ion batteries (LIBs) have long been the preferred tech-nology for mobile and stationary applications due to their highenergy density, challenges including safety risks, high cost, andlimited lithium availability have spurred the search for alternativeD. Setiawan, O. Falyouna, T. MandaiResearch Center for Energy and Environmental Materials (GREEN)National Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: setiawan.dedy@nims.go.jp;mandai.toshihiko@nims.go.jpThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/advs.202511416© 2025 The Author(s). Advanced Science published by Wiley-VCHGmbH. This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properly cited.DOI: 10.1002/advs.202511416energy storage solutions.[5,6] Consequently,research efforts have increasingly turnedtoward the development of next-generationbattery systems that incorporate moreabundant, cost-effective, and safer com-ponents, particularly those based onmultivalent metal chemistries, as potentialpost-LIB technologies.[7–10]Among these emerging alternatives,rechargeable magnesium battery (RMB)has attracted significant attention dueto their potential for lower cost, en-hanced safety, and high volumetric energydensity when paired with high-voltagecathodes.[11–13] Magnesium (Mg) metaloffers key advantages, such as lower cost,higher volumetric capacity, and a highermelting point than lithium.[11,14] Despitethese merits, there is still a large gap inRMB research progress, particularly in real-izing a high-voltage RMB full cell. Althoughthere has been considerable progress in thedesign of high-voltage cathodes, compatibleelectrolyte systems, and Mg metal inter-face, most studies have focused primarilyon half-cell configurations.[15–19] Meanwhile, to enable practicalRMB, a reversible high-voltage cathode must be paired with aMgmetal anode under lean electrolyte conditions.However, theircompatibility and interfacial stability in full-cell configurationsremain poorly understood and require further study.On the high-voltage cathode side, several materials such asMgMn2O4, H2V3O8, VO2, and MgMnSiO4 have been extensivelystudied; however, they typically exhibit limited capacity at roomtemperature and often require elevated temperatures to activateMg2+ intercalation due to kinetic constraints.[20–23] Nevertheless,their performance is often promising in half-cells with non-metallic counter electrodes and conventional electrolytes at ele-vated temperatures. In contrast, full-cell studies with Mg metalanodes remain limited due to interfacial instability with conven-tional electrolytes.[17,18,24,25]On the electrolyte front, weakly coordinating anion sys-tems (WCAs), such as those based on fluorinated alkoxyborateand alkoxyaluminate salts in ether-based solvents, have shownpromising compatibility with Mg metal.[26–34] For example, acombination of Mg[Al(hfip)4]2 and diglyme exhibits stable cy-cling against Mg metal with very low overpotential (≈60 mV)and remarkable Coulombic efficiency over 99% after some activa-Adv. Sci. 2025, 12, e11416 e11416 (1 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbHhttp://www.advancedscience.commailto:setiawan.dedy@nims.go.jpmailto:mandai.toshihiko@nims.go.jphttps://doi.org/10.1002/advs.202511416http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadvs.202511416&domain=pdf&date_stamp=2025-08-04www.advancedsciencenews.com www.advancedscience.comtion pre-cycle.[34] This electrolyte system also has relatively highoxidation stability, over 3.5 V versus Mg/Mg2+, thus potentiallycompatible with high-voltage cathodes.[34] However, their perfor-mance in full-cell configurations with such high-voltage cathodeshas not been widely evaluated.In this study, we investigate the performance of an RMB fullcell with the ethereal solutions of the representative WCAs as theelectrolyte and VO2 as a cathode, at 30 and 60 °C. The goal is tobridge the gap between half-cell investigations and full-cell eval-uation, as well as uncovering potential reactions on the cathodeand the anode. Brookite-type VO2 with a monoclinic-type struc-ture was selected as a model cathode due to its reversible Mg2+intercalation capability and operating voltage, which lies withinthe electrochemical stability window of the chosen electrolyte.[21]VO2 also exhibits limited capacity at room temperature and re-quires activation at elevated temperature, making it a suitablemodel for high-voltage cathode with similar characteristics.[21]Electrolyte solution of 0.3 m Mg[Al(hfip)4]2 in diglyme was cho-sen as the representative WCAs-based electrolyte due to its re-markable Mg plating/stripping reversibility. The electrochemicalperformance of the electrolyte was first assessed at 30 and 60 °C,prior to full-cell evaluation. Full cell evaluation was then con-ducted using VO2 as the cathode, 0.3 mMg[Al(hfip)4]2 in diglymeas the electrolyte, and Mg metal as the anode.2. Results2.1. Electrolyte Performance at Elevated TemperatureThe WCA-based electrolyte system has been well developed, andreversible Mg plating/stripping behavior at room temperaturehas been widely reported. However, its performance at elevatedtemperatures remains largely unexplored. To investigate this,asymmetric cells were assembled using a Cu working electrodeand Mg metal as both counter and reference electrodes, andtested at 30 and 60 °C to compare the Mg plating/stripping be-havior. As shown in Figure 1a and S2 (Supporting Information),typical Mg plating/stripping behavior was observed at 30 °C, witha Coulombic efficiency exceeding 96% at a current density of 1.0mA cm−2 and a Mg utilization of 1.0 mAh cm−2, consistent withprevious reports.[34] In contrast, at 60 °C, the characteristic cur-rent spike during the stripping process was not observed, whichsuggests the occurrence of short-circuiting phenomena.[35]To further probe the electrochemical behavior at elevated tem-perature, asymmetric cells were tested at 60 °C undermilder con-ditions (and 0.1 mA cm−2), as shown in Figure 1b and FigureS3 (Supporting Information). Under these conditions, the typi-cal current spike during Mg stripping reappeared. However, theCoulombic efficiency was significantly lower, reaching only 78%in the first cycle and ≈90% in subsequent cycles (Figure S3, Sup-porting Information). This decrease in efficiency is likely due toenhanced electrolyte decomposition at the elevated temperature.TheMg plating/stripping behavior inMgmetal symmetric cellwas also evaluated. Figure 1c compares the overpotentials duringMg plating/stripping at various current densities. At a low cur-rent density of 0.05 mA cm−2, the overpotential remains ≈0.02 Vat both temperatures and is stable over 20 cycles. However, whenthe current density is increased to 0.1 mA cm−2 after 20 cycles,the overpotential at 60 °C rises sharply to over 0.2 V, eventuallyleading to a short circuit.Electrochemical impedance spectroscopy (EIS) results(Figure 1d) support this observation. After cycling at 60 °C,a very small semicircle appears, indicative of a short circuit.[35]In contrast, only a slight increase in impedance is observed aftercycling at 30 °C. The morphological changes on the Mg metalsurface analyzed by scanning electron microscopy (SEM) aftercycling are shown in Figure 1e (pristine), 1f (cycled at 30 °C), and1 g (cycled at 60 °C). It has been previously reported that shortcircuits in Mg symmetric cells using WCAs-based electrolytesare not caused by dendrite formation. Instead, they result fromthe non-uniform 3D Mg deposits, which can penetrate theseparator.[36,37] In this study, such non-uniformity is clearly morepronounced after cycling at 60 °C. At higher temperatures, therate of electrolyte decomposition increases, likely promoting theformation of unstable interphases.[38] This leads to local overpo-tential variations and non-uniform Mg plating.[36] Additionally,the repeated inhomogeneous deposition and stripping cyclesinduce mechanical stress at the Mg-electrolyte interface, poten-tially causing delamination or loss of interfacial contact. Thesecombined effects explain the observed interfacial degradationand accelerated short circuit at higher current density. However,operating at lower current densities can mitigate these issues,though the stability of the interphases under such conditionsstill requires further investigation.2.2. RMB Full CellA full cell was assembled using VO2 as the cathode, Mg metalas the anode, and 0.3 m Mg[Al(hfip)4]2 in diglyme as the elec-trolyte (Figure 2a). Figure 2b presents the galvanostatic dis-charge/charge profile of the RMB full cell cycled at 30 °C with acurrent density of 10mAg−1 based on the cathode activematerial,which corresponds to ≈0.01 mA cm−2 on the Mg metal anode.Under these conditions, the full cell delivers an initial dischargecapacity of 23mAh g−1 and a charge capacity of 20mAh g−1. Afterfive cycles, the capacity remains relatively stable at ≈26 mAh g−1.The limited capacity at 30 °C should correspond to the limitedelectrochemical activity of VO2, as reported previously.[21]Figure 2c shows the corresponding discharge/charge profile at60 °C under identical conditions with 30 °C. At this elevated tem-perature, the RMB full cell exhibits a significantly enhanced andreversible initial capacity of 77 mAh g−1. This improvement islikely due to more efficient Mg2+ diffusion within the VO2 struc-ture at higher temperatures. However, this performance is notsustained over subsequent cycles. By the second cycle, the capac-ity drops sharply to 28 mAh g−1, followed by further declines to16 mAh g−1 and 11 mAh g−1 in the third and fifth cycles, re-spectively. A similar degradation phenomenon after the 1st cy-cle was also observed in another WCAs-based electrolyte, 0.3 mMg[B(hfip)4]2 in diglyme, as shown in Figure S5 (Supporting In-formation). This pronounced capacity fading at elevated temper-ature highlights critical stability issues and motivates a deeperinvestigation into the degradation mechanisms in RMB full cellsoperating at 60 °C.We were initially curious about Mg metal surface composi-tion cycled in a symmetric cell and in a full cell, because theirAdv. Sci. 2025, 12, e11416 e11416 (2 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202511416 by National Institute For, Wiley Online Library on [28/10/2025]. 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://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 1. Galvanostatic discharge–charge profile of a) asymmetric cell test with Cu working electrode at 30 and 60 °C with current density of 1.0 mAcm−2, b) asymmetric cell test with Cu working electrode at 60 °C with current density of 0.1 mA cm−2, c) Mg metal symmetric cell with at various currentdensities at 30 and 60 °C. d) EIS profile of Mg metal symmetric cell before and after cycled at 30 and 60 °C. SEM images of Mg surface e) before cyclef) after cycled at 30 and g) after cycled at 60 °C.Figure 2. a) Illustration of RMB full cell with 0.3 m Mg[Al(hfip)4]2 in diglyme as the electrolyte and VO2 as the cathode. Galvanostatic discharge–chargeprofile of the full cell with a current density of 10 mA g−1 cycled at different temperatures; b) 30 °C and c) 60 °C.Adv. Sci. 2025, 12, e11416 e11416 (3 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202511416 by National Institute For, Wiley Online Library on [28/10/2025]. 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://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 3. ToF-SIMS analysis on Mg metal surface cycled with 0.3 m Mg[Al(hfip)4]2 in diglyme electrolyte in a), c) symmetric cell and b), d) in full cellwith VO2 as a cathode, respectively.performance might indicate a discrepancy. Therefore, time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis wasperformed on the Mg metal surface after cycling in both sym-metric and full cells at 60 °C to investigate possible surfacechanges. Figure 3a,c shows the ToF-SIMS spectra ofMgmetal cy-cled in the symmetric cell. Several organic-based ions, MgOH−,MgO2H−, and fluorinated species were detected, suggesting sol-vent and anion-derived electrolyte decomposition during cycling.Such decomposition is commonly observed in weakly coordinat-ing anion-type electrolytes.[25,39,40] It is worth noting that the de-tected hydroxide-related ions may also result from reactions withtrace amounts of water present in the electrolyte, as previouslyreported.[41]In contrast, the ToF-SIMS spectra of Mg metal from the fullcell (Figure 3b,d) reveals additional signals, including V+ ions,along with the decomposition products observed in the symmet-ric cell. Moreover, the intensities of organic-based decompositionproducts as well as fluorinated species are significantly higher af-ter full cell cycling, indicating a thicker and more complex inter-phase. These results highlight a clear difference in the interfacialchemistry of Mg metal when cycled in a symmetric cell versusa full cell at 60 °C, underscoring the critical impact of cathode-electrolyte interactions in full cell configurations.To investigate the observed discrepancies, we conducted athree-electrode cell test to independently monitor the behaviorsof the VO2 cathode and Mg metal anode during cycling at 60 °C.Figure 4 displays the galvanostatic discharge–charge profiles ofthe VO2 cathode, Mg anode, and the full cell. The first dischargecapacity reached 77 mAh g−1, consistent with the full-cell per-formance shown in Figure 2c. However, unlike the full cell, thecapacity and discharge–charge overpotential of the VO2 cathoderemained relatively stable in subsequent cycles. This result indi-cates good compatibility between the VO2 cathode and the 0.3 mMg[Al(hfip)4]2 in diglyme electrolyte, but translating the perfor-mance into a full cell is non-trivial.In contrast, the Mg anode exhibited unstable behavior. Whilethe initial overpotential was ≈0.3 V, it increased dramatically af-ter the second cycle, exceeding 1 V, and continued to rise in latercycles. This instability suggests that undesirable reactions oc-cur between the cathode and electrolyte during the initial cycle,adversely affecting the Mg plating/stripping process. One likelycause is the severe decomposition of the solvent at high-voltage,which could trigger vanadium dissolution from the cathode andcause crossover effects, as observed in the ToF-SIMS analysis ear-lier (Figure 3). We strongly believe that the rapid increase in Mgplating/stripping overpotential observed after the first cycle inthe full cell is primarily due to the high-voltage operation. As re-ported in a previous study using a WCA-based electrolyte with alow-voltage Mo6S8 cathode, interfacial degradation at the Mg an-ode was also detected, but the full cell remained operational formore than 20 cycles even at 120 °C.[38] In that case, the interfa-cial failure was attributedmainly to the intrinsic thermal instabil-ity of the electrolyte and non-homogeneousMg plating/strippingrather than cathode-driven degradation. Additionally, the rapiddegradation of high-voltage full cell after the 1st cycle due to tran-sitionmetal cross-over seems to be universally applicable to otherhigh-voltage cathodes.[42] The transition metal dissolution at thecathode and subsequentmigration to theMg anode was also gen-Adv. Sci. 2025, 12, e11416 e11416 (4 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202511416 by National Institute For, Wiley Online Library on [28/10/2025]. 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://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 4. The three-electrode cell measurement of VO2 as the workingelectrode, Mg metal as a counter electrode, and Ag/Ag+ as a referenceelectrode. The cycle was conducted with the current density of 10 mA g−1at 60 °C. The horizontal line in the upper profile corresponds to the dis-charge cut-off voltage of the corresponding full cell.erally observed, not limited to only vanadium, but also in the caseof cobalt.[42] We hypothesize that solvent decomposition at high-voltage raises the electrolyte’s acidity, promoting transition metaldissolution, phenomena which are also well recognized in LIBsystems.[43] However, directlymeasuring the pHof the electrolyteduring cycling remains challenging due to the lean electrolyteconditions used in this study and the lack of a suitable in situexperimental setup.While transition metal dissolution typically leads to cathodecapacity fading, the capacity of our VO2 cathode remained rel-atively stable. We believe this is due to the incomplete utiliza-tion of vanadium redox activity in the first cycle. Despite somevanadium loss, the remaining VO2 still retains redox-active sitesthat become more accessible in later cycles. It is worth notingthat the theoretical capacity of VO2, based on the V3+/V4+ redoxcouple, is 323 mAh g−1, whereas the observed first discharge ca-pacity was only 77 mAh g−1. This suggests an activation processoccurs during subsequent cycles, a behavior commonly reportedfor vanadium-based cathodes.[44,45]On the Mg anode side, the parasitic reactions resultingfrom cathode–electrolyte reactivity led to non-uniform plat-ing/stripping, likely due to uneven current distribution acrossthe Mg surface. Figure S6 (Supporting Information) shows themorphology of Mg metal after the first and tenth cycles: notice-able pitting and unevenMg deposition appear after the first cycleand become more pronounced by the tenth. Additionally, SEMcoupled with energy dispersive X-ray spectroscopy(EDX)−1 map-ping and ToF-SIMS analysis on different spots after the first cy-cle (Figure S7, Supporting Information) confirm the presence ofvanadium on the surface of the Mg metal anode.2.3. Cathode CharacterizationTo better understand the cathode–electrolyte reactivity in thefull cell, particularly during the first cycle, which appears totrigger vanadium dissolution, we conducted structural and el-emental analyses on the VO2 cathode. Figure 5a shows X-raydiffraction (XRD) profiles of the VO2 cathode at four stages:pristine, after the first discharge, first charge, and tenth dis-charge. Following the first discharge, the (110) diffraction peakshifts slightly to a lower angle, consistent with previous re-ports, indicating ion insertion into the VO2 structure. Thispeak returns to its original position after the first charge, sug-gesting a reversible process. Interestingly, after the tenth dis-charge, the (110) peak shift is more pronounced than afterthe first discharge, despite their comparable capacities. Thissuggests that more ions were inserted into VO2 by the tenthdischarge.To quantify the extent of Mg2+ insertion, inductively coupledplasma optical emission spectroscopy (ICP-OES) analysis wasperformed. Figure 5b presents the measured Mg/V ratios at thesame four stages, compared with expected values derived fromgalvanostatic discharge–charge capacities in the three-electrodesetup. After the first discharge, the measured Mg/V ratio is only0.03, significantly lower than the expected 0.11, implying thatmost of the initial capacity is not due to Mg2+ intercalation. Aplausible explanation is the contribution of H+ insertion and/orthe formation of cathode–electrolyte interphases. Despite the useof a low water content electrolyte, electrochemical decomposi-tion of the ether-based solvent may generate trace amounts ofH2O, facilitating H+ formation and interphase growth.[40,46] Onthe other hand, the Mg/V ratio after the tenth discharge is 0.14,which is consistent with the expected value of 0.12, support-ing the XRD observation of greater ion insertion in the latercycle.It is worth noting that the high Coulombic efficiency in thefirst cycle appears contradictory to the occurrence of side reac-tions during discharge. However, electrolyte decomposition—and hence side reactions—may occur not only during dischargebut also during charge, since the cathode-electrolyte interphasesare not stable enough to prevent the electron transfer at the cath-ode interface. Although the Coulombic efficiency appears high,it can be inferred that side reactions during the initial chargeprocess contributed a comparable amount of electron transfer asthe discharge process in the first cycle. This gives the false im-pression that the charge and discharge capacities arise from a re-versible reaction within the applied cut-off voltage, when in factthey do not. This interpretation is supported by the consistent ob-servation that the charge capacity exceeds the discharge capacityAdv. Sci. 2025, 12, e11416 e11416 (5 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202511416 by National Institute For, Wiley Online Library on [28/10/2025]. 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://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 5. a) XRD profile of the VO2 cathode at pristine, 1st discharge, 1st charge, and 10th discharge with enlarged view of (110) peak. b) ICP-OESanalysis of the VO2 cathode at pristine, 1st discharge, 1st charge, and 10th discharge. Ex situ XPS of c) V 2p, d) C 1s, e) O 1s, and f) F 1s survey spectraof the VO2 cathode. 1st D, 1st C, and 10th D in the figures represent 1st discharge, 1st charge, and 10th discharge, respectively.in subsequent cycles (Figure 4). Therefore, a portion of the elec-tron transfer during the first charge originates from irreversibleside reactions rather than a reversible process.To further probe the redox mechanism and the cathode–electrolyte interphase (CEI), ex situ X-ray photoelectron spec-troscopy (XPS) was carried out on the VO2 cathode after the firstdischarge, first charge, and tenth discharge. As shown in the V2p spectra (Figure 5c), both V3+ and V4+ signals are present afterthe first and tenth discharges, even though only a small amountof Mg was inserted in the first cycle. This suggests that vanadiumreduction may be partially driven by other species, such as pro-tons. The C 1s, O 1s, and F 1s spectra (Figure 5d–f) reveal vari-ous organic and inorganic decomposition products on the cath-ode surface after the first discharge, corroborating the conclusionfrom XRD and ICP-OES that much of the initial capacity arisesfrom side reactions. After the first charge, some of these decom-position products, particularly fluorinated species, appear to di-minish, likely due to further decomposition at high-voltage andincreased electrolyte acidity, which may dissolve part of the CEI.The dissolution of CEI components and vanadium into the elec-trolyte after the first charge could be a key factor contributingto the increased overpotential observed at the Mg anode. Thesedissolved species may migrate to the Mg anode, disrupting uni-form Mg plating/stripping and increasing interfacial resistance.However, a fraction of the CEI remains intact, which may helpsuppress side reactions in subsequent cycles and promote moreeffective Mg2+ intercalation during discharge.2.4. The Impact of Cut-Off VoltageWe were further interested in the cathode–electrolyte reactivityduring the charging process, as the dissolution of some CEI com-ponents tends to occur at this stage. To investigate this, we stud-ied the impact of the charge cut-off voltage by intentionally in-creasing it to a higher value. Figure 6a shows the galvanostaticdischarge–charge profile of VO2 with a charge cut-off voltage of0.3 V versus Ag/Ag+ (2.79 V vs Mg/Mg2+), while the correspond-ing dQ/dV curve is presented in Figure 6b. At this cut-off volt-age, the discharge–charge capacity remains relatively stable dur-ing the initial cycles. The capacity gradually increases to 90 mAhg−1 by the 27th cycle and then saturates. The dQ/dV plot shows noindication of anodic decomposition of the electrolyte up to 0.3 Vversus Ag/Ag+.In contrast, when the charge cut-off voltage was increased to0.5 V versus Ag/Ag+ (2.99 V vs Mg/Mg2+), the cell exhibited aAdv. Sci. 2025, 12, e11416 e11416 (6 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202511416 by National Institute For, Wiley Online Library on [28/10/2025]. 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://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 6. a) Galvanostatic discharge–charge of VO2 working electrode at three-electrode cell test with a current density of 10mA g−1 cycled with differentcharge cut-off voltage of (a) 0.3 V versus Ag/Ag+ and c) 0.5 V versus Ag/Ag+. The corresponding dQ/dV of the discharge–charge profile are shown inb) for 0.3 V versus Ag/Ag+ and d) for 0.5 V versus Ag/Ag+.higher charge capacity (91 mAh g−1) than discharge capacity (77mAh g−1) in the first cycle. In subsequent cycles, the dischargecapacity rapidly increased, reaching 120 mAh g−1 after 10 cy-cles, with charge capacities consistently higher than dischargecapacities. This suggests the presence of side reactions contribut-ing to the excess charge capacity at 0.5 V. The dQ/dV profile inFigure 6d supports this hypothesis, showing a noticeable currentspike above 0.3 V versus Ag/Ag+. These results indicate that evenwithin the nominal electrochemical stability window of the elec-trolyte, side reactions at the cathode become more pronouncedafter reversible cycling is established.To further explore the effects of the increased cut-off volt-age, various elemental analyses were conducted. ICP-OES results(Figure 7a) show that Mg insertion after the 10th discharge at a0.5 V versus Ag/Ag+ cut-off is only 0.09, significantly lower thanthe expected value of 0.19. This confirms that, unlike at 0.3 V,continuous side reactions occur during cycling when the cut-offvoltage exceeds the VO2 reversible capacity.XPS analysis of the C 1s spectra (Figure 7b) revealed a greateraccumulation of both organic and inorganic CEI species on theVO2 cathode cycled at 0.5 V, particularly O─C═O and CF3 moi-eties. ToF-SIMS analysis (Figure 7c,d) showed similar types ofCEI species for both voltage conditions, but with notably higherintensity after cycling at 0.5 V. A strong signal corresponding toMgH2− was also observed, likely originating from H+ interca-lation during discharge. These CEI components predominantlyaccumulate on the surface of VO2 and may readily dissolve intothe electrolyte, subsequently migrating to the Mg anode and con-tributing to interfacial instability.3. ConclusionWe have demonstrated RMB full cell employing 0.3 mMg[Al(hfip)4]2 in diglyme as an electrolyte and VO2 as a modelcathode, tested at both 30 and 60 °C. Our study reveals thatcathode–electrolyte reactivity is a key factor contributing to celldegradation at elevated temperature (60 °C). During discharge,side reactions occur concurrently with Mg2+ intercalation, lead-ing to the formation of organic and inorganic CEI. Duringthe subsequent charge process, parts of the CEI and vanadiumspecies dissolve from the cathode and migrate to the Mg anode,significantly increasing theMg plating/stripping overpotential inlater cycles. Our finding indicates that the electrochemical stabil-ity window of the electrolyte itself does not guarantee a stable cy-cling of high-voltage cathode under full cell operation, especiallyat elevated temperature.Moreover, we found that the choice of charge cut-off voltagestrongly influences the extent of these cathode-electrolyte side re-actions, affecting both discharge and charge performance of thefull cell. These findings highlight a critical challenge in translat-ing half-cell performance to full-cell systems in RMB develop-ment particularly under elevated temperature. Future researchshould focus on improving the electrochemical stability of elec-trolytes to advance practical RMB applications.Adv. Sci. 2025, 12, e11416 e11416 (7 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202511416 by National Institute For, Wiley Online Library on [28/10/2025]. 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://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 7. a) ICP-OES analysis and b) XPS C1s survey spectra of VO2 at 10th discharge after cycled with the charge cut-off voltage of 0.3 and 0.5 V versusAg/Ag+. ToF-SIMS analysis of VO2 surface cycled with the charge cut-off voltage of c) 0.3 V and d) 0.5 V versus Ag/Ag+.4. Experimental SectionElectrolyte and Cathode Preparation: 0.3 m Mg[Al(hfip)4]2 in diglymeelectrolyte was prepared by dissolving Mg[Al(hfip)4]2 salt into diglymesolvent and stir at room temperature inside an Ar-filled glovebox.Mg[Al(hfip)4]2 salt preparation was reported elsewhere.[34] Diglyme(>99.5%, Kanto Chemical CO., INC., Japan) was treated with molecularsieves for overnight prior to electrolyte preparation. The water content ofthe electrolyte was less than 50 ppm, as measured by Karl-Fischer mois-ture titration (MKC710, KEM).VO2 powder was prepared following the previous method.[21] 0.3638g of V2O5 (>99.0%, Kanto Chemical CO., INC., Japan) was mixed with0.432 g of oxalic acid (≥99.0%, Sigma–Aldrich) in 50 mL of D.I. water at40 °C for 24 h. After 24 h of stirring, the color of the solution transformedfrom light green to dark blue. The solution was subjected to hydrothermalreaction in a 100 mL Teflon-lined stainless-steel autoclave and heated at180 °C for 24 h. The resulting precipitates were filtered, washed with D.I.water, and dried at 80 °C for 6 h under vacuum conditions. The purity ofVO2 was confirmed using powder XRD (SmartLab, Rigaku, Japan) withCu K𝛼 X-ray tube, and refined using the powder X-ray Rietveld refinementprogram GSAS, and shown in Figure S1a (Supporting Information).[47]The morphology of VO2 was also confirmed using SEM (SU8200, Hitachi,Japan), and shown in Figure S1b (Supporting Information).Cathodematerial was prepared bymixing VO2 powder, Acetylene Black,and PVDF binder with a mass ratio of 8: 1: 1, dispersed in N-methyl-2-pyrrolidone (NMP), and casted on to carbon-coated aluminum foil as acurrent collector. The average loadingmass of the cathode was 1mg cm−2.Electrochemical Characterization: Cathode active material (2.01 cm2),a glass fiber separator (GF/D, Whatman) (2.01 cm2), and polished Mgmetal (2.01 cm2) were assembled in a two-electrode cell for full cell test,and a three-electrode cell for half-cell test. In the three-electrode cell test,Ag/Ag+ reference electrode was used, and calibrated as 2.49 V versusMg/Mg2+.[34] The amount of electrolyte for each cell was 200 μl.The full cell discharge–charge measurement was conducted using abattery cycler (HJ1001SD8 C, HD Meiden Hokuto, Japan). While EIS andthe three-electrode cell measurement were conducted using EC-Lab soft-ware on a Biologic VMP3 multichannel potentiostat (Biologic Science In-struments SAS). The cut-off voltage of the full cell was 0.5 to 3.1 V, and thethree-electrode cell was −1.9 to 0.3 V versus Ag/Ag+, which are still in therange of the electrochemical stability window of the electrolyte (Figure S4,Supporting Information).Cathode and Anode Characterization: The surface of the Mg anodeand VO2 cathode after cycles was analyzed using SEM (SU8200, Hitachi,Japan) equipped with EDX, XPS (VersaProbe II, ULVAC-PHI), and ToF-SIMS (ToF-SIMS5-AD-GCIB) with a primary ion gun of 30 kV Bi3++ and4 nm min−1 as SiO2 film sputter rate. The electrochemical cells were dis-assembled inside an Ar-filled glovebox. The electrodes were washed withdimethoxyethane (DME) and dried for several hours inside the glovebox.All interfacial characterizations, including the sample transfer process,were conducted under an air- and moisture-free atmosphere.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was financially supported by the GteX Program Japan (GrantNo. JPMJGX23S1) of the Japan Science and Technology Agency.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementData sharing is not applicable to this article as no new data were createdor analyzed in this study.Adv. Sci. 2025, 12, e11416 e11416 (8 of 9) © 2025 The Author(s). 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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://www.advancedsciencenews.comhttp://www.advancedscience.com Beyond Half-Cell Success: Cathode-Electrolyte Reactivity Driving Magnesium Battery Full-Cell Degradation at Elevated Temperature 1. Introduction 2. Results 2.1. Electrolyte Performance at Elevated Temperature 2.2. RMB Full Cell 2.3. Cathode Characterization 2.4. The Impact of Cut-Off Voltage 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords