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[Naoya Ishida](https://orcid.org/0000-0003-3707-6781), [Keigo Kubota](https://orcid.org/0000-0002-0536-129X), [Toshikatsu Kojima](https://orcid.org/0000-0002-1774-2698), [Toshihiko Mandai](https://orcid.org/0000-0002-2403-7794), [Shin Kiyohara](https://orcid.org/0000-0003-2890-5760), [Yu Kumagai](https://orcid.org/0000-0003-0489-8148), [Tetsu Ichitsubo](https://orcid.org/0000-0002-1127-3034)
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[Cathode properties of MgV 2 O 4 spinel for magnesium rechargeable batteries: effect of synthesis route on structure and electrochemical performance](https://mdr.nims.go.jp/datasets/a91a58d2-2904-48d2-aad3-993b3048daf5)
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Cathode properties of MgV2O4 spinel for magnesium rechargeable batteries: effect of synthesis route on structure and electrochemical performanceJournal ofMaterials Chemistry APAPEROpen Access Article. Published on 11 February 2026. Downloaded on 3/23/2026 4:48:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal | View IssueCathode propertaResearch Institute of Electrochemical EIndustrial Sciences and Technology (AIS563-8577, Japan. E-mail: naoya-ishida@aisbFunctional Electrolyte Synthesis Team, ReseMaterials (GREEN), National Institute forTsukuba, Ibaraki, 305-0044, JapancInstitute for Materials Research (IMR), TohSendai, 980-8577, JapanCite this: J. Mater. Chem. A, 2026, 14,11057Received 25th November 2025Accepted 6th February 2026DOI: 10.1039/d5ta09602brsc.li/materials-aThis journal is © The Royal Society oies of MgV2O4 spinel formagnesium rechargeable batteries: effect ofsynthesis route on structure and electrochemicalperformanceNaoya Ishida, *a Keigo Kubota, a Toshikatsu Kojima, a Toshihiko Mandai, bShin Kiyohara, c Yu Kumagai c and Tetsu Ichitsubo cMagnesium rechargeable batteries (MRBs) are promising candidates for next-generation energy storageowing to their high volumetric capacity and safety. Herein, we investigate spinel-type MgV2O4 (MVO) asa cathode material and elucidate the correlation between its crystal structure and electrochemicalperformance. An ordered spinel was synthesized via a solid-state route and subsequently subjected tomechanical milling (MM), while a solvothermal (ST) method was employed to prepare a comparativesample. Rietveld refinement revealed that MM induces a transition from an ordered to a disorderedspinel structure, accompanied by partial V occupancy at the interstitial 16c site, which obstructs Mg2+migration between 8a sites. Despite particle size reduction, MM-MVO exhibited poor reversibility due tothis structural disorder. Complementary computational analysis confirmed the energetic favorability of Vmigration into the 16c site by MM, explaining the origin of the diffusion barrier. In contrast, ST-MVOretained a relatively ordered structure with minimal V occupation at 16c sites and delivered a reversiblecapacity of 175 mAh g−1 at 2 V when paired with a high-voltage electrolyte. These findings highlight thecritical role of spinel ordering in enabling efficient Mg2+ transport and provide design guidelines for high-performance MRB cathodes.1 IntroductionLithium-ion batteries (LIBs) have dominated the eld ofrechargeable energy storage owing to their high energy densityand excellent cycling stability. However, the limited availabilityand uneven geographical distribution of lithium resources haveraised concerns regarding the long-term sustainability and costof LIB technology. Consequently, the development of next-generation rechargeable batteries that do not rely on lithiumhas become an important global research focus.1 Amongvarious alternatives, the magnesium rechargeable battery(MRB) is a particularly attractive candidate. The rst prototypeMRB was reported by Aurbach et al. in 2000, demonstrating thefeasibility of reversible magnesium deposition and dissolution.2Magnesium offers several intrinsic advantages: (i) as a divalention, Mg2+ can transfer two electrons per ion, enabling a highernergy, National Institute of AdvancedT), 1-8-31 Midorigaoka, Ikeda, Osakat.go.jparch Center for Energy and EnvironmentalMaterials Science (NIMS), 1-1 Namiki,oku University, 2-1-1 Katahira, Aoba-ku,f Chemistry 2026theoretical capacity than monovalent Li+; (ii) its ionic radius(0.72 Å) in six coordination is comparable to that of Li+ (0.76 Å),3suggesting that magnesium-based cathodes could potentiallyachieve twice the specic capacity in LIBs. On the other hand,the strong coulombic interaction between Mg2+ ions and thehost lattice oen leads to sluggish diffusion kinetics, whichremains a key challenge.4 Metallic magnesium itself possessesa high volumetric capacity of 3833 mAh cm−3, approximatelytwice that of lithium metal (2061 mAh cm−3). Furthermore, itshigh melting point (∼650 °C) ensures thermal stability underhigh-temperature operation,5,6 and its dendrite-less depositionbehavior offers signicant safety advantages.7 Thus,temperature-assisted operation may mitigate the sluggish Mg2+ion diffusion to some extent.The cell constructed of chevrel-phase cathodes withmagnesium metal anodes and grignard-based electrolytes hasdemonstrated excellent cyclability, exceeding 2000 cycles,reecting the intrinsic durability of the MRB system.2,8 Never-theless, such systems generally exhibit low operating voltagesand limited capacities, emphasizing the need for improvedcathode materials and more oxidative-stable electrolytes.Recent progress has achieved enhanced cycling performance,yet further capacity improvement remains a major objective. Inthis context, oxide-based cathodes, e.g. MgCo2O4 (ref. 9,10) andJ. Mater. Chem. A, 2026, 14, 11057–11064 | 11057http://crossmark.crossref.org/dialog/?doi=10.1039/d5ta09602b&domain=pdf&date_stamp=2026-03-18http://orcid.org/0000-0003-3707-6781http://orcid.org/0000-0002-0536-129Xhttp://orcid.org/0000-0002-1774-2698http://orcid.org/0000-0002-2403-7794http://orcid.org/0000-0003-2890-5760http://orcid.org/0000-0003-0489-8148http://orcid.org/0000-0002-1127-3034http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09602bhttps://pubs.rsc.org/en/journals/journal/TAhttps://pubs.rsc.org/en/journals/journal/TA?issueid=TA014018Journal of Materials Chemistry A PaperOpen Access Article. Published on 11 February 2026. Downloaded on 3/23/2026 4:48:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinedelithiated Li2FeSiO4 (ref. 11) are of particular interest due totheir superior air stability and high capacities compared tosulde-based materials. Our previous studies in delithiatedlayered cathode-materials12,13 have shown that oxide-type cath-odes can achieve the highest capacities (greater than 400 mAhg−1) among reported oxide cathodes of MRB,14 demonstratingthe theoretical potential for twice the capacity of LIBs, albeitwith remaining cycling limitations. The use of an ionic liquid-based electrolyte incorporating the fully-solvated Mg complex,[Mg(G4)][TFSA]2 (TFSA: bis(triuoromethanesulfonyl)amide), asa conductive salt, which exhibits a wide electrochemicalstability window, is especially effective for extracting theintrinsic potential of oxide cathodes.15–17Spinel-type MgTM2O4 (TM = transition metal) havefrequently been investigated as Li-free cathode materials, owingto their well-ordered 8a–16c–8a Mg2+ conduction pathways andstructural robustness.9,10,18–22 Most reported spinel-typeMgTM2O4 compounds operate as insertion-type (discharge-rst) materials, while deintercalation-type (charge-rst)spinels remain relatively unexplored. Among these, MgV2O4(hereaer denoted as MVO) is a promising candidate becausethe V3+/4+ redox couple may enable reversible Mg dein-tercalation at moderate voltages. In contrast, in vanadiumspinels containing V4+ species (e.g., Mg4V5O12-typecompounds23), oxidation of V3+ is limited, suggesting that MVOwithout V4+ components is more suitable as a high-voltagecathode. Recently, solvothermally (ST) synthesized cactus-likeMVO particles have shown excellent electrochemical perfor-mance, attributed to their shortened Mg2+ diffusion paths andabundant reactive surfaces.24 However, due to the low oxidationresistance of the electrolyte, the electrochemical properties ofST-MVO have been evaluated in the region below 2 V, limitingthe capacity to about 100 mAh g−1 (partial V3+/2+ redox). If thecharacteristics are evaluated using an electrolyte with a higherpotential, it becomes clear that the theoretical capacity of 282mAh g−1 can be achieved by using V3+/4+ redox. Moreover, solid-state synthesized MVO has scarcely been investigated. If a well-ordered normal spinel can be obtained via solid-state synthesisand subsequently rened into ne particles by mechanicalmilling (MM), it may be possible to reproduce or even surpassthe electrochemical properties of cactus-like MVO.In this study, we systematically investigated the effect of MMon the crystal structure and electrochemical properties of solid-state synthesized MVO. Detailed structural renements wereperformed to clarify how cation disorder and crystallite sizeevolve with milling. Furthermore, we compared the structuraland electrochemical characteristics of MM-derived and ST-derived MVO samples to elucidate the relationship betweencrystallography and cathode performance.2 Experimental2.1 SynthesisMVO was synthesized mainly by two methods: one was a solid-state synthesis followed by MM to produce ne particles (MM-MVO), and the other was a ST synthesis followed by calcina-tion of the precursor (ST-MVO). The solid-state method for the11058 | J. Mater. Chem. A, 2026, 14, 11057–11064former was carried out by using MgO (98.0%, FUJIFILM WakoPure Chemical) and V2O3 (99.0%, FUJIFILM Wako Pure Chem-ical) as starting materials, grinding and mixing them in a MMusing Fritsch P-6 planetary ball mill at 450 rpm for 64 h inethanol, and then calcining them in a vacuum at 750 °C for 12 h.Pulverization was carried out by MM at 450 rpm using differentZrO2 ball diameters with ethanol in stages, rst for 64 h witha diameter of 4 5 mm, then for 16 h with a diameter of 4 0.2mm, and nally for 2 h with a diameter of 4 0.1 mm. The STmethod for the latter was carried out in ethylene glycol (99.5%,Tokyo Chemical Industry) using NH4VO3 (99.0%, FUJIFILMWako Pure Chemical) and magnesium acetate hydrate (99.9%,FUJIFILM Wako Pure Chemical) as raw materials (180 °C, 24 h)with reference to.24 The obtained precursor was ltered andwashed with ethanol, and then calcined at 600 °C in a nitrogenow for 2 h to obtain ST-MVO.2.2 Analysis and cathode propertyEach product was identied by powder X-ray diffraction (XRD,Cu Ka radiation, Rigaku Ultima IV) operated at 40 kV and 50mA. The diffraction patterns were collected over 2q ranges of10–90° with a step size of 0.02° and a scan speed of 2° min−1.Rietveld renement of the crystal structures was performedusing RIETAN-FP.25 Assuming the space group Fd�3m witha spinel-type framework, possible occupancies of Mg and Vatoms were examined not only at the conventional 8a and 16dcation sites but also at the 16c site located between two 8a sites.During the renement, a constraint was imposed such that thesum of the 8a occupancy and twice the 16c occupancy did notexceed unity. For the samples most nely milled by MM andthose synthesized via the ST method, additional XRDmeasurements using Mo Ka radiation (Rigaku SmartLab) werecarried out to compare their crystal structures in greater detail.The morphology of the obtained products was observed by eld-emission scanning electron microscopy (FE-SEM, HitachiRegulus SU8220), and the crystallite size was estimated usingthe Halder–Wagner method.26Electrochemical performance was evaluated following theprocedure reported previously,17 except that a rectangular three-electrode laminated cell (100 mm × 70 mm, Fig. S1) wasemployed to suppress side reactions with the stainless-steelcell.27 The cathode was prepared by thoroughly mixing theactive material, conductive carbon (Super C65, Timcal), andPTFE binder at a weight ratio of 5 : 5 : 1, pressing the mixtureinto a 4 15 mm pellet, and attaching it to an aluminum-meshcurrent-collector. Electrical connection was made through analuminum tab (>99%, Nilaco, 0.05 mm thick, 5 mm × 50 mm).A magnesiummetal foil (99.9%, Rikazai, 0.05 mm thick, 30 mm× 20 mm) was used as the anode, connected in the samemanner. The electrolyte consisted of 0.3 M [Mg(G4)][TFSA]2/[Pyr13][TFSA] (approximately 1 mL).15 A silver reference elec-trode (99.98%, Nilaco, 0.05 mm thick, 5 mm × 40 mm) wasxed inside the laminated cell with fusion tape. The cathodeand anode were separated by a GA-100 glass ber lter (30 mm× 25 mm), while the reference electrode was placed in contactwith the separator without touching either working electrode.This journal is © The Royal Society of Chemistry 2026http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09602bPaper Journal of Materials Chemistry AOpen Access Article. Published on 11 February 2026. Downloaded on 3/23/2026 4:48:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThe three-electrode cell was placed in a thermostatic chamberat 90 °C. Aer resting for 12 h, galvanostatic charge–dischargetests were performed using a Hokuto Denko HJ1001SD8csystem. The current density was set to 5 mA g−1. Charging wascarried out up to 1.0–1.4 V vs. Ag/Ag+ (equivalent to 3.6–4.0 V vs.Mg/Mg2+), and discharging down to −1.4 or −1.2 V vs. Ag/Ag+(equivalent to 1.2 V or 1.4 V vs. Mg/Mg2+), for a total of vecycles. A rest period of 10 min was inserted between each chargeand discharge step. The amount of Mg insertion and extractionaer charge and discharge of ST-MVO was evaluated by X-rayuorescence (XRF) analysis aer thoroughly washing the elec-trode with acetonitrile.Fig. 2 XRD patterns and Rietveld refinement of (a) pristine MVOcalcined at 750 °C, (a) pristine MVO calcined at 750 °C, (b) 64 hmechanically milled with 4 5 mm ZrO2, (c) +16 h with 4 0.2 mm ZrO2,and (d) +2 h with 4 0.1 mm ZrO2, resulting in 82 h in total. Metalarrangement (e) of 8a, 16c and 16d site in spinel-type structure.Refined site occupancies (f) at 8a, 16c and 16d sites derived fromRietveld refinement.2.3 Computational detailsFirst-principles calculations were performed using the projectoraugmented-wave (PAW) method28 as implemented in the VASPcode.29 The r2SCAN exchange–correlation functional30 wasemployed. We used PAW data sets with radial cutoffs of 1.06,1.43, and 0.80 Å for Mg, V, and O, respectively, and describedMg-2s and 2p, V-3d and 4s, and O-2s and 2p as valence elec-trons. To reproduce the experimental open circuit potential(2.3 V vs.Mg/Mg2+), we examined the Hubbard U correction31 onthe V-3d orbitals and found the best agreement was obtainedwith Ueff = 0 (i.e., without the Hubbard U correction). The stablemagnetic conguration was determined by considering allferromagnetic and antiferromagnetic congurations in theprimitive cell. Structural relaxation of the pristine was per-formed using a plane-wave cutoff energy of 520 eV, whereasrelaxations of the conventional cell were carried out witha 400 eV cutoff under xed-cell constraints. Mg migration wasevaluated using the Nudged-Elastic band (NEB) method.323 Results and discussionFig. 1 shows SEM images of the MVO samples synthesized bythe solid-state method and subsequently subjected to stepwiseMM. The SEM images reveal that the pristine sample consists ofparticle sizes within 200–500 nm, while the MM-MVO arepulverized into ne particles smaller than 100 nm.Fig. 1 FE-SEM images of (a) pristine MVO calcined at 750 °C, (a)pristine MVO calcined at 750 °C, (b) 64 h mechanically milled with 45 mm ZrO2, (c) +16 h with 4 0.2 mm ZrO2, and (d) +2 h with 4 0.1 mmZrO2, resulting in 82 h in total.This journal is © The Royal Society of Chemistry 2026The XRD patterns and their Rietveld renements are shownin Fig. 2(a–d). All the diffraction patterns can be indexed toa spinel-type structure. The gradual broadening of the diffrac-tion peaks with increasing milling time indicates a reduction incrystallite size accompanied by particle fragmentation. Thecrystallite sizes estimated by the Halder–Wagner method were45.6 nm (pristine), 20.8 nm (MM 64 h, 4 5 mm), 6.41 nm (MM64 h + 16 h, 4 0.2 mm), and 5.32 nm (MM 64 h + 16 h + 2 h, 4 0.1mm), respectively, conrming the progressive reduction of thecrystallites. We note that crystallite sizes obtained from theHalder–Wagner method should be interpreted with caution,particularly when signicant peak broadening is present. Themethod assumes that size and strain broadening can be sepa-rated, but defects, strain variations, and instrumental effectsalso contribute, making the absolute crystallite size uncertain.Therefore, the obtained values should be regarded as qualitativeor comparative indicators rather than precise absolute dimen-sions. On the other hand, variations in the relative peakintensities depending on the milling degree suggest changes inthe cation occupancies within the spinel lattice. In the Rietveldrenements, models assuming cation deciency at the 16d siteresulted in poor tting accuracy; therefore, the total occupancyat the 16d site was xed to unity. In the spinel structure, themetal sites 8a, 16c, and 16d are in the relationship shown inFig. 2(e), with the 16c site located between the 8a sites. If a metaloccupies 16c, the two adjacent 8a sites cannot be occupiedbecause they are too close to the 16c metal. The occupancies at8a and 16c are subject to the constraints set out above. Therened occupancies of Mg and V at the 8a, 16d, and 16c sites aresummarized in Fig. 2(f). The pristine sample exhibited nearlyfull Mg occupancy at the 8a site and V occupancy at the 16d site,indicating an ordered normal spinel conguration. In contrast,all MM samples exhibited increased cation disorder, and in theJ. Mater. Chem. A, 2026, 14, 11057–11064 | 11059http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09602bJournal of Materials Chemistry A PaperOpen Access Article. Published on 11 February 2026. Downloaded on 3/23/2026 4:48:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinemost heavily milled sample, a signicant V occupancy (0.194 ±0.010) was observed at the 16c site, suggesting severe cationredistribution. While V K-edge XAS and neutron techniqueshave excellent accuracy for rening the presence of V at the 16csite, laboratory XRD should also provide sufficient sensitivity, asthe X-ray scattering coefficients of Mg2+ and V3+ are signicantlydifferent, reecting the two-fold difference in electron numbers.This contrast allows for signicant renement of the cationdistribution using the Rietveld renement, with all renedmetal site occupancies having estimated standard deviations(ESDs) ranging from 0.007 to 0.011. Considering the solid-statediffusion of Mg2+ during charge–discharge, smaller particlesizes are generally favourable due to the shorter diffusion paths.However, since the 16c sites act as migration channels for Mg2+,their partial occupation by V could block the conduction path-ways. Therefore, excessive milling does not necessarily improvethe electrochemical performance of the cathode.The electrochemical properties of each sample are shown inFig. 3. Although direct quantitative comparison is limited due todiffering experimental conditions, the pristine sample(Fig. 3(a)) maintained a discharge capacity of approximately 70mAh g−1 over ve cycles. In contrast, the MM samples exhibitedunstable discharge capacities that uctuated between cycles,implying signicant side reactions. All samples except that inFig. 3(c) displayed pronounced side reactions during charging(Fig. S2). The GITT curve for theMM sample (82 h total) revealedthat the charging potential did not reach 3.0 V vs.Mg/Mg2+ aer60 mAh g−1 due to side reactions (Fig. S3a). MgV2O4 nano-crystals have been reported to decompose electrolytes at hightemperatures.33 These results suggest that moderate particlesizes in the range of 20–100 nm are optimal. Nonetheless, theobserved capacities (<90 mAh g−1) were much lower than thetheoretical capacity of 282 mAh g−1, and the polarization wasconsiderably large. Thus, the electrochemical performance didFig. 3 Charge–discharge curves were measured using a three-elec-trode cell with a quasi-reference electrode. The left vertical axisindicates the potential versus the quasi-reference, while the right axisshows the corresponding values converted to the Mg/Mg2+ scale: (a)pristine MVO calcined at 750 °C, (b) 64 h mechanically milled with 45 mm ZrO2, (c) +16 h with 4 0.2 mm ZrO2, and (d) +2 h with 4 0.1 mmZrO2, resulting in 82 h in total.11060 | J. Mater. Chem. A, 2026, 14, 11057–11064not reach the level previously reported for ST synthesizedsamples (∼120 mAh g−1 even at high current density of100 mA g−1).24 For comparison, the GITT curve for ST-MVO(Fig. S3b) shows a smooth increase in potential duringcharging, corroborating the suppression of side reactions.Furthermore, the overpotential during both charge anddischarge is reduced, supporting enhanced Mg2+ ionic trans-port. Because excessive milling was found to deteriorateperformance—likely due to V atoms blocking the 16c diffusionpaths—the most heavily milled sample was further investigatedusing VASP calculations.To support our experimental ndings, we performed rst-principles calculations. We created a single Mg vacancy in theconventional cell of an ordered normal spinel containing 8 Mg(8a site), 16 V (16d site), and 32 O atoms, and calculated the Mgmigration barrier using the NEB method. Fig. 4(a and b) showthat Mg migrates though the 16c site during diffusion with theactivation barrier of 0.62 eV. Thus, occupation of the 16c sitesby V atoms, as observed inMM samples, is expected to hinder theMg migration. Furthermore, to reproduce the crystal structurerened from the MM-MVO, we randomly moved four V atomsfrom 16d sites to 16c sites (deep blue spheres in Fig. 4(c)) andmoved four Mg atoms to the empty 16d sites (yellows spheres inFig. 4(c)) from the ordered spinel structures. Aer structureoptimization, all Mg atoms originally located at the 16d sitesremained at the 16d sites; in contrast, two of the V atoms at the16c sites moved to the 8a sites, while the other two stayed at 16csites in Fig. 4(d). These results demonstrate that V can occupyboth the 16d and 16c sites when they are excited to these posi-tions, in good agreement with our experimental observations. Wealso relaxed the atomic positions of the disordered spinel, i.e.,Mg0.5V0.5 at the 8a site and Mg0.25V0.75 at the 16d sites, andconrmed that all Mg and V atoms remained at their localpositions. We also investigated the V migration barrier from the16d to the 16c sites. For this calculation, two Mg atoms along theFig. 4 Mg migration and disordered structures of MgV2O4 from DFTcalculations. (a) Mg migration pathway from the initial to the finalposition. Red, purple, and orange spheres are O, V, and Mg, respec-tively. Small yellow spheres are the intermediate positions of themigrating Mg atom. (b) Energy profile along the Mg migration path. (cand d) Disordered MgV2O4 structures before and after structureoptimization. Yellow and deep blue spheres are Mg and V which areintentionally located at the 16d and 8a sites (see the documents fordetails), respectively. (e) V migration pathway from the 16d to 16c sites.The gray spheres are removed Mg sites. (f) Energy profile along the Vmigration path shown in (e).This journal is © The Royal Society of Chemistry 2026http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09602bFig. 5 Rietveld refinements for XRD patterns of MVO (a) synthesized at600 °C via solvothermal method and (b) calcined at 750 °C andsubsequently ball-milled for a total of 82 h. (c) SEM image of MVOsynthesized at 600 °C via solvothermal method. (d) Charge anddischarge cycle-curves of MVO synthesized at 600 °C via solvothermalmethod.Paper Journal of Materials Chemistry AOpen Access Article. Published on 11 February 2026. Downloaded on 3/23/2026 4:48:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinemigration pathway were removed as shown in Fig. 4(e) becausetheir strong Mg–V Coulomb interactions destabilized the NEBcalculations. Still, the resultant migration barrier was approxi-mately 2.0 eV, which is considerably larger than theMgmigrationbarrier. These results indicate that thermal activation alone isinsufficient to move V to 16c sites, and mechanical milling isexpected to provide enough energy to facilitate the V migrations.Consequently, these results demonstrate a clear limitation inimproving the electrochemical properties of MgV2O4 throughmechanical milling alone.To overcome this limitation, a sample synthesized via a STroute was evaluated. As shown in theMo Ka XRD data (Fig. 5(a)),the ST-derived and MM-derived (Fig. 5(b)) samples exhibitedsimilar diffraction patterns, indicating that the ST method canproduce ne crystallites without mechanical milling. SEMobservations in Fig. 5(c) revealed cactus-like particles withdiameters of approximately 5 mm, which can be regarded assecondary particles composed of radially assembled needle-likecrystallites. Although the diameter of each needle-like crystalliteis too small to be measured by SEM, XRD analysis using theHalder–Wagner method revealed a crystallite size of 5.38 nm,comparable to that of MM-MVO (5.32 nm). This indicates thatthe short-axis direction of each needle-like crystal providesa sufficiently short Mg2+ diffusion length, while the long-axisdirection remains relatively long, which may help suppressTable 1 Comparison of cathode properties among typical MgTM2O4 spElectrolyte Temp./°CMgV2O4 (this study) [Mg(G4)][TFSA]2/[Pyr13][TFSA] 90MgV2O4 (ref. 24) (PhMgCl)2-AlCl3/THF RTMgCo2O4 (ref. 10) (Mg10/Cs90)-TFSA 150MgFe1.6Mn0.4O4 (ref. 18) [Mg(G4)][TFSA]2/[Pyr13][TFSA] 100MgMn2O4 (ref. 19) [Mg(G4)][TFSA]2/[Pyr13][TFSA] 100MgCrMnO4 (ref. 20) Mg[TPFA]2/G3 60This journal is © The Royal Society of Chemistry 2026side reactions. Considering that the spinel structure enablesthree-dimensional diffusion, the combination of shortened twodimensions and elongated one dimension can still offersignicant benets. Moreover, Rietveld renement of the ST-MVO revealed the following site occupancies:(Mg0.692(8)V0.070(8))8a(V0.119(7))16c(Mg0.154(6)V0.846(6))16d.Compared with MM-MVO (Fig. 2(f)), the ST-MVO exhibitsa lower V occupancy (0.119 ± 0.007) at the 16c site and higherMg and V occupancies at the 8a and 16d sites, respectively,indicating a more ordered spinel structure.The electrochemical performance of this sample, shown inFig. 5(d), demonstrated a reversible charge–discharge capacityof 175 ± 2 mAh g−1. Compared with the MM samples, thepolarization was smaller, and the average discharge potentialreached 1.95 ± 0.04 V vs. Mg/Mg2+. These results highlight theadvantage of the cactus-like microstructure, which maintainslow V occupancy at the 16c site while effectively utilizing necrystallites for Mg2+ insertion and extraction. In previousstudies24 employing less oxidative-stable grignard-based elec-trolytes, charging was limited to 2.0 V vs. Mg/Mg2+, resulting incapacities of ∼100 mAh g−1. In contrast, the present electrolyteallowed charging up to 3.6 V vs. Mg/Mg2+, enabling greater Mgextraction and thus higher capacity. On the other hand, underthe high-temperature cycling at 90 °C, the degradation becomesevident due to the reductive decomposition of the TFSA anionson anode surface. Moreover, the coordinated G4 molecules inthe electrolyte would undergo thermal decomposition via beingcontact with MgV2O4 at elevated temperature.33 The cross-overfrom cathode-electrolyte interface to Mg anode can alsoinduce impedance growth and consequent cell failure, espe-cially under high temperature operation.34 To gain deeperunderstanding of the events taking place at both electrode–electrolyte interface, further investigations beyond the vecycles are necessary. Quantitative XRF analysis of the electrodesaer washing with acetonitrile showed Mg/V ratios of 0.63(charged) and 1.29 (discharged), conrming the reversibleinsertion/extraction of approximately 0.66 Mg2+ per formulaunit. This corresponds to a calculated capacity of 186 mAh g−1,quite consistent with the measured capacity. These ndingssuggest that the ST-MVO exhibits both deintercalation-typebehavior similar to that of LiMn2O4 spinel and insertion-typebehavior resembling Li4Mn5O12 spinel,35 resulting inenhanced capacity. Further improvement may be achievable bydeveloping electrolytes or solid electrolytes with higher oxida-tive stability, which would enable deeper charging andinel seriesPotential/V Capacity/mAh g−1 Starting reaction Reversibility2 175 Deintercalation 106% (5th/1st)99% (5th/2nd)z0.7 132 Deintercalation 85% (300th/1st)z2.5 170 Insertion 71% (2nd/1st)z1.4 z200 Insertion 45% (10th/1st)2 214 Insertion 46% (5th/1st)z1.8 75 Deintercalation 59% (2nd/1st)J. Mater. Chem. A, 2026, 14, 11057–11064 | 11061http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d5ta09602bJournal of Materials Chemistry A PaperOpen Access Article. Published on 11 February 2026. Downloaded on 3/23/2026 4:48:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineapproach the theoretical capacity. Previous spinel-typeMgTM2O4 (TM = Co,9,10 Mn,19,22 Fe,18 Cr,20 etc.) cathodes havegenerally shown insertion-type behavior upon discharge, andtheir capacity fade has been attributed to irreversible transitionto a rock-salt Mg2TM2O4 phase. The cathode properties oftypical MgTM2O4 spinel series are summarized in Table 1. TheST-MVO developed in this study exhibits a high potential andcapacity, suggesting its superior reversibility. This behavior isattributed to a reaction pathway initiated by demagnesiation,combined with the favorable Mg2+ conductions inherent to thematerial. Further optimization of the synthesis conditions mayallow additional modication of the ball cactus-likemorphology and crystallographic order in cation sites.Combined with the use of high oxidative-stability electrolytessuitable for long-term cycling, ST-MVO has the potential toevolve into a highly practical cathode material in MRBs.4 ConclusionsIt was generally believed that ne particles produced by MMexhibited high cathode properties, but it was found that thiswas not applicable to MVO because the MM caused V to moveinto the Mg2+ conduction site, 16c, hindering Mg dein-tercalation and intercalation. However, in the eld of MRBspinel cathodes, which have previously operated in aninsertion-type manner, the fact that ST-MVO is an importantcathode material capable of starting charging is a major stepforward. By using the ST method, MVO can be synthesized atlow temperature of 600 °C, and it is possible to obtain MVO thatis closer to the order arrangement while still having nano-crystallites. When charged to a potential equivalent to 3.6 V vs.Mg/Mg2+, the cell delivered 175 mAh g−1, 62% of the theoreticalcapacity, representing an improvement of more than 40% overthe capacity reported in the previous study. With further opti-mization of the solvothermal synthesis conditions, MgV2O4with improved morphology and structural ordering may achievecapacities closer to the theoretical value of 282 mAh g−1. At thesame time, the development of electrolytes with higher oxida-tive and thermal stability, particularly beyond the knowndegradation limits of TFSA/G4 at 90 °C, will be essential for fullyrealizing the practical potential of MVO-based MRBs.Author contributionsN. Ishida: conceptualization, data curation, formal analysis,investigation, project administration, supervision, writing –original dra, writing – review & editing. K. Kubota: investiga-tion, methodology, resources, validation. T. Kojima: investiga-tion, methodology, resources. T. Mandai: investigation,resources, writing – review & editing. S. Kiyohara and Y.Kumagai: conceptualization, formal analysis, investigation,visualization, writing – original dra. T. Ichitsubo: fundingacquisition, project administration, super vision.Conflicts of interestThere are no conicts to declare.11062 | J. Mater. Chem. A, 2026, 14, 11057–11064Data availabilityThe data supporting this article have been included as part ofthe supplementary information (SI). Supplementary informa-tion: laminated three-electrode cell, charge/discharge curves,GITT curves, and tting of PDF. 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