High Capacity Rechargeable Magnesium-Ion Batteries Based on a

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High Capacity Rechargeable Magnesium-Ion Batteries Based on a Microporous Molybdenum−Vanadium Oxide Cathode Watchareeya Kaveevivitchai and Allan J. Jacobson* Texas Center for Superconductivity and Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States ABSTRACT: The three-dimensional microporous framework of the oxide Mo2.5+yVO9+δ (containing Mo5+/6+ and V4+/5+) is defined by three-, six-, and seven-membered ring channels. Due to the oxidation−reduction properties of Mo and V ions and the large open tunnels that can provide a diffusion pathway for small ions, Mo2.5+yVO9+δ has been found to intercalate both Li+ ions, reported previously, and also Mg2+ ions in Mg-ion batteries. Cathodes composed of Mo2.5+yVO9+δ were discharged and charged in Mg-ion cells at current densities ranging from 2 mA/g (C/70) to 10 mA/g (C/12). Mg2+ ions can be inserted into and extracted from MgxMo2.5+yVO9+δ between ∼3.33 and 1.73 V vs Mg/Mg2+ at room temperature, with up to 3.49 Mg2+ ions per formula unit intercalated into the framework, corresponding to a capacity of 397 mAh/g (1st discharge) which is among the highest capacities reported for Mg-based batteries. Powder X-ray diffraction was used to determine the lattice parameters of MgxMo2.5+yVO9+δ (0 < x ≤ 3) compositions prepared by chemical insertion using (C4H9)2Mg. Our findings suggest that in order to overcome the significant shortcomings observed in the traditional Mg-ion Chevrel cathodes (low operating voltage and specific capacity), oxide-based intercalation cathodes can be used instead of the chalcogenides. The slow Mg diffusion kinetics caused by strong Coulombic interactions of the multivalent cations are offset by using a microporous oxide to allow easy migration and minimize steric effects. The combination of Mo and V ions, which can change oxidation state by more than one, can reduce volume expansion and help with charge redistribution during insertion of Mg2+ ions to maintain electroneutrality. This approach based on a microporous molybdenum−vanadium oxide suggests the possibility of using other porous mixed metal oxides for the development of advanced secondary multivalent-ion batteries.

1. INTRODUCTION For the past two decades, Li-ion batteries have contributed to the commercial success of portable electronic devices, and are considered one of the most successful advanced electrochemical power sources. The application of Li-ion batteries in higher-volume applications, such as large-scale energy storage units for the power grid and sustainable vehicles, namely, plugin hybrid electric vehicles and ultimately full electric vehicles, however, is still limited by high prices, resource scarcity, safety issues, and energy density.1,2 An alternate solution to achieve the energy density and cost requirements for large-scale applications is to use systems that are based on elements more abundant than lithium. Magnesium batteries have been suggested as one promising candidate.3,4 Magnesium (with divalent Mg2+ as the charge carrier) is expected to possess a high volumetric capacity of 3833 mAh/cm3 (2046 mAh/cm3 for Li) with a low reduction potential (−2.356 V vs standard hydrogen electrode, compared to Li, − 3.04 V). Mg is the fifth most abundant element in the earth’s crust, 24 times cheaper than Li,5−8 and Mg metal anodes are safer than Li when exposed to air due to oxide surface passivation, and do not show dendrite formation.7−9 Despite the potential advantages of magnesium over lithium as an anode material, the development of Mg-based batteries © 2016 American Chemical Society

has been difficult due to complications arising from Mg metal anodes and Mg-ion intercalation cathodes.10,11 Passivation of Mg metal in most nonaqueous polar organic solvents commonly used as electrolytes is an important problem.12,13 Unlike the case of Li metal anodes, when Mg metal is in contact with the electrolytes, the surface films (composed of ionic compounds) that are formed on Mg anodes as a result of reduction reactions, are electronic and ionic insulators. Therefore, Mg metal can only behave as a reversible anode when there is no surface passivation.14−16 The first electrolytes that allowed Mg electrodes to behave reversibly were Grignard reagents in ethers, and magnesium organohaloaluminates with a formula of Mg(AlCl2BuEt)2 in THF. The electrochemical stability window of these electrolyte solutions is between 1.5 and 2.4 V, which limits the use of cathode materials with a higher redox potential.17,18 In 2000, the first successful rechargeable Mg batteries were developed, based on Mg metal anodes and Mo6S8 (Chevrel phase) cathodes with the electrolyte Mg(AlCl2BuEt)2/THF (MgxMo6S8, 0 < x < 2 with reversible specific capacity of ∼80 mAh/g at 0.3 mA/cm2).19 Received: March 28, 2016 Revised: June 10, 2016 Published: June 10, 2016 4593

DOI: 10.1021/acs.chemmater.6b01245 Chem. Mater. 2016, 28, 4593−4601

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Chemistry of Materials Since then, other electrolytes with improved electrochemical stability which allow passivation-free reversible Mg electrodes have been reported, such as the all-phenyl-complex (APC)based electrolyte, hexamethyldisilazide magnesium chloride (HMDSMgCl), and magnesium borohydride (Mg(BH4)2).3,7,17,20 Many new electrolytes reported for magnesium batteries have limited electrochemical stability and therefore are not compatible with many potential cathode materials.7,21 Moreover, intercalation of Mg ions into most materials which are excellent hosts for Li ions is found to be kinetically limited, even though these cations have similar ionic radii (0.76 Å for Li+ and 0.72 Å for Mg2+).3,5 The reason for the slow kinetics of Mg-ion insertion into inorganic hosts is due to slow solid-state diffusion of the divalent cations. Slow diffusion arises because Mg2+ cations have a high charge/radius ratio and strongly interact with the host lattice. Insertion of a divalent cation also requires a more complex redistribution of charge within the local crystal structure to maintain electroneutrality.3,13,22 In addition to the Chevrel phase, Mo6S8, used in the prototype magnesium rechargeable batteries by Aurbach et al.,19 several other types of Chevrel phase (MxMo6T8, M = metal, T = S, Se) cathodes have also shown good cyclability and intercalation kinetics in magnesium batteries.13,18,23−28 The relatively low ionicity of chalcogenides leads to weak electrostatic interactions between Mg2+ cations and the negatively charged frameworks, helping to increase Mg2+ ion mobility. The Chevrel phases contain Mo 6 octahedral clusters surrounded by eight chalcogenide ions and are metallic.13 Due to the electron delocalization, as the Mg-ion insertion takes place, the Chevrel phase can easily tolerate changes in electron density, thus allowing rapid redistribution of charge to maintain local charge neutrality.3 The practical application of Chevrel cathodes, however, is limited by the low operating voltage (∼1.1 V vs Mg/Mg2+) and specific capacity.21 Finding electrode materials that can reversibly insert multivalent cations is difficult. Various cathode materials have been proposed for Mg-ion batteries, such as transition metal chalcogenides (o-Mo9Se11,29 TiS2,30,31 graphene-like MoS2,32 CoS,33 NbS3,34 TiSe2,35 WSe2 nanowires),36 transition metal oxides (MnO2,37−39 V2O5,40−44 MoO3,45 RuO2,46 Co3O4,47 Mn2.15Co0.37O4),48,49 polyanionic compounds (olivine-type MgxMSiO4 (M = Fe, Mn, Co),50−52 MgFePO4F,53 NASICON-type Mg 0. 5 Ti 2 (PO 4 ) 3 and its derivatives, 5 4 , 5 5 V2(PO4)3),56 spinel-related materials (MgMn2O4, MgNiO2, MgCo 2 O 4 , Mg 0.67 Ni 1.33 O 2 ), 57,58 Prussian blue analogs (PBAs),59 and organic compounds.21,60 Nevertheless, these materials either show poor electrochemical behavior or are difficult to make.21 To overcome the low operating voltage observed in the Chevrel phases, oxide-based compounds are an alternative to the chalcogenides. The high electronegativity of oxygen compared to a chalcogen leads to high degree of ionic character of the metal−oxygen bond in oxides. This generally results in higher electrochemical potentials of metal-ion intercalation reactions.13 The higher voltage together with the lower molecular weight increase the specific energy of the battery.13 Stronger transition metal oxide bonds compared to those found for chalcogenides result in higher chemical and thermal stabilities. Transition metal oxide cathodes are less susceptible to structural degradation on reaction with Mg and may have longer cycle life as a result.10

The stronger Coulombic interactions between Mg ions and oxide host lattices, however, may result in slow Mg diffusion.13 To some extent, this problem can be managed by decreasing the diffusion distance by using nanomaterials as cathodes; using appropriate ligands complexed to Mg ions to help shield the charge; and using open-tunnel structures or layered compounds with large interlayer distances to facilitate intercalation/ deintercalation and minimize steric effects.13,61 Examples of such materials that have been successfully used as cathodes in multivalent-ion battery systems are Hollandite α-MnO2 (2 × 2 tunnels),62 Todorokite MnO2 (3 × 3 tunnels),61,63 W18O49 (tunnels formed by six-membered ring units of MO 6 octahedra),64 and V2O5 aerogels or nanowires (layered structure).40,65 The insertion of Mg2+ into a host lattice is associated with a change in the oxidation state of the host transition metal ions. Local electroneutrality requires that the two charges associated with insertion of one Mg2+ ion are compensated by two electron reduction of nearby transition metal ions.22 Transition metal ions that can change by two or more oxidation states, such as molybdenum or vanadium, can facilitate achieving local electroneutrality and lower barriers to Mg-ion diffusion.22,66 To enhance solid-state Mg-ion diffusion, the binding energy between the Mg ions and the anionic host lattice can be weakened by reducing the effective negative charge of lattice oxygen ions by competitive binding to high oxidation state metal ions in the framework. Ternary compounds such as molybdates, tungstates, and vanadates often show enhanced ionic conductivities.67 Structural changes induced by intercalation are an important aspect in determining the performance of multivalent-ion conducting cathode materials. Generally, reduction of the transition metal ions on Mg2+ insertion will lead to an increase in volume. Change in oxidation state of transition metal ions may lead to significant local deformations of the crystal structure resulting in poor electrochemical performance.22 The magnitude of the structural change will depend on the transition metal ion coordination and the nature of the d orbitals being filled. For example, in an octahedral environment, filling nonbonding orbitals causes only a small volume change, whereas filling antibonding orbitals leads to a larger increase in volume. As a result, the early transition metals which have several unoccupied nonbonding orbitals, such as Ti and V, generally show smaller volume changes and are considered to be a better choice than late transition metals where one or more antibonding orbitals are filled.68,69 Taking into account these various criteria, we have investigated the suitability of a microporous molybdenum− vanadium oxide molecular sieve Mo2.5+yVO9+δ as a cathode material for rechargeable Mg-ion batteries. The large open channels constructed by three-, six-, and seven-membered rings of MO6 octahedra (M = Mo5+/6+ or V4+/5+), which can provide diffusion pathways for small guest molecules and the redox properties of Mo2.5+yVO9+δ suggest the potential of this material as an intercalation cathode for rechargeable Mg-ion batteries.70,71 Recently, this microporous molybdenum−vanadium oxide has been reported to reversibly intercalate Li+ ions in lithium batteries up to 6 Li per formula unit with a reversible capacity exceeding 300 mAh/g.72

2. EXPERIMENTAL SECTION 2.1. Synthesis. Mo2.5+yVO9+δ was synthesized under hydrothermal conditions as previously reported.72 To describe briefly the procedure, 4594


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Chemistry of Materials a mixture of (NH4)6Mo7O24·4H2O and VOSO4·nH2O dissolved in H2O was stirred for 10 min at room temperature. The resulting solution was placed in a 23 mL Teflon-lined Parr stainless steel autoclave, purged with nitrogen, and then heated at 190 °C for 48 h. The dark gray solid product was filtered, washed with water, and dried at 80 °C. The product was then stirred in 0.4 M oxalic acid solution at 60 °C for 30 min in order to remove an amorphous impurity, washed with water, and dried at 80 °C overnight. Finally, the micropores were emptied by calcining the solid at 400 °C in air. MgxMo2.5+yVO9+δ (0 < x ≤ 3) was prepared by two different methods: (1) in electrochemical cells and (2) by chemical intercalation of Mo2.5+yVO9+δ using di-n-butylmagnesium, (C4H9)2Mg, in heptane as the reductant at room temperature in an argon atmosphere.11,73,74 2.2. Electrochemical Studies. The electrochemical properties of Mo2.5+yVO9+δ were obtained using a MacPile system (Biologic SA, Claix, France) multichannel potentiostat-galvanostat at different discharge−charge rates between ∼3.33 and 1.73 V vs Mg/Mg2+ in three-electrode cells. Three-Electrode Cells. In a three-electrode cell, the working electrode consisted of 75 wt % Mo2.5+yVO9+δ, 15 wt % acetylene carbon black, and 10 wt % polyvinylidene fluoride (PVdF) binder. The mixture was premixed in an agate mortar, and then ground using a high-energy ball mill (SPEX mill 8000M) in an agate vial set for 10 min. The solid mixture was stirred in N-methyl-2-pyrrolidinone (NMP) solvent to form a slurry, which was then coated onto gold mesh strips to give 1 cm2 of active area. The electrodes were dried overnight under vacuum at 120 °C to remove the solvent (∼1 mg of active material), followed by hot-pressing to ensure uniform thickness. The electrodes were finally dried under vacuum at 90 °C for 12 h, and then transferred to an argon-filled glovebox. Two pieces of activated carbon (AC) cloth were used as a counter electrode and a quasireference electrode (QRE).5,7,75 All three electrodes were attached to gold wires and suspended inside a sealed glass cell, which was filled with a 0.5 M magnesium(II) bis(trifluoromethanesulfonyl)imide, Mg[N(SO2CF3)2]2, (Mg(TFSI)2) in acetonitrile (AN) electrolyte solution. 2.3. Chemical Intercalation. Samples of MgxMo2.5+yVO9+δ were prepared by stirring Mo2.5+yVO9+δ powder in an appropriate amount of 1 M di-n-butylmagnesium ((C4H9)2Mg) in heptane inside an argonfilled glovebox at ambient temperature for 3−12 d. The reaction times depend on the degree of the Mg insertion (the larger the value of x, the longer the time needed). The products were filtered, washed with dry heptane to remove any unreacted reagent and impurities, and then dried under vacuum. The amounts of Mg ions intercalated into the host structure were determined by titrating the liquid mixture (the filtrate and washings mixed with 25 mL of water) with a standardized aqueous HCl solution. The solid was then equilibrated to ensure magnesium homogeneity by stirring in a nonaqueous electrolyte solution containing Mg2+ ions, 0.5 M Mg(TFSI)2 in AN, for 48 h inside an argon-filled glovebox.5,50,53,76 2.4. Materials Characterization. Powder X-ray diffraction (PXRD) patterns were collected at room temperature on a Phillips PANalytical X’Pert PRO diffractometer with Cu Kα radiation (λ = 1.54046 Å) for determination of the unit cell parameters. Phase analyses of MgxMo2.5+yVO9+δ were performed without exposure to air. GSAS was used for profile refinements of PXRD data.77 Scanning electron microscopy (SEM) was carried out with a JSM6330F (JEOL) microscope. X-ray photoelectron spectroscopy (XPS) measurements were made with a Physical Electronics PHI 5700 spectrometer, equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operated at 350 W.

microporous framework with three-, six-, and seven-membered ring tunnels with orthorhombic symmetry, Pba2 (a = 21.0505(3) Å, b = 26.3766(5) Å, and c = 4.0144(1) Å). The diameters of the six-membered and seven-membered ring channels are approximately 3 and 5 Å, respectively (Figure 1).

Figure 1. (a) Structure of Mo2.5+yVO9+δ viewed down the c-axis: MO6 octahedra and MO7 pentagonal bipyramids (M = Mo and V cations predicted theoretically with different oxidation states and occupancies: green, Mo5+/V4+; red, Mo6+/V5+; blue, Mo6+/Mo5+; orange, Mo5+; and purple, Mo6+); (b) Projection of the framework along the b-axis showing the layer connections.

The oxygen, z, in Mo2.5+yVOz was determined to be 9.93 and the Mo:V ratio was 2.48:1 per formula unit.72 Mo2.5+yVO9+δ crystals are rod-like with the size of up to 200 nm in diameter and tens of micrometers in length (Figure 2a). The channels that can provide facile diffusion of guest ions, such as Mg2+, are in the c direction, which is along the length of the crystal.72 3.2. Electrochemical Behavior of Mo2.5+yVO9+δ. To investigate the electrochemical properties, Mo 2.5+y VO 9+δ (Mo2.48VO9.93) was used as the cathode by mixing with conducting acetylene carbon black and polymer binder. The morphology of the ground cathode mixture is shown in Figure 2b,c with the particle size of 80 nm up to 2 μm. For galvanostatic measurements, three-electrode Mg-ion cells were used with activated carbon cloth as counter and quasireference electrodes. Due to complications that may arise from other Mg-battery electrolytes, an electrolyte containing the simple salt Mg(TFSI)2 was used. This salt does not contain any corrosive species such as halides, is highly resistive to oxidation, and is compatible with most cathodes. Mg(TFSI)2 also allows a wide voltage operation and can easily be dissolved in different organic solvents, providing high ionic conductivity.3,50 Acetonitrile was used as the solvent to prepare 0.5 M Mg(TFSI)2 electrolyte solution due to its high oxidation stability.3,5 Because it is not certain if Mg(TFSI)2 allows reversible Mg deposition/stripping of Mg anodes,3 activated carbon cloth which is inert and capable of reversible charge storage via electrical double-layer capacitance (EDLC) due to its high

3. RESULTS AND DISCUSSION 3.1. Description of Mo2.5+yVO9+δ. The detailed discussion of the synthesis has been given previously.72 The structure of Mo2.5+yVO9+δ contains layers made from corner-sharing MO6 octahedra and pentagonal [(Mo)Mo5O27] units, which contain an MoO7 pentagonal bipyramid with five edge-sharing MoO6 octahedra. The layers are stacked by corner-sharing to form a 4595


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Figure 2. SEM images of as-synthesized rod-like Mo2.5+yVO9+δ crystals with the size of up to 200 nm in diameter taken from ref 72 (a), and electrode mixture (Mo2.5+yVO9+δ, carbon black, and PVdF) after grinding by using a high-energy ball mill for 10 min (b and c).

surface area, was chosen instead of Mg metal as the counter electrode.5,17,21 The first electrochemical discharge obtained from a threeelectrode cell at a rate of 1 Mg in 70 h or C/70 (2 mA/g) is shown in Figure 3 (black).72,78 The profile shows that 3.49

Figure 3. Electrochemical discharge−charge profiles of an AC/ Mo2.48VO9.93 cell at a rate of C/70 (2 mA/g): 1st cycle, black; 2nd cycle, red; 5th cycle, blue; 10th cycle, green. Figure 4. (a) Cycling performance of an AC/Mo2.48VO9.93 cell between 3.33 and 1.73 V at a rate of C/70 (2 mA/g). (b) dQ/dV plot for the first cycle.

Mg2+ ions per formula unit can be inserted into Mo2.48VO9.93 with short plateaus at 2.15 and 1.75 V vs Mg/Mg2+. The total capacity corresponds to 397 mAh/g between ∼3.20 and 1.73 V. Due to the fact that molybdenum and vanadium ions in Mo2.5+yVO9+δ have mixed oxidation states (Mo5+/6+ or V4+/5+) and occupancies, it is unclear which redox couples are involved during discharge. Presumably, all of the metal ions (Mo6+, Mo5+, V5+, and V4+) actively participate during the Mg insertion, where the metal ions are reduced as Mg2+ ions are intercalated into the channels. XPS spectra of the Mo 3d and V 2p levels after Mg-ion insertion show significant shifts in binding energies from MoO3, V2O5, and VO2, indicating metalion reduction.79 The initial Mg insertion is relatively slow with a large IR drop, which can be attributed to low ionic conductivity due to the low concentration of the mobile charge carriers in the solid.21 On subsequent charge, not all of the Mg ions can be extracted. This behavior was also observed when Mo2.48VO9.93 was used as cathode in Li batteries at high current densities.72 Cycling performance from a galvanostatic measurement between 3.33 and 1.73 V of an AC/Mo2.48VO9.93 cell at C/70 is shown in Figure 4a, with a dQ/dV plot in Figure 4b. The capacity declines on the first few cycles followed by stable capacity retention at ∼235 mAh/g for up to 15 cycles. The initial capacity loss may be attributed to some Mg2+ ions that are trapped in the three-membered ring tunnels of the

framework. When these small sites are filled, Mg2+ ions can easily diffuse inside the large six- and seven-membered ring channels. Possibly, the initial capacity fade, especially for such a deep cycle, could also be attributed to contact problems due to volume changes. The discharge−charge voltage profiles of Mo2.48VO9.93 (MgxMo2.48VO9.93, 0 ≤ x ≤ 1) at a current density of C/40 (4 mA/g) are shown in Figure 5a. Compared to the discharge profile at 2 mA/g in Figure 3 (1st cycle), as the discharge rate increases, the discharge potential is lower due to kinetic effects. Upon charge, all of the inserted Mg2+ ions (1 Mg per formula unit) can be removed and the cell is reversible with a discharge capacity of 114 mAh/g at 4 mA/g (C/40). The large open tunnel structure of Mo2.48VO9.93 allows rapid transport and provides a large number of vacant sites for Mg2+ ions, thus explaining the rate capability and capacity. The large surface area of small rod-like Mo2.48VO9.93 particles leads to short diffusion paths for the electroactive species. Capacity retention data of AC/Mo 2.48 VO 9.93 cells (MgxMo2.48VO9.93, 0 ≤ x ≤ 1) at current densities of 4 mA/g (C/40) and 10 mA/g (C/12), are shown in Figure 5b. At 4 mA/g, the plot shows stable capacity retention of ∼114 mAh/g. 4596


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PXRD patterns were collected on MgxMo2.48VO9.93 samples under Kapton without exposure to air. The X-ray diffraction data of 0 ≤ x ≤ 3 in MgxMo2.48VO9.93 (x values determined by the titration) are shown in Figure 6. The unit cell parameters of

Figure 5. (a) Electrochemical discharge−charge profile of an AC/ Mo2.48VO9.93 cell (MgxMo2.48VO9.93, 0 ≤ x ≤ 1) at a rate of C/40 (4 mA/g). (b) Capacity retention data for galvanostatic cycling of AC/ Mo2.48VO9.93 cells (MgxMo2.48VO9.93, 0 ≤ x ≤ 1) at different current densities: black, 4 mA/g (C/40); red, 10 mA/g (C/12).

Figure 6. PXRD patterns of chemically prepared MgxMo2.48VO9.93 (x = 0 → 3) in a selected 2θ range (25−30°).

MgxMo2.48VO9.93 determined by profile fitting using GSAS are shown as a function of the degree of Mg insertion in Figure 7. The Le Bail refinement of the pristine Mo2.48VO9.93 (that was equilibrated in the electrolyte) is shown in Figure 8 for comparison with MgxMo2.48VO9.93 (x = 1.5). The lattice parameters of chemically prepared MgxMo2.48VO9.93, 0 < x ≤ 3, compounds are consistent with the voltage profile shown in Figure 3 (black). From x = 0.2 in MgxMo2.48VO9.93, the Mg intercalation occurs with a voltage

When the Mg-ion cell is cycled at 10 mA/g, the capacity declines during the initial cycles, but then equilibrates and cycles reversibly at ∼90 mAh/g without further capacity loss. Interestingly, the capacity retention data of the cell cycled in the range of 0 ≤ x ≤ 1, MgxMo2.48VO9.93 at 4 mA/g (Figure 5b in black) where all of the inserted Mg ions can be completely removed, are compared to those (with initial capacity loss) cycled in the range of 0 ≤ x ≤ 3.49 at 2 mA/g (Figure 4a). The result suggests that when x is small, the preferred sites for Mg ions in the host structure are in the six- and seven-membered ring channels. The large tunnels permit rapid diffusion of the guest species (both in and out); therefore, all of the Mg ions can be extracted even at a high current density. Only when x is large are Mg ions forced to enter three-membered ring channels from which they cannot be removed easily. 3.3. Chemical Redox Intercalation of Mo2.48VO9.93 Using a Reducing Agent. To examine the structural evolution of Mo2.48VO9.93 during Mg intercalation, powder Xray diffraction was performed on chemically reduced Mo2.48VO9.93 (MgxMo2.48VO9.93, 0 < x ≤ 3). A reducing agent, (C4H9)2Mg, was used to prepare MgxMo2.48VO9.93 according to the following reaction: Mo 2.48 VO 9.93 + x(C4H9)2Mg → MgxMo2.48VO9.93 + xC8H18.11 The degree of Mg insertion (x) was obtained by titration to determine the amount of unreacted (C4H9)2Mg from the reaction. To obtain homogeneous composition of the solid particles, the chemically prepared MgxMo2.48VO9.93 was equilibrated in the electrolyte solution containing Mg2+ ions (0.5 M Mg(TFSI)2 in AN) by stirring for 48 h at room temperature under inert atmosphere.

Figure 7. Lattice parameters and unit cell volume of chemically prepared MgxMo2.48VO9.93 from profile refinements (error bars in red). 4597


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shows that the unit cell volume is only ∼5% larger than that of the pristine compound as a result of Mg-ion insertion. Overall, as x in MgxMo2.48VO9.93 increases, a and b which correspond to the in-plane dimensions of the layers significantly increase, whereas the c parameter remains relatively constant (layer stacking direction). Upon charge, magnesium deintercalation occurs, and the voltage profile is sloping over the entire charge curve corresponding to a solid-solution-like electrochemical behavior (Figures 3 and 5a). As indicated by the electrochemical profiles and PXRD data, some structural reorganization does occur; however, Mg insertion into Mo2.48VO9.93 is reversible. The ability of Mo2.5+yVO9+δ to act as a Mg intercalation cathode may be due to (1) the unique nature of Mo2.5+yVO9+δ framework in which molybdenum and vanadium ions (Mo5+/6+ and V4+/5+) have mixed occupation of sites (resulting from the similarities in coordination chemistry between molybdenum and vanadium)80 and variation of local oxygen environments, leading to excellent oxidation−reduction behavior as apparent in the outstanding catalytic properties previously reported.72 As Mg2+ ions intercalate into the Mo2.5+yVO9+δ host, in order to maintain local charge neutrality, the adjacent transition metal ion must reduce its oxidation state by two (to compensate for the two electrons donated by Mg2+). Mo and V ions can easily change by more than one valence state and can redistribute the charge and help lower Mg-ion migration barriers; (2) the large open tunnels along the c-axis of the Mo2.5+yVO9+δ structure providing a large number of vacant sites that can allow transport of the strongly polarizing Mg2+ cations. The size of the migration channels is a crucial factor in determining migration barriers and accounts for the magnesium storage capacity. The fact that the porous host has three-dimensional framework also leads to high structural stability during cycling;67 (3) the nature of the d orbitals of molybdenum and vanadium. As Mg2+ ions are inserted, reduction of the transition metal ions by filling of the nonbonding t2g orbitals present in octahedrally coordinated Mo and V (Mo6+, 4d0; Mo5+, 4d1; V5+, 3d0; V4+, 3d1) causes only a small volume increase. Moreover, the open structure of Mo2.5+yVO9+δ also helps accommodate volume change as more Mg ions are inserted, thus reducing lattice strain and helping with intercalation reversibility; (4) the good electronic conductivity of the electron delocalized Mo2.5+yVO9+δ framework. As reported previously, the compound can be used as an electrode material in lithium cells without conducting additives, and the same specific capacity as those with carbon additives still can be delivered.72

Figure 8. Le Bail refinements of (a) Mo2.48VO9.93 and (b) Mg1.5Mo2.48VO9.93 (prepared by the reduction of Mo2.48VO9.93 using (C4H9)2Mg) in Pba2, wRp = 3.65%: a = 21.2595(8) Å, b = 26.7235(8) Å, and c = 4.0807(2) Å (measured, orange; calculated, green; difference, pink; Bragg reflections, vertical tick marks).

plateau at 2.15 V which is characteristic of a two-phase equilibrium. From x = 0 to ca. 0.2, a sloping potential is observed corresponding to a single-phase solid solution, consistent with the PXRD data (slight increase in cell volume). From x = 0.2 to ca. 1, MgxMo2.48VO9.93 (x = 0−0.2) is in equilibrium with another solid solution (1 ≤ x ≤ 2) as shown in Figure 3 (black). The unit cell parameters show a distinct change at x = 1. According to the profile refinements, MgxMo2.48VO9.93 (1 ≤ x ≤ 2) has a similar structure to the pristine Mo2.48VO9.93 compound. The structure with Pba2 symmetry is maintained throughout the intercalation; a and b significantly increase whereas c only increases very slightly (Figure 7). It is worth noting that the second solid solution (MgxMo2.48VO9.93, 1 ≤ x ≤ 2) as shown by the dramatic change in the unit cell parameters at x ≥ 1, is consistent with the second solid solution phase obtained during lithium intercalation of Mo2.48VO9.93 (2 ≤ x ≤ 4 in LixMo2.48VO9.93).72 However, unlike the lithiated compound LixMo2.48VO9.93 where an obvious phase transition can be seen in PXRD data for 2 ≤ x ≤ 4, no distinct change is observed for Mg insertion (only a significant shift in peak positions as shown in Figure 6). The shift of the peak positions in the X-ray diffraction patterns of MgxMo2.48VO9.93 is indicative of an intercalation mechanism. This is in contrast to a conversion reaction which was observed in α-MnO2.37,38 As shown in Figure 3 (black), further intercalation leads to another voltage plateau at ca. x = 3 and it is unclear what takes place at this point. The refinement of MgxMo2.48VO9.93 (x = 3)

4. CONCLUSIONS The microporous oxide Mo2.5+yVO9+δ can be used as an intercalation cathode not only in rechargeable Li-based batteries (previously reported) but also in Mg-ion batteries with a specific capacity among the highest reported for Mgbased batteries. The oxide host can be used as an electrode material to intercalate reversibly Mg2+ ions in Mg-ion cells even at room temperature, despite the fact that oxides have high ionicity, thus leading to strong interactions between the Mg2+ guest and ions of the host. The use of the oxide with an opentunnel structure in combination with mixed transition metal ions has been shown to help with Mg-ion insertion. This may pave the way for other oxide-based materials as insertion electrodes, and open up new opportunities for the development of advanced energy storage materials for the use not only in the 4598


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Mg-based batteries but also in other challenging rechargeable multivalent-ion systems.

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AUTHOR INFORMATION

Corresponding Author

*A. J. Jacobson. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Drs. Hyun Deog Yoo and Yan Yao for the guidance on three-electrode cells, and Dr. Boris Makarenko for the assistance with XPS analysis. We acknowledge the Robert A. Welch foundation for the support of this project through Grant No. E-0024.

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REFERENCES

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