Reversible Magnesium Intercalation into a Layered OxyfluorideCathodeJared T. Incorvati,†,∥ Liwen F. Wan,‡,∥ Baris Key,§,∥ Dehua Zhou,§ Chen Liao,§,∥ Lindsay Fuoco,†,∥

Michael Holland,† Hao Wang,§,∥ David Prendergast,‡,∥ Kenneth R. Poeppelmeier,†,§,∥

and John T. Vaughey*,§,∥

†Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States‡The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States§Chemical Sciences and Engineering, Argonne National Laboratory, Lemont, Illinois 60439, United States∥The Joint Center for Energy Storage Research, Argonne National Laboratory, Lemont, Illinois 60439, United States

*S Supporting Information

Modern portable devices play a large role in everyday lives,and electric vehicles are becoming more commercially

viable with every passing year. Although lithium-ion batterieshave been the premier technology in these and other secondarybattery applications, they have certain limitations.1,2 Lithium-ion batteries use carbon anodes, rather than metallic lithium,which limit full cell lithium-ion energy density. In general, metalanodes make cell design and manufacture easier; however, agenerally accepted strategy for using metallic lithium anodesdoes not exist for several reasons including surface instabilitiesand dendrite formation on cycling, which combine to limitoverall cell safety and cycle life.3,4 Because Mg metal does nothave the propensity to form dendritic structures on electro-deposition, is more abundant than lithium, has a reductionpotential of −2.372 V vs SHE, and has a higher volumetriccapacity than lithium metal, alternative energy storage systemsbased on magnesium should have certain advantages.5

Since the discovery by Aurbach et al.6 of sulfide-basedChevrel Phase cathodes compatible with electrolytes based onorgano-magnesium halide complexes, work with Mg-ionbatteries has been focused on identifying higher voltagecathodes and more stable electrolytes. Although electrolytedevelopment has focused on removing chloride ions in order tosupport reversible electrochemistry at the higher voltages,multivalent cathode development has focused on oxidecathodes.7 Several materials, including MnO2, V2O5, andother traditional lithium-ion oxide cathodes, have beenreported but the transition to magnesium has been complicatedfor several reasons including higher self-discharge, irreversibleMg insertion, and instability toward electrolytes.8−11 Many ofthese failure mechanisms involve side reactions that rely onoxide abstraction as a pathway to render the cathode materialinactive. Because it is well established that first row transitionmetal oxides are more prone to oxygen loss than second rowmetals as a form of charge compensation on reduction, weundertook a study to evaluate second row metal oxides ascathode materials.12,13 An early candidate is the layered metaloxide α-MoO3. Although it is known in lithium-ion systems tobe electrochemically active, earlier studies in Mg-based cellsshowed that it had limited stability and cyclability, even atelevated temperatures.14 Using synthetic strategies previouslyemployed to synthesize several new metal oxyfluoride

compounds for medical battery applications, we studied themild fluorination of α-MoO3 to slightly reduce the frameworkand increase the electronic conductivity.15,16 Of the materialsisolated, MoO2.8F0.2 was the only one that maintains the layeredstructure of the parent oxide.17−19 In this report, we investigateits stability as a Mg battery cathode, the mechanism ofinsertion, and model its structure to show that by moving to asecond row transition metal, oxide abstraction has beeneliminated as a failure mechanism and a potential new classof Mg battery cathodes has been identified.Synthetic procedures are included in the Supporting

Information. Phase purity was determined by powder X-raydiffraction (PXRD) using a Rigaku Ultima diffractometer. Allproducts were found to be phase pure by powder X-raydiffraction methods (PXRD) when compared to literaturereports (see SI, Figure S1). PXRD diffractograms of the assynthesized MoO2.8F0.2 were indexed on the basis of anorthorhombic cell with a = 3.877, b = 14.043, c = 3.724 Å.Small variations in unit cell volume and some peak broadeningin larger sample batches may be evidence of a small variabilityin the material’s fluoride content. Although this will likely notaffect the electrochemical properties of the material, it may havea small effect on the electronic conductivity and any possiblesupercell that forms on Mg insertion. Scanning electronmicroscopy images show MoO2.8F0.2 crystals form as platestens of micrometers across (see SI, Figure S2). Cathode andcell preparation are described in the Supporting Information.Galvanostatic charge/discharge cycling is shown in Figure 1

and shows the cycling capacity of MoO2.8F0.2 using a 0.2 MMg(TFSI)2 in propylene carbonate (PC) electrolyte.MoO2.8F0.2 shows dramatically higher capacity and bettercapacity retention when compared to the isostructural parentcompound α-MoO3. Similar cycling capacities were found for0.2 M Mg(TFSI)2 and Mg(Triflate)2 in either PC, diglyme ordimethylformamide. The only exception noted was cells basedon acetonitrile, as they showed significant cycling inefficienciesand poor cycling symptomatic of solvent stability issues. Theobserved capacities in Figure 1 correspond to approximately

Received: July 28, 2015Revised: December 7, 2015Published: December 18, 2015

Communication

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© 2015 American Chemical Society 17 DOI: 10.1021/acs.chemmater.5b02746Chem. Mater. 2016, 28, 17−20

0.25Mg2+ inserted into the host lattice per formula unit. Thesloping discharge curves are indicative of a single phase solidsolution process at lower levels of insertion.Powder X-ray diffraction (PXRD) studies were performed on

laminated cathodes before and after cycling, as shown in Figure2. After processing to make an electrode, the laminates of

uncycled MoO2.8F0.2 show significant preferred orientationconsistent with the roll pressing used to reduce the porosity ofthe formed electrode and the material’s plate-like morphology.After cycling, the structure of MoO2.8F0.2 appears to change.Reflections from MoO2.8F0.2 shift to higher d-spacing, which isconsistent with insertion of Mg into the interlayer galleries. Ondischarge, peaks that are symmetry forbidden in the fluoro-bronze appear, including the (200). The high intesity of the

(200) peak suggests Bragg reflection from molybdenum atoms.This peak indicates a symmetry-breaking shift in the relativeordering of MoO2.8F0.2 layers. The discharged phase wasindexed using JADE software package’s whole pattern fitting.Figure 2 highlights that at very low levels of insertion, a two-phase insertion is evident as the middle pattern (approximately0.05 Mg2+ inserted) shows peaks from the initial host materialand the 0.09 Mg2+ endmember pattern in an approximate 1:1ratio. This indicates that the solid solution indicated by theelectrochemical evaluation may have Mg0.09[MoO2.8F0.2] as anendmember or that the Mg insertion kinetics are very slow atthe rates the samples were evaluated (2 μA/mg). In light of thestructural reversibility and the slow rate used, we infer that fullrange cycling shown in Figure 1 includes a region where two-phases coexist above 0.09 Mg2+ inserted and is the active phasefor the higher insertion levels.In addition to the PXRD and electrochemical studies, we

utilized 25Mg MAS NMR spectroscopy on the dischargedcathode samples to get a better understanding of the Mgenvironment in the host lattice. In Figure 3, 25Mg NMR spectra

of electrochemically magnesiated molybdenum oxyfluoride iscompared to a magnesium molybdenum oxide modelcompound (Nolanite-type Mg2Mo3O8) with a symmetrictetrahedral and an asymmetric Mg site. The small sharppeaks at higher frequency in Mg2Mo3O8 spectra (26 ppm) aredue to MgO from exposure to air. The NMR signal ofintercalated Mg is expected to shift to much higher or lowerresonance frequencies owing to increased paramagneticcontribution from Mo-d electrons (i.e., Fermi-contact shiftsdue to Mo4+, d2 for Mg2Mo3O8). However, in the 25Mg NMRspectrum of the model compound Mg2Mo3O8, the quadrupolar25Mg NMR lineshapes for the two lattice Mg sites appear ataround −60 ppm and are due Mg ions coordinated to themolybdenum oxide structure. For the electrochemically cycledsample Mgx[MoO2.8F0.2] (x ∼ 0.16, from electrochemicaltesting), a broad peak appears at a similar shift of −50 ppm,which is most likely due to disordered Mg intercalation in theMoO2.8F0.2 structure. The small peak at 26 ppm was assigned toMgO impurities and a large sideband envelope in the

Figure 1. (A) Voltage Profile for MoO2.8F0.2 over the first 18 cyclesand (B) capacity versus cycle number. Electrolyte is 0.2 M Mg(TFSI)2in PC.

Figure 2. PXRD of pristine, synthesized MoO2.8F0.2, MoO2.8F0.2laminate, laminate cathodes cycled 12 times to 15 and 30 mAh/g,and a recharged cathode. * indicates an Al substrate peak.

Figure 3. 25Mg MAS NMR spectra of the solid state synthesizedMg2Mo3O8 (red and magenta) and the electrochemical cycled sampleMgxMoO2.8F0.2 (blue) collected at 11.7 T (red) and 19.89T (blue andmagenta). Signal is in reference to saturated MgCl2(aq) at zero ppmand a recycle delay of 1s is used. Spinning speeds of 16, 20, and 14kHz are used respectively for blue, red and magenta spectral. Thespectra intensities are not normalized. * spinning sidebands.

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electrochemical sample may be from air exposure duringtransfers or surface reactions with adventitious water from theelectrolyte. The position of the peak is therefore found to beconsistent with the location of the Mg-ion in the lattice asidentified from literature structures and models.20 19F MASNMR suggests no coordination changes for lattice fluorinessuch as formation of MgF2 upon magnesiation (see SI, FigureS3). No intercalation of protons or solvent species was detectedvia 1H MAS NMR (see SI, Figure S4).It is not clear exactly where fluoride resides in MoO2.8F0.2;

however, there are three types of oxygen in the pristine α-MoO3 structure, as shown in Figure 4. When fluoride is

introduced, it is expected to substitute for oxygen and as aconsequence modify its electronic interactions with theneighboring Mo. Here we perform ab initio electronic structurecalculations using density functional theory (DFT)21 toinvestigate the structural variations of fluorine substituted α-MoO3. Calculation methods are described in the SupportingInformation.To study the preferential site for fluorine substitution, we

place one fluorine atom (at different oxygen sites) into anapproximately cubic 3 × 1 × 3 supercell of α-MoO3 andcompare the resulting formation energies. The calculatedfluorine at the O1 or O3 site will result in at least a 0.5 eVhigher energetic cost, significantly reducing their probability(see SI, Table 1). When fluorine is substituted into the O2 site,it induces strong lattice distortions. Unlike the pristine α-MoO3structure where O2 is shared by two neighboring Mo withdramatically different bond lengths (Mo−O1, 1.70 Å; Mo−O2,1.73 and 2.225 Å; Mo−O3, 1.95 and 2.34 Å), the Mo−F bondlengths are approximately the same, 2.13 Å. This means thecation−cation distortion along the a-axis is significantlyreduced. In addition to the lattice distortion, the introductionof fluorine also liberates an electron that induces defect statesinside the electronic band gap as shown in Figure 5.This extra electron is delocalized over the entire Mo−O layer

in the ac plane where the fluorine defect resides and fills up Modxz states that are hybridized with O2 pz states and O3 px states.It is observed that upon fluorine substitution, the Mo−F bondlength is notably longer than the original Mo−O2 bond, whichindicates a destabilized π* antibonding interaction between Modxz states and anion pz states. In the above simulations, onlyone fluorine atom is introduced into the 3 × 1 × 3 supercell ofα-MoO3. We also test the possibility of forming F−F pairs, i.e.,two fluorine atoms within a distance of ∼4 Å. It is found thatthe lowest energy is always achieved by separating the two

fluorine atoms far apart within the supercell, with eachoccupying an O2 site. Therefore, we conclude that at higherfluorine concentration, such as the experimental MoO2.8F0.2composition, the fluorine atoms are most likely to be randomlyand homogeneously distributed among the O2 sites, with littlechance of clustering. This is consistent with the lack of changesobserved via 19F NMR of lattice fluorine sites uponmagnesiation of interlayer galleries which should heavily affectO1 sites. In addition, when the F concentration is increasedfrom 0.028 (1 F per (α-MoO3)3×1×3 supercell) to 0.2 (7 F per(α-MoO3)3×1×3 supercell), the density of states within the bandgap (shown in Figure 5) grows and broadens significantly, asmight be expected for this molybdenum oxide bronze, therebyreducing the optical band gap of the material.On the basis of literature precedent and experimental

observations, we have identified several Mg battery cathodematerial failure mechanisms are attributable to the ease of lossof oxygen (oxide) from the host lattice. By moving to second-row transitional metals, we can mitigate this as a significanteffect. We studied several Mo oxides and have identifiedMoO2.8F0.2 as a promising new Mg-ion cathode material, as itpossesses a layered structure with a measured band gap of ∼0.2eV.19 A combination of electrochemical testing, X-raydiffraction, 25Mg NMR, and first-principles electronic andatomic structural modeling has been used to show that, unlikeprevious nonaqueous systems, this system works by anintercalation mechanism versus the usually observed dispro-portionation or conversion reactions.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.chemma-ter.5b02746.

S1: Rietveld refinement of MoO2.8F0.2. S2: SEM imagesof MoO2.8F0.2 before and after grinding. S3: 19F MASNMR spectra. S4: 1H MAS NMR spectra. Table 1:Formation energies of fluorine substitution. Synthetic

Figure 4. Crystal structure of α-MoO3. The three types of oxygens areshown in different colors (O1: light blue. O2: gold. O3: red). The Moatoms are shown in purple.

Figure 5. Total density of states for single fluorine substituted (α-MoO3)3×1×3 structure. Inset renders the charge distribution (yellowregion) around the Fermi level. The top frame presents the entiresupercell (in ab plane) that shows the charge is localized on a singleMo−O layer where fluorine (shown in blue) is substituted. Thebottom frame plots the charge spread in this single Mo−O layer (in acplane).

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procedures, elemental analysis, tables of DFT results, andnotes on cell design and construction (PDF).

■ AUTHOR INFORMATIONCorresponding Author*J. T. Vaughey. Email: Vaughey@anl.gov.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported as part of the Joint Center for EnergyStorage Research, an Energy Innovation Hub funded by theU.S. Department of Energy, Office of Science, Basic EnergySciences. We acknowledge the use of the Center for NanoscaleMaterials, supported by the U.S. Department of Energy, Officeof Science, Office of Basic Energy Sciences, under Contract No.DE-AC02-06CH11357. The computational work is performedthrough a user project at the Molecular Foundry using the localcluster (vulcan and catamount) that is managed by the HighPerformance Computing Services Group at Lawrence BerkeleyNational Laboratory supported by the Office of Science of theU.S. Department of Energy under Contract DE-AC02-05CH11231. Use of the Advanced Photon Source at ArgonneNational Laboratory was supported by the U.S. Department ofEnergy, Office of Science, Office of Basic Energy Sciences,under Contract No. DE-AC02-06CH11357. High field NMRaccess at Environmental Molecular Sciences Laboratory atPacific Northwest National Laboratory is gratefully acknowl-edged. This work made use of the J.B.Cohen X-ray DiffractionFacility supported by the MRSEC program of the NationalScience Foundation (DMR-1121262) at the Materials ResearchCenter of Northwestern University. We thank Dr. A. Lipsonand Dr. D. Proffit for their electrochemical insight and Dr. N.Sa and Dr. V. Duffort for providing materials.

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