Applicability of Molybdite as an Electrode Material in Calcium Batteries

Article Cite This: Chem. Mater. 2018, 30, 5853−5861

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Applicability of Molybdite as an Electrode Material in Calcium Batteries: A Structural Study of Layer-type CaxMoO3

Marta Cabello, Francisco Nacimiento, Ricardo Alcań tara,* Pedro Lavela, Carlos Peŕ ez Vicente, and Jose ́ L. Tirado Departamento de Química Inorgánica e Ingeniería Química, Instituto Universitario de Investigación en Química Fina y Nanoquímica IUIQFN, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie, Córdoba E-14071, Spain

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S Supporting Information *

ABSTRACT: Calcium batteries could be an alternative to lithium analogues, but this technology is still in its infancy. It is previously known that layered-type molybdite (α-MoO3) can intercalate hydrated calcium ions in aqueous electrolyte, and this intercalation process increases the interlayer spacing. In this work, we have found that α-MoO3 is electrochemically active in calcium cell with nonaqueous electrolyte. The mechanism of intercalation has been explored by using XRD, Raman, and XPS. The layered structure of α-MoO3 is preserved upon electrochemical intercalation of unsolvated calcium, and the perovskite-type structure of CaMoO3 is not formed. The experimental length of the lattice parameter perpendicular to the slab increases from 13.85 to 14.07 Å in the first stages of intercalation. This limited increase can be optimum for achieving good electrochemical cycling. The model for calcium intercalation in the interlayer space was optimized by theoretical calculations based on the density functional theory. The resulting experimental reversible capacity is about 80−100 mA h g−1, and the average voltage is ca. 1.3 V vs Ca. Further improvement of the electrolyte composition and particle size and morphology could render molybdite as a suitable electrode for rechargeable calcium batteries. The slow diffusion of calcium ion, the side-reactions, and the competing conversion reaction could be drawbacks, particularly at deep discharge and low voltages.

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INTRODUCTION

The toxicity of molybdenum and molybdenum oxide is very low, and the multiplicity of the oxidation states of molybdenum can be very useful in batteries. The perovskitetype CaMoO3 was recently found to be not suitable because of the low mobility of calcium in its framework.10 Molybdite (MoO3) is well-known as a host material for intercalation of lithium15−18 and sodium.19,20 The structure of the orthorhombic α-MoO3 phase consists of nonplanar double-layers of MoO6-octahedra separated by a van der Waals gap. This framework is particularly suitable for intercalation reactions of monovalent and divalent cations in both aqueous (protic) and nonaqueous (aprotic) media.21,22 Electrochemical intercalation of magnesium into MoO3 using nonaqueous solvent has also been reported.3,23,24 Wan et al. reported that the nature of the magnesium salt counteranions is very important for the

Lithium ion batteries have been successfully commercialized since the 1990s; however, lithium resources are limited, and these batteries are not free from safety issues. Consequently, developing batteries based on other metallic elements could be advantageous.1−4 Calcium batteries could be an alternative to lithium batteries.5−13 Calcium is very promising in terms of voltage (only 0.17 V beneath lithium), safety (no dendrites formation), abundance, and high volumetric capacity (2073 mA h mL−1). There are two main problems for the future development of rechargeable calcium batteries: the difficulty of the reversible electrodeposition of Ca in common electrolytes at room temperature6 and without byproducts,14 and finding suitable host materials because of the difficult insertion and low mobility of calcium ion within the lattice. Only a few materials have been reported as reversible calcium-intercalating hosts, such as V2O5,13 Prussian blue analogues (e.g., MnFe(CN)6),8 CaCo2O4,5 and Na2FePO4F.11 © 2018 American Chemical Society

Received: March 15, 2018 Revised: August 16, 2018 Published: August 20, 2018 5853

DOI: 10.1021/acs.chemmater.8b01116 Chem. Mater. 2018, 30, 5853−5861

Article

Chemistry of Materials effective intercalation; the “desolvation” of Mg-trifluoromethylsulfonylimide (TFSI) at the MoO3/solution interface is easier than Mg−Cl desolvation, and, consequently, magnesium intercalation is favored with TFSI-salt.24 Irrespective of that, this salt is not capable of reversible Mg plating/stripping.4 The study of calcium intercalation in nonaqueous cells has been mainly hindered by the difficulty of finding electrolytes compatible with Ca metal. Because of its stability, Ca(TFSI)2 dissolved in acetonitrile25 or PC:EC solvent mixture6,26 has been recently proposed as electrolyte for calcium batteries. Barde et al. recently patented the use of molybdenum oxides in calcium batteries.27 In that visionary invention, several electrode materials and electrolytes are proposed, theoretical calculations were given for several host materials, and the result of an electrochemical experiment was also shown; thus, galvanostatic discharge−charge carried out above temperature and using Ca(BF4)2 in PC:EC as electrolyte was provided for CaMoO3 and α-MoO3. In this work, we study experimentally and theoretically the electrochemical intercalation at room temperature of calcium into α-MoO3 using Ca (or activated carbon) as a negative electrode and using a nonaqueous electrolyte.

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Scheffler (TS)32 and Grimme33 methods. Finally, the Hubbard correction (GGA+U) was introduced, using the simplified, rotationally invariant approach as implemented in Castep.34,35 The energy cutoff was kept fix at 800 eV. The pseudo atomic calculations were performed using the following valence electrons: O2s 2 2p 4 , Ca3s23p64s2, and Mo4s24p64d55s1. Spin polarized calculations were performed in all cases. The selected k-point mesh (determined by the Monkhorst−Pack scheme) was selected as “fine” (as defined in the Castep code; i.e., a k-point separation of ca. 0.07 Å−1). The convergence conditions were energy, 5 × 10−6 eV/atom; max force, 0.01 eV/Å; max stress, 0.01 GPa; and max displacement, 10−4 Å.

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RESULTS AND DISCUSSION Electrochemistry. To study the redox stability of the electrolyte solution, blank information was collected by performing cyclic voltammograms (CV) in a symmetric cell without active material. The CV results evidence the good stability of the Ca-based nonaqueous electrolyte solution within the window of potential between ca. 0.3 and ca. 3.5 V vs Ca2+/Ca (Figure 1). Beyond this potential window, irreversible

MATERIALS AND METHODS

Experimental Methods. X-ray diffraction (XRD) patterns were recorded using Bruker D8 Discover A25 equipment provided with Cu Kα radiation, Ge monochromator, and Lynxeye detector. The XRD pattern of raw MoO3 for Rietveld refinement was recorded from 10° to 90° (2θ) using a step of 0.02° (2θ). The Rietveld refinement was carried out using Topas software. To record the XRD patterns of the electrodes retrieved from the electrochemical cells, Kapton-tape was used to isolate from the moisture and air. The ex situ XRD patterns of the electrodes were scanned between 2° and 75° (2θ) at a step scan mode (0.04°/4 s). The refinement of unit cell parameters was carried out using Topas software and the Le Bail procedure. X-ray photoelectron spectroscopy (XPS) measurements were implemented in a SPECS Phoibos 150 MCD instrument, equipped with Mg Kα source, to study the elements at the sample surface. The electrochemical experiments were carried out at room temperature using Swagelok-type three-electrode cells and a VMP galvanostat/potentiostat instrument. The cells were mounted in a glovebox under Ar atmosphere. The working electrode was MoO3 (99.97% purity) supplied by Sigma-Aldrich (80%) mixed with carbon black (10%) and PVDF (10%) as binder deposited on Ti foil. Negative and (pseudo)reference electrodes consisted of a piece of Ca metal pressed onto a steel grid used as a current collector. The electrolyte solution was 0.5 M Ca(TFSI)2 in 1,2-dimethoxyethane (DME). The electrodes retrieved from the cells for XPS measurements were previously rinsed with DME and dried under vacuum. Alternatively, for selected experiments, high surface area activated charcoal (Honeywell Fluka) was used as a counter electrode (carbon:PVDF = 90:10 deposited on Ti foil) in two-electrode cell configuration. In this last case, the electrolyte solution was 0.1 M Ca(TFSI)2 in acetonitrile. After the open circuit voltage of the activated carbon was measured versus calcium, the voltage of the working electrode (MoO3) was referred to as Ca2+/Ca. Computational Methods. The geometry optimization and total energy calculations were performed within the density functional theory (DFT), as implemented in the Castep code.28 We used the Generalized-Gradient Approximation (GGA) using the exchange correlation potential by Perdew, Burke, and Ernzerhof (PBE),29 with “on-the-fly” generated pseudopotentials. A density-mixing scheme with a conjugate-gradient Pulay solver30 was used for the energy minimization and a Broyden−Fletcher−Goldfarb−Shanno (BFGS) algorithm31 for the internal coordinate optimization. The van der Waals interactions were taken into account via Tkatchenko and

Figure 1. Typical cyclic voltammograms of the Ca(TFSI)2-DME electrolyte solution using Ca as negative electrode, Ti as positive electrode, and Ca as (pseudo)reference electrode (blank experiment). The resulting electrical current is normalized by the area.

electrolyte decomposition can take place. Thus, the main reaction of the working electrode would be competing against side-reactions of the electrolyte solution below ca. 0.3 V, making ambiguous the study of the fully reduced working electrode. The platting/striping of calcium is irreversible with this electrolyte solution, as expected. Although the electrolyte solution is not completely satisfactory, it can serve to check the validity of molybdite as a working electrode in calcium battery, unless within certain limits. The electrochemical behavior of MoO3 in nonaqueous electrolyte solution is summarized in Figures 2−4. In the CV experiment (Figure 2), the main redox peaks are placed at 1.9 V in the anodic sweep and at ca. 0.8 V in the reduction sweep. A smaller anodic peak is also observed at ca. 1.4 V. As it can be seen, the main operational voltage of MoO3 is within the stability window of the electrolyte solution. The current intensity does not go to zero at the lower limit of the reduction sweep, and other electrochemical processes may take place below 0.8 V. However, imposing a lower voltage limit near 0 V may involve irreversible side-reactions. In addition, assuming that calcium diffusion is slow, the kinetics of the discharge and charge process can be different, depending on the scan rate of current density, and several processes could be overlapping at 5854


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Chemistry of Materials

373 mA h g−1), it is necessary to discharge near 0 V, the electrolyte solution can be irreversibly decomposed at the working electrode, and the resulting capacity retention was even poorer (not shown). The electrochemical intercalation of calcium into molybdite can be described according to the following idealized reaction: [Mo6 +]O3 + xCa 2 + + 2x e− = Cax[Mo(6 − 2x) +]O3

(1)

Reaction 1 is only partially reversible for x < 0.3. Nevertheless, one could think that a conversion reaction such as reaction 2 could compete with the intercalation: [Mo6 +]O3 + xCa 2 + + 2x e− = xCaOy + [Mo(6 − 2x) +]O3 − xy (2)

Figure 2. CV results using Ca(TFSI)2-DME electrolyte, Ca as reference electrode, molybdite as working electrode, and another piece of Ca as counter electrode. Scan rate: 1 mV s−1. The resulting electrical current is normalized by the mass of the active material.

It is known that, after true lithium intercalation into MoO3, Mo metal and Li2O can be formed at lower voltages throughout the conversion reaction.36 According to the literature,21,22 calcium can be truly intercalated into MoO3 using aqueous electrolyte and, according to our results, also using nonaqueous solution as discussed below. In case of an extended conversion reaction without calcium intercalation, one would expect to see the first reduction process being irreversible and very different as compared to the second and successive reductions, but this kind of behavior is not observed in Figures 2 and 3. However, reactions 1 and 2 may be concomitant, and it is not discarded that, after intercalation of calcium near the surface of the molybdite particles, a certain contribution of conversion reaction can take place, particularly for deep discharge (x > 0.3). Thus, reaction 1 prevails over reaction 2, at least when the depth of the discharge is limited. The slow diffusion rate of calcium could enhance the relative contribution of reaction 2 near the surface of the molybdite particles, and particularly for deep discharge. Taking into account the failure of the plating/stripping of Ca at the negative electrode and that Ca is a pseudoreference electrode,26 alternatively activated carbon was used as a counter electrode instead of Ca metal in a two-electrode cell, and acetonitrile solvent was used instead of DME, similar to the procedure used by Tojo et al.25 In this hybrid battery/ supercapacitor device, it is expected that adsorption/ desorption takes place in the electrode of activated carbon, and insertion/deinsertion of calcium in the working electrode. The resulting electrochemical cycling experiments show that the stability is improved (Figure 4). Although the voltage given for MoO3 is referred to as the Ca2+/Ca redox couple, the voltage−capacity curves (Figure 4A) can be affected by the contribution of the activated carbon electrode. In fact, both activated carbon and Ca are (pseudo)reference electrodes and may affect the measured voltage.26 The discharge capacity is about 100 mA h g−1 after 12 cycles (Figure 4B). These electrochemical results can serve as a proof of concept for the validity of molybdite in calcium battery. Nanostructuring of MoO3 and further improving of the electrolyte solution could allow one to achieve superior electrochemical performance. The slow diffusion of calcium and the sidereactions are drawbacks. XPS. The XPS results (Figure 5) can help to unveil the electrochemical behavior of molybdite. For raw MoO3, the Mo 3d core level spectrum (Figure 5A,a) is in good agreement with the literature.37−39 The spin−orbit coupling splits the 3d core level into 3d3/2 and 3d5/2 with a splitting energy of 3.1 eV. The binding energy difference between the O 1s and Mo 3d5/2

the same voltage. The (pseudo)reference electrode influences the results. The experimental average voltage of the discharge/ charge (ca. 1.3 V) is lower than the theoretically calculated value (2.7 V) for perovskite-type CaMoO3 by other authors27 and also lower than the voltage range (2.2−5.0 V) experimentally found for MoO3 above temperature by the same authors. It is clear that the experimental conditions, such as electrolyte solution, imposed kinetics, and reference electrode,26 may modify the apparent voltage. Definitively, not the redox reaction of MoO3 but the experimentally measured voltage of MoO3 (and other working electrodes) depends on the other electrode materials, in this work and in the work of other authors. In the galvanostatic experiments versus Ca metal (Figure 3), the observed voltage (pseudo)plateaus can be ascribed to the

Figure 3. Galvanostatic results using Ca(TFSI)2-DME electrolyte, Ca as (pseudo)reference electrode, molybdite as working electrode, and another piece of Ca as counter electrode. Current density: 10 mA g−1 (equivalent to 0.05C).

coexistence of MoO3 and CaxMoO3 phases. The reversible capacity is about 80−100 mA h g−1. Only a few charge− discharge cycles can be carried out. The instability of Ca metal in the electrolyte solution and the inefficient plating of calcium can contribute to the loss of capacity upon cycling. In the case of a deep discharge down to x = 1 in CaxMoO3 (equivalent to 5855


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Figure 5. A detailed view of the XPS results in the Mo 3d region: (A) using Ca(TFSI)2-DME as electrolyte for (a) bare MoO3, (b−d) discharged electrodes, and (e−g) x-discharged and y-charged electrodes (Cax,CayMoO3), and (B) using Ca(TFSI)2-acetonitrile as electrolyte. The atomic contributions are given (%). Figure 4. Galvanostatic results using Ca(TFSI)2-acetonitrile electrolyte, molybdite as working electrode, and activated carbon as counter electrode. (A) Voltage−capacity plot. (B) Specific capacity as a function of cycle number. Current density = 2 mA g−1 (equivalent to 0.01C).

electrolyte solution, and the XPS result is affected by the irreversible decomposition of the electrolyte. The experimental atomic ratio Mo6+/Mo5+ continuously decreases when the discharge capacity increases, although Mo4+ is not detected (Figure 5A,a−d). After the discharge to x = 0.5 and partial charge (Ca0.3MoO3 in Figure 5A,e), the relative contribution of the Mo5+ peaks decreases as compared to the discharged electrode (Figure 5A,c,d). After discharge to x = 0.1 (Figure 5A,f) or x = 0.3 (Figure 5A,g) and then total charge to x = 0.0, the resulting spectra are very similar to the raw MoO3 spectrum. In Figure 5B, the XP spectrum was recorded for a totally discharged electrode (nominal composition CaMoO3) but using acetonitrile instead of DME as solvent and activated carbon instead of Ca as counter electrode (similarly to Figure 4). In this last case, it is clearly observed that Mo4+ occurs, and the average oxidation state is Mo4.89+, suggesting that this solvent is more stable on the surface of intercalated molybdite. Thus, the XPS results probe neither reaction 1 nor reaction 2, but just a reversible redox process. The surface analysis (∼20 Å thickness), reactions with the solvent, and formation of surface films prevent measurements of bulk CaxMoO3, and this can be the reason for finding higher oxidation states of molybdenum than expected. Raman Spectra. Raman spectroscopy was applied to investigate the change of MoO3 after calcium intercalation (Figure 6). For raw molybdite (Figure 6a), and according to the literature,15,23 the stretching frequencies at 994 and 817 cm−1 are attributed to the shortest Mo−O bond and to the intermediate bridging O−Mo−O bond, respectively. The bending modes appear in the medium frequency range. Below 200 cm−1, the external modes are observed. The

core level peaks is equal to ΔB.E.(O1s − Mo3d5/2) = 297.77 eV, and this value agrees well with Mo6+.38 Peaks with very small intensity corresponding to traces of Mo5+ (2.5% atomic) are also observed. In the case of the electrodes retrieved from the electrochemical cells, the surface is unavoidably contaminated by traces of electrolyte solution, and this fact makes ambiguous the study of the oxygen region. The contribution at ca. 238.4 eV (overlapping with Mo 3d region) is ascribed to S 2s from TFSI−.39,40 After the first discharge down to CaxMoO3 (0.05 < x < 1.0), the spectra of the Mo 3d core level tend to become broadened because of the coexistence of several oxidation states and traces of electrolyte (Figure 5A,b−d). The broadening of the Mo 3d spectrum toward lower energies is in good agreement with the partial reduction of molybdenum ion, as compared to raw MoO3. The experimental contribution of Mo6+ is overestimated, probably due to fortuitous oxidation of the electrode surface. Similarly, according to the literature about XPS of molybdenum oxides,38 even for MoO2 and Mo2O5 the bulk composition exhibits much more contribution of Mo(VI) ion as compared to Mo(V) and Mo(IV) in the surface of the particles. Thus, the exact determination of the bulk oxidation state of molybdenum by XPS is uncertain. Tentatively, the average oxidation state in the surface of the totally discharged electrode is Mo5.4+ (Figure 5A,d); however, it is worth remembering here that, to achieve the totally discharged electrode (CaMoO3), it is necessary to discharge down to 0 V, this voltage value is below the stability of the 5856


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Figure 7. Ex situ XRD patterns for (a) raw MoO3, (b−g) discharged CaxMoO3 electrodes from x = 0.05 to 1.0, (h) electrode discharged down to x = 0.5 and then charged up to x = 0.3, and (i) electrode discharged to x = 0.1 and then charged up to x = 0.04.

Figure 6. Raman spectra for bare MoO3 and CaxMoO3 electrodes. X0 represents the composition of the previous discharge, and x is the nominal composition of the analyzed electrode.

edges along one direction in the slabs (here “c”) and corners along the other direction (here “a”). The lattice parameters calculated by the Rietveld method (Table 1 and Figure S1) are a = 3.96156(8) Å, b = 13.8550(3) Å, and c = 3.6942(8) Å.

intercalation of calcium has a strong influence on the Raman spectra, similar to magnesium intercalation.23 New peaks emerge at 879, 845, 792, 390, 322, and 143 cm−1 (Figure 6b− e), which are ascribed to the calciated phase CaxMoO3, and the relative intensities of the peaks ascribed to the calciated phase tend to increase with the depth of discharge. For the spectrum of Ca0.5MoO3 (Figure 6e), the small band at 845 cm−1 may be tentatively ascribed to the O−O stretching vibration in peroxide.41,42 Thus, the presence of a small amount of CaO2 in the surface of the particle and a limited contribution of reaction 2 cannot be discarded, similarl to other intercalation electrodes.43,44 For the totally discharged electrode (x = 1.0), the observed spectrum (Figure 6f) is equivalent to pristine MoO3 spectrum, and there is not a completely satisfactory explanation for this experimental fact. A possible influence on this phenomenon could be that, in the fully discharged electrode, the surface of the particle is amorphous, and the Raman spectrum corresponds to the calcium-free core of the particles; however, the amorphous phase would not be transparent, but the Raman selection rules could be broken down, the spectrum broadened, and the intensities decreased. In addition, the latter composition seems to be affected by the energy of the laser used in Raman measurements; the energy of the laser used in the Raman spectrometer changes the structure of the metastable CaMoO3 phase, and the thermodynamically stable α-MoO3 phase is formed during the recording of the Raman spectrum. This last explanation seems more plausible. These results are in line with the limited discharge imposed in Figures 2−4. After the first discharge−partial charge cycle, the spectrum in Figure 6g is more similar to that of Figure 6a, but the peaks of the calciated phase are also observed. After the complete charge, the spectrum of raw MoO3 is almost completely recuperated, except for a very small contribution of the peak near 879 cm−1 (Figure 6h). The results suggest that the structure of molybdite is changed upon calcium intercalation/deintercalation, although the change is not completely reversible, in good agreement with the XRD results as discussed below. XRD. The XRD pattern of raw MoO3 sample (Figure 7a) agrees well with the JCPDS file number 05-0508 and confirms the Pbnm structure of layered MoO3, based on a sequence of double-layer sheets of distorted [MoO6] octahedral sharing

Table 1. Unit Cell Parameters of the Phases MoO3 (Space Group Pbnm), Obtained by Rietveld Method, and CaxMoO3 (Space Group Cmcm), Obtained by Le Bail Method unit cell parameters, Å nominal Ca content x in CaxMoO3

a

b

c

0 0.05 0.1 0.2

3.96156(8) 3.8849(8) 3.8843(4) 3.8919(7)

13.8550(3) 14.073(2) 14.071(1) 14.072(1)

3.69642(8) 3.7306(8) 3.7304(5) 3.7345(6)

To verify the above suggested electrochemical reaction and to explore the reaction mechanism, ex situ XRD patterns were recorded by using electrodes retrieved from the electrochemical cells (Figure 7b−i). The change of the XRD patterns is evidenced from the beginning of the discharge process by the appearance of new diffraction peaks. These new diffractions cannot be attributed to any undesirable side reaction at open circuit such as ion exchange, because they were not present in an electrode retrieved from a cell at open circuit voltage for 19 h (not shown). Upon discharging, new diffraction peaks emerge, in good agreement with the coexistence of two phases (calcium-free MoO3 and calcium-poor CaxMoO3). The XRD results for the calciated molybdite with 0.05 < x < 0.3 (Figure 7b−e) are very similar to those reported for magnesiated molybdite.23 Thus, Aurbach’s group reported a shift of 0.4° 2θ for the (020) diffraction of Mg0.5MoO3 as compared to raw MoO3. The same authors associated this result with the increase of the interlayer distance.23 In addition, Tsumura and Inagaki reported that the intercalation of solvated lithium drives to larger spacing values.17 At the beginning of the discharge process, the new diffraction peaks can be indexed using orthorhombic axis similarly to calcium-free MoO3, but the space group Cmcm was more reliable for the calcium-poor phase CaxMoO3 than was Pbnm. It is worth noting that Cmcm was previously used by other authors to index the protonated phases HxMoO345 There 5857


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Table 2. Unit Cell Parameters of Layered MoO3 (Space Group Pbnm) Obtained by Rietveld Refinement of the XRD Pattern, and Theoretical Values Calculated by Geometry Optimization Using the Approximations GGA-PBE, TS, Grimme, and LDA+U method

a, Å

b, Å

c, Å

V, Å3

Rietveld GGA-PBE +TS +Grimme +Ueff = 3

3.96156(8) 3.948 (−0.34%) 3.929 (−0.82%) 3.916 (−1.15%) 3.903 (−1.48%)

13.8550(3) 15.773 (+13.8%) 14.183 (+2.37%) 14.030 (+1.26%) 14.018 (+1.18)

3.69642(8) 3.683 (−0.36%) 3.680 (−0.45%) 3.679 (−0.47%) 3.675 (−0.58%)

202.888(7) 229.34 (+13.0%) 205.07 (+1.08%) 202.13 (−0.37%) 201.07 (−0.90%)

are two types of feasible calcium intercalation regions in αMoO3: the channels within the double-layers of MoO6octahedra and the van der Waals gaps between the layers. Our results agree well with intercalation of calcium in the interlayer space for 0.05 < x < 0.2 (Figure 7b−d); meanwhile, the relative amount of the Cmcm phase increases. The slab slightly increases from 13.8550 Å (x = 0, space group Pbnm) to a maximum of ca. 14.073 Å (0.05 < x < 0.2, space group Cmcm) in the first stages of intercalation (see Table 1). The lattice parameters of the calciated phase remains nearly unchanged during the discharge process (CaxMoO3 in Table 1), but the relative amount of this phase increases as it can be seen in relative intensities of the diffractions in Figure 7a−d. From x > 0.2 (Figure 7e−g), new peaks emerge at about 12.2°, 24.4°, and 36.6°, which tentatively could be ascribed to (0k0) diffractions of a third phase (calcium-rich CaxMoO3) with longer enlarged interlayer spacing. These diffraction peaks coexist with those of the other two phases, this fact being indicative of heterogeneity in the composition in the electrode (nonequilibrium state) due to the slow diffusion of calcium. In general, the (020) diffraction tends to be shifted or broadened to lower angles for the calcium inserted samples. The largest shift of the (020) diffraction is 0.33 Å (from 6.92 for x = 0.00 to 7.25 Å for x = 1.0), indicating expansion of the interlayer distance (van der Waals gap). The relative contribution of the phase calcium-rich CaxMoO3 with longer interlayer spacing continuously increases from x = 0.2 to x = 1.0. According to the literature, much larger spacing (9.31 Å) is observed after the cointercalation of water molecules.21,22 In the case of solvent cointercalation in nonaqueous lithium cells, even larger spacing (ca. 12 Å) was reported.17 Consequently, it seems that solvent molecules are not cointercalated in our experiments, similar to experiments with magnesium intercalation.23 For the fully discharged electrode (x = 1), the diffraction peaks more clearly observed are (020), (040), and (060), and these diffraction peaks are broadened, suggesting loss of crystallinity. It is worth remembering here that, to achieve the nominal composition CaMoO3, it is necessary to discharge down to voltages where the electrolyte is not stable. The XRD pattern of the initial MoO3 sample with the (020) diffraction at 12.8° 2θ has not completely vanished for any composition. The superstructure of perovskite-type CaMoO3 is not detected upon electrochemical discharge, suggesting that the intercalation process in MoO3 is rather topotactic. This result is in good agreement with those of Arroyo-de Dompablo et al., which found very slow diffusion of calcium in perovskite-type structure and also reported the theoretically limited electrochemical activity of CaMO3 (with M = Mo, Cr, Mn, Fe, Co, Ni).10 Alternatively, the diffraction peaks that emerge for deep discharge (third phase) may be due to the products of the conversion reaction 2, or other side-reactions such as electrolyte decomposition, although we could not ascribe

unambiguously the XRD peaks to any phase expected for a conversion reaction (Mo, MoO2, etc.). In the case of the electrode first discharged down to x = 0.5 and then charged up to Ca0.3MoO3, the XRD pattern (Figure 7h) shows diffraction peaks of the three phases, similar to the electrode only discharged to x = 0.3 (Figure 7e). For the electrode partially discharged to Ca0.1MoO3 and then completely charged up to Ca0.04MoO3, the resulting XRD pattern (Figure 7i) shows diffraction peaks of two phases, both raw MoO3 and CaxMoO3 phases, but the third phase is not observed as expected. According to the XRD results, some calcium ions seem to remain trapped in the framework, and hence the single phase of calcium-free MoO3 is not retrieved. It is worth noting here that the atomic scattering factor of calcium is relatively large as compared to that of oxygen. Similarly, according to Li et al., the lithium ions that are inserted into the α-MoO3 during the discharge process cannot be fully deinserted during the subsequent charge process because of the structural changes.16 Geometry Optimization and Voltage of CaxMoO3. The van der Waals interactions (vdW) are strongly present in layered MoO3.46,47 The density functionals are unable to describe correctly vdW, resulting from dynamical correlations between fluctuating charge distributions. Thus, the geometrical optimization gives theoretical values of the unit cell parameters “a” and “c” (intraslabs) close to the experimental values (Table 2). On the contrary, for the unit cell parameter “b” (perpendicular to slabs), it gives a value of ca. 15.8 Å, far from the experimental value 13.855 Å. Two approaches were considered to take into account the vdW interactions: TS and Grimme. Although both approaches give similar results for “a” and “c” cell parameters, Grimme gives a better result for “b” parameter, with a deviation from the experimental value of only 1.2%, which is one-half of that calculated by the TS approach (2.3%). Also, the average dispersion of the deviation from experimental values is lower for the Grimme approach. Finally, the Hubbard approximation was applied. On the basis of a previous study of the Ca insertion into MoO3 with perovskite-related structure, the used value was Ueffective = 3 eV.10 The values of the calculated unit cell parameters are very close to those with U = 0, and the deviation is clearly smaller than when using the TS approach. For the sake of comparison, the Rietveld refined atomic coordinates and those optimized are included in Table S1. To analyze the possible stability of Ca-inserted MoO3, with CaMoO3 nominal stoichiometry, we assumed that Ca atoms occupy a 4c site, whose coordinates are (x, y, 1/4). The MoO3 layers are centered at ca. y = 0 and 0.5. Thus, Ca atoms were located in the interlayer spacing. The value of the fractional coordinate “y” was systematically varied from 0.15 to 0.35, while the value of “x” varied within the range 0.05−0.50, as illustrated in Figure S2. The values of “y” from 0.65 to 0.85, and those of “x” from 0.55 to 1.00, are generated by symmetry 5858


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During the structure optimization, the x coordinate of all atoms in the unit cell reached values very close to 0 or 0.5 (Δx < 0.001). Thus, a new space group was assumed, Cmcm, in agreement with these values. Figure 8 shows a comparison between the optimized structures of CaMoO3 and pristine MoO3. The [MoO6] octahedral and [MoO3] layers are slightly modified, although the main difference is the presence of Ca in the interlayer spacing, increasing the distance between [MoO3] layers, and thus also increasing of the corresponding unit cell parameter. Ca atoms in the interlayer spacing occupy a double layer, as depicted in Figure 8. The coordination polyhedron of Ca atoms is represented in Figure 9. These Ca atoms are

operations. With these starting positions, the geometrical optimization was carried out. Figure 8 shows the most favorable CaMoO3 structure (other local minima are included in Figure S2 and Table S2). It results in the lower free energy, and thus in the higher voltage of the reaction 1 for x = 1.0.

Figure 9. Coordination of Ca in the hypothetical layered CaMoO3 structure.

coordinated by nine oxygen atoms, eight of them belonging to the near [MoO3] layer and only one to the farthest [MoO3] layer. The distances to eight O atoms in the same interlayer spacing are similar to those found in the perovskite-type CaMoO3, while the distance to the ninth oxygen is very short, 2.19 Å, even shorter than that found in CaO (ca. 2.4 Å). This Ca distribution can make very difficult the diffusion of Ca ions through the interlayer spacing, fully hindering the proposed diffusion path in layered CaxMoO3.48 This fact can justify the difficulties found above in experimentally obtaining a Ca-rich sample. Although there are different possibilities for ordering Ca cations for intermediate compositions (i.e., Ca content lower than 1 per formula unit), to have a first idea about the profile of the voltage versus Ca content during the electrochemical insertion curve, two intermediate composition/orderings were analyzed: Ca0.5MoO3 and Ca0.25MoO3. For the calculation of the intermediate Ca0.5MoO3 composition, we kept only one Ca atom in each interlayer spacing of the unit cell (i.e., 2 Ca per 4 Mo atoms). The distance between Ca atoms of the two layers in the same VdW gap (see Figure 8, planes ab and bc) is ca. 3.70 Å, while in the rectangular lattice of Ca atoms in the same layer (see Figure, 8, plane ac) the shorter distances are along the axes, ca. 3.8 and 4.00 Å. Thus, to minimize interactions, we assumed that only one of the two layers is occupied, resulting in a Ca/Mo ratio of 0.5. For Ca0.25MoO3, we kept only one over two Ca atoms, along the diagonal of a 2 × 2 superlattice. In both cases, the approaches converged to a similar solution, where Ca atoms are located in the middle of the interlayer spacing. After geometrical optimization, Ca0.5MoO3 with U = 0 was shown to be unstable, with the disproportion being more favorable (−0.11 eV/Mo) according to

Figure 8. Comparative projection in the three planes of (left) the optimized structure of CaMoO3 (space group Cmcm) and (right) pristine MoO3 (space group Pbnm). The gray-blue circles represent Ca atoms, and the distorted octahedra correspond to [MoO6] units. 5859


Article

Chemistry of Materials 3Ca 0.5MoO3 → 2Ca 0.25MoO3 + CaMoO3

transition metal such as molybdenum is tremendous. Irrespective of the fact that other electrolyte should be developed to achieve reversible deposition of Ca, these results are valid as a proof of concept. We think that MoO3 is very promising as an electrode material for batteries based on multivalent metals.

(3)

For the other compositions, Figure 10 shows a schematic representation of Ca ordering and the calculated voltage. For U

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01116. Rietveld refinement for pristine MoO3 (Figure S1), structural parameters of MoO3 refined by Rietveld method and after geometry optimization (Table S1), geometrical optimization of the CaMoO3 phase using different coordinates of Ca atoms (Figure S2), and calculated voltage of the calciation reaction for the different minima obtained during the geometry optimization process (Table S2) (PDF)

Figure 10. Schema of the calculated voltage versus composition curve for layered CaxMoO3.

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= 0, the calculated voltage to obtain CaMoO3 is 2.23 V, considerably higher than that observed in our experiment. For U = 3, two partially inserted phases are expected: Ca0.25MoO3 and Ca0.5MoO3. This result agrees with the XRD observations, with the presence of the pristine MoO3 and one Ca inserted phase for x < 0.3, and with two Ca-inserted phases for x ≥ 0.3. At this point, the presence of the pristine MoO3 is probably due to kinetic aspects, related to the low mobility of Ca. Also, a related multivalent system such as MgxMoO323 shows a profile similar to that predicted for CaxMoO3 with U = 3. Yet the calculated voltage values of Ca insertion do not match with those observed in our case. This may be indicative that these phases are not actually obtained, more probably because of kinetic aspects (low mobility of Ca), in a similar way to perovskite-type CaMoO3.10 Instead, a metastable phase could be formed. Taking into account the electrochemical data, the voltage of the reaction for this phase should be close 1.2 V. Yet the slow diffusion of calcium, resulting in possible metastable phases, and the competing conversion reaction, could be at the origin of the observed experimental results.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ricardo Alcántara: 0000-0002-6364-6728 Pedro Lavela: 0000-0002-5182-3440 Carlos Pérez Vicente: 0000-0003-3507-3923 José L. Tirado: 0000-0002-8317-2726 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Ministerio de Economia,́ Industria y Competitividad (MINECO) (MAT2014-56470-R and MAT2017-84002-C2-1-R), ERDF funds, and Junta de Andaluciá for financial support (group FQM288). We also thank SCAI (UCO Central Service for Research Support, XPS) and the research group of Prof. C. Jiménez for use of Raman facilities.

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CONCLUSIONS Reversible storage of calcium in α-MoO3 using Ca(TFSI)2 in DME as electrolyte solution and Ca metal as negative electrode is demonstrated. In nonaqueous solution, nonsolvated calcium ion is intercalated into the molybdite structure; the layer structure is preserved, but the space group is changed. The observed small change of the interlayer spacing as compared to lithium cell can be advantageous in terms of structure stability to achieve good electrochemical cycling. Apparently, the composition CaxMoO3 should be limited to around x = 0.3 for achieving reversible electrochemical cycling. This fact can be due to the redox instability of the electrolytes solutions and/or to the mestastable character of the calcium-rich compositions. Part of the observed capacity may be due to surface contribution and/or conversion reaction. The electrolyte composition should be further optimized to gain stability at the Ca electrode and then larger capacity may be achieved, although the slow diffusion of calcium and the competing conversion reaction can be other drawbacks. The potential applicability of a Ca battery assembled with a multivalent

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