This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article Cite This: ACS Omega 2019, 4, 8943−8952 http://pubs.acs.org/journal/acsodf On the Study of Ca and Mg Deintercalation from Ternary Tantalum Nitrides Roberta Verrelli,†,‡ Ashley Philip Black,†,‡ Carlos Frontera,† Judith Oro-́ Sole,́ † Maria Elena Arroyo-de Dompablo,§ Amparo Fuertes,*,† and M. Rosa Palacín*,†,‡ † Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) Campus UAB, E-08193 Bellaterra, Catalonia, Spain ‡ ALISTORE-ERI European Research Institute, CNRS FR 3104, Hub de l’Energie, Rue Baudelocque, 80039 Amiens Cedex, France § Departamento de Química Inorgánica, Facultad de Cc. Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain * S Supporting Information ABSTRACT: Layered CaTaN2 and MgTa2N3 and cubic Mg2Ta2N4 were prepared by direct solid state reaction from the binary nitrides Ta3N5 and A3N2 (A: Mg, Ca). CaTaN2 showed a slight Ca deficiency (0.11 moles per formula), and a monoclinic distortion from previously reported R3̅m symmetry, with space group C2/m and cell parameters a = 5.4011(2), b = 3.1434(1), c = 5.9464(2) Å and β = 107.91(3)°. Ca 2+ and Mg 2+ deintercalation was investigated in the three compounds both chemically and electrochemically. No significant Mg2+ extraction could be inferred for MgTa2N3 and Mg2Ta2N4, neither after reaction with NO2BF4 nor after electrochemical oxidation at 100 °C in alkyl carbonate electrolytes. Rietveld refinement of the X-ray powder diffraction pattern of chemically oxidized Ca0.89TaN2 indicates a decrease of the Ca content to 0.34 concomitant to the disappearance of the monoclinic distortion and expansion of the interlayer space from 5.658 to 5.762 Å, space group R3̅m and cell parameters a = 3.1103(1) and c = 17.287(1) Å. Deintercalation in this compound was also achieved electrochemically at 100 °C. Results of density functional theory calculations seem to indicate that reaction mechanisms for CaTaN2 oxidation additional and/or alternative to deintercalation are taking place, which is likely related to the loss of crystallinity observed upon oxidation and the irreversibility of the process. 1. INTRODUCTION and even a reliable electrochemical full cell setup is missing.3 The transition from mono- to multivalent ion-based batteries is On the crystal chemistry side, unraveling materials allowing a key strategy to substantially enhance the energy density of fast and reversible ion mobility is a major challenge to address. currently available battery technologies, especially if metal The main issues arise from the high polarizing character of anodes can be used.1 This concept becomes particularly multivalent ions inducing strong Coulombic interactions attractive if earth-abundant and nontoxic metals are used as the within the electrode host framework and high (de)-solvation overall battery technology would benefit from reduced cost energy barriers, which dramatically affect the electrode and lower environmental impact. Ca and Mg are the fifth and intercalation kinetics and power performances. The use of eighth most abundant elements on the earth’s crust, Mo6S8−ySey (y = 1, 2) Chevrel phases exhibiting fast and respectively, and the metals display theoretical volumetric reversible Mg2+ migration as positive electrode materials energy densities of 3833 and 2073 mA h/mL, significantly represents a milestone in Mg battery chemistry,4 despite its larger than that of graphite, which is used as the negative redox potential being rather low. A vast array of other electrode material in most commercial lithium ion batteries materials, including layered and spinel oxides, chalcogenides, (∼800 mA h/mL).2 On the positive electrode side, divalent Prussian blue analogues, vanadates, silicates, and polyanionic charge carriers would enable to double the capacities when phosphates, have been studied, but none of them has resulted compared to single valent ones for the same amount of reacted in suitable performance.1 Calcium systems are even less ions. Alternatively, reaction of only a half of the ions would be explored, as the feasibility of reversible calcium plating/ needed to achieve the same capacity. These prospects have stripping got only recently unveiled.5,6 Encouraging results been prompting increased interest from the scientific have been achieved with traditional layered materials such as community, with Mg batteries having deserved the most TiS2, for which reversible insertion of Ca2+ at 100 °C has been attention so far.1 Progress in this topic is difficult, as significant recently demonstrated.7 Traditional layered oxides such as bottlenecks exist both at a fundamental and technical level, and the understanding of the multivalent battery chemistry is still Received: March 20, 2019 elusive in many aspects. Indeed, know-how gained in the Accepted: May 9, 2019 development of Li-ion batteries cannot be directly imported Published: May 22, 2019 © 2019 American Chemical Society 8943 DOI: 10.1021/acsomega.9b00770 ACS Omega 2019, 4, 8943−8952 ACS Omega Article Table 1. Synthesis Conditions for Tantalum Alkaline Earth Ternary Nitrides phase reactants A/Ta (A: Ca, Mg) T (°C) reaction time(h) atmosphere sample holder refs CaTaN2 Ca3N2; Ta3N5 1.2/1 1080 20 N2 flow, Mo crucible this work CaTaN2 Ca(NH2)2; Ta3N5 1 to 1.15/1 1450 3 N2, Nb crucible 18 MgTa2N3 Mg3N2; Ta3N5 1.2/2 850 20 N2 flow, Mo crucible this work MgTa2N3 Mg(NH)2; Ta3N5 1.2/2 1200 24 N2 flow, Ni crucible 31 Mg2Ta2N4 Mg3N2; Ta3N5 1/1 850 20 N2 flow, Mo crucible this work Mg2Ta2N4 Mg(NH)2 or Mg3N2; Ta3N5 6/1 1050 24 N2 flow, Mo crucible 31 Figure 1. (A) Refined crystal structure of MgTa2N3; N, Mg, and Ta atoms are represented as orange, pink, and gray spheres. (B) Backscattered electron SEM micrograph of MgTa2N3 powder. (C) Characteristic EPS potential profile vs specific capacity (bottom axis) and vs moles of virtually extracted Mg2+ (top axis), calculated considering that this is the only redox phenomenon taking place, for MgTa2N3 electrode (with 15% wt of Super P carbon) in Li cell, with 1 m LiBOB, EC/PC (1:1 vol) electrolyte at 100 °C and C/100 rate and corresponding ex situ X-ray powder diffraction patterns (D). Diffraction patterns of the pristine and chemically oxidized samples are also shown in (D) for comparison. V2O5 and MoO3 have been recently revisited for applications metal nitrides can be conceived as a tool to mitigate coulombic in Ca batteries: while poor (if any) Ca2+ intercalation has been interactions with multivalent ions and hence facilitate their proved in V2O5,8 reversible electrochemical activity has been solid state migration. Furthermore, the high electronic reported for MoO3.9 Calcium transition metal ternary oxides conductivity exhibited by several transition metal nitrides have also drawn increasing attention: while too high Ca2+ could play a key role in promoting fast multivalent ion migration barriers were determined for CaMoO3 and migration kinetics, in agreement with what has been recently CaMn2O4,10 electrochemical extraction of Ca2+ from 1D evidenced for the MgxZr2S4 thiospinel phase,13 exhibiting, Ca3Co2O6 was achieved at high potential (3.2−3.6 V vs Ca), despite its large unit cell volume and almost zero-strain upon encouraging further optimization of the cathode and electro- insertion, lower Mg2+ diffusion coefficient than the MgxTi2S4 lyte compositions to achieve reversibility of the process.11 phase14 as a consequence of its much lower electronic Within this research context, we decided to further explore conductivity. While covalency is also expected to result in a non-oxidic compounds, turning our attention to ternary penalty in the redox operating voltage, this can be in principle AxTayNz (A: Mg, Ca) tantalum nitrides. As compared to compensated by the proper selection of transition metal redox transition metal oxides, nitrides exhibit analogous crystal centers. Although few reports on the study of ternary alkali chemistry and a higher degree of covalency of the metal− transition metal nitrides as Li-ion battery electrode materials anion bonds, which results into unique chemical, magnetic, exist,12,15 such compounds have not been considered for and optical properties.12 The high covalency of transition multivalent technologies, the only exception being MgMoN2, 8944 DOI: 10.1021/acsomega.9b00770 ACS Omega 2019, 4, 8943−8952 ACS Omega Article Figure 2. Refined crystal structure (A) and backscattered electron SEM micrographs (B) of Mg2Ta2N431 prepared at 1080 °C from binary nitrides. (C) X-ray powder diffraction patterns of pristine (a), chemically (b) and electrochemically oxidized (c) and reduced (d) Mg2Ta2N4 samples. for which Mg2+ extraction was not possible.16 Among the treatments may result in significant losses of the most available alkaline earth metal transition metal nitrides, the electropositive metal, which trigger the control over the ternary A−Ta−N (A: Ca, Mg) system represents an interesting product composition. The list of the ternary Ca and Mg playground for research, as the electronic configuration of tantalum nitrides studied in this work and of the experimental tantalum allows in principle to explore a broad spectrum of conditions employed for their synthesis is given in Table 1, phase compositions, with potentially high operating voltage together with the synthetic methods previously reported. and high specific capacities. Hexagonal Mg1−xTa2+xN3 and 2.1. Magnesium Tantalum Nitrides. 2.1.1. MgTa2N3. cubic Mg2.6−xTa2+xN3 phases were originally prepared by Attempts to extract Mg2+ from this phase were carried out both Brokamp and Jacobs by solid state reaction between by chemical oxidation with NO2BF4 and by electrochemical Mg(NH2)2 or Mg3N2 and Ta3N5.17 Analogously, CaTaN2 methods at room temperature and 100 °C. The characteristic was synthesized by Disalvo et al.18 from Ca(NH2)2 and potential versus capacity signature of the MgTa2N3 electrode Ta3N5 and suggested to have rhombohedral symmetry, being (15% wt of Super P carbon) upon extended oxidation using an isostructural to α-NaFeO2. electrochemical potential spectroscopy (EPS) protocol at 100 Herein, we present a study of such phases prepared from °C in Li three-electrode cell [in 1 m LiBOB, ethylene Ta3N5 and binary nitrides A3N2 (A: Mg, Ca) instead of the carbonate (EC)/propylene carbonate (PC) 1:1 vol electrolyte] alkaline earth amides as reactants, and determination of the is shown in Figure 1C. A plateau-like region centered at about crystal structure for the calcium compound. The feasibility of 4.2 V versus Li+/Li is observed in the oxidation profile, with a Ca and Mg ion extraction from CaTaN2, MgTa2N3, and capacity approaching 100 mA h g−1 (corresponding to about MgxTa2N4 was studied both chemically and electrochemically. 0.8 mol of virtually extracted Mg2+ considering this as the only Finally, in light of the results achieved, the feasibility of Ca redox reaction taking place). deintercalation form CaTaN2 is also investigated using first MgTa2N3 was obtained by reacting Mg3N2 and Ta3N5 in 1:2 principles calculations within the density functional theory mol ratio at 850 °C for 20 h (see the Experimental Section for (DFT) approximation. details). The as-prepared sample consists of homogenous micrometric aggregates of sub-micrometric and nanometric 2. RESULTS AND DISCUSSION particles (see Figure 2B). The Mg/Ta ratio determined from The achievement of ternary transition metal nitrides is far from energy-dispersive X-ray (EDX) spectroscopy was 0.45 with being trivial, as high temperatures and prolonged treatments distribution of both elements being homogeneous (see Figure are needed in order to obtain crystalline samples and nitrides S1D). The Rietveld refinement using synchrotron X-ray are inherently less stable than oxides due to their lower free diffraction (SXRD) patterns of the as-prepared sample is energies of formation.26−30 In the synthesis of alkaline or depicted in Figure S1A. Refinement has been done starting alkaline earth transition metal nitrides, these long thermal from previously reported structure with P63/mcm space 8945 DOI: 10.1021/acsomega.9b00770 ACS Omega 2019, 4, 8943−8952 ACS Omega Article group.31 The refined cell parameters are a = 5.2106(2) Å and c observed, neither after electrochemical nor after chemical = 10.4328(4) Å, which are in agreement with those previously oxidation, which points at the amount of Mg2+ deintercalated reported for Mg1−xTa2+xN3 (ca. 0 < x < 0.06). A Mg/Ta ratio being insignificant, if any, and the electrochemical capacity slightly lower than 1/2 could be inferred by the refinement of observed being again related to side reactions. Attempts to the occupation factors, while a nitrogen content of 9.6% wt was electrochemically reduce Mg2Ta2N4 in Mg cell were not obtained by elemental analysis (i.e., approaching theoretical successful either. 9.8% wt value for MgTa2N3). The crystal structure of 2.2. Magnesium Calcium Nitrides. CaTaN 2 was MgTa2N3 is represented in Figure 1A. It consists of prepared by solid state reaction at 1080 °C between Ca3N2 (TaN2)3− layers between which both Mg2+ and Ta5+ ions and Ta3N5 (see the Experimental Section for details) and was occupy octahedral holes. The structure exhibits partial cation found to be stable in ambient air. We initially attempted to fit ordering, as Wyckoff position 4d is mainly occupied by Mg2+ the X-ray powder diffraction pattern using the structural model ions (i.e., for ca. 90% by Mg and 10% by Ta) while positions previously reported by Balbarin et al.18 in space group R3̅m.33 2a and 6g are occupied only by tantalum. Some of the peaks in However, several diffraction peaks show a clear splitting [e.g., the SXRD patterns of the as-prepared sample (such as the 104 reflections 012, 015, 018 of R3̅m] indicating lower symmetry. reflection at d = 2.258 Å corresponding to sin(θ)/λ = 0.2214 As an alternative, we used the structural model with Å−1, see Figure S1B in the Supporting Information) exhibited a monoclinic space group C2/m that was reported for broad, asymmetric shape which prevented proper fitting by NaxFeO2.25 This provided a better fitting of the X-ray Rietveld refinement, neither by considering micro-absorption diffraction data and split reflections. The observed and nor anisotropic strain effects. Such anisotropic broadening calculated X-ray diffraction patterns are plotted in Figure 3. might be due to the presence of stacking faults, in line with observations in other layered compounds reported in the literature.32 This is also consistent with the observation of streaking along [001]* in the electron diffraction patterns (see Figure S1F in the Supporting Information), which indicate stacking disorder along c. The ex situ X-ray powder diffraction patterns of the MgTa2N3 sample collected after the electrochemical and chemical oxidation tests are shown in Figure 1D. Compared with the pristine phase, neither peak shifts nor formation of extra phases are observed, this pointing to insignificant (if any) Mg de-insertion from MgTa2N3, even under the severe oxidizing conditions employed and the electrochemical capacity observed being related to side reactions most likely Figure 3. Observed and calculated SXRPD patterns of Ca0.89TaN2 (λ involving electrolyte decomposition. Attempts to insert = 0.4133 Å) in space group C2/m. Vertical ticks denote the Bragg additional Mg2+ ions in the MgTa2N3 structure were carried positions. Inset: Zoom of the (1̅13) and (202) doublet of C2/m SG, a out by electrochemical reduction in Mg cells at 100 °C, using a single reflection (015) would be present in this region for R3̅m SG. 0.3 M Mg(TFSI)2, EC/PC 1:1 electrolyte, which resulted in very low capacities and no changes in the X-ray powder The inset shows in detail the peak at ca. 0.236 Å−1 diffraction patterns. corresponding to 1̅12/201 reflections in C2/m group and 2.1.2. Mg2Ta2N4. Mg2Ta2N4 was obtained by solid state 015 reflection in R3̅m, emphasizing the need of reducing the reaction at 1080 °C from Ta3N5 and Mg3N2 in 1:1 molar ratio symmetry from rhombohedral to monoclinic. Results of the (see the Experimental Section for details). The as-prepared Rietveld refinement are listed in Table 2. This monoclinic Mg2Ta2N4 exhibits a cation-disordered cubic crystal structure (Figure 2A) with Mg and Ta in octahedral coordination and Table 2. Atomic Coordinates in Space Group C2/m for cell parameter a = 4.35329(6) Å, as determined by Rietveld Ca0.89TaN2 from the Refinement of X-ray Diffraction Data at refinement of the SXRD data (see Figure S2A in the 298 K Using Radiation with λ = 0.4133 Åa Supporting Information. Refinement was made using Fm3̅m atom Wyckoff site x y z Biso Occ. factor space group as reported in ref 26). An Mg/Ta ratio of about Ca 2d 0 0.5 0.5 2.5(2) 0.888(4) 0.8 and 0.7 was obtained by EDX analysis, and N content of Ta 2a 0 0 0 0.39(3) 1 9.5% wt was obtained from chemical analysis, in agreement N 4i 0.278 0 0.795 0.8(3) 1 with the nominal composition. A typical scanning electron a microscopy (SEM) micrograph of the Mg2Ta2N4 sample is Refined cell parameters were a = 5.40108(2), b = 3.1434(1), c = shown in Figure 2B, displaying nanometric particles forming 5.9464(2) Å and β = 107.908(3)°. Agreement factors: χ2 = 7.34; Rwp = 8.75%; RBragg = 3.23%. Average bond distances: ⟨dTa−N⟩ = 2.15(3) micrometric size aggregates with distribution of Mg and Ta Å; ⟨dCa−N⟩ = 2.46(1) Å. being homogeneous (see Figure S2B). When subjected to oxidation using an electrochemical potential spectroscopy (EPS) protocol at 100 °C in Li three- distortion might arise from Jahn−Teller effect (as reported in electrode cells in 1 m LiBOB, EC/PC 1:1 vol electrolyte, the other layered oxides34) or, most likely in view of DFT results electrodes containing Mg2Ta2N4 exhibited a potential profile (see below), to vacancy/calcium ordering related to Ca displaying a plateau around 4.2 V versus Li+/Li and specific deficiency25,35,36 as typically occurs in the triangular lattice of capacities close to 90 mA h g−1 corresponding to Δx of Na sites in α-NaxFeO2-type compounds.37,38 Besides, the virtually de-inserted Mg2+ of about 0.7 mol. However, no background of the pattern indicates presence of a relevant significant change in the X-ray powder diffraction pattern was amount of amorphous phase, which was quantified (by adding 8946 DOI: 10.1021/acsomega.9b00770 ACS Omega 2019, 4, 8943−8952 ACS Omega Article Figure 4. (A) Projection of the refined crystal structure of Ca0.89TaN2 along the [210] direction. N, Ca, and Ta atoms are represented, respectively, as orange, magenta, and gray spheres. (B) Backscattered electron SEM image of CaTaN2 sample. (C) Characteristic GCPL voltage profiles vs specific capacity (bottom) and vs moles of virtually de-inserted Ca2+ (Δx) (top) of Ca/0.45 M Ca(BF4)2, EC/PC (1:1 vol)/CaTaN2 (with 15% wt of Super P carbon) cells at 100 °C and C/100 rate and corresponding ex situ XRD patterns (b,c). (D) X-ray powder diffraction patterns of the pristine (a) and chemically oxidized samples are also displayed in (D). (*) denotes reflections corresponding to CaF2. a 20% wt of SiO2) to be about 25% wt (see Supporting by chemical oxidation reaction with NO2BF4 and by Information). electrochemical methods. For the electrochemical experiments, The Ca−N and Ta−N bond distances (see Table 2) are Ca0.89TaN2 was mixed with conductive Super P carbon in consistent with ionic radii values. The refinement of the 85:15% wt ratio and subjected to electrochemical oxidation occupation factors resulted in a slight Ca deficiency, with Ca/ (de-insertion) tests in Ca metal cells under various conditions. Ta ratio being 0.89(1). The presence of a minor impurity As no electrochemical activity could be observed at RT, phase with main diffraction peaks at sin(θ)/λ of ca. 0.208 and additional experiments were carried out at moderate temper- 0.246 Å−1 was also detected, which may correspond either to ature (100 °C). Figure 4C displays the characteristic potential CaO or TaN. A Ca/Ta ratio close to 1 was also inferred from versus capacity profiles observed for Ca0.89TaN2 electrode at EDX with distribution of both elements being homogeneous 100 °C and C/100 rate in Ca three-electrode cell with Ca (see Figure S5A), while a nitrogen content of 10.5% wt was reference and counter electrodes, in dry 0.45 M Ca(BF4)2, EC/ determined by combustion analysis which is close (1.82 N per PC 1:1 vol electrolyte. The potential signature displayed is the formula) to that expected for the stoichiometry Ca0.89TaN2. measured potential difference between the working and the These results are consistent with previous findings by Balbarin counter electrodes (each measured vs passivated Ca reference). et al. who observed slight calcium deficiency and presence of A full oxidation up to capacity values of about 170 mA h g−1 TaN, TaN0.1, or Ca2TaN3 as secondary phases.18 (corresponding to Δx mol of virtually extracted Ca2+ ions of The crystal structure of Ca0.89TaN2 is represented in Figure about 0.7) is reported in Figure 4C, together with an 4A. Ca and Ta occupy alternating octahedral sites between oxidation−reduction limited to lower values of delivered close packed nitrogen slabs. The ordered distribution of specific capacity (i.e. about 0.3 mol of virtually exchange cations within the layers has been reported to be at the origin Ca2+ ions). A plateau centered at about 4.5 V is observed upon of anisotropy of electrical resistivity.18 oxidation with high polarization upon further reduction, both The as-prepared sample consists of micrometric aggregates using tape or powder electrodes. A variation of the powder of nanometric and sub-micrometric particles (Figure 4B) with sample color from dark gray to light brown was detected both homogeneous composition, as confirmed by EDX analysis. after the chemical and electrochemical oxidation tests, Attempts to extract Ca2+ from this phase were carried out, both reasonably suggesting a variation of the tantalum oxidation 8947 DOI: 10.1021/acsomega.9b00770 ACS Omega 2019, 4, 8943−8952 ACS Omega Article state. The X-ray powder diffraction patterns of Ca0.89TaN2 at particular Li-vacancy orderings and subsequent crystal the end of the oxidation (de-insertion) process and after symmetry changes.39,40 The changes in the cell parameters oxidation−reduction are reported in Figure 4D, together with are similar to those observed in α-NaFeO2 and NaVO2 during the patterns corresponding to the pristine phase and the sodium deintercalation,41,42 which lead to layered phases with powder recovered after 10 h of reaction with NO2BF4 at 80 °C composition Na0.5MO2. Expanded c axis (as mentioned, in acetonitrile (ACN). interlayer distance in Ca0.89TaN2, 5.658 Å would correspond Significant loss of crystallinity is observed, especially for the to a hexagonal c axis of 16.974 Å) resulting from increasing sample oxidized for longer time. An increase of the amorphous coulombic repulsions as the amount of ions in the interslab content from about 25% wt in the pristine sample to 50% wt space decreases. The refined calcium occupancy for the after oxidation was estimated from Rietveld refinement of the chemically oxidized sample is 0.34(2), which agrees with the XRD data, by using an internal standard of SiO2 quartz (see Ca/Ta ratio of 0.4 obtained from XESD analysis. The poor Figures S3 and S4 in the Supporting Information). A strong crystallinity of the electrochemically oxidized sample prevents decrease of the integrated intensity of the 2̅02/111 reflections reliable refinement of the Ca occupancy. Nonetheless, the (104 in the R3̅m cell) [sin(θ)/λ = 0.219 Å−1, d = 2.285 Å] is pattern is quite similar to the one of the chemically oxidized clearly detected after the oxidation (see Figures 4D and 5), sample with R3̅m symmetry and cell parameters a = 3.107(1) and c = 17.363(5) Å, indicating a slightly larger expansion along c. The X-ray diffraction patterns of samples obtained by reduction of oxidized Ca0.89TaN2 did not show any additional change, which is a clear indication that calcium deintercalation is an irreversible process. Interestingly, very similar behavior has been reported for NaTaN243 which was also oxidized chemically (in that case using NO2PF6) with significant amorphization and decrease in the relative intensity of some peaks with respect to others. In that case, no Rietveld refinement was possible even for pristine NaTaN2, despite a hexagonal cell being suggested and hence no values for sodium occupancy were available either. Yet, the c contraction together with a gradual loss of crystallinity upon progressive oxidation Figure 5. Observed and calculated X-ray powder diffraction patterns was clearly established. Moreover, all attempts to reintroduce of the chemically oxidized Ca0.89TaN2 phase. Vertical ticks denote the sodium in the structure were also unsuccessful. As for NaTaN2, Bragg positions for CaxTaN2 and for CaF2 impurity phase, tantalum is in its highest oxidation state, the authors do respectively, in green (top) and in orange (bottom). speculate that the redox reaction may involve nitrogen as well, despite not being able to confirm this hypothesis. The together with an increase in the intensity of the 001 reflection elemental analysis of the Ca0.89TaN2 sample after chemical (003 in R3̅m cell) [sin(θ)/λ = 0.089Å−1, d = 5.762 Å] and oxidation revealed a nitrogen content of 7.7% wt, that is changes in the relative intensities of 110/2̅01 [sin(θ)/λ = slightly lower than the one determined for the pristine phase 0.187 Å−1, d = 2.660 Å, 101 in the R3̅m cell] and 1̅11/200 (10.5% wt) while distribution of Ca and Ta remains [sin(θ)/λ = 0.194 Å−1, d = 2.570 Å, 102 in the R3̅m cell] homogeneous (see Figure S5B). reflections. An additional peak is observed at sin(θ)/λ around At this point, a legitimate question was whether reversibility 0.158 Å−1, which is assigned to CaF2 derived from BF4− of the deintercalation process may be prevented by large decomposition side reactions. The patterns corresponding to calcium desolvation energies in the electrolyte used, as was the chemically oxidized sample exhibit main peaks at values of suggested in the case of Ca3Co2O6,11 or else the mechanism sin(θ)/λ of 0.087, 0.188, 0.195, and 0.219 Å−1 and can be was more complex and involved some sort of decomposition fitted using a structural model with space group R3̅m, with upon oxidation, which may well explain the progressive refined cell parameters a = 3.1103(1) and c = 17.287(1) Å (see amorphization. Figure 5 and Table 3). The stabilization of R3̅m structure in In order to shed some light into this issue, calculations have the chemically oxidized sample is consistent with the been performed for the stoichiometric CaTaN2 and the disappearance of the ordered vacancy-Ca configuration in the hypothetical de-inserted phase Ca0.5TaN2 (see relevant results interlayer space, a feature well described in layered-LiMO2 in Table 4). For CaTaN2, the monoclinic and hexagonal materials where the lithium content drives the stabilization of models yield similar total energies, with a negligible energy difference of 3 meV/f.u. in favor of the latter (note that the Table 3. Atomic Coordinates in Space Group R3̅m for energy difference is within the error range of the calculation). Chemically Oxidized CaxTaN2 from the Refinement of X- Interestingly, in both cases the optimized structure shows six ray Diffraction Data at 298 K Using Radiation with λ = almost equivalent Ta−N distances excluding the presence of a 0.41338 Åa Jahn−Teller distorted Ta d1 cation. This is in line with detailed atom Wyckoff site x y z Occ. factor band structure calculations,44−46 showing that in CaTaN2 the Ca 3a 0 0 0 0.34(2) three t2g-like Ta-based bands are practically degenerate, due to Ta 3b 0 0 0.5 1 the very regular octahedral coordination for the Ta atoms. N 6c 0 0 0.252(1) 1 In order to probe the role of the Ca-vacancy ordering in deviations from the R3̅m symmetry, we have considered a a Refined cell parameters were a = 3.1103(1) and c = 17.287(1) Å. virtual composition Ca0.5TaN2 with C2/m symmetry Ca and Overall temperature factor B = 0.40(1) Å2. Agreement factors: χ2 = vacancies ordered in rows (see Figure 6A). Noteworthy, in the 8.26; Rwp = 12.7%; RBragg = 4.43%. optimized structure of this material, both the TaN and CaN 8948 DOI: 10.1021/acsomega.9b00770 ACS Omega 2019, 4, 8943−8952 ACS Omega Article Table 4. Calculated Lattice Parameters and Selected Bond Distances for CaTaN2 and Ca0.5TaN2 CaTaN2 Ca0.5TaN2 R3̅m C2/m C2/m lattice a = 3.181 Å, c = 16.723 Å a = 5.506 Å, b = 3.179 Å, c = 5.874 Å, β = 108.216° a = 5.343Å, b = 3.155Å, c = 6.004Å, β = 107.492° parameters Ca−N distance 2.4365 × 6 2.4351 × 4, 2.4366 × 2 2.4920 × 4, 2.5285 × 2 (Å) Ta−N distance 2.1862 × 6 2.1863 × 2, 2.1878 × 4 2.0848 × 2, 2.1445 × 4 (Å) Figure 6. (A) Structural model used for the calculation of C2/m-Ca0.5TaN2. (B) Calculated DOS of monoclinic CaxTaN2 with x = 1 and x = 0.5. The Fermi level is set as the zero of energy. (C) Diffusion pathway and calculated energy migration barrier for CaTaN2 (SG C2/m). octahedra are distorted (see Table 3), which is consistent with (or NaTaN2 for the analogous phase), Ca de-insertion would the monoclinic distortion observed experimentally in the reduce the occupancy of the lower N(2p)-band producing a pristine sample being driven by the Ca-vacancy ordering.47 large amount of holes and in the end nitrogen oxidation with Confrontation of the calculated data given in Table 3 to the concomitant structural degradation. experimentally achieved values, (a = 5.4182(4), b = 3.1416(2), Yet, and in line with the experimental findings discussed c = 5.9428(4) Å and β = 108.18(3)°) d(Ta−N) = 2.18(3) Å, above, a mechanism for CaTaN2 oxidation additional and/or d(Ca−N) = 2.47(2), are consistent with the Ca content (x) in alternative to deintercalation should also be considered. Thus, the as-prepared CaxTaN2 being between 0.5 and 1 as one can postulate possible oxidation/decomposition reactions experimentally observed. for CaxTaN2 yielding other known phases in the Ca−Ta−N Upon Ca deintercalation from CaTaN2, electrons should be system, which are the potential stable phases according to the removed from the band structure. The calculated density of calculated phase diagram in the Materials Project database.48,49 states (DOSs) for CaTaN2 (Figure 6B) shows that the N(2p) The calculated total energies for such reactions are and Ta(5d) states largely hybridize below the Fermi level CaTaN2 → 1/2Ca 2TaN3 + 1/2TaN E = 0.27 eV suggesting strong covalent bonding in within the TaN2 layers. (1) As previously reported,43−45 CaTaN2 is a bidimensional metal (no localized JT active Ta d1 cations) with the Ca ions Ca 0.5TaN2 → 1/4Ca 2TaN3 + 1/4Ta3N5 E = −0.023 eV ionically bonded to the (TaN2)2− layers. Two wide bands (2) appear in the DOS: the upper and partially filled band While CaTaN2 is predicted as a stable phase in the ternary dominated by the Ta d-states and the lower band (between −2 Ca−Ta−N phase diagram at 0 K, the negative total energy of and −7 eV) which is mostly of N p character. Ca reaction 2 reflects the thermodynamic trend of Ca0.5TaN2 to deintercalation from CaTaN2 to form Ca0.5TaN2 would decompose. Moreover, under the severe oxidizing conditions imply the upper band, this is to say, the oxidation from the employed, the formation of oxygen containing alternative formal state Ta4+ to Ta5+. Beyond the Ca0.5TaN2 composition phases cannot be completely excluded either. 8949 DOI: 10.1021/acsomega.9b00770 ACS Omega 2019, 4, 8943−8952 ACS Omega Article Finally, it should be noted that deintercalation of Ca ions atures of 1080, 850, and 1100 °C were employed for the from CaTaN2 would require them to migrate in the space synthesis of CaTaN2, MgTa2N3, and Mg2Ta2N4, respectively, between the (TaN2)2− layers. The simplest pathway for Ca with 300 °C/h heating rate. After the thermal treatment, the diffusion in the structure is the octahedral−tetrahedral− samples were naturally cooled in the furnace. All the thermal octahedral hopping mechanism in the Ca layer (see Figure treatments were carried out in Mo (Alfa Aesar, 99.95%) 6C). The large calculated energy barrier (1.7 eV) is indicative crucibles inside a silica tube, using Zr foil (Goodfellow, 99.2%) of a hampered Ca diffusion. Thus, it seems that other as sample holder caps and tube inlet traps in order to avoid oxidation/decomposition reactions which may involve evolu- water and oxygen contaminations. tion of N2 (as suggested by the decrease in N content from 4.2. Structural, Morphological, and Composition 10.5 to 7.7% revealed by EDX) are likely to occur instead or Characterization. X-ray powder diffraction data were concomitant to deintercalation. These would most likely be acquired using borosilicate glass capillaries of 0.3 or 0.5 mm related to the progressive amorphization observed upon diameter as sample holders. Laboratory X-ray powder oxidation. diffraction data were collected using a Bruker D8 Advance A25 diffractometer in a Debye Scherrer configuration equipped 3. CONCLUSIONS with Mo Kα1 radiation source (λ = 0.7093 Å) and Johansson monochromator. Synchrotron X-ray powder diffraction Ternary phases in the Ca−Mg−Ta−N system with different (SXRPD) patterns were collected at the MSPD beamline19 compositions and crystal structures can be easily prepared by a of ALBA Synchrotron light source (Cerdanyola del Vallès, proper tuning of the binary nitride precursors ratio and by Spain) using mythen bank of detectors with λ = 0.6199 Å using excess amounts of the alkaline earth metal in order to (MgTa2N3 and Mg2Ta2N4) or 0.41338 Å (CaxTaN2). The compensate its losses during the high temperature treatments. capillaries were rotated during data acquisition. Rietveld The extraction of Ca and Mg ions from the ternary nitrides was refinements were performed using the program FullProf.20 attempted both by chemically and electrochemically. Oxidation The amorphous content of CaxTaN2 was determined by full- of MgTa2N3 and Mg2Ta2N4 could not be achieved, with the quantitative analysis, adding to the samples a known amount observed electrochemical capacity arising from parasitic side (20% wt) of purely crystalline SiO2 standard.21 reactions involving the electrolyte, exacerbated at 100 °C when N contents were determined by combustion analysis in compared to RT. In contrast, oxidation of Ca0.89TaN2 was oxygen in a Thermo Fisher Scientific instrument, heating the found to be possible both chemically and electrochemically. samples in oxygen up to 1060 °C. Unfortunately, this process was found to be irreversible. The SEM images and EDX analyses were performed by using an differences observed in the behavior of Ca and Mg phases FEI Quanta 200 FEG microscope under high vacuum cannot easily be rationalized, as the crystal structures are operating at 20 kV; secondary and backscattered electron significantly different. According to DFT calculations, the detectors were used. EDX elemental mappings of Mg, Ca, and covalent (TaN2)2− in the CaxTaN2 crystal structure might be Ta were carried out over a representative area of the samples oxidized to some extent, while for Mg2Ta2N4, the disorder of and quantitative atomic ratios were inferred accordingly. Mg and Ta ions would most likely disrupt the formation of an Selected area electron diffraction was carried out by using a analogous band structure. JEOL 1210 transmission electron microscope operating at 120 Overall, the present work represents an additional step in kV, equipped with a Gatan double tilt sample holder, with multivalent cathode exploration. Indeed, despite the un- typically 20 crystals being examined per sample. The powder successful results, ternary nitrides constitute an interesting samples were finely dispersed in ACN and deposited as alternative to oxides, as the electrons compensating multivalent droplets on carbon-coated films supported on copper grids. intercalation are donated to a wider band arising from the 4.3. Chemical and Electrochemical Oxidation Tests. strong covalent interaction between the N p states and the Chemical oxidation tests were performed by reacting 200 mg transition metal ion, which, as inferred from DFT calculations, may result in redox processes and hence lead to high energy of the sample with NO2BF4 (Alfa Aesar, 96%) in 1:4 mol/mol densities. ratio, in 50 mL of anhydrous ACN (Alfa Aesar, 98%). The reaction was carried out under reflux, with continuous Ar bubbling within the reaction vessel. After 10 h, the obtained 4. EXPERIMENTAL SECTION dispersion was vacuum-filtered and the collected powder was 4.1. Synthesis. All samples were prepared in batches of rinsed with anhydrous ACN and dried under vacuum before 250 mg. Commercial Mg3N2 (Sigma-Aldrich, >99.5%) and being subjected to XRD analysis. Ca3N2 (Sigma-Aldrich, >99.5%) and Ta3N5 were used as Electrochemical oxidation tests were carried out in three- reactants. Ta3N5 was prepared by ammonolysis of Ta2O5 electrode Swagelok cells, by using metal discs (Li, Mg, or Ca) (Sigma-Aldrich, >99.5%), through two repeated treatments at as counter and reference electrode and the active phase as 850 °C for 15 h, under a NH3 flow of 600 cm3/min, using 300 working electrode either as a powder mixture (85:15% w/w) °C/h heating rate and natural cooling in the furnace. All the with Super P carbon or as tape (with 80% wt of active material, manipulations of the nitride reactants were carried out in a 10% wt of Super P carbon and 10% wt of polyvinylidene glovebox under an Ar atmosphere. fluoride binder). Al and stainless steel current collectors were CaTaN2 was prepared by mixing Ca3N2 and Ta3N5 using a used for the working and reference/counter electrodes, 15% (w/w) excess of Ca3N2. For the synthesis of Mg2Ta2N4, respectively, and Whatman glass fiber discs soaked in 0.6 mL Mg3N2 and Ta3N5 were mixed in 1:1 stoichiometric ratios of electrolyte solution were used as separators. while a 1.2:2 ratio was used for the preparation of MgTa2N3. LiPF6 (1 M) in EC/dimethyl carbonate (DMC) (1:1 vol, After grinding, the reactant powders were pressed into pellets LP30 Sigma-Aldrich) and home-prepared 1 m LiBOB in EC/ at 10 ton and annealed for 20 h under flowing N2 (Air PC (1:1 vol) electrolytes were used as electrolyte solutions for Products, ALPHAGAZ, >99.9999%) atmosphere. Temper- the tests carried out in Li cells at RT and 100 °C, respectively. 8950 DOI: 10.1021/acsomega.9b00770 ACS Omega 2019, 4, 8943−8952 ACS Omega ■ Article The use of Li cell configurations for the tests aiming to oxidize ASSOCIATED CONTENT magnesium ternary nitride phases stems from the impossibility * S Supporting Information to achieve Mg plating at the counter electrode in the alkyl The Supporting Information is available free of charge on the carbonate-based electrolyte employed. In contrast, the Li and ACS Publications website at DOI: 10.1021/acsome- Ca electrolytes used enabled, respectively, Li and Ca metal ga.9b00770. plating at the counter electrode, thus allowing reliable Observed and calculated SXRPD patterns; SEM micro- measurements. Dry (<50 ppm H2O), 0.3 M magnesium bis graphs; and EDX mappings (PDF) ■ trifluoromethane sulphonyl imide (Mg(TFSI)2, Solvionic, 99.5%) in a 1:1 vol mixture of EC (Sigma-Aldrich, 99%) and AUTHOR INFORMATION PC (Solvionic, 99.9%) was used as electrolyte for the reduction Corresponding Authors (magnesiation) tests carried out in Mg cells. Ca metal three- *E-mail: amparo.fuertes@icmab.es (A.F.). electrode cells and 0.45 M Ca(BF4)2 in EC/PC (1:1 vol) *E-mail: rosa.palacin@icmab.es (M.R.P.). electrolyte with H2O content <25 ppm were used for the ORCID electrochemical tests on CaTaN2. Galvanostatic cycling with Carlos Frontera: 0000-0002-0091-4756 potential limitation (GCPL) and EPS steps tests were run on a Maria Elena Arroyo-de Dompablo: 0000-0001-5249-3562 Bio-Logic VMP3 and MPG2 potentiostat/galvanostat. Poten- Amparo Fuertes: 0000-0001-5338-9724 tial steps of 5 mV and current threshold of C/100 (equivalent M. Rosa Palacín: 0000-0001-7351-2005 to intercalation of 1 mol of divalent ions in 100 h) and C/100 rate were used in the EPS and GCPL tests, respectively. A Notes The authors declare no competing financial interest. ■ temperature equilibration of 5 h at the open circuit potential was completed prior to measurements. The active material ACKNOWLEDGMENTS mass loadings were of about 3−4 mg. Once tested, the cells were disassembled in an Ar-filled dry The authors acknowledge funding from Ministry of Science, Innovation and Universities through grant MAT2017-86616-R, box and the electrodes or active material powder were washed from the “Severo Ochoa” Programme for Centres of Excellence with DMC solvent (Sigma-Aldrich, ≥99%) before being dried in R&D (SEV-2015-0496), and from Generalitat de Catalunya and sealed in borosilicate glass capillaries for the ex situ XRD (grant 2017SGR581). Funding from the European Union’s measurements. Horizon 2020 research and innovation program H2020 4.4. Computational. First-principles calculations have FETOPEN-1-2016-2017 (CARBAT, grant agreement no. been performed using the ab initio total-energy and molecular 766617) is also acknowledged. We are grateful to ALBA dynamics program Vienna Ab initio Simulation Program synchrotron for beamtime allocation at MSPD beamline (VASP) developed at the Universität Wien.22 Total energy (proposal 2016021576) and to Dr. François Fauth for his assistance during data collection. ■ calculations based on DFT were performed for CaTaN2 within the general gradient approximation, with the exchange and correlation functional form developed by Perdew, Burke, and REFERENCES Ernzerhof.23 The interaction of core electrons with the nuclei (1) Canepa, P.; Sai Gautam, G.; Hannah, D. C.; Malik, R.; Liu, M.; Gallagher, K. G.; Persson, K. A.; Ceder, G. Odyssey of Multivalent is described by the projector augmented wave method.24 The Cathode Materials: Open Questions and Future Challenges. 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