Synthesis and Structural, Electrical, and Magnetic Properties of New

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

Synthesis and Structural, Electrical, and Magnetic Properties of New Iron−Aluminum Alluaudite Phases β‑Na2Ni2M(PO4)3 (M = Fe and Al) Douha Harbaoui,†,⊥ Moustafa M. S. Sanad,†,‡,§ Cécile Rossignol,†,‡ El Kebir Hlil,†,∥ Noureddine Amdouni,⊥ and Saïd Obbade*,†,‡ †

Université Grenoble Alpes, F-38000 Grenoble, France LEPMI, CNRS, Grenoble INPF-38000 Grenoble, France § Chemical & Electrometallurgy Division, Central Metallurgical R & D Institute, P.O. Box 87, Helwan, Cairo, Egypt ∥ Institut Néel, Université Grenoble Alpes, CNRS, BP 166, F-38042 Grenoble cedex 9, France ⊥ U.R. Physico-Chimie des Matériaux Solides, Université El Manar, 1060 Tunis, Tunisia ‡

ABSTRACT: Herein we report the studies of different physical properties (structural, magnetic, thermal, morphologic, electrical, and electrochemical) of two new allotropic β-Na2Ni2M(PO4)3 (NNMP) phosphates, with M = Fe and Al. Pure orthorhombic single-phase powders were prepared under air, using an autocombustion synthesis method. They crystallize in the orthorhombic Imma space group with similar unit cell parameters [a = 10.1592(2), b = 13.0321(3), c = 6.4864(2) Å] and [a = 10.3993(1), b = 13.1966(1), c = 6.4955(1) Å] for β-Na2Ni2M(PO4)3 (NNAP) and β-Na2Ni2Fe(PO4)3 (NNFP), respectively. Crystal structures of both compounds were determined using Xray powder diffraction and Rietveld method refinements, which indicate the occurrence of Ni2+ in the 8g site, and of M3+ in the 4a site of the structure. The structure consists of a three-dimensional anionic framework obtained by the association on MO6, NiO6, and PO4 polyhedra, sharing edges and corners. The resulting three-dimensional structure creates monodimensional channels along the [100] and [010] directions formed by face-shared oxygen polyhedra and occupied by Na+ cations. This nondisordered cationic distribution is confirmed by a significant change of magnetic properties. Thus, both NNAP and NNFP samples show paramagnetic to ferromagnetic transition at 14 and 19 K, respectively. For the two compounds, thermal stability, electrical conductivity, and electrochemical properties have been also investigated. The intercalation/desintercalation properties of NNMP compounds as positive electrode were tested in sodium-ion batteries. The first cycling curves exhibit a significant polarization for both prepared samples.

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INTRODUCTION Since the 1990s, lithium-ion batteries have been developed, optimized, and marketed by Sony and are now the de facto standard for rechargeable batteries in electrical consumer applications. As the accessibility of electric and hybrid vehicles continues to increase, their market share is growing rapidly, pushing the battery market to continue to grow at least as rapidly in the coming years. Today’s lithium-ion batteries are one of the most popular battery technologies, but these face several difficulties; one of the biggest is the scarcity of resources. Therefore, it is clear that if all the vehicles of the future were powered by lithium-ion batteries, there would not be enough lithium in the world to support even a few months of production, and it is therefore necessary to find an alternative to lithium. The sodium element represents a promising solution; it offers a good compromise if one judges the performance of an electrical battery just by the underlying properties of the active ion, which are the reduction potential, atomic weight, ion size, valence, and abundance. © XXXX American Chemical Society

However, sodium-ion batteries can hardly compete with lithium-ion ones due to the intrinsic properties of sodium: a less negative standard reduction potential [−2.7 V vs SHE for the Na+(aq)/Na against −3.04 V for the Li+(aq)/Li one] and a higher molecular weight. Sodium-ion technology can be addressed for targeted applications. Therefore, Na-ion batteries became much more interesting, as alternatives for lithium-ion batteries in different applications, due to the abundance and cost effectiveness of sodium metal.1−3 This need for alternatives is also being driven by extending the energy storage for new applications, in order to store power from renewable sources such as solar and wind energy for use on the grid power systems.4,5 Thus, since the discovery of highly interesting properties of olivine phase LiFePO4 as a positive electrode material, the search of novel phosphate, vanadate, sulfate, and silicate polyanion-based insertion hosts has been intensiReceived: July 23, 2017

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DOI: 10.1021/acs.inorgchem.7b01880 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry fied.6−15 Since then, several polyphosphate-based compounds have been extensively studied as electro-positive materials in Na-ion batteries.16−20 Between the large number of existing sodium phosphate compounds, alluaudites have been among the more studied phases; thus, different authors have reported preliminary studies of some sodium alluaudite compounds containing biand/or trivalent metals, with a monoclinic symmetry and C2/c space group,21−34 identical to the crystal structure of natural alluaudite mineral determined by Fisher,35,36 where the crystal structure adopts an anionic framework containing open channels that favor the mobility and diffusion of Na + monovalent cations along these tunnels. However, different research groups have recently reported diverse alluaudite-type compounds Na1−2MM′2(PO4)3 (M, M′ = transition metals), that crystallize in an orthorhombic crystal structure with Imma space group and a more symmetric polyanionic framework.37−39 Lately, two allomorphic Na2Ni2Fe(PO4)3 and αNa2Ni2Fe(PO4)3 compounds with monoclinic and orthorhombic symmetry, respectively, were recently prepared and tested as positive electrode material for Li-ion battery.33,37 Magnetic measurements of both of these compounds showed two different behaviors, with predominant antiferromagnetic interactions (θ = −114.3 K) in orthorhombic compound and a ferromagnetic behavior for the monoclinic phase, with a Curie temperature (Tc = 46 K). In this context, we were interested in the synthesis of new sodium phosphate phases of type Na2M2M′(PO4)3, with M and M′ as transition metals, to understand the correlation between crystal structure and magnetic behavior. Consequently, in this work we present different investigations of Na2Ni2M(PO4)3 compounds with M = Fe and Al, synthesized by an autocombustion method, using a long annealed time at adequate temperatures to stabilize the orthorhombic phases. Also, this paper deals with the crystal structure determination, the particle morphology, the thermal stability, as well as magnetic proprieties of both compounds. Finally, the influence of iron−aluminum substitution on the electrical conductivity will be discussed in terms of electrochemical performance of proposed materials as positive electrodes.

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For pure phases, the Rietveld method has been used for crystal structure refinement, using powder X-ray diffraction data. Thermal analyses (TG/DTA) were performed under air flow (20 mL/min), using a SETARAM TAG24-16 thermal analyzer, and platinum crucibles. For each sample, thermal measurements were carried out in the range from room temperature to 800 °C, with 5 °C min−1 fixed heating rate, and all the DTA curves were normalized with respect to the sample weight. Surface morphology and elemental analysis were examined by field emission gun-scanning electron microscope (FEGSEM) Carl ZEISS model ULTRA 55 equipped with microanalysis BRUKER AXS, detector SDD 30 mm2, and energy dispersive X-ray spectroscopy analyzer (EDS). The magnetic measurements of the asprepared samples were carried out using the commercial Physical Properties Measurements System device (Quantum Design, PPMS), and magnetization M(T) was measured versus temperature ranging from 2 to 400 K and under a magnetic field of 0.05 T. The field dependence of the magnetization M(H) was measured at 2 K by varying the applied magnetic field between −6 and 6 T for NNFP and in the range −10 to 10 T for the NNAP compound. Electrical conductivity measurements were carried out on cylindrical pellets (diameter, 5 mm; thickness, ca. 3.5 mm) obtained using a conventional cold press and sintered at 890 °C for 2 days, followed by very slow cooling, 5 °C h−1, until room temperature. Gold electrodes were vacuum-deposited on both flat surfaces of the pellets, using a SC7620 mini-sputter-coater. The ac impedance diagrams were recorded with an applied voltage of 50 mV from 300 to 700 °C and under zero dc conditions in the 5 to 1.3 × 107 Hz frequency range, using Hewlett-Packard frequency response analyzer (4192 ALF). The sample densities were measured from the size and weight of sintered pellets. For the electrochemical measurements, the carbon coatings of the as-prepared samples were carried out by mixing the material in sucrose with a amount corresponding to 10 wt % C using ball milling and then calcinating at 650 °C under inert atmospheric conditions. For electrochemical measurements, the working positive electrodes were formulated by mixing 80 wt % of Na2Ni2M(PO4)3 (M = Fe, Al) active material, 10 wt % of Super P carbon, and 10 wt % of the polyvinylidene fluoride (PVDF) as binder dissolved in N-methylpyrrolidone (NMP) solution. For each compound, the formed homogeneous mixture was coated on aluminum foil by using a screen printing technique, and then dried at 70 °C under vacuum. The electrochemical tests were carried out using coin cells (CR2025) assembled in an Ar-filled glovebox. The average mass loading of the active material was around 3 mg/cm2, and Na metal foil acts as the negative electrode with Celgard 2400 as the separator. The cell electrolyte was 1 M NaClO4 in propylene carbonate (PC). Galvanostatic charge−discharge cycling and EIS measurements for the assembled batteries were conducted in the potential range 1.5−4.2 V (vs Na+/Na) under various current densities using a VMP-300 multipotentiostat (BioLogic SAS).

EXPERIMENTAL SECTION

Both samples of β-Na2Ni2Fe(PO4)3 [NNFP] and β-Na2Ni2Al(PO4)3 [NNAP] were synthesized by an autocombustion method using nitrate salts as oxidizers and glycine acid as fuel and complexing agent. In a typical synthesis, stoichiometric proportions of NaNO3 (Alfa-Aesar 98.9%), Ni(NO3)2·6H2O (Sigma-Aldrich 99.9%), Fe(NO3)3·9H2O (Sigma-Aldrich 99.9%), or Al(NO3)3·9H2O (Plus laboratories 100%) were completely dissolved in a solution of glycine acid C2H5NO2 (Alfa-Aesar 99%). The molar ratio of glycine to nitrate amounts was kept constant at 0.4. The required amount of NH4H2PO4 (Acros 99.9%) was then added to the first mixture and stirred for 30 min. The excess water was then evaporated by heating at 110 °C. During the evaporation, viscous liquid foams are formed and fine particles produced as a result of autoignition. The dried ash was directly put in a muffle furnace at 400 °C for 5 h, yielding a pale-yellow precursor for Na2Ni2Fe(PO4)3 and greenish material for Na2Ni2Al(PO4)3. The both precursors were successively ground and calcined at 850 °C for 48 h. The obtained yellow Na2Ni2Fe(PO4)3 and green Na2Ni2Al(PO4)3 powders were subsequently ground to be ready for further characterizations. The powder X-ray diffraction data have been collected with an X’pert PRO PANalytical θ/2θ diffractometer equipped by Cu Kα radiation, using Bragg−Brentano geometry, in steps of 0.02°. The counting time is 40 s per step, within an angular range 10−90° in 2θ.

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RESULTS AND DISCUSSION Crystal Structure Determination. For both compounds, the crystal structure determination in orthorhombic symmetry and Imma space group was conducted using powder X-ray diffraction data. Thus, after the adjustment of pattern profile parameters using the “pattern matching” option of the Fullprof suite,40,41 the crystal structure of each compound was refined by means of the Rietveld method.42 Having refined the profile and unit cell parameters using the pattern matching option of the Fullprof program, the refinement of crystal structure was carried out in the centrosymmetric Imma space group, taking as starting model the powder X-ray crystal structure investigations of Na2Ni2M(PO4)3 with M = Fe and Cr (two Na sites 4e−4b, two metal transition Ni−M sites 8g−4a, two P atomic positions 8g−4e, and four sites of oxygen) of the two last publications.37,39 In our study, the aluminum compound βNa2Ni2Al(PO4)3 has been chosen for the first crystal structure refinement, because Ni and Al are the elements possessing the B


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Inorganic Chemistry highest electronic density difference, and hence better resolution. In the beginning of the refinement, occupation factors of Ni and Al atoms in the mixed 8g and 4a sites were refined with a linear constraint to respect the chemical formula and with an equal repartition of Ni and Al in both sites. After several refinement cycles, it appeared clearly that the occupancy factors in the two transition metal sites corresponded exactly to the nominal chemical formula, with no mixed occupation of Ni and Al atoms in 8g and 4a sites. Thus, the final crystal structure of the β-Na2Ni2Al(PO4)3 compound is characterized by a full occupancy in 8g (1/4, y, 1/4) and 4a (1/2, 1/2, 1/2) sites by Ni and Al atoms, respectively. For iron compound βNa2Ni2Fe(PO4)3, the Ni and Fe occupation factors were fixed in agreement with the formula. After several crystal structure refinements with mixed Ni−Fe sites, only the structural model with the Fe atoms in the crystallographic site 4a (1/2.1/2.1/2) gave the best reliability factors, confirming also the nondisordered cation distribution in the structure β-Na2Ni2Fe(PO4)3. In the final crystal structure refinement of both compounds, isotropic thermal parameters were refined for all atoms. For both compound, crystal structure data and details of refinements are collected in Tables 1 and 2.

Table 2. Atomic Coordinates and Isotropic Displacement Factors Biso for β-Na2Ni2M(PO4)3, M = Fe and Al, Obtained from Powder X-ray Diffraction Data name

Table 1. Crystallographic and Structure Refinement Data for β-Na2Ni2M(PO4)3, M = Fe and Ala Crystal Data chemical formula density (g/cm3) fw (g/mol) crystal syst space group T (K) a (Å) b (Å) c (Å) V (Å3) Z diffractometer radiation type 2θmin, 2θstep, 2θmax (deg) Rp Rwp Rexp R(F) RBragg no. params profile function

Na2Ni2Fe(PO4)3 3.757 504.14 orthorhombic Imma (No. 74) 300 10.3993(1) 13.1966(1) 6.4955(1) 891.35(2) 4 Data Collection

Na2Ni2Al(PO4)3 3.676 475.26 orthorhombic Imma (No. 74) 300 10.1592(2) 13.0321(3) 6.4864(2) 858.77(2) 4

Panalytical λCu K(α1,α2) 10, 0.016, 91 Refinement

Panalytical λCu K(α1,α2) 10, 0.016, 91

0.045 0.064 0.051 0.026 0.034 104 pseudo-Voigt

0.035 0.047 0.010 0.031 0.034 104 pseudo-Voigt

x

site

Na(1) Na(2) Ni Fe P(1) P(2) O(1) O(2) O(3) O(4)

4e 4b 8g 4a 8g 4e 16j 16j 8h 8i

Na(1) Na(2) Ni Al P(1) P(2) O(1) O(2) O(3) O(4)

4e 4b 8g 4a 8g 4e 16j 16j 8h 8i

y

Na2Ni2Fe(PO4)3 0.0000 0.7500 0.0000 0.5000 0.2500 0.6368(1) 0.50000(0) 0.5000 0.25000(0) 0.4282(2) 0.00000(0) 0.7500 0.28925(30) 0.3676(3) 0.13821(31) 0.5069(1) 0.00000(0) 0.6547(4) 0.12000(48) 0.7500 Na2Ni2Al(PO4)3 0.0000 0.7500 0.0000 0.5000 0.2500 0.6343(2) 0.50000(0) 0.5000 0.25000(0) 0.4293(2) 0.00000(0) 0.7500 0.28925(30) 0.3676(3) 0.13821(31) 0.5069(1) 0.00000(0) 0.6547(4) 0.12000(48) 0.7500

z

Biso

0.6091(8) 0.5000 0.2500 0.5000 0.2500 0.0864(3) 0.0632(4) 0.2105(5) 0.9527(2) 0.2268(3)

4.61(19) 4.14(20) 1.05(4) 1.08(7) 0.989(71) 1.05(10) 0.73(10) 0.71(11) 0.84(14) 0.56(15)

0.6067(8) 0.5000 0.2500 0.5000 0.2500 0.0713(7) 0.0724(5) 0.1974(6) 0.9566(6) 0.2179(8)

5.04(17) 4.26(24) 1.25(4) 1.78(7) 1.989(71) 1.85(10) 3.73(10) 3.71(11) 2.84(14) 2.56(15)

Table 3. Selected Bond Lengths (Å) for β-Na2Ni2M(PO4)3, M = Al and Fe distance

Na2Ni2Al(PO4)3

Na2Ni2Fe(PO4)3

Na(1)−O(3) (×2) Na(1)−O(1) (×4) Na(1)−O(4) (×2)

2.633(5) 2.577(4) 2.787(6) ⟨2.644⟩ 2.360(5) 2.740(4) ⟨2.550⟩ 2.035(5) 2.107(5) 2.144(4) ⟨2.096⟩ 1.832(5) 1.944(7) ⟨1.870⟩ 1.487(5) 1.622(5) ⟨1.554⟩ 1.522(8) 1.528(8) ⟨1.525⟩

2.626(9) 2.702(5) 2.783(8) ⟨2.704⟩ 2.369(5) 2.833(4) ⟨2.612⟩ 2.021(5) 2.076(4) 2.086(3) ⟨2.061⟩ 1.986(6) 1.934(7) ⟨1.968⟩ 1.509(4) 1.581(5) ⟨1.545⟩ 1.543(7) 1.640(7) ⟨1.594⟩

Na(2)−O(2) (×4) Na(2)−O(1) (×4) Ni−O(4) (×2) Ni−O(2) (×2) Ni−O(1) (×2) M−O(2) (×4) M−O(3) (×2) P(1)−O(1) (×2) P(1)−O(2) (×2) P(2)−O(3) (×2) P(2)−O(4) (×2)

a Final atomic positions and isotropic thermal coefficients are reported in Table 2.

metals localized in two different crystallographic sites: Ni2+ in site 8g and M3+ in atomic position 4a. In both compounds Na2Ni2M(PO4)3 (M = Al, Fe), the crystal structure is built from an assembly of two individual transition metal atoms in distorted octahedra MO6 and NiO6, and two independent phosphorus atoms P(1) and P(2) bonded to four oxygen atoms to form PO4 tetrahedra, to give a tridimensional anionic arrangement. Therefore, the crystal structure can be easily described considering the association of two anionic building entities: (i) the infinite chains [MPO8]8− running parallel to the b⃗-axis, obtained by MO6 octahedra and

The selected interatomic distances are presented in Table 3. At the end of refinement, the good agreement between the refined model and the real crystal structure was indicated by the comparison between the calculated and observed patterns, given in Figure 1a,b for NNAP and NNFP, respectively. Thus, it should be noted that a long annealing at about 850 °C of the samples allowed stabilizing new orthorhombic alluaudite phases with an ordered structure, with the di- and trivalent transition C


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Inorganic Chemistry

Figure 1. XRD patterns of the as-prepared alluaudite samples (a) Na2Ni2Fe(PO4)3 and (b) Na2Ni2Al(PO4)3.

Figure 2. Polyhedral units in crystal structure of Na2Ni2M(PO4)3: (a) anionic [MPO8]8− infinite chain and [Ni2P2O14]14− nickel phosphates entity and (b) anionic [Ni2M(PO4)3]2− infinite layer.

P(2)O4 tetrahedra sharing two opposite O(3) oxygen corners, and (ii) nickel phosphate entities [Ni2P2O4]14− created by two Ni2O10 dimers and two P(1)O4 tetrahedra sharing symmetrically opposite edges O(2)−O(2), Figure 2a. Each infinite [MPO8]8− chain shares corners O(2), O(4), and O(2) with adjacent [Ni2P2O4]14− units to form an infinite layer [Ni2M(PO4)3]2− parallel to the (101) plane as seen in Figure 2b.

Thus, the crystal structure of these compounds is obtained by the association of adjacent infinite layers [Ni2M(PO4)3]2− sharing corners to form a three-dimensional anionic framework creating two different channels running along the a⃗ and b⃗ directions, presented in Figure 3a,b, respectively. These tunnels contain the Na+ cations in two crystallographic sites, with short average distances ⟨Na−O⟩ in aluminum compound NNAP, as indicated in Table 3. Thus, the channel dimensions in the D


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Inorganic Chemistry

Figure 3. Projection views of Na2Ni2M(PO4)3 crystal structure along (a) [100] axis and (b) [010] direction.

Figure 4. Thermal analysis of Na2Ni2Fe(PO4)3 and Na2Ni2Al(PO4)3 compounds by TGA and DTA measurements.

330−450 °C is assigned to the thermal decomposition and evaporation of H2O, NH3/NO2, and CO2. Between 500 and 540 °C, a secondary exothermic peak (c*) is observed only in the NNAP sample due to the beginning of the crystallization of the monoclinic phase of the NNAP compound, confirmed by powder X-ray diffraction of NNAP samples synthesized at 500 K. It is worth mentioning that single crystals of the iron Na2Ni2Fe(PO4)3 monoclinic phase have also been prepared using a flux of sodium dimolybdate.33 The last step of weight loss (c) observed above 600 °C is mainly attributed to the stabilization of orthorhombic phase in both compounds, with the main crystallization temperatures of orthorhombic NNFP and NNAP phases localized at about 640 and 590 °C, respectively, as illustrated in Table 4.

anionic framework arrangement largely influence Na+ cation mobility and therefore electrical conductivity and electrochemical properties of compounds. Thermal Analysis. Thermal reaction evolution during autocombustion synthesis of the new alluaudite-type βNa2Ni2M(PO4)3 (M = Fe, Al) has been followed by the TG/ DTA analyses using a SETARAM TAG24-16 thermal analyzer. The TG/DTA analysis curves in the thermal range from room temperature to 800 °C are shown in Figure 4a,b, and show three main steps (a, b, and c) with no significant weight loss. The weight loss of the first step at 160−190 °C is entirely related to the beginning of the thermal decomposition of organic precursors, which also corresponds to the first exothermic peak (a) observed in the DTA curve. In addition, the weight loss of the second step (b) at the heating range E


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Inorganic Chemistry Table 4. Different Physical Parameters of β-Na2Ni2M(PO4)3 Compounds (M = Fe and Al) cryst temp (°C) theoretical density pellet density activation energy Ea (eV) Tc (K) Hc (T) Mr (uem/g)

β-Na2Ni2Fe(PO4)3

β-Na2Ni2Al(PO4)3

∼640 3.757 90% 0.63 19 0.12 3.14

∼590 3.676 86.3% 0.84 14 0.77 1.56

decrease in aggregate sizes of the powder without changing the average size of grains. Using powder X-ray diffraction data of both compounds, the profile and unit cell parameter refinements were also carried out using a Thompson−Cox−Hasting profile option, which allows us to obtain simultaneously the average particles size (DFe = 83 nm, DAl = 87 nm) and the strain (ηFe = 0.11%, ηAl = 0.09%), for Na2Ni2Fe(PO4)3 and Na2Ni2Al(PO4)3 compounds, respectively. To verify the presence of different chemical elements in the prepared samples, a semiquantitative analysis of the powders was performed by energy dispersive X-ray spectroscopy (EDS) mounted on a (FEG-SEM) Carl ZEISS-ULTRA 55 scanning electron microscope. The EDS spectra shown in Figure 6a,b confirm the presence of appropriate chemical elements in both samples NNFP and NNAP. It is also found that the experimental molar ratios in both samples are very close with experimental results. Figure 7a,b reveals the morphology of the ground aggregates of NNFP with and without carbon coating. It is seen that the apparent particle size was reduced to about 2 μm, while the carbon fibers were clearly observed after coating the ground powders. Magnetic Properties. The magnetic behavior for Na2Ni2Fe(PO4)3 and Na2Ni2Al(PO4)3 presents interesting features. Thus, Figure 8 shows the corresponding temperature dependence of the magnetization M(T) measured in fieldcooled condition under a magnetic field of 0.05 T, revealing magnetic ordering with a ferromagnetic component below the Curie temperature Tc. Both samples show a sharp ferromagnetic (FM) to paramagnetic (PM) transition with Tc, defined as the temperature corresponding to the minimum of the derivative dM/dT of the magnetization M(T) curve, which is about 14 and 19 K as for NNAP and NNFP, respectively.

Morphological Properties. The studied NNFP and NNAP materials are potentially usable as a positive electrode in sodium-ions batteries, so it is important to study the morphology of the powders, because a smaller grain size is favorable to the intercalation process in the primary particles by reducing the length of the ionic diffusion path. Figure 5a,b shows the SEM images of NNFP powder at two different magnifications. The grains of the NNFP sample are homogeneous with micronic size and appear as regular structures of rectangular prism-like morphologies. In addition, the NNFP sample exhibits compact agglomerates with an average size ranging between 15 and 40 μm (Figure 5a), with prismatic primary grains size about 1−4 μm (Figure 5b). On the other hand, SEM images of the aluminum NNAP sample presented in Figure 5c,d exhibit smaller agglomerated grains with size average ranging between 10 and 25 μm, where each aggregate consists of bipyramidal prismlike structures with sharp edges. One can notice that this kind of NNAP aggregate has a smaller, but with same order primary grain average size as compared to those observed in the NNFP compound. It can thus be concluded that substitution of Fe3+ by of Al3+ ions in orthorhombic alluaudite structure leads to a

Figure 5. FEG-SEM images of pure compounds prepared at 850 °C: (a, b) Na2Ni2Fe(PO4)3 and (c, d) Na2Ni2Al(PO4)3. F


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Inorganic Chemistry

Figure 6. Energy dispersive X-ray spectroscopy (EDS) analysis of (a) Na2Ni2Fe(PO4)3 and (b) Na2Ni2Al(PO4)3 samples.

Figure 7. FEG-SEM images of the ground particles of Na2Ni2Fe(PO4)3: (a) without carbon coating and (b) with carbon coating.

The magnetic hysteresis M(H) loops for representative samples recorded at 5 K, in the ferromagnetic region, are shown in Figure 10, up to 6 and 8 T for NNFP and NNAP compounds, respectively. The coercivity field Hc is found to be equal to 0.12 and 0.77 T, while remnant magnetization is estimated to 3.14 and 1.56 uem/g, for β-Na2Ni2Fe(PO4)3 and β-Na2Ni2Al(PO4)3, respectively. All estimated magnetic parameters are gathered in Table 4. The large slope in the high field

For both compounds, the temperature dependence of the magnetic susceptibility inverse χ−1 was presented in Figure 9, where we can observe a curvature in temperature domain above Tc pointing out that some weak magnetic order persists as was observed in Figure 8, presenting the temperature dependence of magnetization M(T). The intersection of the linear part of the inverse of the magnetic susceptibility with the temperatures axis also leads to Tc values of about 14 and 19 K for NNAP and NNFP, respectively, Figure 9. G


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Inorganic Chemistry

Na2Ni2Fe(PO4)3 and orthorhombic α-Na2Ni2Fe(PO4)3, prepared by other synthesis methods using two different thermal conditions.33,37 In the case of the orthorhombic α-Na2Ni2Fe(PO4)3 phase, prepared by direct solid state reaction at 850 °C, using a stoichiometric mixture of sodium carbonate, transition metal nitrates, and NH4H2PO4, the crystal structure characterization has shown a statistical disorder of nickel and iron atoms over the two crystallographic sites 8g and 4a with Ni/Fe occupation ratios of 0.75/0.25 and 0.5/0.5 in 8g and 4a sites, respectively. This cationic disorder between Fe3+ and Ni2+ in the crystal structure of α-Na2Ni2Fe(PO4)3 leads to an antiferromagnetic to paramagnetic transition at about 18 K and to a paramagnetic behavior in the temperature range 100− 350 K, with a Curie−Weiss law of magnetic susceptibility above 100 K. Such para-antiferromagnetic behavior has also been observed in the orthorhombic NaCoCr2(PO4)3 compound,38 which adopts a similar crystal structure with mixed cobalt/ chromium in the 8g crystallographic site. However, for the monoclinic Na2Ni2Fe(PO4)3 compound,33 where no structural disorder between nickel and iron was observed, the temperature dependence of the magnetization M(T) at 100 Oe revealed a ferromagnetic behavior below the Curie temperature of 46 K similar to that observed in our study for orthorhombic β-Na2Ni2Fe(PO4)3, but with an increase in the Curie temperature from 19 to 46 K. Electrical and Electrochemical Properties. To complete this work, electrical and electrochemical investigations of both compounds have also been carried out. Electrical properties of sintered pellets from milled powders were undertaken by ac impedance spectroscopy in air from 300 to 700 °C. In parallel with electrical measurements, electrochemical tests of both samples as positive electrode materials in sodium-ions batteries were also carried out using galvanostatic cycling, with sodium metal as negative electrodes, at C/20, C/50, and C/100 rates in the potential window 1.5−4.2 V. Electrical Conductivity Properties. To provide evidence for the Na+ cation mobility in the different tunnels of the structure, ionic conductivity measurements were carried out. Typical ac complex impedance results are obtained for both compounds, and an example of NNAP and NNFP samples as solid electrolyte at 400 °C is shown in Figure 11.

Figure 8. Temperature dependence of the magnetization M(T) of NNFP and NNAP samples measured under an applied field of 0.05 T.

Figure 9. Temperature dependence of the magnetic susceptibility χ−1 of NNFP and NNAP.

Figure 10. Magnetic M(H) hysteresis loop of representative NNFP and NNAP samples measured at 5 K.

region reveals that only small components of the magnetic moments are ferromagnetically ordered in our both samples. According to Figures 8 and 9, we can notice that Tc temperature decreases from 19 to 14 K with iron−aluminum substitution in the NNAP compound. This Tc evolution can be correlated with the increase of (Ni−O−Ni) bond angles and of Ni−Ni distances, which induces a decrease of magnetic interactions between nickel atoms, causing such diminution of Tc value in NNAP. Since the Al3+ ion is nonmagnetic, there is also no exchange interaction along Al−O−Ni−O−Al−O− Ni−O−Al chains in the NNAP compound. It is also interesting to compare the magnetic properties of our compound β-Na2Ni2Fe(PO4)3 with the monoclinic

Figure 11. Complex impedance plot of sintered pellets of NNFP and NNAP samples at 400 °C. Inset graph is the magnified complex impedance plot of NNFP. H


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Inorganic Chemistry The comparison of impedance spectra of both compounds at 400 K indicates that the total impedance of NNAP is substantially higher than that of NNFP. This low electrical conductivity for the NNAP compound could be explained by a steric effect due to the contraction of the unit cell and the reduction of its volume. This decrease in volume from about 891 for NNFP to 858 Å3 for NNAP is mainly due to the difference in the size of the atomic radius between the iron and aluminum atoms in an octahedral environment, given by Shannon et al.43,44 (rAl3+ = 0.675 Å and rFe3+ = 0.785 Å). It is evident that the contraction of cell parameters in the NNAP compound leads to a decrease in the size of channels in the alluaudite framework and, therefore, to a lower mobility of Na+ ions in the structure. It may be seen that even though the sodium atoms occupy almost the same atomic positions in the two channels of the structure, the Na−O distances are lower in the NNAP compound (see Table 3), which results in a strong chemical bond between the sodium and oxygen atoms in the tunnels. These strong Na−O bonds can explain the conductivity decrease and the activation energy increase observed in the NNAP compound. In addition, the highest relative density of 90% was observed for the NNFP sample as illustrated in Table 4. Therefore, the sample of NNAP showed the lowest values of conductivity at different temperatures, and the obtained results show an increase of the conductivity with temperature for both samples over the studied temperature range. Finally, for both compounds an Arrhenius law applies for the whole temperature range explored with activation energy of 0.63 and 0.84 eV for NNFP and NNAP compounds, respectively, Figure 12.

Figure 13. Electrochemical cycling curves of the assembled sodiumion batteries measured at C/100 rate. Inset graph about the Nyquist plot of both sample electrodes after the first cycle.

profiles, much better performances are obtained for the cell with NNFP. Approximately, about 0.66 Na+ ion per formula unit could be extracted from the structure at the first charge cycle, corresponding to the reduction of some Fe3+ ions, and about 0.4 Na+ ions per formula unit could be intercalated during the following discharge cycle of NNFP, causing the oxidation of some Fe2+ and/or Ni2+ ions. However, in the NNAP cell, about 0.27 Na+ ion per formula unit could be extracted at the first charge step, associated with the reduction of some Ni3+ ions present in the NNAP structure, and about 0.15 Na+ ion per formula unit could be intercalated during the first discharge cycle, which may be explained by the oxidation of Ni2+ ions. The inset graph of EIS of the cycled sodium batteries after the first cycle provides evidence of the higher difficulty of diffusion of sodium ions in the NNAP cell. The NNAP cell has higher bulk resistance (Rs), charge transfer resistance (Rct), and diffusion resistance, evidenced in the Nyquist plot of Figure 13. Finally, the NNFP battery was cycled at different rates (C/100, C/50, and C/20) to improve the storage capacity of this positive electrode material, as seen in Figure 14.

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CONCLUSION Two new compounds β-Na2Ni2M(PO4)3 with (M = Fe and Al) were synthesized by an autocombustion route, and their crystal structures have been determined in the orthorhombic alluaudite-type structure, using powder X-ray diffraction data and Rietveld method refinement. The crystal structure is built by association of polyhedra units [MPO8]8− and [Ni2P2O4]14− forming infinite layers linked by corners to form a threedimensional anionic framework [Ni2M(PO4)3]2− parallel to the (101) plane, creating two different channels running along the [100] and [010] directions. For both NAMP (M = Fe or Al), no Ni/M transition metal disorder in 8g and 4a crystallographic sites was observed, and the magnetic behavior is similar to that observed in monoclinic Na2Ni2Fe(PO4)3. The ferromagnetic Tc, coercivity field Hc, and remnant magnetization Mr of NAMP are found to be affected by substitution of iron by aluminum in the 4a crystallographic site of alluaudite structure. Electrical and electrochemical studies show better Na+ ion diffusion in the NNFP compound and suggest that optimizations in particle

Figure 12. Arrhenius plots of ionic conductivity of bulk samples NNFP and NNAP.

Electrochemical Properties. In order to investigate the effects of trivalent ion substitution (M3+) in the 4a crystallographic site on the performance of NNFP and NNAP compounds as positive electrode materials in Na-ions batteries, electrochemical properties of both samples were measured for the first time by cycling the assembled batteries at slow rate C/ 100 as indicated in Figure 13. This slow cycling rate was chosen in order to reduce the kinetic effect and approach a thermodynamic equilibrium of sodium exchange. Even if both cells exhibit similar cycling I


Article

Inorganic Chemistry

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Figure 14. Comparison between the electrochemical cycling rate on the intercalation/deintercalation of sodium ions in the assembled batteries of NNFP.

size and morphology, and with different carbon coating, can further improve the potential of β-Na2Ni2M(PO4)3 compounds as electrode materials for Na-ion batteries.

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ASSOCIATED CONTENT

Accession Codes

CCDC 1549905−1549906 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Saïd Obbade: 0000-0002-0172-6757 Notes

The authors declare no competing financial interest.

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REFERENCES

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