Alluaudite NaCoFe2(PO4)3 as a 2.9 V Cathode for Sodium-ion

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Alluaudite NaCoFe2(PO4)3 as a 2.9 V Cathode for Sodiumion Batteries Exhibiting Bifunctional Electrocatalytic Activity Debasmita Dwibedi, Ritambhara Gond, and Prabeer Barpanda Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02220 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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Chemistry of Materials

Alluaudite NaCoFe2(PO4)3 as a 2.9 V Cathode for Sodium-ion Batteries Exhibiting Bifunctional Electrocatalytic Activity Debasmita Dwibedi, Ritambhara Gond and Prabeer Barpanda Faraday Materials Laboratory, Materials Research Center, Indian Institute of Science, C. V. Raman Avenue, Bangalore, 560012, India. ABSTRACT: Developing novel earth-abundant and high energy density cathode materials is pivotal to realize the enduring energy storage revolution. Alluaudites NaxMy(XO4)3 (M = Mn, Fe, Co, Ni; X = Mo, W, P, As, S), as a competent series of sodium insertion cathode contenders, have attracted wide scientific attention in recent years due to their unique open framework geometry, structural flexibility, scalable synthesis and desirable electrochemical performance. Exploring alluaudite family of sodium insertion systems, we herein present a hitherto unknown NaCoFe2(PO4)3 alluaudite prepared by economic solution combustion technique. Rietveld analysis of powder X-ray diffraction pattern identified the formation of alluaudite-type monoclinic C2/c phase with a = 11.750(3) Å, b = 12.459(1) Å, c = 6.383(3) Å, and unique angle β = 113.711(7)°. As confirmed by bond valence site energy calculations, the structure renders two distinct tunnels: Na(1) and Na(2), for the one-dimensional migration of Na+ ions along the c-direction. Computational modeling revealed a migration barrier of Ea~0.31 eV for Na(2), which is one of the lowest values for Na+-conducting materials. Preliminary electrochemical study on the as-synthesized NaCoFe2(PO4)3 alluaudite exhibited reversible sodium intercalation involving a 2.9 V Fe3+/Fe2+ redox activity delivering capacity ∼70 mAh/g with good cyclability over 100 cycles. Taking advantage of transition metal active centers and PO4 linkage, NaCoFe2(PO4)3 further showed efficient bi-functional electrocatalytic activity with near four electron transfer reaction. With favorable diffusional and electrochemical performance, the discovery of alluaudite NaCoFe2(PO4)3 introduces a novel 3 V class of cathode for sodium-ion batteries. It not only enriches the materials database of sodium insertion compounds, but also enables its possible application in metal-air batteries and water splitting.

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Keeping the phosphate-based systems on anvil, our group has recently demonstrated few successful cases of The storage and conversion of energy continue to be the st water electrolysis by the conventional cathodes.5,6 This has prime challenge of the 21 century. Although currently further prompted us to search for suitable cathodes that most of the energy needs relies heavily on carbon-based might exhibit both electrochemical and electrocatalytic resources such as oil, natural gas and coal, the voracious performance. Among various low cost and sustainable energy dependence on fossil fuels and the consequent cathodes, “alluaudite” family of synthetic minerals with its greenhouse effect have created serious environmental conrich crystal chemistry has carve out a niche for itself during cerns.1 Together with the stricter emission standards, these the last few years.10-17 Basically, the mineral originally concerns have provided a strong impetus for worldwide reknown as (Na, Ca)Mn(Fe, Mg)2(PO4)3 phosphates was first search revolving around clean energy over the past two 2 reported by Moore having the general structural formula decades. In this line, high performance energy storage and X(2)X(1)M(1)M(2) 2(PO4)3 (X-eight coordinated large caticonversion processes form two key segments for the pro+ ++ + 3,4 ons: Na , Ca , K and M-octahedral cations).18 The general lific execution of sustainable energy. Hence, developing open framework and vacancy at X sites facilitate easy alsuitable materials competent to perform both these key apkali-ion (de)intercalation in these alluaudites, classifying plications forms a key step to realize energy sustainability. them among various potential promising polyanOf significance, in addition to intercalation properties, few ionic electrode materials for secondary batteries.19,20 The transition metal-based phosphates with superior cathodic subsequent study further extended the anionic part to performance have recently been studied for their bifuncmany sulfates, arsenates, vanadate and molybdates with altional electrocatalytic activity as well.4-9 Essentially, the inluring properties.11-17 Apart from promising electrochemical clusive functionality of phosphates is attributed to its activity, the ability of alluaudite-like structures to accomunique lattice structure geometry, that not only facilitate modate a wide selection of transition metals can render adeasy alkali ion (de)intercalation during electrochemical cyditional attractive features like electronic and/or ionic concling process, but also promote the electrocatalysis via oxductivity, magnetism, catalytic activity; in addition, iron idation of metal atoms during proton coupled electron 3,4 phosphate based alluaudite show high thermal stability.15,19 transfer process. Attracted by the numerous possibilities with alluaudites, ACS Paragon Plus Environment INTRODUCTION

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our previous work unveiled phosphate-based NaMnFe2(PO4)3 as a superior Na+ conductor in addition to its role as potential 2.9 V cathode for Na-ion batteries.21 Dwelling further on the rich phosphate-based alluaudite family, we hereby report the hitherto unknown NaCoFe2(PO4)3 by energy-scrooge solution combustion synthesis. X-ray diffraction technique aided the identification of this material assigning to the alluaudite class. To the best of our knowledge, this is the first report on this material. Although with the same elemental constituents, alluaudite type NaCo2Fe(PO4)3 has been reported as an electrochemically active sodium electrode, it has poor cycling performances undergoing irreversible conversion reactions.22 However, our work with bond valence site energy approach on novel NaCoFe2(PO4)3 revealed a migration barrier as low as 0.31 eV along c-direction. Thus, invoking intercalation chemistry and taking the advantage Na+ migration barrier, without any further cathode optimization, our synthesized alluaudite composition NaCoFe2(PO4)3 delivered a decent average redox potential ~ 3 V (vs. Na/Na+) with reversible capacity over 60 mAh.g−1 at a rate of C/20 with excellent rate kinetics and cycling stability. The fact that cobalt in octahedrally coordinated environments renders superior electrocatalytic activity and presence of iron might further induce a synergetic effect set a tone for the catalytic study.23, 24 Indeed, the newly found alluaudite rendered desirable bifunctional (oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)) electrocatalytic activity with near four electron transfer. It is the first report on electrocatalytic activity of any alluaudite family of cathode materials. Overall, the synergistic computational, structural, diffusional, electrochemical and electrocatalytic investigation unleashes the inclusive functionality of the novel NaCoFe2(PO4)3 alluaudite cathode materials with potential application in Na-ion batteries, metal-air batteries and water splitting.

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EXPERIMENTAL PROCEDURES

Synthesis: The target alluaudite compound [NaCoFe2(PO4)3] was prepared by solution combustion route. A 1:1:2 molar mixture of nitrate oxidizers NaNO3 (Aldrich: 99.0%), Co(NO3)2.9H2O (SDFCL: 98%) and Fe(NO3)3.9H2O (SDFCL: 98%) were dissolved in distilled water. To this, 3 moles of NH4H2PO4 (SDFCL: 99.0%) was added, thereby forming a turbid solution. To ensure the complete dissolution of all precursors, dilute nitric acid (HNO3) was added dropwise. Finally, citric acid (C6H8O7, Merck: 99.0%) was added as combustion agent (fuel), taking the fuel to oxidizer molar ratio as 1. This solution was kept on a pre-heated hot plate at 350 °C to trigger exothermic combustion reaction. After 10~15 minutes, a dark grey colored foamy intermediate complex was obtained that was mildly ground in mortar and pestle to get fine powders. To trace the product formation temperature, around 20 mg of this intermediate powder was used for thermogravimetric analysis (TGA) with a TA Q-50 unit in the temperature range of 25-900 °C (under steady O2 flow). Following the TGA result, the intermediate product was annealed in the

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temperature range of 600~900 °C for 6 h duration inside a muffle furnace. The overall reaction can be expressed as:

NaNO3(aq) + Co(NO3)2.6H2O(aq) + 2 Fe(NO3)3.9H2O(aq) + 3 NH4H2PO4(aq) + 2 C6H8O7(aq) → Intermediate complex → NaCoFe2(PO4)3(s) + 12 NH3 (g)+ 12 CO2 (g)+ 23 H2O(g) + 9 O2(g)

Structural and Physical Characterization. The phase analysis of as-synthesized NaCoFe2(PO4)3 alluaudite compound was performed with Powder X-ray diffraction (XRD) using a PANalytical X'Pert Pro diffractometer equipped with a Cu-Kα target (λ1 = 1.5405 Å) operating at 40 kV/ 30 mA and having a LynxEye positional detector. Typical patterns were acquired in the 2θ range of 5-90° (step size = 0. 026°.s-1) using Bragg-Brentano geometry. The crystal structure was analyzed by the Rietveld refinement method using GSAS program with EXPGUI graphical interface.25-27 The background, scale factor, zero shift, lattice parameter, profile functions and phase parameters were iteratively refined until a proper fit to the observed pattern was achieved. The refined crystal structure was further illustrated using the VESTA software.28 To gauge the oxidation states of the elemental constituents in the proposed material, XPS (X-ray Photoelectron Spectroscopy) was conducted with a Kratos Axis Ultra DLD high resolution instrument with automatic charge neutralization equipped with Mg Kα radiation −1253.5 eV. Data were collected at an accelerating voltage of 13 kV and emission current of 9 mA. The resulting spectra were analyzed using CasaXPS software and binding energy shift corrections were made by calibrating the main line of carbon (C 1s spectrum) to 284.6 eV.29 The morphological studies of the alluaudite powder were performed with an FEI Inspect F50 scanning electron microscope (SEM) operating at an accelerating voltage at 15~20 kV (beam current of 3-4 µA) along with an FEI Tecnai T20 U-Twin Transmission electron microscope (TEM) (operating at 200 kV) equipped with EDAX analyzer. The magnetic susceptibility data of NaCoFe2(PO4)3, sealed in a Teflon capsule, were acquired with a magnetic property measurement system (MPMS) equipped with vibrating sample magnetometer option. Magnetization as a function of temperature in the range of 2–300 K was probed in both Field Cooled (FC) and Zero Field Cooled (ZFC) mode with a constant external magnetic field of 1000 Oe. In ZFC mode, the sample was cooled from 300 K down to 2 K in the absence of any magnetic field. Subsequently, the desired external field was applied, and the data were collected on heating the sample up to 300 K. After reaching 300 K, the data record referred as FC mode, was carried with the same strength of the field on cooling the sample down to 2 K. Sodium-ion diffusion analysis. The topology of the Na+ diffusion pathways and the migration landscapes in the combustion prepared NaCoFe2(PO4)3 alluaudite were calculated by Bond Valence Site Energy (BVSE) analysis.30,31 Calculations were performed using softBV program

2 ACS Paragon Plus Environment

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employing the CIF file generated from Rietveld refinements and using Na+ as a test ion. This BVSE model provides a very simple and reliable way to illustrate the pathways for Na+ ion migration in regions of low bond valence site energy EBVSE(Na) by identifying transport pathways in local structure models. In principle, it considers migration pathways for Na+ with regions of low bond valence site energy EBVSE(Na) in grids spanning the structure model with a resolution of ca. (0.1 Å)3. Referring to local minima and saddle points of EBVSE(Na), the grid analysis utilizes a path finding algorithm to map all low energy paths connecting the local site energy minima for the mobile Na+.31

Electrochemical Characterization. For fabrication of the working electrode, 80 wt % NaCoFe2(PO4)3 active material, 15 wt % conductive carbon black (Super P) and 5 wt % PVDF (polyvinylidene fluoride) binder was mixed with minimal amount of N-methyl-2-pyrrolidone (NMP) solvent. The resulting slurry was then uniformly coated on stainless steel current collector disks (⌀ = 16 mm) and they were dried at 80 °C (for 6-8 h) in a vacuum oven to remove the NMP solvent. CR2032 coin-type electrochemical half cells, of coated discs as working electrode and Na metal foils (Sigma Aldrich) as counter electrode, were assembled in an Ar ambient glove box (MBraun LabStar GmbH) to avoid any O2/moisture contamination. The working and counter electrodes were separated by two sheets of glass fiber separator (Whatman GF/C) soaked with few drops of 1M NaClO4: PC acting as electrolyte. These coin cells were subjected to galvanostatic cycling at different rates (C/20 ~ 1C) in the potential window of 1.54.2 V (at 25°C) with a Bio-Logic BCS-805/810 battery cycler. Further to gauge the redox mechanism, ex-situ XRD (X-ray diffraction) study was performed at various points in galvanostatic charge-discharge process. First, the designated amount of sodium ions was electrochemically extracted from or reinserted into NaCoFe2(PO4)3 alluaudite assembled inside Swagelok type cell and the recuperated powder was washed with propylene carbonate (PC). The vacuum dried powder was ready for further X-ray diffraction measurements. Electrocatalysis. NaCoFe2(PO4)3 alluaudite was then tested for its electrocatalytic ORR and OER performances under alkaline condition by using three electrode configurations consisting of as-synthesized alluaudite loaded on rotating disc electrode (RDE) as working electrode, Hg/HgO as reference and Pt as counter electrodes. At first, a slurry was prepared by mixing the combustion-prepared NaCoFe2(PO4)3 (2 mg) and Super P carbon black (1 mg), in a mixed solution of 0.75 mL of double distilled water and 0.25 ml of isopropyl alcohol. For homogeneous dispersion, this slurry was sonicated for about 5 to 10 min. This followed an addition of 5 µL Nafion as binder and a longer sonication of about 30 min. Then, 2 µL of this slurry was drop casted uniformly on platinum ring glassy carbon disk

electrode and was dried under infrared (IR) lamp for around 30 minutes. The electrochemical properties were tested using a CH instrument 7001 E electrochemical workstation. Cyclic voltammetry, linear sweep voltammetry and stability tests were performed at room temperature using 0.1 m KOH as an electrolyte with a CH7001E bip0tentiostat.

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RESULTS AND DISCUSSION

Structure and Morphology. Given the great advantage of solution combustion technique towards stabilization of new phases, it was employed to fabricate the titled compound cobalt iron mixed-metal phosphate alluaudite. The wet combustion route involves intimate atomic level mixing of individual precursors, which facilitates shorter heat treatment.32 Combustion, a self-sustained exothermic reaction between oxidizer precursors and fuels, involves a low temperature (T < 500 °C) initiated gas-evolving reacti0n resulting in voluminous intermediate complex in just few minutes.33,34

Figure 1 Rietveld refined XRD pattern of the novel NaCoFe2(PO4)3 prepared by solution combustion method. The experimental data points (red dots), simulated patterns (black line), their difference (blue line) and Bragg diffraction positions (black ticks) are shown. (Inset) TGA data of the combustion intermediate powder (black line) shows a steady weight loss till 600 °C, suggesting a probable product formation temperature, while the red line represents the thermal stability of the calcined NaCoFe2(PO4)3 alluaudite phase.

As a result of partial reaction completion, the final phase formation proceeds with much lower annealing time and temperature vis-à-vis conventional solid-state (dry) route. Moreover, (i) the shorter annealing restricts excessive grain growth and (ii) the evolution of gaseous species favors nanometric morphology. Thus, taking advantages of this solvothermal route and thermogravimetric (TGA) behavior of intermediate complex (figure 1 inset), the final heat treatment of the target alluaudite phosphate was carried out at 600 °C for 6h in an air-ambient muffle furnace. This annealing step led to the formation of a grey colored product, which was subjected to further structural characterization. A high-resolution powder X-ray diffraction pattern along with Rietveld refinement is shown in Figure 1. The experimental pattern (red solid spheres) agrees very well (goodness of fit: 1.9) with simulated one and could be



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Table 1. Crystallographic parameters determined by Rietveld refinement of high-resolution XRD data (λ = 1.5418 Å) of NaCoFe2(PO4)3 at 25 °C. Formula [molecular weight] Na1.34Co0.71Fe2.01(PO4)3 [469.10] Crystal system Space group Unit cell parameter (Å)

Monoclinic C2/c (#15) a = 11.750(3), b = 12.459(1), c = 6.383(3) unique angle; β = 113.711(7)°, Z = 4 855.657 2.548 Rp = 8.6 %, RF = 7.8 %, χ2 = 1.978%, GII: 0.1466

Unit cell volume (Å3) Theoretical density (g cm-3) Reliability factors & goodness of fit values Atom Na1 Na2 Co1 Fe1 Fe2 Co2 P1 P2 O1 O2 O3 O4 O5 O6

Site 4b 4e 4e 4e 8f 8f 4e 8f 8f 8f 8f 8f 8f 8f

x 0.5 0.0 0.0 0.0 0.206 0.206 0.0 0.235 0.454 0.098 0.329 0.119 0.224 0.312

y 0.0 0.06 0.276 0.276 0.140 0.140 -0.285 -0.115 0.714 0.640 0.663 0.395 0.822 0.502

z 0.0 0.250 0.250 0.250 0.118 0.118 0.250 0.134 0.532 0.241 0.103 0.311 0.316 0.374

indexed using the monoclinic C2/c space group expected for the alluaudite structure. Using isostructural NaMnFe2(PO4)3 as model, the atomic position, thermal parameter and occupancy of the constituent atoms were refined.21 The refined cell parameters: a= 11.750(3), b= 12.459(1), c= 6.383(3) Å and β = 113.711(7)° are of the same order of magnitude to those of reported natural and synthetic NaMnFe2(PO4)3 alluaudites but former having lesser volume than the Mn analogue (owing to the cationic size trend of Mn > Co). Finally, the global instability index (GII): defined as the deviation of the bond valence sums from the formal valence Vi averaged over all N atoms within the asymmetric unit, was checked for our refined results.35 GII, a measure of plausibility of a crystal structure, typically varies from 0.1 (unstrained structure) to 0.2 (lattice induced strained) valence unit (v.u.) and value above 0.2 v.u. are often found to be incorrect.34 For our refined alluaudite crystal structure, the value was obtained to be 0.14 valence unit (v.u.), confirming a crystallographically reliable and stable structure model. The final crystallographic data along with the reliability factors are summarized in Table 1. The novel alluaudite NaCoFe2(PO4)3 material is structurally illustrated in figure 2 a-e. This compound adopts typical alluaudite type X(2)X(1)M(1)M(2)2(PO4)3 construction, where X(2) and X(1) sites are occupied by Na(1) (yellow site-Figure 2c) and Na(12) (pink site- Figure 2c and 2e) respectively. While the M(1) site is filled by Co (blue octahe-

Occupancy 0.897 0. 445 0.675 0.299 0.860 0.019 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Uiso 0.0006 0.0005 0.0006 0.0009 0.0009 0.0311 0.0164 0.0325 0.0744 0.0240 0.00261 0.0002 0.0002 0.0001

BV Sum 1.30 1.12 2.07 2.44 3.09 2.56 5.20 5.11 2.02 1.99 2.09 1.89 2.15 2.15

Figure 2 Pictorial illustration showing the construction of NaCoFe2(PO4)3 alluaudite framework. a. The basic structural unit built from edge shared CoO6 with two FeO6 octahedra. b. These basic units are connected through phosphate units to form a chain along a-direction. c. The arrangement of phosphates and CoO6 and FeO6 octahedron housing two distinct sodium sites. d. Sheet, parallel to bc plane formed by CoFe2O14 polyhedral clusters and phosphate PO4 units. e. three-dimensional NaCoFe2(PO4)3 structure projected along c-direction. CoO6 and FeO6 octahedra are shaded in blue and cyan respectively, PO4 in brown and sodium atoms are presented in pink solid spheres.


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dra), M(2) sites are occupied by Fe (cyan octahedra). Further, brown tetrahedra refers to the phosphate units and pink solid spheres indicate Na atoms in Figure 2. A polyhedral clusters (CoFe2O14) consisting of CoO6 octahedra edge shared with two FeO6 moieties can be imagined as its basic structural unit (Figure 2a). Each cluster apexes with the phosphate PO4 units to form an undulating chain along [100] direction (Figure 2b). Further, each of this clusters are abridged to other (CoFe2O14) units through edges along [101] and to phosphate units through its open corner, thereby forming a sheet along ac plane (Figure 2d). Overall, the neighboring sheets are connected via PO4 units to form 3D [NaCoFe2(PO4)3]∞ framework (Figure 2e) accommodating two distinct Na(1) and Na(2) in the cavities along [001] direction (Figure 2c). The sites for sodium (Na(1)/ Na(2)) in this alluaudite type framework could be either empty or partially filled. The fact that they could render high theoretical capacity (close to 170 mAh/g), if cycled between CoIIIFeIII2(PO4)3 and Na(Na)2CoIIFeII2(PO4)3 end members, is noteworthy. The oxidation state of constituent elements and purity of NaCoFe2(PO4)3 system was examined by X-ray Photoelectron spectroscopy (XPS) measurements. Each line shapes were carefully fitted to get a fair understanding of the local environment. Figure 3 shows XPS spectra recorded at room temperature. As observed from Figure 3a, the 2p3/2 and 2p1/2 doublet in the Co 2p spectrum have binding energy values of 782.07 and 797.8 eV, respectively. The difference of binding energy between Co 2p3/2 and its satellite peak agrees with the Co2+ environment in NaCoFe2(PO4)3 as cited from previous literature.36 Similarly, Fe 2p3/2 peak at 712.5 symbolizes Fe3+, consistent with literature containing Fe3+ as reference (Figure 3c).37 Further, to get an insight in to the purity and local environment of the sample, we analyzed O 1S and P 2p spectra of the material. The profile fitting with P 2p confirmed the (PO4)3- environment in the system (Figure 3c). Also, a new chemical environment corresponding to P2O5 is identified with additional peaks in the O 1s (533.3 eV) spectra (Figure 3d).36 However, we could not trace any signal of metal oxides which could later affect the electrochemical performance. The influence of combustion synthesis involving evolution of gaseous phases and shorter annealing duration was clearly marked in the porous morphology of the target compound as observed by SEM (Fig. 4a). Surface of the material resembled to that of coral scaly pattern (Fig. 4b). High resolution TEM micrographs with atomic fringes and selected area diffraction the average particles size estimated ~300 nm (Fig. 4d, inset). Energy dispersive spectra show the distribution of sodium, iron, oxygen, cobalt and phosphorous in the pristine material (Figure 4d). All elements demonstrate a uniform distribution throughout the compound. Further, the ultra-porous morphology and small particle size of alluaudite NaCoFe2(PO4)3 is beneficial for electrolyte wetting and smaller Na+ diffusion length, which can positively influence the overall Na+ (de)insertion process and rate capability.

a.

Co 2p3/2

b.

Co 2p1/2 Co 2p1/2-sat Co 2p3/2-sat

c.

Fe 2p3/2 Fe 2p1/2

d.

P 2p

Fe 2p3/2-sat

O 1s

PO4

P2O5

Figure 3 Co 2p (a), Fe 2p (b), P 2p (c) and O 1s (d) fitted XPS spectra of alluaudite NaCoFe2(PO4)3. The data points and enveloped fitting plot are overlaid in black dots and red lines respectively. Each spectrum was carefully fitted to trace the local environment.

a

b

c

d

Figure 4 Morphological insights of the combustion synthesized NaCoFe2(PO4)3 by electron microscopy and elemental mapping. (a., b.) SEM micrographs showing ultra-porous morphology with a scaly surface. c. High resolution TEM images with atomic fringes and SAED patterns attesting the crystallinity of the c0mpound. d. Nanometric electron micrograph of the material with representative elemental composition from EDAX analysis.

Magnetic Properties. Figure 5 shows the magnetic moment (M) and the inverse molar magnetic susceptibility χ−1 (measured in both ZFC and FC modes with an applied field of 1000 Oe) for the NaCoFe2(PO4)3 alluaudite in the temperature range of 2–300 K. The high temperature region (above 50 K) of the inverse magnetic susceptibility curve was fitted by Curie-Weiss law, giving rise to a CurieWeiss constant θ = −30.76 K and a Curie constant C = 14.74. The negative value of Curie-Weiss constant suggests


Chemistry of Materials a predominant antiferromagnetic interaction. The effective magnetic moment of 10.88 μB calculated from the Curie constant, (C ≈ μ2eff /8) is in good agreement with the effective moment μeff = 10.57 μB as expected from two high spin Fe3+ (S = 5/2) and one high spin Co2+ (S = 3/2) ions accounting spin-orbit coupling hypothesis of Hund’s rule. Further, the temperature dependence of magnetization in figure 5 presents remarkable divergence between ZFC and FC sequences below 120 K, suggesting the existence of a net uncompensated magnetic moment (weak ferromagnetism).38, 39 Overall, the small values of the Weiss temperatures combined with the lack of apparent magnetic ordering hint minor interactions between adjacent metal centers. This is consistent with the structure, which can be visualized as isolated transition metal octahedra. FC ZFC 1/c linear fit to 1/c

0.35 0.30

12000

9000

0.25

1/c

Magnetic Moment (emu/gm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.20

6000

0.15 3000

0.10 0.05

0

50

100

150

200

250

300

0

Temperature (K)

Figure 5 Magnetic susceptibility curves of NaCoFe2(PO4)3. The black and green hollow spheres indicate Field Cooled (FC) and Zero Field Cooled (ZFC) sequence in temperature dependent magnetization (M-T) curve respectively. The red hollow sphere represents the inverse magnetic susceptibility measurements and solid blue line denotes its Curie-Weiss fitting.

Sodium-ion diffusion from BVS calculation. Diffusion of sodium ions with lower migration barrier forms a key requirement for a perspective cathode. As an efficient tool, bond valence site energy (BVSE)-based calculation allows the examination of ionic conductivity characteristics of crystalline materials that are tedious to decipher by experimental studies alone. The feasibility of Na+ ionic diffusion/(de)intercalation in alluaudite NaCoFe2(PO4)3 cathode was probed by mapping the Na+ migration pathways using the Rietveld refined structure model (Figure 6). In brief, pathways for mobile Na+ are identified with regions of low bond valence site energy EBVSE (Na+). As observed from the structure, the system renders two sites for sodium namely Na1 and Na2, out of which Na1 occupies the special position. As per the BVSE calculations, the most probable pathways for Na+ migration lie along c-axis between the Na2-Na2 sites offering a very low diffusion barrier of 0.31 eV (Figure 6a). It is worth mentioning sodium diffusion between the neighboring Na1-Na1 sites along the c-direction is feasible but involving a higher migration barrier of 0.95 eV that’s makes the Na1-Na1 site diffusion less probable.

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Nevertheless, with comparatively higher energy (towards cut-off voltages), there is a fair chance for Na1 site to get activated. A slice of the structure parallel to b-c plane containing the isosurfaces of equilibrium sites (dark-pink), local path (intermediate-green) and percolating path (light-green) is shown in figure 6b. The BVSE calculations hints towards efficient Na (de)intercalation and Na+ migration in novel alluaudite NaCoFe2(PO4)3 cathode.

Electrochemical performance of NaCoFe2(PO4)3.

Inspired by the BVSE calculations predicting feasible Na+ migration, electrochemical activity in NaCoFe2(PO4)3 was tested by galvanostatic cycling measurements in sodium half-cell architecture (Figure 7). The alluaudite cathode was utilized without any optimization such as carbon coating or particle downsizing. Figure76a shows the galvanostatic charge discharge curve (GCD) superimposed on galvanostatic intermediate titration technique curve (GITT) at a current rate of C/40. Reversible Na (de)intercalation of alluaudite NaCoFe2(PO4)3 was observed along with a 2.9 V Fe3+/Fe2+ redox potential. The exact redox potential was determined from cyclic voltammogram (figure 6a, inset). Similar to the isostructural NaMnFe2(PO4)3, Na+ (de)insertion occurs involving only Fe3+/Fe2+ redox couple.10,21 Further, the slopping voltage-capacity profile together with broad peak on the CV curves indicated a probable solid solution electrochemical process for sodium (de)insertion. Considering 1 Na insertion (per formula unit, pfu), the theoretical capacity was calculated to be 54.08 mAh/g. However, complete Fe3+/Fe2+ redox activity can also be possible involving 2 electrons transfer leading to the discharged state of Na3CoFe2(PO4)3. On the other hand, a hinderance by the high diffusion barrier at Na1 site (as per BVSE calculations) may limit complete Fe3+/Fe2+ redox process, with major Na+ diffusion expected from the Na2 site having low migration barrier. At C/40 rate, the as-prepared alluaudite delivered a reversible discharge capacity of 70 mAh/g (corresponding to 1.3 electron transfer pfu). Further, the reversibility of NaCoFe2(PO4)3 at different rates is shown with excellent cycling stability and 98 percent of Coulombic efficiency (Figure 7b). When the current rate was increased to 1C, the discharge capacity of the cathode was around 27 mAh/g. The low discharge capacity at high current rates is probably due to less content of sodium participation corresponding to fast kinetics and from the oxidation of the liquid electrolyte at the high charging voltages (~ 4.2 V). A close look on rate capability plot and more than one electron transfer at C/40 rate suggested probability of approaching high theoretical capacity at sufficiently slow rates. Aside from an initial capacity loss, all the cells displayed a stable cycling behavior for the initial 20 cycles. When the current density was put back to the initial C/20 rate, the discharge capacity of the cathode returned to the value of 55 mAh/g, demonstrating a useful rate capability of the cathode. Moreover, irrespective of current rates, very good stability of the material toward the cycling showed an excellent structural flexibility of the material.


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Figure 6 Sodium ion diffusion in alluaudite type NaCoFe2(PO4)3 as derived from bond valence site energy model. (a) Energy profile diagram suggesting probable sodium diffusion pathways with corresponding migration barrier for each jump. (b) The structure and the bond valence isosurfaces for a slice of the structure with 0< x< 0.3 projected on to the b-c plane showing two independent one dimensional sodium diffusion pathways along the c-axis. In this graph both the sodium sites are indicated with arrows. The isosurfaces with dark, intermediate and light colors correspond to equilibrium sites, local paths and percolating pathways respectively.

While the preliminary electrochemical capacity value is moderate, this establishes NaCoFe2(PO4)3 as a new alluaudite insertion host material for sodium-ion batteries. The electrochemical performance can be further improved by cathode optimization methods such as particle downsizing, tailoring homogeneous morphology, and carbon nanopainting. With its potential for multiple electron redox activity and robust cycling stability, alluaudite NaCoFe2(PO4)3 forms a novel 3 V class of insertion material for sodium-ion batteries.

Structural evolution of NaCoFe2(PO4)3. To gauge the structural change of NaCoFe2(PO4)3 alluaudite during extraction/insertion of Na+ ions at different cycling states, ex situ XRD patterns were recorded after the first cycle (Figure 8). For this, recuperated cathodes from Swagelok type cell cycled at 0.1C current rate were used. Irrespective of different charge-discharge states, each pattern could be easily refined with the pristine monoclinic C2/c structural symmetry (Figure 8a). There was no appearance of any significant additional peak which ruled out the formation of new phase. However, there was a slight shift in XRD patterns associated with different charge discharge process. As we go from pattern corresponding of open circuit voltage (OCV) to a fully charged pattern of 4.2 V, there was a slight shift towards higher 2q, referring to structural shrinkage associated with sodium extraction. Likewise, there was a gradual shift towards lower 2q for discharged curves corresponding to sodium insertion and structural expansion. It was also confirmed with the variation of lattice parameters as a function of charge and dis-

charge states (Figure 8b). A decreasing trend in lattice parameter as a function of lower sodium content (charged) to higher content (discharged) was noticed. This study combined with the sloppy discharge profile of NaCoFe2(PO4)3 indicated a solid solution redox mechanism in this novel alluaudite. Further, the participation of sodium from different sites was investigated by its occupancy refinement representative of each cycling profile (figure 8c). It is interesting to note that sodium from Na1 sites also participate in the redox process despite having high diffusion barrier (0.95 eV). While sodium from Na2 site with low migration barrier (0.31 eV) was steadily moving in and out of the structure as a function of discharge and charge, there was a sudden drop in sodium content from Na1 site towards fully charged state. This is probably due to the activation of Na1 sites at high voltage (energy) input to the structure. As evaluated from occupancy refinement, there was involvement of ~0.6 sodium in redox process, which is also in sync with the discharged capacity around 45 mAh/g corresponding of 0.1C rate. Overall, the ex situ study proved (i) the participation of both the sodium sites (Na1 and Na2) accompanying the Fe3+/Fe2+ redox process and (ii) possibility of extracting more than one electron at comparatively slower rate. Electrocatalytic Performance. Various Co-based materials are known to exhibit efficient electrocatalytic (ORR and OER) activity suitable for designing metal-air batteries. In case of alluaudite NaCoFe2(PO4)3, although Co is redox inactive during battery cycling, its presence can render electrocatalytic activity. With this clue, the electrocatalytic properties of NaCoFe2(PO4)3 were studied in alkaline



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b)

Figure 7. Electrochemical performance of the NaCoFe2(PO4)3 cathode in half-cell architected Na cell. (a) Galvanostatic charge and discharge profile in CC (constant current) and GITT (Galvanostatic intermediate titration technique) mode at C/40 current rate and with GITT rest time of 15 minutes. Inset shows the cyclic voltammetry profiles for the first 3 cycles at a scan rate of 1 mVs-1. (b) Coulombic efficiency (red sphere) of combustion prepared NaCoFe2(PO4)3 over a period of 100 cycles. Rate kinetics i.e. discharge capacity as a function of different current rate (from C/40 to C rate, each over 20 cycles) are presented with blue sphere.

for alluaudite sample in the O2- saturated solution was observed, whereas no perceptible voltammetry current was marked for bare electrode. The electrocatalytic activities of NaCoFe2(PO4)3 were further compared with 20 % Pt/C using linear sweep voltammetry (LSV) curves (Figure 9b) obtained with a rotating ring disk electrode (RRDE) in O2saturated 0.1 M KOH solution with a scan rate of 10 mV s-1 and a rotating rate of 1600 rpm. From the LSV curve, the alluaudite system showed promising positive ORR onset potential (~0.75 V relative to the RHE) with similar current

Figure 8. Structural evolution during electrochemical cycling by ex-situ XRD. (a) Rietveld refined XRD patterns with observed (black line), calculated (colored spheres) profiles and Bragg positions (vertical bar) of NaCoFe2(PO4)3 cathode at different states of charge and discharge. (b) Variation of lattice parameters during (de)sodiation process during battery cycling. (c) Variation of sodium sites occupancy during cycling of NaxCoFe2(PO4)3 cathode.

electrolytes for the first time. The electrocatalytic oxygen reduction and evolution behavior of the alluaudite was gauged by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) techniques with rotating disc electrodes (RDE). The CV curves of bare graphite and NaCoFe2(PO4)3, recorded in the potential range of 0.25-0.95 V vs. RHE at O2 saturated atmosphere, are shown in Figure 9a. Noticeable oxygen reduction peak

Figure 9. Bifunctional electrocatalytic activity of alluaudite NaCoFe2(PO4)3. (a) ORR cyclic voltammetry in 0.1 M KOH at a scan rate of 10 mV/s. (b) Comparative LSV of NaCoFe2(PO4)3 and 20% Pt/C at 1600 rpm. (c) Tafel slopes of NaCoFe2(PO4)3 (red) and Pt/C (black line) (d) OER cyclic voltammetry in 0.1 M KOH at scan rate of 10 mV/s. [NFCP refers to NaCoFe2(PO4)3]


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profile as that of commercial Pt/C, however significantly less than its onset potential of 0.918 V. Tafel slope for the alluaudite NaCoFe2(PO4)3 system was calculated to be 108 mV decade-1, while Pt/C showed 98 mV decade-1 in 0.1 M KOH solution (Figure 9c). Qualitatively, Tafel slopes are determined by the magnitude of the change in the activation energy for a given increase in overpotential. Hence, lower Tafel slopes are desirable for a good electrocatalyst. Relative stability of NaCoFe2(PO4)3 catalyst was tested over 10 h with over 75 percent retention (Supporting information, Fig. S1). Although the electrocatalytic performance of this cobalt phosphate is not superior to Pt/C, it can be advantageous for the economic large-scale application. The electron transfer number (n) was calculated by the following equation , 𝜂 = 4 (1)

to have one-dimensional Na+ migration pathways with low (ca. 0.31 eV) energy barrier. Indeed, this novel alluaudite was found to be electrochemically active delivering a reversible capacity of 70 mAh.g-1 with an Fe3+/Fe2+ redox activity centered at 2.9 V (vs. Na/Na+). Beyond the electrochemical Na+ (de)intercalation, this Co-based alluaudite was found to exhibit bifunctional electrocatalytic property. The discovery of alluaudite NaCoFe2(PO4)3 marks a PO4 cathode with both electrochemical and electrocatalytic activity. It further attests the richness of PO4 chemistry to unravel new cathodes for Na-ion and Na-air batteries.

where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the Pt ring, which was determined to be 0.41. The electron transfer number per oxygen molecule (n) for ORR of NaCoFe2(PO4)3 was calculated to be ~3.84, which is close to four electron transfer pathways for ORR suggesting at reversible reaction

Phone: +91- 80 2293 2783. Fax: +91-80 2360 7316.

,- .,/ /1

-

2H2O + O2 + 4e- ↔ 4OH (aq)

(2)

NaCoFe2(PO4)3 sample was then tested for the OER performance under same alkaline medium using three electrode systems. Cyclic voltammograms (CVs) were recorded by cycling the electrodes between 1.0 and 1.65 V (vs RHE) in O2 saturated 0.1 M KOH solution at a scan rate of 10 mVs-1 (Figure 9d). NaCoFe2(PO4)3 exhibited OER electrocatalytical activity as well with decent onset potential (1.55 V vs RHE) with a reasonable current density ~ 7 mA cm-2. Essentially, high catalytic activity could originate from excellent conductivity, large surface area and abundant exposed active sites, facilitating charge transition, water adsorption, oxygen or hydrogen formation and diffusion. In this line, nanostructured synthesis plays a crucial role, due to its large contact area with electrolyte and short transport path for electrons.14 Here, for the first time, we demonstrated the preliminary studies on bifunctional OER and ORR electrocatalysis of NaCoFe2(PO4)3 alluaudite. It showed superior activity, desirable catalytic stability and 3.84 electron transfer number (n) per oxygen molecule, which approaches the ideal n value of 4. To improve the electrocatalytic activity, particle downsizing and atomic structure modification could be followed to enlarge the specific surface area, stabilizing the chemical state of active materials and improving the internal conductivity. Nonetheless, it marks the first example of alluaudite compound showing electrocatalytic activity, which may be useful for metal-air batteries. Conclusions. In summary, a hitherto unknown cobalt based NaCoFe2(PO4)3 alluaudite was synthesized by solution combustion route. Its monoclinic (C2/c) crystal structure was solved by Rietveld analysis. With bond valence site energy (BVSE) calculations, this alluaudite was found

AUTHOR INFORMATION Corresponding Author

Prabeer Barpanda E-mail: [email protected] ORCID: Debasmita Dwibedi: 0000-0003-2366-1429 ORCID: Ritambhara Gond: 0000-0003-3061-7434 ORCID: Prabeer Barpanda: 0000-0003-0902-3690

Author Contributions DD planned and carried out most of the experiments. DD and RG completed the electrocatalytic analysis of material. DD wrote the manuscript. The project was conducted under the supervision of PB.

Funding Sources The current work is in part funded by the Department of Science and Technology (DST), Government of India, under the aegis of Solar Energy Research Initiative (SERI) programme (DST/TMC/SERI/FR/169).

Notes The authors declare no conflict of interest.

Supporting Information Available The electrocatalytic activity of NaCoFe2(PO4)3 alluaudite studied under linear sweep voltammetry (LSV) showing KouteckyLevich plots, LSV at different rotating speed, ORR stability performance and number of electron calculation. This material is available free of charge via the internet at http://pubs.acs.org.

ACKNOWLEDGMENTS The authors are grateful to Prof. Stefan Adams (NUS, Singapore) for scientific help and discussions. PB acknowledges Science and Engineering Research Broad (SERB, Govt. of India) for the financial support under Early Career Research Award (ECR/2015/000525). DD is thankful to the Ministry of Human Resource Development (MHRD) fellowship. RG sincerely thanks University Grant Commission (UGC) for her fellowship. DD and RG thank International Centre for Diffraction Data (ICDD) for a 2017 and 2019 Ludo Frevel Crystallography Scholarship Awards respectively. DD and RG acknowledge the Electrochemical Society (ECS) for ECS Summer Fellowships.



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Alluaudite NaCoFe2(PO4)3 as a 2.9 V Cathode for Sodium-ion

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