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Electrochemical and Diffusional Investigation of Na2FeIIPO4F Fluorophosphate Sodium Insertion Material Obtained from FeIII Precursor Lalit Sharma, Prasant Kumar Nayak, Ezequiel de la Llave, Haomin Chen, Stefan Adams, Doron Aurbach, and Prabeer Barpanda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10637 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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ACS Applied Materials & Interfaces

Electrochemical and Diffusional Investigation of Na2FeIIPO4F Fluorophosphate Sodium Insertion Material Obtained from FeIII Precursor Lalit Sharma,† Prasant Kumar Nayak,‡ Ezequiel de la Llave,‡ Haomin Chen,$ Stefan Adams,$ Doron Aurbach,‡ and Prabeer Barpanda†,* †

Faraday Materials Laboratory, Materials Research Center, Indian Institute of Science, C. V. Raman Avenue, Bangalore, 560012, India. ‡ $

Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel.

Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore. * author for correspondence E-mail: [email protected] Phone: +91-80 2293 2783; Fax: +91-80 2360 7316

Abstract Sodium iron fluorophosphate (Na2FeIIPO4F) was synthesized by economic solvothermal combustion technique using FeIII precursors, developing one-step carbon-coated homogeneous product. Synchrotron diffraction and Mössbauer spectroscopy revealed the formation of singlephase product assuming an orthorhombic structure (s.g. Pbcn) with FeII species. This FeIII precursor derived Na2FeIIPO4F exhibited reversible Na+ (de)intercalation with discharge capacity of 100 mAh/g at a rate of C/10 involving flat FeIII/FeII redox plateaus located at 2.92 V and 3.05 V (vs. Na/Na+). It delivered good cycling stability and rate kinetics at room temperature. The stability of Na2FePO4F cathode was further verified by electrochemical impedance spectroscopy at different stages of galvanostatic analysis. Bond valence site energy (BVSE) calculations revealed the existence of 2-dimensional Na+ percolation pathways in the a-c-plane with a moderate migration barrier of 0.6 eV. Combustion synthesized Na2FeIIPO4F forms an economically viable sodium battery material. While the capacity of this cathode is relatively low, this study continues systematic work, which attempts to broaden the scope of reversible sodium insertion materials. Keywords: Na-ion battery, fluorophosphate, Na2FePO4F, combustion, Bond-valence site energy.

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1. Introduction Since 1991, Li-ion batteries ushered the portable (mobile) electronics era experiencing unprecedented growth in applications and global market for lithium batteries. It is ubiquitous these days from small-scale consumer electronics to large-scale (hybrid) electric vehicles and power grid storage. While the portable electronics sector is more or less saturated, challenges persist in case of automobiles and grid storage sectors. These large-scale applications require economic and ecological batteries with competent electrochemical performance. To realize this, materials economy (and abundance) is crucial, which paves way for chemistry based on earth abundant elements. Following this spirit, sodium-ion batteries (SIBs) are touted as viable alternatives for lithium-ion batteries (LIBs), which can be even more promising if they utilize Fe and/or Mn 3d transition elements.1-4 Variety of Fe-based oxides and polyanionic sodium insertion materials have been reported till date, achieving either high capacity (~200 mAh/g) and/ or high redox voltage (ca. 3.8 V vs. Na/Na+) delivering competent energy-density inching close to that of Li-ion counterparts.5-7 While oxide based cathodes offer low operational voltage with safety concerns involving oxygen loss, polyanionic framework compounds can be designed with wide structural diversity, high redox potential and robust thermal/ chemical stability. From stability point-of-view, various phosphate (PO43-) based frameworks have been reported such as triphylite/ maricite polymorphs of NaFePO4, Na2FeP2O7 pyrophosphate and Na4Fe3(PO4)2(P2O7) mixed phosphates.8-12 One such PO4-based sodium insertion host is layered iron fluorophosphate Na2FePO4F, capable of intercalating both Li and Na.13-15 Na2FePO4F employs electronegative F- species to enhance the operating potential due to inductive effect. Nevertheless, it suffers from low electronic conductivity limiting its (de)sodiation activity, hence demanding cathode optimization like particle nanoscaling to reduce the diffusion length and/ or conducting carbon coating. Implementation of such cathode optimization has been reported using ionothermal route, sol-gel, carbothermal reduction, soft chemical synthesis etc.14-18 While ionothermal synthesis employs expensive ionic liquid reacting media, methods like sol-gel is very time consuming. Pursuing scalable economic synthesis of Na2FePO4F, here we report auto-combustion synthesis starting with low cost Fe(III) precursors. This combustion route involves the formation of an intermediate organometallic complex that undergoes fast carbothermal transformation to yield the target product. This two-step method can restrict the high-temperature annealing step


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within 1 minute to produce carbon-coated nanoscale product phase approaching one-electron theoretical capacity. The current article demonstrates nuances of combustion synthesis and electrochemical performance of Na2FePO4F in sodium half-cell to gauge its Na+ intercalation behavior. It is important to note that the current work does not aim at describing optimized cathode material with practical performance. The paper intends presenting a scalable economic synthesis of Na2FePO4F with focus on synthesis optimization, structural characterization, and basic electrochemical performance. The work described herein is a reasonable starting point for next stage that involves further cathode optimization and bringing the material to a practical capability in terms of cycle life, long term durability and stable capacity during prolong operation.

2. Experimental Section 2.1 Fluorophosphate Synthesis. Na2FeIIPO4F was prepared by autocombustion synthesis involving an economic FeIII precursor. Initially, stoichiometric amounts of NaF (Merck, 99%), FeIII(NO3)3.9H2O (Sigma Aldrich, 98%) and NaH2PO4 (Fluka, 99%) were used as ‘oxidants’ and were dissolved in water. Pinch of L-ascorbic acid (C6H8O6) (SD Fine Chemicals, 99%) was optionally added to reduce FeIII to FeII state. Taking into account the combustion index of each element, the amount of combustion agent (or fuel) was calculated to get the optimum combustion as described in earlier reports.19,20 Various carbonaceous materials (like citric acid, ascorbic acid and urea) were used as fuels. This precursor solution was heated at 110-120 °C with continuous magnetic stirring, which led to atomic level mixing of precursors and removal of excess water to form a viscous gel. Upon steady heating, this gel triggered exothermic combustion reaction leading to a sudden rise in local temperature and the formation of a brownish intermediate compound. The latter was annealed at 500-600 °C (for 1 minute to 6 h) to obtain the final desired product. The annealing can be done in two ways. First, the sample was kept in an alumina boat inside a tubular furnace under steady Ar flow, the furnace was ramped to 600 °C rapidly (at a rate of 20 °C/min) and annealing was completed for desired duration (1 min to 6 h) before cooling down to ambient temperature. Alternately, the sample was sealed inside a quartz tube under vacuum (to avoid any oxidation) and was transferred to a muffle furnace already set at 600 °C. After soaking for stipulated duration, the quartz tube was taken out, cooled down to room temperature and was broken to recover the final product. In both cases, similar end products



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were obtained. However, for safety and ease of handling, the first method was mostly used during the course of the work. 2.2 Physical Characterization. Powder diffraction patterns were acquired by a PANalytical XˈPert Pro diffractometer using a Cu-Kα source (λ1 = 1.5408 Å, λ2 = 1.5443 Å) operating at 40 kV/30 mA. X-ray diffraction patterns were collected in the 2θ range of 10~90° with a step size of 0.026° s-1. To gauge the purity and crystallinity, synchrotron X-ray diffraction pattern of selected samples were acquired at the BL-18B Indian beamline (High Energy Accelerator Research Organization, KEK-Photon Factory, Tsukuba, Japan) using a synchrotron beamline of wavelength λ = 0.7861(2) Å and energy E = 15.77 keV calibrated with Si (640b NIST) standard. Rietveld refinement was carried out with GSAS program with the EXPGUI front-end.21-23 The mean crystallite size was calculated by using Scherrer formula [crystallite size (nm) = k*λ / β*cos θ]. The value of shape factor (k) was taken to be 0.9, while the wavelength (λ) value was 1.5418 Å. Room temperature Mössbauer spectra of selected samples were collected with a spectrometer operated in constant acceleration mode in transmission geometry with a

57

Co

source. The calibration of the velocity scale and isomer shift values was performed using an α57

Fe metal foil. The morphology of the fluorophosphate powder (sprinkled on conducting carbon

tape) was observed using a scanning electron microscope with a field emission gun source (FEI Inspect F50, operating at 5-20 kV/ 3-4 µA) and a transmission electron microscope (FEI Tecnai F30 S-Twin, operating at 200 kV). The TEM samples were prepared by dispersion and dropcasting of the powder sample (in acetone) on a copper grid. Raman spectra of powder samples were acquired by a LabRAM HR Evolution spectrometer (HORIBA, Japan) equipped with a 514.5 nm (green) laser source with a spectral resolution of 1 cm-1 and a spatial resolution of 2.5 nm. Infrared spectroscopy of powder samples (diluted in KBr pellets) was conducted with a Perkin Elmer 1000 FTIR spectrometer in the wavenumber range of 400-4000 cm-1 (cycle number = 4). 2.3 Bond Valence Based Diffusion Pathway Analysis. The topology of the Na+ diffusion pathways and the migration barriers in combustion-prepared Na2FePO4F were investigated employing the bond valence site energy (BVSE) method using the synchrotron structural data. The details of BVSE modeling are described elsewhere.24-26 This modeling of pathways for Na+ ion migration as regions of low bond valence site energy EBVSE(Na) has been demonstrated to be


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a simple and reliable way of identifying transport pathways in local structure models. While bond valences sNa-X = exp[(R0,Na-X – RNa-X )/ bNa-X] and the BV sum mismatch |∆V| are expressed in arbitrary valence units, they can be linked to an absolute energy scale by expressing the bond valence site energy EBVSE of a Na+ cation coordinated by X- anions as a Morse-type interaction energy:  N  s E BVSE ( Na ) = ∑ D ∑   Na − X   s x i =1    min,, Na − X

2

 s  − 2 Na − X  s min, , Na − X 

  + E repulsion  

(1)

The required bond valence parameters are taken from our softBV database (ref. 27). Migration pathways for Na+ are then investigated as regions of low bond valence site energy EBVSE(Na) in grids spanning the structure model with a resolution of ca. (0.1 Å)3. Starting from an analysis of 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+. It has been demonstrated for a wide range of materials that the pathways found from this computationally cheap method are closely approximating pathways observed by ab initio approaches and allow for a prediction of the achievable rate performance.28 Ab initio structure relaxations of the experimental structure models of Na2FePO4F and Na1FePO4F are performed using Vienna Ab initio Simulation Package (VASP),29 with the PBE class of generalized gradient approximation (GGA-PBE)30 exchange-correlation potentials using an energy cutoff of 400 eV with a k-mesh of 1×1×1. The DFT+U scheme (with U= 5.3 eV for Fe) is applied to facilitate convergence and enhance precision.

2.4 Electrochemical Characterization. CR2032-type coin cells were assembled to gauge the electrochemical performance of combustion prepared Na2FePO4F. The working electrode was prepared by mixing 80 wt% of active material, 10 wt% carbon black and 10 wt% of poly vinylidene fluoride (PVdF) in minimal amount of N-methyl-2-pyrolidone (NMP). The mixture was cast on an Al foil and dried at 120 °C in vacuum. Circular disks (φ = 12 mm) were punched out with an electrode mass loading of ~3 mg/cm2. CR2032 type coin cells were assembled inside an Ar-filled MBraun GmbH glove box using these disks as working electrodes, Na metal foil as counter electrode separated by two sheets of glass fiber separator soaked with 0.5 M NaPF6:PC electrolyte. The galvanostatic cycling of these cells were conducted using an Arbin BT-1000 battery cycler in the voltage range of 2.0-4.3 V at different rates of C/15 to 4C (at 30 °C).



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Electrochemical impedance spectroscopy (EIS) was conducted to gauge the stability of fluorophosphate cathode. Impedance spectra were collected at various potentials during charge after completing 3 cycles with an AC amplitude of 10 mV over the frequency range of 100 kHz0.01 Hz using Bio-logic instrument. Also, impedance spectra were measured at open circuit potential after completing 10, 30 and 60 cycles in order to check the stability of the material upon cycling.

3. Results and Discussion 3.1 Solution combustion synthesis. Sodium iron fluorophosphate (Na2FePO4F) is a complex system having combination of PO43- and F- anions, where the presence of F- can trigger hygroscopic nature. In contrast to conventional solid-state synthesis, solvothermal routes can be adopted to favor intimate atomic-level mixing of individual precursors and thereby requiring less aggressive heat treatment. Many synthesis routes employ FeII based precursors to produce Na2FeIIPO4F. However, it is possible to employ low cost FeIII-based precursors and convert the redox state of the Fe atoms with the help of cheap reducing agents (or electron donors) like ascorbic acid and carbon.31 That is also the case with the solution combustion route used in the present study. The method employs a two-step approach involving (i) a low temperature step dealing with solution-assisted mixing of precursors, formation of metal-organic complex and partial reaction completion to form an intermediate complex, and (ii) a final heat-treatment to convert this intermediate complex to the desired end product. Two distinct salient features are unique to the proposed combustion synthesis of the fluorophosphate. First, the conventional solid-state method as well as several solvothermal routes (e.g. solgel) are very time consuming with prolonged mixing of precursors. However, the low temperature step of combustion method particularly employs water-soluble precursors to ensure quick and intimate mixing. The combustion fuels (e.g. urea) act both as complexing agent and as carbon source. Progressive dehydration of the precursor solution leads to a thick viscous gel, which triggers exothermic combustion reaction. The exothermic heat propels the reaction to form an intermediate complex, which can be described as a hydrated citrate-nitrate compound embedded within a carbonaceous matrix. Mössbauer study revealed that in this intermediate complex the Fe is exclusively present as FeIII (Figure 1). Thermogravimetric (TGA) analysis of this complex exhibited steady weight loss between 100-550 ºC involving dehydration,


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complexation and decomposition (Supporting information, Fig. S1). Guided by the TGA study, upon annealing at 600 ºC, this FeIII intermediate complex transformed into the desired FeII fluorophosphate as confirmed by Mössbauer analysis. This FeIII→FeII transformation can be described as a carbothermal reduction involving carbonaceous fuels. It also forms a one-step uniform carbon coating of the active material to form Na2FeIIPO4F/C as described later with the help of electron microscopy and Raman spectroscopy. Secondly, we investigated the kinetics of fluorophosphates formation. While the solidstate method involves slow diffusion of the reacting species demanding energy-intensive prolonged heat-treatment (ca. 12-24 h) at temperatures of 600-800 °C, soft chemistry method like combustion synthesis can lead to rapid phase formation. Upon varying the annealing conditions for the FeIII-rich intermediate complex, we observed the formation of Na2FeIIPO4F even with a short annealing duration of just 1 minute (Figure 2). While the targeted Na2FePO4F compound is successfully obtained, we noticed two small impurity peaks at around 33° and 39°. While the former peak arises from the thermodynamically stable maricite polymorph of NaFePO4, the latter can be assigned to slight amount of unreacted NaF precursor. Longer annealing led to larger particles as revealed by sharper diffraction patterns. The crystallite size was found to be in the range of 0.55 ~0.61 nm in these products. Among all processing methods, solution combustion route benchmarks the shortest annealing duration (ca. 1 min) for fluorophosphate formation. This quick annealing (along with carbon coating) restricts large grain-growth to form nanoscale particles. Overall, combustion route was found to form carboncoated Na2FeIIPO4F from economic FeIII precursors with restricted annealing duration and particle size. It imparts material and process economy. The combustion synthesis, which is easily scalable, can be further optimized by (i) using fuels with varied degree of reactivity and carbon content, (ii) varying the oxidant/ fuel ratio to control exothermicity and (iii) annealing conditions. During high-temperature (ca. 600 ºC) annealing of the intermediate complex, a rapid evolution of various NOx and COx gases promoted the formation of a homogeneous porous morphology. Furthermore, the shorter annealing duration involves less aggressive grain growth, hence forming sub micrometric primary particles (Figure 3). EDX analysis confirmed the uniform distribution of individual elements (supporting information, Fig. S2). The presence of carbon and possible carbon coating also acted as grain growth inhibitor. TEM study revealed



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these primary particles consist of secondary particles in the size range of 100-200 nm. HRTEM study revealed the formation of a uniform thin carbon coating layer (~ 5 nm) around each particle, which retards excessive grain growth and facilitates electronic conductivity. The presence of carbon coating was further verified by Raman spectroscopy, where signature D (~1390 cm-1) and G (~1595 cm-1) bands were captured stemming from elemental carbon (Figure 4). The sharper G bands hints at a graphitic nature of the carbon coating. While the signature PO43- bands (at low wavenumber window) could not be resolved by Raman spectroscopy, they were clearly observed by FT-infrared analysis. The presence of PO43- groups was confirmed by symmetric (ν1) and asymmetric (ν3) stretching modes in the high-wavenumber region (900-1200 cm-1) along with symmetric (ν2) and asymmetric (ν4) bending vibrations in low-wavenumber region (500-600 cm-1) (Fig. 4).32 It is worth reporting that despite the use of aqueous media and ambient storage of combustion product, there was no band ~2600-3200 cm-1 confirming the absence of any trace amount of moisture.

3.2 Structure, morphology and BVSE analyses. Structural analysis of solution combustion synthesized fluorophosphate was carried out by combining high-resolution X-ray diffraction and synchrotron diffraction. The target Na2FeIIPO4F end-member was obtained with no trace of any FeIII precursor (Figure 5). Synchrotron data were collected on selected samples and further confirmed the successful synthesis of fluorophosphate compound. Rietveld refinement analysis of diffraction patterns was carried out and the resulting structural data is summarized in Table 1, which corroborates earlier reports.13-16 Reliable fitness parameters (RF2 = 0.1776, χ2 = 7.388) were obtained. In line with earlier X-ray and Neutron powder diffraction studies of Na2FePO4F,15,33 the resulting product is found to assume the Na2MgPO4F structure type,34 i.e. a layered orthorhombic framework with Pbcn symmetry isostructural to known mineraloids like Na2FePO4OH.35 As illustrated in Fig. 5, the structure consists of adjacent face-sharing FeIIO4F2 octahedral building blocks that form FeII2O7F2 bioctahedral units. These FeII2O7F2 blocks are arranged into parallel chains along a-axis by sharing corners via F atoms. These chains are abridged by PO4 tetrahedra along c-axis to form (FeIIPO4F)∞ infinite slabs delimiting two distinct crystallographic sites for constituent Na atoms (Na1 and Na2), each being surrounded by four neighboring O atoms, two neighboring F atoms and a slightly more distant O atom. The Na1 site is slightly larger than the Na2 site.


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The Rietveld refined structure model as well as the literature data on Na2FeIIPO4F15 and NaFeIIIPO4F36 were employed to study the energy landscape for the deintercalation of Na+ from the fluorophosphate by bond valence site energy (BVSE) analysis. The direct analysis of the two powder X-ray-diffraction-based structure models for Na2FePO4F in this work and from literature15 as well as empirical geometry optimizations based on the softBV forcefield converged to essentially the same pathway model and we will mainly discuss the pathways based on our experimental structure determination of Na2FePO4F and briefly compare it to pathway models that we derived for structure models that were geometry-optimized by DFT for Na2FePO4F and by DFT+U for the desodiated Na1FePO4F (Figure 6). As seen in the comparison of energy landscapes for mobile Na+ in Figure 6(a), the BVSE analysis of the experimental structure model (Fig. 6 (a-c)) suggests that Na2FePO4F is essentially a two-dimensional (2D) ionic conductor with a migration barrier of about 0.61 eV and an only marginally lower barrier of 0.58 eV for one dimensional transport along the a-axis. Similarly, for a DFT-relaxed version of the structure model (see Fig. 6 (a,d)) the same BVSE approach yields an estimated migration barrier of 0.55 eV for a directly 2D pathway model, whereas for a DFT+U-relaxed structure model of the desodiated phase Na1FePO4F the anisotropy within the same 2D pathway network appears to be somewhat more pronounced with a migration barrier of only 0.46 eV along the a-direction, but 0.56 eV along the c-direction (see Fig. 6 (a) and (e)). In summary, 2D Na+ transport remains possible in the fluorophosphate host matrix irrespective of the state of discharge with only moderate migration barriers of 0.55 eV- 0.6 eV and a minor anisotropy for faster transport along the a-axis.

3.3 Sodium storage performance. Galvanostatic sodium (de)insertion analysis of combustion made Na2FePO4F was conducted in sodium half-cell architecture without any further cathode optimization like carbon surface coating and particle nanosizing. The as-synthesized product exhibited efficient Na+ (de)intercalation leading to almost 1 Na removal in the first charge. During subsequent cycling, a reversible discharge capacity of 100 mAh/g, which is ~81% of 1electron theoretical capacity (QTh = 124.2 mAh/g), was obtained (Figure 7). This is comparable to previously reported capacity obtained with cumbersome ionothermal synthesis and solid-state synthesis.13-18 This desirable electrochemical performance can be linked to the formation of homogeneous nanoparticles and uniform carbon coating (via combustion synthesis), which



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facilitate efficient Na+ diffusion and electronic conductivity respectively. It involves two distinct FeIII/FeII redox plateaus centered at 2.92 V and 3.05 V (vs. Na/Na+) with minimal polarization. The stability of cathode during desodiation was examined by EIS, which can shed light on Na+ diffusion, electrode-electrolyte interfacial reaction and charge-transfer resistance.37 Impedance spectra were acquired at several equilibrium potentials (ca. 2.6 ~ 4 V vs. Na/Na+) upon progressive desodiation. It is important to note that while impedance spectra of composite electrodes may reflect all their relevant time constants, the assignment of spectral features to certain time constants may be impossible because of overlapping of responses. Thereby, we are very careful not to extract irrelevant conclusions from the impedance spectra thus obtained. Nevertheless, despite the complexity in their analysis, the impedance spectra obtained herein provide important information, by comparing spectra measured at the same conditions in the course of cycling, as presented below.

Comparative Nyquist plots (Fig. 7, inset) revealed

semicircles at the high-to-medium frequency range that can be assigned to contributions from both interfacial resistance (surface) and charge-transfer resistance (surface and bulk). With progressive charging (or desodiation), these semicircles became smaller indicating lower resistance values as the electrodes potential is higher. The linear tail in the spectra related to the low frequency range can be assigned to the Na+ ions diffusion in the fluorophosphate particles. With higher potential, the slope of these linear tails increases sharply. This change may indicate a faster Na+ ions diffusion at higher potential. Overall, the electrodes impedance spectra are supposed to reflect progressive changes in composition, Na+ diffusivity and interfacial resistance during sodiation and desodiation of the Na2-xFePO4F electrodes. At this stage, further analysis of the various impedance spectral features and their potential dependence is not important. The most important information that can be extracted from EIS of these electrodes related to their stability. Reversible Na+ (de)intercalation was observed involving FeIII/FeII redox activity centered at 3 V (Figure 8). After 60 cycles, ~80% of initial discharge capacity was retained. This result is apparently frustrating and calls for further optimization efforts. However, there are several critically important findings indicating that the active mass itself is intrinsically stable during repeated Na+ ions intercalation/de-intercalation cycling. The cathode stability was checked during galvanostatic cycling and the Coulombic efficiency was 100% and the average charge/discharge redox potentials remained unaltered during cycling (Fig. 8, inset). Most


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important, EIS data of these cathodes were collected at the end of different cycles. There was no change in the EIS spectra affirming excellent material stability of Na2FePO4F during electrochemical cycling (Fig. 8, inset, nearly overlapping impedance measured spectra initially and after 60 cycles). This result reflects the intrinsic stability of the electrodes’ active mass, which is essential for long-term application of battery electrode materials. Finally, the rate kinetics of combustion made fluorophosphate compound was gauged (Figure 9). It retained 65% and 45% of initial capacity (at C/10 rate) at 1C and 4C rate respectively. As seen from the inset in Fig. 9, the variation of the capacity with C-rate closely follows the expected stretched exponential behavior,28 which besides confirming that the rate performance is mainly limited by the Na+ diffusion - allows to characterize the rate performance by a critical C-rate of 1 h-1 in line with the moderate activation barrier and a stretching exponent of ½ that is characteristic of fully ordered structures such as NaxFePO4 with low defect site concentration. The extrapolation to low C rates leaves about 20% of the capacity unavailable (probably due to still imperfect electronic conductivity despite the uniform carbon coating). The cycling stability and rate kinetics are quite promising, and it should be possible to further improve it by careful cathode optimization (further increasing the electronic conductivity and the defect concentration to prevent an early onset of capacity reduction), rate-dependent optimization of cut-off windows and suitable electrolytes.

4. Summary To conclude, sodium iron fluorophosphate Na2FeIIPO4F was successfully synthesized by solution combustion synthesis employing low cost FeIII precursors. The exothermicity during combustion facilitated the formation of carbon-metal-nitrate complex, which rapidly transformed into the final product within just 1 minute upon annealing at 600 °C. This benchmark the shortest annealing duration reported till date. Involving carbothermal reduction, it developed homogeneous and nanometric product phase with a uniform 5 nm carbon coating. The phasepurity and Fe redox state was confirmed by synchrotron diffraction and Mössbauer spectroscopy. Bond valence site energy (BVSE) calculations clarify that the Na+ migration during cycling occurs within a two-dimensionally percolating Na+ pathway network extending in the a-c-plane. Within this 2D network transport along the a-axis has to overcome only moderate migration barriers of 0.55-0.6 eV. The pathway topology remains essentially unchanged when the Na



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content is reduced from 2 to 1 per formula unit, with only a slight increase in the anisotropy of the migration barriers (Preference for transport along a slightly increases with desodiation). Combustion prepared Na2FeIIPO4F delivered a reversible discharge capacity of 100 mAh/g at a rate of C/10 with desirable cycling stability and rate kinetics as indicated by the characteristic Crate of 1 h-1. Overall, solution combustion route is demonstrated as an economic, fast and easily scalable way to prepare fluorophosphate cathode. The resulting Na2FeIIPO4F is found to be an economically viable 3 V insertion material for room-temperature sodium battery applications. While the capacity is moderate, it attests ecofriendly synthesis and realization of robust Fe-based polyanionic sodium insertion host materials.

Notes

The authors declare no competing financial interest.

Acknowledgements The authors thank the Department of Science and Technology (DST), Government of India, for financial support under the aegis of Indo-Israel project (DST/INT/ISR/P-10/2014). LS is thankful to Ministry of Human Resource Development (MHRD) for financial support. PB thanks the Department of Atomic Energy (DAE) for a DAE-BRNS Young Scientists Research Award (YSRA). Rajeev Kumar and Prof. Balaram Sahoo are acknowledged for their help in Mössbauer spectroscopy. The authors acknowledge the Saha Institute of Nuclear Physics (SINP), India, for facilitating the synchrotron experiments at the Indian Beamline (BL-18B), KEK Photon Factory (KEK-PF), Japan. Crystalline structure was illustrated using the VESTA software.38 SA, HC and RPR are grateful to National Research Foundation, Prime Minister’s Office, Singapore for support under the Competitive Research Programme (CRP Award NRF-CRP 10-2012-6) and acknowledge support from the NUS “Centre for Energy Research” seed grant.

Supporting Information Thermogravimetric (TGA) graph of combustion intermediate complex. EDAX spectra of combustion made homogeneous Na2FePO4F particles.


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References 1. Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science. 2011, 334, 928-935. 2. Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-ion Batteries. Chem. Rev. 2014, 114, 11636-11682. 3. Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-ion Batteries. Adv. Funct. Mater. 2012, 23, 947-958. 4. Yabuuchi, N.; Komaba, S. Recent Research Progress on Iron- and Manganese-based Positive Electrode Materials for Rechargeable Sodium Batteries. Sci. Technol. Adv. Mater. 2014, 15, 043501. 5. Barpanda, P. Pursuit of Sustainable Iron-based Sodium Battery Cathodes: Two Case Studies. Chem. Mater. 2016, 28, 1006-1011. 6. Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-type Nax(Fe1/2Mn1/2)O2 Made from Earth-abundant Elements for Rechargeable Na Batteries. Nat. Mater. 2012, 11, 512-517. 7. Barpanda, P.; Oyama, G.; Nishimura, S.; Chung, S. C.; Yamada, A. A 3.8 V Earthabundant Sodium Battery Electrode. Nat. Commun. 2014, 5, 4358. 8. Moreau, P.; Guyomard, D.; Gaubicher, J.; Boucher, F. Structure and Stability of Sodium Intercalated Phases in Olivine FePO4. Chem. Mater. 2010, 22, 4126-4128. 9. Avdeev, M.; Mohamed, Z.; Ling, C. D.; Lu, J.; Tamaru, M.; Yamada, A.; Barpanda, P. Magnetic Structures of NaFePO4 Maricite and Triphylite Polymorphs for Sodium-ion Batteries. Inorg. Chem. 2013, 52, 8685-8693.



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10. Barpanda, P.; Ye, T.; Nishimura, S.; Chung, S. C.; Yamada, Y.; Okubo, M.; Zhou, H.; Yamada, A. Sodium Iron Pyrophosphate: A Novel 3.0 V Iron-based Cathode for Sodiumion Batteries. Electrochem. Commun. 2012, 24, 116-119. 11. Barpanda, P.; Liu, G.; Ling, C. D.; Tamaru, M.; Avdeev, M.; Chung, S. C.; Yamada, Y.; Yamada, A. Na2FeP2O7: A Safe Cathode for Rechargeable Sodium-ion Batteries. Chem. Mater. 2013, 25, 3480-3487. 12. Kim, H.; Park, I.; Seo, D. H.; Lee, S.; Kim, S. W.; Kwon, W. J.; Park, Y. U.; Kim, C. S.; Jeon, S.; Kang, K. New Iron-based Mixed-polyanion Cathodes for Lithium and Sodium Rechargeable Batteries: Combined First Principles Calculations and Experimental Study. J. Am. Chem. Soc. 2012, 134, 10369-10372. 13. Ellis, B. L.; Makahnouk, W. R. M.; Makimura, Y.; Toghill, K.; Nazar, L. F. A Multifunctional 3.5 V Iron-based Phosphate Cathode for Rechargeable Batteries. Nat. Mater. 2007, 6, 749-753. 14. Recham, N.; Chotard, J. N.; Dupont, L.; Djellab, K.; Armand, M.; Tarascon, J. M. Ionothermal Synthesis of Sodium-based Fluorophosphates Cathode Materials. J. Electrochem. Soc. 2009, 156, A993-A999. 15. Ellis, B. L.; Makahnouk, W. R. M.; Rowan-Weetaluktuk, W. N.; Ryan, D. H.; Nazar, L. F. Crystal Structure and Electrochemical Properties of A2MPO4F Fluorophosphates (A = Na, Li; M = Fe, Mn, Co, Ni). Chem. Mater. 2012, 22, 1059-1070. 16. Kawabe, Y.; Yabuuchi, N.; Kajiyama, M.; Fukuhara, N.; Inamasu, T.; Okuyama, R.; Nakai, I.; Komaba, S. Synthesis and Electrode Performance of Carbon Coated Na2FePO4F for Rechargeable Na Batteries. Electrochem. Commun. 2011, 13, 1225-1228. 17. Law, M.; Ramar, V.; Balaya, P. Synthesis, Characterization and Enhanced Electrochemical Performance of Nanostructured Na2FePO4F for Sodium Batteries. RSC Adv. 2015, 5, 50155-50164. 18. Cui, D.; Chen, S.; Han, C.; Ai, C.; Yuan, L. Carbothermal Reduction Synthesis of Carbon Coated Na2FePO4F for Lithium Ion Batteries. J. Power Sources. 2016, 301, 87-92. 19. Barpanda, P.; Ye, T.; Chung, S. C.; Yamada, Y.; Nishimura, S.; Yamada, A. Ecoefficient Splash Combustion Synthesis of Nanoscale Pyrophosphate (Li2FeP2O7) Positive-electrode Using Fe(III) Precursors. J. Mater. Chem. 2012, 22, 13455-13459.


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20. Barpanda, P.; Yamashita, Y.; Yamada, Y.; Yamada, A. High-throughput Solution Combustion Synthesis of High-capacity LiFeBO3 Cathode. J. Electrochem. Soc. 2013, 160, A3095-A3099. 21. Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Cryst. 1969, 2, 65-71. 22. Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report, LAUR 86-748; Los Alamos National Laboratory: Los Alamos, NM, 1994. 23. Toby, B. H. EXPGUI, A Graphical User Interface for GSAS. J. Appl. Cryst. 2001, 34, 210-213. 24. Adams, S.; Prasada Rao, R. in “Bond Valences” (Brown, I. D.; Poeppelmeier, K. R. eds.), Springer Berlin Heidelberg, Structure and Bonding. 2014, 158, 129-159. 25. Adams, S.; Prasada Rao, R. Simulated Defect and Interface Engineering for High Power Li Electrode Materials. Solid State Ionics. 2011, 184, 57-61. 26. Adams, S.; Prasada Rao, R. Transport Pathways for Mobile Ions in Disordered Solids from the Analysis of Energy-scaled Bond-valence Mismatch Landscapes. Phys. Chem. Chem. Phys. 2009, 11, 3210-3216. 27. Adams, S. Bond Valences (Brown, I. D.; Poeppelmeier, K. R. eds.), Springer Berlin Heidelberg, Structure and Bonding. 2014, 158, 91-128. 28. Wong, L.L.; Chen, H.; Adams, S. Design of Fast Ion Conducting Cathode Materials for Grid-scale Sodium-ion Batteries. Phys. Chem. Chem. Phys. 2017, 19, 7506-7523. 29. Kresse, G.; Furthmüller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors using a Plane-wave Basis Set. Comp. Mater. Sci. 1996, 6, 15-50. 30. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 31. Taqui Khan, M. M.; Martell, A. E. Metal Ion and Metal Chelate Catalyzed Oxidation of Ascorbic Acid by Molecular Oxygen. I. Cupric and Ferric Ion Catalyzed Oxidation. J. Am. Chem. Soc. 1967, 89, 4176-4185. 32. Kosova, N. V.; Podugolnikov, V. R.; Devyatkina, E. T.; Slobodyuk, A. B. Structure and Electrochemistry of NaFePO4 and Na2FePO4F Cathode Materials Prepared via Mechanochemical Route. Mater. Res. Bull. 2014, 60, 849.



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33. Avdeev, M.; Ling, C. D.; Tan, T. T.; Li, S.; Oyama, G.; Yamada, A.; Barpanda, P. Magnetic Structure and Properties of the Rechargeable Battery Insertion Compound Na2FePO4F. Inorg. Chem. 2014, 53, 682-684. 34. Swafford, S. H.; Holt, E. M. New Synthesis Approaches to Monophosphate Fluoride Ceramics: Synthesis and Structural Characterization of Na2Mg(PO4)F and Sr5(PO4)3F. Solid State Sci. 2002, 4, 807-812. 35. Kabalov, Y. K.; Simonov, M. A.; Belov, N. V. The Crystal Structure of Sodium Iron Orthophosphate Na2Fe(PO4)(OH). Doklady Akademii Nauk SSSR. 1974, 215, 850-853. 36. Lee, I. K.; Shim, I.B.; Kim, C. S. Phase Transition Studies of Sodium Deintercalated Na2xFePO4F

(0 ≤ x ≤ 1) by Mössbauer Spectroscopy. J. Appl. Phys. 2011, 109, 07E136.

37. Lasia, A. Electrochemical Impedance Spectroscopy and Its Applications, Springer Pub. (Germany), 2014. 38. Momma, K.; Izumi, F. VESTA 3 for Three-dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Cryst. 2011, 44, 1272-1276.


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List of Figures: Figure 1:

(Left) Comparative X-ray diffractograms (λCu = 1.5418 Å) of pristine combustion ash as an intermediate complex and resulting Na2FePO4F products obtained upon annealing at 600 °C for 1 to 6 h. The corresponding ICSD pattern is shown for comparison. (Right) Mössbauer spectra showing the transformation of FeIII rich combustion ash to FeII rich Na2FePO4F final product.

Figure 2:

Comparative powder diffraction patterns (λ= 1.5418 Å) of combustion ash (intermediate complex) and Na2FePO4F products obtained by annealing at 600 °C for soaking duration of 1 minute to 12 h. Profile fitting of selected samples are shown to highlight the formation of desired product even with short annealing of just 1 minute. Further annealing develops larger particles with sharper diffraction peaks.

Figure 3:

Morphology of combustion synthesized Na2FePO4F as observed by electron microscopy. Formation of porous, homogeneous, sub-micrometric particles is observed by SEM (a-c) and TEM (d). HRTEM pattern (e) and formation of uniform ~5 nm thick carbon coating on Na2FePO4F particle is clearly shown (f).

Figure 4:

Vibrational spectroscopy of combustion synthesized Na2FePO4F. Fourier transformed infrared spectrum depicting characteristic P-O-P bands centered around 600 cm-1 and 1100 cm-1 arising from phosphate functional groups. (Inset) Raman spectrum with characteristic D and G bands proving the formation of carbon coating.

Figure 5:

Rietveld refinement of high resolution X-ray diffraction pattern (λ= 1.5418 Å) of Na2FePO4F synthesized at 600 °C in 6 hours having χ2 and R(F2) values of 7.33 and 0.17 respectively. The experimental data points (red dots), calculated diffraction pattern (black line), their difference (blue line) and the Bragg positions (black ticks) are shown. The corresponding profile fitting of synchrotron



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Page 18 of 30

diffraction pattern (λ= 0.786 Å) is shown in the inset. The crystal structure is illustrated in the inset showing the FeO6 octahedra (brown), PO4 tetrahedra (blue) and Na atoms (yellow).

Figure 6:

BVSE analyses of Na+ migration pathways in the experimentally refined structure of combustion-synthesized Na2FePO4F, as well as DFT- or DFT+U-relaxed structure models of Na2FePO4F and Na1FePO4F, respectively. (a) Comparison of the energy landscapes for the mobile Na+ in the three discussed structure models. For the experimental structure, the BVSE analysis suggests an essentially 2D migration pathway in the a-c-plane with a migration barrier of 0.61 eV, while a one-dimensional transport along a becomes possible with a marginally lower energy barrier of 0.58 eV. Graphs (b-e) visualize the pathways as superimposed series of isosurfaces of constant BVSE for Na+. Graph (b) demonstrates the position of the ion-conducting planes in the experimental structure model, while graphs (c-e) reveal details of the migration pathway topology in projections of a single pathway layer on the a-c-plane for the (c) experimental and (d) DFTrelaxed models of Na2FePO4F, as well as the DFT+U relaxed model of Na1FePO4F.

Figure 7:

Galvanostatic potential-capacity profiles of combustion prepared Na2FePO4F at a rate of C/10 (i.e. 1 Na+ in 10 h) at 30 °C. The charge-discharge profiles for different cycles are shown. (Left inset) The corresponding differential capacity (dQ/dV) plot highlighting two distinct reversible FeIII/FeII redox plateaus located at 2.92 V and 3.05 V. (Right inset) EIS spectra (Nyquist plots) of Na2FePO4F cathode at different potentials during Na+ (de)intercalation reaction. The potential values are written next to the corresponding EIS spectra.

Figure 8:

Capacity retention of combustion synthesized Na2FePO4F showing the discharge capacity (black) and Coulombic efficiency (red) over initial 60 cycles. The cells were cycled in the potential window of 2-4.2 V (vs. Na/Na+) at a rate of C/10 (i.e. 1 Na+ in 10 h) at 30 °C. (Left inset) Stability of average charge and discharge


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redox potentials over the first 60 cycles. (Right inset) EIS spectra (Nyquist plots) of Na2FePO4F cathode after different cycles. Figure 9:

The discharge capacity of combustion synthesized Na2FePO4F as a function of cycling rate (from C/15 to 4 C). The cells were cycled in the potential window of 2-4.2 V (vs. Na/Na+) at 30 °C. The inset shows that the variation of the discharge capacity with C-rate can be fitted as a stretched exponential decay function with a characteristic C-rate of C0 = 1 h-1 in line with the moderate migration barriers.



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Table 1.

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Crystallographic data, lattice parameters and atomic coordinates of Na2FePO4F derived from Rietveld Refinement of high resolution X-ray diffractogram (λ= 1.5418 Å) data.

Formula [Molecular Weight]

Na2FePO4F [215.794474 g mol-1]

Crystal System

Orthorhombic

Space Group

Pbcn

Unit Cell Parameters

a= 5.23704(0) Å, b= 13.83755(6) Å, c= 11.771(7) Å

Unit Cell Volume

853.0185(9) Å3

Fitness Parameters (R values)

RF2 = 0.1776; χ2 = 7.388

Atoms

x

y

z

Occ.

Uiso(Å2)

Site

Fe

0.24822(2)

0.01125(3)

0.32851(5)

1.000

0.008

8d

P

0.21080(8)

0.36685(9)

0.08376(8)

1.000

0.012

8d

Na (1)

0.24995(1)

0.25087(9)

0.32974(7)

1.000

0.014

8d

Na (2)

0.24180(8)

0.12741(4)

0.08021(2)

1.000

0.010

8d

F (1)

0.00000(0)

0.14318(2)

0.25000(0)

1.000

0.004

4c

F (2)

0.50000(0)

0.11285(9)

0.25000(0)

1.000

0.004

4c

O (1)

0.33985(9)

0.37196(1)

-0.03225(3)

1.000

0.027

8d

O (2)

0.33709(1)

0.30312(1)

0.14659(6)

1.000

0.068

8d

O (3)

-0.10195(1)

0.39740(4)

0.10878(8)

1.000

0.001

8d

O (4)

0.29184(8)

0.47067(7)

0.13984(8)

1.000

0.003

8d


Page 21 of 30

6h Fe2+ Fe3+ Fit

Intensity (a.u.)

3h 1h

Fe3+ Fit

Combustion ash

Relative Transmission

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


ICSD 10

20

40 30 50 Degree (2θ, λ = 1.5418 Å)

60


70

-4

-2 0 2 Velocity (mm/s)

4

Figure 1


12 h

12 h

6h

Bragg R-factor:- 0.2348 χ2:- 11.9

3h 1h 30 min 15 min

Intensity (a.u.)

5h

Intensity (a.u.)

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

Page 22 of 30

1 min

1h

Bragg R-factor:- 0.2304 χ2:- 7.46

1 min

Bragg R-factor:- 0.7015 χ2:- 4.14

Combustion ash ICSD 10

20 30 Degree (2θ, λ = 1.5418 Å)

40

10


30 50 Degree (2θ, λ = 1.5418 Å)

70

Figure 2

Page 23 of 30

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

a

b


5 µm

d

c

3 µm

e

1 µm

f

d=3.79 Å

200 nm

5 nm ACS Paragon Plus Environment

carbon

20 nm Figure 3


1153

605

580

1095

Transmittance

absence of -OH band

Transmittance

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

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D band

G band

carbon

1040

600

900

1200 1500 1800

Wavenumber (cm-1) 500

1000

1500

2000

2500

3000

3500

Wavenumber (cm-1) ACS Paragon Plus Environment

Figure 4

c

Intensity (a.u.)

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


Intensity (a.u.)

Page 25 of 30

a

10

b

2

20

30

5

14 11 8 17 Degree (2θ, λ = 0.786 Å)

50 60 40 Degree (2θ, λ = 1.5418 Å) ACS Paragon Plus Environment

70

80

Figure 5

(a) 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

(b)


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z y

(c)

(d)

(e)

x z ACS Paragon Plus Environment

Figure 6

Page 27 of 30

1st

60th 4

3

-Z'' (kΩ)

0 2.0

2.8

3.6

Cyclevs. Index Potential

Na/Na+

0

20

40

4.0 V

0 0.8 1.6 2.4 3.2 4

0

-400

1

400

2

dQ/dV (CV-1)

Potential vs. Na/Na+ (V)

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


3.6 V 3.4 V OCP (2.67 V)

0

0.8

(V) 60

80

3.2 V 3.0 V 1.6 2.4

Z'Z'(kΩ) (kΩ)

100

3.2

4

120

Capacity (mAh/g) ACS Paragon Plus Environment

Figure 7


100 80

60

60

0

3.0 discharge

2.8

1.0

charge 40

0.5

20

3.2

-Z'' (kΩ)

40

1.5

80

Potential vs. Na (V)

Capacity (mAh/g)

100

20

2.6 0 10 20 30 40 50 60

0

0.5

10

20

1.5

0

Z' (kΩ)

Cycle Index 0

1.0

Coulombic efficiency (%)

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

Page 28 of 30

30

40

50

60

Cycle Index ACS Paragon Plus Environment

Figure 8

Page 29 of 30

110 100 90 80 70 60 50 40 30 0

100

Capacity (mAh/g)

Capacity (mAh/g)

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


80 60 40 0.05 4

0.5 C rate (h-1) 8

12

5 16

20

24

28

Cycle Index ACS Paragon Plus Environment

Figure 9


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

“For Table of Contents Only”


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Electrochemical and Diffusional Investigation of Na ... - ACS Publications

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