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Revisiting the layered Na3Fe3(PO )  phosphate
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Mater. Res. Express 7 (2020) 014001 https://doi.org/10.1088/2053-1591/ab54f4
PAPER
Revisiting the layered Na3Fe3(PO4)4 phosphate sodium insertion
OPEN ACCESS compound: structure, magnetic and electrochemical study
RECEIVED
10October 2019 Ganesh S Shinde1, RitambharaGond1 ,MaximAvdeev2,3 , ChrisDLing3 , Rayavarapu PrasadaRao4 ,
REVISED StefanAdams5 andPrabeer Barpanda1
26October 2019
1 FaradayMaterials Laboratory,Materials ResearchCenter, Indian Institute of Science, C. V. RamanAvenue, Bangalore 560012, India
ACCEPTED FOR PUBLICATION 2
6November 2019 Bragg Institute, B87, AustralianNuclear Science andTechnologyOrganization, Locked Bag 2001, KirraweeDCNSW2232, Australia
3 School of Chemistry, TheUniversity of Sydney, Sydney, NSW2006, Australia
PUBLISHED 4 Centre forMaterials for Electronics Technology, Pune 411008,Maharashtra, India
18November 2019
5 Department ofMaterials Science and Engineering,National University of Singapore, 9 EngineeringDrive 1, Singapore 117546, Singapore
E-mail: prabeer@iisc.ac.in
Original content from this
workmay be used under
the terms of the Creative Keywords:Na-ion batteries, cathode, Na3Fe3(PO4)4, layered structure, BVSE calculation
CommonsAttribution 3.0
licence.
Any further distribution of
this workmustmaintain Abstract
attribution to the Layered sodium iron phosphate phase [Na3Fe3(PO4)4]was synthesized by solution combustionauthor(s) and the title of
thework, journal citation synthesismethod,marking thefirst attempt of solvothermal synthesis of this phase. Its crystal
andDOI.
structurewas verified by synchrotron and neutron powder diffraction. Rietveld analyses proved the
phase purity and formation ofmonoclinic frameworkwithC2/c symmetry. It undergoes an
antiferromagnetic ordering∼27 K. This combustion prepared nanoscaleNa3Fe3(PO4)4 compound
was found to be electrochemically active with a stepwise voltage pro le involving an Fe3+fi /Fe2+
redox activity centred at 2.43 V vs.Na/Na+. Despite various cathode optimization, only 1.8Na+ per
formula unit could be reversibly inserted into theNa3Fe3(PO4)4 framework leading to capacity close
to 50mAh g−1. This limited electrochemical activity can be rooted to (i) relatively large diffusion
barrier (ca. 0.28 eV) as per Bond valence site energy (BVSE) calculations and (ii) possible structural
instability during (de)sodiation reaction.
1. Introduction
With industrial and technological evolution, global energy consumption is increasing at a very high rate. In this
scenario, energy generationwithminimal CO2 emission is pivotal. Also, efficient energy storage and delivery is a
key sector propellingmyriads of consumer electronics, (hybrid) electric vehicles and stationarymicro-to-mega
grid storage in the 21st century.While several technologies do exist, electrochemical energy storage in general
and rechargeable batteries in particular offer themost pragmatic approachwith feasible large-scale
dissemination. In the last three decades, rechargeable Li-ion batteries have seen unprecedented
commercialization ushering awireless revolution and vying for zero-emission transportation. Batteries with
good combination and energy/power density, reversibility, safe operation andmaterials/process economy are
crucial tomeet the demand of the energy-hungry world [1, 2]. From application point of view, batteries can be
divided into two categories: volume/weight restricted portable batteries and volume/weight independent
stationary batteries.While the former is solely dominated by Li-ion chemistry, the latter category can be catered
by alternate chemistry like lead-acid, Ni-MH, Li-ion aswell asNa-ion batteries.
While the Li-ion andNa-ion batteries work on similar operating principles, sodium-ion batteries are touted
as economic alternatives to Li-ion counterparts owing to the elemental abundance and uniform geographic
distribution of sodium-based precursors. Similar to the story of Li-ion batteries, successful implementation of
Na-ion batteries relies a lot on robust positive insertion (cathode)materials. Over the last decade, Na-ion
batteries have attracted significant research effort to develop new cathodematerials, where layered transition
metal oxides (NaxTMO2,TM=Co, Fe,Mn, Cr etc) have ruled thefield ever since their discovery in 1980s [3–8].
However, these layered have issues with structural and thermal stability. To circumvent these issues,many a
polyanionic insertionmaterials have been unravelled.While the theoretical capacity (QTh) is lower owing to high
©2019TheAuthor(s). Published by IOPPublishing Ltd
Mater. Res. Express 7 (2020) 014001 G S Shinde et al
molecular weight, using inductive effect principle, superior operating voltage can be realized leading to
competitive energy density [9, 10]. Among these polyanionicmaterials, while the SO2−4 -based compounds offer
the highest redox voltage [11, 12], the PO3−4 -based ones render easy synthesis andmaterials stability [13].
Exploring the phosphate chemistry, various polyanionic cathodes have been reported such asNaFePO4,
NaVPO4F,Na3V2(PO4)2F3, Na2FePO4F,Na2FeP2O7,Na4Fe3(PO4)2(P2O7) etc [6]. One such PO4-based cathode
system isNa3Fe3(PO4)4, a layer structured sodium iron phosphate [14, 15]. It offers suitable channels forNa
+
ion insertion between the layers built from corner-sharing FeO6 octahedra and PO4 tetrahedra [16], with a
theoretical insertion ability of 3Na per formula unit. However, the previous report realized the reversible
intercalation of only 1.8Na+ ions per formula unit ofNa3Fe3(PO4)4 framework, that too in very low current rage
of C/100 (1Na+ in 100 h) [14]. The layered structure and high theoretical capacity based on 2 electron transfer
motivated us to re-examine and optimize this compound to achieve better electrochemical performances.
In this current work, we have synthesizedNa3Fe3(PO4)4 framework via wet chemistry route for thefirst time.
Its crystal structure was analysed combining synchrotron and neutron powder diffraction. Na3Fe3(PO4)4
exhibits layered structure assuming amonoclinic systemwithC2/c symmetry. Solution route leads to smaller
particles with improved electrochemical reversible capacity of 50mAh g−1 at a faster rate of C/20 involving a
2.45 VFe3+/Fe2+ activity. Despite particle size optimization and carbon coating, it was impossible to obtain
near theoretical capacity. Here, we report thewet chemical synthesis, structure,magnetic and electrochemical
properties of as-synthesizedNa3Fe3(PO ) . TheNa
+
4 4 migration and diffusional analysis has been calculated to
probe the reason behind electrochemical limitations.
2. Experimental
2.1.Materials synthesis
The target compoundNa3Fe3(PO4)4 was synthesized viawet chemistry route for the first time. Thewet chemical
synthesis was carried out with orwithout using fuels such as glycine (C2H5NO2, 99%,Merck), urea (CH4N2O,
99%, SDFCL) and ascorbic acid (C6H8O6, 99%, SigmaAldrich). Stoichiometric quantities ofNaH2(PO4).H2O
(99%, EMPARTA.ACS), Fe(NO3)3.9H2O (98%, Fisher Scientific) and (NH4)2HPO4 (98%,Merck)were
dissolved in de-ionisedwater separately and thenmixed together to have a final precursor solution having a
pHvalue of 2. This solutionwas placed on a hot plate (maintained at 120 °C)with steadymagnetic stirring to
remove excess water. After complete drying, the solid residuewas ground by an agatemortar and pestle was
pressed into pellets using hydraulic press. These pellets were calcined in a tubular furnace at 400 °C for 6 h,
600 °C for 6 h andfinally at 750 °C for 48 h to get thefinal product. This annealing conditionwas used for
precursormixture without using any fuel. In contrast, when fuels weremixed to precursor solution, the desired
product was obtained by one-step calcination at 750 °C for 6 h. Afire yellow coloured powder wasfinally
obtained.
2.2. Structure andmorphological analysis
The powder diffraction patterns ofNa3Fe3(PO4)4 samples were acquired by a PANalytical X’Pert Pro
diffractometer equippedwith aCu-Kα source (λ1=1.5405 Å,λ2= 1.5443 Å) operating at 40 kV/30 mA.
Typical XRDpatterns were collected using Bragg–Brentano geometry in the 2θwindowof 5°–90°with a step size
of 0.0267° (count time= 110 s per step). Synchrotron diffraction patternwas collected at the BL-18B Indian
beamline (High Energy Accelerator ResearchOrganization, KEK-Photon Factory, Tsukuba, Japan) using a
synchrotron beamline of wavelengthλ= 0.7861(2)Å and energy E= 15.77 keV calibratedwith Si (640bNIST)
standard. Neutron powder diffraction (NPD)patternwas acquiredwith Echidna high-resolution diffractometer
atOPAL facility (LucasHeights, Australia) using neutrons of wavelength 2.4395 Å. Rietveld analysis of the
powder diffraction patternswas conducted usingGSAS [17, 18] or FullProf [19] suite of programs. The
backgroundwas refined using a shiftedChybeshev polynomial function and the diffraction profile was fitted by a
pseudo-Voigt function. Brunauer-Emmett-Teller (BET) surface areameasurements were calculatedwith a
Micromeritics ASAP 2020 instrument using surface adsorption ofN2 (at 77 K). Prior to BET test, the powder
samples were evacuated at 373 Kunder vacuum for 2 h. Themorphology of as-synthesized product was
examined using an FEI Inspect F50 scanning electronmicroscope (10 kV) as well as an FEI Tecnai F30 STwin
transmission electronmicroscope (200 kV). For TEManalysis, few drops of powder sample, soaked in acetone,
were deposited on a copper grid.
2.3.Magnetic characterization
Temperature-dependentmagnetic susceptibility data over a 4–300 K rangewere collected forNa3Fe3(PO4)4
underfield-cooled (FC) and zero field-cooled (ZFC) conditions, in an appliedmagnetic field of 1000 Oe. Field-
dependent datawere collected at 4 K over the range±5000 Oe.Measurements weremade using aQuantum
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Mater. Res. Express 7 (2020) 014001 G S Shinde et al
Figure 1.Comparative X-ray diffraction patterns elucidating the evolution ofNa3Fe3(PO4)4 product phase obtained by solvothermal
synthesis without any fuel. XRDpatterns are shown for (a) intermediate product obtained after dehydration and its derivative
products after (b) one step calcination at 400 °C for 6 h, (c) two step calcination at 400 °C for 6 h followed by 600 °C for 6 h and (d)
three step calcination at 400 °C for 6 h followed by 600 °C for 6 hwith final calcination at 750 °C for 48 h.
Figure 2. Solvothermal synthesis ofNa3Fe3(PO4)4 product phase with one step calcination at 750 °C for 6 h using various fuels like
(a) urea, (b) glycine, (c) ascorbic acid and (d)no fuel. The corresponding surface area values are provided in table 2.
Design Physical PropertyMeasurement System (PPMS) equippedwith aVibrating Sample
Magnetometer (VSM).
2.4. Electrochemical characterization
The electrochemical properties ofNa3Fe3(PO4)4 were tested (at 25 °C) in sodiumhalf-cell architecturedCR2032
coin type cells. Composite positive electrode (slurry)was prepared by thoroughlymixing the activematerial
(85 wt%)with carbon black (10 wt%) and polyvinylidene fluoride binder (5 wt%) inN-methyl-pyrrolidone.
The slurrywas coated uniformlywith brush onto thin circular Al-foil (∅= 16 mm) andwas dried overnight in
vacuumoven at 90 °C. The coin cells were assembled in an argon-filled glove box (MBraun Inc.) usingNametal
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Mater. Res. Express 7 (2020) 014001 G S Shinde et al
Figure 3. (a)Rietveld refinedX-ray powder pattern ofNa3Fe3(PO4)4 product showing the experimental data points (red), calculated
pattern (black), their difference (blue) andBragg diffraction peaks (black ticks). (Inset) Illustration ofNa3Fe3(PO4)4 framework built
with interconnected FeO6 octahedra (pink) and PO4 tetrahedra (green)with large cavities accommodating theNa atoms (blue).
(b)Rietveld refined neutron diffraction pattern (λ=2.4395 Å) ofNa3Fe3(PO4)4.
foils as counter electrode and a sheet ofWhatmanfilter paper as separator soakedwith 1.0 mol.l−1NaClO4 in
propylene carbonate as electrolyte. These cells were galvanostatically cycled between 2–2.55 V (versusNa/Na+)
at a rate of C/50 (i.e. 1Na in 50 h at 25 °C) using aVMP3Biologic battery cycler.
3. Results and discussion
3.1. Synthesis and structure
Phosphate based compounds are important among the polyanionic insertionmaterials as they are durable over
high temperature and providesmore chemical stability alongwith good electrochemical properties. From
elemental point of view, Fe-based cathodes are projected as economic candidates due to the abundance of Fe-
based precursors. Nonetheless, the processing cost plays the spoiler asmost Fe-based cathodematerials are
based on Fe2+ species warranting careful processing and packaging in inert (N2/Ar) atmosphere. Switching
towards Fe3+ based cathodematerials [like layeredNa3Fe
3+
3 (PO4)4] can reduce the processing cost ($/watt-
hour) ofmaterial as whole process can be carried in air instead of inert (Argon) atmosphere. Also, conventional
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Mater. Res. Express 7 (2020) 014001 G S Shinde et al
Figure 4.Electronmicrographs illustrating the particlemorphology. (a)–(c) SEM images, (d), (e)TEM images, (f) selected area
diffraction patternwith indexing, and (g) representativeHRTEM image showing the lattice fringes.
Table 1.Crystallographic structure parameters ofNa3Fe3(PO4)4 calculated fromRietveld analysis of neutron powder
diffraction (λ= 2.4395 Å) at 25 °C.
Formula [molecular weight] Na3Fe3(PO4)4 [616.39] (Z= 4)
Crystal system Monoclinic
Space group C2/c (#15)
a= 19.5552(16), b= 6.3858(5),
c= 10.5712(9)
Unit cell parameters (Å) Unique angleβ= 91.7447(20)°
Unit cell volume (Å3) 1319.48(32)
Fitness parameters Rp= 2.72, Rwp= 3.46, Rexp= 2.0,χ
2= 3.0
Atom Site x y z Occupancy Uiso (Å
2) BVS
Na1 4e 0 0.3652(24) 1/4 1 0.008(4) 0.952
Na2 8 f 0.0838(6) 0.1441(16) 0.9748(10) 1 0.0050(31) 0.942
Fe1 4d 1/4 1/4 1/2 1 0.0025(19) 3.267
Fe2 8 f 0.15615(19) 0.4762(7) 0.7470(4) 1 0.0004(11) 3.109
P1 8 f 0.2022(4) 0.0205(12) 0.2431(6) 1 0.0019(7) 4.856
P2 8 f 0.9131(4) 0.3377(11) 0.0109(7) 1 0.0019(7) 4.904
O1 8 f 0.15897(30) 0.8306(11) 0.2748(6) 1 0.0019(7) 1.955
O2 8 f 0.25639(34) 0.0570(9) 0.3539(7) 1 0.0019(7) 2.122
O3 8 f 0.15826(32) 0.2084(11) 0.2126(6) 1 0.0019(7) 1.866
O4 8 f 0.25427(32) 0.9762(10) 0.1338(6) 1 0.0019(7) 1.920
O5 8 f 0.91589(33) 0.4598(10) 0.8801(6) 1 0.0019(7) 1.945
O6 8 f 0.9036(4) 0.5130(11) 0.1128(6) 1 0.0019(7) 2.139
O7 8 f 0.97486(34) 0.2073(10) 0.0389(6) 1 0.0019(7) 1.980
O8 8 f 0.84857(35) 0.2030(10) 0.0073(7) 1 0.0019(7) 1.995
solid-state synthesis involves high temperature and prolonged calcination timewhich augment the processing
cost ofmaterial. In search of alternative novel synthesis, wet-synthesis and solution combustion synthesis
involving short calcination duration can be employed for scalable production of variety of cathodematerials.
Here, (fuel-assisted) solvothermal routewas employed for energy-savvy synthesis of layeredNa3Fe3(PO4)4.
Without any fuel, wet synthesis was conducted involvingmulti-step calcination at 400 °Cand 600 °C for 6 h
followed by final heat treatment at 750 °C for 48 h. The phase evolutionwith various calcination steps is depicted
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Mater. Res. Express 7 (2020) 014001 G S Shinde et al
Figure 5. Field-cooled and zero field-cooledmagnetic susceptibility forNa3Fe3(PO4)4 in an applied field of 1000 Oe. The red line is a
fit to the Curie-Weiss las over the range 100–300 K. The inset shows thefield dependentmagnetic susceptibility at 4 K.
Table 2.BET surface area andBJHpore sizemeasurements for
Na3Fe3(PO4)4 samples prepared by solvothermal synthesis using various
fuels.
BET Specific BJHdesorption
Surface area average pore
Fuel used (m2 g−1) size (nm)
Without fuel 2.124 13.6751
Ascorbic acid 2.961 16.3634
(C6H8O6)
Glycine (C2H5NO2) 2.605 17.9311
Urea (CH4N2O) 2.292 21.6622
infigure 1.While the intermediate product (after dehydration)was amorphous in nature, progressive phase
evolution occurredwith high temperature annealing. Usage of wet synthesis favouring intimatemixing of
precursors led to reduction of final annealing duration from (previously reported) 72 h to 48 h [14, 15]. Faster
reaction can be realized by triggering exothermic combustion reaction in presence of fuels that leads to intimate
precursormixing/metal-nitrate complexation, exothermic reaction involving high local heating, rapid product
formationwith porous (nanoscale)morphology alongwith one-step carbon coating [20, 21].When combustion
synthesis was employed, facilitated by complexation and partial reaction completion during low temperature
step, thefinal calcination duration (at 750 °C)was drastically reduced to 6 h independent of the fuels used
(figure 2). Structural and phase purity was analysed combining lab/synchrotronXRD and neutron powder
diffraction. Rietveld refinement confirmed the formation of desired target product as shown infigure 3.No trace
of possible impurity likemariciteNaFePO4 orNaFeP2O7was noticed. The structure could be indexed to
monoclinic frameworkwithC2/c symmetry. The unit cell parameters and atomic coordinates derived from
Rietveld analysis of neutron powder diffraction pattern are summarized in table 1. The calculated BVS values are
close to the valency of respective elements.
LayeredNa3Fe3(PO4)4 structure is illustrated in the inset offigure 3. The framework consists of FeO6
octahedral and PO4 tetrahedral building blocks.While adjacent FeO6 octahedra are solely connected by corner-
sharing arrangement, they are abridged by PO4 tetrahedra involving both corner-sharing and edge-sharing
fashion. Fe occupies two distinct crystallographic sites.While Fe(1)–O6 shares oxygen atoms solely by corner-
sharing fashion, the Fe(2)–O6 involves both corner and edge sharing bondingwith PO4 tetrahedra. Owing to the
size difference between FeO6 and PO4 units, FeO6–PO4 edge sharing leads to distortion of Fe(2)–O6 octahedra.
The constituentNa atoms are located in between the FeO6–PO4 layers having two distinct crystallographic sites,
a six-coordinatedNa(1)–O6 and a seven-coordinatedNa(2)–O7 sites, creating three-dimensional Na
+ diffusion
pathways along [010], [110] and [1–10] directions.
The solvothermally preparedNa3Fe3(PO4)4 consists ofmicrometric (1–3 μm) anisotropic platelets which
forms large agglomerates (figures 4(a)–(c)). This aggressive grain growth and large particle size results from
prolonged thermal treatment. BET analysis revealed the overall surface area to be in the range of 2 3 m2 g−1– with
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Mater. Res. Express 7 (2020) 014001 G S Shinde et al
Figure 6. (a)Galvanostatic charge-discharge voltage profiles ofNa3Fe3(PO4)4 cathode (versusNa) cycled in the potential window of
2–2.55 V at the rate of C/50 (at 25 °C) for the initial 10 cycles. (b)Corresponding differential capacity (dQ/dV) profile.
mesoporousmorphology (pore size: 13–22 nm) (table 2). High-resolution TEM study further confirmed the
formation of inhomogeneousmicrometric Na3Fe3(PO4)4 particles in the size range of 1–2 μm (figures 4(d)–(e)).
SAEDpattern could be indexed tomonoclinic structure (figure 4(f)). The crystallinity offinal product was
further confirmed byHRTEMstudy showing lattice planes corresponding tomiller indices (202)with an
interatomic d-spacing of 4.5 Å (figure 4(g)).
3.2.Magnetic properties
The presence of Fe impartsmagnetic characteristic toNa3Fe3(PO4)4, similar tomany Fe-based cathodematerials
with antiferromagnetic ordering at low temperature. Temperature-dependentmagnetic susceptibility data for
Na3Fe3(PO4)4 underfield-cooled (FC) and zero field-cooled (ZFC) conditions are presented infigure 5. An
antiferromagnetic ordering transition is clearly observed atTN= 27 K. Therewas no significant offset between
the FC andZFCdata, consistent with the absence of any opening in the field-dependent data at 4 K (inset to
figure 5). ACurie-Weiss fit to the FCdata over the range 100–300 K (red line infigure 5) yields an effective
magneticmomentμeff= 5.95μB/Fe, consistent with the theoretical spin-only value for high-spin Fe
3+,μs.o.
= 5.92μB/Fe. A detailed analysis of long rangemagnetic ordering and elucidation of antiferromagnetic
structure ofNa3Fe3(PO4)4 from low-temperature neutron diffraction datawill be reported shortly.
3.3. Electrochemical performance ofNa3Fe3(PO4)4
Electrochemical performance of wet chemistry preparedNa3Fe3(PO4)4 was analysed using standardNa half-cell
assembly. Being an Fe3+ insertion compound,Na3Fe3(PO4)4 wasfirst discharged to uptake furtherNa atoms to
realize Fe3+/Fe2+ redox activity. Ideally, it will be great to utilize Fe3+/Fe4+ redox activity by charging
(desodiating) the starting compound.However, the reversible conversion of Fe3+ Fe4+– involving FeO6
octahedral environment is structurally unstable and this redox can occur beyond safe operation limit of
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Mater. Res. Express 7 (2020) 014001 G S Shinde et al
Figure 7. (a)Migration energy barrier profile forNa+ diffusion inNa3Fe3(PO4)4 as derived from the bond valence site energy (BVSE)
model. (b)Na+ pathwayswith interstitial sites inNa3Fe3(PO4)4 displayed as isosurfaces of constant EBVSE(Na) superimposed on a
projection of the crystal structure along ac plane. Black arrows showone possible dimensional hopping pathwaywhile red arrow
indicates three-dimensional hopping pathwayNa+ diffusion.
commonly used electrolytes. Galvanostatic voltage-capacity profiles of layeredNa3Fe3(PO4)4 is shown in
figure 6(a).When tested at a rate of C/50 (1Na+ in 50 h), thefirst discharge capacity of 66mAh g−1 was observed
(theoretical capacity∼130mAh g−1). In subsequent cycles,∼1.6Na+ ions could be reversibly inserted into the
structure delivering a discharge capacity∼48mAh g−1. Similar tomany polyanionic cathodes (like Li2FeP2O7),
the capacity decreased from1st to 2nd cycle hinting at irreversible structural rearrangement during the first
discharge. Afterwards, the cell performedwith steady capacity (ca. 48mAh g−1)with 98%Coulombic efficiency.
A closer look revealed sloping step-wise profiles with several distinct pseudo-plateaus. Themajor Fe3+/Fe2+
redox peak appears∼2.5 V followed by severalminor redox peaks∼2.42 V and 2.14 V (vs. Na/Na+)
(figure 6(b)). This sloping profile is indicative of solid-solution (single-phase) redoxmechanismduring (de)
sodiation reaction.While the original report involving prolonged solid-state synthesis showed poor
electrochemical activity even at a slow rate of C/100 [14, 15], the usage of solvothermal synthesis (delivering
smaller particles) led to similar electrochemical performance at improved rate of C/50.Nonetheless, layered
Na3Fe3(PO4)4 is found to have poorNa
+ diffusion despite all attempts on processing/material optimization.
To understand the reason behind poor electrochemical activity inNa3Fe3(PO4)4, theNa
+ diffusional
kinetics and pathways were explored using bond valence site energy (BVSE) calculations, which is a simple yet
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Mater. Res. Express 7 (2020) 014001 G S Shinde et al
reliable approach to speculate ionmigration pathways in structuralmodels. The detailmethodology of BVSE
analysis has been reported elsewhere [22–24]. In this calculations, the bond valences sNa−X=
exp[(R0,Na−X−RNa−X)/bNa−X] and the BV summismatch |ΔV| can be correlated to an absolute energy scale by
expressing the bond valence site energy EBVSE of aNa
+ cation coordinated byX− anions as aMorse-type
interaction energy:
⎡ N ⎛⎛ ⎞2 ⎞⎤
E (Na) = åD⎢å⎜⎜⎜ sNa-X ⎟ s- 2 Na-X ⎟⎥BVSE + E
x ⎢⎣i=1⎝⎝ repulsions ⎟min ,Na-X ⎠ smin ,Na-X ⎠⎦⎥
The required bond valence parameters were taken from softBV database [25].Migration pathways forNa+
are then investigated as regions of low bond valence site energy EBVSE(Na) in grids spanning the structuremodel
with a resolution of ca. (0.1 Å)3. Using Rietveld refinement of neutron diffraction ofNa3Fe3(PO4)4 sample, the
initial structuremodel was obtained for BVSE calculations. Based upon structuremodel, energy barrier forNa+
migration as hollow spheres connecting theNa and interstitial sites inNa3Fe3(PO4)4 was obtained and
represented as superimposed projection of the crystal structure along ac plane infigure 7.Na3Fe3(PO4)4 has two
sites forNa (Na1 andNa2), whereNa2 has lowest energy barrier of 0.15 eV for hopping to otherNa2 via the
interstitial sites i1- i1with hopping energy barrier of 0.17 eV as illustrated infigure 7 (top). Hopping ofNa2 also
takes place from i2 interstitial site with high energy barrier of 0.28 eV along one dimensional pathway toNa2.
Three-dimensional percolation ofNa2 is possible involvingNa1with highestmigration energy barrier of
0.30 eV from the interstitial sites i3 site. Using VETSA, one-dimensional as well as three-dimensional
percolation pathways are superimposed over crystal structure ofNa3Fe3(PO4)4 along ac plane as shown in
figure 7 (bottom).Migration ofNa inNa3Fe3(PO4)4 follows one-dimensional pathway alongNa2-i1-i1-Na2
(figure 7 black arrows)which aremore feasible than three-dimensional percolation pathway alongNa2-i3-Na1
due less energy barrier (figure 7 red arrows). Overall, it offers feasible one-dimensional Na+ diffusivity with high
energy barrier, which is the root cause behind the poor rate kinetics in this system.
4. Conclusions
Thiswork presents thefirst wet chemistry synthesis and layered sodium iron phosphateNa3Fe3(PO4)4 insertion
system.Using this solvothermal synthesis, the target compoundwas prepared by less aggressive heat treatment
resulting subdued grain growth/Ostwald ripening vis-à-vis solid-state synthesis. Rietveld analysis confirmed the
formation ofmonoclinic layered system.Microstructural analysis showed large agglomerated platelets of
5–7 μmrangewith primary anisotropic platelet shaped particles in the size range of 1–2 μm. It exhibited a long-
range antiferromagnetic ordering below (TN=) 27 K.Wet synthesis improved theNa
+ diffusion kinetics in
Na3Fe3(PO4)4 when tested in sodium cells. Involving a nominal Fe
3+/Fe2+ redox potential centred around
2.43 V, it delivered a reversible capacity of 48mAh g−1. The poorNa+ insertion behaviour inNa3Fe3(PO4)4 can
be rooted to structural disordering during initial cycle and occurrence of one-dimensional Na+ diffusion
pathways along [101] directionwith high energy barrier of 0.28 eV.While far from any practical application,
Na3Fe3(PO4)4 forms an economic Fe
3+-based sodium insertionmaterial.
Acknowledgments
PB thanks theDepartment of Atomic Energy (DAE) for aDAE-BRNSYoung Scientists ResearchAward (YSRA).
RG is grateful to ICDD for a Ludo-Frevel Crystallography Scholarship and ECS for an FMBecket Summer
Fellowship. S A andRPR are grateful toNational Research Foundation, PrimeMinister’sOffice, Singapore
for support under theCompetitive Research Programme (CRPAwardNRF-CRP 10-2012-6) and acknowledge
support from theNUS ‘Centre for Energy Research’ seed grant.MA andCDL acknowledge thefinancial
support from theAustralian ResearchCouncil (DP170100269).
ORCID iDs
RitambharaGond https://orcid.org/0000-0003-3061-7434
MaximAvdeev https://orcid.org/0000-0003-2366-5809
Chris DLing https://orcid.org/0000-0003-2205-3106
Rayavarapu Prasada Rao https://orcid.org/0000-0002-4022-2340
StefanAdams https://orcid.org/0000-0003-0710-135X
Prabeer Barpanda https://orcid.org/0000-0003-0902-3690
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Mater. Res. Express 7 (2020) 014001 G S Shinde et al
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