Na2Fe(C2O4)F2: A New Iron-Based Polyoxyanion Cathode for Li/Na Ion Batteries

Article abstract of DOI:10.1021/acs.chemmater.6b04859

A new mixed anion compound, Na₂Fe(C₂O₄)F₂, has been prepared by hydrothermal synthesis. The crystal structure exhibits infinite chains of corner-linked Fe^II-centered octahedra, with coordination composed of both oxalate and fluoride ligands. This compound exhibits promising reversible lithium and sodium insertion. On extended cycling, Na₂Fe(C₂O₄)F₂ is capable of reversibly inserting 0.67 Li⁺ or 0.56 Na⁺ per formula unit up to 50 cycles at the average discharge voltages of 3.3 and 3.0 V, respectively. This represents arguably the best performance as a prospective cathode material so far observed among oxalates and is comparable to many known iron phosphate-based cathode materials.

Full text of DOI:10.1021/acs.chemmater.6b04859

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Article
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NaFe(CO)F2: A new iron-based polyoxyanion cathode for Li/Na ion batteries
Wenjiao Yao, Moulay-Tahar Sougrati, Khang Hoang,
Jianing Hui, Philip Lightfoot, and Anthony Robert Armstrong
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04859 • Publication Date (Web): 10 Feb 2017
Downloaded from http://pubs.acs.org on February 13, 2017
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Na2Fe(C2O4)F2: A new iron-based polyoxyanion cathode for Li/Na ion batteries
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Wenjiao Yao,† Moulay-Tahar Sougrati,§,‡ Khang Hoang,¶ Jianing Hui,† Philip Lightfoot,†,* An-
thony Robert Armstrong†,‡*
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† School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, UK
§Université de Montpellier 2 Place Eugène Bataillon - CC 1502, 34095 Montpellier CEDEX 5, France
¶ Center for Computationally Assisted Science and Technology, North Dakota State University, Fargo, ND 58108,
USA
‡ALISTORE-ERI, 80039, Amiens Cedex, France
ABSTRACT: A new mixed anion compound, Na2Fe(C2O4)F2 has been prepared by hydrothermal synthesis. The crystal
structure exhibits infinite chains of corner-linked FeII-centred octahedra, with coordination composed of both oxalate and
fluoride ligands. This compound exhibits promising reversible lithium and sodium insertion. On extended cycling,
Na2Fe(C2O4)F2 is capable of reversibly inserting 0.67 Li+ or 0.56 Na+ per formula unit up to 50 cycles at the average dis-
charge voltages of 3.3 V and 3.0 V, respectively. This represents arguably the best performance as a prospective cathode
material so far observed amongst oxalates, and is comparable to many known iron phosphate-based cathode materials.
2.15/7.20/12.32). This suggests that the electronic induc-
tive effect of the oxalate group on the d-states of the tran-
sition metal ions should be similar to (SO4)2- and (PO4)3-,
and hence give rise to similar redox potentials to sulfate
and phosphate. On the other hand, its conjugate base,
(C2O4)2-, is found to be a versatile anion since it may serve
as a mono-dentate, bidentate, tridentate or tetradentate
ligand and may form chains, layers, or three dimensional
networks with metal centers.22 Last, but not least, oxalic
acid is a common starting material in synthetic chemistry,
which is capable of both controlling the valence state of
metals and acting as a template or mineralizer to facilitate
crystal nucleation and growth.23 So far, the exploration of
oxalates as electrode materials is rare. Tarascon and co-
workers have reported a few oxalate-based materials, with
good performance obtained from Fe2(C2O4)3ꢀ4H2O.24 This
material shows a capacity of around 100 mAhg-1 at an av-
erage potential of 3.3 V vs. Li+/Li. It suffers from the
drawback that it is fabricated in the charged state (Fe3+),
whereas positive electrodes are typically prepared in the
discharged state (Fe2+) and act as a lithium/sodium source
for the negative electrode during battery operation. Also,
the structural water molecules in Fe2(C2O4)3ꢀ4H2O add
extra mass, thereby limiting the practical capacity. The
same research group also reported the sodium phases
Na2M2(C2O4)3·2H2O (M = Fe, Mn, Co) but concluded that
these materials showed limited electrochemical activity.25
I. INTRODUCTION
The ever increasing demand for portable energy has driv-
en the search for new electrode materials for rechargeable
battery applications. Until now lithium-ion batteries
(LIBs) have been preferred to alternative chemistries as a
result of their high energy density and operating voltages,
but concerns surrounding Li supply and its rising cost
have encouraged a rapidly growing interest in the more
sustainable sodium-ion batteries (NIBs).1-4 Until recently
the latter have garnered little attention, providing many
opportunities for the exploration of new electrode mate-
rials. Since sodium is highly abundant and therefore
cheap, NIBs may find application in the grid storage sec-
tor, and could be major players in next-generation low-
carbon energy technologies. Low cost and environmental
impact are also strong drivers behind the search for new
polyanion compounds using iron as a redox center.5 The
exploration of cathode materials for Na-ion batteries has
encompassed layered transition metal (TM) oxides
(NaTMO2),6 olivine-type (NaTMPO4),7-8 NASICON-type,9-
11
and a range of other polyoxyanionic structures (e.g.
phosphates and sulfates).12-20 The advantage of polyoxyan-
ion-based compounds lies in their enhanced stability via
strong covalent bonds, which in turn leads to improved
safety characteristics. The incorporation of fluoride into
polyoxyanionic systems is being widely investigated due
to the opportunity to provide a tuneable potential owing
to the larger ionicity of the TM-F bond compared with the
Yamada
and
co-workers
reported
that
K4Na2[Fe(C2O4)2]3·2H2O is electrochemically active but
also with limited capacity.26 No examples of iron fluoro-
oxalate materials have been reported to date.
corresponding
TM-O
bond.
Examples
include
Na2TMPO4F, NaVPO4F, NaFeSO4F, etc.21
In the search for new iron-based cathode materials for
either Li- or Na-ion batteries, we note that oxalic acid is a
relatively strong acid, with acidity (pKα = 1.25/4.14) be-
tween those of H2SO4 (pKα = -3/1.99) and H3PO4 (pKα =
Inspired by these considerations, we have synthesized the
first iron-based alkali fluoro-oxalate, formulated
Na2Fe(C2O4)F2. Herein, we report hydrothermal synthesis,
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structural determination, physical characterization, and ground pristine sample, and 15 kV for ball-milled cathode
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primary electrochemical tests. The results showed that it
could operate on the Fe2+/Fe3+ couple, and exhibit promis-
ing reversible lithium and sodium insertion, and thus is
promising to serve as an environmental friendly cathode
material.
samples. A retractable backscattered electron detector
was applied for atomic number contrast imaging.
2.4 Electrochemistry
The material (0.3 g) was mixed with Super S carbon (0.15
g) for 30 min using a Fritsch Pulverisette 8 mill. Subse-
quently the powder was ground with binder (poltetrafluo-
roethylene, PTFE, 0.05g) until homogeneous. Typically,
cathode pellets with 6-8 mg active material were tested in
coin cells (CR2325, NRC Canada) with Li metal as anode,
LP30 (1M LiPF6 in ethylene carbonate: dimethyl carbonate
= 1:1) as electrolyte for LIB, and Na metal as anode, 1 M
NaClO4 in propylene carbonate with 3% fluoroethylene
carbonate as electrolyte for NIB. Half cells were then test-
ed by galvanostatic cycling in the potential window 2.0-
4.4 V for LIB, and 2.0-4.3 V for NIB at a rate of 10 mAg-1
using a Biologic Macpile II system. For samples prepared
for Mössbauer spectroscopy, Kynar 2801 was used as
binder instead of PTFE.
II. EXPERIMENTS
2.1 Synthesis
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Single crystals of the title compound were synthesised by
a hydrothermal method. Typically, as obtained FeCl-
2ꢀ2H2O, NaBF4, H2C2O4ꢀ2H2O, Na2CO3 were mixed in a
teflon-lined autoclave in the ratio of 1:3:3:4 (1 for 5 mmol),
with 2 mL H2O added as solvent. The autoclave was heat-
ed at 190°C for 4d and then cooled down in air. The syn-
thesised product is a mixture of yellowish green crystals
and colourless NaBF4, which is highly soluble in water.
The resulting products were repeatedly washed with dis-
tilled water and acetone and dried at 60°C. Similar proce-
dures performed in the Co, Ni, Zn systems resulted in
NaCoF3, NaNiF3, NaZnF3, respectively. The Mn system
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2.5 Mössbauer spectra
gave
rise
to
a
totally
different
compound
Room temperature Mössbauer spectra were recorded on
absorbers prepared under argon (coffee-bags). Each ab-
sorber contains 30-40 mg/cm² active material recovered
by washing with dimethyl carbonate (DMC). The spec-
trometer is operating in the constant acceleration trans-
mission geometry. The γ-ray source (57Co/Rd, 850 MBq) is
maintained at room temperature. The isomer shift scale is
calibrated using pure α-Fe standard. The obtained data
are fitted using least-squares method and a combination
of Lorentzian lines with MOSFIT program.
Na4Mn4(C2O4)5F2•4H2O, for which attempts to obtain
pure sample are still ongoing.
2.2 Single crystal X-ray diffraction.
A yellowish green crystal was selected and mounted on
nylon loops in inert oil and SXRD data were collected on
a Rigaku SCX Mini diffractometer using Mo Kα radiation
(λ = 0.710 73 Å) at 173 K. Rigaku CrystalClear 2.0 was em-
ployed to index and process the data. The structure was
then solved by direct methods and refined using SHELX-
201427 incorporated in the WinGX program. Absorption
corrections were performed semi-empirically from
equivalent reflections on the basis of multi-scans. All at-
oms were refined anisotropically.
2.6 Calculations
The calculations were based on density-functional theory
within the Perdew, Burke, and Ernzerhof29 version of the
generalized-gradient approximation and the projector-
augmented wave method,30, 31 as implemented in the Vi-
enna Ab Initio Simulation Package (VASP).32 The on-site
Hubbard correction33 with the effective U value of 4.0 eV
was applied on the Fe 3d states and the van der Waals
correction was taken into account using the approach of
Grimme.34 In all calculations, the energy cutoff was set at
500 eV and spin polarization was included.
2.3 Characterization
The mid-infrared spectrum was obtained at room tem-
perature using a Perkin Elmer Spectrum GX IR spectrom-
eter. The spectra were collected in the range 400 to 4000
cm-1 with resolution of 1 cm-1. PXRD patterns were record-
ed on a Stoe STADI/P diffractometer operating in trans-
mission mode with Fe Kα1 radiation (λ = 1.936 Å) in the 2θ
range 10 °- 100 °, the total data collection time being 16 h.
Data sets were refined by conventional Rietveld methods
using the GSAS package with the EXPGUI interface.28 The
background, scale factor, zero point, lattice parameters
and coefficients for the peak shape function were refined
to convergence. Thermogravimetric analysis (TGA) was
carried out with ground crystals of the title compound
using a NETZSCH TG 209 thermal analyser. A sample
(about 10 mg) of ground crystal was placed in an alumina
crucible and heated from room temperature to 800oC at a
rate of 5oC /min in flowing air atmosphere. The residues
were examined and analysed by X-ray powder diffraction
after the experiments. SEM images of prepared samples
were taken on a JEOL JSM-6700F equipped with a field
emission gun (FEG) electron source. Secondary electron
images were recorded with a tungsten filament electron
source using an accelerating voltage of 5 kV for the hand
III. RESULTS AND DISCUSSION
3.1 Structure
Pure samples were obtained via a mild hydrothermal
method. The yellowish green crystals were easily detected
by the naked eye. The crystallographic data determined
from single crystal XRD on a 0.15ꢁ0.07ꢁ0.04 mm3 crystal
is displayed in Table 1. Atomic coordinates and selected
bond lengths are shown in Tables S1-2. Figure 1 gives a
graphic view of the structure. It clearly shows that the
structure features zig-zag [Fe(C2O4)F2]2- chains running
along the b-axis. Within a [Fe(C2O4)F2]2- chain, each Fe
atom is coordinated by four coplanar O atoms from one
bidentate and two mono-dentate oxalate moieties (d(Fe-
O) = 2.1268(15) -2.2255(18) Å), and two trans-F atoms
(d(Fe-F) = 1.9633(15)-1.9782(15) Å), forming a distorted,
compressed octahedron. The adjacent [FeO4F2] octahedra
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Na2Fe(C2O4)F2 crystallites obtained from the hydrother-
are connected by sharing one O corner from an oxalate
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group and therefore form [FeC2O4F2] infinite chains. Each
oxalate group connects three [FeO4F2] octahedra, leaving
one dangling oxygen atom. Notably, the Fe and oxalate
group lie almost in the same plane, parallel to {2 0 1},
while the Fe-F bond is perpendicular to it. Two Na atoms
located in the channel between the chains possess an
open migration pathway along the a- and b- axes. Na1 is
surrounded by four F atoms with bond lengths 2.2253(19)
Å - 2.3490(16) Å and one O atom with bond length
2.2984(17) Å, while Na2 is coordinated to two F atoms
(2.2624(17) Å – 2.2791(14) Å) and three O atoms
(2.3404(20) Å - 2.5867(20) Å) (Figure S1). Bond Valence
Sum (BVS)35 calculations support the expected oxidation
states (BVS values for Na1, Na2 and Fe of +1.104, +1.022
and +2.022, respectively).
mal synthesis are predominantly in the size range 2-10
μm. A powder X-ray diffraction (PXRD) pattern of the
hand ground material was recorded with Fe Kα1 radiation
(λ = 1.936 Å) to avoid Fe fluorescence. Rietveld analysis of
the PXRD pattern confirmed the sample quality, with
lattice parameters similar to those obtained from single-
crystal analysis, as displayed in Figure 2. Only very minor
peaks are observable from possible impurity phase(s). The
hand ground sample shows an average size of 1-3 μm
(Figure S2). No crystalline impurity could be detected.
The presence of a small quantity of an amorphous impuri-
ty cannot be excluded since the Mössbauer spectrum of
the pristine material (Figure 4d) is fitted with two contri-
butions; the major one (green line, 90%) is assigned to
iron(II) in an octahedral site in the crystalline
Na2Fe(C2O4)F2 and a minor one (blue line, 10%) that could
result from an amorphous iron(II) based impurity such as
Fe(C2O4)·2H2O. This contribution could also be explained
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Table 1 Crystallographic data for Na2Fe(C2O4)F2
Formula
Na2Fe(C2O4)F2
monoclinic
C2/c (No. 15)
15.8349(11), 5.9620(4), 12.2784(8)
111.968(7)
by some structural defects as observed in lithium iron
Crystal system
Space group
a, b, c (Å)
β
37
sulphates and lithium iron phosphates.36,
It is worth
noting that the main doublet shows a slight asymmetry
that can be explained very likely by a preferred orienta-
tion as already observed in some sodium oxalates and
malonates or a Goldanskii-Karyagin effect as reported for
Fe3(CO)12.25, 38
V(Å3)
1075.01(14)
Z
8
F(000)
880
R (all data)
R [I > 2σ(I)]
Goodness-of-fit on F2
0.0298/0.0790
0.0217/0.0692
0.0646
2
2
aR(F) = ∑∣∣Fo∣-∣Fc∣∣/ ∑∣Fo∣ for Fo > 2σ(Fo ).
b Rw(Fo ) = {∑ [w(Fo2 -Fc )2] / ∑wFo } for all data.
2
2
4 ½
Figure 2. Rietveld analysis of hand-ground microcrystalline
powder of synthesised sample. Lattice parameters in C 2/c
are a = 15.8566(3) Å , b = 5.9645(1) Å , c = 12.3198(2) Å, β =
112.003(1). wRp = 0.0868, Rp = 0.0678, χ2 = 3.42.
3.2 Electrochemical properties
In order to compensate for the low electronic conductivi-
ty of the pure sample (Figure S3, Band gap ≈ 2.27 eV), the
title compound was mixed with super S carbon in the
weight ratio of 2:1 for 30 min using a Fritsch Pulverisette 8
mill. After ball milling with Super S carbon, the crystallite
size is on average 100-500 nm, as displayed in Figure 3a-b.
The Super S carbon is very well dispersed among the par-
ent compound after ball milling and adheres to the sur-
face of particles. Therefore the back scattered electron
image (BSE) gives a clearer picture of the title compound
than the secondary electron image (SE). This well dis-
Figure 1. Crystal structure of Na2Fe(C2O4)F2. (a) unit cell
viewed along the b-axis (b) [Fe(C2O4)F2] infinite chains. The
FeO4F2 octahedra are shown in blue.
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persed mixture also implies an improved conductivity 4a displays the voltage profiles of the first cycle at a rate
within the composite electrode.
of 10 mAg-1 (about C/12) in the voltage window of 2.0-4.4
V for LIB and 2.0-4.3 V for NIB. The charge-discharge pro-
files are both quite similar to the sloping curve observed
on oxidation for Na2FePO4F.12 The initial sodium extrac-
tion from Na2Fe(C2O4)F2 exhibits two plateaux, probably
associated with structural phase transitions, with a step
occurring at a composition of Na1.5Fe(C2O4)F2. The limit of
sodium extraction corresponds to a composition of
NaFe(C2O4)F2. The two plateaux on the first charging
curve are situated at 3.6 V and 3.9 V for LIB (Figure S4),
and 3.3 V and 3.6 V for NIB (Figure 4b). An average differ-
ence of 0.3 V agrees well with the difference in electro-
chemical potential between Li and Na metal. Mössbauer
spectra confirm the iron oxidation and reduction in the
charge-discharge process, as indicated in Figure 4(d-f)
and Table S3. The blue line shows the Fe2+ component
(8%), which does not change during the cycling process.
At 4.3 V charged state (Figure 4e), the spectrum is fitted
with four doublets; the green and blue ones are the signa-
ture of a little amount of unreacted material due to the
large amount of powder needed for ex situ analysis (40
mg/cm²). The red and pink doublets are typical of
iron(III) environments and represent 90% of the total
iron at the charged state. This suggests that the
NaFe(C2O4)F2 phase has two crystallographic sites or
some structural defects. In 2.0 V discharged state (Figure
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Figure 3. SEM image of cathode material in NIB (a-b) SE
and BSE prepared as stated in the text (c-d) SE and BSE after
50 electrochemical cycles in the voltage window 2.0-4.3 V.
The electrochemical performance of Na2Fe(C2O4)F2 as a
cathode in LIBs and NIBs was then evaluated galvanostat-
ically in half cells using a Biologic Macpile II system. The
typical loading of active material was 6-8 mg cm-2. Figure
4f),
the
Figure 4. (a) Galvanostatic voltage profiles for Na2Fe(C2O4)F2 cathode in the voltage range of 2.0-4.4 V in LIB (blue), and 2.0-4.3
V in NIB (green) (b) dQ/dE curve of initial charge/discharge profiles in NIB. (c) Discharging capacity evolution for the first 50
cycles. Blue for LIB and green for NIB. Mössbauer spectra of Na2Fe(C2O4)F2 in (d) pristine cathode; (e) charged to 4.3V state; (f)
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discharged to 2.0 V. Observed and calculated patterns are present as black circles and lines, respectively. The blue, green, red
and pink lines are described in the text.
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spectrum mainly consists of Fe2+, with some unreduced
Fe3+ suggesting an incomplete reduction at this voltage.
This is in good agreement with the irreversible capacity.
The large cell polarization probably arises from the con-
ductivity of the pristine material. The capacity faded over
the first few cycles (Figure 5), and then stabilized at
around 90 mAhg-1 at an average discharge potential of 3.3
V for Na2-xLixFe(C2O4)F2, and 70 mAhg-1 at an average dis-
charge potential of 3.0 V for Na2Fe(C2O4)F2 as shown in
Figure 4c. The capacity fading might arise from a little
overcharge that cause irreversible structure change as
indicated by the little convex after 4.0 V in Figure 4b, and
incomplete reduction at 2.0 V as proved by Mössbauer
spectrum (Figure 4f). After 50 cycles, the recovered cath-
ode of NIB showed no change in morphology (Figure 3c-
d). It is worth noting that almost 100% of the theoretical
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capacity (133 mAhg−1 for LIB and 124 mAhg−1 for NIB) was
Figure 6. Total and projected electronic density of states of
extracted on the first charge of both cells.
Na2Fe(C2O4)F2. The zero of the energy (at 0 eV) is set to the
highest occupied state.
the total and projected electronic density of states of
Na2Fe(C2O4)F2. The compound has a calculated band gap
of 1.94 eV, which is in reasonable agreement with the ex-
perimental value. The gap is indirect with the valence-
band maximum at the Γ point and the conduction-band
maximum at the Z point in the Brillouin zone. It can be
seen from Figure 6 that the top of the valence band is
predominantly composed of Fe 3d states. As the extrac-
tion of Na from the NIB cathode involves removing one
electron from the valence-band top, the electronic struc-
Figure 5. The first five cycles of the LIB (a) and NIB (b) using
carbon-coated Na2Fe(C2O4)F2 cathode.
ture indicates that the intrinsic extraction/(re)-insertion
mechanism involves Fe3+/Fe2+ redox. Indeed, we find that
Fe2+ is oxidized into Fe3+ upon Na removal; the calculated
3.3 Calculations
magnetic moment of high-spin Fe3+ is 4.33μB. The average
First-principles calculations were further carried out to
elucidate the mechanism of electrochemical activity. We
redox potential is 3.54 V vs Na between Na2Fe(C2O4)F2
and NaFe(C2O4)F2, which is in good agreement with the
find that the iron ion in Na2Fe(C2O4)F2 is stable as high-
spin Fe2+ with a calculated magnetic moment of 3.77 μB,
average experimental value during first charge (Figure 4b
and e).
in perfect agreement with the Mössbauer isomer shift.
The antiferromagnetic and ferromagnetic spin configura-
tions of the iron array are degenerate in energy. Figure 6
shows
3.4 Further characterization
IR spectroscopy was carried out with the objective of de-
termining the coordination of atoms in the sample and to
confirm the absence of incorporation of H2O or OH-. In
Figure 7a, there are no absorption peaks in the range of
3000-4000 cm-1, verifying no H2O or OH- in the obtained
sample. The detailed assignment of absorption peaks is
stated in Supporting Information. Na2Fe(C2O4)F2 is ther-
mally stable up to 267°C in air, before gradual weight loss
occurs with complete decomposition to NaF and Fe2O3
occurring above 410°C (Figure 7b). This is stable enough
for normal working environments. On immersion in wa-
ter, it undergoes a slow reaction to form Fe(C2O4)ꢀ4H2O,
as indicated in Figure S5. The synthesis of the lithium
analogue of Na2Fe(C2O4)F2 was attempted via a topotactic
ion-exchange of the Na for Li by refluxing in excess
LiBr/LiCl in CH3CN or N-Methyl-2-pyrrolidone. Although
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The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
Na2Fe(C2O4)F2 is quite stable in pure solvent, addition of
LiBr/LiCl produced a yellow solution, which indicated the
dissolution of Fe2+. After total evaporation of the solvent,
the residue contained amorphous Fe2O3 and LiF (Figure
S6).
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ACKNOWLEDGMENT
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Thanks to Mr. Pifu Gong and Prof. Zheshuai Lin (Technical
Institute of Physics and Chemistry, Chinese Academy of Sci-
ences) for helpful discussions. Thanks to Prof. John T. S. Ir-
vine and Dr. Paul Connor (School of Chemistry, University of
St Andrews) for electrochemical test support. W.Y. thanks
the Royal Society for the award of a Newton International
Fellowship (140881). Work at North Dakota State University
(NDSU) was supported by the U.S. Department of Energy
Grant No. DE-SC0001717 and by NDSU's Center for Compu-
tationally Assisted Science and Technology.
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Figure 7. IR spectrum(a) and TG(b) of Na2Fe(C2O4)F2
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IV. CONCLUSION
In conclusion,
a
Fe-based polyoxyanion material
Na2Fe(C2O4)F2 was prepared through hydrothermal syn-
thesis. It is the first fluoro-oxalate cathode material re-
ported to date. The structure was determined through
single crystal X-ray diffraction. Electrochemical tests re-
veal that almost 100% of the theoretical capacity (133
mAh/g for LIB and 124 mAh/g for NIB) was extracted on
the first charge at an average voltage of 3.6 V and 3.3 V,
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formula unit up to 50 cycles in the present work, but
might be improved with optimized condition, such as
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best oxalate positive electrode material reported to date,
with comparable performance to Fe2(C2O4)3ꢀ4H2O but
with the advantage that it is fabricated in the discharged
state. In addition, replacing or doping Fe with Mn or oth-
er multi-valent state ions is beneficial for a higher redox
potential as well as a higher capacity in this system. The
present study of Na2Fe(C2O4)F2 confirms that oxalates
represent a promising class of polyoxyanion positive elec-
trode materials. It appears that oxalates display discharge
voltages lower than those of sulfates but comparable to
those of phosphates, which reflects the electronegativity
of (SO4)2- >(C2O4)2- >(PO4)3-. Further research on oxalates
may give rise to even more interesting potential electrode
materials.
ASSOCIATED CONTENT
Supporting Information. Atomic coordinates, selected
bond length, assignment of IR spectroscopy, UV-Vis Absorp-
tion Spectrum, and crystal information file (CIF) are attached
as the supporting files. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*pl@st-andrews.ac.uk (PL); *ara@st-andrews.ac.uk(ARA)
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