Electrochemistry Communications 38 (2014) 79–81
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Short communication
Electrochemical properties of NaNi1/3Co1/3Fe1/3O2 as a cathode material
for Na-ion batteries
Plousia Vassilaras, Alexandra J. Toumar, Gerbrand Ceder ⁎
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
a r t i c l e
i n f o
Article history:
Received 15 October 2013
Received in revised form 13 November 2013
Accepted 14 November 2013
Available online 21 November 2013
Keywords:
Na batteries
NaNi1/3Fe1/3Co1/3O2
Cathode
Principles calculations
Oxidation state
a b s t r a c t
NaNi1/3Fe1/3Co1/3O2 is synthesized by solid-state methods and investigated as a positive electrode material for sodium ion batteries. Galvanostatic cycling of NaNi1/3Fe1/3Co1/3O2 between 2.0 and 4.2 V provides ~165 mAh/g of
reversible capacity at C/20. This material also demonstrates great rate-capability and is able to de-intercalate
80 mAh/g even at 30C. First principles calculations are used to determine the order of transition metal oxidation
within the system and provide lattice parameters with close proximity to the experimental.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Na-intercalation batteries are appearing as an important alternative
to Li intercalation systems, and rapid progress has been made on developing high capacity cathode materials [1]. It has become clear that the
Na analogues of the successful layered LiMO2 electrodes behave very
differently from their Li equivalents [2]. The large difference in
ionic radius between Li and Na provides a stronger tendency for
the Na compounds to form in the layered structure [3–7], and layered NaxMO2 (M = Ti, V, Cr, Mn, Fe, Co, Ni) [4,8–17], as well as several Na compounds with mixed transition metals, NaxNi0.6Co0.4O2,
NaxNiyMn1 − yO2, NaxTiyMn1 − yO2, NaxFe0.5Mn0.5O2, NaFe0.5Co0.5O2,
NaNi1/3Mn1/3Co1/3O2 and NaNi1/3Fe1/3Mn1/3O2 [18–27], all show electrochemical activity.
In this paper, we report the synthesis and electrochemical performance of NaNi1/3Fe1/3Co1/3O2 as a novel Na intercalation cathode
material. The only layered materials in which three transition
metals are mixed in the literature are NaNi1/3Fe1/3Mn1/3O2 [26]
and NaNi1/3Mn1/3Co1/3O2 [24], and their capacity is limited to about
120 mAh/g at C/10. We show reversible sodium insertion/extraction
with capacities between 165 mAh/g–150 mAh/g when cycled at low
rate. Even though the particle size is N1 μm, very high rate capability
is achieved, retaining 80 mAh/g discharge capacity at 30C rate. This is
promising for high-density high-power Na batteries.
First principles calculations are used to determine the order of transition metal oxidation within the system. In the fully sodiated phase Ni
is in a 2+ oxidation state and Fe/Co behaves as a mixed system. As the
⁎ Corresponding author.
E-mail address: [email protected] (G. Ceder).
1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.elecom.2013.11.015
material is de-sodiated the Fe and Co are first oxidized to 4+ while the
Ni goes through two oxidation steps to reach a final oxidation state in
the fully de-sodiated structure of Co4+, Fe4+ and Ni4+.
2. Experimental methods
NaNi1/3Co1/3Fe1/3O2 was synthesized by solid-state reaction. Excess
amounts of Na2O, NiO, Co3O4 and Fe2O3 were mixed and ball milled
for 4 h at 500 rpm rate, and the resulting material was collected in the
glove box. About 0.5 g of powder was fired at 800 °C under O2 for
14 h before it was quenched to room temperature and moved to a
glove box filled with argon.
X-ray diffraction (XRD) patterns were collected on a PANalytical
X'Pert Pro equipped with Cu Ka radiation in the 2θ range of 5–85°.
All the samples were sealed with Kapton film to avoid air exposure.
Profile matching of the powder diffraction data of the as-prepared
NaNi1/3Co1/3Fe1/3O2 was performed with Highscore Plus using space
group R-3m.
Electrochemical cells were configured with Na/1 M NaPF6 in EC:
DEC = 1:1/NaNi1/3Co1/3Fe1/3O2, Super P carbon black (Timcal, 15 wt.%)
as conductive agent, and polyethylenetetrafluoride (PTFE) (DuPont,
5 wt.%) as binder. The 1 M NaPF6 in EC:DEC electrolyte was prepared
by dissolving anhydrous NaPF6 (98%, Sigma Aldrich) into EC:DEC (anhydrous, 1:1 in volume ratio). Two pieces of glass fiber served as separators
and stainless steel as current collectors in Swagelok cells, assembled in an
argon-filled glove box with the moisture level and oxygen level less than
0.1 ppm. The loading density of the active material used was in the range
of 2.2–3.1 mg/cm2. The cells were cycled at room temperature using a
Solatron 1287 operating in galvanostatic mode and potentiostatic mode.
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P. Vassilaras et al. / Electrochemistry Communications 38 (2014) 79–81
3. Computational methods
Energy and charge calculations were performed using the Vienna
Ab initio Simulation Package (VASP) [28] within the projector
augmented-wave approach [29], using the Perdew–Burke–Ernzerhof
(PBE) generalized-gradient approximation (GGA) [30] functional and
the GGA + U [31] extension to it. All calculations were performed on
a single formula unit cell (NaxNiCoFeO6 with x = 0,1,2,3) with sqrt(3)
xsqrt(3) ordering for transition metals. This ordering was the lowest
energy for the few orderings we considered. Calculations were spin polarized and used a plane wave energy cut-off of 520 eV. The magnetic
moments of Co3+ and Co4+ ions were initialized as low spin, which
was found to improve convergence in mixed transition metal layered
oxide systems.
The U values used for Co, Fe and Ni transition metals are those found
by Jain et al. for high-throughput analysis [32] using Wang et al.'s method [33] of fitting the U parameter to experimental binary formation enthalpies for the oxide/fluoride environment. The U values used in these
calculations were 3.4 for Co, 4.0 for Fe, and 6.0 for Ni. Charge and voltage
analysis was carried out with aid of the pymatgen Python library for materials analysis [34].
Table 1
Structural parameters of O3-type NaNi1/3Co1/3Fe1/3O2 refined by Rietveld analysis.
Unit cell parameters
Na
Ni
Fe
Co
O
x
y
z
0.0
0.0
0.0
0.0
0.0(0)
0.0
0.0
0.0
0.0
0.0(0)
0.0
0.5
0.5
0.5
0.2337(3)
Occupancy
B
1.00
0.33
0.33
0.33
1.00
0.555
0.563
0.571
0.571
0.312
ahex. = 2.95108(6), chex. = 15.8543(8), Rwp = 1.91%, χ2 = 13.76.
4. Results and discussion
The as-prepared NaNi1/3Co1/3Fe1/3O2 has a O3-layered structure
with R-3m symmetry as shown on the XRD pattern in Fig. 1. A SEM
image of the as-prepared NaNi1/3Co1/3Fe1/3O2 is also shown in Fig. 1,
with primary particle size N1 μm. The background and three broad
peaks between 10 and 30° are due to the Kapton film, which is used
to seal the sample. Rietveld refinement gives the lattice parameters
ahex. = 2.951 Å, and chex. = 15.854 Å, which are in reasonable agreement to the calculated ones: ahex. = 2.978 Å, and chex. = 15.723 Å.
The interatomic distances of (Ni/Fe/Co)–O were calculated from the
XRD data to be 2.008 Å (6×) and the transition metals are accommodated at the 3b octahedral sites. The Na–O distance was calculated to be
2.323 Å (6×) and the Na is accommodated at the 3a octahedral sites.
Cell parameters and atomic positions are listed in Table 1.
Galvanostatic measurements of charge and discharge of
NaNi1/3Fe1/3Co/3O2 at C/20 rate (1C = 237 mA/g) at various cycles in
the voltage range of 2.0–4.2 V are shown in Fig. 2a. We observe a reversible voltage profile with ≈165 mAh/g reversible discharge capacity, and
500 Wh/kg measured energy density which is a significant improvement over previous reports of similar systems with three transition
Fig. 1. XRD pattern (red) and refinement (blue) of as-prepared NaNi1/3Co1/3Fe1/3O2. The
broad peaks between 10° and 30° are due to the Kapton film. The red and blue lines represent the experimental and calculated data respectively. The residual discrepancy is
shown in black. The refinement is performed in the R-3m space group and results in
Rwp = 1.91%, and χ2 = 13.76. Particle morphology is observed by SEM analysis.
Fig. 2. (a) Voltage profile of NaNi1/3Co1/3Fe1/3O2 for multiple cycles at C/20 and cyclic performance at C/20, C/10 and C/5 as an inset. The cells are galvanostatically cycled between
2.0 and 4.2 V. (b) Rate-capability of NaNi1/3Co1/3Fe1/3O2 in Na cells. The cells were charged
to 4.2 V at a rate of C/20 and then discharged at different rates; C/20 (11.86 mA/g)–30C
(7116 mA/g). The sample loading of the active material was 2.18 mg/cm2. (c) Comparison
between the experimental second voltage profile (red) and calculated compositionaveraged voltages (black) of NaNi1/3Co1/3Fe1/3O2.
P. Vassilaras et al. / Electrochemistry Communications 38 (2014) 79–81
metals, such as NaNi1/3Fe1/3Mn1/3O2 [26] and NaNi1/3Mn1/3Co1/3O2 [24]
with ≈120 mAh/g, but similar to NaFe1/2Co1/2O2 [27]. The charge/
discharge capacities in the range of 2.0–4.2 V in the first, second, fifth,
and tenth cycles are 170/163 mAh/g, 166/163 mAh/g, 157/155 mAh/g,
and 153/150 mAh/g respectively. The coulombic efficiencies are 96.1%
for the first cycle, 97.9% for the second cycle, 98.6% for the fifth cycle
and 97.9% for the tenth cycle. We also demonstrate large reversible capacities (≈150 mAh/g) with small polarization when the material is cycled in the same voltage range (2.0–4.2 V) at rates of C/10 and C/5. The
capacity retention at both rates, C/10 and C/5, is 95% and 92% respectively, as one can observe in Fig. 2a. It is important to note that unlike many
layered oxides that cannot fully re-insert all sodium, NaNi1/3Co1/3Fe1/3O2
is able to insert up to Na0.99–0.97 even after cycling to 4.2 V, Fig. 2c. The
ability of the material to discharge to a fully sodiated phase could be contributing to the high reversible capacities that are observed.
Rate capability experiments were run for Na//NaNi1/3Co1/3Fe1/3O2
cells at rates of C/20 (11.9 mA/g) to 30C (7.12 A/g) as shown in Fig. 2b
(1C = 237 mAh/g). The cell continues to deliver large reversible capacities when cycled at faster rates, 1C and 5C, and it's important to note that
the capacities are close to 80 mAh/g even at rates as fast as 30C.
The charge and discharge curves are very similar over twenty cycles,
indicating a reversible transformation path upon charge and discharge.
Similar behavior with large steps in the voltage curve has also been
observed for de-intercalation of Na from NaNi1/3Mn1/3Co1/3O2,
NaNi1/2Mn1/2O2, and NaFe1/2Co1/2O2 [22,24,27].
First principles calculations were performed at compositions
NaNi1/3Fe1/3Co1/3O2, Na2/3Ni1/3Fe1/3Co1/3O2, Na1/3Ni1/3Fe1/3Co1/3O2
and Na0Ni1/3Fe1/3Co1/3O2 in order to estimate the average voltages
between these compositions and to determine the order of transition
metal oxidation. We find that in the fully sodiated material Ni is close
to a + 2 oxidation state and the Co and Fe states are mixed in a state
that is partially oxidized to +3.5. Upon de-sodiation, the Fe/Co system
is first oxidized to Fe4+ and Co4+ at 2.72 V followed by Ni2+ oxidation
to 3+ and 4+. The horizontal lines in Fig. 2c represent the calculated
average voltage in each composition domain [35], and should not be
interpreted as an actual voltage curve. While the resulting average voltages are in good agreement with the experimental averages, the somewhat uncommon oxidation state needs experimental confirmation.
NaFeO2 was recently revisited and shows a reversible capacity of
80 mAh/g at the limited cutoff of 3.4 V [36]. It is observed that at higher
voltages NaFeO2 goes through an irreversible structure transition, which
is presumably associated with partial migration of iron [36]. Materials
such as NaNi1/3Fe1/3Co1/3O2, NaNi1/3Fe1/3Mn1/3O2, NaFe1/2Co1/2O2 and
NaFe1/2Mn1/2O2 further demonstrate that the substitution of Fe in
NaFeO2 with different elements such as nickel/cobalt, nickel/manganese,
cobalt and manganese respectively can be very effective in suppressing
iron migration in NaFeO2 and provide high reversible capacities and
great rate capabilities.
Designing innovative cathode materials based on abundant elements such as sodium and iron is very important. As Yoshida et al.
pointed out cobalt is not an abundant element and its utilization should
be reduced [27]. With a careful approach our study demonstrates that
the nickel substitution is reducing the amount of cobalt without altering
the reversible capacities and rate capabilities that these O3-type combinations provide.
5. Conclusions
In this article, we have studied O3-type NaNi1/3Co1/3Fe1/3O2 system
as a positive electrode material for rechargeable sodium batteries. This
81
material delivers ~165 mAh/g of reversible capacity with an energy
density of ~500 Wh/kg at C/20. NaNi1/3Fe1/3Co1/3O2 shows excellent
rate-capability with 80 mAh/g at 30C and with 92% capacity retention
even at C/5 rate. The electrochemical properties and the voltage profile
are very similar to NaFe1/2Co1/2O2, which signifies the importance of Ni
substitution on maintaining the excellent rate capability and reversibility of the Fe–Co based materials while reducing the consumption of Co.
Acknowledgments
We thank Jinhyuk Lee, Jae Chul Kim, Shyue Ping Ong and Rahul
Malik for helpful discussions. We would also like to thank the Samsung
Advanced Institute of Technology for their funding of this research. This
work made use of the Shared Experimental Facilities supported in part
by the MRSEC Program of the National Science Foundation under
award number DMR – 0819762.
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