A high-performance sodium-ion full cell with a layered oxide cathode and a phosphorous-based composite anode

Article abstract of DOI:10.1149/2.0931702jes

A low-cost sodium-ion full cell with a O3-type layered Na[Cu_0.2(Fe_1/3Mn_2/3)_0.8]O₂ cathode and an alloy-type P-TiP₂-C anode is presented. The cathode is synthesized by an oxalate coprecipitation method and optimized cathodes shows a high specific capacity of 135 mAh g^?1 at 0.1C rate with a high rate capability of 90 mAh g^?1 at 1C rate and 70 mAh g^?1 at 2C rate with good cyclability. The full cell exhibits better capacity retention than the half cell with the cathode due to the elimination of the degradation caused by sodium-metal anode. The dramatically enhanced electrochemical performance of the Na[Cu_0.2(Fe_1/3Mn_2/3)_0.8]O₂ / P-TiP₂-C full cell compared to that of the sample with no Cu is attributed to the structural stabilization imparted by Cu by suppressing the phase change from the O3 structure to the P3 structure during cycling.

Full text of DOI:10.1149/2.0931702jes

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Journal of The Electrochemical Society, 164 (2) A321-A326 (2017)
A321
A High-Performance Sodium-Ion Full Cell with a Layered Oxide
Cathode and a Phosphorous-Based Composite Anode
Seung-Min Oh, Pilgun Oh, Sang-Ok Kim,∗ and Arumugam Manthiram∗∗,z
Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin,
Austin, Texas 78712, USA
A low-cost sodium-ion full cell with a O3-type layered Na[Cu0.2(Fe1/3Mn2/3)0.8]O2 cathode and an alloy-type P-TiP2-C anode is
presented. The cathode is synthesized by an oxalate coprecipitation method and optimized cathodes shows a high specific capacity
of 135 mAh g−1 at 0.1C rate with a high rate capability of 90 mAh g−1 at 1C rate and 70 mAh g−1 at 2C rate with good cyclability.
The full cell exhibits better capacity retention than the half cell with the cathode due to the elimination of the degradation caused
by sodium-metal anode. The dramatically enhanced electrochemical performance of the Na[Cu0.2(Fe1/3Mn2/3)0.8]O2 / P-TiP2-C full
cell compared to that of the sample with no Cu is attributed to the structural stabilization imparted by Cu by suppressing the phase
change from the O3 structure to the P3 structure during cycling.
© The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0931702jes] All rights reserved.
Manuscript submitted October 17, 2016; revised manuscript received December 12, 2016. Published December 29, 2016.
As the demand for rechargeable batteries is increasing, the research
activities in the Li-ion battery area had grown exponentially during
the past 25 years.1,2 However, due to the relatively low abundance
of lithium resources and the high cost, there is immense interest to
develop battery chemistries based on low-cost working ions, particu-
larly for large-scale applications. In this regard, Na-ion batteries are
emerging as an alternative due to the high abundance and low cost
of Na. In addition, the larger size of Na+ compared to Li+ helps to
minimize the cation disorder between Na+ and the transition-metal
ions. Therefore, various kinds of Na-based cathode materials have
been investigated with the O3-type layered,3–19 P2-type layered,21–29
and polyanion-type structures.30–32 Among them, the O3-type cath-
ode materials are the most promising due to their similarity to the
well-known LiCoO2 cathode used in lithium-ion industry. Also, as
has been reported in the literature, the O3-type cathodes have much
more stable crystal structure and capacity retention than the P2-type
cathode materials.7,18,19
= 0.2 is assembled into a full cell with a phosphorous-based alloy-
type anode P-TiP2-C developed in our group. The assembled full cell
Na[(Cu0.2(Fe1/3Mn2/3
than the half-cell with the Na[(Cu0.2(Fe1/3Mn2/3
)
0.8]O2 / P-TiP2-C exhibits better performance
0.8]O2 cathode due
)
to the elimination of the degradation caused by sodium-metal anode
during cycling.
Experimental
Material synthesis.—The [Cux(Fe1/3Mn2/3
)
1-x]C2O4 (x = 0, 0.1,
and 0.2) precursors were synthesized via a co-precipitation method.
Cu(NO3)2 · 6H2O, FeSO4 · 7H2O, MnSO4 · H2O, (NH4)2C2O4, and
H2C2O4 were employed as the starting materials. To synthesize
the (Fe1/3Mn2/3)C2O4 · H2O precursor, stoichiometric amounts of
FeSO4 · 7H2O, and MnSO4 · H2O were dissolved in distilled water
to a concentration of 0.5 mol dm−3. This metal solution was then
dropped into a continuously stirring solution of ammonium oxalate.
At the same time, an appropriate amount of NH4OH solution was
added into the beaker to adjust the pH value to 5. The concentration
of the solution, pH, temperature, and stirring speed were carefully
controlled. The co-precipitation solution was continuously stirred
for 3 h while keeping the temperature constant at 70◦C. CuC2O4
was synthesized using Cu(NO3)2 · 6H2O and H2C2O4 with the same
method described above. After the reaction, the precursor powders
were filtered, washed, and dried in a vacuum oven overnight at 110◦C.
With the above perspective, several O3 type cathode compositions
have been introduced, but they are generally based on expensive tran-
sition metal ions such as Ni and Co.7,8,10–15,17 To take the low-cost
advantage of sodium-ion batteries, it is critical to replace these expan-
sive transition-metal ions by low-cost transition-metal ions such as
Fe, Mn and Cu. In this regard, several research groups have focused
on the Fe-based O3-type cathode NaFeO2.4–6 However, the high-spin
2
Fe3+:t2g3eg ions tend to migrate readily from the octahedral sites in
The Na[Cux(Fe1/3Mn2/3
)
1-x]C2O4 (x = 0, 0.1, and 0.2) samples were
the transition-metal layer to the octahedral sites in the sodium layer
via a neighboring tetrahedral site as the Fe3+ ion has no particular
preference for octahedral sites.4 Therefore, to overcome this diffi-
culty, it is necessary to mix Fe with other transition-metal ions such
as in Na(Fe,Co)O2, Na(Ni,Fe,Mn)O2, and Na(Fe,Mn)O2.8,10,11,17,18,35
Through this approach, the Fe migration problem has been suppressed
to some extent. Nevertheless, the use of expensive Co or Ni in those
compositions still works against the low-cost proposition of Na-ion
battery systems. Recently, Mu et al. reported the Cu-containing O3-
type cathode Na0.9[Cu0.22Fe0.30Mn0.48]O2, displaying 98 mAh g−1 at
0.1 C-rate. This material was synthesized by a solid-state reaction that
yielded a particle size of 10–30 μm.20
prepared by thoroughly mixing the precursors (Fe1/3Mn2/3)C2O4 and
CuC2O4 with Na2CO3, heating at 900◦C in air for 15 h with a heating
rate 3◦C min−1, and quenching the sample to room temperature.
Physical properties.—The synthesized samples were character-
ized with powder X-ray diffraction (XRD, MiniFlex 600, Rigaku)
using Cu-Kα radiation. Particle morphologies of the precursor and
the as-synthesized powders were observed by scanning electron mi-
croscopy (SEM, Quanta 650 ESEM, FEI) equipped with energy dis-
persive spectroscopic (EDS) analysis. Chemical compositions of the
prepared powder were analyzed with an inductively-coupled plasma-
optical emission spectroscopy (ICP-OES). The oxidation state of each
ion in the samples was investigated with X-ray photoelectron spec-
troscopy (XPS, Kratos Analytical). High resolution transmission elec-
tron microscopy (TEM: JEOL 2010F) was used to analyze the surface
morphology and crystal structure of the powder samples.
Despite an intensive investigation of cathode and anode hosts for
sodium-ion cells in recent years, full cell data with optimized cath-
ode and anode compositions and optimized particle size or morphol-
ogy are rarely available in the literature. Accordingly, we present
here the synthesis of Na[(Cux(Fe1/3Mn2/3
by an oxalate method and their physicochemical and electrochemi-
cal characterization. Then, an optimized cathode composition with x
)
1-x]O2 with 0 ≤ x ≤ 0.2
Electrochemical properties.—Electrochemical testing was per-
formed with 2032R coin-type cells using Na metal (Alfa Aesar., USA)
as an anode. The cathodes were fabricated by blending the prepared
∗
Electrochemical Society Student Member.
Electrochemical Society Fellow.
Na[Cux(Fe1/3Mn2/3
carbon black (7.5 wt%), and polyvinylidene fluoride (7.5 wt%) in N-
)
1-x]C2O4 (x = 0, 0.1, and 0.2) powders (85 wt%),
∗∗
zE-mail: manth@austin.utexas.edu
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Journal of The Electrochemical Society, 164 (2) A321-A326 (2017)
(a)
X = 0.2
X = 0
X = 0.1
10 µm
10 µm
10 µm
(b)
x = 0.2
x = 0.1
x = 0
20
30
40
50
60
32
34
36
38
40
2
θ / degree
2
θ / degree
X = 0.2
X = 0
(c)
Na
Fe, Mn
Cu
O
Figure 1. (a) SEM images and (b) XRD patterns of Na[Cux(Fe1/3Mn2/3
and 0.2.
)
1-x]O2 (x = 0, 0.1, and 0.2). (c) Schematic crystal structures of the samples with x = 0
methylpyrrolidinone. The slurry was then cast onto an aluminum foil
and dried at 110◦C for 12 h in a vacuum oven, and then disks were
punched out of the foil. The electrolyte solution was 1.0 M NaClO4 in
a 98:2 by volume mixture of propylene carbonate and fluoroethylne
carbonate (Aldrich). The electrolytes were carefully used inside an
Ar-filled dry box due to the air sensitivity of the NaClO4 salt. The
active material loading in the cathode and anode were, respectively,
2 and 0.5 mg cm−2, with corresponding electrode thicknesses of 40
μm and 15 μm. All cells were prepared in an Ar-filled dry box. The
half cells were cycled in a constant current mode at a 0.1C rate in the
voltage range of 1.2–4.3 V versus Na (where 1C = 120 mAh g−1).
For full cell test, P-TiP2-C was used as an anode. The negative and
positive electrode capacity ratio for full cell (N/P ratio) was 1.3.
We also recorded the SEM images and the EDS elemental map-
ping of the Na[Fe1/3Mn2/3]O2 and Na[Cu0.2(Fe1/3Mn2/3)0.8]O2 pow-
ders. As seen in Figure 2a, the Na[Fe1/3Mn2/3]O2 sample with no
Cu shows a uniform distribution of Na, Mn, and Fe. Similarly, the
Na[Cu0.2(Fe1/3Mn2/3)0.8]O2 sample exhibits a uniform distribution of
Na, Mn, Fe, and Cu (Figure 2b).
The electrochemical performances of the samples were assessed
with coin type half cells assembled with Na metal as the anode. To
optimize the charge cutoff voltage, we tested the cells with differ-
ent voltage ranges as shown in Figure S4 (1.2 - 4.3 V and 1.2–4.4
V). However, the gain in capacity on increasing the cutoff voltage to
4.4 V was not significant, so we decided to keep the cutoff voltage
at 4.3 V to avoid aggravated electrolyte decomposition at higher volt-
ages. For the charge-discharge tests, therefore, all cells were charged
to 4.3 V and then discharged to 1.2 V at a constant current of 0.1C
rate at room temperature. As the Cu content increases, the specific
capacity in the first cycle increases dramatically (Figure 3a). The
Results and Discussion
Na[Cux(Fe1/3Mn2/3
)
1-x]O2 sample with x = 0.2 shows the highest dis-
Determination of the chemical composition of the final products by
the inductively coupled plasma analysis (ICP) confirmed that the final
compositions were in agreement with the designed compositions (see
Table S1). Figures 1a–c shows that the morphologies and crystal struc-
tures of the synthesized samples. As seen in Figure 1a, all materials are
round-shaped with a particle size of 2 – 3 μm. No morphological dif-
ferences were found from the SEM results with Cu content. The X-ray
charge capacity of 135 mAh g−1 at 0.1C rate in the first cycle and
the x = 0 sample shows the lowest discharge capacity of 60 mAh g−1
at the same rate and cycle. We also compared the specific capacity
and synthetic condition of the Na[Cu0.2(Fe1/3Mn2/3)0.8]O2 sample with
other Fe-containing materials in the literature as in Table S3. From
this comparison, we concluded that our synthetic method is effective
to make layered materials for sodium-ion battery.
diffraction (XRD) patterns of the Na[Cux(Fe1/3Mn2/3
)
1-x]O2 (x = 0,
The enhanced discharge capacity of the Cu-containing samples is
due to (i) the decreased amount of Fe in the sample and suppression
Fe migration to the Na sites during cycling due to Cu substitution and
(ii) the higher electrochemical activity of the substituted Cu2+ ions.
As has been reported before, Cu2+, Fe3+, and Mn3+ start to oxidize
to Cu3+, Fe4+, and Mn4+ during charge at about 2.4, 3.8, and 3.6 V,
respectively.18,33 Additionally, we confirmed the oxidation and reduc-
tion state of each transition-metal ion with the differential capacity
0.1, and 0.2) samples reveal that all samples have the well-crystalized
O3-type rhombohedral structure with the R-3m space group. The lat-
tice parameters of these samples calculated with the least squares
method are given in Table S2 (see supplementary material). Interest-
ingly, the main peaks that are around 2ꢀ = 30 to 40o are shifted to
lower angle with increasing Cu content due to an increase in the lattice
´
parameters caused by a larger ionic radius of Cu2+ (0.73 Å) compared
´
´
´
to those of Fe3+ (0.645 Å), Mn3+ (0.645 Å), and Mn4+ (0.53 Å).
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A323
Na
Mn
Na
Mn
All elements
(a)
Fe
(b)
All elements
Fe
Cu
Figure 2. SEM and EDX mapping images of each ion in the (a) x = 0 and (b) x = 0.2 samples.
profiles of the first cycle (Figure S5). This means copper has an oxida-
tion state of 2+ at the pristine state and that gets oxidized to 3+ first
during charge compared with the other transition-metal ions. To verify
this assertion, XPS analysis was carried out and the results are shown
in Figure 3b. As seen in Figure 3b, the original oxidation state of each
transition-metal ion in the x = 0.2 sample is 2+ for Cu, 3+ for Fe,
and 3+/4+ for Mn, which are well matched with the compositional
calculation of Cu2+, Fe3+, and Mn3.x+ in the samples. The binding
energy of each transition-metal ion is shifted to higher values when
the cell is charged, indicating that all the three transition-metal ions
are electrochemically active during cycling. Among them, the Cu2+
ions start to oxidize to 3+ first from 2.4 V compared to the Fe3+ and
Mn3+ ions, which is consistent with the previous literature. Fe-ion
migration is a critical problem for O3-type Fe-containing materials,
so the decreased amount of Fe with the Cu substitution also helps
increase the specific capacity. From these data, we confirm our as-
sertion that the copper ions contribute to the increased capacity with
increasing Cu content in the samples.
(SOC) and depths of discharge (DOD). In the case of O3-type cath-
ode materials, the intensity of the main peak at around 2ꢀ = 41o
generally decreases and the peaks detected at 2ꢀ = 30 – 35 shift
to lower angles and the peaks at 2ꢀ = 35 – 40 shift to higher
angles during charge, indicating that the crystal structure changes
from the O3-type rhombohedral structure to the P3-type mono-
clinic structure.18 As we anticipated, the Na[Cu0.2(Fe1/3Mn2/3
)
0.8]O2
electrode with 20 atom% Cu shows less structural change than
the Na[Cu0.1(Fe1/3Mn2/3 0.9]O2 and Na(Fe1/3Mn2/3)O2 electrodes dur-
ing cycling. The Na[Cu0.2(Fe1/3Mn2/3 0.8]O2 electrode slightly trans-
)
)
formed to P3-type phase when the cell was charged to 4.3 V and
then returned to the original O3-type structure when the cell was dis-
charged. On the other hand, the Na(Fe1/3Mn2/3)O2 electrode shows
a rapid phase change from the O3-type rhombohedral to the P3-
monoclinic structure during charge and the peaks become broadened
when the cell was discharged. From these crystal structural change
results, we confirm that the Cu substitution enhances the structural
stability by suppressing the iron migration, which leads to an increase
in specific capacity.
We also evaluated the cycle performances and rate capabilities
of the samples to assess the benefits of Cu substitution (Figures 4a
and 4b). For the cycle performance tests, all cells were tested with
a constant current of 0.2C rate at room temperature. For the rate
capability tests, all cells were charged to 4.3 V with a constant cur-
rent of 0.1C rate and then discharged to 1.2 V with various currents
from 0.1C rate to 2C rate. From these results, we could find that the
As already reported in the literature, iron migration during cycling
is a critical problem with Fe-containing cathode materials for sodium-
ion batteries.4,8,18 Therefore, we recorded the ex-situ XRD patterns of
both the x = 0 and 0.2 samples during the first cycle to explain the im-
provement in the discharge capacity from a structural stability perspec-
tive. Figure 3c and Figure S1 (see supplementary material) show the
ex-situ XRD results of Na(Fe1/3Mn2/3)O2, Na[Cu0.1(Fe1/3Mn2/3
)0.9]O2,
and Na[Cu0.2(Fe1/3Mn2/3 0.8]O2 electrodes at different states of charge
)
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5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
(c)
Pristine
3.8 V charge
4.3 V charge end
2.7 V discharge
1 V discharge end
H. P3
H. O3
Na(Fe1/3Mn2/3)O2
Al-foil
Na(Cu0.1Fe0.3Mn0.6)O2
Na(Cu0.2Fe0.27Mn0.53)O2
(a)
30 32 34 36 38 40
42
44
46
30
35
40
45
50
55
60
65
70
0
30
60
90 120 150 180
Specific Capacity / mAh g-1
2θ / degree
2θ / degree
2
θ / degree
(b)
Mn3+
Pristine
3.8 V charge
4.3 V charge end
2.7 V discharge
1 V discharge end
Mn4+
H. O3
Fe3+
Cu2+
Cu3+
Fe4+
Al-foil
30
40
50
60
70
30 32 34 36 38 40
936 934 932
714 712 710
644 642 640
42
44
46
2θ / degree
2θ / degree
Binding energy
/
eV
2θ / degree
Figure 3. (a) Comparison of the first charge-discharge curves of Na/Na[Cux(Fe1/3Mn2/3
with different states of charge (SOC) in the first cycle (Black: pristine state, Red: middle of charge, Blue: end of charge in Fig. 3(a)). (c) Comparison of the
)
1-x]O2 cells (x = 0, 0.1, and 0.2). (b) XPS results of the x = 0.2 sample
structural change of the x = 0 and 0.2 samples with different SOC and depths of discharge (DOD) in the first cycle.
Na[Cu0.2(Fe1/3Mn2/3
)
0.8]O2 electrode exhibits the best cycle retention
doping. We also confirmed the structural stability of the Cu-doped
samples by ex-situ XRD after 30 cycles. As shown in Figure 4c, the
crystal structure of the Na(Fe1/3Mn2/3)O2 electrode partially changed
of 70% during 30 cycles and good rate capability (124 mAh g−1 at
0.2C rate and 70 mAh g−1 at 2C rate) compared to the other samples.
The Na(Fe1/3Mn2/3)O2 sample shows disappointing results in both
cycle and rate performances.
to P3-type monoclinic phase but the Na[Cu0.2(Fe1/3Mn2/3)0.8]O2 elec-
trode shows a single O3-type rhombohedral phase after 30 cycles. For
further investigation of crystal structural changes after 30 cycles, we
recorded the ex-situ HR-TEM and the corresponding electron diffrac-
We believe that these improved performances originate from the
structural stability and suppression of Fe migration facilitated by Cu
200
200
Na(Fe1/3Mn2/3)O2
Na(Fe1/3Mn2/3)O2
(a)
Na(Cu0.1Fe0.3Mn0.6)O2
0.1C
Na(Cu0.1Fe0.3Mn0.6)O2
150
100
50
150
Na(Cu0.2Fe0.27Mn0.53)O2
Na(Cu0.2Fe0.27Mn0.53)O2
0.2C
1C
2C
100
50
0
0.5C
(b)
0
0
5
10
15
20
25
30
0
2
4
6
8
10
Number of Cycles
Number of Cycles
Na(Fe1/3Mn2/3)O2
Na(Cu0.2Fe0.27Mn0.53)O2
P3 phase of Na(Fe1/3Mn2/3)O2
(c)
Al-foil
O3 phase of Na(Cu0.2Fe0.27Mn0.53)O2
Graphite
P
Graphite
Al-foil
P
P
20
30
40
50
60
70
2θ / degree
Figure 4. (a) Cycling performance and (b) rate capability of the Na/Na[Cux(Fe1/3Mn2/3
)
1-x]O2 cells (x = 0, 0.1 and 0.2) charged with a current density of 12 mA
g−1 (0.1C rate) and discharged with different current densities (0.1C rate for cycle test, 0.1C to 2C rates for rate test) at 25◦C. (c) Comparison of the ex-situ XRD
results of the x = 0 and 0.2 electrodes after 30 cycles.
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