A Practical High-Energy Cathode for Sodium-Ion Batteries Based on Uniform P2-Na0.7CoO2 Microspheres

Article abstract of DOI:10.1002/anie.201702024

Layered metal oxides have attracted increasing attention as cathode materials for sodium-ion batteries (SIBs). However, the application of such cathode materials is still hindered by their poor rate capability and cycling stability. Here, a facile self-templated strategy is developed to synthesize uniform P2-Na_0.7CoO₂ microspheres. Due to the unique microsphere structure, the contact area of the active material with electrolyte is minimized. As expected, the P2-Na_0.7CoO₂ microspheres exhibit enhanced electrochemical performance for sodium storage in terms of high reversible capacity (125 mAh g^?1 at 5 mA g^?1), superior rate capability and long cycle life (86 % capacity retention over 300 cycles). Importantly, the synthesis method can be easily extended to synthesize other layered metal oxide (P2-Na_0.7MnO₂ and O3-NaFeO₂) microspheres.

Full text of DOI:10.1002/anie.201702024

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201702024
German Edition: DOI: 10.1002/ange.201702024
Sodium-Ion Batteries
A Practical High-Energy Cathode for Sodium-Ion Batteries Based on
Uniform P2-Na0.7CoO2 Microspheres
Yongjin Fang, Xin-Yao Yu,* and Xiong Wen (David) Lou*
Abstract: Layered metal oxides have attracted increasing
attention as cathode materials for sodium-ion batteries (SIBs).
However, the application of such cathode materials is still
hindered by their poor rate capability and cycling stability.
Here, a facile self-templated strategy is developed to synthesize
uniform P2-Na0.7CoO2 microspheres. Due to the unique
microsphere structure, the contact area of the active material
with electrolyte is minimized. As expected, the P2-Na0.7CoO2
microspheres exhibit enhanced electrochemical performance
for sodium storage in terms of high reversible capacity
(125 mAhgÀ1 at 5 mAgÀ1), superior rate capability and long
cycle life (86% capacity retention over 300 cycles). Impor-
tantly, the synthesis method can be easily extended to
synthesize other layered metal oxide (P2-Na0.7MnO2 and O3-
NaFeO2) microspheres.
Na+ ion extraction and insertion. As for the development of
next-generation LIBs, the development of practical SIBs is
also limited by the lack of suitable cathode materials. Among
the potential cathode materials, phosphates,[20–22] ferrocya-
nides[23–25] and layered metal oxides[26–29] have been intensively
investigated, but their overall performance is still generally
unsatisfactory for practical use of SIBs. Layered metal oxides
have been considered as a class of promising cathode
materials for SIBs, due to the rich family and various
electrochemical active elements.[14] The layered metal oxides
can be catalogued into two main groups: O3 and P2 type
structures, where Na+ ions tend to locate at octahedral and
prismatic sites, respectively.[27] O3 type oxides possess a stable
structure with reversible Na+ ion insertion/deinsertion, but
the reversible capacity can hardly exceed 120 mAhgÀ1
.
Although P2 type oxides can deliver higher capacity, the
cycling stability is usually less satisfactory due to the multi-
phases transition during electrochemical reactions.[30] Up to
now, the investigation of layered metal oxides mainly focuses
on the adjustment of the active metal center and ratio,
determination of reaction mechanisms and structural analysis.
However, very few attention has been paid to the morpho-
logical effect on electrochemical performance for cathode
materials of SIBs.[15,27] It is known that cathode materials with
regular morphology and microsize could endow the electro-
des with high capacity, long-term cyclability and high energy
density for SIBs.[31–34] For example, Hwang et al. have
reported an O3 type layered compound with a radially
aligned hierarchical columnar structure in spherical particles
showing high specific capacity, good capacity retention and
rate capability for SIBs.[32] Zhang et al. have synthesized
graphene decorated Na3V2(PO4)3 microspheres, which enable
high rate capability with long cycle life for sodium storage.[31]
These results indicate that cathode materials with micro-
sphere structure indeed lead to enhanced sodium storage
performance. On the other hand, microspheres can also
improve the volumetric energy density of the electrodes and
more importantly can be easily processed to obtain uniform
electrode films, which are very favorable for practical
batteries.
R
apid growth of renewable electric energy from solar, wind
and other sources in the global energy markets has spurred
the development of low-cost and effective energy storage
systems.[1–4] As lithium-ion batteries (LIBs) expand their
territory from portable electronics to electric vehicles, a great
concern arises about the widespread availability and rising
price of lithium resources.[5,6] Therefore, exploration of
alternatives to LIBs has been highlighted in recent years
and a number of battery systems such as lithium-sulfur,[7–9]
lithium-air[10–13] and rechargeable sodium-ion batteries
(SIBs)[14–17] have been revisited as possible candidates of
post Li-ion technologies. Among them, SIBs have been
considered as an ideal choice due to their similar chemical/
electrochemical reaction mechanisms to LIBs, low cost and
abundant sodium resources.[16,18,19]
SIBs have attracted increasing attention in the past
decade and many materials have been explored as electrodes
for SIBs. However, due to the larger size of Na+ ion compared
to Li+ ion, it is a more arduous challenge to explore suitable
electrode materials with robust structure for stable reversible
[*] Dr. Y. J. Fang, Dr. X. Y. Yu, Prof. X. W. Lou
School of Chemical and Biomedical Engineering
Nanyang Technological University
62 Nanyang Drive, Singapore 637459 (Singapore)
E-mail: xwlou@ntu.edu.sg
Homepage: http://www.ntu.edu.sg/home/xwlou/
Herein we present a facile strategy to synthesize uniform
P2-Na0.7CoO2 microspheres through a two-step self-templat-
ing method, as schematically illustrated in Figure 1. First,
CoCO3 microspheres synthesized via a modified solvothermal
method are converted to Co3O4 microspheres by calcination
in air. Then, the Co3O4 microspheres react with an appro-
priate amount of Na2CO3 under elevated temperature to
obtain P2-Na0.7CoO2 microspheres. Although high calcination
temperature is used during the synthesis process, the P2-
Na0.7CoO2 particles can still maintain the spherical morphol-
Dr. X. Y. Yu, Prof. X. W. Lou
State Key Laboratory of Silicon Materials
School of Materials Science and Engineering, Zhejiang University
Hangzhou, 310027 (P.R. China)
E-mail: yuxinyao@zju.edu.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.201702024.
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Figure 1. Schematic illustration of the two-step synthesis of
P2-Na0.7CoO2 microspheres.
ogy. Impressively, the resultant P2-Na0.7CoO2 microspheres
exhibit excellent sodium storage properties with high rever-
sible capacity, superior rate capability and cycling stability.
The synthesis method has been also successfully extended to
synthesize P2-Na0.7MnO2 and O3-NaFeO2 microspheres.
The panoramic field-emission scanning electron micro-
scope (FESEM) image (Figure S1a, Supporting Information
(SI)) shows that the as-synthesized CoCO3 precursor consists
of microspheres with an average diameter of about 5 mm. The
magnified FESEM image (Figure 2a) reveals the rough
surface of the CoCO3 microspheres. After being annealed in
air, the CoCO3 precursor microspheres are transformed into
Co3O4 microspheres, as confirmed by FESEM (Figure S1b,
SI) and powder X-ray diffraction (XRD) measurements
(Figure S1c, SI). Compared with the CoCO3 precursor, no
obvious changes in structure and size can be observed for the
Co3O4 microspheres (Figure 2b). After the second step
calcination with Na2CO3 at elevated temperature, phase-
pure Na0.7CoO2 with good crystallinity is obtained (Fig-
ure 2c). All the diffraction peaks of the XRD pattern can be
indexed to a hexagonal P2-structure Na0.7CoO2 with P63/mmc
space group (JCPDS card No. 87-0274).[35] As demonstrated
in the diagrammatic presentation (inset of Figure 2c), the P2
type layered structure exhibits an AB BA oxygen packing
Figure 3. a) Typical TEM image of P2-Na0.7CoO2 microspheres and
corresponding elemental mapping of b) sodium, c) cobalt, and
d) oxygen elements. The inset in (a) is the EDX spectrum of
P2-Na0.7CoO2 microspheres.
with Co ions in the octahedral sites and Na ions in the
prismatic sites.[36] The large-scale FESEM image of the
product shows that the particles exhibit a morphology of
well-maintained microspheres (Figure S2, SI). This sample of
as-obtained Na0.7CoO2 microspheres is denoted as s-NCO.
Clearly, the surface structure of s-NCO (Figure 2d,e) is
different from that of the CoCO3 and Co3O4 microspheres,
which should originate from the
recrystallization and crystal growth
during the high temperature calci-
nation. The high-resolution trans-
mission
electron
microscope
(HRTEM) image (Figure 2 f)
shows a clear lattice with a spacing
of 0.541 nm, which matches well
with the (002) plane of P2-
Na0.7CoO2. The TEM image (Fig-
ure 3a) demonstrates that the sur-
face of the microsphere is rather
rough. The corresponding energy-
dispersive X-ray spectroscopy
(EDS) analysis (inset in Figure 3a)
shows that the microsphere is com-
posed of Na, Co and O elements
with a Na/Co atomic ratio of ca.
0.71, further confirming the chem-
ical composition of Na0.7CoO2.
Elemental mapping of a part of
the microsphere (Figure 3b–d)
reveals that Na, Co and O ele-
ments are uniformly distributed in
the particle. In addition, to under-
Figure 2. FESEM images of a) CoCO3 and b) Co3O4 microspheres. c) XRD pattern of P2-Na0.7CoO2
microspheres; inset is the corresponding crystal structure of P2-Na0.7CoO2. d,e) FESEM images and
f) HRTEM image of P2-Na0.7CoO2 microspheres.
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stand the effect of morphology on the electrochemical
domains which are the characteristic of first-order transitions
performance of P2-Na0.7CoO2 material, P2-Na0.7CoO2 micro-
particles with irregular morphology (donated as i-NCO) are
also synthesized and evaluated as cathode materials for SIBs
(Figure S3, SI).
during Na+ ions insertion/deinsertion. The sloping curves are
related to the solid-solution behavior and potential drops are
the signature of single-phase in a narrow sodium content
range.[36] Importantly, the s-NCO cathode can deliver a rever-
sible discharge capacity of 125 mAhgÀ1, which is higher than
that of most of the P2-Na0.7CoO2 electrodes reported
previously.[35,40–42] The improved specific capacity may result
from the high crystallinity and phase purity of the Na0.7CoO2
microspheres.[43,44] Moreover, the s-NCO cathode offers an
average discharge potential of 2.9 V, possessing a high energy
density of 363 WhkgÀ1.
The sodium storage properties of s-NCO are first
evaluated by cyclic voltammetry (CV). Figure 4a shows
a typical CV curve of s-NCO at a scan rate of 0.1 mVsÀ1 in
the voltage range of 2.0–3.8 V (vs. Na/Na+). Similar to most
P2 type oxides, the s-NCO cathode exhibits multiple pairs of
redox peaks, indicating a series of complex phase transition
reactions.[27,36,37] The complicated phase transition comes
from the changeable states of Na+/vacancy ordering and
MO2 layer gliding during Na+ ions insertion/deinsertion.[36,38]
Figure 4b shows the representative charge/discharge voltage
profiles of s-NCO between 2.0 and 3.8 V (vs. Na/Na+) at
a current rate of 0.04 C (5 mAgÀ1, 1 C = 125 mAgÀ1). The
discharge profiles exhibit multiple potential drops separated
by either potential plateaus or sloping curves in good
agreement with the CV result and reported literatures.[36,38–40]
The voltage plateaus indicate the existence of biphasic
The rate performance of the s-NCO cathode is inves-
tigated by charging and discharging at various current rates
ranging from 0.04 to 16 C (Figure 4c). As can be seen, the
electrode can deliver an average reversible capacity of 125,
121, 119, 114, 112, 109, 106, 98 and 84 mAhgÀ1 at 0.04, 0.08,
0.16, 0.4, 0.8, 1.6, 2.4, 4 and 8 C, respectively. Even at a high
rate of 16 C (charge/discharge within about 2 min), a remark-
able reversible and stable capacity of 64 mAhgÀ1 can still be
preserved. When the current density is reduced back to
0.16 C, a reversible capacity of
117 mAhgÀ1 can be recovered,
revealing the excellent cycling
reversibility of s-NCO. Also
shown in Figure 4c, the discharge
capacity of the i-NCO electrode
fades rapidly as the current rate
rises and the capacity drops to
almost zero at the current rate of
4 C. The excellent rate capability
of the s-NCO is also verified by the
charge/discharge voltage profiles
at various current rates (Fig-
ure 4d).
Moreover, the s-NCO elec-
trode also exhibits excellent
cycling
stability.
Specifically,
a capacity retention of 86% over
300 cycles at a current rate of 0.4 C
is observed (Figure 4e), corre-
sponding to an average capacity
loss of 0.046% per cycle. The
Coulombic efficiency of the elec-
trode remains around 100%
during cycling. For comparison,
the capacity of the i-NCO cathode
drops rapidly with cycling and
a capacity of only 40 mAhgÀ1 is
maintained after 200 cycles, corre-
sponding to a low capacity reten-
tion of 46% (Figure 4e). Impres-
sively, the s-NCO cathode also
exhibits excellent cycling perfor-
mance at other current rates (Fig-
ure S4, SI), retaining more than
88% of the initial capacity after
200 cycles (Table S1, SI). To the
best of our knowledge, such supe-
Figure 4. a) A typical cyclic voltammogram of the s-NCO electrode at a scan rate of 0.1 mVsÀ1
.
b) Typical charge/discharge voltage profiles of the s-NCO electrode at a current rate of 0.04 C. c) Rate
performance of s-NCO and i-NCO, and d) the corresponding charge/discharge voltage profiles of the
s-NCO electrode at various current rates. e) Cycling performance of s-NCO and i-NCO at a current
rate of 0.4 C and the corresponding Coulombic efficiency of s-NCO. (Note: 1C=125 mAgÀ1.)
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rior rate capability and long cycling stability have not been
Conflict of interest
reported before for P2-Na0.7CoO2 cathodes. The superior
sodium storage properties of s-NCO may be attributed to its
high crystallinity and uniform microsphere structure. The
microsphere structure can effectively buffer the mechanical
stress caused by Na+ ions insertion/deinsertion. It can also
minimize the contact area of the electroactive materials with
electrolyte, thus reducing unwanted side reactions and active
materials dissolution and leading to high Coulombic effi-
ciency.[32] It is also worth mentioning that the measured tap
density of s-NCO (1.8 gcmÀ3) is higher than that of i-NCO
(1.45 gcmÀ3), which is expected to improve the volumetric
energy density of SIBs.
The authors declare no conflict of interest.
Keywords: cathode materials · layered metal oxides ·
microspheres · P2-Na0.7CoO2 · sodium-ion batteries
[1] Z. Wang, L. Zhou, X. W. Lou, Adv. Mater. 2012, 24, 1903.
[2] L. Yu, H. Hu, H. B. Wu, X. W. Lou, Adv. Mater. 2017, 29,
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Importantly, this self-templated synthesis method is quite
general and can be also applied to prepare other layered
metal oxide microspheres, such as P2-Na0.7MnO2 and O3-
NaFeO2 microspheres. All the Mn/Fe carbonates, Mn/Fe
oxides, and as-derived Mn/Fe layered oxides show a similar
microsphere structure (Figure S5, SI). The corresponding
XRD patterns (Figure S6a,b, SI) confirm the successful
synthesis of the P2-Na0.7MnO2 and O3-NaFeO2 phases from
corresponding metal carbonate precursors. EDX analysis
(Figure S6c,d, SI) confirms the elemental compositions of the
P2-Na0.7MnO2 and O3-NaFeO2 microspheres. The as-
obtained P2-Na0.7MnO2 and O3-NaFeO2 microspheres also
exhibit enhanced reversible capacity (Figure S7, SI) com-
pared to the materials in previous studies.[45,46] In addition, it
can be seen that the P2-Na0.7MnO2 and O3-NaFeO2 electro-
des exhibit different charge/discharge profiles with the P2-
Na0.7CoO2 electrode. This can be attributed to the different
electron configurations of the active elements (Co, Mn, Fe),
leading to different stable intermediates with complex
changeable states of Na+/vacancy ordering, charge ordering
and MO2 layer gliding during Na+ ions insertion/deinsertion.
In summary, we have successfully developed a facile and
self-templated method to synthesize NaxMO2 (M = Co, Mn,
Fe) layered metal oxide microspheres. This method first
involves the conversion of metal carbonates to metal oxide
microspheres. Then the metal oxides are converted to
NaxMO2 microspheres after reaction with Na2CO3 at elevated
temperature. As an example, the P2-Na0.7CoO2 microspheres
are thoroughly evaluated as a cathode material for sodium-
ion batteries (SIBs). Benefiting from their structural advan-
tages, the P2-Na0.7CoO2 microspheres exhibit high specific
capacity (125 mAhgÀ1 at 5 mAgÀ1), excellent rate capability
and long-term cycling stability (86% retention in capacity
over 300 cycles). This work might offer some inspiration for
development of advanced cathode materials for practical
deployment of SIBs.
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Manuscript received: February 23, 2017
Final Article published: && &&, &&&&
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Communications
Sodium-Ion Batteries
Layered metal oxide NaxMO2 (M=Co,
Mn, Fe) microspheres have been synthe-
sized via a facile self-templated method
as advanced cathode materials for
sodium-ion batteries. As an example, the
P2-Na0.7CoO2 microspheres exhibit
greatly enhanced electrochemical sodium
storage properties with high reversible
capacity (125 mAhgÀ1 at 5 mAgÀ1),
superior rate capability and long-term
cycle life (86% capacity retention over
300 cycles).
Y. J. Fang, X. Y. Yu,*
X. W. Lou*
&&&& — &&&&
A Practical High-Energy Cathode for
Sodium-Ion Batteries Based on Uniform
P2-Na0.7CoO2 Microspheres
6
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