Layered Ca0.28MnO2·0.5H2O as a High Performance Cathode for Aqueous Zinc-Ion Battery

Article abstract of DOI:10.1002/smll.202000597

Aqueous zinc-ion batteries are promising candidates for grid-scale energy storage because of their intrinsic safety, low cost, and high energy intensity. However, lack of suitable cathode materials with both excellent rate performance and cycling stability hinders further practical application of aqueous zinc-ion batteries. Here, a nanoflake-self-assembled nanorod structure of Ca_0.28MnO₂·0.5H₂O as Zn-insertion cathode material is designed. The Ca_0.28MnO₂·0.5H₂O exhibits a reversible capacity of 298 mAh g^?1 at 175 mA g^?1 and long-term cycling stability over 5000 cycles with no obvious capacity fading, which indicates that the per-insertion of Ca ions and water can significantly improve reversible insertion/extraction stability of Zn²⁺ in Mn-based layered type material. Further, its charge storage mechanism, especially hydrogen ions, is elucidated. A comprehensive study suggests that the intercalation of hydrogen ions in the first discharge plat is controled by both pH value and type of anion of electrolyte. Further, it can stabilize the Ca_0.28MnO₂·0.5H₂O cathode and facilitate the following insertion of Zn²⁺ in 1 m ZnSO₄/0.1 m MnSO₄ electrolyte. This work can enlighten and promote the development of high-performance rechargeable aqueous zinc-ion batteries.

Full text of DOI:10.1002/smll.202000597

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Layered Ca0.28MnO2·0.5H2O as a High Performance
Cathode for Aqueous Zinc-Ion Battery
Tianjiang Sun, Qingshun Nian, Shibing Zheng, Jinqiang Shi, and Zhanliang Tao*
include Prussian blue analogs, vanadium-
Aqueous zinc-ion batteries are promising candidates for grid-scale energy
storage because of their intrinsic safety, low cost, and high energy intensity.
However, lack of suitable cathode materials with both excellent rate
performance and cycling stability hinders further practical application of
aqueous zinc-ion batteries. Here, a nanoflake-self-assembled nanorod
structure of Ca0.28MnO2·0.5H2O as Zn-insertion cathode material is designed.
The Ca0.28MnO2·0.5H2O exhibits a reversible capacity of 298 mAh g−1 at
175 mA g−1 and long-term cycling stability over 5000 cycles with no obvious
capacity fading, which indicates that the per-insertion of Ca ions and water
can significantly improve reversible insertion/extraction stability of Zn2+
in Mn-based layered type material. Further, its charge storage mechanism,
especially hydrogen ions, is elucidated. A comprehensive study suggests that
the intercalation of hydrogen ions in the first discharge plat is controled by
both pH value and type of anion of electrolyte. Further, it can stabilize the
Ca0.28MnO2·0.5H2O cathode and facilitate the following insertion of Zn2+ in
1 m ZnSO4/0.1 m MnSO4 electrolyte. This work can enlighten and promote the
development of high-performance rechargeable aqueous zinc-ion batteries.
based materials, and manganese-based
materials.[3] Among the various types of
cathode materials, MnO2, with higher
capacity (>250 mAh g−1), higher voltage
(1.3–1.5 V), and various crystallographic
polymorphs (α-, β-, γ-, δ-, λ-, and ε-type),
is widely researched in ZIBs.[4,5] Espe-
cially, layered-type δ-MnO2 (birnessite-type
manganese oxides) exhibits a large inter-
spacing channel (≈0.7 nm),[6,7] which is
much more suitable for fast and revers-
ible (de-)intercalation of Zn ions. Unfor-
tunately, the Zn//δ-MnO2 battery system
suffers from bad electrochemical per-
formance, including significant capacity
fading and poor rate capability. According
to literatures, the δ-MnO2 exists serious
structural transformation during cycling,
as well as the dissolution of Mn, which
are the key reasons for the poor cycling
stability of Zn//δ-MnO2 battery.[6,8] More-
over, the strong electrostatic interaction
between Zn2+ and host materials leads to the terrible rate capa-
bility of Zn//δ-MnO2 battery.[9]
1. Introduction
Aqueous Zn-ion batteries (ZIBs) are receiving increasing atten-
tion owing to their unique advantages of Zn anode, which
include high theoretical specific capacity (820 mAh g−1),
relatively low redox potential (−0.76 V vs standard hydrogen
electrode), and appropriate stability in mild acidic electrolyte.[1]
In addition, compared with aqueous alkali metal ion battery
(Li+, Na+, K+), the Zn anode of aqueous ZIBs involves a two-
electron transfer during charge/discharge process to ensure
high energy density (5851 mAh mL−1) and a large storage
capacity.[2] Therefore, aqueous ZIBs promise to be the best
candidate for aqueous metal ion battery in grid-scale electro-
chemical energy storage.
Liu et al. find that extra addition of Mn2+ into the electrolyte
is a good strategy to inhibit the dissolution of Mn2+ from Mn3+
disproportionation.[10] In addition, it is a common strategy in
vanadium-based materials to introduce metal ions (Ca2+, Zn2+,
Na+, Mg2+) and crystalline water as pillars between interlayers
to stabilize the layered structure.[11] These cathode materials
have efficient electrochemical performance due to strong ionic
bonds between metal ions and oxygen atoms in lattice, which
effectively hinder interlayer slip and structure collapse. For
example, Nazar et al. report Zn0.25V2O5·nH2O as cathode mate-
rial for aqueous ZIBs, which shows high capacity (300 mAh g−1
at 0.05 A g−1), long cycling life (1000 cycles at 4.5 A g−1), and
excellent rate performance.[12] Similar to vanadium-based mate-
rials, layered manganese oxides can be studied to improve the
electrochemical performance. So far, however, there have been
few efforts to explore layered manganese oxides with pillar
structure in ZIBs system, which is worth trying.
In addition, the aqueous Zn//MnO2 battery exhibits compli-
cated energy storage mechanism. It is generally believed that
the Zn2+ and H+ ions insert into δ-MnO2 by two steps in ZnSO4/
MnSO4 electrolyte.[5,13] In the first step, the H+ ions insert into
δ-MnO2 and form MnOOH and ZnSO4·3Zn(OH)2·xH2O,
then the Zn2+ ions intercalate into δ-MnO2 and form spinel
ZnMn2O4 in the following step.[14] Moreover, Pan et al. also con-
firm the joint charge storage of Zn2+ and H+ ions of δ-MnO2
However, a lack of robust cathode materials remains a funda-
mental bottleneck to further promoting ZIBs practical applica-
tion. The reported cathode materials for aqueous ZIBs mainly
T. Sun, Q. Nian, Dr. S. Zheng, J. Shi, Prof. Z. Tao
Key Laboratory of Advanced Energy Materials Chemistry
(Ministry of Education)
College of Chemistry
Nankai University
Tianjin 300071, P. R. China
E-mail: taozhl@nankai.edu.cn
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202000597.
DOI: 10.1002/smll.202000597
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Figure 1. a) XRD pattern of CaMnO. b) XPS of Ca 2p; c) O 1s. d) TGA curve of the as-prepared CaMnO sample from 30 to 800 °C. e) Structural illustra-
tion of the as-prepared CaMnO. f) SEM; g) TEM; h) HRTEM, SAED, and EDX of CaMnO.
in 1 m Zn(TFSI)2/0.1 m Mn(TFSI)2. However, Kheawhom et al.
suggest that only Zn2+ ions insert into δ-MnO2 and transform
into δ-ZnMnO2 in 0.5 m ZnSO4 electrolyte.[15] Additionally,
Kang et al. study the mechanism of Zn2+ ions intercalation
in α-MnO2, the formation of ZnMn2O4 clearly confirms the
insertion of Zn2+ into α-MnO2 in 0.5 m Zn(NO3)2 electrolyte.[16]
There is no doubt that the Zn2+ ions can be reversibly inserted/
extracted in MnO2. However, the insertion of H+ ions in MnO2
still remains controversial, and is very sensitive to the local elec-
trolyte. The lack of detailed study of H+ ions’ storage mecha-
nism has limited the development of more efficient and reliable
electrode materials for practical applications.
Here, we design a novel aqueous ZIBs cathode material,
which used Ca ion and water as pillars pre-insert into layered
δ-MnO2, namely, Ca0.28MnO2·0.5H2O (CaMnO), and shows
an excellent electrochemical performance in 1 m ZnSO4/0.1 m
MnSO4 electrolyte. The CaMnO delivers an ultralong cycling
life of 5000 cycles with no obvious capacity fading. Even at a
high current density of 3.5 A g−1, the CaMnO still maintains a
large discharge capacity of 124 mAh g−1. We also demonstrate
that the H+ ions storage is controlled by pH value and type of
anion of the electrolyte, and it can significantly improve the
electrochemical performance of the layered CaMnO cathode.
expansion of δ-MnO2. The detailed elements’ composition of
Ca, O, and Mn is investigated through X-ray photoelectron
spectroscopy (XPS). As shown in Figure 1b, the presence of
Ca is confirmed by the appearance of a peak around 346.0 eV
(corresponding to Ca 2p3/2) and 349.6 eV (corresponding to Ca
2p1/2),[17] and the ratio of Ca/Mn was calculated to be 0.24. The
O 1s spectrum (Figure 1c) shows that three obvious peaks with
binding energy of 529.1, 530.7, and 532.4 eV, corresponding
 
 
 
to Mn O Mn bond, Mn O H bond, and H O H bond,
respectively.[7] The presence of Mn O H bond and H O H
bond indicates that a significant amount of crystal water has
been intercalated into CaMnO. In addition, the detailed com-
position of Mn element is investigated by Mn 2p spectrum.
As shown in Figure S2 in the Supporting Information, about
66.2% Mn4+ and 33.8% Mn3+ exist in as-prepared CaMnO.
The inductively couple plasma optical emission spectrometry
(ICP-OES) result confirms the atomic ratio of Ca:Mn = 0.28:1.
Further, as displayed in Figure 1d, the thermogravimetric
analysis (TGA) indicates that the number of lattice water in
the CaMnO is 0.5. Therefore, the molecular formula of the
CaMnO can be expressed as Ca0.28MnO2·0.5H2O. A sche-
matic structure of CaMnO is displayed in Figure 1e, the Ca
ions and water molecules are inserted between the inter-
layer, which is beneficial for stabilizing the layered structure.
As shown in Figure 1f,g and Figures S3 and S4 in the Sup-
porting Information, the scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) images indicate
that the CaMnO is composed of nanorods and nanospheres,
formed by uniform interconnected nanoflakes. The nano-
topography of CaMnO provides abundant contact interfaces
between electrode and electrolyte, which can shorten the Zn2+
ion diffusion channel and enable fast reaction kinetics. The
high-resolution TEM (HRTEM) image (Figure 1h) shows clear
lattice fringes with a spacing of 0.63 nm, corresponding to
the characteristic (001) plane of CaMnO. In addition, the (001)
and (002) planes can be clearly observed in the selected area
 
 
2. Results and Discussion
The birnessite-type CaMnO is synthesized by a simple one-
step hydrothermal method. The as-obtained sample is char-
acterized by X-ray diffraction (XRD). As shown in the XRD
pattern (Figure 1a), the (001) and (002) peaks corresponding to
the layered structure (JCPDS No. 80-1098) of CaMnO, and no
obvious impurity peaks are detected. It is noted that the (001)
peak of CaMnO shifts to lower degree compared to δ-MnO2
(Figure S1, Supporting Information). It may originate from
the big volume effect of hydrate Ca ion, thus causes the layer
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Figure 2. a) CV curves of CaMnO and δ-MnO2 at 0.5 mV s−1 (in the second cycle). b) Charge–discharge curves of CaMnO and δ-MnO2 at 350 mA g−1
(in the fifth cycle). c) Rate performance of CaMnO and δ-MnO2. d) Cycling stability of CaMnO and δ-MnO2 at 1500 mA g−1.
electron diffraction (SAED) ring pattern. The energy-disper-
sive spectroscopy (EDX) elemental mapping images show that
the Ca, Mn, and O elements are evenly distributed in CaMnO
nanoflakes. The Raman spectrum of CaMnO in Figure S5a
in the Supporting Information shows the characteristic peak
are shown in Figure 2b. The plateaus of charge/discharge
can be obviously discerned, and the CaMnO exhibits a high
discharge specific capacity of 277.7 mAh g−1 at 350 mA g−1,
while the δ-MnO2 only has a discharge specific capacity of
179 mAh g−1. It is worth noting that the overpotential of CaMnO
is lower than δ-MnO2, which indicates rapid reaction kinetics
of CaMnO. As shown in Figure S8 in the Supporting Informa-
tion, the CaMnO electrode delivers a higher discharge voltage
of ≈1.5 V versus Zn/Zn2+ and specific capacity of 298 mAh g−1
at 175 mA g−1, which is close to the theoretical specific capacity
of δ-MnO2. Moreover, CaMnO electrode possesses an excel-
lent rate performance than δ-MnO2, and still obtains a high
discharge specific capacity of 124.5 mAh g−1 at consider-
ably high current density of 3500 mA g−1 (Figure 2c). As dis-
played in Figure 2d, both CaMnO and δ-MnO2 undergo an
activation process in the first 50 cycles. The CaMnO electrode
maintains a high discharge specific capacity of 135 mAh g−1
at 1500 mA g−1, while the δ-MnO2 electrode shows the rapid
capacity fading with only 54.9 mAh g−1 after 300 cycles. The
impressive capacity retention of CaMnO electrode demon-
strates that the incorporation of Ca ions and water in layered
δ-MnO2 would maintain the structure stability, thus sustain a
more stable cycle life in aqueous ZIBs.
at 252 cm−1 corresponding to stretching vibration of Ca O,

which cannot be observed in δ-MnO2 (Figure S5b, Supporting
Information).[18] The Raman spectrum further confirms that
the CaMnO sample is successfully synthesized. To confirm
the stability of Ca2+ in CaMnO, the XRD patterns of CaMnO
with different soaking times in 1 m ZnSO4/0.1 m MnSO4
electrolyte are collected. Figure S6 in the Supporting Informa-
tion shows that the (001) peak can be clearly observed even
soaking after 2 weeks, which indicates significant stability of
Ca ions in CaMnO.
To investigate the effect of pre-insertion of Ca ions and water
in δ-MnO2, the electrochemical performance of CaMnO and
δ-MnO2 electrodes is tested in coin cells using Zn foil anode,
glass fiber separator, and 1 m ZnSO4/0.1 m MnSO4 electrolyte.
The addition of manganese salt in electrolyte inhibits the Mn2+
dissolution. The cyclic voltammetry (CV) plots of CaMnO and
δ-MnO2 electrodes are collected within the voltage window
of 0.4–1.9 V versus Zn/Zn2+ at 0.5 mV s−1. The five cycle CV
curves of CaMnO and δ-MnO2 (Figure S7a,b, Supporting Infor-
mation) are collected at 0.5 mV s−1. The difference between
the first cycle and sequent four-turn cycles indicates that the
first cycle is irreversible. Moreover, the increasing response
current represents a pre-activation process of fresh CaMnO
and δ-MnO2. Compared with δ-MnO2, the CaMnO electrode
exhibits greater response current, corresponding to the higher
capacity (Figure 2a). The galvanostatic charge–discharge curves
of CaMnO and δ-MnO2 under a current density of 350 mA g−1
To further understand the intrinsic reasons of excellent rate
performance and long-term cycling stability of CaMnO, we ana-
lyze the electrochemical kinetics by the CV curves at different
scan rates. As shown in Figure 3a, two discharge peaks and
one charge peak are obviously observed, and these peaks show
the same shape. The relationship of scan rate and peak current
obeys the following equation[19]
i = avb
(1)
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Figure 3. a) CV curves of CaMnO at different scan rates. b) Log(peak current) versus Log(scan rate) plots of three peaks in CV curves. c) The charge/
discharge GITT profiles of the Zn//CaMnO. d) Diffusion coefficients calculated from the GITT potential profiles for CaMnO (in terms of charge states
in the second cycle). e) Cycling stability of CaMnO at 3500 mA g−1.
where i refers to the peak current (mA), v refers to the scan
rate (mV s−1), and a and b are adjustable parameters. The
b value reflects the charge storage mechanism, when the b = 1,
corresponding to surface-controlled process, while b = 0.5, sug-
gesting a diffusion-controlled process. The calculated values of
b for peak 1, peak 2, and peak 3 are 0.77, 0.79, and 0.56, respec-
tively, which imply that the solid-state ions diffusion is the rate-
limiting step for charge storage (Figure 3b). Furthermore, the
capacitive contributions of CaMnO electrode at different scan
rates are further calculated by simple CV method. The results
are summarized in Figure S9 in the Supporting Information,
the capacitive contribution is 66% at 0.1 mV s−1 and gradu-
ally increasing to 84% at 0.4 mV s−1, indicates fast reaction
kinetics of CaMnO electrode in aqueous ZIBs. On the other
hand, the galvanostatic intermittent titration technique (GITT)
is applied to estimate the ion diffusion coefficient in CaMnO
electrode. The calculation method is according to the following
equation:[20]
are the change of quasi-equilibrium voltage and battery voltage,
respectively. As shown in detailed drawing of Figure 3c, the
system experiences a pulse of 3 mA for 10 min after a rest of
40 min. The calculation results are displayed in Figure 3d, the
ions diffusion coefficients of CaMnO at discharge state are
ranging from 2.6 × 10−11 to 5.3 × 10−9 cm2 s−1, indicate the fast
ion diffusion kinetics. The ions diffusion coefficients also can
be calculated by reported CV method. As shown in Figure S10
in the Supporting Information, the diffusion coefficients are
calculated to be 2.9 × 10−9, 2.6 × 10−9, and 1.0 × 10−8 cm2 s−1
for peak 1, peak 2, and peak 3, which matched well with GITT.
This property can be further confirmed by the electrochem-
ical impedance spectroscopy (EIS) analysis, the Zn//CaMnO
and Zn//δ-MnO2 cells are rested for 30 min to the interfacial
equilibrium. As shown in Figure S11a in the Supporting Infor-
mation, the two systems have big semicircle, which indicates
the quite high interfacial charge transfer resistances for initial
cathodes. Then, with five-cycle activation process, the interfa-
cial charge transfer resistances for CaMnO and δ-MnO2 rap-
idly decrease (Figure S11b, Supporting Information). Note that,
the interfacial charge transfer resistances of CaMnO are more
lower than δ-MnO2, which is likely benefited from the lowered
Zn-adsorption energy by Ca ions pre-inserted in δ-MnO2.[17]
Benefited by the good rate performance, the long-term
stability of CaMnO is further tested at higher current density.
2



2


4
nmV
S
∆E
∆E
m 
D =


s 
t 
(2)
πτ
where nm, Vm, and S are the moles of active materials, molar
volume, and contact area between electrode and electrolyte,
respectively. τ is the time duration of the pulse. ΔEs and ΔEt
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Figure 4. a) CV curves of CaMnO in ZnSO4/MnSO4 and Zn(CH3COO)2/Mn(CH3COO)2 electrolyte at 0.2 mV s−1 (in the second cycle). b) Charge–
discharge curves of CaMnO in 1 m ZnSO4/0.1 m MnSO4 and 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electrolyte at 350 mA g−1 (in the fifth cycle).
c) Cycling stability of CaMnO in Zn(CH3COO)2/Mn(CH3COO)2 electrolyte at 1500 mA g−1.
As displayed in Figure 3e, the CaMnO electrode achieves a dis-
charge specific capacity of 100.9 mAh g−1 after 5000 cycles at
3500 mA g−1. The remarkable cycling life surpasses most of the
previous reported aqueous ZIBs based on MnO2 (Table S1, Sup-
porting Information).
4.2. As shown in Figure S12a in the Supporting Information,
the CV curves of CaMnO in Zn(CH3COO)2/Mn(CH3COO)2/
CH3COOH electrolyte display two discharge peaks in the
previous three cycles. While the following cycle curve only
has one peak, which may be due to the decrease of activity of
hydrogen ions with complexation between H+ and CH3COO−.
Figure S12b in the Supporting Information indicates that the
addition of H+ ions can improve the discharge specific capacity.
The results indicate that the insertion of H+ ions is controlled
by the pH of the electrolyte. Besides, the cycling life of CaMnO
electrode in 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 elec-
trolyte is tested at 1500 mA g−1. As shown in Figure 4d, the
CaMnO electrode achieves an initial discharge specific capacity
of 107.3 mAh g−1, and only has 39.4% capacity retention over
300 cycles in 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electro-
lyte, while the CaMnO in 1 m ZnSO4/0.1 m MnSO4 electrolyte
has high specific capacity and capacity retention (Figure 2d).
The excellent electrochemical performance of CaMnO in 1 m
ZnSO4/0.1 m MnSO4 electrolyte indicates that the H+ ions can
improve the stability of CaMnO in first step of ions insertion,
and benefits the Zn ions insertion into CaMnO in the following
step, thus maintaining the long-term cycling life.
It is generally believed that the charge storage mechanism
in the Zn//MnO2 system is the cooperative insertion of H+ and
Zn2+ ions in ZnSO4/MnSO4 electrolyte. However, it is unclear
what factors affect the H+ ions’ insertion, and the potential
effect of the insertion of H+ ions on the electrochemical per-
formance of Zn//MnO2 system is uncertain. Thus, the elec-
trochemical performances of CaMnO in different electrolytes
are tested. The 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 elec-
trolyte is used as control group due to its moderate pH (about
5.8), while 1 m ZnSO4/0.1 m MnSO4 electrolyte is mildly acidic
(about 4.2). As shown in Figure 4a, the CaMnO electrode dis-
plays two discharge peaks in 1 m ZnSO4/0.1 m MnSO4 electro-
lyte, corresponding to H+ (1.4 V vs Zn/Zn2+) and Zn2+ (1.2 V vs
Zn/Zn2+) ions insertion, respectively.[13,21] It is worth noting that
only one discharge peak at 1.3 V versus Zn/Zn2+ is discerned
in 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electrolyte, which
may only be the insertion of Zn2+ ions.[16] Simultaneously, the
CaMnO electrode only has one discharge plat (Figure 4b) in
1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electrolyte, and the
discharge specific capacity nearly reduces by half compared
to 1 m ZnSO4/0.1 m MnSO4 electrolyte, only has 127 mAh g−1.
To further understanding whether the pH of electrolyte will
affect the H+ ions insertion, we use CH3COOH to adjust the
pH of 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electrolyte to
To further understanding the charge storage mechanism
of CaMnO electrode, the XRD patterns of CaMnO electrode
at the selected states are collected. As shown in Figure 5a, the
typical ZnSO4·3Zn(OH)2·H2O phase appears during the dis-
charge processes, and gradually disappears during the charge
processes, which suggests that H+ ions can reverse insertion/
extraction.[21] Note that, the CaSO4·2H2O phase is observed,
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Figure 5. a) Ex situ XRD patterns of CaMnO during the first discharge and second charge/discharge cycle at 175 mA g−1 at different discharge and
charge voltages. b,c) Ex situ SEM patterns of CaMnO, b) discharge to 0.4 V; c) charge to 1.9 V. d) HRTEM image of the CaMnO at discharged state
(discharge to 0.4 V). e) STEM-EDS elemental mapping images of the CaMnO at discharged state (discharge to 0.4 V). f,g) The ex situ XPS spectra of
Mn 2p and Zn 2p, f) Mn 2p; g) Zn 2p.
and is not disappeared during cycling processes. It may be due
to the overcharging of initial cycle causing Ca2+ ions to pre-
cipitate from the CaMnO, then the Ca2+ ions form insoluble
precipitate attaches to the CaMnO electrode surface. In addi-
tion, the (001) and (002) peaks of CaMnO phase are insig-
nificant during the cycling processes. It is possible that the
by-products on the surface of the electrode weaken the XRD
intensity of CaMnO. Besides, the ex situ XRD of CaMnO in 1 m
Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electrolyte is measured.
As displayed in Figure S13 in the Supporting Information, the
ZnMn2O4 (JCPDS No. 77–0470) phase exists in the discharge
state, and disappears in charge state. Meanwhile, the CaMnO
phase also is observed during the charge/discharge progress.
The MnOOH phases are not detected in discharge state owing
to without H+ ions insertion. The ex situ SEM analysis further
confirms the charge storage mechanism of CaMnO cathodes
in different electrolytes. As shown in Figure 5b, the CaMnO
electrode is covered by a layer of flake-like compounds at full
discharge. Interestingly, some small-sized dispersed flake-
like compounds are still present on the electrode surface
(Figure 5c), which are the resulting CaSO4·2H2O precipitates
during charge/discharge processes. The SEM elemental map-
ping in Figure S14 in the Supporting Information further
confirms the results of XRD and SEM analysis. Moreover,
no dendrites and by-product are found at Zn foil surface in
either the charged or discharged state by SEM and XRD pat-
terns (Figure S15a–d, Supporting Information), which suggests
the excellent stability of Zn anode in 1 m ZnSO4/0.1 m MnSO4
electrolyte. Meanwhile, we test the ex situ SEM of CaMnO
electrode in 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electro-
lyte. As shown in Figure S16a,b in the Supporting Informa-
tion, no flake-like compounds exist on the electrode surface at
discharge state, which is consistent with the XRD results. The
HRTEM analysis of CaMnO electrode at full discharge in 1 m
ZnSO4/0.1 m MnSO4 electrolyte is shown in Figure 5d. A lattice
fringe with d-placing of 0.65 nm is observed, corresponding
to the (001) place of CaMnO, which has hardly changed
compared with the initial state (0.63 nm). According to the
literature, the Zn2+ ions insertion will cause a lattice contrac-
tion due to the effect of interlayer electrostatic attraction and
the expulsion of water from the interlayer, while the hydrated
H+ ions insertion will result in lattice expansion owing to the
large hydration radius.[22] The two opposite effects cancel each
other out, and maintain constant layer spacing during the
charge/discharge cycles. There are other two types of lattice
fringes in the discharge products, corresponding to (150) d = 0.26 nm
plane of ZnSO4·3Zn(OH)2·H2O and (−1−12)
plane of
d = 0.28 nm
CaSO4·2H2O, respectively. Furthermore, scanning transmission
electron microscope (STEM)-EDS mapping analysis indicates
that the discharge CaMnO cathode is composed of nanoflakes
(CaMnO) and two sizes of flakes (ZnSO4·3Zn(OH)2·H2O and
CaSO4·2H2O). As shown in Figure 5e, the distribution elements
in nanoflakes consist of Ca, Mn, O, and Zn, indicate that the Zn
ions successfully insert into CaMnO. The bigger size of flake
includes Zn, S, O elements and is without single Ca element,
corresponding to ZnSO4·3Zn(OH)2·H2O flaker. In contrast,
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Ca, O, and S are uniformly distributed in the smaller flaky
precipitate (the marked place with circle) with nosignal of Zn,
corresponding to CaSO4·2H2O. The results of STEM-EDS
mapping is evidenced by ex situ XRD and HRTEM. In addi-
tion, the HRTEM of full discharge CaMnO cathode in 1 m
Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electrolyte is shown in
Figure S17 in the Supporting Information. The lattice fringes of
CaMnO and ZnMn2O4 can be observed in the flaker, which con-
firms that there is only Zn ions insertion in CaMnO electrode.
Notably, the lattice spacing of (001) plane of CaMnO shrinks
about 0.05 nm, which may originate only from the insertion
of Zn ion. The STEM-EDS mapping (Figure S18, Supporting
Information) shows the apparent signals of Ca, Mn, O, and Zn
elements, which is consistent with the ex situ XRD analysis. The
Nyquist plots for the CaMnO electrode in different electrolytes
are obtained after five-cycle activation. As shown in Figure S19
in the Supporting Information, the charge transfer imped-
ance of CaMnO in 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2
electrolyte is much larger than that in 1 m ZnSO4/0.1 m
MnSO4 electrolyte, which suggests that the Zn ions in 1 m
ZnSO4/0.1 m MnSO4 electrolyte are more easily diffused into
CaMnO cathode. Note that, the Nyquist plots of CaMnO in 1 m
ZnSO4/0.1 m MnSO4 electrolyte have two semicircles as seen in
the detailed image. The first semicircle may be corresponded
to charge transfer impedance between the interface of electro-
lyte and CaSO4·2H2O precipitation on the electrode surface,
and the second semicircle can be attributed to the interfacial
charge transfer resistances between electrolyte and CaMnO
electrode. To verify whether the CaSO4·2H2O precipitation
has an effect on electrochemical performance of CaMnO, we
design a set of controlled tests. The CaMnO cathode is disas-
sembled from coin cell after five-cycle activation to produce
enough CaSO4·2H2O precipitation on the electrode surface.
Then the processed CaMnO electrode is reassembled to coin
cell in 1 m Zn(CH3COO)2/0.1 m Mn(CH3COO)2 electrolyte. As
shown in Figure S20 in the Supporting Information, compared
to untreated electrode, both the specific discharge capacity and
capacity retention of CaMnO have not improved. It suggests
that the CaSO4·2H2O precipitation has no effect on the electro-
chemical performance of CaMnO electrode. We also investigate
the evolution of Mn valence states during discharge/charge
processes by XPS spectra. As shown in Figure 5f, the CaMnO
includes great amount of Mn4+ in charge state, which signifi-
cantly decrease in discharge state. The results indicate the
decrease of Mn valence due to the intercalation of H+ and Zn2+
during discharge processes. As shown in Figure 5g, two obvious
peaks located at 1022 and 1045.1 eV are, respectively, attributed
to Zn 2 p3/2, and Zn 2 p1/2 in discharge state, and the reduced
amounts in the charged state suggest the reversible de-/inser-
tion of Zn2+ ions. The cycling stability of CaMnO electrode in
1 m ZnSO4/0.1 m MnSO4 electrolyte is further confirmed by
SEM patterns. Figure S21a–f in the Supporting Information
shows the morphology evolution of CaMnO cathode during
charge/discharge cycling. The morphology of CaMnO remains
basically unchanged up to 5000 cycles, only the amount of
nanoflakes is decreasing. Coincidently, the capacity also begins
to slightly drop from 4000th cycle (Figure 3e), accompanying
the gradual disappearance of the hierarchical structure. There-
fore, the per-intercalation of Ca ion and water in δ-MnO2 can
enhance the structural stability and provide more cushion for
relieving the strain generated from the Zn2+ and H+ insertion/
extraction.
3. Conclusion
In summary, we report a calcium ions and water-stabilized
Ca0.28MnO2·0.5H2O as cathode for aqueous ZIBs. We prove that
the incorporation of hydrated calcium ion can intrinsically sta-
bilize the layered structure of CaMnO cathode. Benefiting from
this, the Zn//CaMnO battery achieves long cycling life over
5000 cycles with no obvious capacity fading, and excellent rate
performance with a discharge specific capacity of 124.5 mAh g−1
at 3500 mA g−1. Further, we explore both the factor and effect
of H+ insertion using different electrolytes. The H+ diffusion is
controlled by the pH and type of anion of electrolyte. Moreover,
the insertion of H+ in first step can stabilize the layer struc-
ture of cathode, and is in favor of the intercalation of Zn2+ in
following step. Overall, the significant electrochemical perfor-
mance of CaMnO makes the battery promising for future com-
mercial ZIBs. Meanwhile, the strategy in this work to improve
the performance of Mn-based material is expected to be general
for many other battery material designs.
4. Experimental Section
Preparation of Electrode Materials: CaMnO was synthesized by
hydrothermal method. CaCl2, KMnO4, and MnSO4 with a molar ratio
of 1:6:1 were dissolved in deionized water. The obtained solution was
transferred into a 100 mL Teflon-lined stainless steel autoclave and
maintained at 160 °C for 12 h. Last, the product was centrifuged, washed
with deionized water several times, and dried in a vacuum at 60 °C for
12 h. δ-MnO2 was synthesized with similar methods. Briefly, KMnO4
and MnSO4 were homogenously mixed in deionized water with molar
ratio of 6:1. Then the obtained solution was put into a Teflon-contained
autoclave and heated at 160 °C for 12 h. The resulting black precipitates
were filtered, washed with deionized water several times, and then dried
at 60 °C in a vacuum overnight.
Characterizations: The crystallographic structure of the pristine
materials was analyzed by XRD (RigakuMiniFlex600 X-ray diffractometer
with Cu Ka). The morphologies and microstructures of the powers were
examined by SEM (JEOLJSM-7500F) and HRTEM (FEITalosF200XG2).
XPS (PHI5000VersaProbeESCALAB250xi) was used to characterize
the intermediates at various charge/discharge states. The Raman
spectra of CaMnO and δ-MnO2 were tested by Raman spectroscopy
(Thermo-Fisher Scientific, excitation from an argon-ion laser excitation
wavelength, 532 nm). The structural water content in the as-prepared
CaMnO cathode was measured by TGA from 30 to 800 °C with a heating
rate of 10 °C min−1 in Ar using a TG-DSC (thermogravimetry-differential
scanning calorimetry) analyzer (NETZSCH, STA449F3). The molar ratio
of Ca to Mn was measured using ICP-AES (ICAP 7400, Thermo).
Electrochemical Measurements: The CaMnO electrodes were prepared
by mixing CaMnO, Ketjen black (KB), and polytetrafluoroethylene
(PTFE) at an appropriate weight ratio of 8:1:1, and pressed onto
stainless-steel mesh. Then the electrode films were dried at 80 °C for
12 h under vacuum. The CaMnO mass loading was 1–2 mg cm−2. The
δ-MnO2 electrodes were fabricated by pressing a mixture of a mass
ratio of 80 wt% active material, 80 wt% KB, and 10 wt% PTFE onto a
stainless-steel mesh and dried under vacuum at 80 °C for 12 h. The δ-
MnO2 mass loading was 1–2 mg cm−2. 1 m ZnSO4 + 0.1 m MnSO4 or
1 m Zn(CH3COO)2 + 0.1 m Mn(CH3COO)2 was used as electrolyte for
full-cells test. CV and EIS were tested on Solartron 1400 electrochemical
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Supporting Information
Supporting Information is available from the Wiley Online Library or
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Acknowledgements
This study was supported by the National Key R&D Program of
China (2016YFB0901500); the National Natural Science Foundation
of China (51771094, 21835004, and 21571148), Ministry of Education of
China (B12015 and IRT13R30), and Tianjin Natural Science Foundation
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