A high-energy-density aqueous zinc-manganese battery with a La-Ca co-doped ?-MnO2cathode

Article abstract of DOI:10.1039/d0ta03706k

Aqueous zinc-manganese dioxide batteries (Zn//MnO2) are gaining considerable research attention for energy storage taking advantage of their low cost and high safety. However, the capacity and cycling stability of the state-of-the-art devices are still utterly disappointing because of the inevitable MnO2 dissolution and its low conductivity. In this work, to elevate the energy density of Zn//MnO2, a La-Ca co-doping strategy is proposed to boost the electrochemical performance of the ?-MnO2 cathode. Specifically, the introduction of heteroatoms, La3+ and Ca2+, is achieved via a facile one-step liquid coprecipitation method. Our experimental results reveal that Ca2+ significantly improves the stability of ?-MnO2 while Ca2+ and La3+ both contribute to the capacity and reversibility enhancement. Therefore, the overall performance of the La-Ca co-doped ?-MnO2 cathode exceeds the pristine sample, as demonstrated by its commendable capacity of 297.3 mA h g-1 at 0.2 A g-1, superior cycle stability (up to 76.8% capacity retention after 200 cycles) and excellent rate capability (161 mA h g-1 when the current density increased to 1.6 A g-1). Besides, the assembled Zn//?-MnO2 device delivers a maximum energy and power density of 401.22 W h kg-1 and 5.2 kW kg-1 respectively, outperforming most of the recently reported Zn//MnO2 counterparts. This journal is

Full text of DOI:10.1039/d0ta03706k

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A High-Energy-Density Aqueous Zinc-Manganese Battery with a La-
Ca Co-Doped ε-MnO2 Cathode
Manshu Zhang, ‡a Weixing Wu, ‡a Jiawei Luo, a Haozhe Zhang, a Jie Liu,c Xiaoqing Liu,*a Yangyi Yang
Received 00th January 20xx,
Accepted 00th January 20xx
*a and Xihong Lu *a,b
DOI: 10.1039/x0xx00000x
Aqueous zinc-manganese dioxide batteries (Zn//MnO2) are gaining considerable research attention for energy storage taking
advantages of their low cost and high safety. However, the capacity and cycling stability of the state-of-the-art devices are
still utterly disappointing because of the inevitable MnO2 dissolution and its low conductivity. In this work, to elevate the
energy density of Zn//MnO2, a La-Ca co-doping strategy is proposed to boost the electrochemical performance of the ε-
MnO2 cathode. Specifically, the introduction of heteroatoms, La3+ and Ca2+, is achieved via a facile one-step liquid
coprecipitation method. Our experimental results reveal that Ca2+ significantly improves the stability of ε-MnO2 while Ca2+
and La3+ both contribute to the capacity and reversibility enhancement. Therefore, the overall performance of the La-Ca co-
doped ε-MnO2 cathode exceeds the pristine sample, as demonstrated by its commendable capacity of 297.3 mAh g-1 at 0.2
A g-1, superior cycle stability (up to 76.8% capacity retention after 200 cycles) and excellent rate ability (161 mAh g-1 when
the current density increased to 1.6 A g-1). Besides, the assembled Zn//ε-MnO2 device delivers a maximum energy and power
density of 401.22 Wh kg-1 and 5.2 kW kg-1 respectively, outperforming most of recently reported Zn//MnO2 counterparts.
nonchargeable. In recent years, researchers have turned to
neutral electrolytes to realize the upgradation of primary
Zn//MnO2 to rechargeable ones. Pitifully, because of the
inevitable MnO2 dissolution and its low conductivity (10-5 – 10-6
S cm-1), the cycling stability and capacity of the state-of-the-art
Zn//MnO2 is still utterly disappointing, which significantly
hinders their further applications.
Introduction
The depletion of fossil energy and increasingly serious
environmental problems stimulate the ever-growing demand
for renewable energy utilization, which attaches great
importance to the development of efficient energy storage
device.1 Secondary batteries with high capacity, long life, and
excellent rate performance offer an alternative solution for
aforementioned issues. Lithium-ion batteries are the most
widely used and mature batteries. However, the flammable
electrolyte and the limited lithium resources have hindered
their further development.2 Aqueous zinc ion batteries (ZIBs)
stand out as an ideal candidate owing to the unique advantages
of zinc anode, including its safe nature, high theoretical capacity
(820 mAh g-1), low redox potential (-0.763 V versus standard
hydrogen electrode, SHE), low cost ($1550 per ton) and eco-
friendliness. In contrast to other conventional energy storage
systems like supercapacitors and LIBs, ZIBs hold greater
potential in modulating the intermittency nature of renewable
source like solar and wind.3-4 As the earliest developed ZIBs,
alkaline zinc- manganese dioxide batteries (Zn//MnO2) are
MnO2 of various crystal phases have been reported as ZIBs
cathodes, including α, β, γ, δ and Todorokite-type MnO2.5-9 Sharing
the similar 1*1 and 1*2 tunnel-like structure to γ-MnO2, which can
serve as the Zn2+ storage site, akhtenskite MnO2, also known as ε-
MnO2, is also a decent host for Zn2+ accomondation.10 However,
there is still few discussions about ε-MnO2 as the cathode of ZIBs.
Besides, like other phases of MnO2, the limited existing works show
that the capacity and energy density of ε-MnO2-based ZIBs remain
deficient.11-13 For example, Wang achieved a long-term cycling
stability of Zn//ε-MnO2 (500 cycles with 92% capacity retention)
using an ε-MnO2 cathode derived from Mn3O4 by cyclic voltammetry.
But its capacity and energy density were limited to 221 mA h g-1 and
~288 Wh kg-1 at 0.4 C (0.1 A g-1). Moreover, once the discharge rate
increased to 4.3 C, its capacity further decreased to less than 100
mAh g-1. Another Zn//ε-MnO2 battery with both long lifespan and
high initial capacity (290 mAh g-1 at 0.3 C) was demonstrated by
Sun.14 In their work, the H+ and Zn2+ coinsertion mechanism of ZIBs
was first testified. However, apart from the first cycle, the described
Zn//ε-MnO2 battery still suffered from poor capacity. At the
discharge rate of 6.5 C, the initial capacity of ~130 mAh g-1
immediately decayed to only ~50 mAh g-1 in the second cycle. For
other phases of MnO2, complicated and/or high-cost approaches are
needed to address the capacity issues, such as oxygen defect
engineering,15-20 using aqueous Zn(CF3SO3)2 electrolyte21-22 and
a.
School of Materials Science and Engineering, MOE of the Key Laboratory of
Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chem &
Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-
Sen University, Guangzhou 510275, PR China. E-mail:
luxh6@mail.sysu.edu.cn, cesyyy@mail.sysu.edu.cn,
liuxiaoq5@mail.sysu.edu.cn
b.
School of Applied Physics and Materials, Wuyi University, Jiangmen,
Guangdong, 529020, P. R. China.
College of Chemistry and Chemical Engineering, Yantai University, Yantai
c.
264005, PR China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
‡ The authors contributed to this work equally.
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hierarchy structuring.23-25 Therefore, to achieve better capacity and Electrochemical Measurements: Galvanostatic charge/discharge
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DOI: 10.1039/D0TA03706K
energy density, it is essential to develop a facile and efficient (GCD) curves, cyclic voltammograms, electrochemical impedance
technique for ε-MnO2 modifications.
spectra and galvanostatic intermittent titration technique (GITT)
In this work, we report a La-Ca co-doped ε-MnO2 (denoted as curves were collected by using an electrochemical workstation
LCMO) cathode as a promising candidate for high-energy-density (CHI760). All the electrochemical performance of LCMO based
Zn//ε-MnO2 constructions. By virtue of a facile coprecipitation electrodes was characterized by employing a two-electrode cell with
method,13 the LCMO material was synthesized by simply mixing the commercial Zn foil as counter electrode, the mixture of 1 M ZnSO4
precursor solutions. The specific urchin-like nanostructure of the and 0.4 M MnSO4 as electrolyte. The addition of MnSO4 was used to
LCMO provides numerous active sites for Zn2+ accommodation while suppress MnO2 dissolution caused by John-Teller effect and improve
the dual-ion dopants enlarge the tunnel diameter to reduce energy the stability. The tested area of working electrode is 0.5 cm2.
barrier for Zn2+ diffusion. Similar to other metal oxides doped by
Results and discussion
metal ions, in contrast to the pristine ε-MnO2(denoted as MO), the
LCMO displays better electrical conductivity as well as more stable
The LCMO material was synthesized using
a facile liquid
crystal structure, leading to superior energy storage performance.14
Specifically, the LCMO cathode achieves a commendable capacity of
297.3 mAh g-1 at 0.2 A g-1, good cycle stability (76.8% of initial
capacity after 200 cycle), and excellent rate capability (161 mAh g-1
when the current density increased to 1.6 A g-1). In addition, the
coprecipitation method. La3+ and Ca2+ may co-insert into the lattice
of MO when the comproportionation of MnO4 and Mn2+ takes
-
aqueous Zn//MnO2 batteries with LCMO cathodes deliver
a
maximum energy density of 401.22 Wh kg-1 and a peak power density
of 5.2 kW kg-1, outperforming most of Zn//MnO2 counterparts.
Experimental
Synthesis of MO: All the chemicals are directly used without further
purification. 5 mmol MnSO4·H2O was dissolved into 150 mL H2O at
room temperature with magnetic stirring to obtain precursor
solution. Then 10 mmol KMnO4 was slowly added into the solution.
After 3 hours of magnetic agitation, the as-obtained solution was
centrifuged to get MnO2 (denoted as MO) powder. Finally, using
distilled water and ethanol to wash the powder 3 times separately.
The MO material was air-dried overnight at 60 °C.
Synthesis of LCMO, LMO and CMO: 5 mmol MnSO4·H2O, 4 mmol
CaCl2 and 2 mmol La(NO3)3·6H2O were dissolved into 150 mL H2O at
room temperature with magnetic stirring to obtain precursor
solution. Then 10 mmol KMnO4 was slowly added into the solution.
After 3 hours of magnetic agitation, the as-obtain solution was
centrifuged to get LCMO powder. The powder was washed by
distilled water and ethanol 3 times separately. Then, the LCMO
material was air-dried overnight at 60 °C. To obtain La3+-doped MnO2
(denoted as LMO) and Ca2+-doped MnO2 (denoted as CMO) samples,
the same procedures as above were performed without the addition
of CaCl2 and La(NO3)3·6H2O into the precursor solution, respectively.
Electrode Preparation: Using 1-methyl-2-pyrrolidinone (NMP, AR,
Sigma-Aldrich) as solvent, mix carbon black (AR, Sigma-Aldrich) and
polyvinylidene fluoride (PVDF) binder (AR, Sigma-Aldrich) with LCMO
powder at an 8: 1: 1 weight ratio. The mixture was coated on carbon
paper (Shanghai Hesen Electric Company) and dried overnight in
vacuum at 60 °C. The valid mass loading of the electrode was 2.4 mg
cm-2.
Fig. 1 SEM images of (a) MO sample and (b) LCMO sample. (c-d) TEM
images; (e) SAED pattern and (f) EDS mapping of LCMO sample.
Materials Characterization: The morphology structure and
composition of the electrode materials were characterized by
scanning electron microscopy (SEM, JSM-6330F), X-ray diffraction
(XRD, D-MAX 2200 VPC, RIGAKU), transmission electron microscope
(TEM, FEI Tecnai G 2 F30). The chemical composition of different
samples was measured through photoelectron spectroscopy (XPS,
NEXSA, Thermo VG) and Raman Spectroscopy (Renishaw inVia).
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Ca2+ doping.32-33 It presents a spin-orbit double-peak and each peak
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DOI: 10.1039/D0TA03706K
is further split into doublet. Note that no signal is detected in the Ca
2p or La 3d region for MO. The Mn 2p and 3s spectra of MO and
LCMO are of high similarity, as given in Fig. S5 and 2d. In the absence
of satellite peaks, the double-peak of Mn 2p3/2 and 2p1/2 at the
binding energy of 642.4 and 654.0 eV are accordant with MnO2. The
Mn 3s spectra (Fig. 2d) further confirm this conclusion with the
energy separation of ~4.8 eV between the two peaks at 84.5 and 89.4
eV.34-36 Besides, the bonding energy separation increased from 4.7
eV to 4.9 eV after the doping of Ca2+ and La3+. The higher bonding
energy separation suggests the formation of some trivalent
manganese, which is formed to compensate the extra positive
charge brought by Ca2+ and La3+ in the lattice. For the O 1s spectra
(Fig. S6), the peaks at 530.0 and 531.5 eV refer to lattice oxygen and
-OH group, respectively.34 There is no significant difference between
MO and LCMO in the O 1s region, suggesting that the La3+ and Ca2+
inserted into the tunnel instead of the manganese-oxygen
hexahedron.
To evaluate the electrochemical properties of different materials,
a two-electrode cell was assembled using commercial zinc foil as
anode and MO or LCMO as cathode. Previous studies have proved
that the addition of MnSO4 can suppress MnO2 dissolution and thus
prolong its cycling life.9, 12 Therefore, the mixture of 1 M ZnSO4 and
0.4 M MnSO4 was used as electrolyte in this case. CV measurements
was first performed (Fig. 3a). Within the working window from 0.8 to
Fig. 2 (a) XRD spectra of MO and LCMO; XPS (b) La 3d5 spectra; (c) Ca
2p spectra and (d) Mn 3s spectra of MO and LCMO.
place. Fig. 1a, 1b are the SEM images of MO and LCMO respectively.
Both of the materials have the same urchin-like structure, suggesting
that the introduction of heteroatoms does not cause obvious
morphology changes. The representative TEM image of MO and
LCMO in Fig. 1c and 1d reveals that the urchin-like structure is
agglomerates of thin flakes of MO with a radius of ~220 nm. The low-
crystallinity character of LCMO and MO are also verified by the high-
resolution transmission electron microscope (HRTEM, Fig. S1) and
the selected-area electron diffraction (SAED, Fig. 1e and Fig. S2),
which presents no regular crystal lattice with amorphous diffraction
rings. To confirm the distributions of heteroatoms, the
corresponding energy dispersive X-ray spectroscopy (EDS) elemental
mapping and scanning transmission electron microscopy (STEM)
characterizations were preformed and results are given in Fig. 1f and
Fig. S3. Obviously, both La and Ca are successfully introduced into the
LCMO sample and all elements disperse uniformly in the urchin-like
structure.
1.9
V,
both
electrodes
show
typical
To further investigate the detailed crystallinity information of
LCMO, XRD measurements were performed (Fig. 2a). The diffraction
peaks of LCMO and MO are both in conformity to the ε-MnO2 (JCPDS
Card No. 30-0820).10, 26-28 The broad peak width and low intensity of
LCMO and MO verify their low crystallinity, which is consistent with
the SAED result. Compared with MO, LCMO shows higher peak
intensity, and all of its main peaks slightly shift towards lower
degrees, which implies that LCMO is better crystallized and the ions
inserted in the tunnels force the lattice spacing to expand in all
directions. The enlarged lattice provides larger space for Zn2+
transport and reduces the resistance of Zn2+ insertion/extraction,
which are able to conjointly improve the Zn2+ storage capability.29
In order to characterize the oxidation state and chemical
composition of LCMO and MO, XPS analysis was carried out, and the
survey spectra is shown in the Fig. S4. As expected, LCMO has
multiplet splitting components at 834.8 and 838.4 eV (Fig. 2b)
corresponding to the 3d-3s electron coupling of La 3d5, which
confirms the existence of La3+.30-31 The signal in the Ca 2p region of
LCMO (Fig. 2c) is consistent with previously reported work related to
Fig. 3 (a) CV curves at 1 mV·s-1 and (b) GCD curves at 0.2 A g-1 of the
MO and LCMO electrodes. (c) Discharge capacities of the MO and
LCMO electrodes at different current densities. (d) Capacities of the
MO, LMO, CMO and LCMO electrodes at 0.1 A g-1. (e) Cycling
performance of the MO and LCMO electrodes.
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battery behaviour with one oxidative peak and two reductive peaks, the dual doping strategy can improve both capacity and the lifespan
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DOI: 10.1039/D0TA03706K
implying a two-step insertion process. The higher peak at ~1.27 V is of ε-MnO2. This result can be explained by the optimized kinetic
assigned to H+ insertion and the lower one at ~1.18V is corresponding process of Zn2+ insertion/extraction. That is, ε-MnO2 has a tunnel-
to Zn2+ insertion.14 Apparently, the peak current is significantly structure and the La3+ and Ca2+ dopants with big ion diameter can
enhanced and the reduction potential islifted from 1.17 Vand 1.26 V enlarge the diameter of the tunnel and relieve the strong
to 1.18 V and 1.28 V because of the introduction of La3+ and Ca2+. It electrostatic repulsion. What’s more, Ca2+ experience no valence
is thus proved that the heteroatom insertion can
change during the reaction and have a strong electrostatic force with
improve the capacity and ameliorate the reversibility of the the nearby O atoms, preventing the lattice structure from collapsing
electrochemical reaction. To further corroborate this conclusion, and improving the stability of MnO2。
GCD curves were also collected at the current density of 0.2 A g-1. We
To unravel the root cause behind the improved electrochemical
can see from Fig.3b and Fig. S7 that the LCMO exhibits an optimal performance, the electrochemical impedance spectra (EIS) of LCMO
capacity of 297 mAh g-1, superior to the pristine sample (199 mAh g- and MO were compared (Fig. 4a). The two samples both exhibit a
14
1) and other epsilon MnO2.11, Additionally, LCMO presents two typical semicircle curve in the high-frequency region and an oblique
slightly higher discharge plateaus (increased by 20 mV and 10 mV for straight line in the low-frequency region, which represents the
the first and second plateau) and a lower overpotential gap, which is charge transfer resistance (Rct) and the semi-infinite Warburg
consistent with the CV curves. Predictably, the capacities contributed impedance (Zw) resulting from the diffusion limited electron
by H+ insertion and Zn2+ insertion are both increased to some extent, transferring process respectively. The value of Rct and Zw were
which could be attributed to the lower electrostatic repulsion calculated through the quantitative equivalent circuit model. The
brought by enlarged lattice spacing. Capacities collected at different fitted curves are shown in Fig. S11. The Rct of LCMO decreased from
current densities were compared in Fig. 3c and S8 in order to 198.1 Ω to 29.8 Ω compared with MO, indicating that the doped La3+
evaluate the rate capability of both materials. Although the and Ca2+ can narrow the band gap and enhance the electrical
capacities of LCMO and MO both decay with the increase of the conductivity while the Zw of LCMO is calculated to be lower than MO
current density, LCMO always holds a higher capacity than the either (5.3 Ω vs 203.2 Ω), which signifies the faster Zn2+
pristine MO at all current densities. Obviously, large currents do not transfer/diffusion kinetics. To validate this viewpoint, the ion
cause structural collapse to the LCMO. Besides, even when the diffusion coefficient (D) of both cathodes is qualitatively determined
current density increases by 17 folds, from 0.2 A g-1 to 3.4 A g-1, LCMO by the GITT test (Fig. 4b and S12). In the plateau from 0.85 V to 1.30
still preserves a commendable capacity of 91 mAh g-1, significantly V, during which most of the capacity is contributed, the D value of
outperforming other ε-MnO2 cathodes.37-40
LCMO is much higher than the pristine MO, again confirming the
To explore the respective contributions of different metal ion result of EIS. Moreover, to demonstrate the Zn2+ insertion process,
dopants, namely the La3+ and Ca2+, to the Zn2+ storage capability of we carried out XPS measurements on both charging and discharging
LCMO, we synthesized La3+-doped MnO2 (denoted as LMO), Ca2+- LCMO.
doped MnO2 (denoted as CMO) and pristine MO as control samples
As
shown
in
Fig.
4c,
the
bonding
by simply adjusting the composition of precursor solution during the Fig. 4 (a)Nyquist plots of MO and LCMO cathodes. (b) the
synthetic processes. As illustrated in Fig. 3d, the discharge capacities corresponding diffusivity coefficient (D value) of the Zn2+ ions of MO
of LMO and CMO collected at 0.2 A·g-1 are 227 and 216 mAh g-1,
which are 29 mAh g-1 and 18 mAh g-1 higher than that of the MO
sample (199 mAh g-1), respectively. Considering the capacity
increasement of 95 mAh g-1 by LCMO, there is synergistic effect of
La3+ and Ca2+ ions for boosting the Zn2+ storage. Besides, according
to the GCD profiles of the 4 samples (Fig. S9), the LCMO electrode
shows not only higher discharge capacity, but also higher discharge
voltage, which is benefited from the better reversibility of the
electrode reaction. Similar to the capacity performance, this
phenomenon is also contributed by synergistic effect, since the
overpotential of LCMO is further decreased compared with the
single-doped CMO and LMO. The stability of these materials is then
compared by cycling tests in Fig. 3e. All the cathodes experience an
activation process within the first 15 cycles and then start to decay.
Remarkably, both CMO and LCMO cathodes retain more than 76% of
the initial capacity after 200 cycles whereas the pristine MO cathode
retain only 47%. In addition, it is revealed that the Coulombic
and LCMO cathodes during discharge process. (c) Mn 3s and (d) Zn
efficiency of modified MnO2 (including CMO, LMO and LCMO) is
2p spectra of LCMO in charging and discharging state.
much higher than pristine MnO2, indicating that the doping ions can
significantly improve the reversibility of the cathode (Fig. S10).
Therefore, we believe that Ca-doping can significantly improve the
stability of ε-MnO2 while both Ca2+ and La3+ have a positive effect on
the capacity enhancement. Taking advantages of the synergy effect,
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method. The La3+ and Ca2+ inserted into the tunnels ofVMiewOAertxicpleaOndnleinde
its lattice spacing and the enlarged tunnels bDroOuI:g1h0t.1a0b3o9u/Dt0siTgAn0i3fi7c0a6nKt
improvement on ion transfer kinetics and Zn2+ storage capacity. It is
demonstrated that Ca2+ doping can stabilize the crystal structure of
MnO2 while both Ca2+ and La3+ can enhance its capacity owing to the
synergy effect. Besides, benefiting from the improved conductivity
and accelerated kinetics, the reaction reversibility is also enhanced.
Generally, the overall electrochemical performance of MO is
optimized. The resultant LCMO cathode displays a commendable
capacity of 297.3 mA h g-1 at 0.2 A g-1, good cycling durability (~76.8%
capacity retention after 200 cycles), and favourable rate capability
(161 mA h g-1 at 1.6 A g-1). Particularly, the Zn//LCMO battery delivers
an outstanding energy density of 401.22 W h kg-1 and a peak power
density of 5.2 kW kg-1. Our work provides an alternative way for
designing high-performance ε-MnO2 cathodes in future applications
of ZIBs constructions.
Fig. 5 Ragone plot of the Zn//LCMO aqueous battery. The values of
Conflicts of interest
The authors declare no competing financial interest.
other previously reported aqueous rechargeable batteries are also
43-49
added
for
comparison.37,
energy gap between the double peaks of Mn 3s spectra increased
from 4.7 eV to 5.1 eV during the discharge process, suggesting that
the manganese was reduced from +4 to +3 state. Also, the narrower
Mn 2p3/2 peak without satellite peak, which is the representative
signal of Mn4+, can be seen in the charging LCMO (Fig. S13), again
leading to the same conclusion as Mn 3s spectra.42
In addition, according to the integration of the peak area of Zn 3p
and Mn 2p, the Zn/Mn ratio increased from 0.14 to 3.73 during this
period either (Fig. 4d), which further proves the proposed
mechanism. Note that the appearance of Zn atoms in the discharging
LCMO, could be assigned to the irreversible insertion of Zn2+ during
activation. In Fig. S14, both LCMO and MO electrodes have a strong
Zn2+ insertion peak but no H+ insertion peak in the first cycle. None of
the Zn2+ insertion peak of the following reduction processes could
reach the intensity as the first cycle, implying that part of the Zn2+
inserted during the first cycle cannot de-intercalate and therefore
occupy some sites permanently.
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (21802173, 21822509, U1810110, and
31530009), Science and Technology Planning Project of
Guangdong Province (2018A050506028), and the Open Funds
of the State Key Laboratory of Rare Earth Resource Utilization
(RERU2018013).
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The assembled Zn//LCMO battery delivers an outstanding energy
density and a peak power density of 401.22 Wh kg-1 and 5.2 kW kg-1,
respectively. As presented in Fig. 5, the energy storage performance
of Zn//LCMO is superior to most recently reported devices, such as
Zn//γ-MnO2 battery (0.05 kW kg-1, 147.5 Wh kg-1),43
Li4Ti5O12//LiMn2O4 LIB (0.67 kW kg-1, 27 Wh kg-1),44
MoO3//LiMn2O4@ppy battery (0.25kW kg-1, 60 Wh kg-1),45 Zn//
Polyaniline@lens papers (3.34 kW kg-1, 67.8 Wh kg-1),46 Ni//Fe
battery (1.9 kW kg-1, 179.2 Wh kg-1),47 Zn//β-MnO2 battery (0.10 kW
kg-1, 75.2 Wh kg-1),48 Zn//δ-MnO2 battery (0.11 kW kg-1, 320 Wh kg-
49
1), Zn//ZOV@ graphene (5.1 kW kg-1, 115 Wh kg-1).37 This work
might shed light on the application of co-doping strategy to optimize
the electrochemical properties of metal oxides.
13 K. Wang, X. Zhang, J. Han, X. Zhang, X. Sun, C. Li, W. Liu, Q. Li, Y.
Ma, ACS Appl. Mater. Interfaces, 2018, 10, 24573-24582.
14 W. Sun, F. Wang, S. Hou, C. Yang, X. Fan, Z. Ma, T. Gao, F. Han, R.
Hu, M. Zhu, C. Wang, J. Am. Chem. Soc., 2017, 139, 9775-9778.
Conclusions
In summary, a La-Ca co-doping strategy was developed to promote
the Zn2+ storage capability of MO via a facile liquid coprecipitation
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