Synthesis and research of MnO2–NiO composite as lithium-ion battery anode using spent Zn–Mn batteries as manganese source

Article abstract of DOI:10.1016/j.jallcom.2020.155578

In this work, we have successfully synthesized porous flower-like MnO₂–NiO anode material for lithium ion batteries by facile hydrothermal method using the manganese source that is recovered from spent Zn–Mn batteries through hydrometallurgy recycling technology. Scanning electron microscopy (SEM) reveals that such an architecture provides short paths for lithium diffusion and large material/electrolyte contact area. The electrochemical measurements exhibit that the initial discharge capacity of MnO₂–NiO composite is 2981 mAh·g^?1 at a current density of 100 mA g^?1 and retains at 502 mAh·g^?1 after 100 cycles, much higher than that of pure MnO₂. Even at a high current rate of 400 mA g^?1, the specific capacity still remains at 297 mAh·g^?1 after 50 cycles. The electrode exhibits excellent cycling performances and high capacity. Furthermore, the lithiation and delithiation processes of MnO₂–NiO anode material have been studied in detail by X-ray diffraction characterization. We found that during the lithiation process, MnO₂ reacts with lithium ions to form the intermediate product of Mn₂O₃, which is further reduced to Mn metal. NiO directly reacts with lithium ions to form Ni metal.

Full text of DOI:10.1016/j.jallcom.2020.155578

Journal Pre-proof
Synthesis and research of MnO –NiO composite as lithium-ion battery anode using
2
spent Zn–Mn batteries as manganese source
Ningsheng Zhang, Guanghui Guo, Bowen He, Jiaxin Zhu, Jie Wu, Jianghua Qiu
PII:
S0925-8388(20)31942-3
DOI:
https://doi.org/10.1016/j.jallcom.2020.155578
JALCOM 155578
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 16 January 2020
Revised Date: 18 April 2020
Accepted Date: 9 May 2020
Please cite this article as: N. Zhang, G. Guo, B. He, J. Zhu, J. Wu, J. Qiu, Synthesis and research of
MnO –NiO composite as lithium-ion battery anode using spent Zn–Mn batteries as manganese source,
2
Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.155578.
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Credit Author Statement
Ningsheng Zhang: Conceptualization, Methodology, Investigation, Writing-
Reviewing and Editing.
Guanghui Guo: Supervision, Resources.
Bowen He: Visualization.
Jiaxin Zhu: Visualization.
Jie Wu: Visualization.
Jianghua Qiu: Resources.

Synthesis and Research of MnO2-NiO composite as lithium-ion battery anode
using spent Zn-Mn batteries as manganese source
Ningsheng Zhang, Guanghui Guo , Bowen He, Jiaxin Zhu, Jie Wu, Jianghua Qiu
(Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials,
Wuhan University of Science and Technology, Wuhan 430081, China)
Abstract
In this work, we have successfully synthesized porous flower-like MnO2-NiO anode
material for lithium ion batteries by facile hydrothermal method using the manganese
source that is recovered from spent Zn-Mn batteries through hydrometallurgy
recycling technology. Scanning electron microscopy (SEM) reveals that such an
architecture provides short paths for lithium diffusion and large material/electrolyte
contact area. The electrochemical measurements exhibit that the initial discharge
capacity of MnO2-NiO composite is 2981 mAh·g-1 at a current density of 100 mA·g-1
and retains at 502 mAh·g-1 after 100 cycles, much higher than that of pure MnO2.
Even at a high current rate of 400 mA·g-1, the specific capacity still remains at 297
mAh·g-1 after 50 cycles. The electrode exhibits excellent cycling performances and
high capacity. Furthermore, the lithiation and delithiation processes of MnO2-NiO
anode material have been studied in detail by X-ray diffraction characterization. We
found that during the lithiation process, MnO2 reacts with lithium ions to form the
intermediate product of Mn2O3, which is further reduced to Mn metal. NiO directly
reacts with lithium ions to form Ni metal.
Corresponding author: E-mall address: guoguanghui@wust.edu.cn.
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Keywords: Recycling; MnO2-NiO; Anode; Lithium-ion batteries
1. Introduction
Zn-Mn batteries are widely used in devices that require low quantities of energy
such as watches, calculators, and remote controls [1]. These materials are not
rechargeable and have a very brief life span, leading to high consumption and the
generation of large amounts of electronic waste [2]. The residues are incinerated or
disposed in landfills, leading to contamination of groundwater, soil and atmosphere
[3]. Therefore, the recycling of spent Zn-Mn batteries not only enhances the
environmental protection from reduced amount of hazardous wastes but also realizes
the secondary utilization of resources [4−6].
Lithium-ion batteries (LIBs) have been widely applied to electrical automotive
vehicles and portable electronic products due to their excellent advantages [7−10].
However, the low theoretical capacity of graphite anode materials (372 mAh·g-1) in
LIBs limits their further applications in large-scale energy storage [11]. With the
fast-growing demand of the electrical vehicle market, the new generation LIBs with a
higher energy density are urgently needed [12−14]. For the sake of prominent
performances, numerous kinds of anode materials have been explored. Among
various candidates, nanometer-sized transition metal oxides (TMOs) have attracted
much attention because of their wide availability, low cost, environmental benignity
and especially high theoretical capacities [15−17].
Manganese dioxide (MnO2), as one of TMOs, is a promising material because of its
low toxicity, high specific capacity (1232 mAh·g-1) and natural abundance [18].
2

However, the performance of MnO2-based anodes remains inferior due to two main
issues: (1) the MnO2 material exhibits an extremely low electronic conductivity which
limits the electron transport [19−21]; (2) a large volume change of MnO2 occurs
during the lithiation and delithiation processes, which leads to severe electrode
pulverization, serious safety problems and rapid capacity loss during the cycling
[22−24]. To promote the application of MnO2 anode materials, various efforts have
been devoted to improve their capacity and rate capability, such as fabricating various
kinds of nanostructures or introducing heteroatoms [25]. As a potential anode material
of LIBs, various nickel oxide (NiO) nanostructures have been developed to enhance
the Li-ion storage capacity since nanostructured NiO particles are characterized by
large surface area and lots of micropores, which can provide short distance for Li-ions
transport and large electrode/electrolyte interface for charge transfer [26−28]. Jang et
al. fabricated hollow NiO nanofibers which delivered a reversible specific discharge
capacity of 956 mAh·g-1 after 400 cycles at a high current density of 2 A·g-1 [29].
Shang et al. synthesized interconnected porous NiO@MnO2 nanosheets which
displayed a high capacity of 1000 mAh·g-1 after 160 cycles at a rate of 1.0 C [30]. In
view of the high capacity of MnO2 and the outstanding conductivity of nanostructured
NiO, to synthesize MnO2-NiO composites is in favor of enhancing the overall
conductivity and buffering the volume variation synchronously, thus can lead to
outstanding cycling stability and superior rate capability.
In this paper, we use spent Zn-Mn batteries as manganese source to synthesize
flower-like MnO2-NiO composite by a conventional hydrothermal method. The
3

interwoven micropores are in favor of the infiltration of the electrolyte. The
nanostructured NiO component can suppress the volume expansion of MnO2 during
charge and discharge processes. Stemming from these excellent properties, large
reversible capacity, stable cycling performance and high rate capacity can be realized.
2. Experimental section
2.1. Recovery of manganese from spent Zn-Mn batteries
2.1.1. Battery pretreatment
Spent Zn-Mn batteries were manually dismantled to separate the external metal
cover, zinc casing, separator, metals caps, carbon rod and black powder materials. The
powders mainly contain MnO2 and zinc chloride (ZnCl2) as electrolyte, which account
for about 50% of the total weight of the dismantled batteries. The prepared powders
were first washed with deionized water to remove ZnCl2, dried at 60 °C for 12 h and
then calcined in muffle furnace at 750 °C for 5 h.
2.1.2. Acid leaching
5 g of powders, 30 mL of deionized water and 10 mL of sulfuric acid (H2SO4, 98%)
were added to a 100 mL round-bottomed flask in turn. After spirited stirring for 1 h,
20 mL of hydrogen peroxide (H2O2, 30%) as reducing agent was dropwise added into
the flask, and the suspension was stirred at 50 °C for 6 h. Then the suspension was
filtered, and 1 mol·L-1 sodium hydroxide (NaOH) was added into the filtrate to
remove Fe3+ by adjusting the pH value to 6. After stirring, standing and filtering, 100
mL of 1 mol·L-1 sodium carbonate (Na2CO3) was poured into the filtrate to form
manganese carbonate (MnCO3) precipitate. Finally, the product was filtered, washed
4

with distilled water and dried at 60 °C for 12 h.
2.2. Material synthesis
2.2.1 Synthesis of NiO
2.9 g of Ni(NO3)2·6H2O was dissolved in 50 mL of absolute ethanol. The solution
was transferred into a 100 mL Teflon-lined autoclave, which was heated to 135 °C
and maintained for 12 h. After the autoclave was naturally cooled to room
temperature, the precipitate was collected by filtering. The obtained product was
washed with ethanol and deionized water several times, and then dried in a vacuum
drying oven at 60 °C for 12 h. The obtained sample was heated in a tube furnace at
350 °C for 2 h to obtain NiO.
2.2.2 Synthesis of MnO2-NiO composite
The MnO2-NiO composite was synthesized by hydrothermal method. In a typical
process, 0.69 g of the prepared MnCO3 and 60 mL of 1 mol·L-1 H2SO4 were added to
a 100 mL round-bottomed flask. Then the solution was stirred at 50 °C for 1 h and
transferred into a 100 mL Teflon-lined autoclave. Subsequently, 0.63 g of potassium
permanganate (KMnO4) and 0.75 g of prepared NiO sample were added to the above
autoclave, which was maintained at 130 °C for 12 h. The final product was washed
with ethanol and deionized water several times and dried in the vacuum drying oven
at 60 °C for 12 h. The black solid sample was MnO2-NiO composite. The mole ratio
of MnO2/NiO is 1:1. The mass ratio of MnO2/NiO is 1.16:1. For comparison, pure
MnO2 material was synthesized by the same procedure without NiO.
2.3. Material characterization
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The crystal structure was analyzed by X-ray diffraction (XRD, Bruker D8
Advanced diffractometer, Germany) with Cu Kα1 radiation (λ=1.5418 Å). The
diffraction patterns were recorded from 5° to 85° under operating conditions of 40 kV
and 40 mA. The valence state was implied with X-ray photoelectron spectroscopy
(XPS, Escalab 250Xi electron spectrometer, USA) using 150 W Al Kα radiation. The
morphology and structure of the samples were characterized using a scanning electron
microscope (SEM, Merlin Compact, Germany).
2.4. Electrochemical measurement
The electrochemical performances of the MnO2-NiO composite were evaluated in
LIR2016-type coin cells. The test cells were assembled in an argon-filled glove box.
The working electrodes were prepared by mixing MnO2-NiO composite as the active
material, acetylene black and polyvinylidene fluoride (PVDF) with a mass ratio of
8:1:1. After dissolving in N-methyl pyrrolidone (NMP), the mixture was coated on
copper foils with a diameter of 12 mm and dried in a vacuum oven at 60 °C for 12 h.
Pure lithium foils were used as the counter electrodes. Celgard 2400 membranes were
used as the separator. The electrolyte was made of 1 M LiPF6 in a solvent mixture of
ethylene carbonate (EC) and diethylene carbonate (DEC) (1:1 vol%).
The electrochemical performances for application as battery electrodes were
evaluated by galvanostatic charge-discharge tests, cyclic voltammetry (CV)
experiments and electrochemical impedance spectroscopy (EIS) measurements. The
galvanostatic charge-discharge experiments were performed using a battery tester
(NEWARE CT-4008) at various current densities of 100−500 mA·g-1 in a voltage
6

range between 0.01 and 3.0 V versus Li/Li+. The CV experiments were conducted on
an electrochemical workstation (CHI760E) in the same voltage range with a scan rate
of 0.1 mV·s-1. The EIS measurements were executed on the same electrochemical
workstation in a frequency range of 0.01 Hz to 100 kHz with an alternating-current
voltage of 5 mV.
3. Results and discussion
3.1. Material characterization and analysis
The XRD patterns were used to identify the phase-structural composition of the
MnCO3 product and MnO2-NiO composite (Fig. 1). The intermediate product of
MnCO3 was recycled from spent Zn-Mn batteries. As shown in Fig. 1a, the peaks at
2θ values of 24.251°, 31.361° and 37.522° agree well with (012), (104) and (110)
crystal planes of the rhombohedral phase of MnCO3 (JCPDS 44-1472), respectively.
The diffraction peaks are so strong and no other impurity peaks are observed because
of the great crystallinity and extremely high purity of MnCO3. Moreover, the
intermediate product of MnCO3 with high purity is in favor of synthesizing
MnO2-NiO composite with outstanding electrochemical performances. From Fig. 1b,
the peaks at 37.12°, 42.401° and 56.027° can be ascribed to (100), (101) and (102)
crystal planes of hexagonal MnO2 (JCPDS 30-0820). NiO has five characteristic
diffraction peaks located at 37.248°, 43.275°, 62.878°, 75.414° and 79.407°,
corresponding to the (111), (200), (220), (311) and (222) crystal planes of cubic NiO
(JCPDS 47-1049). It is clear that the diffraction peaks of NiO are relatively sharp
compared with the peaks of MnO2, indicating better crystallinity of NiO in the
7

composite material.
In order to further investigate the chemical compositions and the valence states of
MnO2-NiO composite, XPS was measured and the results are shown in Fig. 2. The
XPS spectrum of MnO2-NiO composite (Fig. 2a) confirms the existence of the Ni, Mn
and O elements. The carbon can be discovered in the XPS spectrum due to surface
environmental contamination [31]. Fig. 2b reveals the high resolution XPS spectrum
for Mn 2p, which is comprised of two peaks known as Mn 2p1/2 and Mn 2p3/2
spin-orbit peaks of MnO2 [32]. The peaks at 653.4 and 642.0 eV are attributed to
Mn4+ 2p1/2 and Mn4+ 2p3/2, respectively, while those at 655.5 and 644.7 eV correspond
to Mn3+ 2p1/2 and Mn3+ 2p3/2 [33]. It is worth mentioning that Mn4+ is the predominant
oxidation state in the composite, which confirms the formation of MnO2. As for Mn
3s spectrum (Fig. 2c), two peaks at 89.15 and 84.35 eV could be found. Besides, the
spin energy separation (ΔE) of 4.8 eV is consistent with that of Mn4+ reported in
literature [34], which further demonstrates the dominant oxidation state of Mn4+ in the
composite. Fig. 2d shows the high-resolution XPS spectrum of Ni 2p, which is
comprised of five fitting peaks. The peaks at 856.0 and 854.8 eV are associated to the
Ni 2p3/2 and their satellite peak appears at 860.7 eV [35]. The peak related to Ni 2p1/2
appears at 872.6 eV and its satellite peak appears at 880.4 eV. The location of each
peak is in good agreement with data in literature, which confirms the formation of
NiO crystallites [36]. As shown in Fig. 2e, the O 1s spectrum exhibits that the peaks
at 531.2 eV and 529.5 eV can be ascribed to Ni−O and Mn−O metal oxygen bonds
[37].
8

The microstructures of the MnO2 and MnO2-NiO were characterized by SEM (Fig.
3). It can be seen from Fig. 3a that the regular spindle-shaped structures of MnO2
were formed at 130 °C during hydrothermal reactions. The length of the
spindle-shaped structures is about 10 μm and the width is about 3 μm. Fig. 3c is a
typical SEM image of MnO2-NiO composite. The composite exhibits flower-like
structures with rough surfaces. The nanosheets that make up the petals are
interconnected to form a highly porous surface morphology, which increases the
specific surface area and accommodates the volume change caused by the
electrochemical reactions to enhance reversible capacity and improve rate capability
[38]. Moreover, the SEM image in Fig. 3d suggests that the porous flower-like
structures are composed of nanospheres with diameters of 200−400 nm.
3.2. Electrochemical and cell performance
The charge-discharge curves of MnO2-NiO and MnO2 electrodes in the potential
range 0.01−3 V at a current density of 100 mA·g-1 are shown in Fig. 4a and b. It is
evident that, as compared to pure MnO2, MnO2-NiO exhibits superior specific
capacity over the cycles. The initial discharge and charge capacities of the MnO2-NiO
are 2982 and 1637 mAh·g-1, respectively. The corresponding irreversible capacity loss
of 45.1% could be attributed to the electrolyte decomposition and irreversible
formation of SEI layer [39−41]. For comparison, the discharge and charge capacities
of MnO2 yielded lower values of 1811 and 452 mAh·g-1 corresponding to an
irreversible capacity loss of 75%. The results demonstrate that the MnO2-NiO has
higher irreversible capacity than that of the MnO2.
9

The cycling performances of MnO2-NiO and MnO2 electrodes are displayed in Fig.
4c. The capacities for MnO2-NiO and MnO2 drop gradually and maintain at 571
mAh·g-1 and 311 mAh·g-1 after 50 cycles at a current density of 100 mA·g-1,
respectively. Although both MnO2-NiO and MnO2 electrodes have high coulombic
efficiency of more than 95%, MnO2-NiO delivers more outstanding lithium storage
capacity.
Fig. 4d exhibits the cycling performance of the MnO2-NiO composite at different
current densities of 100, 200, 300, 400 and 500 mA·g-1 in the potential range of
0.01−3.0 V. The 2nd discharge capacities are 1713, 1491, 1035, 698, 613 mAh·g-1 at
different current densities of 100, 200, 300, 400 and 500 mA·g-1, respectively. Even
after 50 cycles, the discharge capacities possess 571 mAh·g-1, 486 mAh·g-1, 447
mAh·g-1, 297 mAh·g-1 and 210 mAh·g-1, which indicates that the MnO2-NiO
electrode can provide enhanced anode performance for LIBs. As shown in Table 1, we
compared the main electrochemical parameters from literature for MnO2- and
NiO-based composite anodes. It is obvious that the porous flower-like MnO2-NiO
composite shows high reversible capacity as anode for LIBs.
The rate capabilities of the MnO2-NiO and MnO2 electrodes at different rates are
shown in Fig. 4e. The discharge capacities of the MnO2-NiO electrode are 908, 569,
375, 213 and 151 mAh·g-1 at different current densities of 100, 200, 300, 400 and 500
mA·g-1, respectively. When the current density recovers to 100 mA·g-1, the
MnO2-NiO electrode can provide discharge capacity of 488 mAh·g-1. For pure MnO2
electrode, the discharge capacities are 371, 238, 194, 164 and 136 mAh·g-1 at various
10

current densities of 100−500 mA·g-1. After the current density recovers to 100 mA·g-1,
the discharge capacity is maintained at 213 mAh·g-1. As the anode of LIBs, the
MnO2-NiO exhibits higher capacity, cycling performance, and rate capability
compared to the pure MnO2, which suggests that MnO2-NiO anode has potential
applications in high-performance LIBs.
Fig. 4f shows the Cyclic voltammetry (CV) curves of MnO2-NiO for the first three
cycles at a scan rate of 0.1 mV·s-1 between 0.01 V and 3.0 V. In the first cathodic scan,
an irreversible reduction peak is observed at around 0.8 V, which should be ascribed
to the transformation of high valence manganese element from Mn4+ to Mn2+. This
peak disappears from the second cycle, indicating that the reduction of Mn4+ to Mn2+
can’t happen easily during the subsequent cycles [52].The peaks at 0.26 and 0.09 V
can be attributed to the further reduction of Mn2+ and Ni2+ to metal Mn and Ni, the
process of intercalation of lithium-ion and the formation of gel-like solid electrolyte
interphase (SEI) layer [53−55]. From the second cycle, the reduction peak is moved to
0.34 V (Mn2+ and Ni2+ to metallic Mn and Ni), suggesting the irreversible
electrochemical reactions in the first cathodic scan. In the first anodic scan, an
oxidation peak at 1.2 V corresponds to the oxidation processes of metal Mn to Mn4+
and metal Ni to Ni2+. The oxidation peak doesn’t shift in the subsequent cycles,
indicating that the formation of MnO2 and NiO and the process of deintercalation of
lithium-ion are highly reversible. In the subsequent cycles, the cathodic and anodic
peaks tend to be stable with only slight shift, which reveals the superior reversibility
of the electrodes during the redox reactions.
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The cycling performances of MnO2-NiO composite are displayed in Fig. 4g. The
capacity for MnO2-NiO composite maintains at 502 mAh·g-1 after 100 cycles at a
current density of 100 mA·g-1. The MnO2-NiO electrode has high coulombic
efficiency of more than 95% and delivers outstanding cycling performances.
To further study the kinetics of ionic diffusion and charge transfer, electrochemical
impedance spectroscopy (EIS) measurements were conducted in the frequency range
of 0.01 Hz to 100 kHz. Fig. 5a depicts Nyquist plots and the equivalent circuit of
MnO2-NiO and MnO2 electrodes after 50 cycles at the current density of 100 mA·g-1.
All Nyquist plots are composed of a semicircle (at the high frequency and medium
frequency regions) and an inclined line (at the low frequency region). RΩ, Rct and CPE
represent the resistance of electrolyte, charge transfer resistance and double layer
capacitance, respectively. Zw is the Warburg impedance, which is associated with the
lithium-ion diffusion coefficient. The semicircle diameter in the high frequency and
medium frequency regions represents the value of charge-transfer resistance. The
MnO2-NiO electrode exhibits a lower charge-transfer resistance (Rct) of only 318 Ω
compared to MnO2 electrode (Rct=441 Ω), which demonstrates that the MnO2-NiO
composite has a better ability to conduct electrons during the cycling. Furthermore,
the Current−Voltage (I−U) curves of the electrodes were measured on the
electrochemical workstation under the same conditions (the same thickness and area).
Fig.5b indicates that the MnO2-NiO electrode has higher electronic conductivity than
pure MnO2 electrode owing to the porous flower-like nanostructure of MnO2-NiO
composite.
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To investigate the electrochemical reactions during the charge and discharge
processes, the XRD tests for the MnO2-NiO electrode after discharge (Li insertion)
and charge (Li extraction) in the first cycle were performed. As shown in Fig. 6a, after
discharge, the sharp diffraction peaks at 43.407°, 50.556° and 74.297° can be indexed
to Cu (JPCDS 65-9743). The diffraction peaks of Cu come from copper foil. In
addition, the diffraction peaks consist of Mn (JPCDS 17-0910), Ni (JPCDS 45-1027),
Li2O ((JPCDS 12-0254), Mn2O3 (JPCDS 41-1442), LiMnO2 (JPCDS 35-0749) and
LiNi2O4 (JPCDS 41-0890). The diffraction peaks of Mn, Ni and Li2O were observed
after discharge, which confirms the reduction processes of MnO2 to Mn metal and
NiO to Ni metal and the conversion from Li+ to Li2O. The diffraction peaks of
LiMnO2 and LiNi2O4 probably arise from the phase-transition processes of the MnO2
and NiO particles after Li insertion, respectively. In the first lithiation process, the
diffraction peaks of Mn2O3 were observed, which indicates that MnO2 particles were
partly converted to Mn2O3 due to the incomplete transformation from MnO2 to Mn
metal. After charge, as shown in Fig. 6b, the diffraction peaks can be indexed to
MnO2 (JCPDS 30-0820) and NiO (JCPDS 47-1049). It is remarkable that the
diffraction peaks of Li2O are still observed, which confirms that not all of Li2O was
reversibly converted into Li+. During the first charge-discharge process, the
incomplete conversion of Li2O results in the high irreversible capacity loss (45.1%) of
the MnO2-NiO electrode.
During the full lithiation process of MnO2, the lithium ions first intercalate into
MnO2 and then react with MnO2, forming the intermediate product of Mn2O3, which
13

is further reduced to Mn metal. The lithiation process can be expressed as follows:
MnO2 + Li+ + e-
2LiMnO2
(1)
(2)
(3)
LiMnO2
Mn2O3 + Li2O
Mn2O3 + 6Li+ + 6e-
2Mn + 3Li2O
The total electrochemical reactions of the MnO2-NiO composite can be described
as follows:
-
MnO2 + 4Li+ + 4e
(4)
(5)
Mn + 2Li2O
Ni + Li2O
NiO + 2Li+ + 2e-
These reactions yield nano-dispersed particles of Mn and Ni metal. It has been
demonstrated that the small size effect of the Mn and Ni nanoparticles, which arises
from an increased total number of surface atoms, can greatly improve the
electrochemical reactivity and enhance the reversible conversion between Li+ and
Li2O. Moreover, the high lithium-storage performance of the MnO2-NiO electrode
can also be ascribed to the following points. Firstly, the porous flower-like
nanostructure of MnO2-NiO composite can improve the reversible lithium storage
capacity because of its high specific surface area. Secondly, the porous structure can
offer a larger material/electrolyte contact area and accommodate the volume change
during the processes of charge and discharge. Thirdly, nano-dispersed Mn and Ni
metal particles can provide shorter path for lithium diffusion to improve the cycling
performance of MnO2-NiO electrode.
To visually verify the stability of the porous flower-like nanostructure, the
MnO2-NiO electrode after 100 cycles is directly observed by SEM (Fig. 7a−b). It can
14

be found that the MnO2-NiO electrode maintains the integrity of the special structure
after 100 cycles (Fig. 7a). Some interconnected porous nanospheres in the electrode
can be distinctly observed after 100 cycles (Fig. 7b). These observations demonstrate
that the porous flower-like nanostructure plays an important role in ensuring the
structural integrity of the MnO2-NiO electrode during the charge/discharge process.
4. Conclusions
The MnO2-NiO composite with porous flower-like nanostructure could be
synthesized by combining the facile hydrothermal method and hydrometallurgy
recycling technology using spent Zn-Mn batteries as raw materials, which provided a
feasible way to recycle spent Zn-Mn batteries. The MnO2-NiO composite delivered
excellent reversible capacity, good rate performance and superior cycling performance
due to the extraordinary porous flower-like nanostructure. It is also indicated that the
MnO2-NiO composite with superior lithium storage performance is a promising
candidate for LIBs anode materials.
Acknowledgments
This research did not receive any specific grant from funding agencies in the public,
commercial, or not-for-profit sectors.
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23

Table 1 Electrochemical performance comparison of MnO2- and NiO-based anodes
for lithium-ion batteries
Sample
Raw materials
Current
density
(mA·g-1)
100
Cycle
Capacity Refs.
(mAh·g-1)
number
MnO2-NiO
composite
Spent Zn-Mn
batteries
100th
100th
50th
502
440
716
450
300
494
520
702
600
This
work
[42]
Carbon
KMnO4
600
100
100
800
500
72
nanofibers@MnO2
SnO2-MnO2-SnO2 MnSO4
nanofibers@MnO2
[43]
[44]
[45]
[46]
[47]
[48]
[49]
MnO2/CNH
composite
KMnO4
Mn(Ac)2
KMnO4
NiSO4
60th
PEDOT/MnO2
nanocomposite
α-Fe2O3/MnO2
microspheres
NiO/polyaniline
film
100th
500th
50th
NiO/RGO
Ni(NO3)2·6H2O 100
Ni(NO3)2·6H2O 200
100th
100th
nanocomposites
Hollow NiO

nanotubes
NiO/C porous
composites
Ni(NO3)2
100
100th
50th
625
586
[50]
[51]
Graphene/NiO
nanoparticles
Ni(NO3)2·6H2O 100

Fig. 1. XRD patterns of (a) MnCO3 and (b) MnO2-NiO