Effect of pH values on structural, optical, electrical and electrochemical properties of spinel LiMn2O4 cathode materials

Journal of Science: Advanced Materials and Devices 4 (2019) 245e251

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Original Article

Effect of pH values on structural, optical, electrical and
electrochemical properties of spinel LiMn2O4 cathode materials
Prakash Chand a, *, Vivek Bansal a, Sukriti a, Vishal Singh b
a
b

Department of Physics, National Institute of Technology, Kurukshetra, 136119, India
Centre for Materials Science & Engineering, National Institute of Technology, Hamirpur, 177005, India

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 15 October 2018
Received in revised form
16 April 2019
Accepted 21 April 2019
Available online 25 April 2019

In the present work, we have synthesized the spinel LiMn2O4 cathode materials via a sol-gel method at
750  C for 8 h under optimal conditions at different pH values (3, 6 and 9) and studied the effect of

different pH values on the structural, optical, electrical and electrochemical properties. X-ray diffraction
(XRD) analysis identified the synthesized materials as crystallized in the cubic spinel structure (Fd3m)
with slight decrease in the lattice parameters. SEM exhibits the formation of a spongy and fragile
network structure in the synthesized samples. An enhancement in the optical energy band (Eg) leads to
the blue shift in the synthesized samples with reduced crystallite size. Cyclic voltammetry (CV) and
Electrochemical Impedance Spectroscopy (EIS) investigations show that the LiMn2O4 nanostructures
synthesized at pH 9 exhibit the long-term cycle constancy and a superior electrochemical reproducibility
as compared to those synthesized at pH values of 3 and 6. The results revealed that pH plays a significant
role in tuning the structural, optical and electrochemical properties of the LiMn2O4 cathode material,
which is considered a promising substitute of cathode materials for the novel lithium-ion battery
applications.
© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
Nanostructures
LiMn2O4
Cathode materials
Cyclic voltammetry

1. Introduction
In recent years, there is a swift development of digital technologies in diverse fields of technology and production, electric vehicles
and, space applications as well as of portable user electronic devices,
for instance, laptops, cell phones, digital cameras etc. Even the majority of the electronic devices have become instrumental in managing the everyday activities. The prompt augmentation of such
electronic devices obviously stimulates immense attention on cheap,
light-weight, eco-friendly, safe and, high energy density battery
materials, for both economic and environmental benefits [1e4]. To
congregate the increasing energy demands of the modern society
and the potential ecological anxiety, Li-ion battery technology has
the potential to meet the requirements of high energy compactness
and high dominance density applications. Recently, researchers and

the scientific community are interested in the LiMn2O4 cathode
material with a three-dimensional framework for the application of
rechargeable Li-ion batteries. It has numerous advantages, such as
abundant resources, non-toxic in nature, low cost, simple
* Corresponding author.
E-mail address: (P. Chand).
Peer review under responsibility of Vietnam National University, Hanoi.

preparation, environmental friendliness and superior safety in
comparison to some layered oxides, for instance, LiCoO2 and LiNiO2
[2,3]. Spinel LiMn2O4 with the 3D tunnel structure (space group
Fd3m) consists of a cubic close-packed array in which the oxygen
ions are positioned at the 32e sites and the Li ions in the tetrahedral
8a sites, whereas, the Mn3þ and Mn4þ ions are placed at the octahedral 16d sites [1,5]. At present, the prime challenges for the
development of Li-ion batteries for the mass market are price, safety,
energy and, power densities, charging and discharging rate and,
service life. Thus, the development and investigation of LiMn2O4
nanostructured cathode materials are very important, in view of the
future progress in the battery industry. To meet up, for such global
relevance, it has become abundantly apparent that the design and
fabrication of electrode materials of Li-ion batteries (LIBs) play an
important role to adapt the increasing worldwide demand for energy. Various properties such as the crystallite size, the stoichiometry
and the homogeneity govern the electrochemical properties of
electrode materials. The small particle size will improve the recycleability and the rate capability of the cathode materials [6]. Arof et
al. observed that the variation in the synthesis process of LiMn2O4
using tartaric acid introduced impurities that affect the specific capacity of the cell [7]. Santiago et al. reported two reversible cyclic
voltammograms for spinel LiMn2O4, synthesized by the combustion

/>2468-2179/© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license
( />

246

P. Chand et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 245e251

process [8]. However, for the large scale power and energy storage
application of LIBs, the price, safety, environmental friendliness and,
long stability of the electrode materials are of major concerns. The
performance of LIBs depends upon a number of factors, including the
properties of the anode, the cathode, and the electrolyte. Hence, an
improvement in the capacity of the cathode material has a larger
upshot on the volume, and consequently, the energy density of a
lithium-ion battery. Further, for subsequent applications, the
chemio-physical properties of the material are strongly dependent
on its dimension, morphologies, surface area and occasionally, on the
synthesis process and conditions throughout the processing procedures. The augmented convention of materials is significantly
affected by the peripheral conditions, such as temperature, precursor
concentration and pH value of the precursor. The literature investigation stipulates that the pH of the precursor solution appears to be a
significant constraint for the crystal structure development, particles
dimension and morphology of the final product through the sol-gel
synthesis [9,10].
Therefore, in the present work, by keeping in view of the above,
we present a systematic investigation on the structural, optical,
electrical and electrochemical properties of spinel LiMn2O4 cathode
materials synthesized via the sol-gel technique under optimal
conditions at different pH values (3, 6 and 9) without any surfactant
for the application of rechargeable lithium-ion batteries.

Photoluminescence (PL), Fourier Transform Infra Red (FTIR) and
UV-Visible spectra were recorded at room temperature to explore

the optical properties of LiMn2O4 nanostructures, synthesized via
the sol-gel method at different pH values. The current-voltage (IeV)
measurements were carried out to examine the electrical property
of the LiMn2O4 nanostructures on Ag-layered pellets through a setup of Keithley (two-probe Model). In order to assess the cycling
behavior of the synthesized cathode materials, the cyclic voltammetry measurements were done using the Biologic SP-240 Potentiostat considering three electrode configurations. For the
electrochemical performance of LiMn2O4 nanostructures, the
working electrode was prepared by a combination of 80 wt.% of the
prepared LiMn2O4, 10 wt.% of acetylene black as a conductor, and
10 wt.% of polyvinylidene difluoride (PVDF) as a binder in Nmethyl-2-pyrrolidone (NMP). The obtained slurry was extended on
a Ni foil and dehydrated at 120  C for 12 h. The cells consisted of the
LiMn2O4 composites which act as the positive electrode, while a Pt
electrode as the negative electrode and an electrolyte composed of
2M KOH solution in DI water.

3. Results and discussion
3.1. X-ray diffraction studies

2. Experimental
2.1. Chemicals
All the chemical reagents used in the present work for the
synthesis of LiMn2O4 nanostructures were of analytical grade and
were utilized without further purification. For the synthesis of
LiMn2O4 at different pH values, CH3COOLi, Mn(CH3COO)2, C6H8O7
(citric acid) and Zn(CH3COO)2, NaOH, Ammonia solution and deionized H2O were used as precursor materials.
2.2. Synthesis
LiMn2O4 spinel nanostructured cathode materials were synthesized via the sol-gel technique. C6H8O7 was used as a chelating
mediator in the synthesis process. For synthesizing LiMn2O4,
lithium acetate (1 mol), manganese acetate (2 mol), and citric acid
(3 mol) were independently liquified in 50 mL deionized (DI) water.
Further, such solutions were mixed collectively to make a final

solution of 150 mL. The molar ratio of C6H8O7 to metal ions was 1.
Initially, due to the presence of citric acid, the pH value of the solution was low (4e5). To maintain the pH at 3, citric acid was gently
mixed to this solution with constant stirring using a magnetic
stirrer. In order to maintain pH at 6 and 9, NH3 solution was
gradually mixed to this solution with constant stirring. The
resulting solution was heated at 60  C, stirred with a magnetic
stirrer for 5 h until a gel was formed. The gel precursor achieved
was dried in an electric oven for 12 h at 120  C to get rid of the
moisture and thus, obtain the dry powder. The obtained dry fine
particles were then calcined at 450  C for 5 h in a tubular furnace in
air and then, at 750  C for 8 h to get a fine black colored powder of
LiMn2O4 nanoparticles.

XRD study has been carried out on LiMn2O4 nanostructures
prepared at different values of pH to find out the effect of pH on the
structural properties of these materials. Fig. 1 illustrates the XRD
patterns of the LiMn2O4 nanostructures synthesized by the sol-gel
technique. The recorded X-ray diffraction peaks could be indexed as
(111), (311), (222), (400), (331), (511), (440), (531), (533) and (622)
Miller planes which validate the configuration of the single-phase
cubic spinel crystal structure having the space group Fd3m
(JCPDS card no 35-782) [3]. The observed broadening of the XRD
peaks is an indication of the grain size of the synthesized samples in
the nano range. The average crystallite size of the LiMn2O4 nanostructures was determined from the broadening of XRD peaks via
the Scherrer's formula [11]:

2.3. Characterization
The crystal structure and phase purity analysis of the synthesized samples was performed by X-ray diffraction measurement
(Rigaku diffractometer) with Cu (Ka) radiation source of the
wavelength 1.54 Å. The surface morphology investigation was

carried out by Scanning Electron Microscopy (SEM).

Fig. 1. Room temperature XRD patterns of LiMn2O4 nanostructures synthesized at
different pH values.


P. Chand et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 245e251

D¼

kl
b cosq

(1)

where, the shape factor (k), is approximately 0.89, the crystallite
size is denoted as D, and l is the wavelength of the X-ray radiation
(Cu-Ka) used ¼ 1.542 Å, b is the full width at half maxima (FWHM)
and q is the Bragg's angle in radians. The average crystallite size, as
determined from the (111) high intensity reflections, came out to be
46, 38 and 32 nm, respectively, for LiMn2O4 nanostructures synthesized at different pH values of 3, 6 and 9, respectively. It has been
found that the crystallite size decreases as we increase pH values
from 3 to 9. The lattice constant (a) and volume (V) for the LiMn2O4
nanostructures prepared at different pH values were estimated by
using the following equations [12e14]:

l

a¼

2sinq

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
h2 þ k2 þ l2

(2)

and

V ¼ a3

(3)

here, l is the wavelength of the X-ray radiation (Cu-Ka)
used ¼ 1.542 Å, q is the Bragg's angle in radians and, h, k, l are the
Miller indices. The changes in the crystallite size and lattice parameters of the LiMn2O4 nanostructures prepared at different pH
values are depicted in Fig. 2. It is observed that the size decreases
with an increase in the pH values and the lattice constants also vary
with the pH values. It is well known that the specific surface area
(S), the X-ray density (dx) and the bulk density (dB) play an extensive role in the alteration of the structural properties of the cubic
spinel structure. The specific surface area (S) and the X-ray density
(dx) of the LiMn2O4 nanostructures prepared at different pH values
was calculated by using the relation given below [13]:

S¼

6 Â 103
Ddx

dx ¼

8M
6:022 Â 10 23 Â a3

(4)

(5)

here, M is the molecular mass of the sample. The specific surface
area (S) is observed to be increasing with the reduction in the
crystallite size. As the crystallite size decreases, the surface to

247

volume ratio increases and consequently, the specific surface area
(S) increases. The surface area of the electrode material is a significant characteristic constraint that establishes the energy and
power density of a particular battery system. In the present study
the surface area as obtained is higher for the LiMn2O4 cathode
material synthesized at pH 9 because of its smaller crystallite size.
The bulk density (dB) of LiMn2O4 nanostructures was estimated
from the following equation [13]:

dB ¼

m

pr2 h

(6)

The porosity (P) of the LiMn2O4 nanostructures was calculated
by using the formula as below:

p¼1À

dB
dX

(7)

The results of measurements of the crystallite size, the lattice
parameters, the cell volume and the variation in all calculated parameters, i.e. dx, S, dB, and P for the LiMn2O4 nanostructures are
presented in Table 1.
3.2. Scanning electron microscopy study
The surface morphologies of the spinel LiMn2O4 nanostructures were observed by SEM. Fig. 3(aec) shows the SEM
images of the LiMn2O4 nanostructures prepared at the different
pH values of 3, 6 and 9, respectively, which clearly show significant changes in the nanostructures and in the porosity. The
development of the spongy and fragile network structure is easily
visible. The sample consists of round-shaped particles, since these
particles were prepared by the sol-gel technique. The voids and
pores, as manifested in the synthesized nanomaterials, are
endorsed which may be due to the liberation of a huge volume of
gases through the combustion process.
3.3. Photoluminescence (PL) spectroscopy analysis
To study the optical properties of the spinel LiMn2O4 nanostructures, photoluminescence (PL) spectra at room temperature
were recorded by using the Xenon lamp light as the irradiation
source for all the samples prepared at different pH values of 3, 6 and
9. Fig. 4 depicts the PL spectra of the LiMn2O4 spinel nanostructures
prepared at different pH values. For the excitation wavelength of
320 nm, the emission spectrum gives two peaks, one around 376

and the other around 473 nm. The broad peak in the UV emission
region appeared around 376 nm may be endorsed due to the near
band edge (NBE) emission which originates through the free
exciton recombination from the conduction band (CB) to the
valence band (VB). This indicates that the LiMn2O4 nanostructures
have a weak photoluminescence property due to the forbidden spin
of Mn2þ (3d5) [15]. A visible emission peak observed around
473 nm is related to the structural imperfections, present in the
LiMn2O4 nanostructures as well as to the recombination of holes
and electrons in the VB and CB. The PL intensity is the highest for
the samples synthesized at pH ¼ 9 and lowest for that at pH 6 also
indicating the variation in the surface defects with the change in
the crystallinity of the synthesized samples.
3.4. Fourier transform Infra-red (FTIR) studies

Fig. 2. Variation of lattice constant (a) and crystallite size (D) of LiMn2O4 nanostructures with pH values.

In order to investigate the vibrational and functional groups
present in the as synthesized samples, FTIR spectra of the LiMn2O4
nanostructures were recorded at room temperature. Fig. 5 shows
the FTIR spectra of the LiMn2O4 nanostructures prepared at


248

P. Chand et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 245e251

Table 1
Variation of the crystallite size (D), the Lattice parameters (a), Unit cell volume (V), X-ray density (dx), Specific surface area (S), Bulk density (d), Porosity (P) and the optical
energy band gap (Eg) of LiMn2O4 nanostructures synthesized at different pH values.

pH values

(hkl)

Average
Crystallite
Size (D) (nm)

Lattice
Constant (a) (Å)

Volume of
unit cell (V) (Å)3

X-ray
density (dx) (g/cm3)

Specific
surface
area (S) (m2/g)

Bulk density
(dB) (g/cm3)

Porosity (P)

Optical energy
band gap (Eg) (eV)

3

6
9

(111)
(111)
(111)

46
38
32

8.27
8.26
8.25

566
563
561

4.24
4.23
4.27

30.70
37.22
44.18

0.57
0.69
0.71

0.86
0.83
0.83

3.86
3.95
4.06

Fig. 3. (aec): SEM images of LiMn2O4 nanostructures synthesized at different pH values (a) pH ¼ 3 (b) pH ¼ 6 (c) pH ¼ 9.

Fig. 4. Room-temperature photoluminescence (PL) spectra of LiMn2O4 nanostructures
synthesized at different pH values.

Fig. 5. Room-temperature FTIR spectra of the LiMn2O4 nanostructures synthesized at
different pH values.


P. Chand et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 245e251

different pH values of 3, 6 and 9. The spectra were recorded in the
range between 500 and 1200 cmÀ1. It is evident from the FTIR
spectra that two broad infrared spectral bands are observed. One
lies around 565 cmÀ1 and the other around 617 cmÀ1 that can be
assigned to the LieO bending and the LieMneO stretching vibration band, respectively [16]. In the FTIR spectra, the characteristic peaks appearing below 1500 cmÀ1 confirm the presence of
the metal-oxygen vibration band. Hence, the FTIR analysis confirms the phase formation and the functional groups present in
the LiMn2O4 nanostructures, which is in accordance with the XRD
results.

3.5. UV-visible absorption studies

To investigate the optical properties of the spinel lithium
manganese oxide nanostructures, room temperature UVeVisible
absorption spectroscopy was employed. It is well-known that the
absorbance of nanomaterials relates to the energy band gap and
depends on the defects of the surface. The electronic structure of
the material governs its optical properties, which in turn determine the material's light absorption. The absorption data,
therefore, play a vital role in the evaluation of the energy gap.
The energy gap of the as prepared LiMn2O4 nanostructures were
estimated through the absorbance versus wavelength data. The
as-prepared nanomaterial was extensively diluted in distilled
H2O and then, its UVeVisible absorbance spectra were recorded.
Various models were proposed to study the optical properties of
the synthesized samples, although, the most familiar was the
Tauc's model that allows to derive the energy gap (Eg) from the
(ahn)2 versus (hn) plot [17]. Fig. 6 depicts the Tauc plot of the
absorbance spectrum of LiMn2O4 spinel nanostructures recorded
in the range 200e800 nm. From this, the energy gap of the
LiMn2O4 nanostructures, prepared at different pH values of 3, 6
and 9, were found as 3.86, 3.95 and 4.06 eV, respectively. The
estimated band gap values are found to be increased with the
increasing pH values. The enhancement in the energy gap (Eg) of
the LiMn2O4 nanostructures with the increase in the pH values
may be related to the decrease in the crystallite size.

Fig. 6. Plot of (a∙h∙n) 2 versus photon energy (h∙n) for the LiMn2O4 nanostructures
synthesized at different pH values. The insets shows the plot of absorption versus
wavelength spectra of as-synthesized LiMn2O4 nanostructures.

249

3.6. IeV characteristics
To determine the electrical properties of the as synthesized
LiMn 2 O 4 spinel nanostructures, the current-voltage (IeV)
characteristics were performed on the Ag-layered pellets using a Keithley two-probe set-up. Fig. 7 shows the IeV curves of
the LiMn 2 O 4 samples prepared at different pH values. It is seen
that the synthesized samples obey the Ohm's law and show
the conducting nature. From the slope of the IeV graph, we
can determine the resistance of the synthesized samples. The
estimated values of the resistance are 207, 143 and 26 k U for
LiMn 2 O 4 nanostructures synthesized at different pH values of
3, 6 and 9, respectively. These results show that with the
increasing pH values in the synthesis process, the resistance of
the corresponding LiMn 2 O 4 nanostructures is decreased. The
decrease in the sample's resistance is correlating with the
crystallite size also.

3.7. Electrochemical impedance spectrum studies
In order to investigate the effect of pH values on the electrochemical cycling performance of the LiMn2O4 nanostructures, the
electrochemical behavior of as-synthesized LiMn2O4 nanostructures was studied by the Electrochemical Impedance Spectroscopy (EIS) using a potentiostat and the recorded spectrum is
depicted in Fig. 8. EIS was carried out to examine the electrode
resistance and impedance change in the as synthesized LiMn2O4
nanostructures. The Nyquist plots of LiMn2O4 nanostructures prepared at different pH values of 3, 6 and 9 are shown in Fig. 8. The
recorded impedance spectra reveal a depressed and a spike arc in
the high-frequency and the low-frequency region, respectively. The
intercept at the real impedance axis corresponds to the ohmic
resistance, whereas, the arc corresponds to a charge transfer
resistance and a binary layer capacitance of a parallel combination.
The charge transfer resistance value is premeditated through the
real axis by the diameter of the arc. A spike provides information
about the Warburg impedance as attained in the low-frequency

section, that is linked to the diffusion in lithium-ion particles. The
impedance was found to be 298, 225, and 210 U-1, for the LiMn2O4
nanostructures synthesized at the different pH values of 3, 6 and 9,
respectively, reaving clearly that the impedance of the LiMn2O4

Fig. 7. I-V characteristics of LiMn2O4 nanostructures synthesized at different pH values.


250

P. Chand et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 245e251

Fig. 8. Nyquist plots for LiMn2O4 nanostructures synthesized at different pH values.

nanostructures decreases with the increasing pH values in the
synthesis procedure.
3.8. Cyclic voltammetry studies
A study of the electrochemical behavior of the as-synthesized
LiMn2O4 nanostructure was performed by the measurements using a potentiostat and the results are shown in Fig. 9(aec). Cyclic

voltammograms were measured at a scan rate of 1 mV/s in 2M KOH
for the potential window from 0 V to 0.5 V. The anodic peaks
observed in the cyclic voltammograms of the as-synthesized
LiMn2O4 nanostructures correspond to the lithium extraction
whereas the cathodic peaks observed correspond to the lithium
insertion. The anodic peak is the evident for the elimination of the
Li ions from the tetrahedral sites, where the LieLi interactions have
occurred. The possible inconsistency between the oxidation and
reduction peaks may be seen in the values of 80, 90 and 71 mV,
respectively, for the LiMn2O4 spinel nanostructures synthesized at

different pH values of 3, 6 and 9. The LiMn2O4 sample prepared
with pH ¼ 9 shows a smaller potential difference between the
anodic and the cathodic peak as compared to those observed in the
LiMn2O4 samples with pH ¼ 3 and 6, indicating that the reversibility of LiMn2O4 synthesized at pH ¼ 9 is much better than that of
the other samples, as it is shown in Fig. 9. It is clearly to see in this
figure that the reversibility of the synthesized materials increases
with the increasing pH value used in the synthesis procedure. The
peak current values are seen as 77, 90 and 110 mA, respectively, for
the LiMn2O4 samples prepared with different pH values of 3, 6 and
9, indicating that the peak current in the LiMn2O4 nanostructures
increases with the increase in the pH values.

3.9. Efficiency study
The cycling lifetime of the as-synthesized LiMn2O4 electrodes
materials was examined via a galvanic charge/discharge measurement at 5 AgÀ1 in a 2 M KOH electrolyte. Fig. 10 depictes the plots
showing the efficiencies versus the cycling numbers for all the
electrodes in the study (up to 300 cycles). The recorded efficiency
for the LiMn2O4 nanostructures prepared with different pH values

Fig. 9. (aec): Cyclic voltammetry studies of LiMn2O4 nanostructures synthesized at different pH values.


P. Chand et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 245e251

251

and 6. Hence, our the present study has revealed that the pH plays
an important role in tuning the structural, optical, electrical and
electrochemical properties of the spinel LiMn2O4 cathode material.
This material also is considered as a potential alternative of cathode

materials for novel lithium-ion battery applications.

Acknowledgments
The authors would like to thank the Director of the NIT Kurukshetra for providing the facilities in the Physics Department for
this study.

References

Fig. 10. Efficiency studies of LiMn2O4 nanostructures synthesized at different pH
values.

of 3, 6 and 9 at the 50th cycle was 65, 70 and 78%, respectively. The
efficiency as a function of cycle number was estimated using the
following relation [18].

Efficiency ð%Þ ¼

Td
 100
Tc

(8)

here, Td and Tc are discharge and charge temperatures. As it is seen
in Fig. 10, there is an increase in the efficiency recorded over up to
300 cycles observed for the LiMn2O4 nanostructures synthesized
at different pH values of 3, 6 and 9, from 70, 76, 83% respectively.
This imlplies that the LiMn2O4 nanostructures synthesized at pH 9
exhibit the long-term cycle constancy and also superior electrochemical reproducibility as compared to the ones synthesized at
pH values 3 and 6.

4. Conclusion
In summary, the spinel LiMn2O4 cathode materials were successfully prepared via the sol-gel technique. XRD analysis has
revealed that all the samples synthesized at different pH values
were identified as the spinel structure of LiMn2O4 with space group
Fd3m. The lattice parameters have been observed to slightly
decrease with the increasing pH values from 3 to 9. SEM studies
have shown the spongy and fragile network type morphology of
the nanostructures. PL and FTIR spectra also confirm the phase
formation of LiMn2O4. An enhancement in the optical energy band
gap (Eg) from 3.86 eV to 4.06 eV has been observed for the asprepared LiMn2O4 nanostructures with the increase in pH values.
This exhibits the blue shift in the synthesized samples with the
reduction in the crystallite size. The EIS and CV examination studies
have revealed the long-term cycle constancy and superior electrochemical reproducibility of the LiMn2O4 nanostructures synthesized at pH 9 as compared to those samples synthesized at pH 3

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