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Original Article

Effect of pH values on structural, optical, electrical and

electrochemical properties of spinel LiMn

₂

O

₄

cathode materials

Prakash Chand

a,*

, Vivek Bansal

a

, Sukriti

a

, Vishal Singh

b
a_{Department of Physics, National Institute of Technology, Kurukshetra, 136119, India}

b_{Centre for Materials Science}_{& Engineering, National Institute of Technology, Hamirpur, 177005, India}

a r t i c l e i n f o

Article history:

Received 15 October 2018
Received in revised form
16 April 2019

Accepted 21 April 2019
Available online 25 April 2019
Keywords:

Nanostructures
LiMn2O4

Cathode materials
Cyclic voltammetry

a b s t r a c t

In the present work, we have synthesized the spinel LiMn2O4cathode 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 LiMn2O4nanostructures

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 LiMn2O4cathode 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 ( />

1. Introduction

In recent years, there is a swift development of digital
technolo-gies in diversefields 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
ma-jority of the electronic devices have become instrumental in
man-aging 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

preparation, environmental friendliness and superior safety in
comparison to some layered oxides, for instance, LiCoO2and 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 Mn3ỵand Mn4ỵions are placed at the
octa-hedral 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
en-ergy. 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
recy-cleability 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
ca-pacity of the cell[7]. Santiago et al. reported two reversible cyclic
voltammograms for spinel LiMn2O4, synthesized by the combustion

* Corresponding author.

E-mail address:(P. Chand).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j s a m d

/>

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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
pro-cedures. 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
investi-gation stipulates that the pH of the precursor solution appears to be a
significant constraint for the crystal structure development, particles
dimension and morphology of thefinal 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 LiMn2O4cathode

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.

2. Experimental
2.1. Chemicals

All the chemical reagents used in the present work for the
synthesis of LiMn2O4nanostructures were of analytical grade and

were utilized without further purification. For the synthesis of
LiMn2O4at different pH values, CH3COOLi, Mn(CH3COO)2, C6H8O7

(citric acid) and Zn(CH3COO)2, NaOH, Ammonia solution and

de-ionized H2O were used as precursor materials.

2.2. Synthesis

LiMn2O4 spinel nanostructured cathode materials were

syn-thesized via the sol-gel technique. C6H8O7was 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 liqui_{fied in 50 mL deionized (DI) water.}
Further, such solutions were mixed collectively to make a final
solution of 150 mL. The molar ratio of C6H8O7to metal ions was 1.

Initially, due to the presence of citric acid, the pH value of the
so-lution 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 dryfine
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}
LiMn2O4nanoparticles.

2.3. Characterization

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

carried out by Scanning Electron Microscopy (SEM).

Photoluminescence (PL), Fourier Transform Infra Red (FTIR) and
UV-Visible spectra were recorded at room temperature to explore
the optical properties of LiMn2O4nanostructures, 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 LiMn2O4nanostructures on Ag-layered pellets through a

set-up of Keithley (two-probe Model). In order to assess the cycling
behavior of the synthesized cathode materials, the cyclic
voltam-metry measurements were done using the Biologic SP-240

Poten-tiostat 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 di_{fluoride (PVDF) as a binder in}

N-methyl-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
LiMn2O4composites 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

XRD study has been carried out on LiMn2O4 nanostructures

prepared at different values of pH tofind out the effect of pH on the
structural properties of these materials.Fig. 1illustrates the XRD
patterns of the LiMn2O4nanostructures 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

nano-structures was determined from the broadening of XRD peaks via
the Scherrer's formula[11]:

Fig. 1. Room temperature XRD patterns of LiMn2O4nanostructures synthesized at

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D¼

_b

k

l

cos

q

(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-K_a) used¼ 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

syn-thesized 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[12_e14]:

aẳ

l

2sin

q

h2_{ỵ k}2_{ỵ l}2

p

(2)

and

Vẳ a3 ₍₃₎

here,

l

is the wavelength of the X-ray radiation (Cu-K_a)

used¼ 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
pa-rameters of the LiMn2O4nanostructures prepared at different pH

values are depicted inFig. 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

exten-sive 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 LiMn2O4nanostructures prepared at different pH values

was calculated by using the relation given below[13]:

S¼6 103

Ddx (4)

dx¼_{6:022  10}8M₂₃ _{ a}₃ (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

volume ratio increases and consequently, the speci_{fic surface area}
(S) increases. The surface area of the electrode material is a
sig-nificant 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 LiMn2O4nanostructures was estimated

from the following equation[13]:

d_B¼ m

p

r2_h (6)

The porosity (P) of the LiMn2O4nanostructures was calculated

by using the formula as below:

p¼ 1 d_dB

X (7)

The results of measurements of the crystallite size, the lattice
parameters, the cell volume and the variation in all calculated
pa-rameters, i.e. dx, S, dB, and P for the LiMn2O4 nanostructures are

presented inTable 1.

3.2. Scanning electron microscopy study

The surface morphologies of the spinel LiMn2O4

nano-structures were observed by SEM. Fig. 3(aec) shows the SEM

images of the LiMn2O4nanostructures prepared at the different

pH values of 3, 6 and 9, respectively, which clearly show signi
fi-cant 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

nano-structures, 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. 4depicts the PL spectra of the LiMn2O4spinel 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 LiMn2O4nanostructures

have a weak photoluminescence property due to the forbidden spin
of Mn2ỵ (3d5) [15]. A visible emission peak observed around
473 nm is related to the structural imperfections, present in the
LiMn2O4nanostructures 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 pH¼ 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

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. 5shows

the FTIR spectra of the LiMn2O4 nanostructures prepared at

Fig. 2. Variation of lattice constant (a) and crystallite size (D) of LiMn2O4

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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 LiMn2O4nanostructures 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 (d}
x) (g/cm3)

Specific
surface
area (S) (m2_/g)

Bulk density
(dB) (g/cm3)

Porosity (P) Optical energy
band gap (Eg) (eV)

3 (111) 46 8.27 566 4.24 30.70 0.57 0.86 3.86

6 (111) 38 8.26 563 4.23 37.22 0.69 0.83 3.95

9 (111) 32 8.25 561 4.27 44.18 0.71 0.83 4.06

Fig. 3. (aec): SEM images of LiMn2O4nanostructures synthesized at different pH values (a) pH¼ 3 (b) pH ¼ 6 (c) pH ¼ 9.

Fig. 4. Room-temperature photoluminescence (PL) spectra of LiMn2O4nanostructures

synthesized at different pH values.

Fig. 5. Room-temperature FTIR spectra of the LiMn2O4nanostructures synthesized at

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different pH values of 3, 6 and 9. The spectra were recorded in the
range between 500 and 1200 cm1. It is evident from the FTIR
spectra that two broad infrared spectral bands are observed. One
lies around 565 cm1and the other around 617 cm1that can be

assigned to the LieO bending and the LieMneO stretching

vi-bration band, respectively [16]. In the FTIR spectra, the
charac-teristic peaks appearing below 1500 cm1confirm the presence of
the metal-oxygen vibration band. Hence, the FTIR analysis
con-firms the phase formation and the functional groups present in
the LiMn2O4nanostructures, 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
deter-mine 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 LiMn2O4nanostructures 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

(

a

h

n

)2 versus (h

n

) plot [17].Fig. 6 depicts the Tauc plot of the
absorbance spectrum of LiMn2O4spinel 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.

3.6. IeV characteristics

To determine the electrical properties of the as synthesized

LiMn2O4 spinel nanostructures, the current-voltage (IeV)

characteristics were performed on the Ag-layered pellets
us-ing a Keithley two-probe set-up.Fig. 7shows the IeV curves of
the LiMn2O4samples 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
LiMn2O4nanostructures 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 LiMn2O4 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
electro-chemical cycling performance of the LiMn2O4nanostructures, the

electrochemical behavior of as-synthesized LiMn2O4

nano-structures was studied by the Electrochemical Impedance
Spec-troscopy (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 LiMn2O4nanostructures

pre-pared at different pH values of 3, 6 and 9 are shown inFig. 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. 6. Plot of (a∙h∙n)2_{versus photon energy (h∙}_n_{) for the LiMn}

2O4nanostructures

synthesized at different pH values. The insets shows the plot of absorption versus

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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
LiMn2O4nanostructure was performed by the measurements

us-ing a potentiostat and the results are shown inFig. 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 LiMn2O4spinel nanostructures synthesized at

different pH values of 3, 6 and 9. The LiMn2O4sample prepared

with pH¼ 9 shows a smaller potential difference between the

anodic and the cathodic peak as compared to those observed in the
LiMn2O4samples with pH¼ 3 and 6, indicating that the

revers-ibility of LiMn2O4synthesized at pH¼ 9 is much better than that of

the other samples, as it is shown inFig. 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 LiMn2O4samples prepared with different pH values of 3, 6 and

9, indicating that the peak current in the LiMn2O4nanostructures

increases with the increase in the pH values.

3.9. Efficiency study

The cycling lifetime of the as-synthesized LiMn2O4electrodes

materials was examined via a galvanic charge/discharge
measure-ment at 5 Ag1in a 2 M KOH electrolyte.Fig. 10depictes 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 LiMn2O4nanostructures prepared with different pH values

Fig. 8. Nyquist plots for LiMn2O4nanostructures synthesized at different pH values.

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

Efficiency%ị ẳTd

Tc 100 (8)

here, Tdand Tcare discharge and charge temperatures. As it is seen

inFig. 10, there is an increase in the efficiency recorded over up to

300 cycles observed for the LiMn2O4nanostructures synthesized

at different pH values of 3, 6 and 9, from 70, 76, 83% respectively.
This imlplies that the LiMn2O4nanostructures synthesized at pH 9

exhibit the long-term cycle constancy and also superior
electro-chemical reproducibility as compared to the ones synthesized at
pH values 3 and 6.

4. Conclusion

In summary, the spinel LiMn2O4cathode materials were

suc-cessfully 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 LiMn2O4with 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

as-prepared LiMn2O4nanostructures 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

elec-trochemical reproducibility of the LiMn2O4 nanostructures

syn-thesized at pH 9 as compared to those samples synsyn-thesized at pH 3

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 LiMn2O4cathode 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
Kur-ukshetra for providing the facilities in the Physics Department for
this study.

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