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

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 LiMn 2 O 4 cathod[r]

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Original Article

Effect of pH values on structural, optical, electrical and

Prakash Chanda,*, Vivek Bansala, Sukritia, Vishal Singhb

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

LiMn 2 O 4

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

750C 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 (http://creativecommons.org/licenses/by/4.0/)

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 Mn3þand Mn4þions 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: kk_pc2006@yahoo.com (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

https://doi.org/10.1016/j.jsamd.2019.04.005

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

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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 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 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 120C to get rid of the

moisture and thus, obtain the dry powder The obtained dryﬁne

particles were then calcined at 450C for 5 h in a tubular furnace in

air and then, at 750C for 8 h to get aﬁne 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 (Ka) radiation source of the

wavelength 1.54 Å The surface morphology investigation was

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

working electrode was prepared by a combination of 80 wt.% of the prepared LiMn2O4, 10 wt.% of acetylene black as a conductor, and

N-methyl-2-pyrrolidone (NMP) The obtained slurry was extended on

a Ni foil and dehydrated at 120C 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 LiMn 2 O 4 nanostructures synthesized at

P Chand et al / Journal of Science: Advanced Materials and Devices 4 (2019) 245e251 246

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D¼ kl

where, the shape factor (k), is approximately 0.89, the crystallite

size is denoted as D, andlis the wavelength of the X-ray radiation

(Cu-Ka) used¼ 1.542 Å,bis the full width at half maxima (FWHM)

andqis 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[12e14]:

a¼ l

2sinq

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

h2þ k2þ l2

p

(2)

and

used¼ 1.542 Å,qis 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

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 speciﬁc 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]:

dB¼ m

The porosity (P) of the LiMn2O4nanostructures was calculated

by using the formula as below:

p¼ 1 ddB

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

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 Mn2þ (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

Fig 2 Variation of lattice constant (a) and crystallite size (D) of LiMn 2 O 4

nano-structures with pH values.

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Table 1

Variation of the crystallite size (D), the Lattice parameters (a), Unit cell volume (V), X-ray density (d x ), Speciﬁc surface area (S), Bulk density (d), Porosity (P) and the optical energy band gap (E g ) of LiMn 2 O 4 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 (d x ) (g/cm 3 )

Speciﬁc surface area (S) (m 2 /g)

Bulk density (d B ) (g/cm 3 )

Porosity (P) Optical energy

band gap (E g ) (eV)

Fig 3 (aec): SEM images of LiMn 2 O 4 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 LiMn 2 O 4 nanostructures Fig 5 Room-temperature FTIR spectra of the LiMn 2 O 4 nanostructures 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

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

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

(ahn)2 versus (hn) plot [17].Fig 6 depicts the Tauc plot of the

absorbance spectrum of LiMn2O4spinel nanostructures recorded

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

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

can determine the resistance of the synthesized samples The estimated values of the resistance are 207, 143 and 26 kUfor 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

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

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 210U-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 2 O 4 nanostructures

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

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 LiMn 2 O 4 nanostructures synthesized at different pH values.

Fig 9 (aec): Cyclic voltammetry studies of LiMn

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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]

Efficiencyð%Þ ¼Td

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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Fig 10 Efﬁciency studies of LiMn 2 O 4 nanostructures synthesized at different pH

values.

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