The peak frequency shifts to the high fre-quency side with the increasing temperature, and the relaxation occurs over several decades of frequency.. Such a Debye-like relaxation peak in
Trang 1Original Article
Structural, impedance, dielectric and modulus analysis
lithium-ion batteries
N Muralia,b,*, S.J Margaretteb, V Kondala Raob, V Veeraiahb
a Advanced Analytical Laboratory, DST-PURSE Programme, Andhra University, India
b Department of Physics, Andhra University, Visakhapatnam, Andhra Pradesh, 530003, India
a r t i c l e i n f o
Article history:
Received 4 October 2016
Received in revised form
24 April 2017
Accepted 26 April 2017
Available online 5 May 2017
Keywords:
X-ray diffraction
FESEM
Dielectric
Impedance
Electric modulus
a b s t r a c t
Mg, Co and Zn co-substituted layer-structured cathode materials LiNiCoxZnyMg0.02O2(x¼ y ¼ 0.0, 0.02 and 0.04) were prepared by a solid-state reaction method The materials were systematically charac-terized by X-ray diffraction (XRD),field effect scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FT-IR), and electrical impedance spectroscopy (EIS) techniques XRD analyses revealed the formation of a rhombohedral structure in the prepared materials with a typicala-NaFeO2
layered structure within R3m space group The grain size was determined by FESEM in the range from 3.19 to 3.85mm for all materials synthesized The site of the local cation (LieO) and of the transition metal cations (MeO) in the materials were identified by FT-IR The complex impedance and modulus studies suggested the presence of a non-Debye type of multiple relaxations in these materials The dielectric constant was found to increase with increasing Co and Zn concentrations The ac conductivity studies revealed a typical negative temperature coefficient of resistance (NTCR) behavior, and the conductivity values varied from 1.58 105to 8.46 106S cm1 The activation energy determined from the Arrhenius plots at 50 Hz was in the range of 0.23e0.78 eV
© 2017 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
The lithium-ion battery cathode materials are known to be
structured of layered compounds such as lithium transition metal
oxides LiMO2(M¼ Co, Ni and Mn), spinel compounds like LiMn2O4
and olivine compounds like LiMPO4(M¼ Fe, Ni, Co and Mn)[1e4]
cathode material in lithium-ion batteries because of its high
volu-metric energy density, excellent cyclability and the ease with which
it can be synthesized from raw materials LiCoO2 has, however,
three major disadvantages, including the high material cost,
toxicity and low (only 50%) use of the theoretical capacity Layered
LiCoO2and LiNiO2exhibit complementary behaviors; LiCoO2was
easy to synthesize, but it is very expensive when compared to LiNiO2 Moreover, LiNiO2shows the better electrochemical perfor-mance and is a low cost material The nickel containing compound LiNiO2and their doped derivatives have been extensively studied LiNiO2had the advantage of presenting a higher specific capacity for lithium cycling, but it was difficult to prepare in the layered structure due to the tendency of lithium and nickel, leading to the deterioration of their electrochemical performance Many re-searchers have undertaken the search for new cathode materials to overcome these shortcomings To reduce the cost and to improve the cell voltage and specific energy, other transition and non-transition metals, such as Cr, Mn, Fe, Al, Ca, Mg, Zn, Mn, Co, Ga,
Sn, Sr, Ti, Zr, Cu, Rh and rare earth elements Ce and Y were used in LiNiO2 [5,6] These materials have been synthesized by various methods, such as solegel, combustion, co-precipitation and solid-state reaction etc Among them, the latter method was desirable due to its simple and low cost route of fabrication[7] In this case, in order to obtain high performance well-ordered Li[Ni1/3Co1/3Mn1/3]
O2and LiNixMnyCo1xyO2cathode materials, the optimizing and
* Corresponding author Advanced Analytical Laboratory, DST-PURSE Programme,
Andhra University, India.
E-mail address: muraliphdau@gmail.com (N Murali).
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
http://dx.doi.org/10.1016/j.jsamd.2017.04.004
2468-2179/© 2017 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
Journal of Science: Advanced Materials and Devices 2 (2017) 233e244
Trang 2control of material parameters and synthesis procedure are rather
important [8,9] LiNi1xCoxO2, Al doped LiNiO2 and Mg doped
LiNiO2, have also been extensively investigated for their safety
characteristics Thus, electro-inactive non-transition metal ions,
partially Mg2þand Al2þ, have been of great interest as depots for
cathode materials However, to our best knowledge, another
po-tential non-transition metal ion Zn2þ, has not received as much
attention as a dopant for layered cathode materials[10]
In the present work, we report a systematic study of the
morphology, structure, dielectric and impedance performance of
LiNi1xy0.02CoxZnyMg0.02O2(x¼ y ¼ 0.0, 0.02 and 0.04) cathode
materials Here Zn and Mg acted as the electrochemically active
elements
2 Experimental
The cathode compositions were synthesized by a solid-state
reaction method using appropriate stoichiometric amounts of
Li2CO3(Merck 99.9%), MgO (Merck 99.9%), NiO (Merck 99.9%), CoO
(Merck 99.9%) and ZnO (Merck 99.9%)
A slight excess amount of lithium (5%) was used to compensate
for any loss of the metal which might have occurred during the
calcination at high temperatures The mixture of the starting
ma-terials was sufficiently mixed and after grinding the powder, it was
then heat treated in air at 500C for 5 h and it was again ground
and mixed, and calcined at 750C for 20 h Then, this powder was
cooled at the rate of 5C/min Finally, the powder was ground and
mixed, and calcined again at 850C for 20 h in air using a muffle
750C for 20 h After being added with polyvinyl alcohol (PVA) as a
binder, the powder was reground and thenfinally pressed at 5 tons/
cm2 pressure into a circular disk shaped pellet The pellets were
heated up at a heating rate of 5C/min and then sintered at 850C
for 20 h in air Finally they were cooled down at the rate of 5C/min
to room temperature The sintered pellets were carefully polished
on one side to obtain a smooth surface and then washed with
acetone After some proper drying, the pellets were coated with
silver paste on the opposite surface which then acted as an
electrode
The powder XRD data of the sample were collected on a Rigaku
Cu-Kadiffractometer with diffraction angles ranging from 20and
80 in an increment of 0.02 Unit cell lattice parameters were
obtained by the least squarefitting method from the d-spacing and
(hkl) values Further, the crystal size of the sample was obtained by
applying the Scherrer's equation from XRD pattern The particle
scanning electron microscopy image taken from CarlZeiss, EVOMA
15, Oxford Instruments, Inca Penta FETx3.JPG Fourier transform
infrared (FT-IR) spectra was obtained on a Shimadzu FT-IR-8900
spectrometer using a KBr pellet technique in the wave number
performed by a Hioki 3532-50 LCR Hitester in the frequency range
50 Hz to 1 MHz at temperatures between room temperature and
120C
3 Results and discussion
3.1 X-ray diffraction analysis
CoxZnyO2(x¼ y ¼ 0.0, 0.02 and 0.04) calcined at 850C for 20 h are
shown inFig 1 XRD patterns matched to LiNiO2of thea-NaFeO2
structure of the rhombohedral system indicating R3m space group
[12] The formation of single phase compounds is revealed by the
fact that all the observed peaks could be indexed within the
rhombohedral crystal structure in the R3m space group in accor-dance with JCPDS Card No 740919 The lattice constants a, c and the unit cell volume of the synthesized materials were calculated by the Unit-Cell Software (1995)[13]using the XRD data ofFig 1(aec) and the results are listed inTable 1 The lattice constants a and c are identified to be slightly increasing from 2.870 to 2.873 Å and 14.280
to 14.321Å respectively and the c/a ratio also increased from 4.980
to 4.985 as Mg, Co and Zn content was increased The synthesized materials showed a clear splitting of the (006) (102) and (108) (110) Bragg peaks broadening of all other diffraction peaks For the as-synthesized materials, the highest c/a, I(0 0 3)/I(1 0 4)ratios and the lowest R factor have also been found, indicating the least cation mixing and the best hexagonal ordering structure[14]
The average crystallite size was calculated from the full width at half maxima (FWHM) of the diffraction peaks using the Debye Scherrer's equation given by
D¼ kl=bcosq where D is the average crystalline size, k is shape factor,lis the wavelength of X-ray radiation,bis FWHM andqis the Bragg's angle The most intense peak (103) in the XRD pattern was used to calculate the average crystalline size and the results are listed in Table 1 The average crystalline size increased with the increase in
Co and Zn concentrations The layered structure of the synthesized materials was Li (3a: 0, 0, 0), Ni/Mg/Co/Zn (3b: 0, 0, 0.5) and O (6c:
0, 0, ~0.24), due to the absence of cation-mixing, i.e Ni and Li in the 3a- and 3b-sites, respectively[15]
3.2 Field effect scanning electron microscopy study FESEM images of LiNi1 xy0.02Mg0.02CoxZnyO2(x¼ y ¼ 0.0, 0.02 and 0.04) samples are shown inFig 2(aec) The particles have
obtained indicates the good crystallinity The electrochemical per-formance of the Lithium-ion battery depends directly on the
Fig 1 XRD patterns of LiNi 1xy0.02 Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04)
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 234
Trang 3particle size, particle size distribution and the morphology of the
cathode materials as well The grain growth at the higher
calcina-tion temperature was considered as rapid, leading to the larger
grain size The calculated particle sizes for x¼ y ¼ 0.0, 0.02 and 0.04
were 3.19, 3.53 and 3.85mm respectively The size or the texture of
the particles was highly even, which indicates high crystallinity and
the absence of defects in the pristine crystallites[16] The FESEM
images revealed the well crystallized particles with a similar
accumulative morphology[17] The primary particles of the
syn-thesized samples became well shaped and their size increased with
the increase of Co and Zn content The synthesized materials with a
smaller particle size with high capacity and uniform particle size
distribution enhance the overall battery performance by the
uni-form depth of charge of each particle[18]
3.3 Fourier transform infrared spectra analysis
LiNi1xy0.02Mg0.02CoxZnyO2(x¼ y ¼ 0.0, 0.02 and 0.04) calcined
at 850C for 20 h The band found around 536 cm1, is assigned to
LieO stretching vibration, which indicates the formation of LiO6 octahedra[19] The characteristic vibrations of CoeO, NieO, ZneO
respec-tively and are listed inTable 2 In this work, the broadband located
at around 638.69 cm1was attributed to the asymmetric stretching modes of MO6(M¼ Ni, Mg, Co and Zn) group[20]
3.4 Complex impedance spectroscopy analysis Complex impedance spectroscopy (CIS) technique was used
to analyze the electrical properties of a polycrystalline sample and its interface with electronically conducting electrodes in a wide
(30e120C).
Finally, Z0and Z00 are displayed in a Nyquist plot in order to
plotted inFig 4(aec) for LiNi1 xy0.02Mg0.02CoxZnyO2(x¼ y ¼ 0.0,
observed for the rise in temperature at low frequency It also shows
a decrease of Z0with an increase of frequency, forming a plateau like behavior which indicates the increase in conductivity of the material The dispersion at low frequency is attributed to the release of space charge polarization with the increased tempera-ture and frequency This shows that the conduction mechanism has
behavior suggests that the material possesses a negative temper-ature coefficient of resistance (NTCR)[22] It was found that the Z0 values decreased with the increase of temperature, indicating the reduction of the grain size, the grain boundaries and the elec-trode interface resistance The Z0values decreased slowly for the
Fig 2 (a), (b) and (c): FESEM images for LiNi 1xy0.02 Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04).
0
20
40
60
80
100
0
20
40
60
80
100 0
20
40
60
80
100
(c)
(a) (b)
Wavenumber (cm -1 )
Fig 3 FT-IR spectra for LiNi 1xy0.02 Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04)
Table 1
Lattice parameters, unit cell volume, I (003) /I (104) ratios and R-factor of LiNi1xy0.02Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04) compounds.
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 235
Trang 41 2 3 4 5 6 0.00
2.60x10 5
5.20x10 5
7.80x10 5
1.04x10 6
1.30x10 6
1.56x10 6
a 120 110 0 0C C
100 0C
90 0C
80 0C
70 0C
60 0C
50 0C
40 0C
30 0C
log f (Hz)
0.00
b
120 0C
110 0C
100 0C
90 0C
80 0C
70 0C
60 0C
50 0C
40 0C
30 0C
log f (Hz)
0.00
c
120 0C
110 0C
100 0C
90 0C
80 0C
70 0C
60 0C
50 0C
40 0C
30 0C
log f (Hz)
Fig 4 (a), (b) and (c): Real part of impedance as a function of frequency at different temperatures for LiNi Mg Co Zn O (x ¼ y ¼ 0.0, 0.02 and 0.04) materials.
Table 2
FT-IR wavenumbers for LiNi1xy0.02Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials.
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 236
Trang 5frequency depending on the temperature, and continuously with
an increase in frequency
Fig 5(aec) show the imaginary part Z00 of impedance as
LiNi1xy0.02Mg0.02CoxZnyO2(x¼ y ¼ 0.0, 0.02 and 0.04) materials
The relaxation species are possibly attributed to the presence of
electrons at low temperatures and of defects/vacancies at higher
temperatures It is clearly seen that the curves display broad and
low intensity peaks The peak frequency shifts to the high
fre-quency side with the increasing temperature, and the relaxation
occurs over several decades of frequency The plots show that Z00
values initially increased, attained a peak (Z00 max) and then
decreased with frequency at all measured temperatures Such a Debye-like relaxation peak in the frequency dependence of Z00 usually indicates the presence of space charges since the electrical behavior of space charges is dependent on the frequency It is also observed that the peak value of Z00decreased with the increased temperature, and the peak position was shifted to the high fre-quency side Peak broadening with the increase in temperature suggests the presence of a temperature dependent dielectric relaxation phenomenon[23]
The impedance diagram (Nyquist plot) is shown inFig 6(aec) for LiNi1xy0.02Mg0.02CoxZnyO2(x¼ y ¼ 0.0, 0.02 and 0.04) ma-terials for different temperatures All the semicircles exhibited
0.00
a
120 0C
110 0C
100 0C
90 0C
80 0C
70 0C
60 0C
50 0C
40 0C
30 0C
log f (Hz)
0.00 1.10x10 5 2.20x10 5 3.30x10 5 4.40x10 5 5.50x10 5 6.60x10 5
b
120 0C
110 0C
100 0C
90 0C
80 0C
70 0C
60 0C
50 0C
40 0C
30 0C
log f (Hz)
0.0
c
120 0 C
110 0 C
100 0 C
90 0 C
80 0 C
70 0 C
60 0 C
50 0 C
40 0 C
30 0 C
log f (Hz)
Fig 5 (a), (b) and (c): Imaginary part of impedance as a function of frequency at different temperatures for LiNi Mg Co Zn O (x ¼ y ¼ 0.0, 0.02 and 0.04) materials.
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 237
Trang 6some depression degree instead of the one, centered on the x-axis.
This can be referred to as the Non-Debye type of relaxation in which
there is a distribution of relaxation time [24] This non-ideal
behavior can be correlated to several factors, such as grain
orien-tation, grain boundary, stressestrain phenomena and atomic defect
distribution The presence of two semicircles at higher temperature
exhibits the presence of both grain interior (bulk property) and
grain boundary effect The contribution peak positioned at low
frequency corresponds to the grain boundary response and that in
the high frequency range, corresponds to the bulk property of the
material[25] The depression of the semicircle is considered as an additional evidence of the polarization phenomena with a distri-bution of relaxation times The assignment of the two semicircular arcs to the electrical response is due to the grain interior and grain boundary, and considered to be consistent with the“brick-layer model” for polycrystalline samples[26]
The ion transport process in ionic conductors was studied in terms of the electrical modulus spectrum In the present study, the impedance data were converted into electrical modulus by using the relation given by,
Fig 6 (a), (b) and (c): Nyquist plots at different temperatures for LiNi1xy0.02Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04).
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 238
Trang 7M0¼uC0Z0 ðreal partÞ and
M00¼uC0Z00 ðimaginary partÞ;
where M0and M00are respectively real and imaginary parts of the
modulus, and C0¼ ε0A/L, with A as the area of the sample, L as the
thickness of the sample, andε0as the permittivity of the free space
(8:854 1014F/cm)
LiNi1 xy0.02Mg0.02CoxZnyO2(x¼ y ¼ 0.0, 0.02 and 0.04) samples
It is observed that M0showed a constant value at higher frequencies
while at lower ones, it approached to zero for all temperatures, albeit showing a dispersion in the range of intermediate fre-quencies which increased as the temperature was increased It
is seen that at lower frequencies, M0approached zero, indicating that the electrode polarization had a negligible contribution to
relaxation The gradual variation of M0indicates that the relaxation processes are spread over a range of angular frequencies[27] The frequency dependence of the imaginary part of the electric modulus M00at different temperatures is shown inFig 8(aec) for LiNi1xy0.02Mg0.02CoxZnyO2(x¼ y ¼ 0.0, 0.02 and 0.04) materials Well defined peaks are seen in the modulus spectra These peaks represent the re-orientation relaxation process of mobile Liþions The low-frequency side of the peaks as seen in the region where Liþ
Fig 7 (a), (b) and (c): Real part of modulus as a function of frequency at different temperatures for LiNi Mg Co Zn O (x ¼ y ¼ 0.0, 0.02 and 0.04) materials.
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 239
Trang 8ions are capable of performing successful hopping from one site to
the next one, whereas the high frequency side of the peak is where
Liþions perform local motion (reorientation) only[28] The
posi-tion of the peaks in the modulus spectra shifted towards high
fre-quency as the temperature increased, which indicates a thermally
activated relaxation process The most probable conductivity
relaxation time is determined by the frequency of the peak
ac-cording to the relaxation It is clearly seen that the values of M00
increased with frequency at each temperature and were constant
for both high frequencies and low temperatures Between these
plateaus, the polarization effect is evidenced From the M00u), we
can observe a relaxation process with increased temperature that
dispersion region of M0(u)
Fig 9(aec) show the frequency dependence of the dielectric constant (ε) at room temperature for LiNi1xy0.02Mg0.02CoxZnyO2 (x¼ y ¼ 0.0, 0.02 and 0.04) samples The high value of the dielectric constant at low frequency and the low value of the dielectric con-stant at high frequency indicate a large dielectric dispersion due to the MaxwelleWagner type interfacial polarization The dielectric constant decreased with increasing frequency and temperature There is a sharp rise in the dielectric constant at low frequency and the shape of the rise changes with the temperature, due to the conducting ion motion The high value of the dielectric constant
reflects the effect of the space charge polarization and the con-ducting ionic motion When an external electricfield is applied, the electrons reach the grain boundary through hopping If the resis-tance due to the grain boundary is high, the electrons pile up at the Fig 8 (a), (b) and (c): Imaginary part of modulus as a function of frequency at different temperatures for LiNi1xy0.02Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04).
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 240
Trang 9grain boundaries and induce the polarization This is called space
charge polarization[29]
measured by the capacitance method based on the equation
εr¼ CL
ε A
whereε0¼ 8.854 1014F/cm is the permittivity of free space, C is the measured capacitance in Farads, L and A are the sample thick-ness and the electrode area, respectively
LiNi1xy0.02Mg0.02CoxZnyO2(x¼ y ¼ 0.0, 0.02 and 0.04) materials
as a function of frequency at different temperatures The a.c con-ductivity of the synthesized compounds was calculated from the Fig 9 (a), (b) and (c) the frequency dependence of dielectric constant (ε) at room temperature for LiNi1xy0.02Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials.
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 241
Trang 10values of the dielectric constant FromFig 11(aec) it is observed
that the plot shows two regions: one at low frequency dispersion,
which can be ascribed to space charge polarization of blocking
electrodes, and the second region which is independent of
fre-quency The conductivity in this region is almost constant
s¼R At
where t is the thickness, A is the area of cross-section, and R is the
bulk resistance of the sample The R value decreased with increase
in temperature
It can be observed thatsacincreased with increasing frequency
This can be explained in terms of conducting grains separated by
highly resistive grain boundaries According to this model, the a.c
conductivity at low frequencies exhibited the grain boundary
behavior, while the dispersion at high frequency is attributed to the
conductivity of grains[30] The calculated a.c conductivity values
8.67 1007S/cm for x¼ y ¼ 0.0, 0.02 and 0.04 It was observed
Fig 10 (a), (b) and (c): Variation of a.c conductivity of LiNi1xy0.02Mg 0.02 Co x Zn y O 2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials as a function of frequency at different temperatures.
Fig 11 Arrhenius plots of a.c conductivity of LiNi 1xy0.02 Mg 0.02 Co x Zn y O 2
(x ¼ y ¼ 0.0, 0.02 and 0.04) materials.
N Murali et al / Journal of Science: Advanced Materials and Devices 2 (2017) 233e244 242