Structural impedance dielectric and modulus analysis of lini1 x y 0 02mg0 02coxznyo2 cathode materials for lithium ion batteries

Journal of Science: Advanced Materials and Devices (2017) 233e244 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Structural, impedance, dielectric and modulus analysis of LiNi1-x-y-0.02Mg0.02CoxZnyO2 cathode materials for lithium-ion batteries N Murali a, b, *, S.J Margarette b, V Kondala Rao b, V Veeraiah b a b Advanced Analytical Laboratory, DST-PURSE Programme, Andhra University, India Department of Physics, Andhra University, Visakhapatnam, Andhra Pradesh, 530003, India a r t i c l e i n f o a b s t r a c t Article history: Received October 2016 Received in revised form 24 April 2017 Accepted 26 April 2017 Available online May 2017 Mg, Co and Zn co-substituted layer-structured cathode materials LiNiCoxZnyMg0.02O2 (x ¼ y ¼ 0.0, 0.02 and 0.04) were prepared by a solid-state reaction method The materials were systematically characterized 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 typical a-NaFeO2 layered structure within R3m space group The grain size was determined by FESEM in the range from 3.19 to 3.85 mm 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 Â 10À5 to 8.46 Â 10À6 S cmÀ1 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/) Keywords: X-ray diffraction FESEM Dielectric Impedance Electric modulus Introduction The lithium-ion battery cathode materials are known to be structured of layered compounds such as lithium transition metal oxides LiMO2 (M ¼ Co, Ni and Mn), spinel compounds like LiMn2O4 and olivine compounds like LiMPO4 (M ¼ Fe, Ni, Co and Mn) [1e4] The layered compound of LiCoO2 was commonly used as the cathode material in lithium-ion batteries because of its high volumetric 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 LiCoO2 and LiNiO2 exhibit complementary behaviors; LiCoO2 was * 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 easy to synthesize, but it is very expensive when compared to LiNiO2 Moreover, LiNiO2 shows the better electrochemical performance and is a low cost material The nickel containing compound LiNiO2 and their doped derivatives have been extensively studied LiNiO2 had 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 researchers 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 nontransition 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 solidstate 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] O2 and LiNixMnyCo1ÀxÀyO2 cathode materials, the optimizing and 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 (http://creativecommons.org/licenses/by/4.0/) 234 N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 control of material parameters and synthesis procedure are rather important [8,9] LiNi1ÀxCoxO2, Al doped LiNiO2 and Mg doped LiNiO2, have also been extensively investigated for their safety characteristics Thus, electro-inactive non-transition metal ions, partially Mg2ỵ and Al2ỵ, have been of great interest as depots for cathode materials However, to our best knowledge, another potential non-transition metal ion Zn2ỵ, 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 LiNi1ÀxÀyÀ0.02CoxZnyMg0.02O2 (x ¼ y ¼ 0.0, 0.02 and 0.04) cathode materials Here Zn and Mg acted as the electrochemically active elements 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 materials was sufficiently mixed and after grinding the powder, it was then heat treated in air at 500  C for h and it was again ground and mixed, and calcined at 750  C for 20 h Then, this powder was cooled at the rate of  C/min Finally, the powder was ground and mixed, and calcined again at 850  C for 20 h in air using a muffle box furnace [11] The calcined powder was further annealed at 750  C for 20 h After being added with polyvinyl alcohol (PVA) as a binder, the powder was reground and then finally pressed at tons/ cm2 pressure into a circular disk shaped pellet The pellets were heated up at a heating rate of  C/min and then sintered at 850  C for 20 h in air Finally they were cooled down at the rate of  C/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-Ka diffractometer with diffraction angles ranging from 20 and 80 in an increment of 0.02 Unit cell lattice parameters were obtained by the least square fitting 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 morphology of the powders was observed using a field effect 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 range between 400 and 1200 cmÀ1 The impedance study was performed by a Hioki 3532-50 LCR Hitester in the frequency range 50 Hz to MHz at temperatures between room temperature and 120  C rhombohedral crystal structure in the R3m space group in accordance 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 of Fig 1(aec) and the results are listed in Table The lattice constants a and c are identified to be slightly increasing from 2.870 to 2.873 Å and 14.280 to 14.321 Å 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 assynthesized materials, the highest c/a, I(0 3)/I(1 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 D ¼ kl=bcosq where D is the average crystalline size, k is shape factor, l is the wavelength of X-ray radiation, b is FWHM and q is 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 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 LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) samples are shown in Fig 2(aec) The particles have regular shapes with well defined faces and the microstructure obtained indicates the good crystallinity The electrochemical performance of the Lithium-ion battery depends directly on the Results and discussion 3.1 X-ray diffraction analysis XRD patterns of the as-synthesized LiNi1ÀxÀyÀ0.02Mg0.02 CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) calcined at 850  C for 20 h are shown in Fig XRD patterns matched to LiNiO2 of the a-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 Fig XRD patterns of LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 235 Table Lattice parameters, unit cell volume, I(003)/I(104) ratios and R-factor of LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) compounds Compound a (Å) c (Å) c/a Cell volume (Å)3 I(003)/I(104) R-factor Crystallite size (nm) LiNi0.98Mg0.02O2 LiNi0.94Mg0.02Zn0.02Co0.02O2 LiNi0.9Mg0.02Zn0.04Co0.04O2 LiNi0.98Mg0.02O2 [23] Mg doped LiNiO2 [24] 2.870 2.872 2.873 2.8724 2.8775 14.28 14.312 14.321 14.187 14.2144 4.98 4.983 4.985 e 4.9398 101.78 102.339 102.356 e 101.81 1.17 1.16 1.10 e 1.22 0.461 0.508 0.504 e e 62.31 67.289 73.15 e e Fig (a), (b) and (c): FESEM images for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) particle size, particle size distribution and the morphology of the cathode materials as well The grain growth at the higher calcination temperature was considered as rapid, leading to the larger grain size The calculated particle sizes for x ¼ y ¼ 0.0, 0.02 and 0.04 were 3.19, 3.53 and 3.85 mm 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 synthesized 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 uniform depth of charge of each particle [18] 100 (c) 80 60 Transmittance (%) 40 20 100 (b) 80 60 40 20 100 (a) 80 60 40 20 400 600 800 -1 Wavenumber (cm ) 1000 Fig FT-IR spectra for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials 3.3 Fourier transform infrared spectra analysis Fig 3(aec) show the FT-IR spectra of synthesized LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) calcined at 850  C for 20 h The band found around 536 cmÀ1, is assigned to LieO stretching vibration, which indicates the formation of LiO6 octahedra [19] The characteristic vibrations of CoeO, NieO, ZneO and MgeO were 560e590, 579, 611e735 and 630 cmÀ1, respectively and are listed in Table In this work, the broadband located at around 638.69 cmÀ1 was attributed to the asymmetric stretching modes of MO6 (M ¼ 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 frequency range (50 Hze1 MHz) at different temperatures (30 e120  C) Finally, Z0 and Z00 are displayed in a Nyquist plot in order to visualize the influence of the parameters The real impedance spectrum Z0 as a function of the frequency and temperature is plotted in Fig 4(aec) for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials The decreased values Z0 have been observed for the rise in temperature at low frequency It also shows a decrease of Z0 with 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 temperature and frequency This shows that the conduction mechanism has improved with increasing temperature and frequency [21] This behavior suggests that the material possesses a negative temperature 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 electrode interface resistance The Z0 values decreased slowly for the 236 N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 Table FT-IR wavenumbers for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials Compound Wavenumbers (cmÀ1) LiNi0.98Mg0.02O2 LiNi0.94Mg0.02Zn0.02Co0.02O2 LiNi0.9Mg0.02Zn0.04Co0.04O2 406.413 407.96208 407.96208 419.318 430.144 430.144 568.1691 568.1691 568.1691 613.271 613.27189 614.35248 846.128 856.1488 863.56968 876.578 881.5067 882.471 1.56x10 Z' (Ohm) 120 C 110 C 1.30x10 100 C 1.04x10 90 C 80 C 7.80x10 70 C 60 C 50 C 5.20x10 Z' (Ohm) 1.30x10 1.56x10 120 C a 110 C b 1.04x10 100 C 90 C 80 C 70 C 7.80x10 60 C 50 C 5.20x10 40 C 40 C 0 30 C 2.60x10 30 C 2.60x10 0.00 0.00 log f (Hz) log f (Hz) 120 C 6.60x10 c 5.50x10 110 C 100 C 90 C 80 C Z' (Ohm) 4.40x10 70 C 60 C 3.30x10 50 C 40 C 30 C 2.20x10 1.10x10 0.00 log f (Hz) Fig (a), (b) and (c): Real part of impedance as a function of frequency at different temperatures for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 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 Z00 decreased with the increased temperature, and the peak position was shifted to the high frequency 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 in Fig 6(aec) for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials for different temperatures All the semicircles exhibited frequency depending on the temperature, and continuously with an increase in frequency Fig 5(aec) show the imaginary part Z00 of impedance as a function of the frequency and temperature for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ 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 frequency 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 120 C 6.60x10 110 C 6.60x10 237 120 C 100 C Z'' (Ohm) 90 C 5.50x10 Z'' (Ohm) 80 C a 4.40x10 3.30x10 70 C 60 C 50 C 40 C 2.20x10 b 4.40x10 100 C 90 C 80 C 70 C 3.30x10 60 C 50 C 30 C 110 C 5.50x10 2.20x10 40 C 30 C 1.10x10 1.10x10 0.00 0.00 5 6 log f (Hz) log f (Hz) 3.0x10 120 C 110 C Z'' (Ohm) 2.5x10 c 2.0x10 100 C 90 C 80 C 70 C 1.5x10 60 C 50 C 1.0x10 40 C 30 C 5.0x10 0.0 log f (Hz) Fig (a), (b) and (c): Imaginary part of impedance as a function of frequency at different temperatures for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials 238 N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 Fig (a), (b) and (c): Nyquist plots at different temperatures for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) some 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 orientation, 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 distribution 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, N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 M ¼ uC0 Z real partị and 00 00 M ẳ uC0 Z ðimaginary partÞ; where M0 and M00 are respectively real and imaginary parts of the modulus, and C0 ¼ ε0A/L, with A as the area of the sample, L as the thickness of the sample, and ε0 as the permittivity of the free space (8:854 Â 10À14 F/cm) The real part of the modulus spectrum variation at different temperatures is shown in Fig 7(aec) for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) samples It is observed that M0 showed a constant value at higher frequencies 239 while at lower ones, it approached to zero for all temperatures, albeit showing a dispersion in the range of intermediate frequencies which increased as the temperature was increased It is seen that at lower frequencies, M0 approached zero, indicating that the electrode polarization had a negligible contribution to M0 only, and the dispersion is mainly due to the conductivity relaxation The gradual variation of M0 indicates that the relaxation processes are spread over a range of angular frequencies [27] The frequency dependence of the imaginary part of the electric modulus M00 at different temperatures is shown in Fig 8(aec) for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ 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 Liỵ ions The low-frequency side of the peaks as seen in the region where Liỵ Fig (a), (b) and (c): Real part of modulus as a function of frequency at different temperatures for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials 240 N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 Fig (a), (b) and (c): Imaginary part of modulus as a function of frequency at different temperatures for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) ions are capable of performing successful hopping from one site to the next one, whereas the high frequency side of the peak is where Liỵ ions perform local motion (reorientation) only [28] The position of the peaks in the modulus spectra shifted towards high frequency 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 according 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 M00 (u), we can observe a relaxation process with increased temperature that exhibits a maximum value, with M00 max at the center in the dispersion region of M0 (u) Fig 9(aec) show the frequency dependence of the dielectric constant (ε) at room temperature for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ 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 constant 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 conducting ionic motion When an external electric field is applied, the electrons reach the grain boundary through hopping If the resistance due to the grain boundary is high, the electrons pile up at the N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 241 Fig (a), (b) and (c) the frequency dependence of dielectric constant (ε) at room temperature for LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials grain boundaries and induce the polarization This is called space charge polarization [29] The relative dielectric constant εr of all the samples was measured by the capacitance method based on the equation εr ¼ CL ε0 A where ε0 ¼ 8.854 Â 10À14 F/cm is the permittivity of free space, C is the measured capacitance in Farads, L and A are the sample thickness and the electrode area, respectively Fig 10(aec) shows the a.c conductivity of LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials as a function of frequency at different temperatures The a.c conductivity of the synthesized compounds was calculated from the 242 N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 Fig 10 (a), (b) and (c): Variation of a.c conductivity of LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials as a function of frequency at different temperatures values of the dielectric constant From Fig 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 frequency The conductivity in this region is almost constant s¼ t RÂA 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 that sac increased 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 at different temperatures are shown in Table These ionic conductivity values are varied between 1.58 Â 10À5 and 8.67 Â 10À07 S/cm for x ¼ y ¼ 0.0, 0.02 and 0.04 It was observed Fig 11 Arrhenius plots of a.c conductivity of (x ¼ y ¼ 0.0, 0.02 and 0.04) materials LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 N Murali et al / Journal of Science: Advanced Materials and Devices (2017) 233e244 243 Table a.c conductivity values of LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials Temperature ( C) a.c conductivity (S/cm) LiNi0.98Mg0.02O2 30 40 50 60 70 80 90 100 110 120 9.05 2.15 2.62 3.26 4.07 4.96 5.96 6.95 7.59 8.14 Â Â Â Â Â Â Â Â Â Â À07 10 10À06 10À06 10À06 10À06 10À06 10À06 10À06 10À06 10À06 that the sample with x ¼ y ¼ 0.02 possessed good ionic conductivity compared to those with x ¼ y ¼ 0.0 and 0.04 Finally, these results also reveal a high ionic conductivity in samples with low additive concentrations (x ¼ y ¼ 0.02) The temperature dependence of the a.c conductivity of the materials can be expressed by the Arrhenius equation, A T  E kT s ¼ exp À  where s is conductivity, T is the absolute temperature, k is the Boltzmann constant, A is the pre-exponential factor and E is the activation energy When log (s) is plotted against 1/T, a straight line is expected with slope (ÀE/k) and an intercept on the log (s) axis of log A The slope of the line leads us to determine the activation energy The plots between log s and 1000/T K in Fig 11(aec) are found to be almost linear For the activation energy of the synthesized samples, the lowest values of 0.78, 0.23 and 0.25 eV have been derived for the 50 kHz frequency Conclusion We have successfully synthesized LiNi1ÀxÀyÀ0.02Mg0.02CoxZnyO2 (x ¼ y ¼ 0.0, 0.02 and 0.04) materials using the solid-state reaction method The XRD studies have revealed single phase formation of the orthorhombic structure in these materials The FESEM images show the grain size in the range of to mm and the presence of agglomeration FESEM micrographs have revealed that the grain size in the synthesized samples increases with increasing Co and Zn content The complex impedance spectroscopy study of the dielectric, impedance and modulus properties of the samples has suggested the multiple relaxation behavior In addition, at high frequencies, the dielectric constant becomes nearly frequency independent Nyquist plots have shown both grain and grain boundary contributions to the impedance The shape of the imaginary part of the modulus suggests that the relaxation processes in these materials are of non-Debye in nature The good ionic conductivity of 1.58 Â 10À5 S/cmÀ1 has been obtained for LiNi0.9Mg0.02Co0.02Zn0.02O2 at 383 K The lowest value of the activation energy of 0.23 eV has been found at 50 kHz References [1] J.R Dahn, U Von Sacken, C.A Michal, Structure and electrochemistry of Li1±yNiO2 and a new Li2NiO2 phase with the Ni(OH)2 structure, Solid State Ionics 44 (1990) 87e97 [2] C.C Chang, P.N Kumta, Particulate sol-gel synthesis and electrochemical characterization of LiMO2 (M¼Ni, Ni0.75 Co0.25) powders, J Power Sources 75 (1998) 44e55 LiNi0.94Mg0.02Zn0.02Co0.02O2 À07 8.67 Â 10 1.52 Â 10À06 2.13 Â 10À06 3.45 Â 10À06 4.87 Â 10À06 6.88 Â 10À06 9.46 Â 10À06 1.27 Â 10À05 1.58 Â 10À05 1.3 Â 10À05 LiNi0.9Mg0.02Zn0.04Co0.04O2 2.59 3.91 5.59 7.71 1.05 1.38 1.81 2.35 2.62 8.46 Â Â Â Â Â Â Â Â Â Â 10À06 10À06 10À06 10À06 10À06 10À06 10À06 10À06 10À06 10À06 [3] R.V Moshtev, P Zlatilova, V Manev, A Sato, The LiNiO2 solid solution as a 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electrical properties of lithium nickel manganese oxide (LiNi0.5Mn0.5O2), Int J ChemTech Res (2014) 5252e5255  ... 5.2 0x1 0 40 C 40 C 0 30 C 2.6 0x1 0 30 C 2.6 0x1 0 0 .00 0. 00 log f (Hz) log f (Hz) 1 20 C 6.6 0x1 0 c 5.5 0x1 0 1 10 C 100 C 90 C 80 C Z'' (Ohm) 4.4 0x1 0 70 C 60 C 3.3 0x1 0 50 C 40 C 30 C 2.2 0x1 0 1.1 0x1 0 0 .00 ... 2.2 0x1 0 b 4.4 0x1 0 100 C 90 C 80 C 70 C 3.3 0x1 0 60 C 50 C 30 C 1 10 C 5.5 0x1 0 2.2 0x1 0 40 C 30 C 1.1 0x1 0 1.1 0x1 0 0 .00 0. 00 5 6 log f (Hz) log f (Hz) 3. 0x1 0 1 20 C 1 10 C Z'''' (Ohm) 2. 5x1 0 c 2. 0x1 0 100 ... 881. 506 7 882.471 1.5 6x1 0 Z'' (Ohm) 1 20 C 1 10 C 1.3 0x1 0 100 C 1 .0 4x1 0 90 C 80 C 7.8 0x1 0 70 C 60 C 50 C 5.2 0x1 0 Z'' (Ohm) 1.3 0x1 0 1.5 6x1 0 1 20 C a 1 10 C b 1 .0 4x1 0 100 C 90 C 80 C 70 C 7.8 0x1 0 60 C 50