Preparation , dielectric and conductivity studies of LiNi 1x Mg x O 2 cathode materials for lithium-ion batteries

A layered structure cathode materials LiNi1-xMgxO2 (where x = 0.0 and 0.02) were prepared by solid state reaction from Li2CO3, MgO and NiO. The thermal analysis of the precursor materials up to 750 °C showed the weight loss of about 30%. The X-ray diffraction patterns of the synthesized samples confirmed the presence of α-NaFeO2 structure with the rhombohedral-hexagonal symmetry (space group, R3̄m) and no evidence of any impurities. The peaks intensity ratio I003/I104 increased with Mg concentration, which indicates the cation mixing within transition metal layer and lithium layer. The Fourier transform infrared spectroscopy showed the presence of two broad vibrational bands between 400 and 1000 cm and corresponding bands for the 3a and 3b octahedral sites of LiO 6 and MO 6 units. The scanning electron microscopy analyses showed that the prepared samples have porous and relatively uniform distribution of grain sizes in the range 0.4–3μm. The complex impedance spectroscopy studies of the samples sintered at 850 °C were done in the frequency range from 42 Hz to 1 MHz with temperature variation from 30 to 100 °C. The results revealed that the total electrical conductivity increases with temperature and frequency and at 100 °C it is 7.06×10 and 9.00×10 S/cm for the LiNiO2 and LiNi1-xMgxO2 materials, respectively. The obtained relatively high conductivity and low dielectric constant confirmed that the prepared ceramics can be used as cathode materials for Li-ion batteries.


I. Introduction
Ceramic cathode materials are currently used in most of the commercially available lithium-ion batteries.They have wide applications, such as in electronic mobile devices and electric vehicles [1][2][3][4][5][6].The cathode materials are primarily categorized based on their structure, those having layered structures (LiCoO 2 , LiNiO 2 and LiMnO 2 ), olivine structure (LiFePO 4 based materials) and spinel structure (LiMn 2 O 4 with threedimensional framework structure) [7][8][9][10][11].In the past decades, the layered structure LiCoO 2 cathode material was commercially used very successfully.However, LiCoO 2 has a high manufacturing cost and insufficient reversible capacity of 140 mA h/g [12].One of the most promising alternative materials for resolving these is-sues is LiNiO 2 [13].Recently, LiNiO 2 has been the most widely used cathode material in commercial Liion batteries because of its high discharge capacity, low cost and environmentally acceptable properties.However, it is difficult to obtain stoichiometric LiNiO 2 by solid-state reaction method at high temperature in air atmosphere.The difficulty in synthesizing stoichiometric LiNiO 2 is also due to the loss of lithium from the host structure during high temperature calcinations because of the high vapour pressure of lithium, thus leading to the formation of non-stoichiometric structure [14].This results in a lower initial capacity as well as severe capacity loss upon cycling.Another problem is that its capacity fades, observed even for the stoichiometrically pure LiNiO 2 , due to the formation of inactive NiO 2 phase.NiO 2 phase is formed due to the irreversible phase transitions occurring in LiNiO 2 structure when charging up to a high voltage.Hence, for these reasons, LiNiO 2 is represented as Li x NiO 2 where x in this case indicates the non-stoichiometry [15,16].The problem might be solved with substitutions.Thus, in LiNi 0.5 Co 0.5 O 2 material, cobalt substitution can decrease cation mixing and stabilize the layered structure [17].
In the present work, LiNi 1-x Mg x O 2 (where x = 0.0 and 0.02) cathode materials were fabricated at 850 °C for 20 h.An investigation of the structural, morphological and electrical properties of the synthesized materials was performed.

II. Experimental procedure
The cathode materials were synthesized by a solidstate reaction method from stoichiometric amounts of Li 2 CO 3 (Merck, 99.9%), MgO (Merck, 99.9%) and NiO (Merck, 99.9%).A slight excess amount of lithium (5%) was used to compensate for any loss of the metal that might have occurred during the calcination at high temperatures.The starting materials were sufficiently mixed, ground and heat treated in air at 500 °C for 5 h.In the second step, the obtained powder was again ground, mixed and calcined at 750 °C for 20 h.Then, the prepared particles were cooled at the rate of 5 °C/min.Finally, the powder was ground, mixed and calcined at 850 °C for 20 h in air using a muffle box furnace.The pellets (with diameter of 10 mm and thickness of 1.1 to 1.25 mm) were also prepared first by mixing the sample and PVA in an agate mortar and pestle for one hour and then by pressing with hydraulic press at 5 t for 6 minutes.The pressed pellets were then sintered at 850 °C for 20 hours.
TG/DTA measurements were conducted using Mettler Toledo TG/DTA 851e instrument from room temperature to 1000 °C in nitrogen atmosphere at the heating rate of 10 °C/min.The phase identification and crystal structure characteristics were determined by X-ray diffraction (XRD) using a Rigaku Cu-Kα diffractometer with diffraction angles between 20 and 80°at the wavelength of 1.5406 Å.The morphologies of the synthesized materials were studied by scanning electron microscopy (FE-SEM, Carl Zeiss, EVO MA 15, Oxford Instruments).The Fourier transform infrared spectra were recorded using Nicolet 6700 using KBr pressed pellet technique.The impedance study on the sintered pellets was performed by a Hioki 3532-50 LCR Hitester in the frequency range 42 Hz to 1 MHz at temperature range from room temperature to 100 °C.The surface layers of the sintered pellets were carefully polished, washed with acetone and coated with silver paste to obtain electrodes.

Thermal analysis
TG/DTA analyses of the precursor mixtures (LiNi 1-x Mg x O 2 where x = 0.0 and 0.02) are presented in Fig. 1.As it can be seen, there is a little change in the weight loss as the result of Mg substitution.The weight loss from room temperature to 150 °C can be related to the loss of water absorbed on the surfaces and some intercalated water molecules that are bonded to hydroxyl groups.The endothermic peaks around 200-250 °C in nitrogen atmosphere were due to the decomposition of M(OH) 2 (M = Ni and Mg) to spinel phase M 3 O 4 [18,19].The exothermic peaks around 300-400 °C resulted from the decomposition reaction of Li 2 CO 3 to Li 2 O.The hexagonal phased LiNiO 2 and doped cathode material are formed at higher temperatures.The weight loss was almost zero after 750 °C, which indicated that the phase formation of LiNi 1-x Mg x O 2 (where x = 0.0 and 0.02) was completed at this temperature [20].

XRD analysis
X-ray diffraction patterns of the LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 pellets heat treated at 850 °C are shown in Fig. 2. The XRD spectra showed that both samples have a typical LiNiO 2 layered structure without any impurity peaks.The intensity vs. diffraction angle peak data showed the similarity to a hexagonal (α-NaFeO 2 ) structure [21,22].All peaks appearing in the XRD patterns were identified with the characteristic LiNiO 2 peaks of the Joint Committee on Powder Diffraction Standards (JCPDS) as well as previously reported works [23,24].The lattice constants of the synthesized materials were calculated by the Unit-Cell software [25] and listed in Table 1.The Mg substitution resulted in slight changes of the lattice constants, i.e. a increases from 2.865 to 2.870 and c increases from 14.24 to 14.28.The most intensive XRD peaks corresponding to the planes (003), ( 101) and (104) suggested the welldefined layered structure.
Generally, the oxygen sub-lattice in the α-NaFeO 2 hexagonal structure forms a closely packed face centred cubic (fcc) lattice with a distortion in the c-direction, which confirms the compound to be hexagonal [26].The presence of distortion in the c-direction of the LiNiO 2 compound was confirmed by the peak splitting at 2θ ∼37°and ∼64°.This corresponds to (006) (012) and (108) (110) plane doublets, respectively.For the prepared materials, the strongest XRD line was observed for (003) plane.The integrated intensity ratio I 003 /I 104 and the split of XRD lines 012 and 110 might serve as a reliable quantitative criterion for the determination of electrochemical activity of LiNiO 2 .Both materials, LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 , showed a high I 003 /I 104 value and a clear splitting of XRD lines [27].The intensity ratio (I 003 /I 104 ) is an indicator of the degree of displacement of ions between lithium layers (at 3a site) and transition metal layers (at 3b site).If this ratio was in the range of 1.2 to 1.6, it indicated cation mixing/disorder which lead to poor electrochemical performance of the battery [28].In the present work, the observed intensity ratios (I 003 /I 104 ) for the prepared LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 compounds are 1.05 and 1.17, which indicate reasonably good electrochemical performance.The value of (I 006 + I 012 )/I 101 is called Rfactor, which is known to be smaller when the hexagonal ordering is high.This value was useful to measure the cation disorder.As it can be seen from Table 1, the R-factor for LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 are 0.5 and 0.46, respectively, suggesting that the hexagonal ordering is good [29].

SEM analysis
The grain size and surface morphology of the materials sintered at 850 °C were investigated by scanning    electron microscopy (FE-SEM).Figure 3 showed that the prepared samples have porous microstructure and relatively uniform distribution of grain sizes.The sintered LiNi 0.98 Mg 0.02 O 2 sample has a somewhat coarser structure, which could be caused by Mg addition.The average grain sizes were estimated to be between 2.7 µm and 3.2 µm for LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 .This unique morphology is advantageous for electrode materials, because it allows improved electrochemical performances [30,31].

FTIR studies
Typical FTIR spectra of the LiNi 1-x Mg x O 2 (x = 0.0 and 0.02) cathode materials are depicted in Fig. 4. The FTIR spectra display the predominance of the stretching modes and the IR resonance frequency of LiO 6 octahedra located between 400 and 460 cm -1 .There are four infrared active vibrations 2 A 2u + 2 E u for the D 5 3d group based on group theory analysis [32].The Wyckoff sites 3a and 3b consist of Li and transition metal cations (i.e.Ni and Mg), respectively.Thus, the separation into LiO 2 and MO 2 mode layers could be seen.The band observed around 532 cm -1 was assigned to the asymmetric stretching modes of M-O bonds in MO 6 octahedra and the other band around 608.13 cm -1 was attributed to the O-M-O bending modes [33,34].The broadening of FTIR bands is due to the cation mixing in the crystal layers.When Mg substitution increased, the stretching and bending mode frequencies slightly shifted to higher wavenumber regions, which attributed to changes in covalent character of M-O bond.

Impedance spectroscopy studies
The electrochemical impedance spectroscopy (EIS) technique was used to study the electrode materials because it can reveal the relationship between the crystal lattice and the electrochemical properties [35].In complex impedance plot the semi-circle at lower frequencies is due to the contribution of electrode-material interface.Another semicircular arc at intermediate frequencies is due to the contribution of grain boundary and at high frequencies the semicircular arc is due to the bulk response of the material [36,37].
Figure 5 shows Nyquist plots of the LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 electrodes during the course of Li deintercalation.The presence of semicircular arcs is clearly visible.The high Z ′ and Z ′′ values at lower frequency regions are due to the higher polarization caused by space charge.It was also observed that the values of Z ′ and Z ′′ gradually decrease with the increase of frequency and temperature, which indicated an increase in AC conductivity with increasing temperature and frequency.In addition, the observed decrease of Z ′ values at low frequency decreased with the increase in temperature showing negative temperature coefficient of resistance (NTCR) type of behaviour similar to semiconductors [38].
The conductivity of the electrode is considered as a characteristic measure for the cathode performance.However, the electronic properties of cathode materials have not been widely reported.In the previous studies on cathode materials it was established that manufacturing methods affect the conductivity properties [39].In this study the real and imaginary parts of impedance were evaluated as a function of frequency.The data collected were analysed in the form of impedance plots.From the impedance plots, we obtained bulk resistance of the sample.Finally, AC conductivity was calculated by using the relation: where t is the thickness, a is the area of cross-section and R is the bulk resistance of the sample.The frequency dependence of AC conductivity of the LiNi 1-x Mg x O 2 (x = 0.0 and 0.02) compounds were presented in Fig. 6 and only the corresponding σ AC values for different temperatures at frequency of 42 Hz were listed in Table 3.The conductivity of the cathode increased with an increase in the temperature.The highest electrical conductivity for LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 was found to be 7.06 × 10 -6 and 9.00 × 10 -6 S/cm, respectively.The results showed increase of the electrical conductivity in the Mg substituted compound.This was attributed to the weak Ni-O bond strength compared to Mg-O bond.

Dielectric properties
The dielectric properties as a function of frequency reflected the dynamic response of the constituents of the material.The relative dielectric constant ε r of both samples was measured by the capacitance method based on the following equation: where ε 0 = 8.854 × 10 -14 F/cm is the permittivity of free space, C is the measured capacitance, L and A are the sample thickness and electrode area respectively.The frequency dependences of the dielectric constant (ε r ) of the sintered LiNi 1-x Mg x O 2 (x = 0.0 and 0.02) are shown in Fig. 7.The recommended cathode materials for Liion batteries should have a low dielectric constant and high conductivity.As it can be seen (Fig. 7), the dielectric constant of the sintered cathode materials (LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 ) are relatively low.The obtained low value of dielectric constant and relatively low resistance can be used as criteria for choosing the cathode materials for use in Li-ion batteries [42].

IV. Conclusions
LiNi 1-x Mg x O 2 (x = 0.0 and 0.02) cathode materials were synthesized by solid state reaction method and sintered at 850 °C for 20 hours.The microstructure, morphology and bonding nature of the synthesized materials were characterized by TG/DTA, XRD, FESEM and FTIR.The intensity vs. diffraction angle peak data showed presence of the hexagonal α-NaFeO 2 structure (with space group R 3m) without any impurity phase.SEM and FTIR analyses confirm presence of uniform LiNiO 2 layered structure with the average grain size if about 3 µm.The AC conductivity of the LiNiO 2 and LiNi 0.98 Mg 0.02 O 2 compounds at 100 °Cwas found to be 7.06 × 10 -6 and 9.00 × 10 -6 S/cm, respectively.This together with low value of dielectric constant confirms that the prepared ceramics can be used as cathode materials for Li-ion batteries.