COMPARATIVE STRUCTURAL STUDIES OF LINI 0.8 CO 0.1 MN 0.1 O 2 CATHODE MATERIALS PREPARED BY SOL-GEL VERSUS SOLID-STATE-REACTION METHOD.

In the present work, the layered structure oxide LiNi 0.8 Co Mn 2 (NCM 811) is synthesized by sol-gel auto combustion and solid state reaction methods. Microstructural and optical properties are investigated by XRD, FE-SEM with EDS, FT-IR and bonding nature studies. Characterization of the crystalline powders, phase identification, particle size examination, and morphological studies are done using XRD and FE-SEM. From FT-IR and UV-NIR studies, vibrational bands are identified in the range of 400-2000 cm-1 representing the MO 2 (M = Ni, Co & Mn) layers.


ISSN: 2320-5407
Int. J. Adv. Res. 7 (10), 1310-1317 1311 controlled by ammonia solution to 8 ~ 9. The solutions are added together under stirring at 130°C for 10 hours, forming a sol solution. The sol solution is vaporized at 130°C till the dry gel is formed, followed by the heat treatment at 500° C for 6h in the air with a 5°C/ min heating rate to eliminate the organic residues. The powders are thoroughly ground and then sintered at 850 °C for 20 hours in the air to obtain the required compounds.

Solid-state reaction method:
The cathode compositions are synthesized by a solid-state reaction method from stoichiometric amounts of Li 2 CO 3 (Merck 99.9%) and NiO (Merck 99.9%), CoO (Merck 99.9%) and MnO(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 is sufficiently mixed and after grinding the powder it is then heat-treated in the air at 500 ºC for 5 h and it is again ground and mixed, and calcined again at 750 ºC for 20 h. Then, this powder was cooled at a rate of 5 ºC/ min. Finally, the powder was ground and mixed, and calcined again at 850 ºC for 20 h in the air using a muffle box furnace.
The powder X-ray diffraction (XRD) data of the samples are collected on a Rigaku Cu-Kα diffractometer with diffraction angles of 20º and 80º in increments of 0.02º. The unit cell lattice parameter is obtained by the least square fitting method from the d-spacing and (hkl) values. Further, the crystal size of the sample is obtained by applying the Scherrer's equation from the XRD pattern. The particle morphology of the powders is observed using a field effect scanning electron microscopy images taken from CarlZeiss, EVOMA 15, Oxford Instruments, Inca Penta FETx3.JPG. Fourier transform infrared (FT-IR) spectra are obtained on a Shimadzu FT-IR-8900 spectrometer using KBr pellet technique in the wavenumber range between 350 and 800 cm -1 . Figure 1 represents the TG curves of LiNi 0.8 Co 0.1 Mn 0.1 O 2 gel precursors synthesized by sol-gel auto combustion method using citric acid as a chelating agent. The formation of synthesized material takes place around 500 0 C, which is observed from the TG curve. The major weight loss (around 50%) occurs from 290-485•C, which leads to the decomposition of acetates corresponding exothermic peak appears around 485•C. After 485•C TG curves become flat which indicates that no weight loss occurs in this region; further heating only assists the crystallization event of the LiNi 0.8 Co 0.1 Mn 0.1 O 2 material. Similarly, solid state reaction method the first weight loss region from 0 to 100•C corresponds to the loss of absorbed water molecules. The next weight loss region (100-290•C) indicates that the decomposition of the chelating agent. From figure 2 shown from the curves, there is an initial weight loss in the temperature range from room temperature to 310°C. This corresponds to the evaporation of methanol used during grinding to homogenize the mixture and the moisture absorbed during storage [21]. TG curve of the synthesized materials are shows significant weight loss (17.8%) between the temperatures 310°C and 570°C. This loss may be due to the decomposition of the precursors Li 2 CO 3 , NiO 2 , Co 2 O 3 and MnO 2 , and the reaction between the decomposed materials thereby to produce crystalline materials. Though, Li 2 CO 3 is stable in air up to 750°C and the melting point is 723°C, some studies have point out that Li 2 CO 3 already reacts below 300°C [22]. At the proposed temperature, the TG curve becomes more flattened, indicating stable phase is formed.  Figure 3. Both XRD patterns of the powders prepared by sol-gel auto-combustion and solid-state method show X-ray peaks corresponding to LiNi 0.8 Co 0.1 Mn 0.1 O 2 can be indexed to a phase-pure hexagonal structure with the space group of R3m and match well with diffraction file (JCPDS) 74 -0919 [23]. The synthesized material is with no evidence of any impurities, indicating that the uniform solid solution has been formed in the compounds [24]. The crystal lattice parameters of the compounds are calculated by using the unit cell program and presented in Table 1 The a-axis increases linearly from 2.893 to 2.896 Å and the c-axis decreases from 14.238 to 14.23 Å. The ratio of c/a decreases from 4.922 to 4.915Å due to the substitution of Ni ions by Co and Mn ions probably result from the severe cation mixing because it has the lowest intensity ratio of (0 0 3) to (1 0 4) peaks as given in Table 1. The unit-cell volume also increases from 103.205 to 103.371 Å 3 . The crystallite sizes of the cathode powders are prepared from the sol-gel auto-combustion and solid-state reaction methods are shown in 9.8 nm and 12.67 nm. The Bragg intensity ratio of I(0 03)/I(1 04), which is a signature of the cation-mixing and Its values are 1.1602 and 1.1361. It has been reported that if this value is < 1.2 for the undesirable cation mixing would occur in the lattice. The crystallite size can be estimated using Scherrer's formula given in eq. (1). D = 0.9/βcosθ --------------(1) where, λ is the wavelength of X-ray used, which is CuKα radiation (λ= 1.5406 Å), and β is the full width at halfmaximum of the diffraction peak corresponding to 2θ. Using the above equation the sizes of the crystallites are found to be in the range of nanometer [26]. The crystallite size is measured by taking the average of three mainline widths, which are obtained from the XRD patterns. The crystallite size calculated using the Debye Scherrer's formula is listed in the table. 1.   Figure 4 and 5 shows the FE-SEM images of the LiNi 0.8 Co 0.1 Mn 0.1 O 2 compound in sol-gel auto-combustion and solid-state reaction method. Both compounds have a smooth, well-shaped and particle-agglomerated morphology [27]. The average size of the primary particle is about 500 nm and 2 µm. These samples have the accumulation morphology, but the primary particles become well-shaped and their size increases. In this results suggest that foreign metal ion doping could improve the crystallinity of the synthesized samples. The precursor powders with spherical shape and dense morphologies were necessary to prepare the cathode powders with spherical shape and dense structure by both methods with a lithium component. Figure 3 and 4 shows the EDS spectra, qualitative results and elemental analysis that confirms the presence of Ni, Co, Mn and O in LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode material. It is clearly observed that the spectra show the appropriate ratios of the elements. Lithium is not observed in the EDS spectrum because it has too low of an atomic number to be detected with EDS.  Figure 4(b) shows the IR spectra of the synthesized sample prepared by solid state reaction method at air atmosphere. It can be seen that strong absorption peaks are observed in the region 400-1000 cm -1 .

Optical properties
1315 It was carried out by measuring the diffuse reflectance spectroscopy in UV-Vis range. The spectrum was taken in the range of 200-700 nm. Fig.1 [31][32]. The band gap energy was calculated from the diffuse-reflectance spectra by plotting the square of the Kubelka-Munk function given as [33], where K is reflectance transformed according to Kubelka Munk, R is reflectancy (%), hv is the photon energy. The E g were measured with the help of reflectance spectra plotting graphs of (K*hv) n versus (hv).

Conclusions:-
The layered structure material LiNi 0.8 Co 0.1 Mn 0.1 O 2 is synthesized by sol-gel auto-combustion and solid-state reaction method. The cathode material resembled the hexagonal α-NaFeO 2 structure which was confirmed from the XRD pattern. This result strongly supports the results obtained for the preparation met hod is sol-gel.