STRUCTURAL, MORPHOLOGICAL AND VIBRATIONAL STUDIES OF MAGNESIUM DOPED LITHIUM TITANATE ANODE MATERIALS FOR LI-ION BATTERIES

1. Department of Nuclear Physics, Andhra University, Visakhapatnam, 530003, India. 2. Department of Physics, Aditya Engineering College (A), Kakinada, 533005, India. 3. Department of Physics, Dr. B. R. Ambedkar University, Srikakulam, 532410, India. 4. Department of HBS, WISTM Engineering College, Visakhapatnam, 531173, India. 5. Department of ECE, Aditya College of Engineering and Technology, Kakinada, 533005, India. 6. Department of Physics, Andhra University, Visakhapatnam, India -530003. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 05 September 2020 Final Accepted: 10 October 2020 Published: November 2020


ISSN: 2320-5407
Int. J. Adv. Res. 8 (11), 386-394 387 Lithium and Titanium sites can be used as substituents. Among all these, Magnesium is very inexpensive and anticipated to even out the structure, it has been chosen as a dopant for Lithium site [9][10][11][12][13][14]. In the structure, the doping of divalent state of Magnesium for monovalent state of Lithium requires that the charge difference is adjusted by lessening corresponding number of Titanium ions from Ti 4+ to Ti 3+ . This resulted not only in amplification of Li 4 Ti 5 O 12 conductivity but also in the electrochemical efficiency relative to the Li 4 Ti 5 O 12 material.
The cubic spinel structure of LTO is represented as is Li 8a [Ti 5/3 Li 1/3 ] 16d O 4 , in this case 75% of the Li + are placed in tetrahedral 8a sites, while the residual ions are like Li + and Ti 4+ as positioned at the 16c octahedral sites of symmetry group with Fd3m [15][16][17]. In this chemical reaction process the tetrahedral-position of Li + freely moves into neighboring 16c octahedral positions to create a Rock salt structure (Li 2 ) 16c (Ti 5/3 Li 1/3 ) 16d O 4 . Mg is chosen for substituting in LTO as Li and Ti lattice sites are not altered in Li 4-x Mg x Ti 5 O 12 prepared through the solid state method [18][19][20][21]. Here we reported the synthesis and structural properties of the Mg doped Li 4 Ti 5 O 12 anode materials to understand the phase, morphology and vibrational properties.

Preparation and Experimental Characterizations:
The anode material compositions are prepared through solid state route from stoichiometric ratios of Li 2 CO 3 , TiO 2 , and MgO is taken as the raw material from the Himedia with purity of 99.9%.
A three percentage of surplus quantity of Li 2 CO 3 is put to use so as to nullify any sort of loss of the material which might have taken place in the process of heating. Initially the unprocessed materials are methodically mixed by making use of agate and mortar; further methanol is added to obtain homogeneity of the materials and grinded for 8h. The material is calcined for 20 hours in air at 850 °C utilizing a programmable furnace to dry the compounds from any impurities. The samples are cooled to ambient temperature and then ground for 2 hours in the mortar again so as to arrive at the final powder.
The TG and DTG measurements are taken by Mettler Toledo TG/DTG 851 e instrument from 30-1000 °C in N 2 atmosphere at a heating rate of 10°C /minute. The XRD properties of the samples are studied by Rigaku X-ray diffractometer using the CuKα radiation (wavelength = 1.54 Å) containing diffraction angle range from 10º to 90º with successive 0.02º angular increments. The morphology of the material grains is observed using FESEM micrographs taken from CarlZeiss, EVOMA 15, Oxford Instruments, Inca Penta FETx3.JPG. FT-IR spectra are extracted by making use of a Shimadzu IR-Prestige21 spectrometer utilizing the methodology of KBr pellet in the wavenumber ranging between 400 and 4000 cm -1 .

Results and Discussion:-Thermal Studies:
Figures 1(a)-1(c) show TG and DTG plots for Magnesium substituted in LTO compounds. In response to the difference in the ratio of Magnesium doped, there is little difference in the three graphs. From the TG/DTG curves, it is shown that there is no loss of weight from room temperature to 400 ºC. The total mass loss of LTO is approximately above the 20% which is high for the Mg substituted materials in LTO. The Mg substituted Lithium titanate materials of TG profiles show the overall loss of weight is (17%) assessed between 400 ºC and 700 ºC.
The initial mass loss observed from the TG curves from 300 ºC to 500 ºC is approximately 2.5 %. This is corresponding to the loss of moisture content absorbed on the surfaces and some intercalated water molecules during grinding and methanol used in the synthesis process with a little difference in the curves. All the TG/DTG curves obtained from Mg substituted precursors indicated that there is a maximum weight loss of the samples, decreases quickly between 600 ºC and 700 ºC suggesting that the complicated reactions of decomposition of the of inorganic materials such as the precursors of Lithium carbonate, Titanium oxide and Magnesium oxide decomposed materials [18][19][20][21][22].
TG curves of Li 4-x Mg x Ti 5 O 12 series show that the major weight or mass loss (15%) is between 600 ºC and 700 ºC temperatures. The corresponding peaks are seen at ~ 630ºC (for LTO it is observed ~ 730 °C) on the DTG curves. From all the TG curves, a negligible weight loss is seen above 700 ºC to 1000 °C, which indicates that the spinel crystalline structures of Li 4

XRD Analysis:
The XRD graphs of Li 4-x Mg x Ti 5 O 12 (x = 0, 0.1, and 0.3) anode materials, calcined at 850 ºC for 20 hours in air, are shown in figure 2. Every peak of diffraction is quite narrow and sharp, depicting the materials to be of the crystalline type. From the XRD graphs, the diffraction spectra of the un-doped and doped materials of the diffraction peaks are good and consistent with PDF card # 49-0207, which had similar characteristics of cubic LTO crystal system [14,[24][25][26][27]. From the X-ray Diffraction figures, it can be observed that all the peaks of Magnesium substituted materials are comparable to those of pristine LTO. There is no other impurity peak seen from XRD graph, which indicated that Mg 2+ has effectively entered into the Li site. Absence of impurity peak also confirmed the single phase crystalline nature. After Mg 2+ substitution, the main peak belonging to the (111) index progressively moves from 18.30° to 18.32°. The lattice constant is incresed with the dopant composition of Mg [19][20][21]28]. The increase of the lattice volume might provide large space for Li + charging and discharging and also can efficiently improve LTO electrical conductivity. Crystallite sizes of the     Synthesized material particles are present as larger grains of agglomeration and as the value of x increase the size of the grains and agglomeration are increased than the base LTO compound. The size difference in the grains increases the amount of surface area inside the anode material, which is potentially causing a much greater charge transference and initial capacity.
As it is well-known, the kinetic property of Li-ion transportation in the particles is limited by ion diffusion pathway. An electrode material has advantage with this unique morphology for the reason that it aids to elevate the electrochemical efficiency [22][23][24][25]. From figures 3 (a) to 3 (c) it is clearly visible that particles are spherically in shape. Aggregated view of all these small particles is seen on the surface of the powder. With increase of Mg content, the particle size also increased and there is a distortion in the spherical shape. The small grain sizes are beneficial to improve the elctrochemical performance as a result of the shortening of Li + and electron diffusion lengths within the materials [28,29]. The electrochemical performances of composite electrode with increase of Mg content are found to be better and the experiments conducted are also in support with the theory of high Mg content. These results are in good correlation with XRD studies and the crystallinity is well developed. All the compounds exhibit uniform grain distribution and a small porous structure.
The EDS is a generally utilized technique implemented for investigative examination of essential parametric composition of a sample. In this method, the spectroscopic data are plotted as a graph of counts versus energy. The  On Magnesium substitution in Lithium titanate the peaks seem to shift towards the higher wavenumber side with the width of the peak increasing as the Mg composition increases. The Mg substituted Li 4-x Mg x Ti 5 O 12 series compounds are partitioned into LiO 6 and MO 6 layers which are found in the frequency range 400-900 cm -1 [25,26]. Due to limitations associated with the instrument employed, the IR spectrum for Mg doped LTO is not recorded below the wavenumber of 400 cm -1 to observe the vibrations of LiO 6 . With the increase of Mg substitution, the stretching and bending mode vibrations also slightly change to higher frequency sides, due to a variation in M-O covalence bond. The Wyckoff positions 8 and 16c consist of Li + ions and transition metal (Li, Mg and Ti) ions respectively [14,30]. Table 2 shows the vibrational modes and band assignments in Mg doped LTO materials.  From XRD studies the compounds possess a typical structure of the spinel type having a space group of the Fd-3m class. Fractional substitution of Mg in place of Li enhances the lattice parameter. SEM with EDS is employed to study the microstructures and elemental compositions respectively. All the samples show that porosity nature is observed in SEM analysis. From the SEM images, we observe that the structural, morphological features and grain size distributions are in the range from 0.98 to 1.3 µm. The spectra extracted by making use of FTIR reveal that the structural lattice of oxide includes MO 6 tetrahedra and octahedra.