STABILIZATION OF CUBIC SPINEL Li-Mn-0 BY DOPANTS, AS CATHODE MATERIAL IN LITHIUM-ION BATTERY

Abstract

Among the various cathode materials available, materials based on the spinel lithium manganese oxides were chosen as the materials to work upon because of the presence of less toxic and cheaper manganese compared to highly toxic and expensive cobalt or nickel present in other compounds such as LiCo02, LiNi02, etc. The present work is to study the stabilization of cubic spinel Li-Mn-0 by dopants in order to stabilize the cubic spinel structure, various stoichiometric and non-stoichiometric cubic spinels were synthesized. To avoid Jahn-Teller distortion in the structure caused by Mn +(high spin), dopants such as Li, Mg and Fe were chosen. Dopants Li, Mg and Fe are Jahn-Teller inactive ions. The effect of doping on the structure and surface morphology of spinel was examined by X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Thermal analysis (TG/DTA), Impedance spectroscopy (IS) and Infrared spectroscopy (IR). Electrochemical performance of the resultant materials was also carried out. Spinel LiMn204, Lii+5Mn2-604, LiMgxMn2-x04, LiFexMn2-x04, Lii+5Mn2-6-o iMg0.iO4, Lii.o3[Mni.97-xMgx]04, Lii.o3[Mni.97-yFey]04 and Lii.03[Mni.97_x_yMgxFey]O4 were successfully synthesized by sol-gel technique using citric acid as chelating agent. The X-ray diffraction analysis of Lii+6Mn2.804 having 8 = 0.00, 0.03, 0.06, 0.12, 0.18, 0.24, 0.33, 0.42, and 0.50 powders, shows that a single-phase cubic spinel structure is stable only when 8 < 0.24. When lithium content increases to 8 > 0.24, the percentage of monoclinic Li2Mn03 phase increases until 8 = 0.42 and saturates thereafter. On magnesium doping, the range of stability of cubic spinel Lii+5Mn2-8-o.iMgo.i04 decreases to 8 < 0.18, and monoclinic Li2Mn03 starts to appear when 8 > 0.18. In magnesium-doped Lii+gMn2-5-o.iMgo.i04, the ion Mg2+ goes to octahedral sites and decreases the population of Mn3+ in the 16d octahedral sites. Therefore, the lattice parameter of the magnesium-doped cubic spinel for the same 8 is lower than that for undoped Lii+sMn2-5O4. However, at higher 8, the population of Mn3+ may get totally exhausted and the charge balance may be maintained by the creation of defects. The voltage step appearing in the discharge curve is attributed to a transition from two cubic phases to one cubic phase. With increasing 8, the size of the step decreases and it becomes broader. However, this voltage step does not appear to be affected by magnesium doping. Microscopic observations have shown the average particle size of the samples Lii+5Mn2-504 calcined at 750 °C for 15 hours slightly i increases with increasing 8. The average particle size of Lii+5Mn2-804 (8 = 0.0) is about 250 nm, and as the 8 increases the average particle size slightly increases and the particle size is around 500 nm for 8 = 0.5. The average particle size in case of the Mg doped Lii+6Mni.9_5_o.iMgo.i04 samples compared to undoped Lii+5Mn2-504 having same value of 8 is less. For undoped samples Lii+5Mn2-504 particle are in 250-500 nm range and for Mg doped it is 40-300 nm. The particles size increase from nano to submicron as the temperature for recalcination increases. FTIR study shows that upon lithium substitution for manganese in the spinel lattice, Li+ ions are located in the 16d octahedral sites. As 8 increases from 0.03 to 0.42 in Lii+6Mn2-504, high wavenumber band at around 618 cm" shows blue shifting. This blue shifting occurs from 618 to 644 cm"1 in the range of0.03 <8 < 0.42. As 8 increases beyond 8 > 0.33, new infrared bands emerge and splitting of the two high frequency bands starts to appear, and is displayed clearly when 8 = 0.42 and 0.5. Splitting of the two bands and emergence of additional infrared bands are explained by simultaneous occupation of the octahedral sites by the two lithium and manganese ions with different charge and mass and formation of monoclinic Li2Mn03 phase. Coexistence of two phases having space groups of Fd3m and P4}32 in the Mg doped LiMn204 spinel was observed. X-ray powder diffraction (XRD) studies show that the crystal structure of LiMn2-xMgx04 for x < 0.25 is a single phase cubic spinel which has space group of Fd3m. The cubic spinel structures having space group of Fd3m and P4s32 are found to coexist in the compound with x = 0.30. X-ray diffraction studies of the Fe doped materials (LiMn2-xFex04 where x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) have shown that the structure remains cubic spinel with Fd3m space group symmetry. Apart from the increase in lattice parameter of the cubic spinel structure with the increase in Fe content, no major change was observed in the crystal structure. No change in the space group symmetry is observed for the entire doping range of Fe, i.e., LiMn2-xFex04; 0.0 < x < 0.6. It was observed that the average particle size decreases with increase in Mg and Fe content in comparison to the undoped stoichiometric cubic spinel LiMn204. The particle size of LiMn2-xMgx04 (x = 0.3) and LiMn2.xFex04 (x = 0.5) samples are 220 nm and 100 nm, respectively. Fourier transform infrared spectroscopy shows the increase in number of vibrational bands with the increase in Mg content, which indicates ordering of the ions in the case of ordered spinel structure and consequent reduction of the space group symmetry from Oh7 to O7. Butfor Fe doped cubic spinel, no major shift in the positions of the bands at 491 and 60 cm"1 was observed. It shows that Fe3+ ions have effectively replaced the Mn +ions. The symmetry ofthe basic LiMn204 cubic spinel structure (Oh7) is maintained even after the compound is doped with higher Fe content. Thermogravimetric analysis (TG) of the synthesized powder LiMn2.xMx04 (M = Mg, Fe) shows that the transition temperature (T,) which corresponds to the oxygen loss from the structure decreases as the Mg and Fe content in the compound increases, showing clearly that the oxygen loss from the structure begins at lower temperature. Hence, non-stoichiometry in the structure of LiMn2-xMx04.6 (M = Mg, Fe) increases in the air atmosphere. Supression of endothermic peak at Td2 temperature in DTA graph of Mg and Fe doped spinel shows the stabilisation of the orthorhomic LiMn02 and tetragonal Mn304 phases at higher temperture. It slightly decreases with the increase in Fe content in LiMn2-xFex04. From the electrochemical performance -of the materials, it was observed that Mg and Fe suppresses the fall in capacity in the 4 V range, and the voltage step which occurs in the 4 V regions also gets suppressed with the increase in Mg and Fe content, and it gets completely eliminated for LiMni 75Mgo.2504 spinel. Decrease in capacity was observed with the increase in Mg and Fe content. Impedance spectroscopy results show that the bulk impedance increases and therefore electrical conductivity of the material reduces with the increase in Fe concentration. The electrical conductivity of the Fe doped materials was observed to be of the order of 10"5 S/cm. The materials Lii.03Mni.97O4, Li1.03Mn1.77Feo.2O4, Lii.o3Mni.77Mgo.204 and Lii.o3Mni 77Mgo.iFeo.i04 exhibit a phase pure cubic spinel structure as evident from the Xray diffraction analyses. In the non-stoichiometric cubic spinel, Fe and Mg occupy 16d octahedral sites, thereby reducing the concentration of Jahn-Teller active Mn3+ ions. Presence of Mn3+ ions in the structure is the main cause of reduction of capacity during cycling. So, double doping with Fe and Mg stabilises the cubic structure. The crystallinity and average particle size of the material increases by doping with Fe and Mg. In Li1.03Mn1.77Mgo.1Feo.1O4 compound, most of the particles have well-defined truncated octahedral shape (truncation occurred on all the edges), however, some particles have octahedron shape truncated only on two edges. It shows evolution of particles shape from truncated octahedron to octahedron having lower energy in case of Fe and Mg doubledoped compound. This is possible only if the growth rate along the {100} is much faster than that of {111} surfaces. The formation of octahedron is due to the lower energy of