Due to technological developments and the continuous depletion of fossil fuel, the development of new power sources is of great interest. With the recent developments in the area of green energy production such as solar and wind energy, the need for high energy density energy storage devices has become equally important [1] [2] [3] . Li ion batteries have the highest energy densities among the commercially available rechargeable batteries [4] [5] . The commercially available Li ion batteries suffer from various drawbacks such as poor cyclability, rate performances and toxicity [6] . Keeping this in view, the research community is working towards the development of new cathode materials [7] [8] [9] [10] [11] . The preparation of phase pure cathode materials for Li ion rechargeable batteries is very time consuming, since the process involves trying so many combinations of temperature and annealing time. Various authors have analyzed thermal behaviors in conjunction with X-ray diffraction (XRD), to understand the reaction mechanism for the synthesis of LiMn₂O₄ cathode materials. There is the need to continue work in this area to optimize the synthesis of these very important groups of energy storage materials.

In order to get a better understanding of the different possible by-products, Berbenni and Marini [12] studied the thermal decomposition processes taking place in solid state mixtures of Li₂CO₃-MnCO₃(x_Li = 0.10 - 0.50, x_Li = lithium cationic fraction) in air and nitrogen flow by thermogravimetric analysis (TGA) and X-ray powder diffraction studies. They found that the formation reaction of LiMn₂O₄ and Mn₃O₄ was completed by about 720˚C. At higher temperatures, complex reactions take place, resulting in the formation of the compounds Li₂Mn₂O₄ and LiMnO₂ with excess of Mn₃O₄. It was also reported that in the mixture of Li₂CO₃-MnO, formation of LiMn₂O₄ is a two stage process, where Li₂MnO₃ forms first, followed by reaction with excess Mn₂O₃ to yield LiMn₂O₄ [13] . It has been reported that LiMn_1.95M_0.05O₄ (M = Al, Co, Fe, Ni, Y) cathode materials can be synthesized by combustion method using lithium hydroxide, manganese nitrate, M-nitrates (M = Al, Co, Fe, Ni, Y), and urea as precursor materials. The thermal behavior of the reaction mixture and synthesized powder revealed that the spinal phase can be achieved in 1 minute at 280˚C [14] .

Michalska and coworkers [15] have studied the important stages of the syntheses of nanocrystalline lithium-manganese oxide spinels using DSC-TGA mea- surements. They found that DSC/TGA/XRD data are co-related to each other, and all major thermal events, for all precursors occur between 500˚C - 700˚C. The mass loss during the synthesis procedure was between 51% and 64%, depending on the material. Above 700˚C pure spinal phase is obtained, as confirmed by X-ray diffraction studies.

The thermal behavior of LiMn₂O₄ spinal was studied by Molenda and coworkers [16] using DSC/TGA in the temperature range of 300˚C - 900˚C in air atmosphere. They reported that the changes of mass within the studied temperature range are related to arrangement of the structure accompanied by the disappearance of cations vacancies and by the formation of the stoichiometric LiMn₂O₄. In the range of 820˚C - 925˚C, the mass changes corresponds to the formation or disappearance of the oxygen vacancies, while above 925˚C Mn₃O₄ and LiMnO₂ phases were formed and released oxygen.

In this paper, we have synthesized LiMn_2−xFe_xO₄ (x = 0.0, 0.25 and 0.50) spinel cathode materials via sol-gel method. The thermal behavior of the synthesized spinel cathode materials during the calcinations process were studied using DSC/TGA. The results of thermal analysis were correlated with XRD data in order to optimize the synthesis process to obtain the phase pure materials.

LiMn_2−xFe_xO₄ cathode materials were synthesized via sol-gel method. The precursor materials lithium acetate dihydrate (LiOOCCH₃∙2H₂O, 99%), iron (II) acetate anhydrous (C₄H₆FeO₄), and Manganese(II) acetate tetrahydrate [Mn 22% (typical), C₄H₆MnO₄∙4H₂O] were procured from Alfa Aesar and used as received. All of the precursor materials were dissolved in 2-ethylhexanoic acid, followed by stirring for 1 hr at 500 rpm. The final solution was dried drop by drop on a Petri dish at 280˚C. The resultant powders were ground and stored in a glass vial for further analysis/processing.

Simultaneous DSC/TGA measurements were carried out between 50˚C and 1000˚C in alumina crucibles using Q600 SDT (by TA Instruments, USA). The data were analyzed using TA Advantage software. The measurement conditions were as follows: LiMn₂O₄ (sample weight = 17 mg, LiMn_1.75Fe_0.25O₄ (sample weight = 25.5 mg), LiMn_1.5Fe_0.5O₄ (sample weight = 28.5 mg) were heated and cooled at a rate of 10˚C/min., under flow of nitrogen gas. The X-ray diffraction studies were performed using Rigaku Mini flex-II diffractometer (wavelength of X-ray, 1.5406 angstrom.) and CuK_α radiations, at a scan rate of 1˚/min. The data were collected at every 0.02˚.

Figures 1-3 show the thermal behavior of LiMn₂O₄, LiMn_1.75Fe_0.25O₄, and LiMn_1.5Fe_0.5O₄, respectively, obtained from DSC and TGA analysis. The XRD patterns of the synthesized materials are given in Figures 4-6. It can be seen from Figures 4-6 that synthesized materials showed additional peaks, which may be due to the defects in structure. Additionally, the peaks are less intense and broader, which may be due to the lower crystallinity. Furthermore, as the calcinations temperature increases, the peaks become more sharp and intense, which may be due to the increased crystallinity. These results are in agreement as reported earlier by Molenda and coworkers [16] .

It can be seen from Figures 1-3 that there is mass loss starting at about 380˚C and corresponding exothermic peak is observed. This may be due to the organic removal and removal of oxygen. Figure 1 showed various transitions at temperatures 535˚C, 665˚C, 720˚C, 741˚C, and 781˚C. The transitions between 280˚C - 450˚C are due to results of pyrolysis, which can be see seen clearly from the peak obtained in X-ray diffraction pattern of as prepared [Figure 4(a)] and pyrolyzed LiMn₂O₄ [Figure 4(b)]. Upon further annealing at higher temperature, heat flow increases up to 705˚C and corresponding mass loss is observed. This may be due to the oxygen removal from the sample and this corresponds to the decrease in peak intensity at 2 theta values of 33˚, 55˚, and 66˚. These peaks correspond to Mn₃O₄ and Mn₂O₃ phases [13] [14] . The peak intensities corresponding to fd3m structures are increased. At 850˚C, we obtained phase pure LiMn₂O₄ cathode materials. This can be verified from TGA graph [Figure 1], where no significant mass change is observed after 850˚C.

Figure 2 showed the thermal behavior of LiMn_1.75Fe_0.25O₄ cathode materials and corresponding XRD patterns are given in Figure 5. It can be seen from Figure 2 that after pyrolysis between 200˚C - 380˚C, the mass decreases gradually from 380˚C to 750˚C, after that no significant mass loss is observed. The corresponding XRD [Figure 5] showed the decrease in peak intensity of peaks at 33˚ and 55˚ and increasing of spinel characteristic peaks. We obtained phase pure spinel LiMn_1.75Fe_0.25O₄ at 750˚C, which is lower than that of pure LiMn₂O₄ cathode materials. Similar behavior was also obtained for LiMn_1.5Fe_0.5O₄ cathode materials, where there is mass loss up to 750˚C, and after this temperature, there is no significant mass loss [Figure 3]. The corresponding XRD patterns of LiMn_1.5Fe_0.5O₄, obtained at various calcinations temperatures [as prepared, 555˚C, 665˚C, and 755˚C] [Figure 6] showed that phase pure material at 755˚C. Table 1 shows the crystallite size and lattice parameters of LiMn₂O₄, LiMn_1.75Fe_0.25O₄, and LiMn_1.5Fe_0.50O₄ cathode materials, calcined at various temperatures. It can be seen from the table that as the calcined temperature increases, lattice parameter also increases. Similar behavior was reported by Dziembaj and coworkers [17] . The crystallite size was calculated using Scherer’s equation. The average crystallite sizes were found to be in the range of 13 - 40 nm. The crystallite size varies with the temperature and was found to be increased upon increasing annealing temperature. Our results are in agreement as reported earlier [18] .

We have successfully synthesized spinelLiMn₂O₄, LiMn_1.75Fe_0.25O₄, and LiMn_1.5 Fe_0.50O₄, cathode materials via sol-gel method. The thermal behavior of the synthesized materials is in agreement with the results obtained from X-ray diffraction studies. Based on the results obtained from thermal and structural studies, the synthesis conditions for the cathode materials can be optimized. We obtained the optimum calcinations temperatures for LiMn₂O₄, LiMn_1.75Fe_0.25O₄, and LiMn_1.5Fe_0.5O₄ as 850˚C, 750˚C, and 750˚C, respectively. Further characterizations such as X-ray photoelectron spectroscopy and micro-Raman spectroscopy may be carried out to provide better understanding of the reaction mechanism.

The authors are thankful to Dr. Faris Malhas, Dean School of Engineering, Science and Technology, Central Connecticut State University (CCSU) for his support in the acquisition of the Q600 SDT. The financial support received from CCSU AAUP university research grant (No. ARLEMJ), and Connecticut NASA Space Grant are gratefully acknowledged.

Chiovoloni, S., Moran, C., LeMaire, P.K. and Singhal, R. (2017) Optimization of Synthesis Conditions of LiMn_2−xFe_xO₄ Cathode Materials Based on Thermal Characterizations. American Journal of Analytical Chemistry, 8, 51-59. http://dx.doi.org/10.4236/ajac.2017.81004