Preparation Of The High Voltage LiNi_0.5 Mn_1.5 O₄ Cathode Material And In Situ Xrd Study For Its Phase Transition

Lithium ion battery has become a promising power sources for electric vehicle and energy storage system application since LiFePO₄ cathode material is close to commercialization. However, the energy density and power density of lithium ion batteries based on LiFePO₄ cathode materials is still lower than the requirement of the future electric vehicle application. Many automakers hope to develop much higher energy and/or power density lithium ion batteries for next generation electric vehicle. Because of its good electrochemical performance and high operating voltage around 4.7 V, LiNi_0.5Mn_1.5O₄ spinel is one of the most promising high voltage cathode materials to design novel lithium ion batteries with high power density. The electrochemical characteristics and performance of LiNi_0.5Mn_1.5O₄ spinel should be influenced significantly by its crystal structure, which is related to the synthesis process. One of the most important steps to prepare the ideal LiNi_0.5Mn_1.5O₄ spinel is how to mix the raw materials uniformly. Spray drying is an excellent unit operation which can obtain the powder from mixture solution directly. In this dissertation, we tried to apply spray drying or melt salt method to prepare the precursors, and to optimize annealing process. The in situ synchrotron X-ray diffraction technique was also used to study the phase transition of LiNi_0.5Mn_1.5O₄ spinel during cycling so as to scale up the synthesis routes of LiNi_0.5Mn_1.5O₄ spinel.Firstly, the LiNi_0.5Mn_1.5O₄ spinel materials were prepared using spray drying assisted annealing process and melt salt method with different heat treatment conditions, respectively. The spray drying process is prior to melt salt process from the comparison of process complex and stability of product quality. The crystal structures of the prepared LiNi_0.5Mn_1.5O₄ materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transformed infrared spectroscopy (FT-IR). It has been found that the average particle size of the prepared LiNi_0.5Mn_1.5O₄ powders is estimated to be 1 to 2μm. All of the prepared LiNi_0.5Mn_1.5O₄ powders have couple of high vibration frequency (⁶27 cm^-1) and low vibration frequency (⁵00 cm^-1) in their FT-IR spectrum. XRD pattern indicates that the prepared LiNi_0.5Mn_1.5O₄ powders show phase-pure cubic spinel of Fd3m structure and are consist of single crystal of octahedral shape with (111) planes.Secondly, the electrochemical characteristics of the LiNi_0.5Mn_1.5O₄ spinel materials were measured using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemical performances of the prepared LiNi_0.5Mn_1.5O₄ cathode materials were tested at different charge/discharge rates between the potential limit of 3.5– 5.0 V. It can be seen, all of the LiNi_0.5Mn_1.5O₄ spinel materials exhibited a flat voltage profile at around 4.7 V that is attributed to the Ni²⁺/Ni⁴⁺ redox couple and another flat voltage profile at around 4.0 V region that arises from the Mn³⁺/Mn⁴⁺ redox couple. The electrochemical performance of the prepared LiNi_0.5Mn_1.5O₄ cathode materials were affected by the heat treatment temperature. As for the spray drying assisted annealing process, three heat treatment conditions were set up. The precursor obtained from spray drying were calined at 700℃for 4 h and 900℃for 8 h in air (sample (a)), 700℃for 6 h and 900℃for 6 h in air (sample (b)) and 700℃for 8 h and 900℃for 4 h in air (sample (c)), respectively. It is found that the initial discharge capacity of the LiNi_0.5Mn_1.5O₄ spinel sample (a), (b) and (c) with 0.1 C rate are 132.0, 131.7 and 129.2 mAh/g, respectively. The initial discharge capacity of the sample (a), (b) and (c) with 5 C rate can also reach 109.1, 106.0 and 104.4 mAh/g, respectively, and their discharge capacity still keep at 93.9, 95.7 and 95.2 mAh/g after 300 cycles. In order to understand the structure transition of the LiNi_0.5Mn_1.5O₄ spinel cathode materials thoroughly, the in situ synchrotron X-ray diffraction technique was firstly carried out to study the online phase transition of LiNi_0.5Mn_1.5O₄ spinel during cycling. We found that all of the Bragg peaks refer to the cubic phase of LiNi_0.5Mn_1.5O₄ spinel were shifted to the higher angle as lithium ion extracted from the host materials. From the in situ XRD patterns and charge-discharge profile, it can be found that four phase transitions existed for LiNi_0.5Mn_1.5O₄ spinel during charge process. In the initial charge stage, LiNi_0.5Mn_1.5O₄ spinel with original cubic phases (cubic-I) was observed. The lattice constants of LiNi_0.5Mn_1.5O₄ spinel decreased with the increase of charge potential, indicating that the material experienced cubic phase transition and structure shrinkage. When the charge potential reached 4.78 V, while the X-ray diffraction peak stayed at the same position, the intensity of it was enhanced suddenly. Thus it can be inferred that two phases (cubic-I and cubic-II) co-existed in this region. However, when the voltage was higher than 4.81 V, only cubic-II remained. Each of the (400), (440) and (511) peaks split into two peaks when the charge potential was higher than 4.90 V, indicating that the LiNi_0.5Mn_1.5O₄ spinel structure changed from cubic phase to tetragonal one. During the discharge process, the tetragonal phase transformed back to cubic-II structure in the 4.7 V region, and transformed from cubic-II to cubic-I structure in the 4.5 V region. All of the peaks shifted back to the lower angle when discharge state was lower than 4.1 V. It is a reversible phase transition for LiNi_0.5Mn_1.5O₄ cathode material during lithium ion intercalation process.