Synthesis And Characterizations Of LiMn₂O₄ And LiNi_0.5Mn_1.5O₄ Cathode Materials For Lithium Secondary Batteries

With the social development and progress, the energy crisis and environmental pollution have become two huge obstacles which are more and more difficult to avoid. If we can not solve these two problems, our living environment will get worse till the day we can not survive again. Maybe a few decades later, after the exhaustion of traditional energy sources, the world will back to darkness and confusion as the ancient times, and people will suffer from cold and starvation again. Therefore, environmental protection and new energy are the two central themes of the current world. Lithium-ion battery is an important medium for rational and effective use of energy, it can realize high efficient energy transfer and conversion, especially for the energies that can be used in HEV or EV to replace the fossil fuel. In fact, development of high performance lithium-ion battery technology will pave the way for the extensive use of new energy. Moreover, Lithium-ion battery technology is the most clean and pollution-free one among the current secondary battery technologies, so it means lot to environmental protection. Because of its two features, efficient and environmentally friendly, can contribute to the solution of the huge obstacles we talked before, research and development of lithium-ion battery have become a strategic imperative.Presently, for most of the lithium-ion batteries in use, the cathode material is still LiCoO2, and the cost is very high for the scarcity and expensiveness of cobalt. Also, there are still some safety problems difficult to overcome for LiCoO2. Therefore, to develop alternative materials is very necessary. The manganese resources are abundant, low cost and environmentally friendly, while spinel structure has high ionic conductivity because of its special three-dimensional network. Consequently, we chose spinel LiMn2O4 and LiNi0.5Mn1.5O4 to research in this thesis, and made some valuable results. LiMn2O4 and LiNi0.5Mn1.5O4 cathode materials with different prepared temperatures (600℃; 700℃; 800℃and 700℃; 800℃; 900℃, respectively) were synthesized by emulsion drying method. XRD patterns show that the structures of both are spinel and change little with the heat treatment temperature increased, but lattice constant, grain size and crystallinity were enhanced. The micro-strains also decrease for the release of micro-stress at higher temperatures. The morphologies of these two materials obtained at various temperatures were observed using scanning electron microscopy (SEM), and indicate that the actual grain sizes of both are all grown up with the increase of sintering temperatures. It is also found that the samples calcined at the temperatures higher than 800℃represent clear polyhedral shape, indicating well-developed crystallinity. According to the mean particle sizes (MPS) obtained from the SEM, the growth activation energy of the LiMn2O4 and LiNi0.5Mn1.5O4 cathode materials were calculated to be 69.8 and 144.2 kJ mol-1, respectively.X-ray photoelectron spectroscopy (XPS) has been extensively used to study the electronic structure and chemical composition of the materials. Here, LiMn2O4 and LiNi0.5Mn1.5O4 prepared at 800℃were characterized by XPS spectra. It can be found that the Ni2p3/2 binding energies of the LiNi0.5Mn1.5O4 XPS spectra are located at 845.65 eV, very closed to that of Ni2+, 854.5eV. Indicate that there are still little Ni3+ existed in LiNi0.5Mn1.5O4, but most are Ni2+. For the XPS spectra of LiMn2O4 and LiNi0.5Mn1.5O4, the Mn2p3/2 binding energies are located at 642.2 and 643.05 eV, respectively. The 643.05 is very close to the special value of Mn4+, 643.2 eV, which indicate that there are still little Mn3+ existed in LiNi0.5Mn1.5O4 besides Mn4+.The FTIR and Raman spectroscopy are the effective means to analyze the molecular structure of the cathode materials. In this thesis, LiMn2O4 and LiNi0.5Mn1.5O4 have been studied via these two technologies. First, according to the intensity and number of bands in the Raman spectra of LiMn2O4 and LiNi0.5Mn1.5O4, we can find that LiMn2O4 has a relatively high conductivity. From the research of the two vibration modes, A1g and , we can know that the [MnO6] octahedron in LiMn2O4 got partial distorted for have more Mn3+. While in LiNi0.5Mn1.5O4, almost all Mn-ion are Mn4+ give rise to the sharp and independent A1g mode. According to the Raman spectra and previous reports, we discussed the cation ordering change along with the prepared temperatures of LiNi0.5Mn1.5O4. As the mode in Raman spectra of the sample synthesized at 700℃split into two, it can be concluded that there are Ni2+/Mn4+ cation ordering existed in this material, and it belongs to P4332 space group, while the other five belong to Fd3m. In addition, we calculated the force constant (f) and the average Mn(Ni)-O bond length ? of LiMn2O4 and LiNi0.5Mn1.5O4 cathode materials, and found that the former has smaller force constant and longer Mn(Ni)-O length. The FTIR study of LiMn2O4 and LiNi0.5Mn1.5O4 shows that LiNi0.5Mn1.5O4 has a lower symmetry of the crystal structure compare to LiMn2O4. Meanwhile, we also compared the FTIR spectra of LiNi0.5Mn1.5O4 samples prepared at 700 and 900℃, confirmed that the 700℃sample have a cation ordering structure, and its space group is P4332.The cyclic voltammetry of the first cycle and the charge-discharge curves of the initial five cycles for LiMn2O4 and LiNi0.5Mn1.5O4 cathode were given in this thesis. The cyclic voltammetry shows that the redox reactions of the two materials all occur in two steps during charge and discharge process. But LiNi0.5Mn1.5O4 has a higher voltage platform, close to 5.0 V. However, for this material, the shape and potential of the oxidation peak and reduction peak are very different; indicate that LiNi0.5Mn1.5O4 has a strong polarization and poor electrochemical performance. Through charge and discharge curves, we calculated the specific capacity and coulomb efficiency for the initial five cycles of the two materials. The results show that LiMn2O4 has a high and stable coulomb efficiency, higher than 99 % in all five cycles. While the coulomb efficiency of LiNi0.5Mn1.5O4 is very low, although increase with the number of cycles, 56.8 % for first cycle to 76.7 % for fifth cycle. After the initial five cycles, the capacity fade of LiMn2O4 and LiNi0.5Mn1.5O4 are 2.6 % and 4.8 %, respectively. The discharge capacities of the two cathode materials for the fifth cycle are 112.18 mAh g-1 and 103.05 mAh g-1, respectively.