Studies On The Electronic Structures And Magnetic Properties Of Doped Monoclinic LiMnO₂ System

On one hand, monoclinic LiMnO₂ system is one of the most promising cathode materials for lithium batteries. Nevertheless, this material shows bad stability of crystal structure and electrochemical cycling, therefore, before the truly practical application, doping has to been employed to improve its electrochemical properties. On the other hand, partially delithiated, i.e. hole-doped monoclinic LiMnO₂, monoclinic LixMnO₂ (0 < x < 1) is the transition-metal oxide which belongs to the manganese oxides on the boundary from Mott-Hubbard insulator charge-transfer insulator, and also belongs to the mixtures of eg system and t2g system. As a result, monoclinic LixMnO₂ is very worthy of deep investigation because it may represent rich, new and unique character in the field of condensed-matter physics, such as spin physics, orbital physics and so on. Aiming at these two issues, this dissertation has carried out to investigate the electronic structures, preparation technique and magnetic properties deeply and carefully.In Chapter II, the Effects of Li/Mn disorder on the chemical bonding and electronic structures of monoclinic LiMnO₂ are investigated via first-principles calculations based on density functional theory. It is found that Li/Mn disorder has reduced the ionicity of Li atoms, enhanced ionicity of Mn atoms, enlarged the structural distortion, weakened the layered character and Mn-Mn binding, which greatly benefits the migration of Mn ions from its own octahedral sites into the nearby Li sites, consequently impelling the material to a spinel-like structure, and doing great harm to the electrochemical activity, structural stability and performance of electrochemical cycling for monoclinic LiMnO₂. Therefore, for the application as cathode material in commercial lithium batteries, the Li/Mn disorder in monoclinic LiMnO₂ have to be controlled strictly or eliminated as far as possible. Additionally, the changes induced by the Li/Mn disorder in the band structures and density of states reveal that the material has changed for a semiconductor to a metal, and the metallic character originates from the 3d electrons of all Mn ions, which suggests that the effects of Li/Mn disorder in the electronic structures of monoclinic LiMnO₂ are not localized only around the disorder atoms. Based on total energy analysis, it is found that the Li/Mn disorder is derived from dynamics factors but not from thermodynamics factors. Therefore, it is definitely possible to obtain monoclinic LiMnO₂ with fully ordered Li/Mn atoms or with greatly reduced Li/Mn disorder.In Chapter III, first-principles calculations based on density functional theory were employed to investigate the effects of Cr-doping and Co-doping in monoclinic LiMnO₂. It is found that the doping can shorten the Mn-O bondlength effectively, suppress the Jahn-Teller distortion markedly and decrease the insulating band gap greatly, consequently enhancing the structural stability, improving the stability during electrochemical cycles, and increasing the electrical conductivity of monoclinic LiMnO₂, respectively. Based on carefully comparative studies on the effects between Cr-doping and Co-doping, it is found that Co-doping is more evident in compressing the crystal structures and suppressing the Jahn-Teller distortion, therefore, it is concluded that Co-doping is more effective than Cr-doping to improve the structural stability under equilibrium condition; Nevertheless, Cr-doping is more evident in decreasing the insulating band gap, which can increase the electrical conductivity and thereby the safety of lithium batteries using monoclinic LiMnO₂ as cathode material more effectively. More importantly, contrary to shorten the Li-O bondlength and the shortest Li-Mn distance induced by Co-doping, Cr-doping can stretch the Li-O bondlength, and especially the shortest Li-Mn distance, which make Cr-doping more effectively in improving the electrochemical activity as well as the rate capability, and hindering the migration of Mn ions into the interlayer Li sites, thereby improving the performance of electrochemical cycling, respectively. As a conclusion, Cr-doping is more effective than Co-doping in improving the electrochemical properties of monoclinic LiMnO₂.In Section IV, monoclinic Li_0.33MnO₂ has been successfully synthesized by low-temperature solid state reaction at 360℃. The magnetic property of Li_0.33MnO₂ has been studied by dc magnetization, magnetic hysteresis and ac susceptibility. The research results indicate that the material show paramagnetism at high temperature. The negative Weiss constant indicates the antiferromagnetic interaction between spins in this system. At low temperature, the irreversibility behavior and the dependence on the magnetic field confirm the multiple spin glass behavior. The fitting to the Power law based on the ac susceptibility data gives the characteristic relaxation time as 2×10-13 and 5.1×10-11 for Tf1 and Tf2, respectively, which indicates that monoclinic Li_0.33MnO₂ undergoes a magnetic transition from paramagnetism to atomic-scale spin glass, then to cluster spin glass with the decrease of temperature.In Chapter V, with the same process as indicated in Chapter IV, we have prepared monoclinic Li_0.5MnO₂ and studied its structural, valence state and magnetic properties via XRD, XPS, Raman and SQUID measurement. XPS analysis shows that the 2p3/2 and 2p1/2 of Mn ions in Li_0.5MnO₂ locate at 642.3 eV and 653.6 eV, respectively, and further quantitative analysis reveals that the mole ratio of Mn3+ to Mn4+ ions equals to 1: 1, which means that as same as LiMn2O4, the Mn ions in monoclinic Li_0.5MnO₂ are in the mixed valence state. The results of XRD and Raman indicate that monoclinic Li_0.5MnO₂ and LiMn2O4 are definitely different in the long-range and short-range crystal structure, which induces the different magnetic properties. SQUID measurement shows cluster spin-glass behavior for monoclinic Li_0.5MnO₂ at low temperature. The comparative studies on the magnetic behavior of monoclinic Li_0.5MnO₂ and Li_0.33MnO₂ reveal that the strong geometrical frustration induces the presence of multiple spin-glass behavior in monoclinic Li_0.33MnO₂, while the difference in bond length and angle should be responsible for the difference in the magnetic behavior of monoclinic Li_0.5MnO₂.Finally in Chapter VI, the research work of the whole dissertation has been summed up, and the shortcomings have been pointed out. In addition, ideas and suggestion for further research work have been put forward.