Lithium manganate-based cathode materials for lithium ion batteries prepared by the Pechini process

In this study, I have adapted the Pechini process to the synthesis of LiMn$\sb2$O$\sb4$ and its various modified forms. This synthetic method eliminates the need for long range diffusion during the formation of the oxide, and thus homogeneous, single phase oxides can be formed at relatively low temperatures with precisely controlled stoichiometry. Using the Pechini process, LiMn$\sb2$O$\sb4$ and its doped forms, such as LiNi$\sb{\delta}$Mn$\sb{2-\delta}$O$\sb4$ and LiCO$\sb{\delta}$Mn$\sb{2-\delta}$O$\sb4$ with $\delta =0.04$ or 0.20, were prepared. These materials not only offer a high level of performance, but also distinct and sharp electrochemical behavior which may provide significant analytical advantages in understanding the structural basis of the electrochemical behavior of this group of important intercalation compounds. High initial capacity and cycling behavior can be optimized by adjusting either the firing temperature or the dopant concentration. X-ray diffraction on various cathodes after extensive cycling reveals that the formation of a secondary phase, tetragonal Li$\sb2$Mn$\sb2$O$\sb4$, which is the product of the Jahn-Teller distortion, can be suppressed by either introducing dopants such as Ni and Co or lowering the firing temperature. Structural studies including neutron and ex-situ X-ray diffraction experiments were performed to reveal aspects of the structural changes that occur during lithium ion insertion into various LiMn$\sb2$O$\sb4$ compositions. A mechanism of the electrochemical insertion of lithium into LiMn$\sb2$O$\sb4$ spinels is suggested. The model I propose is as follows. In the pure LiMn$\sb2$O$\sb4$ system, at the fully charged state, the empty spinel host accommodates lithium ions as a single phase from the range of x = 0 to x = 0.10. Then, lithium ions start to intercalate in a way which gives rise to a phase separation with a lithium-poor phase, and a lithium-rich phase. With further lithium insertion, the lithium-poor phase is enriched at a concentration of approximately 0.35 the transformation is complete and the single-phase host structure accepts additional lithium ions in a random order. For lithium concentrations between 0.4 to 0.5, the host structure undergoes a second order phase transformation, resulting in a significant volume expansion. After half of the lithium 8a sites are filled, the ion ordering is destroyed, and lithium ions enter the spinel host randomly, forming a solid solution. When all of the 8a sites are occupied, the host structure goes through a first order phase transformation from spinel to tetragonal lithium manganese oxide in which lithium ions begin to fill in 16d sites. In the case of the Ni and Co doped samples, the two-phase region in the range of x = 0.10 to x = 0.35 was not observed. All of the diffraction peaks gradually shifted toward lower diffraction angles with higher lithium concentration over the entire intercalation range, indicating the existence of a single phase spinel which accommodates the insertion of guest lithium ions only by expanding the unit cells. The Ni and Co doped spinels exhibit better cycling behavior than pure LiMn$\sb2$O$\sb4$. The phase separation that occurs in the pure LiMn$\sb2$O$\sb4$ system may result in destruction of the host structure, which is reflected on capacity fading. Two intrinsic factors are proposed to be responsible for the instability of the spinel host structure upon cycling. One is the Jahn-Teller distortion in the deeply discharged state where the lithium sites are nearly full, and the other is the phase separation that occurs at low lithium concentrations. (Abstract shortened by UMI.