An investigation of the morphological and electrochemical properties of spinel cathode oxide materials used in li-ion batteries

Li-ion batteries have become the more dominant battery type used in portable electronic devices such as cell phones, computers and more recently their application in full electric vehicles (EV). Li-ion batteries have many advantages over the traditional rechargeable systems (Pb-acid and Ni-MH) such as their higher energy density, low self-discharge, long capacity cycle life and relatively maintenance free. Due to their commercial advantages, a lot of research is done in developing new novel Li-ion electrode materials, improving existing ones and to reduce manufacturing costs in order to make them more cost effective in their applications. This study looked at the cathode material chemistry that has a typical spinel manganese oxide (LiMn2O4) type structure. For comparison the study also considered the influence of doping the phase with various metals such as Al, Mg, Co and Ni that were made as precursors using various carboxylic acids (Citric, Ascorbic, Succinic and Poly-acrylic acid) from a sol-gel process. Traditional batch methods of synthesizing the electrode material is costly and do not necessarily provide optimized electrochemical performance. Alternative continuous less energy intensive methods would help reduce the costs of the preparation of the electrode materials. This study investigated the influence of two synthesis techniques on the materials physical and electrochemical characteristics. These synthesis methods included the use of a typical batch sol-gel method and the continuous spray-drying technique. The spinel materials were prepared and characterized by Powder X-Ray Diffraction (PXRD) to confirm the formation of various phases during the synthesis process. In addition, in-situ PXRD techniques were used to track the phase changes that occurred in the typical batch synthesis process from a sol-gel mixture to the final crystalline spinel oxide. The materials were also characterized by thermal gravimetric analysis (TGA), whereby the materials decomposition mechanisms were observed as the precursor was gradually heated to the final oxide. These synthesized materials prepared under various conditions were then used to build suitable Li-ion coin type of cells, whereby their electrochemical properties were tested by simple capacity tests and electrochemical impedance spectroscopy (EIS). EIS measurements were done on the built cells with the various materials at various charge voltages. TG analysis showed that the materials underwent multiple decomposition steps upon heating for the doped lithium manganese oxides, whereas the undoped oxide showed only a single decomposition step. The results showed that all the materials achieved their weight loss below 400 °C, and that the final spinel oxide had already formed. The in-situ PXRD analysis showed the progression of the phase transitions where certain of the materials changed from a crystalline precursor to an amorphous intermediate phase and then finally to the spinel cathode oxide (Li1.03Mg0.2Mn1.77O4, and LiCo1.09Mn0.91O4). For other materials, the precursor would start as an amorphous phase, and then upon heating, convert into an impure intermediate phase (Mn2O3) before forming the final spinel oxide (Li1.03Mn1.97O4 and LiNi0.5Mn1.5O4). The in-situ study also showed the increases in the materials respective lattice parameters of the crystalline unit cells upon heating and the significant increases in their crystallite sizes when heated above 600 °C. Hence the results implied that a type of sintering of the particles would occur at temperatures above 600 °C, thereby increasing the respective crystallite size. The study showed that the cathode active materials made by the sol-gel spray-drying method would give a material that had a significantly larger surface area and a smaller crystallite size when compared to the materials made by the batch process. The electrochemical analysis showed that there was only a slight increase in the discharge capacities of the cells made with the spray-drying technique when compared to the cells made with the materials from the batch sol-gel technique. Whereas, the EIS study showed that there were distinct differences in the charging behavior of the cells made with the various materials using different synthesis techniques. The EIS results showed that there was a general decrease in the cells charge transfer resistance (Rct) as the charge potential increased regardless of the synthesis method used for the various materials. The results also showed that the lithium-ion diffusion coefficient (DLi) obtained from EIS measurements were in most of the samples higher for the cathode materials that had a larger surface area. This implied that the Li-ion could diffuse at a faster rate through the bulk material. The study concluded that by optimizing the synthesis process in terms of the careful control of the thermal parameters, the Li-ion batteries‟ cathode active material of the manganese spinel type could be optimized and be manufactured by using a continuous flow micro spray process