Abstract
The fast increasing requirement of portable electronics, as well of the demanding of electric vehicles as a substitute for the robust engines that burn fossil fuels and cause warning impacts on the environment and of stable large scale energy storage systems for grid, all put a great pressure on the development and commercialization of high performing batteries with high energy capacity, high power density, and long cycle life. Besides the exploration of new chemistries and materials, modifications of current known battery materials is also an important and reliable direction of extracting higher and more stable energy. And lithium-ion batteries have been and will be considered as the most promising chemistry of batteries for further study.
Composition deviation of multicomponent transition metal (TM) oxides active materials have been found to result in decrease of structural stability and energy capacity, and particle morphologies are reported to determine the electrode packing density and thus the energy density. The synthesis process of the active materials has a profound but also complicated influence on the materials structure and electrochemical performances, which requires fundamental knowledge about the synthesis process itself. As a widely used synthesis method in both lab and industrial environment, coprecipitation reaction is well-known for its advantages of homogenous mixing of different ions and particle morphology tunability; the obtained precursor particles from coprecipitation reaction are directly fired with lithium salt to produce final TM oxide as cathode active materials, which means the properties of the precursors are determinant for the performance of the final active materials. While many different particle morphologies have been gained from coprecipitation synthesis to leverage the electrochemical performances of the cell, the knowledge of relationship between solution conditions and particle composition and morphologies is still lacking for a precise design and control of the precursors and final active materials properties, majorly because that coprecipitation is a complex process where many different factors including temperature, reaction initiation method, solution mixing method and status, will influence the reaction. In addition, most synthesis processes for producing battery cathode materials requires multicomponent coprecipitation where more than one TM are involved and co-existing in the solution, which makes the process more complicated with potential interactions between different ions precipitations. More advanced tools are in need to enable the study of the multicomponent coprecipitation reaction, towards a more precise control of the synthesis procedure.
This dissertation addresses this knowledge gap in the literature by studying about the coprecipitation reaction mechanism via in-situ tracking the ion concentration and particle size distribution along the reactions, and also via conducting apparent reaction activation energy among the whole multicomponent composition rang. It also confirms the advantage of local mixing of different TM ions in the precipitates on the final materials phase purity and other properties. Attempt also has been made to design and tune the particle morphology of the precipitate by adding chemical inhibitors, and to align the particles with special shapes in the electrode with an external magnetic field. In these studies, we aimed to synthesize lithium-ion cathode active materials with controllable composition and particle morphology via coprecipitation reaction by using the knowledge gained through the solution chemistry study of coprecipitation reaction. The tools we developed along the process are expected to be useful techniques for studying nucleation and particle growth processes in general and can be applied to other single or multicomponent coprecipitation reactions for applications which requires accurate composition and particle morphology control.
