Numerical investigations on the influence of the electrode microstructure on the performance of lithium-ion batteries.

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Lithium ion batteries are in the focus of the current research and play an important role as a future energy storage device, with application e.g. in electric vehicles as well as an intermediate storage for renewable energies. Lithium ion batteries basically consist of two porous intercalation electrodes, which are separated by a separator. The two electrodes consist of metal oxide or carbon particles, the so-called active particles, and have, due to their particulate nature, a very complex microstructure. This microstructure dependents largely on the particular shape of the particles as well as on the particle size distributions. The performance of modern lithium ion batteries is therefore not only dependent on the material properties but also on the microstructure of the electrodes. Especially the effects of the microstructure are still largely unknown and shall be closer examined in this project. Direct numerical simulations provide a valuable tool for the design and optimization of lithium ion batteries but also to investigate effects which are difficult to measure by experiments. For the mathematical model, one has to consider the movement of the ions in the electrolyte solution as well as the diffusion of lithium in the active particles. Thereby, the intercalation of lithium into the active particles plays an important role, since the intercalation generates mechanical stresses. These so-called intercalation stresses can cause the active particles to break, which has a negative influence on the cell performance. Therefore, the aim of this project is to study the influence of the microstructure, and in particular the influence of the intercalation stresses, on the performance of lithium-ion batteries. In a first step, the intercalation of lithium into a single particle with arbitrary geometry is examined. This allows a detailed investigation of the influence of curvature effects. Furthermore, the behavior of a phase transition inside the active particles, as it occurs e.g. in the case of LiCoO2, can be investigated for different particle geometries. Based on these single-particle calculations, the methods developed can be applied to three-dimensional microstructure simulations in which the lithium-ion battery is modeled using a periodic unit cell. With these simulations, it is possible to examine the influence of the microstructure and especially the influence of particle size distribution. The numerical results should be validated by additional experiments. Therefore, pouch cells are produced, electrochemically characterized and finally compared with the simulations.