title: Chemo-mechanical Modeling of Lithium-Ion Batteries
Creators: name: Bai, Yang
Publisher: name: UNSPECIFIED
Year: 2021

description: Due to the high energy and power density, Lithium-ion batteries (LIBs) have been widely employed in portable electronics and electric vehicles, and have become a promising solution for the storage of renewable energy. Besides electrochemistry, it has been widely recognized that mechanics plays a critical role in the performance and the lifetime of LIBs. In particular, electrode materials with a high theoretical capacity suffer from irreversible mechanical degradation already after few cycles due to high internal stress, which is the well-known dilemma between capacity and cyclability of LIBs. It has motivated a number of chemo-mechanical studies on both the active particle level and the cell level in the last two decades. This thesis presents first a thermodynamically consistent framework to derive a fully coupled electro-chemo-mechanical model for LIB electrode materials. In particular, it regards not only the chemo-mechanical bulk behavior in large deformation regions with phase separation but also the chemo-mechanical interface model, which addresses both the damage-dependent across grain boundary (GB) transport and a chemo-mechanically coupled cohesive zone law for mechanical failure. Although the unique mechanical and transport features of grain boundaries or interfaces in polycrystalline ion conductors have been recognized, the understanding of the chemo-mechanical interplay at the interface and its impact is insufficient. The derived model serves exactly this purpose. Based on the derived model, 3D finite element simulations for LiNixMnyCozO2 meatball particles have been carried out. Results demonstrate that the enhanced intergranular chemical inhomogeneity can weaken the interface mechanical strength and can lead to GB cracking. In contrast, the interface damage can, in turn, influence or even block the across-GB transport, thus enhance further the chemical inhomogeneity. This positive feedback explains the simulated results of chemical hot spots and surface layer delamination, which have been observed experimentally but go beyond the start-of-the-art simulation work. In order to investigate the impact of chemo-mechanical particle behavior and damage on cell performance, a particle-cell two-level finite element model is further developed in this thesis. The widely used Pseudo-Two-Dimensional (P2D) cell model for LIBs is generally based on a simplified lithium diffusion model of active particles with simple geometry. This thesis presents a two-level framework, which extends the P2D cell model and incorporates a chemo-mechanically coupled 3D particle model as mentioned above. The two-level model allows a more detailed diffusion study of particles with general geometry and can include the full coupling among mechanics, phase separation, interface transport, and damage. To improve the computational efficiency, we manage to reduce one degree of freedom at the cell level by treating the ion flux between the electrolyte and active particle as a dependent quantity. The two-level framework is validated against the original one and applied to study the impact of particle geometry, elastic properties, and phase separation on cell performance. For instance, results show that the oblate particle has better cell performance than other spheroidal particles. It is attributed to the mechanical drifting at the higher curvature. The combination of the proposed chemo-mechanical particle model and the extended particle-cell two-level model lay a good foundation for chemo-mechanical simulations of LIBs and other related ion batteries. A series of parameter studies can be carried out, which are helpful to reveal both the mechanism and degradation of the particle and the related cell performance change, and can thus serve as powerful tools for multiphysics battery designs and optimization.