Multi-scale Modeling of Rechargeable Ion Batteries

Rechargeable Li-ion batteries (LIBs) play a crucial role in fulfilling our energy needs during our everyday residential and commercial activities. Thus, there is a growing demand for next-gen LIBs’ with more robust, reliable and long-term operation. One of the ways of advancement in this technology is through the development of high-capacity electrodes with long cycle-life. However, such an advancement is severely hindered, since almost all the high capacity electrodes are accompanied with rapid mechanical degradation due to large volume changes and crack development in the electrodes during charge and discharge cycles. Therefore, a fundamental understanding of the origin of degradation mechanisms, individual processes, and their interplay is necessary to further improve this technology for a reliable and long-term operation. Mechanical deformation and stress are responsible for most degradation mechanisms and ultimate failure of cells. In this regard, the present work addresses the theory, mathematical formulation, numerical implementation, and application of fully-coupled diffusion and structural mechanics theories aimed at capturing the ion transport and the chemo-mechanics both from a particle and interface scales. The multi-scale approach followed in the present work covers atomistic (sub-nanometer) levels through the interface scale (nanometer) up to a particle (meso-scale) scale. The multi-physics approach couples physics between the scales using the calculated and/or derived inter-domain parameters exchange. Thus, this framework allows for physical multi-scale modeling of fundamental processes occurring in rechargeable LIBs. At the particle scale, a continuum scale coupled diffusion-structural mechanics model (CDSM) of a single NW is proposed. The effect of large strains and the concentration-dependent elastic properties are considered, which has been proved in this work to have a great impact on the Li diffusion and the phase separation. From a materials standpoint, two types of nanowires (NW), i.e., silicon (Si) and tin oxide (SnO2) are considered. The results from the CDSM simulations provide a good understanding of the induced stress due to Li insertion and the plastic deformation. possible locations where the electrode material could start to fail. Through the results, it could be understood that the lithiation induced stress leads to formation of dislocations or necking in these high-capacity electrodes, which further translates into degradation and mechanical failure upon further cycling. At the interface scale, random Li electrodeposition, stress generation and dendrites formation in Li-metal batteries are understood by analysing the electrode/electrolyte interfaces. Since the length scales are much smaller compared to the particle scale, an atomistic scale ab-initio density functional theory (DFT) calculation is employed. The results from the DFT calculations identify the dominant diffusion pathways, energetics and the corresponding diffusion coefficients associated with Li diffusion through the polycrystalline SEI. This crucial information provides important physical insights about the localized Li diffusion and dendritic nucleation through the SEI. In addition to providing physical insights into rate-limiting processes occurring in LMBs, further DFT calculations were conducted to understand the stability of these grain structures on Li surface.