Understanding Growth and Performance Drawbacks of the Solid-Electrolyte Interphase in Lithium-Ion Batteries

Kurzfassung

Modern lithium-ion batteries present an important energy storage technology with a reasonable service life, though still suffering from continued capacity fade. Additionally, pure silicon anodes desired for advanced energy densities face efficiency drawbacks, heat generation, and imprecise state-of-charge (SOC) estimation due to the occurrence of a voltage hysteresis. This thesis elucidates the capacity loss and the silicon voltage hysteresis by considering the solid-electrolyte interphase (SEI), which forms at the anode side owing to electrolyte decomposition. To understand the impact of the SEI and facilitate respective improvements, thermodynamically consistent continuum models are further developed and analyzed. First, this thesis focuses on the investigation of SEI growth, which is considered the main cause of capacity fade in lithium-ion batteries with standard graphite anodes. Thus, a detailed comprehension of the SEI can support increased battery lifetimes. Generating valuable insights, this study compares the diffusion of electrons and solvent molecules through the SEI as possible drivers of SEI growth during battery storage. The simulation results aim to reproduce experimental findings regarding the time and SOC dependence of capacity loss as well as their interplay. The careful comparison approves the electron diffusion as the decisive mechanism for continued SEI growth on graphite anodes. For next-generation lithium-ion batteries, silicon is a promising candidate for substituting graphite with a significant capacity increase. However, the enhanced ability of lithiation is accompanied by substantial volume changes during cycling, highlighting mechanical aspects. In particular, the SEI deforms purely mechanically and causes distinct stresses acting on the silicon particle. The arising stresses inside the silicon anode influence the voltage due to chemo-mechanical coupling. Thus, the simulations reveal that the mechanical impact of the SEI causes the observed voltage hysteresis of silicon nanoparticle anodes during cycling and (after) relaxation. Moreover, the chemo-mechanical description fits the observed long-term voltage relaxation. Hence, the model provides a consistent picture of the observed voltage hysteresis phenomena. For ordinary voltage estimations, the study derives a reduced chemo-mechanical hysteresis model, which preserves physical interpretability and covers voltage relaxations contrary to the empirical Plett model. Eventually, the work investigates the effect of the SEI on an elliptical silicon nanowire. The results reveal lithium concentration anomalies due to the anode geometry and the mechanical impact of the SEI. Overall, this work contributes to an improved understanding of the SEI regarding growth mechanisms and the mechanical impact on silicon anodes. Particularly, the chemo-mechanical interaction of silicon and SEI is considered the reason for the voltage hysteresis herein for the first time. Consequently, this thesis aims to facilitate enhanced battery lifetime and efficiency by advancing SEI properties.