Chemo-Mechanical Core-Shell Model Explaining the Silicon Voltage Hysteresis and Long-Term Relaxation

Abstract

Silicon is considered as next-generation anode material for lithium-ion batteries owing to the tenfold increase in theoretical capacity compared to graphite anodes. However, beneath the significant volume expansion of silicon during lithiation, the silicon voltage hysteresis represents a major challenge for the commercial use. The hysteresis causes a reduced efficiency, detrimental heat generation, and complicates the state-of-charge estimation. Our contribution elucidates the reason of the voltage hysteresis phenomenon and identifies approaches to overcome the related limitations. We developed a chemo-mechanical model accounting for the interaction between active silicon and a surrounding inactive phase in a core-shell geometry. The shell can be considered as solid-electrolyte interphase (SEI), inactive silicon domains, or silicon oxide. The volume changes of the active silicon during cycling cause significant stresses inside the shell, resulting in pronounced degradation [1]. Simultaneously, the visco-elastoplastic shell implies stress to the silicon particle, impacting the chemo-mechanical potential. Therefore, our model reproduces the experimentally observed silicon voltage hysteresis during cycling and after short-term relaxation [2]. Moreover, a recent improvement of our mechanical model allows to describe the long-term, logarithmic voltage relaxation over weeks [3]. Hence, our modeling approach reproduces the observed silicon voltage hysteresis and relaxation consistently. In addition, we derived a reduced hysteresis model, which outperforms the empirical Plett model in terms of physical interpretability and voltage predictions during relaxation. In conclusion, we explain the silicon voltage hysteresis and long-term relaxation with a visco-elastoplastic core-shell model. Our physical understanding supports the improvement of the performance and state estimation of pure silicon anodes desired for future applications. References: 1. L. Kolzenberg, A. Latz, B. Horstmann, Batter. Supercaps 5 (2022), 2, e202100216 2. L. Köbbing, A. Latz, B. Horstmann, Adv. Funct. Mater. 34 (2024), 7, 2308818 3. L. Köbbing, Y. Kuhn, B. Horstmann, ACS Appl. Mater. Interfaces 16 (2024), 49, 67609-67619