Slow Voltage Relaxation of Silicon Nanoparticles with a Chemo-Mechanical Core-Shell Model

Area of Science:

• Materials Science
• Electrochemistry
• Mechanical Engineering

Background:

• Silicon anodes offer high capacity for lithium-ion batteries but suffer from significant volume changes during lithiation/delithiation.
• These volume changes lead to voltage hysteresis, reduced efficiency, heat generation, and complex state-of-charge estimation.
• Observed voltage relaxation over days in amorphous silicon nanoparticles lacks a clear physical explanation.

Purpose of the Study:

• To develop a physically-based model explaining the voltage hysteresis and relaxation phenomena in silicon anodes.
• To investigate the role of chemo-mechanical effects in silicon particle behavior during battery cycling.
• To provide a model that can improve state-of-charge estimation and battery performance.

Main Methods:

• Application of a chemo-mechanical continuum model with a core-shell geometry, representing silicon particles coated by a solid-electrolyte interphase.
• Modeling the visco-elastoplastic behavior of the shell to explain hysteresis during cycling and after relaxation periods.
• Comparison of the developed model with the empirical Plett model and introduction of a reduced model for simplified estimations.

Main Results:

• The chemo-mechanical core-shell model successfully explains the observed voltage hysteresis in silicon anodes.
• A logarithmic voltage relaxation, consistent with the Garofalo law, was identified and explained by the shell's visco-elastoplasticity.
• The proposed model outperforms the empirical Plett model in describing the voltage hysteresis phenomena.

Conclusions:

• The mechanical behavior of the silicon anode, specifically the visco-elastoplasticity of the solid-electrolyte interphase in a core-shell structure, is the primary cause of voltage hysteresis.
• The developed chemo-mechanical model provides a robust explanation for silicon anode voltage hysteresis and relaxation.
• Further research into silicon anode mechanics is encouraged to optimize battery design and performance.