Elliptical Silicon Nanowire Covered by the SEI in a 2D Chemo-Mechanical Simulation

TL;DR

This study models the chemo-mechanical behavior of elliptical silicon nanowires with SEI in lithium-ion batteries, revealing stress distributions and potential fracture points to inform better anode design.

Contribution

It introduces a 2D chemo-mechanical simulation of elliptical silicon nanowires with SEI, highlighting the impact of shape and SEI stiffness on stress distribution.

Findings

Maximum stress occurs at the minor half-axis for soft SEI.

Shape influences stress concentration more than SEI properties.

Stiffer SEI reduces stress concentration at the nanowire surface.

Abstract

Understanding the mechanical interplay between silicon anodes and their surrounding solid-electrolyte interphase (SEI) is essential to improve the next generation of lithium-ion batteries. We model and simulate a 2D elliptical silicon nanowire with SEI via a thermodynamically consistent chemo-mechanical continuum ansatz using a higher order finite element method in combination with a variable-step, variable-order time integration scheme. Considering a soft viscoplastic SEI for three half cycles, we see at the minor half-axis the largest stress magnitude at the silicon nanowire surface, leading to a concentration anomaly. This anomaly is caused by the shape of the nanowire itself and not by the SEI. Also for the tangential stress of the SEI, the largest stress magnitudes are at this point, which can lead to SEI fracture. However, for a stiff SEI, the largest stress magnitude inside the nanowire occurs at the major half-axis, causing a reduced concentration distribution in this area. The largest tangential stress of the SEI is still at the minor half-axis. In total, we demonstrate the importance of considering the mechanics of the anode and SEI in silicon anode simulations and encourage further numerical and model improvements.