Localization Phenomenon Due to Mechanically Induced Transport Non-uniformities in Lithium-ion Batteries

Abstract

Lithium-ion batteries are widely used in many applications, ranging from portable electronic devices to electric vehicles. Lithium plating, the formation of metallic lithium, is a side reaction that not only leads to capacity fade but also imposes serious safety concerns. While most of the plating can be prevented under normal cycling conditions, localized plating is difficult to prevent, as any heterogeneity in materials can create non-uniformities in transport and induce localized plating. Unevenly distributed mechanical stresses that arise externally and internally can locally deform battery separators, decreasing the separator porosity and slows down the ion transport. Variations in the speed of transport create locations with high current densities and lead to metallic plating. The change of local behaviors of a battery due to non-uniform transport is called localization.

This dissertation looks into the mechano-electrochemical coupling effect and addresses the localization phenomenon by connecting lithium-ion transport and degradation from both electrochemical and mechanical perspectives. A battery is a mechanically active system. The mechanics of a battery affect its electrochemical performances, which in return can alter the internal stresses inside a battery. This dissertation starts with characterizing the mechanical properties of battery separators and shows that separators play a critical role in connecting mechanical stress and aging of a battery through creep. After establishing the global mechano-electrochemical coupling in lithium-ion batteries, a micro point of view is adopted to understand lithium plating reactions that are locally induced. By mechanically creating variations in transport using separator pore closure, the probability of plating is evaluated under various non-uniform transport scenarios. There exists a strong size dependence of transport non-uniformities on localized plating, and the probability of plating is determined together by size, current density, and patterns of transport non-uniformities. This work helps elucidate the fundamentals behind local plating and mechano-electrochemical coupling, which can provide practical insights into battery safety, quick charging, and product control.