Analysis of Mechano-Electrochemical Coupling in Intercalation Electrodes

Abstract

Lithium ion batteries (LIB), owing to their high energy and power density, have gained popularity in portable electronics and automotive markets. Diffusion induced stress (DIS), due to intercalation of lithium during lithiation/delithiation process is one of the main causes of mechanical degradation in LIB. The microcracks formed hinder the diffusion of lithium inside the active particle. Also, the microcracks linked to the surface of the particle are exposed to the electrolyte and are electrochemically active.

This study investigates the mechano-electrochemical coupling observed in intercalation electrodes. The interdependence between microcrack formation and lithium concentration distribution in the active particle and its effect on the performance of LIB has been analyzed. A microcrack prediction model has been developed that estimates microcrack formation at each time step based on the DIS calculated using the concentration gradients evaluated from the concentration profile. The microcracks affect the transport of lithium within the particle in two opposing ways. On one hand, microcracks decrease the local diffusivity of the active material thereby hindering lithium diffusion. On the other hand, microcracks emanating from the surface of the particle are electrochemically active and enhance lithium diffusion by allowing electrochemical reactions inside the active particle at the microcrack-electrolyte interface, thereby reducing the effective diffusion length. Thus, microcrack formation leads to a change in the electrochemically active surface area of the electrode. Lithium source/sink terms are introduced along the electrochemically active microcracks to simulate the electrochemical reactions. The non-uniform microcrack patterns predicted by the mechano-electrochemically coupled model closely resemble the patterns observed in SEM images of LIB electrodes.

The performance curve obtained can help identify the effect of mechanical degradation on the performance of the battery and thereby provide a guideline for optimizing the physicochemical factors to leverage mechanical degradation for better cell performance.