Topological and Network Analysis of Lithium Ion Battery Components: The Importance of Pore Space Connectivity for Cell Operation

TL;DR

This paper demonstrates that pore space connectivity, beyond porosity and tortuosity, critically influences lithium ion transport in battery separators, impacting cell performance and safety.

Contribution

It introduces topological and network analysis of separator microstructures to better predict and optimize lithium ion transport in batteries.

Findings

Pore connectivity affects lithium ion concentration gradients.

Different separators with similar porosity can perform differently due to pore connectivity.

Designing separators with optimized pore connectivity can enhance safety and charge rates.

Abstract

The structure of lithium ion battery components, such as electrodes and separators, are commonly characterised in terms of their porosity and tortuosity. The ratio of these values gives the effective transport of lithium ions in the electrolyte-filled pore spaces, which can be used to determine the ionic resistivity and corresponding voltage losses. Here, we show that these microstructural characteristics are not sufficient. Analysis of tomographic data of commercial separators reveals that different polyolefin separators have similar porosity and through-plane tortuosity, which, in the homogenised picture of lithium ion cell operation, would imply that these different separators exhibit similar performance. However, numerical diffusion simulations indicate that this is not the case. We demonstrate that the extent to which lithium ion concentration gradients are induced or smoothed by the separator structure is linked to pore space connectivity, a parameter that can be determined by topological or network based analysis of separators. These findings enable us to propose how to design separator microstructures that are safer and accommodate fast charge and discharge.