Compressive behaviour of separators in Li-ion battery at meso/micro-scale level and its influence on the battery

Abstract

This PhD research focuses on understanding the compressive behaviour of Li-ion battery (LIB) separators and its influence on LIB's performance. A 3D meso-/micro-scale model is developed to simulate compression evolution, analyze microstructure characteristics, and explore the correlation between compressed separators and LIB's performance. Experimental and analytical methods support that developed model. Firstly, a 3D finite element model based on polypropylene (PP) separator microstructure is established to study its compressive behaviour at the micro-scale. Fibrils of the separator are incorporated into the model to consider fibrils' contribution to resisting pore closure under compression, as evidenced by experimental data and SEM images. The variations of the porosity and tortuosity under compression are determined, enabling analysis of LIB performance with compressed separators. Secondly, three methods for determining separator electrical tortuosity are introduced and compared: the porosity-tortuosity relationship, electrochemical impedance spectra (EIS) testing, and numerical analysis using 3D microstructure from XCT. The relationship between geometrical and electrical tortuosity is established through an electrical phenomenological factor, aiding the understanding of the separator's microstructure and transport properties. The tortuosity determined by the EIS method is more accurate than those determined by other methods. Thirdly, the commercial separators (PP and polyethene (PE)) with different compressive behaviours are studied. EIS tests reveal higher tortuosity for compressed PP separators than PE under given compression. Capacity performance testing of LIBs with compressed separators identifies the critical compressive onset strain of densification as a key parameter affecting LIB capacity. PE separators with large deformation can provide a better capacity performance of LIB than PP separators due to their higher microstructure connectivity. Experimental findings align with COMSOL modelling. In conclusion, this research addresses the compressive behaviour of separators and its influence on energy storage, providing insights for improving battery durability, reliability, and safety. It contributes to developing more efficient and safer energy storage technologies.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Qing Li (Supervisor), Ping Xiao (Supervisor) & Paola Carbone (Supervisor)