Mechanical behaviour of thin film separators in rechargeable batteries

Abstract

In the past decade, increasing fire accidents have been reported for electric vehicles (EVs) after the EV rechargeable batteries suffered from mechanical abuses. Separator is the most important component in a rechargeable battery in terms of safety. Its mechanical behaviours play a crucial role for the overall safety of the battery, and therefore, should be thoroughly investigated. The aim of this PhD programme is to develop mechanical models under uniaxial tensile stress state and the associated testing techniques for thin film separators commonly used in lithium-ion batteries. The main objectives are to improve the mechanical test method, explore the mechanical characteristics, and establish a time-dependent constitutive model for the separator under tensile stress state. The thesis introduces the background of the research and gives a thorough literature review, based on which the important outstanding issues are outlined. A comprehensive study is carried out by a combination of FEM simulations, experiments, and analytical methods. Firstly, it is demonstrated that wrinkling phenomena, which commonly occurred in standard uniaxial tensile tests because of the ultra-thin thickness characteristics, can adversely influence the test measurement. A numerical study is conducted to improve its measurement accuracy, and a novel critical geometric boundary (CGB) is proposed for the design of samples to meet the uniaxial tensile test standards. Secondly, uniaxial tensile tests are conducted under the quasi-static condition for typical types of separators, including both dry and wet process separators, using 3D digital image correlation (DIC) technique. It shows that the CGB method successfully diminishes the measuring errors for separators whose anisotropy are not high. Typical mechanical properties and 1D strain-dependent porosity variation are discussed and concluded. Inhomogeneous strain distribution is found in dry process separators, but not in wet process separators. It implies that the longitudinal engineering strain can be accurately measured by a virtual extensometer for wet process separators. The valid conditions for the uniaxial tensile tests of dry process separators are determined. Thirdly, viscoelastic behaviour of separators subjected to cyclic loading is an essential characteristic that should be considered for the long-term application of rechargeable batteries. A tension-relaxation test is designed and conducted in a range of low loading strain-rates using a virtual live extensometer. The hyperelastic constitutive and finite-strain viscoelastic models are combined theoretically to establish an analytical hyper-viscoelastic (HVE) model. To determine the HVE model, a novel parametrical calibration method is proposed and compared with other two traditional methods. By comparing with the experimental results, the proposed HVE model can simultaneously describe viscoelastic effects, and large deformation. Finally, several topics, including the change of the electrochemical behaviour with the deformation of separators in unit cell and the relationship between tortuosity/porosity and strain under mechanical abuses, which were planned but not performed due to the impact of Covid-19, are briefly outlined, together with the discussion of future works.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Qing Li (Supervisor) & Lei Ren (Supervisor)