Degradation of Separator after Calendar Ageing in 18650 Li-Ion Battery: Impact on Safety and Performances

Lithium ion batteries (LIBs) are the most popular types of rechargeable battery with their superior gravimetric and volumetric capacities for electric and hybrid vehicles. But more than energy density, cost, lifetime or recyclability, safety issues is the most challenging parameter to investigate especially after ageing. Even if separator is considered as electrochemically inert, it play a major role in all of these properties. It is so crucial to evaluate its evolution in time, to observe its degradation and its impact on safety and performances of LIBs. The aim of this study is to determine the evolution of chemical, morphological and mechanical properties of a polyethylene separator under two different ageing conditions: calendar ageing and fast charge cycling in 18650 LFP/G batteries systems. Its porosity is evaluated by Helium pycnometry, its mechanical properties by tensile test. Surface chemical composition is also investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Surface state is observed by scanning electronic microscopy (SEM) and by atomic-force microscopy (AFM). Electrochemical performances are analyzed by impedance spectroscopy and C-rate tests to investigate the consequences of the separator ageing. Safety tests are also performed by overcharge in an Accelerating Rate Calorimeter (ARC) and by in-situ dendrites growing in coin cells. First results show a decrease of the separators porosity after calendar ageing which causes a decrease of the cell performances at high capacity rate. A deposit is observed on the extreme surface. A mechanistic model of ageing will be proposed which integrate a porosity gradient and an evolution in time.