Multiscale Analysis and Safety Assessment of Fresh and Electrical Aged Lithium-Ion Pouch Cells Focusing on Mechanical Behavior

Abstract

Analyzing the impact of electrical aging on the lithium-ion cell’s mechanical behavior and safety is an important factor to assess the crash safety of electric vehicles during their lifetime. In this study, fresh and electrical aged state-of-the-art NCM pouch cells were investigated. Aged cells, which were cycled electrically to 90% state of health, under laboratory conditions in electric vehicle battery modules were used. The used charging/discharging strategy represents real customer behavior based on accelerated driving profiles. First, it is shown that electrical aging has a significant influence on the anodes’ and separators’ mechanical properties, which had a lower mechanical strength and stiffness under tension. Additionally, quasi-static cylindrical indentation and three-point bending tests were performed to investigate aging effects on cell level at varying state of charge (SOC). Aged cells with 0% SOC showed a right-shifted force–displacement curve and a 29% lower maximum force compared to fresh cells. Fully charged, aged cells reached a similar maximum force to fresh cells, but faster temperature increase and higher temperature peaks after internal short circuit. Inductively coupled plasma optical emission spectrometry analyses confirmed an increased lithium content on the anode surface, which is indicated in literature as a reason for the increased exothermic reaction of the aged cells. The results indicate a higher safety risk for the aged investigated pouch cells under mechanical loads based on their changed mechanical properties and thermal runaway behavior.

Particle Contamination in Commercial Lithium-Ion Cells—Risk Assessment with Focus on Internal Short Circuits and Replication by Currently Discussed Trigger Methods

“…In summary, it can be concluded that the effective impact force on a hypothetical impurity particle increases due to parallel ageing processes and with each charging cycle, as visualised in Figure 1. Moreover, the mechanical stability of the separator also decreases with cell ageing [58][59][60], as indicated by the red curve in Figure 1. For example, X. Zhang et al observed a reduction in strength by up to 30% after 1200 full cycles compared to the new condition in a punch test [59].…”

“…For example, X. Zhang et al observed a reduction in strength by up to 30% after 1200 full cycles compared to the new condition in a punch test [59]. M. Sprenger et al also noticed a decrease by 25% in tensile strength, 50% in failure strain and 30% in Youngs modulus when investigating aged cells at an SOH of 90% [60].…”

Experimental analysis and safety assessment of thermal runaway behavior in lithium iron phosphate batteries under mechanical abuse

“…Furthermore, studies on battery safety assessment have been conducted in response to ISC and thermal runaway caused by mechanical abuse 36 – 39 . Based on mechanical abuse experiments, Ohneseit 40 and Ellersdorfer 41 conducted safety assessments by extracting characteristic parameters of battery failure for 18,650 batteries and pouch cells, respectively.…”