The Sustainability Assessment of Second Life Applications of Automotive Batteries (SASLAB) exploratory research project of the European Commission’s Joint Research Centre (JRC) aims at developing and applying a methodology to analyse the sustainability of deploying electrified vehicles (xEV) batteries in second use applications. A mapping of industrial demonstration and publicly-funded research projects in the area is presented, followed by an experimental assessment of the capacity and impedance change of lithium-ion cells during calendar and cycle ageing. Fresh cells and cells aged in the laboratory, as well as under real-world driving conditions, have been characterised to understand their application-specific remaining lifetime, beyond the 70% to 80% end-of-first-use criterion. For this purpose, pre-aged cells were examined under duty-cycles that resemble those of second use grid-scale applications.
The main goal of this work is to understand the effect of thermal runaway initiation conditions on the severity of thermal runaway (TR) of Graphite—NMC (111) cells. A coupled electrical-thermal model is developed, which includes the initial energy input, the chemical decomposition processes of the anode, cathode and the electrical energy released by an internal short circuit. 780 different thermal runaway events are simulated and the output is analysed by machine learning techniques such as principal component analysis and clustering. It was found that TR events form 5 clusters between no thermal runaway and severe thermal runaway. Sensitivity analysis is applied on the 39 input invariants and the triggering energy input, resistance ratio, the heat convection coefficient, the ratio of activation energy of oxygen liberation and electrolyte evaporation are found to be the most important parameters. The later one determines the amount of electrolyte combustion. The probability of thermal runaway is calculated taking into account the most important parameters and their interactions. Finally, a combination of initiation parameters is suggested, which most likely results in a repeatable and reproducible outcome.
Lithium-ion batteries, containing flammable electrolytes, have become safer in many ways since their invention. As the technology matures, energy and power densities increase: the European Council for Automotive R&D set 2030 (cell level) targets for battery electric vehicle energy density of 1000 Wh L-1, with 450 Wh kg-1 specific energy and 1800 W kg-1 peak specific power. This rise in energy and power densities increases the risk of accidental release of energy from the batteries and thermal runaway (TR). Safety is thus a key concern when designing a lithium-ion battery system for electric vehicles (EV).
TR relates to fast heating of a battery cell caused by exothermic chemical decomposition reactions of the materials inside the cells. It develops in a cell when heat is generated and cannot be dissipated quickly enough to the surroundings, giving rise to an abrupt (exponential) temperature increase and subsequent reaction rate increase. During TR, various exothermic side reactions can occur, leading to temperature increase, accompanied by pressure increase as electrolyte evaporates and venting. This can result in the emission of highly flammable gas and the formation of toxic atmosphere, as well as, in fire and, in very rare circumstances, in explosion. The corresponding temperature increase in adjacent cells (or modules) might then be sufficient to cause them to also go into TR – leading to a process known as thermal runaway propagation (TRP).
Fit-for purpose testing procedures to assess the risks associated with TRP are therefore of utmost importance and the focus should always be the safety of the EV occupants, bystanders, first responders and property. Some tests in current standards and regulations try to simulate internally driven failures (e.g. internal short circuit), but whether these tests are suitable to represent field failures remains an open question.
This work will give an insight into two selected TR initiation methods assessed within JRC’s TR initiation and propagation test campaigns, namely (a) localised rapid external heating, as developed and patented internationally by NRC (National Research Council Canada) and (b) ceramic nail penetration, as described in the IEC TR 62660-4 standard. It examines the response of short-stacks of pouch cells extracted from automotive batteries after TR is triggered in a single cell. Experimental data and analysis to better understand how TR propagates and the potential for TRP inhibition will be reported. The potential of inhibiting TRP is explored on 2- and 5-cell assemblies with a multi-layer, porous, composite insulation material between cells. Separating the cells can delay significantly TRP in adjacent cells in modules constructed with pouch cells, whereas TRP may be slowed and even inhibited as the thickness of the multilayer material varies.
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