Electrical energy storage plays a crucial role in ensuring mobility and reliable energy supply in the future. Li-ion technology is considered as one of the solutions also for large scale applications (e.g. electrification of transport, smart grids). As we move from single cells to modules and packs, failure modes in Li-ion batteries become increasingly complex and their potential damage can be substantial and difficult to deal with. Failure modes due to thermal, mechanical, electrical abuse or cell-internal failure may develop into so-called thermal runaway which is accompanied by hazardous effects.
Cascading of thermal runaway through an entire battery (which can be composed of numerous electrochemical cells, typically from several dozen to several thousand), denoted as thermal propagation, can lead to severe consequences: large heat/gas release, associated hazardous events (e.g. emission of toxic materials, pressure build-up and release, electrolyte leakage, fire, explosion) and financial losses (e.g. property damage). Risks associated with simultaneous thermal runaways in multiple cells occurring can be mitigated by battery design and other measures from material to system level. In order to address the issue of thermal propagation, the Joint Research Centre organised an international workshop titled: 'Safer Li-ion batteries by preventing thermal propagation?' in March 2018 under the umbrella of its Exploratory Research Programme. The workshop offered a platform where leading experts exchanged ideas and research efforts on thermal propagation testing, new methodologies, policy and standardisation issues and brain-stormed on the potential impact of preventing thermal propagation on the safety testing landscape.
Outcomes of the JRC Workshop: "Safer Li-ion batteries by preventing thermal propagation?"
It is common understanding that a severe single cell thermal runaway failure in a battery system, while rather improbable, cannot be completely eliminated. Nevertheless there is a clear need for reliable batteries and components and, as a consequence, such scenarios should be addressed when evaluating battery safety. One approach for assessing related risks further is a thermal propagation test. As there is no single, clearly defined single cell thermal runaway failure scenario, it seems most useful to develop a thermal propagation test of general robustness versus a single cell thermal runaway. No reliable and practical method exists to create on-demand internal shorts in Li-ion cells that produce a response that mimics field failures. In this context the selection or development of a suitable initiation method is crucial. There was agreement that further pre-normative research is required to develop fit-for-purpose testing methods and standards.
There was also general agreement about the need for a harmonised definition of thermal runaway. A more detailed understanding of the mechanisms of thermal runaway might make also feasible to have several sub-definitions for discriminating between different types of thermal runaway.
Another important consideration concerns the significance of the outcome of a thermal propagation test: whether thermal propagation occurs depends mainly on the difference between heat introduced in and heat removed from a neighbouring cell. From a statistical point of view, this is a difficult situation as a relatively small change in heat flow can change the test outcome and it must be evaluated carefully if a tests delivers reliable test results and thereby a relevant confirmation of a certain level of safety. Even though many standardisation efforts have been on-going in the recent past, current standards still typically allow for different initiation methods and test details are not always defined. Further work is required to define standardised abuse testing methods regarding thermal propagation.
It is a common challenge for product developers, that even shorter battery development cycles are required to be competitive in a fast changing market. At the same time the long-term reliability of offered products must be ensured. In battery abuse tests, big differences in test outcomes have been observed - while not consistently - between end-of-life and beginning-of-life cells. Therefore thermal propagation testing seems also advisable on aged systems also in view of potential second use scenarios. On the other hand, such tests are difficult to realise within the rather short development cycles and lead to extra costs. There is a need for more accurate and faster early detection tools, which could allow control of certain safety events at an even earlier stage. Further research efforts in this direction seem justified and promising.
The mitigation of risks related to thermal propagation requires a holistic view of cell, battery and application. Defining measures on one level - independent of the other levels - may lead to high cost and limited increase in safety. For the near and medium term avoiding thermal propagation will be a key challenge for making Li-ion battery systems safer and the development of suitable tests is of high importance.
- JRC technical report: JRC exploratory research: Safer Li-ion batteries by preventing thermal propagation
- JRC workshop event page
- For further information please contact JRC-PTT-BATTERIES@ec.europa.eu
- 14 november 2018