Lithium-Ion Battery fire safety as a task for sustainable transition from research into fossil-fuel propulsion systems to research into electric propulsion system

Research Project, 2023 – 2025

Development of clean, efficient, and safe Electrified Vehicle (EV) technology is widely accepted to be of primary importance for the sustainable transition to a resilient transport system that uses renewable energy sources and operates without contribution to the accumulation of greenhouse gases in the atmosphere. Since transition to EV transportation is accompanied with a rapid growth of the number of Lithium-Ion Batteries (LIBs) used worldwide, the number of fire incidents due to failures of such batteries is likely to be increased, thus, calling urgently for intensifying research into LIB fire safety. As stated in a recent review article by Wang et al. (Prog. Energy Combust. Sci. 73:95, 2019), “safety issue is still the main obstacle to the usage of LIBs in large scale applications, such as EVs and energy storage systems”. The point is that LIB failures can trigger internal exothermic reactions within the LIB cell. Such reactions build flammable gases and increase the cell temperature, thus, causing the cell to undergo thermal runaway. Subsequently, preheated flammable gases can be released into the atmosphere and mixed with the air, followed by ignition and appearance of a jet fire or even explosion, which poses a significant risk to surroundings. While the problem of thermal runaway has been attracting a growing amount of attention, as reviewed in the aforementioned article, research into jet fires caused by thermal runaway in a LIB cell has yet been outside the focus of the battery and combustion communities. As stated in the same review article, “comprehensive modeling of the flames produced by the materials ejected from LIB has not been performed to date”.

To rapidly respond to this topical crucial challenge, deep knowledge on combustion physics is strongly required and research into LIB fires can efficiently be advanced by adopting state-of-the-art tools and methods elaborated by the combustion community. Nevertheless, such a task is not straightforward, because LIB fires involve new specific challenges to be thoroughly addressed. First, “there is little available information characterizing the flammability properties of the gases released after cell thermal runaway” (Baird et al., J. Power Sources 446:227257, 2020) and such properties has yet been reported in a few studies only. While knowledge and numerical tools required for calculating basic combustion characteristics of a single fuel in a gaseous mixture ejected from a LIB after thermal runaway inside it are available for many relevant fuels, such simulations have yet to be done for a wide range of fuel mixtures associated with various LIB failures, e.g., because the ejected gas composition varies significantly with the state of charge of a LIB. Second, at high states of charge, the ejected gases contain significant amount of H2 and burning or explosion of such gases is expected to be strongly affected by difference in molecular diffusivities of H2 and O2. Such differential diffusion effects are not yet well understood even in the classical case of a lean H2/air flame, where the effect magnitude is known to be high, and the present applicant is not aware of any study on the influence of differential diffusion on LIB fires. Third, electrolytes used in LIB cells consist of organic solvents, such as various carbonates, and salts dissolved in the solvents. While these carbonates are flammable, their basic combustion characteristics are still known poorly. Thus, despite great importance of LIB fire safety both for industry and society, the problem is far from being solved and the present proposal aims at bridging this knowledge gap and, hence, at developing safe LIBs for clean and efficient electric propulsion. The proposal is directly relevant to SVT’s research area propulsion systems.

As profound understanding of combustion physics is strongly required to explore LIB fires, this task offers the unique opportunity to apply state-of-the-art knowledge and competence accumulated by the project applicant in combustion over decades to another highly prioritized research field, thus, expanding well established research towards new topics. However, to advance LIB fire research, the routine use of the available knowledge and competence is not sufficient and new knowledge and competence should also be generated by exploring issues specific to LIB fires. While the applicant is less experienced in LIB research, the required knowledge and competence will rapidly be acquired thanks to his established high level of excellence in combustion physics, thus, making (i) distance to the LIB safety research frontier short and (ii) potential for future excellence high. Besides, the problem of LIB fire safety also offers the unique opportunity to bridge the gap between conventional research into fossil-fuel propulsion systems and modern research into electric propulsion systems, thus, moving from competition between the two research communities to collaboration between them, with such a cross-disciplinary collaboration being expected to yield significant synergetic effect. Thus, another goal of the present proposal is expansion of the applicant’s research network to LIB safety community.

The project will consist of four Work Packages (WPs). WP1 aims at numerically simulating common basic combustion characteristics (laminar flame speed, ignition delay time, flammability limits, etc.) of gaseous mixtures ejected from LIBs after thermal runaways under various conditions, e.g., states of charge. The simulations will be performed adopting detailed chemical mechanisms available in the literature and running software CHEMKIN-PRO installed at Chalmers and widely used by the applicant earlier. The simulation conditions, e.g., the range of mixture compositions, will be selected (i) by analyzing the literature and (ii) discussing results of LIB thermal runaway studies performed in RISE (ongoing projects granted by the Energy Agency) and University of Lund (a project sponsored by Swedish Electromobility Centre, SEC). The simulation results will be delivered in a form of database, with the most dangerous conditions being highlighted. In addition, influence of chemical inhibitors, e.g., molecules containing alkali metals such as Na or K, which could be added to electrolytes to mitigate LIB fires by bonding radicals, on the computed combustion characteristics will be explored.

WP2 aims at simulating critically strained laminar flames for mixtures ejected from LIBs after thermal runaways. This research goal stems from the fact that characteristics of such flames are most appropriate for modeling differential diffusion effects in lean hydrogen-air mixtures, as argued in 7 journal papers published by the applicant over the past two years. As he is a well-recognized leader in modeling such effects in flames, WP2 has a clear potential to go well beyond state of the art in the field of LIB fire safety. The aforementioned chemical mechanisms and software will also be used in WP2. Database of computed flame characteristics and knowledge generated on expected magnitudes of differential diffusion effects under conditions associated with LIB fires are major WP2 deliverables.

WP3 aims at simulating fundamental combustion characteristics of carbonates contained in LIB electrolytes. In the beginning, a few chemical mechanisms developed earlier for dimethyl carbonate will be assessed by simulating a few available experimental databases. Subsequently, combustion chemistry will numerically be explored for other relevant carbonates. This work will be performed in collaboration with Prof. Konnov (University of Lund) whose group has been measuring laminar flame speeds of various fuels, including certain carbonates. Database of computed flame characteristics and validated (and eventually improved) chemical mechanisms are major WP3 deliverables.

WP1, WP2, and WP3 will be started one after another (with six-month delays), but each WP will be active till the project end to continuously adapt relevant experimental and numerical data or chemical mechanisms, which will eventually be published in 2024 and 2025. WP4 aims at disseminating the project results and, at least, two conference and one journal papers are planned to be delivered. Databases created during the project will directly be used in subsequent Computational Fluid Dynamics (CFD) research into LIB jet fires within the framework of following-up large-scale projects.

The outlined research program will be implemented by Research Professor Andrei Lipatnikov who will devote 25% of his time to this task. The project will offer him the opportunity to expand well established combustion research towards a new topic of LIB safety, with a distance to research frontier being expected to be short and potential for future excellence being expected to be high. Besides, the project will offer him the opportunity to expand a LIB-safety-research network involving experts from Chalmers, SEC, RISE, University of Lund, Polarium, Volvo AB, etc. This network will bring together not only experts in combustion, fires, and explosions, but also experts in LIB-specific issues, such as, e.g., thermal runaway. The network will be a base for future large-scale applications. Moreover, a wide international network (about 10 universities worldwide) established by the applicant in the field of hydrogen combustion will be adapted and expanded to acquire new fundamental knowledge on LIB-specific issues, such as the influence of differential diffusion on LIB fires and explosions. Knowledge and tools generated when implementing the project and expanding long-term cross-disciplinary national and international collaboration will directly contribute to the transition to SVT’s vision for a sustainable transport system and will be straightforwardly relevant to SEC mission and vision.