• Energy Research

It is widely accepted that Lithium-Iron Phosphate (LFP) cathodes are the safest chemistry for Li-ion cells, however the study of them assembled in to battery modules or packs is lacking. Hence, this work provides the first computational study investigating the potential of thermal runaway propagation (TRP) in packs constructed of LFP 18650 cells. Utilizing a 2D model of a battery pack in which one cell is assumed to experience an internal short circuit, it is found that TRP does not occur even under extreme environmental conditions. This shows the potential that LFP cells have at enabling safe and abuse resilient large scale batteries.

Abstract Overheating by oven exposure testing is a fundamental method to determine the severity of thermal runaway (TR) in lithium-ion cells. The TR behavior of lithium iron phosphate (LFP) cells under convection oven exposure is quantified and a comparison is made of their stability and severity against that of lithium metal oxide cells under similar conditions presented in the literature. The convection oven test is carried out at 180°C and 220°C, the TR response of the LFP cells is shown to be significantly more stable and less severe than lithium cobalt oxide cells tested in the literature. Also, under an oven abuse test a cylindrical cell is shown to have near uniform surface temperature along its length.

Thermal runaway (TR) is a significant safety concern for Li-ion batteries (LIBs), which, through computational modelling can be better understood. However, TR models for LIBs lack a proper representation of the build-up of pressure inside a cell under abuse, which is integral to predicting cell venting. Here, an advanced abuse model (AAM) is developed and compared to a classical TR model, considering a lithium iron phosphate (LFP) cell case study. The AAM accounts for two additional features: 1) venting, with a novel description of the internal cell pressure governed by the bubble point of the electrolyte/decomposition-gas mixture, and 2) simmering reactions. The novel bubble pressure assumption is validated against experimental data, and we show that the AAM significantly improves the predictions\ud of time to TR and of temperatures after TR. Further, it is shown that there is significant uncertainty in the parameters defining the decomposition reactions for LFP cells. Importantly, cell pressurisation is most dependent on the gases released by the solid electrolyte interphase reaction, and venting is dependent on cell burst pressure and reaction activation energies. The AAM is essential for accurate abuse modelling, due to its improved temperature predictions, and considerably enhances the LIB TR field of study.

In this paper, accelerated rate calorimetry (ARC) and oven exposure, are used to investigate thermal runaway (TR) in lithium-ion cells. Previous work shows that lithium iron phosphate (LFP) cells have a lower risk of TR over other Li-ion chemistries. ARC is carried out on cells at various SOC to identify which decomposition reactions are contributing to the TR behaviour of a cell at different SOC. Results show, at SOC of 100% and 110%, the negative and positive electrode reactions are the main contributors to TR, while at lower SOC it is the negative electrode reaction that dominates. Cells at 100% SOC exposed to high temperatures during oven tests show, along with the ARC analysis, that the presence of the cathode and electrolyte reactions leads to an increase in the severity of a TR event for oven temperatures above . By comparing the heat generated in ARC and oven testing, it is shown that ARC does not fully capture the self-heating and TR safety hazard of a cell, unlike oven testing. This work gives new insight into the nature of the decomposition reactions and also provides an essential data set useful for model validation which is of importance to those studying LFP cells computationally.