• Energy Research

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.

A particular safety issue with Lithium-ion (Li-ion) cells is thermal runaway (TR), which is the exothermic decomposition of cell components creating an uncontrollable temperature rise leading to fires and explosions. The modelling of TR is difficult due to the broad range of cell properties and potential conditions. Understanding the effect that thermophysical and heat transfer characteristics have on the TR abuse model output is essential to develop more accurate and robust TR models. This study uses global sensitivity analysis (GSA) to investigate the effect of the cell parameters on the outcome of TR events. Using a Gaussian Process (GP) surrogate model to calculate the Sobol’ indices, it is shown that the emissivity value is the dominant thermo-characteristic throughout the overall abuse scenario. Further analysis, investigating three key TR features shows the conductivity coefficient to be the most important with respect to the maximum temperature reached during TR. Results demonstrate that researchers can confidently estimate some thermo-characteristics but require accurate characterisation of the emissivity and conductivity coefficient to ensure robust predictions. Given the importance of battery technology to aid in global de-carbonisation, these findings are\ud key to increasing their safe design and operation.

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.

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.

Abstract In this paper a novel method to determine the specific heat capacity of lithium-ion cells is proposed. The specific heat capacity is an important parameter for the thermal modelling of lithium-ion batteries and is not generally stated on cell datasheets or available from cell manufacturers. To determine the specific heat capacity can require the use of an expensive (>£100 k) calorimeter or the deconstruction of the cell whereas the method proposed by the authors in this paper uses common equipment found in most battery laboratories. The method is shown to work for cylindrical, prismatic and pouch cells, with capacities between 2.5 Ah and 10 Ah. The results are validated by determining the specific heat capacity of the cells with use of a calorimeter and a maximum error of 3.9% found. Thermal modelling of batteries is important to ensure cell temperatures are kept within specified limits. This is especially true at rates over 1C, such as the fast charging of electric vehicles, where more heat is generated than lower rate applications. The paper ends by demonstrating how the thermal model that underpins the authors' methodology can be used to model the surface temperature of the cells at C-rates greater than 1C.

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.