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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SAFE LIB (Electro-thermal modelling of lithium-ion battery packs from the safety perspective)

Teaser

Summary

The research was aimed at delivering a predictive tool, which will be generic across all Lithium-ion batteries (LIB) types for LIB thermal management from the safety perspective. Such a tool can aid the development of safer LIB cells and the optimisation of LIB packs balancing performance and safety requirement.

Lithium-ion batteries (LIB) are found in many applications such as consumer electronics, electric vehicles (EVs) and airplanes. However, they can be dangerous under some conditions and can pose a safety hazard since they contain a flammable electrolyte and are also kept pressurized. Despite the high safety standards being imposed, there have been many reported accidents and manufacturer recalls. Most accidents can be sourced to runaway reactions, which could happen if the LIBs are overheated or overcharged. This is often accompanied by cell rupture and in extreme cases can lead to ignition, fire and explosions. As an example, the LIB on a Japanese Airlineâ€™s Boeing 787 caught fire in January 2013, resulting in FAA grounding all delivered 787 until the overhaul of the problem.

The specific objectives of the research include:
- Develop and validate a thermal model to predict the onset of runaway reactions (WP1);
- Extend the above model to predict potential ignition (WP2);
- Extend the model to predict possible escalation from cell ignition to pack fire and explosion (WP3);
- Validate the model with full scale test data giving particular emphasis to cell rupture and the propensity from ignition of a single cell to battery packs (WP4); and
- Conduct cases studies to formulate recommendations on LIB safety.

Work performed

A 3D electro-thermal model has been developed within the frame of open source CFD code OpenFOAM by coupling electric conduction with heat transfer and energy balance for a single LIB cell. The model solves a highly coupled system of equations that describe the relations between the electrical and thermal characteristics of the cell, as well as their interactions with the surroundings. The model has been validated with published and newly generated experimental data for the predictions of voltage, current and temperature variation within the cell under various conditions. Reasonably good agreement has been achieved between the predictions and measurements for all these parameters. In addition, the model has reproduced well the evolution process of a cell from normal to abnormal cycling until thermal runaway.

The studies have revealed new insight of thermal runaway (TR) and TR propagation can aid the design of the pre-warning system in applications such as electric vehicles or energy storage systems using LIBs. For example, the model can be used to determine critical temperatures under different air cooling conditions and current rates; and aid the design of BMS to prevent TR propagation.

The proposed electro-thermal model were used to conduct parametric studies of a particular type of LIB (LNCMO/LTO) by evaluating the effects of discharging current rates, airflow quantities, ambient temperatures and thickness of airflow channel on the response of the cell. Faster function losses, earlier TR and higher extreme temperatures were found when cells were discharged under higher current rates. The airflow with specific velocity was found to provide effective mitigation against over-heating when the ambient temperature was below 97oC but less effective when the ambient temperature was higher than the critical value of 152oC. The thickness of airflow channel was also found to have critical influence on the cell tolerance to elevated temperatures. Airflow at high velocities can shorten the time for the cell to reach steady-state and prevent over-heating and hence TR when the surrounding temperature is lower than critical temperature (122 oC). Above this temperature, air cooling was found to be insufficient to prevent TR and more efficient cooling technique should be introduced.

Final results

Although battery thermal behaviour has been studied by published models, the reported modelling normally addresses either normal operation or thermal runaway condition. A comprehensive electro-thermal model which can capture heat generation, voltage and current variation during the whole process from normal cycling to thermal runaway should be of benefit for BMS by evaluating critical factors influencing potential transition to thermal runaway and investigating the evolution process under different cooling and environment conditions. In this study, such a three-dimensional model has been developed within the frame of open source computational fluid dynamics (CFD) code OpenFOAM to study the electrical and thermal behaviour of lithium-ion batteries (LIBs). This represents major advance beyond the state of the art, and can potentially find wide applications for the development of LIBs, LIB management systems and design optimisation and development of mitigation measures for the propagation of TR.

LibFOAM will be actively promoted to potential end users in the LIB industry and regulatory bodies through the planned activities described above to facilitate commercial exploitation. A business consultant from Warwick Ventures within the host will be engaged to seek opportunities, additional approaches and routes to commercially exploit LibFOAM. In particular, we will explore the potential to combine LibFOAM with other in-house CFD models to start a spin out company at University of Warwick Science Park.

The important selling point is that predictions of potential ignition hazards can guide designers to optimise the geometrical layout of the battery pack and improve ventilation arrangement. It can also be linked with some warning system so that the conditions to activate additional cooling can be specified to avoid potential ignition or quench it at early stage to avoid fire and explosions.

Immediate opportunities for knowledge transfer and exploitation include the existing industrial sponsors and the extensive network of the hostâ€™s collaborators from the LIB and automotive industry including Jaguar Land Rover and Tata motor. There will also be opportunities to transfer knowledge through our long term sponsor FM Global which has offices in many different countries in Europe to improve the safety of LIB by their clients such as LIB storage in warehouses.