A combined experimental and modelling study for understanding lithium ion behaviour under complex thermal boundary conditions and cell design optimisation

Abstract

Lithium-ion batteries are currently the technology of choice for energy storage in electric vehicles and stationary storage applications. The lifetime and safety of the battery pack are adversely affected as its operating time deviates from ambient condition. Thermal management plays a critical role in minimising these effects. Cooling the electrical tabs of the cell instead of the lithium-ion cell surfaces has been shown to provide better thermal uniformity within the cell and can potentially enable higher usable capacity over its lifetime. However, tab cooling is currently limited by the inability to remove much heat efficiently. This thesis explores the effect of thermal management on lithium-ion performance and the potential design changes that can be made to improve the heat transfer rate while maintaining high thermal uniformity. The development of a two-dimensional electro-thermal modelling framework is presented. The model was used to simulate cell performance and internal states under thermal management boundary conditions. It provides a spatial description of internal states (temperature, current and state of charge) that is difficult measure through experiment. A complete procedure from the parameterisation to validation was provided. Particular focus was paid to thermal modelling of the non-core components and thermal boundaries. The performance of tab cooling and surface cooling system were assessed over a wide range of conditions. It was found that tab cooling provides a much smaller thermal gradient, but it is limited by its heat removal capability. It was hypothesised that increasing the tab cross-sectional area (width x thickness plane) can potentially to improve the limitation. Subsequently, the custom-made cells with varying tab width and position were used to test the hypothesis. The experiment shows that the wider tabs lead to a 14% improvement in the rate of heat transfer. Modelling of the custom-made cells show that a significant heat transfer bottleneck exists between the tabs and the electrode-stack. It was shown that the bottleneck can be opened with a single modification, increasing cross-sectional area of the tabs. A virtual large-capacity automotive cell was modelled to demonstrate that optimised tab cooling can be as effective in removing heat as surface cooling while maintaining the benefit of better thermal, current and state-of-charge homogeneity. These findings could potentially enable the benefit of tab cooling system, higher usable capacity, power and longer lifetime.