Lithium-ion batteries are well established as energy storage devices for a wide range of mobile applications from cell phones through power tools to electric vehicles. The different types of usage lead to distinct requirements concerning the battery system, resulting in customized dimensioning of the single components. For the sustainable, technically and economically efficient battery production and utilization, a holistic design and optimization of the cells and the whole battery pack is indispensable. Optimal performance and lifetime of the battery cells do not only require careful consideration of the electrical and mechanical properties, but in particular also the thermal properties, with their significant impact on the temperature and its distribution that do in turn strongly interact with the aforementioned properties.
For the development of thermally optimized cells, knowledge of the potential impact of varying thermal cell properties is crucial. Thus, the properties of four commercial and two research cells were reviewed based on in-house measurements and gave an insight into the ranges of the most important influencing factors on the thermal cell behavior. To detect the properties with the greatest impact on the effective values of density, specific heat capacity and thermal conductivity of the homogenized cell stack, a sensitivity analysis was conducted. Hereby a variation of layer thicknesses and material data of the individual components was carried out with a theoretical model to define the possible range of the effective thermal properties of the whole cell stack with or without the outer separator layer and the pouch foil. With the derived information, the sensitivity of the steady-state temperature gradient within a cell with exemplary geometry on the effective perpendicular thermal conductivity was investigated. From an extensive combination of the reviewed property values, a theoretical best and worst case cell composition were refined, whose thermal behavior could then be compared to the behavior of already existing cells. For an adequate comparison the use of the same thermal boundary conditions and outer dimensions and a variation of solely the inner structure and material properties is inevitable. With this approach, the potential for optimization of the existing cells could be identified.
Furthermore, the influence of the thermal properties on the transient thermal behavior of the battery cell was investigated by evaluating the caloric mean temperature over time. In that regard, different cases – from an existing commercial cell over a worst-case scenario towards an optimized inner cell structure – were compared for the exact same thermal boundary conditions.
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