Holistic design of flexible lithium-ion pouch cells – Project AgiloBat

Summary:

Lithium-ion batteries are well established as energy storage devices for a wide range of mobile applications from cell phones to electric vehicles. Depending on the purpose of use, very different requirements are expected of the battery system.
Present battery production is mainly focused on the fabrication of great numbers of uniform rectangular cells, which are assembled to battery systems in the form of large-scale cuboids. This procedure results in a poor space utilization and – for geometrically complex installation spaces – in low values of energy and power density. Furthermore, the whole design of mobile systems needs to be arranged around the battery. In the search of a solution addressing this matter, the project AgiloBat proposes an agile production of battery modules and cells with complete flexibility in format, materials, power and energy characteristics, etc. This enables battery systems to be manufactured specifically tailored to customer requirements.
In order to assess the advantages of a battery system consisting of different scale cells in comparison to standard cells, a reference system was defined. Herein eight submodules comprising five different cell formats were arranged to a system with almost twice the energy density of the standard cell system referred to the whole assembly space. Hereby should be noted that only rectangular cells have been used in the first step and that a further adjustment of the cell geometry would entail an even higher energy density for the flexible system.
According to a first simulation of a parallel connection of all eight 48 V-submodules (consisting of 12 equal cells each, wired in series), the flexible system cannot keep up with the standard system in terms of maximum power density yet. This behavior is due to the different scales and therefore unequal capacities of the cells and might be enhanced by the use of high power cells, which leads to the design of the different sorts of cells.
For the specification of these different cell types, a sensitivity analyses is conducted with an electrical cell model. Hereby the inner structure of high power, high energy and intermediate cells is defined and subsequently verified with the thermal model. This procedure is based on the knowledge that not only the electric specifications must be reached, but thermal, mechanical and safety issues must be considered as well. The parameter sets derived from this comprehensive approach still allow an almost completely free design of the outer dimensions of the battery cell.
By the interconnection between models for cell and battery system behavior, electrochemical, electrical as well as thermal, the complexity and challenges of the battery design are demonstrated and a way of narrowing down the multitude of interacting parameters is shown.