Multifunctionality and Processability of a Thermoplastic Based Gel Electrolyte Cell for the Realization of Structural Batteries.

In this work, a battery layup consisting of a poorly flammable ionic liquid electrolyte and a poly(vinylidene fluoride--hexafluoropropylene) (PVdF-HFP) thermoplastic has been developed along with composite anode and cathode electrodes. The developed gel electrolyte exhibits feasible ionic conductivity of about 1 mS/cm at 30 °C. State-of-the-art active electrode materials, i.e., LiNiMnCoO (NMC811) and graphite, have been employed. Full cells were tested in coin and pouch cell format, obtaining capacities of about 120 and 100 mA h/g, respectively, at a C-rate of C/10. Thereby, it was observed that good contact between the individual cell layers is crucial. Recently, it was shown that the mechanical properties of structural batteries, realized by integrating battery cells into carbon fiber-reinforced polymer (CFRP) laminates, depend significantly on the mechanical properties of the cell itself. Hence, to promote the realization of such a structural battery concept, tensile tests were carried out to investigate the mechanical properties of cells as well as the individual components developed in this work. The full cell showed values of 10 GPa and 49 MPa for the Young's modulus and tensile strength, respectively. Thus, feasible multifunctionality could be verified on the cell level. However, regarding the contributions of the different components, it could be shown that mainly the current collector foils contribute to the mechanical properties, in contrast to the electrode loadings and the gel electrolyte. Additionally, the thermal and chemical stability of the developed system was evaluated, highlighting the importance of these secondary properties for the fabrication of structural batteries, i.e., the integration of cells into load-bearing CFRP laminates. Specifically, it was observed that the developed system is thermally stable up to 150 °C and no HF release was detected upon exposure to ambient conditions.