In metropolitan areas, traffic congestion and pollution are serious challenges due to the continuous rapid growth in the number of vehicles and population. These concerns have prompted a global trend toward the electrification of transportation powertrains. Electric vertical take-off and landing (eVTOL) aircraft is an efficient and safe alternative mode of transportation with a significant impact on urban mobility, which attracts considerable attention [1]. It could avoid traffic on the ground and allow people to commute between two locations in less time while requiring no additional run-up space. Nevertheless, one of the main challenges with such aircraft is to provide electrical storage systems, such as batteries, which are safe, efficient, reliable, and capable of producing the energy and power demand to meet mission requirements [2].

High flammability and risk of thermal runaway in current Li-ion batteries motivate to develop next-generation chemistries such as all-solid-state batteries (ASSB) as alternative electrical storage systems for eVTOL. However, ASSB currently suffer from low solid electrolyte conductivity, as well as non-utilised active material, and significant reaction overpotentials due to a small effective active area [3]. Electrode composition and structure have a crucial impact on three performance-relevant microstructure parameters, i.e. effective active area, effective ionic conductivity, and effective electronic conductivity. However, considering these restrictions experimentally by producing and evaluating a wide range of design parameters to achieve a well-performing design for ASSB is both time- and resource-intensive. To address these issues, we conduct model-based structure evaluation. Here, we extend the P2D model with a microstructure surrogate model based on Laue's work [4] for Li-ASSB (see Fig. 1-a), because electrode microstructure is important for the effective parameters. Further, this link between the P2D model and microstructure modelling could provide a better understanding of the electrode structure and especially the effects of percolation, which cannot be properly covered by classical Bruggeman approaches. This model is then incorporated into a global optimisation algorithm to determine the optimal design of solid-state cathode with respect to eVTOL power demand. Finally, we compare the performance of the battery at the identified optimal electrode design, electrode reference design, and electrode design with that of the Bruggeman approach. Based on our prior research, we identified that hybrid electrolytes with 12.7 vol% LLZTO had the best performance among other types of hybrid electrolytes at a relatively high C-rate, i.e. 1C. Therefore, we employed this electrolyte for the modelling of Li-ASSB for eVTOL application [5].

Fig. 1-b shows an exemplary result from a simpler model without percolation, i.e. Bruggemann approach, and a microstructure surrogate model. As can be observed, taking into account a more realistic structure in the electrode, the ASSB could only meet 32% of the mission requirements. However, optimisation of solid-state cathode structure significantly increases flight time in comparison to the reference electrode.

In summary, we could show that future electrification of transportation powertrains will necessitate optimisation of the solid-state electrode structure and composition in order to meet the high demands for energy and power density. Also, our work highlights the urgent need to consider microstructure physical effects at an early stage of design for a realistic sizing of batteries.

References

1. Jain et.al., Proc. 4th Int. Conf. Electron. Commun. Aerosp. Technol. (2020) 1173–1178
2. Donateo et.al., Aerospace 7, no. 5 (2020): 56
3. Bielefeld et.al., The Journal of Physical Chemistry C 123, no. 3 (2018): 1626-1634
4. Laue et.al., Energy Technology, 8(2) (2020): 1801049
5. Toghyani, et.al., J. Electrochem. Soc (2022).