Linking Microstructure and Ionic/Electronic Conductivity of Sulfide-Based Composite Cathodes with the Cell Performance

Compared to conventional lithium-ion batteries (LIBs) bearing liquid electrolytes, ASSBs (all-solid-state batteries) have the potential to improve safety and achieve higher performance and energy density [1]. However, there are still many challenges to tackle in order to achieve Li-ASSBs with sufficient cell performance. One of those challenges is overcoming the complexity within the composite cathode. Liquid electrolytes can penetrate composite cathodes easily, while solid electrolytes cannot, leading to residual porosity in the cathode, which then blocks the ionic and electronic charge transport pathways. On the other hand, the degradation of the solid electrolyte (SE), cathode active material (CAM), and conductive additive (CA) caused by parasitic side reactions have to be tackled with protective coatings that hinder the degradation but at the same time keep the charge transport pathways open [2,3]. Therefore, the composite cathode microstructure (e.g., optimal composition ratios_CAM:SE:CA, particle characteristics, distribution of the phases) must be designed for different materials systems to ensure a perfect percolation and provide sufficient ions and electrons, thus tortuosity [4-7].<br/>In this study, we aim to deepen the understanding of the correlation between microstructure and ionic/electronic conductivity of composite cathode with the cell performance. For this purpose, a conductive matrix (CM) comprising Li₆PS₅Cl and C65 was developed and used within the composite cathode. Effective ionic and electronic conductivities of different cathode mixtures of the conductive matrix and NMC622, as well as microstructural evolution, were analyzed. Subsequently, these findings were correlated with the rate capability and cycling performance of the cells. We determined the electronic percolation threshold of the conducting matrix (CM) to keep the amount of C65 as low as possible and detected the ionic/electronic limitations for the composite cathode mixtures by varying the active material content. The results of this study will shed light on how to optimize the microstructure to achieve the best cell performance<br/><br/><b>Acknowledgment</b><br/>The authors gratefully acknowledge the support of the German Federal Ministry of Education and Research within the program "FH Impuls" (Project SmartPro, Subproject SMARTBAT, Grant no. 13FH4I07IA) and Dr. Veit Steinbauer (Aalen University).<br/><br/><b>References</b><br/>[1] Janek, Jurgen, and Wolfgang G. Zeier. "A solid future for battery development." Nature Energy 1.9 (2016): 1-4.<br/>[2] Ma, Yuan, et al. "Advanced Nanoparticle Coatings for Stabilizing Layered Ni Rich Oxide Cathodes in Solid State Batteries." Advanced Functional Materials 32.23 (2022): 2111829.<br/>[3] Sun, Shuo, et al. "Multiscale understanding of high energy cathodes in solid-state batteries: from atomic scale to macroscopic scale." Materials Futures 1.1 (2022): 012101.<br/>[4] Ohno, Saneyuki, and Wolfgang G. Zeier. "Toward practical solid-state lithium–sulfur batteries: challenges and perspectives." Accounts of Materials Research 2.10 (2021): 869-880.<br/>[5] Bielefeld, Anja, Dominik A. Weber, and Jurgen Janek. "Modeling effective ionic conductivity and binder influence in composite cathodes for all-solid-state batteries." ACS applied materials & interfaces 12.11 (2020): 12821-12833.<br/>[6] Dixit, Marm, et al. "Implications of Local Cathode Structure in Solid-State Batteries." Solid State Batteries Volume 2: Materials and Advanced Devices. American Chemical Society, 2022. 113-132.<br/>[7] Hendriks, Theodoor A., et al. "Balancing Partial Ionic and Electronic Transport for Optimized Cathode Utilization of High Voltage LiMn₂O₄/Li₃InCl₆Solid State Batteries." Batteries & Supercaps (2023): e202200544