Linking Microstructure and Ionic/Electronic Conductivity in Composite Cathodes for Thiophosphate-Based All-Solid-State Lithium-Ion Batteries
Authors: Papadopoulos Nikolaos , Reisacher Elias , Kaya Pinar , Knoblauch Volker 
Published: February 2024

Compared to conventional lithium-ion batteries (LIBs) with liquid electrolytes, ASSBs (all-solid-state batteries) have the potential to improve safety and achieve higher performance and energy density1. However, there are still many challenges to tackle in order to achieve ASSBs with sufficient cell performance, and one of them is to optimize the microstructure regarding its ionic and electronic conductivity. To achieve a high energy density, the fraction of AM must be as high as possible, and pores must be avoided. At the same time, an ionic conductive network of the SE has to be established; thus, both phases have to be distributed homogeneously with the composite cathode. On the other hand, the degradation of the solid electrolyte (SE) with cathode active material (CAM) and conductive additive (CA) has to be tackled with protective coatings that hinder the degradation but at the same time, keep the charge transport pathways open2,3. Therefore, the composite cathode microstructure (i.e., composition ratios CAM:SE:CA, particle characteristics, distribution of the phases) must be designed for different materials systems to ensure the sufficient percolation of ions and electrons for high performance4-9.
In this study, we aim to understand the effect of the CAM fraction, morphology, and coating on the transport properties within the composite cathode. For this purpose, effective ionic and electronic conductivity of different cathode mixtures of Li6PS5Cl and NMC622 (coated, uncoated, single crystal, polycrystal) were analyzed and correlated with the microstructures. Moreover, a conductive matrix comprising Li6PS5Cl and C65 was introduced to the composite cathode system. Electronic and ionic conductivities were measured via impedance spectroscopy with ion- or electron-blocking electrodes in a symmetric cell configuration under constant pressure. To do so, a specially designed cell setup comprising a force sensor and a spring to adjust the pressure was used. The impedance spectra were fitted using T-type transmission line model (TLM) with equivalent circuit components with the RelaxIS 3 software package. Microstructural characterization was conducted on the powder mixtures and Ar- polished cross sections via scanning electron microscope using a transfer shuttle. We found the ionic and electronic transport limitations for the prepared composite cathode mixtures by varying the abovementioned parameters and optimized the CAM fraction accordingly. The results presented contribute to an improved understanding of the structure-property relationships and provide guidance on how to design a suitable microstructure.for improved cell performance.

Acknowledgement

The authors gratefully acknowledge the support of the German Federal Ministry of Education and Research within the program “FH‐Impuls” (Project SmartPro, Subproject SMART‐BAT, Grant no. 13FH4I07IA), Dr. Veit Steinbauer (Aalen University), Sebastian Puls and Dr. Nella Vargas-Barbosa (Helmholtz Institute Münster (IEK-12) - Forschungszentrum Jülich).

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