Minimizing Carbon Content in Thiophosphate-Based Composite Cathodes for All-Solid-State Batteries

Abstract

To achieve high energy densities with sufficient cycling performance in all-solid-state batteries (ASSBs), the fraction of active material has to be maximized while maintaining ionic and electronic conduction throughout the composite cathode. In a previous study, C65 added to the composite-cathode showed the potential to enhance the capacity and rate capability [1]. In contrast, it is known that carbon additives can lead to faster decomposition of the cathode constituents in thiophosphate-based cells [2]. By determining the electronic percolation threshold of the conducting matrix (CM), the C65 fraction shall be minimized. As a result, not only side reactions could be suppressed, but also the amount of AM and SE can be kept as high as possible [3,4]. Additionally, the role of the CM is discussed with respect to active material coatings, which are used to enhance electrochemically stability in thiophosphate-based ASSBs [5,6].In this study, we systematically investigated the microstructure and the conductive behaviour of a CM containing the sulfide solid electrolyte (SE) Li6PS5Cl and various fractions of C65. The effective conductivity of the different compositions was determined by temperature depended electrochemical impedance spectroscopy (EIS) and the effective conductivity was calculated as a function of temperature and C65 content. The electronic percolation threshold pc was determined to be about 4 wt.-% C65. It is suggested that below pc, the ionic contribution is dominant, which can be seen from the temperature dependency of the conductivity and blocked charge transfer. Above, the impedance of the CM becomes frequency-independent, and the ohmic law applies. In order to differentiate between electronic and ionic charge transport, the powder mixtures near pc were additionally investigated in electron-blocking condition. Furthermore, the adjusted CM was used in composite-cathodes, and rate capability behaviour, as well as the cycling performance of the ASSB cells, was evaluated. 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), Nikolaos Papadopoulos and Dr. Veit Steinbauer (Aalen University).