Microstructure and Charge Transport in Composite Cathodes for Solid State Batteries

Abstract

Rechargeable batteries play a central role in the transformation of the mobility sector from combustion engines to sustainable technologies such as electric motors. In the pursuit of batteries with higher energy density, improved safety and extended service life, solid-state batteries are regarded as promising next generation battery technology. Higher energy density can increase vehicle range potentially leading to greater acceptance of electric vehicles. In solid-state batteries, solid ion-conducting materials replace liquid organic electrolytes providing advantageous properties such as mechanical resistance and a lithium-ion transfer number close to one. These can enable the use of lithium metal anodes or higher charging rates. Among solid-state electrolytes the lithium argyrodite Li6PS5Cl, which is used in this study, is one of the most intensively investigated materials, due to its high conductivity. Despite intensive research activities, solid-state batteries using this material class are not yet ready for commercialization with persisting challenges in electrode architecture and production. In conventional Li-ion batteries, porous cathodes can be easily infiltrated by liquid electrolytes; however, the particulate nature of solid electrolytes requires a homogeneous distribution in the cathode during the manufacturing process. This requirement becomes increasingly challenging as the proportion of active materials increases and has a significant influence on the charge transport within the cathode and, correspondingly, on the kinetics. Thus, the experimental investigation of the charge transport of cathodes was the primary focus of this dissertation. Composite cathodes composed of sulfide based solid electrolytes and Ni-rich active materials were fabricated and characterized using electrochemical measurements. By comparing charge transport data with results obtained by microscopic imaging techniques, microstructure parameters were elucidated and quantified. Findings revealed that, in addition to limitations in percolation networks, the particle size distribution of the solid electrolyte significantly impacts electrode porosity and homogeneity. Overall, this work extended the insight into key electrode microstructure parameters and their effects on cell performance. From these findings, guidelines for electrode optimization can be derived aiming to enhance both energy density and power density.