Partial Transport Optimization and Mechanochemical Characterization of The Cathode Active Material Li₂FeS₂ in all Solid-State Batteries

Solid-state batteries spark tremendous interest given their potential to overcome the limitations of energy density that are present in conventional Li ion batteries. At the same time, with current state-of-the-art cathode active materials (CAM) employing critical elements such as Nickel and Cobalt, concerns regarding their scarcity and cost are raised even before the wide-spread implementation of the technology. To overcome these concerns, in this work, we evaluated the cation- and anion-redox (cathode) active material Li₂FeS₂ as an alternative candidate for solid-state batteries with a promising theoretical specific capacity of 400 mAh/g. By combining electrochemical characterization with studies of the partial conductivities, cathode composites are optimized in regard to their CAM and solid electrolyte composition, leading to an optimum between high initial capacities and capacity retentions. Half cells employing 50 to 60 wt.% CAM are shown as most promising candidates, without the necessity of carbon additives due to the intrinsically high electronic conductivity of Li₂FeS₂ &gt;30 mS/cm. The kinetic limitations of the cells are further elaborated by current density dependent cycling (rate testing), showing that higher electronic conductivities, i.e., higher intermediate CAM loadings, benefit faster charging rates in comparison. This is a promising result given that higher loadings are thought after to reduce the amount of electrochemically inactive materials in solid-state batteries.<br/>The study of kinetic limitations is complemented by mechanochemical characterization of the cells. Tracing the pressure changes during cycling, both with lithium-indium and LTO as reference electrodes, is used to gather insights into the origin of capacity fading. While usage of lithium-indium allows to capture the mechanochemical response of the system including effects of lithium intercalation and deintercalation in the reference anode, the use of LTO, i.e., a zero-strain material during cycling, allows characterizing the response of the cathode specifically. Thereby, the latter approach could be utilized to characterize not only the general mechanochemical response of Li₂FeS₂ cathodes, but also to capture how these change as a function of cathode composition, ultimately providing insights into the failure mechanism of Li₂FeS₂ containing cathodes.<br/>In conclusion, this work represents an initial introduction of Li₂FeS₂ as a cathode active material candidate for all solid-state batteries. Furthermore, partial transport and mechanochemical characterization are utilized to optimize the cathode composition to achieve both, high average specific capacities, i.e., active material utilization, and high capacity retention. With that, this work is a foundation for further optimization of cells employing Li₂FeS₂ as CAM, and allows a general comparison to established solid state battery CAM, e.g., Li(Ni,Mn,Co)O₂ (NCM).