Influence of Carbon Additives on the Decomposition Pathways in Cathodes of Lithium Thiophosphate-Based All-Solid-State Batteries

On the way to a large-scale industrial application of all-solid-state batteries (ASSBs) it is necessary to overcome a number of challenges. An important task is to maximize the utilization of active material in the cathode composite to achieve high capacities. Carbon-based conductive additives are common in cathode composites for conventional lithium-ion batteries based on liquid electrolytes. In all-solid-state batteries, the beneficial effect of carbon additives is often not maintained over a sufficient number of charge/discharge cycles. Thus, ASSB cells often suffer from an increased long-term capacity loss with an enhanced formation of decomposition products. So far, these effects have not been analyzed in depth and are not fully understood because of the complexity of the composite cathode structure. Together with overlap of the occurring degradation paths, this makes a separation of the individual decomposition processes challenging. In this work, we investigate the influence of vapor-grown carbon fibers as carbon-based conductive additives on the degradation of a LiNi0.6Co0.2Mn0.2O2/β-Li3PS4 composite cathode. We use a combination of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS) and combine surface and bulk analyses to separate the overlapping decomposition processes from each other. The results show an initially higher capacity by using vapor-grown carbon fibers due to higher utilization of the active material and an additional capacity contribution caused by redox-active decomposition reactions. The observed capacity fading is associated with the formation of sulfates/sulfites, phosphates, and polysulfides, which are detected directly in LiNi0.6Co0.2Mn0.2O2/β-Li3PS4 composite cathodes with ToF-SIMS for the first time. Overall, the results extend the knowledge and understanding of degradation phenomena in thiophosphate-based composite cathodes considerably, which is an essential step to develop protection concepts more efficiently on the way to long-term stable ASSBs.