Lithium-ion batteries (LIBs) have been the primary choice for rechargeable energy storage in mobile applications since their inception in 1990. Despite continuous improvements, LIBs are approaching their theoretical limits, particularly in the context of the increasing energy demands of electric and hybrid electric vehicles. Lithium sulfur batteries (LSB) with lithium metal anodes address these challenges by offering significantly higher energy density with their specific energy exceeding 800 Wh/kg compared to the 180-280 Wh/kg specific energy of common graphite-transition metal oxide LIBs. Due to the use of sulfur, an abundant and cheap resource, as a key component LSBs also promise to be a moire ecological and economical alternative to transition metal oxides.
However, there are several challenges to overcome for the commercial development of LSBs. One major challenge is the formation of high surface area lithium (HSAL) and the polysulfide (PS) shuttle effect in LSBs with a liquid electrolyte (LE) leading to high self-discharge, poor cycle life and safety concerns. By replacing the LE with a solid-state electrolyte (SSE) these challenges are potentially mitigated due top the suppression of the PS shuttle and HSAL formation while also introducing a new set of challenges. State-of-the-art ceramic SSEs exhibit high ionic conductivities but their implementation into LSBs is challenging due to their brittle nature and poor interface compatibility making it necessary to cycle them under very high pressure.
This research project involves the -depth analysis of cycled SSE LSBs with two different metal polysulfide cathode active materials (CAM) and a state-of-the-art ceramic SSE. After electrochemical aging, postmortem analysis was carried out by combining focused ion beam (FIB) milled crater with time-of-flight-secondary ion-mass spectrometry (ToF-SIMS) imaging to provide new insights into the different aging effects of SSE LSBs. ToF-SIMS is a surface sensitive analysis technique that enables high mass as well as high lateral resolution imaging by combining the fast-imaging mode with delayed ion extraction. Thus, by the ToF-SIMS imaging with FIB cross sections the acquired atomic and molecular distribution give insights into different aging effects of SSE LSBs. This work was able to show among others the formation of an interphase as well as degradation and migration of the CAM. The ultimate goal of this research is to reveal aging mechanisms of SSE LSBs, paving the way for long-lasting, high-capacity, and safe SSE LSBs as a more environmentally friendly alternative to conventional commercial LIBs with graphite anodes and transition metal oxide cathodes.