Degradation Analysis of Composite Electrodes for Sulfide-Based All-Solid-State Batteries during Long-Term Storage

In the realm of energy storage technologies, lithium-ion batteries (LIBs) have marked a significant milestone, especially in powering portable electronic devices. Nonetheless, the quest for batteries with longer life spans and higher energy densities is ever-present, driven by the demand for more durable energy solutions (i.e., electric vehicles and energy storage systems). This pursuit, however, is challenged by the inherent limitations of LIBs including their capacity limit and the safety hazards posed by flammable liquid electrolytes. These issues have encouraged the exploration of next-generation energy storage solutions, and all-solid-state batteries (ASSBs) employing solid-state electrolytes (SEs) is regarded as one of the most prominent solutions, due to their potential for high safety and energy density. Among the various SEs, sulfide-based ASSBs have emerged as leading contenders, owing to their high ionic conductivity and excellent processability. While significant attention has been dedicated to exploring electrochemical degradation at electrode interfaces by numerous studies, little has been investigated regarding durability of sulfide-based ASSBs after long-term storage under various conditions. Given that batteries inevitably undergo a storage period prior to dispatch after manufacturing, investigating the degradation mechanism during this phase and determining the ideal storage conditions are essential for ensuring the performance of ASSBs. Herein, we investigate the degradation of Li6PS5Cl (LPSCl)-based composite cathodes employing conventional layered oxide cathodes during extended calendar aging under various states of charge (SOCs) and temperatures. Cells consisting of a composite cathode, LPSCl separator layer, and a Li-In alloy anode are fabricated. Following an initial pre-conditioning cycle of 0.1C, each cell is subjected to a two-week storage, showing a significant decline in discharge capacity compared to the fresh one. Notably, the cell stored at high-SOC and at an elevated temperature of 60°C exhibits extremely severe degradation after storage, underscoring the critical impact of aging conditions on battery longevity. In-depth chemical and structural analyses, coupled with impedance decoupling, reveal that the storage at high-SOC facilitates detrimental interfacial side reactions between Ni-rich cathode material and LPSCl, which are accelerated at high temperatures. In addition to the oxidative decomposition of LPSCl, the irreversible phase transition of cathode active materials is also caused by calendar aging at high-SOC, as verified by the X-ray diffraction analysis and differential capacity curves. The resulting heterogeneity of the composite cathode also undermines the long-term cyclability of cells. These findings offer a deeper understanding of the performance decay of Ni-rich cathodes in sulfide-based ASSBs during storage and provides new guidelines for the development of cathode-compatible sulfide SEs.