Calendar aging of silicon pouch cells: In-situ ultrasound analysis and polymer electrolyte interfacial engineering

Abstract Silicon-based anodes, as key high-energy-density anode materials for lithium-ion batteries, face limitations in practical application due to long-term calendar aging. This study systematically investigates the calendar aging behavior of commercial pouch cells with varying silicon oxide (SiOx) contents under storage conditions of 100% state of charge (SOC) and 40 °C. Experimental results demonstrate that increasing SiOx content significantly in SiOx-graphite (SG) accelerates capacity degradation. The pouch cells with an anode capacity of 1000 mAh·g−1 (SG-1000) retained only 10% of their original capacity after 4 weeks of storage, while pure graphite (Gr) counterparts maintained 75%. By employing in-situ ultrasonic scanning technology, we achieved high-resolution, non-destructive visualization of internal gas evolution, confirming that SiOx particles intensify interfacial side reactions. Multiscale characterization reveals a unique “lithium migration-solid electrolyte interphase (SEI) destruction-side reaction” vicious cycle mechanism in SG anodes. The electrochemical potential difference between lithiated graphite and SiOx drives spontaneous lithium migration from graphite to SiOx particles, causing excessive volume expansion and repeated SEI rupture. This process is further exacerbated by the SiOx-promoted LiPF6 hydrolysis cycle, which generates HF and yields a porous, unstable interface. A polyurethane (PCL)-based polymer electrolyte (PCL-2-isocyanatoethyl methacrylate (IEM)) was developed via in-situ polymerization. This highly elastic polymer network effectively suppresses SiOx volume expansion and interrupts the Li+ migration pathways. Consequently, the capacity retention of SG-650 (1000 mAh·g−1) cells improved from 64% to 77%, with gassing effectively suppressed. This work provides critical insights into the calendar aging of SiOx-based anodes and offers a robust strategy for extending the life-cycle of high-energy-density pouch cells.