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

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 (SiO_x) contents under storage conditions of 100% state of charge (SOC) and 40 °C. Experimental results demonstrate that increasing SiO_x content significantly in SiO_x-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 SiO_x 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 SiO_x drives spontaneous lithium migration from graphite to SiO_x particles, causing excessive volume expansion and repeated SEI rupture. This process is further exacerbated by the SiO_x-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 SiO_x 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 SiO_x-based anodes and offers a robust strategy for extending the life-cycle of high-energy-density pouch cells.