Understanding Li creep in Li-metal pouch cells and the role of separator integrity

To realize the advantages of high energy density lithium-metal batteries, their cycle life and safety need to be improved to meet practical and commercial demands. External compression has been shown to improve the performance of lithium-metal batteries through suppressing dendrite growth, densifying the lithium deposit, and improving the lithium deposition morphology. Here, we report the behavior of high-capacity Li||LFP pouch cells in pyrrolidinium-based ionic liquid electrolytes at elevated temperature (50 °C) under various levels of compression and discuss the compression-related mechanisms that affect the performance and failure of the cells. Using scanning electron microscopy, we observed more uniform and less dendritic Li deposition at high levels of compression. However, cell pressure evolution data shows significant lithium metal creep above 800 kPa, effectively limiting the maximum applicable compression. Reducing the testing temperature to 25 °C and maintaining high compression led to suppressed Li creep and extended the cell cycle life by 40%. The adverse effect of lithium creep on the separator is identified as an important mechanism on the pouch cell cycling behavior and is further discussed herein.