Optimizing mechanical compression for cycle life and irreversible swelling of high energy and high power lithium-ion pouch cells

Abstract

Efficient cell packaging is crucial to increase the battery energy density and the driving range of modern electric vehicles. However, mechanical compression of the cells during pack assembly has a significant impact on cycle life and cell swelling of the cells. Therefore, this study follows the research question how lithium-ion (li-ion) pouch cells should be integrated, and at which pressure level, in order to optimize cycle life and to reduce irreversible swelling.A high precision compression test bench was utilized to cycle 5Ah NMC622/graphite li-ion pouch cells at 0.075MPa, 0.2MPa, 0.5MPa, 1.0MPa, and 1.75MPa flexible compression at constant pressure and rigid compression at constant cell thickness, respectively. To obtain transferable results, a high energy (HE) and a high power (HP) cell configuration were investigated. The cells were stressed with individual multi-step constant current (MSCC) charging profiles for each cell type, representing electric vehicle (EV) usage profiles with frequent fast charging. After cycling, post-mortem analysis was conducted at 80% capacity retention to evaluate individual electrode aging.In comparison between flexible compression and rigid compression, flexible compression achieved on average 19% increase in cycle life until 80% capacity retention. It was observed that compression could reduce deterioration of electrical contact in the cathode. Simultaneously, higher compression increased the overpotential on the anode during charging and thereby increased the risk of lithium plating. Ranging from 0.2MPa to 0.5MPa flexible compression, cycle life of the HE cells increased by up to 25% in comparison to the lowest compression. In addition, cells reached around double equivalent full cycles (EFC) before reaching 150% DCIR. The irreversible swelling, normalized to the uncompressed cell thickness at begin of life (BoL), was reduced from around 3.2% at the lowest compression to 2.1% at moderate compression. As a result, flexible compression between 0.2MPa to 0.5MPa is recommended for both cell types as a trade-off optimizing capacity fade, direct current internal resistance (DCIR) increase, and irreversible swelling during cycle life.