Analysis on the effect of external press force on the performance of LiNi0.8Co0.1Mn0.1O2/Graphite large pouch cells

Abstract

In application, lithium-ion pouch-format cells undergo expansion during cycling. To ensure the structure integrity of battery pack, preload force and fixed space were performed during pack assembly. In this paper, the swelling force of LiNi0.8Co0.1Mn0.1O2(NCM811)/Graphite large pouch cells is investigated as a function of the different preload force, state of charge (SOC), cycle number and graphite electrode kinds. The swelling force shows a significant dependency on the state of charge, but not follow linear characteristics due to NCM811’s volume extraction at high SOC. The higher SOC, the higher swelling force. 1500–1700 N is increasing from 50% to 100% SOC under 3000 N preload force. Three cells with different electrode design were cycled 1000 times, and each design with different preload force (1000, 3000 and 5000 N). The cell with 3000 N preload force shows better capacity retention for all electrode cells, the capacity retention of three different electrode cell is 93.7%, 90.9% and 91.2%, respectively. The corresponding swelling force increase fast at initial 300 cycles due to multi-factors. After 300 cycles, the swelling force increasing shows linear characteristics. After 1000 cycles, the cell with natural graphite (NG) has much larger swelling force than the cell with artificial graphite (AG), 9030 vs. 6370 N under 3000 N preload force. The functional binder polyacrylic acid (PAA) with -COOH unit and mixed with carboxymethylcellulose sodium (CMC) can form cross-link net at the graphite surface, supplying higher adhesive strength and restraining the electrode expansion. The adhesive strength increased from 7.2 to 20 N/m, corresponding the anode thickness expansion ratio from 25% reduced to 15%. The swelling force reduced from 9030 N to 6930 N, near to the cell with AG+SBR design. Delta voltage (∆V) and average discharge voltage evolution during cycling were also investigated of the different electrode cells. From the study, it can be useful for mechanical design, battery management of module and pack which can be beneficial for long life, safety and reliability of electric vehicle (EV) products.