Safety assessment of electrically cycled cells at high temperatures under mechanical crush loads

An ageing test procedure was developed in this work in order to assess the influence of battery degradation effects, occurring during electrical cycling of lithium-ion batteries at elevated temperatures, on the electrical and mechanical properties of the cells and on their safety behaviour under mechanical quasi-static crush loading. Commercial 41 A h NMC-LMO/graphite pouch cells were charged and discharged at 60 °C for 700 cycles at 1 C in an SOC range between 10 and 90%. Electrical properties of these batteries were evaluated every 100 cycles at room temperature. By the end of the cycling procedure, a 27% reduction of the initial capacity was observed. The behaviour of the fully charged aged cells under a quasi-static mechanical load was examined in a series of indentation tests, using a flat impactor geometry. This behaviour was compared to that of fresh cells. Test results show that all investigated electrically cycled cells exhibited a slight decrease in stiffness. Aged batteries also failed at higher compressive strengths and larger deformations. Post-mortem analysis of the aged cells was performed as a next step using scanning electron microscopy and the occurred degradation effects were evaluated in order to explain the changes observed in the battery electrical and mechanical properties.