Degradation-Safety Analytics in Lithium-Ion Cells and Modules Part II. Overcharge and External Short Circuit Scenarios

Lithium-ion cells can be inadvertently subjected to overcharge or other off-nominal conditions during their use in the field, compromising user safety. Off-nominal tests are typically carried out on fresh cells. The goal of this work is to characterize the interplay between cycle life aging and the off-nominal events. Cylindrical cells aged to 10, 15 and 20% capacity fade (CF) and battery modules aged to 20% CF, both cycled under two operating voltage windows, were subjected to overcharge and external short tests. Additionally, single cells were aged to 20% CF using a drive cycle profile at three temperatures of 10 °C, 25 °C and 40 °C. Under overcharge conditions, the single fresh cells experience slower activation of the current interrupt device (CID) compared to the aged cells and the cathode displayed severe degradation in spite of the CID activation and the anode exhibited lithium plating on the edges of the electrode. At the module level, the fresh module experiences fire while the aged module shows sequential CID activation with no thermal runaway. No major trends were observed with the external short tests of the aged cells compared to the fresh ones due to protection provided by the positive temperature coefficient (PTC).