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 off-nominal conditions such as overcharge and external short circuit during their use in the field, compromising user safety. Off-nominal tests are typically carried out on fresh cells. The goal of this work was to characterize the interplay between cycle life aging and the off-nominal conditions, such as overcharge and external short circuit. 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).