Analysis of overcharge tolerance of aged LMO cells with Examples 2023-32-0108

The capacity of a lithium-ion battery decreases during cycling. This capacity loss or fade occurs due to several different mechanisms associated with unwanted side reactions that occur in these batteries. The same reactions occur during overcharge and cause electrolyte decomposition, passive film formation, active material dissolution, and other phenomena. As the battery ages the accuracy of state of charge prediction decreases and vulnerability to persistent overcharge increases. Moreover, as the battery ages, its tolerance to such unintended overcharge changes. This tolerance depends on the nature of the history of cycle and calendar aging. A map of this tolerance in the BMS can provide awareness of the factor of safety due to overcharge as battery ages. Signatures of early warning signs of incipient thermal runaway due to overcharge can also be very useful features in a BMS. The SwRI EssEs-I consortium conducted aging of two commercially available LMO cell types with different calendar and cycle conditions. These cells were tested with the SAE 2464 3C overcharge procedure at BOL and EOL stages, but at an ambient temperature of 50 °C.

The primary objective of this work was to investigate LMO cells for overcharge tolerance after aging. The secondary objective was to study to notice trends and correlations of overcharge tolerance to capacity, temperature, ΔSOC, charge/discharge power during aging.

A very strong correlation was established between Ah to failure and EOL Ah. This makes sense since amount of life degradation makes the cell more vulnerable to failure. Scatter between samples seem to increase with EOL capacity. Tolerance to overcharge for cell type #2 seem to be closer to 45 °C than 55 °C indicating a need for close thermal management in aged packs. Calendar aging seemed to be more dominant in determining the tolerance to overcharge than cycle aging.