Jan N. Reimers

Three nominally identical cohorts of NMC/graphite automotive Li-ion cells aged zero, one, and two years were obtained from an automotive Li-ion cell producer. The aged cells were stored at 50% state of charge at room temperature without cycling. High precision coulometry and differential voltage analysis (dV/dQ vs. Q) were used to probe the fresh and aged cells to learn about the parasitic reactions occurring. The stored cells had developed a mature SEI which led to virtually no capacity loss during testing but precision coulometry still showed evidence for significant electrolyte oxidation at the positive electrode. The fresh cells showed SEI growth at the negative electrode leading to initial capacity fading which accelerated with temperature. They also showed electrolyte oxidation at the positive electrode which increased dramatically with cycling temperature or upper cutoff potential. All cells showed virtually no evidence for loss of active material during storage or cycling. These results strongly suggest that electrolyte additives which limit electrolyte oxidation at the positive electrode side are required to improve the longevity of these cells.

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