Ultra High-Precision Studies of Degradation Mechanisms in Aged LiCoO2/Graphite Li-Ion Cells

Twenty LiCoO2/Graphite cells with ages between 0 and 12 years having different cycling histories but identical chemistry and construction were obtained from Medtronic Inc. These cells presented a unique opportunity to learn about aging mechanisms in Li-ion cells. The cells were studied using the ultra-high precision chargers (UHPC) at Dalhousie University, electrochemical impedance spectroscopy (EIS) and differential voltage versus capacity (dV/dQ vs Q) measurements. Even after 12 years of operation at 37°C, the cells still retained 80% of their initial capacity and their coulombic efficiency was 0.99985 when measured at C/20 and 40°C. The capacity loss of the aged cells could be explained by loss of lithium inventory through growth of the solid electrolyte interphase (SEI) at the negative electrode which led to impedance increase. There is no evidence of any active material loss due to electrical disconnect in these cells suggesting the cells have excellent electrodes. A low upper cutoff voltage (4.075 V) is crucial to the long lifetime of these cells due to electrolyte oxidation reactions at the positive electrode, revealed by the UHPC experiments. Comparing similar cells that were cycled to either 4.075 or 4.175 V showed that those cycled to higher voltages under the same conditions failed significantly sooner. These results suggest that electrolyte additives that reduce electrolyte oxidation at the positive electrode, will prolong the cycle and calendar life of these Li-ion cells.