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.Li-ion batteries have become widely used in portable consumer electronics such as cell phones, laptops, and tablets due to their high energy density and long calendar/cycle life. These qualities make Liion batteries an ideal candidate for use in electric vehicles (EVs). A Liion battery in a small electronic device need only last a couple of years to outlive the device however, a successful EV battery must have a calendar life of eight or more years. 1-3 It is necessary to understand the degradation processes occurring inside a Li-ion cell on the timescale of years in order to improve long term cycle life and calendar life.The Ultra High Precision Charger (UHPC) at Dalhousie University 4,5 was used to collect high quality cycling data on fresh, one-year, and two-year aged cells from a commercial automotive Liion cell manufacturer, as well as fresh cells cycled to higher potential and at higher temperature. Highly accurate and precise measurements of potential and capacity allow small changes in charge and discharge capacity to be observable within just a few cycles. 6 From the capacities it is possible to calculate the coulombic efficiency and the charge end point capacity slippage 7 both of which are a measure of parasitic reactions occurring within Li-ion cells.Differential voltage (dV/dQ vs Q) analysis 8 is a powerful tool for highlighting small differences in voltage vs. capacity data and attributing these differences to changes in electrode mass and slippage. The quality of the data collected using the Ultra High Precision Charger 9 enables meaningful conclusions to be drawn from dV/dQ vs Q analysis.
ExperimentalPouch-type electric vehicle Li-ion cells (34 Ah) comprised of Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (NMC442) positive electrodes and graphite negative electrodes were obtained from an established automotive Liion cell manufacturer to probe the effects of tim...