Calendar aging tests were performed at different ambient temperatures and SOCs. The forced-air oven was used to control different ambient temperatures. The cell voltage was kept constant, and the current was collected. When the calendar aging time reached a specific value (e.g., 25, 60, 120, and 180 days), the cell was cycled at a charge and discharge rate of C/3 to determine capacity of the cell. Then, impedance and DCR tests were conducted under the same conditions as that for the cycle-aged cells.