Calendar and Cycle Aging of Commercial NCA Lithium-Ion Cells

Abstract

In extensive experimental aging studies, focusing on the application of lithium-ion cells in electric vehicles, we have investigated the calendar and cycle aging of commercial 18650 lithium-ion cells with graphite anode and NCA cathode for almost three years. Calendar aging was examined by storing the cells at various states-of-charge (SoCs) and temperatures. Cycle aging was investigated for different charging and discharging loads at various temperatures, SoCs and cycle depths. For example, the impact of regenerative braking was examined by applying dynamic highway driving load profiles with different maximum recharging currents during braking periods [1]. To identify the dominant aging mechanisms for the different load conditions, the following non-invasive, non-destructive techniques were used:• Constant-current-constant-voltage capacity measurements• Differential voltage analysis (DVA) [2]• Coulometry [3,4]• DC pulse resistance measurements• Electrochemical impedance spectroscopy (EIS) Results for calendar aging In the storage tests, the capacity measurements exhibit SoC regions in which the capacity fade is similar. Hence, reducing the SoC does not reduce calendar aging in these regions. DVA discloses a strong correlation between the plateaus in the capacity fade and the plateaus in the anode potential. Together with the results from coulometry, anode side reactions are identified as the main driver of capacity fade from calendar aging. They cause an irreversible loss of cyclable lithium, which results in a shift in the electrode balancing. This shift also affects the pulse resistances at low SoC. Moreover, coulometry enables to analyze the reversible self-discharge for storage above 80% SoC. Results for cycle aging Larger cycle depths increase the capacity fade from cycle aging. EIS and pulse measurements demonstrate that the resistance increase also aggravates with larger cycle depths. Especially, deep discharging to 0% SoC massively increases the cell impedance. The results from EIS demonstrate that mainly the cathode resistances increase. Whereas calendar aging increases with temperature, the additional degradation from cycle aging decreases with higher temperature. For charging and discharging at different temperatures (0°C and 25°C), Figure 1 shows a strong degradation for the cells discharged with a dynamic highway driving profile at 0°C. The spectra from DVA reveal that the storage capabilities of the anode have remained largely intact. This demonstrates that discharging at low temperatures does not degrade the anode. A certain cathode degradation can be identified by EIS, but the predominant part of the capacity fade origins from a loss of cyclable lithium. From the aging mechanisms identified for calendar and cycle aging, strategies for optimal battery operation are derived.