Long Term Aging of Automotive Type Lithium-Ion Cells

Introduction – As supply and demand for plug-in electric vehicles (PEV) and plug-in hybrid electric vehicles (PHEV) take off worldwide, the market will get more attention. More people start to consider purchasing a PEV or PHEV as a real alternative to a car with a combustion engine. With more electric cars on the roads new aspects will come into focus such as the durability of the battery, especially the aging behavior at run-time before the end-of-life criteria for automotive application of 70 - 80 % state of health (SOH) is exceeded. Furthermore, the possibilities for the second-life usage after reaching the end-of-life criteria are of interest.

In this work we analyze the behavior of state of the art commercial automotive lithium-ion cells over a long time period with realistic scenarios. The investigated cell is a 40 Ah pouch cell with lithium-nickel-manganese-cobalt-dioxide (NMC) cathode and a graphite anode.

Experimental – The cells were cycled at 25°C (298.15 K) and 40°C (313.15 K) with two procedures. The first procedure is a combination of discharging with the Common Artemis Driving Cycle (CADC), which has parts of urban, rural and motorway driving as seen in Figure 1 and a constant current charging with 60 A. In addition, the cycle has energy recuperation whenever the car is braking. The cycling was repeated in a state of charge (SOC) window between 20 % and 80 % for 50 times. The capacity determination in the full SOC window was performed every 50^th cycle. End-of-life criteria were defined as 124 274 miles (200 000 km) or a SOH of 80 %. The applied currents of the CADC are calculated from a virtual car. In addition, cells were aged with constant current cycling with 2 C (80 A) charging and 3 C (120 A) discharging instead of the driving cycle. Supplementary calendar aging and electrochemical impedance experiments were carried out.

Results – The comparison between the constant current and drive cycle aging is shown in Figure 2 for 40°C and in Figure 3 for 25°C. Capacity fading is comparable between the two aging procedures at both temperatures. The 40°C cells have a capacity > 91 % (> 38 Ah of ~41.5 Ah) SOH after 124 274 miles. The cells aged at 25°C have a remaining capacity of > 95 % (> 38 Ah of ~40 Ah) SOH.

Conclusion – The long-term cycling experiments show that the state of the art lithium-ion cells meet easily the requirements of the car manufactures with the end of life criteria of 124 274 miles (200 000 km). As the results between the two aging methods are comparable it is possible to make lifetime predictions for the battery in a plug-in electric vehicle (PEV) and a plug-in hybrid electric vehicle (PHEV) beyond the 124 274 miles. It can be concluded from the results that the cell would have a life-time of more than 400 000 miles (643 000 km) at 25°C and approximately 200 000 miles (322 000 km) at 40°C.

Acknowledgment - The authors would like to thank the German Federal Ministry of Education and Research (BMBF) for funding this work within the project ‘SafeBatt’ (03X4631N).