Impedance spectroscopy characterisation of automotive NMC/graphite Li-ion cells aged with realistic PHEV load profile

Abstract

The ageing behaviour of Li-ion cells in automotive applications is known to be strongly non-linear with respect to operating conditions. In addition, it has a profound impact on performance, cost and reliability of the target vehicle. The work presented in this paper concerns automotive type NMC/graphite Li-ion battery cells aged under realistic and accelerated conditions, analyzed using electrochemical impedance spectroscopy and with the implementation of an electric equivalent circuit model. Two cycle life tests were performed at +22°C and +45°C surrounding temperature, with reference performance tests performed at ±0°C, +22°C and +45°C. The duty cycle applied is derived from speed samples of real-world city driving characteristics for electric passenger cars with fast charging at 2C-rate. Results show that despite a significant decrease in capacity, a decrease in charge transfer resistance can be observed during the first stages of aging which may be correlated to an increased double layer capacitance. Also, an increase in peak power at low temperatures is observed.