Understanding the impact of recent usage on lithium-ion battery impedance through the relaxation phenomena

Impedance-based state estimation techniques for lithium-ion batteries provide valuable insights into the battery states. In existing literature, the influence of factors such as state-of-charge (SOC), temperature, and relaxation period on impedance has been well-documented. However, the impact of recent usage history has largely been overlooked. This study demonstrates that recent usage history significantly affects impedance, even after hours of relaxation and that controlling the usage history can reduce the required relaxation period to 5 min or less. Using electrochemical impedance spectroscopy (EIS) in 36 different usage scenarios, it is found that the characteristics of the usage profile, i.e. current and rests during discharge, have a strong effect on the measured impedance. This history dependency is found to relate to the battery’s relaxation behaviour and the concept of state-of-ionic-diffusion (SOID) is introduced to summarize their combined effects. When the SOID is controlled, the required relaxation period can be reduced to 5 min or less while still obtaining consistent impedance spectra. These observations highlight that, in addition to the relaxation period, the historical usage patterns play a crucial role in determining the battery’s impedance spectrum. In laboratory testing, the usage history needs to be considered alongside the relaxation period to ensure reliable measurements. In dynamic conditions, such as onboard a battery management system (BMS), effectively controlling the SOID can allow significantly shortened test times while the opposite leads to unreliable measurements.