Accelerated equilibration for lithium-ion battery using optimal time control with electrochemical model

The equilibrium state of the lithium-ion battery refers to a state where concentration of lithium ions within the solid and the liquid becomes uniformly distributed, and as a result, no potential difference is present. This steady state is particularly required for measurements of cell properties such as open circuit voltage (OCV), entropy coefficient (dOCV/dT) and impedances by electrochemical impedance spectroscopy (EIS). Currently, it takes hours or even days of relaxation to reach the equilibrium state at a given state of charge (SOC) after charging or discharging, which prolongs the entire preparation time prior to actual measurements. In this work, a new equilibration method is developed using optimal time control based on an electrochemical model. The optimal time control is numerically solved by employing a direct solution approach to obtain an optimal current profile. Experimental results have shown that equilibration can be accelerated, so measurement time can be significantly reduced. For large format pouch type lithium-ion cells, the measurement time is reduced up to 58.3% compared with that by the current pulse-relaxation method at three different SOCs at 25 degrees C. In addition, the method is applied and experimentally tested at 40 degrees C and 10 degrees C and the effectiveness is verified.