State-of-Charge Estimation of LiFePO₄–Li₄Ti₅O₁₂Batteries using History-Dependent Complex-Impedance

The present paper seeks to extract, analyze, and interpret the electrochemical impedance spectra of a LiFePO 4 -Li 4 Ti 5 O 12 battery to supplement state-of-charge estimation. A particular challenge in determining the state-of-charge of a LiFePO 4 -Li 4 Ti 5 O 12 cell is that the open-circuit voltage is flat over a large range of states of charge. Because the voltage in standard batteries is typically used to inform state-of-charge estimation, a different corrective factor needs to be determined for this cell chemistry. Instead of using the battery voltage, the present study uses the battery's dynamic response to assist state of charge estimation. The present work includes at least three novel contributions: 1) the electrochemical impedance of the LiFePO 4 -Li 4 Ti 5 O 12 cell is shown to be strongly history dependent, 2) a novel system-identification technique is implemented to extract the EIS in-situ, and 3) a signature electrochemical impedance spectra attribute is identified that is a strong function of battery state of charge.