Optimized operating range for large-format LiFePO₄/graphite batteries

Long-term cycling performances of LiFePO₄/graphite batteries have been investigated in different state-of-charge (SOC) ranges. It is found that batteries cycled in the medium SOC range exhibit superior cycling stability over those cycled at both ends of the SOC ranges. A variety of characterization techniques, including galvanostatic intermittent titration technique (GITT) analysis, model-based parameter identification, electrochemical impedance spectroscopy analysis, and entropy change test, were used to investigate the performance difference of the batteries cycled in different SOC ranges. The results reveal that batteries at the end of SOC exhibit much higher polarization impedance than those within the medium-SOC range. This result can be attributed to the significant structural change of the cathode and anode materials as revealed by the large entropy change within these SOC regions. Identification of the best operating conditions for LiFePO₄/graphite batteries will significantly extend their cycle life. The general control principle obtained in this work, such as modulating the charge/discharge current to minimize the impedance extremes can also be used in the operation control of other battery systems.