The electrochemical impedance spectroscopy(EIS) characteristics of lithium iron phosphate batteries

following the cycling periods

at different depths of discharge(DOD)

different states of charge(SOC)

and different charge-discharge rates were studied. The results showed that

within a certain charge/discharge period

the ohmic impedance was basically stable

indicating that the batteries do not deteriorate seriously after long-term cycle. The charge transfer impedance decreased at first and then increased

indicating that the batteries were activated at the initial stage of the cycle. However

the performance of batteries gradually deteriorated in the later cycling

which induced the charge transfer difficulty to increase. The solid diffusion impedance decreased significantly only in the early stage of the cycle

and then remained stable

indicating that the activation process in the early stage of the cycle would significantly improve the solid diffusion of lithium ions. Then we can conclude that

if the resistance value of each part of the electrochemical impedance does not increase significantly

there is no serious performance degradation of the batteries

and the change of charge transfer resistance can also accurately evaluate the attenuation performance of the batteries. The increment of the batteries' impedance is mainly due to that of the cathode materials.