The formation of passivating surface films on the electrodes of a lithium-ion polymer battery was investigated at various cycling state using ac impedance spectroscopy and scanning electron microscopy (SEM). A sealed commercial cell (Sony Co.) with a nominal capacity of 840mAh was used for the experiment. An equivalent circuit used to model the impedance spectra show that, with continuous cycling there is a relatively large increase in the interfacial impedance and charge transfer resistances after a few hundred charge–discharge cycles. It was observed that the cell capacity decrease with increase cell impedance. SEM analysis on the electrodes shows that during continuous charge–discharge cycling, the deposition of sub-micro-size particles and dissolution of surface films on the graphite surface. This observation is consistent with increase in cell impedance as a function of charge/discharge cycling.