Since battery performance is the main factor affecting on-orbit satellite operation, it is very important to understand the degradation mechanism of battery performance and operating conditions. In particular, a lithium (Li) ion cell used as a satellite power source may be subjected to high charge and discharge rates, and the cell volume change during the charge-discharge cycle might cause the mechanical degradation of the electrode. In this work, we used two in situ methods, load cell measurement and X-ray observation, to investigate the cell volume change of a commercial Li-ion polymer cell with charging and discharging An excess cell volume change that was delayed with respect to the change induced by the state of charge was observed at high charge and discharge rates. We attributed this phenomenon to the slow diffusion of Li ions in the electrode active materials and slow structural change in the outer region of the electrode particle. Furthermore, we deduced that the outer region of the electrode particle, rather than the inner region, was mainly responsible for the excess cell volume change. (C) 2004 The Electrochemical Society.