After prolonged cycling of a lithium/petroleum coke half-cell, a sudden and reversible voltage drop is often seen during the discharge cycle which leads directly to an apparent decrease in reversible lithium-ion intercalation capacity. Kinetic analysis of cells in this condition has been studied by complex impedance spectroscopy and current interruption. This has shown that the large increase in discharge resistance (similar to 2.5 k Ohm cm(2)) is caused by an enlarged solid electrolyte interfacial (SEI) layer which increases both the migration resistance and the concomitant concentration polarisation in this layer. This effect has been attributed to passivation debris which has become dislodged from the electrode surfaces (probably the lithium counter) and migrates cataphoretically to, and blocks the carbon electrode during discharge. The appearance of this effect is regarded as a diagnostic indicator of the presence of this debris and should not be confused with a loss of reversible intercalation capacity.