We studied irreversible capacity and solid electrolyte interface (SEI) formation in lithium nickel mixed oxide in the first charge and discharge cycle. Initial capacity loss mainly originates from two irreversible processes; structural change and reactions between electrode active materials and electrolyte components. Upon contact of the electrode with electrolyte, the reactions spontaneously take place and result in an instantaneous formation of SEI film, whose ionic conductivity changes considerably in the subsequent charge and discharge cycle. According to the change of ionic conductivity, formation of SEI film during the first charge can be roughly divided into two voltage regions, below 3.4 V, at which a highly resistive SEI film is continuously grown, and between 3.4 and 3.8 V, at which a highly conductive SEI film is formed. The high conductivity of SEI film remains nearly invariant as the cell is cycled between 3.8 and 4.3 V, while falling significantly as the cell is discharged to 3.4 V and lower. (C) 2002 The Electrochemical Society.