The expression for the time-dependent intercalation level is [28]. x(t) = (zedANW) − 1 ∫ t 0 dt′ I(t′) (6) where z is the valence of the intercalated ion, e is the elementary charge, d film thickness, A is the active area of the electrode, NW is the tungsten number density, and I is the current during the measurement.