Understanding the polarization behavior of the battery electrodes is critical to the development of high-performance lithium batteries. However, for lithium | solid polymer electrolyte (SPE) interfaces no kinetic data have been reported beyond the linear regime, which is why it has not been possible to assess the influence of this interface on the overall battery performance at higher current densities. In this work, a technique based on millisecond current pulses is presented, which enables the deconvolution of the interface and bulk effects, allowing the determination of the lithium | polystyrene-block-polyethyleneoxide (PS-b-PEO) interface kinetics over both linear and non/-linear regimes. Our results indicate that the kinetics roughly follow a hyperbolic sine law rather than a Butler-Volmer behavior. The data are described to a high degree of accuracy by a multistep model based on ion transport through a solid electrolyte interphase (SEI) and two charge-transfer steps described by the Butler-Volmer equation. Hence, it is suggested that the kinetics of the lithium | PS-b-PEO interface are controlled by three distinct processes occurring in series.