  In today's modeling and analysis of electrochemical cycling of Li- and Na-ion
batteries, an assumption is often made regarding the interphase that forms
between the active material and liquid electrolyte at low potentials, the
so-called solid-electrolyte interphase (SEI). The SEI is generally assumed to
act like an Ohmic resistor despite its complex chemical composition and
porosity distribution. Here, one reports that this assumption does not hold for
alkali-ion batteries. The SEI possesses a non-linear overpotential
characteristic which saturates already at low current density of 0.1 mAcm-2
giving only 3.3$\pm$1 mV for Li-metal electrodes in different electrolytes. For
Na- and K-metal electrodes, these SEI overpotentials become dominating with 31
mV and 72 mV at the same low current densities giving significant disadvantages
over Li-ion batteries for commercial applications. With the introduction of a
new term, one achieves agreement between the parameters from galvanostatic
cycling and electrochemical impedance spectroscopy for the first time. The
discovery of the non-linear SEI overpotential disrupts the general believes
about the role of the SEI for today's batteries as it is basically negligible
for Li-ion batteries at room temperature.
