<p>Internal self-discharge poses a critical yet often overlooked challenge for the commercialization of solid-state batteries. It arises from the migration of mobile species, typically cations, between the electrodes driven by chemical potential gradients, leading to a gradual loss of charge even during storage, thus ultimately undermining efficiency, reliability and long-term viability. Unlike liquid-electrolyte batteries, self-discharge in solid-state batteries can be attributed to minor electronic leakage currents through the solid separator, which slowly drain the battery’s stored charge. Here we present an analytical model describing reversible internal self-discharge arising from residual electronic conductivity and the chemical potential gradient in the solid separator, depending on its intrinsic thermodynamic stability or artificial stabilization strategies. Our analysis highlights the need for accurate quantification of electronic properties in solid-state separators and the influence of material stability ranges by reviewing reports on different solid separator classes to guide separator selection, materials engineering and overall solid-state cell design.</p>