We study ionic liquid–solvent
mixtures in slit-shaped nanopores
wider than a few ion diameters. Using a continuum theory and generic
thermodynamic reasoning, we reveal that such systems can undergo a
capillary ionization transition. At this transition, the pores spontaneously
ionize or deionize upon infinitesimal changes of temperature, slit
width, or voltage. Our calculations show that a voltage applied to
a pore may induce a capillary ionization, whichcounterintuitivelyis
followed by a re-entrant deionization as the voltage increases. We
find that such ionization transitions produce sharp jumps in the accumulated
charge and stored energy, which may find useful applications in energy
storage and heat-to-energy conversion.