We develop a theory of charge storage in ultranarrow slitlike pores of nanostructured electrodes. Our analysis is based on the Blume-Capel
model in an external field, which we solve analytically on a Bethe lattice. The obtained solutions allow us to explore the complete phase diagram
of confined ionic liquids in terms of the key parameters characterizing the system, such as pore ionophilicity, interionic interaction energy,
and voltage. The phase diagram includes the lines of first- and second-order, direct and re-entrant phase transitions, which are manifested
by singularities in the corresponding capacitance-voltage plots. Testing our predictions experimentally requires monodisperse, conducting
ultranarrow slit pores, to permit only one layer of ions, and thick pore walls, to prevent interionic interactions across the pore walls. However,
some qualitative features, which distinguish the behavior of ionophilic and ionophobic pores and their underlying physics, may emerge in
future experimental studies of more complex electrode structures.