Superionic liquids in conducting nanoslits: A variety of phase transitions and ensuing charging behavior

TL;DR

This paper develops an analytical theory for charge storage in ultra-narrow slit pores of nanostructured electrodes, revealing complex phase transitions and capacitance behaviors of confined ionic liquids.

Contribution

It introduces a theoretical model based on the Blume-Capel model solved on a Bethe lattice to analyze phase diagrams of ionic liquids in nanopores, including various phase transition types.

Findings

Identification of multiple phase transition lines in the system

Prediction of singularities in capacitance-voltage characteristics

Distinction between ionophilic and ionophobic pore behaviors

Abstract

We develop a theory of charge storage in ultra-narrow slit-like pores of nano\-structured electrodes. Our analysis is based on the Blume-Capel model in 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 characterising 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. To test our predictions experimentally requires mono-disperse, conducting, ultra-narrow slit pores, permitting only one layer of ions, and thick pore walls, preventing interionic interactions across the pore walls. However, some qualitative features, which distinguish the behavior of ionophilic and ionophobic pores, and its underlying physics, may emerge in future experimental studies of more complex electrode structures.