5. Conclusion and perspective

increase of the counterion density within the EDL and thus a larger capacitance. These theoretical predictions are in good agreement with our experimental observations and offer guidance for designing RTIL mixtures for EDL supercapacitors with optimal performance [47].

The charging kinetics of EDLs has a pivotal role in the performance of a wide variety of nanostructured devices. Despite the prevalent use of ionic liquids as the electrolyte, relatively little is known on the charging behavior from a microscopic perspective [33, 75, 77, 87–90]. Here, we study the charging kinetics of ionic liquid EDLs using the TDDFT that captures the molecular excluded volume effects and electrostatic correlations [39]. We found that the thermodynamic non-ideality plays a pivotal role in electrodiffusion and such effect cannot be captured by the lattice-gas model for the excluded volume effects. In particular, TDDFT predicts "wave-like" variation of the ionic density profiles that has not been identified in previous investigations [34]. This unusual charging behavior can be explained in terms of the oscillatory structure of ionic liquids near the electrodes. For ion transport in narrow pores with a high gating voltage, in Figure 5, the conductivity shows an oscillatory dependence on the pore size

Besides, our new TDDFT is able to account for the molecular excluded volume effects, electrostatic correlations, and the dispersion interactions. Our results show that the dispersion interaction between ions makes the surface charge be a non-monotonic function of time shown in Figure 6. However, the dispersion interaction between the electrode and ionic-liquid does not

Figure 5. The conductivity shows an oscillatory dependence on the pore size [91]. Copyright 2017 Royal Society of Chemistry.

4.4. Ion-ion interaction on the transport in nanopore

148 Supercapacitors - Theoretical and Practical Solutions

owing to the strong overlap of electric double layers [91].

In our recent works, the CDFT was developed and applied by us to study the electrode/ electrolyte interface behaviors, to understand capacitive energy storage. As a statistical mechanical tool and an alternative to molecular dynamics or Monte Carlo simulation methods, CDFT offers a powerful and efficient mathematical framework to describe the equilibrium and dynamic properties of many-body systems in terms of the one-body density profiles. It allows one to precisely tune the parameters such as ion diameter, solvent dipole, and pore size over a large range and to focus on the most important physical problems to be addressed, using a computationally efficient coarse-grained approach to model real fluids. Through CDFT, we have found novel behaviors of electrolytes inside nanopores, such as capacitance oscillation, optimal dipole moment, and wave-like charging. Further development of CDFT for complex pore structures and charging kinetics would allow us to directly predict power density and energy density for supercapacitors.
