• Lee, A.A.; Perkin S.
    Ion-Image Interactions and Phase-Transition at Electrolyte-Metal Interface
    J. Phys. Chem. Lett., 2016, 7, 2753-2757.

    Abstract: The arrangement of ions near a metallic electrode is crucial to energy storage in electrical double-layer capacitors. Classic Poisson−Boltzmann theory predicts that the charge stored in the double layer is a continuous function of applied voltage. However, recent experiments and simulations strongly suggest the presence of a voltageinduced first-order phase transition in the electrical double layer, leading to a hysteretic response: the capacitance−voltage relation is dependent on whether the voltage is increasing or decreasing. By developing a simple analytical model, we show that ion− image interaction could explain this phase transition. Moreover, our model shows that the presence of phase transition depends on the bulk energy of the ionic liquid. Our results justify mixing ionic liquids with solvents as a way to achieve large capacitance and avoid hysteresis.

  • Smith, A.M.; Lee, A.A.; Perkin S.
    The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration
    J. Phys. Chem. Lett., 2016, 7, 2157-2163.
    Abstract: According to classical electrolyte theories interactions in dilute (low ion density) electrolytes decay exponentially with distance, with the Debye screening length the characteristic length scale. This decay length decreases monotonically with increasing ion concentration due to effective screening of charges over short distances. Thus, within the Debye model no long-range forces are expected in concentrated electrolytes. Here we reveal, using experimental detection of the interaction between two planar charged surfaces across a wide range of electrolytes, that beyond the dilute (Debye–Hückel) regime the screening length increases with increasing concentration. The screening lengths for all electrolytes studied—including aqueous NaCl solutions, ionic liquids diluted with propylene carbonate, and pure ionic liquids—collapse onto a single curve when scaled by the dielectric constant. This nonmonotonic variation of the screening length with concentration, and its generality across ionic liquids and aqueous salt solutions, demonstrates an important characteristic of concentrated electrolytes of substantial relevance from biology to energy storage.



  • Smith, A.M.; Perkin S.
    Influence of Lithium Solutes on Double-Layer Structure of Ionic Liquids
    J. Phys. Chem. Lett., 2015, 6, 4857–4861 

    Abstract: The ionic liquid–electrode interface has attracted much recent interest owing to its importance for development of energy storage devices; however, the important step of adding electro-active ions is not yet well understood at the molecular level. Using direct force measurements across confined electrolyte films, we study the effect of added lithium-ion solute on the double-layer structure of an ionic liquid electrolyte with molecular resolution. We find anionic clusters involving lithium can persist adjacent to the surfaces, and in many cases, this inhibits direct adsorption of lithium ions to the negative surface. Two apparently similar ionic liquid solvents show diverging properties, with one facilitating and the other preventing direct Li-ion adsorption onto the negative surface. The results have implications for the selection of ionic liquids as electrolytes in lithium-ion batteries.


  • Lee, A. A.; Vella, D.; Perkin, S.; Goriely, A.
    Are Room-Temperature Ionic Liquids Dilute Electrolytes?
    J. Phys. Chem. Lett., 2015, 6, 159-163