Abstract
Purely ionic electrolytes—wherein ionic liquids replace neutral solvents—have been proposed to improve lithium-ion-battery performance, on the basis that the unique microscopic characteristics of polarized ionic-liquid/electrode interfaces may improve the selectivity and kinetics of interfacial lithium-exchange reactions. Here we model a “three-ion” ionic-liquid electrolyte, composed of a traditional ionic liquid and a lithium salt with a common anion. Newman's concentrated-solution theory is extended to account for space charging and chemomechanical coupling. We simulate electrolytes in equilibrium and under steady currents. We find that the local conductivity and lithium transference number in the diffuse double layers near interfaces differ considerably from their bulk values. The mechanical coupling causes ion size to play a crucial role in the interface's electrical response. Interfacial kinetics and surface charge on the electrodes both affect the apparent transport properties of purely ionic electrolytes near interfaces. Larger ionic-liquid cations and anions may facilitate interfacial lithium-exchange kinetics.
Highlights
Improved lithium transport in separators can enhance the energy efficiency and power density of batteries
Conventional Nernst–Planck theory is based on an assumption that ions sit in a background of a neutral solvent species that is present in great excess, and cannot be applied to fully ionic electrolytic solutions
Characterization generally involves a series of measurements of individual transport properties (e.g., Hittorf transference number, Fickian lithium-salt diffusivity) whose very definitions are based on the notion that a neutral solvent is present
Summary
Improved lithium transport in separators can enhance the energy efficiency and power density of batteries. The use of ionic-liquid-based electrolytes in lithium-ion batteries has been shown to improve both current tolerance and cycle life (Lu et al, 2014; Piper et al, 2015). Characterization generally involves a series of measurements of individual transport properties (e.g., Hittorf transference number, Fickian lithium-salt diffusivity) whose very definitions are based on the notion that a neutral solvent is present. Even if this ambiguity is ignored, transport properties cannot be measured independently in concentrated solutions, because the thermodynamic properties and transport characteristics of different species are coupled. As shown experimentally by Popovic et al (2015), electrolytes at
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