Dielectric saturation of liquid propylene carbonate in electrical energy storage applications

Dielectric characteristics of a molecular model of liquid propylene carbonate are evaluated for utilization in molecular scale simulation of electrochemical capacitors based on nanotube forests. The linear-response dielectric constant of the bulk liquid, and its temperature dependence, is in good agreement with experiment. Dielectric saturation is studied by simulations with static uniform electric fields as large as 4 V/nm. The observed polarization is well described by the Langevin equation with the low-field/high-field crossover parameter of 0.09 V/nm. Simulation of liquid propylene carbonate confined between charged parallel graphite electrodes yields a capacitance that depends on the electric potential difference across those thin films. An effective dielectric constant inferred from the capacitance is significantly less than the uniform liquid dielectric constant, but is consistent with the nonlinear dielectric response at the strong fields applied to the electrode film. Those saturation effects reduce the weak-field capacitance.