Modeling ionic transport in solid polymer electrolytes and supercapacitors.

摘要

Solid polymer electrolytes (SPEs) can be used to construct all-solid-state rechargeable lithium batteries which deliver high energy density and have design and safety advantages over traditional batteries employing aqueous electrolytes. In this work, we investigate ion association phenomena in solid polymer electrolytes. Ion and ion pair transport under direct and alternating current (DC and AC) excitation in solid polymer electrolytes are considered. We provide a thorough theoretical analysis of the effects of ion association on the conductivity, general current-potential behavior, limiting current density, and cell impedance. We find that ion association causes the effective molar conductivity to decrease if ion-pairs have a diffusivity comparable to those of cations and anions. However, when ion-pairs have a diffusivity larger than cations, the system can have a larger limiting current density with increasing ion association. The effective molar conductivity reaches a maximum as the degree of ion association increases. For solid polymer electrolytes under AC excitation, we find that the salt dissociation degree has a major effect on the system's impedance. The effects of forward or backward reaction rates of salt dissociation reaction on the system's impedance are negligible if the react ion rates are much faster than the diffusion process. The AC model, when combined with DC measurements and other techniques, can be used to characterize ion association and transport properties in the solid polymer electrolyte system.; We also present a mathematical model for charge/discharge of electrochemical capacitors that explicitly accounts for particle packing effects and concentration polarization in a composite electrochemical capacitor consisting of hydrous ruthenium oxide nanoparticles dispersed within porous activated carbon. Among various types of activated carbons, those with large micropore surface areas and low meso- and macropore surface areas are preferred because they give high double layer capacitance and favor efficient packing of RuO₂ nanoparticles. The carbon content can be optimized to minimize the cell cost while achieving acceptable discharge performance.