Impedance spectroscopy and its use in analyzing the steady-state AC response of solid and liquid electrolytes

Abstract

Impedance spectroscopy is defined and some of its applications illustrated for both liquid and solid electrolyte situations. Particular emphasis is placed on complex least squares fitting of small-signal frequency response data at various immittance levels. Most such response data must be fit to an equivalent electrical circuit since a detailed microscopic model of the response is usually lacking. It is found for most ionic response data, as well as that from purely dielectric systems, that one usually must include in the equivalent circuit one or more distributed circuit elements in addition to the usual ideal elements, such as resistors and capacitors. Important distributed circuit elements useful in equivalent circuits are described. A number of actual equivalent circuits used in the past for both liquid and solid electrolytes are presented and compared. Equivalent circuits following from detailed continuum models of the electrical response of blocking and partly conducting systems are compared, and it is shown that some of the same circuits may be used even in the presence of dc bias which usually makes the charge distribution in the system very inhomogeneous. Because frequency response data have often been inadequately analyzed, the author's general and powerful complex nonlinear least squares fitting program is now available for use by others.