The ac impedance of carbon monofluoride (CF) half-cells and Li/CF batteries that contain 1M LiBFJ4-butyrolactone electrolyte was measured as a function of state of charge. The nonfaradaic components of the CF half-cell impedance were resolved with the aid of a one-dimensional macroscopic treatment of a porous electrode. The values of the nonfaradaic components and their variation with charge withdrawn provide information concerning the nature of cathode discharge products, the degree of tortuosity in the cathode and separator matrices, and the cathode failure mechanism. The CF electrode capacitance, as measured by the low frequency quadrature impedance, can serve as a semiquantitative measure of battery state of charge under certain conditions.The electrochemical technique of ac impedance can be used, under appropriate conditions, to evaluate several important parameters of a porous electrode. The interfacial area between the electrode and the electrolyte can be obtained from the quadrature component of the impedance, provided that the double-layer capacitance of the pure e]ectroactive component is known (1-3). Both faradaic components of the electrode impedance (charge transfer and diffusion resistances) and nonfaradaic impedance components (internal electrolyte resistance and interelectrode resistances) can be quantitatively identified from complex-plane plots (3, 4). Analysis of porouselectrode impedance data in terms of a semi-infinite cylindrical-pore model (5, 6) or, as' has been done more recently, in terms of finite cylindrical pores (7), leads to values of pore radius, pore length, and pore number. Such analyses have been attempted for sintered-plate cadmium electrodes (8), gold powder and Raney gold electrodes (2), sintered nickel (2, 7), Raney nickel (4), and zinc powder (9). With Raney nickel, the impedance technique was used to obtain the pore radius, length, and number of micropores inside the catalyst grain (4).Problems remain in characterizing the structure of porous electrodes, particularly technologically important ones, via the impedance method. Although the impedance data of simple structures, such as loosely packed metal powders, can be adequately interpreted in terms of an equivalent cylindrical-pore model (2, 9), for more complex structures, such as Raney catalysts and sintered metals, tortuosity must be introduced in the internal pore network (2, 8). Furthermore, for electrodes made of two or more materials with different particle shapes and sizes (for example, Teflon-bonded electrodes), a single pore radius cannot account for the distribution of pore sizes. Regarding pore shape, the impedance of an individual pore will reflect pore shape ( 7), but the impedance of an assembly of shaped pores will resemble that of cylindricalpore electrode (2). Hence, pore shapes are better elucidated by electron microscopy. Nonetheless, the ac impedance technique can provide useful information concerning structure and mechanism in porous electrodes.We report here the ac impedance of carbon monofluoride (C...