Impedance Spectroscopy of Li Electrodes. 4. A General Simple Model of the Li−Solution Interphase in Polar Aprotic Systems

Abstract

In this work Li electrodes were studied in a variety of electrolye systems using impedance spectroscopy. The Li electrodes were all prepared in situ by shearing the surfaces in the same solutions in which the experiments were performed. The electrolyte systems included propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), tetrahydrofuran (THF), 2Me−THF, and 1,3-dioxolane (DN) as the solvents, LiAsF6, LiClO4, LiBF4, and LiPF6 as the salts, and trace water up to a concentration of 200 ppm. The electrochemical behavior of these systems is controlled by films formed spontaneously on the Li surfaces in solutions through which Li ions migrate under an electrical field, and therefore, Li+ migration in fact determines the interfacial resistance measured. It is assumed that these surface films have a multilayer structure to which a “Voigt” type analog of RC circuits in series can be fitted. All the spectra could be modeled by five RC circuits in series (one of which also contains a “Warburg” type element). From the R and C values, the thickness and resistivity of the various layers comprising the Li−solution interphase formed in all the above electrolyte systems could be calculated. The above model was examined using four different experimental routes: (1) studying the influence of the solution composition (solvent, salt, and the presence of H2O) and of storage time; (2) studying the effect of temperature; (3) experiments in which the solutions were changed during storage; (4) current passage. The significance of the model was explored by following the changes in parameters such as the layers' resistivity, thickness, and activation energy for Li+ migration as a function of the experimental conditions (e.g. solution composition, storage time, temperature, and current passage).

Temperature-Dependence of the Solid-Electrolyte Interphase Overpotential: Part I. Two Parallel Mechanisms, One Phase Transition

“…For several electrolyte systems, the measured conductivity for the pure electrolyte could be added in the 1/R Ohm plots when adapted for the properties of the glassfiber separator and transference number. This was the case for LiPF 6 and LiTFSI in EC:DMC;…”

“…31 They are also observed for all four PC-based Li electrolytes. Furthermore, this sudden jump is detected for 0.5 M NaPF 6 in EC:DMC; however, at much higher T C between 50-60…”

“…These resistances vary for different Li salts and solvents usually between 40 to 100 cm 2 . 1,[3][4][5][6] In contrast, for Nametal electrodes these SEI resistances rise already to 230-670 cm 2 , while they reach 3500 cm 2 for 0.5 M KPF 6 in EC:DMC 1:1. 1 In contrast, galvanostatic cycling of the K-system is possible up to current densities of 56 mAcm −2 , 1 with as little as 1.5 V total overpotential from both electrodes.…”

“…Many studies concentrated on the system of LiAlCl 4 in SOCl 2 solvent leading to a very thick SEI layer on the Li-metal electrode due to the highly reducing character of Li and the good oxidant SOCl 2 . [8][9][10] Also LiClO 4 , LiAsF 6 , and LiBF 4 in PC, and γ-butyrolactone [10][11][12][13] have been analyzed. For these systems two different models of ionic conduction z E-mail: michael@battronics.com based on Young's equation 14,15 and space-charge limited current 10,13 have been successfully applied.…”

“…7 The Young equation seems to be the most prominent and is cited throughout literature from its development in 1961 14 to its proposition for the SEI in 1979 16 and through SEI research and reviews up to today. 6,17 The equation by Young has the form i = prefactor · sinh(zF/RT · a/d · η) 14 with a being the half-jump distance and d the thickness of the film which is intended to describe the hopping of ions through Frenkel and Schottky defects. 14 One can simply challenge the validity of this equation by its limits.…”