Transfer mechanism in solid-electrolyte layers on lithium: influence of temperature and polarization

The kinetics of lithium electrochemical systems is governed by the transport processes in the solid electrolyte interphase (SEI) coating the lithium anode. The present work studies the temperature effect on the electrochemical kinetics of a metallic lithium electrode immersed into a LiClO4 solution in propylene carbonate in a wide polarization range. A series of polarization curves of the Li electrode within a temperature range of - 35 to + 70 degreesC were recorded using the pulse voltammetry method. Any of these symmetrical anodic and cathodic polarization curves looks as a segment of a straight line (the Ohmic current j(Omega) caused by the intrinsic ionic conductivity of SEI) shading, as the overpotential eta rises, into a power curve j(inj) infinity eta (n) (j(inj) being the injection current) with a temperature-dependent exponent n greater than or equal to 2. Similar polarization curves were recorded for the Li electrode in LiClO4 and LiBF4 solutions in gamma -butyrolactone as well. The cause of such a j(eta, T) dependence is assumed to be structural disordering of the SEI material resulting in the appearance of a distribution of jump distances and energy barrier height for charge carriers. The stochastic transport of carriers in a disordered solid with a wide distribution of site-to-site jump times leads, by calculation of the current-voltage dependence, to the above power function j(inj) infinity eta (n) with an exponent depending on the absolute temperature T as n = 1 + (a(1) - b(1)/T)(-2). Our experimental data are in good agreement with this model. Comparing the experimental j - eta curves with the theoretical equations, one could estimate a set of the microscopic parameters of transfer, including the mean jump distance, the effective radius of charge localization, the jumping attempt frequency, and the mean height of energy barriers. (C) 2001 Elsevier Science Ltd. All rights reserved.