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

The kinetics of lithium electrochemical systems is governed by the transpor t processes in the solid electrolyte interphase (SEI) coating the lithium a node. The present work studies the temperature effect on the electrochemica l kinetics of a metallic lithium electrode immersed into a LiClO4 solution in propylene carbonate in a wide polarization range. A series of polarizati on curves of the Li electrode within a temperature range of - 35 to + 70 de greesC were recorded using the pulse voltammetry method. Any of these symme trical anodic and cathodic polarization curves looks as a segment of a stra ight line (the Ohmic current j(Omega) caused by the intrinsic ionic conduct ivity 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 temperat ure-dependent exponent n greater than or equal to 2. Similar polarization c urves were recorded for the Li electrode in LiClO4 and LiBF4 solutions in g amma -butyrolactone as well. The cause of such a j(eta, T) dependence is as sumed to be structural disordering of the SEI material resulting in the app earance of a distribution of jump distances and energy barrier height for c harge carriers. The stochastic transport of carriers in a disordered solid with a wide distribution of site-to-site jump times leads, by calculation o f the current-voltage dependence, to the above power function j(inj) infini ty 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 t his model. Comparing the experimental j - eta curves with the theoretical e quations, one could estimate a set of the microscopic parameters of transfe r, including the mean jump distance, the effective radius of charge localiz ation, the jumping attempt frequency, and the mean height of energy barrier s. (C) 2001 Elsevier Science Ltd. All rights reserved.