Spurious potential dependence of diffusion coefficients in Li+ insertion electrodes measured with PITT

Diffusion coefficients of Li+ in insertion electrodes determined by the potentiostatic intermittent titration technique (PITT) have been reported in the literature as being potential-dependent (and thus dependent on the degree of insertion). With this PITT method the diffusion coefficients D-PITT are determined from the current response of potential-step experiments using an approach based on the Cottrell equation. This equation assumes as boundary conditions infinitely fast kinetics and a linear system with semi-infinite thickness. Using the example of a spinel-type LideltaMn2O4 electrode it will be demonstrated that this potential dependence of D-PITT is not real but occurs, because the inadmissible boundary condition of infinitely fast kinetics which is not fulfilled has been adopted. This will be demonstrated with the help of numerically simulated PITT data calculated with a constant diffusion coefficient D. Diffusion coefficients D-PITT calculated from these simulated PITT data are potential dependent, even though the PITT experiments were simulated with a constant diffusion coefficient D. The inadmissible boundary condition of a linear system with semi-infinite thickness applied to a bed of spherical active particles with finite radii as represented by a LideltaMn2O4 electrode leads to further deviations of D-PITT which will be investigated. (C) 2002 Elsevier Science Ltd. All rights reserved.