Kinetics of Li_<i>x</i>FePO₄ Lithiation/Delithiation by Ferrocene-Based Redox Mediators: An Electrochemical Approach

Abstract

An electrochemical approach for studying the kinetics of reactions between redox mediators and Li-ion battery electrode materials has been developed. The approach is based on a simple diffusion-reaction model, similar to that used to describe the classical catalytic electrochemical–chemical (EC′) reaction mechanism. Using this approach it is possible to determine the diffusion length of redox mediators in a porous film made from a Li-ion battery electrode material. The rate constant for reaction between redox species and the porous electrode may then be calculated. The approach is applied to determine rate constants for the disappearance of ferrocene and dibromoferrocenium due to reaction with excess pristine and carbon-coated Li_<i>x</i>FePO₄ (0 ≤ <i>x</i> ≤ 1) nanoparticulate films (porosity ∼0.63, BET surface area 20–30 m² g^–1) and excess Li⁺ (0.1 M), which are of relevance to the operation of the recently introduced redox-flow Li-ion battery. Pseudo-first-order volumetric rate constants in the range 1–6 s^–1 were obtained, corresponding to apparent heterogeneous rate constants in the range 2.2 × 10^–6 – 4.4 × 10^–6 cm s^–1, which we show are fast enough not to limit the charge/discharge rate of redox flow Li-ion batteries constructed from these materials