Insight into Lithium Diffusion in Conversion-Type Iron Oxide Negative Electrode

Abstract

In situ electrochemical (by electrochemical impedance spectroscopy, EIS) and ex situ surface (by time-of-flight secondary ions mass spectrometry, ToF-SIMS) analysis were applied to investigate solid-state diffusion coefficient (DLi) into conversion-type α-Fe2O3 negative electrode for Li-ion batteries. DLi values obtained from EIS were in the range of 10–16 to 10–15 cm2 s–1 for electrodes partially and fully lithiated, respectively, showing that pulverization of the converted material promotes Li-ion migration. ToF-SIMS ion depth profiling performed after partial lithiation enabled discriminating the surface solid electrolyte interphase (SEI) region, a converted electrode region (Li2O/Fe0 matrix) of slow diffusion (DLi = 6 × 10–16 cm2 s–1) and an unconverted region (intercalated Fe2O3 matrix) of faster diffusion (DLi = 2 × 10–13 cm2 s–1) ahead of the conversion front. Comparison of the ex situ and in situ results indicates that the electrode conversion kinetics is limited by Li-ion diffusion in the converted matrix and suggests a hindering effect of the passivating SEI layer. ToF-SIMS depth profile analysis appears as a most appropriate and direct methodology to measure Li-ion diffusion solely in electrode materials, excluding SEI layer effects.