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

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 (D-Li) into conversion-type alpha-Fe2O3 negative electrode for Li-ion batteries. DLi values obtained from EIS were in the range of 10(-16) to 10(-15) cm(-2) 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/Fe-0 matrix) of slow diffusion (D-Li = 6 x10(-16) cm(-2) s(-1)) and an unconverted region (intercalated Fe2O3 matrix) of faster diffusion (D-Li = 2 x 10(-13) cm(-2) 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.