Insight into Lithium Diffusion in Conversion-Type Iron Oxide Negative Electrode Bingbing Tian, Jolanta S ́ wiatowska,* Vincent Maurice, Catarina Pereira-Nabais, Antoine Seyeux, and Philippe Marcus Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, PSL Research University, 11 rue Pierre et Marie Curie, 75005 Paris, France ABSTRACT: In situ electrochemical (by electrochemical impedance spectros- copy, 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 α-Fe 2 O 3 negative electrode for Li-ion batteries. D Li 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 (Li 2 O/Fe 0 matrix) of slow diffusion (D Li =6 × 10 −16 cm 2 s −1 ) and an unconverted region (intercalated Fe 2 O 3 matrix) of faster diffusion (D Li =2 × 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. 1. INTRODUCTION The application of transition metal oxides as negative electrode materials in lithium-ion batteries (LiBs) depends on their electrochemical performances, which are associated with electrode reactions, interphase chemistry and ion mobility (i.e., lithium diffusion). 1,2 As nanosized particles, such materials (CoO, Co 3 O 4 , NiO, CuO, Cu 2 O, and FeO) can exhibit reversible capacities up to three times higher than commercially used graphite anodes as reported by Poizot et al. 3−5 Among them, hematite iron oxide (α-Fe 2 O 3 ) is one of the most interesting and important candidate, for its high theoretical capacity (1007 mAh g −1 ), abundance and low cost, low toxicity, and environmental friendliness. Since reported as a conversion- type material (Fe 2 O 3 + 6Li ↔ 3Li 2 O + 2Fe), 6,7 it has been shown to suffer from poor electronic/ionic conductivity, 1,8 the main obstacle for improving rate capability which is one of a primary demands of LiBs. Relatively little attention has been devoted to understanding the diffusion processes in which Li + ions migrate through the solid electrolyte interphase (SEI) layer and into the bulk electrode material during the discharge/charge process and the determination of the apparent diffusion coefficient of lithium (D Li ). Li + ions migration into graphite (intercalation-type) and silicon (alloying-type) anodes has been studied by various techniques including electrochemical impedance spectroscopy (EIS), 9−14 cyclic voltammetry (CV), 12,15,16 potentiostatic intermittent titration technique (PITT), 10,12,17 galvanostatic intermittent titration technique (GITT), 12,18 and potential relax technique (PRT). 19 However, for conversion-type material (e.g., iron oxide), ionic migration has been rarely studied due to lack of physical model accounting for the phase transition associated with the (de)conversion (delithiation) reaction. It is generally believed that the kinetic regime is a conversion- controlled rather than a diffusion-controlled process. In our previous work, conversion was shown to proceed mostly in the outer part of the iron oxide thin film electrode during the first lithiation owing to mass transport limitation, 20 which was also observed for conversion-type Cr 2 O 3 thin films. 21 In this case, Li + ions migration primarily through the SEI and the converted matrix (Li 2 O/Fe 0 ), and through the unconverted bulk oxide material (Fe 2 O 3 ) ahead of the conversion front, can be assumed as a primarily one-dimensional diffusion process. Here, we report on the in situ EIS and ex situ time-of-flight secondary ions mass spectrometry (ToF-SIMS) methods employed for evaluating the apparent diffusion coefficients of lithium (D Li ) in order to better understand the lithiation kinetics of conversion-type iron oxide. α-Fe 2 O 3 thin film electrodes were prepared by thermal oxidation of pure iron substrate. The application of thin film electrodes with large surface-to-volume ratio and without carbon and polymeric binder additives can obtain clearer insight into ionic transport in iron oxide electrode material and thus also bulk and Received: October 11, 2014 Revised: December 11, 2014 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/jp510269e J. Phys. Chem. C XXXX, XXX, XXX−XXX