Direct Evaluation of Lithium Diffusion Kinetics in Electrodes for Lithium-Ion Batteries By Tof-SIMS
Authors: Tian Bingbing , Swiatowska Jolanta , Maurice Vincent , Nabais Catarina Pereira , Seyeux Antoine , Marcus Philippe 
Published: February 2020

The electrochemical performance of electrode materials in lithium-ion, sodium-ion, sulfur or metal-air batteries can be significantly influenced by the diffusion properties of the exchanged ions. The application of new types of electrodes or battery components (i.e. separators) thus requires a good understanding of the diffusion mechanisms during the discharge/charge process.
For lithium-ion batteries (LiBs), the Li-ion distribution in a host electrode material is difficult to measure due to the lack of sensitive and precise techniques. In common practice, the variation of the Li-ion concentration in the host material is deduced from the potential distribution using the Nernst equation and electrochemical characterization techniques such as electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), potentiostatic intermittent titration (PITT), galvanostatic intermittent titration (GITT) and potential relax techniques (PRT). However, it is necessary to develop new and more direct methods for precise measurement of Li-ion diffusion in bulk electrode materials based on precise depth profiling. In the present work, Time-of-Flight Secondary Ions Mass Spectrometry (ToF-SIMS) has been applied for the first time to the measurement of the apparent diffusion coefficient of lithium ions into a conversion-type thin film electrode material (α-Fe2O3). Hematite (α-Fe2O3) is an interesting and important candidate transition metal oxides for application as anode material in LiBs due to its high theoretical capacity (1007 mAh g-1), abundance and environmental friendliness [i,ii,iii,iv]. However, this material suffers from poor electronic/ionic conductivity, which is the main obstacle for improving the charge/discharge rate capability of the battery. So the future of such a conversion-type material in LiBs applications relies on mastering the kinetics of the electrode lithiation reaction [v,vi
].
The analysis of the diffusion coefficient of Li ions in the bulk of this electrode material was based on the in-depth variation of the Li-ion concentration obtained by ToF-SIMS depth profiling a partially lithiated sample (Figure 1). The apparent diffusion coefficient calculated from the infinite integration of Fick’s second law for one-dimensional diffusion was of the order of 10-15 cm2 s−1. This value is in agreement with values obtained from cyclic voltammetry and impedance data. This low value for Li-ion diffusion evidences that the rate of lithiation in this conversion-type material is slower than in intercalation or alloying-type materials. It should be noted that the direct methodology used here for determining the diffusion coefficient can be applied to any type of electrode material or component of the electrochemical cell. It enlarges the horizon for the study of Li diffusion kinetics into electrode for LiBs.

Figure 1. (a) ToF-SIMS negative ion depth profiles of Li- ions on the sample discharge to 0.84 V; (b) Normalized in-depth variation of the Li-ion concentration.

Acknowledgements: Region Ile-de-France is acknowledged for partial support for the ToF-SIMS equipment.
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