Tracer Diffusion Coefficients of Li Ions in Li_xMn₂O₄ Thin Films Observed by Isotope Exchange Secondary Ion Mass Spectrometry

Abstract

Tracer diffusion coefficients D* of lithium ions in Li x Mn2O4 (0.2 < x < 1) thin films were measured as a function of the composition x by using secondary ion mass spectrometry. For this purpose, a new “step-isotope-exchange method” was developed to observe the time dependence of the 6Li isotope concentration ratio in the Li x Mn2O4 film which is in contact with a 6Li-enriched electrolyte to exchange Li+ ions. A steep decrease in D* depending on the Li composition was observed for Li x Mn2O4, with D* = 8 × 10–13 cm2 s–1 for x = 0.2 and decreasing to 1.5 × 10–17 cm2 s–1 for x = 1.0 (bulk diffusion coefficient, D b *). This behavior is well explained by a vacancy diffusion model for the α phase on Li x Mn2O4 (0.77 < x < 1.0). Chemical diffusion coefficients D̃ were also measured in the range of 0.2 < x < 1.0 by an electrochemical method, which was compared with the D* to evaluate the effect of thermodynamic factors. The thermodynamic factors and interactions between Li+ ions were found to strongly influence the chemical diffusion coefficient. The tracer diffusion measurements are important to understand the charge–discharge mechanism in the electrodes of lithium-ion batteries.

Tracer diffusion coefficients of Li+ ions in c-axis oriented LixCoO2 thin films measured by secondary ion mass spectrometry

“…The electrochemical techniques used include the potentiostatic intermittent titration technique (PITT), the galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS) For the isotope ion-exchange, 6 Li-enriched LiClO 4 was synthesized according to a previously reported procedure. 32,58 An aqueous solution of HClO 4 (Wako Pure Chemical) was added dropwise to 6 Li 2 CO 3 (95% 6 Li, 5% 7 Li, Cambridge Isotope Laboratories. Inc.) until the white solid dissolved.…”

“…A recently developed 'step-isotope-exchange method' 32 was used to measure the tracer diffusion coefficient. Fig.…”

“…The principles of the step-isotope-exchange method have been explained in detail in a previous paper, 32 so we will briefly describe them here. The isotope ratio in the Li x CoO 2 thin film depends on the distance z from the surface and the time t. 23,59 Assuming the diffusion-controlled process, the isotope ratio in the film is described by the diffusion equation,…”

“…We have developed a new technique to observe Li diffusion coefficients in electrode materials and solid electrolytes by combining isotope exchange and secondary ion mass spectrometry (SIMS). [29][30][31][32] The tracer diffusion coefficient (D*) has been determined by analysing the distribution of Li isotopes. In particular, SIMS diffusion measurements have the following advantages: (1) application to electron-ion mixed conductors, 33 (2) determination of interfacial exchange rates and bulk diffusion coefficients, 34,35 and (3) application to grain boundary diffusion.…”

“…In this study, we have conducted a detailed measurement on the composition dependence of D* and D in c-axis oriented Li x CoO 2 thin films. We used a technique called the 'stepisotope-exchange method', 32 which enables the tracer diffusion measurements on thin-film electrodes even at room temperature. Instead of the space profile measurement used so far, 29,30 the time-dependence of the 6 Li isotope concentration in the Li x CoO 2 thin film, which comes into contact with a 6 Li-enriched electrolyte to exchange Li ions, was measured.…”