Tracer Diffusion Coefficients Of Li Ions In Lixmn2o4 Thin Films Observed By Isotope Exchange Secondary Ion Mass Spectrometry

Tracer diffusion coefficients D* of lithium ions in LixMn2O4 (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 Li-6 isotope concentration ratio in the LixMn2O4 film which is in contact with a Li-6-enriched electrolyte to exchange Li+ ions. A steep decrease in D* depending on the Li composition was observed for LixMn2O4, with D* = 8 x 10(-13) cm(2) s(-1) for x = 0.2 and decreasing to 1.5 x 10(-17) cm(2) s(-1) for x = 1.0 (bulk diffusion coefficient, D-b*). This behavior is well explained by a vacancy diffusion model for the a phase on LixMn2O4 (0.77 < x < 1.0). Chemical diffusion coefficients (D) over tilde 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.