Tracer Diffusion Coefficients of Li Ions in Li_xMn₂O₄Thin 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 6Li isotope concentration ratio in the LixMn2O4 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 LixMn2O4, 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, Db*). This behavior is well explained by a vacancy diffusion model for the α phase on LixMn2O4 (0.77 < x < 1.0). Chemical diffusion coefficients D 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.