Semi-empirical approach to diffusion kinetics in biphasic materials

In this study, a novel semi-empirical approach was proposed to analyze the diffusion behaviors of lithium in electrode materials in which phase transitions accompany the intercalation and deintercalation of lithium. The existing methods obtain diffusion coefficients by mechanically fitting electrochemical results to equations that are based on the assumption of single-phase diffusion. In contrast, the method proposed herein quantifies the amount of lithium that participates in a real reaction and numerically obtains the diffusion coefficient value relative to that extent of lithium reaction, thereby achieving practical diffusion behavior quantification. This method was applied to the analysis of lithium diffusion behavior in a Li1+δ[Ti5/3Li1/3]O4 electrode that has a wide voltage plateau (two-phase coexistence region). The apparent diffusion coefficient was estimated as 10−15–10−14 cm2 s−1, and the apparent activation energy was determined as ca. 40 kJ mol−1, which was a similar value as previously reported theoretical values.