Apparent diffusion constant and electrochemical reaction in LiFe1-chi Mn chi PO4 olivine cathodes

Olivine compounds LiFe1-xMnxPO4 (0.0 < x < 0.2) were prepared by solid-statereaction and their properties of Fe2+/Fe3+ electrochemical redox reaction were studied. The Mn2+ substitution strongly affected the electrochemical properties, such as initial capacity, capacity fading, and polarization. From the relationship between the redox peak current and the sweeping rate in the cyclic voltammetry, the apparent diffusion constants were numerically evaluated. They increased monotonically with the Mn2+ substitution and were always larger in the charging (delithiation) process than that in the discharging (lithiation) process. Additionally, the open-circuit voltage profiles in the beginning of the charge-discharge process were modified with Mn2+ substitution; the profile was changed from a flat voltage vs the reaction degree to a sloped profile, and the sloped region became wider as the substitution degree was increased. This implied that one-phase reaction, rather than the expected two-phase reaction, was attained and expanded with the substitution. It was found that there existed a relationship between the apparent diffusion constant and the one-phase reaction width; the apparent diffusion constant was enhanced with an increase in the one-phase reaction width. (c) 2007 The Electrochemical Society.