Rate dependence of the reaction mechanism in olivine NaFePO4 Na-ion cathode material

When electrode materials are driven away from equilibrium conditions, significant differences occur in their (dis)charge mechanism. In this paper, the changes in the phase transformations of the olivine NaFePO4 are examined as a function of the cycling rate from the thorough analyses of both the voltage-composition profiles of galvanostatic cycling, and operando laboratory and synchrotron X-ray diffraction experiments. While the charge mechanism remains unchanged at all C-rates, high cycling rates enable to unveil 2 concomitant biphasic reactions appearing as a 3-phase state at lower rates. This finding allows rationalising the asymmetries observed at low rates, which are rooted in different reaction kinetics.