Phase Transition Kinetics of LiFePO<sub>4</sub> Biphasic Systems in Aqueous and Non-aqueous Electrolytes

Power characteristics become one of the important performance measures of lithium-ion batteries as high-power applications such as electric vehicles are emerging.Among several electrochemical steps that limit the power characteristics, phase transition kinetics is known as the limiting step for twophase coexisting (biphasic) materials.In this study, we used LiFePO4 as a model biphasic material and investigated the intrinsic factor that limits the phase transition behavior.When the same LiFePO4 electrodes were tested in non-aqueous and aqueous electrolytes, the activation energy for the aqueous system was lower.In addition, impedance measurements using 4-electrode cells show that the chargetransfer resistance at the electrode/electrolyte interface in the aqueous media is also lower than that in the non-aqueous media, suggesting more facile solvation/de-solvation process in the aqueous media.This indicates that the rearrangement of the phase transition boundary (LiFePO4/FePO4) is sufficiently fast and other factors such as charge-transfer at the electrode/electrolyte interface affects the whole reaction rate.