Phase Transition Kinetics of LiFePO4 Biphasic Systems in Aqueous and Non-aqueous Electrolytes

Abstract

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 two-phase 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 charge-transfer resistance at the electrode/electrolyte interface in the aqueous media is also lower than that in the nonaqueous 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.