Lithium-ion (Li-ion) batteries have been widely used in various kinds of electronic devices in our daily life. The use of aqueous electrolyte in Li-ion battery would be an alternative way to develop low cost and environmentally friendly batteries. In this paper, the lithium iron phosphate (LiFePO₄) thin film cathode for the aqueous rechargeable Li-ion battery is prepared by radio frequency magnetron sputtering deposition method. The XRD, SEM, and AFM results show that the film is composed of LiFePO₄ grains with olivine structure and the average size of 100 nm. Charge-discharge measurements at current density of 10 μAh cm⁻² between 0 and 1 V show that the LiFePO₄ thin film electrode is able to deliver an initial discharge capacity of 113 mAh g⁻¹. Specially, the morphological changes of the LiFePO₄ film electrode during charge and discharge processes were investigated in aqueous environment by in situ EC-AFM, which is combined AFM with chronopotentiometry method. The changes in grain area are measured, and the results show that the size of the grains decreases and increases during the charge and discharge, respectively; the relevant mechanism is discussed.