Kinetics of Li[sup +] Ion Diffusion into FePO[sub 4] and FePON Thin Films Characterized by AC Impedance Spectroscopy

Amorphous iron phosphate ( Fe P O 4 ) and nitrided iron phosphate (FePON) thin films have been fabricated by radio-frequency sputtering in ambience of oxygen and nitrogen, respectively. These thin film electrodes were applied in a variety of sandwich structures and all-solid-state thin film batteries of Li ∕ Li P O N ∕ Fe P O N ( Fe P O 4 ) to examine the electrochemical kinetics by alternate-current impedance. Through the insertion of N, the ionic conductivity of FePON was greatly enhanced, but the electronic conductivity of Fe P O 4 was reduced. The charge-transfer resistance of FePON was dramatically decreased and the chemical diffusion coefficient of Li + ions into FePON was increased by the doping of N. A different electrochemical performance of FePON and Fe P O 4 was observed from cyclic voltammetry and charge/discharge measurements of all-solid-state batteries. The FePON, where 1.1 Li per FePON was inserted at the end of discharge, exhibited higher capacity than Fe P O 4 . The polarization of FePON was observed because of the asymmetrical redox peaks below 2.5 V .