Amorphous iron phosphate (FePO4) 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/LiPON/FePON (FePO4) 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 FePO4 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 FePO4 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 FePO4. The polarization of FePON was observed because of the asymmetrical redox peaks below 2.5 V. (c) 2007 The Electrochemical Society.