Amorphous FePO4 as a Cathode Material for Sodium ion Batteries

Abstract

Lithium-ion batteries are one of the great successes of modern electrochemistry and have been researching portable devices, energy storage system (ESS) batteries and electrical vehicle (xEVs) batteries in order to solve for energy problems in the world. Among various post-lithium ion batteries, sodium ion batteries are especially able to have a competitive in the market of large amounts of energy storage system owing to inexpensive sodium resources also applied to electrode materials for lithium ion batteries due to similar chemical characteristics. Olivine LiFePO4 material is one of cathode materials for lithium ion batteries. Although LiFePO4 has problems of kinetics such as low electrical/ionic conductivity, it has continuously been studying because of high specific capacity (170 mA h g-1), low price and stability of structure and high temperature. However, many trials of LiFePO4 applied to sodium ion batteries have trouble utilizing for cathode material. NaFePO4 maricite is electrochemically inactive because the different connectivity of Fe and Na octahedra unlike olivine which interrupts sodium ion migration pathways during sodium ion insertion/de-insertion. Amorphous FePO4 has been reported to overcome drawbacks of LiFePO4 and NaFePO4. Amorphous structure is able to one-phase reaction with a lot of charge carriers and 3-dimensional migration of Li ions. Therefore it is expected to development of battery performances. In this research, possibility of cathode material for sodium ion batteries was confirmed by using amorphous FePO4. To improve electrochemical performances of amorphous FePO4 material, carbon nanotube (CNT) and fluoro ethylene carbonate (FEC) additive were used and analyzed by using XRD, XPS, FE-SEM and Electrochemical impedance spectroscopy measurements.