Fluorine Doping of Layered NaxCoO2 Structure

Abstract

The room temperature Na-ion secondary battery has been under focus lately due to its feasibility to compete against the already well-established Li-ion secondary battery. Transition metal oxides of general formula NaxMO2 have been investigated as potential cathode materials for sodium batteries. Layered NaxCoO2 is synthesized via solid-state method at 900 ºC in air atmosphere. Fluorine doping of the as-prepared powder is established by the use of ammonium hydrogen difluoride (NH4HF2) as a fluorinating agent. The fluorination takes place only at low temperature (200 ºC), while the treatment at higher temperatures (≥ 400 ºC) facilitates the formation of NaF. It is shown that various and controllable amounts of fluorine can be successfully incorporated into the structure. Finally, the effects of fluorine doping on both structural and electrochemical properties are examined.