Effects of fluorination on the structure, magnetic and electrochemical properties of the P2-type NaxCoO2 powder

Abstract

The main goal of this research has been to investigate for the first time the effects of fluorination on the crystal structure, magnetic, and electrochemical properties of the P2-type NaxCoO2 powder. Sodium cobalt oxide with a P2-type structure is synthesized by a modified solid-state reaction consisting of alternating processes of rapid heating up to 750 °C and rapid cooling to the room temperature. The obtained powder is fluorinated using a gas-solid reaction with NH4HF2 as fluorinating agent. Fluorination causes a decrease of sodium content in the parent phase with the concurrent formation of the minor phases of Na2CO3 and NaF. The structure of NaxCoO2 in both powders is refined in P63/mmc space group. The results of the Rietveld refinement combined with the findings from the XPS measurements confirm the Na0.76CoO2 and Na0.44CoO1.96F0.04 stoichiometries for the pristine and fluorinated powders, respectively, which indicates that 4 at.% of fluorine ions per formula unit are incorporated in the structure. Preliminary electrochemical investigations have revealed an improved charge/discharge performance. The influence of fluorination on morphology and magnetic properties has also been examined.