Synthesis and Properties of a Na3V2(PO4)2F3-Based Cathode Material from a Precursor Mechanoactivated with Carbon Additives

The authors study the behavior of a mixture of NH4VO3, NH4H2PO4, and NaF in a hexane medium during short-term mechanical activation (10 min) in a Pulverisette 7 planetary mill. It is shown that mechanochemical reactions proceed intensely under these conditions to produce a highly aggregated powder containing a mixture of different sodium vanadates and ammonium phosphate vanadates. Monophasic Na3V2(PO4)2F3 forms when the powder is subjected to heat treatment. Introducing the disaggregating additive SPAN-80 prevent intensive mechanochemical reactions, thereby leading to a reduction of the content of the Na3V2(PO4)2F3 phase in the product to 75 wt %. A content of the target phase as high as 92 wt % was achieved by introducing paraffin into the reagent mixture along with SPAN-80 and subsequent heat treatment in a two-stage scheme that includes compacting the intermediate product. The optimum contents of the additive are selected, and the morphology of the products synthesized using one- and two-stage schemes are studied along with the electrochemical properties of the resulting cathode material.