Dual-role magnesium oxide: Enhancing phosphate cathodes for sodium-ion batteries

NASICON-type Na3VFe(PO4)3 is selected as a cathode material to be coated with a MgO protective film, intended to delay the degradation and improve charge transfer. Replacing vanadium with iron reduces toxicity and enhances sustainability. A scalable procedure is employed to prepare the material with three distinct coating levels (1 %, 3 %, and 5 %), followed by electrochemical testing against a bare reference sample. Unexpectedly, Mg incorporation triggers a phase transition accompanied by an expansion of the unit cell, with Mg preferentially occupying Fe sites. DFT results indicate that the Na3VFe(PO4)3-(001) / MgO interface is thermodynamically stable, and that formation of surface Mg/Fe antisite defects is possible and energetically favorable. Electrochemical tests showed a loss of Fe3+/Fe2+ redox activity at low voltage plateaus, reducing the cell capacity in 3 % and 5 % Mg samples. The charge compensation mechanism exerted by Fe2+/Fe3+ and V3+/V4+/V5+ couples is also confirmed by DFT calculations. The sample containing 1 % Mg delivers the highest capacity at high rates and retains capacity better than the bare sample after extended cycling. The improved electrochemical behavior was confirmed by galvanostatic cycling at −15 °C and 50 °C.