Operando XRD studies on Bi 2 MoO 6 as anode material for Na-ion batteries.

Based on the same rocking-chair principle as rechargeable Li-ion batteries, Na-ion batteries are promising solutions for energy storage benefiting from low-cost materials comprised of abundant elements. However, despite the mechanistic similarities, Na-ion batteries require a different set of active materials than Li-ion batteries. Bismuth molybdate (Bi 2 MoO 6 ) is a promising NIB anode material operating through a combined conversion/alloying mechanism. We report an operando x-ray diffraction (XRD) investigation of Bi 2 MoO 6 -based anodes over 34 (de)sodiation cycles revealing both basic operating mechanisms and potential pathways for capacity degradation. Irreversible conversion of Bi 2 MoO 6 to Bi nanoparticles occurs through the first sodiation, allowing Bi to reversibly alloy with Na forming the cubic Na 3 Bi phase. Preliminary electrochemical evaluation in half-cells versus Na metal demonstrated specific capacities for Bi 2 MoO 6 to be close to 300 mAh g -1 during the initial 10 cycles, followed by a rapid capacity decay. Operando XRD characterisation revealed that the increased irreversibility of the sodiation reactions and the formation of hexagonal Na 3 Bi are the main causes of the capacity loss. This is initiated by an increase in crystallite sizes of the Bi particles accompanied by structural changes in the electronically insulating Na-Mo-O matrix leading to poor conductivity in the electrode. The poor electronic conductivity of the matrix deactivates the Na x Bi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.