Operando XRD studies on Bi₂MoO₆as 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 (Bi₂MoO₆) is a promising NIB anode material operating through a combined conversion/alloying mechanism. We report anoperandox-ray diffraction (XRD) investigation of Bi₂MoO₆-based anodes over 34 (de)sodiation cycles revealing both basic operating mechanisms and potential pathways for capacity degradation. Irreversible conversion of Bi₂MoO₆to Bi nanoparticles occurs through the first sodiation, allowing Bi to reversibly alloy with Na forming the cubic Na₃Bi phase. Preliminary electrochemical evaluation in half-cellsversusNa metal demonstrated specific capacities for Bi₂MoO₆to be close to 300 mAh g^-1during the initial 10 cycles, followed by a rapid capacity decay.OperandoXRD characterisation revealed that the increased irreversibility of the sodiation reactions and the formation of hexagonal Na₃Bi 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 NaBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.