Operando XRD studies on Bi₂MoO₆ as anode material for Na-ion batteries

Abstract

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 an operando X-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 Bi2MoO6 to Bi nanoparticles occurs through the first sodiation, allowing Bi to reversibly alloy with Na forming the cubic Na3Bi phase. Preliminary electrochemical evaluation in half-cells vs Na metal demonstrated specific capacities for Bi2MoO6 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 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 electrically insulating Na-Mo-O matrix leading to poor conductivity in the electrode. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

5D Total Scattering Computed Tomography Reveals the Full Reaction Mechanism of a bismuth vanadate lithium ion battery anode

“…8c), presumably LiyVO4, but possibly carbon or some kind of SEI as observed for Bi2MoO6. 19 In the lower magnification EDXS-STEM maps (Fig. 8b), the vanadium and oxygen signals cover the whole sample area uniformly, supporting the theory that LiyVO4 forms an amorphous mass without long range order, while the Bi map is more mottled, showing a mixture of LixBi cluster sizes spread over the area.…”

“…8,20 Both area-summed data (increasing maximum r in the PDFs and appearance of Bragg peaks in conventional XRD) and maps from TSCT show that crystallite size increases with cycling. This has been observed in other bismuth metallate anodes (in both LIBs and NIBs) 12,19,21 , as well as for other conversion/alloying anode materials e.g. bismuth sulphide 22 .…”

“…This appears to be dependent on the level of crystallinity, with crystalline LixBi more likely to follow the route via LiBi during lithiation, perhaps due to slower kinetics in the larger crystallites. The mechanism seems to fit somewhere between those observed in operando studies of NIBs with bismuth metal 24 and Bi2MoO6 19 anodes, in which NaBi is formed as an intermediate during sodiation and desodiation; and phosphorus 8 anodes, where NaP is formed during sodiation but not during desodiation and (dis)charge rate may also influence the route followed.…”

“…This could be caused by crystal growth fed by the amorphous material (which is used up and shifted to the crystalline areas) or dissolution of the smaller particles of active material into the electrolyte (leaving only the crystalline areas). Reactions with the electrolyte have been suggested as reasons for longer term degradation of electrode materials including Prussian blue analogues 23 and Bi2MoO6 19 Post-mortem TEM shows that 5-20 nm LixBi particles sit in a LiyVO4 matrix. TSCT shows that this has no observable or fit-able order beyond the first V-O bond.…”

Benefits and Development Challenges for Conversion-Alloying Anode Materials in Na-Ion Batteries

“…Compared to binary CAMs, ternary materials are significantly less studied, however, several examples based on Bi-and Sb-based oxometallates of transition metals (BiFeO 3 , Bi 2 (MoO 4 ) 3 , BiVO 4 , Sb 2 MoO 6 , SbVO 4 , Bi 2 MoO 6 ) were reported (Durai et al, 2017;Sottmann et al, 2017;Lu et al, 2019;Pan et al, 2019;Brennhagen et al, 2022;Surendran et al, 2022). The choice of the second cation (Fe, Mo, V, etc.)…”