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 Na_xBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

Zero Lithium Miscibility Gap Enables High-Rate Equimolar Li(Mn,Fe)PO4 Solid Solution

Forming olivine-structured Li(Mn,Fe)PO₄ solid solution is theoretically a feasible way to improve the energy density of the solid solutions for lithium ion batteries. However, the Jahn-Teller active Mn³⁺ in the solid solution restricts their energy density and rate performance. Here, as demonstrated by operando X-ray diffraction, we show that equimolar LiMn_0.5Fe_0.5PO₄ solid solution nanocrystals undergo a single-phase transition during the whole (de)lithiation process, with a feature of zero lithium miscibility gap, which endows the nanocrystals with excellent electrochemical properties. Specifically, the energy density of LiMn_0.5Fe_0.5PO₄ reaches 625 Wh kg^-1, which is 16% higher than that of LiFePO₄. Moreover, the high-performance LiMn_0.5Fe_0.5PO₄ nanocrystals are prepared by a microwave-assisted hydrothermal synthesis in pure water.

The Role of Adsorbed and Subsurface Carbon Species for the Selective Alkyne Hydrogenation Over a Pd-Black Catalyst: An Operando Study of Bulk and Surface

The selective hydrogenation of propyne over a Pd-black model catalyst was investigated under operando conditions at 1 bar making use of advanced X-ray diffraction (bulk sensitive) and photo-electron spectroscopy (surface sensitive) techniques. It was found that the population of subsurface species controls the selective catalytic semi-hydrogenation of propyne to propylene due to the formation of surface and near-surface PdC_x that inhibits the participation of more reactive bulk hydrogen in the hydrogenation reaction. However, increasing the partial pressure of hydrogen reduces the population of PdC_x with the concomitant formation of a β-PdH_x phase up to the surface, which is accompanied by a lattice expansion, allowing the participation of more active bulk hydrogen which is responsible for the unselective total alkyne hydrogenation. Therefore, controlling the surface and subsurface catalyst chemistry is crucial to control the selective alkyne semi-hydrogenation.

Electrochemical Mechanism and Effect of Carbon Nanotubes on the Electrochemical Performance of Fe1.19(PO4)(OH)0.57(H2O)0.43 Cathode Material for Li-Ion Batteries

A hydrothermal synthesis route was used to synthesize iron(III) phosphate hydroxide hydrate-carbon nanotube composites. Carbon nanotubes (CNT) were mixed in solution with Fe_1.19(PO₄)(OH)_0.57(H₂O)_0.43 (FPHH) precursors for one-pot hydrothermal reaction leading to the FPHH/CNT composite. This produces a highly electronic conductive material to be used as a cathode material for Li-ion battery. The galvanostatic cycling analysis shows that the material delivers a specific capacity of 160 mAh g^-1 at 0.2 C (0.2 Li per fu in 1 h), slightly decreasing with increasing current density. A high charge-discharge cyclability is observed, showing that a capacity of 120 mAh g^-1 at 1 C is maintained after 500 cycles. This may be attributed to the microspherical morphology of the particles and electronic percolation due to CNT but also to the unusual insertion mechanism resulting from the peculiar structure of FPHH formed by chains of partially occupied FeO₆ octahedra connected by PO₄ tetrahedra. The mechanism of the first discharge-charge cycle was investigated by combining operando X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy. FPHH undergoes a monophasic reaction with up to 10% volume changes based on the Fe³⁺/Fe²⁺ redox process. However, the variations of the FPHH lattice parameters and the ⁵⁷Fe quadrupole splitting distributions during the Li insertion-deinsertion process show a two-step behavior. We propose that such mechanism could be due to the existence of different types of vacant sites in FPHH, including vacant "octahedral" sites (Fe vacancies) that improve diffusion of Li by connecting the one-dimensional channels.