The reaction mechanism occurring during the (de)intercalation of sodium into the host olivine FePO4 structure is thoroughly analysed through a combination of structural and electrochemical methods. In situ XRD experiments have confirmed that the charge and discharge reaction mechanisms are different and have revealed the existence of a solid solution domain from 1 < x < 2/3 in Na(x)FePO4 upon charge. The second part of the charge proceeds through a 2-phase reaction between Na(2/3)FePO4 and FePO4 with strongly varying solubility limits. The strong cell mismatch between Na(2/3)FePO4 and FePO4 enhances the effects of the diffuse interface and therefore varying solubility limits are first observed here in micrometric materials.

The solid state synthesis and electrochemical characterization of pure P2- and O3-Na2/3Fe2/3Mn1/3O2have been carried out.

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We report a manganese-rich P2/O3-cathode material for Na-ion batteries demonstrating suppression of the P2 to OP4 phase transition upon charge.

Fundamental understanding of the physical phenomena and electrochemical reactions occurring in metalair batteries is critical for developing rational approaches towards high-performing Na-O 2 battery cathodes. In this context, air cathode porosity plays a key role in battery performance, influencing oxygen supply and hence oxygen reduction and evolution reaction kinetics (ORR/OER). Graphene-based aerogels offer great versatility as air-cathodes due to their low density, high electronic conductivity and adjustable porosity. Reduced graphene aerogels with different porosities are examined where high meso-macroporosity and a narrow macropore size arrangement exhibit the best electrode performance among all studied materials (6.61 mA h cm À2 ). This is ascribed to the particular macroporous 3D structure of graphene-based electrodes, which favours the diffusion of oxygen to the defect sites in graphene sheets. An outstanding cycle life is achieved by using the pore-tuned cathode, leading to 39 cycles (486 h) at 0.5 mA h cm À2 with very low overpotential (250 mV) and efficiency over 95%. The cyclability is further increased to 745 h (128 cycles) by decreasing the capacity cut-off. This study shows that tuning of material porosity opens a new avenue of research for achieving Na-O 2 batteries with high performance by maximizing the effective area of the electrodes for the ORR/OER. Fig. 1 SEM images of the rGO (a) film and aerogels, (b) ArGO_U and (c) ArGO_N. (Inset a) Cross-sectional SEM image of rGO film.Scheme 1 Schematic illustration of the proposed mechanism as a function of the different 2D and 3D arrangements of GSs on the graphenederived cathodes.This journal is

NaFePO 4 is known as a promising intercalation cathode material for sodium-ion batteries. During the electrochemical reaction, an intermediate phase forms with a composition close to Na ≈2/3 FePO 4 , whose crystal structure is defined with a supercell that results from both Na/vacancy and charge ordering. In this work, we present a detailed study of this superstructure through synchrotron powder X-ray diffraction and electron microscopy studies.

Olivine NaFePO4 has recently attracted the attention of the scientific community as a promising cathode material for Na-ion batteries. In this work we combine density functional theory (DFT) calculations and high resolution synchrotron X-ray diffraction (HRXRD) experiments to study the phase stability of NaxFePO4 along the whole range of sodium compositions (0 ≤x≤ 1). DFT calculations reveal the existence of two intermediate structures governing the phase stability at x = 2/3 and x = 5/6. This is in contrast to isostructural LiFePO4, which is a broadly used cathode in Li-ion batteries. Na2/3FePO4 and Na5/6FePO4 ground states both align vacancies diagonally within the ab plane, coupled to a Fe(2+)/Fe(3+) alignment. HRXRD data for NaxFePO4 (2/3 < x < 1) materials show common superstructure reflections up to x = 5/6 within the studied compositions. The computed intercalation voltage profile shows a voltage difference of 0.16 V between NaFePO4 and Na2/3FePO4 in agreement with the voltage discontinuity observed experimentally during electrochemical insertion.