New Aspects on the Decomposition of Sodium Alanate Revealed by Small-Angle X-ray Scattering

Transition metals added to sodium aluminum hydride by high-energy ball milling have been shown to significantly enhance its absorption and desorption properties. In the present study, we have used small-angle X-ray scattering to elucidate how TiCl₃ affects the nanostructure of NaAlH₄ particles. Scattering data from as-purchased and ball milled NaAlH₄ for 6 and 15 min are compared with NaAlH₄ ball milled for the same time with 4 mol % TiCl₃. Drastic differences were noticed in the two systems which cast a new light on the decomposition of NaAlH₄, in particular on the effect of ball milling and of TiCl₃ on the morphology, grain size, and distribution of the phases. The particle morphology of pure NaAlH₄ showed significant evolution/changes during heating from room temperature to 290 °C, as evidenced by the variations in the power-low scattering parameter, α. Drastic changes were noticed in the particle surface structure during the phase transformation from NaAlH₄ to Na₃AlH₆ + Al, when the system becomes less compact and the particle surface rougher. The addition of TiCl₃ induces a different effect on both surface and mass structure, at least in the nanometer length scale considered in this study: the particles retain their surface morphology at all temperatures. Furthermore, even after short ball milling times the addition of TiCl₃ increases the system compactness with reduction of internal voids

Operando SAXS/WAXS on the a‑P/C as the Anode for Na-Ion Batteries

A complete chemical and morphological analysis of the evolution of battery electrode materials can be achieved combining different and complementary techniques. Operando small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were combined to investigate structural and electrochemical performances of an Na-ion battery, with amorphous red phosphorus in a carbon matrix (a-P/C) as the active anode material in a Swagelok-type cell. The charging process results in the formation of crystalline Na₃P, while during discharging, the anode material returns to the initial a-P/C. From the analysis of the WAXS curves, the formation of crystalline phases appears only at the end of charging. However, SAXS data show that partial reorganization of the material during charging occurs at length scales nonaccessible with conventional X-ray diffraction, corresponding to a real space ordering distance of 4.6 nm. Furthermore, the analysis of the SAXS data shows that the electrode remains dense during charging, while it develops some porosity during the discharge phase. The presented results indicate that the combination of SAXS/WAXS adopted simultaneously, and nondestructively, on a working electrochemical cell can highlight new mechanisms of reactions otherwise undetected. This method can be applied for the study of any other solid electrode material for batteries

Stability and Phase Formation in the (Li/Na)₆C₆₀–H Systems Studied by Neutron Scattering

Small-angle neutron scattering combined with powder neutron diffraction was employed to study the phase formation upon absorption/desorption of hydrogen gas in Li- and Na-intercalated fullerides. The comparative system analysis, addressing changes in the sample phase compositions and the morphology of powders, revealed a higher degree of complexity in the Na-based samples. Results also showed that the disappearance of crystalline lithium/sodium hydride during desorption was accompanied by an increased surface roughness in both systems, most likely because of the formation of a local surface strain. Furthermore, an increased specific internal surface of the studied materials upon subsequent deuterium absorption/desorption cycles was demonstrated. This could indicate moderate fracturing of crystallites