RESUMO
The disodium cobalt(ii) orthosilicate material (NCS) has been synthesized using improved solid-state (NCS-SS) and co-precipitation (NCS-CP) methods of synthesis. The Rietveld refinement of the XRD pattern of Na2CoSiO4 has demonstrated an orthorhombic crystal system with the space groups Pna21 and Pbca for NCS-SS and NCS-CP respectively. The elemental mapping of microstructures by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) showed the porous morphology and the homogenous particles of the Na2CoSiO4 powders. Their dielectric properties were measured in the frequency and temperature ranges of 0.1-106 Hz and 383-613 K respectively. Different dielectric relaxation phenomena associated with the Na+-ion migration through different paths were displayed in relation with the temperature and frequency. The decrease and increase in the dielectric properties were found to be dependent on the formation of short-range ordered structure formed after the migration of Na+-ions. In the present work, an attempt has been made to study the relation between the structural properties and the dielectric process. Thus, interesting insights into the transport behavior of Na+-ions in different chemical environments were obtained. This in turn provides an effective procedure to probe the relationship between the diffusion pathway of Na+-ions and the dielectric response.
RESUMO
A hydrothermal synthesis route was used to synthesize iron(III) phosphate hydroxide hydrate-carbon nanotube composites. Carbon nanotubes (CNT) were mixed in solution with Fe1.19(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.
RESUMO
The performance of electrode materials in lithium-ion (Li-ion), sodium-ion (Na-ion) and related batteries depends not only on their chemical composition but also on their microstructure. The choice of a synthesis method is therefore of paramount importance. Amongst the wide variety of synthesis or shaping routes reported for an ever-increasing panel of compositions, spray-drying stands out as a versatile tool offering demonstrated potential for up-scaling to industrial quantities. In this review, we provide an overview of the rapidly increasing literature including both spray-drying of solutions and spray-drying of suspensions. We focus, in particular, on the chemical aspects of the formulation of the solution/suspension to be spray-dried. We also consider the post-processing of the spray-dried precursors and the resulting morphologies of granules. The review references more than 300 publications in tables where entries are listed based on final compound composition, starting materials, sources of carbon etc.