The development and industrial application of advanced lithium based energy-storage materials are directly related to the innovative production techniques and the usage of inexpensive precursor materials. Flame spray pyrolysis (FSP) is a promising technique that overcomes the challenges in the production processes such as scalability, process control, material versatility, and cost. In the present study, phase pure anode material LiTiO (LTO) was designed using FSP via extensive systematic screening of lithium and titanium precursors dissolved in five different organic solvents. The effect of precursor and solvent parameters such as chemical reactivity, boiling point, and combustion enthalpy on the particle formation either via gas-to-particle (evaporation/nucleation/growth) or via droplet-to-particle (precipitation/incomplete evaporation) is discussed. The presence of carboxylic acid in the precursor solution resulted in pure (>95 mass %) and homogeneous LTO nanoparticles of size 4-9 nm, attributed to two reasons: (1) stabilization of water sensitive metal alkoxides precursor and (2) formation of volatile carboxylates from lithium nitrate evidenced by attenuated total reflection Fourier transform infrared spectroscopy and single droplet combustion experiments. In contrast, the absence of carboxylic acids resulted in larger inhomogeneous crystalline titanium dioxide (TiO) particles with significant reduction of LTO content as low as ∼34 mass %. In-depth particle characterization was performed using X-ray diffraction with Rietveld refinement, thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry, gas adsorption, and vibrational spectroscopy. High-resolution transmission electron microscopy of the LTO product revealed excellent quality of the particles obtained at high temperature. In addition, high rate capability and efficient charge reversibility of LTO nanoparticles demonstrate the vast potential of inexpensive gas-phase synthesis for energy-storage materials.
We study the transformation that WO3 nanoparticles produced by Flame Spray Pyrolysis undergo when subjected to heating and cooling cycles by means of accurate in situ XRD and HRTEM investigations supported by atomistic modeling at the level of Density Functional Theory. As-deposited particles, initially in the monoclinic phase, reversibly transform into the high-temperature orthogonal and tetragonal phases (and vice versa) irrespective of the direction of the temperature gradient. However, upon heating the particles experience an irreversible change of morphology as a consequence of evaporation/reprecipitation processes leading to the formation of larger particles, some of which are elongated (aspect ratios of up to 3 : 1) along various crystallographic directions of the tetragonal unit cell. On the basis of the results of extensive ab initio thermodynamic calculations of surface energies and first-principles molecular dynamics simulations of the growth process, we rationalize these findings in terms of different surface stabilities and reactivities towards gas-phase deposition of molecular WO3 presented by the nanocrystal facets
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