title: Developing nebulized spray pyrolysis as a synthesis route for energy materials: Composite electrolytes and mixed electron-proton-conductors
Creators: name: Waidha, Aamir Iqbal
Publisher: name: UNSPECIFIED
Year: 2022

description: This thesis demonstrates work related to the development and understanding of composite solid electrolytes for all-solid-state lithium-ion batteries and the role of perovskite materials as bifunctional catalyst towards oxygen reduction as well as oxygen evolution reactions for their application in fuel cells and metal-air batteries. The material synthesis of lithium garnets and barium-rich cobaltates and ferrates is carried out via nebulized spray pyrolysis which provides particle morphologies and sizes that may prove to be advantageous for their use in such energy related applications. All-solid-state lithium-ion batteries consisting of solid electrolytes have the capability for meeting mobile high energy density storage demands as required by electric vehicles in addition to their higher safety. Although variety of different inorganic and organic solid electrolytes have been researched over the years for their application within such batteries, each of these electrolytes have inherent issues like high charge transfer resistance at the electrode/electrolyte interface, mechanical and chemical instabilities at the electrode/electrolyte interface, lower ionic conductivity compared to that of liquid electrolytes and most importantly susceptibility to moisture. In this respect, composite electrolytes comprising of embedded inorganic ceramic fillers and a polymer plus a Li salt matrix can benefit from high Li-ion conductivity and wide electrochemical operational window of the inorganic fillers whereas the polymer can provide good mechanical properties, which may result in low electrode/electrolyte interfacial resistance. Most importantly composite electrolytes may provide mechanical stability against a metallic lithium anode, which is the “holy grail anode” for all-solid-state lithium-ion batteries. Within this thesis, both Li-ion active and inert ceramic fillers were used to prepare composite electrolytes via a solvent free approach as opposed to conventional solvent-based approaches, where a ceramic filler is first dispersed in a solvent like acetonitrile followed by the addition of Li-salt and the polymer. On the other hand, the use of cryo-milling as a solvent free approach helps in preventing the exposure of sensitive ceramic fillers like garnets to moisture as well as limiting the post membrane fabrication heating steps for solvent removal. Blends of composite electrolytes ranging from “ceramic in polymer” to “polymer in ceramic” were studied to establish the role of filler size, filler composition and filler to polymer weight fraction on the Li-ion conductivity via the use of electrochemical impedance spectroscopy. In general, the ionic conductivity is found to decrease upon the increase in ceramic weight fraction and alternative Li-ion transport pathways become accessible depending upon the filler to polymer weight fraction. An attempt is made to investigate the Li-ion transport pathways within composite electrolytes via electrochemical impedance spectroscopy. The results indicate that within polymer-rich composite electrolytes the composition of the filler is of less relevance compared to that of particle size and morphology, whereas for ceramic-rich composite electrolytes the intrinsic conductivity of the ceramic plays a significant role towards the ionic conductivity. In addition, interfacial chemical compatibility between the garnet/PEO interface is studied via electrochemical impedance spectroscopy and hints towards the need for improving the chemical stability of this interface in order to realize the garnet-rich composite electrolytes. To improve the ionic conductivity within the garnet-rich composite electrolytes, a novel idea based on PEO infiltration of pre-heat treated porous aluminum-doped garnet network is demonstrated. Such a porous thin film network is obtained via the use of as-synthesized garnet powders obtained from nebulized spray pyrolysis followed by spin coating. A pre-heat treated backbone would in principle limit the particle/particle interfacial Li-ion transport resistances compared to that of loose mechanical contact of garnet particles within the particulate-based composite electrolytes. The porous microstructure of the garnet thin films along with the successful polymer infiltration is confirmed via scanning electron microscopy. The polymer infiltration results in a conductivity of 5.3 x 10-7 S cm-1 at 298 K which is six orders of magnitude higher than pristine aluminum-doped garnet thin film and an order of magnitude higher than the particulate based aluminum-doped garnet containing composite electrolyte with similar filler to polymer weight ratios. The powder synthesis versatility of nebulized spray pyrolysis also allows for the preparation of new perovskite-based barium-rich cobaltates and ferrates, that can be used as bifunctional catalyst towards oxygen reduction reaction and oxygen evolution reaction. Developing new bifunctional catalyst is important to overcome sluggish kinetics of oxygen reduction reaction and oxygen evolution reaction. Herein, nebulized spray pyrolysis offers advantage of synthesizing nano-particles that offer high surface area that can prove to be beneficial towards catalytic performance. Using nebulized spray pyrolysis as the synthesis technique series of compounds with the composition BaFe1−xCoxO3−y−δ(OH)y were synthesized and investigated for the first time. The whole series was found to crystallize in the orthorhombic space group Cmcm. Iodometric titration and Mössbauer spectroscopy indicated that Co is more flexible with respect to its oxidation state and is present in 2+/3+ state depending on the Co content whereas Fe was found to maintain 3+ oxidation state irrespective of the composition. Impedance studies indicate an enhanced electronic conductivity as the Co content increases with the x=1 composition demonstrating a conductivity of 10-4 S cm-1 at 298 K that is four orders of magnitude higher than the x=0 member. Further, the whole series was investigated for its bifunctional catalytic performance towards oxygen reduction reaction and oxygen evolution reaction. To highlight the importance of nebulized spray pyrolysis towards the synthesis of such oxide hydroxide phases, attempts were made to synthesize similar compositions via solid-state route. In this context two new phases were found which had not been reported so far. i) BaCoO2.67 with Co in mixed 3+/4+ oxidation state and occupying three different coordination environments i.e. 4-, 5- and 6-fold. This phase is synthesized via topochemical oxidation of BaCoO2.46. The structure is found to be isotypic to BaFeO2.33F0.33 and BaFeO2.67, both crystallizing in the monoclinic space group P21/m, which is related to the cubic aristotype structure with space group Pm-3m via group-subgroup relations. The mixed valence of Co results in the electronic conductivity of an order of 10-4 S cm-1 at 298 K which similar to that of the x=1 member of the series BaFe1−xCoxO3−y−δ(OH)y. Interestingly, oxygen reduction and oxygen evolution activity of this compound was found to be comparable to that of benchmark perovskite catalyst Ba0.5Sr0.5Co0.8Fe0.2O3–δ, which could be due to the intermediate spin state of Co3+ resulting in the electronic configuration of t2g5eg1. ii) A new highly oxygen deficient tetragonal BaCoO2+δ phase with square planar coordination of Co2+ is synthesized for the first time. The structure and its magnetic and electronic properties are discussed. This new modification is different compared to already known triclinic modification of BaCoO2 with four-fold tetrahedral coordination of Co2+. Although the compound could not be characterized for its catalytic activity due to its metastable nature, the magnetic moment (3.7 μ_B) obtained from the neutron data suggests a high spin state for Co2+ which implies an eg occupancy of 2 for this compound whereas 1 is desired for a good bifunctional catalyst. Interestingly, this is the first report on the high temperature synthesis of such highly oxygen deficient perovskite phase, which traditionally have been synthesized via the low temperature hydride reduction method.