PEO Infiltration of Porous Garnet-Type Lithium-Conducting Solid Electrolyte Thin Films

Composite electrolytes containing lithium ion conducting polymer matrix and ceramic filler are promising solid-state electrolytes for all solid-state lithium ion batteries due to their wide electrochemical stability window, high lithium ion conductivity and low electrode/electrolyte interfacial resistance. In this study, we report on the polymer infiltration of porous thin films of aluminum-doped cubic garnet fabricated via a combination of nebulized spray pyrolysis and spin coating with subsequent post annealing at 1173 K. This method offers a simple and easy route for the fabrication of a three-dimensional porous garnet network with a thickness in the range of 50 to 100 µm, which could be used as the ceramic backbone providing a continuous pathway for lithium ion transport in composite electrolytes. The porous microstructure of the fabricated thin films is confirmed via scanning electron microscopy. Ionic conductivity of the pristine films is determined via electrochemical impedance spectroscopy. We show that annealing times have a significant impact on the ionic conductivity of the films. The subsequent polymer infiltration of the porous garnet films shows a maximum ionic conductivity of 5.3 × 10⁻⁷ S cm⁻¹ at 298 K, which is six orders of magnitude higher than the pristine porous garnet film

On the Surface Modification of LLZTO with LiF via a Gas-Phase Approach and the Characterization of the Interfaces of LiF with LLZTO as Well as PEO+LiTFSI

In this study we present gas-phase fluorination as a method to create a thin LiF layer on Li₆.₅La₃Zr₁.₅Ta₀.₅O₁₂ (LLZTO). We compared these fluorinated films with LiF films produced by RF-magnetron sputtering, where we investigated the interface between the LLZTO and the deposited LiF showing no formation of a reaction layer. Furthermore, we investigated the ability of this LiF layer as a protection layer against Li₂CO₃ formation in ambient air. By this, we show that Li₂CO₃ formation is absent at the LLZTO surface after 24 h in ambient air, supporting the protective character of the formed LiF films, and hence potentially enhancing the handling of LLZTO in air for battery production. With respect to the use within hybrid electrolytes consisting of LLZTO and a mixture of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), we also investigated the interface between the formed LiF films and a mixture of PEO+LiTFSI by X-ray photoelectron spectroscopy (XPS), showing decomposition of the LiTFSI at the interface

Developing nebulized spray pyrolysis as a synthesis route for energy materials: Composite electrolytes and mixed electron-proton-conductors

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

PEO Infiltration of Porous Garnet-Type Lithium-Conducting Solid Electrolyte Thin Films

Composite electrolytes containing lithium ion conducting polymer matrix and ceramic filler are promising solid-state electrolytes for all solid-state lithium ion batteries due to their wide electrochemical stability window, high lithium ion conductivity and low electrode/electrolyte interfacial resistance. In this study, we report on the polymer infiltration of porous thin films of aluminum-doped cubic garnet fabricated via a combination of nebulized spray pyrolysis and spin coating with subsequent post annealing at 1173 K. This method offers a simple and easy route for the fabrication of a three-dimensional porous garnet network with a thickness in the range of 50 to 100 µm, which could be used as the ceramic backbone providing a continuous pathway for lithium ion transport in composite electrolytes. The porous microstructure of the fabricated thin films is confirmed via scanning electron microscopy. Ionic conductivity of the pristine films is determined via electrochemical impedance spectroscopy. We show that annealing times have a significant impact on the ionic conductivity of the films. The subsequent polymer infiltration of the porous garnet films shows a maximum ionic conductivity of 5.3 × 10−7 S cm−1 at 298 K, which is six orders of magnitude higher than the pristine porous garnet film

Compositionally driven relaxor to ferroelectriccrossover in (1-x)Na0.5Bi0.5TiO3–xBiFeO3(0rxr0.60)

Na0.5Bi0.5TiO3-based materials have been widely investigated over the past two decades due to their giant strain and stable mechanical quality factor, which render them potential alternatives to lead-based materials. One of the limiting factors of these materials has been the relatively lower depolarization temperatures, which restrict their use in high power ultrasonics. This work explores the phase evolution and electromechanical properties of (1 − x)Na0.5Bi0.5TiO3–xBiFeO3 (x = 0–0.60, labelled as NBT–100xBFO) and demonstrates a composition-induced relaxor-to-ferroelectric crossover. With increasing BFO content, the samples exhibit R3c symmetry and the unit cell volume increases monotonously. The electromechanical properties indicate a criticality at x = 0.30, beyond which, NBT–100xBFO becomes ferroelectric. The unit cell distortions reflect the changes observed in the electrical response. These results are further corroborated by the temperature dependent strain-field hysteresis and lamellar domain structure in the TEM images of unpoled NBT–50BFO, indicating a stable, long-range ferroelectric order. The outcome of this study indicates that NBT–100xBFO could be a potential high temperature ferroelectric, which can be further tailored to surpass the existing temperature limits of the NBT class of materials

Conductivity enhancement within garnet-rich polymer composite electrolytes via the addition of succinonitrile

All-solid-state lithium-ion batteries (ASSLIBs) are promising alternatives to conventional organic electrolyte-based batteries due to their higher safety and higher energy densities. Despite advantages, ASSLIBs suffer from issues like high charge transfer resistances due to the brittleness of the inorganic solid electrolyte and chemical instabilities at the lithium/electrolyte interface. Within this work, we investigate composite electrolytes based on garnet-type Li6.4La3Zr1.4Ta0.6O12 (LLZTO), polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTSFI), prepared via a solvent free cryo-milling approach in contrast to conventional solvent mediated synthesis. Compositions ranging from polymer-rich to garnet-rich systems are investigated via X-ray diffraction, Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) in order to determine the compatibility of the cryo-milling process towards membrane fabrication along with the possible chemical interactions between the composite membrane components. Electrochemical impedance spectroscopy (EIS) is used to study the role of ceramic to Polymer weight fraction on ionic conductivity. It is shown that the addition of succinonitrile (SCN) to the garnet-rich composite electrolytes can significantly improve the ionic conductivity compared to the SCN free composite electrolytes

Conductivity enhancement within garnet‐rich polymer composite electrolytes via the addition of succinonitrile

All‐solid‐state lithium‐ion batteries (ASSLIBs) are promising alternatives to conventional organic electrolyte‐based batteries due to their higher safety and higher energy densities. Despite advantages, ASSLIBs suffer from issues like high charge transfer resistances due to the brittleness of the inorganic solid electrolyte and chemical instabilities at the lithium/electrolyte interface. Within this work, we investigate composite electrolytes (CEs) based on garnet‐type Li₆.₄La₃Zr₁.₄Ta₀.₆O₁₂ (LLZTO), polyethylene oxide, and lithium bis(trifluoromethanesulfonyl)imide, prepared via a solvent‐free cryo‐milling approach in contrast to conventional solvent‐mediated synthesis. Compositions ranging from polymer‐rich to garnet‐rich systems are investigated via X‐ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy in order to determine the compatibility of the cryo‐milling process toward membrane fabrication along with the possible chemical interactions between the composite membrane components. Electrochemical impedance spectroscopy is used to study the role of ceramic to polymer weight fraction on ionic conductivity. It is shown that the addition of succinonitrile (SCN) to the garnet‐rich CEs can significantly improve the ionic conductivity compared to the SCN‐free CEs

Reversible Tuning of Magnetization in a Ferromagnetic Ruddlesden–Popper‐Type Manganite by Electrochemical Fluoride‐Ion Intercalation

Electrical tuning of materials' magnetic properties is of great technological interest, and in particular reversible on/off switching of ferromagnetism can enable various new applications. Reversible magnetization tuning in the ferromagnetic Ruddlesden–Popper manganite La₂₋₂ₓSr₁₊₂ₓMn₂O₇ by electrochemical fluoride‐ion (de)intercalation in an all‐solid‐state system is demonstrated for the first time. A 67% change in relative magnetization is observed with a low operating potential of <1 V, negligible capacity fading, and high Coulombic efficiency. This system offers a high magnetoelectric voltage coefficient, indicating high energy efficiency. This method can also be extended to tune other materials' properties in various perovskite‐related materials

Reversible Tuning of Magnetization in a Ferromagnetic Ruddlesden–Popper‐Type Manganite by Electrochemical Fluoride‐Ion Intercalation

Electrical tuning of materials' magnetic properties is of great technological interest, and in particular reversible on/off switching of ferromagnetism can enable various new applications. Reversible magnetization tuning in the ferromagnetic Ruddlesden-Popper manganite La2-2xSr1+2xMn2O7 by electrochemical fluoride-ion (de)intercalation in an all-solid-state system is demonstrated for the first time. A 67% change in relative magnetization is observed with a low operating potential of <1 V, negligible capacity fading, and high Coulombic efficiency. This system offers a high magnetoelectric voltage coefficient, indicating high energy efficiency. This method can also be extended to tune other materials' properties in various perovskite-related materials

Synergistic effects of Eu and Nb dual substitution on improving the thermoelectric performance of the natural perovskite CaTiO3

Driven by the development of sustainable and regenerable energy conversion materials, the mineral perovskite (CaTiO3) is considered to be a potential candidate for large-scale high-temperature applications owing to its abundance, light-weight, non-toxicity, and low cost. A series of compounds with the nominal composition Ca1–xEuxTi0.9Nb0.1O3 (0 ≤ x ≤ 0.4) has been synthesized and studied in this paper. The phase purities and crystal structures were evaluated by powder X-ray diffraction (XRD) and subsequent Rietveld analysis. Through X-ray photoelectron spectroscopy (XPS) characterization, the by far dominating valence states of Ti and Nb are confirmed to be +4 and + 5 in Ca1-xEuxTi0.9Nb0.1O3 compounds, respectively. Dual substitution by Nb and Eu yields a synergistic effect of improving electrical transport properties and simultaneously suppressing thermal conductivity. The former is mainly attributed to the d–f electron exchange induced by the strong hybridization of Eu 4f, Nb 4d, and Ti 3d orbitals. The latter is mostly attributed to the dominant phonon scattering by the mass fluctuation originating from the large mass contrast of Eu and Ca. The results demonstrate the evolution of insulating CaTiO3 to metallic-like conduction performance with increasing Eu content. Due to the largest power factor and lowest thermal conductivity, the sample Ca0.8Eu0.2Ti0.9Nb0.1O3 exhibits the maximum ZT of up to 0.3 at around 1173 K