Hartmut Wiggers

Solid-state sodium batteries are attractive next-generation energy-storage systems, combining the safety of inorganic solid electrolytes with the resource and cost benefits of sodium-based chemistries. Among Na+ conductors, NaSICON-type Na3Zr2Si2PO12 (NZSP) is particularly promising due to its high conductivity and stability, yet its practical implementation is challenged by energy-intensive ceramic processing, typically requiring prolonged high-temperature sintering for densification. Here, spray-flame synthesis (SFS) is introduced as a scalable route to nanoparticulate NZSP precursors, offering nanometer-scale compositional homogeneity beyond conventional solid-state synthesis and greater compositional flexibility than sol–gel approaches. Rather than forming crystalline NZSP directly, SFS yields a nanostructured intermediate consisting of ∼5 nm ZrO2 domains embedded in an amorphous Na–Si–P–O matrix, enabling NaSICON formation at ∼980 °C and densification to ∼97% within 2 h at 1140 °C. Systematic variation of the Si/P ratio and substitution of Zr4+ with Mg2+ or Ca2+ increase the Na content per formula unit, resulting in reduced lattice distortion and lower activation energies. Si-rich and Mg-substituted compositions reach conductivities up to 1.16 mS cm–1 with activation energies of 0.299(2)–0.327(6) eV. We have discovered that macroscopic conductivity is governed not by composition alone but by the interplay of the crystal structure, phase stability, densification, and grain-boundary chemistry.

This study provides insights into the relationship between particle properties and dispersion stability of graphene oxide (GO) and reduced graphene oxide (rGO), focusing on the roles of initial oxidation and exfoliation by different expansion of powders. A series of graphene-related materials with controlled C/O ratios and specific surface areas was prepared via thermal and plasma-assisted reduction. The dispersion behavior was systematically analyzed using space- and time-resolved analytical centrifugation across a wide range of organic solvents and aqueous formulations with polymer and surfactant additives. The results reveal that dispersion stability is governed by different particle properties depending on the medium. In aqueous dispersions without additives, stability is primarily controlled by the particles’ degree of oxidation, where the presence of hydroxyl groups in GO enables stabilization. In organic liquids, the exfoliation, as determined by the powder expansion, is the dominant factor: highly exfoliated rGO exhibits significantly improved dispersion stability across multiple solvent systems, whereas rGO with limited exfoliation remains unstable regardless of formulation. Additive effects in aqueous media further depend on particle properties. Anionic polymers and surfactants with extended hydrophobic chains enhance stability, while non-ionic additives are generally less effective. Overall, the results demonstrate that neither oxidation nor expansion alone is sufficient to predict dispersion behavior; instead, their interplay must be considered in the context of the surrounding medium. The findings gained address conflicts which arose in the past on the strategy to prepare liquid dispersions of graphene-related materials and thereby facilitate the technological integration of this class of two-dimensional materials.

In lithium‐ion battery silicon anodes, achieving reliable electrode processing, consistent electrochemical performance, and mechanically stable electrodes remains a challenge. One strategy to overcome these limitations is to assemble silicon–carbon (Si/C) nanoparticles with alloy‐like Si–C bonding into micrometer‐sized secondary particles (“supraparticles”) by spray drying. Without additives, the supraparticles are irregular and fragile; therefore, a polymeric binder is introduced to enable their controlled formation. This study establishes a foundational methodological framework to investigate the interactions of poly (acrylic acid) (PAA) with Si/C during supraparticle formation. In aqueous dispersion, PAA adsorbs onto the Si/C surface via weak hydrogen bonding. Subsequent ionization of carboxyl groups generates electrostatic repulsion between particles, stabilizing the dispersion. Upon spray drying, PAA forms a flexible, hydrogen‐bonded polymer network that crosslinks particles without covalent attachment to silanols. Drying electrodes at 120°C preserves this network, enabling volume‐change accommodation and high‐capacity retention (1996 mAh g ⁻¹ at the 100th cycle). In contrast, drying at 200°C generates PAA anhydrides and ester linkages with Si–OH groups, which reduce compliance, leading to lower Coulombic efficiency and faster fading (1353 mAh g ⁻¹ at the 100th cycle). These findings highlight binder chemistry as a key lever in designing durable Si/C electrodes.

Electrofluids, concentrated suspensions of electrically conductive particles, have recently attracted attention for their potential as soft conductors. In this study, we investigated the influence of morphological differences in 2D filler materials (graphene‐like) on the electromechanical properties of glycerol‐based electrofluids. Plasma synthesized few‐layer graphene (FLG), chemically exfoliated multi‐layer graphene (MLG), and bulk graphite were used as 2D fillers, which differed in specific surface area, aspect ratio, and intrinsic stiffness. Results showed that both electrical and mechanical percolation thresholds decreased with increasing filler aspect ratio. The high aspect ratio of the FLG promotes network formation at low filler concentrations, conferring its electrofluids with the lowest electrical (0.16 wt%) and mechanical (0.63 wt%) percolation threshold. Rheological amplitude sweeps revealed a larger linear viscoelastic region for electrofluids containing few‐layer graphene, caused by their reduced internal stiffness and higher aspect ratio. This large capacity for storing elastic energy of FLG‐electrofluids makes them almost insensitive to uniaxial tensile strain when encapsulated in elastomers, leading to gauge factors below 1, ideal for soft electrical resistors. In contrast, MLG‐electrofluids exhibited good sensing properties as strain gauges. The presented study on structure‐property relationships helps for rational design of electrofluids with tailored electromechanical properties that can be tuned for different use cases.

Amorphous, substoichiometric silicon carbide (a‐SiC _x ) is explored as an anode material for lithium‐ion batteries. Silicon‐rich a‐SiC _x nanopowders with different carbon concentrations yet otherwise comparable properties are produced and processed into electrodes. By electrochemical testing, performance data for the different materials are obtained, and in a second step, correlated with further information from detailed analysis of the lithiation and delithiation behavior as well as electrode morphology evolution dependent on the carbon concentration. An increased lithium loss in the first cycle can be linked to the carbon content in the sample, suggesting a matrix‐phase‐like behavior. In the first cycles, roughly 1 atom of lithium is lost per atom of carbon in the sample, and additionally, each carbon atom inertises up to 2 Si atoms. Cycling stability, on the other hand, is highest for 25.9 at.% C, with no capacity fade after 200 cycles and excellent rate capability. With a lower anode potential and reduced voltage hysteresis, especially during fast‐charging, the carbon‐rich samples show further benefits. Data from fast‐charging and impedance analysis indicate that the solid electrolyte interphase (SEI) remains stable over long‐term cycling for the carbon‐rich materials. Postmortem analysis confirms the assumption that the material is operating around a rather high lithiation state.

Supraparticles are hierarchically structured assemblies of nanoparticles whose size and structural features such as shape, surface roughness, and porosity critically govern their functionality in applications such as energy storage and catalysis. While size and porosity can be measured reliably using established techniques, there is a lack of robust and statistically meaningful methods for assessing supraparticle shape. In this work, we present a novel methodology for estimating supraparticle shape based on mercury intrusion porosimetry (MIP). First, we were able to unravel that the characteristic two-stage intrusion profile of supraparticles corresponds to inter- and intra-supraparticle pores. Then, deconvolution of the MIP pore size distributions enabled separation of these contributions, while assignment of their physical origin was realized by comparison of the extracted intra-supraparticle peaks with the pore size distributions of constituent nanoparticles. By employing a design of experiment that enabled varying supraparticle size and shape independently, we established that the extent of bimodality in MIP pore size distributions is strongly correlated with supraparticle shape. Quantitative shape descriptors obtained from scanning electron microscope-based circularity measurements were compared with statistical metrics of bimodality (Ashman’s D and peak separation) derived from deconvoluted distributions. A clear relationship emerged: supraparticles with higher circularity exhibited more pronounced bimodality, whereas irregular particles showed diminished or poorly resolved bimodal features. This work therefore introduces a statistically reliable approach for simultaneous evaluation of supraparticle porosity and shape, with direct implications for scale up, process optimization and quality control in industrial spray-drying processes.

The transition to a sustainable chemical industry necessitates processes that utilize renewable feedstocks and energy sources. The selective oxidation of bio-based ethylene glycol (EG) to valuable chemicals like glycolic acid as a precursor for bioplastics and formic acid as a hydrogen carrier represents a promising pathway. However, conventional methods often rely on noble metals or unsustainable oxidants. This study explores the potential of non-noble Co3O4 catalysts, synthesized via hard templating (HT), co-precipitation (CP), and spray-flame synthesis (SF), for the thermal and photothermal oxidation of EG. We demonstrate that the synthesis method profoundly impacts catalytic performance and catalyst stability. The SF-derived catalyst exhibited superior initial activity, while the HT catalyst showed exceptional reusability without or only minor deactivation. In addition, it is highlighted that the introduction of light as an additional stimulus significantly enhanced EG conversion by direct utilization of photogenerated charge carriers. Moreover, using illumination, C–C bond cleavage leading to the formation of formic acid appears to be less dominant compared to rate-enhancements obtained by higher reaction temperatures. Thus, higher selectivity for the formation of glycolic acid is maintained. Post-catalytic characterization of the catalyst revealed that catalyst deactivation is linked to structural changes and the catalyst's inability to reoxidize Co²⁺ sites back to active Co3+. This work provides fundamental insight into the rational design of robust, non-noble metal oxide catalysts and demonstrates photothermal catalysis as a powerful strategy for advancing sustainable liquid-phase oxidation reactions.