Using Hierarchically Structured, Nanoporous Particles as Building Blocks for NCM111 Cathodes.

Nanoparticles have many advantages as active materials, such as a short diffusion length, low charge transfer resistance, or a reduced probability of cracking. However, their low packing density makes them unsuitable for commercial battery applications. Hierarchically structured microparticles are synthesized from nanoscale primary particles by targeted aggregation. Due to their open accessible porosity, they retain the advantages of nanomaterials but can be packed much more densely. However, the intrinsic porosity of the secondary particles leads to limitations in processing properties and increases the overall porosity of the electrode, which must be balanced against the improved rate stability and increased lifetime. This is demonstrated for an established cathode material for lithium-ion batteries (LiNi0.33Co0.33Mn0.33O2, NCM111). For active materials with low electrical or ionic conductivity, especially post-lithium systems, hierarchically structured particles are often the only way to produce competitive electrodes.

Mass Transport in the Stefan-Knudsen Transition Region during Vacuum Drying at Different Pressures in a Porous Structure Resembling Battery Electrodes.

A precise understanding of the mass transport kinetics of water inside the porous structure of battery electrodes is crucial to understanding and optimizing their post-drying process. This process and the moisture management during the production of Li-ion battery electrodes adjust and remove residual water from electrodes and are cost intensive. Furthermore, the amount of residual moisture in the electrode affects device performance. Mass transport phenomena in the Stefan and Knudsen transition affect these processes. In this manuscript, we investigate the mass transport in the interparticle gas phase of a porous structure gravimetrically by a magnetic suspension balance with a conditioned (humidity, temperature, and pressure) measurement cell. Emphasis lies on the pressure, porosity, and mass transport distance dependency of the desorption process. Comparing experimental data with a simulation of the interparticle gas phase shows that the mass transport close to ambient pressure can be described by Stefan diffusion through the porous structure. The experiments show the significance of Knudsen diffusion during the mass transport toward lower pressure. A proposed diffusion-coefficient model describes the Stefan and Knudsen region with a transition function, taking the mass transport phenomena overlap into account by lower and upper limits as transition values.

Drying and Coating of Perovskite Thin Films: How to Control the Thin Film Morphology in Scalable Dynamic Coating Systems.

Hybrid perovskite photovoltaics combine high performance with the ease of solution processing. However, to date, a poor understanding of morphology formation in coated perovskite precursor thin films casts doubt on the feasibility of scaling-up laboratory-scale solution processes. Oblique slot jet drying is a widely used scalable method to induce fast crystallization in perovskite thin films, but deep knowledge and explicit guidance on how to control this dynamic method are missing. In response, we present a quantitative model of the drying dynamics under oblique slot jets. Using this model, we identify a simple criterion for successful scaling of perovskite solution printing and predict coating windows in terms of air velocity and web speed for reproducible fabrication of perovskite solar cells of ∼15% in power conversion efficiency─in direct correlation with the morphology of fabricated thin films. These findings are a corner stone toward scaling perovskite fabrication from simple principles instead of trial and error optimization.

Critical Solutal Marangoni Number Correlation for Short-Scale Convective Instabilities in Drying Poly(vinyl acetate)-Methanol Thin Films.

A new empiric correlation for the critical solutal Marangoni number as function of the Péclet and Schmidt numbers is proposed. It is based on previously published experimental flow field data in drying poly(vinyl acetate)-methanol films with an initial thickness in the range of 20-100 µm and an initial solvent load of 1 to 2 gMeOH/gPVAc, as well as newly derived concentration profile measurements and 1D drying simulations. The analysis accounts for realistic transient material properties and describes the occurrence of short-scale convective Marangoni (in)stabilities during the entire drying process with an accuracy of 9%. In addition, the proposed correlation qualitatively follows trends known from theory. As convective Marangoni instabilities in drying polymer films may induce surface deformations, which persist in the dry film, the correlation may facilitate future process design for either thin films with uniform thickness or deliberate self-assembly.

Drying Kinetics from Micrometer- to Nanometer-Scale Polymer Films: A Study on Solvent Diffusion, Polymer Relaxation, and Substrate Interaction Effects.

The drying behavior of two different polymers [polyvinyl pyrrolidone (PVP) and polyisobutylene (PIB)] with different glass transition temperatures are investigated and compared as a function of film thickness from micrometer (∼3 µm) to nanometer scale (∼10 nm). The focus of this study is to distinguish between solvent diffusion, polymer relaxation, and substrate confinement of polymer chain mobility toward the interface as the dominating mechanism of drying kinetics. Relaxation kinetics becomes more dominant when the film thickness is reduced, which is shown experimentally for the first time for nanometer-scale film thicknesses. Identical drying curves regardless of the film thickness of PVP/methanol indicate the limitation of solvent transport by relaxation kinetics. The viscoelastic relaxation behavior of the polymer/solvent film is modeled by a Maxwell element. The results are in accordance with the experimental drying curves and allow for the determination of the characteristic relaxation time. Relaxation limitation becomes relevant at high diffusion Deborah numbers when the relaxation time-which is a function of the deployed material and the polymer/solvent composition-is higher than the characteristic diffusion time in the film. The latter is a function of the polymer/solvent composition and the thickness of the film. Drying curves of PIB/toluene films show additional effect in a substrate-near region of about 5 nm in which polymer chain mobility is confined, resulting in decelerated solvent diffusion. Although this effect near the substrate interface is expected to be present regardless of the film thickness, it becomes more dominant when the substrate-near region represents a significant fraction of the total film thickness. The key to the derived methodology for characterization of the polymer/solvent drying process is to vary dry film thickness from micrometers to a few nanometers which allows us to determine the dominating mechanism of drying kinetics.

Transient Three-Dimensional Flow Field Measurements by Means of 3D µPTV in Drying Poly(Vinyl Acetate)-Methanol Thin Films Subject to Short-Scale Marangoni Instabilities.

Convective Marangoni instabilities in drying polymer films may induce surface deformations, which persist in the dry film, deteriorating product performance. While theoretic stability analyses are abundantly available, experimental data are scarce. We report transient three-dimensional flow field measurements in thin poly(vinyl acetate)-methanol films, drying under ambient conditions with several films exhibiting short-scale Marangoni convection cells. An initial assessment of the upper limit of thermal and solutal Marangoni numbers reveals that the solutal effect is likely to be the dominant cause for the observed instabilities.

Hysteresis Behavior in the Sorption Equilibrium of Water in Anodes for Li-Ion Batteries.

Hysteresis in the sorption equilibrium influences the production process of many multicomponent material systems. Electrodes for Li-ion batteries consist of several materials, some of which exhibit hysteresis in their sorption equilibrium with water. The moisture content adsorbed and absorbed in the electrodes of the Li-ion battery turned out to be an issue for its electrochemical performance and is reduced in the post-drying process. During this process, hysteresis in the sorption equilibrium needs to be overcome in order to achieve a low residual moisture content of the electrode. Modeling the post-drying process requires a description of the sorption equilibria of water in the components of the battery. This paper builds on previous research about the sorption equilibria and examines the hysteresis behavior of typical graphite anodes, with the active material graphite, carbon black as the conductive additive, and sodium carboxymethyl cellulose as well as styrene butadiene rubber as polymeric binders. Moreover, the mechanisms for the occurrence of hysteresis are presented, and how sorption equilibria during drying can be described is shown by applying models from the literature on the materials of battery electrodes. Theoretical deliberations on hysteresis mechanisms are validated, investigating graphite anodes of different material compositions and their materials.

Influence of Particle Shape on the Drying Regime Maps for Platelike Particle-Polymer Composites.

In previous works, the influence of drying conditions on the particle distribution in filled polymer films was investigated using spherical silica particles and polystyrene pellets. The results showed that changing drying conditions can influence the particle distribution in a dry film. These were summarized in drying regime maps to predict the distribution of the particles in the final film; the particles accumulate at the top of the coating (evaporation regime) or sink to the bottom (sedimentation regime) or remain homogeneously distributed (diffusion regime). In the present study, the particle geometry was oblate to determine the changes in the particle distribution in comparison to spherical particles. A simulation was developed by changing the sedimentation and diffusion models. Concentration-dependent sedimentation rates and the particle orientation in the dry film were experimentally determined. New drying regime maps were plotted and compared. The simulation results show an expansion of the diffusion regime for the platelike particles in comparison with spherical particles. The regime maps were validated using platelike glass particles dispersed in poly(vinyl alcohol) (PVA) with water. The particle concentration profiles in the dry film and the film morphology were obtained via three-dimensional (3-D) Raman spectroscopy.

Commentary regarding: "Activity determination of FAD-dependent glucose dehydrogenase immobilized in PEDOT: PSS-PVA composite films for biosensor applications".

In the light of continuous improvement and optimization, recent experiments that the authors conducted give new insights into the applied evaluation method of Riegel et al. [1]: Thorough investigations of the previous results regarding the usage of the Lowry Assay showed discrepancies in the determination of the released amount of protein in the analysis solution. The accurate quantification of this parameter is crucial as it directly influences the calculation of the residual enzymatic activity. In concrete terms, this finding has a major impact on the presented and discussed results in the article "Activity determination of FAD-dependent glucose dehydrogenase immobilized in PEDOT: PSS-PVA composite films for biosensor applications" [1]. Thus, this commentary addresses the new insights concerning the applied evaluation method, explains necessary revisions and discusses new conclusions derived from the adjusted evaluation method.

Experimental investigation into battery electrode surfaces: The distribution of liquid at the surface and the emptying of pores during drying.

HYPOTHESIS: Drying constitutes a key step in the production of thin, particulate films as the complex microstructure develops and functional additives redistribute throughout the film, thus shaping the film properties. The onset of pore emptying constitutes a paramount marker in the film formation process as capillary liquid transport emerges. EXPERIMENTS: Film shrinkage, development of surface liquid content and liquid depletion in surface pores of thin films are studied by means of a novel experimental approach, using the example of lithium-ion battery anodes. An optical brightener is incorporated into the films, which are produced in a convective slot-nozzle dryer. After various drying times, images of the film, which emit light fluorescently during exposure to UV-A radiation, are captured and analyzed by image processing. Film shrinkage is observed by means of a laser displacement sensor. FINDINGS: As soon as the first pores empty, pore fluid is transported convectively. By exerting an external force, i.e. a pressure surge, on the film, the transition from a fully saturated to a partially emptied film is revealed. The significance of drying rate and particle shape are studied. We show that liquid depletion in surface pores can occur both prior and concurrent with the end of film shrinkage.

Digitally Printed Dewetting Patterns for Self-Organized Microelectronics.

Self-organization of functional materials induced by low surface-energetic direct printed structures is presented. This study investigates fundamental fluid and substrate interactions and fabricates all-printed small area organic photodetectors with On-Off ratios of ≈10(5) and dark current densities of ≈10(-4) mA cm(-2) , as well as ring oscillators based on n-type organic field-effect transistors showing working frequencies up to 400 Hz.

Evaporation from open microchannel grooves.

The evaporation of water from open u-shaped microchannel grooves was investigated with particular emphasis on the roles of channel width and air flow conditions. Given the small dimensions of the microchannels, all measurements were conducted in a range where convection and diffusion are of equal importance and known correlations for the calculation of mass transfer coefficients cannot be applied. The evaporation rates were measured using a new optical method and a gravimetric method. Both measurement methods yielded mass transfer coefficients that are in agreement with each other. The observed relation between mass transfer coefficient, air velocity and channel width vastly differs from the predictions obtained from macroscopic structures. With respect to diagnostic devices we conclude that analyte concentration in an open microchannel groove strongly increases even within short times due to the evaporation process and we show that wider channels are more favourable in terms of minimizing the relative evaporation rate.

Moving through the phase diagram: morphology formation in solution cast polymer-fullerene blend films for organic solar cells.

The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the "unfriendly" solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.