Enabling High-Performance Battery Electrodes by Surface-Structuring of Current Collectors and Crack Formation in Electrodes: A Proof-of-Concept.

Today's society and economy demand high-performance energy storage systems with large battery capacities and super-fast charging. However, a common problematic consequence is the delamination of the mass loading (including, active materials, binder and conductive carbon) from the current collector at high C-rates and also after certain cycle tests. In this work, surface structuring of aluminum (Al) foils (as a current collector) is developed to overcome the aforementioned delamination process for sulfur (S)/carbon composite cathodes of Li-S batteries (LSBs). The structuring process allows a mechanical interlocking of the loaded mass with the structured current collector, thus increasing its electrode adhesion and its general stability. Through directed crack formation within the mass loading, this also allows an enhanced electrolyte wetting in deeper layers, which in turn improves ion transport at increased areal loadings. Moreover, the interfacial resistance of this composite is reduced leading to an improved battery performance. In addition, surface structuring improves the wettability of water-based pastes, eliminating the need for additional primer coatings and simplifying the electrode fabrication process. Compared to the cells made with untreated current collectors, the cells made with structured current collectors significantly improved rate capability and cycling stability with a capacity of over 1000mAhg-1. At the same time, the concept of mechanical interlocking offers the potential of transfer to other battery and supercapacitor electrodes.

Electrochemical Surface Structuring for Strong SMA Wire-Polymer Interface Adhesion.

Active hybrid composites represent a novel class of smart materials used to design morphing surfaces, opening up new applications in the aircraft and automotive industries. The bending of the active hybrid composite is induced by the contraction of electrically activated shape memory alloy (SMA) wires, which are placed with an offset to the neutral axis of the composite. Therefore, the adhesion strength between the SMA wire and the surrounding polymer matrix is crucial to the load transfer and the functionality of the composite. Thus, the interface adhesion strength is of great importance for the performance and the actuation potential of active hybrid composites. In this work, the surface of a commercially available one-way effect NiTi SMA wire with a diameter of 1 mm was structured by selective electrochemical etching that preferably starts at defect sites, leaving the most thermodynamically stable surfaces of the wire intact. The created etch pits lead to an increase in the surface area of the wire and a mechanical interlocking with the polymer, resulting in a combination of adhesive and cohesive failure modes after a pull-out test. Consequently, the force of the first failure determined by an optical stress measurement was increased by more than 3 times when compared to the as-delivered SMA wire. The actuation characterization test showed that approximately the same work capacity could be retrieved from structured SMA wires. Moreover, structured SMA wires exhibited the same shape of the stress-strain curve as the as-delivered SMA wire, and the mechanical performance was not influenced by the structuring process. The austenite start As and austenite finish Af transformation temperatures were also not found to be affected by the structuring process. The formation of etching pits with different geometries and densities was discussed with regard to the kinetics of oxide formation and dissolution.

Conversionless efficient and broadband laser light diffusers for high brightness illumination applications.

Laser diodes are efficient light sources. However, state-of-the-art laser diode-based lighting systems rely on light-converting inorganic phosphor materials, which strongly limit the efficiency and lifetime, as well as achievable light output due to energy losses, saturation, thermal degradation, and low irradiance levels. Here, we demonstrate a macroscopically expanded, three-dimensional diffuser composed of interconnected hollow hexagonal boron nitride microtubes with nanoscopic wall-thickness, acting as an artificial solid fog, capable of withstanding ~10 times the irradiance level of remote phosphors. In contrast to phosphors, no light conversion is required as the diffuser relies solely on strong broadband (full visible range) lossless multiple light scattering events, enabled by a highly porous (>99.99%) non-absorbing nanoarchitecture, resulting in efficiencies of ~98%. This can unleash the potential of lasers for high-brightness lighting applications, such as automotive headlights, projection technology or lighting for large spaces.

Concept and modelling of memsensors as two terminal devices with enhanced capabilities in neuromorphic engineering.

We report on memsensors, a class of two terminal devices that combines features of memristive and sensor devices. Apart from a pinched hysteresis (memristive property) and stimulus dependent electrical resistance (sensing property) further properties like dynamic adaptation to an external stimulus emerge. We propose a three component equivalent circuit to model the memsensor electrical behaviour. In this model we find stimulus dependent hysteresis, a delayed response to the sensory signal and adaptation. Stimulus dependent IV hysteresis as a fingerprint of a memsensor device is experimentally shown for memristive ZnO microrods. Adaptation in memsensor devices as found in our simulations resembles striking similarities to the biology. Especially the stimulus dependency of the IV hysteresis and the adaptation to external stimuli are superior features for application of memsensors in neuromorphic engineering. Based on the simulations and experimental findings we propose design rules for memsensors that will facilitate further research on memsensitive systems.

Scalable processing and capacity of Si microwire array anodes for Li ion batteries.

Si microwire array anodes have been prepared by an economical, microelectronics compatible method based on macropore etching. In the present report, evidence of the scalability of the process and the areal capacity of the anodes is presented. The anodes exhibit record areal capacities for Si-based anodes. The gravimetric capacity of longer anodes is comparable to the one of shorter anodes at moderate lithiation/delithiation rates. The diffusion limitation of the lithium ions through the electrolyte in depth among the wires is the limiting factor for cycling longer wires at high rates. PACS: 82.47.Aa; 82.45.Vp; 81.16.-c.

Electrochemical growth of Co nanowires in ultra-high aspect ratio InP membranes: FFT-impedance spectroscopy of the growth process and magnetic properties.

The electrochemical growth of Co nanowires in ultra-high aspect ratio InP membranes has been investigated by fast Fourier transform-impedance spectroscopy (FFT-IS) in the frequency range from 75 Hz to 18.5 kHz. The impedance data could be fitted very well using an electric circuit equivalent model with a series resistance connected in series to a simple resistor-capacitor (RC) element and a Maxwell element. Based on the impedance data, the Co deposition in ultra-high aspect ratio InP membranes can be divided into two different Co deposition processes. The corresponding share of each process on the overall Co deposition can be determined directly from the transfer resistances of the two processes. The impedance data clearly show the beneficial impact of boric acid on the Co deposition and also indicate a diffusion limitation of boric acid in ultra-high aspect ratio InP membranes. The grown Co nanowires are polycrystalline with a very small grain size. They show a narrow hysteresis loop with a preferential orientation of the easy magnetization direction along the long nanowire axis due to the arising shape anisotropy of the Co nanowires.

Pore size modulation in electrochemically etched macroporous p-type silicon monitored by FFT impedance spectroscopy and Raman scattering.

The understanding of the mechanisms of macropore formation in p-type Si with respect to modulation of the pore diameter is still in its infancy. In the present work, macropores with significantly modulated diameters have been produced electrochemically in p-type Si. The effect of the current density and the amount of surfactant in the etching solution are shown to influence the modulation in pore diameter and morphology. Data obtained during the etching process by in situ FFT impedance spectroscopy correlate the pore diameter variation with certain time constants found in the kinetics of the dissolution process. Raman scattering and electron microscopy confirm the mesoscopic structure and roughening of the pore walls. Spectroscopic and microscopic methods confirm that the pore wall morphology is correlated with the conditions of pore modulation.

Structural and Electrochemical Investigation during the First Charging Cycles of Silicon Microwire Array Anodes for High Capacity Lithium Ion Batteries.

Silicon microwire arrays embedded in Cu present exceptional performance as anode material in Li ion batteries. The processes occurring during the first charging cycles of batteries with this anode are essential for good performance. This paper sheds light on the electrochemical and structural properties of the anodes during the first charging cycles. Scanning Electron Microscopy, X-ray diffractommetry, and fast Fourier transformation impedance spectroscopy are used for the characterization. It was found that crystalline phases with high Li content are obtained after the first lithiation cycle, while for the second lithiation just crystalline phases with less Li are observable, indicating that the lithiated wires become amorphous upon cycling. The formation of a solid electrolyte interface of around 250 nm during the first lithiation cycle is evidenced, and is considered a necessary component for the good cycling performance of the wires. Analog to voltammetric techniques, impedance spectroscopy is confirmed as a powerful tool to identify the formation of the different Si-Li phases.

Electrochemical and galvanic fabrication of a magnetoelectric composite sensor based on InP.

: A process chain for a magnetoelectric device based on porous InP will be presented using only chemical, electrochemical, photoelectrochemical, photochemical treatments and the galvanic deposition of metals in high-aspect-ratio structures. All relevant process steps starting with the formation of a self-ordered array of current-line oriented pores followed by the membrane fabrication and a post-etching step, as well as the galvanic metal filling of membrane structures are presented and discussed. The resistivity of a porous InP structure could be drastically increased and, thus, the piezoelectric performance of the porous InP structure. The developed galvanic Ni filling process is capable to homogeneously fill high aspect-ratio membranes.

Analysis of p-Si macropore etching using FFT-impedance spectroscopy.

The dependence of the etch mechanism of lithographically seeded macropores in low-doped p-type silicon on water and hydrofluoric acid (HF) concentrations has been investigated. Using different HF concentrations (prepared from 48 and 73 wt.% HF) in organic electrolytes, the pore morphologies of etched samples have been related to in situ impedance spectra (IS) obtained by Fast Fourier Transform (FFT) technique. It will be shown that most of the data can be fitted with a simple equivalent circuit model. The model predicts that the HF concentration is responsible for the net silicon dissolution rate, while the dissolution rate selectivity at the pore tips and walls that ultimately enables pore etching depends on the water content. The 'quality' of the pores increases with decreasing water content in HF/organic electrolytes.

New Applications of Electrochemically Produced Porous Semiconductors and Nanowire Arrays.

The growing demand for electro mobility together with advancing concepts for renewable energy as primary power sources requires sophisticated methods of energy storage. In this work, we present a Li ion battery based on Si nanowires, which can be produced reliable and cheaply and which shows superior properties, such as a largely increased capacity and cycle stability. Sophisticated methods based on electrochemical pore etching allow to produce optimized regular arrays of nanowires, which can be stabilized by intrinsic cross-links, which serve to avoid unwanted stiction effects and allow easy processing.

Pores in n-Type InP: A Model System for Electrochemical Pore Etching.

The growth mechanism of currentline-oriented pores in n-type InP has been studied by Fast-Fourier-Transform Impedance Spectroscopy (FFT IS) applied in situ during pore etching and by theoretical calculations. Several pore growth parameters could thus be extracted in situ that are otherwise not obtainable. These include the space-charge-region (SCR) width, the SCR potential, the capacitance at the pore tips, and the avalanche breakdown field strength. It could be demonstrated that the system adjusts itself in such a way that the potential across the space-charge-region at the pore tips is kept constant within a certain bandwidth of the applied potential. This provides for a constant field strength at the pore tips, ensuring that avalanche breakdown occurs, generating the necessary holes for the electrochemical dissolution of InP.