Frieder Scheiba

XPS investigations of electrolyte/electrode interactions for various Li-ion battery materials

AbstractFor future Li-ion battery applications the search for both new design concepts and materials is necessary. The electrodes of the batteries are always in contact with electrolytes, which are responsible for the transport of Li ions during the charging and discharging process. A broad range of materials is considered for both electrolytes and electrodes so that very different chemical interactions between them can occur, while good cycling behavior can only be obtained for stable solid-electrolyte interfaces. X-ray photoelectron spectroscopy (XPS) was used to study the most relevant interactions between various electrode materials in contact with different electrolyte solutions. It is shown how XPS can provide useful information on reactivities and thus preselect suitable electrode/electrolyte combinations, prior to electrochemical performance tests. FigureCharacteristic changes of the Li1s XP-spectra at Li2O2 powder after storage in LiPF6 for various time point to a LiF formation

Advances in in situ powder diffraction of battery materials - a case study of the new beamline P02.1 at DESY Hamburg

A brief review of in situ powder diffraction methods for battery materials is given. Furthermore, it is demonstrated that the new beamline P02.1 at the synchrotron source PETRA III (DESY, Hamburg), equipped with a new electrochemical test cell design and a fast two-dimensional area detector, enables outstanding conditions for in situ diffraction studies on battery materials with complex crystal structures. For instance, the time necessary to measure a pattern can be reduced to the region of milliseconds accompanied by an excellent pattern quality. It is shown that even at medium detector distances the instrumental resolution is suitable for crystallite size refinements. Additional crucial issues like contributions to the background and available q range are determined.

Limitation of Discharge Capacity and Mechanisms of Air- Electrode Deactivation in Silicon-Air Batteries

The electrocatalytical process at the air cathode in novel silicon-air batteries using the room-temperature ionic liquid hydrophilic 1-ethyl-3-methylimidazolium oligofluorohydrogenate [EMI⋅2.3 HF⋅F] as electrolyte and highly doped silicon wafers as anodes is investigated by electrochemical means, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy. The results obtained by XPS and EPR provide a model to describe the limited discharge capacity by means of a mechanism of air-electrode deactivation. In that respect, upon discharge the silicon-air batterys cathode is not only blocked by silicon oxide reduction products, but also experiences a major modification in the MnO₂ catalyst nature. The proposed modification of the MnO₂ catalyst by means of a MnF₂ surface layer greatly impacts the Si-air performance and describes a mechanism relevant for other metal-air batteries, such as the lithium-air. Moreover, the ability for this deactivation layer to form is greatly impacted by water in the electrolyte.

Lithium–air battery cathode modification via an unconventional thermal method employing borax

A novel, unconventional thermal treatment employing borax for preparing porous carbon materials is presented. The new method was used to prepare carbon felt electrodes for use in lithium–air batteries. The etching of the carbon fiber surface was found to be highly controllable by the amount of borax. The resulting felts were characterized by cyclic voltammetry (CV), secondary electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The borax treatment resulted in a change of the size, shape and orientation of the Li2O2 crystals formed during discharge.

Surface Functionalization of Silicon, HOPG, and Graphite Electrodes: Toward an Artificial Solid Electrolyte Interface

Electrografting of diazonium salts containing a protected alkyne moiety was used for the first functionalization of silicon and highly ordered pyrolytic graphite model surfaces. After deprotection with tetrabutylammonium fluoride, further layers were added by the thiol-yne click chemistry. The composition of each layer was characterized via X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. The same approach was then used to functionalize graphite powder electrodes, which are classically used as negative electrode in lithium-ion batteries. The effect of the coating on the formation of the solid electrolyte layer was investigated electrochemically by cyclovoltammetry and galvanostatic measurements. The modified graphite electrodes showed different reduction peaks in the first cycle, indicating reduced and altered decomposition processes of the components. Most importantly, the electrochemical investigations show a remarkable reduction of irreversible capacity loss of the battery.

Sequence-controlled molecular layers on surfaces by thiol–ene chemistry: synthesis and multitechnique characterization

Silicon surfaces were functionalized by thiol–ene chemistry using sequential reactions of different α,ω-dienes and α,ω-dithiols bearing marker moieties. A system of six individual layers was achieved and exhaustively characterized by combining independent techniques such as X-ray Photoelectron Spectroscopy, Time-of-Flight Secondary Ion Mass Spectrometry and Atomic Force Microscopy using a molecular ruler system.

Electron microscopy techniques for the detailed study of fuel cell electrodes and components

Transport processes play a significant role for the proper operation of polymer electrolyte membrane fuel cells (PEMFC). In PEMFCs the reactant gases must have access to the catalytically active sites, protons and electrons must be conducted through the electrode and the reaction product water must be removed from the pore system to avoid blocking of the gas diffusion paths. In the current standard electrode design each transport process is realized by a different component. Since the various components influence each other and therefore the electrode properties in a nonconstructive manner, optimization of the electrode structure is far from being trivial. Porosity and the distribution of the polymer electrolyte appear to be the key parameters for the electrode performance. For the detailed investigation of these parameters transmission electron microscopy (TEM) has been chosen as a suitable tool. However, the sample preparation is crucial to the success of the experiment: the membrane-electrode assembly (MEA) is embedded in an epoxy resin and cut into thin sections by ultramicrotomy using a diamond knife. Details of the procedure can be found in [1] and references therein. For the analysis of the polymer electrolyte distribution, infiltration of the sample with an epoxy resin has a significant drawback. Since the polymer electrolyte and the embedding resin have almost identical scattering contrast, the polymer electrolyte cannot be distinguished directly. As pores in the electrode structure may be filled by the polymer electrolyte or the embedding resin, it is also not possible to distinguish between open and closed pores (i.e. those filled by the polymer electrolyte). In this paper, we present different approaches to solve the contrast problem and suggest methods to characterize the polymer electrolyte distribution and electrode porosity. A new imaging routine has been developed in order to avoid significant beam damage of the sensitive sample. By this technique 200 x 200 nm sized parts of the electrode can be imaged and the polymer electrolyte distribution analyzed with the help of the F signal in energy filtered transmission electron microscopy (EFTEM). This approach was very successful in contrast to staining techniques used earlier, and respective results for different MEAs before and after operation will be presented. In addition, first results applying a novel technique using Wood’s alloy will be reported, which will help us to image both MEA and gas diffusion layer (GDL) at once in the future.