FG Physikalische Chemie
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The present study deals with the phenomenological observation of the corrosion of the positive electrode foil of lithium-ion batteries containing LiNi0.6Co0.2Mn0.2O2 (NMC) as cathode material. Due to the presence of moisture, localized water accumulation is formed on the NMC surface. The water absorbed by the electrolyte reacts with the NMC under Li+/H+ exchange and the resulting pH increase leads to dissolution of the carrier foil and characteristic salt-like blooms on the NMC surface. With the increase in the relative area occupied by the holes in the aluminum foil per time, a sufficiently suitable parameter was found with which to quantitatively determine the extent of corrosion. The degree of degradation depends on time and ambient humidity. It was shown that functional recycling with the water jet method is no longer applicable for degraded foils, since the mechanical stability of the foils decreases as corrosion progresses. Lithium, aluminum, sulfur and oxygen were detected in the blooms using SEM–EDX and Laser-Induced-Breakdown-Spectroscopy (LIBS). The underlying NMC layer was found to contain mainly aluminum and significantly lower lithium content than the non-degraded material. SEM and Raman microscopy analyses also showed that the active material is also locally degraded and therefore no longer suitable for functional recycling.
The exact and precise determination of the boron concentration in silicon is still a challenge. A systematic investigation dealing with the digestions of 60 silicon samples with HF-HNO3 and subsequent boron determination by ICP-OES revealed that the concentration found could be up to 60% lower than the actual boron concentration depending on the composition of the sample solution. As the original boron–silicon compound that was identified was colloidally precipitated in the presence of an excess of hydrofluoric acid and then partially retained by filtration or by the sample introduction system, systematic lower boron concentrations were determined. In acidic, HF-free digestion solutions, this compound existed in a soluble form parallel to the borate in B(OH)4−. In an excess of hydrofluoric acid, the compound was converted into the colloidal form and, in parallel, B(OH)4− was converted to tetrafluoroborate, BF4−. For the composition of the colloidal compound, a molar ratio of boron to silicon of 1 : 4 could be determined. 11B-ss-NMR analysis revealed a tetrahedral geometry compound with a central boron atom surrounded by four silicon atoms. It is assumed that a soluble form with four –Si(OH)3 groups was present in the hydrofluoric acid-free solutions, while an insoluble form with four –SiF3 groups was present in HF-containing solution.
The effect of strain through mechanical processing on the formation of silicon dioxide, one of the most important chemical reactions for the fabrication of semiconductors, biosensors or photovoltaics, has not yet been studied in detail. In this study, we use the surface modification of silicon by alkylsilanes and Raman microscopy techniques to visualise where different oxidants react preferentially on mechanically processed surfaces. We found that HNO3, H2O2 as well as H2SiF6 only oxidise tensile strained silicon areas and do not oxidise unstrained silicon even after long reaction times. Furthermore, a comparison between H2O2 and HNO3 in the presence of HF was also carried out and it was shown that H2O2/HF only etches away tensile strained areas, whereas HNO3/HF initially attacks the tensile strained areas but also forms NOx species. These NOx species then lead to a strain unselective, geometry-based etching mechanism. These results lead to new possibilities in strain lithography,high-precision etching, as well as in the structuring of biosensors and localisation of surface modifications.
Metal deposition on silicon in hydrofluoric acid (HF) solutions is a well-established process for the surface patterning of silicon. The reactions behind this process, especially the formation or the absence of molecular hydrogen (H2) are controversially discussed in the literature. In this study, several batch experiments with Ag+, Cu2+, AuCl4– and PtCl62– in HF matrix and multi-crystalline silicon were performed. The stoichiometric amounts of the metal depositions, the silicon dissolution and the molecular hydrogen formation were determined analytically. Based on these data and theoretical considerations of the valence transfer, four reasons for the formation of H2 could be identified. Firstly, H2 is generated in a consecutive reaction after a monovalent hole transfer (h+) to a Si–Si bond. Secondly, H2 is produced as a result of a monovalent hole transfer to the Si–H bonds. Thirdly, H2 occurs if Si–Si back bonds of the hydrogen-terminated silicon are attacked by Cu2+ reduction resulting in the intermediate species HSiF3, which is further degraded to H2 and SiF62–. The fourth H2-forming reaction is the reduction of oxonium ions (H3O+) on the silver/, copper/ and gold/silicon contacts via monovalent hole transfer to silicon. In the case of (cumulative) even-numbered valence transfers to silicon, no H2 is produced. The formation of H2 also fails to appear if the equilibrium potential of the 2H3O+/H2 half-cell does not reach the energetic level of the valence bands of the bulk or hydrogen-terminated silicon. Non-hydrogen-forming reactions in silver, copper and gold deposition always occur with at least one H2-forming process. The PtCl62– reduction to Pt proceeds exclusively via even-numbered valence transfers to silicon. This also applies to the reaction of H3O+ at the platinum/silicon contact. Consequently, no H2 is formed during platinum deposition.