This article presents a comprehensive quantification of the gaseous and dissolved reaction products formed during the etching of Si in HF/HNO3 mixtures and provides a revised model of HNO3 reduction during the oxidation of Si.

The stoichiometry of the wet chemical etching of silicon in concentrated binary and ternary mixtures of HF, HNO3 and H2SiF6 was comprehensively investigated. A complete quantification of both dissolved and...

The stoichiometry of the wet-chemical etching of silicon in concentrated HF/HNO 3 mixtures has been studied. By quantifying the major reaction products in solution, the established model that 3 mol of Si are oxidized by 4 mol of HNO 3 to yield 4 mol of NO could not be confirmed. In HNO 3 -rich HF/HNO 3 mixtures, approximately 1.1 mol of HNO 3 are required to oxidize 1 mol of Si. Excess HNO 3 leads to massive accumulation of N(III) species in the etchants and massive formation of nitrous oxides due to incomplete reduction of the HNO 3 . An excess of HNO 3 leads to higher consumption and poorer utilization indicated by the massive accumulation of N(III) species in the etchants. In HF-rich mixtures, only 0.9 mol of HNO 3 are needed to oxidize 1 mol of Si yielding a lower accumulation of N(III) species and a higher utilization of the HNO 3 . Two parallel pathways contribute to the oxidation of silicon in such solutions: (i) via the oxidation by HNO 3 and reactive intermediates generated by the reduction of HNO 3 and (ii) via the formation of hydrogen. A comprehensive treatment covering alkaline etching, electrochemical etching in HF media, and etching in concentrated HF/HNO 3 mixtures is proposed based on the reactivity of the hydrogen terminated silicon surface against the applied oxidizing agent.

Nickel–manganese–cobalt oxides, with LiNi0.33Mn0.33Co0.33O2 (NMC) as the most prominent compound, are state-of-the-art cathode materials for lithium-ion batteries in electric vehicles. The growing market for electro mobility has led to a growing global demand for Li, Co, Ni, and Mn, making spent lithium-ion batteries a valuable secondary resource. Going forward, energy- and resource-inefficient pyrometallurgical and hydrometallurgical recycling strategies must be avoided. We presented an approach to recover NMC particles from spent lithium-ion battery cathodes while preserving their chemical and morphological properties, with a minimal use of chemicals. The key task was the separation of the cathode coating layer consisting of NMC, an organic binder, and carbon black, from the Al substrate foil. This can be performed in water under strong agitation to support the slow detachment process. However, the contact of the NMC cathode with water leads to a release of Li+ ions and a fast increase in the pH. Unwanted side reactions may occur as the Al substrate foil starts to dissolve and Al(OH)3 precipitates on the NMC. These side reactions are avoided using pH-adjusted solutions with sufficiently high buffer capacities to separate the coating layer from the Al substrate, without precipitations and without degradation of the NMC particles.

The oxidizing effect of nitric acid in aqueous solutions depends on the concentration of undissociated nitric acid. This makes the concentration of undissociated nitric acid an essential parameter to monitor and control the quality of silicon etching in the industrial manufacturing of solar cells. In the present study, a method known already is extended in such a way that the degree of dissociation of nitric acid can be determined by Raman spectroscopy in HF/HNO3/H2SiF6 acid mixtures over a broad concentration range for the first time and without using an internal or external standard to compensate the typical time‐dependent drift of a Raman spectrometer. The method developed requires the calculation of a peak area ratio from the areas of the unimpeded Raman signals assigned to nitrate (νN − O) at 1,048 cm−1 and to undissociated HNO3 (νN − OH) at 957 cm−1. The correlation between the peak ratio and the degree of dissociation of nitric acid revealed can be described by a simple empirical equation. Using this equation, the degree of dissociation of nitric acid can be determined over a broad concentration range in binary and ternary mixtures of HNO3 with HF and H2SiF6. The impact of the acids HF and H2SiF6 and the total water content in the degree of dissociation of nitric acid is discussed.