Kinetic studies on acidic wet chemical etching of silicon in binary and ternary mixtures of HF, HNO3 and H2SiF6.

The etching of silicon with mixtures of hydrofluoric acid (HF), nitric acid (HNO3) and hexafluorosilicic acid (H2SiF6) proceeds in a complex reaction scenario consisting of interacting side reactions. Almost no other dissolution reaction is so massively dependent on the reaction conditions that influence the etching rate and the mechanism of the individual reactions. Extensive studies of the reaction rate of silicon etching in binary and ternary acid mixtures have allowed the transition point between the reaction-controlled and diffusion-controlled reaction regimes to be determined as a function of the composition of the etching mixture. It was verified that the reaction mechanism for binary and ternary mixtures does not differ and only the lower water content in ternary mixtures favours an enhanced formation of the reactive N(III) intermediate HNO2 in side reactions. Based on the exact knowledge of the point of mechanism change, determination of the reaction rate under quasi-isothermal conditions in the bulk etching range allows, for the first time, deriving formal kinetic terms from the kinetic data to describe the dissolution rate in both the reaction-controlled and diffusion-controlled regimes. The formal kinetic terms were designed both for the kinetically correct quasi-isothermal approach to the dissolution rate in the Si bulk and for the application-oriented approach that includes induction phases and temperature increases in the considered dissolution period as well as influences of the surface properties. Moreover, by using the water content of the etching mixtures as a proxy variable, uniform calculation of the etching rates in HF/HNO3 as well as in HF/HNO3/H2SiF6 mixtures in the entire composition range of the application can be formulated.

A phenomenological and quantitative view on the degradation of positive electrodes from spent lithium-ion batteries in humid atmosphere.

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.

About determining reliable etching rates and the role of temperature in kinetic experiments on acidic wet chemical etching of silicon.

A characteristic of the wet chemical etching of silicon in concentrated HF-HNO3 and HF-HNO3-H2SiF6 mixtures is the release of a high reaction heat, without its numerical value being known. This liberated heat can lead to a significant temperature increase during the etching process, especially when the volume of etching solution provided is low. A noticeable increase in temperature not only leads to an increase in the etching rate, it simultaneously changes the concentrations of dissolved nitrogen oxides (e.g. NO, N2O4 and N2O3) and intermediary species (HNO2), resulting in a change in the overall reaction process. The same parameters also influence the experimental determination of the etching rate. Further factors affecting the determination of the etching rate are transport phenomena as a result of the wafer positioning in the reaction medium and the surface properties of the Si used. As a result, etching rates determined from the mass difference of a silicon sample before and after etching are highly uncertain. This work describes a new method for the valid determination of etching rates using turnover-time curves that are calculated from the time-dependent temperature increase in the etching solution during the dissolution process. If only a slight increase in temperature is caused by the choice of proper reaction conditions, bulk etching rates representative for the etching mixture are obtained. Based on these investigations, the activation energy of Si etching was determined as a function of the concentration of the reactive species in the initial reaction step, the undissociated nitric acid (HNO3, undiss). Based on a total of 111 investigated etching mixtures, a process enthalpy for acidic etching of Si was determined for the first time from the calculated adiabatic temperature increases. With a value of -(739 ± 52) kJ mol-1, the determined enthalpy underlines the strongly exothermic character of the reaction.

Comprehensive stoichiometric studies on the reaction of silicon in HF/HNO3 and HF/HNO3/H2SiF6 mixtures.

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 gaseous reaction products was carried out for a variety of different acid mixtures. It could be shown that the total nitric acid consumption is directly determined by the concentration of undissociated HNO3 in the mixture and can be attributed to the consumption in subsequent reactions with increasing concentration. Furthermore, a critical minimum concentration of undissociated HNO3 of q(HNO3, undiss) ≥ 0.35 mol kg-1 could be determined, which is required to start the reaction at 20 °C with agitation, irrespective of the composition of the mixture (binary/tertiary). The simultaneous determination of the nitrogen oxides in the gas phase supports the theory that NO is the only direct reduction product of HNO3 in the reaction with Si. Furthermore, the amount of formed hydrogen is determined by both the HF and the HNO3 concentration in the mixture. For binary mixtures, the H2 formation can be quantitatively described as a function of the concentration of HNO3, HF and H2O. The most important finding from comparative investigations between binary and ternary mixtures is that the overall reaction is largely determined by the formation of the reactive intermediate HNO2 as a result of complex reaction pathways. Both the formation and the accumulation of this intermediate are determined by the water content of the etching mixture. The consumption of HNO3 and also the formation of the reaction products NOx and H2 can therefore be functionally described on the basis of the H2O content in the etching mixture, regardless of a binary or ternary mixture.

A revised model of silicon oxidation during the dissolution of silicon in HF/HNO3 mixtures.

The stoichiometry of wet chemical etching of silicon in concentrated HF/HNO3 mixtures was investigated. The formation of nitrogen species enriched in the etching mixture and their reactivity during the etching process was studied. The main focus of the investigations was the comprehensive quantification of the gaseous reaction products using mass spectrometry. Whereas previously it could only be speculated that nitrogen was a product, its formation was detected for the first time. The formation of hydrogen, N2, N2O and NH4+ showed a dependence on the etching bath volume used, which indicates the formation of nitrogen compounds by side reactions. Simultaneously, the ratio of the nitrogen oxides, NO and NO2, formed decreases with increasing etching bath volume, while nitric acid consumption increases, so that the formation of NO2 could also be identified as a side reaction. Based on the stoichiometries obtained, a new reaction scheme for the reduction of nitric acid during etching in HF/HNO3 mixtures and an electron balance for the oxidation of silicon is presented.

Recovery of Li(Ni0.33Mn0.33Co0.33)O2 from Lithium-Ion Battery Cathodes: Aspects of Degradation.

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.