Metal-Oxygen Batteries

Abstract

In this study some new findings on metal-oxygen batteries are presented. Tackling climate change requires a transition from the use of fossil fuels as energy sources to renewable energies. This is associated with the need to store electricity for different applications. Especially for the use in cars the current Li-ion technology cannot guarantee comparable ranges of the car as fossil fuels allow. In addition, the circumstances surrounding the mining of cobalt and lithium for Li-ion technology are questionable. Therefore, research on new battery technologies is necessary and metal-oxygen batteries appear to be the most promising candidate with respect to the higher energy densities compared to Li-ion technology. <br /> The results published prior to this study have shown some difficulties in Li-O₂ technology regarding the storage of the discharge product Li₂O₂ on the cathode surface, the stability of the electrolyte against reactive oxygen species and the kinetics during battery charging. Redox mediators for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as an additive to the electrolyte can help to solve these problems. Nevertheless, the redox mediators, mostly organic molecules, are also exposed to the reactive oxygen species and it has to be checked if the redox mediators are stable. Therefore, a new electrochemical cell for differential electrochemical mass spectrometry (DEMS) was developed. This cell has a high electrode surface to the electrolyte volume and is therefore comparable to battery architectures. Additionally, high sensitivities for the O₂ signal in the mass spectrometer could be achieved. Thus it was possible to demonstrate that the redox mediators for the OER N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) and tetrathiafulvalene (TTF) known from the Li-O₂ literature also undergo side reactions in the OER. Furthermore, the kinetics of the oxidation of Li₂O₂ by a redox mediator was investigated and a model was derived. It was shown that the electron transfer from Li₂O₂ to the oxidized redox mediator can be understood by an outer sphere mechanism. Furthermore, it could be shown that an increase of the mediator concentration in the electrolyte leads to a shift of the potential of the OER through the mediator to more negative potentials. This finding is especially interesting for practical purpose as it shows that the charging voltage of the Li-O₂ battery could be lowered by increasing the redox mediator concentration. <br /> The kinetics of ORR by the redox mediator 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) was extensively studied using DEMS and rotating ring disc electrode (RRDE). For this purpose a variation of the solvent (dimethyl sulfoxide and tetraglyme) as well as variations of the conducting salt concentration and the cation of the conducting salt were performed. It was shown that the ORR mediated by DBBQ is faster if the interaction of the cation of the electrolyte and the reduced DBBQ increases. These investigations were supported by kinetic modelling of the experiments with a finite difference algorithm. Additionally, by increasing this interaction, the ORR potential can be shifted to higher electrode potentials. This was explained by the thermodynamic potential shift of the DBBQ reduction due to a higher ion pair formation with increasing cation concentration. In general this finding could help to increase the discharge voltage of the metal-O₂ battery. Further experiments also showed the effectiveness of DBBQ as a redox mediator in Ca²⁺ and Mg²⁺ ontaining DMSO. <br /> Special attention was paid to the investigation of ORR and OER in Ca²⁺ containing DMSO. There are almost no fundamental investigations published and only one first study has revealed a high reversibility of the OER charge to the ORR charge compared to other metal O₂ systems. Thus, we were able to prove by mass spectrometry that the superoxide formed in the ORR disproportionates to peroxide and oxygen in the resence of Ca²⁺ in the electrolyte. Carbonates as well as volatile organic compounds were detected as by products. This observation is consistent with the recently reported proportionate formation of singlet oxygen during the disproportionation of superoxide. Due to its reactivity, the formed singlet oxygen is considered responsible for a large part of the side reactions in metal-O₂ batteries. Furthermore, the electrocatalysts Au and Pt were investigated after ORR using photoelectron spectroscopy (XPS). It was found that although a large part of the ORR products are soluble in the Ca²⁺ containing electrolyte, a thin layer is deposited on the electrode surface. This consists of decomposition products of the electrolyte on the surface, Ca-peroxide and Ca-superoxide in deeper layers and a thin layer of Ca-oxide directly in front of the electrode. If the potential of the electrode is cycled into the OER, this layer can be oxidized again to O₂ and Ca²⁺. Based on further investigations with the RRDE, a model of the ORR in the Ca²⁺ containing DMSO can be discussed. In this model, a transition from peroxide formation to superoxide formation during ORR is attributed to a poisoning of the electrocatalyst by calcium oxide or strongly adsorbed calcium peroxide. <br /> Furthermore, the Mg deposition on Pt and the Mg insertion into a Sb electrode is investigated using XPS. As electrolyte system a mixture of MgCl₂ and AlCl₃ in tetraglyme was used (MACC electrolyte). It could be shown that an irreversible Al deposition occurs both during the deposition of Mg and during the insertion of Mg. In addition, the surfaces accumulate with Cl-containing species. The importance of Cl in metal deposition is discussed. The electrochemical fabrication of Sb electrodes is also investigated with XPS. It has been shown that these electrodes oxidize to Sb₂O₃ by air contact. In the insertion process, a reduction of Sb₂O₃ takes place which proves that the insertion material is Sb and not Sb₂O₃. <br /> This study also describes the construction of new electrochemical cells and the design of a sample transfer system between the electrochemical experiment and the XPS apparatus.,Furthermore, this study provides a new experimental technique for the determination of diffusion coefficients and solubility of gases in liquids by time resolved tracking of the mass signal of the gas through a thin liquid film by a mass spectrometer. The collection of these data is essential for the optimization of metal-O₂ batteries in the case of oxygen, as well as for kinetic studies of the ORR.