Modelling alkaline silicon-air batteries

Abstract

Renewable energy sources such as solar and wind energy rely on climate­ and weather conditions, like sun irradiation in the case of solar energy, and wind speed in the case of wind energy. These change throughout the day and with the seasons. There are periods of little wind, and during the night there is no sunlight. During periods of no sunlight and little to no wind, there is still a demand for energy. This leads to a shortage of energy. On the other hand, there are periods when the amount of available wind­ and solar energy will surpass the demand for energy, leading to an energy excess. To mitigate this mismatch between energy production and energy demand the excess energy can be stored to be used during periods of shortage. Many different solutions for this have been investigated in recent years. One of the storage technologies that is currently quite dominant is battery storage. Lithium-ion batteries are used quite widely, among others in battery electric vehicles. However, the use of batteries as a storage device to overcome energy mismatch is not yet implemented on a large scale, as most battery technologies are still quite novel, making them
uneconomical for this use compared to traditional hydrocarbon fired power plants. Furthermore, many battery technologies depend on scarce and expensive minerals. Recently, a battery utilizing silicon as its anode and oxygen from the air at the cathode has been proposed. This so­called silicon­-air battery utilizes mainly silicon and oxygen, which are the two most common elements on earth. Furthermore, the theoretical energy density of this battery type was shown to be significantly higher than the energy density of lithium­-ion batteries. Because of this, the silicon­-air battery has been a growing area of research in the last years.

Battery models help to simulate batteries based on empirical data and electrochemical systems. These models are a powerful tool in the evaluation of the performance of batteries. Parameters of the battery can be altered quickly and specifically. This can provide a powerful analysis tool to determine weaknesses in a batteries. They can also help in further developing an understanding of the operating principles of the battery technology. A specific type of model is the finite element model. In this type of model the object that is modeled is divided into small pieces and for each piece a set of (partial) differential equations is evaluated. Different electrochemical, chemical, physical and mathematical models can be modelled and combined in this tool. For this thesis a finite element model of an alkaline silicon­-air battery is developed in COMSOL. The model is based on an earlier model that was developed in 2020.

Besides the discharge mechanism, alkaline silicon-­air batteries are subject to two secondary reactions that hinder the performance of the battery: corrosion and passivation. Corrosion consumes a large part of the silicon without contributing to the discharge. Passivation creates an oxide layer on the surface of the silicon electrode, stopping the discharge reaction. Both these reactions have been implemented in the model. Besides that, a metal contact on the silicon anode is implemented in the model. The parameters used in this model are supported by empirical values for these parameters. Finally, the model was compared to experimental results.

The simulation of the discharge of the alkaline silicon­air battery was improved in several ways compared to the pre-­existing model. The corrosion was shown in the simulations, although the mechanism is somewhat simplified because of the 1D nature of the model. The passivation reaction was shown in the simulations as well, and was improved on compared to the previous model by breaking it up into two steps. Using this model, experimentally observed trends could be simulated reasonably well. The simulated discharge potential was a close representation of the experimental data, although the open circuit potential was somewhat higher, and for higher current densities the potential was somewhat lower. For different electrolyte concentrations the model showed results similar to what was found in experiments.