|Ph.D Student||Cohn Gil|
|Subject||Study and Development of Silicon-Air Battery|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Yair Ein-Eli|
|Full Thesis text|
In this research a rather unique type of metal - air battery, non-aqueous primary silicon - air battery, is investigated. This system is comprised of a silicon wafer as the anode active material, and a highly conductive hydrophilic room temperature ionic liquid electrolyte named 1-ethyl-3-methylimidazolium oligofluorohydrogenate [EMI∙(HF)2.3F]. Cathodic oxygen reduction takes place on a carbon-based air electrode. Electrochemical studies performed on various silicon types revealed the superior performance of heavily doped n-type silicon. Cell discharges at different constant current densities in ambient atmosphere show working voltages of 0.8 to 1.1 V. The anodic dissolution of silicon and the formation of SiO2 as the reaction product are described in a multistep reaction process involving the formation of H2O in the electrolyte.
One of our goals in this work is to identify and understand the constricting factors of battery operation and to enhance its performance by overcoming these limitations. Scanning electron microscopy and X-ray photoelectron spectroscopy examinations show that as discharge advances, the "wet" face of the air electrode is gradually covered by discharge products which prevent the continuous diffusion of oxygen to the electrode-electrolyte interface. Our studies show that battery performance demonstrably improves with the addition of water to the electrolyte. That is due to shifting of the generation zone of SiO2 discharge product from the air cathode surface into the bulk region of the liquid electrolyte. Such addition of 15 wt% water lead to an increase of 35% in the discharge capacity. Electron paramagnetic resonance studies at the air electrode reveal the degradation of MnO2 oxygen reduction catalyst during discharge. The results show that during discharge the air electrode undergoes deactivation due to MnO?2 transformation into MnF2 through electrochemical reactions involving the ionic liquid electrolyte, which results in discharge capacity losses.
Electrochemical impedance spectroscopy studies were carried out in order to examine not only the air cathode but anode evolution as well during discharge. Such inspections indicate that the interfacial impedance between the electrolyte and the silicon wafer increases upon continuous discharge. Equivalent circuit fitting parameters indicate the difference in the anode - electrolyte interface characteristics for different types of silicon wafer.
The results of this research imply that the silicon - air battery can find immediate applications in sectors such as micro electro-mechanical systems and sensors, as it can provide an internal, autonomous and self sustained energy source.