|Ph.D Student||Balaish Moran|
|Subject||The Development of a Perfluorocarbon-Modified Air|
Cathode/Non-Aqueous Electrolyte System for Li-Air
|Department||Department of Energy||Supervisor||Professor Yair Ein-Eli|
|Full Thesis text|
The worldwide growing demand for energy impelled global' effort to develop clean and renewable energy sources and storage technologies with extremely high energy density. One of the main ﬁelds being explored for the next-generation power source refers to metal-air batteries. Electricity generation through a redox reaction between metal anode and oxygen, not stored in the system but accessed from the environment, represents a promising high specific energy density power source.
Lithium-oxygen (Li-O2) battery, which possesses outstanding specific capacity, involves oxygen reduction reaction (ORR) at the lightweight carbon-based air-cathode and oxidation of the light and highly oxidizable lithium metal, during discharge. The ORR takes place at a two-phase reaction zone as all organic electrolytes wet and flood the carbon surface leaving only the dissolved oxygen to participate in the ORR. Low oxygen concentration and diffusion in most organic electrolyte led us to seek for additives, which can possibly ease oxygen transport of to the reaction sites.
In this research, better oxygen accessibility in non-aqueous Li-O2 battery was investigated through the addition of perfluorocarbons (PFCs), which are known for their excellent oxygen solubility and diffusion properties, to porous carbonaceous air-cathodes. The addition of PFCs to two different carbon-based air-electrodes: Carbon nanotubes (CNT) and activated-carbon, revealed a diverse behavior, manifested both in discharge capacity and products morphology. While in the case of CNT electrode, PFCs addition showed superior performance at higher current, the addition of PFCs to activated-carbon electrode showed an increase in discharge capacity only at lower current density. In order to identify and understand the key factors determining the observed battery behavior, several scientific questions were addressed, upon their resolution, the mechanism behind the behavior of PFC- Li-O2 system was suggested. Our study showed, that the incorporation PFCs with higher superoxide solubility, but more importantly higher PFCs/electrolyte miscibility, in an air-electrode with meso-pores structure, rather than an interconnected meso and micro-pore structure, allows to better exploit PFCs potential and utilization of air-electrode' surface area via the formation of an artificial three phase reaction zones.
In addition to the conventional liquid-based non-aqueous Li-O2 battery, a solid polymer electrolyte (SPE)-based battery system, operated at a temperature higher than the melting point of the polymer electrolyte, where useful conductivity is easily achievable, was developed. Our SPE-based cell was compared to Glyme-based Li-O2 cell through potentiodynamic and galvanostatic experiments showing 80 mV higher discharge voltage and ∼400 mV lower charge voltage, and an almost comparable discharge specific capacity. The polymer electrolyte degradation study using FT-IR combined with quantitative 1H and qualitative 13C liquid NMR spectroscopy complemented with quantitative 1H, 7Li and 19F solid state NMR spectroscopy, revealed that upon cycling, the formation of formates, together with the presence of Li ions, led to the accumulation of lithium formate-polymeric-species on the cathode surface, ultimately impairing the cycle performance of the battery.
The results of this research pin-point several key factors affecting both liquid- and solid-based Li-O2 systems. Challenges in Li-O2 battery were identified, results were analyzed, and mechanisms were provided along with insights into an improved Li-O2 battery system.