Next Generation Rechargeable Zn-Air Batteries: Sustainable and Abundant Materials

Abstract

Due to the increased focus on implementing renewable energy and the consequences of the intermittent aspects of these technologies, energy storage has received a lot of attention. Heavy technological progression in recent decades resulted in an apparent solution in the form of batteries. Despite the success of the Li-ion batteries and its variants, there is a push towards developing high capacity, low-cost batteries due to the sheer amount of energy that needs to be stored if the energy sector commits to a full transition to renewable energy generation. A candidate for this role is the Zn-air battery, a battery with high energy density made from cheap and abundant materials that are already available commercially, but only as a primary battery. Aside from difficulties in recharging, the Zn-air batteries suffer form self-corrosion of the anode in alkaline media, dendrite formation. The aim of this thesis was to investigate Zn-air batteries using in situ techniques and thereby provide the information required to meet these challenges. A flow cell setup was developed to investigate dissolution of typical Zn-air anode current collectors. It was found that Ti, Cu, Sn and W are all suitable current collectors in alkaline media. Ti appeared to be the most stable, while Cu, Sn and W all dissolve quickly above the dissolution potential for Zn, with the two latter materials dissolving the fastest. Using a Differential Electrochemical Mass Spectrometer (DEMS), Znair batteries with four different electrolytes were investigated with the goal of correlating electrochemical properties to gas evolution. Electron numbers were determined for a KOH electrolyte and an optimised KOH electrolyte with the additives KF and K2CO3. Furthermore, a ZnCl2 ¯ electrolyte at pH 8 and a neutral electrolyte composed of Zn(TFSI) and Li(TFSI) were found not to evolve O2.Using the same technique, the effect of dopants on the Hydrogen Evolution Reaction (HER) at a Zn surface was investigated. It was determined that a combination of In and Bi was superior to the dopants by themselves. It was also shown how Ag has detrimental effects, as it enhances the HER while it inhibits the Oxygen Evolution Reaction (OER). These results were used to support a Density Functional Theory (DFT) model that explains why these dopants affect these properties. Finally, a Zn-air capillary cell was developed. Using spatially and time-resolved X-Ray Diffraction (XRD) synchrotron radiation, it was demonstrated that a Zn/ZnO paste anode developed to promote homogeneous ZnO deposition over time actually lead to inhomogeneous distribution of Zn and ZnO, with the former being preferred in the interface between the anode and electrolyte. The results were supported by time-resolved X-ray computed tomography.