Ultrathin titanium dioxide coatings on carbon nanotubes for stable lithium oxygen battery cathodes

Abstract

Fossil fuels hold the biggest share in energy sources for a very long time, especially in transportation, because of their appealing properties like very high energy efficiency, easy transport to any place in the world, very straightforward usage principle and they used to be quite abundant. However fossil fuel consumption results into release of harmful greenhouse gasses that causes global warming. On the other hand fossil fuels are not very abundant anymore and as a product that is formed in millions of years, the increasing energy demand worsens the situation. That is why renewable energy sources are more and more pronounced each day in the last half century. Nonetheless, irregular nature of the renewable energy sources makes them highly unpractical. Energy can only be harvested from renewable energy sources in specific time or specific locations, for instance, it is not possible to harvest energy from sun all day long or wind turbines can only be efficient in the places that there is sufficient wind power. This being the case, a clever approach is needed in order to be able to benefit from such convenient energy sources. Energy storage systems are the saviour in this picture since they can be used to store the energy that is produced from renewable energy sources and available when needed. For instance, lithium oxygen (Li-O2) batteries are a very promising candidates for a replacement of fossil fuels in transportation due to their very high theoretical gravimetric energy density. Oxygen is used as active cathode material unwanted side product formations on cathode-electrolyte interface. These side products are accumulating on the cathode surface upon battery cycling and result into drastic capacity fading. Especially carbon based materials are not stable against battery cycling in Li-O2 batteries even tough they have quite profitable features as a cathode material for Li-O2 batteries, such as; high surface area, low weight, high electrical conductivity, good oxygen reduction reaction activity etc. In this thesis study, the motivation is to increase the stability of carbon nanotubes (CNTs) while benefiting from their aforementioned advantages in Li-O2 batteries. In order to achieve this, an ultrathin and uniform titanium dioxide (TiO2) layer is coated on CNT surface by atomic layer deposition method. Prior to TiO2 coating an effective functionalization method is introduced to CNT surfaces to facilitate a uniform coating. Transmission electron microscopy imaging and x-ray diffractometer analysis are performed to observe coating properties. Xray photoelectron spectroscopy analysis and scanning electron microscopy imaging show the subsided side reactions, proving the stability of the TiO2 coated CNT cathode. TiO2 protective layer significantly prevents side product formation due to reduced cathode degradation and shows superior capacity retention compared to pristine CNT cathode upon full capacity battery cycling.