Transition metal fluorides for lithium-fluoride ions battery
Teng, Yin Ting
Date of Issue2014
School of Materials Science and Engineering
This thesis presents an in-depth investigation of the conversion reaction mechanism of cobalt difluoride (CoF2) as the cathode material in lithium half-cell and the synthesis of nickel-cobalt difluoride [NixCo(1-x)F2]. The use of CoF2 and thorough understanding of the conversion reaction mechanism of CoF2 was first examined by mixing CoF2 with conductive carbon. It is shown in the work that the reversibility of the system depends on the local electronic connectivity of the CoF2 to the conductive carbon, which is largely affected by the morphology of the conductive carbon. In addition, techniques such as XRD, SQUID, EELS, XPS, and TEM have been used to investigate the oxidation, structural and morphology change upon discharging and recharging. In order to have a better understanding of the factors affecting the reversibility of conversion mechanism, a systematic study involving different aspects such as varying weight percentage of conductive carbon, different type of conductive carbon, surface area of CoF2-carbon nanocomposite, and degree of close proximity of the active material to the carbon nanostructured support were also investigated. Lastly, the synthesis of mixed nickel-cobalt fluoride [NixCo(1-x)F2], with x ranging from 0.25 to 0.75, has been successfully demonstrated. Use of mixed nickel-cobalt fluoride in lithium half-cell was demonstrated by galvanostatic and cyclic voltammetry experiments. Results from the galvanostatic and cyclic voltammetry tests have shown that tuning of the discharge voltage is possible via nickel doping. All of the nickel based fluorides have a higher discharge voltage than pure cobalt fluoride.