Date of Award
Redox activity in intercalation cathodes is commonly attributed to changes in oxidation states of transition metal ions. Recently, a growing body of evidence revealed that the redox activity of oxygen contributes to the capacity of some known cathodes. We focused on designing an oxygen free intercalation cathode with redox-active anions. In order to overcome the entropic driving force for the decomposition of a charged redox active anion cathode, we looked for chemistries with solid (no gases) possible decomposition products. Additionally, the involvement of anions in a network of covalent bonds with shared electrons was considered as a possible way for the structure stabilization on anion oxidation. Rather than forming a localized hole on an anion, the whole network would be oxidized. Secondly, the involvement of an anion in multiple strong covalent bonds can hinder the decomposition kinetically. The ternary hexaboride type sodium pentaborocarbide both, chalcogenide and transition metal free, was cycled reversibly electrochemically and systematic changes of unit cell parameters were observed on the framework oxidation. For the first time, a hexaboride type carboboride anion framework was reversibly electrochemically cycled almost at elevated room temperatures. In the search for chemistries beyond lithium ion batteries the synthesis of magnesium pentaborocarbide was carried out by aliovalent ion exchange of the parent sodium pentaborocarbide structure from which density functional theory predicts excellent magnesium ion conductivities. Trilithium nonaboride was synthesized and electrochemically characterized as a weak anode material which supports a small amount of intercalation and deintercalation of lithium at room temperature demonstrated by charge discharge and cyclic voltammetry.
Shabetai, Michael R., "Synthesis and Electrochemical Characterization of Alkali Metal Borides as Redox Active Anion Frameworks" (2022). University of New Orleans Theses and Dissertations. 3045.