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 of O-free intercalation cathode with redox-active anions (RAA). In order to overcome the entropic driving force for the decomposition of a charged RAA 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. Firstly, 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. By chemically and electrochemically oxidizing the anion framework, NaB5C was cycled reversibly between 1.5 and 3 V. Systematic changes of unit cell parameters were observed on the framework oxidation. Boron and carbon K-edge XANES spectroscopy measurements of the chemically oxidized framework provide insight on B5C octahedra distortion as a function of sodium vacancy content. For the first time, a transition-metal-free boron framework is reversibly electrochemically cycled. The proposed Mg0.5B5C compound is isoelectronic with NaB5C and can potentially be prepared by the aliovalent ion exchange of Mg2+ for 2 Na1+ ions. Using DFT, the activation energies for Mg2+ diffusion in [100] and in [001] directions were determined. Activation energy value reached a maximum of 0.92 eV for a jump in the [100] direction. However, the value for the activation energy for the Mg2+ diffusion in the [001] direction is very low, just 0.39 eV. The presence of carbon atoms along the diffusion pathway leads to a lower activation energy of diffusion. The ionic conductivity in [001] direction was estimated to be 4●10-4 S/cm. The average voltage for the reaction corresponding to a cathode discharge, 0.5 Mg + B5C ↔ Mg0.5B5C was found to be 1.56 V. Thus, the reaction is spontaneous and the concept of hexaborides as RAA cathode was confirmed by DFT calculations.

Carbaboride, NaB5C and Mg0.5B5C, as intercalation cathodes with redox-active anions

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.