Facile Chemical Routes to Assess the Feasibility of Electrode Hosts for Multi-Valent-Ion Insertion/Extraction

Abstract

The drive to increase the energy density, lower the cost, and enhance the safety is creating enormous interest in rechargeable battery systems based on multi-valent cations, such as magnesium, zinc, and aluminum as these metals are relatively abundant and inexpensive and offer superior safety compared to lithium when exposed to air and cycled in the cell. However, identifying potential electrode hosts for reversible reaction of multi-valent ions is a great challenge. Also, the lack of well-established electrolytes to assess the true behavior of the electrode materials for multi-valent working ions further hampers the efforts. To overcome this difficulty, this presentation will focus on the development and use of simple, facile chemical methods to assess the reaction/insertion/extraction of multi-valent ions into potential hosts without requiring the electrolytes for multi-valent working ions or the assembly of the electrochemical cells. For example, extraction of multi-valent ions from potential electrode hosts with nitronium tetrafluoroborate in acetonitrile medium and insertion of multi-valent ions into potential hosts with a microwave-assisted solvothermal process will be presented. Use of these insertion/extraction methodologies with example systems based on both close-packed structures and open structures will be presented. Particularly, the challenges associated with close-packed systems such as spinel hosts due to the electrostatic repulsion between the working multi-valent ions and the highly-charge host transition-metal ions will be demonstrated in comparison to the lithium analogues. Furthermore, tuning the reducing power during the microwave-assisted solvothermal process by choosing appropriate solvents will be highlighted. Based on the chemical methods presented, the identification of a few new hosts for reversible reaction of multi-valent ions will be presented.