Looming concerns regarding scarcity, high prices, and safety threaten the long-term use of lithium in energy storage devices. This concern has prompted a team of researchers from Rensselaer Polytechnic Institute to propose a greener, more efficient, and less expensive energy storage alternative.
In their research, the team asserts that calcium ions could be used as an alternative to lithium ions in batteries because of their abundance and low cost. While the larger size and higher charge density of calcium ions relative to lithium impair diffusion kinetics and cyclic stability, the research team offers oxide structures containing big open spaces (heptagonal and hexagonal channels) as a prospective solution.
In their work, they demonstrated aqueous calcium–ion batteries using orthorhombic, trigonal, and tetragonal polymorphs of molybdenum vanadium oxide (MoVO) as a host for calcium ions. Orthorhombic and trigonal MoVOs outperform the tetragonal structure because large hexagonal and heptagonal tunnels are ubiquitous in such crystals, providing facile pathways for calcium–ion diffusion.
“The calcium ion is divalent, and hence one ion insertion will deliver two electrons per ion during battery operation,” explains Nikhil Koratkar, the corresponding author of the paper. “This allows for a highly efficient battery with reduced mass and volume of calcium ions. However, the higher ionic charge and the larger size of calcium ions relative to lithium make it very challenging to insert calcium ions into the battery electrodes. We overcome this problem by developing a special class of materials called molybdenum vanadium oxides that contain large hexagonal and heptagonal shaped channels or tunnels that run through the material.”
Researchers demonstrated that calcium ions could be rapidly inserted and extracted from the material, with these tunnels acting as “conduits” for reversible and fast ion transport. The findings indicate that MoVO provides one of the best performances reported to date for the storage of calcium ions.
“Calcium-ion batteries might one day, in the not-so-distant future, replace lithium-ion technology as the battery chemistry of choice that powers our society,” explains Dr. Koratkar. “This work can lead to a new class of high-performing calcium-based batteries that use Earth-abundant and safe materials and are therefore affordable and sustainable. Such batteries could find widespread use in portable and consumer electronics, electric vehicles, as well as grid and renewable energy storage.”