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New York: Calcium ions could be used as a greener, more efficient, and less expensive energy storage alternative to lithium ions in batteries because of their abundance and low cost, according to a study.

The researchers from Rensselaer Polytechnic Institute in the US noted that long-term use of lithium-ion batteries comes with issues such as scarcity, high prices, and safety concerns.

“The vast majority of rechargeable battery products are based on lithium-ion technology, which is the gold standard in terms of performance,” said Nikhil Koratkar, a professor at Rensselaer Polytechnic Institute.

“However, the Achilles’ heel for lithium-ion technology is cost. Lithium is a limited resource on the planet, and its price has increased drastically in recent years,” said Koratkar, corresponding author of the study published recently in the journal PNAS.

The researchers are working on an inexpensive, abundant, safe, and sustainable battery chemistry that uses calcium ions in an aqueous, water-based electrolyte.

While the larger size and higher charge density of calcium ions relative to lithium impair diffusion kinetics and cyclic stability, Koratkar and his team offer oxide structures containing big open spaces as a prospective solution.

In the study, the researchers demonstrated an aqueous calcium-ion battery using molybdenum vanadium oxide (MoVO) as a host for calcium ions.

“The calcium ion is divalent, and hence one ion insertion will deliver two electrons per ion during battery operation,” Koratkar explained.

“This allows for a highly efficient battery with reduced mass and volume of calcium ions,” he added.

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, the researchers said.

They overcame 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.

The team demonstrated that calcium ions can 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, the researchers said.

“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,” said 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,” he added.

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