Schematic of a calcium-ion battery. (Source: Wikimedia/Synergistik.)

Lithium-ion batteries are everywhere, and for good reason. They are the most advanced battery technology commercially available today, offering high energy density, long life, light weight, fast charging and low maintenance. Lithium ions power everything from small consumer electronic devices all the way up to electric vehicles and battery energy storage systems.

However, lithium is a limited resource, and it’s getting harder to find. The price of lithium has skyrocketed over the past year, and many automakers—in the midst of an electric vehicle transition—are struggling to adapt.

The price of lithium carbonate has more than doubled since the beginning of 2022, according to trading on a contract for difference (CFD), which tracks the benchmark market for the commodity. The price is shown in Chinese yuan (CNY) per ton. (Source: Trading Economics.)

There is an urgent need to develop affordable and sustainable battery technologies, and researchers are exploring other elements on the periodic table. Earth abundant materials like sodium, magnesium, potassium and aluminum are among the most promising options.

But according to researchers from Rensselaer Polytechnic Institute (RPI) in New York, the best choice may be the one you can feel in your bones: calcium.

The Challenges of Calcium-ion Batteries

Calcium-ion is a promising alternative battery technology with the potential to be cost-efficient, sustainable and high performing. And unlike lithium, calcium is widely available at a low price. But the technology still has technical challenges that must be overcome.

For one thing, calcium ions are bigger and have a higher charge density than lithium ions. Calcium has multivalent metal ions, meaning that during battery operation, one ion will deliver two or more electrons. This increases the battery’s specific capacity, or charge density, a measure of electric charge per unit area of a surface (or per unit volume of a body). Higher charge density weakens diffusion kinetics and cyclic stability during battery operation. This problem doesn’t solely affect calcium-ion batteries—magnesium and aluminum ions have the same problem, but to an even greater extent. The reduction potential of calcium ions is also lower than for these other multivalent ions, which enables higher cell voltage of the battery.

One big obstacle to constructing a calcium-ion battery is finding suitable electrodes that can host the large and charge-dense ions. This is the challenge that researcher Nikhil Koratkar and his coauthors from RPI addressed in a paper published in Proceedings of the National Academy of Sciences (PNAS) in July 2022 called “Reversible and rapid calcium intercalation into molybdenum vanadium oxides.”

Big Channels for Big Ions

There are many ways to configure layers of molybdenum and vanadium oxides, which makes them a useful host for intercalating lithium ions. With their larger size, however, calcium ions don’t squeeze in quite as readily.

The RPI researchers experimented with several different molybdenum vanadium oxide (MoVO) structures to find a good fit for calcium ions. So-called orthorhombic and trigonal MoVOs proved the most effective, as they contain large hexagonal and heptagonal tunnels that present effective pathways for calcium diffusion.

Essentially, the big MoVO channels provide an easy way for calcium ions to go back and forth between the electrode and a water-based electrolyte.

“The higher ionic charge and the larger size of calcium ions relative to lithium makes it very challenging to insert calcium ions into the battery electrodes,” explained Koratkar in a news release issued by RPI. “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.”

In their experiments, the researchers demonstrated a specific capacity of 203 mAh/g at a charge rate of 0.2C and 60 mAh/g at 20C. The calcium-ion battery had a capacity fade rate of only ~0.15 percent per cycle.

The work is a promising step toward a much-needed alternative to lithium-ion batteries.

“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. 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,” Koratkar said.