Why some researchers think calcium is the future of batteries

Lithium has reigned in the battery world for decades. Researchers have tried for years to make a cheaper, more sustainable successor to lithium-ion batteries by using the element’s more common neighbors on the periodic table. Sodium batteries are gaining ground, and commercialization is underway. Research on potassium and magnesium batteries is also making steady progress.

Yet when it comes to abundance and cost, calcium takes the crown. The third-most abundant metal, after iron and aluminum, calcium is present most everywhere in Earth’s crust. By contrast, the main metals used in lithium-ion batteries—lithium, nickel, and cobalt—are concentrated in certain geographical regions, which makes them expensive and where mining them is often hard on people and the environment.

Scientists first toyed with calcium-based batteries in the 1960s. But they worked only at high temperatures and fizzled out after just a handful of charge cycles. “It’s very difficult to get calcium to do the things that lithium does,” says Ian D. Hosein, a chemical engineer at Syracuse University. So although the calcium battery concept seemed promising, researchers hit a roadblock. “And at that point the field kind of went to sleep,” Hosein says.

Then, about 5 years ago, a few research groups found the right materials to take up and release calcium at room temperature without decomposing. Interest in calcium batteries saw a resurgence. There has since been a flurry of studies on anodes, cathodes, and electrolytes for viable calcium batteries.

This year, scientists in China have pushed the envelope further by using a novel chemistry approach to rechargeable calcium batteries. One group has developed a calcium-chlorine battery that shows much higher voltage and energy capacity than has been possible so far (Nat. Commun. 2024, DOI: 10.1038/s41467-024-45347-3). Another team reported on a calcium-oxygen battery that can be recharged more than 700 times (Nature 2024, DOI: 10.1038/s41586-023-06949-x). That research group made calcium-oxygen batteries in the form of flexible fibers and wove them into textiles that can charge electronic gadgets.

The new work proves that calcium can be a viable competitor to lithium in batteries, Hosein says. “These studies show good performance and nice chemistry, and they’re very exciting.”

Batteries store and release energy by moving ions between two electrodes through an electrolyte. Lithium’s advantages are that it has a small atomic radius and it’s monovalent, with a single electron that can be donated to form chemical bonds. The light, mobile ion is relatively easy to pack into and get out of the atomic structure of an electrode material. “The number of materials and electrolytes that work for it is vast,” Hosein says.

Calcium atoms are bigger and heavier than those of lithium. Their heft makes them sluggish, and it is hard to find compounds that can hold and release calcium ions without disintegrating after a few charge cycles.

Like magnesium, calcium is divalent: that is, it happily donates two electrons to a variety of acceptors—and that makes it extremely reactive. This causes anodes made of calcium metal to react with various chemical species and typically form a resistive layer on the surface, which degrades battery performance. In addition, doubly charged calcium ions interact strongly with charged species in the electrolyte, thus impeding the ions’ movement.

On the flip side, calcium batteries should in principle be able to match or possibly exceed the energy density of lithium-ion batteries, which stands today at 200–300 W h/kg. Even if calcium batteries don’t reach that level of performance, the element’s abundance and low cost make the batteries promising for grid storage, where weight is not a major concern, as it is for electric vehicles.

But there is a lot of fundamental chemistry sleuthing needed to make electrodes and electrolytes that work with each other, says Alexandre Ponrouch, a researcher at the Institute of Materials Science of Barcelona (ICMAB-CSIC). “I’m always skeptical of performance numbers until we have a definitive technology,” he says. Battery performance “strongly depends on the cathode, the anode, the chemistry, and design.”

On the anode side of a calcium battery, calcium metal wins for highest capacity, Hosein says. Finding the best cathode and electrolyte materials has been a bigger challenge.

Many teams are pursuing cathodes made of calcium oxides and calcium sulfides. Other scientists have tried Prussian blue–like materials, which have large, open crystal structures that provide conduits for ion transport. Yet others, including Ponrouch, are developing cathodes based on porous materials, including polymers and metal-organic frameworks.

Hosein, meanwhile, believes that the first viable calcium batteries will have cathodes made of some type of porous carbonaceous material, such as activated carbon or carbon nanotubes. He is developing an alternative—which he declines to disclose—that offers the benefits of carbon materials. “The heaviest part of a lithium-ion battery is the cathode, which is usually a ceramic of lithium metal oxide,” Hosein says. “The beautiful thing here is that there are some types of cathodes you can use for calcium that are significantly lighter.”

Finding a good electrolyte could prove more difficult. Specifically, calcium batteries need stable electrolyte materials that readily dissolve calcium ions from calcium metal anodes during half of the charge cycle and just as easily put them back into the cathode during the other half.

The breakthroughs that rekindled the field in the past decade were based on electrolyte advances. In 2016, Ponrouch and colleagues at ICMAB took the first step toward rechargeable batteries by using conventional organic electrolytes such as those used in lithium-ion technology (Nat. Mater. 2016, DOI: 10.1038/nmat4462). But the batteries had to be heated to 75–100 °C to work well.

Then, in 2018 and 2019, researchers from the University of Oxford and the University of Waterloo reported electrolytes based on calcium and boron salts that worked at room temperature. “Those were major breakthroughs,” says Brian J. Ingram, a materials scientist at Argonne National Laboratory. “In the last 5 years, there have been several new developments on highly stable electrolytes stemming from those papers.”

For example, Ingram and colleagues at Argonne found that electrolytes containing a mix of multiple positively and negatively charged ions work well. Ponrouch’s group is working to improve calcium salt–based electrolytes by carefully selecting solvents and fine-tuning salt concentrations. And others, including Hosein, are studying polymer-based solid and gel electrolytes.

Despite all the research in recent years, calcium batteries still have unacceptably short cycle lives. That’s why researchers in China are trying an alternative route to improving these devices. Rather than attempting to emulate the intercalation chemistry that drives lithium-ion batteries, these researchers are turning to conversion chemistry.

“The common conversion chemistry we are all aware of is lead acid, which involves conversion between lead dioxide and lead sulfate,” Ingram says.

One team taking this tack is based at Shanghai Jiao Tong University. Led by Hao Sun, a professor of chemistry and chemical engineering, the researchers used a calcium metal anode, a graphite cathode, and an electrolyte made of calcium, aluminum, and lithium salts. The battery relies on reversible reactions between calcium and chlorine at the cathode to form calcium chloride.

The Shanghai Jiao Tong cathodes can hold a charge of up to 1,000 mA h/g, five times that of typical intercalation cathodes. A prototype coin-cell battery based on this design delivers 3 V, enough to power a light-emitting diode light bulb. In principle, the device can reach an energy density of 1,449 W h/kg, Sun says. But it lasts just 100 cycles.

Meanwhile, the new 700-cycle Ca–O₂ battery reported earlier this year was made by researchers at Fudan, Nanjing, and Zhejiang Universities, some of whom were also involved in the Ca–Cl₂ battery work. The device has a calcium metal anode, carbon nanotube cathode, and a gel electrolyte based on an ionic liquid. Oxygen is the fuel at the cathode.

“The oxygen is derived from air rather than stored in the battery, as in most previous calcium batteries, which provides a new solution for achieving high-energy-density batteries,” says Lei Ye, a graduate student in the laboratories of Fudan University chemists Bingjie Wang and Huisheng Peng.

Ca–O₂ batteries typically form a stable calcium oxide layer on the cathode. But in the new battery, the gel electrolyte and the carbon nanotubes foster a reaction that forms more reactive calcium peroxide, which easily releases calcium ions.

Ye says the battery’s safe chemistry makes it promising for powering wearable electronics. To that end, the researchers made a fiber battery by coating a carbon nanotube fiber with calcium, surrounding it with the gel electrolyte, and wrapping the product with a carbon nanotube sheet. The team now plans to replace the ionic liquid with a less expensive solvent and develop more-stable electrolytes to increase the battery’s longevity.

“These papers are exciting because they open new doors to thinking about how to utilize calcium as an energy storage medium,” Argonne’s Ingram says. “But we’re still at a very early stage.”

Several problems remain. For instance, the Ca–O₂ battery loses much of its charge-storage capacity with every charge cycle. And according to Ponrouch, the Ca–Cl₂ battery electrolyte contains as much lithium as calcium, making it unclear which element is playing the lead role.

Translating the performance of any laboratory prototype to real-world conditions is also difficult, Hosein cautions. “There is still a long road ahead to commercialization.”

But he believes that solving the myriad problems with calcium batteries will pay off, given the raw materials’ affordability and availability. Researchers have been focusing on sodium as a successor to lithium because the chemistry is a natural extension of lithium, Hosein says. But very few researchers are studying calcium batteries, at least in the US. If the world is truly committed to electrifying infrastructure, “we need to take a quantum leap to something that is absolutely, incredibly abundant and sustainable,” he says. “The solution is calcium.”