Lithium-ion batteries power everything from smartphones to electric vehicles, but growing demand is increasing pressure on supplies of critical minerals while raising concerns about cost volatility and long-term sustainability.

To diversify battery chemistries, researchers have been investigating calcium and magnesium as alternatives. Like lithium, both metals could deliver high energy densities, but they are much more geologically abundant and potentially less expensive.

Despite their potential to meet the growing rechargeable battery demand, calcium and magnesium batteries have struggled to leave the laboratory. The main issue stalling their practical application is finding positive electrode materials that can reversibly host these metal ions at room temperature while maintaining high performance.

A team of researchers based in Japan and the United States may have found a promising solution to one of the biggest challenges facing calcium and magnesium batteries. Because calcium and magnesium ions are divalent—each carrying two positive charges—they interact much more strongly with battery materials than monovalent lithium ions. As these divalent ions repeatedly enter and leave an electrode during charging and discharging, they can distort the crystal structure of the electrode material, trigger irreversible phase changes, and ultimately degrade battery performance.

Rather than relying on an electrode whose crystal lattice expands and contracts dramatically, the researchers developed a tunnel-structured molybdenum oxide whose framework accommodates calcium and magnesium ions with less than 2% lattice expansion. This design minimizes the mechanical stress that normally damages electrode materials during repeated charging and discharging.

“We were really surprised,” says first author Reona Iimura of the National Institute for Materials Science in Tsukuba, Japan.

“In general, multivalent ions interact strongly with the oxygen ions in the host structure, causing not only large lattice distortion but sometimes substantial structural transformation. Therefore, exceptionally small lattice expansion—less than 2%—without significant structural transformation during cycling was an unexpectedly positive result.”

Much of the research on calcium and magnesium batteries has focused on discovering novel electrode materials. Instead, the team focused on something more fundamental: preventing the crystal structure from becoming damaged in the first place.

After analyzing different electrode materials, they identified repeated structural distortion during charging and discharging as a major bottleneck limiting battery lifetime. Rather than searching for another battery chemistry, they designed a material whose framework could absorb the stress of ion insertion while remaining largely intact.

Their solution was a nanosized, tunnel-structured form of molybdenum trioxide (MoO₃). Unlike conventional layered materials, the crystal contains one-dimensional channels that provide pathways for ions to move through while maintaining a robust framework. The researchers found that the material stores calcium and magnesium ions through an unusual mechanism. Rather than the entire crystal expanding and rearranging, only the lengths of individual metal–oxygen bonds adjust as ions enter and leave.

“The crystal is like a sturdy tunnel lined with flexible springs,” Iimura explains. “The tunnel keeps its overall shape, while the springs locally stretch or contract to accommodate calcium and magnesium ions.”

This subtle rearrangement allows the crystal to absorb local strain without undergoing the large-scale structural transformations commonly seen in other oxide electrodes.

Although both calcium and magnesium ions were accommodated within the same tunnel structure, the researchers discovered that they behave differently once inside.

Computational modelling revealed that calcium ions move relatively easily along the tunnel walls, whereas the smaller magnesium ions bind more strongly to surrounding oxygen atoms due to their higher charge density, slowing their movement through the material.

“The contrasting behavior reflects a balance between ionic size and charge density,” says Iimura. “Ion mobility is governed not simply by ionic size, but by how the ion’s size and charge density match the geometry and oxygen environment of the host structure.”

The agreement between computational predictions and multiple experimental techniques gave the researchers confidence that they had uncovered a genuinely new storage mechanism rather than simply identifying another promising electrode material.

Although the new electrode represents an important advance, some obstacles remain before practical application.

“For calcium batteries, the highest priority is developing electrolytes that are stable and compatible with a calcium-metal anode, enabling efficient and reversible calcium plating and stripping,” says Iimura. “For magnesium batteries, one of the biggest challenges remains the positive electrode, particularly accelerating Mg²⁺ diffusion within the host structure.”

Success will depend on optimizing the entire battery cell, where the electrode materials and electrolyte work together as an integrated system.

The team believes their design strategy could extend beyond molybdenum oxide. By combining open diffusion pathways with crystal frameworks capable of relieving local strain through reversible bond-length changes, similar principles may guide the development of future multivalent batteries.

Next, the researchers plan to apply the concept to higher-voltage materials containing transition metals such as vanadium, iron, and nickel, with the goal of increasing energy density while preserving the same structural resilience. They also hope to collaborate with groups developing compatible electrolytes and metal anodes, recognizing that practical multivalent batteries will require a coordinated effort.

“Our final milestone is to develop a next-generation prototype multivalent battery,” says co-corresponding author Hiroaki Kobayashi of the University of Tokyo.

Reference: Reona Iimura et al., Ultra-Low-Strain Calcium and Magnesium Ion Storage Enabled by Tunnel-Structured MoO₃ Positive Electrode. Advanced Energy Materials (2026), DOI: 10.1002/aenm.71006.

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