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Calcium and magnesium batteries have a stubborn weakness: the very ions that make them attractive are the ones that destroy their electrodes. A US–Japan research team reports in Advanced Energy Materials on May 9, 2026, that a tunnel-structured molybdenum oxide electrode can host both calcium and magnesium ions with less than 2 percent lattice expansion, quieting the mechanical strain that has kept these multivalent batteries stuck in the lab. If the results hold up in complete cells, the design could remove one of the biggest structural barriers on the road to post-lithium storage.
Lithium-ion cells have become the default power source for everything from pocket electronics to electric vehicles. But the world's appetite for them has exposed the fragility of lithium supply chains, the geopolitical weight of critical minerals, and the price volatility that makes long-term planning hard for manufacturers.
Calcium and magnesium look like natural fallbacks. Both are geologically abundant, widely dispersed, and cheap, and both can in principle deliver energy densities comparable to lithium. Yet after more than a decade of research, calcium and magnesium cells have failed to leave the laboratory. The bottleneck is almost always the same: the positive electrode.
Most known cathode materials cannot reversibly absorb these large, doubly charged ions at room temperature. When they try, they pay a heavy structural price.
The trouble begins with electrostatics. A calcium or magnesium ion carries twice the positive charge of a lithium ion, so it pulls far harder on the oxygen ions in a cathode crystal. With every charge–discharge cycle, these divalent ions force their way into and out of the host structure, stretching bonds, displacing atoms, and sometimes triggering irreversible phase transformations. Over time, the electrode loses capacity and the cell dies.
The usual research response has been to hunt for novel electrode compositions. The team behind the new paper took a different route: instead of searching for a tougher material, they engineered a host framework that barely needs to move in the first place.
Their candidate is a nanosized, tunnel-structured form of molybdenum trioxide (MoO3). Conventional layered oxides swell and contract as ions enter and exit; this material instead has rigid one-dimensional channels that guide ions through the crystal while holding the overall framework steady.
The results were better than the team expected. Over repeated calcium and magnesium insertion, lattice expansion stayed below 2 percent, with no meaningful structural rearrangement. First author Reona Iimura of the National Institute for Materials Science in Tsukuba, Japan, said the team was genuinely surprised. Multivalent ions normally interact so forcefully with oxygen in the host lattice that they distort it substantially, and sometimes transform it entirely. Watching the MoO3 electrode absorb repeated insertion with barely a ripple in its crystal structure, she said, was an unexpectedly positive outcome.
The reason appears to be a mechanism that does not rely on bulk expansion. As ions enter the MoO3 channels, the crystal does not inflate like a sponge. Instead, individual molybdenum–oxygen bonds stretch or shrink locally to accommodate the guests, while the channel architecture remains essentially static.
Iimura likens the structure to a rail tunnel whose walls carry a line of shock absorbers: the bore never changes shape, but each absorber flexes inward or outward as ions pass. That distinction matters for battery life. A material that absorbs strain locally, rather than spreading it across the whole lattice, can in principle cycle far longer without cracking or converting into electrochemically dead phases.
The tunnel may be the same, but the two ions experience it differently. Computational modeling by the group shows calcium moves relatively easily along the channel walls, while magnesium, which is smaller but carries a higher charge density, binds more tightly to surrounding oxygen atoms and diffuses more slowly.
That contrast, Iimura said, reflects the interplay of size and charge. A smaller ion concentrates its charge and clings more strongly to oxygen; the host's channel geometry then determines how easily each species can slip through. The same passage can be welcoming to one ion and grudging to another.
The model was cross-checked against multiple experimental techniques, and the agreement between computation and measurement gave the researchers confidence that they had identified a genuinely new storage mechanism, not just another promising electrode.
Solving the electrode problem does not solve the battery. Iimura said the top priority for calcium chemistry is a stable electrolyte compatible with a calcium-metal anode, enabling efficient and reversible calcium plating and stripping. For magnesium, the biggest hurdle remains the positive electrode itself, specifically accelerating Mg2+ diffusion through the host structure.
Co-corresponding author Hiroaki Kobayashi of the University of Tokyo emphasized that electrode materials and electrolytes must be developed as one integrated package, not in isolation. The endgame, he said, is a working multivalent battery prototype designed as a complete system rather than a stack of promising components.
The principle is deliberately general: combine open diffusion channels with a framework that relieves strain through reversible bond-length changes rather than global expansion. The team plans to apply it to higher-voltage oxides containing vanadium, iron, and nickel, aiming to increase energy density while preserving structural resilience. The researchers also want partners working on compatible electrolytes and metal anodes, acknowledging that practical multivalent cells will require coordination across the entire supply chain of materials science.
For US energy planners, the appeal of multivalent chemistry is straightforward. Calcium and magnesium reserves are large, geographically broad, and geopolitically unremarkable. Neither metal carries the supply-chain concentration risk associated with lithium, cobalt, or nickel. Even a partial shift of stationary storage to multivalent cells could ease pressure on lithium supply chains and dampen price shocks.
It is worth being sober about the timeline. A laboratory electrode demonstration, however clean the cycling data, is years away from a commercial cell. Electrolyte development, anode compatibility, and manufacturability all remain unresolved. But the significance of the new paper is that it reframes the problem: if the mechanical-strain barrier can be engineered away, the remaining obstacles become defined engineering challenges with clear research paths rather than a fundamental wall. That is an unusually tractable position for the multivalent battery field.
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.
