Researchers in Japan and the United States have developed a novel tunnel-structured molybdenum oxide electrode that enables ultra-low-strain calcium and magnesium ion storage, according to a study published in Advanced Energy Materials. The breakthrough addresses a central limitation in alternative battery chemistries, offering a potential path past the severe crystal distortion that has historically stalled multivalent rechargeable batteries outside the laboratory.
The Challenge of Divalent Ions in Battery Electrodes
Lithium-ion batteries currently power everything from smartphones to electric vehicles, but growing market demand places heavy pressure on critical mineral supplies while driving cost volatility. To diversify battery options, scientists have increasingly investigated calcium and magnesium. Both metals offer high energy densities comparable to lithium, alongside much greater geological abundance and lower potential costs.
However, practical application has stalled because calcium and magnesium ions are divalent—meaning each carries two positive charges. According to the research team, these ions interact much more strongly with battery materials than monovalent lithium ions. As divalent ions repeatedly enter and leave an electrode during charging and discharging cycles, they trigger irreversible phase changes and degrade performance by distorting the material’s crystal structure.
Tunnel-Structured Molybdenum Oxide Minimizes Strain
Rather than searching for entirely new battery chemistries or relying on conventional layered materials that expand and contract dramatically, the researchers designed a nanosized, tunnel-structured form of molybdenum trioxide (MoO₃). This specialized crystal framework accommodates calcium and magnesium ions with less than 2% lattice expansion, successfully minimizing the mechanical stress that normally ruins electrode materials over time.
“We were really surprised,” said first author Reona Iimura of the National Institute for Materials Science in Tsukuba, Japan, noting that multivalent ions generally cause substantial structural transformations due to strong interactions with host oxygen ions.
Instead of the entire crystal expanding and rearranging, the material stores ions through an unusual mechanism where only the lengths of individual metal-oxygen bonds adjust. “The crystal is like a sturdy tunnel lined with flexible springs,” Iimura explained. “The tunnel keeps its overall shape, while the springs locally stretch or contract to accommodate calcium and magnesium ions.”
Did You Know?
Calcium and magnesium behave differently once inside the same tunnel structure. Computational modeling conducted by the team revealed that calcium ions move relatively easily along the tunnel walls, whereas smaller magnesium ions bind more strongly to surrounding oxygen atoms due to a higher charge density, which slows their movement.
Next Steps Toward Next-Generation Prototypes
Despite the success of the new positive electrode, significant obstacles remain before practical application. According to Iimura, calcium batteries require the highest priority development of stable electrolytes compatible with calcium-metal anodes to enable efficient and reversible plating and stripping. For magnesium batteries, accelerating ion diffusion within the host structure remains a primary hurdle.
The research team plans to apply their design strategy to higher-voltage materials containing transition metals such as vanadium, iron, and nickel. Their ultimate goal is to increase energy density while preserving structural resilience, working alongside groups developing compatible electrolytes to build a next-generation prototype multivalent battery, according to co-corresponding author Hiroaki Kobayashi of the University of Tokyo.
Frequently Asked Questions
Why are calcium and magnesium considered alternatives to lithium?
Calcium and magnesium are much more geologically abundant and potentially less expensive than lithium, while still delivering high energy densities suitable for rechargeable batteries.
What causes calcium and magnesium batteries to degrade?
Because calcium and magnesium ions are divalent—carrying two positive charges—they interact strongly with electrode materials. Repeatedly entering and leaving the electrode during cycling causes crystal distortion, structural phase changes, and mechanical degradation.
How does the new MoO₃ electrode prevent structural damage?
The tunnel-structured molybdenum trioxide features one-dimensional channels. As ions enter and leave, only the individual metal-oxygen bonds adjust locally, resulting in less than 2% overall lattice expansion and protecting the crystal framework from large-scale stress.
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