HKUST Researchers Build a Calcium-Ion Battery That Retains 75 Percent Capacity After 1,000 Cycles, Challenging Lithium's Dominance
A novel covalent organic framework electrolyte solves calcium-ion transport problems that have stalled the technology for decades, opening a path to cheaper, more abundant battery chemistry.
Overview
Scientists at the Hong Kong University of Science and Technology have demonstrated a calcium-ion battery that delivers a reversible capacity of 155.9 milliampere-hours per gram and retains more than 74.6 percent of that capacity after 1,000 charge-discharge cycles. The results, published in Advanced Science, mark a significant step toward a battery chemistry built on one of the most abundant metals on Earth rather than increasingly contested lithium supply chains.
The key innovation is a quasi-solid-state electrolyte made from redox-active covalent organic frameworks (COFs) — engineered porous materials whose carbonyl-rich channels guide calcium ions through the battery far more efficiently than previous electrolyte designs allowed.
What We Know
Calcium is the third-most abundant metal in Earth’s crust, trailing only iron and aluminum. Its raw material cost is a fraction of lithium’s, and it is not concentrated in a handful of politically sensitive regions. In principle, calcium-ion batteries could match or exceed lithium-ion energy densities of 200–300 Wh/kg while costing as little as $40–$50 per kilowatt-hour, according to projections cited by Chemical & Engineering News.
The problem has always been making the chemistry work in practice. Calcium ions are larger and heavier than lithium ions, and their divalent nature — each atom donates two electrons — makes them highly reactive. They tend to form resistive surface layers on anodes that degrade performance, and finding electrode materials that can absorb and release calcium ions without disintegrating after repeated cycles has proved difficult.
The HKUST team, led by Prof. Yoonseob Kim of the Department of Chemical and Biological Engineering, attacked the electrolyte side of the problem. As reported by ScienceDaily, the researchers designed COFs with aligned carbonyl groups that create ordered channels inside their porous structures. Calcium ions travel rapidly along these channels, achieving an ionic conductivity of 0.46 millisiemens per centimeter and a calcium-ion transport number above 0.53 at room temperature.
The quasi-solid-state design also addresses safety concerns. Traditional liquid electrolytes risk leakage and thermal runaway; the COF-based electrolyte is mechanically stable and far less prone to such failures, according to the HKUST research announcement.
The full battery cell, assembled with the COF electrolyte, delivered its 155.9 mAh/g capacity at a current density of 0.15 A/g. When pushed to a higher rate of 1 A/g over 1,000 cycles, it still retained more than 74.6 percent of capacity — a durability figure that, while below the best lithium-ion cells, represents a major improvement over prior calcium-ion attempts. The work was a collaboration between HKUST and Shanghai Jiao Tong University, with PhD student Yin Zhuoyu as first author.
“Our research highlights the transformative potential of calcium-ion batteries as a sustainable alternative to lithium-ion technology,” Prof. Kim said in the university’s announcement. “By leveraging the unique properties of redox covalent organic frameworks, we have taken a significant step towards realizing high-performance energy storage solutions that can meet the demands of a greener future.”
What We Don’t Know
The research demonstrates strong laboratory performance, but the path from lab cell to commercial product remains long and uncertain. The study does not report volumetric energy density, a critical metric for applications like electric vehicles where physical size matters as much as weight. It also does not address manufacturing scalability — covalent organic frameworks are synthesized through specialized chemical processes that have not yet been proven at industrial volumes.
Calcium-ion battery research is still in its early stages compared to sodium-ion technology, which has already reached commercial production through companies like CATL. Whether calcium can close that gap depends on sustained funding and breakthroughs in anode materials, which the HKUST study does not directly address.
The 74.6 percent capacity retention after 1,000 cycles, while a milestone for calcium-ion chemistry, would need to improve substantially to compete with lithium-ion cells that routinely exceed 80 percent retention after several thousand cycles in commercial applications.
Analysis
The HKUST result is best understood not as a lithium-ion killer but as proof that calcium-ion chemistry is no longer stuck at the starting line. For decades, researchers have noted that calcium’s theoretical properties look excellent on paper — high volumetric capacity of 2,072 mAh/cm³, a low redox potential of -2.87 V, and near-universal availability — but the engineering reality has lagged far behind. The field only began to regain momentum around 2018-2019 with the development of room-temperature calcium-boron salt electrolytes.
The COF electrolyte approach is notable because it tackles multiple problems simultaneously: ion transport speed, cycling stability, and safety. If the framework can be adapted to work with improved anode and cathode materials, it could form the basis of a complete battery system rather than an isolated component advance.
Grid-scale energy storage, where weight is less critical and cost per kilowatt-hour matters most, may prove the most natural first market for calcium-ion technology. At projected costs of $40–$50/kWh, calcium batteries would undercut even the most optimistic lithium-iron-phosphate forecasts — assuming the chemistry can be manufactured at scale.