HKUST Engineers a Calcium-Ion Battery That Sustains 74% Capacity After 1,000 Charge Cycles

Lithium-ion batteries power most of the world's portable electronics and are central to the electric vehicle transition. They work well, but two pressures are building against them. Lithium resources are geographically concentrated and facing increasing demand. And at current energy densities, the technology is approaching physical limits that incremental engineering improvements cannot push far beyond.

Calcium offers an alternative that looks attractive on paper. It is the fifth most abundant element in the earth's crust, widely distributed geographically, and its electrochemical properties - particularly its voltage window - are comparable to lithium's. Calcium-ion batteries could, in principle, deliver similar performance at lower cost and with fewer resource constraints.

The problem has been getting calcium ions to actually move through a battery efficiently. Calcium's divalent charge - Ca2+, carrying two positive charges versus lithium's single charge - makes it much harder to transport through electrolyte materials. It tends to bind strongly to electrode surfaces, move slowly, and degrade cycling performance rapidly. These bottlenecks have kept calcium-ion batteries far behind their lithium counterparts in practical performance.

A Framework That Channels Calcium Ions

A research team led by Professor Yoonseob Kim, associate professor of chemical and biological engineering at the Hong Kong University of Science and Technology, has developed an electrolyte approach that overcomes these transport barriers. The solution uses redox-active covalent organic frameworks - ordered, porous organic materials with precisely arranged chemical groups - as quasi-solid-state electrolytes.

Covalent organic frameworks are constructed from molecular building blocks connected through covalent bonds, forming predictable crystalline pore structures. By engineering these frameworks to be rich in carbonyl groups - carbon-oxygen double bonds - the team created materials that facilitate rapid calcium ion transport along predictable pathways within the ordered pores.

The resulting electrolytes demonstrated ionic conductivity of 0.46 mS/cm and a calcium transference number above 0.53 at room temperature. Both figures are meaningfully better than what conventional calcium electrolytes achieve. Computational simulations confirmed the mechanism: Ca2+ ions move rapidly along aligned carbonyl groups within the COF pores, channeled by the framework's ordered geometry rather than having to navigate disordered, resistive pathways.

Cell Performance

The full calcium-ion cell built with this electrolyte achieved a reversible specific capacity of 155.9 milliampere-hours per gram at a current density of 0.15 A/g. At the higher current density of 1 A/g - relevant to faster charge and discharge applications - the cell retained 74.6% of that capacity after 1,000 cycles. That retention figure addresses one of the most persistent criticisms of calcium-ion technology: that it degrades too quickly under real operating conditions to be practically useful.

The study was conducted in collaboration with researchers at Shanghai Jiao Tong University and was published in Advanced Science under the title "High-Performance Quasi-Solid-State Calcium-Ion Batteries from Redox-Active Covalent Organic Framework Electrolytes."

What Comes Next

The results demonstrate a working solution to calcium-ion batteries' fundamental ion transport challenge, at least at laboratory scale. Translating this into commercially viable cells requires additional work: scaling the synthesis of COF electrolyte materials, demonstrating performance in larger format cells, testing across a wider range of temperatures and charging conditions, and evaluating whether the manufacturing costs are competitive with lithium-ion alternatives.

The 74.6% retention after 1,000 cycles is encouraging but still below the retention figures expected from mature lithium-ion technology. Whether further optimization of the COF structure, electrode materials, or cell design can improve this is an open question. The broader point the study establishes is that calcium transport - previously the field's central bottleneck - can be managed effectively through careful electrolyte design, removing the primary conceptual barrier to calcium-ion battery development.