Calcium‑Ion Batteries: The Next Frontier in Energy Storage

Andrew Corselli

Prof. Yoonseob KIM (right), Associate Professor in the Department of Chemical and Biological Engineering and Corresponding Author, and his Ph.D. student YIN Zhuoyu (left), First Author, who is holding an electrochemical cell mold. They are pictured beside a battery cell cycler. (Image: HKUST)

Researchers at The Hong Kong University of Science and Technology (HKUST) have achieved a breakthrough in calcium-ion battery (CIB) technology, which could transform energy storage solutions in everyday life. Utilizing quasi-solid-state electrolytes (QSSEs), these innovative CIBs promise to enhance the efficiency and sustainability of energy storage, impacting a wide range of applications from renewable energy systems to electric vehicles. The findings are published in the international journal Advanced Science titled “High-Performance Quasi-Solid-State Calcium-Ion Batteries from Redox-Active Covalent Organic Framework Electrolytes.”

The urgency for sustainable energy storage solutions is growing critical worldwide. As the world accelerates its shift to green energy, the demand for efficient and stable battery systems has never been more pressing. Today’s mainstream Li-ion batteries (LIBs) face challenges due to resource scarcity and near-limited energy density, making the exploration of alternatives like CIBs essential for a sustainable future.

CIBs hold great promise due to their electrochemical window comparable to that of LIBs and their abundance on Earth. However, they have struggles, particularly in achieving efficient cation transport and maintaining stable cycling performance. These obstacles currently limit the competitiveness of CIBs against commercially available LIBs.

To overcome these challenges, the research team led by Prof. Yoonseob KIM, Associate Professor, Department of Chemical and Biological Engineering, HKUST, has developed redox covalent organic frameworks to serve as QSSEs. These carbonyl-rich QSSEs demonstrated remarkable ionic conductivity (0.46 mS cm-1) and Ca2+ transport capability (>0.53) at room temperature. Combining experimental and simulation studies, the team revealed that Ca2+rapidly transports along the aligned carbonyl groups within the ordered COFs pores.

This innovative approach led to the creation of a full calcium-ion cell that exhibited a reversible specific capacity of 155.9 mAh g-1 at 0.15 A g-1 and maintained over 74.6 percent capacity retention at 1 A g-1 after 1,000 cycles, showcasing the potential of redox COFs to advance CIB technology.

Schematics showing the synthetic processes of making covalent organic framework-based quasi-solid-state electrolytes and working full cell realized in this work. (Image: HKUST)

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Kim.

Tech Briefs: What was the biggest technical challenge you faced while developing this CIB technology breakthrough?

Kim: The biggest challenge was the inherently slow movement of calcium ions. Compared to lithium ions, calcium ions have a larger size and stronger charge, which makes their diffusion much slower — especially in quasi-solid-state electrolytes, where conductivity can be over ten times lower than that of lithium. Overcoming this significant drop in conductivity was critical to making calcium-ion batteries viable.

To address this, we explored the use of highly crystalline porous materials to construct vertically aligned ion transport pathways. By engineering these structures, we aimed to create continuous channels that facilitate ion movement. Additionally, we introduced strategically placed active sites along these pathways to promote and sustain efficient calcium ion transport. This approach was not only innovative but also exceptionally challenging to implement, as it required precise control over material architecture and surface chemistry at the nanoscale.

Tech Briefs: Can you explain in simple terms how it works please?

Kim: Imagine a traditional battery as two containers connected by a bridge. Ions — charged carriers — need to cross that bridge to generate electricity. In calcium batteries, the challenge is that calcium ions are bigger and "stickier" than the lithium ions used in most batteries today. They tend to slow down or get stuck along the way, especially in our quasi-solid electrolyte — which is like a solid rather than a liquid.

So, we built a special highway system inside our battery. Using porous materials, we created clear lanes that guide the calcium ions in the right direction. We also added "service stations" along the way — spots that give the ions a little boost to keep them moving. This design helps the big calcium ions travel efficiently, so the battery can store and deliver energy effectively using abundant calcium instead of scarce lithium.

Tech Briefs: Do you have any set plans for further research/work/etc.? If not, what are your next steps?

Kim: Yes, we have clear next steps. First, we plan to further optimize the ion transport channels to achieve even higher calcium ion conductivity. Our goal is to enable single calcium ion conduction through the covalent organic frameworks (COFs) materials — meaning each channel carries ions more efficiently, like a dedicated express lane.

Second, we'll work on optimizing the cathode and anode materials and structures. Ultimately, we aim to develop a fully solid-state calcium ion battery, which would further improve energy density and safety. This brings us closer to a practical, high-performance battery using abundant calcium.

Tech Briefs: Is there anything else you’d like to add that I didn’t touch upon?

Kim: I am going to talk about the promising direction of rechargeable batteries, one important direction we're excited about: right now, we're focused on building batteries with anode active materials, but our ultimate goal is to develop anode-free batteries.

Imagine a battery where you don't need to carry around the anode material — it forms itself during charging. This would significantly increase energy density because you're using every bit of material efficiently. It's like packing more luggage into the same suitcase.

Anode-free design is considered the next generation for rechargeable batteries, and we believe calcium chemistry offers a unique opportunity to get there. It's ambitious, but that's what we're working on.

Tech Briefs: Do you have any advice for researchers aiming to bring their ideas to fruition?

Kim: Here are some suggestions:

First, don't avoid hard problems — embrace them. If a challenge is difficult, it means the solution will be valuable. Calcium ions are inherently slower than lithium but working through that fundamental challenge is what makes this breakthrough meaningful.

Second, think structurally, not just chemically. Sometimes the answer isn't new material but how you arrange it. Our porous channel design came from asking not just "what" but "how" — how do ions actually move through space?

And finally, stay patient but persistent. Breakthroughs rarely happen overnight. They come from small, incremental wins — like improving conductivity by a few percent, then a few more, until suddenly you've crossed a threshold.

The problems worth solving are the ones that don't give up easily.