Researchers at the Hong Kong University of Science and Technology have built a calcium-ion battery that retains roughly three-quarters of its capacity after 1,000 charge-discharge cycles, a result that could reframe how the energy-storage industry thinks about alternatives to lithium. The achievement addresses two problems that have stalled calcium battery development for years: poor ion transport through the electrolyte and rapid capacity fade during cycling. If the performance holds up beyond the lab, calcium’s abundance and low cost could make it a serious competitor for grid-scale storage and, eventually, consumer electronics.
Calcium is attractive because it is far more abundant in the Earth’s crust than lithium and can, in principle, carry two charges per ion instead of one. That higher charge density means a well-designed calcium cell could store comparable or greater energy using cheaper raw materials. The stumbling blocks have been sluggish ion movement and unstable interfaces that degrade within a few hundred cycles. The HKUST team’s work, detailed in an institutional release and follow-up coverage on ScienceDaily, suggests those barriers are no longer insurmountable.
How a Covalent Organic Framework Solved the Electrolyte Problem
Calcium ions carry a double positive charge, which means they interact far more strongly with surrounding electrolyte molecules than lithium ions do. That strong interaction slows the ions down and degrades the materials they pass through, which is why earlier calcium cells lost capacity quickly. The HKUST team, led by Prof. Yoonseob Kim in collaboration with Shanghai Jiao Tong University, tackled the issue by designing a quasi-solid-state electrolyte built from redox-active covalent frameworks. These are porous, crystalline polymer networks whose internal channels can be tuned at the molecular level to guide calcium ions along organized pathways rather than forcing them to diffuse randomly through a liquid.
The structured channels inside the COF electrolyte do two things at once. They speed up ion mobility, which improves power output, and they limit the side reactions that typically eat away at electrode surfaces. According to HKUST’s announcement, the resulting full cell delivered a reversible specific capacity of 155.9 mAh g-1 at 0.15 A g-1 and retained approximately 74.6% of that capacity after 1,000 cycles at a higher current of 1 A g-1. A related summary on ScienceDaily’s report emphasizes that the team achieved this performance at room temperature, an important milestone because many earlier calcium systems only functioned well when heated.
Earlier Calcium Cells and Why 1,000 Cycles Matters
To appreciate the scale of improvement, consider where calcium batteries stood before this work. A peer-reviewed study published in Angewandte Chemie International Edition described a calcium-based dual-ion battery using a concentrated Ca(FSI)₂ electrolyte paired with a PTCDA anode and graphite cathode. That cell achieved a specific capacity of approximately 63.9 mAh g-1 and held onto about 84.7% of its capacity after 350 cycles at 0.1 A g-1. Those were respectable figures at the time, but 350 cycles falls well short of the thousands of cycles that lithium-ion cells routinely deliver in commercial products like electric vehicles and home storage units.
The HKUST design nearly triples the cycle count while also more than doubling the specific capacity compared with that earlier dual-ion architecture. That combination matters because cycle life determines how many years a battery can serve before replacement, and capacity determines how much energy it stores per gram of active material. A grid-storage operator evaluating battery chemistries cares deeply about both metrics, since they directly affect the levelized cost of stored energy. Separately, researchers at Tohoku University have reported a prototype calcium rechargeable cell optimized for long life, underscoring that multiple groups worldwide are converging on the same durability target from different design angles.
Fluorine-Free Electrolytes Add Another Path Forward
The COF approach is not the only electrolyte strategy gaining traction. A study in Scientific Reports demonstrated a fluorine-free calcium salt, Ca[CB₁₁H₁₂]₂, that achieved a wide electrochemical window of roughly 4 V versus Ca²⁺/Ca and ionic conductivity of approximately 4 mS cm-1. Critically, the researchers showed room-temperature calcium plating and stripping, which is the basic electrochemical process needed for a rechargeable metal anode. The paper also cited Ca[B(hfip)₄]₂ as a related electrolyte candidate, pointing to a small but growing family of salts that can support reversible calcium metal behavior without relying on conventional fluorinated anions.
What makes these parallel efforts interesting is that they could eventually be combined. A hybrid cell pairing the monocarborane electrolyte’s wide voltage window with the COF framework’s organized ion channels might, in principle, push capacity beyond 200 mAh g-1 while extending retention past 1,500 cycles. That remains speculative, and no published study has attempted such a combination yet. Still, the fact that one line of research targets voltage stability while another targets ion transport suggests the design space for calcium batteries is much broader than skeptics assumed even a few years ago.
Where Calcium Still Falls Short of Lithium
Healthy skepticism is warranted. The 155.9 mAh g-1 capacity reported for the HKUST cell, while a record for calcium-ion systems according to the university’s own comparison, is still below the range that commercial lithium-ion cathodes deliver. Lithium nickel manganese cobalt oxide cathodes in today’s electric vehicles typically exceed 200 mAh g-1 at the materials level, and advanced anodes are pushing cell-level energy density even higher. Calcium batteries would need to close much of that gap before automakers consider them for vehicles where weight and volume are tightly constrained and performance expectations are already defined by lithium-based packs.
Manufacturing cost is the other open question. Covalent organic frameworks are synthesized through carefully controlled organic reactions and must be processed into mechanically robust, defect-minimized membranes. Scaling that chemistry from gram-scale lab samples to ton-scale industrial production could prove challenging. The fluorine-free salts highlighted in the Scientific Reports work are also specialty chemicals rather than commodities, and their cost structure is not yet clear. Even if calcium metal and common cathode materials are cheap, the total system cost will depend on how quickly these advanced electrolytes can be manufactured at scale and integrated into standard cell formats without sacrificing safety or reliability.
What This Means for Grid Storage and Beyond
In the nearer term, calcium’s most compelling applications may lie in stationary storage rather than electric vehicles. For grid-scale batteries that smooth out solar and wind fluctuations, weight and volume matter less than raw cost, safety, and longevity. A calcium system that cycles 1,000 to 2,000 times with acceptable capacity retention could already be competitive in applications like peak shaving or backup power, especially if it avoids the supply-chain constraints and price volatility associated with lithium, cobalt, and nickel. The room-temperature operation demonstrated by the HKUST group and by fluorine-free electrolyte researchers strengthens the case that such systems could run reliably in standard containerized enclosures.
Longer term, if researchers can combine high-voltage electrolytes, robust COF-based ion conductors, and stable high-capacity cathodes, calcium batteries might begin to encroach on markets now dominated by lithium-ion, including consumer electronics and light-duty vehicles. Any such transition would take years, given the extensive safety testing, certification, and manufacturing retooling required. For now, the HKUST results serve primarily as a proof of concept that calcium’s fundamental hurdles—sluggish ion transport and short cycle life—are technically solvable. That alone marks a turning point for a chemistry that many in the field once wrote off as too slow and too unstable for practical rechargeable batteries.
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*This article was researched with the help of AI, with human editors creating the final content.
