A team of researchers at The Hong Kong University of Science and Technology (HKUST) has achieved a significant breakthrough in calcium-ion battery (CIB) technology, potentially offering a more sustainable and resource-efficient alternative to the lithium-ion batteries that currently dominate the energy storage market. The work, published in the journal Advanced Science, details a novel battery design incorporating quasi-solid-state electrolytes (QSSEs) that dramatically improves ion transport and battery stability.
The global demand for energy storage is rapidly increasing, driven by the expansion of renewable energy sources like solar and wind power. While lithium-ion batteries have become ubiquitous in applications ranging from electric vehicles to grid-scale storage, concerns about the limited availability of lithium and the environmental impact of its extraction are prompting a search for alternative battery chemistries. Calcium, being far more abundant in the Earth’s crust than lithium, presents a compelling option.
However, calcium-ion batteries have historically faced significant technical hurdles. Calcium ions are larger and move less readily than lithium ions within a battery’s electrolyte, leading to lower performance and shorter lifespans. Maintaining stable performance through repeated charge and discharge cycles has also proven challenging. These limitations have prevented CIBs from effectively competing with their lithium-ion counterparts.
The HKUST team, led by Associate Professor Yoonseob Kim of the Department of Chemical and Biological Engineering, tackled these challenges by focusing on the electrolyte – the medium through which ions travel between the battery’s electrodes. They engineered a new type of QSSE using redox covalent organic frameworks (COFs). These materials, characterized by their highly structured pores and carbonyl-rich composition, facilitate faster and more efficient calcium ion transport.
“We designed these materials to act as highways for calcium ions,” explained Prof. Kim. “The aligned carbonyl groups within the COF structure provide a clear pathway for the ions to move quickly, and efficiently.” Laboratory experiments and computer simulations confirmed that Ca2+ ions indeed move rapidly along these aligned pathways, resulting in significantly improved ionic conductivity (0.46 mS cm-1) and Ca2+ transport capability (>0.53) at room temperature.
The researchers constructed a full calcium-ion battery cell using this new QSSE and achieved impressive results. The battery demonstrated a reversible specific capacity of 155.9 mAh g-1 at a current density of 0.15 A g-1. Crucially, the battery retained over 74.6% of its initial capacity after 1,000 charge and discharge cycles, even at a higher current density of 1 A g-1. This level of stability represents a substantial improvement over previous CIB designs.
The success of this approach hinges on the unique properties of the redox covalent organic frameworks. The COFs not only enhance ion transport but also contribute to the overall structural stability of the electrolyte, preventing degradation during repeated cycling. This addresses a key limitation of earlier CIB prototypes.
The implications of this research extend beyond simply finding an alternative to lithium. Calcium-ion batteries, if successfully scaled, could offer several advantages. Calcium is significantly cheaper and more readily available than lithium, potentially lowering the cost of energy storage. CIBs have the potential for higher energy density than current lithium-ion technology, although further research is needed to fully realize this potential.
While the HKUST team’s work represents a major step forward, several challenges remain before calcium-ion batteries can become commercially viable. Scaling up the production of the redox covalent organic frameworks to meet industrial demand will require significant engineering effort. Further optimization of the battery’s components, including the electrodes, is also necessary to maximize performance and lifespan.
The research was a collaborative effort between HKUST and Shanghai Jiao Tong University, highlighting the growing international focus on developing next-generation battery technologies. Prof. Kim emphasized the broader significance of the work, stating, “Our research highlights the transformative potential of calcium-ion batteries as a sustainable alternative to lithium-ion technology. 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.”
The development of efficient and sustainable energy storage solutions is critical for addressing climate change and transitioning to a cleaner energy future. The advancements made by the HKUST team offer a promising pathway towards achieving this goal, potentially reshaping the landscape of energy storage in the years to come.
