New calcium-ion battery design delivers high performance without lithium

As countries expand renewable energy production, the need for dependable and efficient battery storage continues to grow. Lithium-ion batteries (LIBs) currently dominate the market, but concerns remain about limited lithium resources and the practical limits of their energy density. These constraints have intensified the search for alternative battery chemistries that can meet long-term global energy demands.

Calcium-ion batteries are attracting attention because calcium is abundant and offers an electrochemical window comparable to that of LIBs. However, technical barriers have slowed progress. In particular, calcium ions can be difficult to move efficiently within a battery, and maintaining stable performance over repeated charge and discharge cycles has proven challenging. These issues have kept CIBs from competing directly with established lithium-based systems.

Quasi-Solid-State Electrolytes Improve Ion Transport

To address these problems, a team led by Prof. Yoonseob KIM, Associate Professor in the Department of Chemical and Biological Engineering at HKUST, engineered redox covalent organic frameworks to function as QSSEs. These carbonyl-rich materials achieved strong ionic conductivity (0.46 mS cm^-1) and Ca²⁺ transport capability (>0.53) at room temperature.

Through both laboratory experiments and computer simulations, the researchers discovered that Ca²⁺ ions move quickly along aligned carbonyl groups inside the structured pores of the covalent organic frameworks. This organized internal pathway helps explain the improved ion mobility and overall battery performance.

Strong Performance Over 1,000 Cycles

Using this design, the team assembled a full calcium-ion battery cell that delivered a reversible specific capacity of 155.9 mAh g^-1 at 0.15 A g^-1. Even at 1 A g^-1, the battery retained more than 74.6% of its capacity after 1,000 charge and discharge cycles. These results demonstrate the potential of redox covalent organic frameworks to significantly strengthen CIB technology.

Prof. Kim said, "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 research was carried out through a collaboration between HKUST and Shanghai Jiao Tong University.