Calcium-ion battery breakthrough may rival lithium-ion systems

Researchers at The Hong Kong University of Science and Technology (HKUST) have developed a novel calcium-ion battery featuring quasi-solid-state electrolytes made from redox-active covalent organic frameworks (COFs). This design addresses a major challenge in calcium-ion batteries—poor ion transport and unstable cycling—by creating aligned channels within the COFs that facilitate rapid calcium ion (Ca2+) movement. The new electrolyte achieved an ionic conductivity of 0.46 mS cm–1 and a Ca2+ transport number greater than 0.53 at room temperature. The assembled battery demonstrated a reversible specific capacity of 155.9 mAh g–1 at 0.15 A g–1 and retained over 74.6% capacity at 1 A g–1 after 1,000 cycles, indicating improved stability and performance nearing practical application. This breakthrough is significant because calcium is abundant and inexpensive compared to lithium, which faces resource limitations and challenges in further energy density improvements. The quasi-solid-state