High‐Performance Quasi‐Solid‐State Calcium‐Ion Batteries from Redox‐Active Covalent Organic Framework Electrolytes

Calcium ion batteries (CIBs) are promising for energy storage with volumetric capacity and reduction potential comparable to lithium, while richer in earth abundance. However, sluggish cation transport and unstable cycling performance primarily due to anode surface passivation remain vital challenges for realizing high-performance CIBs. Herein, two kinds of redox covalent organic frameworks (PT-COFs and PQ-COFs) with different-density carbonyl groups are prepared as quasi-solid-state electrolytes (QSSEs) to address those challenges. In particular, PT-COFs exhibit ionic conductivity of 0.46 and 5.05 mS cm^-1 at room temperature and 80 °C, respectively, and Ca²⁺ transference number of 0.532. Due to the efficient ionic conduction and intrinsic stability of PT-COFs structure, the prepared full calcium ion cell with PT-COFs demonstrates the highest reversible specific capacity of 155.9 mAh g^-1 at 0.15 A g^-1 (1 C), and stable cycle performance (capacity retention over 74.6% at 1 A g^-1 after 1000 cycles). This work shows the effectiveness of the redox COFs and their promising potential as SSE for the development of high-performance CIBs.