Amorphous coordination polymers for versatile Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, and Zn²⁺ cation storage

Divalent metal-ion batteries hold immense promise for electrochemical energy storage applications, offering advantages in terms of volumetric capacity, cost-efficiency, sustainability, and safety. Despite advances, the lack of high-voltage and high-performance positive electrode materials remains a critical obstacle. Here, we disclose a family of amorphous coordination polymers capable of reversibly storing Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, and Zn²⁺ cations. For Ca²⁺ and Mg²⁺ systems, the highest reported working potentials of >3.2 V vs. Ca²⁺/Ca and 2.8 V vs. Mg²⁺/Mg are measured, along with fast, stable, and low-hysteresis storage without solvent or ion pair storage. These characteristics stem from the amorphous structure, delocalized anionic charge, and disordered, long bond-distance coordination, enabling weak binding and fast cation diffusion. Using sustainable elements and demonstrating universal divalent cation storage capacity by achieving the first-ever reversible storage of Sr²⁺ and Ba²⁺ ions, this work establishes key design principles for divalent cation storage materials and systems.