Proton‐Calcium Dual Cation Assisted Redox of Phenazine Molecular Crystals with Coplanar Stacking

ABSTRACT Aqueous calcium ion batteries (ACIBs) are emerging as promising next‐generation sustainable energy storage systems due to the high abundance of calcium and the inherent safety of aqueous electrolytes. However, the development of aqueous CIBs remains challenging, primarily due to sluggish Ca ²⁺ diffusion kinetics and the scarcity of suitable electrode materials. Herein, we report, for the first time, phenazine as a small conjugated aromatic organic anode for reversible electrochemical storage of calcium ions. The phenazine anode exhibits a specific calciation capacity of 130 mAh g ⁻¹ at 0.5 A g ⁻¹ , high‐rate capability (50 mAh g ⁻¹ at 20 A g ⁻¹ ), and good cycling stability over 8000 charge‐discharge cycles. Systematic studies reveal that co‐insertion of protons across phenazine planes is strongly dependent on salt concentration and anion identity of the electrolytes. Further, 3D Bode analysis probes the charge storage dynamics based on synergistic proton‐assisted Ca ²⁺ ion storage in phenazine. As a proof of concept, an all‐organic phenazine || triphenylamine full cell is constructed, which operates at 1.8 V with a typical energy density of 36 Wh kg ⁻¹ at a power density of 455 W kg ⁻¹ , besides exhibiting long‐term cycling stability. By deployment of organic electrodes, the design of next‐generation energy storage devices ensures sustainability over inorganic materials.