A π‐Conjugated Porphyrin Complex as Cathode Material Allows Fast and Stable Energy Storage in Calcium Batteries

Rechargeable calcium batteries (RCB) are prospective candidates for sustainable energy storage, as they hold the promise of the high energy density of lithium-ion batteries (LIBs) while simultaneously combining it with highly abundant raw materials. However, for long time, calcium batteries have faced severe issues with regard to cycling stability, until recently developments demonstrated improved battery cycle life when employing CaSn alloy anodes with fluorinated alkoxyborate electrolytes. These findings opened up the possibility to study cathode materials for RCBs not only in a more comparable manner, but also in a practical full cell design. As representative of emerging organic electrode materials (OEMs), we investigated tetrakis(4-pyridyl) porphyrin as both free ligand (H$_2$TPyP) and in the form of its copper MOF complex (CuTPyP−MOF) as active cathode species in RCBs. The cells demonstrated high capacities and excellent cycling stability at the same time. Even at elevated current densities of e. g., 2000 mA/g the full cells delivered stable capacities of ~90 mAh/g proving its excellent rate capability. This study explores the electrochemical performance of porphyrin active materials in calcium batteries and represents a significant step forward in the progress toward organic electrodes for multivalent energy storage systems.