Air Stable Organic Metal Reservoirs: Extended Conjugation Enabled Multi-Electron Redox-Based Cathode Crystals for Metal Ion Batteries

Air-stable organic crystal-based metal ion batteries (MIBs) are important for cost-effective renewable energy storage. In the dearth of such crystalline ‘organic metal reservoirs’, a family of porphyrin-fused extended conjugated sulfonamide compounds (ECSA) has been demonstrated here for the MIBs of lithium (LIBs). The ECSA forms a class of metalloporphyrins (M4ZnSOTAPP) having different metal ion possibilities (M = Li+/Na+/K+) with four redox meso positions. The discharge voltage of Li4ZnSOTAPP crystal-based LIB is found to be 2.5 V vs Li/Li+, in line with the theoretical calculations, having a capacity of ~100 mAhg-1 for a cell constructed in practically relevant conditions having 1M electrolyte. Further to the demonstration of stable performance of Li4ZnSOTAPP in LIB (83 % capacity retention till 500 cycles), its crystal structure and the charge-discharge mechanism have been unraveled. The extension of this synthetic strategy to K4ZnSOTAPP crystals further establishes the avenue for π-extended structures having innate conductivity and high metal ion capacity as the next-generation battery cathodes for organic batteries.