Stable Triphenolamine Radical Cathode with High Electron Conductivity for High-Rate Aqueous Zinc-Ion Batteries

Compared with conventional lithium-ion batteries with organic electrolytes, aqueous zinc-ion batteries with low cost, sustainable, and environmentally friendly organic cathodes exhibit promising potential to meet the continuously increasing demand of safe and large-scale energy storage. The representative organic cathodes, including quinoidal polycyclic aromatic hydrocarbons (PAHs) and 2,2,6,6-tetramethylpiperidinyl-<i>N</i>-oxyl (TEMPO) polymers, suffer from challenging synthesis and limited theoretical specific capacity. Herein, different from PAHs and TEMPO polymers, stable 4,4',4″-nitrilotriphenol [TPA-(OH)₃] is explored as the organic radical cathode material by extremely low-cost raw materials and two simple reactions. Further, an open-shell TPA-O₃ radical is obtained by facile oxidation with air. Interestingly, TPA-O₃ showed unexpected higher electron conductivity of 3.35 × 10^-4 S cm^-1 than the precursor [2.73 × 10^-6 S cm^-1 for TPA-(OMe)₃]. The nitro-like nitroxide resonance structures of TPA-O₃ contribute to its high electrochemical stability during the 200-cycle cyclic voltammetry test in air and thus exhibit good reversibility when used as a cathode material. Moreover, TPA-O₃ exhibits a reduction voltage of ∼ 1.0 V vs Zn/Zn²⁺ at 0.5 mV s^-1, which is higher than those of most quinone-based cathodes and comparable to TEMPO-type radical cathodes. It also demonstrates a stable capacity of 123.7 mA h g^-1 and a high-capacity retention of 95.87% after 2000 stable cycles at 5 A g^-1, marking its superiority to those of previously reported organic radical cathodes.