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

Comparing with conventional lithium-ion batteries with organic electrolyte, aqueous zinc-ion batteries with low cost, sustainable and environment-friendly organic cathodes exhibit promising potential to meet the continuously increasing demand on safe and large-scale energy storage. The representative organic cathodes, including quinoidal polycyclic aromatic hydrocarbons (PAHs) and 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) polymers, suffer from the challenging synthesis and limited theoretical specific capacity. Herein, different from PAHs and TEMPO polymers, a stable 4,4',4''-nitrilotriphenol (TPA-(OH)3) is explored as organic radical cathode material by extremely low-cost raw materials and two simple reactions. Furtherly, an open-shell TPA-O3 radical is obtained by a 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 exhibits good reversibility when used as a cathode material. Moreover, TPA-O3 exhibit high reduction voltage of 1.02~1.33 V vs, Zn/Zn2+, stable capacities of 123.7 mAh g-1 and high-capacity retention of 95.87% after 2000 stable cycles at 5 A g−1, which is superior to previously reported organic radical cathodes.