A small inorganic-organic material based on anthraquinone-decorated cyclophosphazene as cathode for aqueous electrolyte zinc-ion batteries

The small inorganic-organic material (hexakis(2-anthraquinonyloxy)cyclotriphosphazene [THAQ]) is synthesized from the commercially available starting materials in a one-step reaction, and its relevance as an aqueous zinc-ion battery cathode material is assessed. The THAQ structure is verified using appropriate standard spectroscopic methods, such as 31P and 1H nuclear magnetic resonance spectroscopy and matrix-assisted laser desorption/ionization-time-of-flight. The charge storage mechanism and the evolution of the interfacial properties of the THAQ are investigated through several ex situ analyses (Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy) and electrochemical quartz crystal microbalance, indicating both Zn2+ and H+ participation. The insoluble THAQ electrode demonstrates a remarkable electrochemical performance with over 150 mAh/g at 30C, as well as an ultra-long-term cycling (>30,000 cycles) stability at ultra-high current rate (100C). The outstanding electrochemical performance turns out to be governed by the multiple nucleophilic carbonyl active sites and increased π-π interaction of THAQ and its lower band gap compared with the anthraquinone counterparts, verified by density functional theory calculations. Overall, this work is the first report revealing the nature of charge carries of inorganic-organic material system, specifically anthraquinone decorated cyclophosphazene, obtained by a facile and cost-effective method that further demonstrates excellent electrochemical performance.