Charge Storage Mechanism of a Quinone Polymer Electrode for Zinc-Ion Batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) offer a potentially stable alternative to alkaline rechargeable zinc batteries in utilizing zinc metals for low-cost and safe energy storage. Two fundamental features of AZIBs are (1) near-neutral aqueous electrolytes, in which zinc dendrites growth is apparently suppressed compared with that in alkaline electrolytes, and (2) Zn2+ -storing cathode materials, which do not store protons in cathodes and maintain the near-neutral pH of an electrolyte throughout charge-discharge processes. Because of the importance of stored species at the cathode for the viability of a battery, the charge storage mechanism in cathode materials has been under scrutinization. Among cathode materials proposed for high-energy AZIBs, many vanadium oxides seem to store zinc-ions, while manganese oxides more often than not involve proton storage which raises concerns of pH variation of the electrolyte. Quinones have been recently proposed as versatile electrode materials for aqueous batteries. Early attempts of quinones for AZIBs have resulted in promising specific energy and cycling stability. It is therefore interesting to investigate the charge storage mechanism in these materials and assess their viability for practical applications.This work focuses on the identification of stored ions in a quinone polymer, poly(benzoquinonyl sulfide) (PBQS), as an AZIB cathode material. Possible ions to be involved in the electrode reaction include zinc ions, protons, counter anions (in this case triflate), and their hydrated forms. Spectroscopic technologies suitable for qualitative and quantitative analyses of the electrode reaction of organic materials have undergone major development in recent years. Electrochemical quartz crystal microbalance (EQCM) is an established tool to quantitatively probe the stored ions in redox-active polymers and carbon materials. Recent advancement with dissipation monitoring (EQCM-D) further strengthened the technique for characterizing softer electrodes. On the other hand, Fourier transform infrared spectroscopy (FTIR) is a standard technique for qualitative detection of chemical groups, thus a useful tool to compliment EQCM-D which may lack chemical information. In-situ FTIR has been proven effective in monitoring the electrode reactions in organic carbonyl electrodes, though the use for detection of stored ions has not been reported. Here we combine EQCM-D and FTIR techniques for a comprehensive understanding of the charge storage mechanism of PBQS in a near-neutral zinc electrolyte.