Study of multi-electron redox mechanism via electrochromic behavior in hexaazatrinaphthylene-based polymer as the cathode of lithium–organic batteries

Abstract

The charging and discharging mechanism of lithium–organic batteries has always been a challenging issue due to the lack of popularity of in situ characterization technology during the charge/discharge process. In this article, a novel open lithium battery test combining the in situ electrochemical and UV spectral tests is firstly studied to explore the multi-electron redox mechanism of hexaazatrinaphthylene-based polymers as a cathode of lithium–organic batteries. Two triphenylamine-hexanazonaphthalene structure conjugated microporous polymers (PTPA-HATN and PDTPA-HATN) have been synthesized as models to study the charging and discharging mechanism of hexazanaphthalene. Both possess similar nanosphere morphology and high surface area (∼450 mA h g⁻¹) as well as abundant microporous structure. Using them as cathodes to explore the lithium battery performance, an obvious charge–discharge process with multi-electron redox characteristics can be observed in both, and the one with PTPA-HATN shows a higher discharge capacity (∼168 mA h g⁻¹) and better rate performance than those of PDTPA-HATN in LiClO₄ electrolyte. Combining the in situ electrochemical and UV spectral results, a new multi-electron redox mechanism of hexanazonaphthalene-based polymer cathodes in lithium–organic batteries is proposed. Different from the previously reported mechanisms, the N and N lone electron pairs of the hexanazonaphthalene group in the polymer film will adsorb Li⁺ to form the N–Li–N bond in the electrolytes with low HF acid content accompanied with the redshift of absorption peak in the spectra under the open circuit voltage. During the discharge process, the N–Li–N bond will break and combine with the newly entered Li⁺ to form the N₂–Li₂ bond, resulting in a new large blueshift in the spectra. This work may provide new insight into the redox mechanism of functional groups of organic cathode materials via electrochromic behavior on the performance of lithium batteries.