Unveiling Phenoxazine's Unique Reversible Two-Electron Transfer Process and Stable Redox Intermediates for High-Performance Aqueous Zinc-ion Batteries

The low specific capacity determined by the limited electron transfer of p-type cathode materials is the main obstruction to their application towards high-performance aqueous zinc-ion batteries (ZIBs). To overcome this challenge, boosting multi-electron transfer is essential for improving the charge storage capacity. Here, as a typical heteroaromatic p-type material, we unveil the unique reversible two-electron redox properties of phenoxazine in the aqueous electrolytes for the first time. The second oxidation process is stabilized in the aqueous electrolytes, a notable contrast to its less reversibility in the non-aqueous electrolytes. A comprehensive investigation of the redox chemistry mechanism demonstrates remarkably stable redox intermediates, including a stable cation radical PNO⋅⁺ characterized by effective electron delocalization and a closed-shell state dication PNO²⁺. Meanwhile, the heightened aromaticity contributes to superior structural stability during the redox process, distinguishing it from phenazine, which features a non-equivalent hybridized sp²-N motif. Leveraging these synergistic advantages, the PNO electrodes deliver a high capacity of 215 mAh g^-1 compared to other p-type materials, and impressive long cycling stability with 100 % capacity retention over 3500 cycles. This work marks a crucial step forward in advanced organic electrodes based on multi-electron transfer phenoxazine moieties for high-performance aqueous ZIBs.