A facile self-saturation process enabling the stable cycling of a small molecule menaquinone cathode in aqueous zinc...

Small quinone molecules are promising cathode materials for aqueous zinc batteries. However, they experience fast capacity decay due to dissolution in electrolytes. Herein, we introduce a simple methyl group to a naphthoquinone (NQ) cathode and demonstrate a facile self-saturation strategy. The methyl group exhibits hydrophobic properties together with light weight and a weak electron-donation effect, which allows a good balance among cycling stability, capacity and voltage for cathode materials. The resulting menadione (Me-NQ) presents around one-third solubility of NQ. The former thus rapidly reaches saturation in the electrolyte during cycling, which suppresses subsequent dissolution. Thanks to this process, the Me-NQ cathode preserves 146 mA h g^-1 capacity after 3500 cycles at 5 A g^-1, far exceeding 88 mA h g^-1 for NQ. Me-NQ also delivers a stabilized capacity of 316 mA h g^-1 at 0.1 A g^-1 with only 0.05 V lower average redox voltage than NQ. The co-storage of Zn²⁺ and H⁺ with the redox reactions on the carbonyl sites of Me-NQ is revealed.