A cyano cobalt “electron transfer bridge” boosting the two-electron reaction of a MnO2 cathode with long lifespan in aqueous zinc batteries

Abstract

The dissolution/deposition reaction of MnO₂ cathode materials in aqueous zinc batteries provides two-electron transfers and delivers high theoretical capacity. However, this process usually requires an acidic environment and pre-added Mn²⁺ salts in electrolytes. Herein, we present an “electron transfer bridge” at the cathode to enhance the two-electron transfer contribution in MnO₂ in conventional zinc cells. A composite material with MnO₂ and CoO_x connected by Mn–NC–Co interactions was synthesized. Mechanism studies demonstrate that the facile redox center of Co³⁺/Co²⁺ receives/donates electrons during discharge/charge processes, and electrons effectively transport to the Mn centers thanks to the narrowed band gap and charge delocalization. The dolosse-like hollow morphology formed through the inside-out Ostwald ripening process during the discharge further provides abundant reaction sites. These factors largely activate the dissolution/deposition process of MnO₂ material. In aqueous zinc cells with the conventional ZnSO₄ electrolyte, the composite cathode achieves a high capacity of 425 mA h g⁻¹ at 0.1 A g⁻¹, and an ultra-long life of over 25 000 cycles is realized at 2 A g⁻¹. This work presents an effective strategy to promote the high-capacity reaction path of MnO₂ in zinc batteries.