Stabilized Co3+/Co4+ Redox Pair in In Situ Produced CoSe2−x‐Derived Cobalt Oxides for Alkaline Zn Batteries with 10 000‐Cycle Lifespan and 1.9‐V Voltage Plateau

Abstract In aqueous alkaline Zn batteries (AZBs), the Co 3+ /Co 4+ redox pair offers a higher voltage plateau than its Co 2+ /Co 3+ counterpart. However, related studies are scarce, due to two challenges: the Co 3+ /Co 4+ redox pair is more difficult to activate than Co 2+ /Co 3+ ; once activated, the Co 3+ /Co 4+ redox pair is unstable, owing to the rapid reduction of surplus Co 3+ to Co 2+ . Herein, CoSe 2− x is employed as a cathode material in AZBs. Electrochemical analysis recognizes the principal contributions of the Co 3+ /Co 4+ redox pair to the capacity and voltage plateau. Mechanistic studies reveal that CoSe 2− x initially undergoes a phase transformation to derived Co x O y Se z , which has not been observed in other Zn//cobalt oxide batteries. The Se doping effect is conducive to sustaining abundant and stable Co 3+ species in Co x O y Se z . As a result, the battery achieves a 10 000‐cycle ultralong lifespan with 0.02% cycle −1 capacity decay, a 1.9‐V voltage plateau, and an immense areal specific capacity compared to its low‐valence oxide counterparts. When used in a quasi‐solid‐state electrolyte, as‐assembled AZB delivers 4200 cycles and excellent tailorability, a promising result for wearable applications. The presented effective strategy for obtaining long‐cyclability cathodes via a phase transformation‐induced heteroatom doping effect may promote high‐valence metal species mediation toward highly stable electrodes.